The stem gasket dimensions are closely controlled—particularly the hole that fits around the valve stem—but also the thickness. The surface finish may be shiny or matte, but must be free of flash, blanking gouges, cuts, tears, excessive porosity or other imperfections. Dimensions are always specified, although they vary considerably between valve models, with stem orifice diameter and stem shank geometry. A typical set of gasket dimensions would be: o.d. 0.335"(8.51mm), i.d. 0.110" (2.79mm) and thickness 0.044" (1.12mm). Gasket thickness can range from about 0.035" to 0.062" (0.89 to 1.57mm), mainly in response to the stem orifice diameter, which can be as large as about 0.041" +/- 0.002" (1.04 +/- 0.05mm). The valve stem shank, or slotted shank, diameter is normally about the same as that of the stem gasket i.d. This might seem like a rather loose fit at first and subject to “blow-by when the valve is crimped onto the can and the unit pressurized. However, the valve mounting cup that surrounds the gasket is always very accurately clinched, holding the interior components in place and vertically squeezing the gasket so that the gasket hole tightens around the stem shank. The junction then becomes hermetic or gas tight. Almost immediately, solvent and propellant vapors begin a gasket swelling process, making the seal even tighter. Those few aerosol solvents and propellants that may shrink the gasket can loosen the stem-to-gasket connection, sometimes to the extent of causing valve leakage. Any molded stem imperfection, like a parting line of over 0.003" (0.076mm), can exacerbate the possibility of leakage. This aspect must be explored during product development, using immersion gasket swell (or shrink) tests. Remedies include using a different gasket composition or adding a suitable percentage of a gasket swelling ingredient to the formulation. Stem Gasket Swell (or Shrinkage) Testing Ideally, stem gaskets should swell from about 3.5 to 6%, although some valves will tolerate up to about 10%. With excessive swelling, valve operation will become mushy and the delivery rate may decrease or even shut off altogether. Many strong solvents can swell buna gaskets up to about 35%. Examples for a particular buna gasket are acetone (22.5%), methyl ethyl ketone (27.7%), xylene (11.0%) and ethyl acetate (19.3%). They are often used in aerosol paint products, but usually when diluted with water, propylene glycol, hydrocarbon propellants or other low swell components. If buna gasket swelling is still excessive, another buna or an entirely different composition may be used. A number of chemical ingredients have been identified as “shrinkage agents,” including n.butane, n.hexane, VM&P Naptha, n.heptane, methanol, ethanol, iso.propanol, oleyl alcohol, light mineral oil and triethanol-n.amine. Perhaps, fortunately, the amount of shrinkage is almost always 1% or less. Ethanol produces a 0.5% shrinkage—n.nonane is at 0.4%. One of the buna N rubbers swells 1.5% after 21 days of immersion in iso-butane, while another buna N rubber shrinks by 2.9%. Lastly, a 50:50 mixture of ethanol and n.hexane swells still another buna N rubber by 6.1% in six hours, 8.8% in one day, 6.7% in seven days and 3.9% in 50 days. This shows how a solution of two rubber shrinking agents can cause swelling. It also points out the need for relatively long-term immersion testing. Today, much of this testing is done by valve companies as a customer service. Typical time periods are five, 30 and sometimes 60 days. Occasionally, there may be surprises, such as certain gaskets being disintegrated by immersion in dilute hydrogen peroxide. Two methods for determining stem gasket swell (or contraction) are currently in use. In Europe, many firms use the Standard FEA 644E Method, but in North America, no standard method has yet been developed. However, some simple procedures appear to work quite well. The results of the two methods are often comparable. The apparatus for U.S. tests is generally a 1" wide, 8" (200mm) long heavy glass compatibility tube. Stack three test stem gaskets and measure their combined height using a dial gauge with 50 gram compression and accurate to 0.0004" (0.01mm). Record the height. Add the product concentrate to the glass tube, drop in the three gaskets, seal and add the propellant. Store at about 73°F (23°C) for the predetermined time period (i.e. five days). Spray or foam out the contents. Remove the valve gaskets. Quickly wipe them dry; then stack and measure their combined height as before. Determine change by the equation: % Swell (or shrinkage) = (h final – h initial)/h initial Where h = height or thickness of the stacked gaskets The test is then continued (typically up to 25 more days). If the short term and long term swell (or shrinkage) results are comparable, the test may be terminated. However, if they are significantly different, a still longer immersion study may be justified. Tilt valve (l.) Standard valve (r.) December 2016 Spray 39
Spray Decemberr 2016
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