Since PET alone, in its pure form, is relatively permeable to gases, plain PET bottles are unsuitable for products susceptible to quality problems that may arise from gases entering or leaving the contents of a bottle (such as the oxidative deterioration of beer resulting from the ingress of atmospheric oxygen). To solve this problem, many different methods for combining PET with impermeable materials (to form a physical barrier against gas diffusion) and/or substances intended to trap or block specific gases (e.g. oxygen scavengers) have been developed. Most such substances have been applied in separate layers, either as external or internal coatings on plain PET bottles or as components of laminated composite bottles in which the gas barrier layers are sandwiched between layers of PET. However, it has also been proposed to blend certain substances with PET to form more or less homogeneous mixtures combining most of the properties of PET with the relative impermeability and/or oxygen scavenging capability of the other ingredients. Trials are described in which the suitability of "PolyShield", a blend of PET with a form of nylon called MXD6, as a material for beer bottles was evaluated by comparing the stability of beer stored in bottles made of 3 versions of "PolyShield" (with MXD6 contents of 2, 5 and 7% respectively) with that of identical beer in bottles made of various forms of coated and laminated PET as well as plain PET and glass. Chemical and sensory analyses showed that beer in the "PolyShield" bottles with 5 or 7% MXD6 retained its fresh flavour for at least 7 months with no sign of deterioration, which was better than most of the other PET bottle types and equal to the best of them (a laminated type). The "PolyShield" material is therefore concluded to be among the most suitable.
