Wood fiber composites processing, structure and properties

Sammanfattning: The goal of this work has been to investigate how to make wood polymer composite materials with good mechanical properties, which also could be tailored for different applications. The fundamental idea behind the work was that good mechanical properties of the composite material require that the stiffness and strength of the wood cell wall material are really utilized. Furthermore, the volume fraction of fibers has to be high and the fibers have to be oriented in the loading direction. Different kinds of experimental materials were manufactured and characterized. In the first paper, 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 to a level which indicates a good utilization of stiffness of the wood cell wall material. In the second paper experiments were made in order to study the manufacturing process for wood fiber composites with high volume fraction of fibers. Commercial papers were impregnated with a 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. It was also shown that values of stiffness (Young's modulus >19GPa) and strength (tensile strength >190MPa) are obtainable with this type of material. The general conclusions are that to make a wood composite material with good mechanical properties in combination with ability to tailor properties, is to use carefully separated single wood fiber in the form of dense fiber mats or paper with high degree of fiber orientation as reinforcement. Composite materials are then made by impregnation with a compatible low viscosity resin and curing in a hot press.