Arttu Miettinen

Three-Dimensional Microstructural Properties of Nanofibrillated Cellulose Films

Nanofibrillated cellulose (NFC) films have potential as oxygen barriers for, e.g., food packaging applications, but their use is limited by their hygroscopic characteristics. The three-dimensional microstructure of NFC films made of Pinus radiata (Radiata Pine) kraft pulp fibres has been assessed in this study, considering the structural development as a function of relative humidity (RH). The surface roughness, micro-porosity, thickness and their correlations were analyzed using X-ray microtomography (X–μCT) and computerized image analysis. The results are compared to those from scanning electron microscopy and laser profilometry. Based on a series of films having varying amounts of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidated nanofibrils, it was demonstrated that X–μCT is suitable for assessing the surface and bulk 3D microstructure of the cellulose films. Additionally, one of the series was assessed at varying humidity levels, using the non-destructive capabilities of X–μCT and a newly developed humidity chamber for in-situ characterization. The oxygen transmission rate (OTR) of the films (20 g/m2) was below 3.7mLm−2 day−1 at humidity levels below 60% RH. However, the OTR increased considerably to 12.4mLm−2 day−1 when the humidity level increased to 80% RH. The increase in OTR was attributed to a change of the film porosity, which was reflected as an increase in local thickness. Hence, the characterization techniques applied in this study shed more light on the structures of NFC films and how they are affected by varying humidity levels. It was demonstrated that in increasing relative humidity the films swelled and the oxygen barrier properties decreased.

Strength of cellulosic fiber/starch acetate composites with variable fiber and plasticizer content

In this experimental study, the performance of injection-molded short flax and hemp fibers in plasticized starch acetate were analyzed in terms of strength. Parameters involved in the analysis are a variable fiber and plasticizer content. The measured strength of the composites varies in the range of 12–51 MPa for flax fibers and 11–42 MPa for hemp fibers, which is significantly higher than the properties of the unreinforced starch acetate matrix. The micro-structural parameters used in modeling of composite strength were obtained from optical observations and indirect measurements. Some of these parameters were qualitatively verified by X-ray microtomography.

Structural Characterisation of Kraft Pulp Fibres and Their Nanofibrillated Materials for Biodegradable Composite Applications

Structural Characterisation of Kraft Pulp Fibres and Their Nanofibrillated Materials for Biodegradable Composite Applications

Curvelet-based method for orientation estimation of particles from optical images

Abstract. A method based on the curvelet transform is introduced to estimate the orientation distribution from two-dimensional images of small anisotropic particles. Orientation of fibers in paper is considered as a particular application of the method. Theoretical aspects of the suitability of this method are discussed and its efficiency is demonstrated with simulated and real images of fibrous systems. Comparison is made with two traditionally used methods of orientation analysis, and the new curvelet-based method is shown to perform better than these traditional methods.

Curvelet-based method for orientation estimation of particles

A method based on the curvelet transform is introduced for estimating from two-dimensional images the orientation distribution of small anisotropic particles. Orientation of fibers in paper is considered as a particular application of the method. Theoretical aspects of the suitability of this method are discussed and its efficiency is demonstrated with simulated and real images of fibrous systems. Comparison is made with two traditionally used methods of orientation analysis, and the new curvelet-based method is shown to perform clearly better than these traditional methods.

Measuring intrinsic thickness of rough membranes: application to nanofibrillated cellulose films

Adequate measurement of thickness of sheet-like materials or membranes is most important for quantifying their properties such as density, barrier properties and mechanical strength. Depending on the surface roughness of the membrane, the thickness measured by standard micrometre devices (apparent thickness) may considerably overestimate the actual geometrical mean thickness (intrinsic thickness) required for such purposes. In this work, we present a method for correcting the measured apparent thickness value of thin membranes for their surface roughness, thereby obtaining an improved estimate of the intrinsic thickness. The surface roughness data required for the correction can be obtained by common surface profiling techniques. The method includes a calibration parameter, the value of which can be found experimentally by independent measurements, or can be estimated theoretically using results from standard mechanical contact theory. The method is tested on a set of nanofibrillated cellulose films with varying roughness levels controlled by pulp fibre content. The surface topography of film samples was measured using laser profilometry, and the method was calibrated experimentally using data from X-ray microtomographic images for one type of film. The intrinsic thickness estimates given by the new method are generally in good accordance with independent results obtained from X-ray microtomography.

Predicting stiffness and strength of birch pulp – Polylactic acid composites

This paper studies failure of birch pulp–polylactic acid composites. Stiffness and strength are calculated using the theory of short fibre composites and the results are compared to experimental data. The results differed from the experimental values by 0–6%. With less aligned fibres the short fibre theory is not feasible. The performance of the 40 wt% birch pulp – polylactic acid composite is predicted with X-ray microtomography based finite element modelling, and the results are compared with experiments. Stiffness results differed from experiments by 1–17% . By adding into the models a third material phase representing the interface between the fibres and the matrix, the stress–strain curve of the composite was obtained with good accuracy. The work presents finite element modelling methodology of wood plastic composites and the critical further steps needed in order to assess the stress–strain behaviour, strength and stiffness. Tools for comparing different wood plastic composite microstructures are also presented.

Time-resolved X-ray microtomographic measurement of water transport in wood-fibre reinforced composite material

Natural fibre composites are prone to absorb moisture from the environment which may lead to dimensional changes, mold growth, degradation of mechanical properties or other adverse effects. In this work we develop a method for direct non-intrusive measurement of local moisture content inside a material sample. The method is based on X-ray microtomography, digital image correlation and image analysis. As a first application of the method we study axial transport of water in a cylindrical polylactic acid/birch pulp composite material sample with one end exposed to water. Based on the results, the method seems to give plausible estimates of water content profiles inside the cylindrical sample. The results may be used, e.g., in developing and validating models of moisture transport in biocomposites.

Use of micro-tomography for validation of method to identify interfacial shear strength from tensile tests of short regenerated cellulose fibre composites

The interfacial shear strength of short regenerated cellulose fibre/polylactide composites was characterized by means of an industry-friendly adhesion test method. The interfacial shear strength wa ...