Swelling of vulcanizate

McCathern and Thomp n (69) studied the swelling of vulcanizates in various media in relation to the GC determined interaction parameters. It was found that for over 50 polymer-solute pairs a most satisfactory correlation was observed between the volume increase and the x parameter. Considering the concentration range covered (volume increases of up to 400%) and the concentration dependence of the... 

This equation calls attention to the well-established inverse relationship between the degree of equilibrium swelling of a series of rubber vulcanizates in a given solvent and the forces of retraction, or moduli, which they exhibit on stretching. The indicated approximate dependence of qm on the inverse three-fifths power of the modulus has been confirmed. 

The description of the physical properties of fluoroelastomers is necessarily less precise than that of fluoroplastics because of the major effect of adding curatives and fillers to achieve useful cross-linked materials of a given hardness and specific mechanical properties Generally, two parameters are varied increasing cross-link density increases modulus and decreases elongation, and raising filler levels increases hardness and decreases solvent swell because of the decreased volume fraction of the elastomer In addition to these two major vanables, the major determinants of vulcanizate behavior are the chemical and thermal stabilities of its cross-links The selection of elastomer, of course, places limits on the overall resistance to fluids and chemicals and on its service temperature range... 

X parameter, a perfect correlation was not to be expected. From the correlation curve so-established the swelling of experimental vulcanizates could be predicted with reasonable accuracy from the easily determined x parameter. The GC method diould be particularly valuable for testing experimental samples, whenever only small amounts are available. For non-crosslinked materials, the magnitude of the x parameter is a direct indication of the solubility of the polymer in any given solvent. The dividing line between solvents and non-K>lvent of the polymer can be drawn at approximately x = 0.5, the smaller the values of x below this limit, the better the solvent. It is a simple matter to measure the retention characteristics of a series of probes and thus determine a suitable >lvent for any pdymer. It should be noted, however, that the temperature at which tfie GC determination is possible (T>Tg+ 50) may sometimes ermeed the temperature of interest and the dependence of x on temperature may have to be assessed. 

Figure 15.33 shows benzene uptake by natural rubber samples. Filled samples absorb less solvent (lower swelling). The carbon black containing sample had a lower benzene uptake than the silica filled sample. The lower swelling of the carbon black containing sample is due to high bound rubber content, the crosslink density of the black filled vulcanizate, and a strong rubber-filler interaction. 

Experimental Results. Equation 5 cannot be used to calculate C unless the values of the constants are known. An explicit relation for C is given by equation 6 in which the effect of the rubber pressure has been neglected. Equation 6 has been used to calculate the equilibrium amount of water absorbed by natural rubber (vulcanizate B) from a salt solution containing 10% sodium chloride. The value of A cannot be calculated since the nature and concentration of the impurity in rubber is unknown but its value was estimated from the observed equilibrium swelling of rubber in a saturated salt solution. The value of A was found to be 1.14 x 10" and the calculated value of the concentration of water in the rubber at equilibrium in the 107. salt solution was 0.0140 gm cm which was in excellent agreement with the experimental value of... 

The following data were collected on the swelling of polybutadiene vulcanizates of different sulfur contents in toluene at 25° C ... 

Natural rubber of Mn = 2.23x10 was vulcanized with 2% sulfur. Calculate an estimate of the extent of swelling of the vulcanizate in benzene at equilibrium, given that the interaction parameter of rubber and benzene is 0.41. Assume that all sulfur is used in crosslinldng and that there is, on the average, one crosslink for every eight sulfur atoms. [Data Rubber density = 0.91 g/cm molar volume of benzene = 89.4 cm- /moL]... 

FIGURE 14.7 The swelling of butyl vulcanizates by cyclohexane plotted as a function of the elastic modulus using Equation 14.24. (From Flory, P.J., Ind. Eng. Chem., 38, 417, 1946. With permission.)... 

Swelling of aciylonitrile-butadiene rubber vulcanizates in organic liquids... 

In addition to use of organic chemicals, mbbers can be devulcanized by means of inorganic compounds. Discarded tires and tire factory waste were devulcanized by desulfurization of suspended mbber vulcanizate cmmb (typically 10 30 mesh) in a solvent such as toluene, naphtha, benzene, or cyclohexane in the presence of sodium (Myers et al., 1997). The alkali metal cleaves mono-, di-, and polysulfidic crosslinks of the swollen and suspended vulcanized mbber crumb at around 300°C in the absence of oxygen. However, this process may not be economical because the process involves swelling of the vulcanized mbber crumb in an organic solvent where the metallic sodium in molten condition should reach the sulfidic crosslink sites in the mbber cmmb. Also, solvent may cause pollution and be hazardous. A technology was also proposed to reclaim powder mbbers using an iron oxide phenyl hydrazine based catalyst (Kawabata et al., 1981) and copper(I) chloride-tributylamine catalyst (Kawabata et al., 1979). 

From the above it can be concluded that, in general, the nanofibers in the future can be used as a nanofiller affects on the properties of vulcanizates. Fibers obtained by Co-catalyst not strong effect on the kinetics of binding, well-swelled samples in toluene and improve the physical and mechanical properties, molecular mobility. Thus, fiber, adjusted from the metal increases the strength of vulcanized rubber by 40%, but lowers the value of elongation at break, as opposed to a vulcanized fiber, crude from the metal. These results are tiie basis for further research. 

The apparent structural parameters of the three-dimensional networks are presented in Table 3. Cross-link density analysis showed that vulcanizates contained both ionic bonds and covalent bonds. As we expected, addition of co-agents led to an increase of cross-Unk density (vj) of vulcanizates. The highest value was obtained for the sample containing acrylic acid and zinc oxide (7.55 10 mol/cm ) in contrary to the sample without co-agents (4.77 10 mol/ cm ). Thus, also the cross-link efficiency increased. It was foimd that non-covalent bonds are formed in the network structure and then break under the influence of ammonia vapoms. It reveals in a decrease of cross-link density measured on the basis of equilibrium swelling in toluene and ammonia vapours (vt+a)-... 

The crosslinking density of vulcanizates was determined by equilibrium swelling in toluene and toluene under ammonia vapours, based on the Hory-Rehner equation. The parameter of rubber-toluene interaction at a temperature of 20°C was p = 0.45 [8]. 

Kraus G (1963) Swelling of filler-reinforced vulcanizates. J Appl Polym Sci 7 861-871... 

Since we are dealing with a niunber of different effects superimposed on each other, according to Kraus, it is natural to observe a lack of correlation between the bound rubber content and reinforcement. The nature of polymer-filler bonds and their role in the reinforcement of elastomers was considered by Rehner. On the basis of analysis of the deformation properties, it was estab-hshed that the strength of bonds present in vulcanizates is characterized by a very broad spectnim of forces. From the data on the swelling of filled systems, it is possible to determine the concentration of physical bonds (or attachments) of macromolecides to the surface, which restrict the degree of swelhng and the niunber of chemical crosshnks. 

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