Thickness variability of cellulose nanofibril films: Measurement and implications for mechanical characterization
(English)Article in journal (Other academic) Submitted
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.
Engineering and Technology
IdentifiersURN: urn:nbn:se:uu:diva-240248OAI: oai:DiVA.org:uu-240248DiVA: diva2:776074