Degradative agents

On its own, PVC is an extremely unstable polymer, in fact, almost certainly the least naturally stable polymer in commercial use. As Grassie has noted, Had this polymer been discovered at the present stage of development of the plastics industry, it would almost certainly have been eliminated as useless because of its general instability to all common degradative agents [2]. 

Generation of a faster acting thrombolytic (clot degrading) agent... 

In situations where inappropriate clot formation results in the blockage of a blood vessel, the tissue damage that ensues depends, to a point, upon how long the clot blocks blood flow. Rapid removal of the clot can often minimize the severity of tissue damage. Thus, several thrombolytic (clot-degrading) agents have found medical application (Table 12.5). The market for an effective thrombolytic agent is substantial. In the USA alone, it is estimated that 1.5 million people suffer acute myocardial infarction each year, and there are another 0.5 million suffer strokes. 

Table 4.1 lists the principal degradation agents and their consequences. The sections that follow explain the mechanisms and their effects in more detail. The rates at which degradation occur, which provide the basis for life prediction, are discussed in Chapter 8. 

Usually, there is more than one degradation agent present, giving a more complicated situation. For the purposes of life prediction it is desirable, wherever possible, to identify the critical degradation agent for the particular application or the particular objective. 

The purpose of the trial also affects the choice of degradation agents and the parameters used to monitor degradation. For comparison and quality control purposes, single agents are most frequently used. For prediction purposes multiple agents are more likely to be representative of service, but at the same time they make extrapolation rules more complicated. The parameters measured in trials to predict lifetime must be those critical to service, but in many instances of comparison or quality checks the choice of parameter can be heavily influenced by experimental convenience. 

Where multiple degradation agents apply, the extrapolation rules for each have to be combined in such a way that synergistic actions are accounted for. These procedures are likely to be very complicated and add greatly to the uncertainty. 

Regardless of the validity of the extrapolation procedure, the intrinsic experimental uncertainty of the measurements will be magnified as the degree of extrapolation increases. In addition, the difficulties associated with knowing the critical degradation agents and the critical properties for the application have been discussed earlier. 

Decide whether whole products or test pieces (or a combination) are to be exposed to the degradation agents (see Section 5.2.2). 

The effect of degradation agents on low temperature behaviour must be relevant in many applications but is virtually never specifically measured. There are low temperature tests for flexible materials (ISO 458 [36] and ISO 974 [37]), but generally DMTA or impact methods might be more appropriate. 

Using these relationships the change in the property for longer times and lower levels of the degrading agent can be predicted. Clearly, the success of the process is critically dependent... 

When the form of the change in a parameter with time has been established and a suitable measure to represent that form selected, the relation with the level of the degradation agent is needed to allow extrapolation to the service level. Generally, measurements need to be made at several agent levels to establish a model with reasonable confidence. Typically five levels are considered satisfactory. However, it should be noted that when extrapolation is to be made over several decades of time the uncertainty of the prediction will be large, even if the measured data looked very consistent. Estimates of uncertainty should always be made (see Section 9.3). 

Before applying any model it is essential to have confidence that the input data is valid. There are repeated warnings in the literature of data being invalid because it is obtained at such accelerated levels of the degradation agent that reactions occur which are not seen at lower levels. 

The best known and most widely used model is the Arrhenius relationship which, in particular, is applied to the permanent effects of temperature as the degrading agent. 

All the treatments discussed above have been concerned with constant conditions, i.e., where in the accelerated tests the level of the degrading agents has been held constant throughout one exposure, and any extrapolation to service implicitly assumes that conditions there will also be constant. In real life, however, it is much more likely that service conditions will be variable or cyclic. Generally, therefore, further approximations have to be made. 

In ageing trials the most important considerations are the numbers of test pieces, exposure times and levels of the degradation agent, together with ensuring that the test pieces used are representative. Ideally, the number of experimental points should be maximised but in practice cost considerations lead to restrictions. 

The degradation agents used are not the ones critical to service performance. 

ISO 15686-2 describes the methodology of estimating service lives based on experience, short-term measurement and accelerated testing, in that order. All possible degradation agents and their intensities are listed. Methods for evaluation and for pre-testing (control... 

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