Film Dictionary

PET plastic

PET – The Most Widely Produced Plastic Worldwide

The abbreviation PET stands for polyethylene terephthalate, a thermoplastic material belonging to the polyester family. Most people are familiar with the term through PET bottles, since a considerable portion of global PET production is used to manufacture hollow containers such as bottles and other vessels.

Originally, the material was developed in the 1940s for the production of textile fibers intended to replace silk fibers. Even today, PET fibers are still widely used in the textile industry, most commonly in functional clothing.

Since the 1960s, this plastic has also been used for the production of plastic films.

The Production of PET

The raw materials used to produce PET are the petroleum-based chemicals p-dimethylbenzene and ethylene. From the first, dimethyl terephthalate is produced, and from the second, ethylene glycol is obtained.

There are two different processes for further processing into PET. One method is polycondensation using terephthalic acid and diethylene glycol, during which water is released as a by-product.

The second and most commonly used process is the transesterification of dimethyl terephthalate with ethylene glycol. In this reaction, methanol is split off. The desired reaction takes place under the input of heat. Temperatures between 260 and 270 °C are required to initiate the process.

In subsequent steps, any remaining water content is reduced to a residual value below 0.01 percent by several hours of additional heating. This improves the mechanical properties and also prevents the hydrolytic degradation of the finished product.

PET is often chemically modified in order to influence its crystallization behavior and thereby change its melting point.

New Developments in the Production of PET

Various processes have been developed that make it possible to modify PET production in terms of melting point and further processability.

Since 2010, PET has also been available on the market in which the ethylene glycol used is derived from renewable resources rather than from petrochemicals.

However, the production of the required terephthalic acid from renewable raw materials has not yet become established. Therefore, only partially bio-based PET is currently available on the market.

The Use of PET

In addition to the already mentioned use of PET bottles produced by injection molding or stretch blow molding, there are many other fields of application for PET materials.

The largest share of PET consumption in terms of volume still belongs to the textile industry. The wrinkle-resistant and tear-resistant fabrics made from PET fibers are now widely used in sportswear because of their low water absorption and fast drying properties.

In the field of films, PET offers many areas of application. Packaging, especially for food products, is often made using PET films, and cardboard packaging is frequently equipped with transparent viewing windows made of PET.

Carrier films, cable insulation, as well as the insulation of capacitors and other electronic components are also produced using PET films.

The Manufacturing Process of Plastic Films Made from PET

PET films are usually produced by flat film extrusion using a wide-slot die. The molten film is pressed onto the casting roll by electrostatic pinning. In this process, the air beneath the PET film is displaced and the film is cooled until it reaches a temperature below the glass transition temperature of PET, i.e. approximately 65 to 80 °C.

This ensures uniform and rapid cooling, resulting in a fine-crystalline pre-film structure. This step is often followed by a stretching process, in which the film is first drawn in the longitudinal direction and then in the transverse direction. This increases the crystallinity of the film and creates biaxial orientation.

After this processing step, the film is referred to as BO-PET or BOPET film. In particular, technical films and films used in the food sector are produced from BOPET.

By reheating the film, further crystallization and fixation of the film structure are achieved. The processing of PET films into BOPET films leads to improved mechanical properties: high tear resistance, impact resistance, and abrasion resistance are the result, along with increased toughness of the film.

Even during long-term use, the treated film can withstand temperatures from –70 °C up to +150 °C.

The Recycling of PET

Through pure-grade recycling, food-grade PET can be produced again from shredded PET bottles. PET bottles containing a proportion of recycled material can already be manufactured. However, bottles made entirely from recycled material are not yet possible, although research is being carried out on suitable processes.

A large portion of recycled PET is processed into textile fibers. About one quarter of the recycled material is used in film production.

In order to also integrate used textiles made from polyester fibers into the recycling process, methods are currently being tested that perform enzymatic depolymerization of PET products. The monomers obtained in this way can be processed into completely new, high-quality products.

Many different research approaches are currently being pursued to ensure a sustainable future development in the production of PET materials.