Polymeric Compositions and Methods of Using Them

It is well known to thicken a liquid by addition of a polymeric viscosifier. In some instances it is desired to obtain viscosity that is resistant to applied shear but in many instances it is desired to obtain a viscosity that reduces with applied shear, i.e., a shear thinning composition. This is particularly useful in, for instance, drilling fluids and other compositions for use downhole.

When the liquid is aqueous, the viscosifier is often a water soluble polymeric material. It may be a natural, modified natural or synthetic polymer, for instance hydroxyethyl cellulose or high molecular weight polyacrylic acid. In U.S. Pat. No. 4,059,552 Zweigle proposed that an aqueous medium could be given high viscosity that is resistant to high shear by the use, as viscosifier, of micro beads having a size 0.2 to 4 microns and that are of a water soluble, water swellable polymer formed of water soluble vinyl monomer with a specified amount of cross linking agent. In the examples, Zweigle forms polymers of acrylamide, often with sodium acrylate, by reverse phase polymerisation in the presence of a cross linker.

When the liquid that is to be thickened is a water immiscible liquid, for instance a hydrocarbon, the viscosifier is usually soluble in the liquid. For instance in EP No. 22982, mineral spirits are thickened by dissolving a particular acrylate copolymer therein. In Chemical Abstracts Vol. 78 30364Z, the viscosity of toluene is increased by dissolving into it a copolymer of isobutylmethacrylate with methacrylic acid. In Chemical Abstracts Vol. 83 134617D, it is proposed to dissolve various other polymers, including butadiene styrene polymer, in mineral oils to increase viscosity. The use of natural rubber latex in benzene is proposed for increasing viscosity of lubricating oil in Chemical Abstracts Vol. 82 17587N.

In GB No. 2003904 it is proposed to thicken an organic solvent by dissolving therein a copolymer of certain hydrophobic and hydrophilic monomers. The polymer must be such that it is soluble in the solvent and has a glass transition temperature of at least 30.degree. C. Most of the examples show satisfactory copolymers of 99 parts isobutylmethacrylate with 1 part methacrylic acid but the examples also show that these proportions can lead to unsatisfactory copolymers, depending upon the polymerisation conditions. A wide variety of possible monomers are mentioned in the specification but many of them are clearly unsuitable for use as the main component of the polymer as they would give a very low glass transition temperature, far below the specified minimum of 30.degree. C. The polymers may be made by bulk, suspension or, preferably, emulsion polymerisation and the polymer is separated as a dry product.

It is generally desired for soluble polymer thickeners to have the highest possible molecular weight as this increases their thickening effect but the viscosity attainable by the use of such polymers is still rather low.

In EP No. 106528 it is said that a latex of a neutralised sulphonate-containing styrene polymer can be used as a viscosifier in an oil-based drilling mud and that the addition of a polar cosolvent can be added to increase the solubility of the sulphonated polymer. The described products are copolymers of styrene and styrene sulphonate containing 18 to 100 meq sulphonate groups per 100 grammes of polymer. It is stated that the latex can be used as an emulsion or the polymer can be precipitated from the latex prior to use.

Doverstrand Limited of Harlow, Essex, England in their publications POL 050 and 051 September 1983 describe products under the trade names Polymul HV and Polymul LV. These products are each said to be aqueous polymer dispersions of inert hydrocarbon polymer that is soluble or swollen by aliphatic or aromatic solvents. They are each said to be fluid loss reducers in oil muds and Polymul HV is also described as a secondary viscosifier. However, the data in POL 050 shows that it is an inferior viscosifier compared to organophilic clay, amine treated lignite, and asphalt. Although these products are apparently swellable by certain aliphatic or aromatic solvents, it appears that their swellability in oil based drilling muds is inadequate, their particle size is mainly below 0.5 .mu.m and their viscosifying properties are poor.

In GB No. 2,131,067, which was published after the priority date of the present application, there are described natural rubber latices and other aqueous dispersions of polymers which are preferably hydrocarbon polymers or comprise a major proportion of copolymerised hydrocarbon. The polymers are said to dissolve in oil or swell highly in the oil. It is said that if the polymer is obtained in the form of a fine particle dispersion it is desirable to agglomerate this since coarse particle emulsions are preferred to fine particle emulsions. In one example a copolymer of 55 parts 2-ethyl hexy acrylate and 45 parts vinyl acetate is used as a fluid loss additive in an aqueous mud.

Despite the many proposals to thicken water immiscible liquids, for instance the oil phase of drilling muds, by polymeric viscosifiers none of them have been found to compete satisfactorily with, for instance, amine modified bentonite. In particular none have been found to have the desired combination of good viscosification with the desired shear thinning properties and the required thixotropy giving gel strength for particle support that are particularly desired in, for instance, drilling muds and oil-based paints.

Although the prior art does mention the possibility of achieving primary thickening of a water-immiscible liquid by introducing the viscosifying polymer as a latex (for instance in EP No. 106528), general practice has been to introduce it in dry form, for instance as recommended in GB No. 2,003,904. However we have surprisingly found that when utilising a viscosifying polymer made by oil-in-water emulsion polymerisation, better viscosifying properties are achieved when the viscosifying polymer is introduced as the emulsion, i.e., without previous separation of dry particle from the emulsion, than when dry polymer particles are introduced. This is especially the case when not more than 30% of the monomers from which the polymer is formed are hydrophobic aromatic hydrocarbon monomers, especially when 70 to 100% are acrylic monomers, preferably as discussed below.

A particular problem arises in the viscosification of a water-immiscible liquid that is to be the continuous phase of a dispersion of water-soluble or water-swellable polymer particles. It is important to achieve optimum viscosification and stabilisation of the dispersion so as to prevent settlement of the water-soluble or swellable polymer particles. Usually it is necessary to use viscosifiers such as amine modified bentonite for this purpose but the use of bentonite alone is undesirable as, for instance, bentonite is a natural material of variable quality and of limited availability. It would therefore be desirable to be able to viscosify the water-immiscible liquid effectively by using a viscosifying polymer. However none of the existing techniques have proved entirely satisfactory for the stabilisation of dispersions in water-immiscible liquid of water-soluble or water-swellable polymer particles.

The use of an aqueous emulsion of viscosifying polymer can given good results, especially if it is one of the preferred polymers discussed in more detail below, but we hve found that optimum viscosification does not occur upon mere addition of the aqueous emulsion to the water-immiscible liquid. Instead it is necessary to take deliberate steps to remove water from around the particles of the viscosifying polymer of the emulsion by dehydration or agitation of the water-immiscible liquid. There are various ways of achieving this, in full or in part, but it is particularly desirable to utilise the water-soluble or water-swellable polymer particles to remove some or all of the water from around the viscosifying polymer particles.

In one aspect of the invention, a stable dispersion of water-swellable or soluble polymer particles in a thickened water-immiscible liquid is made by adding an aqueous emulsion of particles of water insoluble viscosifying polymer formed by oil-in-water emulsion polymerisation and then removing water from around the particles of viscosifying polymer by dehydration or agitation of the water-immiscible liquid, whereby the water-immiscible liquid becomes thickened by the viscosifying polymer, and water swellable or soluble polymer particles are provided as a dispersion in the water-immiscible liquid.

The viscosifying polymer particles preferably have a hydrophilic coating, generally for instance as a result of being made by copolymerisation of ethylenically unsaturated monomers including a small amount of a hydrophilic ethylenically unsaturated monomer, and it appears that this promotes the retention of a protective water layer. The preferred hydrophilic monomer is acrylic acid but other carboxylic (meth) acrylic monomers, or other hydrophilic (meth) acrylic monomers can be used. The water in the emulsion, for instance held by this coating, appears to inhibit viscosification of the water-immiscible liquid unless it is removed. It may be removed by dehydration or agitation.

The agitation may lead to emulsification. Thus mechanical agitation may be sufficient to achieve optimum viscosification. The addition of water-in-oil emulsifier, that may be anionic, non-ionic, cationic or a blend, may be useful. This emulsifier may be present either in the emulsion or in the water-immiscible liquid. The use of a cosolvent, alone or in conjunction with surfactants, may also accelerate the removal of the protective water layer as a result of agitation.

Instead of or in addition to removing the water layer by agitation, the water-immiscible liquid may be dehydrated by distillation.

Preferably the water layer is removed by dehydration by a dehydration agent that is in the water-immiscible liquid, the dehydration generally being promoted by applying sufficient agitation to promote contact between the emulsion particles and the dehydration agent.

The dehydration agent may be calcium chloride, sodium sulphate or any other inorganic salt or suitable compound that can be introduced in an anhydrous form or in the form of a low hydrate and that can chemically absorb water to form a high hydrate. Preferably however the soluble or swellable polymer particles serve as the dehydration agent and so these are preferably introduced to the water-immiscible liquid in the form of dry polymer particles but they can, alternatively, be aqueous polymer gel particles provided the aqueous gel particles have a water content such that they can still act as chemical absorbent to absorb water from around the particles of viscosifying polymer.

Preferably the water-swellable or water-soluble polymer particles are introduced into the water-immiscible liquid before viscosification by the addition of the emulsion of viscosifying polymer particles. However if desired some or all of the viscosification may be effected before the water-soluble or swellable polymer particles are incorporated. In particular, the emulsion of viscosifying polymer may be blended with the water-immiscible liquid, possibly to given some degree of viscosification, and the water-soluble or swellable polymer particles may then be incorporated with agitation sufficient to cause absorption of water into the particles, with consequential viscosification of water-immiscible liquid and stabilisation of the gel. In the literature mentioned above, the polymer is preferably formed by polymerisation of (meth) acrylic monomers.

The amount of dispersed polymer may be, e.g., 10 to 90%, preferably 40 to 80%, by weight of the final composition.

The viscosifying polymer must be made by oil-in-water emulsion polymerisation, for instance in known manner as described in GB No. 2,003,904 and can be any of the resultant viscosifying emulsion polymers, or made from the monomers, discussed in the prior art mentioned above, all of which is hereby incorporated by reference. At least 70%, and preferably 100%, of the monomers are preferably aliphatic. The monomers can include allylic monomers but it is particularly preferred for at least 70% of the monomers from which the viscosifying polymer is formed to be (meth) acrylic monomers. The viscosifying polymer particles may be soluble in the water-immiscible liquid and are normally below 10 .mu.m dry size.

Preferably however the water immiscible liquid is thickened by a polymeric viscosifier that comprises water insoluble polymer particles having a dry, weight average, particle size of below 10 .mu.m and that are insoluble in the water immiscible liquid but that are swollen substantially by the liquid. The viscosifying polymer should be relatively soft, i.e., it should have a relatively low Tg value.

Thus in the preferred aspect of the invention it is essential that the viscosifying particles do not dissolve but instead retain a discrete particulate structure.

Particularly good thickening effects are achieved when inorganic particulate solid is present in the resultant composition, and in particular when such solid is dispersed in water-in-oil emulsion to which the polymer dispersion is added. Organophilic bentonite or other clays, e.g., fatty-amine-modified bentonite, is a conventional thickener for such systems and the combination of amine-modified bentonite with the polymer dispersions gives particularly good thickening results, both when water-soluble or swellable polymer is dispersed in the water-immiscible liquid and when the liquid is free of such polymer. Other particulate solids that give good results include barytes and drilled rock, such as is present in drilling mud compositions.

Thickened liquids according to the invention have particularly desirable shear-thinning rheology. They are highly viscous at low shear but are less viscous, but still have useful viscosity, at high shear. A gel strength is provided by a developed thixotropy which usefully supports suspended particles. They are therefore extremely valuable as viscosifiers in invert or oil based drilling fluids, completion fluids, packer fluids, stimulation fluids, fracturing fluids lost circulation and pipeline pigging fluids. The compositions are also useful for thickening the oil phase of various dispersions of particles in oil, especially dispersions of dry polymer particles or aqueous polymer gel particles.

We have surprisingly found that when adding the aqueous dispersion of viscosifying polymer particles to the water-immiscible liquid optimum viscosification sometimes may not occur unless deliberate steps are taken to ensure contact between the water-immiscible liquid and the polymer particles. It appears that the desired hydrophilic coating around the particles carries a protective water layer and that this may inhibit viscosification of the water-immiscible liquid unless it is removed. It may be removed by dehydration, agitation or emulsification. Thus mechanical agitation may be sufficient to achieve optimum viscosification. Alternatively the addition of water-in-oil emulsifier, that may be anionic, non-ionic or cationic, for instance a blend of anionic and cationic emulsifiers, may be useful. This emulsifier may be present either in the dispersion or in the water-immiscible liquid. Dehydration agent that may be used may be present in the water-immiscible liquid, for instance calcium chloride may be present. Instead of chemical absorption, the water may also be removed by distillation. Cosolvent can be used to promote thickening, alone or in conjunction with surfactants.

The preferred swellable but insoluble viscosifying particles that are used as viscosifier in the invention must swell substantially in the liquid and in practice this means that their swollen volume should generally be at least 4 times, and usually at least 6 times, their dry volume. If they swell so much that the polymer tends towards full solubility the benefits of the invention may be lessened and so generally the particles do not swell by more than 30 times, or at the most 50 times, their dry volume. Best results are generally obtained when the particles swell by from 10 to 25 times their dry volume.

The viscosifying properties are partly due to the imbibition and immobilisation of liquid in the swollen particles and partly due to particle-particle interactions and so the specified small particle size is essential. The weight average particle size is preferably below 5 .mu.m but is normally above 0.01 or 0.02, preferably above 0.05, .mu.m. Best results are generally achieved with a weight average particle size of from 0.02 to 2 .mu.m, often 0.05 to 0.3 .mu.m. Preferably at least 80%, and most preferably at least 90%, by weight of the particles have a size below 10 .mu.m, preferably 0.05 to 5 .mu.m and most preferably 0.1 to 4 .mu.m.

The particles that contribute most effectively to the viscosifying effect appear to be those that are below 5 .mu.m. Preferably at least 30%, most preferably at least 50% by weight, of the particles are in the size range 0.03 to 5, most preferably 0.01 to 1.mu.m.

The polymers are best made by dispersing polymerisable monomer or monomers in a liquid in which they are substantially insoluble and polymerising the monomer or monomers in that liquid to form an emulsion in the liquid of polymer particles having the desired particle size. This particle size can be controlled in known manner, for instance by appropriate choice of emulsifier and the amount of emulsifier. The process is conveniently referred to as emulsion polymerisation, and the final product is an emulsion, since the final particle size is always small.

The liquid in which the emulsion polymerisation is conducted is aqueous, generally water, and so the resultant dispersion is a dispersion of particles in a continuous aqueous phase. The water may include various additives to improve physical properties. For instance ethylene glycol may be included to improve freeze-thaw stability.

At least 20, usually at least 30 and preferably at least 50%, by weight of the recurring groups in the polymer should be polar, and thus when the polymer is made by copolymerisation of monomers, at least 50% by weight of the monomers should be polar. This is surprising since the water immiscible liquid is generally non-polar. By referring to polar monomers in groups we mean groups having a significant polarity as a result of including polar groups, generally caused by the inclusion of a carboxylic and/or amide group in the side chain substituted on to a hydrocarbon backbone. The presence of a substantial proportion of polar groups in the polymer within the non-polar or other water immiscible liquid appears to promote the particle-particle interactions in a beneficial manner.

The presence in the polymer of a high proportion of aromatic hydrocarbon groups tends to make it difficult, or impossible, to obtain the desired combination of particle-particle interaction, insolubility and swellability and so the monomers from which the polymer is formed should not contain above 30% aromatic hydrocarbon monomers and preferably are substantially free, and most usually totally free, of aromatic hydrocarbons.

Although it appears necessary to include polar monomers for best results, it is necessary that at least most of the polar monomers are hydrophobic, as otherwise the polymer will be water soluble and/or non-swellable in the water immiscible liquid. However the presence of a small amount of hydrophilic monomers appears desirable. By hydrophilic we mean that the monomer has a solubility of above 3% by weight in water at room temperature and by hydrophobic we mean that it has solubility of below 3% by weight in water at room temperature (20.degree. C.). The preferred polymers are formed from 0 to 20% by weight hydrophilic monomers, 25 to 100% by weight polar hydrophobic monomers and 0 to 75% be weight non-polar hydrophobic monomers. If the amount of non-polar hydrophobic monomer is above 30% then it will generally consist of ethylene or haolgenated ethylene and preferably the only non-polar hydrophobic monomers included in the polymers of the invention is ethylene or halogenated ethylene. Best results are generally obtained when the amount of polar hydrophobic monomers is from 50 to 100%, most preferably 80, and often 90, to 100% by weight.

Suitable polar hydrophobic monomers include alkyl (meth) acrylates, the monoesters or diesters of dibasic ethylenically unsaturated acids such as alkyl itaconates, alkyl maleates (e.g., the half ester of stearyl alcohol with maleic anhydride) and alkyl fumarates and vinyl esters of long chain acids, such as vinyl stearate, laurate and versatate. Hydrophobic N-alkyl substituted (meth) acrylamides may be used. Preferably at least 50% by weight, and most preferably at least 80% by weight of the monomers are (meth) acrylic monomers, generally alkyl acrylate monomers. The hydrophobic monomers preferably include an alkyl group that contains up to 30 carbon atoms, preferably 6 to 18 and most preferably 6 to 10 carbon atoms. Particularly preferred are copolymers of 990 to 100% by weight C.sub.1-30 alkyl (meth) acrylate and 0 to 10% by weight hydrophilic monomer, especially those wherein the polymer contains 40 to 100% by weight C.sub.6-10 alkyl acrylate. Naturally the monomers must be chosen such that the viscosifying polymer has the desired characteristics. Since, e.g., methyl methacrylate homopolymer does not form a swellable polymer and has a high Tg, methyl methacrylate and other low alkyl (meth) acrylates can only be used in a minor proportion.

A particularly preferred monomer is 2-ethyl hexyl acrylate. Valuable polymers are formed from 40-100% of this, 0-10% hydrophilic groups and other C.sub.6-30 alkyl (meth) acrylate, preferably 0-50% C.sub.12-18 alkyl (meth) acrylate.

Other polar hydrophobic monomers that may be used include vinyl acetate, for instance copolymerised with ethylene and/or vinyl versatate.

The viscosifying polymer particles used in the invention (irrespective of whether they are soluble or swellable) preferably have a relatively hydrophilic coating as this seems to promote particle-particle interactions within the water immiscible and generally non-polar liquid. The hydrophilic coating may be an external coating of, for instance, an emulsifier or some other material present during the polymerisation but preferably is formed by copolymerisation of hydrophilic monomer. The hydrophilic monomer may be any monomer that will provide polar oxygen containing or nitrogen containing groups on the surfaces during the polymerisation or in some other way. Suitable hydrophilic comonomers that may be used include dialkyl amino alkyl (meth) acrylates and their quaternary addition and acid salts, acrylamide, N-(dialkyl amino alkyl) acrylamide and meth acrylamides and their quaternary addition and acid salts, hydroxy alkyl (meth) acrylates and sulphonic acids such as vinyl sulphonic acid (asthe free acid or, preferably, the sodium or ammonium or alkyl-amino salt) or unsaturated carboxylic acids such as methacrylic acid or, preferably acrylic acid, generally as the free acid but possibly as a sodium, ammonium or alkylamino salt. Other preferred monomers include hydroxyethyl (or propyl) (meth) acrylate, acrylamide and dimethylaminoethylmethacrylate (generally as a quaternary salt).

If the hydrophilic monomers are present in too great an amount, it may be difficult to obtain the desired small particle size. Accordingly the amount of the hydrophilic polymer is generally below 20% and preferably below 5%. The amount is normally at least 0.05%, preferably at least 0.1% and most preferably at least 0.5%. Particularly preferred polymers are those obtained using acrylic acid, preferably in an amount of 0.3 to 3%, as a copolymer with other monomers that are mainly alkyl (meth) acrylate, preferably mainly C.sub.6-10 alkyl acrylate monomers.

The swellable viscosifying polymers useful in the invention preferably have a glass transition temperature (Tg) below about 25.degree. C., often below about 10.degree. C. and most preferably below about 0.degree. C. Best results are generally obtained when Tg is below about -20.degree. C., most preferably in the range -40.degree. to -70.degree. or -75.degree. C.

The swellable viscosifying polymers used in the invention are preferably film forming at 20.degree. C., that is to say if the dispersion of the polymer is cast on a surface at room temperature the polymer will form a film.

The preferred swellable viscosifying polymers have >30% particles in the range 0.05 to 5.mu., are film forming at 20.degree. C., swell to at leat 4 times, preferably 10 to 20 times, their volume in the water immiscible liquid, and are formed mainly of hydrophobic polar monomers, optionally with ethylene, and preferably with 0.2 to 5% of a water soluble, hydrophilic, monomer. Emulsions containing these are novel.

The viscosifying polymer particles remain in the aqueous dispersion in which they are formed prior to incorporation with the water immiscible liquid and should not be dried or otherwise separated from the dispersion. With the preferred film forming, low Tg, polymers it would be difficult or impossible to separate the particles while retaining the desired small particle size. If polymers having higher Tg values were obtained, separation might be possible but the resultant product would be very dusty and difficult to handle. However, even apart from this, we find that the viscosifying characteristics of the polymer are reduced if the polymer is separated as a dry product from the aqueous dispersion prior to use.

The swellable viscosifying polymer is preferably formed from monomers such that, without additional cross linking agent, it has the desired combination of swellability and insolubility in the water immiscible liquid. However if a chosen combination of, for instance, polar acrylate monomers and acrylic acid results in the polymer being too swellable, or perhaps even soluble, then multi-functional cross linking monomer may be added so as to render the polymer truly cross linked. THe cross linking monomer will generally be a hydrophobic cross linking monomer such as diallyl phthalate. Any of the known multi-ethylenically unsaturated hydrophobic cross linking agents can be used in place of diallyl phthalate. The amount of cross linking agent, if added, is generally in the range 0.01 to 2%, preferably 0.2 to 0.8% by weight of the monomers. In general we find best results are obtained without the addition of cross linking agent. It is surprising that substantially linear polymers formed mainly of acrylic or other polar groups are superior to, for instance, cross linked polymers formed mainly or wholly or hydrophobic hydrocarbon groups since it might have been expected that these cross linked hydrocarbon polymers would have a far greater effectiveness as thickening agents in hydrocarbon or other non-polar liquids.

The aqueous dispersions of soluble or swellable viscosifying polymer used in the invention may be made by conventional oil-in-water emulsion polymerisation, for instance by dispersing the monomer or blend of monomers into water in the presence of an oil-in-water emulsifier or surfactant, for instance using a high speed Silverson mixer, and may contain an appropriate polymerisation initiator. Polymerisation may be initiated in conventional manner, for instance by thermal or redox initiator. Although it is often considered that anionic emulsifiers are the most effective in such polymerisations, we have found in the invention that it is particularly desirable to use non-ionic emulsifiers. These seem to promote the formation of the preferred particle size distribution. Preferably the composition contains 2 to 20%, by weight based on the weight of polymer, of a non-ionic surfactant.

Suitable non-ionic surfactants are alkyleneoxy derivatives of long chain alcohols or, preferably, alkyl phenols, wherein the alkyleneoxy groups are propyleneoxy or, preferably, ethyleneoxy and the alkyl groups are preferably nonyl or octyl.

The water-immiscible liquid thickened in the invention can be any hydrophobic solvent having a solubility in water of less than about 5% by weight. it may be a chlorinated hydrocarbon but is generally a mineral oil or other hydrocarbon. It may be a refined hydrocarbon such as kerosene, white spirit or aviation fuel or may be an unrefined hydrocarbon such as crude oil. It may be the oil of an oil based paint, in order to provide a paint of improved rheology. It is generally non-polar. It may be the continuous phase of a water in oil emulsion, for example the continuous phase of a dispersion in oil of an aqueous polymer, that may be soluble or swellable in water. This dispersion may itself be dehydrated and so the water-immiscible liquid can be the continuous phase of a dispersion in oil of dry particles of polymer that is preferably soluble or swellable in water.

The amount of viscosifying polymer particles used in the invention is generally in the range 0.3 to 10%, preferably 0.5 to 5% by weight of the total composition.

Oil-in-water emulsifier may be incorporated in conventional manner to facilitate distribution of the final product into water.

The following are examples of the invention. Examples 1 to 6 demonstrate the use of the preferred swellable viscosifying polymers for thickening various non-aqueous liquids and examples 7 to 9 demonstrate the production of stable dispersions of a water swellable or soluble polymer in water-immiscible liquid that has been thickened in accordance with the invention.

An emulsion of 297 parts by weight 2-ethylhexyl acrylate and 3 parts by weight glacial acrylic acid was formed in 300 parts water and 24 parts oil in water surfactant (nonyl phenol ethylene oxide condensate). 165 parts water containing 6 parts of the surfactant and 0.4 parts thermally decomposing free radical initiator (ammonium persulphate) were heated in a rection vessel to 80.degree. C. and the emulsion and a further 0.4 parts free radical initiator in 30 parts water were separately added to the vessel over a period of 3 hours. After the additions were complete the vessel was held at 80.degree. C. for a further 30 minutes and the product was then allowed to cool to room temperature. The product was a dispersion in water of polymer particles having weight average dry size of about 0.5 .mu.m,. The product was designated Sample A in the following examples.

Various amounts of product A were mixed into kerosene in the presence of a suitable activator. The results are given in Table 1.

The results show the remarkable thickening effect which such polymers have in hydrophobic solvents. The apparent viscosity is also very dependent upon the shear rate used during viscosity measurement.

A drilling fluid is formed of

The mud characteristics of 3 such drilling fluids were determined. One fluid had no additive. Another fluid included 1.5% active polymer of Product A. Another fluid contained 3% organophilic clay, but this suffered from the disadvantage that it required extended mixing to achieve full activation. The results are given in Table 2.

The beneficial effect of Product A is clearly apparent.

A drilling fluid was prepared in the manner described in Example 3. Two products, based on styrene/butadiene resins, which are recommended as viscosity modifiers for oil based drilling fluids were evaluated in comparison with Product B. This product B was prepared in the manner described in Example 1 for Product A except that only 12 parts of surfactant were used during the preparative stage. When examined under an optical microscope, the Polymul products appeared substantially free of particles above 0.5 .mu.m and appeared not to be swollen significantly.

Each product was evaluated at a dose of 1.5% activate polymer. The results are given in Table 3.

The superior effectiveness of Product B can clearly be seen.

A drilling fluid was prepared in the manner described in Example 3. The characteristics of drilling fluids containing the following viscosifiers at doses of 5 ppb active polymer were determined.

C is a copolymer of 2-ethyl hexyl acrylate: acrylic acid 99:1

D is a copolymer of 2-ethyl hexyl acrylate: acrylic acid 99:1, cross-linked with 500 ppm diallyl phthalate.

E is a terpolymer of 2-ethyl hexyl acrylate: n-butyl-acrylate: acrylic acid 84:15:1

F is a terpolymer of 2-ethyl hexyl acrylate:lauryl methacrylate:acrylic acid 79:20:1

G is a terpolymer of 2-ethyl hexyl acrylate:stearyl methacrylate:acrylic acid 79:20:1

H is a copolymer of N-decyl acrylamide: acrylic acid 99:1

I is 2-ethyl hexyl acrylate

J is a terpolymer of 2 ethyl hexyl acrylate: lauryl methacrylate: acrylic acid 50:49:1

K is 19% vinyl versatate, 80% 2 EHA, 1% acrylic acid

L is 39% vinyl versatate, 60% 2 EHA, 1% acrylic acid

M is 99% 2 EHA, 1% acrylamide

N is 99% 2 EHA, 1% hydroxypropyl acrylate

Comparison of C with I shows the benefit of including acrylic acid. Comparison of C with D shows that when the polymer is otherwise satisfactory it is undesirable to cross link. Comparison of J with F shows that including too much of a very long chain methacrylate is undesirable as it appears to make the product too swellable and tending towards solubility. The best results are those for products C, F and J to N. However all the products shown in the table do give useful properties.

Solid polymer was extracted from Product A by drying overnight at 95.degree. C. The resultant polymer dried into a lump or stick mass.

Three drilling fluids were prepared according to the recipe in Example 3. Product A was added to one of them. The dried polymer obtained from Product A was added to another. To a third the dried polymer was incorporated into the mineral oil of the drilling fluid for 24 hours before incorporating the other ingredients of the drilling fluid (to give time for swelling of the polymer by the oil). The properties of the drilling fluid to which Product A had been added were very satisfactory. The properties of the drilling fluid prepared by adding dried polymer to the oil for 24 hours before adding the other ingredients were very much worse, and the properties of the drilling fluid to which dried polymer had been added at the time of mixing the other ingredients were even worse. This demonstrates the value of maintaining the polymer as discrete particles in the aqueous dispersion in which they are formed, in preference to separating the particles from the dispersion.

A further use of the invention is to increase the stability due to settlement of dispersions of water soluble polymer particles such as polyacrylamides in an organic continuous phase, such as mineral oil. Thus, 500 g of a 50% w/w slurry of polyacrylamide microbeads (20-50 .mu.m in diameter) in Pale Oil 60 was prepared by suspension polymerisation. 32.9 g of 38% oil-in-water emulsion of a 99/1 2-ethylhexyl acrylate/acrylic acid copolymer and 10 g Tween 81 (Trade mark, sorbitan monooleate with 5 ethylene oxide units) was added to the slurry with vigorous stirring. The polyacrylamide beads acted as dehydration agent and the resultant product was a mobile viscous slurry which rapidly dispersed in water to give a 1% polyacrylamide solution. No signs of settlement or separation were observed over a period of 21 days compared with marked settlement within 24 hours of a product prepared in the absence of the polymer emulsion.

100 parts of an emulsion polymer formed broadly as described in Example 1 was mixed into a low toxicity mineral oil. Initially there was little or no thickening effect but the system was then subjected to distillation under reduced pressure (14 mm Hg) to a maximum temperature of 100.degree. C. On cooling the clear dispersion of the viscosifying polymer particles in oil set to a weak gel. This dispersion had a 10% viscosifying polymer content and was then diluted with sufficient additional low toxicity mineral oil to give it the 6% content.

Into 490 parts of this thickened water-immiscible liquid was added 500 parts of a finely ground industrial grade xanthan gum powder having 90% less than 90 .mu.m with thorough mixing. A surfactant (Ethylan 77) was added to make the oil phase emulsifiable when added to water.

The resulting suspension was smooth and free flowing and showed no settlement after storage for several months. The xanthan gum hydrated rapidly and easily with no lump formation when the suspension was mixed into sea water under low shear conditions.

6.25 g of an emulsion of sample C was blended with 18.75 g light mineral oil SPO 60 and 25 g carboxy methyl cellulose having a size below 90 .mu.m (Carbocel PS 70) was added slowly with stirring by a Heidolph stirrer. A paste was formed and it was observed that the carboxy methyl cellulose particles became swollen, presumably as a result of absorbing water from the viscosifying emulsion particles.