WO1994019443A1

WO1994019443A1 - Stable aqueous acid compositions thickened with polyacrylamide

Info

Publication number: WO1994019443A1
Application number: PCT/US1994/001730
Authority: WO
Inventors: Timothy E. Janota; James A. Krogh; John C. Miller
Original assignee: Tomah Products, Inc.
Priority date: 1993-02-16
Filing date: 1994-02-15
Publication date: 1994-09-01
Also published as: AU6243094A

Abstract

An acid composition comprising an acid having a pKA value greater than or equal to about 0.5 and a homopolymer less than about 5.0 having the repeating monomeric structure -CH2CH-(CONH2-)-, and water in an amount sufficient to provide the composition a stable viscosity of about 25 - 20,000 centipoise. The polymer of this invention may be utilized at concentration levels much lower than previously thought possible and acts synergistically in the prsence of surfactants to stabilize the viscosity and pH of the thickened acids for extended periods of time.

Description

Title: STABLE AQUEOUS ACID COMPOSITIONS THICKENED WITH POLYACRYLAMIDE

This application is a continuation-in-part of Application Serial No. 08/018,435, filed on February 16, 1993, now abandoned.

Field of the Invention

This invention is related generally to viscous acid solutions and, more particularly to stabilizing the viscosities and pH's of acid compositions thickened with a ho opolymer of acrylamide for prolonged periods of time.

Background of the Invention Acidic compositions have found wide utility in a variety of household and industrial settings, such as hard surface cleaning agents for everything from coffee urns to well-drilling equipment. In many applications, it is desirable to prolong contact time to enhance cleaning efficiency. Not unexpectedly, vertical and sloped surfaces induce a "run-off" condition, thereby presenting unique formulation challenges.

Early concerns over undesirable run-off fostered the use of thickening agents such as xanthan, locust bean, and guar gums, cellulose ethers, and the like. To a certain extent, these vegetative thickeners prolonged contact time, but created other problems. They are relatively expensive and significantly increase the overall cost of the acid composition. Low water- solubility presents formulation problems, and the solutions' viscosities, once obtained, are unstable over time and across a useable temperature range. Specifically, guar gums show a drop in viscocity resulting from acid hydrolysis below a pH of 3. Xanthan gums also exhibit such a drop in viscocity below a pH of 4. These drops in viscocity are experienced rather rapidly over short periods of time. In some applications of the prior art in fact, the change of the viscosities is actually the basis of the invention. Furthermore, thickeners of this type are generally incompatible with many additives commonly found in acid compositions. For instance, the presence of quaternary a ine salts exacerbates the inadequate solubility and viscosity stability.

The search for an efficient, effective acid thickener meeting the requirements outlined above has been an on-going concern in the art. One approach, which has been used with certain success, involves use of synthetic polymers. In particular, water-soluble homopolyacids and their alkali and alkaline earth metal salts have been employed with mineral or carboxylic acids -vi¬

and, in some applications, are used instead of vegetative thickeners.

Clearly, the prior art has associated with it a number of significant problems and deficiencies. Most are related to inadequate and/or unacceptable viscosity stability parameters, which result from the compounds currently used.

One major problem relates to the fact that thickeners of the prior art represent a significant portion of the overall cost of an acid composition, and must be used at relatively-high concentrations to provide adequate viscosities. In addition, there is little correlation between thickener concentration and viscosity because the viscocity is dependent on how the batch is prepared. As a result, cost and efficiency are compromised as the viscosity obtained often exceeds that appropriate for a given application.

Another significant concern is that certain thickeners of the prior art exhibit instability over time and upon exposure to high-temperature conditions.

Incompatibility of the thickener with surfactants, and perfumes/dyes further destabilizes the viscosity of the acid composition.

Another significant deficiency of the prior art is the required incorporation of a low molecular weight alcohol to a polymer to achieve optimal viscosity. Alternatively, high viscosities may require mixing the polymer with a vegetative thickener. The presence of a volatile alcohol component is often undesirable from both a formulation and application perspective, while the drawbacks of using gums and the like are as noted above. Furthermore, addition of either increases the overall cost of the resulting acid composition.

In the prior art, a change in pH is often found desirable in particular applications. However, in a variety of industrial and household applications, a stable pH is critical. The thickeners of the prior art do not allow for the maintenance of a stable pH in acidic media, thus resulting in undesirable changes in the characteristics of household and industrial products.

In summary, a considerable number of drawbacks and problems exist in the art relating to acid cleaners. There is a need for a thickener which maximizes the function, utility, viscosity stability and pH stability of such compositions.

Objects of the Invention It is an object of this invention to provide a thickened acid cleaning composition, overcoming some of the problems and shortcomings of the prior art.

Another object of this invention is to provide a thickener which is efficient, effective, and may be utilized in low concentrations.

Another object of this invention is to provide a thickener such that the compositional viscosities desired may be achieved through control of polymer concentration, affording compositions which lend themselves to a greater range of applications.

Another object of this invention is to provide a thickener which maintains stable viscosities over time and upon exposure to high temperature conditions.

Another object of this invention is to provide a thickener which is compatible with surfactants, perfumes, dyes, and other such components which are typically present in acid compositions. Another object of this invention is to provide a thickener which will give optimal viscosity and viscosity stability without addition of low molecular weight alcohols or vegetative thickeners.

Another object of this invention is to provide a thickener which maintains a stable pH over prolonged periods of time.

These and other important objects will be apparent from the descriptions of this invention which follow.

Summary of the Invention

This invention is an acid composition useful for a variety of household and industrial cleaning applications. It overcomes certain well-known problems and deficiencies, including those outlined above. An important aspect of this invention is a water-soluble polyacrylamide which is admixed with the acid to be thickened. The inventive thickener is less expensive than those of the prior art and may be used effectively at lower concentrations, resulting in a significant decrease in the overall cost of the acid composition.

For the purpose of this discussion, the term "polymer" is used generally and, unless otherwise indicated, includes only homopolymers of acrylamide. Therefore, and as used herein, the term polyacrylamide encompasses homopolymers prepared from acrylamide monomer only. These stable polyacrylamide solutions are prepared either by dissolving the solid polyacrylamide in water or inverting the polyacrylamide that is contained in a water-in-oil emulsion.

This invention is a composition including: (1) an acid having a pK_A value greater than or equal to about 0.5 and less than about 5.0, in an amount sufficient to provide the composition a pH of greater than or equal to about 1.0 and less than about 5.0; 2) a polymer having the repeating monomeric structure -CH₂CH-(CONH₂-)-, wherein the monomer is acrylamide; and (3) water in an amount sufficient to provide the composition a viscosity of about 25 - 20,000 centipoise. Highly preferred embodiments have a viscosity of about 50 - 2,000 centipoise. Most preferred embodiments have a viscosity of about 100 - 1,000 centipoise.

In preferred embodiments, the polymer has a molecular weight of about 500,000 - 25,000,000, and the polymer is about 0.01 to 10 percent by weight of the composition. In highly preferred embodiments, the polymer has a molecular weight of about 3,000,000 - 20,000,000 and is about 0.2 to 5.0 percent by weight of the composition. In most prefeired embodiments the polymer has a molecular weight of about 5,000,000 - 15,000,000 and is about 0.75 to 2.0 percent by weight of the composition.

In preferred embodiments, the acid component of the composition is about 0.5 to 20 percent by weight of the composition, whereas in highly preferred embodiments, it is about 2.0 - 5.0 percent by weight of the composition. The concentration of the acid component can vary depending on the desired acid strength. However, the acid component should provide that, preferably, the pH of the acid composition has a range of 1.0 - less than 5.0. In highly preferred embodiments the pH of the acid composition has a range of 1.0 - 4.5. In the most preferred embodiments the pH of the acid composition is between 1.0 - 4.0.

This invention may further comprise at least one additive which functions with the polyacrylamide component to synergistically stabilize the viscosity and pH of the thickened acid composition. Cationic, anionic, amphoteric, or nonionic surfactants may be used. When present, the additive is about 0.1 to 5.0 percent by weight of the composition and is, preferably, a quaternary amine salt and/or a surfactant mixture.

The preferred polyacrylamide thickener is one that undergoes no hydrolysis to produce polyacrylic acid, commercially available from a number of sources including Allied Colloid Corporation of Suffolk, Virginia; Delta Chemical Corporation of Baltimore, Maryland; Hychem, Inc. of Tampa, Florida; and American Cyanamid of Wayne, New Jersey. Alternatively, the monomeric precursor may be obtained from a variety of sources and polymerized by methods well-known to those skilled in the art. Regardless, to achieve the preferred high molecular weights, care must be taken to rigorously exclude the presence of molecular oxygen and iron impurities which would otherwise prematurely terminate polymerization. It is possible that acrylamide that has undergone slight hydrolysis, i.e. less than 1.5%, may be utilized. However, it is preferable to use acrylamide which has undergone no hydrolysis. As stated above, in highly-preferred embodiments, the polyacrylamide has a molecular weight of about 3,000,000 - 20,000,000. Through observation, it has been found that at a molecular weight within this range the polyacrylamide disclosed herein provides excellent stable viscosities at concentrations much lower than otherwise thought possible. Typically, within this molecular weight range, thickener concentrations are on the order of about four times lower than that needed through use of prior art thickeners. As a result, acid compositions may be thickened more economically and efficiently. At concentrations below this range, viscosities required for most applications are not achieved. Beyond the upper limit, the polyacrylamide tends to become too viscous and not water-dispersable, creating a variety of handling and related process difficulties.

As shown more explicitly in the examples which follow, polyacrylamides afford acid compositions with excellent, effective viscosities and stable pH immediately upon preparation and which remain essentially undiminished over time. In contrast, the vegetative and polymeric thickeners of the prior art are specifically formulated to account for a steady decrease in viscosity. As a further indication of the benefits available through use of these inventive compositions, stable viscosities and pH's of the prior art are measured in days; comparable parameters of this invention are referenced by months or years. Viscosity control is yet another advantage realized through use of the present invention. While viscosities of prior art compositions appear to bear no direct relation to the amount of thickener present, the compositional viscosities documented herein are a function of polyacrylamide concentration. As a result, the polyacrylamides are extremely versatile and lend themselves to a greater range of use applications. Acid compositions may now be formulated economically, incorporating only that amount of thickener required for a desired stable viscosity.

Viscosity stability of the inventive compositions is not adversely affected by the presence of quaternary amine salts or surfactants (amphoteric, nonionic, cationic, or anionic) . Whereas prior art thickeners exhibit solubility and related deficiencies when such ingredients are introduced, the inventive polyacrylamides perform without significant loss of composition viscosity, even under high-temperature conditions. In fact, quite unexpectedly and as illustrated below, the presence of various surfactants induces a positive synergistic effect on viscosity stability at all temperatures tested. Unlike the prior art, polyacrylamides do not require addition of low molecular weight alcohols to achieve optimal composition viscosities. Indeed, comparative data shows compositions thickened with polyacrylamides remain essentially unchanged when prepared with isopropyl alcohol. Prior art teaches that these additives are thought to function via a complexation mechanism and actually increase viscosity by inducing thickener precipitation, thereby compromising the benefits to be gained through use of a soluble system. Resorting to such measures invariably presents formulation problems not otherwise incurred and increases the overall cost of the acid composition. Furthermore, the volatility associated with many such alcohols is undesired in many applications. This invention also permits access to highly viscous compositions without addition of vegetative co- thickeners. When used in such compositions, various gums and/or the hydroxyalkylether derivatives thereof must be present in amounts approaching the weight percents of their polymeric counterparts. Increased cost and formulation problems are the consequence and, as discussed above, the resulting compositions are further destabilized when surfactant additives are included therein. Utilization of the inventive polyacrylamides completely avoids such alcohol and gum-related problems and affords process and economic benefits previously unavailable through the prior art. The acids used in conjunction with this invention generally include those water-soluble organic and inorganic acids which have a pK_A-value greater than or equal to about 0.5 and less than about 5.0. Polybasic acids having a pK_A ^!-value greater than or equal to about 0.5 and less than about 5.0 may also be employed.

Stronger acids tend to hydrolyze polyacrylamides to the corresponding polyacrylic acids, which do not exhibit stable thickening properties in acidic media. Within these parameters, useful acids include, but are not limited to, acetic, butyric, chloroacetic, chloropropionic, citric, hydroxyacetic, maleic, malonic, oxalic, phosphoric, propionic, and sulfanilic. The amount of acid added to the composition is that required to provide the pH desired for a given application, without inducing conditions detrimental to composition performance and integrity.

Graph 1 demonstrates the relationship between pH and acid concentration for some typical organic and mineral acids. This invention teaches that any acid whose concentration results in an aqueous solution of pH greater than 1.0 and less than about 5.0 can be thickene by polyacrylamides to yield desired stable viscous solutions over extended periods of time. A comprehensiv list of organic and inorganic acids having appropriate pK_A values (disassociations constants) can be found in the "Handbook of Chemistry and Physics," 61st Edition pages D165-D167 1980, CRC Press, which is incorporated by reference herein.

Ctαitiunv ΠCUIE i

Graph 1 This graph is a modified version of the graph found in Synthetic Detergents by A. Davidsohn and B. M. Milwidsky, page 198 (CRC Press 1968) .

Polyacrylamide would not seem appropriate for use as an acid thickener. Polyacrylamides have been reported to hydrolyze at high temperatures and precipitate in the presence of alkali or alkaline earth metal ions. However, the compositions of the present invention maintain excellent viscosity stability over a variety of deleterious conditions. Furthermore, at the molecular weights preferred herein, the use of polyacrylic acid does not result in long term stabilized viscosities in aqueous acidic media, presumably due to a high degree of inter- and/or intramolecular hydrogen-bonding. Inasmuch as amides also exhibit hydrogen-bonding, the excellent thickening stability properties observed were quite unexpected and contrary to the prior art. The fact that viscosity remains essentially unchanged over time in the presence of alcohols suggests a fundamental and unanticipated chemical and/or structural distinction between the polyacrylamides and the polymeric thickeners of the prior art.

Careful observation shows that, within the preferred molecular weight range, the polyacrylamides induce a somewhat striated effect as the acid composition slowly proceeds across a sloped surface. While not wanting to be bound by theoretical considerations, it may be that the high molecular weights employed herein are responsible for a fiber-like consistency observed and the fact that lower thickener concentrations and composition viscosities are required to achieve a desired cleaning effect. The striations may operate in such a way as to control the release of the acidic component and prolong contact time with the surface to be cleaned.

EXAMPLES OF THE INVENTION Examples of the inventive compositions are provided below. The compositions may be prepared by dissolving the polyacrylamide in water, then admixing with acid to achieve a desired pH and viscosity. (The order of addition may be varied depending upon the composition components and the amounts used.) Where provided, viscosity measurements (cps) were obtained using an RVT Brookfield viscometer operating at 100 rpm with a No. 2 spindle at the temperature indicated unless otherwise noted.

EXAMPLE 1 The compositions shown below are representative of those which may be used in accordance with this invention. Relative amounts of the acid and thickener components (weight percent of the total composition) are given, with the balance comprised of water. Acid Polvacrvla ide (Molecular Wt,

12- -15 Million. Wt. % a) Citric (5.0%) 1.0% b) Citric (5.0%) 0.75% c) Hydroxyacetic (2.5%) 0.75% d) Hydroxyacetic (2.5%) 0.50% e) Oxalic (5.0%) 1.0% f) Oxalic (2.5%) 0.10% g) Acetic (5.0%) 0.10% h) Propanoic (2.5%) 0.50% i) Phosphoric (2.5%) 0.75%

* * * EXAMPLE 2 This acid composition (5.0% Citric Acid, 1.0% polyacrylamide-molecular weight 12-15 million) exhibited stable viscosity, at room temperature over the time shown, as is typical through use of the polyacrylamides of this invention. The testing period employed extends typical warehouse storage and/or household use conditions dramatically and demonstrates that excellent viscosity stability is maintained throughout. This graph depicts duplicate runs of the above acid composition.

* * *

Room Temperature

1.00% Polyacrylamldβ(12-15 Million MW), 5.0% Citric acid, 100 rpm , spindle RVT #2

Time, in months EXAMPLE 3 The presence of a low-molecular weight alcohol has essentially no effect on the viscosities exhibited by compositions of this invention. A control (1.0% polyacrylamide-molecular weight 12-15 million in water) was compared to the same solution containing 8% isopropyl alcohol. (Viscosities were measured at 100 rpm, using a No. 3 spindle.) This graph depicts duplicate runs of the above acid composition.

Viscosity (cps) Viscosity (cps) at 24 hr. at 96 hr.

Control 215 211

With 8% isopropyl alcohol 175 175

As shown in Examples 4-9 and 12-20 below, the compositions contemplated by this invention include those containing surfactants and quaternary amine salts. Compositions so formulated (water comprises the balance of each) exhibit suitable stable viscosities over extended storage times, even under prolonged exposure to high-temperature conditions. In Examples 10 and 11, the stability study was conducted at 120° F, while viscosity measurements were recorded at ambient temperatures.

(Maquat is the tradename of a quaternary amine salt available from Mason Chemical Company, Chicago, IL; Empigen Bac 50 Benzalkonium Chloride is the tradename of a quaternary amine salt available from Albright and Wilson, Ltd., of Whitehaven, Cumbria, United Kingdom; Triton N101 is the tradename of a nonionic surfactant available from Union Carbide of Danbury, CT; Amphoteric-L is the tradename of an amphoteric surfactant available from the Tomah Products Division of Exxon Chemical Company, Milton, WI; Neodol 25-7 is a tradename of a nonionic surfactant available from Shell Chemical Co. , of Houston, Texas; Tergitol 15-S-7 and Tergitol 15-S-20 are the tradenames of nonionic surfactants available from Union Carbide of Danbury, Connecticut.)

EXAMPLE 4 Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 12-15 million (0.50%), Maquat (1.0%), and Triton N101 (1.0%). This graph depicts runs of the above acid composition.

Time, in months

EXAMPLE 5

Composition comprised of Citric Acid (5.0%), Polyacrylamide-molecular weight 12-15 million (0.50%), and Amphoteric-L (1.0%). This graph depicts duplicate runs of the above acid composition.

Time, in months

* * * EXAMPLE 6 Composition comprised of Citric Acid (5.0%), polyacrylamide-molecular weight 12-15 million (0.50%) and Triton N-101 (1.0%). This graph depicts duplicate runs

RoomTemperature

0.50% Polyacrylamlde(12-15 Million MW), 5.0% Citric add, 1% Triton N-101, 100 rpm , aplndl* RVT #2

Time, in months EXAMPLE 7

Composition comprised of phosphoric acid (2.5%) and polyacrylamide-molecular weight 12-15 million (1.0%). This graph depicts duplicate runs of the above acid composition.

Time, in months

EXAMPLE 8

Composition comprised of Citric Acid (2.5%), Poly¬ acrylamide molecular weight 12-15 million (0.50%), and Triton N101 (1.0%). This graph depicts duplicate runs of the above acid composition. Room Temperature

0.50% Polyacrylamlde(12-15 Million MW), 2.5% Citric add, 1% Triton N-101, 100 rpm , spindle RVT #2

Time, in months

* * * EXAMPLE 9 Composition comprised of Citric Acid (2.5%), Poly¬ acrylamide molecular weight 12-15 million (1.00%), Maquat (1.0%) and Triton N101 (1.0%). This graph depicts duplicate runs of the above acid composition. RVT #2

Time, in months EXAMPLE 10

Composition comprised of Citric Acid (2.5%), Poly¬ acrylamide molecular weight 12-15 million (1.0%), Maquat (1.0%) and Triton N101 (1.0%) at 120°F. This graph depicts duplicate runs of the above acid composition.

Time, In months

Note: T s data represents IHDI warehouse conditions thai may result In accelerated aging of the sample.

EXAMPLE 11 Composition comprised of Citric Acid (2.5%), Poly¬ acrylamide molecular weight 12-15 million (1.0%) and Amphoteric-L (1.0%) at 120°F. This graph depicts duplicate runs of the above acid composition. -22-

Time, in months

* * *

EXAMPLE 12

_Composition comprised of Citric Acid (2.5%) and

Polyacrylamide-molecular weight 20-23 million (1.0%).

This graph depicts duplicate runs of the above acid composition.

EXAMPLE 13

Composition comprised of Citric Acid (2.5%), Polyacrylamide molecular weight 20-23 million (1.0%) and Triton N-101 (1.0%). This graph depicts duplicate runs of the above acid composition.

RoomTemperature

Time, in days

EXAMPLE 14

Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%) and Triton N-101 (3.0%). This graph depicts duplicate runs of the above acid composition.

Time, in days

* * * EXAMPLE 15 Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%) and Neodol 25-7 (3.0%). This graph depicts duplicate runs of the above acid composition.

* * EXAMPLE 16

Composition comprised of citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%) and Tergitol 15-S-20 (3.0%). This graph depicts duplicate runs of the above acid composition.

Room Temperature

EXAMPLE 17

Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%) and Tergitol 15-S-20 (3.0%). This graph depicts duplicate runs of the above acid composition. Room Temperature

Time, in days

EXAMPLE 18

Composition comprised of Citric Acid (2.5%),

Polyacrylamide-molecular weight 20-23 million (1.0%), and

Neodol 25-7 (3.0%) and E pigen Bac 50 Benzalkonium chloride. This graph depicts duplicate runs of the above acid composition.

Room Temperature

1% Polyacrylamide(20-23 Million MW), 2.5% Citric Add, 3% Neodol 25-7, 1% Quat, RVT #3, 100 rp

Time, in days * * * EXAMPLE 19 Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%), Amphoteric L (3.0%) and Empigen Bac 50 Benzalkonium chloride (1.0%). This graph depicts duplicate runs of the above acid composition.

* * *

EXAMPLE 20 Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 20-23 million (1.0%) and Empigen Bac 50 Benzalkonium chloride (1.0%). This graph depicts duplicate runs of the above acid composition.

For comparison purposes, a composition comprising a copolymer of acrylamide and acrylic acid was evaluated to show lack of initial viscocity as well as long term viscosity stability using such a system.

EXAMPLE 21A Composition comprised of Phosphoric Acid (2.5%) and Polyacrylamide-molecular weight 20-23 million (1.0%). This graph depicts duplicate runs of the above acid composition.

EXAMPLE 2IB Composition comprised of copolymer (acrylamide [70%], acrylic acid [30%]) molecular weight 20-23 million (1.0%), and Phosphoric acid (2.5%). This graph depicts duplicate runs of the above acid composition.

EXAMPLE 22 Composition comprised of Citric Acid (2.5%), Polyacrylamide-molecular weight 12-15 million (1.0%) and Triton N-101 (1.0%). This graph depicts duplicate runs of the above acid composition and points out the excellent pH stability over extended periods of time for typical acid compositions.

Time, in months hile the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention. For instance, the quaternary amine salts which may be utilized through the compositions disclosed herein include benzyltrialkyl- ammonium chlorides, present as ethanolic solutions or otherwise; iminopropionates, betaines, or imidazolines may be used as surfactants. Various perfumes, dyes, and other non-active components are also anticipated. Likewise, the polyacrylamides which may be used are limited only by their water-solubility. As such, this invention also contemplates use of those polyacrylamides commercially-available as latex-emulsions whereby the polymer may be dissolved in water through an inversion process upon introduction of an aqueous surfactant.

Claims

CLAIMS :

1. A composition comprising:

- an acid having a pK_A value greater than or equal to about 0.5 and less than about 5.0, in an amount sufficient to provide the composition a pH of greater than or equal to about 1.0 and less than 5.0;

- a homopolymer having the repeating monomeric structure - CH₂CH-(CONH₂-)-, wherein the monomer is acrylamide; and

- water in an amount sufficient to provide the composition a viscosity of about 25 - 20,000 centipoise such that pH and viscosity are maintained essentially constant for periods approaching 2 years.

2. The composition of claim 1 wherein the homopolymer has a molecular weight of about 500,000 - 25,000,000.

3. The composition of claim 2 wherein the homopolymer has a molecular weight of about 3,000,000 - 20,000,000.

4. The composition of claim 3 wherein the homopolymer is about 0.01 - 10 percent by weight of the composition.

5. The composition of claim 3 wherein the homopolymer is about 0.2 - 2.0 percent by weight of the composition.

6. The composition of claim 1 wherein the acid is about 0.5 - 20 percent by weight of the composition.

7. The composition of claim 6 wherein the acid is about 2.0 - 5.0 percent by weight of the composition.

8. The composition of claim 7 wherein the homopolymer has a molecular weight of about 500,000 - 25,000,000.

9. The composition of claim 8 wherein the homopolymer has a molecular weight of about 3,000,000 - 20,000,000.

10. The composition of claim 9 wherein the homopolymer is about 0.01 - 10 percent by weight of the composition.

11. The composition of claim 9 wherein the homopolymer is about 0.2 - 2.0 percent by weight of the composition.

12. The composition of claim 1 further comprising an additive including at least one surfactant to synergistically stabilize the viscosity and pH of the thickened acid composition.

13. The composition of claim 12 wherein: - a surfactant is selected from the group consisting of cationic, anionic, amphoteric, and nonionic surfactants.

14. The composition of claim 13 wherein the additive is about 0.1 - 5.0 percent by weight of the composition.

15. The composition of claim 14 wherein a surfactant is a quaternary amine salt.

16. A composition comprising: - an acid having a pK_A value greater than or equal to about 0.5 and less than about 5.0, in an amount sufficient to provide the composition a pH of greater than or equal to about 1.0 and less than 5.0; - a polymer having the repeating structure

- CH₂CH-(CONH₂-)-, wherein the polymer is a homopolymer of acrylamide which has undergone less than 1.5% hydrolysis; and - water in an amount sufficient to provide the composition a viscosity of about 25 - 20,000 centipoise such that pH and viscosity are maintained essentially constant for periods approaching 2 years.

17. The composition of claim 1 wherein the polyacrylamide is in a water-in-oil emulsion.

18. The composition of claim 16 wherein the polyacrylamide is in a water-in-oil emulsion.