GB2110161A - Improvements in or relating to plates for planographic printing - Google Patents

Abstract

A planographic printing blank to which a developed toner image on an electrophotographic recording member can be transferred and fixed, comprises a sheet of flexible insulating material and an image receiving etchable layer supported by said sheet having a volume resistivity of at least about 1 x 10<12> ohms/cm (determined as hereinbefore described) and consisting essentially of 30 to 75% by weight unsensitised zinc oxide, 0 to 40% by weight of clay and 10 to 20% by weight of a resin binder.

Description

SPECIFICATION Improvements in or relating to plate for planographic printing The present invention relates to a planographic printing blank and to its use in offset lithography.

The printing process known as offset lithography is not only used in the Print Room but is now popular in the office. This is because of the advent of simplified and automated machines known as table-top offset litho machines specially designed for the office.

One of the factors on which the popularity of such machines depends is the availability of cheap and convenient methods of producing offset litho plates.

Such methods arose from the use of electrophotography as a means of making copies of documents. Electrophotographic processes can be divided into those termed direct and those termed indirect The direct or Electrofax process originally devleoped by RCA involves the imaging onto a coated copy paper (generally coated with zinc oxide in a resin binder) and the development of that image. The indirect process exemplified by Xerox involves the imaging onto an electrophotosensitive material such as amorphous selenium and the development of the image, with a subsequent offset of the image onto plain paper.

In the case of direct electrophotography an offset litho process was developed by the application of a special etch to the zinc oxide coated paper. This etch contained ferrocyanide ions and the zinc oxide was converted to zinc ferrocyanide. The print portions of such a copy were oleophilic and the background portions became hydrophilic. Such a difference provides the basis for an offset litho plate and this process has become a popular and easy method of plate making.

But although it would be advantageous to a customer to have a single copying machine that could be used for both copies and for plate making, direct electrophotographic machines are now rarely used forthese dual functions. One of the reasons is that customers prefer the feel and appearance of plain paper copies to those of zinc oxide coated paper. With the advent of cheap plain paper copiers this preference is easily satisfied.

Thus the indirect electrophotographic copier (or plain papercopier) would be preferable if it were cheap, reliable and could produce both copies and offset litho plates. Offset litho plates can indeed be produced merely by using 'direct image' masters instead of copy paper. Direct image masters are plates designed to be typed on and the typed plate used as a master to make copies on an offset litho machine. If this material is used in the plain paper copier the image area is oleophilic and the backing hydrophilic- and so provides an offset litho plate.

The most popular means of fixing the toner used in plain paper copiers is by heated rollers. One of the problems of the use of offset litho plates on plain paper copiers is that the image is not sufficiently well fixed which has adverse consequences on plate life.

One means of overcoming this is to modify the copier so that the heated rollers can operate at a higher temperature than for copy paper and this produces a better fixed image. The problem then is that if an ordinary copy is made after a plate has been produced, the fixing rollers will char the plain paper until they have cooled sufficiently. They do not char the offset litho plate because it is considerably thicker than plain paper.

More recent technological developments for plain paper copiers are aimed at decreasing the price together with increasing the reliability. The latest generation of copiers use a monocomponent toner powder to develop the latent image by release from a magnetic roller. It has the benefit that there is no imbalance between toner and carrier whether the latter be liquid or coated iron filings. This is being developed into a simple and reliable system. In addition the image may be fixed onto plain paper using high pressure steel rollers and this is known as cold pressure fusing. The copier then has no warm up time as compared with hot roller fusing machines.

However such copiers do not lend themselves to be used for making offset litho plates. This is becase whilst cold pressure fusing is perfectly adequate for copying, the image is not sufficiently well fixed to the direct image plate to give more than a few copies. It is possible to heat fuse the image into the plate subsequent to plate making, but this involves another process and another machine.

In one aspect the invention provides a planographic printing black to which a developed toner image on an electrophotographic recording member can be transferred and fixed, comprising a sheet of fiexible insulating material having an image receiving etchable layer supported by said sheet having a volume resistivity of at least about 1 x 1012 ohms/cm and preferably at least about 1 x 1013 ohmslcm (measured as described below) and consisting essentially of 30 to 75% by weight unsensitised zinc oxide, 0 to by weight of clay and 10 to 20% by weight of a resin binder.

In a further aspect the invention provides a method for preparing a planographic printing plate which comprises forming a pattern of electrostatic charge on an electrophotographic recording member corresponding to an original to be duplicated, developing the charge image with toner, and transferring the toner image to a planographic printing black as set forth above.

All resistivity measurements herein quoted are volume resistivities determined on a Model 6105 Resistivity Adapter made by Kiethley Instruments Inc., 28775 Aurora Road, Cleveland, Ohio 44139, USA. The units are in ohms cms. as they apply to a fixed disc area as used in the instrument.

It is an object of this invention to solve the problem of making long run plates on new plain paper copiers that use mono-component toners and cold pressure fusing development. Moreover it is an object of this InventIon to permit a plain paper copier to be used to produce both copies and offset litho plates without making any adjustments to the copier to switch from one use to another. It is a further object of this invention to provide plates to give a better and more consistent performance on plain paper copiers then has hitherto been achieved on these copiers.

It is a yet further object of the invention to produce a plate that whilst being imaged in the plain paper copier, is otched by an electrostatic conversion etch.

The structure of the plate is shown in Figure 1.A base substrate 'A' may either be a completely insulating material such as a polyester or cellulose acetate film, or may be a paper base which is sealed either on the front surface (layer B) or on both front and back surfaces (layers B and D) to minimise any change in moisture content. Layers B and D may be either the same or different and may be applied as an extrusion - for instance double sided or single sided polythene coated paper-orfrom a solvent carrier. Examples of such layers are thin resin films of polyvinyl acetate, acrylic polymers, nitrocellutose and cellulose acetate butyrate.

The preferred resins for layer B are those that form a good insulating layer, give good impermeability and whilst not being soluble in the solvent used to apply layer C, provide a sufficient key to give good adhesion both to C and A so that the layers withstand the conditions and forces imposed on the plate during offset litho printing.

Layer C must contain zinc oxide or a mixture of zinc oxide and a clay such as Satintone Special (calcined aluminium silicate), coated in a resin layer such as De Soto E312 (modified acrylic). The zinc oxide may be of any commercial grade and is unsensitised. Thus it is preferably of a grade that shows little response to visible light and is not a grade sold for its photoconductive properties. The clay used may be a large variety of substances. The function of the clay is to adjust the volume resistivity of the layer to greater than 1 x 1 o12 ohms/cm. This necessary where the substrate A together with the layers B and D do not give sufficiently high bulk resistivity to the system. For instance, the pre-coated base of Example 2 had a volume resistivity of 10 ohms.cms.

When this was coated with the all zinc oxide top coat described in Example 1 (bellow) the resistivity was unchanged. The resulting plate did not image when passed through a Roneo Alcatel 1005 plain paper copier. When the invention was worked as in Example 2 (below), the volume resistivity was 1013 ohm.cm.

Volume resistivity measurements of various supports are shown in Table I: TABLE I Glorythene 100 gram polythene coated paper 1013 ohm.cm lnveresk SO gram wet strength WF base paper 109 ohm.cm Wet strength paper + Example 2 pre-coat 1011 ohm.cm The performance of various planographic printing blanks is summarisod in Table II: TABLE II karn Volurno Resistivity PPC Copy Quality Glorythene + Example 1 top coat 1013 ohm.cm Good Glorythene + Example 2 top coat 1015 ohm.cm Good Inverts!:: 85 gm. + pm-coat + Example 1 top coat 1011 ohm.cm V.Poor lnveresk 85 gm. + pro-coat + Example 2top coat - 1013 ohm.cm Good From Table II it is apparent that a volume resistiv id of 1012 ohms cms. and preferably at least about 1013 ohms. cms. is desirable for satisfactory per formance of the blank.

The quan itgof zinc oxide present must be sufficient that after conversion by a conventional ferrocy Dnido etch, the resulting plate will produce long runs of tisfaory copy with clean background. It has bean Found that as in Example 1 where the paper is coated seIth polythene as B and D ) a sufficiently high volume rosistivity is achieved to permit a plate with no clay to be imaged in the plain paper copier. The 8 vantage of using such a plate is that where the is is used on a highly automated offset litha- machine, etching can take place on such a machine and the machine may be used to duplicate copies immediately. This is because in general, the higher the zinc oxide concentration, the faster the etching.

One of the problems of the new plain-paper copier technology using mono component toners is that copy quality may vary with the resistivity of the plain paper as the latter property affects the toner transfer from the drum to the plain paper. The resistivity of the plain paper is humidity sensitive and therefore copy quality may vary with ambient conditions. The plate of this invention can be made relatively resistant to ambient conditions by producing the high resistivity material that is sealed from taking up moisture. Thus the actual copy quality obtained with these plates may be higherthan that obtained as ordinary plain paper copies.

The invention is illustrated in the following exam ples: Example 1.

Glorythene 100 gramme mats white polythene coated paper from Wiggins Teape was coated to a dry weight of 30 grammes per square metre with the following composition.

Zinc oxide 240 parts by wt.

Modified acrylic resin (E312 ex Destoto Inc.) 80 parts bywt.

Toluene 150 parts by wt.

Industrial methylated spirits 25 parts by wt.

The resulting material was imaged in a Roneo Alcatel 1005 copier, passed through a bath of electrostatic conversion solution and run to 1,000 copies on a Roneo RV2000 offset machine.

Example 2.

Inveresk 85 gramme Wet Strength WF base paper was precoated on each side to a dry weight of 4 grammes per square metre with the following solution: CA4E 220 Nitrocotton (ex Bergerac) 15 parts bywt.

Industrial methylated spirits 200 parts by wt.

Ethyl acetate 80 parts by wt.

One of the precoated surfaces was then coated to a dry weight of 30 grammes per square metre with the following composition: Zinc oxide 120 parts bywt.

Calcined aluminium silicate 120 parts bywt.

Modified acrylic resin it312 ex De Soto Inc) 80 parts by wt.

Toluene 150 parts by wt.

Industrial methylated spirits 25 parts by wt.

The resulting material was then processed as in Example 1, and run to 1,000 copies on a Roneo RV2000 offset machine.

Claims (12)

1. A planographic printing blank to which a developed toner image on an electrophotographic recording member can be transferred and fixed, comprising a sheet of flexible insulating material and an image receiving etchable layer supported by said sheet having a volume resistivity of at least about 1 x 1012 ohmsfcm (determined as hereinbefore described) and consisting essentially of 30 to 75% by weight unsensitised zinc oxide, 0 to 40% by weight of clay and 10 to 20% by weight of a resin binder.

2. A printing blank according to claim 1, wherein the sheet of flexible insulating material is a film of polyester or cellulose acetete.

3. A printing blank according to Claim 1, wherein the sheet of flexible material is of paper coated on its front face with a liquid impermeable insulating resin underlayer.

4. A printing blank according to Claim 3, wherein the rear face is coated with a liquid impermeable insulating resin layer which may be the same as or different from that on the front face.

5. A printing blank according to Claim 3 or 4, wherein the or each resin layer is of polyethylene.

6. A printing blank according to Claim 3 or 4, wherein the or each resin layer is of polyvinyl acetate, an acrylic polymer, nitrocellulose or cellulose acetate butyrate.

7. A printing blank according to any preceding claim where the clay is calcined aluminium silicate.

8. A printing blank according to any preceding claim wherein the resin binder is a modified acrylic resin.

9. A planographic printing blank substantially as described in either of the Examples.

10. A method for preparing a planographic printing plate which comprises forming a pattern of electrostatic charge on an electrophotographic recording member corresponding to an original to be duplicated, developing the charge image with toner, transferring the toner image to a planographic printing blank as claimed in any preceding claim, fusing the toner image by passing the blank through unheated pressure rollers, and etching the blank by means of an electrostatic conversion solution.

11. A method according to Claim 10, wherein the toner is a mono-component toner.

12. The use of a planographic printing plate as claimed in Claim 10 or 11 in offset lithography.