EP1267824A2

EP1267824A2 - Structured surfactant systems containing pearlisers

Info

Publication number: EP1267824A2
Application number: EP00971316A
Authority: EP
Inventors: Kevan Hatchman; Richard Llewelyn Crombie; John Hawkins
Original assignee: Rhodia Consumer Specialties Ltd
Current assignee: Huntsman International LLC
Priority date: 1999-10-07
Filing date: 2000-10-04
Publication date: 2003-01-02
Also published as: AU1021201A; CA2386532A1; GB9923593D0; TW527419B; JP2003511391A; WO2001025378A2; WO2001025378A3

Abstract

The protolamellar phase (a disc micellar, optically isotropic phase which exhibits transient anisotropy under stress) is capable of suspending solid particles. It is particularly effective for suspending pearlisers.

Description

STRUCTURED SURFACTANT SYSTEMS

The invention relates to a novel type of structured surfactant system. The novel system is especially suitable for suspending pearlising concentrates for incorporation into liquid formulations such as shampoos and toiletries to import a nacreous iridescence which is attractive to consumers, and can mask inhomogeneities in the formulations.

Structured surfactant systems have the ability to suspend solid particles indefinitely in a pourable liquid. While the system is at rest it behaves like a solid gel holding the particles immobile, but the shear forces associated with pouring break the structure causing the suspension to flow like a mobile liquid. Known structured surfactant systems are either opaque interspersions of a surfactant mesophase with an aqueous phase or cloudy opalescent lamellar phases.

Pearlisers typically comprise small, thin, transparent platelet crystals which can be suspended in a parallel configuration. When so suspended light falling on the crystals undergoes complex multiple reflections within the substrate similar to those which occur in a pearl and giving rise to similar optical interference effects.

Natural pearls comprise alternate layers of calcium carbonate and protein. Artificial pearlisers include guanine/hypoxanthine crystals extracted from fish scales, mica, various salts of lead, zinc, mercury and bismuth (e.g. bismuth oxychloride). titanium oxide and various fatty acid derivatives such as magnesium stearate. coconut monoethanolamide, ethylene glycol distearate and ethylene glycol monostearate. Fish scale extracts are too expensive and the inorganic pearlisers are either too toxic for general use in toiletries e.g. lead, mercury, or relatively ineffective e.g. bismuth. The fatty acid derivatives are therefore now the most widely used pearlisers. In addition to the chemical nature and physical form of the pearliser the manner in which it is suspended has an important effect on it's visual impact. Difficulty is sometimes encountered obtaining the desired effect when incorporating pearlisers into aqueous formulations.

Conventional fatty acid derived pearlisers are supplied as solids which are usually added to a heated formulation above their melting point and recrystallised in situ. The conditions of crystallisation and especially the amount and nature of the agitation applied must be carefully controlled in order to obtain an acceptable result. This makes it difficult to obtain consistent effects and renders solid pearlisers inconvenient to use.

Attempts have been made to prepare liquid concentrates or suspensions which can be added directly to shampoo formulations without heating. While more convenient for the user, such concentrates face the manufacturer with problems of obtaining a high and consistent pearl effect, similar to those which confront the user of conventional solid pearlisers. Difficulty is also encountered in maintaining the particles in stable suspension and preventing sedimentation.

We have now discovered that certain surfactants having a high solubility parameter such as alkyl ether sulphates at concentrations just below those which normally give rise to liquid crystal phases, e.g. typically 18 to 30% by weight, will form mobile, clear protolamellar Li phases in the presence of sufficiently high concentrations of electrolyte. These mobile phases are clear and optically isotropic and are typically less viscous than normal G-phases but have the capacity to form stable suspensions of solids such as pearlisers.

The protolamellar phase is apparently unique in being clear and optically isotropic while exhibiting the suspending properties of a structured surfactant system. We believe that protolamellar phases comprise oblate (disc) micelles which can adopt a parallel lamellar-like orientation. They may typically be identified by placing them between crossed polarisers and applying stress, e.g. by tapping or shaking, when flashes are observed as the system transiently forms an anisotropic structure.

When pearlisers are suspended in a protolamellar phase the structure promotes the parallel orientation of the pearlising crystals which maximises the pearlescent effect, even in the absence of agitation. The concentrates are readily pourable but do not generally tend to separate on standing, and can easily be added to shampoo formulations, without heating, to give high and consistent pearlising effects.

The compositions are particularly useful because the preferred surfactant, alkyl ether sulphate, is the most commonly used ingredient of shampoo formulations on account of it's skin mildness and comparatively good foaming properties. Thus the concentrate can be included in shampoo formulations without introducing anything extraneous to the formulator's requirements. The optical clarity of the protolamellar phase permits the pearlescent effect to be clearly observed without being obscured by surfactant structures as occurs when any hitherto known structured surfactant is used. When alkyl ether sulphates or similar water soluble surfactants are dissolved in water at relatively low concentrations they form a clear micellar solution (LI phase) with the surfactant molecules arranged in spherical clusters (micelles). As the concentration is increased the micelles form rods (prolate micelles) of increasing length, and the viscosity increases. Further increases of concentration are accompanied by an anomalous fall in viscosity as the rods become aligned. The aligned rod micellar or protohexagonal system is optically clear, pourable and may exhibit a hexagonal symmetry when examined by small angle x-ray diffraction (SAX), giving a first order peak of momentum transfer vector Q \ , and, if the system is sufficiently well defined, one or more, progressively smaller, higher order peaks at Q values in the ratios:

Q2 = Ql ^3 ; Q₃ = 2Qι

These ratios are diagnostic of hexagonal symmetry.

When the concentration is further increased the length of rods increases indefinitely and the immobile M phase is formed. This is typically observed at concentrations of about 30% by weight surfactant. The M phase also shows SAX peaks in the ratios characteristic of hexagonal symmetry. It typically resembles a curdy or gelatinous solid or very viscous mucous like fluid. The normal lamellar, or G phase is typically formed when the concentration of the alkyl ether sulphate is increased to about 55 to 60% by weight. It is a mobile, birefringent liquid crystal with sheer dependent viscosity and lamellar symmetry, revealed by SAX peaks with the characteristic ratios

2τr

Q2 = 2Q] , ; Q3 = 3Qi - The peaks indicate structure with a repeat spacing (d =QT^~) of about 3 to 5 nm. Although the normai G-phase has a yield point and is relatively mobile compared with the M-phase it is not generally suitable for use as a suspending medium for solids, since the presence of appreciable amounts of suspended solid renders it unpourable. The high surfactant concentration is a further disadvantage.

However, when sufficient electrolyte is added to a micellar or protohexagonal solution of alkyl ether sulphate there appears to be a change from prolate to oblate micelles and corresponding shift from hexagonal to lamellar symmetry with a d spacing typically in the range 6 to 15 nm. Such phases are able to suspend solids and remain readily pourable. Our invention provides the use of a protolamellar aqueous surfactant to suspend solid particles, especially pearliser.

Our invention further provides a suspension of solid particles in an aqueous surfactant, wherein said surfactant is in a protolamellar phase. In a preferred embodiment the suspended particles comprise particles of a pearliser.

The surfactant is preferably a Cjo-is alkyl or alkenyl 1 to 10 mole ethoxy sulphate more usually a C_]2_₁₄ alkyl ethoxy sulphate and preferable an alkyl 1 to 5 mole ethoxy sulphate. Alternatively the ether sulphate may be a mixed propoxy ethoxy sulphate or alkyl glyceryl or alkyl glyceryl polyethoxy sulphate. The cation of the alkyl ether sulphate is preferably sodium but may be potassium, lithium, ammonium, or a C]_-6 amine such as an alkanolamine.

The surfactant may comprise minor amounts of other surfactant, especially anionic, non-ionic or amphoteric surfactants, for example alkyl sulphates, alkyl benzene sulphonates, paraffin sulphonates, olefin sulphonates, alkyl sulpho-succinates, soaps, taurides, isethionates, alkyl ethoxylates, fatty acid ethoxylates, alkyl glyceryl ethoxylates, alkyl carbohydrate ethoxylates, anine oxides or betaines. However it is generally preferred to use essentially only alkyl ether sulphate.

The electrolyte is preferably sodium chloride, but could be, for example, sodium carbonate, sodium citrate, sodium tripolyphosphate, sodium hydroxide or any other salt or base which tends to salt alkyl ether sulphates out of solution. The cation of the electrolyte may be sodium, potassium lithium, ammonium or, less preferably, an amine such as alkanolamine or a mixture of any of the foregoing.

The choice of electrolyte and of auxiliary surfactant, if any, may be conditioned by what ingredients the formulator, to whom the concentrate is to be supplied, wishes to include in the end formulations, or can tolerate.

Generally the surfactant is used in a total concentration which, in the absence of electrolyte, would correspond to a clear Lj phase or protohexagonal phase. Typically the electrolyte-free surfactant/water system would be unstructured or would exhibit hexagonal symmetry under SAX. The concentrations required vary with different surfactants but are generally in the range 17 to 30% based on the total weight of the composition, more usually 19 to 28%. The amount of electrolyte is sufficient to convert the ether sulphate/water mixture from an optically isotropic, clear, \ phase or a protohexagonal phase exhibiting hexagonal symmetry to a clear, protolamellar phase, typically exhibiting lamellar symmetry, under SAX. The amount required will depend on the nature and concentration of the surfactant. Typically it will be between 1 and 10% based on the total weight of the composition, more usually 2 to 5%. Sodium citrate is more expensive than sodium chloride and is most effective at concentrations of 4 to 10% but permits higher loading of pearliser.

The pearliser may be any of those previously discussed herein including natural pearl and inorganic pearlisers but is preferably a fatty acid derivative especially a mixture of ethylene glycol distearate and ethylene glycol monostearate.

The pearliser may be dispersed in the aqueous structure surfactant system e.g. by gently stirring, but in the case of the fatty acid derivatives are preferably prepared in situ by heating above their melting point, e.g. temperatures between 65 and 80^0, dispersing the liquid pearliser in the structured surfactant system, preferably with sufficient stirring to form droplets of from 1 to 60 microns, e.g. 15 to 50 microns, and cooling to ambient temperature. Preferably cooling is relatively slow e.g. the mixture is allowed to cool naturally. The amount of pearliser can be varied considerably, the main constraint on the upper limit being the viscosity.

The amount of pearliser should not be so high as to render the product unpourable, or unacceptably viscous. We prefer on economic grounds that the pearliser is present in amounts greater than suspending surfactant. Generally pearliser may be present in amounts ranging from 5% up to about 60% e.g. 10 to 45% more preferably 15 to 30%, especially 18 to 28% of the total weight of the mixture. The ration of pearliser to surfactant is preferably from 0.6 to 2 e.g. 1.2 to 1.3 by weight.

Ancillary ingredients suitable for inclusion in toiletries could in principle be included, but are usually omitted to avoid unduly constraining the customer with regard to its freedom to formulate. However, it is generally desirable to include small amounts of preservative, such as formaldehyde or other microbiocidally and/or fungicidally active material.

The novel structured, protolamellar phase may be used to suspend solids other than pearliser, e.g. builders such as zeolite or phosphate for use in detergent. They may also suspend bentonite or calcite. They may suspend rock cuttings and/or weighting agents for use a drilling muds. They can suspend agricultural pesticides or dyes or pigments, dicalcium phosphate for use in toothpaste or ammonium polyphosphate for fire retardants.

The invention is illustrated by the following examples :-

The method comprised of the following steps:

i) Charge water and heat to 75-80°C. ii) Add NaCl and dissolve with mixing. iii) Add 80-90 % of SLES charge and mix until homogeneous. iv) Add EGDS and mix until a homogeneous emulsion is formed. Stir for 0.5-1.0 hrs to ensure the EGDS has dissolved, v) Commence cooling cycle and ensure the emulsion is sufficiently agitated, c.a.

60-100 rpm (80 rpm is preferable), vi) When the product has cooled to 30-35°C, add remaining SLES and mix until homogeneous, vii) Check solids and adjust with water into specification, viii) Add preservative and adjust pH accordingly.

The concentrates require the following cooling cycle:

Concentrates prepared using the above method were found to have a consistent pearl size, (20 to 40 microns) freeze/thaw stable and do not separate after one month at

40°C. Example 1

Appearance: White/off white metallic pearl pH (100%): 5.5 -6.5 Odour: Characteristic Solids: 46 - 48 % (typical) Viscosity @ 25°C: < 20000 cps Density @20°C: 0.95 -1.05g cm^"3

Example 2

Appearance: White/off white metallic pearl pH (100%): 5.5 -6.5 Odour: Characteristic Solids: 40 - 42 % (typical)

Viscosity @ 25°C: < 20000 cps Density @20°C: 0.95 -1.05g cm^' Example 3

Appearance: White/off white metallic pearl pH (100%): 5.5 -6.5

Odour: Characteristic Solids: 51-54% (typical)

Viscosity @ 25°C: < 10000 cps (typical) Density @20°C: 0.95 -1.05g cm -3

Example 4

Appearance: White/off white metallic pearl pH (100%): 5.5 -6.5

Odour: Characteristic Solids: 57 - 60 % (typical)

Viscosity @ 25°C: < 10000 cps (typical) Density @20°C: 0.95 -1.05g cm

Claims

1. The use of a protolamellar aqueous surfactant to suspend solid particles.

2. The use according to claim 1 of a protolemellar aqueous surfactant to suspend pearlisers.

3. A composition comprising protolemellar aqueous surfactant and suspended particles of pearliser.

4. A composition comprising water, from 17 to 30% by weight of alkali metal, ammonium or Cι_-6 amine salt of a C_to-is 1 to 10 mole ethoxy sulphate, sufficient electrolyte to form with said sulphate and water an optically isotropic phase which exhibits optically anisotropic properties and lamellar symmetry when subject to shear, from 15 to 60% by weight of particles of ethylene glycol mono and/or distearate having a particle site of from 6 to 60 microns suspended in said composition.

5. A composition comprising water, from 19 to 28% by weight of sodium Cι_2-ι alkyl 1 to 5 mole ethoxy sulphate, from 18 to 28% by weight of pearliser consisting at least predominantly of ethylene glycol distearate, optionally in admixture with a minor proportion of ethylene glycol monostearate and having a particle size of from 10 to 50 microns and from 2 to 5% by weight of sodium chloride.

6. A method of making a pearliser suspension as claimed in any of claims 3 to 5 which comprises forming an emulsion of ethylene glycol distearate, optionally in admixture with a minor proportion of ethylene glycol monostearate, at a temperature above its melting point , in a solution of from 18 to 28% by weight of a sodium C_{J O}-_{I S} alkyl 1 to 10 mole ethoxy sulphate and from 2 to 5% by weight of sodium chloride and cooling said emulsion.