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Description  |
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FIELD OF THE INVENTION
This invention relates to a paper coating composition utilizing nonionic
thickeners and in particular a paper coating composition having a
clay-containing pigment system which shows improved coating efficiency and
runnability and which shows only minimal pigment shock.
BACKGROUND OF THE INVENTION
In order to obtain high quality paper, it is necessary that the surface of
the paper is smooth and substantially free of indentations or valleys.
Smooth papers are a prerequisite for good images printed thereon and also
for good transfer of ink to paper. Smooth papers are obtained by coating
the raw paper surface with a pigment composition. The coating composition
to effect this is an aqueous dispersion comprising mainly of mineral
pigments such as clay, calcium carbonate, or titanium oxide, and pigment
binders of natural protein, for example, casein or soy protein, starch, or
synthetic polymer emulsions. Coating compositions are usually applied to a
continuous web of material by high speed coating machines, such as blade
coaters, air knife coaters, rod coaters and roll coaters. The flow
properties of coating color compositions for paper and boards are of
significant importance with regard to the runnability (or flow) of the
color during the coating operation. These flow properties are often
controlled by a "thickener" or "co-binder", which terms are taken to be
synonymous in the industry.
In preparing the coating color, the thickener is mixed with the pigment
slurry. This may result in what is known in the industry as "pigment
shock", due to a strong transient adsorption of the thickener onto the
pigment. This causes a rapid increase in viscosity during the early stages
of thickener addition. This pigment shock may result in flocculation of
the pigment, pigment agglomeration, difficulty in mixing due to momentary
solidification of such a pigment slurry, and in severe cases, complete
coagulation. Industrial practice cannot tolerate such a phenomenon.
Furthermore, when this occurs, the thickener is rendered partially
inactive, resulting in less water retention and unsatisfactory rheology
before, under, and after the blade. As a consequence, corrective actions
during the coating operation are often necessary.
U.S. Pat. No. 4,879,336 discloses an approach to solving the above
mentioned problem of pigment shock by mixing clay slurries with a
butadiene styrene latex composition when certain poly(vinyl alcohol)
copolymers are present in the latex. Another approach to the problem is
mentioned in U.S. Pat. No. 3,558,543 that discloses a method of reducing
initial thickening (pigment shock) of paper coating when a clay or pigment
slurry is mixed with an adhesive solution. This patent uses polyvinyl
pyrrolidine mixed with poly(vinyl alcohol) adhesive solutions to eliminate
the pigment shock. This reference also discloses that the severity of
pigment shock is particularly pronounced when the adhesive is protein
material or poly(vinyl alcohol) (see column 1, line 61-63.
U.S. Pat. No. 4,994,112 discloses a paper coating composition containing a
water soluble hydrophobically modified hydroxypropylcellulose thickener
that has been modified with an alkyl or aralkyl group having preferably 12
to 16 carbons atoms. This thickener enables the paper coating to be
applied uniformly at high machine speeds. Another approach to providing a
paper coating composition is disclosed in U.S. Pat. No. 5,080,717 which
discloses an aqueous paper coating composition comprising clay, a latex,
and a thickener of a multi polysaccharide suspension of a hydrophobically
modified alkyl hydroxyalkyl cellulose suspended in a low molecular weight
polysaccharide and a salt.
None of this prior art discloses nor suggests the instant invention.
SUMMARY OF THE INVENTION
This invention relates to a method for mixing coating color ingredients
wherein thickeners like nonionic polysaccharides and pigment portions are
combined in a manner which increases thickener efficiency and runnability
and simultaneously eliminates pigment shock by using a second water
soluble polymer having a limited low molecular weight which preferentially
adsorbs onto the pigment, blocking the adsorption of the main thickener.
This second water soluble polymer will be referred to as a "blocker".
This improved method for preventing momentary solidification during
preparation of a clay containing paper coating composition comprises the
preparation of an aqueous coating composition with said co-binder/blocker
combination, pigment and binder. The blocker is selected from the group of
nonylphenol ethoxylates, low molecular weight poly(vinyl alcohol), low
molecular weight poly(ethylene) oxide, or proteins.
This invention also comprehends a process for paper coating comprising
applying the above-mentioned composition to a paper web, removing the
excess composition from the web to provide a uniform coating composition
and drying the coating to produce a paper product.
DETAILED DESCRIPTION OF THE INVENTION
The paper coating industry is always seeking improved productivity. It is
known that nonionic polysaccharides like hydroxyethylcellulose (HEC), when
used in a clay based paper coating give improved results regarding coating
holdout and required dosage when compared to conventional thickeners like
carboxymethylcellulose (CMC) and polyacrylates.
This performance is due to its influence on the structure of the paper
coating because the adsorption of the nonionic cellulosic causes (partial)
flocculation of the clay particles at high paper coating, solids content.
Beyond these advantages, hydrophobically modified hydroxyethylcellulose
(HMHEC), like Natrosol.RTM. Plus grade 330 polymer from the Aqualon
Company, a Division of Hercules Incorporated, provides high thickening
efficiency with higher pseudoplasticity in high solids content coating
compositions due to association between the hydrophobes in the HMHEC and
other ingredients present in the paper coating, e.g., the binder. During
blade coating, a hydrophobically modified cellulosic allows lower blade
pressures, which can result in reduced water loss to the paper stock, web
breaking and streaking, particularly at high speed, as described in U.S.
Pat. No. 4,994,112. Also, the associative character of the thickener gives
a faster immobilization of the paper coating after the blade due to quick
structure reformation and thus a better coating holdout, resulting in
improved optical and printability properties of the coated paper.
However, the degree of thickener adsorption must be limited, as
over-flocculation can occur, initially resulting in a so called "pigment
shock" that is caused by the bridging flocculation of clay particles by
the co-binder molecular. This is often the case when nonionic
polysaccharides are involved, especially in combination when European
kaolin clays are involved, which are known for their strong adsorbing
character. As well as the undesired pigment shock, over flocculation
causes poor water retention and high blade load, thus poor runnability of
the coating systems. In cases where the thickener has an associative
character (HMHEC), a too high level of adsorption has an even more
detrimental effect. The benefit of the associative character is diminished
when an insufficient amount of thickener is present in the water phase of
the coating color due to a high level of adsorption of that thickener onto
the pigment.
It is known from the literature that, in addition to nonionic
polysaccharides like HEC and HMHEC, hydrophilic nonionic polymers such as
poly(vinyl alcohol) (PVOH) and polyethylene oxide (PEO) adsorb in
substantial amounts onto clay surfaces. This is particularly the case when
European kaolin clays are involved.
Equilibrium adsorption experiments for individual polymers in an aqueous
suspension of clay particles have indicated that PVOH is adsorbed to a
greater extent than HEC. When the PVOH is present in a binary mixture with
a cellulosic polymer, the amount of adsorbed cellulosic is diminished in
comparison to the amount adsorbed when no competing polymeric species is
present. For cases in which one polymer is permitted to reach an
equilibrium between its presence in the water phase and on the clay
surface, prior to the addition of a second polymer, the displacement of
the first polymer by the second is dependent upon the particular nature of
both polymers being present. It was found that PVOH is able to displace
HEC and HMHEC significantly from the clay. This happens whether the
thickeners are added as dry powders, as solutions or as fluid suspensions
in aqueous or non-aqueous media.
This indicates that PVOH is preferentially adsorbed onto the clay surface
and the strength of attachment is greater than for HEC and thus prevents
adsorption of the HEC onto the clay surface.
Application of the present invention provides a means of preventing or
minimizing the pigment shock by using so called blockers such as described
above in combination with thickeners which have strong adsorbing
tendencies towards clays. The molecular weight of the blockers should be
low to prevent them from causing flocculation of the clay particles
themselves. This blocking also forces the thickener to remain largely in
the aqueous phase of the coating composition, making it better able to
fill its desired function in the papermaking operation.
An optimal balance of co-binder adsorbed on the pigment surface and
dissolved in the liquid phase is required to give the paper coating its
preferred theology. The present invention enables one to control
flocculation of those paper coatings, leading to substantial improvement
of coating process performance in terms of pigment shock, dynamic water
retention, coating holdout and coating rheology, particularly at high
shear rates. In addition to blocking, the blockers might contribute their
own beneficial properties to the coating property balance; PVA, for
example, is known for its positive influence on the brightness of the
coated paper and its positive effect on the boosting of optical
brightening agents (OBA).
As to the materials, the pigment portion is generally an aqueous dispersion
of coating grade clays such as kaolin clays. In conjunction with the clays
there may also be added one or more of the following: Titanium dioxide,
calcium carbonate, barium sulfate, talc, zinc sulfate, aluminum sulfate,
calcium oxide reaction products and other similarly used materials.
Suitable thickeners for this invention are water soluble alkylhydroxyalkyl
cellulose or hydroxyalkyl cellulose or a combination thereof as well as
their hydrophobically modified analogues, the hydrophobically modified
derivates being the most effective. A preferred hydrophobically modified
cellulosic is Natrosol.RTM. Plus, a hydrophobically modified
hydroxyethylcellulose, produced by the Aqualon Company, a Division of
Hercules Incorporated. Depending upon the needs of the paper manufacturer,
it may be desirable to use one or more hydrophobically modified
polysaccharides in combination with HEC or CMC.
As to the blocker, low molecular weight polyols may be used, like PVOH,
PEO, polypropylene glycol (PPG), polyvinyl pyrrolidone, lower molecular
weight water soluble alkylhydroxyalkyl cellulosics and nonionic
polyacrylamide and salts of polyacrylic acid and polymethyacrylic acid are
also effective. In order to obtain the full advantage of blocking, the
blocker should not exceed a certain molecular weight, as it may function
as a flocculant itself at higher molecular weight. For that reason, the
optimum molecular weight for PEO is in the range of 1000-50,000. The
optimum range for PVA is in the range of 5000-50,000. For the practice of
the invention, the PVA can be 70-90, preferably 85-90 and most preferably
87-89 mol. % hydrolyzed.
In preparing the coating material, an aqueous slurry of the pigment is
prepared by admixing the clay and other additives in a water system. pH is
preferably in the alkaline range, between about 7.2-12. The pigment slurry
is generally prepared as a dispersion of solids in the range of about
40-80% by weight, the higher range being preferred as in the range 60-70%
solids for reasons including economy of handling.
The blocker can be added before or together with the thickener, as a
powder, a fluid suspension or as a solution. In order to prevent or
minimize pigment shock, it is important that the blocker reaches the
pigment surface in a dissolved state before the thickener. Depending upon
factors like pigment composition, type and molecular weight of the blocker
and the type of thickener, the blocker is used in weight portion of from
0.005-2% on the weight of pigment solids (clay and other pigments).
Proportions outside of this range are considered either inoperative below
the lower range or uneconomic above the upper range.
EXAMPLE 1
This example illustrates the effect of several cellulosic co-binders on the
pigment shock related to the percentage co-binder being adsorbed onto the
clay surface. Pigment slurries containing 60% solids were prepared based
on formulation 1. The data in Table 1 show that application of nonionic
hydroxyl-rich water soluble polysaccharides can result in severe pigment
shock, being related to the amount of adsorbed polysaccharide. Pigment
shock was quantified by measuring the maximum torque onto the stirrer upon
addition of a 7.5% thickener solution in water onto the slurry. The
relative torque values are used to indicate whether or not the blocker is
effective by controlling the flocculation. Four hours after preparation,
the Brookfield RVT viscosity was measured at 100 rpm and 25.degree. C. The
amount of adsorbed thickener was established by determination of the
thickener amount being present in the water phase after centrifuging the
system 24 hours later for 2 hours at 30,000 g. Analyses were done
according to the anthrone colorimetric method as described in Hercules
Bulletin VC 507.
______________________________________
Formulation 1
Parts by Weight
(based on dry or 100%
Ingredient active materials)
______________________________________
SPS 100
Dispex N40 0.25
NaOH 0.1
Co-binder Variable
______________________________________
SPS Pigment, kaolin clay, ECCI
Dispex N40 Clay dispersant, allied colloids
TABLE 1
______________________________________
Viscosity
Torque Adsorbed
Co-binder Amount* (mPa .multidot. s)
(mNm) (%)
______________________________________
Natrosol .RTM. 250 LR
0.3 1200 >>100 99
Natrosol Plus .RTM.
0.3 1500 >>100 100
grade 330
CMHEC 37L 0.8 1300 35 39
Blanose 7L2C 1.0 1200 22 9
______________________________________
Natrosol .RTM. 250 LR Hydroxyethylcellulose, Aqualon BV
Natrosol Plus .RTM. grade 330 Hydrophobically modified
hydroxyethylcellulose, Aqualon BV
CMHEC 37L Carboxymethylhydroxyethylcellulose, Aqualon
Blanose 7L2C Carboxymethylcellulose, Aqualon France SA
*Amount of thickener is expressed as parts on 100 parts of pigment
EXAMPLE 2
This example illustrates that PVOH acts as a blocker by preferential
adsorption in a pigment system which includes a strongly adsorbing kaolin
clay. By using the formulation and procedure as described in Example 1,
Table 2 shows that both intensity and duration of the pigment shock caused
by strong adsorption of HMHEC onto SPS clay can be significantly reduced
by using PVOH, indicating that the degree of flocculation is controlled.
Natrosol Plus.RTM. grade 330 was used at a level of 0.35 parts on 100
parts clay.
TABLE 2
______________________________________
Poly(vinyl Mw Torque T1 T2
alcohol) (min.) Amount (mNm) (s) (min.)
______________________________________
None (control >>100 130 >10
Airvol 203 10,000 0.1 >100 50 6
Airvol 203 10,000 0.5 92 1 3
Poly(vinyl 13,000 0.1 >100 7
alcohol)
Poly(vinyl 13,000 0.5 95 3
alcohol)
Polyviol M13/140
49,000 0.1 >100 >10
Polyviol M13/140
49,000 0.5 >100 >10
Polyviol W25/140
79,000 0.1 >>100 >10
Polyviol W25/140
79,000 0.5 >>100 >10
______________________________________
Airvol 203 Air Products
Poly(vinyl alcohol) Aldrich Chemicals, PVA being 87-89 mol. % hydrolyzed
Polyviol M13/140 Wacker
Polyviol M25/140 Wacker
T1 expresses the time at which the maximum torque has been measured,
calculated from the moment of co-binder/blocker addition.
T2 gives the time it takes before the viscosity curve has leveled off,
indicating that the system has reached its equilibrium.
These data clearly show that optimum results are found in the lower Mw
ranges.
EXAMPLE 3
By using the formulation and procedure mentioned in Example 2 and replacing
SPS by a less strongly adsorbing kaolin clay like DB Plate, the use of PVA
as blocker is even more pronounced. See data in Table 3.
TABLE 3
______________________________________
Poly(vinyl Mw Torque T1 T2
alcohol) (min.) Amount (mNm) (s) (min.)
______________________________________
None (control 39 25 2
Airvol 203 10,000 0.1 10 1 <1
Airvol 203 10,000 0.5 8 1 <1
Poly(vinyl alcohol)
49,000 0.1 27 1 <1
Poly(vinyl alcohol)
49,000 0.5 14 1 <1
______________________________________
SPS has been replaced by DB Plate, a Kaolin clay delivered by Euroclay, at
the same dosage.
EXAMPLE 4
This example shows that PVOH diminishes the amount of HMHEC being adsorbed
onto kaolin clay, using the formulation and procedure as described in
Example 1. Polyviol M13/140 was used as a blocker at several dosages,
preventing Natrosol Plus.RTM. grade 330 from adsorption onto DB Plate.
TABLE 4
______________________________________
Thickener being
Poly(vinyl alcohol)
adsorbed
(ppH)* (%)
______________________________________
0.0 control 79
0.1 41
0.3 20
0.5 8
______________________________________
*Amount of poly (vinyl alcohol) is expressed as parts on 100 parts of
pigment.
EXAMPLE 5
The effect of PEO on the adsorption level of HMHEC is shown in this
example, using the same set-up as in Example 4, except that Polyviol
M13/140 was replaced by Lutrol E4000, a PEO produced by BASF, having an
average molecular weight of 4000.
TABLE 5
______________________________________
Thickener being
Poly(vinyl alcohol)
adsorbed
(ppH)* (%)
______________________________________
0.0 control 79
0.1 26
0.3 24
0.5 19
______________________________________
*Amount of poly (vinyl alcohol) is expressed as parts on 100 parts of
pigment.
EXAMPLE 6
This example illustrates that PEO is able to reduce or prevent pigment
shock by preferential adsorption onto kaolin clay. Referring to the
formulation and procedure as described in Example 1, Table 6 shows that
the intensity of the pigment shock caused by strong adsorption of HMHEC
onto DB Plate can be significantly reduced by using PEO. Natrosol
Plus.RTM. HMHEC grade 330 was used at a level of 0.35 parts on 100 parts
of clay. After this, DL 945, a styrene butadiene latex produced by Dow
Chemical Co., was added as the binder to the pigment slurry, prior to the
addition of the thickener/blocker combination. This was done at a binder
level of 10 parts on 100 parts DB Plate, based on dry material.
TABLE 6
______________________________________
PEO Level Torque
Mw PEO (ppH)* (mNm)
______________________________________
-- 0.0 control
23
4000 0.1 12
4000 0.5 8
6000 0.1 11
6000 0.1 7
8000 0.5 7
15000 0.1 11
______________________________________
*Amount of PEO is expressed as parts on 100 parts of pigment.
EXAMPLE 7
This example illustrates that the blocking principle is also very effective
with PEO in combination with a strongly adsorbing clay by using the
description of Example 6, except that DB Plate has been replaced by SPS
clay.
TABLE 7
______________________________________
PEO Level Torque
Mw PEO (ppH)* (mNm)
______________________________________
-- 0.0 control
100
4000 0.5 13
______________________________________
*Amount of PEO is expressed as parts on 100 parts of pigment.
EXAMPLE 8
This example illustrates that the blocking principle is effective in
controlling the flocculation caused by polymer adsorption. Paper coatings
at a solids content of 60% were prepared, based on formulation 2.
Thickener dosage was adjusted to end up at a viscosity of 1000 mPa.s (see
Table 8). CLC coater trial results reveal that the addition of PVOH led to
reduced blade pressure, despite a higher Hercules viscosity. The reduction
is explained as a result of an improved dynamic water retention due to
controlled flocculation. This improvement is already indicated by the S.
D. Warren retention time results (see Table 8).
______________________________________
Formulation 2
Components Parts (w/w)
______________________________________
Delaminated clay 50
American clay no. 2
50
Dispersant 0.15
SB latex 7
Nopcote 104 1
Foamaster VF 0.1
Thickener varied
______________________________________
TABLE 8
______________________________________
Thickener.sup.(1)
Dose.sup.(2)
Hercules.sup.(3)
WRT.sup.(4)
BP.sup.I(5)
______________________________________
HMHEC/PVOH, 100/0
0.38 81.5 7 27.0/32.0
HMHEC//PVOH, 0.43 87.5 8 25.0/28.0
75/25
______________________________________
.sup.(1) HMHEC Natrosol Plus .RTM. Grade 330 ex Aqualon BV
PVOH Airvol 803 ex Air Products
.sup.(2) Parts per 100 parts of pigment.
.sup.(3) Hercules high shear viscosity (mPa .multidot. s).
.sup.(4) S. D. Warren water retention time(s).
.sup.(5) Blade pressure index, which indicates the amount of blade runin
required to give the target coat weight, 7.4 g/m.sup.2. The lower BPI
values on the left side were measured at 920 m/min., while the values on
the right side were measured at 1220 m/min.
EXAMPLE 9
This example illustrates that the blocking principle is also applicable to
hydrophobically modified ethylhydroxyethyl cellulose (HMEHEC), using the
same set up as mentioned in Example 6, except that Natrosol Plus.RTM. Grad
330 polymer is replaced by Bermocoll EHM 100 polymer, an HMEHEC produced
by Berol Nobel.
TABLE 9
______________________________________
Thickener
being
Amount*** Torque T1****
T2.sup.a
adsorbed
Blocker (ppH) (mNm) (s) (min.)
(%)
______________________________________
None 19 35 2 75
(control)
Airvol 203*
0.1 8 1 <1 22
Airvol 203
0.5 6 1 <1 0
Lutrol E 0.1 7 1 <1
4000**
Lutrol E 4000
0.5 5 1 <1
______________________________________
*Airvol 203 Air Products (PVA)
**Lutrol E 4000 HASF (PEO)
***Amount of blocker is calculated on the amount of clay
****T1 expresses the time at which the maximum torque has been measured,
calculated from the moment of thickener addition
.sup.a T2 gives the time it takes before the viscosity curves have
levelled off, indicating that the system has reached its equilibrium.
EXAMPLE 10
This example shows that the blocking principle is also very effective with
alkylaryl ethoxylates. Referring to the formulation and procedure in
Example 6, Table 10 visualizes that the intensity of the pigment shock
caused by strong adsorption of HMHEC onto DB Plate can be significantly
reduced by Antarox CO 970 polymer, a nonylphenol ethoxylate (50 EO units)
produced by GAF.
TABLE 10
______________________________________
Amount Torque T1 T2
Blocker (ppH) (mNm) (s) (min.)
______________________________________
None (control) >>100 130 >10
Antarox CO 970
0.5 81 1 5
______________________________________
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