Composites: Past, Present Future: Phenolics Revisited

Phenolics were the first commercialized thermosetting resins made from synthetic components. The earliest phenolic resin — polyoxybenzylmethylenglycolanhydride, formed from phenol (carbolic acid) and formaldehyde — is credited to Dr. Leo Baekeland, a Belgian immigrant to the U.S., although German chemist Adolf von Baeyer had experimented with phenolic/formaldehyde reactions as early as 1872, and history might have been quite different had he followed through with his work. Baekeland patented his material in 1907, one day before British electrical engineer James Swinburne intended to announce an identical discovery. In 1916, Baekeland and Swinburne met and became business partners. As a result of this association, phenolic resin, often combined with a wood flour filler, found use under the tradename Bakelite in a wide variety of products. Based on its heat resistance and electrical nonconductivity, the compound was used widely in radios, telephones and electrical insulators.

The first fiber-reinforced phenolic compo-sites were prepregs and molding compounds. The prepregs were (and still are) expensive and require autoclave cure, finding use primarily in aircraft (as early as the 1930s) and space applications, where the cost was acceptable. (It was used in the Space Shuttle from that program’s beginnings.) Molding compounds found use in some automotive applications, but made their biggest splash in the electrical industry. Early circuit boards (circa. 1940) were made from phenolic/paper and phenolic/glass fiber laminates.

In the 1980s, newer, more user-friendly phenolic formulations — developed in part to meet the more stringent British Standard mass-transit fire code (BS 6853) put in place after the tragic King’s Cross fire in the London Underground — produced composites at much lower cost and opened the door to applications where the old prepregs and compounds weren’t practical. With their high HDT (heat deflection temperature), low viscosity, and great flame/smoke/toxicity (FST) properties, phenolics soon were reducing weight and meeting FST standards in mass transit car interiors as well as architectural and marine components pressure-piping systems and many other applications. While filled polyester, vinyl ester and acrylic resins can meet the less-stringent specs in the U.S., processors who use them pay great weight and strength penalties.

Despite cost and performance advantages, there are lingering concerns about some of the “downsides” of phenolics:

Temperature-controlled storage. If un-cured phenolics are subject to long-term storage, temperature must be maintained at ±44°F/±6.67°C.

Color and appearance. Unlike polyesters and vinyl esters, phenolics are not optically clear. While phenolics can be pigmented, the resin’s natural brown color limits hues to the brown/black range. For that reason, parts typically require post mold finishing — surface prep and priming/painting.

Phenolic resin requires heat for cure. This makes them more expensive to process than resin systems that can be ambiently cured — a deterrent to some processors.

Outgassing. Large amounts of H₂O are produced in the phenolic reaction. As the process temperature approaches 200°F/93°C, the water vaporizes, causing micro-voids at the part surface. In my work, however, I’ve found that this can be minimized by adding surfactants to the resin formulation and then keeping the mold temperature below 180°F/82°C. Outgassing seems to be less problematic in closed mold infusion processes, because the vacuum systems tend to extract the outgassed moisture.

Moldmaking materials are limited. The acid catalysts used in today’s formulations tend to attack molds of aluminum, plain steel and some other metals. Processors of phenolic composites can select from 316 stainless steel, chrome- or nickel-plated steel or composite tooling made with either vinyl ester or epoxy resins.

Worker safety issues. Phenolics fell out of favor when the dangers of the free formaldehyde given off from the reaction of the phenol and aldehyde became apparent and had to be taken more seriously in the workplace. Phenolics processors installed ventilation equipment, which worked for a time to enhance employee safety. But the fact that these systems swept formaldehyde from the workplace and into the environment outside the factory became problematic. As a result, Hazmat and regulatory agencies set forth daily time-averaged limits for workers in such shops, making fume-control compliance even more difficult and air-movement equipment more costly. As processors have adopted resin transfer molding and other closed-mold infusion processes, fumes are easier to confine, but are still present in the workplace.

None of these challenges is insurmountable, of course. Tight control of cure temperature, for example, can be maintained with integrally heated tooling, which today is available not only in stainless and plated steel, but also in acid catalyst-resistant epoxy composite construction. Further, a new method exists for dealing with the formaldehyde outgassing issue that involves no expensive ventilation equipment. This method involves a passive-reactor (in which I have no financial stake, just to be clear) that, when placed in the effluent-stream within infusion processes, changes the CH₂O to CO₂ and water, thus eliminating the formaldehyde from the workplace. The passive-reactor is easily constructed from mostly generic components. Moreover, a single passive-reactor system can be connected to multiple mold stations on a production floor.

Challenges aside, phenolics should enjoy a promising future in the composites industry. Their price today runs about half that of vinyl esters. Further, their superior fire-safety and mechanical properties still make them the material of choice in applications where fire/smoke/toxicity levels are tightly controlled. Further, as gasoline prices move well past $3/gal, mass transit applications should multiply. And we can expect to see phenolics move into previously untapped applications. A case in point — and a particularly notable milestone for phenolic resins — was the completion, in 2004, of the first-ever phenolic composite helipad, at Cooper University Hospital in Camden, N.J. (see “Related Content,” at left). Previously, most helipads had been made of steel or aluminum. The 3,140 ft²/292m² pad was fabricated via vacuum-assisted resin transfer molding (VARTM). Pads were reinforced with E-glass fabrics, and phenolic was selected not only for its fire rating but also because the resulting lightweight composite panels for the pad deck kept helipad weight low enough that contractors did not have to reinforce the hospital roof beneath the structure.