The standout property of polyethylene is its low water vapour permeability. On the other hand, gas and aroma diffusion are relatively high. Due to its higher density, PE-HD is less permeable against water vapour, gases and odours than PE-LD. Polyethylene is not susceptible to water, acids and bases or conventional chemical solvents. In its uncoloured state it exhibits a milky haze, becoming transparent when it undergoes further processing into thin films. Polyethylene is ductile and elastic, cold resistant to -50°C and virtually unbreakable. At high temperatures, the threshold is 70 to 80°C for PE-LD and over 100°C for PE-HD due to its higher density. The welding range for films is 120 to 135°C for PE-LD and slightly higher for PE-HD. Generally however mechanical properties such as hardness at room temperature, ductility, as well as transparency and melting temperature range, are strongly dependent on the different production processes, catalysts and the monomers and copolymers used, which in turn determine the polyethylene chain length and branching level. Because the ethylene monomer is non-toxic, polyethylene represents no hazard to health. Additives can however be toxic. On incineration polyethylene breaks down completely into carbon dioxide, carbon monoxide and water. It is largely resistant to environmental conditions, becomes brittle from UV radiation and degrades very slowly as landfill. Unmixed PE can be reprocessed and reused easily. Reasons for its extensive use and the high proportion of polyethylene in plastic packaging are its favourable properties and the low cost of its raw materials.
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