Natural rubber terms Links

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. 

A critical requirement for obtaining engineering properties from a rubbery material is its existence in a network structure. Charles Goodyear s discovery of vulcanization changed natural rubber from a material that became sticky when hot and brittle when cold into a material that could be used over a wide range of conditions. Basically, he had found a way to chemically connect the individual polymer chains into a three-dimensional network. Chains that previously could flow past one another under stress now had only limited extensibility, which allowed for the support of considerable stress and retraction upon release of the stress. The terms vulcanization, rubber cure, and cross-linking all refer to the same general phenomenon. 

An estimate of the number of monomer units per equivalent random link can be obtained by dividing the value of Aa calculated from the stress-optical coefficient by the anisotropy of the polarisability of the monomer unit calculated from bond polarisabilities. This number can more interestingly be expressed in terms of the number of single bonds in the equivalent random link and is found to be about 5 for natural rubber, about 10 for gutta percha and about 18 for polyethylene. (For the last two the values are extrapolated from measurements at elevated temperature.) The number for polyethylene is considerably higher than the value of 3 suggested by the assumption of totally free rotation around the backbone bonds (see section 3.3.3 and problem 3.7). 

Figure 37-1. Network density ft in amount of network chains per unit volume vulcanizate as a function of the vulcanization time for the vulcanization of natural rubber with 2 phr sulfur, 5 phr zinc oxide, 1 phr 2>mercaptobenzthiazole, and 1 phr stearic acid at 140 C. Cl, main cross-linking reaction Cii, long-term crosslinking A chain degradation. (After D. A. Smith.)...

In the most general sense, an elastomer may be defined as an amorphous, cross-linked polymer above its glass transition temperature (see Section 8.12). The two terms rubber and elastomer mean nearly the same thing. The term rubber comes from the rubbing out action of an eraser. Originally, of course, rubber was natural rubber, c -polyisoprene. The term elastomer is more general and refers to the elastic-bearing properties of the materials. 

Natural rubber is a mixture of sol and gel rubber, two rather imprecise terms. Sol rubber refers to that portion of the rubber that is soluble in carbon disulfide, chlorinated hydrocarbons, and aliphatic and aromatic hydrocarbons, but insoluble in lower alcohols and ketones. Gel rubber is insoluble in such solvents. The gel portion is highly branched and lightly cross-linked. With a high gel content the rubber is difficult to work because of the high shearing resistance in roll mills or internal mixers. 

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