Manufacture of cylindrical LVL by spiral-winding method II. Fast-setting adhesives for cylindrical LVL manufacture

To increase the yield and improve efficiency in the manufacture of cylindri cal LVL using the spiral winding process, various methods are being conside red. The simplest and most effective is the use of fast-setting adhesives ( FSA) which can be cured at low temperatures and short press times. These ca n be mixed with additive resins (AR) which have special properties and/or s maller costs. Two types of adhesive mixtures were studied, 1) Urea formalde hyde resin in combination with isocyanate resin (UF-mixture), and 2) Fast s etting resorcinol meta-aminophenol in combination with ordinary resorcinol resin (RF-mixture). Sawed Veneers of katsura (Cercidiphyllum japonicums S. et Z.) were used as test pieces. The physical properties, adhesive strength , and durability of the adhesives and adhesive mixtures were studied. The a dhesive strengths were determined through shear tests in tension. 1) After the first 20 minutes of pressing for either of the two types of ad hesive mixtures, the development in adhesive strength was slow when the FSA content was less than 50%. For the UF -mixture, the strength value leveled off after 100 minutes. On the other hand, the;strength of the RF -mixture did not level off even after 24 hours. 2) In the case of the UF-mixture, the addition of isocyanate resin hardly i mproved the durability of the bond. 3) The durability of RF-mixture was improved by the addition of ordinary re sorcinol resin. Especially, the mixing ratio of 2/1 (AR/FSA) resulted in go od performance of glued veneers in the durability tests, enough for structu ral uses. 4) Press time (1-5 min) and pressure (0.3-1.2 MPa) hardly influenced the ad hesive strength and wood failure of the RF-mixture at 2/1 mixing ratio.