This invention relates to the production of paper
which is strengthened by starch.
It is standard practice to make paper on a paper-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 WO95/02088.
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 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 for instance U.S. 4,388,150). The amount required
for this purpose is usually not more than about 0.3% (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 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
1dl/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
performance 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 1% by weight improve drainage and retention
but that at higher levels drainage is adversely affected.
It is stated that 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 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
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 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 the pulp or to the thick stock. In all the examples
the pulp 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 cellulosic suspensions. 90 to 98% of the fibre
weight is provided by a first cellulosic suspension,
usually of 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 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 is 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 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 preflocculated
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%
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 40im 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 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, the problem to be solved by 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, we make paper on
a paper-making machine by a process 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 IV
above 4dl/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
thereby forming a reflocculated suspension, draining the flocculated or re-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, wherein insoluble particles of starch are added to
the cellulosic suspension as a slurry of substantially
freely dispersed particles in part or all of the aqueous
solution of the polymeric retention aid or in part or all
of the aqueous suspension of micro-particulate 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 can be conducted
with or without the shearing and reflocculation with
micro-particulate anionic material. If 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 at least 4dl/g and thereby forming a
flocculated suspension, shearing the flocculated suspension and
reflocculating the sheared suspension by adding an aqueous
suspension of micro-particulate 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 in part or all 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
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 4dl/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 wherein the insoluble particles of starch are heated
during the drying and release soluble starch into the
sheet in the 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
reflocculating 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 4dl/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
thereby 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
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.
In these processes the retention aid and starch are
usually 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
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
merely as a result of turbulent flow from the point at
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
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
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
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 cellulosic 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
cellulosic fibres in the suspension. Selection of an
appropriate retention aid that is substantive to the
cellulosic 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 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 3dl/g and often below 1dl/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.
The preferred retention aids for use in the
invention are polymers which have intrinsic viscosity
above 4dl/g and usually above 6dl/g, for instance 8-15dl/g
or 8-20dl/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
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 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 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 to have intrinsic viscosity above 8dl/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
4dl/g, even though it may not be practicable to
manufacture such a polymer to the IV 8dl/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 4dl/g (or cationic
starch) and the amount of it which is used in the process
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
anionic 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
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 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 still 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
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.
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 of the 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 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
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.
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
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 100ìm, preferably below 50ìm, often 5 to
50ìm. The starch particles may have a size of at leat 90%
by weight up to 10ìm, generally 5-10ìm. 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
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 2ìm, preferably
below 1ìm and most preferably below 0.1ìm.
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.
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 12dl/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 acrylamide with 10 mol % dimethylaminoethyl
acrylate quaternary salt [DMAEAq], having IV 12dl/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 it was found that the starch content in the
sheet, with no deliberate addition of starch, fluctuated
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.
500mls of thin stock was placed in a Britt Dynamic
Drainage jar fitted with a machine wire with the stirrer
set at 1500rpm. 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 800rpm 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
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
10 mol % dimethylaminoethyl acrylate quaternary salt, IV
12dl/g) was added at 750 grams per tonne fibre, bentonite
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 bentonite. Addition of the
starch by itself, before or after the polymer, gives poor
results.
Example 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 2,000g/t bentonite.
Starch Addition | Retention System | Starch Retention |
4% | No | 57.9 |
4% | Yes | 99.4 |
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 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
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.
The results were as follows:
Product | Dosage | Starch |
| | Retention |
Polyethylene imine | 1,000g/t | 72.6 |
Polyamine epichlorhydrin | 1,000g/t | 78.3 |
10 mol % DMAEAq/90 mol % acrylamide copolymer IV 12 | 750g/t | 92.5 |
10 mol % DMAEAq/90 mol % acrylamide IV 12 followed by polysilicic acid | 750g/t plus 500g/t | 91.7 |
These results clearly demonstrate the greatly
improved 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 cationic polyacrylamide is not
wholly reliable because of the different conditions used
for the tests.