GB2029324A - Duplicating Stencils - Google Patents

Abstract

For producing a stencil from an electrophotographic copy by heat bonding the toner image of the copy to an ink-impermeable layer of a stencil master and thereafter stripping the copy from the master the latter is composed of a conventional tissue sheet to which an ink-impermeable layer is attached by adhesive. The ink- impermeable layer comprises finely divided zinc oxide in a resin binder, the proportion of pigment to resin preferably being between 3:1 and 12:1. The resin binder may have two components, one of polyurethane or polyvinyl chloride type giving a tough film, the other of cellulose ester type giving a weak film.

Description

SPECIFICATION Duplicating Stencils This invention relates to the preparation of stencils for use on a stencil duplicator for purposes of reproduction of multiple copies of an original document.

The principle of stencil duplicating is well known and involves the use of an impermeable stencil master which is perforated or cut in areas corresponding to the image to be duplicated so that when the stencil is used in a duplicating machine ink can pass through those areas of the stencil and produce duplicate images on the copy paper employed. The stencil can be cut manually by using a stylus or the type of a typewriter.

Numerous proposals have been made for automatic or non-manual production of stencils directly from an original document.

At present, however, there are only two systems commercially available for the nonmanual production of stencils. The first process involves the electrical cutting of a carbon-loaded layer on a porous stencil sheet. A spark discharge cutting-head is synchronised with an optical head which views the original and directs the cuttinghead to cut the stencil, point by point. This system produces good results but is relatively slow, requiring a period of 5 to 1 5 minutes to complete, and since the process involves burning off the carbon loaded regions it is dirty and produces an unpleasant odour.

The second process is a thermographic process known as "thermal imaging". It uses a stencil master consisting of porous tissue with a thin thermoplastic coating which is placed against the original document and heated by an nfra-red lamp. The temperature rise of the coating is greatest in the black image areas and it melts in these areas to expose the porous tissue and thus produce a stencil. The process is very quick, the exposure time being of the order of 10 seconds, but it requires practice and skill to determine the length of exposure. The resolution is poor and the process is blind to many colours of the original, including some blacks. There is also a risk of damage to the original.

In connection with the thermal imaging process it has been proposed in U.S. Patent Specification No. 3,871,899 of Andersen to attach the thermoplastic coating to the tissue by means of an adhesive and to apply to the surface of the thermoplastic coating a layer including talc.

Another process for non-manual production of stencils was proposed in U.S. Patent Specification No. 2,955,531 of Bogdonoff. This proposal dates from 1956 and is believed not to have resulted in any commercial product. The proposal was to produce a powder image of the original by xerography, render the powder image tacky, place it in contact with a master carrying a wax coating, allow the image to solidify in contact with the master and then strip it away so that it removed corresponding parts of the wax coating from the master to produce a stencil. One method of rendering the powder image tacky, which is suggested by Bogdonoff as an alternative to his preferred method of applying solvent vapours is the use of heated rollers to press the powder image against the master.

Bogdonoff was concerned solely with stencil masters having a wax or wax-like coating. The wax was in direct contact with the permeable tissue and must have permeated the tissue at least in part. This would mean that it was difficult to separate the wax cleanly from the tissue in order to obtain a sharply cut stencil. The waxes used were liable to melt at temperatures of around 1 000C, which could easily be reached during the softening of the toner image. If the toner image was softened by the use of solvent vapour there would have been a danger of softening of the wax by residual solvent. In each case softening of the wax would cause degradation of the image quality.

The present invention results from appreciation of the fact that a sharply cut stencil can be obtained by using a heat to secure adhesion of the image to the impermeable layer on the stencil master and relying upon that adhesion to tear away the impermeable layer. To achieve good results, equal to or better than the available commercial processes, atttention must be directed to the structure of the impermeable layer of the stencil master and to the way in which it is bonded to the base sheet.

In accordance with the present invention there is provided a stencil sheet for the production of a stencil from an electro-photographic image by bonding the image to the stencil sheet and then tearing away parts of the stencil sheet adhering to the image, the stencil sheet comprising a porous tissue sheet, an ink-impervious layer comprising a synthetic resin composition containing finelydispersed zinc oxide, and an adhesive bonding the ink-impervious layer to the porous tissue sheet.

It has been found that the attachment of the ink-impervious layer to the base layer by adhesive allows clean separation of the layers when the substrate and master are separated. Such a requirement does not exist in the thermal imaging process and the teaching of Andersen 3,871,899 is thus largely irrelevant. Further the use of an ink-impermeable layer comprising zinc oxide dispersed in a synthetic resin gives the layer a coherence and stability which would not be possessed by a wax layer if the latter were only attached to the porous base layer by adhesive and at the same time assists the ink-impermeable layer to break cleanly and sharply as it is torn away by the image on the substrate.

British Patent Specificafion No. 1,508,009 discloses a stencil blank including an inkimpermeable material comprising a synthetic resin containing photo-conductive particles, for example particles of zinc oxide. However that stencil blank comprises a base layer of perforated or porous foil of synthetic plastics material or of metal and the ink-impermeable material occludes the perforations or pores in the foil. The process ol formation of the stencil can employ an electrostatic image but this is formed on the stencil blank itself using the photo-conductive properties of the zinc oxide particles. The image is developed by means of a toner in conventional manner.A heat-sealing adhesive layer is applied to the surface of the stencil blank and bonded to the toner image by the application of infra-red radiation, after which the image is stripped away to form the stencil. Clearly this is a different process and more complex than one in which a conventional electrophotographic copy is applied to the stencil master with the application of heat and is then stripped away to form the stencil. The invention enables this simpler process to be used successfully by reason of the particular structure of the stencil sheet.

The stencil sheet of the invention comprises firstly a porous tissue sheet and this can be a conventional stencil tissue such as Yoshino Type 602 Standard stencil tissue.

The ink-impervious layer may be formed from a range of resin binders which includes acrylics, polyurethanes, polyvinylchlorides, cellulose esters, vinyl acetates, etc, etc. The ratio of zinc oxide to resin binder can vary from 3:1 to 12:1.

The resin acts as an adhesive to bind the zinc oxide and to form a bond with the toner image.

The zinc oxide assists the bonding of the toner and interrupts the continuity of the resin film, allowing the layer to break rather than stretch during the separation of the bonded sheets, thus producing the perforation mechanism. The suspension of zinc oxide in resin is produced by milling, for example ball-milling, for a long time so that a small particle size is obtained because this tends to improve the sharpness of the cut stencil.

A dye such as Rose Bengal, Crystal violet, Methylene Blue, Malachite Green, or a colouring pigment such as Irgalite Yellow (CibaGeigy} or Monastral Blue (ICI) etc. is included in the milled suspension to improve the visual appearance of the perforated stencil. The prepared suspension is coated onto a casting paper or a release paper to give a dry coating weight of 20-35 gsm. The degree of release is selected so that the cast layer remains bonded to the paper and can only be delaminated once adhered to the tissue.

The function of the adhesive is to laminate the ink-imperious layer to the tissue but it must selectively release the regions of the layer that have been bonded to electrostatically produced image on the substrate. Thus the bond strength of the adhesive must be sufficient to provide good lamination to the tissue allowing multiple copies to be taken on the duplicator without stencil damage but must be sufficiently weak to be overcome by the bond between the layer and the image on the substrate. Various types of adhesive resins can be used such as polyvinyl-acetates, cellulose derivatives, acrylic emulsions, etc. The bond formed between the tissue and the layer depends on the type of adhesive selected but this may be a dry-bond, pressure or heat-sensitive bond.The adhesive is applied to the inkimpervious layer, which has been previously coated onto the release paper, and the tissue is laminated to the adhesive layer in its wet or dry state depending on the type of adhesive employed. The quantity of adhesive applied is that which provides the correct bond strength when tested.

The above laminate forms the stencil blank which may now be stripped from the release paper or retained on the release paper depending on the particular stencil format required.

The electrostatic image used with the stencil blank in the process of the invention may be produced by direct or indirect electrophotography. The imaging processes, which are well-known, involve producing a latent electrostatic image on a substrate and then developing the latent image with a toner powder and fixing to produce a visible, stable, image.

Various types of toner are used, and for the present process a dry heat-fixed toner with a melting point in the range of 80" to 1500C is preferred. For example a one component, magnetic heatfix toner can be applied to the latent image by means of a magnetic roller.

The fusing stage in which the electrostatic image is bonded to the surface layer of the stencil blank is preferably carried out using infra-red heating because in this way radiation is preferentially absorbed by the black areas of the electrostatic master copy thus providing maximum heat in the image areas where bonding is required.

The following is a more detailed example of the process in accordance with the invention:- Example I The stencil sheet was manufactured as follows: A finely-divided suspension was prepared by taking 200 parts by weight zinc oxide (Durham 100, from Durham Chemicals), 66 parts by weight of 50% solution of an acrylic resin in xylene (QR45 1 from Rohm 8 Hass), 100 parts by weight toluene, mixing and ball-milling for 1 6 hours. The suspension was coated onto casting paper (Multithane 700 from Wiggins Teape) to a dry coating weight of 24 g/m2. The dry cast layer was overcoated with an aqueous, pressuresensitive adhesive layer (Revertex A3 12 from Harco Chemicals) and the water dried off. Yoshino tissue was applied by a laminating roller providing sufficient pressure to give firm adhesion.

A copy of the original was taken on a Roneo Vickers Type DB6 electrostatic copier which uses a zinc oxide copy paper and a one component, magnetic toner powder. The toner used was a mixture of carbon black, iron oxide as a magnetic pigment, and a thermoplastic acrylic resin Synocryl 4003 available from Cray Valley Products. The resin had a melting range of 950 to 1 300 C. The image was pre-fixed by cold pressure to the copy paper.

The imaged sheet was placed in contact with the surface layer of the stencil blank and the combination passed through an external fusing unit containing a 500 watt infra-red source in a pyrex glass cylinder with the imaged side of the sheet towards the source.

The bonded composite was mounted on a Roneo Vickers Type 870 Duplicator and the copy paper peeled back. The stencil thus produced gave satisfactory duplicating performance. The duplicated copies had good definition and the stencil produced long runs.

Whereas in Example I a single-component resin binder was used the resin composition preferably comprises a strong flexible film forming resin of the polyurethane or polyvinyl chloride type combined with weak-film-forming, lowmelting-point resin of the cellulose ester type with a ratio of strong-film resin to weak-film resin between 0.5 to 1 and 3 to 1. The zinc oxide is preferably in the proportion of 10 to 4 parts by weight to one part of resin and very good results are obtained with 8-parts by weight of zinc oxide to one part of resin. All grades of zinc oxide tried have been found to work but the preferred material is Grade 100 supplied by Durham Raw Materials Limited.

The formulation of the ink-impermable layer is designed to give a combination of properties.

Firstly it must have the right properties for manufacture by a process in which it is coated from a solvent onto a release paper and after drying in coated with the adhesive, after which the stencil tissue is laminated to the adhesive.

This process minimizes penetration of the tissue by the material of the ink-impermeable layer because the latter is formed as a coherent layer before the adhesive film and tissue sheet are applied. The release paper is removed after drying of the assembly and the ink-impermeable layer must have the necessary properties in relation to the release paper to enable it to release easily at this stage but remain firmly attached during manufacture of the stencil sheet.

Secondly the ink-impermeable layer must have good strength in the plane of the layer but must be easily breakable when pulled in a direction perpendicular to the layer by the adhering toner image. This is achieved by the use of the two resin components loaded with the zinc oxide pigment.

If the proportion of strong flexible film forming resin to weak film resin is less than 0.5 to 1.0 there is insufficient film strength to give adequate duplicator life whereas if it exceeds 3 to 1 there is too great a film strength to allow clean breakage when the toner image is pulled away. The weak film resin may be, for example, cellulose proprionate or cellulose acetate butyrate. The weak film resin should have a melting point not higher than 2500C. The combination of resins does not melt below its own decomposition point, which is higher than the charring point of the tissue substrate, that charring point being about 280"C.

The weak-film resin, if mixed with zinc oxide in a proportion of 10 parts by weight of zinc oxide to one of resin, would produce a film which on creasing will crumble. On the contrary the strongfilm resin is much tougher and if loaded with zinc oxide in the same proportion will show no deterioration in the form of crumbling or pinholding when creased.

The following is an example of the use of a two-component resin binder.

Example II A finely-divided suspension of zinc oxide in binder resins was prepared by taking 240 p.b.w.

(parts by weight) zinc oxide (Grade 100, ex Durham Raw Materials Ltd.,), 50 p.b.w. of 30% solution of a polyurethane in toluene/xylene/isopropanol (Witcote 344 ex Witch Chemicals), 75 p.b.w. of a 20% solution of cellulose acetate butyrate (CAB 381-2 ex Eastman Chemical) in toluene/lndustrial Methylated Spirit, 130 p.b.w. xylene, 35 p.b.w. of 2% solution of methyl violet in Industrial Methylated Spirit, mixing and ball-milling for 1 6 hours. The suspension was coated onto release paper (Quiklease ex Jointine) to a dry coating weight of 28 g.s.m. The dry layer was over-coated with an aqueous based polyvinyl acetate adhesive (ex Monarch Adhesives). Yoshino tissue was applied by laminating roller to the adhesive while still wet and the composite dried.

In Example I the ratio of zinc oxide to resin is 6:1. In Example II the ratio is 8:1 with equal parts of the two types of resin.

Further examples of the composition of the inkimpervious layer are given below.

Example III A finely divided suspension of pigment in binder resins was prepared by taking 240 p.t.w.

zinc oxide, 100 p.b.w. of 30% solution of a vinyl chloride/vinyl acetate copolymer (VYHH ex Union Carbide) in methyl ethyl ketone, 100 p.b.w. of a 30% solution of cellulose acetate butyrate (CAB 381-2 ex Eastman Chemicals) in methyl ethyl ketone, 35 p.b.w. of 2% solution of methylene blue in Industrial Methylated Spirit, 130 mls methyl ethyl ketone and ball-milling for 1 6 hours - to a grind value of 8 on a Hegman gauge. The suspension was coated onto release paper to a dry coating weight of 30 gsm and laminated as previously describe. The proportion of Zinc Oxide to Resin is 4:1.

Example IV The formulation comprises 300 p.b.w. zinc oxide, 66.6 p.b.w. Witcote 344 (30% solution), 66.6 p.b.w. of a 15% solution of cellulose propionate (CAP 482-20 ex Eastman Chemicals) in 1:1 toluene/l.M.S., 39 p.b.w. of 2% solution of rose bengal in l.M.S. and 100 p.b.w.

toluene. (Zinc Oxide to Resin 10:1).

Example V The formulation comprises 300 p.b.w. zinc oxide, 50 p.b.w. of 25% solution of a polyurethane in methyl ethyl ketone/dimethylformamide (Impranol EHC-DL ex Bayer), 50 p.b.w. of 25% solution of CAB 381-2 in methyl ethyl ketone and 200 p.b.w.

methyl ethyl ketone. (Zinc Oxide to Resin 8:1).

Example Vl The formulation comprises 1 80 p.b.w. zinc oxide, 33.3 p.b.w. Witcote 344 (30% solution), 100 p.b.w. of 20% solution of CAB 381-2 in 1:1 toluene/lMS and 70 p.b.w. toluene. (Zinc Oxide to Resin 6:1).

Example VII The formulation comprises 240 p.b.w. zinc oxide, 20 p.b.w. of vinyl chloride/vinyl acetate copolymer (VYNS ex Union Carbide), and 20 p.b.w. of cellulose propionate (PR900 ex Bayer) in methyl ethyl ketone. (Zinc Oxide to Resin 6:1).

In example IV the proportion of the strong-film resin to weak film resin is 2 to 1. In Example VI the proportions are reversed. In Examples III, V and VI there are equal parts of strong and weak resin.

The stencil master comprising the porous tissue sheet with the ink-impermeable laver attached to it by adhesive can be removed from the release paper after its formation or can be supplied to the user with the release paper continuing to act as a protective covering.

Conventionaliy a stencil sheet is supplied attached to a backing sheet which includes a punched or perforated heading for attachment of the stencil to the duplicating machine, the rest of the backing sheet being removed immediately before the stencil is placed on the machine. When using a stencil sheet which is to be bonded to an electro-photographic image and then separated to form the stencil it is possible to form the image directly on the backing sheet of the stencil master assembly and bond the ink-impermeable layer to it. The user then forms the stencil by the process of removing the backing sheet which tears away the parts of the ink-impermeable layer attached to the image on the backing sheet.

Such an assembly can be produced very conveniently by using a machine which first forms an electro-photographic image on a backing sheet and then attaches a stencil sheet to the backing sheet and applies heat to bond the image to the coating on the stencil sheet. The machine can be similar to a conventional electro-photographic copier and can indeed be used for this purpose also but when a duplicating stencil is required a backing sheet is fed instead of the normal copy paper and the special stencil sheet is supplied for attachment to the backing sheet. The stencil sheet assembly consisting of backing sheet and stencil sheet can then be stored or transported to the duplicating machine, the backing sheet providing sufficient stiffness for easy handling and protecting the stencil sheet.Before mounting on the duplicating machine, the stencil sheet is stripped from the backing sheet, thereby forming the stencil, the original of which can still be seen on the backing sheet.

it is preferred to attach the backing sheet to the stencil sheet along a line close to one edge.

Normally the backing sheet includes a pre-printed heading with punchings to fit on to projections on the duplicating machine. The stencil sheet remains attached to this heading region while the body of the backing sheet is removed by tearing along a line of perforations. The attachment of the stencil sheet along a line above the perforations is conveniently effected by means of heat-sensitive resin carried by the backing sheet and activated when the stencil sheet and backing sheet are heated together to bond the image on the backing sheet to the ink-impermeable layer of the stencil.

The heat-sensitive resin for this purpose may be the same toner material which forms the image if a line is imaged onto the backing sheet or if the backing sheet has a line which is chargeable to attract the toner during formation of the electrophotographic image. Alternatively a line of heat-sensitive resin can be applied to the backing sheet before it is fed into the copying machine.

The electrophotographic image can be produced on copy paper by most of the conventional methods. Whether the copy paper serves also as a backing sheet for the stencil assembly is, of course, immaterial but as this is the preferred arrangement the following description refers to a backing sheet as the support on which the electrophotographic toner image is formed.

Where the backing sheet is a conventional plain white paper for example 100 g.s.m., having the usual punched and perforated heading, the toner image can be produced on commercial plain paper copiers, such as those sold under the trade names Toshiba BD-909, Agfa X10, U-BIX Mk.

Xerox 3103, and Sharp 721. These use a dry two component magnetic toner which is heat fused, for example by hot rollers. Such a toner image has strong adhesion to the backing sheet and can be made to adhere to the ink-impermeable layer of the stencil sheet sufficiently strongly to tear the layer away when the stencil sheet is removed from the backing sheet.

In the case where the backing sheet has an electrophotographic zinc oxide coating care has to be taken that the adhesion of this coating to the backing sheet is great enough to prevent it tearing away when the stencil sheet and backing sheet are separated. While certain commercially available papers can be used for this purpose, best results are obtained by the production of a special paper, for example with the following formulation:- Photox 80 400 pts (ex New Jersey Zinc) E312 (50% nv) 132 pts (ex De Soto Inc) Toluene 200 pts Sensitizer Soln. 5 pts.

(dyes in Methanol) Photox 80 is an electrophotographic grade of zinc oxide made by what is known as the French process. E 312 is modified acrylic resin and is supplied as a solution containing 50% nonvolatile material. The Hegman fineness of grind gauge is used to assess the degree of dispersion of pigment in printing inks. It consists of a block with a wedge-shaped groove tapering from a depth of 25 ym or more at one end to zero at the other. The liquid is placed in the deepest part of the groove and spread by a scraper which pulls forward any large particle and creates a scratch iri the surface of the liquid. The particle size is assessed by the position at which the scratch starts.A reading of 5 on the gauge indicates that there are no particles of a size greater than about 38 um, 6 none greater than 25,us, 7 none greater than 1 5 ym and 8 that the particles are too small to be measured by this method and certainly less than 10 ,um.

The Photox 80 is dispersed in the E3 12 resin and toluene and ground to a hegman reading of 5--53. The sensitizing dyes are added and stirred and the resulting coating is applied to E226 Electrofax base (ex Felix Scholle) at coating weight of 20 gsm with a wire wound coating bar.

The coating is then dried in an oven. The coated sheet is then kept in the dark for not less than 2 hours and imaged in an Electrostatic copier e.g.

Roneo Vickers D.B.6.

For use with a coated backing sheet a onecomponent magnetic toner powder is preferred which comprises a synthetic resin component, a magnetic material such as a ferrite, and a conductive carbon black. Selected commercially available materials with high proportions of synthetic resin with appropriate pressure and heat adhesive properties can be used but ayain a special formulation of toner is preferred, of which the following is an example Parts Synocryl 4003 (Cray Valley Products) 25 Versamid 940 (Cray Valley Products) 12 Mapico Black (Columbian) 52 Acetylene Black (Shawinigan) 11 Nigrosine (Spirit Soluble) (Holliday) 1 Whichever type of backing sheet is used, the stencil sheet is placed with its ink-impermeable layer in contact with the toner image on the backing sheet and the assembly is passed through a heating zone to bond the image to the surface layer of the stencil sheet. This heating step can be subsequent to the fusing of the toner image or may serve simultaneously to fuse the toner image. Infra-red heating by means of rollers is preferred in the heating zone whereas the initial fusing of the image can be done by cold pressure.

Claims (12)

Claims

1. A stencil sheet for the production of a stencil from an electro-photographic image by bonding the image to the stencil sheet and then tearing away parts of the stencil sheet adhering to the image, the stencil sheet comprising a porous tissue sheet, an ink impervious layer comprising a synthetic resin composition containing finelydispersed zinc oxide, and an adhesive bonding the ink-imprevious layer to the porous tissue sheet.

2. A stencil sheet as claimed in claim 1 in which the ratio of zinc oxide to resin is between 3 to 1 and 12 to 1 by weight.

3. A stencil sheet as claimed in claim 1 in which the ink-impervious layer comprises between 4 and 10 parts by weight of zinc oxide to one part of synthetic resin.

4. A stencil sheet as claimed in any of claims 1 to 3 in which the synthetic resin composition of the ink imperious layer comprises a first component capable of forming a tough film and a second component capable of forming only a weak film.

5. A stencil sheet as claimed in claim 4 in which the first component is a polyurethane or polyvinylchloride resin.

6. A stencil sheet as claimed in claim 4 or 5 in which the second component is a cellulose ester.

7. A stencil sheet as claimed in claim 6 in which the second component has a melting point below 2500C.

8. A stencil sheet as claimed in any of claims 4 to 7 in which the proportion of the first component to the second component is between 0.5 to 1 and 3 te 1 by weight.

9. A method of making a stencil sheet as claimed in any of the preceding claims in which the ink-impermeable layer is formed on a release surface, adhesive is applied to the inkimpermeable layer and the tissue is laminated to the adhesive layer by heat and/or pressure.

10. A stencil sheet as claimed in any of claims 1 to 8 having the ink impervious layer bonded to an electrophotographic toner image formed on a support which acts as a backing sheet for the stencil sheet, the backing sheet including a heading for attachment of the stencil to a duplicating machine.

1 A stencil sheet as claimed in claim 10 in which the stencil sheet is attached to the heading by a line of heat-cured resin.

12. A stencil sheet as claimed in claim 11 in which the line attaching the stencil sheet to the heading is of the same material asthe image.