Thermosets natural rubber

Ebonite is also made from natural rubber, but with a much higher amount of sulphur it belongs, therefore, to the family of thermosets. 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Compression molding is an old and common method of molding thermoset (TS). It now processes TS plastics as well as other plastics such as thermoplastics (TP), elastomers (TS and TP), and natural rubbers (TS). By this method, plastic raw materials are converted into finished products by simply compressing them into the desired shapes... 

Both linear and branched polymers are thermoplastics. However, cross-linked three-dimensional, or network-polymers are thermoset polymers. The cross-linked density may vary from the low cross-linked density in vulcanized rubber to high cross-linked density observed in ebonite (hard rubber highly cross-linked natural rubber). 

The major use of polymers has been as replacements for naturally occurring materials. Synthetic fibers such as nylon and polyester have substantially replaced natural textiles synthetic rubber is vastly superior to natural rubber, and the wide variety of engineering polymers (both thermosets and thermoplastics) have replaced traditional, naturally occurring materials such as metals and cellulosic compounds in many applications. 

The concept of traditional thermoset elastomers was pioneered by Goodyear s discovery in 1839 that heating natural rubber with some sulfur converted the material from one that was tacky when warm and brittle when cold into a vulcanized rubber that was conveniently useful over a wide temperature range. Crosslinking of the macromolecules of rubber with sulfur bonds endowed the naturally occurring material with some elastic memory and caused it to behave as we have come to expect elastomers to behave. Excessive sulfur crosslinking converts the stretchable, compressible, bouncy rubber into hard rubber such as the material found in the heads of mallets used in machine shops to pound sheet metal into desired shapes. A small dose of crosslinking prevents the macromolecules of natural rubber to crystallize at low temperatures and turn into a brittle solid and to become a tacky, sticky semifluid at elevated temperatures. 

Paradoxically, neither Goodyear nor any of his family members or de-scendents were involved with the Goodyear Tire and Rubber Company, whose founder, Frank A. eiberling, named it to honor one of America s most famous inventors and the founder of an industry that is indispensable to modern life. In 1851 Goodyear s brother Nelson used sulfur to convert natural rubber into ebonite, the first thermosetting plastic. 

Chain flexibility also affects the crystalhzabihty of a polymer. Excessive flexibility in a polymer chain, as in natural rubber and polysiloxanes, gives rise to difficulty in chain packing, with the result that such polymers remain almost completely in the amorphous state. In the other extreme, excessive rigidity in polymers due to extensive cross-hnking, as in thermosetting resins like phenol-formaldehyde and urea—formaldehyde, also results in an inabihty to crystallize. 

Elastomeric 1, Natural rubber. 2, Neoprene. 3, Nitrile. 4, Urethane. 5, Styrene-butadiene. Thermoplastic 6, Poly(vinyl acetate). 7, Polyamide. Thermosetting 8, Phenol-formaldehyde. 9, Resorcinol, Phenol-resorcinol/formaldehyde. 10, Epoxy. 11, urea-formaldehyde. Resin 12, Phenolic-poly(vinyl butyral). 13, Polyeser. Other 14, Cyanoacrylate. 15, Solvent. 

The proportion of bio-based materials in each of the sectors of elastomers and fibres accounted for almost 40% due to the use of 290 000 t of natural rubber and 300 000 t of cellulosic fibres. The market size for thermoplastics and thermosets amounted to circa 15.8 million t, of which circa 12.5 million t accounted for rigid materials, mainly in packaging, building and construction, automotive and electronics industries as well as for furniture and consumer goods. A volume of 3.3 million t is attributed to adhesives, paints and lacquers, binders and other polymeric additives. In these areas it is estimated that bio-based materials... 

This book focuses on bio-based plastics. It emphasizes materials that are presently in use or that show a significant potential for future applications. It presents a broad, up-to-date but concise overview of basic and applied aspects of bioplastics. The main focus is on thermoplastic polymers for material use. Elastomers, thermosets and coating applications, Uke, for example, natural rubber or alkyd resins, will be covered in other volumes of the series. 

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