NO321456B1 - Method of making paper on a paper machine - Google Patents

Abstract

Paper strengthened by starch, typically in amounts of 3 to 15% dry weight, is made by flocculating a cellulosic suspension using a polymeric retention aid which can be dissolved cationic starch but is preferably a synthetic polymer having IV above 4dl/g, shearing the resultant flocculated suspension and reflocculating it by adding bentonite or other microparticulate anionic material, draining the flocculated or reflocculated suspension and drying the resultant wet sheet, 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 polymeric retention aid.

Description

This invention relates to the production of paper which is reinforced by starch.

It is common practice to produce paper on a paper machine by providing a cellulosic thin pulp suspension, flocculating the suspension by adding a solution of polymeric retention agent thereby forming a flocculated suspension, draining the flocculated suspension through a moving screen to form a moist sheet, and advancing the sheet through a heated drying zone and thereby forming a dry sheet. The retention agent can be dissolved cationic starch, but is often a synthetic polymer material. Although the use of fairly low molecular weight polymer may provide some improvement in retention, the polymer is preferably of high or very high molecular weight, generally with an intrinsic viscosity of over 4 dl/g.

A common alternative to this method involves cutting off the flocculated suspension so that the flocs are broken down and then adding an aqueous suspension of microparticulate anionic material thereby reflocculating the suspension, and then draining the flocculated suspension through the sieve. Such methods using cationic starch and colloidal silica are described in U.S. Pat. patent no. 4388150, and methods using cationic synthetic polymer and bentonite are described in EP-A-235893. Methods where glue is added after the flocculation with the cationic polymer are described in EP-A-499448. Methods using other polymers and suspensions suitable for these are described in WO95/02088.

The cellulosic thin pulp is often formed in part from recycled paper which may contain soluble starch (cationic or anionic or non-ionic) and so the thin pulp, and the finished sheet, often contain soluble starch. E.g. the dry sheet may contain as much as 1% starch derived from recycled paper. However, it is often desirable to add starch to the thin mass.

Accordingly, water-soluble cationic starch may be added as part or as the entirety of the solution of the polymeric retention agent (see, e.g., U.S. Patent No. 4,388,150). The quantity required for this purpose is usually no more than approx. 0.3%

(dry weight of starch based on the dry weight of paper).

It is often desirable to add starch to strengthen the paper. It is e.g. it is particularly desirable to include significant amounts of starch in corrugated paper and covering paper. These materials are usually largely 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 because the inclusion of significant amounts of filler would otherwise tend to reduce the sheet's strength!

In order to maximize strength, it is desirable to include starch in amounts of as much as 5 or 10% or even more, but attempts to achieve this tend to make the process less efficient in terms of energy consumption and/or rate of manufacture, or may involve the risk of unacceptable increase in the chemical oxygen demand of the effluent from the process, due to increased starch in the effluent.

Various grades of starch are conveniently commercially available

and includes qualities which are usually insoluble in the cellulosic suspension. They can be used either unmodified or chemically modified. In general, the starch is predissolved at a high temperature to make the starch soluble in the cellulosic suspension.

When it is said in this description that a starch is insoluble, it is meant that it

is insoluble in the cellulosic suspension and remains undiluted in the cellulosic suspension. When it is said that a starch is soluble, it is meant that it is soluble in the cellulosic suspension.

Soluble cationic starch is quite essential for the cellulose fibers in amounts of up to approx. 1 to 1.5 percent starch by weight, 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 retained in the paper, and instead only an increase in the amount of soluble cationic starch in the tailwater dewatered through the screen. This is undesirable because it must be removed before discharge as effluent, due to the high chemical oxygen demand it can create in the effluent from the factory.

The soluble cationic starch can be produced by chemical modification of starch or only by boiling raw starch and adding a low molecular weight cationic polymer before, during or after boiling. Suitable cationic polymers with a low molecular weight have an intrinsic viscosity of less than 1 dl/g! Examples of such systems are provided in CA Patent No. 787294 and U.S. Pat. patent no. 3930877.

When starch is used as a reinforcing agent, it is usually necessary in practice to also include a polymeric retention agent, and various publications have been published regarding the addition of combinations of materials.

E.g. in Tappi, June 1976, 59, 6, pages 120 to 122, the performance of various two-polymer systems, including the performance of a mixture of a soluble cationic starch and hydrolyzed polyacrylamide, is investigated. In CA patent no. 1232713, up to 1.5% soluble cationic starch is used in combination with polyethylene oxide or a cationic, non-ionic or anionic polyacrylamide retention agent with a molecular weight of over 1 million.

In Tappi Journal, February 1984, pages 104 to 108, the effect of various mixtures of soluble cationic starch and polymers is examined, and it is stated that cationic starches at 1% by weight improve drainage and retention, but that drainage is adversely affected at higher levels. It has been stated that an ideal polymer for a paperboard factory in the case of silent cutting appears to be a cationic polymer with a low molecular weight and high charge density, particularly polyethyleneimine.

In normal commercial practice, it has been found that if the amount of cationic starch is increased above approx. 1 or 1.5%, there is an increased risk that the cationic starch will interfere with the effectiveness of the polymeric retention agent. As a result, retention and drainage may be impaired with the consequence that the machine must be operated more slowly or that product quality is degraded.

When it is desired to include a greater amount of starch than 1 to 1.5%, common technique involves applying an unmodified starch solution to a sizing press at the end of the paper machine, i.e. after partial or complete drying of the sheet. The application of a solution of starch at this point can lead to high pick-up (eg up to 7 or 10% is common). However, this can lead to the starch being concentrated more on the surface than in the middle of the sheet, and it has the particular disadvantage that it necessitates re-drying the sheet, which entails a waste of heat energy and/or a reduction in the process speed. It would therefore be desirable to be able to achieve these or higher starch levels without supplying the tailwater with unacceptable levels of soluble starch and without having to re-dry the sheet.

Another known method of providing substantial stocks of starch in the paper involves applying a shower or foam containing undissolved starch particles to the moist sheet before passing it through the dryer, followed by boiling the starch during drying. This method also has the advantage that it seeks to produce a higher concentration of starch on the surface than in the middle of the sheet. Its particular disadvantage, however, is that it is very difficult to achieve even application of the starch when applied with a shower or foam for longer periods due to the tendency of the starch composition to cause blockages in the shower or foam application devices.

Attempts to include cold water-insoluble particulate starch in the suspension prior to drainage have been suggested in the literature, but have not met with success. E.g. Fowler, in Paper, 1978, pages 74 and 93, outlined the general technique for adding starch. He discussed the above techniques and also stated that if raw, uncooked starch is added to the suspension followed by the addition of retentive agent, only minimal retention of starch can be achieved. He suggested that better retention is achieved if the starch is slurried with bentonite and added to the suspension before the retention agent, and he also suggested that retention can be increased further by including in the slurry a polymer with a charge opposite to the charge of the retention agent.

In the U.S. patent no. 4347100 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 gummy precipitate that 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 to achieve the desired stoichiometric amount. He also believes that optimum strength can be achieved by including ungelatinized starch which is gelatinized during subsequent heat drying.

He states that the reaction of the cationic and anionic polymer to produce a gummy precipitate carries the particles of starch and retains the starch in the wood fibers. He says that the pulp composition is then fed to the paper machine where it is formed into a sheet and heat-dried. This suggests that the starch is added to the wood pulp or the thick pulp. In all examples the wood pulp had a consistency of 2.3%, but Brucato suggests that higher consistencies are desirable. All strengths are measured on hand sheets. He gives no information on whether the method could have been carried out on a paper machine, nor how this could have been carried out, nor what degree of retention can be achieved with starch.

Brucato describes in the U.S. patent no. 4609432 another method for obtaining reinforced paper, this time using two different cellulosic suspensions. 90 to 98% of the fiber weight is provided by a first cellulosic suspension, usually of refined fiber, and 2 to 10% of the fiber weight is provided by adding to this first suspension a second cellulosic suspension containing a heat-sensitive binder (such as uncooked starch) for binding the fibers, and a polymer for gluing the binder to the fibers in the second suspension. The second suspension can e.g. contain the other cellulosic fibers together with 20 to 200% uncooked dry starch and 0.01 to 0.1% cationic polymer. The cationic polymer is said to coat the particles of starch and stick them to the fibers in the second suspension. A typical process uses a first suspension containing 95% of all fibers and a second suspension containing 5% of the fibers, 0.012% polyethyleneimine and 20% starch. A hand sheet was formed from this and then dried, and it turns out that the starch is activated during drying. Again, nothing is implied either about how the method is to be carried out on a machine or about retention.

Brucato mentions the same list of cationic polymers in both patents, namely polyethyleneimines (preferred in U.S. Patent No. 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 because all the polymer classes he mentions except the polyacrylamides are inevitably very low molecular weight, and the polyacrylamide he uses as an example is Separan CP7, a trademark of Dow Chemical Co., and it is believed that this material also has a relatively low molecular weight of approximately 1 million.

It is not suggested in any of Brucato's patents that something should be used. additional retention agent. The stoichiometric reaction to form a precipitate in the U.S. patent no. 4347100 will prevent the cationic polymer from acting as an effective retention agent. The total amount of polyethyleneimine used in the examples in the U.S. patent no. 4609432 may be sufficient to effect flocculation of the second suspension, but will be far too small to effect flocculation of the combined suspension. E.g. is the highest dose in the examples approximately 0.002% based on total fiber weight.

Brucato's methods therefore require particular interaction between low molecular weight cationic polymer and other material in the suspension, and do not lead to the formation of a flocculated or re-flocculated suspension of the kind that can be achieved using high molecular weight synthetic polymers or cationic starch alternatively followed by anionic microparticulate matter.

It is desirable to reinforce largely unfilled paper sheets (including cardboard) to be used as packaging, but there is also a particular need to include starch as a reinforcing agent in sheets that are heavily filled, because the use of a large amount of filler tends to weaken the sheet. The filler may be pre-flocculated before it is added to the cellulosic suspension. Although this has some advantages, it can cause particular weakening of the sheet. It is therefore known technique to include water-soluble starch in the pre-flocculated filler composition, but this creates difficulties when handling the flocculated suspension.

In GB patent no. 2223038 filler is included in a cellulosic suspension by adding a slurry of filler, insoluble starch particles and flocculant. Although many of the flocculating agents mentioned have very low molecular weight (eg Magnafioc 1597 is a polyamide), some have a moderate molecular weight. Suspending agent such as a rubber, a synthetic organic polymer, or a swelling clay (eg bentonite) can be included and the suspending agent is preferably chosen so that the net charge in the composition is reduced to near zero. If it e.g. If a cationic flocculant is used, 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, the amount of starch in the final paper being said to be typical 0.05 to 1.5%. The resulting flocculated suspension will contain the particles of starch trapped in the filler flocs, and it is added to the cellulosic suspension which is then dewatered and heated, with subsequent boiling 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 an appropriate technique which utilizes readily available starch, which does not lead to undesirable contamination of the effluent, which is capable of providing a very high pick-up of starch in the paper and which does not involve the problems of adhesive press application or shower or foam application onto the damp sheet.

To the best of our knowledge, the proposals of Fowler, Brucato and in GB Patent No. 2223038 have not led to satisfactory methods of making sheets containing a large amount of starch as a result of incorporating all the starch in suspension prior to drainage. Consequently, the problem remains that if large amounts of starch are to be incorporated, this must be added to the moist sheet by showering or foaming or by the glue press, and there remains an important need to find a way to incorporate starch into the thin mass to thereby allow efficient and environmentally acceptable production of paper with a high starch content.

Support for the opinion that such a method is not known comes from the fact that, after the priority date of this application, in the Nordic Pulp and Paper Research Journal No. 4 1994, pages 237 to 241 it is stated that because starch has come into shape with a diameter of about. 1 to 40 u, the retention of the starch comets is very low when added directly to pulp without dissolution or swelling in water. According to the proposals in this article, it is possible to include large amounts of starch in hand sheets in the laboratory by including in the cellulose-containing suspension starch with a specific flake shape and which is produced by precipitation in mineral salts and processing of the precipitation. It is commercially undesirable to have to undergo this particular process and it would be much more appropriate to be able to achieve high starch levels in paper produced on a conventional paper machine using common granular starches and without creating significant effluent problems due to too much drainage of starch through the strainer.

The problem to be solved with this invention is therefore the provision of a method where it is possible to include starch in the thin mass of

such a way that relatively large amounts of starch can be retained in the paper without significantly disrupting the efficient production of the paper and without creating unacceptable effluent discharges.

The objectives of the present invention are achieved by a method for producing paper on a paper machine comprising

providing a cellulosic thin pulp suspension,

flocculation of the suspension by adding an aqueous solution of polymeric retention agent selected from dissolved cationic starch and synthetic polymer with an intrinsic viscosity above 4 dl/g and thereby forming a flocculated suspension,

shearing off the flocculated suspension and re-flocculating the sheared suspension by adding an aqueous suspension of microparticulate anionic material thereby forming a re-flocculated suspension,

draining the refioculated suspension through a moving screen to form a moist sheet, and

advancing the sheet through a heated drying zone and thereby forming a dry sheet, characterized in that the insoluble starch particles are added to the cellulosic suspension as a slurry of largely freely dispersed particles in all or part of the aqueous solution of the polymeric retention agent or in all or portions of the aqueous suspension of microparticulate anionic material, and

the insoluble starch particles are heated during drying and release soluble starch in the sheet in the presence of moisture.

Preferred embodiments of the method are further elaborated in claims 2 to 11 inclusive.

The invention may be practiced with or without the cutoff and reflocculation with microparticulate anionic material. If the reflocculation step is used, the particulate starch can be included in this suspension of microparticulate anionic material, alternatively also with polymeric retention agent.

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 cellulose fibers and, if present, the anionic microparticulate material. It is therefore desirable that the particles of starch are added as a slurry of largely independent particles so that the particles can interact with the fibers or microparticulate anionic material largely independently of each other.

The best results are obtained in the invention when the method includes the described cut-off and re-flocculation stages.

According to the invention, good retention of fibre, starch particles (and filler if present) is achieved, i.a. the reflocculation stage. The application of cutting off the flocculated suspension containing the cellulose fibers and particles of starch leads to the breakdown of flocs in the flocculated suspension and redispersion of the previously flocculated material. As a result, flocks of starch particles or of fiber free of starch particles will tend to break up at the cut. The consequence of this is that a very even distribution of the individual starch particles is achieved in the refioculated suspension, and consequently in the unwatered sheet. As a result of this uniformity, the gelatinization during drying can be carried out more efficiently and the distribution of the starch in the sheet both before gelatinization and after gelatinization can be more uniform than if the process is carried out without the cut-off and reflocculation.

Although it is preferred that the slurry of the polymeric retention agent be added in a form in which the starch particles are substantially freely dispersed therein, some flocculation of the starch slurry may be acceptable when the resulting flocculated cellulosic suspension is sheared and then reflocculated, because this shearing will break up initial flocs in the original slurry. The slurry may include some filler or fiber. In all methods according to the invention, the slurry largely consists only of the polymeric retention agent and the insoluble starch particles.

The paper produced can be filled, and an advantage of the invention is that paper with good strength can be obtained even when it contains large amounts of filler, e.g. more than 20% by weight or more than 40% by weight and up to 60% by weight based on the dry weight of the paper. Conventional fillers such as calcium carbonate, sulphate, talc, kaolin or other clays can be used.

Another very important feature of the invention is that it enables the production of unfilled paper, i.e. paper with little or no intentional addition of filler. This largely unfilled paper generally has a filler content of no more than 15, and usually no more than 10, percent by weight of the dry sheet. Generally, any filler included will be derived from recycled paper used to form the cellulosic suspension, but if desired small amounts may be deliberately added to the suspension, e.g. up to 5 or perhaps 10 percent by weight based on the dry weight of the suspension. The invention is therefore of particular value in the production of corrugated paper or covering paper.

A distinctive characteristic of the invention is that a high starch content can be achieved in the dry sheet as a result of the inclusion of the undissolved starch in the cellulosic suspension without causing contamination problems. Consequently, a content of at least 2 or 3 and typically 5 and even up to 10 or 15 percent by weight of starch in the dry sheet can easily be achieved.

A high retention of the starch particles is preferably achieved (e.g. over 80% or 90% or more), and starch particles that are dewatered in the tail water can be tolerated as they may be insoluble in the tail water and thus can be recovered and collected during a subsequent passage through the machine. Alternatively, they can be removed by filtration before draining.

When the method is carried out by draining the flocculated suspension, this suspension may be formed in a conventional manner (except for the addition of starch). It can e.g. be produced from an abrasive, mechanical or thermomechanical wood pulp and the thin pulp, or the thick pulp from which it is formed, can be treated with bentonite before adding the retention agent. In such processes, the retention agent is often largely non-ionic, e.g. formed from 0 to 10 mole percent anionic and/or cationic monomers and 90 to 100 mole percent nonionic monomers. In this aspect, however, the invention is not limited to the use of dirty wood pulps and includes the use of any suitable combination of wood pulp and high molecular weight retention agent (anionic, non-ionic or cationic) or dissolved cationic starch retention agent.

In these processes, the retention agent and the starch are usually added after

last point for high intercept, e.g. in or immediately before the inlet box.

In the preferred method according to the invention, the flocculated suspension is subjected to shearing so that the original flocs are broken down, and is then reflocculated or subjected to supercoagulation by the addition of anionic microparticulate material. The cut-off can only be achieved as a result of turbulent flow from the point where the retention agent is added to the point where the microparticulate material is added, but often the cut-off is applied by passing through a device such as e.g. a center strainer, mixing pump or other intentional cut-off mixing stage. The cut-off leads to a reduction in the size of the flocks, e.g. as described in EP-A-235893.

The starch particles can then be added with the anionic microparticulate material. As a result of intimate mixing of the starch particles and this material, the starch particles appear to become trapped in the supercoagulation that takes place upon addition of the microparticulate material, and as a result. good retention of the starch particles is achieved. When the starch is added with the microparticulate material, the slurry of starch and microparticulate material is usually free of other substantial solids phase and generally consists largely only of water, the microparticulate material, the starch and dispersant or other additives necessarily associated with the microparticulate material. The ratio of dry weight of starch to microparticulate matter is generally in the range of 5:1 to 100:1, often around 10:1 to 50:1, by weight.

The starch particles are typically injected into a slurry of the microparticulate material, or the microparticulate material is injected into a slurry of the starch particles, just prior to addition to the cellulosic suspension, although if desired the materials may be premixed and the resulting slurry pumped from the mixing station towards the point of addition. The point of addition is usually in the inlet box or at another position after the last point of significant cutoff because it is usually desired that the refioculated or supercoagulated structure should not be too degraded by subsequent cutoff prior to drainage.

It is usually preferred to introduce the starch particles as a slurry in all or part of the aqueous solution of retention agent. This enables the retention agent to be absorbed or otherwise attached to the surface of the starch particles before the particles are mixed with the cellulosic suspension. As a result of using a high molecular weight retentive agent, or the less preferred dissolved cationic starch, the absorbed retentive agent promotes bridging between the starch particles and the cellulosic fibers, thereby promoting retention.

The starch can be pre-suspended in the aqueous solution of. retention agent, but it is generally convenient to mix the insoluble starch (usually as an aqueous slurry) and the aqueous retention agent as they flow toward a point where the retention agent is added to the cellulosic suspension. The starch can e.g. is introduced into the polymer stream at a point between the polymeric additive supply and the point where the solution is added to the cellulosic suspension. It is often appropriate to mix the starch particles in the solution just before the point where the solution is added to the cellulosic suspension.

Often the starch is initially provided as a slurry of 10 to 40 and often around 20 percent by weight starch in water, and this slurry is added to the polymeric solution in the amounts required to provide the selected dose of polymer and starch. The dry weight ratio of starch:polymer is often in the range of 50:1 to 500:1. Often the slurry contains from 1 to 50 (preferably 10 to 30) weight percent starch particles and 0.01 to 2 weight percent polymer.

Although the slurry added to the cellulosic thin pulp suspension may include other materials, it is generally preferred and appropriate that the slurry consists substantially only of the polymer, the starch and water. The amount of polymer is generally significantly above the amount which, under relatively static conditions, could have a significant flocculating effect on the starch particles. Accordingly, if the selected amount of polymer is gradually added to an aqueous medium containing the selected amount of starch with gentle 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 largely freely dispersed in that they do not cluster together as significant flocks. In practice, the addition of the slurry of starch and polymer is normally accompanied by cut-off at the point of addition, and this will further promote the independent nature 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 clumping of the particles is acceptable. Confinement of the starch particles in flocks of filler or fiber in the slurry is, however, undesirable.

It is important that the polymer that is added is an effective retention agent for the cellulose-containing suspension, so that the polymer that is absorbed onto the starch particles will have the appropriate materiality for the cellulose fibers in the suspension. Selection of a suitable retention agent which is material to the cellulosic suspension can be carried out in the usual way. It can be anionic, nonionic or cationic. The best results are usually obtained when the retention agent is cationic and the suspension is preferably one on which the selected cationic retention agent is material.

It is usually appropriate and preferred that the starch is added as. a slurry with all the retention agent to be used for flocculation of the suspension, alternatively prior to cut-off and re-flocculation, but if desired the slurry can only be mixed with a part, e.g. at least 5 and often at least 25 percent by weight, typically up to 50 or 75 percent by weight, of the total amount of retention agent. If retentive agent is added partially mixed with particulate starch and partially free of starch, different high molecular weight retentive agents may be used for the two additions provided they are compatible, or the same material may be used for each addition.

Polymer of the coagulating type with a low molecular weight can, if necessary, be added at an earlier stage in a known manner, but this is not considered a retention agent in connection with the present invention.

Such coagulating polymers usually have an intrinsic viscosity below 3 dl/g and often below 1 dl/g. They may have a high cationic charge density, preferably above 4, and often above 5, meg/g. The low molecular weight polymer is preferably formed from recurring units of which at least 70%, and generally at least 90%, are cationic. Preferred polymers are low molecular weight homopolymers of diallyldimethylammonium chloride and copolymers thereof with a small amount (usually below 30% and preferably below 10%) of 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%) of acrylamide, polyethyleneimines, polyamines, heptchlorohydrindiamine condensation products, dicyandiamide polymers and other conventional low molecular weight cationic coagulant polymers.

The retention agent with which the particulate starch is mixed prior to addition to the cellulosic suspension may be soluble cationic starch, and thus the system may consist of insoluble starch particles (usually chemically unmodified insoluble starch particles) suspended in a solution of cationic starch. However, it is generally preferred that the retention agent is a synthetic polymer.

The preferred retention agents for use in the invention are polymers with an intrinsic viscosity of over 4 dl/g and usually over 6 dl/g, for example 8-15 dl/g or 8-20 dl/g or higher.

In this specification, intrinsic viscosity is measured at 25°C in 1M sodium chloride buffered at phi 7 using a "suspended liver viscometer".

Nonionic retention agents that can be used include polyacrylamide or other polymer of water-soluble ethylenically unsaturated monomer or monomer mixture, and polyethylene oxide.

Suitable anionic retention agents are polymers of anionic ethylenically unsaturated sulfonic ether carboxylic monomer such as acrylic acid (usually as a sodium salt or other water-soluble salt) alternatively copolymerized with nonionic ethylenically unsaturated monomer such as e.g. acrylamide. Consequently, the anionic polymer can be formed from e.g. 3 to 50 mole percent, often 3 to 20 mole percent anionic monomer such as sodium acrylate, the balance being acrylamide.

Amphoteric polymers containing both anionic and cationic monomer units can be used, usually with acrylamide or other nonionic monomer.

Cationic polymers are preferred.

The or each of the high molecular weight cationic polymers is usually a copolymer of ethylenically unsaturated cationic monomer, the balance being another water soluble, largely nonionic ethylenically unsaturated monomer such as acrylamide. The amount of cationic monomer is usually at least 2 or 3 mole percent. Generally, it is not more than 20 mole percent, but it can be up to 50 mole percent or more. The polymer may be completely water-soluble or it may be in the form of small particles of partially soluble cross-linked polymer as described in EP-A-202780.

The or each of the high molecular weight cationic polymeric retention agents typically has a theoretical cationic charge density of no more than about 3 meq/g, often no more than about 2 meq/g. Generally, it is at least about 0.1, or usually at least about 0.5, meg/g. In this description, the theoretical cationic charge density is the charge density obtained by calculation from the monomer composition that is intended to be used to form the polymer.

Suitable cationic monomers include dialkylaminoalkyl(meth)acrylates and acrylamides as acid addition or quaternary salts. Each of the alkyl groups can contain 1-4 carbon atoms and the aminoalkyl group can 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 that the retention agent has an intrinsic viscosity of more than 8 dl/g, in some cases it may be desirable to use as the retention agent a copolymer of diallyldimethylammonium chloride and acrylamide and which has an intrinsic viscosity of at least 4 dl/g, although it may be practical unfeasible to produce such a polymer with the value for limit viscosity (IV) of 8 dl/g and higher which is preferred for other polymers.

The total amount of polymeric retention agent is usually 0.01 to 1%, generally 0.02 to 0.1% (200 to 1000 grams per ton dry weight of suspension). When the process includes cut-off and re-flocculation with microparticulate material, the amount of retention agent is generally in the range of 0.01 to 0.06% or 0.1%, but when the process is carried out only with fiocculation followed by drainage, i.e. without the cutoff and reflocculation, the amount is usually in the range of 0.04 to 0.15%, often 0.06 to 0.1%.

The amount depends on, among other things of the choice of cellulose-containing thin pulp. This can be formed from a suitable wood pulp or mixture of wood pulps. The thin pulp typically has a cellulose fiber content of 0.2 to 2.0, usually 0.3 to 1.5 percent by weight.

The retention agent with a limit viscosity of over 4 dl/g (or cationic starch) and the quantity of it used in the process must be such that good retention of fine fiber particles and filler (if present) is provided. Selection of the retention agent and its amount can be accomplished in a conventional manner by carrying out the process in the absence of starch with different amounts of different retention agents, thereby selecting an effective combination of retention agent and its amount for the particular cellulosic suspension being treated. This test should of course be carried out with the subsequent addition of microparticulate anionic material when the overall process includes the use of this material. When the original cellulosic suspension includes anionic waste, it may be desirable to initially treat the suspension with a cationic coagulant and/or bentonite, thereby reducing the amount of polymeric retention agent required.

The amount of retention agent will always be greater than the amount required for precipitation or interaction with anionic soluble material in the cellulosic suspension. If the retention performance is plotted against polymer dose in a typical combination, one will see that retention will become poor as the dose increases, and will increase only gradually at low values, but will then increase significantly above a . relatively small dose range, and will not increase further to any significant extent. The dose where retention was markedly improved is an indication of the need in this suspension for this retention agent, and according to the invention the total amount of this retention agent should be at or above the amount where retention has increased significantly. Accordingly, this amount is above the stoichiometric amount required for reaction with an anionic polymeric material in the cellulosic suspension and a wood pulp from which it is formed. In general, the suspension is prepared without the intentional addition of anionic polymer materials.

By saying that the cellulosic suspension is flocculated, it is meant that it has the condition typical of a cellulosic suspension that has been treated with an effective high molecular weight retention agent in an effective amount.

In preferred methods, the retention system is selected and optimized (using high intrinsic viscosity polymer or dissolved cationic starch) for retention, drainage and drying properties in a conventional manner, and the particulate starch is injected into the polymer solution with no significant change in the optimal retention system.

The starch in the particles must remain largely undissolved before starting to drain the suspension, because dissolved starch is otherwise likely to be dewatered from the suspension. A simple way to determine whether or not the particles are maintained largely undissolved is to titrate the drainage water for dissolved starch. If the amount of dissolved starch in the drainage water is sufficiently low (after taking into account dissolved starch introduced with the fibers from e.g. recycled paper), this indicates that the particles are maintained largely undissolved. E.g. the amount of dissolved starch in the drainage water should preferably represent less than 20%, preferably less than 10% and preferably less than 5% of the amount of particulate starch in the suspension after deduction of soluble starch originating elsewhere.

One way to ensure that the particles remain largely undissolved prior to drainage is to introduce the starch in ungelatinized, largely water-insoluble form and to maintain the conditions in the suspension so that significant gelatinization does not take place prior to the start of drainage. In such a process, it is necessary to gelatinize the starch during the draining and drying stages.

In conventional processes, drainage is completed at temperatures above ambient, and drying is accomplished with the application of heat. By appropriate selection of the drainage and drying conditions and of the quality of ungelatinized starch, it is possible to achieve appropriate gelatinization during the drying stage, while the sheet is still moist. It may be desirable to supply the moist sheet with intentional heating, even before final drainage is completed, so that it is preheated prior to entering the drying stages. The damp sheet can e.g. performed under a fume hood or heater, e.g. a Devroniser (trademark), and this can facilitate full gelatinization and dissolution of the starch.

Cutting off the flocculated suspension prior to reflocculation will necessarily tend to break up flocks or aggregates of starch particles, and so this preferred process will tend to result in the starch particles being more evenly distributed as monoparticles throughout the sheet. As a result, more thorough gelatinization of these particles will take place than when clusters of particles are present in the sheet, and this is an important advantage of the preferred methods of the invention, which include cutting off and reflocculating the flocculated suspension.

It is necessary to gelatinize the starch particles while there is still some moisture in the sheet to allow gelatinization to proceed satisfactorily and to allow the particles to disperse in the sheet to thereby tend to form a film in the sheet, as opposed to just point bonds. As a result of the starch gelatinizing in the presence of moisture, it will seek to migrate between the fibers so that more uniform distribution of the starch on, around and between the paper fibers is achieved. The amount of moisture that should remain in the sheet when the starch dissolves can be quite small, and need only be sufficient to allow migration of the gelatinized starch sufficient to provide adequate distribution of the starch throughout the sheet.

To facilitate the achievement of rapid gelatinization, it may be desirable to use a starch which naturally has a low gelatinization temperature or which has been modified to reduce its gelatinization temperature, provided that it remains substantially undissolved prior to drainage.

Usually, the starch is an uncooked, raw starch such as e.g. raw corn, potato, com, wheat or tapioca starch.

Pregelatinized or precooked (and therefore soluble) starch may be included as insoluble particles. Accordingly, instead of relying on the insolubility of ungelatinized starch particles and the subsequent boiling that takes place in the process, the dissolution of preboiled starch in the particles of the suspension can be prevented by protecting the starch with a water-impermeable mantle or matrix that dissolves during the subsequent drainage or drying. Any material that provides sufficient water impermeability to prevent substantial dissolution of the starch prior to drainage may be used, provided that the jacket or matrix will dissolve to release the starch during drainage and/or drying.

The mantle or matrix need not provide long-term water impermeability. E.g. a slowly dissolving jacket or matrix may be sufficient to protect the starch because there may still be insufficient time for dissolution of the entrapped starch particle in the inlet box even if the jacket partially dissolves in the inlet box.

The sheath or matrix may be a thermoplastic material with a melting point such that premature dissolution of the sheath or matrix is prevented. The normal temperature of the suspension leading to the inlet box is e.g. typically in the range 40-50°C and the ambient temperature around the drainage strainer is typically in the same range. If the particles are provided with a coating or matrix having a melting temperature approximately at or above the inlet box temperature, substantially no melting will occur before the inlet box and most of the melting and substantially all of the dissolution of the starch will not occur until most of the drainage is complete. Suitable thermoplastic materials that can be used include hydrocarbon waxes.

Instead of using a thermoplastic sheath or matrix, a pH-sensitive sheath or matrix can be used. The cooked starch can e.g. be encapsulated or otherwise protected by polymer which is water-insoluble and non-swellable at the pH of the starch dispersion supplied to the factory, and this dispersion is added to the inlet box which is at a pH where the polymer swells or dissolves. E.g. the protective polymer may 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 preparation of pH-sensitive polymers of this general nature by oil-in-water emulsion polymerization is well known.

Methods for incorporating an active ingredient into particles of a protective matrix or into a mantle are well known and can be used in the invention. E.g. the mixture of the starch and protective material can be shower dried or a "coacervate" coating can form around starch particles.

The amount of starch 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 quantity is above 2 or 3, e.g. 5.10 or even up to 12 or 15 percent by weight. An advantage of the method according to the invention, however, is that the process can be operated either at high starch loads or low starch loads only by changing the amount of starch, without making significant changes to the rest of the process.

The size of the particles is mostly at least 90 percent by weight below 100 p, preferably below 50 u, often 5 to 50 u. The starch particles can have a size of at least 90 percent by weight up to 10 u, generally 5-10 u. The starch is preferably comet, so that all three dimensions can be roughly the same.

The anionic microparticulate or colloidal material (when used) is preferably bentonite, i.e. an inorganic swelling clay, e.g. as described in EP-A-235893. However, colloidal silica (such as described in U.S. 4643801), polysilicate microgel (such as described in EP-A-359552), polysilicic acid microgel as described in EP-A-348366, or aluminum- modified versions of these. Instead of using inorganic anionic colloidal material, organic material can be used. It is therefore possible to use an anionic organic polymer-containing emulsion. The emulsified polymer particles can be insoluble because they are formed from a copolymer of e.g. a water-soluble anionic polymer and one or more insoluble monomers such as ethyl acrylate, but preferably the polymer-containing emulsion is a cross-linked microemulsion of water-soluble monomer material. The particle size of the colloidal material is generally below 2 µm, preferably below 1 µm and most preferably below 0.1 u.

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 e.g. 2%, but is mostly below 1%. The selection and amount of the anionic colloidal material should be such as to effect what is often referred to as "supercoagulation".

The anionic microparticulate or colloidal material is preferably added to the suspension after the last high cut-off point, e.g. at the inlet box, and the suspension can then be dewatered in a conventional manner.

The initial choice of suitable materials can be made on the basis of experiments with conventional laboratory equipment such as e.g. a "Britt" jar and a hand-sheet technique, but commercial operation of the process is carried out on a paper machine where the cellulosic thin pulp is provided in a conventional manner, generally by diluting thick pulp with tail water, and is fed towards a head box through suitable device such as a mixing pump and center strainer, and is emptied from the inlet box onto a movable strainer.

This sieve can be moved at conventional sieve speeds which normally exceed 100 meters per second. minute and which is typically in the range of 700 to 1500 meters per minute.

The machine will include a drying zone in a conventional manner, but an advantage of the invention is that it is not necessary for the machine to be equipped with a glue press or other device for supplying starch to the moist sheet or to the dried sheet.

If desired, however, additional starch may be added to the moist sheet or the dried sheet in a conventional manner.

The following are examples.

Example 1

A factory trial was carried out on a Fourdrinier machine which produces corrugated sheet paper at 600 m/min from 100% recycled pulp composition. A cationic polymer of acrylamide with 10 mole percent cationic acrylate, IV 12 dl/g, was added to the thin mass in front of the center sieve at a dosage level of 800 g/ton. A 20% slurry of raw starch was added to the polymer feed just prior to the addition of the polymer to the thin stock, in sufficient amounts to provide 5% starch by dry weight of paper. Bentonite was added to the thin mass after the center sieve and just before the inlet box, at a dosage 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 gelatinize during drying.

Example 2

Covering paper with a weight of approximately 140 grams per square meters was produced on a Fourdrinier machine in a process where an aqueous solution of a polymer of acrylamide with 10 mole percent dimethylaminoethyl acrylate quaternary salt [DMAEAq], with IV 12 dl/g, was used as a retention agent, in a dose of 850 g/ton in the top layer and 790 g/tonne in the bottom layer, added before the center sieve, as well as bentonite in a dose of 5 kg/h in both the top and bottom layers added after the center sieve. The suspension included recycled paper and it was found that the starch content of the sheet, without the intentional addition of starch, varied between approximately 0.9 and 1.2%.

Particulate raw potato starch was then injected as a slurry into the polymer feed line at a dose of 1.42% based on the dry weight of the suspension. When stable conditions were restored, 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%, which again indicates largely complete retention.

The explosive strength was increased by approx. 35% and the CMT value by approx. 20%.

Example 3

To perform temporary screening of suitable material combinations, a recycled pulp composition was formed 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 pulp suspension at 25°C. It was left to condition for 24 hours. It had a pH of 7.5 to 7.7.

500 ml of thin stock was placed in a "Britt Dynamic Drainage Jar" fitted with a machine wire with the stirrer set at 1500 rpm. 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 1500 rpm, the stirrer was slowed to 800 rpm and the required amount of bentonite slurry was added. After 10 seconds of mixing, the tailwater was collected for 30 seconds.

The collected tailwater was boiled at 100°C for 30 minutes, the volume returned to the original volume and the sample centrifuged to remove fiber. 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 water's starch content. Because of. the particular analysis 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 mole percent dimethylaminoethyl acrylate quaternary salt, IV 12 dl/g) was added at 750 grains per tonnes of fibre, bentonite at 2000 grams per tonnes of fiber and starch 80 kg per tonnes of fiber (8%). The following results were obtained.

These results indicate that the best results are obtained when starch is mixed with the polymer (followed by the bentonite). Usable retention is also achieved when the polymer is added separately and the starch is then added with the bentonite. Adding the starch alone before or after the polymer gives poor results.

Example 4

A process largely as in Example 3 was repeated, where a comparison was made of the retention (measured as in Example 3) at 4%, 6% and 8% starch when there is no polymer and bentonite (regulation) or when the starch is added with 750 g/t polymer followed by 2000 g/t bentonite.

The amount of added starch based on the volume of the suspension at the amounts of 4%, 6% and 8% based on the weight of fiber was 200, 300 and 400 ppm respectively.

Example 5

A process was used broadly as described in Example 3, except that in the three tests conducted using polymer in the absence of anionic microparticulate matter, the starch was added with the polymer solution in the drain pan with the stirrer set at 800° /min, and after 10 seconds of mixing, the tailwater was collected for 30 seconds. Starch retention was measured as in Example 3.

The results were as follows:

These results clearly demonstrate the greatly improved retention that can be achieved using high IV cationic polymers compared to low molecular weight cationic polymers. They also show that good results can be achieved using polysilicic acid as the anionic microparticulate material, but direct comparison between the two tests with the cationic polyacrylamide is not fully reliable due to the various conditions used during the tests.

Claims (11)

1. Process for producing paper on a paper machine, comprising providing a cellulosic, thin pulp suspension, flocculation of the suspension by adding an aqueous solution of polymeric retention agent selected from dissolved cationic starch and synthetic polymer with an intrinsic viscosity (IV) above 4 dl/g and thereby forming a flocculated suspension, shearing off the flocculated suspension and re-flocculating the sheared suspension by adding an aqueous suspension of microparticulate anionic material thereby forming a re-flocculated suspension, draining the refioculated suspension through a moving screen to form a moist sheet, and advancing the sheet through a heated drying zone and thereby forming a dry sheet, characterized in that the insoluble starch particles are added to the cellulosic suspension as a slurry of largely freely dispersed particles in all or part of the aqueous solution of the polymeric retention agent or in all or part of the aqueous suspension of microparticulate anionic material, and the insoluble starch particles are heated during drying and release soluble starch in the sheet in the presence of moisture. 2. Method according to claim 1, characterized in that the anionic microparticulate material is selected from inorganic swelling clays, colloidal silicon, polysilicate microgels, polysilicic acid microgels and aluminum-modified colloidal silicon, aluminum-modified polysilicate microgel and aluminum-modified polysilicic acid microgel. 3. Method according to claim 1 or 2, characterized in that the paper is a filled paper. 4. Method according to claim 1 or 2, characterized by the fact that the paper is largely unfilled paper. 5. Method according to one of the preceding claims, characterized in that the starch in the particles is ungelatinized so that virtually no dissolution of starch in the suspension takes place prior to drainage, and the starch is gelatinized during the drainage and/or drying. 6. Method according to one of the preceding claims, characterized in that the retention agent is a synthetic polymer material with a limit viscosity of at least 4 dl/g. 7. Method according to one of the preceding claims, characterized in that the retention agent is a synthetic polymer material with an intrinsic viscosity of at least 8 dl/g. 8. Method according to one of the preceding claims, characterized in that the polymeric retention agent is cationic. 9. Method according to one of claims 1 to 3 or claims 5 to 8, characterized in that the sheet contains 20 to 60 percent by weight filler. 10. Method according to one of claims 1,2,4-8, characterized in that the sheet is a largely unfilled sheet which is corrugated paper or cover paper. 11. Method according to one of the preceding claims, characterized in that the amount of starch in the sheet is from 2 to 15%.