Printable bi-luminescent pigment for security ink formulation and process for the preparation thereof

Abstract

A new concept of bi-luminescent security pigments includes lanthanide doped rare-earth compound with rare-earth free compound and its ink formulation. The unique features of this bi-luminescent security ink is that it emits two different colors when it is illuminated by using two different excitation wavelengths. This important feature makes it most suitable for printing of security codes or QR codes/security images on currency, important official documents, food and medicinal packaging etc. The prospective use of this bi-luminescent security ink provides a ground-breaking opening for easily printable, highly stable and unclonable bi-luminescent security codes for anti-counterfeiting applications.

Claims

1. A process for the preparation of a bi-luminescent security pigment, the process comprising the steps of: i. preparing a solution of ZnCl.sub.2 and a solution of Na.sub.2S separately in water; ii. adding 1-3% by weight N-cetyl-N,N,N trimethylammonium bromide (CTAB) in the ZnCl.sub.2 solution as prepared in step (i) with continuous stirring at a rate in a range of 400 to 500 rpm fora period in the range of 50 to 70 minutes at a temperature in a range of 25 to 35° C. to obtain a solution; iii. dropwise adding Na.sub.2S solution as prepared in step (i) to the solution as obtained in step (ii) with constant stirring at a rate in a range of 400 to 500 rpm followed by centrifuging at a rate in the range of 5000 to 6000 rpm to obtain a milky white precipitate; iv. washing and drying the precipitate as obtained in step (iii) at a temperature in a range of 70 to 80° C. to obtain a ZnS powder; v. mixing the ZnS powder as obtained in step (iv) with CuCl.sub.2 followed by heating at a temperature in a range of 700 to 750° C. for a period in the range of 50 to 70 minutes to obtain Zn.sub.1-xS:Cu.sub.x wherein x is 0.01-0.03; vi. mixing Gd.sub.2O.sub.3, V.sub.2O.sub.5 and Eu.sub.2O.sub.3 with HNO.sub.3 to form a homogeneous mixture; vii. heating the mixture as obtained in step (vi) at a temperature in the range of 800 to 900° C. for a period in a range of 6 to 7 hours followed by cooling at a temperature in a range of 25 to 35° C. to obtain Gd.sub.1-yVO.sub.4Eu.sub.y wherein y is 0.29-0.39; viii. mixing Zn.sub.1-xS:Cu.sub.x as obtained in step (v) and Gd.sub.1-yVO.sub.4Eu.sub.y as obtained in step (vii) separately in ethanol to form Zn.sub.1-xS:Cu.sub.x and Gd.sub.1-yVO.sub.4Eu.sub.y slurries; and ix. mixing the slurries as obtained in step (viii) in a ratio ranging between 1.5:1 to 2:1 by volume followed by drying at a temperature in a range of 50 to 60° C. for a period in a range of 22 to 24 hours to obtain the bi-luminescent security pigment.

2. The process of claim 1, wherein mixing of the ZnS powder as obtained in step (iv) with CuCl.sub.2 in step (v) is carried out in an agate mortar.

3. The process of claim 1, wherein the ZnS obtained in step (iv) is in the form of a white powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 represents schematic diagram showing steps for the preparation of ZnS:Cu.sup.2+ phosphor.

(3) FIG. 2 represents schematic diagram showing steps for the preparation of GdVO.sub.4:Eu.sup.3+ phosphor.

(4) FIG. 3 represents schematic diagram showing steps for the preparation of bi-luminescent pigment.

(5) FIG. 4 represents X-ray powder diffraction (XRD) spectra of bi-luminescent pigment.

(6) FIG. 5 represents (a) Excitation spectrum of bi-luminescent pigment at emission 526 nm, (b) Emission spectrum of bi-luminescent pigment at excitation 338 nm, (c) Excitation spectrum of bi-luminescent pigment at emission 617 nm and (d) Emission spectrum of bi-luminescent pigment at excitation 316 nm.

(7) FIG. 6 represents (a) and (b) Time Resolved photoluminescence (TRPL) decay profile of bi-luminescent pigment recorded emission 526 nm and excitation wavelength of 338 nm and exponential fitting of decay profile and the parameters generated by the exponential fitting. (c) and (d) TRPL decay profile of bi-luminescent pigment recorded at emission 617 nm at an excitation wavelength of 316 nm and exponential fitting of decay profile parameters generated by the exponential fitting.

(8) FIG. 7 represents Scanning Electron Microscope (SEM) image of bi-luminescent pigment.

(9) FIG. 8 represents the schematic for screen printing technique.

(10) FIG. 9 represents demonstration of bi-luminescent security ink for anti-counterfeiting applications.

(11) FIG. 10 represents CIE color coordination for green and red emission.

DETAILED DESCRIPTION OF THE INVENTION

(12) Present invention provides a printable bi-luminescent pigment having high quantum yield for security ink formulation. However, several strategies like admixing of two different downshift materials together or augmented activator in a single host lattice were tried for the development of bi-luminescent ink but each suffered either from the luminescence quenching or low quantum yield because of the conversion of radiative transitions to non-radiative transitions due to coupling of multiple rare-earth activator ions. These drawbacks were overcome by the introduction of a totally new and innovative concept of using highly bi-luminescent materials by adopting the strategy of combinatory admixing of lanthanide doped rare-earth compounds with rare-earth free compounds easily available at low cost for ink formulation which has capability of dual mode excitations in UV wavelengths and emits red and green colours.

EXAMPLES

(13) Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1

Synthesis of Zn.SUB.0.97.S:Cu.SUB.0.03..SUP.2+ phosphor

(14) The synthesis of Zn.sub.0.97S:Cu.sub.0.03.sup.2+ phosphor is shown in schematic given in FIG. 1. Solutions of 1.36 gm ZnCl.sub.2 and 0.78 gm Na.sub.2S were prepared separately in water in a beaker. Further the capping agent CTAB (1% by weight) added to ZnCl.sub.2 solution with continuous stirring rate of 400 rpm for 1 hr at 30° C. Na.sub.2S was added drop wise in the solution. The solution was vigorously stirred under room temperature (27° C.) with constant stirring rate of 400 rpm for 1 hour for proper formation of milky white precipitates which were then collected by centrifugation at 5000 rpm. The former collected white precipitate were washed 2-3 times with water and was further transferred to an electrical oven to heat for 24 hour at 80° C. and the white powder of ZnS was formed.

(15) Further, the white powder of 0.940 gm ZnS was mixed with 0.051 gm CuCl.sub.2 in agate mortar by taking their stoichiometric amount according to empirical formula shown above which is further heated at a temperature 700° C. for 1 hour in box furnace to obtain Zn.sub.0.97S:Cu.sub.0.03.sup.2+ phosphor.

Example 2

Synthesis of Zn.SUB.0.98.S:Cu.SUB.0.02..SUP.2+ phosphor

(16) The synthesis of Zn.sub.0.98S:Cu.sub.0.02.sup.2+ phosphor is shown in schematic given in FIG. 1. Solutions of 6.81 gm ZnCl.sub.2 and 3.90 gm Na.sub.2S were prepared separately in water in a beaker. Further the capping agent CTAB (1% by weight) added to ZnCl.sub.2 solution with continuous stirring rate of 400 rpm for 1 hr at 30° C. Na.sub.2S was added drop wise the solution. The solution was vigorously stirred under room temperature (27° C.) with constant stirring rate of 400 rpm for 1 hour for proper formation of milky white precipitates which were then collected by centrifugation at 5000 rpm. The former collected white precipitate were washed 2-3 times with water and was further transferred to an electrical oven to heat for 24 hour at 80° C. and the white powder of ZnS was formed.

(17) Further, the white powder of 0.95 gm ZnS was mixed with 0.03 gm CuCl.sub.2 in agate mortar by taking their stoichiometric amount which is further heated at a temperature 700° C. for 1 hour in box furnace to obtain ZnS:Cu.sup.2+ phosphor.

Example 3

Synthesis of Zn.SUB.0.99.S:Cu.SUB.0.01..SUP.2+ phosphor

(18) The synthesis of Zn.sub.0.99S:Cu.sub.0.01.sup.2+ phosphor is shown in schematic given in FIG. 1. Solutions of 1.36 gm ZnCl.sub.2 and 0.78 gm Na.sub.2S were prepared separately in water in a beaker. Further the capping agent CTAB (1% by weight) added to ZnCl.sub.2 solution with continuous stirring rate of 400 rpm for 1 hr at 30° C. Na.sub.2S was added drop wise in the solution. The solution was vigorously stirred under room temperature (27° C.) with constant stirring rate of 400 rpm for 1 hour for proper formation of milky white precipitates which were then collected by centrifugation at 5000 rpm. The former collected white precipitate were washed 2-3 times with water and was further transferred to an electrical oven to heat for 24 hour at 80° C. and the white powder of ZnS was formed.

(19) Further, the white powder of 0.965 gm ZnS was mixed with 0.017 gm CuCl.sub.2 in agate mortar by taking their stoichiometric amount according to empirical formula shown above which is further heated at a temperature 700° C. for 1 hour in box furnace to obtain Zn.sub.0.99S:Cu.sub.0.01.sup.2+ phosphor.

Example 4

Synthesis of Gd.SUB.0.63.VO.SUB.4.:Eu.SUB.0.37..SUP.3+ phosphor

(20) The synthesis of GdVO.sub.4:Eu.sup.3− phosphor is show in FIG. 2. 1.17 gm Gd.sub.2O.sub.3, 0.91 gm V.sub.2O.sub.5 and 0.62 gm Eu.sub.2O.sub.3 were mixed thoroughly in agate mortar to form a homogeneous mixture, while mixing 0.5 ml of HNO.sub.3 was added. After proper mixing material was heated in a box furnace for 7 hour at 900° C. followed by natural cooling to obtain Gd.sub.0.63VO.sub.4:Eu.sub.0.37.sup.3+ phosphor.

Example 5

Synthesis of Gd.SUB.0.71.VO.SUB.4.:Eu.SUB.0.29..SUP.3+ phosphor

(21) The synthesis of GdVO.sub.4:Eu.sup.3− phosphor is show in FIG. 2. 1.28 gm Gd.sub.2O.sub.3, 0.91 gm V.sub.2O.sub.5 and 0.51 gm Eu.sub.2O.sub.3 were mixed thoroughly in agate mortar to form a homogeneous mixture, while mixing 0.5 ml of HNO.sub.3 was added. After proper mixing material was heated in a box furnace for 7 hour at 900° C. followed by natural cooling to obtain Gd.sub.0.71VO.sub.4:Eu.sub.0.29.sup.3+ phosphor.

Example 6

Synthesis of Gd.SUB.0.66.VO.SUB.4.:Eu.SUB.0.34..SUP.3+ phosphor

(22) The synthesis of GdVO.sub.4:Eu.sup.3− phosphor is show in FIG. 2. 1.19 gm Gd.sub.2O.sub.3, 0.91 gm V.sub.2O.sub.5 and 0.59 gm Eu.sub.2O.sub.3 were mixed thoroughly in agate mortar to form a homogeneous mixture, while mixing 0.5 ml of HNO.sub.3 was added. After proper mixing material was heated in a box furnace for 7 hour at 900° C. followed by natural cooling to obtain Gd.sub.0.66VO.sub.4:Eu.sub.0.34.sup.3+ phosphor.

Example 7

Synthesis of Gd.SUB.0.61.VO.SUB.4.:Eu.SUB.0.39..SUP.3+ phosphor

(23) The synthesis of GdVO.sub.4:Eu.sup.3− phosphor is show in FIG. 2. 1.10 gm Gd.sub.2O.sub.3, 0.91 gm V.sub.2O.sub.5 and 0.68 gm Eu.sub.2O.sub.3 were mixed thoroughly in agate mortar to form a homogeneous mixture, while mixing 0.5 ml of HNO.sub.3 was added. After proper mixing material was heated in a box furnace for 7 hour at 900° C. followed by natural cooling to obtain Gd.sub.0.61VO.sub.4:Eu.sub.0.39.sup.3+ phosphor.

Example 8

Synthesis of Bi-Luminescent Pigment

(24) The synthesis of bi-luminescent pigments is show in FIG. 3. Making slurry of pre-synthesized 10 mg Zn.sub.0.98S:Cu.sub.0.02.sup.2+ and 40 mg Gd.sub.0.63VO.sub.4:Eu.sub.0.37.sup.3+ (ratio of 1:4 by weight) in 10 ml ethanol. Dry for 24 hours at 60° C. and bi-luminescent pigment of Gd.sub.0.63VO.sub.4:EU.sub.0.37.sup.3+ @Zn.sub.0.98S:CU.sub.0.02.sup.2+ was formed.

Example 9

Synthesis of Bi-Luminescent Pigment

(25) The synthesis of bi-luminescent pigments is show in FIG. 3. Making slurry of pre-synthesized 15 gm Zn.sub.0.98S:Cu.sub.0.02.sup.2+ and 52.5 gm Gd.sub.0.63VO.sub.4:Eu.sub.0.37.sup.3+ (ratio of 1:3.5 by weight) in 25 ml ethanol. Dry for 24 hours at 55° C. and bi-luminescent pigment of Gd.sub.0.63VO.sub.4:Eu.sub.0.37.sup.3+ @Zn.sub.0.98S:Cu.sub.0.02.sup.2+ was formed.

Example 10

Synthesis of Bi-Luminescent Pigment

(26) The synthesis of bi-luminescent pigments is show in FIG. 3. Making slurry of pre-synthesized 15 gm Zn.sub.0.98S:Cu.sub.0.02.sup.2+ and 45 gm Gd.sub.0.63VO.sub.4:Eu.sub.0.37.sup.3+ (ratio of 1:3 by weight) in 30 ml ethanol. Dry for 24 hours at 60° C. and bi-luminescent pigment of Gd0.63VO4:Eu0.373+ @Zn0.98S:Cu0.022+ was formed.

Example 11

Characterization of Bi-Luminescent Pigment

(27) (i) X-ray diffraction (XRD) FIG. 4 shows the XRD pattern of the bi-luminescent pigment powder reveals the presence of cubic sphalerite phase of ZnS (JCPDS #80-0020) and cubic phase of GdVO.sub.4 crystals (JCPDS #74-1987). (ii) Photoluminescence (PL) spectroscopy FIGS. 5 and 6 represent the photoluminescence and time-resolved result of bi-luminescent pigment recorded by using an Edinburgh Instruments spectrometer, where a xenon lamp and flesh lamp act as the sources of excitations. The colour-coordinate were estimated from emission spectra of red and green emission, respectively. To estimate the absolute luminescence quantum efficiency of bi-luminescent pigment, an integrating sphere equipped with a spectrometer FLS900 (Edinburgh Instruments, UK) has been used for measuring the integrated fraction of the luminous flux and the radiant flux using the standard method. The estimated quantum efficiencies for green and red emissions are 68%, and 85%, respectively. In FIG. 10, CIE color coordination for green emission are x=0.29 and 0.68 and for red emission are x=0.64 and 0.33. The bi-luminescent pigment have broad excitation in the range of 234-350 nm centered at 316 nm which is originated from the charge transfer (CT) between O.sup.2−.fwdarw.Eu.sup.3+ and other excitations peaks at 362 nm, 395 nm and 466 nm are due to the f-f transitions within 4F.sup.6 electron shell of the Eu.sup.3+ ion. The bi-luminescent pigment have emission peaks at 592 nm, 607 nm, 613 nm 617 nm and 696 nm which is ascribed to the .sup.5D.sub.0-.sup.7F.sub.j (j=1, 2, 3) radiative transitions in Eu.sup.3+ ion. The emission peak ate 617 nm have highest emission intensity. (iii) Scanning electron microscope (SEM) The surface morphology of bi-luminescent pigment was examined by using field emission scanning electron microscope (FESEM) Carl ZEISS-SUPRA 40 VP. The surface morphology of bi-luminescent pigment is show in FIG. 7 and results reveal that the average particle size of the pigment is in the range of 1-5 μm.

Example 12

Bi-Luminescent Security Ink Formulation and Screen Printing Technique

(28) Polyvinyl chloride (PVC) gold medium was used to uniformly disperse the as-synthesized bi-luminescent pigment. Initially, 200 mg of bi-luminescent pigment was dispersed in 50 ml PVC gold medium while vigorous stirring with glass rod and then mixed ultrasonically at 45 kHz for 30 mins to obtain the ink. To print different patterns on black papers, a standard screen printing technique was used. The schematic for screen printing technique of bi-luminescent pigment is shown in FIG. 8.

(29) The FIG. 9 shows the photographs of printed pattern with bi-luminescent security ink which glows in prominent red and green colors when illuminated by two different excitation sources. Color figures are used to demonstrate bi-luminescent images.

ADVANTAGES OF THE INVENTION

(30) Cost effective & Environment friendly. Bi-luminescent security ink, excitable by two different wavelength sources, as a unique anti-counterfeiting feature. Invention provides an indigenous development at an industrial scale of bi-luminescent security pigments for ink formulation. The bi-luminescent security ink technology, with unique security feature in the currency notes, important documents, data etc. It is easily printable with commercial available screen printing technique.