UPCYCLING PROCESS FOR PROCESSING SILICONE WASTES

Abstract

Upcycling process for producing acidic, end-equilibrated siloxanes bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms from end-of-life silicones by thermal digestion in an acidic reaction medium comprising acetic anhydride, acetic acid and at least one further Brønsted acid having a pKa of <4, the digestion taking place in a reactor having a volume of at least 1 liter.

Claims

1. An upcycling process for producing acidic, end-equilibrated siloxanes bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms from end-of-life silicones, by thermal digestion of the end-of-life silicones in an acidic reaction medium comprising acetic anhydride, acetic acid and at least one further Brønsted acid having a pKa of <4, the digestion taking place in a reactor having a volume of at least 1 liter.

2. The upcycling process according to claim 1, wherein the digestion is carried out without removal of water.

3. The upcycling process according to claim 1, wherein the digestion takes place in a reactor having a volume of at least 5 liters.

4. The upcycling process according to claim 1, wherein the digestion is executed at temperatures between 50° C. and 200° C.

5. The upcycling process according to claim 1, wherein the Brønsted acid having a pKa of <4 is used in amounts of 0.1 to 1.5 percent by mass based on the total proportion of silicone in the reaction system.

6. The upcycling process according to claim 1, wherein the acetic acid is used in amounts of 0.5 to 6.0 percent by mass based on the total proportion of silicone in the reaction system.

7. The upcycling process according to claim 1, wherein the acetic anhydride is used in amounts of 0.13 to 33 percent by mass based on the total proportion of silicone in the reaction system.

8. The upcycling process according to claim 1, wherein the Brønsted acids used are protic acids having a pKa of less than −1.30.

9. The upcycling process according to claim 1, wherein the thermal digestion is carried out at standard pressure (1013 hPa), elevated pressure or reduced pressure.

10. The upcycling process according to claim 1, wherein the end-of-life silicones comprise silicone adhesives and/or silicone sealants.

11. The upcycling process according to claim 1 in which silicone-contaminated polyethylene is recycled to provide acidic, end-equilibrated siloxanes bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms with the essentially single-product recovery of polyethylene.

12. A polyether siloxane comprising the acidic, end-equilibrated siloxanes bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms produced from end-of-life silicones using an upcycling process, as set out in claim 1.

13. A silicone-based adhesive compound and/or sealant compound comprising the acidic, end-equilibrated siloxanes bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms produced from end-of-life silicones using an upcycling process, as set out in claim 1.

14. The silicone-based adhesive compound and/or sealant compound according to claim 13, wherein, prior to production of silicone-based adhesive and/or sealant compounds, the acidic, end-equilibrated siloxane bearing acetoxy groups and having chain lengths of greater than 3 silicon atoms is freed of Brønsted acid originating from the upcycling process and still present therein.

15. The upcycling process according to claim 1, wherein the end-of-life silicones are selected from the group consisting of silicone adhesives, silicone sealants, silicone rubber wastes, and silicone oil wastes.

16. The upcycling process according to claim 1, wherein the digestion takes place in a reactor having a volume of at least 10 liters and not more than 500 000 liters.

17. The upcycling process according to claim 1, wherein the digestion is executed at temperatures between 80° C. and 160° C.

18. The upcycling process according to claim 1, wherein the Brønsted acid having a pKa of <4 is used in amounts of 0.2 to 0.8 percent by mass, based on the total proportion of silicone in the reaction system.

19. The upcycling process according to claim 1, wherein the acetic acid is used in amounts of 1.5 to 3.5 percent by mass, based on the total proportion of silicone in the reaction system.

20. The upcycling process according to claim 1, wherein the acetic anhydride is used in amounts of 0.69 to 6.9 percent by mass, based on the total proportion of silicone in the reaction system.

Description

EXAMPLES

[0118] The examples that follow serve solely to elucidate this invention to those skilled in the art and do not constitute any restriction at all of the claimed process. The determination of water contents according to the invention is performed in principle by the Karl Fischer method based on DIN 51777, DGF E-III 10 and DGF C-III 13a. .sup.29Si-NMR spectroscopy was used for reaction monitoring in all examples.

[0119] In the context of the present invention the .sup.29Si NMR samples are analysed at a measurement frequency of 79.49 MHz in a Bruker Avance III spectrometer equipped with a 287430 probe head with slit width of 10 mm, at 22° C. in CDCl.sub.3 solution, and against a tetramethylsilane (TMS) external standard [δ(.sup.29Si)=0.0 ppm].

[0120] The gas chromatograms are recorded on an Agilent Technologies GC 7890B GC instrument fitted with an HP-1 column having dimensions of 30 m×0.32 mm ID×0.25 μm dF (Agilent Technologies No. 19091Z-413E) using hydrogen as a carrier gas and employing the following parameters:

Detector: FID; 310° C.

Injector: Split; 290° C.

[0121] Mode: constant flow, 2 ml/min
Temperature program: 60° C. at 8° C./min—150° C. at 40° C./min—300° C. 10 min.

[0122] The point at which equilibrium has been reached is indicated by the total cycles content determined by gas chromatography, defined as the sum of the D.sub.4, D.sub.5 and D.sub.6 contents based on the siloxane matrix and determined after derivatization to the corresponding α,ω-diisopropoxypolydimethylsiloxanes of the α,ω-diacetoxypolydimethylsiloxanes obtained from the digestion according to the invention. Derivatization to the α,ω-diisopropoxypolydimethylsiloxanes chosen here with the express intention of preventing a thermally induced retrocleavage reaction of the α,ω-diacetoxypolydimethylsiloxanes that could take place under the conditions of the gas chromatography analysis (for information on the retrocleavage reaction, see inter alia J. Pola et al., Collect. Czech. Chem. Commun. 1974, 39(5), 1169-1176 and also W. Simmler, Houben-Weyl, Methods of Organic Chemistry, vol. VI/2, 4th edition, O-Metal Derivatives of Organic Hydroxy Compounds p. 162 ff)).

[0123] The employed polyether diols have water contents of about 0.2% by mass and are predried before use. Toluene and alkylbenzene (C10-C13) used have a water content of 0.03% by mass and are used without predrying.

[0124] The OH value of the polyether diols is determined according to DGF C-V 17 a (53) or according to Ph. Eur. 2.5.3 Method A, wherein the hydroxyl groups of the sample to be analysed are firstly acetylated with acetic anhydride in the presence of pyridine, followed by a differential titration (blank sample, taking into account the excess acetic anhydride) in which the liberated acetic acid is titrated as consumption of KOH in mg per gram of polyether diol.

Example 1 (Inventive)

Production of an End-Equilibrated, Acetoxy-Terminated, Linear Polydimethylsiloxane

[0125] A 1000 ml four-necked flask with precision glass stirrer, internal thermometer and fitted reflux condenser is charged, while stirring, with 120.0 g of a transparent, hardened silicone compound (Care Sanitär Profisilikon, from Conel GmbH) cut into pieces of approx. 4 to 6 mm edge length together with 50.0 g (0.489 mol) of acetic anhydride and 280.0 g (0.752 mol) of decamethylcyclopentasiloxane (D.sub.5) and also 13.6 g of acetic acid (3.0 percent by weight based on the total mass of the reactants), mixed with 0.92 g (0.54 ml) of trifluoromethanesulfonic acid (0.2 percent by mass based on the total mixture), and quickly heated to 80° C. The reaction mixture, which is initially interspersed with proportions of visibly coarser solid material, is held at this temperature for 6 hours with continued stirring.

[0126] On cooling the mixture, a colorless, clear, freely mobile liquid separates as the supernatant of a deposited white solid that is removed by filtration through a filter press. The .sup.29Si NMR spectrum of the clear colorless filtrate shows the presence of Si-acetoxy groups in a yield of approx. 94% based on acetic anhydride used, corresponding to an α,ω-diacetoxypolydimethylsiloxane having an average total chain length of about 14.

Conversion of the α,ω-Diacetoxypolydimethylsiloxane into the Corresponding α,ω-Diisopropoxypolydimethylsiloxane for Analytical Characterization

[0127] Immediately after the synthesis, 50.0 g of this trifluoromethanesulfonic acid-acidified, equilibrated α,ω-diacetoxypolydimethylsiloxane in a 250 ml four-necked round-bottomed flask equipped with a precision glass stirrer, an internal thermometer and fitted reflux condenser is mixed at 22° C., with stirring, with 11.3 g of isopropanol dried over molecular sieves. The reaction mixture is then charged with gaseous ammonia (NH.sub.3), which is passed in until alkaline (moist universal indicator paper), and then stirred at this temperature for a further 45 minutes. The precipitated salts are removed using a fluted filter.

[0128] A colorless, clear liquid is isolated, the .sup.29Si NMR spectrum of which demonstrates quantitative conversion of the α,ω-diacetoxypolydimethylsiloxane into an α,ω-diisopropoxypolydimethylsiloxane.

[0129] An aliquot of this α,ω-diisopropoxypolydimethylsiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents (stated in percent by mass):

TABLE-US-00001 Total Isopropanol D.sub.4 D.sub.5 D.sub.6 (D.sub.4-D.sub.6) content 2.90% 2.20% 0.70% 5.80% 1.50%

[0130] After taking into account the isopropanol excess, the contents of cyclosiloxanes (D.sub.4, D.sub.5 and D.sub.6) are calculated here solely based on the siloxane fraction.

Example 2 (Inventive)

[0131] Conversion of the α,ω-diacetoxypolydimethylsiloxane obtained in example 1 into an SiOC-linked, linear polydimethylsiloxane-polyoxyalkylene block copolymer of the ABA structural type in toluene with ammonia as auxiliary base.

[0132] A 500 ml four-necked flask with precision glass stirrer, internal thermometer and fitted reflux condenser is charged, while stirring, with 96.0 g of a butanol-started, polypropylenoxy-group-containing polyetherol having an average molar mass of 1935 g/mol (determined according to the OH value) together with 126 ml of toluene. To this is then added 30.0 g of the α,ω-diacetoxypolydimethylsiloxane produced in example 1. Stirring of the reaction matrix is continued and a moderate stream of gaseous ammonia is passed into it via a gas-inlet tube for a period of 45 minutes until a drop on moist universal indicator paper shows it to be clearly alkaline.

[0133] Passage of a reduced flow of ammonia is continued for a further 45 minutes and the reaction mixture is heated to 50° C. Gas introduction is terminated and the mixture is allowed to cool to 23° C., after which the salts present therein are separated from the liquid using a fluted filter. The clear filtrate thus obtained is freed from volatiles on a rotary evaporator at a bath temperature of 70° C. and an applied vacuum of <1 mbar.

[0134] A colorless, clear ABA-structured polydimethylsiloxane-polyoxyalkylene block copolymer is isolated, the .sup.29Si NMR spectrum of which confirms the target structure.