What Is It?
Bakelite, commercialized in 1909, triggered a revolution in product design. It was stiff, fairly strong, could, to a muted degree, be colored, and – above all – was easy to mold. Products that, earlier, were hand-crafted from woods, metals, or exotics such as ivory, could now be molded quickly and cheaply. At one time the production of phenolics exceeded that of PE, PS, and PVC combined. Now, although the ratio has changed, phenolics still have a unique role. They are stiff, chemically stable, have good electrical properties, are fire-resistant and easy to mold – and they are cheap.
Phenolic resins are hard, tolerate heat, and resist most chemicals except the strong alkalis. Phenolic laminates with paper have excellent electrical and mechanical properties and are cheap; filled with cotton the mechanical strength is increased and a machined surface is finer; filled with glass the mechanical strength increases again and there is improved chemical resistance.
Fillers play three roles:
- extenders (such as wood flour and mica) are inexpensive and reduce cost;
- functional fillers add stiffness, impact resistance and limit shrinkage;
- reinforcements (such as glass, graphite and polymer fibers) increase strength, but cost increases too.
Unfilled phenolics are susceptible to shrinkage when exposed to heat over time, glass-filled resins are less susceptible. They have good creep resistance, and they self-extinguish in a fire.
Phenolics can be cast (household light and switch fittings) and are available as rod and sheet. Impregnated into paper (Nomex) and cloth (Tufnol), they have exceptional durability, chemical resistance and bearing properties. Phenolics accept paint,
Switchboards, insulating washers, intricate punched parts (phenolic with paper laminate), gears, pinions, bearings, bushings (phenolic with cotton laminate), gaskets and seals (phenolic with glass), used to bond friction materials for automotive brake linings, beads, knife handles, paperweights, billiard balls, domestic plugs and switches, telephones, fuse box covers, distributor heads, saucepan handles and knobs, golf ball heads for typewriters, toilet seats. As a foam, phenolic resin is used in paneling for building work; its fine resistance is a particular attraction.
Phenolics, like all thermosets, cannot be recycled.
Phenolic resins are formed by a condensation, generating water in the process, involving a reaction between phenol and formaldehyde to form the A-stage resin. Fillers, colorants, lubricants, and chemicals to cause cross-linking are added to form the B-stage resin. This resin is then fused under heat and pressure converting to the final product – a c-stage resin – or completely cross-linked polymer.