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•  How do PVC pipes behave under constant stress?

All plastic materials submitted to a constant load, suffer a progressive deformation over time. This phenomenon, caused by the displacement of molecular chains among themselves, is commonly called Creep. This phenomenon depends principally on the type of plastic, its molecular structure, the operating temperature and time (It can for example take several hundred years for PVC pressure pipes to fail as a result of Creep).

For non-pressure pipes, standards describe the relation comparing short-term and long-term creep: this is called the Creep Ratio¹ . This ratio is also used in designing plastic pipes.

Among plastic pipes, PVC pipes have the lowest creep ratio.

As an example, in the European project for structured wall pipe standards² , the ratios demanded for different materials are:

PVC -U < 2.5 and PP, PE < 4

A lower creep ratio indicates that in the long term, the material maintains similar properties to those it initially had.

1 ISO EN 9967

2 prEN 13476.