How Long Will It Last – a Study of Glue Line Durability

Unprotected exposure tests included: 1) two phenol-formaldehyde resins, both in liquid and spray-dried powder form, 2) two resorcinol-formaldehyde resins, a straight resorcinol and phenol-resorcinol modification, and 3) two fortified urea- formaldehyde resins, a liquid and spray-dried powder. Three-ply , 3/16-inch panels and five-ply 5/8-inch panels composed of yellow birch, sweetgum, yellow-poplar, and Douglas-fir veneers were bonded with the various adhesive mixes. Tensile shear specimens were cut for testing dry, testing wet after 48 hours’ soaking at room temperature, and testing after 3-1/2 cycles composed of 16 hours’ soaking at room temperature and 8 hours’ soaking at 145?F. The remainder of the panel was mounted on the outdoor exposure rack. Observations were made at 6-month intervals for the first 2 years and annually thereafter. Test results showed phenol exhibited excellent durability on Douglas-fir, but it showed erratic behavior on medium and low-density species and poor durability on dense species. The spray-dried phenol exhibited excellent adhesion with little difference between unextended and extended mixes. The straight resorcinol exhibited excellent durability, but thicker constructions of high density species deteriorate more readily than thin constructions. Phenol-resorcinol exhibited outstanding durability regardless of construction. Urea-formaldehyde resins lasted only a brief time; if fortified, the durability properties were improved. Protected exposure tests included four types of urea-formaldehyde adhesive mixes: two cold-press and two hot-press formulations. Three-ply 3/16-inch yellow birch panels were bonded with each adhesive mix. Test intervals of 7-1/2 and 10 years were selected. Panel segments were stored in an unheated open shed. Test results showed the hot-press formulations exhibited superior bond strength to the cold-press mixtures after 10 years’ aging. Controlled accelerated tests included: 1) urea-formaldehyde resins, a cold-press unextended mix, and several hot-press formulations extended with 50, 100, and 150 parts of wheat flour; 2) fortified-urea resins, three mixes moderately fortified with 13 parts of resorcinol; 3) phenol-formaldehyde resins, resin mixes extended with 50, 100, and 150 parts of wheat flour; these mixes were cured at a temperature of 280?F. Yellow birch three-ply 3/16-inch panels were bonded with the various mixtures. Specimens were exposed to an atmosphere of 160?F. and 60 percent relative humidity for periods from zero to 12 months. Test results showed that panels bonded with a urea-formaldehyde adhesive failed rapidly under conditions of high heat and moderate humidity. Fortifications of the urea bond by addition of mild resorcinol addition improved performance. The phenol resin exhibited excellent durability under high temperature and moderate humidity conditions.

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