Desulfurization additives, related desulfurization methods and desulfurization products

Abstract

A devulcanization additive for vulcanized elastomers is described, which additive has improved efficiency and selectivity along with related continuous and batch devulcanization processes, and the devulcanized products obtained by said devulcanization processes of vulcanized elastomers.

Description

[Technical field]

[0001] The present invention relates to a devulcanization additive for vulcanized elastomers having improved efficiency and selectivity, the related continuous and batch devulcanization processes, and the devulcanized products obtained by the devulcanization process for vulcanized elastomers. [Background technology]

[0002] It is well known that vulcanized elastomers are products obtained by the crosslinking process of elastomeric polymers. The crosslinking process creates a three-dimensional crosslinked structure that gives the final product specific physical-mechanical properties (e.g. hardness, elasticity, abrasion resistance and durability). Thanks to these inherent properties, the vulcanizates thus obtained are suitable for a variety of civil and industrial applications.

[0003] There are various vulcanization methods. The primary method involves the use of sulfur and its derivatives (sulfur or sulfur-cured vulcanization), while the second most widely used vulcanization method involves the use of peroxides (peroxide or peroxide-cured vulcanization). For each type of vulcanization, numerous subclasses are identified depending on the efficiency of the reaction, the kinetics of the reaction, and the additives used.

[0004] Thus, vulcanization forms a network of bonds between molecular chains with three types of bonds: SS (sulfur-sulfur), CS (carbon-sulfur), and CC (carbon-carbon between different molecular chains).

[0005] Desulfurization involves breaking crosslinks and restoring the original non-crosslinked structure of the elastomer, whereas breaking C-C bonds in the same molecular chain increases the degree of desulfurization significantly, accompanied by a decrease in the average molecular weight of the polymer and a dramatic decrease in the physical-mechanical properties of the resulting end product. The polymer practically does not return to its initial properties, but is randomly degraded, resulting in linear fragmentation of the polymer chains, which results in an ineffective reuse of the resulting product. On the other hand, the more selective the desulfurization is with respect to crosslinks, the more it is possible to preserve the physical-mechanical properties of the polymer, and thus to reuse the desulfurized products obtained in new products.

[0006] The broadest definition of devulcanization is given by standard ASTM D6814-02, which defines devulcanization as "the process of breaking down chemical crosslinks in cured rubber." This regulation then defines standards for measuring the degree of devulcanization.

[0007] For vulcanized blends with high filler loadings (e.g., carbon black and silica) typical of products recovered from end-of-life tires (ELT), the Kraus correction can be useful, which takes into full account the inert parts of the mixture to calculate the degree of desulfurization with greater accuracy (KRAUS, G. (1963) Swelling of Filler Reinforced Vulcanisates. J. appl. Polym. Sci., 7, 861-871).

[0008] The selectivity of the reaction, i.e. the specificity of desulfurization with respect to the cleavage of only crosslink bonds, is calculated by the Horikx equation and the corresponding Horikx diagram (Seghar, Said; Asaro, Lucia; Ait Hocine, Nourredine; Experimental Validation of the Horikx Theory to be Used in the Rubber Devulcanization Analysis; Springer / Plenum Publishers; Journal of Polymers and the Environment; 27; 10; 7-2019; 2318-2323).

[0009] This figure calculates a theoretical curve associated with the selective cleavage of cross-links only, and values ​​for the resulting desulfurized products are then calculated. The more these values ​​can be superimposed on the theoretical curve, the more they reflect the selectivity of the theoretical curve in cleaving only cross-links.

[0010] Selective desulfurization of elastomers has been actively investigated since the middle of the last century, however, these have generally been ineffective and crude approaches aimed at recovering raw materials for reuse as combustibles for energy recovery, inerts for cementitious products, or non-reactive fillers in thermoplastic or elastomeric blends.

[0011] The simplest mechanical and thermal methods are limited to a simple mechanical treatment by grinding that only reduces the particle size of the vulcanized product, without actually providing an effective devulcanization. All methods known in practice for recovering devulcanized elastomers provide a first grinding step of the starting vulcanizate, which is usually oversized, and then, for example, tires are reduced by grinding into irregular pieces of a few centimeters or millimeters, and then separated into their main components according to density.

[0012] Other methods include the use of high temperatures obtained by mechanical stress in open or closed mixers or co-rotating or counter-rotating twin screw extruders. These methods impart energy to the vulcanized elastomer in an indiscriminate manner, causing non-selective devulcanization and a significant loss of molecular weight and mechanical properties. These methods often cause large emissions of gases, including sulfur dioxide or other gases resulting from the thermal decomposition of the products contained in the treated elastomer. Treatment of the mass undergoing this treatment with a vacuum suction system reduces odors, but does not prevent mechanical degradation of the product.

[0013] Alternatively, the energy is partially and selectively supplied by microwaves or ultrasound, again resulting in suboptimal desulfurization and a significant loss of molecular weight and mechanical properties.

[0014] Some chemical or mechanochemical methods use organic solvents at room temperature or below the boiling point of the solvent to treat the vulcanized elastomers. The long times and difficulties in handling the solvents make these methods less applicable.

[0015] Alternatively, methods are known that use the chemical properties of supercritical CO2, for example applied by addition to the molten mass, but this method provides a high temperature extrusion process, which results in a loss of desulfurization selectivity.

[0016] Modern mechanochemical methods show how some of the raw materials used to trigger, regulate or terminate the vulcanization reaction, especially in the case of sulfur vulcanized elastomers, can contribute to efficient devulcanization if applied to the vulcanized elastomer in the right conditions and amounts. Chemical methods using dimethyldithiocarbamates and mercaptobenzothiazoles, or methods using primary and secondary amines and imines, which are also aromatic, belong to this line of research.

[0017] Another method uses derivatives of urea mixed with dicarboxylic acids, which are processed at low temperatures in open or closed mixers.

[0018] These more recent mechanochemical methods improve the selectivity of the reaction but have a number of drawbacks. The first problem is related to the fact that some of these reagents do not react completely during devulcanization, especially when applied in low-temperature processes, and therefore remain unreacted in the desulfurized product. Since these reagents belong to chemical classes that are usually used for vulcanization or are derived from these chemical classes, if they remain unreacted in the desulfurized mixture, they can cause a subsequent re-vulcanization of the desulfurized mixture, which is already in the storage stage of the material, especially in the presence of strong temperature changes, effectively making the desulfurization process completely or partially ineffective. Moreover, these reagents affect the overall stoichiometric balance of the subsequent vulcanization reaction, changing the rheology of the new blend and the desulfurized product is used in a blend with a virgin elastomer to obtain a second vulcanized product.

[0019] More recently, research has been carried out on the use of heterocyclic compounds (e.g., 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-ocyl (4-hydroxy-TEMPO)) used as polymerization inhibitors to prevent the phenomenon of regurgitation of desulfurized products associated with mechanical methods.

[0020] Another drawback of the more recent mechanochemical methods is that due to the low molecular weight of the reactants, they can migrate to the surface of the desulfurized product when it is used in a new blend or new product, resulting in unsightly and potentially dangerous deposits for the health of the user, especially in the case of products containing reagents such as amines and imines, aromatic products or sulfur and its precursors.

[0021] These surface migration phenomena, especially of amines, occur in fresh blends with virgin elastomers and also when the desulfurized products are used in low proportions in blends. Subsequent reprocessing in blends and subsequent regulative curing are therefore often not sufficient to eliminate the problem of migration of potentially harmful compounds on the surface of the manufactured products.

[0022] Moreover, all the described chemical and mechanochemical methods do not substantially take into account the kinetics of the desulfurization process, and in fact no desulfurization method is known in the state of the art that aims to effectively control the desulfurization reaction from the kinetics point of view. Another drawback of the modern mechanochemical methods is that the chemical reagents involved react in a substantially uncontrolled manner around the area where they are deposited and are therefore either consumed very quickly at the surface of the elastomer particles or, due to the applied frictional dynamics, the material undergoes a more rapid temperature increase. In the interior of the particles or in the depressions and cracks typical of the surface resulting from crushing, by contrast, these reagents arrive in smaller quantities and, being protected from mechanical friction and heat (as elastomers are very poor thermal conductors), they do not exert any action and remain unreacted. This effect is particularly detrimental to desulfurization considering that the majority of the sulfur bonds are directed towards the interior of the particles, where sulfur is thermodynamically arranged in a state of lower energy, whereas at the surface of the cold particles, sulfur is almost absent (JS Dick, Rubber Technology, page 353, 3rd Edition, 2020).

[0023] As a result, from a kinetic point of view, the reagents remain distributed mainly on the surface of the elastomer, where they are not very rich in sulfur bonds, until the minimum temperature that allows the reaction is reached. They are then rapidly expelled or are incorporated and accumulated inside the particles undergoing treatment, not allowing their effective reaction at lower temperatures. Even processes that require reaction at room temperature are in fact processes that utilize temperature, the peak value of which is only localized on the surface of the particles for a very short time, such as to make the entire mass appear at room temperature. This statement is evidenced by the fact that simple mixing without friction, for example by mixing the vulcanized elastomer with the reagents in a beaker at room temperature, does not cause the reaction. Thus, the surface reactions are too fast for known chemical and mechanochemical methods to allow an effective desulfurization that gradually allows the new inner layer of the particles to be desulfurized.

[0024] Even in desulfurization methods involving the use of open multi-stage roll mills, most of the steps only serve to increase the temperature, and only the final step results in rapid reaction of reagents that may be consumed very quickly at the surface and remain unreacted in the very interior.

[0025] Thus, from the point of view of physical, macroscopic and mechanical behavior, all the methods described have the disadvantage that they only result in comminution or mainly in surface devulcanization, since they are unable to penetrate the vulcanized network and thus to selectively cleave the crosslinks inside the vulcanized elastomer particles.

[0026] Therefore, in order to achieve homogeneous desulfurization, it is first necessary to carry out an optimal grinding of the product with a very regular particle size curve, avoiding in particular those particles with dimensions larger than the others that may have a very low degree of desulfurization due to their unfavorable surface / volume ratio. These difficult to desulfurize particles are in fact characterized by poor interfacial properties when inserted into new blends, thus creating weak points for the propagation of mechanical tears, which are responsible for a dramatic decrease in the performance of desulfurized products derived from raw materials not selected for their homogeneity and physical morphology.

[0027] In the most common case of truck tires, where the tread is rich in vulcanized natural rubber (NR), an elastomer that is economically interesting for the devulcanization process, they can weigh up to several tens of kg and are composed of many different parts for their structure and chemical composition: the tread part is made of a more inert material such as natural rubber (NR), while the more structural internal parts are made of less valuable materials, even metal or synthetic fiber reinforcements.

[0028] In the end-of-life tire (ELT) recovery process, the tire then undergoes a process of surface buffing of the tread, followed by crushing and screening for separation of the metals. The buffed product, called "buff", is very coarse and variable in size. The buff is then further crushed and screened to produce the so-called "powder" or micronized rubber powder (MRP), with the quality depending on the particle size and homogeneity of the composition.

[0029] At present, the diversity of the starting products strongly influences the properties of the desulfurized end products processed with known devulcanizzazione technologies.

[0030] Thus, in addition to the previously indicated limitations of the currently used desulfurization methods, which are unable to obtain selective desulfurization, a further factor that contributes to making it even more difficult to obtain a high quality desulfurized end product is also related to obtaining a sufficiently homogeneous desulfurization.

[0031] In this respect, in addition to the important factors of fine grinding and optimal particle size distribution in order to obtain a homogeneous devulcanization, the variability present in the crosslink density of the devulcanized material and in the type of intermolecular bonds formed in the crosslinked network contributes depending on the vulcanization method to which the vulcanizate has been subjected.

[0032] In the case of sulfur-based vulcanization, there may be several vulcanization systems defined, for example, as conventional vulcanization (CV), semi-efficient vulcanization (Semi EV) and efficient vulcanization (EV), which are exemplary and not exhaustive, starting from the same polymer base and resulting in vulcanizates with different properties, for example with different prevalence of polysulfide, disulfide or monosulfide bonds.

[0033] Sulfur cured rubbers are indeed known to have different ratios between covalent C-C (carbon-carbon) and S-S (sulfur-sulfur) bonds and, with regard to the bonds of this second group, different ratios between polysulfide, monosulfide and disulfide bonds, depending on the vulcanization technique used. They also have different crosslink densities.

[0034] Vulcanization depends on the amount of sulfur, type of accelerator, accelerator / sulfur ratio, and vulcanization time. High accelerator / sulfur ratio and longer vulcanization time result in the formation of a greater number of monosulfide and disulfide bonds compared to polysulfide bonds. Monosulfide and disulfide bonds confer better resistance to heat and solvents, but are generally stiffer and have lower tensile strength. On the other hand, polysulfide bonds have better tensile strength, better fracture resistance, but lower chemical and heat resistance.

[0035] It is clear that in the devulcanization reaction, this variability results in a diversity of behavior, especially in products derived from end-of-life tires. The polysulfide bonds are more reactive and dissociate first with the help of an increase in the temperature of the system. It is indeed known that only thermomechanical treatments can obtain a small percentage of devulcanization, affecting the polysulfide bonds.

[0036] In the presence of rubbers in which monosulfide bonds are prevalent or in the presence of high crosslink density, the use of devulcanization additives based on a single chemical species is often ineffective. For example, polysulfide bonds are susceptible to nucleophilic attack by thiolate ions, while mono- and disulfide bonds remain intact. On the other hand, the use of more aggressive chemical species may be excessive, causing too many bonds to be simultaneously broken, resulting in loss of selectivity. Moreover, too many broken bonds tend to react with each other, both radically and ionically, and reorganize themselves into a newly crosslinked structure, essentially ineffectively desulfurizing and negatively reducing the average molecular weight of the product.

[0037] All of the above aspects relate to the current desulfurization methods, whether mechanical, mechano-thermal, chemical or mechano-chemical, which: - less selective for effective cleavage of only cross-links, - Partially ineffective in producing desulfurization products that are stable over time during storage and transportation, and these products are exposed to the risk of migration of harmful residues; the constancy of the physical-mechanical properties of the final devulcanized product and of the mixtures in which this devulcanized product is mixed with virgin elastomer and subjected to a subsequent second vulcanization is highly dependent on the vulcanization method, the grinding and the particle size distribution of the starting products; This proves that.

[0038] Concerning the machines and plants used in the known desulfurization methods, three categories of equipment can be mainly found, the first and simpler category includes mills, which chop by direct cutting, such as blade mills, or by rubbing under pressure, such as disk mills, or equipment with various ingenious shapes reminiscent of graters or squeezers. The second category is represented by batch systems such as open and closed mixers, the open mixers are generally mixers with two counter-rotating steel rolls (Roll-Mill) with different absolute and relative rotational speeds, the span between the rolls ranges from 0 millimeters to several millimeters, and the temperature of the rolls is adjustable. The closed type mixers are generally of the "Bambury" type, consisting of counter-rotating steel blades in an hourglass-shaped chamber. The third category is generally represented by continuous systems, generally by extruders, the most common type of extruder in the desulfurization process being twin-screw, co-rotating or counter-rotating.

[0039] Examples of prior art documents are WO 2020 / 169589, U.S. Pat. No. 4,305,850, WO 01 / 29122, U.S. Pat. No. 9,175,155, U.S. Patent Application Publication No. 2017 / 362407, and Chinese Patent Application Publication No. 112458445.

[0040] These documents illustrate methods for the synthesis of polymers by devulcanization, rubber devulcanization processes, acid-base adducts, and methods for functionalization of elastomeric materials, but do not solve the problems identified above. Summary of the Invention [Problem to be solved by the invention]

[0041] The present invention aims to overcome the limitations of known devulcanization methods and to define a devulcanization additive for vulcanized elastomers, as well as an associated devulcanization method that overcomes the shortcomings of the prior art. [Means for solving the problem]

[0042] The present invention is first directed to a devulcanization additive for vulcanized elastomers, particularly sulfur vulcanized rubber, comprising: an acid-base adduct obtained from a mono- or dicarboxylic organic acid having a carbon number ranging from 2 to 18 and urea or a mono-, di- or tri-substituted derivative of urea having the formula: [ka] [wherein R1, R2 and R3 are the same or different and can be hydrogen, a linear alkyl chain having 2 to 18 carbon atoms] Peroxide and comprising or consisting of the acid-base adduct is obtained starting from an organic mono- or dicarboxylic acid and urea or a mono-, di- or tri-substituted derivative of urea in a molar ratio ranging from 1:1 to 1:2, The present invention relates to a devulcanization additive for vulcanized elastomers, particularly sulfur vulcanized rubber.

[0043] The present invention further relates to a process for the batch devulcanization of vulcanized elastomers, in particular sulfur vulcanized rubber, comprising the following steps: i) mixing a devulcanizing additive with the vulcanized elastomer to be devulcanized, where the devulcanizing additive comprises an acid-base adduct obtained starting from a mono- or dicarboxylic organic acid having a carbon number ranging from 2 to 18 and urea or a mono-, di- or tri-substituted derivative of urea having the formula: [ka] [wherein R1, R2 and R3 are the same or different and can be hydrogen, a linear alkyl chain having 2 to 18 carbon atoms] Peroxide and comprising or consisting of ii) optionally adding a compatibilizer and further additives; iii) optionally heating the mixture thus obtained, preferably to a temperature in the range of 20° C. to 80° C.; iv) compression and mechanical stretching steps, in particular in open or closed mixers, and v) repeating the compression and mechanical stretching step iv) a number of times ranging from 0 to 40; The present invention relates to a method comprising the steps of:

[0044] Step iv) and step v) are preferably carried out by controlling the temperature within the range of 20°C to 110°C.

[0045] The present invention further relates to a process for the devulcanization of vulcanized elastomers, in particular sulfur vulcanized rubber, in a continuous manner, comprising the following steps: i) mixing a devulcanizing additive with the vulcanized elastomer to be devulcanized, where the devulcanizing additive comprises an acid-base adduct obtained starting from a mono- or dicarboxylic organic acid having a carbon number ranging from 2 to 18 and urea or a mono-, di- or tri-substituted derivative of urea having the formula: [ka] [wherein R1, R2 and R3 are the same or different and can be hydrogen, a linear alkyl chain having 2 to 18 carbon atoms] Peroxide and comprising or consisting of ii) optionally adding a compatibilizer and further additives; iii) optionally heating the mixture thus obtained, preferably to a temperature in the range of 20° C. to 80° C.; and iv) extrusion of the desulfurized product; The present invention relates to a method comprising the steps of:

[0046] Finally, the present invention relates to a desulfurized product obtainable by the process according to the invention, suitable for use as a raw material for blending with virgin elastomers to obtain re-vulcanized elastomers, in particular sulfur vulcanized rubbers, in a proportion ranging from 5% to 100% by weight, preferably from 20% to 60% by weight relative to the weight of the blend.

[0047] The devulcanization additive for vulcanized elastomers is particularly effective in the case of sulfur vulcanized rubber.

[0048] The acid-base adducts are obtained starting from an organic mono- or dicarboxylic acid having a number of carbon atoms ranging from 2 to 18, and urea or a mono-, di- or tri-substituted derivative of urea, in a molar ratio ranging from 1:1 to 1:2, preferably equal to 1:2.

[0049] The desulfurization additive is a solid, preferably in the form of a powder.

[0050] The acid-base adducts are obtained by precipitation of organic mono- or dicarboxylic acids with urea or mono-, di- or trisubstituted derivatives of urea in a solution or by reaction between the solids intimately mixed in the presence of moisture.

[0051] The organic mono- or dicarboxylic acid having a carbon atom number in the range of 2 to 18, preferably 2 to 10, is more preferably selected from oxalic acid, tartaric acid, and malic acid, and even more preferably is oxalic acid.

[0052] The base is urea or a mono-, di- or tri-substituted derivative of urea having the formula [ka] wherein R1, R2 and R3 are the same or different and can be hydrogen, a straight alkyl chain having 2 to 18 carbon atoms, preferably 2 to 10 carbon atoms; and the base is preferably urea.

[0053] More preferably, the acid-base adduct is an oxalic acid-urea adduct in a 1:2 molar ratio.

[0054] The peroxide may be an organic peroxide or an inorganic peroxide, preferably an organic peroxide.

[0055] The organic peroxide is preferably selected from dicumyl peroxide, 1,3-1,4-bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butylcumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne, n-butyl-4,4-di(tert-butylperoxy)valerate, 1,1-di(tert-butylperoxy)-3,3,5 trimethylcyclohexane, di(2,4-dichloro-benzoyl)peroxide and mixtures thereof.

[0056] The organic peroxide is preferably selected from dicumyl peroxide and 1,1-di(tert-butylperoxy)-3,3,5 trimethylcyclohexane, more preferably dicumyl peroxide.

[0057] The organic peroxide is preferably mixed and absorbed into an inert inorganic filler (e.g., silica, calcium carbonate, kaolin, aluminum silicate and clay), more preferably absorbed into silica, calcium carbonate, aluminum silicate, kaolin or related mixtures, even more preferably absorbed into silica, calcium carbonate or related mixtures.

[0058] A preferred organic peroxide is dicumyl peroxide absorbed on calcium carbonate and silica. This formulation of the peroxide makes it easier to store, handle and administer.

[0059] It is interesting to note that peroxides, also commonly used as vulcanizing agents, represent effective co-agents for the desulfurization process and have been proven to be synergistic for obtaining selective cleavage of intermolecular bonds and for the subsequent chain end stabilization, thus preventing re-vulcanization.

[0060] It is also possible to use organic peroxides with scorch protection to prevent excessively rapid reaction rates and mainly surface reactions on the exterior of the vulcanized elastomer particles. These are organic peroxides mixed with suitable agents that retard the availability of peroxide radicals in the reaction, promoting a more uniform reaction. Examples of these suitable agents are nitroxides and their derivatives (US Pat. No. 7,829,634). Other possible agents are hydroquinone and its derivatives mixed with sulfur donors and coagents such as monofunctional vinyl monomers, monofunctional allylic monomers, polyfunctional vinyl monomers, polyfunctional allylic monomers and mixtures thereof (US Pat. No. 5,849,214).

[0061] In the case of commercially available peroxides, this scorch protection is already intimately incorporated into the peroxide by the manufacturer; one example of these peroxides is the commercial peroxide Luperox SP Scorch Protection marketed by Arkema.

[0062] Also, in the case of peroxides with scorch protection, the peroxide is preferably absorbed on an inert inorganic filler selected from silica, calcium carbonate and related mixtures.

[0063] The peroxide is preferably in granular or powder form and may be burned on a polymeric support.

[0064] The devulcanizing additive of the present invention may also include a compatibilizer selected from ethylene-vinyl acetate copolymers, dimethyl zinc acrylate, NR-g-PDMMMP (a grafted copolymer of natural rubber (NR) and poly(dimethyl(methacryloxymethyl)phosphonate (PDMMMP)), butadiene-isoprene liquid copolymer rubber, GMA and MAH (grafted polyolefin polymers), epoxidized natural rubber, trans-polyoctenamer (TOR), and mixtures thereof.

[0065] The compatibilizer is preferably trans-polyoctenamer (TOR). The trans-polyoctenamer (TOR) is even more preferably in the form of a pure powder or granules or a solution of said granules comprising 10% to 40% by weight of granules in mineral, naphthenic, paraffinic oil or mixtures thereof, i.e. oils commonly used in tires. The powder form has been found to be particularly suitable for the purpose for ease of administration, mixing and uniformity of results. A solution of TOR in oil is also particularly practical and effective.

[0066] The compatibilizer makes it possible to improve the behavior of the not fully devulcanized particles in the blend. The compatibilizer has properties such as a low melting point and a viscosity that allows it to effectively penetrate the porosities of the not fully devulcanized particles, forming an interfacial film with the blend with a thickness ranging from a few microns to hundreds of microns. The unique chemical structure of the compatibilizer is very similar to that of the new blend, and it can also have reactive sites in the new vulcanization reaction of the blend consisting of the desulfurized products, possibly mixed with virgin elastomer, effectively incorporating the not fully devulcanized particles in the network.

[0067] The devulcanization additive according to the invention can be prepared before it is added to the elastomer to be devulcanized and then mixed with the elastomer powder to be devulcanized, or the different components of the devulcanization additive can be mixed directly with the elastomer to be devulcanized simultaneously or in a subsequent stage.

[0068] Compatibilizers can be added to the already desulfurized product, both in a single step and in subsequent steps using, for example, acid-base adducts and peroxides, which also act as viscosity modifiers.

[0069] This mechanism allows for improved mechanical and rheological properties of the desulfurized product compared to a product without compatibilizer.

[0070] There is also an improvement in the aesthetics of the finished desulfurized product, which is smoother, more uniform, and free of surface defects caused by non-desulfurized particles emerging from the surface.

[0071] The use of compatibilizers in grounds derived from end-of-life tires (Ground Tire Rubber or GTR) is known from the state of the art, but its use in combination with devulcanization additives, especially acid-base adducts and peroxide-based devulcanization additives, is entirely new.

[0072] In some cases, depending on the type of rubber being devulcanized, it may be useful to provide that the devulcanization additive may also include long chain fatty acids or their associated salts. This further component, together with the acid-base adduct, the peroxide, and optionally the compatibilizer, may be used in the pretreatment or added to the mixture for devulcanization.

[0073] Preferred long chain fatty acids are carboxylic acids having an aliphatic chain longer than 10, more preferably longer than 16. These acids or related salts prepare the rubber to be devulcanized by optimizing and stabilizing the pH within the range favorable for the devulcanization reaction acting in overall synergy with the acid-base adduct and peroxide.

[0074] The pH is measured before the devulcanization reaction by immersing 10 g of additive-containing rubber in 100 ml of hot water and allowing the entire mixture to stand for 1 hour. If an acid is used, the pH of the water must be below 2, preferably below 0.5.

[0075] When the devulcanization additive comprises an adduct and a peroxide, the adduct is added in an amount ranging from 0.5% to 5% by weight based on the weight of the elastomer to be devulcanized, the adduct is added in an amount ranging from 0.4% to 4.5% by weight, and the peroxide is added in an amount ranging from 0.1% to 3% by weight based on the weight of the elastomer to be devulcanized.

[0076] When the devulcanization additive comprises an adduct, a peroxide and a compatibilizer, the adduct is added in an amount ranging from 1% to 20% by weight, the peroxide is added in an amount ranging from 0.5% to 4.5% by weight, the peroxide is added in an amount ranging from 0.1% to 3% by weight, and the compatibilizer is added in an amount ranging from 0.4% to 19.4% by weight, based on the weight of the elastomer being devulcanized.

[0077] Thanks to the simultaneous use of at least an acid-base adduct and a peroxide, and optionally a compatibilizer and / or a fatty acid, the devulcanization additive according to the invention allows for better management of the devulcanization of all the different sulfur crosslinked rubbers that may be present in products originating from post-consumer or processing wastes, such as wastes from the processing of ELT or footwear soles.

[0078] The use of compatibilizers, as already mentioned, also makes it possible to optimize the behavior of the non-desulfurized residues in the blend, incorporating them and covering them with reactive functional interfaces that compatibilize them in an effective and chemically active manner in the subsequent recuring, regardless of the composition and the initial curing mode.

[0079] An apparatus for desulfurization is also described, comprising a mixing and heating device and a reaction device.

[0080] In the desulfurization unit, the mixing and heating device is selected from a high speed mixer (turbo mixer) or hot air or infrared or microwave device.

[0081] In the mixing and heating unit, the devulcanization is not started, but the vulcanized elastomer to be devulcanized is preheated and mixed with the devulcanization additive according to the invention prior to the devulcanization process, and then devulcanized in the reactor.

[0082] In the desulfurization apparatus, the reactor may be a single-stage roll mill or a multi-stage roll mill, a co-rotating twin-screw extruder having a screw profile suitable for maintaining the temperature as low as possible below the depolymerization value of the elastomer, a counter-rotating twin-screw extruder having a screw profile suitable for maintaining the temperature as low as possible below the depolymerization value of the elastomer, a multi-screw extruder (ring extruder or planetary extruder) suitable for maintaining the temperature essentially below the depolymerization value of the elastomer, a single-screw extruder suitable for maintaining the temperature below the depolymerization value of the elastomer, preferably a roll mill, a single-stage or multi-stage screw extruder, or a multi-screw extruder (ring extruder or planetary extruder).

[0083] In particular, the screw profile has a predominant proportion of transport elements relative to mixing and grinding elements, which mainly contribute to the temperature rise of the material due to high mechanical stresses. The number of inverse elements, which contribute to increase the pressure of the melt and create a recessed area suitable for the insertion of an atmospheric or vacuum degassing device, is also appropriately applied and positioned to induce not excessive mechanical stresses in the melt and to exceed the degradation temperature of the elastomer.

[0084] If desired, the compatibilizer may be added to the initial mix, i.e. both in the mixer, or more effectively to the melt, by means of a suitable side extruder attached to the reactor.

[0085] The invention will also become more apparent from the accompanying drawings. [Brief description of the drawings]

[0086] [Figure 1] FIG. 1 shows the degree of desulfurization of the desulfurized products obtained in the following Examples 1, 2, 4, 5 and 6, measured according to standard ASTM D6814. [Diagram 2] FIG. 2 shows the Horikx diagram analysis of the desulfurized products obtained in Examples 1, 2, 4, 5 and 6 below. [Diagram 3] FIG. 3 shows a diagram of a multi-screw extruder, planetary extruder or ring extruder with 12 co-rotating screws. [Figure 4] FIG. 4 is a schematic cross-sectional view of a multi-screw extruder to demonstrate that cooling is effective both inside and outside the planetary arrangement of the screws. [Diagram 5] FIG. 5 shows that the "elongational flow" values ​​of a multi-screw extruder are higher than those of a conventional twin-screw extruder. [Figure 6] FIG. 6 is a table showing the physical and mechanical properties measured for the technical article (eg "Bushing") subjected to the test. [Figure 7] FIG. 7 is a table showing the physical and mechanical properties measured for the tire tested, Truck Tread - Premium Truck Tire. [Figure 8] FIG. 8 shows a visual comparison of the "raw" product, i.e. the appearance of the state of the art product, with that of the product according to the present invention, where the desulfurized product was added "on top" at 10% by weight to a base blend of an industrial article (a bushing) also containing vulcanized and accelerated products, and the blend was then processed and photographed prior to vulcanization. [Figure 9] FIG. 9 shows a visual comparison of the appearance of the "cured" products obtained according to the method shown in FIG. 8 starting from a base blend for truck treads - premium truck tires, i.e. the appearance of the state of the art products and the product according to the invention, the products were photographed after vulcanization. [Figure 10]FIG. 10 is a table showing the physical and mechanical properties measured for the tire tested, Cartread - Passenger Tire. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0087] As non-limiting examples of the present invention, some representative examples of the present invention are provided below.

[0088] In the following Examples 1 to 3, an ELT powder was used having the following properties derived from thermogravimetric analysis (TGA): [Table 1]

[0089] <Example 1> An example of desulfurization by a batch plant using a roll mill was carried out using the desulfurization additive according to the present invention as follows.

[0090] 100 kg of ELT powder originating from truck treads, with particle sizes ranging from 0.05 mm to 3 mm and mainly equal to 0.8 mm, was weighed.

[0091] 3 wt % of oxalic acid-urea adduct in a 1:2 molar ratio and 0.25 wt % of dicumyl peroxide adsorbed on a 40% mixture of silica and calcium carbonate were added to the powder.

[0092] The mixture thus obtained was homogenized for 5 minutes and brought to a surface temperature of 70°C (measured by an infrared thermometer) by a high-speed turbo mixer using a suitable mixing tool profile and several blades to transfer momentum to the mixture in order to form a precise mixing "cone" and at the same time transfer sufficient energy for a consistent temperature rise within a defined time.

[0093] A suitable mixing tool for this purpose would have one to four mixing steps, in this case three, with a blunt or rounded blade profile to allow for proper fluidization of the material.

[0094] Temperature is a parameter that can be subject to errors of interpretation: the rubber actually heats up due to mechanical action and its temperature, measured by an infrared thermometer, drops very rapidly over time. For this reason, placing an infrared probe too far downstream from the roll can result in a significant drop in the temperature reading. The product becomes tepid in a very short time, thus falsely inducing the effects of a low temperature process.

[0095] The mixture thus homogenized and heated is passed through a roll mill for 6 minutes with a relative speed of 1:1.2, the span between the rolls set to a minimum (practically zero and compatible with roll grinding) and appropriate temperature regulation of the rolls. The rolls are kept at a temperature below 35° C. More specifically, the span between the rolls is 0.1 mm.

[0096] 2% by weight of TOR in powder form was added to the thus treated mixture and distributed evenly throughout the mixture mass.

[0097] The mixture was then processed again on the roll mill for an additional 4 minutes until the mixture was completely melted and the compatibilizer was incorporated into the mixture.

[0098] The mixture was then discharged and sent to packaging before proceeding with a new processing batch.

[0099] The preheat temperature, number of steps in the mixer, time, rotational speed and relative speed of the rolls, as well as the proportion of modifiers and compatibilizers, operating temperature may vary depending on the type of powder, which must be analyzed in advance to determine the best process conditions.

[0100] <Example 2> A second example of desulfurization with the desulfurization additive according to the present invention was carried out according to exactly the same method as in Example 1, except that the peroxide used was dicumyl peroxide with a scorch protection system (Luperox with Scorch Protection SP2, sold by Arkema).

[0101] <Example 3> A third example of devulcanization with the devulcanization additive according to the invention was carried out according to exactly the same method as in Example 1, except that the material to be devulcanized was SBR (styrene butadiene rubber) intended for reuse in the same applications, originating from waste disposal of the footwear industry, in particular from crushing of shoe soles, and 100 kg of SBR rubber powder was subsequently weighed out with a particle size ranging from 0.5 mm to 3 mm, mainly equal to 1, 2 mm.

[0102] <Example 4> A fourth example of desulfurization using the desulfurization additive according to the present invention was carried out as follows. Unlike the previous examples, 100 kg of ELT powder originating from truck treads, of a production batch with particle sizes ranging from 0.5 mm to 3 mm, mainly equal to 1.2 mm, not previously analyzed, was weighed.

[0103] 3.5 wt% of oxalic acid-urea adduct in a molar ratio of 1:2 and 0.30 wt% of dicumyl peroxide adsorbed on a mixture of silica and calcium carbonate at 40% were added to the powder.

[0104] The mixture thus obtained was homogenized in a low speed horizontal mixer for 8 minutes under cold conditions.

[0105] The mixture thus homogenized was passed 25 times in a roll mill with a span between the rolls equal to 0.2 mm to accommodate the larger particle size of the product, while the speed ratio was 1:1.3 to increase the elongation of the product. Proper thermostatic regulation of the rolls was maintained at all times, keeping them at a temperature below 35° C.

[0106] The process took 12 minutes to reach a temperature of 95°C as measured on the rubber using an infrared thermometer.

[0107] The mixture was then discharged and sent to packaging before proceeding with a new processing batch.

[0108] In this example, the mixture, due to its characteristics, is processed in more steps and does not require the use of a compatibilizer or preheating, therefore, in this case, it was possible to homogenize the ingredients under cold conditions in a horizontal mixer with an internal mixing reel rotating at a slow speed and process the mixture thus obtained directly in a roll mill. In particular, the horizontal mixer is a ribbon blender.

[0109] <Example 5> Further examples of desulfurization with the desulfurization additive of the present invention were carried out using the same method as in Example 4, except that the peroxide used was an inorganic peroxide.

[0110] To a previously unanalyzed ELT powder with a particle size ranging from 0.5 mm to 3 mm, mainly equal to 1.2 mm, 3.5% by weight of oxalic acid-urea adduct in a molar ratio of 1:2 and 0.15% by weight of potassium peroxymonosulfate (in the form of a granulated powder as a triple salt, called Oxone) were added.

[0111] Again, no compatibilizer or preheat was required.

[0112] <Example 6> A sixth example of desulfurization using the desulfurization additive according to the present invention was carried out as follows.

[0113] 100 kg of Rubber Buffing was weighed from end-of-life tires, with very irregular grain sizes ranging from 0.5 mm to 10 mm. This is a product derived from the coarse grinding of tire treads. This product, called Rubber Buffing, comes directly from buffing and has a very irregular, elongated and frayed shape with dimensions of several centimeters.

[0114] Three phr (parts per hundred parts of rubber) of oxalic acid-urea adduct in a 1:2 molar ratio and 2.5 phr of 1,1-di(tert-butylperoxy)-3,3,5 trimethylcyclohexane adsorbed onto a mixture of calcium carbonate and sodium aluminum silicate at 40% were added to the rubber buff.

[0115] The mixture thus obtained was homogenized in a low speed horizontal mixer for 8 minutes under cold conditions.

[0116] The homogenized mixture was passed 20 times through a roll mill with the span between the rolls set at 0.1 mm and the speed ratio set at 1:1.5. Proper thermostatic regulation of the rolls was maintained at all times by a heating / cooling unit or by a refrigeration room only to keep the temperature of the rolls below 40° C.

[0117] The roller speed was set at 80 RPM and the process continued until the buff morphology dissolved and produced the standard "flake" morphology obtained by desulfurization.

[0118] The mixture was then discharged and sent to packaging before proceeding with a new processing batch.

[0119] <Example 7> Further examples were carried out in the same way as in Example 4, except that the mixture was processed in a multi-screw, planetary or ring extruder with 12 co-rotating screws (as shown in Figure 3) with a profile suited to the purpose. The unique feature of this extruder is that it allows optimal temperature control to avoid depolymerization phenomena, thanks to the high contact area with the cooled parts of the rolls. Cooling is also effective both outside and inside the planetary arrangement of the screws (Figure 4).

[0120] This extruder has been found to be particularly suitable for devulcanization reactions due to its properties of optimal thermal control. The screw profile used essentially provides a large proportion of conveying elements and a reduction in the amount of mixing elements concentrated in the final part, which would cause greater mechanical stresses and temperature rise. The speeds and pressures are similar to those of the normal extrusion process, but the specific geometry of the ring extruder itself has a larger intermeshing area and therefore achieves better mixing while keeping the temperature lower. In addition, this extruder makes it possible to obtain much higher "elongational flow" values ​​(Figure 5), with values ​​even 50% higher, compared to the normal twin-screw extruder. This allows more compression and decompression movements of the rubber, similar to what takes place in roll mills, and this type of mechanical stress has proven to be very effective for the highly selective devulcanization process.

[0121] The use of these extruders for preparing rubber blends, in which this particular temperature regime is utilized, is known from the state of the art (DE 10 2015 120 586 A1), but in contrast, the use for devulcanization is innovative.

[0122] 200 kg of ELT powder originating from truck treads, with particle sizes ranging from 0.5 mm to 3 mm, mainly equal to 1.2 mm, was weighed.

[0123] 2.5 wt % of oxalic acid-urea adduct in a 1:2 molar ratio and 0.25 wt % of dicumyl peroxide adsorbed on a 40% mixture of silica and calcium carbonate were added to the powder.

[0124] The mixture thus obtained was homogenized in a turbo mixer for 2 minutes without significantly increasing the temperature, which reached 40°C.

[0125] The homogenized mixture was extruded in a multi-screw extruder, also called a ring extruder or planetary extruder, always maintaining a temperature below the polymer depolymerization temperature. The operating temperature was maintained at values ​​ranging from 100°C to 150°C depending on the zone, the screw speed was equal to 600 RPM, the screw diameter was 30 mm and the L / D ratio was 55. The productivity was 350 kg / h.

[0126] The special configuration of this equipment allowed optimal control of the process. The extruded mixture was then sent for packaging.

[0127] As clearly explained, this particular extruder, unlike other types, allows efficient processing, providing optimal temperature control with the same applied stresses, excellent productivity and effective degassing if required.

[0128] <Example 8> This example was carried out in the same way as Example 7, except that the mixture was processed through a co-rotating twin screw extruder with specially profiled screws adapted for this purpose, in particular by increasing the number of conveying screw elements and balancing the number of comminuting and mixing elements in order to effectively control excessive temperature rise.

[0129] The process temperature was maintained at values ​​ranging from 80° C. to 270° C. depending on the zone, with a vacuum degassing of −850 mbar and a pressure of 7 bar. The extruder screw diameter was 35 mm, the L / D ratio was 48, the screw speed was about 300 RPM, and the productivity was equal to about 50 kg / h.

[0130] The presence of a sufficient number of elements with an inverse profile allowed for precise management of the melt pressure and made it possible to insert vacuum degassing devices for effective removal of gases. It was possible to deploy a final degassing in a particularly efficient way, at the front of the supply chain.

[0131] <Result analysis> The desulfurized products obtained at the end of Examples 1 to 8 were analyzed and were characterized by a degree of desulfurization ranging from 74% to 45%, a percentage increase of +80% to +32% relative to the product treated alone with the oxalic acid-urea adduct in a molar ratio of 1:2.

[0132] In particular, the desulfurization degree measured for the product treated only with the oxalic acid-urea adduct in a molar ratio of 1:2 was equal to 56% ± 1, whereas that of the product obtained according to Example 1 was equal to 74% ± 3, an increase of +32% (see FIG. 1), demonstrating a considerable improvement in terms of the desulfurization efficacy obtained with the desulfurization additive according to the invention compared to the state of the art.

[0133] In Example 2, the degree of desulfurization measured on the product treated only with the oxalic acid-urea adduct in a molar ratio of 1:2 was equal to 56% ± 1, whereas the degree of desulfurization of the product obtained according to Example 2 was equal to 64% ± 1, an improvement of + 14%.

[0134] In Example 3, the degree of desulfurization was not measured, but the processability of the blends and the performance obtained in the manufacture of new shoe soles resulting from scrap were measured. The processability of the mixtures treated with the desulfurization additive according to the invention turned out to be optimal, resulting in a percentage of reuse in new blends up to 66% higher than the state of the art.

[0135] The degree of desulfurization measured for the product treated only with the oxalic acid-urea adduct in a molar ratio of 1:2 at low temperature in Example 4 was equal to 13% ± 3%, whereas the degree of desulfurization of the product obtained according to Example 4 was equal to 63%, which is significantly improved.

[0136] In Example 5, the desulfurization degree was equal to 13% ± 3 using only the oxalic acid-urea adduct in a molar ratio of 1:2 at low temperature, as in Example 4, whereas the desulfurization degree of the product according to Example 5 is equal to 60%, which is significantly improved, also demonstrating the effectiveness and possibility of using inorganic peroxides for the purposes of the present invention.

[0137] In Example 6, the desulfurization measured on the product treated only with the oxalic acid-urea adduct in a molar ratio of 1:2 is equal to 31% ± 18, the wide scatter of the results being the result of the uneven grain size of the buffs, whereas the desulfurization of the product obtained according to Example 6 is equal to 45% ± 7, an improvement of + 45% in this example, but with a smaller scatter of results with the same buffing grain size, as well as for the desulfurization.

[0138] The results of examples 7 and 8 in multi-screw and twin-screw extrusion confirm the improvements of the previous examples carried out in the roll mill. To take into account the different processing methods with larger temperature fluctuations and a larger fraction of volatile gases removed by degassing, the degree of desulfurization is not evaluated, but rather the behavior during blending, resulting in a percentage of reuse in new blends up to 50% higher than the state of the art.

[0139] Analysis of the Horikx diagram (see FIG. 2) also indicates highly selective desulfurization with respect to both polysulfide, monosulfide and disulfide sulfur bonds, protecting the carbon-carbon bonds of the molecular chain from cleavage, thus preserving the molecular weight of the elastomer.

[0140] It can therefore be concluded that the desulfurization additive according to the invention makes it possible to obtain a desulfurized product characterized by a high degree of desulfurization compared to the prior art and at the same time a homogeneous and high quality of desulfurization.

[0141] In this patent application, base blend refers to a blend made using only elastomers and virgin polymers, fillers, additives and a specific vulcanizate package.

[0142] In this patent application, final blend refers to the base blend to which an amount of the desulfurization product according to the present invention has been added.

[0143] As indicated above, the thus obtained desulfurized product can be mixed with a base blend, i.e., a blend of elastomers and virgin polymers containing additives, fillers, vulcanizing agents and accelerators, with the desulfurized product being added in an amount ranging from 10% to 90% by weight, intended as a weight percentage relative to the weight of the final blend.

[0144] The final blends, when vulcanized, have mechanical and chemical properties very similar to the respective starting base blends made without the devulcanized products, i.e., using only elastomers and virgin polymers, additives, fillers, vulcanizing agents and accelerators, demonstrating that the devulcanized products of the present invention can be effectively devulcanized to obtain final blends that behave very similarly to blends made using only primary virgin components.

[0145] After analysing the results using the Horikx diagrams, which measure the degree of desulfurisation and the selectivity of desulfurisation according to standard ASTM D6814-02, a test campaign was carried out to verify the physico-mechanical properties of the final blends of the desulfurised products of the invention, in which they are used instead of elastomers and virgin polymers in proportions that depend on the application sector for which the final blends are intended.

[0146] According to the present invention, it is not important and may not even be possible to test the physical-mechanical properties of the devulcanized rubber / product itself. These properties must in fact be tested on test specimens obtained starting from a non-vulcanized blend to which the devulcanized rubber is added in a proportion ranging from 5% to 90% by weight (more generally 10% to 30% by weight) relative to the total weight of the final compound, the specific vulcanizate fillers and packages are added, and then the entire mixture is vulcanized, and then a test specimen is obtained from the final vulcanized and molded blend whose physical-mechanical properties are measured, which are then compared with the corresponding properties of a test specimen obtained in the same way from the corresponding base blend, but without the devulcanized rubber.

[0147] Therefore, the physical-mechanical properties of the devulcanized rubber are more important when tested in the specific final blends for various application areas, and the devulcanized rubber is added in various proportions depending on the criticality of the application. For example, in tires, it is preferable to limit the amount of devulcanized products present in the blend to a range of 10% to 30% by weight relative to the weight of the final blend, while in footwear, which has less stringent technical requirements, it is possible to use an amount of devulcanized products in the blend ranging from 50% to 80% by weight relative to the weight of the final blend.

[0148] More specifically, the use of devulcanized rubber from the ELT according to the invention was tested in blends for the following application areas:

[0149] - technical articles ("bushings", for example bushings for the automotive sector); - Tires, truck treads (examples of premium truck tires), - Tires, car treads (example of passenger car tires).

[0150] The use of reclaimed rubber from treated waste devulcanized according to the invention was also tested in the following areas: -Footwear (an example of a shoe sole).

[0151] Devulcanized rubber was added in amounts ranging from 10% to 50% by weight based on the total weight of the final blend depending on the application.

[0152] The final blends used in the various application areas vary greatly in chemical composition, mixing and vulcanization methods, and the addition of devulcanized rubber according to the invention allows its optimal reuse and has different effects depending on the type of blend.

[0153] The base blends made exclusively of elastomers and virgin polymers, fillers, additives and vulcanizates packages specific to the various application areas can be illustrated as shown in the table below.

[0154] [Table 2]

[0155] [Table 3]

[0156] [Table 4]

[0157] [Table 5]

[0158] The base blend (devulcanized product / no rubber) shown in the previous table represents the standard blend recipe for validation of physical mechanical tests in the blend. And many manufacturers of rubber blends use other products or additives or ingredients, often stored and at different concentrations compared to the examples. However, it should be noted that these differences will produce results consistent with those specified herein.

[0159] Therefore, a base blend (without devulcanized product / rubber as a reference) was prepared for each application field, and further, for comparison, a blend "urea-oxalic acid" was prepared containing different amounts of devulcanized product obtained with an oxalic acid-urea adduct, and finally, a blend containing different amounts of devulcanized product according to the invention, i.e. a blend obtained with a devulcanizing agent according to the invention ("Patent" blend).

[0160] The improvements in mechanical properties are given as a percentage and the results obtained for the base blend are normalized to 100.

[0161] The physical-mechanical properties presented were selected by identifying those that were most significant for a particular reference sector.

[0162] The standard methods used to measure these properties, which are shown in the accompanying figures, are as follows.

[0163] ASTM D6814-02(2018) % Desulfurization degree ASTM D2240-15(2021) Shore A Hardness ASTM D412-16(2021) Mpa Tensile ASTM D624-00(2020) N / mm tear UNI7716: 2000% rebound UNI9185:1988 mm3 wear ASTM D395-18(2018) % Compression Set ASTM D5992-96(2018) Mpa Dynamic ASTM D1646-19a(2019) MU Mooney Viscosity ASTM D5289-19a(2019) minute cure

[0164] As is evident from the attached Figure 6, for technical articles (e.g. "bushings"), significant increases in tear strength, tensile strength, rebound, scorch (t5) and cure efficiency (t90) were found. Furthermore, a crucial aspect for this application, the normalization of the Mooney viscosity and an improvement in the dynamic properties essential for the absorption of vibrations, were observed.

[0165] As is evident from the attached Figure 7, in the tire, truck tread - premium truck tire, a significant increase in tensile strength, tear resistance, modulus at 300% elongation, and normalization of wear were observed. Moreover, in an aspect of great importance for both these applications, an improvement in the appearance of the blends was observed (as shown in Figures 8 and 9) both in the raw blends (before vulcanization) and in the vulcanized blends, giving clear evidence of a better and more intense devulcanization. Comparing the photographs in Figures 8 and 9, it is evident that, in fact, the ELT particles are no longer visible in the blend bodies of the invention, whereas they are still visible in the state-of-the-art blends obtained with the oxalic acid-urea adduct.

[0166] As is evident from Figure 10, for the tire, Cartread - passenger tire, a significant increase in tensile strength, modulus at 100% elongation, in toughness was detected. Of particular interest to the sector is the increase over the criteria in tear resistance.

[0167] For footwear-soles, the desulfurized product obtained from the processing waste was added to the base blend in an amount of 50% by weight.

[0168] This sector does not offer particularly important technical-mechanical requirements, but rather requires that processability and recolorability (understood as the possibility to massively recolor blends containing the desulphurization products) be optimal.

[0169] It has been found that the reuse of the devulcanized products according to the state of the art leads to vulcanized blends that have an orange peel appearance and are difficult to re-pigment. The non-re-pigmentability is due to the fact that dyes do not penetrate the devulcanized products according to the state of the art well, so that the vulcanized blends show very pronounced streaks and aesthetic defects, even with a low percentage of devulcanized product.

[0170] On the other hand, the desulfurized product according to the invention has improved processability and colorability, e.g. recolorability to black or gray, is uniform and defect-free.

Claims

Claim 1: A devulcanizing additive for sulfur-vulcanized rubber, comprising: a carboxylic acid-base adduct obtained from a dicarboxylic organic acid having 2 to 18 carbon atoms and urea or a mono-substituted derivative, di-substituted derivative, or tri-substituted derivative of urea having the following formula; 【Chemical 1】 [wherein, R １ , R ２ and R ３ are the same as or different from each other, and may be hydrogen or a linear alkyl chain having 2 to 18 carbon atoms) a peroxide, where the peroxide is optionally mixed and absorbed in an inert inorganic filler; and optionally, a compatibilizer selected from ethylene-vinyl acetate copolymer, zinc dimethylacrylate, NR-g-PDMMMP (a graft copolymer of natural rubber (NR) and poly(dimethyl(methacryloyloxymethyl)phosphonate) (PDMMMP)), liquid butadiene-isoprene copolymer rubber, GMA and MAH (grafted polyolefin polymers), epoxidized natural rubber, trans-polyoctenamer (TOR), and mixtures thereof; wherein the carboxylic acid-base adduct is obtained from an organic dicarboxylic acid and urea or a mono-substituted derivative, di-substituted derivative, or tri-substituted derivative of urea in a molar ratio in the range of 1:1 to 1:2; the additive. Claim 2: The additive according to claim 1, wherein the carboxylic acid-base adduct is obtained from a dicarboxylic organic acid having 2 to 10 carbon atoms and urea or a mono-substituted derivative, di-substituted derivative, or tri-substituted derivative of urea in a molar ratio in the range of 1:1 to 1:

2. Claim 3: The additive according to claim 1, wherein the devulcanizing additive is solid. Claim 4: The additive according to claim 1, wherein the organic dicarboxylic acid is selected from oxalic acid, tartaric acid, and malic acid, and the base is urea. Claim 5: The additive according to claim 1, wherein the carboxylic acid-base adduct is an oxalic acid-urea adduct in a molar ratio of 1:

2. Claim 6: The additive according to claim 1, wherein the peroxide is selected from organic peroxides or inorganic peroxides. Claim 7: The additive according to claim 1, wherein the peroxide is mixed and absorbed in an inert inorganic filler selected from the group consisting of silica, calcium carbonate, kaolin, aluminum silicate, and clay. Claim 8: The additive according to claim 1, wherein the peroxide is dicumyl peroxide absorbed in calcium carbonate and silica. Claim 9: The additive according to claim 1, wherein the compatibilizer is trans-polyoctenamer (TOR). Claim 10: The additive according to claim 9, wherein the trans-polyoctenamer (TOR) is a powder or a pure granular material.

11. Use of the additive according to claim 1 for the desulfurization of sulfur-vulcanized rubber, wherein when the desulfurization additive consists of an adduct and a peroxide, the additive is added in an amount in the range of 0.5% to 5% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized, the adduct is added in an amount in the range of 0.4% to 4.5% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized, and the peroxide is added in an amount in the range of 0.1% to 3% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized. The use.

12. Use of the additive according to claim 9 for the desulfurization of sulfur-vulcanized rubber, wherein when the desulfurization additive consists of an adduct, a peroxide, and a compatibilizer, the additive is added in an amount in the range of 1% to 20% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized, the adduct is added in an amount in the range of 0.5% to 4.5% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized, the peroxide is added in an amount in the range of 0.1% to 3% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized, and the compatibilizer is added in an amount in the range of 0.4% to 19.4% by weight based on the weight of the sulfur-vulcanized rubber to be desulfurized. The use.

13. A batch process for the desulfurization of sulfur-vulcanized rubber, comprising the following steps: i) A step of mixing a desulfurization additive with the sulfur-vulcanized rubber to be desulfurized, wherein the desulfurization additive is an acid-base adduct obtained starting from a dicarboxylic organic acid having 2 to 18 carbon atoms and urea or a mono-substituted derivative, di-substituted derivative, or tri-substituted derivative of urea having the following formula, 【Chemical Formula 2】 [wherein, R １ , R ２ and R ３ are the same as or different from each other and may be hydrogen or a linear alkyl chain having 2 to 18 carbon atoms) and a peroxide, wherein the acid-base adduct is obtained starting from a dicarboxylic organic acid and urea or a mono-substituted derivative, di-substituted derivative, or tri-substituted derivative of urea in a molar ratio in the range of 1:1 to 1:2, and optionally a compatibilizer is added. ii) Optionally, a step of heating the mixture thus obtained. iii) A compression and mechanical stretching step, and iv) A step of repeating the compression and mechanical stretching step iii) a number of times in the range of 0 to 40. A method comprising the above steps.

14. The method according to claim 13, wherein steps iii) and iv) are carried out by controlling the temperature in the range of 20°C to 110°C.

15. A continuous process for the desulfurization of sulfur-vulcanized rubber, comprising the following steps: i) A step of mixing a desulfurization additive with the sulfur-vulcanized rubber to be desulfurized, wherein the desulfurization additive is an acid-base adduct obtained starting from a dicarboxylic organic acid having 2 to 18 carbon atoms, and urea or a mono-substituted derivative, di-substituted derivative or tri-substituted derivative of urea having the following formula, [Chemical Formula 3] [wherein, R １ , R ２ and R ３ are the same as or different from each other, and may be hydrogen or a linear alkyl chain having 2 to 18 carbon atoms) a peroxide, and the acid-base adduct is obtained starting from a dicarboxylic organic acid and urea or a mono-substituted derivative, di-substituted derivative or tri-substituted derivative of urea in a molar ratio in the range of 1:1 to 1:2, and optionally a compatibilizer is added, said step; ii) Optionally, a step of heating the mixture thus obtained, and iii) A step of extruding the desulfurization product thus obtained. A method comprising the above steps.

16. A method according to any one of claims 13 to 15, which provides for the use of the desulfurization additive according to any one of claims 2 to 12 in step i).

17. A desulfurization product in a proportion in the range of 5% to 100% by weight based on the weight of the blend, suitable for use as a raw material for obtaining a sulfur-vulcanized rubber by blending with a virgin elastomer, obtainable using the method according to claim 13.

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