2. recent advances in the recycling of rubber waste

T Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 47-100 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

2. Recent advances in the recycling of rubber waste

Eldho Abraham1,3, Bibin M Cherian2, Elbi P A1,

Laly A Pothen3 and Sabu Thomas4 1Post Graduate Department of Chemistry, CMS College, Kottayam, Kerala, India

2Department of Natural Science, College of Agricultural Sciences, São Paulo State University (UNESP), Botucatu 18610-307, São Paulo, Brazil; 3Post Graduate Department of Chemistry

Bishop Moore College, Mavelikara 690110, Kerala, India; 4School of Chemical Sciences Mahatma Gandhi University, Kottayam, Kerala, India

Abstract. An overview of reclamation of cured rubber with special emphasis on waste latex reclamation is presented in this chapter. Recycling of rubber waste poses a challenging environmental, economical, and social problem. The latex industry has expanded over the years to meet the world demands for gloves, condoms, latex thread, etc. The waste rubber formed in latex-based industries is around 10–15% of the rubber consumed. The formation of a higher percentage of waste latex rubber (WLR) in latex factories is due to the unstable nature of the latex compound and the strict specifications in the quality of latex products. As waste latex rubber (WLR) represents a source of high-quality rubber hydrocarbon, it is a potential candidate for generating reclaimed rubber of superior quality. The role of the different components in the reclamation recipe is explained and the reaction mechanism and chemistry during reclamation are discussed in detail. Different types of reclaiming processes are described with special reference to

Correspondence/Reprint request: Dr. Eldho Abraham, Post Graduate Department of Chemistry, CMS College Kottayam, Kerala, India. E-mail: [email protected]

Eldho Abraham et al. 48

processes, which selectively cleave the cross links in the vulcanized rubber. The state-of-the-art techniques of reclamation with special attention on latex treatment are reviewed. An overview of the latest development concerning the fundamental studies in the field of rubber recycling by means of low-molecular weight compounds is described. A mathematical model description of main-chain and crosslink scission during devulcanization of a rubber vulcanizate is also given. 1. Introduction The white sap of the South American tree Hevea brasiliensis forms the basis of a large and global industry, the rubber industry. The South American Indians were familiar with the material and used it for various purposes already when the first Europeans came in the late 15th century. The Indians called the tree Ca-hu-chu (kautschuk!) or ‘the crying tree’. Long before Colombus arrived in the Americas, the native South Americans were using rubber to produce a number of water-resistant products. The Spaniards tried in vain to copy these products (shoes, coats and capes), and it was not until the 18th century that European scientists and manufacturers began to use rubber successfully on a commercial basis. The British inventor and chemist Charles Macintosh, in 1823, established a plant in Glasgow for the manufacture of waterproof cloth and the rainproof garments with which his name has become synonymous. A major breakthrough came in the mid 19th century with the development of the process of vulcanisation. In 1839 Charles Goodyear discovered the process of sulfur vulcanization by combining masticated rubber chemically with sulphur, an irreversible process, that he called vulcanization. This process gives increased strength, elasticity, and resistance to changes in temperature. It also renders rubber impermeable to gases and resistant to heat, electricity, chemical action and abrasion. Vulcanised rubber also exhibits frictional properties highly desired for pneumatic tyre application. Crude latex rubber has few uses. The major uses for vulcanised rubber are for vehicle tyres and conveyor belts, shock absorbers and anti-vibration mountings, pipes and hoses. It also serves some other specialist applications such as in pump housings and pipes for handling of abrasive sludges, power transmission belting, diving gear, water lubricated bearings, etc. Thus the discovery of vulcanization in 1850 meant that a whole range of rubber products became available. Virtually all of the stages of manufacturing, including masticating, compounding, milling, vulcanizing and finishing, are the same today as they were in 1900, although the machinery has been continuously refined and improved. The greatest change in the rubber industry occurred in the early 20th century with the development of the first synthetic rubber called Buna

Recent advances in the recycling of rubber waste 49

and later synthetic styrene butadine rubber (SBR). However, the economics of rubber production have undergone considerable change. This industry has always been at the mercy of rapid and drastic changes, both in the cost of raw rubber and the prices of finished goods. At present, natural rubber is commercially produced in Indonesia, Thailand, Malaysia, Brazil, India, Sri Lanka and in some African countries like Nigeria and Ivory Coast. 2. Use and reuse of rubber products 2.1. Rubber in everyday life Not only in industrialized countries but also in less developed nations, rubber products are everywhere to be found, though few people recognize rubber in all of its applications. Since 1920, demand for rubber manufacturing has been largely dependent on the automobile industry, the biggest consumer of rubber products. Rubber is used in radio and T.V sets and in telephones. Electric wires are made safe by rubber insulation. Rubber forms a part of many mechanical devices in the kitchen. It helps to exclude draughts and to insulate against noise. Sofas and chairs may be upholstered with foam rubber cushions, and beds may have natural rubber pillows and mattresses. Clothing and footwear may contain rubber: e.g. elasticized threads in undergarments or shoe soles. Most sports equipment, virtually all balls, and many mechanical toys contain rubber in some or all of their parts. Still other applications have been developed due to special properties of certain types of synthetic rubber, and there are now more than 100,000 types of articles in which rubber is used as a raw material. 2.2. Waste rubber as landfills The use of rubber in so many applications results in a growing volume of rubber waste. With the increase in demands, the manufacturing and use of rubber and the rubber products has increased tremendously both in the developed and less developed countries. About 242 million tyres are discarded every year in the United States alone. Less than 7 percent are recycled. 11 percent are incinerated for their fuel value and another 5 percent are exported. The remaining 78 percent are either landfilled, or are illegally dumped. According to a recent report of the US Environmental Protection Agency (U.S EPA), this has resulted in a national stockpile of over 2 billion waste tyres. But with the decreasing scope of available sites and due to the corresponding cost explosion [1] this process of waste rubber disposal is no


longer feasible. Land filling with waste tire is, also the most unwanted due to environmental problems and has no future possibility. The current rubber waste situation of developed countries is presented in Table I. At the end of 1950s, only about one fifth of the rubber hydrocarbon used by the United States and Europe was reclaimed. By the middle of 1980s less than 1% of the worldwide polymer consumption was in the form of reclaim. At the beginning of 20th century half of the rubber consumed was in the form of reclaim. It is expected that in 21st century most of the scrap rubber will be recycled in the form of reclaim because of day to day increase in environmental awareness. From Table I it is seen that except USA [2], France and Italy [3] the tendency to use scrap rubber as a landfill is decreased. Some countries have already banned the use of discarded tire for land filling. Tires discarded in landfills tend to float on top causing: mosquito breeding and illegal tire disposal are creating problems which can be minimized by recycling. Different ingredients such as stabilizers, flame retardants, colorants, plasticizers etc. were mixed with rubber during compounding. After discarding the tires for landfilling there is a probability of leaching small molecular weight additives from bulk to the surface and from surface to the environment. These small molecular weight additives are not eco-friendly and may kill advantageous bacteria of soil. In this way landfill causes serious environmental problem. Among various methods the least desirable disposal method is discarding the article (or material). This is a situation where not only no value is added to the waste material, but in fact, the value added is negative because of the implicit cost of: (i) transporting the material to the landfill site; and (ii) establishing and maintaining the landfill to satisfy environmental requirements.

Table I. Rubber waste situation in 2006 on developed nations.

Treatment USA (%)

UK (%)

Germany (%)

France(%)

Italy(%)

Belgium (%)

Netherlands (%)

Japan (%)

Sweden(%)

Land fill 58 23 9 45 40 10 - 12 5 Retreading 19 31 18 20 22 20 60 24 12 Energy 11 27 45 15 23 30 28 39 64 Export 5 3 16 4 2 25 - 6 7 Recycling 7 16 12 16 12 15 12 19 12

2.3. Waste rubber as fuel source Sometimes scrap rubber is used as a fuel. Pyrolysis is one of the thermal approaches to recovering energy and basic materials from waste rubber.


Technically, the term 'pyrolysis' covers all forms of heat decomposition, including combustion, although in practice it is normally understood to mean thermal decomposition in a non-reactive (anaerobic, or in the absence of oxygen) atmosphere. Generally 25-50% of the weight of rubber pyrolysed can be recovered in the form of a distillate of approximately 42 MJ/kg calorific value, which is higher than when burning tyres (37.5 MJ/kg) and even higher than coal (29 MJ/kg). The elemental constituents of a rubber compound are almost equal to those found in coal. Indeed coal provides about 30 MJ/kg and rubber provides about 32.5 MJ/kg of heat energy, which compares with liquid fuels at about 56 MJ/kg [4]. Shredded tire chips have been burnt in boilers, but tire grinding and size reduction problem have set back the use of tire chips in boiler. Transportation of tire scrap can cost $0.05/kg, exclusive of grinding costs. The cost of burning one metric ton of tires per hour in an incinerator was ca. $0.20–0.40 per tire in 1974, which increased to $0.35–$0.70 per tire in 1987, and now it is increased further. The Oxford Energy Company incinerates tires and produces electricity. The facility generates 14.4 mW of electricity and costs $38 × 10. Thus in the incineration process discarded rubber is used as a fuel to generate electricity, steam etc. This process is still in use. But it creates new problem of air pollution and is also a low value recovery process of the waste rubber. An environmentally friendly process [5] was developed for recycling rubber waste materials such as waste tires to generate valuable fuels or chemical feedstocks in a closed oxidation process which is free of hazardous emissions. The process involves breakdown of rubber polymer materials by selective oxidation decoupling of C–C, C–S and S–S bonds by water as a solvent at or near its supercritical condition. Adkins [6] invented a method of processing used tires for the recovery of oil, steel, vinyl chloride and carbon. The process includes adding a shredded automobile tire to a batch of isocyanide, polyurethane, latex and soybean oil. The resultant mixture was then heated at 700oF for 10 min to obtain the products. The addition of soybean oil to the bath mixture provides a safer and more economical process. 3. Reclamation of rubber products One of the various problems which mankind faces as it enters into the 21st century is the problem of waste disposal management. Since polymeric materials do not decompose easily, disposal of waste polymers is a serious environmental problem. Large amounts of rubbers are used as tires for aeroplanes, trucks, cars, two wheelers etc. Reclaimed rubber is the product


resulting when waste vulcanized scrap rubber is treated to produce a plastic material which can be easily processed, compounded and vulcanized with or without the addition of either natural or synthetic rubbers. Regeneration can occur either by breaking the existing cross links in the vulcanized polymer or by promoting scission of the main chain of the polymer or a combination of both processes. Reclaiming of the waste rubber can be a difficult process. There are many reasons, however why waste rubber should be reclaimed or recovered; • Recovered rubber can cost half that of natural or synthetic rubber. • Recovered rubber has some properties that are better than those of

virgin rubber. • Producing rubber from reclaim requires less energy in the total

production process than does virgin material. • It is an excellent way to dispose of unwanted rubber products, which

is often difficult. • It conserves non-renewable petroleum products, which are used to

produce synthetic rubbers. • Recycling activities can generate work in developing countries. • Many useful products are derived from reused tyres and other rubber

products. • If tyres are incinerated to reclaim embodied energy then they can

yield substantial quantities of useful power. In Australia, some cement factories use waste tyres as a fuel source.

Reclaiming of scrap rubber is, therefore, the most desirable approach to solve the disposal problem. Reclaim is produced from vulcanized rubber granules by breaking down the vulcanized structure using heat, chemicals and mechanical techniques. Reclaim has the plasticity of a new unvulcanized rubber compound, however, the molecular weight is reduced so reclaim compounds have poorer physical properties when compared to new rubber. The main reasons for their use are price and improved processing of rubber compounds. The main processing advantages claimed can be summarised as: - shorter mixing times - lower energy consumption - lower heat development - faster processing on extruder and calenders - lower die swell of the unvulcanized compound - faster curing of the compounds


In many rubber products 5-10% reclaim can be added to the new rubber content without serious effects to the physical properties. Far higher percentages (20-40%) are used in products like car mats. Traditionally, however, compounds used in the production of tyre carcasses have been the main outlet for reclaim, due to its processing advantages. In spite of this, the proportion of reclaim in radial tyres is limited to around 2-5%. Reclaiming of scrap rubber products, e.g. used automobile tires and tubes, hoses, conveyor belts etc. is the conversion of a three dimensionally interlinked, insoluble and infusible strong thermoset polymer to a two dimensional, soft, plastic, more tacky, low modulus, processable and vulcanizable essentially thermoplastic product simulating many of the properties of virgin rubber. Recovery and recycle of rubber from used and scrap rubber products can, therefore, save some precious petroleum resources as well as solve scrap/waste rubber disposal problems. Reclaiming processes may be broadly classified into two groups: physical reclaiming processes and chemical reclaiming processes. In a review, Warner [7] has summarized various methods of devulcanization using chemical and physical processes. 3.1. Reclaiming agents, oils and catalysts Many attempts have been made since the beginning of the 20th century for reclaiming of scrap rubber products. As a result, a large number of chemical reclaiming agents for natural and synthetic rubbers, viz. diphenyl disulfide, dibenzyl disulfide, diamyl disulfide [8-9], bis(alkoxy aryl) disulfides [10], butyl mercaptan and thiophenols [11], xylene thiols [12] and other mercaptans [13], phenol sulfides and disulfides [14] have been developed. The following definition was adopted in 1981 by the Rubber Recycling Division of the National Association of Recycling Industries, Inc.: Reclaimed rubber is the product resulting when waste vulcanized scrap rubber is treated to produce a plastic material which can be easily processed, compounded and vulcanized with or without the addition of either natural or synthetic rubbers [15]. Regeneration can occur either by breaking the existing cross links in the vulcanized polymer or by promoting scission of the main chain of the polymer or a combination of both processes. A definition of reclaiming agents, catalysts and reclaiming oils depends on their reaction and function within the process. Yamashita [16] published a review on the different reactions that might occur during reclaiming. The auto-oxidation accelerated degradation reactions are described and these reactions occur particularly in reclaiming processes that involve shearing actions. Without the addition of reclaiming agents, auto-oxidation reactions will progress through hydroperoxides formed by the attack of oxygen, which is always present in the rubber. Utilization of reclaiming


agents speeds up and introduces new pathways for the reclamation reactions. Reclaiming catalysts are chemical compounds, which are effective in only small amounts during reclamation. Reclaiming oil has several roles apart from raising the plasticity of reclaimed rubber, such as an accelerating action on the oxidation of the rubber and a gel preventing action by acting as a radical acceptor. It also facilitates the dispersion of the reclaiming agent into the rubber matrix. Hence reclaiming oil with a high compatibility with the rubber should be used. The reclaiming oils often have active double bonds or methylene groups in the molecules, through which they are more easily oxidized than the rubber molecule. It is assumed that these activated molecules formed by the auto-oxidation reaction, accelerate the oxidation of the rubber. 3.2. Proposed reaction mechanism of the reclamation of vulcanized rubber Inspite of a large volume of work on the reclaiming of waste rubbers very little information on the mechanism of reclaiming of rubber supported by straightforward evidences is available. Reclamation can occur by breaking the existing cross links in the vulcanized rubber (scission of the crosslink) or by promoting scission of the main chain of the polymer or by both processes [17]. Amberlang and Smith [18] suggested indirectly the oxidative scission at sulfur crosslinks partly based on speculation and partly based on experimental results. Bennett and Smith [19] reported that an alkyl phenol sulfide reclaiming agent had little activity in the absence of oxygen. The oxygen and reclaiming agent showed exceptional activity in attacking a sulfur cured GR-S gum vulcanizate to produce soluble, low molecular weight fragments under relatively mild experimental conditions. ASTM STP 184 A [20] defined “devulcanization as a combination of depolymerization oxidation and increased plasticity” as they usually occur during the process of reclaiming. But actually devulcanization should be the reverse process of vulcanization. In sulfur vulcanization formation of both the C–S and S–S bond takes place and, therefore, it is expected that during devulcanization only the C–S and S–S bond cleavage should occur. In view of these arguments the conversion of scrap or waste rubbers into usable form by all these above physical and chemical processes may be called reclaiming processes. 3.2.1. Scission of the main chain As an example of this mechanism, the phenyl hydrazine–iron (II) chloride system (PH–FeCl2) is very effective for the oxidative degradation of


the rubber molecules at low temperatures. In the degradation reaction of polyisoprene rubber with the PH–FeCl2 system in air, the phenyl hydrazine is the main reagent and FeCl2 acts as catalyst: the rate of degradation of the rubber is determined by the phenyl hydrazine concentration. Phenyl hydrazine is itself easily degraded by oxygen and it is known that nitrogen gas is liberated in this reaction [21]. The rate of degradation is very high in the presence of a metal salt. The initial oxidative degradation of rubber molecules with the PH–FeCl2 system is outlined below (Fig. 1). If sufficient oxygen is present, the various radicals formed by this reaction degrade the rubber molecules as indicated in Fig. 2 [22]. The hydroperoxide is decomposed in the presence of transition metals as shown in Fig. 3.

Figure 1. Oxidation mechanism for the PH–FeCl2 system [adapted from Ref. 21].

Figure 2. Bolland oxidation mechanism (RH = rubber hydrocarbon) [Reproduced with permission from Ref. 22].


Figure 3. Decomposition of peroxides by ions of metals (redox mechanism) [Reproduced with permission from Ref. 22]. 3.2.2. Crosslink scission (triphenyl phosphine) Triphenyl phosphine is known to open the sulfur cross links by a nucleophilic reaction [23] as shown in Fig. 4. The radicals can recombine, but a reaction with a double bond is also possible, with the net effect that the crosslink is broken.

Figure 4. Opening of sulfur crosslinks by triphenyl phosphine [Reproduced with permission from Ref. 23]. 3.2.3. Main-chain and crosslink scission Thiols and disulfides interact with radicals formed during the degradation of the rubber network. It is assumed that they initiate an oxidative breakdown of sulfur cross links and a degradation of rubber vulcanized [24] and inhibit gel formation by combination with the radicals. A vulcanizate that is recycled with thiols and disulfides shows a larger degree of network breakdown. In thermo-mechanical processes disulfides and thiols are equally reactive [26]. Aliphatic thiols are found to be less active than aromatic thiols. The efficiency of aromatic compounds seems to increase when alkyl groups or halogens are substituted on the benzene ring. A mechanism that is often proposed for the reaction of disulfides with sulfur vulcanizates is the opening of cross links or the scission of chains by heat and shearing forces and their reaction with disulfides, which prevents recombination. Atmospheric oxygen is also said to prevent recombination by stabilizing the radical sites. Other compounds with a stabilizing effect are antioxidants [26]. The result is a drop in molecular weight of the polymer. During the thermal degradation hydrogen sulfide and thiols are produced.


3.2.4. Opening of sulfur crosslinks Various chemical probes are already used in crosslink structure analysis for opening the sulfur cross links [27]. The reactions described in Figs. 5–9 have been proposed as typical methods for rupturing the sulfur cross links of rubbers [16]. These reactions take place under the given reaction conditions, but the complete reaction is not as simple as shown. Hydrogen addition and reduction reactions are also possible but these are to be avoided from the point of view of reclaiming. During the actual reclaiming process it is likely that the thermally generated polymer radicals are scavenged by the sulfur radicals thereby preventing the recombination of polymer radicals shown in fig 5-9. This may be supported by the peptizing action of organic thiol compounds during mastication of raw rubbers where shear generated polymer radicals are prevented from recombination by the action of thiol radicals [28]. The sulfur analysis of rubber before and after the treatment of diallyl disulfide (DADS) in this study [29] has shown the increase of combined sulfur of the treated rubber. This may be explained due to the attachment of DADS fragments with the rubber molecules. It also appears that above sequences of reactions may occur irrespective of any reclaiming temperature or any reclaiming agent.

Figure 5. Opening of sulfur crosslinks by oxidation; ROOH = organic hydroperoxide [Reproduced with permission from Ref. 16].

Figure 6. Opening of sulfur crosslinks by heat or shear [Reproduced with permission from Ref. 16].


Figure 7. Opening of sulfur crosslinks by nucleophilic reagents [Reproduced with permission from Ref. 16].

Figure 8. Opening of sulfur crosslinks by rearrangement [Reproduced with permission from Ref. 16].

Figure 9. Opening of sulfur crosslinks by substitution [Reproduced with permission from Ref. 16]. The two principal methods to obtain a re-usable recycled rubber material are: (i) grinding of the rubber and reusing it in the form of a granulate or surface activated powder; (ii) treating the material in a reclaiming process to generate a visco-elastic reclaim. Different processes are developed in order to reclaim vulcanized rubber. 4. Different types of reclaiming processes Rubber may be converted into reclaim by means of a number of processes. In all these the scrap rubber must first be shredded and ground into crumb to permit chemicals and swelling agents to react adequately with the


vulcanized structure, to promote good heat transfer, and to remove the fibres by mechanical or chemical action. Each of the processes described below is followed by final processing. 4.1. Mechanical reclaiming process Several processes are used in mechanical reclaiming process, all of which are continuous processes. Fine (fabric-free) scrap rubber is mixed with reclaiming chemicals and fed continuously into an extruder in which the rubber is devulcanized at about 200oC for about 5-10 minutes. The heat is partially generated by the electrical heating of the extruder and the friction of the crumb. The devulcanized rubber is extruded from the machine in a dry form ready for refining. In mechanical reclaiming process crumb rubber is placed in an open two-roll mixing mill and milling is carried out at high temperatures. In this process drastic molecular weight breakdown takes place due to mechanical shearing at high temperatures (above 200oC). A physical process of reclaiming of vulcanized rubber and refining of the reclaimed rubber are described in a US patent by Maxwell [30]. Particulate form of the vulcanized rubber is reclaimed with reclaiming agents by passing the rubber between an essentially smooth stator and an essentially cylindrical rotor arranged to provide an axial shear zone in which the rubber is frictionally propelled by the rotor action. The action may be assisted by mixing a suitable amount of previously reclaimed rubber or of vulcanized rubber with or in advance of the particulate vulcanized rubber, and/or by supplemental heating. In other aspects of the invention previously reclaimed and vulcanized rubber is similarly fed and acted upon as substitute for conventional refining operation. The mechanical reclaiming process of vulcanized natural rubber is also reported by De et al [31]. The reclaimed natural rubber was prepared by milling vulcanized sheets at about 80oC. On a two roll laboratory mill it formed a band on the roll. Next, it was mixed with various rubber additives. In another case, mixing of reclaim rubber (RR) with fresh rubber in various proportions and study of their curing characteristics, mechanical properties etc. were done. But the Mooney viscosity of the reclaimed rubber was very high (>200, i.e. out of scale) indicating that the plasticity of rubber was very low due to the presence of higher percentage of crosslinked rubber. A comparative study of the curing characteristics of the blends of fresh rubber with reclaim rubber indicated that with increase in the reclaim rubber content the cure rate increased but the scorch time, optimum cure time and


reversion resistance decreased. As the proportion of reclaim rubber in the blends increases modulus, abrasion loss, compression set and hardness increase while tensile strength, elongation at break, tear strength, resilience and flex resistance decrease which shows that increase in the proportion of reclaim rubber, increases the crosslink density. As crosslink density is very high for the natural rubber/reclaim rubber (25/75) blend, so the modulus is high but tensile strength and flex properties are low. Thus reclaim rubber appeared to perform as a non-reinforcing filler in this study. 4.2. Thermal processes Here reclaiming of the rubber that make use of heat and possibly chemicals to plasticize the rubber scrap are summarized [32]. 4.2.1. Digester process The digester process is a batch process where the ground scrap material is mixed with fiber dissolving agents, water, plasticizing oils and, if needed, reclaiming agents. They are heated in an autoclave with steam 15 bar (=180oC) for 8-12 hours. The fabric dissolves and the rubber softens. The aqueous phase is separated and the regenerate is dried for final refining. Significant amounts of residual fabric may remain with the reclaim if the digester processes is being used. The reclaiming chemicals used in this process include zinc and calcium chlorides (to dissolve the fibres) together with a very complex mixture of solvents, softening oils, hydrocarbon resins, pine tar and reclaim catalysts. 4.2.2. Pan process The heater or pan process is one of the oldest and most simple processes used in the rubber reclaim industry. Finely ground scrap rubber that has been freed of fibres by mechanical cyclones is mixed with reclaiming chemicals, then placed in open pans in an autoclave and heated with 'live' steam at 15 bar (=180oC) for 4-12 hours. Any residual water is removed and the reclaim is ready for final processing. Reclaiming chemicals are aromatic thiols, disulfides and aromatic oils. Their use allows lower temperatures and shorter reclaiming times and produces a product with superior mechanical properties. The reclaiming chemicals used in the pan process are similar to those used in the digester process except that no zinc or calcium chloride are used.


4.2.3. Alkaline process The fiber in the scrap is digested by the use of sodium hydroxide in a high concentration (up to 7%): the cellulose in the fiber-containing scrap is hydrolyzed. After recycling, a washing procedure is required to remove excess de-fiberizing agent. The crumb is then dried and refined. The process is found to be detrimental to SBR containing rubber material, as hardening takes place. Some N,N-dialkyl aryl amine sulfides [14, 33] were shown to be highly active reclaiming agents for vulcanized SBR in alkaline reclaiming processes, and the state of reclaiming was manually assessed through thickness, body and tack evaluation. 4.2.4. Neutral process An improvement to overcome the hardening problem was the development of the neutral process. In this process, zinc chloride and pine oil are used [34]. Calcium chloride or zinc chloride is added for the hydrolysis of the textile. The heater, digester, alkaline and neutral processes use long reaction cycles and are characterized by long recycling time. N,N-dialkyl aryl amine sulfides were shown to be highly active reclaiming agents for vulcanized SBR in neutral reclaiming processes too. 4.2.5. High-pressure steam process More recent developments aimed at shorter reaction times. In the high-pressure steam process [34], fiber-free, coarse ground rubber scrap is mixed with reclaiming agents and reclaimed in a high pressure autoclave at a temperature around 280oC for 1–10 min. 4.2.6. Engelke process In the Engelke process [35] an autoclave is used, in which coarse ground rubber scrap, mixed with plasticizing oil and peptizers. This mixture is heated to very high temperatures (> 250 oC) for 15 min, after which refining and straining takes place. 4.2.7. Continuous steam process A continuous steam process uses temperatures around 260oC and high pressures in a hydraulic column. The rubber is ground and water is used as a carrying medium and to seal the material from extraneous oxygen, because otherwise heat and pressure would cause combustion. Heat and pressure


combined with the injected chemical agents cause a substantial breakdown of the rubber in suspension. 4.3. Thermo-mechanical reclaiming process During most mechanical processes a strong rise in temperature occurs that aids in degrading the rubber network. Thermo-mechanical recycling of rubber is assumed to be a combination of breaking carbon-to-carbon bonds and sulfur cross links. This results in the formation of soluble branched structures and fragments of gel. The modern material recycling processes all use thermo-mechanical regeneration methods. Recycling chemicals and oils are frequently used in addition to the thermal and mechanical breakdown: disulfides, thiols, amines and unsaturated compounds are the most common recycling chemicals. They are added in quantities of around 1wt% [34]. Softeners lower the thermal degradation resistance of a vulcanizate by weakening the interaction between filler and rubber chains. 4.3.1. Milling process This process [36] involves the thermo-mechanical degradation of the rubber vulcanizate network. The vulcanizate is swollen in a suitable solvent and then transferred to a mill to form a fine powder (~20 µm diameter). This powder rubber is revulcanized with curing ingredients. The products thus obtained show slightly inferior properties to those of the original vulcanizates. 4.3.2. High-speed mixing A fast thermo-mechanical recycling process is the high-speed mixing process. The rubber is stirred at a speed of 500 rpm and the temperature rises to 200oC [16]. The process takes 15–20 min. 4.4. Mechano-chemical methods Plasticization can be improved by using a reclaiming accelerator while applying a mechanical force to the rubber powder in the presence of air at room temperature. In general, at low temperatures a reclaiming catalyst, reclaiming oil and process oil are used jointly with the reclaiming agent. Typical agents are shown in Table II [16]. Peroxide in combination with methyl halide (material 1) is a powerful radical initiator for a redox system. Phenyl hydrazine and ferrous chloride as well as tributylamine and cuprous chloride (materials 2 and 3) form a complex with each other. This complex is easily degraded by oxygen, under formation of the oxidation initiators. These systems degrade diene-based rubber


in the presence of oxygen at room temperature. Dioxylyldisulfide and 2,20-dibenzamidodiphenyldisulfide (materials 4 and 5) are used as peptizing agents. N-cyclohexylbenzothiazole-2-sulfenamide (material 6) is commonly used as vulcanization accelerator and Nisopropyl- N0-phenyl-p-phenylene diamine (material 7) is used as an antioxidant. Thiophenol and nbutylamine (material 8), toluene sulfonic acid and 1, 8-diazabicyclo [5.4.0] undec-7-ene (material 9) as well as the rubber accelerators tetraethylthiuram disulfide and triphenyl phosphine (material 10) were found to increase the plasticity of rubber reclaim. The fact that these reagents behave as reclaiming agents for rubber is explained by their function as a radical acceptor for the rubber radicals that are formed in the mechano-chemical reaction. Table II. Reclaiming agents used in mechano-chemical methods [Reproduced with permission from Ref. 16].


4.4.1. Trelleborg cold reclaiming (TCR) process In the TCR process, small quantities of recycling agents are mixed into cryogenically ground rubber powder. A short treatment is carried out in a powder mixer at room temperature or at a slightly higher temperature. Phenyl hydrazine-methyl halide or diphenyl guanidine is used to react with the vulcanizates [16]. 4.4.2. De-Link process Rudi Kohler [37] reported a new technology for the devulcanization of sulfur cured scrap elastomers using a material termed De-Vulc developed by Sekhar [38]. Such technique of devulcanization was designated as De-Link process. The recycling of rubber crumb with vulcanization accelerators and sulfur on a mill is used in the patented de-link process. The process is not only suitable for NR, but also for EPDM. In this process 100 parts of 40 mesh or finer crumb is mixed with 2–6 parts of De-Vulc reactant in an open two roll mixing mill for approximately 10 min at temperature below 50oC. De-Vulc reactant is a proprietary material and its nature and composition is not disclosed. The added chemical mixture is prepared from the zinc salt of dimethyldithiocarbamate and mercaptobenzothiazole with stearic acid, sulfur and zinc oxide dispersed in diols [39]. Tetramethyl thiuram disulfide can also be used. It is assumed that the process is based on a proton transfer reaction. The process is more effective for conventional sulfur vulcanizates than semi-efficient and efficient sulfur vulcanizates, the number of cross links is decreased by a factor of 2. As the nip opening of the mill was found to have a significant effect on properties, the vulcanizate breakdown is probably caused by mechanical breakdown. Table III. Mechanical properties of reclaimed rubber [Reproduced with permission from Ref. 40].


In Table III, NR indicates natural rubber made form virgin materials, NR-D indicates natural rubber with 30% devulcanized rubber added to the blend. Similarly SBR is virgin material, SBR-D indicates SBR with 30% devulcanized SBR added to the blend. It is evident from Table III which was compiled from Ref. [41] that in 30% blend with devulcanized rubber and virgin rubber for NR and SBR, Mooney viscosity, tan δ and 300% modulus are high whereas tensile strength, elongation at break and tear resistance are low [37]. But it was claimed [37] that those tensile properties, tear resistance etc. were very similar to those for the virgin materials. 4.4.3. Swelling in benzene with a sulfoxide Natural rubber vulcanizates are attacked by swelling in benzene with a sulfoxide compound like dimethyl sulfoxide (DMSO), di-n-propyl sulfoxide (DPSO) or a mixture of these with thiophenol, methyl iodide or n-butyl amine in a mechano-chemical process on a mill [42]. A thiol and DMSO react to form a disulfide and a nucleophilic agent CH3-S-CH3. It is reported that these reagents cause selective scission of sulfur bonds. NR is completely degraded by the combination of DMSO and thiophenol. If we apply this technique to SBR, a much less reactivity is reported. Although the degree of swelling in an organic solvent increased, only 2% of sol fraction was formed. The low sol fraction and high swelling ratio is in agreement with the theory of selective crosslink scission [43]. A disadvantage of this process is that solvents like DMSO and methyl iodides are highly toxic. 4.5. Cryomechanical reclaiming process In the mid 1980s, the technique of grinding scrap rubber in cryomechanical process [44] was developed. This reclaiming process involves placing small pieces (1''× 1''× 1/2'') of vulcanized rubber into liquid nitrogen which are transferred to a ball mill and ground in presence of liquid nitrogen to form a fine powder. The particle size of the cryo-ground rubber varies from 30 to 100 mesh for most products. The particle size is controlled by the immersion time in the liquid nitrogen and by the mesh size of screens used in the grinding chamber of the mill. Generally, the cost of the ground rubber increases as the particle size decreases. The cost of 40 mesh ground rubber is usually in the mid to high twenty cents per pound area, while smaller particle sizes like 80–100 mesh cost $0.30–$0.40 per pound [45-46]. It has been reported that using 5–10 phr cryogenically ground rubber in various passenger and truck tire compounds


shows some economic advantage [47]. The economic benefit for a modest usage (5%) in passenger tires and truck tires has been estimated at approximately $0.10 and $0.54 per tire, respectively. At the 10% usage level the economic benefit correspondingly doubles as shown in Table IV.

Table IV. Cost savings per tire using cryogenically ground rubber. Cost savings in $ Usage (%)

In passenger tires In truck tires 5 10

0.0980 0.1861

0.5424 1.0310

Klingensmith [48] has evaluated the performances of cryogenically ground butyl rubber in the tire inner liner. He showed the effect of mesh size on percent retention of physical properties. 4.5.1. Processing and mixing of cryogenically ground rubber [48] In the processing of cryogenically ground rubber certain particle sizes are more suitable in specific applications. Extrusion: 80–100 mesh cryogenically ground rubber is needed to avoid fracturing and rough edges. In extrusion of thick section 50–60 mesh cryogenically ground rubber can be used depending on the surface smoothness of the final product. The optimum level of cryogenically ground rubber to be added to fresh rubber is 5%. Calendering: for optimum surface smoothness of products which are 0.060" or less thick, the compound requires 80–100 mesh cryogenically ground rubber. Where smoothness is not so important 30–60 mesh can be used. The optimum level of cryogenically ground rubber in calendering is 10%. Molding: the cryogenically ground rubber in all mesh sizes can be used because all mesh sizes help in removing trapped air during molding. The cured rubber particles provide a path for the air to escape by bleeding air from the part. Mold flow: cryogenically ground rubber generally improves mold flow. Shrinkage is usually less for compounds containing cryogenically ground rubber. The shrinkage reduction is proportional to the amount of cryogenically ground rubber in the compound. So less mold flashing was found with increase in the percentage of cryogenically ground rubber.


4.5.2. Advantages of using cryogenically ground rubber

In the cryogrinding process the equipment cost is less, operating costs are lower, productivity is increased, and the product has better flow characteristics than ambient ground rubber. The unique nature of the surface morphology of the cryoground rubber particle facilitates the ventilation of trapped air in unvulcanized rubber laminate products, particularly tires, thus reducing tendency for cure blistering. Surface oxidation of the cryoground particle is of little concern because of its inherent low surface area, thus differentiating itself from ultrafine, high surface area ambient ground filler (10–30 mesh). The particle sizes of cryoground rubber are shown in Table V. Table V. Particle sizes of different cryogenically ground rubbers [Reproduced with permission from Ref. 40].

Mesh size Particle size (µm)

40 80 100 200 325 400 500

388 177 149 73.7 44.5 38 20

4.6. Other ground rubber processes Other two types of grinding of rubber are dry ambient grinding and solution or wet ambient grinding. The first step in the manufacture of reclaim is grinding [49-50], of the rubber part to be reclaimed. It is necessary to increase the surface area of the rubber particle which increases the rate of the chemical reaction in reclaiming and also to produce more uniform product. It was found that small particle size vulcanized ground rubber could be added to the rubber compound to reduce the cost. Ground rubber used in compounding varies from 10 to 200 mesh. 4.6.1. Dry ambient grinding This is nothing but a mechanical grinding technique [49]. In this process vulcanized rubber pieces are placed in a serrated grinder for preparation of ground rubber of particle sizes from 10 to 30 mesh. Ambient ground rubber is largely used in tires and mechanical goods. Generally, 5–20 phr ground rubber is used. With higher particle size of ambient ground rubber,


smoothness of the product decreases. Although the name of the process is ambient grinding the grinding in fact generates some heat during processing. With high modulus or aged compounds the amount of heat generated can be higher resulting in heating of the rubber and, therefore, degradation of polymer chain takes place. So the name “ambient grinding” is not appropriate. In ambient grinding process some pendant groups are generated which become attached to the virgin rubber matrix producing an intimately bonded mixture [49]. 4.6.2. Wet or solution grinding This is a modified ambient grinding process [49] that reduces the particle size of rubber by grinding in a liquid medium. The process involves putting a coarse ground rubber crumbs (10–20 mesh) into a liquid medium, usually water, and grinding between two closely spaced grinding wheels. Here the particle size is controlled by the time spent in the grinding process. In this process particle sizes of 400–500 mesh have been reported. The advantage of the fine particle wet ground rubber is that it allows good processing producing relatively smooth extrudates and calendered sheets. But whether this process helps in heat dissipation through water and reduces polymer chain breakdown is not mentioned. 4.7. Microwave recycling In the microwave recycling method, a controlled dose of microwave electromagnetic energy at specified frequency is used to break the sulfur-sulfur or carbon-carbon bonds in the crosslinked rubber powder [51]. Thus in this process elastomer waste can be reclaimed without depolymerization to a material capable of being recompounded and revulcanized having physical properties essentially equivalent to the original vulcanizate. By using microwaves, the temperature of the material increases very fast to reach finally 260–350oC [52]. A pre-condition to reach this temperature level for devulcanization is that the vulcanizates should contain carbon black, making them suitable for this method. The waste material for reclamation must have some polarity so that the microwave energy will generate the heat necessary to devulcanize. The devulcanized rubber is not degraded when the material being recycled [53] which normally take place in the usual commercial processes currently being practiced. In this process they claimed that sulfur vulcanized elastomer containing polar groups is suitable for microwave devulcanization. Tyler et al [54] have claimed their microwave


devulcanization process as a method of pollution controlled reclaiming of sulfur vulcanized elastomer containing polar groups. Carbon black containing rubber is susceptible to ultra high frequency in a microwave chamber due to interface or ion polarization: accumulation of free electrons at the interface of different phases with a different conductivity and dielectric constant. Microwave energy between 915 and 2450 MHz and between 41 and 177 W h per pound is sufficient to sever all crosslink bonds but insufficient to cleave polymer chain degradation. The microwave energy devulcanization device generates heat at a temperature in excess of 260oC to yield a mass which is fed to an extruder which extrudes the rubber at a temperature of 90–125oC. The extrudate can be used per se as a compounding stock. Another process was developed for reclaiming waste elastomers by microwave radiation. The process involves the impregnation of the waste rubber with an essential oil and then heat treating the impregnated material under reduced pressure with microwave radiation [55]. The tensile properties of microwave devulcanized EPDM rubber, EPDM hose and IIR are shown in Table VI. From the Table VI it has been found that the tensile properties of devulcanized rubber and virgin rubber-devulcanized rubber blend is almost comparable. The cost of devulcanized hose and inner tube material by microwave method is only a fraction of the cost of the original compound. The transformation from waste to refined stock ready for remixing takes place in only five minutes with usually 90–95% recovery of the rubber. Therefore, it appears that this microwave technique is a unique method of reclaiming in terms of properties and fastness of the process. This method is very much useful because it provides an economical, ecologically sound method of reusing elastomeric waste to return it to the same process and products in which it was originally generated and it produces Table VI. Physical properties of microwave devulcanized product [Reproduced with permission from Ref. 40].


a similar product with equivalent physical properties. The properties of the reclaim are reported to be better than those of rubber obtained by other reclaiming methods, and EPDM and IIR are particularly suitable for the process. The process is able to convert vulcanized waste such as EPDM (automotive coolant hoses) into a usable compound in just 5 min [56-57]. The process has the disadvantage that it is difficult to control. 4.8. Ultrasound recycling Next to the microwave radiation, ultrasonic energy is used for the devulcanization of cross linked rubber. The first work with ultrasonic energy was reported by Pelofsky [58] in 1973. In this patented process, solid rubber articles are immersed into a liquid, and then ultrasonic energy is applied whereby the bulk rubber effectively disintegrates and dissolves into the liquid. In this process ultrasonic radiation is in the range of about 20 kHz and at a power intensity of larger than 100W. But in the patent information they did not mention the ultimate properties of the devulcanized rubber. The ultrasonic reclaiming of natural rubber vulcanizate was again reported by Okuda and Hatano [59] in 1987 which was also patented. They subjected the NR vulcanizate to 50 kHz ultrasonic energy for 20 min to achieve devulcanization. Mangaraj and Senapathi [60] indicated in their patent on vulcanization of rubber by ultrasonic radiation a possibility of rubber degradation and devulcanization by ultrasonic energy. Later Levin and coworkers reported [61] the phenomenon of devulcanization by ultrasonic energy. The devulcanization process requires a high energy level to break carbon–sulfur and sulfur–sulfur bonds. An ultrasonic field creates high frequency extension–contraction stresses in various media [62]. Isayev and his group have made a percolation simulation of the network degradation during ultrasound devulcanization in which they have claimed an excellent agreement of experimental data for SBR and GRT (ground rubber tyre) with the predicted dependence of the gel fraction of devulcanized rubber on crosslink density. At the University of Akron, Ohio, they designed an ultrasonic reactor of a 38.1mm rubber extruder with a length to diameter ratio of 11 and a co-axial cone-shaped ultrasonic die attachment equipped with three temperature-controlled zones. The screw is heated electrically or cooled by water. The scrap rubber is fed into the extruder by a conveyor belt with adjustable output. A 3kW ultrasonic power supply, an acoustic converter and a 76.2mm cone-tipped horn is used. The horn vibrates longitudinally at a 20 kHz frequency and 5-10 mm amplitude. The scrap rubber particles are transported within the extruder to the chamber with the ultrasound horn, and the recycled rubber can exit this chamber through a die. When SBR is


recycled by ultrasound extrusion [63], cleavages of intermolecular bonds such as C–S and S–S as well as C–C bonds in the main chain take place. An increase in ultrasound amplitude is accompanied by significant decrease in molecular weight of the sol fraction and a decrease of the gel content. Curing behavior, rheological properties, structural characteristics of devulcanized rubber from model SBR and GRT rubbers as well as mechanical properties of vulcanized rubber samples were studied and a possible mechanism of devulcanization was also discussed. They characterized the degree of devulcanization by the measurement of crosslink density and gel fraction of the devulcanized rubber. Later they have published two more papers on the ultrasound devulcanization of sulfur vulcanized SBR and on vulcanization of ultrasonically devulcanized SBR elastomers [64]. An increase in the extruder temperature also results in a higher sol fraction. Conventional sulfur vulcanizates are more susceptible to sol production. This process is characterized by substantial main-chain scission. A continuous ultrasonic devulcanization of unfilled EPDM rubber was also carried out by Yan and Isayev [65], and the mechanical properties of revulcanized EPDM rubber were measured. Gel fraction, crosslink density and dynamic properties were also determined for the virgin vulcanizate, the ultrasonically devulcanized rubber and the revulcanized rubber. The tensile strength of the revulcanized EPDM was much higher than that of the original vulcanizate with elongation at break values being practically unchanged. It is proposed that the improvement in mechanical properties of revulcanized EPDM is mainly due to the extent of non-affine deformation of the bimodal network that possibly appears in the process of revulcanization of ultrasonically devulcanized rubber. For dynamic visco-elastic properties, it is found that devulcanized EPDM is a more elastic material than uncured virgin EPDM and that revulcanized EPDM is less elastic material than the virgin EPDM vulcanizate at the same modulus level. Such devulcanization experiments were carried out by an ultrasound devulcanization reactor developed for the purpose. The ultrasonic reactor is a 1.5 in. rubber extruder with L/D = 11 with a co-axial cone shaped ultrasonic die attachment. There are three temperature-controlled zones. The screw is heated electrically or cooled by water pumped from a thermostat. The die and the ultrasonic horn have sealed inner cavities for running cooling water. A flush mounted thermocouple and a pressure gauge are inserted into the barrel. The temperature and pressure of the rubber at the entrance to the die are measured by a thermocouple and pressure gauge inserted into the barrel. The scrap rubber is fed into the extruder by a conveyor belt with adjustable output. A 3 kW ultrasonic power supply, an acoustic converter, a 1:1 booster and a 3 in. cone-tipped horn are used. The horn vibrates longitudinally at a 20 kHz frequency and 5–10 mm amplitude. The whole unit is mounted on four rigid bars fixed to


the extruder flange. Isayev and co-workers [66-67], studied the devulcanization of SBR vulcanizate using the above ultrasonic reactor at various temperatures viz. 121, 149 and 176oC, different clearances at various flow rates and the ultrasonic oscillation amplitudes. The extent of devulcanization was studied by measuring percentage and crosslink density of the gel fraction. It was reported that both crosslink density and the gel fraction decrease in the devulcanization process. For original ground rubber tire the measured gel fraction is 83% and crosslink density of gel is 0.21 kmol/m3, but after ultrasound treatment at 121oC barrel temperature it reduces to 64–65% with crosslink density of 0.02 kmol/m3 The crosslink density also decreases with higher residence time in the treatment zone and with higher specific ultrasonic energy. The mechanical properties of the revulcanized sample were also studied. With decrease in the crosslink density of the devulcanized rubber, the tensile strength of revulcanized samples varies from 1.5 to 10.5 MPa and elongation at break varies from 130 to 250%. Based on the results of mechanical properties Isayev et al [66]. proposed that the devulcanized sample having a crosslink density lower than 0.06 k mol/m3 can be regarded as over treated, and samples with crosslink density higher than 0.10 kmol/m3 can be regarded as undertreated. Thus over treatment causes main chain breakage and undertreatment causes insufficient devulcanization. They also reported that ultrasound treatment of SBR results in low molecular weights of the sol fraction: Mn = 2-4X103. Ultrasonic devulcanization, therefore, causes significant degradation of polymer chains. A simple model based on a purely topological consideration was proposed and simulation of the process was carried out [68–69]. In the model they have assumed a breakup of the main chain bond and crosslink bonds as independent random events. Such random scission of crosslinks and main chain results in the formation of soluble branched rubber chains regarded as fragmented gel structure or microgel. It is found that during ultrasound devulcanization molecular weight of sol fraction decreases from which it may be understood that during ultrasound treatment not only C–S or S–S bonds but also C–C bonds break. Levin et al [70] suggested a revulcanization scheme. They concluded that devulcanized rubber contained a larger amount of sulfidized molecules which were responsible for crosslinking during revulcanization. 4.9. Reclaiming by organic reagents Many attempts have been made since the beginning of 20th century for reclaiming of scrap rubber products. Some chemicals that cause selective scission of sulfur bonds are used as reagents to determine the relative amounts of mono-, di-, and polysulfidic cross links. These reagents are called chemical probes. Thiols in combination with organic bases can selectively cleave sulfur cross links. Hexanethiol was found to cleave di- and polysulfidic cross links,


while 2-propane thiol selectively cleaves polysulfidic cross links in a nucleophilic displacement reaction with piperidine as base [71-72]. Thus a large number of chemical reclaiming agents for natural and synthetic rubbers, viz. diphenyl disulfide, dibenzyl disulfide, diamyl disulfide [73-74] bis(alkoxy aryl) disulfides[75], butyl mercaptan and thiophenols [76-77] xylene thiols [78] and other mercaptans [13], phenol sulfides and disulfides [14] have been developed. Cook and co-workers [79] reported the preparation, evaluation and structural correlation of alkyl phenol sulfides as reclaiming agents for styrene butadiene rubber (SBR). The effect of these alkyl phenol sulfides as reclaiming agent was compared with that of many aromatic thiols. Reclaiming of neoprene and nitrile rubbers was also evaluated using alkyl phenol sulfides. The extent of reclaiming of these two rubbers was extensively evaluated and monitored by Mooney plasticity and manual observations, such as sheet thickness, body and tackiness by numerical ratings. The reclaiming was done at 188oC (by 175 psi superheated steam) for 4 h using 5 mesh vulcanized rubber powder. Both the total and combined sulfur in the reclaimed stock were determined, but the method of sulfur estimation was not reported. The thiol–amine combination gives a complex, possibly a piperidinium propane-2-thiolate ion pair, in which the sulfur atom has enhanced nucleophilic properties, and is capable of cleaving organic trisulfides and higher polysulfides within 30 min at 20oC, according to a mechanism as shown in Fig. 10 [80,81]. Disulfides react at a rate, which is slower by a factor of 1000. The polysulfide cleavage is faster due to Pπ–dπ delocalization of the displaced s-electron pair of RSS-- as outlined in Fig. 11:

Figure 10. Cleavage of polysulfide bonds by 2-propane thiol/piperidine probe; iPr = isopropyl and R = rubber polymer [Reproduced with permission from Ref. 80-81].

Figure 11. Cleavage of polysulfide crosslinks and Pπ–dπ delocalization; iPrS- = nucleophilic thiol–amine associate [Reproduced with permission from Ref. 80-81].


Some N,N-dialkyl aryl amine sulfides [78] were shown to be highly active reclaiming agents for vulcanized SBR in both neutral and alkaline reclaiming processes, and the state of reclaiming was manually assessed through thickness, body and tack evaluation. A review of science and technology of reclaimed rubber was published by Le Beau [82] in 1967. Knorr [26] has shown the action of diaryl disulfide on the natural and synthetic rubber scraps of technical goods. In this process the finely ground fabric free scrap is heated in saturated steam at a very high temperature (150–180oC) with the addition of reclaim oil and Aktiplast 6 (contain disulfides). First finely ground scrap was thoroughly mixed with diaryl disulfide and reclaim oil and allowed to swell for at least 12 h. The material was then placed on talcum powder trays. The layer thickness should not exceed 2–3 cm to allow the oxygen to penetrate the material well. A devulcanizer (or autoclave) is needed of approximately 1.5 m diameter and 3.8 m in length. The trays with scrap are rolled into the devulcanizer. Good circulation of air and steam is necessary for the process. After having sealed the lid, the autoclave is pressurized with 4 bar of air–steam and the fan was turned on for good air circulation. The compressed air valve was then closed and 8–9 bar steam was forced in until the temperature reached 190oC. Depending on the kind of scrap the reclaiming time varied between 3–5 h per charge (200 kg). Rubber network can easily be swollen by methyl iodide which can be removed therefrom by warming under vacuum. Meyer and Hohenemser [83] introduced the use of methyl iodide to estimate monosulfide linkages in vulcanized natural rubber. The level of network bound iodine after reaction for two to three days would reflect the concentration of monosulfide groups since simple saturated monosulfide group reacts as follows.

R2S + CH3I → R+2SCH3I--

Anderson [84] patented the reclaiming of sulfur vulcanized rubber in the presence of oil, water vapor and aryl disulfide peptizer at elevated temperature in the range of about 175–195oC and at a pressure in the range of about 230–260 psi for 1–4 h. Here aryl disulfide is a mixture of diphenyl disulfide, dicresyl disulfide and dixylyl disulfide. In another attempt rubber like product from rubber scrap was prepared by mixing rubber scrap with sulfur, antioxidant and antiozonants in an apparatus. The reclaiming was carried out at temperature of about 250–450oF and pressure of about 1000–3000 psi for 1–10 min. Sulfur bearing compounds as vulcanizing agents was added in place of free sulfur. The process is particularly suitable for making roofing products [85]. From physical property data (Table VII) it is clear that reclaim rubber possesses very poor mechanical properties. However, the author [26] did not


mention whether any curative was used with the reclaim rubber. Schnecko [86] has reviewed the present aspects of elastomers recycling and reported the development of some chemical probes for devulcanization of crosslinked rubber. These chemical probes selectively cleave carbon– sulfur and sulfur–sulfur bonds but they do not cleave carbon–carbon bonds. Table VII. Physical properties of reclaimed rubber products [Reproduced with permission from Ref. 26].

In a recent study by Bilgili et al [87] propose a new two stage recycling process to reuse a rubber waste. First, the granulates of the waste were pulverized into small particles using a single screw extruder in the Solid State Shear Extrusion (SSSE) process. Then, the produced powder was compression molded in the absence of virgin rubber. The slabs prepared at various molding conditions were subjected to mechanical, chemical, and microscopic tests. It is found that the slabs have high extensibility with low– medium tensile strength. Compressive creep of the powder, self-adhesion of rubber molecules, and interchange reactions of polysulfidic crosslinks are proposed as the basis of particle bonding. They have demonstrated that rubber slabs can be produced by pulverizing rubber waste using the solid state shear extrusion (SSSE) process and subsequently molding the powder in the absence of virgin rubber. The molding conditions and the powder characteristics significantly affected the failure properties and the crosslink density of the slabs. Higher temperatures and pressures generally improve the bonding of the particles, whereas a high crosslink density and the presence of large amount of coarse particles inhibit the particle bonding. The inferior failure properties of the slabs compared with those of the original vulcanizate can be attributed to a lack of sufficient particle bonding. Compressive creep of the powder, self-adhesion of the rubber molecules, and interchange reactions of polysulfidic crosslinks appear to govern the particle bonding.


The rubber slabs can be used in the manufacture of noncritical items such as mats, pads, carpet underlay, etc. It is, of course, possible to produce recycled items with any desired shape using different mold geometry. Considering that we could use the pulverized by-product of a rubber company to manufacture these items in the absence of virgin rubber, the process of converting large quantities of factory by-product into useful items using the above two-stage process could be economically feasible. The powder produced by the SSSE process consists of small particles with relatively large surface area. Therefore, by using the above process, the powder could also be used as filler in various applications such as tire manufacturing and production of polymer composites. 4.10. Reclaiming by inorganic compounds Discarded tires and tire factory waste were devulcanized by desulfurization, in presence of sodium, of suspended rubber vulcanizate crumb (10–30 mesh) in a solvent such as toluene, naphtha, benzene, cyclohexane etc [87]. The alkali metal cleaves mono, di and polysulfidic cross linkages of the swelled and suspended vulcanized rubber crumb at around 300oC in absence of oxygen. As claimed by authors such treatment yielded a rubber polymer having a molecular weight substantially equal to that of rubber prior to vulcanization. Carbon black may also be recovered for reuse and the devulcanized rubber may be subjected to revulcanization without separation of the polymer from the solvent by addition of an appropriate curing composition. Although it appears from the patent that the developed process is a direct reversal of vulcanization without affecting the molecular weight of the polymer. The process may not be economically convenient. Because the process involves swelling of the vulcanized rubber crumb in an organic solvent where in the metallic sodium in molten condition at the process temperature should reach the sulfidic crosslink sites in the bulk of the rubber crumb. Further to this, isolation of the devulcanized product from the solvent may be hazardous and cause pollution. Although the patent has not described the vulcanization characteristics of the devulcanized rubber, the presence of NaS may decrease the scorch safety of the product. Yamashita and co-workers [88-89] have successfully reclaimed powder rubbers using an iron oxide phenyl hydrazine based catalyst. In this process powder rubber from waste tires is treated with phenyl hydrazine and FeCl2, ozonized and treated with H2O2 to give liquid rubber having higher viscosity and better yield than those of similar products without the phenylhydrazine–FeCl2 treatment. For example, they reported that 100 g powder rubber from waste tires, 0.5 g phenyl hydrazine in 10 ml benzene and 0.25 g FeCl2 in 5 ml


MeOH were mixed and kept for a day at room temperature and rolled for 10 min. The above plasticized rubber was ozonized and treated with H2O2 to give liquid rubber having intrinsic viscosity (30oC in benzene) 0.05–0.11 dl/g in 13–25% yield compared with 0.03– 0.05 dl/g and 15–20% yield for liquid rubber obtained without the phenylhydrazine–FeCl2 treatment. Thus from intrinsic viscosity data it is understood that molecular weight of reclaim rubber is very low. During reclaiming by the above process severe breakdown of rubber chains takes place. Kawabata and co-workers [90] have also reclaimed powder rubber using copper (I) chloride–tributyl amine catalyst. They successfully noted that the rate of degradation of isoprene rubber by copper (I) chloride–tributyl amine (Cu2Cl2–Bu3N) mixtures decreased in the order of S-vulcanized rubber> ZnO and tetramethyl thiuram disulfide vulcanized rubber> organic peroxide vulcanized rubber. The sol content and crosslink density of the degradation products indicated that scission in the main chain was as important as breakdown at the crosslinking sites for the sulfur vulcanized and ZnO–TMTD vulcanized samples. But in case of peroxide cured vulcanizates the scission in the main chain was predominant. Therefore, it conclusively proves that during reclaiming process not only the cleavage of carbon–sulfur or sulfur– sulfur bonds takes place but also scission of carbon–carbon bonds of the polymer chain occurs. A novel chemical reclaiming process has been patented wherein reclaiming of pulverized scrap rubber is carried out by a reclaiming composition consisting of reducing agent such as phenyl hydrazine (0.2– 1.0 wt%) and diphenyl guanidine (0.2–0.8 wt%), ferrous chloride and a plasticizer [91]. The reclaiming occurs in a solid phase in oxygenic gas at a temperature of at most 100oC by agitation in a powder mixture for about 30 min. 4.11. Reclaiming by miscellaneous chemicals Vehicle tire scraps containing polyisoprene rubber, SBR, PBR was devulcanized by low temperature phase transfer catalyst. Both the devulcanizing agent composition and the process were patented. The novelty of this process lies in the use of low temperature phase transfer catalyst and a process temperature lower than 150oC. The devulcanized rubber of this invention is distinguishable from conventional reclaimed rubber in that the devulcanized rubber is substantially free from polysulfide crosslink which are selectively broken during the process with negligible main chain scission [92]. Kasai and co-workers [93] reported the use of thiocarboxylic acid as reclaiming agent for crosslinked rubber. In this process waste rubber powder of 30 mesh particle size is mixed with 0.5–10.0 wt% (based on rubber) of


10% thioacetic acid solution in benzene and left one day at room temperature; stripped of benzene and rolled at <120oC in air to give reclaimed rubber with good mechanical strength. A compound of rubber: 200, ZnO:5; stearic acid:1; sulfur:3; and vulcanizing accelerators: 0.7 parts was heated for 30 min at 145oC to give a vulcanizate having tensile strength 9.0 MPa and elongation at break 410%. Cervinschi and coworkers in 1990 developed a process of reclaiming by hydrocarbon solvent [94]. They have made the waste tires which are swollen by treating at 188–200oC for 24-36h in a mixture of aromatic hydrocarbons 38–48%, naphthenic hydrocarbons 12–28%, and paraffinic hydrocarbon 35–45%. The swollen rubber in the mixed solvent is passed through a 2 mm orifice of an extruder at 2–6 kN/cm2 pressure to give a paste which is useful in the manufacturing of tire treads. A compound of rubber containing 40–60 parts of above prepared paste and 100–150 parts isoprene rubber, butadiene rubber and SBR was heated for 15 min at 160oC to give a vulcanizate having 300% modulus 3 MPa, elongation at break 500% and Shore A hardness 62. As a solution to waste tire disposal problem used tire was recycled by soaking the tire in an organic solvent e.g. 1,3,5-trimethyl benzene for a sufficient time while reducing its tensile strength by about 50%. Then the soaked rubber was disintegrated by applying a shear force to give a recycled rubber [95]. Scrap rubber containing natural and synthetic can be reclaimed by digester process with the use of a reclaiming oil having molecular weight between 200 and 1000 consisting of benzene, alkyl benzene and alkylated indanes. The composition of this reclaiming oil and the improved digester process using such reclaiming oil has been patented by Bryson [96]. Vulcanized rubber was also reclaimed by the action of transition metal alloys and derivatives [97]. In this process vulcanized rubber was swollen in an organic solvent and then treated with a size reduction agent e.g. transition metal alloy and their derivatives. In a typical composition 150 g of SBR rubber vulcanizate was allowed to swell in 1000 ml benzene for two days, placed in a ball mill in the presence of 2 g powder Mn for 20 h, then beaten in the presence of 0.1 (N) H2SO4 for 4 h, neutralized, washed, dried, homogenized and then compounded to evaluate the properties. The mixing formulation containing reclaim rubber 100; ZnO:0.3; stearic acid:0.2; aromatic oil:0.6; N-phenyl-N-cyclohexyl-p-phenylene diamine (antioxidant):0.1; accelerator:0.1; diphenyl guanidine accelerator:0.5, and sulfur:2.0 parts was vulcanized at 150oC for 45 min to give a vulcanizate having tensile strength 12.6 MPa, elongation at break 220%, Shore A hardness 70 and abrasion loss 5%. 2-Mercaptobenzothiazole was also found to be effective as reclaiming agent [98]. In this process powder rubber from waste tires was kneaded with process oil in the presence of 2-mercapto benzothiazole or its


cyclohexylamine salt to give reclaim rubber, having Mooney viscosity 31 or 22, respectively. In a typical recipe a mixture of powder rubber:100; reclaiming agent:1; and process oil:10 g was rolled for 30 min. 100 g of the above product was mixed with N-cyclohexyl-2-benzothiozole sulfenamide:1.5, dibenzothiazolyl disulfide:0.5, and sulfur:1.5 g, and vulcanized at 160oC for 10 min to give a vulcanizate having JIS hardness 53, tensile strength 11.9 MPa, and elongation at break 360%. 4.12. Pyrolysis of waste rubber Pyrolysis is one of two thermal approaches to recovering energy and basic materials from waste rubber. Pyrolysis is a controversial, complex, large scale, capital intensive, high-technology approach which is still considered experimental as a method for processing tyres. Technically, the term 'pyrolysis' covers all forms of heat decomposition, including combustion, although in practice it is normally understood to mean thermal decomposition in a non-reactive (anaerobic, or in the absence of oxygen) atmosphere. Pyrolysis is used to describe processes geared towards the recovery of such materials as carbon black, metal, oils, and gasses, as well as those involving the use of waste material for its energy value. 4.12.1. Process Pyrolysis should be understood as the process of heating the rubber under pressure until it decomposes. Three processes are used, involving different thermal ranges: above 600°C; between 400°C and 600°C and below 300°C. The processes differ from each other in production of carbon black, oil and gasses. At high temperatures less carbon black is produced, and more gasses. 4.12.2. Products Whereas the compounds resulting from combustion are relatively predictable, products of pyrolysis differ widely depending on the conditions under which the process is performed. The main products are metals, oils, carbon black, plus a wide range of gaseous and liquid hydrocarbon mixtures, together with varying amounts of residual char. Generally 25-50% of the weight of rubber pyrolysed can be recovered in the form of a distillate of approximately 42 MJ/kg calorific value, which is higher than when burning tyres (37.5 MJ/kg) and even higher than coal (29 MJ/kg). Tests have indicated that this distillate is an excellent low sulphur (less than 0.5%) fuel which can be distilled into a variety of light and heavy fractions, all having similar heating


values. The quantity of char produced varies from 40 to 50% of the weight of rubber pyrolysed. The char generally has a calorific value of 33 MJ/kg, which is higher than coal, but it contains most of the sulphur. The char is also being investigated for its use (after processing) as an activated carbon which could be used in the tertiary treatment of industrial waste water. The gas produced from tyre pyrolysis ranges from 5-20% by weight of the scrap rubber treated. The composition of the gas varies widely with the pyrolysis conditions, but in all cases a readily combustible high calorific value fuel is produced. The gasses are usually burned to maintain the pyrolysis process and to enable the purification and separation of other fractions. The calorific value depends very much on the percentage of carbon dioxides (CO2) and nitrogen (N2) and may vary between 26,250 and 46,200 kJ/m3. The oil fraction can also be used as fuel. Its caloric value is at the level of heating oil: 40 MJ/kg. This oil is also considered by some to be suitable for use as process oil and as an input material for chemical manufacturing. Even though the process of thermal decomposition is theoretically simple, the process factors need to be very well controlled and the products require extensive processing to produce marketable commodities. The capital investment costs of pyrolysis plants run to several millions of dollars, and cannot take place on a small scale. Although in many laboratories, pilot plant and even commercial attempts have been made to establish economical units over the last 25 years (e.g. Kobe Steel in Japan, Tosco in USA, Tyrolysis in UK, Ebenhausen in Germany and many more) [99] but none has survived. The product spectrum is well known [100]. There are variations by vacuum or in presence of H2, N2 [101] or in molten salts like NaAlO2 [102]. Texaco opened an experimental liquefaction unit about 15 years ago for a mild cracking process below 370oC at atmospheric pressure resulting in light and heavy oil fractions [103]. Finally, it should be emphasized that there are few successful examples of operating pyrolysis plants in the world. The products appear so attractive that attempts are continually being made to get this technical approach going, but they consistently fail either financially or technically. 4.13. Reclaiming of rubbers by the use of a renewable resource material (RRM) In the chemical reclaiming process a large number of chemical reclaiming agents viz. different disulfides, monosulfides, thiols etc. have been used for treatment of scrap ground rubber crumbs or powders at an elevated temperature and under pressure. No report is available on reclaiming of rubber at around ambient temperatures. Almost all disulfides and thiols have very repelling smell and are hazardous. So handling of these disulfides and


thiols are not very desirable. Being expensive also the use of such chemicals may not be economic for reclaim rubber production. In all the above physical and chemical reclaiming processes except the ultrasound method the extent of reclaiming had not been evaluated or reported. The product of such reclaiming processes was soft and weak mass which was neither characterized nor analyzed for the composition of reclaim. Probably as a result of biased concept such reclaiming was thought to occur by scission of carbon–sulfur and sulfur– sulfur crosslink bonds. The prospect of carbon–carbon bond scission during reclaiming was not investigated. It is either apparent or appropriate to believe that both the physical and chemical reclaiming processes involve polymer chain scission due to mechanical shearing at low or high temperatures, chemical action at high temperatures, thermal scission, or by ultrasound energy at high temperatures. The chain scission of vulcanized rubber during reclaiming is, therefore, supposed to increase plasticity as well as the sol content. The amount of sol as well as the molecular weight of the sol portion of reclaim rubber is supposed to contribute to a great extent to the properties of the reclaim rubber. But except in ultrasound process none has reported the molecular weight of sol. Such information might help in selecting a suitable reclaiming agent as well as a process. In view of the above state of the art in the reclaiming of waste rubbers, De [104] on his PhD work along with Adhikari and coworkers [105-107] have developed a simple process for reclaiming of rubbers with a vegetable product which is ecofriendly and renewable resource material (RRM). The major constituent of RRM is diallyl disulfide. Other constituents of RRM are different disulfides, monosulfides, polysulfides and thiol compounds [108] The reclaiming activity of RRM was studied in natural rubber, styrene butadiene rubber and natural rubber–polybutadiene rubber blend system and compared with the reclaiming activity of synthetic diallyl disulfide [104]. Reclaiming experiments were done using rubbers of known formulations in order to study the chemical and morphological changes occurred during reclaiming operation. The extent of reclaiming was assessed through the measurement of sol content, molecular weight of sol and Mooney viscosity of the reclaim rubber. Tensile properties of this reclaim rubber in a blend with above mentioned virgin rubbers were also evaluated before and after accelerated aging in air. 4.13.1. Reclaiming process using RRM Vulcanized and aged ground rubber of known composition was milled in a two roll mixing mill with simultaneous addition of the RRM and spindle oil or diallyl disulfide (DADS) and spindle oil separately. The reclaiming was


carried out with different concentrations of reclaiming agents for different milling times at two different temperatures. With progressive milling, band formation took place on the roll surface and the entire mass became sticky. The results in Table VIII showed that 15 min milling of vulcanized rubber with RRM or DADS produced lowest sol content, lowest molecular weight of sol and highest Mooney viscosity of the reclaim rubber whereas 35 min milling provided the highest sol content with the highest molecular weight and the lowest Mooney viscosity. It was found that 10 g of RRM or 2 g DADS was sufficient to obtain reasonable amount of sol with highest molecular weight as well as lowest Mooney viscosity of the reclaim rubber obtained after 35 min milling at 60oC. It is found that the sol fraction gradually increases with the increase in milling time and the highest sol fraction is obtained at 35 min milling showing a major dependence of sol content on milling time because during milling vulcanized rubber samples undergo tremendous mechanical shearing resulting in random polymer chain breakdown. Whatever may be the process of reclaiming, either mechanical or thermal, maximum sol fraction is desirable, the molecular weight of the sol should be as high as possible for improved properties of reclaim. So in any reclaiming process proper care is necessary in setting the reclaiming conditions so that thermal or mechanical Table VIII. Effects of reclaiming agents on sol content, molecular weight of sol and Mooney viscosity of reclaim rubber [Reproduced with permission from Ref. 40].


shearing has minimum effect on fragmentation of the sol. The reason for increase in sol content with progressive milling lies on the action of DADS either added externally or present as the major constituent of RRM. The DADS breaks into radicals as the temperature rises due to mechanical shearing [33], such radicals combine with the broken polymer chain radical and thereby prevent the recombination of these polymer radicals [104–106] which explains the increase of sol fraction with increase in milling time. 5. Biotechnological possibilities for reclaiming of rubber One possible way of getting rid of spent rubber could be to degrade it using microorganisms. Biological attack of natural rubber latex is quite facile [109]. Man has tried time and again to consider elastomeric articles as source for microbial attack [110]. Obviously nature is able to take care of its own waste problems but as soon as man become involved and convert the natural rubber polymer into a technical material by sulfur and numerous other ingredients, biological attack is minimized [111]. Spent rubber could be used as substrate for microorganisms provided the structure can be efficiently degraded [112]. Many studies have been made on microbial degradation of rubber materials aiming to either prevent or enhance mineralization [113-114]. Most studies are dealing with micro-organisms belonging to the Actinomycetes [115-116]. An interesting recent approach was reported in a German patent [117] to utilize a chemolithiotrope bacterium in aqueous suspension for attacking powder elastomers on the surface only, so that after mixing with virgin rubber diffusion of soluble polymer chains is facilitated and bonding during vulcanization becomes again possible. A biotechnological process was developed by Straube et al [118] for the devulcanization of scrap rubber by holding the comminuted scrap rubber in a bacterial suspension of chemolithotropic microorganisms with a supply of air until elemental sulfur or sulfuric acid is separated. This seems to be an interesting process which obtains reclaim rubber and sulfur in a simplified manner. Steinbuchel has reported solubilisation of natural rubber of both pure rubber and vulcanized latex by using species of Gordona [119] Biosurfactants were believed to facilitate degradation of the rubber during adhesive growth [120]. Adaptation of microbial enrichment cultures with tire crumb material for several months resulted in enhanced growth of micro organisms especially for natural rubber. Vulcanized rubber in the form of latex is rather easily degraded while rubbers from spent truck tyres are more resistant. Microbial adaptation to growth on polymer material and adhesion of co-substrate increase degradation. The mechanisms behind microbial rubber degradation are often oxidation and chain scission of the polymer


backbone [121], which does not result in materials suitable for reuse. In a typical process rubber powder, with 1.6% sulfur, was treated with different species of Thiobacillus i.e. T. ferrooxidans, T. thiooxidans, T. thioparus in shake flasks and in a laboratory reactor. The sulfur oxidation depends to a large extent on the particle size. The best results were obtained with T. thioparus with a particle size of 100–200 mm. 4.7% of the total sulfur of the rubber powder was oxidized to sulfate within 40 days [122]. The effect of compounding chemicals on the microbial breakdown of vulcanized natural rubber by a Nocardia sp. has been investigated and found that increased amount of additives such as carbon black (filler) and cyclohexylbenzothiazole sulphenamide (CBS, an accelerator) as well as elemental sulphur increased the resistance towards microbial degradation [123]. The addition of these compounds is most certainly causing a decreased access for the microorganisms to the rubber matrix. 6. Devulcanization in supercritical carbon dioxide A new devulcanization process was developed in which supercritical carbon dioxide (scCO2) was used along with devulcanizing reagents [124-125]. Unfilled polyisoprene rubber vulcanizates with different crosslink distributions were prepared by controlling the cure time and the amount of curatives. Each of the vulcanizates was subjected to Soxhlet extraction using azeotropic acetone/chloroform to remove residual curatives. The devulcanization was performed at various temperatures (140–200oC) in the presence of scCO2 for 60 min. The product was fractionated into sol and gel components, and the molecular weight of the sol component and the crosslink density of the gel component were determined. The thiol–amine reagent was found to be the most effective one among several devulcanizing reagents; the molecular weight of the resultant sol component was determined and the crosslink density of the gel component decreased substantially from the initial ones. The yield of the sol component increased with increase in CO2 pressure. In the supercritical fluid state of CO2, the vulcanizate was more efficiently devulcanized than in an ordinary gaseous state of CO2. The sol fraction depended considerably on the crosslink distribution in the vulcanizate. These results suggest that the devulcanizing reagents penetrate and diffuse better into the vulcanizate in the presence of scCO2. 7. Comparative study of recent reclaiming processes Out of these reclaiming processes mainly three of them have been accepted by the industrial community for preparation of reclaim rubber from


waste rubber products, viz. ultrasound devulcanization, devulcanization by De-Link process and reclaiming by renewable resource material (RRM) and diallyl disulfide. A comparative evaluation of these three processes is presented in Table IX. Thus from the above table it is clear that very high Table IX. Comparative study of recent reclaiming processes [Reproduced with permission from Ref. 40].


temperature is required for ultrasound devulcanization. But Isayev and co-workers have not mentioned the effect of temperature only without ultrasound energy on devulcanization. But in De-Link process and RRM process nearly ambient temperature is required for reclaiming which is advantageous because at low temperature less energy is consumed for reclaiming and simultaneously polymer degradation can be minimized. For ultrasound devulcanization a special type of ultrasonic reactor is required whereas in De-Link and RRM process only internal mixer or two roll mixing mill is sufficient. Again sol content and molecular weight of sol obtained by ultrasonic devulcanization and RRM process are almost same. 8. Latex products reclaiming The latex industry expanded over the years to meet the world demand for examination gloves, condoms, latex thread, etc. Scrap latex products contain rubber hydrocarbon of very high quality, which is only lightly cross linked. These waste materials were modified into processable materials by novel economic processes and can be effectively utilized by reclaiming processes and may be blended with other polymers. Due to strict specifications for latex products, as much as 15% of the products are sometimes rejected, and these rejects create a major disposal problem for the rubber industry. At the same time, the local authorities prohibit open burning of this waste due to environmental pollution. As latex product waste represents a source of high-quality rubber hydrocarbon it is a good candidate for generating reclaimed rubber of superior quality. Thomas et al [126] reported a method for reclaiming latex products. In this process, waste condoms were powdered initially by passing them through a hot two roll mill at 80–90oC to a size of about 40 mesh. The powdered material was admixed with 10 phr of naphthenic oil and 1 phr of pentachlorothiophenol (PCTP) on a cold mill. The resulting compound was heated in an air oven at 140oC for 30 min. The reclaimed rubber obtained by this process was found to form a smooth band on the mill and contained about 82% of rubber hydrocarbon. However, it was found that only a small amount of this reclaim could be added to raw rubber without adversely affecting the mechanical properties. The addition of 25% reclaimed rubber to filled NR caused a decrease in tensile strength, elongation at break, resilience, tear strength and abrasion resistance. The compression set of the raw rubber was not much affected, but an increase in heat built up and hardness was observed. A compound containing this reclaim showed better processing characteristics.


The utilisation of cross-linked waste natural rubber as a potential filler in epoxidised natural rubber (ENR) is reported by Mathew et al [127]. The crosslinked waste rubber has been powdered and sieved into different particle sizes. They found that, as the filler content increases, the curing characteristics like optimum cure time, scorch time and induction time decrease. The cure-activating nature of the filler is clear from the increase in cure rate index and rate constant values. The filler helps the compounder by reducing the sticky nature of epoxidised natural rubber compound during mixing. These observations are advantageous as far as processability and productivity are concerned. In the case of the conventional vulcanization system, where sulphur migration is absent, finer filler shows superior tensile performance than size 4 and the mill-sheeted form of the filler. However, in efficient vulcanization systems, where sulphur migration plays a role, the order of performance is inverted. Anderson [128] patented the reclaiming of sulfur vulcanized latex in the presence of oil, water vapor and aryl disulfide peptizer at elevated temperature in the range of about 175–195oC and at a pressure in the range of about 230–260 psi for 1–4 h. Here aryl disulfide is a mixture of diphenyl disulfide, dicresyl disulfide and dixylyl disulfide. In another study diphenyldisulfide is found to be effective for the reclamation of WLR in a thermomechanical process [129]. A stronger reduction of the crosslink density was observed at temperatures of 170 and 180oC when reclaimed with diphenyldisulfide compared to hexadecylamine. In the present study all poly- and disulfidic cross links were broken during reclaiming with disulfide at the temperatures mentioned above, indicating that after reclamation the cross links present in WLR are mainly the monosulfides. Main-chain scission to crosslink scission studies showed that reclamation has mainly occurred through the scission of cross links rather than by main-chain scission. The crosslink distribution studies showed that some of the polysulfidic cross links remain in the sample after the treatment with hexadecylamine whereas no polysulfides were found in the samples treated with diphenyldisulfide. This again proves the fact that diphenyldisulfide more effectively breaks the polysulfidic cross links in latex vulcanizate compared to hexadecylamine. Hexadecylamine is found to be ineffective as reclaiming agent, because it results in the formation of additional cross links rather than reclaiming. 9. Applications of recycled/reclaimed rubbers

9.1. Uses of cryogenically ground rubber Cryogenically ground rubber is used in tires, hoses, belts and mechanical goods, wire and cable and in various other applications. It is especially useful


in producing a product for tire inner liners. The particle size chosen is controlled by cost and fineness needed to produce the desired processing. The finer the particle size, the smoother the calendared sheets and the finer an edge that can be produced on extrusions. 9.1.1. The TAK system The TAK System originated after its inventor. This process was developed by Takallou in 1986 [130]. In this process rubber modified asphalt concrete paving mixture is prepared by adding 3% by weight of fine and coarse rubber particles to a dense graded aggregate mixture. The rubber granulate is prepared from whole tire recycling. Perhaps one of the greatest potential markets for scrap tire generated rubber is as an additive to asphalt pavement. Ground scrap tire rubber can be added to hot mix asphalt in a variety of manners. The use of ground rubber in asphalt pavement is not meant as a means of disposal. Rather, the addition of scrap tire rubber improves certain characteristics of asphalt. In general, scrap tire rubber provides added flexibility, reduces cracking, enhances aging properties, aids in reducing ice formation, facilitates water removal from pavement and reduces road noise. Scrap tire rubber has been used in crack/joint sealers, surface/interlayer membranes, and as an aggregate substitution in hot mix binder. While the positive aspects of rubber-modified asphalt are numerous, it must be recognized that this technology has not been universally accepted by the paving industry. The average net yield of rubber from a used passenger car tire is about 12 lb (after steel and fabric removal). Hence five tires are required to obtain 60 lb of granulated tire rubber which is necessary for production of one ton of rubber modified asphalt concrete mix. Therefore, rubber obtained from 16,000 tires is consumed per mile in a two lane highway with 3 in. of rubber modified asphalt concrete pavement. 9.1.2. Ground rubber in civil engineering applications The civil engineering market encompasses a wide range of uses for scrap tires. In almost all applications, scrap tire material replaces some other material currently used in construction such as lightweight fill materials like expanded shale or polystyrene insulation blocks, drainage aggregate, or even soil or clean fill. A considerable amount of tire shreds for civil engineering applications come from stockpile abatement projects. Tires that are reclaimed from stockpiles are usually dirtier than other sources of scrap tires and are typically rough shredded. Rough tire shreds can be used as embankment fill and in landfill projects.


Figure 12. Road embankment constructed with shredded tires in El Paso, Texas.

Tire shreds can be used to construct embankments on weak, compressible foundation soils. Tire shreds are viable in this application due to their light weight. For most projects, using tire shreds as a lightweight fill material is significantly cheaper than alternatives. Other uses of tire shreds: subgrade fill and embankments (Fig. 12) include retaining forest roads, protecting coastal roads from erosion, enhancing the stability of steep slopes along highways, and reinforcing shoulder areas. Tire shreds are cost-effective substitutes for traditional materials when they are used to stabilize weak soil, such as constructing road embankments or as a subgrade fill. Additionally, tire shreds provide effective subgrade insulation for roads, walls and bridge abutments. Since its beginnings in the early 1990s, the use of scrap tires in civil engineering applications has had a roller coaster-like existence. Recently, new information has become available --information that should answer many of the doubts, concerns and uncertainties that previously limited the expansion of this market. This market for scrap tires now is poised to provide a myriad of possible uses that can consumer large quantities of scrap tires in a positive manner. Recycled scrap tires play a meaningful role in civil engineering processes, consuming 16 percent of the scrap tire available in 2005. A "civil engineering application" is the use of scrap tires in place of some conventional construction material such as clean fill, aggregate and rock. Scrap tires have been used as lightweight road embankment fill, backfill behind walls, insulation to limit frost penetration beneath roads (Fig. 13), aggregate in leachate and gas collection systems in landfills, and the drainage


Figure 13. Shredded scrap tires used as road base in Odessa, Texas. bed for residential septic system. Scrap tires usually are shredded for use in these applications, with the actual size a function of the intended use. The required size can range from a refined two-inch or three-inch square shred to a coarser, three-by twelve-inch shred. In northern climates, excess water is released when subgrade soils thaw in the spring. Placing a 6 to 12-inch thick tire shred layer under the road can prevent the subgrade soils from freezing in the first place. In addition, the high permeability of tire shreds allows water to drain from beneath the roads, preventing damage to road surfaces. Landfill construction and operation is a growing market application for tire shreds. Scrap tire shreds can replace other construction materials that would have to be purchased. Scrap tires may be used as a lightweight backfill in gas venting systems, in leachate collection systems, and in operational liners. They may also be used in landfill capping and closures, and as a material for daily cover. Some states-Alabama, Florida, Georgia, South Carolina, and Virginia-allow tire shreds to be used in construction of drain fields for septic systems. Tire-derived material replaces traditional stone backfill material, but reduces the expense and labor to build the drain fields. Tire chips can also hold more water than stone and can be transported more easily due to their light weight. In a related application, a study was conducted in Indianapolis, Indiana to evaluate the use of scrap tire shreds as replacement for stone in septic systems. The septic system consisted of two in ground trenches, three inches wide and 25 feet long, with shredded scrap tires placed six inches below and two inches above the gravity distribution pipe. One trench contained one inch nominal chips and other trench two inch chips. This system was loaded alternately each month from a three bedroom home. In alternate months the effluent was directed in a standard stone trench 150 feet in total length. The


system was installed in 1987, and since July 1989 water samples from the stone system and the tire system have been taken each month. Even though the application is one-third smaller in the tire system, results indicate that there does not appear to be significant differences between samples taken from the scrap tire and stone systems. Whole scrap tires can be used in the construction of artificial reefs and breakwaters. Artificial reefs are designed to prevent scouring, project coastal roads and provide habitat to aquatic life, such as filter feeders. Tire reefs are constructed by bundling punctured tires that have been weighed down with concrete and anchoring them to the ocean floor. The largest know scrap tire reef manufactured from approximately 12 million scrap tires is located in Florida. Other states with reef programs include California, Maryland, New Jersey, New York, Virginia and Washington. Breakwaters are used to reduce shoreline erosion. Scrap tire breakwaters are made by tying together tires with rubber strips and nylon bolts. Georgia and New Jersey both have scrap tire breakwaters and report no significant technical difficulties. Floating breakwater designs utilizing scrap tires date back to 1969 with the development the “Wave Maze”. Major advances took place in 1972 when the Goodyear Tire and Rubber Company refined the design concept. This modular concept has since been the most practical and most utilized design for floating breakwaters. In 1981, the Lorain Port Authority selected the Goodyaer design floating tire breakwater to control wave activity and expand recreational boating opportunities in the east harbor basin of the Port of Lorain (Ohio). Scrap tire material can be used in other various civil enginnering applications including roadway crash barriers and railway crossings. Three states, Alaska, Florida and Texas have reported using scrap tires in such applications without significant technical difficulties. Challenges to using tire shreds in drain fields include tire chip quality (tire chips must be clean cut and be of uniform size) and economics-in some areas, stone is abundant and cheap; tire shreds must be cheaper than stone to be used readily.In defining the various uses for scrap tires, civil engineering applications generally are considered to be those that require a shred no smaller than two inches in any dimension. Applications for smaller-sized, scrap tire derived material, such as the use of particulate rubber as a soil amendment, as a turf top dressing or as an additive to asphalt paving materials, are classified as ground rubber markets. The defining characteristic of a civil engineering application is that the tire-derived material produces a cost effective engineering benefit. In other words, civil engineering applications should not be used just to bury tires.


The use of scrap tires in civil engineering applications is based upon the unique characteristics of tire shreds, namely lightweight, good insulation properties, very high ability to transmit water, good long-term durability and high compressibility. With these properties, engineers can use tire shreds to solve many of the construction problems that cause them to lie awake at night, and just as important, tire shreds can save their clients money. Not to mention, applications also can consume a very large quantity of scrap tires. The first use of scrap tires in civil engineering applications can be traced back to the mid-1970, when they were used to build breakwaters and artificial reefs. Few scrap tires were used in civil engineering applications between the mid-1970's and 1991, perhaps two to three hundred thousand. (In comparison, more than 240 million scrap tires were generated annually in the U.S.). By 1992, tire shreds were being used in road embankments and being tested as a lightweight backfill for walls. These uses were offered as alternatives to the federal mandate for the use of rubber-modified asphalt, but neither was readily accepted by the highway construction community. The other uses of scrap tire include,

• Playground surface material • Gravel substitute • Drainage around building foundations and building foundation

insulation • Erosion control/rainwater runoff barriers (whole tires) • Wetlands/marsh establishment (whole tires) • Crash barriers around race tracks (whole tires) • Boat bumpers at marinas (whole tires)

The number of scrap tires going into civil engineering applications had increased to almost 10 million a year by the end of 1995. Estimates at that time about this market segment were that civil engineering applications would be consuming upwards of 15 million to 20 million scrap tires by 1997. Then came the burning roads. In December 1995 and January 1996, reports about hot spots in two road bed embankments built with scrap tire fill in Washington state and one wall with scrap tire backfill in Colorado were announced to the world. At first, it was steam emanating from vents in the embankments. Soon, there were reports of glowing embers within the embankments. Finally, flames were shooting out of the fills. This news spread; pardon the expression, like wildfire. The impact of these events was dramatic, immediate and profound. In quick order, the majority of civil engineering applications for scrap tires came to an abrupt halt. In 1996, the only major use of scrap tires in civil engineering applications was in landfill construction and operation. The market shrunk to five million tires.


9.1.3. The wet process This process (Arizona Refinery System) [134] was developed to overcome the problem of fatigue cracking in resurfaced asphalt pavements. The idea is based on using a composite material of hot asphalt cement with 1% by weight of total mixture of ground crumb rubber and diluted with an oil extender for ease of application. At elevated temperatures (300–4008F) for periods of one-half hour to one hour this reaction forms a thick elastic type material which is then diluted with 5% kerosene to aid in application. At room temperature this asphalt rubber composition is a tough rubbery and elastic binder material. The elastic quality of this mixture is most probably maintained by undissolved rubber particles that serve as units of elastic interference to the propagation of cracking. When a crack begins to propagate through the membrane then it encounters with elastic rubber particle and is stopped or its path of propagation is changed where it will encounter with another elastic rubber particle and so on. Thus propagation of crack is not possible. 10. Advantages of using reclaimed rubber Although reclaim rubber is a product of discarded rubber articles it has gained much importance as additive in various rubber article formulations. It is true that mechanical properties like tensile strength, modulus, resilience, tear resistances etc. are all reduced with the increasing amounts of reclaim rubber in fresh rubber formulation. But at the same time the reclaim rubber provides many advantages if incorporated in fresh rubber. 10.1. Easy breakdown and mixing time During reclaiming process reclaimed rubber has already been plasticized due to a large amount of mechanical working, Therefore, in the consumer’s hands it mixes easily than new rubber at lower mixing time with less heat generation. This is particularly advantageous with compounds containing high carbon black loading. In the mixing of tire carcas and side wall stocks also this property is very advantageous because during first banbury pass reclaim rubber is not added rather added during second banbury pass along with the curing agents to a position of the master batch obtained from the first banbury pass. The second pass is much shorter than the first, therefore, an increase in mixing capacity of as much as 40% occurs with a 30% banbury cost saving per pound of rubber. With increase in the ratio of reclaimed RHC to new RHC, the mixing cycle decreases. Furthermore, an all reclaim stock mixes in just one half the time required for an all new rubber stock.


10.2. Low power consumption during breakdown and mixing Reclaimed rubber consumes less power during breakdown and mixing than new rubber. Rubber Reclaimers Association has done a series of experiments to study the power saving during mixing with reclaim rubber. The first series compared whole tire reclaimed rubber with natural rubber and SBR 1712. Each was mixed with black, filler and oil in proportions to stimulate the composition of the reclaim. Banbury time was kept constant. The savings in power cost per 1000 pounds of reclaim were: 20% vs Natural Rubber 34% vs SBR 1712. The second series show that a mixture of SBR 1712 and BR (without any additives) plus a small proportion of reclaim rubber shows 12% less power consumption than by SBR 1712 alone and 14% less power consumption than for the combination of SBR 1712 1 BR, the mixing time being constant in all the cases. The third series shows that SBR 1712 alone, and SBR 1712 plus increasing proportions of tire reclaimed rubber up to 50% on RHC basis, result in increasing power savings for a constant mixing time. 10.3. Advantages in calendering and extrusion Reclaimed rubber stocks can usually be processed at a lower temperature than those containing virgin rubber alone. It provides generally faster processing during extruding and calendering. Due to the presence of crosslinked gel in reclaimed rubber, it is less thermoplastic than new rubber compounds. Thus when extruded and cured in open steam they tend to hold their shape better. Extruder die swell and calender shrinkage reduce with a proper use of reclaim rubber due to its lower nerve. Fresh rubber calendered sheets show 6–10% shrinkage. Using of reclaim rubber in tire carcass stocks permits high speed calendering and results in smooth uniform coating. The use of substantial proportion of reclaim rubber in automobile floor mat stocks permits maximum calender speeds which is sometimes twice as large as when very high proportions of SBR are used. Reclaim rubber in tire carcass compound gives better penetration in the fabric and chord than a non-reclaim compound. 10.4. Influence on curing and aging Reclaim rubber containing compounds help to retard and reduce sulfur bloom from both uncured and cured stocks. It cures faster than virgin rubber


compound, probably due to its combined sulfur and active crosslinking sites. Energy savings thus obtained constitute its usefulness in commercial purpose. During vulcanization reclaim rubber containing stocks show less tendency to revert indicating better aging resistance. Ball and Randall [131], Adhikari et al. [40, 106] and Dierkes [132] observed antiaging characteristics of reclaim rubber. Adhikari et al. observed around 90% retention of tensile properties of NR, SBR and NR-PBR reclaim rubber without using any antioxidant. As per Ball and Randall such aging resistance of reclaim rubber is due to the severe treatment of oxidation, heating, digestion and mechanical shearing which appear to stabilize the hydrocarbon against further changes. 10.5. Influence on tack behaviour The tack of a non-reclaim compound may disappear within 24 h after calendaring whereas, reclaim rubber compound tend to maintain their tack longer than non reclaim compound. Non-reclaim compounds become more tacky in hot weather and dry in cold weather. On the other hand, reclaim rubber compounds are less influenced in tack variation in hot and cold weather. This characteristic of reclaim rubber is exploited for its usefulness in pressure sensitive tape. 10.6. Cost and energy savings Finally, it may be stated that incorporation of reclaim rubber into new rubber compound, not only reduces the cost of the finished product but also saves our united resource of fossil feed stock. Energy consumption in reclaim production from truck treads is 0.09 l of oil equivalent/kg and 0.12 l equivalent/kg from whole tire. These data show that negligible amount of energy in terms of oil equivalent is consumed for reclaim production. Energy consumption in the tire production is 25 l of oil equivalent/tire. But much less energy is consumed in the production and utilization of recycled rubber products than direct production of rubber articles from the virgin raw materials. Conclusions remarks The increase in the awareness of waste management and environment-related issues has led to substantial progress in the utilization of rubber waste. Recycling materials back into its initial use often are more sustainable rather than finding new applications. This paper has presented various aspects of the reclamation and the reuse of different kinds of rubber products. It is expected that the newly developed rubber recycling technologies described in this


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2. recent advances in the recycling of rubber waste

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