Corrugated PP sheet is a popular sign material: cheap, light, printable, weatherproof in the sense that it does not absorb water. Left outdoors it still degrades, and the degradation is almost entirely down to ultraviolet exposure rather than to rain or cold.
What UV does to polypropylene
UV radiation breaks polymer chains at the surface. The visible results are chalking – a white powdery residue that rubs off – then colour fading, then surface cracking, then loss of strength. Once it reaches the cracking stage the sheet is finished.
The important detail is that this is a surface phenomenon for quite a long time. The sheet can look bad while still being structurally sound, which is why chalked signs stay up for years rather than falling apart.
What changes the lifespan
- UV stabiliser package. This is the big one. Unstabilised PP can start chalking within a season of continuous exposure; properly stabilised sheet can last several years. It is a specification item, not a premium grade.
- Pigment. Some colours hold up better than others. Darker colours absorb more heat and run hotter, which accelerates other degradation. White and light colours generally last better.
- Orientation and latitude. A sign facing north in a temperate climate and the same sign facing south in a subtropical one are not comparable.
- Whether it is printed and with what. The print often fails before the sheet does.
- Mechanical stress. A sign that flexes in the wind fails at the fixings long before the material gives up.
Realistic expectations
For a stabilised sheet in a temperate climate, a few years of acceptable appearance is a reasonable expectation, and longer where appearance is not critical. For an unstabilised sheet used outdoors, expect visible chalking within a year. Anyone quoting a number without asking where the sign is going is guessing.
The print usually goes first
Inks and laminates have their own UV resistance, and it is often worse than the substrate. A common failure is a sheet that is still sound carrying a print that has faded to nothing. If the sign has to last, specify UV-stable inks and consider an overlaminate – and ask for the expected life of the print separately from the sheet.
Fixings and flexing
A sign that flutters in the wind works at the fixing holes and tears out. This is usually the actual end of life for outdoor signs. Use more fixings than seems necessary, spread the load with washers or a backing plate, and keep fixings away from the edges. A framed sign lasts far longer than a screwed one for this reason alone.
What to specify
- UV stabilised – say it explicitly, and ask for it on the order confirmation
- Expected outdoor life and location, so the stabiliser package can be matched
- Colour with the above in mind
- Print method and ink rated for outdoor use
- Thickness sufficient that the sign does not flex
Cleaning and what not to use
Outdoor signs get cleaned, and the cleaning agent can do more damage than a season of weather. Solvent-based cleaners attack the print and can craze the sheet surface. Abrasive pads scratch it, and a scratched surface holds dirt and looks worse than before.
Soapy water and a soft cloth is the right answer for almost everything. Where there is graffiti or stubborn marks, test the cleaner on an offcut first – and expect that removing the mark may remove the print underneath it.
It is also worth noting that a chalked surface cannot be restored. Once UV degradation has started, cleaning will not bring the colour back, and no coating applied over a chalked surface will adhere properly.
What we supply
We supply corrugated PP sheet with a UV stabiliser package for outdoor signage. Tell us the location and how long it needs to look acceptable, and we will tell you what to expect – including where the honest answer is that a sign needs replacing on a cycle rather than specified to last indefinitely.