Compressibility factor typical values

As with pure substances, the properties of mixtures can be tabulated for future reference. However, such tabulations quickly become impractical as the number of components increase. It is therefore of great practical value to be able to obtain the properties of mixture by calculation from an equation of state, as done for pure fluids. In general, equations of state developed for pure fluids maybe extended to mixtures provided that composition is properly accounted for. Cubic equations such as the Soave-Redlich-Kwong and the Peng-Robinson equation require two parameters, a and b (see Chapter 2). For mixtures, these parameters are typically calculated from the corresponding parameters of the individual components. In this approach, the effect of composition is reflected in the mixture parameters a and b. Once these parameters have been determined, the calculation is identical to that for pure components. This includes the calculation of the compressibility factor, molar volume, residual enthalpy, and residual entropy. 

The parameter Cj for the point j is unity when the deviation between the value, in this case pressure, calculated from the equation of state py calc, T (meas), p (meas), n and the experimental value p jmeas, T (meas), p (meas) is equal to the experimental uncertainty Opj. Typically, for a reference equation of state, the should be less than unity for almost all data (typically > 95 % of the points if a is considered to be equal to two times the standard deviation as appropriate for an expanded uncertainty at a confidence interval of 0.95). In eq 12.1 the calculated pressure depends on the parameter vector n, thus on the coefficients of the equation of state that are fitted. In practice, the dimensionless compression factor Z is commonly used instead of the pressure. Thus, the residual becomes... 

Table 8.1 Typical Values for the Critical Compressibility Factor...

Since about half of the applications for fluoroelastomers are O-rings and gaskets, compression set resistance is a key property compounder that typically aim for a value of 20% after 70 h at 200°C for a 75 Shore A formulation. For higher fluorine types, compression set resistance is usually a bit worse, in the 30%-40% range. The major factor for optimizing compression set is the selection of the type and level of curative. Amine curatives are the least effective and led to the development of bisphenol curatives. Until recently, peroxide curatives could not provide the excellent compression set resistance offered by bisphenol cure systems. However, recent advances in development of new cure site monomers have addressed these issues, even offering the capability to reduce the post-cure cycles to only 1 h [15-18]. 

Results of the hardness test regression are summarized in Table 4. The p-value and adjusted R squared indicate a highly significant regression that explains 63.5% of the variation present. Both factors and the interaction between them are statistically significant. As expected, an increase in mold temperature resulted in a drop in hardness, which is due to the thinner skin thickness. As expected, an increase in the lubricant content resulted in an increase in the hardness, which is due to the thicker skin. The interaction shows that at higher mold temperatures, the increased lubricant content resulted in a lower hardness than at lower lubricant levels. However, neither of the factors showed a substantial increase in hardness the largest factor effect was a decrease of approximately 2 points on the Rockwell R hardness scale. This is not unexpected since the skin is relatively thin (typically 500-600 microns) and a hardness test compresses the entire cross-section of the sample. 

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