A process for the commercial production of transparent aluminium oxide coated barrier polyester film, price competitive with PVdC coated polyester film, has been developed using the low cost technology of reactive evaporation of aluminium in oxygen. The oxide coated product has significantly better oxygen and moisture barrier in flat sheet form than PvdC coated film, but like other oxide coated barrier films, is prone to barrier loss during the packing operation caused by cracking of the oxide due to substrate elongation. Robustness can be improved by reducing oxide coat weight and the use of suitable plasma enhancement, the product then retaining oxygen barrier up to an elongation of 4.0 - 4.5% and moisture barrier up to an elongation of 2.0 - 2.5%. Increasing Al : O ratio produces a nanocomposite of aluminium oxide and finely divided aluminium with improved resistance to barrier loss, but lower transparency. The cracked oxide layer shows a significant recovery in oxygen barrier (but not moisture barrier) if exposed to high humidity, due to swelling of the oxide and reduction in cross-sectional area of the cracks. Barrier to oxygen is significantly increased, and to moisture slightly increased, by laminating the oxide coated film using polyurethane adhesives, final barrier being dependent on the adhesive used. Laminates have been developed and commercialised for lidding and vacuum pouches for modified atmosphere packaging of fresh foods, and further penetration of the premium end of this market is anticipated. Laminates have also been successfully developed for vertical form-fill-seal packaging of oxygen sensitive dry foods, vacuum packaging of coffee, and bag-in-box liquid packaging, but commercialisation in these markets will probably depend on the balance between environmental and cost market drivers. Novel products for metal detector transparent packing and microwave susceptors have also been developed.
Research by Camvac (Europe) Ltd indicates that by reacting aluminium vapour with oxygen in a conventional film vacuum metalliser, it is possible to apply transparent barrier coatings of aluminium oxide to plastic films at high rate. Process costs are potentially an order of magnitude less than competitive systems developed in Japan and the USA for coating films with other transparent barrier materials such as SiOx, and subject to development of suitable substrates, should be viable for production of filmfor packaging.
The project comprises 4 tasks.
1.Background investigation of the interdependence between process conditions and raw materials, composition of the aluminium oxide coating conversion technologies for the films and final product performance properties such as barrier, adhesion and flexibility, together with fundamental research into the barrier and adhesion mechanisms.
2.Development of control techniques for the process and products.
3.Development of polyester and polypropylene film substrates to give the combination of cost, adhesion, moisture resistance and handling properties required for packaging applications.