1. Field of the Invention
The invention relates generally to the treatment of sulfur-cured or sulfur vulcanized rubber compounds by chemical and/or biochemical means in order to allow reuse of the sulfur-cured rubber compounds without appreciable loss in performance of the resulting rubber compound. More particularly, the invention is directed to a method, and product thereof, for devulcanizing sulfur-cured rubber compounds that utilizes an organic solvent to swell the vulcanized rubber compound combined with the introduction of microbes and/or enzymes which break the sulfur-sulfur (S—S) and sulfur-carbon (S—C) bonds of the vulcanized rubber compound in order to devulcanize the rubber compound.
2. Background Art
The rubber compounds used in vehicle tires are composite materials that include polymers such as natural rubber or synthetic rubbers that have been blended with a variety of additives such as sulfur and fillers such as carbon black or silica. The tire is molded into the basic circular shape and then cured. The curing process of the tire is well known in the industry as vulcanization and results in the creation of a three dimensional network of sulfur cross-links which link the various polymer chains to form a dimensionally stable thermoset composite which cannot be reshaped once it has been formed and cured. As a result, the recycling and reuse of vulcanized rubber products, such as worn rubber tires, is difficult because the vulcanized rubber cannot be reshaped or simply reconstituted by dissolving it in a solvent in order to form the vulcanized rubber into a new shape.
However, because of the ever increasing cost of oil derived raw materials such as synthetic rubbers and carbon black fillers, there is considerable interest in the reuse of products such as worn vulcanized rubber tires made from these materials. Millions of used tires and other rubber products are discarded annually and only a minor fraction of them are used in any manner. The small amount that are reused are usually first broken up to remove the non-rubber components of the tire, such as the steel cords, beads etc., and the remaining rubber compound is ground-up into rubber particles of different sizes for use in a wide variety of applications, such as synthetic turf for football, soccer, and other sport related playing surfaces. Additional applications of these ground-up vulcanized rubber particles include use of the products in molded or extruded materials such as floor mats etc. Reuse of the vulcanized ground rubber particles in high performance products such as tires is limited to exceedingly small quantities because the ground rubber particles adversely affect key properties vital to the performance of the end product.
More particularly, these adverse affects result because the vulcanized ground rubber particles do not dissolve in the fresh rubber compounds on a molecular scale but they stay intact and act as defects once the product is processed by molding and vulcanization of the new composition. There are two types of defects that typically result from incorporating the vulcanized ground rubber particles into fresh rubber compounds. The first relates to a defect generated because of poor bonding between the ground rubber particles and the fresh new matrix rubber and arises mainly because of insufficient molecular interdiffusion. Therefore, when stress is applied to the resulting end product, small gaps will form where the new matrix fresh rubber polymer separates from the vulcanized ground rubber particle which will subsequently grow into larger propagating cracks and ultimately cause failure. The second type of defect arises because the vulcanized ground rubber particles generally will have a much higher cross-link density in the final product than the fresh new matrix rubber. More specifically, some of the sulfur and cure accelerators added to the fresh compound for vulcanization will diffuse into the ground rubber particles during the mixing, shaping and vulcanization steps, and cause the vulcanized ground rubber particles to experience a second vulcanization. The second vulcanization results in a much higher crosslink density, a higher modulus and a significantly lower extension to break in the ground rubber phase. Thus, when the final product is deformed during use, premature failure will occur in the ground rubber particles because this phase will reach conditions to failure at a much lower strain compared to the new matrix rubber phase. Micro-cracks formed in the process will at first propagate through the particle and then continue to propagate through the product causing it to fail during use. These defects manifest themselves even at relatively low vulcanized ground rubber concentrations resulting in a reduced tensile strength, poor cut growth performance and reduced wear. Therefore, there is a great reluctance on the part of many tire manufacturers to incorporate tire buffings and ground rubber particles from whole tires into fresh tire compounds due to the problems described above.
Recognizing these limitations that the vulcanized state of ground rubber has on many different applications, many different processes have been explored to devulcanize ground rubber prior to its use. The overall objective of the devulcanization process is to break up the S—S (sulfur-sulfur) and C—S (carbon-sulfur) bonds originally formed during vulcanization and to remove or deactivate the chemicals involved in the first vulcanization process as well as during the devulcanization so that they cannot initiate any new cross-links during subsequent use of the devulcanized particles in the fresh rubber compounds. Moreover, it is imperative that the devulcanization process does not involve any breakage of main chain carbon-carbon (C—C) links as this will reduce the molecular weight of the rubber in the ground rubber particles and significantly affect the performance of the compositions comprising the devulcanized rubber. The prior art patents listed below describe different approaches for devulcanization ranging from the use of ultrasonic and microwave exposure to the treatment with certain chemicals or bio-chemical agents. However, no devulcanization process has at present proven to be technically or commercially viable on a large scale.
U.S. Pat. No. 5,284,625 discloses a continuous ultrasonic method for breaking the carbon-sulfur, sulfur-sulfur and, if desired, the carbon-carbon bonds in a vulcanized elastomer. Through the application of certain predetermined levels of ultrasonic amplitudes and the presence of pressure and optional heat, it is reported that cured or vulcanized rubber can be broken down. Using this process, the rubber becomes soft, thereby enabling it to be reprocessed and reshaped in a manner similar to that employed with previously uncured elastomers.
U.S. Pat. No. 4,104,205 discloses a means to devulcanize sulfur-vulcanized elastomers comprising polar groups which includes exposure to microwave energy of between 915 and 2450 megahertz and between 41 and 177 watt-hours/pound sufficient to break substantially all C—S and S—C bonds and insufficient to break up the C—C bonds.
U.S. Pat. No. 6,420,457 sets forth a process and a resulting product of the process in which a vulcanized solid particulate, such as vulcanized crumb rubber, has select chemical bonds broken by microwave radiation. The direct application of microwaves in combination with uniform heating of the crumb rubber renders the treated crumb rubber more suitable for use in new rubber formulations. As a result, larger particle sizes and/or living levels of the treated crumb rubber can be used in new rubber mixtures to produce recycled composite products with good performance properties.
U.S. Pat. No. 6,380,269 discloses a process for devulcanization of the surface of reclaimed rubber crumb into surface devulcanized reclaimed rubber crumb that is suitable for being re-compounded and recurred into high performance rubber products. The process includes the steps of heating the reclaimed rubber crumb to a temperature which is within the range of about 150° Celsius to about 300° Celsius under a pressure of at least about 3.4×106 Pascal, in the presence of 2-butenol to devulcanize the surface of the rubber crumb thereby producing a slurry of the surface devulcanized reclaimed rubber crumb in the 2-butenol, wherein the reclaimed rubber crumb has a particle size which is within the range of about 325 mesh to about 20 mesh. The surface devulcanized reclaimed rubber crumb is then separated from the 2-butenol.
U.S. Pat. No. 5,602,186 discloses a process for devulcanizing cured rubber by de-sulfurization which includes the steps of: contacting rubber vulcanized crumb with a solvent and an alkaline metal to form a reaction mixture, heating the reaction mixture in the absence of oxygen and with mixing to a temperature sufficient to cause the alkaline metal to react with the sulfur and the rubber will vulcanizate, and maintaining the temperature below that at which the thermal cracking of the rubber occurs, thereby devulcanizing the rubber vulcanizate. The patent indicates that it preferred to control the temperate below about 300° Celsius or where thermal cracking of the rubber is initiated.
U.S. Pat. No. 7,425,584 relates generally to the devulcanization of rubber. More specifically, the invention relates to a process for the hydro-devulcanization of vulcanized rubber utilizing a rubber swelling solvent, a source of reactive hydrogen and elevated temperature and, optimally, also uses a disbursed molecular-scale hydrogenation catalyst that is introduced into the vulcanized rubber and a catalytically inert form that is soluble in the rubber swelling solvent.
U.S. Pat. No. 5,891,926 discloses a process for devulcanizing cured rubber into devulcanized rubber that is capable of being re-compounded and recurred into useful rubber product and for extracting the devulcanized rubber from the cured rubber. The process includes: heating the cured rubber to a temperature which is within the range of about 150° Celsius to about 300° Celsius under pressure of at least about 3.4×106 Pascal in 2-butenol in order to devulcanize the cured rubber into devulcanized rubber, thereby producing a mixture of solid cured rubber, solid devulcanized rubber and a solution of the devulcanized rubber in the 2-butenol, removing the solution of the devulcanized rubber from the solid cured rubber and the solid devulcanized rubber, cooling the solution of the devulcanized rubber in the 2-butenol to a temperature of less than about 100° Celsius and separating the devulcanized rubber from the 2-butenol.
U.S. Pat. No. 5,597,851 sets forth a method of using enzymes from thiophilic microbes for selectively breaking the sulfur rubber crosslink bonds in vulcanized rubber. The process is halted at the sulfoxide or sulfone step so that a devulcanized layer is reactivated with virgin rubber.
U.S. Pat. No. 6,407,144 sets forth a process and product where a vulcanized solid particulate, such as vulcanized crumb rubber, has select chemical bonds altered by biotreatment with thermophilic microorganisms selected from natural isolates from hot sulfur springs. Following the bio-treatment, microwave radiation is used to further treat the surface and to treat the bulk interior of the crumb rubber. The resulting combined treatments render the treated crumb rubber more suitable for use in new rubber formulations. As a result, larger loading levels and sizes of the treated crumb rubber can be used in new rubber mixtures and good performance properties are obtained from the new recycled products.
U.S. Pat. No. 6,479,558 describes a process and resulting product where a vulcanized solid particulate, such as vulcanized crumb rubber, has selected chemical bonds broken by bio-treatment with hemophilic microorganisms selected from naturally occurring isolates derived from hot sulfur springs. The bio-treatment of the crumb rubber renders the treated crumb rubber more suitable for use in new rubber formulations. As a result, larger loading levels and sizes of the treated crumb rubber can be used in new rubber mixtures.
While microbes have been used before for the devulcanization of vulcanized rubber crumb, the rates observed are impractical for industrial applications and any devulcanization observed was limited to a very thin surface layer of the treated ground rubber particles. At least three factors are responsible for this: 1) a slow and very shallow penetration of the biological agents (microbes and/or enzymes) into the bulk of the particle, 2) slow rates of the devulcanization reactions, and 3) a partial deactivation of the biological agents by certain chemical compounds present in the ground rubber particle. These problems are described in greater detail below.
The first problem originates from the difficulty that microbes and enzymes have in diffusing into the filler reinforced sulfur cross-linked polymeric matrix of the vulcanized rubber compound in order to catalyze the sulfur oxidation reaction. This is due in part to the essentially hydrophilic nature of microbes and enzymes which are generally incompatible with the mostly hydrophobic elastomers used in cured rubber compounds. Therefore, the hydrophilic microbes and enzymes have a very small mutual solubility in the vulcanized crumb rubber particle. Moreover, any interdiffusion of the microbes/enzymes into the sulfur cross-linked polymeric matrix of the vulcanized rubber crumb particles is also effected by the de facto pores which were formed during the vulcanization of the product from which the ground rubber particles were produced. At that time, the rubber molecules existed in the form of coiled structures which became linked to each other through sulfur cross-links with the average molecular weight between cross-links generally being about 10 kilograms/mole. The resulting fisherman-like net acts as a potential barrier to interdiffusion of the microbes/enzymes into the sulfur cross-link polymeric matrix of the vulcanized rubber crumb particles because the net size is often smaller than the dimensions of most microbes (about 1-30 micrometers). The second problem relates to the use of microbes and enzymes which only show a limited activity toward sulfur. With regard to the third issue it is generally known that certain chemicals such as zinc oxide, 2-mercaptobenzothiozole (MBT) and others added to rubber compounds as stabilizers or to facilitate and control the vulcanization process are toxic to many microorganisms.
Because of the problems set forth above, no devulcanization process which utilizes microbes and/or enzymes at present has proven to be technically or commercially viable on a large scale in order to devulcanize crumb rubber so that it can be readily combined with new rubber without a significant loss in product performance.
The present invention overcomes the problems associated with prior art microbial and/or enzymatic devulcanization methods by providing a process and the resulting product of the process in which a previously vulcanized rubber may be incorporated into new polymer compositions for tires and other products at much greater levels than used before in the prior art without any significant loss in product performance. More specifically, the present invention provides a method for devulcanizing a previously sulfur vulcanized rubber compound which includes treatment of the vulcanized ground rubber compound to reduce or break up all chemical bonds between sulfur (S—S) and sulfur and carbon (S—C) atoms and to deactivate or remove all or some of the active chemical residue generated by the initial vulcanization and the devulcanization step from the rubber particles. The treatment involves exposure of the vulcanized rubber matrix to certain enzyme releasing microbes or microorganisms, or such enzymes by themselves, which react with and break up the S—S and S—C bonds of the vulcanized rubber crumb in the presence of an organic solvent which swells the vulcanized rubber crumb particles but does not interfere with the biological activity of the microbes, microorganisms or the enzymes chosen. The method for microbial and/or enzymatic devulcanization of rubber of the present invention overcomes the problems associated with prior devulcanization methods that utilize microbial or enzymatic agents set forth above by providing a method for devulcanization of vulcanized rubber crumb that ensures that every part of the ground rubber particle will be exposed to the microbe and/or enzyme and thus devulcanized. Therefore, the method of microbial and/or enzymatic rubber devulcanization of the present invention causes most S—S and S—C bonds to be broken in the rubber crumb, resulting in a rubber composition that can easily be combined in large quantities with fresh rubber compound with a uniform dispersion of the rubber as well as the filler particle content of the devulcanized rubber particles in fresh rubber compound on a molecular scale.