HK1006061B - Grinding and/or dispersion aid based on polymers and/or copolymers partially neutralised by magnesium for aqueous suspensions of mineral materials which are convenient for pigmentary applications - Google Patents

Description

The present invention relates to an improved crushing and/or dispersion agent in aqueous suspension of coarse mineral materials to produce an aqueous suspension of refined mineral particles for pigment applications such as an aqueous suspension of calcium carbonate with a dry matter content of at least 70% by weight, of which at least 90% of the constituent particles after crushing are less than two microns in size, while at least 60% of them are less than one micron in size.

The invention also relates to a process for grinding in aqueous medium of coarse mineral materials in the presence of this water-soluble grinding agent. The process leads to aqueous suspensions of mineral materials whose viscosity remains stable over time even if the suspensions are not kept under agitation, thus providing great ease of handling and application. The process is particularly applicable to the grinding of an aqueous suspension of calcium carbonate with a dry matter content of at least 70% by weight, of which at least 90% of the constituent particles after grinding have a dimension of less than two microns, while at least 60% of them have a dimension of less than one micron. Furthermore, the invention relates to aqueous suspensions of mineral particles obtained by the process and their applications.

Mineral substances such as calcium carbonates and titanium dioxide have long been known to be used in the preparation of industrial products for paints, paper-induction, rubber and synthetic resin fillings and other applications.

But since these mineral substances do not have a natural laminar or laminated structure which facilitates their decomposition, as is the case with certain substances such as aluminium silicates commonly known as kaolin, the artisan must, in order to use them in the pigment field, grind them into a highly fine aqueous suspension whose constituent grains have the smallest possible dimension, i.e. less than a few microns.

The number of publications in this field shows the importance and complexity of the aqueous milling of mineral substances to obtain a partially refined quality suitable for pigment application. In the particular case of paper induction, it is well known that the induction mass formed from mineral pigments such as kaolins, calcium carbonate and titanium dioxide suspended in water also contains binding and dispersing agents and other adjuvants such as thickeners and colouring agents.

Thus, up to now, the man of art has known the use of grinding agents and/or aqueous dispersion of mineral particles consisting of acrylic polymers and/or copolymers totally or partially neutralized by various neutralizing agents (patents FR2539137, FR2531444, FR2603042).

The user is also familiar with the use of a grinding and/or dispersion agent consisting of the fraction of alkali acrylic polymers and/or copolymers totally neutralized by a single cation and having a specific viscosity between 0.3 and 0.8 (patent FR2488814) or partially neutralized by a single cation such as sodium, potassium, lithium or ammonium (EP0127388; EP0185458).

These various types of prior art grinding and/or dispersion agents and other treatments or grinding processes known to the art (EP0278880; FR 2642415) make it possible to obtain aqueous suspensions of refined mineral particles, which have hitherto been considered stable over time but for which the user has some difficulties in handling because he is required to store these suspensions for several days or weeks without stirring and thus is confronted with the increase in the viscosity of the unstirred suspension.

In the past, the viscosity of the aqueous suspension of mineral particles was usually measured by means of a Brookfield viscosimeter of the RVT type at a temperature of 20°C and a rotation rate of the mobile number 3 of 100 rpm: (a) after leaving the crusher, (b) after eight days' rest in the vial and after a prior agitation of the suspended solution.

Err1:Expecting ',' delimiter: line 1 column 112 (char 111)

Faced with these drawbacks, the applicant then found that the mineral aqueous suspensions obtained by the previous method had high Brookfield viscosities if the measurement was made without prior agitation of the suspension, which is usually the case for the user of those suspensions.

On the basis of that finding, the applicant then developed a grinding and/or dispersion agent and a grinding process in the presence of that agent which surprisingly made it possible to produce aqueous mineral suspensions meeting the quality criteria mentioned above.

Surprisingly, the purposes of the invention are achieved when the percentage of neutralization of the active acid sites of the polymer by a neutralizing agent containing the magnesium ion is between 40 and 60%, including terminals, while the percentage of neutralization of the active acid sites of the polymer by a neutralizing agent containing the sodium ion is less than or equal to 60%.

Thus, one of the purposes of the invention is to provide an improved crushing and/or dispersion agent in aqueous suspension of coarse mineral materials of substantially identical viscosity over time.

Another purpose of the invention is to provide a process for grinding mineral particles in aqueous suspension.

These suspensions, intended for pigment applications where at least 90% of the particles are less than two microns in size and at least 60% of the particles are less than one micron in size, according to the invention, have a low viscosity and stable viscosity over time, even without agitation, i.e. a viscosity after eight days at rest in the same order as that obtained when the mill is finished.

Finally, another purpose of the invention concerns the use of these refined mineral aqueous suspensions in the field of bulk loading and coating of paper and in the field of paints and polymers after possibly eliminating the aqueous phase.

These purposes are achieved by the use in the grinding process of the invention of the agent of the invention, which results from the polymerisation and/or copolymerisation of acrylic and/or vinyl monomers, followed by fractionation by one or more polar solvents using processes known to the art.

The agent of the invention is characterised by the fact that it is formed from the fraction of the said polymers and/or acid copolymers whose active acid sites are partially or completely neutralized with a neutralization rate of the active acid sites by a neutralizing agent with magnesium ion ranging from 40 to 55% including the terminals and a neutralization rate of the active acid sites by a neutralizing agent with sodium ion up to 60%. These acrylic polymers and/or copolymers are obtained by known radial polymerization processes, in the presence of polymerization regulators such as hydroxylamine-based organic compounds and in the presence of polymerization initiators such as peroxides and persulfides,For example, oxygenated water, persulphate, sodium hypophosphite, hypophosphoric acid, of at least one of the following monomers and/or comonomers: acrylic and/or methacrylic acid, itaconic, crotonic, fumaric, maleic anhydride, or isocrotonic, aconic, mesaconic, synapic, undecylenic, angelic, canellitic, hydroxyacrylic, in acid or partially neutralized form, acrylic acid, acrylamide, acrylonitrile, esters of acrylamic and vinyl acrylic acids, and in particular vinyl vinyl acetate, dimethyl acetate, dimethyl methacrylate, methyl methacrylate, methyl methyl methyl methyl acetate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl methacrylate, butyl butyl butyl butylDimethylsulfoxide, tetrahydrofuran, acetone, methylethyl ketone, ethyl acetate, butyl acetate, hexane, heptane, benzene, toluene, xylene, mercaptoethanol, tertiodecylmercaptan, thioglycolic acid and its esters, n-dodocylmercaptan, acetic, tartaric, lactic, citric, gluconic, glucoheptonic acids, 2-mercaptopropionic acid, thiodiethyl ethanol, halogenated solvents such as methacrylate, chlorophenol, chlorophenol, methacrylate or their glycol ethers.

The resulting acid polymerase solution may be partially or totally neutralized at a rate of 40 to 55% by a neutralizing agent containing magnesium ion and at a rate of 60% or less by a neutralizing agent containing sodium ion.

The solution of the acid polymerisate or neutralised polymerisate is then treated by static or dynamic processes known to the art, by one or more polar solvents belonging in particular to the group consisting of methanol, ethanol, propanol, isopropanol, butanols, acetone, tetrahydrofuran, thus producing a two-phase separation: the least dense phase, comprising the major fraction of the polar solvent and the low molecular weight acrylic polymer and/or copolymer fraction,the densest aqueous phase, comprising the higher molecular weight acrylic polymer and/or copolymer fraction.

The desired fraction of the acrylic polymers and/or copolymers of the invention with a specific viscosity of between 0,3 and 0,8 is collected.

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The capillary tube is generally chosen so that the flow time of the polymer and/or copolymer free NaCl solution is approximately 90 to 100 seconds, giving very accurate specific viscosity measurements.

The temperature at which the selection treatment of the acid or alkaline acrylic polymer and/or copolymer fraction is carried out is not critical in itself, since it influences the only partition coefficient: it is between 10 and 80 °C, and preferably between 20 and 60 °C.

The ratio of the quantities of dilution water to polar solvents, on the other hand, is predominant, since it directly influences the separation.

It is also possible and desirable in some cases to further refine the selection of the acrylic polymer and/or copolymer fraction by reprocessing the densest aqueous phase previously collected with a new quantity of polar solvent, which may be different from the original solvent (s) used, or which may still be a mixture of polar solvents.

The liquid phase obtained after treatment may be distilled to remove the solvent (s) used for treatment.

The aqueous phase containing the fraction of acrylic polymers and/or copolymers with a specific viscosity between 0,3 and 0,8, corresponding to an average molecular weight ( $\overline{\text{Mw}}$ It may also be treated by any known means to remove the aqueous phase and to isolate the polymer and/or acrylic copolymer into a fine powder which can be used in this other form as a grinding agent.

In practice, the milling operation of the mineral substance to be refined consists in grinding the mineral substance with a milling body into very fine particles in an aqueous medium containing the milling agent. An aqueous suspension of the mineral substance to be grinding, the grains of which have an initial dimension not exceeding 50 microns, is formed in such quantity that the dry matter concentration of the suspension is at least 70% by weight.

The grinding body is generally in the form of particles of materials as diverse as silicon oxide, aluminium oxide, zirconium oxide or mixtures thereof as well as high-hardness synthetic resins, steels, or others. An example of the composition of such grinding bodies is given by patent FR2203681 which describes grinding elements consisting of 30 to 70% by weight of zirconium oxide, 0.1 to 5% by weight of aluminium oxide and 5 to 20% by weight of grinding silicon oxide. The grinding body is added in such a way that the amount of suspension is between 2/1, and the ratio of grinding material is preferably between 5 and 3/1.

The mixture of suspension and grinding body is then subjected to the mechanical action of brewing, such as occurs in a conventional microelement grinder.

The grinding and/or dispersion agent of the invention is also introduced into the mixture formed by the aqueous suspension of mineral substances and the grinding body at a rate of 0.2 to 2% by weight of the dried fraction of these polymers in relation to the dry mass of the mineral substance to be refined.

The time required to achieve excellent fineness of the mineral substance after grinding varies according to the nature and quantity of the mineral substances to be porphyrised, and according to the agitation method used and the temperature of the medium during the grinding operation.

The mineral substances to be refined can be of a wide variety of origins such as calcium carbonate and dolomite, calcium sulphate, kaolin, titanium dioxide, i.e. all the mineral substances which must be crushed to be used in such diverse applications as paper-induction, pigmentation of paints and coatings, loading of synthetic rubbers or resins, mating of synthetic textiles, etc.

Thus, the application of the grinding and/or dispersion agent of the invention allows the grinding of coarse mineral substances into very fine particles, of which at least 90% have a dimension of less than two microns and at least 60% have a dimension of less than one micron, and to obtain a suspension of very fine mineral substances with a low viscosity and stable over time.

The scope and interest of the invention will be better understood by the following examples.

Example 1

This example concerns the preparation of a coarse calcium carbonate suspension which has been ground to a fine microparticulate suspension. for test No 1 illustrating the previous art, the densest fraction of a sodium polyacrylate resulting from an isopropanol fraction of a sodium polyacrylate of specific viscosity 0,525 obtained by radical polymerisation and total neutralization (100% of active acid sites) by sodium hydroxide,for test No 2 also illustrating the previous art,the densest fraction of a mixed sodium-calcium polyacrylate resulting from the isopropanol fractionation of a mixed sodium-calcium polyacrylate of specific viscosity 0,525 obtained by radical polymerization and total neutralization by sodium and lime in a ratio corresponding to neutralization of the active acid sites at 70% sodium-30% calcium,for test No 3 still illustrating the previous art, the densest fraction of a sodium polyacrylate resulting from the isopropanol fractionation of a sodium polyacrylate of specific viscosity 0,525 obtained by polymerization and partial neutralization of the active acid sites (66%) by sodium viscoxide,for test No 4 illustrating the invention, the densest fraction of a mixed sodium-magnesium polyacrylate resulting from an isopropanol fraction of a mixed sodium-magnesium polyacrylate of specific viscosity 0,525 obtained by total neutralization (100% of active acid sites) by sodium and magnesium in a ratio corresponding to a neutralization of the active acid sites at a rate of 50% by sodium - 50% by magnesium.

For each test, an aqueous suspension of calcium carbonate from the Orgon deposit in France with a particle size of less than 10 microns was prepared.

The aqueous suspension has a dry matter concentration of 76% by weight of the total mass.

The grinding agent shall be added to this suspension in the quantities shown in the table below, expressed as a percentage by dry weight of the dry calcium carbonate to be ground.

The suspension is moved in a Dyno-Mill type mill with a fixed cylinder and a rotating impeller, the grinding body of which consists of corundum balls of a diameter of between 0,6 mm and 1,0 mm.

The total volume of the grinding body is 1150 cubic centimetres while its mass is 2900 g.

The milling chamber has a volume of 1400 cubic centimetres.

The circumferential velocity of the crusher is 10 meters per second.

The calcium carbonate suspension is recycled at a rate of 18 litres per hour.

The output of the Dyno-Mill is equipped with a 200 micron mesh separator to separate the suspension resulting from the grinding and the grinding body.

The temperature during each grinding test shall be maintained at approximately 60°C.

At the end of the milling (To), a sample of the pigment suspension is taken from a vial, 80% of which is less than one micron in size, and the viscosity is measured with a Brookfield viscosimeter of the RVT type at 20°C and a rotation rate of 10 rpm with the appropriate mobile.

After a rest period of 8 days in the vial, the viscosity of the suspension is measured by introducing into the vial the appropriate mobility of the RVT Brookfield viscosimeter at a temperature of 20°C and a rotation rate of 10 revolutions per minute (AVAG viscosity = before agitation).

All experimental results are recorded in Table 1.

Table 1 shows that only the suspension of calcium carbonate crushed by the agent of the invention (Test 4) has a Brookfield viscosity measured at 10 rpm after 8 days of rest and without prior agitation of well below 2 000 mPa.s, whereas all tests of the samples by means of a state-of-the-art agent give viscosities above 2 000 mPa.s.

Thus, reading Table 1 shows the increase in viscosity in the calcium carbonate suspensions crushed by means of agents of the previous state of the art, while the crushing and/or dispersion agent, according to the invention, allows the stability of viscosity of the calcium carbonate suspensions to be obtained even before any agitation.

Example 2

A second group of tests (tests Nos 5 to 12) involved the preparation and grinding of the aqueous suspension of the same calcium carbonate at the same dry matter concentration in the presence of the densest fraction obtained by isopropanol fractionation of the same polyacrylic acid of specific viscosity 0,525 neutralized by varying levels of magnesium.

These tests were carried out according to the same experimental criteria as in Example 1.

All experimental results are recorded in Table 2.

The exceptional rheological results of the invention can be seen from Table 2 and the fact that the use of the agent according to the invention allows a threshold of profitability to be reached in practical tests.

Two criteria are taken into account by the user, the economic and the rheological aspects.

Thus, the consumption of a crushing and/or dispersion agent in the crushing operation according to the invention must not exceed about 1.1% of dry dispersion agent by dry weight of calcium carbonate to obtain a suspension with 80% particles less than 1 micron and a dry matter concentration of 76%.

As regards rheology, the user who has to handle concentrated suspensions stored for several days without stirring wants suspensions with a viscosity almost identical to that of suspensions leaving the mill and always below 2000 mPa.s. This exceptional result is obtained for the first time by the use of the agent according to the invention (tests Nos 6 to 9 and 12, and in particular tests 7 to 9).

Example 3

This example (tests Nos 13 to 14) concerns the preparation and grinding of the aqueous suspension of the same calcium carbonate at the same dry matter concentration in the presence of the densest fraction obtained by isopropanol fractionation of the same polyacrylic acid of specific viscosity 0.525 neutralized by various neutralizing agents containing multifunction ions other than magnesium and such as calcium.

Thus, the milling of test No 13 is carried out using the same fraction of polyacrylic acid neutralized by soda and lime in a ratio corresponding to a neutralization of the active acid sites at 70% by sodium - 30% by calcium, while test No 14 involves the same fraction of polyacrylic acid neutralized by soda and lime in a ratio corresponding to a neutralization of the active acid sites at 55% by sodium - 45% by calcium.

These tests were carried out according to the same experimental criteria as in Example 1.

All results are recorded in Table 3.

Table 3 shows that the use of another polyvalent neutralising cation such as calcium, even at different levels, does not result in a Brookfield viscosity refined calcium carbonate suspension measured at 10 rpm after 8 days of rest and without prior agitation below 2 000 mPa.s. (tests 13 and 14).

Example 4

This example (tests 15-17) concerns the preparation and grinding of the aqueous suspension of the same calcium carbonate at the same dry matter concentration in the presence of the densest fraction obtained by isopropanol fractionation of the same polyacrylic acid of specific viscosity 0.525 neutralized 50% by magnesium ion and 50% by various monovalent neutralizing agents.

These different monovalent neutralising agents are ammonium for test 15, lithium for test 16, potassium for test 17.

These tests were carried out according to the same experimental criteria as in Example 1.

All results are recorded in Table 4.

It is clear from Table 4 that the sodium ion cannot be replaced with another monovalent ion.

Claims (13)

1. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension, intended for applications involving pigments, consisting of acrylic and/or vinyl polymers and/or copolymers comprising partially or fully neutralised acid groups obtained by treating acrylic and/or vinyl polymers and/or copolymers with one or more polar solvents using static or dynamic methods and selecting the fraction having the requisite molecular weight, characterised in that the active acid sites of the said agent are fully or partially neutralised by a neutralising agent containing the magnesium ion such that the percentage of neutralisation of the active acid sites by the magnesium ion varies between 40 and 55 %, boundary areas included, and by a neutralising agent having the sodium ion capable of neutralising up to 60 % of the active acid sites of the said agent.
2. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claim 1, characterised in that the active acid sites of the said agent are partially neutralised by a neutralising agent containing the magnesium ion such that the percentage of neutralisation of the active acid sites by the magnesium ion varies between 40 and 55 %, boundary areas included, and by a neutralising agent having the sodium ion capable of neutralising up to 60 % of the active acid sites of the said agent.
3. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claim 1, characterised in that the active acid sites of the said agent are fully neutralised by a neutralising agent containing the magnesium ion such that the percentage of neutralisation of the active acid sites by the magnesium ion varies between 40 and 55 %, boundary areas included, and by a neutralising agent having the sodium ion to neutralise the remaining 60 % to 45 % of active acid sites.
4. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claim 1, characterised in that the active acid sites of the said agent are fully neutralised by a neutralising agent containing the magnesium ion such that the percentage of neutralisation of the active acid sites by the magnesium ion varies between 45 % and 55 %, boundary areas included, and by a neutralising agent having the sodium ion to neutralise the remaining 55 % to 45 % of active acid sites.
5. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claims 1 to 4, characterised in that the said agent has a specific viscosity ranging between 0.3 and 0.8.
6. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claim 5, characterised in that it is a solution.
7. An agent for grinding and/or dispersing coarse mineral substances in aqueous suspension as claimed in claim 5, characterised in that it is a powder.
8. A method of grinding coarse mineral substances in aqueous suspension intended for applications involving pigments, which consists in preparing an aqueous suspension of these substances, introducing a grinding agent consisting of acrylic and/or vinyl polymers and/or copolymers comprising partially or fully neutralised acid groups obtained by treating acrylic and/or vinyl polymers and/or copolymers with one or more polar solvents using static or dynamic methods and selecting the fraction with the requisite molecular weight, adding to the suspension a grinding medium and subjecting the mixture so obtained to a grinding action, characterised in that the said grinding agent is as described in any one of claims 1 to 7.
9. A method of grinding coarse mineral substances in aqueous suspension as claimed in claim 8, characterised in that the crushing agent is introduced into the suspension in a quantity of from 0.2 % to 2 % by dry weight of the said polymer relative to the dry weight of the mineral substance to be ground.
10. A method of grinding coarse mineral substances in aqueous suspension as claimed in any one of claims 8 or 9, characterised in that the aqueous suspension of mineral substances to be ground comprises at least 70 % by weight of dry substances.
11. An aqueous suspension of mineral substances refined by means of the grinding method as claimed in any one of claims 8 to 10, characterised in that 90 % at least of the particles of the ground substance are of a dimension smaller than two microns and at least 60 % thereof are of a dimension smaller than one micron.
12. An aqueous suspension of refined mineral substances as claimed in claim 11, characterised in that the Brookfield viscosity of the said suspension, measured without agitation after storing for at least 8 days, is not more than 2000 mPa.s at 10 revolutions per minute.
13. Application of the aqueous suspension of ground mineral substances as claimed in claims 11 and 12 as a substance filler in the field of paper coating, paints and plastics and paper manufacturing.