Natural rubber Density

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

If natural rubber is treated with proton donors a product is formed which has the same empirical formula. (CjHjj), and is soluble in hydrocarbon solvents but which has a higher density, is inelastic and whose unsaturation is only 51% that of natural rubber. It is believed that intramolecular ring formation occurs to give products containing the segments shown in Figure 30.5. Known as cyclised rubber it may be prepared by treating rubber, on a mill, in solvent or in a latex with materials such as sulphuric acid or stannic chloride. 

Both side groups and carbon-carbon double bonds can be incorporated into the polymer structure to produce highly resilient rubbers. Two typical examples are polyisoprene and polychloroprene rubbers. On the other hand, the incorporation of polar side groups into the rubber structure imparts a dipolar nature which provides oil resistance to these rubbers. Oil resistance is not found in rubber containing only carbon and hydrogen atoms (e.g. natural rubber). Increasing the number of polar substituents in the rubber usually increases density, reduces gas permeability, increases oil resistance and gives poorer low-temperature properties. 

In all the compositions, the DCP-cured blends showed better properties than the corresponding unvulcanized samples. Choudhary et al. [30] further demonstrated the use of EPDM, chlorinated PE, chlorosulfo-nated PE, maleic anhydride modified polyethylene, and blends of epoxidized natural rubber-sulfonated EPDM as compatibilizers in NR-LDPE (low-density PE) blends. 

Fig 9 Density effect of stretched and unstretched explosives curves 1—3 refer to unstressed sheets 1 = 15% natural rubber, 2 = 15% depolymerized rubber, 3 = 20% depolymerized rubber. Curve 4 refers to a stretched 15% rubber containing explosive of 1,4g/cc initial density (Ref 45)... 

Detonation velocity of sheets containing 15% natural rubber is given in Figs 8 9 as functions of sheet thickness and density. These data are taken from Kegier Schaii (Ref 45, p 499), who show that the decrease in D produced by stretching the sheet is due to a decrease in sheet density... 

Chattopadhyay S., Chaki T.K., and Bhowmick A.K., New thermoplastic elastomers from poly(ethyle-neoctene) (engage), poly(ethylene-vinyl acetate) and low-density polyethylene by electron beam technology structural characterization and mechanical properties. Rubber Chem. TechnoL, 74, 815, 2001. Roy Choudhury N. and Dutta N.K., Thermoplastic elastomeric natural rubber-polypropylene blends with reference to interaction between the components. Advances in Polymer Blends and Alloys Technology, Vol. 5 (K. Finlayson, ed.), Technomic Publishers, Pensylvania, 1994, 161. 

Fig. 98.— r/(a —l/a ) for natural rubber, cross-linked to the densities (pXlOO) indicated with each curve using a bis-azo cross-linking agent. (Flory, Rabjohn, and Shaffer. )... 

Storage stability Store DF in lead and wax-lined carboys, high-density polyethylene bottles, or nickel-lined containers in well-ventilated areas. Never store DF with alcohols DF will react with alcohols to form lethal chemicals, such as crude GB. Incompatible with water, glass, concrete, most metals, natural rubber, leather, and organic materials like glycols. The acidic corrosive hydrolysis products may react with metals, such as Al, Pb, and Fe, to give off hydrogen gas, a potential fire and explosive hazard. 

Time-crosslink density superposition. Work of Plazek (6) and Chasset and Thirion (3, 4) on cured rubbers suggests that there is one universal relaxation function in the terminal region, independent of the crosslink density. Their results indicate that the molar mass between crosslinks might be considered as a reducing variable. However, these findings were obtained from compliance measurements on natural rubber vulcanizates,... 

When we compared the viscosities of solutions of natural rubber and of guttapercha and of other elastomers and later of polyethylene vs.(poly)cis-butadiene, with such bulk properties as moduli, densities, X-ray structures, and adhesiveness, we were greatly helped in understanding these behavioral differences by the studies of Wood (6) on the temperature and stress dependent, melting and freezing,hysteresis of natural rubber, and by the work of Treloar (7) and of Flory (8) on the elasticity and crystallinity of elastomers on stretching. Molecular symmetry and stiffness among closely similar chemical structures, as they affect the enthalpy, the entropy, and phase transitions (perhaps best expressed by AHm and by Clapeyron s... 

Figure 3. Cross-link density versus curing temperature. Straight lines are drawn through data points of nature rubber (NR). ...

The latex may consist entirely of natural latex or synthetic SBR latex or maybe a mixture of both. In the Dunlop process, natural rubber foams shrink more than SBR foams during washing and drying. The load-bearing capacity of the foams at a given density falls significandy as SBR is used in place of natural rubber. 

PIB exhibits a comparatively low gas permeation (56). In Table 6.5, gas permeation coefficients of some polyolefins are given. Oppanol B 200 is compared with natural rubber, high density polyethylene) and low density poly(ethylene). Certain other Oppanol types have roughly the same permeability to gases as Oppanol B 200. 

Fig. 23. Mooney-Rivlin plots of extended natural rubber vulcanizates. The upswing at small A 1 values, i.e. large strains, is due to finite extensibility [Mullins (72S)]. At high crosslinking densities (upper curves) the upswing occurs at a smaller strain...

BIO, Bristow, G. M., and W. F. Watson Viscosity-equilibrium swelling correlations for natural rubber cohesive energy densities of polymers. Part 2. Cohesive energy densities from viscosity measurements. Trans. Faraday Soc. 54, 1567, 1742 (1958). 

The importance of crosslink density has already been encountered in terms of the vulcanisation (i.e. sulphur-crosslinking) of natural rubber. With low crosslink densities (i.e. low levels of sulphur) the product is a flexible elastomer, whereas it is a rigid material when the crosslink density is high. 

Consider first the signal labeled As discussed in the previous section, for the incident sound energy to penetrate the outer surface of the coating without reflection, the material used must have an acoustic impedance close to that of water. Materials having impedance values near that of water include many common rubbers. For example, arbitrarily selecting four commercial rubber samples from stock, we found the particular natural rubber sample had a surface reflectivity of 24 dB (6% reflective), the urethane sample measured 20 dB (10% reflective), and both the neoprene and nitrile rubbers were 12 dB (25% reflective). If lower reflectivity were required, the impedance of any of these rubbers could be altered simply by adding low density or low sound speed filler material. 

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