NITROSOLS, Continued from preceding page loss. Since the amount of nitrogen (or compressed air) is always less than one gram, small weight losses can result in major decreases in pressure. In fact, pressure checks, always using a pre-pressurized gauge, are the preferred methods of establishing hermetic integrity of test packs. For the marketer, the nitrosol option presents both advantages and disadvantages. A major advantage is that the new, onerous California Air Resources Board (CARB) 20% VOC limit for two-phase air fresheners can be circumvented. Many other safety, production and economic advantages can be cited. FIGURE TWO Advantages of Nitrosols • Nitrogen is totally eco-compatible (78% nitrogen in air). • Nitrogen is non-flammable, a non-volatile organic compound (VOC) and has no ODP or GWP. • Nitrogen is completely inert—not harmful to aerosol formulations. • Cost of nitrogen per aerosol is negligible. • Pressures increase only slightly to moderately when heated (Charles Law) • Nitrosol sprays are almost silent (not frightening to animals). • Nitrogen is accurately filled by all gassers (use only an underthe cup gasser for BOVs). • Typical 0 to 0.6% nitrogen in formulas allow more product to be delivered. • Nitrosol can ruptures are non-hazardous (like opening a beverage can). • Hot water baths may be eliminated—currently by U.S. Dept. of Transportation (DOT) exemption. • If nitrogen is the sole propellant: Explosion-proof equipment need not be used. Highly protected gas houses are not necessary. Propellant bulk tanks can be replaced with manifolded cylinders. While these numerous advantages may seem panegyric, they must be weighed against an equally impressive collection of disadvantages. There are a large number of aerosol products (such as hair sprays, antiperspirants and paints) where nitrosol technology cannot be utilized. In products where the dispenser is likely to be tilted beyond the horizontal, such as bathroom cleaners, the incidence of gas loss and depressurization would be unacceptable. For products such as general purpose cleaners and starch, the increase in cost-per-ounce would be intolerable. The extra cost of BOV packaging components will tend to limit them to cosmetics, toiletries and similar products that command higher, more flexible retail prices. FIGURE THREE Disadvantages of Nitrosols • Loss of nitrogen by overly tilting or inverting the dispenser during use. • Loss of nitrogen over time (use extra tight crimps and lathecut gaskets). • Produces a 40 to 90 micron “wet” spray with MBU valve actuators. • Delivery rate decreases slightly to moderately during consumer use. • Very slight solubility of nitrogen in products. • Pressure may change properties of some products (rare). • Product fills are limited to 70 volume-% to prevent excessive pressure loss. • Flammable aspects if dimethyl ether (DME) is added to reduce particle size. 20 Spray January 2014 • Up to 30% yield loss of nitrogen, mostly due to high cylinder heel pressure. • Extra cost, if the BOV option is utilized. • For each product, the percent fill volume and pressure must be optimized. • In production, weight checks will not accurately assure pressure compliance. • Products that are unsuitable in BOV formats: Emulsions that exhibit coalescence or creaming (phase separation). Sedimentary suspensions of solid particles (agglomerates). Viscous liquids or gels that slowly develop gas bubbles upon release. Heating Effects Since nitrogen and purified compressed air are virtually insoluble in aerosol products, the gases are found almost entirely in the dispenser head space. They closely follow Charles Law when heated. For example, when warmed in the hot water bath from 70°F to 130°F, the pressure only rises by about 12%. FIGURE FOUR Nitrosol Pressure Increases in a Hot Water Bath 70°F Pressures 130°F Pressures 70 psi-g 79.5 psi-g 85 96.3 112 126.4 145 163.1 150 168.6 The safety of nitrosols when overheated can be compared with a low pressure propellant such as iso-butane (A-31): FIGURE FIVE Pressure-Temperature Comparisons Temperature °F Higher Pressure Iso-butane Nitrosol (Air Free) 70°F 140 psi-g 31.0 psi-g 104 150 66 130 158 97 160 166 139 180 172 198 212 181 >284 If the dispensers were to be tossed into boiling water (212°F) a DOT-2Q nitrosol can would almost certainly show no deformation, while that containing the iso-butane (A-31) would probably have exploded, releasing a large cloud of extremely flammable vapor. Pressure Effects The pressure changes in a nitrosol dispenser closely follow Boyle’s Law: the absolute pressure is inversely proportional to the volume. During its service life the gauge pressure in a nitrosol can may drop from about 65% to 100%, depending upon the percent liquid fill volume. For an extreme example, if a nitrosol product were to be filled to 85% of the can volume and then pressurized to 100 psi-g., when 95% had been dispensed the pressure would have dropped to a mere 3 psi-g. For a spray product, the spray would have collapsed when 20% to 25% of the product still remained in the can. A strenuous consumer complaint would surely follow. This shows the importance of limiting nitrosol product fills to about 55 to 70% of the dispenser volume. The use of reasonably higher initial pressures is also beneficial. It is convenient to calculate pressure drops during use.
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