Children are surrounded by plastic; at home, school, their packed lunch...
How much thought do they give to this excess - not a lot I imagine!
But both the production of plastics - the petrochemical industry - and the relatively recent business of recycling/disposal of plastic is worth talking about with them.
Working with plastics is a great way to open up discussion - so look at Plastics Playtime for some great activities.
These activities are aimed at 8-10 year olds but can easily be adapted for other primary ages.