Method for purifying α-sulfofatty acid salts

Abstract

The present invention relates to a method for purifying a crude α-sulfofatty acid salt obtained by reacting a fatty acid with a sulfur oxide, comprising the steps of: a) diluting the crude α-sulfofatty acid in a solvent and adjusting the pH to 2-4; b) heating the slurry of α-sulfofatty acid salt obtained in step a) until it is completely dissolved; c) gradually cooling the solution of α-sulfofatty acid salt obtained in step b) to room temperature to obtain crystals; and d) filtering the crystals obtained in step c) to remove the mother liquor and rinsing the filter cake with an acidic aqueous solution at least twice to obtain a purified α-sulfofatty acid salt.

Description

[Technical field]

[0001] The present invention relates to a method for purifying crude α-sulfo fatty acid salts obtained by reacting fatty acids with sulfur oxides, the method being capable of effectively and efficiently removing sulfate impurities. [Background technology]

[0002] Sulfonation of organic acids or alcohols to produce anionic surfactants has been known for a long time. For example, chlorosulfonic acid, sulfur trioxide, or concentrated sulfuric acid can be used as sulfonating agents. It is known that α-sulfo fatty acid salts (i.e. mixtures of salt forms, typically mono- and di-salts), especially di-salts based on fatty acids with a chain length of 12 to 18 carbon atoms, stand out for their good cleaning properties. In the past, α-sulfo fatty acids or their salts have been prepared by sulfonation of fatty acids using anhydrous sulfur trioxide or sulfur dioxide as sulfonating agents. The acids have been recovered as such or converted to salts by neutralization or by precipitation in the presence of more soluble alkaline salts, such as sulfates. The products obtained in these processes are generally colored due to the presence of sulfates, and to reduce the color of the product, a bleaching step, for example with sodium hypochlorite or a compound that generates hypochlorite ions in solution, is necessary, but the product still contains sulfates that are undesirable for the consumer. Personal care products are driven by the needs of the consumer. Due to growing market concerns over the adverse effects of sulfate-containing compositions, which are widely regarded as potential irritants to the eyes and skin, the demand for no additives continues to grow, with sulfate-free being a major demand in this trend.

[0003] GB 1214714A disclosed a method for preparing monosodium α-sulfocarboxylate. In this process, purified α-sulfo fatty acid salt is obtained by evaporating the solvent and filtering with water and air. Even though the product obtained by this process is a white filter cake and does not need to undergo a bleaching step, the product still contains sulfate salts that cannot be removed by using this process.

[0004] US 2020 / 0100998 A1 discloses a method for producing light-colored α-sulfo fatty acid di-salts, which comprises bleaching a neutralized aqueous paste of a sulfonation product of fatty acids with hydrogen peroxide. However, the method is directed to obtaining α-sulfo fatty acid salts with acceptable color levels without any process or treatment to purify the crude α-sulfo fatty acid salts to lower levels of sulfate content. Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the prior art, there is no readily available method for obtaining purified α-sulfo fatty acid salts having an acceptable level of sulfate content. Therefore, the object of the present invention is to develop a method for purifying crude α-sulfo fatty acid salts obtained by reaction of fatty acids with sulfur oxides. [Means for solving the problem]

[0006] In accordance with the present invention, it has been surprisingly discovered that the purification process of the present invention can result in purified α-sulfo fatty acid salts having a very low sulfate content.

[0007] In one aspect, the present invention relates to a method for purifying crude α-sulfo fatty acid salts obtained by reaction of fatty acids with sulfur oxides, the method comprising the steps of: a) diluting the crude α-sulfofatty acid salt in a solvent and adjusting the pH to 2 to 4; b) heating the slurry of α-sulfo fatty acid salt obtained in step a) until it is completely dissolved; c) gradually cooling the solution of α-sulfofatty acid salt obtained in step b) to room temperature to obtain crystals; d) filtering the crystals obtained in step c) to remove the mother liquor; e) rinsing the filter cake with an aqueous acid solution at least twice to obtain purified α-sulfo fatty acid salts.

[0008] Preferably, in step a), the pH value is adjusted to 2-4, preferably 2.5-3.5, using hydrochloric acid.

[0009] Preferably, the weight ratio of α-sulfofatty acid salt to solvent in step a) is in the range of 1:20 to 1:4, preferably in the range of 1:18 to 1:5, preferably in the range of 1:16 to 1:6, more preferably in the range of 1:15 to 1:7, even more preferably in the range of 1:13 to 1:8.

[0010] Preferably, the aqueous acid solution used in step e) comprises citric acid in an amount of at least 0.001% to 5% by weight, preferably in an amount of 0.01% to 3% by weight, even more preferably in an amount of 0.03% to 2.5% by weight, and more preferably in an amount of 0.05% to 2% by weight.

[0011] Preferably, the filter cake is rinsed twice in step (e). Preferably, the weight ratio of the aqueous acid solutions used in the two rinses is 1:1 to 1:3.5, preferably 1:1.5 to 1:3, more preferably 1:1.8 to 1:2.5.

[0012] In another aspect, the present invention relates to purified α-sulfofatty acid salts obtained or obtainable by the purification process.

[0013] In yet another aspect, the present invention relates to personal care compositions comprising the purified α-sulfofatty acid salts obtained by the purification process of the present invention.

[0014] Surprisingly and unexpectedly, the inventors have now found that by using the present purification method, purified α-sulfo fatty acid salts having a very low sulfate content can be obtained, thus achieving the objectives outlined above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Throughout this specification, including the claims, the terms "comprising one" or "comprising a" should be understood as being synonymous with the term "comprising at least one," unless otherwise specified, and the term "between" should be understood to be inclusive of the limits.

[0016] The indefinite articles "a", "an" and "the" are used to refer to one or to more than one (ie, to at least one) of the grammatical object of the article.

[0017] The term "and / or" encompasses the meaning of "and", "or" as well as all other possible combinations of the components to which this term is associated.

[0018] As used herein, the term "wt. %" or "weight percent" refers to the ratio of the weight of a particular ingredient to the total weight of the entire item, multiplied by 100.

[0019] As used herein, the term "substantially" refers to at least 80%, 90%, 95%, 99% or 100%.

[0020] It should be noted that when specifying any range of concentrations, weight ratios or amounts, any particular upper concentration, weight ratio or amount may be associated with any particular lower concentration, weight ratio or amount limit, respectively.

[0021] The present invention relates to a method for purifying crude α-sulfo fatty acid salts obtained by reaction of fatty acids with sulfur oxides, the method comprising the steps of: a) diluting the crude α-sulfofatty acid salt in a solvent and adjusting the pH to 2 to 4; b) heating the slurry of α-sulfo fatty acid salt obtained in step a) until it is completely dissolved; c) gradually cooling the solution of α-sulfofatty acid salt obtained in step b) to room temperature to obtain crystals; d) filtering the crystals obtained in step c) to remove the mother liquor; e) rinsing the filter cake with an aqueous acid solution at least twice to obtain purified α-sulfo fatty acid salts.

[0022] According to the state of the art, the process for producing α-sulfocarboxylic acids by reaction of substantially saturated or unsaturated fatty acids having 8 to 22 carbon atoms with sulfur dioxide or sulfur trioxide comprises the steps of: (a) mixing a solution of sulfur oxide and fatty acid (in a substantially inert solvent) in an amount such that there is a molar excess of sulfur oxide (usually 1.1 to 1.5 moles of sulfur oxide per mole of fatty acid, as would be expected by a person skilled in the art, would be appropriate); (b) maintaining the mixture at a reaction temperature up to the boiling point of the mixture for a time sufficient for the reaction to be substantially complete; (c) then partially neutralizing the reaction mixture by mixing with an aqueous solution of an alkali hydroxide, preferably sodium hydroxide; and (d) removing the solvent from the neutralized solution to obtain crude α-sulfo fatty acid salt.

[0023] In principle, the sulfonation can be carried out by all known processes. However, it is preferred to carry out the sulfonation in a fallfill reactor using sulfur dioxide or sulfur trioxide as the sulfonating agent. Such a process is described in detail, for example, in DE-A-4035935.

[0024] Fatty acids suitable for use in the production process are substantially linear or branched, saturated or unsaturated fatty acids having 6 to 22 carbon atoms, in particular 8 to 18 carbon atoms. Particularly suitable are fatty acids which are at least 90% by weight lauric, stearic, myristic or palmitic acid, or mixtures thereof. Preferably, technical fatty acid mixtures are present, such as coconut, palm kernel or tallow fatty acid mixtures, where unsaturated fatty acids, for example oleic acid, may be present in small amounts.

[0025] The sulfur oxide can be either sulfur dioxide or sulfur trioxide, which can be used in gas form, but is most conveniently in the form of a stabilized liquid. The solvent can be any that is substantially inert to the sulfur oxide, the fatty acid, and the products of the reaction, and is a solvent for both the sulfur oxide and the fatty acid. Preferred solvents include, for example, dichloromethane, as described in GB 1214714A. The sulfonation of the fatty acid can be carried out in a reaction vessel equipped with a stirrer, for example, with heating to 65° C. at atmospheric pressure or above. The reaction time to achieve a substantially complete reaction can be very short, for example, 5 minutes to 30 minutes. In some embodiments, the lower the reaction temperature, the longer the reaction time may be required.

[0026] According to the state of the art, the process for the production of α-sulfo fatty carboxylates can be carried out using gaseous sulfur oxides in the absence of a solvent. For example, fatty acids are first reacted with gaseous sulfur trioxide. In this case, sulfur trioxide is converted into SO 3 It is preferred to use an amount in which the molar ratio of fatty acid to fatty acid is in the range of 1.0:1.0 to 1.5:1.0. In this case, the fatty acid is introduced into the reactor at a reservoir temperature in the range of 70 to 95° C. Preferably, the liquid sulfonation product obtained after sulfonation is maintained at this temperature for 5 to 20 minutes in a temperature-controlled postreaction coil and exposed to aging.

[0027] The thus obtained crude sulfonated fatty acid product / sulfonation product, which is an acidic sulfonation product, is then partially or completely neutralized.

[0028] The neutralizing agent is used so that the sulfonic acid and carboxylic acid groups are converted or partially converted into their salt forms, depending on the amount of neutralizing agent. Organic bases such as ammonia, amines, especially alkali metal and alkaline earth metal hydroxides, as well as alkali metal carbonates and alkali metal bicarbonates are suitable as neutralizing agents. The acidic mixture obtained by sulfonation of fatty acids can be neutralized by adding an alkaline solution containing an alkali hydroxide, such as sodium hydroxide. In particular, it is preferred to use the neutralizing agent in the form of an aqueous solution. Concentrated sodium hydroxide and potassium hydroxide solutions are particularly preferred. The amount of alkali hydroxide or the pH of the neutralization solution can be adjusted to obtain monosodium salts of sulfocarboxylic acids or disodium salts of sulfocarboxylic acids. Usually, when the pH is adjusted to less than 5, preferably less than 4.5, the majority of the salts obtained are monosodium salts of sulfocarboxylic acids, and when the pH is adjusted to more than 4.5, disodium salts of sulfocarboxylic acids can be obtained. It is also possible to use sodium or potassium hypochlorite as neutralizing agent, preferably in aqueous solution or suspension. In this way, the desired bleaching effect of the product can be achieved simultaneously. However, it is preferred to add the bleaching agent to the monosalt paste, especially only after the monosalt paste has been produced. It is preferred to bleach the monosalt with aqueous hydrogen peroxide, perboric acid monohydrate, perboric acid tetrahydrate or hypochlorite, preferably with concentrated aqueous hydrogen peroxide.

[0029] The crude α-sulfo fatty acid salt is obtained after neutralization and filtration to remove liquid, and sodium sulfate salt is present as an impurity. According to any one embodiment of the present invention, the crude α-sulfo fatty acid salt contains more than 4 wt. %, in particular 6 wt. % to 14 wt. %, or 8 wt. % to 12 wt. % of sulfate salt (e.g., sodium sulfate) as an impurity.

[0030] In order to achieve the object of the present invention, a method for purifying crude α-sulfo fatty acid salts obtained by reaction of fatty acids with sulfur oxides has been discovered by the inventors, the method comprising the following steps: a) diluting the crude α-sulfofatty acid salt in a solvent and adjusting the pH to 2 to 4; b) heating the slurry of α-sulfo fatty acid salt obtained in step a) until it is completely dissolved; c) gradually cooling the solution of α-sulfofatty acid salt obtained in step b) to room temperature to obtain crystals; d) filtering the crystals obtained in step c) to remove the mother liquor; e) rinsing the filter cake with an aqueous acid solution at least twice to obtain purified α-sulfo fatty acid salts.

[0031] Process (a) The crude α-sulfo fatty acid salt is diluted in a solvent and the pH is adjusted to 2-4, preferably 2.5-3.5, more preferably about 3, in particular the pH is preferably adjusted with hydrochloric acid (e.g., as commercially available, concentration is within 28-36% by weight). A suitable solvent for diluting the crude α-sulfo fatty acid salt can be water or a water-miscible solvent such as acetic acid, acetaldehyde, acetone, acetonitrile, butanediol, ethanol, ethylamine, ethylene glycol, formic acid, glycerol, methanol, propanol, propylene glycol, pentanediol, ethylamine, etc. In some embodiments, the solvent can be an acidic solution comprising one or more acids including sulfuric acid, hydrochloric acid, acetic acid, citric acid, methanesulfonic acid, p-toluenesulfonic acid, or sulfonic acid. Preferably, the solvent comprises water, more preferably the solvent is substantially water.

[0032] In some embodiments, the weight ratio of α-sulfofatty acid to solvent is 1:20 to 1:4, preferably 1:18 to 1:5, preferably 1:16 to 1:6, more preferably 1:15 to 1:7, and even more preferably 1:13 to 1:8.

[0033] Step (b) and step (c) A slurry of α-sulfo fatty acid salt is obtained in step (a), then heated until the α-sulfo fatty acid salt is completely dissolved (step (b)), and then gradually cooled to room temperature (step (c)) to obtain crystals of the α-sulfo fatty acid salt.

[0034] In some embodiments of the present invention, the solution of α-sulfo fatty acid salt obtained in step (b) is cooled to room temperature at a cooling rate of 5-40 minutes per degree Celsius, preferably 5-30 minutes per degree Celsius. In some embodiments of the present invention, in step (c), the solution of α-sulfo fatty acid salt is cooled to 40-60°C, preferably 45-55°C, and this temperature is maintained for at least 1 hour, preferably 1-2 or 1-2.5 hours. The solution of α-sulfo fatty acid salt is then continued to be cooled to room temperature to obtain crystals.

[0035] Process (d) The slurry containing the crystals of the α-sulfo fatty acid salt obtained in step c) is transferred to a filter press, and the mother liquor is removed under a pressure of, for example, 2 to 4 bar to collect the crystals of the α-sulfo fatty acid salt.

[0036] Process (e) The filter cake containing the crystals of α-sulfo fatty acid salt obtained in step (d) was rinsed at least twice with an aqueous acid solution using a filter press operating under pressure (e.g., 2 to 4 bar) to obtain purified α-sulfo fatty acid salt.

[0037] In some embodiments of the present invention, the aqueous acid solution may contain citric acid, succinic acid, oxalic acid, glycolic acid, tartaric acid, or phosphoric acid, and preferably, the aqueous acid solution used in step e) contains at least citric acid, and more preferably, the aqueous acid solution contains citric acid in an amount of 0.001% to 5% by weight, preferably 0.01% to 3% by weight, even more preferably 0.03% to 2.5% by weight, and more preferably 0.05% to 2% by weight. In some embodiments of the present invention, the weight ratio of the aqueous acid solution used in step e) to the α-sulfo fatty acid salt may be 40:1 to 1:1, for example, 30:1 to 2:1, 20:1 to 2.5:1, preferably 15:1 to 3:1, more preferably 11:1 to 4:1, even more preferably 8:1 to 4.5:1, even more preferably 7.8:1 to 5:1, and most preferably 7.8:1 to 5.6:1.

[0038] In some preferred embodiments, the filter cake is rinsed twice with an aqueous acid solution, and the weight ratio of the aqueous acid solutions used in the two rinses is 1:1 to 1:3.5, preferably 1:1.5 to 1:3, more preferably 1:1.8 to 1:2.5, and preferably the aqueous acid solution contains citric acid. In some preferred embodiments, the aqueous acid solution used in the first rinse is a citric acid solution having a pH of 2 to 4, preferably 2.5 to 3.5, more preferably a pH of about 3, and the aqueous acid solution used in the second rinse is a citric acid solution with a concentration of 0.5% by weight to 4% by weight, preferably 1% by weight to 3.5% by weight, more preferably 1.2% by weight to 3% by weight, and most preferably 1.5% by weight to 2.5% by weight. After filtering to remove the rinse solution, a purified α-sulfofatty acid salt is obtained.

[0039] The purified α-sulfo fatty acid salts obtained by the process of the present invention contain less than 2% by weight of sulfate impurities, preferably less than 2% by weight, more preferably less than 1% by weight of sulfate impurities.

[0040] The present invention also relates to the use of the α-sulfofatty acid salts obtained or obtainable by the process according to the present purification method for personal care or cosmetic compositions, as well as detergents and cleaning agents.

[0041] The present invention also relates to a personal care composition or cosmetic composition comprising the purified α-sulfofatty acid salt obtained by the purification method of the present invention. The personal care composition is intended for topical application to skin or hair. The personal care composition may be a rinse-off formulation, in which the product is applied topically to the skin or hair and then the skin or hair may be rinsed with water within a few seconds to a few minutes thereafter. A base may also be used to wipe off the product. The personal care composition may be, for example, in the form of a liquid, semi-liquid cream, lotion, gel, solid, or a combination thereof. Preferably, the composition is in the form of a hair shampoo, shower gel, soap, syndet, cleaning paste, cleaning lotion, scrub formulation, foam bath, oil bath, shower bath, shaving foam, shaving lotion, shaving cream, toothpaste, mouthwash, and dental care product.

[0042] Examples of personal care compositions may include, but are not limited to, bar soaps, shampoos, conditioning shampoos, body washes, moisturizing body washes, shower gels, skin cleansers, cleansing milks, in-shower body moisturizers, pet shampoos, shaving preparations, and cleaning compositions used in combination with disposable cleansing cloths.With regard to detergent and cleaning compositions, the compositions may be, for example, bath and toilet cleaners, and may also be cleaning and / or fragrance gels for use in sanitary facilities.

[0043] The personal care compositions of the present invention may further comprise surfactants, such as anionic, nonionic, cationic, amphoteric surfactants, skin active agents, skin conditioning agents, and thickening agents.

[0044] The personal care compositions of the present invention are generally prepared by conventional methods as known in the art of making rinse-off personal care compositions. Such methods typically involve mixing the ingredients in one or more steps to a relatively homogeneous state, with or without heating, cooling, application of vacuum, etc. EXAMPLES

[0045] material: Crude α-sulfofatty acid salt: Sulfopon® SFA (45% active by weight), available from BASF

[0046] Citric acid monohydrate: Sinopharm AR, >99.5%

[0047] measurement: Composition of α-sulfo fatty acid salts 10 mg of test sample was dissolved in 10 mL of methanol / water (50:50 v / v) and analyzed by liquid chromatography / mass spectrometry coupling. The concentrations of components were determined by calculating the area percent distribution in the chromatography.

[0048] Sulfate Content 100 mg of test sample was weighed individually into a 100 mL volumetric flask and topped up to the mark with deionized water. The test solutions were analyzed by ion chromatography. Quantification is performed using peak areas using the external standard method.

[0049] Color (Hazen) The α-sulfo fatty acid salts were diluted with a mixture of deionized water (95 wt%) and isopropanol (5 wt%) to obtain a dry residue of 4 wt% based on the total content of the diluted solution. The color values ​​of this solution were measured in an 11 mm circular cuvette on a commercial colorimeter from Hach Lange GmbH (Lico 500). The color values ​​are expressed in Hazen units.

[0050] The details of the purification method are as follows:

[0051] Crude α-sulfo fatty acid salts (Sulfopon® SFA) were diluted with water to obtain 700 g of slurry (α-sulfo fatty acid salts concentration 8%-15% by weight), and the pH value was adjusted to pH 3 with hydrochloric acid or citric acid. The slurry was then heated to completely dissolve the α-sulfo fatty acid salts and then cooled to room temperature using the following program: - Heat from room temperature to 70°C over 3.5 hours and maintain at 70°C until the crude α-sulfo fatty acid salts are completely dissolved; -Cool to 60°C at a rate of 6 minutes per degree Celsius and hold at 60°C for 1 hour; -Then cool to 50°C at a rate of 30 minutes per degree Celsius and hold at 50°C for 1 hour; -Then cool to 40°C at a rate of 30 minutes per degree Celsius and hold at 40°C for 1 hour; -Then cool to room temperature at a rate of 6 minutes per degree Celsius.

[0052] The slurry containing the α-sulfo fatty acid salt crystals was transferred to a filter press (SHXB-BZ-0.5L) and the mother liquor was removed at 2 bar for 10 minutes. The filter cake was rinsed and filtered as detailed in Table 1 below. Samples were prepared using different pH adjusters and rinsing programs.

[0053] [Table 1]

[0054] [Table 2]

[0055] The results show that when hydrochloric acid was used as a pH adjuster to prepare the crude α-sulfo fatty acid salts slurry and citric acid solution was used to rinse the filter cake twice (i.e., Sample 9), the α-sulfo fatty acid salts were obtained with a lower Hazen color number, which indicates less impurities and higher purity.

[0056] A comparison of the compositions of the refined α-sulfofatty acid salt (sample 8) and the crude α-sulfofatty acid salt is shown in Table 2.

[0057] [Table 3]

Claims

1. A method for purifying a crude α-sulfofatty acid salt obtained by reacting a fatty acid with a sulfur oxide, comprising the steps of: a) diluting the crude α-sulfofatty acid salt in a solvent and adjusting the pH to 2-4; b) heating the slurry of α-sulfofatty acid salt obtained in step a) until it is completely dissolved; c) gradually cooling the solution of α-sulfofatty acid salt obtained in step b) to room temperature to obtain crystals; d) filtering the crystals obtained in step c) to remove the mother liquor; e) rinsing the filter cake with an aqueous acid solution at least twice to obtain purified α-sulfofatty acid salts.

2. 2. The method of claim 1, wherein the weight ratio of the α-sulfofatty acid salt to the solvent in step a) is from 1:20 to 1:4, preferably from 1:18 to 1:5, preferably from 1:16 to 1:6, more preferably from 1:15 to 1:7, and even more preferably from 1:13 to 1:

8.

3. 2. The method of claim 1, wherein in step a) the solvent is water or a water-miscible solvent, preferably the solvent comprises water, more preferably the solvent is substantially water.

4. 2. The method according to claim 1, wherein in step a) the pH value is adjusted to 2-4, preferably 2.5-3.5, using hydrochloric acid.

5. 2. The method of claim 1, wherein in step c), the solution of α-sulfofatty acid salt is cooled to 40-60°C, preferably 45-55°C, and maintained in step c) for at least 1 hour.

6. 2. The method of claim 1, wherein the aqueous acid solution used in step e) comprises at least citric acid in an amount of 0.001 wt. % to 5 wt. %, preferably in an amount of 0.01 wt. % to 3 wt. %, even more preferably in an amount of 0.03 wt. % to 2.5 wt. %, and even more preferably in an amount of 0.05 wt. % to 2 wt. %.

7. 2. The method of claim 1, wherein in step e), the weight ratio of the aqueous acid solution to the α-sulfofatty acid salt is from 40:1 to 1:1, for example, from 30:1 to 2:1, from 20:1 to 2.5:1, preferably from 15:1 to 3:1, more preferably from 11:1 to 4:1, even more preferably from 8:1 to 4.5:1, even more preferably from 7.8:1 to 5:1, and most preferably from 7.8:1 to 5.6:

1.

8. 10. The method of claim 1, wherein the filter cake is rinsed twice with aqueous acid in step e).

9. 9. The method of claim 8, wherein the weight ratio of the aqueous acid solution in the at least two rinses is from 1:1 to 1:3.5, preferably from 1:1.5 to 1:3, more preferably from 1:1.8 to 1:2.

5.

10. The method according to claim 1, wherein the crude α-sulfofatty acid salt contains sulfate as an impurity.

11. A purified α-sulfofatty acid salt obtained by the method according to any one of claims 1 to 10.

12. A personal care composition comprising a purified α-sulfofatty acid salt obtained by the method of any one of claims 1 to 10.

13. 13. The personal care composition of claim 12, wherein the composition is in the form of a hair shampoo, shower gel, soap, syndet, cleaning paste, cleaning lotion, scrub formulation, foam bath, oil bath, shower bath, shaving foam, shaving lotion, shaving cream, and dental care product.