Characterisation Of A Thermochromic Compound For A Temperature Indicator

Abstract

The present invention relates to a coating of the surface of an electrical household appliance comprising a decoration (a) comprising a pigmentary compound B1VO4 having a ΔE* in the coating greater than or equal to 11 between ambient temperature and 150° C., ΔE* being defined by the formula CIE1976 in the CIELAB colour space: Formula (I): L1*, a1* and b1* characterising the values L*a*b* of said compound at ambient temperature; L2*, a2* and b2* characterising the values L*a*b* of said compound at 150° C.

Claims

1. A surface coating of a household article comprising a decoration (a) comprising a BiVO.sub.4 pigment compound having a ΔE* in said coating greater than or equal to 11 between room temperature and 150° C., wherein ΔE* is defined by the formula CIE1976 in the CIELAB colour space:
ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)} L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 150° C.

2. A surface coating of a household article comprising a decoration (a) comprising a BiVO.sub.4 pigment compound having a ΔE* in said coating greater than or equal to 15 between room temperature and 200° C., wherein ΔE* is defined by the formula CIE1976 in the CIELAB colour space:
ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)} L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 200° C.

3. The coating according to claim 1 comprising at least the decoration (a) such as defined in claim 1 and a decoration (b) arranged between or in its coats, said decoration (b) comprising a temperature reference pigment composition.

4-6. (canceled)

7. The coating according to claim 1, comprising one or more finish coats.

8. The coating according to claim 1, wherein the decoration is applied by screen printing or pad printing.

9. The coating according to claim 1, wherein the amount of BiVO.sub.4 pigment compound in decoration (a) is comprised from 0.1 to 100% by weight relative to the weight of said coat in the dry state.

10. A household article comprising a substrate, completely or partially covered with a coating according to claim 1.

11. The household article according to claim 7, characterised in that it is a cookware and in that the coating completely or partially covers the substrate on the face receiving the food.

12. The household article according to claim 7, wherein the substrate is plastic, metal, glass, ceramic or terracotta, aluminium or aluminium alloy, anodised or not, or of polished, brushed or bead-blasted, sandblasted, chemically treated aluminium or aluminium alloy, or polished, brushed or bead-blasted stainless steel, or cast iron or aluminium, or titanium or hammered or polished copper.

13. A cookware according to claim 8, wherein said cookware is selected from the group consisting of a saucepan, frying pan, stew pot, wok, sauté pan, crepe maker, grill, plancha grill, raclette grill, marmite pot or casserole dish.

14. A BiVO.sub.4 pigment compound having a ΔE* greater than or equal to 22 between room temperature and 150° C., wherein ΔE* is defined by the formula CIE1976 in the CIELAB colour space:
ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)} L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 150° C.

15. A BiVO.sub.4 pigment compound having a ΔE* greater than or equal to 30 between room temperature and 200° C., wherein ΔE* is defined by the formula CIE1976 in the CIELAB colour space:
ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)} L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 200° C.

16. The compound according to claim 11, comprising a monoclinic scheelite crystalline structure at room temperature.

17. The coating according to claim 3, wherein the amount of BiVO.sub.4 pigment compound in decoration (a) is comprised from 0.2 to 80% by weight.

18. The household article according to claim 7, wherein the household article is cookware, and wherein the coating comprises at least the decoration (a) and a decoration (b) arranged between or in its coats, said decoration (b) comprising a temperature reference pigment composition, and wherein the coating completely or partially covers the substrate on the face receiving the food.

19. The coating according to claim 2 comprising at least the decoration (a) such as defined in claim 2 and a decoration (b) arranged between or in its coats, said decoration (b) comprising a temperature reference pigment composition.

20. The coating according to claim 2, comprising one or more finish coats.

21. The coating according to claim 2, wherein the decoration is applied by screen printing or pad printing.

22. The coating according to claim 2, wherein the amount of BiVO.sub.4 pigment compound in decoration (a) is comprised from 0.1 to 100% by weight relative to the weight of said coat in the dry state.

23. A household article comprising a substrate, completely or partially covered with a coating according to claim 2.

24. The household article according to claim 20, characterised in that it is a cookware and in that the coating completely or partially covers the substrate on the face receiving the food.

25. The compound according to claim 12, comprising a monoclinic scheelite crystalline structure at room temperature.

26. The household article according to claim 20, wherein the household article is cookware, and wherein the coating comprises at least the decoration (a) and a decoration (b) arranged between or in its coats, said decoration (b) comprising a temperature reference pigment composition, and wherein the coating completely or partially covers the substrate on the face receiving the food.

Description

FIGURES

[0069] FIG. 1: The factors influencing colour

[0070] FIG. 2: Focused sphere representing CIELAB space

[0071] FIG. 3: Diagram illustrating the energy band theory

[0072] FIG. 4: Diagram illustrating the connection between the forbidden band (or gap) and the colour observed

[0073] FIG. 5: General diagram of computer vision method

[0074] FIG. 6: Production of a coating integrating a temperature indicator

[0075] FIG. 7: Diagram illustrating the different types of samples and light booth installation+camera

[0076] FIG. 8: Comparison of pigments in the form of powders

[0077] FIG. 9: Comparison of pigments in the form of pad-printed decoration on a frying pan

[0078] FIG. 10: diagram of the pattern distribution. 10A=non-overlapping adjacent patterns. 10B=partially overlapping patterns. 10C=overlapping patterns.

DETAILED DESCRIPTION OF THE INVENTION

[0079] A first subject-matter of the invention relates to a surface coating of a household appliance comprising a decoration (a) comprising a BiVO.sub.4 pigment compound having a ΔE* in said coating greater than or equal to 11 between room temperature and 150° C., ΔE* being defined by the formula CIE1976 in the CIELAB colour space:

ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)}

[0080] L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature

[0081] L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 150° C.

[0082] Preferably, the BiVO.sub.4 pigment compound has a ΔE* in said coating between room temperature and 150° C. greater than or equal to 13, more preferably, greater than or equal to 15.

[0083] Another subject-matter of the invention relates to a surface coating of a household appliance comprising a decoration (a) comprising a BiVO.sub.4 pigment compound having a ΔE* in said coating greater than or equal to 15 between room temperature and 200° C., ΔE* being defined by the formula CIE1976 in the CIELAB colour space:

ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)}

[0084] L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature

[0085] L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 200° C.

[0086] Preferably, the BiVO.sub.4 pigment compound has a ΔE* in said coating between room temperature and 200° C. greater than or equal to 17, more preferably, greater than or equal to 20.

[0087] Advantageously, the evolution of the colour change ΔE* as a function of temperature is linear. Preferably, this linear evolution has a slope comprised from 0.05 to 0.1, preferably greater than or equal to 0.1.

[0088] The BiVO.sub.4 pigment compound is advantageously present in the form of particles consisting of BiVO.sub.4. These particles are therefore not coated. Advantageously they are rough.

[0089] The colour can be measured and characterised by means of a colour classification. This classification is only possible if the colours are defined by numbers and this conversion can be done by means of colour spaces.

[0090] “A colour space is a three-dimensional mathematical model representing the set of colours perceptible, usable or reproducible by a human or a device.” Numerous spaces exist with their own distribution of colours having precise coordinates (for example: the RGB space, widely used by television systems or the CIELAB space which takes into account the logarithmic response of the eye).

[0091] The CIELAB colour space or L*a*b* CIE 1976 serves to characterise the colours of different surfaces. This space can be represented according to a geometric model bearing 3 orthogonal axes representing the L*a*b* values (FIG. 2). Each colour thus has clearly specified and unique L*a*b* coordinates: [0092] L* represents the lightness along an axis expressed in percentage, or from black: 0 to white: 100 [0093] a* the axis ranging from green: −120 to red: +120 [0094] b* the axis ranging from blue: −120 to yellow: +120

[0095] These coordinates unique to each colour make it possible to calculate several parameters such as the lightness difference ΔL*, the hue difference ΔH* or the colour difference ΔE*.

[0096] The parameter which interests us in this invention is the colour difference ΔE* representing the measurement of the distance between two different colours located in the CIELAB space. The colour difference ΔE* does not have a unit.

TABLE-US-00001 Symbol CIELAB relation Hue angle h arctan (b*/a*) Hue angle difference Δh h.sub.2 − h.sub.1 Chroma C* √{square root over (a*.sup.2 + b*.sup.2)} Chroma difference ΔC* C.sub.2* − C.sub.1* Lightness difference ΔL* L.sub.2* − L.sub.1* Hue difference ΔH* √{square root over ((ΔE*).sup.2 − (ΔL*).sup.2 − (ΔC*).sup.2)} Colour difference ΔE* √{square root over ((ΔL*).sup.2 + (Δa*).sup.2 + (Δb*).sup.2)} (CIE1976 formula)

Formulas of Deviations and Differences in Hue and Colour

[0097] The thermochromism phenomenon is defined as the ability of a compound to change colour as a function of the temperature to which it is subjected.

[0098] The BiVO.sub.4 compound is yellow at room temperature and changes colour continuously when the temperature increases, passing through orange to red.

[0099] The BiVO.sub.4 compound is part of the family of semiconductor oxides: this category has a colour mechanism also responsible for their thermochromic properties.

[0100] Indeed, semiconductor materials are characterised by an energy band theory representing the interactions of atoms. It is a model in which the core electrons are assumed to be localised on the atom to which they belong and are thus present in discrete atomic orbitals and therefore do not appear on the energy bands of the model. The valence electrons, on the other hand, are able to be delocalised throughout the crystalline network of the solid; they constitute the valence band. The conduction band is the first empty energy band which can be occupied by the free electrons. The valence band and the conduction band are separated by a forbidden band whose width (also called “gap”) is equal to the energy difference present between the energy levels associated with the valence and conduction bands (FIG. 3).

[0101] The colour of semiconductor materials is related to the width of the forbidden band which separates the valence bands and the conduction bands of the material in question. It is the electronic transitions of energy equal to or greater than the width of the forbidden band which are responsible for the colour of the material. The width of a forbidden band ranging from 1.7 eV to 3 eV can create colours ranging from black to white through a colour palette extending to colours ranging from light yellow to red, thus passing through oranges (red corresponding to low energies and light yellow to high energies). (FIG. 4)

[0102] Under the influence of temperature, the interatomic anion-cation bonds expand, causing a decrease in the orbital overlap. This results in a drop of the covalence of the bonds and therefore a reduction of the gap; the transfer of electrons is then facilitated between the valence band and the conduction band between two atoms.

[0103] The semiconductor materials are therefore thermochromic due to the size change of their crystalline structure under the influence of a temperature increase. If the crystalline structure changes, the interactions within the network are not the same; the width of the forbidden band will then vary and, as a consequence, the colour changes.

[0104] The majority of semiconductor materials when they are subjected to a temperature increase, have a reduction of the width of their forbidden band. This explains the colour change of BiVO.sub.4, yellow at room temperature, to red when it is heated.

[0105] Preferably, the coating according to the invention comprises at least two decorations (a) and (b) arranged between or in the coats of said coating: [0106] (a) a decoration comprising at least the BiVO.sub.4 pigment compound such as defined above, [0107] (b) a decoration comprising a temperature reference pigment composition.

[0108] Preferably, the amount of the BiVO.sub.4 pigment compound such as defined above in decoration (a) is comprised from 0.1 to 100% by weight relative to the weight of said coat in the dry state, preferably 0.2 to 80% by weight, more preferably 0.5 to 70% by weight.

[0109] Preferably, the amount of temperature reference pigment composition in decoration (b) is comprised from 0.1 to 100% by weight relative to the weight of said coat in the dry state, preferably 0.2 to 80% by weight, more preferably 0.5 to 70% by weight.

[0110] According to one embodiment, each of the decorations (a) and (b) are present in the form of non-overlapping adjacent patterns. For example, each decoration is represented by different geometric patterns uniformly distributed over the entire surface and alternating with one another (see FIG. 10A).

[0111] According to another embodiment, the two decorations (a) and (b) are partially overlapping. For example, each decoration is represented by different geometric patterns uniformly distributed over the entire surface and partially overlapping (see FIG. 10B).

[0112] Preferably, the two decorations (a) and (b) are overlapping, either because one of the two decorations is a continuous coat and the other decoration covers it in the form of patterns, or because the two decorations (a) and (b) are present in the form of two overlapping patterns (see FIG. 10C).

[0113] Preferably, the BiVO.sub.4 pigment compound as defined above exhibits a monoclinic scheelite crystallographic form at room temperature.

[0114] Preferably, the coating according to the invention comprises one or more finish coats applied over the decoration(s).

[0115] According to a first embodiment, the coating according to the invention comprises one or more primer coats applied onto the support. The decorations are then applied onto the last primer coat.

[0116] Preferably, the coating according to the invention comprises, in the following order from one of the faces of the substrate of the cookware: two primer coats, the two decorative coats (a) and (b), and a finish coat.

[0117] According to a second embodiment, the decorations are directly applied onto the substrate.

[0118] The decoration can be applied by any method well known to the skilled person such as, for example, by screen printing or pad printing.

[0119] The temperature reference pigment composition can be chosen in the group consisting of: [0120] Titanium rutile yellow pigments, [0121] Yellow pigments derived from bismuth, for example selected from stabilised bismuth vanadates (Py184), [0122] Red pigments, for example selected from perylene red or iron oxide, [0123] Bismuth oxyhalide orange pigments (PO85), [0124] Bismuth vanadate orange pigments (PO86), [0125] Tin titanium zinc orange pigment (PO82), [0126] Cerium sulfide orange pigment (PO75; PO78), [0127] Chrome antimony titanium yellow-orange rutile pigment (PBr24), [0128] Tin and zinc yellow-orange rutile pigment (Py216), [0129] Zinc tin sulfide niobium oxide yellow-orange pigment (Py227), [0130] Double oxides of tin and niobium yellow-orange pigment, [0131] and mixtures thereof.

[0132] According to one embodiment, the coating according to the invention is a fluoropolymer-based coating.

[0133] The fluoropolymer(s) can be present in the form of powder or aqueous dispersion or mixtures thereof.

[0134] Advantageously, the fluoropolymer(s) can be chosen in the group comprising polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene and perfluoropropyl vinyl ether (PFA), copolymers of tetrafluoroethylene and hexafluoropropene (FEP), polyvinylidene fluoride (PVDF), copolymers of tetrafluoroethylene and polymethyl vinyl ether (MVA), terpolymers of tetrafluoroethylene, polymethyl vinyl ether and fluoroalkyl vinyl ether (TFE/PMVE/FAVE), ethylene tetrafluoroethylene (ETFE) and mixtures thereof.

[0135] Advantageously, the fluoropolymer(s) can be chosen from polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene and perfluoropropyl vinyl ether (PFA), copolymers of tetrafluoroethylene and hexafluoropropene (FEP), a mixture of PTFE and PFA (PTFE/PFA) and a mixture of PTFE and FEP (PTFE/FEP).

[0136] Preferably, the fluoropolymer(s) can represent from 10 to 99% by mass, preferably from 50 to 98% by mass of the total dry mass of the non-stick coating composition.

[0137] According to another embodiment, the coating according to the invention is an organo-mineral or entirely mineral sol-gel coating. These coatings synthesised by the sol-gel pathway from precursors of the metal poly alkoxylate type have a hybrid network, generally of silica with grafted alkyl groups. A sol-gel (SG) composition comprises at least one colloidal metal oxide and at least one metal alkoxide type precursor.

[0138] The metal alkoxide is preferably a colloidal metal oxide chosen from colloidal silica and/or colloidal alumina.

[0139] A metal alkoxide is preferably used as a precursor chosen in the group consisting of: [0140] the precursors corresponding to the general formula M.sub.1(OR.sub.1).sub.n, [0141] the precursors corresponding to the general formula M.sub.2(OR.sub.2).sub.(n-1)R.sub.2′, and [0142] the precursors corresponding to the general formula M.sub.3(OR.sub.3).sub.(n-2)R.sub.3′.sub.2, with: [0143] R.sub.1, R.sub.2, R.sub.3 or R.sub.3′ designating an alkyl group, [0144] R.sub.2′ designating an alkyl or phenyl group, [0145] n being a whole number corresponding to the maximum valence of the metals M.sub.1, M.sub.2 or M.sub.3, [0146] M.sub.1, M.sub.2 or M.sub.3 designating a metal chosen from Si, Zr, Ti, Sn, Al, Ce, V, Nb, Hf, Mg or Ln.

[0147] Advantageously, the metal alkoxide of the SG solution is an alkoxysilane.

[0148] Alkoxysilanes which can be used in the SG solution of the method of the invention can particularly include methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), dimethyldimethoxysilane, and mixtures thereof.

[0149] Preferably, the alkoxysilanes MTES and TEOS will be used, because they have the advantage of not containing methoxy groups. Indeed, methoxy hydrolysis leads to the formation of methanol in the sol-gel formulation, which, given its toxic class, requires additional precautions during application. In contrast, hydrolysis of ethoxy groups only generates ethanol, having a more favourable class and therefore less restrictive using requirements for the sol-gel coating.

[0150] The formation of this sol-gel coating consists of mixing an aqueous composition A comprising the colloidal metal oxide and a solution B comprising the metal alkoxide. The mixture is advantageously done in a ratio of 40 to 75% by weight of the aqueous composition relative to the weight of the sol-gel composition (A+B), so that the quantity of colloidal metal oxide represents 5 to 30% by weight of the sol-gel composition (A+B) in the dry state.

[0151] Aqueous composition A can also comprise a solvent, in particular a solvent comprising at least one alcohol.

[0152] Aqueous composition A can also comprise at least one silicone oil.

[0153] Aqueous composition A can also comprise a pigment.

[0154] Aqueous composition A can also comprise a mineral filler.

[0155] Aqueous composition A can also comprise fumed silica, whose function is to regulate the viscosity of the sol-gel composition and/or the gloss of the dry coating.

[0156] Aqueous composition A typically comprises for a primer coat: [0157] i) 5 to 30% by weight relative to the total weight of aqueous composition A of at least one colloidal metal oxide; [0158] ii) 0 to 20% by weight relative to the weight of composition A of a solvent comprising at least one alcohol; [0159] iii) optionally, 0.05 to 3% by weight relative to the total weight of said aqueous composition A of at least one silicone oil; [0160] iv) 5 to 30% of pigment; [0161] v) 2 to 30% of mineral filler.

[0162] Aqueous composition A typically comprises for a finish coat: [0163] i) 5 to 30% by weight relative to the total weight of aqueous composition A of at least one colloidal metal oxide; [0164] ii) 0 to 20% by weight relative to the weight of composition A of a solvent comprising at least one alcohol; [0165] iii) optionally, 0.05 to 3% by weight relative to the total weight of said aqueous composition A of at least one silicone oil; [0166] iv) 0.1 to 1% of metallic glitter.

[0167] Solution B can also comprise a Bronsted or Lewis acid. Advantageously, the metal alkoxide precursor of solution B is mixed with an organic or mineral Lewis acid which represents from 0.01 to 10% by weight of the total weight of solution B.

[0168] Particular examples of acids usable for mixture with the metal alkoxide precursor are acetic acid, citric acid, ethyl acetoacetate, hydrochloric acid or formic acid.

[0169] Solution B can also comprise a solvent, in particular a solvent comprising at least one alcohol.

[0170] Solution B can also comprise at least one silicone oil.

[0171] Solution B can also comprise metallic glitter.

[0172] According to an advantageous embodiment of the process of the invention, solution B can comprise a mixture of one of the alkoxysilanes such as defined above and an aluminium alcoholate.

[0173] According to this sol-gel embodiment, the coating according to the invention comprises, in this order from said surface: [0174] One or more sol-gel primer coats, [0175] A decoration over at least a part of the last primer coat comprising the BiVO.sub.4 pigment compound such as defined above.

[0176] Another subject-matter of the invention concerns a household article comprising a substrate, preferably metal, completely or partially covered with a coating according to the invention.

[0177] The colour change of the BiVO.sub.4 pigment compound allows the user to be notified, on the one hand, that the article is hot and therefore presents a risk of burns and, on the other hand, that the surface of the article has reached the appropriate temperature for its use.

[0178] Advantageously, the article substrate can be plastic, metal, glass, ceramic or terracotta. Metal substrates usable in the context of the present invention advantageously include substrates of aluminium or aluminium alloy, anodised or not, or of polished, brushed or bead-blasted, sandblasted, chemically treated aluminium or aluminium alloy, or polished, brushed or bead-blasted stainless steel, or cast iron or aluminium, or titanium or hammered or polished copper.

[0179] Examples of household articles usable in the context of the present invention can notably include deep fryer bowls, fondue or raclette pans or pots, the bowl of a deep fryer or bread machine, the jar of a blender, the plates of a straightening iron and the iron soleplates.

[0180] For a better adhesion of the coating, the surface of the substrate can be treated so as to increase its specific surface; for aluminium, this treatment can be done by anodization (creation of a tubular alumina structure), by chemical etching, by sandblasting, etc. The other metal substrates can also be polished, sandblasted, brushed or bead-blasted.

[0181] The primer coat(s) can comprise a bonding resin, especially when the substrate is mechanically treated.

[0182] Preferably, the bonding resin(s) is (are) chosen in the group consisting of polyamide imides (PAI), polyether imides (PEI), polyamides (PA), polyimides (PI), polyetherketones (PEK), polyetheretherketones (PEEK), polyaryletherketones (PAEK), polyethersulfones (PES), polyphenylene sulfide (PPS), polybenzimidazoles (PBI) and tannins.

[0183] Advantageously, the article according to the invention is a cookware and the coating according to the invention completely or partially covers the substrate on the face receiving the food.

[0184] Advantageously, the article according to the invention is a hair straightener and the coating according to the invention completely or partially covers its plates.

[0185] Advantageously, the article according to the invention is a clothing iron and the coating according to the invention completely or partially covers its soleplate.

[0186] Advantageously, the article is a cookware of which one of the faces of the support is a concave inner face intended to be in contact with the food placed inside said cookware and the other support face of the cookware is a convex outer face intended to be in contact with a heat source.

[0187] Preferably, said household article is an article of cookware, preferably chosen in the group consisting of saucepan, frying pan, stew pot, wok, sauté pan, crepe maker, grill, plancha grill, raclette grill, marmite pot or casserole dish, and said coating is intended to come into contact with food.

[0188] In the fields of application considered for the present invention, an article to be heated of the cookware type or a heating article of the iron type is typically used in a range of temperatures comprised from 10° C. to 300° C.

[0189] Another subject-matter of the invention relates to a BiVO.sub.4 pigment compound having a ΔE* greater than or equal to 22 between room temperature and 150° C., ΔE* being defined by the formula CIE1976 in the CIELAB colour space:

ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)}

[0190] L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature

[0191] L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 150° C.

[0192] Advantageously, the compound according to the invention has a ΔE* between room temperature and 150° C. greater than or equal to 24, preferably greater than or equal to 26, still more preferably greater than or equal to 28, in a particularly preferred manner, greater than or equal to 30.

[0193] Advantageously, the compound of the invention is of monoclinic scheelite crystalline structure at room temperature.

[0194] Another subject-matter of the invention relates to a BiVO.sub.4 pigment compound having a ΔE* greater than or equal to 30 between room temperature and 200° C., ΔE* being defined by the formula CIE1976 in the CIELAB colour space:

ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)}

[0195] L.sub.1*, a.sub.1* and b.sub.1* characterising the L*a*b* values of said compound at room temperature

[0196] L.sub.2*, a.sub.2* and b.sub.2* characterising the L*a*b* values of said compound at 200° C.

[0197] Advantageously, the compound according to the invention has a ΔE* between room temperature and 200° C. greater than or equal to 32, preferably greater than or equal to 35, still more preferably greater than or equal to 37, in a particularly preferred manner, greater than or equal to 40.

[0198] Advantageously, the evolution of the colour change ΔE* as a function of temperature is linear. Preferably, this linear evolution has a slope comprised from 0.05 to 0.1, preferably even greater than or equal to 0.1.

EXAMPLES OF EMBODIMENT

Example 1: Pigment Selection

[0199] The pigments used are inorganic compound powders:

[0200] Thermochromic pigments: [0201] BiVO.sub.4, process described in Example 2, batches 2b and 2d [0202] Bi.sub.2O.sub.3, Varistor Fine grade, sold by the company 5N Plus [0203] Sicopal® Yellow K1120FG pigment, sold by the company BASF [0204] Bayferrox® 130 pigment, sold by the company Bayferrox

[0205] Thermostable pigments (temperature reference pigments): [0206] Yellow 10C242 pigment, sold by the Shepherd Color Company [0207] Yellow 6716B pigment, sold by the company FERRO

[0208] These compounds may be used singly or in mixture.

Example 2: Process for Synthesising a BiVO.SUB.4 .Pigment Compound According to the Invention

Process 2.1

[0209] To a solution of bismuth nitrate (0.1 M) in 1 M nitric acid is added stoichiometrically a solution of ammonium vanadate (0.1 M) in 1 M nitric acid. The mixture is stirred overnight, filtered, washed with water and then dried. The powder is then annealed at 450° C. for 3 hours.

[0210] The bismuth vanadate is then obtained in the form of a bright yellow powder of monoclinic scheelite structure characterised by X-ray diffraction analysis.

[0211] The process takes place at pH<1 with no addition of alkaline agent.

Process 2.2

[0212] To a solution of bismuth nitrate (0.4 M) in 1 M nitric acid is added a stoichiometric quantity of sodium metavanadate in the powder form. The mixture is stirred for 2 hours at 80° C. The precipitate is then filtered and washed with water to obtain a yellow BiVO.sub.4 powder in the monoclinic scheelite form. The powder is then annealed at 500° C. for 3 hours.

[0213] The process takes place at pH<1 with no addition of alkaline agent.

[0214] The monoclinic scheelite BiVO.sub.4 thus has a ΔE=40 between room temperature and 200° C.

Example 3: Process for Synthesising a BiVO.SUB.4 .Compound not According to the Invention (Comparative Example)

[0215] The Applicant has reproduced Example 1 of patent CN 101 070 435.

[0216] In this process, an alkaline agent is added to bring the pH to 2 and a calcination is carried out in a second step to modify the hue.

[0217] To a solution of bismuth nitrate (0.1 M) in a nitric acid solution (4 mol.Math.L.sup.−1) is added stoichiometrically a solution of ammonium metavanadate (0.1 M) in nitric acid (4 mol.Math.L.sup.−1). The pH is adjusted to 2 with an ammonia solution.

[0218] The mixture is stirred for 2 h. The precipitate is then filtered and washed with water to obtain a light yellow powder. This powder has a quadratic (tetragonal) zirconium silicate structure.

[0219] It is calcined at 500° C. for 2 h to obtain a dark yellow powder of monoclinic scheelite structure.

[0220] This monoclinic scheelite BiVO.sub.4 thus has a ΔE=20 between room temperature and 200° C.

Example 4: Integration of the Compounds into a Black Fluorinated Non-Stick Coating

[0221] In order to offer the consumer a cookware with an inner non-stick coating provided with a decoration guiding him during its cooking, compounds with improved thermochromic properties are introduced into a non-stick coating having several coats: [0222] The primer coat(s), of opaque colour, [0223] The thermochromic compound integrated into a decorative coat, [0224] The translucent finish coat(s).
i. Preparation of Two Primer Coats

[0225] A first formulation based on an aqueous PTFE dispersion is prepared.

[0226] (a) Primer 1/Formula 1a

TABLE-US-00002 weight % weight % Component wet formula dry film fluorinated polymer aqueous dispersion 24 60 carbon black aqueous dispersion 4 4 colloidal silica 16 20 bonding resin solvent dispersion 41 16 additives (base, wetting agents, etc.) 6 — water 9 —

[0227] A second formulation based on an aqueous PTFE dispersion is prepared.

[0228] (b) Primer 2/Formula 2a

TABLE-US-00003 weight % weight % Component wet formula dry film fluorinated polymer aqueous dispersion 48 88 carbon black aqueous dispersion 5 4 colloidal silica 3 3 bonding resin solvent dispersion 19 5 additives (base, wetting agents, etc.) 12 — water 13 —

[0229] Formulations 1a and 2a are filtered before being applied with an air spray gun onto the inside of a preformed aluminium cap. This support is at least degreased and dedusted beforehand. For a better adhesion of the coating, the support surface is treated by sandblasting so as to increase its specific surface.

[0230] The primer is applied in at least one coat of 5 to 50 microns thick. In the case of an application in several coats, each coat is dried before application of the next one.

ii. Preparation of a Thermochromic Decoration

[0231] A pad printing paste containing a thermochromic pigment such as described in Example 1 is prepared according to the formulation described below.

TABLE-US-00004 PTFE 5035Z PTFE dispersion 58.06 pigment Thermochromic 27.42 SD15 Thickener 1.89 WATER 2.13 AMMONIA pH adjuster 0.48 PPG Solvent 9.72 K7 Antifoaming agent 0.30 Total 100.00

[0232] This paste is applied in the form of patterns by pad printing on the dried primer coat.

iii. Preparation of a Reference Decoration

[0233] A pad printing paste containing a coloured reference material such as described in Example 1 is prepared according to the formulation described below.

TABLE-US-00005 PTFE 5035Z PTFE dispersion 59.06 Pigment Thermostable 23.62 SD15 Thickener 2.19 WATER 3.13 AMMONIA pH adjuster 0.48 PPG Solvent 11.22 K7 Antifoaming agent 0.30 Total 100.00

[0234] This paste is applied in the form of a pattern onto the dried thermochromic decoration.

iv. Preparation of the Finish Coat and Curing

[0235] The finish coat is produced in the same way as the first primer coat, the only difference being that it must remain transparent, so it does not comprise pigments, but optionally glitter.

TABLE-US-00006 % wt % wt Component wet formula dry film fluorinated polymer aqueous dispersion 93 99 Glitter 1 1 additives (base, wetting agents, etc.) 3 — ethylene glycol 2 — Water 1 —

[0236] Once all the coats are applied and dried, the article is cured at 430° C. for 11 min.

Example 5: Method for Characterising Sample Colours

[0237] A light booth consisting of a closed enclosure equipped with controlled lighting is used. A camera is directly installed above the booth and is connected to a photo processing software making it possible to obtain the colour-related characteristics of the samples analysed. The light booth is lit with a D50 illuminant (corresponding to daylight).

[0238] The principle of this characterisation is to position the material to be analysed in the booth, to take a photo of it and to extract colour data from the photo using appropriate software.

[0239] By means of the L*a*b* values obtained, a colour difference (or colour variation) ΔE* is obtained at each temperature relative to the reference temperature (RT=25° C.) for a given sample, ΔE* being defined by the formula CIE1976 in the CIELAB colour space:

ΔE*=√{square root over ((L.sub.2*−L.sub.1*).sup.2+(a.sub.2*−a.sub.1*).sup.2+(b.sub.2*−b.sub.1*).sup.2)}

[0240] with: [0241] L.sub.1*, a.sub.1*, b.sub.1*: coordinates of the pigment colour at the standard temperature (25° C. or RT) or of the reference pigment. [0242] L.sub.2*, a.sub.2*, b.sub.2*: coordinates of the pigment colour at a chosen temperature (25° C./100° C./150° C./180° C./200° C./250° C.) or of the chosen pigment.

[0243] The protocols described below have been developed and make it possible to control at best the following elements: no possible heating directly in the booth (so as not to disrupt the atmosphere and the camera) but taking photo at a target temperature, using timing of the transfer phases.

i. Equipment [0244] Thermometer with contact sensor [0245] Stopwatch [0246] Hotplate [0247] Photo light booth [0248] CANON EOS 13000D camera (Canon 28-mm wide-angle lens) [0249] Glass slides [0250] Data acquisition softwares: Capture One and Photoshop
ii. Protocol for Samples in the Form of Pad-Printed Decoration on a Frying Pan (FIG. 7) [0251] Measure the necessary time to move the frying pan containing the pad stamped pigment circles from the hotplate to the booth, close the booth, activate the camera. (t=15 sec). [0252] Heat and stabilise the sample around the target temperature. [0253] Test the temperature which at the end of 15 sec adjusts to the desired temperature. [0254] Perform a simulation 3 times with this temperature to confirm or adjust depending on the sample temperature obtained after 15 sec. [0255] Initiate the acquisition. [0256] Perform the characterisation by selecting the photo surface to be analysed using the software and then read the L*a*b* values. [0257] The Capture One software is used for the image capture in RAW format, which can subsequently be converted into any other file format. [0258] The Photoshop software used then allows the raw file to be converted (conversion to TIFF format). It then makes it possible to perform the conversion of an RGB colour space to a CIE L*a*b* colour space with a D50 illuminant.
iii. Protocol for Samples in the Form of Powder (FIG. 7) [0259] Make 3 separate heaps of different powders in a frying pan (about a teaspoon). [0260] Flatten the powder heaps with a glass plate in order to have smooth, homogenous surfaces with the same thicknesses. [0261] Then, perform the same protocol as previously.

Example 6: Exploitation of Results

[0262] i. Calculations of the ΔE* of a Sample at Several Temperatures

[0263] Take, for the example, the pigment Fe.sub.2O.sub.3 in the powder form.

[0264] The L*a*b* coordinates and ΔE* results for Fe.sub.2O.sub.3 for each temperature appear below:

TABLE-US-00007 Lens T ° C. Room temperature = 25° C. 100° C. 150° C. Coordinates L* a* b* L* a* b* ΔE* (RT) L* a* b* ΔE* (RT) Fe.sub.2O.sub.3 35.9 34.2 23.1 32.5 30.4 18.8 6.7 29.2 26.4 15.5 12.8 Lens T ° C. 180° C. 200° C. 250° C. Coordinates L* a* b* ΔE* (RT) L* a* b* ΔE* (RT) L* a* b* ΔE* (RT) Fe.sub.2O.sub.3 30.0 25.4 14.6 13.5 30.0 24.2 13.9 14.8 28.7 21.9 12.2 18.0 ΔE*.sub.100° C. = {square root over ((L*.sub.100 − L*.sub.25).sup.2 + (a*.sub.100 − a*.sub.25).sup.2 + (b*.sub.100 − b*.sub.25).sup.2)} ΔE*.sub.100° C. = {square root over ((32.5 − 35.9).sup.2 + (30.4 − 34.2).sup.2 + (18.8 − 23.1).sup.2)} = 6.7104

TABLE-US-00008 ΔE* values calculated for each sample at given temperatures T ° C. 25 100 150 180 200 250 Powders BiVO.sub.4 2d ΔE* (RT) 0 14.3 31.4 39.2 43.6 59.5 BiVO.sub.4 2b 0 21.2 37 41.8 45.2 53.4 Bi.sub.2O.sub.3 0 18.3 30 37 42.1 52.2 Sicopal Yellow 0 12.6 19.9 20.9 19.1 23.6 K1120FG Bayferrox 130 0 6.7 12.8 13.5 14.8 18 Yellow 10C242 0 1.7 3.2 3.6 5.3 6.4 Yellow 6716B 0 1.7 3.9 4.3 5.6 6.7 Frying pans with pad printed decoration BiVO.sub.4 2d 0 9 17.6 22.8 25.4 34.9 BiVO.sub.4 2b 0 9 15.6 20.8 23.4 32.1 Bayferrox 130 0 6.1 9.4 12 14.1 18.4 Yellow 10C242 0 1.7 2.3 2.8 3.2 4.2
ii. Graphic Interpretations

[0265] The ΔE* values calculated for each sample at different temperatures make it possible to visualise by means of graphs (ΔE*=f (Temperature ° C.)) the evolution of the colour difference relative to the colour at room temperature.

[0266] The graph (FIG. 8) shows the colour variations of each pigment in the powder form.

[0267] The pigments studied in this case all have a fairly linear colour change as a function of temperature.

[0268] The “very good” thermochromic pigments have a colour difference ΔE* between 25° C. and 200° C. greater than or equal to 40 and a linear change in the colour difference ΔE* as a function of temperature with a slope greater than or equal to 0.2.

[0269] The “good” thermochromic pigments have a colour difference ΔE* between 25° C. and 200° C. comprised from 30 to 40 and a linear change in the colour difference ΔE* as a function of temperature with a slope greater than or equal to 0.2.

[0270] The “average” thermochromic pigments have a colour difference ΔE* between 25° C. and 200° C. comprised from 10 to 30 and a linear change in the colour difference ΔE* as a function of temperature with a slope comprised from 0.05 to 0.2.

[0271] The thermostable pigments have a colour difference ΔE* between 25° C. and 200° C. less than 10 and a linear change in the colour difference ΔE* as a function of temperature with a slope less than 0.05.

[0272] The graph (FIG. 9) shows the colour variations of each pigment in the form of pad printed decoration in a non-stick coating such as previously described.

[0273] The coloured materials integrated into a non-stick coating studied in this case all have a fairly linear colour change as a function of temperature.

[0274] The colour change behaviours as a function of temperature and the classification of the compounds with respect to each other are very similar to the observations made with the materials in powder form. However, a clear reduction in the colour differences is observed, related to the integration in the non-stick coating and most probably related to the “filter” effect of the finish coat which is not perfectly transparent.

[0275] The “very good” thermochromic pigments integrated into a non-stick coating have a colour difference ΔE* between 25° C. and 200° C. greater than or equal to 20 and a linear change in the colour difference ΔE* as a function of temperature with a slope greater than or equal to 0.1.

[0276] The “good” thermochromic pigments integrated into a non-stick coating have a colour difference ΔE* between 25° C. and 200° C. comprised between 15 and 20 and a linear change in the colour difference ΔE* as a function of temperature with a slope greater than or equal to 0.1.

[0277] The “average” thermochromic pigments integrated into a non-stick coating have a colour difference ΔE* between 25° C. and 200° C. comprised between 7 and 15 and a linear change in the colour difference ΔE* as a function of temperature with a slope comprised from 0.05 to 0.1.

[0278] The thermostable pigments integrated into a non-stick coating have a colour difference ΔE* between 25° C. and 200° C. less than 7 and a linear change in the colour difference ΔE* as a function of temperature with a slope less than 0.05.