Closure fit and function viability can depend on selecting the proper liner.
Because the liner facing is in direct contact with the product, chemical compatibility is crucial. It is important to provide a good liner system to maintain the integrity of the product for its required shelf life. The incredible variety of packaged products has led to the development of many liner systems, and modern research is constantly developing new concepts.
Liners are either die cut and inserted, molded or flowed in for lug, continuous thread, pressed to seal/twist to open and various specialty closures.
Whether termed a liner, innerseal or gasket, all liners are made of a material designed to seal a container and protect the product after the closure is applied to insure that the packaged product reaches the customer in an acceptable condition. In addition, some liner types, such as innerseals, are designed to add evidence of product tampering. Liners vary in type and material according to specific customer and product requirements.
Liners for metal closures consist of several types. These basic classifications are:
- Paper composites
- Plastisols, synthetic foams, and rubber ring gaskets
- Innerseals; e.g., glassine, heat induction liners, pressure sensitive
- Liner materials for plastic closures can be grouped into two categories:
- Extruded polymers
These categories are not always sharply defined, and many combinations of various materials are used to accommodate specific requirements.
The five general areas of sealability on the container neck finish are:
- The top sealing surface
- The interior vertical surface
- The inside edge
- The exterior edge
- The exterior vertical surface
The seal is of the utmost importance when considering the total package. If the seal is not achieved and maintained by the sealing system, the retention and integrity of the product may be jeopardized.
Proper closure/container system selection needs to address many issues and concerns, and should be discussed. To illustrate the magnitude of problems surrounding liner selection, here are some factors that may need to be confronted:
If the product contains acid, or caustic materials it will attack some liner materials, but others will withstand this readily.
Solvents will easily permeate some materials, but not metal foils, or some plastics. Essential oils used for fragrance can also react in a similar manner.
Some products are a mixture that requires protection in several directions. This situation may call for a composite of liner facings that when laminated together provide the right combination of protective factors.
Autoclaving, or exposure to high heat, will require a liner facing to withstand these temperatures, and moisture conditions.
If the product is a food, or drug, or an injectable, the liner must meet government regulations for sanitation and safety regarding ingestion, or injection into the body.
Creating a barrier against oxygen, or other gas permeation, may be critical. The liner facing is an important area of defense to assure this barrier.
O. BERK, through contact with many manufacturers, is the source for up-to-date liner information.