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Spray Decemberr 2016

The ingenious (and insidious) valve stem gasket During product development, formulators are faced with up to three parameters that require long-term testing. They are weight loss, corrosion (if water is present) and gradual decomposition of one or more ingredients. Until about 1951, an aerosol product that showed no leakage bubbles during hot waterbathing was presumed to have a zero weight loss over long time periods. The fallacy was dispelled when methylene chloride began to be used as a solvent for certain insecticides. We now know that all aerosols suffer weight loss to at least some degree, ranging from about 0.03 grams per year to the point where the dispenser becomes completely depressurized. There are up to three sources of leakage. By far the least important is double seam leakage from two and threepiece tinplate cans containing such aggressive solvents as N,N-diethyl-m. toluamide (DEET) and 100% methyl bromide fumigant. Then there can be significant leakage past the cup gasket, especially if faulty crimp dimensions are used, or if the aluminum can curl has deep radial grooves requiring cut gaskets and considerable download pressure for an hermetic seal. Lastly, there can be gas leakage through the interface between the valve stem and the stem gasket for a variety of reasons. This can be insidious in that it may take several days to develop and by that time, defective cans may have been shipped to distant warehouses or to retailers. Except for varying degrees of weight loss, there will be no evidence that the units are defective until the consumer tries to spray the contents. Over the years, gasket and valve-makers have made great progress toward minimizing the valve stem leakage problem. Better gasket compositions, curing techniques and final treatments have been developed. More precise valve stem and assembly specifications are now in use and advanced quality control programs are rigorously utilized. Formulators are increasingly aware of gasket swell and Durometer and other tests are available to effectively match gaskets to their products. Still, to some degree, the problem of stem gasket leakage continues, most often as the result of a lack of complete compatibility testing by formulators. Stem Gasket Compositions: Buna We often think (erroneously) that there are just a limited number of stem gaskets to choose from. Names such as buna, neoprene, butyl, Viton and so forth come to mind, but these are actually just the major categories. Within each one there are a virtually unlimited number of sub-categories, not only in chemical composition but in average molecular weight, the degree of cross-linking, lubricant impregnation and so forth. As might be expected, only a small number of these gasket combinations fit our compatibility requirements to the extent that they have become commercial as aerosol valve stem gasket candidates. The buna compositions—also called GR-S, Buna N and Buna P—are by far the most popular. At least a dozen are in active use by valve-making firms and they all display somewhat different properties. The term “buna,” which dates back to at least 1945, is a composite of “BUtadiene and NAtrium.” Chemically, it is a polymer of 1,3-butadiene and acrylonitrile, although the acrylonitrile may be partly or totally replaced by styrene and other chemicals in some formulas. In Europe, the term “nitrile rubber” refers to gasket polymers that contain acrylonitrile. In addition to the primary ingredients, all buna gaskets contain a mélange of modifying substances. They include fillers, softeners, resins, cross-linking polymerization accelerants, curing agents, lampblack and more. The resulting synthetic thermoplastic rubbers will have a number of important physical attributes, ranging from a density of about 0.94 g/mL, a Brittle Point of about -60°C and an Elongation of 200 to 500% (typically 380%). Other parameters include Tensile Resistance (100% Modulus = 500 to 900 psi-g.), Compressive Deformation (50 to 90%) and sometimes resistance to a leaching test to ensure that the carbon black will not discolor aerosol formulations. The identity and concentration of these various additives are considered to constitute confidential business information. The concentrations of butadiene and acrylonitrile must be balanced for optimum results. If the acrylonitrile is increased the elasticity, solvent resistance and low temperature flexibility will be reduced. In contrast, its oil resistance, thermoplasticity, polar solvent compatibility and strength moduli will all increase. Similar assessments can be made for the various additives. Montfort A. Johnsen, Ph.D. Technical Ed itor Typical male aerosol valve 38 Spray December 2016


Spray Decemberr 2016
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