Polymer chemistry vulcanization

Polymer technology is quite old compared to polymer science. For example, natural rubber was first masticated to render it suitable for dissolution or spreading on cloth in 1820. and the first patents on vulcanization appeared some twenty years later. About another one hundred years were to elapse, however, before it was generally accepted that natural rubber and other polymers are composed of giant covalently bonded molecules that differ from ordinary molecules primarily only in size. (The historical development of modern ideas of polymer constitution is traced by Flory in his classical book on polymer chemistry [ I ], while Brydson [2] reviews the history of polymer technology.) Since some of the terms we are going to review derive from technology, they are less precisely defined than those the... 

The earliest applications of polymer chemistry involved chemical modification designed to improve the physical properties of naturally occurring polymers. In 1839, Charles Goodyear transformed natural rubber, which is brittle when cold and tacky when warm, to a substance that maintains its elasticity over a wider temperature range by heating it with sulfur (vulcanization). The first synthetic fibers— called rayons—were made by chemical modification of cellulose near the end of the nineteenth century. 

The basic chemistry of vulcanization is simple. Sulfur is added to the polymer, and the mixture is heated under controlled conditions. Some of the polymer C—bonds break and are replaced by C—S bonds. The cross-links... 

The process that makes the chemistry, properties, and applications of elastomers so different from other polymers is cross-linking with sulfur, commonly called vulcanization. The modem method of cross-linking elastomers involves using a mixture of sulfur and some vulcanization accelerator. Those derived from benzothiazole account for a large part of the market today. Temperatures of 100-160°C are typical. 

However, for construction purposes, solid ebonites were chosen. As is known from rubber chemistry, solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds). The sulfur and polydienes are combined with some auxiliary additives and heated to produce vulcanization. Typical mass ratios of sulfur to rubber are 2 100 for elastomeric rubber and 40 100 for hard rubber. Due to the large degree of sulfide cross linking formed in the vulcanization process, solid ebonite is a hard, non-flexible, plastic-like material possessed of... 

Soft flexible rubbery behavior depends on long flexible polymer molecules in the form of random coils. Strength, heat and chemical resistance depend on attachment between the coils. Conventional rubber chemistry uses vulcanization, permanent thermoset primary covalent cross-links, usually by sulfiir plus metal oxides, to hold the coils together but this makes processing more difficult, and recycling very difficult. In the past 40 years, this technology has been supplemented by the... 

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

Summary Two-component room temperature-vulcanizing, condensation-curing systems (RTV-2) are well known in silicone chemistry. Even silicone-based materials caimot fulfill all requirements in diverse applications. It is therefore desirable to combine the curing properties of silicone-based systems with those of other polymer backbones. The use of isocyanatomethyl-dimethylmonomethoxysilane allows the straightforward derivatization of, e.g., hydroxyl-terminated polymers, which yield mono-silanol-terminated polymers upon hydrolysis. 

Reactions of Antioxidants with Polymers During Processing. One of the earliest polymer-bound antioxidants was obtained by reaction of nitroso antioxidants (ANO) with rubbers during vulcanization (29). The chemistry of this process is complex, but its discoverers proposed an "ene" reaction with the unsaturation in the polymer. 

Reinforcement by carbon black is remarkably non-specific in regard to the chemistry of widely different cross-linking reactions (17). This lack of specificity alone is evidence that interactions with polymer through the vulcanization recipe are not a primary cause of reinforcement. However, such interactions do occur and undoubtedly can contribute to reinforcement phenomena... 

Three-dimensional (3-D) plot of the shape memory cycle for (a) a shape memory polymer (SMP) and (b) vulcanized natural rubber. The star indicates the start of the experiment (initial sample dimensions, temperature, and load). Both the SMP and the rubber were deformed under constant loading rate at constant temperature. The deformation step was then followed by a cooling step under constant load. At low temperature, the load was removed and shape fixing was observed for the SMP, but an instant recovery was seen for natural rubber. Shape recovery of the primary equilibrium shape was obtained by heating the SMP. (Adapted from Liu, C., Qin, H., and Mather, P. T. 2007. Review of progress in shape-memory polymers, journal of Materials Chemistry 17 1543-1558. Copyright Royal Society of Chemistry. Reproduced with permission.)... 

---