Hot gas welds in unplasticised polyvinyl chloride.

Low weld tensile strengths are shown to be a feature of hot gas welded clear unplasticised polyvinyl chloride (UPVC), Typical weld strengths are 20-30 MPa compared to 45 MPa for hot gas welded filled UPVC and 61 MPa for hot tool welded clear UPVC. A short mechanical, microstructural and chemical analysis revealed only minor differences between clear sheet and rod. An examination of weld fracture surfaces revealed the presence of unfused regions (flaws) which act as fracture initiation sites. In over 80% of the surfaces studied fracture occurred at such sites, at or close to the weld surface. These regions were observed for both hot gas welded clear and filled UPVC but npt in hot tool welded clear UPVC. It was concluded that these regions were a feature of hot gas welding and that clear UPVC was more sensitive to them than filled UPVC. A range of linear elastic fracture mechanics (LEFM) test techniques,were used to establish that,clear UPVC had a K of 2.1 MPam2 , which dropped to 1.5 MPam 2 after welding. The filled UPVC examined was too tough to allow K to be measured. However, embrittlement of the welded filled UPVC was indicated, as only brittle fracture, with no plastic zone, occurred. A simplistic LEFM analysis based on measured flaw sizes and failure stresses for welds confirmed the embrittlement of hot gas welded clear UPVC. Two approaches were tried to improve weld strengths: 1) to reduce the size and number of flaws and 2) to increase the toughness of the weld rod. An initial study indicated that changes to the rod lubricant system and the inclusion of 10 phr methyl methacrylate-butadiene-styrene in the rod composition led to improved and more consistent weld strengths. However, improvements were limited to weld strengths of 32 MPa and to achieve any further improvement the clear sheet toughness would need to be increased.