The Shelf Life Shuffle

To suggest that anything will last forever is just not realistic. A trip to the grocery store will confirm this. Everything, not just the milk and the eggs, but cans of beans, containers of Pam, bottles of aspirin all will have a "use before" date.

Apparently the only thing that lasts forever is "Baloney" because that's how this all started. An aerosol marketer began to advertise that his product had an indefinite shelf life. When the manufacturer of that product questioned the marketer, the marketer stated he felt that it was necessary in the promotion of the devices, that the cans would likely be used up in two years and that if any can failed after the two years the marketer would replace them at his expense.

This is not a good plan. It means that an officer who is posted to a "quiet" zone, or one who has office duties, and would have only rare occasions to use his spray unit might find it to be less than effective. These aerosol devices need to be replaced two years from the date of manufacture. That is not to say that the sprays will fail after two years, it means they can not be depended upon after two years.

The problem is not with the liquid in the can. Most modern formulations are fairly stable. The problem is with the propellant, that is with the gas which pressurizes the aerosol. Even with the best suitable gasket material, even with the most perfect crimp, no one can guarantee a perfect seal. The gas will leak out in infinitesimal amounts, this process is called "Migration". The term means an amount too small to measure, too slow to show in a hot water bath, but an amount which will accumulate into enough loss to effect performance.

Here is an example. There are about 28 grams in an ounce; there are one thousand milligrams in a gram. It follows that a milligram is one twenty eight thousandths of an ounce. This is an amount too small for manufacturers labs to measure. So in our example, if a can were to lose two milligrams per day in 730 days (two years) the can would have lost 1460 milligrams or 1.4 grams. That would be the entire propellant contents of a half ounce unit. If we reduce the propellant loss to one milligram per day, the unit would still lose .7 grams in two years. That would reduce the propellant amount to about one half of its previous level and would severely impact the ballistic characteristics of the unit.

These examples were worked with a half ounce can which is 7/8 of an inch in diameter and about 2 3/4 inches tall. Devices of this type are sealed with a 20 mm valve. It is because of these mathematics that the Association of Self Defense Manufacturers through its Standards Committee is currently studying a proposal to reduce the recommended shelf life of 20 mm valve cans to 18 months.

Granted the preceding examples were worked with a smaller size device for dramatic effect, but the same principle holds for the larger cans. A two ounce unit, the standard police size, would contain a little over six grams of propellant. If the device were used for thirty one second shots over a two year period and had a 2 milligram migration factor the amount of propellant would be reduced to 1.5 grams. That would be sufficient reduction to effect the ballistics of the unit.

Since the loss is not as dramatic, the A. D. S. M. Standards Committee is currently studying a proposal to extend the recommended shelf life of aerosols using a one inch valve to 30 or 36 months.

In conclusion, no matter how stable the liquid contents of the device may be, the nature of propellant gas is the determining factor in shelf life of an aerosol spray. While the data cited may not be significant when applied to a can of hair spray or oven cleaner, it is very significant when applied to self defense sprays. When a police officer pulls a weapon from his holster, any weapon, he needs to be confident that the weapon will function at it's highest level of effectiveness. Tight controls on shelf life will give him that confidence.