The present invention refers to a concentrated aqueous solution of an amphoteric surfactant, particularly betaine, and a method for the preparation thereof.

Amphoteric surfactants exhibit excellent foaming and detergent properties, combined with a very good dermatological compatibility and an ability to decrease the irritating power of anionic surfactants conventionally used in the cosmetic field, such as sodium lauryl/laurylethGxy sulfate. Amphoteric surfactants are used as raw materials for preparing both detergents, such as for example washing-up liquids, and cosmetic products, such as for example shampoos and bath foams.

Among amphoteric surfactants, alkyl betaines, and particularly alkyl amidobetaines, acquired a particular importance from the commercial point of view. Among these, the most important is doubtlessly cocamidopropyl betaine.

Amphoteric surfactants are marketed in the form of concentrated aqueoas solutions. The tendency in this area is of increasing as much as possible the concentration of the active substance in the aqueous solutions to be marketed. However, limitations exist to the possibility of increasing the active substance, as at high concentrations the active substances tend to form an extremely viscous lamellar gel not easy to handle, The increase in viscosity or gelation can likewise occur during storage. For this reason, conventional amphoteric surfactant aqueous solutions, particularly alkylamido propyl betaine, are characterized by a concentration of about 30% of active matter and 35% as the dry residue.

Several attempts to obtain low viscosity betaine concentrated aqueous solutions are described in the prior art.

European patent EP 0560114 Bl describes adding a desired amount of free fatty acids, optionally in combination with glycerol. However, fatty acids are quite expensive additives and their presence in a detergent or cosmetic product not always is considered as acceptable, since frequently fatty acids form as processing residues, which implies that limitations may be imposed to the concentration thereof in the finished product, Moreover, a sensible reduction in the viscosity of surfactant solutions is only observed at free fatty acid concentration values higher than 1% by weight.

US patent 5962708 intends to reduce the viscosity of a betaine solution by adding hydroxycar- boxylic acids or salts thereof, more particuiariy sodium citrate. Such additives are even more expensive than fatty acids, which makes this technical solution very little attractive,

US patent application US 20060110354 describes the use of at least one sulfate, particularly sodium sulfate from 0.01 % to 5% by weight. However, sulfates are also considered as impurities, the presence of which not always is desirable in finished formulations.

One object of the present invention is thus of providing a concentrated aqueous solution of an amphoteric surfactant, preferably betaine, which does not exhibit the drawbacks of the prior art.

This and other objects are achieved through a concentrated amphoteric surfactant aqueous solution as defined in the preamble of claim 1, characterised in that it comprises urea at a concentration of from 0.01% to 5% by weight of the total weight of the solution.

Further features of the solution according to the invention are defined in the appended subordinate claims that form an integral part of the specification.

The inventors have surprisingly found that adding a small amount of urea to a concentrated amphoteric surfactant aqueous solution is sufficient to reduce the viscosity at values equal to or lower than 3000 cps, preferably equal to or lower than 1000 cps (viscosity as measured with a model LV Brookfseld viscosimeter at 2O ⁰ C, 20 r.p.m., spindle 3).

Urea is added in a suitable amount during or after the synthesis of the amphoteric surfactant. The urea concentration ranges from 0.01% to 5% by weight, preferably from 0.1% to 3% by weight, more preferably from 0.1% to 0.5% by weight, of the total weight of the surfactant solution.

Ln a preferred embodiment, the amphoteric surfactant is selected from the group consisting of alky] betaines, alkylamido betaines, aminopropionates, aminoglycinates, imidazolinum betaines. sulfobetaines and any mixture thereof.

More preferably, the aSkylamido betaines are alkylamidopropyl betaines of formula (1):

CHi R - CO - NH - CH ₃ CH ₂ CH ₂ - N ^÷ - CH ₂ CCKJ

wherein R is a linear or branched, saturated or unsaturated alkyl group having from 6 to 22 carbon atoms, or a mixture of linear or branched, saturated or unsaturated alkyl groups having an average of 6 to 22 carbon atoms. A preferred value for R is the alkyl group from coconut fatty acids. The average composition of coconut fatty acids, derived from coconut oil. is specifically described in US 5 354 906. Such a description is hereby incorporated as reference,

Solutions of alkylamido propyl betaines of formula (I) contain electrolyte salts that neutralize the charges and are preferably selected from alkaline or alkaline earth metal chlorides or chlorides with other monovalent (such as for example Cu+), divalent (such as for example Pb2+), trivalent (such as for example A13+), polyvalent (such as for example Sn4+ or NH4+) cations or with amine bases selected from triethanolamine, monoethanolamine, diethanolamine. monoiso- prøpanolamine, triisopropanolamine, 2-aminobutanol, aminoethylpropanediol, arginine, lysine, ornithine, aminomethylpropanol, aminomethylpropanediol, 2-amino-2-hydroxymethyl-l ,3- propanediol. These electrolyte salts may be used individually or as a mixture.

The concentration of the amphoteric surfactant in the concentrated aqueous solution of the invention ranges from 25% to 60% by weight, more preferably from 35% to 48% by weight, of the total weight of the solution. According to another embodiment of the invention, the concentrated amphoteric surfactant aqueous solution comprises sodium phosphate as a further component capable of decreasing the viscosity and prevent gelation of the solution during storage. The concentration of sodium phosphate ranges between 0 and 5%, preferably between 0.01 % and 5%, even more preferably between 0.1% and 3% and even more preferably between 0.1% and 0.5% by weight, of the total weight of the solution. Sodium phosphate is used in the form of Na3PO4, NaH2PO4, Na2HPO4 or any combination thereof.

A preferred composition according to the invention comprises the following ingredients:

- amphoteric surfactant (preferably alkylamido propyl betaines of formula (I)): from 25% to 60% by weight;

- urea: from 0.01% to 5% by weight;

- sodium phosphate: from 0.01% to 5%; and

- water: up to 100% by weight.

The aqueous solution of the invention does not include further active substances. However, manufacturing residues may be present, such as for instance electrolyte salts as mentioned previously (particularly sodium chloride) the amount of which, if present, generally adds up to 5- 10% by weight, unreacted raw materials and small amounts of free fatty acids or salts thereof. The quantity of free fatty acids or salts thereof is maintained as much as possible at very low levels.

The aqueous solution of the invention can be acidic, neutral or basic, the pH value thereof being preferably comprised between 1 and 9.

Since the problem of the increase in viscosity of a concentrated amphoteric surfactant aqueous solution is mostly encountered with acidic solutions at pH < 5, the use of urea in order to reduce viscosity results particularly advantageous with solutions having such pH values. Consequently, a preferred embodiment of the invention is a concentrated amphoteric surfactant aqueous solution as previously defined, having a pH value comprised between 1 and 5, more preferably comprised between 4.0 and 4,9. Further preferred pH ranges are the following: from 4.0 to 4.5; from 4,0 to 4,4; from 4.0 to 4,3; from 4,0 to 4.2: and from 4.0 to 4.1.

As previously indicated, one advantage of the present invention is that urea can be added during the course of the synthesis reaction for the surfactant or. alternatively, it can be added to the already-synthesized surfactant, during the processing of the final product. Adding urea to the already-synthesized surfactant is to be preferred when the surfactant viscosity must be adjusted at an extremely precise value. Sodium phosphate instead is added at the end of the synthesis reaction for the surfactant.

Thus, another object of the present invention is a method according to the preamble of claim 10, characterized in that it comprises synthesizing the said amphoteric surfactant in the presence of a quantity of urea suitable to provide a concentration from 0.01% to 5% by weight, preferably from 0.1% to 3% by weight preferably from 0.1% to 0.5% by weight, of the total weight of the solution.

In an alternative embodiment, the method of the invention is characterized in that it comprises adding, to said amphoteric smfactant, a quantity of urea suitable to provide a concentration from 0,01% to 5% by weight preferably from 0,3% to 3% by weight, preferably from 0.1% to 0.5% by weight, of the total weight of the solution.

Both the above-described method embodiments may include adding, to the amphoteric surfactant, a quantity of sodium phosphate suitable to provide a concentration comprised between 0 and 5% by weight, preferably comprised between 0.01% and 5% by weight, even more preferably comprised between 0.1% and 3% by weight and even more preferably comprised between 0.1% and 0.5% by weight, of the total weight of the solution.

Examples of synthesis reactions for the amphoteric surfactant are quatemization of fatty amines or fatty acid amϊdoammes with halocarboxylic acids or salts thereof, to give alkyl betames or al- kylamido betaines.

The following examples are provided merely by way of illustration and not limitation of the scope of the invention as defined in the appended claims. Example 1

200 g (0,78 mol) of a coconut fatty acid and 79.7 g (0.78 rnol) of N ₅ N -dimethyl arninopropyia- rnine were inserted into a 2-liter 4-neeked round-bottomed flask fitted with a thermometer, a condenser, a water separator, a nitrogen aspiration tube and a stirrer and were heated under nitrogen at 180 ⁰ C. The condensation water was continuously removed. The reaction went on until the acidic value of the mixture dropped to under 5. The amine excess was then removed under vacuum. This way, 250 g of coconut fatty acid amidopropyl dimethylamine were obtained and adjusted with water at a concentration of active substance of 45% by weight. The obtained product was basically solid.

Example 2

Example 3 was repeated, with the exception that the reaction of the coconut fatty acid with N ₅ N- dimethyl aminopropylarnine was carried out in the presence of 0.81 g (corresponding to 0.3% by weight, on the basis of the final composition) urea. The mixture was adjusted with water at a concentration of active substance of 45% by weight with a Brooklield viscosity at 20 ⁰ C of 210 cps,

Example 3

Amounts of 0.3% and 0.5% urea by weight were successively added to a eocoarnidopropyi be- taine aqueous preparation with a concentration of the active substance of 45% by weight obtained as described in Example 1. The appearance and viscosity of the pastes were then determined at 20 ⁰ C. The results are reported in Table 1.

Appearance and viscosity of the Amount of added urea

cocoamidopropyl betaine (% by weight) pastes

0.5

Appearance at 2O ⁰ C ssel

5ity at 20 ⁰ C 305 cps

The influence of pH was also assessed on the formulation containing 0.5% urea by weight. The results are reported in Table 2,

E _. xamp]e 4

Amounts of 0.3% and 0.5% Na3PO4 by weight were successively added to a cocoamidopropyl betaine aqueous preparation with a concentration of the active substance of 45% by weight obtained as described in Example 1. The appearance and viscosity of the pastes were then determined at 20 ⁰ C. The results are reported in Table 3,

Appearance and viscosity of the Amount of addec Na3PO4

cocoamidopropy] bexaine (% by weig≠)

pastes 0 J 0.3 0.5

Appearance at 20 ⁰ C gel liquid liquid

Viscosity at 20 ⁰ C I 360 cps 351 cps

The influence of pH was also assessed on the formulation containing 0.5% Na3PO4 by weight. The results are reported in Table 4. Table 4

pH 4.2 4.6 4.9 5.5 7.0

Viscosity at 20°C solid

305 306 310 325 (cps) tends to separate