RUBBER POWDERS AND METHODS OF PRODUCING AND USING RUBBER POWDERS

Abstract: Rubber powders comprising unvulcanized rubber parti­ cles with an anti-tack agent covering are disclosed. These powders are made directly from rubber and anti-tack agent mixtures by mechanical action at a temperature above the glass transition temperature of the rubber. Typical rubbers used include NR, SBR, EPDM, BR and blends thereof and typical anti-tack agents include carbon black, silica and the like.

Description: RUBBER POWDERS AND METHODS FOR PRODUCING AND USING RUBBER POWDERS Field of the Invention: This invention relates to rubber powders and methods for making and using rubber powders. More particularly, it relates to powders made of rubber particles coated with non-tacky materials and methods of converting bale rubber by mechanical action to rubber powderstcomprised of such coated particles. Background and Pertinent Prior Art: The advantages of rubber powders in storage, handling and processing of rubber are many and have been long apparent. In such a form rubber can be easily handled and blended. Powders are easily adapted to such production techniques as injection molding and extrusion which is useful in making, for example, tires, cables, gaskets, and hoses. The demonstrated or predicted advantages of rubber powder technology include energy savings, process automation and shear history reduction. Generally, however, this potential has yet to be realized because of the lack of convenient, efficient, and inexpensive techniques for making rubber powder. Most of the activity in the rubber powder field has focused on materials made from emulsion polymers, particularly nitrile rubbers and emulsion SBR. A variety of techniques, including grinding, spray drying and coagulation of elastomer latices in the presence of anti-tack agents and/or surface active materials have been reported. See, for example, the following references: 1. T. R. Goshorn, A. H. Jorgenson, M. E. Woods, Rubber World, 161 (1), 66 (1969). 2. W. H. Whittington, M. E. Woods, paper presented at a meeting of The Rubber Division, American Chemical Society, Cleveland, Ohio, October, 1971. 3. M. W. Woods, R. P. Krosky, Rubber Age, 105 (4), 33 (1973). 4. M. E. Woods, R. J. Morsek, W. H. Whittington, Rubber World, 167 (6),-42 (1973). 5. P. L. Blerjie, Rubber Chem. Technol., 48 (2), 254 (1975). 6. T. P. Abbott, W. M. Doane, C. R. Russell, Rubber Age (N.Y.), 105 (8), 43 (1973). 7. H. Widmer, P. Milner, Rubber Age (N.Y.), 106 (11), 41 (1974). 8. R. E. Dixon, E. B. Bagley, J. Apt1. Polym. Sci.- 19, 1491 (1975). 9. Chemische Werke Huls, British Patent 1,387,605 (March, 1975). 10. K. H. Nordsiek (to Chemische Werke Huls), Ger. Offen. 2,214,121 (September, 1973); Chem. Abstr., 80, 28193 r. (1974). 11. K. H. Nordsiek, G. Berg, Polymer Age, 6 (56), 60 (1975). 12. N. Doak, Eur. Rubber J. 156 (10), 52 (1974). Patents have also issued in the general area of rubber powder; see, for example, U.S. Patent 4,032,501 to Schulz which describes a method for producing rubber powders from hydrocarbon solution rubber, carbon black and foam inhibiting surfactants. Other patents dealing generally with this field include U.S. Patent Nos. 2,777,008; 3,245,945; 3,203,922; 3,060,145 and 3,251,798. Summary of the Invention: The rubber powders of this invention comprise substantially nonvulcanized rubber particles having an average particle size of less than about 10 mm and coated with an effective amount of an anti-tack agent. An effective amount of anti-tack agent is an amount rendering the surface of the particle non-tacky; on a macro scale such an amount manifests itself as rendering the rubber powder resistant to agglomeration and cold flow. Thus the powders of this invention are free flowing and do not require substantial post-blending shear to achieve acceptable filler dispersion. The invention also includes a direct mechanical method for producing uncured rubber powder which comprises mixing bale or slab rubber and an anti-tack agent at a temperature above the glass transition temperature of the rubber under conditions of high mechanical action for a time sufficient to produce the powder of the invention. An important aspect of this method is that the bale rubber is first subdivided in a premixing step into segments having a maximum particle size of less than about 20cm and combined with the anti-tack agent to form a homogeneous aggregate before it is subjected to mechanical action and high shear. Often the anti-tack agent used comprises a component to be later compounded with the rubber to make a vulcanizate such as carbon black, talc, silica, clay and the like. It is a characteristic and advantage of the method of this invention that rubber powder is formed without the need of latex or solution steps. In other words, the powder is formed directly and mechanically from bale or slab rubber without the intervention of latex or solution procedures. It is also an advantage of this method that it is carried out at a temperature above the glass transition temperature of the rubber and thus does not require cryogenic techniques. Such techniques demand large investments of energy tq maintain severely reduced temperatures. Methods of using the rubber powders made by the methods of this invention and articles of manufacture made from the compounded powders are also within the scope of the invention. Detailed Description of the Invention: The rubbers used in the present invention generally include elastomers of both the natural and synthetic type. Natural elastomers include Hevea and Guayule rubbers and other rubbers isolated from plant sources as well as blends thereof. Chemically modified natural rubbers such as hydrogenated, chlorinated and sulphonated natural rubbers can also be useld, Synthetic rubbers include a wide range of materials such as those designated in the ASTM standard recommended practice for rubber and rubber latice nomenclature; see ANSI/ASTM DY1418-79a. These include saturated, polymethylene synthetic rubbers such as those made from acrylates and alpha-olefin polymers, as well as related co-polymers such as ethylene propylene types (EPM or EPR), terpolymers of various types such as EPDM, perfloro rubber (FKM and FFKM), oxygenated rubbers containing oxygen in the polymer chain, such as those made from ethylene and propylene oxide (CO, ECO, and GPO types), the sulphur analogs thereof, phosphazene rubbers and silicone rubbers. Rubbers containing carbon, oxygen and nitrogen in the polymer chain such as the urethane types can also be used in the invention. Hydrogenated and other types of chemically treated synthetic rubbers are also useful, such as hydrogenated BR, chlorinated BR, bominated BR, etc. A particularly useful class of rubbers are the "R" class, which contain unsaturation in the main chain and include both natural rubber types and synthetic rubber types These R class rubbers include the following: :- ABR Acrylate-butadiene BHR - Bromo-isobutene-isoprene BR Butadiene CHR Chloro-isobutene-isoprene CR Chloroprene HR Isobutene-isoprene IR Isoprene, synthetic NBR Ni tri le-butadi ene NCR Nitrile-chloroprene NIR Nitrile-isoprene NR Natural rubber PBR Pyridine-butadi ene PSBR Pyridine- styrene-butadiene SBR Styrene-butadiene SCR Styrene-chloroprene SIR Styrene-isoprene rubbers XSBR Carboxylic-sytrene-butadiene XNBR Carboxylic-nitri le-butadiene Both solution and emulsion prepared rubbers can be used in making the powders of this invention. Blends of two or more of the various rubber types noted may also be used. Non-limiting examples of such blends are SBR/NR (50/50), Guayule/Hevea (20/80), IR/hydrogenated BR (33/67), EPDM/BD (60/40), CHR/BBR (35/65), etc. The rubber used to make the powders of this invention is substantially non-vulcanized (i.e., substantially uncured or uncross-linked). This means that the rubber is not vulcanized at all or is vulcanized to such a low degree that it behaves as if it were not vulcanized, particularly with respect to its tack and facile processing properties. Such lightly vulcanized materials are sometimes said to be prevulcanized. Such prewlcanization can be effected by low levels of high energy radiation. As noted above the particles which comprise the rubber powders of this invention are coated with an effective amount of an anti-tack agent. Tack is a well-known term referring to that property of rubber which causes it to adhere to itself and, in the case of small particles such as powders, agglomerate. Thus, an effective amount of an anti-tack agent in the context of this invention is that amount which prevents agglomeration of the powder under the conditions of its formation and storage. Typically, this amount will be about 10-300 parts anti-tack agent per 100 parts rubber (phr). Usually the amount of anti-tack agent will vary directly with the inherent tack of the rubber used to make the powder and inversely with the average size of the rubber particles comprising the powder since the rubber surface area will also vary inversely with particle size. Often about 30-100 phr anti-tack agent is used. While it is possible to use any material as an anti-tack agent which suppresses or eliminates tack interactions between the rubber particles comprising the powder of the invention, it is usually desirable to use a material that is ultimately to be incorporated in the final rubber compound. Convenient materials of this type are the well known rubber fillers or reinforcing agents such as carbon black, silica, talc, various clays and the like. These materials increase the hardness, stiffness, strength, and resistance to tear and abrasion of the vulcanized rubber compound and include a wide range of carbon blacks, zinc oxide, fine particles of percipitated calcium carbonates, hard clays, hydrated silica compounds, such as calcium silicate and silicon dioxide and, in some specialized instances, organic materials such as finely divided phenolic resins. In certain instances abrasives such as pumice, tripoli and carborundum are used as filler components and can also be used as a3ti-tack agents in the powders of this invention. Similarly, in specialized applications, white and colored material such as mineral pigments and organic dyes may be useful. Specific examples include titanium dioxide, the aforementioned zinc oxide, zinc sulphide, iron oxide and various organic materials such as ultramarine blue. Other filler-type materials include whiting (by-product calcium carbonate or ground limestone), barytes and materials such as highly reduced and blown asphalts. Generally, however, the anti-tack agent of choice is one of the well known carbon blacks which are used as fillers and reinforcing agents in rubber compounds intended for ultimate use in tire and rubber article manufacture. The rubber powders of the present invention are generally produced by direct mechanical means without the intervention of emulsions or solutions. That means that there is no need to go through a latex-forming or solution step as is used in many processes of the prior art. Similarly, cryogen' techniques are not used necessarily in making the powders of this invention. Thus, the powders of this invention are made by mechanical, non-solvent, non-emulsion, non-cryogenic, non-chemical treatment, non-special additive techniques. One convenient technique for producing the rubber powders of this invention is by mechanical shearing of rubber and anti-tack agents under specified conditions of high shear, preceded by a premixing stage in which a homogeneous aggregate of subdivided slab or baled rubber and anti-tack agent is formed. While it is not necessary to understand any theoretical concept or explanation of the invention to practice it, it may be interesting to consider a hypothetical analysis of the process. In this analysis any mixing process is considered to involve two simultaneous events. One, entails the shear-induced subdivision of the material and the other, its coalescence after subdivision. If the latter co & escense or agglomeration can be prevented by one means or anther, the subdivided materials continue to be subdivided during mixing and a powder of increasing fineness results. It is possible to conceive of various coalescence-preventing techniques. One is cooling in which the material such as rubber becomes a rigid plastic whose surface ceases to be tacky. Another method of preventing coalescence of rubber particles would be to cover them with an anti-tack agent, a procedure somewhat analogous to-the well-known technique of microencapsulation. In this instance, however, small particles of anti-tack agent are used to surround and prevent coalescence of the relatively larger rubber particles by acting as a barrier. Thus the absence of such substantial coalescence is achieved through the presence of small anti-tack particles on the rubber particle surface which produces, on a macro scale, free-flowing, non-agglomerating powder. Thus, if rubber is first subdivided into fine particles and, either simultaneously or shortly thereafter, these particles are covered with an anti-tack agent, such as carbon black, free-flowable rubber powder is formed. In one aspect of this invention the bale or slab rubber from which the powder is to be made is first subdivided and more or less intimately mixed with the anti-tack agent before the resulting homogeneous aggregate is subjected to mechanical action which affects the final subdivision of the rubber to particles of increasing fineness. Generally, this initial subdivision of the rubber is carried out so as to produce particles of an average size of about 0.2-20 cm, say about 0.5-10 cm, usually about 0.75-5.0 cm. Subdivision initially can be carried out by any convenient technigue such as low shear extrusion and chopping, rough grinding, cutting, stamping and the like. Simultaneously or subsequently, the anti-tack agent is combined with the initially subdivided rubber and mixing is carried out to provide a homogeneous aggregate. This homogeneous aggregate can then be processed directly and mechanically under conditions which progressively break down the rubber particles to a size of less than about 1 cm. Generally, the resultant rubber powder has an average particle size of about 0.01-20 mm, typically about 0.1-10 mm. It should be noted that not all the anti-tack agent present in the rubber powder need be attached to the rubber particle surface. Free unassociated anti-tack agent may be present. One means of accomplishing the aforesaid final subdivision of the bale or slab rubber/anti-tack agent (for example, -filler) aggregate to the inventive powder is through the use of the mixing and plasticating machine described by Nielander in U.S. Patent 3,623,703, which is incorporated by reference in its entirety in this disclosure for its description of said machine and means of using same. Briefly, this machine comprises a mixing and plasticating cylinder and a mixing and plasticating rotor with passages therein, the shaft of said mixing and plasticating rotor being connected to a rotary drive outside said cylinder. The mixing and plasticating cylinder and the mixing and plasticating rotor are axially movable in relation to each other. By means of this relative axial motion of rotor and cylinder, the inside wall of the mixing and plasticating cylinder is scraped off by the mixing and plasticating rotor and the material in the cylinder is pressed through said passages at high shear and friction rates; abrasion may also occur during these processes. The Nielander machine accomplishes mixing and formation of the powders of this invention at controlled pressure, friction and shear conditions. The actual reduction of rubber particles to powder may occur by cutting, abrasion or shear-induced elongation and subsequent rupture of the particles or two or more of these in concert. Thus, one method of making the powders of this invention is wherein the mixing is accomplished by a machine for mixing rubber and other highly viscous materials that control pressure, friction and shear conditions, which comprises a mixing and plasticating cylinder and a mixing and plasticating rotor with aperture passages, the shaft of said mixing and plasticating rotor being connected to a rotary drive outside the mixing and plasticating cylinder, said mixing and plasticating rotor and said mixing and plasticatingcylinder being axially movable in relation to each other, during said relative motion of said mixing and plasticating rotor and said mixing and plasticating cylinder, the internal wall area of said mixing and plasticating cylinder being scrapped off by said mixing and plasticating rotor and the contents (i.e., the rubber/anti-tack agent aggregate) of said mixing and plasticating cylinder being pressed through the passages of said mixing and plasticating rotor at friction and shear. Other means for reducing the particle -size: of the rubber/anti-tack agent aggregate can be used and in its broader aspects the invention is not necessarily dependent on such means. Generally, the direct mechanical conversion of bale rubber to rubber powder according to this invention occurs at ambient or slightly elevated temperatures (the elevation is usually the result of the mixing or related processes). These temperatures range from about -20-1500 C, typically about 0-80 C. The conversion process is carried out for a time sufficient to produce the powder; usually this time 9 about 0.1-10 minutes. Excessively long conversion times sometimes leads to compaction of this powder and should be avoided. It should be pointed out that application of shear to the admix of rubber and carbon black such as occurs in internal mixers, of the Banbury and IMI type, usually lead to formation of a compacted stock in which the carbon black and rubber are well distributed. In such compacted stocks, the carbon black is also dispersed in the rubber on a microscale. When, however, the herein disclosed conditions are4present rubber powders are formed. The abrasive quality of the anti-tack agent such as carbon black may contribute to breakdown of the rubber particles. Similarly, apertures such as the slots found in the piston of the IMI machine may cause breakup of the particles and/or function to allow removal of the particles from the zone of their formation. Description of Preferred Embodiments and Examples Rubber powders, according to this invention, were made in a 3 litre Nielander mixer according to U.S. Patent 3,623,703 and available from Schiesser AG of Switzerland. The mixer has a cylindrical steel chamber having a capacity of about 3 litres. A close-fitting round piston having several radial slots can be moved back and forth in the chamber with speeds ranging from approximately 0 to Sm/minute. This piston can be simultaneously rotated at speeds up to 300 rpm. To make powder, the chamber is charged with rubber, anti-tack agent (such as carbon black) and any other desired ingredients, closed and the piston activated. Motion of the piston results in pumping of the rubber, anti-tack agent and other ingredients present from one side of the chamber to the other. This process results in repeated subdivision of the rubber into increasingly smaller particles which, as they are formed and through the accompanying mixing process, become coated with anti-tack agent. It is theorized that the rotary motion of the piston serves to distribute the rubber, anti-tack agent and other ingredients within the batch while the pumping action forces all of these components through the radial slots, thus subjecting them to a high level of relatively uniform shear. The heat generated by this mixing/shearing process is removed through the cooling system of the mixing machine. Cooling water at room temperature is run through various portions of the machine at rates of approximately 3-11 litres per minute. Under these conditions the temperature of the rubber/anti-tack agent charge during operation is slightly above the temperature at which it is added to the mixer. Example A The aforedescribed mixer was charged with 1100 grams of SBR. The piston speed was set at 1.5-2m/minutes and the rotation at 280 rpm. The rubber was premixed for one complete piston cycle. Then 504 grams of N 339 carbon black were added. Water flow to the cooling units at the chamber, back and front cover and the rotor was adjusted to rates of approximately 3/3/3 and 7 litres per minute, respectively. Mixing was then continued under these conditions for two minutes to produce a free flowing rubber powder. Repetition of this procedure, under seemingly identical conditions, led to variable results. Sometimes powder was formed and some times compaction of the rubber stock occurred. The latter compacted material resembled that obtained with conventional mixing. These results are believed to be due to unintended variations in charging the rubber and carbon black anti-tack agent, and their assemblage in the chamber. Examples B-J A series of powder preparations is made varying carbon black type, rubber type, and the size of the rubber pieces in the premix stage. These variations are described in Table I. A standard procedure is used as follows: Rubber of the indicated type, 1100 grams, is divided into rough cubes of the indicated size (on an edge). These cubes are combined with the carbon black of the indicated type, 504 grams, in an appropriately sized bag and tumbled at 250 C by hand to thoroughly mix the ingredients. Tkz combined mixture is then charged to the above-described Nielander machine and processed under essentially the same conditions. TABLE I Carbon Rubber Cube Example Black Type Rubber, Type Size, Inch B N339 SBR 0.5 C N339 SBR 1.0 D N339 SBR 2.0 E1 N339 SBR ' 0.5 F N110 SBR 0.5 G N990 SBR 0.5 H N339 BR2 0,5 I N339 EPDM' 0.5 J N339 NR 0.5 400 grams carbon black is used Diene 55 available from The Firestone Tire & Rubber Company of Akron, Ohio Epsyn 55 available from The Copolymer Corp., of Baton Rouge, LA. Comparative Example K The cooling rate for-the rotor was increased by a factor of approximately two-fold over that in Example A. The piston speed and rotation rate were kept the same. Five hundred fifty grams of SBR was charged to the chamber of the Nielander machine and premixed for one complete piston cycle. Then 504 grams of carbon black was added and mixing was carried out for one minute. A rubber powder resulted To this powder was added 275 grams of additional rubber. Mixing was continued for another 0.5 minute, then an additional portion of 275 grams of SBR was added, and mixing continued for 0.5 minutes. At this point the rubber/carbon black charge was compacted. Further mixing at a piston speed of 3-3.5m/minutes for two more minutes produced a fully compacted rubber whose temperature upon removal from the machine was 1600C. It is clear from a comparison of the results of Example A and Example K that prolonged mixing of the rubber powder under conditions of high shear leads to compaction of the powder. Thus, it is seen that it is only necessary to subject the rubber/anti-tack agent aggregate to mechanical action for a period sufficient to produce powder. Typically, this period ranges from about 0.1-10 minutes. It may vary somewhat depending on the design of the equipment and conditions applied during the mechanical action. To compare the performance of process stocks prepared from the rubber powder of this invention with stocks made by conventional processing, the powder prepared according to this invention was further mixed in a Brabender Plasticorder for two, four and eight minutes (1200C jacket temperature and 60rpm) and the materials subsequently molded into 6x 6x 0.080" plaques. Stocks of identical composition were prepared from the same basic raw materials in the Brabender under essentially identical conditions using mixing times of six and eight minutes, respectively. These stocks were also molded into 6x 6x 0.080" plaques and radiation vulcanized along with the former plaques to a dose of 10 Mrad. The carbon black dispersion and physical properties of these plaques are shown in Table II. It should be noted that the stocks made from the powder and mixed for six minutes had essentially identical carbon black dispersion and physical properties, that is modulus, tensile strength and energyeto break, as those of stocks prepared from raw ingrediento using a total mixing time of eight minutes. TABLE II PHYSICAL PROPERTIES OF COMPOUND MADE UP OF 100 PER SBR CONTAINING 37.5 WT X AROMATIC OIL AND 73 PBR CARBON BLACK (N339) Mixing Carbon Black 300% M B EB3 Time dispersion ) (MPa) (MPa) (MJ/m) Prepared 2 55 3.4 6.0 16.0 from Powder 4 86 3.1 6.8 20.2 8 93 2.5 7.8 28.4 Prepared 6 85 3.9 6.9 19.6 from Raw 8 91 3.1 7.2 23.8 Ingredients The powders of EiX inventien can be shaped and vulcanized into a variety of useful rubber articles using known techniques such as extrusion, molding and the like ., Among the articles that can be made from the powders are tires, hoses, seals, cables, belts and similar goods. It will be clear that obvious variations and modifications of this invention may be made without departing from the spirit and scope of the invention as described in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A rubber powder comprising substantially unvulcanized rubber particles having an average particle size of less than about 10 mm coated with an effective amount of at least one anti-tack agent. 2. The powder of claim 1 therein the anti-tack agent is carbon back, zinc oxide, fine particles of percipitated calcium carbonate, fine and hard clay particles, calcium silicate, silicon dioxide, or finely divided phenolic resin. 3. The powder of claim 2 wherein the powder is comprised of rubber particles having an average size of about 0.1-10 mm, the rubber is an "R" class rubber and the anti-tack agent is carbon black. 4. The powder of claim 3 wherein the rubber is NR, IR, SBR, BR, hydrogenated BR, chlorinated BR, brominated BR, EPR, EPDM, CHR, BHR or blends thereof. 5. A direct method for producing substantially unvulcanized powder which comprises mixing bale rubber and anti-tack agent at a temperature above the glass transition temperature of the rubber under high shear conditions for a time sufficient to produce non-tacky powder. 6. The method of claim 5 in which the bale rubber iE premixing first subdivided into segments having a particle size of about 0.2-20 cm and is combined with the anti-tack agent to tv3 a hom.¯g^..^¯ - aggregate before being mixed under high shear conditions. 7. The e method of claim 6 wherein the rubber is NR, IR, SBR, BR, hydrogenated BR, chlorinated BR, brominated BR, EPR,-EPDM, CHR, BHR or blends thereof and the anti-tack agent is carbon black. 8. The method of claim 7 in which 100 parts by weight rubber and about 10-300 parts carbon black is present, the mixing time is about 0.1-10 minutes and the temperature is about 0-1000 C. C F 9. The method of claim 6 wherein the high shear mixing is accomplished by a machine for high shear mixing of rubber and other highly viscous materials at controlled pressure, friction and shear conditions, and the premixing is carried out either in said machine or outside of said machine. 10. The method of claim 9 in which mixing is carried out until the rubber powder has an average particle size of about 0.1-10 mm. 11. The method of claim 10 where the premixing is carried out in said machine. 12. The method of claim 8 where the rubber segments are subjected to cutting, shear and/or abrasive forces to produce the powder. 13. A rubber powder made by the method of any one of claims 5-12. 14. A cured rubber composition made from the rubber powder of any one of claims 1-4 and 13. 15. An article of manufacture made from a cured rubber composition made from the rubber powder of any one of claims 1-4 and 13.

Inventors: BOHM, GEORG G. A., Böhm, Georg G. A.
Applicants: THE FIRESTONE TIRE & RUBBER COMPANY
Application year: 1983
Main classification: C08J3
IPC codes: C08L101, C08L21, C08L27, C08L23, C08L33, C08L1, C08L7, C08J3, C08K3