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

Abstract

In the case of panels bonded with adhesives capable to tolerate high to very high percentages M.C. (20%±24% MC) of the mat during pressing, the highest OSB core layer temperature will be reached at longer press closing times. This will result in better plasticization of the core layer. The press temperature will in¯uence the steam front transfer time to the core layer. Furthermore, contrary to the traditional lower moisture content bonding, it was possible to deduce the existence of capillary water and that such a residual liquid water can contribute to heat transfer during hot pressing. Increasing 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 pressure in the core layer. These conclusions are valid for both laboratory and industrial boards, with the latter providing much faster heating rates and higher values of temperature and steam pressure. Press closing time, mat permeability and strength development during densi®cation were also found to be determining parameters as regards board densi®cation and consequently as regards boards mechanical performance in the case of panels bonded with adhesives, such as procyanidin-type tannin adhesives, capable to tolerate high to very high percentages M.C. of the mat. Directional mat permeability in relation to steam movement was found to be the cause for the different characteristics of board edges in relation to board centre, and a theory for quanti®cation of the edge effect was advanced. The results also underlined one of the essential characteristics of adhesives capable of bonding at higher moisture content, namely their capacity of developing more quickly a hardened network resistant to dissolving in water.

Tannins: Major Sources, Properties and Applications

“…A considerable advantage is the much higher moisture content of the resinated chips tolerable with these adhesives than with any of the synthetic phenolic and aminoresin counterparts. In the case of wood particleboard and of oriented strandboard (OSB) panels, the technology so developed allows hot-pressing at moisture contents of around 24 per cent, against values of 12 per cent for traditional synthetic adhesives, and presents other advantages as well [58,66,67] .…”

Monitoramento da temperatura no interior de chapas aglomeradas durante o processo de prensagem.

“…Concluiu descrevendo que, entre as variáveis analisadas, o aumento da temperatura de prensagem se destacou por proporcionar redução do tempo de prensagem e melhora das propriedades físico-mecânicas. Pichelin et al (2001) e Calegari et al (2000 descreveram o perfil da temperatura de modo semelhante a Zombori (2001) e Bolton et al(1989). Pichelin et al (2001), entretanto, descreveram que, quando o colchão é prensado sob alto teor de umidade (20 a 24%), o perfil da temperatura tende a se comportar de maneira diferenciada de algumas fases descritas por Zombori (2001).…”

“…Pichelin et al (2001) e Calegari et al (2000 descreveram o perfil da temperatura de modo semelhante a Zombori (2001) e Bolton et al(1989). Pichelin et al (2001), entretanto, descreveram que, quando o colchão é prensado sob alto teor de umidade (20 a 24%), o perfil da temperatura tende a se comportar de maneira diferenciada de algumas fases descritas por Zombori (2001). Na segunda fase, não ocorre elevação linear da temperatura, mas, sim, um rápido aumento no início da fase e, posteriormente, um aumento muito mais gradativo.…”

“…Pichelin et al(2001) descreveram também sobre a importância da temperatura de prensagem. Quanto maior esse parâmetro, mais rápido é a condução de calor, conseqüência do maior gradiente de vapor e calor pelo colchão.…”

“…Entretanto, a temperatura de cura do adesivo foi atingido anteriormente quando utilizado a maior temperatura de prensagem (Figura 4). Conforme descrito por Pichelin et al (2001), esse comportamento se deve ao maior gradiente de calor produzido entre as faces e o centro do colchão.…”