CA1276754C - Aqueous pigment suspensions based on alkyl (meth)acrylates as dispersants for pigments - Google Patents

Abstract

Aqueous pigment suspensions which are added to the stock suspension during papermaking and contain, as a dispersant, from 0.1 to 2% by weight, based on the pigment, of a copolymer which contains, as copolymerized units, a) from 40 to 95% by weight of an ester of (meth)acrylic acid with a saturated monohydric C1-C8-alcohol or with a saturated dihydric C2-C4-alcohol and b) from 60 to 5% by weight of an ethylenically unsaturated C3-C5-carboxylic acid and has a K value of from 10 to 65 (determined according to H. Fikentscher in 1% aqueous solution in the form of the ammonium salt at pH 9 and at 20.degree.C), their preparation and their use.

Description

lZ7~75~
- 1 - O.Z. 0050/37623 Aqueous pigment suspensions based on alkyl (meth)acrylates as dispersants for pigments Concentrated aqueous pigment suspensions are re-quired in large amounts in industry for the preparation of paper-coating mixtures. In order to produce highly concentrated aqueous pigment suspensions, finely milled pigments are dispersed in ~ater in the presence of from 0.1 to 2% by weight, based on pigments, of a dispersant.
Lo~ molecular weight polymers of acrylic acid, which are prepared, for example, by the process described in European Patent 27,771, are particularly effective dis-persants. Furthermore, according to German Laid-Open Application DOS 3,123,732, copolymers of acrylic acid and an acrylamidopropanesulfonic acid are used for the preparation of highly concentrated aqueous pigment sus-pensions. U.S. Patent 3,840,487, moreover, discloses that copolymers vhich contain from 40.to 95 molX of an ethylenically unsaturated carboxylic acid, such as acrylic acid or methacrylic acid, and from S to 60 molX of an ester of an ethylenically unsaturated carboxylic acid, eg. methyl acrylate, as copolymerized units are dispersants for pigments. The pigment suspensions prepared in this manner contain the pigments in a finely divided state, have a louer viscosity than dispersant-free suspensions and are used exclusively for the preparation of paper-coating mixtures.
Ho~ever, aqueous pigment suspensions prepared using the dispersants described above are unsuitable for the preparation of filler-containing papers in ~hich such aqueous pigment suspensions are added to the paper stock and the sheet is then formed on the paper machine from the stock treated in this manner, because even the small amounts of dispersants interfere to a very considerable extent uith the drainage of the paper stock and the efficiency of the retention aids during papermaking.
It is an object of the present invention to pro-vide aqueous pigment sUsPensions and a dispersant for lZ7~754

- 2 - O.Z. 0050/3762t pigments, which are added to the stock suspension during papermaking.
~ e have found that this object is achieved, according to the invention, by using copolymers which S contain as copolymerized units a) from 40 to 95X by weight of an ester of acrylic acid or methacrylic acid and a saturated monohydric C1-C8-alcohol or a saturated dihydric C2-C4-alcohol and b) from 60 to 5% by weight of an ethylenically unsaturated C3-Cs-carboxylic acid, and have a K value of from 10 to 65 (determined according to H. Fikentscher in 1~ strength aqueous solution as the ammonium salt at pH 9 and at 20C) as dispersants for pigments which are added to the stock suspension during papermaking. The copolymers are prefer-ably-employed in a form partially or completely neutralized with an alkali metal hydroxide solution, ammonia or an amine.
Copolymers of the composition stated above are known. They contain, as component a), from 40 to 95, preferably from 65 to 90, ~ by weight of an ester of acrylic acid or methacrylic acid with a saturated mono-hydric C1-Cg-alcohol or with a saturated dihydric C2-t4-alcohol. Examples of suitable esters are methyl acrylate, ethyl acrylate, n-propyl acrylate, iso-propyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-pentyl acrylate, neopentyl acrylate, n-hexyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acryl-ate, methyl methacrylate, ethyl methacrylate, propyl meth-acrylate, n-, iso- and tert-butyl methacrylate, neopentyl methacrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 2- and 3-hydroxypropyl meth-acrylate and 2-hydroxyethyl methacrylate. It is of course also possible to use mixtures of different acrylates as component a), as well as mixtures of different methacrylates lZ7~754

- 3 - O.Z. 0~50/37623 and mixtures of acrylates and methacrylates. Esters of acrylic acid and methacrylic acid derived from isomeric alcohols, for examples of n-propyl acrylate and isopropyl acrylate, can aLso be used. The copolymers preferably S contain, as component a), n-butyl acrylate or isobutyl acrylate or mixtures of the t~o isomeric butyl acrylates as copolymerized units.
rhe copolymers contain, as component b), from 60 to S, preferably from 35 to 10, ~ by ~eight of an ethylenically unsaturated C3-Cs-carboxylic acid as copolymerized units. Examples of suitable ethylenically unsaturated acids are acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid and itaconic acid, and maleic anhydride is also suitable and can be converted to the acid or salt form after being incorporated as polymerized units. From this group of monomers, acrylic acid is preferably used for the preparation of the copolymers.
The copolymers are prepared by subjecting the monomers a) and b) to copolymerization, for example in aqueous solution in the presence of polymerization initi-ators and polymeriza~ion regulators. Preferred polymer-ization regulators are thio compounds, these being used in an amount of from 0.5 to 12X by weight, based on the monomers.
Examples of suitable thio compounds are thiogly-colic acid, dodecylmercaptan and 2-mercaptoethanol.
Hydroxylamine and its salts, eg. hydroxylammonium chlor-ide, and alcohols can also be used as polymerization regulators. The monomers a) and b) are preferably sub-jected to solution copolymerization in an alcohol at from 50 to 200C in the presence of a polymerization initiator.
Examples of alcohols suitable for the solution polymeriza-tion are methanol, ethanol, n-propanol and isopropanol.
The copolymerization of acrylic acid and n- or isobutyl acrylate is preferably carried out in isopropanol as the solvent. After the polymerization, it is preferable to lZ7~754

- 4 - O.Z. 0050/37623 neutralize some or all of the carboxyl groups of the co-polymer. If the polymerization is carried out in an organic solvent, this solvent is removed from the reac-tion mixture before the neutralization step and is reused for a further polymerization. The distillat;on residue is then neutralized with an alkali metal hydroxide solu-tiOn, eg. sodium hydroxide solution or potassium hydrox-ide solution, ammonia or an amine, eg. ethylamine, ethanolamine, trimethylamine or morpholine. The degree 1û of neutralization of the copolymers is preferably from 60 to 100X. lf the polymerization is carried out in an aqueous med;um, the resulting copolymer is neutralized directly in the form obtained in the copolymerization, using one of the above bases or a mixture of these.
This procedure gives copolymers ~hich have a K
value of from 1û to 65 (determined according to H. Fikentscher in 1X strength aqueous solution in the form of the ammo-nium salt at pH 9 and at 20C). The K value of the copoly-mers is preferably from 15 to 35.
The copolymers thus prepared are used as disper-sants for pigments which are added to the stock suspension during papermaking. To date, the pigments have been introduced directly into the stock suspension when making filler-containing papers. However, it is more advantage-ous to disperse the pigments beforehand with a dispersant and then to incorporate them into the stock suspension, since this results in the pigments being dispersed more finely in the paper and hence also gives papers possess-ing better optical properties. ~hen the copolymers are used as dispersants for pigments, an aqueous pigment suspension is first prepared; this suspension contains from 40 to 80X by weight of a pigment in which from 40 to 90X of the particles have a diameter of < 2,um and from 0.1 to 2X by weight, based on the pigment, of coPoly-mer which contains, as copolymerized units,a) from 40 to 95X by ~eight of an ester of acrylic acid or methacrylic acid with a saturated monohydric i754 - S - O.Z. 0050/37623 C1-Cg-alcohol or with a saturated dihydric Cz-C4-alcohol and b) from 60 to 5~ by weight of an ethylenically unsaturated C3-Cs-carboxylic acid S and has a K value of from 10 to 65 (determined according to H. Fikentscher in 1~ aqueous solution in the form of the ammonium salt at pH 9 and at 20C) and may be par-tially or completely neutraLized with an alkali metal hydroxide solution, ammonia or an amine. The pigment suspensions preferably contain from 0.2 to 0.7~ by weight, based on pigment, of copolymer of the components a) and b). They are prepared, for example, by a method in which the copolymer, preferably one which has been completely neutralized with ammonia, is first dissolved in water, and the pigment is added to the stirred solution. The pigment suspensions in the stated concentration ranges are capable of being pumped and can therefore be added to the paper stock in a simple manner.
The copolymers of the components a) and b) differ from other polymers used as dispersants in particular in that neither the drainage nor the action of retention aids conventionally used in papermaking is adversely affected. Moreover, there is no adverse effect on paper sizing, as is found when conventional dispersants are present. This is a great advantage in papermaking, since the production rate is not adversely affected and the amount of retention aids required is not increased.
~ hcn the copolymers of the components a) and b) are used for dispersing pigments for coating paper, these copolymers, although not as effective as the polyacrylic acids conventionally used for this purpose and disclosed in, for example, European Patent 2,771, do have the great advantage that coated paper rejects, which are always encountered in paper coating, can also be reused for papermaking, without adversely affecting the drainage rate or the action of the retention aids.
The copolymers of the components a) and b) are 1Z'7~754 - 6 - O.Z. 0050/37623 used for dispersing clay, calcium carbonate (lime, chalk or precipitated chalk), talc, bentonite and titanium di-oxide. The pigments may be both filler pigments and coat-ing pigments. In the pigments, from 4û to 90% of the S particles have a diameter of < 2 ~um. The pigment suspen-sions are prepared at room temperature in a conventional apparatus, for example in a deliteur, a dissolver or a caddy mill. The temperature of the pigment suspen-SiGn may increase somewhat during preparation owing to the effect of high shear forces during dispersing. The aqueous pigment suspension is then added to the paper stock, prior to sheet formation, in an amount such that the papers produced contain from 2 to S0, preferably from S to 30, X by weight of pigments.
Testing of the paper sheets for the Examples described below was carried out after the sheets had been conditioned for 24 hours at 23C and a relative humidity of 50%.
The acceleration of the drainage procedure was determined from the increase in freeness in degrees SR
(Schopper-Riegler) according to data sheet 107 of the Verein der Zellstoff- und Papierchemiker und Ingenieure.
The drainage time was measured with the aid of a Rapid-Kothen apparatus.
In the Examples, parts and percentages are by weight. The viscosities were measured in a Erookfield-type rotational viscometer at a speed of 100 min 1 and at 20C.

The following 52Z strength aqueous chalk suspen-sions were first prepared from a finely divided chalk in which 70X of the particles had a diameter of < 2 ,um:
a) Comparison Finely divided chalk was stirred into water without the use of a dispersant. An aqueous suspension having a pH of 9.0 and a viscosity of 2350 mPa.s was obtained.

127~754 - 7 - O.Z. OOSO/37623 b) Comparison A commercial dispersant (sodium salt of a homopolymer of acrylic acid, having a K value of 24 at pH 9.0) was first dissolved in water, after which the above finely divided chalk was added to give a 52% aqueous chalk suspension which contained 0.2X, based on chalk, of the dispersant. This chalk suspension had a vis-cosity of 85 mPa.s at pH 9Ø
c) Example according to the invention The ammonium salt of a solution copolymer of 80X by weight of a mixture of n- and isobutyl acrylate in a weight ratio of 1:1 and 20X by we;ght of acrylic acid ~K value 24 at pH 9.0) was dissolved in water, after vhich finely divided chalk was added to give a 52X strength a~ueous chalk suspension which contained O.ZX, based on pigment, of the copolymer. The suspen-sion had a pH of 9.0 and a viscosity of 260 mPa.s.
Three 0.5Z strength stock suspensions in water were then prepared from pure pine sulfite pulp, and one of the chalk suspensions a) to c) described above was added to each of the suspensions so that in each case a pigment content of 40X, based on dry fiber, was obtained. The resulting stock suspensions a) to c) each had a pH of 7.5 and a Schopper-Riegler freeness of from 54 to SS.
Each stock suspension was then divided into three parts, and its behavior with regard to sheet formation was tested in a Rapid-Kothen laboratory sheet former.
The first part of the stock suspension was used directly for sheet formation, while 0.2X and 0.4Z of a commercial cationic retention aid was added to the second and third parts, respectively. The retention aid was a condensa-tion product of adipic acid and diethylenetriamine, grafted with ethyleneimine and then crosslinked with a polyalkylene oxide whose terminal OH groups had been reacted with equi-valent amounts of epichlorohydrin, according to Example 1 of German Patent 2,434,816. The results obtained are summarized in Table 1.

lZ7~754 - 8 - O.Z. 0050/37623 -E ~ 0~ N N
C~ O `O 1~ O`
`O `O `O

Cl "~ N X ~ N
~ C O U~ O ~
`O `O `O
00 U~ O
U~ U~ `O
-._ ~: E o o~ o o ~ o ~ U~ ~
X

X ~ N
~_ O Il~ U~
U~ o 1/~ U~ U~
~ .
~X `O `O ~
~ O ~ ~0 ~ N V~ N ~J
O _ O~ ~, U~ ~ U~
~ ~ ~O N
O O O` I~

O ~ ~ ~
0 D L~
~h _ _ lZ7~754 - 9 - O.Z. 0050/376Z3 As shown in Table 1, the presence of the commer-cial polyacrylic acid as a dispersant interferes to a very great extent with the drainage and the retention process for stock suspension b), in comparison with the S dispersant-free suspension a). For example, the drainage time is increased, the freeness is less greatly increased and the basis weight is reduced. On the other hand, when stock suspension c) is used, no such interference is observed.

A O.SX strength a~ueous stock suspension is first prepared from 80Z of groundwood and 20~ of bleached sul-fate puLp, and the said suspension is then divided into three parts, each of which was mixed with the a~ueous chalk suspensions a) to c) described in Example 1, so that the resulting pigment content was 40X, based on dry fiber.
The pH of the stock suspensions 2a) to 2c) thus obtained was 7Ø These stock suspensions were then divided into three parts, one part being tested directly and the other two parts being tested after the addition of 0.08 and 0.16X, respectively, of a commercial cationic retention aid. The retention aid used was a water-in-oil emulsion of a copolymer of 70X of acrylamide and 30X of diethyl-aminoethyl acrylate. The results are shown in Table 2.
In this case too, it is clear that the copolymer used according to the invention does not interfere with the drainage of the paper stock whereas the commercial dis-persant has a very adverse effect.

iZ7~754 - 10 - 0.~. 0050/37623 Stock Drainage time tsec~ 8asis weight ~g/m2 suspension after the addition of ... % of a commer-cial retention aid 0 0.08 0.16 0 0.08 0.16 (a) Comparison99 45.5 34.3 61.7 73.1 77.16 (b) Comparison98.4 56.3 42.3 60.8 71.3 73.9 (c) According93.5 46.3 34.5 61.4 73.9 77.3 to the 10invention Three 52% strength suspensions having a pH of 7 were prepared by dispersing clay in water. In the clay, 45X of the particles had a size of < 2 ,um.
a) Comparison A 52Z strength c~ay suspension in water was prepared, the suspension having a viscosity of 4150 mPa.s.
b) Comparison A 52X strength aqueous clay suspension was prepared using 0.2X of the polyacrylic acid described in Example 1b). A clay suspension having a viscosity of 40 mPa.s.
was obtained.
c) Example according to the invention A clay suspension having a viscosity of 1400 mia.s was prepared using, as a dispersant, 0.2X, based on pigment, of a solution copolymer of 80X of isobutyl acrylate and 20X of acrylic acid, the copolymer being prepared in isopropanol and having a K value of 23.
A O.5Z strength stock susPension was then prepared from pure pine sulfite pulp, the suspension containing 1% of alum and having a pH of 5. This suspension was divided into three parts, and the pigment suspensions described above under a) to c) were added to each of the parts so that the pigment content of the resulting stock suspensions was 40X, based on dry fiber. The three dif-ferent stock suspensions a) to c) thus obtained were - 11 - 0.~. 0050/37623 divided into three parts, one of these parts being tested without any further additives while the second part was tested after the addition of 0.2~ of a retention aid and the third part after the addition of 0.4% of a retention aid, the tests be;ng carried out in a Rapid-Kothen labora-tory sheet former. The retention aid used was the commer-cial product described in Example 1. The results, which are given in Table 3, show that the copolymer used accord-ing to the invention ~stock suspension c)) does not inter-fere with the drainage and retention, whereas the commer-cial dispersant (stock suspension b)) results in poorer retention.

Stock Drainage time Csec] Lasis weight ~g/m2]
15 suspension after the addition of ... ~ of a commer-retention aid 0 0 2 0.4 0 0.2 0.4 (a) 86.4 61.4 53.7 64.8 67.0 69.1 (b) 88.3 65.6 59.4 65.5 65.3 68.4 (c) 83.3 60.2 50.4 63.1 67.0 70.3

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of a paper containing from 2 to 50% by weight of a pigment, which comprises adding an aqueous pigment suspension to paper stock prior to sheet formation, wherein said aqueous pigment suspension contains from 40 to 80% by weight of a pigment in which from 40 to 90% of the particles thereof have a diameter of less than two microns and, at a dispersant, from 0.1 to 2% by weight, based on the pigment, of a copolymer which contains, as copolymerized units, (a) from 40 to 95% by weight of an ester of acrylic acid or methacrylid acid with a saturated monohydric C1-C8-alcohol or with a saturated dihydric C2-C4-alcohol, and (b) from 60 to 5% by weight of an ethylenically unsaturated C3-C5-carboxylic acid, and wherein said copolymer has a K value of from 10 to 65 (determined according to the method of H. Fikentscher in 1% strength aqueous solution in the form of an ammonium salt at pH 9 and at 20°C).

2. A process as claimed in claim 1 wherein the copolymer is us-d in the form partially or completely neutralised with an alkali metal hydroxide solution, ammonia or an amine.

3. A process as claimed in claim 1 wherein the copolymer used contains, as copolymerized units, a) from 65 to 90% by weight of an ester of acrylic acid or methacrylic acid with a saturated monohydric C1-C8-alcohol or with a saturated dihydric C2-C4-alcohol and b) from 40 to 10% by weight of an ethylenically unsaturated C3-C5-carboxylic acid and has a K value of from 15 to 35.

4. A process as claimed in claim 1 wherein the copolymer used contains, as copolymerized units, a) from 65 to 10% by weight of an ester of acrylic acid with a saturated monohydric C4-alcohol and b) from 35 to 10% by weight of acrylic acid, methacrylic acid, maleic acid or fumaric acid and has a K value of from 15 to 35.

5. A process as claimed in claim 1 wherein an ammonium salt of a copolymer which contains, as copolymer-ized units a) from 65 to 90% by weight of a butyl acrylate and b) from 35 to 10% by weight of acrylic acid or methacrylic acid and has a K value of from 15 to 35 is used.

6. A process as claimed in claim 1 wherein an ammonium salt of a copolymer which is prepared by solu-tion polymerization of the monomers (a) and (b) at from 50 to 200°C in the presence of polymerization initiators and polymerization regulators, evaporation of the solvent and neutralization with ammonia is used.