Systems and methods for processing guayule rubber

Abstract

The guayule rubber processing system (100) and method (200) include mixing guayule rubber and at least one solvent blend in an extruder (102) to remove resin found in the guayule miscella (104) and precipitate the guayule miscella (104) to form a precipitate. The at least one solvent blend (138, 140, 142) includes a first solvent configured to precipitate the guayule rubber and a second solvent configured to swell the resulting precipitate. In particular, the series of solvent blends (138, 140, 142) may be used at different locations along the length of the extruder (102) and may further include separate blends of the first and second solvents introduced at different locations and having different ratios of the first and second solvents.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 890,924, filed August 23, 2019, the entire disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to systems and methods for obtaining natural rubber from non-Hevea plants, and more particularly to extrusion-based systems and methods for isolating and purifying natural rubber from guayule shrubs. [Background technology]

[0003] The plant Hevea brasiliensis, also known as the "Hevea brasiliensis" or "rubber tree," is a well-known source of natural rubber. Natural rubber consumption in the United States comes primarily from Hevea brasiliensis. Rubber sources such as Hevea brasiliensis, as well as other plants such as Ficus elastica or the "rubber tree" and Cryptostegia grandiflora or the "Madagascar rubber vine," all produce natural rubber in the form of a sap containing rubber latex, which flows freely and can be recovered by tapping the plants.

[0004] Various non-Hevea species are also known to contain natural rubber. An example of this is Parthenium argentatum, commonly known as the guayule shrub. Unlike Hevea species, which produce rubber in lacteal ducts, guayule rubber is produced in parenchyma cells of the bark, stems, and roots. Therefore, rubber must be obtained from guayule plants by mechanically disrupting the cells, rather than by beating, as with Hevea. For this reason, the process of extracting rubber from non-Hevea species is generally more complex than harvesting rubber from sap-producing sources such as Hevea trees. Another difference between guayule and para rubber is the amount of resinous material: while Hevea trees have a very low resin content, guayule shrubs contain resin, which is the major component and must be removed for applications such as tires. It should be understood that the resin in guayule plants adversely affects the curing behavior and compounding properties of rubber.

[0005] Historically, the vast majority of Hevea-derived natural rubber imported by the United States originated in Indonesia, Vietnam, and Thailand. However, natural rubber derived from the guayule shrub can be grown in the southwestern United States and northern Mexico and is therefore desirable in the United States as a domestically produced alternative to Hevea-derived natural rubber.

[0006] There are several methodologies and associated variations for isolating guayule rubber from the guayule shrub. The methodologies can be divided into three categories: flotation, latex, and solvent processes.

[0007] An example of a flotation process can be found in U.S. Patent No. 2,408,853 to Hoover et al., which describes a method in which crushed or cut deciduous guayule shrub is pebbled in a slurry to form "rubber worms" that are then separated by flotation. Hoover et al. also describe the purification of rubber worms by a fermentation process. An improved flotation plant was installed and operated by Mexico in Saltillo, Mexico, in the late 1970s, and a description of this process can be found in Chapter 6 of "An Alternative Source of Natural Rubber," published by the U.S. National Science Foundation in 1977.

[0008] The latex process is described in U.S. Patent No. 5,580,942 to Cornish. The Cornish patent describes a method for preparing guayule latex by homogenizing the plant material in an aqueous medium, filtering the homogenate, and separating the rubber-containing phase from the aqueous phase by centrifugation. The resulting latex can be coagulated using conventional techniques with materials such as formic acid, citric acid, acetic acid, or metal salts. Calcium salts are typically used. The polymer is typically stabilized with phenolic or amine antioxidants to prevent molecular weight loss and degradation.

[0009] Alternatively, the latex can be processed with a solvent mixture to isolate the rubber, as in U.S. Patent No. 8,815,965 to Cole et al., which describes contacting a non-Hevea latex with an organic solvent to obtain a rubber-rich organic phase and a rubber-poor aqueous phase. The organic phase is further processed (e.g., extracted and dried) to obtain solid guayule rubber.

[0010] Further, for example, U.S. Patent No. 9,273,153 to Martin et al. describes a process in which latex is coagulated in an extruder using shear, acid, metal salts, or a combination thereof. Optionally, the coagulated rubber may be washed with a solvent within the extruder. Martin et al. describe feeding guayule latex into an extruder where the rubber is precipitated, washed, and dried. While Martin et al. mentions solvents and solvent blends, there are no examples and no recognition of the importance of selecting a solvent system (blend) in which a first solvent is configured to coagulate the latex and a second solvent is configured to swell the resulting coagulum.

[0011] In a further example, U.S. Patent Application Publication No. 2018 / 0230243 to Sauty et al. discloses a multi-solvent based precipitation and washing extruder process for guayule latex that substantially reduces the resin content in the rubber.

[0012] For large market uses of guayule, where rubber must compete on price with Hevea brasiliensis, extraction processes for rubber isolation are preferred. In this technique, guayule is reduced in size by chopping and / or grinding, and then extracted with one or more solvents. The chopped guayule contains both rubber and resin components, which are desirably separated during the process.

[0013] For example, U.S. Patent No. 4,136,131 to Buchanan discloses a solvent process for extracting guayule rubber from guayule shrubs. The process disclosed in Buchanan involves reducing the size of the guayule shrubs and shaping the resulting particles, followed by sequential extraction of the resin and rubber with a selected solvent.

[0014] In a further example, a continuous extraction process using either toluene, xylene, and a pentane / perchloroethylene blend as the solvent, in combination with methyl alcohol as the precipitating agent, was developed at Texas A&M University (J. Wagner and D. Parma (1988) Polymer-Plastics Technology and Engineering - "Continuous solvent extraction process for recovery of natural rubber from guayule" 27:3, 33 5-3), which is incorporated herein by reference. Although labeled a "continuous" process, the process only used continuous extraction by precipitation of rubber from extracted guayule, which was done in batches.

[0015] As mentioned above, guayule typically contains large amounts of resin, which often rivals the amount of rubber contained in the guayule shrub.

[0016] For example, U.S. Patent No. 2,618,670 to Clark, the entire disclosure of which is incorporated herein by reference, describes how guayule resin can be extracted from guayule rubber-containing materials by using a methyl ethyl ketone / water azeotrope as the extraction medium. Another example is U.S. Patent No. 4,684,715 to Kay, the entire disclosure of which is incorporated herein by reference. Kay discloses the extraction of both resin and rubber by using a single-phase solvent system composed of a polar and non-polar solvent blend.

[0017] Thus, there is a continuing need for more efficient and effective systems and methods for obtaining rubber from non-Hevea sources, such as guayule shrubs, that would result in superior natural rubber quality and utility. Desirably, the systems and methods would facilitate both the isolation of the rubber and the separation of the rubber from the resin with a minimum number of steps. Summary of the Invention

[0018] Consistent with the present disclosure, a more efficient and effective system and method for obtaining rubber from non-Hevea sources, such as guayule shrubs, has surprisingly been discovered that results in superior natural rubber quality and utility and facilitates precipitation and extraction of natural rubber with a minimal number of steps.

[0019] As with any industrial process, costs can be reduced if the number of process steps can be reduced or simplified. In the isolation of guayule from the extraction process, the guayule is extracted (continuously or batchwise), and then the resulting miscella is treated batchwise with a non-solvent in a separate process to isolate the rubber. The rubber can then be separated from the solvent, stabilized, and dried. Depending on the solvent composition, the percentage of guayule recovered and the amount of resin remaining will vary. In the extrusion process of the present invention, rubber precipitation and de-resinization occur simultaneously, and the solvent is separated from the stabilized and dried rubber. Thus, in one process step, the miscella is converted into a finished rubber product ready for compounding.

[0020] In one embodiment of the present disclosure, a natural rubber miscella processing method includes mixing natural rubber miscella with at least one solvent blend in an extruder. Resin and solvent are removed from the guayule in the extruder, while the rubber remains in the extruder. The natural rubber miscella may be "as extracted" from the guayule plant, typically containing less than 5% rubber, or more preferably may be concentrated by removing solvent to increase the amount of rubber contained per unit volume. The at least one solvent blend is configured to precipitate the natural rubber and remove resin found in the precipitated natural rubber. Advantageously, it has been found that more natural rubber can be obtained from the natural rubber miscella, as opposed to conventional processes using latex, where more rubber tends to be retained in the plant during processing.

[0021] At least one solvent blend may include a first polar solvent and a second non-polar solvent. The first polar solvent is configured to precipitate the natural rubber, and the second non-polar solvent is configured to swell the resulting precipitated rubber. The solvent blend achieves different solubility parameters so that the rubber is immiscible in the solvent phase and swelling is carefully controlled.

[0022] Specifically, the solvent blend can be a mixture of polar and non-polar solvents, where the non-polar solvent is a hydrocarbon solvent selected from the group consisting of alkanes having 5 to 9 carbon atoms, cycloalkanes having 5 to 10 carbon atoms, aromatic or alkyl-substituted aromatic compounds having 6 to 12 carbon atoms, chloroaromatics having 6 to 12 carbon atoms, chlorocycloalkanes having 5 to 10 carbon atoms, chloroalkanes containing 2 to 8 carbon atoms, and combinations thereof, and the polar solvent is selected from the group consisting of alcohols having 1 to 8 carbon atoms, esters having 3 to 8 carbon atoms, ketones having 3 to 8 carbon atoms, ethers having 2 to 8 carbon atoms, and combinations thereof, and the solvent blend is capable of precipitating the guayule from the miscella and removing the resin.

[0023] In another embodiment, a guayule rubber miscella processing method includes mixing guayule rubber miscella with a series of solvent blends along the length of an extruder to precipitate the guayule rubber and remove resins found in the precipitated guayule rubber. The series of solvent blends includes a first polar solvent configured to precipitate the natural rubber and a second non-polar solvent configured to swell the resulting precipitated guayule rubber. The solvent blends achieve different solubility parameters so that the precipitated rubber is immiscible in the solvent phase and swelling is carefully controlled. The series of solvent blends includes separate blends of the first polar solvent and the second non-polar solvent, introduced at different locations and with different ratios of the first polar solvent to the second non-polar solvent.

[0024] In yet another embodiment, the natural rubber miscella processing system includes an extruder having at least one filter. Multiple filters may be used because the solvent blend may change along the length of the extruder and more resin is subsequently extracted. For example, the at least one filter may include a first mechanical filter, a second mechanical filter, and a third mechanical filter. The extruder has a first end and a second end and at least one screw. While a single screw may be used, a twin-screw extruder may be preferred in certain embodiments. The extruder further has multiple process zones disposed between the first end and the second end. The multiple process zones include a precipitation zone, a first washing zone, and optionally a second washing zone. The precipitation zone is configured to receive the natural rubber miscella and the first solvent blend and precipitate the natural rubber miscella into a precipitate. The first washing zone is in communication with the precipitation zone and is configured to receive the precipitate and the second solvent blend. The second washing zone is in communication with the first washing zone and is configured to receive the precipitate and the third solvent blend. The first mechanical filter is in communication with the precipitation zone and is configured to remove at least a portion of the first solvent blend from the precipitate in the precipitation zone. The second mechanical filter is in communication with the first washing zone and is configured to remove at least a portion of the second solvent blend from the precipitate in the first washing zone. The third mechanical filter is in communication with the second washing zone and is configured to remove at least a portion of the third solvent blend from the precipitate in the second washing zone.

[0025] In a further embodiment, a guayule rubber processing method includes mixing guayule miscella and a first solvent blend in a precipitation zone of an extruder to form a precipitate. A portion of the first solvent blend in the precipitation zone is then removed using a first mechanical filter. The precipitate is then washed with a second solvent blend in a first washing zone. A portion of the second solvent blend is then removed in a first washing zone using a second mechanical filter. The precipitate is then washed with a third solvent blend in a second washing zone using a third mechanical filter. A vacuum is then applied to the extruder downstream of the third mechanical filter to remove any residual amounts of the first, second, and third solvent blends. The precipitate is then extruded from the second end of the extruder to obtain guayule rubber. A polymeric stabilizer may be added after the final washing zone and before the application of the vacuum.

[0026] In an exemplary embodiment, the present disclosure includes a method for preparing guayule natural rubber from guayule miscella using a screw extruder and solvent blending to remove the resin. The method involves concentrating the natural rubber miscella to about 10% rubber or more and mixing the concentrated miscella with a polar / non-polar solvent blend (e.g., ethanol / cyclopentane) in a precipitation zone of the extruder. Guayule natural rubber is soluble in cyclopentane but insoluble in ethanol. Ethanol is used to precipitate the guayule miscella into guayule natural rubber. Cyclopentane is used to swell the resulting guayule natural rubber to make it more flexible for more efficient resin extraction and to increase mechanical interaction with the extruder. The resin is removed along with most of the solvent by a mechanical filter after the precipitation zone. A vacuum is used to reduce total volatiles to less than 1% before the rubber exits the extruder for final use.

[0027] Advantageously, the ratio of polar to nonpolar solvents is selected so that the precipitate is soft enough to be processed (i.e., the resin can be extracted), but not so soft that it dissolves in the solvent or is extruded through a mechanical filter. Thus, in the case of an ethanol / cyclopentane mixture, the cyclopentane percentage in the solvent blend is high enough to induce the desired amount of swelling, but the extruder conditions are such that the swollen rubber does not extrude through the filter. Variables that can affect the desired mass swelling ratio include the type of mechanical filter, screw speed, screw flight height, screw flight length, barrel segment temperature, and the distance between the screw and the barrel. It should be noted that not only the mass swelling ratio of the solvent blend introduced into the extruder, but also the mass swelling ratio of the particular solvent composition experienced by the rubber in the extruder, controls the performance of the rubber in the extruder. For example, when moving from the coagulation zone to the washing zone, some of the solvent from the coagulation zone migrates to the washing zone, increasing the mass swelling ratio of the solvent blend experienced by the rubber in the washing zone.

[0028] Surprisingly, it was found that the weight fraction of cyclopentane for this purpose should be about 0% to about 50%, more specifically about 20% to about 30%, and most specifically about 25%. The mass swelling ratio, which increases nonlinearly with increasing cyclopentane content in the solvent blend, is also about 1.09 to about 3.00, more specifically about 1.47 to about 1.69, and most specifically about 1.5. In a specific example, this mass swelling ratio is obtained with a ratio of 75% ethanol to 25% cyclopentane. These swelling ratios are applicable regardless of the polar / nonpolar solvent mixture selected in this process, but the ratio of polar to nonpolar solvents will vary depending on the solvent blend selected. This is illustrated by a comparison of Figures 3 and 4, which compare MSR values ​​for different combinations of polar and nonpolar solvents. An illustration of a method for determining MSR is also provided in Example 1.

[0029] Mass swelling ratio was introduced by Sauty in U.S. Patent Application Publication No. 2018 / 0230243, where its importance to latex coagulation and extraction was discussed. However, it should be noted that the water in the latex used by Sauty makes calculating the true mass swelling ratio difficult. The polar solvents used in Sauty's application are soluble in the water contained in the latex. Therefore, since the water present in the process absorbs at least a portion of the polar solvent, it is difficult to determine the actual ratio of polar to nonpolar solvents experienced by the coagulated latex. Sauty points out that the specified conditions are "optimal" for a latex containing 45% water, and that it is expected that the solvent ratio required for guayule purification may change as the latex concentration decreases. Therefore, the target mass swelling ratio required for coagulation / washing of guayule latex differs from that required for precipitation / washing of guayule micelles described herein.

[0030] Additionally, the extruder temperature is also controlled during processing to minimize the loss of precipitate through the filter. It should be understood that the temperature is the barrel temperature, not the solvent temperature. The barrel temperature may be determined, for example, by thermocouples in the extruder barrel in each zone. It should also be understood that the lower the temperature, the less likely the precipitate will be inadvertently extruded through the filter.

[0031] Those skilled in the art can select an appropriate temperature or temperature range for the extruder during the extrusion process, as desired. In certain embodiments, the natural rubber miscella may be processed in the precipitation zone at a temperature below ambient, specifically from 0°C to about 8°C, and most specifically about 4°C. In a further example, the precipitation may be processed in the first wash zone at a temperature below ambient, specifically from 0°C to about 8°C, and most specifically about 4°C. It should be appreciated that lower than ambient temperatures in these locations facilitate the use of volatile solvent blends and help prevent the undesirable discharge of the natural rubber miscella through filters.

[0032] In a further example, the natural rubber miscella may be processed in the second wash zone at about 100° C. to about 200° C., most specifically at about 150° C. It should be understood that at the location of the second wash zone, the contents of the extruder are under pressure and therefore the volatile solvent blend may be processed at these temperatures.

[0033] The ratio of polar to nonpolar solvents is also varied along the length of the extruder to optimize processing and resin removal. For example, in the precipitation zone of an ethanol / cyclopentane system, a 75 / 25 ethanol / cyclopentane ratio is used to promote precipitation and allow some resin to be removed by the solvent; in the first wash zone, a 50 / 50 ethanol / cyclopentane ratio is used to further remove resin and maintain adequate swelling; and in the second wash zone, a 95 / 5 ethanol / cyclopentane ratio is used to complete resin removal while maintaining minimal swelling to prevent excessive softening or dissolution of the rubber (which would otherwise result in loss of rubber through the filter). The polar solvent concentration increases in subsequent wash zones; therefore, the MSR of the solvent blend used in any subsequent wash steps is equal to or lower than that used in either the preceding wash step or the precipitation zone.

[0034] These and other advantages of the present disclosure will become readily apparent to those skilled in the art from the following detailed description, particularly when considered in light of the drawings described herein. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram of an extruder system for the removal of guayule resin and precipitation of guayule miscella into natural rubber according to one embodiment of the present disclosure.

[0036] [Figure 2] FIG. 1 is a flow diagram illustrating a method for the removal of guayule resin and precipitation of guayule micelles into natural rubber according to one embodiment of the present disclosure.

[0037] [Figure 3] 1 is a graph of the relationship of mass swelling ratio as a function of weight percent of ethanol in cyclopentane / ethanol blends.

[0038] [Figure 4] 1 is a graph of the relationship of mass swelling ratio as a function of weight percent for several polar / hexane solvent blends. DETAILED DESCRIPTION OF THE INVENTION

[0039] The following description is merely exemplary in nature and is not intended to limit the disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. With respect to the disclosed methods, the order of steps presented is exemplary in nature and, thus, is not necessary or critical unless otherwise disclosed.

[0040] As used herein, the term "miscella" refers to a solution or mixture containing natural rubber and a hydrocarbon solvent without added water. The term "miscella" is also distinct from "latex," which specifically includes an emulsion or mixture containing natural rubber and water.

[0041] The present disclosure includes a system 100 and method 200 for both resin removal and natural rubber precipitation in an extruder 102. In a non-limiting example, the natural rubber may be guayule rubber. However, other types of natural rubber may be selected by one skilled in the art as desired. Additionally, the present disclosure includes applying an unexpected and surprising discovered criticality of solvent blends for precipitation and solvent washing in the extruder 102 to effectively and efficiently remove guayule resin during the process.

[0042] In certain embodiments, the concentration of the guayule micella 104 added to the extruder 102 can be increased by removing the solvent to increase the amount of rubber per unit volume. For example, by removing the solvent from raw guayule micella, the concentration of the guayule micella 104 can be increased to about 10% rubber or more.

[0043] 1 , a system 100 according to one embodiment of the present disclosure includes an extruder 102 that can receive guayule miscella 104 at an inlet port 106 of the extruder 102. The extruder 102 can have a first end 108 and a second end 110, and at least one screw 112.

[0044] An illustrative description of the extrusion, coagulation, and devolatilization operations, including a typical extruder configuration for a latex process, is described in "Direct Extrusion of Polymer Latex Emulsions," Advances in Polymer Technology, vol. 3, n 1, pp. 41-49, 1983, by Russell Nichols, Richard Senn, and Farokh Kheradi, the entire disclosure of which is incorporated herein by reference. U.S. Patent No. 4,198,265 to Johnson discloses an extruder process for the devolatilization of solutions of elastomers in organic solvents, the entire disclosure of which is incorporated herein by reference.

[0045] In particular, and continuing to refer to Figure 1, the extruder 102 can have multiple process zones disposed between a first end 108 and a second end 110. The multiple process zones of the extruder 102 can include a precipitation zone 114, in which the guayule miscella 104 is processed to form a precipitate, a first washing zone 116, and a second washing zone 118.

[0046] Each of the precipitation zone 114, first wash zone 116, and second wash zone 118 may also have an associated filter 120, 122, 124. While various filter types may be used, mechanical filters may be particularly useful in this application. Exemplary mechanical filters 120, 122, 124 are described in U.S. Pat. No. 4,110,843 to Skidmore and in "Direct Extrusion of Polymer Latex Emulsions," Advances in Polymer Technology, vol. 3, n 1, pp. 41-49, 1983, by Russell Nichols, Richard Senn, and Farokh Kheradi, the entire disclosures of which are incorporated herein by reference. For example, the mechanical filters 120, 122, 124 may be small twin-screw extruders configured to function as an outlet for excess solvent but also to push entrained solids back into the main barrel of the extruder 102. Those skilled in the art may also use other suitable types of filters within the scope of this disclosure, if desired.

[0047] The extruder 102 may also have at least one port 126, 128, 130 located after the process zone. Additional ingredients, such as an antioxidant 132, may be injected into the extruder 102 through the ports 126, 128, 130. Alternatively, vacuums 134, 136 can be pulled through the ports 126, 128, 130 to further extract the solvent and dry the precipitate formed from the guayule miscella 104.

[0048] 1, the guayule micella 104 may be introduced into the precipitation zone 114 through the inlet port 106 with a first solvent blend 138. The first solvent blend 138 is configured to precipitate the guayule micella 104 to form a precipitate, remove resins naturally present in the guayule micella 104, and induce swelling in the resulting precipitate to facilitate processing of the precipitate in the extruder 102. A second solvent blend 140 and a third solvent blend 142 are introduced through ports 144, 146 in the first washing zone 116 and the second washing zone 118, respectively.

[0049] Specifically, the solvent blends 138, 140, 142 can be a mixture of polar and non-polar solvents, where the non-polar solvent is a hydrocarbon solvent selected from the group consisting of alkanes having 5 to 9 carbon atoms, cycloalkanes having 5 to 10 carbon atoms, aromatic or alkyl-substituted aromatic compounds having 6 to 12 carbon atoms, chloroaromatics having 6 to 12 carbon atoms, chlorocycloalkanes having 5 to 10 carbon atoms, chloroalkanes containing 2 to 8 carbon atoms, and combinations thereof, and the polar solvent is selected from the group consisting of alcohols having 1 to 8 carbon atoms, esters having 3 to 8 carbon atoms, ketones having 3 to 8 carbon atoms, ethers having 2 to 8 carbon atoms, and combinations thereof, and the solvent blend is capable of precipitating the guayule from the miscella and removing the resin.

[0050] The first solvent blend 138, the second solvent blend 140, and the third solvent blend 142 can be further configured to remove residual resin from the precipitate produced by precipitation of the guayule micella 104 while maintaining swelling of the precipitate at a level that facilitates its processing. The first solvent blend 138, the second solvent blend 140, and the third solvent blend 142 can also prevent the precipitate from overly softening and inadvertently transferring through the mechanical filters 120, 122, 126 and out of the extruder 102.

[0051] In a particular example, the first, second, and third solvent blends 138, 140, 142 may include mixtures of ethanol and cyclopentane in various predetermined ratios. However, other types of solvents and blends are contemplated and considered within the scope of this disclosure. Another example of a solvent blend for this process is shown in Figure 4, where the effect of solvent composition can be seen on the MSR.

[0052] Regardless of which polar / non-polar solvent system is selected, at least two separate solvent blends with different ratios of polar to non-polar solvents are also contemplated in certain embodiments. The solvent blend for any washing zone may include a solvent blend that provides a guayule rubber mass swelling rate that is lower than the mass swelling rate of the precipitation zone 114. Similarly, the guayule rubber mass swelling rate of any subsequent washing zone may be equal to or lower than the guayule mass swelling rate of the preceding washing zone. Those skilled in the art may select additional appropriate solvent compositions for the first, second, and third solvent blends 138, 140, 142, as well as other blending / extruder configurations, as desired.

[0053] Surprisingly, it has been discovered that the use of a particular combination of ethanol and cyclopentane effectively removes resin from the rubber contained in guayule miscella 104. This criticality of the solvent blend composition is shown in Table 1 below and explained with reference to FIG. 1. It should be understood that the details shown in Table 1 are non-limiting and are provided only as an example of optimal operating ranges for a particular extruder type. Those skilled in the art will understand that the actual optimal operating ranges are a function of the particular extruder 102 selected for the precipitation and resin removal process. [Table 1]

[0054] The materials for the above samples were prepared using a 30 mm twin-screw non-intermeshing extruder equipped with three mechanical filters, two vacuum vents, and a die face cutter, configured as shown in Figure 1. The same miscella prepared in Sample I was used in all tests. The extrusion conditions are listed in Table I.

[0055] As used herein, the term "extraction efficiency" refers to the ratio of resin removed by extraction to the total resin content. In the example of Table 1, the initial resin content in the model guayule micella was approximately 50% by weight of the total solids.

[0056] Without being bound by any particular theory, it is understood that guayule natural rubber is soluble in cyclopentane and insoluble in ethanol. There are ethanol / cyclopentane blends in which guayule natural rubber is soluble, and blends in which guayule natural rubber is insoluble.

[0057] In certain embodiments, as shown in Table 1, the first solvent blend 138 may have a weight fraction of cyclopentane in ethanol of about 0% to about 50%, more specifically, a weight fraction of cyclopentane in ethanol of about 15% to about 35%, and most specifically, a weight fraction of cyclopentane in ethanol of 25%.

[0058] In a further example, the second solvent blend 140 may have a cyclopentane weight fraction of about 25% to about 75% in ethanol, more specifically, a cyclopentane weight fraction of about 40% to about 60% in ethanol, and most specifically, a cyclopentane weight fraction of about 50%.

[0059] In a further example, third solvent blend 142 may have a cyclopentane weight fraction in ethanol of about 0% to about 10%, more specifically, a cyclopentane weight fraction in ethanol of about 3% to about 7%, and most specifically, a cyclopentane weight fraction of 5%. However, one skilled in the art may select other suitable weight fractions of cyclopentane in ethanol for each of first solvent blend 138, second solvent blend 140, and third solvent blend 142, as desired.

[0060] Further, with continued reference to Figure 1, in certain embodiments, the guayule miscella 104 may be processed in the precipitation zone 114 at a temperature below ambient temperature. In a particular example, processing is completed at a temperature of about 0°C to about 8°C, more specifically about 2°C to about 6°C, and most specifically about 4°C. As a further example, the precipitation may be processed in the first wash zone 116 at a temperature of about 0°C to about 8°C, more specifically about 2°C to about 6°C, and most specifically about 4°C.

[0061] In a further example, the precipitate may be processed in the second wash zone 118 at about 100°C to about 200°C, more specifically about 125°C to about 175°C, and most specifically about 150°C.

[0062] It should also be appreciated that as the mass swelling ratio increases, the hardness and viscosity of the material decreases, thereby affecting the mechanical interaction of the material within the extruder 102. The various mass swelling ratios associated with solvent blends of ethanol and cyclopentane are shown in Table 2 below and further depicted in the graph shown in FIG. [Table 2]

[0063] To efficiently extract the resin from the precipitate, the latter must be soft enough that the mechanical action of the extruder 102 screw induces extensive surface renewal for efficient mass transfer. However, if the precipitate is too soft, the mechanical filters 120, 122, and 124 will not be able to retain the precipitate within the extruder barrel. Specifically, the mass swelling ratio should be a minimum of about 1.00 and a maximum of about 5.00. Desirable mass swelling ratios in the precipitation zone 114 have been determined to be between about 1.00 and about 3.00, more specifically between about 1.47 and about 1.69, and most specifically about 1.5. For example, a mass swelling ratio of 1.5 can be obtained by using a ratio of about 75% ethanol to about 25% cyclopentane.

[0064] 2, the guayule rubber processing method 200 of the present disclosure includes a first step 202 in which the guayule micella 104 may be mixed with a first solvent blend 138 at a first end 108 of the extruder 102. The first solvent blend 138 may be configured to precipitate the guayule micella 104 to form a precipitate. Advantageously, the first solvent blend 138 is also configured to extract at least a portion of the resin naturally present in the guayule micella 104.

[0065] The resulting mixture of precipitate and first solvent blend 138 is then advanced along the length of extruder 102 through precipitation zone 114 to first mechanical filter 120. At least a portion of first solvent blend 138, now containing resin extracted from guayule miscella 104, is then removed from extruder 102 through first mechanical filter 120 in step 204.

[0066] The precipitate and residue volume of the first solvent blend 138 is then advanced through the extruder 102 to the first washing zone 116. In step 206, the precipitate is mixed with, and thereby "washed" by, the second solvent blend 140 in the first washing zone 116. It should be appreciated that the mechanical action of the screw within the extruder 102 functions to interact with the swollen precipitate to further agitate or break down the precipitate during washing. The speed of the extruder screw is also controlled to optimize mechanical agitation during this stage. This mechanical agitation of the precipitate facilitates further removal of residual resin in the precipitate, allowing it to be extracted by the second solvent blend 140.

[0067] The resulting mixture of precipitate and second solvent blend 140 is then advanced along the length of extruder 102 through first washing zone 116 to second mechanical filter 122. At least a portion of second solvent blend 140, now containing the resin extracted from the precipitate, is then removed from extruder 102 through second mechanical filter 122 in step 208.

[0068] The precipitate and residue volume of the second solvent blend 140 then proceeds through the extruder 102 to the second washing zone 118. In step 210, the precipitate is mixed with, and thereby "washed" by, the third solvent blend 142 in the second washing zone 118. It should be appreciated that, similar to the washing zone of step 206, the mechanical action of the screw within the extruder 102 functions to interact with the swollen precipitate and further agitate the precipitate during the washing of step 210. The speed of the extruder screw is also controlled to optimize mechanical agitation during this stage. This mechanical agitation of the precipitate facilitates further removal of residual resin in the precipitate, allowing it to be extracted by the third solvent blend 142.

[0069] The resulting mixture of precipitate and third solvent blend 142 is then advanced along the length of extruder 102 through second washing zone 118 to third mechanical filter 124. At least a portion of third solvent blend 142, now containing the resin extracted from the precipitate, is then removed from extruder 102 through third mechanical filter 124 in step 212.

[0070] It should be appreciated that at this stage the precipitate is substantially free of resins found naturally in guayule miscella 104, particularly as shown in Examples 4 and 5 in Table 1 above.

[0071] Additional ingredients to modify or adjust the properties of the resulting precipitate, such as antioxidants, also shown in Table 1 above as non-limiting examples, are optionally injected into extruder 102 in step 214.

[0072] In step 216, a vacuum 134, 136 can be applied to the extruder 102 to extract any remaining solvent and further dry the precipitate prior to extrusion through a die face cutter in step 218. The precipitate or natural rubber may then be delivered to a fluidized air bed, for example, to complete solvent removal.

[0073] In a further embodiment, concentrated guayule miscella may be advantageously prepared having a higher natural rubber content, as illustrated in Examples 1 and 2 detailed below.

[0074] Example 1: Determination of guayule mass swelling ratio Guayule rubber (200 mg) was immersed in a single solvent or solvent blend (20 mL) and allowed to swell for 24 hours under gentle agitation. The swollen rubber was then removed from the solvent, and excess solvent on the surface of the swollen rubber was removed with absorbent paper. The weight of the swollen rubber was recorded. The mass swelling ratio was calculated by dividing the swollen rubber weight by the initial dry weight.

[0075] Example 2: Preparation of model guayule micelles To create a model concentrated guayule rubber for addition to precipitation zone 114, 190 lbs of solid guayule rubber swollen with a mass equivalent of cyclopentane was used to prepare a 30 wt% guayule miscella (15 wt% resin-free purified guayule rubber, 15 wt% rosin). In a 300-gallon mixing tank, 510 lbs of cyclopentane and 186.7 lbs of denatured ethanol were added and thoroughly mixed. A portion of the resulting solvent mixture (190 lbs) was added to the rosin (190 lbs), and the mixture was stirred until uniform. The swollen guayule rubber and diluted rosin were then added to the 300-gallon mixing tank and stirred until the rubber was completely dissolved. The amount of rubber compared to the amount of resin in the guayule miscella is shown in Table 3 below. [Table 3]

[0076] Advantageously, the system 100 and method 200 of the present disclosure are more efficient and effective in extracting rubber from non-Hevea sources, such as guayule shrubs, which results in superior natural rubber quality and utility.

[0077] While certain representative embodiments and details have been shown for the purpose of illustrating the present invention, it will be apparent to those skilled in the art that various modifications can be made without departing from the scope of the present disclosure, as further set forth in the appended claims below.

Claims

1. 1. A method for isolating guayule rubber, comprising: mixing the guayule miscella in an extruder with at least one solvent blend comprising a first polar solvent that precipitates the guayule rubber and a second non-polar solvent that swells the resulting precipitate; and removing resin and the solvent from the guayule rubber in the extruder; the first polar solvent is ethanol and the second non-polar solvent is cyclopentane; method.

2. 10. The method of claim 1, wherein the at least one solvent blend is capable of precipitating the guayule rubber from the guayule miscella and removing resin.

3. 10. The method of isolating guayule rubber according to claim 1, wherein said at least one solvent blend comprises a first solvent blend having a weight fraction of cyclopentane in said ethanol of from 15% to 50%.

4. 4. The method of isolating guayule rubber according to claim 3, wherein in said first solvent blend, the weight fraction of cyclopentane in said ethanol is 25%.

5. 10. The method for isolating guayule rubber according to claim 1, wherein said at least one solvent blend comprises a second solvent blend in which the weight fraction of cyclopentane in said ethanol is between 25% and 75%.

6. 6. The method of isolating guayule rubber according to claim 5, wherein in said second solvent blend, said weight fraction of cyclopentane in said ethanol is 50%.

7. 10. The method of isolating guayule rubber according to claim 1, wherein said at least one solvent blend comprises a first solvent blend having a weight fraction of cyclopentane in said ethanol of from 0.1% to 10%.

8. 8. The method of isolating guayule rubber according to claim 7, wherein in said first solvent blend, said weight fraction of cyclopentane in said ethanol is 5%.

9. 1. A system for isolating guayule rubber, comprising: an extruder including a first end and a second end and at least one screw, the extruder including a plurality of process zones disposed between the first end and the second end, the plurality of process zones including a precipitation zone and at least one washing zone, the precipitation zone configured to receive guayule micella and a first solvent blend and to precipitate the guayule micella into a precipitate, a first washing zone in communication with the precipitation zone and configured to receive the precipitate and a second solvent blend, and a subsequent washing zone in communication with a preceding washing zone and configured to receive the precipitate; a first filter in communication with the precipitation zone, the first filter configured to remove at least a portion of the first solvent blend from the precipitate in the precipitation zone; each of the first solvent blend and the second solvent blend comprises ethanol and cyclopentane; system.

10. 10. The system of claim 9, wherein the solvent blend for the precipitation zone comprises a solvent blend providing a rubber mass swell ratio of 1.00 to 3.00, and the solvent blend for any of the washing zones comprises a solvent blend providing a rubber mass swell ratio lower than the rubber mass swell ratio in the precipitation zone, and the rubber mass swell ratio for any subsequent of the washing zones is equal to or lower than the rubber mass swell ratio in one of the preceding washing zones.

11. 10. The system of claim 9, further comprising a second filter in communication with the first washing zone, the second filter configured to remove at least a portion of the second solvent blend from precipitate in the at least one washing zone.

12. 10. The system of claim 9, wherein the solvent blend for the precipitation zone, the first wash zone, and any subsequent wash zones is capable of precipitating the guayule rubber and removing resin from the guayule miscella.

13. 10. The system of claim 9, wherein in the first solvent blend, the weight fraction of cyclopentane in the ethanol is between 15% and 50%.

14. The system of claim 9, wherein the second solvent blend comprises a weight fraction of 25% to 75% cyclopentane in the ethanol.

15. 1. A method for isolating guayule rubber, comprising: An extruder including a first end and a second end and at least one screw, the extruder including a plurality of process zones disposed between the first end and the second end, the plurality of process zones including a precipitation zone, a first washing zone, and a second washing zone, the precipitation zone configured to receive guayule micella and a first solvent blend and to precipitate the guayule micella into a precipitate, the first washing zone in communication with the precipitation zone and configured to receive the precipitate and a second solvent blend, and the second washing zone in communication with the first washing zone and configured to wash the precipitate and a third solvent blend. a first mechanical filter in communication with the precipitation zone, the first mechanical filter configured to remove at least a portion of the first solvent blend from the precipitate in the precipitation zone; a second mechanical filter in communication with the first washing zone, the second mechanical filter configured to remove at least a portion of the second solvent blend from the precipitate in the first washing zone; and a third mechanical filter in communication with the second washing zone, the third mechanical filter configured to remove at least a portion of the third solvent blend from the precipitate in the second washing zone; mixing the guayule micella and the first solvent blend in the precipitation zone of the extruder to form a precipitate; removing a portion of the first solvent blend in the precipitation zone using the first mechanical filter; washing said precipitate with said second solvent blend in said first wash zone; removing a portion of the second solvent blend in the first wash zone using the second mechanical filter; washing said precipitate with said third solvent blend in said second wash zone; removing a portion of the third solvent blend in the second wash zone using the third mechanical filter; applying a vacuum to the extruder at a location upstream of the third mechanical filter to extract the remaining amount of the first solvent blend, the second solvent blend, and the third solvent blend to remove resin and solvent from the guayule rubber in the extruder and retain the precipitate in the extruder; and extruding the precipitate through the second end of the extruder to obtain guayule rubber; the solvent blend for the precipitation zone comprises a solvent blend that provides a rubber mass swell ratio of 1.00 to 3.00, the solvent blend for any of the washing zones comprises a solvent blend that provides a rubber mass swell ratio that is lower than the rubber mass swell ratio in the precipitation zone, and the rubber mass swell ratio for any subsequent of the washing zones is equal to or lower than the rubber mass swell ratio in one of the preceding washing zones; each of the first solvent blend, the second solvent blend, and the third solvent blend comprises ethanol and cyclopentane; A method for isolating guayule rubber.

16. 16. The method of claim 15, wherein the guayule miscella is mixed in the precipitation zone at a barrel temperature below ambient temperature.

17. 16. The method of claim 15, wherein the precipitate is washed with the second solvent blend at below ambient temperature.

18. 16. The method of claim 15, wherein the precipitate is washed with the third solvent blend at a barrel temperature of 150°C.

19. 16. The method of claim 15, wherein the third solvent blend comprises a weight fraction of cyclopentane in the ethanol of from 3% to 10%.

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