A last note about "change." The potential confusion is that an unwary non-native speaker of English might think it refers to the plastic, and not to the subject of the sentence, "it." The potential confusion exists because "change" might be thought to refer to a "change of state" in the substance exalted noun, "plastics," which is not the subject of the sentence, and not to the lowly pronoun, "it", which is.

A further potential for confusion is that since the phrase "the hardness of plastic" refers to a physical quality of plastic, the temptation to attach the verb "change" to it, and not to the subject, "it," is reinforced.

Whew. I've been trying to make this clearer, but I'm afraid I'm just too tired.

I think the sentences were well chosen to demonstrate proficiency in dealing with specific difficulties a translator is likely to encounter in everyday English writing. It ain't easy. Fortunately, in everyday circumstances, there are usually people you can ask.