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This study has examined in detail the effects of relative humidity, board type and a heat treatment on the physical and mechanical properties of medium density fibreboard (MDF).
Moisture content and thickness swelling of MDF increased with increasing relative humidity as the boards adsorbed moisture macro-molecularly (within fibres) and poly-molecularly (in the resin matrix, between fibres). Such increases may be due an increase in the number of moisture accessible sites and a breakdown of the resin matrix.
SEM analysis of fracture surfaces of MDF specimens after mechanical testing using revealed that fibre pull out or inter fibre shear was the primary failure mechanism of specimens tested at high humidities. The thermosetting resin-cellulose bond is known to be quite strong and it is suggested that weakening of the resin itself, rather than the resin-fibre bond was responsible for failure. At low humidities the fibre resin matrix was still intact and fibre failure was the primary failure mechanism.
Heat treatment of MDF was unsuccessful in decreasing the moisture sorption characteristics or reducing the effect of moisture on the mechanical properties of boards. There are many other heat treatments and variations of the heat treatment conducted here, that may prove to be more successful.
It is likely that advances in the betterment of the physical and mechanical properties of MDF on exposure to high RH's will result from developments in adhesive technology as failure at high humidities appeared to result from poor inter-fibre adhesion.
Two controlled experiments were performed, one to show that a six week conditioning period was sufficient time for board samples to reach EMC, and another to show that oven drying of board samples did significantly affect their properties. Results of both experiments validated the methodologies used in the thesis.
Powder coating of MDF was shown to be possible, but it did not appear from preliminary observations, to greatly increase the resistance of the samples to weathering. The powder coating of MDF and other wood products may depend on the arrival of technology that can cure the coatings at lower temperatures. There are several ASTM tests to measure the performance of coatings and paints (eg.. pencil hardness tests, gloss tests and paint-substrate adhesion tests [ASTM 1580.4-]) that could compare the performance of the powder coating of MDF with a) the performance of other coatings used on MDF and b) the performance of powder coatings on other materials.