Physical properties and their meaning.

(This is the first installment of a seven-part series - ed.)

Rubber has unique and wide-ranging properties. With an understanding of the meaning of these properties, rubber technologists can effectively incorporate them in a variety of products and applications. Obvious examples of products are tires, heels and erasers. Other examples are not so obvious: rubber bearings which support bridges and large buildings, or rubber insulation used in solid rockets to protect rocket cases against the extremely high temperature of burning propellant.

The many and varied applications of rubber require development of specific properties, or a difficult-to-achieve combination of properties. After development, materials and processes must be controlled to insure that the desired properties are reproduced. These activities are aided substantially not only by familiarity with rubber properties, but also by understanding their meaning. This installment describes physical properties of rubber, their meaning, some methods for determining them, and difficulties associated with some of these methods. Virtually all discussion is on dense rubber; for a discussion of properties of cellular rubber, see Blow (ref. 1).

Many materials behave elastically if the strain is limited to low values. For steel, this value is about 0.2% or less. Rubber behaves elastically even after it is stretched several hundred percent. This property, long-range elasticity, is the single most important property of rubber. Second importance is the glass transition temperature (Tg). This temperature critically affects how rubbery a composition will be at a given temperature. The ability to crystallize is yet another important feature for certain types of rubber.

Before considering specific rubber properties like tensile strength, tear strength and ozone resistance, the three important general properties, long range elasticity, Tg and crystallization, are discussed.

Long-range elasticity

Rubber is composed of extremely long molecules or chains with low inter-chain attraction. When the temperature is sufficiently high, segments of chains rotate about links in these chains. This behavior accounts for a unique rubber property. That is, rubber can be repeatedly stretched to several times its original length and then return to nearly its original length.

For uncrosslinked rubbers stretched slowly, the ability to stretch occurs partly because long molecules or chains slip past one another. When released, the force of retraction is inadequate to restore the rubber to nearly its original length.

Thus, uncrosslinked rubber, when stretched slowly, normally shows less long-range elasticity than crosslinked rubber. Uncrosslinked rubber shows the greatest long-range elasticity when stretched rapidly. Under this condition, there is less time for slippage to occur between chains. The chains entangle and these entanglements act as temporary crosslinks and reduce slippage.

Rubber with permanent crosslinks demonstrates long-range elasticity for both slow and rapid rates of stretching. Permanent crosslinks prevent slippage between chains. Hence, the energy of retraction nearly equals the energy of stretching for many rubbers containing permanent crosslinks (especially unfilled rubbers), providing the temperature is sufficiently above Tg.

Glass transition temperature (Tg)

Any rubber will become hard and brittle if cooled to a sufficiently low temperature, called the glass transition temperature, Tg. This effect is shown in figure I where the log of Young's modulus (E) is shown as a function of temperature for a typical rubber (ref. 2). In figure 1, a letter/number combination identifies different zones.

To the area left of the vertical boundary line between m and IV, behavior is nearly identical for uncrosslinked (dashed line) and crosslinked rubber (solid line). To the right of this boundary, E for uncrosslinked rubber drops sharply with increasing temperature; crosslinked rubber is relatively unaffected.

E is maximum in zone ID where rubber is glass-like. In this zone, E is about one thousand times higher than E in zones III and IVB, where rubbery behavior occurs. The extremely high modulus in zone ID is caused by molecular immobility at low temperature. Immobility occurs because chain segments no longer rotate around links in chains.

As temperature increases, rotation commences and mobility of chain segments increases. About midway down the curve in zone IIC, leathery behavior is observed. The chains are not completely set in position as in the glassy state, nor do they have the complete mobility of the ideal rubber.

As temperature is further increased into zone IIIB, modulus decreases at a lower rate and uncrosslinked and crosslinked rubber behave almost identically. Mobility in zone IIIB is still not sufficient to permit bulk slippage of molecules in uncrosslinked rubber. With higher temperatures in zone IVA, increased mobility causes a sharp drop in E in the uncrosslinked rubber due to bulk slippage. Because of this slippage, the uncrosslinked rubber in zone IVA would show long-range elasticity only if it was stretched at extremely high rates. In other words, stretching would have to be so rapid that entangled molecules did not have time to disentangle.

In contrast, crosslinked rubber demonstrates long-range elasticity in zone IVB because permanent crosslinks prevent bulk slippage of chains. Hence, modulus is relatively unaffected even though chain mobility is high. With still higher temperatures (not shown), rubber modulus increases and this effect is discussed later.

Most elastomers of industrial importance have Tg values of -50[degrees]C (-58[degrees]F) and lower (ref. 3). This means that most rubbers are about 70[degrees]C (158[degrees]F) or more above their Tg for a typical use temperature of about 20[degrees]C (68[degrees]F). For example, the Tg of natural rubber (NR) is -70[degrees]C (-94[degrees]F), while that of silicone rubber is -123[degrees]C (-189[degrees]F). Hence at -70[degrees]C, NR will be glass-like but the silicone will still be quite rubbery. While all rubbers become glass-like at a sufficiently low temperature, only certain rubbers crystallize.

Crystallization

The long molecules in most rubbers are irregular in structure. When one molecule is adjacent to another, the molecules cannot nest tightly because of this irregularity.

But some rubbers, for instance NR, are composed of molecules with an extremely regular structure. Under proper conditions, NR molecules nest together tightly or crystallize; crystallization is accompanied by a decrease in volume. It is possible for crystalline and non-crystalline (amorphous) regions to coexist in the same piece of rubber. Figure 2 shows this phenomenon for uncrosslinked NR which has been partially crystalline for at least 15 years (ref. 4).

To the left and below the broken boundary line in figure 2 the rubber is amorphous and soft (33 Shore A hardness). To the right and above the broken line, the rubber is lighter in color and hard (87 Shore A) because it is crystalline (ref. 4). Density of the crystalline portion is 0.932 gram/cm3 while that of the amorphous portion is 0.912. These differences show the substantial property changes caused by crystallization.

The crystalline region of the rubber in figure 2 is similar to crystallized small molecules, like ice crystals, or crystals formed from cyclohexane. There are also significant differences. The curves in figure 3 show the difference in melting behavior between crystalline NR and crystalline cyclohexane (ref 4). These curves were obtained by differential scanning calorimetry (DSC) using a heating rate of 1.25[degrees]C [min.sup.-1]. With DSC, the power required to melt crystals is determined as a function of temperature. The crosshatched area shows the associated thermal energy needed to melt crystals.

Again referring to figure 3, melting behavior is significantly different for cyclohexane and NR. A sample taken from the crystalline NR of figure 2 melts over a wide temperature range. Crystalline cyclohexane melts over a narrow range because it is composed of small molecules. These small molecules can interchange freely and are free to move to any point on the surface of crystalline cyclohexane. Hence, crystalline and liquid cyclohexane are in equilibrium and a narrow melting range is observed.

A broad melting range is observed for crystallized NR because there is not a true state of equilibrium between crystalline and amorphous phases (ref. 3). Chain segments in the NR are physically connected to crystallises. These segments do not have the freedom of movement to interchange as do the molecules of cyclohexane.

The melting point of the crystalline NR in figure 3 is about 45[degrees]C (113[degrees]F); this is 17[degrees]C (31[degrees]F) above the normal melting point for crystalline NR, which is 28[degrees]C (82.4[degrees]F). Long storage time (ref. 3), strain during storage, or a combination of these factors accounts for the higher melting temperature for the NR in figure 2.

The above discussion on crystallinity is limited to uncross linked NR . Normally cry stallization in uncross linked NR is undesirable because crystalline regions must be melted before rubber can be processed satisfactorily on two-roll mills or in internal mixers. Crystalline rubber is much harder than its amorphous counterpart.

Crystallization also occurs in vulcanized NR. The degree to which it occurs depends upon factors such as the level of strain in the rubber, temperature and the nature of the crosslinking or curing system. If a high-sulfur curing system is used, enough sulfur reacts with the NR to reduce the regularity of the NR molecules. Molecular irregularity reduces the capability of the NR vulcanizate to crystallize. Under appropriate conditions, crosslinked NR will crystallize and this desirable property is discussed later.

Hence, vulcanized NR becomes stiff at low temperature, either because of reduced chain mobility (effect of Tg), or because it crystallizes. These two effects can be separated by measuring the bending stiffness of a rubber beam at low temperature, as described by ASTM D 797. By this method, the increase in beam stiffness related to Tg depends only upon establishing thermal equilibrium.

In contrast, increase in beam stiffness caused by crystallinity is time dependent because of the time required for individual chains to align with one another. To shorten this time, crystallizing rubbers can be exposed to temperatures where their crystallization rate is maximum. For NR this temperature is near -25[degrees]C (-13[degrees]F). An increase in stiffening after 72 hours exposure at this temperature indicates crystallization. Crystallization-caused-stiffening is additive to stiffening caused by reduced chain mobility (effect of Tg).

Another method for separating these effects is ASTM D 1329, a temperature-retraction procedure. It is based on the principle that stretched amorphous rubber below its Tg retracts rapidly at temperatures above Tg. Stretched crystallized rubber retracts less rapidly because time is required to melt crystallises. Therefore, time for retraction differentiates effects caused by Tg and crystallization.

[FIGURES 1 to 3 ILLUSTRATION OMITTED]

References

[1.] J.M. Webster, "Cellular rubber," in Rubber Technology and Manufacture, C.M. Blow, Ed., Newnes-Butterworths, London, 1977, p. 405. [2.] J.J. Aklonis, J. Chem. Education, 58, 892 (1981). [3.] L.R.G. Treloar, Introduction to Polymer Science, Wykeham Publications, London, 1970. [4.] J.G. Sommer, Unpublished data, The General Tire & Rubber Company.

(Part two will appear in the June issue)