Masking films

Abstract: Masking films for use with photographic colour-printing media in the preparation of colour proofs, in multi-colour printing, have a peelable masking film which has light-absorption properties which substantially match the spectral sensitivity of the photographic colour-printing media with which the masking film is to be used.

Description: This invention relates to masking films, namely laminated sheet products of the kind comprising a stable transparent base film, which carries a coloured masking film or membrane in the form of a synthetic resin layer or coating. When the sheet product is to be used, the coloured membrane or film is cut through and can be peeled from the base film. The resin layer or coating is dyed or pigmented, so that it has a high density to actinic light, but it is very desirable to maintain a good visual density. A layer of an adhesive sub-coat may be provided between the peelable masking membrane or film and the base film, in order to facilitate the removal of the cut mask. By cutting through the membrane and peeling parts of it away from the base, the product can be used as a photographic mask. In colour printing, normally only four coloured inks are used. To reproduce coloured originals, the normal procedure is to produce from the original a set of four film negatives, either by camera techniques using carefully controlled colour filters or, more usually, by electronic scanning techniques. These negatives each represent one of the four colours, viz. cyan, magenta, yellow and black, which will ultimately be printed in register in order to produce the final colour print. It is very desirable to be able to "proof" the final print from this set of photographic negatives (or positives), without going to the expense of actually making plates and printing from them in four colours, and a considerable number of processes have been developed to achieve this objective. In many cases, a completely accurate colour rendition is not essential and, recently, several processes have been developed which exploit the use of standard photographic colour printing paper (photo-paper). This is exposed sequentially to the colour separation set, using carefully controlled and filtered lights. The end result is a "colour proof" which is not completely accurate with regard to the printing colours, but is a good approximation and is very adequate for most uses. It has the advantage of cheapness and speed, not least because standard colour paper processing is all that is required. For certain applications, it is desirable to use a masking film during the exposure sequence, for example when it is necessary to introduce blocks of text into the final colour proof. Owing to the panchromatic sensitivity of the photo-paper, standard masking films which are heavily dyed red, orange or yellow are not suitable. Neutral density (black) masking films are also not suitable: if pigmented to give a sufficient photographic density, the "see-through" properties are unacceptable, and vice versa. This problem is exacerbated by the fact that, during each exposure, there is a small but finite amount of light which passes through the masking film. This may not be sufficient to fog the photo-paper being protected, but does have the effect of increasing the photosensitivity of the photo-paper, so that, during the multiple exposures necessary for producing the colour proof, the sensitivity of the photo-paper is increased to the point where fogging does take place. Standard colour papers have a panchromatic sensitivity which is not even over the whole region of the spectrum. Because of the nature of dye sensitising agents currently used, the spectral sensitivity of such papers is at a minimum between 570 nm and 610 nm. It is an object of the present invention to provide a masking film in which the absorption properties of the mask can be made closely to match the uneven spectral sensitivity of the photo-paper. With currently available photo-papers, this means that the mask should therefore have a low absorption of light in the 570 to 610 nm region of the spectrum and a high absorption of light outside that region. In other words, light in the aforesaid spectral range of 570 to 610 nm is permitted to pass through the mask, but light of other wavelengths is effectively blocked. Fortuitously, this 570 to 610 nm range corresponds with the maximum sensitivity of the human eye. The net result is that adequate "see-through" properties, together with very safe masking properties for multiple exposures, could thus be achieved. It has now been discovered that this desirable selective transparency of the masking film can be achieved by the selection and use of appropriate dyes. The present invention provides a masking film for use with a photographic colour-printing medium in the preparation of colour proofs by sequential exposure of the medium to a colour separation set, the film comprising a stable base film which is transparent to visible light and a coloured masking layer in the form of a film or membrane which is carried by the film base and is peelable therefrom, the coloured masking layer having a maximum transparency to visible light within the wavelength range from 570 nm to 610 nm, whilst being substantially opaque over the portions of the visible spectrum lying outside the said wavelength range. Preferably, the light-absorption properties of the membrane are such that it has minimum transparency to light within the spectral range from 300 to 570 nm. According to preferred embodiments of the invention, the masking layer has an optical density of 2 or greater over the spectral range from 300 to 570 nm and an optical density of less than 2 over the spectral range from 570 to 610 nm. The optical density is preferably substantially higher than 2 towards the 300 nm end of the 300 to 570 nm range. The thickness of the membrane is preferably in the range from 5µm to 50µm. An adhesive layer may be provided between the membrane and the base film. The supporting base film is preferably selected from polyethyleneterephthalate, polybutyleneterephthalate, other polyester films, and polycarbonate. The masking membrane or film can be any of the dyed polymer mixtures disclosed for use as masking films. These include nitrocellulose heavily plasticised with phthalate esters, mixtures of PVC/VA with polyurethanes (EP-168924-A), nitrocellulose with nitrile rubber (EP-188292-A) and cellulose ester-PVC/VA -nitrile rubber (JP-87-17748 B). The membrane may be coated directly on to the base film or, optionally, a sub-coat may be applied between the base film and the dyed membrane. The purpose of this sub-coat is to improve or change the peel properties of the membrane. Typically, such a sub-coat consists of tackified natural rubber. In order that the invention may be fully understood and appreciated, the following specific Examples are given, by way of illustration only. EXAMPLE 1 A panchromatic masking film was made from a base film in the form of a subbed polyester film, part (a), upon which was deposited, on the sub-coat side, a masking lacquer, formulated as described in part (b): (a) Sub Layer An untreated graphic arts quality polyester film (e.g. Du Pont "Mylar® D") of 125µm (0.005in) thickness, was coated with an adhesive solution of the composition: Component Quantity "Levapren® 452" 10g Toluene 990g 1000 g The solution was applied to the polyester film by means of a 250µm (0.010in) wire wound bar (Meyer bar) and dried by warm air, to produce the desired subbed polyester base film. (b) Masking Lacquer The subbed polyester base film was overcoated on the sub layer with a coloured lacquer formulation by means of a 2-roll reverse coating technique and dried in a warm air oven. The composition of the lacquer was as follows:- Component Quantity Methyl Ethyl Ketone (Butanone) 390g Dowanol® PM (Propylene Glycol Mono-Methyl-Ether) 80g Cellulose Acetate Propionate (CAP) 482-0.5 85g Cellulose Acetate Propionate (CAP) 482-20 25g Permuthane® U4715 400g Degussa® OK412 3.9g Savinyl® Orange RLSE 7.0g Savinyl® Blue GLS 7.0g Savinyl® Yellow RLS 0.40g Neozapon® Red 355 1.70g 1000.00 g The coating was applied to gave an overall dry thickness of 30µm. Film Testing The masking film so produced was a laminated sheet product having a brown-yellow appearance and showing good visual transparency. Examination of the film on a Perkin-Elmer Lambda 7 spectrophotometer showed the film to have UV, blue and green densities in excess of 3.00 (the spectral region 350 to 550 nm) and a red density of average density of 3.0 (600 to 700 nm). Over the range of 550 to 600 nm a dip is recorded in spectral absorbance giving a D min at 580 nm of 2.1. Exposure Tests To assess the film's performance as a panchromatic mask, exposure tests were conducted using Kodak Double Check colour paper and a Carlson Proofmaster exposure unit. A sheet of mask along with a black card control was placed over a sheet of Double Check paper (in a lights-out darkroom) and given an exposure cycle to filtered red, green and blue light. Each exposure was of 200 units duration. The paper was then developed. The developed paper was placed on a densitometer. The D min values of the paper masked both by card and film of this example were measured. The process was repeated at 2x, 4x and 6x the initial exposure level. D min values were recorded as follows:- Number of Exposure Multiples 1x level 2x level 4x level 6x level R G B R G B R G B R G B Control 0.09 0.13 0.14 0.09 0.13 0.14 0.10 0.13 0.14 0.10 0.13 0.14 Example 1 film 0.09 0.13 0.14 0.09 0.13 0.14 0.10 0.13 0.14 0.10 0.13 0.14 As the results show, D min values remain unchanged. There was no fogging due to inadequacy of the mask. EXAMPLE 2 Polyester of 125µm (0.005in) thickness was subbed as in Example 1. The subbed layer was then overcoated with a masking lacquer of the following constitution to a thickness of 30µm Component Quantity CAP 482-0.5 106g Permutane® U4715 333g Degussa® OK412 4.6g M.E.K. 475g Stahl UP3771 Green 60g Savinyl® Yellow RLS 19g Neozapon® Red 355 2.4g 1000.0 g Testing The film produced was of a deep green shade with good visual transparency. Examination of the spectrogram of this film showed densities in the 350 to 500 nm region to be in excess of 3.00. Densities in the red spectral area (600 to 700 nm) were in excess of 4.00. The D min occurred in the 540 to 580 nm region and was measured at 2.1 Exposure Tests The tests outlined in Example 1 were repeated. Results were again as per control proving the formulation to have adequate shielding density for this process.

Claims: A masking film for use with a photographic colour-printing medium in the preparation of colour proofs by sequential exposure of the medium to a colour separation set, the film comprising a stable base film which is transparent to visible light and a coloured masking layer in the form of a film or membrane which is carried by the film base and is peelable therefrom, the coloured masking layer having a maximum transparency to visible light within the wavelength range from 570 nm to 610 nm, whilst being substantially opaque over the portions of the visible spectrum lying outside the said wavelength range. A masking film according to claim 1, wherein the masking layer has an optical density of less than 2 over the wavelength range from 570 nm to 610 nm. A masking film according to claim 2, wherein the masking layer has an optical density of 2 or greater over the wavelength range from 300 nm to 570 nm. A masking film according to any preceding claim, wherein the optical density of the masking layer is substantially greater than 2 towards the 300 nm end of the wavelength range from 300 nm to 570 nm. A masking film according to any preceding claim, wherein the film base is selected from polyethyleneterephthalate, polybutyleneterephthalate, other polyester films and polycarbonates and wherein the masking layer is selected from phthalate-ester-plasticised nitrocelluloses, polyvinylchloride/acetate mixtures with polyurethanes, nitrocellulose/nitrile rubber mixtures and cellulose ester/polyvinylchloride/acetate/nitrile rubber mixtures. A masking film according to any preceding claim, wherein the masking layer is applied directly on to the base film. A masking film according to any of claims 1 to 5, having an adhesive subbing layer between the base film and the masking layer.

Inventors: NAISBY ANDREW, SPERRY JOHN ARTHUR, STEEL JOHN, NAISBY, ANDREW, SPERRY, JOHN ARTHUR, STEEL, JOHN
Applicants: AUTOTYPE INT LTD, AUTOTYPE INTERNATIONAL LIMITED
Application year: 1989
Main classification: G03F1
IPC codes: G03F3, G03F1