The manufacturing process for closures involves processing raw materials and auxiliary substances; weighing and mixing; followed
by vulcanization. Vulcanization is a chemical process for converting rubber or related polymers into more durable materials
via the addition of sulfur (or another equivalent curative) together with an accelerating agent such as 2-mercaptobenzothiazole;
an activator, usually zinc oxide; fillers such as carbon black or limestone; antioxidants; and lubricants. Following vulcanization,
molding and compressing occur.
There are two types of molding: compression and injection, of which the former is the most common. Compression molding is
a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The
mold is closed with a top force or plug member, and pressure is applied to force the material into contact with all mold areas,
while heat and pressure are maintained until the molding material has cured. Injection molding is a manufacturing process
for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed,
and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity.
After molding, the stages are: coating, washing, siliconization (if required, using specific, high-viscosity silicon oil),
and packaging. Siliconization has several advantages in that it prevents stoppers from sticking together or onto other surfaces
and can assist with the insertion of a needle through the stopper. The siliconization step is, however, a potential source
of contamination. Silicone used in the preparation of rubber stoppers should meet appropriate quality control criteria and
not have an adverse effect on the safety, quality, or purity of the drug product.
The mixing of raw materials and auxiliary substances involves the formulation of the stopper. A stopper is typically made
up of 60% rubber, 30% fillers (which protect the physical properties of the rubber) and pigments, 5% plasticizers (which provide
flexibility), 5% additional chemicals including accelerators (which help to create the cross-linkages which give the stopper
its strength and hardness), activators (which are a function of the efficiency of the cross-linkages), and antioxidants (which
help to avoid the degradation of the rubber).
There are different types of rubber, such as natural rubber (latex), isoprene rubber (a chemical copy of natural rubber),
styrol-butadine rubber, ethylene propylene dyes monomers, silicone (polysiloxane) rubber, and halogenized butyl rubber.