Ultraviolet filter

Abstract

The present invention relates to Zinc chalcogenides core-shell nanoparticles having specific light absorbing properties, in particular light filtering material comprising said Zinc chalcogenides core-shell nanoparticles.

Description

ULTRAVIOLET FILTER FIELD OF INVENTION

[0001] The present invention relates to the field of Zinc chalcogenides core-shell nanoparticles having specific light absorbing properties; and to the field of light filtering material comprising said Zinc chalcogenides core-shell nanoparticles. BACKGROUND OF INVENTION

[0002] It is common knowledge that UV light and blue light, also known as high-energy visible (HEV) light, corresponding to visible light in the blue-violet band between 380 and 450 nm, can have deleterious effects on humans or goods.

[0003] For example, prolonged exposure to blue light emitted from digital devices such as television, laptops, tablets and smartphones and fluorescent and LED lighting is harmful to the human eye as blue light is able to reach the retina. Some specific ranges of blue light have been shown to cause photoretinitis; digital eyestrain, or computer vision syndrome which includes blurry vision, difficulty focusing, dry and irritated eyes, headaches, neck and back pain; disruption of the circadian rhythm; decreased melanin production; age-related macular degeneration; glaucoma; retinal degenerative diseases. Similarly, prolonged exposure to UV light – from natural or artificial sources – is harmful to the human skin and may cause skin color changes, actinic keratosis or skin cancers such as basal cell carcinoma, squamous cell carcinoma or malignant melanoma.

[0004] It is also known and commonly observed that the flavor / fragrance quality and / or colour of some kind of food or cosmetics products may be compromised when the product is exposed to light. In the spirit industry it has been known for centuries that light, and in particular sunlight, may negatively affect the flavor of many types of spirits. For fragrances and perfumes, degradation of colour and / or fragrance has been also observed.

[0005] Various UV filter have been developed, either in the eyeglasses industry, or in cosmetic industry, or in glass / packaging industry. However, usual UV filter have the drawback to lead to a yellowing effect. This yellowing effect has two origins. First, the filters may degrade upon UV exposure, thus forming chemical byproducts with a yellow aspect. Second, the spectral width over which the filter switches from absorbent – in UV range – to transparent – in visible range – is usually large. Therefore, an efficient UV filter necessarily has a residual absorption in blue range of visible light, leading to a yellow perception by human eye.

[0006] There is thus a need for a light filtering material for the manufacture of eyeglasses or glass containers allowing for efficient filtration of UV light in the range of wavelength up to 380-420 nm, and especially up to 380-400 nm – while keeping a very low colour. Such an achromatic filter allows to design eyeglasses having a well-defined absorbance spectrum able to protect eye against high energy radiations and optimize colour perception or to design white glass containers without the risk of flavor degradation.

[0007] The Applicant has found that some of these needs could be met with Zinc chalcogenides core-shell nanoparticles having specific light absorbing properties, in particular in the range from 350 nm to 450 nm SUMMARY

[0008] This invention thus relates to a core-shell nanoparticle comprising • A core of ZnSexS(1-x) material where x is in a range from 0.80 to 0.98 and having - a thickness of less than 10 nm, preferably less than 5 nm, for a core having a nanometric size in one dimension; - a section of less than 100 nm², preferably less than 50 nm², for a core having nanometric sizes in two dimensions; - a diameter of less than 20 nm, preferably less than 15 nm, more preferably less than 10 nm, for a core having nanometric sizes in three dimensions; and• A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

[0009] In an embodiment, sulfur is distributed in the core with a gradient of concentration, preferably the concentration of sulfur in the center of the core is less than the concentration of sulfur in the periphery of the core.

[0010] In an embodiment, sulfur is distributed in an external layer of the core.

[0011] In an embodiment, the core has a diameter of less than 20 nm, and x is in a range from 0.80 to 0.90. Preferably, the shell has a thickness greater than 0.5 nm and less than 3 nm, more preferably greater than 0.6 nm and less than 1.5 nm.

[0012] In an embodiment, the core has a thickness of less than 10 nm, and x is in a range from 0.80 to 0.90. Preferably, the shell has a thickness greater than 0.3 nm and less than 3 nm, preferably greater than 0.3 nm and less than 1.5 nm.

[0013] In an embodiment, core-shell nanoparticles are embedded in an encapsulating material, preferably a metal oxide encapsulating material.

[0014] Preferably, the loading charge of the core-shell nanoparticles in the composite particle is at least 1%, preferably at least 2.5%, more preferably at least 5%, said loading charge being the mass ratio between the mass of core-shell nanoparticles comprised in a composite particle and the mass of said composite particle.

[0015] Preferably, the mean size of the composite particles is preferably in a range from 50 nm to 500 nm, more preferably from 50 nm to 250 nm.

[0016] This invention also relates to a light filtering material comprising a matrix material, and core-shell nanoparticles or at least one population of composite particles as disclosed hereabove.

[0017] In an embodiment, absorbance of said light filtering material has:• a local maximum absorbance of highest wavelength in the range from 350 to 450 nm, said local maximum having an absorbance value Amax for a wavelength λmax, • a value of 0.9Amax for a wavelength λ0.9, λ0.9 being greater than λmax; • a value of A+20 for a wavelength λ0.9+20 nm; and wherein0.9^^^^^^^^^is greater than or equal to 25, preferably greater than or equal to 50, more preferably greater than or equal to 100.

[0018] In an embodiment, λmaxis in a range from 370 nm to 420 nm, preferably from 375 nm to 400 nm, more preferably from 380 nm to 395 nm.

[0019] In an embodiment, the light filtering material is obtained by curing of a polymerizable composition comprising: • 0.1%wt to 10%wt of Zinc chalcogenide core-shell nanoparticles or composite particles as disclosed hereabove, • 0.1%wt to 10%wt of anti-UV organic compounds, • optionally stabilizers and / or antioxidants, and • 15%wt to 40%wt of a polymeric matrix material, the polymerizable composition having a dry extract in a range from 15%wt to 70%wt.

[0020] The invention also concerns a core-shell nanoparticle comprising • A core of ZnSexS(1-x) material where x is in a range from 0.60 to 0.98, and having - a thickness of in a range of 3 nm to 10 nm, for a core having a nanometric size in one dimension; - a section in a range of 9 nm² to 100 nm², for a core having nanometric sizes in two dimensions; - a diameter in a range of 3 nm to 20 nm, for a core having nanometric sizes in three dimensions; and • A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

[0021] These particles present higher cutoff wavelength and are able absorbing UV light and a bit of visible blue light (up to 450 nm). They are particularly interesting for the packaging of liquids that are altered by UV. For instance, these particles may be used in packaging of polymerizable liquids, such as glues or nail polish. DEFINITIONS

[0022] In the present invention, the following terms have the following meanings:

[0023] “Absorbance” is the decimal logarithm of ratio I0 / I, where I0is the intensity of light incident on a sample and I is the intensity of light transmitted through said sample. In this disclosure, absorbance of liquid dispersions is measured for a 2-millimeter-thick sample with correction of the absorbance of the pure solvent – blank solvent. Absorbance is measured for wavelengths in UV and visible range from 350 nm to 780 nm.

[0024] “Core-shell” refers to heterogeneous nanostructure comprising an inner part: the core, overcoated on its surface by a layer of at least one atom thick material different from the core: the shell. Core-shell structures are noted as follows: core material-shell material. Core-shell nanostructure also include nanoparticles in which the central part: the core, is embedded – or encapsulated – by a layer of material disposed on the core: the shell, said shell having a gradient of composition from the core to the outside of the shell. In this case, the composition of the nanoparticle is changing smoothly – for instance continuously – from the core composition to the outside composition of the shell. There is no precise boundary between core and shell but properties in centre of the core are different from properties on the outer boundary of shell. In addition, core and shell may have different shapes, for instance a dot – a nanosphere or a nanocube or any other nanocluster – is provided as a core and shell is grown laterally around the core, yielding an heterostructure with shape of a nanoplate but comprising a dot inside the nanoplate.

[0025] “Mean size” refers to a size of a population of particles, obtained by a mathematical mean of sizes of each individual particle of the population. Practically, the mean size may be determined by electronic microscopy: size of each particle visible inthe microscopy is evaluated by fitting each particle with a circle whose diameter defines the size of the particle, then computing the mean of all individual sizes to obtain the mean size. Other methods, such as light scattering may be used to determine indirectly the mean size of the population of particles. Experimentally, particles are always obtained in the form of population of particles. By extension in this disclosure, the mean size of a particle is the mean size of the population of particles which has been synthesized. For the sake of clarity, a particle having a mean size between 100 nm and 250 nm is a particle representative of a population of particles having a mean size between 100 nm and 250 nm.

[0026] “Monodisperse” refers to a population of particles, wherein the distribution of size populations has a PolyDispersity Index (PDI) lower than 0.3, preferably lower than 0.2.

[0027] “Nanometric size” refers to a size of matter in which quantum effects appear due to confinement. For semi-conductive nanoparticles, nanometric size has to be defined with the average Bohr radius of an electron / hole pair. Confinement is effective for size in at least one dimension of nanoplates below 10 nm, preferably below 5 nm. Confinement is effective for section of nanorods below 100 nm², preferably below 50 nm². Confinement is effective for diameter of nanospheres below 20 nm, preferably below 15 nm, more preferably below 10 nm.

[0028] “Nanoparticle” refers to a particle having a size in at least one of its dimensions below 100 nm. For a nanosphere, diameter should be below 100 nm. For a nanoplate, thickness should be below 100 nm. For a nanorod, diameter should be below 100 nm.

[0029] “Nanoplate” refers to a two-dimensional shaped nanoparticle, wherein the smallest dimension of said nanoplate – the thickness – is smaller than the largest dimension of said nanoplate by a factor (aspect ratio) of at least 1.5, at least 2, at least 2.5, at least 3, at least 3.5, at least 4, at least 4.5, at least 5, at least 5.5, at least 6, at least 6.5, at least 7, at least 7.5, at least 8, at least 8.5, at least 9, at least 9.5 or at least 10. Nanoplates include nanosheets, nanoribbons or nanodisks.

[0030] “Nanorod” refers to a one-dimensional shaped nanoparticle, wherein the average dimension of the section of said nanorod is smaller than the length of the nanorod by a factor (aspect ratio) of at least 1.5, at least 2, at least 2.5, at least 3, at least 3.5, at least 4, at least 4.5, at least 5, at least 5.5, at least 6, at least 6.5, at least 7, at least 7.5, at least 8, at least 8.5, at least 9, at least 9.5 or at least 10. Nanorods include nanowires and nanorings.

[0031] “Nanosphere” refers to a three-dimensional shaped nanoparticle having similar dimensions in the three directions. Nanospheres also include nanocubes or nanodots.

[0032] “%wt” refers to the weight fraction of a component in a blend or a formulation. DETAILED DESCRIPTION Core-shell nanoparticles

[0033] This disclosure relates to core-shell nanoparticle comprising a core of ZnSexS(1-x) material and a shell of ZnS material. The core is rich in Selenium, with x ranging from from 0.80 to 0.98.

[0034] The size of the core is small in order to present a nanometric size leading to confinement of exciton created in the core-shell nanoparticle. Core may have a nanometric size in one dimension, the other dimensions being larger: excitons are confined in one spatial dimension only – see figure 1 right. Such nanoplates cores have a thickness of less than 10 nm, preferably less than 5 nm. Core may have a nanometric sizes in two dimensions, the third dimension being larger: excitons are confined in two spatial dimensions – see figure 1 centre. Such nanorods cores have a section of less than 100 nm², preferably less than 50 nm². Last, core may have nanometric sizes in three dimensions, allowing confinement of excitons in all three spatial dimensions – see figure 1 left. Such nanospheres cores have a diameter of less than 20 nm, preferably less than 15 nm, more preferably less than 10 nm.

[0035] The thickness of the zinc sulfide (ZnS) shell is greater than 0.3 nm – which represent two monolayers of ZnS – and less than 5 nm. Preferably, the thickness of the zinc sulfide (ZnS) shell is greater than 0.5 nm and less than 5 nm. More preferably, the thickness of the zinc sulfide (ZnS) shell is greater than 0.6 nm and less than 1.5 nm.

[0036] The dimensions and composition of the core-shell nanoparticles define semi- conductive nanoparticles having a band gap allowing absorption of light in the UV range. Indeed, light having a wavelength more energetic than band gap may be absorbed by the semi-conductive material, yielding an electron / hole pair, an exciton, which later recombine in the material and dissipate heat, or emit light, or both. On the contrary, light having a wavelength less energetic than band gap cannot be absorbed: semi-conductive material is transparent for these wavelengths. Finally, semi-conductive materials behave as high pass filters. When semi-conductive particles have a nanometric size, confinement - i.e., shape and nanometric size - governs electronic structure following the rules of quantum mechanics and light absorption may be limited to UV range or UV and high energy visible light. Within this disclosure, semi-conductive nanoparticles absorb light having a wavelength below a threshold, this threshold being in the range of 350 nm – 450 nm.

[0037] In addition, simple ZnSe nanoparticles – nanospheres, nanorods or nanoplates – are usually degraded upon light exposure when they are included in organic matrices, such as varnishes or coatings. Without being bound by theory, Applicant believe that organic compositions in which ZnSe nanoparticles are well dispersed can extract some selenium atoms and possibly modify their oxidation state, leading to organo-selenide compounds which have a residual yellow colour. Addition of a ZnS shell provides a protection against this phenomenon, but crystallographic structure of zinc selenide and zinc sulfide are slightly different and better properties are obtained when the core is doped with sulfur.

[0038] The distribution of sulfur inside the core of the core-shell nanoparticles may be uniform or uneven.

[0039] According to an embodiment, sulfur is distributed in the core with a gradient of concentration. Preferably, the concentration of sulfur in the center of the core is less than the concentration of sulfur in the periphery of the core. The periphery of the core being enriched in sulfur element leads to a better match with crystallographic structure of the Zinc sulfide shell, the latter being more easily deposited.

[0040] According to an embodiment, sulfur is distributed in an external layer of the core. In this embodiment, the core comprises a central part having a composition of Zinc selenide (ZnSe) and an external layer surrounding the central part. The external layer has a Zinc selenide / sulfide composition (ZnSexS(1-x)). And globally, the atomic percentage of selenide ions in the whole core is ranging from 60%at to 98%at – the atomic percentage for chalcogenide element of the core is given without considering Zinc. Suitable core- shell nanoparticles may be represented by formula ZnSe / ZnSexS(1-x) / ZnS where the core is ZnSe / ZnSexS(1-x). The diameter of the central part of the core is preferably in a range from 2 nm to 7 nm, the thickness of the external layer of the core is preferably in a range from 0.3 nm to 3 nm and the thickness of the Zinc sulfide shell is preferably in a range from 0.3 nm to 3 nm. Core-shell nanoparticles with a diameter of the central part of the core of 3 nm, thickness of the external layer of the core of 1.5 nm and thickness of the Zinc sulfide shell of 1.5 nm are especially suitable.

[0041] According to an embodiment, selenium is distributed in an external layer of the core. In this embodiment, the core comprises a central part having a composition of Zinc sulfide (ZnS) and an external layer surrounding the central part. The external layer has a Zinc selenide / sulfide composition (ZnSexS(1-x)). And globally, the atomic percentage of selenide ions in the whole core is ranging from 60%at to 98%at – the atomic percentage for chalcogenide element of the core is given without considering Zinc. Suitable core- shell nanoparticles may be represented by formula ZnS / ZnSexS(1-x) / ZnS where the core is ZnS / ZnSexS(1-x).

[0042] In an embodiment, the core-shell nanoparticles are nanospheres, wherein the core has a diameter of less than 20 nm, and x is in a range from 0.70 to 0.95. Preferably, the shell has a thickness greater than 0.5 nm and less than 3 nm, more preferably greater than0.6 nm and less than 1.5 nm. Nanospheric core-shell nanoparticles with a diameter of the core of 3.5 nm and a thickness of the Zinc sulfide shell of 1.5 nm are especially suitable.

[0043] In an embodiment, the core-shell nanoparticles are nanoplates, wherein the core has a thickness of less than 10 nm, and x is in a range from 0.70 to 0.95. Preferably, the shell has a thickness greater than 0.5 nm and less than 3 nm, more preferably greater than 0.6 nm and less than 1.5 nm. In a preferred embodiment, the core has a thickness of less than 5 nm, x is in a range from 0.70 to 0.95, and the shell has a thickness greater than 0.5 nm and less than 3 nm.

[0044] In an embodiment, the core-shell nanoparticles may be overcoated on their surface by a layer of at least one atom thick material different from the core and / or from the shell, leading to a core / first shell / second shell nanoparticle. Preferred compositions for the second shell are zinc oxides. Preferred thicknesses for the second shell are ranging from 0.25 nm to 10 nm. Suitable core-shell nanoparticles may be represented by formula ZnSexS(1-x) / ZnS / ZnO.

[0045] The disclosure also relates to a core-shell nanoparticle comprising • a core of ZnSexS(1-x) material where x is in a range from 0.60 to 0.98, and having: - a thickness of in a range of 3 nm to 10 nm, for a core having a nanometric size in one dimension; - a section in a range of 9 nm² to 100 nm², for a core having nanometric sizes in two dimensions; - a diameter in a range of 3 nm to 20 nm, for a core having nanometric sizes in three dimensions; and • A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

[0046] In another embodiment, the core-shell nanoparticle comprises • a core of ZnSexS(1-x) material where x is in a range from 0.60 to 0.98, and have: - a thickness of in a range of 4 nm to 10 nm, for a core having a nanometric size in one dimension;- a section in a range of 16 nm² to 100 nm², for a core having nanometric sizes in two dimensions; - a diameter in a range of 4 nm to 20 nm, for a core having nanometric sizes in three dimensions; and • A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

[0047] All core-shell nanoparticles disclosed hereabove are compatible with the following description of composite particles. Composite particles

[0048] This disclosure relates also to composite particles comprising core-shell nanoparticles disclosed hereabove, said core-shell nanoparticles being embedded in an encapsulating material. By encapsulating material, it is meant a material that covers all surface of Zinc chalcogenide core-shell nanoparticles. In other words, encapsulating material forms a barrier around the nanoparticles. Such a barrier as several advantages. In particular, said nanoparticles may be protected against chemicals, e.g., moisture, oxidants. Besides, Zinc chalcogenide core-shell nanoparticles that are not dispersible in a medium may be encapsulated in a material whose compatibility with said medium is good: the barrier behaves as a compatibilization agent. In addition, encapsulated Zinc chalcogenide core-shell nanoparticles may be under the form of a powder dispersible in a medium instead of a dispersion in a solvent, thereby providing with easier handling in current processes. Last, the encapsulating material may have a role of refractive index matching, in order to lower diffusion or haze: indeed, when core-shell nanoparticles are dispersed in a matrix, haze is proportional to the difference of refractive index between the matrix and the dispersed nanoparticles. Adding an encapsulating material with an intermediate refractive index mitigates this effect and lowers haze.

[0049] Encapsulating material may be organic, in particular organic polymers. Alternatively, encapsulating material may be an inorganic material, such as a metal oxide or mixture of metal oxides. Suitable metal oxides are SiO2, Al2O3, TiO2, ZrO2, FeO, ZnO, MgO, SnO2, Nb2O5, CeO2, BeO, IrO2, CaO, Sc2O3, Na2O, BaO, K2O, TeO2, MnO, B2O3,GeO2, As2O3, Ta2O5, Li2O, SrO, Y2O3, HfO2, MoO2, Tc2O7, ReO2, Co3O4, OsO, RhO2, Rh2O3, CdO, HgO, Tl2O, Ga2O3, In2O3, Bi2O3, Sb2O3, PoO2, SeO2, Cs2O, La2O3, Pr6O11, Nd2O3, La2O3, Sm2O3, Eu2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, Gd2O3, or a mixture thereof. Preferred metal oxides are SiO2, Al2O3, TiO2, ZrO2, HfO2, GeO2, SnO2, or a mixture thereof, including for instance AlyZrzO with^^ ^ + 2^ = 1. In an embodiment, the encapsulating material does not consist of pure SiO2.

[0050] In an embodiment, the loading charge of the core-shell nanoparticles in the composite particle is at least 1%, preferably at least 2.5%, more preferably at least 5%, said loading charge being the mass ratio between the mass of core-shell nanoparticles comprised in a composite particle and the mass of said composite particle. Indeed, the performance of composite particles is proportional to the concentration of core-shell nanoparticles they contain. Therefore, a high concentration of core-shell nanoparticles is advantageous. It has to be noted however that increasing concentration of core-shell nanoparticles without degrading their properties – as a consequence of aggregation or manufacturing process for instance – is not easy.

[0051] The composite particles may be in the form of a monodisperse population. Monodisperse composite particles are advantageous for various reasons, depending on the domain of application. When composite particles are used in optical elements – especially filters – a homogeneous size distribution avoids uncontrolled light diffusion and ensures spatial homogeneity of the optical element.

[0052] The mean size of the composite particles is preferably in a range from 50 nm to 500 nm, more preferably from 50 nm to 250 nm. Composite particles having a mean size from 50 nm to 250 nm, preferably from 50 nm to 100 nm are especially suitable light filtering materials with high transparency and low haze.

[0053] The composite particles may be chemically modified on their surface. Chemical modification may be obtained by grafting, by adsorption of molecules or by physical processes – heat, vacuum or gaseous treatment. Chemical modification may use compatibilization agents, allowing to mix composite particles in complex formulations –such as resins, varnishes, paints, colloidal dispersion… – without aggregation or phase separation of the composite particles. Light filtering material

[0054] The disclosure also relates to a light filtering material comprising a matrix material and at least one population of core-shell nanoparticles – encapsulated in the form of composite particles or not – disclosed hereabove.

[0055] The matrix may be obtained by curing a polymerizable composition, as soon as the polymerizable composition is sufficiently transparent to visible light and allows for dispersion of the core-shell nanoparticles. Suitable polymerizable composition comprise monomers or oligomers selected from allylic compounds, (meth)acrylic compounds, epoxy compounds, compounds used to prepare polyurethane or polythiourethane materials, compounds used to prepare polyester-melamine compounds, compounds used to prepare melamine-formaldehyde resins. Mixtures of these compounds – in particular epoxy / acrylic mixtures or polyester-melamine / melamine-formaldehyde – are also suitable. Besides, compounds used to prepare materials usually known as Sol-Gels are suitable as polymerizable compositions.

[0056] Especially suitable matrices are waterborne polyester–melamine paints made from saturated polyesters containing an increased amount of carboxyl groups, having acid numbers between 45 mg KOH / g and 55 mg KOH / g and molecular weights of approximately 2000 g / mol. The polyesters are combined with water-soluble melamine-formaldehyde resins such as hexamethoxymethyl melamine in a weight ratio of polyester 70 : melamine-formaldehyde 30 to polyester 85 : melamine-formaldehyde 15.

[0057] In an embodiment, the amount of core-shell nanoparticles in the matrix is ranging from 10 ppm to 1% by weight, in particular from 20 ppm to 0.5% by weight, more particularly from 25 ppm to 0.25% by weight, based on the weight of said light filtering material. This embodiment is especially suitable for thick films, having a thickness larger than 50 µm.

[0058] In an embodiment, the amount of core-shell nanoparticles in the matrix is ranging from 1% to 30% by weight, in particular from 3% to 25% by weight, more particularly from 5% to 20% by weight, based on the weight of said light filtering material. This embodiment is especially suitable for thin films, having a thickness less than 20 µm.

[0059] The light filtering material is characterized by its absorbance, as shown in Figure 2. As Zinc chalcogenide core-shell nanoparticles are semi-conductive, they impart to the light filtering material a high-pass property. Looking more precisely to Absorbance spectrum, one can observe a local maximum of absorbance in the range from 350 nm to 450 nm. This local maximum has an absorbance value Amaxfor a wavelength λmax. The Zinc chalcogenide core-shell nanoparticles present a very sharp transition from a highly absorbing state to a non-absorbing state. This transition is defined as follows. The next wavelength greater than λmax where absorbance equals 0.9Amax – thus 90% of Amax value – is defined as λ0.9. Then, absorbance at a wavelength of λ0.9+20 nm is measured and defined as A+20. The ratio0.9^^^^^^^^^thus defines the relative decrease of absorbance over 20 nm. It must be noted that absorbance is a decimal logarithm. It means that the decrease in transmittance of the light filtering material is even more dramatic.

[0060] For usual organic filters whose absorption peak shows a full width at half- maximum of typically 80 nm to 100 nm, the ration would be of the order of few unities.

[0061] According to this disclosure, the ratiois greater than or equal to 25. Preferably, the ratiois greater than or equal to 50. More preferably, the ratiois greater than or equal to 100.

[0062] In an embodiment, the wavelength λmax is in a range from 370 nm to 420 nm, preferably from 375 nm to 400 nm, more preferably from 380 nm to 395 nm. In this range, the light filtering material absorbs strongly UV light – defined as light with wavelength less than 380 nm – and optionally High Energy Visible light, typically up to 400 nm. However, visible light with wavelength greater than 410 nm – for λmax about 380 nm – or greater than 420 nm – for λmax about 390 nm – is not absorbed at all. Finally,such filters protect efficiently against UV and / or HEV light without filtering out blue light contributing to colour perception. In other words, colour perception through such a light filtering material is not altered, while UV and / or HEV light is absorbed. Additives

[0063] Besides core-shell Zinc chalcogenide nanoparticles, various additives may be added to the light filtering material.

[0064] In an embodiment, the light filtering material further comprises organic anti-UV compounds. Suitable anti-UV compounds are benzotriazoles, in particular derivatives of (2H-benzotriazol-2-yl)-4-hydroxybenzene such as Sodium 3-(2H-benzotriazol-2-yl)-5- sec-butyl-4-hydroxybenzenesulfonate – CAS number 92484-48-5 – or Polyethylene glycol mono-3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl)-1-oxopropyl ether – CAS number 104810-48-2 – or Polyethylene glycol di[3-[3-(2H- benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl]-1-oxopropyl] ether – CAS number 104810-47-1 – or Benzenepropanoic acid, 3-(2H-benzotriazol-2-yl)-5-(1,1- dimethylethyl)-4-hydroxy-, C7-9-branched and linear alkyl esters – CAS number 127519-17-9 – or 2-(2H-Benzotriazol-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3- tetramethylbutyl)phenol – CAS number 73936-91-1. Other suitable anti-UV compounds are triazines, such as reaction products of 1,3-Benzenediol, 4-[4,6-bis(2,4- dimethylphenyl)-1,3,5-triazin-2-yl] with [(dodecyloxy)methyl]oxirane and oxirane mono[(C10-16-alkyloxy)methyl] derivatives – CAS number 153519-44-9 – or Isooctyl 2-[4-[4,6-bis[(1,1'-biphenyl)-4-yl]-1,3,5-triazin-2-yl]-3-hydroxyphenoxy]propanoate – CAS number 204848-45-3 – or triazine know as TINUVIN®477 supplied by BASF. Other suitable anti-UV compounds are avobenzones, such as 1,3-Propanedione, 1-[4- (1,1-dimethylethyl)phenyl]-3-(4-methoxyphenyl)- - CAS number 70356-09-1.

[0065] In an embodiment, the light filtering material further comprises stabilizers. Suitable stabilizers are Hindered Amine Light Stabilizers – HALS compounds – such as Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate – CAS number 41556-26-7 – or Methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate – CAS number 82919-37-7 – or Bis-(1- octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate – CAS number 129757-67-1 – orreaction products of 2-Aminoethanol with cyclohexane and peroxidized N-butyl-2,2,6,6- tetramethyl-4-piperidinamine-2,4,6-trichloro-1,3,5-triazine – CAS number 191743-75-6 – or Di-(1,2,2,6,6-pentamethyl-4-piperidyl)-2-butyl-2-(3,5-di-tert-butyl-4- hydroxybenzyl)malonate – CAS number 63843-89-0 – or Tinuvin®249 supplied by BASF.

[0066] Specific mixtures of additives are also suitable, such as blend of CAS numbers : 104810-48-2, 104810-47-1, 41556-26-7 and 82919-37-7 in polyethyleneglycol, or blend of a benzotriazole and a HALS of CAS number 127519-17-9 . Commercial products supplied by BASF under the tradenames TINUVIN®5050, TINUVIN®5060, TINUVIN®5151 or TINUVIN®5333DW are also suitable.

[0067] In an embodiment, the light filtering material further comprises antioxidants or oxygen scavengers, for instance selected from the range of commercial products supplied by BASF under the tradenames IRGANOX®.

[0068] In an embodiment, the light filtering material further comprises cosolvents, such as short chain alcohols – especially propanol or butanol – ketones – especially butanones, methylethylketone or gammabutyrolactones – or glycols – especially ethyl glycol or diethylene glycol butyl ether – or esters – especially butyl acetate – or polar solvent such as dimethylsulfoxide – DMSO – or xylene.

[0069] The light filtering material is preferably obtained by curing of a polymerizable composition comprising: • 0.1%wt to 10%wt of Zinc chalcogenide core-shell nanoparticles disclosed hereabove, • 0.1%wt to 10%wt of anti-UV organic compounds, • optionally stabilizers and / or antioxidants, and • 15%wt to 40%wt of a polymeric matrix material, the polymerizable composition having a dry extract in a range from 15%wt to 70%wt.

[0070] Preferably, the polymerizable composition is a waterborne polymerizable composition.

[0071] In some embodiments, the light filtering material is obtained by curing of a polymerizable composition further comprising 0.1%wt to 5%wt of stabilizers.

[0072] When blends of anti-UV organic compounds and stabilizers are used, the weight composition refers to the individual components of the blend.

[0073] In some embodiments, the light filtering material is obtained by curing of a polymerizable composition further comprising 0.1%wt to 5%wt of antioxidants or oxygen scavengers.

[0074] Light filtering material with an improved balance between UV absorption, weathering resistance and low color is obtained by curing of a polymerizable composition comprising: • 2%wt to 8%wt of Zinc chalcogenide core-shell nanoparticles disclosed hereabove, • 2%wt to 8%wt of anti-UV organic compounds, • optionally stabilizers and / or antioxidants, and • 15%wt to 40%wt of a polymeric matrix material, the polymerizable composition having a dry extract in a range from 20%wt to 60%wt. Blends of core-shell Zinc chalcogenides particles

[0075] The light filtering material may comprise a bend of various core-shell Zinc chalcogenides particles, in order to tune finely optical properties, in particular the balance between UV absorption and residual color of the light filtering material.

[0076] Blend my comprise core-shell Zinc chalcogenides particles whose dimensions - for core and shell – and / or composition – parameter x for the core – are different.

[0077] Blend may comprise core-shell Zinc chalcogenides particles directly dispersed in the matrix material or composite particles comprising core-shell Zinc chalcogenides nanoparticles. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 is a schematic representation – sectional view – of various shapes of core-shell Zinc chalcogenide nanoparticles: nanosphere (left), nanorod (centre) and nanoplate (right). Core of composition ZnSexS(1-x) is represented by hatched surface. Shell of composition ZnS is represented by dotted surface. The arrow on each shape shows the nanometric size of the core-shell Zinc chalcogenide nanoparticle.

[0079] Figure 2 is a graph showing the transition from a highly absorbing state to a non-absorbing state, and how parameters Amax, λmax, λ0.9, A+20 and λ0.9+20 nm are evaluated: absorbance A (Vertical axis, logarithmic scale, arbitrary unit) as a function of light wavelength (λ, in nm)

[0080] Figure 3 is a graph showing absorbance A (Vertical axis, logarithmic scale, arbitrary unit) of various particles as a function of light wavelength (λ, in nm). Solid line corresponds to Example 1 using Zinc chalcogenide core-shell nanoparticles. Double line corresponds to Example 2 using Zinc chalcogenide core-shell nanoparticles. Dotted line corresponds to Comparative Example using ZnSe nanoparticles. EXAMPLES

[0081] The present invention is further illustrated by the following examples. Example 1:

[0082] 5.9 g of zinc stearate (zinc precursor for core), 136 g of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 2.07 g of N,N,N’-tricyclohexylselenourea (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 260°C for 15 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then the mixture is cooled down to room temperature and 10.6 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 280°C. During temperature increase, 20 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPSand 2 hours at 280°C, the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 3.0 nm and formula ZnSexS(1-x) with x about 0.94 and a shell of mean thickness 1.3 nm.

[0083] Absorbance of a dispersion of 2 mg of core-shell nanospheres in 3 mL heptane is measured with a Jasco V770 UV-visible-NIR instrument with increments of 1 nm, and reported on figure 3 (solid line). Absorbance shows a maximum Amax=0.41 at 403 nm. Absorbance 0.9Amax=0.37 is observed for λ0.9=406.5 nm. And absorbance at λ0.9+20=426.5 nm is A+20=0.0029. The ratiois estimated at 124. Absorbance decreases by more than two orders of magnitude in 20 nm: absorption is almost complete at 403 nm and almost no absorption is observed at 426.5 nm.

[0084] 40 mg of core-shell nanospheres – in alkaline water – are mixed in a waterborne polyester resin (75 parts) – hexamethoxymethyl melamine (25 parts) polymerizable composition and applied on a glass substrate to yield a 10 µm thick coating after cure, with a concentration of core-shell nanospheres of 75 ppm.

[0085] Ageing of the coating is made in the following conditions: constant illumination corresponding to D65 illuminant, with 500W / m2total power during 8 hours – referred to as SUNTEST. No appreciable yellowing is observed after ageing. Example 2:

[0086] Example 1 is reproduced at a larger scale in a 10 L reactor. The nanospheres obtained have a core of mean diameter 2.8 nm and a shell of mean thickness 1.2 nm.

[0087] Absorbance of a dispersion of 2 mg of core-shell nanospheres in 3 mL heptane is measured with a Jasco V770 UV-visible-NIR instrument with increments of 1 nm, and reported on figure 3 (double line). Absorbance shows a maximum Amax=0.42 at 383 nm. Absorbance 0.9Amax=0.38 is observed for λ0.9=389 nm. And absorbance at λ0.9+20=409 nm is A+20=0.0145. The ratiois estimated at 26. Thelarge-scale production of core-shell nanospheres leads to different sizes – evidenced by a shift to lower wavelength of band gap – and a higher dispersity in sizes and structure, leading to a broader transition zone between almost complete absorption and almost no absorption.

[0088] 40 mg of core-shell nanospheres – in alkaline water – are mixed in a waterborne polyester resin (75 parts) – hexamethoxymethyl melamine (25 parts) polymerizable composition and applied on a glass substrate to yield a 10 µm thick coating after cure, with a concentration of core-shell nanospheres of 75 ppm.

[0089] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 3:

[0090] 360 mg of zinc nitrate (zinc precursor for core), 6 mg of sulfur, 270 mg of selenium, 40 mL of oleylamine and 20 mL of octylamine are mixed in a 100 mL round bottom flask. After degassing under vacuum, the mixture is heated to 180°C for 35 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then 2 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS, the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean thickness 1.4 nm – lateral dimensions are about 20 nm width and 60 nm length – and formula ZnSexS(1-x) with x about 0.85 and a shell of mean thickness 1 nm.

[0091] Absorption properties similar to example 1 are observed.

[0092] 40 mg of core-shell nanoplates – in alkaline water – are mixed in a waterborne polyester resin (75 parts) – hexamethoxymethyl melamine (25 parts) polymerizable composition and applied on a glass substrate to yield a 10 µm thick coating after cure, with a concentration of core-shell nanoplates of 75 ppm.

[0093] Ageing of the coating is made in the SUNTEST conditions. No appreciableyellowing is observed after ageing. Example 4:

[0094] 136 mg of N-dodecyl-N'-phenylthiourea (sulfur precursor for core), 2.07 g of N,N,N’-tricyclohexylselenourea (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 260°C for 10 min under nitrogen flow and magnetic stirring. Then 2.15 mg of zinc stearate (zinc precursor for core) is added and heat is maintained for an additional 5 min. A zinc chalcogenide core with structure ZnSe / ZnSexS(1-x)is obtained. Then the mixture is cooled down to room temperature and 10.6 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 280°C. During temperature increase, 20 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 2 hours at 280°C, the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core- shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a structure ZnSe / ZnSexS(1-x) / ZnS. The central part of the core has a mean diameter 2.4 nm, the external layer of the core has a thickness of 1 nm, the core having globally a formula ZnSexS(1-x)with x about 0.95 and a shell of mean thickness 1.3 nm.

[0095] Absorption properties similar to example 1 are observed.

[0096] 40 mg of core-shell nanospheres – in alkaline water – are mixed in a waterborne polyester resin (75 parts) – hexamethoxymethyl melamine (25 parts) polymerizable composition and applied on a glass substrate to yield a 10 µm thick coating after cure, with a concentration of core-shell nanospheres of 75 ppm.

[0097] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 5:

[0098] 40 mg of core-shell nanoplates prepared as in example 1 are mixed in water:DMSO (6:1 in weight) with 50 mg of TINOGARD®HS (supplier BASF, CAS92484-48-5), 30 mg of TINUVIN®292 (supplier BASF, CAS 41556-26-7) and 0.7 g of a blend of waterborne polyester resin (85 parts) – hexamethoxymethyl melamine (15 parts), yielding upon curing the polymeric matrix. The dry content of the composition is 65%wt. The polymerizable composition is applied on a glass substrate to yield a 10 µm thick coating after cure.

[0099] Absorption properties similar to example 1 are observed.

[0100] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 6:

[0101] 40 mg of core-shell nanoplates prepared as in example 4 are mixed in water:DMSO (6:1 in weight) with 40 mg of TINUVIN®1130 (supplier BASF, CAS 104810-48-2), 20 mg of TINUVIN®249 (supplier BASF) and 0.6 g of a blend of waterborne polyester resin (85 parts) – hexamethoxymethyl melamine (15 parts), yielding upon curing the polymeric matrix. The dry content of the composition is 60%wt. The polymerizable composition is applied on a glass substrate to yield a 10 µm thick coating after cure.

[0102] Absorption properties similar to example 4 are observed.

[0103] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 7:

[0104] 40 mg of core-shell nanoplates prepared as in example 1 are mixed in water:DMSO (6:1 in weight) with 32 mg of TINUVIN®9945DW (supplier BASF, CAS 127519-17-9), 40 mg of TINUVIN®144 (supplier BASF, CAS 63843-89-0) and 0.8 g of a blend of waterborne polyester resin (85 parts) – hexamethoxymethyl melamine (15 parts), yielding upon curing the polymeric matrix. The dry content of the composition is 67%wt. The polymerizable composition is applied on a glass substrate to yield a 10 µm thick coating after cure.

[0105] Absorption properties similar to example 1 are observed.

[0106] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 8:

[0107] 40 mg of core-shell nanoplates prepared as in example 4 are mixed in water:DMSO (6:1 in weight) with 50 mg of TINUVIN®384-2 (supplier BASF, CAS 127519-17-9 ), 40 mg of TINUVIN®152 (supplier BASF, CAS 191743-75-6) and 0.55 g of a blend of waterborne polyester resin (85 parts) – hexamethoxymethyl melamine (15 parts), yielding upon curing the polymeric matrix. The dry content of the composition is 55%wt. The polymerizable composition is applied on a glass substrate to yield a 10 µm thick coating after cure.

[0108] Absorption properties similar to example 4 are observed.

[0109] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing. Example 9:

[0110] 25 mg of core-shell nanoplates prepared as in example 1 and 15 mg of core-shell nanoplates prepared as in example 4 are mixed in water:DMSO (6:1 in weight) with 20 mg of TINUVIN®1130 (supplier BASF, CAS 104810-48-2), 20 mg of TINUVIN®9945DW (supplier BASF, CAS 127519-17-9), 40 mg of TINUVIN®152 (supplier BASF, CAS 191743-75-6) and 0.65 g of a blend of waterborne polyester resin (85 parts) – hexamethoxymethyl melamine (15 parts), yielding upon curing the polymeric matrix. The dry content of the composition is 65%wt. The polymerizable composition is applied on a glass substrate to yield a 10 µm thick coating after cure.

[0111] Absorption properties similar to example 4 are observed.

[0112] Ageing of the coating is made in the SUNTEST conditions. No appreciable yellowing is observed after ageing.Example 10:

[0113] 5.69 g of zinc stearate (zinc precursor for core), 250 mg of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 412 mg of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 230°C for 90 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then the mixture is cooled down to room temperature and 7.95 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 260°C. During temperature increase, 20 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 260°C, 24 mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 3.0 nm and formula ZnSexS(1x) with x about 0.94 and a shell of mean thickness 0.8 nm. Example 11:

[0114] 17.07 g of zinc stearate (zinc precursor for core), 1.04 g of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 1.17 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 220°C for 90 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then the mixture is cooled down to room temperature and 21.5 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 250°C. During temperature increase, 54 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 250°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 3.5 nm and formula ZnSexS(1x)with x about 0.90 and a shell of mean thickness 0.7 nm.Example 12:

[0115] 29.08 g of zinc stearate (zinc precursor for core), 1.83 g of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 2.46 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 220°C for 30 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. 37.26 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 260°C. During temperature increase, 93 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 30 min at 260°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 6.0 nm and formula ZnSexS(1-x) with x about 0.88 and a shell of mean thickness 0.9 nm. Example 13:

[0116] 29.08 g of zinc stearate (zinc precursor for core), 1.83 g of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 2.45 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed and solubilized to the previous solution. The solution is then pumped by an HPLC pump at a flow rate of 0.75 ml / min is placed in an oven heated to 220°C where is placed the tubing in which passes the solution. The cores are collected in a round bottom flask. 37.2 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 250°C. During temperature increase, 93 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 250°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 2.8 nm and formula ZnSexS(1-x)with x about 0.97 and a shell of mean thickness 0.7 nmExample 14:

[0117] 50 g of zinc stearate (zinc precursor for core), 2.25 g g of N-dodecyl-N'- phenylthiourea (sulfur precursor for core), 3.71 g of selenium powder (selenium precursor for core) and 360 mL of octadecene are mixed in a 1-liter reactor. After degassing under vacuum, the mixture is heated to 230°C for 30 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then the mixture is cooled down to room temperature and 51.21 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 260°C. During temperature increase, 126 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 30 min at 260°C, 150mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 9.3 nm and formula ZnSexS(1-x) with x about 0.81 and a shell of mean thickness 0.8 nm. Example 15:

[0118] 17.07 g of zinc stearate (zinc precursor for core), 3.23 mL of dodecanethiol, 1.42 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 220°C for 90 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. Then the mixture is cooled down to room temperature and 21.5 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 250°C. During temperature increase, 54 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 250°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core-shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 7.0 nm and formula ZnSexS(1-x)with x about 0.91 and a shell of mean thickness 0.8 nm.Example 16:

[0119] 29.08 g of zinc stearate (zinc precursor for core), 5.6 mL of dodecanethiol, 2.45 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed in a round bottom flask. After degassing under vacuum, the mixture is heated to 220°C for 90 min under nitrogen flow and magnetic stirring, to synthesize the zinc chalcogenide core. 37.2 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 250°C. During temperature increase, 93 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 250°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core- shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 2.7 nm and formula ZnSexS(1-x) with x about 0.96 and a shell of mean thickness 0.7 nm. Example 17:

[0120] 29.08 g of zinc stearate (zinc precursor for core), 5.6 mL of dodecanethiol, 2.45 g of selenium powder (selenium precursor for core) and 120 mL of octadecene are mixed and solubilized to the previous solution. The solution is then pumped by an HPLC pump at a flow rate of 0.75 ml / min and placed in an oven heated to 250°C. Indeed, the oven includes a tubing in which passes the solution. The cores are collected in a round bottom flask. 37.2 g of zinc stearate (zinc precursor for shell) are introduced into the flask. The mixture is then heated to 250°C. During temperature increase, 93 mL of 1.0 M trioctylphosphine sulfide (TOPS, sulfur precursor for shell) is added dropwise, leading to the formation of a shell. After injection of TOPS and 3 hours at 250°C, 72mL of oleic acid are added and the mixture is cooled down to room temperature. The obtained core- shell nanoparticles were purified by adding isopropanol and centrifugation twice. The core-shell nanoparticles are finally dispersed in heptane. The core-shell nanoparticles have a core of mean diameter 3.2 nm and formula ZnSexS(1-x)with x about 0.92 and a shell of mean thickness 0.8 nm.Comparative example:

[0121] Example 1 is reproduced, but no shell is deposited on the nanoparticules. Finally Zinc chalcogenide nanospheres of formula ZnSexS(1-x) with x about 0.94 are obtained, with a mean diameter of 3 nm.

[0122] Absorbance of a dispersion of 2 mg of nanospheres in 3 mL heptane is measured with a Jasco V770 UV-visible-NIR instrument with increments of 1 nm, and reported on figure 3 (dotted line). Absorbance shows a maximum Amax =0.49 at 370 nm. Absorbance 0.9Amax=0.43 is observed for λ0.9=375 nm. And absorbance at λ0.9+20=395 nm isA+20=0.0214. The ratio0.9^^^^^ ^^^^ is estimated at 20.

[0123] 40 mg of nanospheres – in alkaline water – are mixed in a waterborne polyester resin (75 parts) – hexamethoxymethyl melamine (25 parts) polymerizable composition and applied on a glass substrate to yield a 10 µm thick coating after cure, with a concentration of nanospheres of 75 ppm.

[0124] Ageing of the coating is made in the SUNTEST conditions. A slight yellow colour appears after ageing. nanospheres without shell are prone to degradation as compared to core-shell nanospheres. Comparison of the diameter of the ZnSexS(1-x)cores

[0125] The following table shows the core radius for different composition of quantum dots having a ZnSexS(1-x) composition. Range of absorption Range of diameter of the core Composition (x value) wavelength (nm) (nm) 0.90-0.95 350-400 2.2-4.3 400-450 4.3-13.5 0.85-0.90 350-400 2.8-5.0400-450 5.0-15.0 0.80-0.85 350-400 3.7-6.5 400-450 6.5-17.0

[0126] It has been observed that the more sulfur there is in the ZnSexS(1-x) core, the lower is the highest absorption wavelength absorbed by the quantum dot. On the other hand, the larger the quantum dot is, the greater is the highest absorption wavelength absorbed by the quantum dot. Accordingly, aiming a constant highest absorption wavelength at 400- 450 nm, the more sulfur there is in the ZnSexS(1-x)core, the larger should be the quantum dot.

[0127] Moreover, it appears that the large particles (i.e., particles with core diameter superior to 4 nm) are particularly adapted to absorb wavelengths up to 450 nm. Thus, such particles are preferred for coatings protecting from UV. For instance, particles with large diameters may be used in packaging of polymerizable liquids that polymerize under UV or violet visible light.

Claims

CLAIMS 1. A core-shell nanoparticle comprising • A core of ZnSexS(1-x) material where x is in a range from 0.80 to 0.98 and having - a thickness of less than 10 nm, preferably less than 5 nm, for a core having a nanometric size in one dimension; - a section of less than 100 nm², preferably less than 50 nm², for a core having nanometric sizes in two dimensions; - a diameter of less than 20 nm, preferably less than 15 nm, more preferably less than 10 nm, for a core having nanometric sizes in three dimensions; and • A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

2. The core-shell nanoparticle according to claim 1, wherein sulfur is distributed in the core with a gradient of concentration, preferably the concentration of sulfur in the center of the core is less than the concentration of sulfur in the periphery of the core.

3. The core-shell nanoparticle according to claim 1, wherein sulfur is distributed in an external layer of the core.

4. The core-shell nanoparticle according to claim 3, wherein the core comprises a central part, an external layer and a Zinc sulfide shell, and wherein diameter of the central part of the core is in a range from 2 nm to 7 nm, the thickness of the external layer of the core is in a range from 0.3 nm to 3 nm and the thickness of the Zinc sulfide shell is in a range from 0.3 nm to 3 nm.

5. The core-shell nanoparticle according to any one of claim 1 to 4, wherein the core has a diameter of less than 20 nm, and x is in a range from 0.80 to 0.

90.

6. The core-shell nanoparticle according to claim 5, wherein the shell has a thickness greater than 0.3 nm and less than 3 nm, preferably greater than 0.3 nm and less than 1.5 nm.

7. The core-shell nanoparticle according to any one of claim 1 to 3, wherein the core has a thickness of less than 10 nm, and x is in a range from 0.80 to 0.

90.

8. The core-shell nanoparticle according to claim 7, wherein the shell has a thickness greater than 0.5 nm and less than 3 nm, preferably greater than 0.6 nm and less than 1.5 nm.

9. A core-shell nanoparticle comprising • A core of ZnSexS(1-x) material where x is in a range from 0.60 to 0.98, and having - a thickness of in a range of 3 nm to 10 nm, for a core having a nanometric size in one dimension; - a section in a range of 9 nm² to 100 nm², for a core having nanometric sizes in two dimensions; - a diameter in a range of 3 nm to 20 nm, for a core having nanometric sizes in three dimensions; and • A shell of ZnS material, said shell having a thickness greater than 0.3 nm and less than 5 nm, preferably greater than 0.3 nm and less than 1.5 nm.

10. A composite particle comprising core-shell nanoparticles according to any one of claims 1 to 9, said core-shell nanoparticles being embedded in an encapsulating material, preferably a metal oxide encapsulating material.

11. The composite particle according to claim 10, wherein the loading charge of the core-shell nanoparticles in the composite particle is at least 1%, preferably at least 2.5%, more preferably at least 5%, said loading charge being the mass ratio between the mass of core-shell nanoparticles comprised in a composite particle and the mass of said composite particle.

12. The composite particle according to claim 10 or 11, wherein the mean size of the composite particles is preferably in a range from 50 nm to 500 nm, more preferably from 50 nm to 250 nm.

13. A light filtering material comprising a matrix material and at least one population of core-shell nanoparticles according to any one of claims 1 to 9 dispersed in saidmatrix material; or at least one population of composite particles according to any one of claims 9 to 10 dispersed in said matrix material.

14. The light filtering material according to claim 13, wherein absorbance of said light filtering material has: • a local maximum absorbance of highest wavelength in the range from 350 to 450 nm, said local maximum having an absorbance value Amax for a wavelength λmax, • a value of 0.9Amax for a wavelength λ0.9, λ0.9 being greater than λmax; • a value of A+20 for a wavelength λ0.9+20 nm; and wherein0.9^^^^^^^^^is greater than or equal to 25, preferably greater than or equal to 50, more preferably greater than or equal to 100.

15. The light filtering material according to claim 14, wherein λmaxis in a range from 370 nm to 420 nm, preferably from 375 nm to 400 nm, more preferably from 380 nm to 395 nm.

16. The light filtering material according to claim 13, wherein the light filtering material is obtained by curing of a polymerizable composition comprising: • 0.1%wt to 10%wt of Zinc chalcogenide core-shell nanoparticles according to any one of claims 1 to 9 or composite particles according to any one of claims 10 to 12, • 0.1%wt to 10%wt of anti-UV organic compounds, • optionally stabilizers and / or antioxidants, and • 15%wt to 40%wt of a polymeric matrix material, the polymerizable composition having a dry extract in a range from 15%wt to 70%wt.

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