Set, permanent

When a rubber or elastomeric component is under stress or strain for a period of time the entangled network of polymer chains can begin to undergo viscous non-returnable flow. This permanent flow or deformation is known as set . The resistance to permanent set is important to many sealing applications of rubber materials. 

Permanent set tests can be carried out in tension by applying a tensile load for a time period and measuring the permanent change in gauge length after the load is removed. Care must be taken when setting up such tests, as temperatures, sample shape, time periods of loading and relaxation will clearly all affect results. 

More commonly, compression-set tests are used on rubber-type materials. Rubber-seal type materials are used in compression because the test apparatus is simple and inexpensive to construct. Despite this popularity, stress-relaxation testing gives a more useful insight into the suitability of a material for seals [5]. 

Constant-stress tests use a heavy metal spring applied to a disc-shaped sample. The spring has a precalibrated stress-strain behaviour, and so, by means of a screw, various stress levels can be applied to the sample and maintained over a long period of time. 

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The elongation of a stretched fiber is best described as a combination of instantaneous extension and a time-dependent extension or creep. This viscoelastic behavior is common to many textile fibers, including acetate. Conversely, recovery of viscoelastic fibers is typically described as a combination of immediate elastic recovery, delayed recovery, and permanent set or secondary creep. The permanent set is the residual extension that is not recoverable. These three components of recovery for acetate are given in Table 1 (4). The elastic recovery of acetate fibers alone and in blends has also been reported (5). In textile processing strains of more than 10% are avoided in order to produce a fabric of acceptable dimensional or shape stabiUty. 

Fiber Immediate elastic recovery, % Delayed recovery, % Permanent set, %... 

Permanent Set. When an elastomer is stretched and then allowed to relax, it will not completely recover its original dimensions. This divergence from its original form is called its permanent set. It is principally affected by the affinity of the elastomer for the filler surface and is, therefore, primarily a function of the surface energy or wetting of the filler. 

Typical stress—strain curves are shown in Figure 3 (181). The stress— strain curve has three regions. At low strains, below about 10%, these materials are considered to be essentially elastic. At strains up to 300%, orientation occurs which degrades the crystalline regions causing substantial permanent set. 

At strains over 300% the stress occurs mostiy in the amorphous regions up to the point where the sample breaks. AH of the grades exhibit permanent set, and the curves of grades with a Shore Hardness of 55 and higher exhibit a yield point. This means that parts have to be designed for low strains to stay within the area of elastic recovery. Special grades of elastomer are available to provide hydrolysis resistance (194), improved heat aging (195), and improved uv-stabihty (196). 

Permanent set and low hysteresis properties depend on minimizing the viscous or plastic component of modulus. Because cross-linking increases elasticity, a high state of cure typically provides the best compression set and heat buildup properties. 

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

Three forms of set may be conferred to wool fibers. (/) Cohesive set is imparted when the fibers are dried under strain or set in steam and is lost when the fibers are relaxed in water at room temperature. (2) Temporary set is imparted at higher temperatures and is lost when the fibers are wet out in hot (70°C) water. (2) Permanent set is imparted in boiling water and in high pressure steam, and is stable to release in hot water. 

In addition to the restrictions on their mobiHty caused by steric and polar interactions between chemical groups, the protein molecules in wool fibers are covalentiy cross-linked by disulfide bonds. Permanent setting only occurs if these disulfide bonds are also rearranged to be in equiHbrium with the new shape of the fiber. Disulfide bond rearrangement occurs only at high temperature (>70° C) in wet wool and at even higher temperatures (above 100°C) in... 

The amount of set imparted in steam-setting depends on the pH and moisture content of the wool as well as the time and temperature of setting. In continuous machines the conditions are mild and Htde permanent set is imparted. In batch machines, the wool is often set at temperature up to 136°C for 3—5 min. This imparts large amounts of permanent set to the fibers. Impregnation of the wool with reducing agents increases the rate of permanent setting. The amount of cohesive set imparted in both batch and continuous machines depends on the temperature of the fibers when the fabric is released from the wrapper cloth. The cooler the fabric, the more cohesive the set imparted. 

Although designed to color the fiber, dyeing operations also impart large amounts of permanent set to wool. 

Now, to be successful, a spring must not undergo a permanent set during use it must always spring back. The condition for this is that the maximum stress (eqn. (12.3)) always be less than the yield stress ... 

Properties such as low permanent set, low creep and low hysteresis are really measures of the efficiency of the heat fugitive network system. This is a complex function of the morphology. As a very general statement, the problem would seem to be less important with the harder grades of thermoplastic elastomer. 

Polychloroprene polymers also vary in the degree of branching in the polymer. Polychloroprenes with little or no branching are called sol polymers, whereas those with considerable branching are referred to as gel polymers. Sol polymers are soluble in aromatic solvents. All of the solvent-grade polychloroprene polymers (except Neoprene AG) are sol polymers. The gel content in the polychloroprene affects the cohesive strength, resilience, elongation, open tack time, resistance to permanent set, and oil swell. 

Neoprene 571. It contains a very high-gel polymer which allows to produce high-strength films with low permanent set. 

Rubbery materials are defined by ASTM (D1566) as those which will have less than 50% permanent set after one minute when recovering from a strain of 100% applied for one minute. Of the many rubbery materials available, block copolymers are by far the most common used in hot melts. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

The important vulcanizate properties demanded by this application, low heat buildup, and low dynamic set have been determined in the Goodrich Flexometer test. The compound containing the antireversion agent exhibits a marked decrease in heat buildup and dynamic permanent set (Figures 14.21 and 14.22). 

FIGURE 14.22 Goodrich flexometer dynamic permanent set at 100°C after 30 min (load 108 N, stroke 4.45 mm, frequency 30 Hz). 

Both dynamic permanent set and oil swell resistance are improved on addition of Perkalink 900, indicative of the greater thermal and mechanochemical strength of the cross-links formed from Perkalink 900 compared to sulfidic cross-links. Data are shown in Figures 14.25 and 14.26. 

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