Degradable plastic

[modeller]

Why is it that all white plastic flower pots crumble to bits after a couple of years whilst the brown and black ones ones seem to last for ever.

On the other hand white plastic window frames seem to be OK.

[wildwood]

Oh, I don't think the windows are made of just plastic. I am not sure but they are probably made of a PVC material.

Talking about degradable plastic.... A couple of years I heard/read that there was a material that was made of rice paper and was as strong as plastic shopping bags. what ever happened to that?

[modeller]

wildwood PVC is a plastic , its polyvinyl.

What I find puzzling is why white flower pots, seed trays, garden ornaments and these imitation Greek urns all break down in sunlight but the dark colours don't. You would have thought that the white objects would have reflected the sunlight and been less affected by UV light.
I believe most garden plastic is made from polypropylene but only the white ones seem to break down in the short term.
I've looked on Google but cant find the answer.

[heathfield]

PVC in itself is a white brittle plastic. You wouldn't believe the number of additives it needs before it becomes suitable for the many purposes it's used for, like underground drains, windows and doors, cable sheathing. waterproof clothing...the list goes on and on. But additives cost money, so manufacturers will look at the durability needed for their finished product before including them. Flowerpots would be looked upon as being fairly expendable. Window frames have to me made to last.

[jomifl]

Broadly speaking the dark pigments in plastics absorb the damaging light so that it does not penetrate very far and only degrades the surface. Some 'white' plastics do not have any pigments at all or white pigments which do not absorb the light so well as the dark ones. With these the light can penetrate deeper and degrade more of the plastic.