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What kind of plastic is used?

High-density polyethylene (aka HDPE or PEHD). Same material used in milk jugs.

I'm pretty sure it's LDPE --- HDPE is not transparent and glossy like that.

But rarely recyclable, not due to chemical composition but because the bags get caught in the machinery.

Milk bags are still more environmentally friendly than cartons or jugs, even if the bags are landfilled and the cartons and jugs are recycled.