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POLYPROCESSING Chemical Resistance Providinq Solutionsihrough Innovallon Chemical Resistance Chemical "resistance" and "compatibility" are synonymous terms used in relation to the ability of a plastic to function in different environments. In regards to polyethylene chemical storage tanks, chemical resistance encompasses the total effect a product would have on a tank. The factors that make up the overall compatibility of a chemical to a rotomolded tank are (1) chemical attack, (2) absorption or permeation, and (3) solubility and stress crack resistance. Chemical Attack By definition, chemical attack involves an actual chemical reaction with the plastic. This can be a breaking of molecular chains and/or an addition of chemical groups to the molecule. For example, in the case of an oxidation reaction with polyethylene, both occur with the addition of carbonyl groups. This causes and eventual loss of properties to the point that a tank would not be serviceable. Polyethylene in general is one of the most inert plastics available. Very few chemicals react with polyethylene and with those that do, the rate is relatively slow. The ultra high molecular weight characteristics of high density crosslinked polyethylene resins after crosslinking makes these particular polyethylenes even more resistant than other grades. Permeation This involves the physical absorption of the chemical into the polyethylene. If this is a volatile chemical, then an actual loss of the product can occur as the chemical vaporizes from the outer wall of the tank. The amount of absorption is generally limited to 3 to 7 percent by weight of the polyethylene. Also, the loss of volatile products is relatively small. For example, a 25-gallon tank with a 50-mil wall will only lose between 5 and 6 grams of gasoline per day due to permeation. The thicker the wall, the lower the rate of loss. The absorption of a product into the wall of a tank will cause more property changes. The tensile strength is reduced approximately 15 to 20 percent and stiffness approximately 20 percent. Normally, this does not affect the utility of a tank or prohibit the application. The property losses due to permeation are offset by increasing the design wall thickness of the tank. Elevated Temperature The effects of elevated temperature (100° F or greater) on polyethylene tank are predictable and expected. Polyethylene, which is a flexible material, becomes even more flexible when heated. It will, therefore, bulge more at an elevated temperature than at room temperature. By looking at the design hoop stress values for various temperatures, one can see the effects of increased temperature services. The values remain relatively constant up to 100° F, after which they begin to decrease. 100° F........................................ 600 psi 130° F........................................ 450 psi 110° F........................................ 550 psi 140° F........................................ 400 psi 120° F........................................ 500 psi 150° F........................................ 300 psi A rise in temperature of 50° F (100° F to 150° F) reduces the design hoop stress value from 600 psi to 300 psi and doubles the required wall thickness. It follows that simply increasing the service rating of a tank from 1.35 S.G. to 1.65 S.G., or even to 1.90 S.G. is not necessarily sufficient. The proper wall thickness must be calculated for the temperature of service. The maximum temperature rating for crosslinked polyethylene material is 150° F. Above that temperature the thermal stabilizers in the plastic are more rapidly consumed. Continued use will cause embrittlement and a reduction in the useful life of the part. Temperature services from 100° F to 150° F are acceptable applications, however, thicker tank walls are required to maintain a safe design. Consult the factory when application such as these arise. Authorized Distributor Toll-Free 1-866-590-6845 www.polyprocessina.com