This invention relates to protective overlays for printed matter and in particular to such overlays for use with printed matter produced by dye diffusion thermal transfer printing 5 (hereinafter referred to as D2T2 printing)

D2T2 printing is a process in which one or more thermally transferable dyes are caused to transfer from selected areas of a dyesheet to a receiver by thermal stimuli, thereby to form an image Using a dyesheet comprising a thin substrate supporting a dyecoat containing one or more uniformly spread dyes, printing is effected by heating selected discrete o areas of the dyesheet while the dyecoat is pressed against a dye-receptive surface of a receiver sheet, thereby causing dye to transfer to corresponding areas of the receiver The shape of the image transferred is determined by the number and locations of the discrete areas which are subjected to heating Full colour prints can be produced by printing with different coloured dyecoats sequentially in like manner, and the different coloured dyecoats 5 are usually provided as discrete uniform panels arranged in a repeated sequence along a ribbon-shaped dyesheet High resolution photograph-like prints can be produced by using appropriate printing equipment, such as a programmable thermal print head or laser printer, controlled by electronic signals derived from a video, computer, electronic still camera, or similar signal generating apparatus A typical thermal print head has a row of tiny selectively 0 energizable heaters, spaced to print six or more pixels per millimetre, often with two heaters per pixel Laser printers require absorbers to convert the laser radiation to heat, usually in or under the dyecoat, and similarly produce the print by transferring dyes to the receiver pixel by pixel

Official documents such as passports conventionally contain a photograph of the 5 holder and it is becoming common practice to laminate a cover sheet on to the page of the passport on which the photograph is mounted in order to make it difficult to substitute one photograph for another, iβ to render the passport tamper proof The cover sheet consists of a transparent film having a layer of melt adhesive on one side and lamination is carried out under heat and pressure In order not to damage the passport itself lamination temperatures 0 of around 100° C are generally used and the adhesive must be such as to give sufficient strength of adhesion at such relatively low lamination temperature to prevent the cover sheet being removed in such a way that the photograph can be substituted Although D2T2 generated prints are particularly suitable as an alternative to the conventional silver halide prints, the low temperature adhesive used can damage the print particularly by causing the

dyes making up the image to migrate out of position and/or bleed into the cover sheet Moreover, it is a requirement that passport images must be capable of withstanding a temperature of 80° C without deterioration and dye migration/bleeding are exacerbated at such a temperature Although cover sheets which are compatible with D2T2 prints are known, they do not meet the specifications laid down by passport authorities for various reasons including being too expensive

In order for D2 T2 prints to be used in passports, it is necessary, therefore, to provide a barrier layer over the surface of the image In addition to preventing image degradation at the specified temperature, such a layer must also adhere strongly to both the surface of the image and the adhesive layer of the cover sheet

Although the situation outlined above is particularly relevant to passports, the problem applies to any laminate assembly in which a D2T2 image is overlain by a cover sheet made of materials which can cause image degradation Thus in addition to passports and other documents containing a personal image such as driving licences and security cards, the assembly may comprise a D2T2 portrait or landscape image mounted for display purposes According to one aspect of the present invention, there is provided a laminate assembly comprising a support, a dye diffusion thermal transfer print superimposed on said support , a barrier overlay on the surface of the print, the overlay comprising a polymeric material having a Tg of between 67 and 85°C, and a cover sheet superimposed on said support so as to overlie the print

The Tg of a polymer is that temperature at which the polymer undergoes transition from a glassy to a rubbery state or alternatively the temperature above which segmented chain movement within the polymer chains can take place In this specification such temperature is measured by Differential Scanning Caloπmetry (DSC) According to a preferred feature, the polymeric material of the barrier overlay is an amorphous polyester containing an alicyclic diol or dicarboxylic acid residue A particularly preferred polyester is Vylon GK-640 (available from Toyobo) which is believed to be a copolymer of terephthalic acid, isophthalic acid, ethylene glycol and 1 ,4-cyclohexylenedιmethanol It is desirable, and in the case of some passport authorities essential, that the image reaches certain standards of light fastness, particularly fastness to uv light

Such light fastness can be achieved, according to a preferred aspect of the invention, by the incorporation in the barrier overlay of a material capable of absorbing and/or scattering uv light Organic uv absorbers such as benzophenones and benzotπazoles are

suitable but a preferred material is zinc oxide or surface stabilised titanium dioxide as disclosed in UK Application No 9617285 3

The barrier overlay is preferably 2- 5 μm in thickness, which causes problems in handling without some form of support A donor sheet is preferably formed comprising a temporary carrier base sheet on which the barrier overlay is coated, the base sheet being removed after the overlay has been transferred to the printed receiver The transfer can be effected simultaneously over the whole print, and the carrier is then removed after the transfer is complete Alternatively, transfer may be progressive, e g using heated rollers or a thermal head to transfer the overlay line by line, and it is then generally more convenient to remove the carrier progressively as it emerges from the rolls or thermal head

The carrier sheet must be formed of material able to withstand the transfer temperatures Paper can be used, but the thicker the sheet, the more transfer energy is required, and polymer films, such as PET film, typically less than 30 μm thick according to the manner in which the barrier composition is to be transferred is preferred A carrier sheet of about 12 μm thickness is suitable when using a hot roller laminator unit, but a heat-resistant back-coated film of 5-7 μm thickness is preferred when using a thermal head To assist in release of the cover material from a thermoplastic carrier sheet, the latter may be primed with a cross-linked resin, to prevent fusion between the carrier and the transferring overlay Such primes, applied effectively in known manner, remain on the carrier as it is stripped off Other coatings featuring one or more of the many known release agents or releasing binders, can be provided instead or in addition to the cross-linked prime, but with such materials there is a chance that at least some will transfer with the cover material This can be undesirable in a number of applications, especially those requiring lamination of the print to a security cover sheet, in the passports, driving licences, medical cards and security passes referred to above, for example In general, therefore, the transferable overlay is placed directly onto the primed surface of the carrier base sheet The overlay can be separate from the dyesheet used to prepare the image, although it is often convenient to have this packaged in a form which enables it to be used in the same apparatus as that which prints the image To have the dyesheet ribbon and the overlay as separate entities, whether used in the same apparatus or not, enables a first printed image to be covered with overlay while a further image is being formed, thereby saving time However, a preferred overlay is one which is incorporated into a dyesheet ribbon, suitably that used to form the image, comprising a substrate supporting different coloured dyecoats provided as discrete uniform print-size panels arranged in a repeated sequence along the ribbon, the carrier sheet of the overlay

being provided by a part of the dyesheet substrate between repeated sequences of the dyecoat panels Thus each sequence of print-size coloured dyecoats also has a further print-size panel of the thermally transferable overlay

The invention is illustrated by the following example Example 1

A dye sheet was prepared by coating 6μm thick polyester film with magenta, cyan and yellow dyecoat panels in repeating sequence in conventional manner Between each sequence, the film was coated from a 20% solids solution of Vylon GK640 in ethyl methyl ketone to form a barrier overlay panel The Tg of the Vylon GK640 is 79°C The dyesheet and a receiver sheet coated with a 4 8μm thick receiver layer based on

Vylon 200 containing a silicone release agent (Tergomer HSi 231 1 ) crossslinked by Cymel 303 were passed through a Mitsubishi CP15EP thermal printer to produce an imaged receiver having a barrier overlay thereon A 30 mm wide strip of cover sheet material as used by the British Passport Office was laminated to the image at 99 - 104 °C and the assembly subjected to a 180° peel test on a 6021 Instron strain gauge at a peel rate of 5 mm/s

The peel strength was measured at 29 4N In contrast, a similarly prepared sample using a material having a Tg of 105°C (Dynapol L912) only had a peel strength of 6 3 N

Example 2 Laminated samples prepared as in Example land a further sample prepared in the same way but using a material for the barrier layer having a Tg of 72° (Vylon 290) were placed in an oven at 80°C for 96 hours After this time, the images were assessed visually for the extent of dye bleed in terms of reduction in the optical density and blurring of the image caused bilateral spreading of the dye The samples based on Vylon GK640 and Dynapol L912 were completely satisfactory but the sample based on Vylon 290 wasjust acceptable having a slight blurring of the image