STRUCTURE AND STRENGTH OF A POLYETHYLENE PIPE GRADE CONTAINING GLASS SPHERES AND LOW-MOLAR-MASS LINEAR POLYETHYLENE

The resistance to slow crack growth and to internal over-pressure of p ipes based on a commercial polyethylene gas pipe grade (BPE; 0.6 mol% butyl branches and (M) over bar(w) = 197,000 g/mol) blended with both an injection molding grade Linear polyethylene (LPE; (M) over bar(w) = 42,000 g/mol) and 0.5 +/- 0.1 mm glass spheres was studied, The blend s and the pure polymer components were characterized by density measur ements, differential scanning calorimetry, small-angle X-ray scatterin g, and transmission electron microscopy. The fracture properties of ex truded pipes mere measured using hydrostatic pressure testing and notc hed uniaxial testing on samples cut from the pipe wall. The polymer co mponents were macroscopically uniformly blended and differential scann ing calorimetry indicated the occurrence of partial co-crystallization between the branched and linear components. Transmission electron mic roscopy showed molar mass segregation on a 100 nm level. Notched uniax ial testing showed that the slow crack growth resistance of pure BPE w as considerably higher than that of the LPE/BPE blend with 30% LPE. Fr actography indicated that the fracture-initiating particles were large r in pipes failing after shorter period of time in the hydrostatic pre ssure testing. The lifetimes of hydrostatic pressure tested pipes base d on the BPE grade containing glass spheres were similar to these of p ipes based on the LPE/BPE blend with 30% LPE.