Structural characterisation of wood pulp fibres and their nanofibrillated materials for biodegradable composite applications

Introduction: The utilization of wood pulp fibres in composite materials has gained major interest during the last years. One of the major motivations has been the potential of wood pulp fibres and their nanofibrillated derivatives for increasing the mechanical properties of some materials. However, in order to exploit the full potential of wood pulp fibres and cellulose nanofibrils as reinforcement in hydrophilic and hydrophobic matrices, several characteristics of fibres and their interactions with a given matrix need to be understood. With the increasing capabilities of novel microscopy techniques and computerized image analysis, structural analysis is moving forward from visual and subjective evaluations to automatic quantification. In addition, several microscopy techniques for obtaining 2D and 3D images of a given composite material, including field-emission scanning electron microscopy (FESEM) and X-ray micro-computed tomography (X-µCT), have evolved considerably during the last years. X-µCT is a non-destructive method for obtaining the three-dimensional structure of a physical material sample. It is well suited for structural analysis of complex heterogeneous materials such as paper, biological materials and fibrous composites (Samuelsen et al., 2001; Holmstad et al., 2005; Axelsson, 2008). In addition, FESEM is a powerful technique for assessment of a variety of materials. One of the major advantages of FESEM is its versatility and high-resolution power (Chinga-Carrasco et al., 2011). Structures down to 1-2 nm can thus be visualized and quantified. In this work we will focus on practical and complementary imaging and image analysis techniques. We will also give a brief introduction to SEM, X-µCT and to 3D image analysis methods, emphasizing topics that are relevant for characterisation of composite materials. Selected case studies of wood pulp fibre-reinforced composite materials and their corresponding microstructure-property relationships will be discussed.peerReviewe