Relationship Between Chemical Characteristics of Phenol-Formaldehyde Resins and Adhesive Performance

Selected resin formulations representing extremes in chemical characteristics were evaluated as adhesives either without additives (neat resins) or with blended extenders and fillers (mixed adhesives). These formulations were used to prepare parallel-laminated panels, which were subjected either to vacuum-pressure or boil-cycle aging. Both breaking load and percent of wood failure in shear were recorded. Estimated wood failure of specimens subjected to vacuum-pressure aging proved to be most useful for evaluating resin performance as a function of chemical characteristics. Wood failure was inversely related to weight-average molecular weight (Mw) of neat resins, primarily because resins of high molecular weight flowed poorly at long assembly times, allowing moisture to migrate away from the resin and into the wood. However, the effect of Mw on wood failure was counteracted by a high relative branching index of the resin polymer and by high percentages of free formaldehyde, phenol, and orthomethylolphenol (OMP) in the resin formulation. When neat resins were blended into an adhesive mix and aged by vacuum-pressure, their intrinsic properties were masked. Because an extended mix retains moisture in the glueline, such resins with a high Mw flowed better at long assembly times than did those with a low Mw and were also associated with higher percentages of wood failure. Resins with a low Mw and those with high contents of monomeric phenol and OMP performed poorly because condensation was inhibited. With boil-cycle aging, the performance of these mixed adhesives with low Mw and high contents of phenol and OMP improved because of the additional cure imparted by the aging process.

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