Effect of solvent extraction and planing of veneers from Southeast Asian species upon glueline characteristics and bond durability Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1v53k007j

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  • Using Southeast Asian hardwood veneers for exterior plywood siding has caused problems because some species produced quality plywood while other species produced plywood which delaminated soon after being put in service in exterior exposure. This study attempted to improve the durability of glue bonds of some Southeast Asian hardwood veneers by treating the veneer surfaces prior to gluing with a phenolic adhesive that has the advantage of shorter press time in plywood production. The species selected were known to produce plywood panels having a range of bond durabilities for untreated veneer glued with this adhesive. Kapur (Dryobalanops aromatica) was chosen because it has consistently glued poorly. A low density Meranti (Shorea curtisii) having good gluability and a high density Balau (Shorea ochropholia) having poor gluability were selected. Also six Keruing species (Dipterocarpus spp.) having variable bond durabilities were included. The two treatments were planing the veneer surface arid extracting the veneer surface with a one percent caustic solution. A statistical analysis was performed on wood failure data from plywood shear tests using a split plot experimental design. The design factors were three treatments (planing, extraction, control), ten species including the Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] control, two phenolic adhesives (A and B), two assembly times (20 and 45 minutes), two test methods (vacuum-pressure and automatic boil) and four replications. A second phase of the study focused on anatomical observation of the veneer surfaces and plywood gluelines of treated and untreated veneer. Stereo light microscopy, incident fluorescence microscopy and scanning electron microscopy were used. Plywood made of untreated veneer using adhesive A produced the variable bond quality previously observed for these species, many being below the commercial standard. Planing the veneer surface prior to gluing was an effective treatment. All of the hardwood species producing unacceptable plywood bonds when made with untreated veneer produced plywood panels with acceptable bonds when made of planed veneer. Planing altered the veneer surface anatomically. The surface appeared smooth with cellular debris deposited in the vessel lumens. Planing appeared to produce veneer surfaces with larger amounts of intra-wall failures rather than cross-wall failures common for unplaned veneer surfaces. The plywood gluelines were of uniform thickness and appeared to cause an even adhesive distribution. Extracting the veneer surface with a caustic solution prior to gluing was not an effective treatment. The variable bond quality of the species was not eliminated. Extraction produced a darker colored veneer surface and it appeared that new extractives might have migrated to the surface following the extraction sequence.
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