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Nanostructured Cellulose Biocomposites: Effects from dispersion, network and interface
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0001-7870-6327
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The major load bearing component in native wood, cellulose nanofibrils, are potential candidates for use as reinforcement in polymer matrices. This study is based on nanocellulose composites and attempts to prepare and characterize biocomposites with high nanocellulose content and investigate the influence of nanostructure on macroscopic properties.

In an initial study, effects from cellulose nanocrystal (CNC) dispersion on optical and mechanical properties of CNC composites are studied in a model system using polyvinylacetate (PVAc) as the polymer. CNC surface modification is used as an aid to improve dispersion, and nanocomposites with up to 20 wt% of modified and unmodified CNC are characterized. Strong influence of CNC as reinforcement and on polymer matrix characteristics were observed with well-dispersed CNCs, resulting in nanocomposites with significantly improved mechanical properties.

In the subsequent parts, an impregnation-based processing strategy is used to prepare cellulose nanofibril (CNF) based thermoset (epoxy and unsaturated polyester) composites with high CNF content (15 - 50 vol%). Influence of CNF surface hydroxyls on epoxy curing is discussed. A mono-epoxy compound is used to confirm covalent epoxy/CNF reaction and the implications of this modification on mechanical properties of wet CNF network are illustrated. Mechanical properties of thermoset composites are characterized at different relative humidities to evaluate their hygromechanical stability. The role of the CNF-thermoset interface is investigated by comparing composites with epoxy and unsaturated polyester matrices. Unique effects due to the nanostructure of composites are discussed with respect to CNF dispersion, CNF network characteristics and CNF/matrix interface. Additionally, pulp fiber composites, where the fiber wall itself is impregnated with resin, are designed and differences between nanocellulose (nanoscale network) and pulp fibers (microscale diameter) as reinforcements are analyzed.

Abstract [sv]

Trä och träbaserade material erbjuder ett hållbart alternativ till petroleumbaserade material. Mikrofibriller, eller som de också kallas i materialsammanhang, nanofibriller från cellulosa (CNF) är den huvudsakliga lastbärande komponenten i trä. Den har stor potential som förstärkande tillsats i polymerer. Denna studie handlar om kompositer baserade på nanocellulosa. Biokompositer med företrädesvis hög andel nanocellulosa framställs och karakteriseras. Huvudsyftet är att förklara inverkan av nanostruktur på makroskopiska egenskaper.

I en första studie används polyvinylacetat (PVAc) som modellpolymer för att undersöka effekten av dispergeringgrad hos cellulosa-nanokristaller (CNC) för optiska och mekaniska egenskaper. En miljövänlig form av modifiering av CNC används för att förbättra dispergeringsgraden. Nanokompositer med upp till 20 vikts-% modifierad eller icke modifierad CNC studerades. Nanokompositer med väldispergerad CNC visade dramatiskt förbättrade egenskaper, eftersom nanopartiklarna kraftigt påskyndade fysikalisk åldring av PVAc så att den fick högre E-modul och hållfasthet.

I avhandlingens andra del framställdes härdplastkompositer genom harts-impregnering av porösa CNF-nätverk, följt av härdning. Epoxi (EP) och omättade polyester (UP) användes som härdplaster. Volymsfraktionen CNF var mellan 15-50%. Reaktionshastigheten under härdning ökade kraftigt i närvaro av nanocellulosa med hög specifik yta. Troligen reagerade epoxigrupper med hydroxylgrupper på cellulosaytan, under förutsättning att katalytiska aminföreningar fanns tillgängliga. Hypotesen fick ytterligare stöd i en studie av monoepoxi + nanocellulosa. Den modifieringen visade sig också göra CNF-nätverket mindre hygroskopiskt, så att de hygromekaniska egenskaperna förbättrades. Mekaniska egenskaper hos EP- och UP-kompositer bestämdes med hjälp av dragprov vid olika luftfuktighet. EP-kompositerna visade bättre egenskaper vid hög fuktighet, främst pga de kovalenta bindningarna mellan EP och CNF. Effekter av CNF-Dispergering, CNF-nätverkets egenskaper och gränsytan CNF-härdplast analyserades. Slutligen jämfördes EP-kompositer baserade på CNF med kompositer från blekta massafibrer. CNF-kompositerna hade högre hållfasthet till följd av den låga diametern (inga problem med fiber-matris separation och sprickbildning vid låga töjningar) och kraftigt töjningshårdnande orsakad av CNF nätverket.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:19
Keyword [en]
Wood, cellulose nanofibrils, hygromechanical properties, network structure, thermoset, polymer matrix composites, natural fiber composites, pulp fiber, strain hardening, humidity
National Category
Materials Engineering
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-185723ISBN: 978-91-7595-924-5OAI: oai:DiVA.org:kth-185723DiVA: diva2:923026
Public defence
2016-05-25, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

Qc 20160427

Available from: 2016-04-27 Created: 2016-04-25 Last updated: 2016-04-29Bibliographically approved
List of papers
1. Strong surface treatment effects on reinforcement efficiency in biocomposites based on cellulose nanocrystals in poly(vinyl acetate) matrix
Open this publication in new window or tab >>Strong surface treatment effects on reinforcement efficiency in biocomposites based on cellulose nanocrystals in poly(vinyl acetate) matrix
2015 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 16, no 12, 3916-3924 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the problem to disperse cellulose nanocrystals (CNC) in hydrophobic polymer matrices has been addressed through application of an environmentally friendly chemical modification approach inspired by clay chemistry. The objective is to compare the effects of unmodified CNC and modified CNC (modCNC) reinforcement, where degree of CNC dispersion is of interest. Hydrophobic functionalization made it possible to disperse wood-based modCNC in organic solvent and cast well-dispersed nanocomposite films of poly(vinyl acetate) (PVAc) with 1-20 wt % CNC. Composite films were studied by infrared spectroscopy (FT-IR), UV-vis spectroscopy, dynamic mechanical thermal analysis (DMTA), tensile testing, and field-emission scanning electron microscopy (FE-SEM). Strongly increased mechanical properties were observed for modCNC nanocomposites. The reinforcement efficiency was much lower in unmodified CNC composites, and specific mechanisms causing the differences are discussed.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2015
National Category
Biochemistry and Molecular Biology Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-131466 (URN)10.1021/acs.biomac.5b01245 (DOI)000366616700021 ()2-s2.0-84949844757 (ScopusID)
Note

QC 20160115

Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2016-04-27Bibliographically approved
2. Cellulose nanofiber network for moisture stable, strong and ductile biocomposites and increased epoxy curing rate
Open this publication in new window or tab >>Cellulose nanofiber network for moisture stable, strong and ductile biocomposites and increased epoxy curing rate
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2014 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, Vol. 63, 35-44 p.Article in journal (Refereed) Published
Abstract [en]

Nanocomposites with high volume fractions (15-50 vol%) of nanofibrillated cellulose (NFC) were prepared by impregnation of a wet porous NFC network with acetone/epoxy/amine solution. Infrared spectroscopy studies revealed a significant increase in curing rate of epoxy (EP) in the presence of NFC. The NFC provided extremely efficient reinforcement (at 15 vol%: 3-fold increase in stiffness and strength to 5.9 GPa and 109 MPa, respectively), and ductility was preserved. Besides, the glass transition temperature increased with increasing NFC content (from 68 degrees C in neat epoxy to 86 degrees C in 50 vol% composite). Most interestingly, the moisture sorption values were low and even comparable to neat epoxy for the 15 vol% NFC/EP. This material did not change mechanical properties at increased relative humidity (90% RH). Thus, NFC/EP provides a unique combination of high strength, modulus, ductility, and moisture stability for a cellulose-based biocomposite. Effects from nanostructural and interfacial tailoring are discussed.

Keyword
Nano-structures, Cure behavior, Interface, Nanocellulose biocomposite
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-148343 (URN)10.1016/j.compositesa.2014.03.017 (DOI)000337878300005 ()2-s2.0-84899572056 (ScopusID)
Funder
Swedish Research Council Formas
Note

QC 20140805

Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2016-04-27Bibliographically approved
3. Nanostructured biocomposites based on unsaturated polyester resin and a cellulose nanofiber network
Open this publication in new window or tab >>Nanostructured biocomposites based on unsaturated polyester resin and a cellulose nanofiber network
2015 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 117, 298-306 p.Article in journal (Refereed) Published
Abstract [en]

Biocomposites reinforced by natural plant fibers tend to be brittle, moisture sensitive and have limited strength. Wood cellulose nanofibers (CNF) were therefore used to reinforce an unsaturated polyester matrix (UP) without the need of coupling agents or CNF surface modification. The nanostructured CNF network reinforcement strongly improves modulus and strength of UP but also ductility and toughness. A template-based prepreg processing approach of industrial potential is adopted, which combines high CNF content (up to 45 vol%) with nanoscale CNF dispersion. The CNF/UP composites are subjected to moisture sorption, dynamic thermal analysis, tensile tests at different humidities, fracture toughness tests and fractography. The glass transition temperature (T-g) increases substantially with CNF content. Modulus and strength of UP increase about 3 times at 45 vol% CNF whereas ductility and apparent fracture toughness are doubled. Tensile properties at high humidity are compared with other bio-composites and interpreted based on differences in molecular interactions at the interface.

Place, publisher, year, edition, pages
Pergamon Press, 2015
National Category
Polymer Technologies Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-176037 (URN)10.1016/j.compscitech.2015.07.004 (DOI)000362132900037 ()2-s2.0-84937901929 (ScopusID)
Funder
VINNOVAKnut and Alice Wallenberg Foundation
Note

QC 20151029

Available from: 2015-10-28 Created: 2015-10-28 Last updated: 2016-04-27Bibliographically approved
4. Interface tailoring through covalent hydroxyl-epoxy bonds improves  hygromechanical stability in nanocellulose materials
Open this publication in new window or tab >>Interface tailoring through covalent hydroxyl-epoxy bonds improves  hygromechanical stability in nanocellulose materials
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Wide-spread use of cellulose nanofibril (CNF) biocomposites and nanomaterials is limited by CNF moisture sensitivity due to surface hydration. We report on a versatile and scalable interface tailoring route for CNF to address this, based on technically important epoxide chemistry. Bulk impregnation of epoxide-amine containing liquids is used to show that CNF hydroxyls can react with epoxides at high rates and high degree of conversion to form covalent bonds. Reactions take place inside nanostructured CNF networks under benign conditions, and are verified by solid state NMR. Epoxide modified CNF nanopaper shows significantly improved mechanical properties under moist and wet conditions. High resolution microscopy is used in fractography studies to relate the property differences to structural change. The cellulose-epoxide interface tailoring concept is versatile in that the functionality of molecules with epoxide end-groups can be varied over a wide range. Furthermore, epoxide reactions with nanocellulose can be readily implemented for processing of moisture-stable, tailored interface biocomposites in the form of coatings, adhesives and molded composites.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-185720 (URN)
Note

QS 2016

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-04-27Bibliographically approved
5. Hierarchical wood cellulose fiber/epoxy biocomposites: Materials design of fiber porosity and nanostructure
Open this publication in new window or tab >>Hierarchical wood cellulose fiber/epoxy biocomposites: Materials design of fiber porosity and nanostructure
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2015 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 74, 60-68 p.Article in journal (Refereed) Published
Abstract [en]

Delignified chemical wood pulp fibers can be designed to have a controlled structure of cellulose fibril aggregates to serve as porous templates in biocomposites with unique properties. The potential of these fibers as reinforcement for an epoxy matrix (EP) was investigated in this work. Networks of porous wood fibers were impregnated with monomeric epoxy and cured. Microscopy images from ultramicrotomed cross sections and tensile fractured surfaces were used to study the distribution of matrix inside and around the fibers - at two different length scales. Mechanical characterization at different relative humidity showed much improved mechanical properties of biocomposites based on epoxy-impregnated fibers and they were rather insensitive to surrounding humidity. Furthermore, the mechanical properties of cellulose-fiber biocomposites were compared with those of cellulose-nanofibril (CNF) composites; strong similarities were found between the two materials. The reasons for this, some limitations and the role of specific surface area of the fiber are discussed.

Keyword
A. Nano-structures, B. Interface, D. Fractography, Moisture stability, Cellulose, Characterization, Composite materials, Fibers, Fracture mechanics, Mechanical properties, Nanostructures, Pulp materials, Textile fibers, Wood products, Cellulose fibrils, Controlled structures, Different length scale, Fractured surfaces, Mechanical characterizations, Microscopy images, Wood cellulose fibers, Wood, Moisture, Pulps, Stability, Surfaces
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-167694 (URN)10.1016/j.compositesa.2015.03.024 (DOI)2-s2.0-84927161110 (ScopusID)
Note

QC 20150602

Available from: 2015-06-02 Created: 2015-05-22 Last updated: 2016-08-19Bibliographically approved

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