Caoutchouc Properties

Isoprene is of especial importance in connection with the terpenes as it has been shown to be the mother hydrocarbon of caoutchouc or rubber. Its constitution and properties will be discussed again later on. 

Source.—The common substance which is known as rubber is the product obtained by the coagulation of the juice or latex which is present, usually in the bark, but sometimes in the woody tissue, of certain tropical or sub-tropical trees, shrubs and vines. Gutta-percha is a variety of rubber differing in physical properties. The chemical individual present in rubber is a terpene hydrocarbon known as caoutchouc. 

Properties of Pure Caoutchouc.—Pure caoutchouc may be obtained by dissolving rubber in certain solvents, after first removing resins by solution in acetone. The rubber free from resins is treated with chloroform, benzene, or carbon tetra-chloride, all of which are solvents of caoutchouc. Evaporation of the solvent leaves j ure caoutchouc. 

Caoutchouc so obtained is a colorless, transparent hydrocarbon of the composition CsHg or better (CsHg) . It is an emulsion colloid of a density approximately 0.90. It is a non-conductor of electricity and this is one of its important properties. It takes up liquids and swells. It is moderately resistant to the diffusion of gases and can be used for balloons but is not as good as other materials. Pure caoutchouc is a soft, sticky, gummy mass of low elasticity and in this condition possesses almost no desirable technical properties. In order to give it such properties it is very definitely changed in the process of manufacture. 

Vulcanization.—The most important treatment of rubber, in the process of converting it into a technically valuable product, is that known as vulcanization. This consists in the addition of sulphur which produces a very definite change in properties. The sticky or adhesive character of pure caoutchouc is entirely lost and it becomes very elastic and does not set when stretched. Even with wide range in temperature it neither hardens nor softens and it becomes insoluble in caoutchouc solvents. The presence of sulphur, usually in small... 

The polymerization of isoprene to caoutchouc has been accomplished by two general methods First the production of what is termed normal caoidchouc by an auto-polymerization in the presence of acid, alkali, amides, urea, etc. Second, the production of sodium caoutchouc by polymerization with sodium or metallic amalgams in the cold or by heat. The different hydrocarbons possible of polymerization to caoutchouc differ as to which of these methods produces the best caoutchouc and also the caoutchouc obtained varies as to its ability to properly vulcanize and yield a satisfactory rubber with proper physical properties. 

The preparation of composite materials in general is a very important appHca-tion of the mechanical properties of nanodiamond. With many polymers like caoutchouc, polysiloxanes, fluoroelastomers polymethacrylates, epoxy resins, etc., composites with markedly improved mechanical characteristics have already been obtained from the noncovalent incorporation of nanodiamond by simple admixing during polymerization. The modulus of elasticity, the tensile strength, and the maximal elongation of the material all increase upon this modification. Depending on the basic polymer, just 0.1-0.5% (w/w) of nanodiamond are required to achieve this effect (Table 5.3). Polymer films can also be reinforced by the addition of nanodiamond. For a teflon film with ca. 2% of nanodiamond added, for example, friction is reduced at least 20%, and scratches inflicted by mechanical means are only half as deep as in neat teflon. 

The jnost valuable property of india-rubber, apart from its elasticity, is that which it possesses of entering into combination with S to form what is known as vulcanized rubber, which is produced by heating together the normal caoutchouc and S to 130°-150° (26(i°-303° P.). Ordinary vulcanized rubber differs materially from the natural gum in its properties its elasticity and flexibility are much increased it docs not harden when exposed to cold it only fuses at 300° (393° P.) finally, it resists the action of reagents, of solvents, and of the atmosphere mucli better than does the natural gum. 

Rubber is obtained from the juice of various tropical trees, mainly the tree Hevea brasiliensis. The juice is a latex consisting of a dispersion of polymer phase at a concentration of about 35% by mass, together with traces of proteins, sterols, fats, and. salts. The rubber is obtained either by coagulation of the latex with acid, either elhanoic or methanoic, or by evaporation in air or over a flame. The material that results from this process is a crumbly, cheese-like substance, sometimes called raw rubber or caoutchouc. In order to develop the mechanical properties that are considered characteristic of rubber, Le. so-called rubberlike elasticity, this raw rubber needs further processing, and in particular lightly crosslinking. This is achieved in the process known as vulcanisation, as will be discussed later. 

Rubber [Indian rubber, caoutchouc, from (South American) Indianic language caa=tears and ochu= tree or cahuchu=crying tree]. R. is the name (according to DIN 53501, 11/1980) for non-cross-linked but cross-linkable (vulcanizable) polymers with rubber-elastic properties at room temperature. At higher temperatures or under the action of deforming forces R. show viscous flow. Thus, under suitable conditions, R. can be processed into specific shapes. They are the starting materials for the manufacture of elastomers and other rubber products. 

Gough, J. (1805) A description of a property of caoutchouc (india rubber), Memoirs Literary Phil. Soc. Manchester, Second Series, 1, 288-295. 

What makes artificial plastics so attractive compared with long-used natural polymers such as wood, paper, cotton, wool, silk, horn, or natural rabber (caoutchouc) Synthesized plastics can be easily formed into almost any shape, they are resistant to environmental effects, heat, chemicals, and they are inexpensive (these properties, of course, differ depending on the type of plastics). Natural polymers have some advantages, too, primarily that they are typically more biodegradable than synthetic materials. As environmental pollution worsens, this property is becoming increasingly important. Humankind must use resources efficiently and must try to prevent unnecessary problems in the enviromnent. 

The study of the physical and chemical properties of rubber has received a decided impetus as a result of recent developments in colloidal chemistry, for it cannot be denied that in caoutchouc, the fundamental substance from which commercial rubber is ma de, we have a typical colloidal body. Many of the processes in the manufacture of both the crude rubber, caoutchouc, and the finished product, rubber, such as the coagulation of the latex, find no explanation from the purely crystalloidal chemical standpoint. Unfortunately in the discussions on the subject it has not always been recognized that in most instances both the colloidal and crystalloidal processes take place simultaneously. Consequently important facts are often ignored by the extreme advocates of the colloidal and the purely chemical schools. Only by a proper perspective involving both views, can we arrive at the true explanation of many of the phenomena connected with the chemistry of rubber. 

The latex from which caoutchouc is obtained is a milk-like fluid differing somewhat in its properties according to its origin. Biologically it is the sap of certain trees or shrubs chemically it is a disperse system consisting of globules of caoutchouc suspended in a watery liquid. 

When raw caoutchouc is mixed with sulfur and the temperature raised sufficiently a remarkable change of chemical and physical properties takes place. The mass loses its adhesiveness, called tackiness in practice the elasticity may vary between great extremes differences of temperature over a comparatively wide range have little effect it is rendered insoluble in any liquid that does not permanently destroy it and finally it is much more resistant to oxidation, and therefore less liable to perish. The process is known as the hot cure or hot vulcanization. Similar alterations in properties, differing only in degree, may be brought about by what is termed the cold cure, or cold vulcanization. The hot cure is much more widely applied in practice. 

The amount of sulfur necessary to give certain properties, such as a desired elasticity, depends a great deal upon the phyacal state of the caoutchouc. In general if the average value of n is lowered, by mastication, for example, more sulfur will be necessary to produce a deared result. 

A great deal of work has been done of recent years on the process of vulcanization. Unfortunately the results of one experimenter often seem to contradict those of another consequently we have few undisputed facts upon which to base our theories. This want of accord among the experimental results is probably not due so much to inaccurate work as it is to the fact that raw caoutchouc is a complex product, varying in properties to a condderable degree with its source, method of preparation, a e, etc. A short summary and not an extended dis-cusdon of the two prindpal theories will be ven here. For a more comprehensive review of the subject the reader is referred to the original literature, or to Der Kautschuk by Ditmar. 

Hinrichsen further remarks that other derivatives of caoutchouc such as the tetrabromide are known. Vulcanized rubber can be changed quantitatively into this bromine derivative where the sulfur is almost exactly equivalent to the bromine. Furthermore the changes in properties during vulcanization are too fimdameutal to be explained on the basis of the adsorption theory. He concludes from the evidence that some of the sulfur, doubtless the free sulfur, is adsorbed, while the remainder is combined chemically. 

Caoutchouc began to appear in large compilations of chemical knowledge such as J. Murray s A System of Chemistry [6]. It was noted that the tree sap from the Hevea guianensis (a type of Euphorbia) could be separated into a firm elastic coagulum and a watery liquid. The specific article on caoutchouc contains a clear declaration The most remarkable physical property of which this substance is... 

Another remarkable property of the block of purified caoutchouc occurred when it was stretched even farther. The block became hard and very tough. This form could be heated and the block returned to its initial shape and elasticity. It is now known that the phenomenon being observed is crystallization, but at the time it was noted and puzzled about. Rubber has been a source of amazement and amusement ever since its discovery. 

The present volume follows the initial study A Prehistory of Polymer Science [1]. The origins and development of the field were surveyed up to the time of the Faraday Discussion of 1935 on Polymerization. From early studies of caoutchouc at the French Academy of Sciences to seminal syntheses of polyamides at DuPont, the concept of covalent macromolecules was developed. This fruitful concept was extended to a large number of observable properties in the period 1935-1953. 

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