and Jin Kuk
a Indian Institute of Engineering Science and Technology, Shibpur, Dr. M.N. Dastur School of Materials Science and Engineering, Howrah, 711103, India. E-mail: firstname.lastname@example.org
b Gyeongsang National University, Department of Materials Engineering and Convergence Technology, Jinju, 660701, South Korea. E-mail: email@example.com
Powdered rubber has several potential application fields, starting with blending in virgin rubber and thermoplastics and up to incorporation in several civil engineering applications such as in concrete and asphalt. Apart from the molecular weight, the filler concentration and grinding technology also has a crucial role in modifying the properties of the processed composites. The grinding method has a direct impact on the surface properties of the particles and hence the polymer–filler interaction, mixing behavior, curing characteristics, and mechanical properties can be tailored by altering the grinding method. For example, cryogenically ground rubber particles have a smoother surface as compared to grind rubber particles. Hence, powder obtained through ambient grinding offers a better miscibility with the polymer system. Sometimes surface modification is performed on cryogenically ground rubber by a revulcanization method with the addition of sulfur in the system in order to achieve better filler matrix interactions. Each of the processes are distinct to each other and offer several advantages, as well as each of them having several limitations. Thus, the grinding method is selected according to the intended application. Moreover, grinding is the preferred waste recycling route of rubbers due to its eco-friendly nature and reasonably moderate cost.
The word rubber originates from the South American Indian word, meaning 'weeping wood' and dictionary definition of rubber is, ‘a material that when stretched returns quickly to its approximate original shape’.1 A three dimensional insoluble and infusible chemical network is formed in the manufacturing of rubber products. This network is formed through an irreversible chemical reaction, known as vulcanization chemistry, between the elastomer, sulfur and other chemicals. After cross-linking the material becomes solid, insoluble and infusible. Hence, direct recycling, or rather reclamation of rubber, is a difficult process which ends up costing half that of natural or synthetic rubbers. However, recycled rubber possesses some properties that are better than the virgin rubber.2 Reprocessing by melting is also not feasible, because of the three-dimensionally crosslinked molecular structure of rubber.3 Moreover, conservation of petroleum products has led to a drive towards a cleaner environment and greener waste disposal of unwanted rubber products serves as an added advantage for the rubber recycling process. Waste tire rubbers in stock piles can allow good breeding conditions for mosquitoes and cause several fire related issues. Waste tire fires can spread rapidly and they are very difficult to extinguish, in addition, the fumes emitted from waste rubber contain several harmful volatile compounds. Land filling of waste rubber deteriorates soil properties by killing useful bacteria in the soil.4 Henceforth, recycling becomes inevitable. Nowadays, the amount of used tire recycling by grinding in the EU27, U.S., Japan and China is estimated to be about 3.6 million tons/year.5 The utilization of powdered rubber in the field of rubber composites has been increasing over the years and now they are no longer considered as a mere filler with an economic advantage, as they have wide applications in several molded, extruded products. Moreover, these composites have a variety of usages such as in playgrounds, in the preparation of artificial turf for several outdoor sports, mulching applications, and in the fields of animal breeding and automobiles.6 Grounded waste rubber has various uses in the field of composites, in particular for reinforcing cement like products. The sulfur vulcanization process was invented by Goodyear, who also initiated efforts to recycle cured rubber waste through a grinding method. Utilization of waste rubber in a vulcanized state requires a reduction in particle size. Sometimes, reduction in surface area also plays a critical role in the utilization of waste rubber in the vulcanized state. One of the widely used industrial processes for achieving this with scrap tires and rubber waste is grinding of the waste rubber. Force can be exerted on the rubber chips in various ways, these are impact, compression, shear and attrition. Attrition is the rubbing of rubber chips against a solid face. The grinding processes vary according to the requirements of the final product, which are cutting, shredding and impact. Moreover, the process is dependent on several grinding conditions such as ambient, wet, or cryogenic grinding.4 Reduction in particle size can also be obtained with shredding or cutting applications, however, shredding creates surfaces with smooth edges. On the other hand, particles obtained by the grinding method have larger surface areas therefore the compatibility of rubber granulates with matrices is increased as a better adherence with the matrix implies a strong reinforcing ability. Under stretching the chances of separation of a particle from the matrices is reduced with increased compatibility.6 The benefits of the grinding method of rubber for material recycling have been described by Karger-Kocsis et al. in their review paper. They accumulated data that described how 1 kg of tire accounts for 128 MJ of energy and that regeneration of this 1 kg rubber yields only 30 MJ of energy. Whereas an additional 6.8 MJ of energy is required to produce 1–1.5 kg of ground tire rubber. ASTM 5603 is the measured standard for characterizing particulate rubber, however, the ASTM 5603 does not provide any information about the physical properties of the powdered sample or provide conclusions about the grinding technology involved in the process. ASTM D 5644 is used to determine the particle size measured in mm or mesh, the surface appearance of the particle is determined by microscopy analysis.5
Early methods for the disposal of waste rubber products were through land filling or sometimes to be reused as fuel sources. Scrap worn out tires from various automobile industries are the major source of reclamation of waste rubber.7 Apart from tires, rubbers are utilized in the automobile industry in several other aspects such as in conveyer belts, hoses, shock absorbing dampers, and so forth. Rubber based products are utilized in several civil engineering applications, agricultural products, clothing and latex goods. Products discarded during the manufacturing of these goods and utilized wastes are also a source of reclamation for waste rubbers.8 There are various processes available for recycling these waste rubber products and the details of the processes have been provided in Table 1.1.9
|1||Recycling energy as fuel||One method is reclaiming the energy through direct burning, this method is simple but it will cause air pollution. The other is to make fuel, that is, waste rubber is blended with other burnable discard for making rubbish solid fuel which is used in a cement revolving tubular kiln instead of coal, and moreover, it can be used in generating electricity. This method can obtain another material, charcoal black. It can be used as activated charcoal black after activating|
|2||Thermal decomposing||Making use of the products of thermal decomposition of waste rubber, such as: coal gas, oil and charcoal black and so on. However, the cost of facilities and operation is quite high|
|3||Modification||Tires are renewed, making man-made fish shelters, conservation materials, and lifebuoys. These materials consume a little energy and make the best use of waste rubber, therefore this is a valuable recycling approach|
|4||Modification asphalt||Roadway materials|
|5||Regenerated rubber||Making regenerative rubber through desulfurization of waste rubber. Due to the cost of cryo-regenerating, rubber products will decrease, if regenerative rubber is blended with rubber. Currently there are some new regenerating methods, such as, normal temperature regenerating, cryogenic phase transition catalytic desulfurizing, microwave regenerating, irradiation regenerating and extrusion regenerating|
|6||Powdered rubber||The main recycling approach of waste rubber is to make PR or FPR. In general, cryogenic crushing is the main process of FPR. The temperature in the process is lower than the glass transition temperature (Tg). Of the polymer in rubber. For example, the temperature of the blends of natural rubber (NR) and styrene-butadiene rubber (SBR) is under −90 °C, to −67 °C at the least, usually the freezing agent is liquid nitrogen. Normal temperature continuous crushing and high pressure water impacting are the crushing approaches used for waste rubber|
The major source of waste rubber comes from the tire industry. Generally, tires contain several steel wires and fibers for reinforcement purposes. After the grinding and shredding operation, the obtained product is a rubber crumb and the byproducts are separated out from the tires. Representative figures of the crumbs and fibers are depicted in Figure 1.1.10
Until the early 1990s high-rate tire shredding was the most popular technique for reclamation of waste rubber. However, it was a very slow process utilizing a popular digester process in the rubber processing industry. Processing of waste tires largely depends on the type and quality of the tires. The processing becomes difficult with dirty tires and stockpiles which have been stored for prolonged periods. Presently, the common grinding methods include three processing stages: primary, secondary and tertiary, to achieve the fine grade of crumbed rubber, for example the formation of rubber powders.
The primary processing stage reduces down the whole tire to a convenient processing size by utilizing any of the following three instruments: the cracker mill, the hammer mill (high impact), or the rotary shearer. The guillotine is another type of instrument which merely cuts the waste rubber. Generally, the rubbers are first shredded and then subjected to a grinding process. The elasticity of rubbers creates more problems in grinding as compared to the shredding process, and cracker mills are engaged to overcome the problems related to grinding. The feed of rubber material is supplied to the nip point of the two corrugated rollers rotating at different speeds. The materials are caught at the nip point and exposed to a shearing force which helps to grind the rubber particles. Hammer mills are the next alternative instrument to break down the size of rubber material. Hammer mills can be of horizontal or vertical type according to their design considerations., and utilize impact force to break down the materials. However, high energy consumption limits their usage in the processing industry. Sometimes, high energy water jets are used as an impact grinder for the breaking down of materials. Altering the architecture of knives and hooks varies the chopping efficiency of the rotary shearer. Power consumption depends on the size and type of the unit.
The secondary stage converts the coarse waste rubber chips to granular rubber material. The two roll mill is similar to the cracker mill, but it is smaller in size and hence works at a lower energy. Granulators are another type of rugged machine which produces finer particle sizes. These are high speed and high loading machines which work with either close or open rotor assemblies. Water mist sprays or cooling jackets are required for granulators due to the high loading of materials. Another popular method to break down waste rubber material is to convert rubber chips into a brittle, glassy state by exposing rubber chips to a liquid nitrogen atmosphere, the frozen particles are then fed to the milling unit. The whole assembly is termed as a cryo mill. The extrusion technique is also utilized in the rubber reclaiming industry, in which tire chips experience a shearing action while they are transported through a screw. The chips are fed into the hopper and travel through a rotating screw which pushes the material in a forward direction. The screw design alters the applied shearing force on the material. In this shearing process a significant amount of heat is generated which must be removed to ensure that the properties of the end product remain unaffected. In the extrusion process, particles are obtained with high surface areas.
Tertiary grinding is the ultimate breaking down of crumb rubbers to obtained a fine powder, for these 100% conversion in a single unit can be achieved with wet grinding techniques.
Generally, coarse size reduction is achieved by crushing and shredding operations and finer grade size reduction is achieved using the grinding method. Ultrafine particle size reduction is carried out through pulverizing mills.11
Grinding can be performed either at room temperature or in the glass transition region through the use of liquid nitrogen in a cryogenic system.
Ambient grinding can be one of two types, using air impact or a water jet. Vulcanized rubbers are passed though the nip gap of a shear mill or two roll mill at room temperature for reduction of the particle size.12 Ambient grinding does not mean processing at room temperature, the milling temperature may rise up to 130 °C when using this grinding technology.5 The particle size reduces with the increase in the number of passes through the nip zone. The principle of fragmentation of particles is by acquiring sufficient kinetic energy by which they can collide with each other and with several parts of the instruments and finally disintegrate into smaller particles. In the ambient grinding method this kinetic energy is achieved by accelerating the particles through the air jet. Turbulence in the mill can also split up particles. Overall, the turbulence, upside-downside air current and energy stored by rubber particles generates heat in the system and therefore cooling of the system is necessary.12 A schematic representation of the overall process has been provided in Figure 1.2.13
The standard granulation process uses knives in a granulator. The knife type granulator assembly (Figure 1.3)14 is a variation of the ambient grinding process. Elastic and thermoplastic wastes can be ground using this process. The key attribute of this process is its high efficiency with a low vibration and noise level.
Another variation on ambient grinding is the flat-die granulation process (Figure 1.4)14 in which rubber products are first shredded and then pressed through flat dies. The material is passed through the openings of the perforated flat die of the granulator. In this process there is no cutting process involved using the knives.
Kinetic energy can be introduced to the rubber particles through the use of a water jet stream. The impact energy exerted by the water jet disintegrates the particles into smaller sizes. The water jets provide the added advantage of cooling down the system during the operation. The major disadvantage of this process is the addition of a processing stage, for example subsequent drying of the particles before finishing. Hence, this process adds to the total energy consumption, making it a costlier process. Moreover, proper separation of the reinforcement material cannot be achieved using this process.10
The basic principle of cryogenic grinding is the conversion of elastic rubbery chips to a brittle material by exposing them to liquid gases. Generally, liquid nitrogen is used to cool down the rubbery material to below its glass transition region, which is followed by crushing of the frozen brittle material through impact force utilizing a hammer mill. Cooling of the rubber chips can be performed prior to their grinding or during grinding time. The high production rate and low energy utilization makes cryogenic grinding a fascinating process.12 The process outline is represented in Figure 1.5.13 However, a pre-grinding and drying operation is required for cryogenically ground material which adds to the overall processing cost. Nitrogen is an inert gas which helps to reduce the degree of surface oxidation of the rubber chips.12
Cryogenically ground rubber powders possess a very poor physical binding ability with other polymers due to their smoother surface and low surface area. The particle morphology is represented in Figure 1.6.13 In this respect ambient ground powder is superior to cryogenically ground powder. The physical binding of ambient ground rubber particles with other polymers can be explained by the surface cavities in which polymer chains can easily infiltrate. Generally, the particle size is controlled by the number of grinding cycles or the time spent during the grinding process. However, in cryogenic grinding the immersion time in liquid nitrogen is also a controlling factor for the determination of particle size.12
Grains of rubber granulates obtained through cryogenic grinding have a smoother surface as compared to the grains of rubber granulates obtained through the ambient grinding method. A smoother surface offers almost no occlusion to the elastomers prepared from this rubber compound, on the contrary ambiently ground rubber granulates offers obstacle to the flowability of the prepared elastomers with these granulates. Ambiently ground granulates are spongy in nature, therefore, when measuring the rheological properties such as the Mooney viscosity, ambiently ground granulates offer a higher viscosity over cryogenically ground granulates. Incorporation of ambiently ground rubber granulates into fresh rubber decreases the mechanical strength and reduces the ultimate elongation. However, resistance to wear is improved with the addition of ambiently ground rubber granulates to the fresh rubber. Reduction in mechanical strength can be explained by the reduction in the cross link density of the overall compound. Sulfur migrates from the elastomer matrix into the granulate and the cross link density becomes reduced.6 Properties of the composites can be tailored with variation of the particle size. Composites prepared with rubber ground rubber granulates also experiences some altering properties due to varying particle size and the type of grinding. Ambiently ground granulates offer better interaction with the matrix due to their irregular shape and fuzzy surface. Thus, the mechanical strength of the composites reinforced with ambiently ground rubber granulates have a higher mechanical strength compared to the cryogenically ground granulates. The smooth surfaces of the granulates obtained from cryogenic grinding show limited physical binding with the matrix material and the mechanical properties of the composites deteriorate. Ambiently ground rubber granulates also help to achieve better impact properties through their porous and rougher surfaces. Overall, the process ability of ambient grinding is better than cryogenic grinding.12
In ambient grinding, large particles are obtained when the rubber is ground in a toothed-wheel mill and hence the produced powder is restricted to low-strength applications. Cryogenically ground rubber is often roughened by ambient milling to enhance the surface area.15 The morphologies of the powdered samples obtained through different milling methodologies and different milling instruments are represented in Figure 1.7.5
The basic principle of grinding remains the same when compared with the other processes, however, in this method rubber chips are first swelled in a solvent, or typically in fatty acids and are then pressed against a finer mesh or subjected to the conventional grinding process.
In the ozone cracking process, the waste tire particles are first exposed to a higher concentration of ozone. The material degrades during the exposure to ozone and subsequently mechanical grinding is required for further processing and to obtain a fine powder. However, the obtained fine powder shows a low surface activity due to the oxidation of ozone.12 Similarly the converse process is also applicable, in which cryogenic grinding is coupled with a subsequent ozone cracking process. Monomers can be recovered in this environmental friendly process for hydrocarbon feed stocks. The initial break down of the vulcanized or cross-linked network through ozone cracking promotes subsequent thermal depolymerization techniques.16
The reactivity of the ozone is controlled by the temperature. At low temperatures (near 0 °C) even though the ozone concentration is high; the chemical attack on the double bonds of the rubber become hindered by the low reaction speed. The solubility and diffusion rate of the ozone reduces at low temperatures. Similarly, at higher temperatures (beyond 60–65 °C) ozone becomes unstable and degrades in air. Therefore, the reactivity of ozone cracking reduces. A practical approach is to mimic the actual working conditions of a tire, in which a tire is subjected to sunshine and the vicinity of the air remains cool. Under this condition ozone diffuses quickly through the surface of the rubber and degrades the material. Therefore, a hot rubber surface with cool air in the surrounding environment is best for ozone reactivity.17
Ozone attack on rubber granulate is also dependent on the tensile stress values. Over the past few decades several studies have investigated the stress cracking behavior of rubbers. Cracks grow above a certain threshold of strain values, the threshold value depends on the type of polymer. Generally, cracks grow perpendicular to the direction of the tensile stress, crack growth also depends on the ozone concentration. In dry ozone conditions, rapid crack growth is observed initially on the surface of rubber and subsequently the crack grows at a uniform rate. Ozonization in moist environment occurs rapidly by hydrolysis of the rubbery material.18
The concept of elastic deformation grinding (EDG) originates from the principle of a high shear Banbury mixer. A group of Russian researchers observed that under high pressure and shear force, low-density polyethylene (LDPE) forms a fine powder, when it was subjected to cooling instead of heating. In this process elastic energy was forced into the polymer solution and under shear deformation this elastic energy continues to flow through the polymer until the stored energy of the system is released. The energy stored in the elastomers acts as the energy required for fracturing of the material. Thus, a new surface is formed utilizing this energy. The process was initially designed for the grinding of thermoplastics but later it was also used in the rubber recycling industry. Deformability of the elastomers can be varied by altering the process parameters such as temperature, pressure, feed rate and so forth. The consumption of energy in the EDG process is very minimal. Almost two to three times less energy is consumed in the EDG process as compared to the conventional grinding method. Traditionally, high elasticity rubbers are ground in the conventional cryo-grinding method. The processing cost increases as the coolant consumption is also of concern, in addition to its energy utilization. Therefore, the EDG method is advantageous for rubbers such as isoprene rubber, butyl rubber, and siloxane rubbers. The grinding process is carried out either in a single or twin screw extruder. The principle of the fracture mechanism of EDG can be explained by Griffith's theory of fracture. The fracture mechanism deals with initiation of a single crack and subsequently propagation of the crack, which leads to the overall fracture of the material. Every material consists of several pre-existing flaws which act as stress concentration sites. Microcracks initiate in the vicinity of these stress concentration sites and propagate through the flaws. Eventually it turns into a crack and at some point results in the fracture of the overall material.4 Griffith proposed the forming criterion for the propagation of crack, that is: “A crack will propagate when the decrease in elastic strain energy is at least equal to the energy required to create the new crack surface”.19
In the case of polymers, a chain scission occurs under the application of stress and free radicals are formed at the rupture end. These free radicals are created due to the application of mechanical load and henceforth they are termed as mechanoradicals. Molecular chain scission depends on the bonding energies associated with the atoms. Chain scission occurs when the applied stress exceeds the critical value required for breaking the covalent bonds. Physical and chemical structures of the polymer become modified and a change in melt rheology is observed. Grinding methodology has an advantage in modifying the surface properties in addition to a reduction in particle size contrary to the conventional milling processes in which only size reduction can be achieved. A change in the physical property is achieved through several sequential stages. Fracture occurs through the weak bonds and the concentration of mechanoradicals decreases exponentially with distance from the fracture surface. The critical value of stress required for chain scission is directly dependent on the temperature, chain length and critical strain rate. In the EDG process the effect of shear can cause a change in the molecular weight distribution (MWD). The rate of mechanochemical reactions is generally higher at low temperature regions.4,20
The EDG process is often included in several other process names such as extrusion grinding and solid-state shear extrusion (SSSE). The beauty of this SSSE process is the elimination of the cryogenic coolant. In most cases a twin screw extruder is used in SSSE and a modified version of the twin screw extruder was developed by researchers at the Center of Excellence in Polymer Science and Engineering at the Illinois Institute of Technology, USA. An effective, yet cheaper SSSE process was developed using a single screw extruder by Arastoopour in his patented work, and Bilgili et al. also used the single screw version of SSSE in their work and identified the effect of different dependent and independent variables on the pulverization of rubber granulates using SSSE.15,21
A schematic diagram of the single screw driven SSSE process has been provided in Figure 1.8.15 The single screw extruder uses a screw feeder to feed the rubber granulates. The temperature in the extruder zone is adjusted by an electrical heating system and by an air or water cooling system. Generally, the extruder zone is divided into several functional zones with varying amounts of compression ratios. Rotating square-pitched screws are used in the extruder for pushing the material forward into the next zone and differentiating the extruder in several zones. The minimum compression ratio is set to the zone near to the inlet and granulates are dragged along by the rotating screw with minimum compression. The maximum compression is exerted on the rubber granulates in the last zone, near to the outlet. For a specific design of the rotating screw, the compression ratio is dependent on the channel depth. Apart from the compression ratio, the particle size is controlled by several other variables such as temperature, feed rate, rotation rate, and so forth. Development of the shear stresses on rubber granulates are caused by the relative motion of the screw with respect to the barrel wall. Tensile stresses develop on the material as rubber granulates experience high compressive shear strain in the extruder. A significant amount of strain energy is stored in the process, which leads to the formation of new surfaces through the crack opening mechanism (Mode I). The minimum size of the particle is controlled by the design of the extruder and by varying the processing conditions. Granulates can be fragmented several times in a single processing cycle or in a multi-processing cycle, in which the extruded product is fed into the granulator again. The reduction in particle size can be continued until their size becomes so small, that a high compressive strain and consequent high stresses can no longer be applied. Simultaneously, a higher fraction of finer particle enhances the possibility of agglomeration. Agglomeration in the produced powder can also be observed with a greater degree of compaction, and a higher temperature of the pulverization zone. The extent of fragmentation can be enhanced by the storage of a larger amount of strain energy. An enhancement in the stored energy can be achieved by the conversion of rubber to a more elastic or solid-like material. This can be achieved with a screw with a higher compression ratio and a sufficient cooling system in the pulverizing zone. Consumption of more mechanical energy in the SSSE process is necessary to obtain a rubber powder with a smaller particle size.15
The concept of EDG of polymeric material using the extrusion process was initiated by Enikolopian who extended the idea proposed by Bridgman for pulverizing metals.22,23 Bridgman proposed that metals can be pulverized by applying a high pressure and shear force simultaneously. Bridgman established an apparatus with two disks that could function for both hydrostatic pressure and shearing action. The Bridgman anvil is also used to study the pulverization mechanism of the rubbers and a prototype was designed at the Center of Excellence in Polymer Science and Engineering at the Illinois Institute of Technology, USA. In the Bridgman anvil, the desired shearing effect is achieved by coupling two hardened steel surfaces in which one of the surfaces is held stationary while the other surface is free to rotate. Hydrostatic pressure is applied on top of the upper anvil in order to exert pressure on the material by squeezing the two surfaces. The lower anvil has no temperature controller unit and is allowed to rotate only. The desired shearing effect can be achieved by rotating this lower anvil. A schematic diagram of the major components of the apparatus is given in Figure 1.9.24
The potential pulverization ability of a rubber disk in a Bridgman anvil based SSSE method can be estimated by mapping the strain energy distribution of the material using the computer aided programming software ANSYS. The process is optimized by using different compression and shear forces. An increment in the normal loading on the rubber sample reduces the residence time in the Bridgman anvil, as the stored elastic energy is increased by an increase in hydrostatic pressure. Similarly, an increment in the rotation rate reduces the residence time of rubber particles in the anvil. Shearing is related to the angular displacement of the lower anvil. Hence, energy is stored at a faster rate in the material at a higher rotation rate. Subsequently, dissipation of this stored energy occurs at a faster rate through creation of a new surface (i.e., pulverization). Normal force can be minimized by increasing the rotation rates or increasing the residence time. However, a minimal normal force is required for the initiation of the pulverization process. This minimal normal force is required to generate the necessary friction for storing the strain energy.24
However, heat generation remains a serious concern in controlling the processing parameters of SSSE. The generated heat can cause partial degradation of the material. Fine particles also tend to agglomerate under exposure to this heat. Heat is generated through the conversion of dissipated elastic deformation energy into heat energy. Hence, a modified version of the single screw extruder based SSSE system was designed at Illinois Institute of Technology; USA by Shahidi et al. primarily to provide an isothermal condition at the pulverization zone and to provide an efficient heat removal system. A schematic of the modified SSSE process is provided in Figure 1.10 in which the major working system has been divided into two separate portions; namely, the pulverization section and the extrusion section.25
In this model an additional screw is enclosed in the cylindrical housing to increase the efficiency of the extrusion and pulverization processes. In this elongated cylindrical housing multicomponent screws are attached which rotate independently of each other. Each multicomponent screw has an extrusion portion and a pulverization portion. A higher compression ratio and increment in the shearing forces at the pulverization zone is achieved by reducing the clearance between the cylindrical housing and the screw. Simultaneously, the overall production efficiency is enhanced.25
Size reduction of waste rubber is carried out in two steps. In the grinding process, the breakdown of particles is the primary objective so that the obtained crumb rubber can be fed into the subsequent processing stages. The successive stage is breaking of the chemical bonds (primarily sulfur bonds) in a process called devulcanization. Incorporation of grounded rubber particles into polymer blends is often not suitable due to the presence of a sulfur crosslink network. Compatibility becomes affected by sulfur crosslinks which leads to a weak interface and deterioration in the properties of the final product, therefore, the necessity of devulcanization arises. Devulcanization can be achieved by utilizing several means such as chemical, ultrasonic, microwave, biological, and other methods.
The initial devulcanization method for the reclamation of waste rubber is based on the utilization of several chemicals. Other means of devulcanization for the purpose of breaking rubber chains involve external energy by means of a two roll mill, or by utilizing microwave or ultrasonic forces. However, these processes cannot distinguish between breaking the crosslinks and the main carbon–carbon bonds that form the backbone of the rubber. Therefore, the properties of reclaimed rubber deteriorate with devulcanization. Organic diallyl disulfide,26 diphenyl disulfide,27 inorganic phenylhydrazine–ferrous chloride28,29 and so forth can be used for production of devulcanized rubber. Organic solvents such as toluene and benzene are generally used for swelling of the rubbers as a first step towards devulcanization. The use of hazardous solvents is a serious draw back in terms of environmental issues. Moreover, solvent recovery after the reaction is a serious challenge which imposes additional costs to the end product.30 A green medium, in the form of supercritical CO2 can be used as a reaction medium for some of the chemicals, such as diphenyl disulfide. Supercritical CO2 is able to swell rubbers properly and can easily distribute the cleaving chemicals. It is non toxic in nature and can be removed easily after devulcanization. Moreover, supercritical CO2 is cheaper and non flammable, which makes it suitable for industrial processing.31 Devulcanization can be achieved by controlled oxidation of the carbon framework of rubbers, forming COOH and NO2 groups. Rios et al. achieved this oxidation with nitric acid.32 Nitrobenzene,33 benzoyl peroxide,34 aliphatic amines35 and so forth are some of the other few chemical agents that have been studied by researchers over the years as a devulcanizing agent for rubbers.
Ultrasonic vibration creates a localized energy density that is sufficient to break the cross links of S–S and C–S bonds. Hence, ultrasonic vibration induces cavitations around impurities or voids in the cured rubber. The bond energy of C–C is on the higher side compared to S–S and C–S bonds. Therefore, occurrence of cleavage in the principal backbone of rubber is minimal. Degradation and mechanical property loss is minimal in the ultrasonic devulcanization method. It is a continuous process suitable for industrial applications, as the ultrasonic device is attached in the extrusion path itself. Moreover, no chemicals are used in this process making the process a cleaner and environmentally friendly process. The arrangement and positioning of the ultrasonic device can be varied in the extrusion path. Key variable parameters in the ultrasonic devulcanization method are the devulcanization temperature and amplitude of the ultrasonic waves. Particle size depends on the above mentioned parameters, along with the die pressure in the twin screw extruder.36
In microwave cracking, electromagnetic energy is used to break the cross-links of the sulfur−sulfur (S–S) and carbon−sulfur (C–S) in the rubber with an aim to restoring the conformation ability of the rubber. In microwave cracking a large amount of material can be processed rapidly in continuous processing. Batch processing is also possible with microwave cracking. The power source and time of processing can be varied to achieve the desired amount of cracking. Molecular interaction with the electromagnetic field is necessary in order to break the crosslinks with microwave energy. In this method rubbers are exposed to a specific amount of microwave energy at a specific frequency in order to cleave the carbon–carbon bonds. Generally, microwave cracking is feasible for rubbers with polar groups. However, for non polar rubbers such as styrene-butadiene rubber (SBR), microwave cracking can be achieved with the use of conducting fillers in the rubber compositions. Carbon black is the most popular reinforcing filler for non polar rubbers in this regard. Depolymerization of rubber wastes can be avoided in the microwave method. Therefore, the physical properties of the final product remain almost equivalent to the original vulcanizate. In molecular interactions the electromagnetic energy is converted into heat energy. Temperature in the range of 260–350 °C can be achieved using microwave treatment and a 300 MHz to 300 GHz frequency is generally used in the microwave method. Higher efficiency of microwave synthesis can be achieved with a high temperature and high pressure sintering process. This process applies the heat energy very quickly and uniformly to the waste rubber.37,38
The negative impact of the common devulcanization process involves the emission of CO2 and other hazardous byproducts such as SOx which overall affects the global warming of the environment. In this scenario, the biological process is an environmental friendly process with the utilization of the least energy. The microbial devulcanization processes uses sulfur-oxidizing and sulfur-reducing bacteria.39 However, vulcanized rubbers can resist bacterial attack. Several studies have been conducted on the devulcanization technique using different types of microorganisms. The sulfur bonds of the vulcanized elastomers are cleaved with the use of microorganisms. A temperature controlled bioreactor is needed, in which finely ground rubber powders are mixed with media containing the appropriate bacterium. The cleaving mechanism through the bacterium varies with the species of bacterium used in the process. Starting with approximately ten days to a few hundred days, the contact time may vary by a wide range. After processing, the newly formed devulcanized rubber materials are rinsed off properly in order to remove the microorganisms.40
Actinomadura sp., Actinomycessp., Gordonia sp., Dactylosporangium sp., Micromonospora sp., Streptomyces sp., Thermomonospora sp., Xanthomonas sp. and so forth are some of the rubber degrading bacteria used. Bacteria from the CNM (Corynebacterium, Nocardia, Mycobacterium) group are some of the most effective rubber-degrading strains. Similarly, species of Penicillium, Aspergillus, Cladosporium and so forth. are some of the rubber degrading fungi. In microbial attack the whole carbon skeleton is exposed to the microbes and there remains a possibility of degradation of the whole structure.40
The concept of biodesulfurization of the sulfur–carbon bonds of dibenzothiophene (DBT) without disrupting the main carbon structure is applied in case of biodesulfurization of tire rubber. Rubber products contain several curing and anti-ageing agents which reduce the efficacy of bacterial treatment. Therefore, additives need to be removed from the rubber before applying any bacterial treatments. Cryo-grinding and successive removal of toxic additives by extraction with ethanol is a common approach used before the microbial devulcanization method.41
Devulcanization through physical deformation is achieved by repeated deformation of rubber particles, temperature and pressure are the main process variables. Apart from varying the process parameters, the design parameters can also be altered to enhance the efficacy of mechanical devulcanization. The efficacy of the process can be enhanced with the utilization of steam for devulcanization. Sometimes chemicals such as caustic soda are used in combination with steam to achieve a pronounced devulcanization effect.
Shredding and breaking down of waste tire materials produces newer surface areas and the energy consumed during this breaking down process is directly proportional to the newly formed surface areas. For a given quantity of materials with a given reduction ratio (initial particle size to the final particle size) the energy consumption requirement is always constant. Apart from this the required energy for shredding and chopping is also dependent on the generated heat and pressure during the grinding process. Sometimes, heating can induce a positive effect on the overall grinding efficiency. However, excess heat generation can lead to the depolymerization of rubber particles and production fumes, which give the potential risk of fire hazards, in addition too much heat deteriorates the mechanical properties of the ground materials. Surfactants are introduced in the spray method to reduce the particle size, which is similar to liquid cutting additives for metal forming technology.11
In grinding two processes occurs simultaneously: (i) formation of smaller particles with fracturing; and (ii) aggregation of particles. A model has been proposed by Prut et al. to control the parameters regarding the aggregation of particles. The proposed model assumes a spherical geometry of the ground particle.42
The term k1n predicts the grinding rate, and the term k1 is dependent on the ratio of material deformation to the particle fracture energy. The term k2n is associated with particle aggregation rate. With an increase in the grinding time particle formation approaches the aggregation rate and the radius of the formed particles depends on shear deformation, fracture energy and surface energy.42
|Coarse PR||1400–500||12–30||Grinding mill, rolling mill, rotary crushing mill||Ballast mats, the raw material of degrading regenerated rubber|
|Tiny PR||500–300||30–47||Rolling mill, rotary crushing mill||The raw material of oiling regenerated rubber|
|Fine PR||300–75||47–200||Cryogenic mill, freezing crushing mill||Products of molding and extrusion, rubber mats for crossties, soft pipes for irrigating vases, and modified asphalt for paving|
|Ultrafine PR||Under 75||Above 200||Rotary colloid mill||Used in renewing of tires (under 20 µm, 30 phr)|
The grinding of rubber products is the preferred waste recycling route due to its economic and environmental advantages. Reduction in particle size can be achieved only by grinding of waste rubber products. However, compatibility with the matrix can be enhanced by breaking the C–S crosslinks. Therefore, grinding of rubber is followed by a devulcanization process. Grinding of rubber can be achieved in ambient conditions, as well as at a sub zero temperature known as ambient grinding. Ambient grinding does not necessarily confine the temperature limit to a room temperature range. The processing temperature can reach up to 130 °C in the ambient grinding process. Ambient grinding imposes a rougher surface on the processed powder and hence physical binding of the rubber particles with other matrix material is observed when using this grinding methodology. Kinetic energy to break the particles can be achieved by applying a water stream jet. Elastic deformation grinding is yet another fascinating technique for size reduction of the particles. Similarly, the devulcanization method is also carried out in several processes. Each of them has their own limitations and advantages. Therefore, selection of the process is purely dependent on the end application and the cost of the product.
© The Royal Society of Chemistry 2019 (2018)