Exterior and accelerated aging of an acid-phenolic molding resin in RF-cured Douglas-fir joints

The durability of adhesive bonds of an acid-catalyzed phenol-formaldehyde molding resin to Douglas-fir was evaluated by two outdoor and two accelerated aging procedures. The resin’s performance was judged by comparisons with three standards: bonds of the highly durable phenol-resorcinol-formaldehyde, a less durable melamine-urea-formaldehyde, and the solid wood of Douglas-fir itself. Durabilities were determined by periodically measuring loss in wet shear strength of bonded and solid wood specimens as they were exposed to the same four aging environments: 1) 40 days of moist-heat aging; 2) 800 cycles of boiling and drying in an automatic boil test; 3) 3 years of sheltered outdoor exposure; and 4) 3 years of direct outdoor weathering; both outdoor exposures were in Athens, Ga. The acid-phenolic resin performed as well as the highly durable phenolic-resorcinol in all four aging tests. Its performance was also comparable to that of Douglas-fir solid wood in the two accelerated procedures and sheltered outdoor exposure, but began to lag behind the solid wood after 2 years of outdoor weathering. Neither the automatic boil nor moist-heat aging procedure could be used to realistically estimate adhesive-bond performance during outdoor weathering. The acid-phenolic resin is potentially a low-cost alternative to the durable phenol-resorcinol and melamine adhesives that are commonly used with radio-frequency curing for laminating wood products for exterior use.

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