Note: Descriptions are shown in the official language in which they were submitted.
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Case 150-5~86
PAPER CC~IINGS
The invention relates to microcrystalline cellulose for use in
paper-coating applications. The invention further relates to an
improved coating composition which contains a "carrier" substance for
additives, particularly optical brighteners.
Conventionally, coated printing papers are prepared using
paper-coating compositions which comprise a pigment (for example
kaolin, calcium carbonate or titanium dioxide) which is dispersed in
water, and a binder.
In the past, the binders used were exclusively high mo].ecular weight,
naturally-occurring products, such as starch or casein. Nowadays,
these have largely been replaced entirely or partially by synthetic
high molecular weight copolymers in the form of aqueous dispersions.
Other additives commonly found in coating compositions are
dispersants, water-soluble polymers, optical brighteners and alkali
for pH adjustment.
Because of the very high level of pigment solids required for a
coating composition, anionic dispersants are used as deflocculants,
causing particle repulsion and reduced viscosity.
The water-soluble polymers previously mentioned act as cobinders and
impart water retention to the paper coating composition. T~ey may be
naturally-occurring polymers such as starch and casein or
alternatively synthetic compounds such as polyvinyl alcohol (PVOH),
carboxymethylcellulose ~CMC) and water soluble polyacrylates.
Commercial optical brightening agents are added to increase the level
of whiteness of the coated paper and the efficiency of these compounds
depends very much on the coating composition. It is known that if the
brightener molecule can bond to a substance by means of adsorption or
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Case 150-5fi86
hydrogen bond formation, the brightener per:Eormance is appreciably
enhanced.
Some water-soluble polymers used in coating compositions, such as PVOH
and CMC, do adsorb optical brighteners to some extent, but the water-
insoluble binders, in aqueous dispersion, have a negative influence on
brightener performance.
me invention relates to a paper coating composition containing a
substance which allows the easy addition of additives to a paper, and
which in particular greatly enhances the performance of commercial
optical brighteners.
me substance is cellulose, preferably microcrystalline cellulose,
having a particle size of 1 to 10 microns, preferably l-to 3 microns,
in aqueous suspension. It is well known that cellulose swells in
water and that the "dry" particle size of a cellulose particle will be
considerably s~naller than the "wet" particle size.
Commercial optical brighteners are very substantive to
microcrystalline cellulose and once adsorbed they appreciably improve
the whiteness of the coated paper.
Microcrystalline cellulose, sold in powder form, has been commercially
available for many years and is used extensively in the pharmaceutical
industry. It has been used to a limited extent in the paper industry
but its potential has been restricted by its large particle size and
lack of surface area.
m e present invention provides a method for reducing the particle size
of cellulose (preferably microcrystalline cellulose) in water and/or
an organic solvent (preferably water) comprising
a) forming a slurry or paste of cellulose material in water, the
cellulose material having a dry weight average particle size of
1 to 100 rnicrons (preferably 1 to 20 microns); and
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b) reducing the average wet particle size of the cellulose
material to 1 to 10 microns (preferably 1 to 3 microns),
preferably by milling, to give a carrier;
an additive being optionally added to the water prior to addition of
cellulose or at the end of operations a) or b) above.
The present invention enables the additive to be adsorbed on to the
carrier substance.
Preferably the process according to the invention comprises
a) slurrying microcrystalline cellulose powder in water to give
dispersion of 2 to 15%, preferably 5 to 10%, weight solids,
preferably at a temperature of 5 to 60C, preferably 20-~0C;
and
b) reducing the wet particle size of the dispersion, preferably
using a bead mill; and
c) adding the additive to the dispersion of reduced particle size.
In one embcdiment of the invention, the additive may be a surface
sizing agent and/or a dyestuff. However, the preferred additive is an
optical brightener. m is particular aspect of the invention will be
discussed further hereinunder.
The present invention also provides an aqueous slurry or paste (a
carrier), for use in a paper-coating composition, containing
microcrystalline cellulose having an average particle size of 1 to 10
microns, preferably 1 to 3 microns. This greatly enhances the
performance of commercial optical brighteners that are often used in
paper coating compositions.
The slurry, paste or dispersion may be given several passes through
the mill until the wet particle size averages 1 to 10 microns,
preferably 1 to 3 microns.
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Case ]50-5486
Cor[mercial optical brighteners are very substantive to
microcrystalline cellulose, and once adsorbed they improve the
whiteness of the coated paper substantially. It is a particular
feature of this invention that the use of optical brighteners results
in an unexpectedly high increase in whiteness.
A summary of optical brighteners may be found, for example, in the
article by H. Gold in Venkataraman, "me Chemistry of Synthetic Dyes",
Academic Press, New York and London 1971, Vol. 5, chapter 8, pp.
536-679 (which is herewith incorporated by reference). Cptical
brighteners are commercially available and therefore do not require
more detailed colrment here. However, a more detailed characterization
of this category of compounds may be found in German Laid-Cpen
Application DOS No. 2,628,878 and German Published Application DAS No.
1,795,047 (which are incorporated herein by reference).
mese publications describe optical brighteners based on stilbene
derivatives which are the preferred optical brighteners for the
purposes of this invention.
The viscosity of the cellulose increases during milling and, if
deslred, compounds may be added which reduce viscosi-ty. For example,
the cellulose may be milled in the presence of dispersing agent,
suitable examples of which are mentioned hereinunder in connection
with pigments, optionally in the presence of a filler such as calcium
carbonate or clay. Dispersants may be used in a quantity of up to 10
by weight (on dry cellulose). An especially effective and preferred
dispersant for the purposes of this invention is carboxymethyl
cellulose (CMC)
To avoid colouration of the cellulose by metal contamination during
milling, sequestering agents may be added. Depending on the
construction of the bead mill, a grey colour resulting from metal
pick-up may be observed in the milled cellulose. If sequestering
agents are used, they are used in a quantity of up to 0.4~6 by weight,
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Case 150-5486
(based on the weight of dry cellulose), usually in -the form of
commercially-availab]e material which has a ~0% concentration.
Preferred sequestering agents are ethylene diamine tetraacetic acid
(EDTA), diethylene triamine pentaacetic acid (DT~A), diethylene
triamine pentamethylene phosphonic aci.d (DTPM~) and nitrilotriac:etic
acid (NTA) or their sodium salts.
By creating a carrier according to -the invention, the surface area of
the cellulose is greatly increased, creating much higher adsorption
capacities for the additive. The additive, which may be in liqui.d or
powder form, is then added to the carrier.
Preferably 1 to 50%, more preferably 8 to 25% additive, based on the
dry weight of cellulose, is added to a slurry according to the
invention.
The amount used depends on the substantivity of the additive and
should be adjusted so that once the cellulose particle size is
reduced, almost all the adsorption sites are taken up and very little
additive is left free in the water. If required, hardness salts, such
as magnesium sulfate may be used to increase the substantivity of the
additive. Normally an addition of 10 to 100 ppm magnesium sulfate to
the water is sufficient.
me preparation of the aqueous carrier according to the invention may
also be carried out by adding -the additive and any dispersant and/or
hardness salts to the water, prior to the addition of the cellulose
powder. The mixture is then milled as before.
The preferred dry microcrystalline cellulose powder that is added to
water to form the carrier has an average particle size of 2 to 100
microns. It is relatively simple to bring the size distribution to
the required level using classification techniques. Once mixed with
water, however, the particles swell and their size is increased by a
factor of 10 to 100. It is therefore important that the particle size
be reduced after the powder has been mixed with the water.
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Further according to-the invention there is provided a process for
coating paper comprising applying the carrier defined above,
optionally containing one or more additives, to the surface oE paper.
Still further according to the invention there is provided paper-to
which a carrier according to the invention has been applied.
It is found that the fine particle size cellulose carrier, after
application to the paper, dries to form a film. As a result, the
surface properties of the paper are enhanced in such a way that if the
coated paper is to be printed, the printing properties are far
superior to those of paper which has not received this surface
treatment.
The cellulose particles shrink on drying to give a srnooth, tough film.The interstices in the paper surface are filled with the carrier, and
when dry the paper surface has a smooth feel and appearance. The
large hydrogen bonding capacity of the rnicronised cellulose ensures a
high surface strength for the coated paper. This surface modification
is particularly noticeable if the cellulose carrier is applied on its
own or together with the addi-tive, using any cornmercial coating
technique or a size press.
Any cornmercial optical brightener, dyestuff or other art-recognised
additive, having affinity for cellulose can then be added to the
rnodified microcrystalline cellulose carrier. The amounts of additive
are adjusted so that almost total adsorption is ahieved. The carrier
with additive is then added to the coating composition, thereby
imparting to the coated paper whatever property the additive confers,
for example, high whiteness, high colour yield or surface wet/dry
strength.
Yet still further according to the invention there is provided a
pigment-containing composition for use in paper coating comprising per
lO0 parts by weight of pigment,
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Case 150-5~86
a) from 5 to 25 parts by weight of binder (in the Eorm of cm
aqueous dispersion),
b) from 0.1 to 10 parts by weight of water soluble cobinder and/or
water retention aid; and
c) 0.1 to 5 parts by weight of the carrier to which 0.1 to 50%,
preferably 0.1 to 20% by weight of additive may optionally be
adsorbed according to the invention.
Suitable pigments for use in the coating composition include, china
clay (kaolin), calcium carbonate, sulfate or silicate, titanium
dioxide, talc, bari.um sulfate, zirconium oxide and alumina. These
pigments may be used independently or in admixture.
Appropriate copolymers for use in the binder mixture are all
commercial synthetic binders which are available in the form of an
aqueous dispersion. These polymers have a glass transition
temperature of from -40 to +50C. Examples of typical monomers from
which these copolymers may be prepared are esters of acrylic acid and
of methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide,
methacrylamide, vinyl chloride, vinylidene chloride, ethylenically
monounsaturated or polyunsaturated hydrocarbons, e.g. ethylene,
propylene, butylene, styrene, butadiene, isoprene and chloroprene,
vinyl esters, vinylsulphonic acid and esters of ethylenically
unsaturated carboxylic acids, e.g. hydroxypropyl acrylate and
hydroxypropyl methacrylate.
Where appropriate, other assistants may also be added to the paper
coating composition, for example alkalis, e.g. sodium hydroxide,
potassium hydroxide or ammonia. These compounds are used primarily to
adjust the pH of the mixture to around 8.5 but they also act as
viscosity modifiers, keeping the coating mix fluid.
To achieve good dispersion of the ingredients, from 0.2 to 5~ by
weight of a dispersant, e.g. a low molecular weight polymer of acrylic
acid and preferably an ammonium salt or an alkali metal (e.g. sodium)
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Case 150-5486
salt of a polyacrylic acid having a K value of from 10 to 35, may be
employed. The K values of the polymers are measured by the method of
H. Fikentscher, Cellulose-Chemie 13 (1932), 58-6~ and 71-74, in 0.5%
strength aqueous solution at 25~C.
The water-soluble polymers may be regarded as co-binders because they
themselves possess pigment binding properties. I'he synthetic
products, and also the natural cobinders, act as thickeners, i.e. they
increase the viscosity of the aqueous paper-coating compositions. In
addition they increase the water retention of the coating composition
and some such polymers activate additives to some extent. Thei.r use
in the latter respect has always been limited because even small
amounts affect the rheology of the mix and these low levels are not
sufficient for efficient brightener development.
To prepare, for example, a paper-coating composition according to the
invention which comprises an optical brightener, the carrier (as
herein defined) is mixed with the other coating composition
ingredients, preferably once the pigment/binder mixture has been
prepared, in a manner as described below.
With the aid of a powerful dispersing blender, 100 parts of selected
pigments are dispersed in water containing 0.1 to 0.5 parts sodium
hydroxide and 0.1 to 0.5 parts of a commercial dispersant based on a
low molecular weight polyacrylic acid, so that the solids of the mix
are 50-70%.
To this are added 5 to 20 parts of a commercial synthetic copolymer
binder and optionally 0.2 to 2.0 parts of a water-soluble polymer as
cobinder. Finally, 0.2 to 5 parts of a brightened cellulose carrier
are added. The pH of the coating composition is adjusted to 8.0 to
9.0 with sodium hydroxide or ammonia.
me final solids of the coating composition is 50 to 70%.
Once applied to paper, using for example an air-knife coater, a blade
coater, a metering bar coater or a size press, the whiteness of the
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Case 150-5486
sheet is far superior to that where cellulose is not used.
Traditional water-soluble polymers, used as "carriers'7 for additives,
are not able to generate the increased levels of whiteness seen with
the present invention.
This method may be used with any other suitable additive replacing the
optical brightener.
The traditional method of preparation for coating compositions is to
slurry the pigments in water containing dispersant and then add
copolymer binder, water soluble cobinder and, say, opti.cal brightener.
After pH adjustment to 8.0 to 9.0, the mix is ready for use.
With this traditional method, the additive is added as a liquid and,
since very little of the additve is actually adsorbed on to a
composition component, st of it migrates with the water phase into
the base sheet or is left in the coating after drying. In the
particular case of optical brighteners, high levels of brightener are
used to produce low levels of whiteness, showing that the traditional
use of additives in coating compositions is far from efficient.
In a conventional coating process, the amount of optical brightener-
containing coating mix applied to the paper surface has a direct
influence on the level of whiteness, measured on the finished paper.
For example, a coat weight i.e., the amount of dry weight added to the
original base paper and measured in grams per square meter (gsm) of
say 5 gsm almost always develops a higher measured whiteness than one
with an 18 gsm coat weight. This is due to the "free" optical
brightener migrating into and adsorbing on the base sheet. If the
coat weight is low, e.g. 5 gsm, the whiteness of the base sheet shows
through the coating and contributes to the measured value. On the
other hand, a coat weight of 18gsm tends to mask the base sheet,
resulting in a lower measured whiteness.
The present invention using the novel carrier substance allows a high
whiteness to be achieved, even with high coat weights.
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Case 150-5~86
Although -the advantages of the present invention are particularly
apparent when the additive is a commercial optical brightening agent,
many more additives may be used in conjunction wi-th micronised
microcrystalline cellulose.
For example, direct dyestuffs, both anionic and cationic in nature,
adsorb quite readily on to the carrier paste. Colour can therefore be
introduced into coatings and the surface treatment o paper.
Much work has been carried out in recent years on the application of
dyestuffs to the surface of paper, using the size press. This
particular application has difficulties, mainly due to the uneven
absorption of liquid dye on the paper surface, leading to a mottling
or "orange peel" effect.
With this invention, the carrier paste is dyed to the required colour
and then applied to the paper surface. m is proposal has the same
effect as coating using coloured pigments. me colouration media is
particulate, albeit in a very fine form. With dyestuf however,
recycling of paper is much easier due to the ease with which
decolourization can be carried out using oxidative bleach. This is
not the case where coloured pigments have been used; many are
resistant to oxidative bleaches.
Other additives which can also be successfully applied to the
cellulose carrier include cationic si~es, cationic water repellent
agents, dry and wet strength resins and other surface modification
chemicals, which, even if not substantive to cellulose, can be "fixed"
using a cationic chemical.
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Case 150-54~6
The invention will now be illustrated by the following examples.
l~PIE 1
The preparation of a carrier for additives.
In a suitable container, 20 g microcrystalline cellulose ("Avicel"
PH-105) having an average dry particle size of 20 ~m, is mixed with
380 g water and 0.8g carboxymethylcellulose (CMC)using a high shear
laboratory blender. The resulting dispersion is then passed through a
laboratory bead mill having a chamber size of 250 ml and a glass
grinding bead size of 1 mm and with a shaft rotation speed of 4000
rpm, until the average wet particle size of the carrier is 1 to 5 ~m.
(measured with a Horiba LA 500 particle analyzer usiny laser
diffraction): Yield is 400g.
The resul-ting white viscous paste, hereinafter referred to as the
carrier paste, is now ready for mixing with an additive.
EX~MeLE 2
20 g of carrier paste, at 5% solids, and 0.1 g "Leucophor" AP Liquid,
a commercial optical brightening agent based on stiIbene, are mixed in
a 50 ml beaker at 20C and left for 10 minutes.
In a separate laboratory high shear blended, 75 g of SPS coating clay
and 25 g of calcium carbonate are added slowly to 42.8 g water
containing 0.5g "Dispex" N 40 (40% active). Once homogeneous, 20 g
"Acronal" S360, a 50% active styrene/acrylate copolymer binder, is
added followed by the 20.1 g brightened Carrier paste made earlier.
The pH of the coating mix is adjusted to 8.5 with sodium hydroxide
(30% solution). The solids content of the coating compositi.on is
approximately 60%. After mixing for 10 minutes, the coating
composition is applied to paper, using a metering bar coater.
The results of this application are shown in the Table below.
EXaMeLE 3
In a laboratory high shear blender, 75 g of SPS coating clay and 25 g
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Case 150-5~86
calcium carbonate are added slowly to 42.8 g water containlng 0.5 g
"Dispex" N 40 (40% active). Once homogeneous, 20 g "Acronal" S 360, a
50% active styrene/acrylate copolymer binder, is added followed by 20
g of a 2.5% CMC solution ("Finnfix" 5) and l.0 g "Leucophor" A~
Liquid. The pH of the coating m~.x is adjusted to 8.5 with sodium
hydroxide (30~ solution).
The solid content of the coating composition is approxlmately 60%.
After mixing for 10 minutes, the coating composition is applied to
paper, using a metering bar coater.
me results of this application are shown in the Table.
EX~MPLE 4
30g of carrier paste, at 5 % solids, and 0.3 g "Leucophor" U Liquid, a
commercial optical brightening agent based on stilbene, are mixed in a
50 ml beaker at 30C and left for 30 minutes.
In a separate laboratory high shear blender, 70 g of SPS coating clay
and 30 g calcium carbonate are added slowly to 51.8 g water containing
0.5g "Dispex" N 40 (40% active). Once homogeneous, 20 g "Acronal" S
360, a 50~ active styrene/acrylate copolymer binder, is added followed
by the 30.3 g brightened carrier paste made earlier. me pH of the
coating mix is adjusted to 8.5 with sodium hydroxide (30% solution).
me solids content of the coating composition is approximately 55%.
After mixing for 10 m mutes, the coating composition is applied to
paper,using a metering bar coater.
m e results of this application are shown in the Table below.
EX2MPLE 5
In a laboratory high shear blender, 70 g of SPS coating clay and 30 g
calcium carbonate are added slowly to 61.8 g water containing 0.5 g
"Dispex" N 40 (40% active). Once homogeneous, 20 g '~Acronal" S 360, a
50 % active styrene/acrylate copolymer binder, is added followed by 20
g of a 2.5% PVOH solution ("Poval" PVA 205) and 1.5 g "Leucophor" U
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Case 150-5486
Liquid. The pH of the coating mix is adjusted to 8.5 with sodium
hydroxide (30% solution).
The solids con-tent of the coating composition is approximately 55~.
After mixing for 10 minutes, the coating composition is applied to
paper, using a metering bar coater.
The results of this application are shown i.n the Table below.
TABLE
EXPMPLEVISCOSITY COATING WHITENESS DELTA
NO (mPas)WEIGHT (R457)REFLEC'rANCE
( s m )
g-
2 820 5 89.4 5.2
2 820 22 94.0 9.7
3 1900 5 86.7 2.6
3 1900 22 84.9 0.9
4 670 5 90.6 6.1
4 670 22 95.3 11.0
1480 5 87.2 3.3
1480 22 85.3 1.4
The viscosity is measured using a Brookfield viscometer, spindle No 4
speed 100 rpm.
Whiteness is measured using a Elrepho 2000 spectrophotometer, with the
W filter out, at 457 nm.
Delta reflectance is a measure of the difference in whiteness with the
W filter in and the W filter out. With the W filter inr the effect
of additive is not observed and therefore the measurement can be taken
to be that of the base coating.
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Case 150-5486
EX~MeLE 6
To 100 g of carrier pas-te as prepared in Example 1 but ~rom which the
CMC has been omitted, 0.75 g "Cartasol" Red K-2BN (a cationic direct
dye) is then added and stirred for 10 minutes. Paper is then coa-ted
with this coloured paste using a metering bar coater. Once dried, the
sheet of paper is assessed for colour uniformity and shade.
No mottle is evident, due to the even distribution of dyed cellulose
and a medium to deep red shade is observed.
As a comparison, a solution of dye was applied, mixed with an 8%
aqueous solution of "Amylox" P 45 oxidized starch at 40C. In
contrast, a severe mottle was noted due to uneven dyestuff penetration
into the paper sheet.
EXaMæIE 7
To 100 g of carrier paste, as prepared in Example 1, 0.95 g of
"Cartasol" Red 3BF (an anioninc direct dye) is added and stirred in
for 10 minutes. This mixture is then added to 200 g of a 12% aqueous
solution of "Amylox" P 45 oxidized starch. 100ml of this composition
is added to the nip of a laboratory size press. The nip pressure is
adjusted so as to obtain a dry pick-up weight on the paper of 7.5% at
a speed of 60 m/min.
A medium even shade of red is produced on the paper, compared to a
severe mottle if the liquid dye is mixed with 8% ~mylox P 45 and
applied to the paper using the size press.
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