Thermal Analysis to Evaluate Ageing Process in Heated Tool and Electrofusion Welding of Polymer Pipes

Abstract

Heated tool and electrofusion welding are the most used joining processes of higher than 2 mm thick polymer pipes. The two welding processes have different heating-cooling cycles and they produce different influences on the properties of the polymers. Exploitation of polymer pipes for water and gas distribution revealed ageing behaviour of the material in the welding area. The modification of the behaviour depends on the base material, on the applied welding process and on the used parameters. Thermal analysis can be used as tool to reveal and to evaluate the modification of the physical and mechanical properties. Such knowledge is important when prediction of the in use life is necessary to be predicted. Experimental programme was applied to HDPE 100 and HDPE 80, both welded using heated tool and electrofusion processes and different sets of parameters (factorial experiment principles were used to establish the welding parameters). Plasticity characteristics of the welds material, as elongation and relaxation modulus, were determined by using thermal analysis. Burst stress test, applied to the pipe, was considered. It has been observed important rate of the heating process of the surface in contact with the heater. The DSC analysis revealed a decreasing of the elongation with about 12% and decreasing of the relaxation modulus with amount up to 14%, for the material located at the interface between pipe and the heater. At 0.5 mm from the interface it was revealed an intensity of the modification up to half of the values recorded for the interface. That was explained on the poor thermal conductivity specific to the both materials. By using DSC thermal analysis it has been revealed that the polyethylene has high rate crystallization during welding cycle, after the heating to the viscous state. Such crystallization, together with potential non-uniformity of the heating provides modifications in the geometrical characteristics of the weld. For high energy input, the material experience large quantity of fluid material with important plastic distortion. That means high possibility to reject material during pressing step of the welding cycle. 10% increasing of the temperature, for the same heating pressure, involves 5-8% increasing of the dimensions of the fluid ring in the interface. About 10% difference between the relaxation modulus of the heated and non-heated HDPE and that means local ageing transformation of the HDPE. The material becomes more fragile than before the welding process.