US 3376162 A
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Description (OCR text may contain errors)
April 2, 1968 SPENCER ET AL 3,376,162
ELECTROGRAPHIC TRANSFER SHEET I Fiied Nov. 12, 1965 United States Patent ()fi ice 3,37 ,162 Patented Apr. 2, 1968 3,376,162 ELECTROGRAPHEC TRANSFER SHEET Alexander Spencer and Alan Francis Blake, Tottenham,
London, England, assignors to Gestetner Limited, Tottenham, London, England, a British company Filed Nov. 12, 1965, Ser. No. 508,462 Claims priority, application Great Britain, Nov. 23, 1964, 47,601/64 Claims. (Cl. 117-216) ABSTRACT OF THE DISCLQSURE The invention provides an electrographic sheet for the electrical imaging of lithographic plates comprising a sheet of electrically conductive thin impervious paper having on one side an electrically conductive layer and, in said layer, an outer layer of oleophobic material transferable to a planographic plate by electric discharge.
This invention relates to electrographic sheets useful for the imaging of planographic offset plates.
It has been proposed (see United States patent specification No. 3,113,511) to make planographic offset plates which can be imaged electronically in which the planographic plate, having a smooth, water-insoluble, hydrophilic, planographic surface, has, strippably attached to said surface, a non-conductive plastic film rendered electrically conductive by the incorporation therein of conductive carbon particles. In use, an electric discharge-producing potential is applied to the plastic film in a pattern corresponding to the design to be reproduced, and the electric sparks cause the transfer of oleophilic material to the hydrophilic surface of the planographic plate. After treatment in this way and stripping of the plastic film from the plate, the latter becomes a planographic offset plate imaged with oleophilic areas corresponding to the image to be reproduced.
This arrangement has enjoyed a certain success but is subject to certain inherent disadvantages. In the first place, plasticiser tends to migrate from the film on to the planographic surface leading both to the production of images with scummy backgrounds in use, and to tendering of the film, which becomes liable to tear during stripping after it has been cut. The transferred image does not, moreover, adhere well to the planographic plate, and the image produced on the latter is not consequently very durable. Thirdly, the transferred image is in practice insufliciently oleophilic to give an entirely faithful copy. Fourthly, during the passage of the electric discharge through the plastic film, a major proportion of the plastic substance is dissipated by volatilization and only a minor part is transferred on to the planographic surface as image-forming material.
The present invention provides a novel type of electrographic sheet for use in the electronic imaging of pianographic offset plates, which avoids or mitigates these disadvantages.
The present invention provides an electrographic sheet for the electronic imaging of a planographic plate comprising an electrically conductive sheet of impervious paper less than 0.005 inch, and preferably less than 0.00125 inch, thick carrying an electrically conductive layer on one side, said layer having a conductivity of 4 10- to 2X10 mhos when determined in the manner hereinafter described, and having an outer face of oleophilic material transferable by electric discharge and so formulated as to form a firm, permanent key to a planographic plate after transfer thereto. The electrically conductive layer may be either unitary, i.e. a homogeneous layer which is both conductive and comprises oleophilic material transferable by electric discharge, or composite, i.e. a double layer of which the part adjacent the paper is primarily to confer the necessary electrical properties on the sheet and the remainder including the outer face consists of material transferable by the electric discharge and having a high degree of oleophility. It is also possible to provide a second electrically conductive layer on the side of the paper sheet opposite the transferable layer. When this is done, the second conductive layer should have a substantially lower conductivity than that of the other conductive layer.
The electrographic sheet of the invention is generally very thin, so that, while it is possible for it to be supplied without any kind of support, it is generally preferred to mount it on a sheet of thicker paper of the flexible material for ease in handling, and to prevent damage in storage or transport. The mounting is effected in the usual way by attaching one edge of the electrographic sheet to one edge of a similarly shaped support, e.g. in the manner used for known duplicating stencil blanks. If desired, the support may be the planographic plate, e.g. of paper, plastic or metal (and of kind known per se), which is to be imaged using the electrographic sheet. Alternatively'the support may be, for example, a sheet of thicker paper in which case the planographic plate is inserted behind the electrographic sheet immediately before the latter is electrically cut. Where the support is the planographic plate, it may be desirable to include a sheet of oil-resisting paper between the electrographic sheet and the plate to prevent damage of the latter during storage by migration of oily material from the electrographic sheet.
In use, the new electrographic sheet is placed with the transferable face adjacent the hydrophilic, water-insoluble planographic surface of a planographic offset plate and subjected to the action of a stylus-applied electrical discharge having a pattern corresponding to the subject matter to be reproduced. The electrographic sheet is perforated by the discharge and oleophilic material is transferred from the transferable layer to the surface of the planographic plate, so that the latter is imaged with the desired pattern. It is to be noted that the electrographic sheet is perforated in such a manner that there is little loss of the transferable layer, a major proportion of which is transferred to the planographic plate at the point of action of the electric discharge. This is believed to be due to the absence of the very sudden, very intense heating of the transferable layer which is apparently inevitable in the use of the prior art combination of planographic plate and plastic film.
As compared with the known type of planographic plate incorporating a plastic film described above, the new electrographic sheets enable better quality and more durable results to be obtained. Transfer of oleophilic material on to the planographic plate is more regular and more complete, and migration of plasticiser into the planographic sheet can be easily prevented ,(if, as is not necessarily the case, the oleophilic layer contains plasticiser) by inserting the thin sheet of oil-resisting paper already referred to between the planographic offset plate and the electrographic sheet, or simply by storing the electrographic sheets out of contact with the planographic plates. Any such removable oil-resisting sheet must, of course, be removed before the electrographic sheet is used.
jThe oleophilic transferable material of the electrically conductive layer may, in general, consist of any oleophilic, water-insoluble, film-forming, natural or synthetic, thermoplastic resin or wax, which does not lose any of its oleophilic character during transfer. Bitumens and gilsonites are satisfactory and, because of their cheapness, preferred, but synthetic products such as polyvinyl acetate can also be used if desired. When bitumen or gilsonite is used, the hardness of the layer may be improved by incorporating a small proportion of a suitable harder, compatible natural or synthetic material e.g. 10-30% by weight of carnauba wax. The electrically conductive layer will ordinarily be not more than 0.0002 inch thick. As already stated, it may be unitary or composite. When it is unitary, it is conveniently made conductive by the inclusion of an appropriate quantity of electrically conductive particles, e.g. carbon black. Alternatively, a composite double layer may consist of an outer layer of oleophilic transferable material and an inner layer of conductive particles dispersed in a filmforming binder, which may be plasticised. The binder is preferably a polyvinyl chloride/ polyvinyl acetate copolymer, but cellulose nitrate, polyvinyl formal, a polyvinyl acetate copolymer, and polymethylmethacrylate and its copolymers can, for example, be used.
The paper sheet is preferably 0.001 inch thick, the maximum practical thickness being generally about 0.00125 inch. With very thin papers, say of a thickness about 0.0005 inch or even less, the paper used must be such as to give sheets of the requisite mechanical strength for ease of handling. The paper must be impervious to the materials coated thereon, in particular the electrically conductive layer. This can in general be achieved by using a paper having a porosity not greater than 250 (determined by the Gurley-Hill SPS method), but it is preferred that the porosity of the paper is about 1500. As already stated, the paper must be electrically conductive. While this can be achieved by, for example, impregnating the paper with an electrolyte, it is most convenient to incorporate electrically conductive particles, especially of carbon, in the paper during its manufacture, about to 30% by weight of the paper being added. About by weight of carbon particles is the preferred quantity.
The optional layer of lower electrical conductivity on the opposite side of the paper to the electrically conduc' tive layer may be similar in composition to the inner conductive layer of the previously mentioned composite electrically conductive layer, except that its conductivity is reduced by using either less of the conductive particles or particles of lower intrinsic conductivity, or 'both.
The conductivity of the electrically conductive layer (or the inner conductive layer forming part thereof when a composite layer is used) is 4 10 to 2 10- mhos, preferably 4X 10* to 1X 10' mhos, as determined using two brass electrodes one cm. apart, each 1 cm. square and pressed on to the electrographic sheet by a load of 2 kilograms, the two electrodes being connected directly to a suitable conductivity meter. The conductivities of the paper and the optional conductive layer on the opposite side of the paper are not critical but are ordinarly much lower than that of the electrically conductive layer. Thus the paper has a conductivity, measured in the same way as that mentioned above but on paper having the electrically conductive layer on the reverse side, which is usually 1.6 10- to 2 l0- mhos or even lower, preferably 1x10 to 6 10 mhos. Similarly the optional layer on the opposite side of the paper has a conduc tivity, measured in the same way on a complete electrographic sheet, i.e. in the presence of both paper and electrically conductive layer, which is less than that of the paper and is ordinarily 1.4 10- to 2X10" mhos and is preferably 6 1()- to 5 x l0 mhos. It should be borne in mind that it may sometimes be desirable to vary the conductance of the upper (optional) layer so as to match the output of the electronic scanning device used to cut the electrographic sheet. The figures given, however, suit the type of machine most commonly in use.
The one or two electrically conductive layers are conveniently coated on the base paper by dispersing the coating composition in a solvent for the binder, for example acetone when the binder is a polyvinyl chloride/polyvinyl acetate copolymer, and applying the dispersion to 4 the paper by a known coating method, e.g. by roller coating or slot coating.
The two electrically conductive layers (when used) should preferably be of approximately equal thickness, their combined thickness being 0.0003 to 0.0004 inch, so
that the whole electrogr'aphic sheet is preferably not more than 0.00175 inch thick. Thicker electrographic sheets could be used but machines suitable for cutting them are not at present commercially available, although there is nothing in principle to prevent their being made.
When the new electrographic sheets are combined with.
a planographic plate as supporting sheet, the latter may be any paper or other flexible sheet materialof suitable strength and dimensional stability, having a hydrophilic water-insoluble planographic surface, and of a weight and strength suitable for the number of copies to be produced from it. Ordinarily wet-strengthened paper weighing about 45 lbs. (Double Crown) is used coated to provide the necessary water-insoluble hydrophilic surface, with or without a barrier coating between the paper and the hydrophilic outer coating to prevent water saturation of the 1 paper in use, and with or without a backing coating to prevent curling.
The hydrophilic surface may be composed of a hydrophilic polymeric material such as sodium carboxymethyl cellulose or sodium alginate rendered insoluble in water by treatment with heavy rnetal ions, e.g., aluminium, zinc, iron, or copper, or by cross-linking with a synthetic resin, e.g. a methylolated polyamide. Fillers such as titanium dioxide, zinc oxide and china clay may be included in the layer is desired. The weight of the hydrophilic surface layer is suitably 5 to 15 grams per square metre, of which at least 10% by weight is the aforesaid polymeric material.
When used, the barrier layer conveniently consists of a layer of chin-a clay bonded with 15-25% of casein,
based on the total weight of the barrier layer, and the weight of the layer is preferably 5 to 15 grams per square metre.
The following example illustrates the invention. The parts are by weight.
EXAMPLE Electro graphic sheet A sheet of thin paper weighing 11 g./sq. m. having a 1 porosity of 1600 and a thickness of 0.0009 inch in which has been incorporated during manufacture 10% by weight of highly conductive carbon (Vulcan C carbon black of Cabot Carbon) is coated on one side with a. composition consisting of:
Parts Polyvinyl chloride/ polyvinyl acetate copolymer (S 46/70 of I.C.I.) Dioctyl phthalate 20 Carbon black (Vulcan XC72 of Cabot Carbon) 12 Methyl ethyl ketone 400 and on the other side, to give a relatively highly conductive layer, with a composition consisting of:
Parts Polyvinyl chloride/polyvinyl acetate copolymer (S 46/70 of I.C.'I.) 100 Dioctyl phthalate 35 Carbon black (Vulcan XC72 of Cabot Carbon) 6O Methyl ethyl ketone 500 In both cases the dry weight of the coated layer is 10 grams per square metre.
An oleophilic layer transferable by electric discharge The dry weight of the coating is again grams per square metre.
This electrographic sheet may be used to image any planographic plate of known kind. Thus, if the electrographic sheet is subjected to the action of a stylus-applied electric discharge from an electronic scanning machine with the planographic plate immediately adjacent the transferable layer, the planographic plate is imaged with a pattern corresponding to that of the applied discharge.
The electrographic sheet may be applied as such without any kind of support, or it may be (or is generally preferred) attached along one edge to a flexible support, e.g. of paper of the kind used to support conventional duplicating stencil blanks. The support may also be a flexible planographic offset, e.g. one of the following.
Planographic offset plate Wet strengthened paper weighing 45 lbs. (Double Crown) is coated on one or both sides with The same paper with or without the barrier coating is then provided with a planographic surface by coating in one of the following ways.
I. A composition consisting of Parts Sodium carboxymethyl cellulose 5 Water 95 China clay Glycerine 3 is applied to the barrier coating and dried to give a coating having a weight of 5-15 g./m. An aqueous insolubilizing wash containing:
Percent Copper sulphate 3 Aluminium sulphate 1 Ferric chloride /2 Chromium trioxide /2 is then applied.
II. The aforesaid barrier coating is omitted, and the paper is coated directly with the following alternative barrier coating:
Parts China clay 100 Casein (in aqueous ammonium solution) 17.5 Dimethylolurea 1.75
to a coating weight of 515 g./m.
After drying at 120-140 F., a second coating is applied of Parts Sodium alginate 1 Water 95 Butanol 4 Finally, the coating is insolubilized with a wash coat of Parts Zinc acetate Water 72 Butanol 3 III. The base paper with barrier coating is coated with 7 Parts Sodium carboxymethyl cellulose 5 Water Clay 24 Titanium dioxide 14 to which has been added Kymene Resin 709 4 Kymene Resin 647 0.4
to a dry coating weight, after curing at 200 F., of 5-15 g./m.
IV. The base paper with barrier coating is coated with a composition made by mixing together the following mixtures:
The composition is applied to a dry coating weight of 5-15 g./rn.
In place of the aforesaid paper planographic plates, a grained aluminium or zinc lithographic plate can be used.
The electrographic sheet is then attached along one edge to any one of the planographic offset plates prepared in the aforesaid manner.
A preferred combination of electrographic sheet and planographic offset plate is shown diagrammatically in the accompanying drawing, which is a partial cross-section of exaggerated thickness of such a combination. In the drawing, the electrographic sheet, indicated generally as 7, consists of a sheet of thin impervious paper 2 having on its upper side a layer 1 of relatively low electrical conductivity and, on its lower side, a relatively highly conductive layer 3 and an oleophilic layer 4 transferable by an electric discharge. Along one edge, the electrographic sheet is attached to a planographic offset plate, indicated generally as 8, and consisting of a sheet of wet-strengthened paper 6 carrying a planographic surface coating 5.
1. An electrographic sheet comprising an electrically conductive sheet of impervious paper less than 0.005 inch thick, an electrically conductive layer having a conductivity of 4 10 to 2 10- mhos, on one side of said paper, and on the said electrically conductive layer an outer layer of bitumen containing up to 30% of carnauba wax transferable by electric discharge and so formulated as to adhere firmly to a planographic plate after transfer thereto.
2. An electrographic sheet according to claim 1 which also comprises a second, electrically conductive layer on the side of the paper sheet opposite the electrically conductive layer already specified, the said second electrically conductive layer having a substantially lower electrical conductivity than the said other electrically conductive layer.
3. An electrographic sheet according to claim 2, in which the conductivity of the second electrically conductive layer is 1.4 10 to 2 10 mhos.
4. An electrographic sheet according to claim 1, in which the bitumen contains 10-30% by weight of carnauba wax.
5. An electrographic sheet according to claim 1, in which the said electrically conductive layer between the paper sheet and the outer layer of oleophilic material consists of conductive particles dispersed in a film-forming binder.
6. An electrographic sheet according to claim 5, in which the total thickness of the electrically conductive layer is not more than 0.002 inch.
7. An electrographic sheet according to claim 1, in which the paper sheet is less than 0.00125 inch thick and has a porosity not greater than 250 determined by the Gurley-Hill SPS method.
8. An electrographic sheet according to claim 1, in which the paper sheet comprises 5-30% by weight of carbon particles.
9. An electrographic sheet according to claim 1, in
which the paper has a conductivity of 1.6 10- to 2 1O- mhos.
10. An electrographic sheet according to claim 1 having a total thickness not more than 0.00175 inch.
References Cited UNITED STATES PATENTS 2,328,198 8/1943 Davenport et al. 2,398,779 4/1946 Dalton et a1. 2,664,044 12/1953 Dalton 2.. 101--l49.2 2,713,822 7/1955 Newman 101-1492 3,048,515 8/1962 Dalton 204-2 X 3,151,548 10/1964 Picking.
DAVID KLEIN, Primary Examiner.