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Can water vapor go through plastic?

Water, as a vapour, can pass through some plastic packaging considerably more easily than expected. It is one of the most damaging contaminants, since in the vast majority of pharmaceutical products the water content has to be accurately maintained.

versaperm.com - A taste of your own medicine? Water vapour and your packaging

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A taste of your own medicine?

Water vapour and your packaging – the truth behind the myths

By Christopher Roberts, Director, Versaperm

The use of plastics and related materials in pharmaceutical and medical packaging is widespread (US$18.2 billion by 2005) and growing – especially for blister packs and flexibles. This is because everyone knows that plastic packaging is impervious - it acts as a vital barrier to prevent contamination and the ingress or exit of water vapour, which is both omnipresent and insidious. Or that’s what people would like to believe! Unfortunately it’s not true. Water, as a vapour, can pass through some plastic packaging considerably more easily than expected. It is one of the most damaging contaminants, since in the vast majority of pharmaceutical products the water content has to be accurately maintained. Water can also be a major factor in product’s shelf life. To complicate matters still further, even an excellent barrier material can lose up to 50% of its effectiveness through thermoforming, and another 50% through heat sealing. Water and other contaminants can and do pass through packaging. Chemical and biological reactions do occur, physical changes do take place and quality and shelf life are degraded. This article runs through many of the problems, benefits, materials and package types that are used. Because there is no reliable way of predicting properties of a finished pack from its constituent materials (however good) and manufacturing process, medical and pharmaceutical packages must be tested if the efficacy of the drug, or the sterility of the product is not to be compromised.

Packaging testing

Existing legislation regarding the permeability of pharmaceutical packaging involves a number of tests – most of which are gravimetric based. These tests often require several days or weeks to produce an accurate reading – even under ideal conditions. Anecdotal evidence suggests that they are more sensitive to the operator than is desirable. Instrumental techniques, such as WVTR meters, can be far more versatile – they are fast, reliable and can be used to test a wide range of packaging types. In many cases accurate data can be gained in as little as 30 minutes. They also allow the temperature and humidity during the testing to be accurately controlled. Many drugs are sensitive to moisture and need high value vapour barrier films. Most polymers offer very good resistance to liquid water, with the exception of a few such as EVOH, PVOH and cellulose – but there is little correlation between resistance to liquid water and water vapour. A material that is good in one case might have little effect on the other. Some of the best polymeric barriers to water vapour include PVDC (polyvinylidenechloride) , PCTFE (polychlorotrifluoroethylene,commonly known by the trade name, Aclar), and COC (cyclic olefin copolymers)To achieve all the properties required for the application, materials are typically multilayered laminates. The best laminates for barrier properties include a component of aluminium, either as a discrete layer or as a result of a metalisation process.

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PVC or Polyvinyl Chloride is the most widely used blister material – though it only provides a nominal or zero barrier to moisture and can only be used when the content do not require this protection. PP or Polypropylene is an economical medium-moisture barrier material, though it is more common in Europe than the USA. PVC/PCTFE Laminations is a thermoplastic film laminated to the PVC by adhesive or heat sealing, sometimes with an intermediate polyethylene layer to assist lamination. Different thickness of the composite offer average-to-extreme moisture barriers. PVC/PVdC is a film where the PVC is coated with polyvinylidene chloride (PVdC). The coating provides a medium to high vapour barrier depending on the amount used. As with PVC/PCTFE it is supplied either with or without a middle polyethylene layer and this can effect the thermoforming characteristics of the blister. COC or Cyclic Olefin Copolymers are a relatively new family of polymers that offer excellent barrier for applications where halogen free packaging is required. To maintain halogen free status, COC’s are normally laminated between layers of PE or PP Cold Form Foil is used for medicines, drugs and other products that are extremely sensitive to either water or light. It provides an extreme moisture barrier and is constructed using three layers: PVC, aluminium foil, and nylon.

Table 1 Comparison of Typical WVTR’s

(Relative to PVC at 100%)

PVC

100 PP

15 PVDC

1.25 – 1.5 CTFE

1.0-1.1 COC 1.5-1.75 Cold Form Foil 0

versaperm.com - A taste of your own medicine? Water vapour and your packaging

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