CA1043193A

CA1043193A - Coating of paper

Info

Publication number: CA1043193A
Application number: CA220,182A
Authority: CA
Inventors: Douglas C. Edwards
Original assignee: Polysar Ltd
Current assignee: Polysar Ltd
Priority date: 1975-02-14
Filing date: 1975-02-14
Publication date: 1978-11-28
Also published as: DE2605575A1; FR2300843B1; GB1499235A; FR2300843A1; IT1057155B; SE428710B; CH611957A5; DE2605575C2; SE7601424L

Abstract

COATING OF PAPER
Abstract of the Disclosure A coating composition comprising granules of unmodified rice starch dispersed in a latex of a flexible polymer was applied to bank note paper. Only the surface fibres of the paper were penetrated, the body fibres being left unaffected. The paper had improved resistance to soiling, humidity and deterioration due to folding. It also had improved detail and fine line fidelity when sub-jected to the conventional printing operation. There was no discernible change to the appearance of the paper and it retained its characteristic crackle.

Description

^ " 1043~ 9~
The present invention relates to compositions for coating paper. It is particularly concerned with im-proved bank note paper.
Bank note paper is comprised of various fibres and pigments held together by a binder. The paper possesses certain attributes such as dimensional`stability, non-glare surface, opacity, a reasonable durability and a character-istic crispness or crackle which the public has come to associate with paper of this kind. However, the surface of bank note paper is suficiently rough that it is a poor vehicle for high quality printing and it provides an un-desirably large number of sites for the accumulation of dirt. In addition, the fibres of the paper tend to loosen and break on repeated folding and the paper is undesirably sensitive to variations in humidity.
It has now been found that bank note paper having improved printability, durability and resistance to soiling -and humidity variations can be obtained while retaining its desirable properties by coating the paper with a composition comprising as its essential components a latex of a flexible film-forming polymer and ungelatinized starch granules, the granules having an average diameter smaller than the average interstitial distance between the surface fibres of the paper, removing any excess coating composition and drying the coated paper.
The present invention therefore provides a method of coating a sheet of paper comprising spreading over the surface of the paper a composition com-prising a latex of a flexible polymer and ungelatinized starch granules, said granules having an average diameter smaller than the average interstitial distance between the surface fibres of the paper, removing any excess coating composition and drying the coated paper. ~

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-1~43~93 In another aspect the present invention provides a sheet of paper having a surface coated with a composition comprising a latex of a flexible polymer and ungelatinized starch granules, said granules having an average diameter smaller than the average interstitial distance between the surface fibres of the paper.
In a further aspect the present invention provides a latex coating composition comprising a latex of a flexible polymer and ungelatinized starch granules.
In a still further aspect this invention provides a method of 10 preparing bank note or security document paper composed of wood, cotton or flax fibers or any mixture thereof, comprising spreading over the surface of the uncoated paper as the sole coating agent a composition comprising a latex of a rubbery polymer containing about 50 weight percent or more of total solids, and ungelatinized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0. 5 to 3:1, removing any excess coating composition and drying the coated paper.
In a still further aspect this invention provides a sheet of coated 20 bank note or security document paper composed of wood, cotton, or flax fibers or any mixture thereof coated with a composition comprising a latex of a rubbery polymer containing about 50 weight percent or more of total solids, and ungelatinized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0. 5 to 3:1, said composition being the sole coating agent.
The latices which may be used for preparing the coating compo-sitions of the present invention are those in which the polymer particles are 30 flexible and film-forming in characterO The polymers may be homopolymers of C4-Clo dienes such as butadiene, 2-methyl butadiene, pentadiene-1,3, -43~93

2,3-dimethyl pentadiene-1,3, 2,5-dimethyl hexadiene-1,5, norbornadiene, ethylidene norbornene, dicyclopentadiene and halo-substituted derivatives of these compounds. The polymers also may be copolymers of the C4-C10 dienes with each other or with one or more copolymerizable monomers - containing a CH2 ~ C group. Examples of these monomers are acrylic acid and its esters, nitriles and amides such as methyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, acrolein, alpha and beta methyl acroleins, alpha-chloroacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, cinnamic acid, cinnamic aldehyde, vinyl acetate, vinyl chloride, vinylidene chloride, isobutylene, divinyl benzene, and methyl vinyl ketone. The polymers can also be homopolymers --of vinyl acetate and vinyl alcohol, copolymers such as ethylene-vinyl acetate, ethylene-vinyl chloride, vinyl acetate-methyl methacrylate-acrylic acid-styrene, styrene-- vinyl pyrrolidone, ethyl acrylate-vinyl pyrrolidone or anyof the other rubbery polymers proposed as binders for paper coating applications. If desired, the polymer latices may be blended with minor proportions of latices of resinous polymers such as polystyrene, polyacrylonitrile, polymethyl methacrylate, copolymers of the monomers of these resinous polymers such as styrene-acrylonitrile resins and resinous copolymers of these monomers with other copolymerizable monomers such as copolymers of styrene with butadiene in which styrene forms more than 70 weight % of the polymer.
Preferred are latices of copolymers of about 20-60 weight %
of a C4-C6 conjugated diolefine, 40-80% of ~ styrene ant 0.1-5% of a C3-C6 mono- or dicarboxylic acid, thè total of 1~ 31 ~ 3 the percentages adding up to 100. The total solids content of the latices should be over 40% by weight and normally about 50% or more prior to compounding.
Starch granules which may be used in the present invention are cold water-insoluble, ungelatinized particles ~- having an average diameter smaller than the average inter-stitial distance between the surface fibres. The inter-stitial distance will depend among other things on the type of fibres used in producing the paper and the closeness with which they are packed. The usual bank note paper is characterized by interstitial distances such that rice starch granules are the preferred specie. Substantially all natural rice starch granules have a particle size fall-ing in the range of about l-10ju (microns). Starch granules obtained from other sources have particle size ranges which either overlap or totally exceed the 1-10 ,u range e.g. wheat 2-39,u, corn 2-25,u, sorghum 3-26,u, sago 24-41 ~, tapioca 5-35,u and potato 15-100,u. They can be used in place of the rice starch if their particle sizes are reduced sufficiently to permit their free entry into the smaller interstitial spacing of the usual bank note paper. They can, of course, be used without particle size reduction with papers having suitably large interstitial spacing between the surface fibres. The amount of starch used in the coating composition can vary considerably e.g.
0.25-25 parts by weight per 1.0 part by weight of flexible polymer. It will normal~y be in the range of 0.5-10 parts.
Other additives may also be included in the latex-starch mix in preparing the coating composition. Thus, minor amounts of dispersing agents e.g. sodium hexameta-.~ ~

1~43~93 phosphate, binders e.g. cooked and oxldized starches, , .
casein and soya protein, fillers e.g. clay, calcium car-bonate, silica, alumina, titania and satin white, alkaline compounds e.g. sodium hydroxide, viscosity modifiers e.g.
sodium polyacrylate, ammonium carbonate, defoamers, etc.
The final coating composition will normally have a total solids content of about 30-75% by weight, usually 30-60%.
The coating composition can be applied to the paper substrate by conventional techniques such as coating ` 10 roll, trailing blade, air knife, etc. After the coating has been applied and the excess remoyed, the coated paper is dried and calendered to the desired degree by known means. Coating weight will normally be 1-20 gms./m2.
The following examples are provided to illustrate the invention.
EXAMPLE I
A latex containing 50 per cent by weight total solids of a carboxylated styrene-butadiene copolymer, the ratio of unsaturated carboxylic acid/styrene/butadiene 20 being 3/75/22, was prepared by conventional free radical ;~ emulsion polymerization. The concentration of the buta-diene component was made greater at the surface of the polymer particles without altering the overall composition, by incremental addition. 60 grams of this latex were mixed ~- 25 with 45 milliliters of a 1 per cent aqueous solution of sodium hexametaphosphate. To this mixture, 60 grams of rice starch were added at room temperature. The mixture was agitated thoroughly to disperse the starch granules.
A strip of bond paper was coated with this com-position by means of a laboratory trailing blade coater.

In this operation, one end of the paper strip was in-serted between a smooth vertical surface and a flexible steel blade, the angle between the blade and the vertical - surface being about 60 and the pressure on the blade being sufficient to hold the paper firmly. An excess of the latex-starch composition was introduced along the channel formed by the conjunction of paper and blade; the paper was then drawn downwards by hand at a rate of about 2 ft.
per second until the strip was completely drawn through.
The sheet was allowed to dry at room temperature and then coated on the other side in the same manner. After further drying at room temperature, it was heated in an air oven at 140C for 3 minutes to assure thorough fusion of the polymer particles with each other and with the other com-ponents in the coating and with the paper.
The average thickness of four sheets prepared inthis manner was 117 microns, compared to 112 microns for an untreated control.
Upon examination under a microscope by reflected light, the untreated paper was observed to have a very un-even surface, with individual fibres clearly visible and with deep and irregular recessed areas present between the fibres. In the case of the treated paper, only the sur-faces of the fibres could be seen, and the spaces between were observed to be smoothly filled to the level of the top-most fibre surfaces. Examination of sections of the treated papervshowed that only the surfaces had been filled, the body of the matrix being unaffected.
The treated paper was not visually different from the untreated paper as judged by the naked eye, nor were 1~43$~3 the tactile properties noticeably affected. Subsequent examination with a scanning electron microscope showed that the treated surface consisted mainly of a smooth binding film, forming a level plain between the fibre surfaces, with the starch granules projecting above this plain to give a micro-roughness effect. This slight surface rough-ness is desirable in order to preserve the "flat" or non-glossy appearance of the paper.
When subjected to a conventional printing opera-tion, the treated paper was found to provide greatly im-proved line fidelity and detail as compared to the untreated paper.
EXAMPLES II and III
_ A latex containing 50 per cent by weight of total solids of a carboxylated styrene-butadiene copolymer, the ratio of unsaturated carboxylic acid/styrene/butadiene being 2/60/38 was compounded with sodium hexametaphosphate and rice starch granules as in Example I.
Bank note paper was coated with this compounded latex composition. The paper.was unwound from a supply roll and fed continuously at a rate of about 14 ft./minute by means of a series of rolls to a bath containing the coating composition where one surface of the paper was coated.
Uniform application and removal of excess coating composi-tion was obtained by means of doctor and scraper blades.The paper was next passed through a hot air circulation oven maintained at 135C to dry it. Average re~idence time in the drier was about 6.7 minutes. On leaving the oven, the dried paper was collected o~ a wind-up roll. The other surface of the paper was coated in the same way.

1~43~93 The foregoing procedure was repeated but in this case the rate of travel of the paper was 25 ft./minute and the average residence time in the drier was about 3.7 minutes. At the 14 ft./minute speed, the total coating weight applied (i.e. both sides coated) was 9.3 grams per square meter of paper. This resulted in a 2.6% increase in the thickness of the paper. At the 25 ft./minute speed, the total coating weight applied was 11.4 grams per square meter and the increase in thickness of the paper was 4.0%.
The smoothness of the papers was i.mproved as shown by electron and optical microscopic examination al-though to the unaided eye there was no readily discernible change to the appearance of the paper. Nor was there any obvious change to the feel or handle of the paper. Print-ing trials showed the improvements obtained in Example I.
It is desirable to avoid excessive calendering of the coated paper in order to maintain its characteristic handling properties.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of preparing bank note or security document paper composed of wood, cotton or flax fibers or any mixture thereof, comprising spreading over the surface of the uncoated paper as the sole coating agent a composition comprising a latex of a rubbery polymer containing about 50 weight percent or more of total solids, and ungelatinized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0.5 to 3:1, removing any excess coating composition and drying the coated paper.

2. A method as in Claim 1 of preparing bank note or security document paper composed of wood, cotton or flax fibers or any mixture thereof, comprising spreading over the surface of the uncoated paper as the sole coating agent 1 to 20 grams per square meter (dry weight) of a composition compris-ing a latex of a rubbery polymer containing about 50 weight percent or more of total solids and ungelatinized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0.5 to 3:1, removing any excess coating composition and drying the coated paper, and wherein the coated paper is not subjected during its production of a calendering step.

3. A process according to Claim 1 or 2 in which the surface of the paper is scraped with a blade after the coating composition has been applied to its surface.

4. A process according to Claim 1 or 2 wherein the rubbery polymer in the latex is a polymer of a monomer con-taining a CH2 = C ? structure in its molecular configuration.

5. A process according to Claim 1 or 2 wherein the rubbery polymer is a carboxy polymer.

6. A process according to Claim 1 or 2 wherein the rubbery polymer is a copolymer of 30-60 weight % of a C4-C6 conjugated diolefin, 35-70 weight % of a styrene and 0.1-5 weight % of a C3-C6 mono- or dicarboxylic acid.

7. A process according to Claim 1 or 2 in which the rubbery polymer is a polymer of an acrylic monomer.

8. A sheet of coated bank note or security document paper composed of wood, cotton, or flax fibers or any mixture thereof coated with a composition comprising a latex of a rubbery polymer containing about 50 weight percent or more of total solids, and ungelatinized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0.5 to 3:1, said composition being the sole coating agent.

9. An uncalendered sheet of coated bank note or security document paper as in Claim 8, composed of wood, cotton, or flax fibers or any mixture thereof coated with a composition comprising a latex of a rubbery polymer containing about 50 weight percent or more of total solids, and ungelati-nized rice starch granules having an average diameter of about 1-10 microns which is an average diameter smaller than the average interstitial distance between the surface fibers of the paper, the ratio of starch to latex solids being 0.5 to 3:1, said composition being the sole coating agent, and said coating being present on the surface of the paper in an amount of 1 to 20 grams per square meter (dry weight).

10. A sheet of paper as in Claim 8 or 9, wherein the rubbery polymer is a carboxy polymer.

11. A sheet of paper as in Claim 8 or 9, wherein the rubbery polymer is a copolymer of 30-60 weight % of a C4-C6 conjugated diolefin, 35-70 weight % of a styrene and 0.1-5 weight % of a C3-C6 mono- or dicarboxylic acid.