CR vulcanizates

General Vuleanizate Properties. Table 4 smmnaiizes the properties of CR vulcanizates that ate impoitant to the designers of rubber parts, and compares these properties to those of two competitive materials. The comparison is general and varies depending on compound design (90). The... 

Better cross-linking with the latter also improves post Tg viscoelastic responses of the rubber vulcanizates. Similar effect has also been observed with polychloroprene as investigated by Sahoo and Bhowmick [41]. Figure 4.8 represents the comparative tensile stress-strain behavior of polychloroprene rubber (CR) vulcanizates, highlighting superiority of the nanosized ZnO over conventional rubber grade ZnO [41]. 

The influence of clay and modified clay on the physical properties of the CR vulcanizates at 5 phr filler concentration is given in Table 5. As is obvious, the modulus values (stresses at 100%, 200% and 300% elongation) increase in all cases after filler loading. Again, the extent of improvement depends on the nature of the nanofillers. In the case of unmodified clay, the increase in 100% and 300% modulus is more with LDH, whereas MMT shows similar enhancement as that observed with... 

Table 5 Curing characteristics and physical properties of CR vulcanizates reinforced with different types of clay...

Fig. 15 Effect of different kinds of nanofiller on the dynamic loss tangent of CR vulcanizates with temperature...

Fig. 16 Variation of storage modulus with temperature in CR vulcanizates containing different kinds of nanofillers (a). Circled region is shown in more detail in (b)...

Fig. 18 Strain dependence of G of CR vulcanizates filled with different kinds of nanofillers. The measurements were done by a moving die rheometer (Scarabaeus SIS V50) applying sinusoidal strain at constant frequency of 0.1 Hz at 60°C...

Properties of Chloroprene Rubber Yulcanizates. Unfilled (nonreinforced) gnm vulcanizates are mnch stronger for CR than for other synthetic rubbers (except for synthetic polyisoprene) becanse of the tendency for strain-induced crystallization (similar to the case of isoprene mbbers). Similarly, resistance to tearing is very good for CR vulcanizates. 

Because of the increased polarity of CR over other (i.e., hydrocarbon) rubbers, CR vulcanizates are sufficiently oil resistant for many uses. Also, CR vulcanizates are fairly resistant to chemicals such as concentrated alkalis, dilute acids, and aqueous salt solutions. 

Uses of CR. CR vulcanizates are used in many mbber products that are flame resistant, resistant to fats and oils, and resistant to weathering and ozone (when compounded with proper antidegradants). Products include moldings, extrusions, seals, hoses, rolls, belts, shoe soles, bearings, rubberized fabrics, linings, and cable jackets. However, in some areas, the use of CRs has been reduced by competition form nitrUe rubbers (for better oil resistance) and EPDM compositions because of price. 

Synthetic Plasticizers. The most important types of synthetic plasticizers are esters. Phthalate esters are used to improve elasticity and low-temperature flexibility, especially in NBR and CR vulcanizates. Common examples are dibutyl phthalate (DBP), di(2-ethyl-hexyl) phthalate (DOP), diisooctyl phthalate (DlOP), and diisononyl phthalate (DINP). They are generally used at levels of 5 to 30 phr. 

Sae-oui el al. analysed NR/CR vulcanizates by atomic force microscopy (AFM) of microtomed samples. CR appeared to be the continuous phase at weight ratios of 25/75 and 50/50 NR/CR. However, this situation is reversed... 

Chloroprene rubber (CR) vulcanizates can be made using fillers, plasticizers, antioxidants, and processing aids commonly used in diene rubber compounding. 

Levels of 0.5-1.0 pphr methyl mercapto benzimidazole have been shown to improve resistance to flex cracking of CR vulcanizates, but tend to be scorchy. The zinc salt of MMBI (ZMMBI, Vulkanox ZMB-2) is more effective in this respect. 

It must be added that ozone protection is improved if antiozonants are used together with microcrystalline waxes. Optimized CR vulcanizates have been shown to resist outdoor conditions for several years. Long-term tests with several antio-zonant/wax combinations in an outdoor test yielded the results presented in Figure 1.11. 

FIGURE 1.10 PPDA Type antioxidant influence of flex fatigue resistance of CR vulcanizates. 

The attraction of CR lies in its combination of technical properties, which are difficult to match with other types of rubber for a comparable price. With the correct compound formulation, CR vulcanizates are capable of yielding a broad range of excellent properties as shown below ... 

Tear resistance of CR vulcanizates is better than that of SBR. Tear propagation resistance of CR vulcanizates containing active silica may be greater than that of those with natural rubber. CR vulcanizates show good elasticity, although they do... 

A more relevant description of heat resistance is shown in Figure 1.14, where the Arrhenius equation is applied to the thermal aging of optimized CR vulcanizates. 

FIGURE 1.14 Arrhenius plot of air aged CR vulcanizates (VDE 0304/Part 2/7.59). 

Synthetic low-temperature plasticizers allows CR vulcanizates to exhibit elastic behavior down to temperatures of around —45°C to —50°C (depending on the formulation). 

CR vulcanizates are inherently flame resistant. The good flame resistance properties of the polymer itself mean that stringent end-user specifications can be fulfilled by the use of appropriate compounding materials such as chlorinated paraffins instead of mineral oils, mineral fillers plus aluminum trihydrate, zinc borate, and antimony trioxide. 

However, like all organic substances, CR vulcanizates will decompose at high temperatures such as those encountered in open fires. In addition to decomposition products such as carbon dioxide and water, corrosive hydrogen chloride gas is also formed. 

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