Process for the production of polyacrylic acids

The invention relates to a process for producing polyacrylic acids and copolymers of acrylic acid with up to 50 mol-% of comonomers. These compounds have a molecular weight of less than 100,000, measured at a pH-value of 8, and 10 to 60 mol-% are neutralized with calcium hydroxide. The invention also relates to the use of these products as grinding and dispersing aids for calcium carbonate.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for producing polyacrylic acids 
and copolymers of acrylic acid with up to 50 mol-% of comonomers. These 
compounds have a molecular weight of less than 100,000, measured at a 
pH-value of 8, and 10 to 60 mol-% are neutralized with calcium hydroxide. 
The present invention further relates to the use of such products as 
grinding and dispersing aids for calcium carbonate. 
Polyacrylic acids may be applied in many fields. They may be used, e.g., as 
anti-deposition agent in water cycles and as grinding and/or dispersing 
aid in the production of pigment suspensions. In this connection the 
grinding and dispersion of calcium carbonate is of particular interest. 
Finely ground calcium carbonate is used as filler in the paper industry. 
Polyacrylic acids are manufactured by polymerizing acrylic acid. In 
general, the polymerization is carried out in a solvent, preferably water 
or mixtures of water and isopropanol are used. 
German Patent Application No. 34 32 082 describes the production of 
alkaline-earth acrylate from acrylic acid and alkaline-earth carbonate. 
Alkali acrylate is obtained by further reaction with alkali bicarbonate. 
European Patent No. 0100948 relates to the use of polymers of acrylic acid 
as dispersing aids. For this purpose the polyacrylic acid is neutralized 
only partially. 
European Patent No. 0046573 mentions that the polyacrylic acids claimed as 
grinding aids may be neutralized with the hydroxides of sodium, potassium, 
ammonium, zinc or calcium, or with primary, secondary, or tertiary amines. 
According to European Patent No. 0127388 a phase separation, which can be 
used to purify the polymer, may be achieved by adding C.sub.1 to C.sub.5 
alcohols to the neutralized aqueous polyacrylic acid solution. Sodium, 
potassium, lithium and ammonium are mentioned as cations suitable for the 
neutralization. 
European Patent No. 0129329 deals with the same problem and the same 
cations. 
In most cases the polyacrylic acid must be present in neutralized form. 
Sodium, ammonium, potassium, calcium, magnesium, amines or combinations 
thereof are usually employed as couterions. The maximum proportion of ions 
which--when used alone--form an insoluble polyacrylate may naturally only 
be so high that the polyacrylic acid is not yet precipitated. For this 
reason the maximum possible amount of calcium ions, for example, is 60 
mol-%. 
According to the teaching of European Patent No. 0100947 it is appropriate 
to use polyacrylic acids which are partially neutralized with calcium as 
grinding aids. The polymerization takes place in mixtures of isopropanol 
and water. On the other hand, acrylic acid partially neutralized with 
calcium cannot be polymerized in the presence of isopropanol without 
causing the precipitation of the polymer. The neutralization with calcium 
may only be carried out when the isopropanol has been distilled off the 
polyacrylic acid solution, since, otherwise, precipitation of the polymers 
occurs. Neutralizing the polyacrylic acid with calcium hydroxide after the 
polymerization has taken place involves numerous serious disadvantages. 
For example, it takes a long time until the calcium hydroxide dissolves in 
the polymer solution, in addition, the polymer solution is extremely 
turbid even after the neutralization. 
SUMMARY OF THE INVENTION 
It is the object of the present invention of provide polymers of the 
acrylic acid which are partially neutralized with calcium and do not 
exhibit the above mentioned disadvantages of working according to EP 
0100947. 
This object is achieved by a process for the production of polyacrylic 
acids and copolymers of acrylic acid, respectively, with up to 50 mol-% of 
comonomers which, measured at a pH of 8.0, have a molecular weight of less 
than 100,000 and are neutralized with calcium hydroxide by 10 to 60 mol-%. 
This process is characterized by the fact that the acrylic acid is 
partially neutralized with calcium hydroxide prior to polymerization and 
that the further neutralization is carried out with monovalent bases after 
the polymerization. 
Surprisingly, it was found that the neutralization of the monomeric acrylic 
acid with calcium hydroxide takes place considerably faster and leads to 
improved results as compared to the neutralization of polyacrylic acids 
after polymerization thereof. 
The polymers according to the present invention preferably have a molecular 
weight of 500 to 50,000, determined according to the method of 
gel-permeationchromatography, in particular, they have a molecular weight 
of 3,000 to 9,000. 
N-alkyl substituted acrylamides or methacrylamides having a tertiary amino 
group at the alkyl residue may be used as comonomers. 
Examples thereof include: dimethylaminopropyl acrylamide, 
dimethylaminopropyl methacrylamide, dimethylamino-2,2-dimethylpropyl 
acrylamide, dimethylamino-2,2-dimethylpropyl methacrylamide, and 
dimethylaminoethyl acrylamide. These monomers may also be present in 
quaternized form, such as the trimethylacrylamidopropyl ammonium chloride. 
Further examples of monomers include acrylamide, methacrylamide, 
methacrylic acid, 2-acrylamido-2-propane sulfonic acid, methyl acrylate, 
hydroxypropyl acrylate, and acrylonitrile. 
By using said comonomers a modification of the properties of the copolymers 
is to be achieved. After the polymerization, neutralization of the 
polyacrylic acid is completed with an aqueous monovalent base, e.g., 
sodium hydroxide solution, ammonia, or potassium hydroxide solution. 
The polymers according to the present invention may be used for many 
purposes. The production of concentrated suspensions of solids is a 
preferred application. Pigment suspensions manufactured by using the 
copolymers according to the present invention exhibit a considerably 
improved storage stability as compared to the products of EP 0100947. 
Thus the present invention further relates to the use of the process 
products, in particular as grinding and dispersing aids for calcium 
carbonate. The quantity applied as grinding and dispersing aid amounts to 
between 0.1 and 1.0%-wt., each relative to solid (polymer) to solid 
(calcium carbonate).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The polyacrylic acids according to the present invention may be obtained by 
polymerization methods known per se. Polymerization is carried out in an 
aqueous solution and is initiated by means of radical polymerization 
catalysts. 
Redox systems and/or thermally decomposing radical formers of the type of 
azo compounds, e.g., azoisobutyric acid dinitrile can be used. Suitable 
redox systems include: combinations of hydrogen peroxide, salts of the 
peroxy disulfuric acid or tert.-butyl-hydroperoxide with sodium sulfite or 
sodium dithionite as reduction component. 
The molecular weights of less than 100,000 according to the present 
invention are suitably achieved by carrying out the polymerization in the 
presence of polymerization regulators. Effective regulators include 
hydroxylammonium salts as well as mercaptans, e.g., mercaptoethanol or 
thioglycollic acid. 
The polymerization is advantageously carried out under adiabatic 
conditions. In this case, the polymerization is suitably started at low 
temperatures, e.g., at 20.degree. C. The final temperature achieved by the 
liberated polymerization heat depends on the monomers used and on the 
concentration ratios; in case of an adequate pressure it may amount, for 
example, to up to 180.degree. C. 
Another method of carrying out the polymerization is to prepare a portion 
of the monomer mixture, start the copolymerization and then continuously 
charge the monomer mixture. The temperature during the copolymerization 
may vary within wide ranges. 
Depending on the catalysts employed temperatures between 50.degree. C. and 
200.degree. C. may be optimum temperatures. It is also possible to carry 
out the polymerization at the boiling point of the solvent, or under 
pressure at temperatures above that. 
The monomer solution is partially neutralized with calcium hydroxide prior 
to polymerization. It is suitable to produce a suspension of calcium 
hydroxide in water in which the acrylic acid, and possibly the comonomers, 
are subsequently introduced. It is, however, also possible to add the 
calcium hydroxide to the prepared monomer solution. According to the 
present invention 10 to 60 mol-% of the acrylic acid are neutralized with 
calcium. If 60 mol-% is exceeded, precipitation of the insoluble calcium 
polyacrylate occurs during the polymerization. 
EXAMPLE 1 
38 g calcium hydroxide are suspended in 700 g water, then 250 g acrylic 
acid are added. Within a period of 15 minutes the calcium hydroxide is 
dissolved, the temperature rising from 20.degree. C. to 32.degree. C. 
The turbidimetric measurement results in a value of 3.5 TU(F) (turbidity 
units relative to formazine standard). After cooling again to 20.degree. 
C., the polymerization is initiated by stirring into the mixture: 16 ml 
mercaptoethanol, 1 ml 1% iron (II) sulfate solution and 5 ml 35% hydrogen 
peroxide. After addition of the catalysts, a final temperature of 
approximately 90.degree. C. is achieved within 2 minutes. 2 ml 40% 
hydroxylamine hydrochloride solution and 4.5 ml 35% hydrogen peroxide are 
added to complete the reaction. The post-reaction period amounts to 1 
hour. 
After cooling to 60.degree. C., neutralization to pH 7 is effected with 
approximately 220 g 45% caustic lye of soda within a period of 20 minutes, 
cooling to 20.degree. C. follows. The final product exhibits a turbidity 
of 10.5 TU(F). The molecular weight (Mw) amounts to 5491 (Table 2). 
COMATIVE EXAMPLE 1 
A mixture of 700 g water and 250 g acrylic acid is manufactured. In 
accordance with Example 1, the polymerization is initiated at 20.degree. 
C. and a final temperature of approximately 90.degree. C. achieved after 2 
minutes. Post-reaction takes place within 1 hour as in Example 1. 
Subsequently the mixture is cooled to 60.degree. C. and 38 g calcium 
hydroxide are introduced. 
After a period of approximately 2.15 hours the calcium hydroxide is 
dissolved, the turbidity of the partially neutralized product amounts to 
37 TU(F). Within 20 minutes the mixture is neutralized to pH 7 using 
approximately 220 g 45% caustic lye of soda and cooled to 20.degree. C. 
The final product exhibits a turbidity of 21.2 TU(F). The molecular mass 
(Mw) amounts to 5913 (Table 2). 
EXAMPLE 2/COMATIVE EXAMPLE 2 
The same method as in Example 1 and Comparative Example 1, respectively, is 
used. 225 g acrylic acid and 41.7 g 60% trimethylacryloylamidopropyl 
ammonium chloride solution are used as monomers. Neutralization is 
effected with 34.7 g calcium hydroxide and 188 g 45% caustic-soda 
solution. 
EXAMPLE 3/COMATIVE EXAMPLE 3 
The methods of Example 1 and Comparative Example 1 is used respectively. 
200 g acrylic acid and 50 g dimethylaminopropyl acrylamide are used as 
monomers. Neutralization is carried out with 30.8 g calcium hydroxide and 
145 g 45% sodium hydroxide solution. 
EXAMPLE 4/COMATIVE EXAMPLE 4 
The methods are in accordance with in Example 1 and Comparative Example 1 
respectively. 175 g acrylic acid and 187.5 g 40% acrylamide solution are 
used as monomers. Neutralization is carried out with 27.0 g calcium 
hydroxide and 146 caustic-soda solution. 
EXAMPLE 5/COMATIVE EXAMPLE 5 
The same method as in Example 1 and Comparative Example 1 respectively. 150 
g acrylic acid and 100 g methacrylic acid are used as monomers. 
Neutralization is carried out with 23.1 g calcium hydroxide and 230 g 45% 
caustic-soda solution. 
EXAMPLE 6/COMATIVE EXAMPLE 6 
The same method as in Example 1 and Comparative Example 1, respectively, is 
used. 125 g acrylic acid and 125 g 2-acrylamido-2-propane sulfonic acid 
are used as monomers. 
Neutralization is effected with 19.3 g calcium hydroxide and 164 g 45% 
caustic-soda solution. 
TABLE 1 
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Results of turbidimetry: 
Turbidity after 
Turbidity 
Ca(OH).sub.2 -neutral- 
addition of in final 
ization time Ca(OH).sub.2 product 
[min] [TU(F)] [TU(F)] 
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Example 1 
15 3.5 1O.5 
Comparison 1 
135 37 21 
Example 2 
15 5.5 8 
Comparison 2 
150 40.6 23.5 
Example 3 
25 16.5 4.5 
Comparison 3 
160 81.5 22 
Example 4 
20 24 9.5 
Comparison 4 
140 26.9 25 
Example 5 
20 18 8.1 
Comparison 5 
115 29 15.2 
Example 6 
15 26.3 8.3 
Comparison 6 
35 18 34 
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TABLE 2 
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Molecular weight (Mw) 
Example Comparative Example 
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1 5491 5913 
2 4317 4124 
3 10655 11250 
4 18397 17891 
5 36320 35563 
6 13257 13814 
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APPLICATION--EXAMPLE 1 
The polymer of Example 1 is used as grinding and dispersing aid for calcium 
carbonate. 17.1 g dry polymer (0.9%-wt. solid matter (polymer) to solid 
matter (calcium carbonate)) are dissolved in approximately 500 g water, 
and 1900 g calcium carbonate are then suspended therein. After grinding 
with a laboratory pearl mill, the suspension contains approximately 77% 
solid matter, 80% thereof exhibit a degree of grinding of smaller than 1 
.mu.m. 
The viscosities obtained are listed in Table 3. 
APPLICATION--COMATIVE EXAMPLE 1 
The polymer of Comparative Example 1 is used in accordance with 
Application-Example 1. 
TABLE 3 
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Application 
Application- 
Comparative- 
Example 1 Example 1 
Time d after Viscosity Viscosity 
grinding [mPa.multidot.s] 
[mPa.multidot.s] 
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0 439 346 
1 744 764 
7 710 980 
14 610 1060 
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