Vulcanized rubber, structure

This sulfur radical can attack another chain to give a cross-link or bite back to give a link within the same chain. Many different sulfur links are formed and the next diagram summarizes a part of the vulcanized rubber structure. There is some license here in reality the links would not be as dense as this, and more than two chains would be involved. 

Ic. Cross-Linking of Polymer Chains.—Formation of chemical bonds between linear polymer molecules, commonly referred to as cross-linking, also may lead to the formation of infinite networks. Vulcanization of rubber is the most prominent example of a process of this sort. Through the action of sulfur, accelerators, and other ingredients present in the vulcanization recipe, sulfide cross-linkages are created by a mechanism not fully understood (see Chap. XI). Vulcanized rubbers, being typical network structures, are insoluble in all solvents which do not disrupt the chemical structure, and they do not undergo appreciable plastic, or viscous, flow. 

Covalent polymeric networks which are completely disordered. Continuity of structure is provided by an irregular three-dimensional network of covalent links, some of which are crosslinks. The network is uninterrupted and has an infinite molecular weight. Examples are vulcanized rubbers, condensation polymers, vinyl-divinyl copolymers, alkyd and phenolic resins. 

It is possible to explain the decrease of ZnCFO efficiency as component of various vulcanization systems for rubbers of general and special assignment in the earlier submitted line (fig. 10) also by character of formed morphology of compositions. So, at use of ZnCFO as the activator of sulfur vulcanization the structure of rubbers with the minimal value of parameter r is formed, and at transition from sulfur to peroxide vulcanization of elastomeric compositions the particles size of heterophase is increased (fig. 11 b). 

Flory,P.J. Network structure and the elastic properties of vulcanized rubber. Chem. Rev. 35, 51-75 (1944). 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

Unsaturated and Vulcanized Rubbers. Oxidation occurs most readily at polymers with structural double bonds, such as natural rubber, polybutadiene, or polyisoprene. Aromatic amines and sterically hindered phenols are effective antioxidants. From the rubber antioxidants, 96.8 million pounds were amines, and 20 million pounds were phenols. Amines act also as antiozonants whereas phenols are not effective. Furukawa shows that amines have a lower oxidation potential which is a prerequisite for antiozonant action. 

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. 

The macromolecular structure of (vulcanized) rubber can be degraded using thermal, mechanical, and chemical means [14]. The resulting reclaim was once incorporated in limited amounts in new tyres. Safety and quality considerations dried up this outlet. 

This chapter is devoted to the properties of polymeric gel-forming liquids. Particulate gels are discussed in Chapter 7. The structure of a polymeric gel is sketched in Fig. 5-1. Since this book is devoted to materials that are in some sense liquid, or at least liquefiable, we shall not say much about hard, irreversible, chemical gels such as cured epoxies or vulcanized rubber, but shall focus instead on chemical pre-gels and thermally reversible physical gels, both of which can be considered borderline fluids. This chapter is confined to a brief overview. Much more detail can be found in Winter and Mours (1997), and volume 101 of the Faraday Discussions. ... 

Saville, B. Watson, A.A. Structural characterization of sulfur-vulcanized rubber networks. Rubber Chem. Technol. 1967, 40, 100. 

Acid bottles, rubber Air-supported rubber structures Aprons, vulcanized rubber and rubberized fabric—mitse Bags, rubber or rubberized fabric... 

The efficiency of vulcanization and structure formation of rubber concrete can be considerably increased by supplementary thermal treatment of the outside of the covering, thereby creating a more uniform thermal field along the surface and volume of the heated article. 

In 1961 Crouch and Short (38) discussed the use of S-B block polymers, although not identified as such, in vulcanized rubber applications. Commercial production of the S-B block polymer commenced late in 1962. Identification of the S-B block structure was presented by Railsback, Beard, and Haws (39) in 1964. 

The reticulated structures are made up of clusters. If all the clusters have a finite size, the system is soluble and the solution is called sol. On the other hand, if the structure contains a cluster of infinite size, the system is a gel which is not soluble but which may swell in a solvent. The same reaction may lead either to sols or to gels according to the final branching rate. The sol-gel transition may be considered as a percolation transition. Note that an infinite cluster can be made either by chemical binding or (partially) by topological trapping [see Fig. 1.8]. From a mechanical point of view, a sol is viscous, a gel is elastic. Thus a piece of vulcanized rubber can be considered as a gel. 

Polymer seienee is a relative yoimg science, and the concept of macromolecules for understanding the properties of polymeric materials was introduced by H. Staudinger in 1919 and developed in the 1920s, in particular on cellulosic materials. The industry had certainly previously produced vulcanized rubber, a process invented by Goodyear in 1839, celluloid in 1865 with the Hayatt brothers, bakelite in 1910 developed by Baekeland, but the tme nature of the ehemieal species was at that time not yet revealed. From 1930, a better understanding of the macromolecular structure led to the fast development of different ehemieal famihes low-density polyethylene by radical polymerization synthesized in 1933, and the works of Carothers on polycondensation led to polyamides in 1938. 

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