Gap-Filling Phenol-Resorcinol Resin Adhesives for Construction

The consistency of a conventional phenol-resorcinol-formaldehyde resin adhesive was increased to the mastic-like consistency of an elastomeric construction adhesive by adding fillers of asbestos, walnut shell flour, or wood flour. The resulting systems were capable of being extruded in non-sagging beads to form gap-filling gluelines between poorly fitting structural members of lumber and plywood. Tests on lumber-to-lumber joints of southern pine indicated that these modified adhesives in thick gluelines did not possess the strength and serviceability of joints prepared from a conventionally formulated adhesive in thin gluelines. However, those adhesives filled with either asbestos or walnut shell flour had sufficient strength, even after cyclic soaking and drying treatment, to support shear loads of almost l,000 psi and more in gluelines up to 9.060 inch thick. None of the adhesives was capable of developing 80 percent wood failure on southern pine lumber. Only the asbestos-filled adhesive yielded less than 5 percent delamination in all glueline thicknesses. Less than 10 percent delamination developed in all glueline thicknesses with the adhesive filled with walnut shell flour. Statistical comparisons between the three modified adhesives indicated some important differences in strength properties. The primary influencing factors on strength properties were type of filler and whether specimens were tested dry or after cyclic soaking and drying treatment; glueline thickness proved to be a relatively insignificant influence. Plywood-to-lumber joints made with adhesives containing asbestos, walnut shell flour, or wood flour had sufficient strength and serviceability to utilize nearly the full rolling-shear strength of southern pine plywood, as indicated by a high degree of wood failure. The adhesive containing asbestos filler developed acceptable levels of dry shear strength and resistance to delamination, although percentage of wood failure was somewhat below standards for southern pine lumber. Such filled phenol-resorcinol resin adhesives should be seriously considered for factory assembly of structural building components. Despite disadvantages of thick and variable-thickness gluelines, this type of adhesive can possibly offer strength stiffness, and serviceability which cannot now be achieved with mastic adhesives in construction lumber and plywood

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