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Physical properties and their meaning.

Viscoelastic properties

ASTM D 1566 defines viscoelasticity as a combination of viscous and elastic properties in a material. The relative contribution of each is dependent upon time. temperature, stress and strain rate. Creep. stress relaxation and set are important manifestations of the viscoelastic behavior of rubber.

Creep

Creep refers to the increase in deformation of rubber under a constant force (ref. 13) and it can be measured in tension. compression and shear. For example, creep is measured in compression and shear by ASTM D945. The extent of creep arising from the viscoelastic nature of rubber is proportional to the logarithm of time (ref. 39). This type of creep is known as primary creep since it predominates at short times. Secondary creep may be physical or chemical in origin. Oxygen and ozone are chemical agents which often affect creep. The design of some rubber products limits the rate at which these agents react with rubber and accelerate creep. For example, the small area of exposed rubber in a rubber/metal bearing limits the reaction of these agents with rubber (ref. 28).

Buildings sometimes are mounted on bearings similar to the one in figure 20 (Oct. 1996, p. 27) to isolate them from vibration caused by railroads and subways. Obviously, settling of a building caused by creep must be minimized in this application (ref. 40). Short term laboratory tests were used to predict creep. So far, building creep has followed these predictions exactly; a building mounted on rubber bearings is not expected to settle onto its foundation for over 100 years.

Stress relaxation

Stress relaxation is the decrease in stress with time at a constant deformation. It can be measured in compression by ASTM D 1390, where a rubber specimen is normally compressed 25%. Measurements like these are useful in understanding rubber properties in sealing applications, e.g. gaskets. The decay of contact stress with time sets a limit on the useful life of a gasket (ref.41).

Set

Set refers to the residual deformation in a rubber specimen after completely releasing the load causing the deformation (ref. 13). Set in compressed rubber can be measured by ASTM D 395 using a constant force or a constant deflection. Set results obtained by compression are sometimes quite erratic (ref. 42). A probable reason for erratic results is slippage that occurs between a rubber specimen and the metal plate squeezing it (ref. 19). Reducing slippage causes the formation of concave faces in a specimen. Concavity on these faces makes it difficult to measure specimen thickness accurately.

The presence of oxygen is yet another variable (ref. 43). Compression set for styrene-butadiene rubber (SBR) was significantly higher in oxygen than in nitrogen. These different gases caused little difference in set for IIR. This result is attributed to the much higher rate of oxygen diffusion into SBR, compared to IIR.

Set after break in tension can be measured by ASTM D 412. By this method, the ruptured faces of a tensile specimen are carefully fitted together and the bench marks measured. Set after break is calculated from these bench marks and the original marks.

Set and many other rubber properties are determined under essentially static conditions. Because rubber is used so frequently in dynamic applications, dynamic properties are important.

Dynamic properties

The functioning of many rubber products in service depends critically upon proper control of rubber dynamic properties. When broadly examined, the factors controlling the dynamic properties of a rubber product fall into four categories (ref. 26): composition, processing, design (product size, shape and configuration) and testing (strain rate, strain magnitude and temperature).

Figure 28 shows the effect of strain and temperature at 20 Hz on the elastic component of shear modulus (G') for an NR composition. The figure shows the simultaneous decrease in G' as strain and temperature increase. The decrease in G' is quite significant over the range of variables shown. Dynamic properties are important because they affect the performance of many rubber products in service.

[Figure 28 ILLUSTRATION OMITTED]

Effect of test temperature

Rubber normally is tested at room temperature because of its prevalent use at or near this temperature. Also, testing at room temperature is quite convenient. Because rubber is used over a wide range of temperatures, its properties at low and high temperatures are important.

Low temperature

Rubber parts on airplanes must perform at the low temperatures associated with high altitudes. Standard tests on low-temperature resistance depend on the fact that progressive cooling of rubber increases its modulus and ultimately makes it brittle or glassy.

Change in Shore A hardness (ASTM D 2240) with temperature is frequently measured. This test is both simple to conduct and convenient. Other tests like ASTM methods D 1053 and D 797 are also used. With both these methods, continued stiffening at a given temperature indicates other phenomena are occurring, e.g. crystallization or plasticizer incompatibility (ref. 13).

With D 1053, torsional stress is applied to a small rubber sample by a calibrated torsion wire. Plots of relative modulus are made as a function of test temperature. Relative modulus is the ratio of modulus obtained at test temperature to that obtained at room temperature. Typically, temperatures are reported for relative moduli of 2,5, 10 and 100.

ASTM D 797 was discussed in the section on crystallization along with ASTM D 1329, another test for low-temperature stiffness. In the above low-temperature tests, measurements are made on rubber at low strain rates. Brittleness tests involve testing at high strain rates.

Brittleness properties are relevant to some rubber products, like fuel cells for military aircraft (ref. 11). These rubber cells are used to store fuel and they must not shatter when a bullet hits them at the low temperatures encountered at high altitudes. A widely used brittleness test is ASTM D 746. With this method, a cantilevered specimen is struck at about 2 m/s (6 ft./sec.) at low temperatures. The brittleness temperature may be calculated by two different means described by this ASTM test.

In this test either a gas or a liquid cools a specimen. The longer time required for a gas to cool the specimen is a disadvantage, especially if a specimen is prone to crystallize (ref. 13). Liquids, of course, cool a sample more rapidly. They should not swell or otherwise adversely affect a rubber test specimen.

The ruptured surface of a rubber specimen which fails by embrittlment typically exhibits a glass-like appearance. The failed surface of a tensile specimen is normally ragged after the specimen ruptures at room temperature.

The brittleness test does not provide useful information about the stiffness, nor the crystallization behavior of rubber. This test is sometimes supplemented by stiffness tests over a range of temperatures, or over a period of time at constant temperature.

High temperature

The unusual effect of high temperature on a rubber vulcanizate can be demonstrated easily with an ordinary rubber band. The band is suspended from a hook and a weight is hung onto the lower portion of the band. Upon heating the band, the weight rises because the higher temperature raises modulus of the rubber.

The increase in modulus with heating is known as the Joule effect. The weight rises because higher temperature excites rubber molecules in a way that tends to reduce distance between ends of molecules. At constant length, increased tension results.

This behavior has practical implications, in o-rings for example (ref. 44). O-rings in contact with rotating shafts are often used to contain (seal) fluids. As temperature increases, an o-ring tends to contract, causing increased tension in it. If an o-ring grips a shaft too tightly, it will ultimately fail.

This behavior is similar to that observed upon heating a gas in a closed container (ref. 3). With heating, gas pressure increases because gas molecules impact the container walls with increasing frequency. Hence, the source of increased gas pressure and increased rubber modulus with higher temperature is kinetic in origin.

Fatigue and cut growth

Fatigue failure of rubber results from the growth of one or more cracks (ref. 45). These cracks develop from small flaws during repeated deformation of rubber. Growth of cracks is very slow at first, but may accelerate rapidly as flaw size increases. In severe cases, a crack propagates completely through a rubber component.

Cut growth is the growth of a deliberately-introduced cut caused by repeated deformation of rubber. Generally, the deliberately-introduced cut is much larger than the natural flaws that occur in rubber. Natural flaws are about 40 gm (0.002 in.) in size (ref. 23). Flaws caused by nicks, surface irregularities, cut edges, undispersed particles, etc., are often much larger.

Flaws cause local stress concentrations in rubber. If the stress is sufficiently high, as in tensile testing, failure occurs in a single cycle. With typical cut growth tests, the cut growth per cycle is low because stress is relatively low. Therefore the time required for cut growth tests is relatively long because crack length increases only slightly per cycle.

Fatigue and cut growth properties deserve considerable attention because rubber is repeatedly deformed in many applications. In some of these applications rubber deforms at constant strain, and modulus of the rubber is therefore important. The energy available to propagate cracks is partly determined by the modulus. Higher rubber modulus in a constant strain cycle provides more energy to cause crack growth.

For a constant stress cycle, higher modulus in rubber reduces the deformation per cycle. For these reasons, fatigue test methods should be considered in terms of the end use application. These considerations are often complicated by factors such as non-uniform strain cycles, frequencies, temperatures, loads, and combinations of these factors.

A number of machines are available for crack growth and fatigue testing. Testing by the DeMattia machine (essentially a constant strain test) is the only test considered here. ASTM D 430 (method B) describes the use of this machine for crack initiation or fatigue tests; ASTM D 813 describes the use of DeMattia machine for crack growth tests. Crack growth tests are run only in the bending mode. Crack initiation tests (D 430) are run in either bending or extension.

Specimen size in the bending mode is 150 mm (6 in.) long x 25 mm (1 in.) wide x 6.4 mm (0.25 in.) thick; a transverse, semi-circular groove is molded in the specimen center. ASTM D 430 mentions that the surfaces of specimens for crack initiation tests must be smooth and free of irregularities (unintended flaws). Otherwise these flaws might initiate a crack prematurely. Specimen thickness is important because it affects stress in the specimen surface.

Unintended flaws are less important in crack growth tests by ASTM D 813. Prior to testing, a dominant flaw of controlled size is placed in the groove of specimens by piercing. When a specimen is flexed in bending, cracks propagate from this flaw. The resulting crack growth data can be reported in the several ways described by this ASTM standard.

Rubber fatigue and cut growth are affected by other factors such as rubber composition, crosslink density (state of cure) and crystallite formation. There is an optimum crosslink density for maximum fatigue life (figure 27). Long fatigue life is favored by rubbers which crystallize when stressed, e.g. NR. Crystallites form at the tip of a growing crack in NR and reduce the rate of crack growth, thus extending fatigue life (ref. 46). The formation of these crystallites is a factor affecting local hysteresis (ref. 34) at the tip of a crack.

[Figure 27 ILLUSTRATION OMITTED]

Hysteresis and resilience

Hysteresis in the bulk of a specimen is defined simply as the energy lost when a rubber specimen is deformed and then released (ref. 36). It is the result of internal friction and is evident as heat. Hysteresis is commonly measured as a temperature increase in a flexed specimen (heat buildup) as described in ASTM D 623.

In Method A of D 623, a Goodrich Flexometer subjects a specimen to rapidly oscillating compressive stresses superimposed on a constant compressive load. In Method B (Firestone Flexometer), a rotary motion is applied to a specimen while it is under a constant compressive load. Additional details of these tests are described in the ASTM standards.

Resilience is the ratio of energy output to energy input in a specimen, after the specimen has recovered from deformation. The input energy to deform a sample is therefore equal to the sum of the energies for hysteresis and resilience.

Several ASTM methods are available to determine resilience. With the Bashore Resiliometer (ASTM D 2632), a guided plunger falls freely onto the horizontal surface of a rubber specimen. The height that the plunger bounces is a measure of specimen resilience.

With ASTM D 1054, a free-swinging pendulum impacts the vertical surface of a rubber specimen. The extent of pendulum rebound determines resilience. While this test is simple to run, it is affected by factors such as the presence or absence of mold release on the specimen surface. For this and other reasons, good correlation should not be expected among different resilience methods (ref. 13). Yet another resilience method is ASTM D 945 (mechanical oscillograph), where resilience is obtained on a specimen in compression or shear.

Resilience measurements are sensitive to specimen temperature; minimum resilience for any rubber occurs midway up the curve in zone IIC of figure 1. Here, viscous dissipation of energy (damping) is maximum.

References

(3.) L.R.G. Treloar, Introduction to Polymer Science, Wykeham Publications, London, 1970.

(11.) J.R. Beatty, "Physical testing," presented at the Tenth Annual Lecture Series (Akron Rubber Group), Akron, OH, Feb. 5, 1973.

(13.) F.S. Conant, "Physical testing of vulcanizates," chapter 5 in Rubber Technology, M. Morton, Ed., Van Nostrand Reinhold Company, Second Edition, 1973, p. 114.

(19.) J.R. Scott, "Testing procedures and standards," chapter 11 in ref. 1, p. 46.

(23.) A.N. Gent, "Strength of elastomers," chapter 10 in ref. 17, p. 419.

(26.) J.G. Sommer and D.A. Meyer, Society of Automotive Engineers paper 730267; J. Elastomers and Plastics, 6, 49 (1974).

(28.) D.A. Meyer and J.A. Welch, Rubber Chem. Technol. 50, 145 (1977).

(34.) A.G. Thomas, Rubber Developments, 19, 14 (1966).

(36.) J. R. Beatty and M.L. Studebaker, Elastomerics 109, 33 (1977).

(39.) C. Metherell, Journal Polymer Science: Polymer Physics Edition, 16, 813 (1978).

(40.) C. J. Derham, paper F in "Rubber in engineering - 1973 Conference," Natural Rubber Producers Research Association, p. F/1.

John Sommer, an educator ad consultant, is president of Elastech, Inc., a firm Specializing in elastomer technology. His publications on elastomers include 30 technical papers and book chapters, in addition to 16 U.S. patents. He is a registered professional engineer in Ohio. (41.) P.K. Freakley and A.R. Payne, "Theory and Practice of Engineering with Rubber," Applied Science Publishers Ltd., London, 1978, p.36.

(42.) R.D. Stiehler, ASTM Special Technical Publication 553, 1974, p. 100.

(43.) M. Lowman, Elastomerics 112, 32 (1980).

(44.) T.J. McCuistion, India Rubber World 125, 575 (1952).

(45.) P.B. Lindley, Rubber Developments 19 (4) 168 (1966).

(46.) E.H. Andrews, J. Mech. Phys. solids 11, 231 (1963).
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Title Annotation:part 6; rubber
Author:Sommer, John G.
Publication:Rubber World
Date:Feb 1, 1997
Words:2521
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