What are the graphite impregnants? What are their respective features?

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The properties of the graphite impregnant determine the chemical stability, thermal stability, mechanical strength, and operating temperature of the impregnated graphite. The commonly used impregnants at present can be divided into two main categories: synthetic resins and metals. Under normal circumstances, it is mostly impregnated with synthetic resin, while graphite impregnated with metal is used less frequently. This is because synthetic resins do not have a tendency to melt, and when the operating temperature exceeds the allowable limit, they do not fail quickly like graphite impregnated with metal; when the metal in the impregnated graphite melts, it often sticks to the metal with which it is in contact. Graphite impregnated with synthetic resins also has superior corrosion resistance compared to graphite impregnated with metals. When choosing a metal as an impregnant, two issues need to be considered: the melting point of the metal must not be too low; experience has shown that metals with low melting points such as lead, tin, and babbitt cannot be used as impregnants. Another issue is that the metal used for impregnation must be resistant to corrosion by the medium. In media whose chemical properties are not well understood or where the chemical composition varies significantly, graphite with embedded metal should not be used rashly. For example, in the sealing of crude oil pumps and bottom pumps in atmospheric and vacuum distillation towers, babbitt and graphite impregnated with copper (including copper alloys) cannot be used; instead, graphite impregnated with tin can be selected. Currently, various synthetic resin impregnants are widely used both domestically and internationally. These include phenolic resins, novolac resins, and epoxy resins, etc.; different impregnating agents are selected depending on the properties of the medium. Phenolic resins have good acid resistance, epoxy resins are resistant to corrosion in alkaline media, and phenyl resin has good resistance to both acids and alkalis; therefore, phenyl resin is widely used. In addition, there are those that use impregnated polytetrafluoroethylene suspensions, which offer better corrosion resistance; however, their production process is not very stable, so they have not been widely adopted. As the operating temperature rises, especially when it exceeds 200°C, the synthetic resin decomposes and carbonizes, resulting in a decrease in the hardness and strength of graphite and thereby exacerbating wear. Figure 53 shows the wear amount of this material at different temperatures; the wear amount increases rapidly above 100°C. To overcome the aforementioned drawbacks, graphite impregnated with asphalt was developed. In this process, asphalt is heated to a certain temperature and then impregnated under pressure; subsequently, the graphite is heated again to cause the asphalt to volatilize, decompose, and carbonize, thereby filling the pores in the graphite. This process is repeated multiple times until high-quality graphite is obtained. Table 5 lists the relevant properties of some graphite materials for reference.