chains, the polymers align, thus allowing embrittlement. ESC generally occurs at the surface of a plastic and doesn't require the secondary chemical agent to penetrate the material significantly, which leaves the bulk properties unmodified. Another theory for the mechanism of craze propagation in amorphous polymers is proposed by Kramer. According to his theory, the formation of internal surfaces in polymers is facilitated by polymeric surface tension that is determined by both secondary interactions and the contribution of load-bearing chains that must undergo fracture or slippage to form a surface. This theory provides and explanation for the decrease in the stress needed to propagate the craze in the presence of surface-active reagents such as detergents and high temperature. ESC mechanism in polyethylene Semi-crystalline polymers such as polyethylene show brittle fracture under stress if exposed to stress cracking agents. In such polymers, the crystallites are connected by the tie molecules through the amorphous phase. The tie molecules play an important role in the mechanical properties of the polymer through the transferring of load. Stress cracking agents, such as detergents, act to lower the cohesive forces which maintain the tie molecules in the crystallites, thus facilitating their “pull-out” and disentanglement from the lamellae. As a result, cracking is initiated at stress values lower than the critical stress level of the material. In general, the mechanism of environmental stress cracking in polyethylene involves the disentanglement of the tie molecules from the crystals. The number of tie molecules and the strength of the crystals that anchor them are considered the controlling factors in determining the polymer resistance to ESC. Characterizing ESC A number of different methods are used to evaluate a polymer's resistance to environmental stress cracking. A common method in the polymer industry is use of the Bergen jig, which subjects the sample to variable strain during a single test. The results of this test indicate the critical strain to cracking, using only one sample. Another widely used test is the Bell Telephone test where bent strips are exposed to fluids of interest under controlled conditions. Further, new tests have been developed where the time for crack initiation under transverse loading and an aggressive solvent (10% Igepal CO-630 solution) is evaluated. These methods rely on an indentor to stress the material biaxially, while preventing a radial stress concentration. The stressed polymer sits in the aggressive agent and the stressed plastic around the indentor is