Majari Magazine

CHE Around Us: Aerosol-Can

by Inra Sumahamijaya on 07/12/08 at 2:10 am | 3 Comments | |

Liquefied Propellants

Liquefied propellants are gases that exist as liquids under pressure. Because the aerosol is under pressure the propellant exists mainly as a liquid, but it will also be in the head space as a vapor.  As the product is used up as the valve is opened, some of the liquid propellant turns to vapor and keeps the head space full of vapor. In this way the pressure in the can remains essentially constant and the spray performance is maintained throughout the life of the aerosol. The propellant is an essential element in the formulation.

When the liquid propellant emerges from the actuator the droplets immediately vaporize, and if the propellant is intimately mixed with droplets of the product, these will be ‘blown’ into smaller droplets. The ultimate size of the droplets can be controlled by adjusting the amount of propellant, and its pressure, in the aerosol. The design of the actuator is also important, as this will have a significant affect on the droplet size due to the mechanical action on the liquid, as it passes through the small holes and channels within the actuator.

Compressed Gas Propellants

Aerosol-can Based on Propellants

Compressed gas propellants really only occupy the head space above the liquid in the can. When the valve is open, the pressure on the liquid propellant is instantly reduced. Particles break free, forming a gas layer at the top of the can. This compressed gas layer pushes the liquid product, as well as some of the liquid propellant, up the tube to the nozzle. Some cans, such as spray-paint cans, have a ball bearing inside. If you shake the can, the rattling ball bearing helps to mix up the propellant and the product, so the product is pushed out in a fine mist. Unlike liquefied propellants, there is NO liquid to instantly vaporize when the product emerges from the actuator, and only the product is sprayed out.

In both of the system above, the can is not full to the top with liquid.  This is for safety reasons, as there must always be sufficient space for the propellant gas to occupy, under all likely storage conditions.  If all the space in the can was full of liquid, there would be the possible danger of the can bursting. The actual amount of liquid that can be filled into an aerosol can is controlled by legislation, and there has to be a greater head space when using compressed gas propellants.

Typical Propellants

Below are propellants typically used for aerosol-cans:

1. Liquefied Petroleum Gas (LPG)
   Aerosol propellant grade LPG consists of high purity hydrocarbons derived directly from oil wells, and as a by-product from the petroleum industry. They consist of a mixture of propane, isobutane and n-butane.  These propellants are used in most aerosols today, and have been used for many years in household aerosol products. These gases are flammable, and this is reflected in the classification of aerosols which contain them.
2. Dimethyl Ether
   This is an alternative liquefied propellant, and is more common in personal care products, and some air fresheners.
3. Chlorofluorocarbons (CFC-11, or F-11 and CFC-12, or F-12)
   Often called Freons, were used extensively as propellants in aerosol-spray products manufactured in the United States until 1978, when the federal government banned most uses of those compounds because of their potentially harmful environmental effect. Scientific studies indicated that chlorofluorocarbons released into the air rise up to the stratosphere, where they catalyze the decomposition of ozone molecules. The stratospheric ozone helps shield animal life from the Sun’s intense ultraviolet radiation, and it was feared that a significant reduction of atmospheric ozone by chlorofluorocarbons could lead to higher rates of radiation-induced skin cancer in humans. They are however permitted in inhalation aerosols, as used in the treatment of asthma.
4. Non-soluble compressed Gasses (e.g. Compressed Air and Nitrogen)
   These are sometimes seen in consumer products, and are an environmental alternative to LPG.
5. Soluble compressed Gasses (e.g. Carbon Dioxide)
   This is another alternative to LPG, but has limited use, mainly with alcoholic systems, such as air treatment products, deodorants and personal care products.

Aerosol Cans

Metal aerosol cans are usually made from tin-plated steel, or aluminum. Tinplate aerosol cans are mostly made up of three components – a top containing the valve opening, a body and a bottom.  Some two piece cans are also now available.  Aluminum cans are usually made from single piece of aluminium metal.

The tinplate used to make aerosol cans is low carbon mild steel sheet, coated with tin applied by electrodeposition.  The thickness of the tinplate sheet used to make aerosol cans will vary, depending on the size of the can, the pressure specification, and whether it is for can bodies or end components.  For can bodies the thickness will range from 0.18mm to 0.25mm, and for tops / bottoms they will be 0.28mm to 0.43mm.

Unless otherwise specified, the layer of tin on the steel is the same thickness on both sides.  Again the amount of tin will vary, ranging between 2.0 g/m² and 2.8g/m².  The internal surfaces of the tinplate will be either uncoated, or have a lacquer, or other material applied to give the metal better corrosion resistance.

Tinplate combines the strength and formability of steel together with the corrosion resistance and good appearance of tin.  Tin is a very soft metal and because the tin coating is very adherent it follows the movement of the steel base when tinplate is formed into the various components of an aerosol container.

Aerosol containers must be capable of withstanding the internal pressures generated during filling, and the subsequent transport, warehousing, and usage in the consumer’s hands.  They must also safely contain the product throughout the life of the aerosol. Aerosols are pressurized systems, and as such they are governed by legislation.  This not only covers the manufacture of the empty can, but also its subsequent filling.

Aerosol Filling Process

Here explained the aerosol filling proces:

1. Start with an empty aerosol container.  This will be made of tinplate or aluminium, or perhaps glass. The capacity of the container will be greater than that which is declared on the pack.
2. The product, usually in the form of a liquid, is now added. This contains all the active ingredients, except for the propellant.
3. The Aerosol valve is now fitted (crimped) to the can. This is a very critical operation and the crimping machinery has to be carefully set up to ensure that the can/valve seal does not leak.


Aerosol-Can Filling Process

4. The propellant is now injected under pressure, through the valve. The propellant may be in the form of a liquified gas, or a compressed gas. If a liquified gas is used it will exist as both a liquid, and vapour in the aerosol can head space.  The volume of liquid in the can will increase, as shown in this example. If a compressed gas is used, it will usually only be in the head space, above the liquid in the can, and there will be little or no increase in liquid volume.
5. The aerosol is now in a pressurised state due to the addition of the propellant.  The cans are now immersed in a water bath at 50°C to check for any leaks.  Any cans that leak are rejected.
6. If a large, or special, actuator is required it is fitted now.  Where necessary, a dust cap is also fitted. Finally the can will be date/batch coded, and shrink wrapped or boxed as required.

References:

Harris, Tom. “How Aerosol Cans Work.” 08 November 2001.  HowStuffWorks.com, 2008
Budhikarjo, Kusno. Lecture Handout “Desain Bejana”. Chem-Eng ITS, 2007.
Federation of European Aerosol Associations. www.aerosol.org
“Chlorofluorocarbon (CFC) .” Encyclopædia Britannica. Ultimate Reference Suite.  Chicago: Encyclopædia Britannica, 2008.
“Aerosol.” Encyclopædia Britannica. Ultimate Reference Suite.  Chicago: Encyclopædia Britannica, 2008.