Это водостойкие и прочные полимеры, liner тут в смысле футеровка/облицовка.
Вот цитата из http://www.iea-shc.org/data/sites/1/publications/Task39-C7.3...
Common polymeric liner materials are semicrystalline polyolefin-based polyethylene or polypropylene grades developed for applications with water contact (piping, tunnel and landfill construction, etc.). The relevant material classes are PE-RT for polyethylene-based liners and PP-R and PP-RCT for polypropylene based liners. These polymeric materials exhibit a ductile material behavior in the service temperature range between about 40°C to 95°C, they are waterproof, resistant to hydrolysis and weldable. For applications which require a low water vapour transmissivity, polyolefin-based lining materials are available as multilayered films containing a vapour barrier.
У нас про полимерные облицовочные материалы можно посмотреть, например, здесь http://www.arhplan.ru/materials/information/polymer-lining-m...