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Thickness variability of cellulose nanofibril films: Measurement and implications for mechanical characterization
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
Paper and Fiber Research Institute (PFI AS), Trondheim, Norway.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
(English)Article in journal (Other academic) Submitted
Abstract [en]

To investigate the properties of different types of cellulose nanofibrils (CNFs), films are often produced. CNF films are easy to manufacture and relatively straightforward to characterize. Accurate measurements of the tensile properties of the films are often performed from which the mechanical properties of the films are determined. However, accurate measurment of the film thickness is often neglected which is an important property when defining the elastic modulus and strength of a CNF film. Many papers dealing with CNF films measure the film thickness with a micrometer screw gauge. The thicknesses that are measured in such way are the maximum thicknesses resulting in a significant error when determine mechanical properties of films with a high variability of local thickness, such as unsmooth CNF films. In this paper, the statistical distribution of CNF film thickness has been investigated by scanning electron microscopy of film cross-sections. Scale factors are proposed to relate the average thickness to the maximum thickness over a cross-section segment for films of different CNFs. Such scale factors are applicable to estimate the strength and stiffness, which are usally underestimated when using thickness measurements undertaken with a micrometer screw gauge. The maximum thickness, measured with a micrometer screw gauge, was as much as 95% higher than the average thickness. This effect is significant for films that show uneven surfaces resulting from inefficient CNF fibrillation. For chemical pretreatment and several passes through the homogenizer in the CNF manufacturing process, the effect is considerably smaller, but not negligible in characterization of mechanical properties. For further investigation of the thickness distribution, various three-parametric distributions with a lower bound and an upper tail all gave a fitting approximation to the experimental data.

National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-240248OAI: oai:DiVA.org:uu-240248DiVA: diva2:776074
Available from: 2015-01-06 Created: 2015-01-06 Last updated: 2015-03-09
In thesis
1. Elasticity of Cellulose Nanofibril Materials
Open this publication in new window or tab >>Elasticity of Cellulose Nanofibril Materials
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The demand for renewable load-carrying materials is increasing with increasing environmental awareness. Alternative sources for materials manufacturing and design have to be investigated in order to replace the non-biodegradable materials. The work presented in this thesis investigates structure-property relations of such renewable materials based on cellulose nanofibrils. Cellulose is the most abundant polymer on earth and exists in both ordered and disordered phases, where the ordered crystalline cellulose shows excellent mechanical properties. The celluloses nanofibril is composed of partly crystalline cellulose where the stiff crystal regions, or crystallites, are orientated in the axial direction of the fibrils. The cellulose nanofibrils have a high aspect ratio, i.e. length to diameter ratio, with a diameter of less than 100 nm and a length of some micrometres. In the presented work, different properties of the cellulose nanofibril were studied, e.g. elastic properties, structure, and its potential as a reinforcement constituent. The properties and behaviour of the fibrils were studied with respect to different length scales, from the internal structure of the cellulose nanofibril, based on molecular dynamic simulations, to the macroscopic properties of cellulose nanofibril based materials. Films and composite materials with in-plane randomly oriented fibrils were produced. Properties of the cellulose nanofibril based materials, such as stiffness, thickness variation, and fibril orientation distribution, were investigated, from which the effective elastic properties of the fibrils were determined. The studies showed that a typical softwood based cellulose nanofibril has an axial stiffness of around 65 GPa. The properties of the cellulose nanofibril based materials are highly affected by the dispersion and orientation of the fibrils. To use the full potential of the stiff fibrils, well dispersed and oriented fibrils are essential. The orientation distribution of fibrils in hydrogels subjected to a strain was therefore investigated. The study showed that the cellulose nanofibrils have high ability to align, where the alignment increased with increased applied strain.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 60 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1215
Keyword
Cellulose nanofibrils, Elastic properties, Micromechanics, Composites
National Category
Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-240250 (URN)978-91-554-9135-2 (ISBN)
Public defence
2015-02-13, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2015-01-22 Created: 2015-01-06 Last updated: 2015-03-09

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