The reliability of underground polyethylene pipeline systems is limited by the durability of the welded joints. If the welding and installation technology were up to standards, then the durabili~ of the joints is determined by the properties of the original material.The durability of ethylene homopolymers is assumed to be connected with its molecular characteristics [I]. Moreover, it has been shown that for a sufficiently high polyethylene molecular weight, the effect of the molecular characteristics oh the durability is less appreciable than the effect of the supermolecular characteristics [2]. This is explained by the fact that the probability u of formation of tie molecules between crystals according to [3] is connected with the width of the amorphous interlayers m by the relationwhere a, b are constants; n is the length of the tie molecule. Modern pipe grades of low-pressure polyethylene contain comonomer units of higher olefm, hindering the formation of large crystals, so we should expect a substantial effect from the comonomer concentration on the durability of welded joints.We have investigated welded joim between pipes of size and type 32t and 63t, manufactured at the Kazan Orgsintez Association from different batches of high-density polyethylene (HDPE) compounded from base grades 273, 289, 295 (see Table 1).The pipes were welded using the standard procedure in [4]. We cut I0 specimens (type I according to GOST 11262-80), with the weld seam in the middle, from each of the welded pipe connections. Tests of the weld specimens for crack resistance under constant tensile load were done in a medium of 10% OP-10 aqueous solution at 80"C. The initial stress r was selected in the range from 3.55 to 5.7 MPa. Under these conditions, quasibrittle fracture of the specimens occurred over the flash region of the butt joint, and cracks began to propagate from the outer surface of the pipe. For stress above 4.6 MPa, the brittle nature of the fracture seemed less definite.The molecular weight of polyethylene was estimated from the value of the melt index MI 5, while the molecular weight distribution was estimated from the ratio MI2t.s/MIs, as done in industrial practice.The degree of r o~ was calculated using the formula " (t~p -t~,)/(t'c -,~,),Kazan State Technological University, Kazan, Tatarstan. Russia.
