Process for stabilization of dry cleaning solutions

[0002] The present invention is directed to a process, more specifically, to a process for stabilizing silicone dry cleaning solvents containing basic impurities.

[0003] Current dry cleaning technology uses perchloroethylene (“PERC”) or petroleum-based materials as the cleaning solvent. PERC suffers from toxicity and odor issues. The petroleum-based products are not as effective as PERC in cleaning garments. Volatile siloxanes are being introduced into the dry cleaning industry as an alternative to PERC. However, there exists a need to stabilize the siloxane solvents to prevent undesirable cyclic siloxane (D₄) formation and polymerization.

[0004] Methods for the purification of organopolysiloxanes have previously been reported, but they have not been reported for the purification of certain cyclic siloxanes (D₅). Methods for purifying organopolysiloxanes utilizing elemental metals has been reported (see U.S. Pat. No. 5,245,067). Other patents disclose the purification of polyether silicones by contacting with an aqueous acid and removing the odorous materials formed (see U.S. Pat. No. 5,118,764), or the reaction with hydrogen and a hydrogenation catalyst (see U.S. Pat. No. 5,225,509). Hexamethyldisiloxane has been purified by successive treatments with a condensation catalyst, washing with water, separating the phases, distilling the siloxane, treating with acid clay and then treating with activated carbon (see U.S. Pat. No. 4,774,346). Siloxanes have also been purified by contacting with steam and distilling out the impurities (see EP 543 665). A deodorization method utilizing active carbon to which a functional group has been fixed through a silanol bond has been reported (see U.S. Pat. No. 5,238,899). Finally, a method was reported for purifying silicone oil by adding a drying agent and an adsorption agent to silicone and passing a low water vapor inert gas through the system (see U.S. Pat. No. 4,661,612).

[0005] What is needed in the art is an aqueous extraction system that renders the base catalyst inactive, stabilizes the siloxane and suppresses reequilibration and polymerization.

[0006] In a first aspect, the present invention is directed to a method for stabilizing silicone dry cleaning solvents that may contain an undesirable basic impurity capable of causing cyclic siloxane formation, comprising contacting the silicone solvent with an aqueous solution to purify the solvent and then separating the silicone solvent.

[0007] The process of the present invention is effective in preventing formation of certain cyclic siloxanes (i.e., D₄) that are undesirable in the silicone solvent.

[0008] As used herein, the terms D₄, D₅and D₆refer to cyclic siloxanes having the formula: —(R₂SiO)_x— where x is 4, 5 or 6 (i.e., D₅is decamethylcyclopentasiloxane).

[0009] Preferably, the first preferred embodiment of the method of the present invention comprises, contacting a silicone dry cleaning solvent that may contain an undesirable basic impurity capable of causing cyclic siloxane formation with an aqueous solution, agitating to ensure good mixing of the solvent and the aqueous solution, and separating the silicone solvent. The aqueous solution can be a salt solution or a series of dilute aqueous acid and base solutions.

[0010] Base catalysts that promote reequilibration and redistribution include, but are not limited to, potassium silanolate, potassium hydroxide, tetramethylammonium hydroxide, tetrabutylphosphonium hydroxide and the like.

[0011] Solutions suitable as extractants are aqueous solutions of the formula:

[0012] MeX

[0013] wherein Me refers to alkali metals and alkaline earth metals, such as sodium, magnesium, potassium, calcium and the like, and X refers to halogens, such as chlorine, bromine, fluorine and the like, or chalcogen derivatives, such as sulfates, carbonates, bicarbonates, acetates and the like. Examples of suitable solutions include but are not limited to sodium chloride, potassium chloride, sodium bromide, magnesium sulfate and the like.

[0014] Also suitable are dilute aqueous acid mixtures and base mixtures such as acetic acid, citric acid, sorbic acid, sodium carbonate and sodium bicarbonate.

[0015] After extraction, the silicone solvent may be dried with a suitable drying agent such as 4A molecular sieves, 13X molecular sieves, magnesium sulfate, calcium chloride and calcium sulfate.

[0016] Preferably, the silicone dry cleaning solvent is a volatile linear, branched, cyclic or a combination thereof, siloxane.

[0017] Compounds suitable as the linear or branched, volatile siloxane solvent of the present invention are those containing a polysiloxane structure that includes from 2 to 20 silicon atoms. Preferably, the linear or branched, volatile siloxanes are relatively volatile materials, having, for example, a boiling of below about 300° C. point at a pressure of 760 millimeters of mercury (“mm Hg”).

[0018] In a preferred embodiment, the linear or branched, volatile siloxane comprises one or more compounds of the structural formula (I):

[0019] M_2+y+2zD_xT_yQ_z  (I)

[0020] wherein:

[0021] M is R¹₃SiO_1/2;

[0022] D is R²₂SiO_2/2;

[0023] T is R³SiO_3/2;

[0024] and Q is SiO_4/2

[0025] R¹, R²and R³are each independently a monovalent hydrocarbon radical; and

[0026] x and y are each integers, wherein 0≦x≦10 and 0≦y≦10 and 0≦z≦10.

[0027] Suitable monovalent hydrocarbon groups include acyclic hydrocarbon radicals, monovalent alicyclic hydrocarbon radicals, monovalent and aromatic hydrocarbon radicals. Preferred monovalent hydrocarbon radicals are monovalent alkyl radicals, monovalent aryl radicals and monovalent aralkyl radicals.

[0028] As used herein, the term “(C₁-C₆)alkyl” means a linear or branched alkyl group containing from 1 to 6 carbons per group, such as, for example, methyl, ethyl, propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, pentyl, hexyl, preferably methyl.

[0029] As used herein, the term “aryl” means a monovalent unsaturated hydrocarbon ring system containing one or more aromatic rings per group, which may optionally be substituted on the one or more aromatic rings, preferably with one or more (C₁-C₆)alkyl groups and which, in the case of two or more rings, may be fused rings, including, for example, phenyl, 2,4,6-trimethylphenyl, 2-isopropylmethylphenyl, 1-pentalenyl, naphthyl, anthryl, preferably phenyl.

[0030] As used herein, the term “aralkyl” means an aryl derivative of an alkyl group, preferably a (C₂-C₆)alkyl group, wherein the alkyl portion of the aryl derivative may, optionally, be interrupted by an oxygen atom, such as, for example, phenylethyl, phenylpropyl, 2-(1-naphthyl)ethyl, preferably phenylpropyl, phenyoxypropyl, biphenyloxypropyl.

[0031] In a preferred embodiment, the monovalent hydrocarbon radical is a monovalent (C₁-C₆)alkyl radical, most preferably, methyl.

[0032] In a preferred embodiment, the linear or branched, volatile siloxane comprises one or more of, hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane, dodecamethylpentasiloxane, tetradecamethylhexasiloxane or hexadecamethylheptasiloxane or methyltris(trimethylsiloxy)silane. In a more highly preferred embodiment, the linear or branched, volatile siloxane of the present invention comprises octamethyltrisiloxane, decamethyltetrasiloxane, or dodecamethylpentasiloxane or methyltris(trimethylsiloxy)silane. In a highly preferred embodiment, the siloxane component of the composition of the present invention consists essentially of decamethyltetrasiloxane.

[0033] Suitable linear or branched volatile siloxanes are made by known methods, such as, for example, hydrolysis and condensation of one or more of tetrachlorosilane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, or by isolation of the desired fraction of an equilibrate mixture of hexamethyldisiloxane and octamethylcyclotetrasiloxane or the like and are commercially available.

[0034] Compounds suitable as the cyclic siloxane component of the present invention are those containing a polysiloxane ring structure that includes from 2 to 20 silicon atoms in the ring. Preferably, the linear, volatile siloxanes and cyclic siloxanes are relatively volatile materials, having, for example, a boiling point of below about 300° C. at a pressure of 760 millimeters of mercury (“mm Hg”).

[0035] In a preferred embodiment, the cyclic siloxane component comprises one or more compounds of the structural formula (II):

[0036] wherein:

[0037] R⁵, R⁶, R⁷and R⁸are each independently a monovalent hydrocarbon group; and

[0038] a and b are each integers wherein 0≦a≦10 and 0≦b≦10, provided that 3≦(a+b)≦10.

[0039] In a preferred embodiment, the cyclic siloxane comprises one or more of, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, tetradecamethylcycloheptasiloxane. In a more highly preferred embodiment, the cyclic siloxane of the present invention comprises octamethylcyclotetrasiloxane or decamethylcyclopentasiloxane. In a highly preferred embodiment, the cyclic siloxane component of the composition of the present invention consists essentially of decamethylcyclopentasiloxane.

[0040] Suitable cyclic siloxanes are made by known methods, such as, for example, hydrolysis and condensation of dimethyldichlorosilane and are commercially available.

[0041] It is believed that those dry cleaning solvents useful in the present invention that lack a cyclic siloxane component would be more stable than those which include a cyclic siloxane component, in that cyclic siloxanes are known to ring open and polymerize under acidic and basic conditions.

[0042] In a first embodiment of the method of the present invention, approximately 100 parts by weight (“pbw”) of siloxane solvent is contacted with up to 1000, more preferably up to 500, even more preferably up to 100 pbw of an aqueous salt or weak acid mixture which could be followed by a weak base mixture treatment or a combination thereof for 0.001 to 6 hours at 10 to 80° C. After the siloxane solvent has contacted the aqueous solution for the appropriate time and has been purified, the siloxane solvent is separated from the mixture and dried if necessary, and then the solvent can be recycled in the dry cleaning apparatus. The method of the present invention effectively reduces the level of impurities in the silicone solvent.

[0043] In a second embodiment of the method of the present invention, a dry cleaning fluid is treated by the method of the present invention.

[0044] The method of the present invention also comprises a dry cleaning process comprising the steps of: contacting an article with a silicone solvent, and removing the silicone solvent, then treating the silicone solvent that has been removed with an aqueous solution, agitating to ensure good mixing of the solvent and the aqueous solution, and separating the silicone solvent, then reusing the treated silicone solvent in the dry cleaning process.

[0045] The following examples illustrate the process of the present invention. They are illustrative and the claims are not to be construed as limited to the examples.

[0046] Decamethylcyclopentasiloxane (D₅) was mixed with a base catalyst (potassium silanolate with a potassium hydroxide equivalence of approximately 4%) to make a stock solution containing 200 parts per million (“ppm”) potassium hydroxide. Aliquots of the stock solution were shaken with various concentrations of NaCl solutions and then separated or separated and dried at ambient temperature. The silicone solvent was then heated to 100° C. and cyclic levels were monitored over time as shown in table.

[0047] The base catalyst readily initiated polymerization and reequilibration of the D₅as shown by experiment 1. Contacting the contaminated silicone solvent with water or salt solutions and then separating the solvent gave a stable silicone mixture that did not polymerize or reequilibrate. The salt solutions served to more efficiently allow separation of the aqueous and silicone phases.

[0048] The same stock solution from Example 1 was treated with a dilute solution of acid followed by separation of the siloxane layer and then treatment with dilute base and then separation and drying as shown in table 2. The silicone solvent was heated to 100° C. for 2 hours and cyclic levels were measured. Heat was continued until 18 hours, and cyclic levels were again measured.

[0049] Treatment with dilute solutions of weak acids and bases effectively prevented undesired reequilibration and polymerization.

[0050] The same experiment as Example 1 was conducted using a linear siloxane solvent (MD₂M). Results of the experiment are shown in Table 3.

[0051] Table 3 shows that contacting linear siloxane dry cleaning solvents with water and salt solutions is also effective at stabilizing the dry cleaning solvent with respect to base catalysis.

[0052] Another experiment was conducted in the same manner as Example 2 using a linear silicone solvent (MD₂M). The results are shown below in Table 4.

[0053] Table 4 illustrates that treatment with dilute solutions of weak acids and bases effectively prevented undesired reequilibration and polymerization.

[0054] Cyclic silicone solvent (D₅) containing varying concentrations of base were treated with salt solutions. The results are shown in Table 5.

[0055] Table 5 illustrates that up to 1000 ppm base in a silicone solvent can be rendered unreactive when contacted with aqueous salt solutions.

[0056] Cyclic silicone solvent (D₅) containing varying concentrations of base were treated with varying amounts of water. The results are shown in Table 6.

[0057] Table 6 illustrates that contacting base containing silicone solvents with aqueous systems at various ratios of silicone solvent to water are effective in suppressing polymerization and reequilibration.