The ingenious (and insidious) valve stem gasket Stem Gasket Hardness Testing No polymer has 100% elasticity or 100% plasticity. About 80–90% of these nirvanas is the best that can be obtained. Stem gasket elasticity increases with rising temperature, while low temperatures increase stiffness or hardness. Useful polymers must display acceptable elasticity across temperatures from about -20°C to 60°C. (-4°F to 140°F). If the gasket becomes stiff or brittle at low temperatures, it will not spring back fully after the aerosol is used, resulting in a leaking product. Prolonged exposure to certain solvents may cause leaching and other gasket problems, resulting in increased plasticity or reduced resilience. Various devices have been developed to detect any increase in hardness upon exposure to aerosol formulations. Hardness, usually called “Durometer” in this case, can be measured by the depth to which an indenter penetrates the stem gasket when it is driven into the surface at a constant low velocity and force. The basic technique is described in Standard Test Method ASTM D2240, but improvements have been made in the apparatus since then. The Shore Digital Microtester, Model No. 717, has a suitable flat surface accessory. The so-called “A” Scale is commonly used; the instrument can be used with elastomers as thin as 0.030" (0.77mm). Ideally, penetrations should be taken at three locations, each well away from the edge of the sample. The sample should be equilibrated to about 73°F (23°C) before testing. Typical Shore “A” Durometers are shown as follows: Stem Gasket Compositions: Neoprene Neoprene (or polychloroprene) gaskets are produced by polymerization of 2-chloro-1,3-butadiene. Because of the presence of a chlorine atom in the monomer, the density is unusually high—typically about 1.23g/mL at 70°F (21°C). Although it swells remarkably in aliphatic and aromatic solvents it is relatively unaffected by acetone, ethanol, mineral oils, vegetable oils and only moderately by chlorinated solvents. It became popular about 1953 when large amounts of methylene chloride were added to insecticide and air freshener aerosol formulas. The selection of additives can have an important effect upon both swelling and hardness (Durometer) properties. It is somewhat more susceptible to long term hardening than most other gaskets. Its popularity has waned with the systematic regulatory restrictions and banning of all the chlorinated solvents, due to their suspected carcinogenicity. Probably second in gasket elastomer popularity, it finds its greatest application in water-based aerosol systems. As with buna and the other valve stem gaskets, there are a number of neoprene compositions. They typically swell a mere 1% in HFO-1234ze and only 2% in iso-butane/propane mixtures. This is less than the usual 3% minimum recommendation. Durometers are reduced by about 3–6 points. The relatively loose (non-interference) fit around the valve stem can cause “blow-by” leakage in some dispensers. The problem is corrected by including swelling-type solvents in the formulas. Neoprene gaskets can be differentiated from other common gaskets by their ability to sink in water, or better, their ability, once ignited with a match, to sputter and extinguish the flame. Stem Gasket Compositions: Butyl The “butyls” (actually “iso-butyls”) are created by the polymerization of iso-butylene and iso-pentadiene, causing them to have a paraffinic structure blistered with methyl groups. Additives include fillers, plasticizers and sulfurbased vulcanizing agents. As expected, they are swelled and softened by other hydrocarbons. A 50:50 mixture of iso-butane and propane swells them intolerably by about 14%, while mixed hexanes swell them by about 36%. On 40 Spray December 2016 TYPICAL SHORE “A” HARDNESS FOR ELASTOMERS Elastomer Hardness Buna N 71–73 Neoprene 73 Butyl 70–72 Chlorobutyl 72 Viton A 69 Standard EPDM 72 Butyl (In ethanol, for 8 weeks (23°C) +2 Butyl (In DME, for 8 weeks (23°C) -11 Butyl (in HFO1234ze, for 8 weeks (23°C)-1 -1 Butyl (In Hexane, for 8 weeks (23°C) -26 For Butyl 21B5. Other Butyls are sometimes different.
Spray Decemberr 2016
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