Natural rubber compounds

Manganese, copper, iron, cobalt and nickel ions can all initiate oxidation. Untinned copper wire can have a catastrophic effect on natural rubber compounds with which it comes into contact. Inert fillers for use in rubbers are usually tested for traces of such metal ions, particularly copper and manganese. The problem is perhaps less serious in saturated hydrocarbon polymers but still exists. 

This lower has a number of ramifications on the properties of polybutadiene. For example, at room temperature polybutadiene compounds generally have a higher resilience than similar natural rubber compounds. In turn this means that the polybutadiene rubbers have a lower heat build-up and this is important in tyre applications. On the other hand, these rubbers have poor tear resistance, poor tack and poor tensile strength. For this reason, the polybutadiene rubbers are seldom used on their own but more commonly in conjunction with other materials. For example, they are blended with natural rubber in the manufacture of truck tyres and, widely, with SBR in the manufacture of passenger car tyres. The rubbers are also widely used in the manufacture of high-impact polystyrene. 

Effect of Antirad Agents on the Scission Yield of a Natural Rubber Compound... 

Figure 20 GC chromatogram of the dichloromethane partition solution from the distilled water extract of the natural rubber compound NR2 (24 h at 40°C).

An isomeric derivative of natural rubber produced by a variety of chemical treatments. The products are resinous and find application as reinforcing agents in natural rubber compounds (particularly in shoe soles) and as rubber-to-metal adhesives. 

Oils of the three types are offered in a range of viscosities and this will influence their processing character to some extent, although there is little evidence that it will have much influence on the ultimate compound physical properties, at least in natural rubber compounds. The small additions of oil to a compound help with filler dispersion by lubricating the polymer molecular chains and thus increasing their mobility. There will also be some wetting out of the filler particles which enables them to achieve earlier compatibility with the rubber and improve their distribution and dispersion speed. 

Insoluble Sulfur. In natural rubber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of rubber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

If natural rubber compounds are subjected to thermal aging plus fatigue, the conventional systems perform no better than EV systems. The compromise obtained by using semi-EV systems involves the balance between heat aging and flex life. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems in a simple, carbon black-filled natural rubber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found in Figure 9 (24). 

The most commonly reported physical properties of radiation cross-linked natural rubber and compounds made from it are modulus and tensile strength, obtained from stress-strain measurements. Figure 5.5 illustrates some of the results obtained from gum rubber and from a natural rubber compound reinforced by HAF carbon black. In Figure 5.6 the tensile strength of radiation cured gum is compared to that of vulcanizates cured by sulfur and by peroxide. ... 

Under Behavior Towards Metals and Other Substances. AN very definitely attacks shellac, baked oils and natural rubber compounds if applied as a coating. The only materials that we know of which can be used as coatings for AN and AN solutions are certain polyvinyl chloride coatings and a number of epoxy resins. The Ordnance Department used acid-proof black paint, shellac, baked oil and rubber paints. All were quite unsuccessful over long periods of time... 

A cure which is longer than optimum is over cure. Over cures may be of two types. In one type the rubber continues to harden, the modulus rises and tensile strength and elongation fall. In certain cases, including most natural rubber compounds, reversion occurs with over cure leading to decrease in tensile strength and modulus. 

Antiozonents protect the rubber surface by the formation of a protection layer, the ozonides on the surface of rubber by reaction of the antiozonents with ozone. Certain polymers also provide good ozone protection. The use of 10-20 parts of EPDM, a low diene rubber, in natural rubber compound significantly increases ozone resistance. 

Iron ore with a solid content of 25%-30% in water was being handled in these feed launders. The velocity of the slurry was 2 ft per second. A 6 mm thick soft natural rubber compound of hardness 40°A was found to be well suited for this application. 

For wet grinding of copper ore in the ball mills at ambient temperature a 6 mm thick natural rubber compound of 45°-50°A was used successfully as an abrasion resistant layer. 

G 4. — Tensile strengths of pure gum natural rubber compounds. J. Polymer Sci. 2, 451 (1947). 

All three properties of carbon black affect hardness in natural rubber compounds (Fig. 18). Larger particle size reduces hardness higher structure increases hardness and increased surface area reduces hardness. Quite the contrary in SBR (Fig. 19), only structure is significant in regard to hardness. As DBP increases, hardness increases. 

Cleaning of the two-roll mill is a simple operation since the compound only contacts the exposed external surface of the rolls. Typically a natural rubber compound may be used for surface cleaning and/or the first batch of polymer compound might be used for this purpose. 

Diamine salts of fatty acids are used as multifunctional additives in natural rubber compounds filled with carbon black.They affect the elastomer-carbon black interface. With an increased concentration of multifunctional additive, the concentration of bound rubber decreases but dispersion of carbon black is improved. In silica filled rubber, multifunctional additive also improves the dispersion of silica, but in addition, it decreases the negative influence of silica filler on vulcanization rate. 

The proven compounding practice for the natural rubber layer is to load it with 30 phr of graphite, 30 phr of semi-reinforcing carbon black filler and 50 phr of inert filler, china clay. A low sulfur curing system is adopted for heat resistance with a suitable antioxidant to prevent flex-cracking. A typical chlorine resistant soft natural rubber compound formula is given in Table 3.2. 

Table 3.2 Typical chlorine resistant soft natural rubber compound ...

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