I hope some answers your question. Your suggestion sounds valid.

Plastics are materials which can be very contaminated in THEIR production. The word 'their' refer to plastics', which is plural in this case. Had it been singular, we could say: "A (isang) plastic contains many contaminants in its production". The 'its' is not plural, even though it ends in an 's'. The word THEIR can refer to gender neutral objects, even though THEIR can be applied to gender specific plural ownerships: his bottle and her bottle -> their bottles. The word THEIR, however, is not in the possessive case, here. The word "their" applies to both examples you have provided. Grammatically, you can't say "very contaminants" or "less contaminants". Instead, you could say "more contaminated" or less contaminated". "Contaminants" is a noun. What is called for here is an adjective; hence, "contaminated", a describing word, and NOT an 'object' or noun, which is the case for the word "contaminants"