WO1997039101A1

WO1997039101A1 - Dicalcium phosphate slurries

Info

Publication number: WO1997039101A1
Application number: PCT/EP1997/001776
Authority: WO
Inventors: Richard Malcolm Clapperton
Original assignee: Albright & Wilson Uk Limited
Priority date: 1996-04-13
Filing date: 1997-04-10
Publication date: 1997-10-23
Also published as: GB9707384D0; GB2311939B; AU2695197A; AU2384297A; WO1997039083A1; GB2311939A

Abstract

Dicalcium phosphate is suspended in a structured surfactant, to provide a mobile, stable, dust-free slurry.

Description

DICALCIUM PHOSPHATE SLURRIES

Introduction

The present invention provides pourable, stable, concentrated aqueous based slurries of dicalcium phosphate (herein referred to as DCP) comprising, typically, at least 35% of DCP. The slurries find particular application in the manufacture of toothpaste, and also in animal feeds and pharmaceuticals as an easily handleable form in which the DCP may be supplied to e.g. toothpaste manufacturers.

Problem

Previously, DCP has always been supplied in the form of a solid powder of very fine particle size, which is difficult and costly to handle. This is undesirable for safety reasons, as dust particles may be inhaled. It is difficult for the phosphate manufacturer to dry and for the user to wet. A liquid would be substantially easier to handle than the solids, both for the maker and the user, however, it has proved impossible to provide a concentrated solution or suspension of DCP due to the restrictions imposed by its very low solubility and the instability and/or viscosity of concentrated suspensions.

There is a need to provide concentrated, pourable, stable, aqueous DCP slurries which can supply high levels of phosphate in a convenient form to avoid the problems typically associated with drying, handling and rewetting the solid.

The problem is therefore to provide DCP in a form :

(a) that is more easily handled than the conventional powder,

(b) which is pourable at ambient temperature and stable; (c) which contains an economically viable concentration of DCP; and

(d) which contains only ingredients which are acceptable to the end user.

In particular it is a specific object of the invention to obtain products meeting some or preferably all ofthe following criteria :

(i) They contain more than 35% w/w of DCP, and preferably the maximum amount consistent with (ii) below;

(ii) They have a viscosity less than 3 Pas preferably less than 2 Pas at ambient temperature (e.g. 20°C) and 21 sec^'1 shear;

(iii) They do not sediment on standing e.g. for 24 hours, preferably for 1 week most preferably for 3 months, at 20°C or at normal storage temperatures;

(iv) They do not contain ingredients which are extraneous to the desired end formulation or which are in proportions relative to the DCP which would be greater than is desirable in the end formulation;

Discovery

We have now discovered that pourable, stable, concentrated, aqueous slurries which solve the above problem and which meet at least some ofthe above criteria may be prepared by suspending the DCP in a structured surfactant wherein the concentration of surfactant and of any electrolyte are sufficient to provide a solid-suspending (e.g. spherulitic) system. Prior Art

Structured surfactants and in particular spherulitic systems are known in the detergent art. Detergent compositions typically require the presence of high concentrations of surfactant and may also require a solid such as an abrasive and or an insoluble builder.

It has been found (see, for example, GB 2 153 380) that surfactants, when present in sufficiently high concentrations and in the presence of sufficient quantities of dissolved electrolyte, form structured systems comprising a surfactant mesophase interspersed with an aqueous electrolyte phase. The surfactant mesophase is typically present, at least partly, in the form of spherulites having a diametre ofthe order of 0.05 to 20 microns (usually 0.1 to 10 microns) and comprising a plurality of concentric shells of surfactant, alternating with shells of aqueous phase. The surfactant shells are in the form of bilayers of surfactant molecules all arranged with their hydrophilic groups on the outer faces ofthe bilayer and their hydrophobic groups in the interior.

The spherulites can interact with one another to form weak structures which will immobilise and suspend solid particles indefinitely in the manner of a gel, but which break under the shear stresses imposed when the suspension is poured, permitting it to flow like a mobile liquid. Such systems are referred to as spherulitic. They may usually be identified: by optical microscopy, using a polarising microscope, when the spherulites typically appear as bright discs bearing dark extinction crosses; by scanning electron microscopy when the spherulites are usually directly visible; and/or by small angle x-ray diffraction, when they often exhibit a peak corresponding to the repeat spacing between successive surfactant bilayers. This usually corresponds to a repeat spacing between 3 and 15nm, typically 5 to lOnm, often with a smaller second order peak at double the angle ofthe first, indicating lamellar symmetry.

Further details of structured surfactant systems are to be found in GB 2259519 and GB 2279080 which describes the use of certain surfactants to avoid flocculation of the spherulites. EP 0414549 describes the preparation of spherulitic structured surfactant systems in the absence of dissolved electrolytes.

The structured surfactants hitherto used to suspend abrasives and builders in the detergent industry have generally required high concentrations of anionic surfactants which confer a repulsive soapy taste to the product. Such systems are obviously not suitable for suspension of DCP for use in toothpaste.

The Invention

The present invention provides a stable, pourable, aqueous dicalcium phosphate slurry comprising: water; at least 35% w/w of dicalcium phosphate, said phosphate being present as suspended solid particles; and a surfactant system, adapted either alone or in the presence of any surfactant-desolubilising electrolyte which may optionally be present in the slurry, to form a spherulitic solid-supporting structure.

Preferably the amount of surfactant is sufficient, with the dissolved electrolyte, if any, to form a structure capable of suspending, or inhibiting the sedimentation of, the particles, and in particular of preventing sedimentation over periods of at least three months.

According to a particular embodiment the present invention provides a stable, pourable, phosphate slurry comprising: (i) at least 20% by weight based on the total weight of the composition water; (ii) at least 38% by weight based on the total weight ofthe composition of dicalcium phosphate; (iii) 1% to 10% by weight based on the total weight ofthe composition of surfactants; and (iv) from 0 to 2% by weight based on the total weight of the composition of foam inhibitor.

In general the total amount of solid is preferably the maximum which can be slurried using the selected surfactant system, without the viscosity exceeding a desired level, e.g. 3 Pas, more preferably 2 Pas at 20°C and 21 sec^'1. It is preferred that the slurries comprise at least 45% w/w total phosphate, most preferably at least 50% w/w, for example at least 52% w/w. In some cases levels of up to 70% w/w total phosphate may be used.

The phosphate preferably has a particle size of less than 75 microns especially less than 40 micron eg less than 30 micron, most preferably less than 25 micron.

Surfactant

The surfactant is generally present in a total concentration of from 0.5 to 10% by weight, especially 1 to 8%, preferably 1.5 to 7% e.g. 2 to 3% by weight based on the total weight of slurry. We prefer that the weight ratio of phosphate to surfactant is greater than 4, more preferably greater than 5, especially 10 to 30 eg. 15 to 25.

The surfactant is preferably selected to form a spherulitic structured system. According to the conventional technology, hitherto employed in preparing liquid detergents a surfactant system has been selected such that when the concentration of dissolved electrolyte in an aqueous micellar solution ofthe surfactant is progressively increased from zero, keeping the surfactant to water ratio constant, the electrical conductivity of the system first rises to a maximum and then falls, passing, if the electrolyte is sufficiently soluble, through a minimum. The conductivity may subsequently pass through a second maximum. The two maxima define a trough within which the system is turbid and forms optically anisotropic spherulites. A space filling system has been selected within the conductivity trough. The behaviour is described in detail in GB 2 153 380.

For systems containing low levels of dissolved electrolyte it has sometimes been preferred to use surfactants of a type which form spherulites in the absence of electrolyte or in the presence of relatively low concentrations of electrolyte. Such systems are described in more detail in EP 0 414 549. When electrolytes are added to such surfactants the conductivity does not increase to an initial maximum, but instead may show an initial fall in conductivity on adding electrolyte, followed by a rise, if the electrolyte is sufficiently soluble. For convenience, where the conductivity ofthe aqueous surfactant passes through a minimum on addition of a sufficiently soluble electrolyte, the range of electrolyte concentrations containing the minimum and extending between the next lowest maximum, (or in the case where there is no initial maximum, zero) and the next highest maximum (where present) will be referred to as the conductivity trough. Some surfactants may exhibit a plurality of troughs, associated with different, stable, suspending structures.

When an aqueous surfactant which exhibits a conductivity trough, is mixed with an electrolyte which causes a fall in conductivity, but reaches saturation before the minimum value is reached the saturation value is deemed for the purposes of this specification to fall within the conductivity trough which would notionally be formed if the concentration ofthe dissolved electrolyte could be increased further.

The conventional spherulitic systems as described in the above prior art are not generally applicable to suspending relatively large amounts of DCP in the presence of relatively small amounts of surfactant as required in the present invention. Because ofthe large volume of solid phosphate to be suspended, it is not necessary to have such a large volume fraction of surfactant mesophase present in order to form a space filling system. We have found that, at the concentrations which are preferred according to the present invention, many ofthe preferred surfactants do not exhibit a conductivity minimum on addition of dissolved electrolyte. Moreover adjusting the electrolyte content is not generally the most convenient way of obtaining a stable phase, since electrolyte is preferably absent or present in a small amount.

One way of identifying suitable surfactant combinations is to make up a mixture of two surfactants. Typically one surfactant is relatively water-insoluble, e.g. a sorbitan ester or glycerol ester, and the other is relatively water-soluble, e.g. an alkyl polyglycoside. A series of compositions may be prepared at a constant total surfactant concentration and a constant (saturated) dissolved phosphate concentration, but containing the two surfactants in varying proportions. At sufficiently high concentrations, a plot of conductivity will usually reveal a minimum corresponding to the optimum mixture of surfactants.

Taking the optimised surfactant mixture, compositions spanning a range of total surfactant concentrations are prepared, in order to determine the optimum surfactant concentration.

The surfactant concentration may be adjusted such that the concentration of any electrolyte present in the slurry is greater than the concentration corresponding to the first maximum (where present) but below that which would be required to pass any second maximum and also, preferably, below that required to pass the minimum. Thus if addition ofthe electrolyte to the aqueous surfactant system causes the conductivity to pass through the conductivity minimum before the saturation concentration is reached, the surfactant level may be decreased until, on addition ofthe electrolyte, the saturation concentration ofthe electrolyte is reached before the conductivity passes through the minimum. Conversely if on addition to the surfactant of any electrolyte desired in the aqueous phase of the slurry, one reaches the saturation point before the first conductivity maximum value has been passed, the concentration of surfactant may be increased, tending to shift any conductivity features to lower electrolyte concentrations, until the position ofthe trough is such that the electrolyte concentration present in the slurry is sufficient to take the conductivity past the first maximum but not past the first minimum.

Instead of increasing the surfactant concentration the same effect can be achieved by increasing the electrolyte concentration, eg by adding a more soluble salt to the slurry. Alternatively the composition ofthe surfactant mixture may be adjusted using the same amount of a less soluble surfactant to achieve the same effect as increasing the surfactant (or electrolyte) concentration, or of a more soluble surfactant to achieve the same effect as lowering the surfactant concentration.

The more soluble surfactants typically include, alcohol higher ethoxylates and alkyl polyglycosides. In order to achieve a non-flocculated surfactant system, especially where the dissolved electrolyte concentration is high, it is sometimes desirable to use small amounts of a deflocculant, which is preferably a deflocculating surfactant. The deflocculant is typically an alkyl polyglycoside. Deflocculating surfactants are described in detail in EP 0 623 670. Alternatively deflocculating polymers may be used, such as are described for instance in EPO 301 883 EP A 0 346 993 EP A 0 346 994 EP A 0 41568 WO91/09102 and WO91/09932. Generally only small amounts, e.g. less than 0.5% by weight or less of such deflocculants are required. Conventional deflocculants for the suspended solids are not normally required, however, since the surfactant itself is normally sufficient to prevent or inhibit the flocculation ofthe phosphate, to the extent required to form a stable, mobile slurry.

In general the surfactant system is usually a mixture of two or more surfactants selected from anionic, amphoteric and/or nonionic surfactants.

The active system may comprise at least one anionic surfactant, especially a sodium linear alkyl benzene sulphonate. Other anionic surfactants include alkyl sulphates, alkylether sulphates, parafin sulphonates, olefin sulphonates, fatty ester sulphonates, isethionates, soaps, taurides, salts of alkyl phosphonic acids, sulphosuccinates and sulphosuccinamates. In each case the surfactant has at least C₈.₂₀ aliphatic hydrocarbon group e.g. a straight or branched chain alkyl or alkenyl group and the counter ion is preferably sodium but could also be potassium, lithium, an alkaline earth metal such as calcium, ammonium or an organic base such as isopropylamine or an alkanolamine. References to "ether" herein imply, unless the context requires otherwise, polyoxyethylene group containing from 1 to 80 ethylene oxy groups more usually 2 to 50 groups e.g. 3 to 20 groups, said groups optionally containing a minor proportion of oxypropylene groups and/or a glyceryl group.

The surfactant may additionally or alternatively contain one or more non-ionic surfactants

I especially straight or branched chain Cg.₂₀ acolhol ethoxylates having from 1 to 80 preferably

2 to 50 e.g. 3 to 20 ethyleneoxy groups, βther non ionics which may be present include ethoxylated C₈.₂₀ carboxylic acids, ethoxylated Cg.₂₀ alkyl glycerides, ethoxylated C_g_₂₀ sorbitan esters, alkyl polyglycosides, alkanolamides such as coconut mono and diethanolamide, sugar esters and amine oxides.

The surfactants may optionally comprise amphoteric surfactants, for example betaines, which are typically tertiary amines having one C₈..₂₀ alkyl group, which have been quaternised with chloracetic acid, other surfactants which may be used includes, sulphobetaines and carboxymethylated imidazolines, or amidoamines. Less preferably the surfactant may comprise cationic surfactants such as quaternary ammonium surfactants. The counter-ions of the cationic surfactants are typically chloride, bromide, iodide, hydrosulphate, phosphate, citrate, acetate, formate, lactate or tartrate.

We normally prefer that cationic surfactants should not be used in conjunction with anionic surfactants.

For suspending DCP for use in toothpaste we prefer to use surfactants which are approved for food use and which do not adversely affect flavour, such as ethoxylated sorbitan or glyceryl esters, sugar esters, APGs, proteins such as albumin, phospolipids such as lethicin sarcosinates and taurides.

The surfactant is preferably present in an amount sufficient to provide a surfactant structure which inhibits or preferably prevents the sedimentation f the particles. The structures preferably comprise spherulites, which are formed of concentrically arranged bilayers of surfactant, or domains of lamellar or other mesophase dispersed in the aqueous medium. The spherulites typically have a size in the range 0.05 to 20 microns, e.g. 0.1 to 10 microns.

Electrolyte

The electrolyte is preferably the minimum necessary to obtain a structured system i.e. we prefer a substantially electrolyte free system (apart from the trace of DCP). Any electrolyte that may be required should be non-toxic. Electrolytes which may be added include sodium monofluorophosphate, sodium chloride, sodium phosphate, sodium citrate, sodium acetate and sodium tartrate.

The system preferably includes a small amount of a foam inhibitor which helps to lower the viscosity by assisting the disentrainment of air from the slurry. For example a silicone foam inhibitor may be included in effective amounts of, typically 0.005 to 2% by weight based on the total weight ofthe slurry more usually 0.01 to 1%, especially 0.05 to 0.5% e.g. 0.1 to 0.2%. The composition may also contain some sodium carboxymethyl cellulose and glycerol.

The invention is illustrated by the following Examples:-

Example 1

Four DCP suspensions were prepared in accordance with the following formulation wherein the surfactant consisted in each case of a mixture, in varying proportions, of C,₂.n alkyl polyglucoside, having a mean degree of polymerisation of 1.5 (APG) with sorbitan monolaurate (SML).

Water 40%

Silicone antifoam 0.15%

Surfactant 5.6%

DCP 54.25%

Viscosity and yield point were determined and the product was examined after 1 month storage. The results are noted in Table 1. This indicates an optimum proportion of APG based on total surfactant of 0.46. The proportion of APG is given by>

■^■'* = ΓAPGI

[APG] + [SML]

Table 1

Example 2

To demonstrate how surfactant loading affects slurry stability in samples containing the optimised surfactant mixture composition (ie. Packed spherulitic phase).

The example is based on DCP slurried in an APG/sorbitan manolaurate structured system in which APG has an active proportion of 0.46.

Example 3 - Stable Slurry Formulations of DCP (Balance water + 0.15% antifoam)

Is.

Claims

1. A stable, purable dicalcium phosphate slurry comprising: water; at least 35% by weight of dicalcium phosphate present as suspended solid particles; and a surfactant system, adapted either alone or in the presence of any surfactant-desolubilising electrolyte which may optionally be present to form a spherulitic, solid-supporting structure.

2. A composition according to claim 1 consisting essentially of: at least 20% by weight based on the total weight ofthe composition of water; at least 38% by weight based on the total weight ofthe composition of dicalcium phosphate; from 1 to 10% by weight based on the total weight ofthe composition of surfactants; and from 0 to 2% by weight based on the total weight ofthe composition of foam inhibitor.

3. A composition according to either of claims 1 and 2 containing from 2 to 5% surfactant.