Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 91/08341 PCI /GB90/01883
Z1~7;~541.
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PAPER COATING
This invention relates to a paper coating
composition, a method for preparing a paper coating
composition, a method for coating a paper with a paper
coating composition and coated paper. This invention
also relates to a paper recycling process in which a
coated paper of the invention is employed as "broke" in
a paper making process. "Broke" i~ the term used in
the art for paper, cardboard, or the like which is to
be recycled.
Calcium carbonate is known as a paper coating
pigment and, because it normally carries a positive
charge, it is conventionally dispersed with an anionic
dispersing agent. Other paper coating pigments which
carry a neutral or positive charge exist, such as
gypsum, talc, calcined kaolin clay, and these must also
be dispersed using an anionic dispersing agent. (These
minerals are also recognised as having a deficiency in
negative sites).
A full discussion of the constituents of paper
coating compositions and of the methods of applying
such compositions to paper i9 given in Chapter XIX,
Volume III of the second edition of the book by James
P. Casey entitled "Pulp and Paper: Chemistry and
Technologyn. A further discussion is given in "An
Operator's Guide to Aqueous Coating for Paper and
Board", edited by T.W.R. Dean, The British Paper and
Board Industry Federation, London, 1979.
DE-3707221 and EP-0307795 disclose a cationic
pigment dispersion. The pigment is first given a
protective colloid cover using a cationised polymer and
then, under certain circumstances, i8 di~per~ed with a
- cationic polymer. --
TAPPI, vol. 65, no. 4, April 1982, pages 123-125,
Atlanta, Georgia, U.S.A.; A.J. Sharpe, Jr. et al.:
"Improved Cationic Conductive Polymer Displays
Wosl/08~1 PCT/CB90/01883
z~7~64~ -2-
Outstanding Fflmability describes a polysalt formed
from the interaction of a strongly cationic polymer,
such a~ a poly(diallyl dimethyl ammonium chloride), and
a weakly anionic polymer, ~uch as polyacrylic acid.
The thus-formed polysalt is added in substantial
quantities (of the order of 50% by weight, based on the
weight of the pigment) to a predispersed, low ~olids
pigment slurry in order to provide a conductive coating
;j colour which is used to prepare a paper having a
conductive surface.
~'.
According to a first aspect of the present
invention there is provided an aqueou~ paper coating
; composition which comprises (i) at least 45% by weight
~:.
of a particulate inorganic pigment dispersed with a
dispersing agent and (ii) an adhesive; characterised in
that said dispersing agent comprises an anionic
polyelectrolyte and a cationic polyelectrolye~ the
cationic polyelectrolyte being present in an amount`
~; sufficient to render the pa,ticles cationic; in that
said adhesive is a cationic or non-ionic adhesive; and
in that said particulate pigment i~ one which i~ not
;3 capable of being dispersed in water at high solids, and
following vigorous mixing, in the sole presence of said
~- cationic polyeletrolyte.
Z5 According to a second aspect of the present
invention there is provided a method of coating a sheet
member comprising the step of coating the sheet member
with a paper coating composition in accordance with the
first aspect of this invention.
According to a third aspect of the present
invention, there is provided a coated paper produced by
the method of the second aspect of this invention.
The particulate pigment used in the present
invention is one which i8 not capable of being
dispersed in water at high solids (~uch as greater than
60% by weight) and following vigorous mixing, (for
example sufficient to dissipate at least 10kJ
of energy per kg of dry piqment), in the
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W~3 91/08341 PCI/GB90/01883
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sole presence of the cationic polyelectrolyte. This
means that the pigment surface should have a neutral,
or overall positive, charge. This i8 true of inorganic
pigmentn such as calcium carbonate, calcium sulphste,
talc and calcined kaolin clay, for example. Most
preferably, the pigment i8 calcium carbonate, in any
form, natural or synthetic. Most preferred is ground
or crushed marble, but chalk, or precipitated calcium
- carbonate (PCC) may also be used. In this respect, it
should be noted that whilst raw chalk is capable of
being dispersed using a cationic polyelectrolyte in the
; absence of vigorous mixing, this is not true if the
chalk is subject to vigorous mixing. It i~ believed
that this is because the vigorous mixing strips off the
anionic aluminosiliceous layer normally present on raw
chalk. In the absence of vigorous mixing, the
aluminosiliceous layer is able to confer a negative
charge on the surface of the chalk particlee and this
enables dispersion by a cationic polyelectrolyte to be
achieved.
~i It is preferred that the ground pigment has a
particle size distribution such that at lea~t 50%
percent by weight has an equivalent spherical diameter
smaller than two microns. More preferably, at least
60% percent by weight has an equivalent spherical
diameter smaller than two microns. _
Ground marble for use in the present invention is
preferably formed by crushing batches of marble in
aqueous suspension in the absence of a chemical
dispersing agent using a particulate grinding medium.
; Any agglomerates formed may be broken up by dewatering
the suspension of ground marble, for example by
filtration in the absence of a flocculating agent and
;~ then drying the pigment, and pulverising the dried
product in a conventional mill.
The particulate pigment is dispersed with the
.
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WO91/0834l PCT/GB90/01883
2Q7~164~ r
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combination of an anionic polyelectrolyte and a
cationic polyelectrolyte. Preferably, the anionic
polyeletrolyte is a water-soluble vinyl polymer, an
alkali metal or ammonium ~alt thereof or an alkali
S metal or ammonium salt of polysilicic acid. Most
; preferably, the anionic polyeletrolyte is a
poly(acrylic acid), a poly(methacrylic acid), a
substituted poly~acrylic acid) or a substituted
poly(methacrylic acid), or an alkali metal or ammonium
10 salt of any of these acids. The substituted
poly(acrylic acid) may be a partially sulphonated
polymer. An especially effective anionic
polyelectrolyte is an alkali metal or ammonium salt of
a copolymer of acrylic acid and a sulphonic acid
15 derivative of acrylic acid, in which the proportion of
the sulphonic acid derivative monomer is preferably
from 5% to 20% of the total number of monomer units.
The number average molecular weight of the anionic
polyelectrolyte is preferably at lea~t 500, and
~i 20 preferably no greater than lO0,000. The amount u~ed is
generally in the range of from about 0.01% to about
0.5% by weight based on the weight of dry pigment,
preferably in the range of from about 0.1 to 0.2% by
weight.
The cationic polyelectrolyte may be a water-
soluble substituted polyolefine containing quaternary
ammonium groups. The quaternary ammonium group~ may be
in the linear polymer chain or may be in branche~ of
the polymer chain. The number average molecular weight
of the substituted polyolefine i8 preferably at least
1500 and preferably no greater than 1,000,000, and is
more preferably in the range of from 50,000 to 500,000.
- The quantity required i~ generally in the range of from
about 0.01% to about 1.5% by weight based on the weight
of dry pigment. Advantageou~ results have been
obtained when the substituted polyolefine is a poly
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W 0 91/08341 `2~7~S41 P~rJGB90/01883
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. (diallyl di(hydrogen or lower alkyl)ammonium salt).
The lower alkyl groups, which may be the same or
different, may for example, have up to four carbon
. atom~ and e~ch i~ preferably methyl. The ammonium ~alt
` 5 may be, for example, a chloride, bromide, iodide, ~S04-,
CH3S04- or nitrite. Preferably the salt is a chloride.
Most preferably, the cationic polyelectrolyte is poly
(diallyl dimethyl ammonium chloride). Alternatively,
.~ the water-soluble substituted polyolefin may be the
. 10 product of co-polymerising epichlorohydrin and an
aliphatic secondary amine, said product having the
~-. formula
.~--CEI2 c~2 C~2 ~
: 15 X' lR' l~
;: in which R and R', which may be the same of different,
.i are each hydrogen or a lower alkyl group having from
one to four carbon atom~, preferably methyl or ethyl
`. and ~ i5 Cl-~ Hr~' I-~ EIS0~ C~3SO"- or nitrite. The
preferred number average molecular weight of this
epichlorohydrin product i5 in the range of from 50,000
~ to 300,000.
: Alternatively, the cationic polyelectrolyte may be
a water-soluble organic compound having a plurality of
basic groups and preferably having a number average
molecular weight of at least 10,000 and preferably no
greater than 1,000,000. Most preferably, the number
~:: average molecular weight is at least 50,000. These
: water-soluble organic compounds may be described as
polyacidic organic bases, and are preferably compounds
of carbon, hydrogen and nitrogen only and are free of
other functional groups, such as hydroxy or carboxylic
acid groups, which would increase their solubility in
water and thus increase the likelihood of their being
desorbed from the clay mineral in an aqueous
suspension. Preferably, the organic compound is
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WO91/08~1 PCT/GB90/01883
~Q7~641. ~ :--
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polyethyleneimine (PEI) having a number average
molecular weight in the range 50,000 to l,000,000. A
further example of a water-soluble organic compound
~ which may be employed is a polyethylene diamine which
.1 5 may be a copolymer of ethylene diamine with an ethylene
dihalide or with formaldehyde.
: The cationic polyelectrolyte is employed in an
: amount sufficient to render the mineral particles
cationic. Experiments have shown that the zeta
potential of the particles will normally be at least
~2~mV after treatment, typically in the range of from
~30 to ~40 mV and usually no greater than +50 to ~60mV.
These potentials have been measured using a dilute
(0.02 weight %) solids suspension using a supporting
lS electrolyte of potassium chloride (lO~M) with a "Pen
Kem Laser Z" meter.
: The ratio of the weight of cationic
polyelectrolyte to the weight of anionic
polyelectrolyte used is preferably in the range of from
20 2:1 to 20:1, when the calcium carbonate is a ground
marble.
According to a fourth aspect of the pre~ent
invention, there is provided a process for preparing a
paper coating composition comprising the steps of:
(i) dispersing in aqueous suspension a
particulate pigment; and ~
(ii) combining the dispersed aqueous suspension
with an adhesive and, if necessary, adjusting the
dilution such that the particulate material constitutes
at least 45% by weight of the composition;
characterised in that said pigment is dispersed
using a dispersing agent comprising a combination of an
anionic polyelectrolyte and a cationic polyelectrolyte;
in that said cationic polyelectrolyte is used in an
amount sufficient to render the pigment particles
cationic; in that said adhesive is a cationic or non-
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~ ` ~'0 91/08341 PC~r/G B90/01883
2~2~
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-ionic adhesive; and in that ~aid particulate pigment
is one which is not capable of being dispersed in water
at high solids, and following vigorous mixing, in the
~ole presence of ~aid cationic polyeletrolyte.
In the method of the invention, it is normally the
case that the raw pigment is received a~ a filter cake
having a relatively high solids content. To thi~ i8
added the dispersing agent in order to provide a
dispersed high solids slurry ~45-80% by weight ~olids)
: 10 which may then be subjected to vigorou6 mixing. This
slurry is then "made down" into a paper coating
composition by dilution and addition of the required
quantity of the cationic or non-ionic adhe~ive and
other conventional paper coating composition adjuvants.
Preferably, the pigment is mixed with the anionic
polyeletrolyte before mixing with the cationic
polyelectrolyte. This appears to enable a more fluid
suspension to be obtained at a higher solids
concentration.
The aqueous dispersion of the pigment may also
include other conventional paper coating composition
adjuvants such as an insolubilising agent (e.g. a
` melamine formaldehyde re~in), a lubricant such as
- calcium stearate and a catalyst to catalyze cross-
linking of the cationic latex if present: a ~uitable
such catalyst is sodium bicarbonate. The quantities of
; these adjuvants required are known to those skilled in
the art.
The adhesive used in the present invention should
be a non-ionic or a cationic adhesive. Such adhesives
- contrast with the anionic adhesives which are normally
used in paper coating compositions in which the pigment
is anionic. Thus, cationic guar gum and cationic
starch adhesives can be used as well as cationic or
non-ionic latices. Such cationic and non-ionic
adhesives are readily commercially available. The
particular cationic or non-ionic adhesive used will
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~ WO91/08341 PCT/GB90/018X3
2Q7~641.
depend, for example, on the printing proce~a to be
used, e.g. offset lythography requires the adhesive to
be water-insoluble. For paper to be used in an offset
printing technique, the amount of adhesive should
` 5 preferably be of the order of from 7 to 25~ by weight,
based on the weight of pigment whil~t, for graw re
printing paper, the adhesive should be used in an
amount of 4-15% by weight, based on the weight of
pigment. The precise quantity of adhesive required
}0 will depend upon the nature of the adhesive and the
material being coated, but this can readily be
determined by the person skilled in the art.
The suspe~sion of the pigment for incorporation
into the paper coating composition of the present
invention should preferably be subjected to vigorous
mixing before or after dispersion. Typically, the
vigorous mixing should be sufficient to impart at least
lOkJ energy per kg of pigment, and preferably no more
than about 50kJ per kg. Normally, the amount of energy
input will be in the range of from 18-36kJ per kg of
pigment.
The coating composition may be coated on to a
sheet member using normal paper coating machinery and
under normal paper coating conditions. It has been
found that the paper coated with a cationic composition
in accordance with the present invention provides
broadly similar results to that obtained with a
conventional anionic system.
The coated paper of the present invention is of
advantage when it is employed as "broke" or recycled
paper in a paper making process. Commonly, large
quantities of paper are recycled at the point of
manufacture for one reason or another, and the
advantages of the paper of the present invention in
recycling are most important to the paper manufacturer.
Thus, in accordance with a fourth aspect of the present
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:~ WO 91tQ8341 PCI/GB90/01883
2~7:~641.
, 9
invention, there is provided a method for recycling
paper including the step of reducing paper in
accordance with the third aspect of the present
invention to a fibrous recyclable state and
incorporating said fibre in a paper-making composition.
Such a paper-making composition may include
conventional paper-making pulp, such as a bleached
sulphite pulp and, typically, the broke fibre and the
. pulp will be employed in a ratio of from 10:90 to
: 10 60: 40 .
~` Also included in the paper making composition will
be a filler, for instance a calcium carbonate filler
and also a retention aid. Since the broke fibre will
include a proportion of calcium carbonate from the
coating, it is possible to reduce the amount of calcium
carbonate filler employed to give a total quantity of
filler in the range of from 5 to 20 percent by weight
of the total paper-making composition. The weight of
dried broke added (fibre and filler) should preferably
be in the range of from about 5 to 30 percent by weight
of fihre.
It has been found that, when the broke fibre
employed is derived from a coated paper in accordance
with the present invention, this enables the amount of
retention aid employed in the paper making composition
to be reduced.
The present invention will now be illustrated by
the following examples: -
Three batches of raw cru~hed marble were ground in anaqueous suspension containing 30% by weight of dry
solids and in the absence of chemical dispersing agent,
by means of a particulate grinding medium. The
duration of grinding wàs different in each case 50 as
~` 35 to yield three different ground products having
p~rticle size distributions such that 50%, 68~ and 87
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WO91/08~1 PCT/GB90/01883
,.,.~
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- ~y weight, respectively, had an equivalent spherical
diameter smaller than 2 microns. In each case the
suspension of ground marble wa~ dewatered by filtration
in a tube pressure filter in the absence of a
flocculating agent and the filter cake was dried and
; pulverised in a laboratory hammer mill.
Samples of each of the three ground marble powders
were mixed with water and with two different dispersing
; age~ts by each of two different methods described
below. The dispersing agents were:-
(E) an anionic polyelectrolyte which was a sodium
polyacrylate having a number average molecular
weight of 4,000; and
(F) a cationic polyelectrolyte which was a poly
(diallyl dimethyl ammonium chloride) having a
number average molecular weight of 50,000.
In each case the ratio of weight of (F) to the
weight of (~) was 4:1 but the optimum total quantity of
dispersing agents were determined for each ground
marble powder and was found to differ in each case.
The two methods for preparing the aqueous suspension of
the marble powders were:-
(i) the powder was mixed with water containing the
reguired quantity of (~) and after thorough mixing
the required quantity of (F) was added, followed
by further mixing; and
(ii) the powder was mixed with water containing the
required quantities of both (E) and (F) together.
In each case the viscosity of the suspension was
measured by means of a Brookfield Vi~cometer at a
spindle speed of lO0 rpm. and the percentage by weight
of dr,v solids was determined by completely drying a
known weight of the suspension and weighing the dried
residue. The suspension wa~ then diluted with a small
quantity of water and further determinations of
viscosity and percentage by weight of dry solids were
, .. . ~ ,.. , .. ... . . .. , . . .. , . ., ,, , . "=
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WO91/08~1 PCT/GB90/01883
1 1- 2Q7~
made. A graph was plotted of vi~cosity against
percentage by weight of dry solids and the solids
concentration for a suspension having a vi~cosity of
500 mPa.s was determined by interpolation.
The results obtained are set forth in Table I
below:-
TABLE 1
by wt. of dry
solids for a
% by wt. based on viscosity of
% by wt. smaller wt. of dry 500 mPa.s
than 2 microns marble of by method
in marble ~E~ (F) (il (ii)
0.02 0.083 72.0 71.3
; 15 ~8 0.075 0.289 71.4 65.6
87 0.166 0.65 64.6 60.3
These results show that fluid suspension can be
obtained at a higher solids concentration by method (i)
(mixing the powder firstly with the anionic
polyelectrolyte and secondly with the cationic
polyelectrolyte) than by method (ii) (mixing the powder
with both dispersing agents together). This effect is
more marked with finely ground marble powders than with
a coarser product.
~XAMPI~ 2
- A further batch of finely ground marble powder was
prepared by the process described in Example 1, the
particle size distribution of the ground product being
such that 87~ by weight consisted of particles having
an equivalent spherical diameter smaller than 2
microns.
Samples of this marble powder were incorporated
- into paper coating compositions prepared according to
,
the followinq recipes:-
. ~ .
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~.:. , .
:
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WO91/08341 PCT/GB90/018~3
Z07~Ç,4~.
IngredientParts by_weiqht
` Composition (a) Marble powder100
~ Quaternary cationic
:~. 5 acrylic latex12
~' Cationic guar gum 0.5
Inqredient~Parts by weiqht
10 Composition (b) Marble powder 100
Quaternary cationic
acrylic latex 8
Cationic starch 4
Three different compositions of type (b) were
prepared containing the following three different
cationic starches:- .
(i) a cationic starch of low degree of
substitution
(ii) another cationic starch of low degree of
substitution
(iii) a cationic starch of high degree of
substitution
The three compositions were identified as
compositions (b)(i), (b)(ii) ant (b)(iii), respectively
InqredientParts by wei~ht
Composition (c) Marble powder 100
Styrene butadiene
rubber latex 12 =
Sodium carboxymethyl
cellulose 0.75 'i
InqredientParts ~v ~eiaht
Composition (d) Marble powder 100
Styrene butadiene
' rubber latex 8
Oxidi~ed starch 4
In the case of the cationic compositions (a) and (b)
an aqueous suspension of the marble powder wa~ first
prepared using as the dispersing agents 0.16% by
- - .,
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WO91/08341 PCT/GB90/01883
~ 2C7~641
-13-
weight, based on the weight of dry marble, of anionic
dispersing agent (E) and 0.65% by weight, based on the
weight of dry marble, of cationic dispersing agent (F),
by the method de~cribed under (i) in Example 1 ~bove.
In the case of the anlonic compositions (c) and (d)
the marble powder was treated with 0.30~ by weight,
based on the weight of dry marble, of dispersing agent
(E) alone.
In addition there were added to each of compositions
(a), (b), (c) and (d) 0.8 part by weight of a melamine
formaldehyde resin, as an insolubilising agent, and 0.5
part by weight of calcium stearate. There was also
added to each of the cationic compositions (a) and (b)
0.2 part by weight of sodium bicarbonate to catalyse
the cross linking reaction of the cationic latex.
Each paper coating composition was diluted with
water to give a high-shear viscosity as measured by a
Ferranti-Shirley Viscometer at a shear rate of 12,800 s~
1 in the range 60-70 mPa.s if possible. The high-shear
viscosities and the percentage by weight of solids in
the diluted compositions are set forth in Table II
below.
Each composition was coated on to a lightweight
offset base paper of substance weight 48 g~m~2 by means
of a laboratory paper coating machine of the type
described in British Patent Specification No. 1032536.
:, ,
The coated paper samples were then supercalendered
under a pressure of lO00 psi ~6.89 MPa) and a
temperature of 65C with lO passes through the nip of
the calender rolls at a speed of 36m.min~1.
Each sample of calendered, coated paper wa~ tested
~- for gloss by the TAPPI Standard method, for smoothness
by the Parker Print Surf at lO Rgf., for percentage
reflectance to light of wavelength 457nm and for
percentage opacity. In each case the determinations of
gloss, smoothness, ~ reflectance and % opacity were
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WO91/08~1 PCT/GB90/01883
-:` 2Q7~41 `
-14- -
obtained by coating samples of paper at a range of
different coat weights, measuring these quantities for
each coat weight, plotting a graph of the quantity
against coat weight and interpolating to find the value
of quantity for a coat weight of 8g.m~2. The results
are set forth in Table II below:-
TABL~ II
% by High ~ reflectance
wt. of shear Gloss to light of
dry viscosity (TAPPI 457nn.
ComPosition solids (mPa.s) units) Smoothness wavelength ~ oPacity
(a) 62.0 62 45 0.7Z 78.1 90.1
(b)(i) 60.6 66 45 1.05 7-4 9 4
(b)(ii) 51.2 70 43 0.68 78.4 89.8
(b)(iii) 59 4 72 38 0.71 78.4 90.1
(c) 65.8 73 46 0.67 78.1 90.1
(d) 64.9 104 39 0.76 77-8 89.7
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W~91/08~1 PCT/GB90/01883
2137
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When the results for cationic composition (a) are
compared with those for the corresponding anionic
composition (c), and the results for the cationic
compositions of type (b) are compared with those for
5 the corresponding anionic composition (d), it will be
noted that the coated paper properties for a cationic
system in accordance with the invention are broadly
similar to those obtained with a conventional anionic
system .
Samples of paper coated with the above coating
compositions were used as recycled paper or "broke" in
a paper making process. Bleached sulphite pulp was
; beaten in accordance with TAPPI Standard T200 to a
degree of freeness of 45 SR or 270 Canadian Standard
lS Freeness and paper ma~ing compositions were prepared
consisting of suspensions in water of the following
ingredients:-
Inqredient Parts of weiqht
Bleached sulphite pulp 70
Broke fibre 30
Calcium carbonate filler 50
Polyacrylamide retention aid 0.03
` 25 The calcium carbonate filler had a particle size
;) distribution such that 43% by weight consisted of
` particles having an equivalent spherical diameter
smaller than 2 microns. Since the broke çontained
``~ about 20~ by weight of inorganic filler material the
quantity of fresh calcium carbonate filler added was
reduced to give a total quantity of filler of 50 parts
- by weight. Similarly the weight of dry broke added
(fibre + filler) was such as to provide 30 parts by
` weight of fibre.
- The retention of the calcium carbonate filler in
paper prepared from compo~itions containing broke which
had been coated with each of the coating composition~
(a), (b)(ii), (c) and (d) above wa~ measured by means
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W091/08~1 PCT/GB90/018~3
2Q7~64~ - 16-
of a retention jar with the stirrer set at speed 5
(lO50 rpm) and a stirring duration of 30 seconds. As a
comparison the retention of the same calcium carbonate
filler in a paper making composition containing no
broke was also measured. The results are set forth in
Table III below:-
TABLB III
Broke containing % by wt. retention
coatinq composition of calcium carbonate
Non~ 74.3
(a) 83.9
(b)(ii) 79.2
(c) 49.8
lS (d) 46.2
These results show that although the incorporation
into paper making composition of broke which has been
coated with an anionic composition (c) and (d) reduces
the retention of a calcium carbonate filler as compared
with a paper ma~ing composition which contains no
broke, the incorporation of broke coated with a
cationic co~position actually improves the retention of
the filler.
~XAMPI~ 3
A batch of raw cru~hed marble was ground by the
method described in Bxample l above to give a ground
product having a particle size distribution such that
60% by weigh~ consisted of particles having an
equivalent spherical diameter smaller then 2 microns.
The suspension of ground marble was dewatered by means
of a centrifuge and the centrifuge cake which contained
68% by weight of dry solids was used Sn the following
experiments.
Samples of the centrifuge cake of ground marble were
mixed first with an anionic dispersing agent, and then,
-- after thorough mixing, with a cationic dispersing
agent. In each ca~e a predetermined quantity of the
anionic di~persing ~gent was used, but, for the
:
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WO 91/08341 PCI/GB90/01883
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cationic dispersing agent, a small quantity of the
dispersing agent was first added, the suspension was
vigorously mixed for 1 minute, and the viscosity of the
su~pension was mea~ured by meane of a Brookfield
Viscometer at a spindle speed of 100 rpm. The amount
of dispersing agent and the viscosity were recorded and
a further small quantity of the dispersing agent was
added and the procedure repeated. Further small
increments of the dispersing agent were added until the
viscosity of the suspension reached a minimum, at which
point the total amount of the cationic dispersing agent
which had been added was regarded as the optimum.
The dispersing agents were:-
(G) an anionic polyelectrolyte which was a sodium
15 polyacrylate having a number average molecular
weight of 70,000
(H) sodium silicate which is a sodium salt of a
polysilicic acid and acts a~ an anionic
dispersing agent;
(I) a cationic polyelectrolyte which was a poly
(diallyl dimethyl ammonium chloride) having a
number average molecular weight of 50,000.
(J) a cationic polyelectrolyte whlch was a
' polyethyleneimine.
When dispersing agent (J) was used it was also
necessary to add sufficient sulphuric acid to adjust
the pH to 7.8 since polyethyleneimines are sensitive to
pH and do not act efficiently as dispersing agents at
p~ values greater than 8.
As a comparison one sample of the centrifuge cake
was t~eated with disper3ing agent (J) at p~ 7.8 alone,
no anionic dispersing agent being used.
For each combination of disper~ing agents the
percentage by weight of dry marble in the ~uspension,
the minimum viscosity of the suspension and the
quantities of anionic and cationic dispersing agent
::
.: . . ~ . .. ... .. . . ., . , _ j , . . . .. . . . . . . . . .....
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Wo91/08~1 PCT/GB90/01883
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were recorded, and the results are set forth in Table
IV below:-
TABLE IV
Anionic Amount CAtionic Amount Minimum
dispers- used dispers- used % by wt. viscos-
ing (% by ing (~ by of dryity
aqent wt.) agent wt.) solids ~mPa.s)
G 0.10 I 0.48 67.23000
G 0.10 J 0.40 67.5 252
H 0.16 J 0.39 65.4 224
none - J 0.37 67.1>10,000
These results show that, in general, lower
viscosities are obtainable with polyethyleneimine as
the cationic dispersing agent, rather than with poly
(di~llyl dimethyl ammonium chloride), but the use of
the cationic dispersing agent must be preceded by the
~- addition of an anionic di~persing agent.
EXAMPT.R 4
A batch of marble flour having a particle size
distribution such that substantially all of the
particles passed through a No. 300 mesh British
Standard sieve (nominal aperture 53 microns) was
subjected to attrition grinding in a concentrated,
deflocculated aqueous suspension, the quantities of
marble flour, water and grinding sand being:-
615g marble flour
330g water + anionic and cationic dispersing
agents
1500g sand
The grain size of the sand was smaller than No. 18
mesh ~ritish Standard sieve (nominal aperture 0.850mm)
and larger than No. 30 mesh British Standard sieve
(nominal aperture 0.500mm). The anionic disper~ing
agent used was (E) and the cationic dispersing agent
was (F), both as described in Example 1 above.
Portions of marble flour were ground using different
.,
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WO91/08~1 PCT/GB90/01883
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total quantities of (E) and (F) but in each case the
weight ratio of (F):(E) was 4:l. In each case the
grinding was continued for a time sufficient to
dissipate in the ~u~pension 396 kJ of energy per kg of
dry marble and in each case the product had a particle
size distribution such that about 50% by weight
consisted of particles having an equivalent spherical
diameter smaller than 2 microns. On completion of
grinding the suspension of ground marble was separated
from the sand and the viscosity of the suspension was
measured by means of a ~rookfield Viscometer at a
spindle speed of l00 rpm. The suspension was then
diluted with a smaIl quantity of water and the
viscosity was measured again. The percentage by weight
, 15 of dry marble in the suspension was also determined by
drying a ~nown small weight of the suspension and
weighing the dried residue. The steps of diluting with
water and measuring the viscosity and percentage by
weight of dry marble were repeated several tLmes. A
graph was drawn of viscosity against percentage by
weight of dry marble and the percentage by weight of
dry marble in the suspension which had a viscosity of
500 mPa.s was found by interpolation. The results are
set forth in Table V below:-
TABLE V
Total % by % by wt. of dry
wt. of marble for a
% by wt. % by wt. dispersing viscosity of
of (E) of (F) aqents 500 mPa.
0.05 0.20 0.25 63.9
0.06 0.24 0.30 64.5
~` 0.07 0.28 0.35 65.0
EXAMPIæ 5
Further batche8 of the same marble flour as was uaed
in Example 4 were ground by the method described in
:~ .
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WO91/08~1 PCT/GB90!01883
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Example 4, there being used as the anionic dispersing
agent 0.07% by weight, based on the weight of dry
marble, of (E), and as the cationic di~persing agent
0.28~ by weight, based on the weight of dry marble, of
S one of a selection of poly (diallyl dimethyl ammonium
chloridet polyelectrolytes of different molecular
weights. In each case the percentage by weight of dry
marble in a suspension having a viscosity of 500 mPa.s
was measured as described in Example 4 above and the
results are set forth in Table VI below:-
~ABL~ VI
Number average ~ by wt. of dry
molecular weight marble for a
of cationic viscosity of
disPersinq aqent 500 mPa.s
9,500 58.4
26,000 61.8
50,000 65.0
71,500 64.9
These results show that the poly (diallyl dimethyl
ammonium chloride) should have a number average
molecular weight of at least 50,000 if a marble
suspension of acceptable fluidity is to be obtained.
~XAMPIE 6
A batch of raw crushed marble was ground by themethod described in Example l to give a ground product
having a particle size distribution such that 60~ by
weight consisted of particles having an equivalent
spherical diameter smaller than 2 microns. The
suspension of ground marble was dewatered by means of
centrifuge and the centrifuge cake which contained 73%
by weight of dry solid~ was used in the following
experiments:
Samples of the ground marble centrifuge cake were
first mixed with 0.1% by weight, based on the weight of
dry marble, of anionic dispersing agent ~ as described
in Example 3 ~i.e. a sodium polyacrylate having a
.
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WO91/08341 PCTtGB90/01883
2~64~
number average molecular weight of 70,000). After
thorough mixing there was added to each sample of
aqueous suspension of anionically dispersed ground
marble a quantity of one of the four following cationic
dispersing agents:-
(I) a cationic polyelectrolyte which was apoly(diallyl dimethyl ammoniùm chloride) having a
number average molecular weight of 50,000 - 100,000;
(J) a cationic polyelectrolyte which was a
polyethyleimine;
(K) a polyethyleneimine of number average molecular
weight lower than that of (J);
`~ (L) a polyethyleneimine of number average molecular
weight lower than that of tR).
The quantity of each cationic dispersing agent was
that which was found by experiment to give the lowest
` viscosity for a suspension of given solids content.
For dispersing agent (I) this quantity was found to be
0.45% by weight, based on the weight of dry marble, and
for dispersing agents (J), (K) and (L) the quantity was
found to be 0.40% by weight, based on the weight of dry
marble.
In the case of the polyethyleneimine dispersing
agents (J), (K) and (L) there was alRo added sufficient
sulphuric acid to adjust the p~ to 7.8. In each case
the viscosity of the suspensio~ was measured by means
of a Brookfield Viscometer at a spindle speed of 100
rpm and the percentage by weight of dry solids in the
suspension was determined by completely drying a known
weight of the suspension and weighing the dried
residue. The suspension was then diluted with a small
quantity of water and further determinations of
viscosity and percentage by weight of dry solids were
made. The procedure of diluting the suspension and
measuring the viscosity and percentage by weight of dry
i" solids was repeated two or three tLmes and a graph was
.. , . .. .... , . , ~ .. . .. ..
Wo91/08~1 PCT/GB90/01883
2C~7~G41
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plotted of viscosity against percentage by weight o f
dry solids. The solids concentration for a suspension
having- a viscosity of 300 mPa. 8 was determined by
interplotation and the results are set forth in TablQ
VII:-
TABLE VII
Cationic % by weight of dry
dispersing marble for a viscosity
aqent of 300 mPa.s
I 61.0
; J 68.5
K 71.8
L 73.0
EXAMPLR 7
15 A batch of raw crushed marble was ground in an
aqueous suspension containing 30~ by weight of dry
solids and in the absence of chemical dispersing agent,
by means of a particulate grinding medium to yield a
ground calcium carbonate product of paper coating grade
having a particle size distribution such that 90~ byweight of the particles had an equivalent spherical
diameter smaller than 2 microns. The suspension of
ground marble was dewàtered by filtration in the
absence of a flocculating agent and the filter cake was
dried and pulverised in a laboratory hammer mill.
Samples of the finely ground marble powder were
mixed with water to form a suspension containing 60% by
weight of dry solids and varying quantities of an
anionic and of a cationic dispersing agent. The
anionic dispersing agent was a 30dium polyacrylate
dispersing agent having a number average molecular
weight of 4000 and the cationic dispersing agent was a
poly~(diallyl dimethyl ammonium chloride) having a
number average molecular weight of about 50,000.
; 35 In each experiment the anionic dispersing agent was
added first to the suspension of ground marble and the
.:
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W~9t/08~1 PCT/GB90/01883
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mixture stirred by 9,400 revolutions of an impeller
rotating at l,420 rpm. The cationic dispersing agent
was then added and the mixing procedure was repeated.
The visco~ity was measured immediately on completion of
the second mixing procedure by means of a Brookfield
Viscometer.
The results obtained are set forth in Table VIII
; below:
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WO 91/08341 PCT/GB90/Ol883
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These results show that the optimum dispersion was
-- obtained when the ratio of the weight of cationic
~- dispersing agent to the weight of anionic dispersing
r,!, agent was about 4:1. When the ratio was increased to
6.9:1 it was still possible to obtain a very fluid
suspension but at the expense of a slightly higher dose
of mixed dispersing agents.
~XAMPLE 8
A batch of raw crushed marble was ground in an
aqueous suspension containing 30% of dry solids and in
the absence of chemical dispersing agents by means of a
particulate grinding medium to yield a ground product
having a particle size distribution such that 78% by
weight of the particles had an equivalent spherical
diameter smaller than 2 microns. The suspension of
ground marble was dewatered in the absence of a
flocculating agent on a vacu~m drum filter to a dry
solids content of 64~ by weight. Some of the filter
cake thus formed was thermally dried and mixed back
` 20 with the moist filter cake to give a mixture having a
dry solids content of 70% by weight.
This mixture was divided into three portions to be
treated with cationic poly(diallyl dimethyl ammonium
chloride) dispersing agents having three different
number average molecular weights. ~ach of the three
portions were further subdivided into three ~maller -:
portions which were treated with different doses of
anionic dispersing agent (E) as described in Example 1.
In each case the anionic dispersing agent was added
first to the cake of ground marble and well mixed
therewith, and the cationic dispersing agent was then
added and mixed in. The dose of the cationic
dispersing agent used was in each case about 3.5 times
- the dose of the anionic dispersing agent.
In each case the viscosity of the resultant
suspension was measured by means of a Brookfield
.,
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WO91/08~1 PCTtGB90/018~3
.:~.
2Q7 26 4~ -26-
Viscometer at a spindle speed of lO0 rpm and the
percentage by weight of dry solids was determined by
completely drying a known weight of the suspension and
weighing the dried residue. The suspension was then
diluted with a small quantity of water and further
determinations of viscosity and percentage by weight of
dry solids were made. A graph was plotted of viscosity
against percentage by weight of dry solids and the
solids content for a suspension having a visc08ity of
300 mPa.s was determined by interpolation.
The results obtained are set forth in Table IX
bolow:-
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WO 91/08341 PCr/GB90/01883
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Table ~C
~ Mol wt % by ut Z by wt % by ut of dry
:. dispersing dispersing dispersing 300 mPa.s
~i 100 000 o,074 o.265 66666~4~ 7l
200,000 000,1o755 '45 6677.. 9
500 000 0.100 o.360 68.
.. 500 000 0.125 0.450 67.6
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W091/08~1 PCT~GB90/01883
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The results show that slightly more fluid
suspensions for a given solid~ content were obtained
when the cationic dispersing agent having a number
average molecular weight of 500,000 was used.
. . ,
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