CA1134160A - Suspension medium - Google Patents

Abstract

A B S T R A C T
The invention relates to a suspension of seeds and a method for forming such a suspension which comprises dispersing the seeds in a pseudo-plastic, substantially non-thixotropic biodegradable aqueous medium containing in solution xanthan gum and at least one non-ionic water-soluble polymer. The medium is useful in the planting of seeds which may be pre-germinated.

Description

- 1~3416~

The present in~ention relates to suspensions of dispersed seeds, and to methods of forming stable suspensions of seeds. ~ore particularly the invention relates to the use of an aqueous suspension medium containing certain branched polysaccharide guns in solution, for suspending seeds and especially for suspending seeds for fluid drilling.
Our copending application Serial No. 299,202 filed on March 17, 1978, from which this application i5 divided, relates to suspensions of solid particulate ~atter other than seeds and to methods of forming such suspensions.
The capacity of a medium for suspending solid particulate matter is of commercial concern to many industries.
~mong the ~ethods now in use by ~hese industries for the suspension of solid particulate matter in aqueous media is the use of water soluble strongly ionically charged chemicals which tend to adsorb 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. Com-binations of these two methods are often used. Disadvantages in the use of these methods is that the ormer is liable to give a system which is unstable in the presence of other electrolytes, and the latter presents problems in the handling of high viscosity liquids. Many of the thickening agents in general use are su~iciently thixotropic to allow considerable settlement of the sus-pended 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 akove, 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 .

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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 abovementioned require-ments and useful for suspending seeds, which may be germinated, prior to and during planting.
Nutrients, fungicides and other adjuvants could be included in such a suspending medium in intimate contact with the seeds. A high degree of pseudo-plasticity of the medium is important to permit the seeds to be readily dispersed in the medium which is otherwise ~iscous, and to permit the resulting seed dispersion to be readily pumped with a lo~ consumption of energy through fairly small orifices in planting. For use in the planting of germinated seeds, the suspending medium must also be biodegradable.
We have discovered that an aqueous medium containing xanthan gum and a non-ionic, water-soluble organic polymer, exhibits synergism in respect of ;
yield s*ress of the medium.
According to one aspect o$ the present inyentiQn there is provided a suspension of seeds in a pseudoplastic, substantially non-thixotropic biodegrad-able aqueous medium containing in solution ~anthan gum and at least one non-ionic water-solu~le polymer.
According to another aspect of the present invention there is provided a method of forming a suspension of seeds which comprises dispersing the seeds in a pseudoplastic, substantially non-thixotropic biodegradable aqueous medium containing in solution xanthan gum and at least one non-ionic ~ater-soluble polymer.

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~341fi~1 By pseudoplastic, substantially~non-thlxotropic 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.
Xanthan gum is the dried product resulting from the fermentation of simple sugars with a member of the ~anthomonas family, preferably Xanthomonas compestris (NRRL - B 1459).
Examples of such water soluble polymers include non-ionic soluble starches, non-ionic cellulose ethers such as hydroxyethyl cellulose, galacto-mannans, and non-ionic synthetic polymers, e.g. polyacrylamides.
While the invention is particularly described with reference to galactomannans, for example guar gum, it is applicable to a ~ide range of poly-mers provided the criteria defined above are observed, that is, provided the polymers are non-ionic and water-soluble and form a pseudoplastic, substantiallynon-thixotropic blend with xanthan gum in aqueous medium, such that the medium is biodegradable.
~anthan gu~ 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 Yiscosity are not effective in suspending particles. The suspending powerof these solutions of xanthan gum is believed to be due to the gelatinous texture which in ~olecular terms may involve a network structure. In rheologicalterms it may be said that the xanthan gum solutions have a definite yield stress.
A medium with the re~uisite pseudoplasticity and other rheological phenomena can be prepared by dissol~ing about 1 part of xanthan gum in 100 partsof water, but such solutions do not have the necessary degree of biodegradability :~139~160 when placed in or on the soil and hence would not be suitable for the seed planting application described above.
When pseudoplastic, substantially non-thixotropic solutions of blends ~ -of xanthan gum with non-ionic, water-soluble polymers, 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.
euar 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 aq~eous solutions of 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 relative-ly low viscosity when the yield stress has been exceeded, i.e. they exhibit pseudoplasticity which allows them to be more readily stirred, pumped, poured orextruded 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, elas-ticity 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 usingthe blends in place of xanthan alone in solutions for suspending solid particles.

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Two other galactomannans ~hich may be used in placc of guar gums or locust bean gum in blends with xanthan gum in this invention are tara and espina corona.
The invention further contemplates a method of sowing seeds which comprises dispersing seeds, especially germinated seeds in a biodegradable pseudoplastic aqueous medium containing in solution xanthan gum and at least one non-ionic, water-soluble polymer, preferably a galactomannan, to form a stable suspension of the seeds, and distributing said medium on the land in such a manner as to plant the seeds in or on the soil.
In another embodimen~ of thi$ 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 polymer 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/polymer concentration.
This may permit the use of less total gum and polymer to achieve a desired suspension.
The suspending~medium accordingly preferably CQmpriSes an aqueous solution of ~rom 1 to 30 parts by weight of xanthan gum with from 99 to 7a parts by weight of polymer especially 2 to 20 parts by weight of xanthan gum with 98 to 80 parts by weight of polymer. In the case where the non-icnic polymer is guar gum the more preferable proportions are 3 to 10 parts by weight of xanthan gum, and 97 to 90 parts by weight of polymer.
In the most preferred embodiments the proportion o polymer used with the xanthan gum is at least as high as that which will give the maximum sus-pending power in the resulting medium. In the case of guar gum as the polymer 3416~

this proportion is about 92.5 parts of guar gum to 7.5 parts of xanthan gum.
The mixtures of ~anthan gum and polymer may be dissolved for purposes of the invention in water at concentrations ranging from 0.05 per cent by weight, preferably Q.3 per cent, up to lQ per cent by weight, preferably 1 per cent, and yield solutions that embody the requirements o an ideal universal suspending medium.
These solutions exhibit substantially no thixotropy. ~oreover the solutions are 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.
The application of the suspension medium of t~e present inYention, to the suspension of pre-germinated seeds intended for fluid drilling of the seeds, is a particularly good illustration of its convenience and efficiency. Such use calls or a fluid with the ability to support various si~es and densities of seeds and yet to ~oye easily through narrow bore tubing, and the properties should be maintained under field conditions, i.e. with yarious water supplies and in an environment permitting only limited facilities~ for control hy the far~er.
The gum/polymer mixtures of the invention meet these requirements. In particular they can he pumped without incurring damage to the delicate suspended matter constituted by~the germinated seeds. ~oreover the mixtures do not form a hard skin but in the soiI break down at a suitable rate to avoid damage to the seeds by local accumulation of their own toxic metabolic products. They also advantageously retain moisture.
Additives ~f yarious klnds may be incorporated in the suspensions as desired, for ins*ance preservatives, seed ~rowth agents, colouring and so forth.
The following general procedure may be used for germinating and plant-, ` , ~; ' ~ , ' ' ~3~6~

ing the seeds. The seeds are suspended in a glass column about 1 metre high filled with water. The column has openings at the bottom for the addition of both air and water. The air is added as fine bubbles at such a rate as to keep the seeds suspended. Water is added at the bottom at a slow rate and overflows at the top. The column is exposed to light and kept at about 20C during the germination period. The germinated seeds are filtered off and added to a gently agitated 1 per cent aqueous solution of e.g. guar - xanthan gum 92.5 :
7.5. Agitation is continued until the seeds are uniformly dispersed. When agitation is stopped, the seeds remain suspended for several days.
The suspension is poured or pumped into the reservoir of a planter.
As the planter moves, the suspension is pumped by means of a peristaltic pump through a nozzle into the furrow. The pump is set in such a manner that substantially no damage is done to the seeds. A rate of extrusion of 20 to 100 ml per metre of row has proved satisfactory~. -The concentration of suspended seeds will be adjusted to give the desired spacing which will depend on the seeds.
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. This is illustrated in the following Examples.

-100 oil-seed rape seeds ~non-germinated~ of specific gravity approxi-mately 1.10 were suspended in a solution consisting of 1 per cent w/v guar/
xanthan gum blend, such blend being made with 5 parts by weight xanthan gum and 95 parts by weight commercial guar gum. The solution had been preserved withpentachlorometaxylenol to prevent biodegradation of the components and to de-activate any enzymes present which may have caused degradation of the poly-' ', ~39~

saccharides.
After 7 days the seeds had shown no sign of settlement whereas similar seed suspended in 1.0 per cent guar gum solution had settled out com-pletely after only one day. Settlement had also occurred with rapc seeds in 1.0 per cent solutions o other polysaccharides such as hydroxyethyl cellulose and sodium carboxy ~ethyl cellulose.

A ~ixture of 2 parts xanthan gum to 98 parts guar was treated as in Example 1. The seeds remained in suspension for a~out 48 hours after which they were considered to show distinct signs o settlement.
The relationship between suspending power of guar/xanthan blends and rheological parameters which are readily measured with standard laboratory equipment, is illustrated by the data in the following Table, which also shows the synergistic effects on Yiscosity and yield points when the xanthan proportion of the gum blend is below 10 per cent.
Yield point ~YP) is the limiting stress in dynes sec cm 2 calculated hy extrapolation of the stress ~Fann Readingl/shear rate (revolutions per minute~
of the fluid, using the Fann viscometer as recommended in the Standard procedure for Tes~ting Drilling ~luids; issued by the American Petroleum Institute of Dallas~ Texas, U.S.A.

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1~L3~6~) Notes with reference to the foregQing Table~
(a) Solutions (1 per centl ~ere prepared by adding the dry powders together to the water, stirring for 10 minutes and ageing for 24 hours before testing;
(b) ~easured Qn a Fann Y.C. 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- s~ettled 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 hy the viscosity of that fluid, so that in the long term suspension of giyen particles in a given sus-pending fluid, a low value of yelocity of the particles through the liquid is obtained when viscosity is greatest.
yisco$ity is the resistance to movement exhi~ited by the fluid. If the fluid requires a finite amount of force to be applied before any moYement occurs it is said to possess a yield stress. This phenomenon is difficult to measure directly~with existing instruments ~ut may be inferred from o~serYation of the ~oYement of relatively dense particles through the fluid; if the par-ticles remain s*ationary it is inferred that the cohesiye orces 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 ~ann visco-meter procedure referred to in connection with the Table and, by extrapolation of the dial reading/r.p.m. curYe, obtain a "dial reading" value for zero r.p.m.
This is expressed as the yield point (YP~. ~hilst the yield point is not an accurate measurement of the yield stress (since the yield point is deduced on - lQ -,, ~, ', ' ' :

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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 viscosi~y" 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 viscosit~ 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 seeds in suspension for long periods of time but which can also be readily moYed by the operations of stirring, pumping or pour~
ing.
The foregoing indicates that the qptimum fluid will be one with a high yield point and low plastic viscosity~. It is well known that 1 per cent aqueous solutions of xanthan gum will provide the desired properties. In order to better relate the yield point and plastic viscosity, the ratio ~P/P~ shown in the Table has been calculated from the results of the measurements made by a Pann yiscometer on the series of 1 per cent solutions prepared from a range of blends of xanthan gum and guar gum in the proportions indicated ~n the Table.
For the purposes of the present invention it is desirable that the ratio YP!PY be high. The greatest values for the ratio are obtained with blend ratios o$ guar gu~ to:xanthan gum o$ from 98/2 to 8Q/20, and with xanthan gum on its own.
Direct obserYation of the suspending power of each blend solution in the series is rep~rted at the bottom of the Table; hlends of guar gum and xanthan gum with ratios between ~5/5 and qO/lQ gave almost limitless suspension time, similar to a solution o$ xanthan gum alone.
There is accordingly a close relationship between the instrumental , , : , . .

- ~3416~) data and the observed suspending power. The marked increase in the YP/PV ratio around the 92.5/7.5 guar/xanthan blend point is surprising. Just as surprising is the low value for the YP/PV ratio and the suspending power when the blend composition is in the region of 50/50 guar/xanthan.
~ iscosity 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 well-known suspending power of xanthan gum itself would not have heen predicted 12 from the data presented.

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Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A suspension of seeds in a pseudoplastic, substantially non-thixo-tropic biodegradable aqueous medium containing in solution xanthan gum and at least one non-ionic water-soluble polymer.

2. A suspension according to claim 1, wherein the non-ionic water-soluble polymer is guar gum.

3. A suspension according to claim 2, wherein the ratio by weight of non-ionic water-soluble polymer to xanthan gum is in the range from 99:1 to 70:30.

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

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

6. A suspension according to claim 2, wherein the ratio by weight of guar gum to xanthan gum is about 92.5 parts guar gum to 7.5 parts xantban gum.

7. A suspension according to claim 1, 2 or 3, wherein the total concen-tration of gum and polymer in the water is from 0.05 to 10 per cent by weight.

8. A suspension according to claim 1, 2 or 3, wherein the total concen-tration of gum and polymer in the water is from 0.3 to 1 per cent by weight.

9. A suspension according to claim 1, 2 or 3, wherein the medium has a ratio of yield point to plastic viscosity of at least 4.2.

10. A method of forming a suspension of seeds which comprises dispersing the seeds in a pseudoplastic, substantially non-thixotropic hiodegradable aqueous medium containing in solution xanthan gum and at least one non-ionic water-soluble polymer.

11. A method according to claim 10, wherein the non-ionic water-soluble polymer is guar gum.

12. A method according to claim 10, wherein the ratio by weight of non-ionic water-soluble polymer to xanthan gum is in the range from 99:1 to 70:30.

13. A method according to claim 12, wherein the said ratio is in the range from 98:2 to 80:20.

14. A method according to claim 12, wherein the said ratio is in the range from 97:3 to 90:10.

15. A method according to claim 11, wherein the ratio by weight of guar gum to xanthan gum is about 92.5 parts guar gum to 7.5 parts xanthan gum.

16. A method according to claim 10, 11 or 12, wherein the total concen-tration of gum and polymer in the water is from 0.05 to 10 per cent by weight.

17. A method according to claim 10, 11 or 12, wherein the total concen-tration of gum and polymer in the water is from 0.3 to 1 per cent by weight.

18. A method according to claim 10, 11 or 12, wherein the medium has a ratio of yield point to plastic viscosity of at least 4.2.

19. A method of sowing seeds which comprises distributing a suspension according to claim 1, 2 or 3, on the land in such a manner as to plant the seeds in or on the soil.

20. A method of sowing seeds which comprises distributing a suspension according to claim 6, on the land in such a manner as to plant the seeds in or on the soil.

21. A method of sowing germinated seeds which comprises distributing a suspension according to claim 1, 2 or 3 on the land in such a manner as to plant the seeds on the soil.

22. A method of sowing germinated seeds which comprises distributing a suspension according to claim 6 on the land in such a manner as to plant the seeds on the soil.