Brittle Greenhouse Polythene

greenhouse polythene cracking

When I built my dome greenhouse earlier this year I took my time reading about coverings, so how did I end up with brittle greenhouse polythene? The greenhouse covering looked rather good when it was first complete.

rear of the greenhouse

I had bought Allplas 500 polythene, recommended for small greenhouses and UV resistant.

Clear Polythene sheeting 2m wide with many lengths to choose from ideal for many outdoor applications, UV Stabilised and should last 5 seasons in the UK

This UVI Sheeting is 500 gauge and has great properties to help with the longevity of the product.

allplas.co.uk

So what had gone wrong for this polythene sheeting to fail after just one summer?

brittle polythene greenhouse covering

The polythene covering first started failing on the dome, cracking was most pronounced where the covering was on top of beams.

What makes polythene go brittle?

  • sunlight – carbon black is usually used as a UV stabilizer.
  • extreme temperatures – chemical failure occurs as a result of exposure to extreme temperatures. Interesting as this plastic has gone brittle over the summer and before we have got to the extreme colds of winter.

It might be the wood treatment that reacted with the polythene to speed the process up. However, the plastic around the back of the greenhouse has remained flexible even where it overlaps the wooden frame. Also, the lower frame was treated last and so I would expect the polythene covering this area to have failed first. But it was the dome that failed first.

My conclusion is that the Allplas UV polythene is just not UV protected and certainly not able to withstand 5 years, it barely lasted 2 seasons. This is a very brittle greenhouse polythene.

Just a quick look at altenatives:

  1. Copolymer Plastic – a grade above polythene, the general view is this can last up to 3 years. Makes me wonder if the polythene I bought ever had a chance of lasting 5 years as stated.
  2. Polyvinyl Plastic – a stronger and more durable plastic than either polythene or copolymer. It is though more expensive.
  3. Polycarbonate Plastic – the most durable plastic and you can buy it in sheets sheets of various thickness and as multi-wall sheets. The multi-wall sheets can increase the thermal properties significantly and extend your growing season. Not so easy though to fit to a dome.
About Nigel 338 Articles
I've been making models since I was around 7 years old and using a lathe from the age of 11, a self taught engineer with a passion for making model engines.

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