Details
Material hierarchy: Polymer - Thermoset
Phenolics were among the earliest commercial synthetic materials and have been the workhorse of the thermoset molding compound family. In general, they provide low cost, good electrical properties, excellent heat resistance, and fair mechanical properties (they suffer from low impact strength), coupled with excellent moldability. They are generally limited in color (usually black or dark brown) and color stability. Many phenolic applications have been converted to high-temperature thermoplastics, which have advantages in processing efficiencies.
General-purpose grades are usually wood-flour, fabric, and/or fiber-filled. They provide a good all-around combination of moderately good mechanical, electrical, and physical properties at low cost. They are generally suitable for use at temperatures up to 300°F.
The materials are severely attacked by strong acids and alkalis, whereas the effects of dilute acids and alkalis and organic solvents vary with the reagents and with the resin formulation.
Impact grades vary with the type and the level of reinforcement. Phenolic molding compounds can be reinforced or filled at levels higher than 70%. In order of increasing impact strength, paper, chopped fabric or cord, and glass fibers are used. Glass-fiber grades also provide substantial improvement in strength and rigidity. Glass-containing grades can be combined with heat-resistant resin binders to provide a combination of impact and heat resistance. Their dimensional stability also is substantially improved by glass.
Electrical grades are generally mineral- or flock-filled materials designed for improved retention of electrical properties at high temperatures and high humidities.
Many specialty-grade phenolics are available. These include chemical-resistant grades in which the resin is formulated particularly for improved stability to certain chemicals; self-lubricating grades incorporating dry lubricants such as graphite or molybdenum disulfide; grades with improved resistance to moisture and detergents; rubber-modified phenolics for improved toughness, resilience, and resistance to repeated impact; and ultra-high-strength grades (glass content over 60% by weight), designed to provide mechanical strengths comparable to those of cloth-reinforced plastics laminates.
Phenolics are processed by thermoset injection molding, compression molding, and transfer molding, and have recently been formed by pultrusion and made into sheet molding compound. Their fire resistance and low smoke emission have found phenolics new applications in panels for buildings and transport facilities and in aircraft parts. For the same reasons, phenolic foams are gaining use as fire-resistant insulation materials.