Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to aqueous dispersions of
polymers with thereon hydrophilic monomers grafted
and to a process for their preparation. These dispersions
are suitable for the preparation of improved coating
compositions for flat substrates, in particular for
coated printing carriers.
Synthetic binders based on copolymers of acrylic
acid esters, styrene, butadiene and unsaturated carbo~-
ylic acids are nowadays widely used for the manufacture
of coated paper used for printing. The copolymers are
used in the form of aqueous dispersions. However, these
binders only partly satisfy the stringent requirements of
high-speed coating machines. Their viscosity characteristics,
water retention, migration in the pigment matrix and
resistance to picking o~ten need to be adjusted by the
addition of so-called co-binders. The co-binders used
may be, for example, copolymers o$ vinyl acetate and acryl-
amide (German Offenlegungsschri$t No. 24 50 039). However,
these co-binders are only able to improve individual
characteristics but cannot provide an overall improvement
in quality for the multiplicity of properties requ~ed.
German O$~enlegungsschrift No. 27 03 418 discloses
pigmented paper coating compounds which, in addition to
the usual binder consisting of a synthetic late~ based
on polyacrylates, polymethacrylates, styrene-butadiene
polymers of polymers of vinyl acetate in the form of
aqueous dispersions, also contain a high molecular weight
water-soluble polymer as co-binder. An essential feature
of these paper-coating compounds is that the water-soluble
polymer is prepared by polymerisation in a water-in-oil
emulsion. This enables the viscosity characteristics~
water-retention and resistance to picking to be improved
but the problem 9 till remains to obtain high- $ilm
hardness together with good binding capacity, low
tendency to migration in the pigment matri~ sufficient
covering o~ the fibres, high opacity and, particularly
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in LWC paper (low weight per unit area), little or no tendency of
the printing ink to soa~ through the paper.
German Offenlegungsschrift No. 1 495 722 discloses a
process for the preparation of graft polymers by the reaction of
acrylamide with substantially amorphous olefine copolymers contain-
ing tertiary carbon atoms in aqueous dispersion by radical
activation, characterised in that the olefine copolymers containing
tertiary carbon atoms are ternary unsaturated copolymers of (1)
ethylene, (2) propylene or butene and (3) a diene or triene.
Although the products of this process have been
recommended for refining paper, the disadvantages mentioned above
are still inherent.
It has now been found that improved paper-coating com-
pounds which do not require an additional co-binder may be obtained
by using certain dispersions as binders.
This invention therefore provides an aqueous dispersion
having a solids content of 35 to 65 % by weight of a graft
copolymer, said graft copolymer having been obtained from a graft
substrate of an aqueous dispersion polymer of at least one
compound selected from the group consisting of styrene, butadiene,
isoprene, chloroprene, acrylonitrile, ~methlacrylic acid-Cl-C12-
alkyl esters, (meth)acrylic acid, itaconic acid, fumaric acid,
maleic acid, ~inyl acetate and vinyl chloride, on to which graft
substrate has been grated 0.5 to 15 ~ by weight, based on the
graft substrate, of at least one hydrophilic mo~omer selected from
the group co~sisti~g of acrylamide, methacrylamide, vi~yl
pyrrolidone and ethylene oxide with up to 60 ~ by ~eight o said
grafted hydrophilic monomer having been replaced by at least one
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hydrophobic monomer selected from styrene, butadiene, isoprene
and (meth)acrylic acid-Cl-Cl2-alkyl esters.
The graft substrates preferably consist of styrene/
butadiene copolymers containing 40 to 75% by weight of styrene,
21 to 59~ by weight of butadiene and l to 4% by weight of one or
more unsaturated carboxylic acids. In another preferred
embodiment, the graft substrates consist of copolymers containing
40 to 75% by weight of styrene, 21 to 59% by weight of acrylic
acid-Cl-Cl2-alkyl esters and 1 to 4% by weight of one or more
unsaturated carboxylic acids. In a particularly preferred
embodiment, the graft substrates consist of copolymers containing
40 to 75% by weight of styrene, 21 to 59~ by weight of butyl
acrylate and l to 4% by weight of unsaturated carboxylic acids.
The unsaturated carboxylic acids used in the preferred embodiments
may be fumaric, itaconic, maleic or in particular (meth)acrylic
acid.
According to the invention, from 0.5 to 15~ by weight,
based on the graft substrate, of water-soluble or at least
hydrophilic monomers are used for grafting, (meth)acrylamide being
preferred. It is particularly preferred to use for grafting from
0.5 to 6% by weight of (meth)acrylamide, based on the polymer
substrate.
The graft substrates are prepared in the form of aqueous
dispersions by the conventional techniques of emulsion polymerisa-
tion. These dispersions are used as starting substances for th~
subsequent grafting reaction which must be carried out undex certain
conditions in order to obtain reproducible dispersions according
to the invention.
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The invention therefore also provides an improved paper
coating compound which comprises an aqueous dispersion of pigment
and as the sole binder therefor, a dispersion as deined above.
The polymerisation initiators used may be, for example,
inorganic or organic peroxides. The grafting yield in the graft
polymerisation according to the invention, that is to say the
percentage of monomer which i~ chemically bound to the graft
subs~rate, may be from 10 to 95%.
The grafting reaction is preferably carried out at a
temperature of 70 to 100Co
In one embodiment given here by way of examplel a 52%
styrene-butadiene latex is used as graft substrate to which 0.9 to
; ~ : 1.7% by weight of acrylamide, based on the graft substrate, with
0.9 to 1.7% by weight of water, based on the graft substra-te, are
added at 95C, either all at once or in batch as with stirring.
The reaction is started by the addition of 0.1% of ammonium
~peroxydisulphat:e dissolved in 1% by weight of water, both
quantities bas~d on the graft substrate, and the temperature is
maintained at 95C with stirring until the reaction is completed.
Most of the paper-coating characteristics required
including the mo~t important, can be simultaneously optimised by
the aqueous dispersions according to the present invention so that
better results are obtained than with the cobinder methods
hitherto used~ The aqueous dispersions according to the invention
are preferably used for the preparation of paper-coating compounds
although other applications are also possible, e.g. the preparation
of coating compounds for other substrates, such as pasteboard,
cardboard, textiles and leather.
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The process according to the invention not only
ena~les many of the praperties to be optimised simul-
taneously but also provides the possibility of e~cellent
controlled optimisation of specific characteristics of
5 coatings, such as their compressibility~ microporosity,
colour absorption, smoothness and binding power.
It was surprisingly found that when the
aqueous dispersions, i.e. the modified latices
according to the present invention, are used for
10 the manufacture of lightweight printing paper, the
latex does not pass through the paper, covering of
the fibres is improved and a homogeneous~ uniform coating
matrix is obtained. The opacity, degree of whiteness
and smoothness are thereby improved and uniform and
15 rapid printing dye uptake are obtained together with
an e~ually good or improved binding power. The further
advantage of the binder according to the invention is
that the irreversible thickening which is liable to
occur in the substances hitherto used, e.g. polyvinyl
? 20 alcohol in staining colours, is now prevented.
Compared with the state of the art, the latices
modified according to the invention provide more
bulky and consistently more microporous coatings. In
offset printing, this ensures rapid and unifor~ uptake
25 of colour, which is particularly advantageous for wet_
in-wet multicolour printing.
For intaglio printing, latices modified according to
the invention provide high compressibïlity and smoothness
of the coating with less calendering work. The coating
30 structure described provides high and rapid colour-
absorption and high printing-gloss~
There ~s a demand for stiffer paper both for offset
printing and for intaglio printing. It has h~therto
been -~mpossible to produce stiff paper because the use
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o~ latices with hard films was accompanied by a marked
deterioration in the printing characteristics, i.e.
uneven porosity, reduced compressi~ility and missing
aots. However, due to the special structure of the
coating, latices with hard ~ilms modified according to
the invention enable stiff paper to preserve all the
characteristics necessary for intaglio printing so
that it is possible for the first time to use stif~
paper in the intaglio printing process.
Due to the uniform embodiment o~ the modified binder
in the coating matri~ and its high binding power, the
proportion of binder in the coating compound may be
reduced, whereby a further improvement in the printing
characteristics is obtained.
Example 1
a) Preparation of the ~raft latex_
Carbo~ylated styrene-butadiene late~ containing
600~ styrene, 36~ butadiene and 4% (meth) acrylic acid
and having solids content of 50%, the usual film
hardness of 50 to 60 Shore A and a high pigment binding
power is used as graft substrate which is reaoted with
a) 2% acrylamide = gra~t latex A and
b) 2~ methacrylamide = graft late~ B,
the percentages being based on the grait substrate.
The latex characteristics are entered in Table 1 below.
Table 1
Late~ A B Untreated
graft substrate
Solid content % 50 50 50
PE 5.0 5.3 5.2
Viscosity mPa.s 210 140 95
Film hardnessSh A 82-71 80-~9 56-48
b) Preparation of the coating compound and method of
carryin~ out the coatin~ test
A paper-coating compound suitable ~or the ma~ufacture
of paper for of~set printing îs prepared by dispersing
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100 g of china clay in an approximately 650/o aqueous
slurry using 0.2 g of ~mmoniacal water ànd 0.3 g o~
sodium polyphosphate. Th0 latices described under a)
are added to the homogeneous suspension, in each case in
a quantity amounting to 10~ (polymer solid content,
based on the clay). The PH is adjusted to 8.5 with
ammoniacal water after the final solids content of the
staining colour has been adjusted to 55% with water,
The compositions are shown in Table 2.
Table 2
Composition of paper-coatin~ _ompounds (~)
Experiment _1 2 3
Water for dispersion I 53.8 -
15 Water for final adjustment 1 26.2
Sodium polyphosphate I O.3
China clay Dinkie A 1 100
Graft latex A 20 - ~
Graft latex B - 20 t
20 Graft substrate - - 20
pH I 8.5
Solids content 1 55
The properties of the coating compQunds are shown
25 in Table 3 below.
When the coating compounds prepared as described
above have been paci~ied and freed from air, they are
applied to both sides of a bleached body paper of 55~
mechanical wood pulp and 45~ bleached sulphite cellulose
30 (pine wood) having an ash content of 9.9~ and a weight
of 50 g/m2, using an experimental doctor blad0 spread
coating machine at a speed of 200 m/min.~ the dry
weight of the coating applied being 8 to 10 g/m on
each side~
When the coating compounds with grafted late~ are used,
no coagulate or deposit is formed either on the doctor
blade or on the counter roller even aYter the machine
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has been in operation for a considerable time, and no
coating compound if sound to pass through the-p~per
to the counter roller. ~he coated paper is subse~uently
satinised by two passages through super glazing rollers,
and the paper is tested after it has been air conditioned
at 20C and 650/o relative humidity.
The results of the tests are summarized in Table 4
Table 3
Properties of the paper coatin~ compounds
Experiment 1 2 3
Solids c~ntent (%) 54.9 55.1 54.8
pH 8.4 8.5 8.4
~iscosity (mPa,s)
Brookfield (immediately) 594 670 390
Brookfield (after 24 hours) 682 743 430
Rheomat
at D = 3.5 . lol 320 342 376
a~ D = 1.103 42 44 49
Setting point oh 76.8 75.2 79.1
Water retention capacity slightly consid-
(comparison with ungrafted improved erably
late~) improved
Table 4
Coatin~ characteristics
Experiment _ _ 1 2 3
Application oi coating
(g/m on each side) I _ 10
Thickness (mm) o,o75 0.079 o.o6g
Water absorption (g/m2)
according to Cobb (60 sec.) 21 22 20.5
Smoothness according to
Bekk. (sec./ml) 1110 1190 640
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Table 4 (Cont.)
Roughness according to Bendtseen
(ml/min) 25 22 35
Bending strength un~er i~p2cts
according to Schlenker, 40 62 58 47
IGT picking resistance
(cm/sec.) 124 120 116
E and N wiping test (/0) 28 26 22
Intaglio printing sample pring 1 2 3
Micro contour test 1 2 4
Gloss (%) 2~ 25 22
Opacity (/0) 93.9 92.6 91.1
Degree of whiteness (/0) 82.3 82.4 81.6
Fibre covering (o,b) 78.0 7~.6 69.7
Dynamic sheet stiffness 0.16 0.16 0.14
c) Assessment
The coated paper samples from ~2~periments 1 and 2~ in
which the coated compounds contained the graft latices
according to the invention, show significant improvements
in hardness of the coating, opacity, printing character-
istics and covering of the fibres compared with thepaper coated with conventional latex which has not
been grafted (Experiment 3). The gloss was also somewhat
increased. The setting point of these coating compounds `
was reduced so that the coating can be rapidly immobilised
before the drying process proper and migration of the
binder can be suppressed. This enabled the tendency to
mottling to be also significantly reduced.
The marked increase in picking-resistance should be
3 particularly mentioned.
d) Test methods
.
The ~ollowing test methods were employed:
(atmosphere under which tests were carried out, unless
otherwise indicated: 20C, 65~6 relati~e humidity)
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Electrometric according to
DIN 53 785
Solid content: a) for latices: according to
DIN 53 189
b) for coating compounds: diff-
erence weighing after drying
at llO~C
Viscosity: According to DIN 53 788
a) measuring instrument: Brookfield
viscosimeter LVT~ spindle 3 or 4,
60 revs/min
Test temperature: 20C
b) measuring instrument for taking
rheographs: Rheomat 15, measur-
ing systems A-C, test temperature
20C.
Settin~ uoint of the coatin~ compounds:
100 g o~ the coating compound are
poured on an unglazed clay plate
which has been dried under con-
stant conditions. As soon as
the shine due to moisture has
disappeared from the surface of
the coating compound, a sample
is remo~ed to determine the solids
contents.
The solids content in percent
determined by this method defines
the setting point.
Water~ LL~ Y--bL~_
10 cc of the coating compound
are spread-coated over a sample
area of the paper to be coated,
using a small doctor blade~ height
of gap 0.2 mm, at a constant
~ coating speed for a few tenths of
a second. Water penetrating the
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~aper from the coating compound
causes the relative humidity of
the air to`rise on the underside
of the pape~, The relationship
o~ this moisture increase with
time is continously registered
- by means of a hygroscopic
humidity meter. The greater the
increase in moisture per unit
time, the lower is the water
retention capacity of the coating
compound. Humidity meter:
VAISALA (equipment HN 11)
measurements in air conditioned
chamber ( 650/o relative humity).
Application wei~ht: Di~ferences in weight per unit
area between air conditioned
samples o~ coated and uncoated
paper (according to DIN 53 111).
Thick_ess: Determination according to DIN 53 105
Water absorption accordin~ to Coob:
Determination according to
DIN 53 132 60 sec.
25 Smoothness accordin~ to Bekk:
Determination according to
DIN 5 3 107
Rou~hness accordin~ to.Bendtsen:
Determination according to
3 DIN 53 108
Fluxural strength according to Schlenker:
Determination according to
DIN 53 121 bending angle 40~,
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Dynamic sheet stiffness: Test method according to
Kodak-Pathe DIN 5~ 123
Pickin~ resistance: Determination according to
Tappi standard T 499 Su 64
using the IGT instrument,
Model AC2, and a crushing stress
of 35 kg/cm . The picking stain
used was low viscosity IGT
picking test stain.
10 K 1 N test: Colour absorption of coated
(printing ink absorption) paper determined by the usual
Tappi routine control method
RC 19, exposure time 3 min.
The K I N value is the differ-
ence in degree of whiteness
between untreated paper and
paper treat~d with test ink.
Measuring instrument: Elrepho,
Filter R 457. Testing colour:
K + N testing ink of K ~ N
laboratories Inc., Melrose Park
Micro contour test: Test method ~or assessing the
uniformity of the coating,
procedure similar to the K I N
~5 wiping test but time of e posure
to test only 2 minutes. Test
colour: blue test colour 3811 of
Lorilleux Druck~arben Gmb~,
Ratingen. The assessment is made
~isually, using a five-stage
assessment scale ~rom Grade 1 for
an absolutely homogeneous test
surface free ~rom stipples, gaps
and streaks, to Grade 5 ~or a
very turbulent, uneven sur~ace with
many faults.
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printin test: ~ntaglio printing sample test
on sample printing machine,
system Dr. Durner.
The printing results were assessed
visually ana graded according
to sharp~ess of the raster rep-
roduction, uni~ormity of colour
absorption, freedom from missing
dots, shining through and the
like. 3 Grades were used:
1 = very good
2 = good
3 = poor.
Gloss- Determination of the gloss
1~
according to DIN 67 530 against
- a comparison standard at a
measuring angle of 60~.
Measuring instrument: Lange
Reflectometer.
Opacity: Determination according to
DIN 53 146
Degree of whiteness: Determination with remission
photometer Elrepho, Filter R 457.
25 Fibre covering: The coating stain is painted
on black cardboard (application
8-10 g/m2 dry) and carefully
dried. The degr~e of w~iteness
(Elrepho, Filter R 457) of the
3 painted side of the cardboard
is then measured. The more the
coating covers the black surface,
the higher is the degree of
whiteness
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The ~ollowing is another method for assessing the
covering of fibre:
~ ~ery concentrated beam of a spectral photometer is
swept over the coated cardboard surface and the remission
is continuously recorded over a length of cardboard of
about 10 cm. A measure of the uniformity o~ fibre covering
is obtained by stochastic assessment of the fluctuations
in remission. Cross-section of the illuminating beam:
O ~ 2 . 2 ~ rectangular shutter.
10 E~ample 2
a) Preparation of the ~raft late~
As described under a) of Example 19 2% acrylamide =
gra~t latex C are grafted on a styrene-butadiene latex
similar to that of E~ample 1 but with a greater film
15 hardness of ca. 80 Shore A, a slightly lower pigment
binding power.
The properties of the late~ are entered in Table 5,
below
Table 5
20 Late~ C untreated
_ _ _ _ ~ra~t substrate
Solids content % 50 50
pH 5~3 5.1
~ Viscosity mPa.s 180 90
25 Film hardness Sh A 90-79 88-61
b) Preparation of the coating compound and method of
carryin~ out ,the coating experiment
The same conditions w~re employed as described in
Example 1 b). In addition, e~periments were carried
out (E~periments 6 and 7) using a binder content of
5~ under otherwise identical conditions to test the
printing ch&racteristics in the intaglio printing process.
The compositions are shown in Table 6 below, the properties
35 of the coating compounds in Table 7 below.
No coagulate or deposit on the doctor blade was formed
in these e~periments even after a considerable operation
time. The coating stain did not seep through the paper
(no deposit on counter roller).
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The samples of paper coated on both sides were
subsequently gla~ed over superglazing rollers and tested
after they had been air conditioned. The properties of
the coatings are summeri~ed in ~able 8 below.
Table 6
Composition of Paper coating compounds in ~
Experiment 4 5 ~ 7
Water for dispersion 1 53.~ -
Water for final adjustment ~ 26.2 27.1
10 Sodium polyphosphate 1 0.3
China clay l 100
Graft latex C 20 - 10
Graft substrate - 20 - 10
pH 8., ~ -
15 Solids content (%) ~ 55
Table 7
Properties of paper coating compounds
E~periment 4 5 6 7
20 Solids content (%) 55.2 55.1 55.1 54.9
pH 8.5 8.6 8.4 8.5
Viscosity (mPa.s)
Brookfield, immediately385 110 234 105
Brookfield after 24 hours 445 170 310 180
Rheomat
at D = 3.5.101 472 165 345 160
at D = 1.103 38 27 43 25
Setting point (%) 76.2 79.8 75.4 77.6
Water retention capacity slightly slightly
3 (compared with latex improved improved
which has not been grafted)
.
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Table 8
Coatin~ ProPerties
Experiment 4 5 6 7
Coating application (g/m2
5 ~n each side 10
~hic~ness (mm) 0.075 o.o68 0.077 0~067
Water absorption (g/m2)
according to Cobb (60 sec.) 21 19.5 24 22.5
Smoothness according to `
10 Bekk (sec./ml) 1100 680 980 640
Roughness according to
Bend~sen ~ml/min) 24 32 25 35
Bending strength under impacts
according to Schlenker, 40 40 38 29 26
15 IGT picking resistance
(cm/sec) 116 98 35 27
K & N wiping test (/) 27 23 32 28
Intaglio printing
. sample print - - 1 3
20 Micro contour test 1 2 1 3
Gloss (~0) 30 27 38 34
Opacity (/0) 93.2 92.7 92.0 91.6
Degree of whiteness (/0) 83.7 83.5 84.0 83.8
Fibre covering (%) 78.4 73.3 80.0 75.g
dyn. sheet strength 0.17 0.14 0.13 0.11
c) Assessment
As described in E~ample 1, the experiments carried out
in this E~ample also result in improvement of the coating
properties when late~ modified according to the invention
3 is used, even when ~ilm hardness is increased, so that
very good properties for the purposes of printing process-
es are obtained, including intaglio printing. A paper
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which is light in weight but stiff can thus be obtained,
which is particularly suitable for intaglio printing
in the LWC range,
Example 3
A relatively hard styrene-butadiene late~ having
a film hardness of about 95 Shore A and low pigment
binding power is grafted according to Example 1 a) with
1. 5% acrylamide = graft late~ D.
This e~ample is provided to illustrate the marked
increase in binding power obtained by the treatment
according to the invention.
A coating egperiment using graft latex D was carried
out as described in Example 1 and t~e coated paper was
tested,
The following Tables show the lateæ properties,
coating stain composition and data and test results.
This e2periment con~irms the possibility of increasing
the binding power of a paper-coating latex by treating
this latex in accordance with the invention.
Table 9
Latex D Untreated graft
substrate
Solid content (%) 50 50
pH 5.3 5.1
Viscosity mPaOs 140 82
Film hardness Sh A 97_95 89-86
Table 10
Composition of paper coatin~ compounds in
Experiment _ 8 9
30 Water for dispersion 53.8
Water for final adjustment 1 26.~ - J
Sodium polyphosphate 1 0.3
China clay Dinkie A 1 100
Graft lateg D 20
35 Gra~t substrate - 20
pH I _~.5
Solid content (/0) . 1 55 -
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Table 11
Properties of paper-coatin~ compounds
Experiment 8 9
Solids content (%) 55 .054 . 9
5 pH 8.4 8.5
Viscosity (mPa.s)
Brookfield, immediately 185 90
Brookfield, after 24 hours 265 110
Rheomat
10 at D = 3.5 . 101 `210 85
at D = 1.103 26 21
Setting point (/0) 78.2 78.6
Water-retention capacity as
(comparison with ungrafted comparison
15 latex)
Table 12
-
Properties of_coatin~
E~periment - 2 8 9
20 Coating application (g/m! on each
side ¦ 10 -
Thickness (mm) 0.75 0.75
Water absorption (g/m )
according to Cobb (60 sec.) 21.5 21
25 Smoothness according to Bekk.
(sec~/ml) 660 590
Roughness according to
Bendtsen (ml/min.) 26 27
Bending strength under ~cts
accor ~ g to 5chlenker, 4~ 62 59
IGT picking resistance (cm/sec.) 85 27
K & N wiping test (%) 26 28
Intaglio printing sample print 2 3
Micro contour test 2 3
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Gloss (o,b) 26 26
Opacity (/0) 92.5 92.6
De~ree cf whi~eness (/0) 81.8 81.9
Co~ering of fibres (%) 71.1 69.7
dynamic stiffness of sheets 0.19 0.18
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