Wood fiber composites : from processing and structure to mechanical performance

Sammanfattning: The work presented in this thesis has been focused on the mechanical properties of wood fiber composites. One goal has been to investigate the processing of wood polymer composite materials with good mechanical properties, and how to tailor these properties for different applications. Different kinds of composites were manufactured and characterized regarding microstructure and mechanical properties. In the first study, it was investigated how to utilize the high stiffness and strength of the wood cell wall material to get good mechanical properties of the composite material. Wood chips were cooked with a sulfite technique for different cooking times in an attempt to extract relatively undamaged fibers with high stiffness and strength and study the influence of different degree of fiber separation. Composites were made by impregnating oriented fiber mats with an epoxy resin and curing in a laboratory press. The best properties were achieved with completely separated fibers. The apparent Young’s modulus of the fibers was calculated by a micromechanical model to approximately the same level as is measured on single wood fibers, which indicates a good utilization of stiffness of the wood cell wall material. Recognizing that high stiffness and strength requires high volume fraction of fibers without getting porosity in the material, the manufacturing process was developed for high volume fraction of fibers. Commercial kraftliner and saturating papers were impregnated with low viscosity phenol- formaldehyde resin and laminates were made by pressing a stack of these papers in a hot press. Fiber volume fractions of about 70% were achieved, which is high compared to for example conventional glass fiber composites. It was also shown that high values of stiffness (Young’s modulus >19GPa) and strength (tensile strength >190MPa) are obtainable with this type of material. By further development of the paper, higher strength and stiffness could probably be achieved The general conclusions are that it is favorable to use carefully separated single wood fibers to make a wood composite material with good mechanical properties. To be able to tailor properties, fibers in the form of thin and relatively dense fiber mats or paper with high degree of orientation could be used. Composite materials could then be made from these mats by impregnation with a pre-polymer with low viscosity and good compatibility with wood to get a material free from porosity. Since the mechanical properties of both wood fibers and polymer matrix material are time dependent, wood fiber composites have complex time dependent stress-strain response. For wood fiber composites to be used in load bearing applications, their time dependent behavior has to be known and be able to predict. Starting from a general non-linear viscoelastic and non- linear viscoplastic constitutive law derived from thermodynamics, a methodology for determination of the included nonlinearity functions for a specific material has been developed. Creep - strain recovery tests have been performed and the developed methodology was used to determine a constitutive model. An incremental form of the model was also developed to simulate arbitrary loading conditions and to implement the model in finite element software. The material model was verified by simulating a linear load ramp and compared with results from a corresponding experiment. The material tested was characterized as a nonlinear viscoelastic viscoplastic material, with no stiffness reducing micro damage detected in the load range tested.