Phenolic resins are the condensation products of phenol and formaldehyde and are an important class of adhesives. They are relatively inexpensive and are manufactured as liquid compositions and films. Thermosetting phenolics withstand high temperatures both under mechanical load and in severe environments with minimal deformation and creep. In other words, cured phenolic resins maintain structural integrity and dimensional stability even under severe conditions.
Phenolic resins can be divided into two different types:
1) Novolacs - These resins are not reactive and need a cross-linking agent to fully polymerize.
2) Resoles - These resins are self-curing due to the presence of reactive side groups.
Novaolacs are prepared by acid-catalyzed addition polymerization of phenol with formaldehydeand and resoles by base-catalyzed addition. A brief thermal exposure of these resins completes the cross-linking (thermosetting) and results in the final properties.
The primary use of phenolic resins is as a bonding agent. Phenolic resins readily penetrate and adhere to many organic and inorganic fillers and reinforcements, and when cross-linked throughout the fillers and reinforcements, provide excellent mechanical, thermal, and chemically resistant properties. Their exceptional compatibility with cellulose fillers makes them the ideal binder for particleboard, plywood, hardboard, and oriented strand board (OSB). However, they are more expensive than UF resins, and therefore, are only used when superior heat and environmental stability is required like in outdoor and more demanding indoor applications. For example they are used for durable laminates and for decking applications. Liquid phenolic resins also penetrate and saturate paper and other substrates to provide good mechanical strength, electrical properties.
Many other products benefit from the hardness, good dimensional stability and heat and chemical resistance of phenolic resins. Applications include abrasive grinding wheels, sandpaper, friction or brake linings, clutch and transmission papers, metal bases of light bulbs, electrical laminates, refractory products, and other molded parts used in high temperature or aggressive media. Most of these products are solvent solutions and blends of phenolic resins filled with clay and other fillers. The bond is rather brittle, however, and tends to break under impact or vibration. For this reason, phenolic resins are often modified by the addition of elastomeric resins to improve toughness and peel strength. The modified phenolic adhesives are well suited for structural bonding of metals and many other substrates (composite structures) and find many applications in the aerospace and automotive industry.
A major draw back of phenolic structural adhesives is that the cure process requires both heat and pressure. During the cure water is released from the adhesive which would be present as vapor due to the high cure temperature of > 100°C / 212°F. To avoid foaming, phenolic resins have to be cured under pressure.
The most common type of modified phenolic adhesives are nitrile-phenolic, vinyl-phenolic, and neoprene-phenolic adhesives. They are supplied in solvent solutions and as supported and unsupported films. The solvent has to evaporate prior cure.
When modified with nitrile rubber, the adhesives can withstand temperatures up to 260°C (500°F). Typical formulations may contain up to 50 percent by weight nitrile rubber. These adhesives have improved peel and impact strength and good fatigue properties. They are ideal for metal-to-metal bonding in high temperature applications. Like epoxies, nitrile-phenolics have excellent chemical and heat resistance and typically outperform epoxies at high temperatures. For example, they have greater strength retention at temperatures above 200°C (400°F). However, they require heat-pressure curing at temperature between 150 and 260°C (300 - 500°F) and pressures of up to 14 bar (200 psi). The nitrilephenolic structural adhesives with the highest curing temperature have also the greatest resistance to elevated temperatures. The typical service temperature is between -55 to 260°C (-65 to 500°F).
Another important class of phenolic structural adhesives are vinyl-phenolic adhesives. They are blends of phenolic resin and polyvinyl formal or polyvinyl butyral resins. Because of their high shear and peel strength, vinylphenolic adhesives find many applications as structural adhesives for demanding metal bonding applications in the aerospace industry. Room temperature shear strength can be more than 35 MPa (>5,000 psi). However, the maximum operating temperature is only about 90°C (200°F). Like nitrile-phenolics, these adhesives are cured at elevated temperatures between 170 and 230°C (325-450°F) under constant pressure around 3.5 bar (50 psi). They are generally used to bond metals, rubbers, composites and plastics to themselves.
Neoprene-phenolic adhesives are often used to bond different rubbers and metal substrates. Typical service temperatures are from -20 to 95°C (-70 to 200°F). They have high creep and fatigue resistance and can withstand prolonged stress. They also have excellent impact strengths. However, the shear strength is lower than that of the other modified phenolic adhesives but is still high. Shear strength values in the order of 12 MPa (1750 PSI) for steel bonds and peel values of about 10kN/m for rubber-to-rubber (SBS) bonds have been reported. Temperatures over 150°C (300°F) and pressure greater than 3.5 bar (50 psi) are usually needed to fully cure these adhesives.