CLASSIFICATION OF AEROSOL
Aerosols are classified on the basis of the particle size. They are:
Space sprays: these are having particle size of dispersed phase not larger than 50pm in diameter, e.g., insecticides etc.Surface coating sprays: These sprays possess dispersed particles having size not larger than 200 um in diameter, e.g./hair sprays, power sprays etc.Foams: these are produced by rapid expansion of propellants through an emulsion and profuse in the foam of foam or froth, e.g., shaving cream, vaginal products etc.
The pressure of an aerosol is controlled by the type and amount of propellant, the nature and mount of product concentrate and the solubility of the medicaments in the propellants. Various additives such as solvents, antioxidants, surface active agents and flavoring agents are also added in the formulation.
FORMS OF AEROSOL
Aerosols are available in two forms:
(a) Two phase system: It is comprised of two phases. One phase is the liquid phase containing product concentrate (liquid and/or solid) dissolved or suspended in liquefied propellant and the second phase is the vapour phase.
(b) Three Phase system: It is comprised of three phases. One phase is the layer of water-immiscible liquid propellant, second one is a layer of highly aqueous product concentrate and the third phase is the vapour phase.
Aerosols container is mainly divided into two parts:
1. Container body
2. Valve assembly
Container for aerosol is made up of various materials like glass, uncoated or plastic coated metal, tin plated steel, aluminium, stainless steel and plastics.
Glass containers are preferred for most aerosols because they present fewer problems with respect to chemical compatibility with ingredients of the product and also not subjected to corrosion. They should be designed to provide the maximum in pressure safety and impact
resistance. Plastic coatings are applied on the outer surface of the glass to make it strong and resistant to accidental breakage. Class containers are considered quite safe when the total pressure of an aerosol system is below 25 pig and not more than 50% propellant is used.
Sometimes inner surface of glass container may be coated to more resistant to formulation ingredients. Now a day, tin-plated steel containers are the most widely used metal containers for aerosols with their surfaces coated by suitable coatings to protect against any incompatibility formulation materials and careful examination before filling to avoid any defect in the seam or in the protective coating.
The widely used seamless containers are made up of aluminium by extrusion or by other methods. They have greater safety against leakage, incompatibility, and corrosion. Stainless steel containers are rarely used whenever required because of its high cost. Plastic containers for aerosols are also not very much popular due to its problem of vapor permeation which may reduce the stability and efficacy of the product.
- Actuator: it is a button which activates the valve assembly for the emission of the product when pressed. The actuator permits the opening and closing of the valve.
II. Stem: It supports the actuator and delivers the formulation in the proper form to the
chamber of the actuator.
III.Gasket: The gasket serves to prevent leakage of the formulation when the valve is in the closed position.
IV.Spring: It holds the gasket in proper place. By its function, the actuator retracts when pressure is released, thereby returning the valve to the closed position.
V.Mounting cup: it is the part of aerosol container which keeps the valve in place. The Inner side of mounting cup may be coated with epoxy resin to prevent an undesired interaction with the materials of formulation.
VI.Housing: It is placed just below the mounting cup, serves as the link between the dip
tube and the stem and actuator.
VII.Dip tube: this tube is fitted to the housing and extended down into the product. It serves to being the formulation from the container to the valve.