Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
  • D21H23/06—Controlling the addition
  • D21H23/14—Controlling the addition by selecting point of addition or time of contact between components
  • D21H23/16—Addition before or during pulp beating or refining
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/21—Macromolecular organic compounds of natural origin; Derivatives thereof
  • D21H17/24—Polysaccharides
  • D21H17/28—Starch

Definitions

• This invention relates to the production of paper which is strengthened by starch.
• a common alternative to this process involves shearing the flocculated suspension so as to degrade the flocs and then adding an aqueous suspension of micro-particulate anionic material and thereby reflocculating the suspension, and then draining the reflocculated suspension through the screen.
• Such processes using cationic starch and colloidal silica are described in U.S. 4,388,150 and processes using cationic synthetic polymer and bentonite are described in EP-A-235,893.
• Processes in which size is added after the flocculation with the cationic polymer are described in EP-A-499,448.
• Processes using other polymers and suspensions suitable for these are described in WO95/02088.
• the cellulosic thin stock is often formed in part from recycled paper which may include soluble starch (cationic or anionic or non-ionic) and so the thin stock, and the final sheet, often includes soluble starch.
• the dry sheet may contain as much as 1% starch derived from recycled paper. It is, however, often desired to add starch to the thin stock.
• water soluble cationic starch may be added as part or all of the solution of polymeric retention aid (see for instance U.S. 4,388,150).
• the amount required for this purpose is usually not more than about 0.3% (dry weight starch based on the dry weight of paper).
• starch it is often desired to add starch in order to strengthen the paper. For instance it is particularly desirable to include significant amounts of starch in fluting medium and liner board. These materials are usually substantially unfilled and increasing their strength makes them more suitable for use as packaging materials. It is also desirable to include significant amounts of starch in filled sheets as the inclusion of significant amounts of filler would otherwise tend to reduce the strength of the sheet.
• starch in amounts of as much as 5 or 10% or even higher, but attempting to achieve this tends to make the process less efficient as regards energy consumption and/or rate of production, or can incur the risk of unacceptable increase in the chemical oxygen demand of the effluent from the process, because of increased starch in the effluent.
• starch Various grades of starch are conveniently commercially available and include grades which are usually insoluble in the cellulosic suspension. They can be used either unmodified or chemically modified. Generally the starch is pre-solubilised at high temperature to render the starch soluble in the cellulosic suspension.
• a starch insoluble we mean that it is insoluble in the cellulosic suspension and remains undissolved in the cellulosic suspension.
• a starch is soluble we mean it is soluble in the cellulosic suspension.
• Soluble cationic starch is reasonably substantive to the cellulosic fibres in amounts up to about 1 to 1.5% by weight of the starch, based on the dry weight of the paper. If the amount of cationic starch in the suspension is increased significantly above this, there may be little or no increase in the amount of starch which is retained in the paper and, instead, there is merely an increase in the amount of soluble cationic starch which is in the white water which drains through the screen. This is undesirable since it has to be removed before discharge as effluent, because of the high chemical oxygen demand that it may create in the effluent from the mill.
• the soluble cationic starch can be made by chemical modification of starch or merely by cooking raw starch and adding a low molecular weight cationic polymer before, during or after the cooking.
• Suitable low molecular weight cationic polymers have intrinsic viscosity below 1dl/g. Examples of such systems are in CA 787,294 and U.S 3,930,877.
• an ideal polymer for a board mill at low shear appears to be a cationic, low molecular weight, high charge density polymer, in particular polyethylene imine.
• the usual technique involves applying an unmodified starch solution on a size press at the end of the paper-making machine, i.e., after partial or complete drying of the sheet.
• the application of a solution of starch at this point can result in high pick-up (for instance up to 7 or 10% is common).
• the starch being concentrated more on the surface than in the centre of the sheet and it has the particular disadvantage that it necessitates redrying of the sheet, thus wasting heat energy and/or slowing down the process. It would therefore be desirable to be able to achieve these or higher levels of starch without providing unacceptable levels of soluble starch in the white water and without having to redry the sheet.
• Another known method for providing significant loadings of starch in the paper involves applying a spray or a foam containing undissolved starch particles on to the wet sheet before it is carried through the driers, followed by cooking the starch during drying.
• This process also has the disadvantage of tending to produce a higher concentration of starch on the surface than in the centre of the sheet.
• its particular disadvantage is that it is very difficult to achieve uniform application of the starch by spraying or foam application for prolonged periods because of the tendency of the starch composition to cause blockages in the spray or foam applicators.
• Fowler reviewed the general techniques of adding starch in Paper 1978 pages 74 and 93. He discussed the techniques mentioned above and also stated that if raw uncooked starch is added to the suspension followed by the addition of retention aid only minimal retention of starch can be achieved. He proposed that better retention is achieved if the starch is slurried with bentonite and added to the suspension prior to the retention aid, and he also proposed that retention can be increased further by including in the slurry a polymer having a charge opposite to the charge of the retention aid.
• Brucato describes in U.S. 4,609,432 another method of obtaining strengthened paper, this time using two different cellulosic suspensions.
• 90 to 98% of the fibre weight is provided by a first cellulosic suspension, usually of refined fibres, and 2 to 10% of the fibre weight is provided by adding to this first suspension a second cellulosic suspension which contains a heat-sensitive bonding agent (such as uncooked starch) for bonding the fibres and a polymer for adhering the bonding agent to the fibres of the second suspension.
• a heat-sensitive bonding agent such as uncooked starch
• the second suspension can contain the second cellulosic fibres together with 20 to 200% uncooked dry starch and 0.01 to 0.1% cationic polymer.
• the cationic polymer is said to coat the starch particles and adhere them to the fibres of the second suspension.
• a typical process uses a first suspension containing 95% of the total fibres and a second suspension containing 5% of the fibres, 0.012% polyethylene imine and 20% starch. A hand sheet was formed from this and was then dried and it appears that the starch is activated during the drying. Again there is no indication about how to conduct the process on a machine nor about retention.
• Brucato quotes the same list of cationic polymers in both patents, namely polyethylene imines (which are preferred in U.S. 4,609,432), polyamide polyamine resins, urea formaldehyde resins, melamine formaldehyde resins and polyacrylamides. It seems that Brucato wants to use low molecular weight polymers since all the classes of polymers he mentions except for the polyacrylamides inevitably have very low molecular weight and the polyacrylamide he exemplifies is Separan CP7, a trade mark of Dow Chemical Co., and we believe that this material also has a relatively low molecular weight, of about 1 million.
• the stoichiometric reaction to form a precipitate in U.S. 4,347,100 will prevent the cationic polymer acting as an effective retention aid.
• the total amount of polyethylene imine used in the examples of U.S. 4,609,432 may be sufficient to cause flocculation of the second suspension but will be much too low to cause flocculation of the combined suspension. For instance the highest dosage which is exemplified is around 0.002% based on total fibre weight.
• the Brucato methods therefore require particular interaction between low molecular weight cationic polymer and other material within the suspension and do not result in the production of a flocculated or reflocculated suspension of the type that is attainable by the use of high molecular weight synthetic polymers or cationic starch optionally followed by anionic microparticulate material.
• filler is included in a cellulosic suspension by adding a slurry of filler, insoluble starch particles and flocculating agent.
• flocculating agents which are mentioned have very low molecular weight (for instance Magnafloc 1597 is a polyamine) some have a moderate molecular weight.
• Suspending agent such as a gum, a synthetic organic polymer, or a swelling clay (e.g., bentonite) can be included and preferably the suspending agent is chosen so as to reduce the net charge in the composition close to zero. For instance if a cationic flocculant is used then an anionic suspending agent is usually required.
• the amount of filler in the composition is preferably 30 to 40%, and the amounts of starch and flocculant (based on filler) are preferably 1 to 5% and 0.05 to 0.2% respectively, with the amount of starch in the final paper being said to be typically 0.05 to 1.5%.
• the resultant flocculated suspension will contain the starch particles trapped in the filler flocs, and it is added to the cellulosic suspension which is then drained and heated, with consequential cooking of the starch.
• 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.
• the problem to be solved by this invention is the provision of a method in which it is possible to include starch in the thin stock in such a way that relatively large amounts of starch can be retained in the paper without interfering significantly with efficient production of the paper and without creating unacceptable effluent discharges.
• This first aspect of the invention can be conducted with or without the shearing and reflocculation with micro-particulate anionic material. If the reflocculation step is being used, then the particulate starch can be included in that suspension of microparticulate anionic material, optionally also with polymeric retention aid.
• the particles of the starch should be able to interact with the surfaces of the cellulosic fibres and, if present, the anionic microparticulate material. It is therefore desirable for the starch particles to be added as a slurry of substantially independent particles so that the particles can interact with the fibres or microparticulate anionic material substantially independent of each other.
• a preferred, second, aspect of the invention is a process for making paper on a paper-making machine which comprises
• starch particles (and filler if present) is achieved by the reflocculation stage.
• the application of shear to the flocculated suspension containing the cellulosic fibres and the starch particles results in degradation of flocs in the flocculated suspension and redispersion of the previously flocculated material.
• any flocs of starch particles, or of fibres free of starch particles tend to be broken up by the shearing.
• the slurry of polymeric retention aid is added in a form wherein the starch particles are substantially freely dispersed in it, some flocculation of the starch slurry can be acceptable when the resultant flocculated cellulosic suspension is sheared and then reflocculated since this shearing will break up any initial flocs in the initial slurry. It is possible for the slurry to include some filler or fibres. Generally, in all processes of the invention, the slurry consists essentially only of the polymeric retention aid and the insoluble starch particles.
• the paper that is produced can be filled, and an advantage of the invention is that papers having good strength can be obtained even when they contain high amounts of filler, for instance more than 20% by weight or more than 40% by weight and even up to 60% by weight based on the dry weight of the paper.
• Conventional fillers such as calcium carbonate or sulphate or talc or kaolin or other clays can be used.
• Another very important feature of the invention is that it permits the production of unfilled paper, that is to say paper to which little or no deliberate addition of filler is made.
• This substantially unfilled paper generally has a filler content of not more than 15%, and usually not more than 10% by weight of the dry sheet.
• any filler which is included originates from recycled paper which is used in forming the cellulosic suspension but if desired small amounts, for instance up to 5% or perhaps 10% by weight based on the dry weight of the suspension can be deliberately added to the suspension.
• the invention is therefore of particular value for the manufacture of fluting medium or liner board.
• the process can be performed by draining the flocculated suspension which results from the addition of the polymeric retention aid or by shearing that flocculated suspension and reflocculating it by the addition of an aqueous suspension of micro-particulate anionic material, and then draining the resultant reflocculated suspension.
• a unique characteristic of the invention is that we can achieve a high starch content in the dry sheet as a consequence of the inclusion of the undissolved starch in the cellulosic suspension without causing pollution problems. Thus we can easily obtain a content of at least 2% or 3% and typically 5% and even up to 10 or 15% by weight starch in the dry sheet.
• starch particles e.g., above 80% or 90% or more
• any starch particles that do drain into the white water can be tolerated as they can be insoluble in the white water and so can be recycled and trapped on a subsequent pass through the machine.
• they can be removed by filtration before discharge.
• the preferred way of performing this fourth aspect of the invention is by including the starch as a slurry in part or all of the aqueous solution of polymeric retention aid or in part or all of the aqueous suspension of microparticulate anionic material.
• the particles may be sprayed or otherwise coated with a solution of the retention aid and added to the cellulosic suspension before or after adding the remainder of the retention aid.
• this suspension may have been formed in conventional manner (apart from the addition of starch).
• starch for instance it may have been made from a groundwood, mechanical or thermomechanical pulp and the thin stock, or the thick stock from which it is formed, may have been treated with bentonite before the addition of the retention aid.
• the retention aid is often substantially non-ionic, for instance being formed from 0 to 10 mole percent anionic and/or cationic monomers and 90 to 100 mole percent non-ionic monomers.
• the invention in this aspect, is not limited to the use of dirty pulps and includes the use of any suitable combination of pulp and high molecular weight retention aid (anionic, non-ionic or cationic) or dissolved cationic starch retention aid.
• the retention aid and starch are usually added after the last point of high shear, e.g., in or immediately prior to the head box.
• the flocculated suspension is subjected to shear so as to degrade the initial flocs and is then reflocculated, or subjected to super-coagulation, by the addition of anionic microparticulate material.
• the shearing can be achieved merely as a result of turbulent flow from the point at which retention aid is added to the point at which the microparticulate material is added, but often the shearing is applied by passage through a device such as a centriscreen, fan pump or other deliberate shear mixing stage.
• the shearing results in reduction of the size of the flocs, for instance as described in EP-A-235,893.
• the starch particles can then be added with the anionic microparticulate material.
• the starch particles appear to become entrapped within the supercoagulation that occurs upon the addition of the microparticulate material and as a result good retention of the starch particles is obtained.
• the slurry of starch and microparticulate material is usually free of any other significant solid phase and usually consists essentially only of water, the microparticulate material, the starch and any dispersing agent or other additives necessarily associated with the microparticulate material.
• the ratio dry weight of starch to microparticulate material is generally in the range 5:1 to 100:1, often around 10:1 to 50:1, by weight.
• the starch particles are injected into a slurry of the microparticulate material, or the microparticulate material is injected into a slurry of the starch particles, just before addition to the cellulosic suspension, although if desired the materials may be premixed and the resultant slurry pumped from the mixing station towards the addition point.
• the addition point is usually in the headbox or at some other position after the last point of substantial shear since it is usually desirable that the reflocculated or supercoagulated structure should not be degraded excessively by subsequent shear prior to drainage.
• the starch particles are usually preferred to introduce the starch particles as a slurry in part or all of the aqueous solution of retention aid. This allows for the retention aid to be absorbed or otherwise attached to the surfaces of the starch particles before the particles are mixed with the cellulosic suspension. As a result of using a high molecular weight retention aid or, less preferably, dissolved cationic starch the absorbed retention aid promotes bridging between the starch particles and the cellulosic fibres, and thus promotes retention.
• the starch may be preslurried in the aqueous solution of retention aid but it is generally adequate to mix the insoluble starch (usually as an aqueous slurry) and the aqueous retention aid as they flow towards a point at which the retention aid is added to the cellulosic suspension.
• the starch may be injected into the polymer stream at some point between the polymer make-up supply and the point where the solution is added to the cellulosic suspension. Often it is adequate to mix the starch particles into the solution just prior to the point at which the solution is added to the cellulosic suspension.
• the starch is provided initially as a slurry of 10 to 40%, often around 20%, by weight starch in water and this slurry is added into the polymer solution in the amounts required to give the chosen dosage of polymer and starch.
• the dry weight ratio of starch:polymer is often in the range 50:1 to 500:1.
• the slurry contains from 1 to 50% (preferably 10 to 30%) by weight starch particles and 0.01 to 2% by weight polymer.
• the slurry that is added to the cellulosic thin stock suspension can include other materials, it is generally preferred and convenient for the slurry to consist substantially only of the polymer and the starch and water.
• the amount of polymer is generally considerably above the amount which might, under relatively static conditions, have a significant flocculating effect on the starch particles.
• some flocculation may initially be visible to the naked eye but further addition of the polymer, accompanied by further mixing, will result in the starch particles becoming substantially freely dispersed in that they do not cling together as significant flocs.
• the addition of the slurry of starch and polymer is normally accompanied by shear at the addition point and this will further promote the independent character of the particles.
• some aggregation of the particles is acceptable.
• trapping the starch particles in flocs of filler or fibre in the slurry is undesirable.
• the polymer that is added is an effective retention aid for the cellulosic suspension in order that the polymer which is absorbed onto the starch particles will have adequate substantivity to the cellulosic fibres in the suspension.
• Selection of an appropriate retention aid that is substantive to the cellulosic suspension can be conducted in conventional manner. It can be anionic, non-ionic or cationic. Best results are usually obtained when the retention aid is cationic and so preferably the suspension is one onto which the selected cationic retention aid is substantive.
• the starch it is usually convenient and preferred for the starch to be added as a slurry with the entire retention aid that is to be used for flocculating the suspension, optionally prior to shearing and reflocculation, but if desired the slurry may be mixed with part only, for instance at least 5% and often at least 25% by weight, typically up to 50 or 75% by weight, of the total amount of retention aid. If retention aid is being added partly mixed with particulate starch and partly free of starch, different, high molecular weight, retention aids may be used for the two additions provided they are compatible, or the same material may be used for each addition.
• Low molecular weight, coagulant-type polymer may be added at an earlier stage if required, in known manner but this is not considered as a retention aid in the context of the present invention.
• Such coagulant polymers usually have intrinsic viscosity below 3dl/g and often below 1dl/g. They can have high cationic charge density, preferably above 4, and often above 5, meg/g.
• the low molecular weight polymer is preferably formed of recurring units of which at least 70%, and generally at least 90%, are cationic.
• Preferred polymers are homopolymers of diallyl dimethyl ammonium chloride and low molecular weight co-polymers of this with a minor amount (usually below 30% and preferably below 10%) acrylamide, low molecular weight homopolymers of dialkylaminoalkyl (meth) -acrylamide or -acrylate quaternary salt or acid addition salt and copolymers of these with small amounts (generally below 30% and preferably below 10%) acrylamide, polyethylene imines, polyamines, epichlorhydrin diamine condensation products, dicyandiamide polymers and other conventional low molecular weight cationic coagulant polymers.
• the retention aid with which the particulate starch is mixed prior to addition to the cellulosic suspension can be soluble cationic starch and thus the system can consist of insoluble starch particles (usually chemically unmodified insoluble starch particles) slurried in a solution of cationic starch.
• the retention aid is a synthetic polymer.
• the preferred retention aids for use in the invention are polymers which have intrinsic viscosity above 4dl/g and usually above 6dl/g, for instance 8-15dl/g or 8-20dl/g or higher.
• intrinsic viscosity is measured at 25°C in 1M sodium chloride buffered at pH7 using a suspended level viscosmeter.
• Non-ionic retention aids that can be used include polyacrylamide or other polymer of water soluble ethylenically unsaturated monomer or monomer blend, and polyethylene oxide.
• Suitable anionic retention aids are polymers of anionic ethylenically unsaturated sulphonic or carboxylic monomer such as acrylic acid (usually as a sodium or other water soluble salt) optionally copolymerised with non-ionic ethylenically unsaturated monomer such as acrylamide.
• the anionic polymer may be formed from, for instance, 3 to 50 mole percent, often 3 to 20 mole percent anionic monomer such as sodium acrylate with the balance being acrylamide.
• Amphoteric polymers containing both anionic and cationic monomer units usually with acrylamide or other non-ionic monomer, can be used.
• Cationic polymers are preferred.
• the or each cationic high molecular weight polymer is usually a copolymer of ethylenically unsaturated cationic monomer, with the balance being other water soluble, generally non-ionic, ethylenically unsaturated monomer such as acrylamide.
• the amount of cationic monomer is usually at least 2 or 3 mole%. Generally it is not more than 20 mole% but it can be up to 50 mole % or more.
• the polymer can be wholly water soluble or it can be in the form of small particles of partially soluble cross-linked polymer as described in EP-A-202,780.
• the or each high molecular weight cationic polymeric retention aid typically has a theoretical cationic charge density of not more than about 3meq/g, often not more than about 2meq/g. Generally it is at least about 0.1, or usually at least about 0.5, meg/g. In this specification, the theoretical cationic charge density is the charge density obtained by calculation from the monomeric composition which is intended to be used for forming the polymer.
• Suitable cationic monomers include dialkyl aminoalkyl (meth) -acrylates and -acrylamides as acid addition or quaternary salts.
• the alkyl groups may each contain 1-4 carbon atoms and the aminoalkyl group may contain 1-8 carbon atoms.
• Particularly preferred are dialkylaminoethyl (meth) acrylates or acrylamides and dialkylamino-1,3-propyl (meth) acrylamides.
• the retention aid Although it is usually preferred for the retention aid to have intrinsic viscosity above 8dl/g, in some instances it can be desirable to use as the retention aid a copolymer of diallyl dimethyl ammonium chloride and acrylamide and which has intrinsic viscosity at least 4dl/g, even though it may not be practicable to manufacture such a polymer to the IV 8dl/g and higher values that are preferred for other polymers.
• the total amount of polymeric retention aid is usually 0.01 to 1%, generally 0.02 to 0.1% (200 to 1,000 gram per tonne dry weight of suspension).
• the amount of retention aid is generally in the range 0.01 to 0.06% or 0.1% but when the process is conducted merely with flocculation followed by drainage, i.e., without the shearing and reflocculation, the amount is usually in the range 0.04 to 0.15%, often 0.06 to 0.1%.
• the amount depends, inter alia, on the choice of cellulosic thin stock. This may be formed from any convenient pulp or mixture of pulps.
• the thin stock typically has a cellulosic fibre content of 0.2 to 2.0%, usually 0.3 to 1.5% by weight.
• the retention aid of IV above 4dl/g (or cationic starch) and the amount of it which is used in the process must be such as to give good retention of fibre fines and filler (if present). Selection of the retention aid and its amount can be conducted in conventional manner by performing the process in the absence of starch with different amounts of different retention aids so as to select an effective combination of retention aid and its amount for the particular cellulosic suspension that is being treated. Naturally this test should be conducted with the subsequent addition of microparticulate anionic material when the overall process involves the use of that material. When the initial cellulosic suspension includes anionic trash, it can be desirable to treat the suspension initially with a cationic coagulant and/or bentonite so as to reduce the amount of polymeric retention aid that is required.
• the amount of retention aid will always be greater than the amount required to precipitate or interact with anionic soluble material in the cellulosic suspension. If the retention performance is plotted against dosage of polymer in a typical combination it will be seen that as the dosage increases retention will be poor and will increase only gradually at low values, but will then increase significantly over a relatively small dosage range, and will not then increase further to any significant extent.
• the dosage at which retention improved markedly is an indication of the demand of that suspension for that retention aid and in the invention the total amount of that retention aid should be at or above the amount at which retention has increased significantly. Accordingly this amount is above the stoichiometric amount required to react with any anionic polymeric material in the cellulosic suspension and any pulp from which it is formed. Generally the suspension is made without deliberate addition of anionic, polymeric materials.
• the cellulosic suspension is flocculated we mean that it has the state which is typical of a cellulosic suspension which has been treated with an effective high molecular weight retention agent in an effective amount.
• the retention system is selected and optimised (using high IV polymer or dissolved cationic starch) for retention, drainage and drying properties in conventional manner, and the particulate starch is injected into the polymer solution with no substantial change in the optimum retention system.
• the starch in the particles must remain substantially undissolved prior to the start of drainage of the suspension, since otherwise dissolved starch is likely to drain from the suspension.
• a simple way of determining whether or not the particles have remained substantially undissolved is to titrate the drainage water for dissolved starch. If the amount of dissolved starch in the drainage water is sufficiently low (after allowing for any dissolved starch introduced with the fibres from, for instance, recycled paper), this indicates that the particles have remained substantially undissolved.
• the amount of dissolved starch in the drainage water should represent less than 20%, preferably less than 10% and most preferably less than 5% of the amount of particulate starch in the suspension after discounting soluble starch originating elsewhere.
• One way of providing that the particles remain substantially undissolved prior to drainage is to introduce the starch in ungelatinised, substantially water insoluble, form and to maintain the conditions in the suspension such that significant gelatinisation does not occur prior to the start of drainage. In such a process, it is necessary to gelatinise the starch during the draining and drying stages.
• draining is completed at temperatures above ambient, and drying is conducted with the application of heat.
• the draining and drying conditions and of the grade of ungelatinised starch it is possible to achieve appropriate gelatinisation during the drying stage, while the sheet is still moist. It can be desirable to apply deliberate heating to the wet sheet, even before final drainage is completed, so as to pre-warm it before entry to the drying stages.
• the wet sheet may be passed under a steam hood or heater such as a Devroniser (trade mark), and this can facilitate full gelatinisation and dissolution of the starch.
• the starch particles need to gelatinise while there is still some moisture in the sheet in order to allow gelatinisation to proceed satisfactorily and in order to allow the particles to spread in the sheet so as to tend to provide a film within the sheet, in contrast to mere spot bonds.
• the starch gelatinising in the presence of moisture it will tend to migrate between the fibres so as to obtain more uniform distribution of the starch on and around and between the paper fibres.
• the amount of moisture that should remain in the sheet when the starch is dissolving can be quite low, and only needs to be sufficient to allow migration of the gelatinised starch sufficient to give adequate distribution of the starch through the sheet.
• a starch that has naturally a low temperature gelatinisation or that has been modified to reduce its temperature of gelatinisation, provided it remains substantially undissolved prior to drainage.
• starch is an uncooked, raw starch such as raw maize, potato, corn, wheat or tapioca starch.
• Pregelatinised or precooked (and therefore soluble) starch can be included as insoluble particles.
• the dissolution of precooked starch in the particles of the suspension can be prevented by protecting the starch with a water impermeable shell or matrix that disintegrates during the subsequent draining or drying. Any material which provides sufficient water impermeability to prevent significant dissolution of the starch prior to draining can be used provided the shell or matrix will disintegrate to release the starch during draining and/or drying.
• the shell or matrix does not have to provide long term water-impermeability. For instance a slow dissolving shell or matrix may be sufficient to protect the starch since even if the shell disintegrates partially within the headbox there may still be inadequate time for the enclosed starch particle to dissolve in the headbox.
• the shell or matrix may be a thermoplastic material having a melting point such as to prevent premature disintegration of the shell or matrix.
• a melting point such as to prevent premature disintegration of the shell or matrix.
• the normal temperature of the suspension leading to the headbox is typically in the range 40-50°C and the ambient temperature around the drainage screen is typically in the same range.
• the particles are provided with a coating or matrix which has a melting temperature at about or above the temperature of the headbox, substantially no melting will occur until the headbox and most of the melting and substantially all the dissolution of the starch will not occur until most of the draining has been completed.
• Suitable thermoplastic materials that can be used include hydrocarbon waxes.
• a pH sensitive shell or matrix may be used.
• the cooked starch may be encapsulated or otherwise protected by polymer that is water insoluble and non-swellable at the pH of the starch dispersion which is provided to the mill, and this dispersion is added to the headbox which is at a pH at which the polymer swells or dissolves.
• the protective polymer can be a copolymer of water soluble and water insoluble ethylenically unsaturated monomers such as methacrylic acid or other water soluble monomer and ethyl acrylate or other water insoluble monomer.
• Methods of incorporating an active ingredient within particles of a protective matrix or within a shell are well known and can be used in the invention.
• the mixture of the starch and protective material may be spray dried or a coacervate coating may be formed around starch particles.
• the amount of starch that is included in the sheet will normally be at least 0.05% and usually at least 0.2% dry weight.
• the greatest advantages of the process are achieved when the amount is above 2 or 3%, for instance 5%, 10% or even up to 12 or 15% by weight.
• an advantage of the process of the invention is that the process can be operated either at high starch loadings or low starch loadings merely by altering the amount of starch, without making any significant changes in the remainder of the process.
• the size of the particles is generally at least 90% by weight below 100 ⁇ m, preferably below 50 ⁇ m, often 5 to 50 ⁇ m.
• the starch particles may have a size of at leat 90% by weight up to 10 ⁇ m, generally 5-10 ⁇ m.
• the starch is preferably granular, so that all three dimensions may be broadly similar.
• the anionic microparticulate or colloidal material is preferably bentonite, that is to say an inorganic swelling clay, for instance as described in EP-A-235,893.
• it can be colloidal silica (such as described in U.S. 4,643,801), polysilicate microgel (such as described in EP-A-359,552), polysilicic acid microgel as described in EP-A-348,366, or aluminum modified versions of any of these.
• organic material can be used.
• an anionic organic polymeric emulsion can be used.
• the emulsified polymer particles may be insoluble due to being formed of a copolymer of, for instance, a water soluble anionic polymer and one or more insoluble monomers such as ethyl acrylate, but preferably the polymeric emulsion is a crosslinked microemulsion of water soluble monomeric material.
• the particle size of the colloidal material is generally below 2 ⁇ m, preferably below 1 ⁇ m and most preferably below 0.1 ⁇ m.
• the amount of colloidal material (dry weight based on the dry weight of the cellulosic suspension) is generally at least 0.03% and usually at least 0.1%. It can be up to, for instance 2% but is generally below 1%.
• the choice and the amount of the anionic colloidal material should be such as to cause what is frequently referred to as "super coagulation".
• the anionic microparticulate or colloidal material is preferably added to the suspension after the last point of high shear, for instance at the headbox, and the suspension can then be drained in conventional manner.
• This screen may travel at conventional screen speeds which are normally in excess of 100 metres per minute and typically are in the range 700 to 1500 metres per minute.
• the machine will include a drying zone in conventional manner but an advantage of the invention is that it is not necessary for the machine to be equipped with a size press or with any other means of applying starch to the wet sheet or to the dried sheet.
• starch can be applied to the wet sheet or the dried sheet in conventional manner.
• a mill trial was carried out on a Fourdrinier machine producing fluting medium at 600m/min from 100% waste furnish.
• a 20% slurry of raw starch was added to the polymer line just prior to the addition of the polymer to the thin stock, in sufficient quantities to provide 5% starch on dry weight of paper.
• Bentonite was added to the thin stock after the centriscreen and just before the head box, at a dose level of 0.5%.
• Liner board having a weight of about 140 grams per square metre was made on a Fourdrinier machine in a process using as retention aid an aqueous solution of a polymer of acrylamide with 10 mol % dimethylaminoethyl acrylate quaternary salt [DMAEAq], having IV 12dl/g, at a dosage of 850g/tonne in the top ply and 790g/tonne in the bottom ply, added before the centriscreen and bentonite at a dosage of 5kg/t in both the top an dbottom ply added after the centriscreen.
• the suspension included recycled paper and it was found that the starch content in the sheet, with no deliberate addition of starch, fluctuated between about 0.9 and 1.2%.
• Particulate raw potato starch was then injected as a slurry into the polymer feed line at a dosage of 1.42% based on the dry weight of the suspension.
• the amount of starch in the sheet was 2.49%, indicating substantially complete retention of the particulate starch.
• the burst strength was increased by about 35% and the CMT value by about 20%.
• a waste furnish was prepared from 60% newsprint, 30% cardboard and 10% magazine and was pulped in a laboratory disintegrator for 20 minutes and then diluted to form a 0.5% thin stock suspension at 25°C. It was left to condition for 24 hours. It had pH 7.5 to 7.7.
• the collected backwater was cooked at 100°C for 30 minutes, the volume re-adjusted to the original volume and the sample centrifuged to remove fibres.
• Acidified potassium iodide/iodine reagent was added and the blue starch/iodine complex was assessed optically and compared to a calibration graph to give an indication of the starch content of the water. Due to the particular analytical techniques used the values are more indicative of relative values than absolute values, but increasing the value indicates increased retention.
• Example 3 A process broadly as in Example 3 was repeated comparing the retention (measured as in Example 3) at 4%, 6% and 8% starch when there is no polymer and bentonite (control) or when the starch is added with 750g/t polymer followed by 2,000g/t bentonite.
• Starch Addition Retention System Starch Retention 4% No 57.9 4% Yes 99.4 6% No 63.1 6% Yes 83.7 8% No 71.2 8% Yes 90.5
• the amount of starch added based on the volume of the suspension at the 4%, 6% and 8% amounts based on the weight of fibre was 200, 300 and 400ppm respectively.
• Example 3 A process broadly as described in Example 3 was used except that in the three tests conducted using polymer in the absence of anionic microparticulate material the starch was added with the polymer solution to the drainage jar with the stirrer set at 800rpm and after 10 seconds mixing the backwater was collected for 30 seconds. Starch retention was measured as in Example 3.

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