11

Biotechnological Processesfor Recycling of RubberProducts

Dr. Katarina Bredberg1, Dr. Magdalena Christiansson2, Dr. Bengt Stenberg3,Dr. Olle Holst4

1 Department of Biotechnology, Center for Chemistry and Chemical Engineering,Lund University, P.O. Box 124, 221 00 Lund, Sweden; Tel:�46-462224626,Fax:�46-462224713, E-mail : katarina.bredberg@biotek.lu.se

2 Department of Biotechnology, Center for Chemistry and Chemical Engineering,Lund University, P.O. Box 124, 221 00 Lund, Sweden; Tel:�46-462224626,Fax:�46-462224713

3 Polymer Technology, Royal Institute of Technology, 100 44 Stockholm, Sweden;Tel:�46-87908269, Fax:�46-8208856, E-mail: stenberg@polymer.kth.se

4 Department of Biotechnology, Center for Chemistry and Chemical Engineering,Lund University, P.O. Box 124, 221 00 Lund, Sweden; Tel:�46-462229844,Fax:�46-462224713, E-mail : olle.holst@biotek.lu.se

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3621.1 Grinding of Rubber Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3641.2 Recycling of Rubber Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3651.3 Biotechnological Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

2 Historical Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

3 Rubber Products to which Biotechnological Recycling is Applied . . . . . . . . 3673.1 Microbial Degradation of Rubber . . . . . . . . . . . . . . . . . . . . . . . . . . 3673.2 Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3693.2.1 Cleavage of the Hydrocarbon Chains . . . . . . . . . . . . . . . . . . . . . . . . 3693.2.2 Desulfurization of the Rubber Material . . . . . . . . . . . . . . . . . . . . . . 3693.3 Microbial Detoxification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

4 Current State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

5 Rubber as a Carbon Source for Fermentative Processes . . . . . . . . . . . . . . 371

6 Putative Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

361

7 Outlook and Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372

8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

CGTR cryo-ground tire rubberCBS cyclohexylbenzothiazole sulfenamideSSSE solid state shear extrusion

1

Introduction

Rubber material as we know it is a materialwith properties that differ widely from thoseof the viscous sap of the rubber tree, Heveabrasiliensis. Elasticity and extensibility are thefeatures most characteristic for rubber (Blowand Hepburn, 1982; Brydson, 1988; Franta,1989; Gent, 1992). To gain these desirablerubberyproperties±whichmakeitpossible toproducetires,hoses,andawiderangeofotherrubber products ± it is necessary to vulcanizethe raw rubber. By adding a few percent ofsulfur and heating the mixture under pres-sure, sulfur bridges will be formed betweenthe hydrocarbon chains (Figure 1). Thisirreversible reaction will produce a materialwith unique mechanical and thermoelasticproperties, which is stable over a widetemperature range. Unfortunately, the mate-rial will also be almost impossible to reshape.

The synthetic rubbers differ from naturalrubber, and from each other in chemicalstructure as well as physical and chemicalproperties. The molecular weight of naturalrubber is several millions, while the molec-ular weight of synthetic rubbers is muchlower. Rubber materials can also gain differ-ent properties by adding for example accel-erators, activators, retarders, and fillers. Inthis way, a wide range of properties can beobtained. The chemical structures of naturaland synthetic rubbers are shown in Figures2±4.

In 1999, the natural rubber consumptionworldwide was 6.7 million tons, while syn-thetic rubber consumption was 10 million

11 Biotechnological Processes for Recycling of Rubber Products362

Fig. 1 Sulfur cross-links between rubber molecules.The exact structures of the sulfur bridges are notknown, though several different suggestions havebeen presented (Roberts, 1988). The molecularweights between the cross-links (Mc) vary between500 and 20,000, depending on the cross-link density.

Fig. 2 Chemical structure of natural rubber, poly-cis-1,4-isoprene.

Fig. 3 Chemical structure of butadiene rubber con-sistingof 1,4-butadiene monomers and1,2-butadienemonomers.

Fig. 4 Chemical structure of styrene-butadiene rub-ber consisting of styrene and butadiene monomers.

tons (IRSG, 1999). The ratio between naturaland synthetic rubber consumption has in-creased by a few percent during the pastdecade. The major part of the producedrubber is used in the manufacture of tires,but different areas of rubber use are shown inFigure 5.

As a consequence of the difficulties inreusing rubber material and of the wide-spread use of rubber products (mainly in the

form of tires), huge numbers of used tires arestockpiled all over the world, with 2 ± 3 billiontires currently estimated to be stockpiled inthe US alone (Jang et al., 1998). Those tons ofmaterial could, instead of being environ-mental and health threats, provide a greatasset of raw material, if efficient methods forrecycling rubber could be developed. Theproblem with the reuse of rubber is caused bythe strong sulfur bridges that occur betweenthe hydrocarbon chains, these making itimpossible to melt and reshape the materialas can be done with thermoplastic andthermoelastic materials. In general, a priorityorder for decreasing the amount of wasteshould be: (1) reduction of consumption; (2)reuse of the product; (3) recycling of materi-als; (4) energy recovery; and (5) as a lastpossibility, deposition of the waste. In prac-tice, themostenvironmentally friendlyway toreuse scrap tires is of course dependent uponthe costs, energy demands and pollution thatsuch reuse creates. In part, the reduction ofconsumption and reuse of the product is, inthe case of scrap tires, obtained by retreading.

A wide range of different methods forreusing and recycling tires and rubber mate-rial in general has been developed over theyears (Figure 6). The most common method

1 Introduction 363

Fig. 5 Global synthetic rubber use in 1990. Totalsynthetic rubber consumption in 1990 was 7.1 milliontons (Schnecko, 1998).

Fig. 6 Use of scrap tires worldwide in 1996 (Scheirs, 1998).

is to burn the tires in cement kilns and powerplants as tire-derived fuel, or to use them asland-fill or as artificial reefs, for example.Ground rubber powder is used as fillingmaterial in a wide range of applications,together with different matrices such as newrubber material, plastics and cement. Shoesoles, plant pots and swings for children'splaygrounds are other possible applicationareas, though at present these are limited innumber.

1.1

Grinding of Rubber Material

Some type of size reduction is a prerequisitefor most applications of waste rubber materi-al, such as revulcanization with fresh rubbermaterial, blends with other plastic materials,applications in civil engineering, soil im-provements, running tracks and even energyrecovery. Several such methods have beendescribed (Astafan, 1995; Klingensmith,1991; Kohler and O'Neill, 1996; Kohler andJackson, 1998). In general, several steps areneeded in the grinding process. The tires arecut or chopped into large pieces, and these arethen ground to particles of millimeter sizeduring the removal of textile, dirt and metals.For special applications, crumbs or particles<1 mm are produced. The requirements ofenergy increase drastically with decreasingparticle size. The energy consumption con-stitutes themajorpartof thecost in therubbergrinding process. When using crumb rubberin the manufacture of new rubber products,the price of the ground rubber should bearound half the price of raw rubber materialor less, in order to make the process cost-efficient.

Cutting and grinding of the rubber materi-al requires the use of special equipment dueto its elasticity. Most processes are based on acombinationofcuttingandtearing.However,a general problem of grinding technology is

the heat build-up in the rubber material,which means that cooling has to be applied inall grinding techniques. Cutting with hy-draulically agitated knives is used to producerubber chips of a large size. Shreddingequipment is mainly used for rough grind-ing, the device consisting of low-speed rotat-ing shafts with overlapping knives (Astafan,1995). Granulators or hammer mills areequipped with rotor knives or rotating ham-mers, the counterparts being stationaryknives and a grate, respectively. These cangenerate further size reduction. The wasterubber is exposed to large shear forces duringgrinding, and these ± together with the heatbuild-up ± leads to the problem of surfacedegradation. Ground particles tend to re-agglomerate, which further limits through-put. Water slurries of rubber particles and aflour-grinding mill can be used to produce awet ground rubber, an advantage of which isits uniformity and cleanliness (Klingensmithand Baranwal, 1998).

Another way to avoid degradation is to coolthe rubber under the glass transition temper-ature and then apply impact forces; this leadsto cracking of the brittle material. Liquidnitrogen has to be used as cooling mediumas the temperature required is ±70 8C to±130 8C. Cryo-grinding (as this procedure iscalled) generates a powder with particle sizesdown to 0.2 mm which can be producedwithout degradation and oxidation of thesurface. Easier fiber and metal liberation isalso reported (Astafan, 1995; Klingensmith,1991; Kohler and O'Neill, 1996; Kohler andJackson, 1998). The surface structure ofground rubber differs between cryo-grindingand ambient milling. Cryo-ground rubberhas a smooth surface with relatively littlesurface area as compared with the `cauli-flower' structure obtained with ambient mill-ing processes (Kohler and Jackson, 1998).Considerations on how to handle spentmaterial and a cryo-grinding process have

11 Biotechnological Processes for Recycling of Rubber Products364

beendescribed byLiaskos (1994).Cryo-grind-ing isarelativelyexpensivemethod,duetothecost of the liquid nitrogen. Another approachof interest is solid state shear extrusion(SSSE). Waste rubber chips are fed to acooledco-rotating twin-screwextruder,whichpermits pulverized material of controlledparticle size (down to 0.05 mm) to be ob-tained (Khait, 1997).

1.2

Recycling of Rubber Material

One way to recycle rubber material is to shredit, grind it, mix the crumbs with virgin rubber,and then to vulcanize it to new rubberproducts. Rubber materials for certain appli-cations, e.g. the tread on off-the-road tires,contain a few percent of recycled groundrubber which, in addition to giving a cost-reducing effect, also increases stiffness andtear strength. Furthermore, the ground rub-ber improves mixing and processing charac-teristics, facilitates the removal of enclosedair, and is an efficient way to control die swellin extrusion operations (Franta, 1989; Man-uel and Dierkes, 1997). However, the materi-al's mechanical properties, such as tensilestrength and elongation at break, declinedrastically when the concentration of groundrubber is higher than a few percent (Asplund,

1996). The reason for this decline is probablythe low cross-linking in the interface betweenthe virgin material and the ground spentmaterial, that is due to a low molecularinteractionbetweenthepolymerchains intheinterface. Only a few new sulfur bonds arethen formed during the vulcanization proc-ess (Myhre and MacKillop, 1996). Anotherreason could be that sulfur migrates from thenew rubber matrix in to the ground rubberparticles during vulcanization. This makesthe new rubber matrix less cross-linked(Gibala et al., 1999). The fracture areas oftest pieces of rubber containing cryo-groundtire rubber (CGTR) have been studied withscanning electron microscopy (SEM). Thosepictures clearly show that the rubber crumbsare discrete particles in a surrounding matrix(Figure 7).

Different methods for surface activation ofthe rubber crumbs have been developed toenhance the interactionsbetweenthecrumbsand the new rubber matrix, and in this wayimprove the mechanical properties. Thoseinclude mechanical, chemical and a fewbiotechnological methods.

Other ways to recycle rubber include, forexample, biotechnological processes such asmicrobial degradation of rubber into prod-ucts that are in one way or another useful.There are generally some specific advantages

1 Introduction 365

Fig. 7 Scanning electron micro-graph of fracture area of rubbermaterial containing 15% cryo-ground tire rubber (original mag-nification, �150).

with biotechnological processes comparedwith chemical and physical processes. Bio-technology does not include any harmful ortoxic chemicals, and is normally not energyintensive. Furthermore, the specificity ofmicroorganisms and enzymes results in lessunwanted degradation of the material. How-ever, there are also some drawbacks, mostnotably that microorganisms are sensitivetowards many chemical substances, includ-ing rubber additives. When manufacturing atire for example, many different substancesare added to the polymer to obtain a productthat is stable and functional over a widetemperature range and in different applica-tions. Except for sulfur, a vulcanizationsystemalsoconsistsofaccelerators, activators(e.g. zinc oxide and stearic acid), and retard-ers.Antidegradants, processingaids,mineraloil, pigments and fungicides are also added.Most of those additives are toxic towardsmany microorganisms. Some additives, forexample fungicides, are added because of justthat property, i.e. to prevent microbial attackof the product during use.

Zyska (1981) suggests that, in the manu-facture of vulcanized rubbers, compoundingingredients which have a satisfactory level ofmicrobiological resistance must be selected.Finding ingredients that are resistant tomicroorganisms implies fewer difficultiesthan finding an adequate fungicide forrubber. Such compounding ingredients willcause problems when biotechnology is to beused for rubber recycling.

1.3

Biotechnological Processes

Biotechnological processes can be definedvery generally as ªcontrolled and deliberateapplication of simple biological agents ±living or dead cells or cell components ± intechnologically useful operationsº (Bu'Lockand Kristiansen, 1987). They have also been

described as ªthe integration of naturalscience and engineering science in order toachieve the application of organisms, cells,parts thereof and molecular analogues forproducts and servicesº (European Federationof Biotechnology, 1995). These kinds ofprocesses include such diverse operationsas the production of animal feed, citric acidand antibiotics, waste treatment, water puri-fication and agricultural plant improvement.When designing a biotechnological process,one must consider a variety of differentmethods and reactor designs, for examplewhether to use an aerobic or anaerobicprocedure; to use whole cells or pure en-zymes; to use a continuous reactor or a batch-reactor. These different procedures have theirspecific advantages and drawbacks depend-ing on the process to be accomplished.

2

Historical Outline

Rubber has been used by man for thousandsof years (Blow and Hepburn, 1982). Colum-bus and his crew were the first Europeans toencounter rubber material when they discov-eredSouthAmerica inthe1490s.At that time,the material was already well known and usedby the Indians, not only as rubber balls fordifferent games but also to make bottles,shoes and waterproof hats. Despite this earlyencounter, it took many years before rubberbecame widespread outside South America,and it was not until the middle of thenineteenth century that scientists first beganto experiment with rubber. This led to thedevelopment of a variety of different rubberproducts, including the rubber eraser, whichwas invented by Joseph Priestly in 1770 andconsidered at that time to be an expensiveluxury. In fact, the name `rubber' was derivedfrom its use as an eraser.

11 Biotechnological Processes for Recycling of Rubber Products366

The problem with rubber at this time wasthat the material was sticky and impervious.This made the use of rubber products rela-tively limited until Charles Goodyear hap-penedto leaveapieceofrubbersprinkledwithsulfur on a warm oven in 1839. The result ofthis was, to his surprise, a stable non-stickymaterial, which in the next few years becameone of the most used materials in the world.Fifty years later, Dunlop invented the pneu-matic tire, which is at present the mostimportantrubberproduct.DuringWorldWarI, Germany began to produce syntheticrubber, and they developed the techniquefurther during the 1930s. Ten years later,during World War II, the USA was forced tostart producing synthetic rubber due to theJapanese occupation of the rubber planta-tions. Today, about 60% of the manufacturedrubber is synthetic.

When it comes to the recyclingofpolymers,rubber lags significantly behind the muchmore recently developed thermoplastic ma-terials. This is in spite of the fact that rubberhas been widely used, and recycled, for over150 years (Rader et al. , 1995). The use ofrecycled rubber was more prevalent 30 yearsago than it is now. During the 1960s, recycledrubber constituted about 20% of the rubberindustry's raw material. However, due to themore widespread use of synthetic rubber,production liability concerns, and the intro-duction of the steel-belted radial tires, the useof recycled rubber has fallen to~2%. In recentyears, significant technical innovations havebeen made in the tire-recycling field, and itwas only between 1994 and 1996 that the useof tire-derived fuel increased by 70%. The useof ground tires in the production of newrubber products has increased from 4.5million tires to 10 million tires, while thenumber of tires used for civil engineeringpurposes has been constant over those years(Scheirs,1998).During theyearsofexpansiveuse of rubber products, there has always been

the problem of microbial deterioration, andextensive studies have been carried out aimedat its prevention. One idea is to use microbialdeterioration to dispose of rubber material,and perhaps even to gain a useful end productas a result.

3

Rubber Products to which BiotechnologicalRecycling is Applied

Thenumberofbiotechnologicalprocesses forrecycling of rubber material is still verylimited. Recycling through microbial desul-furization is today probably the most inves-tigated option in this area, but is yet not incommercial use (see Chapter 10, `Biotechno-logical Desulfurization of Rubber Products').Biotechnological methods in general presentpromising prospects for finding solutions tothe future demands of increased rubberrecycling.

3.1

Microbial Degradation of Rubber

Microbial deterioration of rubber productshas attracted much interest, and many stud-ies have been carried out on the degradationof both pure rubber elastomers and vulcan-ized rubber products. The main purpose ofthose studies has been to reduce rubberdeterioration during use of rubber products,but the degradation of rubber products mightrepresent a possible opening in the search forefficient biotechnological methods to recyclerubber products.

In 1955, Rook wrote a survey of theprincipal publications on microbial attackon rubber. He concluded that the data con-cerning degradation of vulcanized rubber inmost cases are very vague and should beinterpreted with great care (Rook, 1955). Thesame conclusions were drawn by Heap and

3 Rubber Products to which Biotechnological Recycling is Applied 367

Morrel (1968) some years later. Several differ-ent rubber-degrading microorganisms havebeen isolated. Heisey and Papadatos (1995)isolated 10 strains belonging to the generaStreptomyces, Amycolatopsis and Nocardia,which reduced the weight of rubber latexgloves by more than 10% in 6 weeks.Jendrossek and co-workers (1997) used nat-ural rubber latex as sole source of carbon andenergy to find 50 isolates of rubber- degradingbacteria. A total of 1220 strains from differentculture collections was also screened, andrevealed 46 positive strains; both their ownisolates and strains from the culture collec-tions were Actinomycetes. Linos and Stein-büchel (1998) isolated a bacterium belongingto the genus Gordona that used rubber as solecarbon source. Tsuchii and coworkers madeextensive studies on rubber degradation andstudied the degradation products from sim-ple natural rubber materials after treatmentwith Nocardia 835 A. They also found that anincreased amount of carbon black, sulfur andaddition of the accelerator cyclohexylbenzo-thiazole sulfenamide (CBS) increased theresistance towards microbial degradation,and that the shape of the particles is ofimportance for degradation. With Nocardiastrain 835 A, a 51% weight loss of truck tirewas obtained when pieces of truck tires wereincubated with pieces of rubber gloves ascosubstrate,but thedegradationratewasslow(Tsuchii et al., 1985, 1990, 1996, 1997). Thecommon observations reported in the differ-ent studies are that the isolates capable ofrubber degradation generally belong to Acti-nomycetes, and that natural rubber is moreeasily degraded than synthetic rubbers.

Linos and Steinbüchel (1996) studied thegrowth of adapted isolates on natural andsynthetic rubber elastomers. The growthbehavior of one isolate indicates productionof biosurfactants. Tsuchii and Takeda (1990)reported on extracellular rubber-degradingenzymes from the Gram-negative bacterium

Xanthomonas sp. strain 35 Y. The enzymeswere shown to degrade natural rubber latex tocreate the same degradation products asNocardia strain 835 A. When more complexrubber materials were tested, little successwas obtained. Linos and coworkers (2000)isolated the first Gram-negative bacteriumexhibiting strong rubber-decomposing prop-erties. Interestingly, the most efficient deg-radation has been obtained with naturalrubber and almost only with low-wear prod-uctsorrubberswith lowcomplexity.Thereareprobably two explanations for this: (1) thatmicroorganisms find it difficult to degradecomplex high-molecular weight compounds;and (2) the addition of toxic compounds to therubber materials. Cundell and Mulcock(1973) investigated the effect of curing agentconcentrationsonthemicrobialdeteriorationof vulcanized rubber, and concluded that thecross-link densities did not affect the deteri-oration rate, while the nature of the cross-links did affect the rate of microbial deterio-ration.

Loomis and coworkers (1991) used con-trolled microbial chemostats to evaluate thepolymer degradation with emphasis on plas-tics, while Steinbüchel and Linos (1997) usednatural and synthetic polyisoprenes as modelcompounds for studies on microbial actionon rubber materials. Keursten and Groene-velt (1996) concluded after a series of experi-ments with unknown microorganisms insoil-rubber mixtures, that the degradation ofrubber particles in soil could be describedby first-order kinetics. Williams (1986) hassuggested that microorganisms preferential-ly attack the stearic acid content in vulcanizedrubber, which is an interesting idea and mustbe considered in the efforts to prevent rubberdeterioration.

The time scales for microbial deteriorationin the reports mentioned above are weeks andlonger. Tsuchii and coworkers (1985) report-ed a 100% rubber weight loss of unvulcanized

11 Biotechnological Processes for Recycling of Rubber Products368

natural rubber in 8 weeks, but only 7 ± 17%weight loss of tire tread in the same time.Linos and Steinbüchel (1998) noted a weightloss of 20% after 20 weeks of incubation. Thisindicates that efficient methods for degrada-tion of rubber products such as tires on a largescale are still far from being applicable on aregular basis.

3.2

Surface Modification

A wide range of different methods for surfacemodification of ground rubber particles hasbeen suggested over the years. Most of theseare chemical or physical methods, but somebiotechnological approaches have been pro-posed. The purpose of surface modification isto increase the interactions between groundrubber particles and a new matrix whenrubber crumbs are mixed with a new materi-al. This is done to improve the properties ofmaterials that consist partly of ground rubbermaterial.

3.2.1

Cleavage of the Hydrocarbon ChainsA great number of microorganisms havebeen studied in the context of degradation ofrubber (see above). Most studies were aimedat preventing rubber deterioration, though itmight be possible that cleavage of the hydro-carbon chains on the surface of rubbercrumbs can enhance the interaction with asurrounding matrix material. Limited cleav-age of the hydrocarbon chains on the surfaceof vulcanized rubber could make the chainsmore flexible, and a larger number of unsa-turated bonds would be available to form newbonds with the surrounding material. Thiscould for example apply to CGTR in a newmatrix of unvulcanized rubber material orother polymers. Romine suggested that sur-face-modified rubber can bind with polarspecies in asphalt to establish a rubber-

asphalt phase (Romine and Snowden-Swan,1993)

3.2.2

Desulfurization of the Rubber MaterialThe possibility of enhancing the interactionsbetweenoldgroundrubbercrumbsandanewrubber matrix by desulfurization of thevulcanized rubber hasbeenstudied by severalgroups (Torma and Raghavan, 1990; Löffleret al. , 1993; Christiansson et al. , 1998; Ro-mineandRomine,1998;KimandPark,1999;Bredberg et al. , 2001; Bredberg et al. , inpreparation). The principal effects would beabout the same as for cleavage of the hydro-carbon chains, but possibly more efficient.Cleft sulfur bonds make the hydrocarbonchains more flexible and a produce a largernumber of active sites available for newbonding, but do not affect the quality of thepolymers. This topic is extensively discussedin Chapter 10.

3.3

Microbial Detoxification

The largest single rubber product ± tires ±normally contain a wide range of differentrubber additives such as accelerators, retard-ers, and antioxidants, and this applies also tomany other rubber products. Many of therubber additives used in different applica-tions have been shown to be toxic to micro-organisms (Zyska et al. , 1971; Christianssonet al. , 2000;Bredberget al. , 2001). Ithasbeenshown that most toxic rubber additives can beleached from the rubber with organic sol-vents, but from an environmental viewpointthis is not the best solution. This situation hasalso become a problem in other applicationsofusedtires.Forexample, therearereasons tobelieve that artificial reefs made from usedtires pose a threat to the aquatic environment,while ground tires incorporated into road-ways and athletic tracks may leak toxic

3 Rubber Products to which Biotechnological Recycling is Applied 369

substances (Evans, 1997; Galbraith andBurns, 1997). This is of course a majorobstacle in the biotechnological processesthat include rubber material, but it might be aproblem that can be overcome with microbialdetoxification. The possibility of using micro-organisms capable of degrading for examplearomatic substances in order to detoxify usedrubber has immense appeal. Rhodococcus, forexample, is reported to be able to metabolizearomatic compounds and a wide range ofother compounds (Warhurst and Fewson,1994). Extensive studies have been carriedout with different species of fungi in the areaof soil remediation, but there are no reports

about this application in the context of rubberrecycling.

Some possible ways to reuse rubber mate-rial are shown in Figure 8.

4

Current State

At present, a very limited number of bio-technological processes exist for the reuse ofwaste rubber material. A number of patentsexist for the biotechnological desulfurizationofrubbermaterial,but toourknowledgenoneof these processes is currently in commercial

11 Biotechnological Processes for Recycling of Rubber Products370

Fig. 8 Schedule of possible biotechnological methods for reuse of rubber material.