Dimensional instability of cement-bonded particleboard: The effect of surface coating

Abstract

Cement-bonded particleboard (CBPB) as a composite of wood chips and reacted Portland cement is dimensionally unstable in service in the presence of changes in relative humidity (RH). One solution to this deficiency is the application of surface coatings to reduce its magnitude. The work reported here first evaluated the compatibility of various sealers to the highly alkaline surface of CBPB using small coupons of material. Three proprietary sealers were then applied to large-sized samples, and the most promising sealer was then subjected to long-term exposure. The test results indicated the following: (1) The ranking in effectiveness of sealers was proprietary system>model systems, and solvent-borne sealers>water-borne sealers. (2) The behaviour of coated CBPB reflected the change of RH, although the amplitude of the change was much reduced. CBPB 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 samples on moving from 65% to 90% RH compared with uncoated CBPB. (3) For the most effective sealer, doubling of the number of coats reduced mass change to about 60% that of a single coat during cyclic exposure and 20% to 70% for dimensions, but the level of reduction decreased as the number of coats increased. (4) All coatings showed a strong resistance to carbonation. Coated CBPB did not exhibit a consistent increase in mass and decrease in dimensions with cycles. (5) Under long-term exposure, there was a slight deterioration of the coatings. (6) The hysteresis loops for both mass and dimensional changes of coated CBPB moved upwards as the number of cycles increased; this is in contrast to that for dimensional change of uncoated CBPB.