Introduction

Nowadays, plastics and rubbers are extensively used in different fields and thus the recent period is termed sometimes “Plastic Age.” Because of polymeric materials do not decompose easily, the disposal of their wastes causes a serious environmental problem. The recycling (reutilization) of post-consumer and post-industrial polymeric wastes poses a great challenge. This note holds especially for worn (discarded, scrap) tyres. The matrix material of tyres is composed of vulcanized (crosslinked) rubbers which cannot be reprocessed owning to their crosslinked structure. It is, therefore, of paramount importance to develop technologically feasible and cost-effective methods for recycling of rubber from scrap tyres. The best way would be to devulcanize the rubber and reuse it in new rubber products. Although such processes are worked out, they often remained in preindustrial/precommercial stage, mostly due to economic reasons. Alternative solutions, following the material recycling routes “back to the feedstock”, focus on the production of oils (to be used as plasticizer, extender in rubber mixes) and carbon black (CB) [1]. The latter can be used as reinforcement and coloring additive in various polymers and rubbers.

The most straightforward option is, however, to combine (to blend) particulate rubber with a polymeric material having the ability to flow under certain conditions (supported usually by the action of heat and/or pressure), so that it can be shaped into products at acceptable cost [2]. This can be achieved using thermoplastics, thermosetting resins and rubber compounds as potential matrices.

Next, an overview will be given on how ground tyre rubber (GTR), with and without surface treatment, can be used in thermoplastic, thermoset and rubber formulations. This treatise will cover also other ground scrap rubbers when the related recycling strategy can be adapted for GTR. It is the right place to underline that GTR is termed differently in the literature. The other widely used terms are: particulate (tyre) rubber, scrap (tyre) rubber, crumb (tyre) rubber, rubber granulate, rubber powder, size-reduced rubber, pulverized rubber.

Worn tyres—quantity, composition

The world consumption of rubber (natural and synthetic) in 2010 was 24.845 kt. Most of this material (~ 65 %) is used in the tyre industry, from which, sooner or later becomes waste [3, 4].

In the 27 member states of the European Union (EU) completed with Norway and Switzerland, 3273 kt post-consumer tyres were accumulated during 2010. The ranking of states according to the quantity of the arisen used tyres is shown in Fig. 1.

Fig. 1
figure 1

The quantity of used tyres arisen in the EU countries completed with Norway and Switzerland in 2010 [5]

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Tyres have a four-phase life cycle: new, part-worn (road-worthy tyres), retreadable (casing suitable for retreading), and recyclable.

The composition of tyres for passenger cars (7.5–9 kg) and trucks/buses (50–80 kg each) is summarized in Table 1.

Table 1 Composition of tyres in the EU (wt%) [6]
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The major disposal routes for post-consumer tyres in the EU are depicted in Fig. 2.

Fig. 2
figure 2

Average breakdown in post-consumer tyre disposal in the EU [7]

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The export of part-worn tyres is directed toward less developed countries. Retreaded tyres meet the same standards as new ones. Note that during retreading 2–3 kg fresh rubber is applied to rebuild a tyre, the rubber content of which is 48 % for a passenger and 43 % for a truck tyre. Energy recovery covers apart the use of tyres as non-fossil fuel (in cement kilns, paper mills, electricity generation plants) also energy recovery/material recycling (pyrolysis, production of synthesis gas, etc.) [7–9]. In the latter case, the products (oils, aromatics, gaseous products, steel, ZnO, and CB) are marketable. Land filling is no more a viable option in the EU as the Landfill Directive (1999/31/EC) forbids the disposal of entire tyres from 2003 and shredded ones (size between 50 and 300 mm) from 2006.

Material recycling is strongly favored due to several reasons

  • Legislative actions 2000/53/EC End of Life Directive requires that from 2006 80 % and from 2015 85 % of all cars should be recyclable. This has a strong impact on rubber products built in passenger cars. Note that a compact car contains ca. 60 kg rubber; the major part (ca. 70 %) is given by tyres [9]. A further EU directive, viz. Incineration of Waste 2000/76/EC, also supports the material recycling via lowering the emission standards of cement kilns and the like.

  • Energy balance the energy equivalent of 1 kg of tyre is ca. 128 MJ. Its recovery by energy generation yields, however, only 30 MJ. On the other hand, ca. 6.8 MJ of additional energy is required to produce 1–1.5 kg GTR [10]. This is likely the major driving force to develop advanced grinding techniques for worn tyres.

Material recycling of post-consumer tyres goes in two directions

  • Civil engineering is using greater quantity and larger pieces, such as whole tyres, shred, and chips (particle size between 10 and 50 mm) in various applications (sound deadening, insulation, artificial reefs, soil stabilization, dam/road constructions, etc.).

  • Product/material manufactures are using smaller quantities (at the present) and smaller pieces (granulates lying in the range of 0.5–15 mm and powders with a particle size <0.5 mm).

Some of these applications often appear under the heading “downcycling” as the recycled product has a different application than the initial one (i.e., tyre) [9]. Nowadays under “downcycling” less demanding, inexpensive applications are meant whose costs are controlled by the rubber granule or powder themselves. Further, the related applications have been saturated with the major exception of road paving.

Accordingly, there is a considerable demand to find new, value-added use for GTR fractions.

Worn tyres—recycling

Recycling of tyres may follow different ways such as [11]:

  • retreading (truck and passenger tyres)

  • use of tyres as a whole (artificial rafts, cover foil weights) or in parts (building blocks)

  • grinding

  • pyrolysis (to oils, monomers, CB)

  • reclaiming (decrosslinking for mixing into fresh rubbers).

Note that landfill disposal is not even mentioned above being environmentally problematic (leakage of pollutants, breeding grounds for rats, mosquitoes) [12, 13] and thus in many countries banned.

Our survey deals mostly with grinding and reclaiming as they represent the major material recycling options. Moreover, they are often combined to guarantee the necessary compatibility between GTR and matrix polymer as discussed later. Rubber recycling usually concentrates on discarded tyres.

This is due to the fact that almost 70 % of all natural and synthetic rubbers are “consumed” in the tyre manufacture [11, 14]. The related recycling methods were already a topics of review articles [14–16] and book chapters [17, 18] which are, however, dated back by one decade.

In the EU, the Revised Framework Directive on Waste 2008/98/EC focuses on the EU becoming a “recycling society” whereby considering tyres as a target with specific criteria for end of waste status [5].

Ground tyre rubber (GTR)

Production, properties

As underlined before, grinding (size reduction) is the preferred recycling route for waste tyres being associated with obvious economic and social benefits [16].

To convert the whole tyre into GTR the related technology comprises the following steps: shredding, separation (steel, textile), granulation, and classification. GTRs are produced by mechanical grinding at ambient temperature, at ambient temperature under wet condition, at high temperature and at cryogenic temperature [11, 15]. Prior to grinding to higher mesh sizes, i.e., smaller particle sizes, the tyre is cut into relatively large and then shredded into smaller pieces. Ambient grinding is usually practiced in two-roll cracker-type mill. Though termed “ambient” the temperature may rise up to 130 °C during milling [11]. The achievable particle size and particle size distribution of GTR depend on the milling sequences and mill type. By finishing mills a mean particle size of ca. 200 μm can be reached. Under wet (ambient) grinding the crumb rubber is cooled by water spraying. Afterward, water is separated from the GTR and the latter is dried [14].

High-temperature grinding (T ~ 130 °C), accompanied with substantial devulcanization is seldom followed. This method results in granules of 1–6 mm. This limitation is given by the viscoelastic nature of rubber along with its low heat conductivity. In order to reduce the temperature loading of the rubber during grinding water may be added. This wet process may produce GTR of <100 μm mean particle size.

Cooling the rubber below its glass transition temperature (which is type dependent and lies between −30 and −80 °C) the energy needed for grinding can be substantially reduced. At this cryogenic grounding, the “frozen” pieces pass an impact-type mill causing their shattering. The GTR is dried afterward; fibers and metals are separated and then classified into the required mesh sizes [11, 14, 15]. Unlike ambient grinding, the surface of the cryogenic GTR is not “oxidized”. The cryogenic process results in “cleaner” GTR in terms of fiber and steel residues than the ambient one. Major benefit of the ambient grinding is that the surface/mass ratio of the GTR is almost double that of the cryogenic one at equal mesh size [11]. On the other hand, the cooling via liquid nitrogen is very costly.

Figure 3 shows the differences between GTR powders produced by different grinding methods. With an ambient mechanical grinding relative high specific area can be achieved (Fig. 3a, d). At ambient temperature, water-jet process can also be applied (Fig. 3b) resulting in higher area/volume ratio. Needless to say, specific area has a great influence on the adherence of GTR to a given matrix polymer. Cryogenic GTR particles are “edgy” with flat surfaces (Fig. 3c, e) compared to the rough, irregular-shaped ambient ground powders.

Fig. 3
figure 3

SEM pictures of GTR produced in different ways: ambient (mechanically ground) (a) [19]; ambient (ground by high pressure water) (b) [19]; cryogenically ground in pin mill (c) [20]; ambient ground in rotary mill (d) [20]; cryogenically ground in rotary mill [20] (reproduced with permission)

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Properties of GTR

It was early recognized that the quality of GTR has to be determined and guaranteed accordingly. The major quality properties of GTR are the particle size range (particle size distribution) and level of contamination (steel, textile).

The standard released on how to characterize “particulate rubber” is the ASTM D 5603 (1996). According to this standard rubber powders are classified with respect to feed stock and sieve analysis results. Accordingly, GTR should meet a range of chemical and physical properties including the maximum allowable concentration of fibers and metal. Manuel [11] pointed out that the ASTM D 5603 cannot be used in its current form in Europe as the raw materials and their composition in the tyres in USA differ from those in Europe.

The specifications of commercial GTR offered by Vredestein Rubber Resources contain the following characteristics: rubber origin, grinding technology, chemical composition, particle distribution, physical properties, and impurities. Recall that the ASTM D 5603 does not inform about the grinding technology, physical properties, and impurity leaching to the environment. Among the chemical composition, the acetone extract (ASTM D 297), ash content (ASTM D 297), CB content (ASTM E 1131), rubber content (ASTM E 1131), natural rubber (NR)/synthetic rubber composition (ASTM D 3452), and heat loss (ASTM D 1278) are listed. The particle size fraction is usually determined according to the ASTM D 5644 and the particle size distribution is given in millimeter (or mesh). Among the physical properties, the Vredestein specification reports about the surface appearance (rough/edgy) determined by microscopy and various densities of the rubber (specific density—ASTM D 297; pour density—ISO 1306, and compacted density—ISO 787). Among the impurities, the maximum allowable values of the steel, textile, and others (all of them according to ASTM D 5603) are also indicated [11].

However, the determination of the above parameters is labor intensive and/or hardly reproducible. For example, the specific surface values are highly influenced by outgassing phenomena [21, 22].

As mentioned before, the appearance of GTR strongly depends on the grinding technique. Researchers tried to describe the uniformity of the powder [23] along with its surface roughness also by other means, e.g., fractal dimensions [24]. Nonetheless, it has to be stated that the mean particle size and particle size distribution are of great practical relevance because the properties of compounds with GTR are mostly controlled by them ([24] and references therein). Unfortunately, in many scientific papers the characteristics of the GTR used were not adequately disclosed.

As in many of cited works the same GTR was used, namely fractions produced by the Genan Technology, their properties are listed in Table 2. Note that in this Genan Technology [25] the grinding to fine fractions occurs at ambient temperature in an air-gab at supersonic speeds. As GTR is made from very different tyres, only guiding values can be given on the elastomer composition of the powders. So, the rubber components are: NR—ca. 30 %, styrene/butadiene rubber (SBR)—ca. 40 %, butadiene rubber (BR)—ca. 20 %, butyl and halogenated butyl rubbers—ca. 10 %.

Table 2 Characteristics of GTR produced by the Genan Technology at Scanrub (Viborg, Denmark) [25]
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Compatibilization strategies

The oldest technique to recycle GTR is simply to “rebond” the GTR particles using adhesive binders. These binders are usually liquid thermosetting resins, fresh (virgin) thermoplastic polymers or rubbers, as well as rubber curatives. However, this rebond approach often fails to ensure the necessary adherence of the GTR particles to the corresponding matrix. For that purpose, namely to establish a smooth stress transfer between the GTR and the matrix, the surface of GTR should be modified. This modification, termed compatibilization, makes the interfaces of the phases similar to each other or provides specific interaction sites between the phases [2]. The result is a “technologically compatible” system with useful mechanical properties. In general, compatibilization can be achieved by physico-mechanical and chemical methods. In many surface modification procedures, both physical and chemical aspects are involved. Similar to mineral-filled polymers, the introduction of dispersing aids helps to reduce the interfacial tension and to prevent agglomeration. On the other hand, this is less straightforward than chemical treatment as argued later.

Chemical compatibilization is aimed at creating good bonding between the GTR particle and the matrix polymer. This is achieved by reduction of the interfacial tension yielding improved wetting, and formation of a “thick” interphase. Good bonding can be guaranteed by both non-reactive and reactive methods. In the former case, copolymers and graft copolymers, showing compatibility toward both GTR and matrix, are preferentially used. According to the reactive route, the GTR and the matrix—mostly in presence of further additives—react with each other, thereby linking these phases chemically [2].

The efficiency of compatibilization can strongly be improved when the GTR surface is (partly) devulcanized, or its surface is activated by various treatments. Devulcanization may support the molecular entanglement with thermoplastics and curable rubbers. In case of thermosets, a thick interphase may be formed when the resin swells the partially decrosslinked GTR particle. Surface activation by various routes, on the other hand, may trigger entanglement and/or chemical coupling with the matrix or additional polymeric compatibilizer. Therefore, next the various devulcanization and surface activation strategies will be briefly introduced.

Devulcanization, reclamation

Devulcanization and reclaiming are related but quite different processes. The outcome is practically the same: a rubber compound that can be compounded and revulcanized similar to fresh gum. The difference between reclamation and devulcanization is given by the target of the chemical attack. Devulcanization targets the sulfuric crosslinks in the vulcanized rubber and thus C–S and S–S bonds are selectively cleaved. Note that devulcanization requires high energy to break the –C–S-C– (285 kJ/mol), –C–S–S–C– (268 kJ/mol) or –C–S x –C– (251 kJ/mol) bonds [26]. On the other hand, reclaiming is usually accompanied with considerable scission along the polymeric chains resulting in lower molecular mass fractions. Irrespective to the above clear distinction between reclamation and devulcanization [8], the processes adapted can hardly be grouped into devulcanization or reclamation. So, in the brief survey below the processes will be classified as follows: thermomechanical, thermochemical, ultrasonic, microwave, and other processes.

Themomechanical processes

In the corresponding processes, crumb rubber is subjected to shear and/or elongational stresses on suitable equipments such as mills, twin-screw extruders, etc. Milling and extrusion are usually carried out at ambient and high temperatures. This may result in a prominent decrease in the molecular mass because even at ambient process considerable heat is generated. The characteristics of the products strongly depend on the processing equipment, its characteristics (e.g., local shear rate), and processing parameters (e.g., residence time). Earlier, for that purpose discontinuously operating open mills were used ([26] and references therein). Nowadays, there is a clear tendency to prefer continuous operations using extruders [27–29]. To find the optimal processing parameters, various strategies of experimental design are followed, such as response surface method [29]. Moreover, the design and the construction of devulcanization extruders is a topic of ongoing research [27]. It is worth of noting that such thermomechanical processes are often performed in the presence of reclaiming agents (e.g., [25, 27, 29, 30] and references therein) and thus they belong already to the category of thermochemical decomposition processes.

Themochemical processes

To manufacture reclaim rubbers usually chemical reclaiming agents are used. There is a wide range of inorganic and organic reclaiming compounds as reviewed by Adhikari et al. [15]. Those chemical agents work exclusively at elevated temperatures and the related processes usually require mechanical mixing/kneading. Because of hazardous and costly reclaiming agents, attempts were made to avoid them, for example, using phase transfer catalyst [31].

Interestingly, one of the oldest methods, the “pan process” is a static one [15, 25]. In the “pan process”, GTR is heated in saturated steam at a high temperature (150–180 °C) in the presence of “catalysts” (peptizer) and reclaim oil. First, the mixture is allowed to swell for at least 12 h. Note that GTR is retaining its “free powder flow” characteristics when containing up to 35 parts per hundred resin (phr) paraffinic oil [25]. The material is then placed in 2–3 mm thickness in pans to allow oxygen to penetrate. The trays are autoclaved in air/steam atmosphere at ca. 190 °C for ca. 5 h. After that, the material should be strained/refined to get a homogeneous mixture from the more degraded skin (outer layer) and less degraded bulk (inner layer) [8, 15]. The related claim is plastic dough generally used in fresh rubber mixes.

The role of the reclaiming agents is to cleave the sulfuric crosslinks between the rubber chains and terminate the free radicals formed. The digester processes reclaim the rubber crumb in a steam vessels equipped with agitators, which continually stir the crumb rubber while steam applied.

It is noteworthy that the vulcanization system used strongly affects the formation of the sulfuric crosslinks. Poly- and disulfidic crosslinks are mostly formed by conventional, whereas monosulphidic ones are by efficient vulcanization systems. The grouping between conventional, semi-efficient, and efficient vulcanization systems depends on the actual accelerator/sulfur ratio [32]. The devulcanization of rubbers which were vulcanized by conventional curing system is the easiest.

Like to the pan process “softeners” (processing oil, reclaim oil) are frequently used in GTR reclaiming. The primary function is to stretch the bonds between rubber chains and thus to reduce the mechanical energy required for mixing, kneading. As softener, asphalts [33, 34] and bitumens [35, 36] can also be used. It is noteworthy that GTR is very frequently introduced in pavement mixes containing bituminous or asphaltene-rich binders. The recent research strategy is to use processing oils from renewable resources [37]. It is the right place to call the attention that repeated reclaiming is associated with some changes in the properties [38].

Partial devulcanization of GTR may occur in both batch and continuous processes. The former procedure keeps the powder appearance of GTR which can be incorporated afterward in thermoplastics and rubber mixes [39, 40]. By contrast, the output of continuous extrusion is a plastic mass which is either incorporated in fresh curable rubbers or revulcanized by adding curatives [41–43].

Devulcanization may be performed also in solid state. This was shown by Cavalieri et al. [44] making use of high energy ball milling (HEBM). It was found that HEBM led to preferential breakage of sulfur crosslinks. It was also observed that NR can be grafted onto the surface of GTR via this mechanochemical milling.

Fukumori et al. [45] pinpointed a new concept: combination of devulcanization with dynamic vulcanization to produce a thermoplastic elastomer (TPE) on line continuously. The related concept is depicted in Fig. 4.

Fig. 4
figure 4

Dynamic devulcanization and dynamic vulcanization processes for the product of TPE based on EPDM waste [45] (reproduced with permission)

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The process is composed of pulverization, devulcanization, blending with a thermoplastic polymer (polypropylene, PP), and dynamic vulcanization. Attention was paid also to the deodorization of the final TPE compound. Thermoplastic dynamic vulcanizate (TDV) is a new member of the TPE family. TDVs are produced by dynamic curing of blends composed of a thermoplastic resin and a crosslinkable rubber. Dynamic curing means the selective curing of the rubber in the molten thermoplastic under intensive shearing. The matrix of TDV is given by the thermoplastic phase in which (sub)micron-sized crosslinked rubber particles are dispersed. Note that TDVs containing other rubbers than ethylene–propylene–diene rubber (EPDM, i.e., NR and other synthetic rubbers) can also be produced. On the other hand, differences in the devulcanization mechanisms of the related rubbers [45] should be taken into account.

Ultrasonic, microwave

A viable alternative to decompose crosslinked rubber is the use of powerful ultrasound (low frequency range of the ultrasound defined by the interval 20 kHz to 500 MHz). The basis of the process is that the energy of sulfur–sulfur bond is on the level that can be broken up by the ultrasonic weaves [14]. The chemical and mechanical effects of the ultrasound are caused by cavitation of bubbles (formed during the treatment) during the negative pressure period of the ultrasound. Two competing theories exist to explain the cavitation-induced chemical changes: the “hot spot” and “electrical theory.” The former postulates that the localized bubbles may have a temperature of 5000 K and a pressure of 500 bar. The electrical theory assumes that on the surface of the bubble locally generated electrical field gradient is enough to break chemical bonds [46, 47]. Ultrasonic devulcanization of rubber proved to be a useful recycling strategy. Isayev and co-workers [48, 49] studied the continuous devulcanization of various rubbers using an extruder coupled to an ultrasonic generator. In that case, the plasticized rubber was pumped through a narrow gap between the stationary die and the vibrating horn (coaxial). Another solution is possible when the horn is placed exactly after the screw and then the treated material goes through the die (barrel). Both the types of treatment are effective, the main characteristic parameters are the residence time and the ultrasound amplitude. Of course the temperature and the pressure also affect the rate of the devulcanization. After revulcanization, the mechanical properties are close to the original ones [50–52]. Ultrasonic treatment was successfully used to produce silicon rubber reclaim that was incorporated in both fresh and ultrasonically decomposed SBR to modify the surface and bulk properties of the latter [53].

The use of microwave allows the energy required for the devulcanization of the GTR to be easily obtained and controlled. The energy required for the devulcanization of rubber is around 180 Wh for 1 kg rubber [54, 55]. In the microwave technique, the waste rubber must be polar to generate the heat (>250 °C) necessary for devulcanization. The presence of CB, having high thermal conductivity and heat capacity, ensures the accumulation of internal energy and its uniform distribution in the material [54]. Softeners, when apolar, have a negative impact on the devulcanization. Nevertheless, guide lines for a highly selective and uniform devulcanization are not yet deduced. Microwaving is often coupled with extrusion and mixing. The transformation to refined, devulcanized stock is rapid and cost-efficient. In addition, the corresponding compound may serve as raw material for the same product that has been produced from ([15] and references therein). Pistor et al. [54] used the microwave-aided devulcanization in EPDM systems. They also investigated it in the presence of paraffinic oil (apolar). They concluded that the devulcanization is best controlled when paraffinic oil is extracted from the rubber and when the samples are exposed to microwaves for short periods. Zanchet et al. [56] added microwave-devulcanized SBR waste powder up to 80 phr in a sulfur-curable SBR and studied the mechanical thermooxidative and photooxidative properties of the resulting blends. The sol content of the waste SBR increased from the initial 7 to 67 % after 3 min microwave treatment in a modified domestic-type oven with 900 W magnetron capacity.

Others

There are many other methods of reclamation/devulcanization, which, however, represent some combination of the above-listed ones. The microbiological devulcanization deserves a separate entry. Encouraged by the fact that NR-latex is target of microbiological attack in the nature, researchers looked for bacteria able to scission the polysulfide linkages [57]. It has been reported that several bacteria are able to “digest” (break) the sulfide linkages and the extent of reaction can be regulated via the temperature, pH, etc. [8, 15, 57–60]. The result is a “surface modified” GTR particle, the surface layer of which is no more crosslinked. In this case, the devulcanization can be solved only partially because the bacteria can work just on the surface of the rubber. So the results also depend on the type of the grinding. Another problem could be the toxicity of rubber additives that can inhibit the function of bacteria. Microbiological devulcanization processes may become important issues in the future [3].

Quality of the reclaim/devulcanizate

Recall that the major task of the rubber (material) recycling is to produce from the initial infusible, insoluble, three-dimensioning crosslinked structure (100 % gel content) a melt processable one. This means that the initial crosslinked structure should be destroyed, at least partially. As a consequence, the material becomes partially soluble (sol content >0 %) and will show a lower crosslink density than initially. It is worth of noting that the sol content of most rubbers is higher than 0 due to different low molecular mass additives, processing aids which can be removed by extraction. The (sol) gel content is exclusively determined by extraction (in Soxhlet apparatus). To determine the crosslink density, usually the Flory–Rehner [61] equation is used.

The melt processability can be characterized by the Mooney viscosity (ISO 289, ASTM D 1646), torque (apparent viscosity) values or torque (apparent viscosity)—time curves, assessed by various torque rheometers and characterized by plasticity parameters (Defo), change in hardness, etc.

Surface activation

For the surface modification of GTR, physical and chemical methods can be used.

Physical methods

For the surface modification of GTR, treatments in different environments (dry, wet) are used which result, however, in chemical changes in the GTR surface. As a consequence, the above heading, viz. “physical methods”, may be ambiguous. To modify the GTR surface reactive gas (a mixture of oxygen and chlorine [8, 62]), ozone, plasma, corona, and electron beam (EB) irradiation procedures [22] were explored. It is known that irradiation of organic systems in air leads to their oxidation which is manifested in the appearance of peroxy, hydroperoxy, hydroxyl, and carbonyl groups [63]. The presence of these groups guarantees the compatibilization of the modified GTR with polar polymers.

Chlorination is considered as an effective way to make polar the GTR surface [64, 65]. The chlorinated GTR proved to be excellent filler in polyvinylchloride (PVC) compounds. Oxidation of GTR was performed also by H2O2, HNO3, HClO4, and H2SO4 solutions to enhance the compatibility to NR [66] and to high density polyethylene (HDPE), respectively [67].

Colom et al. [68] improved the compatibility of GTR toward HDPE by chlorination, oxidation using H2SO4, and silane coupling agent. Except chlorination, the other two surface activations improved the adhesion between GTR and HDPE, which was reflected in enhanced mechanical properties. Sadaka et al. [69] explored the possibility of controlled chemical degradation of both an NR-like model compound and GTR using periodic acid (H5IO6). The latter is able to efficiently cleave the –C=C– bonds.

As ozone is a well-known degrading agent for rubbers, it can be used for the surface modification of GTR. This was shown by Cataldo et al. [70] who functionalized GTR in a fluidized bed reactor. It was demonstrated by pyrolysis chromatography that ozonation took place exclusively at the surface of GTR.

Chemical methods

These methods focus on various grafting procedures. Grafting of unsaturated monomers and oligomers on the GTR surface which may participate in the subsequent crosslinking with the fresh rubber or at least entangle with the matrix macromolecules is a very promising approach. This has been followed by several research groups making use various grafting procedures. The monomers preferred were: styrene, glycidyl methacrylate (GMA), acrylic and methacrylic acids (MAA), and the like.

Naskar et al. [71] produced maleic anhydride (MA) grafted GTR (GTR-g-MA) by free radical-induced process in an internal mixer at T = 160 °C. It was demonstrated that the properties of GTR-g-MA-containing TPE were superior to the reference sample with the same amount of GTR.

Coiai et al. [72] grafted styrene onto the surface of GTR via free radical polymerization. The cited authors demonstrated that reactive double bonds are present on the GTR surface which may act as “anchoring” sites for styrene grafting. The grafting efficiency, i.e., the percentage of grafted styrene over the total weight of polymer formed reached almost 40 % when dibenzoyl-peroxide was used as initiator. Grafting was, however, negligible with azobisisobutyronitrile initiator. This was attributed to the difference in the H-abstraction capability of the radical formed from the mentioned initiators. A similar approach, namely styrene grafting onto GTR surface in dibenzoyl-peroxide-induced bulk polymerization, was followed by Zhang et al. [73].

Tolstov et al. [74] grafted MA and acrylamide by free radical-induced grafting and γ-irradiation onto GTR. The corresponding grafted GTRs were used in TPE formulations. The presence of MA groups (MA grafting at around 1 wt %) was exploited in reactions with amine (–NH2) functional groups. Fuhrmann and Karger-Kocsis [75, 76] functionalized GTR with MAA, and GMA through photoinitiated grafting. GTR surface was modified by allylamine through UV-induced photografting by Shanmugharaj et al. [77–79]. The mechanical properties of PP/PP-g-MA/GTR blends were better when the GTR was functionalized with allylamine than without. This was attributed to the reaction between –NH2 and anhydride groups of the graft and PP-g-MA compatibilizer, respectively [77, 78].

Du et al. [80] grafted bismaleimide by thermally and UV-induced polymerization onto cryogenic GTR. It was aimed at producing rubbers from NR/GTR blend at high GTR content. As optimum curing condition for GTR/NR/bismaleimide (85/15/5 part) system 10 min at T = 110 °C was found.

Abdel-Bary et al. [81] grafted waste rubber powder by various vinyl monomers (acrylamide, acrylic acid, crylonitrile) using γ-irradiation. The graft yield depended on the type of the monomer. EB irradiation was used to graft MA onto sheets produced from reclaimed rubber powder and NR with and without additional short glass fiber [82]. Yamazaki et al. [83] converted polyisoprene (IR), and vulcanized NR with and without CB into lower molecular weight (MW) compounds by subjecting them to heat treatment at 300 °C in air and N2, respectively. Afterward these thermally decomposed rubbers were copolymerized with styrene in bulk via free radical initiation. Fan and Lu [84] introduced “immobile” reactive hydroperoxide groups through ozonization on the GTR surface prior to start with the grafting of methyl methacrylate. Variation in the ozone treatment and polymerization temperature resulted in a grafting degree of 20 wt %. Shahidi et al. [85, 86] developed an “impregnation” process to produce GTR particles with semi-interpenetrating network (semi-IPN) structure. This was achieved by polymerization of a reaction mixture composed of toluene (swelling agent), acrylic acid monomer, and an oil-soluble radical initiator. The final product was foreseen as carrier for wastewater purification. Amash et al. [87] pinpointed in a study which are the morphological and compositional characteristics of rubber powders which are best suited for grafting by MA or to perform other functionalizations (epoxidation, hydroxylation).

GTR in thermoplastics

The use of GTR in thermoplastics was the often preferred way of recycling besides rubbers. This was fueled by the following aspects:

  • a small percentage of GTR in thermoplastics (<10 wt%) is associated with a very large consumption of GTR owning to the market share of thermoplastics [14],

  • like rubbers, the thermoplastic may act as binder allowing the use of GTR as major component in the related blends,

  • recycled thermoplastics are available in the market which can be modified by GTR whereby reducing the cost further,

  • GTR incorporation may yield enhanced toughness in the corresponding blends. This expectation is based on the fact that the majority of toughened thermoplastics are rubber modified by whatever means achieved.

To improve the toughness, and eventual further properties, suitable compatibilization strategies should be followed. This is accompanied with increased costs impairing the production of low-cost GTR-containing compounds. Therefore, it is a straightforward to incorporate GTR in recycled, post-consumer thermoplastic wastes. One major source of the latter is the agri- and horticulture. Note that recycled PEs from agricultural films (soil and greenhouse covers) are marketed for long time. Their macromolecules are naturally “oxidized” (usually quantified by the “carbonyl index”) which supports the adherence of PE to GTR particles as mentioned before.

Next, we shall summarize the result of R&D activities devoted to thermoplastic/GTR blends in tabular form. The selected grouping is as follows: commodity thermoplastics (Table 3), engineering thermoplastics (Table 4), and TPEs (Table 5) as this selection covers the overwhelming majority of the works done in this field.

Table 3 Commodity thermoplastics with GTR content—effects of GTR type and size, polymer/GTR blend ratio, compatibilization, and processing on selected properties
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Table 4 Engineering thermoplastics with GTR content—effects of GTR type and size, polymer/GTR blend ratio, compatibilization, and processing on selected properties
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Table 5 TPEs with GTR content—effects of GTR type and size, additional rubber, composition, compatibilization, and processing on selected properties
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Commodity thermoplastics

Commodity or high-volume or low-cost resins cover polyolefins (PEs and PPs), polystyrene (PS), and PVC systems—the related results are listed in Table 3. Sometimes acrylics, such as acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate and related blends are also listed here but we treat the latter among the engineering polymers.

It has to be mentioned here that some polyolefin/GTR combinations can also be treated as TPEs. However, when their rubbery characteristics were not disclosed, or failing to reach 100 % elongation at break, they were incorporated in the commodity thermoplastics heading. Besides GTR, some other powdered rubber wastes, blended with commodity thermoplastics, were also considered.

Based on the results, summarized in Table 3, the following conclusion can be drawn:

  • To moderate the degradation in the mechanical properties of blends with increasing GTR content it is indispensable to perform some surface treatment of the GTR and/or use compatibilizers. Reclamation of GTR, also on its surface, seems to be cost-effective and efficient way that can be performed continuously in-line during blending. Compatibilization via surface treatments (oxidation, chlorination, corona treatment, etc.) is less efficient than using block copolymer rubbers with and without functionalization (e.g., SEBS, SEBS-g-MA). Unfortunately, the latter polymers are, however, expensive additives. The reactive compatibilization strategies followed so far resulted only in small property improvements.

  • Because GTR incorporation is accompanied with reduction in ductility, the additional reinforcement of the corresponding blends through adding fibrous fillers (e.g., SGF) represents a promising strategy. Here, the target is the development of injection moldable granules which may compete with some engineering thermoplastics.

  • The production of microcellular thermoplastics with GTR content is not only a straightforward strategy (if the adhesion between the matrix and the GTR phases can hardly be improved then why not avoid this by foaming?) but also contributes to find novel application fields that GTR badly needs.

  • Until now, R&D works focused on the processability and mechanical performance of thermoplastic/GTR blends. To support the market penetration and acceptance of GTR other properties, such as mechanical and acoustic damping, should also be considered next.

Engineering thermoplastics

Engineering thermoplastics are qualified for continuous use at temperatures above 100 °C, and showing tensile strength higher than 40 MPa [143]. Table 4 lists those works in which engineering polymers were combined with GTR.

Data in Table 4 suggest that modification of engineering thermoplastics with GTR is possibly not the right way even if waste thermoplastic materials are used. Recall that the property level of the virgin material can only be reached by sophisticated compatibilization methods if the GTR amount is 20 wt% or less in the corresponding blend.

Thermoplastic elastomers

TPEs are thermoplastic polymers which can be melt-processed at elevated temperatures while possessing elastomeric behavior at their service temperature. TPEs thus contain a thermoreversible network (also called physical network) structure. This is formed by phase separation in all cases. In segmented (multi)block copolymers, the “knots” of the physical network are given either by glassy or by crystalline domains. They are referred as “hard phase.” The hard phase domains are dispersed in the “soft” rubbery phase. The latter forms the matrix in block copolymers with the flexible segments.

Upon cooling from the melt, the glassy and the crystalline domains (knots) are reformed by phase segregation and crystallization processes, respectively. A physical network can be generated also by ionic interaction. In the so-called ionomers the clusters, acting as “knots”, are held together by ionic bonds. This is usually achieved by saponification of acidic groups in copolymers.

TPE character can be received by blending of thermoplastics and elastomers whose final morphology is co-continuous, i.e., the related phases are intermingled. Blending of polyolefins with rubbers (NR, EPDM, etc.) to produce TPEs has a long history [150]. A further impetus to the related R&D activities was given by adaption of the dynamic vulcanization ([151–153] and references therein).

TDVs are new members of the family of TPEs. TDVs are produced by dynamic curing of blends composed of thermoplastics and crosslinkable rubbers. The term “dynamic curing” means the selective curing of the rubber and its fine dispersion in the molten thermoplastic via intensive mixing/kneading [151, 153–155]. The microstructure of TDV fundamentally differs from the physical networks formed by phase segregation. Note that matrix phase here is given by the “hard” thermoplastic which accounts per se the melt processability. The rubbery properties (recovery) are guaranteed by the matrix ligaments (between the crosslinked rubber particles) undergoing inhomogeneous deformation but not yielding, upon loading ([152, 156]) and references therein). It is important to emphasize that the rubber particle dispersion in the thermoplastic matrix of the TDV is very fine, i.e., few microns and below.

It is most likely that the TDV technology was the driving force to develop TPE compounds with GTR content. Recall that TDV involves some kind of reactive compatibilization. Though the mean particle size requirement for rubber inclusions in TDV can hardly reached with GTR particles (not even by cryogenic milling), nevertheless, the GTR-containing blends possess rubbery behavior, albeit with some limitation in the stress–strain behavior [152]. That is the reason why we define TPEs as such compounds whose tensile strain under the usual testing conditions is at least 100 %. The other important parameters are the set properties. The requirement for TPEs that their compression set should be lower than 50 %.

One of the first reports on TPE composed of PP or PP/LDPE blend and GTR was published in 1989 [157]. Al-Malaika and Amir [157] reported that reclaimed tyre rubber should be used together with fresh NR to improve the properties of the corresponding blend. Dynamic vulcanization of the latter by peroxide yielded further property improvements. Many recent works target the development of TPEs which represents an “upcycling” strategy for GTR use. Upcycling is related to fact that TPEs are “costly” products. Considering the large body of works devoted to polyolefin/GTR combinations (cf. Table 3) and the fact that TDVs are usually of polyolefinic nature, the development of GTR-containing TPEs focused on systems containing polyolefins as matrices. In order to improve the adhesion between the phases various compatibilization methods have been followed (swelling, reclamation, reactive and non-reactive compatibilization, etc.). Nonetheless, additional use of rubber (fresh, devulcanized, reclaimed) in the thermoplastic/GTR blend to produce TPE is inevitable. Table 5 lists the activities performed on GTR-containing TPEs.

Results listed in Table 5 clearly suggest that the development of GTR-containing TPEs is a very promising research strategy. The related products may have comparable properties with competing TPEs, however, at lower costs. The key issue is to improve the interfacial adhesion between GTR particles and matrix polymer. As matrix polymers, polyolefins are most suited being inexpensive, and even available as wastes without problematic contaminants. To increase the interphase bonding, different strategies were followed which can be grouped into: in situ, ex situ, and compatibilizing methods. They differ from one another whether the interphase is created during blending or partial destruction of the initial crosslinked structure takes place in a separate process. They cover different chemical (decrosslinking, grafting) and physical reactions (molecular entanglement). In this respect, the role of dynamic curing should be emphasized. The goal of compatibilization strategies is to encapsulate the GTR particles. This is usually achieved by additional compatibilizers which are mostly rubbers. The reason behind the selection of fresh rubbers is that rubbers, being amorphous, can highly be filled. Moreover, many of them are well compatible with polyolefins, such as ethylene–propylene rubber (EPR), EPDM, polyoctane ethylene (POE). The encapsulation strategy is most straightforward when a co-continuous phase structure can be set. Note that this is thermodynamically less stable in blends whose structure turns generally into a dispersed one. On the other hand, GTR particles may promote the preservation of the co-continuity of the phases. This aspect is not always articulated (except [2]) though the composition range thermoplastic/GTR = 30/70…70/30 already suggests the possible formation of dual-phase continuity. Large body of R&D works addressed the improvement of UE. According to our feeling, the best way to reach it that besides encapsulating phase (not necessarily rubber), a dynamically cured rubber phase should also be present. The corresponding morphology is outlined in Fig. 5.

Fig. 5
figure 5

Morphology of thermoplastic/GTR blends: dispersed (a) and co-continuous (b)

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In case of TPE with high GTR content the creation of a finely dispersed crosslinked rubber phase is not necessary when the GTR surface is well reclaimed. It has to be underlined that the surface reclamation of GTR, by whatever method achieved, is very beneficial with respect to the final properties of the TPE. Here, the bitumen treatment, proposed by Lievana and Karger-Kocsis [6, 161] may be the right way, however, for some limited applications (dilatation parts in building/construction). The development to produce GTR-containing TPEs requires the adaption of novel experimental design techniques (already emphasized in the notes of Tables 3, 4), as well as modeling studies [188] considering the optimal morphology. For GTR reclaiming, thermomechanical methods will be favored instead of thermochemical ones. Nevertheless, thermochemical reclamation will further be explored [45]. This is predicted based on the fact that cost-efficiency triggers the set-up of on-line compounding lines performing the reclamation of GTR and its reactive compounding with polyolefins.

GTR in thermosets

The use of GTR particles in thermoset resins usually targeted their toughness improvement. It is well known that rubber particles when dispersed in micron-scale range may improve the fracture toughness and energy of the corresponding thermoset matrix. These particles are created in situ via phase separation or mixed in particle form into the resin [189]. The latter approach can be adapted for GTR which was followed, in fact. Next, we shall list those works which were devoted to the toughening of thermosets by adding GTR. It is the right place to mention that the R&D works addressing thermoset/GTR combinations in which the role of thermoset (especially polyurethane) just a binder of GTR particle will not be covered. Interested reader in this field is directed to Refs. [190–192].

Most of the works performed on GTR/thermoset combinations addressed the modification of epoxy resins (EPs)—cf. Table 6. The GTR particles were introduced in the corresponding resin by mixing.

Table 6 GTR-containing thermosets—effects of GTR (type, size and amount), and compatibilizers/additives on selected properties
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The results summarized in Table 6 clearly suggest that the modification of thermosets with GTR is less promising because the desired toughening cannot be achieved. This note holds also for using surface-modified GTRs. The scenario may change when ultrafine GTR particles (≤10 μm) are incorporated. It is interesting to note that researchers did not consider other effects which may be of interest, such as changes in the residual stress state and shrinkage, when incorporating GTR in thermosets.

GTR in curable rubbers

Earlier, it was accepted that GTR could be extensively recycled into traditional rubber products. However, the incorporation of GTR in rubber mixes in appreciable amounts alters the processability and it is associated with some reduction in the mechanical properties, such as tensile and tear strength. Nonetheless, there has been always considerable interest to recycle GTR in rubber stocks [7–9, 203–213]. From the earlier studies, the following rule of thumb was deduced: incorporation of 1 wt% of GTR result in 1 % deterioration in the mechanical properties. Accordingly, the GTR content of vulcanizable rubbers generally did not surpass the 10-wt% threshold [214]. Further, learning from these studies was that the smaller the property degradation, the finer the used GTR fraction is. The reports are mostly in harmony about the reason of the property (ultimate tensile and tear properties) drop. They locate it in the missing interfacial bonding between the GTR and the rubber matrix. This argument is supported by the fact that the mechanical properties of GTR-containing rubbers are less affected when GTR particles are smaller. Klüppel et al. [209] considered the GTR-containing rubber as a rubber with locally varying network density and filler (CB) content. This may be beneficial with respect to some properties (oil-resistance, compression set, acoustic damping), whereas detrimental for others (fatigue, tensile, and tear characteristics).

The R&D activities in this field have been running in two directions: (a) to use GTR without adding fresh rubber and (b) to incorporate GTR as filler in curable rubber stocks. Needless to say that partial reclamation/devulcanization of GTR, by whatever method achieved, is preferred in both the above-mentioned recycling strategies.

On the other hand, devulcanization is exactly that mechanism, which allows us to produce rubber products from 100 % GTR. Employing heat and pressure, the different sulfide linkages (mono-, di-, polysulfide) can be cleaved in situ during molding. Recall that the sulfide-, sulfur-bonds are the weakest with respect to the bond energies among the chemical crosslinks. Nowadays, considerable efforts are devoted to shed light on the decrosslinking reactions upon heating and in the presence of reclaiming agent [213].

Next, we shall split the works done on this field whether GTR was used alone (cf. Table 7) or in combination with other rubber(s)—cf. Table 8.

Table 7 Revulcanization of GTR—effects of GTR type and size, crosslinking, additives, and processing on selected properties
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Table 8 GTR in virgin rubbers—effects of GTR (type, size and treatment), composition, crosslinking, and processing on selected properties
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GTR particle bonding (without fresh rubber)

A recent work was devoted to the “direct powder molding” or direct molding of GTR particles. Gugliemotti et al. [225] studied the effects of particle size, particle size distribution, and processing conditions on the mechanical performance of the related products made only from GTR without any additives. Major advantage of the strategy of “GTR particle bonding” is that a high amount of GTR can be recycled by this way. On the other hand, mostly “noncritical” rubber items (mats, pads, carpet underlays, walkway tiles, sport field surface, etc.) can be manufactured, whose market is limited. Incorporation of fibrous reinforcements of different origins, including waste products, is a straightforward approach. Further investigations are needed to find devulcanization/reclaiming additives, which “activate” the surface of GTR prior to the “sintering” (bonding) process.

GTR with fresh rubber

GTR with and without reclaiming have been introduced in virgin rubber recipes to reduce material costs. This was accompanied with sacrificing some properties and had a negative impact on the processability, as well. A large body of works was dedicated to clarify what is that GTR amount which does not influence the requested property profile markedly, and how the GTR content of the rubber stocks can be increased. Next, we shall review the related works to conclude the related achievements and to forecast future developments. To provide the reader with a broad overview, the use of pulverized rubbers (processing or post-consumer waste), other than GTR, will also be considered (cf. Table 8). This is due to our belief that the related recycling technique can be adapted, or serve as example for GTR recycling.

The results summarized in Table 8 suggest that incorporation of GTR in fresh rubber stocks remains a promising recycling route for GTR. In order to enhance the GTR loading in the recipes without sacrificing the basic property requirements, it is inevitable to subject the GTR to devulcanization/reclaiming. Otherwise, the GTR amount, that can be incorporated, remains below 10 phr. For devulcanization/reclaiming continuous processes should be preferred. This suggestion is based on two basic aspects. First, continuously operating devulcanizing lines do not require finely powdered GTR which contributes to cost saving. On the other hand, for that purpose most probably special extruders should be developed. Second, during breakage of crosslinks (devulcanization) and molecular chains (reclaiming) macroradicals develop and their recombination, when not associated with reduction in the crosslink density, should be avoided. This can be efficiently done during extrusion by introducing suitable reclaiming additives, diluting with fresh rubber, etc. Accordingly, the macroradicals generated during devulcanization/reclaiming should be involved in co-reactions with suitable “diluting additives.” By this way, even an accelerator-containing, ready-to-incorporate mix can be produced. Accordingly, preparation of rubber mixes in traditional batch mixers can be circumvented.

Conclusion and outlook

Many arguments support the material recycling of worn tyres via production/use of GTR. The use of GTR in thermosets to improve some properties of the latter is practically not feasible though researchers still follow this route. Thermosets (especially PU based) will play further on, however, a role as binders for GTR in construction and civil engineering applications. Incorporation of GTR in thermoplastics is also not rewarding unless waste materials (e.g., agricultural films, discarded transportation crates) are used as matrices. However, even in that case new markets should be found. The above survey clearly articulates that GTR recycling should target the production of TPEs, and its (re)use in the rubber industry. The yearly consumption of TPEs is increasing. So, the development of GTR-containing TPEs with useful characteristics would contribute to the recycling of a large amount of GTR. The same note holds for the GTR use in rubber industry provided that GTR devulcanization/reclaiming is economically solved. According to the authors’ feeling, it may happen by preferring thermomechanical, eventual (thermo) mechanochemical, decomposition routes than others.

One can also recognize a new trend in GTR recycling: instead of the mechanical other properties become under spot of interest. In this respect, acoustic and vibration damping should be mentioned. It is well known that rubbers and rubber-like materials with very inhomogeneous crosslinked structure are excellent sound absorbers. GTR particles are predestinated to generate such a structure in different matrices [253–255].