Note: Descriptions are shown in the official language in which they were submitted.
9130
--1--
IMPROVING THE OUALITY QF ~OATED PAPER
This lnvention relates to the illl~lUV~ t of the
quality of a coated cellulosic sheet material which is
prepared by applying, to a cellulosic sheet material
base, two or more mineral pigment-containing coating
compositions .
If a high quality printed image is to be applied
to a cellulosic sheet material such as paper or
cardboard it is generally necessary to apply to the
lû surface at least one coating composition containing one
or more mineral r;~ --t, such as kaolin clay, calcium
carbonate, calcium sulphate, titanium dioxide, barium
sulphate, satin white and the like. The application of
such a coating composition improves the smoothness,
gloss, whiteness and opacity of ~he surface to which
the printed image is to be applied In many cases, in
order to obtain a final coated surface of the desired
quality, it is necessary to apply two or more layers of
pigment-containing coating compositions.
Generally, in order to receive the maximum return
from capital invested in paper coating rn--h1n~y, it is
desirable to run the coating machine at the highest
practicable web speed. Also, since the coating
compositions consist of pigment, adhesives, and
possibly other solid ingredients in suspension in
water, it is necessary to remove the water content of
the composltion by thermal ~vaL,u~ lon in order to dry
the coating. In order to minimise the consumption of
energy for thermal evaporation it is desirable to
operate with coating compositions having the highest
pos~1 hl (~ solids concentrations . However, it is found
that when a f inal coating composition of relatively
high solids concentration is applied to a base sheet of
relatively high water absorbency, there is a tendency,
when a doctor blade is used to remove excess coating
composition and smooth the coating, for this final
. ~- 21~9~3~
--2--
coating to be marred by scratches and other defects.
An ob~ ect of this invention is to provide a method
in which two or more coatings are applied to a
cellulosic sheet base, and in which it is possible to
5 achieve a final surface of good sluality at a relatively
high web speed in the coating machine and with a f inal
coating composition of high solids concentration
It ls well known to use water-repellant chemical
reagents in the production of ~P~ l Q5i r sheet
10 products. The absorption of water into the sheet
product may be controlled by a process known as
"sizing". There are generally two main types of sizing
process, internal sizing and surface sizing. In
internal sizing a water-repellant reagent is mixed with
15 the pulp of cellulosic fibres prior to the formation of
the sheet. In surface sizing, the reagent, which is
usually starch in this case, is applied to a web of
ready formed sheet material, by passing the web through
a coating trough known as a "size press". Generally,
20 in surface sizing, both sides of the web are coated
with a film of the reagent.
The most commonly used sizing reagent for internal
sizing is rosin which is precipitated in a suspension
of cellulosic fibres by the addition of aluminium
25 sulphate. E~owever, this treatment reduces the pH of
the suspension of fibres to about 4-6 which renders it
unsuitable for use when an ;~lk; 1 inP filler, such as
calcium carbonate, is incorporated into the suspension.
In order to size a cellulosic sheet material
30 containing an ~lk~l ~n~ filler, sizing agents have been
developed which confer water-repellant properties to
the n~ n~1n fibres at neutral or Alk~l inP pH values.
Examples of these sizing agents include those of the
alkyl ketene dimer type which are described in United
35 States Patent Specification No. 2627477, and those of
the alkenyl succinic anhydride type which are described
2~913~
in United States Patent Specificatlon No. 3102064.
The present invention provides a method for
improving the ease of application of a final inorganic
pigment-containing coating to a cellulosic sheet
5 material which has been coated with at least one
pr~l i m~ ni~ry inorganic pigment-containing coating, which
method comprises incorporating into the pr-~l ;min;~ry
coating or into at least one of the prGl im;ni:lry
coatings, up to 2% by weight, based upon the weight of
10 the inorganic pigment, of a sizing reagent.
The present invention also provides a method of
coating a .~ l l . ! 1 r-s; C sheet material, which method
comprises coating the ~ l1 n~:; C sheet material with at
least one pr-l i m i n~ry inorganic pigment-containing
15 coating and then with a final inorganic pigment-
containing coating, wherein there is present in the
pr-~l ;m; n;:lry coating or in at least one of the
prPl ~m;n~ry coatings, up to 296 by weight, based upon
the weight of the inorganic pigment, of a sizing
20 reagent.
The present invention further provides a
composition for forming a pr~l;m;n~ry inorganic
pigment-containing coating on a cellulosic sheet
material, the composition comprising an inorganic
25 pigment and up to 2~ by weight, based on the weight of
the inorganic pigment, of a si~ing reagent.
The sizing reagent is conveniently an alkyl ketene
dimer of the type which is used as an internal size for
paper under ~ l k~ l; n~ or neutral conditions . Such an
30 alkyl ketene dimer has the general formula:-
~-CN-C=O
O= C - CH-R
where R is an alkyl group having from 8 to 20 carbon
atoms .
The sizing reagent may also be, for example, a
~ 91~
--4--
styrene-maleic anhydride copolymer, an alkenyl succinic
anhydride or an anionic polyurethane.
The inorganic pigment in both the pr~l lm~n~ry and
the final coating compositions may be chosen from the
group consisting of kaolin clay, calcium carbonate,
titanium dioxide, calcium sulphate, barium sulphate and
satin white. In addition to the inorganic pigment,
both the final and the pr~l imin~ry coating compositions
will also include from 4% to 20% by weight, based on
the weight of the inorganic pigment, of an adhesive for
the pigment which is generally chosen from the group
consisting of starches, proteinaceous adhesives such as
casein, and latices of, for example, styrene butadiene
rubber or an acrylic polymer. The coating compositions
will also generally contain from 0.02% to 1% by weight,
based on the weight of the pigment, of a dispersing
agent for the inorganic pigment.
The amount of the sizing reagent used in the
prF~1 imin~ry coating composition is preferably in the
range from 0.01% to 0.30% by weight, based on the
weight o~ the inorganic pigment.
The invention will now be illustrated by the
following Examples, wherein reference is made to
Figures 1 to 4 of the ~ nying drawings.
EXAMPLE 1
Three coating compositions were prepared for use
in providing the first of two coatings applied to a
base paper to form a double coated paper. Each
composition was prepared according to the general
recipe:-
Inaredient Parts bY Weiqht
Calcium carbonate pigment 100
Adhesive 15
The calcium carbonate pigment was a comminuted
natural marble which had a particle size distribution
such that 6096 by weight consisted of particles having
~ ~159130
--5--
an equivalent spherical diameter smaller than 2um.
The adhesive used ln each of the three
compositions was:
A 15 parts by weight of starch
5 B 12 parts by weight of starch and 3 parts by weight
of latex solids
C 10 parts by weight of starch and 5 parts by weight
of latex solids
The starch was an nx;~l;c~-9 corn starch which is
lO marketed by Cerestar under the trade name "AMISOL
05591". The latex contained 5096 by weight of styrene
butadiene rubber polymer and is marketed by The Dow
rh~m;c;ll Company under the trade name "DOW 950". The
amounts of latex used in the recipes given above are
15 expressed in terms of the weight of dry polymer solids.
Each composition was divided into two portions, 1
and 2 . To Portion 1 there was added 0. 25 parts by
weight, on a dry weight basis, per hundred parts by
weight of calcium carbonate pigment, of a weakly
20 cationic alkyl ketene dimer which is marketed by the
Hercules Corporation under the trade name "AQUAPEL
C519 n, No alkyl ketene dimer was added to Portion 2 .
Each composition was applied to an unsized,
absorbent, woodfree base paper by means of a laboratory
25 paper coating machine of the type described in British
Patent Specification No. 1032536 at a paper speed of
400m.min~1. In each case the coating was dried by
blowing air over it for 2 minutes.
Each sample of paper coated with a first coating
30 composition was then coated by means of a laboratory
bench blade coating apparatus with a second composition
having the general formula:-
In~redient Par~s bY Weicht
Fine calcium carbonate pigment 100
35 Latex adhesive 12
Sodium caLb~,~y l llyl cellulose
159~30
--6--
The fine calcium carbonate pigment was a
comminuted natural marble having a particle size
distribution such that 95% by weight consisted of
particles having an e~auivalent spherical diameter
5 smaller than 2um. The latex adhesive was the same as
that used for the first coat, and the sodium
ualbu~y l,llyl cellulose was that marketed by Metsa
Serla under the trade name "FINNFIX 5". Each sample of
paper coated with each of the six different first
10 coating compositions was divided into a number of
portions and the blade pressure in the coating
apparatus was varied to give a series of dif ferent
weights per unit area of the second composition in the
range from 8 to 20g.m~2.
The coating apparatus was provided with a device
which monitored the rate of drying of the second
coating compositLon by measuring the intensity of light
reflected from the surface of the coated paper. The
coating immediately after passing beneath the blade was
20 uniformly wet and was therefore highly reflective to
light. However, as the coating dried it became duller
in appearance . An ad~ ustable light source and a
detector were mounted above the coating apparatus to
give an incident light beam and a measured beam both at
25 an angle of 75 to the normal to the paper. The signal
from the detector was applied by way of a voltage
mea,~ul~ ~ interface to an input of a personal
computer. The computer was capable of recording up to
15, 000 measurements of light intensity per second, so,
30 in order to provide 150 data points per second, the
average of 100 such mea:iul~ t~ was calculated to give
each data point.
For each sample of paper to be coated,
measurements of reflected light intensity were made for
35 about 2 seconds on dry paper. The coating apparatus
was then started and a drying curve of light intensity
21~9130
against time was recorded by the computer. A typical
drying curve takes the form of an initial drop in light
intensity to near zero as the blade passes beneath the
source and detector, followed by a rapid increase in
intensity as the wet coating film is exposed. The
intensity will then begin to decrease as the coating
dries and will eventually become constant to give a
measure of the reflectance of light from the dry coated
paper. For each sample of paper, the rate of drying
was expressed as a drying parameter, ô. The value of ô
was detPrmi nP~ precisely by plotting the rate of change
of light intensity with time and reading as ~ the time
interval between the maximum positive gradient of the
light intensity/time curve, when the wet coating is
first sensed, and the greatest negative gradient of
this curve, when the rate of change of light intensity
is at a maximum and the coating is in a partially dried
state .
For each of the six f irst coating compositions a
graph was plotted of drying parameter, ô, against coat
weight and the results are shown in Figures 1-3
Figure 1 gives the results for first coating
compositions A1 and A2, with and without the alkyl
ketene dimer, respectively; Figure 2 gives the results
for first coating compositions B1 and B2 and Figure 3
gives the results for first coating compositions Cl and
C2 .
It will be noted that in every case the drying
parameter, ~, and hence the drying time, increases with
coat weight, but that, for each adhesive system used in
the f irst coating composition, the second coat dries
more slowly when the alkyl ketene dimer is added to the
first coating composition.
EXAMPLE 2
Batches of an absorbent woodfree base paper of
weight 83g.m 2 were precoated with four different first
13~
--8--
coating compositions, each of which was prepared to the
general recipe:-
Ingredient Parts by weight
5 Calcium carbonate pigment lOO
Oxidised starch adhesive 18
Sodium hydroxide to pH 8 . 5
Alkyl ketene dimer see below
Water to 62% sQlids
The calcium carbonate pigment was the same as that
used in the first coating compositlon in Example l.
The amounts of the alkyl ketene dimer in the four
compositions were, respectively, 0, 0.02, 0.05 and 0.l
parts by weight of active alkyl ketene dimer per lO0
15 parts by weight of the pigment. The alkyl ketene dimer
was the same as that used ln Example l. The alkyl
ketene dimer, when used, was added into the composition
after the starch adhesive at a I " ~Lal ul~ below 40C.
The f irst coating composition was applied to the
20 base paper in each case by means of a pilot-scale paper
coating machine fitted with a roll applicator and a
doctor blade at a web speed of 600m.min~l and a blade
angle of 49 . The blade pressure was ad~ usted to give
a coat weight of l0g.m 2 In order to m1nlm~ce curl of
25 the paper during the second coating step, a first
coating composition was also applied to the reverse
side of the paper web at a coat weight of 8, 5g . m~2 .
Af ter the f irst coating composition had been
applied to each batch of paper, the surface of the
30 coated paper was r~ nrl~red by passing it through two
nips of a superr~lPn-l~r at a line pressure of 50kN.m~
at 60C and at a speed of 600m.min~l.
Each sample of paper coated with a f irst coating
composition was then coated with a second coating
35 composition having the general formula:-
~ 21~13~
Ingredient Parts by Weight
.
Fine calcium carbonate pigment 100
Styrene butadiene latex 12
Sodium carboxymethyl cellulose
Optical brightening agent 0. 5
The fine calcium carbonate pigment, the latex andthe sodium carboxymethyl cellulose were the same as
those used in Example 1.
The second coat was applied to each batch of
precoated paper by means of a pilot-scale paper coating
machine fitted with a roll applicator and a doctor
blade. The blade pressure was kept constant at a
suitable value which would give a coat weight of the
15 second coating of 9g.m~2.
The runnability, or resistance to scratching, of
the second coat was investigated by the following
procadure: -
A second coat was applied f irst to a batch of base
2b paper which had been precoated with a f irst coatingcomposition containing no alkyl ketene dimer. The
second coating composition was applied at a web speed
of 300m.min~l at the highest solids ~u~ LatiOn which
would permit the application of a coat weight of
25 9g . m 2 The web speed was then increased until
scratching was observed at the blade. When scratching
was observed, the base paper was changed to a base
paper which had been precoated with a f irst coating
composition which contained some alkyl ketene dimer,
30 and a second coat was applied to this paper keeping the
second coating composition and the coating machine
settings unchanged. If an illl~)LUV~ - ~ in the
resistance to scratching was observed, the web speed
was increased until scratching was observed, or until a
35 web speed of 800m.min 1 was reached.
The second coating composition was then diluted
~ 21~gl3~
--10--
with water by about 1-2% by weight of solids and coated
on to the base paper which had been precoated with a
f irst coating containing no alkyl ketene dimer . The
procedure was then repeated until no scratching could
S be detected during the application of a second coat to
the base paper which had been precoated with the f irst
coating which contained no alkyl ketene dimer.
The whole procedure was then repeated using the
base papers which had been precoated with the f irst
lO coating compositions which contained different amounts
of the alkyl ketene dimer.
The results are set forth in Table l below:
2159~.3~
Table
Amount of alkyl 96 by weight web speeld Observations
ketene dimer ln solids in ( m . min~
first coat (pph) second coat
5 0 69 . 0 300 Frequent
scratching
0 67 . 4 300 Slight
scratching
0 65 . 7 300 Slight
scratching
0 63. 9 300 No
scratching
O . 02 68 . 4 300 Slight
scratching
0 . 02 66 . 8 300 No
scratching
0 . 02 66 . 8 800 No
scratching
0.05 66.6 800 No
scratching
O. 10 68 . 6 300 No
scratching
O .10 68 . 6 600 Scratching
reappeared
0 .10 66 . 9 600 No
scratching
Note:- "pph" means parts by weight per 100 parts by
weight of pigment.
These results show that scratching was most
20 pronounced when a second coating composition was being
applied at a high solids concentration ( 68-69% by
weight ) on to a base paper which had been precoated
with a f irst coating composition containing no alkyl
ketene dimer . The inclusion of only O . 02 parts by
25 weight of alkyl ketene dimer into the first coating
composition was suf f icient to reduce scratching
markedly when a second coating composition was applied
at a solids concentration of 68-6996 by weight. When a
--1 2--
first coating containing 0.1 parts by weight of alkyl
ketene dimer was applied, scratching during the
application of a second coating composition was
completely eliminated under the same condltions.
EXAMPLE 3
Batches of an absorbent woodfree base paper of
weight 94g,m~2 were precoated with three different
first coating compositions having the following
recipes: -
l. lO0 parts by weight calcium carbonate pigment A; 15
parts by weight nYifl~RDfl corn starch.
2. lO0 parts by weight calcium carbonate pigment B; 15
parts by weight oxidised corn starch; 0. 6 part by
weight sodium salt of styrenc ~ acid copolymer.
3. lO0 parts by weight calcium carbonate pigment A; 15
parts by weight oxidised corn starch; 0 . 25 part by
weight alkenyl succinic anhydride.
Calcium carbonate pigment A was the same as that
used in the first coating composition in Example 1.
Calcium carbonate pigment B was a natural marble
which was comminuted to a similar particle size
distribution as that of calcium carbonate pigment A,
but in an aqueous suspension of lower solids
concentration and in the absence of a dispersing agent.
The oxidised corn starch was the same as that used
in Example 1.
The sodium salt of the styrenc ~~ c acid
copolymer was supplied by Atochem under the trade name
n SMA 3000 " .
The alkenyl succinic anhydride was supplied by
Claymore (~hc~m~rz~l c Limited under the trade name
"CLAYSIZE PR4".
In each case the oxidised corn starch was added to
the coating composition in the form of a 30% by weight
solution which was cooked at 90C for 20 minutes before
addition .
-13- 215913~
In the case of compositions 1 and 3, the cooked
starch solution was added to an aqueous suspension
containing 78% by weight of calcium carbonate pigment A
and a sodium polyacrylate dispersing agent. In the
5 case of composition 3, the alkenyl succinic anhydride
was mixed in to the composition immediately before
coating .
Composition 2 was prepared by mixing 1333g of a
cake containing 75% by weight of the dry calcium
10 carbonate pigment E~ with 6g of the sodium salt of the
styrene-maleic acid copolymer. A deflocculated
suspension of the calcium carbonate pigment at a solids
concentration of 74 . 4% by weight was obtained.
Each first coating composition was applied to the
15 base paper by means of the laboratory paper coating
machine described in Example 1 at a paper speed of
400m.min~l and a blade angle of 35. The blade angle
was ad~usted, if nPrPqC~ry~ to give a coat weight of
8.0+0.5g.m 2 for each first composition. The coatings
20 were dried by infrared heating for 25 seconds with a
current of hot air followed by 25 seconds during which
cold air was blown over the coated surface.
Each sample of paper coated with a f irst coating
composition was then coated by means of the laboratory
25 bench blade coating apparatus with a second coating
composition having the general formula:-
Ingredient Parts by Weight
Fine calcium carbonate pigment 100
Latex adhesive 12
30Sodium u~LLu~Ly thyl cellulose
The fine calcium carbonate pigment, the late~adhesive and the sodium call,u--y ~llyl cellulose were
the same as those used in Example 1. Each sample of
35 paper coated with each of the three different first
coating compositions was divided into a number of
~ 21~913~
--14--
portions and the blade pressure in the bench coating
apparatus was varied to give a series of different
~eights per unit area of the second coating composition
in the range from 8 to 20g.m 2 For each first coating
5 composition a graph was drawn of drying parameter, ô,
against coat weight, and the results are shown in
Figure 4. It will be seen that for the control first
coating composltion l the drying parameter remains
virtually constant with second composition coat weight,
10 while in the case of first compositions 2 and 3, in
accordance with the invention, the drying parameter
increases steeply with second composition coat weight,
indicating that the second coat dries more slowly when
a sizing reagent is added to the first composition.
EXAMPLE 4
Batches of an absorbent woodfree base paper of
weight 83g.m 2 were precoated with three different
first coating compositions, each of which was prepared
according to the general recipe:-
InGredient Parts bY weiqht
Calcium carbonate pigment lOO
Oxidised starch adhesive 18
Sodium hydroxide to pH 8 . 5
Sizing reagent see below
Water to 62% solids
The calcium carbonate pigment was the same as thatused in the first coating composition in Example l.
The sizing reagent was an anionic polyu~ alle
30 marketed by Eka Nobel under the trade name "CYCLOPAL A"
and the amounts used in the three compositions were,
respectively, O, O . l and 0 . 2 parts by weight of active
anionic polyurethane per lOO parts by weight of the
pigment. The anionic polyurethane was added into the
35 composition before the starch adhesive.
The first coating composition was applied to the
~ 21~913V
--15--
base paper in each case by means of a pilot-scale paper
coating machine fitted with a roll applicator and a
doctor blade at a web speed of 600m.min 1 and a blade
angle of 49. The blade pressure was ad~usted to give
5 a coat weight of lOg.m 2, It was not nPrPqqAry in this
case to apply a first coating to the reverse side of
the paper web, as the paper coated on one side only was
found, after rqlPn~lPring, to have sufficient resistance
to curling.
After the first coating had been applied to each
batch of paper, the surface of the coated paper was
r~ 1 Pn~lPred by passing it through two nips of a
superr~lpnrlpr at a line ~.as:~ult: of 50kN.m~l at 60C
and at a speed of 600m.min~l.
Each sample of paper coated with a f irst coating
composltion was then coated with a second coating
composition having the general formula:-
Inqredient Parts bY Weiqht
Fine calcium carbonate pigment 100
20Styrene butadiene latex 12
Sodium ~ LbU~y ~llyl cellulose
ûptical brightening agent 0. 5
The fine calcium carbonate pigment, the latex and
the sodium ~Lb~ y l llyl cellulose were the same as
25 those used in Example 1.
The second coat was applied to each batch of
precoated paper by means of a pilot-scale paper coating
machine fitted with a roll applicator and a doctor
blade. The blade pressure was kept constant at a
30 suitable value which would give a coat weight of the
second coating of 9g.m~2.
The rl~nn~hlllty, or resistance to scratching, of
the second coating was investigated by the procedure
described in Example 2. The results are set forth in
35 Table 2 below:
21~13~
--16--
Tabie 2
Amount of % by web speed Ob:~LvaL~ons
anionie weight (m.min~l)
polyurethane in solids in
5first coat seeond
(pph) eoat
069 . 2 500 Frequent
seratehing
lO 0 69 . 2 1000 Frequent
seratehing
067 . 2 500 No
seratching
0 .1 69 . 0 500 Slight
scratehing
0 .1 69 . 0 1000 No
seratehing
0 . 2 69 .1 300 No
seratching
250 . 2 69 .1 1000 No
scratching
These results show that the inelusion of the
anionic polyuL~ al~e into the first coating eomposition
30 at a level of 0.1 part by weight per lO0 parts by
weight of pigment greatly reduces seratehing when the
seeond eoating eomposition is applied at a solids
C~ La~ion of about 69% by weight.
