CA1296894C - Anti-tack adhesive surface for thermal print elements - Google Patents

Abstract

- o -ANTI-TACK ADHESIVE SURFACE
FOR THERMAL PRINT ELEMENTS
Abstract of the Disclosure A thermal print element comprising a support having thereon a layer containing a thermally-transferred dye image, the element having at least one layer of adhesive thereon having an anti-tack surface, the anti-tack surface comprising at least about 0.2 g/m2 of particulate material, such as fumed silica, silica, alumina or polystyrene matte beads, having a particle size up to about 20 µm.
In a preferred embodiment, the adhesive layer comprises a linear, random copolyester of one or more aromatic dibasic acids and one or more aliphatic diols, modified with up to 30 mole % of one or more aliphatic dibasic acids, having a melt viscosity of between about 1,000 and about 20,000 poise at 150°C.
The adhesive is used to laminate a cover sheet to one or both surfaces of the thermal print element. The anti-tack surface prevents the adhesive on the cover sheet from sticking to another cover sheet having an adhesive layer thereon, prior to having a thermal transfer print inserted therebetween to form the laminate.

Description

lZ96894 ANTI-TACK ADHESIVE SURFACE
FOR THERMAL PRINT ELEMENTS
This invention relates to the use of certain anti-tack adhesive surfaces on cover sheets used to 5 laminate thermal print elements for protective and ~ security purposes.
In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color 10 video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into elec-trical signals. These signals are then operated on 15 to produce cyan, magenta and yellow electrical sig-nals. These signals are then transmitted ~o a ther-mal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted 20 between a thermal printing head and a platen roller.
A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and i8 heated up sequentially in response to the cyan, 25 magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are 30 contained in U.S. Patent No. 4,621,271 by Brownstein entitled "Apparatus and Method For Controlling A
Thermal Printer Apparatus," issued November 4, 1986.

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The use of the above process to produce identification (ID~ cards is described in U.S. Patent 4,738,949 issued April 19, 1988. It would be desirable to provide protection and additional 5 tamper-proofing of such ID cards.
Heat-lamination of a cover sheet of polymeric film, such as polycarbonate or a polyester such as poly(ethyleneterephthalate), over the entire surface and extended sides of an ID card is an 10 excellent way to increase protection from surface abrasion and to minimize tampering. Attempts to remove the cover sheet ideally should result in its destruction to prevent alteration and reuse.
U. S. Patent 4,713,365 issued December 15, 15 1987, relates to an adhesive, meltable at a controlled temperature, that adheres rapidly, firmly and uniformly, to the ID card receiver layer surface, card support stock, and a protective cover sheet.
That adhesive also does not adversely affect 20 dye-stability nor alter definition of the thermal dye-transfer image or any auxiliary information on the card.
A problem has developed with the use of such adhesives on a llbutterflyl' pouch used to laminate an 25 ID card. The "butterflyl' pouch is formed by laminating two sheets of adhesive-coated cover sheet together at one edge, the adhesive surfaces being face-to-face, to form a V-shaped larninate.
Alternatively, one piece of adhesive-coated cover 30 sheet could be used if folded in the middle.
There is a problem in that the adhesion of the adhesive-coated surfaces is so great that the pouch cannot be readily opened to insert the thermal transfer print. It would be desirable to find a way 35 to lower the adhesivity of the adhesive-coated surfaces prior to the insertion of the thermal transfer print, yet not destroy the adhesivity of the adhesive layers when they are heat-sealed together to form an integral laminate.
These and other ob;ects are achieved in accordance with this invention which comprlses a thermal print element comprising a support having thereon a layer containing a thermally-transferred dye imsge, the element having at least one layer of adhesive thereon having an anti-tack surface, the anti-tack surface comprising at least about 0.2 g/m of particulate material having a particle size up to about 20 ~m.
Any particulate material can be used in the invention provided it has a particle size up to about 20 ~m and performs the desired function. In general, good results have been obtained with fumed silica, silica, alumina or polystyrene matte beads.
In a preferred embodiment of the invention, the particulate material is fumed silica.
The particulate material can be present in any amount which is effective for the intended purpose. In general, good results have been obtained when the particulate material is present from about 0.2 to about 0.3 g/m .
Any adhesive used to laminate ID materials which has the tackiness problem described above would be useful in the invention. Such adhesives include polyvinyl acetates, polyalkyl-acrylates, polyalkyl-methacrylstes, polyalkyl-diacrylates, polyalkyl-dimethacrylates, polyvinyl chlorides, urea-formaldehydes, phenol-formaldehydes, polyurethanes, polyamides, polyimides, polysiloxanes, polysulfides, epoxy resins, natural rubbers, chloroprene rubbers, nitrile rubbers and other thermoplastic rubbers. In a preferred embodiment of the invention, the adhesive comprises a linear, random copolyester of one or more aromatic dibasic acids and one or more aliphatic diols, modified with ~4~,~`'' ' '' iZ96894 up to 30 mole % of one or more aliphatic dlbasic acids, the copolyester having a melt vi~co~ity of between about 1,000 and about 20,000 poise at 150 C.
In another preferred embodiment of the invention, the aromstic dibasic acid of the copolyester adhesive is terephthalic acid, isophthalic acid, dipicolinic acid or 2,2-bis(p-carboxyphenyl)propane. In another preferred embodiment, the aliphatic diol of the copolyester adhesive is ethyleneglycol, diethyleneglycol, triethyleneglycol, 1,4-butanediol, 1,2-propanediol, 1,3-propanediol, 1,6-hexanediol, cyclohexanedimethanol, or 1,4-cyclohexanediol. In yet another preferred embodiment, the aliphatic dibasic acid of the copolyester adhesive is sebacic acid, glutaric acid, adipic acid, azelaic acid, or 1,4-cyclohexanedicarboxylic acid.
The adhesive layer may be solvent-coated to the cover sheet or may be thermally-applied as a self-supporting layer to either or both sides of the thermal print element or the cover sheet. Thus, another embodiment of the invention comprises a laminate adapted to receive a thermal transfer print comprising two plastic film supports each having thereon an adhesive layer in face-to-face contact, at least one of the adhesive layers having an anti-tack surface so that the thermal transfer print can be readily inserted into the laminate, yet retains enough adhesivity so that the adhesive surfaces can be heat-sealed to the thermal transfer print to form an integral laminate.
Any thickness of adhesive may be used provided it provides a secure bond to the cover sheet. In general, good results have been obtained using thicknesses of from about 5 to about 100 ~m.
The layer containing the dye image employed in the invention may comprise, for example, a ~Z96894 polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene-co-acrylonitrile), poly(caprolactone) or mixtures thereof. The dye image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results have been obtained st a concentration of from about 1 to about 5 g/m .
In a preferred embodiment, a polycarbonate layer containing the dye image is used which has 8 number average molecular weight of at least about 25,000. The term "polycarbonate" as used herein means a polyester of carbonic acid and a glycol or an aromatic diol. Examples of such glycols or aromatic diols are p-xylene glycol, 2,2-bis(4-oxyphenyl)-propane, bis(4-oxyphenyl)methane, 1,1-bis(4--oxy-phenyl)ethane, l,l-bis(oxyphenyl)butane, l,l-bis(oxyphenyl)cyclohexane, 2,2-bis(oxy-phenyl)butane, etc.
In an especially preferred embodiment of the invention, the above-described polycarbonate is a bisphenol A polycarbonate. In another preferred embodiment of the invention, the bisphenol A
! polycarbonate comprises recurring units having the formula:

~ ~ _ ~ C(CH3)2--\~-{) C

wherein n is from about 100 to about 500.
Examples of such polycarbonates include:
General Electric Lexan~ Polycarbonate Resin #ML-4735 (Number average molecular weight app.
36,000), and Bayer AG, Makrolon #5705~ (Number ~ average molecular weight app. 58,000).
;~ 35 As noted above, the sdhesives of the invention are used to laminate a cover sheet to one ~; or both surfaces of the thermal print element. There .,`~
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lZ96894 can be used as the cover sheet, for example, varlous polymeric transparent films such as poly(ethylene terephthalate), polycarbonate, polystyrene, polyethylene, cellulose acetate, poly(vinyl alcohol-co-acetal), etc.
Specific copolyesters useful as the adheslve in the invention include the following materials:

Compound 1): a random copolyester formed from 1,4-butanediol; 1,6-hexanediol; and terephthalic acid (mole ratio of diols: 80% C6, 20% C4) Compound 2): a random copolyester formed from 1,4-butanediol; 1,6-hexanediol; terephthalic acid;
and isophthalic acid (mole ratio of diols: 65% C6, 35% C4; mole ratio of acids: 90% terephthalic, 10%
isophthalic) Compound 3): a random copolyester formed from diethyleneglycol; 1,4-butanediol; terephthalic acid;
and glutaric acld (mole ratio of diols: 55~ C4, 45%
glycol; mole ratio of acids: 70% terephthalic, 30%
glutaric) Compound 4): a blend of a random copolyester formed from 3) and a random copolyester formed from cyclohexanedimethanol; ethyleneglycol; and terephthalic acid (mole ratio of diols: 69% C2, 31%
cyclohexanedimethanol) Compound 5): a random copolyester formed from 1,4-butanediol; 1,6-hexanediol; terephthalic acid;
and isophthalic acid (mole ratio of diols: 80% C6, 20% C4; mole ratio of acids: 80% terephthalic, 20%
isophthalic) lZ96894 ~ ` `

Compound 6): a random copolyester formed from 1,4-butanediol; sebacic acid; terephthalic acid; and isophthalic acid (mole ratio of acids: 15% sebacic, 35% terephthalic, 50% isophthalic) sold commercially 5 as Bostik 7962TM (Bostik Chemical Group, Emhart Corp.).
; The support for the thermal print element of the invention may be a transparent film such as a poly(ether sulfone), a polyimide, a cellulose ester 10 such as cellulose acetate, a poly(yinyl alcohol-co-acetal) or a poly(ethylene terephthalate). The sup-port may also be reflective such as baryta-coated paper, polyethylene-coated paper, white polyester (polyester with white pigment incorporated therein), 15 an ivory paper, a condenser paper or a synthetic paper such as duPont Tyvek~. In a preferred embodiment, polyester with a white pigment incorporated therein is employed. It may be employed ~ at any thickness desired, usually from about 50 ~m ; 20 to about 1000 ~m.
; A dye-donor element that iB used to form the : thermal print element of the invention comprises a support having thereon a dye layer. Any dye can be used in such a layer provided it is transferable to 25 the dye image-receiving layer of the dye-receiving element by the action of heat to provide the thermal print. Especially good results have been obtained : with sublimable dyes. Examples of sublimable dyes include anthraquinone dyes, e.g., Sumikalon Violet 30 RS~ (product of Sumitomo Chemical Co., Ltd.), Dianix Fast Violet 3R-FS~ (product of Mitsubishi Chemical Industries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM~ and KST Black 146~
(products of Nippon Kayaku Co., Ltd.); azo dyes such ~`~ 35 as Kayalon Polyol Brilliant Blue BM~, Kayalon Polyol Dark~Blue 2BM~, and KST Black KR3 (products of Nippon Kayaku Co., Ltd.), Sumickaron Diazo Black 5Go (product of Sumitomo Chemical Co., ~

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Ltd.), and Miktazol Black 5GH~ (product of Mitsui Toatsu Chemicals, Inc.); direct dyes such as Direct Dark Green B~ (product of Mitsubishi Chemical Industries, Ltd.) and Direct Brown M~ and Direct Fast Black D~ (products of Nippon Kayaku Co. Ltd.);
acid dyes such as Kayanol Milling Cyanine 5R~ (pro-duct of Nippon Kayaku Co. Ltd.); basic dyes such as Sumicacryl Blue 6G~ (product of Sumitomo Chemical Co., Ltd.), and Aizen Malachite Green~ (product of Hodogaya Chemical Co., Ltd.);

N\ /~-N=N~ --N(C3H7)2 (magenta) CN C~3/ ~ / \CH33 (yellow) o ~.\ ~!~ /CONHCH3 ~ O O (cyan) i--~ ~ ---N ( C2H5 ) 2 or any of the dyes disclosed in U.S. Patent 4,541,830. The above dyes may be employed singly or in combination to obtain a monochrome. The dyes may be used at a coverage of from about 0.05 to about 1 g/m and are preferably hydrophobic.
The dye in the dye-donor element is dis-persed in a polymeric binder such as a cellulose . .
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derivative, e.g., cellulose acetate hydrogen phthal-ate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate; a polycarbonate; poly(styrene-co-acrylonitrile), a poly(sulfone) or a poly(phenylene oxide). The binder may be used at a coverage of from about 0.1 to about 5 glm .
The dye layer of the dye-donor element may be coated on the support or printed thereon by a printing technique such as a gravure process.
Any material can be used as the support for the dye-donor element provided it is dimensionally stable and can withstand the heat of the thermal printing heads. Such materials include polyesters such as poly(ethylene terephthalate); polyamides;
polycarbonates; glassine paper; condenser paper;
cellulose esters such as cellulose acetate; fluorine polymers such as polyvinylidene fluoride or poly-(tetrafluoroethylene-co-hexafluoropropylene); poly-ethers such as polyoxymethylene; polyacetals; poly-olefins such as polystyrene, polyethylene, poly-propylene or methylpentane polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from about 2 to about 30 ~m. It may also be coated with a subbing layer, if desired.
A dye-barrier layer comprising a hydrophilic polymer may also be employed in the dye-donor element between its support and the dye layer which provides improved dye transfer densities. Such dye-barrier layer materials include those described and claimed in U.S. Patent 4,700,208 issued October 13, 1987.
The reverse side of the dye-donor element may be coated with a slipping layer to prevent the ~' :` ~ f ;;:
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lZ96894 printing head from sticking to the dye-donor ele-ment. Such a slipping layer would comprise a lubricating material such as a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder.
Preferred lubricating materials include oils or semi-crystalline organic solids that melt below 100C
such as poly(vinyl stearate), beeswax, perfluorinated alkyl ester polyethers, poly(caprolactone), carbowax or poly(ethylene glycols). Suitable polymeric binders for the slipping layer include poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal), poly(styrene), poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate or ethyl cellulose.
The amount of the lubricating material to be used in the slipping layer depends largely on the type of lubricating material, but is generally in the range of about .001 to about 2 g/m . If a poly-meric binder is employed, the lubricating material is present in the range of 0.1 to 50 weight %, prefer-ably 0.5 to 40, of the polymeric binder employed.
As noted above, dye-donor elements are used to form a dye transfer image in the thermal print.
Such a process comprises imagewise-heating a dye-donor element and transferring a dye image to a dye---receiving element as described above to form the dye transfer image in a thermal print element.
The dye-donor element employed with the thermal print elements of the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have only one dye thereon or may have alternating areas of different dyes such as cyan, magenta, yellow, black, etc., as disclosed in U. S. Patent 4,541,830.
In a preferred embodiment of the invention, a dye-donor element is employed which comprises a poly(ethylene terephthalate) support coated with ~296894 sequentlal repeating areas of cyan, ma8enta and yel-low dye, and the above process steps are sequentlally performed for each color to obtain a three-color dye transfer image. Of course, when the process is only performed for a single color, then a monochrome dye transfer image is obtained.
Thermal printing heads which can be used to transfer dye from the dye-donor elements employed in the invention are available commercially. There can be employed, for example, a Fu~itsu Thermal Head (FTP-040 MCSOOl), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3.
The following example is provided to illustrate the invention.

ExamPle A cover sheet laminate was prepared by coating 175 ~m thick poly(ethylene terephthalate) with Compound 6, Bostik 7962~ polyester adhesive degcribed above, (0.076 g/m2) from dichloro-methane. Before the adhesive layer was dry, a suspension of Cabogrip IIA~ (Cabot Corp.), supplied as a 20% aqueous dispersion of colloidal (<1 ~m) fumed silica, was coated at the level indicated in the Table over the adhesive layer. The fumed silica was coated from a methanol and water mixture without a binder and is thus a "wash overcoat" rather than a discrete overcoat.
Testing for tackiness was done by first clarifying the coated adhesive by heating with a hot air gun for approximately three seconds. This removes the crystallinity by melting and brings the adhesive to its tackiest state. Two clarified sheets were then laminated, adhesive-face to adhesive-face, at room temperature with rollers. The level of tack between the two sheets was then estimated using an Instron Universal Testing Machine Model TM-1122. A
one-inch wide sample was cut from the composite and :

12968g4 formed into a T-peel ~oint for testing at R peel rate of 20 inches/min. The results obtained are tabulated in the Table.
The adhesion testing was done by laminating two cover sheets, adhesive-face to adhesive-face in a Kodak Readyprint Laminator (ad~usted so that the adhesive reached lO0 C). The laminated sheets were allowed to come to room temperature and the adhesion was measured using the same test as described above.
In practice, an imaged thermal transfer print would be placed between the two cover sheets. The following results were obtained:

Table Fumed Silica Tack Adhesion (glm2) (g/in.) (g/in.) 0 >2000 >5000 .01 >2000 >5000 05 >2000 >5000 .10 >2000 >5000 .20 <100 >5000 .30 <lO0 >5000 .40 <100 >1000 .50 <lO0 >1000 .60 <100 >1000 The above results show that at least about 0.2 g/m of the anti-tack agent is required in order to reduce the tackiness from >2000 to <lO0 g/in. as measured by the Instron Testing Machine.
The adhesion between sheets is also reduced at levels greater than 0.4 g/m .
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and qcope of the invention.

Claims (20)

1. A thermal print element comprising a support having thereon a layer containing a thermally-transferred dye image, said element having at least one layer of adhesive thereon having an anti-tack surface, said anti-tack surface comprising at least about 0.2 g/m2 Of particulate material having a particle size up to about 20 µm.

2. The element of Claim 1 wherein said particulate material is fumed silica, silica, alumina or polystyrene matte beads.

3. The element of Claim 1 wherein said particulate material is fumed silica.

4. The element of Claim 1 wherein said particulate material is present from about 0.2 to about 0.3 g/m2.

5. The element of Claim 1 wherein said adhesive comprises a linear, random copolyester of one or more aromatic dibasic acids and one or more aliphatic diols, modified with up to about 30 mole %
of one or more aliphatic dibasic acids, said copolyester having a melt viscosity of between about 1,000 and about 20,000 poise at 150 C.

6. The element of Claim 5 wherein said aromatic dibasic acid is terephthalic acid, isophthalic acid, dipicolinic acid or 2,2-bis(p-carboxyphenyl)propane.

7. The element of Claim 5 wherein said aliphatic diol is ethyleneglycol, diethyleneglycol, triethyleneglycol, 1,4-butanediol, 1,2-propanediol, 1,3-propanediol, 1,6-hexanediol, cyclohexane-dimethanol, or 1,4-cyclohexanediol.

8. The element of Claim 5 wherein said aliphatic dibasic acid is sebacic acid, glutaric acid, adipic acid, azelaic acid, or 1,4-cyclohexanedicarboxylic acid.

9. The element of Claim 5 wherein said copolyester is formed from 1,4-butanediol, sebacic acid, terephthalic acid, and isophthalic acid.

10. The element of Claim 1 wherein said layer containing said dye image is a polycarbonate having a number average molecular weight of at least about 25,000.

11. The element of Claim 10 wherein said polycarbonate is a bisphenol A polycarbonate comprising recurring units having the formula:

wherein n is from about 100 to about 500.

12. The element of Claim 1 wherein said support is poly(ethylene terephthalate) having a white pigment incorporated therein.

13. The element of Claim 1 which has a cover sheet laminated to at least one outer surface by said adhesive.

14. The element of Claim 13 wherein said cover sheet is poly(ethylene terephthalate).

15. The element of Claim 1 which has a poly(ethylene terephthalate) cover sheet laminated to both surfaces thereof by said adhesive.

16. A laminate adapted to receive a thermal transfer print comprising two plastic film supports each having thereon an adhesive layer in face-to-face contact, at least one of said adhesive layers having an anti-tack surface so that said thermal transfer print can be readily inserted into said laminate, yet retains enough adhesivity so that said adhesive surfaces can be heat-sealed to said thermal transfer print to form an integral laminate.

17. The laminate of Claim 16 wherein said anti-tack surface comprises particulate material having a particle size up to about 20 µm.

18. The laminate of Claim 17 wherein said particulate material is fumed silica, silica, alumina or polystyrene matte beads.

19. The laminate of Claim 18 wherein said particulate material is present from about 0.2 to about 0.3 g/m2 .

20. The laminate of Claim 16 wherein said adhesive comprises a linear, random copolyester of one or more aromatic dibasic acids and one or more aliphatic diols, modified with up to 30 mole % of one or more aliphatic dibasic acids, said copolyester having a melt viscosity of between about 1,000 and about 20,000 poise at 150°C.