Phenolic resins, or resins formed by an acid- or base-catalyzed condensation reaction of formaldehyde and phenol, were the first commercially available synthetic plastic materials. During the early part of the 20th century nearly every home was stocked with a variety of everyday household items, from combs to bowls to radios, made from Bakelite, a mixture of phenolic resin and wood flour. The fully or nearly-fully saturated crosslinking of every phenol group via methylene bridges yields a material with exceptional hardness, good thermal stability and superior fire, smoke and smoke-toxicity properties.
The superior physical properties of phenolic-based composites in industrial and architectural applications requiring corrosion and/or heat resistance have long been recognized and touted. How superior? According to one veteran industry consultant, some engineers believe that phenolic-coated steel beams, had they been used, might have prevented the collapse of the World Trade Center twin towers after the 9/11 terrorist attacks. The perceived downside to phenolics, however, has been that the resins have a relatively short shelf life and are more difficult to process compared to other thermosets such as polyesters, epoxies and vinyl ester.
Over the past few decades, however, several newer, easier-to-process grades of phenolics have gradually gained market share in applications requiring stringent corrosion resistance and/or superior flame and smoke retardant properties. One commercial line, Cellobond Phenolics, was originally developed by BP Chemicals and introduced to North America in 1990. The product line is now owned by Momentive Specialty Chemicals (Columbus, OH) and distributed in North America by Mektech Composites, Inc. (Hillsdale, NJ). Aram Mekjian, president of Mektech, reports earlier generation phenolic-based composites could only be processed in one of two ways: as a prepreg in an autoclave at temperatures at or above 300°F/149°C; or injection molded in pellet form. Both the autoclave and injection molding tooling added cost; as well, the mechanical properties of parts made from pellets were limited because of the short length of the fiber. Phenolic resins developed by BP Chemicals were modified to have a lower molecular weight distribution that allows them to be processed much like vinyl esters and polyesters, i.e. via hand layup, vacuum infusion, filament winding, compression molding and pultrusion. The Cellobond phenolics do, however, require a two-step cure: 140°F to 160°F (60°C to 71°C) for 20 to 30 minutes, followed by a demolded post-cure of 140°F to 180°F (60°C to 82°C) for 2 to 3 hours.
Mektech sells a variety of Cellobond grades targeted for different processes. One of the most commonly used, Cellobond J2027L, has a viscosity of 350 cps, water content of 11.5 percent and a specific gravity of 1.225. It is used for hand lay-up, filament winding, RTM and vacuum infusion. A processor can tailor the resin gel time for a particular process by choosing one of three acid catalyst systems. Phencat 15, very fast, is used for spray-up; Phencat 10, with a pot life of about 30 minutes, is applicable for hand layup; and Phencat 382, with a pot life of 4 hours, is used for filament winding, RTM and vacuum infusion. The shelf life of neat Cellobond phenolic resin at room temperature (70°F/21°C) is about one month. The shelf life can be extended to 5 to 6 months by storing the resin at 40°F/4.4°C.
FiberSystems Inc. (FSI, Dayton, OH) fabricated all the large diameter piping used in the National 911 Memorial Fountains at New York City’s World Trade Center. The memorial is designed with two fountains that have the approximate footprint of the original twin towers. The fountains hold about 600,000 gal/2.27 million liters of water; the water for each fountain is pumped and re-circulated at a rate of about 30,000 gal/min (113,562 liters/min). As the water is circulated it is also purified via chemical treatment and filtration.
FSI fabricated approximately 4,300 lineal ft (1,311m) of 12-inch and 16-inch (305-mm and 406-mm) diameter pipe via wet-filament winding using a “dual wound” construction comprising an inner layer (or liner) of roughly 30 percent glass-reinforced vinyl ester and an outer layer of about 60 percent glass-reinforced phenolic. Wall thickness throughout the entire piping system ranges from 0.230 inch (5.8 mm) to 0.290 inch (7.3 mm). The high-performance vinyl ester, supplied by Interplastic Corp. (St. Paul, MN) provides good hydrolysis and corrosion resistance, as well as structural support, and the outer phenolic layer, made from Cellobond J2027L, contributes structural support and meets New York City’s stringent fire and smoke toxicity codes. The inner and outer layers are reinforced with Owens Corning’s boron-free Advantex E-CR Glass.
David Dean, vice president, new business development at FSI, says one of the reasons project engineers specified the composite piping for larger diameter sections is that it is about one-third the weight of stainless steel pipe, and thus easier to install. Filament winding also enabled IFS to manufacture the pipe in a net-like shape with all the flanges and fittings. “The space under the memorial is quite tight,” Dean says. “The contractors gave us a drawing of the piping network and we literally created the sections so that all the installers would have to do is line up the flanges and bolt them together,” Dean says.
Georgia-Pacific Chemcials LLC (Atlanta, Ga.) manufactures phenolic resins for industrial applications under a number of trade names. These include GP resins, LEAF FREE formaldehyde-free resins, RESI-SHELL and RESI-FLAKE foundry resins, RESI-MAT resins for glass mat and RESI-LAM laminating resins. Chris Lee, sales manager, industrial resins, says the GP portfolio includes acid- and base-catalyzed products. Pot life of the phenolic resins is typically in the range of one hour, but can be extended or shortened by a selection from GP’s line of catalysts. Lee reports that the grade-specific resins can be processed by the entire range of options available to fabricators, including resin transfer molding (RTM), vacuum-assisted RTM (VARTM), hand layup, filament winding, pultrusion and prepreg.
Phenolics move into mass transit
Not only is fiber-reinforced phenolic lighter than steel, it is 20 to 30 percent lighter than flame- and smoke-retardant polyester and vinyl ester, which are typically filled with aluminum tri-hydrate. “Phenolics have inherently better flame and smoke toxicity properties even without the additives, which add weight,” says Mekjian. In the U.S. the flame spread requirement for mass transit trains, as specified by ASTM E-162, is 35 or less. Whereas FRP manufactured with fire-retardant grades of vinyl ester and polyester meets the requirement with a flame spread in the range of 25, flame spread for painted Cellobond FRP is 0.85, roughly 30 times less.
Yet it is the low smoke density of phenolics, which impart an extra, significant margin of safety compared to other resins. “Smoke is usually more fatal than flames,” says T.R. Morton, CEO of FSI. In the U.S. the smoke density requirement, as specified by ASTM E-662, is 100 or less. While smoke density values for polyester and vinyl ester typically lie in the range of 70 to 80, independent tests on painted Cellobond FRP yield a smoke density value of about 0.6.
Unlike phenolics, polyester and vinyl ester FRPs do not require heat to process, thus parts made from the resins are slightly cheaper to purchase. Although underperforming in flame and smoke tests compared to phenolics, polyester and vinyl ester still meet the letter of the U.S. requirements. Consequently, phenolic FRP has seen much wider use in mass transit applications in Europe, which has more stringent fire and smoke requirements.
For example, much of the interior and exterior components of the locomotives and passenger train cars servicing the 31.4-mile/50.5-km undersea rail tunnel (Chunnel) between England and France are manufactured by hand layup from Cellobond J2027L (supplied by BP) with the Phencat 10 catalyst. More recently, however, phenolic FRP is increasingly being used in North America and elsewhere to manufacture flooring in trains and buses as a replacement for traditional plywood-metal (ply-metal) constructions, Mektech Composites’ Mekjian reports. Typical FRP flooring construction in these applications is a sandwich panel comprising outer layers of 45- to 65-percent glass-filled Cellobond J2027L with either a balsa wood or isocyanate foam core. The panels are manufactured via VARTM or RTM. The flooring meets all flame- and smoke-requirements, but the other critical selling points are a 50 percent weight reduction compared to ply-metal, as well as greater water- and corrosion-resistance and enhanced durability.
Other promising applications
Apache Pipe & Plastic fabricates a variety of custom piping, vessels, ducting and inserts, much of it from phenolics, for the petro-chemical industry. Apache’s operations manager, Thomas Oldham, reports that while piping and inserts for reaction vessels comprise the bulk of the company’s business, customers continue to find new applications for phenolic FRP in these highly corrosive manufacturing environments. For example a few years ago a petro-chemical company had ordered a generator transition component built from carbon fiber. The component, which is about 5 ft wide by 3 ft high (1.5m by 0.9m), directs heat from a flame arrester to a reaction vessel used for making vinyl with an operating temperature as high as 500°F/260°C. When the carbon part arrived at the plant, however, maintenance workers discovered it had shattered during shipping. The company requested Apache to build a replacement from phenolic FRP. Apache hand-laid the part using Cellobond J2027L phenolic resin, Phencat 10 catalyst and 30 percent chopped-strand EC-R glass from Owens-Corning. Since then, the company has ordered a second generator transition component from Apache.
While declining to be specific, Georgia-Pacific’s Lee likewise reports good growth for the company’s phenolic resin in mass transit, marine, aerospace and offshore drilling. Over the past 20 to 30 years phenolics have made significant headway as a replacement for metal in grating on offshore oil and gas platforms. Phenolic FRP is lighter and more corrosion-resistant than metal, but also has the edge over fire-retardant vinyl ester for this structural application, which must meet stringent fire-performance standards established by the U.S. Coast Guard: At 130°C/266°F, vinyl ester FRP loses about 50 percent of its flexural modulus relative to room temperature; however, phenolic FRP retains more than 90 percent of its room-temperature flexural modulus at temperatures as high as 200°C/392°F.
Mekjian reports one application in which filament-wound pressure pipe made from Cellbond J2027L used on offshore platforms for water sprinklers had to pass the Jet Fire Test. This involves exposing the pipes, joints and fittings to burning jet fuel (2000°F/1,093°C flame temperature) for 20 minutes, after which the pipes are still required to hold water (stationary) at high pressure with minimal leakage.
Despite such success stories in offshore applications, however, it appears that some North American manufacturers of FRP are witnessing a strong downturn in the same arena. “We had a $3 million dollar order that was put on hold and never renewed,” says Morton, citing restrictive U.S. government policy toward oil and gas exploration as the cause. “We haven’t built an off-shore platform in years — all that work is going to Brazil.” Morton notes the company has also seen a significant decrease in duct and piping fabrication for coal-fired plants as new government regulations are forcing many of the plants out of business.
Morton says more extensive corrosion test data, as well as better marketing, is needed to grow the sales of phenolics. He reports that chemical companies selling resins that compete with phenolics have invested time and money to develop complete case histories documenting the corrosion resistance of their resins to hosts of corrosive agents at a wide range of temperatures. “Electronics and chip manufacturing is potentially a huge market for phenolics, but till this day there is no coordinated marketing approach, nor is there corrosion data on phenolics to the extent there is for other resins,” Morton says.
It appears that the best strategy for marketing and growing phenolic FRP applications would be to sell end users on the material’s clear, long-term benefits. These are, according to Morton, unparalleled corrosion resistance and low smoke toxicity. If the industry can harness the resources to produce the necessary test data, phenolics may be able to compete in a nearly unlimited way for new markets and fresh applications.