15-06-2013, 03:45 PM
ASESSMENT OF LIFE OF RUBBER
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Degradation Mechanism for life of Rubber
Degradation Agents
Polymer degradation is generally taken to mean the irreversible change of chemicalor physical properties of the polymer, most common is a reduction in strength. Themanner and speed with which this occurs will depend on the material itself and theadditives it contains, on the environmental conditions such as temperature, ultravioletlight or the presence of chemicals, and on the mechanical and electrical stresses relatedto its use. During its lifetime a plastic will be exposed to one, or usually several,types of degradation due to these various agents. In many cases there will be morethan one agent present. It is essential that at least a general appreciation of thedegradation reactions of the polymers being tested is obtained, before an estimate ismade of lifetime planning or the test conditions are chosen for a trial.
At the molecular level, degradation comprises the deterioration of macromoleculescaused by bond scissions in the polymer backbone and chemical reactions in theside-chains, as well as intermolecular crosslinking with formation of new chemicalbonds between different molecules. There can also be changes to ingredients such asextraction of plasticisers or attack by acids on reinforcing fibres. Degradation alsoincludes physical changes such as set, creep and relaxation, and the stiffening effectsof low temperatures.
This section will outline the agents and basic mechanisms of degradation and identifythose which are most significant for life prediction.
Thermo-Oxidation
The intrinsic resistance to oxidation of an unstabilised polymer will depend on the polymer
structure . Ranking polymers in this way becomes academic, however, since theirpractical resistance to oxidation depends on the package of stabilising additives.
Oxidative degradation can be the most serious problem in the use of plastics at highertemperatures. At ambient temperature oxidation proceeds relatively slowly on its own,but can be stimulated by light (photo-oxidation), ionising radiation (radio-oxidation),certain gaseous and liquid environments and by the presence of transition metals. Therate at which oxidation occurs will therefore depend on the intensity of these agents, ontemperature, and on the availability of oxygen, which in turn depends upon its solubility,its rate of diffusion and the rate at which it is consumed.The process of oxidation has three stages: initiation, chain reaction and termination.Initiation is produced by external stimuli as described and activates the polymer to forma reactive radical. Initiation continues only for as long as the external stimulus persists.Removal of the energy required to form the initiator, for example the heat or light, willresult in the rate of degradation decaying to zero.
Critical Factors
Temperature
For products intended for operation at elevated temperatures it would be expectedthat the temperatures would be known. Where the operating temperature is cyclic, themaximum might be used or an equivalent temperature dose estimated on the basis ofthe Arrhenius relation.For operation indoors at ambient temperature it is usual practice to take 20 or 23 °C,
largely because these are taken to be the normal temperature in laboratories. The actualtemperature in factories, warehouses and homes could clearly be somewhat different,particularly in different parts of the world.
Outdoor temperatures are less easy to quantify, not only because of different climaticconditions in different places, but because temperature is very dependent on the degreeto which the product is exposed to sunlight, whether or not it is enclosed and its colour.In temperate climates it is common to again take 20 or 23 °C as average and this isprobably sufficiently accurate in many cases.
However, omitting to take account of the temperature of products used or stored insunlight when selecting materials can lead to unexpected rates of deterioration or failure.Conversely, the temperatures reached during weathering tests, both natural andaccelerated, need to be considered when assessing the results.Because of the great variation in practical conditions and the fact that accurate surfacetemperature measurement is not easy, there is some spread in reported figures for naturalexposure. Any estimate of temperatures likely to be reached is approximate but thefollowing can be taken as a useful guide.
Solar Irradiation
Warnings are often given that acceleration factors for relating artificial light sourceswith service are meaningless, because of both the variation in solar irradiation and thevariation in spectral distribution. Regardless of this, acceleration factors are estimated,and indeed have to be if any extrapolation from accelerated tests is to be made.CIE Publication No. 85 provides data on solar spectral irradiance for typicalatmospheric conditions. A condensed version of a table for maximum global irradianceat the equator is given in ISO 4892-1 . Reference solar spectral irradiance can befound in ISO 9845-1 and analytical expressions for daily solar profiles are given inIEC 61725 , but this sort of data cannot generally be used to provide simplistic averageacceleration factors.
It can, however, be noted that both total irradiation and UV content vary with the location,the time of year, the atmospheric conditions and the angle of the sun.There does not seem to be one definitive collection of measured data for various locationsworld wide, although quite a lot have been collected by Wypych . Figures for totalirradiation cannot be sensibly used because it is necessary to work with the irradiation atthe more important UV wavelengths to make comparison with the artificial light sources.Figures were obtained for the 295-385 nm band of 280 and 333 MJ/m2/year for Floridaand Arizona respectively. Combining these with figures given by Davis and Sims fortotal irradiance in London and Phoenix (75 and 175 kcalories/cm2/year, respectively)and making several assumptions, rough acceleration factors were calculated for artificiallight sources:
Taking a total xenon irradiance of 1,000 W/m2, and comparing it to a typical quotedfigure of 3.5 GJ/m2 for the UK gives an acceleration factor of 9.In calculating exposure times, adjustment needs to be made for light/dark cycles in artificialweathering apparatus.
Fluids
For most indoor applications humidity and moisture can be ignored, although it is possiblethat in some cases an abnormally high level in service can be predicted. Out of doors it isquite impossible to suggest any typical level and if moisture is expected to be a problemthe worst case should be considered. The worst case may well be intermittent precipitationand drying.The same is true of fluids generally. The particular chemical(s) of importance need to beidentified and tests based on total immersion. The exceptions are if service is known toinvolve intermittent or one sided exposure, which can be simulated. Pollutants are aspecial case of chemical exposure.