Natural rubber peroxide-curing

Fig. 32. The VL function (98) derivative vs stretch for natural rubber samples cured with 1, 5, and 15 parts per hundred dicumyl peroxide, as indicated. The VL function was obtained from torque and normal force measurements, o APHRl A APHR5 0 APHR15. After McKenna et al. (102).

FIGURE 5.15 Failure envelope of various mixes A, natural rubber-polyethylene (NR-PE) vul-canizate (peroxide cured) , NR-PE vulcanizate (sulfur cured) , NR-PE vulcanizate with CPE as compati-bilizer V, EPDM-PE vulcanizate o, EPDM-PP vulcanizate (sulfur cured) NR-ENR-PE -PE. (Erom Roy Choudhury, N. and Bhowmick, A.K., J. Mat. Sci., 25, 161, 1990. With permission from Chapman HaU.)... 

Deformulation of vulcanised rubbers and rubber compounds at Dunlop (1988) is given in Scheme 2.3. Schnecko and Angerer [72] have reviewed the effectiveness of NMR, MS, TG and DSC for the analysis of rubber and rubber compounds containing curing agents, fillers, accelerators and other additives. PyGC has been widely used for the analysis of elastomers, e.g. in the determination of the vulcanisation mode (peroxide or sulfur) of natural rubbers. 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

SBR can be cured by the use of sulphur, sulphur donor systems and peroxides. Sulphur cures generally require less sulphur (1.5-2.0 phr) and more accelerator than is normally required to cure natural rubber. 

Ethylene/propylene products reign supreme among the copolymers. They are elastomers. Plastics containing about 20% or more propylene perform like natural rubber and can be cured by peroxide cross-linking. They are faster to chemical and to ageing than other types of natural rubber. 

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. ... 

Tensile strength of radiation cured purified natural rubber, o, sulfur A, peroxide , EB irradiation in nitrogen at 2.5 kGy/s. (Bohm, G. G. A., and Tveekrem, J. O., Rubb. Chem. Technol., 55 3, p. 620. Reprinted with permission from Rubber Division, ACS.)... 

FIGURE 5.7 Tensile strength of radiation-cured purified natural rubber. Legend = sulfur T = peroxide = EB irradiation. (Bohm, G.G.A. and Tveekram, J.ORubber Chem. Technol. Vol. 55, No. 3 (1982). With permission.)... 

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. 

Figure 5-17. Chemical stress relaxation of Natural Rubber cured with dicumyl peroxide. [Data from Y. Takahashi and A. V. Tobolsky, Technical report No. 125, Office of Naval Research, N00014-67-A-0151-0011 (1970).]...

Infrared spectroscopy of peroxide cured rubbers has revealed only minimal spectroscopic information on the new cross-linked structure. What has been observed is the decrease in the intensities of the C-H out-of-plane bending modes of the olefin double bond which absorb at 837 cm l for the natural rubber and at 740 cm for cis-1,4-polybutadiene. While these bands reflect losses in the amount of unsaturation in the final material, when compared to the starting material, no evidence of the network carbon-carbon single bond absorption bands has been reported. 

In spite of the relative weakness of the cross-link bands, the course of the peroxide curing process can be followed to extract kinetic data. Examining Figure 3, where the infrared spectra of (A) natural rubber, (B) dlcumyl peroxide, (C) dlcumyl... 

Figure 6. Superposed spectra of natural rubber cured with dlcumyl peroxide. Amount of peroxide Indicated at the high field side of the spectra.

Figure 12. CP-MASS spectra of natural rubber cured with dlcumyl peroxide. Oleflnlc region shown only. The amount of peroxide Indicated on the high field side of spectra. The arrow Indicates an artifact of the carrier. Greek letters represent Identical carbons as In Figure 7.

Figure 1. Natural rubber cured with 30 phr dlcumyl peroxide. Spectrum (A) obtained under CP-MASS. Spectrum (B) obtained with the CP-MASS experiment with a delay between acquisition of 100 yiAsec, the quartenary carbons are distinguishable at 135 ppm and 45 ppm.

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