We encounter polyethylene every day, it accounts for more than a third of the world's total consumption of plastics. It can be used in a variety of ways: from wafer-thin films to sturdy barrels or bottles.
From naphtha to ethylene
Crude petrol – also known as naphtha – is extracted from crude oil. Gaseous ethylene is produced by strongly heating ("cracking") the naphtha.
From ethylene to polyethylene
In the next step, the gaseous ethylene is fed into a reactor. At a fixed temperature and under varying pressure depending on the manufacturing process, white, powdery polyethylene is produced. Solid polyethylene is formed by melting and cooling. Crushed into granules, it becomes the basic material for the manufacture of a wide variety of products.
In material recycling, used plastics are reprocessed into new raw materials for production. In the process, the chemical structure of the material remains unchanged. The resulting granulate is called regranulate. This regranulate is obtained in several steps:
In the first step, sorted plastic packaging is shredded. The ground material is then washed and foreign matter is sorted out in water basins according to the principle of float/sink separation. As polyethylene has a lower density than water and floats on top, it can be easily separated from other types of plastic. The now pure ground material is washed, dried and then granulated in an extruder. For this purpose, the plastic is first melted in a screw shaft. The resulting moulding compound is pressed through a perforated plate – as in a meat grinder. The emerging strands are cooled under water and cut. This is how the regranulate is produced.
The extruder is again used for the production of plastic film. First, the (re-)granulate is fed through the extruder screw and heated externally thus creating a moulding material. A tool placed before the screw cylinder then forms the grouted mass into a film. This takes tubular form and is ejected balloon-like via a blower. The resulting plastic-film tube is then cooled, laid flat, and finally wound up onto a roll.
A special finishing machine turns the "endless" roll of plastic film into garbage bags. Depending on the type of end product, the plastic film tubing is folded (side gusset, star bottom etc.) before a combined welding and perforation machine adds the bottom and tear-off seams. Finally, a set number of connected bags or liners is wound up onto a roll.
