Many pre-insulated pipes use rigid polyurethane foam and a HDPE casing. Over time, the insulation efficiency of the pipe structure decreases, caused by the permeation of air (oxygen, nitrogen) into the foam cells and the diffusion of cell gases (blowing agent, carbondioxide) out of the foam cells. The insulation efficiency of PUR foam decreases as air permeates in and cell gasses permeate out of the pipe structure, increasing thermal conductivity. Oxygen permeation can also lead to oxidative degradation of the PUR-foam insulation itself. This reduces the adhesion between the PUR-foam and inner pipe, weakening the structure and compromising the mechanical integrity of the entire pipe system.

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