Dynamic properties of rubber.

Influence of compounding ingredients on the dynamic properties of rubber compounds

Mechanical blending

The dynamic properties of blends depends on the miscibility of the elastomers (ref. 21). Elastomers having close solubility parameters are completely miscible and have a single narrow Tg. One of the few examples of this is a blend of nitrile rubber and PVC. A blend that is slightly immiscible still has a single Tg; but it is broader or extends over a wider temperature range. At the other extreme are two elastomers that are completely immiscible. In this case, there are two Tgs, at temperatures corresponding to the unblended elastomers. If they are slightly miscible, these Tgs will shift closer together. The elastomer constituting the continuous phase usually has the greater influence on dynamic properties.

Typical intenal or mill mixing is frequently not capable of mixing two elastomers intimately enough to form a miscible blend. However, the high temperature of vulcanization will sometimes increase mutual solubility enough to form a miscible blend.

It is also possible to polymerize an elastomer containing two immiscible portions. An example of this is nitrile rubber. It has been reported to have two separate Tgs when the nitrile content is within a certain range (ref. 22).

Effect of molecular weight

The molecular weight distribution affects the dynamic properties of a polymer only if there are chain fragments with molecular weights too small to form entanglements (ref. 23). These short chains improve molecular slippage and essentially act as plasticizers. Their large proportion of dangling chain ends increases the % free volume of the total polymer. This in turn decreases the glass transition temperature and increases the width of the glass transition region. Crosslinking such a polymer can reduce the molecular slippage of these low molecular weight fragments; but has little effect on the free volume contributed by the dangling ends. The effect on the glass transition temperature and width therefore does not change with crosslinking. Although the much higher molecular weight chains have a large influence on a polymer's long term creep and stress relaxation properties, they have little effect on the short term stress relaxations associated with dynamic properties.

Effect of branching

Long chain branching can also affect a polymer's Tg. If this branching is spaced close enough, the rotational motion of the backbone is impaired with a resulting increase in Tg. With very large distances between branching sites, the effect on rotational motion is minimal. The exact opposite is found when a short flexible molecule is added to the polymer backbone. These shorter molecules do not become entangled with the surrounding polymer, and increase the free volume of the system with a consequent lowering of the Tg. They function as lubricants that reduce the resistance to rotational motion. Polystyrene for example has a Tg of 100[degrees]C; while the addition of an n-octyl group to the para position reduces the Tg to -45[degrees]C.

Typical elastomers

It is interesting to review the effect of structure on the Tg and resulting hysteresis properties of some typical elastomers. Polybutadiene has cis, trans and 1,2-vinyl structures. The effect of the level of these structures on the polymer's Tg is shown in table 7. The absence of side groups to hinder rotational motion in the cis and trans structures cause them to have low Tgs. Polybutadiene containing 98% cis structure (standard BR-01) has a Tg of -102[degrees]C. On the other hand, a polybutadiene containing 70% vinyl, 11% cis and 19% trans structure (Nippon Zeon BR 1240) has a Tg of -30[degrees]C.

Table 7 - polybutadiene polymer
characteristics
Polymer/supplier         Cis   Trans   Vinyl   Tg([degrees]C)
Standard BR-01           98       1       1     -102
Nippon Zeon BR1240       11      19      70      -30
Ameripol 491394          13      70      17      -80
Huls BP 1949             23      28      49      -50
Polysar XC993            54       4      42      -60
Goodyear Budene MV1255   20      35      45      -62

In styrene butadiene rubber, both the bulky aromatic ring and the 1,2-vinyl structures offer resistance to rotational motion. By varying the level of these structures a wide range of Tgs is possible. The wide variation in 1,2-vinyl content, however, is only possible with solution polymerized SBR. In emulsion polymerized SBR, the styrene content can be varied; but the cis, trans and 1,2-vinyl content are fairly constant. Emulsion SBR has a wider molecular weight distribution (more low molecular weight chains), and a fair amount of branching. Also, the styrene content can vary from one polymer chain to another. Solution SBR, on the other hand, has an extremely narrow molecular weight distribution, less branching and more homogeneity. Solution SBR therefore has a narrower glass transition region that emulsion SBR. With a given Tg, the solution SBR has lower hysteresis properties than an emulsion SBR. However, because of the wide variations in structure that are possible, it can be custom polymerized to have low or high hysteresis, depending on its Tg.

Natural rubber contains a pendant methyl group which offers some resistance to rotational motion. This methyl group is not near as bulky as the 1,2-vinyl group in poly 1,2-butadiene. Natural rubber therefore, has a fairly low Tg (-60[degrees]C). It also has a very homogeneous structure and little if any low molecular weight fractions. Consequently, it has a fairly narrow Tg region.

The polar chlorine group in neoprene rubber offers more resistance to rotational motion than does the methyl group in natural rubber. Neoprene, therefore, has a slightly higher Tg (-50[degrees]C).

The Tg of EPDM rubber depends on the ratio of ethylene, propylene and the third monomer. The methyl group on the propylene unit restricts rotational motion (polypropylene has a Tg of -10[degrees]C and polyethylene has a Tg of -120[degrees]C). The bulky third monomer has a similar effect. EPDM rubbers typically have Tgs ranging from about -60[degrees]C to -35[degrees]C.

Butyl and halobutyl rubbers are somewhat unique. The high symmetry imparted by the two methyl groups causes them to have a Tg (-65[degrees]C) much lower than polypropylene (-10[degrees]C). Because of the low Tg, they should have low hysteresis properties at room temperature. However, the exact opposite is found. The reason is due to the wide Tg region which extends above room temperature. This wide Tg region is believed to be associated with the geometrical restrictions to rotational motion caused by the closeness of the dual methyl groups attached to alternate carbon atoms. The rotation of the polymer molecule starts at a low temperature (-65[degrees]C); but the resistance to rotational motion continues to well above room temperature. The chlorine or bromine contents of halobutyl rubbers are less than 2% and their effect on the Tg is minimal.

The intermolecular attraction associated with the polar nitrile group in acrylonitrile-butadiene rubber resists rotational motion. Increasing the nitrile content from 22% to 51% increases the Tg from -43[degrees]C to -12[degrees]C (figure 22).

The polyacrylate rubbers consist of an ethylene backbone with pendant ester groups. The size and flexibility of the side group determine the polymer's Tg. Polyethyl acrylate, for example, has a Tg of -15[degrees]C. Replacing the ethyl with an n-butyl group imparts lubricity and reduces the Tg to -46[degrees]C. The addition of flexible ether groups as in poly 2-methoxyethyl acrylate and poly 2-ethoxyethyl acrylate gives Tgs of -36[degrees]C and -41[degrees]C, respectively (ref. 24).

The highly flexible ether linkage has a major effect on the Tg of polyether elastomers. In spite of polyepichlorohydrin having a bulky and polar chloromethyl side group, it still has a relatively low Tg (-20[degrees]C). Copolymerizing it with ethylene oxide reduces the Tg to -40[degrees]C. Polypropylene oxide is a good example of the effect of the flexible ether linkage. It has a Tg of -65[degrees]C, compared to a Tg of -10[degrees]C for polypropylene. Polypropylene oxide consequently has excellent low damping properties.

In silicone rubber, the high flexibility of the Si-O group in the backbone imparts very low Tgs. Polydimethylsiloxane has a Tg of -123[degrees]C (ref. 25). Replacing one methyl with a phenyl group increses the Tg to -113[degrees]C. Replacing it with a large fluoro containing group increases it appreciabley, as shown in polymethyl 3, 3, 3-trifluoropropylsiloxane, which has a Tg of -65[degrees]C.

Due to an increase in bulkiness and polarity, halogen containing polymers typically have higher Tgs than the corresponding olefin polymer. The fluorine atom has a smaller atomic radius than the chlorine atom. When attached to a polymer it therefore has less bulkiness and a lower dipole moment (reduced polarity) offering less resistance to rotational motion. This is evident when comparing the Tgs of polyvinyl fluoride and polyvinyl chloride. The polyvinyl fluoride has a 100[degrees]C lower Tg. The Tgs of some typical fluorocarbon elastomers are -20[degrees]C for poly (vinylidene fluoride-cohexafluoropropylene) and -28[degrees]C for poly (vinylidene fluoride-co-tetrafluoroethylene-co-hexafluoropropylene).

Chlorosulfonated polyethylene is produced by treating polyethylene with a mixture of chlorine and sulfur dioxide. The resulting chloride and sulfonyl chloride groups attached to the backbone reduce crystallinity and give elastomeric properties to the polymer. They of course also restrict rotational motion and increase the polymer's Tg. The effect of increasing chlorine level is seen in table 8 (ref. 26).

Table 8
             %Cl   Tg
Hypalon 45   24    -30
Hypalon 40   36    -20
Hypalon 48   43      0

[Figure 22 ILLUSTRATION OMITTED]

References

(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, Encyclopedia of Polymer 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.