Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HYDROXYALKYLCELLULOSE AS ADDITIVE
IN PIGMENTED METERING SIZE PRESS COATINGS
FIELD OF THE INVENTION
The present invention relates to a process for preparing coated paper
wherein the coating is applied by a roller or set of rollers as a film layer
to a
moving paper substrate. More specifically, this invention relates to the
adding of
a specific type of rheology modifier to a paper coating to enhance the film
transfer process so as to effect improved coated paper properties.
BACKGROUND OF THE INVENTION
It is desired in various industries to use rheology modifiers to achieve
thickening, flow control, water retention and other properties in aqueous
systems. A number of rheology modifiers are commercially available, such as
sodium carboxymethylcellulose (CMC), guar gum, sodium alginate,
hydroxyethylcellulose (HEC), alkali-soluble lattices, starch or starch
derivatives,
and the like.
Coated paper manufacturers have traditionally used one or more of the,
above rheology modifiers as additives for paper coating compositions in order
to
2o attain key application properties. Paper coatings are typically composed of
a
pigment, such as kaolin clay and calcium carbonate, binders, such as starch
and
styrene butadiene latex, water, various specialty additives, and at least one
of
the above mentioned rheology modifiers
It has been determined in prior industrial practice that the above rheology
modifiers are often interchangeable when employed in the application of paper
coating thickening, where the coating has been applied by traditional methods
such as blade metering. For general use, CMC is established as one of the
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most prevalent rheology modifiers for traditional blade metered paper-coating
application.
In recent years, a new method of applying aqueous pigment containing
paper coatings onto the surface of paper has been devised that is known as
"pigmented metering size press" or sometimes "film transfer coating". In this
method of coating paper, a wet coating is first metered onto a roller that is
typically composed of stainless steel or synthetic substrate. The wet coating
film
layer is then transferred by contact to a moving web of paper. Typically this
io operation is performed on both sides of the paper simultaneously.
Unfortunately, in the pigmented metering size press application of paper
coating to a moving paper web, it is often observed that an aerosol mist of
the
wet paper coating is formed as the paper web leaves the nip of the adjacent
rollers. It is, thus, generally known in the paper industry that "misting" of
pigmented paper coatings is a severe problem that is often observed when these
are applied by means of metering size press equipment. Misting refers to the
tendency of pigmented coatings to form an aerosol on the exit side of the film
press during application to the paper web. This problematic coating mist can
2o deposit on surfaces in the mill, and is a severe cleanup problem, as well
as
depleting expensive coating in a wasteful manner. Because of the prevalence of
this problem, the large-scale industrial use of pigmented metering size press
has
been slow to develop commercially. The producers wanting to use this
technology has had to develop coating formulations that are very closely
formulated with equipment operations that are strictly controlled in order to
minimize the adverse misting effect. The use of most rheology modifiers in
pigmented metering size press application is ineffective for reducing misting
under many conditions.
Since, In the past, adjustments of equipment, processing conditions,
and/or coating color formulations have been made in an attempt to improve
runnability and reduce misting. Hence, a need exists in the paper industry to
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develop a pigmented metering size press coating process to minimize the above
mentioned problems.
SUMMARY OF THE INVENTION
The present invention is directed to a process for preparing coated paper
comprising a) adding a water-soluble hydroxyalkylcellulose (HAC) with an alkyl
moiety of 1 to 4 carbons, or water soluble alkyl hyd roxyal kylcel lu lose
(AHAC) with
an alkyl hydrophobe of 1 to 24 carbons to a wet coating formulation in a n
amount equal to or greater than 0.15 parts HAC or AHAC based on pigment
io content of the coating and b) applying the pigmented wet coating
formulation to a
moving paper web via roller metering (film transfer) size press, wherein the
use
of HAC or AHAC in the pigmented coating significantly reduce misting of the
coating during application as compared to the control condition with no HAC or
AHAC added.
Examples of the hydroxyalkylcellulose (HAC) are hydroxyethylcellulose
(HEC) and hyd roxybutylcellu lose (or n-butylhydroxyethylcellulose )(BHEC).
Examples of the alkylhydroxyalkylcellulose (AHAC) are
methylhydroxyethylcellulose (MHEC), ethylhydroxyethylcellulose (EHEC) and
2o hydrophobically modified hydroxyethylcellulose (HMHEC) wherein the
hydrophobe has 1 to 24 carbons.
BRIEF DESCRIPTION OF DRAWINGS
In the accompanying drawings:
Figure 1 is metering size press misting panels with HEC thickened coating
at various coater speeds; and
Figure 2 is metering size press misting panels with CMC coating at
various coater speeds.
3o DETAILED DESCRIPTION OF THE INVENTION
It has been unexpectedly found that the use of HAC and AHAC in the
pigmented metering size press process reduces misting of a general paper
coating recipe and indicates significant potential for commercial utility of
this
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process. Thus, the combination of a specific rheology modifier with pre-
existing
metering size press apparatus and coating formulations represent a
significantly
improved process for making coated paper.
This invention has advantages in preventing the misting that has been
traditionally observed in the prior art pigmented metering size press
operations.
In contrast to the prior art metering size press process, the present
invention has
unexpectedly been found to produce a balance of additional performance
properties, such as enhanced fiber coverage, opacity, and brightness that are
io significantly useful for the end product of the metering size press
application.
The present invention represents a more detailed process of:
1) Preparing a coating mixture of water, standard paper coating
pigments, binders, and at least one hydroxyalkylcellulose such as
hydroxyethylcellulose, or derivative thereof;
2) Applying the coating by metering the coating layer onto a set of
moving rollers;
3) Transferring the wet coating layer onto a paper web by passing the
paper web between the rollers on which the wet paper coating was previously
2o applied; and
4) Drying the wet coated paper to form a final dried coating layer. The
runnability and paper optical properties are improved by this process as
opposed
to prior art processes that do not use HAC or derivatives thereof.
The cellulose ethers operable in the present invention include all water
soluble nonionic derivatives of cellulose containing hydroxyethyl
functionality and
that express aqueous solution viscosities in excess of 100 centipoise at a
concentration of 5% in water. The water-soluble polymer may also in addition
to
hydroxyethyl functionality also be derivatized with other substituents such as
a
methyl group, butyl group, or longer hydrophobes having up to 24 carbon atoms.
In the case where any one of these types of water-soluble polymers are
employed as a rheology modifier in a pigmented metering size press paper
coatings they would be dosed into the coating at a level to attain a runnable
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coating viscosity typically of 100-4000 cps. The dosage of rheology modifier
employed in practice is typically 0.2-0.8 parts of rheology modifier based
upon
100 parts of pigment in the coating. It is reasonable to suggest that a range
of
0.05 to 2 parts of the rheology modifier of the present invention could be
effective for conveying desirable runnability features to pigmented metering
size
press coatings.
In accordance with the present invention, a typical coating formulation
would contain about 100 parts pigment, about 10 to about 18 parts binder
(i.e.,
io starch and/or latex), about 20 to about 60 % solids. Preferably, the solids
content can be 50 to 60 %. The HAC or AHAC content in this composition is 0.2
to 0.8 part, which is included in the solids percentage. Examples of the
binder
that is used in this composition is starch and latex (which can be styrene or
vinyl
based). Examples of the pigment in this composition are calcium carbonatP and
kaolin clay.
The present invention is further illustrated in the following examples,
wherein all parts or percentages mentioned are by weight unless otherwise
indicated. These examples are given only by way of illustration and are not
intended to limit the invention except as set forth in the claims.
THE EXAMPLE
In a series of pilot scale pigmented metering size press tests, paper
coatings were prepared with identical pigments, binders, and other additives,
at
equal percent solids, then thickened to a constant viscosity target range by
means of various rheology modifiers, including hydroxyethylcellulose and
sodium
carboxymethylcellulose of different molecular weights. These various coating
samples were then, in turn, applied in successive tests to a paper substrate
by
means of a high-speed film press pilot apparatus. Variable speeds were used to
surface coat the paper in these tests so that comparisons of runnability and
coated paper properties could be assessed. Monitoring the nip exit of the
metering size press apparatus assessed misting tendency of each coating test.
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TABLE 1
COATING FORMULATION FOR PIGMENTED MSP TRIAL
Order of Addition Dosage
Parts by weight
Dispex N-40 Dispersant 0.1
H dras erse #2 kaolin clay 50
Hydracarb 90 calcium carbonate 50
Nopcote C104 Calcium stearate 0.5
Dow 692 SBR latex 10
Tinopal PT Optical brightener agent 0.5
Adjust H to 9.0
Solids target 61.2 + 0.2
Thickener dosage adjusted to pr2duce 500 + 100 cps Brookfield RVT 100 RPM
TABLE 2
SUMMARY OF PIGMENTED METERING SIZE PRESS TRIAL TESTS
Dosage Coating
Test Metering Size Press
Rheological Modifier (per 100 # of Viscosity,
Condition i ment %Solids Runnability Observed
p
Ambergum 770 low 0.43 active 470 cps, Little misting on lowest coater
1 Molecular weight CMC parts 61.2% speed, but misting increased to
severe at 4500 foot/minute.
AQU D-3556,
2 30% active 0.62 wet parts 528 cps, Severe misting at highest
High Molecular weight 61.5% coater speed.
CMC
Admiral FPS 3089, 0.54 active 560 cps, Very low misting at all
3 25% active parts 61.1% metering size press speeds
H drox eth Icellulose
TABLE 3
SUMMARY OF PIGMENTED METERING SIZE PRESS TRIAL
COATED PAPER PHYSICAL TEST RESULTS @ 4500 FPM COATER SPEED
Test Rheological Modifier Average K+N Ink Absorption Average Brightness
Condition opacity
Ambergum 770 low 88.1
1 Molecular weight 93.1 18.2
CMC
AQU D-3556,
2 30% active 93.5 18.7 87.9
High Molecular weight
CMC
Admiral FPS 3089,
3 25% active 93.8 19.2 88.3
H drox eth Icellulose
The paper coatings tested and runnability results are described in Tables
I and 2. Misting panel results showing the effects results of incorporating
Admiral 3089FS Fluidized Polymer Suspension, a 25% active suspension of
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hydroxyethylcellulose, into the pigmented metering size press process as
compared to CMC are shown in Figures 1 and 2. In this test method, a black
panel was placed near the web exit of the pigmented metering size press for a
standard length of time. The coating misting tendency was indicated by the
quantity of white coating spots that deposit on the test panel. It was clearly
shown that the white coating deposits of the HEC coating were much lower than
was observed with CMC coating. This beneficial effect of the HEC incorporation
was particularly pronounced at higher coater speeds.
- io It was observed during this series of trials that the paper coating
containing hydroxyethylcellulose (ADMIRAL 3089FS fluid polymer suspension
(FPS)) desirably exhibited the least/best misting of any coatings tested in
the
metering size press at high speeds. In subsequent physical testing of the
coated
paper from these trials, it was then also found that the coated paper made
with
ADMIRAL 3089 FS FPS gave one of the highest opacity, Tappi brightness, and
higher K&N Ink absorption, when compared with carboxymethylcellulose (CMC)
rheology modifiers in the trial (see Table 3). On visual inspection the coated
paper from the ADMIRAL 3089FS FPS trials also clearly exhibited the most
complete fiber coverage, indicative of a bulking effect of this thickener in
paper
coatings.
It was concluded from this trial comparison that incorporating
hydroxyethylcellulose into the pigmented metering size press trial process
produces unexpected valuable runnability performance. It is, therefore,
reasonable to expect that nonionic cellulose derivatives that are similar to
HEC
would also show this same performance, if included into the metering size
press
application. These latter materials are thought to include water-soluble
alkylhydroxyethylcellulose derivatives in general and
methylhydroxyethylcellulose (MHEC) ethylhydroxyethylcellulose, (EHEC) and
3o hydrophobically-modified hydroxyethylcellulose (HMHEC) in particular.
Although the invention has been described with reference to preferred
embodiments, it is to be understood that variations and modifications in form
and
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detail thereof may be made without departing from the spirit and scope of the
claimed invention. Such variations and modifications are to be considered
within
the purview and scope of the claims appended hereto.
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