Physical properties and their meaning.

(This is the seventh installment of an eight-part series)

Abrasion

Abrasion consists of the rupture of small particles of rubber under frictional forces, when sliding occurs between a rubber surface and a substrate (ref. 23). Abrasion of rubber occurs under a variety of conditions (ref.35).

For example, in sandblast hose, lightweight sand particles impact rubber at a high speed. A tire tread might scrub over a road surface over a range of speeds while it is under a heavy load.

A number of laboratory tests are available to estimate abrasion properties of rubber in different applications. These tests depend upon producing relative motion between surfaces of rubber and an abrasive (ref. 19). These surfaces are pressed together by a controlled force during testing. Only two tests are considered here - ASTM Methods D 1630 and D 2228.

* D 1630 - In this method, a rubber test piece presses against a rotating cylinder that is covered with abrasive paper. This test primarily determines the abrasion resistance of vulcanized rubber compounds for use in soles and heels of footwear. Vulcanizates examined should be at least 2.5 mm (0.1 in.) thick; the abrasive paper is a 40 grit, No. 1-1/2 garnet paper. Because some compounds tend to clog the abrasive paper, a suitable suction device or air pressure may be used to remove abraded rubber particles.

* Abraded particles are handled differently by the Pico Abrader (ASTM D 2228), where a pair of knives rub over a rubber specimen in a rotary manner. The resulting abraded rubber particles are engulfed by a dust applied at the interface between knives and specimen. The knives have a specified geometry and are made of tungsten carbide to resist wear. The Pico Abrader is a popular instrument because it shows good agreement, both within and among laboratories (ref. 13). It has predicted tire performance reasonably well (ref. 11).

It was earlier mentioned that friction is a necessary element in abrasion; friction is considered now.

Friction

Rubber does not obey the classical law of friction (ref. 47). This law states that the ratio, frictional force/normal load, is a constant called the coefficient of friction ([micro]). For rubber, decreases with increasing normal pressure.

Another difference with rubber friction is stick-slip that occurs when friction falls rapidly as velocity increases. The associated changes in force (often rapid) complicate the interpretation of friction measurements.

As expected, boundary conditions at a sliding rubber surface greatly affect frictional behavior. This effect was shown in figure 15 where boundary conditions significantly altered the mechanical behavior of rubber squeezed between plates. If reproducible friction measurements are to be obtained, experimental conditions must be carefully controlled.

[Figure 15 ILLUSTRATION OMITTED]

Friction properties for rubber vary widely. For waterlubricated rubber bearings, levels of [micro] are low. With tires, high levels of [micro] are needed to provide good directional control of vehicles.

There are few standardized tests for measuring [micro]. Probably the most common is ASTM E 303, which uses the British Portable Skid Tester. A rubber slider (the test specimen) is attached to the base of a pendulum (figure 29). As the pendulum swings, the rubber surface of the slider contacts and slides across a test surface. Friction is determined from the degree of swing of the pendulum after contact. This tester is portable and friction can be determined against either dry or wet pavement surfaces. These features are significant advantages.

Adhesion

Adhesion depends upon factors other than the composition of a rubber part. Adhesion involves mating rubber with other materials to form composites like rubber/textile and rubber/ steel. Examples of composites are rubber products such as tires, hose, belting and bridge bearings. In service, these products are subjected to a variety of deformations.

To approximate these deformations and to accommodate measurement of adhesion properties, a number of different adhesion tests are available. Some of these are listed in table 1. Only the last two standards are considered further

[Table 1 ILLUSTRATION OMITTED]

The specimen used in Method A of D 429 was discussed earlier under biaxial tension (p. 27 October, 1996). In Method B of D 429, bonded rubber is peeled from metal using the fixture shown in figure 30. Rubber peels from the metal at approximately a 90[degree] angle because the fixture rotates about a pin. This pin holds the top portion of the fixture (not shown) in the test machine.

[Figure 30 ILLUSTRATION OMITTED]

When rubber peels from the metal, peeling force (F) often varies rapidly and substantially. While minimum force or maximum force may be reported, most often the average force is reported.

As with Methods A and B of D 429, Method C is useful for development and control of rubber adhesives and compounds. Method C is also useful for evaluating test specimens prepared with two separate adhesives or different preparation methods. Adhesion can also be examined in shear.

The shear specimen shown in figure 24 is tested to failure by BS903, Part A28, to determine bond strength (ref. 19). This quadruple specimen is advantageous because its bonded faces remain essentially parallel to one another as a force (F) is applied; also, the shearing forces for this specimen share a common axis. Hence, the quadruple specimen (figure 24) does not tend to rotate during test, as does the single-lap specimen (figure 23).

[Figure 23,24 ILLUSTRATION OMITTED]

Permeability

Permeability tests measure the ease with which a gas or liquid passes through a rubber sheet. When measuring permeability, it is important to establish equilibrium flow. For liquids, permeability can be measured by ASTM D 814 for a sheet of moderate thickness; ASTM D 815 is used to measure permeability of hydrogen through rubber. Permeability is important because rubber is often used as a seal, e.g. to contain a gas for long periods at high pressures.

The driving force for permeation is the concentration gradient of a material (gas or liquid) across a rubber sheet. Permeation consists of the material going into solution on one side of the sheet. The material then diffuses through the sheet and evaporates on the other side. Two important characteristics of a permeating material are its solubility and diffusion rate. Permeation behavior is described for a variety of materials (ref.48).

Deterioration

Rubber encounters a number of different exposure conditions during service in various applications. These conditions deteriorate rubber properties at different rates and to different degrees. Conditions include exposure to: weather, ozone, radiation, flame, high and extremely high temperatures.

Weather

The change in rubber properties with outdoor exposure is quite variable, depending upon location of exposure. In some locations, intense sunshine and high temperature are the major cause for deterioration; in others, ozone might be the major cause. Laboratory tests are available to measure deterioration and simulate weathering under controlled conditions. Chief among these tests is ASTM D 750.

In this test, rubber specimens are placed in a jig or holder, with or without elongation. The specimens are then exposed to light richer in the ultraviolet range than sunlight, but otherwise similar to sunlight.

This exposure generally deteriorates the rubber surface and causes flaws on the specimen surface. Since surface flaws reduce tensile strength and breaking elongation, there is a concern about this measure of deterioration. Estimation of the extent of surface crazing and cracking is an alternate method for measuring weathering. A reference specimen should be included in weathering tests to make results more meaningful.

For an overall assessment of weathering, specimens are often exposed under the different climatic conditions that occur in different locations. Commercial services are available for testing in these different locations, e.g. in the south and southwest.

Ozone

Ozone occurs generally in the atmosphere in extremely small amounts, usually only several parts per hundred million (pphm); even these low amounts can cause stressed rubber to crack. The cracks occur at right angles to the direction of the applied stress. Stress is important because it affects crack size and severity (ref. 23). The most harmful condition occurs just above the critical stress level (about 0.06 MPa or 8.7 psi), corresponding to a critical strain of about 3%.

Just above the critical stress level, widely-separated deep cracks form that are more likely to cause product failure than small cracks (ref. 23). Increasing the stress or strain favors formation of smaller but more frequent cracks. Another factor affecting crack size and frequency is crazing of a rubber surface caused by sunlight. This crazing pattern is similar to the mud-cracking pattern observed on a river bank after the mud dries rapidly. The effect of this factor was observed upon comparing the shaded side and the exposed side of an inflated tire exposed to sunlight (ref. 49). On the exposed side, crazing was observed along with a multitude of small ozone cracks. On the shaded side, there were far fewer but considerably deeper cracks. These differences were attributed to the formation of a resinous film on the exposed side of the tire (ref. 49). This result is consistent with observations by other workers (ref.35).

ASTM D 1149 is probably the most widely-used accelerated test for ozone cracking. This test permits use of three different types of specimens, all of which are deformed during testing. Test results are reported by two different methods. The time is given for cracks to appear or the character of the cracks is described for various periods of exposure.

Ozone concentrations were formerly expressed as pphm, on a volume-per-volume basis. Concentrations are expressed now as ozone partial pressure by ASTM D 1149-78 and the standard partial pressure is 50 mPa.

Partial pressure is used now because atmospheric pressure fluctuations affect the rubber cracking rate. The use of the partial pressure of ozone takes atmospheric pressure into account. The conversion at standard conditions of atmospheric pressure (760 mm) and temperature (273 K) is: I pphm = 1.01 mPa.

High-energy radiation

Rubber requires resistance to gamma radiation when it is used in applications like nuclear reactors or nuclear submarines. Radiation deteriorates initially optimized rubber properties (ref. 50). For instance, radiation usually causes increased modulus and hardness, and decreased tensile strength and breaking elongation. For those rubbers which predominantly crosslink during irradiation, a 100 megarad radiation dose about doubles or trebles the original S-100 value (ref.50).

Tests for measuring radiation effects are:

* ASTM D 1672 - Exposure of Polymeric Materials to High-Energy Radiation

* ASTM D 1519 - Rubber Chemicals-Melting Range

In some applications a rubber component must have both radiation resistance and flame resistance.

Flame

A nuclear power cable is an example of a rubber component with both radiation- and flame-resistant properties (ref. 51). Physical properties of the EPDM insulation used in a cable changed only slightly after exposure to [10.sup.7] reds; the cable showed good flame retardant properties by horizontal and vertical flame tests.

Flammability properties of rubber are described by its response to a specific ignition stimulus (ref. 51). Typically the flammability of a material is expressed in terms of:

* its ease of ignition;

* surface flame spread;

* amounts and rates of production of heat, fuel, smoke and toxic gases.

A number of fire response tests are available to measure these properties under strictly specified conditions. Several of these are listed in table 2.

Table 2-flammability tests used for rubber products

Test                 Used for
DOC FF-4-72          Mattresses and upholstery
  (cigarette test)
DOC FF-1-70          Carpet backing and rug underlay
  (methenamine pill test)
ASTM E 84            Carpet backing and rug underlay
  (Steiner tunnel test)
ASTM E 162           Interior of mass transit cars
(radiant-panel test)
IEEE 383             Wire and cable
(ribbon burning)
MIL R 200 92 G       Shipboard mattress and rubber
(hot-bolt test)       sheets
ASTM D 2863          Laboratory research test
(oxygen index test)

High temperature

Rubber test specimens and components are frequently aged at high temperatures to increase their rate of deterioration. This rate is critically affected by a number of factors, including:

availability of oxygen, specimen size and shape, nature of the rubber (saturated vs. unsaturated backbone, e.g. EPDM vs. NR) and velocity of air across a rubber surface during aging. Because of these factors, a specimen of known performance characteristics should be included in aging tests as a control.

Two tests frequently used for accelerated aging are described in ASTM Standards D 573 and D 865. D 573 compares the degradation resistance of different vulcanizates when they are heated in air at atmospheric pressure. The change in several different rubber properties is determined with aging time, for example changes in tensile and elongation. A potential problem with D 573 is the transfer of volatiles (especially antioxidants) from one vulcanizate to another during aging.

This problem is avoided by D 865. Each vulcanizate is heated in its own container, each with its own air circulating system. This technique prevents transfer of volatiles between different vulcanizates. Firmer conclusions can be drawn from this aging test.

Of course aged properties of rubber depend upon the stability of the rubber involved. A typical aging temperature for rubber like NBR and SBR is 100[degrees]C (212[degrees]F). Sometimes, rubber is tested and used at extreme temperatures.

Extremely high temperatures

Rubber used as thermal insulation for solid rocket motors experiences temperatures of about 2,760[degrees]C (5,000[degrees]F) (ref. 10). This rubber insulation protects rocket cases from the intense heat of burning propellant. Insulation was tested with an oxyacetylene torch with a slightly reducing flame to simulate service conditions (ref. 10). During test and in service, the rubber degrades to form intermediate materials (ultimately char) which also provide insulation.

After propellant ignites, pressurized gases from burning propellant expand a rocket case. Rubber insulation, bonded to the case, accommodates this expansion. The rubber then degrades as it sees progressively higher temperatures. In this unique application, both the original rubber and its degradation products serve as insulation.

References

[11.] J.R. Beatty, "Physical testing," presented at the Tenth Annual Lecture Series (Akron Rubber Group), Akron, OH, February 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.

[35.] A.E. Juve, chapter 19, "Physical testing" in Introduction to Rubber Technology, M. Morton, ea., Reinhold Publishing Corp., New York, 1959, p. 462.

[47.] A. Schallamach, chapter 13 in The chemistry and physics of rubber-like substances, L. Bateman, ed, Maclaren and Sons Ltd., 1963, p.355.

[48.] G. J. van Amerongen, Rubber Chem. Technol. 37,1065 (1964).

[49.] D.A. Meyer and J.G. Sommer, A.S.T.I.A. report no. AD 143312, June 30,1957 p.13.

[50.] J.W. Born, chapter 7 in Radiation effects on organic materials, R.O. Bolt and J.G. Carroll, editors, Academic Press, 1963, p.245.

[51.] H.J. Fabris and J.G. Sommer, Rubber Chem. Technol. 50,523 (1977).