Process for the production of paper

A process for the production of paper by forming and dewatering a suspension of cellulose containing fibers and optional fillers on a wire in the presence of a cationic silica based sol and a cationic polymeric retention agent. The cationic polymeric retention agent is cationic guar gum or a cationic synthetic polymer such as cationic polyacrylamide. The combination of cationic silica sol and cationic polymer retention agent gives improved retention of fines and fillers and eases drainage.

The present invention relates to a process for the production of paper 
utilizing a combination of substances for improving retention and 
dewatering. More particularly the invention relates to the use of a 
combination of a cationic silica based sol and a cationic, organic, 
polymeric retention agent in papermaking. 
It is previously known to use combinations of inorganic silica sols and 
cationic retention agents in papermaking. In these cases anionic silica 
sols have been used in combination with cationic polymeric retention 
agents, such as for example cationic starch and cationic polyacrylamide. 
Such systems are disclosed for example in the European patent No. 41056 
and the European patent application No. 218674. The effect of systems 
comprising an anionic silica sol and a cationic component is based on the 
interaction of the two differently charged substances and it is assumed 
that the sol particles with their strong anionic charges to some degree 
produce a cross-linking of the polymeric retention agent. 
Cationic inorganic silica based colloids are per se known and their use in 
specific paper making processes is also known. Thus the U.S. Pat. Nos. 
4,309,247 and 4,366,068 disclose the use of cationic inorganic silica 
colloids in the preparation of filter media based on cellulose fibers. It 
is also known from the Japanese patent application No. 85260377 to use 
cationic collodial silica in ink jet recording paper to improve water 
resistance of water soluble dyes and to improve light resistance. In an 
example in the Japanese application the preparation of the ink jet 
recording paper from a pulp slurry containing talcum, cationic starch and 
cationic colloidal silica is shown. 
According to the present invention it has unexpectedly been found that a 
combination of a cationic silica based sol and a cationic, organic, 
polymeric retention agent can be used in papermaking and that the 
combination of the two components of the same charge gives improved 
retention and dewatering. The combination according to the invention gives 
an improved retention of fine fibers and optional fillers and eases 
drainage and thereby makes the papermaking process more efficient. 
The present invention thus relates to a process for the production of paper 
by forming and dewatering a suspension of cellulose containing fibers and 
optionally fillers on a wire whereby said formation and dewatering takes 
place in the presence of a cationic silica based sol and a cationic 
polymer retention agent selected from the groups cationic guar gum and 
cationic synthetic polymers. 
Silica sols with positively charged particles are, as stated above, known 
per se and their preparation is disclosed for example in the U.S. Pat. 
Nos. 3,007,878, 3,620,978 and 3,719,607. The general methods for preparing 
cationic silica sols start from aqueous sols of silica which are reacted 
with a basic salt of a polyvalent metal to give the sol particles a 
positive surface charge and stabilizers such as boric acid, alkali metal 
bases, alkaline earth metal bases, ammonia etc are often used in the 
processes. The polyvalent metal salt is usually an aluminum salt, due to 
availability and lower costs, although it is of course also possible to 
use basic salts of other polyvalent metals for preparing cationic silica 
based sols, such as chromium, zirconium and others. Any basic salt which 
is water soluble and gives the desired positively charged surface can be 
used and generally the cationic sols are prepared using chlorides, 
nitrates or acetates of the metal. 
The particles of the cationic sols have a small average particle size, 
usually below 100 nm and the size is generally in the range of from 2 nm 
to 100 nm, more often in the range of 2 nm to 80 nm. Suitably the particle 
size is within the range of from 3 to 20, and preferably from 3.5 to 14 
nm. The cationic silica particles will have positively charged species of 
the polyvalent metal, preferably of aluminum, on their surfaces and the 
mole ratio of aluminum to surface silica can be within the range of from 
1:8 to 4:1, suitably within the range of from 1:6 to 4:1 and preferably 
within the range of from 1:4 to 4:1. Most preferably the ratio is within 
the range of 1:2 to 4:1. The mole ratio of aluminum to surface silica has 
here been calculated as in U.S. Pat. No. 3,956,171, i.e. on basis of 8 
silicon atoms per square nm of silica surface whereby the fraction of 
total silica occurring in the surface becomes 8.times.10.sup.-4 .times.A, 
where A is the specific surface area of the sol particles in m.sup.2 /g. 
The cationic silica sols used according to the present invention can be 
prepared from any anionic silica sol by reaction with a basic salt of a 
polyvalent metal salt as above. They can thus be prepared from commercial 
sols of collodial silica and from silica sols consisting of polymeric 
silicic acid prepared by acidification of alkali metal silicate, for 
example by mixing mineral acid and water glass or by using acid ion 
exchange resins. The cationic silica is added to the stock in the form of 
an aqueous sol. The concentration in the cationic sol can be up to about 
50 per cent by weight for sols made from commercial anionic silica sols 
and up to about 10 percent by weight when made from polysilicic acid. The 
stability of the last mentioned type of sols is limited and thus 
concentrations about or lower than 5 percent are suitable. The stability 
is generally higher if more aluminum is present, within the above ratios. 
From a practical point of view it is anyhow suitable to dilute the sols to 
a concentration of from 0.05 to 5.0 percent by weight of the cationic 
particles, preferably from 0.1 to 2 percent by weight, before addition to 
the stock. 
The cationic retention agents which are used in combination with the 
cationic silica sols are at papermaking conventional organic, polymeric 
retention agents, which have a cationic net charge at the pH at which they 
are used, and they are either cationic guar gum or synthetic cationic 
polymers. Examples of suitable synthetic cationic polymers are cationic 
polyacrylamides, polyethyleneimines and polyamidoamines. A mixture of two 
or more cationic retention agents as above can also be used, and any of 
these can also be used in combination with cationic starch. Synthetic 
cationic retention agents are preferred, and particularly cationic 
polyacrylamide. 
The amounts of cationic silica and of cationic retention agent which are 
used will of course depend on the particular stock, presence of fillers 
and other papermaking conditions. Usually amounts of from 0.005 to 2.0 
percent by weight of the cationic silica, as dry, based on dry fibers and 
optional fillers give good results and the amounts suitably used are from 
0.005 to 1 percent by weight. Amounts in the range of from 0.03 to 0.3 
percent are preferred. The ratio of cationic retention agent to cationic 
silica will vary widely depending on for example the papermaking 
conditions, the particular cationic polymer and on other effects desired 
from this. Usually the weight ratio of cationic retention agent to 
cationic silica should be at least 0.01:1 and suitably at least 0.2:1. The 
upper limit of the cationic retention agent with lower cationicity such as 
guar gum is not critical and can for such cationic polymers be very high, 
up to a ratio of 100:1, and higher, and the limit is usually set by 
economical reasons. Ratios of cationic retention agent to cationic silica 
within the range of 0.2:1 to 20:1 are suitable for most systems. 
The two-component system of the present invention can be used in 
papermaking from different types of stocks of papermaking fibers, suitably 
from stocks containing at least 50 percent by weight of cellulose 
containing fibers. The components can for example be used as additives to 
stocks from fibers from chemical pulp, such as sulphate and sulphite pulp, 
thermo-mechanical pulp, refiner mechanical pulp or groundwood pulp, from 
as well hardwood as softwood. The system of the invention can also 
advantageously be used for recycled fibers. As mentioned, the stock can 
also contain mineral fillers of conventional types, such as e.g. kaolin, 
titanium dioxide, gypsum, chalk and talcum. Particularly good results have 
been obtained with pulps which are generally considered as difficult and 
which contain fairly high amounts of non-cellulose substances such as 
lignin, i.e. different types of mechanical pulp such as groundwood pulp. 
The two component system of the invention is particularly suitable for 
stocks made up from at least 25 percent by weight of mechanical pulp and 
give a much improved effect in such systems compared with sols of anionic 
silica and a cationic retention agent. The terms paper and papermaking, 
which are used herein, do of course not only include paper and its 
production, but also other cellulose fiber containing sheet or web form 
products, such as pulp sheet, board and cardboard and their production. 
The cationic silica sol and the cationic polymeric retention agent can be 
added to the stock separately, simultaneously or premixed. They can also 
be added in two or more increments. It is preferred that the two 
components are added separately. It seems that the order of addition of 
the sol and the cationic retention agent has some influence on the 
obtained effect and that when the sols contain smaller particles a better 
effect is obtained if the cationic retention agent is added before the sol 
of cationic silica, while for sols of larger particles a better effect 
generally is obtained when the cationic silica is added first and the 
cationic retention agent is added subsequently. The addition of cationic 
silica and cationic retention agent according to the invention 
considerably improves the retention of fines and fillers, when present, 
and also considerably improves the dewatering, in comparison with the use 
of solely the cationic retention agent. Smaller amounts of cationic 
polymer can thus be used for obtaining a desired effect and for expensive 
cationic polymers, such as polyacrylamide, important cost-savings can thus 
be made. Using the system of the invention the papermaking process can 
thus be made more efficient without negative effects on the strength and 
other important properties of the produced paper. The mechanisms 
contributing to the positive effect of the two component, which have the 
same charge, have not been entirely established, but it is believed that 
the cationic silica of the sol at least partly neutralizes dissolved 
anionic wood substances and that it also improves the strength of flock, 
formed from dissolved and solid components of the stock by the added 
cationic retention agent, by its capability of penetrating and chargewise 
neutralizing the flocks. 
In the present process for production of paper conventional additives can 
of course be used in addition to the two additives according to the 
invention. Fillers have been discussed above, and as examples of other 
additives can be mentioned sizing agents, rosin based or synthetic sizing 
agents, cationic starch, wet strength resins and aluminum based compounds, 
such as alum, aluminate, aluminum chloride or polyaluminum compounds, can 
thus be used. The papermaking process using the present combination of 
substances for improved retention and dewatering can be carried out in a 
wide pH range, from about 4 to about 9. It is a special advantage that 
wood containing papers with high levels of fines content can be produced 
at high retention with the present system without adverse effects on paper 
formation. 
The invention is further illustrated in the following examples which, 
however, are not intended to limit the same. Parts and percent relate to 
parts by weight and percent by weight respectively, unless otherwise 
stated.

EXAMPLE 1 
The cationic silica sols used in Examples 1 and 2 were prepared as follows. 
Aluminum chlorohydrate, with the formula Al.sub.2 (OH).sub.5 Cl.2H.sub.2 
O, was heated to 47.degree. C. under stirring in a flask equipped with a 
heating jacket. When the temperature had been reached anionic silica sols, 
deionized with regard to sodium ions, which has been diluted with 
deionized water were added for a certain time to allow reaction with the 
aluminum chlorohydrate. As a more specific preparation procedure the 
following is typical: 408 g 50% Al.sub.2 (OH).sub.5 Cl.2H.sub.2 O solution 
was warmed to 47.degree. C. 657 g of anionic silica sol containing 15.21% 
SiO.sub.2 were diluted with 928 g deionized water. The particles of this 
sol had a size of about 7 nm. The sol was added for 90 minutes at 
47.degree. C. and the obtained cationic sol was then allowed to cool to 
room temperature. 
In the following tests the dewatering was evaluated with a "Canadian 
Freeness Tester" which is the usual method for characterizing the 
dewatering or drainage capability according to SCAN-C 21:65. 
The stock system was composed of 60% bleached birch sulphate pulp and 40% 
bleached pin sulphate pulp and 30% of China clay had been added to the 
system. The chemical additions are calculated in kg per ton dry stock 
system (fibre+filler) and the amounts of sols and cationic polymers are 
given as dry substnace. All chemical additions were made with a mixing 
speed of 800 rpm in a Britt Dynamic Drainage Jar with a blocked outlet for 
45 seconds and the stock systems were then added to the Canadian Freeness 
Tester. In all tests the sol was added before the polymer. 
Different sols were used: 
(a) Cationic aluminum modified silica sol with a mole ratio of aluminum to 
surface silica groups of 1.30:1 and a particle size of about 7.5 nm. 
(b) Cationic aluminum modified silica sol with a mole ratio of aluminum to 
surface silica groups of 2.95:1 and a particle size of about 7 nm. 
(c) Cationic aluminum modified silica sol with a mole ratio of aluminum to 
surface silica groups of 3.25:1 and a particle size of about 6 nm. 
(d) Cationic aluminum modified silica sol with a mole ratio of aluminum to 
surface silica groups of 2.40:1 and a particle size of about 14 nm. 
The following cationic polymers were used: 
(A) Cationic polyacrylamide, PAM 1, of medium cationicity, sold by Allied 
Colloids under the name of Percol 292. 
(B) Polyethyleneimine, PEI, sold by BASF AG under the name of Polymin. 
(C) Cationic polyacrylamide, PAM 2, of low cationicity, sold by Allied 
Colloids under the name of Percol 140. 
In the table below the results of the freeness tests are given in ml CSF. 
Comparisons with addition of solely the respective cationic polymers are 
given. A comparison was also made with an anionic aluminum modified silica 
sol with a particle size of about 5.5 nm. 
______________________________________ 
Sol/amount 
Cat. polymer/amount CSF 
kg/ton kg/ton stock pH ml 
______________________________________ 
a/1.0 PAM1/0.5 4.5 560 
a/1.0 PAM1/1.0 4.5 620 
a/1.0 PAM1/2.0 4.5 675 
b/1.0 PAM1/0.5 4.5 540 
b/1.0 PAM1/1.0 4.5 605 
b/1.0 PAM1/2.0 4.5 640 
c/1.0 PAM1/0.5 4.5 565 
c/1.0 PAM1/1.0 4.5 620 
c/1.0 PAM1/2.0 4.5 660 
d/1.0 PAM1/0.5 4.5 530 
d/1.0 PAM1/1.0 4.5 590 
d/1.0 PAM1/2.0 4.5 640 
-- PAM1/0.5 4.5 430 
-- PAM1/1.0 4.5 515 
-- PAM1/2.0 4.5 570 
Anionic/1.0 
PAM1/0.5 4.5 305 
Anionic/1.0 
PAM1/1.0 4.5 495 
Anionic/1.0 
PAM1/2.0 4.5 580 
a/1.0 PEI/0.6 7.0 430 
a/1.0 PEI/1.0 7.0 470 
a/2.0 PEI/2.0 7.0 485 
-- PEI/0.6 7.0 350 
-- PEI/1.0 7.0 410 
-- PEI/2.0 7.0 435 
a/1.0 PAM2/0.5 4.5 555 
a/1.0 PAM2/1.0 4.5 625 
a/1.0 PAM2/2.0 4.5 690 
-- PAM2/0.5 4.5 410 
-- PAM2/1.0 4.5 505 
-- PAM2/2.0 4.5 575 
______________________________________ 
EXAMPLE 2 
In this test the dewatering effect of a system of the cationic aluminum 
modified silica sol designated as (a) in Example 1 and a polyacrylamide, 
Percol 292, was measured and a comparison was made with a system of an 
anionic aluminum modified silica sol, with a particle size of about 5.5 
nm, and the polyacrylamide. The stock was made up from groundwood pulp 
beaten to 130 ml CSF and the pH was adjusted to 5 with H.sub.2 SO.sub.4. 
In the tests with the cationic sol this was added to the stock before the 
polyacrylamide, except in one experiment when the order of dosage was 
reversed. In the tests with the anionic sol this was added to the stock 
after the polymer. The added amounts given in kg/ton are calculated as dry 
chemicals on dry pulp. 
______________________________________ 
Cationic sol 
Anionic sol Polyacryl- CSF 
kg/ton kg/ton amide kg/ton 
ml 
______________________________________ 
-- -- -- 130 
-- -- 0.5 210 
-- -- 1.0 230 
-- -- 2.0 250 
-- -- 3.0 245 
1.0 -- 0.25 290 
1.0 -- 0.5 325 
1.0 -- 0.75 340 
1.0 -- 1.0 355 
1.0 -- 2.0 360 
1.0 (reversed dosage order) 
1.0 270 
3.0 -- 0.25 330 
3.0 -- 0.5 375 
3.0 -- 1.0 425 
3.0 -- 2.0 405 
-- 1.0 0.5 190 
-- 1.0 0.75 230 
-- 1.0 1.0 255 
-- 1.0 2.0 280 
-- 3.0 0.5 190 
-- 3.0 1.0 240 
-- 3.0 2.0 320 
-- 3.0 3.0 360 
-- 3.0 4.0 350 
______________________________________ 
As evident from the table maximum CSF level is reached at a much lower 
addition of polyacrylamide in the system with the cationic sol, compared 
with the system with the anionic sol. 
EXAMPLE 3 
Some different cationic silica sols [(a), (b), (c) and (d)] were used in 
this example. 
Sols (a) and (d) had been prepared according to the following: 19.49 g of a 
50% solution of polyaluminum chloride [Al.sub.2 (OH).sub.5 Cl.2H.sub.2 
O].sub.x was diluted to 200 g. Into this solution 1000 g of a 1% 
polysilicic acid were pumped slowly during 45 minutes at room temperature. 
The polymeric silicic acid had been prepared according to the following: 
Water glass (Na.sub.2 O.3SiO.sub.2) was diluted with water to a SiO.sub.2 
content of 5 percent by weight. The aqueous solution was ion exchanged 
using ion exchange resin Amberlite IR-120 to a pH of 2.3. The specific 
surface area of the obtained acid polymeric silicic acid was measured by 
titration according to the method disclosed by Sears in Analytical 
Chemistry 28 (1956) 1981 and was found to be 1450 m.sup.2 /g. This 
polymeric silicic acid which was later treated with polyaluminum chloride 
consisted of particles of a size of the order of about 1 nm, to some 
degree aggregated into chains and networks. The obtained cationic silica 
sol had the following analysis: 0.39% Al.sub.2 O.sub.3 and 0.84% SiO.sub.2 
and thus a mole ratio of Al to surface silica of about 1:2. Sol (a) was 
made from a freshly prepared polysilicic acid and sol (c) from a 
polysilicic acid which had been aged for 1 day. 
Sols (b) and (d) were prepared as follows: 9.75 g of a 50% polyaluminum 
chloride, [Al.sub.2 (OH).sub.5 Cl.5H.sub.2 O].sub.x, solution was diluted 
to 200 g and 1000 g of a 1% polysilicic acid, prepared as described above, 
were added to the solution. The resulting product had the following 
analysis: 0.20% Al.sub.2 O.sub.3 and 0.83% SiO.sub.2 and thus a mole ratio 
Al to surface Si of about 1:4. Sol (b) was made from a freshly prepared 
polysilicic acid and sol (d) from a polysilicic acid which had been aged 
for 1 day. 
Sols (a) to (d) were used together with a cationic polyacrylamide (PAM) 
sold under the designation Percol 292 by Allied Colloids in a stock made 
up from 60% birch sulphate pulp and 40% pine sulphate pulp. The stock 
further contained 30% calcium carbonate and 1 g/l of Na.sub.2 
SO.sub.4.10H.sub.2 O. The pH of the stock was 8.5. The polyacrylamide was 
added to the stock before the cationic silica sol, except were otherwise 
indicated. The dewatering was evaluated as disclosed earlier using a 
Canadian Freeness Tester. The results are given in the following Table. 
______________________________________ 
PAM Sol;amount CSF 
kg/ton kg/ton ml 
______________________________________ 
-- -- 390 
0.5 -- 475 
-- (b); 1 395 
0.5 (a); 1 595 
0.5 (b); 1 605 
0.5 (c); 1 590 
0.5 (d); 1 600 
0.5 (b); 1 505 
(reversed 
dosage order) 
______________________________________ 
A comparison was also made with anionic aluminum modified silica sol with a 
particle size of about 5.5 nm and this, in an amount of 1 kg/ton together 
with 0.5 kg/ton of PAM gave a CSF of 520. 
EXAMPLE 4 
Sols (a) and (b) of Example 3 and also sols (e) and (f) were investigated 
in combination with cationic polyacrylamide for a stock made up from 
groundwood pulp. Sol (e) had been prepared according to the following: 
27.84 g of a 50% solution of polyaluminum chloride [Al.sub.2 (OH).sub.5 
Cl.2H.sub.2 O].sub.x was diluted to 200 g. 1000 g of a 1% polysilicic 
acid, as in Example 3, was added to the polyaluminum chloride solution and 
the obtained product contained 0.56% Al and 0.83% SiO.sub.2 and thus had a 
mole ratio of Al to surface silica of about 1:1.5. Sol (f) had been 
prepared according to the following: 34.80 g of a 50% polyaluminum 
chloride, [Al.sub.2 (OH).sub.5 Cl.5H.sub.2 O].sub.x, solution was diluted 
to 200 g and 1000 g of a 1% polysilicic acid was added to the solution. 
The product contained 0.70% Al.sub.2 O.sub.3 and 0.83% SiO.sub.2 and the 
mole ratio of Al to surface Si thus was about 1:1.2. 
The groundwood pulp stock contained 2 g/l of Na.sub.2 SO.sub.4.10H.sub.2 O 
and had a pH of 7.0. The dewatering effect was investigated as described 
earlier. In most cases the cationic polyacrylamide was added to the stock 
before the addition of the sol, if not reversed dosage order (rdo) has 
been indicated. The dosage of the cationic polyacrylamide was 1.0 kg/ton 
which has been found to be the optimum amount for this stock when it was 
used alone. In the tests it was noted that the water collected from the 
freeness tester was much more clear when combinations of sol and cationic 
polyacrylamide were used than when the polyacrylamide was used alone and 
this is an indication of very good fines retention. 
______________________________________ 
PAM Sol;amount CSF 
kg/ton kg/ton ml 
______________________________________ 
-- -- 120 
1.0 -- 195 
-- (b); 1.0 120 
1.0 (a); 1.0 400 
1.0 (a); 1.5 445 
1.0 (a); 2.0 485 
1.0 (a); 2.5 510 
1.0 (a); 1,0 (rdo) 
330 
1.0 (a); 1.5 (rdo) 
345 
1.0 (a); 2.0 (rdo) 
355 
1.0 (a); 2.5 (rdo) 
360 
1.0 (b); 1.0 420 
1.0 (b); 1.5 480 
1.0 (b); 2.0 505 
1.0 (b); 2.5 530 
1.0 (e); 1.5 400 
1.0 (e); 2.0 440 
1.0 (e); 2.5 435 
1.0 (f); 1.5 390 
1.0 (f); 2.0 425 
1.0 (f); 2.5 435 
______________________________________ 
EXAMPLE 5 
In this example the filler and fines retention was evaluated in a mill 
test. The stock was made up from 30% of chemical pulp, 24% of groundwood 
pulp and 46% of CaCO.sub.3 filler. The concentration of the stock was 0.5% 
and the pH was 8.3. The measured fillers and fines content was 76.9%. 
A Britt Dynamic Drainage Jar was used to evaluate retention. The stirrer 
speed was set to 800 rpm and the wire used was of 200 mesh. 
The cationic silica sol used was sol (a) according to Example 1 and this 
was added before the cationic retention agent. The following cationic 
retention agents were used in the different runs: 
(A) Cationic polyacrylamide, Percol 292 manufactured by Allied Colloids. 
(B) Cationic guar gum. 
The results of the tests are shown in the table below. The filler and fines 
retention (FF ret.) is given in percent at different dosages of the 
respective cationic polymers. The dosage is calculated as dry polymer on 
dry pulp plus filler. The cationic silica sol was used in an amount of 1 
kg/ton of dry pulp filler. Comparisons were made with addition of solely 
the cationic polymer. 
______________________________________ 
Added cationic 
Added amount Added sol FF ret. 
polymer kg/ton kg/ton % 
______________________________________ 
A 0.25 1 75 
A 0.50 1 97 
A 0.75 1 100 
A 0.25 -- 43 
A 0.50 -- 61 
A 0.75 -- 80 
B 2 1 95 
B 4 1 95 
B 6 1 95 
B 2 -- 45 
B 4 -- 83 
B 6 -- 90 
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EXAMPLE 6 
In this example the system of a cationic silica sol (a) according to 
Example 1 and cationic polyacrylamide, was tested in a mill producing 
magazine paper. The stock consisted of 19% sulphate pulp, 37% groundwood 
pulp, 20% thermomechanical pulp and 24% clay, i.e. a stock with high 
amounts of non-cellulosic substances. The pH was 4.45. Retention was 
measured with a Britt Dynamic Drainage Jar and freeness with a Canadian 
Freeness Tester. 
______________________________________ 
Additions 
kg/ton 
Sol PAM Retention % 
Freeness ml 
______________________________________ 
-- 0.25 26.4 110 
-- 0.50 44.6 140 
-- 1.0 57.6 190 
2.0 0.25 41.7 150 
2.0 0.50 65.0 200 
2.0 1.0 85.6 305 
______________________________________