SUBSTRATES WITH REDUCED TENDENCY TO CURL

Abstract: Photographic base paper (10) with a polyethylene coated (11) face side is given a ''wire'' side coating which includes an anti-curl layer (14) of polystyrene, polypropylene or polyethyleneterephthalate.

Description: SUBSTRATES WITH REDUCED TENDENCY TO CURL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a substrate material adapted to carry a sensitive coating in which an image may be recorded, the substrate comprising a core medium having a front side and a back side, a polyethylene (PE) front side coating and a waterproof polymeric back side coating. 2. Background .of Invention Photographic prints are normally produced in an emulsion layer coated on a gelatine layer on the face side of the paper substrate (commonly called "raw photobase") made waterproof by a coating on both sides. Conventionally, the side to carry -the photo emulsion (the "face side") is coated with a polyolefin, pigmented with an opaque pigment such as titanium dioxide and including other additives to produce the correct colour shade characteristics. The back "wire" side of the raw photobase is normally coated with an unpigmented polyolefin. Polymers shrink on cooling. Thus, when the raw paper core medium of photographic base paper is given a waterproof polymeric coating on its front ("face") side and back ("wire") side by extrusion of the molten polymers it is likely that the resultant layered structure will have a tendency to curl, unless the front and back side coatings are identical. Another, difficulty with such substrates is that, once coated with gelatine, they have a tendency to curl when there is a change in the relative humidity (RH) of their surroundings. This is because gelatine expands as it absorbs moisture, so it expands as RH increases, and loses moisture and shrinks as RH falls. Companies who apply a photographic emulsion to the coated face side of such photographic base papers are interested in controlling and minimising the amount of curl which the sensitised base paper exhibits. Any gelatine layer on the face side, for example, will expand and shrink with changes in temperature and humidity. Until now it has been customary to control curling by laying down on the paper base a back side coating similar enough to the face side coating (possibly including a gelatine layer, for example) that the effect of the thick back side coating(s) will adequately resist the curling tendency created by the face side coatings. It is of course expensive to provide such thick back side layers, especially since their only function is to resist curling of the coated paper. It is an object of the present invention to discover back side coatings which can be used to balance more economically than at present the curling tendency of any particular face side coating prescribed by a manufacturer of photographically sensitive base paper. Another object of the present invention is to achieve adequate control of the tendency to curl without the need for the additional process step of applying the wire side anti-curl gelatine coating. In this specification, the expression "face-in" curl is used to describe a curl of the paper in which the face side is on the inside (concave) surface of the curled sheet. "Face-out" curl is curl in the opposite sense. With a PE coating on the face side of the paper core, and no coating on the back side, the extruded product when cool exhibits face in curl. This can be neutralised by providing a PE coating of equal thickness on the back side. It was therefore thought that savings in back side coating weight should be achievable by selecting for the back side coating a polymer which exhibits a shrinkage on cooling which is more pronounced than that of PE. One measure of linear shrinkage on cooling is the so-called "mould shrinkage" that is, the amount of shrinkage (expressed as length/length) shown by the polymer when a bulk object of it, such as a rod, is cast in a mould and cooled to ambient temperature. For the range of well-known polymers listed below is given a range of characteristic mould shrinkages for each one - PMMA - polymethylmethacrylate: 0.002 to 0.008 NYLON - polyamides: 0.005 to 0.015 PC - polycarbonates: 0.005 to 0.007 PET - polyethylene terephthalate: 0.020 to 0.025 PBT - polybutylene terephthalate: 0.015 to 0.020 LDPE - low density polyethylene: 0.015 to 0.050 HOPE - high density polyethylene: 0.020 to 0.050 PP - polypropylene: 0.010 to 0.025 PS - polystyrene: 0.001 to 0.006 These data are taken from "Guide to Plastics" 1978 pages 6 to 25 available from Modern Plastics Encyclopaedia. (By "high density" polyethylene is meant PE with a density _3 of more than 0.94 gem ). Prima facie, those polymers with the greatest mould shrinkage should have greatest potential as a back side coating to resist face in curl. SUMMARY OF .THE INVENTION According to one aspect of the present invention there is provided co-extruded photobase having a face side and a back side, intended to receive a face side coating of photographic emulsion, and comprising a sheet substrate with a waterproof polymeric surface on both of its faces, and characterized in that at least one of the co-extruded coating layers is an anti-curl coating of a polymer which exhibits a change in dimensions with change in ambient relative humidity, the anti-curl coating being responsive to humidity changes to induce a degree of curl in the photobase, whereby the said degree of curl may be chosen such as to be equal and opposite to a degree of emulsion-induced curl which would otherwise occur with changes of humidity after the photobase has subsequently received its face side coating of photographic emulsion. According to a second aspect of the present invention the substrate is characterised in that the back side polymeric coating is constituted by mutually adhesive layers of at least two different compositions, comprising an adhesive layer next adjacent the core medium and a shrink .layer of a polymeric composition which exhibits dimensional shrinkage on cooling wh-ich is substantially greater than the equivalent shrinkage in polyethylene. According to a third aspect of the present invention, the substrate material is characterised in that the back side is a multi-layer structure which comprises a shrink layer of at least one of polystyrene (PS) polyethylene terephthalate (PET) or polypropylene (PP) and a tie layer for adhering the shrink layer to the underlying substrate. There have been various proposals to replace a single layer coating on the face side of the paper with a coating comprising two (or more) layers. See for example GB.1339045, 2061131A and EP.0183467 Al. The technique of co-extrusion, as mentioned in the above-numbered patent publications, is a convenient way of providing a multi-layer back-side coating. By such a technique, individual layers with as little as 2 g/m 2 can be provided. Many polymers useful as a shrink layer have limited adhesion to PE and paper. The purpose of the tie layer between the core medium and the shrink layer is therefore to form a strong bond between the shrink layer and the underlying material, which may be, for example, a back side coating of PE on the core medium. Polystyrene has an amount of mould shrinkage which is about an order of magnitude less than that of LDPE yet is capable of producing substantial face-out curl at coating weights on the wire side substantially lower than those of the LDPE on the face side of the substrate. PET and PP wire side coatings also created face out curl but not to such a marked extent despite their having higher mould shrinkage values than PS. With the invention, coating weights on the wire side are contemplated to fall generally within a range of from 10 g/m 2 to 40 g/m 2 . The invention opens up good possibilities for economies by reducing the wire side coating weight from what is provided conventionally. DETAILED DESCRIPTION For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying Figures 1 to 4 of the drawings, in which each of Figures 1, 3 and 4 is a section through the thickness of one embodiment of photobase in accordance with the invention, and Figure 2 shows a prior art structure. In Figure 1, the paper substrate 10 has a face side polymeric coating 11 which may be of a plurality of layers, co-extruded onto the paper substrate 10. The paper 10 also has a -wire side polymeric coating, co-extruded onto the paper, and consisting of an adhesive layer 12, a tie layer 13 and a shrink layer 14. The paper may be any paper conventionally used for photobase. The face side coating 11, while it may have individual layers of other polymers, is largely composed of low density polyethylene, that is, polyethylene with a density of less than about 0.94 g/cm 3 . The adhesive layer 12 has a coating weight of not more than 5 g/m 2 and preferably a coating weight of around 2 g/m 2 . The tie layer 13 has a coating weight of not more than 5 g/m 2 and preferably around 2 g/m 2 . It has been found that the thinner the tie layer, the better it functions to bond together the layers above it and below it. The shrink layer has a coating weight of up to 20 g/m 2 . Applicants have conducted tests (as specified by Agfa) to reveal what degree of curl may be expected from coatings of particular polymers on paper photobase when ambient humidity values of 50% and 80% are established. The results are shown in the Table below:- B L E KEY W = The wire side is on the inside of the arc produced upon curling of the sample F = The face side is on the inside of the arc produced upon curling of the sample O = No significant curling of sample In each case, the anti-curl polymer was coated on the face side of raw paper photobase and an intermediate tie layer was used to ensure adequate mutual adhesion. The Control sample has as its face side coating a layer of low density polyethylene identical to the wire side coating used in all the tests. It can be seen that some polymers cause curling with face side inwards and some to produce a curled sample with face side outwards. To counteract the tendency of gelatine to cause curl with face side outwards it is therefore reasonable to coat on the face side with those polymers which cause face-in curl, for example, polyamides, EMA and EVAL. Conversely, for those which when coated on the face side cause wire side in curl, coating on the wire side is indicated. Examples of these polymers are polypropylene, polystyrene, PET and PMMA. Coated photobase is invariably manufactured in accordance with the unique specification of a particular customer (who will be applying the emulsion to it). Thus, the specification of the anti-curl layer will be drawn up with a specific gelatine layer structure already prescribed, and so the specification will be chosen so as to produce the desired amount of curl in opposition to that which derives from the gelatine layer. Using co-extrusion technology to lay down simultaneously all (or a substantial number) of the polymeric layers which coat the substrate of the photobase, it is relatively easy to provide as many tie layers as are necessary to ensure adequate adhesion of the anti-curl layer to the other layers adjacent to it, or elsewhere within the photobase structure. Referring again to the drawings, in which: FIGURE 2 shows a prior art structure; FIGURE 3 shows a second embodiment of the invention; and FIGURE 4 shows a third embodiment In Figure 2, raw paper photobase 15 has on its face side a low density polyethylene (LDPE) coating 16 and then a photosensitive layer of emulsion 18. An LDPE layer 17 on the wire side is covered with an anti-curl gelatine layer 19. In Figure 3, the face side structure is the same. However, on the wire side there is no gelatine. Instead, a layer of 21 of polystyrene is adhered to the base 15 through a thin polymeric tie layer 20. In Figure 4, the face side of the base 15 carries a tie layer 27, an anti-curl layer 26 and a second tie layer 25. Above this is the LDPE layer 16 adapted to receive emulsion, and the gelatine emulsion layer 18 itself. The layer 26 replaces gelatine layer 19. It can be of, for example, polyamide, EVAL or EMA. A further set of experiments was conducted, measuring the amount of curl exhibited at humidities of 50% and 80%. In the results which follow, the extent of curl is expressed i-n units of h of the circumference of a circle. Thus, if a sample disc does not curl at all, but remains perfectly flat, the amount of curl is zero. If, however, the sample disc curls to a point where its opposite edges just touch one another, so that the disc describes a complete circle, then the amount of curl is 8 of the units, that is, "8.0". For each result, the average of at least two sample discs is taken. A negative quantity denotes face-in curl, and vice versa. The following Examples illustrate the present invention. EXAMPLE 1 Experiments were conducted on a layer structure made up from a core medium of photographic grade paper with a _2 weight of 110 gm coated on its front (face) side with 19 gm of low density polyethylene (LDPE). On its back "wire" side was a co-extruded multi-layer coating of A _2 _2 _2 gm LDPE, B gm of a tie layer and C gm of polymer D. Table 1 lists the layer structures investigated and the amounts of curl obtained when samples were maintained at the stated relative humidities and ambient temperature. The polymeric compositions used to make up the various coating layers, the manufacturers and product names actually used in the experiments, are set out below: Polymer Manufacturer Product Name EMA Chevron 2207 (ethylene methacrylate) EVAL Kuraray H 103A (ethylene vinyl alcohol) 1017 8207F POCAN 7300 9663 4G7DP 615 AT507 * Omitted in Part Nos.l3A/8 to 16A/8 EXAMPLE 2 Experiments similar to those of Example 1 were conducted using a core medium a heavier photographic paper with a _2 _2 weight of 178 gm " Its side face coating was 39 gm of LDPE. Table 2 lists the three multi-layer back (wire) side structures investigated and the results obtained. The results in Tables 1 and 2 reveal the following points: 1. With PS, the extent of face-out curl becomes increasingly large with increasing ambient relative humidity, from 50% to 80% humidity. 2. At 50% RH there is little effect on the extent of curl from variations in the weight of a _2 polystyrene back side coating (5 gm LDPE and 2 _2 gm tie layer being below the PS on -the back side). Curiously, the Table 1 results suggest that at 80% relative humidity a thin PS coating gives a greater curling effect than a somewhat thicker PS layer. 3. At relative humidities of 50% and more, a back _2 _2 side coating structure of 5 gm LDPE, 2 gm tie _2 layer and 16 gm of PS is sufficient to resist the curling effect of a front side coating of 39 _2 gm of LDPE. 4. The mould shrinkage value of PET is only half that of LDPE but PET as shrink layer can nevertheless generate marked face out curl particularly at high humidities. The curling effect increases with increasing weight of the coating of PET. 5. PP, like PET, offers potential for achieving face out curl, which improves with the coating weight of the PP and with ambient humidity. INDUSTRIAL APPLICATION The results given above reveal three materials (PS, PP, PET) which exhibit a curling effect strong enough to compensate fully for that of the face side PE coating of photographic base paper but at back side coating weights low enough to offer significant financial savings over the conventional product with its relatively thick back side PE coating. In addition, since the performance of PS in this respect varies with changing humidity differently from that of PP and PET, a combination of PS and PP or PET can be used to engineer in a photographic base paper a curl behaviour (through a range of temperatures and humidities) tailored to the particular specification of a manufacturer of sensitised photographic printing materials. It is known to treat wire side photo base coatings in various ways. For example, pigments or optical brightening agents can be included in the coating compositions, and conductivising or aqueous silica backwriting coatings can be laid down on top of the polymeric coating. These same treatments are not excluded by the present invention, and the layer structures of the present invention may be selected such as to accommodate any required by the eventual customer. While the above Examples are with papers of weights 110 and _2 178 gm , other photographic base papers may use papers of _2 _2 around 80 gm and 140 gm . The present invention has application to these products also. AB LE 1 AB E 2

Claims: C L A I M S 1. A substrate material adapted to carry a sensitive coating in which an image may be recorded and with a reduced tendency to curl, the substrate comprising a core medium having a front side and a back side, a polyethylene (PE) front side coating and a waterproof polymeric back side coating, and characterised in that the back side coating is a structure of at least two layers and which comprises a shrink layer of at least one of polystyrene (PS), polyethylene terephthalate (PET) or polypropylene (PP) and a tie layer for adhering the shrink layer to the underlying substrate. 2. A substrate material as claimed in Claim 1, wherein the shrink layer and the tie layer are co-extruded. 3. A substrate material as claimed in Claim 1, wherein the back side coating includes a layer of PE, with the tie layer adhering to the PE layer. 4. A substrate material as claimed in Claim 3, wherein the PE layer of the back side coating is co-extruded with the shrink layer and tie layer. 5. A substrate material as claimed in Claim 1, wherein the core medium is of paper. 6. A substrate material as claimed in Claim 5, wherein _2 the paper core medium has a weight of around 110 gm 7. A substrate material as claimed in Claim 6, wherein _2 the front side coating has a weight of around 19 gm 8. A substrate material as claimed in Claim 7, wherein the back side coating has a .weight in a range of from 10 to _2 27 gm 9. A substrate material as claimed in Claim 8, wherein the shrink layer has a weight in a range of from 10 to 20 _2 gm 10. A substrate as claimed in Claim 5, wherein the paper _2 core medium has a weight of around 178 gm 11. A substrate as claimed in Claim 10, wherein the _2 front side coating has a weight of around 39 gm 12. A substrate material as claimed in Claim 11, wherein _2 the back side coating has a weight of around 24 gm 13. A substrate material as claimed in Claim 12, wherein _2 the shrink layer has a weight of around 16 gm 14. A substrate material as claimed in any one of the preceding claims wherein the shrink layer is of polystyrene. 15. A substrate material as claimed in any one of the preceding claims which is a photographic base paper.

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