Sulfur-vulcanized Natural Rubber

Linear and carbon-filled, sulfur vulcanized natural rubber were investigated using the eight-pulse sequence MREV-8, which has the following structure 182,183) ... 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

By using this method, the chemical shifts of the resonances in the spectra of a sulfur vulcanized natural rubber (Fig. 32 expanded aliphatic region in shown in Fig. 33 [top]) are assigned to various units of the polymer network, which arise from structural modifications induced by the vulcanization 194,196 200). Different sulfidic structures are found for unaccelerated and accelerated sulfur vulcanizations, respectively. With increasing amount of accelerator (as compared to the sulfur), the network structure exhibits less crosslinking, fewer main chain structural modifications, and fewer cyclic sulfide structures 197). 

Andreis M, Liu J, Koenig JL (submitted to J. Polym. Sci., Polym. Phys. Edn. for publ.) Solid state carbon-13 NMR studies of vulcanized elastomers. V. Observation of new structures in sulfur vulcanized natural rubber... 

It is known that sulfur-vulcanized Natural Rubber (NR) can be completely recycled at 200 to 225°C by using diphenyldisulphide [41]. Recently, the effi-ciacy of various disulphides as recycling agents for NR and EPDM vulcan-izates were reported [42]. While complete de vulcanization was observed on sulfur-cured NR at 20b°C, a decrease in crosslink density of 90% was found when EPDM sulfur vulcanizates with diphenyldisulphide were heated to 275°C in a closed mold for 2 hours. At the same time, EPDM cured by peroxide showed a decrease in crosslink density of about 40% under the same conditions. 

Fig. 10. Stress-temperature curves for sulfur-vulcanized natural rubber (99,103). Courtesy of the American Chemical Society.

Figure 9.18 Mooney-Rivlin plot for sulfur-vulcanized natural rubber. Solid line, equation (9.48) dotted line, equation (9.63).

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

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. 

Natural rubber comes from tropical trees. It is soft and sticky, so it has little practical use. However, while experimenting with rubber in 1839, Charles Goodyear dropped a mixture of sulfur and natural rubber on a hot stove by mistake. The heated rubber became tough and elastic because of the formation of covalent bonds. The resulting compound was vulcanized rubber, which is strong enough to make up a basketball that can take a lot of hard bounces. 

Problem 1.6 Assuming that each cross-link produced by vulcanization [Fig. 1.4(b)] contains an average of two sulfur atoms, calculate the sulfur content of vulcanized natural rubber that is 50% cross-linked. (Neglect sulfur other than that... 

Peroxides are highly efficient vulcanizing agents for elastomers and plastics. In addition they are used as polymerization catalysts in the coatings industry. Tliese materials are commercially available in several physical forms and container types to suit the needs of users. Sulfur vulcanization of rubber was discovered in the early 1800s. The cross-linking of natural rubber with a peroxide was discovered in 1914 by a Russian, who used benzoyl peroxide. However, benzoyl peroxide vulcanizates have lower strength and poorer heat resistance than do sulfur vulcanizates. In 1950... 

Accelerated-sulfur vulcanization is the most widely used method. For many applications, it is the only rapid crossUnking technique that can, in a practical manner, give the delayed action required for processing, shaping, and forming before the formation of the intractable vulcanized network. It is used to vulcanize natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber (HR), chlorobutyl rubber (ClIR), bromobutyl rubber (BUR), and ethylene-propylene-diene-monomer rubber (EPDM). The reactive moiety for all of these elastomers can be represented by... 

Pure natural rubber is soft and tacky. To be useful, natural rubber has to be vulcanized. In vulcanization, natural rubber is heated with sulfur. A reaction takes place that produces cross-links between the dr-polyisoprene chains and makes the rubber much harder. Sulfur reacts both at the double bonds and at allylic hydrogen atoms ... 

The serendipitous discovery in 1839 that a mixture of sulfur and natural rubber could be crossHnked (vulcanized) when dropped accidently on his wife s hot kitchen stove was the breakthrough that made the use of rubber practical for applications other than erasers. 

Sulfenamide accelerators generally require lower levels of fatty acid because they release an amine during the vulcanization process which acts to solubihze the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. A study of the effect of stearic acid and zinc oxide on a sulfenamide-accelerated, sulfur-cured natural rubber compoimd showed the need for both zinc and fatty acid activators is presented in Figure 11 (52). 

In a further study [63], inversion recovery spin-relaxation time measurements and computer fitting of overlapping spectra regions were used to identify additional peaks in vulcanized natural rubber cured with 1, 3, and 5% sulfur for different time intervals. Those overlapping crosslinked carbons with different T times showed different peak intensities which could be determined by computer fitting of the peaks. Two new peaks at 37.8 ppm and 25.5 were identified. 

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