Plastic containers are convenient to handle but the major disadvantage is the two-way permeation or 'breathing' through the container walls. Volatile oils, perfumes and flavouring agents can permeate through plastics to some extent. Components of emulsions and creams have been reported to migrate through the walls of some plastics causing either a deleterious change in the formulation or collapse of the container. Loss of moisture from a formulation is also common. It must not react chemically nor physically with the formulation.
Rubber is a common component of stoppers, cap liners, and parts of dropper assemblies.
Absorption of the active ingredients, preservatives into the rubber and the extraction of one or more components of rubber into the formulation is a common problem. The application of an epoxy lining to the rubber closure reduces the amount of leached extractives but has no effect on the absorption of the preservative from the solution. Teflon-coated rubber stoppers may prevent most of the leaching and absorption. The process is repeated until all the diluent has been added. Expiration date is defined as the time in months and years after which a drug product is expected to be subpotent or to be degraded under designated storage condition.