Technical field

The invention relates to a method of manufacturing laminate having at least two layers of thermoplastic materials, with optical diffractive element incorporated between them; the method comprises in covering the carrying layer of the thermoplastic material with a layer having different refraction index from that of the carrying and/or cover layer of the thermoplastic material and applied on the carrying layer. The invention also pertains to laminate having at least two layers of thermoplastic materials with optical diffractive element incorporated between these layers.

Prior Art

There exist various laminates, comprising of at least two layers of thermoplastics, that are transparent at least in some places, and with a diffractive optical element between these layers. The diffractive optical element serves for protection against falsification. The examples are identification cards, credit/debit cards, passports etc.

Incorporation of the diffractive optical element between two layers of thermoplastic materials protects the element against mechanical damage, destruction by chemical agents, moisture and also eliminates the influence of UV radiation.

At first, diffractive optical element is placed on the surface of the thermoplastic foil which is subsequently placed between other thermoplastic foils and the final products will be made by lamination of the foils under increased temperature.

There are several options of how to place the diffractive optical element on the surface of thermoplastic foil. For instance, by hot stamping process, where a thin polymer foil, having thickness of several microns, is applied on the thermoplastic foil surface carrying the diffractive optical element.

Another option is placing/sticking-in a thin carrying foil, typically having thickness of several microns, on which a polymer layer, having thickness of several microns and containing diffractive optical element is applied. However, both methods above require both sides of polymer layers incorporated or applied in any other manner to be treated with an adhesive suitable for subsequent lamination with thermoplastic material the final structure is made of or the material of these incorporated or applied layers should be easy to laminate with other thermoplastic layers in the course of final lamination.

Generally, lamination of thermoplastic structure is conducted at temperatures up to 200°C. The materials of the final laminate must not be affected in terms of strength, colours etc. after completion of the lamination process. Therefore, selection of suitable materials used for production of protective elements is limited. This is why some manufacturers use protection elements made of materials resistant to the conditions of lamination (e.g. metal) for their thermoplastic products; at the same time, protective element structure is not compact, i.e. is made with holes or channels. In the course of final structure lamination, melted thermoplastic material flows through these holes in the protective element, hence the entire structure is good fixed within the thermoplastic product, without additional adhesives. This procedure is described, for instance, in EP2331323A, WO2008/060918, US4313984, FR2638120 and US 3802101. For increased safety, protective diffractive optical elements can be produced in such a manner that the entire protective element surface is made of small segments placed so close to each other that visual impact of the entire protection element is not significantly affected and, at the same time, the space between the segments provides channels for thermoplastic material flow during the lamination process. In this way, the final protective element is appropriately fixed within the final structure and cannot be taken off, replaced or handled without being damaged.

Protective element of this type is described in EP2393653A. According to this description, the entire pattern of diffractive protective element is imprinted and/or embossed in one layer of the thermoplastic foil, and then only small surfaces of individual segments are covered with fixing material (e.g. metal). Consequently, diffraction structure of the protective element is only preserved on small surfaces of individual segments, with thermoplastic material flowing through spaces between the segments, thus providing for fixation of the protective element.

However, diffraction structure of the protective element is damaged in spaces between the segments, thus reducing optical value of the protective element, particularly its brightness and sharp contours. The contours of the protective element are more or less visually affected by spaces between segments, which also can adversely affect size of protective element details and, hence, place increased requirements on visual observation quality, e.g. more intensive light needed. The size and position of spaces between the segments must also be considered in composition of the protective element so that its image can be easily identified.

The visual deterioration effect can be suppressed by reduction of the space between the segments, however, this effect cannot be entirely eliminated, due to protective element segmentation.

The objective of this invention is design of manufacturing procedure of the thermoplastic laminate with diffractive optical element eliminates the drawbacks of existing technology. Disclosure of the Invention

The objective specified above is achieved by a method to manufacture the laminate having at least two layers of thermoplastic materials, with optical diffractive element incorporated between them. The method comprises in covering surface of the carrying layer of thermoplastic material with a layer which refraction index differs from that of the carrying/cover layer of the thermoplastic material, thus covering the carrying layer, according to the invention, which nature comprises in putting a layer with different refraction index only in some places of the carrying layer, subsequent imprinting and/or embossing a stamp with optical diffractive element in the layer with different refraction index, putting a cover layer of thermoplastic material on the layer with different refraction index and, finally, heating the material to a temperature allowing cover layer and the carrying layer being bonded together.

In this application, the layer which refraction index differs from that of the carrying layer and/or the cover layer of the thermoplastic material is designated as a "layer with different refraction index". After imprinting and/or embossing the stamp, this layer with different refraction index provides the diffractive optical element.

The advantage of this invention is elimination of the requirement to put ready- made diffractive optical element between carrying and cover layer, as this element could be damaged by high temperature necessary for subsequent bonding of both layers (i.e. carrying and cover layer) and also improvement of visual quality of the diffractive optical element, despite segmentation of its surface.

In order to achieve perfect fixation of stamped diffractive optical element, it is good if optical diffractive relief on the stamp imprinted and/or embossed in the layer with a different refraction index is, at least in some places, high enough to produce micro-fissures in the layer with a different refraction index by imprinting and/or embossing the stamp. Another advantageous embodiment is one having the layer with a different refraction index perforated (at least in some places) in accordance with the relief of the stamp by imprinting and/or embossing the stamp with optical diffraction relief, thus allowing perfect fixation of the final optical diffraction element.

Another advantageous embodiment is one having a part of the material with different refraction index detached from the layer with different refraction index and stamped optical diffraction relief by imprinting and/or embossing the stamp with optical diffraction relief, and this detached material of the layer with different refraction index embossed deeper into the carrying layer.

Height segmentation of the resulting diffractive optical element cannot be seen with the naked eye after finalization of the lamination process, however, multi-level structure of diffractive optical elements of this type is of advantage, since it can be distinguished on the final product when looking at it closely, so that it can be utilized as an additional protection element.

Another advantageous embodiment is one with stamping the layer with a different refraction index being performed in several steps, using several different stamps.

The objective as specified above can also be achieved by production of the laminate comprising of at least two thermoplastic layers, with intermediate layer having different refraction index from that of the carrying and/or the support layer of thermoplastic material, the layer with different refraction index constituting diffractive optical element in accordance with the invention, which nature comprises in the layer with different refraction index having micro-fissures and/or perforations and/or detached layer resulting from imprinting and/or embossing of at least one stamp with optical diffractive relief, thus enabling thermoplastic material of the cover layer to flow through and provide for bonding cover layer and carrying layer. Brief Description of Drawings

Figures 1 up 3 show individual steps of production of the laminate according to the invention. Fig. 4 shows imprinting and/or embossing of the stamp arranged for perforation of the layer with different refraction index. Fig. 5 shows imprinting and/or embossing of the stamp, arranged for making parts with a different refraction index that are detached and stepped. Fig. 6 shows an example of the laminate produced in accordance with the invention.

Description of Preferred Embodiments

Figures 1 , 2 and 3 show individual steps in production of the final laminate comprising of two layers 1 , 5 of thermoplastic materials with diffractive optical element fixed between them.

At first, layer 2, which refraction index differs from that of layer 1 and/or layer 5 of thermoplastic material, will be put on pre-selected places of layer 1 , as shown in Fig. 1 (for reasons of simplification, only one of these places is shown in Fig. 1). In the application, this layer is designated as "layer 2 with a different refraction index".

Layer 2 with different refraction index is created by selective deposition of thin layers of materials having different refraction index from that of thermoplastic laminate foils, such as metals (silver, aluminium, copper, chrome, gold etc.) metal alloys or non-metal materials (ZnS, ΤΊΟ2 etc.).

Then, stamp 4 with optical diffraction relief in the places of layer 2 with a different refraction index 6 will be imprinted and/or embossed in the layer 2 with different refraction index and in the carrying layer 1. By imprinting and/or embossing the stamp 4, its relief will be transferred to layer 2 with different refraction index (see Fig.2), thus creating diffractive optical element which profile corresponds with that of the optical diffraction relief 6 on the lower side of the stamp 4. In the next stage, carrying layer 1 with configured layers 2 having different refraction index will be covered with cover layer 5 of thermoplastic material and resulting laminate will be heated to 190°C, thus bonding cover layer 5 of thermoplastic material with carrying layer 1 of thermoplastic material.

According to an advantageous embodiment, such a stamp 4 will be used, which optical diffractive relief 6 is, at least in some places, high enough to produce micro-fissures 3 (see Fig. 2) by imprinting and/or embossing the stamp 4 in layer 2 with different refraction index. Cover layer 5 being in place and the entire laminate structure being heated up, micro-fissures 3 allow thermoplastic material flowing through and bonding cover layer 5 with carrying layer 1 (see Fig. 3).

According to another embodiment, such a stamp 4 will be used, which optical diffractive relief 6 has, at least in some places, recesses to produce perforations 8 in layer 2 with a different refraction index, thus enabling easier bonding of thermoplastic material of cover layer 5 with material of the carrying layer 1 (see Fig.4). According to another embodiment, such a stamp 4 will be used, which optical diffractive relief 6 is appropriately shaped to detach part 7 of material of layer 2 having different refraction index with imprinted and/or embossed optical diffraction relief from layer 2 with different refraction index at some places of layer 2 and to emboss this part 7 of the material of layer 2 deeper into carrying layer 1 (see Fig. 5). Owing to different heights of layer 2 with different index of diffraction, constituting the resulting optical diffractive relief, thermoplastic material of the cover layer 5 and the carrying layer 1 (see Fig. 6) flows through during lamination. Height segmentation of the resulting diffractive optical element cannot be seen with the naked eye after finalization of the lamination process. Moreover, the surfaces being at different heights after lamination, will come slightly closer together, thus producing optical homogenization of the resulting holographic image, which is not optically segmented, thus avoiding visible fragmentation of the image of diffractive optical element. Consequently, maximum visual quality of individual diffractive optical elements fixed at selected points between the carrying surface 1 and the cover surface 5 remains unaffected. Nevertheless, multi-level structure of diffractive optical elements can be distinguished when looking at it closely, hence it can be utilized as an additional protection element. This procedure is of advantage, for instance, in manufacturing polycarbonate data pages of passports.

The experts know that processing of layer 2, having a different refraction index, can be made using one stamp 4 in one step or using several different stamps 4 in several steps. Several stamp and several steps allow, particularly, making perforations 8 in layer 2 with a different refraction index or detached, as well as embossing parts 7 of the material of layer 2 with different refraction index deeper into carrying layer 1. Example of the laminate produced according to the method as described above is shown in Fig. 6. This laminate consist of at least two layers 1 , 5 of thermoplastic materials with intermediate layer 2 with a different refraction index, which layer 2 creates diffractive optical element. Layer 2 with different refraction index has micro-fissures 3, perforations 8 and detached part 7 by imprinting and/or embossing shaped stamp 4 with optical diffractive relief 6. Micro-fissures 3, perforation 8 and detached part 7 allowed thermoplastic material of layer 5 to flow through, perfectly bonding thermoplastic materials of cover layer 5 and carrying layer 1. Industrial applicability

Manufacturing procedure of the laminate in accordance with the invention as well as the resulting laminate structure can be used for production of various identification cards with protective elements provided as diffractive optical elements, credit and debit cards, bank notes, polycarbonate data pages of passports and more.