Kinetics of the non-isothermal fusion-welding of unlike ethylene copolymers over a wide crystallinity range

Abstract

The non-isothermal fusion-welding of unlike polyethylene materials has been studied using three linear ethylene copolymers covering the crystallinity range 16–77% and displaying partial miscibility of the binary blends. Welding was carried out by putting into intimate contact under slight pressure the molten surfaces of two flat beams quickly heated up far above the melting point by means of infrared radiations. Particular attention was paid to the wetting of the beams using ultrasonic measurements. The interface adhesion strength was determined by means of double cantilever beam method. Homo- as well as hetero-welding proved to be highly efficient for only a few seconds of contact of the two beams in the molten state. The critical strain energy release rate of the interface reached values G1C ≥ 6 kJ/m2 for contact time less than 10 s. The time window of efficient welding proved to be intermediate between the number-average and weight-average values of the terminal relaxation time according to melt rheology. This is consistent with Wool's criterion assuming that perfect self-welding of amorphous polymer requires reptation of chains over their whole length through the interface. The longer chains yet seemed not to be able to achieve complete tube renewal during the experimental time window of efficient welding. It is suggested that the reptation of the shortest chains contributes to the restoration of the entanglement network of the longest chains within a time scale much shorter than the reptation time of the latter ones. The surprising efficiency of hetero-welding in agreement with Wool's criterion is attributed to the interfacial miscibility of the unlike copolymers. The concomitant role of cocrystallization in the process is pointed out for such semi-crystalline polymers.