Cross-linking, natural rubber

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. )... 

Comparison of Stress-Strain Properties of Natural Rubber Cross-Linked by Sulfur and Radiation... 

Figure 4. Aliphatic region of natural rubber cross-linked with dicumyl peroxide. Spectra were taken under normal FT conditions. The loading of peroxide is Indicated at the high field side of the spectra.

In trying to increase the resolution of the CP-MASS spectra of the highly cross-linked rubber networks, the samples were swollen in benzene to equilibrium. The gel was then packed into the rotor and spectra accumulated as if the sample was a dry solid. Figure 16 shows the spectra of natural rubber cross-linked with 25 phr peroxide, obtained from the CP-MASS experiment... 

Figure 16. Spectra of natural rubber cross-linked with 25 phr ROOR. Spectrum (A) swollen in benzene to equilibrium swelling. Spectrum obtained under conditions of NFT experiment. Spectrum (B) same sample as (A), obtained under CP-MASS. The asterisk marks resonance of benzene solvent. Spectrum (C) the difference between (A-B).

Dependence of the shear modulus on temperature for three representative engineering polymers natural rubber (cross-linked) PVC (essentially amorphous and not cross-linkied) and nylon 6 (crystalline). The temperatures at which these polymefS are used in technology are indicated ( - - - - ) (after WolO. 

Fig. 7.1 Plot of melting point depression (AT) as a function of fraction of units cross-linked, p, for polymer networks formed from random chains. A molten polyethylene cross-linked by ionizing radiation o natural rubber cross-linked with sulfur natural mbber cross-linked with di-t-butyl peroxide. (Data from Refs. (13) and (14))...

Peroxides were first introduced as cross-linking agents for diene rubbers as long ago as 1915 by Ostrosmislenskii, benzoyl peroxide being used with natural rubber. The materials remained laboratory curiosities until the introduction of dicumyl peroxide (Braden et al, 1954 Braden and Fletcher, 1955). Natural rubber cross-linked by this peroxide shows low compression set and creep, may be compounded for high transparency and does not exhibit the staining problems (e.g. 

Fig. 10.38 Plot of normalized crystallinity level as a function of log time for natural rubber cross-linked to varying extents with sulfur at 2 °C. Curve derived Avrami equation with = 3. Combined sulfur content in percent o zero 0.1 V 0.2 T 0.3 0.35 0.40 A 0.43 A 0.46 O 0.5. (Data from Bekkedahl and Wood (92))...

PMMA degradation determines a general decrease of mechanical performances of the material while the other phenomena may have beneficial effects even if rubber cross-linking can produce a decrease in the strain at break. These mechanisms, however they work, determine a decrease in the strain at break and a slight increase of the yield stress upon further irradiation. The different responses of the system varying the rubber nature is clearly attributable to the different effects of ionizing radiation on the rubber, which affect the final structure of PMMA-rubber blend. 

Natural rubber, silicone rubber Cross link 50... 

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

Figure 3.3 Comparison of experiment (points) and theory [Eq. (3.39)] for the entropy elasticity of a sample of cross-linked natural rubber. [From L. R. G. Treloar, Trans. Faraday Soc. 40 59 (1944).]...

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

In order to "cure" or "vulcanize" an elastomer, ie, cross-link the macromolecular chains (Fig. 2), certain chemical ingredients are mixed or compounded with the mbber, depending on its nature (4,5). The mixing process depends on the type of elastomer a high viscosity type, eg, natural mbber, requires powerhil mixers (such as the Banbury type or mbber mills), while the more Hquid polymers can be handled by ordinary rotary mixers, etc (see Rubber... 

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. 

The vulcanisation of natural rubber, a long chain polyisoprene, with sulphur involves a similar type of cross-linking. 

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

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