Patent Publication Number: US-2003225171-A1

Title: Process for recycling rubber

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] This application is closely related to and claims benefit from U.S. Provisional Patent Application Serial No. 60/385,353 which was filed on May 31, 2002. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates, in general, to a useable mixture made from a rubber blend utilizing recycled rubber and, more particularly, the present invention relates to a process for producing a mixture made from a rubber blend utilizing crumb rubber from used tires to form such mixture, and most particularly, the present invention relates to a process wherein useable products are made from a mixture in which significant quantities of crumb rubber are utilized.  
       BACKGROUND OF THE INVENTION  
       [0003] It has often been desired in the past to reclaim or recycle vulcanized rubber to reduce cost of a product. Today it is even more desirable to recycle used vulcanized rubber, particularly rubber from automobile and truck tires, because of the large quantities of used tires and the difficulty in the disposal of such tires. The overwhelming quantities of these tires presents an environmental problem.  
       [0004] Used tires can be burned or pyrolyzed as a means of disposal. However, because of the hazardous gases that can be produced by pyrolysis or burning as well as the basic pollution problems that are incurred with virtually every combustion process, unless the process is performed under very stringent conditions, the detrimental environmental effect presents a very real problem. To provide a process that is not environmentally harmful can be very costly. This is particularly true when the process only deals with waste products and where there is no economic gain for the process. The environmental problem keeps growing as the number of vehicles in this country and throughout the world keep growing.  
       [0005] As is well known in the tire industry significant quantities of waste rubber called “tire buffings” are formed when tires are prepared in a re-treading process where worn tires are re-treaded to make usable tires. These buffings are also waste materials that also present a disposal problem similar to the disposal problem encountered with used tires.  
       [0006] Presently in this country there is a great deal of attention given to the disposal, or the reuse, of these used tires because of the large volume of tires and the disposal problems, particularly with automobile tires. Most processes at the present time that utilize recycle rubber or crumb rubber involve binding the crumb rubber with a binder which include carcinogenic or otherwise hazardous materials. An isocyanate compound is used as the binder in many of the processes currently in use. Thus, a process that could economically utilize these used tires to make useful products and also be environmentally safe would be very welcome to not only the rubber industry but also for anyone that is the least bit concerned with the environment.  
       SUMMARY OF THE INVENTION  
       [0007] Thus, the present invention provides in one embodiment a process for manufacturing an article using a predetermined amount of recycled rubber. The process comprises the steps of selecting an article to be manufactured and determining a maximum amount by weight of recycled (crumb) rubber which can be used in manufacturing the article selected. A step in the process includes determining a particle size for the crumb rubber to be used and another step is selecting a binder to be used for binding the crumb rubber. Another step involves determining an amount of the binder selected that is required to be added to the crumb rubber for molding the desired article. A further step involves selecting curing agents to be used in the process and determining the weight of these curing agents to be added. This is followed by mixing the crumb rubber, the binder and the curing agents and heating this mixture in a shear mixer to a predetermined temperature. The mixture is removed from the mixer and a mold is filled with this mixture. The mixture is then molded at a predetermined temperature and a predetermined pressure being at least sufficient to cure the mixture so as to form the article previously selected and then the molded product is removed from the mold.  
       [0008] In another embodiment of the invention there is provided a substantially dry, particulate molding material. This material comprises a mixture of crumb rubber particles, the crumb rubber particles being present in a predetermined concentration by weight and a binder that is present in a concentration at least sufficient to provide predetermined properties of the substantially dry, particulate molding material after molding. Sulfur is present in a concentration of between about 0.4 and about 0.6 percent by weight and trimethylthiuram disulfide is added in a concentration of between about 0.07 and about 0.12 percent by weight.  
       [0009] In yet another embodiment of the invention there is a process for forming a substantially dry, particulate material using a predetermined amount of recycled rubber. The process comprises the steps of determining a maximum amount by weight of recycled rubber which can be used to form the substantially dry, particulate material, determining a particle size for the recycled rubber to be used in a previous step and selecting a binder for binding the recycled rubber. Another step in the process is determining an amount of binder, that was selected previously, required to be added to the recycled rubber. Other steps involve selecting curing agents to be used and determining weight of the curing agents selected. The process includes the steps of mixing, in a shear mixer, the recycled rubber, the binder and the curing agents that were selected previously and heating the mixture of recycled rubber, binder and curing agents to a predetermined temperature, then removing the blend of recycled rubber, binder and curing agents that were mixed and heated in previous steps.  
       OBJECTS OF THE INVENTION  
       [0010] Is one of the primary objects of the present invention to provide a process in which recycled rubber can be utilized to produce useful products economically.  
       [0011] It is another object of the present invention to provide a process for utilizing recycled rubber that does not use any carcinogenic or otherwise hazardous materials as a binding agent.  
       [0012] Another object of the invention is to provide a process in which larger quantities of recycled rubber can be utilized when compared to prior art processes to produce useful products.  
       [0013] Yet another object of the present invention is to provide a dry, particulate, moldable material utilizing crumb rubber as a main ingredient.  
       [0014] Still another object of the invention is to provide a dry, particulate, moldable material utilizing crumb rubber as a main ingredient that can be molded to make useful products that are economically feasible to manufacture.  
       [0015] Another object of the present invention is to provide a dry, particulate, moldable material that utilizes a blend of crumb rubber and tire buffings to form usable products.  
       [0016] Yet another object of the invention is to provide a dry, particulate, moldable material that has a relatively long shelf life.  
       [0017] It is another object of the present invention to provide a dry, particulate, moldable material that can be tailored to fit the end product by varying the ratio of the concentrations of the ingredients.  
       [0018] It is still another object of the present invention to provide a process for making dry, particulate, moldable material that utilizes used tires without producing hazardous materials or materials that are detrimental to the environment.  
       [0019] Yet another object of the present invention is to provide a process in which recycled rubber can be utilized that does require frequent cleaning because of the substantially non-hazardous nature of the ingredients.  
       [0020] These and various other objects and advantages of this invention will become apparent after a full reading of the following detailed description, particularly, when read in conjunction with the description below and the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE PRESENTLY PREFERRED AND ALTERNATE EMBODIMENTS OF THE INVENTION  
     [0021] The present invention provides, in a first embodiment, a process for manufacturing an article using a predetermined amount of recycled rubber. The process comprises the steps of selecting an article to be manufactured and determining a maximum amount, by weight, of recycled rubber which can be used in manufacturing the article selected. In this process any recycled rubber may be used; however, in a preferred embodiment of the invention such recycled rubber is crumb rubber and the description of the process will refer to crumb rubber but it must be remembered that this does not preclude the use of other forms of recycle rubber. The amount of crumb rubber used will generally be a function of the end product or article to be formed. Generally the crumb rubber being used will be between about 5 and 98 percent. When crumb rubber is used in quantities beyond 98% there is insufficient binder present to for a stable product and crumb rubber concentrations below 5% are economically unfeasible. To make the process economically feasible large quantities of crumb rubber are used (&gt;50%); however, for some products the amount of crumb rubber may be substantially lower than 50%. It is preferred that-the amount of crumb rubber utilized in the process is between about 40 and about 95%. In a more presently preferred embodiment of the invention such crumb rubber is present between about 60 and about 92%. While in the most preferred embodiment of the invention such crumb rubber is used between about 75 and about 90%.  
     [0022] A step in the process involves determining a particle size for the crumb rubber to be used and further in selecting a binder for binding the crumb rubber previously determined. It is presently preferred that such binder be selected from natural rubber and styrene butadiene rubber (SBR). In a presently preferred embodiment of the invention natural rubber is used.  
     [0023] Also it is presently preferred that such crumb rubber be generally in a range of generally between about 5 mesh and about 30 mesh. It is even more desirable if the crumb rubber is present in particle sizes of 20 mesh and greater. If ultrafine particulate matter is present the product generally has a nicer appearance; however, if too high a concentration of fines are used considerably more binder is required for the material to properly blend together. This is probably because the surface area with the ultrafine particles is increased dramatically and thus more binder is needed to coat all of the particles.  
     [0024] Another step involves determining an amount of binder, previously selected, that is required to be added to the crumb rubber for molding the desired article that was selected. Generally, for many useful products the binder will be present in a range of between about 2 and about 95 percent by weight. The amount of binder will be dependent on the amount of crumb rubber used to make the selected product. The more crumb rubber that is used in the blend the smaller the quantity of binder that is used.  
     [0025] Generally, the product selected that utilizes the largest quantities of crumb rubber will also require the least amount of natural rubber. This type of product will be one that does not require exceptional tensile strength or elongation properties. As a corollary a product that only utilizes about 50% of crumb rubber will generally use about the same quantity of natural rubber and will provide a product which has relatively high tensile strength and elongation properties. As the amount of crumb rubber decreases and the amount of binder is increased, the product that is formed will have higher tensile strengths and better elongation properties.  
     [0026] It is preferred that the amount of binder (natural rubber) utilized in the process is between about 5 and about 60%. In a more presently preferred embodiment of the invention such binder is present between about 8 and about 40%. While in the most preferred embodiment of the invention such binder is used between about 10 and about 25%.  
     [0027] A further step in the process involves selecting curing agents to be used in the process and determining the weight of these curing agents to be added. At present sulfur and tetramethylthiuram disulfide (Methyl Tuads) are used in all of the mixtures. Methyl Tuads is a trade name for the product supplied by R. T. Vanderbilt.  
     [0028] It is presently preferred that such sulfur be added in a concentration of between 0.4% and 0.6% by weight and that Methyl Tuads be added in a concentration of between about 0.07% and 0.12%.  
     [0029] In a presently preferred embodiment of the invention sulfur is present at about 0.5% and Methyl Tuads at about 0.1%. These concentrations are the same regardless of the amount of crumb rubber and natural rubber being used. Additional sulfur beyond about 0.6% does not appear to provide any additional benefit. Methyl Tuads is an ultra accelerator. If less than 0.1% is used curing takes longer; however, if much greater amounts beyond 0.12% are used then curing may become too fast and the product dries out and becomes brittle.  
     [0030] It is also presently preferred that N-t-butyl benzothiazole sulfenamide and 2-mercaptobenzothiazole are added as additives. N-t-butyl benzothiazole sulfenamide is also referred to as Santocure TBBS and is sold under the trade name of Vanex NS supplied by R. T. Vanderbilt.  
     [0031] 2-Mercaptobenzothiazole is sold under the trade name of Captex by R. T. Vanderbilt. In a preferred embodiment of the invention Santocure is added in a concentration of between about 0.08% and 0.12% and Captex is added in a concentration of between about 0.12% and 0.17%. The curing process will take place without the addition of Santocure but the cure will take longer and quantities beyond 0.12% may cause the product to become brittle. Additional Captex beyond 0.17% only adds to the cost without any apparent benefit. Again as with the sulfur and Methyl Tuads additions, the quantities of Santocure and Captex that are added remain substantially the same regardless of the relative concentrations of crumb rubber and binder.  
     [0032] The weighed components of crumb rubber, natural rubber and the additives are then mixed in a commercial shear mixer such as a Banbury type mixer. The mixture is then heated in the mixer to a predetermined temperature. It is presently preferred that such mixing take place until the temperature of the mixer reaches about 220F.  
     [0033] The mixture is removed from the mixer and a mold may be filled with this mixture. The mixture after being removed from the mixer may be stored as a relatively dry, particulate material until one is ready for molding. The shelf life for the dry particulate will vary depending on the care of storage but left uncovered in an open room, such particulate material can be stored for approximately six months without any apparent deleterious effect. Product has been stored for up to a year and still produced useful products.  
     [0034] When the mold is filled with the dry, particulate mixture it is then molded at a predetermined temperature and a predetermined pressure being at least sufficient to cure the mixture so as to form the article that was previously selected. It is presently preferred that such temperature generally be between about 300 and 390F and at a pressure of approximately 250 psi. The pressure may range between 200 and 300 psi. However, increased pressure does not increase the pressure on the particulate material—just increases the pressure on the mold. The molded product is then removed from the mold and is ready for whatever application the article was selected for.  
     [0035] The process for forming the substantially dry particulate material further includes a step of cooling the product after it is removed from the mixer. Since the material was heated to approximately 220F. in the mixer, this step becomes necessary since the produced material is not going to be used immediately and must be stored. It becomes important to cool the material to a temperature of 90F or less at which, temperature the material is cool enough that a spontaneous fire is not a possibility. The product is generally air cooled by the use of fans.  
     [0036] The process, in another embodiment, further includes the step of preheating the mixture before it is placed into the mold. This step may not be necessary if the mixture can be put into the mold directly after mixing but since this is a rarity the mixture is placed into the mold from storage and, thus, the preheating step is useful since the mixture has been cooled prior to being stored.  
     [0037] In still another embodiment of the process for manufacturing a useful article using a mixture of crumb rubber and a natural rubber binder, the mixture is additionally blended with tire buffings to form the mixture that is added to the Banbury mixer. In this embodiment of the invention crumb rubber is mixed with tire buffings prior to be being put in the Banbury mixer. In this embodiment crumb rubber is added in a concentration of between about 50 and about 75% (of the maximum amount by weight of recycled rubber determined in the second step) while the tire buffings is utilized at a concentration of between about 25 and about 50%. This mixture is then blended with the natural rubber binder, and the curing agents and placed in the mixer. The tire buffings essentially replaces a portion of the crumb rubber that would normally be used in the process. This embodiment further utilizes waste rubber material that otherwise would present an additional disposal problem.  
     [0038] In another embodiment of the invention there is provided a substantially dry, particulate molding material. This material comprises a mixture of crumb rubber particles, such crumb rubber particles being present in a predetermined concentration by weight, and a binder that is present in a concentration at least sufficient to provide predetermined properties of the substantially dry, particulate molding material. Crumb rubber is generally added in a concentration of between about 5 weight % and 98 weight % depending on the final use of the particulate material. It is preferable that the binder is natural rubber and that the concentration of natural rubber in the mixture be between about 2% and 95% by weight.  
     [0039] Sulfur is present in a concentration of between about 0.4 and about 0.6 percent by weight and trimethylthiuram disulfide is added in a concentration of between about 0.07 and about 0.12 percent by weight. It is also presently preferred that such curing additives as N-t-butyl benzothiazole sulfenamide and 2-mercaptobenzothiazole are also added. N-t-butyl benzothiazole sulfenamide is also referred to as Santocure TBBS and is sold under the trade name of Vanex NS supplied by R. T. Vanderbilt. 2-Mercaptobenzothiazole is sold under the trade name of Captex by R. T. Vanderbilt. It is preferred that Santocure be added in a concentration of between about 0.08% and 0.12% and that Captex be used in a concentration of between 0.12% and 0.17%.  
     [0040] As mentioned previously, with the increased percentage of crumb rubber in the blend the product has lower strength. The ratio of recycle rubber (crumb rubber) to natural rubber depends on the end application. The following table shows some of the variations in tensile strength and percent elongation with a few of the various ranges of crumb rubber that are used.  
                       TABLE I                       % Crumb Rubber   Tensile Strength, (lbs/sq.in.)   % Elongation                                            70   1057   205       80   678   207       90   660   174       95   550   104                  
 
     [0041] It should be noted that the process for making the product of the dry, particulate molding material containing crumb rubber is substantially a non-hazardous process in that the crumb rubber is not blended with any carcinogenic or otherwise hazardous material. Unlike products that are formed utilizing hazardous materials the present product can be scraped, bent or twisted and no hazardous materials are emitted. Such cannot be said of the processes that involve suspect chemicals, such as isocyanates, as binders in converting the crumb rubber to a usable product.  
     [0042] In yet another embodiment of the invention there is a process for forming a substantially dry, particulate material utilizing a predetermined amount of recycled rubber. The process comprises the steps of determining a maximum amount by weight of recycled rubber which can be used to form the substantially dry, particulate material, determining a particle size for the recycled rubber to be used in the previous step and selecting a binder for binding the recycled rubber. Another step in the process involves determining an amount of binder, that was selected previously, that is required to be added to the recycled rubber. Other steps involve selecting curing agents to be used and determining weight of the curing agents selected to be added. The process includes the steps of mixing the recycled rubber, the binder and the curing agents that were selected previously and heating the mixture of recycled rubber, binder and curing agents to a predetermined temperature. The mixing involves shear mixing and is performed in a Banbury type mixer. It is presently preferred that such mixing take place until the temperature of the mixer reaches about 220F. The final step in the process is removing the blend of recycled rubber, binder and curing agents that were mixed and heated in previous steps. This process produces the substantially dry, particulate material that can be stored and used as needed. The process utilizes the curing agents that were described previously.  
     [0043] In still another embodiment of the process for forming such dry particulate material utilizing a mixture of crumb rubber and natural rubber binder, the mixture is additionally blended with tire buffings to form the mixture that is added to the Banbury mixer. In this embodiment of the invention where such crumb rubber is mixed with tire buffings the crumb rubber is added in a concentration of between about 50 and about 75% (of the maximum amount by weight of recycled rubber determined in the first step) while the tire buffings are utilized at a concentration of between about 25 and about 50%. This mixture is blended with the natural rubber binder and the curing agents and placed in the Banbury mixer. The tire buffings essentially replaces a portion of the crumb rubber that would normally be used in the process. This embodiment further utilizes waste rubber material.  
     [0044] The process for forming the substantially dry particulate material further includes a step of cooling the product after it is removed from the mixer. Since the material was heated to approximately 220F. in the mixer, this step becomes necessary since the produced material is not going to be used immediately and must be stored. It becomes important to cool the material to a temperature of 90F or less at which temperature the material is cool enough that a spontaneous fire is not a possibility. The product is generally air cooled by the use of fans.  
     [0045] While both the presently preferred and a number of alternative embodiments of the present invention have been described in detail above it should be understood that various other adaptations and modifications of the present invention can be envisioned by those persons who are skilled in the relevant art without departing from either the spirit of the invention or the scope of the appended claims.