The principle of air bubble defect of polyurethane coating of ductile iron pipe was discussed that carbon dioxide created by the reactivity of the water of manufacture process and isocyanate is the key factor.

Through the comparative studies on the tar-based polyurethane which comes into common use at present and the polyurethane modified with organosilicon which is developed in the laboratory,a difference of performance in them is obtained,which provides a reference for the joint sealants of cement concrete pavement to upgrade.