Field of Invention

The invention is directed to improving the color of sulfonated or sulfated surfactant compositions, particularly alpha-sulfo fatty acid alkyl ester compositions, without the need for bleaching.

Background of the Invention

The manufacture of alkali metal salts of alpha-sulfo fatty acid alkyl esters (hereinafter "ester sulfonates") by neutralization of fatty acid ester sulfonic acids with aqueous caustic is well known. Such ester sulfonates are predominantly used as surfactants in washing and cleansing agents and products.

The known processes for making these ester sulfonates in good yields suffer from the formation of dark-colored impurities. The ester sulfonic acids, from which the ester sulfonates are derived, are obtained by sulfonation of fatty acid esters or, less preferably, by sulfonation and esterification of fatty acids. In order to obtain high sulfonation yields, excess sulfonating agent in combination with greater processing times and/or temperatures is required. These conditions can result in undesirable side reactions including the formation of the dark-colored impurities. Examples of such sulfonation processes are described in U.S.

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3,485,856 and "The Journal of the American Oil Chemists Society". 52 (1975), pp. 323-329.

For aesthetic and other reasons, the dark-colored ester sulfonate ₅ compositions are not suitable for use directly in washing or cleansing agents and products. Therefore, the dark ester sulfonate products have heretofore been bleached in order to lighten their color. Typically the dark products are treated with an aqueous bleaching agent, such as hydrogen peroxide or _Q hypochlorite, before and/or after neutralization. Such bleaching processes are described in U.S. 3,159,657; 3,452,064; 4,547,318 and 4,617,900.

The art has recognized certain problems inherent to the bleaching _^ 5 process, particularly handling difficulties and hydrolysis of the ester group. Heretofore these problems have been dealt with, inter alia, by the optimizing bleaching process itself, or by modifying the ester sulfonation process itself to deliver an ester sulfonate with less color, thus allowing the use of milder 0 bleaching conditions. Such processes are described in U.S. 3,997,576; 4,080,372; 4,547,318; and 4,617,900. However, none of these references disclose a process for making ester sulfonate surfactant which is completely satisfactory.

5 A method of improving the color of dark-colored ester sulfonate compositions without the need for bleaching has now been discovered. More specifically, it has been discovered that the dark-colored impurities can be separated by known separation methods from a solution comprising the ester sulfonate 0 substantially dissolved in a suitable solvent. Separation of the dark-colored impurities from the solution can be enhanced with an adsorbent material. After removal of the dark-colored impurities, the ester sulfonate can be recovered from the solvent to yield a product with improved, i.e. lighter, color. The process also provides a particulate ester sulfonate surfactant having improved physical properties', relative to that obtained by drying of the surfactant from aqueous pastes and improved surfactant odor.

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The inventive process can also be applied to other sulf(on)ated surfactants whose preparation may result in the formation of dark-colored impurities during and/or after the sulfation or sulfonation reaction. Such surfactants include, but are not limited to, alkylbenzene sulfonates, linear alkane sulfonates, alpha-olefin sulfonates, fatty alcohol sulfates (i.e. alkyl sulfates), and alkyl ether sulfates.

Summary of the Invention

The present invention involves a novel process for improving the color of a sulfonated or sulfated (hereinafter sulf(on)ated) surfactant composition, said surfactant composition comprising:

(i) a sulf(on)ated surfactant, preferably selected from the group consisting of alkylbenzene sulfonates, linear alkane sulfonates, alpha-olefin sulfonates, ester sulfonates, fatty alcohol sulfates, alkyl ether sulfates and mixtures thereof; and

(ii) dark-colored impurities formed during the preparation of said sulf(on)ated surfactant;

said process comprising the steps of:

(1) forming a solution comprising:

(a) said surfactant composition comprising said sulf(on)ated surfactant and said dark-colored impurities; and

(b) a suitable sol vent, preferably a Cj -Cs al cohol , in an amount suffi ci ent to substanti al ly di ssol ve said sul f(on) ated surfactant ;

(2) separating said dark-colored impurities from said -solution; followed by

(3) recovering said sulf(on)ated surfactant from said _c solution,

wherein the amount of water present in said solution in step (1) is sufficiently low to avoid interference with effective separation of the dark-colored impurities from said solution.

10

The invention is particularly significant for improving the color of ester sulfonate compositions, since methods have been discovered for making the other sulf(on)ated surfactants with low levels of dark impurities.* The following disclosure is therefore

■c directed to improving the color of ester sulfonate compositions.

DETAILED DESCRIPTION OF THE INVENTION 0

The ester sulfonate compositions which are improved in color by the invention comprise an ester sulfonate having the preferred general formula (I) 5

Rl - CH -- C00R2

SO3X 0 wherein Rl is a C4-C22 linear or branched chain alkyl,

R2 is a C1-C8 alkyl, and

X is a water-soluble salt-forming cation.

Particularly useful ester sulfonates are those wherein R2 is -CH3, i.e. methyl ester sulfonates, and more particularly methyl ester sulfonates wherein R-* is Cjo-Ciβ-

The cation X is obtained from the agent used to neutralize the ester sulfonic acid to form the ester sulfonate. Suitable X cations are monovalent cations, including alkali metals such as sodium, potassium, and lithium; substituted or unsubstituted ammonium; and cations derived from lower alkanolamines, for example monoethanola ine, diethanola ine, and triethanola ine; and mixtures thereof. Particularly suitable cations are sodium, potassium, lithium, and those derived from lower alkanolamines. It is contemplated that the neutralization agent can also provide a cation having a valence number greater than one, for example, alkaline earth metals such as magnesium and calcium. In this case, the general formula (I) would be modified to reflect the greater number of moles of ester sulfonic acid associated with the cation in the salt (ester sulfonate) form, said number being equal to the cation valence number.

The ester sulfonic acids, from which the ester sulfonates are prepared, can be obtained by sulfonating and then esterifying natural or synthetic fatty acids, or by sulfonating synthetic fatty acid esters. For commercial reasons the ester sulfonic acids are preferably prepared by sulfonating fatty acid esters.

Examples of suitable fatty acid esters include, but are not limited to, methyl laurate, ethyl laurate, propyl laurate, methyl pal itate, ethyl palmitate, methyl stearate, ethyl stearate, methyl hydrogenated tallow fatty acid ester, ethyl hydrogenated tallow fatty acid ester, methyl hydrogenated coco fatty acid ester, ethyl hydrogenated coco fatty acid ester, methyl hydrogenated palm fatty acid ester, and mixtures thereof. Preferred are hydrogenated tallow fatty acid methyl esters. hydrogenated palm oil fatty acid methyl esters, hydrogenateα coconut oil fatty acid methyl esters, and mixtures thereof.

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The fatty acid esters can be sulfonated to the sulfofatty acid esters by known processes, for example, by thin layer or batch sulfonation. Suitable sulfonating agents include anhydrous SO3, SO3 diluted with nitrogen or dry air, and the like. As an example, linear esters of C8-C20 carboxylic acids can be ^* sulfonated with gaseous SO3 according to "The Journal of the American Oil Chemists Society", 52 (1975), pp. 323-329.

The sulfonation of the fatty acids or fatty acid esters can result in the formation of dark-colored impurities in the ester sulfonic acid product. Neutralization of the ester sulfonic acid with an agent providing the water-soluble cation X results in ester sulfonate comprising dark-colored impurities. In accordance with the present invention, these dark-colored impurities can be separated from the ester sulfonate to provide a product having significantly lighter color.

The process comprises the step of forming a solution of the dark ester sulfonate product in a suitable solvent. Suitable solvents include any which are capable of substantially dissolving the ester sulfonate under appropriate temperatures and pressures. Mixtures of solvents can be used provided that the mixture is capable o ^* f substantially dissolving the ester sulfonate under appropriate process conditions. Particularly suitable solvents for processing ester sulfonates are Ci-Cs alcohols and lower esters thereof. Preferably Cχ-C8 alcohols are used; therefore, the process is further described in terms of lower alcohol solvents. However, it is contemplated that any solvent as broadly defined above is usable in the process. More preferably methanol, ethanol and mixtures thereof are used, and most preferably methanol is used.

Sufficient lower alcohol, in an amount relative to the ester sulfonate, must be present to solubilize the ester sulfonate at practical processing temperatures and pressures. For best results, concentrations and process conditions are selected to substantially dissolve, and most preferably to wholly dissolve.

the ester sulfonate prior to separation of the dark-colored impurities. The selection of suitable solubilization conditions is considered to be within the ability of one of ordinary skill in the art, However, the weight ratio of lower alcohol to ester sulfonate will generally be from 10:1 to 0.75:1 at temperatures of between about 10 ^β C to about 110 ^β C, more preferably from 5:1 to 0.75:1, even more preferably from 3:1 to 0.75:1, and most preferably from 2:1 to 1:1. Higher ratios of lower alcohol to ester sulfonate (greater than 10:1) can be used, but are probably not any more effective. The amount of lower alcohol required to solubilize the ester sulfonate can be added at any or all of the points before, during and after neutralization of the ester sulfonic acid to form the ester sulfonate, further discussed herein. Advantageously some or all of the lower alcohol is added during neutralization. Preferably substantially all of the lower alcohol is added during the neutralization step.

The amount of water in the solution must be sufficiently low to avoid interference with effective separation of the dark-colored impurities from the solution. Without intending to limit the invention, it is theorized that too much water, in relation to the amount of lower alcohol and ester sulfonate present, can result in the formation of a separate ester sulfonate/water phase which can make it difficult to separate the dark-colored impurities from the lower alcohol solvent-containing phase. It is further theorized that in the presence of sufficient water, the ester sulfonate can act as a surfactant to effectively solubilize at least a portion of the dark-colored impurities.

It is also theorized that some portion of the dark-colored impurities are soluble or suspendable in the lower alcohol and/or the solution. It is believed that such soluble or suspendable impurities can be separated by adsorbents,- such as activated carbon. The other portion of the dark impurities, which are not soluble or suspendable in the lower alcohol and/or solution, can be separated from the solution by other separation methods as described herein.

The selection of suitable ratios of lower alcohol to water relative to a given amount of ester sulfonate is considered to be within the experimental ability of one having ordinary skill in the art. Preferably the weight ratio of alcohol to water is at least 3:1, more preferably 10:1, and even more preferably 30:1. ^' Most preferably the solution is essentially free of water in order to achieve more effective separation of the dark-colored impurities.

It is highly preferred that the neutralization process used to prepare the ester sulfonate minimize the amount of water in the ester sulfonate product. In this way, the alcohol: water ratios required for effective separation of the dark-colored impurities can be obtained without the -need for excessively large amounts of alcohol or the need for a separate dehydration step for the ester sulfonate, prior to dissolving with lower alcohol. According to a preferred embodiment, neutralization is performed with substantially anhydrous solutions of the neutralization agent in a lower alcohol solvent. The selection of the particular alcohol solvent depends upon the desired ester, since transesterification may occur during neutralization. For example, where methyl ester sulfonates are desired, methanol is the preferred alcohol solvent.

According to a particularly preferred embodiment, neutralization is performed by addition of the ester sulfonic acid to a solution of alkoxide in alcohol, said alkoxide having the formula R*-*0X, wherein R ³ is C -Cs alkyl and X is a water-soluble salt-forming cation as hereinbefore defined. (See Japanese Laid-Open Patent

Publication No. 290842/1990). Alternatively the alkoxide in alcohol solution can be added to the ester sulfonic acid, or the two solutions can be mixed together simultaneously, as which occurs in an in-live mixer. The alkoxide solution can be prepared by known methods, for example, by dissolving an alkali- or alkaline earth- metal Ci-Cs alkoxide in the respective alcohol to directly provide an essentially anhydrous neutralization system.

The alkoxide solution can also be formed by dissolving a solid alkali- or alkaline earth- metal hydroxide in the alcohol,

although this method of forming the alkoxide solution is less preferred since one mole of water is formed for each mole of alkoxide generated. If this latter method is used, the water from alkoxide generation can optionally be removed by known methods to provide an essentially anhydrous neutralization system.

An amount of lower alcohol sufficient to dissolve the resultant neutralized ester sulfonate can be used in the preferred substantially anhydrous neutralization. Additional solvent can be added after neutralization as required to dissolve the ester sulfonate prior to separating the dark-colored impurities. Such additional solvent can be any as defined above in forming the solution comprising the ester sulfonate surfactant and the lower alcohol solvent.

Of course, the ester sulfonate can also be prepared by neutralizing the ester sulfonic acid by well-known processes, including conventional neutralization processes involving aqueous 5-50% caustic solutions. Any residual water in the ester sulfonate product can then be removed as necessary by known methods, such as drying, before proceeding to separate the dark-colored impurities.

Where water is present during neutralization, preferably at least a portion of the alcohol required to substantially dissolve the ester sulfonate is mixed with the ester sulfonic acid prior to neutralization. By premixing the ester sulfonic acid with alcohol, it has been shown in the art (See, for example, U.S. 4,404,143, Sekiguchi et al , September 13, 1983, incorporated herein by reference) that reduced levels of fatty acid disalts are formed during neutralization, relative to systems where no premixing with alcohol is performed. Additionally, premixing suppresses the formation of an ester sulfonate/water phase, thereby providing better mixing and improved neutralization.

The process of the present invention further comprises the step of separating dark-colored impurities from the solution of the ester

sulfonate product in alcohol. Separation can be achieved by conventional ^' methods such as settling/clarification, centrifugation, filtration, adsorption, or a combination thereof. The particular separation method or methods employed will depend upon a number of factors, such as the amount and proportion of dark-colored impurities which are insoluble in the solvent, versus those that are soluble or suspendable in the solvent, and the amount and proportion of water relative to the amount of solvent and ester sulfonate surfactant.

Clarification can be accomplished by simple gravitation; on an industrial scale the use of conventional equipment, such as revolving plows or rakes, can be used to aid separation. Centrifugation can be by either a batch method or a continuous method, involving decantation of the supernatant from the sedimented dark-colored impurities.

Filtration can be performed through conventional filters. For example, on a laboratory scale, filtration through paper, diatomaceous earth, or adsorbent are suitable. On an industrial scale, suitable filtration equipment includes pressure filters of the piate-and-frame or shell-and-leaf construction, or of the rotating drum or disk type; vacuum or suction filters of the rotating drum or disk type; edge filters; clarification filters; etc.

In a preferred embodiment, the solution is treated with an adsorbent material, such as activated carbon, activated alumina, or silica gel. Such adsorbent material is believed to be particularly effective at separating that portion of the dark-colored impurities which are theorized to be soluble or suspendable in the alcohol and/or solution, as discussed herein above. Preferably activated carbon is used as the adsorbent.

Where an adsorbent filter is used for separation, adsorbent treatment occurs during filtration. Adsorbent treatment can alternatively occur either before or, preferably, after separation

of dark impurities by a non-adsorbent method. For example, the solution can be mixed with a suitable amount of adsorbent particulate, such that dark-colored impurities adsorb onto the particulate, followed by separating the adsorbent particulate by, for example, centrifugation and/or non-adsorbent filtration. Treatment can also, and most preferably, occur by passing the liquor obtained after an initial separation of dark-colored impurities by, for example, centrifugation and/or non-adsorbent filtration, through an adsorbent bed. Alternatively, the liquor can be mixed with fresh adsorbent particulate such that dark impurities adsorb onto the particulate, followed by separating the spent or used adsorbent particulate by any suitable method, such as those previously described.

Preferably, the temperature required to solubilize the ester sulfonate (in the step of forming the solvent solution of the ester sulfonate composition) is maintained throughout the separation and through to the final product recovery step. Additional lower alcohol can be added as needed to solubilize any ester sulfonate which may precipitate during the separation.

In commercial applications, the ester sulfonate dissolving and impurity separation steps would preferably be conducted in suitable pressurized, enclosed equipment and equipment systems to avoid evaporating the solvent at the selected solvent temperature. Such evaporation can result in undesirable evaporation cooling, and loss of solvent vapors to the environment.

After separation of the dark-colored impurities from the solution, the ester sulfonate product having improved color can be recovered from the solvent solution by known methods. Such recovery methods include, for example, precipitation of the ester sulfonate from the solution, evaporation of the lower alcohol solvent from the solution or a combination thereof. Precipitation of the ester sulfonate can be achieved by reducing the temperature of the solution, and thereby the solubility of the ester sulfonate in the lower alcohol. The precipitated ester sulfonate can then be

recovered by known methods, for example, filtration followed by evaporation of essentially all of any residual solvent. Evaporation may occur under normal or reduced pressure and with or without heating to yield a solid or molten ester sulfonate that can be processed by known methods to any desired form, such as- powder, flake, chunk or granulate.

On an industrial scale, the lower alcohol, and any water which may be present, can be removed by heating the solution and flashing or evaporating the alcohol (and water, if present). This can be done by any suitable method, including conventional processes, such as spray drying, atmospheric flash drying, vacuum flash drying, drum drying, wiped film evaporation, or a combination thereof. Spray drying can be used to direct-ly yield an ester sulfonate product in powdered or granular form. The other methods yield ester sulfonate products in a chunk, noodle, or large particulate form, which can be further processed by known methods to any desired form, for example, milling to a granular form, or flaking and then chopping or milling to a granular form. The alcohol which is removed to recover the ester sulfonate is advantageously condensed, recovered and recycled for re-use in any of the alcohol addition steps described herein. Where the ester sulfonate is recovered by filtration of precipitated (re-crystallized) ester sulfonate, the resultant liquor obtained from filtration, which contains lower alcohol and some amount of dissolved ester sulfonate, can be recycled to any preceding step in the process, preferably back to the step where fresh, dark-colored ester sulfonate is dissolved in lower alcohol to form a solution.

As a result of this process, ester sulfonates of improved, i.e. lighter, color are obtained. Where the process has also involved effective adsorbent treatment of the solution, the resultant product is near-white in color and can be used directly in cleansing and washing agents and products. The resultant product may also be converted into a light-colored paste by addition of water after separation of the dark impurities. Effective adsorbent treatment further results in improved surfactant odor.

Improved particle physical properties are also achieved by the present process, relative to those obtained by drying of a conventional aqueous system. The amount of water relative to alcohol in the separated solution is believed to have an important effect on the physical properties of the resultant light-colored ester sulfonate product. As the ratio of water: alcohol increases, it becomes increasingly difficult to remove the solvent comprising alcohol and water. Therefore, where water is present in the solution, removal of the solvent with heating is preferred for improved physical properties.

The process further allows for greater flexibility in the raw materials and process conditions of sulfonation. For example, starting materials having* a greater degree of impurities themselves which can result in the formation of the dark-colored impurities, or processing conditions for obtaining greater rates of conversion to the ester sulfonate, may be used without the concerns heretofore associated with the need for bleaching. Impurities which can result in the formation of dark-colored impurities are known in the art, and include glycerine, glyceride (mono-, di- or tri-) and unsaturated fatty acid ester. By avoiding the need for bleaching, the process may also avoid the formation of sensitizers, such as those described in D. Connor et al . ; Identification of Certain Sultones as the Sensitizers in an Alkyl Ethoxy Sulfate. "Fette Seifen Anstrichmittel " 77, 25-29 (1975).

The ester sulfonates obtained by the method of the present invention are useful as an active ingredient for cleansing and washing agents and products, and which can be employed either independently or in admixture with other surfactants. For example, in detergent compositions, suitable co-surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, zwitterionic surfactants or amphoteric surfactants. Other ingredients conventionally used in detergent formulations may also be used. Such ingredients include those generally used as builders, enzymes, bleaching agents and activators, soil

release agents, chelating agents, soil -removal ana anti-redeposition agents, dispersing agents, brighteners, suds suppressors, etc.

The invention is illustrated by the following non-limiting- examples. All parts and percentages herein are by weight unless otherwise stated. All color measurements were carried out using a Hunter Colorimeter providing L, a, b readings.

EXAMPLES

Example I

Ester sulfonic acid was produced by conventional sulfonation of palm stearin fatty acid methyl ester. The acid component of the methyl ester consisted essentially of saturated fatty acids with an Iodine Value of 0.28 and the following chainlength distribution (by weight percent):

C12 - 0.23

C14 - 1.55

C15 - 0.08

C16 - 66.75

C17 - 0.15 C18 - 31.28

C20 - 0.19

The sulfonation reaction was carried out at 80 ^β C to 95 ^β C in an annular falling film reactor using a mixture of sulfur trioxide and air (SO3 content: 3-4% by volume; SO3 excess: 15-30 mole percent). The sulfonated methyl ester acid mix was then digested in a closed vessel for 35 to 40 minutes at a temperature of 80°C to 95*C. The degree of sulfonation after digestion was 95%.

A portion of the sulfonated methyl ester acid mix was cooled to about 20'C and ground into a powder. This powder gave a color reading of L=14.9, a=0, b=0.8. Percent volatiles of the sample was 2-3% (by Cenco drying; consisting essentially of incidental moisture).

Under high shear mixing, 500 g of the acid mix was mixed with methanol (lOOg) at 45 ^β C to 55'C _. and neutralized by adding 25% w/w

solution of CH3θNa in methanol. (More preferably, the acid mix can be added into the 25% w/w solution of CH3θNa in methanol .)The volatiles content of the neutralized product was determined (by Cenco drying an aliquat) to be 32%, consisting of methanol and incidental moisture. A portion of the neutralized product was air dried to a 7% volatiles content (Cenco) and ground to a powder. The color of this dried sample was L=28.8, a=2.5, b=7.1. A second portion of the neutralized product (73.5g; Cenco volatiles 32%) was dissolved into 176.5g of methanol. 2g of activated carbon (decolorizing activated carbon, Aldrich Chemical Co., catalog #16,155-1) was added to the solution, which was then stirred for 60 minutes at a temperature of 40'C to 50'C. This solution was vacuum filtered through 15cm-diameter Whatman #41 paper onto which was added an additional 2g -activated carbon. The filtered liquor was passed a second time through the same filtration assembly. The liquor was then air dried to a 10.4% volatiles content (Cenco) and ground to a powder. The color of this powder was L=81.5, a=-0.7, b=13.6.

Example 2 (comparative)

Sulfonated methyl ester acid mix was prepared from the same palm stearin fatty acid methyl ester stock used in Example 1, using substantially the same sulfonation conditions. The color of a ground, powdered sample of the sulfonated methyl ester acid was L = 19.8, a = 0.7, and b = 1.4.

Following digestion of the acid mix, 20% (w/w sulfonated ester acid) methanol and 8% (w/w sulfonated ester acid) hydrogen peroxide solution (50% H2O2 in water) were added to and mixed with the acid mix. This mixture was then further digested with negligible additional mixing or back-mixing for about 50 minutes at 70 ^* C. The bleached acid mix was then neutralized with 12.5% NaOH (aq.) solution at 63°C to an aqueous (55-57% moisture) paste. Water was evaporated from the paste (plate and frame heat exchanger, exit paste temperature 295-305 ^β F, flash to atmosphere), followed by chilling to yield bleached, ester sulfonate flakes having 3-5% moisture (Cenco analysis). The flakes were ground into a powder having a color of L ^*** 90.7, a = -3.1, and b = 10.8.