Service-induced degradation

The main features of low-temperature oxidation resemble that of the high-temperature processes described previously. Nevertheless, being thermally activated, low-temperature degradation proceeds at a much lower rate. It is relatively slow in semi-crystalline polymers and in polymers used below their glass transition. 

A typical service-induced degradation process is photo-oxidation which usually occurs through a mechanism similar to thermal oxidation. The difference being that initiation is through UV radiation. Photo-oxidation only attacks the superficial layers of a part with serious cosmetic consequences. Outdoor weathering combines thermal and photo oxidation, heat being generated by IR radiation. 

Ozone attacks double bonds within polymer chains to form unstable ozonides which further degrade the chains. This type of degradation is predominant for under-the-bonnet applications where ozone formation can be significant. 

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The analytical methods developed within the project are used to map process-and service-induced degradation for the material. Knock-down factors (KD) can be determined for property degradation as a function of process and service parameters. 

The staff concluded that, in view of the past experience and the results of the augmented reactor inspections, there is no indication of service-induced degradation (IGSCC) in the reactor tanks. 

Over the service life of a PO, heat-induced degradation can cause losses of multiple desired mechanical properties and inaeased embrittlement. This is shown in Figure 3.3, which compares long-term heat aging (LTHA) of heat-stabilized... 

The durability and reliability of a polymer-based product are determined by a number of factors inherent to the material itself (for example, crystallinity, average molecular weight), to its processing (for example, shear-induced degradation, process-induced thermal degradation), and to its service environment (such as, temperature, humidity and the presence of vibrations) [11]-... 

Light stabilisers (photostabilisers) are chemical compounds that interfere in the physical and chemical processes of light-induced degradation. The service life of unprotected PE in the dark at 20 °C is 8-10 years, and that of stabilised or crosslinked PE in moderate climates is 15-20 years. In tropical climates the service life is only two to five years, even if the products (pipes, tubes) contain stabilisers ( 2%) the lifetime of films is only three months. 

The combination of the multiple pertinent physics in the battery cell, incorporating the interplay of manufacturing- and service-induced loads and failure mechanisms, will surely inform advanced modeling. Of the commonly recognized battery performance degradation mechanisms, namely electrolyte decomposition, active material dissolution, passive layer formation, lithium deposition, and mechanical failure of battery materials, mechanical failure is the least understood at present, although there is ample evidence of the importance of these effects, and they are the subject of this chapter. 

Another problem occurs when some fire retardant formulations ate exposed to elevated temperatures (eg, when used as roof tmsses or as roof sheathing) thermal-induced strength reductions can occur in-service. The thermo-chemical factors were discussed by LeVan and Winandy (26), and a kinetic degrade model was developed (27). The treater should be consulted to obtain appropriate in-service modifications for specific fire retardant treatments. 

With rubber base adhesives, it is necessary to prevent their properties from changing during service life. Oxidative changes induced by thermal, ozone exposure and UV light can dramatically affect service life of rubber base adhesives. More precisely, the rubber and the resin are quite susceptible to oxidative degradation. Environmental and physical factors exert detrimental effects on rubber base adhesive performance. These effects can be mitigated by the incorporation of low levels of stabilizers during the fabrication process of the adhesive. 

Ultraviolet stabilisers for epoxies. The function of ultraviolet (UV) stabilisers is to retard light-induced polymer degradation which is caused by electromagnetic radiation emitted by the sun. Commonly used UV stabilisers for epoxy systems are given in Table 2.6 (Dexter, 1982). Only chemical names along with chemical abstracts services (CAS) registry numbers are mentioned commercially they are known by trade names. 

Areas requiring special attention in the future include improved stabilization processes for existing polymers and the synthesis of new polymeric materials which are more resistant against environmental corrosion than those presently used. The implications of the stability of natural polymers for the prediction of service life of synthetic polymers,the general effects of hydrolysis, the light-induced hydrolytic degradation are relevant research topics. 

Where X R" and C R" ea e vectors composed, respectively, of the level of inoperability and external failures associated with each one of theudififerent infrastructures considered in the scenario. A e R is the Leontief matrix, in which entry a,y represents the level of influence that the inoperability of the i-th infrastructure has on the y -th one (interoperability). Notice that in the model, atj = 1 means that the y-th infrastructure is completely dependent on the i-th one, because a given service reduction in the latter will directly induce an equal level of service degradation into they -th one. Applicative examples and results of this approach have been already proposed (Setola 2007 Haimes Jiang 2001). 

In general, a variety of mechanisms may contribute to the failure of actual components in service. These may include chemical degradation or oxidation a chemical mechanism that may induce cross-linking and chain-scission. Alternately, other physical processes may alter the state of the polsrmer (eg, surface active agents in the presence of stress may induce crazes due to local diflfiision of the agents near defects). These aspects are not discussed in this article. 

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