Paper poly bag construction is by bonding kraft paper to the inner polyethylene film. The Kraft paper gives the bag persistence and hardness, and is able to maintain shape and also be handled. In the meantime, the poly layer makes the bag water, oil, and grease-resistant, which is why the things you carry inside it can remain intact when you are transporting them or even when you store them. This two-ply construction implies that such bags are meant to pack products that need protection and appearance. Manufacturers usually decide on either bleached or unbleached kraft in order to obtain the required finish.