The Pharmaceutics and Compounding Laboratory
Aerosols

The Valve Assembly

The effectiveness of a pharmaceutical aerosol depends on achieving the proper combination of product concentrate formulation, container, and valve assembly. The valve mechanism is the part of the product package through which the contents of the container are emitted. The valve must withstand the pressure required by the product concentrate and the container, be corrosive resistant, and must contribute to the form of the emitted product concentrate.

The primary purpose of the valve is to regulate the flow of product concentrate from the container. But the valve must also be multifunctional and regulate the amount of emitted material (metered valves), be capable of delivering the product concentrate in the desired form, and be easy to turn on and off. Among the materials used in the manufacture of the various valve parts are plastic, rubber, aluminum, and stainless steel.

The basic parts of a valve assembly can be described as:

  1. Actuator - the actuator is the button which the user presses to activate the valve assembly and provides an easy mechanism of turning the valve on and off. In some actuators, mechanical breakup devices are also included. It is the combination of the type and quantity of propellant used and the actuator design and dimensions that determine the physical form of the emitted product concentrate.
  2. Stem - the stem supports the actuator and delivers the formulation in the proper form to the chamber of the actuator.
  3. Gasket - the gasket, placed snugly with the stem, serves to prevent leakage of the formulation of the valve is in the closed position.
  4. Spring - the spring holds the gasket in place and also is the mechanism by which the actuator retracts when pressure is released thereby returning the valve to the closed position.
  5. Mounting Cup - the mounting cup which is attached to the aerosol container serves to hold the valve in place. Because the undersigned of the mounting cup is exposed to the formulation, it must receive the same consideration as the inner part of the container with respect to meeting criteria of compatibility. If necessary, it may be coated with an inert material to prevent an undesired interaction.
  6. Housing - the housing located directly below the mounting cup serves as the link between the dip tube and the stem and actuator. With the stem, its orifice helps to determine the delivery rate and the form in which the product is emitted.
  7. Dip Tube - the dip tube which extends from the housing down into the product concentrate serves to bring the formulation from the container to the valve. The viscosity of the product and its intended delivery to rate dictate the inner dimensions of the dip tube and housing for a particular product.

Spray valves are used to obtain fine to coarse wet sprays. Depending on the formulation and the design of the valve and actuator, the particle size of the emitted spray can be varied. The spray is produced as an aerosol solution passes through a series of small orifices which open into chambers that allow the product concentrate to expand into the proper particle size.

Vapor tap valves are used with powder aerosols, water based aerosols, aerosols containing suspended materials, and other agents that would tend to clog a standard valve. This valve is basically a standard valve except that a small hole has been placed into the valve housing. This allows vaporized propellant to be emitted along with the product concentrate and produces a spray with greater dispersion. These valves are used with aqueous and hydroalcoholic product concentrates and hydrocarbon propellants.

Foam valves have only one orifice that leads to a single expansion chamber. The expansion chamber also serves as the delivery nozzle or applicator. The chamber is the appropriate volume to allow the product concentrate to expand into a ball of foam. Foam valves are used for viscous product concentrates such as creams and ointments because of the large orifice and chamber. Foam valves also are used to dispense rectal and vaginal foams. If the size of the orifice and expansion chamber are appropriately reduced, a product concentrate that would produce a foam will be emitted as a solid stream. In this case, the ball of foam begins to develop where the stream impinges on a surface.

Metered dose inhaler (MDI) valves (metering valves) are used to accurately deliver a dose of medication. Metered valves are used for all oral, inhalation, and nasal aerosols. The metered valves reproducibly deliver an amount of product concentrate accurately from the same package and also allow for the same accuracy between different packages.

The amount of material emitted is regulated by an auxiliary valve chamber of fixed capacity and dimensions. This metering chamber volume can be varied so that about 25 to 150 mL of product concentrate is delivered per actuation. Access in and out of the metering chamber is controlled by a dual valve mechanism. When the actuator is closed, a seal blocks emission from the chamber to the atmosphere. However, the chamber is open to the contents of the container and it is filled. When the actuator is depressed, the seals reverse function; the chamber becomes open to the atmosphere and releases its contents and at the same time becomes sealed from the contents of the container. When the actuator is again closed, the system prepares for the next dose.

Two basic types of metering valves are available; one for inverted use and the other for upright use. Generally the valves for upright use are used with solution type aerosols and contain a thin capillary dip tube. Suspension or dispersion aerosols use the valve intended for inverted use that does not contain a dip tube.

In general, valves should retain the material in the metering chamber for fairly long periods. However, it is possible for the material in the chamber to slowly return back to the container. The degree to which this occurs depends on the construction of the valve and length of time between actuations of the valves. Some valves have been fitted with a "drain tank" to overcome this problem.

Containers

Aerosol containers are generally made of glass, metals (e.g., tin plated steel, aluminum, and stainless steel), and plastics. The selection of the container for a particular aerosol product is based on its adaptability to production methods, compatibility with the formulation, ability to sustain the pressure necessary for the product, the design and aesthetic appeal, and the cost.

Pressure Limitations of Aerosol Containers
Container
Material
Maximum
Pressure (psig)
Temperature
(oF)
Tin-plated steel
180
130
Uncoated glass
< 18
70
Coated glass
< 25
70
Aluminum
180
130
Stainless Steel
180
130
Plastic
< 25
70

Glass containers would be the preferred container for most aerosols. Glass presents fewer problems with respect to chemical compatibility with the formulation compared to metal containers and is not subject to corrosion. Glass is also more adaptive to design creativity and allows the user to view the level of contents in the container.

However, glass containers must be precisely engineered to provide the maximum pressure safety and impact resistance. Therefore, glass containers are used in products that have lower pressures and lower percentages of propellants. When the pressure is below 25 psig and less than 50% propellant is used, coated glass containers are considered safe.

To increase the resistance to breakage, plastic coatings are commonly applied to the outer surface of glass containers. These plastic coatings serve many purposes: 1) prevent the glass from shattering into fragments if broken; 2) absorb shock from the crimping operation during production thus decreasing the danger of breakage around the neck; 3) protect the contents from ultraviolet light; 4) act as a means of identification since the coatings are available in various colors.

Glass containers range in size from 15 to 30 mL and are used primarily with solution aerosols. Glass containers are generally not used with suspension aerosols because the visibility of the suspended particles presents an aesthetic problem. All commercially available containers have a 20 mm neck finish which adapts easily to metered valves.

Tin-plated steel containers are light weight and relatively inexpensive. For some products the tin provides all the necessary protection. However when required, special protective coatings are applied to the tin sheets prior to fabrication so that the inside of the container will be protected from corrosion and interaction between the tin and the formulation. The coating usually is an oleoresin, phenolic, vinyl, or epoxy coating. The tin plated steel containers are used in topical aerosols.

Aluminum is used in most MDIs and many topical aerosols. This material is extremely light weight and is less reactive than other metals. Aluminum containers can coated with epoxy, vinyl, or phenolic resins to decrease the interaction between the aluminum and the formulation. The aluminum can also be anodized to form a stable coating of aluminum oxide. Most aluminum containers are manufactured by an impact extrusion process that make them seamless. Therefore, they have a greater safety against leakage, incompatibility, and corrosion.

Aluminum containers are made with a 20 mm neck finish that adapts to the metered valves. For special purposes and applications, containers are also available that have neck finishes ranging from 15 to 20 mm. The container themselves available in sizes ranging from 10 mL to over 1,000 mL.

Stainless steel is used when the container must be chemically resistant to the product concentrate. The main limitation of these containers is their high cost.

Plastic containers have had limited success because of their inherent permeability problems to the vapor phase inside the container. Also, some drug-plastic interactions have limited the efficacy of the product.