WO2009137585A2

WO2009137585A2 - Cryo-mechanical selective grinding and activation

Info

Publication number: WO2009137585A2
Application number: PCT/US2009/042992
Authority: WO
Inventors: Anthony M. Cialone
Original assignee: Lehigh Technologies, Inc.
Priority date: 2008-05-06
Filing date: 2009-05-06
Publication date: 2009-11-12
Also published as: WO2009137585A3

Abstract

A process of producing powder rubber having a particle size in the range of between about 50 microns and 180 microns is described. The process removes metal, fiber and other foreign materials from ground used tire rubber particles. The processed used tire rubber particles are embrittled by contact with a cryogenic liquid. The embrittled rubber particles are milled in a conical mill. The comminuted rubber particles exit the conical mill at a temperature in the range of between -15 degrees C and -30 degrees C. The powder rubber produced can vulcanize as easily or even more easily than virgin rubber.

Description

CRYO-MECHANICAL SELECTIVE GRINDING AND ACTIVATION CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Serial No.61/050,735, filed on May 6, 2008, which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH Not applicable.

REFERENCE TO MICROFICHE APPENDIX Not applicable.

FIELD OF THE INVENTION

The present invention is directed a process of comminuting used rubber tire particles ground into very fine powder which effects a chemical change in the fine rubber powder. More particularly, the present invention is directed to a process in which used rubber tire powder rubber is subjected to cryogenic grinding under inert conditions to produce mechanochemical devulcanized rubber. Furthermore, the present invention includes the fine powder rubber product of this process.

BACKGROUND OF THE PRIOR ART

The ever increasing number of scrap vehicle tires generated around the world is a major cause for environmental concern. Although many uses for used rubber tires have been identified, a significant number of those tires remain unprocessed as solid waste, despoiling the environment. For example, in the last year for which statistics have been accumulated, there were 299 million tires scrapped in the United States. Of these about 26 million of them remain as solid waste.

Obviously, most of the scrapped tires were utilized or recycled. A significant quantity of these used vehicle rubber tires were employed as fuel. This use is a mixed blessing given its contribution to air pollution. A large number of used tires are recycled and employed in civil engineering applications. For example, vehicle tires are ground into crumb rubber particles for use in rubber-modified asphalt or recycled into other rubber-containing products. Surprisingly, used rubber tires are known for not being utilized in the most important use of rubber, the manufacture of new vehicle tires. This is a serious deficiency insofar as that utility would significantly increase the recycling of used rubber tires and, if produced efficiently, could even reduce costs of vehicle tires. In those applications where used rubber is employed in the manufacture of vehicle tires, only between about 1 % to 5% of the constituency of such vehicle tires comes from recycled rubber. At this concentration level, the type of recycled used rubber particles prepared in the art suffices in the production of vehicle tires. That is, used rubber particles, as presently processed, can be added, at these low concentration levels, to virgin rubber to produce acceptable tires. However, in order to utilize used rubber tire particles at higher loading levels, functionalization or surface activation treatment, usually in the form of cryo- mechanical processing, is needed to compatibilize that rubber with virgin rubber with which it was mixed. Studies have shown that current reground rubber compounded with virgin rubber requires added sulfur and carbon black. When such modification is made, it is found that ground rubber may be mixed with natural rubber to produce vehicle tires, having acceptable physical and mechanical properties. However, the additional costs of such processing of used rubber particles make such processes non-commercial.

Another difficulty associated with used rubber tire particles is that their size is not fine enough to be compatible with tire compositions in the manufacture of new rubber tires. That is, the particles have too broad a particle size distribution and, more significantly, cannot be ground to the ultrafine powder necessary to be formulated into vehicle tires.

United States Patent Publication No. US 2005/0107484, published May 19, 2005, assigned to the assignee of the present application, describes a process and apparatus for manufacturing powder rubber from rubber vehicle tires heretofore not often produced commercially. Although that disclosure, the contents of which are incorporated by reference herein, produces powder rubber of a particle size required in the manufacture of new vehicle tires, compatibility with virgin rubber and problems associated with vulcanization of such rubber discouraged its use. Thus, although the process described in the aforementioned '484 publication represents a significant advance in the art, still there is a need for a process that not only provides powder rubber in the fine particle size range obtained in the process and apparatus of that disclosure but, in addition, a powder rubber which can vulcanize as easily or even more easily than virgin rubber.

BRIEF SUMMARY OF THE INVENTION

A new powder rubber has been developed which meets the requirements of utilization in the manufacture of new vehicle tires. That new powder rubber is very fine particle powder rubber having a narrow particle size distribution range and which, due to the novel processing conditions utilized in its manufacture, produces free radicals that vulcanize more rapidly in the presence of accelerators than do virgin rubbers. When compounded with a fresh virgin rubber matrix, these radicals, present in the used rubber tire powder product, combine to form bonds between the fresh polymer matrix and the recycled rubber powder that make the particle-matrix interface chemically indistinguishable from the virgin rubber matrix.

In accordance with the present invention, powder rubber is provided in a process which includes the steps of removing metal, fiber and other foreign materials from ground used rubber tire particles. These rubber particles are thereupon embrittled by contact with an inert cryogenic liquid. The embrittled rubber particles are milled in a conical mill wherein the particles are comminuted between an adjustable gap formed between rotating blades of a rotor and a stationary conical grinding track. Not only is the gap adjustable but, in addition, the rotational velocity of the rotor is variable. The comminuted rubber particles exits the conical mill at a temperature in the range of between about -15 degrees C and about -30 degrees C. After warming to ambient temperature, the final recovered powder rubber product has a particle size in the range of between about 50 microns and about 180 microns.

In further accordance with the present invention, a process of producing powder rubber is provided. In this process metal, fiber and other foreign materials are removed from ground used tire rubber particles. The thus processed used tire rubber particles are embrittled by contact with a cryogenic liquid. The embrittled rubber particles are thereupon milled in a conical mill wherein the particles are comminuted in an adjustable gap between rotating blades of a rotor, whose rotational velocity is variable, and a stationary conical grinding track. The comminuted rubber particles exit the conical mill at a temperature in the range of between about -15 degrees C and about -30 degrees C. The comminuted rubber particles are warmed to ambient temperature and recovered as powder rubber having a particle size in the range of between about 50 microns and 180 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by reference to the accompanying drawings of which:

Figure 1 is a graph of DSC thermographs of used rubber feedstocks within and outside the scope of the present invention;

Figure 2 is a graph of TGA thermographs of used rubber feedstocks within and outside the scope of the present invention;

Figure 3 is a graph of ATR-FTlR spectra of used rubber feedstocks within and outside the scope of the present invention; Figure 4a and 4b are EPR spectra of ambient and cryogenically ground 80 mesh used rubber, respectively; and

Figure 5 are absorption spectra of ambient and cryogenically ground 80 mesh used rubber; and

Figures 6a, 6b and 6c are histograms of particle size distributions of 80, 140 and 200 mesh ground used rubber.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention involves an initial step of removing metal, fibers and other foreign materials from rubber particles ground from used vehicle rubber tires. Although the process encompasses an initial step of processing used vehicle rubber tires, it is often preferred to start with preground used vehicle rubber tire particles. In either case, initial processing begins with whole vehicle rubber tires or a supply of used vehicle rubber particles, usually having a particle size of no more than about U.S. Sieve No. 4 mesh previously processed to remove tramp metal. Tramp metal is defined as large metal objects easily removed from the rubber particles. Tramp metal is distinguished from small metal particles resulting from comminution of used steel belt-containing vehicle rubber tires.

The aforementioned rubber particles, having a size no greater than about U.S. Sieve No. 4 and no larger than particles passing through U.S. Sieve No. 30, e.g. particles having a particle size of 600 microns, are processed through a series of redundant magnetic separations, destoners, gravity tables, air classifiers and the like to remove metal, fiber and other foreign materials. The thus processed rubber particle feedstock is then embrittled by contact with a cryogenic liquid. Although any inert cryogenic liquid may be utilized, the preferred cryogenic liquid is nitrogen insofar as liquid nitrogen is more economical than are any of the other commercially available cryogenic liquids, e.g. inert gases of Group VIII of the Periodic Table of the Elements.

The embrittlement step, in which the rubber particles are chilled to below their glass transition temperature, takes place in a chilling means of the type well-known in the art. The cryogenic liquid, preferably nitrogen liquid, is introduced in a manner permitting contact between the rubber particles and the cryogenic liquid, such as by spray nozzles, ports, manifolds or the like. As will be discussed below, control of the temperature of the embrittled particles is significant insofar as the temperature of the particles need only be below the glass transition temperature. That is, the rubber particles need not be chilled significantly below the glass transition temperature to provide effective comminution. The most significant variables defining particle temperature are the mass velocity of the particles in the chilling means and the mass velocity of the cryogenic liquid introduced into the chilling means.

The embrittled vehicle rubber tire particles upon exiting the cryogenic chilling apparatus, which constitute the chilling means, are immediately introduced into a comminution zone provided by a conical mill utilized in the process of the present invention. Upon introduction into the conical mill, the cryogenic liquid, usually nitrogen although it may also be liquefied argon, neon or helium, is vaporized. This vaporization has a significant effect on the particles. This is so insofar as the same mass of nitrogen in the gaseous state occupies 650 times the volume occupied by the same mass of liquid nitrogen. Thus, vaporization of the nitrogen liquid expands that fluid at a volumetric ratio of 650:1. This instantaneous expansion in volume accelerates the rubber particles entering the conical mill. When this phenomenum is combined with the rotational velocity imparted by the rotor blades, the rubber particles are accelerated within the milling area. The milling action is thus a combination of particle acceleration and physical impact against a hard surface or another particle.

The above remarks establish that particle comminution in the conical mill result in part from high velocity impact between particles, as occurs in a jet mill. However, an even more significant reduction in particle size results from continued particle impact between the rotor blades and the conical grinding track. Thus, the capability of adjusting the gap between the rotor blades and the outer track casing, when combined with the rotational velocity variability, imparted by the variable speed rotor blades, provides control over the resultant particle size of the pulverized rubber particles. Indeed, these particles are better defined as a powder by the time the particles exit from the conical mill.

It has been found that the unique chemical and physical properties of the resultant comminuted rubber powder are critically dependent upon the temperature of the pulverized rubber product exiting the conical mill. The comminuted pulverized rubber powder product that leaves the conical mill is at a temperature in the range of between - 10 degrees C and -60 degrees C, preferably about -15 degrees C and about -30 degrees C. More preferably, this pulverized rubber product temperature is in the range of between about -20 degrees C and - 25 degrees C. The pulverized rubber powder exiting the conical mill is thereupon warmed to ambient temperature.

The ambient temperature rubber powder is subsequently again further stripped of minor amounts of ferrous metal and fiber by contacting the powder rubber product stream on a drum magnet and centrifugal separators, respectively. The rubber powder product has a particle size range of from between about 50 microns to about 180 microns. This particle size distribution of the rubber product may be further narrowed by separation by screening into different mesh sizes, typically 80 mesh (180 microns); 140 mesh (107 microns); 200 mesh (74 microns); and 325 mesh (43 microns).

Although the present invention is independent of any theory explaining its operation, it is believed that the process practiced in the present invention involves cryo- mechanochemical selective grinding and activation. This cryo-mechanochemical effect, wherein the unique cryogenic turbo-shearing process provides the necessary mechanical energy to cause bond rupture, results in partial devulcanization of used vehicle vulcanized rubber powder.

The mechanical energy to which the rubber particles are subjected includes impact, the effect of a single force on the rubber particle. The impact component is principally manifested at the entrance to the conical mill wherein the pressure and velocity of gaseous nitrogen is calculated and controlled to maximize this effect. A second mechanical force to which the rubber particles are subjected is compressive force resulting from the force of compression between the rotor blades and the conical grinding track. In addition, a third mechanical force imposed upon the rubber particles is shear, the wrenching apart of the particle. Finally, a fourth mechanical force imposed on the rubber particles is attrition. Attrition is created by impact between particles.

Compressive force is controlled by calculating and manipulating pressure and velocity of gaseous nitrogen, which, in turn, becomes a function of the amount of liquid nitrogen utilized in the embrittlement step which occurs in the chilling means. Compression is also controlled by the rotational velocity of the grinding rotor and the resulting centripetal force of the grinding elements in the compression zone. Shearing force is controlled by the gap between the grinding track and the blades of the rotor. That gap may be adjusted thus controlling the shearing effect. Attrition force is controlled by the volumetric flow of rubber particles into the conical mill. That is, the greater the flow of rubber particles into the conical mill, the greater is the particle volume and attrition effect.

The mechanical forces imposed on the rubber particles in the process of the present invention are sufficient to overcome the critical value requires for cleavage of primary covalent bonds, e.g. C-C, C-H and C-O, secondary van der Waals forces and hydrogen bonds. This cleavage results in chain scission and the formation of free radicals at the ruptured end. It is appreciated that the rupturing of the particles create concentrated heat at the surface of the particle. Under less than cryogenic conditions, this heat build-up would result in rubber scorching and degradation. However, since the present process occurs under cryogenic conditions, e.g. temperatures well below 0 degrees C, this heat build-up is eliminated.

Furthermore, under ambient conditions, the air in the environment could cause oxidation to reverse the positive effects of free radical generation. However, the process of the present invention occurs in an atmosphere of nitrogen or another inert gas, which prevents oxidation from occurring.

Although the present invention is independent of any theory explaining its operability, it is believed that free radicals are formed at crosslinking sites in the process of the present invention. This permits the ultrafine rubber powder product of the present invention to be easily revulcanized with the addition of small amounts of curatives. This theory is borne out by the fact that the rubber product of the present invention vulcanizes more rapidly, in the presence of accelerators, than do virgin rubbers.

Prior art mechanical grinding mills, it is theorized, could not generate enough energy to (1) create enough free radicals, (2) reduce particle size, (3) control particle size distribution and (4) increase surface area per unit volume. Moreover, the elimination of heat build-up, by cryogenic milling, it is believed, serves to selectively sever S-S bonds initially and C-S bonds secondarily to create radical receptor sites and, equally importantly, limit or eliminate C-C bond cleavage.

It is not known that the combination of high temperature and energy (mechanical, chemical or ultrasound), especially under high pressure, serve to unselectively break all chemical bonds, resulting in polymer deformation and resultant low physical-mechanical properties as a consequence of mechanochemical degradation. This unselective scission of the main chain creates shorter polymer chains and causes a deterioration of mechanical properties compared to virgin rubber.

The aforementioned theory explains how the process of the present invention produces the improved results permitting the used rubber of the present invention to be combined with virgin rubber in tire compositions. It is known that difference in elastic constant is most important in combining virgin and used rubber. Since the elastic constant for S-S bonds is estimated to be only about 1/30 of that for C-C bonds, it is apparent that bond scission is critical to compatibility with virgin rubber. By bringing the rubber to its glass transition point, the elastic constant of S-S bonds become more susceptible to cleavage at lower energy levels when energy is imparted to the rubber particles in the comminution step. This explains how the comminution step focuses successfully on selective breakage of S-S bonds creating crosslinking sites on the surface where physical and chemical bonding and adhesion occurs. Thus, this targeted bond scission, at selective crosslinking positions of the used rubber particles, it is theorized, importantly contributes to the successful utilization of the used rubber particles of the present invention.

For the above reason, the rubber product of the present invention is easily compounded with fresh virgin polymer matrix to form bonds between the two phases. That is, the particle-matrix interface is chemically indistinguishable from the virgin rubber matrix. Thus, whereas in the past it was known that incorporating used rubber powder into a rubber or plastic composition required little or no formulation modifications at low loadings, e.g. 1% to 5%, maximization of the economic and environmental benefits associated with recycled rubber requires higher loadings of that recycled rubber. At these higher loadings, formulation adjustments must be made. This formulation adjustment is illustrated in the examples below wherein the rubber product of the present invention is utilized in the formation of new vehicle tires.

Not only does the process of the present invention produce a rubber product that it can be easily revulcanized and reutilized with virgin rubber but, in addition, that rubber product is provided in very fine particles, in the micron particle size range, and in a narrow particle size distribution. This ultrafine particle size is characterized by a high surface area to volume ratio. This, in turn, results in better interaction and bonding with the matrix compounds resulting in better mechanical properties.

The following examples are given to illustrate the present invention. Because these examples are given for illustrative purposes only, the present invention should not be deemed limited thereto.

Example 1

A typical charge of used vehicle tires was processed in accordance with the present invention. The product of this process was recovered and screened to determine its particle size distribution. That is, the particles were screened on successively finer screens and the percentage retention on each screen was employed to define the particle size distribution. The results of this test are summarized in Table 1. Table 1

Analysis of Table 1

The results summarized in Table 1 indicate that 85% of the particles were smaller than 80 mesh (180 microns). Moreover, 21% of the particles were ultrafϊne 325 mesh particles having a maximum particle size of 43.2 microns. Finally, it is noted that only a small percentage, 5%, were smaller than 325 mesh. This indicates that the process of the present invention not only provides ultrafϊne particles in a narrow particle size distribution but that the number of excessively fine particles is minimized. This confirms that excessive pulverization is avoided in the process of the present invention. Histograms of the product samples retained on US Standard Mesh Nos. 80, 140 and 200 are presented in Figures 6a, 6b and 6c, respectively. These figures show a Gaussian distribution of the particle size. The diameters at 75% (d75%) are 165 microns, 93 microns and 74 microns for 80, 140 and 200 mesh samples, respectively. This data establishes that the ultrafine rubber particle product of the present invention is achieved with a narrow size distribution.

Example 2

A used rubber tire feedstock, hereinafter referred to as UTR, having a size range of between 3/8 inch and 40 mesh, e.g. 425 microns, was processed in accordance with the process of the present invention to produce streams of powder rubber within and outside the scope of the present invention. That is, a first stream, outside the scope of the present invention, included particles having a size of between 30 and 60 mesh, e.g. between 250 and 600 microns. A second stream of powder rubber, within the scope of the present invention, encompassed particles of 80 mesh, having a particle size of 180 microns. A third stream included rubber particles having a size range of between 80 and 140 mesh, that is, between 107 and 180 microns. A fourth stream, within the scope of the present invention, included powder rubber of 140 mesh. Finally, powder rubber of 200 mesh, 73.7 microns, was obtained. In addition, as a comparison, ambient ground used rubber particles having an 80 mesh size (180 microns), was obtained from a vendor of that product. Samples of each of these rubber particles were thermally analyzed by differential scanning calorimetry (DSC). DSC scans of the UTR feed and the aforementioned used rubber particle samples were conducted in a TA Q- 1000 Series differential scanning calorimeter. The glass transition temperature (T_g) of each of these samples were measured on second heating. The temperature range studied was -75 degrees C to 80 degrees C and the heating and cooling rate employed was 10 degrees C per minute with nitrogen as the purge gas. The resulting DSC thermograms are depicted in Figure 1.

As shown in Figure 1, a broad T_g for the UTR feed sample is seen from -65 degrees C to -30 degrees C. This broad range in T_g is attributed to the resulting glass transitions of the different types of rubber present in the sample.

The rubber sample having an 80 mesh size, within the scope of the present invention, defines a T_g in the range of 40 degrees C to 70 degrees C. The same glass transition temperature is observed for the 140 mesh and 200 mesh samples, but these transition temperatures are shifted higher by about 10 degrees C. It is noted that the glass transition temperature in this range for the UTR sample is extremely broad and is hardly perceptible, compared to the samples within the scope of the present invention.

The conclusion reached, after analysis of the glass transition temperatures by means of DSC, is that the rubber particle samples within the scope of the present invention possess more chain mobility, which is theorized to be the result of devulcanization and scission during the pulverization process, than rubber particle samples outside the scope of the present invention.

Thermogravimetric analysis (TGA) was conducted on the UTR feed and the rubber samples produced in accordance with the process of the present invention. This thermogravimetric analysis involved monitoring of weight loss with increasing temperature in a TA Q-5000 Series thermogravimetric analyzer. The heating rate employed was 20 degrees C per minute. Nitrogen was employed as the purge gas in the first segment from 25 degrees C to 550 degrees C and was changed to air for the second segment from 550 degrees C to 900 degrees C.

The result of the thermogravimetric analysis were TGA thermograms, depicted in Figure 2, of the UTR feed, the 80 mesh powder rubber sample, the 140 mesh powder rubber sample and the 200 mesh powder rubber sample.

The TGA thermograms depicted in Figure 2 show a weight loss of about 10% measured for all samples starting at 200 degrees C. This expected loss is attributed to the loss of low molecular weight acetone-extractable volatiles present in rubber. For all samples, the onset of pyrolysis began at around 350 degrees C. This similarity in pyrolysis onset indicates that the thermal stability of the rubber was not affected by the pulverization step. The rate of pyrolysis is, however, higher for rubber samples within the scope of the present invention compared to the UTR sample. It is believed that this is due to partial devulcanization or reduction in molecular weight or both, which is believed to happen during the process of the present invention.

The second segment, from 550 degrees C to 900 degrees C, shows that the onset of oxidation of carbon black and pyrolysis began at about 560 degrees C for all samples. However, the rate of oxidation was higher for the UTR feed as compared to the powder rubber samples within the scope of the present invention. This phenomenon is believed due to varying and higher carbon black content among the powder rubber samples of the present invention. Finally, silica or ash content is higher in the samples within the scope of the present invention compared to the UTR feed sample. This is due to the higher silica content of finer mesh sizes obtained in the process of the present invention. This was confirmed by the fact that the rubber sample having an 80 mesh size had a lower silica content than both the 140 mesh and 200 mesh rubber particle samples.

The samples were analyzed to obtain spectroscopic characterization. In this analysis, all samples were analyzed by a Nicolet Magna 550 FTIR spectrometer with a high endurance diamond attenuated total reflectance (ATR) attachment. Each sample utilized 32 scans at a resolution of 4 cm^"1 to obtain this spectra.

Figure 3 shows the ATR-FTIR spectra for all the samples. It is noted that the area under the hydroxyl peak in the region from 3,000 to 3,600 cm^"1 of the powder rubber samples of 80 mesh, 140 mesh and 200 mesh increased in comparison to the UTR feedstock sample. This increase in functionality is attributed to oxidation, which cannot be eliminated completely, during the process of the present invention.

Another observation noted in Figure 3 is the broad signal at 1050 cm^"1 with a shoulder at 1,200 cm^"1. This signal and shoulder is due to Si-O-Si vibrations of the silica present in the rubber powders. This trend, attributed to silica content, is consistent with the observations obtained in the thermogravimetric analysis.

Electron Paramagnetic Resonance (EPG) spectroscopic characterization involved analysis of the comparison between the used rubber sample having an 80 mesh size, ground at ambient temperature, with the ground rubber sample of the present invention also having a particle size of 80 mesh. These samples are compared in Figures 4a and 4b, respectively. As noted therein, a sharp singlet superimposed on a broader feature was observed in both cases. With a g-value of 2.0017, close to the free electron g-value of 2.0023, the signal is in the range indicative of carbon- or hydrocarbon-based radicals. The broader feature can be explained by a combination of all signals caused by many different radical structures that are present in the polymer samples. Since the observed signals are actually first derivatives of absorption, the area under the curve after integration can be used as a indicator of relative radical sample density. This is illustrated in Figure 5, which shows that the 80 mesh rubber sample, prepared in accordance with the present invention, has a greater radical density, a ratio of 1.76, than that of the prior art ambient-ground rubber sample.

Example 3

The ultrafine ground rubber product of the present invention was utilized in the formation of rubber compositions employed in the manufacture of commercial vehicle tires. In those formulations a first rubber composition was employed as the tread composition of the tire. These formulations appear below in Table 2.

Five tread compositions were prepared. These compositions are denoted as Tread Compositions A-E in Table 2. The major variation in Compositions A-E was the constituency of the powder rubber component, the subject of the present invention. Six powder rubbers, within the contemplation of the present invention, the powder rubber components of Compositions B, D and F, having a particle size of 80 mesh, and the powder rubber components of Compositions C, E and G, having a particle size of 80-140 mesh, were utilized in the tread compositions of Table 2. Composition A was a control wherein no powder rubber was employed. The powder rubber component having a particle size of 80 mesh was utilized in components B, D and F at concentrations of 5%, 10% and 15%, respectively, said percentage being by weight, based on the total weight of the composition. Components C, E and G, utilized the powder rubber having an 80-140 mesh particle size range, were present in concentrations of 5%, 10% and 15% by weight. Table 2 appears below:

Table 2

¹ Natural Rubber

² Polybutadiene

³ Styrene Butadiene

⁴ Carbon Black

⁵ Acting as a viscosity reducing agent

⁶ Diphenyl Guanidine

⁷ N-tert-Butyl Benzothiazole Sulfonamide

⁸ Dibenzothiazole Disulfide

⁹ Acting as a vulcanizing agent

Three important characteristics of tread compositions were tested for each of the aforementioned samples. The first of these properties was heat build-up. Heat build-up was measured in accordance with ASTM Standard Test No. D623. Three tread composition samples were tested in accordance with the heat build-up test. The first, Composition A, the control, utilized no used rubber powder. That sample produced a heat build-up of about 21 degrees C. The second sample tested, Composition B, the tread composition which included 5% by weight of the used rubber powder of the present invention having an 80 mesh size, produced the same result, a heat build-up of 21 degrees C. The third tested sample, Composition C, the rubber powder of the present invention having a mesh size of between 80 and 140, also present in a concentration of 5% by weight, produced significantly less heat build-up, about 16 degrees C.

The second set of heat build-up tests involved a comparison between the used rubber powder of the present invention present in a concentration of 10% by weight. The first of these samples was Composition D, the 80 mesh sample, and the second, Composition E, the powder rubber in a particle size range of 80 mesh to 140 mesh. Composition D produced a heat build-up of about 21 degrees C while Composition E produced a heat build-up of 16 degrees C. The final heat build-up test involved a similar comparison between the tread compositions employing powder rubber having particle size of 80 mesh, Composition F, and powder rubber having a particle size range of 80 to 140 mesh, Composition G. Compositions F and G both included powder rubber loadings of 15% by weight, based on the total weight of the tread composition. Composition F produced a heat build-up of 22 degrees C while Composition G resulted in heat build-up of 20 degrees C.

The results of these tests lead to the conclusion that powder rubber addition, at levels of up to 15%, results in reduced heat build-up when the powder rubber has a particle size range of between 80 and 140 mesh compared to the absence of powder rubber or powder rubber in the narrow 80 mesh particle size range.

A second test was conducted to determine tear resistance of the tread compositions. Tear resistance was obtained in accordance with ASTM Standard Test No. D624. Tear resistance was measured in terms of kiloNewtons per meter (kN/m.). The same compositions compared for heat build-up were compared for tear resistance. That is, in a first test, the control composition, Composition A, was compared to Compositions B and C, the compositions containing 5% of used rubber powder present in a 80 mesh size and a particle size ranging between 80 mesh and 140 mesh, respectively.

The results of this comparison indicated that Composition A had a tear strength of about 57 kN/m; Composition B had a tear strength of about 54 kN/m and Composition C had a tear strength of about 58 kN/m. In a second test comparing tear strength of tread compositions which contained the used rubber powder of the present invention at a concentration of 10%, tear strength of the 80 mesh-containing sample, Composition D, produced a tear strength of about 58 kN/m while the 80 to 140 mesh containing sample, Composition E, produced a tear strength of about 62 kN/m. Finally, the same comparison between the composition containing used powder rubber, present in a concentration of 15%, produced a tear strength for Composition F of about 60 kN/m while Composition G produced a tear strength of about 67 kN/m.

The results of this comparison establish that tear strength is improved for tread compositions when powder rubber is present in a concentration of at least 10%. Indeed, increasing the powder rubber of the present invention concentration to 15% by weight in tread compositions further increases tear strength. The most important result is that the inclusion of used rubber powder in a concentration of 10% by weight or more increases tear strength over tread rubber compositions free of reground rubber powder. The third test conducted on the tread rubber compositions was a test to determine abrasion resistance. Abrasion resistance is determined in accordance with DIN Standard Test No. 53516. Abrasion resistance for the control, Composition A, was about 107 mm³. Abrasion resistance of the ground rubber powders of the present invention, present in a concentration of 5%, Compositions B and C, were about 104 mm³ and about 70 mm³, respectively. At a powder rubber loading of 10%, Compositions D and E produced abrasion resistances of about 110 mm³ and about 72 mm³, respectively. The abrasion resistance of the tread compositions at a loading of 15% by weight were about 110 mm³ and about 75 mm³ for Compositions F and G, respectively.

It is thus seen that the abrasion resistance of tread compositions containing ground powder rubber have a particle size of 80 mesh is little changed from tread compositions free of used rubber powder. However, abrasion resistance is increased significantly when used rubber particles in a particle size range of 80 to 140 mesh is employed in tread compositions.

It is noted that although abrasion resistance is improved when tread compositions include used powder rubber having a particle size in the range of between 80 to 140 mesh, this improvement is maximized at 5% (Composition C). At higher loadings of this powder rubber, abrasion resistance decreases albeit still representative of marked improvement over tread compositions free of this powder rubber.

The tread compositions were tested for tensile strength in accordance with ASTM Test Procedure No. D412. The control, Composition A, the tread composition free of used rubber powder, was compared with tread compositions which contained 5% of the powder rubber of the present invention, Compositions B and C. The result of that test yielded a tensile strength of the control, Composition A, of 18.35 MPa; the tread composition containing 5% powder rubber having a mesh size of 80, Composition B, provided a tensile strength of 16.6 MPa; and Composition C, the tread composition containing 5% powder rubber in the range of 80 to 140 mesh, produced a tensile strength of 17.8 MPa.

In an identical comparison of tread compositions containing 10% powder rubber of either 80 mesh or 80 to 140 mesh, Composition D produced a tensile strength of 16.7 MPa while Composition E produced a tensile strength of 16.85 MPa. Finally, in the same test wherein the loadings were 15% powder rubber, the tensile strength of the powder rubber having 80 mesh, Composition F, was 15.5 MPa while the same loading of the composition containing powder rubber in the range of 80 to 140 mesh, Composition G, produced a tensile strength of 17.6 MPa. These results show that tensile strength of otherwise identical tread compositions are significantly effected by the addition of powder rubber. Although the invention is independent of any theory explaining its effectiveness, it is speculated that there is migration of sulfur to the powder rubber matrix, thereby reducing the crosslinked density in the fresh rubber matrix, which causes loss of tensile strength. To confirm this theory, two additional compositions were produced wherein the sulfur concentration was increased somewhat in terms of dibenzothiazole disulfide (MBTS) and pure sulfur. The consistency of these tread compositions, Compositions H and I, is defined in Table 2.

Compositions H and I were identical to each other and to Compositions D and E, in that they all included powder rubber in a concentration of 10% by weight, but for the mesh size distinction. The only distinction between the two sets of tread compositions were minor increases in sulfur concentrations for Compositions H and I compared to Compositions D and E. Whereas Compositions D and E, produced tensile strengths of 16.7 MPa and 16.8 MPa, respectively, tensile strengths of both Compositions H and I were dramatically increased to 19.0 MPa additions of sulfur.

The tread compositions were additionally analyzed to determine dynamic mechanical properties. Specifically, tan β at 0 degrees C, an indicator of tire tread wet traction, was determined for tread compositions employing used powder rubber in accordance with the present invention compared to tread compositions which included no used rubber powder. Tan β at 0 degrees C was obtained in accordance with ASTM Standard Test No. F424.

As in the aforementioned tests, tan β for the control tread composition, Composition A, the composition free of used rubber powder, was compared with tread compositions which included 5%, 10% and 15% of powder rubber at 80 mesh and 80-140 mesh. At 0 degrees C, the control, Composition A, produced a tan β of 0.51. Tread Compositions B and C, which included powder rubber present in a loading of 5%, produced tan β's of 0.526 and 0.48, respectively. Powder rubber loadings at 10% for 80 mesh and 80-140 mesh, respectively, e.g. Compositions D and E, produced tan β's of 0.532 and 0.45, respectively. Compositions F and G, representing powder rubber loadings of 15% by weight, produced tan β's of 0.534 and 0.49, respectively.

These results indicate improvement in wet traction obtained when used powder rubber of 80 mesh, e.g. Compositions B, D and F, is introduced into tread compositions. That is, higher tan β values are indicative of improved wet traction. However, it is noted that there is a significant drop off in wet traction when loadings of used rubber powder having a mesh size in the range of 80 to 140 was utilized. An identical test to determine tan β at 60 degreesC was also conducted, as set forth in ASTM Standard Test No. F424. Tan β at 60 degreesC is an indicator of rolling resistance and hence decreased values are preferred. At 60 degrees C, tan β for Composition A, the control, was 0.28. Compositions B and C, wherein 5% of the used powder rubber at particle size ranges of 80 mesh and 80-140 mesh, respectively, produced tan β's of 0.27 and 0.335, respectively. When the loadings were increased to 10%, Compositions D and E produced tan β's of 0.26 and 0.27, respectively. Compositions F and G, wherein the respective used powder rubber was present in the loading of 15%, produced tan β's of 0.27 and 0.4, respectively.

These results establish that rolling resistance is slightly improved over the control when used powder rubber having a 80 mesh size is introduced into tread compositions. However, when the wider particle size distribution powder rubber of 80-140 mesh is utilized, undesirable rolling resistance is increased.

It is noted that the tan β of tread compositions measured at room temperature, e.g. 23 degrees C, produced results summarized in Table 3 below. It is emphasized that measurements of tan β at room temperature do not significantly effect rolling resistance of tread compositions. Table 3 summarizes rolling resistance, measured as tan β at 0 degrees, 23 degrees and 60 degrees C, for all tread compositions.

Table 3

Example 4

An inner liner composition, utilized in the formation of tire liners, was prepared in accordance with usual practice. That composition included a control, a virgin composition free of used rubber powder, denoted as Composition K, and six additional compositions which included loadings of 5%, 7.5% and 10% by weight of used rubber powder having a size of 80 to 140 mesh and a particle size distribution of 140 mesh. These seven compositions are summarized in Table 4. Table 4

Natural Rubber

² Chloronated Isobutene Isoprene

³ Brominated Isobutene Isoprene

⁴ Carbon Black

The tensile strength of the inner liner compositions summarized in Table 4 were tested and it was determined that all samples within the scope of the present invention were reduced in strength by an average of 8 to 14% compared to the powder rubber free control. This conforms to results obtained in the testing of the tread composition. However, increasing sulfur concentration made up this deficit.

Tan δ at 0 degrees C was, for all of Compositions L to Q, increased compared to control Composition K indicating improved traction. Moreover, tan δ at 60 degrees C produced 85% and 12% lower values for certain of the compositions which include loadings of 80-140 and 140 mesh powder rubber compared to the control, indicative of reduced rolling resistance.

Air permeability of inner liner compositions were determined in accordance with ASTM Standard Test No. D7476. Inner liner Composition K, the control, was compared to Compositions L-Q which were identical to Composition K but for the inclusion of 5%, 7.5% and 10% by weight of used powder rubber at a particle size range ranging from 80 to 140 mesh or at particle size of 140 mesh only. The results are summarized in Table 5.

As demonstrated in the Table 5, the air permeability of inner liner compositions which included used powder rubber were each significantly improved over an inner liner composition devoid of any used rubber particles. It should be appreciated that the lower the air permeability, the more effective is the operation of a vehicle tire. As demonstrated in Table 5, air permeability was reduced by as much as 45% to 50%.

Table 5

What is claimed:

Claims

1. A process of making powder rubber comprising: a. removing ferrous metal from a stream of granulated used rubber particles; b. screening and removing fiber from said stream of granulated used rubber particles; c. chilling said stream of screened granulated used rubber particles with a cryogenic liquid wherein the final chilled temperature of said rubber particles is below glass transition temperature of screened granulated used rubber particles; d. grinding said stream of chilled granulated used rubber particles by a conical mill wherein the exit temperature of the comminuted rubber particles is in the range of from -10 to - 60 degrees C; and e. screening said ground rubber particle stream into desired powder rubber having a particle size in the range of between 50 microns and 180 microns.

2. The process of claim 1 wherein said used rubber particles are particles of used vehicle tires.

3. The process of claim 1 wherein said powder rubber particles have a particle size range of from between 50 microns to 180 microns.

4. The process of claim 3 wherein said powder rubber particles are further screened into different mesh sizes comprising 80 mesh (180 microns); 140 mesh (107 microns); 200 mesh (74 microns) and 325 mesh (43 microns).

5. The process of claim 1 wherein the cryogenic liquid is liquid nitrogen.

6. The process of claim 1 wherein the cryogenic liquid is vaporized upon entry into the conical mill.

7. The process of claim 6 wherein the ground rubber particle size is controlled by controlling the pressure and velocity of the cryogenic vapor.

8. The process of claim 1 wherein the ground rubber particle size is controlled by the rotational velocity of the conical mill.

9. The process of claim 1 wherein the ground rubber particle size is controlled by the volumetric flow rate of the screened granulated used rubber particles.

10. The process of claim 1 wherein the ground rubber particle size is controlled by the operation of the conical mill, the conical mill having a grinding track and one or more blades on the rotor including the gap between the grinding track, wherein a gap between the grinding track and the one or more blades may be adjusted.

11. The process of claim 1 wherein said control of said particle size distribution further comprise varying the space between an impact surface and a rebound surface.

12. The process of claim 1 wherein said stream of ground cryogenically cooled rubber particles are dried to ambient temperature after said step (d).

13. The process of claim 1 wherein said granulated used rubber particles are obtained from an initial charge of used rubber particles which are subjected to the steps of: (i) removing any tramp metal from said initial charge of used rubber particles; and (ii) granulating said product of said step (i).

14. The process of claim 1 wherein said stream of granulated used rubber particles is provided by a charge of preprocessed used rubber particles.

15. The process of claim 1 wherein the exit temperature of the comminuted rubber particles is in the range of from -15 to - 30 degrees C.

16. The process of claim 1 wherein the exit temperature of the comminuted rubber particles is in the range of from -20 to - 25 degrees C.