WO1998007573A1

WO1998007573A1 - Thermal transfer printing receiver sheet

Info

Publication number: WO1998007573A1
Application number: PCT/GB1997/002186
Authority: WO
Inventors: Ian Stephenson; Gary John Jenno; Christopher Bennett
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1996-08-16
Filing date: 1997-08-14
Publication date: 1998-02-26
Anticipated expiration: 1999-02-16
Also published as: JP2001505493A; EP0918644A1; GB9617237D0

Abstract

A thermal transfer printing receiver sheet comprising a substrate, a receiver coat on one surface of the substrate and between the substrate and the receiver coat, a subbing layer comprising an amorphous polyester and a poly vinylchloride/poly vinylacetate copolymer, the ratio of polyester to copolymer being preferably within the range 400:1 to 100:1. A laminate assembly comprising a support, a subbing layer comprising an amorphous polyester and a poly vinylchloride/poly vinylacetate copolymer on the support, a dye diffusion thermal transfer print on said subbing layer, 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.

Description

Thermal Transfer Printing Receiver Sheet

This invention relates to thermal transfer printing and more particularly to receiver sheets for use in such printing

Dye diffusion thermal transfer printing (hereinafter referred to as 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 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 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

Receiver sheets conventionally comprise a substrate with a dye receiving surface on one side into which a dye ts thermally transferable and retainable The dye-receiving surface may be provided by one side of the substrate. However, receiver sheets typically comprise a substrate supporting a receiver layer which layer presents a dye-receiving surface The receiver layer typically comprises a dye-receptive polymer such as an amorphous polyester and generally contains a release agent to prevent the dye sheet adhering to the receiver layer

Substrates conventionally employed in thermal transfer printing include thermoplastic films, for example polyethylene terephthalate, and laminated paper substrates

Receiver sheets bearing a D2T2 image have found wide usage in security applications, for example documents such as credit cards, charge cards, identification cards, driving licences and passports The security of such documents can be enhanced by lamination to a cover sheet to make it difficult to substitute the image Security can be further enhanced by ensuring that, even if separation does occur, neither the image or the cover sheet can be re-used This can be achieved by controlling the adhesion such that the plane of failure is image (iβ receiver layer) from substrate resulting in the destruction of the image and at least part of the image remaining adhered to the cover sheet

According to one aspect of the invention, there is provided a thermal transfer printing receiver sheet comprising a substrate, a receiver coat on one surface of the substrate and between the substrate and the receiver coat, a subbing layer comprising an amorphous polyester and a poly vinylchloride/poly vinylacetate copolymer Preferably, the ratio of polyester to copolymer is within the range 400 1 to 100 1

Surprisingly it has been found that within this range, the force needed to separate the receiver layer from the substrate remains substantially constant at a value of between 15 and 20 N Outside this range , the force decreases and increases sharply and is either too low or so high that there is a risk that the plane of failure will be between the cover sheet and the image surface This is particularly so in the case of passports where the temperature used to laminate the cover sheet to the image is relatively low, iβ 100°C

It is well known that the dyes used in D2T2 printing 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 material from which cover sheets are made can cause havoc with unprotected D2T2 images Particularly bad in this respect is dioctylphthalate, used as a plasticiser in polyvinyl chloride from which such cover sheets are commonly made It is also well known to protect thermal transfer prints against such effects by means of a protective barrier layer Clearly, to provide the required plane of failure.the adhesion between the image and such a barrier layer must be greater than the adhesion between the receiver layer and the substrate Barrier layers which are particularly suitable for use with the receiver sheet of the invention are described in Applicant's co-pending applications nos UK 9617285 and 9617286 The invention will be more readily understood from the following example Exam le

A series of receiver samples were prepared by coating a white Melinex ® D935 polyester substrate with 1 μm thick subcoats made up of blends of Vylon® 200 amorphous polyester and VAGH poly vinylchloride/poly vinylacetate copolymer at ratios between 100% Vylon 200 through to 80% Vylon 200 20% VAGH Coating was carried out from a 4% solids solution in ethyl methyl ketone Each sample was then coated with a 4 8μm thick receiver layer based on Vylon 200 containing a silicone release agent (Tergomer HSi 231 1 ) crosslinked by Cymel 303 Standard test images were then prepared using a Mitsubishi CP15EP thermal printer and the resultant images were laminated to 30mm strips of cover sheet at a temperature of 99 to 104 °C and subjected to a 180° peel test on a 6021 Instron strain gauge at peel rates of 0 5, 5 and 10 mm/sec The results are shown in the graph

To ascertain the visible degree of tamper evidence produced, further samples were manually peeled and examined by eye It was found that samples giving satisfactory performance contained amounts of VAGH lying between the vertical lines in the graph

Ratio of Vytoπ 200 to VAGH in a 4% T/S subcoat solution.

Claims

1 A thermal transfer printing receiver sheet comprising a substrate, a receiver coat on one surface of the substrate and between the substrate and the receiver coat, a subbing layer comprising an amorphous polyester and a poly vinylchloride/poly vinylacetate copolymer 2 A thermal transfer printing receiver sheet according to claim 1 , in which the ratio of polyester to copolymer is within the range 400 1 to 100 1

3 A thermal transfer printing receiver sheet according to claim 1 , in which the force needed to separate the receiver layer from the substrate is substantially constant at a value of between 15 and 20 N 4 A laminate assembly comprising a support, a subbing layer comprising an amorphous polyester and a poly vinylchloride/poly vinylacetate copolymer on the support, a dye diffusion thermal transfer print on said subbing layer , 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