Let's ignore the confused grammar of those sentences for a moment and concentrate on the pronouns. If the subject is male ( i.e. a man, boy, or possibly a male animal), the possessive pronoun is HIS. If the subject is female ( ie a woman, girl or possibly a female animal), the possessive pronoun is HER. If the subject is an inanimate object or substance, or an animal whose gender is unknown or unimportant, the pronoun is ITS. If the subject is plural, the pronoun is always THEIR. The subjects of both your sentences are plural (bottles, plastics), so the pronoun should be 'their'.

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"

Ok. It's true, my English is very bad. :) I'm trying The Su.Ki.'s answer clears my doubt. Thank you.

Absolutely. Please can you give us some examples of CORRECT English to work with? Thank you.

Your examples are not correct English. Neither pronoun fits. Are you sure you have written them correctly?