Note: Descriptions are shown in the official language in which they were submitted.
~S6238
HIGH STRENGTH PIGMENT
BINDERS FOR PAPER COATINGS CONTAINING
CARBOXYLATED VINYL ESTER ETHYLENE INTERPOLYMERS
The present invention is directed to high strength pigment binders
for paper coating having increased water retention and stability. The
coating compositions comprise a synthe-tic polymer latex and pigment and
may include other additives used in the art of pigmented paper coating.
The latex comprises a dispersed interpolymer of a vinyl ester, ethylene,
a polyethylenically unsaturated comonomer and an ethylenically unsatur-
ated mono- or dicarboxylic acid.
In the preparation of a coated paper web there is used a pigment,
such as clay or the like, which is then compounded with a latex binder
1~ or adhesive material to produce a composition known in the art as a
coating "color" for use in coating a cellulose web, e.g. a paper or
paperboard web. Substantial quantities of the binder are used, and
accordingly, the composition and characteristics of the binder are of
great importance in determining the qualities of the finished coated
web.
It has been recognized by the paper industry that increased dry
strength properties may be provided to these latex binders by the in-
clusion therein of carboxylate functionalities. There has however been
difficu`lty encountered in providing carboxylic functionality in excess
of about 2% by weight to vinyl ester containing latex polymer com-
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positions due to excessive alkaline swellabili-ty of the resultant latex
particles. This swellability, in turn, produces unacceptable latex
thickening at these pH values and consequent problems in the transport
of such materials in conventional latex handling equipmert where vis-
cosities less than about 1 Pa.s (1000 cps.) are generally employed.
It has now been found that substantially higher levels of carboxy-
lation, with a synergistic improvement in coating strength, may be ach-
ieved with reduction in alkaline swellability by incorporation in the
interpolymer of one of a specific class of polyethylenically unsa-turat-
ed comonomers.
The present invention provides a pigmented paper coating composi-
tion comprising an aqueous synthetic polymer latex binder, pigment, and
sufficient alkali to achieve a pH of 6 to 10, the latex comprising dis-
persed therein an interpolymer which consists essentially of:
a~ a vinyl ester of an alkanoic acid having 1 to 13 carbon atoms
interpolymerized with the following comonomers:
b) from 15 to 40% by weight ethylene, sufficient to achieve a Tg
of 0 t~ -40C.;
c) from 2 to 15 milliequivalents per 100 grams (a) and (b) of a
polyethylenically unsaturated comonomer selected from the group con-
sisting of triallyl cyanurate, triallyl isocyanurate, diallyl maleate,
diallyl fumarate, divinyl benzene, and diallyl phthalate;
d) from 25 to 65 milliequivalents per 100 grams (a) and (b) of an
ethylenically unsaturated mono- or dicarboxylic acid or a half ester
thereof characterized in that the alkali response of the polymer latex
is substantially reduced wi-th respect to latices lacking component (c)
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while the coating strength of the resultant pigmented paper coa-ting is
substantially improved thereover.
The vinyl ester monomers which may be utilized herein include the
vinyl esters of alkanoic acids having from 1 to about 13 carbon atoms.
Typical examples include: vinyl formate, vinyl acetate, vinyl propion-
ate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, vinyl 2-ethyl-
hexanoate, vinyl isooctanoate, vinyl nonoate, vinyl decanoate, vinyl
pivalate, vinyl versatate, etc. Of the foregoing, vinyl acetate is the
preferred monorner because of its ready availability and low cost.
Generally, any ethylenically unsaturated mono or dicarboxylic acid
may be used to provide the carboxyl functionality. Examples of suit-
able acids include the monocarboxylic ethylenically unsaturated acids
such as acrylic, vinyl acetic, crotonic, methacrylic, tiglic; the di-
carboxylic ethylenically unsaturated acids such as maleic, fumaric,
itaconic, maleic, citraconic, hydromuconic, allylmalonic; as ~ell as
the corresponding half ester of those dicarboxylic acids, such as mono-
(2-ethyl hexyl) maleate, monoethyl maleate, monobutyl maleate. Substan-
tial improvements in strength are shown at levels of greater than 25
milliequivalents per 100 grams of the ethylene comonomer with amounts
greater than 40 milliequivalents being preferred.
The resultant paper coating latex compositions are characterized by
reduced alkali response and increased water retention in the latex
state with improved properties of dry strength imparted to the final
paper sheets coated therewith. Thus, while latex compositions prepared
with levels of carboxylic functionality in excess of about 25 milliequi-
valents per hundred and particularly in excess of about 40 milliequiva-
lents per hundred will generally exhibit substantial increase in vis-
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cosity a-t alkaline pH's, compositions prepared with 2 to 15, preferably
4 to 7, milliequivalents of polyethylenically unsaturated monomer per 100
part comonomer as taught herein exhibit little or no increase in vis-
cosity under the same conditions. Moreover, in addition to the increase
in coating strength which may be expected as a result of the increase in
carboxyl functionali-ty, further increases in coating strength are observ-
ed when the pigmented coating binders are prepared with the latices
taught herein. It will be recognized by one skilled in the art tha-t the
preferred level of polyethylenically unsaturated comonomer will vary pro-
portionately depending upon the particular type and level of acid used.
To prepare the interpolymer latices used in the coating compositionsherein the vinyl ester, ethylene, the polyetnylenically unsaturated
monomer and -the carboxylic acid are interpolymerized in an aqueous medium
under pressures less than about 130 atmospheres in the presence of a
catalyst, and an emulsion stabilizing amount of an anionic or a nonionic
surfactant or mixtures thereof, the aqueous system being maintained by
a suitable buffering agent, if necessary, at a pH of 2 to 6. The poly-
merization is performed at conventional temperatures from about 21-107C
(70-225F), preferably from 49-79C (120-175F), for sufficient time
to achieve a low monomer content, e.g. from 1 to about 8 hours, prefer-
ably from 3 to about 7 hours, to produce a latex having less than 1.5
percent, preferably less than 0.5 weight percent, free monomer. Conven-
tional batch, semi-continueous or continuous polymerization procedures
may be employed and are taught, for example in U,S. Patents 3,708,388
issued Jan. 2, 1973 to M. K. Lindemann et al.; 3,404,112 issued Oct. 1,
1968 to M. K. Lindemann et al.; 3,71~,099 issued Jan. 30, 1973 to
G. Biale; and 4,164,488 issued Aug. 14, 1979 to B. V. Gregorovich et al.
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The polymeriza-tion is initiated by a water soluble free radical in-
itia-tor such as water soluble peracid or salt therof, e.g. hydrogen per-
oxide, sodium peroxide, lithium peroxide, peracetic acid, persulfuric
acid or the ammonium and alkali metal salts thereof, e.g. ammonium per-
sulfate, sodium peracetate, lithium persulfate, potassium persulfate,sodium persulfate, etc. A suitable concentration of the initiator is from
0.05 to 5.0 weight percent and preferaby from 0.1 to 3 weiyht percent.
The free radical initiator can be used alone and thermally decompos-
ed to release the free radical initiating species or can be used in com-
bination with a suitable reducing agent in a redox couple. The reduc-
ing agent is typically an oxidizable sulfur compound such as an alkali
metal metabisulfite and pyrosulfite, e.g. sodium metabisulfite, sodium
formaldehyde sulfoxalate, potassium metabisulfite, sodium pyrosulfite,
etc. The amount of reducing agent which can be employed throughout the
copolymerization generally varies from about 0.1 to 3 weight percent of
the amount of polymer.
The emulsifying agent can be any of the nonionic or anionic oil-in-
water surface active agents or mixtures thereof as are generally em-
ployed in emulsion polymerization procedures. When combinations of emul-
sifying agents are used, it is advantageous to use a relatively hydro-
phobic emulsifying agent in combination with a relatively hydrophilic
agent. The amount of emulsifying agent is generally from about 1 to
about 10, preferably from about 2 to about 8, weight percent of the mono-
mers used in the polymerization. Latex stabilizers such as sodium vinyl
sufonate may also be present in amounts up to about 0.5% by weight. It
will also be recognized that some portion of the carboxylic acid monomer
may provide the stabilizing function in addition to the strength increase.
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The emulsifier usecl in the polymerizaton can also be added, in
it entirety, -to the initial charge to the polymerization zone or a
portion of the emulsifier, e.g. from 90 to 25 percent thereof, can be
added continuously or intermittently during polymerization.
The preferred in-terpolymerization procedure is a modified batch
process wherein the major amounts of some or all the comonomers and em-
ulsifier are charged to -the reaction vessel after polymerization has
been initiated. In this manner, control over the copolymerization of
monomers having widely varied degrees of reactivity can be achieved.
It is preferred to add a small portion of the vinyl ester initially and
-the remainder o-f the vinyl ester and other comonomers intermittently or
continuously over the polymerization period which can be from 0.5 to
about 10 hours, preferably frorn about 2 to about 6 hours.
The ethylene content of the interpolymer depends on the ethylene
content of the aqueous polymerization medium. Factors which control
~he ethylene content of the polymerization medium include the partial
pressure of ethylene in the vapor phase over the medium, the tempera-
ture of polymerization and the degree of mixing between the vapor phase
and the liquid medium. Generally, the polymerization is performed at
temperatures from 49-79C l120-175F) and, at these temperatures,
ethylene partial pressures from 345-6895 x 103 Pa (50-1,000 psig)
preferably from 1724-5171 x 103 Pa (250-750 psig.) are sufficient to
incorporate from 1 to 30, preferably from 5 to 25, weight percent -
ethylene in the polymer. The reaction medium is preferably agitated
with a stirrer, however, other agitation can be used as sparging the
liquid with recirculated ethylene from the vapor phase. In the pre-
ferred procedure, the ethylene partial pressures is maintained con-
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stant throughout the polymerization period so that this monomer is con-
tinuously supplied at a constant rate.
The latices are produced and used at relatively high solids con-
tents, e.g. between 35 and 70% although they may be diluted with water
if desired. The preferred contents of solids are from 40 to 60, and,
most preferred, from 50 to about 60 weight percent.
The particle size of the latex can be regulated by the quantity of
non-ionic or anionic emulsifying agent or agents employed. To obtain
smaller particles sizes, greater amounts of emulsifying agents are used.
As a general rule, the greater the amount of the emulsifying agent em-
ployed, the smaller the average particle size.
The actual paper coating composition comprises the interpolymer
latex together with pigment, such as clay and the usual paper coating
additives which may include other cobinders, such as polyvinyl alcohol,
protein, e.g. casein or soy protein, or starch, as is well known to
those skilled in the art.
The pigment used in the papermaking compositions may be any of
those conventionally employed. Generally, at least a portion of the
pigment comprises clay and for this portion, any of the clays custom-
arily used for coating paper, including the hydrous aluminum silicatesof kaolin group clays, hydrated silica clays, and the specific types of
clays recommended in Chapters 10-16 of "Kaolin Clays and their Indus-
trial Uses," by J. M. Huber Corp., New York, NY (1949). In addition
to clay itself, there may be utilized other paper pigments such as, for
example~ calcium carbonate, titanium dioxide, blanc fixe, lithopone,
zinc sulfide, or other coating pigments including plastics, for example
polystyrene, in various ratios, e.g. up to 50%, preferably up to 35%,
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by weigh-t of the clay. Adclitionally, the composition may also contain
other additives such as zinc oxide and/or a small amount oF a dispersing
or stabilizing agent such as tetrasodium pyrophosphate. In general, the
paper coating composition comprises 100 parts pigment containing 65-100
parts clay and 0-35 parts secondary pigment; 0.01-0.5 parts dispersing or
stabilizing agent; 3-30 parts interpolymer latex (solids basis); 0-25
parts co-binder; 0-0.2 parts defoamer and sufficient water to provide the
desired level of solids. The modification of the coating color using
these materials will be within the knowledge of those skilled in the art.
The coating compositions produced herein may be applied to fibrous
paper webs using any of the conventional coating devices including, but
not limited to, those referred to as trailing blade coaters, air knife
coaters, roll coaters and the like.
In the examples which follow all parts of polyethylenically unsat-
urated comonomers and carboxylic acid are based on parts per 100 parts
by weight of the combined vinyl ester and ethylene component. In test-
ing the latices and coating colors, the following test procedures were
followed:
75 Gloss was meausured using a Gardner Glossmeter.
Brookfield viscosity values were obtained using Spindle ~2 at 20
rpm and/or 100 rpm as indicated.
Dry Strength values on paperboard were determined using an IGT
Dynamic Pick Tester or an AIC 2-5 Tester, the ink as indicated, a "B"
spring setting and 35 kg. load.
EXAMPLE I
Two latices were prepared using an interpolymer of 85% vinyl ace-
tate, 15% ethylene, and 5 parts monoethyl maleate (35 milliequivalents
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per 100). To one of the interpolymers was added 0.~ parts diallyl mal-
eate (6 milliequivalents per 100) in accordance wiih the teachings herein.
The Brookfield viscosity of the resultant latices ~50% solids)
were recorded at varying pH ranges in order to test the alkali response
of the latices. Viscosity values are shown in Table I.
TABLE I
Brookfield Viscosity (Latex)
Parts DiallylPa.s (cPs.)
Maleate pH 6.0 7.0 ~.0 9.0
0.4 0.06 2.000 20.150 60.000 -
(60)(2000) (20,150)(60,000)
0 0.0612.000 67.000
(60)(12,000) (67,000) agglomerated ~ -
As can be seen from the above results, the presence of the diallyl
maleate served to substantially reduce the alkali sensitivity of the car-
boxylated interpolymer latex. One set of colors (designated Group A)
were prepared using 100 parts clay, 16 parts latex (dry resin weight),
0.3 parts carboxymethyl cellulose and 0.1 part tetrasodium pyrophosphate.
Another set ~Group B) were prepared from 100 parts clay, 5 parts latex,
7 parts ethylated starch co-binder and 0.1 part tetrasodium pyrophos-
phate. In both sets, the resultant coating colors, which at 55% solids
level had a pH of 8.0, were compounded using conventional techniques
known in the art of paper coating such as are described by R. H. Mosher
in "The Technology of Coated and Process Papers" (Chemical Publishing
Company, Inc., New York, 1952).
In order to better illustrate the reduction in alkali sensitivity,
Brookfield viscosity measurement of the coating colors were also made
at pH 9.5.
The coating colors were then applied to the wire side of several
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sheets of 56.7 kg./278.7 m2 125 lb./3000 ft.2 bleached board to a
final weight of 4.5 kg. (10 lb.) per 278~7 m.2 (3000 ft.2) square
feet. The sheets were machine calendered by 1 pass at 77C (170F),
90.7 kg. per 2.54 cm. (200 lb. per linear inch) and then conditioned
overnight before testing. The test results are shown in Table II.
TABLE II
pH 8 pH 9.5
Parts Brookfield Brookfield 75
Diallyl MaleateViscosity Viscosity Gloss IGT
Pa.s (cps.) Pa.s (cps.) (#5 Ink)
20 rpm100 rpm 20 rpm100 rpm
Group A:
0.4 2.050 0.670 2.750 0.915
(20~0)(670) (2750)(915) 43 400
0 2.150 0.730 6.600 2.180
(2150)(730) (6600)(2180) 44 320
Group B:
0.4 2.650 0.940 2.750 0.990
(265Q)(940) (2750)(990) 52 215
0 2.700 0.970 4.150 1.550
(2700)(970) (4150)(1550) 53 95
As the above results show, the dry strength of the coating color ~as
measured by IGT values) is substantially increased by the use of both
the diallyl maleate and the monoethyl maleate.
EXAMPLE II
Another series of latices were prepared using an interpolymer of 70%
vinyl acetate, 30% ethylene and varying amounts of acrylic acid and dial-
lyl maleate. In each case a small amount of sodium vinyl sulfonate (SVS)
was also added as latex stabilizers. The Brookfield viscosity of the re-
sultant latice (50% solids) of varying pH ranges and the coating strength
of colors prepared with the corresponding latices are shown in Table III.
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TABLE III
pH (latex)
Diallyl Acrylic Pa.s (crs.) IGT
Latex Maleate Acid SVS 7.0 8;0 9 0 #6 Ink
A <0.100 0.100 0.100
- 0.75 O.S (<100) (100) (100) 230
B <0.100 0.100 0.100
0.35 0.75 0.5 (<100) (100) (100) 370
C <0.100 0.200 0.350
~ 2.0 0.5 (<100) (200) (350) 310
D <0.100 0.100 0.200
0.35 2.0 0.5 (<100) (100) (200) 420
E 1.000 2.500 2.500
- 3cO 1.0 (1000) (2500) (2500) 330
F 0.200 0.350 0.400
0.35 3.0 1.0 (200) (350) (400) 480
G <0.100 <0.100 <0.100
- - 3.0 (<100) (<100) (<100) 190
H 0.35 - 3.0 not measured 250
(ConYersions to milliequivalents: 0.35 parts diallyl maleate = 5~4 meq.,
0.75, 2.0 ml 3.0 parts acrylic acid = 10.4, 27.8 and 4.2 meq. respect-
i vely . ~
As the above results show, the addition of acrylic acid and diallyl
ma7eate substantially improves the coating strength of pigment binders
while effectively suppressing the alkali response of the polymer latex.
Thus, a comparison of samples C and E shows that when the levels of car-
boxyl functionality are increased, with an increase in IGT strength, the
alkali response of the latex is also increased. Surprisingly, when the
diallyl maleate is added (Samples D and F) the strength values are even
further increased yet the alkali response is substantially reduced. As
a control to illustrate the necessity of using carboxyl functionality,
a sample (G) was prepared using high le~els of sodium vinyl sulfonate.
No substantial increase in wet strength was obtained thereby. Sample H
shows some increase as a result of the addition of the diallyl maleate.
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A conventionally employed acrylate paper coa-ting latex was tested
as a control with the aforementioned series and found to give an IGT
strength value of 480 thus showing that the use of higher levels of
carboxylic functionality will provide an ethylene vinyl acetate based
paper coating binder comparable to those binders conventionally employ-
ed in the industry.
When interpolymers are prepared using other vinyl esters, and/or
triallyl cyanurate, triallyl isocyanurate, diallyl fumarate, divinyl
benzene or diallyl phthalate and/or other carboxylating agents compar-
able results will be obtained.
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