Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF COATING SUBSTRATES UTILIZING
AN ALKALI-FUNCTIONA~ ASSOCIATIVE
THI~K~N~K CONTAINING COATING COMPOSITION
~echnical Field
This invent$on relates to a method of coating
~ubstrates utilizing an agueous coating composition that
is thickened with an alkali-swellable associative
thickener.
Backqround Of The Invention
Coating compositions are applied to substrates
for ~any reasons, as for example to modify the color,
brightness, whiteness, ~moothness, gloss and/or porosity
of the substrate. They can also be used to improve the
ability of the surface of the 6ubstrate to receive
additional coatings, ink and the like, and to increase
the stiffness, strength, and/or weight of the substrate.
One problem with conventional roll applied
coatings is incomplete transfer of the composition from
an applicator roller to the substrate which can ruin the
coated substrate surface. Transfer can be improved by
lowering the solids content of the composition but this
is undesirable because the excess water utilized to
lower the solids content ~ust subsequently be removed.
The excess water can also weaken the substrate and
permit the coating to be w$cked into the ~ubstrate
rather than remain on the surface.
A high solids content is desirable ~o that a
single, thick coating can be obtained. A high solids
content can also cAuse inadequate flow and leveling o~
the composition wh$ch result~ in uneven coating
thickness and can cause webs to be formed between the
composition appl$cator and the substrate.
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Conventional (non-associative) thickeners can
be added to ~ttempt to solve these problems but coating
compositions containinq such thickeners also have
processing problems.
For example, conventional thickener-containing
compositions can have a high extensional viscosity which
causes misting/spattering or pattern formation that can
ruin the coated substrate.
The high extensional viscosity may also result
in high blade pressure when the conventional
thickener-containing cGlu~osition is applied by a blade
coater. High blade pressure requires a slower line
~peed or results in paper breaks and/or ~cratching of
the substrate. ~urthermore, the high blade pressure can
undesirably force water and coating from the substrate.
Non-ionic associative thickener-containing
compositions also exhibit low extensional viscosity, but
because they are lower in molecular weight they are
absorbed into the cubstrate and thus do not remain on
the surface to improve it.
Conventionally coated substrates typically are
of relatively poor quality and do not have the desired
gloss, uniformity of appearance and print density.
These problems are particularly apparent in the
production of paper of newsprint guality.
U.S. Patent No. 4,514,552 to Shay et al.
discloses alkali soluble latex thickenerc suitable for
use as associative thickener~. However, the Shay Patent
does not discloce or suggest the use of these thickeners
in a method of coating substrates.
Summary Of The Invent~on
The method of the present invention comprises
the step of applying to a ubstrate an aqueous coating
composition comprising an alkali-swellable associative
thickener that compr1ses tA) about 2 to about 70 weight
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percent of a monoethylenically uncaturated car~oxylic
acid, (B) about 20 to 80 weight percent of ~
monoethylenically unsaturated monomer lacking surfactant
capacity, (C) about 0.5 to about 50 weight percent of a
non-ionic urethane monomer that is the urethane reaction
product of a monohydric non-ionic ~urfactant and a
monoethylenically unsaturated monoisocyanate, and (D) 0
to about 2 weight percent of a polyethylenically
unsaturated monomer. The weight percents of the
components of the associative thickener are based on the
total weight of associative thickener.
The method of the present invention provides
improved transfer, flow and leveling characteristics and
reduced webbing, misting/spattering and pattern
formation.
Furthermore, the method of the present
invention enhances the properties of the coated
substrate by providing a thicker film, a higher gloss, a
more uniform appearance and a superior print density as
compared to substrates coated by a conventional method.
When a blade coater ~s utilized to apply the
composition, lower blade pressure can be utilized which
results in increased line gpeed without ccratching the
gubstrate. Also, lower blade pressure does not force
2S water into gubstrates ~uch as paper.
The size coating composition utilized in the
method of the present invention cAn be produced by
admixing the associative thickener with a conventional
aqueous composition that can contain an elastomer,
pigment, filler, binder, and wAter. Preferably, the
conventional agueous cGh.ycsition ig latex based.
~ he associative thickeners utilized in the
method of the present invention are anionic and have a
relatively high number average molecular weight thereby
retaining water better than conventional thickener6.
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Thus, the associative thickener containing compositions are not
absorbed into the substrate and produce improved coatings
compared to nonionic associative thickeners.
The coating compositions utilized in the
present invention can possess a viscosity higher than can
conventionally be utilized and still achieve good transfer of the
composition to the substrate. This permits an increase in the
solids content of the coating composition and/or an increase in
the line speed. Lower solids coatings can also be applied,
if desired, by using more of the alkali-swellable associative
thickener to increase the viscosity.
Detailed Description Of The Preferred Embodiments
The method of the present invention comprises
the step of applying to a substrate an aqueous coating
composition comprising an alkali-swellable associative
thickener comprising (A) about 20 to about 70 weight percent,
based on the weight of the associative thickener, of an alpha,
beta-monoethylenically unsaturated carboxylic acid; (B) about
20 to about 80 weight percent of a monoethylenically
unsaturated monomer lacking surfactant capacity; (C) about
0.5 to about 60 weight percent of a non-ionic urethane
monomer that is the urethane reaction product of a
monohydric non-ionic surfactant, and a monoethylenically
unsaturated monoisocyanate; and (D) 0 to about 2 weight
percent of a polyethylenically unsaturated monomer to a
substrate. The above components of the alkali-swellable
thickener are copolymerized by conventional emulsion
copolymerization processes and are anionic. Alkali-swellable
associative thickeners that can be used herein are
disclosed in U.S. Patent No. 4,514,552 to Shay et al. (the
Shay Patent).
~,,
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,
The term "associative thickener", as used in
its various grammatical forms, indicates that the
thickening mechanism is predominantly associative.
In contrast, the thickening mechanism of
non-associAtive thickeners, i.e., conventional
thickeners, thickens on~y the aqueous phase by a
hydrodynamic mechanism.
Compositions containing associative thickeners
generally have a relatively low extensional viscosity
while compositions containing non-associative thickeners
have a relatively high extensional viscosity. This
difference in extensional viscosity is due to the
difference in thickening mechanisms of the associative
and non-associative thickeners. The low extensional
viscosity of the coating compositions utilized in the
method of the present invention is presently believed to
result in improved transfer and reduced webbing,
misting/spattering and pattern formation of the coating
composition and the ability to utilize a low blade
pressure when a blade coater is utilized to apply the
composition.
Extensional viscosity is a measure of the
resistance to elongational flow that oCcuru when a
fluid, e.g., a coating composition, is forced through an
aperture of the applicator, such as the nip between
rollers, the space between a blade and a substrate, or
the orifice of a spray nozzle. The fluid is extended
which results in an increase in viscosity as it
approaches the aperture. Most viscometer6, e.g., a
Brookfield vi~cometer, impart rotational flow on the
liquid whose viscosity is being measured and hence only
measure rotational chear viscosity. Extensional
viscosity is important when roller, blade or spray
applicators are utilized because it can be as much as a
1000 times greater than the rotational shear viscosity.
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The associative thickener-containing coating
compositions have a relatively high viscosity when
subjected to a high-rotational shear force such as the
- force imparted on the composition by the applicator.
This relatively high viscosity i8 presently believed to
result in the formation of thicker films.
The associative thickener-containing coating
compositions have a relatively low viscosity when
sub~ected to a low-rotational shear force such as the
force imparted on the composition by gravity. This
rel~tively low viscosity is presently believed to result
in improved flow and leveling of the composition.
Preferably, the unsaturated carboxylic acid is
present in an amount in the range of about 25 to about
55 weight percent; the unsaturated monomer lacking
surfactant capacity i~ present in an amount in the range
of 30 to about 65 weight percent; and the non-ionic
urethane monomer is present in an amount in the range of
about 10 to about 50 weight percent.
The unsaturated carboxylic acid enables the
emul~ion copolymerized particles of the associative
thickener to be disper~ed in an agueous medium, e.g., a
conventional coating composition, without the assistance
of the ~urfactant that is used to stabilize the emulsion
during copolymerization.
The unsaturated carboxylic acid is present in
an amount sufficient to di~perse the associative
thickener in an agueous medium having a pH of at least
6.0, preferably at least about 7.0, and most preferably
a pH in the range of about 7.5 to about 10.5.
Although acrylic acid or methacrylic acid are
preferred unsaturated carboxylic acids, other useful
acids are crotonic acid, itaconic acid, maleic acid,
fumaric ~cid and the like.
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The unsaturated monomer lacking ~urfactant
capacity can be any monoethylenically unsaturated
monomer that is copolymerizable with the carboxylic acid
and the other monomere in the compocition and that
provides a copolymer that i~ ~ubstantially insoluble in
water in the absence of alkali conditions, i.e., a pH of
at least about 7Ø
It is preferred to utilize unsaturated
~onomers lacking ~urfactant capacity in which the single
ethylenic group is the only group which is reactive
under the conditions of copolymerization. The
unsaturated monomers lackinq surfactant capacity can
have groups that are reactive during cure of the
composition, e.g., hydroxyethyl acrylate.
These unsaturated monomers are usually
acrylate or methacrylate esterC, e.g., ethyl acrylate,
butyl acrylate, ~ethyl acrylate and the like, and the
corresponding ~ethacrylates. ~ther unsaturated
monomers, e.g., styrene, vinyl acetate, vinyl toluene,
vinylidene chlorides, acrylonitrile and the like, are
also useful. Admixtures of the unsaturated monomers
can also be utilized.
The non-ionic urethane monomer is a
conventional urethane reaction product of a monohydric
non-ionic surfactant with a monoethylenically
unsaturated monoisocyanate. As disclosed in the Shay
Patent, the surfactant is a hydrophobe carrying a
polyalkoxylate chain and ie terminated with a ~ingle
hydroxy group. ~he polyalkoxylate chain preferably has
at least 2 carbon atoms. When the hydroxy group of the
~urfactant is reacted with the unsaturated
monoisocyanate a urethane linkage is formed and the
hydrophobe is linked with the ethylenically unsaturated
group which ie available for copolymerization.
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- The preferred ~urfactants have the formula:
RO- ( CH2CHR ~ ) m~ ( CH2C~H20) n~H
wherein R is an alkyl group containing 6 to 22 carbon
atoms (typically dodecyl) or an alkaryl group containing
8 to 22 carbon atoms (typically octyl or nonyl phenol),
R' is a C~ to C4 alkyl group (typically methyl), n is an
average number from about 6 to 150, and m is an average
number of from 0 to 50 provided n is at least as great
as ~ and n I m ~ 6 to 150.
A preferred monohydric non-ionic ~urfactant is
an ethoxylated aliphatic alcohol or alkyl phenol wherein
a carbon chain containing at least 6 carbon atoms
provides the hydrophobic portion of the ~urfactant,
e.g., an ethoxylate of nonyl phenol.
The monoethylenically unsaturated
monoisocyanate can be produced from the reaction of a
copolymerizable unsaturation-containing group and an
organic diisocyanate.
The copolymerizable unsaturation-containing
group can have ethylenic unsaturation provided by an
acrylate, methacrylate or allylic group (as provided by
allyl alcohol). Preferably, the copolymerizable
unsaturation-containing group is hydroxy-functional and
can be produced by reacting a C2 to C4 monoepoxide,
e.g., ethylene oxide, propylene oxide, and butylene
oxide, with acrylic or methacrylic acid to produce a
hydroxy ester.
An organic diisocyanate is reacted in
equimolar proportions with the copolymerizable
unsaturation-containing group. Representative of the
organic dii~ocyanates are toluene diisocyanate and
isophorone diisocyanate.
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A preferred ethylenically unsaturated
monoi60cyanate is styryl, as in alpha,
alpha-dimethyl-m-isopropenyl benzyl isocyanate which is
commercially available.
The polyethylenically unsaturated monomer is
conventionally utilized in alkali soluble emulsion
copolymer~. Representative of these polyethylenically
unsaturated monomers are ethylene glycol diacrylate,
ethylene glycol dimethacrylate, 1,6-hexanediol
diacrylate, 1,6-hexanediol dimethylacrylate, diallyl
be~zene, and the like.
The associative thickeners of the present
invention are produced by emul ion copolymerization of
the ~onoethylenically unsaturated carboxylic acid, the
unsaturated monomer lacking surfactant capacity, the
non-ionic urethane monomer, and the optional
polyethylenically unsaturated ~onomer.
A representative associative thickener for use
in the method of the present invention is the Thickener
Example 7 of the Shay Patent which is produced by
copolymerizing 40 parts of methacrylic acid, 35 parts of
ethyl acrylate 25 parts of a urethane monomer, all parts
being by weight.
The aqueous composition typically include an
elastomer, pigment, filler, binder, and water.
Preferably the elastomer i5 a latex ~uch as a
styrene-butadiene latex. The aqueous composition can
produce a non-adhesive, water-activated adhesive or
pressure activated adhesive coating.
The active associative thickener is pre~ent in
the coating composition in an amount in the range of
about 0.005 to about 2, preferably about 0.01 to about
0.5, weight percent based on the solids content of the
coating composition.
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The ~olids content of the aqueous coating
composition utilized in the present method is preferably
in the range of about 30 to about 65 percent, most
- preferably about 35 to about 60 percent by weight.
After the associative thickener i~ admixed
with the conventional agueous composition the resultant
coating composition is neutralized with an alkali euch
as ammonium, ~odium or potassium hydroxide or an amine.
Alternatively, the associative thickener can be
neutralized before it is added to the conventional
aq~eous composition.
The coating compositions of the present
invention cAn be applied to a substrate capable of
receiving the coating compositions by conventional
coating techniques such as blade, roll, curtain, air
knife, spray, etc. After the composition i~ applied, it
is cured using heat to remove the water which results in
coagulation that produces a cured coating.
Representative eubstrates are paper, e.g.,
paper made from vegetation and/or plastic, cardboard,
films, e.g., polyethylene films and MylarR films, yarns,
fabrics, leather and the like. Preferred ~ubstrates are
selected from the group consisting of paper, cardboard,
films, yarns, fabrics and leather.
Paper coated with the cured coating
compositions can be utilized as newsprint stock,
packaging ~aterial, wallpaper stock, printers ~tock, and
the like.
The following Examples are presented by way of
illustration of preferred emho~iments of the present
invention, and not limitation.
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EXAMPLE 1: COATlNG COMPOSITION AND PAPER
COATING METHOD UTILIZING A SIZE PRESS
An aqueous size coating composition for use in the method of
the present invention was prepared by admixing the
alkali-swellable associative thickener PP-101, commercially
available from DeSoto, Inc., Des Plaines, IL, and a conventional
aqueous composition including a styrene-butadiene latex, clay,
starch and calcium carbonate. The associative thickener
concentration was 0.014 weight percent based on the solids content
of the size coating composition. The rotational shear viscosity of
the size coating composition was about 200 centipoise (cp). The
solids content of the size coating composition was about 40 weight
percent.
This size coating composition was applied to paper
utilizing a size press. The line speed during coating was about
1,200 to about 1,750 feet per minute. The paper utilized was high
quality Gravure #1 having a weight of 60 pounds per 3,300 square
feet. The weight of the coating applied to the paper was 7 pounds
per 3,300 square feet.
The amount of associative thickener utilized is about l/5th
the amount of the convention thickener hydroxy propyl methyl
cellulose, commercially available from Dow under the trade
designation Methocel(~, conventionally utilized and the
line speed was higher than can be obtained utilizing a composition
cont~ining this conventional thickener.
The paper exhibited improved surface smoothness and
gloss and had a print quality comparable to that which can be
attained utilizing an off-machine coater, e.g., a roll or blade
coater.
.
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EXAMPLE 2: COATING COMPOSITION AND PAPER COATING
~ETHOD UTILIZING A REVERSE ROT~ COAT~R
An agueous ~ize coating composition for use in
the ~ethod of the present invention was prepared by
admixing the alkali-swellable associative thickener
PP-102, commercially available from DeSoto, Inc., Des
Plaines, IL, and a conventional agueous composition
including a styrene-butadiene latex, clay, starch,
calcium carbonate and ink capsules. The associative
thickener concentration was 0.16 weight percent based on
the ~olids content of the size coating composition. The
rotational ~hear vi~cosity of the size coating
composition was about 3000 cp. The solids content of
the size coating composition was about 50 weight
lS percent.
This size coating composition was applied to
paper utilizing a rever~e roll coater. The coating
~peed was about 1,000 to about 2,000 feet per minute.
The paper utilized was newsprint having a weight of 28
pounds per 3,300 ~quare feet. The weight of the coating
applied to the paper was 5 pounds per 3,300 square feet.
Unlike conventionally thickened coating
compositions, this size coating composition transferred
completely from the roller to the substrate with no
2S webbing, misting/spattering or pattern formation.
EXAMPLE 3: COATING COMPOSITION AND PAPER COATING
METHOD UTILIZING A SHORT DWELL BLADE
COATER
An agueous size coating co~position for use in
the ~ethod of the present invention was prepared by
admixing 0.3 parts, on a solids basis, of t~e
alkali-~wellable associative thickener PP-101
commercially available from DeSoto, lnc., Des Plaines,
IL, and ~ conventional aqueous composition including 6
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parts of a styrene-~utadiene latex and 6 parts clay.
The associative thickener concentration utilized was
0.17 weight percent based on the solids content of the
~i2e coating composition. The rotational shear
S viscosity of the ~i2e coating composition was about
3,500 cp. The solids content of the ~ize coating
composition was about 60 weight percent.
This size coating composition was applied to
paper utilizing a short dwell blade coater. The coating
speed was about 3,200 to about 4,000 feet per minute.
The paper utilized was fine Gravure having a weight of
28 pounds per 3,300 square feet. The weight of the
coating applied to the paper was about S.S to about 6.5
pounds per 3,300 square feet.
lS Very good coatability was obtained. The
present method utilized a lower blade pressure as
compared to compositions containing a conventional
thickener which is desirable for faster coater ~peeds
without scratching and for improved water retention of
the paper.
