CN104812958A - Filler suspension and its use in the manufacture of paper - Google Patents

Abstract

Provided herein are filler suspensions comprising swollen cationic starch, anionic, water-soluble polymer and filler particles. Pulp furnishes comprising the filler suspensions and paper comprising the pulp furnish are also provided. Processes for making the filler suspensions and processes for their use in manufacturing paper are also provided.

Description

Filler Suspension and Its Application in Papermaking

technical field

Provided herein is a filler suspension comprising swollen cationic starch and anionic water-soluble polymer, its use in a process for making paper, and paper and paper products comprising said suspension.

Background technique

In the manufacture of filler paper, the filler slurry is conventionally added to the pulp suspension before it is transferred to the forming section of the paper machine. Usually a retention aid or a retention aid system comprising several components is added to the pulp/filler suspension (also called furnish) in order to retain the filler in the resulting paper.

Adding fillers to paper can lead to many improvements in paper properties, including improved opacity, brightness, texture and print clarity. In addition, adding filler to paper results in cost savings as filler replaces pulp fibers when filler is less expensive than pulp. Such savings can be substantial when using low cost fillers such as precipitated calcium carbonate (PCC) instead of expensive chemical pulp fibers. In addition, filled papers can be dried more easily than unfilled papers, with the result that paper machines can run faster and consume less steam, which further reduces costs and improves productivity.

However, there are various limitations on the amount of filler that can be added for a given paper weight. The strength of the resulting paper is the main factor which, together with the efficiency of the paper machine, limits the filler content, although other factors such as retention, drainage and sizing are also considered.

Manufacturing paper with high filler content requires an effective retention system. Retention aids should provide good filler retention under the high shear and turbulence that occurs during papermaking and should improve dewatering without compromising formation. Retention aid chemicals are usually added to the furnish before or at the inlet of the headbox of the paper machine. The retention aids are one, two or three component chemical additives that improve filler and fines retention through bridging and/or flocculation mechanisms. The chemicals help to attach the filler particles and fines (fine fiber fragments) to the long fibers or cause them to agglomerate into larger floc particles that are more easily retained in the web. In order for the attachment and flocculation to occur, the chemicals must adsorb on the surface of fillers, fines and fibers. The degree of adsorption and adhesion of the chemical is affected by many factors including furnish cleanliness and chemical characteristics of the furnish, the nature of the added chemical, the level of shear during the papermaking process, and the interaction between the retention aid and the furnish components. Contact time.

Replacement of fibers by fillers inevitably reduces paper strength, not only because there are fewer fibers in the paper, reducing the number of fiber-fiber bonds, but also because the presence of said fillers reduces the remaining fiber-to-fiber contacts. The filler particles do not bond themselves, and their location in the fiber-fiber bonding region prevents hydrogen bonding between the pulp fibers. As a result, the high amount of filler that remains produces a weaker paper that may break more easily on the paper machine, size press, coater, winder, and printing press. Weaker fiber-fiber bonds also reduce the surface strength of the paper, resulting in reduced peel resistance and increased fuzzing. Poor incorporation of filler particles in the fibrous structure can also increase dust in the printroom.

In general, all inorganic fillers commonly used in papermaking such as, but not limited to, clay, ground calcium carbonate (GCC), PCC, chalk, talc, titanium dioxide, and precipitated calcium sulfate (PCS) are known to impair the strength of paper and increase resistance to chemical Material needs. Fillers with high surface area such as scalenohedral PCC have a substantial negative impact on strength and increase the chemical requirements on additives for strength, sizing and retention. Due to its shape, narrow particle size distribution, and high surface area, PCC has the tendency to reduce incorporation in paper more than other common papermaking fillers such as chalk, GCC, and clay, and also to render paper too permeable or porous. open structure. As the level of PCC in the formulation increases, the need for sizing chemicals such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) increases to maintain the desired degree of sizing or hydrophobicity. This is because a disproportionate portion of the sizing chemistry is adsorbed on the high surface area PCC. Poor sizing efficiency and loss of hydrophobicity over time (sizing reversal) are common problems associated with the use of PCC in highly filled woodfree papers sized with AKD and ASA. In recent years, many paper mills manufacturing wood-containing grades have converted to neutral papermaking to allow the use of bright calcium carbonate fillers such as GCC and PCC; the main concern with the use of calcium carbonate in these grades remains Yield, paper strength and print operation.

The current industry trend is to reduce paper grammage to reduce costs. Unfortunately, nearly all paper properties deteriorate as grammage decreases, including the limiting factors of opacity, flexural stiffness and permeability. Reducing grammage may also reduce filler retention during papermaking and increase the frequency of paper breakage on the paper machine and during converting and printing. To overcome the loss of paper opacity, papermakers can add more fillers with high opacity, but this may lead to further deterioration of paper strength. The industry needs cost-effective technologies to produce lightweight paper grades with good filler retention and dewatering, as well as acceptable strength, composition, optical properties and printing performance.

Contents of the invention

Accordingly, one aspect of the present invention relates to a filler suspension for papermaking comprising filler particles, swollen cationic starch and an anionic water-soluble polymer.

In one aspect of the invention, the anionic water soluble polymer is lightly crosslinked.

In one aspect of the invention, the anionic, lightly cross-linked, water-soluble polymer has a tan delta rheological oscillation value of at least 0.5 in 1.5% aqueous solution at 0.005 Hz.

In one aspect of the invention, the anionic, lightly cross-linked, water-soluble polymers include sodium acrylate, acrylamide, and methylenebisacrylamide.

In one aspect of the present invention, the anionic water-soluble polymer is M305.

In one aspect of the invention, the filler particles are selected from the group consisting of clay, talc, ground calcium carbonate, chalk, precipitated calcium carbonate, precipitated calcium sulfate, and combinations thereof.

In one aspect of the present invention, the cationic starch granules are swelled with hot water at a gel point temperature of the cationic starch ±10°C without cooking the cationic starch.

In one aspect of the present invention, the temperature at which the cationic starch granules are swollen is gel point temperature + 10°C.

In one aspect of the invention, the size of the filler particles is 1-10 microns.

In one aspect of the present invention, the size of the swollen cationic starch granules is 25-100 microns.

In one aspect of the invention, said filler particles are present in an amount of 60% to 99.5% by weight, said swollen cationic starch is present in an amount of 35% to 0.499% by weight, based on the total solids content of said filler suspension and the anionic water-soluble polymer is present in an amount of 5.0% to 0.001% by weight.

One aspect of the invention is a pulp furnish comprising pulp fibers and the filler suspension herein.

In one aspect of the invention, the pulp furnish further comprises a co-additive selected from the group consisting of sizing agents, wet strength agents, retention aids and combinations thereof.

One aspect of the present invention is a paper comprising the above pulp furnish.

One aspect of the invention is a method of producing a filler suspension for papermaking comprising contacting filler particles with swollen cationic starch and an anionic water soluble polymer.

In one aspect of the invention, in the above method, the anionic water-soluble polymer is lightly cross-linked.

One aspect of the invention is a method of making paper comprising: contacting pulp fibers with the filler suspension herein to form a pulp furnish; discharging said pulp furnish through a line to form paper; and drying said paper.

In one aspect of the present invention, in the above method, the pulp furnish further comprises a co-additive selected from the group consisting of sizing agents, wet strength agents, dry strength agents, retention aids and combinations thereof.

Description of drawings

Figure 1 is a schematic illustration of a papermaking process in which anionic water soluble polymer, cationic swollen starch and filler are mixed together substantially simultaneously.

Figure 2 is a schematic illustration of a papermaking process in which the anionic water soluble polymer is premixed with the swollen cationic starch prior to mixing with the filler.

Figure 3 is a schematic illustration of a papermaking process in which the anionic water soluble polymer and filler are premixed prior to mixing with the swollen cationic starch.

Figure 4 is a schematic illustration of a papermaking process in which the swollen cationic starch and filler are premixed prior to mixing with the anionic water soluble polymer.

Figure 5 is a graph of the viscosity response of corn starch when heated to and above the gelation temperature of corn starch.

Figure 6 includes microscopic images of cornstarch granules at different temperatures when heated in water.

Detailed ways

overview

As used herein, the singular includes the plural and vice versa unless expressly stated otherwise. That is, "a" and "the" refer to one or more of what the word modifies. For example, "a lightly crosslinked polymer" includes one such polymer, two such polymers, or, where appropriate, even more such polymers. Again, by the same token, words such as, but not limited to, "sizing agent" may refer to a plurality of such agents or to only one unless expressly stated otherwise or otherwise evident from the context. such reagents.

Approximate words such as, but not limited to, "about", "substantially", "substantially" and "approximately" indicate that the modified characteristic need not be exactly that explicitly described, but may differ to some extent from written description. The extent to which this differs from that described will depend on how much change is possible and would allow one of ordinary skill in the art to recognize that the modified features still have the desired properties and functions of the unmodified features. Generally, and subject to the foregoing discussion, expressed or implied values herein modified by words of approximation may differ by ±15% from the set value.

combination

In one embodiment herein, there is provided a filler suspension for papermaking comprising filler particles, swollen cationic starch and an anionic water-soluble polymer in a liquid medium, typically water.

In another embodiment there is provided a pulp furnish comprising a filler suspension as described herein and pulp fibers in an aqueous medium. The furnish may also contain other papermaking agents.

In yet another embodiment there is provided a method of producing paper by adding the filler suspension of the present invention to a pulp fiber stock to form a pulp furnish and then making paper from said furnish. Anionic and cationic agents can be added to the formulation comprising the filler suspension to enhance retention and improve dewatering. As previously mentioned, the furnish may also contain other papermaking agents.

The present invention also provides methods for producing swollen starch/polymer compositions and combinations thereof to form filler suspensions with filler particles.

As used herein, "swollen" starch refers to starch in which the raw starch granules have absorbed water and have swollen (preferably herein) to a state in which reabsorption of water could rupture the swollen granules. To achieve this, starch swelling is performed under carefully controlled conditions of temperature, pH, mixing and time. These parameters will vary from starch type to type and are usually determined empirically for each type of starch prior to use in mill scale paper production. The general procedure is simply to suspend raw starch in cold water and then heat the suspension until the starch is swollen. The swollen starch is then mixed with the anionic water-soluble polymer and filler particles in any desired order to form a filler suspension. For example, in Figure 1, swollen cationic starch 1, anionic polymer 8a and filler particles 2 are mixed together in mixer 4 to form a filler suspension, which is then transferred to mixer 5 where it is mixed with pulp fibers 3 Mix to form the topping. In Fig. 2, swollen cationic starch 1 and anionic water-soluble polymer 8a are premixed, then this combination is mixed with filler particles 2 in a mixer 4, after which the formed filler suspension is mixed with pulp fibers 3 Mix in mixer 5. In Fig. 3, anionic water-soluble polymer 8a is premixed with filler particles 2, then this combination is mixed with swollen cationic starch 1 in mixer 4, and the filler suspension formed is transferred to mixer 5, where Here it is mixed with pulp fibers 3 to form a furnish. In Figure 4, the swollen cationic starch 1 is premixed with the fibrous particles 2 in the mixer 4, and the premix is transferred to the mixer 5, basically anionic Polymer 8a was added to the premix. In each case, the furnish comprising the filler suspension is then transferred to a paper machine 6 to form paper 9 . Co-additives can optionally be added. During the paper drying operation, the remaining swollen starch granules will rupture, releasing the amylopectin macromolecules and amylose macromolecules, which serve to bind the solid components of the paper.

The combination of swollen cationic starch, anionic polymer and filler particles can be used to make paper under acid, neutral or alkaline conditions. The composition is primarily used to ensure that fillers and starches are well retained in the paper during papermaking with minimal negative impact on paper strength. Compositions using swollen cationic starch/anionic water soluble polymer/filler particles tend to give greater retention and strength than using swollen cationic starch or anionic polymer with filler particles alone.

When the filler is mixed with the swollen cationic starch/anionic water soluble polymer and added to the pulp slurry, the filler particles agglomerate and adsorb onto the surface of the slurry fines and fibers, resulting in rapid flocculation of the furnish. This results in good filler and fines retention and improved web drainage even without the addition of retention aids. However, under high shear levels, turbulence, and vacuum, filler retention may be reduced due to deflocculation and separation of the filler from the fiber surface. Addition of anionic microparticles (e.g. colloidal silicic acid) to the papermaking furnish containing the filler suspension at or before the headbox of the paper machine, and here preferably before the pressure screen of the paper machine, further improves retention and dehydration rate.

filler

The filler particles may be any filler known to those skilled in the art, and the filler suspension may comprise a single filler or more than one filler. The filler particles are typically inorganic materials with an average particle size in the range of 0.5 μm to 30 μm, more typically 1 μm to 10 μm, such as, but not limited to, clay, ground calcium carbonate (GCC), chalk, precipitated calcium carbonate (PCC), talc, and precipitated Calcium sulfate (PCS) and their blends. Currently, PCC is the preferred filler. The pulp slurry to which the filler suspension is added may consist of mechanical pulp, chemical pulp, recycled pulp and mixtures thereof.

cationic starch

Starches suitable for use in the present invention include, but are not limited to, those derived from corn, waxy corn, potato, wheat, tapioca, sorghum, waxy sorghum, and rice. Starch is usually rendered cationic by including quaternary ammonium cations in the starch. Cationic starches are commercially available, and their preparation is well known to those skilled in the art of papermaking, and thus need not be further described herein. The average specific size of most raw starch granules is approximately 5 μm to 45 μm.

Starch granules are insoluble in cold water. To disperse or "cook" starch, the starch is heated in an aqueous suspension. As heating proceeds, the starch granules first undergo a mild, reversible swelling phase up to a critical temperature known as the "gelatinization" temperature, the "gelation" temperature, or simply the "gelation" temperature, Massive swelling occurs, resulting in a large increase in viscosity. If held above the gel temperature for a sufficient time, the viscosity will return to a lower level due to rupture of the swollen particles. Each starch has its own gelling temperature. The gelation temperature of many starches can be found in the existing literature, or it can be easily determined empirically by heating a given starch suspension while monitoring the viscosity. Swollen starch granules are distinguished from cooked starch. Cooked starch results when swollen starch granules rupture at temperatures above the gelation temperature and thereby release amylose and amylopectin which dissolve in the aqueous medium.

For the purposes of the present invention, the swelling of the starch granules is carefully controlled to result in a swollen starch in which a minimal amount of rupture of the swollen granules occurs. Depending on the starch source, the final particle size of the swollen starch granules ranges from about 25 μm to about 100 μm. Representative but non-limiting examples of the preparation of swollen starches are given in the Examples.

anionic water soluble polymer

Anionic water-soluble polymers can be linear or lightly crosslinked. Lightly crosslinked means that the crosslinked polymer is still water soluble. That is, lightly cross-linked polymers behave more like "branched" polymers than fully cross-linked polymers in which the polymer chains are inextricably intertwined to render them insoluble in water. As used herein, lightly crosslinked and branched are used interchangeably to refer to polymers that are crosslinked but still water soluble. Examples of anionic, branched, water-soluble polymers are described in U.S. Pat. Nos. 5,958,188; They are incorporated herein in their entirety.

In certain embodiments, the anionic, branched, water-soluble polymer:

(a) is prepared by adding a crosslinking or branching agent to the monomer feed;

(b) have an intrinsic viscosity greater than about 3 dl/g; and

(c) has a tan delta rheological vibration value of at least 0.5 at 0.005 Hz, or, alternatively, has a deionized SLV viscosity value of a polymerization prepared under the same conditions from the same monomer feed without the addition of a branching agent At least three times the salinized SLV viscosity value of the compound.

Polymers can be prepared by conventionally reacting a monomer or monomer blend with a crosslinking or branching agent included in the monomer feed under polymerization conditions such as inverse emulsion polymerization. The amount of branching agent and the polymerization conditions are chosen such that the polymerization results in a water soluble polymer rather than a water insoluble crosslinked polymer.

For purposes of the present invention, it is not necessary to specify a numerical range (ie, percentage) of crosslinking that will result in a useful water-soluble branched polymer. Rather, the properties of the resulting polymer are monitored until certain ranges of those properties are achieved. One indication that a branched polymer behaves as a solution polymer rather than a particulate polymer is the tan delta value. While not being bound by any particular theory, it is believed that at low frequencies (eg, 0.005 Hz) the rate of deformation of the polymer sample is slow enough that the linear or branched entangled chains can be unwound to give high tan δ values. On the other hand, network polymers or highly cross-linked polymers are permanently entangled and exhibit low tan δ values over a wide frequency range. Therefore, by determining the tan δ at low frequency (ie 0.005), a measure of the degree of branching can be determined. For the purposes of the present invention, tan delta values are higher than 0.5, preferably higher than 0.7, even more preferably higher than 0.9, 1.3 or even higher. Tan delta values below 0.5 generally indicate polymers that are too highly crosslinked to be true solution polymers and are preferably avoided.

The tan delta value at 0.005 Hz can be obtained using a controlled stress rheometer in oscillatory mode for a 1.5% by weight solution of the polymer in deionized water. The value of tan δ is the ratio of the loss (viscous) modulus G" to the storage (elastic) modulus G'.

It is preferred that the branched polymers herein have tan delta values at 0.005 Hz that are reasonably close to the values of the corresponding unbranched polymers; that is, polymers prepared without branching agents under the same conditions give higher intrinsic viscosity. For example, the tan δ of a branched polymer is preferably at least 50%, and typically at least 80%, eg up to 120% or more, of the tan δ of a corresponding unbranched polymer.

Another indication that the polymer is in solution rather than particulate form is that the deionized SLV viscosity values for branched polymers were determined under the same polymerization conditions by reacting the same monomer feed without the addition of branching agents. The prepared polymer has a viscosity value at least three times that of the salified SLV. This is referred to as the "corresponding unbranched polymer".

The "same monomer feed" and the "same polymerization conditions" represent constant feeds and conditions that can reasonably be obtained in commercial production except for deliberate changes in the amount of branching agent and optional chain transfer agent .

The branching agent can be any crosslinking entity capable of reacting with side groups on the polymer backbone such as but not limited to the carboxylic acid groups of acrylic acid, but preferably the branching agent is a polyethylenically unsaturated monomer (polyethylenically unsaturated monomer). monomer). The polyolefinic branching agent can be a difunctional material, such as methylenebisacrylamide, or it can be a trifunctional, tetrafunctional, or higher functional branching agent, such as a tetraalkyl chloride ammonium chloride. Preferably the branching agent itself is soluble in water.

The amount of polyolefinic branching agent is generally below 10 ppm (mole), preferably below 5 ppm (mole). Useful results can be obtained using 0.5 ppm (mole) to 3.8 ppm (mole) but in some cases amounts of 4.1 ppm (mole) up to 7 ppm (mole) or even 10 ppm (mole) can also be used. In fact, amounts up to 20 ppm (mole) or even up to 30 ppm (mole) or 40 ppm (mole) can sometimes be used (usually in the presence of chain transfer agents), but lower amounts are usually required to meet the tan delta limit . The designation "ppm (mole)" refers to the number of moles of branching agent per million moles of monomer (ie ppm (mole)).

Branched polymers can be prepared under polymerization conditions in which no chain transfer agent is intentionally present during the reaction. The amounts of branching agent given above, in particular 1 ppm (mole) to 10 ppm (mole) and preferably 0.5 ppm (mole) to 3.8 ppm (mole) are especially suitable when no chain transfer agent is added. However, it may be necessary to add some chain transfer agent, in which case the amount of branching agent can be increased up to 20 ppm (mole) or 30 ppm (mole) or even 40 ppm (mole) while still maintaining the properties and performance of the polymer. The choice of the amount of chain transfer agent depends on the particular material used as well as the amount of branching agent, monomer feed, and polymerization conditions.

Although relatively large amounts of branching agents can be used, small amounts are preferred since it has been observed that small amounts of chain transfer agents give the best results. A preferred chain transfer agent is sodium hypophosphite. While large amounts can be used, best results generally require (by weight based on monomer weight) amounts below 50 ppm, preferably below 20 ppm. Usually the best results will be obtained with no higher than 10ppm. However, if the amount is too low, for example below about 2 ppm, the full benefit from the use of the chain transfer agent may not be obtained.

Any chain transfer agent suitable for aqueous polymerization of water-soluble acrylic monomers, such as but not limited to isopropanol or mercapto-containing compounds, can be used as a hypophosphite replacement. If a non-hypophosphite material is used, it should be used in an amount that produces about the same chain transfer effect as the amount of hypophosphite it replaces.

While the use of small amounts of chain transfer agent is preferred, larger amounts such as 100 ppm or more can be used, often with suboptimal results, provided the combination of materials and polymerization conditions is such that the polymer has the desired physical properties.

One of the desired physical properties is the intrinsic viscosity of the polymer. The intrinsic viscosity is measured using a suspension viscometer in 1M NaCl buffered to pH 7.5 at 25°C. This value is usually at least 3dl/g or 4dl/g, preferably at least 6dl/g. It can be as high as eg 18dl/g, but is usually below 12dl/g and often below 10dl/g.

Suitable branched polymers can also be characterized by comparison with corresponding unbranched polymers. The intrinsic viscosity of the unbranched polymer is generally at least 6 dl/g, and preferably at least 8 dl/g. Frequently, it is 16dl/g to 30dl/g. The amount of branching agent is usually such that the intrinsic viscosity is reduced by at least 10%, usually at least 25% or 40%, up to 70%, or sometimes up to 90% of the value of the unbranched polymer.

Instead of, or in addition to, intrinsic viscosity, a polymer may also be characterized by its brine Brookfield viscosity. The brine Brookfield viscosity was measured by preparing a 0.1% by weight solution of the polymer in 1M aqueous NaCl at 25°C. A 60 rpm Brookfield viscometer equipped with a UL adapter can be used. Therefore, the powdered polymer was added to a 1M NaCl aqueous solution, or the inverse emulsion polymer was added to this solution. The viscosity of the saline solution is usually above 2.0 mPa.s., and usually at least 2.2, preferably at least 2.5 mPa.s. Usually, the viscosity value is not greater than 5 mPa.s, and a value of 3 to 4 is often preferred.

Instead of or in addition to the tan delta value, the ratio between the deionized SLV viscosity value and the salinized SLV viscosity value can also be used as an indication of the absence of insoluble crosslinked microparticles.

The SLV viscosity value is determined by using a glass suspension viscometer at 25°C, which is selected according to the viscosity of the solution. Viscosity values are η−η ₀ /η ₀ , where η and η ₀ are the viscosity results for aqueous polymer solutions and blank solvents, respectively. Said viscosity value may also be referred to as specific viscosity. Deionized SLV viscosity values are those obtained for 0.05% polymer solutions in deionized water. Salted SLV viscosity values are the viscosity values obtained for 0.05% polymer in 1 M sodium chloride in water.

Deionized SLV viscosity numbers are preferably at least 3, usually at least 4. Best results are obtained when the deionized SLV viscosity number is above 5, such as 7 or 8. Preferably, this value is higher than the deionized SLV viscosity value of the unbranched polymer. If the deionized SLV viscosity value is not greater than the deionized SLV viscosity value of the unbranched polymer, it is preferably at least 50%, typically at least 75%, of the deionized SLV viscosity value of the unbranched polymer. Salted SLV viscosity numbers are usually below 1. The viscosity value of the deionized SLV is generally at least five times, preferably at least eight times, that of the salinized SLV.

The polymers may be obtained from commercial sources or may be prepared by any conventional suitable polymerization method known for the preparation of water soluble acrylic polymers and other addition polymers such as bead polymerization method or gel polymerization method. A preferred type of polymerization process is inverse emulsion polymerization to form an inverse emulsion of water-soluble polymer particles in an anhydrous liquid. Typically at least 95% by weight of such products have a primary particle size below 10 μm, preferably at least 90% by weight of such products have a primary particle size below 2 μm, for example down to 0.1 μm or 0.5 μm . As such, it may be a conventional inverse emulsion or microemulsion and may be prepared by any of the known techniques for the preparation of such materials. The number average particle size of the microemulsion can typically be as low as 0.05 μm or 0.1 μm, for example, if desired.

The emulsion can be provided in the form in which the emulsion is made (as an emulsion of aqueous polymer droplets in an oil or other water-immiscible liquid), or if desired, can be substantially dehydrated to form a substantially anhydrous polymer A stable dispersion of liquid droplets dispersed in oil. Conventional surfactants and optional polymeric amphoteric stabilizers may be included in known manner to stabilize the emulsion.

Reverse phase or other polymerizations are performed on a feed of the desired monomer or monomer blend, usually in aqueous solution.

It is generally preferred that the anionic branched polymer is 5-97% by weight of acrylamide or other water-soluble, nonionic, ethylenically unsaturated monomer and 95-3% by weight of ethylenically unsaturated carboxylic acid monomer, sulfonic acid Monomers or copolymers of other anionic monomers. Any conventional water-soluble carboxylic and sulfonic acid monomers may be used, such as, but not limited to, acrylic acid, methacrylic acid, crotonic acid, vinyl sulfonate, and AMPS. The presently preferred anionic monomer is acrylic acid, usually as sodium acrylate or other water soluble salts. Preferred copolymers comprise 20-80%, typically 40-75%, by weight acrylic acid, with the balance being acrylamide.

In a particular embodiment, the anionic aqueous polymer is M305 (BASF, commercially available).

Swollen cationic starch/anionic polymer/filler particle suspension

Typically, the suspension comprises 60-99.5% by weight of filler, 35-0.499% by weight of swollen cationic starch and 5.0- 0.001% anionic polymer. It is understood that the suspension contains a complex of anionic polymer bound to swollen cationic starch, but may also contain free swollen cationic starch and free anionic polymer particles.

The swollen cationic starch and anionic polymer are suitably applied as dry solids in amounts of 0.5-10% by weight, based on the weight of the filler particles.

Papermaking agent

Compositions, suspensions and furnishes herein may additionally include conventional papermaking agents such as, but not limited to, sizing agents such as alkyl ketene dimers, alkenyl succinic anhydrides, and rosins; wet strength agents, and cationic polymeric retention aids or anionic polymeric retention aids. The composition may include retention aids which may be a single chemical such as cationic microparticles (colloidal silicic acid, bentonite), anionic polyacrylamides, cationic polymers (cationic polyacrylamide, cationic starch), dual chemical systems (cationic polymer/anionic microparticle, cationic polymer/anionic polymer) or a three-component system (cationic polymer/anionic microparticle/anionic polymer, cationic polymer/anionic micropolymer/anionic polymer). The choice of retention aid chemistry and its point of incorporation during paper formation depends on the nature of the filler slurry being treated and the ionic charge of the papermaking furnish.

Typically, the filler suspensions herein are used as dry solids in amounts of 5% to 60% based on the dry weight of the pulp in the furnish.

Papers made with fillers can exhibit greater internal bond strength than control papers without fillers, as measured by the Scott bond technique. At equal filler content, the wet and dry strength properties of paper made using the filler suspensions described herein can be stronger than those made with filler alone.

The use of the filler suspension according to the invention allows the production of filled papers such as coated and uncoated fine papers, supercalendered papers and newsprint with minimal strength loss and good optical properties. Thus, the use of the filler suspension according to the invention allows the papermaker to produce filled paper with a higher filler content in the paper. In general, potential benefits of using the treated filler suspensions of the present invention include improved sizing, wet strength, dry strength, opacity, and print quality, as well as reduced use of expensive reinforcing chemical pulp fibers.

Under certain conditions, combinations of swollen cationic starches and anionic polymers can be used to reinforce other grades of paper that do not contain fillers, such as sack paper and board products.

Example

overview

The suspensions and methods of the present invention can be described and understood by means of the following illustrative examples. In the Examples, results were obtained using laboratory scale techniques. However, these examples are not intended or construed as limiting in any way.

Starch (2-20% solids in slurry at room temperature) can be swollen at temperatures near the gel point of starch in batch cookers, steam autoclaves or by mixing with hot water. The preferred method is to swell the granules by mixing a starch slurry prepared in cold water with hot water. The temperature of the hot water used depends on the consistency of the initial starch slurry in the cold water, the final target temperature of the swollen starch, the temperature of the cold water, the pH and the residence time. The temperature and reaction time for preparing the swollen starch/polymer composition depend on the type of starch used, the pH of the starch slurry and the heating time. The following is an example of a process for the preparation of swollen starch for the purposes of the present invention.

Example 1

The raw starch dispersion mixed with polymer in cold water is swollen and the swollen starch/polymer composition is added to the agitated filler suspension. In this method, the starch powder is first dispersed in cold water and then the polymer is incorporated into the dispersion under shear. The starch/anionic polymer mixture is mixed with hot water or heated to a temperature close to the gel point of the starch. The swollen starch/anionic polymer composition is then mixed rapidly with the filler suspension at a temperature below the starch gelation temperature.

Example 2

The cationic starch dispersion is first swollen and then added to the agitated filler suspension before the anionic polymer is introduced. In this method, starch powder is dispersed in cold water and then mixed with hot water or heated to a temperature close to the gel point of starch. The swollen starch is then mixed rapidly with the filler suspension at a temperature below the gel point of the starch, followed by the addition of the anionic polymer.

The combination of swollen cationic starch, anionic water-soluble polymer and filler is carried out under good mixing conditions. Additional anionic or cationic agents may be added during the preparation of the swollen cationic starch/anionic water soluble polymer composition to form a complex prior to addition of the filler, or they may be added to the shear treated filler suspension in order to form bridging filler particles. These handling strategies can produce a homogeneous filler suspension that is stable during long-term storage.

The treated filler suspension can be introduced directly into the pulp stock or, if desired, can also be diluted before the paper forming process, e.g. at the inlet of the mixing chest, machine chest or fan pump And added to paper machine pulp stock. Introducing the treated filler to the pulp suspension results in flocculation of the pulp slurry. However, shear level and residence time can affect the degree of flocculation. In general, the treated filler suspension tends to retain its flocculation properties over time when added to the papermaking pulp slurry. To enhance filler retention, anionic particles such as but not limited to silica, anionic polymers such as but not limited to CMC, or traditional polymer flow aids such as but not limited to polyacrylamide can be preferably placed in the headbox or pressure Add to furnish before screen or at headbox or pressure screen. Retention and dewatering are greatly improved once silica or CMC is added to the pulp stock containing treated filler.

Example 3

A 0.3% concentration stock was prepared by mixing the internal cationic starch with the pulp furnish, followed by pre-prepared PCC and finally with the retention aid. Wood free handsheets of 80 g/ ^m2 were produced using a dynamic sheet former (DSF) followed by dynamic sheet pressing and drying at 120°C. Before the paper test, the paper was calendered under the same conditions, then controlled conditions at 50% RH and 22°C.

The raw materials used in paper manufacturing are as follows:

Fiber: 100% eucalyptus used as pulp, refined to SR 30 (at 20°C) using a Valley Beater laboratory refiner.

Filler: Precipitated calcium carbonate (Albacar LO PCC) from Specialty Minerals Inc., average particle size 2.3 μm. The PCC content in the paper varied between 19.3% and 25.9% by weight.

Swollen cationic starch for PCC pretreatment: cationic potato starch. Swelled cationic starch was prepared by mixing dry cationic starch powder with water to make a 3% solids slurry, then heating the 3% solids slurry to 63°C with mixing. Swollen cationic starch was used for pretreatment of PCC by mixing 5 kg per metric ton (ton) of paper with PCC at 20% solids. Some filler samples were pretreated with swollen cationic starch only, and some filler samples were pretreated with swollen starch and anionic co-additives.

Co-additive for PCC pretreatment with swollen cationic starch: anionic micropolymer (trademark of BASF ). Treatment of PCC is accomplished by mixing swollen cationic starch and anionic micropolymer with PCC. A different order of addition was used and at the same time the dosage of swollen cationic starch was fixed at 5 kg/ton of paper, the dosage of anionic micropolymer was 0.05% incoming material/dry PCC (wt) and 0.1% incoming material/dry PCC (wt) ).

Internal Starch: Cationic Potato Starch. The dry starch powder was mixed with water to obtain a 1% solids slurry, which was then aged at 97°C with mixing. Cooked cationic starch was used by mixing it with the pulp furnish at 8 kg/ton of paper.

Retention aid: 0.2 kg/ton of paper cationic polyacrylamide (CPAM) was used for retention.

Example 4

Table 1 shows the properties of papers made from PCC treated with swollen starch alone and PCC treated with swollen starch followed by anionic micropolymer. The dosage of anionic micropolymer is 0.05-0.1% incoming material/dry PCC (weight). Papers using anionic micropolymers in PCC pretreatment showed better strength properties - tensile, internal bond, flexural stiffness - compared to papers treated with PCC and swollen starch alone. The best strength properties were obtained with dosages of up to 0.1%/PCC anionic micropolymer. This would give a 6% increase in unit filler without loss of strength properties.

Table 1

PCC: Precipitated Calcium Carbonate, SST: Cationic Swellable Starch, AMP: Anionic Micropolymer.

The tensile and hardness values are the geometric mean of the longitudinal and transverse directions.

Example 5

Table 2 shows PCC treated with swollen starch only and PCC treated with swollen starch and anionic micropolymer (different order of addition: PCC treated with swollen starch followed by anionic micropolymer; and anionic micropolymer treated then Properties of paper made from swollen starch-treated PCC). The presence of anionic micropolymers improved the strength properties of paper prepared from swollen starch-treated PCC, regardless of the order of addition.

Table 2

PCC: Precipitated Calcium Carbonate, SST: Cationic Swellable Starch, AMP: Anionic Micropolymer.

The tensile and hardness values are the geometric mean of the longitudinal and transverse directions.

Example 6

These microscope images in Figure 6 illustrate how the starch granules swell and increase in viscosity until they begin to drop due to rupture of the swollen starch granules. Images represent cornstarch samples at 25°C, 56°C, 60°C, 66°C and 95°C.

For the purposes of the present invention, swollen starch is characterized as the majority of the granules have started to swell as in the 56°C image until large swollen granules are still visible as in the 66°C image. Thus, viscosity curves combined with microscopic images can be used to determine when starch swelling is used to prepare filler suspensions of the present invention. The region of maximum viscosity in Figure 5 is when most starch granules are swollen but not ruptured. The temperature range in which useful swollen starch granules can be obtained is in the +/- 10°C peak region of Figure 5, where most starch granules are swollen but not yet completely ruptured. Preferably, the suspension of raw starch granules is heated to a temperature of peak +10°C, where all starch granules are swollen and all unswollen granules are eliminated.

All patents and patent applications cited in this specification are hereby incorporated by reference as if each individual patent or patent application were specifically and individually indicated to be fully set forth herein. While the claimed subject matter has been described in terms of various embodiments, those skilled in the art will understand that various modifications, substitutions, deletions and changes are possible without departing from the spirit of the invention. Accordingly, it is intended that the scope of the subject matter be limited only by the scope of the following claims to include equivalents thereof.

Claims (19)

1. A filler suspension for papermaking, comprising: filler particles; swollen cationic starch; and Anionic water soluble polymer. 2. The filler suspension of claim 1, wherein the anionic water-soluble polymer is lightly cross-linked. 3. The filler suspension of claim 2, wherein the anionic, lightly cross-linked, water-soluble polymer has a tan delta rheological oscillation value of at least 0.5 in a 1.5% aqueous solution at 0.005 Hz. 4. The filler suspension of any one of claims 1-3, wherein the anionic, lightly cross-linked, water-soluble polymer comprises sodium acrylate, acrylamide and methylenebisacrylamide. 5. The filler suspension of claim 1, wherein the anionic water-soluble polymer is M305. 6. The filler suspension of claim 2, wherein the anionic water-soluble polymer is M305. 7. The filler suspension of claim 1, wherein the filler particles are selected from the group consisting of clay, talc, ground calcium carbonate, chalk, precipitated calcium carbonate, precipitated calcium sulfate, and combinations thereof. 8. The filler suspension of claim 1, wherein the cationic starch granules are swelled with hot water at a gel point temperature of cationic starch ± 10° C. without ripening the cationic starch. 9. The filler suspension according to claim 8, wherein the temperature at which the cationic starch granules are swollen is the gel point temperature + 10°C. 10. The filler suspension of claim 1, wherein the filler particles have a size of 1-10 microns. 11. The filler suspension of claim 1, wherein the swollen cationic starch granules have a size of 25-100 microns. 12. The filler suspension of claim 1, wherein the filler particles are present in an amount of 60% to 99.5% by weight based on the total solids content of the filler suspension, the swollen cationic starch is present by weight is present in an amount of 35% to 0.499%, and the anionic water-soluble polymer is present in an amount of 5.0% to 0.001% by weight. 13. A pulp furnish comprising pulp fibers and the filler suspension of claim 1. 14. The pulp furnish of claim 12, further comprising a co-additive selected from the group consisting of sizing agents, wet strength agents, retention aids, and combinations thereof. 15. A paper comprising the pulp furnish of any one of claims 13 or 14. 16. A method of producing a filler suspension for papermaking comprising contacting filler particles with swollen cationic starch and an anionic water soluble polymer. 17. The method of claim 16, wherein the anionic water-soluble polymer is lightly cross-linked. 18. A method of making paper comprising: contacting pulp fibers with the filler suspension of claim 1 to form a pulp furnish; discharging said pulp furnish through a line to form paper; and drying said paper. 19. The method of claim 18, wherein the pulp furnish further comprises a co-additive selected from the group consisting of sizing agents, wet strength agents, dry strength agents, retention aids, and combinations thereof.