Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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of approximately 150O F (66O C). Type I PVC has outstanding structural integrity attributable to its high ten- sile, compressive, and flexural properties. It is one of the easiest plastics from which to construct tanks, tank liners, dipping baskets, 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 mate- rial from which to fabricate corrosion-resistant equipment. Polypropylene has arrived. Its popularity is attributable to its excellent chemi- cal resistance and surpasses PVC because of its superior physical and thermal resis- tance. It is available as flame and nonflame retardant homopolymer and copoly- mer 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 metal- finishing 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 prop- erties 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 appli- cations for many years. The earliest fabrications utilized furan and phenolic resin binder systems. The more popular resin binder systems in use today are poly- ester, 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 corro- sion 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 bisphe- nol 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 630

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