Food packaging, smart phones and solar panels: Biaxially oriented polyester film is being used worldwide. But the material is under scrutiny from a sustainability perspective. Line improvements have the potential to make production more sustainable in many ways.
Films made of biaxially stretched PET (BOPET) have a high tensile strength as well as chemical, mechanical and thermal stability. According to Wood Mackenzie's "BOPET Film Global Supply Demand Report" published in May 2019, this specialty film has recorded annual growth rates of 6.7 % since 2013. An overall quantity of 5 million tonnes was produced in 2018 alone, which corresponds to a turnover of almost 11 billion euros. In the next five years, another 1.1 million tonnes are expected to be produced.
At Lindauer DORNIER GmbH, a contribution to higher energy efficiency is made by a patented air guidance system for transverse stretching (TDO – Transverse Direction Orienter) of the film. In the up to 150 m long and up to 26 m wide BOPET lines, the line component stretches the film, which is up to 240 °C hot during the production process, transversely to the running direction to a final film width of up to 10.6 m. The company claims that the innovative air flow system can reduce the consumption of thermal energy in this process step by up to 35 % compared to the latest technology by using the heated air several times in the various oven zones while maintaining the temperature requirements. Because the TDO clip systems, which stretch the film in the area of the air flow, can be overhauled many times in the course of their product life, even older systems can be adapted to modern environmental requirements.
Material can already be saved through recycling in the manufacturing process. "Film stretching lines from DORNIER recycle all production waste inline", says Bernhard Wandinger, Head of Product Line Specialty Machines. The separated film edge, for example, is fed back to extrusion without re-granulation. Since the regranulating system is no longer required, film producers save an entire machine part. In future, the energy input for inline recycling and recirculation is to be reduced further. Recycling management: the R-PET recyclate obtained from used PET bottles is also becoming increasingly important. "At the K in Düsseldorf, discussions with customers often focused on how a higher proportion of used material can be used in future with the same product quality," says Wandinger. A material input of 50 % post-consumer recyclate (PCR) and more is now technically possible, Wandinger assures. Due to the special properties of PET, such as its low contamination migration rate, it is even suitable for reuse as food film.
The recyclability of the BOPET film itself is also becoming more and more important. DORNIER is confident that polyester films can make a significant contribution to achieving the continuously increasing ambitious targets for the use of recycled material in packaging. The special characteristics of PET also influence this: because the film itself has a good intrinsic barrier against oxygen, Wandinger says that even with extremely thin coatings of aluminum oxide, the material can be quickly and cost-effectively brought to a level that meets even ultra high barrier requirements.
Another important factor for improving the ecological balance of BOPET film is its thickness, i.e. its weight. "Our BOPET film has the lowest grammage on the market," says Wandinger. And DORNIER intends to further exploit this parameter, the reduction of which has a positive effect on the ecological balance during film use, especially during transport. Although BOPET films have a low basis weight anyway due to their high stiffness, the special design of the extrusion and the temperature uniformity of the TDO allow low film thicknesses to be implemented particularly efficiently on DORNIER lines, according to Wandinger. For applications with high heat exposure, which have recently been in increasing demand – for example as separator film in batteries of electric cars or in photovoltaic storage systems – the further developed MD-Relax system for the transverse stretch offers higher stability: film shrinkage, especially MD shrinkage (machine direction), can be better adjusted. The resulting dimensional stability of the film in both the longitudinal (MDO) and transverse (TDO) directions, even at high process temperatures, ensures that the film does not shrink later when heat is applied. This is intended to prevent electric car batteries from failing due to shrinking separator film.