Thermochromic ink composition

Abstract

A thermochromic ink composition includes, in a liquid medium, known as solvent, a dispersion of thermochromic microcapsules incorporating at least one leuco-dye and a thermoplastic binder selected from the group containing polyacrylic polyesters, polyurethanes and copolymers thereof. The composition is characterized in that the solvent is aprotic. A a method for producing a card and such a card incorporating at least one thermochromic pattern produced with a thermochromic ink composition are also described.

Claims

1. A thermochromic ink composition comprising in a liquid solvent: a dispersion of thermochromic microcapsules incorporating at least one leuco dye, and a thermoplastic binder chosen from the group formed of polyacrylic polyesters, polyurethanes, and copolymers thereof, wherein said solvent is aprotic and comprises at least one compound chosen from the group formed of n-alkyl propanoates, propylene glycols, 1-methoxy-2-propyl acetate, 1-propoxy-2-propanol, 1-propoxy-2-propyl acetate, 1-butoxy-2-propanol, 1-(2-methoxy-1-methylethoxy)-2-propanol, 1-(2-methoxy-1-methylethoxy)-2-propyl acetate, 1-(2-propoxy-1-methylethoxy)-2-propanol and [2-(2-methoxymethyl-ethoxy)methylethoxy]-2-propanol.

2. The composition as claimed in claim 1, wherein said solvent is chosen from the group of aprotic solvents having a dipole moment of from 0 to 1.9 C.Math.m.

3. The composition as claimed in claim 2, wherein said solvent is chosen from the group of polar aprotic solvents.

4. The composition as claimed in claim 2, comprising thermochromic microcapsules of melamine formaldehyde.

5. The composition as claimed in claim 2, comprising as binder a thermoplastic resin chosen from the group formed of alkyl poly(meth)acrylates and copolymers thereof.

6. The composition as claimed in claim 2, further comprising at least one anti-UV additive chosen from the group formed of ultraviolet absorbers and light stabilizers.

7. The composition as claimed in claim 1, wherein said solvent is chosen from the group of aprotic solvents having a dielectric constant of from 1.5 to 11.

8. The composition as claimed in claim 7, wherein said solvent is chosen from the group of polar aprotic solvents.

9. The composition as claimed in claim 7, comprising thermochromic microcapsules of melamine formaldehyde.

10. The composition as claimed in claim 7, further comprising at least one anti-UV additive chosen from the group formed of ultraviolet absorbers and light stabilizers.

11. The composition as claimed in claim 1, wherein said solvent is chosen from the group of polar aprotic solvents.

12. The composition as claimed in claim 1, comprising thermochromic microcapsules of melamine formaldehyde.

13. The composition as claimed in claim 1, comprising as binder a thermoplastic resin chosen from the group formed of alkyl poly(meth)acrylates and copolymers thereof.

14. The composition as claimed in claim 1, further comprising at least one anti-UV additive chosen from the group formed of ultraviolet absorbers and light stabilizers.

15. The composition as claimed in claim 1, comprising from 1 wt. % to 10 wt. % of an anti-UV additive.

16. The composition as claimed in claim 1, comprising in said solvent from 5 to 35 wt. % thermochromic microcapsules, from 30 to 45 wt. % thermoplastic binder, from 1 to 10 wt. % of at least one anti-UV additive.

17. The composition as claimed in claim 1, wherein said solvent is chosen from the group of aprotic solvents having a dielectric constant of from 1.5 to 11.

18. The composition as claimed in claim 1, wherein the n-alkyl propanoates are chosen from the group consisting of n-propyl propanoate, n-butyl propanoate, n-pentyl propanoate and ethyl 3-ethoxypropanoate.

19. A method for producing a card of thermoplastic material comprising at least one step of hot lamination under pressure of a plurality of layers of thermoplastic material, wherein, before the at least one step of hot lamination under pressure, at least one thermochromic motif is printed on at least one layer of thermoplastic material using a thermochromic ink composition comprising in a liquid solvent said solvent being aprotic: a dispersion of thermochromic microcapsules incorporating at least one leuco dye, and a thermoplastic binder chosen from the group formed of polyacrylic polyesters, polyurethanes, and copolymers thereof.

20. The method as claimed in claim 19, wherein the at least one thermochromic motif is printed so as to be interposed between a surface layer of the card and a layer immediately underlying said surface layer.

21. The method as claimed in claim 19, wherein the at least one thermochromic motif is printed on at least one layer of polycarbonate.

22. A card comprising a plurality of superposed layers of at least one thermoplastic material comprising at least one thermochromic motif formed by printing an ink composition comprising in a liquid solvent said solvent being aprotic: a dispersion of thermochromic microcapsules incorporating at least one leuco dye, and a thermoplastic binder chosen from the group formed of polyacrylic polyesters, polyurethanes, and copolymers thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The inventors have found that the use of an aprotic solvent in a thermochromic ink composition according to the invention allows thermochromic printing to be obtained which is subsequently compatible with hot lamination under pressure, which has a thermochromic effect which lasts over many years and is sufficiently reactive to be used through at least one thickness of thermoplastic material—especially polycarbonate—, which has excellent adhesion to thermoplastic materials, more particularly to rigid thermoplastic materials at ambient temperature—especially to polycarbonate—and does not impair the adhesion of layers of such plastics materials—especially of polycarbonate—when they are interposed between such layers. The invention also makes it possible to obtain thermochromic printing which is subsequently compatible with synthetic papers and security papers. No clear explanation can be given for this surprising phenomenon which has been found by the inventors. However, it is possible that the aprotic nature of the solvent, although the solvent is normally no longer present when the printing has cured (the solvent having been evaporated off), plays a part in the later resistance of the thermochromic microcapsules to hot lamination under pressure and/or in the working of the leuco dyes. More precisely, it is possible that the protic nature of the aqueous solvents used hitherto in the known thermochromic ink compositions has a considerable effect on the later resistance of the thermochromic microcapsules to hot lamination under pressure and/or on the working of the leuco dyes.

(2) Said aprotic solvent must also be chosen to be compatible with the microcapsules and with the binder, and to be compatible with the printing technique used. Advantageously and according to the invention, the aprotic solvent is chosen from the group of solvents of compositions that are printable by screen printing. Advantageously and according to the invention, said aprotic solvent has at least one of the following features: it is chosen from the group of aprotic solvents having a dipole moment of from 0 and 1.9 C.Math.m, it is chosen from the group of aprotic solvents having a dielectric constant (vacuum permittivity) of from 1.5 to 11, it is chosen from the group of polar aprotic solvents, it comprises at least one compound chosen from the group formed of n-alkyl propanoates and propylene glycols (solvents which are particularly compatible with thermochromic microcapsules of melamine formaldehyde).

(3) More particularly, in an advantageous embodiment a thermochromic ink composition according to the invention comprises as solvent a compound chosen from the group formed of n-propyl propanoate, n-butyl propanoate, n-pentyl propanoate, ethyl 3-ethoxypropanoate, 1-methoxy-2-propyl acetate, 1-propoxy-2-propanol, 1-propoxy-2-propyl acetate, 1-butoxy-2-propanol, 1-(2-methoxy-1-methylethoxy)-2-propanol, 1-(2-methoxy-1-methylethoxy)-2-propyl acetate, 1-(2-propoxy-1-methylethoxy)-2-propanol and [2-(2-methoxymethyl-ethoxy)methylethoxy]propanol. Other examples are possible.

(4) In addition, the thermoplastic binder is chosen in particular in dependence on the material serving as the print support for the thermochromic ink composition according to the invention. It is an advantage of the invention that it allows the above-mentioned problems to be solved with a large number of print supports which can be used to receive the thermochromic printing. Accordingly, a thermochromic ink composition according to the invention can be optimized for printing on a support chosen from the group of thermoplastic materials, synthetic papers and security papers. It is to be noted in this respect that a thermochromic ink composition according to the invention can be optimized for printing on a support that is sensitive to solvents, such as polycarbonate supports and security papers.

(5) The thermoplastic binder of a thermochromic ink composition according to the invention can also be optimized for printing on a support which is to be hot laminated under pressure (for example a laminate of thermoplastic layers forming a security and/or identification card such as a bank card).

(6) The thermoplastic binder of a thermochromic ink composition according to the invention can also be optimized to have a long lifetime, especially of several years, for example of at least ten years.

(7) In particular, by way of example, the thermoplastic binder of a thermochromic ink composition according to the invention can be optimized for printing on a support of a thermoplastic material chosen from the group consisting of polycarbonate, PVC and polyesters (polyethylene terephthalate PET, PETG). Other examples are possible.

(8) Advantageously, a thermochromic ink composition according to the invention which is intended more particularly for printing on a support of polycarbonate comprises as binder a thermoplastic resin chosen from the group formed of alkyl poly(meth)acrylates and copolymers thereof.

(9) In particular, by way of example, the thermoplastic binder of a thermochromic ink composition according to the invention can be optimized for printing on a support of synthetic paper chosen from the paper marketed under the trademark Teslin® by PPG, Monroeville, USA, the paper marketed under the trademark Neobond® by Neenah Lahnstein, Lahnstein, Germany, and the paper marketed under the trademark Polyart® by Arjobex, Boulogne, France. Other examples are possible.

(10) In particular, by way of example, the thermoplastic binder of a thermochromic ink composition according to the invention can be optimized for printing on a support of security paper, such as paper for passports, banknote paper, identity card paper, driving license paper, visa paper, birth certificate paper, customs seal paper, etc. Other examples are possible.

(11) In the security and/or identification applications of the invention, the thermoplastic binder of a thermochromic ink composition according to the invention can likewise advantageously be chosen so as to reveal attempts at forgery with solvents (chemical attack), and in particular a binder incorporating an agent that is soluble in the solvents of an adhesive (when the thermochromic ink composition is used to print a thermochromic motif on an adhesive card) as described, for example, by EP 1109675.

(12) In addition, another advantage of a thermochromic ink composition according to the invention is that various additives can be added to the solvent without interfering with the curing of the composition after printing.

(13) In particular, a thermochromic ink composition according to the invention advantageously further comprises at least one anti-UV additive chosen from the group formed of ultraviolet absorbers and light stabilizers (especially of the HALS (“hindered amine light stabilizers”) type). Such an ultraviolet absorber and/or light stabilizer of a thermochromic ink composition according to the invention is incorporated into said aprotic solvent and is in particular separate from any ultraviolet absorbers and/or light stabilizers incorporated into the thermochromic microcapsules. Advantageously, a thermochromic ink composition according to the invention comprises from 1 wt. % to 10 wt. %—especially from 2 wt. % to 5 wt. %—of an anti-UV additive.

(14) As the anti-UV additive of a thermochromic ink composition according to the invention there can be used any ultraviolet absorber and/or light stabilizer which is compatible with the other constituents of the composition. There may be mentioned by way of example the group of the 2-(hydroxyphenyl)benzotriazoles (see especially “Ultraviolet stabilizers of the 2-(hydroxyphenyl)benzotriazole class: influence of substituents on structure and spectra” J. Phys. Chem., 1992, 96 (25), pp. 10225-10234); the group of the 2-hydroxyphenyl-s-triazines (see especially U.S. Pat. No. 5,096,489); and the group of the 2,2,6,6-tetramethylpiperidines (TMP) (see especially “oxidation inhibition in organic materials” Jan Pospisil, Peter P. Klemchuk CRC Press, 1989). Advantageously and according to the invention, an ultraviolet absorber can be chosen from the group 2-(2-hydroxyphenyl)-benzotriazole (BTZ) and 2-hydroxyphenyl-s-triazine (HDT).

(15) In an embodiment which is advantageous and according to the invention, the thermochromic ink composition comprises in said solvent from 15 to 35 wt. % thermochromic microcapsules, from 30 to 45 wt. % thermoplastic binder, from 1 to 10 wt. % of at least one anti-UV additive.

(16) The invention extends to a method for producing a card of thermoplastic material—especially of polycarbonate—comprising at least one step of hot lamination under pressure of a plurality of layers of thermoplastic material—especially of polycarbonate—,

(17) wherein, before the at least one step of hot lamination under pressure, at least one thermochromic motif is printed on at least one layer of thermoplastic material—especially of polycarbonate—using a thermochromic ink composition according to the invention.

(18) The thermochromic motif can be of any kind: text(s), drawing(s), line(s), etc. It can also be solid print (continuous printing on at least one surface portion of a layer of the card).

(19) Advantageously and according to the invention, at least one thermochromic motif is printed using a thermochromic ink composition according to the invention so as to be interposed between a surface layer of the card and a layer immediately underlying said surface layer. To that end, at least one thermochromic motif is printed using a thermochromic ink composition according to the invention on a surface layer of the card, that is to say on a layer of thermoplastic material—especially of polycarbonate—one face of which constitutes a free face of the card after hot lamination under pressure. Such a thermochromic motif is advantageously printed on a face of said surface layer that is opposite the face of the layer that constitutes the outer free face of the card. In a variant or in combination, at least one thermochromic motif is printed using a thermochromic ink composition according to the invention on a layer immediately underlying a surface layer of the card, especially on a face of the underlying layer that comes into contact with said surface layer after hot lamination under pressure.

(20) The invention extends to a card comprising a plurality of superposed layers of at least one thermoplastic material—especially of polycarbonate—comprising at least one thermochromic motif formed by printing an ink composition according to the invention.

(21) The invention relates also to a thermochromic ink composition, a method for producing a card and a card which are characterized in combination by all or some of the features mentioned hereinabove or hereinbelow.

(22) Other objects, features and advantages of the invention will become apparent upon reading the following examples.

Example 1: Production of a Thermochromic Ink Composition

(23) A thermochromic ink composition is produced by dispersing 20 wt. % of thermochromic microcapsules in a liquid mixture formed of a polar aprotic solvent and a thermoplastic resin.

(24) The thermochromic ink composition comprises: 20 wt. % thermochromic pigments (microcapsules), 40 wt. % methyl methacrylate copolymer as transparent binder, 26 wt. % 1-(2-methoxy-1-methylethoxy)-2-propanol, 10 wt. % ethyl 3-ethoxypropanoate, 4 wt. % 2-hydroxyphenyl-s-triazine (ultraviolet absorber).

(25) The thermochromic microcapsules are microcapsules marketed under the trademark CHAMELEON® by Polychrom Co LTD, Ansan City, Korea.

(26) The ultraviolet absorber is marketed under reference TINUVIN® 400 by BASF, Ludwigshafen, Germany.

(27) The ink has a viscosity of 7.0 Pa.Math.s+/−1 Pa.Math.s.

(28) The ink can be printed using screen-printing screens of 120 threads/cm, permitting fine printing of the type with a matrix of 80 dpi.

Example 2: Production of a Card of Polycarbonate Incorporating Thermochromic Printing

(29) An ink composition obtained according to Example 1 is printed in the form of solid print on one of the faces of two sheets of transparent polycarbonate having a thickness of approximately 50 μm.

(30) The ink is dried with hot air.

(31) The printed sheets of polycarbonate are combined with five other sheets of polycarbonate which are interposed between the printed sheets of polycarbonate so as to form a stack, the printed faces being placed in contact with the underlying sheets of the stack, that is to say not constituting the outer faces thereof. The five sheets of polycarbonate which, with the two printed sheets, form said stack are, in succession, as follows: a 100 μm sheet of transparent polycarbonate; a 200 μm sheet of white polycarbonate; a 200 μm sheet of white polycarbonate; a 200 μm sheet of white polycarbonate; a 100 μm sheet of transparent polycarbonate.

(32) The assembly of the sheets forming the stack is laminated in a laminating press under the following conditions: increase to 180° C. for 1 minute 12 minutes at 180° C. under 30 N/cm.sup.2 5 minutes at 180° C. under 150 N/cm.sup.2 cooling at 26° C. for 17.5 minutes under 180 N/cm.sup.2.

(33) There is obtained a card whose total thickness is equal to the sum of the thicknesses of the sheets of polycarbonate used, reduced by about 10%.

(34) The thermochromic printing produced using the thermochromic ink according to the invention is thus incorporated into the thickness of the card.

Example 3: Adhesion and Durability Tests

(35) Durability tests are carried out on cards of polycarbonate as produced according to Example 2.

(36) The adhesion between the different layers of which the card is composed is evaluated by different test protocols: measurement of peeling according to standard ISO 10373—90° at 300 mm/minute, measurement of peeling after weathering according to standards ISO 24789-1 and 2, flexural test according to standards ISO 24789-1 and 2, 50,000 flexes lengthwise, 50,000 flexes widthwise, flexural test after weathering according to standards ISO 24789-1 and 2, flexural test after exposure to artificial light according to standards ISO 24789-1 and 2.

(37) The measurement of peeling shows that the adhesive force between the layers is greater than 3.5 N/cm.

(38) In the flexural tests, no delamination occurs in the region of the layers of polycarbonate surrounding the thermochromic printing.

Example 4: Light Fastness

(39) The light fastness of the cards of polycarbonate as produced according to Example 2 is evaluated by measuring the color difference (DE*.sub.Lab) of the thermochromic printing before and after exposure to artificial light. The color measurement is carried out when the thermochromic printing is in its first non-activated state, that is to say in the colored version of the thermochromic microcapsules.

(40) The color difference is measured using a spectro-colorimeter (Spectropen® (Dr. Lange, Düsseldorf, Germany)) according to standard ISO 7724-3.

(41) The thermochromic printing is exposed according to three test protocols: exposure to UVA: 20 h; UVA 340 nm; 60° C.; 0.72 W/m.sup.2/nm (standard ISO 4892), exposure to artificial light according to standard OACI 3.2—2006-paragraph 5-14, exposure to arc xenon light according to standard ISO 24789-1 and 2.

(42) The color difference DE is less than 10 in the case of exposure to UVA and less than 5 in the case of exposure to artificial light of the xenon arc type.

(43) It is found that the thermochromic activity of the exposed samples, with reversible thermochromic microcapsules which are colored in the non-activated state and transparent in the activated state in terms of the decoloration temperature range and the recoloration temperature range is identical to that of samples that have not been exposed.

Example 5: Resistance to Weathering

(44) The resistance to weathering of the cards of polycarbonate as produced according to Example 2 is evaluated according to the following test protocols: exposure to thermal cycles and to stressful storage conditions according to standard OACI 3.2-2006 paragraph 5.2 (“thermal cycling method”) and paragraph 5.3 (“storage temperature stress method”), exposure to high temperatures and humidity, thermal shock and thermal cycling according to standard ISO 24789-1 and 2, paragraph 5-7 (“temperature and humidity ageing”), paragraph 5-8 (“temperature shock”), paragraph 5-9 (“temperature and humidity cycling”).

(45) For the different test protocols. there is no loss of thermochromic activity, in terms of decoloration, change in the level of the decoloration range and the recoloration range.