Vinyl Esters Make Tough Parts For Highly Corrosive Applications

Source: C.P.F. DualamA dual-laminate dechlorination tower incorporates a thermoplastic liner and a fiberglass structure for very severe chemical applications. This installation in Venezuela replaces a metal and rubber-lined unit that required frequent maintenance.

The direct cost of metallic corrosion in the U.S. is staggeringly high - $276 billion per year, or about 3 percent of the country's gross domestic product (GDP), according to a 2002 report entitled Corrosion Costs and Preventive Strategies in the United States, issued by the U.S. Federal Highway Administration and written by CC Technologies Laboratories Inc. (Dublin, Ohio, U.S.A.), with support from NACE International (National Association of Corrosion Engineers). Every sector of the economy pays the price, including civil infrastructure (highways and bridges), pipelines, water and sewer systems, electrical utilities, ships and aircraft, industrial plants and military installations.

While there are many strategies for fighting the problem - better inspection methods, improved management practices and life-prediction computer models - one of the most obvious solutions is to replace corrosion-prone metal materials with composites. Fiber-reinforced polymer composites made with chemical-resistant resins are virtually corrosion-free and can provide decades of maintenance-free performance, as documented by countless case studies.

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In general, unsaturated resins, that is, resins with a higher number of double-bonded carbon atoms, can break down in the presence of polar solvents, strong acids and strong bases, because the carbon atoms offer attachment sites for the reacting molecules. More highly crosslinked, higher-weight saturated resins are more resistant because any reactive sites are nested tightly between benzene rings and are difficult to access - a phenomenon that resin manufacturers call "stearic hindrance."

While the majority of resins used for corrosion-resistant applications are polyesters, well suited for most chemical applications, vinyl esters are increasingly specified for aggressive chemical environments. The epoxide backbone chemistry, high degree of crosslinking and higher molecular weight of vinyl ester resins gives them epoxy-like performance, with the processing ease of polyesters. Bisphenol-A-based epoxy vinyl esters are generally selected for use in environments where composites will be exposed to inorganic acids, alkaline materials or salt, particularly where greater resin strength is needed to resist vibration loads. Novolac epoxy-based vinyl esters contain phenol or cresol and have even higher cross-linking density, providing outstanding thermal stability and mechanical properties for handling a wide range of chemicals, especially organic solvents.

Ashland Specialty Chemical Co.'s Composite Polymers business (Columbus, Ohio, U.S.A.) makes HETRON epoxy vinyl ester resins, a brand first commercialized in 1953 for pulp and paper bleaching applications. Ashland's new HETRON FR 992-Sb fire-resistant resin was recently selected for a 142.6m/468-ft high chimney stack liner, one of two liners planned for the Santee Cooper Power facility in Cross, S.C., U.S.A., says project engineer Al Newberry of FEMech Engineering (Harrison, Ark., U.S.A.). The resin met the project requirement of the American Society of Testing and Materials' (ASTM) E84 Class I flame spread rating of 25 or less. The resin's new Sb version has the antimony (for this project, antimony pentoxide) already mixed in, which relieves the fabricator of the responsibility for mixing the additive into the resin. In addition, HETRON's patented "F-Cat" technology makes the resin stable for long shelf life, eliminates foaming when catalysts are added and keeps exotherm low so that fabricators can work with thick laminates, if necessary, says Ashland's Ike Shank.