Performance improvement of PVAc glue in wood finger joints by using Isocyanate as a hardener

In this study the effect of open time and hardener content on bending strength of finger jointed made of three wood species; Beech, Fir and Poplar was investigated. For this purpose wooden samples with dimension 25mm width, 35 mm thickness and 350 mm length from three species were prepared. After three weeks storage in climate room, in order to form finger joint pattern, samples were cut at the half of length. Then the finger joint pattern was done in new exposed cross sections of samples. After that the joints were bonded with three PVAc glue types; pure, with 10% and with 20 % hardener. The second variable of study was open time that was controlled in two levels of 2 and 4 minutes. The clos time and press time were constant which were adjusted on 2 and 30 minutes respectively. The joints were pressed in a special wooden frame by hand clamp. After making joints, they were stored in climate room for three weeks and then the three point bending test was carried out according to ISO 10983. The results showed that by adding isocyanate hardener up to 20% the bending strength increases significantly. The longer open time could not improve bending strength even it caused strength dropping in some treats but this changes were not significant. The strength of joints made with poplar wood was lower than beech and fir although this difference was not significant statistically. Finally the best condition for making this joint was determined by application of 2 min open time and using 10 % PU resin as hardener.