Swelling Volume

Fig. 1. SAE J200 Classification system for ASTM No. 3 oil where in volume swell nr = no requirement. EPDM is ethylene—propylene—diene monomer HR, butyl mbber SBR, styrene—butadiene mbber NR, natural mbber VMQ, methyl vinyl siUcone CR, chloroprene FKM, fluoroelastomer FVMQ, fluorovinyl methyl siUcone ACM, acryUc elastomers HSN, hydrogenated nitrile ECO, epichlorohydrin and NBR, nitrile mbber.

Structure—Property Relationships The modem approach to the development of new elastomers is to satisfy specific appHcation requirements. AcryUc elastomers are very powerhil in this respect, because they can be tailor-made to meet certain performance requirements. Even though the stmcture—property studies are proprietary knowledge of each acryUc elastomer manufacturer, some significant information can be found in the Hterature (18,41). Figure 3a shows the predicted according to GCT, and the volume swell in reference duid, ASTM No. 3 oil (42), related to each monomer composition. Figure 3b shows thermal aging resistance of acryHc elastomers as a function of backbone monomer composition. 

Fig. 3. The percent volume swell in benzene after seven days at 21°C compared with the wt % of fluorine on standard recommended compounds. A, copolymers of vinyUdene fluoride—hexafluoropropylene B, terpolymers of vinyUdene fluoride—hexafluoropropylene—tetrafluoroethylene C, terpolymers of vinyhdene fluoride—hexafluoropropylene—tetrafluoroethylene-cure site monomer D, copolymer of tetrafluoroethylene—perfluoro(methyl vinyl ether)-cure...

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C—H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at 1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer. 

Volume swell can be quantified during absorption measurements by weighing in water as well as in air, to apply the Archimedes approach. 

FIGURE 38.3 Volume swelling (%) of composites against volume fraction of fiber. (Reprinted from Naskar, A.K., Bhowmick, A.K., and De, S.K., J. Appl. Polym. Sci., 84, 622, 2002. With permission from Wiley InterScience.)... 

The degree of vulcanisation of a rubber compound is assessed technically by the indefinite terms of undercure, correct cure, optimum cure and overcure. It may be given precision by (a) measurement of stress-strain relationship of a range of cures, (b) measurement of the modulus at 100% elongation, (c) measurement of the volume swelling in benzene, or (d) by the use of instruments such as the oscillating disc rheometer and the moving die rheometer. 

This is the percentage increase in volume attained by immersing a vulcanised sample of rubber in a specified solvent. Volume swelling measurements are useful in determining the resistance of rubbers to solvents, but are also of value in measuring the crosslink densities of rubber vulcanisates. 

Now the post-gelation period of the copolymerization of divinyl/vinyl monomers in the presence of a good solvent as a diluent will be considered. Let Qv(x) be the equilibrium volume swelling ratio of the gel formed at conversion x, and Q°(x) its degree of dilution in the reaction system, i.e.,... 

Swelling profile determinations of the hydrogel films are provided elsewhere [35]. The results were given as the volume swelling ratio plotted... 

P2,13 permeability coefficient of solute in water swollen polymer Q equilibrium volume swelling ratio... 

Epichlorohydrin (ECO) has excellent resistance to fuel and oil swell. The ECOs show a volume swell of 35% at room temperature compared to 70% for a medium ACN—nitrile rubber in ASTM Reference Fuel C. The copolymer has a low temperature brittle point of —40°C and the homopolymer,... 

Volume swelling measurements have produced erratic results even under the most carefully controlled conditions. One important contribution in this regard is the work of Bills and Salcedo (8). These investigations showed that the binder-filler bond could be completely released with certain solvent systems and that the volume swelling ratio is independent of the filler content when complete release is achieved. Some thermodynamic problems exist, however, when such techniques are used to measure crosslink density quantitatively. First, equilibrium swelling is difficult to achieve since the fragile swollen gel tends to deteriorate with time even under the best conditions. Second, the solubility of the filler (ammonium perchlorate) and other additives tends to alter the solution thermodynamics of the system in an uncontrollable manner. Nonreproducible polymer-solvent interaction results, and replicate value of crosslink density are not obtained. 

The volume swelling ratios (Vr) are inversely proportional to the equilibrium volume fraction of the rubber in the corresponding swollen systems (filled and gum, represented by subscripts f and 0, respectively), such that ... 

Bristow, G.M. Relation between stress-strain behavior and equilibrium volume swelling for peroxide vulcanizates of natural rubber and cis-1,4-polyisoprene. J. Appl. Polymer Sci. 9, 1571-1578 (1965). 

Another approach to this problem arises from the fact that the number of crosslinks in a network can be evaluated directly. The overall concentration of polymer in the swollen network being v (the reciprocal of the volume swelling degree Q), the number of elastic chains per unit volume of swollen network is given by ... 

When the equilibrium volume swelling degree is low - as it is for PDMS networks obtained at very high polymer concentration (Fig. 6) - it is found experimentally that Q is a linear function ofM3 8. To account for this result one has to develop the logarithmic term in Eq. (11) to the third order. 

The most important result obtained on the structure of model networks is the existence of a rather well-defined correlation distance between first neighbor crosslinks. The affine character of deformations induced by swelling processes is clearly shown by the fact that, for a given gel, the correlation distance d increases as the cubic root of the volume swelling degree Q up to values of Q of the order of 10. 

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