What are the differences in gas barrier properties between plastic types?

The gas barrier properties of plastics are critical for applications like food packaging, where preventing oxygen and moisture ingress is essential for preserving product quality and shelf life. Different plastic types exhibit vastly different permeability characteristics due to their unique molecular structures and densities.

Ethylene Vinyl Alcohol (EVOH) offers an exceptionally high barrier to oxygen, nitrogen, and carbon dioxide, making it the gold standard for sensitive food packaging, though it is highly sensitive to moisture. Polyethylene Terephthalate (PET) provides a good balance of clarity and moderate gas barrier properties, ideal for beverage bottles. High-Density Polyethylene (HDPE) has low permeability to water vapor but is relatively porous to gases like oxygen and carbon dioxide. Polypropylene (PP) shows moderate barrier properties, better than HDPE but inferior to PET. Low-Density Polyethylene (LDPE) has poor gas barrier properties but excellent resistance to water vapor, making it suitable for moisture-sensitive products.

The key metric for comparison is the Oxygen Transmission Rate (OTR), measured in cm³/m²/day. EVOH can have an OTR as low as 0.1, while LDPE can be as high as 8000. This immense variation dictates material selection, often leading to the use of multi-layer co-extruded films that combine the strengths of different plastics to achieve the desired protective barrier.