Physical properties of Pinus radiata veneers modified with hexamethoxymethyl melamine prepolymers
C Molloy, W Rae, S Connor, T Henderson, A Siraa
Prepolymers containing hexamethoxymethyl melamine and either sucrose (Suc-HMMM) or polyvinyl alcohol (PVA-HMMM) were prepared by acid catalysis under reaction conditions optimised with respect to stiffening effect when introduced into Pinus radiata veneers by vacuum impregnation and cured by hot pressing at 150C. Maximum increases in MOE were 20% for Suc-HMMM (50% WPG), 22% for PVA-HMMM (14% WPG). A 1:1 mixture of the two prepolymerisation reactions (P-H:S-H) produced a maximum 27% increase in MOE (40% WPG) and was stable at ambient temperature for at least 54 days. Veneers vacuum-impregnated with P-H:S-H increased the flatwise MOE and MOR by 32% and 19%, respectively, when used as the two superficial veneers on both sides of 12-ply LVL. Edgewise MOE and MOR increased by 19% and 11% respectively. Smaller increases in stiffness and strength were achieved when four treated veneers were distributed throughout the LVL, or when LVL was produced using twelve treated veneers. Veneers treated by immersion in P-H:S-H at various concentrations, then used as a face veneer in plywood increased surface hardness significantly but formaldehyde emissions of the most useful plywood products exceeded the limit of the F*** classification. MDF manufactured from Pinus radiata fibres treated with P-H:S-H at 10%, 30% and 50% by fibre weight showed a decrease in water-induced weight gain and swelling proportionate to the prepolymer loadings.