Polyethylene Polyethylene, a thermoplastic, was discovered about 50 years ago. It has a good resistance to moisture and impact over wide temperature ranges. Polyethylene is noted for its toughness and chemical resistance. Although Polyethylene is the least flexible of liner material, it is very popular among pondkeepers. Just Liners offers 4 types of polyethylene pond liners, JPL16, JPL20, JPL24, JPL30 and Permalon.