Both LDPE and HDPE are polyethylene types. The difference between the two types is that LDPE is made under high pressure, in contrast to HDPE. LDPE has many more side branches in its molecular structure, which gives LDPE a good tear strength in the transverse direction. The welding temperature of LDPE is somewhat lower than that of HDPE (around 10 degrees).
Because the molecular structure has more side branches, the water vapour permeability of LDPE is less good than that of HDPE. This is about a factor of 2.5.
The advantage of the molecular structure with side branches is that LDPE is therefore suitable for shrink films. When LDPE films are reheated, the molecules want to return to their original state, which means that in addition to longitudinal shrinkage, shrinkage in the width direction can also occur. HDPE only shrinks in the longitudinal direction.
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