WO1993008230A1

WO1993008230A1 - Cellulose ether thickening compositions

Info

Publication number: WO1993008230A1
Application number: PCT/GB1992/001914
Authority: WO
Inventors: Scott Rangus; David Brian Shaw; Patrick Jenness
Original assignee: Laporte Industries Limited
Priority date: 1991-10-24
Filing date: 1992-10-19
Publication date: 1993-04-29
Also published as: GB9122607D0

Abstract

The combination of a cellulose ether thickening agent having an average degree of substitution of from 0.7 to 1.1 with a swelling silicate thickening agent such as smectite clay or a synthetic clay having a structure resembling hectorite gives excellent thickening performance in electrolyte containing compositions such as, for example, tooothpastes. Other potential applications are in drilling muds, concrete mixes and emulsion paints.

Description

Cellulose ether thickening compositions

This invention relates to cellulose ether-containing thickening compositions, a method for their production, and to thickened compositions containing them.

Cellulose ethers are widely used to thicken aqueous or organic liquids. It has been found that combinations of cellulose ethers with other thickening agents show a synergistic thickening effect and there are a number of publications which disclose particular synergistic thickening compositions. Examples of such compositions are mixtures of cellulose ethers with polyacrylamidomethyl propane sulphonic acid as disclosed in United States Patent Specification No. 4540510, with Xanthan gum as disclosed in US Patent Specification No. 4,313,765, with a^'maleic anhydride/olefin copolymer as disclosed in US Patent Specification No. 4172055, with a cross-linked acrylic copolymer as disclosed in US Patent Specification No. 4798682 or with hydroxyethyl starch as disclosed in GB Patent Specification No. 2110698 and 2110699.

The presence of any substantial quantity of electrolyte in a polar aqueous and/or organic medium which requires to be thickened can in at least some instances greatly disturb the thickening effect of cellulose ethers and particularly the synergistic effect mentioned above. The Applicants have found however that a combination of a suitably selected carboxymethyl cellulose thickening agent in combination with a swelling silicate thickening agent can give useful thickening or even synergistically enhanced thickening when

SUBSTITUTESHEET the medium to be thickened is a high electrolyte composition.

The present invention therefore relates to cellulose ether thickening compositions which are suitable for use to thicken high-electrolyte compositions. For the purposes of the present invention a high electrolyte concentration is regarded as one above which the performance of a cellulose ether thickener other than as claimed is impaired. In a low solids medium consisting only of water thickener and electrolyte this effect may show itself at around 0.02 to 0.2 meq/g of electrolyte in the total solution. In other compositions containing a high solids concentration such as a toothpaste formulation it may show itself around 0.5-0.8 meq/g of the total composition. In other compositions of intermediate constitution the high electrolyte effect may occur between these figures. The term high electrolyte concentration should accordingly be interpreted from the point of view of effect on a cellulose ether thickener having a degree of substitution other than that now claimed. The use of the invention in relation to compositions containing up to 1 or even up to 2 or 2-5 m.eq/g of electrolyte is also envisaged.

The present invention provides a composition suitable for use to thicken electrolyte containing compositions comprising a cellulose ether thickening agent and another thickening agent, the composition being characterised in that the cellulose ether is a substituted cellulose ether having an average degree of substitution of at least from 0.7 and not more than l.l, and in that the other thickening agent is a swelling silicate thickening agent.

In the manufacture of the substituted cellulose ethers usually used for thickening purposes the cellulose molecule is reacted with alkali and then with a reagent such as chloroacetic acid to cause conversion of one or more of the three hyroxyl groups present in each anhydroglucose unit in the cellulose chain, first to the alkali metal salt and then ro tne alkali metal acetate ether. The chloroacetic acid may be replaced by other reagents to produce other cellulose ethers. The degree of substitution can be controlled by, amongst other factors, the concentration of the alkali. The degree of substitution is the number of hydroxyl groups per anhydroglucose unit which are substituted and is usually stated as a range defining the minimum and maximum degree of substitution. The average degree of substitution is the weight mean average appropriate to a range. It is envisaged that to practical purposes a cellulose ether product will be outside the invention if any substantial part of the range, for example 20% or more is outside the range selected according to the invention.

It is found that while acceptable thickening properties may be attained over the above stated range of degree of substitution it is advantageous for the average degree of substitution of the cellulose ether to be above 0.7 and up to 1.1, Cellulose ethers having a range of substitution which extends substantially below 0.65 are undesirably difficult to dissolve while those having a degree of substitution which extends substantially above 1.1 or even above 1.0, show a diminution of synergism to some degree. Particularly preferably the cellulose ether has a minimum degree of substitution of above 0.7 for example of at least 0.8.

The cellulose ether utilised herein is preferably on anionic cellulose ether such as for example sodium carboxymethyl cellulose.

Many silicates have swelling or thickening properties. These are layered silicates, which swell by the entry of water or other polar liquids between the layers thereby causing an increase in the basal spacing, and are typified by the layered clay minerals of the smectite series such as the saponites, including saponite and hectorite or the montmorillonites. There also exists a range of synthetic silicates having structures of the smectite type which show greatly enhanced swelling properties. Detail concerning the identification of structure types in clay minerals is given in "X-ray identification and crystal structure of clay minerals" Minerlogical Society (G. Brown) 1961. Suitable synthetic silicates are described in US Patent ^'Specification No. 4049780 together with a process for their preparation and are hydrous magnesium silicates having a crystal structure similar to that of a hectorite and a composition

[Si₈(Mg_aLi_bH_c)O₂₀(OH) -_yF_y]^Z"ZM+

wherein a,b,c,y,z and M have the values: a is > 0 to 6 preferably 4.95 to 5.7 b is from 0 to <6 preferably 0 to 1.05 c is from 0 to <2 preferably 0 to 2 a-r-b+c is from >4 to <8 preferably 4 to 8 y is from 0 to <4 z = 12-2a-b-c M is Na⁺ or Li⁺

Such hydrous magnesium silicates are available under the LAPONITE Trade Mark from Laporte Industries Limited.

It is postulated without intending to limit the invention to the correctness of the following theory, that the particular effectiveness of the combination according to the invention may be connected to an inter-reaction between the anionic sites on the cellulose ether and hydroxyl sites on the silicate and that an extended cage type of structure is thereby obtained when the degree of substitution of the cellulose ether is as selected herein which structure is sufficiently stable to resist degradation in the presence of high electrolyte concentrations. This theory is supported by the fact that certain non-ionic cellulose ethers such as methyl cellulose and hydroxyethyl cellulose have been found not to be operative in the present invention. The silicates preferably contains at least 1 hydroxyl group for each 8 silicon atoms in the framework e.g. "y" in the above general formula is preferably 3 or less.. To maximise the opportunities for inter-reaction between the silicate and the cellulose ether it is preferred that the number of hydroxyl groups on the silicate is as high as possible. Preferably therefore in the above general formula "y" is less than 1 or 0.

The relative weights of the cellulose ether and the clay may, for example, be from 5:95 to 95:5, preferably from 30:70 to 70:30.

The present invention may be utilised in the production of thickened high-electrolyte compositions by including the cellulose ether and the silicate, together or separately in an aqueous or suitable polar medium containing the other components of the composition to be thickened. According to the invention however it is preferred that the cellulose ether and the 'silicate' are both included in a liquid medium before contact with a high concentration of electrolyte. Thus the cellulose ether and the silicate may be premixed into a liquid medium which is then added to the main composition or they may be added to the main composition before any substantial quantity of electrolyte, sufficient to give a high electrolyte concentration, is included with the main composition. It is believed that an interaction between the cellulose ether and the electrolyte could prevent the desired cellulose ether/silicate structure from developing.

The present invention may be utilised in many differing applications in which high electrolyte concentration liquids require to be thickened. Dentrifice compositions intended for special "purposes, for example antiplaque dentrifices, can contain high concentrations of electrolyte in the form of sodium pyro- or poly-phosphates. Other examples of high electrolyte systems which require to be thickened are drilling muds in situations where high salt strata are penetrated, concrete mixes, aqueous emulsion paints and coating mixes which may contain highly charged polymers such as polyacrylic copolymers, and the like.

The invention will now be illustrated by the following examples thereof which are not intended to be limiting on the general scope of the invention.

Example 1

Samples of water containing varying quantities of sodium sulphate were thickened with 2% by weight of the total composition of a 50/50- weight blend of a silicate thickener (Laponite RD) and one of 3 different carboxy methyl cellulose (CMC) ethers having degrees of substitution of 0.65-0.9, 0.8-0.95 and 1.15-1.45 denoted hereafter for convenience CMC/.65, CMC/.8 and CMC/1.15. The solutions were stored for 24 hours and the viscosities of the samples was determined using a Brookfield LVT (T-bar) viscometer at 0.6 rpm. The results are depicted in Figure 1 attached hereto.

Examples 2-11

The subject of these Examples is a typical high electrolyte dentrifice composition. To carry out the Example numbered 6 herein 495 g of sorbitol syrup (70% aq) was added to a polythene beaker, stirred under high shear whilst adding 6.0 g of a sodium carboxymethyl cellulose (CMC .65 or CMC .8) and mixing was continued for 15 minutes until the cellulose was completely dispersed.

In a separate beaker 4.0 g of synthetic silicate thickening agent (Laponite D) was added to 149.5 g of demineralised water with stirring. The mixture was heated to 40°C and mixing continued for 15 minutes.

The Laponite dispersion was added to the cellulose/Sorbitol mucilage and mixed thoroughly. Then 15.0 g sodium saccharin solution (10% aq) , 10.0 g sodium fluoride solution (25% aq) and 50.0 g tetrasodiu pyrophosphate was added to the mucilage which was transferred to a vacuum mixer and to which was added 200 g of dentifrice grade silica abrasive and 10.0 g of titanium dioxide. Mixing under vacuum was continued for 5 minutes to deaerate the composition.

Lastly 10.0 g of flavour oil, 0.5 mg of preservative and 50.0 g of sodium lauryl sulphate solution (30% aq} were added and the resulting product was then further mixed under vacuum until completely deaerated.

After storage for 24 hours the rheological properties of the toothpaste were measured on a Brookfield viscometer.

To carry out Examples 2-5 and 7-11 the same procedure was followed with variations in the identity and quantity of the carboxymethyl cellulose, the quantity of the Laponite gelling agent and the identity and quantity of the electrolyte as indicated in Table l. The rheological results for each shear rate used are set out in Table 2.

SUBSTITUTESHEET Ex Laponite

0.6 II

1.0 II

0.5 100% Sorbitol/ 0.6 II II

0.6 II II

0.5 0.75 100% Sorbitol

0.6 1.0 " "

0 0.35 0.5 " " 10 SrCl₂.6H₂0 1 0 35 0.5 " " " "

TSPP = Tetrasodium pyrophosphate

TKPP = Tetrapotassium pyrophosphate

CMC = Carboxymethyl cellulose

CMC .65 = Grade 7M4XF available from Aqualon Corp.

CMC .8 = Grade 9M31XF available from Aqualon Corp.

DS = Degree of Substitution Table 2

Viscosity (Poise) at given

Shear Rate after 24 hours

In comparison tests using the same procedure but in which the Laponite gellant and/or the CMC were replaced by other products an inferior rheology was found.

1.8% Laponite combined with 0.6% xanthan gum in a formulation otherwise identical to Examples 8 and 9 gave viscosities of 1438, 270, 61, 21 respectively despite this relatively very large quantity of Laponite while the same quantity of xanthan gum in the absence of any Laponite gave viscosities of 1010, 206, 50 and 18 respectively. The replacement of the Laponite by 0.2% of polyacrylic acid polymer gave viscosities of 1450, 286, 59 and 18 respectively.

SUBSTITUTE SHEET

Claims

1. A composition, suitable for use to thicken electrolyte containing compositions, comprising a cellulose ether thickening agent and another thickening agent, the composition being characterised in that the cellulose ether is a substituted cellulose ether having an average degree of substitution of from 0.7 to 1.1 and in that the other thickening agent is a swelling silicate thickening agent.

2. A composition suitable for use to thicken electrolyte- containing compositions comprising a cellulose ether thickening agent and another thickening agent, the composition being characterised in that the cellulose ether is a substituted cellulose ether having a degree of substitution confined to a range having a minimum of 0.65 and a maximum of 1.1 and in that the other thickening agent is a swelling silicate thickening agent.

3. A composition as claimed in claim l or 2 wherein the cellulose ether is an anionic cellulose ether.

4. A composition as claimed in any preceding claim wherein the swelling silicate thickening agent is a layered natural or synthetic layered silicate of the smectite series.

5. A composition as claimed in claim 4 wherein the swelling silicate is a synthetic layered silicate having a crystal structure similar to that of hectorite.

6. A composition as claimed in any preceding claim wherein the swelling silicate thickening agent comprises hydroxyl groups.

7. A composition as claimed in any preceding claim wherein the relative weights of the cellulose ether and the other thickening agent are from 5 to 95 to 95 to 5.

8. A composition as claimed in any preceding claim also comprising a liquid polar medium.

9. A composition as claimed in claim 8 wherein the polar medium comprises water.

10. A thickening composition comprising a composition as claimed in any one of claims 8 or 9 also containing at least 0.02 m.eq/g of electrolyte.

11. A composition as claimed in claim 10 containing not more than 2.5 m.eq/g electrolyte.

12. A process for the production of a thickened composition as claimed in claim 10 or 11 by mixing the thickening agents, a source of electrolyte and a polar liquid medium characterised in that the cellulose ether thickening agent and the swelling silicate thickening agent are mixed together, before contact with the source of electrolyte.

13. A composition as claimed in any preceding claim and substantially as described in the Examples herein.