Aerosol spray
From Wikipedia, the free encyclopedia
Aerosol Spray is the name given to type of Spray Can with what one might call Aerosol in it.
This type of Spray Can is a canister containing liquid under pressure from pressurized vapor in equilibrium with another liquid, which often also dissolves the payload (see propellant below). When the can's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol, or mist. As gas expands to drive out the payload, some propellant evaporates inside the can to maintain an even pressure. Outside the can, the droplets of propellant evaporate rapidly, leaving the payload suspended as very fine particles or droplets. Typical liquids dispensed in this way are insecticides, deodorants and paints. An atomiser is a similar device that is pressurised by a hand-operated pump rather than by stored gas.
The modern aerosol spray can was invented in Oslo in 1926 by Erik Rotheim, a Norwegian chemical engineer. The patent was sold to a US company for 100,000 Norwegian kroners, but it wasn't until 1941 that it was first put to good use by Americans Lyle Goodhue and William Sullivan. They turned it into an instrument for the US military to fight the malaria mosquito in the Pacific during World War II.
[edit] Propellant
If the can was simply filled with compressed gas, either it would need to be at a dangerously high pressure, or the amount of gas in the can would be small, and it would soon run out. Hence, usually, the gas is the vapour of a liquid with boiling point slightly lower than room temperature. This means that inside the pressurised can, the vapour can exist in equilibrium with its bulk liquid at a pressure that is higher than atmospheric pressure (and able to expel the payload), but not dangerously high; yet, as gas escapes it is immediately replaced by more evaporating liquid. Since the propellant exists in liquid form in the can it is desirable that it be miscible with or dissolved in the payload.
Chlorofluorocarbons (CFCs) were once often used but since the Montreal Protocol came into force in 1989 they have been replaced, in nearly every country, due to the negative effects CFCs have on Earth's ozone layer. The most common replacements are mixtures of volatile hydrocarbons, typically propane, n-butane and isobutane. Dimethyl ether (DME) and methylethyl ether are also used. All these have the principle disadvantage of being quite flammable. Nitrous oxide is also used as a propellant to deliver foodstuffs (for example, whipped cream). Medicinal aerosols such as asthma inhalers use hydrofluoroalkanes (HFA): either HFA 134a (1,1,1,2,-tetrafluoroethane) or HFA 227 (1,1,1,2,3,3,3-heptafluoropropane) or combinations of the two.
[edit] Packaging
Aerosol spray products have three major parts; the can, the valve and the actuator or button. The can is most commonly lacquered tin plate and may be made of 2 or 3 pieces of metal crimped together. Aluminum cans are also common and are generally used for more expensive products. The valve is crimped to the rig of the can, the design of this component is important in determining the spray rate. The actuator is depressed by the user to open the valve; the shape and size of the nozzle in the actuator controls the spread of the aerosol spray.
