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2 Supply and demand of natural rubber (NR) in the twenty-first century

2 Supply and demand of natural rubber (NR) in the twenty-first century

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388



Chemistry, Manufacture and Applications of Natural Rubber



Thirdly, the changes of the numbers of cars shown in Table 15.1 suggest

that an upward trend of income results in a larger increase of car numbers (100

times) than that of internet users (50 times). This trend, is clearly observed

and is likely to continue.4 This may be due to the fact that possession of a car

is an important status symbol. However, a recent example may offer another

reason: the political and social changes in Libya and Egypt that began in

2011 were thought to be greatly influenced by social media such as Twitter

and Facebook. But this was only the first step. Many people travelled to the

gathering places, on foot, by bicycles, motor bikes, cars, buses or trains. This

was a more decisive factor than merely recognising information for social

change. It could therefore be said that information itself is neutral and that

physical movement is decisive in modern society. The present author thinks

this to be the era of transportation networks, rather than that of information

technology.

Lastly, an improvement in incomes would most probably cause an increase

of carbon dioxide in the atmosphere. Growth in automobile numbers is

therefore a major contributor to the gases causing global warming. This issue

is also essential when considering rubber consumption for automobile tyres,

which are the main end products of natural rubber. The author believes that

it may be possible to limit the maximum number of aircraft and heavy-duty

automobiles sometime during this century through the amount of available

natural rubber used in the manufacture of tyres.4

It is estimated that around one third of carbon dioxide is absorbed by the

ocean, resulting in an increase in seawater acidity. This acidification may

cause changes to the sustainability of the hydrosphere which occupies 2.5

times the surface area of land. Although this problem is not directly related

to natural rubber, it affects human and marine life. Chapter 11 is relevant

to this issue.

To summarise: the number of automobiles is likely to increase throughout

this century. We are still living in an age of transportation networks.4

Therefore the demand for natural rubber will increase accordingly. Global

warming due to the exhaust from automobiles, and the increasing number

of traffic accidents are significant concerns and their control or regulation

is a challenge to sustainability.4

When discussing natural rubber, the expansion of production must be

considered. Increasing the number of plantations, estates and small holders

involved in Hevea cultivation is essential in some tropical countries. It may

become necessary to extend them in some African countries. However,

natural rubber cultivation competes with the need to produce more food for a

growing population. The scientific and technological rationalisation of rubber

cultivation is an urgent requirement if agriculture is to be improved, particularly

in tropical countries where Hevea trees can be cultivated. The author’s belief

that a limit to automobile numbers might be determined by the availability



Improving the sustainable development of natural rubber



389



of tyres, which in turn is limited by the quantity of natural rubber, has to

be examined with regard to sustainability. New technological innovations

are expected and are briefly described in the following sections.



15.3



Biodiversity



Biodiversity is now a widely used concept in biology. However, it is used

here in accordance with the FAO (Food and Agriculture Organisation) of

the United Nations (http://www.fao.org/biodiversity/en/):

Biodiversity for food and agriculture includes the components of biological

diversity that are essential for feeding human populations and improving

the quality of life.

Such diversity is the result of thousands of years of farmers’ and

breeders’ activities, land and forest utilisation, and fisheries and aquaculture

activities combined with millions of years of natural selection.

Natural rubber has been assumed to be a typical example of insufficient

biodiversity. The origin of most of the natural rubber in the world-wide

markets comes from only one species, Hevea brasiliensis. As noted in the

Introduction, there are more than 2,000 plant species capable of producing

rubber. Table 15.2 is a comprehensive list of rubber yielding trees so far

reported.4–19

In Table 15.2, botanical names without a native region and habitat are

simply mentioned as names in the references. Siphonia may be found in

literatures of the nineteenth or early twentieth centuries.9,20 In Ref. 20,

Hancock described Hevea trees using the name Siphonia.13

Goldenrods,21 which are common weeds (see Introduction), Lactuca sativa

(a shrub) and Helianthus annus (a grass) are found in some literature, but the

authors have failed to list their families. It should be noted that the author

has not been critical in listing names from the literature and that the table

is not exhaustive.

In spite of these extensive studies and a large number of expeditions in

search of better rubber-yielding plants or vegetables, no plants have so far been

reported as superior to Hevea brasiliensis. Quite a number of the studies and

expeditions were officially supported by public organisations, including local

and central governments, and some were internationally supported. Attempts

to collect better wild species in tropical forests, including the Amazon Valley,

were in vain. These historical results4,9,10,13–17,21–23 clearly show that among

a large number of rubber-yielding plants, only Hevea brasiliensis has so far

successfully been domesticated and cultured, affording a supply of natural

rubber to the world market. It is also noted that the domestication of this

species has not been successful in Brazil, which is the original habitat of

wild Hevea.4,5,8–10,13–17,22,23 See Section 15.5 of this chapter.



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Chemistry, Manufacture and Applications of Natural Rubber



Table 15.2 Rubber yielding trees and shrubs so far reported in the literature4–19

Family



Botanical name



Region



Habitat



Common name



Apocynaceae



Clitandra sp.



Tropical Africa



Vine



Black Congo rubber



Funtumia elastica Tropical Africa



Tree



Lagos silk rubber



Landolphia florida West Africa



Vine



L. heudelotii



Africa



Vine



L. kirkii



Africa



Vine



L. madagascariensis



Madagascar



Vine



L. owariensis



West Africa



Vine



Red Congo rubber



L. perrieri

L. thollonii

Asclepiadaceae Calotropis procera Venezuela

Cryptostegia

grandiflora



Madagascar



Vine



Palay rubber



Hancornia

speciosa



Eastern Brazil



Vine



Mangabeira rubber



Madagas cariensis Madagascar



Asteraceae



Shrub



Vine



Urceola elastica



Northwest Bengal Tree



White Assam

rubber



Parthenium

argentatum*



North Mexico



Shrub



Guayule



Scorzonera tausaghyz



Russia



Grass



Taraxacum koksaghyz*



South Russia



Grass



Euphorbiaceae Euphorbia intisy



Madagascar



Tree



E. resinifera



Morocco



Cactoid



Hevea

benthamiana



Amazon, Orinoco Tree



H. brasiliensis



South Amazon



Tree



H. camargoana



Marajo island



Tree



H. camporum



Madeira river



Small

tree



H. guianensis



Wide in Amazon

valley



Tree



H. microphylla



Venezuela



Small

tree



H. nitida



Upper Rio Negro Small

tree



H. pauciflora



North & West

Amazon



Tree



H. rigidifolia



Rio Negro



Tree



H. spruceana



Banks of Amazon Tree



H. paludosa



Iquitos, Peru



Tree



Russian dandelion

rubber



Para rubber



Improving the sustainable development of natural rubber



391



Table 15.2 Continued

Family



Botanical name



Region



Habitat



Common name



Northeast Brazil



Tree



Ceara



Colombia



Tree



Central America



Tree



Manihot

dichotoma

M. glaziovii

M. heptaphylla

Sapium tapuru

S. thompsonii

S. verum

Moraceae



Castilla elastica



Castilloa



C. panamenisis

C. ulei



Upper Amazon



Tree



Ficus elastica



India, Southeast

Asia



Tree



Assam rubber,

Rambong tree,

(Indian) Rubber tree



*Some sources give the family name ‘Compositae’ for these.



With regard to the question of future supply, it remains reasonable to

consider possibilities other than Hevea from the point of view of biodiversity.

Scientists are working on Guayule and other possible sources of natural

rubber (see Section 1 of the Introduction and Chapter 1). Since the end uses

of natural rubber are not restricted to automobile tyres,24,25 the excellent

elasticity of natural rubber4,24–26 may necessitate the development of nonHevea rubbers. Chapter 1 presents some aspects of this topic, and Chapter

2 describes the possibilities of synthetic polymer chemistry.



15.4



Applications of state-of-the-art biotechnology



In the agricultural technology field of natural rubber, the production of new

clones by breeding for higher yield performance has historically played

the most important role in increasing output.12,15–17,19 There have also

been several trials on the in vitro biosynthesis of natural rubber. The tissue

culture of natural rubber is a possible biological means of production (see

Chapters 1 and 2).

The greatest challenge is the application of genome analysis to the sustainable

development of natural rubber.4 The human genome project has already been

completed, and other applications are in progress. Similar genome projects

on Hevea may be in progress, including a potential new design for natural

rubber production.4,16,18,27 Recently, there has been a partial disclosure

of some results of Hevea genome analysis.28 The present author believes

that these Hevea genome projects may soon achieve a breakthrough. More

specific applications, for example, the development of a Hevea species with



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Chemistry, Manufacture and Applications of Natural Rubber



improved resistance to South American leaf blight (SALB) disease4,12,15–17

are urgent objectives for the stabilisation and improvement of natural rubber

production. (The following section on bio-safety is in relation to SALB.)

It is possible that this kind of innovative biotechnology will be slower to

develop than initially expected. The Hevea genome projects will eventually

produce results, and it is assumed that some applications may be in practice

by the end of this century.



15.5



Biosafety



Consideration of bio-safety is of the greatest importance for the sustainable

development of natural rubber production. SALB disease must be contained

within central and southern America and its spread to Asian countries where

Hevea trees are cultivated must be prevented. The destruction caused by

SALB has been well documented through many failures of Hevea cultivation

in the American continents.4,22,23 None of the Hevea plantations maintained

stable production of natural rubber in the Amazon Valley, which is the

original habitant of the wild Hevea tree.4 The experiences at Fordlandia

demonstrated the extreme difficulty of controlling the disease in Hevea

plantations in Brazil.4,23

At present, the best method of preventing the spread of SALB disease has

been quarantine control at sea and airports. The RRIM (Rubber Research

Institute of Malaysia) has published guidance on quarantine and to date,

SALB has been successfully contained within the American continents.4

Future quarantine control of Hevea plants must be strict and managed

internationally in the same manner as measures to counter the smuggling

of mass-destruction technology. At present, control is assumed to be the

responsibility of the rubber-producing countries. If the supply of natural

rubber were to be disrupted, the effect on global transportation systems

would be calamitous and all countries must therefore take a share of the

responsibility.4 It will also be necessary to develop pathology techniques

which will enable control of SALB by the end of this century.



15.6



Conclusion and future trends



This chapter emphasises the practical scientific and social aspects of sustainable

development in relation to natural rubber. Chemical modifications (Chapter

3) and sulphur vulcanisation (Chapter 4) are indispensable for the sustainable

development of natural rubber. The chapters on fillers (Chapters 6, 7, 10

and 11) are concerned with non-carbon or renewable rubbers. The unique

elasticity of natural rubber (Chapter 5) is the basis of sustainable applications

(Chapters 1, 12, 13 and 14) and the simulation technique (Chapter 8) will

become indispensable for sustainability.



Improving the sustainable development of natural rubber



393



The present author advises a re-reading of the Introduction, particularly the

historical summary by R. E. Schultes13 which has scientific and social value

for the future. To conclude this chapter, the author cites a great historian on

natural rubber:

…the rubber of the Amazon natives transformed the whole technology

of the New World, from transportation to contraception. Within their

own narrower ambit, primitive peoples had usually preserved, better than

those who had submitted to civilization, their contact with the central

modes of life: respect for sexuality and for the phases of bodily growth,

communication with their own unconscious resources, welling up in dream

and myth, not least the innate joy of being in a harmonious relationship

to nature. Had New World man shown more understanding of the whole

range of primitive gifts, too often despised and cast aside, he would have

left mankind as a whole both wiser and richer.29

Here, ‘the New World’ refers to the post Industrial Revolution era. This

historical recapitulation was presented over half a century ago. However,

it remains fresh and instructive when considering sustainability for future

generations.

Future sustainable development must be practical if it is to be the rule

of everyday social and personal actions. This is recommended to anyone

who are interested in and/or working on natural rubber. Past achievements

should be studied carefully, and among textbooks on natural rubber, Refs.

15 and 16 are recommended for biology-oriented readers, and Refs. 23, 24

and 25 for those more concerned with chemistry and physics. Ref. 4, a book

on history of natural rubber, is for general readers, while the present book

is more technical in nature.

(Postscript: After completing this chapter, the author found the following

report: Cock, M. J. W., Kenis M. and Wittenberg, R., ‘Biosecurity and

Forests: An introduction’, Food and Agricultural Organization (FAO) of

the United Nations, Forestry Department, Rome (2003). The concept of

biosafety in this chapter is fully compatible with the concept of biosecurity

reviewed in this report).



15.7



References



1. World Commission on Environment and Development: Our Common Future, Oxford

University Press, Oxford (1987).

2. Barbier, E. B.: Natural Resources and Economic Development, Cambridge University

Press, Cambridge (2005).

3. The World of Seven Billion, Supplement to National Geographic, March 2011.

4. Kohjiya, S.: ‘Ten-nen Gomu no Rekisi’, History of Natural Rubber, Kyoto University

Press, (2013), (in Japanese).



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Chemistry, Manufacture and Applications of Natural Rubber



5. Collins, J.: Report of the Caoutchouc of Commerce, printed by the order of Her

Majesty’s Secretary of State for India in Council, London (1872).

6. Ferguson, J.: All about Rubber: All Varieties in all Countries, with harvesting and

preparation, 3rd edn, A. M. & J. Ferguson, Colombo (1899).

7. Wright, H.: Hevea brasiliensis or Para Rubber – Its Botany, Cultivation, Chemistry

and Diseases, 3rd edn, A. M. & J. Ferguson, Colombo (1908).

8. Wickham, H. A.: On the Plantation, Cultivation, and Curing of Para Indian Rubber,

Kegan Paul, Trench, Trubner & Co., London (1908).

9. Spruce, R., ed. and condensed by Wallace, A. R.: Notes of a Botanist on the Amazon

& Andes, Macmillan and Co., London (1908).

10. Barker, P. W.: Rubber: History, Production, and Manufacture, US Department of

Commerce, Washington, DC (1940).

11. Le Bras, J.: Rubber: Fundamentals of its Science and Technology, Chemical

Publishing, New York (1957).

12. Peries, O. S. ed.: A Handbook of Rubber Culture and Processing, Rubber Research

Institute of Ceylon, Agalawatta (1970).

13. Schultes, R. E.: Botanical Review, 36, 197–276 (1970).

14. Schultes, R. E.: Economic Botany, 41, 125–147 (1987).

15. Webster, C. C. and Baulkwill, W. J., eds.: Rubber, Longman Science & Technical,

Harlow (1989).

16. Sethuraj, M. R. and Mathew, N. M., eds.: Natural Rubber: Biology, Cultivation

and Technology, Elsevier, Amsterdam (1992).

17. George, P. J. and Jacob, C. K., eds.: Natural Rubber: Agromanagement and Crop

Processing, Rubber Research Institute of India, Kottayam (2000).

18. Liyanage, K. K.: Bulletin of the Rubber Research Institute of Sri Lanka, 48, 16

(2007).

19. Barlow, C.: The Natural Rubber Industry: Its Development, Technology, and

Economy in Malaysia, Oxford University Press, Petaling Jaya (1978).

20. Hancock, T.: Personal Narrative of the Origin and Progress of the Caoutchouc

or India-Rubber Manufacture in England, Longman, Brown, Green, Longmans, &

Roberts, London (1857).

21. Vanderbilt, B. M.: Thomas Edison, Chemist, American Chemical Society, Washington,

DC (1971).

22. Dean, W.: Brazil and the Struggle for Rubber: A Study in Environmental History,

Cambridge University Press, Cambridge (1987).

23. Grandin, G.: Fordlandia: The Rise and Fall of Henry Ford’s Forgotten Jungle

City, Metropolitan Books, New York (2009).

24. Bateman, L., ed.: The Chemistry and Physics of Rubber-Like Substances, Maclaren

& Sons, London (1963).

25. Roberts, A. D., ed.: Natural Rubber Science and Technology, Oxford University

Press, Oxford (1988).

26. Treloar, L. R. G.: The Physics of Rubber Elasticity, 3rd edn, Clarendon Press,

Oxford (1975).

27. Okumura, A., Hayashi, Y. and Kato N.: Nippon Gomu Kyokaishi, 82, 424 (2009)

(in Japanese).

28. News (no. 114) released from Bridgestone Corp. Ltd., on July 10, 2012. (http://

www.bridgestone.co.jp/corporate/news/2012071002.html).

29. Mumford, L.: The Transformations of Man, Harper & Row, New York (1956).



16



Recycling of natural and synthetic isoprene

rubbers

A. I. I s a y e v, University of Akron, USA

DOI: 10.1533/9780857096913.3.395

Abstract: This chapter provides an up-to-date account of reuse, recycling

and devulcanization of natural and synthetic isoprene rubbers. Landfilling

and waste utilization, grinding and pulverization, high pressure high

temperature sintering, pyrolysis, microwave, ultrasonic, chemical,

mechanochemical and biochemical devulcanization are discussed. Attention

is paid to properties of the products made from recycled and devulcanized

rubbers. Blending of natural and isoprene rubbers with virgin rubbers is

discussed. Structural changes occurring in rubbers during devulcanization are

elucidated. The importance of recycling of rubbers without adding chemicals

for their devulcanization and reuse of devulcanized rubbers in a factory floor

are stressed. Future trends in rubber recycling are discussed.

Key words: natural rubber (NR), isoprene rubber, recycling, reuse,

devulcanization, properties.



16.1



Introduction



Vulcanization or curing, creating a three-dimentional chemical network,

is the reason that gum natural rubber (NR) is useful in the tire industry.

Unfortunately, vulcanization has also created a serious environmental problem

as tremendous amounts of waste rubbers are dumped and stockpiled. Unlike

the thermoplastic polymers which can be easily reprocessed by heating,

the thermoset polymers, such as vulcanized NR cannot be simply reused

once they form the three-dimensional network. Therefore, the development

of technologies for recycling of such materials is important due to the

environmental and economic factors resulting from the increasing amount

of waste rubbers, especially scrap tires.

Among various rubbers, NR is one of the most widely used elastomers.

It supplies about one-third of the world demand for elastomers and is the

standard by which the performance of many synthetic rubbers is judged.1 The

majority of NR usage is in transportation. In particular, 63.9% and 11.5% of

NR is, respectively, utilized in tires and non-tire applications.2 Accordingly,

along with SBR, NR forms a major component of tire rubbers.

Developed almost simultaneously with the discovery of vulcanization,

reclaimed or devulcanized NR has played an important part in the growth

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Chemistry, Manufacture and Applications of Natural Rubber



of the rubber industry.3 Charles Goodyear’s discovery4 of NR vulcanization

in 1839 was not only the real start of the rubber industry, but of the rubber

recycling industry as well.5 Soon after the discovery of vulcanization,

utilization of scrap and waste vulcanized NR was initiated. Gradually, a

number of recycling procedures was developed for converting ground waste

NR vulcanizates to reclaimed or devulcanized NR.

Devulcanization of sulfur-cured NR can be defined as the process of

cleaving, totally or partially, the poly-, di- and mono-sulfidic crosslinks which

are formed during the initial vulcanization.6 The rubber industry faces a major

challenge to find a satisfactory way to deal with the enormous quantity of

rubber goods, including tires which reach the end of useful life each year

and their concomitant stockpiles, all of which pose a major ecological threat

to this planet.

Recycling of vulcanized elastomers has always intrigued the industry and

the scientific community. In the past, it was possible for waste disposal sites

and reclaiming plants to burn the rubbers and tires for energy and recover

the metal or dump them at a landfill.7 However, these traditional methods

are a major source of air and land pollution. Chemical processing8 is a

possible method for reversing the vulcanized network through the use of

chemical agents that attack carbon–sulfur or sulfur–sulfur bonds. However,

it may create a problem with the removal of solvents and additional wastes

that is generated in the form of sludge. Other suggested processes include

mechanical,9 thermo-mechanical10 and cryo-mechanical11 methods, which

only comminute the vulcanizates and do not devulcanize them. Besides, each

method possesses certain disadvantages concerning product quality, time

of treatment and production cost. As a result, there is a continuous search

to find more effective ways of reclaiming waste NR and other rubbers. In

order to carry out successful recycling of vulcanized NR, it is necessary to

preferentially break the carbon–sulfur and sulfur–sulfur crosslinks in the

three-dimensional network.

The first method for the utilization of scrap tire and rubber wastes from

production was a grinding method to make granulates or even powder with

subsequent utilization into new compounds.11 This purely mechanical method

is still used to some extent for recycling of rubber vulcanizates and hard

rubber waste. The addition of ground hard rubber powder to virgin rubbers

can simultaneously solve some difficulties that occur in the reprocessing of

hard rubber.

As early as 1858, Hall found that the ground NR waste could be softened

by prolonged treatment with live steam.12 The cured ground NR was subjected

to steam pressure for 48 hours. However, this finding did not solve the

problem of the processing of waste NR completely, because at that time

the waste mainly consisted of footwear containing considerable amounts of

textiles, which restricted the utilization of the reclaim. The textiles from waste



Recycling of natural and synthetic isoprene rubbers



397



rubber could be eliminated mechanically or chemically. The first chemical

method for the elimination of textile fibers was proposed and patented in

1881 by Mitchell. According to this patent, cellulose present in the waste was

destroyed by dilute sulfuric acid, the residue was then thoroughly washed

and pure rubber was reclaimed with steam. However, the chemical methods

decreased the yield and also modified the character of the reclaimed NR.

Important progress in reclaiming procedures was the proposal of an alkaline

process, patented in 1899 by Marks.12 In this procedure, the softening of

the NR and elimination of the textile and free sulfur were carried out in a

single operation. The procedure was discovered at the time when the main

raw material for reclaiming was tires, containing a considerable amount of

textile material. Good quality of the reclaim obtained in this manner was

also helped by the fact that the tires of various origins at that time contained

almost exclusively NR. The alkaline reclaiming process spread rapidly and

represented the most important method up to 1940.

World War II marked the beginning of several events in the rubber

reclaiming industry. Large amounts of synthetic rubber (SR) waste came for

reclaiming. The introduction of the oil-extended SR in the manufacture of

tires was a significant event in the history of the rubber reclaiming industry.

Also, the so-called neutral procedure started to predominate. In principle,

the technology was the same, but instead of an alkaline medium, which is

less suitable for the degradation of the vulcanizates from SR, a solution of

zinc chloride or calcium chloride was used as the medium. The method was

called neutral because, in contrast to earlier procedures, neither acids nor

alkalis were used.11

After the war, the so-called digester processes were complemented by

an aqueous neutral method in which the softening took place in water as

the medium and the waste must be fiber free. In all digestion methods, the

softening effect of heat was supported by the effect of various reclaiming

oils. During the war various chemical reclaim agents that accelerate and

control the degradation processes began to be used. However, a poor quality

of the reclaimed rubber was a common problem in this process. Also, these

methods created chemical sludges and residues. Some of them were not

only impossible to be properly disposed of, but also were dangerous to both

human life and the environment.

With the advent of radial tires in the 1960s, the importance of reclaimed

rubbers in the tire industry diminished because little reclaimed rubber was

used in radial tires due to its poor flex cracking and abrasion resistance.

After World War II, the consumption of reclaimed rubbers has decreased.

This decrease was also affected by the low price of oil-extended styrenebutadiene rubber (SBR).12 However, since the reclaimed rubber could be

easily used in many products, companies continued to develop reclaiming

processes.



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Chemistry, Manufacture and Applications of Natural Rubber



The importance of the reclaimed rubber does not lie solely in its relatively

low and stable price, but also in its technical advantages. Reclaimed rubbers

are used as compounding ingredients due to their positive influence on

processing behavior and the reduction of compound cost.13 Recycled ground

rubber is being used today in a wide range of products and applications

ranging from rubber compounding to brake linings to asphalt rubber for

roads.14 Numerous small, medium and large rubber companies are trying to

find a way to deal with their scrap and are looking for ways to utilize their

own scrap. In fact, the latter is the best way to avoid production of waste

on a factory floor. Also, the interest in recycling has been increasing due to

the environmental problems created by discarded tires and waste rubbers.

Increasing legislation restricting the disposal of used tires and waste rubber

has demanded the search for economical and environmentally sound methods

of recycling.

Many methods to recycle waste rubbers were developed during the past

several decades.15,16 These methods can be generally divided into two

categories. The first is at the physical level. It involves grinding the material

mechanically into smaller pieces with little breakage of the chemical bonds. The

end result is the size reduction to various levels of fineness. Another category of

methods attempts to break the three-dimensional network mechanochemically

with the aid of various forms of energy. These include mechanical and

thermal treatment, chemical and biological treatment, microwave and

ultrasonic waves. Such treatments convert the three-dimensional, insoluble

and infusible thermoset into a soft, tacky, reprocessable and revulcanizable

elastomer simulating the properties of the virgin rubber. Recovery and

recycling of vulcanized NR from the used products and production waste will

save precious petroleum resources as well as solve the waste rubber disposal

problem. Accordingly, the present chapter describes the state-of-the-art in

recycling NR vulcanizates. This includes reclaiming, landfilling, mechanical

grinding, pulverization, mechanochemical, burning and pyrolytic, chemical,

microwave, biotechnological and ultrasonic techniques for recycling of NR

rubber. It also describes some efforts in recycling of the synthetic isoprene

rubber (IR) using various methods since IR is similar to NR in its chemical

structure.



16.2



Approaches to the reuse and recycling of

natural rubber (NR)



Before World War II, only NR was used and reclaiming was simple. Various

plasticizers were used to evenly penetrate NR, leading to uniform bond

cleavage. The reclaimed products were soft and consistent. However, these

plasticizers could not penetrate SRs as efficiently as NR. In addition, SRs

are hardened under pressure and heat.17



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