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Structural changes during cellulose composite processing
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Two approaches for creating a new all-cellulose composite material have been studied: the biosynthesis of compartmentalised bacterial cellulose fibril aggregates and the compression moulding of commercial chemical wood pulps processed with only water. The objective was to study the structural changes during processing and the complexity of relating the mechanical properties of the final biocomposites to the nanoscale structure was highlighted.

Solid-state CP/MAS 13C NMR spectroscopy was utilised to determine both the fibril aggregate width and the content of the different crystalline cellulose forms, cellulose I and cellulose II. Using this method, the quantities of hemicellulose present inside the fibre wall and localised at the fibre surfaces could be determined.

The formation of cellulose fibrils was affected by the addition of hydroxyethylcellulose (HEC) to a culture medium of Acetobacter aceti, and the fibrils were coated with a thin layer of HEC, which resulted in loose bundles of fibril aggregates. The HEC coating, improved the fibril dispersion in the films and prevented fractures, resulting in a biocomposite with remarkable mechanical properties including improved strength (289 MPa), modulus (12.5 GPa) and toughness (6%).

The effect of press temperature was studied during compression moulding of sulphite dissolving-grade pulps at 45 MPa. A higher press temperature yielded increases in the fibril aggregation, water resistance (measured as the water retention value) and Young’s modulus (12 GPa) in the final biocomposite. The high pressure was important for fibril aggregation, possibly including cellulose-cellulose fusion bonds, i.e., fibril aggregation in the fibre-fibre bond region. The optimal press temperature was found to be 170°C because cellulose undergoes thermal degradation at higher temperatures.

The effect of hemicellulose was studied by comparing a softwood kraft paper-grade pulp with a softwood sulphite paper and a softwood sulphite dissolving-grade pulp. A significant fibril aggregation of the sulphite pulps suggested that the content and distribution of hemicellulose affected the fibril aggregation. In addition, the hemicellulose structure could influence the ability of the hemicellulose to co-aggregate with cellulose fibrils. Both sulphite pulp biocomposites exhibited Young’s moduli of approximately 12 GPa, whereas that of the kraft pulp was approximately half that value at 6 GPa. This result can be explained by a higher sensitivity to beating in the sulphite pulps.

The effect of mercerisation, which introduces disordered cellulose, on the softwood sulphite dissolving-grade pulp was also studied under compression moulding at 170°C and 45 MPa. The mechanisms causing an incomplete transformation of cellulose I to II in a 12 wt% NaOH solution were discussed. The lower modulus of cellulose II and/or the higher quantity of disordered cellulose likely account for the decrease in Young’s modulus in the mercerised biocomposites (6.0 versus 3.9 GPa).

Abstract [sv]

Två metoder för att skapa ett nytt kompositmaterial baserat på enbart cellulosa har studerats, biosyntes av fibrillaggregat bestående av bakteriecellulosa och varmpressning av kommersiella träfiberbaserade massor med vatten som den enda processkemikalien. Målet var att studera de strukturella förändringarna som sker under tillverkningsprocessen. Även komplexiteten i att relatera strukturen på nanonivå till de mekaniska egenskaperna hos de slutliga biokompositerna belystes.

Med fastfas CP/MAS 13C NMR-spektroskopi var det möjligt att bestämma både fibrillaggregattjockleken och mängden av cellulosakristallformerna; cellulosa I och cellulosa II. Det var också möjligt att bestämma mängden hemicellulosa dels närvarande inuti fiberväggen och dels mängden lokaliserad på fiberytor.

Tillsats av hydroxyetylcellulosa (HEC) i odlingsmediet för Acetobacter aceti påverkade bildandet av cellulosafibriller som blev belagda med ett tunt skikt av HEC, vilket också resulterade i löst bundna fibrillaggregat. HEC-beläggningen förbättrade fibrilldispersionen i filmerna och minskade sprickbildningen, vilket gav en biokomposit med mycket goda mekaniska egenskaper med kombinerad hög styrka (289 MPa), styvhet (12.5 GPa) och seghet (6%).

Effekten av presstemperatur vid varmpressning (45 MPa tryck) studerades på sulfit dissolvingmassor. Högre presstemperatur gav ökad fibrillaggregering, ökat vattenmotstånd (mätt som vattenretentionsvärde) och högre styvhet (12 GPa) för biokompositen. Det höga trycket var också viktigt för fibrillaggregeringen, som troligen omfattar cellulosa-cellulosa samkristallisation dvs. fibrillaggregering i fiber-fiber-bindningsregionen. Den optimala presstemperaturen föreslogs vara 170° C pga. termisk nedbrytning av cellulosa vid högre temperaturer.

Effekten av hemicellulosa studerades genom att jämföra sulfat pappersmassa med sulfit pappersmassa och sulfit dissolvingmassa. Mängden och fördelningen av hemicellulosa föreslogs ligga till grund för skillnaden i fibrillaggregering, som var mera uttalad i sulfitmassorna. Även hemicellulosans struktur kan påverka förmågan hos hemicellulosa att sam-aggregera med cellulosafibriller. Biokompositerna baserade på sulfitmassorna hade en styvhet på ca. 12 GPa, medan sulfatmassan bara hade hälften av den nivån ca. 6 GPa, vilket förklarades av sulfitmassornas högre känslighet för malning.

Även effekten av mercerisering av sulfit dissolvingmassa varmpressad vid 170° C och 45 MPa studerades. Mercerisering introducerar oordnad cellulosa och mekanismerna som endast ger en partiell omvandling av cellulosa I till II i en 12 vikt% NaOH-lösning diskuterades. Den sämre styvheten hos den merceriserade biokompositen (6.0 resp. 3.9 GPa) förklaras troligen genom cellulosa II kristallens lägre styvhet och/eller den högre mängden av oordnad cellulosa.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xi, 48 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2012:46
Keyword [en]
CP/MAS 13C NMR, cellulose, fibril aggregation, biocomposite, compression moulding, supramolecular structure
National Category
Chemical Sciences Materials Chemistry
URN: urn:nbn:se:kth:diva-104568ISBN: 978-91-7501-518-7OAI: diva2:565072
Public defence
2012-11-30, K1, Teknikringen 56, KTH, Stockholm, 10:00 (English)
Wallenberg Wood Science CenterBiomime
Knut and Alice Wallenberg Foundation

QC 20121106

Available from: 2012-11-06 Created: 2012-11-06 Last updated: 2012-11-09Bibliographically approved
List of papers
1. Nanostructured biocomposites based on bacterial cellulosic nanofibers compartmentalized by a soft hydroxyethylcellulose matrix coating
Open this publication in new window or tab >>Nanostructured biocomposites based on bacterial cellulosic nanofibers compartmentalized by a soft hydroxyethylcellulose matrix coating
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2009 (English)In: Soft Matter, ISSN 1744-683X, Vol. 5, no 21, 4124-4130 p.Article in journal (Refereed) Published
Abstract [en]

Biomimetic approaches involving environmentally-friendly synthetic pathways provide an opportunity to elaborate novel high-performance biocomposites. Here we have developed a low-energy biosynthetic system for the production of a high-strength composite material consisting of self-assembled and nanostructured cellulosic nanofibers. This biocomposite is analogous to natural composite materials with high strength and hierarchical organization such as wood or tendon. It was generated by growing the bacterium Acetobacter, which naturally produces cellulosic nanofibers, in the presence of hydroxyethylcellulose (HEC). Individual cellulose fibrils were coated by HEC and exhibited a smaller lateral dimension than pure bacterial cellulose (BC) fibrils. They self-assembled to form compartmentalized nanofibers and larger cellulose fibril aggregates compared to pure BC. The tensile strength of nanocomposite films prepared from the compartmentalized cellulosic nanofibers was 20% higher than that of pure BC sheets and wood cellulose nanopapers, and 60% higher than that of conventional BC/HEC blends, while no strain-to-failure decrease was observed. The thin nanoscale coating consisting of hydrated HEC significantly increased the mechanical performance of the nanocomposite films by provoking compartmentalization of individual fibrils.

cp/mas c-13 nmr, different polymeric additives, cell-wall, polysaccharides, in-situ crystallization, acetobacter-xylinum, native, cellulose, mechanical-properties, composites, microfibrils, spectroscopy
National Category
Chemical Sciences
urn:nbn:se:kth:diva-18869 (URN)10.1039/b907838j (DOI)000270837900008 ()2-s2.0-70350117835 (ScopusID)

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2015-05-26Bibliographically approved
2. Biomimetic design of cellulose-based nanostructured composites using bacterial cultures
Open this publication in new window or tab >>Biomimetic design of cellulose-based nanostructured composites using bacterial cultures
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2009 (English)In: Polymer Preprints, ISSN 0032-3934, Vol. 50, no 2, 7-8 p.Article in journal (Refereed) Published
National Category
Engineering and Technology
urn:nbn:se:kth:diva-82672 (URN)

QC 20120427

Available from: 2012-02-12 Created: 2012-02-12 Last updated: 2014-01-22Bibliographically approved
3. A non-solvent approach for high-stiffness all-cellulose biocomposites based on pure wood cellulose
Open this publication in new window or tab >>A non-solvent approach for high-stiffness all-cellulose biocomposites based on pure wood cellulose
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2010 (English)In: Composites Science And Technology, ISSN 0266-3538, Vol. 70, no 12, 1704-1712 p.Article in journal (Refereed) Published
Abstract [en]

All-cellulose composites are commonly prepared using cellulose solvents. In this study, moldable all-cellulose I wood fiber materials of high cellulose purity (97%) were successfully compression molded. Water is the only processing aid. The material is interesting as a "green" biocomposite for industrial applications. Dissolving wood fiber pulps (Eucalyptus hardwood and conifer softwood) are used and the influence of pulp origin, beating and pressing temperature (20-180 degrees C) on supramolecular cellulose nanostructure is studied by solid state CP/MAS C-13 NMR. Average molar mass is determined by SEC to assess process degradation effects. Mechanical properties are determined in tensile tests. High-density composites were obtained with a Young's modulus of up to 13 GPa. In addition, nanoscale cellulose fibril aggregation was confirmed due to processing, and resulted in a less moisture sensitive material.

Wood, Mechanical properties, CP/MAS C-13 NMR, Heat treatment, Isostatic pressing
National Category
Chemical Sciences Polymer Technologies
urn:nbn:se:kth:diva-26690 (URN)10.1016/j.compscitech.2010.06.016 (DOI)000281998000006 ()2-s2.0-77955846808 (ScopusID)
QC 20101130Available from: 2010-11-30 Created: 2010-11-26 Last updated: 2013-11-11Bibliographically approved
4. Compression molded wood pulp biocomposites: A study of hemicellulose influence on cellulose supramolecular structure and material properties
Open this publication in new window or tab >>Compression molded wood pulp biocomposites: A study of hemicellulose influence on cellulose supramolecular structure and material properties
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2012 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 3, 751-760 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the importance of hemicellulose content and structure in chemical pulps on the property relationships in compression molded wood pulp biocomposites is examined. Three different softwood pulps are compared; an acid sulfite dissolving grade pulp with high cellulose purity, an acid sulfite paper grade pulp and a paper grade kraft pulp, the latter two both containing higher amounts of hemicelluloses. Biocomposites based the acid sulfite pulps exhibit twice as high Young's modulus as the composite based on paper grade kraft pulp, 11-12 and 6 GPa, respectively, and the explanation is most likely the difference in beating response of the pulps. Also the water retention value (WRV) is similarly low for the two molded sulfite pulps (0.5 g/g) as compared to the molded kraft pulp (0.9 g/g). The carbohydrate composition is determined by neutral sugar analysis and average molar masses by SEC. The cellulose supramolecular structure (cellulose fibril aggregation) is studied by solid state CP/MAS 13C-NMR and two forms of hemicellulose are assigned. During compression molding, cellulose fibril aggregation occurs to higher extent in the acid sulfite pulps as compared to the kraft pulp. In conclusion, the most important observation from this study is that the difference in hemicellulose content and structure seems to affect the aggregation behaviour and WRV of the investigated biocomposites.

Chemical pulps, Composites, Compression molding, CP/MAS 13C-NMR, Fibril aggregation, Hemicelluloses, Mechanical properties, Aggregation behaviours, Average molar mass, Bio-composites, Content and structure, CP/MAS <sup>13</sup>C-NMR, Material property, Neutral sugar analysis, Paper grade pulp, Softwood Pulps, Supramolecular structure, Water retention value, Wood pulp, Young's Modulus, Chemical pulp, Composite materials, Kraft process, Kraft pulp, Paper and pulp mills, Paper products, Pulp beating, Sugars, Sulfite pulp, Wood, Cellulose, Agglomeration, Sulfite Pulps
National Category
Paper, Pulp and Fiber Technology
urn:nbn:se:kth:diva-95695 (URN)10.1007/s10570-012-9688-2 (DOI)000303459200017 ()2-s2.0-84860387690 (ScopusID)

QC 20150630

Available from: 2012-05-30 Created: 2012-05-29 Last updated: 2015-06-30Bibliographically approved
5. Mercerized cellulose biocomposites: A study of influence of mercerization on cellulose supramolecular structure, water retention value and tensile properties
Open this publication in new window or tab >>Mercerized cellulose biocomposites: A study of influence of mercerization on cellulose supramolecular structure, water retention value and tensile properties
2013 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 20, no 1, 57-65 p.Article in journal (Refereed) Published
Abstract [en]

In this study the effect of the mercerization degree on the water retention value (WRV) and tensile properties of compression molded sulphite dissolving pulp was evaluated. The pulp was treated with 9, 10, or 11 % aqueous NaOH solution for 1 h before compression molding. To study the time dependence of mercerization the pulp was treated with 12 wt% aqueous NaOH for 1, 6 or 48 h. The cellulose I and II contents of the biocomposites were determined by solid state cross polarization/magic angle spinning carbon 13 nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy. By spectral fitting of the C6 and C1 region the cellulose I and II content, respectively, could be determined. Mercerization decreased the total crystallinity (sum of cellulose I and cellulose II content) and it was not possible to convert all cellulose I to cellulose II in the NaOH range investigated. Neither increased the conversion significantly with 12 wt% NaOH at longer treatment times. The slowdown of the cellulose I conversion was suggested as being the result from the formation of cellulose II as a consequence of coalescence of anti-parallel surfaces of neighboring fibrils (Blackwell et al. in Tappi 61:71–72, 1978; Revol and Goring in J Appl Polym Sci 26:1275–1282, 1981; Okano and Sarko in J Appl Polym Sci 30:325–332, 1985). Compression molding of the partially mercerized dissolving pulps yielded biocomposites with tensile properties that could be correlated to the decrease in cellulose I content in the pulps. Mercerization introduces cellulose II and disordered cellulose and lowered the total crystallinity reflected as higher water sensitivity (higher WRV values) and poorer stiffness of the mercerized biocomposites.

Cellulose II, Compression molding, CP/MAS 13C NMR, Mercerization, Supramolecular structure
National Category
Polymer Chemistry
urn:nbn:se:kth:diva-104593 (URN)10.1007/s10570-012-9801-6 (DOI)000313365700006 ()2-s2.0-84872286468 (ScopusID)

QC 20130205

Available from: 2012-11-06 Created: 2012-11-06 Last updated: 2013-02-11Bibliographically approved

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