The Use of Propellants in Aerosol Manufacturing

Definition and Function

The propellant is the component that provides the pressure necessary to expel the product from the aerosol container. It is the element that provides energy to the system, allowing the product to be released in the form of a spray, foam, gel, or powder.

Propellants can be in a gaseous or liquid state inside the container, and their selection depends on the type of product, the desired application method, and environmental and safety considerations.

Classification of Propellants

Liquefied Gases (Hydrocarbons)

Hydrocarbon propellants (HCP) are the most commonly used today. They include:

• Propane (C₃H₈): High vapor pressure, used in blends.
• Butane (C₄H₁₀): Medium pressure, very common.
• Isobutane: Similar to butane but with a different molecular structure.

Main characteristics: They are flammable, odorless, and colorless. Heavier than air, they tend to accumulate in low areas. They have good solubility with many products.

Liquefied Gases (Halocarbons)

Historically, they included CFCs (chlorofluorocarbons), which were banned by the Montreal Protocol due to their effect on the ozone layer. Currently used are:

• HFCs (hydrofluorocarbons): They do not damage the ozone layer but contribute to the greenhouse effect.
• HFOs (hydrofluoroolefins): New generation with lower environmental impact.

Main characteristics: They are non-flammable, have good chemical stability, and compatibility with many products. Their use is increasingly restricted by environmental regulations.

Liquefied Gases (Ethers)

Include compounds such as dimethyl ether (DME), which combines properties of hydrocarbons and halocarbons.

Main characteristics: Partially soluble in water, which allows for the formulation of water-based products. It is flammable but less so than hydrocarbons. It has good compatibility with many ingredients.

Compressed Gases

They do not change state inside the container; they always remain as a gas.

• Nitrogen (N₂): Inert, non-flammable.
• Carbon dioxide (CO₂): Non-flammable, slightly soluble in water.
• Compressed air: Used in products where flammability is a concern.

Main characteristics: Pressure decreases as the product is used. They are non-flammable. They generally produce a coarser and less fine spray than liquefied gases.

Technical Characteristics

Vapor Pressure

It is the pressure exerted by the propellant in a gaseous state on the liquid at a given temperature. It determines the force with which the product will be expelled. It is measured in bars or psi (pounds per square inch). Propellants can be mixed to obtain intermediate pressures.

Solubility

The ability of the propellant to dissolve in the product. It affects spray quality, emulsion stability, and overall aerosol behavior. Hydrocarbons have good solubility in oily products, while DME is partially soluble in water.

Density

Affects the behavior of the propellant inside the container. Propellants denser than the product will settle at the bottom, while less dense ones will float on top.

Flammability

The ability of the propellant to ignite. It is defined by its lower and upper flammability limits, which indicate the minimum and maximum percentage of propellant in air for the mixture to be flammable. For example, propane has a lower limit of 2.4% and an upper limit of 9.5%.

Compatibility

The ability of the propellant to coexist with the product and the container materials without adverse reactions. It is crucial for the stability and shelf life of the aerosol.

Environmental Considerations

Montreal Protocol

Signed in 1987 by 70 countries, it established the reduction in the use of CFCs due to their harmful effect on the ozone layer. Currently, CFCs are banned in most applications, being allowed only in certain medicinal products where no substitute has been found.

Global Warming Potential (GWP)

Measures the capacity of a gas to contribute to the greenhouse effect. HFCs, although they do not damage the ozone layer, have a high GWP, so their use is also being progressively limited by agreements such as the Kigali Amendment to the Montreal Protocol.

Sustainable Alternatives

The aerosol manufacturing industry is developing propellants with lower environmental impact, such as hydrofluoroolefins (HFOs), and exploring new formulations with compressed gases or alternative dispensing systems that do not require propellants.

Selecting the Right Propellant

The choice of propellant depends on multiple factors:

• Product type: Determines chemical compatibility and the necessary solubility.
• Desired application form: Fine spray, foam, gel, etc.
• Safety considerations: Flammability, toxicity.
• Environmental impact: Local and international regulations.
• Cost: Some propellants are significantly more expensive than others.
• Required pressure: Determined by the type of application and the viscosity of the product.

In many cases, propellant blends are used to obtain the desired characteristics, combining, for example, propane and isobutane in different proportions to adjust the pressure.