EP0204916A1

EP0204916A1 - High bulk calendered paper containing vesiculated beads

Info

Publication number: EP0204916A1
Application number: EP86104826A
Authority: EP
Inventors: Ferdinand Bates, Jr.; Roman Berka
Original assignee: DeSoto Inc
Current assignee: DeSoto Inc
Priority date: 1985-06-05
Filing date: 1986-04-09
Publication date: 1986-12-17
Also published as: FI862389A0; JPS61282496A; FI862389L

Abstract

A high bulk calendered paper comprises from 0.5 to 15 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 microns up to about 35 microns. These vesiculated beads preferably have a volume average particle size in the range of from 8 to 30 microns and are present in an amount of from 2% to 10%.

Description

Technical Field

This invention relates to high bulk 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 micron. 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.
While titanium dioxide pigments are quite effective in increasing the desirable properties of opacity and brightness of any paper, they are expensive. It will be understood that 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 and 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 the bulk which they provided was lost. The calendering force is normally at least about 150 pounds per linear inch, and may be as high as about 2000 pounds per linear inch, and the temperature is at least about 100°F., more usually at least about 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.
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 for 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

In accordance with this invention, all or a portion of the usual inorganic pigment, such as the titanium dioxide, anatase, normally employed in the paper furnish, is replaced by highly cross-linked, titanium dioxide-pigmented, polymeric vesiculated beads having a volume average particle size of at least about 5 microns. The final paper contains from 0.5 to 15 weight percent of such beads, preferably from 2% to 10% thereof, based on the total weight of the paper. This paper is calendered at a pressure of at least about 150 pounds per linear inch and at a temperature of at least about 100°F to provide a calendered paper product having improved bulk.
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. With small proportions of vesiculated beads, this effect is proportional to the amount used. While proportions larger than 10% can be used, little further improvement is seen, so the use of amounts in excess of 10% are not very cost effective.
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 microns, with larger beads providing even better bulking than the smaller ones. Beads having a diameter of up to about 35 microns can be used, but it is preferred to employ a volume average particle size in the range of from 8 to 30 microns. 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 improvement 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 35 pounds per ream (typically 30 pounds per ream), the smaller beads having a particle size below about 15 microns appear to perform more efficiently. With paper having a higher basis weight, e.g., above about 35 pounds per ream (typically 40 pounds per ream), beads having an average particle size above about 15 microns perform better (more of the bulking action is retained on calendering).
Typical machine calendering conditions which have been investigated involved calender stack pressures of 150 and 900 pounds per linear inch. at normal papermaking temperatures at this point in the process, e.g., about 150°F. Calendering temperatures of less than about 200°F. are preferred.
In summary, it is found that the vesiculated beads are effectively retained in the paper as the water and some of the fines and mineral filler pass through the Fourdrinier wire in conventional papermaking. Bead retention is enhanced when conventional retention aids are employed. Both anionic and cationic retention aids are useful, but anionic retention aids are more efficient. The vesiculated beads withstand the heat and pressure of calendering, and despite their much larger size (in comparison with mineral filler), the vesiculated beads serve to maintain the opacity and brightness while the smoothness of the calendered paper is improved, and 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 microns 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.
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 8% pigmented vesiculated beads in the absence of the titanium pigment. Using 8% of titanium dioxide pigmented vesiculated beads in the absence of additional 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 because 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 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. Pat. No. 3,879,314. The size of the vesiculated beads under consideration may vary considerably, but is generally larger than a volume average of about 5 microns in diameter. For example, beads having a volume average diameter of from 8 to 30 microns are conveniently prepared and fully useful in this invention. As the bead size increases above about 30 microns, it becomes progressively harder to produce beads of good quality.
Various other patents are of interest to the formation of vesiculated beads useful in this invention, particular attention being directed to U.S. patents Nos. 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. It is preferred that the polyester have an acid value of 10 to 45 mgm KOH per gm.
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. Pat. No. 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 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. Patent No. 3,879,314 issued April 22, 1975, see particularly Example II. By proceeding in accordance with said Example 11 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 titantium dioxide, anatase, to contain about 43.2% pigment are provided. These beads have an average size of about 25 microns and contain an average of more than 10 cells per bead.
These beads are incorporated into a paper fumish in which 88 pounds of softwood kraft, 132 pounds of hardwood kraft, 44 pounds of clay, 5 pounds of calcined clay, and 3.4 pounds of alum is mixed into water to provide 500 gallons of mixture to which is added 300 cc of concentrated sulfuric to provide a pH in the range of 4.5 to 6.5. This basic fumish 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 37 to 40 pounds per ream and was calendered at either 150 or 900 pounds per linear inch at a temperature of about 150°F. Compared with the same fumish 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.
While printing papers are particularly contemplated, papers calendered under minimal pressure and temperature are particularly useful for xerographic reproduction, and such papers having a basis weight of about 50 pounds have been prepared and found to have greater bulk, greater smoothness, less one-sidedness (the two sides of the paper are more comparable) and greater resistance to print show through.

Claims

1. A high bulk calendered 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 microns up to 35 microns.

2. A high bulk calendered paper as recited in claim 1 in which said vesiculated beads have a volume average particle size in the range of from 10 to 30 microns.

3. A high bulk calendered paper as recited in claim 1 in which said vesiculated beads are present in an amount of from 2% to 10% and contain an average of at least 5 cells per bead.

4. A high bulk calendered paper as recited in claim 3 in which said vesiculated beads have 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.

5. A high bulk calendered paper as recited in claim 1 in which said vesiculated beads are incorporated in the furnish and retained with the aid of an anionic retention aid.

6. A high bulk calendered paper as recited in claim 5 in which the polymeric body of said beads is constituted by a carboxyl-functional unsaturated polyester resin cross-linked by an ethylenically unsaturated monomer copolymerizable therewith.

7. A high bulk calendered paper as recited in claim 6 in which from 35% to 45% of maleate- unsaturated polyester is cross-linked with from 55% to 65% of styrene.

8. A high bulk calendered paper as recited in claim 1 in which said paper has a basis weight below about 35 pounds per ream and said beads have a particle size below about 15 microns or said paper has a basis weight above about 35 pounds per ream, and said beads have a particle size above about 15 microns.

. 9. A high bulk calendered paper as recited in claim 1 in which said paper is calendered at a pressure of at least about 150 pounds per linear inch using a temperature of at least about 150°F.

10. A high bulk calendered printing paper as recited in claim 9 in which the proportion of titanium dioxide, anatase, not in said beads is about 0.5% to 5% of the weight of the paper.