Stress Corrosion Cracking of Polymers

Stress corrosion cracking of polymers occms in a corrosive environment and also under stress [23,24]. This kind of crack starts at the surface and proceeds at right angles to the direction of stress. The amount... 

The stress corrosion resistance of polymers depends on the magnitude of the stress, the nature of the environment, the temperature, and the molecular weight of the specimen. Ozone cracking is a typical example of stress corrosion cracking of polymers. The critical energy for crack propagation (Tc) in ozone cracking varies very little from one polymer to another and is about 100 erg/cm (0.1 J/m ). This value is much lower that the Tc values for mechanical fracture, which are about 10 erg/cm (10 J/m ). 

Some direct experience of polymers with tritium has been obtained. Teflon, Viton, or Kel-F exposure in tritium produces the acid TF, noted as SiF4, gas in a glass system. Because of this acid production, tritium + moisture + Teflon in a stainless steel system at pressures of approximately 1300 atm caused catastrophic stress corrosion cracking of 0.76-mm thick stainless steel tube walls in 16 hours. Substituting deuterium for tritium or removing Teflon or moisture caused no failure. Radiation damage to Teflon is more severe than to all other thermoplastics. Teflon is therefore not recommended in the presence of concentrated tritium streams. 

Environmental stress cracking is similar, but not identical to, stress corrosion cracking of metals. Corrosion involves chemical reactions that produce corrosion products, whereas, in ESC, a liquid is absorbed by the polymer, promoting crazing and crack formation. Corrosion reactions are rare in polymers. ESC can typically cause a factor-of-ten reduction in strength. The two conditions for it to occur are that... 

The type of corrosive environment responsible for brittle fracture of suspension composite insulators was established. A series of FTIR experiments was performed to identify chemical functionalities formed during the degradation process of composite insulators affected by brittle fracture. It was shown that the brittle fracture process was caused by the formation of nitric acid either outside or inside an insulator leading to stress corrosion cracking of the glass/polymer composite rod material. Nitrate was detected on the composite fracture surfaces inside a 115 kV suspension composite insulator which failed in service by brittle fracture. 17 refs. 

In metal alloys the combination of stress and environment can also lead to premature failures, indicated as Stress Corrosion Cracking, SCC [1]. The influence of the environment on SCC is generally of a chemical nature a chemical reaction occurs between the metal and the environment. Most of the research published on the ESC of polymers focuses on ESC in which the environment influences the material only physically [2-8]. In such cases the mechanism of ESC is studied and models are established for ESC prediction [9]. These models for physical ESC are based predominantly on the solubility parameters of the considered polymer/environment combination. In other words, ESC is mainly a consequence of polymer softening, i.e. it is a reduction of the interaction between the polymer chains that lowers the yield stress. 

WILLIAM H. SMYRL is Professor of Chemical Engineering and Materials Sciences and Associate Director of the Center for Corrosion Research at the University of Minnesota. He received his Ph.D. (chemistry) at the University of California, Berkeley, and spent 3 years at the Boeing Scientific Research Laboratories and 11 years at Sandia National Laboratories. He joined the faculty of the University of Minnesota in 1984. His research interests are modeling of corrosion processes, in situ techniques for metal-metal oxide interface studies, digital impedance for faradaic analysis, stress corrosion cracking, polymer-metal interfaces, and electrochemical processes. 

Polymers, unlike metals, are subject to swelling rather than corrosion. Chloride-induced pitting corrosion, to which austenitic Cr-Ni steels are particularly susceptible, is unknown in polymeric materials. Stress corrosion cracking, however, is possible under conditions that cause the medium to attack the polymer chemically. Stress corrosion cracking can only occur at the same time that stresses resulting from manufacture, coating or lining, or the conditions of use are present. 

Fig. 4.181 Crack speed as a function of the applied stress intensity factor at static long-term conditions for thermoplastic polymers, A iscc - onset value of stress corrosion cracking (SCC), Kic - fracture toughness at quasi-static loading conditions.

Sandwich construction is used to yield a surface with specific properties. For example in refrigerator panels there is a problem with stress corrosion cracking in the presence of milk, fats, and oils. One solution is a surface of expensive, glossy and oil-resistant high-acrylonitrile ABS, with a cheaper low acrylonitrile ABS centre. The sandwich is coextruded (7.N.7). Coextrusion is not the only method for upgrading performance laminates, heavily-filled and foamed polymers are also thermoformed. 

Evans AG (1972) A method for evaluating the time-dependent failure characteristics of brittle materials - and its application to polycrystalline alumina. J Mater Sci 7 1137-1146 French MA, Pritchard G (1991) Strength retention of glass/carbon hybrid laminates in aqueous media. In Cardon AH, Verchery G (eds) Durability of polymer based composite systems for structural applications. Elsevier Applied Science, New York, pp 345-354 Friedrich K (1981) Stress corrosion crack propagation in glass fibre reinforced/thermoplastic PET. J Mater Sci 16(12) 3292-3302... 

Pomies F, Carlsson LA, Gillespie JW Jr (1995) Marine environmental effects on polymer matrix composites. In Martin RH (ed) Composite materials fatigue and fracture, vol 5, ASTM STP 1230. American Society for Testing and Materials, Philadelphia, pp 28-303 Price JN (1989) Stress corrosion cracking in glass reinforced composites. In Roulin-Moloney AC (ed) Fractography and failure mechanisms of polymers and composites. Elsevier Applied Science, New York, pp 495-531... 

Karbhaii VM, Zhang S (2003) E-glass/vinylester composites in aqueous environments - I experimental results. AppI Compos Mater 10(l) 19-48 Megel M, Kumosa L, Ely T, Armentrout D, Kumosa M (2001) Initiation of stress-corrosion cracking in unidirectional glass/polymer composite materials. Compos Sci Technol 61 (2) 231-246... 

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