Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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
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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.
EXAMPLE 1
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.
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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.
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-
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 _ _
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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.
EXAMPLE 2
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.
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TA3LE 2
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
EXAMPLE 3
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
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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.
TABLE 3
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
