Development and application of superfine tire powders for rubber compounding.

Today's scientists are looking for cost effective high performance fillers to be used in a wide variety of compounds and emulsions. The major emphasis is on fillers that provide lighter specific gravities and are cost effective. The use of fine rubber powders produced principally from tires and other single source thermoset polymers can offer a very cost effective and engineering solution for roofing, automotive, coatings, sealants and a wide variety of compound applications. The development of super fine rubber powders (particles passing an 80 mesh or 177 micron screen opening) with surface morphologies approaching carbon blacks in surface area and reinforcing characteristics can be functional through weight reductions and improved performance parameters. Future developments call for improving the interfacial adhesion between the fine tire particles surface and the Parent compound by the addition of selected chemicals. These chemicals should improve strength properties including bonding.

This article is limited to a general introduction to the production, development and application of thermoset powders; in particular, tire rubber, as light weight resilient fillers. These fillers perform like the virgin polymers themselves and when used in the proper proportions can, in many cases, show improvement in certain physical properties.

Fine powder processing

Initial preparation

Normally, most recycled polymers are received in bulky form and first must be reduced to a minus four to six inch piece upon arrival at the processing plant for ease of storage and material handling retrieval. The second step involves reducing these six inch pieces into minus 5/8-inch chunks for further reduction to a minus 10 mesh (2,000 microns). If the received materials contain inherent reinforcing fibers, steel or other contaminates, these contaminates are removed by a combination of separation processes during the various stages of size reduction. The minus 10 mesh granules are then sent to the wet grinding department where the particles are reduced to the desired fine particle specification.

The finished powders can then be packaged into paper or polyethylene bags, gaylord boxes, super bags or bulk handled in pneumatic transport trailers.

Grinding technologies

The development of an ambiently produced, high surface area fine rubber powder (< 80 mesh particle size) is relatively new. These powders include tire rubber (principally SBR and SBR-NR blends), EPDM, neoprene, nitrile and natural rubber. Historically, the production of tire powders in the 80 to 325 mesh range was not economically available and the only common method for production of fine rubber powders was produced by cryogenic methods. The cryogenic method of freezing the rubber below its embrittlement point followed by fracturing in a hammermill, limits the particle range distribution and surface morphology of the final sized product. The UltraFine powders particle size range is not limited to a narrow band or particle size range when passing through a specified opening during milling.

Today, ambient ground tire powders are available in a wide range of sizes in the 80 to 325 particle range. A comparison of relative particle size ranges produced by the UltraFine process is listed in table 1.

Of importance is the predicted surface morphology of the fine tire particle. In a limited number of cases, particles with minimal surface area to volume are desirable as in the case of bonding urethane with EPDM or SBR in sport surfaces or developing a porous like material where bond strength is not a factor. Many problems have been encountered in calculating the specific surface area from size distribution using such equations as [S.sub.v] = ([[K.sub.8]/[d.sub.80]).sub.n]. The more direct measurements using air permeability, gas absorption, photosedimentometry and laser granulometery are preferred to defining more quantifiable values. Figure 1 is a summary of these values developed for various type grinds.

Regression analysis indicates fairly good curve fits can be obtained from laboratory data.

The wet fine grind process is unique in that very intense and rigorous cutting paths are required for the particle to travel while being reduced. This particle size reduction process results in highly fractured particle surfaces with fairly precise particle size distributions. More work is required to help explain the sinusoidal looking cure whose cure fit (3rd degree polynomial) is obtained by the surface area measurement procedures noted. From the work performed to date, the normally milling area measurements tend to fall into a linear line. However, more testing and data need to be obtained to verify these early values.

In addition to the high surface area produced with the wet ambient grinding process, the cost of grinding can also be graphically illustrated (figure 2) for processing tire rubber below the 40 mesh level. As expected, particle reduction by liquid cryogens is very dependent on heat transfer and maintaining a very constant, low isotherm condition during the intense material reduction mechanisms encountered with cryogenic grinding.

Coarse versus fine particles 

Conventional mechanical particle size reduction methods are generally limited to the minus 40 mesh range (420 microns). This limitation is caused by the tendency of the rubber surface to thermally degenerate and become tacky when heated. As the thermal degeneration begins, particle reduction ceases and the total process may be reversed as the particles begin to lump, mass or sheet. This is not the case with cryogenics, however, the cost is higher and the surface are-as are significantly lower.

Conventional regrind has found applications in rubberized athletic surfaces, rubber asphalt surfaces, resilient fillers, brake friction linings and many other applications. These "coarse regrinds" can be used as fillers in pneumatic tire production when used in moderation. In fact, one major tire company was using 40 mesh regrind in tread, carcass and sidewall compounds as early as 1960.

Figure 3 is an illustration of a minus 80 mesh ground fine tire rubber (GF-80) against a standard 30 mesh groun rubber. Test data demonstrate that the UltraFine GF-80 is suitable for most rubber compound applications. Homogeneous blend is achieved when UltraFine GF-80 is added to conventional compounds which reduces the degrading of the physical properties normally associated with coarser crumb products. Aesthetic defects which are normally associated with the usage of 30-40 mesh rubber particles in rubber compounds have been nearly eliminated when UltraFine GF-80 is used.

To date, most commercial powders (< 80 mesh or 74 microns) have been produced by milling or cryogenic techniques. In the case of milling, high temperatures are developed (> 360-degrees-F) and cause a degree of thermal degradation to the polymer being reduced. In the wet ambient grinding process, the temperature of the grinding medium, normally water, does not exceed 180-degrees-F. This temperature is well below 300-degrees-F, which is the critical temperature when thermal degradation begins to occur for most rubber materials. Figure 4 reflects the relative surface areas of various types of commonly used fillers in meters (ref. 2) per centimeter (ref. 3) ([m.sub.2]/[cm.sub.3]) available in today's market. This grinding technique maintains the integrity of the particular polymer's rheological and chemical properties. Additionally, the wet slurry processing provides the additional benefit of producing a very clean polymer particle with negligible impurities. Table 2 lists the chemical and physical properties of several UltraFine powders available for filler applications produced by the wet ambient grinding process.

Discussion

Thermosets as light weight, high resilient fillers

Many thermoset compounds containing SBR, NR and EPDM are lightweight organic materials that can add resilience and impact resistance when used as a filler material. Tire powders, such as the UltraFine GF 80, contain antioxidants, antiozonants, carbon black and other materials to improve resistance to atmospheric conditions. The fine particle size ranges available allow the compounder to use these powders in combination with other polymers. The development of specific surface treatments and coupling agents opens up the new area of interest for using thermosets with thermoplastics, called composites. The rubber component in the new composites can add impact resistance, which are the physical properties of the rubber compound, and reduce the overall weight of the new compound.

Traditionally, the cost of a filler was based on its cost per pound. In order to accurately determine the true cost of a given filler in a given compound, the fillers must be compared on their pound volume differences to their competing filler additive. These differences are to be discussed.

A relative comparison of the specific gravity of the most commonly used fillers are listed in figure 5. The thermoset compounds are lighter in weight when compared to the naturally occuring coals (specific gravity of 1.12 versus 1.22). However, the coals perform more like inorganic fillers without resilient and other physical and chemical property benefits of the rubber powders.

Figure 6 shows the relative cost comparison between the various organic and inorganic fillers on a cents per pound basis.

When combining the average cost of a filler with its physical properties, a surface area to price ratio can be developed for these materials, as illustrated in figure 6. Of all the materials, with the exception of the carbon blacks which are more expensive on a cost per pound and volume cost per pound basis, the UltraFine powders are the only ones that can provide the additional reinforcing properties when compounding.

The compound specific gravity (using a theoretical compound with a 0.95 specific gravity to a specific filler loading) can be calculated and compared to other various filler loadings as illustrated in figure 7. The compound specific gravity increases with increasing filler loadings. The more dramatic the increase, the worse the performance of a given filler. Thus, the less expensive fillers with the highest specific gravities are not always the best choice in selecting a filler to give a lower system specific gravity. Pet coke was used in place of a specific coal, since pet coke's chemical and physical composition can be more closely controlled and predicted than with the extreme variation in naturally occurring mined coals such as lignite, subbituminous and bituminous coals. Normally, bituminous coals have a high percentage of sand and other inerts in their composition. Pet coke can be recognized for its low volatile, moisture and ash values, but has a much greater specific gravity than bituminous coal. Thus, bituminous coal is not considered to be a true filler candidate.

Figure 8 reflects the net effect on the total system cost per pound when the filler loadings are increased and compared. The real performance of a lightweight, highly resilient filler when compared to an expensive filler, such as silica, is dramatically illustrated.

Figure 9 shows the relationship between the cost per pound volume as filler loading is increased. Again, UltraFine reflects the lowest dollar per pound volume cost compared to whiting or clay fillers.

On the basis of a generic filler only, UltraFine is equal or slightly superior in cost per pound and volume performance to an expensive clay or whiting, clearly providing many more beneficial properties to the desired product application.

Therefore, predicting performance on filler cost alone is insufficient information in determining the optimum filler ingredient when compounding for a particular coating or mixing application. Enhancement of thermoset powders interfacial adhesion can be achieved with chemical surface treatment during the powder grinding process. This area of development offers a great step forward in utilizing the majority of the primary and secondary waste polymers for reuse into compounds.

Tire compound applications

The reincorporation of tire powders back into their original compounds is being met with increasing interest and use. The bias and radial tire is precisely engineered to ride quietly, smoothly and safely. Its engineering helps conserve energy, provides dimensional stability, assures cornering and road handling ability, supports loads and cushions bumps.

The new superfine homogeneous powders of today can be used in many component parts of the tire. Table 3 lists the key areas in which various fine tire powders can be included in the tire formulation. The tire components include both bias and radial tires for passenger, agricultural, light commercial trucks, medium and heavy duty trucks, and off-the-road equipment. The exact percentages of powder and reclaim will vary by tire type, performance, and economic requirements. Proper use of GF-80 in tire compounds could replace as much as 15% by weight of costly virgin materials in overall tire weight.

In review of the data in table 4, the compound using a coarser or minus 30 mesh rubber (right hand column) shows the typical reduction in tensile, elongation and tear properties that are expected when conventional regrind is added to a sidewall compound. This table contains the physical data fora radial sidewall compound at various substitutions, demonstrates that superior sidewall compounds can be obtained when UltraFine (GF-80) is substituted at 10 parts for 10 parts of oil extended black master batch. A "rule of thumb" guide is to expect a 3-4% reduction in physical properties for each one part of ground rubber added above the three part level with a coarser rubber material. Surface textures of the compound will deteriorate very fast with the 30 mesh rubber. With UltraFine powders, the integrity and texture of the compounds are retained.

The superfine mesh rubber product obtained by the UltraFine process permits its usage in more critical applications and at much higher substitution levels than possible with the conventional 30-40 mesh rubber crumb. This UltraFine rubber acts as a reinforcing filler and processing aid and is an option for controlling compound costs. Development work has shown that minimal loss of properties can be expected when substituting at the 25 part level. The 10 part substitution results in no changes in the critical properties of the final compound. Experience has shown additional processing oils and proportionately less accelerators are required, thus reducing the overall cost of the compound.

Table 5 contains the physical data for a radial passenger tread compound and shows the same substitution level in passenger tread compounds.

Reviewing the data in tables 4 and 5, the GF-80 performs more like a reinforcing carbon black than an inert rubber filler. This phenomenon can be partially explained in analyzing the surface morphology of the GF-80 particle. The particle has an approximate surface area of 2.0 [meters.sub.2]/gram, and particle size range from minus 80 mesh to minus 325 mesh with a mean particle size being 200 mesh (74 microns). Various mesh sizes and particle ranges can be produced with the process and tailored to specific compound requirements.

Other applications

Now that economic light weight polymer fillers are available to industry, many new areas of product development are available, especially for tire manufacturers. The fine tire powders with mean particle sizes of 200 mesh (74 micron) or less also provides the mechanism for these tire rubber particles to be dispersed into the matrix of many other compounds such as asphalt cements. This is accomplished by stoichiometric addition, blending and controlled temperature cycles to impart the properties of tire rubber into the overall composition of asphalt for improving road binders. This area of development, called rubber modified asphalt cement (RMAC), is especially important with our deteriorating highway system and the Federal Highway Administration directive to implement a system of performance-based pavements. The use of 10% tire rubber in theasphalt-cement alone would conservatively consume all the tires generated annually by the U.S. and eventually useup all the stockpiled tires in an environmentally sound manner.

Summary

In conclusion, UltraFine powders are an excellent choice when selecting a filler from weight, cost, and volume aspects. The UltraFine family of thermoset powders provides: high resilience, low specific gravity, non-friable properties, reinforcing properties, high specific surface areas, and contains antioxidants, antiozonants and UV inhibitors.

The development of fine surface treated rubber for rubber compounding and in the development of composites will significantly reduce our waste stream and allow us to capture and utilize our resources to their highest value without being a burden to our environment.

References

(1). "Comparison of UltraFine to cryogenic grinding for cured thermosets ad selected theromoplastics," Rouse Rubber Industries, brochure no. 507, August 1988.

(2). "New views on using ground coal fillers," Michael J. Trojan, Keystone Filler and Manufacturing Co. and William Klingensmith, Akron Consulting Co., Rubber World, Vol. 202, No. 5, August 1990, Page 22.

(3). "UltraFine powders," Rouse Rubber Industries, brochure no. 508, January 1989.

(4). "Recent developments in ground and reclaim rubber for compounding," Michael Wm. Rouse, Rouse Rubber Industries, Inc., January 25, 1991

(5). "Rubber technology handbook," Werner Hofman, Hanser Publishers, Oxford University Press, Canada, 1989.

(6). "Crushing and grinding process handbook," C.L. Prasher, John Wiley & Sons Limited, New York, New York, 1987.

(7). "Chemistry of accelerated sulfur vulcanization: an overview," Aubert Y. Coran and David J. Sikora, 1989 Fall technical meeting, American Chemical Society.

(8). "A guide to truck tire retreading," Bandag, Inc., 1988.

(9). "A major breakthrough in rubber reclaiming technology," B.D. LaGrone and Jerry Lynch, U.S. Rubber Reclaiming Co., August 1985.