EXTRACTION OF SILOXANE CYCLES

Description

[0001] The invention lies in the field of silicones and silicone recycling. In particular, the invention relates to a process for obtaining siloxane cycles from waste silicones.

[0002] In line with the concept of sustainability, the importance of recycling and reusing waste products from economic cycles is increasing. This also applies to the recycling of silicone waste, which in the context of this invention is also referred to as "end-of-life" silicones. Therefore, there is a desire to make progress in this field as well. Irrespective of this, there is a growing need and thus a steady demand for organically functionalized siloxanes for applications in areas such as construction, electrical and electronics, automotive, healthcare, and cosmetics, as well as numerous other fields. Siloxane cycles are of considerable importance as base materials for the production of a wide variety of organically functionalized siloxanes.

[0003] Siloxane cycles can be obtained, for example, as secondary products from the industrial Müller-Rochow synthesis, which, however, is a high-temperature process with considerable energy requirements.

[0004] Siloxane cycles in the context of this invention are the monocyclic, ring-shaped siloxane oligomers consisting of directly linked D units (= dimethylsiloxy units), in particular D 3 (hexamethylcyclotrisiloxane), D 4 (octamethylcyclotetrasiloxane), D 5 (decamethylcyclopentasiloxane) and / or D 6 (dodecamethylcyclohexasiloxane), where D 4 (octamethylcyclotetrasiloxane) and / or D 5 (decamethylcyclopentasiloxane) are the most preferred siloxane cycles in the context of this invention.

[0005] As a reference to the M, D, T, Q nomenclature used in this document to describe the building blocks of organopolysiloxanes, see W. Noll, Chemistry and Technology of Silicones, Verlag Chemie GmbH, Weinheim (1960), page 2 ff.

[0006] In a review article, Rupasinghe and Furgal attempted to compile all available studies on the degradation and depolymerization of polysiloxanes up to July 2021 (Polymer international, 71(5), 521-531).

[0007] Recycling silicone rubber waste presents a particular challenge because it requires the constant separation of the fillers incorporated into the silicone rubber from the silicone matrix. This separation task is made more difficult by the strong interactions between the filler particles and the polydimethylsiloxane chains.

[0008] For example, W. Huang et al. (Polymer 43, 7295-7300 (2002)) discuss the difficulty of finding an effective solvent that also promotes the depolymerization of the silicone matrix in the context of the recovery of monomers and fillers from high-temperature crosslinked silicone rubber (HTV) through the combined action of solvent, base, and fillers. Accordingly, it is not easy to remove siloxane cycles from an equilibrated depolymerization mixture, leaving behind solvent and fillers. If the solvent is removed before the fillers are separated, the remaining siloxane cannot be easily depolymerized, which inevitably results in low yields of siloxane cycles.

[0009] The difficulty of depolymerizing the siloxane to be recycled after removing the solvent previously used to swell the siloxane can be seen, for example, in DE 19502393 A1, which states in its Example 1 that the solvents are distilled off from an addition-crosslinked polydimethylsiloxane rubber vulcanizate after it has swelled under reflux conditions in a mixture consisting of hexane, dichloromethane and PNCl 2 and the remaining gel is pyrolyzed at 650°C for one hour, so that a mixture of cyclic siloxanes is formed as the pyrolysis product.

[0010] Huang et al. (Ic) report that a mixture of diethylamine, methanol, and hexane is very effective both in inducing the KOH-catalyzed depolymerization of a HTV silicone rubber filled with SiO 2 and Al 2 O 3 and in completely removing said fillers before stripping the siloxane cycles and the solvents.

[0011] The authors describe the difficult handling of this complex solvent mixture based on the experimental finding that a specific sequence of solvents must be followed to achieve successful depolymerization. If the HTV silicone rubber is heated in methanol with the addition of potassium hydroxide and then diethylamine is added, the rubber dissolves after a short time, but forms an emulsion rather than a dispersion. In contrast, the only effective method is refluxing the silicone rubber in diethylamine with the addition of potassium hydroxide, followed by the addition of methanol, which quickly forms a suspension of the filler particles in the liquefied siloxane.

[0012] The authors also face the fundamental necessity of removing the filler particles from the resulting suspension by filtration. Without removing the solids, vacuum distillation yields only 7 percent of distillable siloxanes based on the silicone rubber used, compared to 32.1 percent after removing the solids.

[0013] With the aim of completely separating the filler particles from the siloxane matrix, Huang et al. then added methanol to achieve the aggregation of the finely dispersed polar filler particles and facilitate their separation by filtration.

[0014] Without further explanation, the expert is aware that this complicated method with its multiple influencing parameters is not suitable for successful implementation in industrial practice.

[0015] Ikeda et al. (Green Chemistry, 2003,5, 508-511) take up the results of Huang et al. and substitute potassium hydroxide with tetramethylammonium hydroxide in the complex solvent system consisting of hexane, diethylamine and methanol, with the intention of being able to reuse the filler components (silicon dioxide and aluminum oxide) isolated from the depolymerization of silicone rubber, because Huang's work had further shown that potassium hydroxide covers the surface of the solid particles, which leads to a reduction in the thermal stability of fresh silicone rubber formulations filled with it and is therefore prohibitive for their direct reuse.

[0016] Oku et al. (Polymer 43, 7289-7293 (2002)) also focus on the depolymerization of the radically crosslinked, SiO 2 and Al 2 O 3 -filled silicone rubber used in the studies by Huang and Ikeda to form siloxane cycles. They employ potassium hydroxide in toluene and potassium hydroxide combined with buffer acids without solvent. Good cycle yields of up to 84 percent are achieved, particularly through the stoichiometric combination of KOH with certain buffer acids such as potassium dihydrogen phosphate KH 2 PO 4 and the monopotassium salt of terephthalic acid p-KOOC(C 6 H 4 )COOH. The buffer acids are added only after the silicone rubber has dissolved and before the siloxane cycles are distilled off.

[0017] Taking into account the ideal laboratory conditions underlying Okus' work, it is easy to see that real silicone waste, with its range of completely different fillers than the acidic or neutral fillers SiO 2 and Al 2 O 3 investigated there, i.e., in particular with basic fillers such as basic oxides or carbonates, and also with different content proportions, which in practice makes the stoichiometric adjustment of the proposed buffer mixtures difficult or usually impossible.

[0018] Vornokov and Shabarova describe in Zh. Obshch. Khim.1959,29,1528-1534 (Russian Journal of General Chemistry 1959, 29, 1528-1534) the preparation of organoalkoxysilanes by cleavage of organosiloxanes with C4 to C12 alcohols under basic conditions, using alkali metal hydroxides, alkali metal alcoholates, or the alkali metals themselves. The authors emphasize the need to remove water from the reaction system using water-insoluble but azeotrope-forming solvents or, even more preferably, by using additional dehydrating agents.Vornokov produces organoalkoxysilanes from linear, branched, or cyclic organosiloxanes by reaction with primary or secondary alcohols having boiling points above 100°C in the presence of 1 to 10 mole percent of a hydroxide or alkoxide under continuous azeotropic distillation conditions using inert, water-immiscible solvents such as toluene or benzene. The continuous azeotropic distillation serves to remove water using a water separator.

[0019] However, Vornokov cannot convert alcohols such as methanol and especially ethanol, which have boiling points below 90°C, in this way. Instead, he uses dehydrating agents, preferably the corresponding tetraalkoxysilanes (tetramethoxysilane or tetraethoxysilane), which, according to Vornokov, react with water to form silicon dioxide and the corresponding alcohol.

[0020] However, the publication does not provide a solution for the production of alkoxysiloxanes, because, as is understandable to the person skilled in the art, the retention of high-boiling tetraalkoxysilanes (e.g. tetraethoxysilane boiling point 168°C) and the resulting condensation products would entail considerable separation and purification effort.

[0021] Chang et al. (J. Polym. Res. 2005, 12, 433-438) investigated the nucleophilic cleavage of cross-linked polysiloxanes to obtain cyclic siloxane monomers. They first allowed a cross-linked, filled polydimethylsiloxane to swell overnight at room temperature in either 4 to 5 times the amount of tetrahydrofuran, toluene, or diethylamine. The swollen samples were then treated with separately prepared, homogeneous solutions of potassium hydroxide in dimethylamine and dissolved with stirring at room temperature. The authors observed complete dissolution of the silicone rubber components within periods of 0.4 to 4 hours. The yields of cyclic products determined after 25 hours of reaction time ranged from 10 wt% to 77 wt%, with the yield of the dissolution experiment conducted in diethylamine exceeding those obtained in the tetrahydrofuran, tetrahydrofuran / toluene, and toluene solvent systems. Vu et al. ( ChemRxiv (2023) 1-8, 2023, CODEN: CHEMWF; ISSN: 2573-2293

[0022] URL: https: / / chemrxiv.org / engage / chemrxiv / public-dashboard) describe the recycling of silicone waste to produce cyclic siloxanes. Industrial silicone waste consists of low-viscosity, unfilled silicone oils consisting primarily of D units and containing impurities such as Si-H, H 2 O, HCl, and AlCl 3 , among others. Multidentate ligand potassium silanolate complexes are assumed as depolymerization reagents, among which an 18-crown-6-crown ether potassium silanolate complex and an end-methylated polyethylene glycol potassium silanolate complex, in which the polyether has an average molecular weight of 500 g / mol, are preferably used. Since the viscosity of the reaction matrices increases so much during the course of depolymerization, which is carried out under vacuum distillation conditions between 150 and 170°C and 5 mbar pressure, that stirring of the reaction mass is impossible, the authors add 10% by weight of octadecanol.It is speculated that the addition of fatty alcohol effectively counteracts the formation of siloxane bodies linked via T units, which can arise from the dehydrogenative condensation between SiH and SiOH groups, by n-octadecanol itself dehydrogenatively reacting with the existing SiH groups.

[0023] While all of the above findings are interesting, they do not offer a solution to the problem of depolymerizing any waste silicones, especially filled end-of-life silicone rubbers, and preferably systems filled with basic fillers, into siloxane cycles in an economical and resource-saving manner. In particular, the use of amine bases in the depolymerization step proves to be a technical challenge, as it is important to avoid their entrainment into the siloxane cycles, even in the smallest quantities. A large number of silicone intermediates and end products are obtained by acid-catalyzed equilibration, which, because it is optimized for minimal acid use, is highly sensitive to even small concentrations of bases. The methodology described by Vu et al. should also be viewed from this perspective.To avoid the crown ether, which the authors classify as highly effective but hazardous, they use a terminally methylated polyethylene glycol with an average molecular weight of 500 g / mol as a substitute. This means that the low-molecular-weight range of this polyether contains ethoxylate components that are volatile along with the siloxane cycles under the harsh conditions of the vacuum distillation described there. However, polyethers and lower ethoxylates, like amine bases, interfere with the technically important acid-catalyzed equilibration. The authors therefore only use the base-catalyzed (specifically the KOSiMe 3 -catalyzed) equilibration as experimental evidence for the repolymerizability of the resulting siloxane cycles.

[0024] Against this background, the technical task arose to find a new process that would allow siloxane cycles to be obtained in the purest possible form on a production scale from any range of waste silicones, be it silicone oils or, much more challenging, filled, high-temperature crosslinked silicone rubbers, and thereby avoid the use of complex solvent systems and extreme reaction temperatures, as described, for example, in DE 19502393 A1.

[0025] The teaching of the not yet published EP patent application with the reference 22190105.1 relates to a process for the preparation of one or more alkoxysiloxanes by thermal reaction of at least one waste silicone with at least one alkali metal alcoholate and at least one alcohol, wherein (a) in a first step, the at least one waste silicone is reacted with at least one alcohol and at least one alkali metal alkoxide while introducing heat, without removing any water that may occur from the reaction mixture, and (b) in a second step, the reaction mixture resulting from this reaction is neutralized with the aid of at least one Brönsted acid, optionally with the addition of at least one solvent, the solid components are separated off, in particular filtered off, and (c) the alkoxysiloxane(s) are then isolated by thermal separation of volatile compounds.

[0026] Surprisingly, it has now been found that waste silicones can also be depolymerized to siloxane cycles in high yields based on the technical teaching of the aforementioned not yet published EP patent application with the file number 22190105.1.

[0027] The subject of the invention is a process for the depolymerization of waste silicones to siloxane cycles, wherein (a) in a first step, at least one waste silicone is reacted with at least one alcohol and at least one alkali metal alkoxide under heat input, without removing any water that may occur from the reaction mixture, in particular without the use of inert, water-insoluble solvents that form azeotropes with water and / or without the use of further dehydrating agents, and (b) thereafter, in a second step, the reaction mixture resulting from this reaction is neutralised with the aid of at least one Brönsted acid, optionally with the addition of at least one solvent, the solid components are separated off, in particular filtered off, and then any solvent previously added and the excess alcohol originating from step (a) are distilled off from the alkoxysiloxane(s) obtained, and(c) then, in a third step, the alkoxysiloxane(s) thus obtained are heated with at least one fatty alcohol and at least one alkali metal alcoholate while mixing and, preferably by applying an auxiliary vacuum, the siloxane cycles formed are thermally separated, preferably by distillation.

[0028] "Inert solvents" in the first step (a) are understood to mean all solvents that are chemically inert in the first step (a), i.e., they do not participate in the reaction itself. Examples of inert, water-insoluble solvents that form azeotropes with water are benzene and toluene.

[0029] Heat input in the first step (a) according to the invention is understood to mean the targeted heating of the reaction mixture consisting of at least one waste silicone with at least one alcohol and at least one alkali metal alkoxide. The heating can be carried out in a known manner. For example, the heat input can preferably be carried out via the jacket surfaces of a reaction vessel or reactor used and optionally using a suitable heat transfer medium, such as hot steam, water or oil, or also, for example, by electric heating. It is also possible according to the invention, for example, to utilize autogenous heating when using vigorously stirring machines, which is explained by the mechanical work (frictional heat) performed by the machine in the more or less viscous reaction mixture. For the basic design options for heating, see Wilhelm RA Vauck / Hermann A.Müller, Basic Operations of Chemical Process Engineering, 8th edition, Weinheim, New York, VCH Verlagsgesellschaft mbH, 1990, pages 421 - 471.

[0030] In the context of this invention, auxiliary vacuum preferably means a pressure range of less than 300 hPa, in particular a pressure range of 0.001 hPa to 250 hPa.

[0031] The first step (a) according to the invention provides that at least one waste silicone is reacted with at least one alcohol and at least one alkali metal alkoxide while mixing with heat, without removing any water that may occur from the reaction mixture, in particular without the use of inert, water-insoluble solvents which form azeotropes with water and / or without the use of further dehydrating agents.

[0032] Depending on the degree of drying of the alcohol used, which can range, for example, from technical grade to an absolute solvent, and the highly hygroscopic alkali metal alcoholates, the first step (a) according to the invention may, in addition to alkoxysiloxanes, possibly also produce proportions of hydroxyl-bearing siloxanes (see inventive example 1 with ethanol). However, their possible presence alongside the alkoxysiloxane is irrelevant for the successful implementation of the process according to the invention and, in particular, for completing steps (b) and (c).

[0033] If the reaction in the first step (a) of the process according to the invention is carried out without the use of water-binding silicic acid esters, in particular without the use of tetraalkoxysilanes, this corresponds to a particularly preferred embodiment of the invention.

[0034] The second step (b) according to the invention provides that the reaction mixture resulting from the first step (a) is neutralized with the aid of at least one Brönsted acid, optionally with the addition of at least one solvent, the solid components are separated off, in particular filtered off, and then the solvent added beforehand, if appropriate, and the excess alcohol originating from the first step (a) are distilled off from the alkoxysiloxane(s) obtained.

[0035] The third step (c) according to the invention provides that the alkoxysiloxane(s) obtained from the second step (b) are heated with at least one fatty alcohol and at least one alkali metal alkoxide while mixing, and the siloxane cycles formed are separated thermally, preferably by distillation, in particular by applying an auxiliary vacuum.

[0036] According to a particularly preferred embodiment of the invention, the process according to the invention can be carried out semi-continuously in the third step (c) by adding a fresh amount of alkoxysiloxane continuously or in portions discontinuously to the distillation bottoms which are depleted or depleted in alkoxysiloxane and siloxane cycles and still contain fatty alcohol and alkali metal alkoxide, and restarting or continuing the formation of siloxane cycles in this way.

[0037] The thermal, preferably distillative, separation of the volatile compounds resulting from the third step (c) yields a distillate containing siloxane cycles, preferably a distillate comprising, in particular consisting of, the siloxane cycles octamethylcyclotetrasiloxane (D 4 ) and decamethylcyclopentasiloxane (D 5 ).

[0038] When using the particularly preferred alcohols methanol and / or ethanol in the first step (a) of the process according to the invention, it is furthermore possible in the third step (c) to preferably collect a low-boiling fraction containing alkoxy-Si bodies, which, as analytically proven, consists essentially of dialkoxydimethylsilane and dialkoxytetramethyldisiloxane in addition to entrained portions of siloxane cycles.

[0039] For the purposes of the present invention, alkoxy-Si bodies are those silicon-containing silanes or siloxanes which have two alkoxy-Si bonds per molecule and which, due to their volatility, can be separated thermally, preferably by distillation, from the third step (c) according to the invention.

[0040] For the purposes of the present invention, "low-boiling fraction" means the material that, due to its increased volatility, can be obtained by thermal separation before the thermal separation of the bulk of the siloxane cycles in the third step (c). It preferably comprises, and in particular consists of, the alkoxy-Si compounds dialkoxydimethylsilane and dialkoxytetramethyldisiloxane, along with entrained portions of the siloxane cycles.

[0041] For the purposes of this invention, "volatile" means, in particular, that these substances evaporate (volatize) quickly due to their low boiling point or high vapor pressure. According to the definition of the World Health Organization (WHO), VOCs (volatile organic compounds) are organic substances with a boiling range of 60°C to 250°C. Quote taken from "Volatile Organic Compounds," https: / / wissenwiki.de.

[0042] Only to illustrate the relative volatilities of the substances preferably involved in the third step (c) of the process according to the invention, the boiling points referred to in W. Noll, Chemie und Technologie der Silicone, Verlag Chemie, Weinheim (1960) on pages 62, 168 and 180 are used here.

[0043] For example, when using the ethanol which is particularly preferred in the first step (a) according to the invention, the following situation is expected: (CH 3 ) 2 Si(OC 2 H 5 ) 2 114°C boiling point at 742 Torr (corresponding to 989.251 hPa) C 2 H 5 O[(CH 3 ) 2 SiO] 2 C 2 H 5 161°C boiling point at 760 Torr (corresponding to 1013.249 hPa) [(CH 3 ) 2 SiO] 3 134°C boiling point at 760 Torr (corresponding to 1013.249 hPa) [(CH 3 ) 2 SiO] 4 175°C boiling point at 760 Torr (corresponding to 1013.249 hPa) [(CH 3 ) 2 SiO] 5 210°C boiling point at 760 Torr (corresponding to 1013.249 hPa) [(CH 3 ) 2 SiO] 6 245°C boiling point at 760 Torr (corresponding to 1013.249 hPa)

[0044] The low-boiling fraction containing alkoxy-Si bodies, which can be optionally separated in the third step (c), preferably separated, can preferably be fed to the first step (a), which is to be restarted or continued, i.e., the depolymerization, if desired. As a substance providing alkoxy groups, it can, if desired, replace portions of the fresh alcohol added otherwise. This corresponds to a particularly preferred embodiment of the invention.

[0045] It thus also corresponds to a particularly preferred embodiment of the invention if excess alcohol originating from the second step (b) and / or the low-boiling fraction containing alkoxy-Si bodies optionally separated in the third step (c) are recycled as reagents to the depolymerization step (a).

[0046] Such a procedure as well as a semi-continuous cyclization process step (c) described above are very advantageous for a recycling process for waste silicones which is particularly preferred from the point of view of material economy in the sense of a preferred embodiment of the invention.

[0047] It corresponds to a preferred embodiment of the invention if the at least one waste silicone is mechanically comminuted before the reaction, ie before carrying out the first step (a).

[0048] Within the scope of the invention, the term "waste silicone" (or synonymously: "end-of-life silicone") encompasses all silicone-based or silicone-containing products, as well as products with silicone adhesions or silicone contamination, that have almost and / or completely reached their technical service life or durability, or that would otherwise be considered for disposal. Durability or service life refers to the period of time a material or object can be used without the replacement of core components or complete failure.Within the scope of the teaching, this also includes those silicone adhesives and silicone sealants, for example in cartridges, whose shelf life or expiration date has almost been reached and / or exceeded (assessed according to the expected and / or already reached stage of curing), as well as, for example, more or less old sprue and / or stamping waste from silicone rubber production or also discarded electronic scrap containing silicone-sealed components / component groups. The term waste silicone within the scope of the teaching of the invention also includes all silicone waste, including production waste. It includes in particular all silicones or silicone-containing parts or parts with silicone adhesions or silicone contamination that are otherwise intended for normal disposal and are therefore considered waste. It therefore includes, for example:This also includes silicone adhesive and / or sealant cartridges intended for disposal, especially used silicone adhesive and / or sealant cartridges in and on which silicone residues are still present or adhered. The terms "waste silicones," "silicone waste," and "end-of-life silicones" are understood synonymously for the purposes of this invention.

[0049] In particular, in the context of this invention, waste silicone is understood to mean corresponding silicone rubbers and / or silicone oils.

[0050] If the at least one waste silicone, in particular corresponding waste silicone oils, is composed of D and M units, then a further preferred embodiment of the invention is present. For the purposes of this invention, alkali metal alcoholate is preferably understood to mean compounds of the general formula: [M +< ][OR -< ], where Mouse is selected from the series of alkali metals Li, Na or K, preferably Na or K, and represents a pure linear, branched or cyclic alkyl radical, preferably having 1 to 10 carbon atoms, particularly preferably having 1 to 6 carbon atoms, most preferably having 1 or 2 carbon atoms.

[0051] According to the invention, at least one alkali metal alkoxide is used. Thus, one or more alkali metal alkoxides may be used. Most preferred is potassium ethanolate, sodium ethanolate, potassium methoxide, and / or sodium methoxide.

[0052] Among the known processes for the production of alcoholates is chlor-alkali electrolysis according to the amalgam process, in which sodium amalgam is reacted with alcohol [see, for example, Chemical and Engineering News 22, 1903-06 (1944)].

[0053] Another known method is the production of alkoxides from an alkali metal and an alcohol, or from an alkali hydroxide and an alcohol. The production of alkoxides from an alkali metal and a tertiary alcohol is known, for example, from DE-23 33 634 (Dynamit Nobel) or DE 26 12 642 (Degussa). The production of an alkoxide from an alkali hydroxide and a tertiary alcohol is also known. The first process variant requires the use of expensive alkali metal, and in the second variant, starting from the alkali hydroxide, the water formed during the reaction must be removed by distillation, which requires a correspondingly high thermal expenditure.

[0054] According to the teaching of DE-A-33 46 131, alkali metal alkoxides can be produced electrolytically from salts using an electrolysis cell in which a cation exchange membrane separates the electrode spaces. DE-42 33 191.9-43 describes a process that enables the production of an alkali metal alkoxide from a salt by electrodialysis.

[0055] Processes for the production of special alcoholates, such as the alcoholates of higher and / or polyhydric alcohols, have also been described in isolated cases. Alcoholates of higher and / or polyhydric alcohols can generally be produced by transalcoholization, i.e., by substituting the alcoholate residue of lower alcoholates ROM by reaction with higher alcohols R'OH (where R and R' are alkyl radicals of different carbon chain lengths and M denotes a metal cation) in a liquid reaction mixture at suitable temperature and pressure conditions. This reaction, which substitutes for the alcoholate residue, is also referred to as "reboiling" in laboratory jargon. The position of the equilibrium ROM + R'OH ⇔ ROH + R'OM depends on the acidity of the two alcohols, which decreases in the order methanol > primary > secondary > tertiary alcohols [RT Mclver and JA Scott, J. American Chem. Soc. 96 (1973) 2706].Therefore, the production of alkoxides of secondary alcohols by this method is only possible in exceptional cases, and the production of alkoxides of tertiary alcohols by transalcoholization is not possible at all ["Methoden der Organischen Chemie" (1963) Vol. 6 / 2, p. 13]. However, the production of alkoxides by reboiling was also described for higher alcohols in DE-1 254 612 and DE-27 26 491 (both from Dynamit Nobel). GB-1 143 897 (Metallgesellschaft) describes the reaction of a monohydric alkali alkoxide with a C 2 - to C 18 -alcohol or phenol containing up to six hydroxyl groups, using an excess of monohydric alcohol and / or a hydrocarbon as the solvent. However, reboiling always leads to the formation of the low-boiling alcohol ROH (e.g.Methanol), which must be separated from the reaction product mixture, sometimes with considerable thermal effort, in order to isolate the desired alcoholate - possibly in addition to the unreacted higher alcohol R'OH.

[0056] Beyond these thermally driven equilibrium shifts, EP 0776995 B1 teaches a process for producing alkoxides under the influence of an electric field. An alcohol is converted into the desired alkoxide by adding metal ions, and the metal ions themselves originate from the electrochemical decomposition of another alkoxide in the electric field. The alkoxide formation and decomposition take place in chambers spatially separated by ion exchange membranes.

[0057] The at least one alkali metal alkoxide used in the first step (a) of the process according to the invention is preferably used in total amounts of 1 to 20 mass%, preferably 5 to 19 mass%, particularly preferably 6 to 18 mass%, based on the total mass of the waste silicone used in the reaction. This corresponds to a preferred embodiment of the invention.

[0058] The process according to the invention is carried out in the first step (a) preferably in the temperature range from 50°C to 200°C, preferably in the temperature range from 60°C to 180°C, in particular between 78°C and 170°C, and over a period of preferably 0.5 to 12 hours, preferably over a period of 1 to 8 hours, and in each case preferably solvent-free. This corresponds to a particularly preferred embodiment of the invention.

[0059] The process according to the invention can preferably be carried out in the first step (a) at a pressure greater than 1,013.25 hPa and less than 12,000 hPa, i.e., at overpressure. Likewise, it can preferably also be carried out at normal ambient pressure.

[0060] According to a preferred embodiment of the invention, the at least one alcohol used in the first step (a) of the process according to the invention is selected from the group consisting of linear, branched, and cyclic C 1 to C 10 alkanols. One or more alcohols, including mixtures of alcohols, can be used; preferably, methanol, ethanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, isobutanol, pentanols, hexanols, heptanols, octanols, nonanols, and / or decanols, and / or their isomers, can be used. Methanol and / or ethanol are particularly preferred, and ethanol is most particularly preferred.

[0061] The process according to the invention provides in the first step (a), inter alia, that the reaction is carried out without removing any water that may occur from the reaction mixture, in particular without the use of inert, water-insoluble solvents which form azeotropes with water and / or without the use of further dehydrating agents.

[0062] The alcohol used in the first step (a) is not considered a solvent within the meaning of this first step (a). This means that the at least one alcohol, preferably selected from the group consisting of linear, branched, and cyclic C 1 to C 10 alkanols, in particular methanol, ethanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, isobutanol, pentanols, hexanols, heptanols, octanols, nonanols, and / or decanols, and / or their isomers, particularly preferably methanol and / or ethanol, very particularly preferably ethanol, is / are not solvents within the meaning of the first step (a) of claim 1.

[0063] According to a preferred embodiment of the invention, the at least one alcohol used in the first step (a) is used in total amounts of 10 to 200 mass%, preferably 20 to 100 mass%, particularly preferably in amounts of 30 to 80 mass%, based on the total mass of the waste silicone used in the reaction.

[0064] Ensuring good stirrability and miscibility in the first step (a) of the process according to the invention can preferably have a positive influence on the ease with which the dissolution process takes place. From this point of view, in order to avoid high shear and stirring performance, particularly when using solid waste silicones with a high degree of polymerization and possible cross-linking, it can be preferable to design the first step a) of the process according to the invention, for example, in a sequenced manner. This means that a portion of optionally previously comminuted waste silicone is first reacted with alkali metal alkoxide(s) and alcohol(s) while applying heat and the consistency of the reaction matrix is ​​then assessed with regard to its stirrability and miscibility. If this reaction matrix proves to be readily stirrable, a further portion of the waste silicone can be added and the process continued according to the invention.This procedure can be continued until the reaction matrix exhibits the desired target rheology. This corresponds to a preferred embodiment of the invention.

[0065] In a further preferred embodiment, and advantageous for the achievable yield in the reaction of the waste silicone with at least one alcohol and at least one alkali metal alkoxide, the reaction according to the invention can also be carried out under superatmospheric pressure conditions in a pressure-resistant reactor. The pressure buildup observed is preferably autogenous in nature and is due to the vapor pressure of the system components involved. If desired, the reactor can also preferably be pressurized with an inert gas cushion.

[0066] For the application of the process according to the invention on an industrial scale, it may be advisable and therefore preferred to first evaluate the respective waste silicone with the help of some preliminary tests on a laboratory scale in order to determine the optimal process parameters in each case.

[0067] As will be understood by those skilled in the art, the degree of polymerization, the degree of crosslinking, and, if present, the type and amount of filler incorporated into the respective waste silicone influence its behavior in the inventive implementation. Among waste silicones, peroxide-post-crosslinked and also tempered silicone rubbers always pose a particular technical challenge for chemical recycling. Tempering silicone rubber parts improves their dimensional stability and prevents the exudation of plasticizers, especially during application at high temperatures.

[0068] According to the invention, the reaction mixture resulting from the first step (a) of the process according to the invention is neutralized in a second step (b) by adding at least one Brønsted acid, optionally with the addition of at least one solvent. One or more Brønsted acids can be used. Preferably, anhydrous mineral acids (such as preferably anhydrous sulfuric acid and / or anhydrous perchloric acid) and / or anhydrous organic acids (such as preferably anhydrous acetic acid) can be used for neutralization. In particular, anhydrous sulfuric acid, anhydrous perchloric acid, and / or anhydrous acetic acid are used.

[0069] When using anhydrous mineral acid(s), their addition amount is preferably calculated to achieve stoichiometric equivalence relative to the total alkali metal alkoxide used. When using the significantly weaker, anhydrous organic acid(s) (such as anhydrous acetic acid), a significant stoichiometric excess of acid relative to the total alkali metal alkoxide used is preferably selected. This is preferably calculated to a stoichiometric excess of up to 50%.

[0070] The amount of Brönsted acid used is preferably chosen so that it ranges from stoichiometric equivalence to a 50% stoichiometric excess, in each case based on the total amount of alkali metal alcoholate used.

[0071] Particularly when the amount of salt expected from the neutralization step according to the invention prevents easy filtration, the use of at least one solvent may be preferred. Optionally, one or more solvents may be used.

[0072] According to the invention, one or more solvents suitable for the second step (b) are preferably those which are themselves chemically inert with respect to the reaction system and promote the dilution or dispersion of the constituents of the neutralization stage. Preferably, the at least one solvent is selected from the group consisting of alkanes, alkylaromatics, alcohols, hexamethylcyclotrisiloxane (D 3 ), octamethylcyclotetrasiloxane (D 4 ), decamethylcyclopentasiloxane (D 5 ), and dodecamethylcyclohexasiloxane (D 6 ).

[0073] Particular preference is given to the use of alkylaromatics, such as toluene and / or xylenes. Also preferred are siloxanes selected from the group consisting of hexamethylcyclotrisiloxane (D 3 ), octamethylcyclotetrasiloxane (D 4 ), decamethylcyclopentasiloxane (D 5 ), and dodecamethylcyclohexasiloxane (D 5 ), as well as mixtures thereof.

[0074] The solid components resulting from the neutralization can then preferably be separated, in particular filtered off, in the second step (b). Then, in particular after filtering off the solid components resulting from the neutralization, the volatile compounds are separated off by distillation in the second step (b), and the alkoxysiloxane, which may also contain portions of hydroxy-functional polydimethylsiloxane, is isolated.

[0075] In the third step (c) of the process according to the invention, at least one fatty alcohol, preferably having a carbon number of C 12 to C 18 , preferably having a carbon number of C 14 to C 18 , in particular having a carbon number of C 16 (preferably stearyl alcohol = hexadecanol), is preferably used. The use of stearyl alcohol is very particularly preferred. It is also particularly preferred to use mixtures of two or more suitable fatty alcohols.

[0076] Preferably, the at least one fatty alcohol used in the third step (c) is used in amounts of 20 to 50 percent by weight, preferably in amounts of 25 to 35 percent by weight, based on the total amount of alkoxysiloxane present.

[0077] Preferably, the amount of alkali metal alcoholate used in the third step (c) is 10 to 20 percent by weight, preferably 12 to 18 percent by weight, based on the total amount of the at least one fatty alcohol used in the third step (c).

[0078] The process according to the invention is carried out in the third step (c) preferably in the temperature range from 100°C to 200°C, preferably in the temperature range from 120°C to 160°C and over a period of preferably 1 to 12 hours, preferably over a period of 2 to 8 hours, and in each case preferably with the application of an auxiliary vacuum, preferably with a pressure range of less than 300 hPa, in particular with a pressure range of 0.001 hPa to 250 hPa. This corresponds to a preferred embodiment of the invention.

[0079] The siloxane cycles resulting from the third step (c) according to the invention are suitable, in particular in optionally distillatively purified form, as base materials for producing a large number of organomodified siloxanes according to all processing routes familiar to the person skilled in the art. Examples:

[0080] The following examples serve solely to explain this invention to the person skilled in the art and do not represent any limitation of the subject matter of the invention. 29< Si NMR spectroscopy was used to monitor the reaction in all examples.

[0081] The 29< Si NMR samples were measured in the context of this invention at a measurement frequency of 79.49 MHz in a Bruker Avance III spectrometer equipped with a probe head 287430 with a 10 mm slit width, at 22°C dissolved in CDCl 3 and against tetramethylsilane (TMS) as an external standard [d( 29< Si) ​​= 0.0 ppm].

[0082] Unless otherwise stated, all percentages are to be understood as percentages by weight. Example 1 (according to the invention)

[0083] In a 1000 ml four-necked round-bottomed flask equipped with a precision glass stirrer, internal thermometer and attached reflux condenser, 300 g of a mixed silicone rubber waste consisting of 150 g of a small-sized, cross-linked silicone extruder waste (particle diameter approx. 6 mm) and 150 g of a bathroom and sanitary silicone (Conel GmbH, Manhattan grey) that had been comminuted to a comparable size and previously cured in air on polyethylene film, together with 300 g of ethanol and 50 g of sodium ethanolate (NaOC 2 H 5 ) were rapidly heated to 80°C with stirring, whereby slight reflux boiling occurred.

[0084] After about 1 hour of reaction time, the silicone pieces had dissolved, forming a homogeneous, yellow-brown suspension. The mixture was allowed to cool to approximately 40°C, and 66.3 g of anhydrous acetic acid (50% excess acid based on the alcoholate used) was added while stirring. Stirring was continued for about 30 minutes, and the solid components were filtered off using a filter press through a Seitz K300 filter disc.

[0085] The resulting filtrate was freed from ethanol using a rotary evaporator at 70°C and an applied vacuum of < 5 mbar. Concentration of the filtrate resulted in a slight salt precipitation, which was removed by filtration through a pleated filter (MN 606 1 / 4).

[0086] According to 29< Si NMR spectroscopy, the yellowish-brownish filtrate consisted of an α,ω-diethoxy-polydimethylsiloxane with an average chain length of N = 20.8 with smaller amounts of a hydroxy-functional polydimethylsiloxane.

[0087] A 30 g portion of said filtrate was heated together with 15 g of stearyl alcohol (hexadecanol) and 2.5 g of potassium methoxide for one hour at 130°C bottom temperature on a rotary evaporator at atmospheric pressure (1,013.25 hPa), whereby no distillate was observed.

[0088] The bottom temperature was then raised to 140°C, and an auxiliary vacuum of <5 mbar was applied. Within the next 2 hours, the majority of the distillate was transferred, and after a total of 4 hours, 18.1 g of a colorless, clear distillate was isolated, corresponding to 60.3% of the alkoxysiloxane used. The accompanying GC analysis assigned a composition of 0.54% D 3 , 83.9% D 4 , 11.0% D 5 , and 1.8% D 6 , with complete freedom from ethanol (<0.01%) (total 97.2%). In addition, 3.2 g of a colorless, clear liquid were collected in a downstream cold trap. This low boiler fraction containing alkoxy-Si bodies corresponded in its amount to 10.7% of the amount of alkoxysiloxane used and, according to accompanying GC analysis, had a composition of 2.4% D 3 , 52.0% D 4 , 0.29% D 5 , and < 0.02% D 6 as well as 1.8% ethanol (sum of the siloxane cycles = 54.7%).The remaining components consisted of diethoxydimethylsilane and diethoxytetramethyldisiloxane according to 29< Si NMR spectroscopy.

[0089] In accordance with the semi-continuous procedure particularly preferred according to the invention, the bottom flask of the rotary evaporator was then again charged with a 30 g portion of the filtrate and the procedure described above was repeated, but a bottom temperature of 140°C was chosen from the beginning.

[0090] 26.2 g of a colorless, clear distillate was isolated (corresponding to 87.3% of the alkoxysiloxane used). The accompanying GC analysis assigned this distillate a composition of 0.6% D 3 , 82.5% D 4 , 12.1% D 5 , and 2.0% D 6 , as well as <0.01% ethanol (total of the siloxane cycles: 97.2%). The contents of the downstream cold trap also consisted of a colorless, clear liquid and weighed 3.0 g (corresponding to 10.0% of the alkoxysilane used). The accompanying GC analysis assigned a composition of 6.4% D 3 , 82.8% D 4 , 1.30% D 5 , and 0.04% D 6 , as well as 0.14% ethanol (total siloxane cycle fraction = 90.2%) to the cold trap contents. The remaining components consisted of diethoxydimethylsilane and diethoxytetramethyldisiloxane, according to 29< Si NMR spectroscopy.

Claims

1. Process for depolymerizing waste silicones to afford siloxane cycles, characterized in that the process comprises (a) a first step of reacting at least one waste silicone by mixing with at least one alcohol and at least one alkali metal alkoxide with heating but without removing any potentially occurring water from the reaction mixture, in particular without the use of inert solvents which are water-insoluble but form azeotropes with water and / or without the use of further dehydrating agents and (b) subsequently neutralizing the reaction mixture resulting from this reaction in a second step using at least one Brønsted acid optionally with addition of at least one solvent, removing, especially filtratively removing, the solid constituents and then distillatively removing the optionally previously added solvent and the excess alcohol deriving from step (a) from the obtained alkoxysiloxane(s) and (c) subsequently in a third step heating the thus-obtained alkoxysiloxane(s) with at least one fatty alcohol and at least one alkali metal alkoxide with mixing and thermally removing, preferably distillatively removing, the siloxane cycles formed.

2. Process according to Claim 1, characterized in that the reaction in the first step (a) is undertaken without the use of water-binding silicic esters, in particular without the use of tetraalkoxysilanes.

3. Process according to either of Claims 1 to 2, characterized in that the at least one alkali metal alkoxide conforms to the general formula [M+] [OR-], wherein M is selected from the group of alkali metals Li, Na or K, preferably Na or K, and R represents a linear, branched or cyclic alkyl radical, preferably having 1 to 10 carbon atoms, particularly preferably having 1 to 6 carbon atoms, very particularly preferably having 1 or 2 carbon atoms.

4. Process according to any of Claims 1 to 3, characterized in that the at least one alcohol employed in the first step (a) is selected from the group consisting of linear, branched and cyclic C1 to C10 alkanols, preferably methanol, ethanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, isobutanol, pentanols, hexanols, heptanols, octanols, nonanols and / or decanols and / or the isomers thereof, particularly preferably methanol and / or ethanol, very particularly preferably ethanol.

5. Process according to any of Claims 1 to 4, characterized in that the at least one alcohol employed in the first step (a) is employed in total amounts of 10% to 200% by mass, preferably 20% to 100% by mass, particularly preferably 30% to 80% by mass, based on the total mass of the waste silicone altogether used in the reaction.

6. Process according to any of Claims 1 to 5, characterized in that the at least one alkali metal alkoxide employed in the first step (a) is employed in a total amount of 1% to 20% by mass, preferably from 5% to 19% by mass, particularly preferably from 6% to 18% by mass, based on the total mass of the waste silicone altogether used in the reaction.

7. Process according to any of Claims 1 to 6, characterized in that the at least one Brønsted acid added in the second process step (b) is an anhydrous mineral acid and / or anhydrous organic acid, in particular anhydrous sulfuric acid, anhydrous perchloric acid and / or anhydrous acetic acid.

8. Process according to any of Claims 1 to 7, characterized in that at least one solvent preferably selected from the group consisting of alkanes, alkylaromatics, alcohols, hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) is added in the second step (b).

9. Process according to any of Claims 1 to 8, characterized in that the reaction of the at least one waste silicone in the first step (a) is undertaken at temperatures between 50°C and 200°C, preferably between 60°C and 180°C, in particular between 78°C and 170°C.

10. Process according to any of Claims 1 to 9, characterized in that the reaction of the at least one waste silicone in the first step (a) is performed over a period of 0.5 to 12 hours, preferably over a period of 1 to 8 hours, and / or, preferably and, at a pressure above 1013.25 hPa and below 12000 hPa.

11. Process according to any of Claims 1 to 10, characterized in that the third step (c) is performed semicontinuously by adding to the distillation bottoms depleting or depleted in alkoxysilane and in siloxane cycles and still containing fatty alcohol and alkali metal alkoxide a fresh amount of alkoxysiloxane continuously or portionwise and discontinuously and thus restarting or continuing the formation of siloxane cycles and / or, preferably and, in the third step (c) removing a low boilers fraction containing alkoxy-Si structures and supplying it to the first step (a) that is to be restarted or continued.

12. Process according to any of Claims 1 to 11, characterized in that the third step (c) employs at least one fatty alcohol having a carbon number of preferably C12 to C18, preferably having a carbon number of C14 to C18, very particularly preferably having a carbon number of C16, in particular hexadecanol.

13. Process according to any of Claims 1 to 12, characterized in that the at least one fatty alcohol used in the third step (c) is employed in total amounts of 20% to 50% by weight, preferably in amounts of 25% to 35% by weight, based on the total amount of alkoxysilane present.

14. Process according to any of Claims 1 to 13, characterized in that the amount of alkali metal alkoxide used in the third step (c) is 10% to 20% by weight, preferably 12% to 18% by weight, based on the total amount of the at least one fatty alcohol used in the third step (c).

15. Process according to any of Claims 1 to 14, characterized in that the third step (c) is performed in the temperature range of 100°C to 200°C, preferably in the temperature range of 120°C to 160°C, and over a period of preferably 1 to 12 hours, preferably over a period of 2 to 8 hours, preferably with application of an auxiliary vacuum, preferably at a pressure range below 300 hPa, in particular at a pressure range of 0.001 hPa to 250 hPa.