Сначала я решила, что это название компании, но по-крайней мере один производитель (Bradshaw) использует его как название материала (см последнюю ссылку)


http://www.bradshawsponds.co.uk/pondproducts/polyexlinersize...
What is POLYEX and Why is it So Good?
Once you have dug your pond, the area must be lined with a waterproof membrane; without this the water will simply seep away. A quality liner like Polyex is one of the most important factors and will help create the best foundation for your garden pond. Only Bradshaws sell Polyex and it was developed by them 20 years ago.

Flexible liners are the most popular way of lining a garden pond. Polyex liners are very flexible so they allow you to design a pond to any shape you require. Our Polyex liners are the biggest selling pond liners in Britain and provide outstanding value for money and we are proud to say that these top quality liners are only available from Bradshaws Direct.

http://www.garden-pond-filters.com/Bradshaws/bradshaws_polye...



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Note added at 24 mins (2008-12-29 15:58:30 GMT)
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скорее всего, оно также является названием производителя, но я не нашла пока прямых ссылок