Stresses, plastics degradation

In past studies (3), Electron Spin Resonance, ESR, has been used to investigate bond rupture associated with stress-ozone degradation in unsaturated rubbers. It was believed that it might be enlightening to perform similar experiments on plastics ( 5, 8, 10, 11). In the study reported here the combined effects of environment (O3, N02 and S02) and sustained stress on the ultimate properties of Nylon 6 fibers are presented. These agents were found to have a profound effect on strength, toughness and the deformation-bond rupture kinetics. 

Briefly, antioxidants quench free radicals that are formed in the process of plastic degradation by oxygen and initiated by temperature and UV light, and assisted by moisture, stress, presence of metals, and other catalysts of plastic oxidation. If not intercepted by antioxidants, the polymeric plastic is degraded (depolymerized) so much that it loses its integrity and ceases to be a plastic anymore. It is converted to a loose powderous material, mainly a filler. 

A. Lustiger, Understanding Environmental Stress Cracking in Polyethylene, in Medical Plastics Degradation, Resistance Failure Analysis ed. R.C. Portnoy (SPE, Plastic Design Library, Rice RC, Tritsch DE, 1998), pp. 66-71... 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Since the mid-1950s several materials have been found effective in combating ozone-initiated degradation, in particular certain p-phenylenediamine derivatives. The actual choice of such antiozonants depends on the type of polymer and on whether or not the polymer is to be subject to dynamic stressing in service. Since antiozonants are not known to have any use in plastics materials, even those which may have certain rubber particles for toughening, they will not be dealt with further here. Anyone interested further should consult references 3-5. 

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Figure 7.1 Typical stress-strain curves for (a) a brittle plastic and (b) a tough plastic with yield point, showing the parameters used for the evaluation of degradation in tests...

Creep behaviour is a commonly used and very important measure of the effect of mechanical stress on plastics, but it is less used as a means of monitoring degradation due to environmental agents. At shorter times, the measured creep is predominantly due to physical effects and it is only at longer times that environmental effects will be apparent. It can be noted that creep tests use the same test piece at successive time intervals which is advantageous from the point of view of reproducibility. 

It is necessary to remember that the combination of several factors often has a synergistic effect a plastic resistant to a chemical in the absence of mechanical stress and at ambient temperature can crack quickly when exposed to the under load and be degraded more or less quickly in the event of a temperature rise. 

Plastics have to be stabilized to withstand chemical and physical stresses during different phases of their lifetime. Stabilizers protecting plastics against particular degradation processes were developed and commercialized by various companies. According to their principal activity mechanisms, polymer stabilizers are conventionally classified as antioxidants, photoantioxidants, photostabilizers, heat stabilizers and... 

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