44 Spray October 2014 the propellant would be delivered at pressure of about 450 to 625 psi-g. This required a special pump to raise the typical pipeline pressure from about 150 psi-g to the desired range. Excessive pressures had to be avoided or they could “dish” depress the flat-bottomed valve mounting cups of those days. A refinement that took a long time before coming into general use was to control the propellant temperature, and thus its density. This could be accomplished with a small heat exchanger. Otherwise, if the propellant slowly warmed up during sunny days, the density would decrease and less weight would be injected into cans. The machine could always be stopped and the cylinder volume re-set, but this would be time consuming. A major problem with these early TTV gassers was the throughput rate of the aerosol valve. This was most serious when attempting to fill a large quantity of propellant through a valve with a small stem or body tailpiece orifice, such as one with a 0.013" diameter. The gasser would then become the rate-determining operation of the production line, slowing it down and thus reducing the all-important number of filled cans produced per shift. Product development chemists were advised to concentrate on specifying valves with larger orifices, even though they might deliver the product faster than desired. In these early days, a number of aerosol products utilized carbon dioxide or nitrous oxide. There was even White’s Propellant: a blend of 85% nitrous oxide (“sweet”) and 15% carbon dioxide (“tart”), where the two taste elements cancelled each other out and did not change the sensory properties of natural or synthetic whipped creams. These high pressure propellants were stored in manifolded cylinders or receptacle tanks. Since vapors would be withdrawn, a cooling process, the containers were heated electrically to maintain a pre-set temperature range. At some facilities, cylinders were warmed with hot water. The gas, at pressures of about 130 to 180 psi-g, was then piped to a gassershaker machine. Both in-line and (later) rotary designs have been utilized. In the in-line machines, some 10 to 18 cans (depending upon diameter) are led into a long slot, placing each of them under a gassing head. The heads descend, with adapters depressing the valve stems. Then, gas pressure is applied; at the same time, the machine agitates the cans using a pre-set amplitude, frequency and time. Lower injection pressures are needed to add the desired amount of carbon dioxide or nitrous oxide to low viscosity products, such as windshield de-icers and those containing high levels of petroleum distillates. In the case of cookware lubricants, which contain high levels of relatively viscous vegetable oils, higher injection pressures are needed to adequately achieve the desired amount of propellant. Viscosities are often checked, preferentially by determining the Ostwald Coefficient (Zahn Cup Method, et al.). The gassed aerosol cans leave the gasser-shaker with various “over-pressures,” typically of about 10 to 50 psi. Further shaking or just standing overnight will cause a pressure reduction to (or close to) the equilibrium value. A little known fact is that the gas injections will increase the content temperature, typically by about 3° to 6°F, depending on the amount absorbed, the specific heat of the product and other factors. After propellant injection, the cans are normally led directly to the hot water bath, where the pressure is raised still further. The jostling on the conveyor belts and the act of heating both cause some of the “over-pressure” to dwindle. There are reports of injection pressures as high as 260 psi-g. If this much pressure was actually transmitted into an aerosol can, it would at least become distorted and possibly burst. However, the pressure in the can relates to the rate-limiting orifice size of the valve and to the rate of absorption by the product. Because a given valve specification will produce a rather significant range of delivery rates, it follows that gas injection rates will also be variable. As a general rule, higher injection pressures will result in a wider range of aerosol equilibrium pressures than lower ones. The solubility of carbon dioxide and nitrous oxide varies considerably, depending on the solvent. In contrast, the solubility of nitrogen and compressed air (CAIR) is almost negligible. At 70°F and 100 psi-g., 100mL of water will dissolve a mere 0.478 g. of the gas. Solubility data in some solvents is now illustrated. Modern Propellant Injection In the U.S., most liquid propellants are now filled by either TTV or Under-the-Cup (UTC) gassers. The latter machine is available in one-head, rotary nine head or rotary 18-head models. The rotary machines accurately position the aerosol cans (with valves inserted but not crimped) under a complex head and adapter. As Table 2 Solubility in Various Solvents at 100 psi-g and 70°F SOLVENT CARBON DIOXIDE (%) NITROUS OXIDE (%) Water 1.50 0.78 Acetone 12.08 10.21 Ethanol (Anhydrous) 5.57 5.66 Isopropanol 4.7 4.5 Methanal 18.5 17.7 iso-Pentane 5.39 5.38 Petroleum Distillates 3.32 3.52 Vegetable Oils 4.48 4.87 NOTE: Methylal is CH3-O-CH2-O-CH3; a very strong solvent with a sharp odor. Vegetable oils include corn, canola and soybean oils. TTV gassing station. Photo provided by MBC Aerosol.
Spray Oct 2014
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