Cards

Details Cards Cards

1999-05-14

2001-03-06

Hess, Bruce H. (Department: 1774)

Record receiver having plural interactive leaves or a colorless

Having plural interactive leaves

C156S235000, C427S152000, C428S195100, C428S480000, C428S500000, C428S913000, C428S914000

Reexamination Certificate

active

06197726

ABSTRACT:

The invention relates to secure cards having images formed by thermal transfer printing on at least one side, and especially to thermally transferable protective topcoats for securing such images.
Thermal transfer 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 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 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 thermal transfer printing 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 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.
The transfer mechanism is believed to depend very much on the conditions under which printing is carried out. Thus for example, when using a thermal head, the dyesheet and receiver are pressed together between the head and a platen roller, giving conditions favouring diffusion of the dyes from the dyesheet directly into the receiver, virtually precluding any sublimation. Where a small gap is provided between the dyesheet and receiver, as favoured in some laser driven printers for example, the transfer mechanism appears to be exclusively sublimation. However, in both cases the dyes are mobile molecules which can diffuse into and out of the receiver when warmed, or in the presence of various lyophilic liquids. In particular, grease from a finger holding a print can lead to migration of the dye to the surface, making the print seem dirty or causing smearing of the dyes, and plasticisers in plastic pouches can cause havoc with unprotected thermal transfer images. Particularly bad in this respect is dioctylphthalate, commonly used as a plasticiser in polyvinyl chloride.
For many years various protective covers have been proposed to protect thermal transfer prints against abrasion, loss of dyes by migration to the surface, and protection against UV-induced fading, for example. Very thin covers are generally preferred, typically 4 &mgr;m, which are difficult to handle without some form of support, and in the past it has been proposed first to prepare a donor sheet comprising a temporary carrier base sheet having a surface coated with a layer of transparent thermally transferable cover material, then thermally transferring the coating onto the printed receiver and removing the carrier, thereby leaving the transferred material to form a topcoat. 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 topcoat line by line, and it is then generally more convenient to remove the carrier progressively as it emerges from the rolls or thermal head.
It has been recognized that polymeric compositions having higher Tg values generally provide better protective coatings, but higher Tg values can lose some of the advantages of the lower Tg materials. Thus for example, good barrier materials of high Tg are not always good adhesives, and to overcome this problem, complex coatings consisting of a plurality of layers of differing functions have previously been proposed. Thus for example, multilayer polymeric coatings comprising a layer of barrier material, laminated to a layer of more adhesive material on one side for providing better adhesion to the receiver, and on the other a layer of a less adhesive material to assist in its release from the carrier, has been described in U.S. Pat. No. 4,977,136.
Because a thermal transfer image corresponds to the electronic signal fed to the thermal head, laser printer or other thermal transfer driving means, each image can be readily customized as required, and this has been made use of in producing wallet size cards with personalized images. These include, for example, credit cards, driving licenses and identification cards, all of which can have images incorporating electronic photographs, signatures and/or personal data to provide a card unique to the user. Such cards are frequently carried in plastic pouches, but plasticisers in the pouches are a particular problem because they are generally good solvents for thermal transfer dyes. A heavily plasticised PVC pouch, for example, can extract virtually all the colour from an unprotected image, and it has become the custom to protect such images with a thermally transferred polymer topcoat, typically of a polymethyl methacrylate based formulation, usually containing a small loading of filler.
The topcoat makes the card more secure by giving the image some degree of protection against abrasion and attack by plasticisers, and cards having such protective topcoats are referred to herein as secure cards, to distinguish them from cards having no topcoat.
However, presently used topcoats only provide a degree of protection. We have seen many examples of cards showing severe fading of the image with use, particularly in the more heavily printed areas. After microscopic examination of the failing cards, we believe we have found a cause for such failure, and provide herein a means for improving the useful lifespan of protected cards. Thus we found that when the above known cards are flexed, e.g. by subjecting them to unconstrained hand bending without permanent deformation, microscopic cracks formed in the topcoat over both the heavily printed areas and lightly primed areas.
According to one aspect of the invention, a method for manufacturing secure cards, each consisting essentially of a card base and a topcoat, comprises forming a thermal transfer image in a dye-receptive surface of the card base, and thermally transferring the topcoat onto the image-containing surface; wherein to improve protection against plasticiser degradation of the thermal transfer image, the topcoat comprises at least one barrier layer which is formed of a polymer composition having a Tg>70° C., and which is resistant to the formation of microscopic cracks in the topcoat under tensile bending that is insufficient to cause macroscopic permanent deformation.
A preferred method is one wherein the level of tensile bending is that achieved by supporting the ends of the secure card, flexing the card to displace by 2 cm the portion the card equidistant from its supported ends, and repeating to complete 100 such displacements; and wherein the microscopic cracks are of a size to be visible when viewed at a magnification of 400×.
In practice, this may be achieved by preparing a plurality of sample secure cards of which each is topcoated with a different barrier layer composition, flexing each card as above, selecting a thus flexed sample card for which no cracks were evident in the surface of the topcoat, and carrying out the manufacture of secure cards using a topcoat composition corresponding to that used in the selected sample. In more detail, we prefer to carry out these steps as below.
Sample Card Preparati

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