High-Performance Composites from Modified Wood Fiber
Doctoral thesis, 1998
The objective of this work was to produce and evaluate high-performance wood fiber composites. High-performance was defined as: high dimensional stability and a high durability, including resistance to wood-decaying organisms (fungi, bacteria, and insects).
The main method, used to accomplish these properties, was acetylation of the wood fibers prior to the composite production. Acetylation involves a chemical modification of wood components, which was carried out by reaction with acetic anhydride. Another method applied was impregnation with kraft lignin followed by fixation of the lignin with an aluminum salt solution. Thus, lignin metal complexes are formed in situ - complexes that are insoluble under normal conditions in the environment.
Test results show that both methods lead to more dimensionally stable composites, i.e. reduced swelling and shrinkage due to a varying moisture load. Kraft lignin treatment of wood fiber was estimated to be an economical method for obtaining a certain degree of stabilization of composites (fiberboards), although the composites cannot be labeled "high-performance" since they do not meet all requirements, as defined above. However, the use of acetylated fibers leads to a very high degree of dimensional stability as well as biological decay resistance and, furthermore, the mechanical properties are maintained during and after cyclic climate aging. Regardless of the lignocellulosic fiber source used, the thickness swelling of acetylated fiberboards in water is reduced by approximately 90% and results from cyclic testing according to EN 321 (three cycles, each comprising 72h water immersion, 24h freezing at -18°C and 72h drying at 70°C) show that more than 85% of the original internal bond strength, IBS, remains after testing. The latter figure should be compared with the corresponding value of 20-40% obtained for fiberboards made from unmodified fibers. Acetylated wood fiber composites have also been tested for resistance to biological decay in a worldwide field test where 30 cm long fiberboard stakes were half-buried in soil. After three years of testing, most of the control stakes (made from unmodified fiber) had failed due to heavy decay while most of the acetylated composite stakes were perfectly sound, showing no sign of decay.
To determine whether the acetylation, and other methods of chemical modification, cause any degradation of the cellulose, .alfa.-cellulose was isolated from unmodified and chemically modified fibers after which the intrinsic viscosity and molecular mass distribution (determined by size exclusion chromatography) of the cellulose samples were analyzed. The main part of the cellulose from acetylated wood fiber seems to be unaffected by the modification method, whereas the other modification methods appear to yield more or less degraded cellulose.
In order to manufacture products with complex shapes, moldable flexible fibermats were produced on both a small pilot scale (kilograms) and on a large scale (over one metric ton). A variety of industrial products were manufactured from the acetylated fibermats: exterior doors, automotive panels, façade claddings, flooring boards, wet-room wall panels, toilet lids and roof tiles. Generally, it was found that the dimensional stability of the products was high and that other important properties, such as mechanical and weathering properties, were also very satisfactory.
The conclusion is that composites based on acetylated wood fiber are truly high-performance, as defined above.