CA2040451C - Process for preparing soap-acyl isethionate compositions - Google Patents

Abstract

A batch and continuous process is disclosed for the production of a composition comprising alkali metal salts of C8-C22 alkyl fatty monocarboxylic acid and C8-C22 acyl isethionate in a ratio of 20:1 to 1:0.98. Both batch and continuous routes require that the saponifying aqueous caustic solution includes sodium hydroxide and sodium isethionate added as a hot solution. For the batch route, the sequence of steps requires the caustic solution to be added slowly to the fatty acid. In the continuous process, it is advantageous to introduce fatty acid upstream from the point where the caustic solution stream enters the mixing chamber.
Addition of the acyl isethionate either precedes addition of caustic solution, which then is added at a temperature of at least 80°C, or else is subsequent to addition of caustic solution.

Description

20~4~
PROCESS FOR PREPARING SOAP-ACYL ISETHIONATE
COMPOSITIONS

The invention relates to a process for preparing compositions comprising a major amount of soap and a minor amount of acyl isethionate.
Soap is an excellent cleaning agent but is quite harsh to the skin. A study by Frosh ~ ~ligr~n, J. Amer. Acaderm. pp. 35 (1979), revealed that substantial replAc ~t of soap with an alternative detergent such as an acyl fatty isethionate would provide a more skin compatible ~y~ . Unfortunately, this alternative is expensive. Less costly solutions are nee~e~ to provide the consumer with an econ~ ical, yet mild, product.
One approach to resolving the problem has been reported in US Patent 4,695,395 (Caswell et al.).
The patent reports that bars cont~; n; ng a major amount of soap and a minor amount of acyl isethionate can be rendered relatively non-irritating by incorporation of non-acylated sodium isethionate. Consequent upon this discovery, there was a need for a process to prepare such compositions.
US Patent 4,663,070 (Dobrovolny et al.) discloses a batch process wherein a reactor ContAi ni ng a major amount of soap, a minor amount of C10-Cl 6 acyl 2~

isethionate, sodium isethionate, water, stearic acid, sodium chloride and certain minor additives are heated at 99~C-lO4~C under agitation. The reaction was judged as complete and terminated when the blend had passed a ~econd peak in viscosity.
A related case, US Patent 4,707,288 (Irlam et al.) reports an essentially identical formulation prepared in a reactor under conditions of shear maintaining a temperature of from 60~C to about 90~C.
Thereafter, the composition is fed to a plodder and extruded to form a detergent bar.
Each of the foregoing processes begin with soap as a starting material. A necessary condition for the soap/acyl i~ethionate based i~1ng is the need for ~5 certain initial levels of water. Without a minimum water level in the raw materials, blen~ing would be difficult and a gritty product would result. A
disadvantage of the aforementioned processes cont~i ni ng water is that moisture must be reduced through evaporation to arrive at an acceptable end product.
There is a critical window of moisture beyond which bar physical properties are adversely affected. A second problem with the afo~- ?ntioned processes is the time required in bl~n~ing soap with acyl isethionate before there can be achieved the appropriate product viscosity.

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Elimination of a drying step has long been known in the soap making art. For instance, US Patent 2,578,366 (Mills) meters an aqueous sodium hydro~ide stream and a fatty acid slurry stream into a ;~ing reactor. Typically, the aqueous solution of caustic soda is maintained at about 27~C-35~C. Temperatures for the neutralization and subsequent soap i~in~ range from about 55~C to 102~C.
Along similar lines, US Patent 3,657,146 (F - qon et al.) reveals a method for the direct production of soap from fatty acid under reaction temperatures of 120~C-180~C. Separate streams of tallow/coconut (80/20) fatty acids, of a stoi~hi~_ Ptric amount of aqueous sodium hydl~ide, and of sodium chloride are pumped into a reactor vessel.
In principle, it would appear attractive to form soap in situ, neutralizing fatty acid, while simultaneously fee~ng acyl isethionate into the blend.
However, there is a problem. Acyl isethionate is susceptible to hydrolysis. This route would therefore not be perceived as feasible.
It is an ob;ect of the present invention to provide a process for preparing toilet bars cont~1n~ng a ma;or àmount of soap and a minor amount of acyl isethionate.
Further objects of at least some forms of this invention is to provide a method for producing a soap/acyl isethionate bar which substantially eli inates the need for drying and thereby increases production rates, and/or provides a process route which 5 ~ ni i zes hydrolysis of the acyl isethionate component, and/or yields a soap/acyl isethionate composition having consumer use and toilet bar processing properties that fall within commercially acceptable parameters.
The present invention provides a process for the production of a composition comprising alkali metal salts of C8 -C2 2 alkyl fatty monocarboxylic acid and C~-C2 2 acyl isethionate in a weight ratio of 20:1 to 1:0.98, said process comprising the steps of:
(i) forming hot aqueous caustic solution comprising sodium hydroxide and alkali metal isethionate.
(ii) charging C~-Cz 2 alkyl fatty ~ oc~rboxylic acid to a reactor and maintaining said fatty monocarboxylic acid at an elevated temperature while xi n~;
~ ng said hot caustic solution to said fatty monocarboxylic acid in said reactor; and (iv) either (a) fee~; ng said acyl isethionate salt to said reactor at a time prior to the addition of said caustic solution in step (iii) or subsequent to step (iii) and adding said hot caustic solution at a temperature of at least 80~C in step (iii) or (b) feeding said acyl isethionate salt to said :reactor at a time subsequent to step (iii).
The caustic solution is preferably added at a temperature of at least 80~C in any event. More preferably it is maintained and added at a temperature of 90~C-95~C, desirably at about 93~C. The amount of sodium hydroxide should of cour~e be substantially stoichiometric, i.e. sufficient to bring about substantially complete neutralization of the fatty acid. The caustic solution will typically contain at least 30~ by weight solids (sodium hydroxide and sodium isethionate).
The process may be carried out as a batch process, in which the hot caustic solution is added slowly to the fatty -r~oc~rboxylic acid in the reactor.
There is also provided a process for continuous production o~ a composition comprising alkali metal salts of Ca -C2 2 alkyl fatty acid and CA ~
C2 2 acyl isethionate in a weight ratio of about 20:1 to 1:0.98, said process comprising the steps of:
( i ) foL ; n~ a hot aqueous caustic solution comprising sodium hydroxide and alkali metal isethiona-te.
(ii) separately and simultaneously 6 ~4~
charging a first feed stream of said C8 -C2 2 alkyl fatty monocarboxylic acid and a second feed stream of said hot caustic solution into a mixing chamber to form said alkali metal C8 -C2 2 alkyl fatty monocarboxylic acid salt; and ( iii ) ~ xi ng said formed alkali metal C8 C2 2 alkyl fatty monocarboxylic acid salt with said alkali metal C8-C2z acyl isethionate salt to form said composition, and wherein either said acyl isethionate salt is introduced to said mixing chamber as a slurry in said fatty monocarboxylic acid in which case said feed stream of said hot caustic solution is intrs~uc~d into said i~1 ng chamber at a temperature of at least 80~C, or said acyl isethionate salt is introduced subsequent to step (ii).
Embo~1 ~rLs of the invention will now be described. Parts and pelcen~ages are by weight, unless the contrary is indicated. Reference will be made to the accompanying drawing which is a schematic flow diagram for a continuous process utilizing an extruder as reactor.
Both a batch and continuous method for the production of soap/acyl isethionate compositions will now be described. These methods utilize fatty acids as a starting material for the soap. Unless otherwise stated, parameters found for the batch route are 2 ~

equally relevant to that of the continuous one.
Broadly, a batch process embodying this Lnvention involves ixi n~ together acyl isethionate and distilled fatty acids to produce a slurry in a reactor vessel. Under agitation, the fatty acids in the slurry are neutralized by slow addition to the vessel of a hot caustic solution comprising sodium hydroxide, sodium isethionate and water. After a ixing period, e.g.
about 30 minutes, the resultant blend is discharged from the vessel for further processing including cooling on chill rolls, ill~n~ plodding and stamping operations to form the cr ,osition into bars.
A factor of importance in obt~i n i n~ a composition that i ni i ~es the hydrolysis of the acyl isethionate co~cçrns that of the caustic solution t- -lature. The temperature of this solution must be maintained at a temperature of at least B~~C, preferably about 93~C. Lower temperatures result in a substantial crystallization of caustic and increased acyl isethionate hydrolysis.
Other parameters can also have some effect upon acyl isethionate hydrolysis. Best yields are obtained where there is sufficient electrolyte present in the caustic solution to achieve a saturated state.
Typical electrolytes are alkali metal and alkaline earth chloride and sulfate salts, especially sodium 2 ~

chloride. Amounts of electrolyte that will normally achieve the saturation level will ran~e from about 0.4%
to about 2~, preferably from 0.4~ to 1.5~, optimally about 0.~% by weight based on the weight of the final product composition.
The amount of alkali metal isethionate may be from 1% to 20~, preferably from 2% to 10% of the final product composition. Sodium isethionate is convenient.
It has also been found desirable that there be slow addition, ~cc- ,Janied by high rotational speed ;~ng, of caustic solution to the fatty acid/acyl isethionate c_ Lonents.
If the process is carried out as a continuous process, a feed of fatty acid is generally injected into a reactor upstream from where the caustic solution enters. An extruder is a suitable reactor vessel for thls process. Fatty acids are normally held in their feed vessel at about 93~C and thus are fed in the molten state to the extruder. Acyl isethionate may be introduced in combination with the molten fatty acid.
Alternatively, acyl isethionate may be introduced into the reactor at a point downstream from where the caustic solution enters. Further in~redients such as stearic acid should be dosed to the reactor at a point downstream from the aqueous caustic entry point.

2 ~

Batch Process In a batch process, processing commenced with molten tallow fatty acid and cocoyl isethionate being charged into a Patterson mixer. These two ingredients were mixed and heated for no longer than 10 minutes.
Mixing and heating produced an off-white suspension or slurry of cocoyl isethionate/tallow fatty acid.
Neutralization of the fatty acids was conducted at about 93~C. The Patterson mixer was heated with steam to this temperature. Upon achieving this temperature, a caustic solution of sodium hydlo8ide, sodium isethionate and water which had been preheated to 93~C, was metered by peristaltic pump into the Patterson mixer ContR~ nl ng the cocoyl isethionate/tallow fatty acid batch. Addition was performed at a rate such that the caustic solution was completely added within 4 to 5 minutes. During the latter part of this time interval, normally after about 4.5 minutes, viscosity of the mixture increased to the point where it bç~ semi-solid. Phase transition of material in the reactor was ~ )panied by a substantial energy release. A brief temperature rise of around 11~C occurred but temperature returned to the set point within 3-5 minutes.

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~ ' 2~40~1 After addition of the caustic solution, the batch was mixed at 93~C for at least 30 minutes.
Stearic acid was then added and ;~, ng continued for an iadditional 5 minutes. Thereafter, the product was removed from the reactor and chill rolled into ribbons.
Perfume, preservatives and other minor ingredients were surface coated onto the ribbons. Further processing included conventipnal milling, plodding and stamping to obtain final bars. The finished, saponified formulation, ignoring hydrolysis of cocoyl isethionate, is outlined in Table I.

TABLE I
Theoretical Final Composition (before hydrolysis) Ingredient Weight Sodium fatty monocarboxylic acid salt (82/18 tallow/coconut soap) 51.17 Sodium cocoyl isethionate21.93 Stearic/palmitic acid 6.19 Coconut fatty acid 1.33 Sodium isethionate 5.00 Water 10.50 Fragrance 1.50 Titanium dioxide 1.00 Sodium chloride 0.43 Miscellaneous minor ingredients 0.22 .

~ o ~
ll The effects of temperature were evaluated in a series of experiments outlined in Table II. Process and composition were essentially identical to that described in Example 1, except for variations of the caustic solution temperature.

TABLE II
Percent Cocoyl Isethionate Loss by Varying Caustic Solution Temperatures Elecbx~yte ~ Cx~yl ~q~r; ~ (Sodium rhl~r;dp) Caustic ~nlut;~n TCp~h;nn~te No. (% WtfWt Pn~ct) .. , ~re (~C) Active LLSS
1 0 93 5.4 2 0 65 24.1 3 0.43 93 3.8 4 0.43 71 15.6 0.86 93 2.8 6 0.86 65 12.2 From the Table, it is seen that the caustic solution ~-nperature radically affects the hydrolysis (loss) of cocoyl isethionate. Experiments 1, 3 and 5 exhibit active loss of 5.4~ or less. By contrast, at 65~C-71~C the loss is more than tripled falling within the range 12.2 to 24.1~. See Experiments 2, 4 and 6.

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Electrolyte level also has some effect upon limiting the amount of cocoyl isethionate lost through hydrolysis. Table III sets forth relationship of electrolyte concentration to that of % cocoyl isethionate loss.

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TAsLE III

% Cocoyl Isethionate Loss by ~arying ~lectrolyte Concentration Initial El~lyte Batch (&x~um Caustic % Cbooyl ~r1 ' Mbisture ~hl~r~. Solution T~Pth~nn~te No. (%) (% Wt/Wt P~x~ct) ~ , ~re Active Loss*
107a 14 0 Ambient 11.53 8a 14 0.43 P ''~nt 10.74 9a 14 0.86 Ambient 9.60 15lOa 14 0.97 P ~nt 9.65 lla 14 1.08 P' ent 8.96 2012a 14 1.3 P' ~nt 8.98 7b 18 0 P ~nt 11.63 8b 18 0.43 Ambient 10.61 259b 18 0.86 Ambient 9.79 lOb 18 0.97 Ambient 9.01 30llb 18 1.08 P ~Pnt 8.97 12b 18 1.3 Ambient 9.29 13 18 0 P-'~nt 11.78 18 0 93~C 8.82 18 1.3 93~C 3.86 40 ~e: ~r; ' No. 7b-12b are the av~xye of at least 2 runs.

From Table III, it is evident that cocoyl isethionate hydrolysis is slowed in the presence of ' ': : ' ' '. ',, 2 ~

certain amounts of salt. Yield improvement occurs up to a level of about 91.3~ by weight electrolyte.
Beyond levels of 2% electrolyte, other physical properties become evident such as that of unacceptable mush values. A combination of a 1.3% salt level with a caustic temperature of 93~C was particularly effective in experiment No. 15 where only 3.86% hydrolysis occurred.

TABLE IV

% Cocoyl Isethionate Loss by Varying the Type of Electrolyte Ele~lyte % GK~yl ~ri ~ G~Ihd~lon T~e~h1~nate Nb. Ele~lyte(% Wt/Wt Pnxh~) Active Loss 16 ~sium rhl ~ri ~P 0. 86 8.93 17 rAl~,il ~hl~ri~P. 0.86 8.98 18 Lithium ~hl ~ri AP. O.86 9.29 2519 ÇAlril sulfate 0.86 8.98 From Table IV, it is evident that the particular type of elec~x~yte is n~t criti~l. Any in~nic salt that readily 30 dissolves in the caustic 5nlllti~n will be ~x~k~le.

20404~

The following experiment was run similar to that described in Example 1 except that the caustic solution was added to the batch reactor concurrently with cocoyl isethionate/tallow fatty acid. Table V summarizes the parameters of these experiments.
From Table V, it is evident that by the concurrent addition of caustic solution with the other reactants, the amount of cocoyl isethionate loss is quite significant, ranging from 22.8 to 33.2.

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A procedure similar to Example 1 was again used, although without close regulation of processing temperature. In a control experiment, addition to the reactor followed the same sequence as in Example 1. In further experiments the addition of acyl isethionate was delayed until after the addition of caustic solution and consequen~ neutralization of the fatty acid to soap.
This led to reduced isethionate loss, as shown by the following Table VI

204o4~l TAsLE VI

Batch Process Studying Effects of Cocoyl Isethionate Point of Addition Isethionate~ Cocoyl Experiment AdditionIsethionate No. Method Loss*
24 Control 23.0 Modified 9.9 26 Reverse 3.7 *Control = cocoyl isethionate/tallow fatty acid added prior to caustic addition.
Modified = tallow fatty acid followed by slight excess caustic addition (to neutralize the coco fatty acid impurity in cocoyl isethionate) and cocoyl isethionate added last after soap formation.
Reverse = tallow fatty acid followed by equivalent weight caustic addltion and cocoyl isethionate added last after soap formation.

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This Example illustrates the continuous process outlined schematically in the sole drawing.
The drawing shows liquid streams being fed to a twin screw extruder lO driven by an electric motor 11. More particularly, the extruder was a Werner and Pfleiderer Corporation 32L/D twin screw extruder having a 40 mm screw flight ~ er. The first feed stream which entered along line 12 was a slurry of cocoyl isethionate/tallow fatty acid. Downstream therefrom was introduced along line 14 a second feed stream.
This was a hot caustic solution comprising sodium hydlo~ide, sodium isethionate and water. Screw configuration of the extruder was such that caustic and fatty acid streams were rapidly completely neutralized and continue i~i ng as they travel towards discharge from the extruder 10. Resi~nce times within the extruder nol ~lly ranged from 3 to 5 minutes, depending upon product throughput, rotational speed and configuration of the extruder screw.
The cocoyl isethionate/tallow fatty acid feed stream was prepared using a Schold intensive mixer (not shown). This stream was prepared by first ~d~ n~
molten tallow fatty acid to the mixer with agitation.
After reaching a temperature of 93~C, cocoyl 2040~1 isethionate was added to the molten tallow fatty acid.
Heating at 93~c with ;xing was continued for 20 minutes whereafter the mixture was transferred to a waiting extruder feed tank 18 in which a temperature of approximately 93~C was maintained.
Caustic solution was prepared by mixing water, 50 weight % sodium hydroxide, and 56 weight %
sodium isethionate together in a feed tank 20. It was found necessary that the water be added first to the tank to provide sufficient solvent for the two solutes ~sodium hydroxide and sodium isethionate) to avoid precipitation of the sodium hydroxide. Temperature maintained in this tank was approximately 93~C.
The extruder had a barrel whose length was divided into five 5/1 length/diameter sections that were controlled with separate dual output, self tuning controllers. These controllers regulated electrical heating or closed loop cooling of the respective barrel sections. Feed rates to the extruder were controlled via a K-tron loss-in-weight liquid feed system. Each feed tank rested on top of a scale 22 which relayed tank weight information to a controller. Changes in tank weight with time were monitored by the controller.
The controller then regulated the rotational speed of a gear pump 24 feeding the extruder. The system automatically compensated for a decreasing suction head ' ~

~ 1 occurring as a result of the liquid height in the feed tank decreasing as feeding progressed. At an entrance to the extruder, each feed line was fitted with an in~ection nozzle allowing feed to enter into the extruder under a specified pressure. Respective lines 26 provide for recycle to the feed tanks.
Upon start up, the caustic line was first opened and then the cocoyl isethionate/tallow fatty acid feed line opened thereafter. Extruder screw speed was set at 400 to 500 rpm. Feed line injection pressures were adjusted to 350 kN/m2. Feed controllers were no- -lly set to achieve a product throughput of 91 Kg per hour.
Stearic acid was maintained at a temperature of 65~C in thlrd feed tank 28. This was metered into the extruder 10 along line 30. The produot was discharged from the extruder in the form of noodles 34.
With the above-identified equipment, a series of continuous runs were conducted. Table VII lists details of these experiments. The continuous process achieved a cocoyl isethionate loss of only 8-10 weight . These results are significant considering that in batch experiments where a total caustic charge ls added in a single shot, cocoyl isethionate hydrolysis is approximately 20-30~, and sandy bars result. The neutralization temperatures were measured values of the 4 ~ 1 extruder barrel jacket temperature.

TABLE VII

Continuous Extruder Processing Neutrali- Neutrali-zation zation % Cocoyl MoistureTemperature SampleIsethionate (~) (~C) No. Lo~s 16.36 80 1 3.55 100 2 5.41 120 3 7.74 14.98 80 1 8.03 100 2 8.80 120 3 9.70 20 14.98 80 1 9~86 100 2 8.28 120 3 8.59 15.01 80 1 9.82 100 2 7.25 18.00 80 1 9.87 100 2 9.50 120 3 10.22 ::

Claims (17)

1. A process for the production of a composition comprising alkali metal salts of C8-C2 2 alkyl fatty monocarboxylic acid and C8-C2 2 acyl isethionate in a weight ratio of 20:1 to 1:0.98, said process comprising the steps of:

(i) forming a hot aqueous caustic solution comprising sodium hydroxide and alkali metal isethionate;
(ii) charging C8-C2 2 alkyl fatty monocarboxylic acid to a reactor and maintaining said fatty monocarboxylic acid at an elevated temperature while mixing;
(iii) adding said hot caustic solution to said fatty monocarboxylic acid in said reactor; and (iv) either (a) feeding said acyl isethionate salt to said reactor at a time either prior to the addition of said caustic solution at a temperature of at least 80°C in step (iii) or (b) feeding said acyl isethionate salt to said reactor at a time subsequent to step (iii).

2. A process according to claim 1 wherein the caustic solution is added at a temperature of 90 to 95°C.

3. A process according to claim 1 wherein the acyl isethionate salt is added to the reactor in admixture with said fatty monocarboxylic acid.

4. A process according to claim 1 wherein the aqueous caustic solution is added at a temperature of at least 80°C, and the acyl isethionate salt is added subsequent to step (iii).

5. A process according to claim 1 wherein the aqueous caustic solution contains from 0.4 to 2% by weight of an electrolyte.

6. A process according to claim 1 wherein the aqueous caustic solution contains from 0.8 to 1.5 by weight of an electrolyte.

7. A process for continuous production of a composition comprising alkali metal salts of C8-C22 alkyl fatty acid and C8-C22 acyl isethionate in a weight ratio of about 20:1 to 1:0.98, said process comprising the steps of:

(i) forming a hot aqueous caustic solution comprising sodium hydroxide and alkali metal isethionate;

(ii) separately and simultaneously charging a first feed stream of said C8-C22 alkyl fatty monocarboxylic acid and a second feed stream of said hot caustic solution into a mixing chamber to form said alkali metal C8-C22 alkyl fatty monocarboxylic acid salt; and (iii) mixing said formed alkali metal C8-C22 alkyl fatty monocarboxylic acid salt with said alkali metal C8-C22 acyl isethionate salt to form said composition, and wherein either said acyl isethionate salt is introduced to said mixing chamber as a slurry in said fatty monocarboxylic acid in which case said feed stream of said hot caustic solution is introduced into said mixing chamber at a temperature of at least 80°C, or said acyl isethionate salt is introduced subsequent to step (ii).

8. A process according to claim 7 wherein the caustic solution is added at a temperature of 90-95°C.

9. A process according to claim 7 wherein the acyl isethionate salt is included in the first feed stream in admixture with said fatty monocarboxylic acid.

10. A process according to claim 7 wherein the aqueous caustic solution contains from 0.4 to 2% by weight of an electrolyte.

11. A process according to claim 7 wherein the aqueous caustic solution contains from 0.8 to 1.5%
by weight of an electrolyte.

12. A process according to claim 7 wherein said first feed stream is introduced at a point upstream from where said caustic solution enters said mixing chamber.

13. A process according to claim 7 wherein said acyl isethionate salt is fed into said mixing chamber reactor at a point downstream from the entry points of both said caustic solution and said fatty monocarboxylic acid streams.

14. A process according to claim 7 wherein said mixing chamber is an extruder.

15. A process according to claim 1 wherein the aqueous caustic solution contains sodium hydroxide and alkali metal isethionate in a total amount from 30%
by weight of the solution up to saturation thereof, and in proportions such that addition of solution in an amount substantially stoichiometric to the fatty acid introduces alkali metal isethionate in an amount from 1% to 20% of the overall composition.

16. A process according to claim 1 further comprising forming the composition into bars.

17. A process according to claim 7 further comprising forming the composition into bars.