Closures stress cracking

Polyethylene and polystyrene are examples of plastics subject to environmental stress cracking. Crack resistance tests have shown that surfactants, alcohols, organic acids, vegetable and mineral oils, and ethers provide an active environment for stress cracking of polyethylene. Table 6 lists typical sterile devices and plastic materials used to fabricate them, while Tables 7-9 list the potential effects of sterilization processes on polymeric materials. The effect of gamma irradiation on elastomeric closures has been studied by the Parenteral Drug Association [15]. 

Thompson, R. B., Fiedler, C. J., and Buck, O. (1984). Inference of fatigue crack closure stresses from ultrasonic transmission measurements. In Nondestructive methods for materials property determination (ed. C. O. Ruud and R. B. Thompson), pp. 161-70. Plenum Press, New York. [278]... 

Fig. 12.12 Results of compact tension specimen showing increase in toughness as crack goes past fiber, and crack opening displacement from which closure stresses can be determined.

In the case of a screw closure, it should be evident that a closure which has one turn or more of thread engagement will suffer less thread-to-thread stress than one of, say, turn (i.e. the same force (torque) is spread over less area with the latter, hence stress cracking could be more likely). This torque force is also conveyed to the sealing surface of the container and the contact surface of the closure, hence areas receive and convey a greater force (onto the cap). Studying how a stress occurs with the closure—container system is therefore a useful part of avoiding stress cracking problems. 

The next largest molded product market for PP is packaging, especially closures and containers. Child-resistant, tamperproof, linerless features are important design factors as also inherent chemical resistance, stress-crack resistance, and high productivity at low cost. Housewares utilize random copolymers for refrigerator and shelf-storage containers and lids. Medium-impact copolymers are used for hot/cold thermos containers, lunch boxes, coolers, and picnic ware. 

MDPE can be produced by chromium/silica, Z-N, or metallocene catalysts. The polymer has good shock and drop resistance properties it is less notch sensitive and has higher stress cracking resistance than HOPE. It is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags, and screw closures. 

Designers must be familiar with bottle-performance tests. The Society of the Plastics Industry has recommended 21 standard practices the most important are vertical compression or top-load strength, drop-impact resistance, product compatibility and permeability, closure torque, and top-load stress-crack resistance. 

Meschke, F., Raddatz, O., Kolleck, A., and Schneider, G.A., 2000, R curve behavior and crack closure stresses in barium titanate and (Mg, Y) PSZ ceramics. Journal of the American Ceramic Society 83 353 361. 

The problem of environmental stress cracking can be an important one for many applications in the medical industry. These include medical laboratory equipment, caps, closures, and implant components. 

Injection moulded articles represent only a small proportion (5%) of the consumption of LDPE and LLDPE. This proportion is not insignificant, however, considering the total volume of 300,000 tonnes per year. LDPE and LLDPE parts produced by injection moulding are used in a wide range of applications, for example, for domestic storage containers and closures in cosmetic, food, and pharmaceutical applications. High environmental stress crack resistance and low warpage are required for LLDPE applications. 

Another characteristic feature that may be observed on each single striation, as in Fig. 2.35, is the presence of smaller closely spaced striations. The origin of these closely spaced striations may be explained by the striation mechanism proposed by Gross and schematized in Fig. 2.36. In this mechanism, basically similar to that of Fig. 2.25 and based on two stages process of blunting and closure re-sharpening of the crack tip, compressive closure stress. Fig. 2.36b, may activate a series of slip lines ahead of the crack tip that leave a trace on the main striation. 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

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