Dynamic Mechanical Analysis of Carbon-Black-Filled Rubber Vulcanizates under Swollen Conditions

1. For unswollen vulcanizates, storage modulus (E′) decreases with the increase in temperature and decrease in loading and surface area of carbon-black filler. On partial swelling, these effects are much reduced, and E′ becomes independent of surface area and loading on equilibrium swelling. 2. Loss modulus (E″) of unswollen vulcanizates also follows the same trend. However, on swelling in xylene, E″ increases to a great extent because of contributions from both the solvent and the rubber matrix. 3. Loss tangent (tan δ) also decreases with increase in temperature and decrease in loading of carbon black. On swelling, it optimizes at 30 phr loading. 4. Tan δ increases with ϕΨ for unswollen samples, but swollen vulcanizates show an optimum in tan δ at ϕΨ=2.5. 5. Limiting value of storage modulus has been determined for all the filled samples.

The Correlation between Entanglement and Chemical Crosslinks in EPDM Vulcanizates

Using peroxide cured specimens prepared from a selected EPDM terpolymer containing olefinic residues derived from ethylidene norbornene, and Young's modulus measurements taken in compression on n-heptanc swollen vulcanizates, a correlation between the apparent total, effective network chain density and the chemical network chain density has been derived. The type of correlation equation used may be generally useful.

Micromethod for Estimation of the Degree of Crosslinking in Elastomers from Compression Modulus of Swollen Vulcanizates

The method uses small specimens (of volume less than 0.1 cc) which swell to diffusion equilibrium in hydrocarbon solvents within 8 hr. Measurements of deflection to better than one μ are made with a modified Wallace microhardness tester, which is described. The name “reticulometer” is proposed for this instrument. Using peroxide-crosslinked natural rubbers swollen to equilibrium νr values of about 0.23 in n-decane or in benzene, values for the concentration of physically active network chains determined by the present method were not greatly different from those determined by conventional “equilibrium” stress—strain measurements in tension on the same samples. Detailed differences are discussed. The chief advantages of the microcompression method are: (a) the use of small samples of shape and size convenient for carrying out other conventional measurements of network chain density; (b) the rapid attainment of diffusion equilibrium encouraging the use of fully swollen specimens and minimizing the risk of inhomogeneous distribution of swelling agent; and (c) the rapidity of testing compared with tension measurements.

Carbon Black Reinforcement in Pre-Swollen Rubbers

The mechanism of carbon black reinforcement was studied by examining stress-strain properties in SBR 1500 vulcanizates cured while swollen, then de-swollen. Carbon black-polymer attachments started contributing to the modulus of rubber at much higher elongations (ca. 300 per cent for a swelling ratio of 4.0) than in normally cured, unswollen specimens. The modulus in pre-swollen rubbers is considerably lower than in normal vulcanizates but it can be increased by heating the same specimens 30 minutes at 155° C in their de-swollen state. The observed increase in modulus of vulcanizates containing graphitized and regular carbon blacks after such a treatment was found to be related to carbon black “activity” and loading. Bound rubber does not seem to function as a distinct phase contributing to reinforcement, but it can be used as a measure of the number of attachments of carbon black and polymer. Additional information on the molecular mechanism of the phenomenon was obtained through radioactive tracer analysis. It was also found that regular vulcanizates can be softened by heating swollen samples at elevated temperatures (150-170° C). This softening was related to carbon black “activity” and was fully reversible: the original modulus could be recovered by additional heating of de-swollen vulcanizates for a further thirty minutes at 155° C. The above observations are discussed in terms of mobile adsorption of polymer chains on carbon black surface and the steric effect of fixing carbon black particles into an already extended polymer network.

Oxidative Degradation of Swollen Vulcanizates

1. In the course of a study of the oxidative degradation of vulcanizates with different structures (a thermal vulcanizate, a vulcanizate with Thiuram, one with diphenylguanidine and, one with zinc ethylphenyldithiocarbamate) it was established that the swollen vulcanizate, regardless of the accelerator present, undergoes two simultaneous reactions—destruction and structure formation. 2. It was revealed that the dominance of the destructive process, in the oxidation of swollen vulcanizates, can occur only in the presence of antioxidants; and that only after a considerable consumption of antioxidant does the rate of structure formation equal or exceed the rate of destruction. 3. It was shown that the behavior of antioxidants in swollen vulcanizates is governed not only by the nature of the rubber, but also in large degree by the nature of the solvent.

Transformation by the Action of Heat and Oxygen of the Sulfur Bonds Which Compose a Vulcanization Network

1. A study of the effect of combined sulfur on the oxidation of swollen and unswollen polybutadiene vulcanizates shows that stocks vulcanized with different types of accelerators (tetramethylthiuram disulfide, diphenylguanidine, and zinc ethylphenyldithiocarbamate) form sulfur bonds which differ in their content of long polysulfide groups. 2. It is shown that sulfur liberated by the polysulfide bonds in vulcanizates reduces the speed of the oxidation reaction and the equilibrium concentration of stable peroxides. The sulfur content also governs the structure-forming process which develops when a raw polymer containing sulfur is oxidized at 90° C. The decomposition products of the polysulfide groups likewise lower the speed of oxidation in swollen vulcanizates at 150°. 3. When swollen vulcanizates are heated in the absence of oxygen at 150°, only the polysulfide bonds decompose. Heating unswollen vulcanizates under the same conditions, however, brings about the reverse effect, viz., structure formation. 4. Sodium sulfite and alcoholic alkali do not remove completely the polysulfide sulfur from vulcanizates. 5. In the process of vulcanization a rearrangement of the sulfur bonds takes place, and this is accompanied by a reduction of the polysulfide content.

Changes of Fatigue Resistance of Vulcanized Natural Rubbers during Swelling

A method of fatiguing swollen vulcanizates of natural rubber while maintaining a minimum deforming load, independent of the change of modulus of elasticity of the experimental specimens, was devised. The effect of swelling vulcanized natural rubber in petrolatum and in dibutyl phthalate on fatigue resistance was studied. With increase of swelling of a vulcanizate in dibutyl phthalate and in petrolatum, an initial increase of fatigue resistance was observed up to 1.8 times the fatigue resistance of the unswollen vulcanizate. Following this, a decrease of fatigue resistance was observed. The equation obtained for the change of fatigue resistance as a function of the degree of swelling is explained by two simultaneous processes: (1) increase of fatigue resistance because of decrease of energy loss in the destructionof intermolecular bonds, and (2) initiation of chemical processes leading to destruction, and a smaller difference between the deformation and the relative elongation at rupture. The effect of the character of the solvent used for swelling on the nature of the fatigue resistance-swelling relation is shown.