Method of dispersing inorganic materials in aqueous systems with low molecular weight acrylic acid copolymers

BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to the use of water soluble, low molecular weight copolymers of acrylic acid to disperse inorganic particulate matter and to inhibit the precipitation of common hardness ion salts in aqueous systems. More particularly, the invention relates to the use of certain copolymers, having a weight average molecular weight of from about 2,000 to about 5,000 formed from about 85 weight percent to about 97 weight percent acrylic acid monomer and from about 3 weight percent to about 15 weight percent of a hydrophobic monomer, to disperse suspended inorganic particulate matter and to inhibit the precipitation of hardness ion salts in aqueous systems.
Much of the water used in recirculating cooling water systems and water cooled industrial heat exchangers supplied from rivers, lakes, ponds and the like contains high concentrations of suspended inorganic particulate materials such as mud, silt and clay. In cooling tower systems, the cooling effect is achieved by evaporating a portion of the water circulating through the tower. This evaporating results in the concentration of the suspended materials in the water. These materials settle in locations of low flow rates and cause corrosion, frictional losses and inefficient heat transfer.
Water used in cooling systems and water-cooled heat exchangers also contains dissolved salts of hardness ions, such as calcium and magnesium. These salts can precipitate from solution and lead to the formation of scale on the heating surfaces of the cooling systems or exchangers. The most common deposit found in cooling water systems is calcium carbonate. Calcium carbonate has a relatively low solubility in water. This solubility decreases with increasing pH, temperature and concentration. Calcium sulfate is also likely to be encountered in cooling water systems. Calcium sulfate may result from the addition of sulfuric acid to raw water to control calcium carbonate scale. While calcium sulfate is more soluble than calcium carbonate, the solubility limit of calcium sulfate can also be easily exceeded resulting in its precipitation from solution.
High molecular weight polyacrylates and polyacrylamides have been used to agglomerate fine particles of mud and silt into a loose floc to reduce the accumulation of these materials in pipes and heat exchanger tubes. However, these flocs tend to settle in cooling tower basins which then require frequent cleaning for their removal.
Many other compounds have been developed and are being used with varying degrees of success to disperse suspended particulate matter and to inhibit the precipitation of hardness ion salts and the resultant formation of scale in aqueous systems.
PRIOR ART
U.S. Pat. No. 2,783,200 is directed to a process for conditioning sludge or precipitates in boiler feed water by adding polyacrylic acid or polymethacrylic acid with, and without, inorganic phosphates. Copolymers of sodium polyacrylate and polymethacrylate, having undisclosed compositions and high molecular weights, are disclosed as being as effective as sodium polyacrylate homopolymer to treat internal boiler water.
U.S. Pat. No. 3,085,916 relates to a method for removing mud and silt from cooling water by using high molecular weight (greater than 100,000) polymers of acrylic acid or methacrylic acid, their salts, or copolymers formed from at least 50 mole percent acrylic or methacrylic acid with other copolymerizable monomers. High, rather than low, molecular weight copolymers are taught as being generally more effective as mud and silt flocculants. The patent is silent, however, on the use of low molecular weight acrylic polymers or copolymers as dispersants for mud and silt in aqueous systems.
U.S. Pat. No. 3,578,589 is directed to a method for treating cooling water deposits using a nonionic surfactant and acrylic acid or methacrylic acid polymers, salts thereof and copolymers of such salts. The molecular weight of the copolymers is disclosed as ranging from about 4,000 to about 95,000 while the preferred polymethacrylic acid or sodium polymethacrylate is disclosed as having a molecular weight ranging from 5,000 to 15,000.
U.S. Pat. No. 4,008,164 relates to inhibiting calcium deposits in water using copolymers of acrylic acid and low levels of methylacrylate where the copolymer has a molecular weight ranging from 3,000 to 20,000 and preferably 6,000 to 8,000. A molar excess of acrylic acid to methylacrylate of at least 3:1 and preferably 4:1 to 5:1 is required. The preferred copolymer has a molecular weight ranging from 6,000 to 8,000.
Canadian Pat. No. 1,097,555 also relates to a process for inhibiting calcium sulfate, calcium carbonate and barium sulfate scale by the addition of copolymers of acrylic acid having molecular weights ranging from 1,000 to 25,000 and preferably 6,000 to 8,000. A molar excess of at least 3:1 and preferably 4:1 to 5:1 acrylic acid to methacrylic acid is disclosed.
U.S. Pat. No. 4,029,577 is directed to a process for controlling the formation of scale and/or suspended solid matter in aqueous systems by introducing copolymers of acrylic acid or a water-soluble salt thereof, and hydroxy lower alkyl (C.sub.2 -C.sub.6) acrylate moieties, or water soluble salts thereof, in a molar ratio of about 34:1 to about 1:4. The copolymers disclosed have molecular weights ranging from 500 to 1,000,000.
Other references relating to methods of treating water with polyacrylic acid-containing additives include: U.S. Pat. Nos. 3,293,152; 3,579,455; 3,663,448; 3,699,048; 3,766,077; 4,004,939; 4,209,398; 4,303,568; 4,326,980; Canadiant Pat. No. 1,057,943; and German Offeng. No. 2,344,498.
None of the above references disclose or suggest that low molecular weight, less than about 5,000 weight average molecular weight, copolymers of acrylic acid and certain hydrophobic comonomers can effectively disperse inorganic particulate matter or inhibit the precipitation of common hardness ion salts in aqueous systems.
SUMMARY OF THE INVENTION
We have unexpectedly found that inorganic particulate matter can be effectively dispersed in aqueous systems by the introduction of water soluble, copolymers of acrylic acid and selected hydrophobic comonomers where the molecular weight of the copolymer ranges from about 2,000 to about 5,000 weight average molecular weight.
In addition, we have found that low molecular weight copolymers formed from about 85 weight percent to about 97 weight percent acrylic acid and from about 3 weight percent to about 15 weight percent of a hydrophobic comonomer, such as a lower alkyl (C.sub.2 -C.sub.8) acrylate or (C.sub.1 -C.sub.8) methacrylate or a vinyl aromatic compound, are superior dispersants for inorganic particulates and effective anti-precipitant scale inhibitors for common hardness ion salts in aqueous systems. Products available heretofore possessing high anti-precipitant activity have generally been deficient in their ability to disperse particulate matter.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 presents a graphical illustration of the kaolin dispersancy of the acrylic acid-hydrophobic copolymers as a function of their weight average molecular weight.
FIG. 2 presents a graphical illustration of the kaolin dispersancy of acrylic acid-ethyl acrylate copolymers, having about one unit of ethyl acrylate per chain of acrylic acid, as a function of their weight average molecular weight.

DETAILED DESCRIPTION OF THE INVENTION
The method of the present invention requires the addition of low molecular weight, water soluble, copolymers of acrylic acid and certain hydrophobic comonomers to aqueous systems containing suspended inorganic particulate materials and/or common hardness ion salts. The hydrophobic comonomers found to be useful in the preparation of the copolymers of this method include lower alkyl acrylates, having 2 to 8 carbon atoms in the alkyl group, and methacrylates, having 1 to 8 carbon atoms in the alkyl group, such as ethyl and butyl acrylate, methyl methacrylate and ethyl methacrylate, and vinyl aromatic monomers such as styrene and alkyl substituted styrene, as well as copolymers formed from mixtures of these hydrophobic comonomers with acrylic acid. These comonomers must be copolymerizable with and soluble in acrylic acid monomer or a mutual solvent.
COPOLYMER SYNTHESIS
In general, the prior art discloses several suitable synthesis methods for preparing low molecular weight copolymers of acrylic acid.
U.S. Pat. No. 4,314,004 is directed to one suitable copolymer synthesis method and the disclosure thereof is incorporated herein by reference. This method requires a specific concentration range of a copolymerization initiator and a specific molar ratio range of the initiator concentration and the concentration of certain metal salts to obtain the desired low molecular weight copolymers useful in the present invention. The preferred copolymerization initiators are peroxide compounds such as ammonium persulfate, potassium persulfate, hydrogen peroxide and t-butyl hydroperoxide. The preferred concentration range of the initiator is between about 1 to about 20 weight percent based on the weight of monomers. The metal salts used to regulate molecular weight preferably include cuprous and cupric chloride or bromide, cupric sulfate, cupric acetate, ferrous and ferric chloride, ferrous sulfate and ferric and ferrous phosphate. The molar ratio of the copolymerization initiator to the metal salt is preferably between about 40:1 to about 80:1. The copolymers of acrylic acid useful in this invention are preferably prepared in water at a copolymer concentration of about 40 to about 50 percent based on total weight of solution.
Another method useful to prepare these low molecular weight copolymers is described in U.S. Pat. No. 4,301,266, the disclosure thereof also being incorporated herein by reference. In this process isopropanol is used as the molecular weight regulator as well as the reaction solvent. The reaction solvent may also be an aqueous mixture of isopropanol containing at least 40 weight percent isopropanol. The copolymerization initiator is a free radical initiator such as hydrogen peroxide, sodium persulfate, potassium persulfate, or benzoyl peroxide. The copolymerization is carried out under pressure at a temperature of 120.degree. to 200.degree. C. The concentration of the copolymer in the solvent is preferably 25 to 45 percent based on the weight of the total solution. When copolymerization is complete, the isopropanol is distilled from the reactor and the copolymer may be neutralized with a base.
Still another method for preparing low molecular weight copolymers useful in this invention is described in U.S. Pat. No. 3,646,099, the disclosure thereof also being incorporated herein by reference. This process is directed to the preparation of cyano-containing oligomers; however, it is also applicable for preparing low molecular weight copolymers useful in the present invention. This process employs a bisulfite salt as the copolymerization molecular weight regulator and the resulting copolymers prepared thereby are sulfonate terminated. The preferred bisulfite salt is sodium bisulfite at a concentration of between 5 and 20 weight percent based on the weight of monomers. The free radical copolymerization initiator is ammonium, sodium or potassium persulfate, hydrogen peroxide or t-butyl hydroperoxide. The preferred concentration of the initiator is between 0.2 and 10 weight percent based on monomers. The polymerization temperature is preferably between 20.degree. and 65.degree. C. and the concentration of the copolymers in the aqueous solvent is between 25 and 55 weight percent based on total solution weight.
EVALUATION OF COPOLYMERS
A number of copolymers were prepared from about 80 weight percent to about 99 weight percent acrylic acid monomer and from about one weight percent to about 20 weight percent hydrophobic comonomer. The weight average molecular weight of the copolymers was varied from about 1,350 to about 8,000 based on polyacrylic acid polymer standards as determined by standard aqueous gel permeation chromatographic techniques. For each copolymer type prepared, the number of comonomer units per acrylic acid polymer chain was calculated. As the weight average molecular weight of the copolymer is decreased, the number of polymer chains per gram of polymer solids increases.
TABLE I______________________________________COMPOSITION OF ACRYLICACID/HYDROPHOBE COPOLYMERS Hydrophobic Comonomer Average Wt. % No. Per CopolymerExample AA.sup.1 Type.sup.1 Wt. % Chain M-w.sup.2______________________________________1 98.1 EA 1.3 0.2 31102 97.6 EA 2.4 0.3 16303 96.0 EA 4.0 0.8 30104 92.8 EA 7.2 0.8 15705 96.0 EA 4.0 1.0 48606 95.3 EA 4.7 1.0 36107 94.7 EA 5.3 1.0 29108 92.8 EA 7.2 1.0 29809 92.2 EA 7.8 1.0 204010 87.7 EA 12.3 1.0 136011 85.0 EA 15.0 1.0 143012 90.6 EA 9.4 2.0 341013 89.4 EA 10.6 2.0 301014 85.6 EA 14.4 2.0 265015 98.7 BA 1.3 0.14 218016 98.2 BA 1.8 0.3 482017 96.7 BA 3.3 0.3 244018 94.9 BA 5.1 1.0 734019 90.8 BA 9.2 1.0 267020 88.5 BA 11.5 1.25 294021 98.7 MMA 1.3 0.14 204022 98.2 MMA 1.8 0.4 528023 91.8 EMA 8.2 1.0 289024 92.5 S 7.5 1.0 5140______________________________________ .sup.1 AA = acrylic acid; EA = ethyl acrylate; BA = butyl acrylate; MMA = methyl methacrylate; EMA = ethyl methacrylate; S = styrene .sup.2 M-w means weight average molecular weight as determined based on polyacrylic acid standards using standard gel permeation chromotographic techniques.
The copolymers were then evaluated for their ability to disperse a common inorganic particulate material according to the following test method.
KAOLIN DISPERANCY SCREENING TEST
To a blender was added 500 ml. of 200 ppm Ca.sup.+2 (as calcium carbonate) hard water and 0.50 grams of Ansilex.RTM. X-1846A kaolin, manufactured by Engelhard Minerals and Chemicals, Inc. (93% having a particle diameter less than 2 microns). The copolymer was then added to the blender. The concentration of the copolymers utilized was 1, 5, 10, and 30 ppm. The blender was then operated at low speed for one minute. The pH was then adjusted to 7.5 with sodium hydroxide. The dispersion was then poured into a 100 ml. graduated cylinder and was allowed to stand undisturbed for 2 hours. 20 ml. of the dispersion was then removed from the top of the cylinder and placed in a HF model DRT 100D turbidimeter. Turbidity was then determined in nephelometric turbidity units (NTU). High NTU's, on the order of about 700, indicate good dispersancy. The turbidity of each sample was determined, and repeated at least twice, using a fresh test sample for each repeat test. The average NTU values for each Example 1-24 are presented in Table II. The dispersancy of the copolymers as a function of weight average molecular weight is illustrated in FIG. 1. The results show that at a one ppm. copolymer concentration dispersancy drops off sharply at below 2,500 weight average molecular weight (Mw). At 10 ppm copolymer concentration, a gradual drop off in performance occurs as the Mw is increased above 3,000.
TABLE II__________________________________________________________________________KAOLIN DISPERSANCY SCREENINGOF AA.sup.1 HYDROPHOBE COPOLYMERS Hydrophobic Avg. KaolinMonomer Copolymer Dispersancy NTUExample Type Wt. % M-w.sup.2 1 ppm 10 ppm__________________________________________________________________________Control No polymer 145 --1 EA.sup.1 1.3 3110 558 6662 EA 2.4 1630 489 7413 EA 4.0 3010 680 7634 EA 7.2 1570 561 7855 EA 4.0 4860 704 6216 EA 4.7 3610 664 6887 EA 5.3 2910 716 7458 EA 7.2 2980 652 7539 EA 7.8 2040 478 70810 EA 12.3 1360 294 73311 EA 15.0 1430 474 78712 EA 9.4 3410 709 71413 EA 10.6 3010 639 72114 EA 14.4 2650 736 77615 BA.sup.1 1.3 2180 -- 758, 220.sup.316 BA 1.8 4820 730 64817 BA 3.3 2440 566 77118 BA 5.1 7340 724 63319 BA 9.2 2670 720 81220 BA 11.5 2940 697 74921 MMA.sup.1 1.3 2040 -- 48222 MMA 1.8 5280 -- 58123 EMA.sup.1 8.2 2890 730 74924 S.sup.1 7.5 5140 743 730__________________________________________________________________________ .sup.1 AA = acrylic acid; EA = ethyl acrylate; BA = butyl acrylate; MMA = methylmethacrylate; EMA = ethyl methacrylate; and S = styrene .sup.2 M-w = Weight average molecular weight, polyacrylic acid as standar .sup.3 220 inaccurate test result. Repetition showed 758 NTU.
FIG. 2 illustrates the dispersancy of the acrylic acid/ethyl acrylate copolymers (Examples 5-11) formed from one unit of ethyl acrylate per chain of acrylic acid. This Figure shows the same drop off in dispersancy at one ppm. copolymer concentration with copolymers having Mw below about 2,500. The composite of dispersancy for the 1 ppm. and 10 ppm. copolymer concentrations shows that a maximum dispersancy occurs at about 3,000 Mw which corresponds to the preferred copolymer composition of about 95 weight percent acrylic acid and about 5 weight percent ethyl acrylate. It is believed that the incorporation of about one unit of hydrophobic comonomer per chain of acrylic acid results in a copolymer having preferred performance as compared to other ratios of hydrophobic comonomer units per chain.
This test method was repeated utilizing various conventional polymers and copolymers for comparison. The compositions of these copolymers and their kaolin dispersancy is presented in Table III. Full dispersancy (approximately 750 NTU) is capable of being achieved with low molecular weight (1000 Mw) polyacrylic acid (Example 26) at concentrations of 10 ppm. and above. However, dispersancy drops off dramatically at low concentration (1 ppm of the homopolymer). Higher molecular weight (Mw 2,200-4,700) polyacrylic polymers (Examples 32, 33, 34, 36, and 37) prepared using a peroxy type initiator and isopropanol gave good dispersancy at both high and low concentrations. A copolymer of polyacrylic acid and hydroxypropyl acrylate (Example 39) (Mw 3,200) also gave good dispersancy at both high and low concentrations.
TABLE III______________________________________KAOLIN DISPERSANCY SCREENINGOF CONVENTIONAL ADDITIVESEx- Copolymer Kaolin Dispersancy, NTUam- Composition 10 30ple Monomers Wt. % M-w 1 ppm 5 ppm ppm ppm______________________________________25 Acrylic Acid 100 600 -- -- 422 --26 Acrylic Acid 100 1120 254 597 759 76727 Acrylic Acid 100 2200 616 640 572 --28 Acrylic Acid 100 5140 624 419 323 26029 Acrylic Acid 100 3000 553 280 250 --30 AMP.sup.1 100 -- 200 -- 236 --31 HEDP.sup.2 100 -- 185 -- 131 --32 Acrylic Acid.sup.3 100 2200 682 771 819 80733 Acrylic Acid.sup.3 100 4700 742 751 764 --34 Acrylic Acid.sup.3 100 4500 762 -- 751 --35 Acrylic Acid.sup.4 100 4300 702 -- 494 --36 Acrylic Acid 100 -- 565 -- 781 --37 Acrylic Acid.sup.3 100 2100 -- -- 788 --38 Acrylic Acid/ 92/8 2300 592 -- 744 -- Arylamide39 Acrylic Acid/ 62/68 3200 775 -- 799 -- HPA.sup.540 Na.sup.+ Styrene 80/20 -- 578 -- 775 -- Sulfonate/ Na.sup.+ Maleate______________________________________ .sup.1 AMP -- aminotrismethylene phosphonic acid .sup.2 HEDP -- 1hydroxyethylidene-1,1-diphosphonic acid .sup.3 Polymerized using peroxytype initiator and isopropanol as chain transfer agent and solvent .sup.4 Polymerized using sodium hypophosphite/H.sub.2 O.sub. 2 /TEA/Cu/Fe initiator system .sup.5 HPA -- hydroxypropyl acrylate
The results of the kaolin screening tests, described above, were then compared with another kaolin dispersancy test as described in U.S. Pat. No. 4,326,980 for the preferred acrylic acid/ethyl acrylate copolymer (Example 7) and the comparative polyacrylic homopolymers of Examples 27 and 33. This test procedure was performed as follows: To a blender was added 500 ml. of 200 ppm. Ca.sup.+2 (as calcium carbonate) solution and 0.5 Hydrite.RTM. UF kaolin. The solution was blended for five minutes. The pH was then adjusted to 7.5 with sodium hydroxide while the solution was constantly stirred. The solution was then blended again for one minute. A 90 ml. aliquot of the solution was placed in a 4 oz. bottle and diluted to 100 ml. with deionized water. After two 4 oz. bottles were prepared, the remaining solution was re-blended for less than 1 minute. The sample polymers were then added to the bottle at concentrations of 0.5, 1, 2, and 3 ppm. The bottle was then capped, inverted about 5 times and placed on a shaker for 15 minutes. The bottle was then allowed to stand for 18 hours. The top 25 ml. of the bottle was pipetted into a 1 oz. vial and the percent transmittance of light was measured at 415 nm. The .DELTA. percent T is calculated as follows: ##EQU1## The results of this test are presented on Table IV.
TABLE IV______________________________________KAOLIN DISPERSANT ACTIVITY .DELTA. % T Polymer Concentration, ppmExample 5 10 20______________________________________7 20 38 4527 13 31 4133 19 37 47______________________________________
The results at 10 ppm. confirm the screening test results presented in Tables II and III. The preferred copolymer of this invention (Example 7) was found to be equivalent to the high molecular weight homopolyer (Example 33) and much better than the low molecular weight homopolymer (Example 27) at 10 ppm. At 5 ppm. and 20 ppm., the copolymer of Example 7 was also better than the low molecular weight homopolymer (Example 27) and about equivalent to the higher molecular weight homopolymer (Example 33).
The anti-precipitation activity of the preferred acrylic acid-ethyl acrylate copolymer for common hardness ion salts was also evaluated and compared with an effective dispersant high Mw homopolyacrylic acid polymer (Example 33) and the best dispersant low Mw polyacrylic acid (Example 27). The test methods used to determine calcium sulfate and calcium carbonate precipitation inhibition are also disclosed in U.S. Pat. No. 4,326,980.
CALCIUM SULFATE ANTIPRECIPITATION TEST
Two stock solutions (A and B) were prepared. Stock Solution A contained 11.1 grams/liter calcium chloride adjusted to pH 7.0. Stock Solution B contained 14.2 grams/liter sodium sulfate adjusted to pH 7.0.
To a 4 oz. jar was added 50 ml. stock solution A and either 0, 0.5, 1 or 2 ppm. of polymer and 50 ml. of stock Solution B. The jar was heated in an oven at 50.degree. C. for 24 hours and was then cooled for one-half hour. The cooled sample was then filtered through a 0.45 micron filter. Five ml. of the filtrate was diluted to 50 ml. with deionized water. Two drops of 50% NaOH was then added followed by a Ca.sup.+2 indicator. The solution was then titrated with EDTA to a purple-violet end point. The percent calcium sulfate inhibition was calculated as follows: ##EQU2##
CALCIUM CARBONATE ANTI-PRECIPITATION TEST
Two Stock Solutions (C and D) were prepared. Stock Solution C contained 2.45 g/l calcium chloride adjusted to pH 8.5. Stock Solution D contained 2.48 g/l Na.sub.2 CO.sub.3 adjusted to pH 8.5.
To a 4 oz. jar was added 50 ml of Stock Solution C, 0, 2.5, 5, or 10 ppm polymer, and 50 ml of Stock Solution D. The sample jar was preheated in a warm water bath at about 70.degree. C. for 5 minutes. The preheated sample was then heated at 70.degree. C. in an oven for 5 hours. The sample was then removed from the oven and cooled to room temperature. The cooled sample was then filtered through a 0.2 micron filter. To 25 ml of filtrate was added 4 ml of concentrated hydrochloric acid. The sample was then allowed to stand for at least 15 minutes. The sample was then diluted to 50 ml with deionized water and then 3 ml of 50% sodium hydroxide was added followed by Ca.sup.+2 indicator. The solution was then titrated with EDTA to a purple-violet end point and the calcium carbonate inhibition was calculated as follows: ##EQU3##
The results of the calcium sulfate and calcium carbonate precipitation inhibition tests are presented in Table V. The preferred acrylic acid-ethylacrylate copolymer performed better than both polyacrylic acid samples at low concentrations and equivalently at high concentrations for both calcium sulfate and calcium carbonate precipitation inhibition.
TABLE V______________________________________AVERAGE ANTIPRECIPITATION ACTIVITY,% INHIBITION % Calcium Sulfate % Calcium Carbonate Inhibition Inhibition Polymer concen- Polymer concen- tration, ppm tration, ppmPolymer 0.5 1.0 2.0 1.0 2.5 5.0______________________________________Example 7 45 71 96 70 72 83Example 27 39 -- 95 67 81 80Example 33 15 33 92 57 69 83______________________________________
Accordingly, the copolymers of this invention exhibit effective dispersancy for inorganic particulate matter and precipitation inhibition for common hardness ion salts in aqueous systems.
The anti-precipitation activity of a number of the other low molecular weight acrylic acid copolymers was also evaluated using the calcium sulfate test described above. The results of the test at 1 ppm copolymer solids concentration are presented in Table VI.
TABLE VI______________________________________CALCIUM SULFATE ANTI-PRECIPITATIONINHIBITION, % INHIBITIONHydrophobic Monomer % CaSO.sub.4 Average Copolymer InhibitionExample Type Number/Chain M-w 1 ppm______________________________________5 EA 1 4860 876 EA 1 3610 937 EA 1 2910 .sup. 97.sup.19 EA 1 2040 10010 EA 1 1360 6811 EA 1 1430 7514 EA 2 2650 10018 BA 1 7340 8219 BA 1 2670 9623 EMA 1 2890 9024 Styrene 1 5140 36______________________________________ .sup.1 Temperature lower than Example 7, Table V.
The test results demonstrate the drop-off in anti-precipitation activity of the acrylic acid copolymers having weight average molecular weights less than about 2500 and greater than about 5000.
I have also found that low molecular weight copolymers of acrylic acid and hydrophobic monomers are effective dispersants for concentrated slurries of calcium carbonate and kaolin clay. Kaolin clay and calcium carbonate are used as fillers in plastics, rubbers, and paper, as pigments for paint and rubber, and as paper coatings to provide brightness and gloss. In many applications, and in particular paper coating applications, kaolin clays and calcium carbonate are shipped, stored, and applied as high solids content aqueous slurries. The clay slurries typically contain about 60 to 70% clay solids. The particle size of the kaolin clays in the slurries is usually about 2 microns or less in diameter. The calcium carbonate slurries typically contain about 60 to 75% or more calcium carbonate solids. For handling purposes it is important that the slurry viscosity be as low as possible to permit pumping and spraying of the slurry and to permit the slurry to flow after extended storage.
The cost to manufacture and the concentration of the dispersant in the slurry should be as low as possible to effectively and economically reduce the viscosity of the slurry to manageable levels. The viscosity of the slurry without the addition of a dispersant would be in the excess of 50,000 centipoise. Low molecular weight polyacrylic acid polymers are effective as dispersants for such slurries. The effectiveness of the preferred acrylic acid-ethylacrylate copolymer formed from about 95 weight percent acrylic acid and about 5 weight percent ethyl acrylate, having a weight average molecular weight of about 3000, was evaluated and compared with conventional polyacrylic acid polymers as dispersants for concentrated slurries according to the following test procedures.
CALCIUM CARBONATE SLURRY DISPERSION TEST
Into a 1 liter stainless steel mixing cup was added 1.88 grams of 10% dispersant polymer or copolymer solution (pH approximately 7.0) and 123.12 grams of deionized water. 375 grams of Snowflake White calcium carbonate, manufactured by Thompson Weinman & Company, was added to the mixing cup to form a slurry. The slurry was then mixed for 15 minutes. A 450 gram aliquot of the slurry was removed from the cup and placed into a one-pint jar. The jar was capped and shaken gently until the slurry cooled to room temperature. The pH and viscosity of the slurry was then measured using a Brookfield RV viscometer at 20 rpm. Following this, the dispersant concentration was raised by 0.025 weight percent while keeping the calcium carbonate solids concentration at 75% by adding 0.84 grams of the 10% dispersant solution and 0.63 grams of calcium carbonate to the slurry in the pint jar. The new slurry was mixed for 2 minutes and the viscosity was measured. This was repeated until a 0.100 weight percent dispersant containing slurry was formed and measured. The results of this test are presented in Table VII.
TABLE VII______________________________________CALCIUM CARBONATE SLURRY DISPERSION TESTPolymer Concentration Brookfield Viscosity (cps) at 20 RPM(Wt. % Based on CaCO.sub.3) PAA.sup.1 PAA.sup.2 AA/EA.sup.3______________________________________0.050 5250 3045 36750.075 600 300 5830.100 390 230 430pH 9.2 9.22 9.10______________________________________ .sup.1 PAA is polyacrylic acid having M-w 3300 and M-n 1600. .sup.2 PAA is DISPEX N40 polyacrylic acid, manufactured by Allied Colloids, Ltd., having a M-w of 3300 and a M-n of 2200. .sup.3 AA/EA is a copolymer of AA (acrylic acid)/EA (ethyl acrylate) (95 weight percent AA, 5 weight percent EA), M-w of 3000 and M-n 1700.
The table illustrates that the preferred acrylic acid-ethyl acrylate copolymer is an effective dispersant for high solids content calcium carbonate slurries.
KAOLIN SLURRY DISPERSION TEST
Into a 1 liter stainless steel mixing cup was added 6.13 grams of a 10% solution of the dispersant polymer or copolymer (pH approximately 7.0), 4.66 grams of 20% Na.sub.2 CO.sub.3 solution and a 50/50 mixture of deionized and tap water to bring the aqueous solution to 210 grams. To the solution was added 490 grams of Astra Glaze.RTM. kaolin clay, manufactured by Georgia Kaolin Company, with low speed mixing, to form a slurry. The slurry was then mixed for 15 minutes at high speed. A 500 gram aliquot of the slurry was then removed from the mixing cup and placed into a pint jar. The jar was capped and shaken gently until the slurry cooled to room temperature. The pH and viscosity (Brookfield RV viscometer at 20 rpm) was then measured. Following this, the concentration of the dispersant was increased by 0.025 weight percent while keeping the concentration of the clay solids constant at 70% by adding 0.88 grams of the 10% dispersant solution and 2.05 grams of kaolin to the slurry in the pint jar. The new slurry was mixed for 2 minutes and the viscosity of the slurry was remeasured. This was repeated until the level of dispersant in the slurry was 0.200 weight percent based on kaolin. The results are provided in Table VIII.
TABLE VIII______________________________________KAOLIN SLURRY DISPERSION TESTPolymer Concentra-tion (Wt. % Based Brookfield Viscosity (cps) at 20 RPMon Kaolin) PAA.sup.1 PAA.sup.2 PAA.sup.3 PAA.sup.4 AA/EA.sup.5______________________________________0.125 738 580 430 476 2970.150 318 500 281 292 2760.175 310 370 284 281 2780.200 355 340 311 328 310______________________________________ .sup.1 PAA is polyacrylic acid polymer having M-w 4200, M-n 2600. .sup.2 PAA is polyacrylic acid polymer having M-w 3000, M-n 1600. .sup.3 PAA is DISPEX N40, polyacrylic acid polymer manufactured by Allied Colloids, Ltd. having M-w 3300, M-n 2200. .sup.4 PAA is Colloid 211 polyacrylic acid polymer manufactured by Colloids, Inc., having M-w 3300 and M-n 2200. .sup.5 AA/EA is a copolymer formed from 95 weight percent acrylic acid (AA) and 5 weight percent ethyl acrylate (EA) having M -w 3000 and M-n 1700.
Table VIII illustrates that the preferred acrylic acid-hydrophobic copolymer of this invention is an effective dispersant for concentrated aqueous kaolin slurries.
While the method of this invention for controlling scale formation, precipitation inhibition, and the dispersion of inorganic materials has been described and exemplified in detail herein, various modifications, alterations, and changes should become readily apparent to those skilled in this art without departing from the spirit and scope of the invention. These modifications may include the blending of the acrylic acid copolymers described herein with other conventional additives to achieve additional benefits. For example, the acrylic acid copolymers may be blended with sulfonated styrene-maleic anhydride copolymers for improved iron dispersion, organic phosphonates for scale control, defoamers, biodispersants, corrosion inhibitors, and the like.

Number	Name	Date
2783200	Crum et al.	Feb 1957
3699048	Krueger et al.	Oct 1972
3791978	Krueger et al.	Feb 1974
4008164	Watson et al.	Feb 1977
4186027	Bell et al.	Jan 1980
4301266	Muenster et al.	Nov 1981
4314004	Stoneberg	Feb 1982
4326980	Snyder et al.	Apr 1982

Number	Date	Country
53-2393	Jan 1978	JPX
1414964	Nov 1975	GBX
604866	Apr 1978	SUX

Method of dispersing inorganic materials in aqueous systems with low molecular weight acrylic acid copolymers

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