Among the major resins considered for the caps and closures are the following:
Accounting for the largest share of plastic used (over 60 percent), the resin is best characterized by good clarity, excellent moisture and chemical resistance, high softening point and outstanding dimensional stability. It has been adapted to a broad assortment of food, pharmaceutical and beverage applications, and dominates the child-resistant and tamper-evident fields with sealing characteristics comparable to metal.Though most polypropylene closures are lined, in controlled circumstances linerless, or unlined closures eliminate the cost of punching and inserting liners into the closures. The economies can be worthwhile. It is important that the neck finish of the container mate properly and consistently with the closure. Pre-testing and "on-going" quality control checks of both the container and closure are important when choosing this option.
Polystyrene exhibits good clarity, low moisture absorption, smooth surface qualities and ease of processing. Principal applications include overcaps, lid and screw caps for non-oxygen sensitive materials.
High density (HDPE) brings strong impact, rigidity, good chemical resistance and excellent drug compatibility to the field and is frequently utilized for threaded screw caps, shrink bands and laminated liners. Meanwhile, low density (LDPE/LLDPE) is known for its impact resistance and flexibility. It is especially useful for closures requiring storage at low temperatures. The increasing penetration of polypropylene is expected to continue to depress future growth in caps and closures for the polyethylenes.
Many end-use applications for polypropylene closures require the resin used be a food grade quality. Code of Federal Regulation 21 CFR 177.1520 of the Food and Drug Administration (FDA) covers the use of certain resin grades and additives for this use. The Drug Master File also lists certain resins which have approval for packaging of this type. These regulations may well narrow the list of suitable materials for a specific application and should be considered early in the search process.
The many variations within the four major headings above can make the material selection process for a specific closure application seem overly complex, when in fact the closure supplier has an abundance of assistance to call on beyond just the experience factor.
Phenolic and Urea compounds are included in thermoset closures. These plastics have proven themselves to have the following characteristics:
- Widest range of chemical compatibility.
- Widest range of temperature tolerance of the plastic closures.
- Will withstand sub-zero temperatures without embrittlement.
- Will withstand temperatures to 300° with proper liner selection.
- Provide the most consistent adherence to close tolerance specifications.
The rigidity of thermosetting plastics helps guard against slipping over threads, which can be a problem with thermoplastic closures, particularly polyethylene. This problem is aggravated with polyethylene bottles when thread depth, or neck diameters are not accurate.
Phenolic closures are available in black and brown. Urea closures are available in white and other light colors. When closures are to be metallized, phenolics seem to give better results. The "heft" of the thermoset closure also adds to the feel of quality and superiority.