Nitrile-Phenolic Adhesives

Nitrile-phenolic adhesives are one of the oldest synthetic structural adhesives being commercialized shortly after World War II. They have a reputation for toughness, heat and chemical resistance, and durability. Nitrile-phenolics are useful up to 120°C, and certain formulations are useful up to 150°C continuously. At temperatures above 175°C, strength properties falloff significantly. Unlike other high temperature resistant structural adhesives, nitrile-phenolics have moderately good peel strength. These properties have made them historically a very useful adhesive for applications such as bonding auto brake linings and other automotive and aircraft applications. They are appropriate for many contemporary uses, but often remain unrecognized because of more modern adhesive developments.

Nitrile-phenolic adhesives are made by blending a nitrile rubber with a phenolic resin. Typical formulations may contain up to 50% by weight nitrile rubber. 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. Other basic components in a nitrile-phenolic structural adhesive include: sulfur, zinc oxide, and accelerators. Nitrile-phenolic adhesives are generally considered to be low cost adhesives, especially given their unique set of properties.

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. These adhesives also have high impact strength. Because of their good peel strength (Table 1) and elevated-temperature properties, nitrile-phenolic adhesives are commonly used for bonding linings to brake shoes. They are also used in the aircraft industry for bonding of aluminum. Other applications include electronics, footwear, and products produced in the assembly of furniture.

Nitrile-phenolic adhesives have a good resistance to high humidity conditions and to exposure to various fuels and solvents. They are resistant to all normal fluids used in the automotive or aerospace industry including phosphate ester based hydraulic fluids.

Nitrile-phenolic adhesives are generally available as solvent solutions as well as supported and unsupported film. Because of their flexibility when cured, nitrile-phenolics are often used for laminating metal sheet stock that must then be cut or formed in secondary operations.

Like most adhesive systems, the nitrile-phenolics have some disadvantages which limit their use for certain applications. They require heat curing at 150°-250°C under pressure of up to 200 psi. The nitrile-phenolic systems with the highest curing temperature have the greatest resistance to elevated temperatures during service. Nitrile-phenolic adhesives also have inherently low flow characteristics. The low-flow characteristic does not usually present any problems in metal-to-metal bonding; however, cure pressure needs to be higher than that used for other adhesives.