The urgent need for revision of the normative test method (EN 253) for the lifetime prediction of district

heating pipes requires a better understanding of the failure mechanisms involved. Therefore, various

methods were used to study thermal degradation characteristics of rigid polyurethane (PUR) foam in

both air and nitrogen atmosphere. Accelerated ageing in nitrogen caused insigni

ficant changes, whereas

ageing in air caused signi

ficant changes in weight, dimensions, chemical structure and cell gas composition,

indicating importance of the thermo-oxidative type of degradation. A clear indication of the

thermo-oxidative type of degradation was the formation of new carbonyl groups in PUR together with

the loss of CH

2 groups after ageing in air. Another result of ageing in air was the loss of pentane and

cyclopentane, and the formation of some new volatile compounds in the cells of PUR foam. However,

despite a large difference in degradation characteristics between the samples aged in air and in nitrogen,

no signi

ficant difference in the flexural strength of PUR foam was recorded during the induction stage of

the degradation process. Furthermore, it is shown that the signi

ficant drop in shear strength, which

re

flects the adhesion force between PUR foam and steel pipe, observed during the early stage of

accelerated ageing of district heating pipes is not caused by thermo-oxidative degradation.