GB2305943A

GB2305943A - Paper

Info

Publication number: GB2305943A
Application number: GB9620558A
Authority: GB
Inventors: John Edward Rose; Nicholas Robin Whittaker; Ivan Gbur
Original assignee: J R Crompton Ltd
Current assignee: J R Crompton Ltd
Priority date: 1995-10-04
Filing date: 1996-10-02
Publication date: 1997-04-23
Also published as: GB9520226D0; GB9620558D0

Abstract

A paper for use in a thermosensitive stencil paper is comprised of cellulose fibres and optionally synthetic polymer fibres. The fibres of the paper are bonded together by a cured resin admixture comprising 0.05 gsm to 1 gsm of dry strength increasing additive. 0.1 gsm to 0.5 gsm of a cationic wet strength increasing agent, and 0.5 gsm to 3 gsm of a polyurethane or ethylene vinyl acetate, the admixture having been applied and cured subsequent to the paper forming step in a single operation.

Description

PAPER The present invention relates to paper intended particularly, but not exclusively, for use in a thermosensitive stencil paper.

Thermosensitive stencil papers generally comprise a layer of a thin paper (usually having a basis weight of 5 to 15 gsm and a thickness of 10 to 50 microns) capable of absorbing ink bonded to a thermosensitive polymeric sheet in which the stencil design (which may for example be comprised text, drawings or the like) is formed by a thermal process (e.g. using a mask and a thermal head or flash lamp). To effect a printing operation, the stencil paper is located on a stencil printing machine and the thin paper is loaded with ink. During printing, the ink passes through the apertures (defining the stencil pattern) in the polymeric film whereby the design may be printed onto a substrate.

The thin paper layer must satisfy a number of criteria, and in particular it must: I) be capable of being securely bonded to the polymeric sheet such that no de lamination occurs, II) have good mechanical strength so that the stencil paper may be used for producing multiple (e.g. at least 3,000) printed copies of the design on the stencil printing machine, and III) must have good dimensional stability so that those areas of the paper which are exposed through the apertures in the polymeric film (and which are not therefore supported in direct face to face contact with the film) do not expand or contract as a result of the absorption of ink into these areas.

EP-A-0 451 269 discloses a resin-treated thin sheet for a heat sensitive stencil printing paper. The thin sheet has a basis weight of 5 to 15 g/m2 and a thickness of 10 to 50m and comprises at least 10% by weight of polyester fibres of defined characteristics. The thin sheet further comprises a urethane resin or epoxy resin which is present at crossing points and surfaces of filaments in an amount of at most 3g/m2 of the thin sheet.

The process disclosed in EP-A-0 451 269 for producing the thin sheet comprises preparing a fibre dispersion which may contain additives (e.g. paper strength increasing additives), preparing paper from the fibrous dispersion, drying the paper and winding it onto a roll. Paper from the roll is then treated with the polyurethane resin.

According to the first aspect of the present invention there is provided a paper comprised of cellulose fibres and optionally synthetic polymer fibres wherein the fibres of the paper are bonded together by a cured resin admixture comprising 0.05 gsm to 1 gsm of dry strength increasing additive, 0.1 gsm to 0.5 gsm of a cationic wet strength increasing agent, and 0.5 gsm to 3 gsm of a polyurethane or ethylene vinyl acetate, the admixture having been added and cured subsequent to the paper forming step in a single operation.

According to the second aspect of the present invention, there is provided a method of producing a paper which is comprised of cellulose fibres and optionally synthetic polymeric fibres wherein after the paper forming step the web is impregnated with an aqueous resin admixture comprised of dry strength increasing additive, a non-ionic dispersion of a polyurethane or non-ionic dispersion of ethylene vinyl acetate and a cationic wet strength increasing agent in amounts such that the paper comprises 0.05 gsm to 1 gsm of the dry strength increasing additive 0.1 gsm to 0.5 gsm of the wet strength increasing agent, and 0.5 gsm to 3 gsm of the polyurethane or ethylene vinyl acetate, and the resin admixture is subsequently cured.

The use (after the paper forming step) of a resin admixture which is comprised of a dry strength increasing additive, a non-ionic dispersion of either a polyurethane or ethylene vinyl acetate polyurethane and a cationic wet strength increasing agent and which is subsequently cured has the advantage that all resin components (including the strength increasing agents) are applied in a single operation. This is rendered possible by the use of non-ionic dispersion of polyurethane or ethylene vinyl acetate and has the advantage that (by virtue of the single resin treatment step) the number of operations which cause creasing or paper damage is minimised.A further advantage is that the various resin components are able to cure with each other as well as with sites on the cellulose fibres so as to produce a cured impregnant which gives the paper good mechanical strength and dimensional stability. The cured dry strength increasing additive and polyurethane or ethylene vinyl acetate, bond the fibres together at their crossing points to give good wet and dry strength characteristics for the finished paper. Addition of the wet strength increasing-additive at the paper forming step would reduce the number of sites on the cellulose available for curing with the dry strength increasing additive and polyurethane.

Papers produced by the invention are eminently suitable for use in stencil papers, < vhich purpose the paper of the invention preferably has a basis weight of 5 to 15 gsm and a thickness of 10 to 50 microns. The paper of the invention can however be used for a variety of applications where dimensional stability and good pore size distribution are required, e.g. aqueous filtration. For such applications it is possible for the paper to have a basis weight above 15 gsm.

When used in stencil papers, the polyurethane of the thin paper provides a good lamination of the thermosensitive polymeric film (usually polyester) to the paper. For use in stencils, the papers of the invention may incorporate polymeric fibres as an aid to dimensional stability. Such fibres would generally be present in an amount of 10% to 50% by weight of the paper and may for example be polyester fibres. Suitable polyester fibres are as described in EP-A-0 451 269 (Asahi). The cured impregnant in the paper of the invention serves to "key" such polymeric fibres into the paper and prevent these fibres being "pulled out" either during lamination to the thermosensitive polymeric film or during actual printing.

The polyurethane or ethylene vinyl acetate is preferably provided as a nonionic aqueous dispersion.

For preference, the polyurethane is a hydrophobic, polyesterpolyurethane. The dispersion may be produced by a forced emulsification technique. A particularly suitable product is available under the name DKS International Superflex E-2500.

The non-ionic dispersion of ethylene vinyl acetate may be provided by Vinamul 3303 which is a non-ionic aqueous dispersion of ethylene vinyl acetate having a pH of 5.5 and a glass transition point (Tg) of 0 C.

The preferred dry strength increasing additive for use in the invention is polyvinyl alcohol which is normally produced by the hydrolysis of polyvinyl acetate or other polyvinyl esters. We prefer to use a polyvinyl alcohol having a degree of hydrolysis of at least 95% and more preferably at least about 98%. Most preferably the polyvinyl alcohol is a fully hydrolysed (98-99% hydrolysis) or a superhydrolysed (99+% hydrolysis) polyvinyl alcohol solution. It is however within the scope of the present invention to use a cold water soluble grade of polyvinyl alcohol, for example one having a degree of hydrolysis of 66 to 68%. Most preferably the polyvinyl alcohol is of relatively high molecular weight exhibiting a solution viscosity (at 4% solids content) of greater than 40 cP, preferably 45-70 cP.

A particularly suitable polyvinyl alcohol for use in the method of the invention is Airvol 165.

In certain circumstances, it may be necessary or desirable to ensure that the polyvinyl alcohol or ethylene vinyl acetate does not form a film and (in such circumstances) it is therefore necessary to use an agent for inhibiting film formation.

This agent may for example be a non-ionic surfactant (e.g. Triton X114) used in an amount of 0.5-2% (e.g. about 1%) by weight of the polyvinyl alcohol or ethylene vinyl acetate present in the aqueous resin admixture.

Other dry strength increasing additives which may be used include amphoteric starch and carboxymethylcellulose (eg. 30EG).

The wet strength increasing agent used in the method of the invention is preferably a cationic epoxidised polyamide-polyamine resin or a cationic aminoformaldehyde resin, e.g. a melamine formaldehyde resin. A particularly suitable product is Kymene SLX2 available from Hercules. Other examples include Kenores from Akzo Nobel and BIP Melamine BC355.

The aqueous resin admixture is preferably formed by the following steps: a) dissolution of the polyvinyl alcohol in water and heating to 950C to form a fully hydrolysed solution; b) cooling the solution obtained from (a) to 15 to 250C followed by addition of the surfactant; c) addition of the wet strength increasing agent to the polyvinyl alcohol solution; and d) addition of an aqueous non-ionic dispersion of the polyurethane resin or ethylene vinyl acetate to the admixture obtained from step (c).

The aqueous resin admixture may be impregnated into the preformed paper in a single pass at a coating station which could for example involve application of the impregnant by means of a size press or gravure roll.

The amount of aqueous resin admixture applied to the paper is such that the latter is treated with 0.05 gsm to 1 gsm of polyvinyl alcohol, 0.1 gsm to 0.5 gsm of the wet strength increasing agent, and 0.5 gsm to 3 gsm of the polyurethane or ethylene vinyl acetate. Impregnation of polyvinyl alcohol in an amount greater than 1 gsm and/or impregnation of polyurethane or ethylene vinyl acetate in an amount greater than 3 gsm of the paper results in film formation of these resins across the voids of the paper resulting (when the paper is to be used in a stencil paper) in insufficient ink transmission, or poor ink fill-in, where the stencil master has poor optical density or poor definition of characters during printing. Use of more than 0.5 gsm of the wet strength increasing additive gives similar disadvantages.Use of amounts of the various resins below specified minima gives rise to problems such as unsatisfactory wet strength and unsatisfactory print durability.

The subsequent cross-linking by curing of the resin admixture may be effected in a single pass through a hot air dryer or an oven. Most preferably this curing step is effected at a temperature of 150 to 200"C. Curing temperatures below 1 500C may result in insufficient bonding of polyvinyl alcohol and polyurethane at the crossing points on the fibre filaments leading to poor wet and dry strengths which in turn lead to poor print durability of the stencil master. A curing temperature above 2000C may result in decomposition of the polyvinyl alcohol and polyurethane leading to poor bonding at the fibre filament crossing points.This will give poor wet and dry strengths again leading to poor print durability. Papers in accordance with the invention may be formed~ into stencil papers in accordance with standard procedures. The advantageous strength and dimensional stability properties of the papers of the invention produce stencils which produce good definition print for several thousand copies.

The paper of the invention comprises cellulose fibres and, optionally, synthetic polymer fibres. The cellulose fibres may be bast natural fibres (such as manila hemp, flax, jute or sisal).

The invention is illustrated by the following non-limiting Examples and Comparative Examples.

Examples and Comparative Examples A range of papers were prepared from vegetable fibres having a length of 1 to 4 mm and polyester fibres (length = 5mm, diameter = 10 microns). Each paper comprised 85% vegetable fibres and 15% of the polyester fibres, these percentages being based on the total weight of the fibres. The papers were formed with different basis weights and after the paper formation step were treated with resin impregnants comprising Polyvinyl Alcohol, epoxidised polyamide - polyamine resin (provided by Kymene SLX 2) and either polyurethane (provided by Superflex E-2500) or ethylene vinyl acetate (provided by Vinamul 3303). The resin impregnant was produced in accordance with steps (a)-(d) above.Various impregnants were used to give the amount of polyvinyl alcohol, epoxidised polyamide-polyamine and either polyurethane or ethylene vinyl alcohol on the finished paper-as shown in Table 1 below.

After impregnation, the resin was cured at 1 800C.

The physical properties of the papers thus obtained are shown in Table 1 together with an assessment of the properties of print optical density and durability of printing when using the papers in stencils.

TABLE 1

RESIN IMPREGNANT EX1 EX2 EX3 EX4 EX5 EX6 EX7 COMP COMP COMP COMP COMP POLYVINYL ALCOHOL 0.25 0.05 0.21 0.21 - - 0.23 EX1 EX2 EX3 EX4 EX5 (g/m2) 0.25 1.25 0.25 0.21 0.10 EPOXIDISED 0.25 0.25 0.10 0.25 0.14 0.14 - 0.05 0.25 0.74 0.15 0.15 POLYAMIDE/AMINE (g/m2) MELAMINE FORMALDEHYDE (g/m2) - - - - - - 0.23 - - - - POLYURETHANE (g/m2) 1.00 1.00 2.50 - 1.00 - 1.10 1.00 1.00 3.52 0.25 ETHYLENE VINYL - - - 1.00 - 0.50 - - - - 0.25 ACETATE (g/m2) CARBOXYMETHYL CELLULOSE - - - - 0.41 0.41 - - - - - PHYSICAL PROPERTIES BASIS 12.0 12.0 11.8 11.3 11.5 11.3 11.3 11.8 12.2 12.4 11.5 11.6 WEIGHT (g/m2) CALIPER (microns) 45 45 44 44 43 43 43 44 46 45 44 45 APPARENT 0.27 0.27 0.27 0.26 0.27 0.26 0.26 0.27 0.26 0.28 0.26 0.26 DENSITY (g/cm3) DRY TENSILE 80 65 75 65 74 60 60 77 95 104 65 55 MD (g/mm) WET TENSILE 30 27 26 25 29 25 24 18 33 36 19 17 MD (g/mm) AIR PERMEABILITY 235 240 240 220 235 230 240 240 180 175 230 250 (m3min/m2) PRINTING CHARACTERISTICS PRINTING GOOD GOOD GOOD GOOD GOOD GOOD GOOD GOOD POOR POOR GOOD GOOD OPTICAL DENSITY PRINTING DURABILITY GOOD GOOD GOOD GOOD GOOD GOOD GOOD POOR GOOD GOOD POOR POOR It will be seen from Table 1 that the products of Examples 1 to 7 (i.e. products in accordance with the invention) had wet strength, printing optical density and printing durability characteristics rendering them suitable for use in a thermosensitive stencil paper. On the other hand, the papers of Comparative Examples 1 to 5 do not have the required characteristics for use in a thermosenstive stencil paper. Thus, reducing the level of epoxidised polyamide/polyamine to 0.05 g/m2 results in unsatisfactory wet tensile strength and poor print durability. Increasing polyvinyl alcohol level to 1.25 g/m2 (Comp Ex2) or increasing the polyurethane level to 3.52 g/m2 (Comp Ex3) both lead to unsatisfactory printing optical density. Decreasing the polyurethane level to 0.25 g/m2 or decreasing the ethylene vinyl acetate level to 0.25 g/m2 both result in unsatisfactory print durability.

Claims

1. A paper comprised of cellulose fibres and optionally synthetic polymer fibres wherein the fibres of the paper are bonded together by a cured resin admixture comprising 0.05 gsm to 1 gsm of dry strength increasing additive, 0.1 gsm to 0.5 gsm of a cationic wet strength increasing agent, and 0.5 gsm to 3 gsm of a polyurethane or ethylene vinyl acetate, the admixture having been applied and cured subsequent to the paper forming step in a single operation.

2. A paper as claimed in claim 1 wherein the dry strength increasing additive is polyvinyl alcohol.

3. A paper as claimed in claim 1 or 2 having a basis weight of 5 to 15 gsm and a thickness of 10 to 50 microns.

4. A paper as claimed in any one of claims 1 to 3 incorporating 10% to 50% by weight of the paper of synthetic polymeric fibres.

5. A paper as claimed in claim 4 wherein the synthetic polymeric fibres are polyester fibres or acrylic fibres.

6. A paper as claimed in any one of claims 1 to 5 wherein the cellulose fibres comprise bast natural fibres.

7. A paper as claimed in claim 6 wherein the bast natural fibres are manila hemp, flax, jute or sisal.

8. A method of producing a paper which is comprised of cellulose fibres and optionally synthetic polymeric fibres wherein after the paper forming step the web is impregnated with an aqueous resin admixture comprised of dry strength increasing additive, a non-ionic dispersion of a polyurethane or non-ionic dispersion of ethylene vinyl acetate and a cationic wet strength increasing agent in amounts such that the paper comprises 0.05 gsm to 1 gsm of the dry strength increasing additive 0.1 gsm to 0.5 gsm of the wet strength increasing agent, and 0.5 gsm to 3 gsm of the polyurethane or ethylene vinyl acetate, and the resin admixture is subsequently cured.

9. A method as claimed in claim 8 wherein the dry strength increasing additive is polyvinyl alcohol and an agent is used to inhibit the film forming characteristics of the polyvinyl alcohol.

10. A method as claimed in claim 9 wherein the polyvinyl alcohol has a degree of hydrolysis of at least 95%.

11. A method as claimed in claim 10 wherein the polyvinyl alcohol has a degree of hydrolysis of at least 98%.

12. A method as claimed in claim 11 wherein the polyvinyl alcohol has a degree of hydrolysis of at least 99%.

13. A method as claimed in any one of claims 9 to 12 wherein the polyvinyl alcohol exhibits a solution viscosity at 4% solid content of greater than 40 cP.

14. A method as claimed in any one of claims 9 to 13 wherein the agent for inhibiting the film forming characteristics of the polyvinyl alcohol is a non-ionic surface active agent.

15. A method as claimed in any one of claims 8 to 14 wherein the wet strength increasing agent is a cationic epoxidised polyamide-polyamine resin, or a cationic malamine formaldehyde resin.

16. A method as claimed in any one of claims 9 to 15 wherein the aqueous resin admixture is formed by the following steps: a) dissolution of polyvinyl alcohol in water to form a fully hydrolysed solution; b) addition of a surface active agent to inhibit film forming characteristics of polyvinyl alcohol; c) addition of the wet strength increasing agent to the polyvinyl alcohol solution; and d) addition of a non ionic aqueous dispersion of the polyurethane resin or ethylene vinyl acetate to the admixture obtained from step (c).

17. A method as claimed in any one of claims 8 to 16 wherein the resin is cured at a temperature of 150 to 2000C.