Phenolic Resin Glue Line as Found in Yellow Birch Plywood C. A.
FARROW,D. H, HAMLY,
AND
E. A. SMITH, Department of Chemical Engineering, University of Toronto, Toronto 5, Canada
A new technique for the analysis of phenolic resin glue lines is described. The method involves complete removal of the veneers by digestion with chromic acid, the glue being loft in an unaltered state rvitablo for microscopic examination. and subsequent rtereophotomicrography. Tho relation of wood structure to the penetration of tho adhesive is discussed with suitable illustrations.
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RODUCING stahle physical and chemical properties in adhesive which ' bonds veneers is important a t the present time, since it appears both desirable and possihle to make strong wood structures possessing both variety in shape and diversity in size. The fundamental theories of specific and mechanical adhesion , were established to a considerable degree by the work of McBain . and bo-workers (S)in England and Browne and Truax (i)in the United States during and after the close of World War I. Although these and suhsequent investigators have established the importance of specimen analysis strain test methods, and also the usefulness ai cross-section analysis, they have left glue line thickness, the relative importance of specific and mechanical sdbesjon theories, and the variahility of strength over limited areas, suhjects of lively interest. These problems are discussed in the present paper in the light of work done with anew technique which permits examination of the surface features and strncture of the phenolic glue line, I n order t o carry out a comprehensivestudy of a plywood bond, factors such as glue line thiokness, moisture content of veneers, eluine: Dressure. and temperature must be considered. The 'glue line has heen descrihed as a continuous film of resin between two veneer surfaces and the thickness of this line has been correlated to the bonding pressure. McBain and Ijee (4) and more recently Maxwell (6) claim that the strength in(:reases with decrease of thiokness. N. V. Poletika states that the strength varies inversely with the thickness (6). Browne and Tmax ( 1 ) disagree with this view and say that the thickness of the film may vaTy within rather wide limits without affecting the strength of the joint. The moisture contents of the veneers and of the resin before bonding are critical: when these are high, blistering results, and when low, the resin does not penetrate the superficial openings of the wood. In hotb cases the resulting adhesion is poor. The bonding pressure employed depends on the flatness and density of the wood. The degree of polymerization depends on the temperature gradient and bonding time (7); hence the control of these conditions is an important factor in plywood manufacturing. The authors have heen unahle to find any reports relating 'dl these factors with the nature of the resin bond under controlled conditions. ~
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has proved capahle of digesting wood and not digesting phenolformaldehyde resin within certain time and temperature limits. The present standard oractiee is to suspend the test specimen in the hot chromio acid solution (100 grams of chromic acid in 100 cc. of water) with.shout 2.5 em. (1 inch) immersed. The remainder of the sueeimen forms the sumort and carries the identifying code in' notches on the upoi/ edm. Dimstion is carried &at 80" to 85" C . for ahout 20 minutes, whkh is sufficient to expose completely thk glue lines in %ply !/,+h birch veneers After careful digestion, thorough wmhmg with hot water is necessarv t o remove the chromic acid and the caramellike DrOduCtS of &dation. which. if present. ohseure glue line detail.. The specimen i s then rinsed with acetone and allowed to dry. Though complete removal of this sludge is very diffioult in the case of fiber oests. normal washine and drvine " _ are raoid and permit immediate examination of t h e k i n .
During th'ese- obsc;va&& a n d i h e suhsei;e
