Dynamic properties of rubber.

Effect of plasticizer 

The addition of a plasticizer to a polymer should modify its Tg. The amount of change depends on the plasticizer's Tg and the level added. The resultant Tg can be calculated from the same equation used for copolymers (ref. 19):

[Tg.sub.o] = [V.sub.1] [Tg.sub.1] + [V.sub.2] [Tg.sub.2] where: [Tg.sub.o] = resultant Tg; [V.sub.1] + volume of polymer; [V.sub.2] = volume of plasticize; [Tg.sub.1] = Tg of polymer ([degree]Kelvin); [Tg.sub.2] = Tg of plasticizer ([degree]Kelvin).

Changing a compound's Tg through the addition of a plasticizer also broadens the Tg region. Its effect on dynamic properties is therefore different from the lowering of the Tg produced through a structural modification of the polymer. In the latter case, the transition region is usually not widened. This effect is demonstrated in figure 23, where the [delta] peak is shown for PVC plasticized with different levels of DOP (ref.27). Increased DOP levels not only decrease the temperature of the tan [delta] peak, but also increase its width. This widening of the glass transition region is a function of the solubility of the plasticizer in the polymer. Decreased solubility produces a wider tan [delta] peak. In figure 24, the tan [delta] peaks are plotted for PVC containing different volumes of three plasticizers with varying solubility (ref. 28).

The Tg decreases with increased volume, with the dioctyl phthalate producing the lowest Tg. The PVC plasticized with diethyl phthalate, which has good solubility in PVC, has a fairly narrow tan [delta] peak. At the other extreme, the PVC plasticized with dioctyl phthalate, which has borderline solubility in PVC, has a wide tan [delta] peak. This widening of the Tg region increases the polymer's tan [delta] at higher temperatures.

The Tg of a plasticizer or oil depends on its structure and molecular weight in the same manner that the Tg of a polymer does. An aromatic or polar oil, having more rotational hindrance than a paraffinic oil, has a higher Tg. The effect of different types of oil on a rubber's Tg is shown in figure 25 (ref. 29). Here, the polynorbornene compound shows a large decrease in Tg in going from an aromatic to a naphthenic oil and to a mixture of a paraffinic oil and dioctyl adipate. The higher polarity of the dioctyl adipate is overshadowed by its low molecular weight. The effect of an oil's molecular weight on the hysteresis properties of butyl rubber is shown in figure 26 (ref. 30). The higher viscosity oils have a higher molecular weight and reduced number of dangling free ends, which in turn reduces the free volume. The higher viscosity oils therefore have a higher Tg, which leads to a higher tan [delta] or reduced resilience (rebound) for the rubber compound.

Effect of vulcanization 

Vulcanization affects the viscoelastic behavior of elastomers in both the rubbery region and the flow region (refs. 31-33). In the rubbery region, the crosslinks function in the same manner as entanglements; except that they are permanent, and are not disentangled by Brownian motion. Long term creep properties are therefore drastically improved. The permanence of the crosslinks extends up into what is normally the flow region. Here the crosslinks prevent molecular slip-page and the flow region is essentially absent.

The main effect of these crosslinks on dynamic properties is to increase the elastic modulus through a reinforcement effect. The effect of cure time, and the accompanying increase in crosslink density, on dynamic properties is shown in figure 27. The loss modulus is little affected by the cure, and the decreasing tan [delta] is a result of the increasing elastic modulus. Once the cure is complete and the crosslink density reaches a plateau level, the elastic modulus does likewise. The effect of crosslink density on elastic modulus is also shown in figure 28 (ref. 31). In this case the crosslink density is varied through cure system modifications.

The crosslinking has only a small effect on the Tg or loss modulus as long as the chain length between crosslink is larger than the length of the chain undergoing rotational motion. When the distance between crosslinks becomes small enough to hinder rotational motion, both the Tg and the

loss modulus increase. The other factor affecting dynamic properties is the formation of cyclic sulfur or accelerator groups off the elastomer backbone. These bulky groups hinder rotational motion. If present at high enough levels, both the crosslinks and the attached groups can affect an elastomer's Tg. A good example of this is the addition of high quantities of sulfur to natural rubber. Once the sulfur level is above 25 to 30 parts, the polymers Tg increases to a value above room temperature. The resultant ebonite is a glassy high modulus material.

References

(19.) L.A. Wood, J. Polvmer Science, 28, p. 319, (1958). (20.) L.E. Nielsen, J. Am. Chem. Soc., 75, p. 1435, (1953). (21.) R.D. Corsaro, L.H. Sperling, "Sound and vibration damping with polymers," ACS Symposium Series (1989), p. 18. (22.) L.A. Chandler and E.A. Collins, Journal Appl. Polymer Science, 13, p 1585, (1969). (23.) E. Maekawa, R. G. Mancke and J. D. Ferry, Journal Phys. Chem., 69, p. 2811, (1965). (24.) J.F. Hagman, J.W. Crory, Enc_yclopedia of Poly,,mer Science and Engineering, Wiley, New York, 1, p. 281. (25.) B. Hardman, A. Torkelson, Encyclopedia of Polymer Science and Engineering, Wiley, New York, 15, p. 281. (26.) M. Morton, Rubber Technology, Van Nostrand Rheinhold, New York, (1987), p. 370. (27.) K. Wolf Kunstoffe, 41, p. 89 (1951). (28.) L.E. Neilsen, R. Buchdahl, R.J. Leureault, Appl. Phys., 21, p. 607 (1950). (29.) R. F. Ohm, Chem. Tech., 10, p. 183, (1980). (30.) E.B. Storay, Rubber Chem. and Technol., 34, p. 1402, (1961). (31.) M.L Studebaker and J.R. Beatty, Rubber Chem. and Technol., 47, p. 803, (1974).