Note: Descriptions are shown in the official language in which they were submitted.
~119332
The present invention relates to an aqueous latcx base
binder of a synthetic latex for the preparation of aqueous
compositions for coating paper and board for printing. It also
pertains to the aqueous compositions containing the aforementioned
binder and the paper and board coated by means of the aforemen~
tioned compositions.
It is known that, in principLe, the aqueous compositions
for coating paper and board consist of pigments and binders.
The most often used pigment is mineral clay. Other pigments
such as calciurn carbonate, titanium oxide, hydrargillite, talc
and barium sulfate are also used in low proportions with respect
to the mineral clay. These pigments are dispersed in water,
generally in an alkaline medium and in the presence of dispers-
in~ agents, the major ones being tetrasodium pyrophosphate,
sodium hexametaphosphate, low-molecular-weight polyacrylates,
employed in ratios between 0.2 and 0 5% by weight with respect
to the pigments. The pigments are fixed to the paper or board
by means of binders. The binders are generally aqueous latexes
of a synthetic polymer, such as a styrene-butadiene copolymer,
carboxylatëd styrene~utadiene copolymer, acrylic pol~mer or
vinyl acetate polymer, used either alone or mlxed with natural
binders such as starches, proteins, casein or with synthetic
binders such as polyvinyl alcohols. It is also possible to
add to the latexes, products such as carboxymethylcelluloses or
alginates, which can improve the water retention properties o~
the coa-king compositions.
Finally, the coating compositions rnay also contain
va-ious o-ther ingredien-ts such as cross-Link;llg agents, anti-
foarning agents, lubricating agents, optical brighteners and
colorants.
~1933Z
Coating paper and board witn aqueous compositions is
accomplished by means of a machine coa-ter, employing various
techniques which use various devices among which -the following,
known industrially under the follo-~ing trade names, can be
cited: air-doc-or coater, size-press, Champion coater, Massey
coater, trailing~blade coater. Arter coating the paper or
board, the coating is dried. :.
The properties desired for the coated paper and board
vary according to the printing method for which they are in-
tended, the major ones being gravure and offset printing. Themore viscous the printing ink, the higher must be the dry tear-
reslstance of the paper, which is a function of how well the
coating adheres to the support. In addition, in four color
offset pxinting, one is led to print.the second, third or fourth
color on a previously wet area. Therefore, to avoid tearing
when wet, which would result in dirtying the blanket and the
offset presses, it is necessary for the paper or board to have
adequate wet tear-resistance. To obtain the dry and wet tear-
resistance levels required by the printing process, it is
20 desirable to use -the minimum amount of binder because, very :.
~enerally speaking, the higher the proportion of binder used,
the more the whiteness and brightness, i.e., the optical pro-
perties specifically provided by the pigment, are degraded.
Among the synthetic polymer latexes used as binders,
those which have the highest binding power are th.erefore the
most desirable in that, since they can be used in the lowect
possible proportion, maximum t-~hiteness and brightness can be
obtained.
The hi.nders of the lnvention ha~e an aqueous latex base
oE a synthetic polymer, and the content in hydrosoluble com-
111933Z
pounds dissolved in the aqueous phase being less -than 0.5~ by
weight in relation to -the polymer.
Hydrosoluble compounds primarily include, for example,
salts formed by catalytic residues, surEace-active agen-ts
5 and macromolecular compounds such as carboxyl compounds.
The applicant has found that, all things bei.ng equal
in other respects, latexes with a sufficiently low content oE
hydrosoluble compounds d-issolved in the aqueous phase have an
improved binding power.
The following can be cited as synthetic polymers usable
as latexes according to the invention: styrene-butadiene
copolymers, carboxyl styrene-butadiene copolymers, vinyl p-oly-
acetates, carboxyl vinyl polyacetates, alkyl acrylate-vinyl
acetate copolymers, carboxyl alkyl acrylate-viny.l acetate co-
15 POlymers.
The content in.the latex of hydrosoluble compounds dis-
.
solved in the aqueous phase can be estimated by using the-method
hereinafter described in which semi-permeable membranes with
high cutoff are used which permit low-molecular-weight compounds
20 or if need be carboxyl macromolecular compounds to diffuse
therethrough while blocking passage of polymer particles.
The latex to be tested, having a dry material content
by weight tl, expressed in percentage by weight, undergoes
ultrafiltration by passing throu~h a laboratory ultrafiltration .
25 module equipped with a semi-permeable membrane mar~eted by
Rhone Poulenc Industries in France under the trade name "Iris
3538". The ponderal dry material content is de-termined from
the first drop of permeate collec_ed tp, e~;pressed in percent-
age by weight, which equals that of the aqueous phase of the
30 latex.
111933Z
The content in the latex of hydrosoluble compounds dis-
solved in aqueous phase t , expressed in percentage by weight
in relation -to the polymer is calculated by the ormula:
tp(100 - tl)
t
s
A latex usable as a binder, according to the invention,
is usually obtained from a latex of a synthetic polymer which
is known to be usable for the preparation of aqueous composi-
tions for coating paper and board for printing and which is
prepared by any known aqueous emulsion polymerization process,
from which a sufficient quantity of the aforementioned hydro- ,.
soluble compounds are eliminated. These çan be eliminated,
for example, by uitrafiltration through a semi-permeable mem- '
brane. As ultrafiltration progresses, the compo,unds alssolved.
in the aqueous phase flow through the membrane in the permeate
while the hydrosoluble compounds adsorbed at the surface of
the latex particles are progressively desorbed and eliminated.
The ultrafiltration operation can be carried out in a classical
unit of a type commonly employed in industry.
A cross-section of this type of unit is shown in the
attached figure. Basically, it consists o~ tank 1 containing
the latex to be processed, tank 2 containing deionized water, .
~5 ultrafilter 3 and pump 4. Tank 1 is fed by tank 2 which supplies
deionized water through ~ipe 5; and automatic valve 6, actuated
by floa-t 7, maintains constant the level of the bath contained
i.n tank 1. Pump 4 in~ures latex flow -through pipe 8 toward
ultrafilter 3. Throllgh pipe 9, on which ~lowmeter 10 is fitted,
the concentrate deliverecl by ultra~ilter 3 is recycled into
i.
~119332
tank 1. Heat exch~nger fluid, circulated through coil 11 im-
mersed in the latex contained in tank 1, maintains it at a
constant temperature. Recirculatiny pipe 12 is used to put
the unit in stable working conditions.
The unit is also equipped with shut-off valves 13, 14,
15, 16 and 17 and manometers 18 and 19.
The maintenance of stable working conditions and the
operation proceed as follows: With valve 14 closed and valve
13 open, pump 4 is turned on; then valves 14 and lS are opened
lO and valve 13 closed. By successive approximations the opening
of valves 14 and 15 is regulated so as to obtain the desired
latex ~low at the desired pressure, the pressure differences
indicated by manometers 18 and 19, representing the.pressure
drop un ultrafilter 3.
To insure good operation of the ultrafiltration techni-
que, it is advantageous, in accordance with the practice of the
., .: ':
invention, to respect the following conditions:
- The semi-permeable membrane must have a high cutoff,
generally between 5,000 and 100,000, expressed as the molecular-
weight-value of standard proteins in a neutral buffered
medium.
- The flow rate of the latex on the membrane must be higher
than 0.5 m/s, and preferably bet~7een 1 and 2 m/s, to pre-
vent the membrane from clogging at the sa~e time it limits
the pressure drop in the ultrafilter and also the shearing
to which the latex .is subjected.
- The pressure differences on each side of the-membrane must
be between 0.1 and 6 bars, and preEerably bet~een 1.5 and
3 bars.
3O - The temperature oE the latex rnust be bet~een 0 ancl 100 C,
~----
11~933Z
and preferably bet~leen 0 and 50 C.
- Since the flow rate of the permea~e decreases as the
ponderal dry material content of the latex increases, it is
preferable to accompl::sh ultrafiltration with a perceptibly
constant ponderal contell-t, generally bet~7een 5 and 70%, and
preferably between 45 and 5';~, possibl~ after dilution,
es~ecially if the latex vi~cosity is very high.
- Unit shutdown must be followed by an adequate cleaning cycle
with pure water to prevent irreversible clogging, hence
destroying the membrane.
If the mechanical stability of the latex is insufficien-t
to permit ultrafiltration without forming agglomerates, one
can proceed by first of all bringing the latex to an alkaline
pH value of usually between 7.5 and 9.5. After ultrafiltra-
tion is completed, the latex can be concentrated, if need be,
untll its dry material content-is suitable for' the intended
usage.
The following examples are provided to illustrate the
invention:
EXAMPLES 1-14:
Examples 1, 3, 5, 7, 9, 11 and 13 are given for compara-
tive purposes. Examples 2, 4, 6, 8, 10, 12 and 14 illustrate
the invention.
Aqueous coating compositions are prepared and a 20
g/m2 layer, expressed in dry material, of each of the afore-
mentioned compositions is applied on a paper by me(lns of an
"air doctor" machine coater. In examples 1-6, 54 g/m2 paper
is used. After coa-ting, the paper is dr:ied i.n a tunnel .fur-
nace at 100 C. It then undergoes caleIldering ~hi.ch consists
f four successive passes bet~ieen two cylinclers at a Eorce
I
~llg33Z
of 80 kg/cm
~ -fter drying and calendering, the coated paper is con
ditioned at 20 C in an atmosphere at 65% relative humidity.
It is then tested for dry and wet tearing-resistance by means
of the following methods:
Dry tearing-resistance: This is determined with the
IGT device, designed by the "Institut von Grafisch Tecknik",
in which the paper is put into contact with an inking roller,
the pressure of which is regulated on the paper at a chosen
value and the speed of which is increased until the coated
layer begins to tear. The dry tearing-resistance is deter-
mined by the value of the speed when tearing begins. "3805"
ink with a graduated indicator, sold by the Lorilleux Lefranc
Establishments, is used,
Wet tearing-resistance: This is also determined with
the IGT device, with the difference that th,e inking is carried
out with a sample o coated paper on which a thin water film
is already deposited. Wet tearing-resistance is determined
visually by comparison with other samples. "3801" ink, sold
by -the Lorilleux Lefranc Establishments, is used. The results
are expressed by a grade ranging from 0 to 10; 0 corresponding
to a paper with very bad resistance and 10 a paper with excel-
lent resistance. Table 1 gives, for each example, the
reference of the aqueous latexes used as binders, the nature o~
2~ the polymer of which they are co~posed, and their content o
hydrosoluble compounds dissolved in the aqueous phase, in
relation to the weight percent of the polymer,
~ --7-
1~933Z
TABLE 1
.,.~ydrosoluble
T~e f e I comp3und conte~t
ren I Tlei~`nt in rela~io
ce I llto ~he polymer (,'
. . , ! . . -
I . I
! . I
xa~les A ~ Carboxyl styrene-buta-
1,3 ant~l 5 I diene copolymer composed ! 4 7
of, by wei~ht, 43~ but-
adiene, 53~ styrene and
4j~ ethylene carboxylic acids
. , .. __, .. .
. . -I
Examples B Carbo~yl styrene-bv.ta~ 1 0.3
2~ 4 and 6 diene copolymer composed 01~ j
l by waight, 43~butadiene, 53
I styrene and 4~ ethylene
. carboxylic acids
",~1 .. , . j , .
l Carboxyl vinyl polyacetate
- - l . -compose~ of, by wei~ht ? 98~ .,
Examples ~ C vinyl acetate and 2,'7 ethyl-- 2.2
7 and 9 ~ I ene carboxylic acid_ ~
.- ' . . . .
Carboxyl vinyl polyacetate
Ex~mples ¦ D composed of, by uei~ht, 98~ j o.48
8 ana 10 1 vinyl acetate and 2~7 e~hyi-
. I ene car~o~-ylic acid
__ I . .. .- . . ...
Carboxyl 2 ethylhexyl
Eva~ples ~ acrylate - ~inyl acetate 1 1.83
li and 13 copolymer composed of, by
wei~ht, 88~ inyl acetate,
10~ acrylate of 2 ethylhe:~yl.
a~d 2% ethy3ene carbo~ylic
acid
.~
I Carbo~yl 2 ethylhexyl
acrylate - vinyl act-~tate
~xa~es I ~ copo].ymer composed ol, by 0.08
12 antl 34 1 wei~ht, ~,' vinyl a.cetate,
! lo~ ~crylatt-~ of 2 ethylhe~yl
antl 2~ ethylene carboxylic
aci.d
___
,0
111933'2
La-texes B, D and F, used in the exarn~les illustratiny
the invention, were obtained from l.atexes A, C and E, re-
spect:ively, used in the comparati-~e examples, with a dry
materlal content of 50% by weiyht; the latter undergoing ultra- :
5 filtration as explained below. In exarnples 2, 4 and 6, the
ultrafiltration process is applied on the latex as is, while
in exam~les 8, 10, 12 and 14, it is applied on the latex
brought before hand to a dry material content by weight of 30%
and a pH of 8 by the addition of an ammoniacal aqueous solution.
Ultrafiltration takes place in a unit with an ultra-
.
filtration surface-area of 0.7 m2. The unit is equipped with ~:
a membrane with a cutoff at 20,000, which is marketed by the
Rhone Poulenc Industries of France under the trade name "Iris
3538", and is fed by a pump capable of suppl~ing 6 m3/h a't a
pressure of 3 bars. During ultrafiltration, the dry material
content in the latex is maintained constant with deionized
.
water. - . .
Table 2 which follows shows. the ultrafiltration time
for each example according to the invention.
TABLE 2
:Ultrafiltration time
(hours)
:
: Examples, 2, 4 and 6: 9
2~. Examples 8 and 10 . 8
: Examples 12 and 14 . 13
- _ _
~ hen ultrafiltration is co!npl2ted, late~es D ancl F are
conc2ntra-ted to their initial dry material content o:E 50~0 by
weight. After clrainage, -the unit i5 flushed cleall with water
111933;~:
~70 clogging of the membrane is noticed.
Tables 3 and 4, ~Jhich follo~J, sho~l the nature and the
~eight content of the solid constituents of the coating
compositions corresponding to each example. They also show
the weight content in dry material ~o which are ad~usted the
coating compositions brought to a pH of 8.5 by the addition
of a~monia,
TABLE 3
.
: Example : 1 : 2 : 3 : 4 : 5 : 6
1 0
: Kaolin : 100 : 100 : 100 : 100 : 100 : 100 :
:,
:
Sodium hexametaphosphate : O. 3 : O.3 : O.3 : O.3 : O.3 : O.3
: Carboxymethylce~ulose 0.3 : 0.3 :
:
: Starch : - : - : 6 : 6
.: :
: . : : ,: : : : :
: Folyvinyl alcohol : - : - : ~ : - : 2 : 2
: Latex A - : 12 : - : 8 : - : 10
:- : : : : : :
~ Latex B : - : 12 : - : 8 : - : 10
: Ponderal content in : 48 : 48 : 48 : 48 : 52 : 52
: dry material t~) : : : : : : :
--1.0--
1~9332
TABLE 4
Exa~ple 7 8 ~ 10 11 12 13 14
Kaolin 00 100 100 100 100 100 100 100
. _ _ _
Sodium .4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
pyrophosphate _ ~ _ . .
Polyvinyl . .
alcohol - 1 1 2 2 1 1 2 2
_' : . .
Latex C 12 10 . .
_ ~
Latex D 12 ~ 10 . .
.. ' _--~ . '
Latex E . 12 10
. __ ~ ';' ' .
Latex F . 12 : 10
_ . __ ... .
Ponderal content
in dFy ~a-er.ial 5050 50 50 50 50 50 50 .
. ., ~ .
Tables 5 and 6, which follow, show the results obtained
for each example.
. TABLE 5
20 I Example 1 2 3 ~ 4 ~ 6 .
. .
Dry tearing-resistance 67 71 50 61 62 63
(cm/s)
_
LWet tearing-resistance 4 7 4 6 5 6
.
:-
1~l1933Z
TABLE 6
Exampl.e 7 8 ~ 11 12 13 14
Dry tearing-res.~stance 42 43 42 43 42 45 52 53
5. -. _ ~ _ . .
Wet tearing-resistance 5.5 5 6 5.5 7 8 9
It can be seen that latexes B, D and F, used as binders
in the invention, lead to coated paper, the dry and wet tearing-
10 resistances of which are superior to paper coated with aqueouscompositions ~ontaining latexes A, C and E.
3o
--]2-
,~
