If you have a list of items separated by commas, can it be used as an appositive if you separate it from the rest of the sentence by yet another by comma?
Here's an example:
The factory produces various types of ropes , such as cotton, acrylic, nylon, etc. , from garment waste.
In the above sentence, I use two commas (in bold here) as appositive commas, whereas the other commas are used to separate the items on the list. Is this correct? If not, why not?
Yes, it's correct.
However, some people could think it was confusing. Stylistically, you can always use different punctuation to indicate the appositive if the commas concern you:
The factory processes different types of ropes (such as cotton, acrylic, nylon, etc.) from garment waste.
The factory processes different types of ropes—such as cotton, acrylic, nylon, and so on—from garment waste.
But there's nothing grammatically wrong with the sentence as you had it.
(Note that in my second revision, I replaced etc. with and so on. This is because some style guides, such as The Chicago Manual of Style, recommend only using etc. inside parentheses.)
<i>,</i>instead, which is another way of doing it, but the formatting on just the commas isn't visible. Perhaps you could try putting them inside square brackets.