PET Films (Polyethylene terephthalate) The petrochemicals p-dimethyl benzene and ethylene are the raw materials. From these, terephthalic acid dimethylester and ethylene glycol are produced. There are two methods of manufacturing PET. PET is obtained by polycondensation from terephthalic acid and diethylene glycol by removing water. The second and most frequently used method involves transesterification of terephthalic acid dimethylester with ethylene glycol by removing methanol. The reaction takes place at temperatures between 260 and 270°C. In a further step, the substance is reheated for several hours until the remaining water is reduced to less than 0.01%. This prevents hydrolytic degradation of the end products while significantly improving mechanical properties thanks to further polycondensation at this stage. PET is often chemically modified to affect crystallization and thereby the melting point.
Different types of PET with different melting points and processibility can be obtained by using specific methods.
There are two major groups. The A-PET (amorph) group exhibits low cristallinity, has a rather low melting point and can be processed across a wide temperature range without the risk of melt fracture, i.e. breakage when forming under heat. A-PET is primarily used for transparent, deep drawn trays, containers or bottles as packaging for foodstuffs, cosmetics or chemical/technical products.
By contrast, the C-PET group exhibits high crystallinity and has a high melting point above 280°C, offering a high gloss finish and good transparency. C-PET films are suitable for hot forming containers, dishes or trays that feature dual ovenability, i.e. they are heatproof for use in fan ovens and microwaves. Polyester films are generally produced via flat film extrusion. After extrusion they are cooled rapidly to create a fine crystalline polymer structure. Further processing is then often performed using the expansion method to form oriented polyester films known as BOPET films (biaxially oriented). This significantly increases the film’s crystallinity. Further crystallization is achieved by reheating, which also fixes the film structure. This process develops mechanical properties well, with the result that oriented polyester films are exceptionally tear- and shockproof and rub-resistant and are highly ductile. Optical properties are adequate, while thermal properties are outstanding in terms of resistance to cold and heat (around -70°C to 150°C), even with extended use. Due to their good temperature stability in particular, polyester films are increasingly used as packaging for use in microwaves. Polyester films repel water vapour, oxygen and aroma substances well, although not sufficiently well for use on their own as barrier film. They contain no plasticizers and are physiologically harmless. In addition, they have no taste or odour and are resistant to most organic chemicals, solvents, fats and oils. The properties of polyester films mean they are ideally suited to many applications and can be adapted for specific purposes by modifying the base material (polyethylene terephthalate) and the production method. Sample applications include photographic film, magnetic tape films for magnetic recording, microfilms and reprographics films, electrical insulation films and condenser films in electrical engineering, plus a wide range of uses in packaging. Due to their good mechanical properties and high resistance to temperature, polyester films are frequently used here in a compound with PE. Additional barriers such as EVOH (ethylene/vinyl alcohol copolymer) can be incorporated into the compound. Important applications for polyester films or compounds include boil-in-the-bag packaging, meal trays featuring dual ovenability, household baking foils or foils for medical packaging. A further advantage of polyester films is that they are highly suitable for metallization and printing. The most prominent features of PET are its excellent resistance to temperature and outstanding mechanical properties. Optical properties are satisfactory and PET is also a good barrier and repels gases, water vapour and aromas. However, the values are not sufficient for use on its own as barrier film. It is suitable for compounds with other plastics, particularly PE, and integrated barriers made of gases or plastics such as EVOH. It is also easy to metallize as an additional coating. What is more, the material is odour and aroma neutral, resistant to organic chemicals, solvents, fats and oils, as well as physiologically harmless. In terms of versatility, PET outperforms all other plastics.
