Exterior OSB preparation technology at high moisture content - Part 1: Transfer mechanisms and pressing parameters

In the case of panels bonded with adhesives capable to tolerate high to ver y high percentages M.C. (20%-24% MC) of the mat during pressing, the highes t OSB core layer temperature will be reached at longer press closing times. This will result in better plasticization of the core layer. The press tem perature will influence the steam front transfer time to the core layer. Fu rthermore, contrary to the traditional lower moisture content bonding, it w as possible to deduce the existence of capillary water and that such a resi dual liquid water can contribute to heat transfer during hot pressing. Incr easing the press temperature will cause the maximum steam pressure peak to appear earlier, but this does not result in a higher core temperature. For long press closing times the initial mat percentage moisture content has a positive effect on the heating rate, but it also leads to high steam pressu re in the core layer. These conclusions are valid for both laboratory and i ndustrial boards, with the latter providing much faster heating rates and h igher values of temperature and steam pressure. Press closing time, mat per meability and strength development during densification were also found to be determining parameters as regards board densification and consequently a s regards boards mechanical performance in the case of panels bonded with a dhesives, such as procyanidin-type tannin adhesives, capable to tolerate hi gh to very high percentages M.C. of the mat. Directional mat permeability i n relation to steam movement was found to be the cause for the different ch aracteristics of board edges in relation to board centre, and a theory for quantification of the edge effect was advanced. The results also underlined one of the essential characteristics of adhesives capable of bonding at hi gher moisture content, namely their capacity of developing more quickly a h ardened network resistant to dissolving in water.