Metal Finishing Guide Book

2012-2013

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and other storage and process equipment. PVC is easily thermoformed, cut, drilled, tapped, machined, and welded, consequently, making it an excellent, versatile, and cost-effective material from which to fabricate corrosion-resistant equipment. Polypropylene has arrived. Its popularity is attributable to its excellent chemical resistance and surpasses PVC because of its superior physical and thermal resistance. It is available as flame and nonflame retardant homopolymer and copolymer sheet stock. Polypropylene utilizes fabricating techniques similar to those used for PVC. Small tanks for pickling and plating, large tanks for continuous strip pickling lines, and pickling line covers have been fabricated of polypropylene. These and similar fabrications are enjoying an enviable record of success in challenging chemical and physical applications. Linear polyethylene fabrications have performed well in the small-parts metalfinishing industry because of their low absorption, high chemical resistance, and outstanding resistance to impact. They do not possess the rigidity and flexural capabilities of PVC or PP and, consequently, the fabrications are significantly smaller. A new generation of PE is making significant inroads into the finishing industry. Crosslinkable, high-density PE for rotational molding is being used for tanks of 5-10,000-gal capacity. These new resins exhibit excellent physical properties and good resistance to weathering. Applications for the most part have been indoor and outdoor storage tanks and portable receiver tanks. Fiberglass-reinforced plastics have been successfully used for a multitude of applications for many years. The earliest fabrications utilized furan and phenolic resin binder systems. The more popular resin binder systems in use today are polyester, epoxy, and vinyl ester. The success of FRP structures is substantially predicated on the proper choice of resin and hardener system most inert to the environment to which the fabrication will be subjected. It is not enough to request an FRP tank, any more than it is to request a flake-glass-reinforced polyester tank. It is important to either provide the fabricator with all chemical, thermal, and physical information pertinent to the process in order that the proper resin and hardener system might be selected, or to have in-house technical competency capable of making the proper selection of the resin-hardener system from which the manufacturer can fabricate the desired equipment. There are numerous polyester resins available; however, for aggressive corrosion environments, such as high concentrations of alkalies and a broad range of acids, the bisphenol-A fumarate resin is the best recommendation. Vinyl esters are epoxy-resin-based, thermosetting resins that provide chemical resistance similar to that of bisphenol A polyesters. They are considered to be slightly better in high concentrations of alkaline hypochlorites than the bisphenol A polyester. Vinyl esters exhibit outstanding physical properties, i.e., tensile, flexural, and elongation that are normally associated with epoxies. The chemical resistance and physical properties of epoxy resins are functions of the resins, but equally important, they are very much functions of the curing systems employed. Aliphatic and aromatic amine curing systems impart better chemical resistance to epoxy resins than do polyamide curing agents. Polyamides, however, impart better impact resistance to epoxies than do aliphatic or aromatic curing systems. The intention of these three examples of the resin systems utilized for constructing fiberglass-reinforced tanks and tank linings is to point out the neces798

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