Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Modification of Paper Coating Rheoloay
This invention relates to paper coating and more particularly the modification
of
paper coating rheology.
Various types of polymeric thickeners are well documented in the prior art.
Often these are in the form of aqueous emulsions which are either alkali
soluble
or alkali swellable.
WO-A-00/34361 describes a comb polymer comprising a backbone of
hydrophilic units and dihalogeno compounds and a moiety contain pendent
hydrophobe, being particularly suited as a thickener for latex paints. The
hydrophobic group is attached to the backbone of the preformed polymer.
WO-A-011 X360 describes an aqueous dispersion of copolymeric
microparticles, useful as an associative thickener in coating compositions
such
as painfis. The thickeners exemplified include at least 10% by weight
associative monomer.
~0 US 54.~~350~ describes coating a substrate by applying an ague ous coating
composition containing an alkali swellable complex hydrophobe associative
thickener. Since the polymer is alkali swellable it must be so substantially
cross-linked that the polymer is not soluble.
~5 It is well known that associative thickeners bring about low high shear
viscosity
at small dosages. They can thus give an economic advantage over other types
of synthetic thickeners. However an equally well known problem with
associative thickeners is that they provide poor water retention. So the
economic advantage of associative thickeners is normally only obtained in
30 conditions where water retention is not important.
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Water retention of coating colours has a profound influence on the runnability
of
the paper coating process. The coating colour loses a large proportion of the
water that it contains as soon as it comes into contact with the surface of
the
paper owing to capillary action, the extent of which depends on the absorbency
of the substrate. The pressure in the nip and under the blade also promotes
dewatering. The release of water from the coating colour can have the
following
consequences:
The solids content of the wet coating increases which modifies the Theology of
the coating before it comes into contact with the blade. In the worst case the
coating can form a thick, immobile "filter cake" which can cause streaking and
might even cause the web to break.
The water taken up by the paper causes a reduction in its internal bonding
1 is strength which causes it to tear more easily under tension.
The solids content of the coating colour re-circulated from the costar head
can
increase by 5% and more over a period of a few hours because the
concentration of water soluble polymer in the c~ating colour is also mach
lower
if too mush water is released. This phenomen~n is often observed at the
precoating stage.
It becomes more difficult to apply an even coat as the solids content of the
coating colour increases. The blade pressure often has to be increased in
order to maintain a constant coat weight.
In the manufacture of paper there is a tendency to seek ever higher coating
speeds and this requires lower high shear viscosity. Although low high shear
viscosity can be provided by associative thickeners the low water retention of
those thickeners can result in water from the coating permeating the paper
thus
weakening the paper. This in turn can place an unacceptable limit on the
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3
coating speed. In addition rapid release of water from the coating may prevent
the desired smoothing of the coating.
When the coating composition does not exhibit low high shear viscosity the rod
pressure of the coating apparatus can increase significantly. On the other
hand
when the coating composition results in poor water retention on the rod
pressure tends to be unstable. In both instances in this can result in damage
to
the coated paper and/or poor runnability of the water.
In order to improve the water retention other products need to be added
together with the associative thickeners and the economic advantage of those
thickeners is no longer obtained. As far as we are aware no proposals have
been made for improving water retention when using associative thickeners in
paper c~ating without the addition of an additional water retention aid so as
to
~btain the ec~nomic benefits of using associative thickeners f~r paper
coating.
The present invention has been made in order t~ address this problem.
Acc~rding t~ the ine~enti~n there is provided a comp~siti~n f~r modifying the
the~logy ~f paper c~atings comprising an ass~ciative thickener characterised
in
that the associative content ~f the thickener is below 10~/~, the molecular
weight
of the thickener is below about 1 million, and the acid content is at least
10~l'o by
weight.
We have discovered that the adjustment of the three parameters, associative
content, molecular weight and acid content within the limits defined above
provides an associative thickener composition which results in both low high
shear viscosity and good water retention without the necessity for the
addition of
a water retention aid.
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It must be understood that while the three above mentioned parameters are
seen as essential for obtaining the desired combination of low high shear
viscosity and good water retention the dosage of the thickener will also have
an
effect on the performance of the thickener. For example lowering of the
molecular weight may require an increase in the dosage if the desired
viscosity
is to be obtained. In addition, lowering the acid content can lower the
efFciency
of the thickener so that an increase in dosage may be needed. Once these
consequences have been appreciated the appropriate adjustment becomes a
matter for a limited amount of experimentation in order to obtain the desired
result.
Associative thickeners for obtaining low high shear viscosity are well known.
They comprise hydrophilic, water soluble polymers with hydrophobic terminal
groups or side chains. The hydrophobic terminal groups may be aliphatic or
aromatic hydrocarbons and they are water insoluble. They can be joined to the
hydrophilic polymer backbone by means of a hydrophilic spacer so that they
remain flexible. Their structure is similar to that of surfactants.
Interaction
between the hydrophobic groups or side chains is what is considered to provide
for very high viscosity at low shear. There are various types of associative
thicl<eners but those which are generally useful for the purpose of the
present
invention are formed from ethylenically unsaturated monomers such as (meth)
acrylic acid, (meth) acrylates, malefic acid or anhydride, maleates, itaconic
acid,
itaconates, allyl ethers and vinyl esters. Ideally the choice of monomers is
such
that the polymer is insoluble in water and at neutral pHs, but dissolves under
alkali conditions, for instance pH ~ to 10 or higher. The polymers are
typically
made by aqueous emulsion polymerisation of the monomers to form an
aqueous emulsion of a polymer. Polymers of particular interest in connection
with the present invention are the hydrophobic alkali-soluble emulsions in
which
the hydrophilic polymer backbone comprises an alkali-soluble polyacrylate
derived from monomers such as alkyl acrylates for example ethyl acrylate,
acrylic acid and methacrylic acid. The hydrophibic side chains are attached to
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the polymer backbone by, for example, a polyoxyethylene oxide spacer.
Normally the hydrophobic side chains are provided by including ethylenically
unsaturated monomers that contain the hydrophobic side chains. Typically the
hydrophobic side chains are Cs-Cso alkyl groups. Thus the hydrophobic side
5 chain moiety can be bonded to ethylenically unsaturated components such as
acrylamido, acrylate or allyloxy etc.The thickening effect can be adjusted by
altering the ratios of the monomers. All this is well known. Examples of
associative thickeners used as paint thickeners, printing pastes and the like
are
disclosed in European Patent Specification 0 216 479 A1. Other associative
thickeners are described in detail in European Patent Specification 0 013 836
A1 and 0 011 806 A1. These can be used in the present invention subject to
their being modified in accordance with the definition of the invention as set
out
above.
ale have found that associative thickeners musf have the special combination
of
an acid content of at least 10°/~ by weight and an associative monomer
content
of below 10% of molecular weight below 1 million. i~llolecular weights of
below
700,000 have been found to be particularly suitable for use in the present
invention. An es~aecially preferred molecular weight range is X0,000 to
150,000,
and most preferably X0,000 to below 100,000.
The polymers may be made by the inclusion of branching or cross-linking
agents andlor chain transfer agents. However, it is preferred that the amounts
of branching or cross-linking agents are used in amounts such that the polymer
is substantially soluble in at least in alkali. Thus the polymer may be
substantially linear, branched or slightly cross-linked. Nevertheless, the
polymers desirably should not be so cross-linked that it is insoluble. The
amount of cross linking agent is generally below 2000 ppm (by weight) and
preferably below 1000 ppm. Typically the amount of cross-linking agent will be
below 500 ppm, for instance in the range of 1 or 2 ppm up to for instance 100
ppm, preferably 5 ppm up to 20 or 30 ppm. The cross-linking agent can be a
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polyethylenically unsaturated monomer, for instance methylene bis acrylamide,
butane diol diacrylate and tetra allyl ammonium chloride.
Preferred thickeners for use in the invention are alkali soluble aqueous
emulsions of ethyl acrylate/methacrylic acid/steareth 10 mole ethoxylate allyl
ether and having a molecular weight below 1 million and preferably in the
range
10,000 to 700,000 and more preferably in the range 50,000 to 200,000 or
250,000, and most preferably 70,000 and to below 100,000. Cross linkers and
initiators can be included if desired.
These polymers are mildly associative their associative content being
preferably
not above 5% and more preferably below 2.5%.
The acid functionality may be provided by any suitable acid, methacrylic acid,
acrylic acid or a mixture thereof being preferred. The acid content can be
from
10 to 70°/~, preferably 20 to 50°/~ by weight (based on the
weight of polymer).
As already indicated the precise values of the above paramefiers are chosen
having regard to the viscosity and water retention that the particular coating
operation demands and to the dosage of the thickener in the coating
composition.
In use the thickener of the invention is included in a conventionally
formulated
coating composition for a pre-coat or top coat. The dosage of the thickener of
the invention in the coating composition is kept as low as possible consistent
with the required viscosity being obtained. For example, in many top coat
formulations a dosage of thickener of 0.2pph will give a target Brookfiield
viscosity of about 1000 to 1400 mPas at 100 rpm (typical for "roll and blade"
coating method) in combination with good water retention of the order of 100
g/m2 or lower. Unless otherwise stated the viscosity is measured at
25°C.
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The target viscosity will to some extent be chosen having regard to the
coating
method that is to be used. Thus for film press a target Brookfield viscosity
is of
the order of 700 to 1000 mPas whereas for a jet applicator the target
Brookfield
viscosity is of the order of 2000 to 2500 mPas. In some instances the dosage
may need to be altered in order to obtain the required viscosity. Of course,
changing the dosage of thickener to obtain the desired viscosity can also
change the water retention. However adjustment of the acid content of the
thickener can, in those circumstances, be used in order to secure good water
retention.
When used the coating composition is preferably mildly alkaline, of the order
of
pH 3 to 10, preferably 3.5 to 9.5. Where necessary the pH can be adjusted for
example by the addition of an alkali such as sodium hydroxide.
The invention also provides a method of modifying the rheology of a paper
coating composition comprising adding to the coating composition a thickener
as defined above.
Further the in vention proe~ides a paper coating method comprising coating
paper
or paperboard with a paper coating composition, said composition including a
thickener as defined above.
The thickeners of the invention can be used in all methods of paper coating
and
at all possible coating weights for example from 2 to 25 g/m2. The thickeners
can be used in coating compositions for pre-coat, middle coat and top coat and
can be used with any pigments and binders.
The following examples further illustrate the invention:
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Example 1
Preparation of associative thickener.
Into a 700m1 resin pot fitted with stirrer, thermometer, nitrogen inlet and
outlet,
condenser and feed lines was placed 157.62gms deionised water and 10.5gms
Disponil FES993 (Alkylether sulphate, sodium salt).
The contents of the resin pot were heated to 85°C and then
degassed with
nitrogen for 30 minutes.
An aqueous emulsion was prepared by mixing 143.75gms ethyl acrylate,
100.OOgms methacrylic acid, 6.25gms steareth-10 ethoxylate allyl ether and
1.25gms n-dodecyl mercaptan into 10.5gms Disponil FES993 in 162.02gms
deionised water.
An initiator feed was prepared by dissolving 0.50gms ammonium persulphate in
37.5gms water.
After degassing, 0.25gms ammonium persulphate dissol~sed in 5gms water vas
added to the reactor followed by the start of the emulsion and initiator feeds
over 2 and 3 hours respectively.
The contents of the reactor were held between 83 and 87°C during
the feed
time and held for a further 1 hour at 85°C to reduce residual monomer
content.
The contents were then cooled to <40°C and filtered.
This resulted in an aqueous emulsion polymer having a molecular weight of
about 150,000 with a solids content of 41.0%.
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Example 2
A top coat was prepared according to the following recipe:
Mix @ 1200cps
60 parts CaCOs (Carbital 95)
40 parts Clay (Suprawhite 95)
parts SB latex (DL 950)
Solids content: 60%
Different thickeners were added to the mix and the pH adjusted by the addition
of 1.0% sodium hydroxide solution. The dosages of thickener and the high
shear viscosity and water refiention obtained are set out in the following
Table 1.
Table 1
Parity Amount AA- Cone
Sample Dose (againstpH 1 /~ BrookfieldGWR(gsm) &
(pph) control) ~IaOH viscosityWater Plate,
(mls) Retention HST
Blanlc - - 8.57 4.50 14.8 232 28
Control 0.60 100 8.67 5.50 1190 155 47
(CMC)
A 0.20 33 8.60 9.00 1150 215 43
B 0.315 53 8.54 11.0 1290 153 40
C 0.33 55 8.C2 11.0 1140 153 48
D 0.34 57 8.52 11.0 1160 157 35
E 0.45 75 8.45 13.0 1270 146 41
The composition of the control and samples A to E were as follows (all
percentages by weight):
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Control: Commercially available carboxymethyl cellulose
A: Commercially available hydrophobic alkali swellable emulsion HASE
B: Ethyl acrylate 55% methacrylic acid 40%; steareth 10 ethoxylate allyl
5 ether 5%
C: Ethyl acrylate 57.5% methacrylic acid 40%; steareth 10 ethoxylate allyl
ether 2.5%
D: Ethyl acrylate 5i3% methacrylic acid 40%; steareth 25 methacrylate 2%
10 E: Ethyl acrylate 53% methacrylic acid 40%; steareth 10 ethoxylate allyl
ether 2%
Compositions B to E also included about 5000 ppm of n-dodecyl mercaptan.
Fcr the evaluati~n ~f the results the c~mpositi~ns of the invention were
judged
by comparison of their viscosity and water retenti~n with fibs c~ntrol sample
where the thickener was carboxymethyl cellulose and HASE thickener A. The
thickeners of the invention gave lower high shear viscosity than c~ntrol and
about the same neater retention. The HASE thickener gave ab~ut the same
high shear viscosity as the thickeners of the inventi~n, but significantly
worse
water retention. Note that the d~sage of HASE thickeners and the thickeners
of the invention was about the same.
Water Retention GWR (gsm)
The water retention was measured in gsm ~n a gravimetric water retenti~n
meter using the parameters of 2 minutes dwell time, 1.5 bar pressure and 5pm
polycarbonate membrane filters.
High Shear Viscosity (HSV)
The high shear viscosity was measured in mPas on a cone and plate
viscometer at 10000s-~~
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Example 3
Certain of the samples used in Example 2 were added to a top coat
composition. The dosages and the resultant viscosity and water retention are
shown in the following Table 2.
Sample Dosage Brookfield ACAV Water Haake
pph Viscosity (100 0.6M Retention3000 1/s
rpm) 1/s glm2 mPas
mPas mPas
Control 0.6 1210 43 83 42
A 0.185 1280 38 123 31
B 0.2 1240 36 100 29
0.25 1400 37 93 31
The results confiml the results of Example 2, i.e. with the thickeners of the
invention (B and C) it is possible to get low high shear and good water
retention.
On the other hand typical HASE thickener (A) also gives low high shear
vise~sity beat p~~r water retenti~n.
ACAV measured with a capillary viscometer in which pressure in a cylinder
forces the sample through a capillary. By measuring pressure in the cylinder
and knowing the flow rate of the sample through the capillary the shear rate
and
viscosity of the sample can be determined.
Water retention was measured on a gravimetric water retention meter using the
parameters of 2.0 minutes dwell time, 0.5 bar pressure and 5 micrometer
membrane filters using a 20 ml sample. (Water retention was measured in the
same way in the following Examples 4 and 5).
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Example 4
A pre-coat formulation was prepared as follows:
100 parts CaCOs
10 parts SB-latex
Solids content: 63.0%
pH: 9
The target Brookfield viscosity (100 rpm): 900 mPas
Samples were added to the pre-coat formulation in the dosages set out in the
following Table 3 which also shows the resultant viscosity and water
retention.
Table 3
Sample ~osage Brookfield ACAV lfVater Haake
pph Viscosity (100rpm)0.6f~il Retention 3000 1/s
mPas 1/s g/m2 mPas
mPas
C~ntr~I 0.7~ 30 a 33 20T 4~.
A 0.13 000 27 202 29
B 0.3 9~5 26 13~' 33
C 0.4.5 1060 27 120 38
The results show that the control sample and the HASE thickener A provide a
poor water retention whereas the thickeners of the invention provide the
required viscosity and good water retention.
Example 5
A pre-coat formulation was made up as follows:
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100 parts CaCOs
6 parts SB latex
6 parts starch
Solids content: 62%
pH: 8.5
Target Brookfield viscosity (100 rpm): 800 mPas
Samples of thickener compositions Were added to the pre-coat formulation in
the dosages shown in Table 4 which also sets out the resultant viscosity and
water retention.
Table 4
Sample ~osage Brool~eld A~AV hater
pph !/iscosity (100rpm)0.6iill Retention
mPas 1Is glm~
mPas
control 0.2 810 52 110
A 0.15 840 58 88
B 0.25 840 54. 64
C 0.29 800 50 65
The results show better water retention for the control and HASE thickener A
than in the pre-coat formulation of Example 4, which does not contain starch.
However the thickeners of the invention have significantly further improved
water retention.
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Example 6
Various coating formulations were prepared employing thickener compositions
selected from a commercial HASE thickener, a commercial ASE thickener and
in a thickener composition according to the invention. The composition of the
coating formulation is a shown in Table 5.
Table 5
Materials 1 2 3 4 8 9 10 14 15 16
Premier 75 75 75 75 75 75 75 75 75 75
Hydrocarb 90 25 25 25 25 25 25 25 25 25 25
~ispex N. 40 0.15 0.150.150.150.15 0.150.15 0.150.150.15
SPP 0.05 0.050.050.050.05 0.050.05 0.050.050.05
~ow620 15 15 16 15 16 16 15 16 16 15
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
Glosscole 50 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
Commercial HASE
thi~h~n~r 0 0 0 0 0 ~ 0 0.1 0.~ o.~~
Commercial ASE
thickener 0 0.1 0.2 0.4 0 0 0 0 0 0
Sample F . 0 0 0 0.1 0.2 0.4 0 0 0
0
,
pH 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5
solids 66 66 66 66 66 65 56 66 56 66
Sample F: Ethyl acrylate 57.5% methacrylic acid 40%; steareth 20
ethoxylate allyl ether 2.5%
The following tests were carried out on the formulations:
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Measure Brookfield viscosity, 20 rpm, 100 rpm at 4 min.
Measure AAGWR at 2 bar pressure for 2 minutes
Measure Hercules High Shear viscosity, { FF bob,6600 rpm, 138,5341/sec).
5 Run twice with 3 minute delay between measurements.
The results are shown in Table 6
Table 6
Results 1 2 3 4 8 9 10 14 15 16
6solids 66.5766.2966.3966.3756.4566.59f6.2 66.3466.4766.67
pH 8.59 8.798.458.85 8.788.758.3 8.358.37 8.32
Brookfield viscosity804 181252001070 199032227500 40801281 1902
4120
Brookfield viscosity289 560 15122880 642 10222244 16263480 5208
4110
Hercules,cps 64.3 61.37~.591.1 60.774.872.2 72.869 77
hysterisis L L L ~ L f~i I~l L I~1 f~l-H
I~i
GWR {g/m2) 276.4187.6143 11~.7195.7164.1115 234 173.3152.4
The results show that sample F provides an excellent combination of low high
shear viscosity and water retention values. In addition sample F exhibits
significantly improved water retention than the conventional associative
thickener (commercial HASE). This is also clear from Figure 1.
Example 7
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Thickener composition sample F was used in and in a coating formulation to
coat paper in a pilot trial.
Details of the conditions used are as follows:
Base paper. 42 g/m2
Speed: , 1820 m/min
Coat weight: 7 g/m2/side
Coating unit: Film press, one-sided
Recipes: Reference Sample F
1 ~ - Pigment mi ,tore 100 parts 100 parts
- SB-latex (DL 920) . 8 parts 8,~ parts
- Starch (Raisamyl 302E) 6 parts 3,5 parts
- FWA (Blankophor P) 0,6 parts 0,6 parts
-Sample F - 0,2 parts
The results are shown in Table 7.
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Table 7
ReferenceSample
F
Coating color properties
- Brookfield, at the 556 540
beginning
- Brookfield, in the 572 574
middle
- Brookfield, at the 574 564
end
- Solids, at the beginning59.3 59.2
- Solids, in the middle59.5 59.3
- Solids, at the end 59.5 59.4
- Mater retention 111 95
- pH 3.3 3.6
- Temperature 35 33
Rod ~aressure, bar
-1 st side
- at fibs beginning 155 153
- at the end 175 160
- 2nd side
- at the beginning 150 130
- at the end 160 130
Qbservations
1. improved runnability
2. lower high shear viscosity (lower rod pressure)
3. better dynamic water retention (no increase of rod pressure)
4. less misting with sample F
5. It would have been possible to increase solids content of coating color
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Associative thickener composition Sample F provides low high shear viscosity
which gives lower rod pressure and better water retention gives more stable
rod
pressure.
Example 8
Thickener composition sample F and various other thickener compositions were
used in coating formulations to coat paper in a pre coating pilot trial.
Base paper: Uncoated, 52 g/m2 (base paper for 90 glm2 grade)
Pre-coating:
Recipe: 100 parts NPS CaC~3 (Covercarb 60-LV)
8 parts SB-latex (~L 920)
4. parts Starch (302 ESP)
0.6 parts F1~VA (Tinopal ABP-~)
Pre-coating conditions: pH: ca. 8.5
Solids: 02/~
Broolfield: 800 - 900 mPas
Temperature: 29 - 31 C
IVlethod: Sym-sizer (double side)
Speed: 1500 mlmin
Coat weight: 9 g/m2/side
Rod diameter:20 mm
Moisture: 3.5%
The results are shown in Table 8
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Table 8
High
BrookfieldWater shear Rod
Thickenermount,Solids, pH (100 retentionviscositypressure
% rpm)
parts mPas glm2 1 M bar
11s
Start Start mPas Start
/ End I End I End
CMC 0.2 61.9 / 8.4 480 / 162 39 1.3 /
62.0 470 1.3
Commercial0.1 61.9162.08.5 370 / 123 41 1.4511.5
395
ASE
Commercial0.2 61.9162.18.6 545 / 103 48 2.412.5
560
ASE
Sample 0.2 62.1 162.28.4 4.301470108 39 1.3/
F 1.3
The results show fihat the thickener according to the present invention
exhibits
the best combination of low high shear viscosity and water retention over CMC
or commercial ASE thickener. It can be seen that the CMC gave low high shear
viscosity but poor water retention. The dose of commercial ASE thickener that
provides low high shear viscosity gives poor water retention whereas a dose
and of the same thickener and that provides good water retention exhibits poor
high shear viscosity. Furthermore, the coating composition made using the
thickener of the present invention provided better runability and no or low
misting.
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Example 9
Thickener composition sample F and various other thickener compositions were
used in coating formulations to coat paper in a top coating pilot trial.
5
Base paper: Pre-coated, 52 g/m2 (pre-coated paper for 65 g/m2 grade)
Recipe: 70 parts fine CaC03 (Covercarb 85}
parts fine Clay (Hydragloss 90)
10 11 parts SB-latex (X~ 9644.5}
0.6 parts PVA {Airvol 103)
0.6 parts Ca-stearate (Raisacote CAS 50}
0.2 parts hardener (Bacote 20)
0.6 parts FV1IA (Tinopal ABP-~)
Coating conditions: pH: ca. 8.5
Solids: 63°/~
Brookfield: 1200 -1500 mPas
Temperature: 27 - 28 C
I~leth~d: Opticoat bet
Speed: 1500 m/min
Coat weight: 9 glm2lside
Blade width: 0.4.57 mm
Moisture: 5.0~Bo
The results are shown in Table 9.
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Table 9
BrookfieldWater High Blade
shea
ThickenerAmount,Solids, pH (100 rpm) retentionviscositypressure
%
parts mPas g/m2 1 M 1Is bar
Start Start I mPas Start
I End End I End
CMC 0.6 C3.0 / 8.2 1180 I 122 44 *0.99
62.8 1220 I 0.99
** 1.07
l 1.03
commercial0.2 63.0 / 8.2 680 / 650 118 44 *0.93
62.8 I 0.95
ASE **0.99
/ 0.97
commercial0.3 63.2 / 8.2 985 I 940 111 47 *1.01
63.0 l 1.03
ASE ** 1.10
I 1.08
Sample 0.3 63.0 I 8.3 750 I 690 110 39 *0.95
F 62.6 I 0.94
**0.96
/ 0.95
= first side
** = second side
This shows that the associative thickener of the present invention provided
the
best combination of low high shear viscosity and good water retention.
