Natural Rubber and SBR

FIGURE 21. Dynamic mechanical properties of natural rubber and its blend (1 /1) with polystyrene (incompatible system) or with polyvinylcyclohexane (compatible system). 

and V2 are the weight fraction of the resin and the volume fraction of the elastomer, respectively and refer to the glass transition temperatures for the blend and pure 

FIGURE 22. Natural rubber/Cs resin effect of concentration on tan 8. 

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The important properties of the rubbers are their temperature stability, retention of elasticity at low temperatures and good electrical properties. They are much more expensive than the conventional rubbers (e.g. natural rubber and SBR) and have inferior mechanical properties at room temperature. 

Class and Chu [34] have studied the tackification of natural rubber and SBR over a wide range of resin concentrations for several tackifiers. From their graphical data it can be estimated that 1 1 tackification (by weight) with a poly(/-butyl styrene) resin, MW 850 and Tg = 59°C, gives a PSA with Tg about — 13°C, and storage modulus, G about 8.8 x 10 Pa, well within the PSA window. 

The balance between natural rubber and SBR is a delicate one. Natural rubber has made a comeback and reversed its downward trend. Developments of rubber farming have raised the yield from 500 Ib/acre/yr to 2,000-3,000. Petrochemical shortages and price increases have hurt SBR. Finally, the trend toward radial-ply tires, which contain a higher proportion of natural rubber, favors this comeback. Fig 18.1 shows the U.S. natural rubber consumption trends vs. U.S. SBR production, where this bounceback of the natural rubber market is very evident from 1980 to the present. The competitive price structure for these two elastomers through the years has been very evident, and their prices are never too far apart. 

The carcass requires better flexing properties than the tread and is a blend of natural rubber and SBR, but at least 60% of natural rubber. The sidewalls have a lower percentage of natural rubber, from 0-50%. The liner is made of butyl rubber because of its extreme impermeability to air. 

Both natural rubber and SBR are used in Scotch tape, masking tape, and adhesive-backed floor tiles. They are used in automobiles to bond fabric, carpets, and tire cord. 

Tensile strength in both natural rubber and SBR is dependent upon structure and particle size as the key variables. Tensile strength increases with lower structure and with smaller particle size (Fig. 16 17). 

Use Accelerator for natural rubber and SBR, especially in cable covers and mechanical items. 

Liquid natural rubber, SBR, acrylonitrile elastomers, neoprene, and chlorosulfonated polyethylene are also used as sealant compositions. Filled depolymerized natural rubber and SBR latex are readily cured at room temperature by the incorporation of ultraaccelerators. 

Polybutadiene Higher resilience than similar natural rubber compounds, good low-temperature behavior and adhesion to metals, but poor tear resistance, poor tack, and poor tensile strength Blends with natural rubber and SBR manufacture of high-impact polystyrene... 

The applications of the rubbers stem from their important properties, which include thermal stabflity, good electrical insulation properties, nonstick properties, physiological inertness, and retention of elasticity at low temperatures. The temperature range of general-purpose material is approximately — 50°C to -l-250°C, and the range maybe extended with special rubbers. Silicone rubbers are, however, used only as special-purpose materials because of their high cost and inferior mechanical properties at room temperature as compared to conventional rubbers (e.g., natural rubber and SBR). 

Elastomers used in water-based systems include various rubber latices, especially natural rubber and SBR, and occasionally, polychloroprene. The bulk of the market in water-based adhesives is now held by acrylic dispersions. Although these are designed for use without modification, it is normal to formulate, especially by addition of tackifiers. Commonly used tackifying resins include soft resins, or hard resins in solution, which may... 

FIGURE 10.31 Fatigue life versus depth of initial cut for test pieces of natural rubber and SBR stretched repeatedly to 50% extension. (From Lake and Lindley (1964).)... 

Toxicology LD50 (oral, rat) > 3 g/kg, (subcut., mouse) 20 g/kg low toxicity by ing. suspected tumorigen, carcinogen TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits very toxic fumes of SOx and NOx Uses Vulcanization accelerator for natural rubber and SBR, esp. in cable covers and mech. goods... 

Such running surfaces consist of, for example, natural rubber and SBR or of cis-BR and SBR. 

FIGURE 3 Shear viscosity of rubber gums SMR natural rubber and SBR (Bridgestone-Firestone Duradene 706) as a function of shear stress. 

General purpose polymers (natural rubber and SBR) pose no difficulty unless they are of high viscosity or conversely are of very low hardness, i.e., s 40 IRHD (NR) and styrene-butadiene rubber (SBR). 

For comparative purposes, the effects of nonblack fillers, both natural rubber and SBR compounds, are shown in Tables (12) and (13). For example it illustates... 

In this chapter we will consider the results of efforts by polymer scientists to emulate the structure and properties of natural rubber with polymers prepared by both laboratory and industrial synthesis as well as the closely related polymers from butadiene and piperylene. Whilst such materials have not yet acquired the commercial importance of natural rubber and SBR they are nevertheless significant general purpose materials. 

The main advantages of these polymers, in common with SBR and natural rubber, are that they are inexpensive, may be cross-linked by standard vulcanizing systems and, in many cases, remain rubbery at lower temperatures than is common with many synthetic rubbers. Like natural rubber and SBR, the synthetic diene homopolymer rubbers have limited oxygen and ozone resistance whilst chemical reactions resulting from the existence of a carbon-carbon double bond can, in some instances, lead to unwanted cleavage or scission. Furthermore as hydrocarbons they do not have good swelling resistance to petrol and other hydrocarbon liquids. 

It is found that if butyl rubber-carbon black compounds are subjected to heat treatment before final processing and vulcanization the vulcanizates exhibit improved tensile strength, modulus and resilience. Although this effect may be observed to a small extent in natural rubber and SBR it is most significant in butyl. 

On economic grounds the cost of the termonomer has been a significant factor as have certain aspects of production costs so that, at the time of writing the EPDMs are significantly more expensive than natural rubber and SBR. Technical problems such as the lack of building tack have proved a deterrent to their use in tyre applications which involve extensive assembly operations whilst problems of compatibility and covulcanization in the case of a far from impossible stock mix-up have also been significant. 

Which roll will the stock stick to—front or back Most mills are arranged with a speed differential (friction ratio) between front and back rolls so one roll runs faster than the other. In general, natural rubbers and SBRs stick to the slower roll and EPDM, neoprene, etc., stick to the faster roll, but there are exceptions. 

The phase contrast microscope is invaluable for examining other junctions formed between rubbers differing only in refractive indices. Figure 3 shows the junction between natural rubber and SBR. There is enough refractive index difference to show the bond line in the phase contrast microscope. Bond lines can be observed even when identical rubbers are used if they have been cured differently. For instance, one can see the bond between peroxide and sulfur cured natural rubbers. 

Femandez-Berridi. M.J.. Gonzalez. N.. Mugica, A., and Bernicot. C. (2006) Pyrolysis-FTIR and TGA techniques as tools in the characterization of blends of natural rubber and SBR. Thermochim. Acta, 444 (1), 65-70. 

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