Recycling of rubber: an overview.

The interest in recycling of rubber has been increasing over the last decade. This has been driven by the concern over scrap tires' and rubber products' effects on the environment. Many tire companies, trade associations, government agencies and private recycling firms have expanded their efforts in solving the problem. In the U.S., an estimated 270 million tires a year are scrapped. It is also estimated that over 800 million used tires are stockpiled in various tire piles all over the U.S. Large piles are reported in Ohio, California and Texas.

In addition, it is estimated that over 350 million pounds or more of rubber are scrapped from the production of non-tire goods in the form of runners, trim and pads. Many landfills are closing to scrap rubbers. Numerous small, medium and large rubber companies we trying to find a way to deal with their scrap and we looking for ways to reuse it. As a result, many rubber companies am looking for ways to utilize their own scrap. In addition, there is an issue on how to deal with scrap flat belts and roofing taken out of service. At the present, most of these are landfilled. In the case of the belts, the fiber and steel are difficult to deal with, and in the case of the EPDM roofing much of this is badly aged

Tire derived fuel

It is estimated that over 172 million tires (64%) of the total were used as tire derived fuel (TDF) in the U.S. in 1997, representing the single largest use of scrap tires. These were used by cement, paper and pulp plants, and power generating boilers at over 100 locations. Figure 1 summarizes the total utilization of scrap fires in the U.S. for 1997 (ref. 1). TDF is expected to continue to grow as more and more cement plants, power generation facilities, pulp and paper plants continue to expand their use of scrap tires for fuel. Rubber has a very high fuel value, higher than most coal. As a result, it is possible to replace or supplement other fossil fuel burning. Figure 2 shows the relative fuel value of some common fuels and compares them to rubber. The purpose of this article is to review the technical uses of scrap tires and the discussion of TDF is beyond its scope. A more thorough review of TDF can be found in reference 1.

[Figures 1-2 ILLUSTRATION OMITTED]

Overview of scrap tire use

With the efforts of so many companies and recycling firms trying to develop new uses for scrap tires, the result has been many new and expanded uses. Table 1 summarizes the market demand for size-reduced rubber.

Table 1 - U.S. market demand for size-reduced rubber from tires

                                    1996        1998(*)

Pneumatic tires                  48 million     140 million
Friction materials                8 million     8.5 million
Molded and extruded goods        18 million      24 million
Rubber/plastic bound products   168 million     200 million
Athletic and recreation          24 million      50 million
Asphalt products                134 million     160 million
Total                           400 million   582.5 million

(*) estimated pounds

Many of the non-tire components using recycled rubber are in static applications. Much work is under way looking at using recycled rubber in more dynamic applications such as body seals, gaskets, goal is to utilize 25% post consumer scrap in the automotive non-tire components and 10% recycle content in tires (refs. 3 and 4).

Robert Pett of Ford reports that the automotive industry's goals are to have no more than 15% of the vehicles retired from service go to landfills by 2002, and this is to be reduced to 5% by 2015 (ref. 5). The automotive and transportation industries are the biggest consumers of rubber goods, using an estimated 70-75% of all rubber articles produced.

Asphalt in roads

Experiments to use ground scrap tires in roads have proven useful in some areas. The product is known as Crumb Rubber Modifier (CRM). California, Arizona, Texas and Florida use crumb rubber modifier in their asphalt roads. Efforts to make this a national standard through legislation were tried in 1991. A bill called the Intermodal Surface Transportation Efficiency Act or ISTEA required incorporating 20 pounds per ton of CRM to portions of roads being paved. Because of the increased cost of doing this, many states resisted, and in 1995 the act was repealed.

Recycling methods

At the present time the major methods of producing recycled rubber are reclaiming, ambient grinding, cryogenic grinding and wet or solution grinding. Reclaiming of silicone and butyl rubbers is common and the resulting recycled products are useful for cost reduction and improved processing when added to virgin compounds.

Reclaiming

In the past, large quantities of whole tire, tread peel, tubes and other products were reclaimed using various reclaiming agents. Aryl sulfides and other rather unpleasant smelling chemicals were used to treat the rubber. The resulting product was used extensively in tire manufacture. The product improved processing and was relatively inexpensive. As much as 700 million pounds per year were reported used in the U.S. However, the radial tire didn't use reclaim, environmental regulations were tightened and SBR rubber prices were lowered-resulting in almost the complete elimination of reclaim in the country. Presently there are two reclaimers in the U.S. - U.S. Rubber in Vicksburg. MS and TRC in Stow, OH. In addition. Vredestein in the Netherlands is reported to sell low odor reclaim from whole tire, natural rubber and butyl. It currently is used in mats, bumpers, chocks and other low dynamic stress rubber articles.

Ambient vs. cryogenic grinding

Vulcanized scrap rubber is first reduced to a 2"x2" or 1"x1" chip. This can then be further reduced using ambient ground mills or ground into fine particles while frozen using cryogenic grinding.

The ambient process often uses a conventional high powered rubber mill set at close nip. The vulcanized rubber is seared and ground into a small particle. It is common to produce 10 to 30 mesh material using this relatively inexpensive method to produce relatively large crumb. Typical yields are 2,000-2,200 pounds per hour for 10-20 mesh and 1,200 pounds per hour for 30-40 mesh. The finer the desired particle the longer the rubber is let run on or in the mill. In addition, multiple grinds can be used to reduce the particle size. The lower practical limit for the process is the production of 40 mesh material. Any fiber and extraneous material must be removed using air separation or an air table. Metal is removed using a magnetic separator. The resulting material is fairly clean.

The process produces a material with an irregular jagged particle shape. In addition, the process generates a significant amount of heat in the rubber during processing. Excess heat can degrade the rubber and if not cooled properly combustion can occur upon storage.

Cryogenic grinding usually starts with chips of a fine crumb. This is cooled using a chiller with liquid nitrogen as the medium. The rubber while frozen is put through a mill. This is often a paddle type mill. The final product is a range of particle sizes which are sorted and either used as is or passed on and further size reduction performed e.g. using a wet grind method. A typical process generates 4,000 to 6,000 pounds per hour.

The cryogenic process produces fairly smooth fracture surfaces. Little or no heat is generated in the process. This results in less degradation of the rubber. In addition the most significant feature of the process is that almost all fiber or steel is liberated from the rubber resulting in a high yield of usable product and little loss of rubber. The price of liquid nitrogen has come down significantly recently and cryogenically ground rubber can compete on a large scale with ambient ground products.

Table 2 compares the properties and benefits of ambient and cryogenically ground rubbers. Table 3 shows the particle size distribution for two typical 60 mesh ground rubbers. One was prepared ambient and the other cryogenically. Table 4 shows the effects of using ambient ground SBR in a compound. In addition, the effects of cryogenically ground butyl in a tire innerliner compound are shown in table 5. Table 6 shows the effects of particle size and loading for cryogenically ground EPDM.

Table 2

Physical property    Ambient ground   Cryogenic ground

Specific gravity          Same              Same
Particle shape          Irregular          Regular
Fiber content              0.5%              nil
Steel content              0.1%              nil
Cost                    Comparable       Comparable
Table 3

Amount retained        Ambient         Cryogenic

30 mesh                   2%              2%
40 mesh                  15%            10-12%
60 mesh                 60-75%          35-40%
80 mesh                  15%            35-40%
100 mesh                  5%             20%
Pan                     5-10%           2-10%
Table 4 - cryogenically ground rubber (20 mesh) in an SBR 1502
compound

Formulation
Ingredient              Level

SBR 1502               100.0
Zinc oxide               5.0
Stearic acid             1.0
TMQ                      2.0
N660 carbon black       90.0
Aromatic oil            50.0
Sulfur                   2.0
MBTS                     1.0
TMTD                     0.5

Properties of compound with 17, 33 and 50% crumb addition

                         Control    17%     33%     50%
                                   crumb   crumb   crumb

Mooney viscosity             40       61      91     111
Rheometer max. torque        59       47      33      34
tc90, min.                  2.5      2.4     1.8     2.0
Tensile strength, psi     1,470    1,150     870     560
Ultimate elongation, %      330      330     300     270
Table 5 - cryogenically ground butyl in the innerline

Formulation
Ingredient             Level

Butyl HT-1066           80.0
RSS #1                  20.0
N-650                   65.0
Mineral rubber           4.0
Durez 29095              4.0
Stearic acid             2.0
Sunthene 410             8.0
Zinc oxide               3.0
Devil A sulfur           0.5
MBTS                     1.5

Cryogenically ground butyl at various levels

                           Control     5%     10%     15%

Masterbatch                   188    178.6   169.2   159.8
Cryo ground butyl              --      9.4    18.8    26.2
Properties
Cure time, Tc90, min.        47.5     46.3    47.0    46.5
Cure rate, lbf. in./min.     0.59     0.58    0.55    0.56
Tensile strength, psi       1,410    1,350   1,290   1,280
300% modulus, psi           1,120    1,040   1,000     950
100% modulus, psi             415      410     365     365
Air permeability, 0 (a)      4.71     4.70    4.47    4.16

(a) Q x 10e3 (cubic ft./0.001 inches/[degrees]F psi/day)

Table 6 - cryogenically ground rubber in an EDPM compound

Formulation
Ingredient              Level

EPDM                    100.0
N-650                    70.0
N-774                   130.0
Paraffinic oil          130.0
Zinc oxide                5.0
Low MW PE                 5.0
Stearic acid              1.0
Antioxidant               1.0
Sulfur                   1.25
Sulfads                   0.8
Methyl tuads              0.8
Ethyl tellurac            0.8
Altax                     1.0

Cryogenically ground rubber at 10% levels

                         Control     40      60      80     100
                                   mesh    mesh    mesh    mesh

Tensile strength, psi     1,410    1,290   1,430   1,470   1,440
Ultimate elongation, %      410      330     340     400     380
300% modulus, psi         1,180    1,220   1,230   1,230   1,220
100% modulus, psi           535      490     530     490     480
Hardness, psi                73       70      70      70      71
Die C tear, ppi             193      175     173     171     172

Cryogenically ground rubber at 20% levels

Tensile strength, psi     1,410    1,230   1,380   1,460   1,410
Ultimate elongation, %      410      320     390     390     390
300% modulus, psi         1,190    1,220   1,300   1,200   1,160
100% modulus, psi           535      450     500     460     460
Hardness, psi                73       72      70      69      68
Die C tear, ppi             193      178     163     165     181

Wet or solution grinding

Using a water suspension of rubber particles and a flour grinding type mill, several companies produce a wet ground rubber. The material is finely ground and mesh sizes from 60 to 120 mesh are commonly made and used. These products are employed in many tire compounds because of its uniformity and cleanliness. Figure 4 shows the particle size distribution of different mesh wet ground rubbers.

[Figures 3-4 ILLUSTRATION OMITTED]

A review of scrap tire processing was published by Astafan in Tire Technology International '95 (ref. 7). In addition, microwave (ref. 8), ultrasonic (ref. 9), chemical devulcanization (ref. 10). microbial degradation and mechanical shear are or have been used to produce recycled rubber. Ultrasonic and chemical devulcanization are discussed in detail in the 1996 issue of Tire Technology International '96 (ref. 11).

In ultrasonic "devulcanization," a coarse crumb rubber is exposed to high intensity ultrasonic vibrations. The resulting energy absorbed by the rubber is theorized to fracture sulfur-sulfur bonds and produce a recycled rubber that can be used as is or compounded back into virgin rubber and recured. This process is being developed by National Feedscrew and Machining Co. in Massillon, OH.

Chemical "devulcanization" uses accelerators and sulfur, added on a mill or in an internal mixer to form a coarse rubber particle. It is theorized to fracture sulfur-sulfur bonds and produce a usable recycled rubber. This process is offered by STI-K in Washington, D.C. The reason for quote marks around devulcanization is that in addition to sulfur-sulfur bonds being broken, there is also a lot of depolymerization likely occurring with the heat and shear used in these processes, and it is difficult to quantify exactly what mechanism of bond failure is actually occurring. In any case, the result of the process is a recycled rubber that can be reused.

Surface treatment and additives for producing recycled rubber

Numerous methods have been and are being used to modify the surface or composition of recycled rubber to make it more compatible or useful. These include halogenation, liquid polymers (ref. 12), thermoplastic polymers, homogenizing agents (ref. 13) and wetting agents. These are too extensive to cover in a review article, but they are reviewed in detail in the CWC Best Practices Manual (ref. 14). Many other processes are also under study to enhance the value of ground scrap rubber.

Testing standards

Recently we summarized the quality, testing and handling issues for dealing with scrap tires and rubber for the CWC Best Practices Manual (ref. 6). In December of 1996, ASTM published two documents. ASTM D-5603-96 (ref. 15) and ASTM D-5644-96 (ref. 16). Lite in 1997, the Chicago Board of Trade (CBOT) also published a document which includes definitions of terms and particle size specifications of recycled rubber for buying and selling materials at the CBOT (ref. 17). Thus, there are specifications available for recycled rubber that vendors and customers should use to ensure material quality.

Material storage

Recycling of rubber from whole tires into chips and crumbs can generate heat up to 220 [degrees]-240 [degrees] F. At these temperatures and in the presence of oxygen, spontaneous combustion can occur. Also. the presence of iron catalyzes oxidation of natural rubber. To minimize/delete any such problem. material should be cooled with air or water before storage or shipping. Make sure the material temperature is much below 200 [degrees] F. Avoid metal pieces, store at ambient temperature and not in metal sheds or warehouses at both the vendor's and customer's place.

Moisture content

Current accepted level of maximum moisture content is 1% (ASTM D-5603-96). Typically, however, it is less than 1%. Too much moisture can cause caking and may inhibit free flow in processing. Anticaking agents such as calcium carbonate can be used. Moisture build-up can lead acidic conditions giving slower cure rates in compounds. Therefore, recycled crumb rubber should be stored in a cool and dry place.

Moisture content is determined by heating a weighed amount of sample at 125[degrees]C for one hour, cooling and weighing again. The difference in sample weight is the heat loss (ASTM D-1509-95).

Bulk density

Because of the particulate nature of crumb rubber, it is rather difficult to measure specific gravity of crumb rubber. Bulk density measurement may be more appropriate. There is no bulk density specification for crumb rubber. However, ASTM D-1513 for carbon black can be used. Our recommendation is that bulk density be part of material specification and a range of values be agreed upon between vendor and customer.

Another way of determining specific gravity of crumb rubber may be done by making solid sheets or pieces by passing crumb rubber through a tight mill nip and measuring density of compressed pieces.

Sampling and QA testing

In crumb manufacturing plants, two samples, each about 125 gms., are taken from each skid (about 1,000 Kg) at the time of bagging. At the customer's site, two samples per truckload are tested for % moisture and bulk density. For ash, carbon black, acetone extract and rubber hydrocarbon content, testing is done once/day/shipment according to ASTM D-5603-96. Specific frequency and sampling procedures may be agreed upon between the vendor and the customer. For ambient ground materials, moisture content and bulk density measurements are made on every skid because of possible moisture content variations in feedstock. Vendors should send material specification conformance data along with shipment to customers.

Chemical analysis and material specifications

ASTM D-5603-96 (ref. 15) lists specification for acetone extractable, ash, moisture, carbon black, natural rubber and rubber hydrocarbon contents for recycled rubber. As mentioned previously, these chemical tests are done according to ASTM D-297. Typical chemical analysis results done in the Akron Rubber Development Laboratory on six different commercially available tire crumb samples are shown in table 7 ( ref. 18). The specification (ref. 15) also fists maximum metal content of 0.1% and fiber content of 0.5% in whole tire crumb. Fiber and metal contents in tread buffing should be zero. See table 8 for specifications. In production of recycled rubber, steel wire pieces are separated by a magnetic separator. Fibers are separated by use of a vibrating screen table and vacuuming off the fabric balls from the top of the screen.

Table 7 - analysis results on six different crumb
rubber samples

% Acetone extractables                  10-14
% Ash                                 5.9-7.1
% Carbon black                      29.8-30.8
% Loss on heating                   0.40-0.58
% Rubber hydrocarbon                    45-52
% Isoprene                           14.1-242
Table 8 - properties for recycled rubber
(grades 1-6) (ref. 15)

Property                   %              Test method

(a) Grade 1-4
Acetone extractables     8-22       D 297, Sec. 17, 18, 19
Ash, max.                   8   D 297, Sec. 34, 35, 36, 37
Carbon black            26-38           D 297, Sec. 38, 39
Loss on heating, max.       1                       D 1509
Natural rubber          10-35           D 297, Sec. 52, 53
Rubber hydrocarbon
  content (RHC), min.      42               D 297, Sec. 11

(b) Grades 1-6
Metal content, max.       0.1                    See 7.3.2
Fiber content, max.
  (grades 1,4,5,6)        0.5                      See 7.4
Fiber content, max.
  (grades 2,3)            Nil                      See 7.4

To determine iron content, a pre-weighed amount of recycled rubber is spread on a non-magnetic flat surface. A small magnet is used to go over the material. The magnet should pick up steel pieces. Weight of the material thus picked up is obtained. However, for very small particles of crumb rubber ([is less than] 100 mesh). magnets may pick up recycled rubber particles as well. In that case, atomic absorption (AA) should be used to determine amounts of iron.

Particle size and distribution

Particle size and distribution of recycled cured rubber particulates are determined by the Ro-Tap method as described in ASTM 5644-96. Six sieves are used in this mechanical shaker. The first two screens are defined in the above document for 10, 20, 30. 40, 60, 80 and 100 mesh particle size designations and shown in table 9 (ref. 15). The remaining four screens are to be decided between the vendor and customer. About 100 gms. of crumb rubber are weighed and put on the top pan with a cover and five other screens. After a fixed time of running the shaker, material in each pan is weighed and plotted as a function of screen size giving particle size distributions. Vibrators and sieves are available from most scientific suppliers. This technique works well for coarser particles ([is greater than] 80 mesh). For 80 mesh and finer, small balls are formed as a result of particle agglomeration on screens giving higher apparent particle sizes than they really are.

Table 9 - recycled rubber product designation (ref. 15)

Nominal          Example       Zero screen [micro]m
product        ASTM D 5603
designation   designation(A)

10 mesh         Class 10-X        2,360 (8 mesh)
20 mesh         Class 20-X        1,180 (16 mesh)
30 mesh         Class 30-X          850 (20 mesh)
40 mesh         Class 40-X          600 (30 mesh)
60 mesh         Class 60-X          300 (50 mesh)
80 mesh         Class 80-X          250 (60 mesh)
100 mesh        Class 100-X         180 (80 mesh)

Nominal         Percent          Size           Max. %
product       retained on     designation     retained on
designation   zero screen   screen [micro]m   designation
                                                 screen

10 mesh            0        2,000 (10 mesh)         5
20 mesh            0          850 (20 mesh)         5
30 mesh            0          600 (30 mesh)         10
40 mesh            0          425 (40 mesh)         10
60 mesh            0          250 (60 mesh)         10
80 mesh            0          180 (80 mesh)         10
100 mesh           0          150 (100 mesh)        10

(A) When specifying materials replace the X with the proper parent material grade designation code. For example, Class 30-2 would indicate a 600 [micro]m (30 mesh) product made from Grade 2 material, car, buck and bus tread rubber. Class 100-6 would indicate a 150 pin (100 mesh) product made from Grade 6 material, non-tire rubber.

Several other techniques for determining particle sizes are being evaluated for finer particles. In our laboratory we have developed an ultrasonic technique where a small quantity of crumb rubber is put in a non-solvent liquid and exposed to low levels of ultrasonic energy. The resulting dispersion is put on a glass slide and dried. Using an image analysis software program on a light optical microscope (LOM), particle size distribution is obtained. Our experience is that this techniques works well even with small particles, i.e., up to 1 to 2 microns. Figures 5 and 6 are particle size distributions of a commercial 80 mesh crumb rubber sample determined by the Ro-Tap method and LOM-ultrasonic techniques, respectively. Obviously, the LOM-ultrasonic technique separates particles much better than the Ro-Tap method.

[Figures 5 and 6 ILLUSTRATION OMITTED]

Some of the other commercially available techniques are: Coulter counter (Coulter Corporation, Miami, FL); Malvern Instruments Ltd. (Southborough, MA); Particle Sizing Systems (Langhorn, PA); and Elcan Industries, Inc. (New Rochelle, N-Y).

References

(1.) "Scrap tire use/disposal study, 1996 update," published by the Scrap Tire Management Council, edited by John Serumgard and Michael Blumenthal, 1996.

(2.) "Scrap tire recovery, an analysis of alternatives," published by Goodyear Tire and Rubber Company, Akron, OH, 1998.

(3.) Rubber and Plastics News, published by Crain Communications. February 23, 1998, pp. 22-23.

(4.) Personal communications - Dr. Robert Pen, Ford Motor Company. 1997.

(5.) Op. cit. (3).

(6.) "Best practices in scrap tires and rubber recycling," published by Clean Washington Center, primary authors William Klingensmith and Dr. Krishna Baranwal, June 1997.

(7.) "Scrap tire processing in the U.S.," Charles Astafan, Tire Technology International '95, 1995.

(8.) U.S. Patent 4,104,205, Microwave devulcanization of rubber. August 1. 1978.

(9.) U.S. Patent 5,258.413, Ultrasonic devulcanization, Not-ember 1993.

(10.) European Patent Application EP 0690 091 A I, Application No. 95301399.2, Filed March 3. 1995.

(11.) "Ultrasonic devulcanization of tire compounds." by Boron, Roberson and Klingensmith, pp. 82-84 and "De Link concept" by Dr. Sekhar and Kromer. pp. 87-88, Tire Technology International '96, 1996.

(12.) U.S. Patent 4.481,335, "Rubber compositions and methods," issued to Fred Stark Nov. 6, 1984.

(13.) U.S. Patent 5,510,419, "Polymer modified surface, issued to Burgoyne, Fisher and Jury, April 23, 1996.

(14.) Op cit. (6).

(15.) ASTM D-5603-96, Standard Classification for Rubber Compounding Materials - Recycled Vulcanizate Particulate Rubber.

(16.) ASTM D-5644-96, Standard Test Method for Rubber Compounding Materials - Determination of Particle Size Distribution of Recycled Vulcanizate Particulate Rubber.

(17.) "Crumb rubber (tire or non-tire)," Crumb Rubber Grades Definitions, by Chicago Board of Trade, 1997.

(18.) K. Baranswal, Analysis Results on Six Different Commercial Crumb Rubber Samples, done at the Akron Rubber Development Laboratory, 1996-97.