Cure of Phenol-Formaldehyde Adhesive in the Presence of CCA-Treated Wood by Differential Scanning Calorimetry

Chromated copper arsenate (CCA) preservatives interfere with bond formation of phenolic-based adhesives on CCA-treated southern pine. Ions of chromium (Cr+3) and copper (Cu+2) are known to complex with phenol and formaldehyde, affecting the rate of cure of the resin. Differential scanning calorimetry (DSC) was used to thermally analyze the cure of a commercial phenol-formaldehyde adhesive in the presence or CCA-treated wood, solutions of CCA preservative, and solutions of model compounds containing ions of Cr+6, Cr+3, Cu+2, and As+5. The DSC thermograms indicated that free metallic ions in solution can accelerate the cure of a portion of the adhesive at less than normal temperatures. However, when the CCA preservative was chemically "fixed" within the southern pine wood, accelerated cure of the adhesive did not occur

Reactivity of Hydroxymethylated Resorcinol Coupling Agent as it Affects Durability of Epoxy Bonds to Douglas-fir

Epoxy adhesives develop strong bonds to wood, but they lack the structural durability to withstand the severe stresses from repeated water soaking and drying. Research at the Forest Products Laboratory led to a discovery that hydroxymethylated resorcinol (HMR) physicochemically couples to both epoxy adhesive and lignocellulosics of wood to produce bonds that are extraordinarily resistant to dclamination. The HMR coupling agent is quite reactive at room temperatures; therefore, the length of its reaction time, or the time between preparing the solution and applying it to the wood surface, strongly influences the durability of adhesion. The experiments in this study defined the optimum range of reaction time when adhesion is maximum for epoxy bonds to HMR-primed Douglas-fir. Heats of reaction (by differential scanning calorimetry), molecular-size distribution (by gel permeation chro-matography), and chemical structures of HMR (by carbon-13 nuclear magnetic resonance spectros-copy) are described for this range of optimum reaction times

Hydroxymethylated Resorcinol Coupling Agent for Enhanced Durability of Bisphenol-A Epoxy Bonds to Sitka Spruce

Epoxy adhesives can develop bonds to wood that are as strong as the wood itself, but only if the bonds remain dry. Once exposed to repeated water soaking and severe stresses from drying, epoxy bonds delaminate and fail to meet requirements for structural wood adhesives intended for exterior exposure. A new hydroxymethylated resorcinol (HMR) coupling agent, applied to lumber surfaces before bonding, chemically couples both epoxy adhesive and lignocellulosics of wood to produce bonds to Sitka spruce that are extraordinarily resistant to delamination. In this report, we explain and demonstrate the nature of this coupling agent and the mechanism by which it enhances the durability of bonds of a bisphenol-A epoxy adhesive to wood. When diluted with benzyl alcohol, an epoxy adhesive developed structural bonds in HMR-primed lumber laminates that met the 5% maximum delamination requirement of ASTM Specification D 2559. The USDA Forest Service has applied for a patent for this invention

Mechanical Interlocking of Adhesive Bonds to CCA-Treated Southern Pine—A Scanning Electron Microscopic Study

New adhesively bonded products made from lumber, veneer, flakes, and fibers that are protected from biological deterioration can play a prominent role in the marketplace if difficulties in bonding preservative-treated wood can be overcome. The purpose of this study was to demonstrate that bonds of extraordinary integrity can be developed in southern pine treated with chromated copper arsenate (CCA) preservative if a durable adhesive penetrates deeply enough to mechanically interlock within the cellular structure of the wood. Scanning electron microscopy and elemental analysis by energy dispersive spectrometry were used to explore the interfacing of adhesive with the cellular structure of wood treated with CCA preservative. The surfaces of cell lumens were thoroughly covered with hemispherically shaped deposits consisting of mixtures of chromium, copper, and arsenic. The micrographs support physicochemical theories of bonding of metals to microfibrils in cell walls. The presence of insoluble metallic deposits was so pervasive that most opportunities for molecular forces of attraction to act between normally polar wood and adhesive were physically blocked. Our results nevertheless show that adhesion by mechanical interlocking of a phenolic adhesive deep in the cellular structure of CCA-treated southern pine can produce delamination-resistant bonds even after severe cyclic aging

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