2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Rubber, 2. Natural
HEINZ-HERMANNGREVE,Bayer AG, Leverkusen, Federal Republic of Germany
1. Introduction. . . 583
2. Rubber Extraction. . . 585
3. Composition of Natural Rubber Latex . . 585
4. Biosynthesis of Natural Rubber. . . 585
5. Commercial Extraction of Natural Latex 586 6. Production of Natural Rubber. . . 587
6.1. Extraction by Evaporation of Water (Evaporation, Spray Drying). . . 587
6.2. Coagulation. . . 587
7. Classification. . . 588
7.1. Technically Classified (TC) Rubber . . . . 588
7.2. Standard Malaysian Rubber (SMR). . . . 588
7.3. Standardized Indonesian Rubber (SIR). . 589
8. Physical and Technological Properties of Solid Rubber. . . 589
9. Uses. . . 590
10. Modification. . . 590
10.1. Hydrogenated Natural Rubber. . . 590
10.2. Chlorinated Natural Rubber . . . 590
10.3. Hydrohalogenated Natural Rubber . . . . 591
10.4. Cyclized Natural Rubber . . . 591
10.5. Resin-Modified Natural Rubber . . . 591
10.6. Poly(Methyl Methacrylate)-Grafted Natural Rubber . . . 592
10.7. Superior-Processing Natural Rubber . . . 592
10.8. N-Phenylcarbamoylazoformate-Modified Natural Rubber . . . 592
10.9. Polystyrene-Grafted Natural Rubber . . . 592
10.10. Epoxidized Natural Rubber (ENR). . . 592
10.11. Degraded Natural Rubber . . . 593
10.12. Thermoplastic Natural Rubber. . . 593
11. Compounding . . . 593
12. Summary . . . 593
References. . . 594
1. Introduction
Many plants contain a milky juice – an aqueous emulsion or dispersion of oils, fats, waxes, resins, starch, and proteins. More than 1000 plant spe- cies contain aqueous dispersions of rubber – or, more precisely, of 1,4-polyisoprene with a high content ofcisdouble bonds.
Rubber occurs only in seed-bearing plants (spermatophytes),which are also dicotyledonous angiosperms. Within one genus the phylogeneti- cally younger forms always produce more rubber than the older ones. Thus, rubber formation appears to be the result of more recent evolution.
With regard to economic importance, rubber plants can be divided into three groups:
1. Plants from which rubber is currently extracted:
Hevea brasiliensis, which provides more than 99 % of the rubber from natural sources.
2. Plants that were used in the past or are being investigated for future use: about 50 species, in particular, Ficus elastica (rubber plant), Funtumia castilla, kok-saghyz (Taraxacum), guayule (Parthenium argentatum), cassava, and tapioca bush. The guayule shrub is in a special situation. Its economic exploitation is constantly being reinvestigated in the United States and Mexico.
3. Rubber plants whose exploitation has not yet been investigated: ca. 1000 species.
All trees and bushes have a layered structure.
From inside out are the heartwood, the sapwood, the cambium, the phloem, and the bark. In the cambium, which consists of only one layer of cells, new cells are continuously formed during the growth period. Those to the inside become wood and those to the outside become phloem.
Latex is produced between the cambium and the
DOI: 10.1002/14356007.a23_225
phloem. Latex can be formed in three types of location: milk vessels, milk tubes, or milk cells.
Milk Vessels consist of an associated system of tubes between the bark and the cambium, to which new cells are being joined continuously.
However, the whole plant does not have a single milk vessel system throughout, but rather many individual ones. Each system extends from the roots via the trunk to the twigs but is not connected to the other systems. Examples of plants with milk vessel systems areHeveaand cassava.
Milk Tubes consist of a branched tube sys- tem permeating the entire plant. They are found, for example, in poinsettias or crown-of-thorns (Euphorbia spendens).
Milk Cells are individual, isolated, elongat- ed cells. Latex exudes only from cut cells, and tapping is not possible. Such cells are present in lettuce, chamomile, and salsify.
Hevea Brasiliensis. The habitat of the rubber tree is the Amazon rain forest. Today, plantations ofHevea brasiliensisare also found in Southeast Asia, principally in Thailand, Indonesia, and Malaysia. When growing in the wild these trees live for more than 100 years and grow to more than 40 m high; on plantations their height is kept to less than 20 m. Hevea brasiliensis requires temperatures of 25 – 30 C, at least 2000 mm of rainfall per year, and high atmospheric humidity.
For this reason it grows only between 15North and 15 South.
More than 99 % of natural rubber currently comes from Hevea brasiliensis. After an initial growth period of 5 – 7 years, a tree gives latex for ca. 25 – 30 years. Hevea brasiliensis is very suitable for rubber extraction for the following reasons:
1. It can be tapped because of its milk vessels.
2. It does not form resins on tapping.
3. Polyisoprene with almost 100 % cis double bonds is produced.
4. It tolerates tapping over a period of many years very well.
In the second half of the 19th century the conditions for a rapid expansion of the rubber industry were created by the discovery of vulca- nization. The development was impeded only by the difficulties in extracting natural rubber in sufficient quantities. The only source was Brazil, where wild rubber was extracted by inadequate and dangerous methods. In tapping expeditions to the often widely scattered trees, deep cuts were made in the trees with axes, which destroyed the milk tube systems. This careless harvesting re- sulted in rapid exhaustion of the sources, so the tappers had to penetrate ever further into the jungle, with the increasing danger of disease and attacks by natives. For these reasons the price of rubber was higher than the price of silver for a time.
On the initiative of Sir CLEMENTS MARKHEM
the India Office and the London Bontanical Garden decided to cultivateHevea brasiliensis. They commissioned HENRYWICKAMto procure the seeds of the plant. In 1874 he collected 70 000 seeds and shipped them to England for a fee of £70. Contrary to all claims, the export of Hevea seeds was legal. However, a problem arose because Brazilian customs operated very slowly andHeveaseeds were capable of germi- nating only for a relatively short period and therefore had to be brought quickly to Kew Gardens near London. WICKAMsolved the prob- lem with the aid of the British consul in Para and several £1 notes. Of the 70 000 seeds, about 2600 germinated and about 1800 grew. Most of them were shipped to India and Ceylon, where they perished because of the unsuitable climate. A few plants (50 in 1876 and 22 in 1877) were taken to Malaysia and Singapore, and of those, about 5 – 7 could later be tapped. They formed the basis for the current world natural rubber production.
Today’sHeveaplantations are predominantly in Southeast Asia: Malaysia (24 % of world pro- duction), Indonesia (24 %), and Thailand (25 %);
some can also be found in Africa (6 %). In Brazil, rubber trees could not be planted for a long time because of a widespread fungal diseaseDothidel- la ulei. In Africa and Asia this disease has thus far not appeared because the climate is slightly dif- ferent from that in Brazil. An attempt is now being made to solve this problem by cross-breeding with resistantHeveaspecies, so thatHevea bra- siliensiscan again be planted in Brazil.
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2. Rubber Extraction
Latex is extracted from Hevea brasiliensis by tapping, which involves making a downward- slanting cut in the bark of the tree with a knife around half the circumference of the trunk at an angle of 30 – 35. The inner cambium must not be damaged in the process because, otherwise, irregular reformation of the bark would lead to difficulties with the next tapping cut. In 2 – 5 h, ca. 50 cm3of latex exudes from the cut (ca. 15 g of rubber). It is collected in a vessel fixed at the lower end of the cut. Later the cut is closed by the drying latex. After two days, this rubber film is removed and tapping is continued.
Although this is the most common tapping method, others also are used (e.g., instead of being cut, the bark is pierced with sharp needles).
Ca. 2 kg of rubber is extracted per tree per year. This corresponds to 1000 kg per hectare per year at 500 trees per hectare. Originally, the annual yield of a plantation was 350 kg per hectare; today, it is 600 – 700 kg per hectare per year, if the entire planted area is considered, including trees that are not ready for tapping, those that are too old, and newly planted areas.
Targeted cultivation has enabled average annual yields of more than 1500 kg per hectare to be achieved on large plantations, particularly in Malaysia. On experimental plantations, yields have even been increased to 2500 kg per hectare.
Ca. 80 % of the production originates from smallholdings. The typical smallholder, with his family, works 1.25 hectares with about 625He- veatrees. With 288 harvesting days, he has to tap 90 000 times per year in a 2-day cycle. He can harvest 625 kg of natural rubber per year from the lower-yield trees (500 kg per hectare). At
$ 0.27 – 0.33/kg, the smallholder earns $ 209 per year. From this he has to feed, on average, 3.2 people, and he is at the minimum subsistence level for a family of three.
In summarizing the problems of natural rub- ber extraction from Hevea (suitable climate;
large area required; abundant, cheap work force needed), it becomes clear that natural rubber plantations are uneconomical in highly devel- oped countries. In the United States and Mexico, attempts are being being made to counteract these problems by extraction of natural rubber from guayule. In guayule the rubber latex occurs
in individual milk cells and therefore cannot be obtained by tapping. In the autumn harvest, shrubs are pulled up by machine and ground;
the rubber is extracted with solvents and separat- ed from resins. The rubber is then processed further in a process that can be automated exten- sively. Guayule grows in arid deserts that could otherwise not be used, so the large area required does not play an inhibitory role.
3. Composition of Natural Rubber Latex
Natural rubber latex is essentially a dispersion of cis-1,4-polyisoprene in water. The average par- ticle size is between 0.15 and 3.0 mm. The parti- cle-size distribution is very broad. The aqueous dispersion contains between 30 and 38 % solid material, depending on the time of the year and the age of the tree. Other components of the latex are 1 – 2 % proteins and phosphoproteins, 2 % resins, 1 % fatty acids, 1 % carbohydrates, and ca.
0.5 % inorganic salts. The rubber particles are surrounded by protein anions and are thus effec- tively negatively charged, which hinders coagu- lation of the latex. These proteins are decom- posed rapidly by bacteria and enzymes when exposed to air, and the rubber then partially coagulates. In addition, in the presence of air, cross-linking of the rubber occurs within the latex particles, with gel formation and subse- quent degradation of the polymer chains.
4. Biosynthesis of Natural Rubber [16–18]
In the plant, natural rubber is not formed by polymerization of unsaturated monomers (e.g., isoprene); instead, a polycondensation occurs, starting from a basic unit of five carbon atoms, isopentenyl pyrophosphate.
If the formation of the basic building block is considered in a highly simplified form (see Fig. 1), a molecule with six carbon atoms (b- hydroxy-b-methylglutaric acid) is built up from three acetyl groups. b-Hydroxy-b-methylgluta- ric acid is then reduced to mevalonic acid. The latter eliminates CO2and water to give isopen-
tenyl alcohol, or more precisely its ester with pyrophosphoric acid. In the second stage of polycondensation, isopentenyl pyrophosphate is condensed with one molecule of dimethylallyl pyrophosphate, which is formed from isopente- nyl pyrophosphate by isomerization. In this way, geranyl pyrophosphate is produced, which reacts with another molecule of isopentenyl pyrophos- phate to give farnesyl pyrophosphate. The con- densation is repeated until the final molecular mass of natural rubber is reached. The entire process is controlled enzymatically.
Why the plant forms latex is thus far not clear:
the milky juice or the rubber precipitated from it does not appear to have any protective function in the plant. Two theories are currently being discussed:
1. The rubber constitutes an energy reserve that the plant can degrade when energy is needed.
2. The rubber is a product of excretion, with which the plant can render toxic metabolites harmless.
Both theories have been supported by a num- ber of investigations, but refuted by an equal number. Possibly, no single reason exists for latex formation.
5. Commercial Extraction of Natural Latex [19–24]
When latex was first extracted, the problem was to stabilize it toward enzymatic and oxidative attack by bacteria. Today, an aqueous ammonia- cal solution (ca. 0.7 %) is generally used to counteract degradation. In some cases, ammonia is used in combination with phenol or alkali phenolates.
Natural latex is concentrated to lower the transportation costs. Three processes are essen- tially available for this:
1. Centrifugation 2. Creaming 3. Evaporation
Electrodecantation is also sometimes used.
Centrifugation of the fresh latex allows extensive separation of the lighter disperse rub- ber phase from the aqueous serum. About 80 % of the rubber phase remains in the concentrate, and the solid content is up to 60 wt %. Most of the other latex components and rubber particles with smaller diameters remain in the serum. Thus the concentrated latex has a higher average particle size than the original latex. The process is con- tinuous and therefore very economical, and is the most commonly used method of concentration.
Creaming Method. In the creaming meth- od, 0.25 wt % ammonium alginate and 0.5 wt % ammonium oleate are added to the latex. In the course of several days the latex separates into two layers, of which the upper has a rubber content of ca. 60 %. After separation of the serum and renewed creaming, a concentration of 65 – 68 % solid materials is achieved.
Evaporation. Latices produced by evapora- tion have a very high solid material content (up to 75 wt %) because the very small particles are still present, unlike centrifuged latex. They also differ from the latex obtained by the other processes in that they contain all the water-soluble solids.
Evaporation is the most expensive and least used process.
Uses. When the transportation problem was solved by stabilization, natural latex could be
Figure 1. Simplified scheme showing synthesis of natural rubber.
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used worldwide and became more and more competitive with synthetically produced latex.
The areas of use of natural rubber latex are those in which high toughness, elongation, and resistance to tear propagation in the vulcanizate and the raw material are required. Natural latex is as yet unsurpassed in the sum of these prop- erties. The main areas of use are in diving gear, rubber thread, adhesives, and foam linings for carpets.
6. Production of Natural Rubber
Most of the natural rubber latex is further pro- cessed to solid rubber; in this case, no concen- tration of the latex is necessary. The rubber can be obtained by evaporation, spray drying, or the most important process, coagulation and subse- quent drying.
6.1. Extraction by Evaporation of Water (Evaporation, Spray Drying)
The tappers in the Brazilian jungle extract the rubber from the milky juice. The latex is poured onto an oarlike piece of wood covered with clay and dried by rotating the wood over a smoky fire.Addition of urikuri nuts to the firewood gives the smoke a high phenolic content. The process is stopped when a large lump has been produced.
This highly smoked wild rubber is called hard- cure fine para. It is qualitatively a very high-value natural rubber, because in this processing method the most valuable parts of the latex solidify while those of low value drop off. The preserving effect of the phenol- containing smoke is so good that the rubber lumps can be stored for years without mold formation.
In spray drying the latex is poured onto a rapidly rotating disk atomizer. From this it falls in finely divided form into a drying tower, into which hot air is blown in countercurrent. The dry material is obtained in the form of small flakes that are later pressed together to form blocks. This type of rubber is called sprayed rubber. It contains all the solid components of the natural rubber latex and has very good me- chanical properties. Unlike coagulated rubber it is very tough and difficult to process.
6.2. Coagulation
On a plantation the rubber is separated from the latex by coagulation. Acids, such as formic or acetic acid, or salts, such as sodium silicofluor- ide, are used as precipitating agents. The latex is first diluted to a solid material content of 15 – 20 % and coagulated at pH 5.1 – 4.8 (isoelectric point). The coagulate must be processed further immediately; otherwise, it will be attacked by bacteria. Most of the water-soluble substances in the natural rubber latex remain in the serum.
Two main processes are used tostabilizethe raw rubber: the smoked-sheet and the cr^epe rubber processes.
Smoked Sheets still contain a large propor- tion of other substances present in the latex. In the production of smoked sheets the diluted latex is treated with 1 part of 0.5 % formic acid to 10 parts of latex in long, rectangular vats with constant stirring. After skimming, aluminum plates are placed upright in the vats about 4 cm apart. The next day the coagulated rubber formed between the plates is removed. These soft, completely homogeneous rubber plates are rolled out with a hand roller and processed further on a sheet mangle. The latter consists of five pairs of press rollers arranged in series.
The resulting sheets of 3 – 4-mm thickness are hung in a smokehouse and treated with phenol- containing smoke, produced by burning fresh wood and nutshells, to protect the rubber against oxidation and mold formation. After the smoking process the temperature is raised to 60C and the sheets are dried for two to three days.
The standard grades ofribbed smoked sheets are rated in terms or purity and quality from 1 (very good) to 5 (poor quality). In addition, an unsmoked gradeair-dried sheets, is an odorless, top-quality, light-brown product of high purity, whose vulcanizate has very good mechanical properties. Air-dried sheets are produced by grinding the moist coagulate, extruding it, and after renewed grinding, washing it in washing mills. The rubber is then air-dried and molded into plates several millimeters thick.
Cr^epe Grades. The cr^epe grades are freed extensively from nonrubber components by washing in washing mills. The quantity of coag- ulating agent is calculated so that, after a few
hours, the 15 – 20 % latex yields a soft, cohesive coagulate from which a clear serum gradually separates. To remove the acidic residues and most of the nonrubber components the coagulate is washed under friction in ribbed mills with powerful water scrubbing. The process is repeat- ed with increasingly closely spaced rollers hav- ing increasingly fine ribbing, until thin cr^epe-like sheets are obtained. These are dried either for 10 to 12 days in air or with a vacuum dryer at 70C for 2 h. With the vacuum dryer the product must be cooled very quickly to avoid surface oxida- tion. The cr^epe grades all have a brownish color and are stabilized with 0.5 – 0.75 wt % sodium hydrogensulfide before coagulation. Here, the grades also differ according to color, purity, and strength of the sheet.
The secondary grades play an additional, economically important role. These generally consist of rubber from small plantations, which has been processed in very different ways and from which a cr^epe is produced by mixing and milling them together. This cr^epe is sold as remilled cr^epe.Flat bark cr^epeis produced from the dried rubber films that form where the tree has been tapped and from waste material.
Special Grades. Two special grades exist:
initial concentration rubber[25], which is coag- ulated from undiluted latex, and Hevea crumb [26]. The latter is a crumblike rubber that gives a moist coagulate on addition of an incompatible oil, such as 0.7 % castor oil. The coagulate is crumbled by passage through conventional cr^epe rollers. Approximately 0.4 % of the castor oil remains in the rubber, but it does not affect the properties significantly. Because of the large surface area of the crumb the rubber can be purified readily by washing with water and is dried in a deep-bed dryer with air circulation at 80 – 100C in 3 to 4 h. The Hevea crumb is molded into compact lumps and classified ac- cording to the standard malaysian rubber scheme (see Section 7.2).
Superior-Processing Rubber. Another form of natural rubber is superior-processing rubber (SP rubber, see Section 10.7) [27–34]. It is produced by coagulation of a mixture of nor- mal and vulcanized latex. The advantages of SP rubber are better processing properties, particu- larly in injection molding and calendering.
There is also premasticated natural rubber, whose molecular mass is degraded significantly through the addition of chemicals, which is very easy to process.
7. Classification
Even with strict working instructions for coagu- lation and processing, the standard grades, such as cr^epe and smoked-sheet rubber, produced from plantations differ in their processibility and rate of vulcanization. Constant quality control is therefore necessary, and guidelines for the clas- sification of natural rubber were worked out at the suggestion of the French Rubber Institute in Indochina in 1949.
Natural rubber that has been classified accord- ing to these guidelines is known as technically classified (TC) rubber. This classification was further improved by Malaysia, and the so- called standard malaysian rubber (SMR) scheme was introduced.
7.1. Technically Classified (TC) Rubber
[35]Natural rubber is classified according to its Mooney viscosity and the tensile modulus of a standard vulcanizate. A blue circle on a lump means, for example, that the natural rubber shows average plasticity during processing and has a high vulcanization rate.
Since 1953 the Mooney viscosity has no longer been quoted by some plantations, because it gives no indication of the processing behavior of the natural rubber. However, the tensile strength of test vulcanizates has proved to be important for calibration of the vulcameter (!
Rubber, 11. Testing).
7.2. Standard Malaysian Rubber (SMR)
[36–42]The quality features of SMR are laid down in a technical specification. They are
1. Dirt content 2. Ash content
588 Rubber, 2. Natural Vol. 31
3. Copper, manganese, and nitrogen content 4. Content of volatile components, and 5. Plasticity retention index (PRI)
In determination of thePRI, rubber is heated to 140C for 30 min and the plasticity is mea- sured before and after. The ratio of the former to the latter number multiplied by 100 is the PRI.
The PRI characterizes the degradation behavior of natural rubber. It gives an indication of the expected aging properties and of the mixing viscosity, which is in turn related to tensile strength, dynamic properties such as rebound resilience, and hysteresis. At the end of 1970 the so- called MOD value was introduced, which is also a measure of vulcanization behavior.
There are also constant-viscosity (CV) and low-viscosity (LV) grades.Constant viscosityis achieved by saturating the aldehyde groups, which are formed in the oxidation of natural rubber, with hydroxylamine, thus hindering hardening. Naphthenic oil is added to the low- viscositygrades to decrease the viscosity.
Another grade isdynat rubber.This is an SMR grade processed by air drying.
Since its introduction in 1965, much success has been achieved in the standardization of nat- ural rubber by use of the classification scheme.
The proportion of natural rubber classified ac- cording to the Malaysian scheme was more than 40 % in 1990.
7.3. Standardized Indonesian Rubber (SIR)
The SIR classification is not widely used and corresponds generally to the SMR classification.
8. Physical and Technological Properties of Solid Rubber
At 20C the density of natural rubber is 0.906 – 0.916 g/cm3, the specific heat is 1.905 kJ kg1 K1, and the refractive index ca. 1.5191. The heat of combustion is45.2 kJ/kg. The UV absorp- tion of thin rubber films begins at 310 nm; com- plete absorption occurs below 225 nm.
Electrical properties are affected mainly by water-soluble nonrubber components. The elec- trical conductance is 21015to 1 – 1013S/m.
Natural rubber is generally processed after mastication, which gives it excellent processing properties. These include rapid sheet formation in open roll mills, high toughness of the un- vulcanized mixture (green strength), and high building tack. The tack and green strength are very important properties for the production of composites, in which various layers must be welded together as in tires or pneumatic springs. Natural rubber also exhibits excellent behavior on extrusion and calendering. Another positive feature is the high vulcanization rate.
Oncooling and subsequent temperingat ca. 10 to35C, natural rubber becomes opaque and inelastic. This effect is caused by partial crystallization.
Onelongationto more than 100 % the forced alignment of the chains also induces crystalli- zation (elongation crystallization), which can be detected by the appearance of diffraction patterns in X-ray diagrams. The elongation crystallization leads to a strengthening effect and a higher tensile strength in the direction of elongation (anisotropy of mechanical proper- ties). The result is a high green strength of unvulcanized and vulcanized mixtures. Crys- tallites formed at greater elongation do not melt when the load is removed. Thus they hinder, for example, propagation of an edge tear. In an intermittent stress –strain experiment the elongation crystallization is responsible for the fact that the loading and unloading curves show sharp differences (i.e., there is high hysteresis).
At low deformation (<10 %), carbon black filled natural rubber exhibits a small loss factor (tand) because of the homogeneous carbon black distribution and the uniform network, so that hysteresis losses are low. This means that on small deformations the vulcanizates undergo low heat buildup.
Raw rubber dissolves (or at least swells very strongly) in many organic liquids such as ben- zene, petroleum ether, crude petroleum, and carbon tetrachloride. In contrast, vulcanized nat- ural rubber can only swell because the chemical cross-linking prevents dissolution.
Compared with synthetic rubber, natural rub- ber has the following advantages: high structural stability with high elasticity, very good cold flexibility, excellent dynamic properties, and acceptable abrasive wear. Because of this
combination of properties, it is superior to syn- thetic rubber in many applications. Its disadvan- tages include low oil resistance and unsatisfac- tory aging properties. In many cases, however, the latter can be improved significantly by the addition of stabilizers.
9. Uses
The main area of use of natural rubber is tires, particularly for trucks, where its property pro- file makes it much more suitable than synthetic rubber. Because of the low heat conductance, heat buildup is important in large tires. When the elasticity of rubber is too low and the hysteresis too high, too much heat is evolved and heat buildup occurs. This leads to inner burning, which causes destruction of the tire.
Because of its high elasticity associated with low hysteresis, natural rubber is also used in spring elements and buffers.
10. Modification
Since 1801, natural rubber (NR) has been modi- fied in many different ways, and modified forms have been available commercially since 1915.
The degree of modification can vary from a few percent to complete modification of the polymer chain.
Modification highly affects physical proper- ties. Even thermoplastic or resinous material can be obtained by modification of rubber.
The most well known types of modification are hydrogenation,chlorination,hydrohalogena- tion, cyclization, resin modification, methyl methacrylate grafting, superior-processing N- phenylcarbonylazoformate modification (see Section 10.8), polystyrene grafting, and epoxidation.
In addition, degraded natural rubber (liquid NR) and thermoplastic natural rubber (TP-NR) exist. Liquid NR is a chemically degraded natural rubber and a true modified rubber, whereas TP – NR is a physical blend of rubber with polypro- pylene. It is a new form of a thermoplastically processible natural rubber and not a real modification.
10.1. Hydrogenated Natural Rubber
[43–48]Hydrogenated natural rubber is of purely aca- demic interest. It was first produced by PUMMERER
and BURKARDin 1922 and HARRIESin 1923, using platinum as hydrogenation catalyst. Further syn- thetic routes were found by STAUDINGERand cow- orkers in 1930. Hydrogenated natural rubber is a colorless, transparent, elastic to waxy solid.
Elastic hydrogenated natural rubber could, in principle, be used in the cable industry because of its good insulating capacity, as well as a raw material for adhesives. However, the relatively high cost of hydrogenation has thus far prevented any commercial applications.
10.2. Chlorinated Natural Rubber
[49–57]Chlorinated natural rubber was the first modified natural rubber for which industrial applications were developed. Chlorination can be carried out in solution, in the latex, or on solid rubber. The chlorination mechanism was first investigated by KRAUSand REYNOLDSand later by BLOOMFIELD.
The glass transition temperature of chlorinat- ed natural rubber increases with increasing chlo- rine content. Up to 35 wt % chlorine the material is still a rubber. Chlorinated natural rubber is more thermally stable and has a clearly higher oil resistance than pure natural rubber. Products with thetrade namesAlloprene (ICI), Parlon (Hercu- les), and Pergut (Bayer) with a chlorine content of ca. 65 % are available commercially. These products are crumbly materials that no longer possess rubbery properties. They are light, cream- colored, thermoplastic powders, which are nonflammable and highly resistant to chemi- cal attack. They are used in paints and coatings that are resistant to chemicals and protect against serious environmental damage. Chlorinated rub- ber is dissolved in a solvent and mixed with plasticizers and pigments. These types of paint contain 10 – 12 % chlorinated rubber and can be painted or sprayed on. They are used to protect wood, steel, and cement, and for underfloor protection in car bodies and other vehicles.
Chlorinated rubber is also used in adhesives and textile coatings. However, in many areas it has been replaced by polychloroprene.
590 Rubber, 2. Natural Vol. 31
10.3. Hydrohalogenated Natural Rubber
[58–63]Hydrohalogenation can be carried out with hy- drogen chloride, bromide, iodide, or fluoride.
Only hydrogen chloride is used industrially. The hydrobrominated product is very unstable, the hydrogen iodide addition has thus far aroused little interest, and HF is highly toxic.
As early as 1900, WEBERfound that natural rubber reacted with hydrogen chloride in chloro- form. He obtained a very hard, white product with the molecular formula (C5H9Cl)x. Hydro- chlorination was further investigated in 1942 by BUNN and GARNER. They discovered that the addition of hydrogen chloride to polyisoprene gives an anti-Markovnikov product. Products with varying degrees of chlorination were syn- thesized by many others. The hardness of the products increases with increasing chlorine con- tent. Completely hydrohalogenated rubber is a highly crystalline material, which assumes high- ly ordered structures when elongated and has a mpof>115C. It is very inert chemically and completely incompatible with natural rubber.
The largest area of use in the past was in a transparent film with the trade name Plio-Film, which was used as a packaging material, partic- ularly for foods. Hydrohalogenated natural rub- ber was used as a binding agent between rubber and metals. In 1950, ca. 3500 t was produced.
Hydrohalogenated natural rubber is, however, no longer available commercially.
10.4. Cyclized Natural Rubber
[63–71]Cyclized natural rubber is probably the oldest known modification of natural rubber. In 1791, LEONHARDIdiscovered that rubber becomes hard and brittle when treated with sulfuric acid. Simi- lar observations were made at the beginning of the 1920s, but only in 1927 did FISCHERinvesti- gate thoroughly the reaction of natural rubber with organic sulfonic acids, sulfonyl chloride, and sulfates. At the same time BRUSON, SEBRELL, and CALVERTfound that cyclized rubber is also obtained from reaction of natural rubber with zinc chloride. The mechanism of this internal cyclization is not completely understood. A sug- gestion based on the results of M. GORDONwas worked out in 1956 by BLOOMFIELDand STOKES.
Two types of cyclized rubber were formerly produced commercially:
Products Obtained by Cyclization with Sulfuric Acid or its Derivatives. These cy- clized products were subdivided into two further classes, depending on the degree of modification.
1. Brown-black, resinous substances similar to balata materials. They were sold by Goodrich under the names Thermoprene and Fenolac, and were used as adhesives, binding agents, and reinforcing resins.
2. Light, transparent, low molecular mass resins.
They were soluble in many solvents and were compatible with many other resins, oils, and plasticizers. The commercial products were Plastoprene from Plastanol and Alpex 450 J from the Societa Italiana Resine. They were used for printing inks and surface varnishes, in particularly for products requiring resistance to chemicals.
Products Obtained by Cyclization with Chlorostannic Acid. The second group of pro- ducts included thermoplastic, fusible materials, which were sold by Goodyear under the name Plioform. Other commercial products were mas- terbatches, which were sold by Durham Raw Materials under the name Cyklite and by Hubron Rubber under the name Cyklatex. They were used for shoe soles and nonblack heavy-duty industrial rollers.
10.5. Resin-Modified Natural Rubber
[72–74]Resin-modified natural rubber is not a modified rubber in the classical sense. It is formed by reaction of natural rubber with phenol – formal- dehyde resins during vulcanization. Very soft vulcanizates to ebonite-type materials (hard rub- ber) can be produced. The products have a wide range of applications and are available, for ex- ample, under the name Cellobond from BP Che- micals. They have a low compression set, excel- lent dynamic stability,and good aging properties.
Moreover, with reinforcing carbon black (!
Carbon, 6. Carbon Black, Chap. 7.) (e.g., N 330) the Mooney viscosity is not increased, which improves processibility.
10.6. Poly(Methyl Methacrylate)- Grafted Natural Rubber
[75–85]Several monomers can be grafted onto natural rubber (e.g., styrene, vinyl acetate, acrylonitrile, and methyl methacrylate). Peroxides or hydro- peroxides are used as initiators. The most impor- tant graft polymers are those with methyl meth- acrylate that are marketed under the trade name Hevea Plus MG. Their main areas of use are in self-reinforcing vulcanizates and adhesives.
Three qualities of Hevea Plus MG are available commercially: MG 30 with a total content of 30 wt % methyl methacrylate, MG 40 with 40 wt %, and MG 49; the latter consists of 80 % graft polymer, 10 % free methyl methacry- late, and 10 % free polyisoprene. MG 49 is mis- cible with natural rubber and common carbon blacks in almost all proportions and can be vulcanized with normal sulfur cross-linking sys- tems. About 300 – 500 t of all types is produced per year.
One use of these materials is for poly(vinyl chloride) adhesion. For this application a free- flowing powder form of Hevea Plus MG 49 is available, which dissolves rapidly in toluene or methyl ethyl ketone.
10.7. Superior-Processing Natural Rubber
[30–33]Superior-processing natural rubber is a mixture of vulcanized and unvulcanized natural rubber.
The cross-linking occurs in the latex and was investigated in detail by the Rubber Research Institute of Malaysia in 1957. Products are avail- able with 20 – 80 wt % vulcanized phase in the mixture. They are characterized by better pro- cessibility, particularly in injection molding.
After mixing the vulcanized and unvulcanized latices, they are coagulated and dried together in the normal way. The products are SP 20, SP 40, SP 50, PA 57, and PA 80. The PA products, which contain more than 50 % of the vulcanized phase, are available only as masterbatches. They are used as process materials in rubber proces- sing. The masterbatches do not contain any fillers but are stretched with oil. PA 57 is a variant of PA 80 stretched with 40 wt % oil, so that there is 57 % cross-linked rubber, 15 % unmodified rub- ber, and 29 % oil in the final product. Unlike other
process materials, SP rubber does not alter the properties of the final vulcanizate. It leads only to improvements in calendering, extrusion, and in- jection molding. In calendering, these improve- ments include smoother surfaces, less shrinkage, and reduced temperature sensitivity of the mix- ture. Also, in continuous salt vulcanization the addition of PA 80 helps to prevent problems of porosity.
10.8.
N-Phenylcarbamoylazoformate-Modified Natural Rubber
[86–93]Like the hydrogenated analogue, natural rubber modified with ethyl N-phenylcarbamoylazofor- mate (ENPCAF) is of scientific interest only.
Many scientists have investigated this modifica- tion and its mechanism of formation. No com- mercial applications have yet been found.
10.9. Polystyrene-Grafted Natural Rubber
[94–101]Polystyrene-modified natural rubber was devel- oped for the production of a thermoplastic elas- tomer. Thermoplastic elastomers frequently have an ABA block structure, where A is styrene (see
!Thermoplastic Elastomers). The desired structures cannot be obtained by a common grafting process, as used for grafting with methyl methacrylate. A longer route via special poly- styrenes having azodicarboxylate as the terminal group must be used. With simultaneous disinte- gration of the azo groups a polymer of defined structure is formed, but not a pure ABA block.
Since thermoplastic natural rubber is also obtained through blends of natural rubber with polyolefins, the graft products with styrene have not gained any commercial importance.
10.10. Epoxidized Natural Rubber (ENR)
[102–115]The trialkylethylene double bonds of natural rubber readily undergo a reaction with peracids to give epoxides in high yield. This is the most economical method of modifying natural rubber at the latex stage. A mixture of hydrogen perox-
592 Rubber, 2. Natural Vol. 31
ide and formic acid, for example, can be used for the epoxidation. The mechanism was extensively investigated by GREENSPANand GONSOVSKAJAaf- ter the reaction had been discovered by PUM- MERERand BURKARDin 1922. A 100 % epoxidized natural rubber is a thermoplastic with a glass transition temperature of ca. 100 C and can be processed by injection molding.
Products epoxidized to 10 – 50 % are com- mercially available under the names ENR 25 and 50. These products can be cross-linked by using normal vulcanization agents.
The glass transition temperature increases by ca. 1C per mole of epoxidation per mole rubber, so that ENR 50 has aTgnear room temperature.
ENR 50 is a highly damped rubber at room temperature and at higher temperature it has the same resilience as natural rubber itself. The oil resistance is improved significantly with increas- ing degree of epoxidation, compared to natural rubber, whereas the gas permeability decreases.
Because of this the use of epoxidized products as inner liners in tires has been investigated. How- ever, the degree of modification must be 70 % to achieve the same gas permeability as butyl rub- ber, and the products are then no longer rubbery and therefore unsuitable for the purpose envis- aged. Another application that is being tested is the reinforcement of NR with light fillers, for which epoxidized natural rubber is used as bind- ing agent. These vulcanizates have the same mechanical properties as natural rubber vulcani- zates with highly reactive carbon blacks, but they exhibit improved resistance to skidding in the wet and a lower resistance to rolling. Another potential area of use is improvement of the impact resistance of polystyrene. No commercial breakthroughs have occurred in any of these areas, however.
10.11. Degraded Natural Rubber
[116–118]Liquid NR is a dark, viscous material obtained by depolymerization – oxidation of natural rubber with peroxides in a kneader. STEVENSregistered the first patent in 1933. The products are sold under the trade names Lorival R 525 and 200, and Hartmann DPR 35, 40, 75, 400, and 01. Their molecular masses vary between 13 000 and 160 000 g/mol.
These products are used as rubber processing materials and are incorporated into the network, so that they do not lead to any significant deteri- oration of the properties of the vulcanizate.
10.12. Thermoplastic Natural Rubber
[119–126]As already mentioned, thermoplastic natural rub- ber can be obtained either by grafting with poly- styrene or by blending with polyolefins. Products based on blends are available commercially (!
Thermoplastic Elastomers, Chap. 1.).
11. Compounding
Like all rubber, for natural rubber the property profile of the vulcanizate depends considerably on compounding. Hardness can be increased as desired by the addition ofinactive fillers(chalk, kaolin, etc.), with decreases of toughness and elasticity. Withsemiactive carbon blacksa good compromise is achieved among good process- ibility, low compression set, relatively high hard- ness, and good toughness. For very high require- ments concerning toughness and abrasive wear, highly active fillersmust be used (highly active carbon black, light fillers with SI 69). To improve the processibility and lower the cost of compounding,plasticizersare added. Because of the sensitivity toward oxidation,agingandozone protection agentsare also necessary.
Vulcanization is carried out by using sulfur and vulcanization accelerators (e.g., dimorphyl disulfide) or with resin systems, depending on the area of use.
Depending on the use, substances can also be added for flameproofing, increasing the building tack, improving tear resistance, etc.
12. Summary [127]
Natural rubber is the most important rubber in terms of quantity produced because of its excel- lent mechanical properties (and will also remain so, at least in the medium term). The annual consumption is ca. 5106t, which is 35 % of
the total world rubber consumption. The growth between 1970 and 1990 amounted to 65 %, and the growth forecast for the next five years is 1.7 % annually.
Since the retail prices have been relatively low, they should be maintained at this level by the International Rubber Agreement or at least be kept within certain limits.
Countries in which natural rubber is cultivated are attempting to achieve higher yields from their production by increasing the refinement of fur- ther processing of NR, in addition to continuous efforts to improve yields (kilograms per hectare).
How successful this will be is not known. If the wage level in the producer countries increases in the long term, NR will be replaced increasingly by synthetic rubber because of rising prices. Only in applications where, for example, extremely high resistance to tear propagation is required (medical gloves, condoms, etc.) will NR not be replaced, even at a much higher price.
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Further Reading
C. S. L. Baker, W. S. Fulton:Rubber, Natural, ‘‘Kirk Othmer Encyclopedia of Chemical Technology’’, John Wiley &
Sons, Hoboken, NJ, online DOI: 10.1002/
0471238961.1821020202011105.a01.
J. E. Mark, B. Erma:Science and Technology of Rubber, 3rd ed., Academic Press, London 2006.
J.-M. Vergneaud, I.-D. Rosca:Rubber Curing and Properties, CRC Press, Boca Raton, FL 2009.
596 Rubber, 2. Natural Vol. 31
