Polyurethanes are used to make products with properties ranging from soft flexible foams such as memory foam mattresses, to rigid foams used for house insulation. They can be made into soft rubber as used for skateboard wheels or into very hard enamels as used in modern automobile paints.
The range of properties can be quite wide, just by changing the chemistry a little, but there's a problem. The traditional chemistry uses materials that have an affinity for water. For applications such as fountains, pond liners, waterproofing, water and waste water tanks, roofing — where long term or constant immersion is an essential requirement — that affinity for water will lead, eventually, to the product swelling or even failing. The reason is in the chemistry, itself. Here’s why.