Organopolysiloxane depolymerization

Abstract

The present application relates to a process for preparing cyclic organopolysiloxane OC by depolymerization of at least one organopolysiloxane O selected from: - linear organopolysiloxanes, - branched organopolysiloxanes, - crosslinked organopolysiloxanes, or mixtures thereof; in the presence of a solvent, catalyzed by an alkali salt SA selected from alkali silanolates, alkali siliconates, or mixtures thereof, in which the amount of alkali salt used is between 0.01 and 2% by weight relative to the weight of organopolysiloxane O.

Description

[0001] DESCRIPTION

[0002] TITLE: Depolymerization of organopolysiloxane

[0003] Technical field:

[0004] The present invention relates to a process for the preparation of cyclic organopolysiloxanes OC by depolymerization of organopolysiloxanes O in the presence of a catalyst system comprising an alkali salt under mild operating conditions.

[0005] For the purposes of the present invention, the term “mild operating conditions” means a reaction carried out at atmospheric pressure and at a temperature less than or equal to 120°C.

[0006] Technological background:

[0007] A major societal and industrial challenge for the coming years is the ecological transition. Thus, from an economic perspective, the so-called "linear" economic model (take, make, use, throw away) must eventually be replaced by a so-called circular model.

[0008] A circular economy describes an economic system based on models that replace the concept of "end-of-life" products with the reduction, alternative reuse, recycling and recovery of these materials in the production / distribution and consumption processes, with the aim of achieving sustainable development for the benefit of current and future generations.

[0009] With the ever-increasing silicone market and the increasing number of applications for such materials, recycling used silicone products is a major challenge in the silicone industry.

[0010] Silicone polymer recycling could reduce gas emissions (CO, CO2) from the silicone industry by 75% and silicone waste by 65%. One of the recycling routes being considered is the depolymerization of organopolysiloxanes to produce cyclic organopolysiloxanes (or cyclosiloxane).

[0011] Historically, in the 1970s, processes for preparing cyclic organopolysiloxanes were developed to meet the growing demand for silicones, as illustrated by patent application FR2419297. This patent application discloses a process for preparing cyclic organopolysiloxane, and more specifically octamethylcyclotetrasiloxane (D4). This method involves a reagent composed mainly of cyclic organopolysiloxanes brought into contact with alcoholic potash. During this reaction, by-products such as hexamethylcyclotrisiloxane and other unwanted cyclic organopolysiloxanes are distilled and separated from octamethylcyclotetrasiloxane. These by-products are then continuously reintroduced into the reaction medium to promote the formation of octamethylcyclotetrasiloxane.

[0012] Other alternative processes were subsequently developed as evidenced by patent application EP0223513. This method relates to the production of cyclic organopolysiloxanes (47 to 75% of a mixture) from the product resulting from the hydrolysis of chlorosilanes which is brought into contact with potassium hydroxide.

[0013] In 2002, patent US6448360 discloses a process for preparing cyclic organopolysiloxanes from a reaction mixture comprising a cyclic organopolysiloxane, a linear organopolysiloxane, an alkaline catalyst such as potassium trimethylsilanolate. This preparation process aims to prepare functionalized cyclic organopolysiloxanes such as hexenylmethylcyclosiloxane.

[0014] It is known, in particular from US5670689, the depolymerization of silanol-terminated polydimethylsiloxane (PDMS) in the presence of strong alkali hydroxide or quaternary hydroxide. The reaction is carried out at 140°C to obtain a 90% yield of a mixture of cyclic organopolysiloxanes. However, a high catalyst loading is required, in particular at least 2% by weight relative to the weight of PDMS-OH.

[0015] Recently, patent application WO2009145837 discloses a method for treating polymeric materials such as silicones, highly charged or not, to recover monomers such as cyclic organopolysiloxanes. The method comprises carrying out the depolymerization reaction under heating and at reduced pressure in a device having a heated side wall, an agitator and at least one additional heated structure, other than the internally heated side wall, and means for forming a thin coating of material treated in the container on said heated side wall. A polymeric material is introduced into this device heated to a sufficient temperature in the presence of about 15% of alkali salt relative to the mass of polymer introduced to cause depolymerization of the polymeric material into monomers.This process is not satisfactory for working under mild conditions and requires continuous distillation during the implementation of this process.

[0016] Furthermore, the batch depolymerization process in the presence of tetra-n-butylammonium fluoride disclosed in patent application WO2022 / 165305 is also known to those skilled in the art. This process allows cyclic organopolysiloxanes to be obtained from different silicones. However, this process requires carrying out the reaction in a very dilute medium for 24 hours, which leads to low productivity.

[0017] Recently, the scientific article Green Chem., 2023,25, 3869-3877, discloses a depolymerization process by reactive distillation (high temperature and reduced pressure) in the presence of a catalytic system comprising an alkali salt and a multidentate complexing agent. This industrially adaptable process makes it possible to obtain a good yield of cyclic siloxanes, in particular D3 (hexamethylcyclotrisiloxane), D4 (octamethylcyclotetrasiloxane) and D5 (decamethylcyclopentasiloxane).

[0018] There is therefore an interest in providing an optimized process allowing the depolymerization of silicones efficiently in a solvent medium, under mild conditions and also allowing a selective yield of D4(octamethylcyclotetrasiloxane) to be obtained.

[0019] Summary of the invention:

[0020] An objective of the present application is therefore to propose a process for depolymerizing silicones which is satisfactory under so-called mild operating conditions.

[0021] Another objective of the present application is to obtain a good yield of cyclic organopolysiloxanes and more precisely to selectively form octamethylcyclotetrasiloxane (D4). Even more precisely to form predominantly D4 at the end of the process of the present invention compared to other cyclic organopolysiloxanes.

[0022] Another objective of this application is to provide a robust and adaptable process on an industrial scale.

[0023] Another objective of the present application is to propose a simple catalytic system allowing good reaction kinetics compatible with industrialization of the process.

[0024] Still other objectives will appear upon reading the description of the invention which follows.

[0025] These objectives are fulfilled by the present application which relates to a method for preparing cyclic organopolysiloxane OC by depolymerization of at least one organopolysiloxane O chosen from:

[0026] - linear organopolysiloxanes,

[0027] - branched oligopolysiloxanes,

[0028] - crosslinked organopolysiloxanes or their mixtures; in the presence of a solvent, catalyzed by an alkali salt SA chosen from alkali silanolates, alkali siliconates or their mixtures, in which the quantity of alkali salt used is between 0.01 and 2% by weight relative to the weight of organopolysiloxane O.

[0029] Thus, the present invention has the advantage of carrying out a depolymerization reaction of silicone waste using a specific alkaline salt and under mild operating conditions. Indeed, this method does not require a distillation step or working under reduced pressure and at very high temperature to form predominantly octamethylcyclotetrasiloxane. This method then positions itself as a relevant solution for the revalorization of silicone waste for the silicone industry. In addition, surprisingly, this method makes it possible to selectively form octamethylcyclotetrasiloxane (D4) directly. In other words, octamethylcyclotetrasiloxane is formed without using distillation steps or re-engaging unwanted by-products in the reaction medium.

[0030] Detailed description of the invention:

[0031] Silicones, otherwise known as organopolysiloxanes, are polymeric materials comprising alternating silicon and oxygen atoms with various organic radicals bonded to the silicon.

[0032] In the context of the present invention, silicone or silicone product or silicone polymer or organopolysiloxane means polymers comprising a siloxane backbone (Si-O-Si) having alternating silicon and oxygen atoms with various organic radicals bonded to the silicon. These silicone polymers can be liquid or solid, depending on the molecular weight and the degree of crosslinking.

[0033] The organopolysiloxanes O of the invention may be of any type, for example, they may be linear organopolysiloxanes such as oils or gums, branched organopolysiloxanes such as resins, crosslinked organopolysiloxanes such as gels or elastomers, or mixtures of such compounds.

[0034] For the purposes of the present invention, organopolysiloxane O means an organopolysiloxane containing little or no cyclic organopolysiloxane.

[0035] In the context of the present invention, cyclic organopolysiloxane is understood to mean mainly the following compounds: hexamethylcyclotrisiloxane, octamethyltetrasiloxane and decamethylcyclopentasiloxane.

[0036] Preferably, the organopolysiloxane O contains a cyclic organopolysiloxane content of less than 15%, preferably less than 10%, preferably less than 5%, even more preferably less than 1%. Preferably, the organopolysiloxane O of the invention is free of cyclic organopolysiloxane.

[0037] The organopolysiloxane O may in particular be an oil and preferably has a dynamic viscosity of between 50 and 600,000 mPa.s at 25°C.

[0038] All the viscosities discussed in this presentation correspond to a dynamic viscosity quantity at 25°C called “Newtonian”, i.e. the dynamic viscosity which is measured, in a manner known per se, with a Brookfield viscometer at a shear rate gradient sufficiently low so that the measured viscosity is independent of the rate gradient.

[0039] The term gum is used for organopolysiloxane compounds with viscosities typically greater than 600,000 mPa.s, which corresponds to a molecular mass greater than 300,000 g / mol.

[0040] These organopolysiloxanes O may comprise one or more functional units such as: - OH;

[0041] - H ;

[0042] - alkenyl in particular comprising from 2 to 6 carbon atoms, preferably vinyl;

[0043] - O-Alk with Alk represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl;

[0044] - (O-Alk)x with Alk representing an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, and x representing an integer between 2 and 200;-a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (C1-C5) alkyl-CF3, the alkyl being linear or branched.

[0045] Preferably, the functional units are chosen from: - OH;

[0046] - H ;

[0047] - alkenyl in particular comprising from 2 to 6 carbon atoms, preferably vinyl;

[0048] - a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (C1-C5) alkyl-CFs, the alkyl being linear or branched. Preferably, the organopolysiloxanes O may comprise one or more functional units such as H, OH, alkenyl (preferably vinyl), aryl, cyclic amine, as defined above. The organopolysiloxanes O may be partially crosslinked. The organopolysiloxanes O may in particular be used organopolysiloxanes, having served for example as a heat transfer fluid, which should be recycled, the process of the invention thus making it possible to generate cyclic organopolysiloxanes OC which can then be directly used in industrial processes.In the case of the use of used organopolysiloxanes O, the organopolysiloxane may then contain other elements such as additives, pigments, etc.

[0049] According to one embodiment of the invention, the organopolysiloxane O comprises:

[0050] - at least 150 units, preferably at least 200 siloxyl units of formula (I): R c SiO(4-c) / 2 (I) in which,

[0051] R, identical or different, represents:

[0052] -an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, or an aryl group comprising from 6 to 10 carbon atoms, preferably phenyl, and c = 0, 1, 2 or 3; and optionally one or more units of formula (II):

[0053] R 1 dReSiO(4-de) / 2 (H) in which,

[0054] R is as defined above;

[0055] R 1 , identical or different, represents:

[0056] -an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl,

[0057] -a hydroxyl group (OH),

[0058] -a group (O-Alk) with Alk representing an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably OCH3 or OC2H5, -a group (O-Alk) x with Alk represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, and x represents an integer between 2 and 200,

[0059] -a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or

[0060] -a hydrogen; d=1, 2 or 3, preferably d=1 or 2, more preferably d=1; e=0, 1 or 2; and the sum d+e=1, 2 or 3.

[0061] It is understood in formulas (I) or (II) above that, if several R groups are present or if several R groups 1 are present, they can be the same or different from each other.

[0062] Preferably, in the above formulas R 1 , identical or different, represents:

[0063] - an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl,

[0064] - a hydroxyl group (OH),

[0065] - a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or

[0066] -a hydrogen.

[0067] In the present invention:

[0068] - a siloxyl unit “M” represents a siloxyl unit of formula Y3SiOi / 2,

[0069] - a siloxyl unit “D” represents a siloxyl unit of formula Y2SiC>2 / 2,

[0070] - a siloxyl unit “T” represents a siloxyl unit of formula YSiOs / 2,

[0071] - a siloxyl unit “Q” represents a siloxyl unit of formula SiC>4 / 2, the symbols Y being R or R 1 as defined above.

[0072] The organopolysiloxane O may optionally be linear or branched and in particular comprise T and Q units.

[0073] According to a preferred embodiment, the organopolysiloxane O is chosen from the compounds of formula (III): R 1 aR(3-a)SiO-(SiR2O) n l-(SiR 1 RO) m l-SiR 1 a R(3-a) (III) in which:

[0074] R, identical or different, represents:

[0075] -an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, or

[0076] -an aryl group comprising from 6 to 10 carbon atoms, preferably phenyl; R 1 , identical or different, represents: - an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl,

[0077] -a hydroxyl group (OH),

[0078] -a group (O-Alk) with Alk representing an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably OCH3 or OC2H5, -a group (O-Alk) x with Alk represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, and x represents an integer between 2 and 200,

[0079] -a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or

[0080] -a hydrogen; a is an integer and represents 0, 1, 2 or 3, preferably 0, 1 or 2, more preferably 0 or 1;

[0081] -n1 represents an integer between 50 and 10,000, preferably between 100 and 5,000, preferably between 500 and 5,000, more preferably between 600 and 2,000;

[0082] -m1 represents an integer between 0 and 100, preferably between 0 and 50, more preferably between 0 and 30, preferably m=0.

[0083] Particularly preferably, the organopolysiloxane O is a compound of formula (III) in which R 1 , identical or different, represents:

[0084] -an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl,

[0085] -a hydroxyl group (OH),

[0086] -a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or

[0087] -a hydrogen.

[0088] Particularly preferably, the organopolysiloxane O is a compound of formula (III) in which, R 1 , identical or different, represents: -CH3, vinyl, H, (Ci-C5)alkyl-CF3 or OH.

[0089] Particularly preferably, the organopolysiloxane O is a compound of formula (III) in which,

[0090] R, identical or different, represents CH3 or phenyl, preferably CH3; R 1 , identical or different, represents:

[0091] -an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl,

[0092] -a hydroxyl group (OH),

[0093] -a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted or not by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or -a hydrogen.

[0094] Particularly preferably, the organopolysiloxane O is a compound of formula (III) in which,

[0095] R, identical or different, represents CH3 or phenyl, preferably CH3, R 1 , identical or different, represents CH3, vinyl, H, (C1-C5) alkyl-CF3 or OH.

[0096] The organopolysiloxanes O of the invention may also be of the crosslinked silicone material type such as gels or elastomers. Crosslinked silicone materials are well known to those skilled in the art. These materials may in particular be obtained by polycondensation, radical polymerization, thermal polyaddition or irradiation under UV.

[0097] Organopolysiloxane-based products or crosslinked silicone materials have a multitude of applications. For example, they can be found in food applications such as baking molds, medical applications, and in the pharmaceutical sector such as baby bottle nipples, catheters, implants, or tubes for medical applications. In technical industrial applications, silicone is often used as a material for seals or membranes. In the automotive sector, it is used for hoses, cable sheathing or insulation, and as a damping material.

[0098] Silicone elastomers are crosslinked silicone materials that include fillers, such as silica, to achieve good mechanical properties.

[0099] By varying the silicone oils, fillers and additives as well as the crosslinking method, silicone elastomers exhibit different properties and colors.

[0100] Silicone elastomers can be divided into three main groups well known to those skilled in the art. Hot vulcanizable elastomers (HTV or heat cured rubber) are silicone elastomers obtained from silicone compositions with very high viscosity including silicone gums and fillers. They are vulcanized at high temperature, generally between 110 °C and 250 °C. Crosslinking is either radical catalyzed by peroxides or obtained by an addition reaction where platinum compounds are used as catalysts.

[0101] Liquid silicone rubber (LSR) is a silicone elastomer made from compositions containing viscous silicone oils and fillers. Crosslinking occurs through an addition reaction at temperatures similar to those of LSR, with crosslinking generally occurring much more rapidly.

[0102] The third group is that of silicones obtained by crosslinking at room temperature of silicone compositions from silicone oils and fillers crosslinking by polycondensation or polyaddition reactions. These elastomers are known in French as cold vulcanizable silicone elastomers (RTV). These compositions are available in one- and two-component systems.

[0103] In the process of the present application, organopolysiloxane O also means silicone-based materials, for example materials comprising at least 0.1% by mass of silicone relative to the total mass of silicone-based material. Conventionally, these materials can be found in silicone-coated textiles (airbags or synthetic leather), silicone coatings (food paper).

[0104] The silicone-based material may comprise up to 100% by mass of silicone, preferably up to 99.9% by mass of silicone relative to the total mass of silicone-based material. These silicone-based materials may also comprise additives or fillers such as colorants, silica, calcium carbonate, calcium oxide, celite, quartz, titanium oxide, cerium hydroxide, magnesium oxide, mica, etc.

[0105] According to one embodiment, the silicone-based material comprises silica, calcium oxide, quartz, titanium oxide, magnesium oxide, mica and mixtures thereof.

[0106] In the context of the present invention, the term “number of moles of (Si-O)” means the number of moles of (Si-O) bonds of the organopolysiloxane O.

[0107] In the case where the starting organopolysiloxane O is of unknown formula, in particular in the case of formulated commercial products, it is possible to estimate the number of moles of (Si-O) bonds. Indeed, for formulated silicone elastomers, the person skilled in the art knows that the quantity of filler is between 20 and 40% by mass, thus, by estimating the quantity of filler at an average of 30% by mass, we have 70% by mass of silicone, which makes it possible to calculate the number of moles of (Si-O) bonds, from the average molar mass of the repeating unit.

[0108] The process of the present invention is carried out in the presence of a solvent. The solvent must be chosen from solvents solubilizing organopolysiloxane O or allowing crosslinked silicone materials to swell. The solvent may be a polar or apolar solvent. The solvent may be, for example, an alcohol, an alkane, an aromatic solvent or an ether derivative.

[0109] For example, the alcohol may be selected from fatty alcohols such as n-lauryl alcohol, n-myristyl alcohol, n-palmitic alcohol, n-stearyl alcohol, n-docosanol, or Guerbet alcohols such as 2-octyl 1-dodecanol, 2-decyl 1-tetradecanol.

[0110] Concerning the alkane, it can be chosen from branched alkanes such as isododecane or isohexadecane or from n-paraffins such as the compounds C12H26, C14H30, C16H34, CisHss, C20H42, C22H46, C24H50.

[0111] Concerning the aromatic solvent, it can be chosen from benzene derivatives such as toluene or xylene.

[0112] For ether derivatives, the solvent may be chosen from tetrahydrofuran (THF), diisopropyl ether (DIPE), MeTHF, cyclopentyl methyl ether (CPME), or ethyl tetrahydrofurfuryl ether (ETE).

[0113] The solvent can also be selected from other solvents such as ethyl acetate, methyl ethyl ketone (MEK) or methyl isopropyl ketone (MIBK).

[0114] The solvent may also be selected from carbonates such as propylene carbonate, dimethyl carbonate, diethyl carbonate and their derivatives.

[0115] For the purposes of the present invention, the mass concentration Cm of the organopolysiloxane O is defined by the ratio between the mass of organopolysiloxane O expressed in grams and the volume of solvent used expressed in liters.

[0116] According to one embodiment, the method of the invention is implemented at a mass concentration Cm of organopolysiloxanes O less than or equal to 2,000 gL -1 , preferably less than or equal to 1000 gL -1 . Indeed, the process of the present invention is in a solvent medium and a minimum quantity of solvent is necessary to solubilize or swell the organopolysiloxane O.

[0117] However, for the sake of efficiency, the method of the invention is implemented at a mass concentration Cm of organopolysiloxanes O greater than or equal to 50 gL -1 .

[0118] According to one embodiment of the invention, the mass concentration Cm of the organopolysiloxanes O is between 50 and 2000 gL -1 , preferably between 50 and 1000 g. L -1 .

[0119] According to one embodiment of the invention, the mass concentration Cm of the organopolysiloxanes O is between 50 and 500 gL -1 , preferably between 50 and 350, preferably between 50 and 250, preferably between 100 and 200 gL -1 .

[0120] For the purposes of the present invention, the alkali salt catalyst is an alkali metal salt chosen from silanolates and siliconates, preferably silanolates. Thus, in the remainder of the present application, the term “alkali salt SA” means an alkali salt catalyst or an alkali metal salt chosen from silanolates and siliconates.

[0121] This alkali metal salt comprises a metal M, the metal M being selected from K, Na, Rb or Cs, preferably K, Rb, Cs.

[0122] In the context of the present invention, when referring to the alkali metal salt, the following terms are understood to mean:

[0123] "silanolate" means a salt derived from a silanol group which is a functional group comprising at least one Si-OH group. Silanolates generally have a formula MOSi(R')3 with M as defined above and R', which may be identical or different, represents a linear or branched alkyl group comprising from 1 to 6 carbon atoms, an alkenyl group comprising from 2 to 6 carbon atoms, or an aryl group such as phenyl; preferably R', which may be identical or different, represents methyl or vinyl.

[0124] "siliconate" a compound of formula M(OSi(R 3 )2) P -OSi(R 3 )3with M as defined above and R 3 represents an alkyl group comprising from 1 to 6 carbon atoms, preferably methyl, and p represents an integer between 1 and 20, preferably between 1 and 10.

[0125] Preferably, the alkaline catalyst of the invention is a catalyst of formula MX with:

[0126] M is selected from K, Na, Rb, Cs, preferably K, Rb, Cs, preferably K; X is selected from OSiR 4 2R 5 , (OSi(R 3 )2) p -OSi(R 3 )3with M as defined above and

[0127] R 3 represents an alkyl group comprising from 1 to 6 carbon atoms, preferably methyl, and p represents an integer between 1 and 20, preferably between 1 and 10,

[0128] R 4 identical or different represents a linear or branched alkyl comprising from 1 to 6 carbon atoms, preferably methyl,

[0129] R 5 represents a linear or branched alkyl comprising from 1 to 6 carbon atoms, preferably methyl, or a vinyl group, preferably X represents OSiMe3 (Me represents methyl), OSiMe2Vi (Vi represents vinyl), OSi(Me)2-(OSi(Me)2) p -OSiMe3 with p as defined above.

[0130] Preferably, M is selected from K, Rb, Cs and X is selected from OSiMe3, OSiMe2Vi (Vi represents vinyl), OSi(Me)2-(OSi(Me)2) p -OSiMe3p represents an integer between 1 and 20, preferably between 1 and 10.

[0131] The alkali salt SA is used in catalytic proportions, preferably in an amount of between 0.01 and 2% by weight relative to the weight of organopolysiloxane O, preferably between 0.05 and 1%, more preferably between 0.05 and 0.8% relative to the weight of organopolysiloxane O.

[0132] In one embodiment, the method of the invention is characterized in that the molar ratio of alkali salt SA relative to organopolysiloxane O is from 0.005% to 2%, preferably from 0.01 to 2%, preferentially from 0.05 to 1% and even more preferentially from 0.1 to 1%.

[0133] The method of the invention is carried out at a temperature between 0 and 120°C, preferably between 10 and 120°C, preferably between 10 and 80°C, preferably between 20 and 60°C.

[0134] According to one embodiment, the method of the invention does not need to be carried out under reduced pressure.

[0135] Preferably, the method of the invention is carried out at atmospheric pressure.

[0136] For the purposes of the present invention, the term "multidentate complex" means a complexing agent of the alkali metal of the alkali salt catalyst, i.e. a complexing agent of K, Na, Rb or Cs, preferably a complexing agent of K, Rb or Cs. Examples of multidentate complexing agents that may be mentioned are crown ethers, cryptands which are diamine or polyamine macroheterocycles, and OH or OCH3 terminated polyethylene glycols (PEGs), such as polyethylene glycol dimethyl ether.

[0137] According to a preferred embodiment, the method of the invention is free of multidentate complex.

[0138] Thus, according to a preferred embodiment, the method of the invention is carried out at a temperature between 10 and 120°C, at atmospheric pressure and is free of multidentate complex.

[0139] According to one embodiment, the method of the invention is implemented at atmospheric pressure and at a mass concentration Cm greater than or equal to 50 gL- 1

[0140] The method according to the invention may further comprise a step of neutralizing the alkali salt. This neutralization step makes it possible to stop the depolymerization reaction.

[0141] Advantageously, this neutralization step is carried out by adding an acid to the reaction medium.

[0142] This step can be implemented by the addition of aqueous solution such as an acidic aqueous solution like phosphoric acid.

[0143] This step can also be implemented by adding other acid solutions such as silicone phosphate for example.

[0144] This neutralization step can be accompanied by a filtration step in order to separate the cyclic organopolysiloxanes OC from any residues or impurities.

[0145] The person skilled in the art will be able to adapt the filtration method according to the size of the reactor and the species used in the process according to the invention.

[0146] The method according to the invention may further comprise a distillation step in order to evaporate and separate the solvent from the various cyclic organopolysiloxanes OC obtained.

[0147] According to one embodiment, this step can also make it possible to separate the different cyclic organopolysiloxanes OC.

[0148] The cyclic organopolysiloxanes OC (or cyclic organopolysiloxanes) obtained by the method of the invention are preferably compounds D4 or D5, and preferably predominantly octamethylcyclotetrasiloxane D4. Compounds D4 and D5 are compounds of the following formulas:

[0149] As mentioned above, the starting organopolysiloxane O may comprise functions described by the group R 1 described above. In these cases, at least one of the methyl groups of compounds D4 and / or D5 may be substituted by at least one of R 1 .

[0150] According to one embodiment of the invention, the cyclic organopolysiloxanes OC obtained are free of hexamethyltrisiloxane (D3).

[0151] According to one embodiment in order to facilitate the recycling of compounds D4 and D5, and their reuse in industrial processes, in particular polymerization, these compounds preferably comprise less than 10%, preferably less than 5%, preferably less than 2% and even more preferably less than 0.2% by weight of functions described by the group R 1 as described above relative to the total weight of compounds D4 and D5.

[0152] The method of the invention preferably makes it possible to obtain a mass yield of cyclic organopolysiloxanes OC greater than 85%, preferably greater than 90% and preferentially greater than 95% by weight.

[0153] Preferably, a mixture of cyclic organopolysiloxane OC is obtained with at least 80% by weight of compounds D4, D5 or their mixtures, preferably from 90 to 99%, relative to the total weight of cyclic organopolysiloxanes OC produced.

[0154] Preferably, the majority compound obtained by the method of the invention is compound D4.

[0155] Preferably, the method of the invention allows a yield of D4 of at least 60% by weight, preferably at least 70% by weight relative to the total weight of cyclic organopolysiloxanes OC produced.

[0156] Optionally, compounds D4 and D5 can be separated, in particular by distillation, for example by distillation with numerous theoretical plates.

[0157] Particularly advantageously, the compounds D4 and D5 obtained by the process of the invention can be directly used in other industrial processes to be polymerized and to manufacture new organopolysiloxanes such as the oils and gums described above.

[0158] Thus, the method of the present invention has the advantages of being able to be implemented under mild operating conditions, of requiring a low catalytic load, of selectively obtaining cyclic organopolysiloxanes (D4, D5 and De) and also of being adaptable to the industrial scale.

[0159] Furthermore, this approach makes it possible to recover numerous and varied industrial waste without requiring pre-treatment or additional steps.

[0160] The method of the present invention also has the advantage of being implemented in a concentrated solvent medium as set out above, which is economically and ecologically advantageous (energy saving, simplification of the reprocessing and recycling processes for the solvent used).

[0161] Alternatively, the method according to the invention may further comprise a reactive distillation step in the presence of a multidentate complex chosen from crown ethers, cryptands which are diamine macroheterocycles and OH or OCH3 terminated polyethylene glycols (PEG).

[0162] This process is detailed in the scientific article Green Chem., 2023, 25, 3869-3877.

[0163] According to one embodiment of the invention, an additional amount of alkali salt is introduced during the optional reactive distillation step.

[0164] The additional alkali salt is used in catalytic proportions, preferably in an amount of between 0.001 and 1.5% by weight relative to the weight of organopolysiloxane O, preferably between 0.005 and 0.9%, more preferably between 0.01 and 0.8%.

[0165] According to one embodiment of the invention, the molar ratio of alkali salt, added during this optional step, relative to the multidentate complexing agent is between 1 / 100 and 100 / 1, preferably between 1 / 50 and 50 / 1, preferably between 1 / 25 and 25 / 1, preferably between 1 / 10 and 10 / 1, preferably between 1 / 4 and 4 / 1, preferably between 1 / 2 and 2 / 1, preferably greater than or equal to 1, preferably between 1 and 2, for example the ratio is equal to 1.

[0166] The present invention also relates to the use of the cyclic organopolysiloxanes obtained OC as raw material for carrying out a ring-opening polymerization process.

[0167] This application is described using the following examples. O implemented in the examples Organopolysiloxane 01 is a silicone gum of formula whose viscosity is greater than 600,000 mPa.s.

[0168] Organopolysiloxane 02 is a silicone gum with a formula whose viscosity is greater than 600,000 mPa.s.

[0169] Organopolysiloxane 03 is an organopolysiloxane mixture comprising 50% by weight of 01 and 50% by weight mixture of an organopolysiloxane type 02.

[0170] Organopolysiloxane 04 is an oil of formula n=70 viscosity 100 mPa.s

[0171] Organopolysiloxane 05 is an RTV2 Gel formulated by ELKEM. This silicone was obtained by polyaddition reaction.

[0172] Organopolysiloxane 06 is an industrial by-product. More specifically, this organopolysiloxane is an oil derived from chlorosilane hydrolysis reaction by-products comprising at least 95% of an organopolysiloxane O comprising about 2% Si-H group, and other reaction products such as reaction product residues, catalyst, etc.

[0173] Alkaline salt used in the examples:

[0174] SA1: KOSiMe3

[0175] SA2: between 1 and 10

[0176] Example 1: General protocol according to the present invention: In a 500mL flask, the solvent (THF in the examples below), the organopolysiloxane O and the alkaline salt SA are introduced, maintained at a temperature T for a period of 2h to 24h.

[0177] This reaction is optionally followed by a neutralization step in the presence of a phosphoric acid or silicone phosphate solution. This step can be followed by a filtration step to remove any solid residues in the reaction mixture.

[0178] The product obtained using the method of the present invention is sampled and then analyzed by 29 Si-NMR or SEC.

[0179] The various examples below mention:

[0180] - the yield of cyclic organopolysiloxane OC was determined by the integration of the NMR spectrum signals in 29 Si-NMR obtained. This yield corresponds to the ratio between the mass of cyclic organopolysiloxane OC formed and the mass of the starting organopolysiloxane O. Alternatively, this yield is measured by size exclusion chromatography (SEC) in the presence of polystyrene standards in a solvent such as toluene at 40°C.

[0181] - the ratio of the different cyclic organopolysiloxanes OC D3 / D4 / D5 / D6 was determined by the integration of the NMR spectrum signals in 29 If obtained. This ratio corresponds to a weight ratio. Alternatively, this yield is measured by size exclusion chromatography (SEC) in the presence of polystyrene standards in a solvent such as toluene at 40°C.

[0182] Example 2: Implementation of the method of the present invention with different SA catalysts:

[0183] The protocol of example 1 is implemented in the table below with organopolysiloxane 03 (98g, 1.32 mol of (Si-O)) catalysts SA1 (0.75mol%) and SA2 (0.75mol%) for 7h at 25°C.

[0184] It is observed that the catalysts of the invention SA1 and SA2 make it possible to obtain a satisfactory yield in OC as well as a very satisfactory selectivity in octamethycyclotetrasiloxane (D4). Example 3: Implementation of the present invention under different operating conditions:

[0185] The protocol of Example 1 is implemented in the table below in the presence of 20g of organopolysiloxane 01 (20g, 270mmol of (Si-O)) and catalyst SA1 (0.75mol%). The other operating conditions are listed in the table below. nd means undetermined

[0186] It is observed that the process of the present invention is robust and makes it possible to obtain satisfactory yields by varying the different operating conditions as set out in the table above.

[0187] Example 4: Implementation of the present invention with different organopolysiloxanes

[0188] O_:

[0189] The protocol of Example 1 is implemented in the table below in the presence of different organopolysiloxanes O and the catalyst SA1 (0.75 mol%). The other operating conditions are listed in the table below. In the context of these examples, a neutralization step in the presence of a phosphoric acid solution followed by a filtration step are implemented.

[0190] This example illustrates that the method of the present invention is adaptable to different organopolysiloxanes O such as oils, gums or even crosslinked polymers.

[0191] Furthermore, this example shows according to the different tests 3, 7, 8 and 9 that the yield is not impacted by the size of the reactor. Thus, this example demonstrates the performance on both small and large scales of the method of the present invention.

[0192] Example 5: Example with an oil from a by-product of chlorosilane hydrolysis reaction

[0193] The protocol of Example 1 is implemented with KOSiMes (1 mol%, 0.2g) (SA1). The reaction mixture comprising SA1, organopolysiloxane 06 (20g, 270mmol of (Si- O)) is placed at 25°C for 18 hours.

[0194] Following the 18 hours of reaction, a neutralization step in the presence of a phosphoric acid solution followed by a filtration step is implemented.

[0195] The method of the present invention resulted in an 82% yield of cyclic organopolysiloxane OC.

[0196] These results show that the catalytic system of the invention allows the production of cyclosiloxane with oils from industrially generated by-products and therefore of complex composition.

[0197] Thus, using the present invention it will be possible to recycle industrial waste that could not be recovered until now and to move a little further towards carbon neutrality. This example also highlights the robustness of the process of the present invention.

Claims

CLAIMS 1. Method for preparing cyclic organopolysiloxanes OC by depolymerization of an organopolysiloxane O chosen from: - linear organopolysiloxanes, - branched oligopolysiloxanes, - crosslinked organopolysiloxanes or their mixtures; in the presence of a solvent, catalyzed by an alkali salt SA chosen from alkali silanolates, alkali siliconates or their mixtures, in which the quantity of alkali salt used is between 0.01 and 2% by weight relative to the weight of organopolysiloxane O.

2. Method according to claim 1, in which the organopolysiloxane O is chosen from the compounds of formula (III): R 1 aR(3-a)SiO-(SiR2O) n l-(SiR 1 RO) m l-SiR 1 aR(3-a) (III) in which: R, identical or different, represents: -an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, or -an aryl group comprising from 6 to 10 carbon atoms, preferably phenyl; R 1 , identical or different, represents: - an alkenyl group comprising from 2 to 6 carbon atoms, preferably vinyl, -a hydroxyl group (OH), -a group (O-Alk) with Alk representing an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably OCH3 or OC2H5, -a group (O-Alk) xwith Alk represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, preferably methyl, and x represents an integer between 2 and 200, -a linear or branched alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, substituted by at least one fluorine atom, for example 1 to 10 fluorine atoms, for example (Ci-C5)alkyl-CF3, the alkyl being linear or branched, or -a hydrogen; a is an integer and represents 0, 1, 2 or 3, preferably 0, 1 or 2, more preferably -n1 represents an integer between 50 and 10,000, preferably between 100 and 5,000, preferably between 500 and 5,000, more preferably between 600 and 2,000; -m1 represents an integer between 0 and 100, preferably between 0 and 50, more preferably between 0 and 30, preferably m=0.

3. A method according to any one of claims 1 or 2, wherein the alkali salt catalyst SA is a catalyst of formula MX with: M is selected from K, Na, Rb, Cs, preferably K, Rb, Cs, preferably K; X is chosen from OSiR 4 2R 5 , (OSi(R 3 )2) p -OSi(R 3 )3; with M as defined above and R 3 represents an alkyl group comprising from 1 to 6 carbon atoms, preferably methyl, and p represents an integer between 1 and 20, preferably between 1 and 10; R 4 identical or different represents a linear or branched alkyl comprising from 1 to 6 carbon atoms, preferably methyl; R 5represents an alkyl, linear or branched, comprising from 1 to 6 carbon atoms, preferably methyl, or a vinyl group.

4. Method according to any one of claims 1 to 3, wherein the method is carried out at a temperature between 0 and 120°C, preferably between 10 and 100°C, preferably between 10 and 80°C, preferably between 20 and 60°C.

5. Method according to any one of claims 1 to 4, in which the mass concentration Cm of the organopolysiloxane O relative to the volume of solvent used is greater than or equal to 50 gL. -1 .

6. Method according to any one of claims 1 to 5, in which the mass concentration Cm of the organopolysiloxane O relative to the volume of solvent used is between 50 and 500 gL -1 , preferably between 50 and 350 gL -1 , preferably between 50 and 250, preferably between 100 and 200 gL -1 .

7. Method according to any one of claims 1 to 6, characterized in that the mass yield of cyclic organopolysiloxane OC is greater than 85%.

8. Method according to any one of claims 1 to 7, in which a mixture of cyclic organopolysiloxane OC is obtained with at least 80% by weight of compounds D4, D5OR of their mixtures, preferably from 90 to 99%, relative to the total weight of cyclic organopolysiloxanes OC produced.

9. Method according to any one of claims 1 to 8, characterized in that the cyclic organopolysiloxanes OC obtained are free of hexamethyltrisiloxane.

10. Use of the cyclic organopolysiloxanes OC obtained according to any one of the preceding claims as raw material for carrying out a ring-opening polymerization process.

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