Polyethylene has already been discovered in the year 1898. At that time the production process was very complex. The first industrial production was established 1940. Under a high pressure (approx. 1400 bar) and with a temperature of 17° C the production of polyethylene succeded. 1953 finally an economical manufacturing method has been found. The chemist could achieve the manufacturing process with a normal pressure.
PE-HD (HDPE): Light branched polymer chains with a high density between 0,94 g/cm³ and 0,97 g/cm³, ("HD" stands for high density").
PE-LD (LDPE): Heavy branched Polymer chains with a low density between 0,915 g/cm³ and 0,935 g/cm³, ("LD" stands for "low density").
Nowadays water pipelines preferably are produced with PE-Hoses because they only emit small up to no particles to the water line. Additionally they have a smoother surface than metal tubes.
Good to know is that for the gas supplying pipelines today HD-PE hoses are preferably used and not the old grey iron pipes. Cable tubes and packaging as well as shrink foils are manufactured out of the same basic material. Foils for example on asparagus fields are made of PE-LLD which is also a polyethelene with a low density and high flexibility.