IL38465A - Duplicating stencils - Google Patents

Description

Duplicating stencils GBSTETNER LIMITED 38465/2 This invention relates to duplicating stencils designed to be cut electrically. %e tera'-stencil as sed herein means a stencil blari¾ except where a stencil is specifically stated to be imaged.

It is well known that duplicating stencils designed to be cut electrically generally have a two-layer structure. She layer adjacent the electric stylus has a higher electrical resistance than the other layer, and is ink-impervious in the uncut (non-imaged) state. It is possible for the two layers to be integral in the form of a self-supporting thermoplastic film, as in the constructio described in United States Patent Hp. 2,€64,043» ©r for the layer of higher electrical resistance to be supported by an ink-pervious porous sheet of stencil tissue, while the layer of Imar electrical resistance is either coated onto the back of the first layer, or supported separately on a backing sheet, as in the constructions described in our British Specification Ho. 1,042,585. In another form of construction, described, for example, in United States Patent So. 3,151,548, the layers of higher and lower electrical resistance are coated on opposite sides of a sheet of thin ink-impervious paper, preferably impregnated with carbon particles, hich prevents mixing of the two layers. However, known duplicating stencils constructed in any of these ways suff r from the disadvantages that they are unable to reproduce accurately continuous tones from photographic originals, and that they give poor response to 38465/2 $he present invention provides a duplicating stencil adapted t© be cut by the use of an electric stylus comprising two thermoplastic layers adapted to be contiguous in use* each layer having electrically conductive particles dispersed therein, that layer adapted to be nearer the stylus being- nk-impervious and having a higher electrical •l i resistance than the other layer and having semi-conductive as well as conductive particles dispersed therein.

Preferably^ the two thermoplastic layers form an integral self-supporting film which is strippably mounted on a sheet of a suitable release paper. The film is then cut in situ while still mounted on the release paper, and the latter except for a strip used to facilitate mounting is stripped from the former immediately before use.

In an alternative construction, the layer of higher electrical resistance is a self-supporting film, while the other layer is coated onto a backing sheet such that in use the two are contiguous. In this connection, it is to be understood that the term "contiguous" as used herein refers to the electrical properties of the two layers, and the presence between them of a thin layer of another material, e.g. paraffin wax, which does not interfere with their desired electrical properties, does not prevent them from being "contiguous" in the present sense.

It has been found that duplicating stencils of this kind are much better able than prior art stencils not containing the semi-conductive particles to reproduce continuous tone originals. It is believed that the presence of the semi-conductor in the thermoplastic film decreases the avalanche breakdown field for the film thereby facilitating the electrical cutting. For example, the avalanche breakdown field for silicon, a preferred semi-conductor for use in this invention, is 10^ volts avalanche in a particle 1/2 micron in diameter requires a potential of 50 volts. In contrast, for a film made of polyvinyl chloride, a suitable thermoplastic film-forming material for use in the present invention, breakdown occurs at a field of 5 x 10 volts per centimetre, so that a 1/2 micron thickness requires a potential of 250 volts. Thus, it is believed that the presence of the semi-conductor particles substantially reduces the potential at which avalanche breakdown of the film occurs.

Other semi-conductor materials which can be used in the present invention, besides silicon, are e.g. germanium, cuprous oxide, cadmium sulphide, cadmium selenide and indium oxide. The average particle size for the semi-conductor particles should be within the range 0.1 to 5 microns, at least 80% of the particles having dimensions within this range. These particles are preferably used in a proportion of 1 to 20% by weight of the resin in the layer in which they are present. The resin itself (together with the plasticiser therefor, when such plasticiser is used) makes up 30 to 85% by weight of the layer, and the conductive particles, e.g. of carbon, make up 8 to about 69% by weight of the layer.

Instead of being part of a self-supporting film, the layer of higher electrical resistance may be impregnated into a conventional sheet of stencil tissue such as is used in the construction of duplicating stencils designed to be cut mechanically, as with a typewriter. Such stencil tissue weighs 6 to 13 grams per sq. metre, is 0.0013 to is substantially pervious to duplicating ink, but provides a support of adequate strength for the ink-impervious thermoplastic layer of higher electrical resistance. In this construction, it is possible to coat the layer of lower electrical resistance, either on the back of the layer of higher electrical resistance, i.e. on the side furthest from the electric stylus, or alternatively on the adjacent face of a sheet of backing paper provided as a temporary support for the sheet of stencil tissue.

According to a feature of the invention the layer of higher electrical resistance is a multi-layer, most conveniently a double layer, and the two or more layers which together make up the layer of higher electrical resistance should have the same overall thickness and weight as the single layer of higher electrical resistance when only the one layer is used.

This construction has the advantage of being capable of being cut at higher speeds, itfhile the stencils with a single layer of higher electrical resistance can be cut satisfactorily on conventional scanning machines having a drum speed of 180 to 240 rpm, , the high speed scanning machines which have recently become available, which have drum speeds of 300 to 400 or even 800 rpm. , do not give such good results with such stencils. Microscopic examina-tion of stencils which have been cut using the new high speed scanning machines shows areas which have not been punctured by the electrical spark.. Thus, the stencils are not sensitive enough to be cut satisfactorily at these stencils involving a single layer of higher electrical resistance, which is aggravated by the higher drum speeds, is poor hole formation and irregular hole size. These disadvantages are avoided by constructing the layer of higher electrical resistance as a multi-layer. It is believed that the improved performance obtained with duplicating stencils constructed in this manner is caused by the higher homogeneity of each individual layer.

It has further been found that when duplicating stencils are constructed in this manner, it is possible to include in the outermost layer, i.e. that layer which is adapted to be nearest to the electric stylus, a white pigment, e.g. titanium dioxide, zinc oxide or zinc sulphide or mixtures thereof, so that the stencil has a grey surface. Such a surface enables the cut image on the duplicating stencil to be more easily seen.

The thermoplastic layers in the duplicating stencils of this invention are preferably based on a thermoplastic vinyl polymer, e.g. polyvinyl chloride, polyvinylidene chloride, polyvinyl butyral, polyvinyl formal, or copolymers of vinyl chloride and vinyl acetate, where self-supporting layers are required. It is also possible to use mixtures of resins, e.g. a mixture of vinyl resins, to confer desirable strength characteristics on the thermoplastic layer. Such polymers may be plasticised in known manner with plasticisers such as dioctyl phthalate, dinonyl phthalate, dioctyl adipate, dioctyl sebacate, or tricresyl phosphate. When the thermo lastic la er is im re nated into a sheet of stencil V tissue as described above, it may be based on, e.g. plasticised nitrocellulose, as in known duplicating stencils designed to be cut by typing, or cellulose acetate propionate. In either case, the total proportion of resin plus plasticiser (if any) is 30 to 85% by weight of the particular thermoplastic layer in question, not including the weight of any stencil tissue used as support for the said layer. These coatings are applied from appropriate organic solvents in known manner, While it is possible to vary the resistance of the layers of different electrical resistance by simply varying the proportion of conductive particles in the layers, preferably particles of higher electrical resistivity are used in the layer of higher electrical resistance, than in the other layer. Thus, it is preferred that the carbon particles present in the layer of higher electrical resistance have an electrical resistivity greater than 2 ohm-inches at an apparent density of 44 pounds per cubic foot. On the other hand, in the layer of lower electrical resistance, the carbon particles preferably have an electric resistivity of 0,03 to 0,1 ohm-inches at the same apparent density. As is well known the electrical conductivity of particulate carbon is dependent on a number of factors including residual o;xygen content and particle structure. Carbon particles having a well developed open structure and a low residual oxygen content have in general relatively high conductivities and are often referred to as "electrically-conductive carbon blacks" Carbon of this kind which v ab 4 commercially for use in structures such as coal mine belting and aircraft tyres) is preferably used in the layer of lower electrical resistance, while other, less conductive carbons are used in the layer of higher electrical resistancei The layer or layers of higher electrical resistance preferably have a resistance of from 30,000 to 2,000,000 ohms, as determined by measuring the resistance between two electrodes each having a surface area of 2 1 cm and 1 cm apart, each pressed onto the said layer by a weight of 2 kilograms. This layer, or layers where the above described multi-layer construction is used, preferably have a total weight of 20 to 80, especially to 50 grams per sq. metre and a total thickness of 0.5 to 2 mils. In the above described multi-layer construction, no one layer should be more than 9 times as thick or as heavy as any one other layer, and preferably both or all the layers have approximately the same thickness and weight.

The layer of lower electrical resistance is substantially lighter, and preferably weighs only 1 to 5 grams per sq. metre. Its electrical resistance is much lower, being preferably 250 to 3500 ohms determined in the same manner as that already mentioned. [The figures given herein for the electrical resistances of the two layers are those for the finished stencil, i.e. the figure for each layer is measured with the other layer in place on the reverse side of the layer to the two electrodes].

The ro ortion of conductive artic e in a er or layers which also contain semi-conductor is^ as already mentioned, 8 to about 69% of the combined weight of the resin, plasticiser if any, conductive particles, and semi-conductive particles. When no semi-conductor is present, i.e. in the layer of lower electrical resistance, the proportion of conductive particles is preferably 10 to 70% of the combined weight of resin, plasticiser if any, and conductive particles. Where a white pigment is present in the outermost layer, it may constitute up to 40% of the weight of that layer.

The novel duplicating stencils of the present invention may be made in exactly the same way as known stencils merely modifying the composition of the layers and/or the number of layers in the manner described above. Thus, the various constituents of each layer may be thoroughly mixed together, e.g. in a ball mill, together with an appropriate volatile solvent for the thermoplastic film-forming material, the suspension so obtained applied to an appropriate support to give a coating of the desired solids content, and the solvent then evaporated. Further layers as defined above can then be coated on top of the first layer or onto a separate support when such is used.

The following Examples illustrate the invention.

EXAMPLE 1 The following composition was coated onto a backing sheet provided with a release layer, e.g.. polyethylene-coated Kraft paper.

Vinylite V.Y.H.H. (a copolymer of vinyl chloride and vinyl acetate containing about % vinyl-acetate residues made by Union Carbide Corp. ) 120 parts by weight XC.72 Carbon black (Cabot Carbon) 90 parts by weight Di-octyl phthalate 36 parts by weight Methyl ethyl ketone 780 parts by weight The resin was dissolved completely in the methyl ethyl ketone and the di-octyl phthalate and carbon black then added. The mixture was milled in a ball mill for 18 hours. The suspension obtained was coated onto the polyethylene coated Kraft paper at a dry coating weight of 1 to 2 grams per sq. metre. The layer obtained had a surface resistance of 1000 ohms measured in the manner previously indicated.

A second layer of higher electrical resistance was coated onto the layer so obtained to form a unitary film strippable from the polyethylene-coated Kraft paper, using the following composition: V.Y.N. S, (a copolymer of vinyl chloride and vinyl acetate containing about 10% of vinyl acetate residues made by Union Carbide Corp. ) 60 parts by weight Vulcan 6F Carbon Black (Cabot Carbon) 13 parts by weight Di-octyl phthalate 17 parts by weight Silicon metal powder 3 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight The resin was dissolved in the mixture of solvents, the other ingredients were added, and the whole mixture was milled in a ball mill for 18 to 24 hours. The suspension obtained was then coated onto the coating previously described in the conventional manner at a dry coating weight of 24 to 28 grams per sq, metre. This layer forms with the first layer a unitary strippable stencil film suitable for the reproduction of full tone photographs using a conventional commercially available scanning machine.

EXAMPLE 2 A duplicating stencil was made in the manner described in Example 1, but replacing the second composition by the following.

V.Y.N. S. 60 parts by weight Vulcan C Carbon Black (Cabot Carbon) 12 parts by weight Di-bctyl phthalate 17 parts by weight Cuprous oxide 4 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight Essentially the same results are obtained as with the duplicating stencil of Example 1.

EXAMPLE 3 A sheet of polyethylene-coated Kraft paper was coated with the composition first given in Example 1 V.Y.N. S. 60 parts by weight Vulcan 6F 12 parts by weight Silicon metal powder 3 parts by weight Di-octyl phthalate 17 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight These materials were made into a dispersion as described in Example 1 and then coated on the base layer to give a dry film 0.7 mil thick. On top of the layer so obtained a further layer 0.3 mil thick (when dry) was coated using the same composition. Essentially the same results are obtained for any ratio of thicknesses of the two outer layers from 1:4 to 4:1, the total thickness being 1 mil. Preferably, however, the two layers are of essentially equal thickness.

A duplicating stencil made in this way is more homogeneous than a conventional duplicating stencil having only two layers and gives a better tone response and a more uniform and regular hole distribution after imaging on a scanning machine.

In Examples 1-3 the V.Y.N. S. resin can be replaced by an equal weight of a mixture of V.Y.N, S., (50 parts by weight) and V.A.G.H. (a copolymer of vinyl chloride with vinyl acetate and vinyl alcohol in the ratio 91I3:6J Union Carbide Corp.) 10 parts by weight, EXAMPLE 4 A duplicating stencil was prepared in the manner described in Example 3 except that the third layer was V.Y.N. S. 40 parts by weight Vulcan XC 72 8 parts by weight Titanium dioxide 32 parts by weight Silicon metal powder 2 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight This composition was coated so as to give an outer layer 0.2 mil thick when dry. The presence of the titanium dioxide does not adversely affect the electrical properties of the duplicating stencil, and has the advantage that it aids viewing of scanned areas. The titanium dioxide can be replaced by an equal weight of zinc oxide and essentially the same result obtained.

EXAMPLE 5 A paper support sheet provided with a release layer was coated with the following composition to give a dry coating weight of 2 grams per sq. metre.

Vinylite V.Y.N. S. 120 parts by weight Di-octyl phthalate 36 parts by weight Acetylene Black 90 parts by weight Methyl ethyl ketone 780 parts by weight The resin was dissolved in the solvent in a high speed disperser and the other ingredients then added and the whole mixture milled in a ball mill for 14 to 16 hours.

The dispersion was then coated onto the paper using a conventional reverse roll applicator. When the solvent had completely evaporated, a second high electrical resistance coating was applied at a dry coating weight of V.Y.N. S. 60 parts by weight Vulcan 6F 14 parts by weight Di-octyl phthalate 18 parts by weight Silicon metal powder 3 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight The resin was dissolved in the solvent, the remaining ingredients then added to the solution obtained, and the whole milled in a ball mill for 20 to 24 hours.

Finally, onto the layer so obtained a third layer was coated at a coating weight of 6 grams per sq. metre using the following compositions V.Y.N, S. 40 parts by weight Vulcan XC 72 8 parts by weight Titanium dioxide 32 parts by weight Silicon metal powder 2 parts by weight Methyl ethyl ketone 120 parts by weight Methyl isobutyl ketone 120 parts by weight The duplicating stencil finally obtained could be used to give good reproduction from continuous tone originals and could be scanned with a high speed scanning machine rotating at 300 to 400 or even 800 rpm. Moreover, because of the presence of the titanium dioxide in the outermost layer, the scanned stencil was easy to examine visually, and no separate masiting layer is required, EXAMPLE 6 A sheet of conventional stencil tissue made of Yoshino fibres was impregnated with the following composition: Nitro-cellulose grade FHM 15/20 (I.C.I. Ltd.) 69 parts by weight Castor oil 5i parts by weight Oleyl alcohol 51 parts by weight Butyl stearate 21 parts by weight Silicon metal powder 12 parts by weight agecol 888 Carbon Black (Columbian Carbon) 75 parts by weight Ethyl acetate 270 parts by weight Methylated Spirits 174° British Proof 540 parts by weight The nitro-cellulose is dissolved in the solvents, the other ingredients are added, and the whole mixture is then placed in a ball mill and milled for about 12 or better 24 hours until a uniform dispersion is obtained. This dispersion is then applied to the stencil tissue using the same stencil coating techniques as are used in the making of conventional stencils designed for use on a typewriter. The solvents are then allowed to evaporate to give a coating weighing 25 grams per sq. metrei The low resistance layer may either be applied directly to the reverse side of this high resistance layer using solvents which will not adversely affect the latter, or it may be applied separately to a sheet of backing paper. If the latter system is used the composition first described in Example 1 may be applied to a sheet of backing paper, in which case the impregnated sheet of stencil tissue is attached to the coating backing paper along one edge. Alternatively, a low resistance coating may be applied directly to the back of the high resistance coating using the following composition: Elvax 40 (vinyl acetate/ethylene copolymer containing 40% of vinyl acetate residues, Du Pont) 90 parts by weight Vulcan XC 72 180 parts by weight Petroleum Distillate (aliphatic hydrocarbons boiling in the range of 135 to 148°C, ) 1000 parts by weight This coating is applied at a coating weight of approximately 2 grams per sq, metre on the back of the coating already described after all solvents have been evaporated from the latter. It has a resistance of 1000 to 2000 ohms when measured by the method already mentioned.

The nitro-cellulose based composition described above may be replaced by the following: Cellulose acetate propionate 40 parts by weight Di-octyl phthalate 42 parts by weight Silicon metal powder 4 parts by weight Pelletex S Carbon Black (Cabot Carbon) 20 parts by weight Methyl ethyl ketone 200 parts by weight in the same coating weight. Essentially the same results are obtained.

Claims (18)

1. WE CLAIM li A duplicating stencil adapted to be cut by the use of an electric stylus comprising two thermoplastic layers adapted to be contiguous in use, each layer having electrically conductive particles dispersed therein, that layer adapted to be nearer the stylus being ink-impervious and having a higher electrical resistance than the other layer and having semi-conductive as well as conductive particles dispersed therein.
2. 2. A stencil according to claim 1 in which the said semi-conductive particles are of siliconj germanium, cuprous oxide, cadmium sulphide, cadmium selenide or indium oxide.
3. 3. A stencil according to claim 1 or 2 in which at least 80% of the semi-conductive particles have a particle size within the range 0.1 to 5 microns.
4. 4. A stencil according to any of claims 1 to 3 in which the layer adapted to be nearer the stylus comprises 1 to 20% of semi-conductive particles by v/eight of the resin in the layer, 30 to 85% of a thermoplastic resin and optional plasticiser therefor by weight of the layer, and 8 to 69% of conductive particles by v/eight of the layer.
5. 5. A stencil according to any of claims 1 to 4 in which the conductive particles in the layer of higher electrical resistance are carbon particles having an electrical resistivity greater than 2 ohm-inches at an apparent density of 44 pounds per cubic foot while the conductive particles in the layer of lower electrical resistance have an electric resistivity of 0,03 to 0.1 ohm-inches at the same apparent density.
6. 6. A stencil according to claim 5 in which the layer of higher electrical resistance has a resistance of 30,000 to 2,000,000 ohms as determined by measuring the resistance between two electrodes each having a surface 2 area of 1 cm and 1 cm apart each pressed onto the said layer by a weight of 2 kilograms, and the layer of lower electrical resistance has a resistance of 250 to 3500 ohms determined in the same manner,
7. 7. A stencil according to any of claims 1 to 6 in which the layer of higher electrical resistance weighs 20 to 80 grams per sq„ metre and the layer of lower electrical resistance weighs 1 to 5 grams per sq. metre.
8. 8. A stencil according to any of claims 1 to 7 in which the two thermoplastic layers form an integral self-supporting film,,
9. 9. A stencil according to claim 8 in which the said film is strippably mounted on a sheet of a release paper.
10. 10. A stencil according to claim 8 or 9 in which the layer of higher electrical resistance is a multi-layer.
11. 11. A stencil according to claim 10 in which the outermost layer includes a white pigment so that the stencil has a grey surface.
12. 12. A stencil according to any of claims 8 to 11 in which the thermoplastic layers comprise polyvinyl chloride, polyvinylidene chloride, polyvinyl butyral, polyvinyl formal, a vinyl chloride/vinyl acetate copolymer, or a mixture of§/nYe«e°¾f tvhineysel resins.
13. 13. A stencil according to any of claims 1 to 7 in which the layer of higher electrical resistance is impregnated into a conventional sheet of stencil tissue weighing 6 to 13 grams per sq. metres 0.0013 to 0,0025 inches thick, and containing 10 to 40% of voids, and the other layer is either coated on the side of the layer of higher electrical resistance or on the adjacent face of a sheet of backing paper*
14. 14. A stencil according to claim 13 in which the thermoplastic layer impregnated into the sheet of stencil tissue comprises plasticised nitro-cellulose or cellulose acetate propionate, and the other layer comprises a thermoplastic resin as defined in claim 12.
15. 15. A stencil according to claim 1 substantially as described in any one of the foregoing Examples.
16. 16. A method of imaging a duplicating stencil as claimed in any one of claims 1 to 15 which comprises cutting the said stencil with an electric stylus to produce an image thereon,
17. 17. A method according to claim 16 in which the said stencil is cut on a scanning machine having a drum speed of 300 to 800 rpm,
18. 18. Imaged duplicating stencils made by the