IL49871A - High definition stencil - Google Patents

Description

HIGH DEFINITION STENCIL *ΠΒ • sao'? ^'οαβο THIS INVENTION relates to duplicating stencils and their manu acture.

Duplicating stencils comprising a sheet of stencil tissue impregnated with an imageable ink-impervious coating are very widely used. The imageable coating may be pressure-sensitive as in duplicating stencils designed to be used on a typewriter (mimeograph stencils) , or heat-sensitive, as in duplicating stencils designed to be imaged by exposure to infra-red rays when in contact with a heat-absorbing image to be copied.

Both types of duplicating stencil are capable of giving copies of high quality in use, but studies of the factors which control the quality of the copies obtained have shown that there are certain disadvantages inherent in the manner in which such duplicating stencils have heretofore been constructed.

In both types of stencil, the proximity of the fibres of the stencil tissue to the surface of the impregnation of ink-impervious material causes the latter to have a surface which is not completely smooth and which is not therefore capable of giving images of such high quality as those which can be obtained using a truly planographic , i.e. very smooth, surface. In the case of the heat-sensitive stencils, this lack of smoothness also impairs the thermal contact of the stencil - 2 - I sheet with the original and thus causes the melted areas not to correspond exactly to the heat-absorbing areas of the image to be copied.

In the manufacture of duplicating stencils of these kinds, a stencil coating dispersed in a volatile solvent is applied to open-textured base tissue (stencil tissue), and more or less complete penetration occurs so that the coating spreads on both sides of the sheet before evaporation of the solvent causes the coating to solidify. With many normal coating techniques, this penetration is further promoted by the pressure of the conventional Meyer bars used to control the weight of the coating. Dy selection and variation of the solids content, the proportion and nature of the solvent, and its speed of evaporation, it is possible to influence to some extent the degree of penetration, but, for all except the lightest coatings, there is a detectable difference between the side of first contact (the 'coated side'.) and the opposite side (the 'tissue side'). In a typical case, a substantial part of the dry coating is held in the interstices of the tissue and the balance is believed to be divided approximately 60 : 40 between the 'coated side' and the 'tissue side' respectively. However, both surfaces of a stencil coated in the conventional manner are somewhat uneven because of the fibre crossings distributed randomly over the tissue area, and this leads to the disadvantages already mentioned.

To achieve a smoother surface on the stencil, it is possible to provide a second coating (say, on the 'coated side0), which may remain entirely on that side and make it somewhat smoother, but to achieve a substantial improvement in smoothness it is generally necessary to apply a second coating at least half the weight of the first. When the film-formers in the two coatings are different, it can be arranged that the solvent used for the second coating will not attack the first. But, when the film-formers are the same, and especially when the solvents are the same in both coatings, it is inevitable that some intermingling of the coatings will occur and in some cases the second coating will actually penetrate the firsto Because of such considerations it has hitherto been impossible to avoid a relatively large increase in the weight of the stencil. This is disadvantageous both because of the substantially increased amounts of coating composition which are required and because in the case of heat-sensitive stencils, the greater quantity of heat required to melt the coating causes lateral diffusion of heat with consequent loss of definition in the image obtained.

These disadvantages of known duplicating stencils can be mitigated or avoided altogether, in accordance with the present invention, by making a duplicating stencil comprising a sheet of oleophobic stencil tissue carrying on one side an imageable ink-impervious coating penetrating into but not through the said tissue and having a smooth outer face, which in use is in contact with the surface receiving the duplicated image, and, on the other side, a second, relatively thin ink-impervious coating covering the surface of the stencil tissueβ The use of oleophobic stencil tissue, e.g. stencil tissue pretreated with a small amount of an oleophobic agent, prevents the material which is to form the ink-impervious coating from penetrating through the sheet of stencil tissue= when the ink-impervious material is applied to an oleophobic sheet of stencil tissue , the ink-impervious coating obtained remains mainly on the side of the sheet of stencil tissue to which it was applied and the coating obtained has a very smooth outer face, much smoother than can be obtained by applying the same weight of coating to a sheet of stencil tissue which is not oleophobic.

The second coating, on the other side of the stencil tissue, protects the tissue from the duplicating ink during use=, Without it, the stencil has a poor working life.

It is an important advantage of the present invention that it makes it possible to estimate accurately, and thus control, the distribution of the ink-impervious coating between the two sides of the stencil tissue. For any particular combination of oleophilic stencil tissue and coating compositions (whether pressure-sensitive or heat-sensitive) it is a matter of simple routine experiment to discover the optimum combination of coating weights to give the best quality copies and longest working life (length of run). '·''■·'·'■ ■"' ■'■·':"·'' As already stated, the stencil tissue may be rendered oleophoblc by treatment with a material known to confer this property. Such compounds are well known and are described, for example, in British Specification Nos. 857,334: 857,335? 857,336 and 1,296,676. In many cases, they confer hydrophobic as well as oleophoblc properties on the stencil tissue, and this is not necessarily undesirable. Examples of commercially available perfluorinated complexes, some of which contain chromium, which are capable of use as oleophoblc agents may be mentioned? Scotchban 805, 807 and 808 (3M), Monflor 31 (I.e. I.). Other materials, which only confer oleophoblc properties on the stencil tissue include copolymers of acrylic acid with fluorinated alkyl acrylates, e.g. as described in "Fluoropolymers'" Ed. L.A. Wall, Vol. 25 of -"High Polymers", Wiley-Interscience , 1972, page 434» Since these materials are well known and are commercially available, they do not require further description.

The stencil tissue used in the duplicating stencils of the present invention is the same as that used in known duplicating stencils. That is to say, it may be made of abaca fibres with or without admixture of jute or viscose rayon or other synthetic fibres, and with or without wet strengthening agents. Such tissue generally weighs 9.5 to 14.5 grams per sq. metre, and may contain 15 to 40% of voids. This tissue may be rendered oleophobic by impregnation, e.g. by roller or dip coating, with a solution of the oleophobic agent in an appropriate solvent, e.g. at a concentration of 0.01% to 2%, preferably 0.02% to 1% by weight. Larger amounts can be used, but are uneconomical. The solvent is preferably isopropyl alcohol, but any other volatile organic solvent for the oleophobic agent may be used. After impregnation, the stencil tissue is dried at a temperature of 70 to 100°C. To prevent loss of fibre strength, the impregnating solution may contain urea in a proportion of 3 parts by weight per part by weight of oleophobic agent.

The nature of the ink-impervious coating applied to the oleophobic stencil tissue, and its weight, depend upon whether the final duplicating stencil is to be pressure-sensitive or heat-sensitive,, In the former case, the coating may comprise highly plasticised nitrocellulose as in known duplicating stencils designed to be cut on a typewriter In the case of heat-sensitive stencils, the ink-impervious coating may comprise a cellulose-based resinous material e0g_ cellulose-acetate-butyrate , mixed with oils which are partially but incompletely compatible therewith* In either case, the ink-impervious coating is of known composition, and known formulations may be used without modification in the present invention,, The ink-impervious material is preferably applied to the oleophobic sheet of stencil tissue from solution in a volatile solvent. Provided the first coating is applied to one side only of the tissue, the actual manner of application is immaterial and any known method can be used.

After the first coating has been applied, a second coating is applied to the opposite side of the stencil tissue which has the effect of covering the 'tissue side" and protecting the fibres from the duplicating ink during use. Since the second coating need not have a smooth surface, it can be lighter than the first, typically by 60 to 85%. Its composition and manner of application may be the same as, or different from, those of the first coatingo In particular, the second coating may Heat-sensitive atencil blanks Generally useful Preferred Range Range g0/sq.m. g./eq.m.

"Abaca tissue preferably wet- 9.5 - 14.5 10 - 11 strengthened. v Tissue + 1st coating. 30 - 40 32 - 38 2nd coating. 2 - 8 3 - 5 In heat-sensitive stencils . an uncoated sheet of porous tissue is usually provided as part of the stencil assembly and placed against the 'tissue side', i.e. the side coated secondly* of the stencil. This receives the fused material corresponding to the image areas of the stencil and permits the apertures to remain clear for the subsequent passage of inko Though the overall density of the copy may be influenced by this sheet, the sharpness of outline is dependent on the combination of intimate imaging contact with the smooth main coating and the use of correct exposure time.

The stencils of this invention give improved copy quality. Both pressure-sensitive and heat-sensitive forms print from the smoother surface. The heat-sensitive form also has this smoother surface next to the original during imaging and give®, because of the better contact, ^x more sharply defined characters» The difference is quite apparent to the naked eye and greatly enhances the aesthetic quality of the print„ Experiments, in which stencils believed to have approximately the same distribution of coating material between the two sides of the base tissue have been made (for example using stencil tissue with a very light coating of oleophobic agent inadequate to render the stencil tissue oleophobic , or by applying two coats to the same side) , have shown in a few cases a marginal improvement in copy quality over prior art stencils, but none of these stencils has given as good results as the preferred forms of the stencils according to the present invention„ The following Examples illustrate the invention. The oleophobic stencil tissues used were prepared as followss Sheets of stencil tissue, lightly wet-strengthened and weighing 10½ g0/squmo , were precoated with solutions in isopropyl alcohol of equal parts by weight of Scotchban 805 (30% solids in isopropyl alcohol) and urea, the concentrations of Scotchban being 0.02%, 0.05%, 0ol% and 1% by weight The sheets were dried at 90®C„ These tissues ^are hereinafter .referred to as D2, D3, D and D5 respectively» Wet-strengthened Manila tissue was precoated similarly to give tissue hereinafter called M2, M3, M4 and M5o Non-wet-strengthened tissue was treated with the 0<,02% solution of oleophobic agent to give tissue hereinafter called T2o While the following Examples refer to the use of particular types of oleophobic stencil tissue, it will be understood that the other types of oleophobic stencil tissue can equally well be used° EXAMPLES 1 to 4 Non-wet-strength tissue T2 was coated on one side with the following composition at a dry coating weight of 55-58 goSqom0 or 44-46 g0sqom. (including stencil tissue) and then dried at 95®C<.

Parts by weight Nitrocellulose FHM 13/20 (dry weight) 10 Coconut oil 4 Diethylene glycol 2„7 Butyl stearate 15»4 Oleic acid 77 „ 6 Titanium dioxide 10 Antioxidant 2 Ethyl acetate 33 Industrial Methylated Spirits (0o3 to 0o5% water by weight) 100 FHM 15/20 is a medium viscosity industrial nitrocellulose having a nitrogen content of 11» 2 to 11„ 7% by weight manufactured by I.C.I. It is supplied damped with methylated spirit in the proportion 70 parts nitrocellulose to 30 parts of methylated spirits 0 Wet-strengthened Manila tissue M2 was coated in the same way at coating weights of 55-58 goSq.m. and 49-51 g0sq0 mo (including stencil tissue).

Tissue T2 carrying a coating and weighing 44-46 g.sq.m. and Tissue M2 carrying a coating and weighing 49-51 g.sq.m. were then given a second coating of the same material on the opposite side, at an additional coating weight of 10-12 g.sq.m0 and 5-7 g_squm0 respectively.

The single coated stencils weighing 55-58 goSq.m. (Examples 1 and 2) and the double coated stencils (Examples 3 and 4 ) were typed and run on a Gestetner 466 duplicator with the following results? Example 1 2 3 4 Tissue T2 M2 T2 M2 Wt, 1st coat 55-58 55-58 44-46 49-51 + tissue, gosq0m.

Wt, 2nd coat ~ - 10-12 5-7 g. sq.m.

Quality of copy VDgood V.good Good Good.

Length of run Poor Fair Medium Medium to good Only the double coated stencils have a satisfactory working life.

EXAMPLES 5 to 8 Tissue D4 was coated on one side with the following materials and dried at 55 to 60eC.

Parts by weight Cellulose acetate-butyrate EAB 500-1 10 Hercolyn D 23 Mobilsol L 19 Methyl ethyl ketone 27 Toluene 50 at weights of coating + tissue of 35-40 , 32-38, 20-25 and 38=43 g0sq0m0 EAB 500-1 is low viscosity cellulose acetate-butyrate manufactured by Eastman-Kodak and having a butyrate content of 48% Hercolyn D is a methyl ester of hydrogenated rosin acids manufactured by Hercules Powder Co Mobilsol L is a refined naphthenic petroleum oil from Socony^Mobil Oil Co If desired the storage life of these stencils may be improved by adding 1% by weight, based on the cellulose ester, of an appropriate antioxidant, e0g0 Nonoxol DCP, Tenamene 3 of Eastman Chemicals Ltd. , or Topanol 0 of I oC d o , which are tertiary butylated para-cresols, or Topanol CA, which is a bisphenol derivative_ The sheets of stencil tissue coated at weights of 32-38, 20-25 and 38-43 gcsq.mo were given a second coating of the same material on the opposite side and again dried at 55 to 60®C_ The weights of the second coatings were 3-5, 8-10 and 8-10 g.sq0m» respectively0 After imaging by exposure to heat in contact with an original to be copied, the stencils were run on a Gestetner 466 duplicator with the following results^-Example 5 6 7 8 1st coat + tissue, 35- 0 32-38 20-25 38-43 g. sq.nu 2nd coat, g.sq.m. - 3-5 8-10 8-10 Quality of copy Excellent Excellent Poor Fair Length of run Poor Good Good Good Example 6 shows that with the correct choice of 1st and 2nd coating weights, the advantages of the smoother surface made available by the new technique may be combined with adequate length of run on the duplicator.

In Example 6, the first coating may alternatively have the following composition; Parts by weight Cellulose acetate/butyrate (17% butyryl, 29 „ 5% acetyl) 24 Vinyl chloride/vinyl acetate (87.13) copolymer 16 Cyclohexyl adipate 60 Dimethoxyethyl phthalate 60 plasticisers Butyl phthalyl butyl glycollate 90 Antioxidant 3 Monastral Green pigment paste 0,4 Methyl ethyl ketone 400 The mixture of plasticisers may be replaced by: Tricresyl phosphate 80 Butyl phthalyl butyl glycollate 120 Cyclohexyl adipate 12 Dimethoxyethyl phthalate 12 in which case the proportion of methyl ethyl ketone is reduced to 300 parts by weight.

The second coating may be the same as the first coating or may be either of the following; Parts by weight Cellulose acetate/butyrate (EAB 500-1) 13,3 Staybelite ester 5 6.7 Mobilsol L 24 Hercolube Ά 9.3 Hercolyn D 6.7 Antioxidant 2 » 1 Monastral Green pigment paste (1.1 in oleyl alcohol) 0.2 Methylated spirit 74® OP 33 Toluene 124 Ethyl acetate 89 Staybelite Ester 5 is a glycerol ester of hydrogenated resin manufactured by Hercules Powder Co* Hercolube A is a pentaerythritol ester of a saturated fatty acid manufactured by Hercules Powder Co„ or Cellulose acetate/butyrate 15 Oleyl-(OCH2CH)2OH 14.9 Cumar R9 or Piccoumaron 410EH 16„ 7 Mobilsol L 28=6 Antioxidant 2β4 Monastral Green pigment paste 0„ 2 Toluene 154 Isopropyl alcohol 42 Ethyl acetate 113 Cumar R9 and Piccoumaron 410EH are coumarone/indene resins of Neville Chemical Co0 Essentially the same results as in Example 6 are obtained in each case*

Claims (1)

1. WE CLAIM: ■•C 1. A duplicating stencil comprising a sheet of oleophobic stencil tissue carrying on one aide an imageable ink-impervious coating penetrating into but not through the said tissue and having a smooth outer face, which in use is in contact with the surface receiving the duplicated image, and, on the other side, a second, relatively thin ink-impervious coating covering the surface of the stencil tissue0 2. A duplicating stencil according to claim 1 which is pressure-sensitive in which the stencil tissue is wet-strengthened abaca tissue weighing 9„5 to 14» 5 g/sq0m, the combined weight of the first coating and the said tissue is 40 to 50 g/sq„mu , and the weight of the second coating 3o A duplicating stencil according to claim 2 in which the said tissue weigh s 10 to 11 g/eq0m, the combined weight of the first coating and the said tissue is 43 to 48 g/sq.m., and the weight of the second coating "is 8 to 10 g/sq<= 4o A duplicating stencil according to claim 1 which is heat-sensitive in which the stencil tissue is abaca tissue weighing 905 to 14.5 g/sq0mu , the combined weight of the first coating and the said tissue is 30 to 40 g/sq0m0 , and the weight of the second coating is 2 to 8 g/sqamo 5. A duplicating stencil according to claim 4 in which the said tissue is wet-strengthened and weighs 10 to 11 g/sq0m0 „ the combined weight of the first, coating and the said tissue is .32 to 38 g/ qoKic, , and the weight of the second coating is 3 to 5 g/sq0m0 6o A duplicating stencil according to claim 2 or 3 in which the first and second coatings both comprise highly plasticized nitrocellulose,. 7. A duplicating stencil according to claim 4 or 5 in which the first and second coatings both comprise a cellulose-based resin mixed, with oils partially but incompletely compatible therewit „ 8, A duplicating stencil according to claim 2 substantially as described in Example 3 or 4. 9o A duplicating stencil according to claim 4 substantially as described in Example 60 ' 10 o Method of duplicating which comprises imaging a duplicating stencil as claimed in any of claims J. to 9 and using the imaged stencil to produce duplicated copies. claim 10 » PBsKRs 19 -