High-temperature aging

The oil companies supplying the rubber industry claim that there is no effect on ageing properties when sulphur is present in an oil (up to 6% is possible). They claim that the refining removes the active mercaptans and sulphides and the remainder of the sulphur is complexed into polycyclic compounds and there is no evidence that this type of sulphur has any effect on cure. This claim may be true in some circumstances. However, it is known from practical experience that with certain polymers and compounds the claim is incorrect, especially when the high sulphur oils are used as test media. Oils supplied to the same oil specification from oil fields in different parts of the world, and meeting all the requirements of the specification may, because of widely different sulphur levels, have serious effects on high temperature ageing. 

The phenyl modified polymers possess the optimum combination of high temperature and elastomeric properties and were used in the study of formulation parameters These variables can have an important effect on the thermal stability and property profile of vulcanized systems For example, the use of reinforcing silicas, peroxide content, and oxidative stabilizers have been shown to be important ( 3, 10, in However, polymer-silica interactions had the most pronounced effect on retaining properties during high temperature aging studies ... 

The retention of the maximum tensile strength at elevated temperatures is greater for radiation cured than for chemically cured natural rubber. The physical properties after high-temperature aging are not improved, however. Lower flex life and higher abrasion resistance of radiation cross-linked NR were reported. Other properties, such as permanent set, hardness, and resilience, were found to be nearly equal. 

TPR is a routinaiy technique in the redox characterization of M/Ce02 and related catalysts. It has been very extensively used in comparative studies aimed at establishing the influence of variables like the chemical composition (281,341,343,344,350-353), or the high temperature ageing treatments (187,288,337,347,354), on the reducibility of ceria-based mixed oxides, both in the presence and in the absence of a supported noble metal. 

It is the ability of epoxies to withstand high temperatures and at the same time maintain specified properties. With high-temperature ageing, epoxy... 

Figure 61. Tailpipe emission of CO, HC and NO c from a gasoline fueled passenger car equipped with a three-way catalytic converter, in the US-FTP 75 vehicle test, as a function of the number of catalysts in the converter at fixed total catalyst volume (monolith catalyst with 62 cells cm three-way formulation with Pt 0.83gl , Rh 0.16gl fresh condition and after high temperature aging for 20 hours on an engine bench).

Figure 67. Influence of the washcoat formulation on the conversion of CO, HC and NO , reached over various engine aged three-way catalysts as a function of the exhaust gas lambda value (monolith catalyst with 62cells cm three-way formulation with Pt 1.16gC, Rh 0.23gl" engine bench test A/F scan at a space velocity of 60000 Nl 1 h exhaust gas temperature 723 K dynamic frequency 1 Hz amplitude 1 A/F engine bench high temperature aging cycle for lOOh). Sample A baseline formulation, samples B-D increasing amounts of stabilizers.

Figure 5. Arrhenius plots of times to fifty-percent failure of high temperature aged wirebonds coated with standard resin Q and experimental resins X0, X1 X2, and X3. (Reproduced with permission from ref. 7. Copyright 1988 IEEE.)...

In this paper we report on some of the factors that affect catalyst performance in the presence of lead and sulfur both before and after high temperature aging. These are total metals loading, catalyst compo-... 

There are no standard aging or stability tests in our industry. Each company or independent formulator has developed its/his own set of standards to assess product stability to higher temperatures and to use high-temperature aging as a means to project longer-term aging effects to speed up product introductions. The best approach is to test a product at multiple temperatures because in some cases (e.g., some emulsions can be more stable at a higher than at a lower temperature). 

Figure 16 illustrates the effects of an unstabilized tackifier on color formation as a result of high-temperature aging of the EVA HMA formulation. In this scenario the tackifier was aged for 18 days at 50° C and then combined with the other components at 177°C (350°F). The final HMA formulation was then aged at 170°C (338°F). Use of the unstabilized tackifier results in a darker initial color and a more rapid rate of discoloration than that of HMA using the stabilized tackifier. 

A consistent viscosity of the EVA HMA during high-temperature aging can be achieved using a stabilized tackifier (Fig. 17). The EVA HMA usig the unstabilized... 

The presence of zinc compounds can also promote the reduction of the sulfur rank of crosslinks during high-temperature aging of the vulcanizate, for example, during reversion (Layer, 1992). In some cases zinc compounds actually promote the decomposition of crosslinks (Morrison and Porter, 1984). 

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