The effects of wetting and scratching pretreatment of veneers on shear strength of binderless plywood made from sugi (Japanese cedar, Cryptomeria japonica)

Abstract

To produce practicable binderless laminated veneer lumber (LVL) and plywood, their water resistance needs to be improved. The objective of this study was to manufacture highly water-resistant binderless plywood from sugi (Japanese cedar, Cryptomeria japonica), basing on a self-bonding mechanism, without any resins or powders. To achieve this, a pretreatment of wetting and scratching veneers was applied. Two types of wetting conditions, soaking for 12 h, and wetting for 1 min, were examined. Sandpaper or a wire brush was used for scratching. Three veneers were laminated in perpendicular directions and hot pressed at 200–220 °C for 20–45 min using two types of pressing schedule: simple 1-step pressing, or 4-step pressing while gradually increasing the pressure. As a result, whichever wetting time was employed, and regardless of the scratching tool used, plywood could be manufactured with both pretreatments. As the wet shear strength showed a comparable value to dry shear strength, the plywood had high water resistance. The highest wet shear strength was 0.6 MPa, for the plywood made by pretreatment wetting of the surface for 1 min and scratching with a wire brush followed by 1-step pressing at 220 °C for 20 min. This was fairly close to the Japanese Agricultural Standard (JAS) requirement. This study indicated that easy pretreatment and mild pressing conditions achieved a binderless plywood with high water resistance.