EP1047834A1

EP1047834A1 - Silica-acid colloid blend in a microparticle system used in papermaking

Info

Publication number: EP1047834A1
Application number: EP99946808A
Authority: EP
Inventors: Paul F. Richardson; Charles R. Hunter
Original assignee: Calgon Corp
Current assignee: Calgon Corp
Priority date: 1998-09-22
Filing date: 1999-09-08
Publication date: 2000-11-02
Anticipated expiration: 2019-09-08
Also published as: BR9907290A; AU761790B2; AU5913399A; CN1231639C; ID25782A; DE69931343D1; NZ504660A; EP1047834A4; JP2002526680A; ATE326579T1; DE69931343T2; WO2000017450A1; CN1287585A; EP1047834B1

Abstract

A microparticle system for use as a retention and drainage aid in the production of alkaline and acid paper products comprises a high molecular weight flocculant polymer (6) and a silica-acid colloid blend (7), and optionally a cationic coagulant or a medium molecular weight flocculant (5). The high molecular weight flocculant polymer (6) may be added to the furnish after the fan pump (1) and prior to the pressure screen (2); the silica-acid colloid blend (7) may be added to the furnish after the pressure screen (2), and optionally, the coagulant or medium molecular weight flocculant (5) may be added prior to the fan pump (1). Addition of the microparticle system to the paper furnish improves retention and drainage during the papermaking process and sheet formation. The acid colloid of the silica-acid colloid blend (7) is a polymer or a copolymer of a melamine aldehyde acid colloid.

Description

"SILICA-ACID COLLOID BLEND IN A MICROPARTICLE SYSTEM USED IN PAPERMAK NG"

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention relates to an improved microparticle system used as an aid in making a paper product, i.e. paper or paperboard, with improved properties in the areas of retention, drainage, and sheet formation. More particularly, it pertains to a microparticle system comprising a silica-acid colloid blend as an inorganic particulate material in a microparticle system.

2. Description Of The Background Art

In the production of paper or paperboard, a dilute aqueous composition known as "furnish" or "stock" is sprayed onto a moving mesh known as a "wire" . Solid components of this composition, such as cellulosic fibers and inorganic particulate filler material, are drained or filtered by the wire to form a paper sheet. The percentage of solid material retained on the wire is known as the "first pass retention" (FPR) of the papermaking process. Drainage, retention and formation (D/R/F) aids are used in the papermaking process.

Retention is believed to be a function of different mechanisms, such as filtration by mechanical entrainment , electrostatic attraction, and bridging between the fibers and the fillers in the furnish. Because both the cellulosic fibers and many common filler materials are negatively charged, they are mutually repellent. Generally, the only factor tending to enhance retention is mechanical entrainment. Therefore, a retention aid is generally used to improve retention of the fibers and fillers on the wire. The retention of fines and fillers is important to the papermaker to insure the capture of colloidally sized particles in the sheet. First pass retention (FPR) measures this ability of the retention program. Colloidal silica has been used in the past as a microparticle in a retention aid for alkaline fine paper. Silica, when used properly, can enhance the retention of fines and fillers by forming microflocs that capture colloidal material and allow the pulp slurry to dewater quickly.

Drainage relates to the rate of removal of water from the furnish as the paper sheet is formed. Drainage usually refers to water removal which takes place before any pressing of the paper sheet subsequent to formation of the sheet. Thus, drainage aids are used to improve the overall efficiency of dewatering in the production of paper or paperboard. Formation relates to the formation of the paper or paperboard sheet produced in the papermaking process. Formation is generally evaluated by the variance of light transmission within a paper sheet. A high variance is indicative of "poor" formation and low variance is generally indicative of "good" formation. Generally, as the retention level increases, the level of formation generally decreases from good formation to poor formation.

It can be appreciated that improvements in retention and drainage and in the formation properties of the paper or paperboard sheet are particularly desirable for several reasons, the most significant of which is productivity. Good retention and good drainage enable a paper machine to run faster and to reduce machine stoppage. Good sheet formation lessens the amount of paper wastage. These improvements are realized by the use of retention and drainage aids. Retention and drainage aids generally are additives which are used to flocculate the fine solid material present in the furnish to improve these parameters in the papermaking process. The use of such additives is limited by the effect of flocculation on the paper sheet formation. If more retention aid is added so the size of the aggregates of the fine solid material is increased, then this generally results in variations in the density of the paper sheet which, as stated hereinabove, may result in what is referred to as "poor" sheet formation. Over-flocculation can also affect drainage as it may eventually lead to holes in the sheet and to a subsequent loss of vacuum pressure in the later stages of dewatering during the papermaking process. Retention and drainage aids are generally added to the furnish in the wet-end of the paper machine, and generally are of three types, viz:

(a) single polymers;

(b) dual polymers; or

(c) microparticle systems which may be used with a flocculant and/or a coagulant.

A microparticle system may generally give the best result as a retention and drainage aid, and has been widely described in the prior art . Examples of publications of microparticle systems include: EP-B-235, 893 wherein bentonite is used as the inorganic material in conjunction with a HMW cationic polymer in a specified addition sequence; WO-A-94/26972 wherein a vinylamide polymer is disclosed for use in conjunction with one of various inorganic materials, such as silica, bentonite, china clay, and organic materials; WO-A-97/16598 wherein kaolin is disclosed for use in conjunction with one of various cationic polymers; and EPO 805234 wherein bentonite, silica, or acrylate polymer is used in conjunction with a cationic dispersion polymer. U.S. Patent Nos . 4,305,781 and 4,753,710 disclose the use of HMW nonionic and ionic polymers in conjunction with bentonite clay to aid in dewatering and retention in papermaking. U.S. Patent Nos. 4,388,150 and 4,385,961 teach the use of cationic starch and colloidal silica. U.S. Patent Nos. 4,643,801 and 4,750,974 describe the use of cationic starch, anionic HMW polymer, and colloidal silica in papermaking. U.S. Patent No. 5,185,062 describes an anionic polymer acting as a microparticle with a HMW cationic flocculant. U.S. Patent No. 5,167,766 teaches the use of charged organic polymeric microbeads as a microparticle in papermaking. The use of melamine-formaldehyde (MF) acid colloids for wet strength in paper is well known. Reference is made to TAPPI Monograph No. 29 "Wet Strength in Paper and Paperboard" by C. S. Maxwell, J. P. Weidner, ed. U.S. Patent No. 2,345,543 describes the preparation of stable melamine-formaldehyde acid colloids, and U.S. Patent No. 2,485,080 includes the incorporation of urea into the condensation products. U.S Patent Nos. 2,559,220 and 2,986,489 teach the use of these colloids to increase the wet strength of paper. U.S. Patent No. 4,845,148 describes the use of amino-aldehyde acid colloid with acrylamide for increasing dry strength of paper. U.S. Patent No. 5,286,347 describes the use of melamine formaldehyde colloid for pitch control in papermaking. U.S. Patent No. 4,461,858 describes the use of polyvinyl alcohol - melamine formaldehyde colloid blends for wet- strength in paper. U.S. Patent No. 4,009,706 teaches the use of MF colloid and anionic HMW polymer to flocculate raw sugar. U.S. Patent No. 5,382,378 describes a composition of colloidal silica and melamine aldehyde, urea aldehyde or melamine-urea aldehyde acid colloid blended together for the purpose of collecting paints, oils and greases, or colors from process waters.

A microparticle system may generally comprise a polymer flocculant with or without a cationic coagulant and a fine particulate material. The fine particulate material may be an inorganic particulate material and improves the efficiency of the flocculant and/or allows smaller, more uniform floes to be produced.

In spite of the several microparticle systems presently available for use in the paper mills to attain better runnability of the paper machine and/or to obtain a specific end use paper property, such as improved sheet formation for better printability, or improved surface strength, there remains a very real and substantial need for an improved microparticle system for improving the paper or paperboard by improving drainage and retention during the papermaking process and formation properties in the formed sheet .

SUMMARY OF THE INVENTION

The present invention has met this above described need. The present invention relates to a microparticle system used as a retention and drainage aid in a papermaking process.

According to a first aspect of the present invention, there is a method of producing paper which comprises adding to a paper stock or furnish a microparticle system as a retention and/or drainage aid which comprises a high molecular weight polymer flocculant and an inorganic particulate material, wherein the inorganic particulate material comprises a silica- acid colloid blend, and which blend comprises colloidal silica and an acid colloid.

According to a second aspect of the present invention, there is an improved microparticle system which is added to a paper stock or furnish as a retention and/or drainage aid, and which microparticle system comprises a high molecular weight polymer flocculant and an inorganic particulate material, wherein the inorganic particulate material comprises a silica-acid colloid blend, and which blend comprises colloidal silica and an acid colloid.

According to a third aspect of the present invention, there is a paper or a paperboard product with improved properties in the area of retention, drainage and formation wherein the paper or paperboard product is made by adding an improved microparticle system to an aqueous cellulosic paper stock or furnish wherein the microparticle system comprises a high molecular weight polymer flocculant and an inorganic particulate material comprising a silica-acid colloid blend, and which blend comprises colloidal silica and an acid colloid.

A fourth aspect of the invention involves a process in which paper or paperboard is made by forming an aqueous cellulosic paper furnish, the steps comprising:

(a) adding to the thin stock flow of a paper furnish a high molecular weight polymer flocculant after a first shearing stage, (b) after a second shearing stage, adding an inorganic particulate material comprising a silica-acid colloid blend comprising colloidal silica and an acid colloid to the paper furnish;

(c) draining the paper furnish to form a sheet; and (d) drying the sheet.

The acid colloid may be comprised of an aqueous solution of a water-soluble polymer which may be melamine aldehyde, urea aldehyde, or melamine-urea aldehyde, and the aldehyde is : o

wherein Rl is selected from the group consisting of a straight and a branched C^ alkyl. Preferably, the acid colloid is a polymer of melamine formaldehyde, but may be a copolymer of melamine-formaldehyde and urea- formaldehyde, or a coploymer comprising melamine aldehyde and condensates, or a copolymer of amine-aldehyde-type and ethylenically unsaturated monomers.

In the silica-acid colloid blend, the acid colloid and the colloidal silica are blended in the ratio range of from about 99.5:0.5 to about 0.5:99.5, respectively, on a total solids basis in an acid environment of pH 3.0 or less. This silica-acid colloid blend is present in an amount of from about 0.0005% to about 0.5% by weight based on the dry weight of the solids in the furnish. The high molecular weight polymer flocculant is present in an amount of from about 0.0025% to about 1.0% by weight based on the dry weight of the solids in the furnish and the silica-acid colloid blend is present in an amount ranging from 0.0005% to 0.5% by weight based on the dry weight of the solids in the furnish. A high charge density cationic coagulant or a medium molecular weight flocculant may be added to the furnish prior to a first shearing stage and may in some instances be added prior to or after the addition of the silica-acid colloid blend. In some instances, it may be more advantageous to change the sequence of the feed points of addition of the chemical additives in the paper machine. That is, the silica-acid colloid blend may be added to the stock or furnish prior to the first shearing stage and the coagulant or medium molecular weight flocculant after or before the second shearing stage and the high molecular weight flocculant after or before the second shearing stage .

BRIEF DESCRIPTION OF THE FIGURES

The single figure is a sketch illustrating a portion of a typical paper machine and the points of addition of the components of the microparticle system of the present invention in a preferred form.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a microparticle system used as a retention and/or drainage aid for particular use in the wet end of a paper machine in the papermaking process for both acid and alkaline fine paper.

As used herein, the term "paper" includes products comprising a cellulosic sheet material including paper sheet, paper board, and the like. The "microparticle system" of the invention refers to the combination of at least one hydrophilic polymer used as a flocculant and an inorganic particulate material which is the microparticle in the system, and optionally, a cationic coagulant or a medium molecular weight flocculant. In the invention, the inorganic particulate material is a silica-acid colloid blend. The components of this combination may be added together to the stock or furnish to be treated, but are preferably added separately in the manner and order described hereinbelow.

The invention can be carried out using a conventional papermaking machine. According to conventional practice, the furnish or "thin stock" that is drained to form the paper sheet is often made by diluting a thick stock which typically has been made in a mixing vessel or chest by blending pigment or filler material, the appropriate fiber, any desired strengthening agent and/or other additives, and water which may be recycled water. The thin stock may be cleaned in a conventional manner, e.g., using a vortex cleaner. Usually the thin stock is cleaned by passage through a centriscreen. The thin stock is usually pumped along the paper machine by one or more centrifugal pumps known as fan pumps. For instance, the thin stock may be pumped to the centriscreen by a first fan pump. The thick stock can be diluted by water to form the thin stock prior to the point of entry to this first fan pump or prior to the first fan pump, e.g., by passing the thick stock and dilution water through a mixing pump. The thin stock may be cleaned further by passage through a second centriscreen or pressure screen and passed through a headbox prior to the sheet forming process of a paper machine .

The sheet forming process may be carried out by use of any conventional paper or paperboard forming machine, for example a flat wire fourdrinier, a twin wire former, or a vat former or any combination of these forming machines . An approach system to a paper machine may comprise the components shown in the single figure. These components include a fan pump 1, a pressure screen 2, and a headbox 3. The thick stock may be diluted by water to form a thin stock prior to the stock's entry into fan pump 1 by passing the thick stock and dilution water through a mixing pump (not shown) . The thin stock is cleaned of contaminants by passage through pressure screen 2 and the thin stock that leaves pressure screen 2 is passed to headbox 3 prior to forming the sheet .

The single figure also illustrates the preferred points of addition for the components of the microparticle system of the present invention. Preferably, if a cationic coagulant or a medium molecular weight (MMW) flocculant is used, it is added to the thin stock prior to the thin stock being passed through fan pump 1 which travel is indicated by arrow 4 and which addition is indicated by arrow 5. The high molecular weight (HMW) flocculant polymer is added to the thin stock as it exits fan pump 1, as indicated by arrow 6, and the silica-acid colloid blend is added to the thin stock as the thin stock exits pressure screen 2, as indicated by arrow 7. Fan pump 1 and pressure screen 2 produce high shear stages in the paper machine.

In the invention, the HMW flocculant polymer of the microparticle system is preferably added before the thin stock reaches the last point of high shear and the resultant thin stock is preferably sheared, e.g., at the last point of high shear, and preferably before adding the silica-acid colloid blend of the microparticle system of the invention. In the single figure, the HMW flocculant is shown as being added before the thin stock travels through pressure screen 2 and the silica-acid colloid blend is shown as being added after the stock passes through pressure screen 2.

Preferably, the HMW flocculant polymer of the microparticle system of the invention is added to the thin stock (i.e. having a solids content of desirably not more than 2% or, at the most, 3% by weight) rather than to the thick stock. Thus, the HMW flocculant polymer may be added directly to the thin stock or it may be added to the dilution water that is used to convert thick stock to thin stock. The HMW flocculant polymer comprises an agent for aggregating the solids, especially the fines, in the papermaking furnish. As used herein, "fines" means fine solid particles and fibers as defined in TAPPI Test Methods T261 and T269, respectively. Flocculation of the fines of the furnish may be brought about by the HMW polymer itself or in combination with a high charge density cationic coagulant or the MMW flocculant. Flocculation of fines gives better retention of the fines in the fiber structure of the forming paper sheet, thereby giving improved dewatering or drainage. The HMW flocculant is a polymer which provides flocculant action, preferably, by itself.

Examples of HMW flocculant polymers suitable for use in the invention are those having a weight average molecular weight of about 100,000 or more, especially 500,000 or more. Preferably, the weight average molecular weight is above about 1 million and often above about 5 million, and most typically in the range of 10 to 30 million or more. These polymers may be linear, branched, cationic, anionic, nonionic, amphoteric, or hydrophobically modified polymers of acrylamide or other nonionic monomers .

The amount of HMW flocculant of the microparticle system added to the stock or furnish in the present invention may be any amount sufficient to give a substantial effect in flocculating the solids, especially the fines, which are present in the furnish. The total amount of water soluble polymer added may be in the range of about 0.0025% to about 1%, more preferably in the range 0.01 wt. % to 0.2 wt . %, and most preferably in the range of about 0.0125 wt . % to about 0.1% by weight (dry weight of polymer based on the dry weight of the solids present in the furnish) . The addition may be carried out in one or more doses at one or more addition sites and, preferably, is carried out in one dose to the thin stock flow after the fan pump, which causes a high shear action.

Desirably, the floes formed by the HMW polymer flocculant are subjected to a shearing action before addition of the silica-acid colloid blend of the microparticle system of the invention. Preferably, this shearing action is induced by a pressure screen which causes a high shear action.

The microparticle particulate material of the invention in a preferred embodiment is comprised of a silica-acid colloid blend such as those compositions disclosed in the aforesaid U.S. Patent 5,382,378, the teachings of which are incorporated herein by reference. For example, a blend of melamine aldehyde, urea aldehyde, or melamine-urea aldehyde and colloidal silica comprise the silica-acid colloid blend of the invention. As U.S. Patent No. 5,382,378 describes, a silica sol is a stable dispersion of discrete, colloidal size particles of amorphous silica in aqueous solutions. Silica sols range from 5 to 50% Si0₂ and have particle sizes less than 1 micron. The stability of the silica sol depends on maintaining a high electrostatic repulsion between the silica particles. The pH must be alkaline to maintain the negative charge on the silica particles to prevent aggregation. The colloidal solutions are much less stable at low pH and tend to gel . Suitable silica sols for use in the invention have a particle size of less than 1 micron, preferably 3 to 20 nm, and having 0.5% to 50% solids by weight.

In a preferred embodiment, for the acid colloid component of the microparticle, any melamine aldehyde type polymer can be used. This polymer is prepared by using a) melamine or a substituted melamine; and b) an aldehyde having the formula: 0

R_I — c — H

wherein Rl is selected from the group consisting of straight and branched C _₄ alkyl . Dialdehydes may also be used. The dialdehyde may be straight-chain or cyclic which contains 2 to 8 carbon atoms and which may be C- substituted and contain heteroatoms. The preferred aldehydes are methanal (formaldehyde), ethanal, propanal, glyoxal, and glutaraldehyde . The most preferred aldehyde is formaldehyde .

The mole ratio of component a) to component b) above should range from about 1:1 to about 1:10, with the preferred ratio being about 1:3 to 1:6. The most preferred mole ratio is about 1 mole of melamine or derivative thereof to about 3 moles of an aldehyde. Thus the most preferred polymer is prepared from melamine and formaldehyde, and the mole ratio of melamine to formaldehyde is about 1:3.

The melamine aldehyde type polymer of the invention is insoluble in water, but can be maintained in a colloidal suspension in acidic solutions. Any acid or compatible combination of acids can be used to prepare the melamine aldehyde acid colloids of the microparticle particulate material of the invention, although hydrochloric acid is preferred. The active content of the melamine aldehyde-type polymer in acidic suspension or solution should range from about 0.1% to about 20%, preferably 1% to about 15%, and most preferably about 4% to about 12%. The pH should be sufficiently low, between 1.0 to 2.5 with an aqueous mineral or organic acid, to keep the melamine aldehyde type polymer in fine colloidal suspension.

Urea aldehyde type polymer solutions suitable for use in the present invention are those wherein the aldehyde is defined as above, most preferably urea- formaldehyde solutions. The mole ratio of urea to aldehyde should range from 1:1 to 1:10 with the most preferred ratio being 1:3 to 1:6. Melamine urea aldehyde copolymer solutions may also be employed in the present invention. These solutions are prepared from an aldehyde component as described above, urea, and melamine or a substituted melamine. Preferred are melamine-urea-formaldehyde copolymer solutions. The melamine-urea-aldehyde copolymer solutions suitable for use in the present invention contain 1 to 70 mole percent urea, 30 to 99 mole percent melamine, and about 1 to 4 moles of aldehyde for each mole of combined melamine and urea in the acidic aqueous medium. The copolymer solution for use in the present invention ranges from 0.1 to 20 percent solids, and preferably 1 to 12 percent solids.

The acid colloid may be a copolymer comprising melamine aldehyde and condensates which include ammeline- aldehyde, dicyandiamidealdehyde, biguanidine-aldehyde, ureaformaldehyde polyalkylene polyamine, and polyureido.

The acid colloid is prepared by reacting the specified aldehydes with the amine and aging the solution under acid conditions, typically using hydrochloric acid. As aging proceeds, the colloidal particles grow to a size of 20 to 200 Angstroms. The average degree of polymerization is from 10 to 20 methylolated melamine units. The particle carries a cationic charge, i.e. some of the secondary amine units are protonated. The colloidal solutions characteristically exhibit a blue haze. The solutions are stored at a concentration of 8- 12% active. The solutions may be composed exclusively of amine and aldehyde, or may be derivatives thereof. The solutions may be partially etherified with an alcohol, glycol, or other hydroxyl containing species. The solutions may be a co-condensate of melamine - formaldehyde and another aminoplast , which can then be etherified. The solutions may be a mixture of such aminoplasts, which are then used to form the acid colloid. The aminoplasts that form the colloid may also be copolymers of ethylenically unsaturated monomers such as acrylamide, dimethylaminoethyl acrylate, diallyl dimethyl ammonium chloride (DADMAC) , or methacrylamidopropyl trimethylammonium chloride and the like.

In a preferred embodiment, the compositions of the microparticle of the present invention are similar to and are prepared similarly to those disclosed in U.S. Patent 5,382,378, which is incorporated herein by reference. The pH of the colloidal silica solution is first dropped to between 1.3 and 2.0, using 10% hydrochloric acid. The acid colloid solution, e.g. melamine urea aldehyde copolymer, is then added with stirring. The resultant blend should have a pH of about 1.0 to 3.0, and preferably a pH of 1.5. The blends can be from 1.0 to 50% total solids. In the invention, these silica-acid colloid blends are applied as part of a microparticle system for a drainage, retention, and sheet formation program with respect to making paper or paperboard.

The amount of silica-acid colloid of the microparticle particulate material of the microparticle system of the invention which is added to the paper or furnish may range between about 0.0005% to about 0.5%, and preferably from about 0.005% to about 0.25% by dry weight based on the dry weight of the solids in the furnish. The addition may be carried out in one or more doses at one or more addition sites, but preferably, in one dose, and preferably after the pressure screen 2 in the single figure, and at least between pressure screen 2 and headbox 3.

The silica-acid colloid blend of the microparticle system of the invention comprises colloidal silica and an acid colloid, preferably melamine formaldehyde acid colloids and derivatives thereof as described hereinabove .

The addition of the HMW flocculant polymer generally will cause the formation of large floes of the suspended solids in the paper or furnish to which the polymer is added. These large floes are immediately or subsequently broken down by high shear to very small floes that are known in the art as "microfIocs" . This "high shear" may be induced by passing the flocced furnish through pressure screen 2 of the single figure.

The water soluble coagulant is generally lower in molecular weight than the HMW flocculant added to the stock before the pressure screen 2, and preferably is added to the stock prior to the stock passing through the fan pump 1 of the single figure. This coagulant, preferably, is a high charge density cationic polymer. For instance, if the coagulant polymer is a nitrogen containing cationic polymer, it may have a charge density of about 0.2, preferably, at least 0.35 and, most preferably, 0.4 to 2.5 or more, equivalents of nitrogen per kilogram of polymer. When the polymer is formed by polymerization of cationic, ethylenically unsaturated, monomer optionally with other monomers, the amount of cationic monomer will normally be about 2 mole % and usually about 5 mole %, and preferably, at least about 10 mole %, based on the total amount of monomers used for forming the polymer.

Suitable cationic coagulants include: polydiallyldimethyl ammonium chloride (p-DADMAC) ; polyalkylamines ; cationic polymers of epichlorohydrin with dimethylamine and/or ammonia or other primary and secondary amines; polyamidoamines ; copolymers of a nonionic monomer, such as acrylamide, with a cationic monomer, such as DADMAC or acryloyloxyethyltrimethyl ammonium chloride; cyanoguanidine modified polymers of urea/formaldehyde resins; melamine/formaldehyde polymers; urea/formaldehyde polymers; polyethylene imines; cationic starches; monomeric and polymers of cationic aluminum salts; amphoteric polymers processing a net cationic charge; and blends of the aforementioned coagulants. The amount of cationic coagulant polymer of the microparticle system of the invention added to the stock or furnish may be any amount sufficient to give a substantial effect in coagulating the solids present in the paper or furnish. The total amount of water soluble coagulant polymer may be in the range of about 0.0025 to 1.0 wt. %, more preferably in the range of about 0.005 wt . % to about 0.50 wt . % dry weight based on the dry weight of the solids present in the furnish.

If a MMW flocculant is to be used instead of the cationic coagulant, this flocculant may be added prior to the stock passing through the fan pump 1. Examples of a MMW flocculant suitable for use in the invention are those having a weight average molecular weight ranging from 500,000 to about between 5 and 6 million. This chemical additive may be a copolymer of an acrylamide or any unsaturated monomer. A suitable MMW flocculant may include the ECCat™ 500 copolymers available from Calgon Corporation, PA.

The amount of MMW flocculant may be any amount sufficient to give a substantial effect in coagulating the solids present in the paper or furnish. The total amount of MMW flocculant may be in the range of about 0.0025 to 1.0 wt . % based on the dry weight of the solids present in the furnish. The dosages would range from 0.01 to 5.0 lb. /ton polymer. As mentioned hereinabove, the cationic coagulant or the MMW flocculant may be added to the thick stock prior to the fan pump, the HMW flocculant polymer may be added to the thin stock after the stock's passage through the fan pump 1, and the silica-acid colloid blend of the invention may be added to the thin stock after the stock's passage through pressure screen 2 of the single figure. Alternatively, these chemical additives may be added to the stock in a different sequence than that shown in the figure. That is, the silica-acid colloid blend may be added before the fan pump 1, the HMW flocculant may be added after the pressure screen 2, and the coagulant or MMW flocculant may be added before pressure screen 2. There may be other sequences for the feed points for the chemical additives in the paper machine .

The initial thick stock can be made from any conventional papermaking stock, such as, traditional chemical pulps, for instance bleached and unbleached sulphate or sulphite pulp; mechanical pulps such as groundwood; thermomechanical pulp; or chemi- thermochemical pulp; or recycled pulp, such as deinked waste, fiber filler composites from aggregating or recycling processes; and any mixtures thereof.

The furnish or stock employed in the invention, and the final paper, can be substantially unfilled (e.g., containing less than 10% and generally less than 5% by weight filler in the final paper) , or filled with a filler which can be provided in an amount of up to 50% based on the dry weight of the solids in the furnish, or up to 40% based on the dry weight of the paper. When filler is used, any conventional white pigment filler such as calcium carbonate, kaolin clay, calcined kaolin, titanium dioxide, or talc, or a combination thereof may be present. The filler (if present) is preferably incorporated into the furnish in a conventional manner, and before addition of the components of the microparticle system of the present invention.

The furnish or stock employed in the invention may include other known optional additives, such as, rosin, alum, neutral sizes or optical brightening agents. It may include a strengthening or binding agent, and this can, for example, comprise a starch, such as cationic starch. The pH of the furnish is generally in the range of from about 4 to about 9. The amounts of fiber, filler, and other additives, such as, strengthening agents or alum can all be conventional. Typically, the thin stock has a solids content of 0.1% to 3% by weight or a fiber content of 0.1% to 2% by weight. The thin stock will usually have a solids content of from 0.1% to 2% by weight. These percentages are based on the dry weight of the solids in the furnish.

As discussed herein above, the silica-acid colloid blend employed as the microparticle particulate material in the microparticle system of the invention comprises colloidal silica and an acid colloid or derivatives thereof. Preferably, the acid colloid is comprised of an aqueous solution of a water-soluble polymer which preferably is a melamine aldehyde, preferably, melamine- formaldehyde . This particulate material is readily dispersed in an aqueous pulp suspension in a papermaking process to enhance the surface characteristics of the final paper product. The inventors have found that silica-acid colloid blends in conjunction with a HMW flocculant by itself or with a coagulant or a MMW flocculant can increase drainage and retention, and improve sheet formation in a papermaking process . Experiments

The following examples demonstrate the invention in greater detail and are not intended to limit the scope of the invention in any way. In these examples the following products were used:

Polymer A: a 25 weight % active acrylamide - acryloyloxyethyltrimethylammonium chloride copolymer available from Calgon Corporation (Pittsburgh, PA) , comprising about 90 mole % acrylamide and about 10 mole % acryloyloxyethyltrimethyl-ammonium chloride . Polymer B: an anionic flocculant - a 28 wt % active anionic acrylamide - acrylic acid copolymer available from Calgon Corporation, (Pittsburgh, PA) , comprising about 70 mole % acrylamide and about 30 mole % acrylic acid.

Polymer C: a medium molecular weight cationic copolymer of acrylamide and diallyldimethylammonium chloride available from Calgon Corporation (Pittsburgh, PA) .

Polymer D: a medium molecular weight terpolymer of acrylamide, diallyldimethylammonium chloride, and acrylic acid available from Calgon Corporation (Pittsburgh, PA) .

Melamine-formaldehyde (MF) acid colloid: an 8% active solution available from Calgon Corporation (Pittsburgh, PA) .

Colloidal Silica: a 15% active solution available from DuPont (Wilmington, DE) .

Carbital 60: a dry, ground calcium carbonate available from ECC International Inc. (Atlanta, GA) .

Stalok^® 400 ( a Federal Trademark of A.E. Staley) and Interbond C: cationic starches available from A.E. Staley.

Hercon 70: an AKD (alkylketene dimer) size available from Hercules, Inc.

Examples 1-16 - Alkaline Fine Paper Furnish Furnish Preparation

A synthetic alkaline fine paper furnish was prepared and used for drainage and retention tests and making handsheets. This furnish was prepared with the following components : Fiber: 50 / 50 wt % bleached hardwood

Kraft / bleached softwood Kraft

Filler: 50 / 50 wt % ground calcium carbonate (Carbital 60) / precipitated calcium carbonate.

Filler loading: 20 wt % based on fiber solids

Starch: 0.5 wt % (Interbond C) based on fiber solids

Size: 0.25 wt % Hercon 70 (AKD)

A dry lap pulp was soaked in tepid water for 10 minutes, diluted with water to a consistency of 2 wt % solids, and refined or beaten with a Laboratory Scale Voith Allis Valley Beater to a Canadian Standard Freeness (CSF) of 590 ml. The starch, size, and filler were added in this sequence to the refined pulp slurry. The pH of the pulp slurry was typically 7.5 + 0.3. The pulp slurry was diluted further with tap water to approximately 1.0 wt % consistency to form thin stock for drainage and retention tests and for making hand sheets. The furnish is representative of a typical alkaline fine paper furnish used to make printing and writing grades of paper.

Drainage Test Procedure

1. 200 ml (2g solids) of furnish at 1 wt % headbox consistency were poured into a square mixing jar and diluted to 500 ml with tap water.

2. These contents were mixed using a standard Britt Jar style propeller mixer (1 inch diameter) under the following mixing time (seconds) and speed (rpm) conditions to simulate chemical addition to the secondary fan pump inlet, fan pump outlet, and pressure screen outlet: Time Speed (rpm) Additive Feed Point t_n 1200 Coagulant Pre-fan t₁₀ 1200 Flocculant Pre-screen t₂₀ 600 D/R/F aid Post-screen t₃₀ Stop

3. The contents in the mixing jar were transferred to a 500 ml graduated drainage tube fitted on the bottom with a 100 mesh screen. The tube was inverted 5 times to ensure that the stock was homogenous. The bottom stopper of the tube was removed and the elution times for 100, 200, and 300 ml elution volumes were measured. The elution time at a volume of 300 ml for an untreated stock blank should preferably be greater than 60 seconds. 4. The improvement in drainage provided by a treatment was calculated as follows based on the drainage time for an untreated, blank sample:

(Drainage Time With No Treatment(s) - Drainage Time With Treatment(s)) % Drainage Improvement = x 100%

Drainage Time With No Treatment(s)

Retention Test Procedure (FPR, FPAR) - TAPPI Test Method T269

1. 500 ml of furnish at headbox consistency (1.0%) were poured into a Britt Jar with a 70 mesh screen while stirring the stock at 1200 rpm.

2. The mixing time (seconds) / speed (rpm) sequence was similar to that used in the drainage test procedure above in order to simulate chemical addition points with the following change: at t₃₀, the bottom stop cock was opened and the first 100 ml of eluate were collected.

3. This eluate was filtered through a Whatman No. 4 filter paper and dried at 105°C. 4. The pad was burned at 600°C for 2 hours to determine ash retention. Hand Sheet Preparation and Testing

Hand sheets were prepared at 70 grams per square meter basis weight using a Noble & Wood Hand Sheet Mold.

This apparatus generates a 20 cm x 20 cm square hand sheet . The mixing time / speed sequence used in preparing hand sheets was the same as the sequence used for the drainage test procedure. The treated furnish sample was poured into the deckle box of the Noble & Wood handsheet machine and the sheet was prepared employing standard techniques well known by those skilled in the art.

Sheet Properties

Formation was tested on the hand sheets using an MK

Systems Formation Tester, Model M/K 950R. Brightness and opacity measurements were done using a Technidyne Color Touch, Model ISO.

Preparation of the Blends

The colloidal silica-acid colloid blends were prepared according to the following procedure:

1) The amounts of the colloidal silica solids and the acid colloid solids in the ratios of Table 1 were calculated and the specified amounts for each solution were weighed into separate beakers.

2) A pH probe attached to a pH meter was placed into the colloidal silica solution. The pH of this solution was dropped to about 1.5 by adding 10% hydrochloric acid.

3) The acid colloid solution was added to the colloidal silica solution while stirring the colloidal silica solution. 4) The pH of the final blend of 3) above was adjusted to 1.5 by adding 10% hydrochloric acid.

Table 1 summarizes the ratio of the compositions for the silica-melamine formaldehyde (MF) blends used in Examples 1 - 16. Table 1 Summary of Blends

Examples 1 - 7

Table 2 shows the drainage results for Examples 1-7, and Table 3 shows the retention results for these same Examples 1-7. Examples 2 and 4 particularly show the effectiveness of using a silica-MF blend ratio of 4:1. Drainage :

The dosages in Examples 1- 7 are expressed as being active based on lb. /ton of dry pulp. The data in Table 2 show the effectiveness of the silica-MF blend ratio of 4:1 in increasing the dewatering rate of furnish. As Example 2 indicates, the use of 1.0 lb. /ton of the 4:1 blend ratio increases drainage, i.e. the drainage rate of Example 2 which used the silica-MF blend ratio of 4:1 shows an increase when compared to Example 1, i.e. a 57% drainage rate for Example 2 compared to a 39% drainage rate for Example 1. As can be seen by Examples 3 and 4 in Table 2 , further increases in the dosages of the silica-MF blend ratio of 4:1 increased the drainage even further, i.e. 63% for Example 3 and 68% for Example 4.

The silica-MF blend ratio of 4:1 (Example 4) also performed better than did silica by itself (Example 7) at similar dosages of 2.0 lb. /ton and with less starch, i.e.5 lbs. /ton for Example 4 compared to 15 lbs. /ton for Example 70, when added to the furnish at the inlet to the fan pump. This is significant in that the papermaker can save money by reducing the amount of starch needed in the papermaking system. Table 2 4:1 Blend -Drainage Improvement

Retention:

Table 3 indicates the ability of the silica-MF blend ratio of 4:1 of the invention to increase the first pass retention (FPR) and the first pass ash retention (FPAR) in a drainage/retention/formation program. Retention was measured using the Britt method (TAPPI Test Method

T269) with the mixing sequences mentioned herein above.

Example 1, which does not involve a microparticle particulate material, has a FPR of 85.2%. When 2 lbs . /ton of the silica-MF blend ratio of 4:1 were added, as shown in Example 4, the FPR increased to 92.0% and the

FPAR increased from 61% (Example 1) to 80.1% (Example 4) .

This increase in FPR and FPAR may be important factors to the papermaker in that the papermaking process may become more efficient, filler usage may be decreased, and sheet properties may be improved. The silica-MF blend ratio of

4:1 of the invention again allows less starch to be used while achieving retention results similar to those examples using only silica, i.e. Example 3 (4:1 blend) compared to Example 6 (silica) . Table 3 4:1 Blend -Retention

Examples 8 - 12 1:1 Blend Table 4 shows the drainage results for Examples 8 - 12 and Table 5 shows the retention results for Examples 8 - 12. These results show the effectiveness in drainage and retention enhancement when increasing the dosage of the silica-MF blend ratio of 1:1 in Table 1 from 1.0 lbs . /ton to 2.0 lbs. /ton. This 1:1 ratio blend, as well as the silica-MF blend ratio of 4:1, performs similarly to silica but at a lower starch level, i.e. 5 lbs . /ton as compared to 15 lbs. /ton for silica.

Table 4 1: 1 Blend -Drainage

Table 5 1 : 1 Blend - Retention

Examples 13 - 16: 1:4 Blend Table 6 shows the drainage results and Table 7 shows the retention results for Examples 13 - 16. These results show a drainage and retention improvement when the silica-MF blend ratio of 1:4 of Table 1 is added to the DRF program. Again, as seen in Tables 6 and 7, the drainage and retention are improved when the silica- MF blend ratio of 1:4 of the invention is used, i.e. Examples 14-16 as compared to Example 13 where no blend was used.

Table 6 1:4 Blend - Drainage

Table 7 1:4 Blend -Retention

[ Stalok^® Is A Federal Trademark of A . E . Staley . )

Stability of the Blends:

As Table 8 shows, the different ratio blends of silica-MF have different stabilities at pH = 2.0, depending on the composition. The following observations were made .

Table 8 Stability of the Blends

As the silica-MF blend ratio of 1:4 shows, the more acid colloid in the blend, the longer it is stable at acidic pH, i.e., greater than 180 days. The 12 % MF was made by concentrating an 8% total solids solution. The blends of the invention have much better stability than either of the components separately, that is, 15 and 90 days versus 150 days or more for the silica-acid colloid blends of the invention. Examples 17 - 21: Lightweight Coated (LWC) Furnish The remainder of the examples used a silica-MF blend ratio of 9:1 (wt . /wt . solids) at 15% total solids. The silica used was a 30% solids silica available from DuPont (Wilmington, DE) . Blends were prepared as described herein above.

Examples 17 - 21 in Tables 9-A and 9-B illustrate the effectiveness of the instant invention in improving drainage, retention, and sheet properties of a synthetic aqueous furnish. This furnish represents a typical furnish used to manufacture a base sheet for lightweight coated grades. The furnish was treated for 15 minutes with 15 lb./ton Stalok 400 starch before retention aids were added.

Furnish Preparation:

The synthetic furnish used for drainage and retention tests and for making hand sheets was prepared with the following components:

Fiber: 45 wt . % bleached softwood kraft

(SWK)

55 wt . % chemithermomechanical pulp (CTMP) Filler: Calcined Clay

Filler Loading: 10 wt . % based on oven dry fiber weight Alum: 0.5 wt . % based on oven dry iber weight

In preparing the furnish, CTMP was soaked in hot water for 15 to20 minutes, diluted to 1.5 wt . % solids in water, and refined or beaten with a Laboratory Scale Voith Allis Valley Beater to a Canadian Standard Freeness (CSF) of 200 ml. The SWK was soaked separately in water, diluted to 1.5 wt . % solids, and refined or beaten to a CSF of 550 ml. The above fibers were then blended to the proportions listed above and the calcined clay was added. The pH of the furnish was adjusted to 5.0 with dilute sulfuric acid and the conductivity was adjusted to 2000 μS/cm with sodium sulfate.

Table 9-A LWC Furnish

Table 9-B LWC Furnish

Examples 18 - 21 in Tables 9-A and 9-B show the effect of adding the microparticle blend to the LWC furnish in a post-screen position. As the dosage of the microparticle blend is increased, drainage, FPR, and FPAR increase significantly. These benefits may be important in that the papermaker may be able to increase the speed of the paper machine in that the sheet may dry faster while retaining more of the fillers and fines of the furnish in the sheet compared to a sheet not treated with the microparticle system of the invention. The results in Tables 9-A and 9-B also show that sheet formation decreases with the use of the invention while sheet brightness and opacity remain relatively unchanged compared to a sheet not treated with the microparticle system of the invention. Examples 22 - 26: Board Furnish

Examples 22 - 26 of Tables 10-A and 10-B illustrate the effectiveness of the invention in improving drainage, retention, and sheet properties of a synthetic aqueous furnish. This furnish represents a typical furnish used to manufacture a base sheet for paperboard.

Furnish Preparation:

A furnish was prepared by disintegrating 360 g of unbleached old corrugated cardboard (OCC) in tepid water and diluting to 23 liters with tap water. The pulp was then refined by a Laboratory Scale beater similar to the previous Examples to a CSF of 300 ml. 18 liters of this stock was diluted to 0.5 wt . % consistency and the following salts were added to adjust the water chemistry to paper mill conditions: 5.61 g Calcium Chloride; 0.96 g Potassium Chloride; 8.17 g Alum (50 wt.%); 15.96 g Sodium Sulfate; 0.59 g Sodium Bicarbonate; and 0.97 g Sodium Silicate. The conductivity measured approximately 2000 μS/cm. The pH was adjusted to 5.0 with dilute sulfuric acid.

Table 10-A Board Furnish

Table 10-B Board Furnish

In Tables 10-A and 10-B, Examples 22, 25, and 26 illustrate the effect of using 20 pounds of starch in conjunction with the microparticle blend of the invention in a paperboard furnish. Also, Tables 10-A and 10-B shows that an increase in the dosage of the microparticle blend of the invention and the starch increase drainage, FPR, and FPAR. These results also show that the microparticle blend also performs well when 10 pounds of starch are used in the furnish. This seems to indicate that the invention may not require high levels of starch in order to be effective in some furnishes. It has been shown that traditional silica programs typically require high dosages of starch in order to be effective, whereas this does not appear to be the case when using the microparticle system of the invention.

Examples 27 - 30: Newsprint Furnish Examples 27 - 30 of Tables 11-A, ll-B and 11-C illustrate the effectiveness of the invention in improving drainage, retention, and sheet properties of a synthetic aqueous furnish. This furnish represents a typical groundwood furnish used to manufacture a base sheet for newsprint . Furnish Preparation:

The synthetic furnish used for drainage and retention tests and for making hand sheets was prepared as follows :

Fiber: 80 wt.% CTMP

10 wt.% SWK

10 wt.% recycled newsprint

Filler: Calcined Clay

Filler Loading: 4 wt.% based on oven dry fiber weight

Alum: 50 lb./ton

In preparing this furnish, CTMP was soaked in hot water (140°F) and defibered m a blender for 15 to 20 minutes . The recycled newsprint was treated separately in the same fashion. The SWK was soaked for two hours in tepid water and defibered in a blender for 15 to 20 minutes .

The CTMP, recycled newsprint, and SWK were blended together and refined at a consistency of 1.5 wt.% by a Laboratory Scale beater similar to the previous examples to a CSF of 50 to 75 ml. The calcined clay and alum were added to make the final pH 4.8 to 5.2. The conductivity of the stock was adjusted to 2000 μS/cm using sodium chloride .

Table 11-A Newsprint Furnish

Table 11-B Newsprint Furnish

Table 11-C Newsprint Furnish

In Tables 11-A - 11-C, Examples 27 and 28 illustrate that there is an increase in drainage, FPR, and FPAR when the microparticle blend of the invention is added to newsprint furnish m the post-screen position. This increase m drainage, FPR and FPAR may be important considerations m a high-speed newsprint machine. These results for Examples 27 and 28 also show that the microparticle system of the invention is effective even without starch. These factors may allow the paper manufacturer to reduce costs, thereby lowering the total cost per ton of paper being produced. Commercial Machine Application Alkaline Fine Paper The performance of the invention was evaluated on a commercial alkaline fine paper machine. The microparticle blend program of the invention was compared to a baseline program. The baseline program was ran on a machine where Polymer B was fed pre-screen (0.15 lb/ton active) and Polymer C was fed to the furnish post-screen (1.0 lb/ton active) . For the microparticle program of the invention, Polymer C was fed to the furnish at the pre-screen stage, and the microparticle blend was fed at the post-screen stage (1.0 lb active) . It is to be noted that dosages are approximate and were changed with differing basis weight and paper grade. The program of the invention was also compared to a typical silica program where polyamine was fed to the tray (1.5 lb/ton active) , HMW anionic polyacrylamide was fed at the pre- screen stage (1.0 lb/ton active), and colloidal silica was fed at the post-screen stage (1.5 lb/ton active) . The results of this comparison are shown in Tables 12 and 13.

Table 12 Paper Machine Comparison - 80 lb. Basis Weight

Table 13 Paper Machine Comparison - 100 lb . Basis Weight

From Table 12 (80 lb. basis weight) it can be seen that the microparticle blend of the invention results in improved sheet formation (lower number is better) , lower steam usage, and higher press solids compared to the other programs. At equivalent steam usage, the invention increased the speed of the machine by 13.6% over the baseline program and 3.1% over the traditional silica program. This factor will allow the papermaker to increase the speed of the machine resulting in a greater production of paper and/or less energy for the production of steam. These results also show better sheet formation when the microparticle blend of the invention is used. This factor may be important in that a more uniform, higher quality sheet of paper can be produced.

Table 13 (100 lb. basis weight) illustrates a comparison between using the microparticle blend of the invention and using the other programs in a 100 lb. basis weight paper. On a heavier weight paper, dewatering is critical in the thicker sheet requires more energy to dry. The invention is shown as providing better sheet formation, higher press solids, and lower steam usage when compared to the baseline program or to the silica program. These factors of the invention may allow the papermaker to make a higher quality sheet at a higher paper machine speed.

Whereas particular embodiments of the present invention have been described for purposes of illustration, it will be evident to those skilled in the art that numerous variations and details of the invention may be made without departing from the invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A microparticle system used as a retention and drainage aid in a paper furnish, comprising: a) a HMW polymer flocculant being present in an amount of from about 0.0025 % to about 1.0% by weight based on the dry weight of the solids in said furnish, and b) a silica-acid colloid blend comprised of:

(1) an acid colloid comprised of an aqueous solution of a water-soluble polymer or a copolymer; and

(2) a colloidal silica, wherein said acid colloid and said colloidal silica are blended in the range of 99.5:0.5 to 0.5:99.5, respectively, on a total solids basis in an acid environment of pH 3.0 or less,

said silica-acid colloid blend being present in an amount of from about 0.0005% to about 0.5% by weight based on the dry weight of the solids in said furnish.

2. The system of Claim 1 wherein said polymer is selected from the group consisting of a melamine aldehyde, melamine is substituted or unsubstituted; and the aldehyde is

O

R, H

wherein Rl is selected from the group consisting of straight and branched C_x_₄ alkyl .

3. The system of Claim 2 wherein said aldehyde is selected from the group consisting of formaldehyde, ethanal, propanal, glyoxal, and glutaraldehyde .

4. The system of Claim 3 wherein said aldehyde is formaldehyde .

5. The system of Claim 4 wherein the water soluble polymer is melamine-formaldehyde .

6. The system of Claim 5 wherein the melamine- formaldehyde is etherified with a linear or branched alcohol .

7. The system of Claim 1 wherein said silica-acid colloid blend comprises colloidal silica and an acid colloid of a copolymer of melamine-formaldehyde and urea- formaldehyde .

8. The system of Claim 1 wherein said silica-acid colloid blend comprises said colloidal silica and an acid colloid of a copolymer comprising said melamine aldehyde and condensates selected from the group consisting of ammeline-aldehyde, dicyandiamidealdehyde, biguanidine- aldehyde, ureaformaldehyde polyalkylene polyamine, and polyureido .

9. The system of Claim 1 wherein said silica-acid colloid blend comprises said colloidal silica and an acid colloid of a copolymer of amine-aldehyde-type and ethylenically unsaturated monomers selected from the group consisting of acrylamide, dimethylaminoethyl acrylate, diallyldimethyl ammonium chloride, and methacrylamidopropyl trimethylammonium chloride .

10. The system of Claim 1 further comprising: a high charge density cationic coagulant being present in an amount of from about 0.005% to about 0.5% by weight based on the dry weight of the solids in said furnish.

11. The system of Claim 1 further comprising: a medium molecular weight flocculant being present in an amount of from about 0.0025 to 1.0 wt . % based on the dry weight of the solids in the furnish.

12. A paper product made with the microparticle system of Claim 1.

13. A paper product made with the microparticle system of Claim 2.

14. A method for producing paper products, the steps comprising: a) after a first high shearing stage and prior to a second high shearing stage, adding a high molecular weight polymer flocculant to a paper furnish in an amount of from about 0.0025% to about 1.0% by weight based on the dry weight of the solids in said furnish; and b) before or after said second high shearing stage, adding to the paper furnish a silica-acid colloid blend being present in the amount of from about 0.0005% to about 0.5% by weight based on the dry weight of the solids in said furnish.

15. The method of Claim 14, the steps further comprising : c) prior to said first high shearing stage, adding a high charge density cationic coagulant to the furnish in an amount of about 0.005% to about 0.5% by weight based on the dry weight of the solids in said furnish.

16. The method of Claim 14 wherein said silica-acid colloid blend is comprised of melamine-formaldehyde acid colloid.

17. The method of Claim 14 the steps further comprising: c) prior to said first high shearing stage, adding a medium molecular weight polymer to the furnish in an amount of about 0.0025% to about 1.0% by weight based on the dry weight of the solids in the furnish.

18. A method for producing paper products, the steps comprising: a) after a first high shearing stage and prior to a second high shearing stage, adding to the paper furnish a silica-acid colloid blend being present in the amount of from about 0.0005% to about 0.5% by weight based on the dry weight of the solids in said furnish; and b) before or after said second high shearing stage, adding to the paper furnish a high molecular weight polymer flocculant in an amount of from about 0.0025% to about 1.0% by weight based on the dry weight of the solids in the furnish.

19. The method of Claim 17, the steps further comprising: c) prior to said first high shearing stage, adding a high charge density cationic coagulant to the furnish in an amount of about 0.005% to about 0.5% by weight based on the dry weight of the solids in said furnish.

20. The method of Claim 17 wherein said silica-acid colloid blend is comprised of melamine-formaldehyde acid colloid.

21. The method of Claim 17 the steps further comprising: c) prior to said first high shearing stage, adding a medium molecular weight polymer to the furnish in an amount of about 0.0025% to about 1.0% by weight based on the dry weight of the solids in the furnish.