PRODUCTION OF HIGH BULK CALENDERED PAPER CONTAINING VESICULATED BEADS

Abstract: A high bulk calendered paper comprises from 2% to about 6% of the weight of the paper of highly cross-linked, titanium dioxide-pigmented, polymeric vesiculated beads having a volume average particle size of at least about 5 microns up to about 35 microns. The calendered paper preferably has a basis weight of from about 20 to about 40 pounds per ream, and the vesiculated beads are preferably used in an amount of from 4% to 6%. As a feature of the invention, it is found that the carboxyl-functional cross-linked beads used herein when used in aqueous suspension thick­ ened with a neutralized polycarboxylic acid polymer thick­ ener, enhance the retention of fines in the papermaking process when used in conjunction with anionic retention aids, especially carboxyl-functional acrylamide copolymers, and particularly when used in an aqueous furnish having a pH of from 4 to 6.

Description: Technical Field This invention relates to a method of making calendered paper containing vesiculated beads to provide brightness, opacity and smoothness while minimizing the density of the paper. Background Art Inorganic pigments are normally introduced into the paper furnish in order that the resulting paper will possess desired brightness, opacity and printability. These inorganic pigments generally have a particle size of less than 1 µm. Unfortunately, these inorganic pigments, and especially titanium dioxide, anatase, are dense and can increase the weight of the paper per unit area which is produced. Also, these very fine pigments, as well as many of the other finely divided components of paper pass through the Fourdrinier wire and are lost with the "white water". While these loses are minimized by recycling, there is a limit to how much recycling can be tolerated before the paper product is damaged, so the loses are substantial and some of the fines find their way back into our rivers and streams to pollute these waterways. More particularly, conventional papermaking processes normally retain only up to about 65% of the finely divided materials which are employed, including titanium dioxide, clay and carbonates, the remaining fines passing through the wire to be recovered or not in accordance with the recycling practices which are employed. In this invention, the retention of fines is considerably improved, usually by an increase in the ash content of from 7-10 percentage points. The mechanism by which this is accomplished is not entirely clear, but it will be understood that improved retention of fines is a serious industrial problem which has plagued the paper industry throughout its history. While titanium dioxide pigments are quite effective in increasing the desirable properties of opacity and brightness of any paper, they are expensive. Printing paper is normally finished in a calendering operation to improve printability and appearance for use in books, magazines, etc. Between the increased density which adds to the cost of producing each unit area, the cost of mailing, and the cost of the pigment, the conventional use of titanium dioxide adds considerably to the cost of producing and using such calendered paper. Attempts have been made to incorporate opaque cellular plastic particles into the paper furnish in order to provide the desired opacity. These cellular plastic particles were thermoplastic materials, and they collapsed under the heat and force of calendering, particularly supercalendering. When the cells collapsed, the opacity and bulk which they provided was lost. The calendering force is normally at least about 26.8 kg/cm (150 pounds per linear inch), and may be as high as about 35.7 kg/cm (2000 pounds per linear inch), and the temperature is at least about 38°C (100°F.), more usually at least about 66°C (150°F). Since these cellular plastic particles lose their bulk and opacifying power when they are collapsed, they are not very useful in making high bulk calendered paper. EP-A-0 204 916 discloses a high bulk caledered paper comprising from 0.5 to 15 weight percent of the weight of the paper of highly cross-linked, titanium dioxide-pigmented, polymeric vesiculated beads having a volume average particle size of at least about 5 µm up to 35 µm. The article in Abstract Bulletin of the Institute of Paper Chemistry, Vol. 53, No. 1, July 1982, page 75, Abstract No. 465 describes the addition of polyacrylamide to permit the increase in the ash content of printing papers. It does not disclose or suggest the use of carboxyl-functional, highly cross-linked, titanium dioxide pigmented, polymeric, vesiculated beads incorporated into paper furnish. This invention is directed toward maintaining the brightness and opacity of calendered paper while increasing its thickness (caliper) and resistance to print show through. It is also desired to minimize the need of inorganic pigments, especially titanium dioxide. As a result, the cost can be contained and the density of the calendered paper product minimized, while maintaining, or improving smoothness and printability. Disclosure of Invention The invention relates to a method of making paper comprising: adding to an aqueous paper furnish an aqueous suspension of carboxyl-functional, highly cross-linked, titanium dioxide-pigmented, polymeric, vesiculated beads having a volume average particle size of at least about 5 µm together with an anionic retention aid, said beads being pre-associated with a neutralized polycarboxylic acid polymer thickener, to thereby reduce the proportion of fines which pass through the wire into the white water; forming a high bulk paper with 2 % to 6 % by weight of polymeric vesiculated beads; and calendering said high bulk paper to produce a calendered paper product with a basis weight of up to about 65 g/m² (40 pounds per ream). Preferred embodiments of the invention are recited in the dependent claims. At about 2 % of highly cross-linked, titanium dioxide-pigmented, polymeric vesiculated beads, in very light paper, a minimal effect is observed. However, practical significant results require a paper having a basis weight in the range of about 32.5 to 65 g/m² (about 20 to about 40 pounds per ream), and for these papers a proportion of bead of from about 3.5 % to about 6 %, preferably about 4 % to 5 %, provides significantly improved practical results. Below 3.5 %, the effect is hard to notice in papers having the basis weight specified. Above about 6 %, the incorporation of beads of the character specified herein is not economical. Below 32.5 g/m² (20 pounds per ream) basis weight, the papers are not quality printing papers to which this invention relates, and above 65 g/m² (40 pounds per ream) basis weight, the bulking effect of the beads used herein becomes more difficult to observe. In order that the calendering operation will produce a paper which is useful as a quality printing paper, the calendering operation must exert some considerable force upon the paper of the character which would crush most beads. This paper is calendered at a pressure of at least about 26.8 kg/cm (150 pounds per linear inch) and at a temperature of at least about 38°C (100°F) to provide a calendered paper product which can qualify as a quality printing paper, and under these conditions the pigmented vesiculated beads used herein provides improved bulk. It is preferred herein to provide papers suitable for offset printing to employ a calendering operation which applies a pressure of from 35.7 to 214 kg/cm (200 to 1200 pounds per linear inch), the improvement provided by this invention being enhanced at these greater loadings. For gravure printing, a pressure of from 125 to 214 kg/cm (700 to 1200 pounds per linear inch) is preferred. More particularly, when the vesiculated beads are formed to contain titanium dioxide, it is found that the calendered paper product is brighter and more opaque per unit of caliper as a result of the presence of these beads. This effect is proportional to the amount used. Moreover, it is particularly surprising to find that there is a significant improvement in printing smoothness, and this superior smoothness improves the ink hold-up. It is found that equivalent quality calendered paper can be made satisfactorily at lower basis weight. "Basis weight" is a commonly used papermaking term identifying the weight per unit area. For a paper of given thickness (caliper), at a lower basis weight, the vesiculated beads have been found to provide better resistance to print show through and a smoother, more uniform surface fron side to side in calendered sheets than a control containing all mineral pigment, such as titanium dioxide. It is stressed that the increased bulk action provided by this invention requires vesiculated beads of at least about 5 µm, with larger beads providing even better bulking than the smaller ones. Beads having a diameter of up to about 35 µm can be used, but it is preferred to employ a volume average particle size in the range of from 8 to 30 µm. The term "vesiculated" denotes the presence of a plurality of vesicles or cells. An average of at least about 5 cells per bead is contemplated herein, preferably at least about 10 cells per bead. It appears that the presence of a large number of cells in each bead is what allows the bead to avoid collapse when paper containing the same is compressed under the considerable force imposed in the calendering operation. Thus, with beads of the size specified, the bulking improvement after calendering is about 5-15%, where the improvment in bulking indicates the extent of thickness increase in paper having the same weight. With paper having a lower basis weight, e.g., below about 57 g/m², typically 48.8 g/m² (about 35 pounds per ream, typically 30 pounds per ream), the smaller beads having a particle size below about 15 µm appear to perform more efficiently. With paper having a higher basis weight, e.g., above about 57 g/m², typically 65 g/m² (about 35 pounds per ream, typically 40 pounds per ream), beads having an average particle size above about 15 µm perform better (more of the bulking action is retained on calendering). Typical machine calendering conditions which have been investigated involved calender stack pressures of 26.8 and 160.7 (150 and 900 pounds per linear inch), at normal papermaking temperatures at this point in the process, e.g., about 66°C (150°F). Calendering temperatures of less than about 93°C (200°F.) are preferred. In the production of the papers which are calendered in this invention, the vesiculated beads may be used in aqueous medium, as produced, or they can be dried prior to use, but in preferred practice the aqueous suspension of beads is modified by the introduction of a neutralized polycarboxylic acid polymer, such as an ammonia-neutralized polyacrylic acid, to thicken the aqueous medium and thereby hold the beads in stable suspension. It is found that when the aqueous bead suspension is stabilized, as described, that the the papermaking process is substantially alterred because there is a reduction in the proportion of fines which pass through the wire into the white water. This action is particularly effective in combination with the inclusion of an effective amount of an anionic retention aid in the furnish, the proportion of such anionic retention aid being of secondary significance because the normal proportions thereof as presently used remain effective herein. It is stressed that the vesiculated beads used herein are cross-linked carboxyl-function beads in order that they might participate in improving the retention of fines. The presence of titanium dioxide pigmentation in these beads is not required for fine retention, but is only an aspect of the capacity of the beads to improve calendered paper. It is of interest to note that the neutralized polycarboxylic acid polymer which thickens the aqueous bead suspension to stabilize the same apparently chemically coacts with the carboxyl functionality in the beads. This is evidenced by the fact that these neutralized polymers are effective to stabilize the suspension at a lower viscosity than other thickeners, so more is involved in providing stability than merely the viscosity of the aqueous medium. Also, there appears to be a chemical coaction with the anionic retention aid. This is evidenced by the fact that the anionic retention aid is selected and used in a proportion which maximizes retention on a practical basis. The amount of neutralized thickener, which is chemically similar to the anionic retention aid, which is added with the bead suspension is far too small to account for any detectable increase in retention. It is thought that the amine or ammonia used to neutralize the carboxyl functionality in the anionic retention aid, the carboxyl functionality In the thickener and the carboxyl functionality in the beads, serves to effectively couple the retention aid to the beads. As a result, when the retention aid couples with a finely divided particle which would normally pass through the wire, that retention aid may also be coupled with a bead. This bead is much larger than the finely divided particle, and this increases the chance that the much larger conglomerate including the bead will be retained on the wire. In summary, it is found that the vesiculated beads, when used in aqueous suspension which has been thickened with a neutralized polycarboxylic acid polymer thickening agent to prevent settling of the suspension, are effectively retained in the paper together with an increased proportion of the fines and mineral filler which normally pass through the Fourdrinier wire in conventional papermaking. Bead retention is enhanced when conventional retention aids are employed, both anionic and cationic retention aids being useful to increase the proportion Of the beads which is retained. However, anionic retention aids are more efficient and the combination of carboxyl-functional cross-linked beads preassociated with a neutralized polycarboxylic acid polymer thickening agent and anionic retention aid functions to also increase the retention of all fines in the papermaking process. The visiculated beads bulk the paper even when they are not pigmented. But the pigmented vesiculated beads withstand the heat and pressure of calendering, and despite their much larger size (in comparison with mineral filler), these beads bulk the calendered paper and serve to maintain its opacity and brightness while the smoothness of the calendered paper is improved. At the same time, the proportion of titanium dioxide pigment can be substantially reduced. This is surprising. While the titanium dioxide pigment need not be reduced, larger proportions of this pigment can be omitted when larger proportions of titanium dioxide are included in the vesiculated beads which are used. The retention aids which have been found to be useful herein are themselves well known, and many are available in commerce. Acrylamide and methacrylamide copolymers with monoethylenic acids or monoethylenic amines are preferred, these generally containing from 5% to 50% of the acid or amine monomer, balance the amide monomer. The anionic copolymers which are preferred will usually contain acrylic or methacrylic acid as the acid component, and these are rendered anionic with the aid of an amine, which is preferably ammonia. Thus, a copolymer of 20% methacrylic acid with 80% acrylamide is illustrative of a preferred anionic retention aid. The cationic amide copolymers are illustrated by copolymers of acrylamide with diethyl aminoethyl methacrylamide. Various other anionic and cationic resins which may be used as the retention aid are discussed in the Kirk Othmer "Encyclopedia of Chemical Technology, third edition" Volume 16, at page 804 which points out that the usual fillers having a size up to 5 µm are not effectively retained because they are so small, thus provoking the use of retention aids in the paper furnish. The beads used herein are generally much larger, but their retention on the Fourdrinier wire is enhanced by the conventional retention agents nonetheless. The preferred retention aids are thus anionic and are constituted by carboxyl functional copolymers which are dispersed in the aqueous furnish because they are solubilized with a base, usually ammonia or a volatile amine. The vesiculated beads which are used herein are constituted by cross-linked polyester resin which carry unreacted carboxyl groups. These carboxyl groups are also associated with a base since a base is always present when the beads are made. The polyacrylic acid thickeners are well known materials and are of relatively high molecular weight as is produced by polymerization in aqueous emulsion. Acrylic acid or methacrylic acid are usually employed, the selected acid, or mixture thereof, being substantially the only monomer, though up to about 10% of other monoethylenically unsaturated monomers, such as ethyl acrylate, may be present. Up to about 2% of a polyethylenic monomer, like divinyl benzene or ethylene glycol diallyl ether, may be present during the copolymerization to increase the molecular weight or to introduce limited cross-link density. Ammonia or volatile amine, like monoethanol amine, can be used for the neutralization which thickens the aqueous medium containing the polymer. Ammonia is the preferred base. Usually 0.1% to 2% of the polyacrylic acid polymer will provide all the thickening which is needed (after neutralization) to stabilize a bead suspension containing from 10% to 50% of free water. Exact neutralization is not required, the achievement of desired thickening determining the amount of base which should be used. It is preferred in the process of this invention to employ an aqueous furnish having a pH of from 4 to 6, an acid such as sulfuric acid being added to achieve this result. This is because this pH range maximizes the retention of fines. It is of interest to note that anionic retention aids are usually added at or near the head box to minimize the agitation they are subjected to, and this conventional procedural device is also preferably used herein (the vesiculated beads are added with the retention aids) in order to maintain the associations between the anionic groups which are added and the fines and fibers which are present in the furnish. More particularly, it is preferred to add the beads and the anionic retention aid to the furnish just prior to the headbox and after pumping and centrifugal screening pressures have been applied so as to minimize the shear which is encountered by the anionic retention aid and the anionic beads which are used herein. In a specific comparison with the use of titanium dioxide, anatase, at the 7.5% level in a typical opacified paper, it is found that the combination of 2. 5% titanium dioxide, anatase, and 5% pigmented vesiculated beads provides a calendered paper of the same basis weight having equal strength, higher bulk, equal opacity and better smoothness. About the same situation applies when one uses only pigmented vesiculated beads in the absence of the titanium pigment. Using only titanium dioxide pigmented vesiculated beads in the absence of additional titanium pigment in place of the same weight proportion of titanium pigment provides, for the same basis weight paper, a bulkier paper having comparable brightness and opacity and a smoother surface than the titanium dioxide control. The titanium dioxide-pigmented beads are used for the further reason that the presence of the finely divided pigment in the cell walls of the vesiculated beads provides greater resistance to collapse during the calendering operation. While pigmented vesiculated beads are important in the production of calendered paper, the effect of increasing the retention of fines in the papermaking process does not appear to require the presence of pigment in the cell walls of the beads, so pigmented beads are not an essential factor in the new process of this invention. While excellent properties are obtained in the absence of free titanium dioxide, it is desirable for some purposes to include a smaller than usual proportion of such inorganic pigment. Thus, the normal proportion of titanium dioxide, anatase, in opacified calendered printing paper is about 7.5%, and this is reduced herein to 0.5% to 5%, preferably from 1% to 4%. The preferred vesiculated beads are styrene-cross-linked unsaturated polyester resins. These are made into a vesiculated bead in conventional fashion, as illustrated by U.S.-A-3,879,314. The size of the vesiculated beads under consideration may vary considerably, but is larger than a volume average of about 5 µm in diameter. For example, beads having a volume average diameter of from 8 to 30 µm are conveniently prepared and fully useful in this invention. As the bead size increases above about 30 µm, it becomes progressively harder to produce beads of good quality. The beads are obtained in aqueous suspension and normally retain within themselves about 55% to 60% of their own weight of water. The suspensions contain more water than the beads can retain in a dry-appearing cake, so that the suspensions will contain 15% to 35% of bead solids, balance water. These are typically thickened with ammonia-neutralized polyacrylic acid or polymethacrylic acid using enough to provide a stable suspension, but exact proportions are not critical because poorly stable suspensions are still useful while excess thickener can be tolerated. Various other patents are of interest to the formation of vesiculated beads useful in this invention, particular attention being directed to U.S.-A-3,822,224, 3,923,704 and 3,933,579. This last-named patent describes the vesiculated beads which are preferred herein, namely, those having a ratio of granular diameter to mean vesicle diameter of at least 5:1, a vesicle volume of from 5% to 95% of the volume of the granule, and not more than about 60% pigment, by volume. The vesiculated beads used herein have a highly cross-linked polymeric body which is preferably constituted by a carboxyl-functional unsaturated polyester resin cross-linked with an ethylenically unsaturated monomer copolymerizable therewith. The unsaturation in the polyester is preferably maleate unsaturation, these polyesters being themselves well known and illustrated hereinafter. These polyesters are carboxyl-functional materials, and it is preferred that the polyester have an acid value of 10 to 45 mg KOH per g., more usually 15 to 25. The unsaturated monomers used for cross-linking are also well known and are water insoluble monomers typically illustrated by styrene or vinyl toluene. The polyesters and monomers are more fully discussed in U.S.-A-3,879,314 which shows the production of vesiculated beads using a water-soluble polyamine containing at least three amine groups per molecule and having a dissociation constant in water (pKa value) of 8.5-10.5, typically illustrated by diethylene triamine. The polyamine is used in a concentration providing 0.3 to 1.4 amine groups per polyester carboxyl group. It is preferred to have from 35% to 45% of the unsaturated polyester cross-linked with from 55% to 65% of styrene. The beads are normally used without drying, and thus contain much of the water and the amines which were used in their production. The invention is illustrated as follows, it being understood that all parts and proportions are by weight, unless otherwise specified. Suitable pigmented vesiculated beads in accordance with this invention are illustrated in U.S.-A-3,879,314 issued April 22, 1975, see particularly Example II. By proceeding in accordance with said Example II and using a polyester of 18% phthalic anhydride, 37% maleic anhydride and 45% propylene glycol dissolved in styrene to form a solution containing 41.8% of the polyester, vesiculated beads pigmented with titanium dioxide, anatase, to contain about 43.2% pigment are provided. The polyester is a carboxyl-functional product and this carboxyl functionality is largely in salt form with the amines used in the production of the beads. These beads have an average size of about 25 µm and contain an average of more than 10 cells per bead. These beads are incorporated into a paper furnish in which 39.9 kg (88 pounds) of softwood kraft, 59.9 kg (132) pounds of hardwood kraft, 20 kg (44 pounds) of clay, 2.3 kg (5 pounds) of calcined clay, and 1.5 kg (3.4 pounds) of alum is mixed into water to provide 1136 l (500 gallons) of mixture to which is added 300 ml of concentrated sulfuric to provide a pH in the range of 4.5 to 6.5. This furnish is modified to include 4% or 6% of the vesiculated beads and 1.5% of titanium dioxide, anatase, based on solids content, and is supplied to the head box by passing the mixture through a valve in which the solids concentration is reduced to 2.5%. The paper made from this furnish had a basis weight in the range of 60.2 to 65.1 g/m² (37 to 40 pounds per ream) and was calendered at either 26.8 or 160.7 kg/cm (150 or 900 pounds per linear inch) at a temperature of about 66°C (150°F). Compared with the same furnish containing 6.5% titanium dioxide, anatase, a 5-15% increase in the caliper of the calendered paper at any given basis weight was observed at both of the 4% and 6% bead inclusion levels which were investigated. The paper produced, with and without the beads, was passed through a size press in conventional fashion and then calendered under the conditions noted to provide the final paper which was tested. The paper containing the pigmented vesiculated beads had greater caliper and greater resistance to print show through than the conventional titanium dioxide-pigmented paper. This establishes that printing paper of lesser weight and the same resistance to print show through can be provided by this invention. The vesiculated beads in the form of an ammonia-neutralized polyacrylic acid-thickened stable aqueous suspension can be added together with an anionic retention aid constituted by a copolymer of 20% methacrylic acid and 80% acrylamide in an amount of 1% based on the weight of solids present in the furnish. These are added to the furnish together with the water used to dilute the furnish to the desired solids content immediately prior to the head box so as to minimize shear. In comparison with the use of the anionic retention aid alone, a substantial reduction in fines passing through the wire into the white water is experienced, as evidenced by the lowering of the zeta potential of that white water. From the standpoint of ash content, it is found that the normal retention as evidenced by comparing the theoretical ash content with the actual ash content increases from about 67-68% to about 76-77% when this aspect of the invention is practiced.

Claims: A method of making paper comprising: adding to an aqueous paper furnish an aqueous suspension of carboxyl-functional, highly cross-linked, titanium dioxide-pigmented, polymeric, vesiculated beads having a volume average particle size of at least about 5 µm together with an anionic retention aid, said beads being pre-associated with a neutralized polycarboxylic acid polymer thickener, to thereby reduce the proportion of fines which pass through the wire into the white water; forming a high bulk paper with 2 % to 6 % by weight of polymeric vesiculated beads; and calendering said high bulk paper to produce a calendered paper product with a basis weight of up to about 65 g/m² (40 pounds per ream). A method of making paper as recited in claim 1 in which said carboxyl-functional, polymeric material in the vesiculated beads has an acid value of 10 to 45 mg KOH per g, said anionic retention aid is a copolymer of acrylamide with 5 to 50 % of a monoethylenically unsaturated carboxylic acid and said thickener is a polyacrylic acid, said anionic retention aid and said thickener being neutralized with ammonia. A method of making paper as recited in claim 1 in which said vesiculated beads are present in an amount of at least about 3.5 %, and said calendered paper has a basis weight of at least 32.5 g/m² (20 pounds per ream) to provide a quality printing paper and said vesiculated beads have a volume average particle size in the range of from 10 to 30 µm. A method of making paper as recited in claim 3 in which said vesiculated beads are present in an amount of 4 to 5 % and said vesiculated beads contain an average of at least 5 cells per bead, have a ratio of granular diameter to mean vesicle diameter of at least 5 : 1, a vesicle volume from 5 to 95 % of the volume of the granule, and not more than about 60 % pigment, by volume. A method of making paper as recited in claim 4 in which the polymeric body of said beads is a polyester resin cross-linked by an ethylenically unsaturated monomer copolymerizable therewith. A method of making paper as recited in claim 4 in which from 35 to 45 % of unsaturated polyester containing maleate unsaturation is cross-linked with from 55 to 65 % of styrene and said polyester has an acid value of 15 to 25 mg of KOH per g. A method of making paper as recited in claim 1 in which said paper is calendered at a pressure of at least about 26.8 kg/cm (150 pounds per linear inch) using a temperature of at least about 38°C (100°F). A method of making paper as recited in claim 1 in which the calendering pressure is in the range of 125 to 214 kg/cm (700 to 1200 pounds per linear inch) and the calendering temperature is at least about 66°C (150°F). A method of making paper as recited in claim 1 in which the proportion of titanium dioxide, anatase, not in said beads is 1 to 4 %.

Inventors: BATES, FERDINAND, JR., BERKA, ROMAN
Applicants: ROHM AND HAAS COMPANY
Application year: 1987
Main classification: D21H21
IPC codes: D21H17, D21H21, D21G1