Acrylic acid polymers neutralized with sodium and calcium ions and having a narrow molecular weight distribution

Abstract

An acrylic acid polymer having a weight average molecular weight M.sub.w in the range from 3500 to 2000 g/mol and a polydispersity index M.sub.w/M.sub.n≤2.5, wherein 30% to 60% of the acid groups of the acrylic acid polymers have been neutralized with calcium ions, 30% to 70% of the acrylic acid polymers have been neutralized with sodium ions and 0% to 10% of the acid groups of the acrylic acid polymers have not been neutralized.

Claims

1. A process for preparing aqueous solutions of acrylic acid polymers comprising polymerizing acrylic acid in feed mode with a free-radical initiator in the presence of a chain transfer agent in water as solvent, which process comprises (i) initially charging water and optionally acrylic acid in acidic, unneutralized form, optionally one or more ethylenically unsaturated comonomer, optionally the chain transfer agent, and optionally initiator, (ii) adding acrylic acid, optionally one or more ethylenically unsaturated comonomers, aqueous free-radical initiator solution, and chain transfer agent, (iii) adding a base to the aqueous solution after termination of the acrylic acid feed, wherein, in step (iii), a base comprising sodium ions and a base comprising calcium ions are added in such amounts that 30% to 60% of the acid groups of the acrylic acid polymers are neutralized with calcium ions, 30% to 70% of the acrylic acid polymers are neutralized with sodium ions and 0% to 10% of the acid groups of the acrylic acid polymers are not neutralized, and wherein the chain transfer agent is hypophosphite.

2. The process according to claim 1, wherein, in step (iii), first the base comprising calcium ions and then the base comprising sodium ions is added.

3. The process according to claim 1, wherein the base comprising sodium ions is selected from the group consisting of sodium hydroxide and sodium carbonate.

4. The process according to claim 1, wherein the base comprising calcium ions is selected from the group consisting of calcium hydroxide and calcium carbonate.

5. An acrylic acid polymer obtained from the process according to claim 1.

6. A method of dispersing and grinding an aqueous dispersion of solids comprising adding a polymer according to claim 5 to the aqueous dispersion.

7. The method of claim 6 wherein the aqueous dispersion is CaCO.sub.3, kaolin, talc, TiO.sub.2, ZnO, ZrO.sub.2, Al.sub.2O.sub.3, or MgO.

8. A dispersant and grinding auxiliary for aqueous dispersions of solids comprising an acrylic acid polymer according to claim 5.

9. The dispersant and grinding auxiliary of claim 8 wherein the aqueous dispersions are dispersions of CaCO.sub.3, kaolin, talc, TiO.sub.2, ZnO, ZrO.sub.2, Al.sub.2O.sub.3, or MgO.

Description

EXAMPLES

Examples 1 to 8 and Comparative Examples C1 to C5

(1) The polymers used in the examples are polyacrylic acids which have been prepared by the feed method in water as solvent from acrylic acid with sodium persulfate as initiator and sodium hypophosphite as chain transfer agent. Subsequently, the acidic polyacrylates have been neutralized or partly neutralized with calcium hydroxide, magnesium hydroxide and/or sodium hydroxide.

(2) The neutralization is conducted in a coolable stirred tank. For this purpose, first the calculated amount of about 20% calcium hydroxide suspension or of a magnesium hydroxide suspension and then the calculated amount of about 50% sodium hydroxide solution is pumped into the aqueous acidic polyacrylic acid solution present in the stirred tank. The mixture is cooled during the operation in order to remove the heat of neutralization released. The cooling is continued until the evolution of heat has abated and a clear solution has formed. To establish the desired final concentration, water is added.

(3) The acrylic acid polymers of the invention were first neutralized with calcium hydroxide and then with sodium hydroxide.

(4) The neutralization level of the polyacrylic acids was determined by means of titration.

(5) The molecular weight and the number average M.sub.n and the weight average M.sub.w of the molecular weight distribution of the polymers were determined by means of gel permeation chromatography (GPC). The molecular weight distribution was determined by means of GPC on aqueous solutions of the acrylic acid polymers buffered to pH 7 using hydroxyethyl methacrylate copolymer network (HEMA) as stationary phase against sodium polyacrylate standards. The (partly) neutralized acrylic acid polymers prepared and used in the examples are shown in table 1.

(6) TABLE-US-00001 TABLE 1 Molecular PDI Active Solids Counterion Poly- weight (M.sub.w/ content content (equivalent %) mer (g/mol) M.sub.n) (wt %) (wt %) Ca Na Mg H 1 5000 3.2 42.0 45 — 96 — 4 2 5000 2.1 41.9 45 — 97 — 3 3 5000 2.2 47.1 45 — 55 — 45 4 5000 2.1 36.7 39 — 60 40 — 5 5000 2.1 38.1 40 — 37 60 3 6 5000 2.8 37.4 40 60 35 — 5 7 5000 2.0 36.1 39 60 37 — 3 8 5000 2.0 33.8 37 60 40 — 0 9 5000 2.1 36.2 39 40 58 — 2 10 5000 2.8 37.4 41 — 56 40 4 11 5000 2.8 37.5 39 — 36 60 4 12 5000 3.2 48 45 — 55 — 45 13 5000 2.1 36.8 39 50 46 — 4 14 5000 2.8 33.3 35 40 56 — 4

(7) The polyacrylate solutions were tested for their suitability as dispersants for production of slurries. For this purpose, grinding of calcium carbonate (Marple Flour, Imerys) was conducted in each case in a Dispermat. For this purpose, 300 g of calcium carbonate and 600 g of ceramic beads in each case were mixed and initially charged in 1 L jacketed tank. Subsequently, 100 g of a dilute aqueous solution of the polyacrylate to be tested were added. Grinding was effected using a grinding assembly of the Dispermat AE04-C1 type (from Getzmann GmbH) with a cross-beam stirrer at a speed of 1300 rpm. As soon as 73% of the pigment had a particle size (PSD) of less than 1 μm (Malvern Mastersizer 3000), the grinding was ended (after about 60 min). After the grinding, the slurry was filtered through a 780 μm filter with the aid of a porcelain suction funnel for removal of the ceramic beads, and the solids content of the slurry was adjusted to 77%. The viscosity of the slurry was measured after one day, one week, two weeks and three weeks.

(8) The starting weight of the polymer is reported in wt % of active dispersant based on solid calcium carbonate (wt/wt %). The results are shown in table 2.

(9) TABLE-US-00002 TABLE 2 Amount of Dynamic viscosity of the Exam- Poly- dispersant CaCO.sub.3 slurry (mPas) ple mer (wt/wt %) 1 day 7 days 14 days 21 days C1 1 0.8 1055 2169 2706 not measurable C2 2 0.8 787 1677 2453 not measurable C3 3 0.8 271 317 353 401 C4 4 0.8 277 355 337 364 C5 6 0.8 237 259 275 315 1 7 0.8 213 216 227 242 2 8 0.8 198 223 240 256 3 9 0.8 230 239 242 251 C6 10 0.8 419 570 604 614 C7 11 0.8 280 365 443 517 C8 12 0.8 345 524 694 518 4 7 0.7 331 329 439 518 5 7 0.6 558 834 1098 1448 6 7 1.0 208 211 215 232 7 13 0.8 227 241 246 262 C9 14 0.8 308 474 479 509 8 7 1.0 218 221 232 261

(10) The viscosity of the calcium carbonate slurries produced with the acrylic acid polymers of the invention is significantly lower than the viscosity of the calcium carbonate slurries produced with the comparative polymers with the same amount of dispersant over the entire period of three weeks.

Example 9 and Comparative Examples C10, C11

(11) In the course of storage, the alkaline calcium carbonate dispersions absorb carbon dioxide from the air. Carbon dioxide reacts with dissolved calcium carbonate to give calcium hydrogencarbonate, which lowers the pH of the dispersion and leads to an increase in viscosity. In order to simulate the effect of the dispersants on the viscosity of a calcium carbonate dispersion on absorption of CO.sub.2, different amounts of sodium hydrogencarbonate were added to the slurries stabilized with various polyacrylates. Immediately after the addition, the viscosity of the slurries was measured. Sodium hydrogencarbonate was added as a 10% by weight solution; the amount is reported in the table as wt % of solid NaHCO.sub.3 based on solid calcium carbonate in the slurry. The slurries were each stabilized with 0.8% (wt/wt %) of polyacrylate. The results are shown in table 3.

(12) TABLE-US-00003 TABLE 3 Viscosity Viscosity Viscosity after after after Viscosity addition of addition of addition of before 0.25 wt/wt % 0.50 wt/wt % 0.90 wt/wt % Polymer addition of NaHCO.sub.3 of NaHCO.sub.3 of NaHCO.sub.3 3 271 330 595 1050 7 213 188 237 474 5 237 219 423 671

(13) Inventive polymer 7 and comparative polymers 3 and 5 had a narrow molecular weight distribution (PDI=2.1). The smallest rise in viscosity after addition of the NaHCO.sub.3 solution was exhibited by the calcium carbonate slurry which was stabilized with the polyacrylate neutralized with calcium and sodium ions (polymer 7).

(14) Polyacrylates neutralized with calcium and sodium ions thus stabilize calcium carbonate slurries better against the aging of the slurries as a result of absorption of CO.sub.2 than polyacrylates neutralized with sodium ions only (polymer 3) or neutralized with sodium and magnesium ions (polymer 5).