BY MONTFORT A. JOHNSEN, PH.D. TECHNICAL EDITOR July 2014 SPRAY 77 As may be anticipated, regulators and the industry have both introduced complexities to the hot tanking process. Certain products that are debilitated by heat are exempted, provided one can in every 2,000 is heated to 130°F without incident. Others that develop pressures as high as 310 psi-g (21.4 bars-g) at 130°F are permitted as aerosols, provided the dispenser carries a pressure-relief fitment. Several Bag-On-Valve (BOV) aerosol products are excused from hot tanking, provided their pressures at ambient temperatures are 100% checked with a can wall deflection device and found to be within a stipulated range. In Germany, the use of rotary machines to determine empty can pressure resistance and filled can seepage rate is approved as an alternative to hot tanking. Regulations related to residence time and seepage rate in hot tanks are under consideration. Plastic aerosols are slowly gaining popularity in the U.S., Europe and Australia and are known in Japan. They will usher in new complexities. Hot Water Bath Mechanics & Safety The simplest hot tanks are “dip tanks”—shallow tanks containing water at about 134°F (56.7°C.), into which open mesh trays of several hundred small aerosol cans are immersed for a minute or two. Typical products are metered-dose inhaler (MDI) pharmaceuticals in deep-drawn aluminum #5052 cans ranging from about 16 to 28.5mm in diameter. There is no regulatory requirement to perform this test in the U.S., since the dispensers always have a net capacity well under four fluid ounces (118.3mL). For larger aerosols (up to the 33.8 fluid ounce or one liter limit), the usual hot tank will be a straight line type, with one to four magnetic rails or conveyor lanes. Lengths can vary from about 12 to 30' (3.7 to 9.2m)—the smallest ones often used on pilot lines. The dispensers are normally fully immersed, but a few fillers adjust the hot water level to cover only about the lower 80% of the can. This satisfies the regulatory heating requirement, but if there are any leaks through the top double seam (tinplate cans only), the crimped seal or through the valve, they will not be detected. The rationale is that the high quality of modern cans and valves is such that leaks in these areas will affect fewer than 0.01% of those cans that pass the usual weightchecker and continue through the hot tank. Since aluminum cans are not affected by magnetic hold-downs, without special arrangements they would fall over on the inclines and also become “floaters.” If they happened to remain near the inlet of very hot make-up water, they could become deformed from excessive internal pressure. One popular solution is to fit a special puck around the bottom of each can. The pucks have a large steel washer in the bottom. The inner wall has a few tangential wing fittings, designed to hold the can firmly, but without marring the surface finish. Pucks are needed for each can diameter. In production, the empty cans are passed through a “pucker” device that attaches the pucks. Later, the finished aerosols go through a “de-pucker” that removes the pucks. Then, they are conveyed back to the beginning of the line for recycling. Alternatively, hot water baths can be purchased in which plastic clips are fastened to aluminum cans and hold them firmly in place as they pass through the water. They do obstruct visual leak detection to some degree and are still there, however unnecessarily, when the hot tank is used for tinplate or steel cans. A variation of the usual water bath is the serpentine type. Here, the immersed cans will be conveyed about 16' (4.9m) in one direction, then turned and moved in the opposite direction. This is done three or four times. This longer immersion time is a debatable advantage. The typical 80 square foot (7.4 square meter) footprint makes it the largest of all the machines on most aerosol production lines. An advantage to linear and serpentine water baths is that many have given good service for over half a century. Two types of Magnetic Rail Water Baths
SprayJuly2014
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