Permeability: what passes through a plastic wall
A closed bottle looks tight. At the molecular scale it never quite is: a little oxygen gets in, a little water vapour passes. The whole question is how much, and whether the product can take it.
By Tecmom editorial team · 2 min read
A wall that lets things through
A plastic has no holes, but its molecular chains leave gaps. A gas can dissolve on one side, diffuse through the thickness and come out on the other. This is called permeation, and the amount that crosses the wall in a given time is its permeability.
Two measurements come up most often: the oxygen transmission rate, OTR, and the water vapour transmission rate, WVTR or MVTR. They are expressed as a quantity of gas per area of wall per day.
How it is measured
The methods are standardised. For water vapour, ASTM F1249 for instance measures transmission through films and sheets, single or multilayer, up to a few millimetres thick. For oxygen, other ASTM methods measure the flow through a film or a whole pack.
Results depend heavily on conditions: temperature, humidity on each side, thickness. A value only means something with its test conditions.
Each material, its own barrier
The usual plastics do not protect against the same things. Polyethylenes slow water vapour fairly well, oxygen much less; PET slows oxygen better. When both are needed, materials are combined in layers, for instance with a thin EVOH layer at the heart of the wall. See our advice page HDPE, PET, PP and multilayer tubes.
Thickness counts too: for the same material, a wall half as thick lets more through.
What it decides
Permeability, together with closure tightness, sets the shelf life of a sensitive product. A tablet that fears moisture, an oil that oxidises, an aroma that escapes: each has its limit. For a medicine, stability studies run in the marketed pack show whether the barrier is enough. When it is not, a desiccant is added. See silica gel.
No plastic is a wall. But a permeability that is known, measured and chosen becomes a parameter like any other: the one that says how long a product will stay what it was at filling.
Glossary
- Permeation
- The passage of a gas through a wall by dissolution and diffusion.
- OTR
- Oxygen transmission rate through a wall.
- WVTR
- Water vapour transmission rate through a wall.
- EVOH
- A copolymer used as a thin oxygen barrier layer.