UV stabilisers: protecting a can that lives outdoors
A can of farm product spends the summer in the yard, in the sun. By autumn, its handle snaps at the first knock. The plastic did not age on its own: ultraviolet light attacked it, and nothing protected it.
By Tecmom editorial team · 2 min read
What the sun does to plastic
The sun's ultraviolet rays carry enough energy to create free radicals on a polymer's chains. These radicals react with oxygen in the air, in a chain reaction: this is photo-oxidation. The chains break, the surface whitens or chalks, the material yellows, then turns brittle.
Polyolefins, polyethylene and polypropylene, are among the most susceptible polymers. Yet they are precisely the materials of cans and jerrycans.
Three ways to protect
A UV stabiliser is added to the material, often as a masterbatch. There are several families. UV absorbers capture the radiation and dissipate it as heat. HALS, hindered amine light stabilisers, do not absorb UV: they continuously trap the radicals formed, and are highly effective in polyolefins.
Some pigments act as a screen. Carbon black is the most effective, but it does not suit a can meant to be white or translucent.
Protecting the container, not the content
This need should not be confused with that of an amber bottle. Amber filters light to protect a light-sensitive content. A UV stabiliser protects the wall itself, so that it keeps its mechanical strength.
The two can be combined, but they answer two different questions: is the product sensitive to light, and will the pack stay in the sun?
An additive like any other
A UV stabiliser is a component of the material. In contact with a medicine or a food, it must be suitable for that use and declared; see food contact and the pharmacopoeia. Its effectiveness is checked by accelerated weathering tests under lamps, on the real material and wall thickness.
On the farm, packaging often lives outdoors, in rain and sun; see veterinary packaging on the farm.
A can that breaks in autumn is a defect from the summer before. It is prevented on the day the material is chosen.
Glossary
- Photo-oxidation
- The degradation of a polymer by light and oxygen.
- UV stabiliser
- An additive that protects a plastic from ultraviolet light.
- HALS
- Hindered amine light stabilisers, which trap free radicals.
- Polyolefins
- The polymer family that includes polyethylene and polypropylene.