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They are an important class of adhesive used in the manufacture of outdoor grade plywood. In most of these applications the adhesive is applied as a solution in alcohol, acetone, or water. It is coated on the substrate, dried, and then cured under heat and pressure. However, several forms of phenolic adhesives are available including spray dried powders that are dissolved in water for application and films. Curing is accomplished at temperature of approximately 140°C for several minutes.

Adhesives for plywood are essentially solutions of low molecular weight condensation products of phenol and formaldehyde in aqueous sodium hydroxide. They provide bond strengths that are generally greater than the strength of the wood substrate. Durability is good, and the bond is essentially unaffected by boiling water, mold, or fungus. These adhesives are suitable for exterior use, and their properties are not appreciably affected by low temperatures. In the presence of alkaline catalysts, crosslinking is brought about at elevated temperatures. Acid catalysts give room temperature cures, but also cause degradation of wood and paper. Phenol formaldehyde resin is available in the form of glue film, carried on tissue paper. Although expensive, this form of adhesive is useful for faying of very thin or highly porous veneers.
Phenolic resins have also been used to bond metal to glass. The bond is somewhat brittle, however, and tends to shatter under impact or vibration. As a result, phenolic resins are often modified by the addition of elastomeric resins to improve toughness and peel strength:

Nitrile-phenolic adhesives: The major advantage of incorporating nitrile into phenolic resin is to improve the peel strength of the phenolic without significant reduction in high temperature strength. On metals, nitrile-phenolics offer shear strength of up to 5 000 psi along with excellent peel and fatigue properties. Good bond strengths can also be achieved on rubber, plastics and glass.

Because of their good peel strength and elevated-temperature properties, nitrile-phenolic adhesives are used commonly for bonding linings to brake shoes. They are also used in the aircraft industry for bonding of aluminum facings to honeycomb cores. Other applications include electronics, footwear, and furniture assembly.

Vinyl-phenolic adhesives are based on a combination of phenolic resin with polyvinyl formal or polyvinyl butyral resins. Because of their excellent shear and peel strength, vinyl-phenolic adhesives are one of the most successful structural adhesives for metal. Room-temperature shear strength as high as 5,000 psi is available. Maximum operating temperature, however, is only 93°C because the thermoplastic constituent softens at elevated temperatures. Chemical resistance and impact strength are excellent. Vinyl-phenolic adhesives are supplied in solvent solutions and as supported and unsupported film. The adhesive cures rapidly at elevated temperatures under pressure. They are generally used to bond metals, rubbers, and plastics to themselves or each other. A major application of vinyl-phenolic adhesive is the bonding of copper sheet to plastic laminate in printed-circuit-board manufacture.

Neoprene-phenolic alloys are used to bond a variety of substrates. Normal service temperature is from 20 to +93°C. Because of high resistance to creep and most service environments, neoprene-phenolic joints can withstand prolonged stress. Fatigue and impact strengths are also excellent. Shear strength, however, is lower than that of other modified phenolic adhesives. Temperatures over 150°C and pressure greater than 50 psi are needed for cure. Neoprene-phenolic adhesives are available as solvent solutions and film. During cure these adhesives are quite sensitive to surface contamination from atmospheric moisture and other processing variables.


 

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