Process for valorizing silicone elastomer waste for the production of recycled silicone materials

Abstract

The present invention relates to a process for treating a material comprising one or more silicone elastomer(s), the process comprising a step of decrosslinking the material in the presence of a metal catalyst and a solvent selected from the group consisting of alcohols, carboxylic acids, esters, ethers, and mixtures thereof, leading to the formation of a mixture comprising fragmented chains of silicone elastomer.

Description

Title of the invention: Process for the recovery of silicone elastomer waste for the production of recycled silicone materials TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method of treating a material comprising one or more silicone elastomer(s). STATE OF THE ART

[0002] Silicones, compounds based on silicone elastomer(s), are ubiquitous in several current industrial sectors, including construction, cosmetics, coatings, transportation, electronics and health. Thanks to their insulating, flexible and heat-resistant properties, they have become essential materials in our daily lives.

[0003] However, despite their prevalence, options for their recovery and recycling remain limited. In the domestic context, silicone waste is generally treated as non-hazardous waste, while the majority of industrial waste is disposed of as non-hazardous industrial waste (NIW), particularly through landfilling. This approach not only has environmental consequences, but also eliminates the possibility of reusing these expensive and high-value materials.

[0004] Other methods, namely the mechanical recovery of crosslinked silicone elastomers by grinding and micronization, have been explored, but they only allow low levels of incorporation in virgin formulations, generally less than 10% or even 5%. These products are therefore poorly represented in the industry.

[0005] The prior art also provides methods for the complete depolymerization of silicones back to cyclic monomers, or cyclosiloxanes, as illustrated in Scheme 1 below, in which a strong acid and then an inorganic base are used successively; other organic and metallic catalysts can also be employed. Two recent reviews summarize the state of the art on the use of electrophiles or nucleophiles to catalyze depolymerization. Using various catalysts and solvents, the degradation of silicones can reach moderate yields, usually around 50-60%. These cyclic monomers can then be reused in the manufacture of new silicones by ring opening polymerization (ROP).

[0006] [Chem.l] 1» R3SO3H ----p [Si(CH3)2~ÔU (I) * 2. MOK Silicone Scrap Cyclics Scheme 1: Example reaction for the production of cyclosiloxanes from silicone. Patent literature

[0007] Patent EP0523323 AlllaJ, from the company Tremco LLC, therefore proposes immersing pieces of silicone in an organic solvent and then carrying out a double step of acid and then basic catalysis at approximately 150°C to recover, after distillation, cyclosiloxanes.

[0008] Patent WO2014130948llbJ by Dow Corning describes a two-step depolymerization of a silicone gum, in the presence of montmorillonite and optionally a thermoplastic polymer. A first step in a screw conveyor initiates the depolymerization, which is completed in a conventional reactor. Cyclosiloxane yields can reach 100%.

[0009] Patent EP 10086 A2 of the company Yazaki Corp. depolymerizes a silicone material in the presence of dimethylcarbonate, a proton donor (methanol) and sulfuric acid, to produce a mixture of oligosiloxanes with a yield of more than 90%. After distillation of the dimethoxysilane, it is transformed into poly(methylhydrogensiloxane) in the presence of a dichlorosilane and a catalyst.

[0010] Patent EP1057855 from Yazaki Corp. proposes hydrolyzing a polysiloxane using a protonic solvent, for example methanol, in the supercritical state (240°C, 8 MPa) possibly in the presence of water. This digestion process is rapid (15 minutes), but the final mixture obtained is not described and this solution is a priori expensive.

[0011] Patent EP3744774 by the company Wacker proposes to 'digest' polysiloxanes via a mixture of weak (acetic acid) and strong (triflic acid) acids. The resulting product is an oligomer with terminal acetate functions. Open literature

[0012] Devulcanization of silicones by ultrasound

[0013] Ultrasonic devulcanization is a decrosslinking method proposed by Shim et al. to recycle vulcanized silicones. Devulcanization of precipitated silica-filled silicone rubber was performed using a continuous ultrasonic reactor. Significant differences were observed between the filled and unfilled systems. A decrease in the gel fraction and crosslink density was observed, allowing reprocessing and revulcanization of the silicone rubber. devulcanized. However, a degradation of the mechanical properties of the revulcanizates was observed for the filled rubbers. In order to improve the properties, the devulcanized filled rubbers were blended with virgin filled rubber, which led to a significant improvement in the performance characteristics of the blend revulcanizates. Usable mechanical properties could be achieved with only 25% virgin material.

[0014] In another study by Shim et al., the recycling of fumed silica-filled PDMS was investigated using an extruder equipped with a high-power ultrasonic sonotrode. The results showed that fumed silica strongly impacts the devulcanization process. Higher power is required to devulcanize fumed silica-filled PDMS compared to precipitated silica-filled PDMS, due to the strong interaction between the fumed silica particles and the PDMS chains. In addition, the influence of filler concentration on devulcanization was investigated, showing that scission of the main chains (Si-O) and crosslinked bridges (Si-C and CC) occurs simultaneously during the process. The nature of the silica and its concentration have major impacts on the properties of the devulcanized materials as well as the recycled materials and are therefore parameters to consider for silicone recycling.

[0015] Pyrolysis of silicones:

[0016] Silicone pyrolysis is a thermal recycling method that uses the chemical decomposition of polymers at high temperatures in the absence of oxygen. A study conducted by ZM. Qin et al. explored the use of titanium dioxide (anatase) as a catalyst for silicone pyrolysis. The results showed that the catalyst promotes the breaking of siloxane bonds and improves pyrolysis efficiency, even at relatively low temperatures (50-100 C). The volatile substances thus obtained are mainly cyclosiloxanes with 4 and 5 silicon atoms.

[0017] Chemical depolymerization of silicones:

[0018] In recent years, several new approaches in catalysis have been presented by different research teams to optimize the depolymerization of silicones.

[0019] In a study conducted by D. Krug and R. Laine, a method for recycling highly crosslinked thermosetting silicone resins was developed. The researchers demonstrated that small amounts of fluoride ions could efficiently depolymerize the silicone resins, producing oligomers that were easily recoverable and reusable.

[0020] V. Monteil's team presents an efficient method for solvent-free depolymerization of linear polydimethylsiloxanes using a [polydentate-silanolate ligand] complex, which promotes the chemical recycling of silicones into monomers. cyclics from many industrial substrates. This method requires only a small amount of catalyst (0.1% wt) for high yields of cyclosiloxanes (up to 98-99%) and at moderate temperatures (60-170 C).

[0021] Solvent-catalyzed chemical degradation has also been studied on rubber-type silicones (HCR for heat cured rubber). In this study, R. Petrus et al. showed the effectiveness of simple solvolysis using fatty acids on silicones with high molar masses. However, the repolymerization of these recyclates proved to be tricky. Indeed, this reaction leads to the formation of small functionalized -OR molecules of MDXM type, with l <x<4. La réaction utilise des aryloxydes métalliques à base de Mg-Na / K. Les rendements présentés vont de 17 à 79%.

[0022] The activity of various additives promoting equilibrium reactions of siloxanes is examined and quantified on model compounds in the work of DZ Khedaioui et al. They found in particular that the "superbase" phosphazene derivative P4-Bu can promote very rapid exchanges (a few seconds at 90°C) with the Si-O-Si bond even at low concentration (<0.1% by mass). This work shows that permanent silicone networks can be transformed into dynamic networks that can be reprocessed and recycled. However, this approach requires thermal evaporation of the introduced additives in order to restore the permanent nature of the silicone network. Indeed, "high temperature annealing degrades the additives and deactivates the dynamic properties of the silicone networks, causing them to revert to permanent networks.A simple rheological experiment and the corresponding model make it possible to extract the critical kinetic parameters to predict and control such deactivations. The properties of the silicones obtained remain limited and the process seems difficult to transpose industrially.

[0023] In their recent work, JL Torgunrud and SA Miller investigated the depolymerization of silicones by diols. Initially, a numerical approach allowed them to determine that hexylene diol (HG) appears to have a very favorable conformation for the chelation of a hydroxy-terminated PDMS. They were thus able to assume a thermodynamically driven reaction, with the product formed being much more stable (a hybrid cyclic monomer, M2HG or 2,2,4,4,6-pentamethyl-1,3-dioxa-2-silacyclohexane). These tests were confirmed in the laboratory, with distillation yields of up to nearly 80%. The authors subsequently investigated the recycling of PDMS from these degradation products by cationic ROP (cROP). The yields of this last step are between 21 and 67% but the technique has the advantage of being able to polymerize Mn chains of the order of 50,000 g / mol. Limitation of the above-mentioned processes

[0024] To date, most chemical approaches to silicone recycling propose recycling by total depolymerization to small monomers / oligomers for repolymerization to new polymer chains by repolymerization of these products. These methods are therefore mainly aimed at the treatment of end-of-life waste.

[0025] Another disadvantage is that the efficiency of these recycling methods is partial, since the fillers introduced into the silicone must be separated from the cyclic monomers; it is not made clear in the open and patent literature whether these fillers are reusable and / or reused.

[0026] A third disadvantage is that this method of completely depolymerizing silicones requires as much energy, and is as expensive, as producing silicone from virgin material.

[0027] In this context, it becomes evident that the silicone field is in urgent need of new recycling solutions. More specifically, there is a clear need for the development of methods that can not only facilitate the treatment of silicone elastomer waste, but also make their recycling energetically and economically viable. Summary of the invention

[0028] The present invention therefore relates to a method for treating a material comprising one or more silicone elastomer(s) and optionally fillers, the method comprising a step of decrosslinking the material in the presence of a metal catalyst and a solvent chosen from the group consisting of alcohols, carboxylic acids, esters, ethers, and mixtures thereof, leading to the formation of a mixture comprising fragmented chains of silicone elastomers and optionally the fillers originally present in the material. DETAILED DESCRIPTION OF THE INVENTION

[0029] Surprisingly, a method has been discovered for treating a material comprising one or more silicone elastomer(s) allowing their decrosslinking into fragmented chains of silicone elastomers. The fragmented chains of elastomers are of different weight-average molar masses and advantageously contain alkoxysilyl functions (ROSi) resulting from the reaction between a siloxane function (SiOSi) and an alcohol (ROH).

[0030] The present invention relates, more particularly, to a treatment method carried out under mild conditions in the presence of non-toxic, bio-sourceable and recyclable reagents, as well as preferably non-toxic metal catalysts.

[0031] The present invention makes it possible to reform silicone materials from one or more silicone elastomer(s) while reducing the energy consumption required. necessary for this synthesis and by limiting the number of post-processing steps required. This invention also makes it possible to recycle the entire original material, including the fillers originally present in the material, into a new material.

[0032] The proposed method allows for simple, rapid and economical treatment. It is also potentially less impactful for the environment and less energy-intensive than current state-of-the-art methods. For example, but without being bound by any limitation, the present invention is a new recovery route for materials comprising one or more silicone elastomer(s) from waste, scrap or production offcuts.

[0033] Material comprising one or more silicone elastomer(s)

[0034] The expression “silicone elastomer(s)” designates one or more polymer materials whose main matrix is ​​a polysiloxane, i.e. a chain of siloxane bonds (-SiR2-O-).

[0035] The material comprising one or more silicone elastomer(s) is typically composed of one or more crosslinked polymers. Typically, the silicone elastomers of the material have Mc, ranging from 1 to 100 kg / mol or from 2 to 50 kg / mol. Mc denotes the average molar mass of the polymer chains between crosslinking nodes. It is determined by swelling in a solvent, typically methylcyclohexane.

[0036] The material comprising one or more silicone elastomer(s) may be obtained by crosslinking by polycondensation, crosslinking by polyaddition or even radical crosslinking.

[0037] The material comprising one or more silicone elastomer(s) may be in the form of, for example, a tube, a pipe, a seal or a foam. Commonly, the material is a solid.

[0038] The material comprises at least 30% by mass of silicone elastomer(s), or at least 50% by mass or even from 30% to 80% by mass or even from 60% to 80% by mass of silicone elastomer(s) relative to the total mass of the material. The material may consist of silicone elastomer(s).

[0039] The silicone elastomers of the material may further comprise inorganic fillers.

[0040] The term "filler" refers to all the solid constituents, typically inorganic, added to the elastomer to provide specific properties and / or reduce the total cost of the elastomer. Generally, the "fillers" can be: reinforcing fillers (for example fumed silica, precipitated silica, etc.), semi-reinforcing fillers (for example calcium carbonate, carbon black, etc.) or non-reinforcing / diluting fillers (for example quartz, mica, etc.), as well as other additives (fire-resistant, anti-oxidant, etc.).

[0041] Typically, the silicone elastomers of the material have a quantity of fillers, typically inorganic, varying from 0 to 70% by mass, typically from 1 to 70% by mass, more frequently from 10 to 60% by mass or even from 20 to 40% by mass relative to the total mass of the material.

[0042] The material comprising one or more silicone elastomer(s) may originate from the production of silicones and may be, for example, waste, scrap, a post-expiration product, a production drop or a mixture of these different classes. The material may also originate from any end-of-life silicone object. These silicone objects may originate from, but are not limited to, the transport, construction, electronics, energy, health, cosmetics and / or special systems sectors.

[0043] Generally, the material is in the form of pieces having a size less than or equal to 10 cm, or even less than 1 cm and typically ranging from 0.1 to 10 cm, or even ranging from 0.1 to 1 cm. The term "size" designates the largest dimension of the fragment or piece. These fragment or piece sizes are typically obtained by mechanical processing.

[0044] Thus, the method according to the invention may comprise, before the step of decrosslinking the material, a mechanical treatment step aimed at homogenizing the physical characteristics of the material. Typically, the mechanical treatment step is carried out for a solid material comprising one or more silicone elastomer(s) in the form of pieces having a size greater than 10 cm with the aim of reducing and homogenizing the sizes of fragments or pieces.

[0045] Advantageously, the mechanical treatment step makes it possible to facilitate the implementation and to optimize the performance of the decrosslinking step.

[0046] The mechanical processing step can be carried out by cutting or grinding or any other methods known to those skilled in the art.

[0047] The pieces can also be washed.

[0048] The material obtained at the end of the mechanical treatment can be in the form of submillimetric particles. Typically, the particles have a size less than or equal to 1000 pm, or varying from 50 to 1000 pm. Decrosslinking step

[0049] The method according to the invention comprises a step of decrosslinking the material comprising one or more silicone elastomer(s) in the presence of a metal catalyst, a solvent, optionally a swelling solvent, leading to the formation of a mixture containing fragmented chains of silicone elastomers and optionally the fillers initially present in the material.

[0050] Commonly, the decrosslinking step is carried out at a temperature lower than or equal to 250°C, typically less than or equal to 200°C, or even less than or equal to 150°C, generally ranging from 40 to 150°C, or even from 50 to 120°C, or even ranging from 50 to 90°C.

[0051] The decrosslinking step is commonly carried out at atmospheric pressure. It can also be carried out at a pressure greater than or equal to 0.1 MPa, or even greater than or equal to 1 Mpa, generally ranging from 0.1 to 10 Mpa, or even from 0.15 to 4 Mpa or even from 0.2 to 2 Mpa.

[0052] Advantageously, the decrosslinking step makes it possible to carry out at low temperature, i.e. a temperature less than or equal to 250°C or even less than or equal to 200°C, a controlled decrosslinking of the material into a mixture of fragmented chains of silicone elastomers and possibly fillers, generally in the form of a paste. Solvent(s)

[0053] The solvent is selected from the group consisting of alcohols, carboxylic acids, esters, ethers and mixtures thereof. The solvent is commonly a protic organic solvent such as an alcohol and / or an acid.

[0054] The term "alcohol" designates a linear or branched molecule comprising from 1 to 20 carbon atoms and at least one (-OH) group. Alcohols can comprise from 1 to 12 carbon atoms, or from 1 to 8 carbon atoms, or even from 1 to 6 carbon atoms, for example from 1 to 4 carbon atoms or even 1, 2, 3 or 4 carbon atoms.

[0055] The alcohols are typically chosen from the group consisting of methanol, ethanol, propanol, iso-propanol, butanol, iso-butanol or sec-butanol and mixtures thereof.

[0056] The expression "carboxylic acid" designates a linear or branched molecule comprising from 2 to 20 carbon atoms and at least one (-COOH) group. Carboxylic acids may comprise from 2 to 12 carbon atoms, or from 2 to 8 carbon atoms, or even from 2 to 6 carbon atoms, for example from 2 to 4 carbon atoms or even 2, 3 or 4 carbon atoms. They may also comprise from 4 to 12 carbon atoms, in particular from 4 to 8 carbon atoms, in particular from 4 to 6 carbon atoms.

[0057] The carboxylic acids are typically selected from the group consisting of acetic acid, propanoic acid, butanoic acid, hexanoic acid, or lauric acid.

[0058] The term “ether” designates an alcohol in which the proton of the (-OH) group is replaced by a linear or branched carbon chain comprising from 1 to 20 carbon atoms and conventionally from 1 to 12 carbon atoms, or from 1 to 8 carbon atoms, or even from 1 to 6 carbon atoms, for example from 1 to 4 carbon atoms or 1, 2, 3 or 4 more carbon atoms

[0059] The ethers are typically selected from the group consisting of, but not limited to, diethyl ether, dimethyl ether, diisopropyl ether, dibutyl ether, diethyl ether, tert-butyl ether, diphenyl ether.

[0060] The term “ester” designates an acid in which the proton of the group (-COOH) is replaced by a linear or branched carbon chain comprising from 1 to 20 carbon atoms and conventionally from 1 to 12 carbon atoms, or from 1 to 8 carbon atoms, or even from 1 to 6 carbon atoms, for example from 1 to 4 carbon atoms or even 1, 2, 3 or 4 carbon atoms

[0061] The esters are typically selected from the group consisting of methyl, ethyl, propyl, butyl, 2-amyl acetates, tert-butyl ethyl acetate, phthalates.

[0062] A mixture of solvents may also be used to carry out in particular the decrosslinking step. Conventionally, the mixture is a mixture of at least two alcohols, one alcohol and one carboxylic acid or two carboxylic acids.

[0063] Commonly, the solvent is used in excess relative to the silicone elastomer(s). Typically, the solvent is introduced at a rate of 20 to 300% by mass of solvent, or 50 to 200% by mass of solvent, or even 80 to 150% by mass of solvent relative to the mass of the material.

[0064] Optionally, the decrosslinking step can be carried out, in addition, in the presence of a swelling solvent. The swelling solvent is commonly an aprotic solvent. It is used as a swelling agent for the material comprising one or more silicone elastomer(s). The swelling solvent can advantageously facilitate the decrosslinking of the material.

[0065] The swelling solvent may be selected from the group consisting of alkanes, cycloalkanes, alkenes, cycloalkenes, aromatics and mixtures thereof.

[0066] The term "alkane" designates a saturated, linear or branched hydrocarbon molecule of general formula CnH2n+2 with n between 1 and 20 carbon atoms. Alkanes in the liquid state at room temperature can comprise from 5 to 12 carbon atoms, or from 5 to 8 carbon atoms, or even 5 or 6 carbon atoms.

[0067] The term “cycloalkanes” designates an alkane as defined above comprising at least one cycle, of general formula CnH2n.

[0068] The term “alkene” designates an unsaturated hydrocarbon molecule corresponding to an alkane as defined above, furthermore having at least one carbon-carbon double bond. Typically, an alkene comprising a carbon-carbon double bond has the formula CnH2n.

[0069] The term “cycloalkene” designates a cycloalkane as defined above which has furthermore at least one carbon-carbon double bond.

[0070] The term “aromatic” designates a molecule comprising from 5 to 20 carbon atoms or from 6 to 12 carbon atoms and at least one aromatic nucleus, such as for example toluene.

[0071] More particularly, the swelling solvent is selected from the group consisting of, but not limited to, pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane, toluene and mixtures thereof.

[0072] The swelling solvent may be present at a mass percentage ranging from 25% to 75% by mass of swelling solvent relative to the total mass of the material. Metal catalyst

[0073] The metal catalyst generally comprises a metal selected from the group consisting of alkalis, metalloids, transition metals, post-transition metals, lanthanides, actinides and mixtures thereof. The metal may be selected from the group consisting of: Bi, Na, K, B, Mg, Al, Ca, Bi, Ti, Mn, Fe, Co, Cu, Ni, Cu, Sn, Zn, Zr, Hf and mixtures thereof or from the group consisting of Sn, Zn, Al, Cu, Co, Ni, Zr, Ti, Hf, Bi and mixtures thereof. Typically, the metal is Zn or Sn. Generally the oxidation state of the metal of the metal catalyst varies from (II) to (IV) depending on the nature of the metal.

[0074] The metal catalyst can be any catalyst that can catalyze the siloxane bond breaking reaction (Si-O-Si).

[0075] The metal catalyst may be selected from the group consisting of tin octoate, dioctyltin dineodecanoate (Sn(neo)2Oct2), dibutyltin dilaurate (DBTL), dibutyltin diacetate (DBTA), zinc bis(2,4-pentanedionato) (Zn(acac)2), bismuth tris(neodecanoate) and a mixture thereof.

[0076] The decrosslinking step is typically carried out in the presence of 0.1 to 20% by mass of metal catalyst, preferably 0.5 to 10% by mass or even 1 to 5% by mass of metal catalyst relative to the initial mass of material comprising one or more silicone elastomer(s).

[0077] Without being bound by any theory, the decrosslinking step consists of cutting the silicone chains in places, and this is not limited to the crosslinking nodes. For example, when the solvent is an alcohol, in the silicone elastomer a di-siloxane bond (Si-O-Si) is replaced by two alkoxysilyl bonds (2Si-OR), as illustrated in diagram 2 below. This makes it possible to move from a solid 3-dimensional chemical network of the elastomer to individual fragmented chains constituting a liquid, in which the presence of the fillers generally induces a significant viscosity.

[0078] [Chem.2]

[0079] Diagram 2: Example of reaction responsible for the decrosslinking of silicone elastomers. Fragmented chains of silicone elastomers

[0080] The decrosslinking step results in obtaining a mixture of fragmented chains of silicone elastomers and possibly fillers initially present. Typically, the fragmented chains of silicone elastomers of the mixture obtained at the end of the decrosslinking have weight-average molar masses, Mw, less than or equal to 1,000 kg / mol, preferably less than or equal to 500 kg / mol or even less than or equal to 300 kg / mol, or even less than or equal to 100 kg / mol.

[0081] Typically, a portion of the fragmented chains of silicone elastomers, designated part A or fragmented chains A, has a weight-average molar mass of less than 5 kg / mol, or even less than or equal to 2 kg / mol, or even less than or equal to 1 kg / mol, as determined by SEC. Conventionally, part A of the fragmented chains has a weight-average molar mass varying from 0.1 to 2 kg / mol or even from 0.1 to 1 kg / mol. Part A of the fragmented chains is commonly soluble and in solution in the solvent(s) used during the decrosslinking step.

[0082] Typically, a portion of the fragmented silicone elastomer chains, designated part B or fragmented silicone elastomer chains B, has a weight-average molar mass varying from 5 to 1000 kg / mol, or even varying from 5 to 500 kg / mol or from 10 to 300 kg / mol or even from 10 to 100 kg / mol. Part B of the fragmented chains is commonly in the form of a viscous phase, also designated viscous paste or pasty phase.

[0083] Generally, the B part of the fragmented chains is insoluble in the solvent, and can be recovered in the form of a viscous phase also containing the filler(s), typically inorganic.

[0084] Generally, the mixture of fragmented silicone chains is typically made up of:

[0085] A) from 0 to 100% by mass, typically from 10 to 80% by mass, or even from 30 to 70% by mass or even from 40 to 60% by mass (relative to the initial mass of the material) of fragmented chains A of silicone elastomer having a weight-average molar mass, Mw, of less than 5 kg / mol;

[0086] and / or

[0087] B) from 0 to 100% by mass, typically from 20 to 90% by mass, or even from 30 to 70% by mass or even from 40 to 60% by mass (relative to the initial mass of the material) of fragmented chains B of silicone elastomers having a weight-average molar mass, Mw, greater than or equal to 5 kg / mol.

[0088] Without being linked to any theory, the proportions between the fragmented chains A and the fragmented chains B can be controlled according to the conditions of the decrosslinking step and in particular the quantity of metal catalyst used. For example, the greater the quantity of metal catalyst, the greater the proportion of fragmented chains of part A with a weight-average molar mass, Mw, less than 1 kg / mol.

[0089] Part B of the fragmented chains obtained at the end of the decrosslinking step may comprise a quantity of filler varying from 0 to 70% by mass, typically from 1 to 70% by mass or from 10 to 60% by mass or even from 20 to 40% by mass relative to the initial mass of material. The fillers are as defined above. Generally, the quantity of fillers in the mixture of fragmented chains of silicone elastomers is identical to the quantity of fillers present in the initial material.

[0090] Commonly, the weight-average molar masses of the fragmented silicone elastomer chains can be determined by size exclusion chromatography (SEC). The nature of the fragment obtained and in particular the nature of the functions resulting from the decrosslinking reactions can be determined by nuclear magnetic resonance analysis of silicon (29Si NMR), carbon (13C NMR) and proton ('H NMR). Optional separation step

[0091] The process may comprise, at the end of the decrosslinking step, a separation step leading to the production of fragmented chains of silicone elastomers in liquid form and / or viscous paste.

[0092] The separation step can be carried out for example by drying, filtration, decantation, distillation or any other method known to those skilled in the art.

[0093] Conventionally, the fragmented chains of recovered silicone elastomers are - on the one hand in part A as defined above, typically in liquid form;

[0094] and - on the other hand in part B as defined above, typically in the form of a viscous paste.

[0095] The fragmented chain paste B obtained by the above process may contain less than 10% by mass of fragmented chains A defined above relative to the total mass of silicone paste, typically less than 1% by mass, or even less than 0.1% by mass or even less than 0.01% by mass.

[0096] The separation step may also comprise a step of recovering part or all of the solvent used during the decrosslinking step. Typically, the solvent may be recovered by distillation, or any other method known to those skilled in the art. The solvent thus recovered may be recycled, for example, in the decrosslinking step of the process according to the invention.

[0097] The fragmented chains of silicone elastomers obtained by the process according to the invention, whether part A or part B, can be used as a base for the formulation of various silicone products such as, for example, sealants, recycled raw material (RPM) or any other silicone-based product, including paints, coatings, adhesion primers, antifoams, solid foams.

[0098] The invention also relates to the paste of fragmented chains of silicone elastomers obtained or capable of being obtained by the above treatment method. It also relates to the solutions containing the fragmented chains A of silicone elastomers.

[0099] Size exclusion chromatography (SEC) and silicon nuclear magnetic resonance (29Si NMR) analyses are used to determine the proportions and weight-average molar masses of the fragmented silicone elastomer chains obtained according to the invention and in the examples below.

[0100] The process according to the invention therefore offers the possibility of effectively degrading by decrosslinking a wide range of silicone elastomers, in particular addition and condensation RTV types, addition and radical HTVs, as well as addition LSRs.

[0101] The fragmented chains of silicone elastomers obtained using the process according to the invention can be used in admixture or not with virgin material for the manufacture of various products such as sealants, adhesion agents, water repellent precursors and coatings. This versatility opens up new perspectives for the recovery of silicone waste and contributes to a more sustainable approach in the production of innovative materials.

[0102] For the purposes of the present invention, the various embodiments presented throughout the description may be used alone or in combination with each other, without limitation of combination. EXAMPLES

[0103] The following examples illustrate a particular embodiment of the invention without limiting its scope. Materials

[0104] The expressions used in the examples below have the following meanings: • The term "condensation silicone" refers to a two-component cold-curable silicone (RTV2) (RTV for 'room temperature vulcanizing', 2 ​​because two components are mixed) from the Elastosil® range marketed by the Wacker company, a material cured by tin-catalyzed polycondensation. This condensation silicone is representative of entry-level silicones used for stuffing and encapsulating electronic equipment, for example. It is a hard silicone with limited mechanical properties (typically 40 Shore A hardness, 1.8 MPa breaking stress and 130% strain at break in tensile testing). • The term “addition silicone” refers to the two-component RTV 3428 silicone from the Bluesil® range marketed by Elkem, a material crosslinked by platinum-catalyzed polyaddition. This addition silicone is a flexible silicone (30 Shore A) with good tear strength (20 kN / m) and high mechanical properties (6 MPa / 600% at break by tensile test). • The term “transparent silicone” refers to Bluesil® RTV 141 silicone marketed by Elkem and crosslinked by platinum-catalyzed polyaddition. This silicone is hard (50 Shore A), unfilled (no silica, composed mainly of silicone resins) and has acceptable mechanical properties (6 MPa, 120% at break in tension). • The term “radical silicone” refers to the Elastosil® R400 silicone marketed by Wacker, which crosslinks by radical reaction in the presence of peroxide. This silicone is representative of rubber-type silicones used for demanding applications. It has very high mechanical properties: hardness of 60 Shore A, 11 MPa / 500% at break in tension and good tear resistance of 26 kN / m.

[0105] The catalysts used in these examples are as follows: • Cat 1 designates Sn(neo)2Oct2: dioctyltin bis neodecanoate (68299-15-0) marketed either in the form of a ready-to-use hardener (T40, Wacker) where it is mixed with a crosslinking agent (tetraethylorthosilicate, TEOS) in respective proportions of 30 / 70% by mass, or pure (TIB KAT 318, TIB Chemicals) for example. • Cat 2 designates Sn(EHO)2: Tin(II) 2-ethylhexanoate (301-10-0), TK 129 (TIB Chemicals) • Cat 3 designates Zn(acac)2: Zinc acetylacetonate hydrate (108503-47-5), Bis(2,4-pentanedionato)zinc (Merck / Sigma Aldrich) • Cat 4 designates Ti(OBu)4: titanium tetrabutoxide (5593-70-4) or orthotitanic acid tetrabutyl ester (TNBT, Tyzor® TBT) or tetrabutyl ortho-titanate (Merck / Sigma Aldrich).

[0106] The post-crosslinking agent used in these examples is as follows: • Crosslinker 1 designates PDMS-OH 4K: dihydroxy-polydimethylsiloxane 4 kg / mol General protocol

[0107] The material comprising one or more silicone elastomer(s) is previously packaged in parallelepiped-shaped pieces of approximately 2 mm on each side.

[0108] The conditioned material (20 g) is introduced into a stainless steel reactor provided with a resistance heating jacket controlled by a regulation unit. The solvent (50 mL) and the metal catalyst (1, 5 or 10% by mass) are then added. The reactor is locked by cross-clamping. The mixture is heated to the decrosslinking temperature (90°C, 120°C or 150°C) for a time varying from 1 to 7 days, and at a pressure of 0.2 MPa, 0.8 MPa or 1.1 MPa respectively depending on the decrosslinking temperature. At the end of the decrosslinking step, the reactor is cooled for 30-45 minutes to approximately 70°C.

[0109] The mixture obtained at the end of the decrosslinking is separated into a liquid phase and a pasty phase. These two phases are analyzed by size exclusion chromatography (SEC) and by silicon nuclear magnetic resonance (29Si NMR). Example 1:

[0110] Tests 1 to 6 were carried out with condensation silicone in accordance with the general protocol under the conditions summarized in Table 1 below. [YES] [Tables 1] Tests Solvent Catalyst (% by mass) Temperature (°C) Time (h) 1 ethanol Cat 2 (10) 90 96 2 ethanol Cat 2(1) 120 36 3 ethanol Cat 2 (10) 150 24 4 isopropanol Cat 1 (5) 90 24 5 isopropanol Cat 3 (5) 90 168 6 butanol Cat 2 (5) 90 24

[0112] The quantities of catalyst are expressed in % by mass relative to the mass of material introduced in the decrosslinking step.

[0113] Tests 1 to 6 above all made it possible to obtain fragmented chains of silicone elastomers after a decrosslinking time varying from 1 to 7 days. These results show that the process according to the invention makes it possible to decrosslink under mild conditions (temperature, % catalyst, solvent) a material comprising a condensation silicone elastomer.

[0114] At the end of the decrosslinking step, the mixture obtained was separated into a liquid phase and a pasty phase.

[0115] As an example for test 2, after separation, the following were obtained:

[0116] - a pasty phase whose mass corresponds to 60% of the initial mass of the material (condensation silicone). The pasty phase is composed of fragmented chains of silicone elastomers having an Mw ranging from 5 to 300 kg / mol. This pasty phase may contain traces (<0.1% by mass) of fragmented chains of silicone elastomers with an Mw less than 1 kg / mol;

[0117] and

[0118] - a liquid phase after evaporation of the solvent whose mass corresponds to 40% of the initial mass of the material (condensation silicone). The liquid phase is composed of fragmented chains of silicone elastomers having an Mw less than 2 kg / mol. Example 2

[0119] Tests 7 and 8 were carried out with addition silicone in accordance with the general protocol under the conditions summarized in Table 2 below.

[0120] [Tables2] Tests Solvent Catalyst (% by mass) Temperature (°C) Time (h) 7 methanol Cat2(l) 90 96 8 ethanol Cat2(10) 150 15

[0121] Tests 7 and 8 above made it possible to obtain fragmented chains of silicone elastomers after a decrosslinking time varying from 1 to 4 days. These results show that the process according to the invention also makes it possible to decrosslink a material comprising an addition silicone elastomer under mild conditions in terms of temperature and quantity of catalyst used.

[0122] At the end of the decrosslinking step, the mixture obtained was separated into a liquid phase and a pasty phase.

[0123] As an example for test 7, after separation, the following were obtained:

[0124] - a pasty phase corresponding to 90% of the initial mass of the material (silicone of condensation). The pasty phase is composed of fragmented chains of silicone elastomers having an Mw varying from 5 to 300 kg / mol. This pasty phase may contain traces of fragmented chains of silicone elastomers with an Mw less than 1 kg / mol;

[0125] and

[0126] - a liquid phase after evaporation of the solvent corresponding to 10% of the mass initial phase of the material (condensation silicone). The liquid phase is composed of fragmented chains of silicone elastomers with an Mw of less than 2 kg / mol.

[0127] Example 3:

[0128] Tests 9 and 10 were carried out with other silicone materials in accordance with the general protocol under the conditions summarized in Table 3 below.

[0129] [Tables3] Tests Silicone Solvent Catalyst (% by weight) Temperature (°C) Time (h) 9 Addition transparent ethanol Cat 2 (10) 150 45 10 Radical ethanol Cat 2 (10) 150 18

[0130] Tests 9 and 10 demonstrate that the process according to the invention also allows the decrosslinking of silicones either crosslinked by radical reaction with peroxide, or uncharged and crosslinked with platinum.

[0131] Thus, the method according to the invention makes it possible to treat all types of materials comprising one or more silicone elastomer(s). Example 4:

[0132] Recycling tests (reformation of a network) were carried out from pastes obtained according to the general protocol described above. Decrosslinking tests 11 to 13 were carried out on the RTV condensation silicone described above. The populations of chains obtained were characterized by SEC toluene.

[0133] [Tables4] Tests Solvent Catalyst (% by mass) Temperature (°C) Time (h) 11 ethanol Cat 2 (1.5) 150 28 12 ethanol Cat 2 (2.5) 150 24 13 ethanol Cat 2 (2) 150 16

[0134] The characteristics of the fragmented chains of silicone elastomers obtained are presented in Table 5 below.

[0135] [Tables5] Tests Mw Fragmented chains B (kg / mol) Mw Fragmented chains A (g / mol) Population ratio B / A (%mass) 11 5-500 <800 75 / 25 12 5-115 <800 82 / 18 13 5-400 <1000 90 / 10

[0136] The fragmented silicone elastomer chains from Part B were used to prepare new silicone elastomers under the conditions shown in Table 6 (tests at a temperature of 80°C, for 48 hours).

[0137] The results presented correspond to a material composed of 100% recycled raw material (RPM), i.e. from parts B of tests 11 to 13 presented above.

[0138] [Tableauxô] Tests Fragmented chains B from Catalyst (% by mass) Crosslinker (% by mass) Time (h) 14 Test 11 Cat 4(5) 0 48 15 Test 11 Cat 2(1) crosslinker 1 (1.5) 48 16 Test 11 Cat 2(1) crosslinker 1 (2) 48 17 Test 12 Cat 2(1) crosslinker 1 (2) 48 18 Test 13 Cat 2(1) crosslinker 1 (2) 48

[0139] [Tables?] Test Strain at break (%) Stress at break (MPa) Initial condensation silicone (Control) 130 1.8 14 60 0.97 15 50 1.16 16 75 0.69 17 130 0.6 18 120+20 0.6+0.2

[0140] Tensile tests, to assess deformation and / or breaking stress, have were carried out on an Instron® machine equipped with a 500 N load cell and a video extensometer at a speed of 100 mm / min on type 5A dumbbell specimens (ISO 527-2). The strain at break and the stress at break are measured according to ASTM D412 with a crosshead speed of 100 mm / min.

[0141] The results obtained show that after a recycling cycle by the present invention, the materials have stress and strain values ​​at break representing 30 to 60% of those of the initial material. REFERENCES

[0142] [1] (a) TW Greenlee, Process for recycling silicone scraps and products relating thereto, EP0523323A1, 1992; (b) DJ Julian, D. Katsoulis, BA Link, TA Peitz, B. Zhu, Method of recycling silicone waste with the use of organic polymer and depoly-merization catalyst, WO2014130948A1, 2014.

[0143] [2] (a) Rupasinghe, B. and Furgal, JC (2022), Degradation of silicone-based materials as a driving force for recyclability. Polym Int, 71, 521-531; (b) B. Rupasinghe, Recycling Silicone-Based Materials: An OverView of Methods, IntechOpen (2023).

[0144] [3] T. Kawamoto, Process for recycling silicone compounds, EP1008621A2, 1999.

[0145] [4] T. Kawamoto, Method of recycling crosslinked silicone compound waste, EP1057855A1, 2000.

[0146] [5] W. Knott, H. Dudzik, D. Schaefer, Method for recycling of silicones, EP3744774B 1,2021.

[0147] [6] S. E. Shim, A. Isayev, Ultrasonic devulcanization of precipitated silica-filled silicone rubber, Rubber Chem. Technol. 2001, 74, 303-316.

[0148] [7] S. E. Shim, V. V. Yashin, A. I. Isayev, Environmentally-friendly physico- chemical rapid ultrasonic recycling of fumed silica-filled poly(dimethyl siloxane) vul-canizate, Green Chemistry, 2004, 6, 291-294.

[0149] [8] Z.-M. Qin, P.-C. Wang, R. Yang, H.-B. Chen, Fast pyrolysis of silicones at low températures catalyzed by anatase titanium dioxide, Polym. Degrad. Stab. 2020, 182, 109387.

[0150] [9] D. J. Krug, M. Z. Asuncion, R. M. Laine, Facile Approach to Recycling Highly Cross-Linked Thermoset Silicone Resins under Ambient Conditions, ACS Oméga 2019, 4, 3782-3789.

[0151]

[10] N. D. Vu, A. Boulegue-Mondière, N. Durand, J. Raynaud, V. Monteil, Back- to-cyclic-monomers-chemical-recycling-of-silicone-wastes-using-a-polydentate-ligand -potassium-silanolate-complex, Green Chem. 2023, 25, 3869-3877.

[0152]

[11] R. Petrus, J. Utko, R. Gnilka, M. G. Fleszar, T. Lis, P. Sobota, Solvothermal Al- coholysis Method for Recycling High-Consistency Silicone Rubber Waste, Macro- molécules 2021, 54, 2449-2465.

[0153]

[12] D. Z. Khedaioui, C. Tribout, J. Bratasanu, F. D'Agosto, C. Boisson, D. Montamal, Deciphering Siloxane Bond Exchanges: From a Molecular Study to Vitri-merization and Recycling of Silicone Elastomers, Angew. Chem. Int. Ed. Engl. 2023, 62, e202300225.

[0154]

[13] J. L. Torgunrud, A. M. Reverôn Pérez, E. B. Spitzberg, S. A. Miller, Entropy- Driven Depolymerization of Poly(dimethylsiloxane), Macromolecules 2023, 56, 10, 3668-3678.

[0155]

[14] ecousarecycling.com / silicone-rubber-recycling-process. Reached on 06 / 23 / 2023

[0156]

[15] E. Delebecq, N. Hermeline, A. Fiers, F. Ganachaud, Looking over Liquid Silicone Rubbers: (2) Mechanic Properties vs Network Topology, ACS Appl. Mater. Interfaces, 2012, 4, 3353-3363.

Claims

Claims

1. A method of treating a material comprising one or more silicone elastomer(s), the method comprising a step of decrosslinking the material in the presence of a metal catalyst and a solvent chosen from the group consisting of alcohols, carboxylic acids, esters, ethers, and mixtures thereof, leading to the formation of a mixture comprising fragmented chains of silicone elastomers.

2. Treatment method according to claim 1, in which the silicone elastomer(s) have(s) average molar masses of the polymer chains between crosslinking nodes, Mc, ranging from 1 to 100 kg / mol, preferably from 2 to 50 kg / mol.

3. A treatment method according to any one of claims 1 or 2, wherein the silicone elastomers of the material further comprise fillers.

4. Treatment method according to any one of claims 1 to 3, wherein the decrosslinking step is carried out at a temperature less than or equal to 250°C, preferably less than or equal to 200°C, preferably less than or equal to 150°C.

5. Treatment method according to any one of claims 1 to 4, in which the decrosslinking step is carried out at a pressure greater than or equal to 0.1 MPa, preferably ranging from 0.2 to 2 MPa.

6. Treatment method according to any one of claims 1 to 5, in which the solvent is chosen from the group consisting of methanol, ethanol, propanol, isopropanol, butanol, iso-butanol or sec-butanol and mixtures thereof.

7. A treatment method according to any one of claims 1 to 6, wherein the decrosslinking step is further carried out in the presence of a swelling solvent selected from the group consisting of alkanes, cycloalkanes, alkenes, cycloalkenes, aromatics and mixtures thereof.

8. A treatment method according to any one of claims 1 to 7, wherein the metal catalyst comprises a metal selected from the group consisting of alkali metals, metalloids, transition metals, post-transition metals, lanthanides, actinides and mixtures thereof, preferably from the group consisting of: Bi, Na, K, B, Mg, Al, Ca, Bi, Ti, Mn, Fe, Co, Cu, Ni, Cu, Sn, Zn, Zr, Hf and mixtures thereof.

9. Treatment method according to any one of claims 1 to 8, in which the fragmented chains of silicone elastomers obtained at the end of the decrosslinking step have weight-average molar masses, Mw, less than or equal to 1000 kg / mol, preferably less than or equal to 500 kg / mol.

10. Treatment method according to any one of claims 1 to 9, in which the mixture of fragmented chains of silicone elastomers resulting from step (1) is composed of: A) from 0 to 100% by mass, typically from 10 to 80% relative to the total mass of the material, of fragmented chains of silicone elastomers having a weight-average molar mass, Mw, less than or equal to 5 kg / mol; and / or B) from 0 to 100% by mass, typically from 20 to 90%, or even from 30 to 70% relative to the total mass of the material, of fragmented chains of silicone elastomers having a weight-average molar mass, Mw, greater than 5 kg / mol.