Microemulsion Process for Producing Acrylamide-Alkyl Acrylamide Copolymers

Polyacrylamide and hydrolyzed polyacrylamide are water soluble polymers that have been previously disclosed in the literature and have found application in the viscosification of aqueous solutions. This is achieved through a combination of high molecular weight and chain expansion due to repulsion of pendant ionic groups along the polymer chain. However, high molecular weight polymers present well known difficulties in manufacture and subsequent processing because of their irreversible degradation when exposed to conditions of high shear such as would be obtained in the usual stirring devices. Moreover, the presence of pendant ionic groups leads to solution properties which are markedly influenced by the presence of dissolved cations. In particular, the viscosity of solutions of these polymers usually decrease strongly upon increasing concentrations of brine.

We have discovered an alternative means for providing polymers which viscosify water or brine at low concentrations. This method relies on the incorporation of a small amount of hydrophobic groups into a polymer with a water soluble backbone. Those hydrophobic groups tend to associate with one another in an aqueous solution, and when the association occurs intermolecularly, the solution viscosity may be increased relative to the polymer without the hydrophobic side groups. An additional benefit is that the solution viscosity is relatively insensitive to salts because the hydrophobic groups are not ionic.

The synthesis of copolymers composed of water soluble and water insoluble monomers presents difficulties. In order for polymerization to be effected, the monomers must obviously come into close proximity to one another. A variety of processes based upon prior art could conceivably achieve this, but have serious deficiencies, necessitating the instant invention. For example, simply dispersing the water insoluble monomer as fine particles in the aqueous medium, while dissolving the water soluble monomer in water would result in poor incorporation of the water insoluble monomer and would lead to a heterogeneous product of particles dispersed in the predominently water soluble polymer. This would therefore require the extra step of separating the unreacted monomer particulates from the reaction product.

Alternatively, both monomers may be dissolved in a solvent or solvent mixture having properties intermediate between water and a hydrocarbon solvent. Although this undoubtably allows the comonomers to come into close proximity to one another, since the dispersion is on a molecular scale, this process presents other difficulties. For example, often the copolymer is insoluble in the mixed solvent which is capable of solubilizing the monomers. This leads to precipitation of the copolymer when it has a molecular weight which is still too low to produce efficient viscosification. The reaction product is usually heterogeneous which therefore requires a disadvantageous additional processing step. Further, the water miscible solvents such as alcohols, acetone, ether and acetic acid are fairly good chain transfer agents and when used in reasonable quantities would lead to decreased molecular weights and hence poor viscosification efficiency.

Conventional emulsion polymerization, which uses a surfactant to disperse the water insoluble monomer into the aqueous medium containing the dissolved water soluble monomer, has other disadvantages. In this process, the bulk of the water insoluble monomer is contained initially in droplets which are at least one micron in diameter. These droplets must be stabilized against coalescence by a combination of agitation and added surfactant. The product copolymer is usually in the form of particulates with diameters on the order of 500 to 2000 .ANG..

It has been found that a process for producing water soluble copolymers of acrylamide and alkyl acrylamide which are efficient viscosifiers may be produced by a process wherein the water insoluble alkyl acrylamide is dissolved into microemulsion droplets which are dispersed in water containing the water soluble acrylamide and a suitable water soluble initiator. The microemulsion droplets are sufficiently small, less than about 1000 .ANG. in diameter, that they form spontaneously upon mixing the components, are stable toward phase separation, and effectively disperse the water insoluble monomer on a very fine scale so that the copolymerization is effected without the formation of latexes or fine particulates of water insoluble polymer.

The microemulsion is composed of an oil or hydrocarbon which is capable of dissolving the water insoluble alkylacrylamide, a surfactant and cosurfactant which act together to enable very small droplets of these components to form which can be dispersed into water containing the water soluble monomer. This microemulsion copolymerization process does not result in any phase separation of the polymer or microemulsion components as the reaction proceeds to completion, but remains clear and homogeneous. The copolymers isolated from the reaction mixture are efficient viscosifiers of water or brine, having molecular weights such that their intrinsic viscosities are greater than about 1 dl/g, but not so high that they are extremely susceptible to shear degradation.

FIG. 1 illustrates a plot of reduced viscosity versus concentration for acrylamide/dodecyl acrylamide copolymers polymerized in a Tween 60 microemulsion.

FIG. 2 illustrates a plot of reduced viscosity versus concentration of acrylamide/dodecyl acrylamide copolymers (polymerized in a Tween-Span microemulsion) and the effect of salt thereupon.

FIG. 3 illustrates a plot of reduced viscosity versus concentration for acrylamide/octyl acrylamide.

FIG. 4 illustrates a plot of reduced viscosity versus concentration of acrylamide/octyl acrylamide copolymers and the effect of salt thereupon.

The process of the present invention overcomes the difficulties experienced in conventional polymerization just described. In particular, it enables the copolymerization of the water soluble monomer, acrylamide, and water insoluble monomers such as a higher alkyl acrylamide. The process relies on cosolubilizing the water insoluble monomer into a predominantly aqueous media by the use of a special mixture of surfactant, cosurfactant, and hydrocarbon. When mixed with an aqueous solution of the water soluble monomer, these special mixtures can disperse on a molecular scale the water insoluble monomer to form isotropic, clear, homogeneous systems, called microemulsions. These microemulsion reaction mixtures are free of visible oil droplets or particulates of the water insoluble monomer. The polymerization can therefore be initiated by water soluble initiators to give, surprisingly, copolymers of the water soluble monomer and water insoluble monomer which are substantially free of visible particulates. The resultant reaction product remains homogeneous. The components of the microemulsion can be chosen so as to give copolymers which are water soluble and which viscosify at low concentrations.

The microemulsion droplets used in this process form spontaneously upon mixing the components together, i.e., they do not require the vigorous mixing conditions that are required for the preparation of macroemulsions such as that used in conventional emulsion polymerization processes. The process is further differentiated from conventional emulsion polymerization processes in that the diameters of the microemulsion droplets is of the order of 100 to 1000 .ANG. in contrast to the diameters of the emulsion droplets of at least 10,000 .ANG.. Thus, the microemulsion reaction mixture is much more stable against demixing than the formulations used in emulsion polymerization. Indeed, no stirring is required during the course of the microemulsion copolymerization--the droplets remain extremely finely dispersed throughout. Moreover, the extremely dispersed nature of the microemulsion droplet containing the water insoluble monomer permits the copolymerization to occur in such a way that a water soluble copolymer is produced which does not contain particulates or latices of water insoluble polymer. These would be detrimental in such applications as secondary oil recovery, which requires a product substantially free of pore-plugging particulates.

The microemulsion formulations which may be used in the process are generally complex mixtures of a surfactant, cosurfactant, hydrocarbon or oil and water or brine. The water or brine is generally the predominant component, comprising from about 99 to 80% of the total mixture. The surfactant used may be any of the water soluble surfactants such as salts of alkyl sulfates, sulfonates, carboxylates and the like, or nonionic such as ethylene oxide-propylene oxide copolymers, or polyoxyethylene alkyl ethers, etc., or cationic surfactants such as primary alkylamines, dialkyl secondary amines, or ethoxylated fatty amines. The cosurfactant used in formulating the microemulsion is often an aliphatic alcohol containing about four to about eight carbon atoms, but may also be any of the surfactants chosen above. The oil used is one that is capable of dissolving the water insoluble monomer. The cosurfactant is then chosen by determining whether a microemulsion can be formed by, e.g., titrating with it the other components which are dispersed into water at the desired concentrations. This dispersion is usually a macroemulsion. If the cosurfactant is capable of making this dispersion transparent (indicating droplet sizes less than about 1000 .ANG.) during the titration then it is a candidate microemulsion reaction mixture. Other methods of choosing the cosurfactants and the relative concentrations of components may also be used. Compositions within the range of 1:2 to 2:1 cosurfactant/surfactant (on a weight basis) and with an oil/(surfactant+cosurfactant) ratio of 4:1 to 1:4 are useful but non-limiting guidelines. The composition limits within which the composition is clear (i.e. the microemulsion droplets have diameters less than about 1000 .ANG.) are often narrow.

Some further guidance on the formulation of microemulsions which can be useful may be found in "Microemulsions-Theory and Practice", L. M. Prince, ed., Academic Press, N.Y., N.Y., (1977) p. 33 ff.

Suitable free radical initiators for the microemulsion copolymerization process are peroxides such as hydrogen peroxide, potassium persulfate, alkyl peroxides and the like. The concentration of the free radical initiator is about 0.01 to about 0.50 grams per hundred grams of acrylamide and alkyl acrylamide monomer. The polymerization is conducted in the absence of oxygen at a temperature of about 20.degree. to about 100.degree. C. The polymer may be recovered from the microemulsion reaction mixture by precipitation by nonsolvents such as acetone.

The water soluble copolymers which are produced by the microemulsion copolymerization process of the instant invention are characterized by the formula ##STR1## wherein R.sub.1 is preferably a C.sub.6 to C.sub.22 straight chained or branched alkyl or cycloalkyl group, more preferably C.sub.6 to C.sub.20 and most preferably C.sub.6 to C.sub.18, and R.sub.2 is the same or different alkyl group as R.sub.1, or hydrogen. Typical, but nonlimiting examples of preferred alkyl groups are hexyl, octyl, decyl, dodecyl and hexadecyl groups; X is preferably about 90.0 to about 99.9 mole %, more preferably about 95.0 to about 99.8 mole %, and most preferably about 97.0 to about 99.5 mole % and y is 1-X. These water soluble copolymers are of a sufficiently high molecular weight that they are efficient viscosifiers of water or brine, but not so high that they are readily susceptible to irreversible shear degradation. That is, their intrinsic viscosity is greater than about 1 dl/g and less than about 10 dl/g.

The following examples explain the invention, which are by way of illustration, but not of limitation.

In the examples and comparative examples, the alkyl acrylamides used were prepared as follows. A 500 ml 4 necked round bottom flask was equipped with a condenser, thermometer, N.sub.2 inlet, magnetic stir bar and dropping funnel. After purging with N.sub.2, the n-octylamine, 14.35 g (0.11 mole) and triethylamine, 12.35 g (0.41 mole) were mixed with 50 ml of toluene and added to the flask. The acryloyl chloride, 10.0 g (0.11 mole) was dissolved in 50 ml of toluene and added to the dropping funnel. The reaction is exothermic so the temperature was controlled via an ice bath and the acryloyl chloride was added dropwise so that the contents of the flask stayed below 40.degree. C. The resulting slurry was stirred for an additional hour and then filtered to remove the triethylamine hydrochloride. The filtrate was stripped in a rotary evaporator to remove toluene. The resulting product was taken up into 240 ml of acetone and then cooled to -70.degree. C. in a dry ice bath. The monomer crystals which resulted were filtered on a coarse filter under N.sub.2 and then vacuum dried at room temperature for one day. A yield of 14.3 g (70%) of white crystals was obtained. A melting range of 36 to 37.degree. C. was observed. The same procedure was used to prepare N-(n-dodecylacrylamide except 20.36 g (0.11 mole) of n-dodecylamine was substituted for the n-octyl amine.

In this copolymerization, a nonionic surfactant, Tween-60, supplied by ICI Americas, was chosen. Tween-60 is a polyoxyethylene-20-sorbitan monostearate having the formula ##STR2## wherein u+v+y+z=20.

A microemulsion concentrate was prepared by mixing 15.85 g of the Tween-60 surfactant, 8.15 g of the cosurfactant, n-pentanol and 1.0 g of hexadecane. A clear homogeneous mixture resulted. Then 0.5 g of dodecyl acrylamide was dissolved into this mixture.

This was placed in the reactor and then 460 ml of deaerated water was added under N.sub.2 and 14.69 g of acrylamide was added. After mixing, the microemulsion reaction mixture was water-clear and had a low viscosity. The clarity was comparable to the Tween-60 microemulsion itself without added comonomers. The initiator, consisting of potassium persulfate (0.01 g) was added when the temperature reached 50.degree. C. The temperature was maintained for 24 hours. The resulting mixture became slightly hazy and slightly foamy, but no phase separation of the components occurred. The polymer was recovered by precipitation into acetone and it was redissolved into water.

In this polymerization, 460 g of deaerated water was placed into a nitrogen purged polymerization reaction vessel and 14.69 g of acrylamide and 0.5 g of solid dodecyl acrylamide were added. Nitrogen was continuously bubbled through the mixture, and it was heated to 50.degree. C. The dodecyl acrylamide became somewhat better dispersed at this temperature. When the temperature reached 50.degree. C., 0.01 g of potassium persulfate was added as initiator and the reaction was left at this temperature for 24 hours. A very viscous mixture resulted but there were small particles dispersed throughout, giving it a very cloudy appearance. The polymer was recovered by precipitation into acetone, dried, and redissolved into water at a concentration of 0.5 wt.%.

Relative amounts of particulates in this solution and one prepared from Example 1 were assessed by determining the volumes of solution that could pass through a Nucleopore.TM. polycarbonate filter which was 13 mm in diameter and had a pore size of 5 m. The copolymer polymerized in water, plugged the filter after only 6 cm.sup.3 of solution passed through it. In contrast, 28 cm.sup.3 of solution of the microemulsion polymerized copolymer of Example 1 was able to pass before plugging. Thus, the microemulsion copolymerization technique provides a product that produces a polymer solution more free of particulates.

One method for ascertaining the incorporation of hydrophobic groups in a water soluble polymer structure involves solubilization of a hydrophobic material which is normally insoluble in the aqueous phase. The solubilization properties of the two copolymers of Example 1 and Comparative Example 1 were compared by measuring the saturation uptake of an oil soluble dye into a 0.5 wt.% solution of the copolymer. The absorption in a one cm cell at 485 nm was 0.424.+-.0.007 for the microemulsion polymerized copolymer and 0.080.+-.0.005 for the copolymer made in the absence of microemulsion. This latter value is indistinguishable from the absorption of water saturated with dye. This is evidence that the water polymerized product did not incorporate the hydrophobic dodecyl acrylamide, while the microemulsion polymerized products of Example 1 did incorporate dodecyl acrylamide.

This polymerization was carried out in the same manner as Example 4, except that 1.0 g (2 mole %) of dodecyl acrylamide was used to make a somewhat more hydrophobic polymer. The reaction product was similar in appearance to Example 1 and had a similar low microgel content. The reaction product did not phase separate and was macroscopically homogeneous. However, when the polymer was separated from the surfactant by repeated acetone precipitation, the product was not completely soluble in water. Approximately 40% of a 1% solution in water contained in a test-tube formed a swollen, turbid gel in the bottom of the tube. A sample of the same polymer added to the Tween-60 microemulsion at the same concentration, however, formed a uniform, slightly turbid phase.

Copolymers of acrylamide and dodecyl acrylamide were prepared, according to the procedure of Example 1, with different mole fractions of dodecyl acrylamide, y, that were initially contained in the polymerization vessel. These copolymers were isolated, purified and redissolved in water to give solutions with known concentrations.

The viscosities of these solutions were measured by means of a Contraves.TM. low shear viscometer model LS30 using a No. 1 cup and No. 1 bob. Temperatures were controlled to .+-.0.1.degree. C., and measurements were made at a rotational speed that gave a shear rate of 1.28 s.sup.-1. The concentration dependence of the solutions are plotted in FIG. 1 for different mole fractions of dodecyl acrylamide. As the mole fraction of hydrophobic dodecyl acrylamide is increased in the copolymers, the specific viscosity per unit mass of polymer also increases. An important feature demonstrated by these data is that the intercepts at zero concentration are independent of the mole fraction of dodecyl acrylamide. This intercept is the intrinsic viscosity, defined as ##EQU1## where .eta..sub.o is the viscosity of the solvent, .eta. is the measured viscosity of the solution with a concentration of polymer c. This intrinsic viscosity is related to the molecular parameters for a homopolymer by ##EQU2## where N.sub.A is the Avagadro number, M.sub.w is the polymer molecular weight and V.sub.h is the hydrodynamic volume. For copolymers with a repeating backbone unit, the hydrodynamic volume is given by: ##EQU3## where R.sub.G is the radius of gyration of the polymer, and .xi. is a dimensionless constant with a theoretical value of 0.875 for polymer having a random chain conformation.

The radius of gyration is given by ##EQU4## where .beta. is an effective bond length and .nu. is the degree of polymerization. The molecular weight of a copolymer with a mole fraction 1-y of acrylamide (M.sub.w =71) and dodecyl acrylamide (M.sub.w =239) is given by ##EQU5##

This formula permits one to compare intrinsic viscosities for acrylamide/dodecyl acrylamide copolymers in a simple way. For example, the copolymer with x=0.98 can be compared with the homopolymer, which has x=1.0: ##EQU6## Thus, if the degree of polymerization for the two polymers are the same, their intrinsic viscosities should differ by only one percent. Conversely, if the intrinsic viscosities of the two polymers differ by only a few percent, then we may infer that their degrees of polymerization are nearly the same. FIG. 1 therefore shows that the degree of polymerization of the copolymers and homopolymer are very nearly all equal, and that introduction of the dodecyl acrylamide produces a beneficial enhancement in viscosity.

Values for the reduced viscosities of some of these systems are shown in Table I at a polymer concentration of one percent, in water and also in a two percent sodium chloride solution. These data show that the viscosities are not lowered by the salt.

A copolymer of 99.5 mole % acrylamide and 0.5 mole % dodecylacrylamide was prepared in a manner similar to that described in Example 1 except that SPAN 20 surfactant was used as a cosurfactant. A microemulsion concentrate consisting of 10 parts by weight of Tween 20 surfactant, 1 part by weight of SPAN 20 surfactant and 1.2 parts by weight of hexadecane was prepared by mixing the components at a temperature of about 80.degree. C. and subsequently cooling it to room temperature to give a homogeneous fluid. No alcohol was used. To 25 g of this mixture, 0.25 g of dodecylacrylamide was added, then after dissolution of it, 14.76 g of acrylamide and 475 ml of water was added. This mixture was purged with nitrogen. The temperature was brought to 70.degree. C. to give a clear, stable microemulsion reaction mixture. Then 0.01 g of potassium persulfate initiator was added. The mixture was stirred under nitrogen for 24 hours. The polymer was precipitated with acetone and vacuum dried.

This polymer was dissolved in water and viscosities as a function of concentration were measured as previously described. Similar concentrations of polymers were prepared in a two weight percent sodium chloride solution and the viscosities were also measured. The results are compared in FIG. 2, which shows that the viscosities are substantially unchanged by the presence of sodium chloride. This contrasts sharply with the well-known acrylamide/sodium acrylate copolymers.

Copolymers of acrylamide/octyl acrylamide were prepared according to the procedure of Example 1. FIG. 3 gives results for their reduced viscosity as a function of concentration in aqueous solutions for the formed copolymers. These plots clearly demonstrate the improved thickening efficiency resulting from the introduction of a small mole fraction of octyl acrylamide into the polymer. The salt independence of the viscosity is also shown in Table I. Plots with and without 2% NaCl are also compared in FIG. 4 for the 98/2 acrylamide/octyl acrylamide copolymer.