The present invention relates to the industrial sector of extracting and processing natural rubber, and other components, from plant material.

In particular, the invention relates to a process for extracting latex, resin and rubber from guayule plants, which includes harvest, preservation, mechanical and chemical treatment of the plant parts, is applicable both in the laboratory and on an industrial scale and is characterised by significantly high yields and high quality of the extracted products.

Guayule (Parthenium argentatum) is a perennial shrub native of the semidesert regions of the south-west of the USA (in particular Texas) and the north of Mexico. This plant accumulates natural rubber, preponderantly composed of the elastomer cis-1 ,4-polyisoprene, in the form of latex (a milky suspension or dispersion in water), above all in the bark of the branches and stem. The content of natural rubber may depend on various factors relating to the environment, cultivation and preservation and it is between 5 and 20% of the total weight of the dry plant.

Extracting natural rubber from the guayule plant, as well as other plants belonging to the Asteraceae, Euphorbiaceae, Campanulaceae, Labiatae and Moraceae genera such as, for example, Euphorbia lathyris, Parthenium incanum, Chrysothamnus nauseosus, Pedilanthus macrocarpus, Cryptostegia grandiflora, Asclepias syriaca, Asclepias speciosa, Asclepias subulata, Solidago altissima, Solidago graminifolia, Solidago rigida, Sonchus arvensis, Silphium spp., Cacalia atriplicifolia, Taraxacum ko -saghyz, Pycnanthemum incanum, Teucrium canadense or Campanula americana (for brevity's sake denoted "guayule-type"), is an important alternative to extracting natural rubber from Hevea brasiliensis, in particular in the light of these species' greater resistance to pathogens which attack Hevea, the lower import costs of the raw material of vegetable origin and due to the lower content, in rubber extracted from these plants , in comparison with the rubber derived from Hevea, of numerous protein contaminants responsible for type I (or IgE-mediated) latex allergies.

For these reasons, over recent years numerous studies have been carried out into the development of methods and techniques for extracting the natural rubber from nor\-Hevea plants such as, for example, guayule or guayule-type plants.

It is important to note that, while the natural rubber of Hevea brasiliensis may be obtained simply by collecting the latex present in the laticiferous ducts along the bark by making cuts through the cortex to said ducts, natural guayule rubber accumulates within the plant's cells (stem, leaves and roots) and may be obtained by crushing the plant material and collecting the cellular components using physical and/or chemical methods.

Prior art processes involve extracting rubber from the guayule plants by grinding the plants and extracting the rubber itself with water or with organic solvents.

International application WO 2014/047176, for example, describes a method for extracting guayule rubber which involves a "post-harvest" (or "pre-extraction") treatment including partial drying of the plant material. WO 2014/047176 in fact demonstrates that when the guayule rubber is extracted from plant material having residual moisture contents below specific values, it is characterised by a significantly lower molecular weight and is consequently of lower quality. Furthermore, extraction efficiency also drops as the residual moisture content falls. The extraction described in the above-stated application is performed with mixtures of nonpolar and polar organic solvents (for example pentane/acetone).

Indeed, from the standpoint of solely recovering the rubber, the majority of processes described in the prior art provide extracting all the main components obtainable from the guayule plant in a single step.

However, for the purpose of meeting quality standards (for example the standards set out by ASTM International), natural rubber must be substantially pure, i.e. must have had other compounds which are extractable from the plant material and other contaminants removed from it: for this reason, methods have been developed which include steps for removing the above-stated compounds which are extractable from the guayule plant, in particular the resin.

International patent application WO 2013/134430 describes a process for extracting the natural rubber from non-Hevea plants which involves harvesting the guayule plants, removing the majority of the foliage and partially drying the plant material.

After crushing and grinding, the plant material is suspended in the presence of a polar organic solvent (for example acetone) and a nonpolar organic solvent (for example hexane). Once the bagasse has been separated, a suspension, or miscella, containing rubber and resin is obtained. Further polar organic solvent is added to this miscella so as to cause the rubber to coagulate into particles which are separated by sedimentation. WO 2013/134430 does not describe recovering the residual resin from the miscella from which the high molecular weight rubber was isolated, since its primary focus is on removing the contaminants from the suspension containing the rubber.

Patent US 4,681 ,929 describes a process for preparing guayule rubber, performed in a single step, in which the plant material is subjected to extraction with polar organic solvents (for example methanol) and nonpolar organic solvents (for example hexane), so giving rise to a miscella containing both rubber and resin. In a preferred aspect of the invention, the process involves a first extraction of the resin with polar organic solvent and a second extraction of the rubber using nonpolar organic solvent. A characteristic feature of the process described in US 4,681 ,929 is the use of the miscella itself as an extracting agent, added to the fresh solvent in a percentage of up to 90%, consequently reducing the quantity of solvent required for extraction.

Furthermore, the solution containing resin and/or rubber is separated from the bagasse by squeezing instead of sedimentation or gravity draining: in this manner, the volumes of bagasse to be rinsed are particularly small, so enabling a further reduction in the use of organic solvent. In this case too, however, the resin is deemed to be a rubber contaminant which is to be removed and disposed of.

For some time, however, guayule resin, produced by the plant in comparable if not higher amount than rubber, has found uses in various applications, such as for example, the manufacture of adhesives and the production of wood panels resistant to pest attack. For this reason, some processes described in the prior art also place importance on isolating this component.

For example, patent US 4,435,337 describes a process for extracting rubber, resin, water-soluble compounds and bagasse based on the "historical" process for extracting guayule rubber, known as the "Saltillo process" (described in the volume "Guayule: An Alternative Source of Natural Rubber", 1977, National Academy of Sciences). The process described in US 4,435,337 comprises a preliminary step of partially drying the plant material, followed by extraction of the resin with anhydrous acetone and a stage of recovering the rubber, the water-soluble compounds and the bagasse by flotation of the rubbery material, said latter stage being carried out similarly to the old Saltillo process. It is noted in US 4,435,337 that resin extraction efficiency is all the greater, the lower is the water content of the extracting solvent; it is furthermore observed that, unexpectedly, it is more advantageous for the purpose of extracting the resin itself from the plant material to use the miscella containing concentrated resin, rather than to use fresh solvent.

Patent application US2014/0288255 describes a process for separating rubber, resin and bagasse, comprising a first stage of homogenising the plant material in the presence of a medium capable of solubilising the resin, which is subsequently separated from the bagasse; a second phase of further homogenisation of the bagasse in the presence of a solvent capable of solubilising the rubber, which is subsequently separated from the bagasse; a final stage of drying the rubber and the bagasse which may include, for example, evaporation of the solvent in wiped film evaporators and extrusion of the rubber.

US2014/0288255 furthermore describes a process in which the plant material is homogenised in the presence of a "milling solvent" able to solubilize the rubber and the resin, which are separated from one another in a subsequent purification step using a fractionation solvent.

It should be noted that US2014/0288255, while placing an emphasis on the importance of the correct timing of plant harvesting, by hedging or pollarding, does not provide any teaching regarding the usefulness of optional seasoning of the plants once harvested. The process described in European patent EP 0 164 137 also does not involve a stage of seasoning the plant material; on the contrary, EP 0 164 137 emphasises the importance of processing the guayule plants immediately after harvest. According to the process of EP 0 164 137, the complete guayule plant is subjected to extraction by countergravity percolation with organic solvents (for example hexane, or the miscella obtained by the above-stated extraction) capable of solubilising both the resin and rubber. In a second stage, a non-solvent for the rubber (for example acetone) is added to the miscella obtained from the extraction, so causing the rubber to precipitate. The deresinated rubber is then recovered in relatively pure form by washing and reprecipitation followed by filtration and/or centrifugation.

As has already been pointed out, guayule rubber is located in the parenchymal cells of this plant in the form of latex, which is a precursor of the natural rubber in suspension form.

Processes which involve the separation of guayule latex have been described in the prior art.

For example, patent US 7,923,039 describes a process which comprises harvesting the guayule plants at the time of maximum accumulation of latex in suspension form, by hedging and/or pollarding, and chopping said biomass, which is maintained in aqueous solution at a basic pH in the presence of additives, in mills. Once the bagasse has been separated by filtration, the resultant aqueous suspension containing latex is subjected to various separation/concentration steps with the purpose of removing water and any contaminants. US 7,923,039 does not provide any teaching regarding the extraction of resin and residual rubber from the bagasse obtained in the process described therein. The applicant has identified various drawbacks in the prior art processes:

1. processes which involve extracting rubber and resin in a single step do not permit quantitative recovery of the resin which, as described above, has an intrinsic commercial value;

2. processes which involve the above-stated extraction of rubber and resin in a single step and subsequent separation of the resin, make it possible to obtain rubber but require further purification steps in order to meet the quality standards demanded by the market;

3. the bagasse obtained after extraction in a single step, whether by processes involving the use of solvents or by processes using basic aqueous solutions, may still contain substantial quantities of rubber and resin, and may for this reason be considered unsuitable for utilisation other than as fuel (for example in agriculture or in animal feeding).

4. none of the processes involving solvent extraction of rubber and resin involve extracting the polymer in latex form, which is essential for producing non-allergenic manufactured articles by film coating (for example latex gloves).

The applicant accordingly set out to solve the problem of finding a new process for preparing all the components of guayule plants, i.e. latex, rubber, resin and bagasse, in such a manner as to obtain all said components with product yields, and quality characteristics, which are improved over prior art processes.

One object of the present invention is accordingly that of providing a process for extracting the latex, resin and rubber from guayule plants which is characterised by measures directed at obtaining the maximum extraction yield and which substantially lacks the drawbacks of the above-stated prior art.

For the purposes of the present description and the appended claims, unless stated otherwise, definitions of numerical ranges always include the extremes.

For the purposes of the present description and the appended claims, unless stated otherwise, percentages are by weight.

In the description of the embodiments of the present invention, use of the terms "comprising" and "containing" indicates that the options described, for example relating to the steps of a method or a process or the components of a product or a device, are not necessarily exhaustive. It is, however, important to note that the present application also provides those embodiments in which, in relation to the described options, for example in relation to the steps of a method or a process or the components of a product or a device, the term "comprising" should be interpreted as "which essentially consists of or "which consists of, even if not explicitly stated.

In the present description and the appended claims, guayule plant is taken to mean generically both the species Parthenium argentatum and the guayule-type plants of the species listed above.

For the purposes of the present invention, the term "plant material" is taken to mean any form (for example, the whole plant, plant parts including roots, branches and/or stem, leaves, any bark, plant fragments obtained by chopping, grinding, etc., briquettes and pellets obtained by compacting the plant fragments) in which the guayule plant is used for the purpose of extracting, by chemical and/or physical methods, the rubber, resin and other components present in said plant.

The term "bagasse" is taken to mean the residual portion of plant material deriving from the extraction process. Bagasse may also include small quantities of non-plant material (e.g. loam, sand, etc.) typically associated with the plant roots and derived from the soil in which they have been grown.

For the purposes of the present invention, the term "miscella" is taken to mean a solution, suspension or emulsion composed of latex, rubber and/or resin, water and/or organic solvents in which the extraction process is performed and which is obtained after separation of the bagasse.

For the purposes of the present invention, "volatile matter" refers to compounds other than rubber which may be present within a sample of rubber in the solid state, but which pass into the vapour phase and may be separated from said sample at temperatures of greater than or equal to 100°C.

The volatile matter present in a sample of rubber in the solid state may be determined, for example, by the ASTM D1278-91 (1977) standard test which is known to a person skilled in the art.

Volatile organic compounds or "VOC" (for example, the nonpolar organic solvent present in a sample of rubber in the solid state) and the residual concentration thereof are furthermore determined by gas chromatography with a flame ionisation detector using a standard solution of the above-stated VOCs of known concentration.

Further features and advantages of the present invention will be apparent from the following detailed description and with reference to the appended figures which are intended to illustrate the general features of the method, the structure and/or the materials used in some embodiments of the invention and to complement the following written description.

In particular, figure 1 is a complete block diagram of one embodiment of the process of the present invention, starting from whole guayule plants.

Figure 2 is a block diagram which represents part of the process according to an alternative embodiment of the present invention, in which defoliation step ("b") and preservation step ("c") are in the reverse order. To ensure greater clarity, the numbering of the steps has not been changed. In said figure, steps "b" and "c" are denoted by bold boxes.

Figure 3 is a block diagram which represents part of the process according to an alternative embodiment of the present invention, in which immersion step ("d") and grinding step ("e") are in the reverse order. To ensure greater clarity, the numbering of the steps has not been changed. In said figure, steps "d" and "e" are denoted by bold boxes. Figure 4 is a block diagram which represents part of the process according to an alternative embodiment of the present invention in which extraction with polar organic solvent ("h") is preceded by a further bagasse grinding step. To ensure greater clarity, the numbering of the steps has not been changed. In said figure, the additional step is represented by a bold box.

Figure 5 shows the distribution of molecular weights (MW) of the natural rubber obtained with the process according to the invention, determined by gel permeation chromatographic analysis (GPC), conveniently performed in accordance with standard method ISO 1 1344:2004, IDT ("Rubber, raw, synthetic - Determination of the molecular-mass distribution of solution polymers by gel permeation chromatography"), using polystyrene as standard. Said method is known to a person skilled in the art. The thin-lined curve relates to the rubber extracted with nonpolar organic solvent; the thick-lined curve relates to the latex rubber extracted with a basic aqueous solution.

It is important to note that the above-stated figures are intended to illustrate the general features of the method, the structure and/or materials used in some embodiments of the invention and to complement the following written description.

These figures are, however, not to scale and may not accurately reflect the precise structural characteristics or performance of a given embodiment, and as such must not be interpreted as defining or limiting a range of values or a property of said embodiment. The use of similar or identical reference numerals in the figures is intended to indicate the presence of one or more similar or identical elements or functions.

The present invention relates to a process for extracting latex, resin and rubber from guayule plants, comprising:

a. harvesting the guayule plants;

b. defoliating said plants;

c. preserving the defoliated plants in an environment under controlled temperature and relative humidity, for a time of between 7 and 21 days, such that the residual moisture present in the plant is maintained in the range of 30-45%;

d. immersing said defoliated plants in a basic aqueous solution comprising a stabilising system;

e. grinding said defoliated plants immersed in said basic aqueous solution to obtain an aqueous suspension of plant material comprising plant fragments;

f. subjecting the aqueous suspension obtained in step "e" to filtration/pressing to separate a first miscella comprising said latex from a first bagasse;

g. recovering the concentrated latex from said first miscella;

h. dispersing said first bagasse in a polar solvent system, comprising at least one polar organic solvent and a stabilising system, to obtain a suspension;

i. subjecting the suspension obtained in step "h" to filtration/pressing to separate a second miscella comprising said resin from a second bagasse;

j. removing the at least one polar organic solvent from said second miscella to obtain the concentrated resin;

k. removing the at least one polar organic solvent from the second bagasse obtained in step "i";

I. dispersing said desolventised second bagasse obtained in step "k" in a nonpolar solvent system, comprising at least one nonpolar organic solvent and a stabilising system, to obtain a suspension;

m. subjecting said suspension obtained in step "I" to filtration/pressing to separate a third miscella comprising said rubber from a third bagasse;

n. removing the at least one nonpolar organic solvent from said third miscella to obtain the rubber in the solid state;

o. removing the at least one nonpolar organic solvent from the third bagasse obtained in step "m".

The steps of the above-stated process may preferably be carried out in sequence from "a" to "o". As described in greater detail below, in some embodiments of the invention, some of the above-stated steps may be carried out simultaneously or in a reverse order to the list mentioned above, without modifying the general principle of the invention.

It is known that the accumulation of rubber in the various parts of guayule plants depends on various factors, such as the age of the plant, the intensity of ambient light, the availability of water, the temperature or the season. For example, in the locations from which the guayule plant originates, it accumulates rubber in the winter months, while during the summer it favours resin production (S. Macrae, M.G. Gilliland, J. Van Staden in "Rubber production in guayule: determination of rubber producing potential" (1986) Plant Physiol., vol. 81 , pp. 1027-1032).

According to the invention, the guayule plants are preferably harvested on resumption of the vegetative period of said plants. This period typically covers spring and early summer.

In a preferred embodiment of the present invention, the whole guayule plants are harvested in the field by making a cut at the neck of the stem (if regrowth by offshoots is intended), or are uprooted (if the plant is not shrubby or regrowth is not desired) (figure 1 , box 10).

In the harvest stage, the plants may be harvested manually or by harvesting machinery, preferably in a size of greater than or equal to 8 cm and less than or equal to 20 cm to facilitate subsequent storage and preservation operations.

In a preferred aspect of the invention, the size may be between 8 and 15 cm and still more preferably between 10 and 12 cm.

For the purposes of the present invention, in the steps subsequent to harvesting, and where not specified otherwise, "plant" is taken to mean both the whole plant and the plant fragments of the harvested size.

Once harvested, the plants may be partially or completely defoliated (20).

Defoliation may be performed manually or mechanically using blade, roller or pneumatic defoliators. In the latter case, the leaves are separated from the stems by vibration and streams of air which exploit the lower density of the leaves in comparison with the branches.

Preferably at least 50% of the leaves are removed from the plants during step "b" of the above-stated process and more preferably at least 90% of the leaves are removed from the plants.

The defoliated plants may then be subjected to a preservation treatment.

The applicant has unexpectedly discovered that, by subjecting the guayule plants, once harvested and defoliated, to an appropriate preservation treatment, and partial drying, in an environment under controlled temperature and relative humidity (30), it is possible to extract high yields of both latex and rubber in which the elastomer is characterised by a high weight-average molecular mass.

Without wishing to be bound by any particular theory, the plant's metabolism may continue for a short period even after cutting and, under particular temperature and humidity conditions, the weight-average molecular mass of the elastomer present in the plant is observed to continue to increase, even increasing as much as threefold relative to the starting value. Furthermore, the rubber extracted after the preservation step according to the invention exhibits a reduced polydispersity index and a distribution of the weight-average molecular masses of said elastomer within a very narrow range, a feature which contributes to defining the overall quality of the natural polymer.

As is known, the polydispersity index, or simply "dispersity", is represented by the ratio between weight-average molecular mass and number-average molecular mass (lUPAC Recommendation "Dispersity in polymer science" (2009), Pure Appl. Chem. vol. 81 , pp. 351-353).

For the purposes of the present invention, an environment under controlled temperature and relative humidity is taken to mean a location in which the temperature and relative humidity of the air are monitored and modified on the basis of specific requirements.

Process for extracting latex, resin and rubber from guayule plants, comprising: a. harvesting the guayule plants;

b. defoliating said plants;

c. preserving the defoliated plants in an environment under controlled temperature and relative humidity, for a time of between 7 and 21 days, such that the residual moisture present in the plant is maintained in the range of 30- 45%;

d. immersing the defoliated plants in a basic aqueous solution which comprises a stabilising system;

e. grinding said defoliated plants immersed in said basic aqueous solution to obtain an aqueous suspension of plant material comprising plant fragments; f. subjecting the aqueous suspension obtained in step "e" to filtration/pressing to separate a first miscella comprising said latex from a first bagasse;

g. recovering the concentrated latex from said first miscella;

h. dispersing said first bagasse in a polar solvent system, comprising at least one polar organic solvent and a stabilising system, to obtain a suspension; i. subjecting the suspension obtained in step "h" to filtration/pressing to separate a second miscella comprising said resin from a second bagasse;

j. removing the at least one polar organic solvent from said second miscella to obtain the concentrated resin;

k. removing the at least one polar organic solvent from the second bagasse obtained in step "i";

I. dispersing said desolventised second bagasse obtained in step "k" in a nonpolar solvent system, comprising at least one nonpolar organic solvent and a stabilising system, to obtain a suspension;

m. subjecting the suspension obtained in step "I" to filtration/pressing to separate a third miscella comprising said rubber from a third bagasse;

n. removing the at least one nonpolar organic solvent from the third miscella to obtain the rubber in the solid state;

o. removing the at least one nonpolar organic solvent from the third bagasse obtained in step "m".

Process according to claim 1 , in which the steps of said process are carried out in sequence from "a" to "o".

3. Process according to claim 1 or 2, in which the guayule plants are harvested on resumption of the vegetative period of said plants.

4. Process according to any one of claims 1 to 3, in which the plants are harvested with a size greater than or equal to 8 cm and less than or equal to 20 cm.

5. Process according to any one of claims 1 to 4, in which step "c" is carried out for a time of between 10 and 15 days, in an environment in which the temperature is constantly maintained between 15 and 40°C, and/or in which the relative humidity is constantly maintained between 80% and 95%.

6. Process according to claim 1 and any one of claims 3 to 5, in which preservation step "c" precedes defoliation step "b".

7. Process according to any one of claims 1 to 6, in which the pH of said basic aqueous solution is greater than or equal to 7.5 and less than or equal to 12.

8. Process according to any one of claims 1 to 7, in which, in step "d" of said process, the ratio of the volume of said basic aqueous solution to the weight of the plant material ranges is between 1 and 10 and preferably between 2 and 5.

9. Process according to any one of claims 1 to 8, in which said basic aqueous solution comprises a base selected from KOH, NaOH, NH4OH, NaHC03, or mixtures thereof, in a final concentration of between 0.1% and 0.5% by weight.

10. Process according to any one of claims 1 to 9, in which grinding step "e" is performed using one or more hammer mills.

11. Process according to any one of claims 1 to 10, in which the fragments of plant material obtained by grinding step "e" have an average size of between 0.5 and 7.5 mm.

12. Process according to claim 1 and any one of claims 3 to 11 , in which grinding step "e" is performed before step "d" of immersion in said basic aqueous solution comprising said stabilising system.

13. Process according to any one of claims 1 to 12, in which extraction step "h" is preceded by an additional grinding step of the first bagasse.

14. Process according to claim 13, in which said additional grinding step is carried out with corrugated roller mills and/or smooth roller mills.

15. Process according to any one of claims 1 to 14, in which, in step "h" of said process, the ratio of the volume of the polar solvent system to the weight of the first bagasse is between 1 and 7 and preferably between 2 and 5.

16. Process according to any one of claims 1 to 15, in which the polar solvent system used in extraction step "h" comprises at least one polar organic solvent, selected from an alcohol having from 1 to 8 carbon atoms, ethers and esters having from 2 to 8 carbon atoms, cyclic ethers having from 4 to 8 carbon atoms, ketones having from 3 to 8 carbon atoms, or mixtures thereof.

17. Process according to claim 16, in which said polar organic solvent is selected from ethanol and acetone.

18. Process according to any one of claims 1 to 17, in which said polar solvent system is brought into contact with said first bagasse in one or more countercurrent steps, with or without stirring, for a time of between 0.1 and 5 hours, at a temperature of between 25°C and the boiling point of the polar organic solvent used.

19. Process according to any one of claims 1 to 18, in which said filtration/pressing step "i" is performed at a temperature of between 25°C and 50°C.

20. Process according to any one of claims 1 to 19, in which, in step "I" of said process, the ratio of the volume of the nonpolar solvent system to the weight of the second bagasse is between 1.5 and 7, preferably between 2 and 5.

21. Process according to any one of claims 1 to 20, in which the nonpolar solvent system used in extraction step "I" comprises at least one hydrocarbon solvent selected from linear or branched alkanes having from 4 to 9 carbon atoms, cycloalkanes or alkyl cycloalkanes having 5 to 10 carbon atoms, aromatic hydrocarbons having 6 to 10 carbon atoms, or mixtures thereof.

22. Process according to claim 21 , in which said hydrocarbon solvent is selected from hexane and cyclohexane.

23. Process according to any one of claims 1 to 22, in which said nonpolar solvent system is brought into contact with the second bagasse in one or more countercurrent steps, with or without stirring, for a time of between 0.1 and 5 hours, at a temperature of between 25°C and the boiling point of the nonpolar organic solvent used.

24. Process according to any one of claims 1 to 23, in which the nonpolar organic solvent is removed in step "n" from the third miscella comprising the natural rubber by steam stripping in the presence of a dispersant system.

25. Process according to claim 24, in which said dispersant system comprises at least one water-soluble salt of a metal selected from Al, Ca and Mg, and at least one water-soluble surfactant belonging to the polycarboxylate family.

26. Process according to any one of claims 1 to 25, in which the yield of rubber extracted from guayule plants is greater than or equal to 80% in relation to the total amount of rubber present in the plants.

27. Guayule rubber obtained by the process according to any one of claims 1 to 26, characterised by a weight-average molecular mass of between 1 - 106 and 2- 106 g/mol, a volatile matter content of less than 0.75% by weight and a residual organic solvent content of less than 4000 ppm.

28. Guayule rubber according to claim 27, in which the content of said organic solvents is less than 4000 ppm and greater than or equal to 50 ppm.

29. Guayule rubber according to claim 27 or 28, wherein the polydispersity index is comprised between 2 and 5, and preferably is comprised between 2.5 and 3.5.