CO-EXTRUSION METHOD

Abstract: Using co-extrusion a sheet product web can be created which has first and second strands bonded together weakly. Post-extrusion parting of the strands, at the weak bond, can produce sheet form products in a variety of fields (notably imaging techniques including photography). In one example, a stiff substrate can be retained through image processing and then peeled away. In another example, a heavily pigmented face side coating of photographic base paper can be extruded beneath a protective top shrouding co-extruded layer which is peeled away before a photographic emulsion is coated on to the face side coating.

Description: CO-EXTRUSION METHOD BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a co-extrusion method of making a sheet product web, such as photographic base paper, which is often referred to as "photobase" . In this specification, the expression "photobase" is to be taken to mean sheet material adapted for use as photographic base material, that is to say, sheet material adapted to receive on one of its major surfaces a thin layer of light-sensitive material in which a photographic image may be formed. Conventionally, photobase consists of a paper substrate, coated on both of its major surfaces with a waterproof polymer coating, usually of polyethylene. Paper made by a Fourdrinier technique has a face side and a wire side. Conventionally, the photographic material is applied to the coated face side of the paper. THE PRIOR ART There have been various proposals to use a co-extrusion method to replace a single layer polymer coating on the face side of a photographic base paper with a coating comprising two or more layers. See for example GB.1339045, 2061131A and EP.0183467 Al. With polyethylene coatings, the requirements of the manufacturers of photographic papers for photobase of high and uniform opacity imposes on the makers of photobase a need to incorporate into the face side polymer coating a high loading of white pigment, usually titanium dioxide or barium sulphate. Difficulties set in, however, whenever the loading of pigment in one or other of the two outermost co-extrusion layers exceeds a relatively low pigment loading, in that pigment deposits are likely to build up on the surface of the extrusion die lip over which the highly-loaded pigmented layer flows. It is an aim of the present invention to provide a method for co-extrusion of photobase with a relatively high pigment loading in the face side surface coating layer of the extruded photobase. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a method of making a sheet product web characterised by co-extruding first and second strands and bringing the strands together, for bonding to each other with a relatively weak bond, thereby adapting the sheet product web for post-extrusion parting by peeling apart, at the weak bond, the first and second strands. According to a second aspect of the present invention there is provided apparatus for making a product web, which comprises a co-extrusion die for creating a multi-layer web in which there are first and second strands bonded to each other with a relatively weak bond; and characterised by means for peeling apart the first and second strands, to produce, from one of the separated strands, said product web. An important application of the invention is to the manufacture of photobase web. The peelable layer would normally be a clear resin layer of a material selected to have relatively good cohesion, but relatively poor adhesion to the adjacent polymer coating layer below it, which underlying layer would normally have the desired high pigment loading. Generally high pigment loadings are those in which pigment particles take up at least 20% of the volume of the pigmented layer. After stripping, the material of the peelable layer could be recycled and reused. It is envisaged that the peelable layer could be formed from an acrylic, polystyrene or polycarbonate synthetic polymeric material. Preferably, the peel apart force for a 2cm strip of the bonded strands, in the length direction of the strands, is not more than 0.9 N, more preferably 0.6 N. Bonded to polyethylene, particularly low density polyethylene, the polymeric materials listed immediately above offer these possibilities. Application of photographic emulsion to photobase is a sophisticated process. It may well be desirable to maintain the strippable layer on the face side of the photobase until immediately prior to the emulsion coating strip. In other processes, the peel apart bond could be between i) an opaque or transparent layer which in use receives an image, and ii) a backing sheet which is removed prior to display of the captured image, but after the backing layer has served its purpose during image processing. One example is to provide a transparent imaging layer weakly bonded to a stiff paper backing layer which is stripped off after imagewise exposure, and image processing of the imaging layer, to yield an imaged transparency of whatever large) size is demanded. BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made by way of example to the accompanying drawings, in which: FIGURE 1 shows a schematic diagram of a first embodiment of apparatus for performing the process of the invention; FIGURE 2 shows a like diagram of part of a second such embodiment; FIGURES 3, 4 and 5 are all sections through the thickness of different sheet product webs. DETAILED DESCRIPTION In Figure 1, a base paper web 10 is fed into the nip 11 between a pressure roll 12 and a chill roll 13 of a co-extrusion die 14 from which is extruded on to the chill roll a two layer polymeric structure as mentioned in Example 1 below. Thus, from the nip 11 emerges a three layer paper/polymer web 15 which is stripped from the chill roll 13 at the nip 16 with a stripper roll 17. The web 15 then advances to the nip 18 between a parting roll 19 and co-operating pressure roll 20. In the roll 19 is a transversely-extending scoring blade 21 which normally resides within the periphery of the roll 19 which can be actuated, when required, to protrude from the roll by a distance which would normally be in a range of from 5 to 40 μm. With any one web, it should normally be necessary to use the blade 21 only once, to initiate the peeling process. From the nip 18, the web advances to the nip 22 between a pair of vacuum rolls 23, 24 each having a segment 25 where the periphery is subject to a vacuum for holding to the drum surface 26 whatever sheet material passes through the nip 22. In this way, the three layer web advancing through the nip 22 is peeled apart, between the two polymer layers, into a product web 27 constituted by the polyolefin-coated paper, and a peeled layer web 28. The product web passes over successive transport rolls, 29, 30 and 31 before being wound on a reel-up roll 32. The peel material 28 may be wound up or its own reel-up reel 33 or returned direct to the melt preparation area (not shown) of the die 14. In operation, a three layer web 15 is created and then the blade 21 is actuated to break the continuiy of the (top) layer of peel material in the web. The vacuum applied at rolls 23, 24 is sufficient for roll 23 to retain the peel layer from the point where its continuity is broken and so, downstream on the web from the transverse knife cut in the web, two separate webs emerge from the vacuum rolls 23, 24. In Figure 2, the equipment is very similar (so the same reference numerals are used whenever appropriate) but instead of a knife inside the parting roll 19, an external knife 34 is used, which acts against a backing roll 35. Downstream of the parting knife handling of the web is substantially unchanged. Penetration of the knife 33 into the web would normally be limited by mechanical stops to from 5 to 40 ym. Figure 3 shows a sheet product with three coating layers on a paper substrate layer 50, the coating layers being a tie layer 51 of unpigmented low density polyethylene with a coating weight of 2 .g/m 2 , a pigmented layer 52 of 17 g/m 2 of low density polyethylene containing 30 wt.% of titanium dioxide pigment particles, and a surface layer 53 of a peelable top coating of polymer X (see Example 1 below). Not shown is a polyethylene coating on the reverse side of the paper substrate 50, for waterproofing. Figure 4 shows a sheet product with three face side coating layers on a stiff paper substrate 60, namely, a first layer 61 of low density polyethylene, a peel layer 62 of polymer X (Example 1 below) and a second layer 63 of an adhesive polymer such as low density polyethylene, which is itself adhered to the back side 64 of a thin paper substrate 65. Not shown are conventional photographic base paper face side coatings on the face side 66 of the thin substrate 65, and a waterproofing back side coating on the back side 67 of the heavy substrate 60. The layers 61 and 63 are called "tie" layers because they adhesively tie together the layers to each side of them. Those skilled in the art of co-extrusion are familiar with the concept of a tie layer. Examples of tie layer materials are: 1) modified low density polyethylenes, such as; BYNEL from DuPont ADMER from Mitsui LOTADER from Atochem PLEXAR from USI 2) modified polypropylenes, such as: PLEXAR from USI OREVAC from Atochem 3) polyolefin co-polymers containing such material as : EVA (ethylene vinyl acetate) EAA (ethylene acrylic acid) EMA (ethylene methyl acrylate) EMMA (ethylene methyl methacrylate) 4) polyethylenes (for example, low, high or medium density, as well as linear low density polyethylene) In use, the multi-layer structure of Figure 4 may serve as photographic base paper, with sensitised emulsion on the face side 66 of the thin substrate 65. After photographic image processing the stiff paper 60, tie layer 61 (and possibly the peel layer 62) can be stripped from the lighter substrate 65 by parting the layer structure at one of the two interfaces of the peel layer 62. This facility may be useful in some situations where paper for photographic proofs are required. Figure 5 proposes to use a top surface layer 70 of an "engineering" polymer such as a polycarbonate as the basis of a photographic transparency. On its face side 71 it carries (not shown) photographic emulsion layers for development of an image therein. To its back face 72 is bonded weakly a peel layer 73 of a polymer such as low density polyethylene, itself rather more strongly bonded to the face side of a paper substrate 74, waterproofed on its back side with a coating layer of polyethylene (not shown). The paper lends stiffness to the structure for handling and photographic processing but, thereafter, the paper 74 and peel layer 73 are stripped from the back of the transparency base 70 to provide a transparency as large as is needed. Very large transparencies would be relatively cheap and easy to produce. Special visual effects could be achieved by imaginative modulations of light directed through selected areas of the transparency . The invention is further illustrated in the following Examples : EXAMPLE 1 Photobase paper was subjected to a co-extrusion process in which it received a coating layer of white pigmented polyolefin and an overlying coating of a polymeric material thought to have potential for removal by peeling. Four materials were tried, namely:- A = Polycarbonate B = Polybutylterephthalate C = NYLON D = Polyethyleneterephthalate these materials were tried at different coatweights CW g/m , with results shown in the Table 1 below: TABLE 1 From the results above it may be that the peeling action generates a surface finish to the prepared photobase which has not been achieved previously, and which is attractive to downstream photographic emulsion manufacturers. With a polycarbonate peelable layer what can best be described as a "misty" or "foggy" gloss surface on the photobase was achieved and such a finish could offer advantages for receipt of subsequent emulsion or other overlying coating layers. EXAMPLE 2 Sample quantities of sheet product web as shown in Figure 3 were co-extruded, with two levels of coating weight W (5 and 10 g/m 2 ) of the surface coating layer, and two different chill rolls, one matt and the other gloss. Five different polymers X were tried as the surface coating layer. These polymers are identified in Table 2 below. Sample strips, 2 cm wide and in the length direction of the web, were taken, and the surface coating layer peeled apart from the underlying polyethylene coated paper substrate. The peel apart force was measured in Newtons at 20°C and the results are shown in Table 2. TABLE 2 Peel Apart Force (N) Top Surface Layer X PC DIAKON PS PET PBT KEY N/A = Not available PC = polycarbonate DIAKON = An acrylic-based polymer from ICI Pic PS = polystyrene PET = polyethylene terephthalate PBT = polybutylene terephthalate The results indicate that the peel apart force with use of a matt roll tends to increase as the coating weight is increased. With a gloss roll, however, the opposite trend is present. In general, forces for stripping are larger with a matt roll than with a gloss roll, possibly because a matt roll has a bigger surface area incident on the 2cm wide test strip than a gloss roll, by virtue of its surface roughness, the greater surface area improving the adherence between the layer of polymer X and the polyethylene layer beneath it. As the layer structure underlying layer X was kept constant, the change from W=5 to W=10 raised slightly the thermal capacity of the co-extruded web. It is thought that an increased thermal capacity should have a tendency to increase the peel apart force. In this respect the results with the matt roll may bear out this proposition but there is no support for it in the results with the gloss roll. INDUSTRIAL APPLICATION Although the description above is concerned largely with conventional photographic processes, the invention is likely to have application to other imaging processes, such as dye diffusion thermal transfer imaging systems, where a synthetic surface carries an image-receiving layer. In the area of pre-press proofing systems, there is a problem that the imaging substrate required is not stiff enough to advance reliably and without wrinkling or creasing through the roller pairs and transport rollers of the processing equipment. With this invention, however, a plurality of colour-separated images could be printed on a face surface of a substrate such as shown in Figure 4. Afterwards, the stiff substrate could be peeled away to leave the image presented on the lighter weight substrate only, in order to give after peeling away of the paper substrate an indication what would be the appearance of the printed image that would be created on light paper by a 4-colour printing press, without the previous processing transport problems .

Claims: C L A I M S 1. A method of making a sheet product web characterised by co-extruding first and second strands and bringing the strands together, for bonding to each other with a relatively weak bond, thereby adapting the sheet product web for post-extrusion parting by peeling apart, at the weak bond, the first and second strands. 2. A method according to claim 1 wherein the peel apart force at 20°C is not more than 0.9 N for 2cm wide strip, taken from the product web in its lengthwise direction. 3. A method according to claim 2 wherein the peel apart force is not more than 0.6 N. 4. A method according to claim 1 wherein the weak bond is made between an extruded layer of polyethylene and an extruded layer of one of an acrylic, polystyrene or polycarbonate synthetic polymeric material. 5. A method according to claim 1 wherein the first strand is adapted for use as photographic base paper and the second strand is a strippable overcoat of the face side surface of the base paper. 6. A method according to claim 5 which is a method of making sensitised photographic base paper and which includes the step of stripping the overcoat immediately prior to coating the face side surface with a photographic emulsion. 7. A method according to claim 5 wherein the photographic base paper has a face side surface which is pigmented, with a volume concentration of at least 10% of particulate pigment within a polymeric matrix. 8. A method according to claim 1 wherein the first strand comprises an imaging layer and the second strand comprises a stiff backing sheet. 9. A method according to claim 8 wherein the imaging layer is opaque. 10. A method according to claim 8 wherein the imaging layer is transparent.

Inventors: DUNK PAUL, REED ROGER JOHN, DUNK, PAUL, REED, ROGER, JOHN
Applicants: GLORY MILL PAPERS LTD, GLORY MILL PAPERS LIMITED
Application year: 1991
Main classification: D21H19
IPC codes: B32B7, D21H19, D21H23, G03C1