Cured rubber properties

Furnace carbon black is one of the most important compounding ingredients used in the rubber industry. It is certainly the most important rubber filler because it imparts such a profound improvement on cured rubber properties such as ultimate tensile strength, hardness, wear resistance, and tear resistance. Carbon black even improves the extrusion process by making the extruded rubber product smoother in appearance. By using carbon black with process oil, the rubber compound s pound-volume costs can be significantly reduced. In fact, if a rubber compound is black in color, chances are that it contains 26 to 32% carbon black. The level of carbon black... 

If stearic acid were not available to the rubber industry, other saturated fatty acids such as lauric acid could possibly be used as a substitute even though some cured rubber properties might be affected. Of course, a formal evaluation would have to be made. Lauric acid is derived from the hydrolysis of coconut oil or palm kernels. Oleic acid is probably not as good a substitute as lauric acid because oleic acid contains relatively high amounts of unsaturation, which can interfere with vulcanization and alter the final state of cure. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Mooney viscometer is also used to measure the time it takes, from initial exposure of the compound to a particular temperature, to the time of onset of cure at that temperature [2]. This is known as the scorch time. Scorch time is an important parameter to the rubber processor, as a short time may lead to problems of premature vulcanization. As the test is taken past the onset of cure, the rotor tears the cured rubber, and therefore this device cannot be used to investigate rheological properties after the scorch time. 

Vulcanizing agent-Any material that can produce in rubber the change in physical properties known as vulcanization, such as Chemcure applied to chemical cure rubber linings. 

Dynamic mechanical analysers, as discussed in chapter 9, can be constructed so that they can be used with unvulcanised materials and, hence, the in phase and out of phase components of modulus and the loss angle measured. The usual test piece geometries for cured rubbers are not convenient for the uncured materials where some form of oscillating shear is probably the best approach. This is the geometry used in cure meters discussed in the next section and such instruments have formed the basis for apparatus which measures dynamic properties from before and through the curing process. 

Thermal and Chemical Stability. In addition to load-bearing properties, tire reinforcement must be able to resist degradation by chemicals in cured rubber and heat generation. The most critical degradant depends on the material in use. Most thermoplastic reinforcements are either modified direedy or stabilized with additives to offset some, mosdy thermal, degradation (32,33). 

High temperature epoxy resins are brittle materials, and one method of improving their fracture properties is to incorporate reactive liquid rubbers in the formulations In situ phase separation occurs during cure the cured rubber-modified epoxy resins consist of finely dispersed rubber-rich domains ( 0.1-S pm) bonded to the epoxy matrix. TTT diagrams can be used to compare different rubber-modified systems. 

Cured properties of elastomers with a very low number of cure functional sites such as butyl and EPDM rubbers are independent of curative (accelerators) levels used. However, varying the levels of unsaturation during polymerisation can alter cured physical properties often without processing difficulties. For butyl and EPDM rubbers only a small fraction of the monomer units in a chain take part in the crosslinking reactions. For example, if an average chain contains 10,000 monomer units only about 50-250 would normally be involved in the crosslinking. 

The adhesive properties of epoxy resins coupled with their dielectric behavior have made them attractive to the electronic industry. The evaluation of thermally cured rubber modified epoxy thermosets has been the subject of recent studies (1, 2), which dealt with the dependence of morphology on the curing parameters, e.g., catalyst, cure schedule, time of gelation, etc. This work utilizes one of the new series of photocationic initiators (PCI) developed by Crivello, et al (3) which are presently commercially available. These onium salts initiate the reaction by absorbing the actinic radiation, generating radicals and producing a protonic acid. The radicals can lead to polymerization of olefinic moieties (4) while the acid initiates the polymerization of the epoxy groups (3). 

Physical test properties on some cured rubber stocks prepared from lithium-catalyzed butadiene polymers are listed in Tables V and VI with appropriate controls. The results are only roughly indicative of the potential properties of rubbers made from lithium-catalyzed butadiene polymers because of the limited quantity of polymer available. The tensile data in Table VI indicate that compounded stocks from the lithium polymers are about equal or slightly inferior to the emulsion and sodium polymer controls in regard to these properties however, a hot tensile (lOO C.) on a cured compound from lithium polybutadiene was 325 pounds per square inch compared to 200 to 250 for an emulsion polybutadiene control. The internal friction of cured stocks from the lithium-catalyzed butadiene polymers is similar in magnitude to the emulsion or sodium polymer controls at 50 C. but higher at 100 °C. All lithium polymers, even those with low Mooney viscosities, gave cured compounds with high values of dynamic modulus. 

As this reaction proceeds, the system begins to crosslink. Subsequent sites react and are tied together as more water from the air enters the system. Deep section cure is relatively slow because it is dependent on the diffusion rate of water, and a half inch bead of sealant will take several days to develop full rubber properties. 

---