oils are only compatible with CRs in low concentrations, and they serve generally only as processing aids, that is, to reduce sticking of compounds to mill rolls. For special formulations of CR, synthetic plasticizers must be used instead of oils. This is especially important if improved flame resistance, low temperature flexibility, and elasticity are required of the vulcanizate. Of particular importance are the flame resistant esters of phosphoric acid to replace oils which burn readily. A further improvement in the flame resistance of CR compounds is achieved through the addition of chlorinated hydrocarbons together with hydrated alumina. Elasticity and low temperature flexibility of CR vulcanizates can be improved by using esterand ether-based plasticizers, same as for NBR.
Various resins, such as xylene formaldehyde resins, Koresin, rosin and coumarone reins, can be used in CR compounds to improve the filler dispersion and the building tack.
Factice. Since CR vulcanizates have a relatively good strength for gum compounds and at low filler loadings, it is possible to formulate very soft vulcanizates with adequate mechanical properties by using factice or plasticizers. Such vulcanizates are required for printing roll covers, seals, and hoses. In addition, as with other rubbers, factices improve the processibility (extrudability, calenderability, green strength, and dimensional stability) of CR compounds, and the compatibility of CRs withoils. Particularly successful are the factices, that were especially developed for CR, to give a high resistance to solvent swell [2.119].
Process Aids. Those particularly suitable for CR compounds are esters of fatty alcohols, to improve flow behavior [3.267, 3.268]. Due to the interaction with zinc ions, zinc soaps strengthen the CR compounds, but reduce their processibility and Ca soaps tend to bloom. Other process aids recommended in the literature, such as emulsion plasticizers, have only little effect. Therefore esters of fatty alcohols are of great importance. Stearic acid and paraffinic oils are also important for reducing the stickiness of CR compounds and to aid in the filler dispersion.
3.3.4.5 Processing of CR
The mixing and processing of CR compounds is done using conventional rubber machinery and processes. Because of their tendency to scorch, the the heat history of CR compounds has to be kept as short as possible during processing.
3.3.4.6 Properties of CR Vulcanizates
Mechanical Properties. Gum compounds or those with non-reinforcing fillers have a higher mechanical strength than those from most types of Srs, because, due to their ability to strain-crystallize, CRs have a certain self-reinforcing ability. The greatest influence on the mechanical properties has, of course, the activity of the filler in the compound, same as with other SRs. With highly reinforcing blacks and silicas, one obtains tensile properties which are only slightly below those of comparable NR vulcanizates. The resistance of CR vulcanizates to tear initiation and propagation is excellent, if low structure blacks and, in particular, active silicas are used. Semi-reinforcing blacks together with ester-and ether-based plasticizers give are high elasticity for CR vulcanizates. To obtain a good resistance to compression set at high temperatures, mercaptan modified CRs are used compounded with semi reinforcing blacks, but, with as little plasticizer of crosslinking. Crystallization can aggrevate the compression set at low temperatures. Therefore, the same compounding measures are to be implemented for good low temperature compression set, as for depressing the freezing point and the tendency towards crystallization.
Heat and Aging Resistance. To obtain a good heat resistance, mercaptan grades should be used instead of the thiuram grades, and a sulphur-free vulcanization. Light coloured fillers, especially talc, and the use of protective agents based on diphenyl amine, such as ODPA or SDPA, give an optimum in heat resistance, namely of the same order as NBR vulcanizates with EV cures. In addition to very good oxidative stability, unprotected CR vulcanizates have already a significantly better weather and ozone resistance than other diene rubbers. Of fillers, carbon blacks have a positive influence and of plasticizers, aromatic oils and unsaturated fatty acid ester have a negative one on the ozone resistance of vulanizates. Since the ozone resistance improves as the vulcanizates become less stressed, any measure tance. The presence of appropriate protective agents gives, however, a high level of ozone resistance, which suffices for many applications.
Depending on the specific compound composition, light coloured vulcanizates tend to discolour under sustained influence of light.
Flammability. [3.307-3.310] Because of the chlorine content, CRs have a favourable flme resistance, and in this respect they are superior to other rubbers. Although CR vulcanizates burn when exposed to high temperatures and open flames, they selfextinguish within a short time after removal of the flame. This requires, however, that the vulcanizate has not been compounded with larger quantities of combustible ingredients, such as oils. Vulcanizates with a high degree of flame resistance can be obtained by using non-burning plasticizers, like esters of phosphoric acid, and it can be improved even further by using chlorinated hydrocarbons in combination with antimony trioxide or hydrated alumina.
When chlorinated polymers, including CR, burn, they develop great quantities of toxic and corrosive fumes, which are extremely dangerous and cannot be tolerated in areas where there is a high demand for safety. Therefore, halogen free vulcanizates have been recently specified for these applications, and examples are oxygen containing polymers, like EVM (see page 103) or EAM (see page112), which are compounded with hydrated alumina. Optimized EVM vulcanizates, which are halogen free and develop little smoke, have oxygen indices of 50% (LOI according to ASTM D 2863), and thus approach those of 55% for the optimum compositions based on CR and chlorinated paraffins [3.310]. Since, in the event of fire, the EVM vulcanizates create less danger regarding toxicity and corrosion, they will be increasingly used in the future, and they will replace CR in one of its major applications – cable jackets.
Low Temperature Properties. The stiffening of CR vulcanizates on cooling is often superposed by crystallization processes. Thus, the freezing temperature of CRs depends on the grade and its tendency to crystallize. Fillers have little effect on the freezing point, but plasticizers determine to a large degree the dynamic freezing point of CR vulcanizates. While aromatic oils have a detrimental effect, ester-and ether-based plasticizers improve the dynamic freezing point to a certain extent. Tendency towards Crystallization. This undesirable property of CR has already been discussed on several occasions. Crystallization is detrimental in the production of certain technical rubber goods, since it reduces the building tack. Also with cold feed extruders, which are almost exclusively used in modem production processes,