fenolhars

Fenolhars, or phenol-formaldehyde resins, are a family of synthetic thermosetting polymers produced by condensations of phenol or substituted phenols with formaldehyde. They form three-dimensional crosslinked networks upon curing, which gives high heat resistance, chemical resistance, and dimensional stability. They are among the earliest commercially produced plastics and have been used in a wide range of applications since their development in the early 20th century.

There are two main subtypes: novolac and resol resins. Novolacs are phenol-formaldehyde resins with limited formaldehyde and require an external curing agent, such as methylene donors, to crosslink during heating. Resols contain excess formaldehyde and can cure thermally without additional agents, forming highly crosslinked networks. The final properties depend on the phenol source, formaldehyde-to-phenol ratio, catalysts, and curing conditions.

History and notable examples: The development of phenolic resins culminated in Bakelite, introduced by Leo Baekeland in 1909, which became a widely used early plastic for electrical insulators, coatings, and molded components. Phenolic resins remain common in electrical laminates and high-temperature coatings, and are used in certain paper-phenolic laminates (FR-2) and is reinforced in some composite materials.

Properties and limitations: Phenol-formaldehyde resins offer excellent thermal stability, flame retardance, and rigidity but can be brittle and difficult to process due to their thermosetting nature. Recycling is limited, and formaldehyde emissions and handling hazards require appropriate controls.