Technical Background:

The present invention relates generally to a composite laminar material, and also to a method of making a composite laminar material to which the present invention relates, comprising generally a plurality of layers bonded, laminated or ^' otherwise applied to a substrate.

A typical application for composite laminar materials formed in accordance with the present invention is as release liners. Release liners are backing layers which are removable from a product, very often a product having an adhesive coating, and such release liners have a wide range of applications useful in industry and elsewhere.

With the modern expansion of knowledge of a wide range of different adhesives, many products or components which were previously fixed in place by releasable or other fixing elements such as bolts, screws, rivets and the

like are now secured in place by very strong and durable adhesives. The use of adhesives has considerable advantages in industrial processes in that the assembly and fixing operations for components and sub-assemblies can be effected very quickly and with a high degree of certainty.

Whereas the use of individual fixing elements required implements for effecting fixing, no such tools or implements are required to fix components by adhesives. The use of adhesives, however, does carry a disadvantage in that it is necessary to take steps to ensure that adhesive contact between a component and any other surface than that to which it is intended to be fitted cannot take place until fixing is effected. This is particularly the case with the so-called self-adhesive components to which an adhesive layer is preliminarily applied before the component is offered up into its desired position. It is known for such self-adhesive components to be produced, bearing their layer of adhesive, some time in advance of the point at which assembly is to take place, and very often at a geographically remote location from that at which assembly is to take place. It is therefore necessary to protect the adhesive layer from inadvertent contact with other surfaces in the time interval between production of the adhesive-coated component and its final assembly in position. This task is achieved by so-called release

liners. Release liners are disposable layers of a substrate, typically paper, which cover the adhesive and prevent it from coming in to contact with any other surface, but which do not adhere to the adhesive so that the release liner can be stripped away quickly and without carrying off any adhesive, leaving the adhesive layer exposed to be applied to the surface to which the component is to be fixed.

Such release liners, because they are disposable items, must be made as economically as possible using the least amount of material of the least expensive type available. Of course, for certain applications, where highly aggressive environments are concerned, the ability of the release liner to resist attack by aggressive chemicals and/or wide temperature variations, means that a high specification is set for the material. Moreover, because the release liners may be used both inside and outside buildings, it is important that it should be possible to form them in such a way as to be able to withstand wide variations in humidity and moisture content of the air, as well as wide temperature ranges.

An important component of a release liner is the release agent against which the adhesive actually contacts, but which does not adhere to the adhesive. Such materials are expensive and it is consequently important to design the production processes for producing release liners in

such a way that the minimum amount of release agent is used commensurate with obtaining a secure release from the adhesive when required. Typically silicone is used for achieving this purpose since it has well known abilities to resist adhesion. Silicone, however, is readily absorbed into paper, and if a relatively low specification paper is to be used as the substrate it is necessary to interpose a barrier layer between the substrate and the silicone in order to prevent absorption which would have the dual disadvantage of consuming large quantities of silicone and reducing the non-adherent effect. A third disadvantage lies in the fact that impregnation of the substrate by silicone would result in an at least partially non-adherent surface on each side of the substrate, and very often it is not desirable for the opposite face from that coated with silicone to have non-adherent properties. Although two-sided release liners are sometimes used for special purposes, in the majority of cases it is essential for the release agent to be located only on one face.

One very common material used as the barrier layer is polyethylene which is extruded in a thin film onto the substrate. Polyethylene is highly resistant to absorption of release agents such as silicone, and thus performs this task well; it does have many disadvantages, however, in particular it has a very low temperature tolerance and polyethylene laminated paper can usually

not withstand temperatures greater than 100°C without serious delamination taking place resulting in the appearance of bubbles and wrinkles on the surface which, in order properly to perform its function as a release liner, must be as smooth and flat as possible. Such bubbles and wrinkles can cause considerable problems not only when attempting to apply an adhesive layer to the release agent, but also when applying the product.

It will be appreciated that, typically, 'in use the release liner is intended to be coated with adhesive. The adhesive is then treated to render it tacky and the product or face stock applied thereto. The adhesive bonds with the surface of the product more strongly than to the release agent so that upon subsequent removal of the release liner the tacky adhesive surface bonded to the product or face stock is left exposed and ready for contact with the surface against which the component (product) is to be fixed. Adhesives may be naturally curing or drying, or may be cured by the application of heat or ultra-violet radiation or the like. The substrate, whatever material is chosen for this layer, must preferably have a high degree of stability, adequate strength, that is burst strength, tear strength and tensile strength, have a high degree of uniformity and resistance to dimensional variation with changes in temperature and humidity. The barrier layer to which the release agent is applied must have a high degree of

consistency, must be highly impermeable to the release agent, must be as economical as possible and should also preferably have a high degree of surface flatness in order to minimise the quantity of release agent necessary in order to achieve the required release effect.

According to one aspect of the present invention, therefore, a composite laminar material suitable for use in the production ^• of a release liner comprises a substrate of at least partly porous material, a barrier layer, on at least one surface of the substrate, of substantially impermeable material having a smooth surface facing away from the substrate, the material of the barrier layer being one to which a layer of low- adhesion release agent such as silicone will bond if applied thereto.

The precise composition of the barrier layer will be discussed hereinbelow; for the moment suffice it to say that the composition from which the barrier layer is made can be rendered thixotropic such as to permit the layer to be spread on to one surface of the substrate without substantial absorption taking place thereby ensuring that the barrier layer, whilst being securely bonded to the substrate upon curing, does not penetrate into the body of the material, and certainly does not appear on the opposite face of the substrate from that to which it is applied.

Preferably the barrier layer is composed of a material which is curable by the action of ultra-violet light rather than one which is merely dried prior to the application of the release agent. This is important in ensuring the quality of the surface finish of the barrier layer and in economising on the production process which will be described in more detail hereinbelow.

In one exemplary embodiment the barrier layer includes a mixture of oligomers and monomers. This mixture of oligomers and monomers may comprise or include epoxide based oligomers and monomers. Alternatively the said mixture may comprise or include oligomers and monomers of acrylic acid.

In a particularly formulation the said ultra-violet curable material includes hydroxy compounds and may further include photo initiators.

The said ultra-violet curable material may also include a modified phenol.

The form of the substrate may be different in different embodiments, and the requirement for economy or the requirement for high tolerance to moisture or temperature variations may predominate in the decision on the precise material selected for this purpose. Conveniently the substrate is a woven or non-woven fibrous web, typically

paper.

Of course, for high performance the material of the barrier layer may be impermeable not only to water, but also to grease, oil and other like such materials, including silicone.

The present invention also comprehends a release liner comprising a composite laminar material such as that defined hereinabove in sheet or web form, having a layer of release agent, such as silicone, bonded to the said barrier layer on at least one face thereof.

In any of the embodiments discussed hereinabove, the substrate, may have a barrier layer formed on each of two opposite major faces and may, likewise, receive respective layers of release agent on respective faces.

According to a second aspect of the present invention a method of producing a composite laminar material as hereinabove defined comprises the steps of applying a barrier layer of substantially impermeable material in thixotropic form (or as part of a thixotropic composition, to at least one face of a web of non-woven fibrous or other laminar material in such a way as to lie on the surface of the web without substantial penetration such as by absorption, the barrier layer being cured by any suitable process, such as irradiation with ultra-

violet light or the application of heat, to form a smooth surface, and subsequent application of a layer of a release agent such as silicone to the said barrier layer in such a way as to form a substantially continuous layer of release agent over at least a part of the area of the substrate covered by the barrier layer.

The barrier layer may be a thixotropic composition which is curable by irradiation with ultra-violet light and preferably includes a mixture of oligomers and monomers, hydroxy compounds and photo initiators.

In performing the method of the invention a composition from which the barrier layer is to be formed may comprise a thixotropic mixture of oligomers and monomers of acrylic acid with photo initiators and modified phenol, or epoxide-based oligomers and monomers, each of which composition is curable by irradiation with ultra-violet light .

The method of the invention includes application of the thixotropic composition for forming the barrier layer to at least one face of the web by an applicator roller coated with the said composition.

The method of the present invention may be performed in an advantageous manner by spreading the thixotropic composition over one or more spreader rollers to form a

uniform film of consistent thickness before the composition is transferred to the applicator roller.

For maximum efficiency and therefore economy in performing the method of the invention it is preferred that the barrier layer composition is coated on to the said substrate, cured, and then the release agent applied in the same pass through a series of roller applicators.

Embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic view showing a composite layer formed as an embodiment of the present invention; Figure 2 is a schematic perspective view, with the thickness exaggerated for clarity, of an exemplary product made using the material of the present invention;

Figure 3 is a schematic view illustrating the performance of one step of the method of the invention; and

Figure 4 is a schematic representation of apparatus usable in performing the method of the invention to produce the material of the invention.

Referring now to the drawings, the multiple layer material illustrated in Figure 1, and generally indicated with the reference numeral 11 comprises a substrate 12 which in this case may be a relatively cheap paper, even

recycled paper. Superimposed over one face of the substrate 12 is a barrier layer 13 which is bonded to the substrate without significantly penetrating the body of the material of the substrate such that the barrier material does not appear on the opposite face of the substrate. The barrier layer in this embodiment is a cured mixture of epoxide based oligomers and monomers with hydroxy compounds and photo initiators. As will be described in more detail below the mixture is applied in a thixotropic form, spread thinly over the surface of the substrate 12 so as to lie only on that surface, and then cured by irradiation with ultra-violet light. Such material has a very much greater temperature resistance than the prior art polyethylene used previously as a barrier layer, and may withstand temperatures of, for example, up to 220°C. The material can bond strongly to the substrate upon curing and remains only on one side of the substrate due to the thixotropic nature of the material upon application and the rapidity of curing. This makes it possible to create a very smooth flat surface which has considerable advantages as will be discussed below when the next layer, namely a silicone layer 14 is applied.

The silicone may be applied in the same way as the barrier layer using any of the known techniques for the application. However it is applied it is important to control the quantity and consistency of the silicone

layer, and at the same time to achieve a high speed of application. Because the surface of the barrier layer 13 is extremely smooth and uniform it is possible to coat this with an extremely thin layer of silicone whilst still achieving the same physical properties and achieving the same release function as conventional release liners having a silicone layer of usual thickness. Another advantage of this smoothness is that it will allow smaller quantities of adhesive to be applied than is conventional with, eg paper-based release liners.

In forming a product, which, as illustrated in Figure 2, may be a sheet of self-adhesive labels 16, the composite sheet material 11 illustrated in Figure 1 is first coated with an adhesive layer 15 and then treated to make the adhesive layer 15 tacky, for example by physical or chemical means. Products, or face stock 16 are then applied thereto in any known way. In Figure 2 there is shown an overlying sheet 17 of printed paper (face stock) which is preliminarily stamped with separation lines 16a defining individual labels 16. The entirety of the sheet 17 is applied to the composite layer 11, however, and upon such application the adhesive layer 15 adheres to the undersurface of the layer 17 to bond this to the composite layer 11. The product is then stable and may be packaged, shipped, stored and sold in this form. In use, when the combined layers 11 and 17 are flexed, for

example by bending down a corner 18 in the direction of the arrow A of Figure 2, to the position shown in the insert circle, a corner, such as the corner 19 of the nearest label 16 separates from the underlying layer at the junction between the adhesive and the silicone which allows ready release of the adhesive. The adhesive remains attached to the label 16 and this can then be peeled from the backing strip comprising the composite strip 11 and placed in position for its intended use where it will remain by the adhesion of the adhesive layer 15.

Of course, other products than labels may be provided with release liners in accordance with the principles of the present invention, including industrial components both of a rigid and flexible nature intended to be secured in place by adhesive and any other of the wide range of known applications for release liners. The material described not only has a high temperature resistance, but is also resistant to aggressive chemicals (such as those appearing in some industrial products such as bitumen sheets) and has a high resistance to dimensional variation, upon changes in humidity so that it can be used in outdoor situations even where moisture precipitation may be present.

In another embodiment (not illustrated) the barrier layer may be a mixture of oligomers and monomers of acrylic

acid with photo initiators and modified phenol . This mixture can also be made thixotropic and is curable by irradiation with ultra-violet light. Particular properties of this material include, as with the material specifically described above, a high resistance to dimensional variation even at relatively high temperatures, and a very good resistance to chemical attack and dimensional variation due to moisture or humidity changes. In experiments conducted with specimen materials it was established that shirlastain, an industrial test material, did not significantly penetrate the barrier layer formed from the above identified material .

Figure 3 illustrates an exemplary process for application of a thixotropic composition to a substrate. In Figure 3 a substrate 12 is passed between two rollers 20, 21 the upper roller 20 of which is a pressure roller, and the lower roller 21 of which is an applicator roller. The applicator roller 21 rolls in contact with a transfer roller 22 which in turn engages a pick-up roller 23 which is partly immersed in a bath of the thixotropic material 24 contained in a reservoir 25. A doctor blade 26 closely positioned against the transfer roller 22 ensures that a uniform film of material of the surface of the transfer roller 22, which is transferred to it from the pick up roller 23 as a result of its partial immersion in the bath 24, is transferred to the applicator roller 21.

Adjustments to the speed of rotation of the rollers and the proximity of the doctor blade 26 to the transfer roller 22 can adjust the thickness of the film of thixotropic material on the surface of the applicator roller 21 and therefore the thickness of the barrier layer 13 applied to the substrate 12 as it passes between the two rollers 20, 21.

Because it is applied by a smooth surface roller 21 the barrier layer 13 itself has a smooth surface which is enhanced by rapid cross linking caused by irradiation with ultra-violet light by an array of lamps 27, closely spaced along the path of the composite substrate/barrier layer web 12, 13.

Figure 4 illustrates apparatus including the applicator apparatus such as that illustrated in Figure 3 , and in which corresponding components are identified with the same reference numerals as in Figure 3. In this apparatus the web 12 is unwound from a roll 40 and turned at a pair of rollers 42 to a vertical path along which it travels past a barrier layer application station including application roller 21 and pressure roller 20 as described in relation to Figure 3. From there the composite web 12, 13, after having passed the ultra¬ violet lamps 27 passes between two rollers 43, 44 of a silicone application station drawing silicone from a bath

35 via a pick-up roller 45 and transfer roller 46 and applying it as a layer 14 by the application roller 43.

Thereafter the composite web 12, 13, 14 passes a further set of ultra-violet lamps 47. The composite web then passes between a pair of rollers 30, 31 to a loop 50 where the web is inverted with a twist (represented schematically) from which it passes to a pair of rollers 33, 34 at which it is turned to a downward vertical run with the coated face facing in the opposite direction, that is towards the previously described application stations. Along this latter run the composite web passes two further, similar, application stations at which a second barrier layer and a second silicone layer are applied to the opposite side of the substrate to provide double-sided release liner. The barrier layer and silicone layer applicator rollers and ultra-violet lamps are identified with the same reference numerals as for the ascending run, raised by 100. Finally, the double- sided release liner passes between two rollers 37, 38 to a take-up reel 51.