Note: Descriptions are shown in the official language in which they were submitted.
WO 95/33096
PCT/GB95/01259
1
Manufacture of Paper
This invention relates. to the production of paper
which is strengthened by starch.
It is standard practice to make paper on a paper-
s making machine by providing a cellulosic thin stock
suspension, flocculating the suspension by adding a
solution of polymeric retention aid and thereby forming
a
flocculated suspension, draining the flocculated suspension
through a moving screen to form a wet sheet, and carrying
the sheet through a heated drying zone and thereby forming
a dry sheet. The retention aid can be dissolved cationic
starch but is often a synthetic polymeric material.
Although the use of polymer of rather low molecular weight
can give some improvement in retention, the polymer is
preferably of high or very high molecular weight, generally
having intrinsic viscosity above 4dl/g.
A common alternative to this process involves shearing
the flocculated suspension so as to degrade the flocs and
then adding an aqueous suspension of micro-particulate
anionic material and thereby reflocculating the suspension,
and then draining the reflocculated suspension through the
screen. Such processes using cationic starch and colloidal
silica are described in U.S. 4,388,150 and processes using
cationic synthetic polymer and bentonite are described in
EP-.A-235,893. Processes in which size is added after the
flocculation with the cationic polymer are described in
EP-
A-499,448. Processes using other polymers and suspensions
suitable for these are described in W095/02088.
The cellulosic thin stock is often formed in part from
recycled paper which may include soluble starch (cationic
or anionic or non-ionic) and so the thin stock, and the
final sheet, often includes soluble starch. For instance
the dry sheet may contain as much as 1% starch derived from
recycled paper. It is, however, often desired to add
r
starch to the thin stock.
Thus, water soluble cationic starch may be added as
part or all of the solution of polymeric retention aid (see
A
WO 95/33096 PCT/GB95/01259
2
f or instance U . S . 4 , 3 8 8 , 15 0 ) . The amount required f or this
purpose is usually not more than about o.3% (dry weight
starch based on the dry weight of paper).
It is often desired to add starch in order to
strengthen the paper. For instance it is particularly
desirable to include significant amounts of starch in
fluting medium and liner board. These materials are
usually substantially unfilled and increasing their
strength makes them more suitable for use as packaging
materials. It is also desirable to include significant
amounts of starch in filled sheets as the inclusion of
significant amounts of filler would otherwise tend to
reduce the strength of the sheet.
In order to maximise strength, it is desirable to
include starch in amounts of as much as 5 or 10% or even
higher, but attempting to achieve this tends to make the
process less efficient as regards energy consumption and/or
rate of production, or can incur the risk of unacceptable
increase in the chemical. oxygen demand of the effluent from
the process, because of increased starch in the effluent.
Various grades of starch are conveniently commercially
available and include grades which are usually insoluble in
the cellulosic suspension. They can be used either
unmodified or chemically modified. Generally the starch is
pre-solubilised at high temperature to render the starch
soluble in the cellulosic suspension.
In this specification when we say a starch is
insoluble we mean that it is insoluble in the cellulosic
suspension and remains undissolved in the cellulosic
suspension. When we say a starch is soluble we mean it is
soluble in the cellulosic suspension.
Soluble cationic starch is reasonably substantive to
the cellulosic fibres in amounts up to about 1 to 1.5% by
weight of the starch, based on the dry weight of the paper.
If the amount of cationic starch in the suspension is
increased significantly above this, there may be little or
no increase in the amount of starch which is retained in
WO 95/33096 . ,, ~
PCT/GB95/01259
3
the paper and, instead, there is merely an increase in the
amount of soluble cationic starch which is in the white
water which drains through the screen. This is undesirable
" since it has to be removed before discharge as effluent,
because of the high chemical oxygen demand that it may
create in the effluent from the mill.
The soluble cationic starch can be made by chemical
modification of starch or merely by cooking raw starch and
adding a low molecular weight cationic polymer before,
during or after the cooking. Suitable low molecular
weight cationic polymers have intrinsic viscosity below
idl/g. Examples of such systems are in CA 787,294 and U.S
3,930,877.
In practice, when starch is being used as a
strengthening aid it is usually necessary also to include
a polymeric retention aid, and there have been various
publications about adding combinations of materials. For
instance in Tappi June 1976, 59, 6, pages 120 to 122 the
per:Eormance of various dual polymer systems is examined,
including the performance of a blend of soluble cationic
starch and hydrolysed polyacrylamide. In CA 1,232,713 up
to 1.5% soluble cationic starch is applied in combination
with polyethylene oxide or cationic, non-ionic or anionic
polyacrylamide retention aid having molecular weight above
1 million.
In Tappi Journal February 1984 pages 104 to 108 the
effect of various blends of soluble cationic starch and
polymers is examined and it is noted that cationic starches
at ~.% by weight improve drainage and retention but that
at
higher levels drainage is adversely affected. It is stated
than an ideal polymer for a board mill at low shear appears
to be a cationic, low molecular weight, high charge density
polymer, in particular polyethylene imine.
In normal commercial practice it is found that if the
amount of cationic starch is increased above about 1 or
1.5% there is increased risk that the cationic starch will
interfere with the effectiveness of the polymeric retention
PCT/GB95/01259
R'O 95/33096
4
aid. As a result retention and drainage may deteriorate
with the result that the machine has to operate more slowly
or product quality deteriorates.
When it is desired to include a greater amount of
starch than 1 to 1.5%, the usual technique involves
applying an unmodified starch solution on a size press at ,
the end of the paper-making machine, i . e. , after partial or
complete drying of the sheet. The application of a
solution of starch at this point can result in high pick-up
(for instance up to 7 or 10% is common). However it can
result in the starch being concentrated more on the surface
than in the centre of the sheet and it has the particular
disadvantage that it necessitates redrying of the sheet,
thus wasting heat energy and/or slowing down the process.
It would therefore be desirable to be able to achieve these
or higher levels of starch without providing unacceptable
levels of soluble starch in the white water and without
having to redry the sheet.
Another known method for providing significant
loadings of starch in the paper involves applying a spray
or a foam containing undissolved starch particles on to the
wet sheet before it is carried through the driers, followed
by cooking the starch during drying. This process also
has the disadvantage of tending to produce a higher
concentration of starch on the surface than in the centre
of the sheet. However its particular disadvantage is that
it is very difficult to achieve uniform application of the
starch by spraying or foam application for prolonged
periods because of the tendency of the starch composition
to cause blockages in the spray or foam applicators.
Attempts to include cold-water insoluble particulate
starch in the suspension before drainage have been proposed .
in the literature but have not achieved success. For
instance Fowler reviewed the general techniques of adding
starch in Paper 1978 pages 74 and 93. He discussed the
techniques mentioned above and also stated that if raw
uncooked starch is added to the suspension followed by the
WO 95/33096
PCT/GB95/01259
addition of retention aid only minimal retention of starch
can be achieved. He proposed that better retention is
achieved if the starch is slurried with bentonite and added
to the suspension prior to the retention aid, and he also
5 proposed that retention can be increased further by
- including in the slurry a polymer having a charge opposite
to the charge of the retention aid.
In U.S. 4,347,100 Brucato describes that mechanical
and thermomechanical pulping processes can be improved by
adding an anionic surfactant or an anionic polymer during
the pulping process. He states that the addition of a
cationic polymer causes reaction with the anionic polymer
and the formation of a gum-like precipitate which
contributes to strength, and he recommends the addition
of
cationic polymer in a stoichiometric amount based on the
anionic polymer. He describes a titration technique for
obtaining the desired stoichiometric amount. He also
proposes that optimum strength can be achieved by including
ungelatinised starch which is gelatinised during subsequent
heat drying.
He states that the reaction of the cationic and
anionic polymers to produce a gum-like precipitate carries
the starch particles and retains the starch in the wood
fibres. He says that the furnish is then supplied to the
paper-making machine where it is formed into a sheet and
heat dried. This suggests that the starch is being added
to t:he pulp or to the thick stock. In all the examples
the
pule had a consistency of 2.3% but Brucato suggests higher
consistencies are desirable. The strengths are all
measured on hand sheets. He gives no information about
whether the process could be conducted on a paper making
machine, nor how this could be done, nor the extent of
retention of starch that can be achieved.
- Brucato describes in U.S. 4,609,432 another method of
obtaining strengthened paper, this time using two different
cel:Lulosic suspensions. 90 to 98% of the fibre weight is
provided by a first cellulosic suspension, usually of
WO 95/33096 PCT/GB95/01259
6
refined fibres, and 2 to 10% of the fibre weight is
provided by adding to this first suspension a second
cellulosic suspension which contains a heat-sensitive
bonding agent (such as uncooked starch) for bonding the
fibres and a polymer for adhering the bonding agent to the
fibres of the second suspension. For instance the second
suspension can contain the second cellulosic fibres
together with 20 to 200% uncooked dry starch and 0.01 to
0.1% cationic polymer. The cationic polymer is said to
l0 coat the starch particles and adhere them to the fibres of
the second suspension. A typical process uses a first
suspension containing 95% of the total fibres and a second
suspension containing 5% of the fibres, 0.012% polyethylene
imine and 20% starch. A hand sheet was formed from this
and was then dried and it appears that the starch is
activated during the drying. Again there is no indication
about how to conduct the process on a machine nor about
retention.
Brucato quotes the same list of cationic polymers in
both patents, namely polyethylene imines (which are
preferred in U.S. 4,609,432), polyamide polyamine resins,
urea formaldehyde resins, melamine formaldehyde resins and
polyacrylamides. It seems that Brucato wants to use low
molecular weight polymers since all the classes of polymers
he mentions except for the polyacrylamides inevitably have
very low molecular weight and the polyacrylamide he
exemplifies is Separan CP7, a trade mark of Dow Chemical
Co., and we believe that this material also has a
relatively low molecular weight, of about 1 million.
There is no suggestion i n, either of the Brucato
patents that any additional retention aid should be used.
The stoichiometric reaction to form a precipitate in U.S.
4,347,100 will prevent the cationic polymer acting as an
effective retention aid. The total amount of polyethylene
imine used in the examples of U.S. 4,609,432 may be
sufficient to cause flocculation of the second suspension
but will be much too low to cause flocculation of the
WO 95/33096 PCT/GB95/01259
7
combined suspension. For instance the highest dosage which
is .exemplified is around 0.002% based on total fibre
weight.
' The Brucato methods therefore require particular
interaction between low molecular weight cationic polymer
and other material within the suspension and do not result
in the production of a flocculated or reflocculated
suspension of the type that is attainable by the use of
high molecular weight synthetic polymers or cationic starch
optionally followed by anionic microparticulate material.
It is desirable to strengthen substantially unfilled
sheets of paper (including paper board) that is to be used
as packaging, but there is also a particular need to
include starch as a strengthening aid in sheets which are
highly filled, since the use of a large amount of filler
tends to weaken the sheet. The filler can be
preflocculated before addition to the cellulosic
suspension. Although this has some advantages, it can
cause particular weakening of the sheet. It is therefore
known to include water-soluble starch in the pre-
flocculated filler composition, but this causes
difficulties in handling the flocculated suspension.
In GB 2,223,038 filler is included in a cellulosic
suspension by adding a slurry of filler, insoluble starch
particles and flocculating agent. Although many of the
flocculating agents which are mentioned have very low
molecular weight (for instance Magnafloc 1597 is a
polyamine) some have a moderate molecular weight.
Suspending agent such as a gum, a synthetic organic
polymer, or a swelling clay (e.g., bentonite) can be
included and preferably the suspending agent is chosen so
as to reduce the net charge in the composition close to
zero. For instance if a cationic flocculant is used then
an .anionic suspending agent is usually required. The
amount of filler in the composition is preferably 30 to
40%, and the amounts of starch and flocculant (based on
filler) are preferably 1 to 5% and 0.05 to 0.2%
WO 95133096 ~" PCT/GB95/01259
k ° ..
8
respectively, with the amount of starch in the final paper
being said to be typically 0.05 to 1.5%. The resultant
flocculated suspension will contain the starch particles
trapped in the filler flocs, and it is added to the
cellulosic suspension which is then drained and heated,
with consequential cooking of the starch. In the examples, .
the amount of filler ranges from 7 to 24% and the amount of
starch is 4% based on filler, i.e., about 0.3 to 1% based
on paper.
Accordingly, none of these detailed methods provide
any practical solution to the problem of providing a
convenient technique which uses readily available starch
and which does not result in undesirable contamination of
effluent and which is capable of giving very high pick-up
of starch in the paper and which does not involve the
problems of size press application or spray or foam
application on to the wet sheet.
So far as we are aware, the proposals of Fowler,
Brucato and in GB 2,223,038 have not resulted in
satisfactory processes for producing sheets containing a
large amount of starch as a result of incorporating all the
starch in the suspension before drainage. Accordingly,
the problem remains that if large amounts of starch are to
be incorporated then they have to be added to the wet sheet
by spraying or foam or at the size press, and there remains
an urgent need to find a way of incorporating starch in the
thin stock so as to allow efficient and environmentally
acceptable production of paper having a high starch
content.
Support for our belief that such a process is not
known arises from the fact that, subsequent to the priority
date of this application, in Nordic Pulp and Paper Research
Journal Number 4 1994 pages 237 to 241 it is stated that
since starch has granular form with diameter of about 1 to
40~m the retention of the starch granules is very low when
added directly to paper stock without dissolution or
swelling in water. According to the proposals in this
CA 02167803 2005-O1-21
9
article it is possible to include high amounts of starch in
laboratory handsheets by including in the cellulosic
suspension starch having a particular flake form and which
has been made by precipitation in mineral salts and
processing the precipitate. It is commercially undesirable
to have to undergo this particular process and it would be
much more convenient to be able to obtain high starch levels
in paper made on a conventional paper-making machine using
conventional granular starches and without incurring
significant effluent problems due to excessive drainage of
starch through the screen.
Accordingly, an aim of this invention is the provision
of a method in which it is possible to include starch in the
thin stock in such a way that relatively large amounts of
starch can be retained in the paper without interfering
significantly with efficient production of the paper and
without creating unacceptable effluent discharges.
In a first aspect of the invention, there is provided a
process for making paper on a paper-making machine
comprising providing a cellulosic thin stock suspension,
flocculating the suspension by adding an aqueous solution of
polymeric retention aid selected from dissolved cationic
starch and synthetic polymer having an intrinsic viscosity
of above 4d1/g and thereby forming a flocculated suspension,
shearing the flocculated suspension and reflocculating
the sheared suspension by adding an aqueous suspension of
microparticulate anionic material and thereby forming a
reflocculated suspension, draining the reflocculated
suspension through a moving screen to form a wet sheet, and
carrying the sheet through a heated drying zone and thereby
forming a dry sheet, wherein insoluble particles of starch
are added to the cellulosic suspension as a slurry of freely
dispersed particles in part or all of the aqueous solution
CA 02167803 2005-O1-21
1
of the polymeric retention aid or in part or all of the
aqueous suspension of microparticulate anionic material, and
the insoluble particles of starch are heated during the
drying and release soluble starch into the sheet in the
presence of moisture.
This first aspect of the invention is conducted with
the shearing and reflocculation with microparticulate
anionic material. When the reflocculation step is being
used, then the particulate starch can be included in that
suspension of microparticulate anionic material, optionally
also with polymeric retention aid.
In order to promote good retention it is necessary that
the particles of the starch should be able to interact with
the surfaces of the cellulosic fibres and, if present, the
anionic microparticulate material, It is therefore desirable
for the starch particles to be added as a slurry of
substantially independent particles so that the particles
can interact with the fibres or microparticulate anionic
material substantially independent of each other.
Best results are obtained in the invention when the
process involves the described shearing and reflocculation
stages.
A preferred, second, aspect of the invention is a
process for making paper on a paper-making machine which
comprises
providing a cellulosic thin stock suspension,
flocculating the suspension by adding an aqueous
solution of polymeric retention aid selected from dissolved
cationic starch and synthetic polymer having intrinsic
viscosity of at least 4dl/g and thereby forming a
flocculated suspension,
CA 02167803 2005-O1-21
l0a
shearing the flocculated suspension and reflocculating
the sheared suspension by adding an aqueous suspension of
microparticulate anionic material and thereby forming a
reflocculated suspension,
draining the reflocculated suspension through a moving
screen to form a wet sheet, and
WO 95/33096 PCT/GB95101259
11
carrying the sheet through a heated drying zone and
thereby forming a dry sheet,
wherein insoluble particles of starch are added to the
' cellulosic suspension as a slurry in part or alI of the
solution of polymeric retention aid, and the insoluble
starch particles are heated during the drying to release
soluble starch into the sheet in the presence of moisture.
In these preferred processes good retention of fibres,
starch particles (and filler if present) is achieved by the
reflocculation stage. The application of shear to the
flocculated suspension containing the cellulosic fibres and
the starch particles results in degradation of flocs in the
flocculated suspension and redispersion of the previously
flocculated material. As a result, any flocs of starch
particles, or of fibres free of starch particles, tend to
be broken up by the shearing. The consequence of this is
that a very uniform distribution of the individual starch
particles is achieved in the reflocculated suspension, and
thus in the drained sheet. As a result of this uniformity,
the gelatinisation during the drying can be conducted more
efficiently and the distribution of the starch within the
sheet both before gelatinisation and after gelatinisation
can be more uniform than if the process is conducted
without the shearing and reflocculation.
Although it is preferred that the slurry of polymeric
retention aid is added in a form wherein the starch
particles are substantially freely dispersed in it, some
flocculation of the starch slurry can be acceptable when
the resultant flocculated cellulosic suspension is sheared
and then reflocculated since this shearing will break up
any initial flocs in the initial slurry. It is possible
for the slurry to include some filler or fibres.
Generally, in all processes of the invention, the slurry
consists essentially only of the polymeric retention aid
and the insoluble starch particles.
The paper that is produced can be filled, and an
advantage of the invention is that papers having good
WO 95/33096 PCT/GB95/01259
12
strength can be obtained even when they contain high
amounts of filler, for instance more than 20% by weight or
more than 40% by weight and even up to 60% by weight based
on the dry weight of the paper. Conventional fillers such
as calcium carbonate or sulphate or talc or kaolin or other
clays can be used.
Another very important feature of the invention is
that it permits the production of unfilled paper, that is
to say paper to which little or no deliberate addition of
filler is made. This substantially unfilled paper
generally has a filler content of not more than 15%, and
usually not more than 10% by weight of the dry sheet.
Usually any filler which is included originates from
recycled paper which is used in forming the cellulosic
suspension but if desired small amounts, for instance up to
5% or perhaps 10% by weight based on the dry weight of the
suspension can be deliberately added to the suspension.
The invention is therefore of particular value for the
manufacture of fluting medium or liner board.
Accordingly, in a third aspect of the invention we
make substantially unfilled fluting medium or liner board
on a paper-making machine by a process comprising
providing a substantially unfilled cellulosic thin
stock suspension,
adding an aqueous solution of polymeric retention aid
selected from dissolved cationic starch and synthetic
polymer having intrinsic viscosity at least 4d1/g,
draining the suspension through a moving screen to
form a wet sheet, and
carrying the sheet through a heated drying zone and
thereby forming a dry sheet, and
wherein insoluble particles of starch are added to the
suspension as a slurry of substantially freely dispersed
particles in part or all of the aqueous solution of the
retention aid, and
WO 95/33096 _ PCT/GB95/01259
~ ~. ~ ~ $ ~ 3 ~~s ~ ~, '~~ .~ :r . .
13
wherein the insoluble particles of starch are heated
during the drying and release soluble starch into the sheet
in l:he presence of moisture.
' In this aspect of the invention, the process can be
performed by draining the flocculated suspension which
results from the addition of the polymeric retention aid or
by shearing that flocculated suspension and ref locculating
it by the addition of an aqueous suspension of micro
particulate anionic material, and then draining the
resultant reflocculated suspension.
A unique characteristic of the invention is that we
can achieve a high starch content in the dry sheet as a
consequence of the inclusion of the undissolved starch in
the cellulosic suspension without causing pollution
problems. Thus we can easily obtain a content of at least
2% ~or 3% and typically 5% and even up to 10 or 15% by
weight starch in the dry sheet.
According to a fourth aspect of the invention, we make
paper on a paper-making machine by a process comprising
providing a cellulosic suspension,
flocculating the suspension by adding an aqueous
solution of polymeric retention aid selected from dissolved
cationic starch and synthetic polymer having intrinsic
viscosity at least 4d1/g and thereby forming a flocculated
suspension,
optionally shearing the flocculated suspension and
reflocculating the sheared suspension by adding an aqueous
suspension of micro-particulate anionic material and
thexeby forming a reflocculated suspension,
draining the flocculated or reflocculated suspension
through a moving screen to form a wet sheet, and
carrying the sheet through a heated drying zone and
thereby forming a drying sheet,
wherein we include in the cellulosic suspension
insoluble starch particles in an amount of above 3% by
weight based on the dry weight of the suspension, and we
retain insoluble starch particles in the wet sheet in an
.4 4
WO 95/33096 . ~ ~ "~ ' PCT/GB95/01259
14
amount of at least 3% based on the dry weight of the sheet
and heat the insoluble particles during the drying and
thereby release soluble starch into the sheet in the
presence of moisture.
Preferably we achieve high retention of the starch
particles (e.g., above 80% or 90% or more), and any starch
particles that do drain into the white water can be
tolerated as they can be insoluble in the white water and
so can be recycled and trapped on a subsequent pass through
the machine. Alternatively they can be removed by
filtration before discharge.
The preferred way of performing this fourth aspect of
the invention is by including the starch as a slurry in
part or all of the aqueous solution of polymeric retention
aid or in part or all of the aqueous suspension of
microparticulate anionic material. However other ways of
including it can be used. For instance the particles may
be sprayed or otherwise coated with a solution of the
retention aid and added to the cellulosic suspension before
or after adding the remainder of the retention aid.
When the process is conducted by draining the
flocculated suspension, this suspension may have been
formed in conventional manner (apart from the addition of
starch). For instance it may have been made from a
groundwood, mechanical or thermomechanical pulp and the
thin stock, or the thick stock from which it is formed, may
have been treated with bentonite before the addition of the
retention aid. In such processes, the retention aid is
often substantially non-ionic, for instance being formed
from 0 to 10 mole percent anionic and/or cationic monomers
and 90 to 100 mole percent non-ionic monomers. However the
invention, in this aspect, is not limited to the use of .
dirty pulps and includes the use of any suitable
combination of pulp and high molecular weight retention aid
(anionic, non-ionic or cationic) or dissolved cationic
starch retention aid.
WO 95/33096 PCT/GB95/01259
;;, ~~~ ;~.
In these processes the retention aid and starch are
usua:Lly added after the last point of high shear, e.g., in
or immediately prior to the head box.
- In the preferred processes of the invention, the
5 flocculated suspension is subjected to shear so as to
- degrade the initial flocs and is then reflocculated, or
subjected to super-coagulation, by the addition of anionic
microparticulate material. The shearing can be achieved
mere:Ly as a result of turbulent flow from the point at
10 which retention aid is added to the point at which the
microparticulate material is added, but often the shearing
is applied by passage through a device such as a
centriscreen, fan pump or other deliberate shear mixing
stage. The shearing results in reduction of the size of
15 the flocs, for instance as described in EP-A-235,893.
The starch particles can then be added with the
anionic microparticulate material. As a result of
intimate admixture of the starch particles and this
material, the starch particles appear to become entrapped
within the supercoagulation that occurs upon the addition
of the microparticulate material and as a result good
retention of the starch particles is obtained. When the
starch is being added with the microparticulate material,
the slurry of starch and microparticulate material is
usually free of any other significant solid phase and
usually consists essentially only of water, the
microparticulate material, the starch and any dispersing
agent or other additives necessarily associated with the
microparticulate material. The ratio dry weight of starch
to microparticulate material is generally in the range 5:1
to 100:1, often around 10:1 to 50:1, by weight.
Typically the starch particles are injected into a
slurry of the microparticulate material, or the
microparticulate material is injected into a slurry of the
starch particles, just before addition to the cellulosic
suspension, although if desired the materials may be
premixed and the resultant slurry pumped from the mixing
WO 95/33096 ' PCT/GB95/01259
16
station towards the addition point. The addition point is
usually in the headbox or at some other position after the
last point of substantial shear since it is usually
desirable that the reflocculated or supercoagulated -
structure should not be degraded excessively by subsequent
shear prior to drainage. '
It is usually preferred to introduce the starch
particles as a slurry in part or all of the aqueous
solution of retention aid. This allows for the retention
aid to be absorbed or otherwise attached to the surfaces of
the starch particles before the particles are mixed with
the cellulosic suspension. As a result of using a high
molecular weight retention aid or, less preferably,
dissolved cationic starch the absorbed retention aid
promotes bridging between the starch particles and the
cellulosic fibres, and thus promotes retention.
The starch may be preslurried in the aqueous solution
of retention aid but it is generally adequate to mix the
insoluble starch (usually as an aqueous slurry) and the
aqueous retention aid as they flow towards a point at which
the retention aid is added to the cellulosic suspension.
For instance the starch may be injected into the polymer
stream at some point between the polymer make-up supply and
the point where the solution is added to the cellulosic
suspension. Often it is adequate to mix the starch
particles into the solution just prior to the point at
which the solution is added to the cellulosic suspension.
Frequently the starch is provided initially as a
slurry of 10 to 40%, often around 20%, by weight starch in
water and this slurry is added into the polymer solution in
the amounts required to give the chosen dosage of polymer
and starch. The dry weight ratio of starch:polymer is.
often in the range 50:1 to 500:1. Often the slurry
contains from 1 to 50% (preferably 10 to 30%) by weight
starch particles and 0.01 to 2% by weight polymer.
Although the slurry that is added to the cellulosic
thin stock suspension can include other materials, it is
WO 95133096 ~. ~ PCT/GB95/01259
17
generally preferred and convenient for the slurry to
consist substantially only of the polymer and the starch
and water. The amount of polymer is generally
considerably above the amount which might, under relatively
static conditions, have a significant flocculating effect
on the starch particles. Thus, if the chosen amount of
polymer is added gradually to an aqueous medium containing
the chosen amount of starch with mild mixing, some
flocculation may initially be visible to the naked eye but
further addition of the polymer, accompanied by further
mixing, will result in the starch particles becoming
substantially freely dispersed in that they do not cling
together as significant flocs. In practice the addition
of the slurry of starch and polymer is normally accompanied
by shear at the addition point and this will further
promote the independent character of the particles. In
view of the shearing that tends to occur during addition,
and in view of the preferred shearing of the flocculated
cel.lulosic suspension that follows the addition of the
polymer and particles, some aggregation of the particles is
acceptable. However trapping the starch particles in
flocs of filler or fibre in the slurry is undesirable.
It is important that the polymer that is added is an
effective retention aid for the cellulosic suspension in
order that the polymer which is absorbed onto the starch
particles will have adequate substantivity to the
cel.lulosic fibres in the suspension. Selection of an
appropriate retention aid that is substantive to the
cel.lulosic suspension can be conducted in conventional
manner. It can be anionic, non-ionic or cationic. Best
results are usually obtained when the retention aid is
. cationic and so preferably the suspension is one onto which
the selected cationic retention aid is substantive.
It is usually convenient and preferred for the starch
to be added as a slurry with the entire retention aid that
is to be used for flocculating the suspension, optionally
prior to shearing and reflocculation, but if desired the
WO 95/33096 ~ ~ . PCT/GB95101259
18
slurry may be mixed with part only, for instance at least
5% and often at least 25% by weight, typically up to 50 or
75% by weight, of the total amount of retention aid. If
retention aid is being added partly mixed with particulate
starch and partly free of starch, different, high molecular
weight, retention aids may be used for the two additions
provided they are compatible, or the same material may be
used for each addition.
Low molecular weight, coagulant-type polymer may be
added at an earlier stage if required, in known manner but
this is not considered as a retention aid in the context of
the present invention.
Such coagulant polymers usually have intrinsic
viscosity below 3d1/g and often below ldl/g. They can have
high cationic charge density, preferably above 4, and
often above 5, meg/g. The low molecular weight polymer is
preferably formed of recurring units of which at least 70%,
and generally at least 90%, are cationic. Preferred
polymers are homopolymers of diallyl dimethyl ammonium
chloride and low molecular weight co-polymers of this with
a minor amount (usually below 30% and preferably below 10%)
acrylamide, low molecular weight homopolymers of
dialkylaminoalkyl (meth) -acrylamide or -acrylate
quaternary salt or acid addition salt and copolymers of
these with small amounts (generally below 30% and
preferably below 10%) acrylamide, polyethylene imines,
polyamines, epichlorhydrin diamine condensation products,
dicyandiamide polymers and other conventional low molecular
weight cationic coagulant polymers.
The retention aid with which the particulate starch is
mixed prior to addition to the cellulosic suspension can be
soluble cationic starch and thus the system can consist of
insoluble starch particles (usually chemically unmodified
insoluble starch particles) slurried in a solution of
cationic starch. However it is generally preferred that
the retention aid is a synthetic polymer.
r ,
WO 95/33096 ' .' ~ ' PCT/GB95/01259
19
The preferred retention aids for use in the invention
are polymers which have intrinsic viscosity above 4d1/g and
usually above 6d1/g, for instance 8-15d1/g or 8-20d1/g or
higher.
In this specification, intrinsic viscosity is measured
at 25°C in 1M sodium chloride buffered at pH7 using a
suspended level viscosmeter.
Non-ionic retention aids that can be used include
polyacrylamide or other polymer of water soluble
ethylenically unsaturated monomer or monomer blend, and
polyethylene oxide.
Suitable anionic retention aids are polymers of
anionic ethylenically unsaturated sulphonic or carboxylic
monomer such as acrylic acid (usually as a sodium or other
water soluble salt) optionally copolymerised with non-ionic
ethylenically unsaturated monomer such as acrylamide.
Thus the anionic polymer may be formed from, for instance,
3 to 50 mole percent, often 3 to 20 mole percent anionic
monomer such as sodium acrylate with the balance being
acrylamide.
Amphoteric polymers containing both anionic and
cationic monomer units, usually with acrylamide or other
non-ionic monomer, can be used.
Cationic polymers are preferred.
The or each cationic high molecular weight polymer is
usually a copolymer of ethylenically unsaturated cationic
monomer, with the balance being other water soluble,
generally non-ionic, ethylenically unsaturated monomer such
as acrylamide. The amount of cationic monomer is usually
at least 2 or 3 mole%. Generally it is not more than 20
mole% but it can be up to 50 mole % or more. The polymer
can be wholly water soluble or it can be in the form of
small particles of partially soluble cross-linked polymer
as described in EP-A-202,780.
The or each high molecular weight cationic polymeric
retention aid typically has a theoretical cationic charge
density of not more than about 3meq/g, often not more than
WO 95/33096 _ PCT/GB95/01259
about 2meq/g. Generally it is at least about 0.1, or
usually at least about 0.5, meg/g. In this specification,
the theoretical cationic charge density is the charge
density obtained by calculation from the monomeric '
5 composition which is intended to be used for forming the
polymer.
Suitable cationic monomers include dialkyl aminoalkyl
(meth) -acrylates and -acrylamides as acid addition or
quaternary salts. The alkyl groups may each contain 1-4
10 carbon atoms and the aminoalkyl group may contain 1-8
carbon atoms. Particularly preferred are dialkylaminoethyl
(meth) acrylates or acrylamides and dialkylamino-1,3-propyl
(meth) acrylamides.
Although it is usually preferred for the retention aid
15 to have intrinsic viscosity above 8d1/g, in some instances
it can be desirable to use as the retention aid a copolymer
of diallyl dimethyl ammonium chloride and acrylamide and
which has intrinsic viscosity at least 4d1/g, even though
it may not be practicable to manufacture such a polymer to
20 the IV 8d1/g and higher values that are preferred for other
polymers.
The total amount of polymeric retention aid is usually
0.01 to 1%, generally 0.02 to 0.1% (200 to 1,000 gram per
tonne dry weight of suspension). When the process involves
shearing and reflocculating with microparticulate material
the amount of retention aid is generally in the range 0.01
to 0.06% or 0.1% but when the process is conducted merely
with flocculation followed by drainage, i.e., without the
shearing and reflocculation, the amount is usually in the
range 0.04 to 0.15%, often 0.06 to 0.1%.
The amount depends, inter alia, on the choice of
cellulosic thin stock. This may may be formed from any
convenient pulp or mixture of pulps. The thin stock
typically has a cellulosic fibre content of 0.2 to 2.0%,
usually 0.3 to 1.5% by weight.
The retention aid of IV above 4d1/g (or cationic
starch) and the amount of it which is used in the process
~~. ~'~~0~
WO 95/33096 PCT/GB95/01259
..
21
must be such as to give good retention of fibre fines and
filler (if present). Selection of the retention aid and
its amount can be conducted in conventional manner by
' performing the process in the absence of starch with
different amounts of different retention aids so as to
select an effective combination of retention aid and its
amount for the particular cellulosic suspension that is
being treated. Naturally this test should be conducted
with the subsequent addition of microparticulate anionic
material when the overall process involves the use of that
material. When the initial cellulosic suspension includes
anionic trash, it can be desirable to treat the suspension
initially with a cationic coagulant and/or bentonite so as
to reduce the amount of polymeric retention aid that is
required.
The amount of retention aid will always be greater
than the amount required to precipitate or interact with
an~.onic soluble material in the cellulosic suspension. If
the retention performance is plotted against dosage of
polymer in a typical combination it will be seen that as
the dosage increases retention will be poor and will
increase only gradually at low values, but will then
increase significantly over a relatively small dosage
range, and will not then increase further to any
significant extent. The dosage at which retention improved
markedly is an indication of the demand of that suspension
for. that retention aid and in the invention the total
amount of that retention aid should be at or above the
amount at which retention has increased significantly.
Accordingly this amount is above the stoichiometric amount
required to react with any anionic polymeric material in
the cellulosic suspension and any pulp from which it is
formed. Generally the suspension is made without
. deliberate addition of anionic, polymeric materials.
By saying that the cellulosic suspension is
flocculated we mean that it has the state which is typical
of a cellulosic suspension which has been treated with an
z~~7~o~
WO 95133096 _ PCT/GB95/01259
22
effective high molecular weight retention agent in an
effective amount.
In preferred processes, the retention system is
selected and optimised (using high IV polymer or dissolved '
cationic starch) for retention, drainage and drying
properties in conventional manner, and the particulate
starch is injected into the polymer solution with no
substantial change in the optimum retention system.
The starch in the particles must remain substantially
undissolved prior to the start of drainage of the
suspension, since otherwise dissolved starch is likely to
drain from the suspension. A simple way of determining
whether or not the particles have remained substantially
undissolved is to titrate the drainage water for dissolved
starch. If the amount of dissolved starch in the drainage
water is sufficiently low (after allowing for any dissolved
starch introduced with the fibres from, for instance,
recycled paper), this indicates that the particles have
remained substantially undissolved_. For instance
preferably the amount of dissolved starch in the drainage
water should represent less than 20%, preferably less than
10% and most preferably less than 5% of the amount of
particulate starch in the suspension after discounting
soluble starch originating elsewhere.
One way of providing that the particles remain
substantially undissolved prior to drainage is to introduce
the starch in ungelatinised, substantially water insoluble,
form and to maintain the conditions in the suspension such
that significant gelatinisation does not occur prior to the
start of drainage. In such a process, it is necessary to
gelatinise the starch during the draining and drying
stages. ,
In conventional processes, draining is completed at
temperatures above ambient, and drying is conducted with
the application of heat. By appropriate choice of the
draining and drying conditions and of the grade of
ungelatinised starch, it is possible to achieve appropriate
WO 95/33096 "' °'~ PCT/GB95/01259
a
23
gelatinisation during the drying stage, while the sheet is
still moist. It can be desirable to apply deliberate
heating to the wet sheet, even before final drainage is
' completed, so as to pre-warm it before entry to the drying
stages. For instance the wet sheet may be passed under a
' steam hood or heater such as a Devroniser (trade mark) , and
this can facilitate full gelatinisation and dissolution of
the starch.
The act of shearing the flocculated suspension prior
to reflocculation will necessarily tend to break up any
flocs or aggregates of starch particles, and so this
preferred process will tend to result in the starch
particles being more uniformly distributed as mono
particles through the sheet. As a result, more thorough
gelatinisation of these particles will occur than when
clusters of particles are present in the sheet, and this is
an important advantage of the preferred processes of the
invention which involve shearing and reflocculation of the
flocculated suspension.
The starch particles need to gelatinise while there is
stall some moisture in the sheet in order to allow
gelatinisation to proceed satisfactorily and in order to
allow the particles to spread in the sheet so as to tend to
provide a film within the sheet, in contrast to mere spot
bonds. As a result of the starch gelatinising in the
presence of moisture, it will tend to migrate between the
fibres so as to obtain more uniform distribution of the
starch on and around and between the paper fibres. The
amount of moisture that should remain in the sheet when the
starch is dissolving can be quite low, and only needs to be
sufficient to allow migration of the gelatinised starch
. sufficient to give adequate distribution of the starch
through the sheet.
To facilitate attainment of rapid gelatinisation, it
may be desirable to use a starch that has naturally a low
temperature gelatinisation or that has been modified to
WO 95/33096 ' - PCT/GB95/01259
24
reduce its temperature of gelatinisation, provided it
remains substantially undissolved prior to drainage.
Usually the starch is an uncooked, raw starch such as
raw maize, potato, corn, wheat or tapioca starch.
Pregelatinised or precooked (and therefore soluble)
starch can be included as insoluble particles. Thus,
instead of relying on the insolubility of ungelatinised
starch particles and the subsequent cooking occurring
during the process, the dissolution of precooked starch in
the particles of the suspension can be prevented by
protecting the starch with a water impermeable shell or
matrix that disintegrates during the subsequent draining or
drying. Any material which provides sufficient water
impermeability to prevent significant dissolution of the
starch prior to draining can be used provided the shell or
matrix will disintegrate to release the starch during
draining and/or drying.
The shell or matrix does not have to provide long term
water-impermeability. For instance a slow dissolving shell
or matrix may be sufficient to protect the starch since
even if the shell disintegrates partially within the
headbox there may still be inadequate time for the enclosed
starch particle to dissolve in the headbox.
The shell or matrix may be a thermoplastic material
having a melting point such as to prevent premature
disintegration of the shell or matrix. For instance the
normal temperature of the suspension leading to the headbox
is typically in the range 40-50°C and the ambient
temperature around the drainage screen is typically in the
same range. If the particles are provided with a coating
or matrix which has a melting temperature at about or above
the temperature of the headbox, substantially no melting ,
will occur until the headbox and most of the melting and
substantially all the dissolution of the starch will not
occur until most of the draining has been completed.
Suitable thermoplastic materials that can be used include
hydrocarbon waxes.
WO 95/33090 PCT/GB95/01259
Instead of using a thermoplastic shell or matrix, a pH
sensitive shell or matrix may be used. For instance the
cooked starch may be encapsulated or otherwise protected by
polymer that is water insoluble and non-swellable at the pH
5 of i~he starch dispersion which is provided to the mill, and
this dispersion is added to the headbox which is at a pH at
which the polymer swells or dissolves. For instance the
protective polymer can be a copolymer of water soluble and
water insoluble ethylenically unsaturated monomers such as
l0 methacrylic acid or other water soluble monomer and ethyl
acrylate or other water insoluble monomer. The manufacture
of pH sensitive polymers of this general type by oil-in-
water emulsion polymerisation is well known.
Methods of incorporating an active ingredient within
15 particles of a protective matrix or within a shell are well
known and can be used in the invention. For instance the
mixture of the starch and protective material may be spray
dried or a coacervate coating may be formed around starch
particles.
20 The amount of starch that is included in the sheet
will normally be at least 0.05% and usually at least 0.2%
dry weight. The greatest advantages of the process are
achieved when the amount is above 2 or 3%, for instance 5%,
10% or even up to 12 or 15% by weight. However an
25 advantage of the process of the invention is that the
process can be operated either at high starch loadings or
low starch loadings merely by altering the amount of
starch, without making any significant changes in the
remainder of the process.
The size of the particles is generally at least 90% by
weight below 100~cm, preferably below 50~,m, often 5 to 50~,m.
The starch particles may have a size of at left 90% by
weight up to 10~m, generally 5-10~m. The starch is
preferably granular, so that all three dimensions may be
broadly similar.
The anionic microparticulate or colloidal material
(when used) is preferably bentonite, that is to say an
WO 95/33096 PCT/GB95/01259
~16'~'~03
26
inorganic swelling clay, for instance as described in EP-A-
235,893. However it can be colloidal silica (such as
described in U.S. 4,643,801), polysilicate microgel (such
as described in EP-A-359,552), polysilicic acid microgel as
described in EP-A-348, 366, or aluminum modified versions of
any of these. Instead of using inorganic anionic colloidal '
material, organic material can be used. Thus it is
possible to use an anionic organic polymeric emulsion. The
emulsified polymer particles may be insoluble due to being
formed of a copolymer of, for instance, a water soluble
anionic polymer and one or more insoluble monomers such as
ethyl acrylate, but preferably the polymeric emulsion is a
crosslinked microemulsion of water soluble monomeric
material. The particle size of the colloidal material is
generally below 2~m, preferably below lam and most
preferably below O.l~m.
The amount of colloidal material (dry weight based on
the dry weight of the cellulosic suspension) is generally
at least 0.03% and usually at least 0.1%. It can be up to,
for instance 2% but is generally below 1%. The choice and
the amount of the anionic colloidal material should be such
as to cause what is frequently referred to as "super
coagulation".
The anionic microparticulate or colloidal material is
preferably added to the suspension after the last point of
high shear, for instance at the headbox, and the suspension
can then be drained in conventional manner.
Initial selection of suitable materials can be made on
the basis of trials with conventional laboratory apparatus
such as a Britt jar and a hand sheet technique, but
commercial operation of the process is conducted on a
paper-making machine wherein the cellulosic thin stock is
provided in conventional manner, generally by dilution of
thick stock with white water, and is fed towards a headbox
through suitable apparatus such as a fan pump and
centriscreen, and is discharged from the headbox onto a
moving screen.
WO 95/33096 PCT/GB95/01259
..
27
This screen may travel at conventional screen speeds
which are normally in excess of 100 metres per minute and
typically are in the range 700 to 1500 metres per minute.
The machine will include a drying zone in conventional
manner but an advantage of the invention is that it is not
necessary for the machine to be equipped with a size press
or with any other means of applying starch to the wet sheet
or to the dried sheet.
If desired, however, further starch can be applied to
the wet sheet or the dried sheet in conventional manner.
The following are examples.
Example 1
A mill trial was carried out on a Fourdrinier machine
producing fluting medium at 600m/min from 100% waste
furnish. A cationic polymer of acrylamide with 10% mole
cationic acrylate, IV 12d1/g, was added to the thin stock
before the centriscreen at a dose level of 800g/tonne. A
20% slurry of raw starch was added to the polymer line just
prior to the addition of the polymer to the thin stock, in
sufficient quantities to provide 5 % starch on dry weight of
paper. Bentonite was added to the thin stock after the
centriscreen and just before the head box, at a dose level
of 0.5%.
Analysis of starch retained in the sheet showed that
over 95% of the added starch was retained in the sheet.
The heating during the drying stages of the machine caused
the starch to be gelatinised during the drying.
Example 2
Liner board having a weight of about 140 grams per
square metre was made on a Fourdrinier machine in a process
using as retention aid an aqueous solution of a polymer of
y acrylamide with 10 mol % dimethylaminoethyl acrylate
quaternary salt [DMAEAq], having IV 12d1/g, at a dosage of
850g/tonne in the top ply and 790g/tonne in the bottom ply,
added before the centriscreen and bentonite at a dosage of
5kg/t in both the top an dbottom ply added after the
centriscreen. The suspension included recycled paper and
WO 95/33096 PCT/GB95/01259
' 28
it was found that the starch content in the sheet, with no
deliberate addition of starch, f luctuated between about 0. 9
and 1.2%.
Particulate raw potato starch was then injected as a
slurry into the polymer feed line at a dosage of 1.42%
based on the dry weight of the suspension. When steady
state conditions had been re-established, the amount of
starch in the sheet was 2.49%, indicating substantially
complete retention of the particulate starch.
When the amount of particulate starch in the
suspension was increased to 3.11%, the amount in the sheet
was raised to 4.34%, and when the amount in the suspension
was raised to 3.50%, the amount in the sheet was raised to
4.55%, again indicating substantially complete retention.
The burst strength was increased by about 35% and the
CMT value by about 20%.
example 3
In order to conduct preliminary screening of suitable
combinations of materials, a waste furnish was prepared
from 60% newsprint, 30% cardboard and 10% magazine and was
pulped in a laboratory disintegrator for 20 minutes and
then diluted to form a 0.5% thin stock suspension at 25°C.
It was left to condition for 24 hours. It had pH 7.5 to
7.7.
500m1s of thin stock was placed in a Britt Dynamic
Drainage jar fitted with a machine wire with the stirrer
set at 150orpm. The required amount of a 20% starch
slurry was mixed with the required amount of a 0.5%
solution of polymer and added to the drainage jar. After
stirring for 60 seconds at 1500rpm the stirrer was slowed
to 80orpm and the required amount of bentonite slurry was
added. After 10 seconds mixing, the backwater was
collected for 30 seconds.
The collected backwater was cooked at 100°C for 30
minutes, the volume re-adjusted to the original volume and
the sample centrifuged to remove fibres. Acidified
potassium iodide/iodine reagent was added and the blue
WO 95/33096 r PCT/GB95/01259
29
starch/iodine complex was assessed optically and compared
to a calibration graph to give an indication of the starch
content of the water. Due to the particular analytical
techniques used the values are more indicative of relative
values than absolute values, but increasing the value
indicates increased retention.
In a first series of tests, polymer (acrylamide with
mol % dimethylaminoethyl acrylate quaternary salt, IV
12d1/g) was added at 750 grams per tonne fibre, bentonite
10 at 2,000 grams per tonne fibre and starch 80kg per ton
fibre (8%). The following results were obtained.
Addition Point Starch
Retention
Starch No Polymer No Bentonite 74
Starch before Polymer 81.4
Starch with Polymer 96.5
Starch after Polymer 82.6
Starch with Bentonite 93.1
These results indicate that best results are obtained
when starch is mixed with the polymer (followed by the
bentonite). Useful retention is also obtained when the
polymer is added separately and the starch is subsequently
added with the bentvnite. Addition of the starch by
itself, before or after the polymer, gives poor results.
Examble 4
A process broadly as in Example 3 was repeated
comparing the retention (measured as in Example 3) at 4%,
6% and 8% starch when there is no polymer and bentonite
(control) or when the starch is added with 750g/t polymer
followed by 2,OOOg/t bentonite.
WO 95/33096 PCT/GB95/01259
Starch Retention i~ Starch
Addition System Retention
4% No 57.9
4% Yes 99.4
5 6% No 63.1
6% Yes 83.7
8% No 71.2
8% Yes 90.5
The amount of starch added based on the volume of the
10 suspension at the 4%, 6% and 8% amounts based on the weight
of fibre was 200, 300 and 400ppm respectively.
Example 5
A process broadly as described in Example 3 was used
except that in the three tests conducted using polymer in
15 the absence of anionic microparticulate material the starch
was added with the polymer solution to the drainage jar
with the stirrer set at 800rpm and after 10 seconds mixing
the backwater was collected for 30 seconds. Starch
retention was measured as in Example 3.
20 The results were as follows:
Product Dosage % Starch
Retention
Polyethylene imine 1,OOOg/t 72.6
Polyamine epichlorhydrin l,OOOg/t 78.3
10 mol % DMAEAq/90 mol %
25 acrylamide copolymer IV 12 750g/t 92.5
10 mol % DMAEAq/90 mol % 750g/t 91.7
acrylamide IV 12 followed plus
by polysilicic acid 50og/t
These results clearly demonstrate the greatly improved
30 retention that is attainable using high IV cationic polymer
compared to low molecular weight cationic polymers. They
also show that good results can be obtained using
polysilicic acid as the anionic microparticulate material
but direct comparison between the two tests with the
WO 95/33096
PCT/GB95/01259
31
cationic polyacrylamide is not wholly reliable because of
the different conditions used for the tests.
