Rubber Recycling

Rubber Recycling

S. N. Chakravarty* 

KPS Consultants & Impex Pvt. Ltd.,

 812, Devika Tower, 6 Nehru Place, New Delhi -110019, (India)

E-mail : kpspltd@gmail.com

Today’s industry and manufacturing process are guided by 4 Rs.

       REDUCE            -   Materials, Resources & Wastage

       REUSE               -   Process waste 

       RECYCLE          -   Up cycle 

       RECOVER         -   Polymers / Rubbers from used / scrap products

Introduction

Discovery of vulcanization by Charles Goodyear in USA and Thomas Hankock in UK established the base of rubber industry and there was an increased demand for rubber.  Initially this demand was met by Natural Rubber and subsequently different types of Synthetic Rubber were introduced and widely used in rubber industry.  However, the price of virgin rubber and it’s fluctuation due to many factors stimulated the interest of inventors and attracted their attention towards the waste vulcanized rubber generated during production of rubber goods.

This waste / scrap rubber had to be de-vulcanized to get to a plastic state so that it can be further processed. Reclaiming is essentially de-polymerization of vulcanized rubber. The sulfur used for vulcanization remains in the product. Reclaimed rubber is primarily made from natural rubber based products.

China and India are the largest manufacturer and consumer of reclaimed rubber. India produces nearly 130,000 MT of reclaim rubber per annum which is also fully consumed.

According to statistics, the production of reclaimed rubber in China increased from 1.1 million ton in 2002 to 2.5 million ton in 2009, accounting for 73% of total recycled rubber products and 81% of total reclaimed rubber worldwide. According to statistics, there are more than 600 companies that produce rubber powder in China, with a total annual production capacity of 5 million ton.

 *President – Elastomer Technology Development Society, India

With the growth of rubber industry and increase in rubber consumption, the demand for reclaimed rubber also increased during past decades.  Reclaimed  rubber offers two prong advantage, namely it reduces the cost of the product and in certain cases gives technical advantage like in calandering & extrusion process.  Of course the proportion of reclaimed rubber in the compound has certain limitation with respect to physical properties and other behaviour.

World Tire production

World total tire sales in 2010 was USD 152 billion. Considering approximately 20% growth in 2 years it is estimated for 2012 at USD 180 billion.

Production of tire of major country are –   China 865 million tires ( USD 79.3 billion ) in 2012, India 124 million tires in 2012 – 13, USA domestic production dropped from 218.4 to 160.3 million tires during 2004 – 2008 ( international economic depression). Considering 20% growth in 2010 if works out to be approx 192 million tires. Japan produced 160 million tires in 2012. European tires production in 2010 was 4.50 million ton (+ 26%, 2008 / 2009 were depressed economic years). European  tire replacement market sales in 2007 was Truck / Bus – 11.6 million, Passenger / LT – 28 million, Motor Cycle 1.6 million.

Annual scrap tire generation

Scrap tires were used for land filling for quite some time but due to detrimental effect on soil restriction to this was introduced. Scrap tires are used as cheap fuel, mainly by the Cement industry. Scrap tires are used as economical alternative to coal as fuel in cement kilns , pulp and paper mills and industrial and utility boilers.

Sector wise used tyre usage in EU in 2010 was

Reclaim rubber                      40%

Energy recovery ( Fuel )       38%

Reuse                                     10%

Retrading                               8%

Landfill / others                      4%     

What is Reclaim Rubber ?

Reclaimed or Re-generated rubber is the product resulting from the treatment of ground vulcanized scrap rubber tires, tubes and miscellaneous waste rubber articles by the application of heat and chemical agents, followed by intense mechanical working. The regenerated rubber has almost the original plasticity of virgin rubber, permitting the same to be compounded, processed and re-vulcanized or reused.

In reclaiming of scrap vulcanized rubber, the scrap is normally ground and is then treated with the application of heat, chemical peptizers and is then intensely worked upon mechanically to partially de-vulcanize (break the bonds between rubber polymer chains) the rubber component. This partially de-vulcanized product is commonly called reclaimed or re-generated rubber and can be compounded, processed and re-vulcanized much like virgin rubber. The availability and processing characteristics of suitable scrap sources has dictated the types of reclaim produced. Reclaimed rubber has become widely accepted as a raw material which possesses processing and economic characteristics that are of great value in the compounding of natural and synthetic rubber compounds.

Different Manufacturing Processes

Reclaimed rubber can be manufactured by different methods of which the most important is the Digester process. The raw material for reclaiming is scrap rubber in a wide variety of forms, mainly tires. The first stage, in all processes, is the cracking and grinding of the scrap rubber to reduce it to a crumb or powder passing through a 20-30 mesh screen

1.         Digester process

2.         Pan process

3.         High pressure steam process

4.         Banbury process

5.         Reclaimator process

Digester Process

Most of the reclaimed rubber produced today is by this process. Tires are cut into smaller pieces after removal of beads of the tire. These pieces are put to the cracker mill (heavy corrugated steel rolls revolving at different speeds thus applying tearing and grinding action on the tire pieces).  Ground scrap is passed through vibrating sieve to remove fine material from the larger pieces which are fed back to the cracker.  This scrap is passed through magnetic separator to remove magnetic metal particles so that no damage to further machinery takes place. 

Reclaiming agent and process oil are added to the weighed quantity of the scrap and put into the digester ( steam heated Autoclave ). High pressure steam and digestion period causes scrap rubber to de-vulcanize.  After digestion, the steam pressure is brought down and water is added / spread to wash the rubber free from digester liquor and compost fiber. This is then conveyed to de-watering press to squeeze out water and then to pass through hot air dryer to reduce the moisture content.  This dried reclaimed rubber is put on refining mill to homogenize the material and disperse it thoroughly by passing through closed tight nip of the refining mill.

Some part of hard improperly de-vulcanized particles remain in the rubber and these are removed which is known as “tailings“. This is then sold as a low price reclaimed rubber used for cheaper molded products.

Fine rubber crumb (20 to 30 mesh) free of fiber and steel is loaded into a digester along with water, chemical reclaiming agents, processing oils, and other additives. The digester is a cylindrical jacketed pressure vessel fitted with a horizontal agitator, and steam or heated thermic fluid can be supplied to both interior and jacket, thus enabling a uniform temperature to be maintained throughout the mass. The contents of the digester are then heated to about             190 ⁰C to 200 ºC at 20 to 22 Kg / cm² pressure and maintained at this temperature for some 4 - 5 hours with continuous mechanical working of the material by means of an agitator. After the process the material is dumped and this is followed by milling, straining and refining in the conventional manner to obtain reclaim rubber in sheet form.

Pan Process

This is a simple reclaiming process. In this case fiber / fabrics are removed from the ground scrap and then reclaiming agents along with oil are added into this finely ground scrap by passing repeatedly through cracker and grinder mills.  This finely ground rubber powder is blended thoroughly with reclaiming agents and process oils  which is then placed on open pans which are put on trolleys at different layers and pushed into an horizontal vulcanizer / autoclave.  High pressure steam ( 14 kg / cm² )  is used for different time as per requirement.

After the process pans are removed and the material is passed through milling operations This is followed by straining and refining in the conventional manner.  Often small amount of China Clay is sprinkled on the mill to avoid sticking of degraded rubber.  Also instead of mills only mixer – extruder type of machine may be used for continuous blending operation.  Straining and finishing of milled reclaimed rubber may be required.

Not only NR and SBR scraps but also IIR (Butyl rubber), CR (Polychloroprene rubber), and NBR (Acrylonitrile Butadiene rubber) compounds are reclaimed by this method. 

The pan process was widely used in the early stages of the reclaim industry before the advent of the wet digester. Its use is now limited to specialty types such as light colored natural rubber reclaim

High pressure steam Process

This process is similar to the above with respect to usage of fiber free coarsely ground scrap mixed with reclaiming agent and process oil etc.  Only difference is the usage of high pressure steam ( 55 to 70 kg / cm² ) in the specially designed autoclave for short period of time ( 10 minutes ). Pressure is reduced suddenly and the reclaimed mass is blown into a cyclone collector where it disintegrates. After drying the mass is processed and refined as described above.

 

Banbury Process

Ground scrap is mixed with reclaiming agent, process oil and small proportion of carbon black which is fed into the banbury having high rpm rotors and high pressure ram.  Intense mixing action increases the temperature substantially       ( above 250°C ) and the de-vulcanization takes place within about 15 minutes. The mass is cooled and discharged on the refiner mill which is further processed as described above.

Reclaimator process

In this process finely ground scrap after removal of the fabric / fiber, is mixed continuously with the reclaiming agent and process oil and conveyed to the Reclaimator.  Reclaimator is a cylindrical unit similar to extruder, having screw for conveying the mass working on it by pressing against the barrel wall. At the clearing end of the unit the clearance between screw and wall can be adjusted.  The process temperature is around 200 °C and the reclaimed rubber level is controlled by the transit time ( to about 4 minutes ). The mass temperature is reduced during discharge and this devulcanized material is further process – refined sheeted etc. as per standard method.

A reclaimator process is used for the continuous reclaiming of whole tire scrap. Fine rubber crumb (30 mesh) free from fiber and steel is mixed with various reclaiming agents and processing oils is subjected to high temperatures with intense mechanical working in a modified extruder for the partial de-vulcanization of the rubber scrap. Depending on the specification of the finished products, fillers may be added to the reclaimed product before further processing. The material from above process is then milled, strained and refined as dictated by the specification of the finished product before being sheeted or extruded into the finished form.

The reclaimator machine, from which the process takes its name, is screw extrusion type with a hopper at one end into which the crumb, previously mixed with oil and chemicals, is fed at a predetermined rate. It generates heat of de-polymerization by mechanical working with the finely ground rubber crumb under pressure, and discharges the mass as partially de-vulcanized rubber. Temperatures in the machine are controlled by alternating oil and water jackets. The discharged material is then milled, strained and refined in the conventional manner to obtain reclaim rubber in sheet form.

Merits & Demerits of each process

The Pan process is an outdated technology. The pan process requires higher process time and hence provides lower productivity, product quality is not uniform, involves higher labour requirement etc. Still Pan process is used only by the small scale manufacturers or specialized synthetic rubber reclaim manufacturers.

Uses of the Reclaimator process is also limited to high capacity plant. The main reasons are

• Suitable mainly for high output operation
• High cost of plant & machinery
• Continuous process – difficult to control various process parameters (residence time of material inside the extruder is 2 to 4 minutes) – higher possibility of wastage.
• More suitable for reclaiming of synthetic rubbers that show hardening during longer recycling period like SBR.

Wet Digester technology is well established and widely used throughout the world especially in China, India and S.E. Asia. In a wet digester process crumb rubber is de-vulcanized at high temperature and pressure along with mechanical working. This gives high level of de-vulcanization and hence produces quality reclaim rubber. The machinery cost is not high making it the preferred process for the production of reclaim rubber. The only drawback of the wet digester process is that it is a batch process.

Manufacturing Process

Crumb Rubber

Processing of scrap tires to make rubber crumb is one of the most common and useful disposal steps followed in the rubber industry. Crumb rubber is widely used in different applications, both in rubber & non-rubber usage. Crumb rubber is the name given to material derived by reducing scrap tyre or other rubber into uniform granules with the inherent reinforcing materials such as steel and fiber removed along with any other type of inert contaminants such as dust, glass, or rock.

Tire Bead Separator

Tire Shredder

Crumb rubber manufacturing machinery is designed to convert whole scrap tires into clean and size reduced particles, usually 20 to 30 mesh size. The first step in the process involves removal or extraction of steel bead wire from the whole scrap tire. The next step is shredding of the whole scrap tire into smaller pieces (say around 60 mm wide pieces). The shredded pieces are then fed by means of a conveyor into a Cracker Mill to reduce size of the pieces further (say around 20 or 30 mm wide pieces). The cracked rubber pieces are fed by means of a conveyor into a Breaker Mill or Granulator to produce rubber powder of 20 to 30 mesh size. Crumb rubber is typically produced through an ambient grinding process. Ambient describes the temperature of the rubber as it is being size reduced. Typically, the material enters the cracker mill at "ambient" or room temperature. The temperature of the rubber will rise significantly during the process due to the friction generated as the material is being "torn apart." Granulator’s size reduction process by means of a cutting and shearing action. Product size is controlled by a screen within the machine. Screens can be changed to vary end product size.

                                                                              

Cracker Mill

Cracker mills - primary, secondary or finishing mills - all are very similar and operate on basically the same principle. The mills have two large rotating rollers with serrations & cuts in one or both of them. The roll configurations are what make them different. These rollers operate face-to-face in close tolerance at different speed in opposite direction. Product size is controlled by the clearance between the rollers. Cracker mills are low speed machines and the rubber is usually passed through 2-3 mills to achieve various particle size reductions and further liberate the steel and fiber components.

Magnetic Metal & Nylon Fiber Separator

The permanent magnetic drum is major cost saving non-electric separator. It is widely used for removing steel pieces etc. from the powder material processed in bulk form. It consists of a stationery permanent magnetic assembly around which the drum shell revolves. The assembly has a uniform magnetic field across the entire drum. It holds ferrous particles to the revolving shell and the non magnetic material falls freely from the shell. Magnetic flow sweepers are used in combination with magnetic drum to remove any particles on the flow. The Nylon fiber separator uses high pressure air flow to separate nylon fabric fluff from the powder rubber by inertial action. The separated scrap nylon fluff is conveyed by high pressure air draft though pipes and collected in bins.                         

Fiber Separator

Rubber Cracker Mill with Magnetic metal separator, Belt Conveyer and Vibrating Sieve

Reclaim Rubber

Following steps are used in the manufacture of crumb & reclaim rubber from whole scrap tire.  

1.      Bead is removed from the tire by De-beading machine, thus removing the steel wire portion from tires. 

2.                  The de-beaded tires are conveyed to a Shredding unit where they are reduced to small pieces (50 to 60 mm).

3.                  These pieces are then conveyed to a Cracker mill to reduce the sizes further (20 to 30 mm).

4.                  These pieces are then conveyed to a Breaker mill to produce fine rubber powder of 20 to 30 mesh size.

5.                  Simultaneously a back-feeding conveyor passes the material through a Metal & Fiber Separator system i.e. through magnets to remove the metal pieces and high pressure air blow to remove the fiber pieces. 

6.                  Simultaneously the material is subjected to Vibratory Sieving separating finer mesh crumb from larger granules. The higher size granules are fed back for further grinding to finer mesh size.  

7.          Such prepared crumb rubber is mixed thoroughly with water, process oil & reclaiming agent and allowed to mature for a while.

8.                  This mixture is fed into a Dynamic De-Vulcanizer and subjected to mechanical working under heat & pressure by steam or other heating medium.

9.                  After a definite period of de-vulcanization, the mass is taken out and fed into a series of mixing mills for homogenization, through mixing and sheeting out.

10.             The material is passed through a Strainer Extruder to eliminate lumps / larger particles / foreign matter etc. and then

11.             Subjected to refining on refining mills followed by sheeting out

12.             It is then packed in bale form for dispatch.

Crumb rubber powder (20 to 30 mesh size) free from fiber and metal is then mixed with process oil (e.g. Pine tar and / or Aromatic oil) 3 - 4 parts and reclaiming agent (like Diaryl Disulphide / Di-Xylene Disulphide) at 1 part level. This is then fed to Dynamic De -vulcanizer or Autoclave heated by Steam / Thermic Fluid etc. to desired temperature (around 200⁰ C) and pressure ( 20 to 22 Kg / cm²) for a definite period of time (around 4 - 5 hours) during which rubber becomes partially de-vulcanized. The material inside the autoclave is continuously mechanically worked upon by means of an agitator (rotating paddle).

Devulcanizer ( Autoclave )

This partially de-vulcanized mass is dumped and then fed onto a Two Roll Mixing Mill followed by preliminary refining on a Kneading Mill (another two roll mixing mill). At this stage often China clay and / or Carbon Black may be added which aid in smoothing the dried stock.

Kneading Mixing Mill

Two Roll Mixing Mill

The mass is then strained using a Strainer extruder to remove any non-magnetic metal or other foreign matter. Finally, this is then subjected to refining using a Refining Mill where the rolls are set with very low nip gap. Any hard, improperly de-vuclanized particles remaining on the rolls are periodically removed to be designated as “tailings” which is used in low priced molded products. Refiner produces paper thin layer which is then collected, multiplied to a thicker sheet and finally made a bale (20-30 Kg), wrapped in polyethylene sheet & put in paper bag.      

        

Strainer Extruder

Strainer Extruder Head

Refining Mill

Typres of Reclaimed Rubber 

There are different types of Reclaimed rubber, primarily based on the rubber scrap source. For example whole tire reclaim ( WTR ) is based on the scrap tire, mainly the tread portion where the filler is carbon black only. This is relatively high grade of reclaim rubber. Other major type of reclaimed rubber is from tubes- based on Natural Rubber (Cycle tubes) and reclaimed rubber from butyl rubber based inner tube and bladder / curing bag.

Now a day’s reclaimed rubber of specialized elastomer like nitrile & silicon are also available in the market. Typical analytical properties of reclaimed rubber is given in the following table.

Reclaim Rubber- Specifications

Properties ( IS 7490 : 1997 )	WTR - R	TRR - N	TRR - I
( Method of Test of RR – IS 6306)
Volatic matter, (%)	1.0	1.0	1.0
Ash (%)	8	7	6
Carbon Black (%)	27±3	25±3	34±3
Acetone Extract (%)	16.0	10.0	10.0
RHC by Difference (%)	50	52	52
Specific Gravity	1.16 ±0.03	1.12 ±0.03	1.13 ±0.03
Tensile Strength (kg / cm²)	8.0	5.0	6.0
Elongation at Break (%)	280	280	450
Hardness at Shore A
Mooney Viscosity ML (1+4 ) @ 100°C.	40±5	40±5	--

WTR – R  Tire rubber from automobiles such as truck, bus and passenger tire

TRR – N  Rubber from cycle tire tubes  principally made of natural rubber

TRR – I    Rubber of tire tubes principally made of isobutylene – isoprene ( Butyl) 

Rubber.

Application and advantages of Reclaimed rubber

Reclaimed rubber is widely used in varieties of rubber products - extruded, calandered, moulded etc.  because of either price advantage and / or technical advantage . Whole tire reclaim (WTR) contain about 50% of rubber hydrocarbon (RHC) by weight and obviously the price is much less than the virgin rubber. Part replacement of such virgin rubber by quality reclaimed rubber works out quite satisfactorily in many compound giving both techno-economic advantage.

Reclaimed rubber having lower Mooney Viscosity compared to virgin rubber are much easily processed i.e faster processing with respect to mixing, extrusion, calendering etc.  resulting in better productivity and lower energy consumption.  The power consumption in mixing reclaimed rubber is lower than that consumed for mixing virgin rubber.  Because of easier processing lower heat development takes place during mixing and subsequent processing compared to virgin rubber.

Incorporation of reclaimed rubber in the compound helps to reduce die swelling and shrinkage in extruded and calendered items. Since reclaimed rubber already contains sulfur it increases the rate of cure of the compound but are generally free from scorching problem and are safe in processing.  Compound with reclaimed rubber has lesser tendency to reversion.  Also reclaimed rubber shows good ageing properties in comparison to virgin rubbers. Reclaimed rubber is used extensively in frictioning where tackiness retention is important like tire carcass.

However, reclaimed rubber have certain disadvantages and negative effect on the rubber products which limits it’s usage in many products and to higher proportion in the compound.  Foremost is lower abrasion resistance of the vulcanizate containing reclaimed rubber thus restricting its usage in such compound which are subjected to wear and tear. The Tear properties of  reclaimed rubber compound is also unsatisfactory.

Major areas where  reclaimed rubber consumption is substantial  are – Tires , Camel back, Inner tubes , Automobile and Floor Mats , Automotive Flaps , Hard Rubber Battery Container , Mechanical Goods, Shoe sector etc.  Obviously the largest quantity of reclaimed rubber consumed is in the tire and related sector.

Price of natural rubber in recent past years have in increase very strongly affecting in finished cost & price. To control cost increase rubber technologist started looking towards reclaim rubber usage. The cost of reclaim rubber is usually around one fourth / one fifth that of virgin natural rubber. Hence, use of reclaim rubber substantially improves economics of the rubber compound and final product. Besides much lower cost as compared to virgin rubber, reclaimed rubber provides various other economic & processing advantages like :

·              Low power consumption during break-down and mixing. During manufacture, reclaimed rubber has already been thoroughly plasticized so that it breaks down and mixes more quickly than the virgin rubber.

·              Lower mixing, calendaring and extrusion temperature.

·              Non-critical calendaring temperature range. Reclaim rubber builds up less internal heat in the calendar bank, imparting more process safety.

·              Fast, Uniform Calendaring and Extrusion.

·              Improves penetration of fabric during coating of nylon tire cord fabric

·              Improves tackiness and holds the same over a broader range of temperature.

·              Reduces and retards blooming of sulfur from both cured and uncured compound.

·              Low swelling and shrinkage on extrusion and calendaring.
·         Low Thermo plasticity. Due to the cross linked structure of reclaimed rubber, its compounds are less thermoplastic than virgin rubber compounds and therefore when extruded and cured in open stream, they tend to hold their shape better.

·              Low volume cost for the product

Broad Types of Reclaim Rubber

·                    Whole Tire Reclaim – Superfine Grade

·                    Whole Tire Reclaim – Fine Grade

·                    Whole Tire Reclaim – Medium Grade

·                    Whole Tire Reclaim – Coarse Grade

·                    Synthetic Reclaim – Butyl / Halo Butyl Rubber Reclaim

·                    Natural Rubber Tube Reclaim

·                    High Tensile Reclaim Rubber

Crumb Rubber is fine mesh rubber powder obtained from scrap rubber tires, tubes and miscellaneous waste rubber articles. Grades of crumb rubber can be

·                    30 – 40 Mesh Size

·                    80 Mesh Size

·                    120 mesh Size  

Composition of Automotive Tire

The composition and properties of tires ultimately affect the range of their potential uses after they are scrapped. The construction and composition of tires vary considerably, depending on their intended application. Table below presents typical materials in a tire and  typical basic composition (% by weight) of different types of tires viz. Cross-ply Truck / Bus tires, all steel Radial Truck / Bus tires and Passenger Steel Belted Radial tires in India.

Typically a Cross-ply truck tire has around 50 % rubber content, around 22 % Carbon Black, 12 % Nylon Fabric, 4 % Steel Bead Wire and various other rubber processing chemicals, process oil, Zinc oxide, Sulfur etc. constitute the remaining 12 % of the tire. Theoretically, the reclaim rubber manufacturing process has 100 percent yield which means 100 tones of input (crumb rubber) would produce 100 tones of output (reclaim rubber). However, in practice there is a very small percentage of output loss to the tune of 1 to 2 %. The output loss is material loss due to factors like evaporation etc. There is no loss in terms of composition of rubber at all.

Typical Material Composition of an Automotive Tire

·              Natural Rubber

·              Synthetic Rubber

·              Carbon Black – Reinforcing Filler

·              Silica – Reinforcing Filler

·              China Clay – Inert Fillers

·              Sulfur – Vulcanizing Agent

·              Processing Oils (Aromatic, Naphthenic, Paraffinic types)

·              Reinforcing Fabric (Polyester, Nylon, Rayon etc.)

·              Waxes

·              Activators : Zinc Oxide , Fatty acids

·              Other Chemicals (Accelerators, Anti-oxidants, Anti-ozonants etc.)  

·              Steel Bead Wire, Steel Cord etc.

·               

Components of a Typical Tire

Component (% by Wt.)	Cross-ply T / B	Radial T / B	Radial Passenger
Natural rubber	42	35	20
Synthetic rubber	8.0	12	30
Carbon black	22	22	23
Fabric (Nylon / Polyester)	12	0.5	3.0
Steel	4.0	18.50	11
Rubber Chemicals, ZnO Sulfur, Process oils, Silica etc.	12	12	13

Source: Standard Input Output Norms in Exim Policy

Above Table shows that around 70 to 75 percent of a waste tire is composed of carbonaceous material. Primary inorganic components of a tire are steel (bead wire, steel cord in carcass / belt), sulfur, silicates and zinc in the form of zinc oxide.

Bulk of goods transport is done by road. Truck tires last much less (even after repeated re-treading) as compared to passenger vehicle tires. Hence, bulk of scrap tires is expected to be Cross-ply Truck / Bus tires. Moreover, due to their lower steel content as compared to All-Steel Radial Truck / Bus tires, Cross-Ply tires are preferred for recycling process such as Reclaim Rubber manufacture and Pyrolysis process.

Compounding

The proportion of reclaimed rubber in a compound to be blended with virgin rubber depends on the product and it’s end usage.  While blending reclaimed rubber with virgin rubber once has to keep in mind about the oil content of reclaimed rubber and accordingly adjust the process oil proportion in the compound in order to maintain the Mooney viscosity of the compound and hardness of the product.

There are some products which are totally based on reclaimed rubber like Hard Rubber Battery container, Automotive, Floor mat. Some virgin rubber however is usually added to reclaimed rubber even if it is at a very low proportion. There are some products were proportion of reclaimed rubber is also quite high like Automotive Flap, Cycle tire carcass etc.  These compounds are in blend with virgin rubber. Reclaimed rubber can as well be reinforced with reinforcing fillers like other general purpose rubber and also the usage of process oil is same.  Same curing agents and accelerators are used with reclaimed rubber as with virgin rubber but the curing is faster than general purpose rubber.  Lower proportion of Zinc oxide and Stearic acid generally used for compounds containing reclaimed rubber.            

 Reclaim Rubber Guiding Specifications

A.        Whole Tire Reclaim - Superfine Grade

Source: Whole Tire Scrap.

Test Methods as per IS 6063

Chemical Properties

1.         Ash (%)                                              7 ± 2

2.         Carbon Black (%)                             27 ± 3

3.         Acetone Extract (%)                          15 ± 3

4.         RHC by Difference (%)                     47 min.

Physical Properties

1.         Specific Gravity                                 1.14 ± 0.02

2.         Tensile Strength (kg / cm²)               40 min.

3.         Elongation at Break (%)                   200 min.

4.         Hardness at Shore A                        59 ± 3

5.         Mooney Viscosity                              25 - 45

           ML (1+4) @ 100°C.

B.        Whole Tire Reclaim - Fine Grade

Source: Whole Tire Scrap.

Test Method as per IS 6063

Chemical Properties

1.         Ash (%)                                              7 ± 2

2.         Carbon Black (%)                             27 ± 3

3.         Acetone Extract (%)                          15 ± 3

4.         RHC by Difference (%)                     47 min.

Physical Properties

1.         Specific Gravity                                 1.14 ± 0.02

2.         Tensile Strength (kg / cm²)               40 min.

3.         Elongation at Break (%)                   200 min.

4.         Hardness at Shore A                        59 ± 3

5.         Mooney Viscosity                              25 - 45

           ML (1+4) @ 100°C.

C.        Whole Tire Reclaim - Medium Grade

Source: Whole tire scrap.

Test Method as per IS 6063  

Chemical Properties

1.         Ash (%)                                              7± 2

2.         Carbon Black (%)                             27 ± 3

3.         Acetone Extract (%)                          15± 3

4.         RHC by Difference (%)                     47 min.

Physical Properties

1.         Specific Gravity                                 1.14 ± 0.02

2.         Tensile Strength (kg / cm²)               35 min.

3.         Elongation at Break (%)                   190 min.

4.         Hardness - Shore A                          59 ± 3

5.         Mooney Viscosity                              25 - 45

           ML (1+4) @ 100°C.

D.        Whole Tire Reclaim - Coarse Grade

Source: Whole Tire Scrap

Testing Specifications as per IS 6063

Chemical Properties

1.         Ash (%)                                              9 max.

2.         Carbon Black (%)                             30 max.

3.         Acetone Extract (%)                          19 max.

4.         RHC by Difference (%)                     46 min.

Physical Properties

1.         Specific Gravity                                 1.14 ± 0.02

2.         Tensile Strength (kg / cm²)               25 min.

3.         Elongation at Break (%)                   150 min.

4.         Hardness at Shore A                        59 ± 3

5.         Mooney Viscosity                              35 - 55

           ML (1+4) @ 100°C.

 Guiding applications of different grades of reclaim rubber

	Super Fine	Fine	Medium	Coarse
Automotive Tire	o	o
Bicycle Tire & Tube	o	o	o
Tread Rubber	o	o
Conveyor belt	o	o	o
Hose	o	o
Moulded Goods		o	o	o
Footwear			o	o
Sheeting / Matting		o	o	o

E.        High Tensile Reclaim

Source: Tire Scrap.

Test Methods as per IS 6063 - 1971

Chemical Properties

1.         Ash (%)                                              4 - 7

2.         Carbon Black (%)                             28 ± 3

3.         Acetone Extract (%)                          14 ± 2

4.         RHC by Difference (%)                     48 min.

Physical Properties

1.         Specific Gravity                                 1.13 ± 0.02

2.         Tensile Strength (kg / cm²)               75 min.

3.         Elongation at Break (%)                   250 min.

4.         Hardness at Shore A                        59 ±  3

5.         Mooney Viscosity                              35 - 60

           ML (1+4) @ 100°C.

E.        Butyl Tube Reclaim Rubber

Source: Butyl Inner Tube  

Test Methods as per IS 6063

Chemical Properties

1.         Ash (%)                                              4 ± 2

2.         Carbon Black (%)                             32 ± 4

3.         Acetone Extract (%)                          9 ± 3

4.         RHC by Difference (%)                     50 min.

Physical Properties

1.         Specific Gravity                                 1.14 ± 0.02

2.         Tensile Strength (kg / cm²)               75 min.

3.         Elongation at Break (%)                   480 min.

4.         Hardness at Shore A                        51 ± 3

5.         Mooney Viscosity                              30 - 45

           ML (1+4) @ 100°C.

Note: Guiding specifications suggested are followed by industry leaders

Product Testing Standards 

Indian Standards

IS: 6306 – 1971 (Reaffirmed 2002) Methods of Test for Reclaimed Rubber.

IS: 7490 – 1997 (Reaffirmed 2008) Reclaimed Rubber - Specification

International Standards

ASTM D 5603-96 Classifies the type of ground rubber. The standard covers:

    - Maximum Particle Size

    - Size Designation

    - Grades

ASTM D 5644-96: Method for Determining Mesh Size for the Particulate Rubber

International Material Property Testing Standards

Property	Unit	Standard
Ash Content	( % )	ASTM D297-18
Carbon Black	( % )	ASTM E 1131
Acetone Extract	( % )	ASTM D297-18
Heat Loss	( % )	ASTM D1278
Rubber Hydrocarbon Content (RHC)	( % )	ASTM E 1131
Specific Gravity		ASTM D297-93
Tensile Strength	Kg / cm2	ASTM D412
Elongation at Break	( % )	ASTM D 412
Hardness	Shore A	ASTM D2240
Mooney Viscosity	@ 100ºC	ASTM D1646

 

Applications of Reclaim & Crumb Rubber

Reclaimed rubber imparts faster processing during extrusion and calendaring than raw virgin rubber, develops less heat during mixing and subsequent processing than virgin rubber, is less affected by continued milling and is less thermoplastic than virgin rubber, minimizes swelling of extruded stock and shrinkage of calenderer stocks, does not tend to revert, has good ageing properties but has limitation with respect to abrasion resistance.

The following list is intended as reference of the kind of products and applications, which can incorporate reclaim & crumb rubber.  

A)        Automotive Tyres, Tubes & Flaps

The different compounds used to make Truck / Bus, Light Truck, Passenger, Agricultural & Implement tires use reclaim rubber added at different levels depending on the application area. Major areas of reclaim rubber usage in tire are Carcass & Bead compounds and that of crumb rubber in Tread compound. Tire Flaps use large proportion of reclaim rubber. Tire inner tubes use butyl reclaim.

B)        Tire Re-treading Material both Pre-cured (Cold) as well as Mould cure (Hot).

C)        Bicycle tires use large proportion of reclaim rubber in different compounds e.g. Carcass, Bead & Tube compounds. Bicycle inner tubes also use large proportion of reclaim rubber.

D)        Hoses & Belting – Conveyor / Transmission etc. use reclaim rubber in friction compounds.

E)        Mechanical Goods / Automobile Floor Mats

F)        Metal Bonded & Molded Products: Solid Tires, Metal to Rubber bonded moulded products, Flooring Tiles, Mud Flaps, Traffic Control Products, Home & Garden Products, Wheels etc. 

G)        Hard Rubber Battery Containers

H)        Shoe Soles / Heels

I)          Athletic & Recreational Applications

Soil amendments, Top dressings, Playground surfacing,Parking lots applications, Indoor / Outdoor Equestrian Footing,Asphalt, Mats, Running Tracks  etc.

J )        Rubber Tiles / Flooring

K)        Crumb rubber used in Asphalt modification / Road construction.

     

Quality Control

A well equipped testing laboratory for quality assurance is necessary . The testing laboratory to have a physical testing section, a chemical testing section and a sample preparation section. Physical properties of the reclaim rubber compound that need to be tested are Specific Gravity, Tensile Strength & Elongation, Hardness, Mooney Viscosity and Rheometry. Chemical properties tested will be Ash Content, Acetone Extract, Carbon Black content, Volatile Matter content and Rubber Hydrocarbon content etc. Crumb rubber mesh size is measured by means of vibratory sieves of different size.

Pyrolysis of scrap tires

The pyrolysis method for recycling used tires is a technique which heats whole or shredded tires in a reactor vessel containing an oxygen free atmosphere and a heat source. In the reactor the rubber is softened after which the rubber polymers continuously breakdown into smaller molecules. These smaller molecules eventually vaporize and exit from the reactor. These vapors can be burned directly to produce power or condensed into an oily type liquid, generally used as a fuel. Portion which do not condense remain as a gas which can be burned as fuel. The minerals that were part of the tire, about 40% by weight, are removed as a solid charred mass.

Pyrolysis is an endothermic process (a process that requires energy input) that induces the thermal decomposition of feed materials without the addition of any reactive gases, such as air or oxygen. The thermal energy used to drive the pyrolysis reaction is applied indirectly by thermal conduction through the walls of a containment reactor. Pyrolysis typically occurs at temperatures between 400° and 800° Centigrade. As the temperature changes, the product distribution (or the form of the product) can be altered. Lower pyrolysis temperatures usually produce more liquid products and higher temperatures produce more gases.

The speed of the process and rate of heat transfer also influences the product distribution. Slow pyrolysis (carbonization) can be used to maximize the yield of solid char. This process requires a slow pyrolytic decomposition at low temperatures. Rapid quenching is often used to maximize the production of liquid products, by condensing the gaseous molecules into a liquid.

Pyrolysis is typically a multi-step processes such as (1) feedstock preparation, (2) introduction of the feedstock into the reactor, (3) the pyrolytic decomposition reaction, and (4) separation and post- processing of the gases, oils, solid char.

The method used to prepare and introduce feedstock into the reactor can vary depending on the specific nature of the scrap tires used or the processing system. One common method of preparing feedstock is shredding it to promote a more favorable reaction for the material after entry into the pyrolyzer. The feedstock can be introduced into the reaction chamber by a number of methods, including gravity feeding, bottom feeding, or through the use of containers. In many cases, the feedstock material is introduced into the reactor using an airlock system to reduce or eliminate the introduction of oxygen into the system.

The reaction vessel is one of the most variable components of the system design for pyrolysis or gasification processes. The reactor type used depends on a number of variables including the type and preparation of the feedstock and the operating conditions required for the appropriate reactions. Reactors can be characterized as either vertical or horizontal types. A rotary kiln is an example of a horizontal reactor. The three main types of vertical reactors are fixed bed, fluidized bed, and entrained bed.

The main part of the scrap tire pyrolysis system is the indirect heated horizontal rotatory kiln. The horizontal rotatory kiln is heated indirectly by means of hot air. The temperature inside the horizontal rotatory kiln is maintained in the range of 600 ºC to 800 ºC. The horizontal rotary kiln has an automatic and continuous charging and discharging system. Tire pieces are pyrolized while moving along a feeding screw channel from low temperature to high temperature zone. The screw speed in the feeding zone and pyrolysis zone is controlled by means of AC Motor and Variable Frequency Drive. Feeding of scrap tire pieces as well as

pyrolysis zone screw speed can be easily adjusted according to the temperature in the rotary kiln and particle size of input raw material.

In the screw feeding zone, residual hot air from the burner is fed to pre-heat the material inside. Hence, it raises the temperature of the material before entering the pyrolysis zone. This increases the pyrolysis process speed. While residual heat of the hot air is absorbed by the material, temperature of the exhaust air is lowered (cooled) before emitting to the atmosphere. Utilization of the waste heat is an effective way to save energy and also a positive answer to the Greenhouse effect.

After preheating the material passes into the pyrolysis zone where it is broken down into gases and coke by the action of heat in the absence of oxygen. The pyrolysis coke after cooling is discharged onto a magnetic separator conveyor belt. The steel pieces are separated from the pyrolysis coke. The pressure screw feeding device ensures an even distribution of heat, complete breakdown of the material and also separation of gas and the pyrolysis coke.

The pyrolysis gas after filtration is passed into a condenser. Part of gas is condensed to pyrolysis oil. The non-condensable part (waste gas) is used to generate steam.

Waste Tire Pyrolysis Process and Typical Output Constituents 

Tire granulates or shredded tires are fed into the pyrolysis reactor (rotary kiln) and indirectly heated to about 600 - 800 ºC in the absence of oxygen. Primary output products are Pyrolysis gas, oils and coke / char (Refer Table 1). The non-condensable gases are used to provide process heat to sustain the pyrolysis process. The pyrolysis oil may be further processed (distillation process) to produce higher grade furnace oil, diesel, gasoline etc. Alternatively it may be used as a feedstock for the production of carbon black. The pyrolysis coke undergoes magnetic separation to remove the steel wire pieces. The pyrolysis coke may be used to manufacture value added products like Activated Carbon, higher grade of carbon black (hence higher price realization) through ash removal and fine grinding etc.

 Output constituents ( % by weight ) for a Typical Waste Tire Pyrolysis process    

Primary Products	Wt. %	Constituents	Secondary Products Or Usage
Pyrolysis Gas	05 - 10	Hydrogen, CO2, CO, Methane, Ethane, Propane, Propene, Butane, Other hydrocarbons,  around 1% of Sulfur	·       Generate process heat for pyrolysis ·       Steam generation ·       Power generation
Pyrolysis Oil	40 - 45	High aromatic MW 300 - 400 Low in sulfur (0.3 - 1.0%) Aromatics, Alkanes, Alkenes, Ketones, Aldehydes	·       Furnace Oil ·       Diesel Oil & Gasoline by distillation process
Pyrolysis Coke	30 - 35	~15 % of Ash (mainly ZnO &      Silicates)  3 - 5 % of Sulfur	·       Activated carbon ·       Ash removal & micro fine grinding for refined carbon black ·       Granulated wet process carbon black
Steel scrap	15		·       Scrap sale

Gases produced from tire pyrolysis are mainly hydrogen, carbon dioxide, carbon monoxide, methane, ethane and butadiene, with lower concentrations of propane, propene, butane and other hydrocarbon gases. The pyrolysis oil is highly aromatic with sulfur content below 1% and average molecular weight in the 300 - 400 range.

Tire pyrolysis performed in an inert environment can produce 33 - 38 wt. % of pyrolysis coke or char. It has been reported that the char yield increases with decreasing pyrolysis temperature and decreasing heating rate. The surface area of a tire char produced by pyrolysis in an inert gas usually ranges from 30 to 90 m² / g. Chars from tire pyrolysis contain as much as 15 wt. % of ash, with the majority of this ash being zinc oxide, silica etc. A means of removing the ash from tire char is an important issue in the process of producing useful carbon black from waste tires.

Carbon as an adsorbent is usually evaluated by its surface area. Measurement of surface area can be obtained by a gas adsorption method, for example nitrogen BET. As mentioned above, the surface area of tire char is in the range of 30 - 90 m² / g, which is comparable with those of carbon blacks used in rubber products. However, these values are too low for use of the char as activated carbon since commercial activated carbons have surface areas of around 800 – 1000 m² / g. Therefore, an activation process is required to produce activated carbon from tire char. Carbons can be activated by mild oxidation with steam or carbon dioxide at high temperatures to develop internal surface area.

    Figure 1: Tire Pyrolysis Production Process Flow

 Pollution Control Norms & Comments

As per Environment (Protection) Rules the Effluent Standards notified by the  Government of India following limits are to be followed. Other countries regulations might be similar to these.

For Rubber Processing & Product Industry

 

Parameters	Limiting value for concentration in mg / L
	Inland Surface Water	Land for irrigation / Public sewer
pH	6.0 – 8.5	6.0 – 8.5
Suspended Solids	100	200
Oil & Grease	10	10
BOD, 3 Days at 27 ºC	30	100
COD	250	---
Total Nitrogen	100	#
Free Ammonia	5	#
Ammonical Nitrogen	50	#
Sulphides as S	2	#
Total Dissolved Solids	2100	2100

# Norms for these standards shall be prescribed by the concerned State Pollution Control Boards

For Rubber Reclamation Unit

 

Parameters	Limiting value for concentration in mg / L
	Inland Surface Water	Land for irrigation / Public sewer
pH	6.0 – 8.5	6.0 – 8.5
Suspended Solids	50	100
Oil & Grease	10	10
BOD, 3 Days at 27 ºC	50	#
Lead	0.1	#
Zinc	5	#

# Norms for these standards shall be prescribed by the concerned State Pollution Control Boards

Air Emission Standard

	Concentration not to exceed mg / Nm³
Particulate Matter	150
Volatile Organic Compounds	50

 All rubber units should channelize their fugitive emissions through a stack having a height of 12 meters or 2 meters of roof top of shed / building whichever is more.

 Modern technology as well as plant & machinery can easily meet the above effluent & emission norms. Reclaim rubber manufacturing does not require any process water and hence effluent discharge is very less. Limited quantity of water is required for cooling of mills etc. and may be required for washing of scrap tires. No dust or particulate matter is generated during manufacture of reclaim rubber. Fugitive emissions during the reclaiming process are limited and controllable.

Carbon Credit

Many countries have system of awarding carbon credit for recycling process and manufacture of recycled material.

New Developments

 Green Carbon L.L.C. and OTR Wheel Engineering in Rome, Ga., said the hybrid recycling process has been developed which can break down tires of any size. Speakers at the 2012 Rubber Recycling Symposium in Toronto said they had yet to see such processes become commercially viable

Green Carbon’s technique processes the whole tire, with automatic separation of carbon black, oil, gas and steel. Unlike pyrolysis, which destroys the structure of carbon black and turns it into char, Green Carbon’s process creates carbon black that can be incorporated into tire manufacturing. An average passenger tire can produce approx.3.4 Kg of carbon black under the Green Carbon process. The Green Carbon recycling process also creates a gas from scrap tires that generates 88 percent of the energy the process needs. Although the Green process differs from pyrolysis, it shares the goal of breaking down tires into their components parts for reuses.

Aubrun University is seeking a license or development partner for an invention that separates nylon from post-industrial or post-consumer carpet waste and makes it available for recycling. This recycling technology has application in

• Carpet and textile industries
• Chemical and polymer industries
• Nylon recycling industry

With the continued increase in the production of polymers, there is an urgent need for recycling for three important reasons: 1) the increasing costs of manufacturing virgin polymers due to rising oil prices; 2)  manufacturing requires the use of precious natural resources that are in diminishing supply; and 3) most polymers are virtually non-biodegradable.

Recycling becomes much more complex and expensive once polymers are combined with other materials, often becoming more expensive than landfill disposal and production of new product from virgin materials. Due to a lack of an economical recycling process, an estimated 4.7 billion pounds ( 2.1 billion Kg ) of carpet were discarded in 2002. A clear need exists for an economical process for recycling of polymers contained in mixed materials.

Continental A.G. is predicting car tires could be made of 100 percent renewable raw materials by 2020. Non-oil-based materials content of modern-day tires already is at about 45 percent, but increasing that to 100 percent is not a straight-forward process.

Also, the widespread replacement of fossil materials with renewable raw materials is not always a solution since requires acreage that might already be used in food production.

Conti are working on include replace fossil oils with rapeseed oils and polyester with rayon for reinforcing materials. In addition, the tire maker is increasing the quantities of recycled rubber from old tires to replace virgin synthetic and natural rubber.

Titan Mining Services business has started Titan Tire Reclamation Corp., a partnership with Green Carbon L.L.C., which is part of Rome, Ga-based OTR Wheel Engineering Ltd. Green Carbon has developed a reclamation process Titan claims is capable of handling mining tires and conveyor belting. System can accommodate tires in sizes up to 59,R63, and each of these size tires yields 450 to 500 gallons (@ 3.785 l @ Rs. 50 / l ) of oil, (1893 l  x 50 = Rs. 95,000 ), approximately 4,000 pounds  (@ 454 gm ) of carbon black (1816 Kg @ 60 / Kg =           Rs. 108,960 ) and about one ton of steel @10 / Kg = Rs. 10,000 . Total = Rs. 213,960. The recycling needs to be brought to the mining company sites where the used tires accumulate. The green Carbon recycling process also creates a gas from scrap tires that generates 88 percent of the energy the process needs.    

Continental A.G. has developed a process to separate steel cord and uncured rubber compound, allowing the tire market to reclaim the compound and thus make manufacturing more sustainable.  Since the material in question is uncured, separation the rubber from the steel cord is achieved by applying mechanical force, Conti said. Once the rubber has been granulated, a series of high-powered magnets remove any remaining pieces of steel from the compound.

A new continuous rubber recycling technology has been developed for crosslinked rubber waste for the purpose of obtaining recycled materials of high quality, as reclaimed raw rubber and thermoplastic elastomer (TPE). A new continuous rubber recycling technology has been developed mainly for crosslinked EPDM waste.

The continuous recycling process for crosslinked rubber waste is performed in a modular screw type reactor as schematically shown in Fig. below In the first pulverizing zone of this process, roughly crushed rubber materials is converted into fine particles by high shear stress, and heated to the de-vulcanization reaction temperature quickly. 

Schematic illustration of the reactor for the product of reclaimed rubber

Application of continuous recycling technology to tire rubber waste

The amount of various rubbers, such natural rubber (NR) , synthetic-butadiene rubber (SBR) and butyl rubber (IIR), consumed for the production of new tires correspond to about 70% of the total weight of raw rubber materials annually consumed in Japan. This result in one million tons of scrap tires generated per year, Hence, the continuous rubber recycling technology in this study was also applied to tire rubber waste, generated from both manufacturing products and scrap tires, to investigate its application potential here.  

This new technology enables to continuously produce high quality reclaimed rubber with a twin screw extruder, while cutting mainly S-S crosslink by applying suitable energy such as heat, pressure and shear stress. The manufacturing process is more simple and smaller space than those of exiting Pan method and the processing time is greatly shrunk by one sixtieth of Pan method. Furthermore, human safety is improved thanks to its automated process. 

The Laccaster-Banbury process is one of the oldest examples in which fiber-free coarse ground scarp in mixed reclaiming agents and sheared in a high speed, high pressure internal batch Mixer. The material reaches high temperatures (around 250°C) in 3-12 minutes and is then cooled, refined, and strained. The Fickler reclaiming process uses the same concept but is carried out in a continuous, twin-screw devulcanizer instead of an internal mixer .

One of the continuous reclaiming processes developed made use of a “reclaimator” which in essence is a single screw extruder with a feedstock of 0.6 mm ground, fiber-free rubber scrap. The rubber is subjected to high shearing action between the screw and the wall of the extruder barrel.  Temperature between175-205°C are applied and the residence time is between 1 and 3 minutes . The end of the extruder is cone shaped. Only the material that has well plasticized will be able to pass this cone.