Note: Descriptions are shown in the official language in which they were submitted.
2~2~ ~5~
The preparation of coated papers and cardboard
employs, to a very large extent, aqueous coating composi~
tions which, besides the customary white pigments, in
particular china clay and calcium carbonate, contain
synthetic dispersions as binders.
As a rule, the optical brightening of these
coating compositions is achieved by the use of substan-
tive snionic whiteners, for example, alkali metal salts ~ -
of bistriazinylaminostilbenedisulphonic acids. However,
these whiteners have only very unsatisfactory optical
- brightening effects and a very low greyness limit (white- ~`
ner concentration at which further addition of the
whitener gives no increase, or even a decrease, in the
degree of whiteness). Furthermore, the abovementioned
types of whitener have a very low light fastness in
coating compositions of that kind.
It i8 generally known to partially eliminate the
abovementioned difficulties in two different ways:
1. By adding to the coating composition minor propor-
tions of hydrophilic cobinders to which the
whiteners can become attached. Cobinders of this
type are for example:
starch, casein, carboxymethylcellulose, alginates,
polyvinyl alcohol, polyacrylate, and melamine- or
ureaformaldehyde resins (cf. ~Das Papier" 36
(1982), 66).
2. By the use of specific water-soluble whiteners which
contain hydrophilic carriers. Carriers of this type
may be, for example, the polyglycols (cf. DE-A-
3,502,038 and EP-A-43,790). -~ ;
Le A 27 045 - 1 - ~
... .. . . . . , ~ . .. -
2 0 2 ~ 9 : ~
Although the brightening effect can be si~nifi-
cantly improved by these methods, the water sensitivity
of the coated papers is at the same time increased, which
causes difficulties in certain printing processes and on
use of the coated papers. Moreover, the use of these
additives in the coating composition often causes rheo-
logical problems on modern fast-running coating equip-
ment.
Furthermore, it is known (cf. DE-A-3,112,435) to
attach water-soluble whiteners to certain plastics ~uch
as, for example, urea resins or methylol resins and to
add these optically brightened plastics in the form of a
dispersion to the paper coating composition. However,
these proposals have hitherto not become established
owing to their excessively high cost or owing to rheo-
logical difficulties.
Surprisingly, it has now been found that composi-
tions for paper coating can be optically brightened using
the abovementioned whiteners and without the abovemen-
tioned disadvantages if optical brighteners, water-
dispersible cellulose powder and optionally coating
pigments or an optical brightener product composed
essentially of cellulose powder coated with optical
brightener and also optionally a coating pigment are
added to the coating compositions in any desired order,
the cellulose powder having in every case an average
particle ~ize of less than 50 ~m.
An example of a suitable cellulose powder is
. .
microcrystalline cellulose which is obtained from cotton ~ -
linters or wood cellulose usually by a combination of ~-
-' .
: .
~e ~ 27 0~5 - 2 -
20;2~59
hydrolytic degradation and mechanical wet crushing of the
particles and also by subsequent spray drying, the
microcrystalline cellulose no longer having any kind of
fibre structure (cf. "Umschau" 77 (1977), 312).
Also suitable are cellulose powders which have
been prepared by dry grinding of customary wood cellulose
and which likewise virtually no longer have any fibre
structure.
Particularly suitable are cellulose powders which
have a high crystallinity, above 30%, preferably above
50%, and also have a diminished hydrophily and are
dispersible in water up to a solids content of about 25%.
Also suitable are water-insoluble substituted
types of cellulose, for example cellulose ethers such as
methyl- or ethyl-cellulose, or cellulose esters ~uch as
cellulose acetate, cellulose acetobutyrate and cellulose
acetopropionate.
Preference is given to cellulose powders of the
type which have particle sizes of 25 - 35, in particular
about 30 ~m.
The coating composition can be prepared by incor-
porating up to 20% of cellulose powder (relative to the
total amount of pigments added).
Suitable cellulose whiteners which are to be used
according to the inven~ion are bistriazolylstilbenedi-
sulphonic acid, bisstilbenedisulphonic acid and in
particular bistriazinyl-aminostilbenedisulphonic acid.
Examples of particularly suitable optical
brighteners are those of the formula
Le A 2? Q~5 - 3 -
: :~ . -: - . . . . .. .
~ 2021~59
X H=CH ~ N ~ 1
~==N N==~ (I) :;:.
X2 S03Z S03Z X2
in which
X1 is amino, methylamino, ethylamino, dimethylamino,
diethylamino, 2-hydroxyethylamino, 3-hydroxy-
propylamino, di-(2-hydroxyethyl)amino, di-(2
hydroxypropyl)amino, 2-sulphoethylamino, morpho-
lino, anilino, chloroanilino, sulphoanilino,
methylanilino or disulphoanilino and
X2 i8 hydroxy, methoxy, ethoxy, methoxyethoxy,
chlorine or Xl,
and also those of the formula
X X3
k N ~ H=CH ~ ~NN ~ (II)
X4 S03Z S03Z X4
in which
X3 and X4 are hydrogen, methyl, ethyl, phenyl or ``
lS sulphophenyl,
and also of the formula .;
X5 ~ H-CH ~ H=CH ~ 5 (III) ~`
S03Z S03Z ;``
~. - . .~
, ~ ' ~"`.'.`'";.'',~",'.,"".
'.`.'~
.:
::
,.:: .:, . - .- -
Le A 27 a4s - 4
: :
::
^ 202~9
in which
X5 iS hydrogen, methyl, ethyl, methoxy, ethoxy,
chlorine or sulpho and in every case -
Z is an alkali metal ion, amine ion or ammonium
ion.
The optical brighteners are preferably present in
the form of their sodium salts.
Suitable coating pigments are clay, kaolin,
silica, aluminium oxide, aluminium silicate, talc and in
particular chalk. These pigments are preferably used in
the form of an aqueous slurry. ~ -
The optical brightener preparations according to
the invention can be simply prepared by dispersing
suitable cellulose powders in water and adding customary
anionic optical brighteners for paper or textiles.
Surprisingly, even at low temperature, the
optical brightener becomes attached to the dispersed
cellulose powder without difficulty, and the optically ~ -
brightened cellulose powder shows no greyness even at
relatively high levels of optical brightener addition.
If the whitener preparations according to the ~ ;~
invention contain customary white pigments, the prepara~
tion can be simply carried out by admixing suitable -~
cellulose powders and also customaxy anionic optical
brighteners for paper or textiles, if necessary together
with a little water, with commercially available filler
slurries with stirring or else by admixing even during
the dispersion and grinding opexations which take place
during the preparation of the filler slurries.
The resulting dispersions may contain customary
:~ :"'' -'.
:::' '~
~ . :..
Le A 27 045 - 5 -
.~:
~- 2021~
anionic or non-ionic dispersants, for example salts of
polycarboxylic acids, sulphated or unsulphated higher
alkanol ethers or alkylphenol polyglycol ethers, lignin-
sulphonates, ~arboxymethylcellulose, hydroxyethylcellu-
lose and the like.
Surprisingly, despite its high hydrophily, the
cellulose powder additive causes only a slight inçrease
in viscosity of the filler slurries, which may conse-
quently contain high solids concentratione of up to over
70%.
The cellulose powders may be used in the whitener
preparations according to the invention in any desired
ratio relative to the white pigment, and the viscosity
can be ad~usted to a desired value by simultaneous
addition of water. Preference is given to a cellulose
powder content of up to 20% relative to white pigment,
and high pigment concentrations in the slurry can be
~elected without causing any significant increase in
viscosity.
The coating composition may be prepared by
incorporating up to 20% of cellulose powder (relative to
the total amount of added-pigment) into the coating -~
composition.
The amount of optical brightener added is governed
by the amount of cellulose powder which is present in the
whitener preparations according to the invention. Gener-
ally, it is possible to use amounts of up to 10% of
active ingredient relative to cellulose powder. Larger
amounts do not have any further effect since the greyness
limit i~ exceeded. Preference is given to the use of
,': ~ : '~
`: :
Le A 27 045 - 6 -
~ . .
^` 2021~9
amounts of from 2.5 to 7% relative to dry cellulose
powder. In this addition, it is advantageous to use the
optical brightener in the form of an approximately 10 to
30% strength, preferably 15 to 25% strength commercially
available liquid formulation.
The optical brightener preparations which are to
be used according to the invention are novel and are
therefore likewise a sub~ect of the present invention.
The optical brightener preparation may be added
in the form of a dispersion to the coating composition
immediately after this has been prepared and may be
processed on customary coating machines.
The aqueous optical brightener products may also
be processed to form powder compositions by conventional
process steps such as filtration and drying.
In another variant according to the invention,
the cellulose powder and the optical brightener are added
separately during the preparation of the coating composi-
tion, the optical brightener immediately becoming
attached to the cellulose powder which has been employed.
The amount of optical brightener or of optical
brightener preparation which is used is governed by the
desired optical brightening effect. Generally, from 0.01
to 0.5~ by weight of the pure active ingredient of the
optical brightener (relative to the pigment of the paper
coating composition which i8 to be brightened) is suffi-
cient. A particular advantage is that, depending on the
constituents of the coating composition, the achievable
greyness limit (up to which further addition of optical
brightener is still effective) is exceptionally high.
Le A 27 045 - 7 -
2 ~ 2 1 ~ ~ ~
It is particularly advantageous to use the
optical brightener in the form of an approximately 10 ~
30% strength, preferably 15 - 25% strength, commercially
available liquid forming (% = percent by weight).
Example Ia
250 parts of microcrystalline cellulose having an
average particle size of 20 ~m, for example Avicel PH 105
from Lehman and Voss, Hamburg) are dispersed in 750 parts
of water and to this dispersion are added 25 parts of an -
optical brightener of the formula (A) ; ;;-
H H ~ :
NaO35~3N ~03Na ,, . ~. ,.:,
N `~ NH ~ CH-C} ~ 3 NH ~ (A)
(HOCH2CH2)2N S03Na 503Na N(cH2cH2oH)2
(in the form of a 25% strength liquid formulation). -~-
This dispersion can be incorporated directly into
the paper coating composition. This gives an excellent
optical brightening effect with comparatively high light
fastness.
Example 2a
250 parts of cellulose powder having an average
particle size of 30 ~m (for example Arbocel BE 600/30
from Rettenmaier, Holzm~hle ~ber Ellwangen) are dispersed
in 750 parts of water and to this dispersion are added
25 parts of an optical brightener of the formula (B)
Le A 27 045 - 8 -
n ' ' ~ . . ~ ,
~21~3~ ;
~CH=CH~3~J3 (B)
S03Na S03Na
(in the form of a 13% strength liquid formulation).
This dispersion can be incorporated directl~ into
the paper coating composition. The resulting paper
coating has an excellent light fastness.
Example 3a
250 parts of cellulose powder (for example Avicel
PH 105 from Lehmann and Voss) are dispersed in 750 parts
of water and to this dispersion are added 50 parts of an
opt~cal brightener of the formula (C)
O~NH~H=CH~}NH~N (C)
( HocH2cH2 ) 2N S03Na S03Na N ( CH2CH20H ~
(in the form of a 25% strength liquid formulation).
After incorporation in coating compositions, this
gives a good optical brightening effect and a compara-
tively high light fastness in the paper coating.
Example 4a
250 parts of cellulose powder having an average
particle size of 30 ~m (for example Technocel 30~ from
Cellulose-F~llstoff-Fabrik, Nonchengladbach) are dispersed
in 750 parts of water and to this disper~ion are added
3.5 parts of an optical brightener of the formula (D)
Le A 27 045 - 9 -
~` 2021~9
..... .
, . -
H H --~
N ~NH ~ H=CH ~ NH ~ N (D)
/---N S03Na 503Na N ~
(in the form of a powder formulation containing ~8% of
pure optical brightener).
The resulting di~persion can be incorporated
directly into coating compositions. It likewise gives
excellent results.
Example 5a
250 part~ of cellulose powder having an average ~:
particle size of 30 ~m (for example Arbocel BE 600/30
10from Rettenmaier) are dispersed in 750 parts of water and :~
to this dispersion are added 5 parts of an optical
brightener of the formula (E)
H H
N N ~ (E)
NH ~ H=CH ~ NH ~ ~ N ::: :
~ ~ ~ N ~ :
H3C-N 503Na 503Na N-CH~ ;:
(in the form of a powder formulation containing 83~ of
active ingredient). : :~
The resulting dispersion is likewise eminently
suitable for incorporation in coating compositions. ~:
Example 6a
250 parts of pulverulent cellulose acetobutyrate
.'..~:
:: . ~ . : -
:: :
'~;: -~ ~ ..
Le A 27 045 - 10 -
~.. .
` 2 ~ 2 1 5 ~ 9 : : ~
are dispersed in 750 parts of water and to this dispersion
are added 25 parts of a 25% strength liquid forming of
the optical brightener (A). The dispersion can be incor~
porated directly into the paper coating composition. This
5 gives an excellent brightening effect with comparatively
good light fastness.
Example 7a
Stirred mixing of ~-
60 parts of china clay SPS
40 parts of calcium carbonate
20 parts of an anionic synthetic dispersion of an
acryloyl-ester-containing copolymer
having a solids content of about 50%
(for example Acronal S 320 D from
BASF)
0.5 parts of polyvinyl alcohol
parts of polyacryloyl ester as co-binder (40%
strength)
(for example Acrosol 40 D from BASF)
and
80 parts of water
is used to prepare a non-brightened paper coating compo-
sition having a solids content of about 55%, whose pH is
ad~usted to 9 using aqueous sodium hydroxide solutLon.
Example 8a (comparison, not according to the invention)
The coating composition from Example 6 is opti-
cally brightened by adding 1.6 parts of the optical
brightener of the formula (A) according to Example 1,
relative to pigment.
- :.
~: :
Le A 27 045 - 11 -
-'~ 2 0 2 1 ~ ~ 9
Example 9a
Stirred mixing of
60 parts of china clay SPS
30 parts of calcium carbonate
40 parts of the cellulose dispersion from Example .
1 containing 25% of optical
brightener
20 parts of an anionic synthetic dispersion of an
acryloyl-ester-containing copolymer
having a solids content of about 50%
(for example Acronal S 320 D~ from :~
BASF)
0.5 parts of polyvinyl alcohol -~
5 parts of polyacrylate binder ~40% strength)
(for example Acrosol 40 D from BASF)
and
50 parts of water :~
is used to prepare a paper coating composition having a ~ :
solids content of about 55% whose pH is ad~usted to 9 :. :~
using aqueous sodium hydroxide solution.
Examples lOa - 13a .
To a coating composition composed of
60 parts of china clays SPS
30 parts of calcium carbonate
~:;:
1~ psrts of cellulose powder (Arboc~l BE 600/30 ;~
from Rettenmaier)
20 part~ of an anionic synthetic dispersion of an ~ -:.
acryloyl-ester-containing copolymer ~ ~:
having a solids content of about 50
(forexample Acronal S 320 D from BASF)
-~: , . .
~':,~' :, ,~
~e A 27 045 - 12 -
: .:: . :
:
2021~9
0.5 parts of polyvinyl alcohol
5 parts of polyacrylate binder (for example
Acrosol 40 D from BASF)
and
80 parts of water
are added the following amounts of the optical brightener ~ ~-
for paper, according to formula (A) of Example la, and
the pH i6 ad~usted to 9 using aqueous sodium hydroxide
solution:
Example lOa: 0.4 parts of optical brightener of the
formula (A)
Example lla: 0.6 parts of optical brightener of the
formula (A)
Example 12a: 0.8 parts of optical brightener of the
formula (A)
Example 13a: 1.6 parts of optical brightener of the
formula (A).
Example 14a
The coated paper is prepared by applying the
coating compositions from Examples 7a - 13a to paper
using a doctor blade or test coating equipment and drying
the product at 80C. ;
Table 1 shows the CIE whiteness of the papers
after preparation and also after exposure to light (1
week in daylight).
Papers which have been prepared by the process
according to the invention have, even at reduced optical
brightener concentration, not only a higher degree of
whitenes~ but also a significantly improved light
fastness.
e A 27 045 - 13 -
~,. ~:: : . . .
: - ..
- . :
w a~ o~ o N 1` CO` O~ O
O 1 ~t~
u~ ~ a~_~
~ ~ O ~
a\ ~ , "
a~
U~ o O~ t~ o
~ a~ o _~ N
W ,~ ~ ~ o t~
u ~ 0 a~ ~ : .
o
a~ ~0 o u~
~ ~ 0~ t~ O~ O C~ O
_
a3 0 ~
¢ ~ a~ ~: a!
P. w a~ ~ a~ a~ a~ ~ a~ ~ a~ a~ a~
~0 a~ C , ~,, .c_l ~ O ~ , " ~,
rl .: :,
a~ ~ ~ ~ a~ A a~ ~ a~
_I U --I u --0l u 0 u ,a u
u u ~ u ~ u ~ u ~ u ~ -
rl O rl O ~ O ~rl O ~1 0' ''
~ æ æ~ æ~ æ~ æ~ æ~
o o a~ o ~ o a~ o a~o a~ . --
0 W W d W rl W X W X W X
rl ~ O O W O W O W O W O W
dP d~
_I _I O O O _I : .:
.: .:
:-'
0 0 ~ 0 0 0 0 :' '.:'.'.' .'
I` Ct~ O~ O _I N ~
a~ a~ a~ a~
x x x x x 0 x0
. --: - -:
Le A 27 045 - 14 ~
.
~- 2 ~ 5 9
Examples 15a - 22a
.
A coating composition is prepared similarly to
Example 7a, but one part of the chalk is replaced by the
same amount of microcrystalline cellulose ~Avicel PH 105)
to which 2% in each case of the optical brightener of the
formula (A) or (B) has previously been fixed.
The degree of whiteness of the coated papers was
examined after preparation and also after exposure to
light (Table 2). Papers which have been prepared by the
proces~ according to the invention have not only a higher
- degree of whiteness but also an improved light fastness.
Le A 27 045 - 15 -
~A ~ . , '
2~
C
U~ ~ : .
u~ la ~ :
a~ o ~ ~ ~ ~ O ~D ~ ~ O
o e P.`~
V ~-,~ o o~
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3 o~ :
~o .,
~U 0 o 1~ H ~r N
~1 ~I.C o ~ o ~
P ~1 co cn o ~ o o o `': "'"~"
a) o _~ ~.
H ~ ~ O ~ : ~
0 ~D ~ . :: .
'
a
C .Oe o~ O ~ N t`~
~D ~ ~ O ~ 1~ ~ N
~J J, (~J ~J~ o o N ~
H ~LI S-l
t~, u ~a P. . .
~. . . ,' ~
E J ~ '~
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~ C m ~r~ m m
~rl Oe ~
~ ~, 4~
~ p t~rl o o o o o o o o
0 rl ~ U~ ~ ~ ~ ~ . ;.
U J,~ '1 Q N N N N N N N N
.rl ~rl ~ a. ` :::
O O
0 ~ U
~ P~ O Pl O p~ O p~ o p~ o p~ o , ~
o ~, tn" u u u u u u ... ' ,~ .
Q Wo Wo Wo Wo Wo Wo~ ';. .`.:
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U 0 U U _I N ~I N`: ~- : : -
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0 0 la 0 ~ 1 0 "'- ' ;' '
0 --~ --I H --I ~I N N N ' ~
Le A 2 7 0 4 5 - 16
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. . -
~-- 2021~9
:,
Example lb
~, 90 parts of microcrystalline cellulose having an
3 average particle size of 20 ~m (for example Avicel PH
1050 from Lehman and Voss, Hamburg) are dispersed in 900
, 5 parts of chalk slurry (for example Omyalite 90~ from
,`~ Omya, having a solids content of 72%) and to this disper-
'~ sion are added 9 parts of an optical brightener of the
formula (A) ~
H H
Na03S ~ N ~ S03Na
N~NI~H- CH~NH~
( HOCH2CH2 ) 2N S03Na S03Na N(CH2CH20H) 2
(in the form of a 25% strength liquid formulation) and
I also 33 parts of water.
This dispersion has a solids content of 72% and
can be incorporated directly into the paper coating
composition. This gives an excellent optical brightening
effect with comparatively high light fastness.
I ~ Example 2b
! ~ 116 par~s of cellulose powder having an average
particle size of 30 ym (for example Arbocel BE 600/30
from Rettenmaier, Holzmuhle Uber Ellwangen) are dispersed
in 900 parts of chalk slurry according to Example 1 and
to this dispersion are added 9 parts of an optical
brightener of the formula (B)
. : .-
.: :
_e A 27 045 - 17 -
- 2Q~ 9
N~H=CH~3~3 (B)
503Na S03Na
(in the form of a 13% strength liquid formulation) and
also 80 parts of water.
This dispersion having a solids content of 70%
can be incorporated directly into the paper coating
composition. The resulting paper coating has an excellent
light fastness. ~
Example 3b ;
90 parts of cellulose powder (for example Avicel
PH 105 from Lehmann and Voss) are dispersed in 900 parts
of chalk slurry according to Example 1 and to this
di~persion are added 18 parts of an optical brightener of
the formula (C)
I~CH=CH~N~;> ( C )
)=N N=<
(HocH2cH2)2N SQ3Na S03Na N(CH2CH20H)3 '''~
lS (in the form of a 25% strength liquid formulation).
After incorporation in coating compositions, this
gives a good optical brightening effect and a compara~
tively high light fastness in the paper coating. ~ ~ -
Example 4b
90 parts of cellulose powder having an average
particle size of 30 ~m (for example Technocel 30 from
Cellulose-FUll~toff-Fabrik, Monchengladbach are dispersed
, ~ .,, :: .
,
Le A 27 045 - 18 -
-~` 2021~9
in 900 parts of chalk slurry according to Example 1 and
to this dispersion are added 1.2 parts of an optical
brightener of the formula (D)
H H
N ~NH~:H-CH~NH~ ~N , (D)
~,~> S03Na 53Na N~ ~:
:: ':'
(in the form of a powder formulation containing 78% of ~-
pure optical brightener) and also 60 parts of water.
The resulting dispersion can be incorporated
directly into coating compositions. It likewise gives
excellent results.
Example 5
90 parts of cellulose powder having an average
psrticle size of 30 ~m (for example Arbocel BE 600/30
from Rettenmaier) are dispersed in 900 parts of chalk slurry
according to Example 1 and to this dispersion are added
1.75 parts of an optical brightenar of the formula (E)
H - H
N ~NH~CH~CH~}NH~ N
H3C -N S03Na S03N~ N - CH3 ~ .r.. ..
This di~persion can be incorporated directly into
a coating composition. The resulting paper coating has a
good optical brightening effect and a comparatively high
light fastness.
. . .
Le A 27 045 - 19 -
2021.559
Example 6b
90 parts of cellulose powder having an average
particle size of 30 ~m (for example Arbocel BE 600/30
from Rettenmaier, HolzmUhle uber Ellwangen) are dispersed
in 900 parts of kaolin slurry (for example clay SPS~) and
to this dispersion are added 9 parts of an optical
brightener of the formula (A) (in the form of a 25%
strength liquid formulation) and also 250 parts of water.
This dispersion can be incorporated directly into
the paper coating composition. This gives an excellent
- optical brightening effect with a comparatively high
light fastness.
Example 7b
Stirred mixing of
60 parts of china clay SPS~
40 parts of calcium carbonate (corresponding to
55 parts of chalk slurry according to
Example 1)
20 parts of an anionic synthetic dispersion of an
acryloyl-ester-containing copolymer
having a solids content of about 50%
(for example Acronal S 320 D from
BASF)
0.5 parts of polyvinyl alcohol
5 parts of polyacryloyl esters as co-binder (40
strength) (for example Acrosol 40 D~
from BASF)
and
65 parts of water
Le A 27 045 - 20 -
2 0 '~
is used to prepare a non-brightened paper coating compo-
sition having a solids content of about 55%, whose pH is
ad~usted to 9 using aqueous sodium hydroxide solution.
Example 8b (comparison, not according to the invention)
The coating composition from Example 7 is opti-
cally brightened by adding 1.6 parts of the optical
brightener of the formula (A) according to Example lb, -~
relative to pigment.
Example 9b
Stirred mixing of
60 parts of china clay SPS~
55 parts of the chalk slurry from Example
containing 72% of optical brightener
20 parts of an anionic synthetic dispersion of an
acryloyl-ester-containing copolymer
having a solids content of about 50%
(forexample Acronal S 320 D~ from BASF)
0.5 parts of polyvinyl alcohol -
5 parts of polyacrylate binder (40% strength)
(for example Acrosol 40 D~ from BASF)
and
65 parts of water
is used to prepare a paper coating composition having a
solids content of about 55~ whose pH i8 ad~usted to 9
using aqueous sodium hydroxide solution.
Exam~le lOb
The coated paper is prepared by applying to paper
the coating compositions from Examples 7b - 9b using a
doctor blade or test coating equipment and the product is
dried at 80C.
Le A 27 045 - 21 - -
.'-~' :'
~ 2~21~9
Table 3 shows the CIE whiteness of the papers
after preparation and also after exposure to light (1
week in daylight).
Papers which have been prepared by the process
S according to the invention have, even at reduced optical ;~
brightener concentration, not only a higher degree of
whiteness but also a significantly improved light
fastness.
.~- ' ,',",':.' '
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