FI120631B - Production of paper - 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

PAPER PRODUCTION

This invention relates to the manufacture of paper reinforced with starch.

5

It is standard practice to make paper in a papermaking machine using a thin cellulose pulp suspension, flocculating the suspension, adding a solution of the polymeric retention aid, and thereby forming a flocculated suspension, such a dry sheet. The retention aid may be a dissolved cationic starch, but is often a synthetic polymeric material. Although the use of a relatively low molecular weight polymer may provide some improvement in retention, the polymer is preferably high or very high molecular weight, generally having an intrinsic viscosity of greater than 4 dl / g.

A common alternative to this method involves cutting the flocculated suspension to disintegrate the flakes, followed by adding an aqueous suspension of microparticulate anionic material, whereby the suspension is flocculated, and then the flocculated suspension is drained through a sieve. These processes using cationic starch and colloidal silica are described in U.S. Patent 4,388,150, and processes using cationic synthetic polymer and bentonite are described in EP-A-235,893. Processes of applying glue after flocculation with a cationic polymer are described in EP-A-499,448. Processes employing other suitable polymers and suspensions are described in WO95 / 02088.

Thin cellulose pulp is often formed from recycled paper which may contain soluble starch (cationic or anionic or nonionic), and thus the thin pulp and final sheet often contain soluble starch. For example, a dry sheet can contain up to 1% starch from 2 recycled papers. However, it is often desired to add starch to a thin paper pulp.

Thus, water-soluble cationic starch may be added as 5 parts of a polymeric retention aid solution or as a whole (see, for example, US 4,388,150). The amount required for this purpose is usually not more than about 0.3% (dry weight of starch based on dry weight of paper).

0 It is often desired to add starch to strengthen the paper. For example, it is particularly desirable to use significant amounts of starch in corrugated board and linerboard. These materials are usually substantially free of fillers and increasing their strength makes them more suitable as packaging materials. It is also desirable to use significant amounts of starch in the filler sheets, since the presence of significant amounts of filler would otherwise tend to reduce the strength of the sheet.

It is desirable to use starch as much as 5 or 10% or even more to maximize strength, but companies to this end tend to render the process less efficient in terms of energy savings and / or production rate, or may risk undue increase in chemical oxygen demand. increase in process effluent because of the higher amount of starch.

Various types of starch are readily available commercially, such as grades which are usually insoluble in a cellulose suspension. They can be used either unmodified or chemically modified. Generally, the starch is pre-solubilized at high temperature to render the starch soluble in the cellulose suspension.

In this specification, when we say starch is insoluble, we mean that it is insoluble in the cellulose suspension and remains insoluble in the cellulose suspension. When the starch is said to be soluble, we mean that it is soluble in a cellulosic suspension.

Soluble cationic starch is relatively substantive to cellulosic fibers in amounts up to 1-1.5% by weight of starch based on the dry weight of the paper. If the amount of cationic starch in the suspension increases significantly above this, there may be little or no increase in the amount of starch remaining in the paper, and instead, only the amount of soluble cationic starch in the circulating water flowing through the sieve will increase. This is not desired because it must be removed before being disposed of as effluent due to the high chemical oxygen demand it may generate in the effluent from the mill.

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Soluble cationic starch can be prepared by chemically modifying the starch or by simply boiling the crude starch and adding low molecular weight cationic polymer before, during or after cooking. So-0 day low molecular weight cationic polymers have an intrinsic viscosity of less than 1 dl / g. Examples of these systems are disclosed in CA 787,294 and US 3,930,877.

In practice, when starch is used as a reinforcing agent, it is usually also necessary to add a polymeric retention aid, and several publications have been reported on the addition of combinations of agents. For example, Tappi, June 1976, 59, 6, pages 120-122 investigates the performance of various binary polymer systems, such as a mixture of soluble cationic 0 starch and hydrolyzed polyacrylamide. CA 1, 232,713 uses up to 1.5% soluble cationic starch in combination with polyethylene oxide or a cationic, nonionic or anionic polyacrylamide retention aid having a molecular weight of more than 1 million.

5

In the Journal of February 1984, pages 104-108, the effect of various soluble cationic starch and polymer blends has been studied, and it has been found that cationic starches at 1% by weight improve runoff and retention, but that higher amounts of castings are adversely affected. It has been found that the ideal polymer for cardboard production at low shear appears to be a cationic, low molecular weight, high charge density polymer, especially polyethyleneimine.

In normal commercial use, it has been found that increasing the amount of cationic starch by more than about 1 or 1.5% increases the risk that the cationic starch will interfere with the effectiveness of the polymeric retention aid. As a result, retention and runoff can be degraded, resulting in slower machine operation or product degradation.

L5 If larger amounts of starch than 1-1.5% are desired, it is a common technique to add an unmodified starch solution to an adhesive press at the end of the papermaking machine, i.e.. after the sheet has been partially or completely dried. Use of the starch lysate solution at this point can result in high accumulation (for example, up to 7 or 10% is common). However, it can result in the starch being more concentrated on the surface of the sheet than in the center, and has the particular disadvantage of requiring the sheet to be re-dried, thereby wasting thermal energy and / or slowing down the process. Therefore, it would be desirable for k5 to be able to achieve these or higher starch levels without using unsuitable amounts of soluble starch in the tap water and without having to re-dry the sheet.

Another known method for obtaining significant amounts of starch on paper is to apply a spray or foam containing insoluble starch particles to a wet sheet before passing through the dryers and then boiling the starch during drying. This process also has the disadvantage that it tends to produce higher starch contents on the sheet surface than its center. However, its particular disadvantage is that it is very difficult to cause the starch to spread evenly by spraying or foaming over a longer period of time because the starch composition tends to clog the spray or foam spreaders.

Attempts have been made in the literature to attempt to add cold water-soluble particulate starch to the suspension prior to dewatering, but attempts have not been successful. For example, Fowler described a general technique for adding starch in Paper 1978 on pages 74 and 93. He described the above technique and also stated that if a raw starch is added to the suspension and then a retention aid is added, only minimal starch retention can be achieved. He suggested that better retention be obtained by slurrying the starch with bentonite and adding to the suspension before the retention aid, and he eh-5 also suggested that the retention can be further increased by using a polymer with a charge opposite to that of the retention aid.

In U.S. Patent No. 4,347,100, Brucato describes that mechanical and thermo-mechanical pulping processes can be improved by adding an anionic surfactant or anionic polymer during the pulping process. He states that the addition of a cationic polymer causes a reaction with the anionic polymer and the formation of a gummy precipitate which affects the strength, and he recommends the addition of a stoichiometric amount based on the anionic polymer. He describes a titration method to obtain the desired stoichiometric amount. He also proposes that optimum strength can be achieved by using non-gelatinized starch which is gelatinized during the subsequent drying step.

He states that the reaction of cationic and anionic polymers to obtain a gummy precipitate transports the starch particles and holds the starch in the wood fibers. He says the pulp is then taken to a papermaking machine where it is formed into a sheet and heat-dried. On this basis, the starch is added to the pulp or to the thick pulp. In all examples, the consistency of the pulp was 2.3%, but higher consistency is desirable by Brucato. The strengths are all measured on hand sheets. He gives no information as to whether the process could be carried out on a papermaking machine, nor how this would be done, nor the amount of reten-5 thion that could be achieved by the starch.

In U.S. Patent 4,609,432, Brucato describes another method for obtaining stronger paper, this time using two different cellulose suspensions. 90 to 98% by weight of the fiber is obtained with a first-grade cellulosic suspension, usually refined fibers, and 2-10% by weight of the fiber is obtained by adding to the first suspension a second cellulosic suspension containing a heat-sensitive binder (such as uncooked starch) sex with the fibers of the second suspension. The second suspension may, for example, contain other cellulose fibers together with 20-200% of uncooked dry starch and 0.01-0.1% of a cationic polymer. The cationic polymer is said to cover the starch particles and glue them to the fibers of the second suspension. In a typical process, a first suspension containing 95% of the total fiber content and a second suspension containing 5% of the fibers, 0.012% of polyethyleneimine and 20% of starch are used. This was used to form a handsheet and then dried to reveal that ak-5 starch is drying during drying. Again, there is no indication of how the process is performed on the machine nor on retention.

Brucato mentions the same list of cationic polymers in both patents, namely polyethyleneimines (which are preferred in U.S. Patent 4,609,432), polyamide-polyamine resins, urea-formaldehyde resins, melanin-formaldehyde resins, and polyacrylamides. It seems that Brucato prefers to use low molecular weight polymers because all the polymer classes he mentions except for polyacrylamides 5 inevitably have a very low molecular weight, and as an example of polyacrylamides, he mentions Separan CP7, a trade name of Dow Chemical Co., and the substance has a relatively low molecular weight of about 1 million.

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There is no suggestion in either patent of Brucato that any additional retention aid should be used. The stoichiometric reaction to form a precipitate in U.S. Patent No. 4,347,100 prevents the cationic polymer from acting as an effective retention aid. The total amount of polyethyleneimine used in the examples of U.S. Patent 4,609,432 may be sufficient to cause flocculation of the second suspension, but too low to cause flocculation of the combined suspension. The highest exemplified dosage is about 0.002% based on the total weight of the fibers 0.

The Brucato methods therefore require special interaction between the low molecular weight cationic polymer and the other agent in suspension, and do not result in the type of vaporized or unflaked suspension obtained by the use of high molecular weight synthetic polymers or cationic starches.

It is desirable to substantially reinforce non-filler sheets of paper (including cardboard) to be used for packaging, but in particular they must also contain starch as a reinforcing agent in sheets containing a large number of fillers, since the use of large amounts of filler tends to weaken the sheets. The filler may be pre-flaked before being added to the cellulose suspension. While this has some benefits, it can cause a particular sheet of deterioration. It is therefore known to use water soluble starch in a pre-flocculated filler composition, but this causes difficulties in handling the flocculated suspension.

In British Patent 2,223,038 the filler is added to the cellulose suspension by adding a filler slurry, insoluble starch particles and a flocculant. While many of the said flocculants have very low molecular weights (for example, Magnaf loc 1597 is a polyamine), some have a moderate molecular weight. A suspending agent such as rubber, a synthetic organic polymer, or a swelling clay (e.g., bentonite) may be used, and preferably the suspending agent is selected to reduce the net charge in the composition to near zero. For example, if a cationic flocculant is used, anionic suspending agent-5 is usually required. The amount of filler in the composition is preferably 30-40%, and the amounts of starch and flocculant (based on the filler) are preferably 1-5% and 0.05-0.2%, respectively, the amount of starch in the final paper is typically said to be 0.05-1. 5%. The resulting flocculated suspension contains .0 starch particles trapped among the filler flakes and added to the cellulose suspension which is then drained and heated to boil the starch. In the examples, the amount of filler is in the range of 7-24% and the amount of starch is 4% based on the filler essence, i. 0.3-1% based on paper. .5

Thus, none of these methods provide any practical solution to the problem of obtaining a convenient method using readily available starch, which does not result in undesirable effluent contamination, and which provides 0 very high starch accumulation in the paper, and which does not involve problems with the adhesive press or or applying foam to a damp sheet.

As far as we know, the proposals in Fowler, Brucato and GB Patent 5,223,038 have not led to satisfactory processes for the preparation of sheets containing high amounts of starch as a result of adding the entire amount of starch to the suspension prior to draining. Thus, the problem remains that if large amounts of starch are to be added, 0 must be added to the wet sheet by spraying or foaming or in an adhesive press, and there remains an urgent need to find a way to add starch to thin pulp to produce an efficient and environmentally acceptable production method Contains a lot of starch.

5

Supporting our belief that such a process is not known is given by the fact that, following the priority date of this application, it is stated in Nordic Pulp and Paper Research 9

Journal, No. 4, 1994, pages 237-241 that since starch has a granular form with a diameter of about 1-40 µπι, the retention of starch granules is very low when added directly to the pulp without dissolving in water or swelling in water. According to the suggestions in this article, it is possible to use large amounts of starch in laboratory hand sheets by using starch in a cellulosic suspension having a special flake-like form, which is prepared by precipitation as mineral salts and by processing the precipitate. It is highly undesirable to go through such a special process, and it would be much more convenient to be able to obtain high levels of starch on paper made in a conventional papermaking machine using conventional granular starches without causing significant effluent starch casting.

Thus, the problem solved by the present invention is to obtain a process in which it is possible to add starch to a thin pulp so that relatively large amounts of starch can be retained in the paper without significantly affecting the efficient production of the paper and without generating inappropriate waste water effluents.

According to a first aspect of the present invention, we prepare a pap-25 in a papermaking machine by a method of forming a thin cellulose pulp suspension, flocculating the suspension by adding an aqueous solution of polymeric retention aid selected from solubilized cationic starch and synthetic polymer suspending, optionally cutting the flocculated suspension and re-flocculating the sheared suspension by adding an aqueous suspension of microparticulate anionic material, thereby forming a flocculated suspension, Thus providing a dry sheet, whereby insoluble starch particles are added to the cellulose suspension with a slurry of substantially freely dispersed particles and / or aqueous suspension of the microparticulate anionic agent, or insoluble starch particles are heated during drying and release the soluble starch into the sheet in the presence of moisture.

The first part of the invention may be carried out using surgery and flocculation with or without these microparticulate anionic agents. If the re-flocculation step is used, the particulate starch can be added to the suspension of the microparticulate anionic agent, optionally also in combination with the polymeric retention aid.

In order to promote good retention, it is necessary for the starch particles to interact with the surfaces of the cellulosic fibers and, if present, with the anionic microparticulate material. Therefore, it is desirable that the starch particles be added to the slurry of the substantially discrete particles so that the particles can interact with the fibers or the microparticulate anionic agent substantially independently of one another.

The best results are obtained in the invention when the process comprises the cutting and re-flaking steps described.

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A preferred second part of the invention is a process for making paper in a papermaking machine which comprises forming a thin suspension of cellulose pulp, flocculating the suspension by adding an aqueous solution of a polymeric retention aid selected from solubilized cationic starch and synthetic g to form a flocculated suspension, cutting the flocculated suspension and re-flocculating the shear suspension by adding an aqueous suspension of microparticulate anionic material 5 to thereby reconstitute the flocculated suspension, drain the flocculated suspension through a dry sieve, such a dry sheet wherein insoluble starch particles are added to the cellulose slurry suspension as a slurry in a portion of a solution of a polymeric retention aid or i throughout the solution, and insoluble starch particles are heated during drying to release the soluble starch into the sheet in the presence of moisture.

In these preferred processes, good retention of the fibers, starch particles (and filler, if present) is achieved by the flocculation step. The use of shear in a flocculated suspension containing saliva fibers and starch particles results in the disintegration of flakes in the flocculated suspension and the redispersion of the previously flocculated material. As a result, starch particle flakes or fiber flakes without starch particles tend to degrade by shearing. As a result, a very uniform distribution of the individual starch particles in the flocculated suspension and also in the drained sheet is achieved. Due to this uniformity, gelatinization during drying can be achieved more efficiently and the distribution of starch in the sheet both before and after gelatinization may be smoother than if the process is carried out without shear and flocculation.

Although it is preferred that the slurry of polymeric retention aid .0 be added in a form where the starch particles are substantially freely dispersed therein, some flocculation of the starch slurry may be acceptable when the resulting vaporized cellulose suspension is cleaved and 5 sludge. It is possible that the slurry contains some filler or fibers. Generally, in all of the processes of the invention, the slurry contains substantially only polymeric retention aid and insoluble starch particles.

The paper to be produced may contain a filler, and it is an advantage of the invention that papers having good strength can be obtained even when they contain large amounts of filler, for example, more than 20% by weight or more than 40% by weight or up to 60% by weight . Conventional excipients such as calcium carbonate or sulfate or talc or kaolin or other clays may be used.

Another very important feature of the invention is that it allows the production of filler-free paper, i.e., paper with little or no filler added. This substantially filler-free paper generally contains no more than 15% filler, and usually no more than 10% by weight of dry sheet. Typically, the filler present is derived from recycled paper used to form a cellulose lozenge suspension, but small amounts, for example up to 5% or perhaps 10% by weight based on the dry weight of the suspension, may be added to the suspension if desired. The invention is therefore particularly valuable in the manufacture of corrugated board or linerboard.

Thus, in the third part of the invention, we make essentially 5 filler-free corrugated board or linerboard on a papermaking machine by the method of forming a substantially filler-free thin cellulose pulp suspension, adding an aqueous polymeric retention aid selected from 15 draining the suspension through a movable screen to form a wet sheet and passing the sheet through a heated drying zone to form a dry sheet; they release soluble starch into the sheet in the presence of moisture.

In this part of the invention, the process can be carried out by draining the flocculated suspension obtained by adding a polymeric retention aid or by cutting and flocculating the flocculated suspension by adding an aqueous suspension of microparticulate anionic material, and then draining the resulting flocculated suspension.

A unique feature of the invention is that a high starch content in a dry sheet due to insoluble starch in the cellulose suspension can be achieved without causing problems of contamination. Thus, a starch content of at least 2% or 3% and typically 5% and even 10 or 15% by weight can easily be obtained in a dry sheet.

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According to a fourth aspect of the invention, paper is made in a papermaking machine by a process of forming a cellulosic suspension, flocculating the suspension by adding an aqueous solution of a polymeric retention aid selected from solubilized cationic starch and a synthetic polymer having an optionally cutting the flocculated suspension and re-flocculating the shear suspension by adding an aqueous suspension of microparticulate anionic material to thereby re-form the flocculated suspension, drain the flocculated or re-flocculated suspension through a mobile sieve to form a wet sieve, such a drying sheet, wherein insoluble starch particles are added to the cellulose suspension over 3% by weight based on the dry weight of the suspension, and and starch particles remain in the wet sheet at least 3% of the dry weight of the sheet and heat the insoluble particles during drying to release the soluble starch into the sheet in the presence of moisture.

Advantageously, we achieve high retention of starch particles (e.g., greater than 80% or 90% or more), and starch particles that do not enter the circulation water may be retained because they may be insoluble in the circulation water and thus trapped and trapped next time they pass through the machine. through. Alternatively, they may be removed by filtration prior to disposal.

A preferred way of carrying out this fourth part of the invention is to add the starch as a slurry in part or all of the aqueous polymeric retention aid solution or part or all of the aqueous microparticulate anionic suspension. However, other ways can be used. For example, the particles may be sprayed or otherwise coated with a solution of the retention aid and added to the cellulose suspension before or after the addition of the remaining retention aid.

When the process is carried out by draining the flocculated suspension, this suspension may be formed in a conventional manner (except for the addition of starch). For example, it may be prepared from ground, mechanical or thermomechanical pulp and thin pulp, or the thick pulp from which it is formed may be treated with bentonite prior to addition of the retention aid. In these processes, the retention aid is often substantially nonionic, for example formed from 0 to 10 mol% anionic and / or cationic monomers and 90 to 100 mol% non-ionic monomers and. However, the invention is not limited in this respect to the use of soiled pulps and includes the use of any suitable pulp combination and high molecular weight retention aid (anionic, nonionic or cationic) or dissolved cationic starch retention aid.

In these processes, the retention aid and starch are usually added at the final stage of high-throughput, e.g., in the headbox or immediately before.

In preferred processes of the invention, the flocculated suspension is subjected to shear, whereupon the original flakes are disintegrated, and then flocculated or supercoagulated by the addition of an anionic microparticulate substance. Cutting may be effected solely by the turbulent flow at the point where the retention aid is added to the point where the microparticulate material is added, but often the cutting is effected by passing through a device such as a centrifugal sieve, impeller pump or other optional shear mixing step. The surgery results in a reduction in the size of the flaps, for example as described in EP-A-235,893.

The starch particles can then be added with the anionic microparticulate material. As a result of thorough mixing of the starch particles and this substance, the starch particle-0. Coils appear to be trapped in the supercoagulation that occurs when microparticulate material is added and a good retention of the starch particles is obtained. When the starch is added together with the microparticulate material, the starch and the microparticulate slurry will usually not contain any other significant solid phases and will usually consist essentially of water, microparticulate material, starch and dispersant or other additives necessary for the microparticulate matter. The dry weight ratio of starch to microparticulate: 0 is generally in the range of 5: 1 to 10: 1, often in steps of 10: 1 to 50: 1 by weight.

Typically, the starch particles are injected into a slurry of microparticulate material, or the microparticulate material is injected into a slurry of starch particles, just prior to addition to the cellulosic suspension, although if desired, the materials may be premixed and pumped from the mixing station. The insertion site is usually the headbox or some other point after the last point of the substantial surgery, since it is usually desirable that the re-flocculated or supercoagulated structure should not be disintegrated by the subsequent subsequent surgery prior to draining.

It is usually advantageous to introduce the starch particles as a slurry in 5 parts of the aqueous retention aid or in the whole solution. This allows the retention aid to be absorbed or otherwise adhered to the surface of the starch particles prior to mixing the particles with the cellulose suspension. As a result of the use of a high molecular weight retention aid or less preferably dissolved cationic starch, the absorbed retention aid promotes crosslinking between the starch particles and the cellulosic fibers and thus promotes retention.

The starch may be pre-slurried in an aqueous solution of the retention aid, but it is generally sufficient to mix insoluble starch (usually in the form of an aqueous slurry) and aqueous retention aid when they flow towards the point where the retention aid is added to the cellulose suspension. For example, starch may be injected into the polymer stream at some point between the polymer formation and the point where the solution is added to the cellulosic suspension. Often, it is sufficient to mix the starch parsules into the solution just before the solution is added to the cellulose suspension.

Often, the starch is initially added as a slurry of 10-40%, often in 20% by weight of starch in water, and this 0 slurry is added to the polymer solution in an amount sufficient to provide the selected dosage of polymer and starch. The dry weight ratio of starch: polymer is often in the range of 50: 1 to 500: 1. Many slurries contain 1-50% (preferably 10-30%) by weight of starch particles and 0.01-2% by weight of polymer.

5

Although the slurry added to the thin cellulose pulp suspension may contain other substances, it is generally advantageous and convenient that the slurry essentially contains only polymer and starch and water. The amount of polymer is generally substantially greater than the amount that under relatively static conditions could have a significant flocculating effect on the starch particles. Thus, if the selected amount of polymer is gradually added with gentle stirring to an aqueous medium containing the selected amount of starch, some flocculation will initially be observed with the naked eye, but when further added and further blended, the starch particles will not substantially disperse, 18 significant fluctuations. In practice, the addition of starch and polymer slurry normally involves shear at the point of addition and this further enhances the particle retention properties. As for the shear which tends to occur at the insertion site and for the preferred shear of the flocculated cellulose suspension following the addition of the polymer and the particles, some degree of particle aggregation is acceptable. However, binding of starch particles to filler or fiber flakes in the slurry is not desirable.

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It is important that the polymer to be added is an effective retention aid for the cellulosic suspension in the sense that the polymer adsorbed on the starch particles has sufficient substance affinity for the cellulosic suspension. Selection of a suitable reten-15 thio excipient, which is a substantive cellulosic suspension, can be carried out in a conventional manner. It may be anionic, nonionic or cationic. The best results are generally obtained when the retention aid is cationic and thus preferably the suspension is one for which the selected cationic retention aid is substantive.

It is usually convenient and advantageous for the starch to be added as a slurry with the entire retention aid to be used to flocculate the suspension, optionally prior to surgery and flocculation, but if desired, the slurry may be mixed with only a portion, e.g. typically up to 50 or 75% by weight of the total amount of retention aid. If the retention aid is added in part mixed with the particulate starch 30 and partially in the absence of starch, separate high molecular weight retention aids may be used for two additions, provided they are compatible, or the same agent may be used for each addition.

The low molecular weight, coagulant-type polymer may be added at an earlier stage, if desired, in a known manner, but this is not considered as a retention aid in the present invention.

These coagulating polymers typically have an intrinsic viscosity of less than 3 dl / g and often less than 1 dl / g. They may have a high cationic charge density, preferably above 4, and often above 5 meq / g. The low molecular weight polymer is preferably composed of repeating units of which at least 70%, and generally at least 90%, are cationic. Preferred polymers are homopolymers of diallyl dimethylammonium chloride and its low molecular weight copolymers, together with a small amount (usually less than 30% and preferably less than 10%) of acrylic O-amide, low molecular weight homopolymers of with amounts (generally less than 30% and preferably less than 10%) of acrylamide, polyethyleneimines, polyamines, epi-5 chlorohydrin diamine condensation products, dicyandiamide polymers and other conventional low molecular weight cationic coagulating polymers.

The retention aid with which the particulate starch is admixed before being added to the cellulosic suspension may be a soluble cationic starch, and thus the system may consist of insoluble starch particles (usually non-chemically modified insoluble starch particle particles).

However, it is generally preferred that the retention aid is a synthetic polymer.

Preferred retention aids for use in the invention are polymers having an intrinsic viscosity of greater than 4 dl / g and typically greater than 6 dl / g, for example 8-15 dl / g or 8-20 dl / g or more.

* In this specification, the intrinsic viscosity is measured at 25 ° C in 1M sodium chloride buffer at pH 7 using a suspension level viscometer.

Nonionic retention aids that may be used include polyacrylamide or other polymer of water-soluble ethylene-unsaturated monomer or mixture of monomers, and polyethylene oxide.

Suitable anionic retention aids are polymers of anion-5 ethylenically unsaturated sulfonic or carboxylic acid monomers, such as acrylic acid (usually a sodium or other water soluble salt) optionally copolymerized with a nonionic ethylene unsaturated monomer such as. Thus, for example, the anionic polymer may be formed from 0 to 3 to 50 mole percent, often 3 to 20 mole percent, of an anionic monomer such as sodium acrylate, the remainder being acrylamide.

Amphoteric polymers containing both anionic and .5 cationic monomer units, usually with acrylamide or other nonionic monomer, may also be used.

Cationic polymers are preferred.

The high molecular weight cationic polymer, or any such, is usually a copolymer of an ethylenically unsaturated cationic monomer in combination with another water soluble, generally nonionic, ethylene unsaturated monomer such as acrylamide. The amount of cationic monomer is usually 5 at least 2 or 3 mol%. Generally, it is not more than 20 mole percent, but may be up to 50 mole percent or more. The polymer may be wholly water soluble or may be in the form of small particles of a partially soluble crosslinked polymer as described in EP-A-202,780.

0

The high molecular weight cationic polymeric retention aid or each of them typically has a cationic charge density of up to about 3 meq / g, often not more than about 2 meq / g. Generally, it is at least about 0.1, 5, or usually at least about 0.5 meq / g. In this disclosure, the theoretical cationic charge density is the charge density obtained from the monomer composition for use in polymer formation.

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Suitable cationic monomers include dialkylaminoalkyl (meth) acrylates and acrylamides, such as acid addition or quaternary salts. The alkyl groups may each contain 1 to 4 carbon atoms and the aminoalkyl group may contain 1 to 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 aid has an intrinsic viscosity of greater than 8 dl / g, in some cases it may be desirable to use a copolymer of diallyl dimethylammonium chloride and acrylamide as an retention aid, even if not it is convenient to prepare such a polymer having a 5 IV of 8 dl / g and above, which is advantageous to other polymers.

The total amount of polymeric retention aid is usually 0.01-1%, usually 0.02-0.1% (200-1000 grams per tonne of dry weight of sus-0 pension). When the process involves shearing and re-flocculation with a microparticulate material, the amount of retention aid is generally in the range of 0.01-0.06% or 0.1%, but when the process is carried out by flocculation alone and then drying, i. without surgery and flushing again, the amount is usually in the range 0.04-0.15%, often 0.06-0.1%.

The amount depends, among other things, on the choice of a thin pulp. This may be formed from any suitable pulp or 0 pulp mixture. The cellulosic fiber content of the thin pulp is typically 0.2 to 2.0%, usually 0.3 to 1.5% by weight.

The retention aid with an IV greater than 4 dl / g (or cationic starch) and the amount used in the process is such that good retention of the fine fibers and excipient (if present) is obtained. Selection of the retention aid and its amount can be carried out in the conventional manner by carrying out a different amount of different retention aids in the absence of starch to select an effective combination of the retention aid and its amount for the particular cellulose suspension being treated. Of course, this test should be performed by the subsequent addition of a microparticulate anionic agent when the entire process involves the use of that agent. When the original cellulose suspension contains an anionic waste, it may be desirable to first treat the suspension with a cationic coagulant and / or bentonite to reduce the amount of polymeric retention aid required.

0

The amount of retention aid is always greater than the amount required to precipitate or interact with the anionic soluble material in the cellulose suspension. If the retention event is graphically represented in the polymer dosage regimen in a typical combination, it is seen that as the dosage rises, the retention deteriorates and rises only at low values, but then rises significantly above relatively small dosage ranges, and then no longer rises significantly. The dosage at which retention 0 was significantly improved indicates the need for that retention aid for that suspension, and in the invention, the total amount of that retention aid should be equal to or greater than the amount at which retention is significantly increased. Thus, this amount is more than the stoichiometric amount required to react with the anionic polymeric material in the cellulose suspension as well as with the pulp from which it is formed. Generally, the suspension is prepared without intentional addition of the anionic polymeric material.

By saying that the cellulose suspension is flocculated, we mean that it is in a typical state for a cellulose suspension treated with an effective high molecular weight retention agent in an effective amount.

In preferred processes, the retention system is selected and optimized (using high IV polymer or dissolved cationic starch) for retention, drainage and drying properties in a conventional manner, and the particulate starch 23 is injected into the polymer solution without any significant change in the optimum retention system.

The starch in the particles must remain substantially insoluble before the suspension begins to drain, otherwise the dissolved starch tends to drain from the suspension. A simple way to determine if the particles have remained essentially insoluble is to titrate the starch dissolved in the runoff water. If the amount of dissolved starch in run-off water is sufficiently low (after allowing the dissolved starch to adhere to the fiber, for example, from recycled paper), this indicates that the particles are substantially insoluble. For example, the amount of starch dissolved in run-off water should preferably be less than 20%, preferably less than 10%, and most preferably less than 5% of the amount of particulate starch in the suspension after deducting soluble starch from elsewhere.

One way of making the particles substantially insoluble prior to draining is to convert the starch to a non-gelatinized, substantially water insoluble form and to maintain the conditions in suspension such that no significant gelatinization occurs before draining begins. In such a process, it is necessary to gelatinize the starch 5 during the draining and drying steps.

In conventional processes, the draining takes place at ambient temperatures and the drying is carried out using heat. By appropriately selecting the drainage and drying conditions and the quality of the non-gelatinized starch, it is possible to achieve suitable gelatinization during the drying step while the sheet is still moist. It may be desirable to use deliberate heating to a wet sheet, even before final draining, whereby it is preheated before passing to the drying stages.

For example, a wet sheet may be transported under a steam hood or heater such as Devroniser (Trademark) and this may facilitate the complete gelatinization and dissolution of the starch.

The shear of the flocculated suspension prior to flocculation will necessarily tend to disintegrate the flakes or aggregates of the starch particles, whereby this preferred process tends to cause the starch particles to be more uniformly distributed throughout the sheet. As a result, these particles are better gelatinised for all than when there are particle lumps in the sheet, and this is an important advantage of the preferred processes of the invention, which is related to cutting and re-flocculating the flocculated suspension.

The starch particles must gelatinize while there is still some moisture in the sheet, allowing the gelatinization to proceed satisfactorily, and allowing the particles to spread into the sheet so as to tend to form a film within the sheet, rather than merely blotching. As a result of the gelatinization of the starch in the presence of moisture, it tends to migrate into the fibers, thereby achieving a more even distribution of the starch on, around and between the paper fibers. The amount of moisture that should remain in the sheet when the starch dissolves can be quite low and should only be sufficient to allow sufficient migration of the gelatinized starch to obtain sufficient starch throughout the sheet.

To facilitate rapid gelatinization, it may be desirable to use starch which is naturally gelatinized at low temperature or modified to lower its gelatinization temperature, provided that it remains substantially insoluble prior to draining.

Typically, the starch is uncooked from raw starch, such as raw corn, potato, corn, wheat or tapioca starch.

The pregelatinised or pregelatinised (and therefore soluble) starch can be incorporated into insoluble particles.

Thus, instead of relying on the insolubility of the non-gelatinized starch particles and subsequent boiling during the process, the dissolution of the pre-cooked starch in the suspension particles can be prevented by protecting the starch with water-impermeable coat or matrix which degrades during subsequent dewatering. Any agent that provides sufficient water impermeability to prevent significant starch 1 opening prior to draining may be used provided that the 0 shell or matrix disintegrates to release starch during draining and / or drying.

The shell or matrix need not provide long-term water impermeability. For example, a slowly dissolving shell or matrix may be sufficient to protect the starch because even if the shell partially decomposes in the headbox, the sealed starch particle will still have insufficient time to dissolve in the headbox.

The shell or matrix may be a thermoplastic material having a melting point such that it prevents premature degradation of the shell or matrix. For example, the normal temperature of the headbox suspension is typically in the range of 40-50 ° C, and the ambient temperature at the stages of the drain screen is typically in the same range. If the particles have a coating or matrix having a melting point of about headbox temperature or higher, there is no substantial melting before the headbox, and most starch melting and substantially all dissolution does not occur until most of the 0 draining has occurred. Suitable thermoplastic materials which may be used are hydrocarbon waxes.

Instead of using a thermoplastic shell or matrix, a pH-sensitive shell or matrix may be used. For example, the cooked starch may be enclosed in a polymer or otherwise protected by a polymer which is water insoluble and non-swellable at the pH of the starch dispersion entering the mill, and this dispersion is added to a headbox at which the polymer swells or dissolves. For example, the protective apolymer may be a copolymer of water-soluble and water-insoluble ethylene-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 type by oil-in-water emulsion polymerization is well known.

Methods of incorporating the active ingredient into the protective matrix particles or shell are well known and can be used in the invention. For example, the mixture of starch and the protective material may be spray dried or a coacervate coating may be formed around the starch particles.

5

The amount of starch in the sheet is normally at least 0.05% and usually at least 0.2% by dry weight. The greatest benefits of the process are achieved when the amount is greater than 2 or 3%, for example 5%, 10% or even 12 or 15% by weight. However, the advantage 0 of the process of the invention is that the process can operate with either high starch loads or low starch loads simply by changing the amount of starch without any significant changes to the rest of the process.

The particle size is generally at least 90% by weight less than 100 μιη, preferably less than 50 μπι, often 5-50 μπι. The starch particles may have a size of at least 90% by weight up to 10 μπι, generally 5-10 μπι. The starch is preferably granular so that all three dimensions can be substantially the same.

The anionic microparticulate or colloidal material (when used) is preferably bentonite, i.e., an inorganic swelling clay as described in Example 5 of EP-A-235,893. However, it can be colloidal silica (as described in U.S. Patent 4,643,801), poly-silicate microgel (as described in EP-A-359,552), polysilicic acid microgel as described in EP-A-348,366, or 27 aluminum-modified versions of any of of these. Instead of using an inorganic anionic colloidal material, an organic material can be used. Thus, it is possible to use an anionic organic polymer emulsion. The emulsion polymer particles may be insoluble due to the fact that they are formed, for example, from a copolymer of a water-soluble anionic polymer and one or more insoluble monomers such as ethyl acrylate, but preferably the polymer emulsion is a cross-linked microemulsion 0 water soluble monomeric material. The colloidal material has a particle size of less than 2 μπι, preferably less than 1 μιη, and most preferably less than 0.1 μπι.

The amount of colloidal substance (dry weight based on dry weight of cellulose content-5 pension) is generally at least 0.03% and usually at least 0.1%. It can be up to 2%, for example, but is usually less than 1%. The choice and amount of the anionic colloidal agent should be such that it results in what is often called "supercoagulation".

0

Preferably, the anionic microparticulate or colloidal substance is added to the suspension after the final cutting site, for example in the headbox, and the suspension can then be drained in a conventional manner.

5

The initial selection of suitable materials can be made on the basis of tests carried out on conventional laboratory equipment such as a Britt's foot, and by hand-sheeting, but commercially the process is carried out in a papermaking machine using thin cellulose pulp in a conventional manner, generally diluting a thick pulp such as the impeller pump and centrifugal screen, and removed from the stern to the moving screen.

This sieve can run at conventional sieve speeds which are normally greater than 100 meters per minute and typically in the range of 700 to 1500 meters per minute.

5 28

The machine has a drying zone in the conventional manner, but the advantage of the invention is that the machine need not be equipped with an adhesive press or other means for applying starch to a dry sheet or a dried sheet.

5

However, if desired, additional starch may be applied to a wet sheet or a dried sheet in a conventional manner.

The following are examples.

0

Example 1

A factory test was carried out on a Fourdrinier machine, which produces corrugated board at a speed of 600 m / min, from 100% waste. The cationic polymer of acrylamide with 10% cationic acrylate, IV 12 dl / g, was added to the thin pulp before the centrifugal screen at a dose of 800 g / ton. Just before the polymer was added to the thin pulp, a 20% slurry of crude starch was added to the polymer line sufficient to provide 0-5% starch by dry weight of the paper. Bentonite was added to the thin mass after the centrifugal screen and just before the headbox at a dose level of 0.5%.

Analysis of the starch retained on the sheet showed that more than 5 95% of the added starch was retained on the sheet. Heating during machine drying steps caused Gela-tin starching during drying.

Example 2 0

Lining board weighing about 140 g / m 2 was prepared on a Fourdrinier machine by a process using 10 mole% aqueous dimethylaminoethyl acrylate quaternary salt 5 [DMAEAq] at 12 dl / g as a retention aid. 850 g / ton for the upper layer and 790 g / ton for the lower layer, added before the center screen and 5 kg / ton for both the upper and lower layers after the center screen. Suspension 29 contained recycled paper and it was found that the starch content in the sheet without intentional starch addition ranged from about 0.9 to 1.2%.

Particulate crude potato starch was then sprayed as a slurry to the polymer feed line at a dosage of 1.42% of the dry weight of the suspension. When the steady state conditions were reestablished, the starch content in the sheet was 2.49%, indicating a substantially complete retention of the particulate starch.

When the amount of particulate starch in the suspension had increased to 3.11%, the amount in the sheet had increased to 4.34%, and when the amount in the suspension had increased to 3.50%, the amount in the suspension had increased to 4.55%, again demonstrating substantially complete retention.

The burst strength had risen by about 35% and the CMT by about 20%.

A

U

Example 3

If desired, pre-screening of suitable combinations of materials was prepared by leaving 60% newsprint, 5 30% carton and 10% magazines, and was prepared in a cellulosic pulp lab diffuser for 20 minutes and then diluted to give a 0.5% thin pulp suspension at 25 ° C. :in. It was left to wet for 24 hours. It had a pH of 7.5 to 7.7.

500 ml of thin pulp was placed in a Britt Dynamic drain pan equipped with a machine wire with a stirrer at 1500 rpm. The required amount of 20% starch slurry was mixed with the required amount of 0.5% polymer solution and added to the drain pan. After stirring for 60 seconds at 1500 rpm, the agitator speed was reduced to 800 rpm and the required amount of bentonite slurry was added. After stirring for 10 seconds, zero water was collected for 30 seconds.

The collected blank water was boiled at 100 ° C for 30 minutes, the volume adjusted again to its original volume, and the sample centrifuged to remove the fibers. The acidified potassium iodide / iodine reagent was added and the blue starch / iodine complex was determined optically and compared to a calibration curve to give an indication of the starch content of water. 10 Due to the analytical technique used, the values are more relative value than absolute values, but the increase in value indicates a higher retention.

In the first set of tests, polymer (acrylamide with 10-15 mol% quaternary salt of dimethylaminoethyl acrylate, IV 12 dl / g) was added at 750 grams / tonne fiber, 2000 g / tonne bentonite and 80 kg / tonne starch (8%). . The following results were obtained.

20 Addition point Starch retention Starch, no polymer, no benton. 74 Starch Before Polymer 81.4 Starch With Polymer 96.5 Starch After Polymer 82.6 25 Starch With Bentonite 93.1 These results show that the best results are obtained when starch is blended with the polymer (after bentonite). A useful retention is also obtained when the polymer 30 is added separately and the starch is then added with the bentonite. Addition of starch as such, before or after the polymer, gives poor results.

35 31

Example 4

A process very similar to Example 3 was repeated comparing the retention (measured as in Example 3) with 4%, 6% and 8% starch in the absence of polymer and bentonite (control), or when 750 g of starch was added. / t of polymer, followed by addition of 2000 g / t of bentonite.

0 Starch Increase Retention System% Starch Retention 4% No 57.9 4% Yes 99.4 6% No 63.1 6% Yes 83.7 5 8% No 71.2 8% Yes 90.5

The amount of starch added was based on the volume of the suspension at 4%, 6% and 8% based on the weight of the fiber and was 0 200, 300 and 400 ppm, respectively.

Example 5 A process similar to that described in Example 5 3 was used, except that in three experiments performed using polymer without anionic microparticulate material, starch was added to the polymer solution in a drain pan at 800 rpm, and after 10 seconds, zero water was collected. The retention of starch 0 was measured as in Example 3.

32

Product Dosage% starch.

retention

Polyethyleneimine 1000 g / ^ 72.6

Polyamine epichlorohydrin 1000 g / t 78.3 5 10 mol% DMAEAq / 90 mol% acrylamide copolymer. IV 12 750 g / t 92.5 10 mol% DMAEAq / 90 mol% 750 g / t 91.7 acrylamide IV 12 followed by 500 g / t polysilicic acid

-------------- --- ......... -__ I

0 ------------------------------ 1 These results clearly show much better retention that can be obtained using a high IV cation polymer as compared to low molecular weight cationic polymers. They also show that good results can be obtained using polysilicic acid as an anionic microparticulate agent, but direct comparison between the two experiments on cationic polyacrylamide is not completely reliable because different conditions were used in the experiments.

0

Claims (14)

A process for making paper in a papermaking machine comprising forming a thin cellulose pulp suspension, the suspension is flocculated by adding an aqueous solution of a polymeric retention aid selected from dissolved cationic starch and synthetic polymer with an internal viscous polymer. site above 4 dl / g, whereby a flocculated suspension is formed, characterized in that the flocculated suspension is cut and the foamed suspension re-flocculated by adding a water suspension of microparticle-shaped anionic material, and thereby forming a flocculated suspension, the re-flocculated suspension is drained through a moving screen, forming a wet sheet, and passing the sheet through a heated drying zone, thereby forming a dry sheet, wherein unsolved starch particles are added to the cellulose suspension as a slurry of substantially freely dispersed particles in some or all of the aqueous polymeric retention solution. solvents or in some or all of the aqueous suspension of microparticulate anionic material, and the unsolvated starch particles are heated during drying and give off soluble starch in the sheet in the presence of moisture. Process according to claim 1, characterized in that the insoluble starch particles are added to the cellulose suspension as a slurry of substantially freely dispersed particles in part or all of the aqueous solution of the polymeric retention aid. 3. A process according to claim 1 or 2, characterized in that the anionic micro-particulate agent is selected from inorganic swelling clays, colloidal silica, polysilicate microgels, polysilicic acid microgels and aluminum-modified colloidal silica-modified silica, aluminum modified 4. A process according to claim 1, characterized in that the flocculated suspension is drained through a sieve and the insoluble starch particles are added to the cellulose suspension as a slurry of substantially freely dispersed particles to some or all of the aqueous solution of the polymeric retention medium. . Process according to any of the preceding claims, characterized in that the paper is a filler containing paper. Process according to any one of claims 1-4, characterized in that the paper is a substantially filler-free paper. 15 Process according to any of the preceding claims, characterized in that the starch in the particles is non-gelatinized, so that essentially no dissolution of starch in the suspension takes place before the draining, and the starch is gelatinized during the draining and / or drying. 20 Process according to any of the preceding claims, characterized in that the retention aid is a synthetic polymer material whose internal viscosity is at least 4 dl / g. Process according to any of the preceding claims, characterized in that the retention aid is a synthetic polymer material whose internal viscosity is at least 8 dl / g. Process according to any of the preceding claims, characterized in that the polymeric retention aid is cationic. 30 11. A process according to claim 10, characterized in that the insoluble starch particles are added as a slurry to substantially the entire solution of polymeric retention agent. Method according to any one of claims 1 to 5 or 7 to 11, characterized in that the sheet contains 20 - 60% by weight of filler. Method according to any of claims 1-4 or 6-11, characterized in that the sheet is a substantially filler-free sheet, which is corrugated cardboard or coverboard. Process according to any of the preceding claims, characterized in that the amount of starch in the sheet is 2-15%.