Abstract:
A method of marking a polymer-based laminate including a core layer, a cover layer on at least one side of said core layer and an adhesive layer acting between the core layer and the cover layer is provided. The core, adhesive and cover layers have different radiation transmission coefficients. The method comprises the step of selectively irradiating the laminate with laser radiation having a fluence sufficient to mark the adhesive layer at selected locations while maintaining the cover layer intact.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of marking polymer-based laminates used during the production of documents such as for example identification cards, credit cards and banknotes. 
     BACKGROUND OF THE INVENTION 
     Identification cards, credit cards etc. as well as some banknotes are often formed of polymer-based laminates. Marking these cards and banknotes is common practice to deter forgery and allow counterfeit cards and banknotes to be readily detected. Ideally, these markings are placed within the laminates without damaging their skins so that security features do not have to be incorporated into the manufacturing process of the polymer. Many techniques have been devised to mark cards using laser radiation to satisfy the above. 
     For example, U.S. Pat. No. 4,507,346 to Maurer et al discloses a multi-layer identification card and method of making the same. The identification card includes an inlay on which a thermosensitive coating is applied. A synthetic layer is disposed on the inlay and on the thermosensitive coating and includes a blowing agent. A laser beam recorder is used to activate the blowing agent and the thermosensitive coating to personalize the identification card. 
     U.S. Pat. No. 4,597,592 to Maurer et al discloses a multi-layer identification card with duplicate data. The identification card includes a backing, an opaque middle layer and a cover film. Characters are burned primarily in the opaque middle layer using a laser. 
     U.S. Pat. Nos. 4,544,181 and 4.579,754 to Maurer et al disclose a multi-layer identification card including an inner opaque layer surrounded by upper and lower cover sheets. A photograph and two data areas are provided on the inner opaque layer. A magnetic strip is laminated on the under surface of the lower cover sheet. The inner opaque layer may be coated with a thermosensitive dye. Alphanumeric indicia is printed on the inner opaque layer by burning or blackening the inner opaque layer using a laser beam. 
     As will be appreciated, since the above described techniques use lasers to mark cards, the marking techniques are non-contact, fast and environmentally safe due to the fact that no consumables are involved in the marking process. However, improvements to marking techniques are continually being sought. 
     It is therefore an object of the present invention to provide a novel apparatus and method of marking a polymer-based laminate. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a method of marking a polymer-based laminate including a core layer, a cover layer on at least one side of said core layer and an adhesive layer acting between said core layer and said cover layer, said core, adhesive and cover layers having different radiation transmission coefficients, said method comprising the step of: 
     selectively irradiating said laminate with laser radiation having a fluence sufficient to mark said adhesive layer at selected locations while maintaining said cover layer intact. 
     Preferably, the fluence is in the range of from about 0.06 mJ/cm 2  to 0.1 2 mJ/cm 2 . During the step of irradiating, it is preferred that the selected areas of the cover layer are irradiated with at least one pulse of laser radiation and that the laser radiation is patterned prior to irradiating the cover layer. It is also preferred that the spatial distribution of the laser radiation is adjusted prior to irradiating the cover layer. 
     According to another aspect of the present invention there is provided an apparatus for marking a polymer-based laminate comprising: 
     a source of laser radiation to generate a beam of laser radiation having a fluence in the range of from about 0.06 mJ/cm 2  to 0.12 mJ/cm 2 ; 
     a beam adjuster to adjust the spatial distribution of said beam; and 
     a focusing lens to focus said adjusted beam onto a polymer-based laminate to be marked. 
     Preferably, the beam adjuster includes a beam homogenizer and a field lens. It is also preferred that the source is an Excimer laser and wherein the beam of laser radiation has a wavelength equal to about 248 nm. 
     According to yet another aspect of the present invention there is provided a polymer-based laminate comprising: 
     a core; 
     a cover surrounding said core; and 
     adhesive acting between said core and said cover, wherein said core, adhesive and cover have different radiation transmission coefficients and wherein said adhesive includes at least one marking thereon resulting from exposure of said laminate to laser radiation having a fluence sufficient to mark said adhesive while maintaining said cover intact. 
     The present invention allows polymer-based laminates used to form valuable documents, to be marked in a secure, non-contact manner after the polymer manufacturing process. This is achieved by marking the adhesive acting between the core and the skin of the laminate using laser radiation. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described more fully with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a polymer-based laminate to be marked using laser radiation; 
     FIG. 2 is a schematic diagram of an apparatus for marking polymer-based laminates in accordance with the present invention; 
     FIG. 3 is a graph showing the spectra of various polymer-based laminates exposed to Excimer laser radiation; 
     FIG. 4 is a table showing threshold and damage fluences for some of the polymer-based laminates illustrated in FIG. 3; 
     FIG. 5 illustrates a polymer-based laminate marked with bar codes using the apparatus of FIG. 2; and 
     FIG. 6 illustrates a polymer-based laminate marked with lettering using the apparatus of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to an apparatus and method for marking a polymer-based laminate used to form documents such as for example, identification cards, credit cards etc. or banknotes. The marking process is performed after the polymer-based laminate is formed so that security features do not have to be incorporated into the manufacturing process of the polymer. 
     In the present embodiment, the polymer-based laminate  10  to be marked is used to form banknotes and includes a transparent film core  12  formed of oreinted polypropylene (OPP) and an outer skin or cover layer  14  formed of high-density polyethylene surrounding the core (see FIG.  1 ). The skin is bonded to the core by high bond strength adhesive  16 . The core  12 , skin  14  and adhesive layer  16  have different radiation transmission coefficients. The films used in the laminate  10  provide high tensile strength and deadfold characteristics. The thickness of the laminate is generally in the range of from about 80 to 110 μm. To facilitate printing, the core  12  and/or skin  14  can be coated with opacifiers and print caps. Antistats and friction reducers can also be provided on the outer surface of the laminate to improve handling. 
     To mark the polymer-based laminate, the polymer-based laminate is exposed to laser radiation R which marks the adhesive layer  16  between the skin  14  and core  12  without damaging the skin  14 . This of course allows security features to be incorporated within the polymer-based laminate  10  after the polymer manufacturing process and without compromising the surface of the laminate. 
     Turning to FIG. 2, an apparatus  18  to mark the polymer-based laminate via pattern imaging is shown. As can be seen, the apparatus  18  includes an Excimer laser  20  generating a beam of laser radiation  22  having a wavelength equal to 248 nm and a fluence in the range of from about 0.06 mJ/cm 2  and 0.12 mJ/cm 2 . A pair of mirrors  24  and  26  redirect the beam of laser radiation  22  to a beam profile adjuster  28  including a beam homogenizer  30  and a field lens  32 . The beam of laser radiation  22  exiting the beam profile adjuster  28  passes through a mask  34  to a focusing lens  36  before impinging on the polymer-based laminate  10  to be marked. In this particular embodiment, the focusing lens  36  is a doublet lens with a focal length equal to approximately 100 mm. 
     The beam profile adjuster  28  reduces variations in the spatial intensity distribution of the beam of laser radiation  22  to produce a generally uniform energy distribution in the beam. The beam homogenizer  30  includes four cylindrical lenses, two in the vertical plane and two in the horizontal plane. The lenses are arranged to focus energy from the outer edges of the beam profile back towards the central region of the beam profile in a manner so that the outer edges of the beam profile become comparable to the central region in intensity. The beam of laser radiation  22  exiting the beam homogenizer  30  is directed towards the field lens  32  to improve the coupling of the apparatus  18  and reduce aberrations. Specifically, the field lens  32  creates a top hat distribution at the plane of the mask  34  and collimates the beam of laser radiation  22  so that basically all of the light exiting the mask  34  passes through the focusing lens  36 . By patterning the laser radiation, the laser radiation irradiates the polymer-based laminate at selected locations to mark the polymer-based laminate as desired. 
     Tests using the apparatus of FIG. 2 and a mask  34  of the letter “E” were performed on samples of polymer-based laminates. In this particular example, the polymer-based laminates included a three ply clear lamination with an inner core  12  made of oriented polypropylene of the type sold under the name Bicor LBW by Mobil Films and having a thickness between about 25 or 30 microns. The core provided the desired tensile strength and deadfold characteristics. The core  12  was sandwiched between two outer high density polyethylene cover films  14  of the type sold under the name Hicor 115 THD by Mobil Films and having a thickness of about 30 microns. The outer cover films also provided good deadfold characteristics. 
     The two outer cover films  14  were adhered to the core  12  via a 100% solids adhesives  16 , made for example by the Fuller Co. The outer cover films  14  were coated with two layers of an opacifying coating to make the laminate opaque and two layers of print caps to make the surface printable by all the standard banknote printing methods. Antistats and coefficient of friction reducers were used to improve handling. The laminate  10  was ready to print and did not require an overlacquer to protect the printing. The adhesive  16  was either clear, silver or gold in appearance as a result of an image printed on the core  12  using metallic inks. 
     FIG. 3 shows the spectra of the polymer-based laminates tested. Threshold fluence and damage fluence were also determined for each of the samples. Threshold fluence represents the fluence where the letter is barely legible and indicates the lower energy limit to achieve marking of the polymer-based laminates. Damage fluence represents the fluence where the laminate begins to bubble. FIG. 4 shows the relevant threshold levels for marking and damage. As can be seen, the fluence thresholds for the various tested samples are similar and are generally in the range of from about 0.06 mJ/cm 2  to 0.12 mJ/cm 2 . 
     FIGS. 5 and 6 show polymer-based laminates marked using the apparatus of FIG.  2 . As can be seen in FIG. 5, the adhesive  16  of the polymer-based laminate was marked with bar codes having a length of 150 microns. The bar codes are spaced apart a distance equal to 75 microns. As will be appreciated, these bar codes can be read with machine vision. 
     FIG. 6 shows images of a clear polymer-based laminate having its adhesive marked with letters. The images were taken with enhanced contrast using phase contrast techniques. Thus, in true visual contrast, the letters appear as being more opaque than the unmarked laminate. The letters are 2 mm in height. 
     To enhance contrast between the markings and the unmarked laminate, photosensitive dyes compatible with the polymers can be incorporated into the polymer-based laminate during the manufacturing process. As is well known, the photosensitive dyes undergo a color change when exposed to laser radiation to enhance the color contrast of the markings and the unmarked laminate. 
     As will be appreciated, since the fluence required to mark the polymer-based laminate is in the range of from about 0.06 mJ/cm 2  to 0.12 mJ/cm 2 , which is low, it is easily attained allowing marks to be made in the adhesive layer of the polymer based laminate in a single laser pulse. The polymer-based laminate can be exposed to multiple pulses to enhance marking contrast; however care must be taken to avoid over exposure which may result in bubbling of the laminate. 
     Although a particular polymer-based laminate has been described those of skill in the art will appreciate that other film types can be used to form the polymer-based laminates provided the tensile strength and deadfold characteristics are maintained. Also, although the adhesive has been described as being a 100% solids adhesive, other adhesives such as UV curable and solvent based adhesives can be used. 
     Those of skill in the art will also appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.