Aerosols refer to products where medicated, emulsified, or suspended liquids are sealed together with a suitable propellant in a pressure-resistant container equipped with a specialized valve system. When used, these products are sprayed out in a misty form with the help of the pressure from the propellant.
Composition of Aerosols
Aerosols consist of four main components: propellant, drug formulation, valve system, and pressure-resistant container.
The propellant serves as the driving force for dispensing the medication and sometimes acts as a solvent or diluent for the medication. The drug formulation is the active ingredient, which can be in liquid, semi-solid, or solid powder form. The valve system includes different types of valves such as standard valves, metered valves, and foam valves, depending on the product. Pressure-resistant containers commonly used include tinplate cans and aluminum cans.
Working Principle of Aerosols
The working principle of aerosols involves simultaneously filling the original liquid (active ingredient) and liquefied gas or compressed gas into the aerosol can and sealing it with a valve. When used, pressing the actuator opens the valve, allowing the liquid-phase substance inside the can to be pressurized upwards through the liquid delivery tube into the valve body under the pressure of the propellant. Then, it enters the valve core through the metering hole in the valve core and is sprayed out through the nozzle. The atomization process when the spray leaves the nozzle is a result of a combination of factors. As it is sprayed out at high speed, the propellant rapidly vaporizes, forming droplets, and the pressure of the liquid-phase propellant contained in the droplets evaporates instantly into gas phase. After dispensing the original liquid, the reduced volume inside the aerosol can be replenished by the vaporization of the liquefied gas, thus continuing the cycle until all the original liquid is dispensed.
If compressed air is used as the driving force for the original liquid, it maintains its original shape when dispensed from the aerosol can because this gas is almost insoluble in the active ingredient. When liquefied gas is used in the aerosol can, some of it vaporizes inside the sealed container, while the rest remains in liquid form, maintaining equilibrium. At this point, the gas phase is at the top of the can, and the liquid phase is at the bottom, with the active ingredient dissolved or dispersed in the liquid phase. The gas phase not only exerts pressure on the can walls but also on the liquid, so when the valve is opened, the active ingredient can be forced from the extending conduit to the valve outlet, forming a spray or foam shape automatically. This may also be in liquid or paste form, depending on the properties of the propellant used as the driving force and the structure of the valve.
If liquefied gas with a boiling point much lower than room temperature or ambient temperature is used, it vaporizes immediately upon release, causing the active ingredient to spray into the air, forming fine mist particles. If the active ingredient needs to be sprayed onto the surface of the target, the aerosol must vaporize near or upon contact with the surface, leaving the active ingredient behind. This can be adjusted appropriately based on the different boiling points, quantities, and viscosities of various liquefied gases used in combination. Similarly, if the dispensed shape of the aerosol can is foam, the main difference lies in the fact that the foam product is a solution rather than an emulsion. When the valve is opened, the expansion of the liquefied gas creates many small bubbles in the emulsion, forming a foam shape. If powdered material is suspended in the liquefied gas, when the valve is opened, the powder and liquefied gas are simultaneously expelled, and the liquefied gas quickly vaporizes and volatilizes, leaving the powder on the surface of the target.
Aerosol Filling
The filling of aerosol products requires specialized aerosol filling equipment, consisting of a liquid filling machine, a sealing machine, and an inflator. First, the original liquid is injected into the pressure-resistant container via the filling machine, then the valve system is installed, the sealing machine seals the valve, and finally, the inflator injects the propellant gas through the valve hole, the nozzle is attached, and a bottle of aerosol product is filled.
