Coating thermal breakdown

Even below the temperature at which thermal breakdown becomes significant, there may be large variations in friction and wear with temperature. The nature and extent of the variations are generally inconsistent and unpredictable, depending on the type of binder, concentration of lubricant, other components present in the coating, completeness of film curing, degree of consolidation and the test method used. 

Figure 8-39. Voltage required for the thermal breakdown of coating (solid curve) and experimental values of the breakdown voltage (points with error bars) as a function of the film thickness.

Dhawan and Trivedi [127] studied the thermal stability of conducting polypyrrole film, grown on the FeCU spray-coated polyvinylacetate film, exposed to pyrrole vapours under mild vacuum. The thermogravi-metric analysis data for the conductive polypyrrole composite showed its stability up to I50°C and after that a continuous weight loss was observed up to 450°C implying the breakdown of the host polymer, polyvinylacetate matrix. The differential thermal analysi.s of the composite also showed first inflection at 205°C followed by a major transformation at 296°C. 

The most interesting example of the use of Sprut-9 took place in Kazakhstan on the Mangyshlak oil field. Very viscous oil is produced on this field, and it has to be preheated for pipehning. Part of a pipeline passed along the bottom of a saline lake, and the water produced rapid breakdown of the thermal insulation and antirust coating of the pipe, sharply decreasing the pipe s transfer capacity. A group of divers... 

PVC has inherently poor stability to heat, even the heat of processing. When thermally decomposed it releases HCl gas and becomes brown and brittle. The acid gas acts as a catalyst to accelerate further breakdown. Calcium carbonate absorbs and neutralizes this acid and therefore acts as a low cost stabilizer. This is a major reason for using this mineral in preference to others. Ultrafme ground grades, coated or uncoated, give the best stabilization. Table 6 shows calcium carbonate usage in PVC. 

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