[0001] Users are increasingly demanding functionalities beyond merely recognizing a touch to the surface of a touch-sensitive device. Such other functionalities include handwriting recognition and direct note taking (using, for example, a stylus). Such functionalities are generally provided in so-called digitizing systems. Digitizing systems may have position-dependent indicia detected by an image sensor in a stylus, such as an electro-optical pen or other image reader, where the position-dependent indicia include an ink.

BRIEF DESCRIPTION OF THE DRAWING

[0002] Features of the present disclosure are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which:

[0003] FIG. 1 is a cross-sectional view illustrating an example reflective hot mirror stack pigment;

[0004] FIG. 2 is a cross-sectional view illustrating another example reflective hot mirror stack pigment;

[0005] FIG. 3 is a line graph of the reflectance of an example reflective hot mirror stack pigment;

[0006] FIG. 4 is a cross-sectional view illustrating another example reflective hot mirror stack pigment;

[0007] FIG. 5 is a flow chart of an example of a method of making a reflective hot mirror stack pigment; and

[0008] FIG. 6 is a flow chart of an example method of making a printed substrate. DETAILED DESCRIPTION

[0009] For simplicity and illustrative purposes, the present disclosure is described by referring mainly to an example thereof. In the following description, numerous specific details are set forth in order to provide a thorough

understanding of the present disclosure. It will be readily apparent however, that the present disclosure may be practiced without limitation to these specific details. In other instances, some methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure. As used herein, the terms "a" and "an" are intended to denote at least one of a particular element, the term "includes" means includes but not limited to, the term "including" means including but not limited to, and the term "based on" means based at least in part on.

[0010] An example ink as disclosed herein may include a carrier and a reflective hot mirror stack pigment. A "reflective hot mirror stack pigment" as used herein is defined as a stack of alternating layers of a first index of refraction material and a second index of refraction material. The reflective hot mirror stack pigment disclosed herein may transmit light in the visible wavelength (about 400 nm to about 700 nm). The reflective hot mirror stack pigment disclosed herein may be color neutral in the visible wavelength spectrum and may not, noticeably, to the human observer, impact color, such as color-shifting, in the visible wavelength spectrum. The reflective hot mirror stack pigment may reflect light in the near infrared (NIR) wavelength spectrum (about 701 nm to about 2000 nm) and may have an average reflectivity in the near infrared wavelength ranging from about 20% to about 95% as measured by air. The NIR reflective properties beyond about 700 nm of the reflective hot mirror stack pigment may be selectively higher at some NIR wavelengths. For example, the described "reflective hot mirror stack pigment" may be tuned with regard to the thin-film stack design to reflect more intensely at a particular NIR wavelength such as about 850 nm to accommodate increased reflectance at a common irradiating source, such as a light emitting diode (LED), wavelength for various electro-optical (EO) applications. [0011] The reflective hot mirror stack pigment may utilize a short wavelength pass filter having a design, such as 1 .6(0.5L H 0.5L) ⁿ , where n is an integer greater than 0, where H is the quarter wave optical thickness (QWOT) of a high index material and L is a QWOT of a low index material. In an aspect, H may be titanium oxide and L may be silicon dioxide. According to an example, the reflective hot mirror stack pigment does not appreciably reflect or absorb visible light and does not appreciably transmit or absorb infrared or NIR light. Reflective hot mirror stack designs that incorporate the use of more than two different index of refraction materials are also possible. It will be noted that different reflective hot mirror stack designs produce off-axis to normal incidence results that may trade-off better performance off-axis for reduced on axis performance. The reflective hot mirror stack for use herein may have improved characteristics both on and off axis.

[0012] "Index of refraction," or "refractive index," refers to the absolute refractive index of a material which is understood to be the ratio of the speed of light in a vacuum to the phase velocity of the light in a medium. Index of refraction may be measured using a refractometer.

[0013] FIG. 1 illustrates an example reflective hot mirror stack pigment 10 including alternating layers of a first index of refraction material 2a, 2b, 2c and a second index of refraction material 4a, 4b. In an aspect, the first index of refraction material may be a high refractive index material or a high refractive index thin film layer. The first index of refraction material may have an index of refraction greater than the second index of refraction material by at least 0.1 . Any material may be used for the first index of refraction material so long as the material possess an index of refraction greater than the second index of refraction material by about 0.1 In an aspect, the first index of refraction material may be greater than or equal to about 1 .6.

[0014] In an aspect, the second index of refraction material may be a low refractive index material or a low refractive index layer. The second index of refraction material may have an index of refraction of less than the first index of refraction layer by about 0.1 or more, i.e., a low index of refraction. Any material may be used for the second index of refraction material so long as the material possesses an index of refraction of less than about 0.1 or more of the first index of refraction material. In an aspect, the second index of refraction material may be less than about 1 .6.

[0015] As shown in FIG. 1 , a layer 4a of second index of refraction material, such as low refractive index layer, may be applied via a thin film deposition method (e.g.- ion beam sputtering, thermal evaporation, physical vapor deposition, magnetron sputtering, etc.) on a layer 2a of a first index of refraction material, such as a high refractive index layer. An additional layer 2b of first index of refraction material may then be deposited on the applied layer 4a of second index of refraction material. This design of alternating layers 2a, 2b, 2c of first index of refractive material and layers 4a, 4b of second index of refractive material may be repeated as many times as necessary or desired to create a reflective hot mirror stack pigment 10 having certain and/or desired properties.

[0016] The alternating layers may be stacked in any sequence, for example, the layers may be stacked in a sequence of (H/L) _n , (H/L) _n H, or L(H/L) _n where H denotes a first index of refraction material, such as a high refractive index layer, and L denotes a second index of refraction material, such as a low refractive index layer. The number of alternating layers (n) may range from about 3 to over about 75, such as from about 3 to about 50 alternating layers, or for example from about 5 to about 21 alternating layers.

[0017] Any number of layers may be deposited using any number of different materials. In this manner, tailoring the optical design of the reflective hot mirror stack pigment is possible by controlling the layer thickness and the refractive index of each alternating layer in the reflective hot mirror stack pigment 10.

[0018] The first index of refraction material and the second index of refraction material may each independently be an inorganic material. The first index of refraction material, such as a high refractive index material, may include, but is not limited to, metal oxides. In an aspect in which the first index of refraction material is a metal oxide, then the second index of refraction material may be a different metal oxide. Non-limiting examples of metal oxides include aluminum oxide, antimony trioxide, antimony tetroxide, antimony pentoxide, arsenic trioxide, arsenic pentoxide, barium oxide, bismuth(iii) oxide, bismuth(v) oxide, calcium oxide, cerium oxide, chromium(ii) oxide, chromium(iii) oxide,

chromium(iv) oxide, chromium(vi) oxide, cobalt(ii) oxide, cobalt(ii,iii) oxide, cobalt(iii) oxide, copper(i) oxide, copper(ii) oxide, indium oxide, iron(ii) oxide, iron(ii,iii) oxide, iron(iii) oxide, lead(ii) oxide, lead(ii.iv) oxide, lead(iv) oxide, lithium oxide, magnesium oxide, manganese(ii) oxide, manganese(iii) oxide,

manganese(iv) oxide, manganese(vii) oxide, mercury(ii) oxide, nickel(ii) oxide, nickel(iii) oxide, rubidium oxide, silicon dioxide, silver(i) oxide, thallium(i) oxide, thallium(iii) oxide, tin(ii) oxide, tin(iv) oxide, zinc oxide, and combinations thereof. In an aspect, the first index of refraction material may include Al ₂ 0 ₃ , Ti0 ₂ , and ZnS. In another aspect, the first index of refraction material may include oxides of elements selected from Si, Ti, Zr, Al, Ba, La, Ta, Zn, Sn, Sb, Zr, Be, Ce, Pb, Ge, Bi Y, Gd and W. In an aspect a material may be selected in which the transmission of the material at 550 nm (center of visible spectrum) for a thickness of 10 nm is greater than 30% and the material may be deposited as a thin film. This materia! may be considered as the first index of refraction material. A second material that also meets the above criteria and that is different from the first index of refraction material by 0.1 may then be considered as a second index of refraction material.

[0019] The second index of refraction material, such as a low refractive index material, may include, but is not limited to silicon oxide, silicon nitride, or magnesium fluoride. Non-limiting combinations of first and second index of refraction materials include, but are not limited to,: Example 1 - first iorm (index of refraction material) is ZnS (n=2.3 @ 850 nm) and 2 ^nd iorm is nMgF ₂ (n=1 .37 @ 850 nm) and with a delta index of Δη=(2.3-1 .37) = 0.93. Example 2 - 1 ^st iorm (index of refraction material) is Nb ₂ 0 ₅ (n=2.27 @ 850 nm) and 2 ^nd iorm is Si0 ₂ (n=1 .47 @ 850 nm) and with a delta index of Δη=(2.27-1 .47) = 0.80.

[0020] In an aspect, the reflective hot mirror stack pigment 10 may further include a layer of a polymer, for example alternating with the first index of refraction material 2 and the second index of refraction material 4a, 4b. As shown in FIG. 2, the reflective hot mirror stack pigment 10 may include alternating layers of a first index of refraction material 2, a second index of refraction material 4a, 4b, and a polymer 8a, 8b. The reflective hot mirror stack pigment 10 may include any variation (random or patterned) of alternating layers of a first index of refraction material 2, a second index of refraction material 4, and/or a polymer 8, including the total numbers of each layer present in the reflective hot mirror stack pigment 10. In an aspect, each layer of first index of refraction material 2, second index of refraction material 4, and polymer 8 should have a different refractive index as compared to the adjacent layers.

[0021] The layers 8a, 8b of polymer may be a thermoplastic polymer, including but not limited to, polypropylene, polyethylene terephthalate, high and low density polyethylene, polycarbonate, polyethylene-2,6-naphthalate, nylon, polylactic acid, polybenzimidazole, polyether sulfone, polyetherether ketone, polytherimide, polystyrene, polyvinylidene difluonde, polyphenylene oxide, and polyphenylene sulfide, and thermoset films that include cellulose derivatives, polyimide, polyimide benzoxazole, and polybenzoaxozole. Non-limiting examples of a polymer with a high refractive index include poly(pentabromophenyl methacrylate), poly(pentabromophenyl acrylate), poly(pentabromobenzyl methacrylate), poly(pentabromobenzyl methacrylate), poly(pentabromobenzyl acrylate), poly(2,4,6-tribromophenyl methacrylate), poly(vinylphenylsulfide), poly(1 -napthyl methacrylate), poly(2-vinylthiophene), poly(2,6-dichlorostyrene), poly(N-vinylphthalimide), poly(2-chlorostyrene) _i poly(pentachlorophenyl methacrylate). Non-limiting examples of a polymer with a low refractive index include poly(1 , 1 , 1 ,3,3,3-hexafluoroisopropyl acrylate),

poly(2,2,3,3,4,4,4-heptafluorobutyl acrylate), poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate), poly(2,2,3,3,3-pentafluoropropyl acrylate),

poly(1 , 1 , 1 ,3,3,3-hexafluoroisopropyl methacrylate),

poly(2,2,3,4,4,4-hexafluorobutyl acrylate), poly(2,2,3,4,4,4-hexafluorobutyl methacrylate), poly(2,2,3,3,3-pentafluoropropyl methacrylate),

poly(2,2,2-trifluoroethyl acrylate), poly(2,2,3,3-tetrafluoropropyl acrylate), poly(2,2,3,3-tetrafluoropropyl methacrylate), and poly(2,2,2-trifluoroethyl methacrylate). The number of layers of polymer present in the reflective hot mirror stack pigment 10 may vary according to the design and property of the reflective hot mirror stack pigment 10. The polymer layer 8 may assist in preventing delamination of the reflective hot mirror stack pigment 10.

[0022] The reflective hot mirror stack pigment 10 may include a layer of a third index of refraction material. To be clear, the reflective hot mirror stack pigment 10 may have alternating layers with each layer independently having a different refraction index, e.g., a first index of refraction material 2, a second index of refraction material 4, a third index of refraction material, a fourth index of refraction material, etc. Any alternate layer design of the reflective hot mirror stack pigment is envisioned so long as the reflective hot mirror stack pigment 10 transmits light in the visible wavelength, reflects light in the NIR wavelength, as shown in FIG. 3, and each layer has a different refractive index relative to adjacent layers.

[0023] With respect to the aspect shown in FIG. 4, in practice, a method for making a reflective hot mirror stack pigment 10, such as the pigment illustrated in FIG. 1 , may include depositing a first index of refraction material 2a on a release layer 6 of a substrate 12. In an aspect, the first index of refraction material 2a may be deposited onto a substrate 12 without a release layer 6. The substrate 12 may be made of a flexible material. Non-limiting examples of suitable substrate materials include polymer web, such as polyethylene terephthalate (PET), glass, silicon wafers, etc.

[0024] The first index of refraction material 2a may be deposited onto the release layer 6 by a deposition process, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), or the like. Once the first index of refraction material 2a has been deposited, a second index of refraction material 4a may be deposited on the first index of refraction material 2a. The above steps may then be repeated one or more times in order to form the reflective hot mirror stack pigment 10.

[0025] The substrate 12 may then be released from the deposited layers to create the reflective hot mirror stack pigment 10, for example as shown in FIG. 1 . In an aspect, the substrate 12 may be cooled to embrittle the associated release layer 6. In another aspect, the release layer 6 may be embrittled through heating to increase the degree of cross-linking, which may enable stripping. The deposited layers may then be stripped mechanically, such as by sharp bending or brushing of the substrate 12.

[0026] In an aspect, as shown in FIG. 5, an example method 200 of making a reflective hot mirror stack pigment 10 may include applying a first metal oxide onto a substrate including a release layer (block 210); applying a first polymer onto the first metal oxide (block 220); applying a second metal oxide onto the first polymer (block 230); and applying a second polymer onto the second metal oxide (block 240). The method 200 may further include applying a third metal oxide onto the second polymer resulting in a reflective hot mirror stack pigment (block 250), such as illustrated in FIG. 2. The third metal oxide may be the same or different from the first and second metal oxides.

[0027] The method 200 may further include mechanically altering the reflective hot mirror stack pigment 10 to achieve a particle size of about 10 microns or less, for example from about 8 microns or less, and as a further example from about 5 microns or less. The particle size may be measured by laser diffraction using a Mastersizer available from Malvern Instruments Ltd. Mechanically altering the reflective hot mirror stack pigment 10 may include milling or perforating followed by milling. In an aspect, the perforations may be in any shape, such as squares, truncated trihexagonal tiling, or three-dimensional raised tiles. The step of mechanically altering the reflective hot mirror stack pigment 10 may result in the pigment being in a shape of a flake.

[0028] A plurality of flakes of reflective hot mirror stack pigment 10 may be combined with a carrier, such as a clear carrier or clear resin, to form the ink. By way of non-limiting examples, the carrier may be polyvinyl alcohol, polyvinyl acetate polyvinylpyrrolidone, poly(ethoxyethylene), poly(methoxyethylene), poly(acrylic) acid, poly(acrylamide), poly(oxyethylene), poly(maleic anhydride), hydroxyethyl cellulose, cellulose acetate, poly(sacchrides) such as gum arabic and pectin, poly(acetals) such as polyvinylbutyral, polyvinyl halides) such as polyvinyl chloride and polyvinylene chloride, poly(dienes) such as polybutadiene, poly(alkenes) such as polyethylene, poly(acrylates) such as polymethyl acrylate, poly(methacrylates) such as poly methylmethacrylate, poly(carbonates) such as poly(oxycarbonyl oxyhexamethylene, poly(esters) such as polyethylene terephthalate, poly(urethanes), poly(siloxanes), poly(sulphides), poly(sulphones), poly(vinylnitriles), poly(acrylonithles), poly(styrene), poly(phenylenes) such as poly(2,5 dihydroxy-1 ,4-phenyleneethylene), poly(amides), natural rubbers, formaldahyde resins, other polymers and mixtures of polymers and polymers with solvents. According to an example, an ink is not visible to the human eye.

[0029] The flakes of reflective hot mirror stack pigment 10 may be present in the clear carrier in any amount so long as the flakes may randomly orient within the clear carrier. In an aspect, the ink may include from about 2% by volume to about 15%, for example, from about 4% to about 12%, and as a further example, from about 6% to about 10%, by volume of the flakes of reflective hot mirror stack pigment 10 in the ink.

[0030] The ink may be applied to a clear substrate, such as a plastic film, to form a display. In an aspect, the ink may be applied to the clear substrate in a pattern of indicia. The pattern of indicia may be a dot array. A user may utilize an electrical-optical reader, such as an electro-optical pen to read the pattern of indicia on the clear substrate. The ink may be present on the clear substrate in any amount so long as a pattern of indicia is formed. In an aspect, the ink may be present at a thickness ranging from about 2 to about 5 microns, such as from about 3 microns to about 5 microns.

[0031] In another aspect, as shown in FIG. 6, an example method 300 of making a printed substrate may include making the reflective hot mirror stack pigment 10 (block 310); mechanically altering the reflective hot mirror stack pigment to achieve a particle size of about 10 microns or less to form flakes (block 320); combining the flakes and a carrier to form an ink (block 330); applying a pattern of indicia onto a clear substrate (block 340) using the ink; and curing the applied pattern (block 350).

[0032] The ink may be cured using ultraviolet light, visible light, infrared, electron beam, or the like. Curing may proceed in an inert or ambient atmosphere. In an aspect, the curing step may utilize an ultraviolet light source having a wavelength of about 395 nm. The ultraviolet light source may be applied to the ink at a dose ranging from about 200 mJ to about 1000 mJ, for example ranging from about 250 mJ to about 900 m J, and as a further example from about 300 mJ to about 850 mJ.

[0033] Although described specifically throughout the entirety of the instant disclosure, representative examples of the present disclosure have utility over a wide range of applications, and the above discussion is not intended and should not be construed to be limiting, but is offered as an illustrative discussion of aspects of the disclosure.

[0034] What has been described and illustrated herein is an example of the disclosure along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the disclosure, which is intended to be defined by the following claims - and their equivalents - in which all terms are meant in their broadest reasonable sense unless otherwise indicated.