High-temperature-resistant adhesives modified phenolics

Structural adhesives are formulated from epoxy resins, phenolic resins, acrylic monomers and resins, high temperature-resistant resins (e.g., polyimidcs), and urethanes. Structural adhesive resins arc often modified by elastomers. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

The normal service temperatures of these adhesives range from -57°C to 93 °C. Because of their high resistance to creep and most severe environments, neoprene-phenolic joints can withstand prolonged stress. Fatigue and impact strengths are excellent. However, shear strength is lower than that of other modified phenolic adhesives. 

Of the common modified phenolic adhesives, the nitrile-phenolic blend has the best resistance to elevated temperatures. Nitrile-phenolics have high shear strengths up to 121-177 C, and their strength retention on aging at these temperatures is very good. These materials are available in solvent solutions and unsupported and supported films. 

Most crosslinked thermosetting adhesives such as epoxies, phenolics, polyurethanes, and modified acrylics are highly resistant to many chemicals, at least at temperatures below their glass transition temperature. 

Resorcinol adhesives are resistant to boiling water, oil, many solvents, and mold growth. Their service temperature ranges from -184 to +177°C. Because of high cost, resorcinol-formaldehyde resins are often modified by the addition of phenolic resins to form phenol resorcinol-formaldehyde. 

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