Stress cracking agent

Stress-cracking from the interaction of a stress-cracking agent, ... 

Environmental stress-cracking is the failure of plastic materials in the presence of certain chemicals types. This failure is not a result of chemical attack, but rather the simultaneous presence of three factors tensile stress, a stress-cracking agent, and inherent susceptibility of the plastic to stresscracking. 

ISO 4600 details a ball or pin impression method for determining the ESCR. In this procedure, a hole of specified diameter is drilled in the plastic. An oversized ball or pin is inserted into the hole, and the polymer is exposed to a stress cracking agent. The applied deformation, given by the diameter of the ball or pin, is constant. The test is multiaxial, relatively easy to perform, and with not very well-defined specimens, and the influence of the surface is limited. Drawbacks are the small testing surface and the undefined stress state. After exposure, tensile or flexural tests may be performed on the specimens. This leads to the determination of either the residual tensile strength or the residual deformation at break. 

A constant tensile-stress method is outlined in ISO 6252, in which a test specimen is exposed to a constant tensile force while immersed in a stress cracking agent so as to determine the time to rupture under a specified stress. This uniaxial test leads to the determination of the lifetime of the specimen with accuracy, but it is time consuming and requires complex equipment. Variations of this test include a tensile creep test that monitors the strain and a monotonic creep test that uses a constant stress rate instead of a fixed stress [1]. 

Another bent-strip method for evaluating the ESCR is presented in ISO 4599. In this test, strips of a plastic are positioned in a fixed flexural strain state and exposed to a stress cracking agent for a predetermined period. The test is uniaxial and simple to perform, and the deformation is constant. Because of the molecular chain relaxations, the stress state is well defined only at the beginning of the test. After exposure to the medium, the strips are removed from the straining rig, examined visually for changes in appearance, and then tested for some indicative property such as tensile strength. 

Qian R, Lu X, Brown N (1993) The effect of concentration of an environmental stress cracking agent on slow crack growth in polyethylenes. Polymer 34(22) 4727-4731... 

Ward AL, Lu X, Huang Y, Brown N (1991) The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent. Polymer 32(12) 2172-2178... 

SBC are generally attacked by many hydrocarbon solutions. Degreasing of parts in many solvents can result in stress whitening, cracking or premature failure of SBC parts. Some bodily fluids contain high levels of lipids and are considered SBC stress cracking agents. Care should be taken any time SBC parts are exposed to any stress crack medium to ensure suitability for use. 

The results are discussed in terms of material and stress-cracking agent characteristics, and their interactions. 

The stress-cracking agent used for ESC testing in the present work was a water solution of the industrial detergent nonylphenoxy poly(ethyleneoxy)ethanol (Antarox CO-630). Concentration was varied between 2% and 100%. 

However, a critical value is found for the applied stress intensity factor, K ic, below which the environmental stress-cracking agent accelerates fracture initiation below this transition value, the slope of the stress intensity factor vs. initiation time plot is higher but still similar for the two materials. 

Trtethylene Glycol 21 60 rasisianl poss. stress crack agent resist, depends on condit. pass, stress cracks 4 4... 

If the separation Into mechanisms Is a valid representation, then. In the region where both are contributing equally, one would expect to see contributions from each. Experimentally, It was found that In this region, the specimens would tend to neck In one place and crack In another. Moreover, since mechanism B In the presence of stress-cracking agent Is depressed In time relative to air, one would expect the specimens to crack Into the region of higher stresses than Is the case In air, a result that Is observed experimentally. 

In Figure 4 Is shown on a double-logarithmic plot the time to fall In solvent or stress-cracking agent versus the time to fall In air. The dashed line I with unity slope corresponds to behavior which Is unaffected by the presence of either type of chemical environment. Deviations from this line represent,... 

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