HYDROPHOBE ASSOCIATIVE COMPOSITION CONTAINING A POLYMER OF A WATER-SOLUBLE MONOMER AND AN AMPHIPHILIC MONOMER

Abstract: The invention comprises a thickening agent comprising an aqueous-soluble polymeri­ zation product of one or more water-soluble mono­ mers which are sufficiently water-soluble to form at least a 10 weight percent solution when dis­ solved in water and which undergo addition polym­ erization to form polymers which are water-solu­ ble; and at least one second monomer of the for­ mula: wherein R is a hydrocarbyl radical which contrib­ utes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or substituted hydrocarbyl radical and Y is an ionic species which contributes hydro­ philic character to the molecule. These compositions are useful for thick­ ening aqueous mediums, in particular, for thicken­ ing equeous media containing water-soluble electro­ lytes. Such thickened aqueous media retain their viscosity when subjected to shear, heat or high electrolyte concentrations. The thickened aqueous solutions may be used in enhanced oil recovery pro­ cesses as fluid mobility control agents, fracturing fluids and drilling muds, as well as hydraulic fluids and lubricants.

Description: This invention relates to hydrophobe associative compositions that increase the vis­ cosity of aqueous media containing water-soluble electrolytes. As taught in Encyclopedia of Polymer Science and Technology , Interscience Publishers, Vol. I, 192 (1964); and Kirk-Othmer's Encyclopedia of Chemical Technology , Vol. 20, 3d Ed., (1982), pp. 207-230, John Wiley & Sons, it is known that the viscosity of an aqueous medium may be increased by the addition of a water-soluble polymer. Such water-soluble polymers include polyacrylamide, acrylamide/acrylic acid copolymer, sodium poly­ acrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharide, as well as naturally occurring gums, such as, for example, guar gum and chemically modified gums such as, for example, hydroxypropyl guar gum. As a result of this thickening capability, there are many existing, as well as potential, industrial applications for aqueous media thickened with such water-swellable polymers. Unfortunately, however, the aforemen­ tioned conventional water-soluble polymers suffer from deficiencies or limitations in actual use in such industrial applications. For example, for reasons of efficiency and economical considera­ tions, it is common to employ very high molecular weight versions of such polymers. However, dur­ ing many industrial applications, the conditions present during the practice of the application involve exposing an aqueous medium containing the high molecular weight water-soluble polymer to high shear. Such shear often causes mechani­ cal degradation of the polymer and thus reduces the viscosity of the aqueous medium. While lower molecular weight polymers are less sensitive to shear degradation, they must be used in much high­ er concentrations in order to achieve the desired level of viscosity. Secondly, while ionic water-soluble polymers such as neutralized acrylamide/acrylic acid copolymer, sodium polyacrylate, polystyrene sulfonate and the like are more efficient thicken­ ers in deionized water than their nonionic counter­ parts, their thickening ability is greatly reduced by the presence of water-soluble electrolytes such as sodium chloride, calcium chloride and magnesium sulfate in the aqueous medium. Such electrolytes are present in the aqueous media employed in most industrial applications, particularly those requir­ ing the use of ground waters in subterranean for­ mations as in enhanced oil recovery. Finally, in many applications, the aqueous medium thickened with water-soluble poly­ mer is exposed to temperatures in the range of 30°C to 100°C which normally causes reduction of viscosity. Such high temperatures are particu­ larly common in enhanced oil recovery applications wherein the aqueous medium is pumped underground to depths of 5,000 to 20,000 feet, as is common for mobility control fluids and packing fluids. In attempts to overcome some of the aforementioned deficiencies of the conventional water-soluble polymers, it has been a common prac­ tice to cross-link the polymer in order to improve resistances to thermal as well as shear degrada­ tion. See, for example, U.S. Patent 3,247,171. Such attempts have generally not been successful. More recently, as taught in U.S. Patent 3,984,333, an aqueous medium has been thickened by dissolving a block copolymer having water-soluble blocks and water-insoluble blocks in the aqueous medium. While such block copolymers apparently exhibit reasonably good resistance to shear degra­ dation, such polymers are difficult and often impractical to prepare. More importantly, such polymers do not exhibit significant tolerance of electrolytes normally present in the aqueous media to be thickened. While the cellulosic derivatives such as hydroxyethyl cellulose and biopolymers exhibit acceptable tolerance to the presence of electro­ lytes, cellulosic derivatives are generally inef­ fective at the low concentrations that are economi­ cal and exhibit poor thermal stability. The bio­ polymers such as xanthan gums exhibit acceptable thermal stability, resistance to shear degradation and electrolytic tolerance. Unfortunately, such biopolymers are generally very expensive and are susceptible to biodegradation. U.S. Patent 4,463,152 discloses a copolymer of acrylamide and nonionic surfactant monomers such as: R-OCH₂CH₂O n -=CH₂ I wherein R is hydrophobic and n is 1-60. The pat­ ent claims that these copolymers are more efficient viscosification agents than homopolymers of acryl­ amide and are different from hydrolyzed polyacryl­ amides (a copolymer of acrylamide and acrylic acid) by virtue of higher thickening ability and concen­ tration dependence of viscosity in a brine environ­ ment and often higher retention of viscosity in the presence of inorganic electrolytes. The pat­ ent states that the small amount of hydrophobic or water-insoluble alkyl group on the surfactant mono­ mer is believed to result in intermolecular aggre­ gation or interaction in solution, resulting in enhanced viscosity relative to a noninteracting polymer such as polyacrylamide. U.S. Patent 3,551,479 discloses a mono­ mer which may be useful in the instant invention; wherein Y is a sulfonate group, m is an integer from 2 to 4, n is 1 or 2, and R is a hydrophobic radical or a nonylphenol ethoxylate having 1 to 40 ethoxy groups and attached to the compound through the terminal ethoxy group. The patent discloses that this monomer may be copolymerized with numerous polymerizable ethylenically unsatu­ rated compounds including at column 4, acrylic acids and acrylic amides. The utility disclosed for the monomer is the capability of polymerically combining with copolymerizable ethylenically unsaturated compounds dispersed in an aqueous medium to provide a very stable latex containing polymer particles compris­ ing the anionic polymerizable surfactant as an immobile integral component of the polymer. U.S. Patent 4,432,881 discloses a thickened aqueous medium having dispersed therein a hydrophobe associative polymer which is a water­ -soluble polymer having pendant hydrophobic groups and a water-dispersible surfactant. These composi­ tions suffer from the shortcoming of having a sur­ factant present which has its own independent char­ acter and properties. These properties may con­ flict with the properties of the polymer. For instance, if the flocculating properties of the polymer are desired, the surfactant may indepen­ dently act as a dispersing agent. While the poly­ mer may not have foaming tendencies, the surfactant may very well have foaming tendencies, making use of the composition less than optimal. In view of the aforementioned defi­ ciencies of conventional water-soluble polymers, it is highly desirable to provide a relatively inexpensive polymer composition capable of increas­ ing the viscosity of aqueous media containing water­ -soluble electrolytes. It is also desirable that such polymer exhibits thermal stability, electro­ lyte tolerance and good resistance to shear and biological degradation. It is further desirable that the aqueous media not be rendered foaming by the presence of surfactants used to aid the polym­ erization reaction. These and other deficiencies of the art are overcome by the present invention which relates to a thickening agent comprising an aqueous-soluble polymerization product of: (A) one or more water-soluble monomers which are sufficiently water-soluble to form at least a 10 weight percent solution when dissolved in water and which undergo addition polymerization to form polymers which are water-soluble; and (B) at least one second monomer of the formula wherein R is a hydrocarbyl radical which may be substituted and which contributes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or sub­ stituted hydrocarbyl radical and Y is an ionic species which contributes hydrophilic character to the second monomer. It is a feature of this invention that monomer 'B' contains both a hydrophobic portion to effectuate the hydrophobe association and the hydrophilic portion to render the monomer solu­ tion polymerizable without addition of surfactant. The group Z preferably hydrates in aqueous media but dehydrates as the temperature increases or the electrolyte concentration increases. This would permit stronger association of the hydrophobic portions of the polymer molecule as the electro­ lyte concentration increases. It is an advantage of this invention that the aqueous media containing the hydrophobe­ -associated copolymer of this invention can be subjected to substantial mechanical shear without a significant loss of viscosity. This increase in viscosity is particularly enhanced by the pres­ ence of a water-soluble electrolyte in an aqueous medium being thickened. Moreover, such hydrophobe­ -associated copolymers may provide a substantial viscosity increase even when the aqueous medium is exposed to temperatures up to 80°C and higher. Consequently, such copolymers can be used for a variety of applications wherein high temperatures, substantial electrolyte concentrations and condi­ tions of higher mechanical shear, such as in high shear pumping of the aqueous medium are required. Hydrophobe-associated copolymers that do not contain the hydrophilic portion Y as a part of one of the comonomers may require the addition of a surface-active agent in order to effectuate the polymerization. The comonomers of this inven­ tion do not require such additional surface-active agents. Accordingly, when conditions are experi­ enced which might tend to cause foaming, solutions of the thickening agent of the invention in water have a lower foaming tendency than many prior art­ -thickened aqueous media while still retaining the advantages of hydrophobe association. Unlike high molecular weight polymers currently available which thicken in aqueous media but which also give solutions which are stringy, the copolymers of this invention, when dispersed in an aqueous medium, may exhibit suitable plastic rheology and short solution characteristics. By "short solution characteristics" is meant that an aqueous medium containing the hydrophobe-associated copolymer does not produce threads or strings of such aqueous medium when surfaces wetted with the medium are contacted and pulled apart. As a result of these and other thick­ ening characteristics of the present invention, these copolymers may be useful in all applications which require aqueous media having increased vis­ cosity, such as drilling mud formulations, fractur­ ing fluids, liquid mobility control agents, aqueous solutions of inorganic salts, hydraulic fluids, lubricants, friction reducing agents, suspending agents, and aqueous suspensions of insoluble par­ ticulates such as paint formulations. Preferably, the hydrophobe associative copolymer of this invention is aqueous-soluble, i.e., if is soluble in an aqueous medium consist­ ing of water and 0.2 weight percent of sodium chloride. For the purposes of this invention, a hydrophobe associative copolymer is soluble in the aforementioned aqueous medium if 0.5 weight parts of the copolymer forms a thermodynamically stable solution when it is intimately mixed with 100 weight parts of the aqueous medium. Such solutions form spontaneously in the aqueous medium and include (1) true solutions in which the indi­ vidual polymer molecules are dispersed, as well as (2) micellar or colloidal solutions wherein the polymer molecules are aggregated to some extent, but wherein such aggregates are no larger than colloidal size. By "hydrophobe associative" is meant that, in aqueous media, the hydrophobic moieties of the copolymer associate thereby increasing the viscosity of the aqueous medium. Additionally, it is observed that this property of hydrophobe association is even further enhanced by the addi­ tion of a water-soluble electrolyte to the aqueous medium. This further enhancement (increase) of viscosity resulting from the addition of the elec­ trolyte occurs in absence of any precipitation or phase separation. In fact, the occurrence of such precipitation or phase separation upon addition of the electrolyte is not desirable in the practice of this invention. The copolymer is predominantly hydro­ philic, but preferably contains a sufficient con­ centration of suitable hydrophobic moieties to enable it to associate with the hydrophobic moi­ eties of other molecules of the copolymer in an aqueous medium and thus cause an increase in vis­ cosity of the aqueous medium. The molecular weight of the copolymer is preferably low enough such that an aqueous medium containing 0.1 weight percent of the copolymer can be subjected to a degree of shear on the order of 10,000 sec⁻¹ without causing the copolymer to degrade significantly. Moreover, the copolymer preferably has a molecular weight such that, when 0.5 weight part of the copolymer is dis­ solved in 100 weight parts of water, the Brookfield viscosity of the water is increased to at least 2 centipoises (.002 Pa.s) (as measured using a Brookfield LVT viscometer equipped with a UL adapter and operated at 6 rpm and 25°C). While copolymers having very high molecular weight, e.g., weight average molecular weight (M w ) greater than 5 million, can be suitably employed, such copolymers tend to degrade when subjected to high shear, e.g., in excess of 10,000 sec⁻¹. Accordingly, such copolymers are less preferred for some applications. Preferably, the copolymers of this invention have weight average molecular weights (M w ) as determined by the intrinsic viscosity method described in the examples in the range from 200,000 to 5 million, most preferably from 800,000 to 2.5 million. The hydrophobic group in the compound III which is designated as 'R' is a hydrocarbyl or substituted hydrocarbyl radical which contrib­ utes hydrophobic character to the molecule. By hydrocarbyl is meant an organic group containing hydrogen and carbon atoms configured in any of the known ways that such atoms may be structured. Substituted hydrocarbyl means that one or more of the carbons and/or hydrogens is replaced by another atom or group. For example, a -CH₂- may be replaced by -O- or -S-. An -H may be replaced with -OH, Cl, Br, I, F, etc. R is preferably a pendant organic group having hydrophobicity comparable to and may be one of the following: aliphatic hydrocarbon groups having at least 8 carbons, such as C₈ to C₂₂ alkyls or cycloalkyls; polynuclear aromatic hydrocarbon groups, such as naphthyls; alkylaryls wherein the alkyl has 2 or more carbons and pref­ erably 8 to 12 carbons; haloalkyls of 4 or more carbons, preferably perfluoroalkyls; polyalkylene­ oxy groups wherein alkylene is propylene or higher alkylene and there is at least one alkyleneoxy unit per hydrophobic moiety. The concentration of the hydrophobic moiety 'R' in the hydrophobe associative copolymer is that which is sufficient to increase the viscos­ ity of an aqueous medium containing an electrolyte such as sodium chloride in concentration as low as 1/10 of a weight percent of the copolymer. Prefer­ ably, the concentration of hydrophobic groups in the copolymer is such that when 0.5 weight percent of the copolymer is dissolved in 100 weight parts of the aqueous medium containing at least 1 weight percent of sodium chloride, the Brookfield viscos­ ity (as defined hereinbefore) of the aqueous medium is substantially higher than, e.g., at least twice that of a similar aqueous medium comprising a copolymer which is identical in all respects to the hydrophobe associative copolymer except that it contains no hydrophobic groups. For example, if an aqueous solution containing 1 weight percent of sodium chloride and 1 weight percent of poly­ acrylamide (hydrophilic polymer) has a Brookfield viscosity of 10 cps (.01 Pa.s), the aqueous solu­ tion containing 1 weight percent of sodium chloride and 1 weight percent of the copolymer of the invention having a M w equivalence of the polyacrylamide will have a Brookfield viscosity of at least 20 cps (.02 Pa.s). The hydrophobic group 'R' is attached to the monomer 'B' through a divalent linking group 'Z' that is preferably capable of hydrating in aque­ ous media. By hydration it is meant that water molecules loosely associate with 'Z'. Upon heat­ ing or upon addition of electrolyte to the solution, the divalent linking group preferably apparently dehydrates or salts out of the solution. That is, the loose association with water molecules is at least partially broken. This apparently renders the -Z-R moiety more hydrophobic and increases the hydrophobe associative properties of the polymer. Suitably, 'Z' is an ethoxy chain, either unsubstituted, substituted or interrupted by other alkoxy groups, such that the preferred hydration/dehydration properties are retained. Preferably, 'Z' is an unsubstituted ethoxy chain containing from 5 to 40 ethoxy groups which may be adjusted to yield appropriate polymer solubility and viscosity. Most preferably, 'Z' contains 10 to 40 ethoxy groups. Alternatively, 'Z' is a poly-2-alkyl-2­ -oxazoline which is a ring-opened polymer of the formula wherein R is hydrogen or lower alkyl, preferably methyl, ethyl or isopropyl. These polymers may be obtained as disclosed in U.S. Patent 3,483,141. The polymer may have repeating units of the formula: wherein x is the number of repeating units. The symbol 'Y' represents a monovalent ionic species which contributes hydrophilic char­ acter to the molecule. The ionic species may be anionic or cationic. If a counterion associated with the ionic species is normally considered a cation, then the ionic species 'Y' is anionic. Examples of anionic 'Y's are: phospho­ nates, sulfonates, carboxylates or sulfates. Sul­ fonates are preferred, and in particular sulfonates wherein the counterion is an alkali metal such as sodium or potassium. Alkaline earth metals, hydro­ gen, ammonia or lower alkyl-substituted ammonias are alternative cations. When the counterion associated with the ionic species is normally considered an anion, then the ionic species is cationic. Examples of cationic 'Y's are the onium salts such as phospho­ nium, ammonium or sulfonium salts. Of the cat­ ionics, quaternary ammonium salts are preferred. Possible counterions include halides, carboxylates, sulfates, and nitrates. The group 'Y' preferably is anionic if the polymer to be made is to be anionic or non­ ionic, and cationic if the polymer is to be cat­ ionic. The group 'Y' is separated from the balance of the second monomer 'B' by a divalent hydrocarbyl radical or substituted hydrocarbyl rad­ ical 'M'. Hydrocarbyl and substituted hydrocarbyl have the same meanings as listed above. Preferably, M is CH₂ x , wherein x represents the number of meth­ ylene groups separating the acid group 'Y' from the balance of the monomer molecule 'B' and may be 2, 3 or 4. Suitable water-soluble monomers 'A' include those which are sufficiently water-solu­ ble to form at least a 10 weight percent solution when dissolved in water and readily undergo addi­ tion polymerization to form polymers which are water-soluble. Exemplary water-soluble monomers include ethylenically unsaturated amides, such as acrylamide, methacrylamide and fumaramide and their N-substituted derivatives such as 2-acrylamido-2-­ methylpropane sulfonic acid, N-(dimethylaminomethyl)­ acrylamide as well as N-(trimethylammoniummethyl)­ acrylamide chloride and N-(trimethylammoniumpropyl)­ methacrylamide chloride; ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid and fumaric acid; and other ethylenically unsaturated quaternary ammonium compounds such as vinylbenzyl trimethyl ammonium chloride, sulfoalkyl esters of unsaturated carboxylic acids such as 2-sulfoethyl methacrylate, aminoalkyl esters of unsaturated carboxylic acids such as 2-aminoethyl methacrylate, vinyl amines such as vinyl pyridine and vinyl morpholine, diallyl amines and diallyl ammonium compounds such as diallyl dimethyl ammonium chloride, vinyl heterocyclic amides such as vinyl pyrrolidone, vinylaryl sul­ fonates such as vinylbenzyl sulfonate, as well as the salts of the foregoing monomers. The water-soluble monomer 'A' and the second monomer 'B' preferably are compatible. In­ compatibility would mean that while one monomer is anionic, the other is cationic. Mixtures such as anionic/anionic, anionic/nonionic, nonionic/non­ ionic, cationic/nonionic and cationic/cationic would be considered compatible. Of the foregoing water-soluble mono­ mers, acrylamide and combinations of acrylamide and acrylic acid, including salts of acrylic acid such as sodium acrylate or ammonium acrylate, are preferred. Acrylamide and combinations thereof with up to 75 mole percent of acrylic acid or salt thereof, based on total water-soluble monomer, are more preferred. Most preferred are polymers wherein the water-soluble monomer is a mixture of acrylamide with from 5 to 50 mole percent, especially from 5 to 30 mole percent of acrylic acid or salt thereof. The hydrophobe associative copolymer of the invention is preferably an addition copolymer of a water-soluble ethylenically unsaturated mono­ mer 'A' and the second monomer 'B', a compound of the formula III, having sufficient concentration of the hydrophobic group 'R' to enable the copoly­ mer to associate with the hydrophobic groups of other molecules of the copolymer and sufficient concentration of nonionic hydrophilic groups to enable the copolymer to control its hydrophilic­ -lyophilic balance. Exemplary preferred polymers include copolymers from 90 to 99.995, more preferably from 98 to 99.9, most preferably from 99 to 99.5, mole percent of one or more water-soluble monomers 'A' with from 0.005 to 10, more preferably from 0.1 to 2, most preferably from 0.5 to 1, mole percent of monomer 'B'. For these copolymers, it is found that preferred amounts of the monomer 'B' will vary with the molecular weight of the copolymer. For example, a hydrophobe associative copolymer, hav­ ing a weight average molecular weight near 200,000 preferably contains from 1 to 2 mole percent of the second monomer 'B'. Alternatively, the copolymer having a weight average molecular weight of 2,000,000 preferably contains from 0.02 to 0.1 mole percent of the monomer 'B', preferably from 0.05 to 0.1 mole per­ cent. Also, the preferred percentage of the sec­ ond monomer 'B' varies with the relative balance of hydrophilic moiety versus the hydrophobic moi­ ety in the monomer 'B'. For example, as the bal­ ance shifts from hydrophilic to hydrophobic, gen­ erally less monomer 'B' is employed. Conversely, if the balance shifts to more hydrophilic, then more of the monomer 'B' is required. The aforementioned hydrophobe associ­ ative copolymers may be advantageously prepared by copolymerizing the water-soluble monomers 'A' with the comonomers 'B' in an aqueous medium which may contain a chelating agent that complexes with any existing polymerization inhibitors. Monomers such as acrylamide often contain around 20 ppm of copper ion to prevent polymerization. A chelating agent such as ethylenediamine tetraacetic acid (EDTA), the pentasodium salt of (carboxymethyl­ imino)bis(ethylenenitrilo)tetraacetic acid, tetra­ sodium ethylenedinitrilotetraacetate or the triso­ dium salt of N-(carboxymethyl)-N′-(2-hydroxyethyl)­ -N,N′-ethylenediglycine is then added to the reac­ tion mixture to complex metallic salts. The copo­ lymerization is then preferably initiated by adding a polymerization initiator capable of generating free radicals. Optionally, a chain-transfer agent may be included in the polymerization reaction mix­ ture. A solubilizing emulsifer may be used if desired to solubilize the monomer 'B' and to subsequently obtain a hydrophobe associative copolymer having a desirable concentration of hydrophobic moieties in the copolymer. However, because of the hydrophilic group on the second monomer 'B', this need for a solubilizing emul­ sifier is preferably eliminated or substantially reduced over a monomer which contains no hydro­ philic group. Thus, the solubilizing emulsifier may be employed in an amount which is below the critical micelle concentration of the emulsifier and is less than that which reduces the concentra­ tion of monomer 'B' in the copolymer to a point that the hydrophobic groups of the copolymer will not associate in the presence of an electrolyte. Preferably, no solubilizing emulsifier is needed or used to solubilize the monomer 'B'. Suitable emulsifiers which may be used when the group Y is anionic, include anionic agents such as alkali metal salts of alkyl sulfates and alkyl and aryl sulfates, e.g., dodecyl alkyl sulfo­ succinates and sodium dodecylbenzene sulfate; fatty acid soaps, e.g. sodium oleate, sodium stearate and potassium oleate; alkali metal salts of sulfo­ nated fatty alcohols, e.g., sodium dodecyl sulfate; sulfates of ethoxylated alcohols; alkyl phosphate esters, e.g., dodecyl hydrogen phosphate; and fluoro emulsifiers, e.g., perfluoroalkyl sulfates. Cationic emulsifers which may be used when the group Y is cationic are alkylamine hydrochlorides, e.g., dodecyl­ amine hydrochloride and tridecylamine hydrochloride; quaternary alkyl or aryl ammonium halides such as dodecyl trimethyl ammonium chloride; and ethoxylated fatty amines. McCutcheon's Detergents and Emulsifers , North American Edition, 1980 Annual discloses addi­ tional emulsifiers which come under the above listed categories. As an additional limit, when the hydro­ philic/hydrophobic polymer is anionic or nonionic, an anionic emulsifier such as an alkali metal alkyl sulfate may be preferably employed as the emulsifier. When the hydrophobe associative copolymer is cat­ ionic, a cationic emulsifier such as dodecylamine hydrochloride may be employed. When the hydrophobe associative copolymer is nonionic or anionic, a nonionic emulsifier such as nonylphenoxy polyeth­ ylene glycol having 10 ethyleneoxy units per mole­ cule or other water-dispersible nonionic surfac­ tant as defined herein is suitably employed. When using metallic ion inhibited water-soluble polymers, one may use chelating agents including those mentioned hereinbefore, with the pentasodium salt of (carboxymethylimino)­ bis(ethylenenitrilo)tetraacetic acid being pre­ ferred. When employed, the chelating agent is used in an amount in the range from 0.1 to 0.2, preferably from 0.1 to 0.15, weight percent based on the weight of total monomer. Exemplary suitable polymerization ini­ tiators include the inorganic persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate, azo oatalyst suoh as azobis­ isobutyronitrile and dimethyl azoisobutyrate; organic peroxygen compounds such as benzyl perox­ ide, t-butyl peroxide, diisopropyl benzene hydro­ peroxide and t-butyl hydroperoxide. Of these ini­ tiators, the oil-soluble types such as the organic peroxides and azo compounds are preferred. It is desirable to employ from 0.01 to 0.1 weight percent of initiator based on the weight of total monomers. The hydrophobe associative copolymers are readily recovered from the aqueous medium when such is desired by conventional techniques includ­ ing precipitation with organic solvents such as methanol or acetone or removal of water under vacuum or by azeotropic distillation or by drum drying. Alternatively, the aqueous medium con­ taining the hydrophobe associative copolymer can be used as such. It is also understood that hydrophobe associative copolymers of acrylamide, acrylic acid and amphiphilic monomer 'B' can be prepared by copolymerizing all three of these monomers or by copolymerizing acrylamide with the amphiphilic monomer 'B' and subsequently hydrolyzing a por­ tion of the copolymerized acrylamide by contact­ ing the copolymer with a base such as sodium hydroxide and/or sodium carbonate. .The amount of hydrophobe associative copolymer in the aqueous medium being used is suf­ ficient to provide the desired increase in viscos­ ity of the aqueous medium. Preferably, such amounts of copolymer range from 0.01 to 1.5 weight percent, most preferably from 0.05 to 1 weight percent, based on the aqueous medium. In addition to the hydrophobe associ­ ative copolymer, the aqueous medium may also con­ tain a nonpolymeric, water-soluble electrolyte including salts of monovalent and divalent cations such as ammonium, alkali metal and alkaline earth metal chlorides, bromides, nitrates, sulfates, car­ bonates, and acetates; monobasic and di- and tribasic acids such as hydrochloric acid, sulfuric acid, phos­ phoric acid and oxalic acid; and monovalent bases such as sodium hydroxide and potassium hydroxide. Such electrolytes may be present in the aqueous media as in the case of brines and other aqueous media containing water-soluble salts of various metals and other water-soluble electrolytes. Such aqueous media often contain from 0.01 to 20 weight percent of salts of alkali metals and alkaline earth metals. These electrolytes generally enhance hydrophobe association.between hydrophobic groups of the hydrophobe associative copolymer. In addition to the hydrophobe associ­ ative copolymer and electrolyte (salt), the aque­ ous medium may contain a variety of other ingre­ dients common to mobility control fluids, fractur­ ing fluids, drilling muds, or other additives depending upon the intended use of the aqueous medium. The aqueous medium may also contain vari­ ous additives such as antioxidants and antimicro­ bials, stabilizers, and fillers. An exemplary fracturing fluid comprises the aforementioned hydrophobe associative copolymer, a stabilizer (antioxidant) such as a water-soluble thiosulfate; a fluid loss additive such as silica flour or a liquid hydrocarbon; a particulate solid proppant such as sand, bauxite, glass beads; and an energizing gas such as nitrogen or carbon diox­ ide. In each instance, the proportions of ingre­ dients are the proportions employed in conventional fracturing fluids. To increase the viscosity of an aqueous medium containing substantial amounts, e.g., up to weight percent based on the aqueous medium of monovalent inorganic salts such as sodium chlo­ ride and up to 1 weight percent, usually from 0.0015.to 0.5 weight percent of salts of polyvalent cations such as calcium and magnesium, it is pre­ ferred to employ the polymerization product of one or more water-soluble monomers 'A', and at least one monomer 'B' of the formula III. The viscos­ ity increase in this aqueous medium is retained or increased at temperatures over the range of from 30°C to 80°C. In many instances this viscosity increase is retained at temperatures substantially higher than 80°C. The preferred polymerization product will be a copolymer of acrylamide and acrylic acid with the compound of the formula III, wherein R is C₄-C₁₈ alkyl, and preferably is C₆-C₁₆ alkyl, wherein 'Z' is an eth­ oxyl chain with 10 to 40 ethoxy groups, 'M' is eth­ ylene or n-propylene and 'Y' is sodium or potas­ sium sulfonate. In general, the type of application and the conditions characteristic of the application will determine the number of carbons in the hydro­ phobic moiety and the value of n in the monomer 'B' to give the desired viscosity response. For exam­ ple, it is found that the monomer 'B' containing from 5 to 10 ethoxy groups in M, and wherein R is a C₁₂ to C₁₆ alkyl group is advantageously employed in an aqueous medium having a relatively low salt concentration and moderate temperature. On the other hand, the monomer 'B' containing from 10 to 40 ethoxy groups in M, and wherein R is a C₆ to C₁₀ alkyl group as a hydrophobic group is employed in an aqueous medium having a relatively high salt concentration and high temperature. Moreover, the desired viscosity response for a given set of conditions can be achieved by using a single monomer 'B', or a mixture of monomers 'B' which combine to provide the desired proper­ ties. The following examples are given to illustrate the invention and should not be con­ strued as limiting its scope. Unless otherwise indicated, all parts and percentages are by weight. Examples 1A to 3C and Comparative Runs A, B and C A. Preparation of the Monomer B The monomer 'B' was prepared by the following procedures. Fifty grams (50 g) of dodecoxy(polyoxy­ ethylene)₂₀ (50 g) with a hydroxyl number of 1.45 percent was dissolved in 40 cm³ of toluene. To this solution, 6.5 g of itaconic acid and 0.5 g of p-toluene sulfonic acid were added. The mix­ ture was heated to reflux and generated water was removed azeotropically using a Dean-Stark separator. The reaction was allowed to continue until water ceased to collect in the separator. After 5 hours, 0.8 cm³ of water was collected. The remaining toluene was removed at 50°C and 5 mm of Hg to give a waxy product. The conversion to the ester was determined by nuclear magnetic resonance comparing the intensity of the double-bond proton at δ equals 6.35 and 5.75 to the intensity of the methylene proton of the ester group at δ equals 4.25. The ratio of the two was 90 percent, which was compatible with the conversion of 89 per­ cent calculated from the collected volume of water. In a second step, 35 g dodecanoxy(poly­ oxyethylene)₂₀-itaconic monoester formed in the first step was dissolved in 70 cm³ of t-butyl alcohol. While stirring, 4.5 g potassium t-butoxide was added to the solution which became a viscous suspension in a short time. The use of potassium t-butoxide was pre­ ferred to potassium hydroxide because the reaction with the free carboxylic acid was more complete and required a shorter time. To this mixture, 4.9 g of warm propane sultone was added slowly. Then the mixture was heated to reflux. After 2 hours of refluxing, the t-butanol was removed in vacuum to yield 41 g of a yellowish waxy compound which was readily soluble in water, and of which a 2 percent aqueous solution stayed clear after being heated to 75°C. This product, an example of the self-emulsifying monomer 'B', was used in the following testing. B. Copolymerization of Monomers 'A' and 'B' In a general procedure for preparing the water-dispersible thickening agents of the present invention, 25 g of monomers consisting of the alkyloxy­ (polyoxyethylene) itaconoxy propane sulfonate, the monomer 'B', and the other water-soluble monomer 'A' were placed in a 300-cm³ citrate botttle. Then a measured amount of VAZO 64 (2.4 g of a 2 percent VAZO -64 in t-butanol solution and 1.8 g of a 2 percent aqueous solution of VERSENEX 80) was added. The total weight of the contents was brought to 250 g with denionized water. The solution was purged with nitrogen and the bottle was capped and transferred to a steam tumbler. The polymerization was carried out at 60°C for 16 hours. The polymers obtained were transparent gels. In cases where the copolymers contained an acid group, their solutions were titrated with dilute NaOH to a pH of about 7.5. A molecular weight of about 700,000 was determined by capillary viscometer based on the Mark-Houwink relation: [η] = 6.31 × 10⁵ × [M] 0.80 , where η is intrinsic viscosity, and M is weight average molecular weight. C. Polymer Composition Copolymers of acrylamide, acrylic acid and the monomer 'B', made above were formed at dif­ ferent levels for each of the comonomers. The weights of each monomer used in the above proce­ dure is listed in Table I. D. Effect of Concentration on the Viscosity A solution of the resulting polymer was made at concentrations listed in Table II. The example designations were the same as those in Table I. Viscosities were measured with a LVF syncro-lec­ tric Brookfield viscometer using an UL adapter at a speed of 6 rpm. The numbers listed in Table II are centipoise (Pascal-second). E. Effect of Electrolyte Concentration on Viscosity Viscosities were measured at a concentra­ tion of 0.3 percent of the copolymer of the inven­ tion using a Brookfield LVF viscometer at a speed of 6 rpm. The electrolyte tested was sodium chlo­ ride at 0, 1, and 2 weight percent. The numbers indicated in Table III are centipoise (Pascal­ -second). F. Effect of Temperature on Viscosity Polymers of the invention were tested at varying temperatures. In all cases the solu­ tion contained 1 weight percent of sodium chloride. The concentration of Comparison 'B' was 0.49 weight percent polymer. The concentration of Example 1B and Example 2B were 0.3 weight percent. Viscosity was measured with a Haake Rotovisco with Adaptor VI at a shear rate of 43.1 seconds ⁻¹. Viscosities are cps (Pa·s).

Claims: 1. A thickening agent comprising an aqueous-soluble polymerization product of: (A) one or more water-soluble mono­ mers which are sufficiently water-soluble to form at least a 10 weight percent solu­ tion when dissolved in water and which undergo addition polymerization to form polymers which are water-soluble; and (B) at least one second monomer of the formula wherein R is a hydrocarbyl radical which may be substituted and which contributes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or substituted hydro­ carbyl radical and Y is an ionic species which contributes hydrophilic character to the sec­ ond monomer. 2. The thickening agent of Claim 1, wherein Z is OCH₂CH₂n and wherein n is 5-40. 3. The thickening agent of Claim 2, wherein n is 10-40. 4. The thickening agent of any of Claims 1 to 3, wherein R is a C6-16 alkyl group. 5. The thickening agent of any of Claims 1 to 3, wherein M is CH₂x and wherein x is 2, 3 or 4. 6. The thickening agent of any of Claims 1 to 3, wherein Y is -SO₃Na or -SO₃K. 7. The thickening agent of Claim 6 wherein the water-soluble monomers are acrylamide, acrylic acid, salts of acrylic acid or mixtures thereof. 8. The thickening agent of Claim 1, wherein the polymer has a weight average molecu­ lar weight of from about 800,000 to about 2.5 mil­ lion as determined by intrinsic viscosity. 9. A composition containing a thickening agent comprising the aqueous-soluble polymerization product of: (A) one or more water-soluble mono­ mers which are sufficiently water-soluble to form at least a 10 weight percent solution when dissolved in water and which undergo addition polymerization to form polymers which are water-soluble; and (B) at least one second monomer of the formula wherein R is a hydrocarbyl radical which may be substituted and which contributes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or substantial hydro­ carbyl radical and Y is an ionic species which contributes hydrophilic character to the sec­ ond monomer; and an aqueous solution of an inorganic salt in an amount such that at a temperature in the range of up to 80°C, the aqueous medium containing the water-dispersible thickening agent exhibits a fur­ ther increase in viscosity with the inorganic salt over a similar solution without the inorganic salt. 10. A composition as in Claim 9 wherein the concentration of hydrophobic moiety "R" in the second monomer (B) is sufficient to increase the viscosity of the aqueous solution to at least twice that achieved by a polymer comprising the polymerization product of 'A' without comonomer 'B' at an equivalent molecular weight and concentration in the aqueous solution. Claims for the following Contracting States: AT, ES 1. A method for the preparation of a thickening agent which comprises copolymerizing in an aqueous medium: (A) one or more water-soluble mono­ mers which are sufficiently water-soluble to form at least a 10 weight percent solu­ tion when dissolved in water and which undergo addition polymerization to form polymers which are water-soluble; with (B) at least one second monomer of the formula wherein R is a hydrocarbyl radical which may be substituted and which contributes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or substituted hydro­ carbyl radical and Y is an ionic species which contributes hydrophilic character to the sec­ ond monomer. 2. The method of Claim 1, wherein Z is OCH₂CH₂ and wherein n is 5-40. 3. The method of Claim 2, wherein n is 10-40. 4. The method of any of Claims 1 to 3, wherein R is a C6-16 alkyl group. 5. The method of any of Claims 1 to 3, wherein M is CH₂x and wherein x is 2, 3 or 4. 6. The method of any of Claims 1 to 3, wherein Y is -SO₃Na or -SO₃K. 7. The method of Claim 6 wherein the water-soluble monomers are acrylamide, acrylic acid, salts of acrylic acid or mixtures thereof. 8. The method of Claim 1, wherein the polymer has a weight average molecu­ lar weight of from about 800,000 to about 2.5 mil­ lion as determined by intrinsic viscosity. 9. A method for the preparation of a composition containing a thickening agent which comprises mixing the aqueous-soluble polymerization product of: (A) one or more water-soluble mono­ mers which are sufficiently water-soluble to form at least a 10 weight percent solution when dissolved in water and which undergo addition polymerization to form polymers which are water-soluble; and (B) at least one second monomer of the formula wherein R is a hydrocarbyl radical which may be substituted and which contributes hydrophobic character to the second monomer, Z is a divalent linking group, M is a divalent hydrocarbyl or substituted hydro­ carbyl radical and Y is an ionic species which contributes hydrophilic character to the sec­ ond monomer; with an aqueous solution of an inorganic salt in an amount such that at a temperature in the range of up to 80°C, the aqueous medium containing the water-dispersible thickening agent exhibits a fur­ ther increase in viscosity with the inorganic salt over a similar solution without the inorganic salt. 10. The method of Claim 9 wherein the concentration of hydrophobic moiety "R" in the second monomer (B) is sufficient to increase the viscosity of the aqueous solution to at least twice that achieved by a polymer comprising the polymerization product of 'A' without comonomer 'B' at an equivalent molecular weight and concentration in the aqueous solution.

Inventors: EVANI, SYAMALARAO, VAN PHUNG, K.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C08F222
IPC codes: C09K8, C10M173, C08F20, C08F220, C08F222