Polyethylene : general
By refining crude oil or gas, ethylene is obtained. This ethylene gas can be polymerised to plastic. Polymerisation is the appending of ethylene molecules into long chains. After polymerisation we obtain the product polyethylene, that is supplied in granules to plastic converting companies. This granule is then melt again in order to give it it's final shape. Whitin the polyethylene family there exists a lot of variations like LDPE, MDPE, HDPE, LLDPE, mLLDPE,...
LDPE stand for low density polyethylene and has a density of 921 kg/m³. It is used for strong and flexible consumption goods such as screwcaps and lids. In the extrusion however the most popular application is the production of highly transparant, clear and glossy films.
HDPE stands for high density polyethylene and has a density of 950 kg/m³. It is used for strong and rigid products like clothes-pins and plastic bottles. In the film extrusion HDPE is mainly used for the production of very thin and rigid films that are very crackling. HDPE film is not transparent but semi-transparant just like tracing paper.
Polyethylene film is produced by melting granulates of polyethylene at about 200°C and pressing them together with compressed air through a circle matrix. The material then rises up like a balloon of plastic film. After cooling, rollers squeeze the film in a dubble layer, after which it is rolled up and ready for further processing.
To add extra properties to the film, we can add additives to the material while extruding. The mostly used properties are : colouring of films, removing static electricity, stabilising against UV-radiation, regulating the coëfficient of friction, obtaining fire safety norms, anti-fog, blocking of UV radiation,....
Polyethylene is a very friendly material for the environment :
- It is an economical material: for the world production, only 1% of the total production of natural oil and natural gas is used.
- The production is clean and efficient : the expulsion of dangerous substances is minimal.
- It is extremely suitable for re-use : it can be melted and re-used indefinitely.
- When it can not be re-used, it is an eminent fuel for energy provision.