WO1992015535A1

WO1992015535A1 - Polyaspartic acid and its salts for dispersing suspended solids

Info

Publication number: WO1992015535A1
Application number: PCT/US1992/001704
Authority: WO
Inventors: Larry Paul Koskan; Kim C. Low
Original assignee: Donlar Corporation
Priority date: 1991-03-06
Filing date: 1992-03-05
Publication date: 1992-09-17

Abstract

Polyaspartic acid salts having a molecular weight range of 1 000 to 5 000 are excellent dispersants.

Description

POLYASPARTIC ACID AND ITS SALTS FOR DISPERSING SUSPENDED SOLIDS INTRODUCTION

The inhibition and dispersion of a broad variety of mineral and metal oxide scales and particulates in water is a common technology used in many different industries. For example, it is used widely in water treatment to prevent scale from forming on heat transfer surfaces and in pipes, in laundry and cleaning products to prevent suspended particles such as dirt from readhering to cleaned surfaces, in toothpastes and mouth washes as an anti-tartar agent, in paints and coatings to suspend pigments for ease of shipping, mixing and uniform application, and in polymer systems where emulsion droplets need to be suspended, to name a few.

Examples of the different types of scale and particulates dispersed would include CaC0₃, CaS0₄, BaSO„, Fe₂0₃, clays such as Kaolin, Ti0₂, Zn(0H)₂, Ca₃(P0)₄, Mg(0H)₂, Mn₂0₃ and many others.

Most dispersants used today are of a synthetic variety, usually a water soluble polymer made from acrylic acid, acrylamide, their derivates, maleic acid, vinyl esters, and the like. These polymers are non-biodegradable and potentially toxic to the environment.

Starch and lignin based dispersants, although biodegradable, tend to be poorer performers compared to their polyacrylate counterparts.

SUMMARY OF THE INVENTION

Water soluble salts of polyaspartic acid are excellent agents for suspending in water a variety of inorganic and organic particles. Due to the biodegradability of polyaspartic acid it is acceptable for use in a variety of industrial products and processes.

DRAWINGS

FIGURE 1 depicts a temperature versus time reaction curve.

Series 1 is the oil temperature. Series 2 is the reaction mixture temperature.

FIGURE 2 depicts a temperature versus time reaction curve.

Series 1 is the oil temperature. Series 2 is the reaction mixture temperature.

FIGURE 3 depicts a temperature versus time reaction curve.

Series 1 is the oil temperature. Series 2 is the reaction temperature.

FIGURE 4 depicts a temperature versus time reaction curve.

Series 1 is the oil temperature. Series 2 is the reaction temperature.

FIGURE 5 depicts a temperature versus time reaction curve.

Series 1 is the oil temperature. Series 2 is the reaction mixture temperature.

The Starting Polysuccinimide

(Anhydropolyaspartic acid)

The starting polysuccinimides from which the polyaspartic acids are synthesized are produced by the thermal condensation of powdered L-aspartic acid to produce polysuccinimide in high yields. The term "yield" means theoretical yield based on the starting molecular weight of the aspartic acid. These yields optimally occur above the initiation temperature of about 370 °F and preferably occurs above 420 °F, and most preferably occurs above 440 °F.

A reactant temperature less than 370 °F may produce polysuccinimide over a period of many hours. Theoretical yields will be low; the conversion of the L-aspartic acid to polysuccinimide will be less than 70% and will require a period of many days.

As the reactant temperature increases above 370 °F, the percent conversion increases to greater than 90% and the reaction times become greatly reduced.

The thermal condensation of L-aspartic acid to polysuccinimide using the above reaction conditions produces a characteristically shaped "temperature vs. time" reaction curve. The curve is characterized by an initial, rapid rise in reactant temperature, followed by an endotherm signally the beginning of the reaction. Immediately following the onset of the endotherm there is evaporative cooling, followed first by a temperature rise, and then by a second endotherm, which is followed by an evaporative cooling plateau. The temperature then rises to a plateau. That plateau is at a constant temperature. The reaction has gone to at least 95% conversion at the temperature midway between the final plateau and the time the temperature begins to rise to that plateau.

In the following examples, the color of each product sample was noted. The color of L-aspartic acid is white. Polysuccinimides vary in color according to the temperature of the sample taken during the course of the reaction. From low temperature to high, the colors vary as follows: light pink, to pink, to tannish pink, to tan, to light yellow, to yellow. These colors generally correspond to the percent conversion of the L- aspartic acid, in the same order with light pink indicating the lowest percent conversion and yellow indicating the highest percent conversion. The pink colors had less than 70 % conversion. The literature has never reported any other color but pink. The polysuccinimides used in the practice of this invention should be free of pure pink color. The polysuccinimides used to prepare the polyaspartic acid dispersants of this invention have a water content less than 1%, and they usually are substantially water-free.

Illustrative Preparation of Polysuccinimide

A series of experiments were conducted to thermally, polymerize solid phase L-aspartic acid. In each instance, the powdered L-aspartic acid was added to a reaction vessel and heated. Samples were taken throughout the polymerization reaction. Those samples were analyzed for percent conversion to the product, polysuccinimide, and the color and temperature of the samples were noted.

Each of these, conversion, color, and production of polyaspartic acid are described below.

The following procedure was utilized to determine the percent conversion of the L-aspartic acid to the product, polysuccinimide: The conversion of L-aspartic acid to polysuccinimide was determined as follows: A specific amount of the reaction mixture or product was dissolved in an aliquot of dimethylformamide (DMF). The dissolution was allowed to proceed for 4 to 5 hours until all of the polysuccinimide dissolved in the DMF leaving unreacted L-aspartic acid which was filtered out.

The amount of unreacted L-aspartic acid was determined by using the following formula:

A - B

% CONVERSION = * 100 %

A

Where: A = weight of initial sample

B = weight of filtrate

The percent conversion of the L-aspartic acid to the polysuccinimide in the reaction can be increased in reduced time periods by increasing the temperatures used.

Where a thermal fluid is used to heat the L-aspartic acid and as its temperature is brought to a maintenance temperature of at least 550 ^βF in a reasonable time period, at least 90% conversion can be effected within 4 hours.

Where the thermal fluid used to heat the L-aspartic acid is brought to a maintenance temperature of at least 550 °F within a reasonable time period, at least 90% conversion can be effected within 2 hours.

Continuous and batch processes can be used. Some process examples include fluidized bed, stirred reactor, and indirectly heated rotary driers.

It may be said, therefore, that, once initiation temperature is achieved, temperatures between 420-520 °F produces polysuccinimide at yields greater than 80%. Typically at temperatures between 420-450 °F, 90% conversions will be obtained. 500 °F will produce a 90% conversion in 4 hours and 510 °F will produce a 90% conversion in 1 1/2-2 hours.

Example 1 A "time vs. temperature" plot of the following reaction is depicted in Figure 1.

A 500 ml covered, stainless steel, beaker charged with 400 grams of powdered, L-aspartic acid was placed in an oil bath. The oil bath was quickly heated to a 450 °F maintenance temperature. The sample was stirred throughout the experiment.

At 30 minutes, the reaction began when the first endotherm was reached. The first endotherm of the reaction mixture peaked at 395 °F at an oil temperature of 439 °F.

Evaporative cooling immediately followed this first endotherm. Water loss was evidenced by the evolution of steam. The reaction mixture temperature dropped to a low of 390 °F during this period and the oil temperature rose to the 450 °F maintenance temperature. Following the temperature drop, the reaction mixture began to heat up. At 1.67 hours, a second endotherm occurred. At this endotherm, the reaction mixture temperature was 420 °F and the oil temperature was 450 °F. Steam coming from the system evidenced water loss.

Evaporative cooling continued to take place until the conclusion of the second endotherm. Water loss was evidenced by the evolution of steam. At the conclusion of this period, the reaction mixture was then heated up and maintained at an equilibrium temperature of 434 °F. Table 1 below provides data developed during this experiment. Samples were taken at the times indicated and analyzed for percent conversion to polysuccinimide.

The color of the reaction mixture is provided. Color was observed to vary with product temperature.

TABLE 1 POLYMERIZATION

A "time vs. temperature" plot of the following reaction is depicted in Figure 2.

A 500 ml covered, stainless steel, beaker charged with 400 grams of powdered, L-aspartic acid was placed in an oil bath. The oil bath was quickly heated to a 500 °F maintenance temperature. The reaction mixture was stirred throughout the experiment.

At 30 minutes, the reaction began when the first endotherm was reached. The first endotherm of the reaction mixture peaked at 405 °F at an oil temperature of 465 °F.

Evaporative cooling immediately followed the first endotherm. Water loss was evidenced by the evolution of steam. The reaction mixture temperature dropped to a low of 390 °F during this period, and the oil temperature rose to 490 °F. At 1.25 hours, a second endotherm occurred. At this second endotherm, the reaction mixture temperature was 438 °F and the oil temperature was 495 °F.

Evaporative cooling continued to take place until the conclusion of the second endotherm. Water loss was evidenced by the evolution of steam. The reaction mixture temperature dropped to a low of 432 ^βF during this period and the oil temperature rose to 599 ^βF.

A diminution in evaporative cooling was evidenced by a steady rise in reaction mixture temperature between approximately 2.65 hours and 3.17 hours. At 3.17 hours a temperature plateau was attained. No further increase in conversion was noted beyond that point.

Table 2 below provides data developed during this experiment. Samples were taken at the times indicated and analyzed for percent conversion to polysuccinimide.

Example 3

A "time vs. temperature" plot of the following reaction is depicted in Figure 3.

A 500 ml covered, stainless steel, beaker charged with 400 grams of powdered, L-aspartic acid was placed in an oil bath. The oil bath was quickly heated to a 550 °F maintenance temperature. The sample was stirred throughout the experiment.

At 24 minutes, the reaction began when the first endotherm was reached. The first endotherm of the reaction mixture peaked at 410 °F at an oil temperature of 470 °F.

Evaporative cooling immediately followed the first endotherm. Water loss was evidenced by the evolution of steam. The reaction mixture temperature dropped to a low of 395 °F during this period.

A second endotherm occurred at 1 hour at a reaction mixture temperature of 442 °F.

Evaporative cooling continued to take place until the conclusion of the second endotherm. The reaction mixture temperature dropped to a low of 440 °F during this period.

A diminution in evaporative cooling was evidenced by a steady rise in reaction mixture temperature between approximately 1.5 hours and 2.06 hours. At 2.06 hours a temperature plateau was attained. No further increase in percent conversion was noted beyond 1.95 hours. Table 3 below provides data developed during this experiment. Samples were taken at the times indicated and analyzed for percent conversion to polysuccinimide.

Production scale product runs were conducted as follows:

Pilot Plant Test Run

A "time vs. temperature" plot of the following reaction is depicted in Figure 4.

A DVT-130 drier, mixer manufactured by the Littleford Brothers, Inc., of Florence, Kentucky was used. The jacketed drier utilizes a thermal fluid (hereinafter called "oil"), a plough blade impeller, a stack open to the atmosphere; and has a heat transfer area of 10 ft². The reactor's oil reservoir was preheated to 550 °F.

The reactor was charged with 110.4 lb of powdered, L-aspartic acid. Hot oil began to flow through the jacket, and the impeller speed was set at 155 rpm. Both the product and oil temperatures rose steadily. At a product temperature of 390 °F, there was a sudden, endothermic reaction which caused the product temperature to drop (see Fig. 4). Water loss was evidenced by the evolution of steam. A sample taken revealed that the powder had changed from white to pink. Three percent of the material was converted to polysuccinimide.

Thereafter, product temperature began to rise steadily until it reached a plateau at 428 °F which continued for an hour. Throughout this whole reaction, steam evolved, and the conversion increased in a linear fashion. At the end of the hour, the product temperature rose to 447 °F at which time the reaction underwent a second endotherm. Immediately after this endotherm, steam ceased to evolve. Shortly after this point, the reaction was at least 88% complete. Following the second endotherm, the product slowly changed from a pink to a yellow color. The final conversion was measured at 97%.

Table 4 below provides data developed during this experiment. Samples were taken at the times indicated and analyzed for percent conversion to polysuccinimide.

The above data and procedures may be summarized as follows: Polysuccinimides may be produced using the steps of a) . heating powdered L-aspartic acid to at least 370 ^βF to initiate the condensation reaction, then b). raising the reaction mixture temperature to at least 420 °F, and c) . maintaining at least the 420 °F temperature until at least 80% conversion has occurred.

When the reaction mixture temperature is raised to at least 430 °F for a sufficient period of time a 90 % conversion can be achieved.

When the reaction mixture temperature is raised to at least 440 °F for a sufficient period of time a 95% conversion can be achieved.

THE POLYASPARTIC ACID

Polyaspartic acid is produced from polysuccinimide using the following hydrolysis procedure: A slurry is made from a measured amount of polysuccinimide and distilled water. Sodium hydroxide is added dropwise to hydrolyze polysuccinimide to polyaspartic acid. The completion of the hydrolysis is attained at pH 9.5.

Bases other than sodium hydroxide can be used. Suitable bases include ammonium hydroxide, potassium hydroxide, and other alkaline and alkaline earth hydroxides.

Generally, base should be added to the slurry until the pH has been raised to 9.5, and a clear solution has been formed.

The pH may be adjusted to higher levels. Between pHs ranging between 11 and 12, the polaspartic acid solutions have a bright yellow color. These higher pH solutions are useful when compatibility with higher pH slurries is required.

Polyaspartic acids are made up of alpha and beta peptide bonds. The polyaspartic acids used as dispersants to practice this invention contain between 50% to about 75% of beta peptide groups. The preferred dispersants usually contain 60% to 75% of beta peptide bonds.

The term polyaspartic acid used herein and in the claims means the salts of polyaspartic acid. Counterions for polyaspartate include, but are not limited to, the alkaline and alkaline earth cations, some examples of which are Na⁺, K⁺, Mg⁺, and Li⁺, Ca⁺⁺, Zn⁺⁺, Ba⁺⁺, Co⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, and NH4⁺. The free acid is very water soluble therefore making it of extended applicability as a dispersant. MOLECULAR WEIGHT DETERMINATION The polyaspartic acid dispersant of this invention has a weight average molecular weight of 1000 to 5000.

Gel permeation chromatography was utilized to determine the molecular weights of the polyaspartic acid produced. The molecular weight determinations were made on the polysuccinimide that was hydrolyzed using the hydrolysis procedure described herein.

Rohm & Haas 2000 Mw polyacrylic acid and Rohm & Haas 4500 Mw polyacrylic acid were utilized as standards. The molecular weights provided for the polyaspartic acids as described herein and in the claims are based on these standards unless otherwise noted, and are reported as weight average molecular weights,(Mw) . This is because molecular weights based on gel permeation chromatography can vary with the standards utilized.

It was found that the molecular weight for the polyaspartic acid produced fell within the range of 1000 Mw to 5000 Mw, regardless of percent conversion.

SUSPENDED SOLIDS The suspended solids capable of being effectively suspended b y the polyaspartic acid salts include a wide variety of both inorganic and organic particles. 1. THE INORGANIC PARTICLES

One group of inorganic particles most effectively treated may be generically described as the alumino-silicates which encompass a wide number of clays. The alumino-silicates also include a large number of inorganic ion exchange materials illustrated by the base exchange clays and the synthetic zeolites illustrated by the molecular seives. It is obvious to those skilled in the art that certain of the alumino-silicates described above contain elements other than aluminum, silicon, and oxygen. When such additional elements are present for instance, magnesium, the solids are considered to be alumino-silicates.

A particlularly broad class of inorganic particles capable of being suspended by the polyaspartic acid salts may be generically described as pigments. Illustrative of such materials are the finely divided particles calcium carbonate, titania, and silica. These materials find use in the form of aqueous suspension in the manufacture of paints, paper, ceramic slurries and many other well known commercial products.

The invention is particularly useful in its ability to produce stabilized iron oxide suspensions.

2. ORGANIC PARTICLES

These particles include a wide variety of organic materials illustrated by such materials as dirt, which includes silt. Other such organic materials are carbon particles and a variety of finely divided water insoluble polymers which are often found in coating compositions in the form of latexes. Illustrative of such latexes would be polystyrene, polyvinylchloride, polyacrylonitrile, synthetic rubbers, e.g., polybutadienes and the like. A particlularly useful application for the suspending agents of the invention is their use in the suspension polymerization of a variety of water insoluble polymers.

PARTICLE SIZE OF THE SUSPENDED SOLIDS The suspended solids that may be suspended using the polyaspartic acid salts described herein will vary between as little as 0.01 micron upto particles as large as about 1 centimeter. Typical particle sizes of this suspended solids will be in the range of 50-500 microns. In describing particle sizes it is understood that they are described with respect to the average particle size of the particular particles present in a given suspension.

DOSAGE The amount of water soluble salt of the poly aspartic acid used to suspend a variety of solids in water may range between 0.5-200 ppm. A typical dosage to suspend clays, iron oxides, dirt and the like is within the range of 1-50 ppm. The optimum dosage will depend upon the particular polyaspartic acid salt used, The ph of aqueous suspension and the nature of the particles with respect to their composition and size.

EXAMPLES Kaolin Dispersion 1 g/L of kaolin was mixed with CaCl-,.2H₂0. The pH of the slurry was adjusted and poured into 100 ml graduated cylinders. Known concentrations of dispersants were then added to the cylinders and thoroughly mixed.

Aliquots were ta'ien from the graduated cylinders at 2 and 24 hours and tested for dispersion by nephelometric method. Higher NTU values correspond to superior kaolin dispersion by the polymers.

The samples were also scanned with UV/VIS from 900 nm to 200 nm and the absorbance recorded at 450 nm.

The following results were obtained for NTU and absorbance, respectively:

Table 5

Polyaspartic Ac

10 50

150 150

45 65

0.95 0.98

ABS (24 hours) 0.03 0.12 0.16

Kaolin Dispersion with Fe³⁺ This assay follows the same procedures as the kaolin dispersion test except 2.5 ppm ferric chloride are added to each graduated cylinder. The following data compares polyaspartic acid with polyacrylic acid. The results are as follows :

Table 6

Control Polyaspartic Acid Polyacrylic Acid ppm 0 10 50 100 10 50 100 NTU (2 hours) 36 150 120 150 150 140 140 ABS (2 hours) O.ll 0.89 0.38 0.92 0.66 0.48 0.47

Ferric Oxide Dispersion 700 ppm Fe as Fe₂0₃ were mixed with 200 ppm Ca²* as CaC0₃. The pH of the slurry was adjusted to a fixed value. The solution was thoroughly mixed. The slurry was transferred to 100 ml graduated cylinders and known concentrations of dispersants were added.

Samples were taken at 4 hours and turbidity was measured by nephelometric method. Higher NTU values correspond to superior ferric oxide dispersion by the polymers. The results are as follows: Table 7 Control Polyaspartic Acid Polyacrylic Acid ppm 0 1 2 100 1 3_ 100

NTU (4 hours) 181 233 222 284 196 209 210

Calcium Carbonate Dispersion Freshly prepared, precipitated CaC0₃ was added to 500 ppm of Ca⁺⁺ as CaC0₃ and 250 ppm of Mg"^-1" as MgCl₅.6H;,0 solutions and were thoroughly mixed. The pH of the solution was adjusted and poured into 100 ml graduated cylinders. Finally, 7 ppm of dispersant were added to each and allowed to stand for 30 minutes and 1 hour.

Supernatant taken from each cylinder was tested for turbidity. Higher NTU values correspond to superior calcium carbonate dispersion by the polymers.

The results are as follows:

Table 8 Control Polyaspartic Acid Polyacrylic Acid NTU(30 minutes) 43.5 71.5 68.3

NTU(1 hour) 24.3 30.8 29.3

Titanium Dioxide Dispersion lg/L of titanium dioxide was mixed with 200 ppm Ca²⁺ as CaC0₃ and adjusted to a fixed pH. The solution was thoroughly stirred and poured into 100 ml graduated cylinders. Known concentrations of inhibitor were added.

Samples were taken at 2 and 24 hours and turbidity was measured. Higher NTU values correspond to superior dispersion by the polymers.

The results are as follows:

Table 9

Control Polyaspartic Acid Polyacrylic Acid ppm 0. 1 10 100 1 10 100 NTU(2 hours) 282 320 412 459 331 480 594 NTU(24 hours) 21 22 36 49 23 41 44

Zinc Hydroxide Dispersion 250 ppm of Ca²⁺ as CaC0₃ and 125 ppm Mg²⁺ as CaC0₃ were made into a solution. An inhibitor was added at this time prior to the addition of O.Olg/L of zinc chloride. Equal amount of sodium hydroxide was added for each test. White precipitate of zinc hydroxide was evident. The ability of the dispersant was tested using nephelometric turbidity. The higher the NTU, the better the dispersant. Here were the results of each testing.

Table 10

Control Polyaspartic Acid Polyacrylic Acid ppm 0_. 5 10 50 5 50 NTU 27 38 39 155 32 37 195

It is hypothecated that aqueous suspensions of other solids could be suspended effectively using polyaspartic acid salts as the suspending agent shows the results that probably would be achieved.

Table 11

The polyaspartic acid used in all the above examples corresponds to the polyaspartic acid produced in the pilot plant test run.

Claims

1. An aqueous suspension of finely divided solid particles which are maintained in suspension by a suspension stabilizing amount of water soluble polyaspartic acid salt having a molecular weight ranging between 1000 to 5000.

2. The aqueous suspension of finely divided solid particles of Claim 1 where the particles are inorganic particles.

3. The aqueous suspension of finely divided solid particles of Claim 2 where the inorganic particles are alumino-silicates.

4. The aqueous suspension of finely divided solid particles of Claim 3 where the alumino-silicate is a clay.

5. The aqueous suspension of finely divided solid particles of Claim 4 where the clay is kaolin.

6. The aqueous suspension of finely divided solid particles of Claim 3 where the alumino-silicate is a zeolite.

7. The aqueous suspension of finely divided solid particles of Claim 2 where the particle is a pigment.

8. The aqueous suspension of finely divided solid particles of Claim 7 where the pigment is calcium carbonate.

9. The aqueous suspension of finely divided solid particles of Claim 7 where the pigment is titanium dioxide.

10. The aqueous suspension of finely divided solid particles of Claim 7 where the pigment is silica.

11. The aqueous suspension of finely divided solid particles of Claim 2 where the inorganic particle is iron oxide.

12. The aqueous suspension of finely divided solid particles of Claim 1 where the finely divided solid particle is an organic particle.

13. The aqueous suspension of finely divided solid particles of Claim 12 where the organic particle is carbon.

14. The aqueous suspension of finely divided solid particles of Claim 12 where the organic particle is dirt.

15. The aqueous suspension of finely divided solid particles of Claim 12 where the organic particle is a water insoluble polymer.