Process for devulcanization of sulfur vulcanized elastomers

Abstract

Process for devulcanization of vulcanized elastomers, comprising the following steps: A. preparing an aqueous mixture of vulcanized elastomers with a devulcanization composition, obtained by: i) mixing urea and at least one dicarboxylic acid in an aqueous solvent at a temperature higher than 40°C for a time comprised between 3 and 8 hours, to obtain an aqueous mixture comprising devulcanization ionic compounds; and ii) adding a plasticizer to said aqueous mixture from step i), to obtain a devulcanization composition; B. carrying out a devulcanization reaction, by subjecting the aqueous mixture from step A to a state of stress, thereby obtaining a devulcanized product.

Description

Technical Field

[0001] The technical field to which this invention pertains is the method for desulfurization of sulfur-cured elastomers. Background Technology

[0002] In the rubber industry, polymers are almost never used in pure form, except in rare cases. They are typically blended with additives and accelerators of varying properties, selected based on the desired final properties of the product and the optimal processing conditions for achieving it. These mixtures of various types of polymers and additives are generally referred to as blends.

[0003] Prior to the vulcanization process, the polymer chains present in the mixture are unconstrained and freely distributed within the mixture itself, thus placing the mixture in a plastic state.

[0004] Vulcanization is a process used to process rubber by heating it to chemically bond it with sulfur. In particular, sulfur allows the formation of bridging (or cross-linking) bonds that bind the polymer chains of the rubber together.

[0005] This process enables the production of elastic materials that are both wear-resistant and tensile-resistant, intended for use in the manufacturing processes of various products and consumer goods.

[0006] From a chemical perspective, rubber vulcanization leads to a change in the molecular conformation of the polymers that make up the rubber. In this regard, several studies have emphasized that, as a result of polymer chain folding, thioether bonds (RSRs) are formed at the intermolecular level and, to a lesser extent, at the intramolecular level.

[0007] In particular, the addition of sulfur causes the loss of two hydrogen atoms, which leads to the formation of H2S; as a result, oxides such as ZnO are conducive to sulfidation, fixing sulfides.

[0008] These changes lead to alterations in the rheological, chemical-physical, and / or mechanical properties of the rubber. In particular, it is emphasized that the resistance to abrasive degradation (a phenomenon accelerated in the presence of environmental agents such as atmospheric oxygen) can be largely attributed to the bonding of sulfur to oxygen-sensitive sites.

[0009] The spatial network of sulfur-cured elastomers typically has three types of chemical bonds: carbon-carbon (CC), sulfur-carbon (SC), and sulfur-sulfur (SS).

[0010] Classification of elastomers

[0011] For the classification of various elastomers commonly used in the rubber industry, refer to the standard DIN / ISO 1629 (derived from the earlier ASTM D 1418-79). Based on this classification, the group to which a polymer belongs is identified by the last letter of its identification code, while the remainder of the code uniquely defines the elastomer and provides more specific information.

[0012] DIN / ISO 1629 classifies synthetic rubber into 5 groups:

[0013] • Group M: Polymers containing saturated polymer chains of polyethylene. This group includes polymers known and identified by the following acronyms: EPDM, ACM, CSM, FEPM, FFPM;

[0014] • Group O: Polymers containing oxygen atoms. This group includes known polymers identified by the following acronyms: CO, ECO;

[0015] • Group Q: Polymers containing both oxygen and silicon atoms. This group includes known polymers identified by the following acronyms: FMQ, MQ, PMQ, PVMQ, VMQ;

[0016] • Group R: Polymers containing unsaturated carbon. This group includes polymers known and identified by the following acronyms: BIIR, BR, IIR, CR, IIR, IR, NBR, SBR;

[0017] • Group U: Polymers containing carbon, oxygen, and nitrogen. This group includes known polymers identified by the following acronyms: AU, EU.

[0018] Desulfurization

[0019] Desulfurization is a process in which vulcanized rubber is transformed back into a plastic material by breaking the cross-linking bonds between the chains, thus making it reprocessable and recyclable.

[0020] In summary, desulfurization attempts to bring vulcanized rubber back from an elastic state to a plastic state.

[0021] The following are some known methods for the desulfurization of sulfur-cured elastomers.

[0022] mechanical process

[0023] US5883140 A discloses a rubber recycling process that involves subjecting a material consisting substantially of finely separated vulcanized rubber components to high-intensity impact forces in specialized mechanical equipment. This process allows for the production of re-vulcanizable rubber materials with a tensile strength of at least 35% relative to the tensile strength of similarly polymerized virgin rubber blends.

[0024] US5731358 A relates to a process for producing high-quality desulfurized rubber from vulcanized waste and waste rubber products. Specifically, the process comprises at least two stages, wherein: in a first stage, a polymeric additive is mixed with vulcanized waste rubber and the resulting mixture is cooled; and in a second stage, unvulcanized virgin rubber is added to the mixture obtained from the first stage.

[0025] Microwave process

[0026] US4104205 A discloses a method for devulcanizing rubber using microwave energy, which allows for the production of materials that can be recompacted and revulcanized into new vulcanized rubber-based articles. The process is carried out by applying microwave energy to vulcanized rubber waste at controlled dosage, dose rate, and temperature.

[0027] Ultrasonic process

[0028] US5258413 A relates to a continuous ultrasonic method for breaking carbon-sulfur (CS) bonds, sulfur-sulfur (SS) bonds, and possibly carbon-carbon (CC) bonds in vulcanized elastomer particles.

[0029] Biotechnology process

[0030] US5677354 A relates to a process for regenerating vulcanized or semi-vulcanized rubber, the process comprising: subjecting the rubber material to be regenerated to a solvent capable of causing it to swell, and passing the swollen rubber material through a flow-limiting device (e.g., a perforated plate) having a small opening, wherein a de-vulcanizing agent is present in at least one of the flow-limiting device and the swollen rubber material.

[0031] US5798394 A discloses a process for desulfurizing and functionalizing rubber vulcanization products by desulfurization, the process comprising: suspending fragments of the rubber vulcanization product in a solvent, the solvent preferably swelling the rubber vulcanization product before or during the desulfurization reaction, and adding an alkali metal, such as sodium, to the resulting suspension. The alkali metal performs the function of breaking the monosulfide, disulfide, and polysulfide crosslinks in the vulcanized rubber.

[0032] US5891926 A relates to a device for passing through a temperature of at least about 150°C and at least 3.4 × 10⁻⁶ ℃. 6 The process of de-vulcanizing vulcanized rubber by heating in the presence of 2-butanol under pressure of Pa.

[0033] Process using solid-based desulfurizing agents

[0034] EP0690091 A1 discloses a solid desulfurizing agent and a relative process for desulfurizing an elastomer material polymerized using elemental sulfur. The process includes treating the polymerized elastomer material with one or more rubber accelerators and one or more activators that are capable of initiating proton exchange at temperatures below 70°C and thus capable of opening or breaking the bonds in the vulcanization network of the elastomer material.

[0035] EP1242520 B1 discloses a modifier for the desulfurization of polymeric elastomers, particularly vulcanized rubber, comprising a first chemical substance and a second substance, the first chemical substance being capable of dissociation and resulting in the formation of organic cations and amines, and the second substance being capable of promoting the dissociation of the first substance by virtue of the presence of the acceptor functional group of the amine.

[0036] Background Art Problems

[0037] Existing desulfurization methods are typically based on techniques that utilize high-temperature heating to vulcanize elastomers. These processes exhibit poor selectivity in breaking crosslinking bonds (CC, CS, and SS), even allowing the breaking of the polymer's strongest carbon-carbon bonds.

[0038] However, the breaking of C-C bonds typically involves a partial loss of the physical and mechanical properties of the desulfurization product relative to the fresh starting product (i.e., the uncured elastomer). Therefore, these processes are particularly destructive, in addition to requiring high energy consumption.

[0039] Furthermore, processes involving handling the mixture at temperatures of at least approximately 300°C also contribute to the generation of unpleasant odors.

[0040] Furthermore, known bio-based desulfurization processes in the prior art (such as those described in US5677354 A, US5798394 A and US5891926 A) typically require complex equipment, large spaces, long operating times, and considerable costs.

[0041] Disadvantages associated with processes employing solid-based desulfurizing agents (especially those described in EP0690091 A1) include:

[0042] i) High cost;

[0043] ii) Short shelf life because at the end of the process, free sulfur in the mixture promotes a slow process of resulfurization of the desulfurization products;

[0044] iii) Protons have poor penetration into the sulfidation products, acting only in the surface region and not at deeper levels. To overcome this problem, it is necessary to pulverize the sulfided material to a particle size of at least 0.4 mm, which results in increased cost and process time.

[0045] iv) Proton treatment did not show good selectivity for the effects of different cross-linking bonds on the desulfurization products, and sometimes even broke the C-C bonds, resulting in a loss of the physical / mechanical properties of the products (such as tensile strength and percentage of elongation at break).

[0046] Regarding patent EP 1242520 B1, the process described therein has the following limitations:

[0047] • It is not selective for saturated and unsaturated elastomers. According to DIN / ISO 1629 classification, it is more effective for elastomers belonging to group R, but less effective for other groups in the same classification.

[0048] • Its side effect is that the product viscosity increases significantly compared to the initial mixture, which makes it more difficult to disperse the obtained desulfurization products in the mixture for subsequent reuse;

[0049] • It operates under certain pressure values ​​and coefficients of friction (applied to the powdered elastomer), resulting in the additive-polymer aggregates not being effectively broken down, especially when the present polymer is saturated. As a result, spots appear on the surface of articles produced using the desulfurization products obtained from this process;

[0050] • It does not prevent the recombination of the molecular chains of rubber that have just been released from crosslinking with sulfur. Summary of the Invention

[0051] Against this backdrop, the fundamental technical objective of this invention is to provide a method for desulfurizing sulfur-cured elastomers, the method comprising the following steps:

[0052] A. An aqueous mixture is prepared by mixing a vulcanized elastomer with a desulfurization composition, the desulfurization composition being obtained as follows:

[0053] i) Mixing urea and at least one dicarboxylic acid in an aqueous solvent at a temperature above 40°C for 3 to 8 hours, preferably equal to 4 hours, to obtain an aqueous mixture containing a desulfurization ionic compound; and

[0054] ii) Adding a peptizing agent to the aqueous mixture from step i) to obtain a desulfurization composition; and

[0055] B. Perform the desulfurization reaction by subjecting the aqueous mixture from step A to a stress state to obtain the desulfurization product.

[0056] Advantages of the present invention

[0057] Compared to the process for preparing the modifier described in EP 1242520 B1 (which involves simply dry-mixing at least two components of the modifier), the desulfurization method according to the present invention specifies the preparation of the desulfurizing agent in an aqueous solvent. This ensures that the composition has better processability not only in the preparation step but also for subsequent use.

[0058] Furthermore, according to the desulfurization method of the present invention, by mixing urea and at least one dicarboxylic acid in an aqueous solvent in step A, crystallization of ionic compounds formed through the interaction of the aforementioned components is prevented. In fact, dry mixing often leads to crystallization of salts formed through the combination of the aforementioned compounds, resulting in reduced efficiency of the desulfurizing agent when used in a relative desulfurization process.

[0059] Note that the desulfurization method according to the present invention allows for the desulfurization of sulfur-cured elastomers.

[0060] In particular, compared to known methods that require high operating temperatures, the desulfurization method of the present invention allows for better selectivity in the breaking of crosslinks in the vulcanized elastomer, maximizes the preservation of the main chain (CC) bonds, and prevents the chains released from crosslinking with sulfur from recombinating with each other, thereby ensuring a longer shelf life.

[0061] Advantageously, compared to known methods using solid-phase desulfurizing agents, the desulfurization method according to the invention allows for ensuring that the desulfurization products have better physical and mechanical properties in terms of tensile strength and percentage of elongation at break.

[0062] In contrast to the method disclosed in patent EP 1242520 B1 (which provides dry desulfurization), the method according to the present invention specifies that the reaction is carried out using a desulfurization composition prepared in an aqueous solvent and containing a plasticizer.

[0063] This advantageously allows for the acquisition of desulfurized products with lower viscosity and therefore greater ease of processing. Specifically, the purpose of using plasticizers is to prevent the elastomers (especially natural rubber and some synthetic rubbers) from recombining after breakage during processing, thereby promoting the absorption of additives present in the elastomers and maintaining the viscosity of the resulting mixture within a range comparable to that of the masterbatch (i.e., the unvulcanized mixture). This facilitates the acquisition of desulfurized products that are similar to the masterbatch and easily dispersed in the mixture for subsequent use.

[0064] Furthermore, advantageously, the obtained desulfurization products can be processed in a specialized mixer to transform them into flakes, thus making them even easier to process and, in particular, easier to disperse in the mixture.

[0065] Advantageously, the desulfurization method of the present invention results in the formation of a desulfurization product, which can be used in combination with uncured masterbatch. The resulting mixture can be subjected to a vulcanization process again, and advantageously, the resulting product (or secondary vulcanization product) has rheological, chemical-physical, and / or mechanical properties, such as tensile strength and / or elongation at break percentage, comparable to those of the masterbatch.

[0066] Therefore, thanks to the aforementioned properties, the secondary vulcanization products can be advantageously used for further applications in the rubber industry, thereby ensuring the continuous recycling of raw materials. Detailed Implementation

[0067] definition

[0068] "Vulcanized elastomers" refers to elastomers that are cross-linked by forming divalent bonds with sulfur.

[0069] "Desulfurization" refers to the selective breaking of the divalent bonds with sulfur, a process that allows solid elastomer objects (typically particles) to return to a processable and moldable form, ready for reuse in the manufacture of new products such as tires or gaskets. In other words, through the desulfurization process, vulcanized elastomers are transformed again into plastic, reprocessable, and / or recyclable materials.

[0070] "Desulfurization composition" refers to a composition suitable for use in the desulfurization process.

[0071] A method for desulfurization of sulfur-cured elastomers.

[0072] A method for desulfurizing sulfur-cured elastomers is claimed, the method comprising the steps described below.

[0073] A. Preparation of aqueous mixtures

[0074] In step A of the desulfurization method according to the present invention, an aqueous mixture is prepared by mixing a vulcanized elastomer with a desulfurization composition, the desulfurization composition being obtained as follows:

[0075] i) Mix urea and at least one dicarboxylic acid in an aqueous solvent.

[0076] In step i) of the desulfurization method according to the invention, urea and at least one dicarboxylic acid (preferably dicarboxylic acid) are mixed in an aqueous solvent for 3 to 8 hours, preferably equal to 4 hours, at a temperature above 40°C, preferably between 40°C and 67°C, to obtain an aqueous mixture containing a desulfurization ionic compound.

[0077] Preferably, step i) is carried out in a dedicated first mixer. Preferably, the first mixer has a capacity between 400 L and 600 L, preferably equal to 500 L. Furthermore, the first mixer is preferably equipped with a dedicated motor having a power between 5 kW and 6 kW, preferably equal to 5.5 kW.

[0078] It should be noted that the mixing of urea with at least one dicarboxylic acid in step i) leads to the interaction between these components, and consequently to the formation of an ionic compound (or salt) formed by an organic cation responsible for the desulfurization properties of the compound.

[0079] For further details regarding the properties and formation mechanism of the aforementioned ionic compounds, please refer to the description in patent EP1242520 B1, which is incorporated herein by reference.

[0080] Preferably, at least one dicarboxylic acid is a dicarboxylic acid with 2 to 8 carbon atoms in its main chain, more preferably 2 to 6, more preferably 2 to 4, and more preferably equal to 2.

[0081] Preferably, at least one dicarboxylic acid is selected from: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, or combinations thereof, with oxalic acid being the preferred choice.

[0082] According to a preferred embodiment, urea is mixed with at least one dicarboxylic acid in step i) at a molar ratio of 1.5:1 to 2.5:1, preferably equal to 2:1.

[0083] Preferably, at least one dicarboxylic acid is mixed in step i) at a concentration of 2 M to 3 M, preferably equal to 2.38 M.

[0084] Preferably, urea is mixed in step i) at a concentration of 6.5 M to 7.5 M, preferably equal to 7.1 M.

[0085] ii) Add a plasticizer.

[0086] In step ii) of the desulfurization method according to the invention, a plasticizer is added to the aqueous mixture from step i) to obtain a desulfurization composition.

[0087] Preferably, the plasticizer comprises a salt of an aliphatic acid, preferably a zinc salt, wherein the aliphatic acid preferably has a main chain length of at least 6 carbon atoms, preferably 6 to 10 carbon atoms.

[0088] According to a preferred embodiment, based on the total weight (w / w) of the desulfurization composition, the plasticizer is mixed in step ii) in an amount of 0.1 wt.% to 0.7 wt.%, preferably equal to 0.5 wt.%.

[0089] It should be noted that, preferably, the aqueous mixture prepared in step A is in the form of a processable and / or moldable aqueous slurry.

[0090] According to a preferred embodiment, the desulfurization composition is mixed in step A in an amount of 1.0 wt.% to 4.0 wt.%, preferably equal to 3.0 wt.%, based on the total weight (w / w) of the vulcanized elastomer.

[0091] The vulcanized elastomer is preferably used in the preparation of the aqueous mixture in step A in the form of powder or granules, i.e., in the form of solid particles, which preferably have an average diameter of 0.3 mm to 1 mm, preferably equal to 0.8 mm.

[0092] Preferably, the elastomer used as a raw material for obtaining the vulcanized elastomer is selected from natural rubber (NR), synthetic rubber, or mixtures thereof.

[0093] Examples of synthetic rubbers suitable for the purposes of this invention are SBR rubber (styrene-butadiene copolymer), SBR1502 rubber, nitrile rubber (NBR), IIR rubber (isoprene-isobutylene rubber or butyl rubber), IR rubber (synthetic polyisoprene), BR rubber (polybutadiene), SBR rubber (styrene-butadiene copolymer), EPM rubber (ethylene-propylene copolymer), CR rubber (polychloroprene), NBR rubber (acrylonitrile-butadiene copolymer), HNBR rubber (hydrogenated nitrile rubber), ACM rubber (ethylene-acrylate copolymer), or combinations thereof.

[0094] It should be noted that, preferably, the elastomer used as a raw material for obtaining the vulcanized elastomer contains additives known to those skilled in the art, and the additives are preferably selected according to the desired properties of the product to be obtained.

[0095] The additive is preferably selected from:

[0096] - Crosslinking agents, such as sulfur and / or organic sulfur derivatives;

[0097] - Accelerators that can increase the speed of crosslinking (vulcanization) with sulfur, such as thiazoles, CBS (N-cyclohexyl-2-benzothiazole sulfenamide), DPG (1,3-diphenylguanidine), TMTM, zinc carbon OZA, TBzTD, or combinations thereof;

[0098] - Activator, an activator that can act as a main promoter, such as an amine or a zinc oxide (ZnO)-fatty acid mixture, preferably a ZnO-stearic acid mixture;

[0099] - Anti-caking agents, such as silica;

[0100] - Fillers, used as inert fillers or to improve the properties of elastomers, such as clay, calcium carbonate and / or talc;

[0101] - Reinforcing agents, such as carbon black;

[0102] - Plasticizers, such as waxes, petrolatum, paraffin oil; and

[0103] - Stabilizer.

[0104] B. Implement the desulfurization reaction.

[0105] In step B of the desulfurization method according to the present invention, the desulfurization reaction is carried out by subjecting the aqueous mixture from step A to a stress state, thereby obtaining the desulfurization product.

[0106] It should be noted that, preferably, the desulfurization reaction occurs because the organic cations of the ionic compounds in the desulfurization composition act on the vulcanized elastomer when subjected to a stress state (i.e., a stressed polymer structure state).

[0107] Advantageously, under such stress conditions, organic cations can selectively break the cross-linking bonds between the chains of the vulcanized elastomer while maximizing the retention of the main chain bonds, namely carbon-carbon (CC) bonds.

[0108] Further details regarding the interaction scheme between salt organic cations and vulcanized elastomers are provided in the description of patent EP1242520 B1, which is incorporated herein by reference.

[0109] Preferably, in step B, the vulcanized elastomer is brought to a high-stress state by conveying the aqueous mixture through a narrow channel between the smooth rolls of a two-roll mill. Note that the narrow channel specifies that the distance between the two rolls is preferably less than 0.2 mm, and more preferably equal to 0.1 mm.

[0110] Preferably, in step B, the water-based mixture is conveyed through the narrow channel between the smooth rolls of the two-roll mill in a cyclic manner, and preferably the water-based mixture passes through the narrow channel 18 to 22 times, more preferably 20 times.

[0111] Preferably, the roller is controlled by a frequency converter that allows the rotational speed to be changed according to procedures known to those skilled in the art.

[0112] It should be noted that the two rollers of the rubber mixing mill preferably rotate at different speeds relative to each other, preferably in opposite directions of rotation, in order to produce a coefficient of friction calculated according to ASTM D 1894, which is less than 1.8, preferably 1.2 to 1.6, and more preferably equal to 1.4.

[0113] Advantageously, friction coefficient values ​​within the above range allow for the avoidance of breakage of the polymer chains (main chain) of the elastomer.

[0114] Preferably, the diameter of each roller is at least 400 mm, more preferably 450 mm to 600 mm. Still preferably, the length of each roller is 600 mm to 1500 mm, more preferably 800 mm.

[0115] Preferably, the steel is coated with a roller, and preferably has a Vickers hardness (H) greater than 40 GPa. V ).

[0116] According to a preferred embodiment, the desulfurization method includes a further step C, namely, processing the desulfurization product in a specialized mixer configured to shred the desulfurization product into flakes.

[0117] It should be noted that, preferably, the second mixer has similar characteristics to the first mixer.

[0118] Advantageously, the flake-shaped desulfurization products are not only easier to disperse in the mixture, but can also be easily packaged for subsequent use.

[0119] Example

[0120] Example 1: "Vulcanization process of elastomer blends based on SBR 1502 rubber"

[0121] Prepare 5 kg of SBR 1502 rubber-based masterbatch. Additives known and commonly used in the prior art are added to this mixture.

[0122] The masterbatch was processed in a closed mixer, and 3 kg of masterbatch was vulcanized in a mold at a temperature of 167°C for 15 minutes. The resulting vulcanized mixture was allowed to stand for 24 hours and then reduced to powder with an average particle size of less than 800 micrometers.

[0123] Example 2: "Process for preparing desulfurization composition"

[0124] In a suitable mixer, dissolve 75 kg of oxalic acid in 350 L of water at a temperature of at least 40°C, preferably between 40°C and 67°C. Keep the system stirred for at least about 5 minutes until the oxalic acid is completely dissolved in the water. Add 150 kg of urea to the aqueous solution of the acid and keep the system stirred for 4 to 8 hours until the reaction between the urea and oxalic acid to form an ionic compound (or salt) is complete.

[0125] When the reaction is complete, a plasticizer based on a mixture of zinc soaps of high molecular weight fatty acids is added in an amount equal to 0.5 wt.% of the weight (w / w) of the desulfurized composition thus obtained.

[0126] Example 3: Devulcanization of vulcanized SBR rubber-based blends

[0127] In a dedicated mixer, 3 kg of the vulcanizing mixture in powder form prepared according to the procedure described in Example 1 is mixed with the desulfurizing composition obtained according to the procedure described in Example 2, the amount of the desulfurizing composition being equal to 3 wt. based on the weight (w / w) of the vulcanizing mixture.

[0128] The resulting mixture is subjected to a stress state by passing it through a narrow channel between two smooth rolls of a two-roll mill. This operation is repeated 20 times in total, at a recorded temperature of 38°C. The resulting desulfurized product is then processed in a specialized mixer to produce flakes.

[0129] Example 4: Characterization of a vulcanized mixture comprising SBR-based masterbatch and desulfurization products

[0130] The mixture was prepared by mixing 2 kg of the unvulcanized masterbatch used in Example 1 with the desulfurization product in sheet form obtained according to the procedure described in Example 3 in a closed mixer, wherein the weight ratio of the desulfurization product to the masterbatch elastomer (and thus excluding additives) was approximately 30:70. The mixing was carried out for a period of 15 minutes.

[0131] After mixing, the resulting mixture is subjected to further vulcanization following the same vulcanization procedure described in Example 1 to obtain the final vulcanized product (or secondary vulcanized product).

[0132] Experimental tests were conducted to characterize the secondary sulfidation products using chemical, physical, and mechanical methods. The results are summarized in Table 1 below.

[0133]

[0134] Table 1: Comparison of Rheological, Chemical-Physical, and Mechanical Properties between SBR Secondary Rubber Vulcanization Products and Masterbatch

[0135] As can be seen from the experimental data reported in Table 1, there are significant improvements in both tear strength and elongation at break.

[0136] Example 5: Characterization of vulcanized mixtures obtained from a mixture of butyl rubber-based masterbatch and desulfurization products

[0137] Following the same vulcanization, desulfurization composition preparation, desulfurization, and re-vulcanization procedures described in Examples 1-4, but using a butyl rubber (IIR rubber)-based compound as the masterbatch, experimental tests were conducted to characterize the final vulcanized (or secondary vulcanized) products rheologically, chemically-physically, and mechanically. The results of these tests are summarized in Table 3 below.

[0138] The composition of the masterbatch and the final vulcanized product is reported in Table 2 below.

[0139]

[0140] Table 2: "Comparison of Masterbatch Composition Based on Butyl Rubber with Secondary Vulcanization Products"

[0141] As noted from Table 2, excluding additives, in 1.800 kg of elastomer (butyl IIR) from the masterbatch, 0.400 kg of desulfurization product (obtained from 0.400 kg of masterbatch elastomer) and 1.400 kg of the remaining elastomer from the masterbatch were used in the secondary vulcanization product, i.e., the weight ratio of desulfurization product to masterbatch elastomer is approximately 29:71. The amount of each additive remained essentially constant between the masterbatch and the secondary vulcanization product.

[0142]

[0143] Table 3: Comparison of Rheological, Chemical-Physical, and Mechanical Properties between Butyl Rubber Secondary Vulcanization Products and Masterbatch

[0144] The data reported in Table 3 show an improvement in tensile strength at break, while other rheological / mechanical properties of the secondary vulcanized product compared to the masterbatch did not exhibit significant changes that could not be corrected by appropriately selecting additives known to those skilled in the art.

[0145] Example 6: "Characteristics of vulcanized mixtures obtained from masterbatch and desulfurization products based on hard NBR rubber"

[0146] Following the same vulcanization, desulfurization composition preparation, desulfurization, and re-vulcanization procedures described in Examples 1-4, but using a hard NBR-based compound as the masterbatch, experimental tests were conducted to characterize the final vulcanized product (or secondary vulcanized product) rheologically, chemically-physically, and mechanically. The results of these tests are summarized in Table 5 below.

[0147] The composition of the masterbatch and the final vulcanization product (secondary vulcanization product) is reported in Table 4 below.

[0148]

[0149] Table 4: "Comparison of Masterbatch Composition Based on NBR Rubber with Secondary Vulcanization Products"

[0150] As noted in Table 4, excluding additives, in 1.800 kg of elastomer (NBR rubber) from the masterbatch, 0.400 kg of desulfurization product (obtained from 0.400 kg of masterbatch elastomer) and 1.400 kg of the remaining elastomer from the masterbatch were used in the secondary vulcanization product, i.e., the weight ratio of desulfurization product to masterbatch elastomer is approximately 29:71. The amount of each additive remained essentially constant between the masterbatch and the secondary vulcanization product.

[0151]

[0152] Table 5: Comparison of rheological, chemical-physical, and mechanical properties between secondary vulcanization products of hard NBR rubber and masterbatch.

[0153] The data reported in Table 5 show that the other rheological / mechanical properties of the secondary vulcanized products did not undergo significant changes compared to the masterbatch that could not be corrected by appropriately selecting additives known to those skilled in the art.

[0154] Example 7: Characterization of vulcanized mixtures obtained from masterbatch and desulfurization products based on hard SBR rubber

[0155] Following the same vulcanization, desulfurization composition preparation, desulfurization, and re-vulcanization procedures described in Examples 1-4, but using a blend based on natural rubber and SBR 1502 rubber as the masterbatch, experimental tests were conducted to characterize the final vulcanized (or secondary vulcanized) products rheologically, chemically-physically, and mechanically. The results of these tests are summarized in Table 7 below.

[0156] The composition of the masterbatch and the final vulcanization product (secondary vulcanization product) is reported in Table 6 below.

[0157]

[0158] Table 6: "Comparison of Secondary Vulcanization Products Based on Masterbatch Composition of Natural Rubber and Hard SBR Rubber"

[0159] As noted in Table 6, excluding additives, in 1.800 kg of elastomer (natural rubber + SBR 1502) from the masterbatch, 0.400 kg of desulfurization product (obtained from 0.400 kg of masterbatch elastomer: 0.100 kg of natural rubber + 0.300 kg of SBR 1502) and 1.500 kg (0.900 kg of natural rubber + 0.600 kg of SBR 1502) of the remaining elastomer from the masterbatch were used in the secondary vulcanization product, i.e., the weight ratio of desulfurization product to masterbatch elastomer is approximately 27:73. The amount of each additive remained essentially constant between the masterbatch and the secondary vulcanization product.

[0160]

[0161] Table 7: Comparison of Rheological, Chemical-Physical, and Mechanical Properties of Secondary Vulcanization Products of Natural Rubber and Hard SBR Rubber with Masterbatch

[0162] Furthermore, from the data reported in Table 7, it can be observed that the other rheological / mechanical properties of the secondary vulcanized products, compared to the masterbatch, did not undergo significant changes that could not be corrected by appropriately selecting additives known to those skilled in the art.

Claims

1. A method for desulfurizing sulfurized elastomers, comprising the following steps: A. An aqueous mixture is prepared by mixing a sulfur-cured elastomer with a desulfurization composition, wherein the desulfurization composition is obtained by: i) Mixing urea and at least one dicarboxylic acid in an aqueous solvent at a temperature above 40°C for 3 to 8 hours, preferably equal to 4 hours, to obtain an aqueous mixture containing a desulfurization ionic compound; and ii) Add a plasticizer to the aqueous mixture from step i) to obtain the desulfurization composition. B. Perform the desulfurization reaction by subjecting the aqueous mixture from step A to a stress state to obtain the desulfurization product.

2. The method according to claim 1, wherein step B is carried out by passing the aqueous mixture through a two-roll mill, the step B being repeated cyclically, preferably the aqueous mixture being passed 18 to 22 times.

3. The method according to claim 1 or 2, wherein in step A, the desulfurization composition is mixed in an amount of 1.0 wt.% to 4.0 wt.% based on the total weight (w / w) of the sulfurized elastomer.

4. The method according to any one of claims 1 to 3, further comprising step C, namely, processing the desulfurization product in a dedicated mixer configured to shred the desulfurization product into flakes.

5. The method according to any one of claims 1 to 4, wherein urea and the at least one dicarboxylic acid are mixed in step i) at a molar ratio of 1.5:1 to 2.5:1, preferably equal to 2:

1.

6. The method according to any one of claims 1 to 5, wherein the at least one dicarboxylic acid is selected from: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, or combinations thereof, preferably oxalic acid.

7. The method according to any one of claims 1 to 6, wherein the plasticizer comprises a salt of an aliphatic acid, preferably a zinc salt, the aliphatic acid having a main chain length of at least 6 carbon atoms.

8. The method according to any one of claims 1 to 7, wherein the plasticizer is added in step ii) in an amount of 0.1 wt.% to 0.7 wt.% based on the total weight (w / w) of the desulfurization composition.

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