[elhenrico]

What kind of plastic is used?

[29athrowaway]

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

[ComputerGuru]

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

[timbit42]

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

Source: https://theconversation.com/milk-jugs-cartons-or-plastic-bag...

[userbinator]

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