GB1604216A - - Google Patents

Description

(54) SUSPENSION OF SOLID PARTICLES (71) We, HERCULES LIMITED, a Company organised under the laws of Great Britain, of 20, Red Lion Street, London, W.C. 1., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to suspensions of dispersed particulate solid matter, and to methods of forming stable suspensions of solid particulate matter.

More particularly the invention relates to the use of an aqueous suspension medium containing certain branched polysaccharide gums in solution, for suspending solid particulate matter.

The capacity of a medium for suspending solid particulate matter is of commerical concern to many industries. Examples of such industries include agriculture, where fertiliser and animal feed distribution are important; the paper and paint industries utilising colour pigment and filler suspensions; the oil industry which utilises drilling muds and fracturing fluids; the mining industry where materials are transported as slurries; the foundry industry which uses mould and sand coatings; the cosmetic and pharmaceutical industries with products such as toothpaste; the printing, paper, textile and similar industries in which a great variety of treating agents are to be applied in suspension; the building industry for instance with gypsum board pastes; the food industry; and many others.

Among the methods now in use by these industries for the suspension of solid particulate matter in aqueous media is the use of water-soluble strongly ionically charged chemicals which tend to absorb onto the surfaces of the suspended particles rendering them mutually repellent, or the use of water-soluble thickening agents which act by increasing the viscosity of the continuous phase, thus reducing the settling rate of the suspended solid particles. Combinations of these two methods are often used. Disadvantages in the use of these methods is that the former is liable to give a system which is unstable in the presence of other electrolytes, and the latter present problems in the handling of high viscosity liquids.Many of the thickening agents in general use are sufficiently thixotropic to allow considerable settlement of the suspending particles before full recovery of the fluid to the limiting viscosity or shear stress.

Ideal requirements of a suspending medium for universal use in the industries mentioned above, are: a non-polar or only weakly ionic nature, chemical inertness, non-toxicity, ease of handling, ease of making-up in cold water, and ability to exhibit the phenomena of pseudoplasticity and yield stress sufficient to overcome the forces tending to settle a given suspended material. The suspensive power of such a medium is not intrinsically due to viscosity as such, but viscosity can enhance the suspensive power of the gel structure characteristic of such systems.

It is a general object of the invention to provide an improved and economically attractive suspending medium fulfilling the above-mentioned requirements.

Pseudoplasticity of the medium has been found important to permit the particles to be readily dispersed in the medium which is otherwise viscous, and to permit the resulting dispersion to be readily pumped with a a low consumption of energy through fairly small orifices.

We have discovered that an aqueous medium containing xanthan gum and a galactomannan exhibits synergism in respect of yield stress of the medium.

According to the invention a method of forming a stable suspension of solid particles comprises dispersing the particles in a pseudoplastic, substantially nonthixotropic aqueous medium containing in solution xanthan gum and at least one galactomannan.

Accordingly the present invention provides a suspension of dispersed solid particles in a pseudoplastic, substantially non-thixotropic aqueous medium containing in solution xanthan gum and at least one galactomannan.

By pseudoplastic, substantially non-thixotropic aqueous medium is meant a viscous aqueous medium which exhibits a fall in viscosity when a shear force is applied to the medium, and a return to substantially the original viscosity when the shear force is removed, such return taking place without hysteresis. By galactomannan is meant any naturally occurring galactomannan.

Xanthan gum is the dried product resulting from the fermentation of simple sugars with a member of the Xanthomonas family, preferably Xanthomonas campestris (NRRL-B 1459).

Xanthan gum solutions at suitable concentrations have the ability to suspend solid particles very effectively. This property is not primarily due to the viscosity of these solutions. Solutions of many polymers exhibiting much higher viscosity are not effective in suspending particles. The suspending power of these solutions of xanthan gum is believed to be due to the gelatinous texture which in molecular terms may involve a network structure. In rheological terms it may be said that the xanthan gum solutions have a definite yield stress.

A medium with the requisite pseudoplasticity and other rheological phenomena can be prepared by dissolving about 1 part of xanthan gum in 100 parts of water, but when solutions of blends of xanthan gum with for example guar gum are examined, it is found that there is a synergistic increase in viscosity and suspending power and moreover also in yield stress, even when the proportion of xanthan gum in the blend is relatively small.

Guar gum by itself exhibits little or no yield stress. It is often used as a suspending agent by virtue of its high solution viscosity, but such solutions are difficult to pump, pour or stir. However, we have found that aqueous solutions or guar gum with minor proportions of xanthan gum exhibit considerable yield stress, apparent in the elastic solid extant under low rates of shear; that the solutions behave as normal non-Newtonian fluids when the yield stress is exceeded by means such as stirring, pumping and pouring; and that the solutions exhibit little or no thixotropy. These solutions have a relatively low viscosity when the yield stress has been exceeded, i.e. they exhibit pseudoplasticity which allows them to be more readily stirred, pumped, poured or extruded than guar gum solutions alone which rely on high viscosity for their suspending action.

Thus solutions of 95 parts guar to 5 parts xanthan (by weight) in water have substantially as good suspending power as solutions of xanthan gum at the same total polymer/gum concentration. Similar behaviour is exhibited by blends of locust bean gum and xanthan gum; the synergistic yield stress, elasticity and viscosity effects are even greater with locust bean gum than with guar gum. In view of the fact that these natural gums are more abundant and less costly than xanthan gum, there is a significant economic advantage in using the blends in place of xanthan alone in solutions for suspending solid particles.

Two other galactomannans which may be used in place of guar gums or locust bean gum in blends with xanthan gum in this invention are tara and espina corona.

Solid particles which may be suspended in the media of the invention include for example soil treating agents, pigments and mineral fillers, polymeric materials, rock cuttings and so forth; and medium may be in the form of a drilling mud or fracturing fluid; a printing paste in which the colour is a pigment dispersion, a foodstuff, medicament, or hygiene or toilet preparation such as toothpaste, or a moulding or casting material for instance in foundry applications.

In another embodiment of this method the aqueous medium may contain a suspended fertiliser or other soil treating substance. Such a composition may also contain biologically active ingredients such as fungicides, insecticides, bactericides, herbicides, nematocides or other pest control agents.

In preferred embodiments the proportions of the xanthan gum and the galactomannan are such that the suspending power of the medium is higher than that of a solution of either component alone at the same total gum/galactomannan concentration. This may permit the use of less total gum and galactomannan to achieve a desired suspension.

The suspending medium accordingly preferably comprises an aqueous solution of from 1 to 30 parts by weight of xanthan gum with from 99 to 70 parts by weight of galactomannan, especially 2 to 20 parts by weight of xanthan gum with 98 to 80 parts by weight of polymer, most preferably 3 to 10 parts by weight of xanthanum gum with 97 to 90 parts by weight of galactomannan.

In the most preferred embodiments the proportion of galactomannan used with the xanthan gum is at least as high as that which will give the maximum suspending power in the resulting medium. In the case of guar gum as the polymer this proportion is about 92.5 parts of guar gum to 7.5 parts of xanthan gum.

The mixtures of xanthan gum and galactomannan may be dissolved for purposes of the invention in water at concentrations ranging from 0.05 per cent by weight, preferably 0.3 per cent, up to 10 per cent by weight, preferably 1 per cent, and yield solutions that embody the requirements of an ideal universal suspending medium.

These solutions exhibit substantially no thioxotropy. Moreover, the solutions are mostly non-toxic, chemically inert, easy to handle, soluble in cold water, and largely biodegradable in spite of the presence of xanthan gum which is reported to be resistant in this respect.

It is contemplated that the solid particles can be encapsulated delicate objects since they will not only be held suspended but the elastic properties of the medium will protect the objects from the effects of external impact.

The suspending power of the solutions of mixtures according to the invention may be utilised in suspending up to 90 per cent by weight of solid matter in the solution, i.e. 90 parts of solid suspended matter to 10 parts of aqueous solution.

The invention is illustrated in the following Examples.

Example 1 About 3 grammes of sand with specific gravity approximately 2.8 were dispersed in 100 ml. of each of two xanthan gum/guar systems. One system was an aqueous solution containing 1 per cent w/v guar gum/xanthan gum blend, such blend being made with 5 parts by weight of xanthan gum and 95 parts by weight of commerical guar gum, the solution being preserved with pentachlorometaxylenol to prevent biodegradation of the components and to deactivate any enzymes which might be present and which might tend to cause degradation of the polysaccharides. The other system was similar to the first but contained 2 parts by weight of xanthan gum and 98 parts by weight of guar gum. It was observed that the suspension in the 5/95 blend was stable over 3 days whereas the sand settled out relatively rapidly from the suspension in the 2/98 blend.

The following Example reports data illustrating the application of the suspension medium of the invention in association with liquid fertiliser material and particulate solid treatment agent for soil, i.e. a compound supplying lime.

Example 2 A stock aqueous solution of 28 per cent by weight urea was prepared. Samples of xanthan gum alone and of dry blends of guar gum and xanthan gum at a weight ratio of 95:5 were prepared. The gums were dissolved in the urea solution and < 200 mesh calcium carbonate was added at a rate of 10 g/50 ml. urea solution. The resulting suspensions were set aside and checked for stability after about 72 hours.

The results were as follows.

Sample Gum Concn. Remarks xanthan 0.3% Separation into packed bottom layer-cloudy supernatant layer 2 xanthan 0.4 Ó Good suspension- small clear layer at top 3 xanthan/guar 0.2% Loosely packed bottom layer cloudy supernatant and clear top layers 4 xanthan/guar 0.4% Good suspension- judged better than 2 above-slight hint of top clear layer In further tests, one sample using the xanthan gum/guar gum blend at 0.3 per cent concentration proved satisfactory, while another showed slight settling at the top of the jar, not excessive for use within a few days.

The relationship between suspending power of the media used in the invention and rheological parameters which are readily measured with standard laboratory equipment is illustrated by the data in the following Table, guar gum being used as the galactomannan. The Table also shows the synergistic effects on viscosity and yield points when the xanthan proportion of the blend is below 10 per cent.

Yield point (YP) is the limiting stress in dynes sec cm-2 calculated by extrapolation of the stress (Fan Reading)/shear rate (revolutions per minute) of the fluid, using the Fann viscometer as recommended in the Standard Procedure for Testing Drilling Fluids issued by the America Petroleum Institute of Dallas, Texas, U.S.A.

TABLE Guar per cent 100 98 95 92.5 90 80 70 60 50 40 20 0 Xanthan per cent 0 2 5 7.5 10 20 30 40 50 60 80 100 Fann Readings (b) 300 rpm 131 137 137 141 131 126 120 107 102 87 73 64 600 rpm 157 164 162 166 154 152 145 131 125 106 88 74 Plastic viscosity 26 27 25 25 23 26 25 24 23 19 15 10 Yield point 105 110 112 116 108 100 95 83 79 68 58 54 YP/PV 4.04 4.07 4.48 4.64 4.70 3.85 3.80 3.46 3.43 3.58 3.87 5.4 Brookfield viscosity 20 rpm 3600 5300 6700 6500 6150 5500 4500 4500 3700 3600 3400 25 C (cps) Contraves Rheomat viscosity 10500 60000 79000 85000 70000 50000 40000 10400 9000 7000 (cps at 1.0 Sec. -1) Sand (c) suspension E C A A C D D D C B A Notes with reference to the foregoing Table:: (a) Solutions (1 per cent) were prepared by adding the dry powders together to the water, stirring for 10 minutes and ageing for 24 hours before testing; (b) Measured on a Fann V.G. viscometer with Spring 250; (c) A-remained suspended indefinitely; B-remained suspended for several days; C-remained suspended for 24 hours; D-settled out overnight; E-settled out in less than one hour.

It is well known from such laws as Stokes' Law that the movement of particles suspended in a fluid medium is governed by the viscosity of that fluid, so that in the long term suspension of given particles in a given suspending fluid, a low value of velocity of the particles through the liquid is obtained when viscosity is greatest.

Viscosity is the resistance to movement exhibited by the fluid. If the fluid requires a finite amount of force to be applied before any movement occurs it is said to possess a yield stress. This phenomenon is difficult to measure directly with existing instruments but may be inferred from observation of the movement of relatively dense particles through the fluid; if the particles remain stationary it is inferred that the cohesive forces within the fluid are greater than the force of gravity which would otherwise lead to settlement or sedimentation.

Alternatively it is possible to obtain data using the Fann viscometer procedure referred to in connection with the Table, and, by extrapolation of the dial reading/r.p.m. curve, obtain a "dial reading" value for zero r.p.m. This is expressed as the yield point (YP). Whilst the yield point is not an accurate measurement of the yield stress (since the yield point is deduced on the assumption that the dial reading/r.p.m. relationship is linear), experience with suspensions such as oil well drilling muds indicates increasing suspending power with increasing yield point.

The "plastic viscosity" is a measure of the increase in resistance as the speed of the instrument is increased; in the case of the foregoing Table, from 300 to 600 r.p.m. If the plastic viscosity is low, the fluid will show little increase in flow resistance with increasing speed (or shear rate).

In the present invention it is an object to provide a suspending medium which will maintain relatively dense particles in suspension for long periods of time but which can also be readily moved by the operations of stirring, pumping or pouring.

The medium may also need to be able to flow readily through a narrow gap such as the nip of a roller or the blade of a doctor-blade, should it be used on one of the many coating operations in various industries. Such a fluid should also be capable of remaining in the bowl of a screen printing machine yet readily flow through the screen when forced through.

The optimum fluid will be one with a high yield point and low plastic viscosity.

It is well known that I per cent aqueous solutions of xanthan gum have such properties. In order to better relate the yield point and plastic viscosity, the ratio YP/PV shown in the Table has been calculated from the results of the measurements made by a Fann viscometer on the series of I per cent solutions prepared from a range of blends of xanthan gum and guar gum in the proportions indicated in the Table.

For the purposes of the present invention it has been found desirable that the ratio YP/PV be high, preferably at least 4.2. The greatest values for the ratio are obtained with blend ratios of guar gum to xanthan gum of from 98/2 to 80/20, and with xanthan gum on its own.

Direct observation of the suspending power of each blend solution in the series is reported at the bottom of the Table; blends of guar gum and xanthan gum with ratios between 95/5 and 90/10 gave almost limitless suspension time, similar to a solution of xanthan gum alone.

There is accordingly a close relationship between the instrumental data and the observed suspending power. The marked increase in the YP/PV ratio around the 92.5/7.5 guarlxanthan blend point is surprising. Just as surprising is the low value for both the YP/PV ratio and the suspending power when the blend composition is in the region of 50/50 guar/xanthan.

Viscosity data obtained with two well-known and widely used methods (Brookfield Synchro-Lectric and Contraves Rheomat) have been included in the Table to show that some degree of viscosity synergism occurs when the blend ratio is in the region between 95/5 and 90/10 but that the remarkably good and wellknown suspending power of xanthan gum itself would not have been predicted from the viscosity data presented.

Our co-pending Application No. 11578/77 (Serial No. 1,604,215) describes and claims a suspension of seeds in a pseudoplastic substantially non-thixotropic biodegradable aqueous medium containing in solution xanthan gum and at least one non-ionic water-soluble polymer, and a method of forming such a suspension.

We make no claim to such suspensions of seeds, or to methods of forming such suspensions.

Subject to the above disclaimer, WHAT WE CLAIM IS: 1. A suspension of solid particles in a pseudoplastic, substantially nonthixotropic aqueous medium containing in solution xanthan gum and at least one galactomannan.

2. A suspension according to claim 1, wherein the ratio by weight of other galactomman to xanthan gum is in the range from 99:1 to 70:30.

3. A suspension according to claim 2, wherein the said ratio is in the range from 98:2 to 80:20.

4. A suspension according to claim 3, wherein the said ratio is from 97:3 to 90:10.

5. A suspension according to any of claims 1A, wherein the galactomannan is guar gum.

6. A suspension according to claim 5, wherein the ratio by weight of guar gum to xanthan gum is about 92.5 parts guar gum to 7.5 parts xanthan gum.

7. A suspension according to any of claims 14, wherein the total concentration of xanthan gum and galactomannan in the water is from 0.05 to 10 per cent by weight.

8. A suspension according to claim 7, wherein the said total concentration is from 0.3 to 1 per cent by weight.

9. A suspension according to any of claims 1--8, wherein the medium has a ratio of yield point to plastic viscosity of at least 4.2.

10. A suspension of solid particles according to claim 1 substantially as hereinbefore described.

11. A method of forming a suspension of solid particles which comprises dispersing the solid particles in a medium as defined in any of claims 1--9.

12. A suspension of solid particles formed by a method according to claim 11.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. Subject to the above disclaimer, WHAT WE CLAIM IS:

1. A suspension of solid particles in a pseudoplastic, substantially nonthixotropic aqueous medium containing in solution xanthan gum and at least one galactomannan.

2. A suspension according to claim 1, wherein the ratio by weight of other galactomman to xanthan gum is in the range from 99:1 to 70:30.

3. A suspension according to claim 2, wherein the said ratio is in the range from 98:2 to 80:20.

4. A suspension according to claim 3, wherein the said ratio is from 97:3 to 90:10.

5. A suspension according to any of claims 1A, wherein the galactomannan is guar gum.

6. A suspension according to claim 5, wherein the ratio by weight of guar gum to xanthan gum is about 92.5 parts guar gum to 7.5 parts xanthan gum.

7. A suspension according to any of claims 14, wherein the total concentration of xanthan gum and galactomannan in the water is from 0.05 to 10 per cent by weight.

8. A suspension according to claim 7, wherein the said total concentration is from 0.3 to 1 per cent by weight.

9. A suspension according to any of claims 1--8, wherein the medium has a ratio of yield point to plastic viscosity of at least 4.2.

10. A suspension of solid particles according to claim 1 substantially as hereinbefore described.

11. A method of forming a suspension of solid particles which comprises dispersing the solid particles in a medium as defined in any of claims 1--9.

12. A suspension of solid particles formed by a method according to claim 11.