[0001] Paper or paper board is made by providing a thick stock, diluting the thick stock
to form a thin stock, draining the thin stock to form a sheet and drying the sheet.
The thick stock can be made either by mixing water into dried pulp or, in an integrated
mill, by diluting a drained pulp.
[0002] It is standard practice to improve the process performance, or the product quality,
by including various additives at one or more of these stages.
[0003] For instance, if the pulp from the which the thick stock is made is impure, the normal
way of preparing it for drainage is by adding inorganic material, such as alum, talc
or bentonite, at the pulping or thick stock stages. These treatments can have the
effect of minimising problems due to pitch and other sticky materials.
[0004] If it is necessary to improve the strength of the final sheet it is common to include
a dry strength resin, for instance a cationic starch, in the stock that is to be drained.
[0005] It is standard practice to include cationic polymers in the stock that is to be drained
in order to improve drainage and/or retention.
[0006] Processes for improving retention are described in U.S. 4,388,150 and involve the
addition of cationic starch and colloidal silicic acid to the stock before drainage.
Such processes have been commercialised under the trade name "Composil" (trade mark).
[0007] Processes that give improved drainage, retention, drying and formation are described
in EP 235893 and involve adding a first synthetic cationic polymer before a shear
stage and bentonite after that shear stage. Such processes have been commercialised
under the trade name "Hydrocol" (trade mark).
[0008] Although this process gives very good results in most instances, there is room for
improvement with some stocks, especially impure stocks, and for some end products,
for instance newsprint and board.
[0009] In the invention, paper or paper board is made by a process comprising providing
a cellulosic suspension, subjecting this to one or more shear stages selected from
cleaning, mixing and pumping stages, adding a main polymer, selected from substantially
linear synthetic cationic polymer having molecular weight above 500,000 and cationic
starch, before one of the shear stages and adding inorganic material selected from
bentonite and colloidal silicic acid after that shear stage, draining the suspension
to form a sheet and drying the sheet, and in this process there is a preliminary polymer
inclusion stage selected from (a) the inclusion in the suspension before the main
polymer of a low molecular weight water soluble synthetic cationic polymer having
molecular weight lower than the molecular weight of the main polymer, and (b) the
inclusion of a water soluble synthetic cationic polymer as a drainage aid for the
drainage of cellulosic pulp when the suspension is made by draining a cellulosic pulp
(in the presence of the drainage aid) and diluting the drained pulp.
[0010] The preferred aspect of the invention comprises the incorporation of the said low
molecular weight water soluble synthetic cationic polymer.
[0011] The inclusion of the low molecular weight cationic polymer in the thin stock before
addition of the main polymer can lead to improvement in the processing and performance
properties obtained by the addition of the main polymer before a shear stage and bentonite
or colloidal silicic acid after that shear stage. For instance, depending upon the
other conditions, it can lead to reduced problems due to pitch and other sticky materials
and can lead to improved wet and/or dry strengths, runability, drainage, linting,
opacity and other paper qualities.
[0012] In this first aspect of the invention, the aqueous cellulosic suspension can be made
either from dried pulp or, in an integrated mill, by diluting a drained pulp, all
in conventional manner.
[0013] In the second aspect of the invention, the cellulosic suspension is made by diluting
a drained pulp in an integrated mill and the drainage of the pulp is promoted by including
a pulp drainage aid in the pulp that is to be drained, this draining aid comprising
a water soluble cationic polymer. The cationic polymer for this purpose can be any
of the synthetic polymers discussed below for use as the main cationic polymer.
[0014] When draining a pulp, in an integrated mill, to form a wet pulp that can then be
diluted to make the thick stock and the thin stock, it is common to include no drainage
aid in the pulp since drainage often occurs adequately without incurring the expense
of a drainage aid. However in this aspect of the invention it is desirable to include
a drainage aid since it promotes drainage and/or retention and provides a drained
pulp that already contains cationic polymer and the inclusion of this cationic polymer
has beneficial effects on the subsequent treatment with the described main polymer
and the inorganic additive. For instance it can reduce the amount of main polymer
that is required for optimum performance and the combined amount of drainage aid and
main polymer may then be approximately the same as the optimum amount of main polymer
if the pulp had not been treated with drainage aid. Thus by applying drainage aid
the process can be improved both at the pulp drainage stage and the sheet formation
stage but the total amount of polymer that is used is substantially unchanged and
the final performance quality can be substantially unchanged.
[0015] The amount of drainage aid polymer is usually at least 0.005 or 0.01%, often at least
0.03 or 0.05%, but it is generally unnecessary for it to be more than 0.3% or, at
the most, 0.5%. Amounts of 0.1 to 0.2% are often preferred. These percentages are
based on the dry weight of the pulp.
[0016] The sunthetic polymeric drainage aid can be a drainage-promoting, relatively low
molecular weight polymer, for instance any of those discussed below as the polymer
having lower molecular weight than the main polymer, but is generally a relatively
high molecular weight polymer for instance having a molecular weight conventional
for dewatering aids and retention aids. For instance the polymer typically is a substantially
linear synthetic cationic polymer having molecular weight above 500,000, and preferably
having intrinsic viscosity above 4dl/g. Thus it may be any of the polymers described
in EP 0235893.
[0017] Intrinsic viscosities herein are derived in standard manner from determination of
solution viscosities by suspended level viscometer of solutions at 25°C in 1 Molar
NaCl buffered to pH about 7 using sodium phosphate.
[0018] Irrespective of whether or not the thick stock is made by dilution of a wet pulp
that has been drained in the presence of a drainage aid, it is preferred in the invention
to incorporate the described low molecular weight weight soluble synthetic cationic
polymer before the main polymer.
[0019] It is preferred that the remainder of the process should be similar to the "Hydrocol"
process and, thus, should be otherwise conducted as in EP 235893, using a synthetic
cationic polymer having molecular weight at least 500,000 before one of the shear
stages and bentonite after. The materials and processing conditions described in EP
235893 can be used in the invention, subject to the modification that the suspension
includes the low molecular weight polymer before addition of the main polymer. Alternatively,
and less preferably, the bentonite can be replaced by colloidal silicic acid or other
suitable fine particulate material or the synthetic polymer can be replaced by cationic
starch.
[0020] Sometimes lower amounts of the main polymer than are recommended in EP235893 can
give good results in the present invention, for instance amounts of less than 300g/t
e.g. 50g/t (0.005%) to 250g/t, especially above 100g/t based on the dry weight of
the stock.
[0021] The process can alternatively be similar to that described in US4388150 with the
addition of cationic starch into the suspension prior to the colloidal silicic acid
(which can be modified as WO86/5826).
[0022] The low molecular weight polymer can be present in the thick stock that is diluted
to form the thin stock or it may be added to the thin stock. For instance generally
the thick stock is diluted to form the thin stock by use of white water. It is desirable
to add the low molecular weight polymer before, or immediately after or during, the
dilution with white water and to add the main polymer to the thin stock, after the
addition of the low molecular weight polymer.
[0023] The low molecular weight polymer should have a molecular weight sufficiently lower
than the molecular weight of the main polymer that it will provide different process
or performance benefits. For instance this aspect of the invention does not include
a process in which both the low molecular weight and high molecular weight polymers
are primarily cationic retention aids. Instead, it is restricted to processes in which
the low molecular weight polymer does provide a different performance benefit. Generally
the low molecular weight polymer has intrinsic viscosity below 2dl/g and usually has
molecular weight below 500,000. The molecular weight is usually above 50,000 and often
above 100,000.
[0024] A preferred relatively low molecular weight polymer is polyethylene imine. A suitable
grade of this type of polymer is the material sold under the trade name Polymin SK.
Other suitable materials are polymers and copolymers of diallyl dimethyl ammonium
chloride, of dialkyl amino alkyl (meth) acrylates and of dialkylaminoalkyl (meth)
acrylamides (both generally as acid addition or quaternary ammonium salts), as well
as polyamines and polydicyandiamides-formaldehyde polymers. Amphoteric synthetic polymers
may be used.
[0025] One preferred process according to the invention utilises a relatively crude stock
containing significant amounts of pitch and/or having high cationic demand. For instance
it may require at least 0.1% Polymin SK to give improved retention when the Polymin
SK is used in conventional manner as retention aid. Polymin is a trade mark. Such
stocks are, for instance, those containing more than 25% by weight, usually more than
50% by weight, of mechanically derived pulps and/or deinked pulps. By mechanically
derived pulps we mean groundwood, pressure refined groundwood, thermo-mechanical,
chemi-thermo mechanical or any other high yield mechanically derived fibres.
[0026] In these instances, the low molecular weight polymer can be selected primarily to
reduce cationic demand and/or avoid pitch problems and/or linting.
[0027] When using these relatively crude pulps, the process is of particular value when
the stock is to be used for the manufacture of newsprint, and for this purpose stock
is generally substantially unfilled or only contains small amounts of filler, for
instance 0 to 15% and often 0 to 10% based on the dry weight of the stock. Benefits
are however also achieved if the stock contains filler in amounts to give up to 30%
filler in the final paper produced.
[0028] The process is also of value in the manufacture of board, again often from similar
crude pulps containing little or no filler. In these instances an alternative or additional
property of the low molecular weight polymer may be to improve the strength of the
board and for this purpose a low molecular weight water soluble synthetic cationic
dry strength resin may be used as the polymer. Amphoteric polymers are particularly
suitable for this purpose.
[0029] The amount of low molecular weight polymer is up to 0.5% generally in the range 0.01
or 0.05 to 0.2%, based on the dry weight of the stock, and the optimum can be found
by routine experimentation. Often the pulp, before treatment with the low molecular
weight polymer, has a cationic demand (as measured by titration with the main cationic
polymer) of above 400g/t and the low molecular weight polymer is included in the stock,
or ahead of the stock, in an amount to reduce the cationic demand of the thin stock
to below 300g/t before adding the main polymer.
[0030] The process of the invention is found to give an improvement in the performance since
it can give improved pitch and/or stickies removal, improved paper quality such as
opacity and linting characteristics improved wet strength or runnability during manufacture.
Furthermore the performance of the process when assessed in terms of the drainage
characteristics is improved by the incorporation of the second polymer, as compared
to a process without that polymer, for instance a process as described in EP235893
or US4388150.
[0031] In the following examples, Polymer A is a polymer of IV 7dl/g formed from 75% acrylamide
and 25% dimethylaminoethyl acrylate, MeCl quaternised, and Polymer B is a modified
polyethyleneimine as sold under the trade name Polymin SK.
Example 1
[0032] A 100% mixed waste stock having a consistency of 0.5% was prepared. Drainage tests
were conducted on the stock using a modified Shopper Riegler freeness tester, the
time for 600mls of backwater to drain from the stock sample being measured. The stock
was subjected to shear and the drainage was measured. In one test no additions were
made before or after the shear. In other tests bentonite was added after the shear
and polymer A and/or polymer B was added before the shear. When both polymers A and
B were added, B was added considerably ahead of polymer A.
[0033] The results are as follows.
Table 1
Polymer B |
Polymer A |
Bentonite |
Drainage |
0 |
0 |
0 |
74 |
0 |
0.04% |
0.2% |
32 |
0.02% |
0.04% |
0.2% |
18 |
0.04% |
0.04% |
0.2% |
13 |
0.04% |
0 |
0.2% |
51 |
Example 2
[0034] A process similar to the preceding example was conducted using a stock having a high
mechanical fibre content, and in particular being a 50:50 groundwood:bleached kraft
pulp having a consistency of 1.0%. In addition to measuring the drainage time as in
the previous example, a pitch count was made (in particles/ml by the Allen method).
The following results were obtained.
Table 2
Polymer B |
Polymer A |
Bentonite |
Drainage |
Pitch Count |
Percentage Pitch Reduction |
0 |
0 |
0 |
80 |
5.8 x 10⁶ |
|
0 |
0.025% |
0.2% |
49 |
1.7 x 10⁶ |
70% |
0.025% |
0.025% |
0.2% |
35 |
1.2 x 10⁶ |
79% |
0.05% |
0.025% |
0.2% |
31 |
5.1 x 10⁶ |
91% |
[0035] These examples clearly demonstrate the value of adding, for instance 0.01 to 0.1%,
generally around 0.02 to 0.07%, polyethylene imine so as to reduce the amount of high
molecular weight (for instance IV above 4) cationic retention aid that is required
for good drainage and retention and so as to counteract the effect of stock having
high cationic demand and, especially, high pitch count.
Example 3
[0036] Newsprint is made using a stock based on 3% kraft, 17% magnefite, 38% thermomechanical
pulp and 42% groundwood, and to which 20% broke has been added. High molecular weight
polymer is added, in some tests, just before the last shear stage and bentonite is
added, in some tests, after the last shear stage. Low molecular weight polymer is
added to the thin stock soon after it is diluted from the thick stock.
[0037] In these tests the low molecular weight polymer is polymer K which is a solution
polymer of about IV 1 dl/g and formed from about 20% acrylamide and 80% by weight
diallyl dimethyl ammonium chloride. The high molecular weight polymers are L, which
is 70% acrylamide, 30% methyl chloride quaternised dimethylaminoethyl acrylate IV
8, and polymer M which is 95% acrylamide and 5% methyl chloride quaternised dimethylaminoethyl
acrylate IV 11. The drainage rate for each of the treated suspensions is measured,
with the best results being those that have the highest drainage figure. The results
are as follows.
Table 3
Polymer K |
High MW Polymer |
Bentonite |
Drainage |
0 |
0 |
0 |
205 |
0.2% |
0 |
0 |
195 |
0.2% |
0 |
0.2% |
300 |
0.2% |
0.05%L |
0.2% |
335 |
0.2% |
0.05%M |
0.2% |
340 |
0 |
0.05%M |
0.2% |
325 |
[0038] These results clearly demonstrate the benefit in the manufacture of newsprint from
adding high molecular weight cationic polymer immediately before shear and bentonite
after shear even when the high molecular weight polymer only has a relatively low
cationic charge, and they also show that a useful result can be obtained when the
high molecular weight polymer is replaced by a lower molecular weight polymer having
molecular weight above 500,000, but that best results are obtained using a combination
of both.
1. A process in which paper or paper board is made by forming an aqueous cellulosic
suspension, passing the suspension through one or more shear stages selected from
cleaning, mixing and pumping stages, adding a main polymer selected from cationic
starch and high molecular weight water soluble cationic polymer to the suspension
before one of the shear stages and adding inorganic material selected from bentonite
and colloidal silica after that shear stage, draining the suspension to form a sheet
and drying the sheet, characterised in that the process includes a preliminary polymer
inclusion stage selected from (a) adding to the suspension, before the addition of
the main polymer, a low molecular weight water soluble synthetic cationic polymer
having molecular weight lower than the molecular weight of the main polymer and (b)
adding a water soluble, cationic, polymeric, drainage aid to a cellulosic pulp and
then draining the pulp and diluting the drained pulp to form the aqueous cellulosic
suspension.
2. A process according to claim 1 in which the main polymer is a high molecular weight
linear water soluble cationic polymer having molecular weight above 500,000.
3. A process according to claim 2 in which the inorganic material is bentonite.
4. A process according to any preceding claim in which at least 25% by weight of the
cellulosic suspension is formed from mechanically derived pulp and/or deinked pulp.
5. A process according to any preceding claim in which the product is newsprint or
board.
6. A process according to any preceding claim in which the main polymer is a synthetic
polymer having intrinsic viscosity at least 4dl/g or is cationic starch and a low
molecular weight water soluble synthetic cationic polymer having lower molecular weight
is incorporated in the suspension before the main polymer.
7. A process according to claim 6 in which the low molecular weight polymer has intrinsic
viscosity below 2dl/g.
8. A process according to claim 6 in which the low molecular weight polymer has molecular
weight 100,000 to 500,000.
9. A process according to any of claims 6 to 8 in which the low molecular weight polymer
is selected from polyethylene imine, polyamines, polycyandiamide formaldehyde polymers,
amphoteric polymers, and polymers of monomers selected from diallyl dimethyl ammonium
chloruide, diallylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides.
10. A process according to any of claims 6 to 9 in which the low molecular weight
polymer is an amphoteric cationic dry strength resin and the product is board.
11. A process according to any of claims 6 to 10 in which the suspension to which
the low molecular weight polymer is added has a cationic demand, as measured on the
main cationic polymer, of at least 400g/t and the amount of low molecular weight polymer
that is added reduces the said cationic demand to below 300g/t.
12. A process according to any of claims 6 to 11 in which the suspension that is drained
to form the paper or paper board is a thin stock formed by dilution of a thick stock
and the main polymer is added to the thin stock and the low molecular weight polymer
is present in the thick stock.
13. A process according to claim 12 in which the suspension that is drained to form
the paper or paper board is a thin stock formed by dilution of a thick stock and the
main polymer is added to the thin stock and the low molecular weight polymer is added
to the thin stock or to the thick stock in an amount of from 0.01 to 0.5% based on
the dry weight of suspension.
14. A process according to any preceding claim in which the suspension that is drained
to form the paper or paper board is made by diluting a drained pulp that has been
made by draining a cellulosic pulp containing a pulp drainage aid and in which the
drainage aid comprises a water soluble, cationic, synthetic, polymeric drainage aid
having intrinsic viscosity above 4dl/g.