THE 4 STAGES OF MAKING A POLYTHENE BAG

Creating bags can be split into 4 stages - (1) raw material (2) film (3) printing and (4) final product. Please use the glossary table to look up any words or phrases contained within this text. A list of technical abbreviations can be found here.

(1) RAW MATERIAL

Polyethylene (PE) - the correct name for 'polythene', which is the trade name given to the material first commercially produced by ICI in the 1930s, is a member of a family of materials called POLYMERS (from the Greek meaning many parts). Unlike most naturally occurring substances, these have long chains of repeating molecular units (known as monomers), which give them strength and other properties.

The main ingredient in most plastics is polyolefins hydrocarbons, mainly crude oil or natural gas, which have one or more double bonds (ethylene [CH2=CH2] being the one). To simplify a complicated process: crude oil or natural gas is CRACKED into several useful components (including petrol and ethylene), ethlene ( a monomer) is then POLYMERISED using heat, pressure and catalysts, into polyethylene in large chemical plants, using a variety of processes developed by individuals and corporations over the years.

POLYMERISATION

The original polyethylene - low density PE (LDPE) is produced by a high pressure process, the newer types of polyethylene - starting with high density PE (HDPE) in the 1950s, are produced with lower pressures. The difference in density between the two is not great - LDPE having a density of approx. 0.92 (8% less than water) and HDPE 0.95 (5% less than water).

To make it suitable for processing into something useful, the molten material is forced (extruded) through a plate containing many small holes (the die), so that it forms continuous strands like spaghetti, which are then cooled and chopped into small pieces, known as granules, which are then conveyed into silos for subsequent transport to the processing factories in bulk tankers or sacks.

raw material | film | printing | final product