Formation of cellulose hybrid films from an alkaline solution

Formation of cellulose hybrid films from an alkaline solution Wouter Vanreyten

Promotors: prof. dr. Gustaaf Schoukens, prof. Pertti Nousiainen Coach: Pieter Samyn, Marianna Vehviläinen, Taina Kamppuri Masters dissertation submitted for the purpose of obtaining the academic degree of Master of Textile Engineering

Department of Textiles Head of department: Prof. dr. Paul Kiekens Faculty of Engineering Academic year 2009-2010

Association of Universities for Textiles

Preface I would like to take this opportunity to thank Prof. Pertti Nousiainen for giving me the possibility of doing my thesis at Tampere University of Technology, Taina Kamppuri and Marianna Vehvil¨ ainen for guidance, support and interest in my work. I also would like to thank Arja Puolakka for helping me with the measurements of the films. Thanks to my parents, who supported my throughout my studies and allowed me to pursue my dreams. Special thanks to Hannele, for her love and support.

Copyright: The author gives admission to make this Master’s thesis available for consultation and to copy parts of the Master’s thesis for personal use. Any other use falls under the limitations of the copyright, especially with regard to the obligation of mentioning the source explicitly on quoting the results of this Master’s thesis.

Author: Wouter Vanreyten May 28, 2010

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Formation of Cellulose Hybrid Films From Alkaline Solution by Wouter Vanreyten Thesis to obtain the degree of Master of science in Textile Engineering Academic year 2009–2010 Promotors: prof. dr. G. Schoukens, prof. dr. P. Nousiainen Supervisors: ir. P. Samyn Faculty of Engineering Ghent University Department of Textiles Head: Prof. Dr. Ir. P. Kiekens

Summary In this work, cellulose hybrid films are prepared from an alkaline solution of enzyme treated cellulose. The used additives are casein, carbonate salts, TiO2 /AgCl particles and Multiwalled Carbon Nanotubes (MWNT). Cellulose/casein films show a uneven distribution of casein. Cellulose porous structure were prepared with CaCO3 as a foaming agent. With increasing amount of CaCO3, increased foaming occurs. Also temperature and salt content of the coagulation bath promote pore formation. Trials to create antibacterial properties with TiO2/AgCl particles were not successfull. This is because of the reaction of AgCl with NaOH to Ag2 O. Cellulose/carbon nanotubes films were succesfully prepared. Diluted cellulose solution of 1wt% can disperse 5 wtc% MWNTs. Cellulose films with 0.5 and 1 wtc% MWNT were prepared. The films were investigated under SEM and showed a homogeneous distribution of the MWNTs.

Keywords Carbon nanotubes, carbonate salt, casein, cellulose, sponge, TiO2 /AgCl

Nederlandstalige samenvatting Inleiding Deze thesis werd geschreven aan de Technische Universiteit van Tampere (TUT, Finland) als een deel van het E-team opleidingsprogramma georganiseerd door de vakgroep textielkunde van de Universiteit Gent. Het meest gebruikte proces voor de productie van rayonvezels is nog steeds het viscose proces . Dit proces is zeer oud, complex en milieuonvriendelijk: het proces gebruikt koolstofdisulfide (CS2 ) en heeft als bijproduct waterstofsulfide (H2 S). Onderzoek naar andere meer milieuvriendelijke processen leverde processen als het Lyocell- en cuppramoniumproces op. Alhoewel deze processen milieuvriendelijker zijn, zijn ze nog steeds erg complex. Aan TUT ontwikkelde men een nieuw en eenvoudiger proces. In dit proces ondergaat cellulose eerst een enzymatische behandeling. Deze behandeling maakt cellulose direct oplosbaar in verdund natriumhydroxide (zie figuur 1 (b)). Het grootste nadeel van dit proces is de gevoeligheid van de cellulose oplossing voor temperatuur en additieven. De oplossing vormt heel snel een gel. Figuur 1 (a) toont het fase diagram van cellulose en natriumhydroxide, hieruit kan men afleiden dat cellulose met een lage polymerisatiegraad direct oplosbaar is tussen -4 en 10◦ C en een concentratie van NaOH tussen 6 en 10 wt%. In deze thesis gebruik ik enzymatische behandelde cellulose als startmateriaal. Het doel van deze thesis is om na te gaan of producten met een toegevoegde waarde kunnen worden geproduceerd met een enzymatisch behandelde cellulose oplossing. Hiervoor bestudeerde ik verschillende additieven: chitosan, carbonaat zouten, case¨ıne, titanium deeltjes met een zilverchloride coating en koolstof nanobuizen. De thesis bestaat uit twee delen. Het eerste deel is de literatuurstudie, waar het viscose proces, het enzymatisch behandelen van cellulose en de verschillende additieven worden besproken. Nadruk wordt gelegd op de toepassingen, het gedrag van de additieven in natriumhydroxide en eventueel gevormde cellulose producten. In het experimenteel gedeelte is het gedrag van de additieven in de cellulose oplossing besproken. Ook bestudeerde ik wat de optimale parameters zijn voor het coagulatiebad.

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Figuur 1: Fase diagram van cellulose-NaOH-water (a) en schematische weergave van het proces voor enzymatische behandeling van cellulose

Literatuurstudie Additieven Chitosan Chitosan is een gedeacetyleerde vorm van chitine. Chitine is, na cellulose, het meest voorkomend natuurlijk polymeer in de wereld. Men treft het in de natuur aan als een component in de celwanden van schimmels en in het exoskelet van geleedpotigen. Figuur 2 (a) toont de chemische structuur van chitosan. De belangrijkste eigenschappen van chitine en chitosan zijn de positieve elektrische lading, de mogelijkheid om met metalen te binden, de biologische afbreekbaarheid en de antimicrobe activiteit. Een mix van cellulose en chitosan vertoont superieure eigenschappen ten opzichte van cellulose, zoals beter mechanische treksterkte en verfbaarheid. In tegenstelling tot de meeste polysacharides, is chitosan een basisch polysacharide. Daardoor is het alleen oplosbaar in een zure omgeving en niet in een basisch milieu Doordat chitosan enkel oplosbaar is in een zuur milieu en cellulose enkel in een basisch milieu, is het ´ en mogelijkheid is het gebruik van een gemeenmoeilijk om een mix van beide te maken. E´ schappelijk solvent zoals trifluorazijnzuur. Een andere manier is het chemisch wijzigen van chitosan met CS2 tot chitosan xanthaat (figuur 2 (b) toont de chemische structuur). Chitosan xanthaat is oplosbaar in een basische milieu en kan worden gemengd met een viscose oplossing. Het gebruik van CS2 is echter niet geëlimineerd en dus is het niet milieuvriendelijk. Een milieuvriendelijkere manier is het gebruik van polyelectroliet complexen (PEC). Chitosan kan een polyelectroliet complex vormen met natriumalginaat. Het resulterende deeltje heeft een kern van chitosan en een schil van natrium alginaat en is oplosbaar in een basische omgeving. Een nieuwe en veelbelovende vorm van chitosan is micro kristallijn chitosan (MCCh). Dit

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Figuur 2: De chemische structuur van chitosan (a) en chitosan xanthaat (b)

is chitosan dat bestaat uit een heel hoge fractie kristallijn materiaal. MCCh polyelectroliet complexen met natrium alginaat werden gemengd met een basische cellulose oplossing. Vezels gevormd van cellulose (viscose) en MCCh vertonen een licht verminderde treksterkte, maar een verhoogde water retentiewaarde en verlenging in vergelijking met standaard rayon vezels. Natriumcarbonaat Natriumcarbonaat is een carbonaatzout en kan gebruikt worden als een additief voor een cellulose oplossing. Het vormt koolstofdioxide als het reageert met zwavelzuur. Dankzij de vorming van CO2 gas kan het gebruikt worden om holle vezels en sponzen te maken. Cellulose sponzen worden gebruikt als biologisch afbreekbare inplantaten. Natriumcarbonaat (Na2 CO3 ) en natriumbicarbonaat (NaHCO3 ) zijn oplosbaar in water. Na2 CO3 is zeer basisch terwijl NaHCO3 amfoterisch is. Door het amfoterische karakter zal NaHCO3 reageren met NaOH en Na2 CO3 vormen (zie reactie 1). Door deze reactie verdwijnt NaOH uit de oplossing en vermindert de oplosbaarheid van cellulose. Hierdoor kan het niet worden gebruikt als een additief voor de basische cellulose oplossing. Natrium carbonaat is stabiel in een natrium hydroxide oplossing, wanneer het in contact komt met zwavelzuur zal het wegreageren en koolstofdioxide vormen (zie reactie 2). NaHCO3 + NaOH −−→ Na2 CO3 ↓

(1)

Na2 CO3 + H2 SO4 −−→ Na2 SO4 + H2 O + CO2 ↑

(2)

Carbonaat zouten worden gebruikt als een schuimingsadditief voor de productie van holle en opgeblazen viscose producten. Uit literatuur blijkt dat elk type viscose oplossing kan worden gebruikt, zolang de cellulose concentratie tussen 6 en 12% en de hoeveelheid NaOH tussen 4 en 10% ligt. Elk type carbonaat zout kan worden gebruikt zolang het de eigenschappen van de cellulose oplossing niet verandert (daarom kan natriumbicarbonaat niet worden gebuikt), hoewel een onoplosbaar carbonaatzout de voorkeur heeft. De optimale hoeveelheid carbonaatzout concentratie ligt tussen 10 en 500 wtc%. Een te kleine hoeveelheid geeft onvoldoende zwelling, een te hoge concentratie zorgt voor slechtere mechanische eigenschappen.

vii Wanneer de cellulose oplossing in het coagulatiebad komt, vormt er zich een dun laagje cellulose aan de buitenkant. Dit laagje dient als barrière voor het zuur. Het zuur gaat door middel van diffusie door dit laagje en reageert met het cellulose xanthaat en het carbonaat zout. De reactie van het zuur met het zout vormt CO2 -gasbellen. Het is belangrijk dat regeneratie en gasvorming gelijktijdig gebeuren. Indien de gasvorming gebeurt na de regeneratie van cellulose, kunnen er geen porieën meer worden gevormd. Indien de gasvorming te vroeg gebeurt, dan kunnen de gasbellen ontsnappen uit de cellulose oplossing en worden er ook geen porieën gevormd. Hierdoor verschilt het coagulatiebad voor de productie van holle cellulose vezels en sponzen van dat van het standaard viscose proces. Algemeen wordt aangenomen dat het coagulatiebad een hogere temperatuur, een hogere concentratie van zuur en een hogere hoeveelheid zout nodig heeft in vergelijking met het standaard viscose proces. Case¨ıne Case¨ıne is de belangrijkste prote¨ıne in melk. Films van een mix van cellulose en case¨ıne hebben een betere mechanische treksterkte dan standaard cellulose films. Vezels bestaande uit cellulose en case¨ıne hebben eigenschappen die vergelijkbaar zijn met deze van wol. Case¨ıne is, net als alle prote¨ınen, opgebouwd uit aminozuren die met elkaar verbonden zijn door een peptidebinding. Prote¨ınen organiseren zich in secondaire, tertiaire en quaternaire structuren . De meest voorkomende secondaire structuren zijn de α-helix en de β-plaat. Case¨ıne vertoont geen tertiaire structuur. Prote¨ınen vertonen een zuur en basisch gedrag. Afhankelijk van de pH van de omgeving zal hun nettolading veranderen. Wanneer de pH gelijk is aan de iso-elektrische pH (pI) is de nettolading nul. In basische omgeving protonizeren de prote¨ınen zodat er een netto negatieve lading is. Deze negatieve lading verhoogt de afstoting tussen de verschillende amino zuren en het prote¨ıne ontvouwt tot een random formatie. Daarom zijn ze oplosbaar in een basisch milieu. Wanneer de pH gelijk is aan de pI is er geen afstoting meer tussen de verschillende amino zuren en de prote¨ıne vouwt toe. In een lagere pH zal de prote¨ıne gedeeltelijk geprotoneerd zijn. Hierdoor ontstaat een toestand tussen compleet gevouwen en random formatie. Onderzoek is verricht naar de vorming van cellulose-case¨ıne films in het NaOH/urea solvent. Het coagulatiebad was 5wt% zwavelzuur. Cellulose was mengbaar met case¨ıne tot een case¨ıneconcentratie van 15%. De mengbaarheid wordt verklaard door waterstofbruggen tussen de hydroxyl groep van cellulose en de peptide binding van casëine. De sterkte van het membraan stijgt omdat casëine de vorming en schikking van cellulose kristallen promoot. Titaniumoxide deeltjes gecoat met zilverchloride ´ en mogelijkheid om antibacteriële eigenschappen in polymeerfilms te bekomen is door E´ gebruik te maken van zilver. De antibacteriële eigenschappen van zilver zijn al heel lang bekend. Recentelijk heeft zilver weer aan populariteit gewonnen in de medische wereld

viii aangezien bacteriën immuun worden tegen antibiotica. Immuniteit tegen zilver is nog bijna niet waargenomen. Zilver wordt ook gebruikt in lucht- en waterzuivering. De antibacteriële deeltjes gebruikt in dit onderzoek zijn titaniumoxide (TiO2 ) deeltjes met een coating van zilverchloride (AgCl) (zie figuur 3 (a) en 3 (b)). De deeltjes hebben een grootte tussen 1 en 15 µm en bestaan uit 80% TiO2 en 20% AgCl. TiO2 is een ideale drager voor AgCl aangezien de TiO2 deeltjes niet oplosbaar zijn in water en een groot oppervlak hebben. AgCl is ook niet oplosbaar in water, hierdoor blijft AgCl sterk gebonden aan het TiO2 deeltje. AgCl komt slechts zeer traag los van het TiO2 -oppervlak. Het loskomen van de Ag+ ionen geeft de antibacteriële eigenschappen, de trage snelheid zorgt voor de duurzaamheid. De composietdeeltjes kunnen in een stabiele dispersie worden gebracht indien de viscositeit van het medium groter is dan 0.0025NSm−1 .

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Figuur 3: Schematisch model (a) en SEM afbeelding (b) van de TiO2 /AgCl composiet deeltjes

Er is amper literatuur verschenen over het gebruik van TiO2 /AgCl deeltjes als een additief voor cellulose. Een patent uit 1997 beschrijft de productie van antibacteriële vezels met het Lyocell proces en de TiO2 /AgCl deeltjes. De vezels vertonen goede antibacteriële eigenschappen, zelfs wanneer de concentratie van TiO2 /AgCl zeer laag is. Een concentratie van 0.0125wtc% is voldoende voor duurzame antibacteriële eigenschappen. De mechanische eigenschappen veranderen niet door toevoeging van de deeltjes. Koolstof nanobuizen Polymeer composieten met koolstof nanobuizen (CNT) hebben superieure mechanische en elektrische eigenschappen in vergelijking met het standaard materiaal. De eerste observaties van CNT’s zijn gedaan in 1991 door Iima. CNT’s zijn allotropen van koolstof die bestaan uit een cilindrische nanostructuur met enkel koolstofatomen die verbonden zijn met sp2 bindingen. De twee meest voorkomende types van CNT’s zijn enkelwandige (SWNT) (figuur 4 (a)) en meerwandige (MWNT) (figuur 4 (b)) koolstofbuizen. CNT’s hebben buitengewone eigenschappen. De elastisiteitsmodulus van CNT’s is ongeveer 1 TPa (dit is 5 keer hoger dan die van staal). De maximale treksterkte is 30 GPa (100 maal hoger dan staal). De elektrische resistiviteit is ongeveer 10−4 Ωcm. CNTs zijn uitermate stabiel bij hoge temperaturen (tot

ix 750◦ C in atmosferische omgeving).

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Figuur 4: Schematisch model van enkelwandige koolstof nanobuizen (a) en meerwandige koolstof nanobuizen (b)

Het gebruik van koolstof nanobuizen in composieten stuit op een aantal moeilijkheden. CNT’s zijn, net als polymeren, allemaal verschillend: ze bestaan in verschillende chiraliteiten, diameters en lengtes. Defecten, onzuiverheden en aggregatie van CNT’s verminderen de mechanische eigenschappen. Ook zijn CNT’s, net omdat ze zo klein zijn, vaak gekruld en gedraaid. Daardoor gebruiken CNT’s slechts een fractie van hun mogelijkheden voor het versterken van een materiaal. Om de eigenschappen van het composietmateriaal te optimaliseren moeten CNT’s een sterke affiniteit vertonen met de polymeermatrix. Cellulose/CNT composiet materialen zijn vervaardigd met het Lyocell- en viscose proces en met ethylmethylimidazolium acetaat (EMIAc). De viscose oplossing is een goed medium voor de dispersie van CNT’s. Dit is omdat cellulose xanthaat een negatieve lading draagt en dus vrije elektronen heeft. CNT koolstofatomen daarentegen hebben een p-orbitaal die niet deelneemt aan een hybridisatie. De vrije elektronen van cellulose xanthaat kunnen binden met dit orbitaal. Cellulose xanthaat zal zich rond de CNT wikkelen (zie figuur 5). De dispersie is stabiel en de CNT’s worden niet vernietigd tijdens coagulatie in zwavelzuur.

Figuur 5: Schematisch model van het wikkelen van cellulos xanthaat rond koolstof nanobuizen

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Experimenten Materialen en methodes Cellulose oplossing Domsjö Fabriker Ab (Zweden) leverde hoog kwalitatieve cellulose pulp bestaande uit een mengsel van den en spar. De pulp werd mechanisch versnipperd gedurende 5 uur met een Baker Perkins Mixer. Nadien werd de pulp enzymatisch behandeld gedurende 3 uur met een commercieel enzym. De enzymatisch behandelde pulp werd opgelost in een waterige oplossing van natrium hydroxide en zink oxide. Tabel 1 toont de eigenschappen van de cellulose oplossing. Tabel 1: Samenstelling van de cellulose oplossing

parameters van de cellulose oplossing Cellulose, %

NaOH, %

ZnO, %

6

6.5

1.3

Filmformatie De films zijn gemaakt met de glasplaat methode. Hierbij wordt de cellulose oplossing tussen 2 glazen plaatjes geperst. Nadien worden de plaatjes over elkaar geschoven zodat er een dun laagje oplossing op beide plaatjes achterblijft. De plaatjes worden geplaatst in een coagulatiebad, nadat de films gevormd zijn worden ze gewassen en gedroogd onder spanning. Het coagulatiebad van de referentiefilms was een 10wt% waterige oplossing van H2 SO4 en de coagulatietijd was 5 minuten. Case¨ıne Case¨ıne van industriële kwaliteit kwam van de reserves van TUT. Case¨ıne werd opgelost in een 6.5 wt% waterige oplossing van NaOH om een concentratie van 100g/l te bekomen. De case¨ıne oplossing werd gemengd met de cellulose oplossing met behulp van een Ika RW20 Mixer (Ika, Duitsland) tot het mengesel homogeen was. Het mengsel rustte een nacht in een ijskast waarna films gemaakt werden. Films met een cellulose:case¨ıne verhouding van 100:0, 95:5, 90:10, 85:15 en 80:20 werden gemaakt. Het coagulatiebad was een 5wt% waterige oplossing van H2 SO4 en de coagulatietijd was 5 minuten. Cellulose sponzen De invloed van de verschillende parameters op de vorming van cellulose sponzen werd bestudeerd. Om de invloed van het type carbonaatzout op de stabiliteit van de cellulose oplossing te kennen, werden het oplosbare natriumcarbonaat en het niet oplosbare calciumcarbonaat gebruikt. Natriumcarbonaat werd eerst opgelost in een 6.5 wt% waterige oplossing van NaOH in verschillende hoeveelheden: 5, 10 en 15g/100ml. Nadien werd deze

xi oplossing al roerend toegevoegd aan de cellulose oplossing en er werd nagegaan wanneer er gelvorming plaatsvindt. Calciumcarbonaat poeder werd direct toegevoegd aan de cellulose oplossing. De invloed van het zuur in het coagulatiebad werd bestudeerd. H2 SO4 en HCl werden gebruikt als coagulanten. Films werden gemaakt in een 10wt% waterige oplossing van het zuur. Ook de invloed van de hoeveelheid carbonaatzout, de temperatuur en de hoeveelheid zout in het coagulatiebad werden bestudeerd. Hiervoor werden steeds 3 parameters constant gehouden en 1 variabel. Tabel 2 toont de parameters van elk experiment. De coagulatietijd was steeds 5 minuten, behalve voor de experimenten waar de invloed van de temperatuur werd bestudeerd. Daarbij was de coagulatietijd de tijd die de films nodige hadden om transparant te worden. Tabel 2: Parameters voor de experimenten met cellulose sponzen

Bestudeerde parameter CaCO3 Temperatuur NaCl

parameters van de cellulose oplossing [CaCO3 ] (wtc%) 0-100-200-300 100 100

[HCl] (wt%) 10 10 10

[NaCl] (wt%) 0 0 0-10-15-20

T(◦ C) 50 22-40-60-70 50

TiO2 /AgCl composiet deeltjes Addmaster (Verenigd Koninkrijk) leverde de TiO2 deeltjes met een coating van AgCl, deze deeltjes zijn commercieel beschikbaar onder de naam Biomaster. Om de stabiliteit van de deeltjes in NaOH te bestuderen, werd 1g Biomaster opgelost in 20ml waterige oplossing van NaOH. Nadien werd aan deze oplossing een grote hoeveelheid waterige oplossing van zwavelzuur toegevoegd. Cellulose films met 2 wtc% deeltjes werden gemaakt door 0.0032g TiO2 /AgCl deeltjes op te lossen in 3ml 6.5 wt% waterige oplossing van NaOH. De oplossing werd gemengd met 30g of een 6wt% cellulose oplossing. Coagulatie gebeurde voor 30 minuten in een 10wt% H2 SO4 oplossing. Koolstof nanobuizen Nanocyl (België) leverde meerwandige koolstof nanobuizen (MWNT) van industriële kwaliteit. De MWNT’s hadden een zuiverheid van 90%, de overige 10% zijn metaal oxides. De MWNT’s werden gemixt met een cellulose oplossing door ultrasoon geluid (Hielscher UP200S dispenser (Hielscher, Duitsland)). Om de stabiliteit van de dispersie te testen, werd een zeer geringe hoeveelheid (mespunt) MWNT’s gemixt in water en in een 1, 2 en 6 wt% cellulose oplossing. Er werd nagegaan hoeveel MWNTs een 1wt% cellulose oplossing in dispersie kan houden. Hiervoor werden dispersies met een cellulose:MWNT verhouding van 20:1, 15:1, 10:1 en 5:1 gemaakt. De dispersies werden onderzocht met behulp van een optische microscoop.

xii Cellulose oplossingen met 0.5, 1 en 2 wtc% MWNTs werden gemaakt door eerst een dispersie te maken van een 1 wt% cellulose oplossing met 5 wtc% MWNTs en deze daarna toe te voegen aan een 6 wt% cellulose oplossing tot de gewenste hoeveelheid MWNT werd bekomen. Films werden gemaakt door middel van de glasplaat methode, het coagulatiebad bestond uit een 10 wt% waterige oplossing van H2 SO4 en de coagulatietijd was 5 minuten. Testen van de films Optische microscoop De films werden bestudeerd met behulp van een optische microscoop ((Leitz Laborlux D)) Fourier Transform Infraroodspectroscopie (FTIR) Het FTIR spectrum van de films met case¨ıne werd genomen (PerkinElmer, VSA) nadat ze werden gedroogd onder spanning. Het spectrum had een golflengte van 4000 tot 650 cm-1 . Om case¨ıne poeder te onderzoeken, werden tablets gemaakt van 1 mg case¨ıne en 200 mg kaliumbromide. Kaliumbromide werd gebruikt omdat het transparant is voor infrarood straling. Zure kleurstoffen De cellulose/case¨ıne films werden geverfd met een rode (rood 266) zure kleurstof. De cellulose films met een case¨ıne fractie van 0, 10, 15 en 20 % werden eerst een uur in een waterbad geplaatst zodat ze voldoende nat zouden zijn. Nadien werd elke film in 200 ml van een 3 wt% waterige oplossing van 3 wt% azijnzuur met 0.5 g zure verfstof geplaatst. De verfbaden werden verhit gedurende 30 minuten. Na het verven werden de films gewassen met water en detergent om niet-bindende kleurstof te verwijderen. De films werden visueel onderzocht. Scanning electron microscope (SEM) SEM-afbeeldingen van de cellulose/MWNT hybride films werden genomen met een Philips XL30 microscoop (Philips, Nederland). Het gebruikte voltage was 15 kV. Alvorens de SEMafbeeldingen werden genomen, werd een laag goud op de samples gesputterd.

Resultaten en bespreking Case¨ıne Case¨ıne werd opgelost in een concentratie van 100 en 200g/l in een 6.5wt% waterige oplossing van NaOH. De 100g/l oplossing had een oranje kleur en was vloeibaar. De 200 g/l oplossing vormde een schuim die niet mixbaar was met een cellulose oplossing. De 100g/l case¨ıne was goed mengbaar met cellulose. Het mengsel had een witte kleur, dit kan wijzen op een gedeeltelijke coagulatie van cellulose. Tijdens de coagulatie in verdund zwavelzuur verkleurde het coagulatiebad niet en werden geen particles waargenomen. Wanneer een kleine hoeveelheid van een 100g/l case¨ıne werd toegevoegd aan een 5 wt% waterige oplossing van H2 SO4 kleurde de oplossing melkwit. Dit is omwille van de lage oplosbaarheid van case¨ıne in een zuur milieu. De afwezigheid van deze witte kleur in het

xiii coagulatiebad tijdens het maken van cellulose-case¨ıne hybride films wijst erop dat case¨ıne tijdens coagulatie in de films blijft. Fourier transform infrarood spectra werden genomen van de films en van case¨ıne poeder (zie figuur 6 (a)). Case¨ıne heeft absorptiepieken op een golflengte van 1650 en 1550 cm-1 , deze zijn toegewezen aan de amide I en amide II trillingen van de peptide binding. Deze pieken zijn niet aanwezig in de spectra van de films. Dit betekent dat er geen case¨ıne is gedetecteerd in de films. Dit betekent echter niet dat case¨ıne niet aanwezig is, het niet detecteren kan zijn omwille van een niet homogene distributie van case¨ıne. Om case¨ıne te detecteren werd zure kleurstof (rood 266) gebruikt. Zure kleurstoffen kunnen gebruikt worden om prote¨ınes te verven aangezien ze ionisch kunnen binden met de amine groep van de peptide binding. Na het verven werden de films uitvoerig gewassen met water. De cellulose films tonen geen kleur meer na het wassen met water. De films met case¨ıne blijven gekleurd (zie figuur 6 (b)). Nadien werden de films gewassen op 75◦ C met een commercieel beschikbaar detergent. De cellulose:case¨ıne films met een case¨ıne fractie van 15% werden bestudeerd met behulp van een microscoop. Hieruit blijkt dat het merendeel van de verf afgewassen is en dat de resterende verf niet homogeen verdeeld is. Dit wijst erop dat er case¨ıne in de films is, maar dat de distributie ervan niet homogeen is. De niet homogene distributie is verklaart door een verschillend coagulatiemoment van cellulose (pH 10-11) en case¨ıne (pH 4-6). Hierdoor zal cellulose voor case¨ıne coaguleren en resulteren in een niet homogene film.

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Figuur 6: FTIR spectra van case¨ıne, cellulose/case¨ıne en cellulose films (a) en foto’s van de films na kleuring en wassen met water (b), cellulose:case¨ıne verhouding van 100:0 (i), 90:10 (ii), 85:15 (iii) en 80:20 (iv)

Cellulose sponzen De invloed van het type carbonaatzout op de stabiliteit van de cellulose oplossing werd bestudeerd. Vooraleer Na2 CO3 toe te voegen aan de cellulose oplossing, werd NaCO3

xiv opgelost in een waterige oplossing van 6.5 wt% natriumhydroxide in een hoeveelheid van 5, 10 en 15g NaCO3 /100ml. Als een 5g/100ml Na2 CO3 oplossing wordt toegevoegd aan de cellulose oplossing, dan verdunt de cellulose oplossing en konden geen sponzen gemaakt worden omdat de viscositeit van de oplossing te laag is. Een 10 g/100ml Na2 CO3 oplossing zorgt voor een lichte gelvorming wanneer er tweemaal zoveel Na2 CO3 in de oplossing is dan cellulose. Wanneer een 15 g/100ml oplossing gebruikt wordt als additief, dan is er al gelvorming wanneer er evenveel cellulose als Na2 CO3 in de oplossing is. De gelvorming is te verklaren door de hydratatie van Na+ ionen. Indien Na2 CO3 toegevoegd wordt aan een cellulose oplossing, dan verhoogt het aantal Na+ ionen. Dit is omdat Na2 CO3 oplosbaar is. Een hoger aantal Na+ ionen zorgt voor een lagere hydratatie van de Na+ ionen. Voor een goede oplosbaarheid van cellulose in natriumhydroxide is het echter belangrijk dat er voldoende hydratatie is van de Na+ -ionen. Dit is verklaard in sectie ?? van deze thesis. Calciumcarbonaat (CaCO3 ) kan in elke concentratie aan de cellulose oplossing worden toegevoegd. Dit komt omdat CaCO3 een niet oplosbaar zout is en dus de hydratatie van de natriumionen in de cellulose oplossing niet verandert. Ik bestudeerde ook de invloed van het type zuur in het coagulatiebad. CaCO3 werd gebruikt als carbonaatzout en zwavelzuur en zoutzuur (HCl) als coagulant. Het zuur reageert met CaCO3 en vormt koolstofdioxide, water en een zout. Wanneer H2 SO4 wordt gebruikt als coagulant dan wordt het onoplosbare CaSO4 gevormd. CaSO4 blijft zelfs na wassen in de cellulose sponzen waardoor ze bros en plaasterig worden. Wanneer HCl het coagulant is, dan wordt het oplosbare CaCl2 gevormd. CaCl2 kan uit de films worden gewassen. De invloed van hoeveelheid CaCO3 werd bestudeerd. Cellulose films werden gemaakt met 100, 200 en 300 wtc% CaCO3 . Figuur 7 (A) toont microscoopafbeeldingen van de films. Een grotere hoeveelheid CaCO3 creëert meer poriën. De invloed van een zout in het coagulatiebad werd bestudeerd. De concentratie van NaCl in het coagulatiebad was 10, 15 en 20 wt%. Figuur 7 (B) toont de films. Een hoger zoutgehalte zorgt voor een structuur met meer poriën. Dit komt omdat zout in het coagulatiebad de coagulatie afremt, hierdoor gebeurt de coagulatie en gasvorming gelijktijdig.

xv

(A)

(B)

Figuur 7: Microscoop afbeeldingen van cellulose films met (A): 0, 100, 200 en 300 wtc% CaCO3 en films met 100 wtc% CaCO3 met (B): 0, 10, 15 en 20 wt% NaCl

De invloed van de temperatuur van het coagulatiebad op de filmformatie werd bestudeerd. De temperaturen van het coagulatiebad waren 22-40-60 en 70◦ C. De coagulatietijd werd vastgelegd op het tijdstip dat de films transparant werden. Tabel 3 toon de gemiddelde coagulatietijd voor de verschillende temperaturen en figuur 8 toont de microscopische afbeeldingen van de cellulose films. Een hogere temperatuur resulteert in een kortere coagulatietijd, dit is omwille van de hogere reactiekinetiek. De films die werden gemaakt in een coagulatiebad op 70◦ C waren niet sterk genoeg om gedroogd te worden onder spanning. Dit is te verklaren door hydrolyse van cellulose door HCl op hoge temperaturen. Wanneer de temperatuur hoger is lijkt het dat de poriën groter zijn. Tabel 3: Gemiddelde coagulatietijd voor cellulose sponzen

temperatuur (◦ C) 22 40 60 70

(a)

coagulatietijd 7 6 3 1

(b)

min 30 s min min 30s min 45s

(c)

Figuur 8: Microscopische afbeeldingen van cellulose films met 100 wtc% CaCO3 . De coagulatietemperatuur was 22 (a), 40 (b) en 60 ◦ C (c)

xvi Pogingen werden ondernomen om dikkere sponzen te creëeren. De invloed van de concentratie cellulose in de oplossing werd bestudeerd. Sponzen werden gemaakt met 2, 3 en 6 wt% cellulose en 200 wtc% CaCO3 . De 2 wt% cellulose oplossing was te vloeibaar en ik kon geen sponzen maken. Een spons gemaakt met een 3 wt% cellulose oplossing was dikker dan een spons gemaakt met een 6 wt% cellulose oplossing maar de mechanische eigenschappen waren veel slechter en de spons brak zeer snel. Cellulose sponzen krimpen enorm tijdens het drogen. Om dit tegen te gaan probeerde ik een solventuitwisseling van water naar ethanol. Nadien werden de films gedroogd in lucht. Er werd geen verminderde krimp waargenomen en de sponzen toonden een gele kleur. TiO2 /AgCl deeltjes De stabiliteit van de TiO2 /AgCl deeltjes in NaOH werd bestudeerd. Wanneer het TiO2 /AgCl poeder werd toegevoegd aan een waterige oplossing van natriumhydroxide kleurde de totale oplossing lichtbruin. Wanneer een grote hoeveelheid waterige oplossing van zwavelzuur werd toegevoegd veranderde de kleur naar melkachtig wit. De eerste kleurverandering gebeurt omdat natriumhydroxide met zilverchloride in een tweestapsreactie reageert tot zilveroxide. In een eerste stap wordt het thermodynamisch onstabiele zilverhydroxide gevormd. In appendix ?? wordt thermodynamisch aangetoond dat de vorming van zilver oxide thermodynamisch stabiel is. Reactie 3 toont de totale reactie. Wanneer een grote hoeveelheid zwavelzuur wordt toegevoegd zal het zilveroxide reageren met het zuur en zilversulfaat (AgSO4 ). Dit verklaart de witte kleur. De reactie is getoond in reactie 4. 2 AgCl + 2 NaOH −−→ Ag2 O + 2 NaCl + H2 O

(3)

Ag2 O + H2 SO4 −−→ Ag2 SO4 + H2 O

(4)

Koolstof nanobuizen Er werd onderzocht of enzymatische behandelde cellulose kan worden gebruikt als een dispergeermiddel voor CNTs. Hiervoor werd een zeer kleine hoeveelheid MWNTs in dispersie gebracht in een 1, 2 en 6 wt% cellulose oplossing en in water door middel van ultrasoon geluid. De 2 en 6 wt% cellulose oplossingen vormden een gel wanneer ze werden blootgesteld aan ultrasoon geluid, de 1 wt% cellulose oplossing was stabiel. De gelvorming van de 2 en 6 wt% cellulose oplossing wordt verklaard door de hitte die vrijkomt tijdens de behandeling met ultrasoon geluid en de instabiliteit van de cellulose oplossing voor hoge temperaturen. Het gedrag van de dispersies in een 1 wt% cellulose oplossing en in water werden bestudeerd (zie figuur 9). De MWNT-water dispersie is onstabiel en na 15 seconden precipiteren de MWNTs al uit de oplossing. De cellulose-MWNT dispersie daarentegen is stabiel. Er werd onderzocht hoeveel MWNTs een 1 wt% cellulose oplossing in dispersie kan houden.

xvii

(a)

(b)

(c)

(d)

Figuur 9: De stabiliteit van dispersies van CNT in een 1 wt% cellulose oplossing (links) en water (rechts) na: 15 seconden (a), 30 seconden (b), 30 minuten (c) en 1 dag (d)

Hiervoor werden MWNTs in dispersie gebracht met de cellulose oplossing in een cellulose:MWNT verhouding van 20:1, 15:1, 10:1 en 5:1. Alleen de oplossing met een cellulose:MWNT verhouding van 20:1 toont een homogene dispersie (zie figuur 10 (a)). In de andere oplossingen is er agglomeratie van MWNTs (figuur 10 (b-d)). De stabiliteit van de dispersie is verklaard door het wikkelen van cellulose moleculen rond individuele CNTs.

(a)

(b)

(a)

(b)

Figuur 10: Microscopische afbeeldingen van een 1wt% cellulose oplossing met een cellulose:CNT verhouding van 20:1 (a), 15:1 (b), 10:1 (c) en 5:1 (d)

Cellulose/MWNT hybdride films werden gemaakt door een dispersie van 1 wt% cellulose: 5 wtc% MWNT te mengen met een 6 wt% cellulose oplossing. Cellulose/MWNT oplossingen met 0.5, 1 en 2 wtc% MWNTs werden gemaakt. Omdat de MWNTs eerst in dispersie moeten worden gebracht in een 1 wt% cellulose oplossing, daalt de eindconcentratie van cellulose in de oplossing met een stijgende concentratie van MWNTs. Hierdooor daalt de viscositeit. Een cellulose oplossing met 2 wtc% MWNTs had een te lage viscositeit om films te maken. SEM-afbeeldingen werden genomen van de films (zie figuur 11). De pure cellulose film

xviii vertoont een vlak oppervlak met een aantal kleine onzuiverheden. Deze onzuiverheden zijn waarschijnlijk tijdens regeneratie gevormde Na2 SO4 -zouten die niet zijn uitgewassen. De films met MWNTs (figuur 11 (b) en (c)) vertonen een ruw maar homogeen oppervlak en onzuiverheden. Dit wijst erop dat er MWNTs in de films zijn en dat deze homogeen verdeeld zijn.

(a)

(b)

(c)

(d)

Figuur 11: SEM-afbeeldingen van cellulose films met 0 (a), 1 (b) en 2 (c) wtc% MWNT. De cellulose films met 1 wtc% MWNT vertonen porien (d)

Conclusies Case¨ıne Basische cellulose waaraan case¨ıne werd toegevoegd waren stabiel, er was een kleurverandering van de oplossing maar de viscositeit veranderde niet. Films werden gemaakt in een 5 wt% waterige oplossing van zwaverlzuur. Er was geen kleurverandering van het coagulatiebad wat erop wijst dat case¨ıne in de films blijft. Films met een case¨ıne fractie hoger dan 20% konden niet gemaakt worden. FTIR-analyse van de cellulose/case¨ıne films vertoonde geen karakteristieke pieken van de amide vibraties van case¨ıne. De films kunnen worden geverfd met zure kleurstoffen, maar na het wassen is de kleur zeer oneven verdeeld. De afwezigheid van de amide vibraties in het FTIR spectra en de oneven kleurverdeling wijzen erop dat case¨ıne aanwezig is in de films maar dat case¨ıne zeer oneven verdeeld is. De niet homogene distributie van case¨ıne is verklaart door een verschillend coagulatiemoment van cellulose (pH 10-11) en case¨ıne (pH 4-6). Hierdoor zal cellulose voor case¨ıne coaguleren en resulteren in een niet homogene film.

Celluloze sponzen Natrium carbonaat kan niet gebruikt worden als een schuimvormer, dit komt omdat een grote hoeveelheid natrium carbonaat de cellulose oplossing onstabiel maakt. In kleine

xix hoeveelheden daarentegen kan Na2 CO3 worden gebruikt. Mogelijk kunnen holle vezels worden gemaakt aangezien het carbonaat tamelijk laag kan zijn voor de productie van holle vezels(5-35 wtc%). Calcium carbonaat is een geschikte vervanger. CaCO3 is niet oplosbaar en maakt daarom de oplossing niet onstabiel. Zwavelzuur is geen goed coagulant omdat het onoplosbaar CaSO4 vormt. HCl is een geschikt zuur, het vormt het oplosbare CaCl2 wanneer het reageert met CaCO3 . Om voldoende poriën te creëeren is een grotere hoeveelheid CaCO3 aan te raden. Hogere temperaturen (50-60 ◦ C) verhogen de snelheid van reactie en creëeren een hogere porositeit. Temperaturen van 70◦ C en meer zijn niet aan te raden omdat dan hydrolyse van cellulose kan plaatsvinden. Een hoger zoutgehalte geeft kleinere, maar een hoger aantal poriën.

TiO2 /AgCl deeltjes Cellulose films met TiO2 /AgCl deeltjes konden niet worden gemaakt. Dit komt omdat de cellulose is opgelost in natriumhydroxide. Zilverchloride reageert met natriumhydroxide en vorm zilveroxide.

Koolstof nanobuizen Een verdunde 1 wt% cellulose oplossing kan worden gebruikt als dispergeermiddel voor koolstof nanobuizen. Een 1 wt% cellulose oplossing kan 5 wtc% MWNTs in dispersie houden. Een hogere MWNT-concentratie veroorzaakt agglomeratie van MWNTs. Cellulose/MWNT hybride films met 0.5 en 1 wtc% MWNT konden worden gemaakt. De oplossing met 2 wtc% MWNT had een te lage viscositeit om films te maken. SEMafbeeldingen van de films tonen de aanwezigheid en de even distributie van de MWNTs.

De conclusie van deze thesis is dat cellulose producten met een toegevoegde waarde kunnen gemaakt worden met enzymatisch behandeld cellulose. Case¨ıne kan worden gebruikt om woleigenschappen te creëren, carbonaatzouten om poreuze structuren te maken en koolstof nanobuizen voor verhoogde sterkte en elektrische geleidbaarheid. Dit onderzoek is slechts een inleiding en verder onderzoek is nodig om de processen te optimaliseren en vezels te vormen. Antibacteriële cellulose producten konden niet gemaakt worden.

Formation of cellulose hybrid films from alkaline solution Wouter Vanreyten Supervisor(s): G. S CHOUKENS and P. N OUSIAINEN Abstract— Cellulose hybrid films were prepared using enzymatically modified cellulose. Cellulose/casein films showed an uneven distribution of casein. Cellulose porous structures were prepared with CaCO3 as a foaming agent and HCl as a coagulant. It was shown that with increasing amount of CaCO3 , increased foaming occured. Also temperature and salt content of the coagulation bath promoted pore formation. Trials with TiO2 /AgCl particles were not successfull because of the reaction of AgCl with NaOH to Ag2 O. A diluted 1wt% cellulose solutio could disperse 5 wtc% of MWNTs. Cellulose/MWNT films with 0.5 and 1 wtc% MWNT could be prepared. The distribution of the MWNTs was homogeneous. Keywords— Carbon nanotubes, carbonate salt, casein, cellulose, sponge, TiO2 /AgCl

[23]. Cellulose/CNT composites have been manufactured using different processes. Cellulose xanthate (viscose) is a good dispersant for CNTs due to the wrapping of cellulose xanthate around the CNT [24]. Rahatekar et. al studied the formation of cellulose/CNT composites using an ionic liquid as a solvent [25]. Lu et. al. functionalized CNTs with sodium dodecylbenzene sulfonate (SDBS) to make them easily dispersible in a lyocell solution [26].

I. I NTRODUCTION

The starting material was a commercial dissolving grade softwood TCF sulphite pulp delivered by Domsjö Fabriker AB, Sweden. The pulp was first shredded mechanically for 5 h with a Baker Perkins Mixer at a cellulose consistency of 20 wt%, and thereafter treated with a commercial enzyme preparation at pH 5, at 50 °C for 3 h. The enzymatically treated pulp was dissolved into an aqueous sodium hydroxide and zinc oxide. The final solution consisted out of 6 wt% cellulose, 6.5 wt% NaOH and 1.3 wt% ZnO. Casein was bovine milk casein from TUT’s reserves. Casein was dissolved in a 6.5 wt% NaOH aqueous solution to obtain a casein concentration of 100 g/l. The casein solution was added to the cellulose solution to obtain a hybrid solution consisting of cellulose:casein fraction of 100:0, 95:5, 90:10 and 85:15. The films were prepared by the glass plate method by coagulation in a 5wt% aqueous solution of H2 SO4 for 5 minutes. Na2 CO3 and CaCO3 were added to the cellulose solution to determine the influence of CaCO3 on the stability of the cellulose solution. Na2 CO3 was first dissolved in 6.5 wt% NaOH in concentrations of 5, 10 and 15g/100ml, CaCO3 was added directly. Porous films were prepared using 100, 200 and 300 wtc% (weigth percentage compared to cellulose) CaCO3 , the coagulants tested were H2 SO4 and HCl. Films using a 10 wt% HCl solution were prepared [Temperature (T) = 22-40-60 °C, salt content (CN aCl ) = 0, 10 , 15 and 20 wt%]. The influence of the amount of carbonate salt was examined at T= 50°C, CN aCl = 0 wt%, the influence of the temperature at CN aCl = 0 wt% and the influence of the salt content at T= 50°C, CCaCO3 = 100 wt%. The silver chloride/ titanium dioxide composite particles were provided by Addmaster (Staffor, UK). They consisted of silver chloride deposited on titanium in a ratio of 20 % to 80 %. 1 g of TiO2 /AgCl particles were dispersed in 20 ml of a 6.5 wt% aqueous solution of NaOH. Afterwards, the solution was added to an excess of a 1 wt% H2 SO4 solution. Multiwalled carbon nanotubes, grade NC700, were purchased

ELLULOSE is one of the most abundant renewable resources, it is biodegradable and an environmentally friendly material. In industry, viscose, lyocell and cuprammonium are still the most used processes despite their environmental hazards [1]. Enzymatically modified cellulose provides a direct and environmentally friendly way to dissolve cellulose in sodium hydroxide [2], [3], [4], [5]. Regenerated cellulose products with added value can be prepared by blending cellulose with other materials. Cellulose/casein films have been prepared [6], [7], [8]. The films containing casein have a higher tensile strength than pure cellulose films. Cellulose and casein are miscible up to 25% of casein when using a cupramonium solution and 15% when using a NaOH/urea solution. Cellulose sponges can be prepared by using a carbonate salt as an foaming agent. Carbonate salts react with acid and form CO2 . Sodium carbonate has been used for manufacturing hollow fibres [9], [10], [11]. The production of porous cellulose sheets with the use of carbonate salts has been investigated [12]. All agree that a higher carbonate content, elevated temperature and a higher acid and salt content of the coagulation bath promote inflation. Antimicrobial properties can be achieved by using silver. The antimicrobial properties of silver are well known [13], [14]. Regenerated antimicrobial cellulose products have been prepared using TiO2 /AgCl composite particles [15]. TiO2 /AgCl particles are characterized by their durability and their effectivenes at very low concentrations [16], [17], [18]. Carbon nanotubes (CNT) are of great interest to polymeric science. CNTs are characterized by their high electrical conductivity, mechanical strength, and chemical stability [19], [20]. Polymer/CNT composites show superior properties [21], [22],

C

W. Vanreyten is a master student at the department of textiles, Ghent University (UGent), Gent, Belgium. E-mail: [email protected] .

II. M ATERIALS AND M ETHODS A. Cellulose solution

from Nanocyl (Belgium). The particles had an average diameter of 9.5 nm and length of 1.5 µm and a purity of 90 %. The particles were dispersed in the cellulose solution by sonication using a Hielscher UP200S dispenser (Hielscher, Germany). A small amount of MWNTs were dispersed in a 1, 2 and 6 wt% cellulose solution. dispersions with a cellulose:MWNT ratio of 20:1, 15:1, 10:1 and 5:1 were prepared with a 1 wt% cellulose solution. Cellulose films containing 0.5, 1 and 2 wtc% MWNT were prepared by dispersing MWNTs in a 1 wt% cellulose solution before adding to a 6 wt% cellulose solution. The films were prepared by the glass plate method in a 10 wt% H2 SO4 coagulation bath for 5 minutes.

casein

cellulose: casein 80:20

cellulose

B. Testing Flms were investigated under an optical microscope (Leitz Laborlux D). Fourier Transform Infrared (FTIR) Spectroscopy (PerkinElmer, USA) was done after the films were dried under tension. The scanning range was 650-4000 cm−1 . To investigate casein powder, tablets were made by compressing a mixture of 1 mg casein and 200 mg Potassium bromide. Potassium bromide is used because it is transparant to infrared radiation between 40000 and 400 ctm−1 . The cellulose/casein films were dyed with a red 266 (cherry red) acid dye. The films were first soaked for 1h to become fully wet. Afterwards, the films were put in 200 ml of a 3wt% aqueous solution of acetic acid with 0.5g of acid dye. The dyebath was heated up and kept at elevated temperatures (85°C) for 30 minutes. Then the films were rinsed and washed with detergent to remove unfixed dyestuff. Visual examination of the films was done. Scanning electron microscope images (SEM) were taken of cellulose films containing CNT using a Philips XL30 microscope (Philips, The Netherlands). The accelerating voltage during imaging was 15 kV. Before examination, the samples were coated with gold using a sputter coater. III. R ESULTS AND DISCUSSION A. Casein When mixing casein and cellulose, the solution turned white. This might be due to a partial coagulation of cellulose. Figure 1 shows the FTIR spectra of casein powder, cellulose films and cellulose/casein hybrid film. The peaks found in the casein sample at a wavelength of 1650 and 1550 cm−1 are assigned to the amide I and amide II vibrations. The absence of these peaks in the cellulose/casein hybdrid sample indicates that no casein is detected. The cellulose and cellulose/casein hybrid films were dyed with acid dyes. Afterwards the films were washed with water and detergent. After a water washing the films that contain casein remain coloured, but the colour is very inhomogeneous, the cellulose films show no colour anymore. After detergent washing, all films reduce in colour. This indicates that there is casein in the films but is badly dispersed. The bad dispersion probably happens during regeneration. Cellulose coagulates at a pH of 10-11 wheras casein only coagulates at pH 4-6. Therefore

1700

1650

1600

1550

1500

1450

1400 cm-1

Fig. 1. FTIR spectra of casein powder, cellulose/casein and reference cellulose films

coagulation of cellulose will occur much faster than casein coagulation and a inhomogeneous mixture will be formed. B. Cellulose sponges The influence of the amount of Na2 CO3 on the stability of the cellulose solution was studied. No gelling occured with a concentration of 5 g/100 ml. With a 10 g/100 ml, the solutions becomes unstable at 200 wtc% of Na2 CO3 , but no real gel formation occurs. With a 15 g/100 ml Na2 CO3 solution, gelling occurs at a Na2 CO3 concentration of 100-125 wtc%. This behaviour can be explained by the hydratation of Na+ ions: Na2 CO3 is a soluble salt and therefore the Na+ concentration ([Ca+ ]) will increase. A higher [Ca+ ] gives rise to a lower hydratation state. The influence of the hydratation state of Na+ on the solubility of cellulose has been explained [27], [28]: a too low or too high hydratation reduces the solubility of cellulose. This explains the gel formation when using a high concentration of Na2 CO3 . CaCO3 could be added in all amounts since it is an insoluble salt and therefore does not alter the hydratation state of the Ca+ ions. When using H2 SO4 as a coagulant, insoluble CaSO4 is formed, which gives the films a plastery feeling. When using HCl as a coagulant soluble CaCl2 is formed. Figure 2 (A) shows optical microscope images of films. It is clear that an increased amount of CaCO3 gives an increased amount of airbubles. The influence of the temperature was investigated. An increasing temperatures resulted in a decreasing coagulation time (see table I). This is explained by the higher kinetics. Films prepared at 70 °C could not be dried under tension. This was probably due to the ability of HCl to hydrolyse cellulose at high temperatures. The influence of the salt of the coagulation bath was investigated. Figure 2 (B) shows microscopic images. A higher salt

TABLE I C OAGULATION TIME OF CELLULOSE SOLUTION WITH C ACO 3

temperature (°C) 22 40 60 70

diluted cellulose solution, even after 1 day. The dispersion in water was not stable, even after 30 minutes the MWNTs sedimented.

time 7 min 30 s 6 min 3 min 30s 1 min 45s

(a)

content resulted in a higher number of pores. This was because the salt slowed down regeneration which normally occured first. Therefore regeneration and gas formation occured simultaneously and air bubbles were formed around which coagulation occured.

(A)

(B)

Fig. 2. Microscopic images of cellulose films containing 0 (A-a), 100 (A-b), 200 (A-c) and 300 (A-d) wtc% of CaCO3 . Cellulose films containing 100 wtc% CaCO3 and a salt content of the coagulation bath of 0 (B-a), 10 (B-b), 15 (B-c) and 20 (B-d) wt% NaCl

C. TiO2 /AgCl particles The stability of TiO2 /AgCl particles in an aqueous solution of NaOH was investigated. When TiO2 /AgCl powder was added to a NaOH solution, the colour changed from white to a milky brown colour. When further stirring, the solution became completely opaque. When adding the solution to an excess of sulphuric acid, the colour changed to a white milky colour. The change of colour from white to milky brown is explained by the reaction of silver chloride to silver oxide which is thermodynamically favourable in alkaline environment. As an intermediate step, silver hydroxide is formed. The overall reaction is given by equation 1. When adding the obtained solution to an excess of H2 SO4 , silver oxide will react with the acid to form silver sulfate (Ag2 SO4 ). 2 AgCl + 2 NaOH → − Ag2 O + 2 NaCl + H2 O

(1)

D. Carbon nanotubes Sonication of a 2 and 6 wt% cellulose solution resulted in gel formation. This is due to heat generation during sonication. Upon sonication of the MWNTs in a 1wt% cellulose solution and in water, dark colored dispersions were formed. Fig. 3 (a)(c) show pictures of MWNTs in a 1 wt% cellulose solution (left) and water (right) after letting the dispersion rest for 15s, 30s, 30 min and 1 day. The MWNTs remained in dispersion in the

(b)

(c)

(d)

Fig. 3. Testing of the stability of the emulsion of MWNT in 1wt% cellulose solution (left) and water (right) after: 15 seconds (a), 30 seconds (b), 30 minutes (c) and 1 day (d)

Figure 4 shows microscopic images of a diluted 1 wt% cellulose solution with 5, 6.5, 10 and 20 wtc% MWNT. Only the dispersion with 5 wt% was well dispersed, the other ones have agglomerations of MWNT. Cellulose-MWNT films with 0.5 and 1 wtc% MWNT could be prepared. The solution with 2 wtc% MWNT had a too low viscosity to make films.

(a)

(b)

(a)

(b)

Fig. 4. microscopic images of a 1 wt% cellulose solution with a cellulose:CNT fraction of 20:1 (a), 15:1 (b), 10:1 (c) and 5:1 (d)

SEM images were taken of the films (see figure 5). The reference cellulose film (figure 5 (a)) showed a very smooth surface with some small impurities. These impurities are probably Na2 SO4 -salts that were formed during regeneration and were not removed during washing. The cellulose films containing MWNTs (figure 5 (b) and (c)) had a very rough, but homogeneous surface and contained the same impurities as the reference cellulose films. This all indicates the presence and homogeneous distribution of MWNTs in the cellulose films.

(a)

(b)

(c)

(d)

Fig. 5. SEM images of a reference cellulose film (a) and cellulose films containing 01.5 (b) and 1 (c) wtc% MWNTs. The films containing 1 wtc% MWNTs show pores in the structure.

IV. C ONCLUSIONS Alkaline cellulose-casein solutions were stable, the colour of the solution changed to white but the viscosity did not alter. Films were produced by coagulation in a 5 wt% solution of

H2 SO4 . The coagulation bath did not change colour during film manufacturing which indicates that casein remains in the films during coagulation. Films with a casein fraction higher than 20 % could not be prepared. FTIR did not proove the presence of casein. This might be due to a inhomogeneous mixture of casein and cellulose. When the films were acid dyed, the colour of the cellulose-casein films remained after washing but was inhomogeneous whereas the colour of the reference cellulose films was completely washed away. This confirms the presence of casein, but the casein was very unevenly distributed. The nonuniformity of the films was probably generated during coagulation. Cellulose coagulated faster than casein and thereby creating a inhomogeneous film structure. Sodium carbonate could not be used as a foaming agent for forming sponges because it induces gel formation in large quantities. However, inflated hollow fibres might be manufactured by using Na2 CO3 since the amount of foaming agent only needs to be 10-35 wtc%. CaCO3 is a suitable foaming agent, it is not soluble and therefore did not change the properties of the solution. H2 SO4 cannot be used as a coagulant since the insoluble salt CaSO4 is formed. When using HCl as a coagulant, the soluble CaCl2 was formed. A higher salt content and temperature of the coagulation bath promoted pore formation. A temperature of 70°C was unstuitable because HCl at high temperatures can hydrolyse the glycosidic linkage in cellulose. During drying of the cellulose sponges, there was a huge shrinkage. Trials to prevent shrinkage by a solvent exchange to ethanol were unsuccessful. Films containing TiO2 /AgCl particles could not be manufactured. This was because sodium hydroxide was used as a solvent for cellulose and silverchloride is not stable in it. AgCl will react with NaOH to form silveroxide. This gave the cellulose solution a very dark colour. During coagulation, silveroxide reacts with the sulphuric acid forming silversulfate. A diluted cellulose solution (1wt%) could be used as a stable dispersant for carbon nanotubes. A 1wt% cellulose solution could disperse 5 wtc% MWNTs. Cellulose/ multiwalled carbon nanotubes hybrid films with 0.5 and 1 wtc% MWNT could be prepared by the glass plate method. The viscosity of a cellulose solution with 2 wtc% MWNT had a too low viscosity to prepare films. Scanning electron microscope images of the films show the presence of MWNT in the films and an even distribution. In summary, Cellulose product with additives could be prepared using casein, carbonate salts and carbon nanotubes. The cellulose/casein hybrid products could give wool like properties due to the introduction of peptide-bonds. Porous structures can be prepared using calcium carbonate. Cellulose/CNT composite structures were succesfully prepared. Antibacterial cellulose products using silver chloride could not be prepared. Further ressearch is needed to form fibres. This would allow to measure properties as strength. Conductivity in case of cellulose/CNT fibres and dyeability of cellulose/casein fibres. R EFERENCES [1] C.R. Woodings, Regenerated cellulose fibres, CRC Press, 2001. [2] L. Rahkamo, M. Siika-Aho, M. Vehviläinen, M. Dolk, L. Viikari, P. Nousiainen, and J. Buchert, “Modification of hardwood dissolving pulp with purified trichoderma reesei cellulases,” Cellulose, vol. 3, pp. 153–163, 1996. [3] N. Le Moigne, K. Jardeby, and P. Navard, “Structural changes and alkaline solubility of wood cellulose fibers after enzymatic peeling treatment,” Carbohydrate Polymers, vol. 79, pp. 325–332, 2010.

[4] Danuta Ciechanska, Dariusz Wawro, Wlodzimierz Steplewski, Janusz Kazimierczak, and Henryk Struszczyk, “Formation of fibres from bio-modified cellulose pulp,” Fibres & Textiles in Eastern Europe, vol. 13, pp. 19–23, 2005. [5] M. Vehviläinen, “Biocelsol publishable final activity report,” Tech. Rep., Tampere University of Technology, 2007. [6] G. Yang, L. Zhang, H. Han, and J. Zhou, “Cellulose/casein blend membranes from naoh/urea solution,” Journal of Applied Polymer Science, vol. 2001, pp. 3260–3267, 81. [7] L. Zhang, G. Yang, and L. Xiao, “Blend membranes of cellulose cuoxam/casein,” Journal of Membrane Science, vol. 103, pp. 65–71, 1995. [8] G. Yang, C. Yamane, T. Matsui, I. Miyamoto, L. Zhang, and K. Okajima, “Morphology and amorphous structure of blend membranes from cellulose and casein recovered from its cuprammonium solution,” Polymer journal, vol. 29, pp. 31–332, 1997. [9] C.R. Woodings and A.J. Bartholomew, “The manufacture, properties and use of inflated viscose fibres,” Lenzinger Berichte, 195. [10] J. Hall, M. Hook, and A. Hunter, “artificial fibre,” 1949. [11] I. Kosuge, “process for producing hollow viscose filaments,” 1958. [12] A. Higashiyama, S. Kanazu-Cho, and H. Saito, “Porous cellulose sheets and method for manufacturing the same,” 2001. [13] L. Kvitek, A. Panacek, J. Soukupova, M. Kolar, R. Vecerova, R. Prucek, M. Holecova, and R. Zboril, “Effect of surfactants and polymers on stability and antibacterial activity of silver nanoparticles (nps),” The Journal of Physical Chemistry C, vol. 112, pp. 5825–5834, 2008. [14] J. Morones, J. Elechiguerra, A. Camacho, K. Holt, J. Kouri, J. Ramirez, and M. Yacaman, “The bactericidal effect of silver nanoparticles,” Nanotechnology, vol. 16, pp. 2346–2353, 2005. [15] H. Vegad and M. Hayhurst, “Anti-microbial fibres and their production,” 2007. [16] N. Edwards, S. Mitchell, and A. Pratt, “Anti-microbial composition,” 1988. [17] K. Brunt, R. Corbett, P. Wood, and D. Murphy, “Biocidal composition,” . [18] R. J. Corbett, “An inorganic biocide using a novel presentation of silver,” International Journal of Cosmetic Science, vol. 18, pp. 151–165, 1996. [19] “http://www.pa.msu.edu/cmp/csc/ntproperties/,” 16 February 2010. [20] M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, 1996. [21] “http://www.personal.rdg.ac.uk/ scsharip/tubes.htm,” 15 February 2010. [22] Z. Spitalskya, D. Tasisb, K. Papagelisb, and C. Galiotis, “Carbon nanotubepolymer composites: Chemistry, processing, mechanical and electrical properties,” Progress in Polymer Science, vol. 35, pp. 357–401, 2010. [23] M. Rosso, “Origins, properties, and applications of carbon, nanotubes and fullerenes,” . [24] B. Wei, P. Guan, L. Zhang, and G. Chen, “Solubilization of carbon nanotubes by cellulose xanthate toward the fabrication of enhanced amperometric detectors,” Carbon, vol. 48, pp. 1380–1387, 2010. [25] S. S. Rahatekar, A. Rasheed, R. Jain, M. Zammarano, K. K. Koziol, A. H. Windle, J. W. Gilman, and S. Kumar, “Solution spinning of cellulose carbon nanotube composites using room temperature ionic liquids,” Polymer 50, vol. 50, pp. 4577–4583, 2009. [26] J. Lu, H. Zhang, H. Shao, and X. Hu, “Preparation and characterization of multiwalled carbon nanotubes/lyocell composite fibers,” Polymer(Korea), vol. 31, pp. 436–441, 2007. [27] M. M. Egal, Structure and properties of cellulose/NaOH aqueous solutions, gels and regenerated objects, Ph.D. thesis, Ecole des Mines de Paris, 2006. [28] Y. Wang, Cellulose fiber dissolution in sodium hydroxide at low temperature: dissolution kinetics and solubility improvement, Ph.D. thesis, Georgia Institute of Technology, 2008.

Contents Preface

ii

Summary

iii

Nederlandstalige Samenvatting

xx

Extended abstract

xx

Abbreviations

xxvi

1 Introduction

1

I

2

Literature Survey

2 Viscose process 2.1 The viscose process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 3

3 Cellulose solution 3.1 Solubility of cellulose in sodium hydroxide . . . . . . . . . . . . . . . . . . . 3.2 Enzymatic hydrolysis pre-treatment to improve solubility . . . . . . . . . . . 3.3 Cellulose product formation . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 6 7 8

4 Additives to cellulose 4.1 Microcrystalline Chitosan (MCCh) . . . . . . . . . . . . . . . . 4.1.1 Solubility of chitosan . . . . . . . . . . . . . . . . . . . . 4.2 Casein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Solubility, miscibility and film formation . . . . . . . . 4.3 Sodium carbonate . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Manufacturing of viscose cellulose sponges using sodium 4.4 Titanium oxide particles coated with silver chloride . . . . . . . 4.4.1 TiO2 /AgCl composite particles . . . . . . . . . . . . . . 4.5 Carbon Nanotubes (CNT) . . . . . . . . . . . . . . . . . . . . . 4.5.1 Polymer/CNT composites . . . . . . . . . . . . . . . . . 4.5.2 Cellulose/CNT composites . . . . . . . . . . . . . . . .

xxiv

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

. . . . . . . . . . .

10 10 11 14 15 16 17 20 21 23 24 25

Contents

II

xxv

Experiments

26

5 Materials and methods 5.1 Enzyme-treated pulp and cellulose solution . . . . 5.1.1 Pure cellulose solution . . . . . . . . . . . . 5.2 Film formation . . . . . . . . . . . . . . . . . . . . 5.2.1 Glass plate method . . . . . . . . . . . . . 5.2.2 Cellulose/casein films . . . . . . . . . . . . 5.2.3 Cellulose sponges . . . . . . . . . . . . . . . 5.2.4 TiO2 /AgCl particles . . . . . . . . . . . . . 5.2.5 Carbon nanotubes . . . . . . . . . . . . . . 5.3 Properties of the films . . . . . . . . . . . . . . . . 5.3.1 Optical microscope . . . . . . . . . . . . . 5.3.2 Fourier Transform Infrared . . . . . . . . . 5.3.3 Acid dyeing of cellulose/casein hybrid films 5.3.4 Scanning electron microscope . . . . . . . .

. . . . . . . . . . . . .

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. . . . . . . . . . . . .

6 Results and discussion 6.1 Casein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Film formation . . . . . . . . . . . . . . . . . . . . 6.1.2 Detection of casein in the cellulose films . . . . . . 6.2 Cellulose sponges . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Influence of a carbonate salt on the stability of the 6.2.2 Influence of the coagulation bath . . . . . . . . . . 6.2.3 Influence of the concentration of foaming agent . . 6.2.4 Influence of the temperature . . . . . . . . . . . . 6.2.5 Influence of salt content of the coagulation bath . 6.2.6 Thickness of the cellulose sponges . . . . . . . . . 6.3 TiO2 /AgCl particles . . . . . . . . . . . . . . . . . . . . . 6.4 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Cellulose as a dispersant for CNT . . . . . . . . . 6.4.2 Cellulose/MWNT hybrid films . . . . . . . . . . . 7 Conclusions 7.1 Casein . . . . . . . . 7.2 Cellulose sponges . . 7.3 TiO2 /AgCl particles 7.4 Carbon nanotubes .

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. . . . . . . . . . . . . . . . . . . . cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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. . . . . . . . . . . . . . . . . . . . solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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27 27 27 28 28 28 29 30 31 32 32 32 32 32

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33 33 33 34 37 37 38 38 39 41 42 43 44 44 46

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48 48 48 49 49

A Theoretical calculation about the solubility of AgCl

50

Bibliography

51

List of Figures

57

List of Tables

59

Abbreviations DP

degree of polymerization

EG

endoglucanases

CBH

cellobiohydrolases

MCCh

MicroCrystalline Chitosan

PEC

polyelectrolyte complex

CRI

crystallinity index

WRV

water retention value

wtc%

Weight percentage based on the amount of cellulose

Pi

isoelectric pH

Pro

proline

MIC

minimum inhibitory concentration

NMMO

N-Methylmorpholine N-oxide

CNT

carbon nanotubes

SWNT

single walled carbon nanotubes

MWNT

multi walled carbon nanotubes

CVD

chemical vapor deposition

EMIAc

ethylmethylimidazolium acetate

SDBS

sodium dodecylbenzene sulfonate

TCF

Totally Chlorine Free

CLB

closed loop bleach plant

FTIR

Fourier Transform InfraRed

SEM

Scanning Electron Microscope

TGA

Thermal Gravimetric Analysis

xxvi

Chapter 1

Introduction This work was carried out as a part of the E-team program at Tampere University of Technology in the Instititute of Fibre Material Science. Most regenerated cellulose is nowadays still manufactured using the viscose process. This is a complex and very environmentally unfriendly process which uses harsh chemicals such as carbon disulphide and where hydrogen sulphide is a by-product. A lot of research has been done to develop alternative processes (e.g. lyocell, cuprammonium process). Even though these processes are more environmentally friendly, they are still very complex. At Tampere University of Technology, a new process was developed to manufacture regenerated cellulose products. Cellulose is made directly soluble in sodium hydroxide by an enzymatic treatment. The process is very simple and can be implemented in existing viscose plants. The biggest drawback of this process is that the cellulose solution is very unstable. This thesis uses this enzymatically modified cellulose as a starting material. The aim of this work was to investigate the possibility of producing cellulose products with added value from the enzymatically modified cellulose. For this purpose, different additives are used: chitosan, casein, carbonate salts, carbon nanotubes and TiO2 /AgCl. The objectives are, first, to investigate the behaviour of the additive in the cellulose solution and the influence on the solution, second, to investigate and optimize the coagulation conditions and, third, the influence on the properties of the films. In the literature study first the viscose process and the Biocelsol process are discussed. Next the different additives are discussed: their behaviour in alkaline environment and, if known, processes for manufacturing cellulose products with these additive. Preferably this is done with viscose since it uses the same solvent and coagulant as the Biocelsol process. In the experimental part, the additives are studied in the cellulose solution and in the coagulation bath. Unfortunatly, chitosan was not used as an additive in the experiments.

1

Part I

Literature Survey

2

Chapter 2

Viscose process In this chapter the viscose process for the manufacturing of regenerated cellulose products will be shortly discussed. The viscose process is interesting within the scope of this thesis because it uses the same solvent (sodium hydroxide) and regeneration bath (sulphuric acid) as the enzyme assisted process. Also most of the additives of the cellulose solution will be discussed in the literature survey as additives of the viscose process.

2.1

The viscose process

The viscose process is a very old process, the first patent on viscose was granted to Cross and Bevan in 1893. Since then, the viscose process has been refined, but the basic chemistry has stayed the same. A schematic representation of the process can be seen in figure 2.1. The most important step in the viscose process is the xanthation. In this step the sodium cellulose reacts with carbon disulphide to form sodium cellulose xanthate (reaction and chemical structure of sodium cellulose xanthate see figure 2.2). Not only sodium cellulose xanthate is formed, but also by-products from the reaction between sodium hydroxide and carbon disulphide. Cellulose xanthate is soluble in dilute sodium hydroxide. Before regeneration, the solution has to be aged to allow the even distribution of CS2 on the cellulose chains. After this the viscose dope has to be filtered to remove undissolved particles and deaired to remove air bubbles.

3

Chapter 2. Viscose process

1.

Viscose is made from cellulose, a constituent of all land-growing plant life. Spruce and eucalyptus yield much of the high-grade cellulose required for viscose

5.

Pre-aging. The schredded alkali cellulose is stored to allow the oxygen in the air to oxidise it. This reduces the molecular size of the cellulose, which is necessary to obtain a spinning solution of the correct viscosity

9.

Filtration. The viscose is filtered to remove any particles and undissolved cellulose which might block holes in the spinning jet (spinneret), which could produce decitex variations

4

2. Cellulose. In the pulp mill the bark, lignin etc. are removed from the trees and the extracted cellulose is pressed and cut into sheets

6.

Xanthation. The alkali cellulose is combined with the carbon disulphide (derived from carbon and sulphur) to produce sodium cellulose xanthate

3.

Slurrying. In the viscose factory the sheets of cellulose are slurried in caustic soda with which it combines to form alkali cellulose

4.

Pressing. The excess soda is then pressed out and drained off for recovery and re-use

7.

Dissolving. The sodium cellulose xanthate is dissolved in caustic soda to form the syrup-like spinning solution known as viscose

8.

Ripening. To improve the spinning qualities, the viscose is allowed to ripen. Meanwhile it is held under vacuum to remove air bubbles

10. Spinning/Washing. The viscose is extruded through the fine holes of a spinneret into a coagulation bath of sulphuric acid and salts which neutralizes the alkali content of the viscose and regenerates the original cellulose as continuous filaments

Figure 2.1: Schematic representation of the viscose process (adapted from [1])

Chapter 2. Viscose process

5

Figure 2.2: Reaction for forming sodium cellulose xanthate from sodium cellulose and carbon disulphide [2]

Regeneration is the process of converting sodium cellulose xanthate back to cellulose and carbon disulphide. This is done in a regeneration bath containing sulphuric acid (see reaction 2.1). The by-products formed in the xanthate step react with the sulphuric acid and form carbon disulphide and hydrogen sulphide. Upon regeneration, sodium sulphate is formed. 2 cellulose−OCS2 Na + H2 SO4 −−→ 2 cellulose−OH + Na2 SO4

(2.1)

Carbon disulphide and hydrogen sulphide are polluting products, and together with the complexity of the process, they are the main drawbacks of the viscose process. Other systems for regenerating cellulose have been studied to reduce the environmental problems of viscose (Lyocell, Cuprammonium and enzyme-treated cellulose). [1, 2, 3]

Chapter 3

Cellulose solution In this paragraph, first the dissolution of cellulose in NaOH and the influence of the temperature and sodium hydroxide concentration will be discussed. Next, the influence of an enzymatic treatment on the cellulose to increase its solubility and the possibility of forming cellulosic products like fibres and films will be discussed. The key to cellulose dissolution is to break the hydrogen bonds between cellulose molecules [4]. Chemical methods for dissolution have been widely used (CS2 , NMMO, NaOH )[1, 2, 3]. The viscose methode is the most commonly used for this.

3.1

Solubility of cellulose in sodium hydroxide

A cellulose-NaOH-water phase diagram has been published in 1944 by Sobue [5] (figure 3.1). The reaction of cellulose with sodium hydroxide is very dependend on the temperature and concentration. At concentrations of 6-10% NaOH and temperature of -10-4◦ C, cellulose is highly swollen. Further research has shown that in this region, cellulose with a low to moderate degree of polymerization (DP) can be directly solved in the NaOH solution. [6, 7] The goal of adding alkali hydroxide is to increase solubility. Alkali hydrates can penetrate the cellulose and so can introduce water into the cellulose to cleave the hydrogen bonding between the cellulose chains, allowing the chains to swell apart from each other. It is assumed that when the concentration of sodium hydroxide is too low, the number of Na+ ions is insufficient to bring water into the cellulose molecules to cleave the hydrogen bonding, resulting in a low solubility. On the other hand, if the concentration is too high, the hydratation of the alkali is insufficient to cleave the hydrogen bonding.[8] The behaviour of the type of alkaline hydroxide on the swelling of cellulose has been studied by Marsh [9]. It appeared that for all alkalis there is a certain concentration of the alkali hydroxide where the swelling is maximum. The degree of swelling decreases with an increase size of the metallic anion ( Li < Na < K < Rb < Cs) (figure 3.2).

6

Chapter 3. Cellulose solution

7

Figure 3.1: cellulose-NaOH-water phase diagram [5]

Kamide et al.[4] showed that for good alkaline solubility of cellulose it is important to destruct the intra molecular hydrogen bonding, the amount of amorphous region is of less importance. Other factors affecting the direct solubility of cellulose were investigated by Isogai et al. [10]. They found the following: 1) cellulose with a DP lower than 200 could be easily dissolved. 2) Cellulose with a higher DP could only be partially dissolved. 3) The presence of hemicelluloses did not affect the alkaline solubility since most of the hemicelluloses fractions are soluble. 4) The presence of lignin reduced the solubility. Wang [7] measured a change in concentration of NaOH after dissolution of cellulose in it. This indicated that there is absorption of NaOH on cellulose.

3.2

Enzymatic hydrolysis pre-treatment to improve solubility

To improve the alkaline solubility of cellulose, enzymatic treatment with cellulases has been investigated as an alternative to modify cellulose. It has been shown that cellulases have the capacity to modify cellulose to become directly alkaline soluble. [11, 12, 13, 14] Cellulases are enzymes that are able to hydrolyze the β-(1→4) linkage in cellulose. Cellulase consists of a multicomponent enzyme system that has three enzymes which act synergisti-


8

Figure 3.2: Change in volume of cotton hairs in five alkalis [9]

cally in the hydrolysis of cellulose: endoglucanases (EG), exoglucanases or cellobiohydrolases (CBH) and cellobiases. EG attack the middle of the chain and so decreases the DP. CBH attack the chain ends of cellulose, releasing cellobiose. This cellobiose is then converted by cellobiases to glucose. [11, 12, 13] The effect of an enzyme-treatment on the solubility of cellulose in an aqueous sodium hydroxide solution has been studied [11, 12, 13, 14]. They observed an increased solubility of enzyme treated cellulose at low temperatures in a 9 wt% sodium hydroxide solution. The alkaline solubility was found to increase when the viscosity of the pulp was enzymatically decreased [12] . Cao et al. [11] suggested that the increased solubility can be explained by a decrease in DP and in the number of hydrogen bonds.

3.3

Cellulose product formation

Once the solution of the cellulose has been made, cellulosic products like fibres and films can be produced of it. Examples can be found in literature [6, 14, 15, 16]. A big advantage of enzymatic treatment to dissolve cellulose is that there is no need for chemical modification of cellulose as in the viscose process (see 2.1) and therefore, the environmental problems are


9

reduced. Then enzymatically modified cellulose can be coagulated in a coagulation bath containing sulphuric acid. In this bath, cellulose becomes solid. The solidification occurs because sodium hydroxide reacts with sulphuric acid in the coagulation bath and forms sodium sulphate (the salt of sodium hydroxide and sulphuric acid). Since sodium hydroxide is no longer available to enable the dissolving of cellulose, the cellulose will precipitate out of the solution. [14, 15] NaOH + H2 SO4 −−→ NaHSO4 + H2 O (3.1) NaOH + NaHSO4 −−→ Na2 SO4 + H2 O

(3.2)

The system of making regenerated cellulose by enzymatically modified direct soluble cellulose has been studied in the BIOCELSOL project [16]. The Biocelsol-process is schematically presented in figure 3.3. This thesis uses solutions that are made according to WO 2009/135875 [17].

DISSOLVING PULP

BIOCELSOL

PRETREATMENT Mech. schredding

ENZYMATIC TREATMENT DISSOLUTION 7.8 wt% NaOH 0.84 wt% ZnO

PRESSING

FILTRATION, DEAERATION, REGENERATION

Figure 3.3: Schematic representation of the process steps of the Biocelsol process [15]

Chapter 4

Additives to cellulose In this chapter different additives to create an added value to cellulose will be discussed (microcrystalline chitosan, casein, sodium carbonate, TiO2 /AgCl particles and carbon nanotubes). Most emphasis will be put on the mixing of the additives with cellulose and the properties of the hybrid products.

4.1

Microcrystalline Chitosan (MCCh)

Chitosan is a chemical derivate of chitin formed by a deacetlyation reaction. Chitin (poly(β(1-4)-N-acetyl-D-glucosamine)) is the second most abundant natural polymer in the world after cellulose. It is found in nature as the main component of cell walls of fungi, exoskeletons of crustaceans (e.g. crabs and shrimps) and insects. In industrial processing, chitin is extracted from crustaceans by an acid treatment to dissolve the calcium carbonate followed by an alkaline extraction to solubilize proteins (see figure 4.1 a). [18, 19, 20] Chitosan poly(2- amino-2-deoxy-b-D-glucopyranose) is deacetylated chitin, but the deacetylation process is almost never completed. Therefore it is not always easy to make a clear distinction between chitin and chitosan. Kohr [21] notes that if the degree of deacetylation is higher than 50% it is called chitosan, if not we remain the name chitin. In the process of deacetylation, the acetyl amine group is substituted by a primary amino-group. (See figure 4.1). The most important properties of chitin and chitosan are the positive electrical charge, ability to bind to metals, biodegradability, biocompatibility, antimicrobial activity and nontoxicity. Chitin and chitosan have a broad range of application in different fields. Medical applications (e.g. absorbable sutures, wound dressing materials, artificial skin), photography, cosmetics, food products, agriculture, waste water treatment, biotechnology, product separation . . . [18, 20, 23] As a blend with cellulose, chitosan showed extraordinary performance, even addition of a small amount of chitosan to cellulose-based products increased significantly the mechanical strength, colorability, bactericidal properties, and other characteristics. [24] 10

Chapter 4. Additives to cellulose

11

Crustacean Cuticle (crab or shrimp shell) Mill

Cold 3N HCl soak

Decolourize (Solvent extraction) Demineralize

Treat with EDTA

Wash Cold 20% NaOH soak Deproteinize Wash/ dry Purified Chitin 50% NaOH at 120°C for 2h under nitrogen Deacetylate (repeated several times)

Chitin deacetylase

Wash/ dry Cold 20% NaOH soak Deproteinize

(a)

(b)

Figure 4.1: Schematic view of the process for making chitosan (addapted from [22]) (a) and the chemical structure of cellulose, chitin and chitosan[18] (b).

4.1.1

Solubility of chitosan

Most natural polysaccharides occurring in nature are neutral or acid, but chitosan is a highly basic polysaccharide. This makes it only soluble in strong acids (formic acid, acetic acid) which have pH lower than 6 [19]. This is because the primary amino groups (pKa 6.3) in the chitosan get protonated and become positively charged, making the chitosan a water soluble cationic poly-electrolyte. In an alkaline environment however, there is no protonization of the amino group destroying the hydrogen bonding, making it insoluble. [19] Blending with cellulose The easiest way to make a cellulose/chitosan blended film would be with the use of a common solvent. Cellulose-chitosan films have been formed from solutions in trifluoroacetic acid and chloral-dimethylformamide mixed solvent. [25] Another possibility is to chemically modify the chitosan in a more soluble form [26, 27]. The modification of chitosan is easier than that of cellulose due to the presence of the reactive amine group. A process worth mentioning here is the xanthate process, which is similar


12

to the viscose process for cellulose [19, 28, 29]. Here chitosan is chemically modified to dithiocarbamate chitosan (see figure 4.2) which is soluble in alkaline environment because the hydrogen bonding between the amino groups is broken. It is miscible with cellulose xanthate. Chitosan can be mixed with cellulose as a raw material, in the step of alkaline addition, xanthate or viscose formation. Although there are a good miscibility and good properties, the use of this process is not advisable since the same environmental problems as the viscose process occur. The use of CS2 is not eliminated. [28, 29]

Figure 4.2: Chemical structure of dithiocarbamate chitosan [28]

A more environmentally friendly and direct way to blend chitosan and cellulose without chemical modification is the use of polyelectrolyte complexes (PEC). PEC’s are formed by the reaction of a polyelectrolyte with an opposite charged electrolyte. Chitosan (polycation) can form complexes with Sodium alginate (polyanion) with different mechanisms. [30, 31] Sodium alginate is a salt of alginate which is found in the cell walls of 3 types of brown algae’s (Laminaria hyperborean, Ascophyllum nodosum and Macrocystis pyrifera). Commercial alginate is a mixed salt of the various cation found in the seawater such as Mg2+ , Sr2+ , Ba2+ and Na+ . Alginate is a linear polysaccharide that exists of blocks of 1-4 linked α-Lguluronic and β-D-mannuronic acid residue . The length and sequence of the blocks, as well as the overall molecular weight, determine the properties of the alginate. [32] When forming a polycomplex, chitosan is not directly dissolved, but instead dispersible microspheres of chitosan with an alginate membrane are created. Saether et al. [30] suggested a core-shell model for the PEC. A schematic representation of the membrane of the PEC at different pH’s can be seen in figure 4.3. At alkaline pH, the protonation of chitosan is suppressed, so the majority of the amine groups can form hydrogen bonding with other chitosan molecules, only a few amine groups are protonized and can react with the alginate. On the other hand, the alginate is completely deprotonized, resulting in carboxylic groups which have not participated in the complex formation with chitosan. These are compatible with the alkaline environment, making the particles dispersible. Since the ionic bonding between the polyelectrolytes are stronger than other secondary bonding (e.g. hydrogen or van der Waals bonding) the particles are stable. [30, 31, 32, 33]


13

Figure 4.3: Schematic representation of membrane formation mation of PEC’s between chitosan and sodium alginate (ALG: sodium alginate; CHD: chitosan). [31]

Microcrystalline Chitosan (MCCh) Chitosan can also be converted to a microcrystalline state (MCCh) which has superior properties compared to standard chitosan [34, 35, 36]. MCCh is a material that exists almost completely of small crystalline regions without amorphous regions. There are two major methods for manufacturing of microcystalline polymers. One way is to extract the non-ordened amourphous region physically and/or chemically. Another way is aggregation of macromolecules from solutions or dispersions by coagulation, precipation or crysallisation. The final product has a lower degree of polymerization (DP), a higher crystallinity index (CRI), a higher energy of the hydrogen bounds (EH ) and a higher water retention value (WRV) than to the initial chitosan. Nousiainen et al. [37, 38] produced a hybrid viscose/MCCh fibre. Since chitosan is insoluble in alkaline environment, they used sodium alginate as a dispersing agent. MCCh/sodium alginate was used in a 2:1 ratio, this creates the microspheres as mentioned earlier. Up to 6 % MCCh was miscible. The resulted fibre showed a slightly reduced tenacity, increased water retention value, titer, and elongation compared to standard viscose fibers.


4.2

14

Casein

In this part, the preparation of cellulose solutions with casein as an additive will be discussed. Casein is a protein found in milk of mammals. Casein represents about 80% of the proteins found in milk. It has the lowest solubility at a the iso-electric pH (pI) which is 4.6. In milk, caseins form a micelle structure [39]. Films prepared from a blend with cellulose and casein have showed to have higher mechanical properties compared to pure cellulose films [40, 41, 42]. Fibres made from the blend showed to have wool like properties [43]. Like all proteins, casein is made of amino acids that are bonded together by peptide-bonds. This peptide bond is formed by a condensation reaction (see figure 4.4). Amino acids show acid and basic behaviour, depending on the pH of the environment, the net charge of the protein changes. At the iso-electric pH (pI), the net charge of the amino acid is zero.

Figure 4.4: formation of peptide bond between two amino acids [44]

In the protein structure different levels can be identified. The primary structure is the sequence of the amino acids. The primary structure organizes in a secondary structure determined by hydrogen bonding. The tertiary structure refers to the complete threedimensional structure of a polypeptide-chain. This structure is determined by the spatial organisation of the different secondary structure of a polypeptide chain. A quaternary structure occurs when two different polypeptide chains interact with each other. Some proteins contain disulphide bonds, this is a bonding between the residues of two cysteines. The two major secondary structures are the α-helix (figure 4.5) and the β-sheet (figure 4.6). In the α-helix, each amino group makes a hydrogen bonding with the third amino group away from it. The side chains are on the exterior of the helix. Amino acids as Ala, Asp, Glu, Ile, Leu and Met favour the formation of α-helices, while Gly and Pro disrupt formation of it. Casein has a high number of proline residues (Pro), so the formation of secondary structures is hindered. Caseins contain no disulphide bonds. Caseins have no tertiary structures but show a miccelar structure. β-sheets are composed of two or more different regions of stretches of at least 5-10 amino acids. The stretches are aligned together and stabilized by hydrogen bonding between amino groups of different stretches. [39, 44, 45]


15

Figure 4.5: α-helix structure [44]

4.2.1

Solubility, miscibility and film formation

The influence of the pH on the structure of casein has been studied by Chakraborty et al. [46]. If the conditions are basic, the proteins are deprotonized, leaving a net negative charge. This negative charge increases the repulsion between the different amino acids and the protein chain unfolds to a random coil conformation. This makes them easily soluble. If the pH is reduced to lower values, the negative charges will be neutralized by the protons. This will cause the protein to fold. The maximum folding happens at the iso-electric pH. The absence of electric charges reduces the repulsion to a minimum. Because there is no repulsion, it can form aggregates with other casein proteins and precipitate out of solution. At pH lower than the pI, the added acid will disturb the electrostatic interactions formed at pI. This will cause the proteins to relax and form an intermediate state between the random coil (alkaline pH) and folded structure (at pI). [46] Fibres made from casein are being produced by dissolving casein in sodium hydroxide. The concentration of sodium hydroxide can be low (0,1-0,5 %) [47, 48]. This is because the pI of casein is rather low (4-6) and adding of a small amount of sodium hydroxide will cause an increase of net negative charge. 20 % of casein is easily dissolved. The regeneration bath is diluted sulphuric acid in a concentration of about 5 wt%. [47, 48] A 1939 patent [43] describes a method for forming a cellulose/casein blend fibre through the viscose process. It states that to form a homogeneous mixture of viscose and casein, casein first has to be chemically decomposed. This decomposition can be done by hydrolysis with steam under pressure, by the use of acid or alkalis or by enzymes. [43] Zhang et. al [40, 41, 42] have studied the formation of blend membranes of cellulose and casein in different solvents. One of these solvents is a 6 wt% NaOH/ 4 wt% urea aqueous solution at 4◦ C. A cellulose solution of 4 wt% cellulose and a casein solution of 23 wt%


16

Figure 4.6: β-sheet structure [44]

casein were prepared. The coagulation bath was diluted sulphuric acid. They found that cellulose and casein were miscible up to 15 wt% casein and that optimal mechanical properties are achieved at 10 wt% casein at coagulation conditions of a 5 wt% sulphuric acid coagulation bath for 5 min. They explained that the blend was miscible because of formation of hydrogen bonding between the hydroxyl group of cellulose and the peptide bond of casein. The strength of the membrane increases because casein promotes the formation and arrangement of hydrogen bonding in cellulose, also the number of crystalline nuclei increases with increased amount of casein. At higher concentration the strength reduces because of the reduced miscibility. The maximum strength at 5 wt% H2 SO4 is explained: at very low concentrations there is almost no regeneration in a reasonable time; at high concentrations of sulphuric acid the coagulation rate is very fast, so it restricts the crystallization and arrangement of cellulose with the casein. [41]

4.3

Sodium carbonate

In this part, sodium carbonate will be discussed as an additive for the cellulose solution. Sodium carbonate is a blowing agent, because when in contact with an acid, it will form carbon dioxide which will create pores in the structure. Cellulosic porous structures have shown to be an excellent product in the field of tissue engineering. Cellulose is biocompatible and sponges have been used in the support of bone tissue and have shown the ability to induce cell migration and promote cell growth. [49, 50, 51, 52, 53, 54, 55, 56] There are several ways to create cellulosic porous structures and some will be discussed here briefly. One method is freeze-drying, a thermal phase separation method in which the temperature of the homogeneous polymeric solution is lowered to induce a phase separation. Two liquid phases will form, a polymer rich phase and a polymer poor phase. Further cooling


17

will cause the polymer poor phase to freeze. By keeping the temperature very low and by creating a vacuum, the solvent will sublimate while the polymer phase stays. The big disadvantage of this technique is that its time and energy consuming (drying takes several days and during this period, vacuum has to be applied and temperature kept low). [56, 57] Another possibility to create porous structures is by adding Glauber’s salt (hydrated Na2 SO4 ) crystals of appropriate size into the viscose solution. This solution is then coagulated by heating (e.g. in water) and in this bath, the majority of the crystals are dissolved, creating a coagulated porous structure. The next step is the regeneration of the structure in an acid precipitation bath. The disadvantage is that Glauber’s salt is soluble in water. If they are dissolved in too large amount into the viscose solution, the pore volume will become smaller and the solution will become less fluid. In order to minimize dissolving, the Glauber’s salt has to be mixed at low temperatures and it’s needed to proceed to the next step quickly. [49, 54, 56]

4.3.1

Manufacturing of viscose cellulose sponges using sodium carbonate

Sodium carbonate Sodium carbonate (aka soda ash) (chemical structure see figure 4.7) is the sodium salt of carbonic acid. Its water solubility is 71 g/l water at 0◦ C, 215 g/l water at 20◦ C and 455 g/l water at 100◦ C. Sodium carbonate is a strong alkaline compound with a pH of 11.6 in a 0.1 M aqueous solution. About 70 % of the world production capacity of sodium carbonate is manufactured by the Solvay process, invented by the Belgian chemist Ernest Solvay. [58, 59]

Figure 4.7: Chemical Structure of Sodium Carbonate [59]

In alkaline solution of NaOH the sodium carbonate will dissolve and no reaction will take place. This is because sodium carbonate is a salt of a weak base and a strong acid. The pKa of CO3 2- is 10.33, which practically means that in the pH range of the alkaline cellulose solution (see previously) there will be exclusively carbonate ions and no buffering action will take place (see reaction 4.1). On the other hand in an acid solution of H2 SO4 , the carbonate ion, which is a strong base, will react with the sulphuric acid forming CO2 , water and the


18

acid salt of the original carbonate salt (see reaction 4.2). 2− + −− * HCO− − − CO3 + H 3 )

(4.1)

Na2 CO3 + H2 SO4 −−→ Na2 SO4 + H2 O + CO2 ↑

(4.2)

Sodium bicarbonate Sodium bicarbonate (aka baking soda) (chemical structure see figure 4.8) is used in the paper and pulp industry as a foaming and swelling agent. Sodium bicarbonate is produced by addition of carbon dioxide to a solution of sodium carbonate, which at sufficiently high concentration will precipitate out of solution (see reaction 4.3). Sodium bicarbonate starts decomposing when heated over 50◦ C, releasing CO2 , H2 O and Na2 CO3 , with total decomposition at 270◦ C, its water solubility is 69 g/l at 0◦ C, 96 g/l at 20◦ C and 165 g/l at 60◦ C. [60] Na2 CO3 + CO2 −−→ 2 NaHCO3 ↓ (4.3)

Figure 4.8: Chemical Structure of sodium bicarbonate [60]

In alkaline solutions of NaOH sodium bicarbonate will react. The bicarbonate ion, which is amphoteric, will act as an acid and react with NaOH and form sodium carbonate (see reaction 4.4). This buffering action will cause the pH to drop and decrease cellulose solubility. In acid solution of H2 SO4 , NaHCO3 will react with the sulphuric acid forming CO2 , water and the acid salt. In this thesis, NaHCO3 would first be dissolved in an alkaline cellulose solution. Sodium bicarbonate will react with NaOH of the cellulose solution. Therefore it could not be used as an additive. NaHCO3 + NaOH −−→ Na2 CO3 ↓

(4.4)

NaHCO3 + H2 SO4 −−→ NaHSO4 + H2 O + CO2 ↑

(4.5)

Miscibility with cellulose Carbonate salts have been used as an foaming agent for the production of hollow and inflated porous viscose products [49, 50, 51, 52]. Figure 4.9 shows the cross-section of a viscose fibre


19

made with sodium carbonate as an additive. Almost any kind of viscose solution can be used with a cellulose content of 6.5 to 12 % and a sodium hydroxide content between 4 and 10 %. Any kind of carbonate salt can be used, as long as it does not affect the properties (that is why bicarbonate cannot be used, see 4.3.1). In this thesis, sodium carbonate is used, as it is soluble in alkaline solution. Generally there is no limitation on the procedure of adding the carbonate salt to the cellulosic solution. It can be added directly, or by first dissolving it in water or a sodium hydroxide solution. Stirring is required to get a uniform mixture. The proportion of carbonate salt may vary between 10 and 500 weight percentage compared to cellulose (wtc%). With a too low amount, the production of gas cells is insufficient to provide inflation, a too high amount reduces the mechanical properties too much.

Figure 4.9: Longitudinal transillumination of a hollow viscose fibre with sodium carbonate as an additive [53]

Regeneration usually takes place in a bath with different properties than in the normal viscose process and is a very delicate process. On entering the regeneration bath, a thin cellulose film is formed at the outside and the acid starts to diffuse through this film to the material, regenerating the cellulose and reacting with the sodium carbonate. This reaction forms CO2 gas which forms gas cells since evasion is prevented by the cellulose film. In general, the regeneration bath requires a higher acid content, temperature, zinc and sodium sulphate content than the standard viscose process. The higher the temperature, the higher the degree of inflation. A higher acid level increases the degree of inflation since the acid has a triple effect: it coagulates, regenerates the cellulose and produces CO2 . The zinc sulphate content of the coagulation bath affects the coagulation: The formation of a denser


20

and less permeable skin gives rise to an increased inflation. However, the effect is tempered by the reduced diffusion of the acid through the membrane. The sodium sulphate affects the coagulation and shrinkage, and so increases the barrier for gas. As initially most acid is used to neutralize the viscose before gas generation, inflation does not start until a high level of sodium sulphate is used (18-20%). [49, 50, 51, 52]

% acid

(a)

Figure 4.10:

4.4

inflation

inflation

inflation

Because of the porous structure, the material has a lower tenacity, higher water absorbency and better insulation (sound and heat) compared to standard viscose. Increased mechanical properties can be achieved by using reinforcement fibres. Different kind of fibres can be used (natural & synthetic) with an amount varying between 5-200 wtc%. A too low amount will not provide any reinforcement, a too high amount will suppress foaming in the regeneration bath. [49, 51]

% zinc sulphate

(b)

% sodium sulphate

(c)

influence of acid (a), zinc sulphate (b) and sodium sulphate (c) content of the regeneration bath on inflation. (adapted from [51]

Titanium oxide particles coated with silver chloride

The manufacturing of antibacterial films will be discussed in this chapter. The antibacterial properties are created by the use of titanium oxide particles coated with silver chloride. The antibacterial properties of silver have already been known for centuries: the ancient Romans knew that water, wine and vinegar stayed longer fresh in silver barrels. In recent years, silver has gained popularity in the medical field as an antimicrobial agent since many bacteria became immune to antibiotics. The bacterial resistance against silver has only been observed rarely [61]. Silver has also been used in air and water filtration to eliminate microorganisms.[61, 62, 63] Silver works against single celled microorganisms by destroying the oxygen metabolism enzyme. However, the exact mechanism remains to be understood, only models have been proposed. The process is shown in figure 4.11. Silver ions can bind to the cell surface,


21

causing disruption of the membrane function and allowing the silver ions to penetrate the cell (1). The silver ions are reactive and can bind with thiol-groups of enzymes (2), causing the enzymes to denature. Since enzymes are responsible for the oxygen supply, the cell will suffocate. Silver can also bind with the base pairs of DNA and so inhibit the replication of DNA (3). [63, 64, 65, 66]

Figure 4.11: Action of silver on microorganism [64]

4.4.1

TiO2 /AgCl composite particles

The antimicrobial agent used in this thesis are titanium oxide particles coated with silver chloride. The product is brought to the market by the JMAC technology of Clariant. The TiO2 /AgCl particles can be formed by producing a slurry of titanium oxide in an aqueous solution of silver nitrate so silver chloride can precipitate on the titanium oxide. The best size of titanium oxide particles is between 1 and 15 µm in diameter and in the end the concentration of silver chloride on the particles will be 20 wt% of TiO2 . Titanium oxide is very suitable as a carrier since it is insoluble and stable in water and has a large surface area. Silver chloride is suitable as the antimicrobial substance since it has a very low solubility in water and the silver is in anionic form. This will cause a limited release of silver ions from the carrier into solution. This very slow rate of release allows the particles to be


22

very stable over time. It is believed that because of the slow release rate of silver chloride, the concentration of silver ions in the solution is low, but effective enough for antibacterial properties without being toxic (figure 4.12). The particles tend to cluster together in spheres with some space between the different particles. This is proven by SEM-microscopy (figure 4.13) . The composite particles can be easily dispersed in high viscous liquids. It has been shown that a 300ppm dispersion will remain suspended when the viscosity is higher than 0.0025 Nsm-1 . When trying to disperse the particles in a low viscosity single phase medium, settling out of the particles may occur. The particles are non-toxic for higher life forms, but are very effective against a variety of micro organism like bacteria, yeast and moulds. [67, 68, 69, 70]

Medium De-ionised water Ag+ 20-40 ppb

Medium sea water Ag+ 2-4 ppm

+ + + ++++

AgCl coating 20% of particle weight

TiO2 + + + ++++

TiO2

+ + + ++++

+

+ + + ++++ + + + + + + + + + +

+ + + ++++

+ + + ++++

TiO2

+ + + ++++

+

+ + + + ++++

TiO2

TiO2

+ + + ++++

+

Saturated solution Of AgCl

+ + + + ++++

+

TiO2 + + + ++++

+ + + ++++

Figure 4.12: Structure of TiO2 particle and principle of controlled release (adapted from [69])

The titanium oxide/silver chloride particles show excellent antibacterial properties. The antibacterial properties of the composite particles can be proved by the Minimum Inhibitory Concentration (MIC). The MIC is the lowest concentration of an antibacterial compound that will inhibit the visual growth of microorganism after an overnight of incubation. The titanium oxide coated particles show a MIC of 160 ppm for bacteria.Table 4.1 shows the MIC values of some microorganism. [68] Antimicrobial cellulose fibres using JMAC have been manufactured using the Lyocell process [71]. The lyocell process for making regenerated cellulose is a well known process and uses N-Methylmorpholine N-oxide (NMMO) as a direct solvent for cellulose [1, 72, 73]. JMAC powder is dispersed in water and then added to the spinning solution. The fibres showed very good antimicrobial properties at very low concentration of JMAC. There was no clear increase in antimicrobial properties of fibres containing 0.0125 wtc% and 0.0250 wtc%. It was


23

Figure 4.13: Scanning electron micrography of the silver chloride/titanium oxide composite particles 3 to 5 µm in size. [70]

Table 4.1: MIC values for some microorganism (adapted from [68])

Microorganism

TiO2 /AgCl (ppm)

Bacteria Yeast Moulds

160 80-160 >160

also shown that the antimicrobial properties were durable, they sustained after dyeing and washing. The mechanical properties did not change much after addition of the particles.[71]

4.5

Carbon Nanotubes (CNT)

Carbon nanotubes (CNT) as an additive to cellulose will be discussed in this chapter. Polymer/CNT composites have shown to have superior properties compared to the reference polymer: higher tensile strength, modulus and conductivity. [74, 75] Carbon nanotubes are allotropes of carbon which consists of a cylindrical nanostructure. They were first observed by Iijima in 1991 [76]. They only consist of carbon atoms connected by sp2 bonds and that is why they are often seen as a layer of graphite rolled up. The two most common types of carbon nanotubes are single walled carbon naotubes (SWNT) and multi-walled carbon nanotubes (MWNT) (see figure 4.14). MWNT consist of multiple rolled layers (concentric tubes) of graphite. There are two models which describe the structures of MWNT: the Matryoshka model where SWNT are arranged as concentric cylinders (see figure


24

4.14 b) and the Parchment model, which consists of 1 layer of graphite rolled up. There are several ways to produce CNTs. The three methods most commonly used are arc-plasma evaporation, chemical vapor deposition (CVD), and laser vaporization. [74, 77, 78, 79]

(a)

(b)

Figure 4.14: Schematic representation of single walled carbon nanotubes (a) and multi walled carbon nano tubes (b) [80]

Carbon nanotubes are characterized by their unique properties. The elastic modulus of SWNTs is close to 1 TPa (this is similar to that of graphite in plane and five times as high as stainless steel). The maximum tensile strength is close to 30 GPa and the elongation at breaking is about 5%. The electrical resistivity is in the order of 10−4 Ωcm. They are stable at high temperature (750◦ C at atmospheric condition, 2800◦ C in vacuum). [78, 79, 81]

4.5.1

Polymer/CNT composites

It is the combination of the unique electrical and mechanical properties of CNT that makes it the ideal reinforcing agent. However there are some difficulties with CNTs. CNTs, just like polymers, are not all the same: they are a mixture of various chiralities, diameters and lengths. Defects, impurities and aggregation of CNTs reduce the mechanical properties of the composite. Also, since CNTs are so small, they are often curled and twisted. Therefore individual CNTs in a polymer matrix only inhibit a fraction of their full potential as a reinforcing agent. To overcome these problems and improve the transfer of properties, CNTs need to have a strong affinity with the polymer matrix. Modified CNTs show to have a better dispersion and stress transfer in a polymer matrix. Modification is done by binding a functional group or a whole polymer to the nanotube in a covalent of non-covalent way. A possible non-covalent way is wrapping of polymers on the surface of the CNTs. The sp2 configuration of the CNT sidewall allows interaction with conjugated polymers or


25

polymers containing heteroatoms with a free electron pair. The advantage of non-covalent functionalization is that it does not alter the structure, and so the properties, of the CNTs. Covalent bonding means grafting the CNT. [75, 82, 83] Moulton et al. [82] studied biomolecules as a selective dispersant for CNTs. They successfully dispersed MWNTs in Salmon sperm DNA, chondroitin sulphate salt and chitosan. They observed that more than 50% of the CNTs remained dispersed after 3 days using chitosan as a dispersant. Impurities in the CNTs did sediment out. The dispersion of CNTs has been explained by wrapping of the biomolecules around the CNTs. Spinks et al. [84] also dispersed CNT in chitosan. Wei et al. [85] successfully used cellulose xanthate as a dispersant for CNTs.

4.5.2

Cellulose/CNT composites

Rahatekar et al. [86] made cellulose/CNT composite fibres using an ionic solvent (ethylmethylimidazolium acetate (EMIAc)). They observed good miscibility of CNT with EMIAc and shear thinning of the solution occurred. To spin the fibres, they had to dilute the spinning dope. The resulted fibre had a much higher strain to break and an improve of tensile strength compared to standard cellulose fibres. Lu et al. [87] made cellulose/CNT composite fibres using the Lyocell process. MWCNT were functionalized with sodium dodecylbenzene sulfonate (SDBS). The use of SDBS improved the dispersion of MWNT in NMMO. They found that the mechanical properties (modulus & strength) of the fibres increased with addition of CNT. The optimal concentration was 1wt%. The thermal stability of the fibres was also improved. Wei et al. [85] have investigated the dispersion of MWNT’s in an aqueous cellulose xanthate solution (viscose solution). MWNTs were homogeneously mixed with the cellulose with the aid of sonication. Afterwards the films were coagulated in a sulphuric acid bath. The dispersion of MWNTs in the viscose dope is explained by the negative charge of the cellulose xanthate. Cellulose xanthate can wrap around individual MWNT (see figure 4.15). Also, since the MWNT’s are now bearing negatively charged cellulose xanthate, there is repulsion between the MWNT’s. This causes a good dispersion. The dispersion was stable over time and the MWNTs were not destroyed during cellulose regeneration.

Figure 4.15: Schematic representation of cellulose xanthate wrapped around CNT [85]

Part II

Experiments

26

Chapter 5

Materials and methods 5.1 5.1.1

Enzyme-treated pulp and cellulose solution Pure cellulose solution

Dissolving grade pulp of high quality softwood was supplied by Domsjö Fabriker Ab (Sweden). The pulp consists of a mixture of spruce and pine (60 %/ 40 %) and is produced through a two-stage sodium based cooking process. The cellulose is chlorine free (TCF) bleached in a closed loop bleach plant (CLB). The pulp was mechanically shredded for 5 hours with a a Baker Perkins Mixer at pulp consistency of 20 %. Afterwards the pulp was enzymatically treated with a commercial endoglucanse-rich enzyme preparation for 3 hours, at 50◦ C and at pH 5. The enzymatically treated pulp was dissolved into an aqueous sodium hydroxide and Zinc oxide according to WO 2009/135875. The values of the cellulose, sodium hydroxide and zinc oxide can be seen in table 5.1 .The final solution was studied under microscope and only very few undissolved particles were observed. Table 5.1: Cellulose solution constitution

Cellulose solution parameters Cellulose, %

NaOH, %

ZnO, %

6

6.5

1.3

27

Chapter 5. Materials and methods

5.2 5.2.1

28

Film formation Glass plate method

The films were prepared by the glass-plate method. This process is shown in figure 5.1. The cellulose solution is pressed between two glass-plates, that are then slided over each other. On both glass plates will be a layer of the cellulose solution. These plates are placed in a coagulation bath of an appropriate acid to regenerate the cellulose. Afterwards the films are washed with water and dried in air when mounted on a cup to prevent shrinkage.

Cellulose solution, which is stored in a freezer, is thawed.

The glass plate is put in the coagulation bath, where the cellulose film is formed

Cellulose solution is spread on a glass plate, a second plate is pressed on the first one

The cellulose film is washed

The two plates are slided over each other, leaving a thin layer of cellulose solution

The formed cellulose film are dried under tension in air

Figure 5.1: Schematic representation of the glass plate method for producing cellulose films

5.2.2

Cellulose/casein films

Casein was Bovine milk casein of industrial grade from TUT’s reserve. Casein was dissolved in a 6.5 wt% NaOH aqueous solution to obtain a casein concentration of 100g/l. The blend of cellulose and casein was obtained by mixing the cellulose solution with the casein solution. Different mixtures were prepared with different weight ratios of cellulose to casein. 100:00, 95:5, 90:10, 85:15, 80:20 and 70:30. The formula to determine the amount of casein solution needed to obtain a cellulose/casein mixture of x:y is given by formula 5.1.


Vcas = Mcel ∗ α ∗

29

y 1 ∗ 1000 ∗ x Ccas

(5.1)

where: Mcell α x y Ccas

= = = = =

mass of the cellulose solution (g) cellulose content of the cellulose solution (%) cellulose fraction of the cellulose/casein mixture casein fraction of the cellulose/casein mixture concentration of the casein solution (g/l)

The mixtures were mixed with a Ika RW20 mixer (Ika, Germany) to obtain a homogeneous blend. The mixture was stored overnight to reduce the amount of air bubbles. Next the films were prepared by the glass plate method (see 5.2.1). The coagulation conditions were 5 and 10 wt% sulphuric acid concentration in the coagulation bath and the coagulation time was 5 minutes. Afterwards the films were washed and dried.

5.2.3

Cellulose sponges

Influence of the carbonate salt on the stability of the cellulose solution The influence of the amount of sodium carbonate on the stability of the cellulose solution was studied. Sodium carbonate was first dissolved in a 6.5 wt% aqueous solution of sodium hydroxide in different quantities: 5 g/100 ml, 10 g/100 ml and 15 g/100 ml. The dissolved sodium carbonate was slowly added to the cellulose solution while stirring. Visual examination was done to examine when gel formation occurs. The influence of calcium carbonate on the stability of the cellulose solution was investigated. Calcium carbonate powder was added to the cellulose solution and mixed with a Ika RW20 mixer (Ika, Germany) until a homogeneous mixture was obtained. Visual examination of the solution was done. Influence of the coagulation bath The influence of the kind of acid of the coagulation bath was studied. H2 SO4 and HCl were the coagulants used. Films were coagulated in a 10 wt% solution of the acid. Afterwards the films were dried. Visual examination was done. Influence of the amount of CaCO3 Films were produced where the cellulose solution contained 0, 100, 200 and 300 wtc% of CaCO3 . The regeneration bath was a 10 wt% aqueous solution of HCl, the salt content was 0 % and the temperature of the coagulation bath was 50◦ C. The films were regenerated untill the formed films were transparent.


30

Influence of the temperature Films were prepared with a cellulose solution where 100 wtc% CaCO3 was added. The regeneration bath was a 10 wt% aqueous solution of HCl and the salt content was 0 %. The different temperatures were room temperature (22◦ C)-40 and 60◦ C. The time was measured to see when the films became completely transparent. Influence of salt content of the coagulation bath Films were prepared with a cellulose solution where 100 wtc% CaCO3 was added. The regeneration bath was a 10 wt% aqueous solution of HCl and the temperature was 50◦ C. The different salt contents of the coagulation bath were 0, 10, 15, 20 and 25 wt% of NaCl. Thickness of the cellulose sponges Trials were done to make thicker cellulose sponges. The coagulation bath was a 15 wt% aqueous solution of HCl with 20 wt% NaCl added. The temperature was 50◦ C. To the cellulose solution, 200 wt% CaCO3 (compared to cellulose) was added. To make thicker sponges, the cellulose solution was added in a little cup made of woven polyester. To test the coagulation mechanism, a 6 wt% cellulose solution with CaCO3 was used. The cross section of the sponge was studied after 10, 30 and 90 minutes. The influence of the cellulose concentration on sponge formation was studied. The total amount of cellulose added to the cup was 1.2 g, the concentration of cellulose was 2, 3 and 6 wt%. Sponge drying After coagulation, the thick sponges were washed in water. Afterwards a solvent exchange was done with ethanol by putting the sponge in 20-50-100 wt% ethanol. Afterwards the films were dried in air.

5.2.4

TiO2 /AgCl particles

TiO2 coated with AgCl was provided by Addmaster (Staffor, UK). The silver chloride/titanium dioxide composite is a white, light-stable, free-flowing powder with a particle size of 3-5 µm, consisting of silver chloride deposited on titanium in a ratio of 20 % to 80 %. To evaluate the stability of TiO2 /AgCl in NaOH, 1 g of TiO2 /AgCl particles were dispersed in 20 ml of a 6.5 wt% aqueous solution of NaOH. Afterwards, the solution was added to an excess of H2 SO4 . Visual examination was done. To obtain a cellulose solution with 2 wtc%, 0.032 g of TiO2 /AgCl particles were dispersed in 3ml 6.5 wt% aqueous solution of NaOH. Afterwards this was added to 30 g of cellulose solution and mixed with a Ika Baker mixer (Germany). Films were prepared with the glass plate method, the coagulation bath was of 10 wt% aqueous solution of H2 SO4 . The regeneration time was 30 minutes.


5.2.5

31

Carbon nanotubes

Multiwalled carbon nanotubes, grade NC700, were purchased from Nanocyl (Belgium). The particles were produced in multi-tons, via the Chemical Vapor Deposition (CVD) process. The properties are shown in table 5.2. Table 5.2: Properties of the carbon nanotubes

Property Average Diameter Average Length Carbon Purity Metal Oxide Surface Area

value 9.5 nm 1.5 µm 90% 10% 250-300 m2 /g

The particles were dispersed in the cellulose solution by sonication using a Hielscher UP200S dispenser (Hielscher, Germany). First it was investigated if a cellulose solution of 1, 2 and 6 wt% cellulose is a good dispersant for CNT. A 1 wt% cellulose solution was prepared by diluting 1.5 g of a 6 wt% with 7.5 ml of a 6.5 wt% aqueous solution of NaOH. A 2 wt% was prepared by diluting 3 g of a 6 wt% cellulose solution with 6 ml. Next a small amount (spatletip) of CNT was dispersed in the solution by sonication. The same amount of CNT was dispersed in 9ml H2 O. The suspensions were kept at room temperature (22◦ C) and visual examination was done. It was investigated how much MWNTs a 1 wt% cellulose solution can disperse. For this cellulose:CNT mixtures of 20:1, 15:1, 10:1 and 5:1 were made by sonication of 6 ml of a 1 wt% cellulose solution with 0.003, 0.004, 0.006 and 0.012 g MWNT. The solutions were investigated under optical microscope. Cellulose films containing 0.5, 1 and 2 wtc% MWNT were prepared. Therefore MWNT’s were dispersed in a 1 wt% cellulose solution to have a cellulose:MWNT ratio of 20:1. Next the dispersion was added to a 6 wt% cellulose solution to obtain cellulose solutions with 0.5, 1 and 2 wtc% MWNT. Table 5.3 shows the amounts of a 6 wt% cellulose solution and of a 1 wt% cellulose: 5 wtc% MWNT dispersion needed to make 0.5, 1 and 2 wtc% MWNT dispersions. Films were prepared with the glassplate method (see 5.2.1), the coagulation bath was a 10 wt% aqueous solution of H2 SO4 and the coagulation time was 5 minutes.


32

Table 5.3: Amount of CNT needed

5.3 5.3.1

MWNT (wtc%)

1wt% cellulose: 5 wtc% MWNT (g)

6 wt% cellulose (g)

0.5 1 2

12 24 48

18 16 12

Properties of the films Optical microscope

The films were investigated under an optical microscope (Leitz Laborlux D).

5.3.2

Fourier Transform Infrared

The cellulose films containing casein were investigated using Fourier Transform Infrared Spectroscopy (FTIR) (PerkinElmer, USA) after they were dried under tension. The scanning range was 650-4000 cm-1 . To investigate casein powder, tablets were made by compressing a mixture of 1 mg casein and 200 mg potassium bromide. Potassium bromide was used because it is transparaet to infrared radiation between 4000 and 400 ctm-1 [88]

5.3.3

Acid dyeing of cellulose/casein hybrid films

The cellulose/casein films were dyed with a red 266 (cherry red) acid dye. The cellulose films with a 0, 10, 15 and 20 % casein fraction were first soaked for 1h to become fully wet. Afterwards, each film was put in 200 ml of a 3wt% aqueous solution of acetic acid with 0.5g of acid dye. The dyebaths were heated up and kept at elevated temperatures (85◦ C) for 30 minutes. Then the films were rinsed and washed with detergent to remove unfixed dyestuff. Visual examination of the films was done.

5.3.4

Scanning electron microscope

Scanning electron microscope images (SEM) were taken of cellulose films containing CNT using a Philips XL30 microscope (Philips, The Netherlands). The accelerating voltage during imaging was 15 kV. Before examination, the samples were coated with gold by using a sputter coater.

Chapter 6

Results and discussion 6.1 6.1.1

Casein Film formation

Casein was first dissolved in a 6.5 wt% aqueous solution of NaOH. 100g/l and 200 g/l casein solutions were prepared. The 100g/l casein solution showed an orange colour (figure 6.1 (a)) and the 200 g/l solution was not liquid, but a foam (figure 6.1 (b)). The 200 g/l casein foam could not be used as an additive for the cellulose solution because it could not be mixed with the cellulose solution. The 100 g/l casein solution was miscible with the cellulose solution. Figure 6.1 (c) shows a picture of a mixture of equal amounts of cellulose and casein. The solution turned white, this might be due to a partial coagulation of the cellulose. Cellulose-casein hybrid films were made by coagulation of a cellulose-casein mixture in a 5 wt% aqueous solution of H2 SO4 for 5 minutes. During regeneration of the films, the coagulation bath remained transparent and no colour change occurred. The films were of sufficient strength to let them dry under tension.

(a)

(b)

(c)

Figure 6.1: Picture of a 100 g/l casein solution (a), a 200 g/l casein solution and a mixture of casein and cellulose solution (c)

33

Chapter 6. Results and discussion

34

When adding a 100 g/l casein solution to a 10 wt% aqueous solution of H2 SO4 , the solution turned milky white. This was due to the reduced solubility of the proteins in acidic environment which is caused by folding of the protein structures (see section 4.2.1). The absence of this white colour in the 5 wt% H2 SO4 coagulation bath of the films indicates that casein stayed in the cellulose films during coagulation.

6.1.2

Detection of casein in the cellulose films

Fourier transform infrared Figure 6.2 shows the Fourier transform infrared spectra of a 100% cellulose film, a cellulose:casein (80:20) film and casein powder. The area of interest is between 1700 and 1400 cm-1 .

casein

cellulose: casein 80:20

cellulose

1700

1650

1600

1550

1500

1450

1400 cm-1

Figure 6.2: FTIR casein

Casein has several absorption bands in the 1450-1650 cm-1 region. The bands at 1650 and 1550 are assigned to the amide 1 and amide 2 vibration of the peptide bond (see figure 6.3). If casein would be detected in the cellulose films, the absorption peak at 1650 cm-1 would become deeper (due to the amide I vibration which has a strong absorption peak) and there would be an absorption peak at 1550 cm-1 because of the amide II vibration. In our samples this is not seen. This indicates that there is no peptide bond detected. However this does not mean that casein is not in the sample. The absence of the peaks of the peptide bond might be due to a non-mixture of casein and cellulose.


35

Figure 6.3: The vibrations responsible for the Amide I and Amide II bands in the infrared spectra of proteins and polypeptides. The Amide I band is due to carbonyl stretching vibrations while the Amide II is due primarily to NH-bending vibration [89]

Acid dyes

C

R

N

H3+

OH N

R’

C

- OS 3

C

H

O

H

To detect casein in the cellulose films, acid dyes were used. Red 266 (cherry red) was used as an acid dye. Acid dyes can be used to dye protein fibres because they are able to bind ionically to the amine group of the peptide bond in the proteins (see figure 6.4) The films were dyed in an aqueous solution of 3 wt% acetic acid at elevated temperature for 30 min. Afterwards the films were washed overnight in water. Figure 6.5 (a)-(d) shows pictures of cellulose films with a casein fraction of 0, 10, 15 and 20 %. The films with casein retain the colour, the film with only cellulose did not have any colour. Then films were washed in detergent at 75◦ C and rinsed with water, the colour of the films almost completely disappeared. Microscopic images of a cellulose film with a casein fraction of 15 % are shown in figure 6.6. The films before detergent washing (figure 6.6 (a)) are completely red, the films after detergent washing (figure 6.6 (b)) are almost colourless, but some shade of red remains.

NH2

N

Cl

3FC

Figure 6.4: Schematic representation of the ionic bonding between the amine group of a peptide bond and red 266 acid dye


36

(a)

(b)

(c)

(d)

Figure 6.5: pictures of dyed cellulose:casein films before washing with detergent: 100:0 (a), 90:10 (b), 85:15 (c) and 80:20 (d)

(a)

(b)

Figure 6.6: microscopic images of a cellulose film with a casein fraction of 15%: before (a) and after (b) detergent washing

The loss of colour of the 100 % cellulose films when washing is explained by the absence of casein, the acid dye is unable to bind with cellulose. The films containing casein stayed coloured, however the film with a casein fraction of 20 % showed very uneven dyeing. After detergent washing the films reduced in colour and the distribution was uneven. The colour


37

indicates that there was some casein in the film but that casein was badly dispersed. The bad dispersion probably occurred during coagulation. During coagulation, the pH will drop from highly basic to acid and pass by the cloud points of cellulose (pH 10-11) and casein (pH 4-6). Because the cloud point of cellulose is at higher pH, coagulation of cellulose will occur before coagulation of casein. This will result in a non homogeneous mixture in solid state.

6.2 6.2.1

Cellulose sponges Influence of a carbonate salt on the stability of the cellulose solution

The influence of sodium carbonate on the stability of a cellulose solution was studied. This was done by dissolving Na2 CO3 in different amounts in an aqueous solution of sodium hydroxide and then adding this to the cellulose solution. When using a 50 g/l sodium carbonate solution, the cellulose solution did get diluted but no gelling occurred. When the amount of sodium carbonate was 200 wtc%, the solution was so diluted that no sponges could be made. With a 100 g/l sodium carbonate solution, the solution seemed to become somewhat unstable when there was 200 wtc% sodium carbonate. But no gel formation occurred, no sponges could be prepared because the sponge broke during coagulation. When a 150 g/l sodium carbonate solution was used, gel formation occurred when the sodium carbonate content was about 100-125 wtc%. When using higher concentrations of sodium carbonate, gel forming occurred much faster than when using a lower concentration. When using a too low concentration of sodium carbonate, the cellulose solution will be too much diluted and as a result, the viscosity will drop. The gelling behaviour at high concentration of Na2 CO3 is explained by the hydratation of sodium ions. In section 3.1, the influence of the amount of sodium hydroxide has been explained. It was proposed that the solubility of cellulose was depended on the hydratation of the sodium ion. Adding sodium carbonate to the solution will increase the number of sodium ions (we can assume that Na2 CO3 completely dissociates), which will result in a lower hydratation of them. When the hydratation is too low, cellulose is less soluble. When using a 50 g/l sodium carbonate solution, the hydratation will not change too much, so cellulose is still soluble. With a 150 g/l sodium carbonate solution, the amount of sodium ions increases so much that the cellulose will precipitate out of solution. Because of the instability of the sodium carbonate/cellulose, calcium carbonate CaCO3 was studied as a blowing agent instead. As an insoluble carbonate salt it will not get hydrataded and so will not change the properties of the solution. CaCO3 has a solubility of 0.015 g/l and was successfully mixed with the cellulose solution in all quantities. No change in viscosity was observed, except with very large quantities (1000 wtc%) the fluidity of the solution worsened. The solution was examined under microscope and an even dispersion of the CaCO3 -particles was observed.


6.2.2

38

Influence of the coagulation bath

First H2 SO4 was used as a coagulant. CaCO3 reacts with the acid generating CO2 and CaSO4 (see equation 6.1). The drawback of using H2 SO4 as a coagulant is that the formed salt is a non soluble salt (the water solubility of CaSO4 is 0.021 g/l at 20◦ C). Therefore, CaSO4 cannot be washed out of the film and the film has a plastery feeling. CaCO3 + H2 SO4 −−→ CaSO4 ↓ + H2 O + CO2 ↑

(6.1)

Secondly, a 10 wt% aqueous solution of HCl was used as a coagulant. Equation 6.2 shows the reaction of CaCO3 with HCl. The formed CaCl2 is soluble in water (solubility is 745 g/l). During regeneration, the white colour of the cellulose solution disappeared. This is a proof that CaCO3 is depleted in the reaction and CaCl2 is formed. After regeneration, the films were washed to remove any remaining CaCl2 . When drying the films under tension, they almost always broke. This is because the air bubbles resulted in decreased mechanical strength. CaCO3 + 2 HCl −−→ 2 CaCl + H2 O + CO2 ↑

6.2.3

(6.2)

Influence of the concentration of foaming agent

The influence of the concentration of CaCO3 (100, 200 and 300 wtc%) on the properties of cellulose films was investigated. Optical microscopy was used to examine the films and the results are shown in figure 6.7. The film without CaCO3 added only shows a very little amount of pores. These pores exist because there are air bubbles in the cellulose solution. In the films where CaCO3 was added, a high amount of pores are observed. From figure 6.7 it can be concluded that with increasing amount of CaCO3 , more pores are formed. This is because more CaCO3 particles are dispersed in the cellulose solution, which causes more CO2 to be formed. The pore size does not change because the CaCO3 powder used is very well dispersed in the cellulose solution.


(a)

(c)

39

(b)

(d)

Figure 6.7: Microscopic images of cellulose films containing CaCO3 as a foaming agent. 0 (a), 100 (b), 200 (c) and 300 (d) wtc% of CaCO3

6.2.4

Influence of the temperature

The influence of the coagulation bath temperature on the film formation was investigated using a 10 wt% aqueous solution of HCl. The different temperatures of the coagulation bath were 22, 40, 60 and 70 ◦ C. The coagulation time was chosen as the time for the films to become transparent. Table 6.1 shows the time needed for the films at different temperatures to become transparent. Figure 6.8 shows microscopic images of the different films. With increasing temperature, the coagulation time decreases. This means that CaCO3 depletes faster at higher temperature. The shorter depletion time of CaCO3 is explained by the higher reaction kinetics. All films except the ones coagulated at 70◦ C were of sufficient mechanical strength to dry them under tension. The reason why the films coagulated at 70 ◦ C were of insufficient strength could be due of hydrolysis of the cellulose. At elevated temperatures, HCl can hydrolyse cellulose [90, 91]. The reaction mechanism is shown in figure 6.9. Due to the breakage of the glycosidic linkages, the molecular weight of cellulose decreases and thereby the strength is lowered. The effect of temperature on pore formation is not very clear as seen in figure 6.8. There is a slight increase in pore size with increasing temperature.


40

Table 6.1: Coagulation time of the cellulose solution with CaCO3 at different temperatures

temperature

(a)

time

22 ◦ C

7 min 30s 8 min 7 min 20s

40

◦C

5 min 40s 9 min 5 min 20s

60

◦C

3 min 30s 3 min 50s 3 min 20s

70

◦C

2 min 1 min 45s 2 min 30s

(b)

(c)

Figure 6.8: Microscopic images of cellulose films containing 100 wt% CaCO3 as a foaming agent. Regeneration temperature is 22◦ C (a), 40◦ C (b) and 60◦ C (c)

Figure 6.9: Reaction mechanism for the acid-catalysed hydrolysis of the glycosidic linkage [91]


6.2.5

41

Influence of salt content of the coagulation bath

The influence of the NaCl content of the coagulation bath on the film formation was investigated. The coagulation temperature was 50◦ C. The salt content did not affect the time needed for CaCO3 to deplete. The coagulation time was around 5 minutes for all salt contents (0-25 wt%). Figure 6.10 shows microscopic images of the films. It can be observed that with increasing salt content, the amount and density of the pores increases. This has been explained by Woodings [51]. When there is no salt in the coagulation bath, the acid will mostly coagulate the cellulose without reacting with CaCO3 . When adding a salt to the coagulation bath, the cellulose coagulation will slow down, therefore air bubbles can be formed simultaneous with coagulation of cellulose. This will decrease the resistance of pore formation and more pores can be formed.

(a)

(b)

(c)

(d)

Figure 6.10: Microscopic images of cellulose films containing 100 wt% CaCO3 as a foaming agent. Regeneration temperature is 50◦ C and the salt content of the regeneration bath is 0wt% (a), 10wt% (b), 15wt% (c) and 20wt% (d)


6.2.6

42

Thickness of the cellulose sponges

To determine a coagulation mechanism, a mixture of 20 g of a 6 wt% cellulose solution and 2.4 g CaCO3 was coagulated in a 10 wt% aqueous solution of HCl and the cross section was studied after 10, 30 and 90 minutes. Figure 6.11 (a)-(c) show images of the cross section and (e)-(h) the proposed coagulation mechanism.

(a)

(c)

(b)

(e)

(f)

(g)

(h)

Figure 6.11: Pictures of the cross section of cellulose sponges after 10 (a), 30 (b) and 90 (c) minutes of coagulation, (e)-(h) show the proposed coagulating mechanism

Upon entering the coagulation bath, a thick shell is quickly formed (figure 6.11 (a) and (e)). This shell grows only very slowly and even after 90 minutes the shell had not increased much in size. It took several hours to complete regeneration and for CaCO3 to react. This can be explained as following: Upon entering the coagulation bath, the formed outer cellulose film will act as a barrier for the acid. The acid has to diffuse through this layer to be able to react with the alkaline cellulose and CaCO3 . This diffusion process is the rate-determining step of coagulation. The influence of the cellulose concentration of the solution was studied. The concentrations of cellulose were 2, 3 and 6 wt% cellulose with 200 wtc% CaCO3 added. The cellulose was allowed to coagulate overnight to ensure complete coagulation. A 2 wt% cellulose solution was too liquid, and therefore when entering the coagulation bath the solution collapsed and no sponges could be prepared. The sponge prepared with a 3 wt% cellulose solution was thicker and more porous but it was very weak and broke instantly. The sponge prepared with a 6 wt% cellulose solution was less thick but was much stronger. To reduce shrinkage of the sponges, a solvent exchange of water to ethanol was done. Afterwards the sponges were dried in air. The sponges dried in ethanol did not have reduced shrinkage and showed a yellow colour.


6.3

43


The stability of TiO2 /AgCl particles in an aqueous solution of NaOH was investigated. When the TiO2 /AgCl powder (figure 6.12 (a)) was added to the NaOH solution (figure 6.12 (b)), the colour changed from white to milky brown. With further stirring the solution became completely opaque (figure 6.12 (c)). When adding the solution to an excess of sulphuric acid (figure 6.12(c)), the colour changed to a white milky colour.

(a)

(b)

(c)

(d)

Figure 6.12: Testing of the stability of TiO2 /AgCl particles: TiO2 /AgCl powder (a), TiO2 /AgCl added to NaOH solution (b), after stirring in NaOH solution (c), after adding to an excess of H2 SO4

The change from powder to milky brown in a NaOH solution is explained by the reaction of silver chloride with NaOH to silver oxide. As an intermediate step, silver hydroxide is formed. Silver hydroxide is however very unstable and will react to form silver oxide. In appendix A, a theoretical calculation is done and it is shown that the formation of silver oxide is thermodynamically favourable. The overall reaction is given by equation 6.3. 2 AgCl + 2 NaOH −−→ Ag2 O + 2 NaCl + H2 O

(6.3)


44

When adding the obtained solution to an excess of H2 SO4 , the silver oxide will react with the acid to form silver sulphate (Ag2 SO4 ). This explains the white milky colour. The reaction is given in equation 6.4. Ag2 O + H2 SO4 −−→ Ag2 SO4 + H2 O

(6.4)

When 0.032 g of TiO2 /AgCl powder was dispersed in 3 ml of an aqueous solution of NaOH and then added to 30 g of cellulose solution, the solution turned black (see figure 6.13). Films were prepared, washed and dried . The dried films still had a brown colour which indicates that not all Ag2 O did react to Ag2 SO4 and there is still some Ag2 O in the films. Even after regeneration for 30 minutes a brown colour remained. The coagulation bath remained transparent.

Figure 6.13: Cellulose solution containing 2wt% (compared to cellulose) of TiO2 /AgCl particles

6.4 6.4.1

Carbon Nanotubes Cellulose as a dispersant for CNT

It was investigated if an alkaline solution of enzyme modified cellulose is a good dispersant for MWNTs. For this, very little amounts of MWNTs were dispersed in a 1, 2 and 6 wt% cellulose solution and in H2 O. The cellulose solution of 2 and 6 wt% cellulose did gel during sonication. This is due to the heat that is generated during sonication. A cooling bath did not reduce gel formation. It was possible to disperse MWNTs in a 1 wt% cellulose solution and in water. The behaviour of the dispersions were visually investigated over time. Figure 6.14 show the dispersions. The dispersion of MWNT in H2 O was not stable, already after 15 seconds the MWNTs agglomerated together. The MWNT-suspension with the diluted cellulose solution remained black, even after letting it stand for 1 week. This indicates that the MWNTs were still dispersed in the cellulose solution. It was investigated how much MWNT can be dispersed in a 1 wt% cellulose solution. For this, cellulose:MWNT mixtures of 20:1, 15:1, 10:1 and 5:1 were prepared by sonication. After sonication, the dispersions were investigated under optical microscope. Figure 6.15


45

shows microscopic images of the solutions. Only a cellulose solution with 5 wtc% MWNT is well dispersed. When more than 5 wtc% MWNT was dispersed, the MWNTs agglomerated together. The stability of the suspension is explained by wrapping of cellulose chains around individual MWNT. There is only a limited amount of MWNT that can be wrapped. The reason why cellulose xanthate can disperse its own weight [85] and why the alkaline cellulose solution used in this thesis only 5 wtc% is explained by the chemical structure of cellulose and cellulose xanthate. In alkaline solution, both cellulose xanthate and cellulose will bear a negative charge. Cellulose xanthate will have this negative charge on a sulphur-atom whereas cellulose will bear it at an oxygen atom. The negative charge on the sulphur atom is easier created and more reactive than the one on the oxygen atom, this is because the electron affinity of sulphur is greater than that of oxygen. The higher availability of electrons in cellulose xanthate results in a stronger bond with MWNTs.

(a)

(b)

(c)

(d)

Figure 6.14: Testing of the stability of the emulsion of MWNT in 1 wt% cellulose solution (left) and water (right) after: 15 seconds (a), 30 seconds (b), 5 minutes (c) and 1 day (d)


46

(a)

(b)

(a)

(b)

Figure 6.15: microscopic images of a 1 wt% cellulose solution with a cellulose:CNT fraction of 20:1 (a), 15:1 (b), 10:1 (c) and 5:1 (d)

6.4.2

Cellulose/MWNT hybrid films

For preparing cellulose film containing MWNT, the MWNTs first had to be dispersed in a 1 wt% cellulose solution in a concentration at 5 wtc% and then be added to a 6 wt% cellulose solution to obtain the desired amount of MWNT. This caused the cellulose solutions to dilute. Cellulose solutions with 0.5, 1 and 2 wtc% MWNTs were prepared. Table 6.2 shows the cellulose concentrations of the cellulose/MWNT solutions in function of the amount of MWNTs. With increasing amount of MWNTs, the cellulose concentration drops. This results in a drop of viscosity. Films containing 0.5 and 1 wtc% MWNTs could be prepared, the solution containing 2 wtc% MWNT had a too low viscosity to make films. During coagulation, the coagulation bath remained transparent, which indicates that the carbon nanotubes remained in the films. Table 6.2: Cellulose concentration of solutions with MWNTs

Concentration MWNT (wtc%)

cellulose concentration

0 0.5 1 2

6 4 3 2


47

Scanning electron microscope images were taken of the cellulose/MWNT hybrid films (see figure 6.16). The reference cellulose film (figure 6.16 (a)) shows a very smooth surface with some small impurities. These impurities are probably Na2 SO4 -salts that were formed during regeneration and were not removed during washing. The cellulose films containing MWNTs (figure 6.16 (b) and (c)) have a very rough, but homogeneous surface. This indicates the presence of carbon nanotubes and the homogeneous distribution of them. The white particles are probably, just as in the reference cellulose films, Na2 SO4 salts. The films containing 1 wt% MWNTs have pores (figure 6.16 (d)). These pores are created because of the low concentration of the cellulose in solution. This low concentration creates a less viscous solution that sticks less to the glass plate.

(a)

(b)

(c)

(d)

Figure 6.16: SEM images of a reference cellulose film (a) and cellulose films containing 0.5 (b) and 1 (c) wtc% MWNTs. The films containing 1 wtc% MWNTs show pores in the structure.

Chapter 7

Conclusions 7.1

Casein

Alkaline cellulose solutions to which casein were added were stable. There was a change in colour of the solution from transparent to white but the viscosity did not alter. Films were produced by coagulation in a 5 wt% solution of H2 SO4 . The coagulation bath did not change colour during film manufacturing which indicates that casein remains in the films during coagulation. Films with a casein fraction higher than 20% could not be prepared. FTIR of the hybrid films did not show the characteristic absorption peaks of the amide vibration of casein. This might be due to a non-homogeneous mixture of casein and cellulose. When the films were acid dyed, the colour of the cellulose-casein films remained after washing whereas the colour of the reference cellulose films was completely washed away. The cellulose-casein films showed a very uneven colour distribution. The colour confirms the presence of casein, but the casein was very unevenly distributed. The non-uniformity of the films is probably generated during coagulation. During coagulation, the pH drops from highly basic to acid. Cellulose coagulutes at higher pH than casein. Therefore cellulose coagulates before casein, and a inhomogeneous solid state mixture is formed.

7.2

Cellulose sponges

Sodium carbonate can not be used as a foaming agent for forming sponges. For creating sponges, a high content of the foaming agent is needed, but Na2 CO3 in large quantities induces gel formation. However, inflated or hollow fibres might be manufactured by using Na2 CO3 since the amount of foaming agent only needs to be 10-35 wt%. CaCO3 is a suitable foaming agent because it is not soluble, and therefore does not change the properties of the solution. H2 SO4 cannot be used as a coagulant since the insoluble salt CaSO4 is formed. When using HCl as a coagulant, CaCl2 was formed, which is soluble.

48

Chapter 7. Conclusions

49

The amount of pores increased with an increasing amount of CaCO3 . A higher salt content resulted in smaller pores, but in an increased number of them. Elevated temperatures increased the reaction rate and bigger pores were formed. If coagulation happens at very high temperatures, the mechanical properties of the films reduce. This is probably due to acidic hydrolysis of the glycosidic linkage in cellulose which reduces the degree of polymerisation. During drying of the cellulose sponges, there was a huge shrinkage. Trials to prevent shrinkage by a solvent exchange with ethanol were unsuccessful.

7.3


Films containing TiO2 /AgCl particles could not be manufactured. This is because sodium hydroxide was used as a solvent for cellulose and silver chloride is not stable in it. AgCl will react with NaOH to form silver oxide. This gives the cellulose solution a very dark colour. During regeneration, the silver oxide reacts with the sulphuric acid forming silver sulfate.

7.4

Carbon nanotubes

A diluted cellulose solution (1wt%) could be used as a stable dispersant for carbon nanotubes. A 1wt% cellulose solution can disperse 5 wtc% MWNTs. Cellulose/multiwalled carbon nanotubes hybrid films with 0.5 and 1 wtc% MWNT could be prepared with the glass plate method. The viscosity of a cellulose solution with 2 wtc% MWNT was too low to prepare films. Scanning electron microscope images of the films showed the presence of MWNT in the films and an even distribution of them. In summary, Cellulose product with additives could be prepared using casein, carbonate salts and carbon nanotubes. The cellulose/casein hybrid products could give wool like properties due to the introduction of peptide-bonds. Porous structures can be prepared using calcium carbonate. Cellulose/CNT composite structures were succesfully prepared. Antibacterial cellulose products using silver chloride could not be prepared. Further ressearch is needed to form fibres. This would allow to measure properties as strength. Conductivity in case of cellulose/CNT fibres and dyeability of cellulose/casein fibres.

Appendix A

Theoretical calculation about the solubility of AgCl In this appendix the theoretical calculation to determine the solubility of silver chloride in sodium hydroxide will be shown. Where:

KspAgCl KspAgOH wt% NaOH MMN aOH

= = = =

1.70 * 10−10 mol2 /l2 1.52 * 10−8 mol2 /l2 6.5% 40g/mol

We can calculate the concentration of hydroxide ions in the sodium hydroxide solution. 6.5wt% NaOH solution means a concentration of 69.7g/l [92]. We can assume that NaOH completely dissociates in water. So we get: [OH− ] = 1.74 mol/l

(A.1)

Although AgCl is considered an insoluble salt, there will be an equilibrium concentration of the ions in the solution, given by: KspAgCl = [Ag+ ][Cl− ]

(A.2a)

[Ag+ ] = 1.3 ∗ 10−5 mol/l

(A.2b)

Since [Ag+ ]=[Cl− ] we get: The ionic product QspAgOH of the [Ag+ ] and [OH− ] ions is given by: QspAgOH = [Ag+ ][OH− ] = 2.26 ∗ 10−5 mol2 /s2

(A.3)

This is higher than the solubility product KspAgOH , which means that AgOH will precipitate out of the solution. AgOH however, is a non stable product and will react to form silver oxide. 2 AgOH −−→ Ag2 O + H2 O (A.4) 50

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List of Figures 2.1 2.2

Schematic representation of the viscose process (adapted from [1]) . . . . . . Reaction for forming sodium cellulose xanthate from sodium cellulose and carbon disulphide [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

3.1 3.2 3.3

cellulose-NaOH-water phase diagram [5] . . . . . . . . . . . . . . . . . . . . . Change in volume of cotton hairs in five alkalis [9] . . . . . . . . . . . . . . . Schematic representation of the process steps of the Biocelsol process [15] . .

7 8 9

4.1

4.15

Schematic view of the process for making chitosan (addapted from [22]) (a) and the chemical structure of cellulose, chitin and chitosan[18] (b). . . . . . Chemical structure of dithiocarbamate chitosan [28] . . . . . . . . . . . . . Schematic representation of membrane formation mation of PEC’s between chitosan and sodium alginate (ALG: sodium alginate; CHD: chitosan). [31] formation of peptide bond between two amino acids [44] . . . . . . . . . . . α-helix structure [44] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . β-sheet structure [44] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Structure of Sodium Carbonate [59] . . . . . . . . . . . . . . . . . Chemical Structure of sodium bicarbonate [60] . . . . . . . . . . . . . . . . Longitudinal transillumination of a hollow viscose fibre with sodium carbonate as an additive [53] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . influence of acid (a), zinc sulphate (b) and sodium sulphate (c) content of the regeneration bath on inflation. (adapted from [51] . . . . . . . . . . . . . . Action of silver on microorganism [64] . . . . . . . . . . . . . . . . . . . . . Structure of TiO2 particle and principle of controlled release (adapted from [69]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning electron micrography of the silver chloride/titanium oxide composite particles 3 to 5 µm in size. [70] . . . . . . . . . . . . . . . . . . . . . . . Schematic representation of single walled carbon nanotubes (a) and multi walled carbon nano tubes (b) [80] . . . . . . . . . . . . . . . . . . . . . . . . Schematic representation of cellulose xanthate wrapped around CNT [85] .

5.1

Schematic representation of the glass plate method for producing cellulose films 28

4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14

57

4

. 11 . 12 . . . . . .

13 14 15 16 17 18

. 19 . 20 . 21 . 22 . 23 . 24 . 25

List of Figures 6.1 6.2 6.3

6.4 6.5 6.6 6.7 6.8 6.9 6.10

6.11 6.12

6.13 6.14

6.15 6.16

Picture of a 100 g/l casein solution (a), a 200 g/l casein solution and a mixture of casein and cellulose solution (c) . . . . . . . . . . . . . . . . . . . . . . . . FTIR casein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The vibrations responsible for the Amide I and Amide II bands in the infrared spectra of proteins and polypeptides. The Amide I band is due to carbonyl stretching vibrations while the Amide II is due primarily to NH-bending vibration [89] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic representation of the ionic bonding between the amine group of a peptide bond and red 266 acid dye . . . . . . . . . . . . . . . . . . . . . . . . pictures of dyed cellulose:casein films before washing with detergent: 100:0 (a), 90:10 (b), 85:15 (c) and 80:20 (d) . . . . . . . . . . . . . . . . . . . . . . microscopic images of a cellulose film with a casein fraction of 15%: before (a) and after (b) detergent washing . . . . . . . . . . . . . . . . . . . . . . . . Microscopic images of cellulose films containing CaCO3 as a foaming agent. 0 (a), 100 (b), 200 (c) and 300 (d) wtc% of CaCO3 . . . . . . . . . . . . . . . Microscopic images of cellulose films containing 100 wt% CaCO3 as a foaming agent. Regeneration temperature is 22◦ C (a), 40◦ C (b) and 60◦ C (c) . . . . . Reaction mechanism for the acid-catalysed hydrolysis of the glycosidic linkage [91] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microscopic images of cellulose films containing 100 wt% CaCO3 as a foaming agent. Regeneration temperature is 50◦ C and the salt content of the regeneration bath is 0wt% (a), 10wt% (b), 15wt% (c) and 20wt% (d) . . . . . Pictures of the cross section of cellulose sponges after 10 (a), 30 (b) and 90 (c) minutes of coagulation, (e)-(h) show the proposed coagulating mechanism Testing of the stability of TiO2 /AgCl particles: TiO2 /AgCl powder (a), TiO2 /AgCl added to NaOH solution (b), after stirring in NaOH solution (c), after adding to an excess of H2 SO4 . . . . . . . . . . . . . . . . . . . . . . Cellulose solution containing 2wt% (compared to cellulose) of TiO2 /AgCl particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Testing of the stability of the emulsion of MWNT in 1 wt% cellulose solution (left) and water (right) after: 15 seconds (a), 30 seconds (b), 5 minutes (c) and 1 day (d) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . microscopic images of a 1 wt% cellulose solution with a cellulose:CNT fraction of 20:1 (a), 15:1 (b), 10:1 (c) and 5:1 (d) . . . . . . . . . . . . . . . . . . . . . SEM images of a reference cellulose film (a) and cellulose films containing 0.5 (b) and 1 (c) wtc% MWNTs. The films containing 1 wtc% MWNTs show pores in the structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58

33 34

35 35 36 36 39 40 40

41 42

43 44

45 46

47

List of Tables 4.1

MIC values for some microorganism (adapted from [68]) . . . . . . . . . . . . 23

5.1 5.2 5.3

Cellulose solution constitution . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Properties of the carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . 31 Amount of CNT needed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.1

Coagulation time of the cellulose solution with CaCO3 at different temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Cellulose concentration of solutions with MWNTs . . . . . . . . . . . . . . . 46

6.2

59

Formation of cellulose hybrid films from an alkaline solution

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