SUBSTRATE FOR IMAGE-RECEIVING SHEET MATERIAL

Abstract: Photographic base paper (10) has a face side waterproof coating which is a co-extruded layer structure of at least a pigmented core layer (P) and top surface layer (E) of polyethylene. The thickness of layer (E) is no more than 12 mu m. The pigment content in the face side coating is at least 3 gm<-2> and it is distributed such that the proportion by weight of pigment in the core layer is at least 1.75 times as great as the proportion by weight of pigment in the top surface layer. For optimum photodefinition (image sharpness) the top surface layer should be free of pigment, but as thin (1.5 to 3 mu m) as possible. The pigment is preferably TiO2.

Description: SUBSTRATE FOR IMAGE-RECEIVING SHEET MATERIAL FIELD OF THE INVENTION This invention relates to opaque substrate for image-receiving sheet material. More particularly, but not exclusively, this invention concerns substrates for carrying photographically sensitive materials such as silver halide emulsions and which are for the making of opaque prints (as opposed to transparent photographic negative film or positive transparencies). BACKGROUND OF THE INVENTION Photographic print material is usually made by coating sensitised photographic emulsion on to a waterproof opaque paper base or substrate (often referred to as "photobase"). The prepared paper base has to meet a rigorous specification not only optically and mechanically but also chemically. Thus, it should be inert to the chemistry of the photographic materials coated on it and used to develop the image, it should resist penetration of such chemicals (edge penetration beneath the waterproof coating at cut edges can be a problem) , and it should provide adequate stiffness to the laminated material for acceptable mechanical processing and manual manipulation of the developed print. The optical qualities of the photobase are, however, extremely important to photographic print sensitiser companies striving to offer the market a product of higher quality than their rivals. The photobase should be uniformly and densely opaque and constant in colour and of a specified degree of surface roughness to achieve the required gloss or matte finish. Clearly, such qualities are required in any substrate for an image. Another optical quality of the image to which the substrate makes a contribution is "image sharpness", which in this specification we have called "photodefinition" . The nature of the surface of the substrate should reduce as little as possible the sharpness of the image actually achieved by the reaction of the sensitive material in the image-forming layer above the substrate to the pattern of energy incident on it. Within the photobase industry it is understood that for highest photodefinition, the photobase should present as densely opaque a surface as possible, and that surface as close up behind the developed image as possible. Experience has shown that photodefinition suffers with any increase in distance between the opaque photobase and the photographic layer, as can occur for example with the use of a layer of gelatine as a means of bonding the photographic layer to the substrate. There is some explanation of this phenomenon in GB-A-1339045 of Fuji Photo Film Co. Ltd. Conventionally photobase comprises a paper sheet (the "raw" photobase) coated on both sides with a waterproof coating, which in recent years is of polyethylene (except for some specialist products). The polyethylene is pigmented with a pigment which is white (except for specialist products) and which is usually titanium dioxide (Ti0 2 ) . The usual method of coating is by extrusion of the coating on to the paper. The paper is identified with a "face side" and "wire side" (from Fourdrinier paper-making terminology) and it is the superior face side which when coated will receive the energy-sensitive imaging layer as a further coating. There is a practical limit to the amount of Ti0 2 which can be incorporated in a polyethylene for extrusion. Above about 15 to 20 wt% of Ti0 2 there is the prospect that the material will tend to accumulate on the lips of the extrusion die and thereby create a heterogeneous product. However, the polyethylene itself is quite capable of carrying much more Ti0 2 pigment. Indeed, it is supplied from pigment suppliers to photobase makers in the form of a "masterbatch" , usually of 50 wt% pigment 50% resin mixture. In US-A-4263080 of Whiting Jr. assigned to Ludlow Corporation there is described how a high opacity packaging material can be created by co-extruding a three-layer structure of which the middle layer has a very high loading of carbon black. By this means, it is stated, "severe rheological and sticking problems" are avoided, and there is "minimal build up of material on the die surfaces". The two outer co-extru.ded layers are of a thickness of 12μm. The use of a co-extrusion method to make photobase has been proposed from time to time. See, for example, Fuji's GB-A-1339045 (mentioned above), Schoeller's GB-A-2061131 and Wiggins Teape's EP-A1-0 183 467. The last-mentioned disclosure is significant in that it discusses many different structures and is a relatively recent publication. It discloses the use of a combination of an upper layer of polycarbonate material, optionally pigmented, with a lower layer of pigmented polyethylene to achieve higher than expected levels of stiffness in the resulting photobase. It contains no mention of the consequences for image sharpness of moving from a mono-extruded to a co-extruded coating structure. SUMMARY OF THE INVENTION It is one object of the present invention to determine whether acceptable photodefinition levels, at least comparable with those achieved in the current mono-extruded photobase products, can be achieved in a co-extruded product and, if so, to specify those structures which are acceptable. According to the present invention there is provided a substrate for an imaging layer which is to carry an image to be viewed by reflected light the substrate comprising a base sheet with a face side and a wire side, the face side having an opaque co-extruded polymeric face side coating comprising at least: i a core layer bearing a particulate opacifying pigment, and ii a top surface layer of polyethylene having first and second faces, the first face being for receipt of the imaging layer and the second face being for contact with the core layer; the substrate being characterised in that: iii the thickness of the top surface layer is no greater than 12um; iv the total content of opacifying pigment in the — _2 face side coating is at least 3gm ; v the proportion by weight of pigment in the core layer is greater than the proportion by weight of the pigment in- the top surface layer by at least 1.75 to 1.00. It is expected that the method of manufacture of the at least two layer face side coating will be by simultaneous co-extrusion of the layer structure. In such a case it is expected that the core layer will be extruded between the top surface layer and a lower layer, with the pigment content of the and lower layer low enough to avoid fouling of the die lips with pigment. The proportion by weight of pigment in the top surface layer, however, would normally be less than the minimum to avoid fouling that is to say, normally less than about 15wt%, probably no more than 10wt% and preferably no more than about 5wt%. Most preferably for highest photodefini.tion, the surface layer contains no pigment at all. This is startlingly unexpected and surprising, directly contrary to conventional wisdom in this art, but it is what has emerged from Applicant's research and experiments. For the time being at least, Applicant can offer no explanation for the phenomenon. The top surface layer is of polyethylene, thereby to optimise acceptability of the photobase of the invention to sensitiser companies. The core layer and any lower layer present will normally be of polyolefin-based composition. For economy, the core layer may also be of polyethylene, but other polymers could be incorporated in a blend, or could replace the polyolefin completely in the core layer, when conditions make it appropriate. Specifically, a blend of low density and high density polyethylenes has been found to optimise pigment dispersion at high pigment levels in the core layer. The weight per unit area of each layer (its "coatweight" ) will depend upon the specific requirements of the photobase user, that is, the customer sensitiser company. For example, a so-called "PTS" paper (phototypesetting paper) _2 conventionally has a face side coating of 15gm polyethylene ("PE") with a pigment content of 15 wt% Ti0 2 . With the invention this could be replaced by a core layer _2 of llgm PE at 66 wt% Ti0 2 flanked by top surface and _2 lower layers each of 2gm PE at 20 wt% Ti0 2 or a core layer of the same thickness carrying 74 wt% Ti0 2 flanked by top surface and lower layers devoid of pigment, and both these structures should according to Applicant's experiments yield a photodefinition quality about 10% up on that of the conventional product. In another example, a conventional .monochrome photobase has _2 a face side coating of 40gm PE with a pigment loading of 10 wt%. Applicant's results given below would suggest that an improvement in photodefinition of no less than 20% is achievable .by the use of an top surface layer and a lower _2 layer each of 2gm coatweight but both devoid of any _2 pigment and with a core layer between them of 36gm coatweight and pigment content of around 26 wt%. The results given below suggest that the benefit to photode inition of a pigment-free top surface layer is progressively stronger as the pigment content of the face side coating rises, in particular above the range of from _2 _2 3gm to lOgm . It had been thought that pigment weight per unit area of coating would be less significant than pigment concentration in the coating (on the basis that photodefinition would be affected by the surface, not bulk, characteristics of the coating) . Yet, no significant correlation between pigment concentration and photodefinition could be found. The results below further indicate that optimum photodefinition is achieved with a thin top surface layer, that is, preferably no more than 8μm, more preferably 5μm or less, and most preferably no more than 3μm. Layers less than 1.5 μm are difficult to co-extrude on a continuous, controlled basis. * The known technology of production of resin-coated substrates for imaging layers includes for example the use of dyes and optical brightening agents, stabilizers and anti-oxidants in the resin compositions, and the use of a corona treatment to improve adhesion between the resin and the base sheet. It is known to use a polymeric tie layer between two co-extruded polymer layers to improve adhesion between the layers. This known technology is to be applied, as appropriate, to the structures of the present invention. DETAILED DESCRIPTION In this specification, numerical values of photodefinition are established by a test procedure which involves exposure of the imaging material in question to a range of strengths of incident radiation, in order to produce a plot of image density against the logarithm (S) of the exposure (X) which the material has suffered. This procedure is repeated with a grid of opaque lines (spacing 2.365 mm ) overlying the imaging material, and this naturally has the effect of requiring greater exposure to the radiation to achieve a given density. From the superimposed plots, the quantity - ΔLog X = Δ S (1.0) - Δ S (0.1) where - Δ Log X = Image Sharpness Δs(l.O) = Increase in log. of exposure on superimposition of grid, measured at image density of 1.0 above fog. Δ S(0.1) . = Increase in log. of exposure on superimpositon of grid, measured at image density of 0.1 above fog. can be established, and it is this value for image sharpness which is given herein as a numerical measure of "photodefinition" . In the Examples which follow, the extruded structures the subject of the tests were of photographic grade paper with a conventional wire side PE coating and one of three different face side coatings. Experimental co-extruded structures, shown in transverse section in the attached Figure 1, comprised a photographic base paper 10, a wire side PE coating 11 and a 3 layer co-extruded face side PE coating comprising a lower layer B a core layer P arid a top surface layer E. The polymer used in the core layer P was a mixture (LD/HD) of low density polyethylene and high density polyethylene. The low density component was of Chevron Oil's Grade 4516 . (or Grade 1017) polyethylene (Chevron Oil, Orange, Texas 77630, USA) . The high density component was of Grade 7250 or 7840 from E.I. DuPont de Nemours & Co. Inc., Polymer Products Dept., Wilmington, Delaware 19898, USA. Pigment was incorporated as a masterbatch of 50 wt% Ti0 2 /50 wt% polyethylene, Grade 11171 from Ampacet International Corporation, 250 South Terrace Avenue, Mount Vernon, New York 10550, USA. The top surface E and lower layers B were of 100% low density PE (as above) whenever unpigmented. When pigmented they were of the LD/HD blend mentioned above. In this specification "low density" PE means PE with a density of less than _3 0.940gcm . "High density" PE's are those with a density of 0.940gcm and above. Coatweights were calculated by microscopic study of the transverse thickness of the extruded product, followed by computation of the product of coating thickness and density to yield coatweight. From the calculated coatweight and the known concentration of pigment in the extrudate the _2 pigment bulk (gm ) in each coating layer was calculated. A photodefinition index PD was calculated by the method described above. The results given in Table 1 (below) are displayed graphically in various ways in Figures 2, 3 and 4. Referring to Figure 2, the results of Table 1 are plotted as three straight lines of best fit together with rectangle A which shows the area occupied by current production material made by mono-extrusion and included in Table 1 as results of samples M. The circular data points are for face side structures with top surface and lower layers devoid of pigment (C-P-C structures) and yields the chain-dotted C-P-C line. The square data points are for s±ructures in which the lower layer is again clear but the top surface layer is pigmented (P-P-C structures). These points yield the full line as the straight lines of best fit. The diamond-shaped data points are for structures in which the top surface layer is clear but the lower layer is pigmented (C-P-P structures). These points yield the dashed line as the straight line of best fit. The following features emerge:- i_ the C-P-P structures give better photodefinition than the P-P-C structures over all pigment weights ii the " C-P-C structures start to give better photodefinition than the P-P-C structures at _2 about 10 gm~ of pigment _2 iii below about 3gm of pigment, the photodefinition performance of the C-P-C structures is not as high as in current production material (although C-P-P material _2 with as little as lgm of pigment can match current production material). Referring now to Figure 3, three different lines of best fit have been established on the same axes and same data points, but only those relating to the structures with unpigmented top and lower layers. The full line is for those points which derive from structures with a top surface layer not more than 2μm thick. The dashed line is for structures with a top layer more than 2μm but not more than 4,um thick. The chain dotted line covers structures with a top surface layer more than 6 μm thick but not thicker than 8 μm. There are not enough data points to permit a meaningful line of best fit to be drawn for 6-8/Lιm clear top surface layer structures. Figure 3 shows how the thinner is .the clear top layer the better is the resulting photodefinition. Figure 4 is a plot of the relevant data used to produce Figure 2, but with the thickness of the clear top layer E as the horizontal axis. The line of best fit gives an indication how quickly the photodefinition level falls with increasing clear top surface layer thickness. A B E FACE SIDE COATING DETAILS continued over. TABLE 1 I ~ continued over. TABLE 1 KEY TO TABLE 1 3/1 = 3 layer co-extruded structure only core layer pigmented 3/2 = 3 layer co-extruded structure with core and either lower or top surface layer pigmented to different levels. E = top surface layer. P = core layer. B = lower layer. M =- mono-extruded control samples taken from current production stock.

Claims: C L A I M S 1. A substrate for an imaging layer which is to carry an image to be viewed by reflected light, the substrate comprising a base sheet with a face side and a wire side, the face side having an opaque co-extruded polymeric face side coating comprising at least: i a core layer bearing a particulate opacifying pigment, and ii a top surface layer of polyethylene having first and second faces, the first face being for receipt of the imaging layer and the second face being for contact with the core layer; the substrate being characterised in that: iii the thickness of the top surface layer is no greater than 12μm; iv the total content of opacifying pigment in the — _ 2 face side coating is at least 3gm ; v the proportion by weight of pigment in the core layer is greater than the proportion by weight of the pigment in the top surface layer by at least 1.75 to 1.00. 2. A substrate as claimed in claim 1, wherein the base sheet is of paper. 3. A substrate as claimed in claim 2, wherein the paper is of photographic quality. 4. A substrate as claimed in claim 1, which is a substrate for photographic prints. 5. A substrate as claimed in claim 1, wherein the thickness of the top surface layer is no greater than 8μm. 6. A substrate as claimed in claim 5, wherein the thickness of the top surface layer is no greater than 5μm. 7. A substrate as claimed in claim 6, wherein the thickness of the top surface layer is in a range of from 1.5μm to 3.0μm. 8. A substrate as claimed in claim 1, wherein the core layer is of a polyolefin. 9. A substrate as claimed in claim 8 wherein the core layer is of a blend of low density and high density polyethylene. 10. A substrate as claimed in claim 1, wherein the concentration of the pigment in the top surface layer Is not more than 10 wt%. 11. A substrate as claimed in claim 10, wherein the said pigment concentration is not more than 5 wt%. 12. A substrate as claimed in claim 11, wherein the top surface layer is substantially devoid of the pigment. 13. A substrate as claimed in claim 1, wherein at least a major proportion of the opacifying pigment is titanium dioxide. 14. A substrate as claimed in claim 1, wherein the co-extruded face side coating includes a lower layer on the face of the core layer remote from .the top surface layer. 15. A substrate as claimed in claim 12, wherein the lower layer is a polyolefin. 16. A substrate as claimed in claim 15 wherein the lower layer is of polyethylene. 17. A substrate as claimed in claim 14, wherein the lower layer carries some of the total content of the opacifying pigment in the face side coating. 18. A substrate as claimed in claim 1, wherein the total weight of the co-extruded face side coating is 'around _2 15gm , the total content of opacifying pigment in the said coating is around 3gm and the substrate is suitable for use as a phototypesetting paper. 19. A substrate as claimed in claim 1, wherein the total weight of the co-extruded face side coating is around _2 40gm , the total content of opacifying pigment in the said coating is around 7.5gm and the substrate is suitable for use as a monochrome photographic base paper. «•* 20. A substrate as claimed in claim 1, wherein the total weight of the co-extruded face side coating is around _2 30gm , the total content of opacifying pigment in the said _2 coating is around 5.5gm and the substrate is suitable for use as a colour photographic base paper.

Inventors: CUSICK CHRISTOPHER, DUNK PAUL, REED ROGER JOHN, CUSICK, CHRISTOPHER, DUNK, PAUL, REED, ROGER, JOHN
Applicants: JAMES RIVER GRAPHICS LTD, JAMES RIVER GRAPHICS LIMITED
Application year: 1989
Main classification: G03C1
IPC codes: G03C1