Rubber and Guttapercha

Rubber or caoutchouc is obtained by coagulation of the latex of numerous plants belonging to different families, principally to the Euphorbiaceae, Artocarpeae and Apocyneae. Whatever its origin and method of preparation, its value depends essentially on the content in hydrocarbons pure rubber) and on the substances accompanying it (resinous matters, various impurities). 

Besides in the raw state, rubber is sold also in a purified condition these are not generally employed as such, but are first vulcanised, i.e., incorporated by suitable processes with sulphur, which imparts special properties. 

Rubber articles often contain admixtures of various substances introduced either to give them certain definite characters or as adulterants. 

Addition of a large amount of sulphur and of other substances, mostly inorganic, under suitable conditions converts rubber into ebonite, which exhibits properties somewhat different from those of ordinary rubber. 

Synthetic rubber has been recently prepared, but it has not yet assumed importance as an industrial product. This must not be confused with rubber substitutes (principally jactis), which have been for long in common use, either alone or more frequently mixed with rubber. 

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Natural rubber and guttapercha consist essentially of polyisoprene in a s-1,4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structures but reduced chain regularity, although... 

Blending technology developed slowly. The third processable polymer of the nineteenth century was cellulose nitrate, developed by Schonbein [20] as an explosive. An 1855 patent by Parkes [21] describes the blending of natural rubber and guttapercha with a solution of cellulose nitrate, and fabricating the resultant sheets for various applications. 

Examine structures of the different forms of rubber provided. Which is natural rubber and which is guttapercha How many monomers are in each strand ... 

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... 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

The infrared spectra of hevea (natural rubber), balata (or guttapercha), the latter both in the crystalline (a) and the amorphous forms, and of synthetic polyisoprene are compared in Fig. 32. The hevea and balata (amorphous) spectra offer calibrations for cfs-1,4 and irans-1,4 structures, respectively, in the synthetic polymer. Owing to the presence of the methyl substituent, however, the spectral difference between the as and trans forms is slight both absorb at about 840... 

Guttapercha (Gummi plasticum or Isonandra gutta), (C10H10)2. A group of hydrocarbons (such as fluavil,. alban) with volatile oil. It consists of purified and coagulated milky exudates of various trees of Palaquium species, native to S. America Asia, colorless rubber-like masses, sticks or sheets,... 

Fig. 4.18. Proton-decoupled 13C NMR spectra (22.63 MHz 100-150 mg/ mL deuteriochloroform) of guttapercha (trans-) (a) and natural rubber (m-polyisoprene) (b) [73 i].

Such differences in structure can have a profound effect on the physical properties of a polymer. Thus natural rubber, which comprises cis-1,4-poly(isoprene), is a soft rubbery material at room temperature, whereas guttapercha, which comprises the corresponding ftms-isomer, is semi-crystalline and hard. The method of polymerisation determines the isomeric form of the polymer. 

Natural rubber is 1,4-polyisoprene and the polymer configuration is cis at each double bond in the chain, as shown , in (XI). Consequently, the polymer molecule has a bent and less symmetrical structure. Natural rubber does not crystallize at room temperature and is amorphous and elastomeric. Balata (guttapercha) is also... 

Scheme 6.10 (a) The structure of an isoprene molecule. (b) Bond redistribution and subsequent polymerisation to form poly(isoprene). (c) The structure of natural rubber, all-cri-poly(isoprene). (d) The structure of guttapercha, all-rrani -poly(isoprene)... 

Rubber is a naturally occurring diene polymer of isoprene (2-methyl-l,3-butadiene) and is produced by more than 400 different plants. The major source is the so-called mbber tree, Hevea brasiliensis, from which the cmde material, called latex, is harvested as it drips from a slice made through the bark. The double bonds of rubber have Z stereochemistry, but gutta-percha, the E isomer of mbber, also occurs naturally. Harder and more brittle than mbber, guttapercha has a variety of minor applications, including occasional use in dentistry and as the covering on golf balls. 

Natural mbber (Section 14.6) is the most common example of an elastomer. Rubber has the long chains and occasional cross-links needed for elasticity, but its irregular geometry prevents close packing of the chains into crystallites. Guttapercha, by contrast, is highly crystalline and is not an elastomer (Figure 31.6). 

If all the units along the chain are trans or if all the units along the chain are ois the polymer is called tactic in the first case trans-tactic, in the second case cis-tactic. It may be noted that, in the case of 1,4 polyisoprene, these two possibilities correspond to guttapercha and natural rubber respectively tactic polymers of isoprene are naturally occurring polymers. 

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