Product shelf life is linked to oxidation and, in carbonated drinks, to loss of carbonation. A feature of plastics is the ability of gases to pass through them. Gas permeation does depend to a degree on the difference between the internal and external pressures, but even a bottle of carbonated drink with an internal pressure of 5 bar will be subject to permeation of oxygen into the bottle from the surrounding atmosphere. It is also the case that CO2 will tend to pass from the bottle to the outside at the same time as oxygen molecules are passing into the product.

The quality of the drink can also be affected by the migration of substances from the plastic itself, although this has been the subject of a great deal of research and no longer presents a problem. Advances in resin production and in the selection of processing parameters ensure that the bottler has PET material or preforms ideally suited to the specific application.

Improving shelf life

The correct thermal conditioning of the preform prior to stretch blow moulding, selective heating of the mould tool, and the degree of mechanical stretching applied to the preform have an influence on the crystallinity of the PET material in the finished bottle, which in turn affects its performance as a barrier against permeation.

Other ways of improving a PET bottle’s barrier properties are:

Wall thickness: clearly a thicker bottle will offer more resistance to gas permeation but there are cost implications all the way down the manufacturing chain.

Bottle design: for a bottle of a given capacity the external surface area (and hence overall permeability) will vary with the shape of the bottle. With traditional bottle shapes it is generally true that larger volume bottles have a lower surface-area-to-capacity ratio than smaller bottles.

Multi-layer bottles: the barrier properties of a bottle can be improved by using a multi-layer preform, i.e. a preform with a sandwich-like design - usually two layers of PET enclosing a thin layer of some other plastic. These preforms can readily be processed on SBM machines but are often excluded for reasons of increased cost or recycling implications.

Barrier coating: what appears to be a practical and economic is offered by barrier coating technologies. The finished PET bottle is given a very thin coating of a material which is relatively impermeable to gases. Both internal and external coating systems are available. The coating system can be set up as continuous process and be fully integrated into the bottle production line, immediately prior to filling. Alternatively barrier coating can be set up as an additional process step which can be switched on and off line depending on the requirements of the product being filled.

There are several advantages of using a secondary barrier coating:

1. The cost of applying a barrier coat to the bottle (including initial investment and operating costs) can add as little as 0.1 % to the cost of the initial preform.
2. The barrier coating is so thin that it is has an insignificant impact on the recyclability of the bottle.
3. It does not detract from the glass-like appearance of the bottle.