What are the differences in vapor transmission rates between plastic types?

The rate at which water vapor passes through plastic materials - known as water vapor transmission rate (WVTR) - varies significantly across polymer types due to differences in their molecular structure and density. This characteristic is crucial for applications requiring moisture barrier properties, particularly in food packaging, pharmaceutical packaging, and construction materials.

High-barrier plastics like polyvinyl chloride (PVC) and polyethylene terephthalate (PET) exhibit low WVTR due to their tightly packed molecular chains and crystalline structures. PET, for instance, is widely used in beverage bottles because it effectively contains carbonation and prevents moisture ingress. Conversely, polyethylenes exhibit higher transmission rates. Low-density polyethylene (LDPE) has a relatively high WVTR because of its branched molecular structure that creates more free volume, making it suitable for breathable packaging applications. High-density polyethylene (HDPE), with its more linear structure and higher crystallinity, provides a better moisture barrier than LDPE but is still generally less effective than PET or PVC.

Polypropylene (PP) falls somewhere in the middle, offering moderate barrier properties. The variation in WVTR is not just material-specific but is also influenced by factors such as material thickness, temperature, and relative humidity. Understanding these differences is essential for selecting the right plastic for specific environmental protection needs, ensuring product integrity, and extending shelf life.