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From the abstract

> The input polymer is polypropylene or polystyrene, and can include discarded plastic containers of these materials, ...

Marked and respectively.

Edit: U+2677 and and U+2678. Obviously no one would ever want to write about plastics on HN, so it's totally OK to strip those symbols.

Before you spend time looking this up, this is it:

U+2677 is the recycling symbol type 5 (polypropylene/"PP") and U+2678 is recycling material type 6 (polystyrene/"PS").