In most plastics processing plants, raw material dryers are standard equipment, except where only polyolefins such as polyethylene and polypropylene are processed. These polymers do not require drying as they are non-hygroscopic and non-polar. Water is a highly polar substance, so polymers that have a certain degree of polarity can absorb moisture from the atmosphere. The amount of moisture that a given polymer can absorb depends on the chemical composition of the polymer and the moisture content of the surrounding atmosphere.