Natural rubber, 1.27

Rubber is a natural product that exists as a colloidal dispersion named latex in the sap of certain plants from various families such as Moraceae, Compositae or Euphorbiaceae. From this last family, Hevea brasiliensis is the most common plant that produces natural rubber for practical use. The latex has rather large colloidal particles (in aqueous medium) with diameters up to 5 m (although the average is 0.5 jim). 

These particles are made from aggregates of 10 to 10 macromolecules of polyisoprene. The presence of the isoprene molecule in the structure of natural rubber makes it part of the polyterpenes family of compounds [1]. 

The stability of latex is due to a thin layer of proteins on particles, which acts as a colloid stabilizer. Natural rubber is practically obtained by the precipitation and drying of the latex. The precipitation is done with acids (acetic acid is commonly used for this purpose) when the isoelectric point of the protecting protein is reached (pH 4.6). The macromolecules have a MW between 5 10 to 3 10 Dalton and contain between 600 to 50,000 units of isopentene. Due to the double bond, both cis and trans isomers are possible for the monomer units. It was determined that natural rubber is an isotactic polymer formed exclusively from cis units and has the following (idealized) structure (in reality the polymer is not perfectly planar)  

A natural polymer formed from trans isopentene units is found in some plants from Sapotaceae family and is known as gutta-percha (or balata when it comes from a South American plant from the same family). 

Other pyrolysis products besides isoprene and its dimer are also formed from rubber. The more volatile compounds with the maximum of five carbon atoms generated from the pyrolysis of natural rubber at 700 C are indicated in Table 6.1.1. 

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 ethanoic or methanoic, or by evaporation in air or over a flame. The material that results from this process is a crumbly, cheeselike substance, sometimes called raw rubber or caoutchouc. In order to 

The polymer in natural rubber consists almost entirely of ci -poly(isoprene) (1.6). The molecules are linear, with relative molar mass typically lying between 300 000 and 500 000. The macromolecular nature of rubber was established mainly by Staudinger in 1922, when he hydrogenated the material and obtained a product that retained its colloidal character, rather than yielding fragments of low relative molar mass. 

Vulcanisation is the term used for the process in which the rubber molecules are lightly crosslinked in order to reduce plasticity and develop elasticity. It was originally applied to the use of sulfur for this purpose, but is now used for any similar process of cross-linking. Sulfur, though, remains the substance most widely used for this purpose. 

Sulfur reacts very slowly with rubber, and so is compounded with rubber in the presence of accelerators and activators. Typical accelerators are thia-zoles and a typical activator is a mixture of zinc oxide and a fatty acid. The chemistry of the vulcanisation reactions is complicated, but generates a three-dimensional network in which rubber molecules are connected by short chains of sulfur atoms, with an average of about five atoms in each chain. 

A much more heavily crosslinked material can be obtained by increasing the amount of sulfur in the mixture, so that it represents about a third of the mass of the product. Heating such a mixture of raw mbber and sulfur at 150 °C until reaction is complete gives a hard, thermoset material that is not at all elastic. This material is called ebonite and is used to make car battery cases. 

Rubber can be obtained from a number of trees, but commercial production is done with tropical plant Hevea brasiliensis. Rubber is a polymer of a single monomer isoprene (2-methyl butadiene). Isoprene itself can chemically (i.e., artificially) be polymerized by various means to produce polyisoprene, but this is not the way the natural rubber is produced in rubber trees. The biological process to produce rubber (i.e., biological polymerization) is shown in Fig. 5.5. This process is a kind of condensation with removal of a pyrophosphate group 

Rubber—an unusual name for an unusual substance—is a naturally occurring alkene polymer produced by more than 400 different plants. The major source is the so-called ruhher tree, Hevea brasiliensis, from which the crude material is harvested as it drips from a slice made through the bark. The name rubber was coined by Joseph Priestley, the discoverer of oxygen and early researcher of rubber chemistry, for the simple reason that one of its early uses was to rub out pencil marks on paper. 

Unlike polyethylene and other simple alkene polymers, natural rubber is a polymer of a conjugated diene, isoprene (2-methylbuta-l,3-diene). The polymerization takes place by 1,4-addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry  

Natural rubber is obtained from the bark of the rubber tree, Hevea brasiliensis, grown on enormous plantations in Southeast Asia. 

23 Name the following alkenes, and predict the products of their reaction with (i) mefa-chloroperoxyhenzoic acid followed by (ii) acid-catalyzed hydrolysis  

24 Draw the structures of alkenes that would yield the following alcohols on hydration (red = O). Tell in each case whether you would use hydro-boration/oxidation or oxymercuration. 

Rubber products are very dependent on the availability of petroleum and natural gas. In the short term, if petroleum were not readily available, it would require a major change in the existing infrastructure to base the rubber industry on coal. Also, it would be very difficult to maintain a modern economy if the only source of rubber were natural rubber. In many ways natural rubber is a wonderful rubber with many advantages such as strength and low hysteresis however, NR does not possess sufficient resistance to oil and heat exposure for many modern-day applications. It is seriously doubted that even special coatings of natural rubber parts would allow NR to be the universal rubber, replacing all synthetic rubber applications. 

General-purpose elastomers represent the work horse rubbers of the rubber industry. They are the lowest cost, most cost-effective rubber polymers available today. The vast majority of these raw elastomers are used in the tire sector however, a large amount is also used in nontire applications as well, such as single-ply roofing, hose, dynamic parts (such as bushings, isolators, and dampers), and conveyor belts. 

The following general-purpose elastomers and their approximate worldwide annual consumption are shown in Table 3.1. 

Elastomer Approx. Annual 2010 Consumption (Billion Pounds) 

The high-volume synthetic rubber produced today represents about 8% (by monetary value) of all the high-volume polymers (including plastics and textiles), which in turn are about 25% of the chemical industry. 

Natural rubber of the best quality is prepared by coagulating the latex of the Hevea brastliensis tree that is primarily cultivated in the Far East. However, there are other sources such as the wild rubbers of the same tree growing in Central America, guyayule rubber coming from shrubs grown mostly in Mexico, and balata. Balata is a resinous material and carmot be tapped like the Hevea tree sap. The balata tree must be cut down and boiled to extract balata that cures to a hard, tough product used as golf ball covers. 

Another source of rubber is the planation leaf gutta-percha. This material is produced from the leaves of trees grown in bush formation. The leaves are picked and the rubber is boiled out as with the balata. Gutta-percha has been used successfully for submarine-cable insulation for more than 40 years. 

Chemically, natural rubber is a pol)nner of methyl butadiene (isoprene)  

Purified raw rubber becomes sticky in hot weather and brittle in cold weather. Its valuable properties become apparent after vulcanization. 

Depending upon the degree of curing, natural rubber is classified as soft, semihard, or hard rubber. Only soft rubber meets the ASTM definition of an elastomer, and therefore, the information that follows pertain only to soft rubber. The properties of semihard and hard rubber differ somewhat, particularly in the area of corrosion resistance. 

NR is the only natural product amongst those in Table 1.1. It is a cis-polyisoprene but contains a variety of impurities such as proteins and resins. It crystallises much more readily upon stretching compared to synthetic cis-polyisoprene, IR, which does not contain the impurities. NR has a very high molecular weight (MW) and contains long branches. 

The prevalence of rubber in wood has been studied by Sandermann et al. (12). Although rubber is found in more than 2000 genera in the stalks, stems, roots, and leaves of 150 wood species investigated, only eight had rubber in the xylem. Of these eight, only four had a rubber content of more than 1%. Rubber is present only in the parenchymatic tissue of the xylem. 

The mechanics of latex formation in the rubber tree has been the subject of considerable study. It is now known that the monomer isopentyl pyrophosphate is the one used in the biosynthesis of natural rubber. Catalysts for these transformation are enzymes in the latex and tissues of Hevea. The structure for natural-rubber is c/5 -l,4-polyisoprene (1) and the trans form for the structure of gutta 

Moisture Acetone extract Protein (calc, from N2) Ash 

Non-rubbers in the latex include fatty acid, protein, sterols, and esters these remain in the dry rubber. Trace elements include copper, manganese, iron, potassium, and magnesium. The proteins and fatty acids help in the vulcanization reaction. Slight variations in these properties can be expected because the non-isoprenic content will vary. 

Latex can be defined as a colloidal dispersion of rubber in a water solution or serum. The dispersed rubber globules are constantly in motion (Brownian motion) and have a diameter of 1 to 2 microns. Latex has moderate stability because the particles all have a negative charge and therefore repel each other. 

As with most elastomers, natural rubber can be readily bonded with cyanoacrylates although in these trials [2] the adhesion achieved with the toughened cyanoacrylates was relatively low (Table 4.7). 

All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. 

The derivatives of butadiene are natural rubber and synthetic rubber. 

Natural rubber is extracted from the hevea brasiliensis tree which is grown in tropical regions. When its bark is slit with a cutter, a liquid, named latex, is obtained. Latex is an emulsion of rubber in water. When acid is added to this emulsion, natural rubber is precipitated. This precipitate is the polymer of a hydrocarbon with the molecular formula CgHg 

The common name of this compound is isoprene and its ICJPAC name is 2-methyl-l, 3-butadiene. Polymerization of isoprene gives polyisoprene, natural rubber. Polymerization may lead to cis or trans versions of polyisoprene. 

Sg is added to the double bond of the alkene in the vulcanization process. 

The major degradation product of natural rubber is l-methyl-4-(l-methylethenyl)cyclo hexene. The presence of this compound as the major degradation product along with 2-methyl-1,3-butadiene (monomer) and groups of compounds containing 15 and 20 carbon atoms (three and four monomer units) in the pyrolysate of a rubber is sufficient to identify it as natural rubber. Similarly, the presence of l-chloro-4-(l-chloroethenyl)cyclohexene and 2-chloro-l, 3-butadiene, the cyclic dimer and monomer of poly(chloroprene) rubber, in the pyrolysate of a rubber identify it as poly(chloroprene) rubber. A correlation between the crosslink density and the product ratio of isoprene dimer species to isoprene formed from pyrolysis of natural rubber vulcanisates has been reported 697436 [a.232]. The major products of the isoprene dimer species were l,4-dimethyl-4-vinylcyclohexene and 

Reprinted from 594526 with permission from Elsevier 

Groves and co-workers [a.237] analysed the oil derived from the pyrolysis of natural rubber in a Py-GC at 500 °G. These researchers showed that the major products were the monomer, isoprene, and the dimer dipentene, with other oligomers up to hexamer also being formed in significant concentrations. It was suggested that the isoprene monomer was formed via a depropagating mechanism in the polymer chain, and that dipentene dimer was formed either by intramolecular cyclisation followed by scission, or by monomer recombination via a Diels-Alder reaction. 

This is the most widely used naturally occurring rubber. The literature search shows that many research groups have prepared nanocomposites based on this rubber [29-32]. Varghese and Karger-Kocsis have prepared natural rubber (NR)-based nanocomposites by melt-intercalation method, which is very useful for practical application. In their study, they have found increase in stiffness, elongation, mechanical strength, and storage modulus. Various minerals like MMT, bentonite, and hectorite have been used. 

FIGURE 2.7 Effect of various clays on natural rubber (NR)-based nanocomposites (at 4 phr loading). (From Bhattacharya and Bhowmick, Unpublished data.) 

Chlorination and hydrochlorination of natural rubber are industrial processes carried out on solutions of uncrosslinked rubber in chlorinated solvents [Allen, 1972 Ceresa, 1978  

Subramaniam, 1988]. Hydrochlorination, usually carried out at about 10°C, proceeds by electrophilic addition to give the Markownikoff product with chlorine on the tertiary carbon (Eq. 9-33) [Golub and Heller, 1964 Tran and Prud homme, 1977]. Some cyclization of the intermediate carbocation (XXVI) also takes place (Sec. 9-7). The product, referred to as rubber hydrochloride, has low permeability to water vapor and is resistant to many aqueous solutions (hut not bases or oxidizing acids). Applications include packaging film laminates with metal foils, paper, and cellulose films, although it has been largely replaced by cheaper packaging materials such as polyethylene. 

Chlorination of natural rubber (NR) is carried out with chlorine in carbon tetrachloride solution at 60-90°C to yield a chlorinated rubber containing about 65% chlorine, which corresponds to 3.5 chlorine atoms per repeat unit. The process is complex and includes chlorine addition to the double bond, substitution at allylic positions, and cyclization. Chlorinated rubber has high moisture resistance and is resistant to most aqueous reagents (including mineral acids and bases). It is used in chemical- and corrosion-resistant paints, printing inks, and textile coatings. Bromination of butyl rubber is also practiced [Parent et al., 2002]. 

Butyl rubber, containing only 0.5-2.5% isoprene units, is not efficiently crosslinked by sulfur. Chlorination of butyl rubber is carried out to improve its vulcanization efficiency by allowing a combination of sulfur and metal oxide vulcanizations. 

Source Chemical and Engineering News, and Chemical Economics Handbook 

Manufacturing (NAICS 326), Rubber Products (NAICS 3262) totals 35.3 billion, of which Tires (NAICS 32621) makes up 15.4 billion, showing the dominance of the automobile tire market in this sector of the chemical industry. The top polymer production summary in Table 1.16 gives a numerical list of important synthetic elastomers. Styrene-butadiene rubber (SBR) dominates the list at 1.93 billion lb for U.S. production. All other synthetic elastomers are much smaller. While elastomers had a slight increase in production from 1980-1990, only 0.5% annually, SBR was down 2.3% per year. From 1990-2000 it was up 1.0% per year. The fastest growing elastomer is ethylene-propylene, up 5.2% annually for 1990-2000. Table 18.1 gives a breakdown in percent production of synthetic elastomers and consumption of natural rubber in the U.S. 

Natural rubber can be found as a colloidal emulsion in a white, milky fluid called latex and is widely distributed in the plant kingdom. The Indians called it wood tears. It was not until 1770 that Joseph Priestly suggested the word rubber for the substance, since by rubbing on paper it could be used to erase pencil marks, instead of the previously used bread crumbs. At one time 98% of the world s natural rubber came from a tree, Hevea brasiliensis, native to the Amazon Basin of Brazil which grows to the height of 120 ft. Today most natural rubber is produced on plantations in Malaysia, Indonesia, Singapore, Thailand, and Sri Lanka. Other rubber-bearing plants 

LAURENT VAYSSE, FREDERIC BONFILS, PHILIPPE THALER,AND JEROME SAINTE-BEUVE  

Rubber is one of the few examples where chemical synthesis succeeded in a nearly identical performance copy of a natural polymer (polyisoprene) - albeit with a completely different chemical composition (styrene-butadiene-rubber, SBR). Regarding sustainable development, the complete imbalance of the early rubber history has emanated during recent years into equilibrium between natural and synthetic rubber. 

Natural rubber from Hevea brasiliensis is a natural polymer composed of an association of poly(cz5-l,4-isoprene) [poly(2-methyl-1,3-butadiene)] and biological elements, giving it highly specific properties. Originating from the Amazon Basin, Hevea was already booming in Asia at the turn of the twentieth 

Both NR and SR are traded in a dry and in a liquid form. The elastomer market (Table 9.5.1) is divided into three major zones (USA, Europe and Southeast Asia) each of which has its own dynamics very closely linked to its internal growth. In the years 2006-2007 China increased its synthetic rubber consumption (-1-19.3%) more quickly than its natural rubber consumption 

Newly planted rubber trees take 5 to 7 years before they can be tapped and reach their peak production in 10 to 20 years. Of the major global challenges, the environment occupies a dominant place for rubber trees and natural rubber, even on smallholdings where Hevea is usually intercropped with other tree crops or food crops, thereby constituting agro-forests, which nowadays have important secondary functions (maintaining biodiversity, environmental conservation, rehabilitation of degraded zones, etc.)  

Elongation of a crosslinked elastomer decreases the entropy of the network chains and the additional decrease in entropy required for crystallization to occur is therefore relatively small. A schematic representation of strain-induced crystallization within a polymer network which has been elongated by a force in the specifled direction is shown in Fig. 33. Crystallites thus formed act as crosslinks of high functionality 

Schematic drawing of strain-iidiiced crystallization in a crosslinked dastcaner 

A linear relationship between [f] and however, holds only at small elongations and an upturn in the reduced stress occurs at small reciprocal elongations less than 0.4. The deviation from the linearity is controversely interpreted on the one hand by the limited chain extensibilityand on the other hand by strain-induced crystallization 

In this context rheo-optical FTIR spectroscopy has proved a valuable technique to study the phenomenon of strain-induced crystallization on-line to the deformation process of the elastomer under investigation. Whith the aid of an appropriate absorption band which is characteristic of the threedimensional order in the crystalline phase the onset and progress of strain-induced crystallization during elongation and its disappearance upon recovery can be unambigously monitored simultaneously to the mechanical measurements. Representative for several rubber-like materials which have been investigated by this technique in our laboratory the results obtained with sulfur-crosslinked (1.8 % S) natural rubber (100% 1,4-ds-polyisoprene) and a radiation-crosslinked synthetic polyisoprene (93% 1,4-ds-isomer) lall be discussed in some detail here. 

Stress-strain diagrams of various crosslinked rubbers (a) sulfur-crosslinked natural rubber (100% 1,4-cis-polyisoprene) at 300 K, (b) sulfur-crosslinked natural rubber at 343 K, (c) radiation-crosslinked synthetic polyisoprene (93% l,4-cis-isomer) at 300 K (see text) 

FIGURE 8. Dynamic mechanical properties of NR (Hartex 103) and milled smoked NR with different Mooney viscosities. 

FIGURE 9. PSA performance of Piccolyte A85/natural rubber aqueous vs. solvent systems. 

The conventional method for characterizing natural rubber, Mooney viscosity, is not sensitive enough to be used to obtain the necessary information for natural rubber-based pressure-sensitive adhesive characterization. The RDS generates better information than the Mooney viscometer. 

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Emulsion polymerisation of a mixture of butadiene and styrene gives a synthetic rubber (Buna S GBS rubber), which is used either alone or blended with natural rubber for automobile tyres and a variety of other articles. 

Sulfur is a component of black gunpowder, and is used in the vulcanization of natural rubber and a fungicide. It is also used extensively in making phosphatic fertilizers. A tremendous tonnage is used to produce sulfuric acid, the most important manufactured chemical. 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

A detailed discussion of the history structure and applications of natural rubber appears in the May 1990 issue of the Journal of Chemical Education... 

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. 

Natural rubber is a polymer of isoprene in which the configuration around each double bond is cis (orZ) ... 

Figure 1.3 shows several repeat units of cis-l,4-polyisoprene and trans-1,4-polyisoprene. Natural rubber is the cis isomer of 1,4-polyisoprene, and gutta-percha is the trans isomer. 

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

Polymers of chloroprene (structure [XII]) are called neoprene and copolymers of butadiene and styrene are called SBR, an acronym for styrene-butadiene rubber. Both are used for many of the same applications as natural rubber. Chloroprene displays the same assortment of possible isomers as isoprene the extra combinations afforded by copolymer composition and structure in SBR offsets the fact that structures [XIIll and [XIV] are identical for butadiene. 

Although the conditions of the polymerization reaction may be chosen to optimize the formation of one specific isomer, it is typical in these systems to have at least some contribution of all possible isomers in the polymeric product, except in the case of polymers of biological origin, like natural rubber and gutta-percha. 

One of our previous complaints was that we had more parameters than we knew what to do with Eq. (2.33) makes this problem even worse. It turns out, however, that using only two or three terms of Eq. (2.33) results in a usable equation with improved curve-fitting ability. Techniques have been developed for extracting acceptable parameters from experimental data in these cases (see Problem 4). Figure 2.9, for example, shows data collected from a sample of natural rubber, analyzed according to a two-term version of Eq. (2.33). The line in Fig. 2.9 is drawn according to the equation... 

Figure 2.9 F /A versus shear rate for natural rubber. The line is drawn according to a two-term version of the Eyring theory. (Redrawn from Ref. 5.)...

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

Figure 3.3 Comparison of experiment (points) and theory [Eq. (3.39)] for the entropy elasticity of a sample of cross-linked natural rubber. [From L. R. G. Treloar, Trans. Faraday Soc. 40 59 (1944).]...

Guajule An Alternative Source of Natural Rubber, National Academy of Sciences, Washington, D.C., 1977. 

Natural Rubber. To obtain natural mbber (NR), the Hevea hrasiliensis tree is tapped for its sap. The off-white sap is collected and coagulated. This process produces a high molecular weight substance which is natural mbber. The principal producing countries are Malaysia, Indonesia, Thailand, India, China, and Sri Lanka (see Rubber, natural). 

There are several systems that define the quality and uniformity of natural mbber. One system of grading natural mbber is based on form and visual observation of color and cleanliness. This is known as the International Natural Rubber Specification. The principal types and grades are as follows. There are five other types of mbber classified by this system and many other grades not Hsted here. 

Fig. 11. Aging properties of cured natural rubber for 70 hours at 70°C. A is the conventional, B the semi-KV, and C the EV system where U shows tensile...

Table 11. Effects of Nonblack Fillers in Natural Rubber ...

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