Composites made from acetylated lignocellulosic fibers of different origin - Part I. Properties of dry-formed fiberboards

Lignocellulosic fibers of different origins were acetylated in large batches. The fibers used were of commercial, medium density fiberboard (MDF) pulp quality produced from softwood, beech, waste wood (low quality residue from an intermediate forest cutting) and wheat straw, respectively. Fiber from de-inked, semi-bleached, recycled paper was also included in the study. Laboratory fiberboards were produced from the acetylated fibers by using powdered phenolic resin of the novolak type as a binding agent. Control boards made from unmodified fiber were also produced. The fiberboards were subjected to mechanical and dimensional stability testing. The acetylation of the fiber, regardless of the source of the lignocellulosic material, has a very positive impact on the performance of fiber composite products. For instance, the thickness swelling for softwood fiberboard in water is reduced by approximately 90%, and the mechanical properties are slightly improved. Results from cyclic testing according to EN 321, (three cycles, each comprising 72 h water immersion, 24 h freezing at -18 degrees C and 72 h drying at 70 degrees C) show that more than 90% of the internal bond strength, IBS, remained after the testing. This value can be compared with the corresponding value of 30-40% obtained for fiberboards made from unmodified fibers. The remaining thickness swelling of fiberboards made from acetylated fibers was independent of fiber source and resin content. The results obtained showed that acetylation can improve the mechanical properties of fiberboards provided that a proper resin is chosen.