Dimensional instability of cement bonded particleboard: stabilisation treatments

Abstract

As part of an extensive programme of research aimed at a better understanding of the dimensional instability of cement bonded particleboard (CBPB) and establishing methods of improving its stability, this paper discusses the treatment processes for stabilisation of CBPB. CBPB as a composite of wood chips and reacted Portland cement is unstable in service. One solution to this deficiency is to treat the CBPB product to reduce its magnitude. Three treatment processes, namely, storage of, pre-drying and application of surface coatings on CBPB, were investigated. The results indicated that all three treatments were able to reduce both dimensional and mass change of CBPB, but the efficacy of the treatments was closely related to their schedules. Prolonged storage resulted in a stabilisation of CBPB due to further hydration and carbonation of CBPB. Mass change of CBPB with 12 month storage was only about 2/3–3/4 that with 2 month storage under changing relative humidity (RH). A pre-drying process resulted in a structure alternation of CBPB and stabilised it permanently. A considerably irreversible mass decrease and dimensional shrinkage occurred when re-wetting at 65%RH: about 0.7% and 3% for mass, 0.02% and 0.12% for length, and 0.08% and 0.19% for thickness, respectively for 35%RH and 105°C pre-drying CBPB. However, mass change was independent of the pre-drying schedules after exposure at 90%RH, while irreversible dimensional shrinkage remained. Surface coating had a significant influence on the behaviour of CBPB due to physical and chemical reaction. The ranking in effectiveness of sealers was: proprietary system greater than model systems, and solvent-borne sealers greater than water-borne sealers. Compared with uncoated CBPB, that which is coated with the most effective sealer showed 70–90% reduction in both mass and dimensions over the whole range of RH exposure with the exception of the range from 65–90%RH. All coatings also showed a strong resistance to carbonation of CBPB.