BOPET – stable plastic films for the packaging industry and industrial applications

The component BO stands for biaxially oriented. In films, this means that after the production process the films are stretched both in the longitudinal and in the transverse direction. The technical term for this process is “stretching”. BO-PET film is therefore a film that has been stretched in both directions. Through the stretching process, the molecular structure of the film changes: disordered polymers are aligned in the direction of tension, which leads to higher mechanical tensile strength. In addition, the desired film thickness can be achieved during the stretching process.

For stretching in the longitudinal direction, the film is guided over several heated rollers that rotate at increasing speeds. This causes the film to be stretched monoaxially. For biaxial stretching, the film is then guided through a transverse stretching unit. Many interconnected tenter clips grip the film at the sides. The chains initially run parallel into a preheating zone in which the film is brought to the appropriate stretching temperature. In the actual stretching zone, the distance between the two chains increases continuously until the film has been stretched to the desired stretching ratio. After the oven, the two chains run parallel again at the increased distance. In this cooling phase, the achieved properties of the film are fixed. From this point on, it can be referred to as BO-PET film.

The stretching process not only changes and improves the mechanical properties of BO-PET films such as tear resistance and impact resistance. Product properties such as diffusion behavior, cold resistance or optical properties can also be adapted to specific applications in this way. In combination with further processes such as the incorporation of fillers or additives, acrylic pre-treatment or the combination with other types of film, additional optimizations of the film properties can be achieved.

BO-PET films are extremely tough, tear-resistant, heat-resistant and are characterized by very good flatness. Therefore, they are used in a wide variety of areas in the printing industry: as mounting films, as carrier film in flexographic printing or as printable film in offset printing and screen printing. Inkjet film made of BO-PET film is used in inkjet printers as a positive film. Various industrial applications and many areas of medical technology rely on the durable BO-PET films. Laminations and labels are produced from them. Electrical insulation films made of BO-PET are installed in transformers, chokes or relay coils as well as in generators, magnets or in cable production. For viewing windows in packaging, especially in the food sector but also in other retail sectors such as tools, craftsmen’s supplies or pet food, the food-safe BO-PET films are used.

Since BO-PET films are produced from monomers derived from crude oil and the films are not compostable, the environmental balance does not look good at first glance. This changes when looking at the recycling efforts in the field of PET. Significant proportions of recycled material can now also be used in film production, which improves the energy balance and reduces water consumption in manufacturing. Research aimed at enzymatically depolymerizing used PET, meaning breaking it down into its basic components, is particularly interesting. This process could also increase the recycling rate for films and textiles. The recovered monomers would be fully suitable for the production of new BO-PET films.

rPET is PET that is obtained from recycling processes. The use of up to 50 percent rPET in the production of BO-PET films is now technically possible. Since PET has only a very low absorption of foreign substances, film with recycled content can even be used for food film. Further increasing the rPET content in the film production process is currently the goal of many different research approaches.

BOPET film is also a PET film, but it is biaxially oriented, which is what the component BO stands for. This means that the films are stretched in the machine direction and transverse to the machine direction after they have been produced as PET film. This process, also referred to as “stretching”, changes the molecular structure of the film. The initially disordered polymers are aligned through the stretching process. This leads to higher mechanical strength of the films. Other properties of the films such as transparency, cold resistance and others can also be influenced by the stretching process. In addition, during the stretching process the film thickness required for the respective application of the BOPET film is achieved.

In addition to tear resistance and good heat and cold resistance, the dimensional stability of BOPET films is an important property for many applications. Through the stretching process, in which the film is controlled and cooled under tension at the end, the dimensional properties of the films are also fixed. This is important for all printing applications and many other fields of application. Increased transparency compared to simple PET films is also an important property of BOPET film. Good insulation properties enable its use in the electrical and electronics industry as well as in cable production.

In the stretching process, the PET film is first heated and then guided over a series of rollers that rotate at increasingly higher speeds. This achieves stretching in the longitudinal direction. In the transverse stretching unit, the film is then gripped at the sides by interconnected tenter clips. In a preheating zone, the chains initially run parallel. Once the film has been reheated to the appropriate temperature, the distance between the chains is continuously increased to the desired level. The chains then run parallel again until the film has cooled and the properties achieved through stretching are fixed. The film can now be wound up as finished BOPET film.

The direct reintroduction of contaminated PET industrial waste and PET flakes from bottle recycling is being improved through the further development of extruders with high degassing and decontamination performance. With melt purification directly in the extruder, upstream cleaning steps are made unnecessary. This means that even heavily contaminated and/or moist residual materials can be processed without preparation. Solid contaminants are removed from the melt after this first step in a rotating melt filter. In this way, different PET qualities can be produced from which BOPET film can at least partially be manufactured again. An important field of research in PET recycling efforts is the enzymatic depolymerization of PET. If it succeeds in breaking down the collected PET into its basic components in this way, nothing stands in the way of processing it into high-quality new products – including BOPET film.