Peeling, gluing and bonding characteristics of Nigerian plantation-grown Gmelina arborea (Roxb.)

The gluing properties of plantation-grown gmelina wood (Gmelina arborea Roxb.) from Nigeria were investigated. Three wood peeling temperatures - 20°C, 50°C and 85°C; two glue types - urea-formaldehyde (UF) and phenol-formaldehyde (PF); two glue spreads - 25 kg/MDGL (55 lb) and 32 kg/MDGL (70 lb); and two closed assembly times - 10 minutes and 20 minutes were used. Veneers from the sample logs were peeled tight and were 1.27 mm (0.05 in) thick. The specific gravity for the logs was determined and its influence on the probable end uses of gmelina plywood discussed. Three 5-ply plywood panels were made, for each treatment combination for 72 in all. A total of 1438 shear test specimens were used. The UF specimens were tested dry and after vacuum-pressure treatment whereas the PF specimens received vacuum-pressure and boil-dry-boil tests. Bond quality was evaluated on the basis of wood shear strength and percentage wood failure. Results were compared to the U.S., British and German Plywood Standards. Wood peeling temperature was highly significant regardless of glue type and bond quality testing method. Heating of gmelina logs prior to peeling did not improve veneer peel-quality. Veneers were of the highest peel-quality (basis: Thickness variation and surface roughness) when logs were peeled at 20°C. The highest peeling temperature yielded the lowest peel-quality. Bond quality, (percentage wood failure), was consistently reduced by increasing peeling temperature and was lowest at 85°C in all the UF and PF treatments, regardless of bond quality testing method. Ignoring glue spreads, panels made from veneers cut at temperatures of 50°C and 85°C gave the highest shear strength values among the UF treatments. On the other hand, panels from veneers cut at temperatures of 20°C and 85°C gave the highest shear strength values among the PF treatments. All factors considered, treatment combination of Spread 55 - Time 20, arising from veneers cut at the control temperature of 20°C, gave an impressive bond quality in all the UF and PF treatments used. Five of the 12 PF treatments used, regardless of type of bond quality testing method, pass the U.S. Plywood Standard; one passes the British Standard; while all pass the German Standard. On the other hand, five of the 12 UF treatments from vacuum-pressure test pass the U.S. Standard; two pass the British Standard; while all pass the German Standard. Furthermore, all the 12 UF treatments from dry test pass the U.S. Standard; six pass the British Standard; while all pass the German Standard. From the results obtained, plantation-grown Gmelina arborea wood from Nigeria, with a specific gravity of 0.41 ± 0.027 (as determined), was found suitable for use as construction plywood, core and crossband veneer for decorative panel as well as container veneer and plywood. The dominant factor accounting for the general trend of low percentage wood failure was attributed to veneer surface inactivation, resulting from surface aging of veneers.Forestry, Faculty ofGraduat