Compression Set Results

Softer materials normally have better compression set results than the harder grades. This is due to the chains being able to move more readily over each other. Compression set values can also be improved by introducing some degree of permanent cross-linking at the prepolymer stage. If polyurethane is placed under a constant stress, it will creep slowly over the course of time due to strain relaxation. 

The main problem with the compression set results is that the time period of testing is short in relation to the real-life situation for example, a gasket seal may be in service for one or more years before it is removed. It is also found that if left for longer periods than the 30 minutes specified, the compression set becomes less. 

The compression set results are tabulated in Appendix 3, Table 1 and two examples of the results displayed graphically are given for compounds N1 and N2 in Figures 1 and 2. Predictions were made from the compression set results by fitting a dose rate equation and these are tabulated in Appendix 3, Table 2 for 1 and 40 years at 23 °C. 

Extrapolation of the accelerated results to longer times at lower temperatures was attempted by two approaches—the Arrhenius relation and the WLF equation—and compression set results analysed using a dose rate equation. These techniques are outlined below and are discussed in more detail in a guide to the assessment of the useful life of rubbers [8]. 

The two examples of the compression set results presented graphically in Figures 1 and 2, Appendix 3 (for compounds N1 and N2, respectively) illustrate how the rate of change with time will vary for different materials and for different temperatures. As set approaches 100% the rate of change with time will become smaller because of the asymptotic nature of set curves. 

Compression set was shown to be a special case in that the direction of change and the general form of the compression set-time curve are always correctly predicted. Relatively few compression set results were obtained in this work and the predictions made from them always underestimated the long-term set found in natural ageing. However, the results raised optimism that if a quantity of results comparable with that obtained for the other properties were to be obtained good predictions could result. 

The performance of elastomers is of major interest and concern to the design engineer. The readily available data concern the tensile-elongation factor, the compression set, results from durometer tests, and information on oil resistance, heat aging, and the static modulus. 

Various grades of factice are used in loadings of 10-150 phr with typical levels around 20 phr. Addition levels above this wiU lead direcdy to increasing compression set results and diminishing stress-stress properties. The addition of NBR to SBR to improve oil extraction resistance may dictate use of low levels of typical NBR plasticizers but it needs to be understood that the synthetic plasticizer has an affinity for NBR and the addition level should relate to NBR only. Some diester may migrate to the random styrene domains in the SBR but mainly to the ACN portions of the NBR. 

Properties The best physical properties are obtained when the materials are exposed to 12-15 Mrads Irradiation. In Table 3 we show the compression set results at various temperatures and the tensile properties at room temperature. The... 

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

Reduction in compression set began to be achieved in the late 1960s when it was found that tropolene and phenanthroline not only accelerated amine cures but were also effective with certain bisnucleophiles such as resorcinol, hydroquinone and bis-phenol AF. In due course even better results were obtained with quaternary ammonium or phosphonium salts being used in conjunction with aromatic dihydroxy compounds. 

By incorporation of some trihalide to give a branched polymer such as Thiokol ST (about 2% of 1,2,3-trichloropropane is used in this instance). The resultant vulcanisates have lower cold flow and compression set than obtained with Thiokol A. 

The pressure is to be identified as the component of stress in the direction of wave propagation if the stress tensor is anisotropic (nonhydrostatic). Through application of Eqs. (2.1) for various experiments, high pressure stress-volume states are directly determined, and, with assumptions on thermal properties and temperature, equations of state can be determined from data analysis. As shown in Fig. 2.3, determination of individual stress-volume states for shock-compressed solids results in a set of single end state points characterized by a line connecting the shock state to the unshocked state. Thus, the observed stress-volume points, the Hugoniot, determined do not represent a stress-volume path for a continuous loading. 

Residual stress There is a condition that develops, particularly in products with thin walls. This is a frozen-in stress, a condition that results from the filling process. The TP flowing along the walls of the mold is chilled by heat transferring to the cold mold walls and the material is essentially set (approaching solidification). The material between the two chilled skins formed continues to flow and, as a result, it will stretch the chilled skins of plastics and subject them to tensile stresses. When the flow ceases, the skins of the product are in tension and the core material is in compression that results in a frozen-in stress condition. This stress level is added to any externally applied load so that a product with the frozen-in stress condition is subject to failure at reduced load levels. 

This type is produced from di-2-chloroethyl formal with a small percentage of 1,2,3-trichloropropane to provide a branch point for improving the cure state obtainable, and hence compression set. A much lower molecular weight polymer, ca. 80,000, is produced with predominantly mercaptan (SH) end groups. The sulphur content of the resultant polymer is 37%. 

These in situ generated silica fillers also give increased resistance to creep or compression set in cyclic deformations, as is illustrated in Figure 8.6.43 The in situ filled PDMS samples are seen to show very little compression set. They can also provide increased thermal stability, as demonstrated by the results presented in Figure 8.7. 44... 

Starch poly(ethyleniminothiourethanes) prepared by the reaction of SX with poly(ethylenimine) were shown to be suitable reinforcing agents for rubber compounds.79,80 Master batches were prepared using SX with DS values ranging from 0.08 to 0.58 and starch contents of 15 to 50 phr. Best results were obtained at a DS of —0.22 and a loading of 25 phr starch, with a PEI stoichiometric ratio of 3.5. Hardness generally increased with DS and starch content, while compression set and abrasion resistance decreased. 

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