WO2024015009A1

WO2024015009A1 - Self-heating coatings

Info

Publication number: WO2024015009A1
Application number: PCT/SE2023/050733
Authority: WO
Inventors: Abhilash SUGUNAN; Kenth Johansson; Mikael JÄRN; Viveca WALLQVIST
Original assignee: Rise Research Institutes of Sweden AB
Priority date: 2022-07-13
Filing date: 2023-07-12
Publication date: 2024-01-18

Abstract

A self-heating coating for a substrate surface, wherein the self-heating coating comprises a carrier matrix and particles dispersed in the carrier matrix to a concentration of 0.05-50 wt.%. The particles having a size of 5-500 nm, and substantially absorbing radiation in the near infrared range between wavelengths of 800 nm – 2500 nm, while being substantially transparent to visible radiation between wavelengths of 400 nm – 800 nm. The particles transferring absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix and the self-heating coating

Description

SELF-HEATING COATINGS

TECHNICAL FIELD

[001] The present disclosure is related to self-heating coatings and anti-icing/de-icing of substrate surfaces.

BACKGROUND ART

[002] In colder areas ice formation on surfaces, such as pavements, zebra-crossings, wind turbine blades etc. is a large and costly problem.

[003] Different approaches have been taken concerning anti-icing or de-icing of pavements to limit the number of ice-related accidents as discussed in a review article by S. Xu, et. al., "Durability of pavement Materials with Exposure to Various Anti-Icing Strategies", Processes 2021, 9, 291. In this review article anti-icing strategies, including elastic surfaces or high-friction overlays, asphalt binders mixed with anti-icing additives, pavement heating technologies, de-icers, and fixed automated spray technology, are compared from different aspects.

[004] One way of trying to solve the icing problem of surfaces is to include in the surface material or a coating of the surface material, materials that emit or absorb infrared radiation. Thereby, heating up the surrounding material, de-icing the surface thereof. Such solutions are discussed for example in W007020355A1, JP2101209A1, US2013040129A, and US20140200301.

[005] In the area of wind turbine rotor blades, different ways of heating the wind turbine blade surface have been tried, such as heating the blade surface by means of a copper wire integrated in the blade/in the blade surface.

SUMMARY OF THE INVENTION

[006] It is an object of the present invention to provide an improved or at least an alternative self-heating or anti-icing/de-icing coating formulation for coating on a substrate surface without changing the visual appearance substantially, wherein the substrate surface can be any kind of surface, such as pavement surface, zebra-crossing, wind turbine rotor blades, clothing, etc. [007] According to a first aspect there is provided a self-heating coating for a substrate surface, wherein the self-heating coating comprises a carrier matrix and particles dispersed in the carrier matrix to a concentration of 0.05-50 wt.%, wherein the particles: have a size of 5- 500 nm, substantially absorb radiation in the near infrared range between wavelengths of 800 nm - 2500 nm, while being substantially transparent to visible radiation between wavelengths of 400 nm - 800 nm, wherein the particles transfer absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix and the coating.

[008] Such a self-heating or anti-icing/de-icing coating may be applied on or constitute a surface on any kind of substrate, such as on a wind turbine rotor blade, on a pavement, on a bicycle track, or on a road as a road marking, zebra crossing line, road-shoulder marking, or divider. The coating may also be used as a coating or smart surface for street signs, light signals or on side markers (sticks that mark the side of the road in winter) etc. In further embodiments, the coating may be a coating applied on an outdoor garment, a coating on handle bars, park benches etc. Hence, the coating formulation may be applied on or constitute a hard, medium hard, or soft substrate surface.

[009] With transparent is here meant that the particles in the coating are not affected by visible light and that the coating/matrix retains its original colour and appearance. Transparent in this context does not mean that the coating is transparent like a glass window. [0010] It has been shown that when used in white road marking paint on asphalt, the presence of the particles in the paint/binder increased the temperature of the white paint only by absorbing sun light. It was confirmed that sunlight during winter in Sweden was strong enough to melt ice on the substrate onto which the coating formulation had been applied. The more sunlight, the more ice-melting. If the surface gets a little warmer, it may be enough to prevent or delay ice build-up.

[0011] For road markings and bicycle tracks, the colored markings are usually cooler than the darker asphalt and tends to retain ice, leading to traffic accidents. With addition of near IR absorbent particles in the paint or coating, these would warm up under ambient sunlight (or IR lamps) leading to ice-melting and improved safety.

[0012] For wind-turbine blades, de-icing is an expensive and complicated process.

Availability of solar (or artificial IR lamp) induced ice-melting could prevent build-up of ice or removal of thin ice layers that affect the efficiency of wind-turbines. [0013] For outdoor clothing, the warmth retained by the clothing depends on the difference in temperature between the inside (body temperature) and the outside that is exposed to the cold. If the outer layer warms up due to sunlight, the wearer will feel more comfortable with thinner layer insulation.

[0014] The particles may have an average diameter of 5 - 500 nm. In one embodiment, the size is 15 - 300 nm. In yet an embodiment, the particles are >100 nm or 150-500 nm. Particles smaller than 5 nm are generally complicated to produce. Particles larger than 500 nm would appear visually non-homogenous and the heating effect would be poor.

[0015] Generally, the particles should have a very low absorption of, i.e. be substantially transparent to, visible light and have a high absorption of near infrared light (wavelength ranging between 800 - 2500 nm). Two such particle examples are cesium-doped tungsten oxide (Cs:WO3) and lanthanum hexaboride. Obviously, there are other alternative particles that absorb mainly near infrared light, while being substantially transparent to visible radiation.

[0016] The particles substantially absorb light within a spectral range of 800 nm - 2500 nm within the near infrared range (IR) and substantially transmit light within a spectral range of 400 nm - 800 nm within the visible range. The particles, hence, absorb wavelengths (i.e. < 2500 nm) which are abundantly available in sunlight and estimated to be about 56% of the total solar energy received on the surface of the earth.

[0017] In US20140200301 is described an application of using doped tin oxide or indium tin oxide particles in a coating formulation together with a water repellent material, which allows ice to melt and 'slip' and fall off from wind-turbine blades. These particles absorbs mid- IR radiation (usually in the range of 10s of micrometres). As most of the mid-IR radiation (and long-IR radiation) from the sun is absorbed in the ionosphere layer, it is very unlikely that sunlight will produce a large heating effect with such particles and the particles are likely to be effective only with man-made IR radiation sources (such as tungsten lamps, IR light emitting diodes (LED), etc.).

[0018] Substantial visible transmission implies that although the particles absorb light to some extent, it is practically considered as a visible transparent material. Vice-versa for IR absorption. The particles will transmit to some extent, but mostly absorb. Actual transmission and absorption numbers can be measured only in terms of certain sample parameters, notably, concentration, thickness (path length), matrix material (refractive index), particle size and/or shape, etc.

[0019] In one embodiment, such a coating has a visible transmission of light of at least 50%, or at least 60%, or at least 70%, or at least 75%, and an absorption of light within the near infrared range of at least 50%, or at least 60%, or at least 70%, or at least 80%.

[0020] In one example, the formed coating comprises white paint as the carrier matrix. This white paint having an average transmission of visible light of 83% and an average absorption of light within the near infrared range of 21%. By adding 0.5 wt.% of particles into such a carrier matrix, the resulting coating had a visible range transmission average of 75% and an infrared range absorption average of 60%. Adding 1.0 wt.% of particles into such carrier matrix, the resulting coating had a visible range transmission average of 70% and an infrared range absorption average of 70%. Adding 2.0 wt.% of particles into such a carrier matrix, the resulting coating had a visible range transmission average of 60% and an infrared range absorption average of 80%. The addition of particles into the carrier matrix clearly increased the absorption of light within the near infrared range of the formed coating, while only nominally decreased the visible range transmission.

[0021] The carrier matrix could be any kind of binder or carrier medium. The binder could be a solution/liquid, be a semi-solid, as long as the particles can be evenly dispersed in the binder.

[0022] A concentration of particles in the carrier matrix is 0.05-50 wt.%. The effective range may be 0.05-25 wt.%, 0.05-20 wt.%, 0.05-15 wt.%, 0.05-10 wt.%, 0.1-50 wt.%, 0.25-50 wt.%, 0.5-50 wt.%, 1-50 wt.%, 5-50 wt.%, 10-50 wt.%, 20-50 wt.%, 0.1-25 wt.%, or 0.5-10 wt.%. A concentration below 0.05% may lead to almost no heating with most common sources of light. A concentration above 50 wt.% or even above 25 wt.% may result in very limited further increase in the heating effect.

[0023] The particles may be selected from doped tungsten oxide, metal borides, and anisotropic nanoparticles of metals selected from gold, silver, platinum, palladium, and copper.

[0024] Particles of tungsten oxide may have a general formula of WyOz, wherein W is tungsten, O is oxygen, satisfying 2.2<z/y<2.999, and/or with a general formula of MxWyOz, wherein M is one or more elements selected from H, He, alkali metal, alkaline earth metal, rare earth element, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Be, Hf, Os, Bi, I, Yb, and W is tungsten, O is oxygen, satisfying 0.001<x/y<l, 2.0<z/y<3.0).

[0025] Particles being metal borides may have a general formula of M_nB_m, wherein M is selected from Sc, Y, La, Ac, Hf, Zr, or W, and m is 1-12 and n is 1 or 2.

[0026] With anisotropic nanoparticles is meant rods, prisms, sheets, etc. The particles used in the self-heating coating formulation may be of a single type of any of the above listed particles in the listed different groups of particles. Alternatively, the particles used in the selfheating coating formulation may comprise two or more particle types of anyone of the above listed particles in the listed different groups of particles. For example, the coating formulation may comprise a first particle and a second, different, particle in a 1:10 to 10:1 proportion. The proportion is chosen to select the range of wavelengths for absorption and intensity of absorption according to specific characteristic spectral range of the input radiation (e.g. incandescent lamps, mercury vapor lamps or Halogen lamps used in artificial lighting, or the sun).

[0027] The above listed particle types all selectively absorb near-IR and not visible radiation. When a material has sufficient 'free-electrons', specific bands of radiation is selectively absorbed due to a phenomenon called localized surface plasmon resonance (SPR). The absorption mechanism here is due to resonance, therefore it is possible to have a band of wavelengths that is strongly absorbed, while allowing the remaining wavelengths (both longer than or shorter than the resonance band) being largely unabsorbed. This is different from band-gap absorption, wherein all radiation with shorter wavelengths, (i.e. with equal or greater energy than the bandgap) are absorbed. Such resonance due to SPR is strongly dependent on the specific material, its crystal structure, presence of impurities, physical size and shape of the material. For example, noble metal nanoparticles (gold, silver, platinum, etc, and their alloys) with highly anisotropic shapes (e.g. in the form of nano-rods, nanometer thick sheets of various shapes) selectively absorb radiation in the near-IR. It is known in the scientific literature that doped tungsten oxides or hexaborides of some rare earth metals (e.g. lanthanum hexaboride) selectively absorb near-IR, in the range of 750 nm till 2500 nm, and is transparent to radiation in the visible wavelength range. For a coating to utilize solar radiation for a heating effect without strongly impacting the overall visible color of the said coating, particles within the family of doped tungsten oxide (e.g. cesium doped tungsten oxide) or halides of rare-earth metals (e.g. lanthanum hexaboride) are ideally suited. [0028] The thickness of the self-heating coating may be 0.001-25 mm.

[0029] The self-heating coating may have a thickness of 0.001-25 mm, or 0.001-10 mm,

0.001-1 mm, 0.001-0.1 mm, 0.001-0.01 mm, 0.01-25 mm, 0.1-25 mm, 1-25 mm, 10- 25 mm, 0.1-10 mm, 0.01-10 mm, or 0.01-5 mm.

[0030] In one embodiment, the self-heating coating may have a thickness of 0.01-10 mm.

[0031] Coatings with thicknesses in the upper thickness range may be obtained by applying the carrier matrix with particles dispersed therein in several subsequent applications onto the substrate. The self-heating coating may be applied on the substrate surface by means of e.g. a coating applicator, a brush or a foam roller. The thickness of the self-heating coating is the dry thickness. When the film dries/hardens, it becomes thinner, as compared to its wetthickness, as some of the solvent dries out and the material cross-links and hardens. The dry thickness of a coating may be at least 50% of the wet-thickness of the coating.

[0032] The above described self-heating coating absorbs near-IR radiation (sunlight or from man-made sources) and provide a heating effect to affect melting of ice formed on the coating, or alternatively prevents ice nucleation and build-up on the coating, during exposure to near-IR radiation. With a coating with a thickness of 0.001-25 mm, most or all of the near IR radiation may be absorbed by the particles in the coating, to maximize the needed surface heating effect. Such a thick coating may or may not be transparent, depending on the matrix used. Especially in the upper range of thickness it is very unlikely that the coating would be transparent. A thick coating ensures that almost all the incident near IR energy is transferred to heating of the coating. An even thicker coating may result in coating complications (longer drying time, difficult to apply, failure such as delamination, cracking, etc ). A thickness below 0.001 mm may also affect the original function of the coating (such as varnish, paint etc). Further, a particle size in the upper range mentioned above may provide more heat efficiency than a similar coating provided with particles of smaller size. Thinner coatings, for applications requiring transparency, i.e. coatings having a maximum thickness of maximum a few hundred nanometers, would not give any noticeable heating effect to e.g. induce ice-melting on the coating.

[0033] The coating may be a more or less even coating, but may have a thickness of at least 0.001 mm. The absorption of radiation by a material is directly related to the distance that the radiation passes through the material, compared to the wavelength of the radiation. A longer path length ensures greater absorption. The need for a coating to be visibly transparent, e.g. on a window, brings inherent limitations to the size of the absorbing particles within the film to less than about 50 nm to avoid visible light scattering. Similarly, the overall film, i.e. particles and the transparent matrix together, needs to be have a thickness of 200 nm or less to avoid the cumulative absorption of individual small particles adding up and significantly reduce the transparency.

[0034] The self-heating coating may comprise or essentially consist of the self-heating coating formulation.

[0035] The carrier matrix may be selected from a paint, a varnish, a lacquer, a dye, or an ink.

[0036] The carrier matrix may not be limited to the list above but may be any system that produces a coating or pattern on a surface. The coating formulation may be a colour, a coating formulation forming a high-visibility pattern on a surface, a coating formulation forming a weather resistant coating on a surface, a coating formulation forming a scratch resistant coating on a surface, a coating formulation forming UV protection, a coating formulation forming a dirt-repellent, a coating formulation forming a Durable-Water-Repellent (DWR), an ice-phobic coating, a coating formulation forming an anti-reflection coating.

[0037] The carrier matrix may comprise a polymeric material and/or an inorganic material.

[0038] The polymeric material may be e.g. polyurethanes, polyesters, polyolefins, acrylates, polyamides, silicones, and epoxys.

[0039] The inorganic material may be e.g. silicates, ceramics, and minerals.

[0040] Alternatively, the matrix material is a hybrid material comprising a polymeric material and an inorganic material.

[0041] The carrier matrix could also be bitumen and future bio-binders for roads such as lignin derivatives etc.

[0042] According to a second aspect there is provided a substrate with a self-heating coating arranged on a surface or portion of a surface thereof, wherein the self-heating coating is as described above.

[0043] The substrate surface or portion of the substrate surface, on which the selfheating coating is arranged, may be made of or covered with asphalt, glass, metal, concrete, stone, brick, composites, plastics, carbon fibres, polymer fibres, wood, or textile. [0044] The substrate surface may be a road surface, a bicycle track surface, a pavement surface, a wind turbine blade surface, an aircraft wing surface, an aircraft control surface, an aircraft propeller surface, an aircraft fuselage surface, a bench surface, a handle bar surface, a traffic sign surface, a road sign surface, an outdoor facade, an outdoor-wear fabric surface, or a stair surface.

[0045] When the substrate surface is a road surface, a bicycle track surface or a pavement surface, the self-heating coating may comprise as the carrier matrix a (white) paint or a component of a paint, used as or in a road marking, zebra crossing line, shoulder marking, divider etc.

[0046] When the substrate is for example a wind turbine blade surface or an aircraft wing surface, the carrier matrix of the self-heating coating could be or comprise a polyurethane or its derivatives.

[0047] When the substrate is an outdoor garment/a textile the self-heating coating could be a coating or an impregnation into which the particles are mixed that for example could be sprayed onto the textile surface, or other commonly used processes such as bath processes.

[0048] The substrate surface may be a road surface, a pavement surface, or a bicycle track surface and the carrier matrix may be a white colored paint.

[0049] The white paint may be paints for white road markings such as zebra crossings, road shoulders, lane markings, bicycle lane markings, etc.

[0050] The substrate surface may be a road surface, a pavement surface, or a bicycle track surface and the carrier matrix may be a non-white colored paint or a transparent paint.

[0051] The non-white paint may for example be a red or green colored paint.

[0052] The substrate surface may be a road sign surface or a traffic sign surface.

[0053] The substrate surface may be an outdoor facade.

[0054] The coating may be a coating/paint for a facade of a building or structure made of concrete, glass, wood, steel or other metal.

[0055] The substrate surface may be a wind turbine blade surface.

[0056] The substrate surface may be an aircraft wing surface, an aircraft control surface, an aircraft propeller surface, an aircraft fuselage surface, or an aircraft propeller surface. [0057] The substrate surface may be a stone, concrete or brick surface. [0058] Such a substrate surface is for example a surface of a pavement, stairs, a bench, etc.

[0059] The substrate surface may be a wooden surface.

[0060] Such a wooden surface may for example be a surface of a bench, a chair, and other furniture surfaces.

[0061] The substrate surface may be a metal surface.

[0062] The metal surface may for example be a surface of an outdoor metal safety rail, handle bars, etc.

[0063] The substrate surface may be an outdoor wear fabric surface.

[0064] For example textiles and garments for outdoor clothing, such as outer jackets, safety and high-visibility vests and jackets, etc.

[0065] The particles may be dispersed in the carrier matrix through for example mixing or stirring using e.g. high-shear mixers, homogenizers, and/or through ultra sonication, etc. [0066] If necessary, the particles may be surface modified with common dispersing agents, (e.g. polyacrylic acid (PAA), PSS) to improve the dispersibility of the particles.

[0067] The particles may be chosen from anyone or more of the above listed particles.

[0068] According to a third aspect there is provided a method of preparing a selfheating coating on a substrate surface or a portion of a substrate surface, the method comprising: a) preparing a self-heating coating formulation by: providing a carrier matrix, providing particles, wherein the particles have a size of 5-500 nm, and substantially absorb radiation in the near infrared range between wavelengths of 800 nm - 2500 nm, while being substantially transparent to visible radiation between wavelengths of 400 nm - 800 nm, and dispersing the particles in the carrier matrix to a concentration of 0.05 - 50 wt.%, the particles transferring absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix. b) providing a substrate onto a surface or a portion of a surface of which the coating is to be applied, c) applying the self-heating coating formulation on said substrate surface or portion of said substrate surface, d) drying the self-heating coating, e) optionally, repeating steps c) and d) one or more times.

[0069] The method may comprise to apply the self-heating coating on the substrate surface or to a portion of the substrate surface to a coating thickness of 0.001 - 25 mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Fig. 1. shows absorption spectra of four different coatings prepared with a carrier matrix consisting of white paint for walls, with 0 wt.%, 0.5 wt.%, 1 wt.%, and 2 wt.% near infrared (IR) absorbent particles blended into it. Within the marked visible range, the absorption of all the four coatings was very low, with a maximum of 38 % absorption. At the same time, the absorption within the near infrared wavelengths was very high for the coatings with 0.5 wt.%, 1 wt.%, and 2 wt.% particles with an average absorption of 63%, 72%, and 79%, respectively, compared to only a 21 % absorption for the carrier matrix only (0 wt.% particles). [0071] Fig. 2 shows the friction coefficient as a function of irradiation time for three different carrier matrices with 0 wt.%, 1 wt.% and 2 wt.% near IR absorbent particles blended into it. For the carrier matrices with IR absorbent particles, the friction coefficient increases rapidly after 10 - 15 minutes of irradiation, while the carrier matrix with 0 wt.% particles shows no significant change even after 20 minutes of irradiation.

[0072] Fig. 3 shows the surface temperature of a coating with 0%, 1%, and 2% by weight of IR particles blended into a carrier matrix comprising white-road markings paint. The starting temperature of all the coatings where - 18°C (freezer). Upon irradiation for 20 minutes with an IR lamp, the coatings with IR absorbent particles heated up to above 0°C after 10 minutes, while the temperature of the coating with 0 wt.% IR particles did not rise above 0°C [0073] Fig. 4 shows the difference between a bare glass substrate and a glass substrate coated with three different coatings comprising only varnish (0 wt.% IR particles), and varnish with 2 wt.% and 10 wt.% IR absorbent particles, respectively. The IR irradiation was adjusted to represent the sun during the months of January, April, July, and October (South-Western Sweden (57.7 N, 13.4 E)).

[0074] Fig. 5 shows the temperature difference between a bare glass substrate and a glass substrate coated with a coating comprising a carrier matrix being a red bicycle track paint with 0 wt.%, 0.5 wt.%, and 2 wt.% IR particles, respectively, blended into it. The IR irradiation was simulating the sun during April (South-Western Sweden (57.7 N, 13.4 E)).

[0075] Fig. 6 shows the coating surface temperature on a glass substrate coated with a coating comprising a carrier matrix being a red bicycle track paint with 0 wt.%, 1 wt.%, and 2 wt.% IR particles, respectively, blended into it. The IR irradiation was simulating the sun during July (South-Western Sweden (57.7 N, 13.4 E)). [0076] Fig. 7 shows the temperature difference between two coatings, red bicycle track paint with 0 wt.% IR particles and 0.5 wt.% IR particles, respectively, blended into it. The coating was applied on an asphalt slab and placed outdoor during morning of May (in Stockholm in Sweden).

DETAILED DESCRIPTION

[0077] Below is described in more detail a self-heating coating formulation for coating on a substrate surface. The coating formulation comprises a carrier matrix and particles dispersed in the carrier matrix to a concentration of 0.05-50 wt.%. The particles have a size of 5-500 nm, substantially absorb radiation in the near infrared range, while being substantially transparent to visible radiation. The particles transfer absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix.

[0078] Such a self-heating or anti-icing/de-icing coating formulation may be applied on or constitute a surface on any kind of substrate, such as on a wind turbine rotor blade, on a pavement, on a bicycle track, or on a road as a road marking, zebra crossing line, roadshoulder marking, divider, etc. The coating may be applied on an outdoor garment, on handle bars, on park benches etc. Hence, the coating formulation may be applied on or constitute a hard, medium hard, or soft substrate surface.

[0079] The particles may be selected from tungsten oxide, metal borides, and/or from, anisotropic nanoparticles of metals selected from gold, silver, platinum, palladium, and copper.

[0080] The carrier-matrix could be any kind of binder or carrier medium. The binder could be a solution/liquid, be a semi-solid, as long as the particles can be evenly dispersed in the binder.

[0081] In the examples below the concept above is shown in different carrier matrices used with two types of near infrared absorbing particles for different applications.

EXPERIMENTAL

[0082] Example 1. Carrier Matrix - white wall paint

[0083] Experimental details

Commercial wall paint (Alcro, Milltex 5 - Tonad vit S0502-Y) was used as a carrier matrix and 0 wt.%, 0.5 wt.%, 1 wt.%, and 2 wt.%, respectively, of infrared absorbing particles (Cs:WO3, Luxacai 202, William Blythe Ltd, 100-200 nm) were blended into it using a high-shear blender (Ultra-Turrax Homogenizer). These carrier matrices were painted on brushed aluminium substrates. The spectral absorption of the coatings prepared from these infrared absorbing coating formulations were measured using a spectrophotometer (Perkin-Elmer Lambda 900), within the wavelength range of 250 nm to 2500 nm. The dry thickness of such coating was approximately 50 pm.

[0084] Results

The absorption spectra of the coated surfaces are shown in Fig.l. The absorption spectra show a substantially increased absorption in the near infrared, with systematically higher absorption for 0.5 wt.%, 1 wt.%, and 2 wt.% infrared absorbent particles in the carrier matrix (63%, 72% and 79% respectively), compared to the carrier-matrix only (21 %). Correspondingly, there was only a nominal increase in the absorption of radiation in the visible spectra with increased proportion of the IR absorbent particles. This indicates that when exposed to 'white light' radiation, e.g. from the sun, the carrier matrices with the IR absorbent particles will absorb more near-IR energy than the carrier matrix only.

Example 2: Carrier matrix - road marking white paint

[0085] Experimental details

[0086] Paint

Commercial road markings paint (Geveko Markings Sweden AB), a waterborne acrylic-based paint was used as a carrier-matrix for modifying road markings paints.

[0087] Near infrared absorbing particles

The following IR-absorbing particles were used: Cesium-doped Tungsten Oxide (Cs: WO3) (Luxacai 202, William Blythe Ltd), particle size: 100-200 nm and Lanthanum hexaboride (LaB6) (4430DX, Sky Spring Nanomaterials, Inc.), particle size: 50-80 nm.

[0088] Preparation of self-heating coating substance for coating

[0089] Direct mixing in paint

Mixing of IR particles in the binder to a concentration of 0 wt.%, 0.01 wt.%, 0.05 wt.%, 0.1 wt.%, 1 wt.%, 2 wt.% and 10 wt.%. The mixing was performed in an Ultra-Turrax Homogenizer.

[0090] Substrates

The binder-particle mixtures were applied to the following substrates:

• Glass plates Asphalt samples

[0091] Application of coating of binder-particle mixtures onto substrates

The mixed color-IR particle samples were applied on the substrate surface using a medium soft color roller. Each paint was applied in 4 coats with 30 minutes drying time between each application to obtain sufficient thickness of the coating. The number of coats is not crucial, and is dependent on e.g. the brush used. The objective is to obtain a continuous coating. The dry thickness of such coating may be several 10s of micometers. It was, however, difficult to determine the coating thickness exactly due to the combination the 10s of mm scale roughness of the asphalt surface and the relatively thin and conformal coating applied on the asphalt.

[0092] Characterisation of the coating on the substrate

The applied coating was characterized as follows: [0093] Heating tests with IR lamp

An IR lamp with a maximum intensity of 1900 W / m2 with an output optical spectrum range of 380-3000 nm was used. The distance between the lamp and the color samples was varied between 40-70 cm. Warm-up tests were performed in the lab (room temperature, screening tests with colors applied to slides). The surface temperature of the road color samples in the screening tests was measured with an IR camera (Optris PI230) and with a contact thermometer. The surface temperature of the paints was recorded as a function of the exposure time. Each sample was irradiated separately with exactly the same position in relation to the IR lamp, eliminating the risk of uneven exposure.

[0094] Friction measurements

The friction measurements were performed using a Mark 111 B Slip Test Instrument The friction is measured from a scale of 0 - 1 where 0 is low friction and 1 is high friction. The surface temperature of the road paint samples in the friction measurements was measured continuously with a contact thermometer [0095] Cold room tests

Friction and temperature measurements were performed at -5°C in a cold room (+ 5°C). [0096] Asphalt slabs (ABT 8, for cycle paths) coated with a regular and a modified road marking color were placed in a freezer at -18°C overnight. The slabs were transferred to the cold room and ice water (deionized water and ice) was applied with a wide brush and the water was immediately frozen to a thin ice film. The measurements were performed as soon as possible.

[0097] Outdoor tests-Exposure to real sunlight

Prior to the outdoor test, asphalt slabs (ABT 8, for cycle paths) coated with a regular and a modified road marking color were stored in a freezer at -18°C. Just before the test, the slabs were brought outdoors into direct sunlight. An IR camera recorded the temperature changes of the paint coating on the asphalt slabs. The experiment was performed during a morning between 09.30-12.30 at a temperature of below 10 degrees. The heating effect was measured on reference and modified paints simultaneously, so any effect from air temperature would apply to both equally.

RESULTS

[0098] Different IR particle concentrations in the carrier-matrix (in this example: white road-paint) were evaluated. Friction properties of road markings paint on asphalt with the addition of 1% and 2% by weight of Cs:WO3 particles and a reference paint (without particles) measured as a function of IR exposure time at -5°C were measured (Fig. 2). The samples were irradiated for 20 minutes. It is shown that the friction increases sharply in the road paint with IR particles after about 15 minutes of IR exposure. This is an indication the created ice layer has melted away due to heating of the surface. At the same time, the reference road-paint without particles remains largely unchanged with low friction.

[0099] The surface temperature of the paint coating (including the reference paint) increases with increasing exposure time. Carrier matrices with 1 wt.% and 2 wt.% Cs:WOs absorbent particles rises to 5°C and 4°C respectively during 20 minutes of exposure to IR radiation, while for the carrier matrix without IR particles the coating temperature remained at -1°C. The temperature curves flatten out towards a plateau during longer radiation times, but the temperature level rises with increased concentration of IR absorbent particles. When the exposure lamp is 'turned-off', the surface temperature of the carrier matrices with 1 wt.% and 2 wt.% of IR particles was lowered, but remained higher than 0°C, while the surface temperature of carrier matrix only coating dropped to about -2°C and remained constant (Fig.3).

[00100] The asphalt block with different carrier matrix coatings was placed outdoors for measuring the heating effect under actual sunlight during April (in Stockholm in Sweden). The surface temperatures after 90 minutes of exposure is tabulated in Table 1. By adding 1 and 2 wt.% of either IR absorbent particle (Cs:W0₃ and La Be), the surface temperature of the coating increased by 7°C and 8.5°C, respectively.

Table 1

[00101] Thus, outdoor exposure with direct sunlight provides a clear heating of road marking colors containing IR-absorbing particles. Addition of Cs:W0₃ and La Be particles, up to 2 wt.%, have equivalent heating properties (kinetics and temperature level), and leads to a temperature rise about 7-8°C higher than reference road paints without particles.

Example 3: Transparent polyurethane varnish

[00102] Experimental details

A commercial off the shelf two-component polyurethane (PU) varnish was purchased from Biltema. This was used as carrier-matrix. Cs:WO3 particles (Luxacai 202, 100-200 nm) was mixed into the carrier-matrix at a final dry content of 2 wt.% and 10 wt.%, respectively. In case of 2 wt.%, 0,094 g of Luxacai was added to 8 mL of component A of the PU varnish and mixed with a Vortex shaker. The mixture was then sonicated in an ultrasonication bath for 1 hour before 2 mL of component B of the PU varnish was added and mixed with a Vortex shaker. The mixture was sonicated for another 30 minutes and then applied to a 4 mm thick float glass substrate with a film applicator with a wet film thickness of 90 pm, which after drying was 19 pm thick. The coated glass was finally cured at 70°C for 2 hours.

[00103] In case of 10 wt.%, 0.49 g of Luxacai was added to 8 mL of component A of the PU varnish and mixed with a Vortex shaker. The mixture was then sonicated in an ultrasonication bath for 1 hour before 2 mL of component B of the PU varnish was added and mixed with a Vortex shaker. The mixture was sonicated for another 30 minutes and then applied to a 4 mm thick float glass substrate with a film applicator with a wet film thickness of 90 pm and a dry thickness of 22 pm The coated glass was finally cured at 70°C for 2 hours. [00104] The average solar irradiance in January, April, July, and October, at a geographic location in South-Western Sweden (57.7 N, 13.4 E) was calculated based on SMHI data for the year 2008, as an example. The IR lamp was adjusted to correspond to these values to estimate the heating effect produced during different months of the year. The temperature of the coated glass under solar simulating irradiation was measured using a thermal camera.

[00105] Results

Under IR irradiation, all surfaces heated up to varying temperatures. The surface temperatures at two locations, one on bare glass (uncoated), adjacent to the coated part and that of the coated part of the substrate were measured. The difference between these measured temperatures for each of the coating were recorded. Three different types of coating were measured;

Sample-1: only carrier-matrix as reference without any particles;

Sample-2: Carrier-matrix with 2 wt.% particles;

Sample-3: Carrier-matrix with 10 wt.% particles.

[00106] The difference between the temperature of the uncoated and coated parts of the glass substrates for each simulated months and each sample is summarized in Fig. 4. In brief, the results are described here:

[00107] For simulated January, the temperature of the Sample-1 increased by only 1.7°C, while the temperature of the Sample-2 and Sample-3 of particles increased by 6.2°C and 8.8°C, respectively.

[00108] For simulated April, the temperature increase under simulated sunlight for Sample-1, Sample-2, and Sample 3 were 3.7°C, 11.3°C, and 18.7°C, respectively.

[00109] For simulated July, the temperature increase under simulated sunlight for Sample-1, Sample-2, and Sample 3 were 7.9°C, 19.6°C, and 18.9°C, respectively.

[00110] For simulated October, the temperature increase under simulated sunlight for Sample-1, Sample-2, and Sample 3 were 3.4°C, 9.2°C, and 15.9°C, respectively.

[00111] These results indicate that e. g. in January at the location in South-Western

Sweden, if the Sample-3 was placed outdoor and the air temperature was equal or above -8°C, then the sunlight would be sufficient to heat up the Sample-3 to produce an ice melting effect. In comparison, for the coating with carrier-matrix only (Sample-1), the outside air temperature in January needs to be as high as -1.7°C for the available sunlight to heat the coating and produce similar ice-melting effect. This improved heating effect is even more pronounced in April and October, when the icing conditions are called 'glaze-ice' or 'black-ice', wherein the ice forms a smooth transparent layer. This type of ice is dangerous in terms of public safety (slippery and invisible) and high energy required to actively de-ice structures such as roadsurfaces or wind-turbine blades makes it uneconomical. A passive form of de-icing can be achieved by using this invention with only sunlight.

Example 4: Red bicycle track paint

[00112] Experimental details

A commercial two-component red paint for bicycle tracks (Base: PlastiRoute RollGrip 3020 traffic red, and initiator: Noviper BP50) obtained from Geveko Markings Sweden AB was used as another example of Carrier-Matrix. IR absorbing particles (Luxacai 202, 100-200 nm) were blended in with 0%, 0.5%, 1% and 2 % by weight were mixed into the base material by means of a high shear homogenizer for 15 min at 1000 rpm. To 40 g of the base, 0.0g, 0.2 g, 0.4 g, or 0.8 g (0%, 0.5%, 1%, or 2%) of the IR absorbent particles were added and then after blending, 0.6 g of the initiator was added. These samples were respectively called Carrier matrix-only, Carrier-matrix-0.5, Carrier-matrix-1, and Carrier-matrix-2.

[00113] The initiator was added and within 5 minutes the mixture was applied on glass slides as well as asphalt slabs and allowed to cure at room temperature for 30 minutes. The dry thickness of such coating was ranging from 1.5 - 2.4 mm. The cured coatings, both on glass slides and asphalt slabs, were placed under an IR lamp and exposed to IR radiation at various distances, corresponding to average sunlight for April and July (as described in Example 2). Furthermore, the coated asphalt slabs were cooled to 5 degrees Celsius and then placed outdoors in early 03 May, 2022 between around 9 am till about 2 pm (Location: Stockholm). The increase in temperature for obtained from these asphalt slab samples were recorded with a thermal camera.

[00114] Results

The IR particles were mixed into an example of carrier-matrix comprising of commercially available red colored road-paint used for marking bicycle tracks in Stockholm city. Using the indoor measurement set up similar to example 2, the glass substrates coated with 0% (Carriermatrix only), 0.5%, 1% and 2% were illuminated under simulated sunlight of April and July. In the case of April, the temperature difference between the bare glass substrate and the different coatings consisting of carrier-matrices with different proportions of IR absorbent particles is shown in Fig. 5. The observed temperature differences were recorded to be 3.9°C, 8.8°C, and 8.5°C, respectively for 0 wt.%, 0.5 wt.% and 2 wt.% of particles. This indicates that one could obtain an ice-melting effect if it were a sunny day in April with an outside temperature of equal or above about -8°C, if the present invention were used. In a real scenario, taking into account the possibility of wind-chill effects and sporadic cloud covers, it is necessary maximize the temperature difference in order to have an ice-melting effect under realistic conditions. At the same time, it is necessary to ensure that the temperature of the modified carrier matrices do not exceed safe levels during summer months when there is an abundance of sunlight. The simulated July conditions revealed a maximum temperature of 70°C for the carrier matrix with 2 wt.% particles (Fig. 6). This is within the accepted levels for typical road surfaces.

[00115] In order to evaluate a real outdoor performance, the samples prepared on asphalt slabs with different types of coatings were pre-cooled to 5°C and then placed outdoor. The weather conditions were mostly sunny, with occasional cloud cover. The changes in temperature over time was recorded with a thermal camera for asphalt slabs coated with carrier-matrices containing 0 wt.%, and 0.5 wt.% particles. The temperature difference between these two coated surfaces are shown in Fig.7. The high-frequency fluctuations in temperature represents wind related cooling effects and the slower but more intense fluctuations in the temperature is due to occasional cloud cover. Due to these interferences, the outdoor tests showed a maximum difference in temperature of 4.3°C, i.e. carrier matrix modified with 0.5 wt.% particles were 4.3°C hotter than unmodified carrier-matrix.

Discussion

[00116] The results above are shown for two different kinds of near infrared absorbing particles being substantially transparent to visible radiation, Cs:WOs and La Be, carrier matrices being wall paint, white road marking paint, transparent polyurethane varnish and red bicycle track paint and substrates being brushed aluminium, glass and asphalt. From these results, it is clear that the concept would give similar results also for the other above listed near infrared absorbing particles being substantially transparent to visible radiation as long as the particles absorb invisible infrared energy from the incident radiation and transmits this absorbed energy into the surrounding carrier matrix. It is clear that the concept would give similar results also for other carrier matrices mentioned above, such as lacquer, dyes or inks, into which infrared absorbing particles have been dispersed, enabling the entire coating to be uniformly heated when it is illuminated with light that contains infrared radiation. The substrate onto which the coating is applied, hard substrates or softer substrates such as textile, is of minor importance as long as an appropriate carrier matrix is chosen. The selection of carrier matrices for blending the infrared absorbing particles into, according to the nature of the substrate, the method of blending the particles, and the method of applying the coating on the substrate, will be known to people skilled in those areas, such as textile dyeing, roadmarkings painting, abrasion protective coatings on wind turbine blades, aircraft surfaces, etc.

Claims

1. A self-heating coating for a substrate surface, wherein the self-heating coating comprises:

- a carrier matrix,

- particles dispersed in the carrier matrix to a concentration of 0.05-50 wt.%, wherein the particles:

- have a size of 5-500 nm,

- substantially absorb radiation in the near infrared range between wavelengths of 800 nm - 2500 nm, while being substantially transparent to visible radiation between wavelengths of 400 nm - 800 nm, wherein the particles transfer absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix and the coating.

2. The self-heating coating of claim 1, wherein the particles are selected from tungsten oxide, metal borides, and anisotropic nanoparticles of metals selected from gold, silver, platinum, palladium, and copper.

3. The self-heating coating of claim 1 or 2, wherein a thickness of the self-heating coating is 0.001 - 25 mm.

4. The self-heating coating of claim 3, wherein the thickness of the self-heating coating is 0.01-10 mm.

5. The self-heating coating of any of claims 1-4, wherein the carrier matrix is selected from a paint, a varnish, a lacquer, a dye, or an ink.

6. The self-heating coating of any of claims 1-5, wherein the carrier matrix comprises a polymeric material and/or an inorganic material.

7. A substrate with a self-heating coating arranged on a surface or portion of a surface thereof, wherein the self-heating coating comprises the self-heating coating of any of claims 1-6.

8. The substrate of claim 7, wherein the substrate surface or portion of the substrate surface on which the self-heating coating is arranged, is made of or covered with asphalt, glass, metal, concrete, stone, brick, composites, plastics, carbon fibres, polymer fibres, wood, or textile.

9. The substrate of any of claim 7 or 8, wherein the substrate surface is a road surface, a bicycle track surface, a pavement surface, a wind turbine blade surface, an aircraft wing surface, an aircraft control surface, an aircraft propeller surface, an aircraft fuselage surface, a bench surface, a handle bar surface, a traffic sign surface, a road sign surface, an outdoor facade, an outdoor-wear fabric surface, or a stair surface.

10. The substrate of claim 9, wherein the substrate surface is a road surface, a pavement surface, or a bicycle track surface and the carrier matrix is a white colored paint.

11. The substrate of claim 9 or 10, wherein the substrate surface is a bicycle track surface and the carrier matrix is a non-white colored or transparent paint.

12. The substrate of claim 9, wherein the substrate surface is a road sign surface or a traffic sign surface.

13. The substrate of claim 9, wherein the substrate surface is an outdoor facade.

14. The substrate of claim 9, wherein the substrate surface is a wind turbine blade surface.

15. The substrate of claim 9, wherein the substrate surface is an aircraft wing surface, an aircraft control surface, an aircraft propeller surface, an aircraft fuselage surface, or an aircraft propeller surface.

16. The substrate of claim 8, wherein the substrate surface is a stone, concrete or brick surface.

17. The substrate of claim 8, wherein the substrate surface is a wooden surface.

18. The substrate of claim 8, wherein the substrate surface is a metal surface.

19. The substrate of claim 9, wherein the substrate surface is an outdoor wear fabric surface.

20. A method of preparing a self-heating coating on a substrate surface or a portion of a substrate surface, the method comprising: a) preparing a self-heating coating formulation by:

- providing a carrier matrix,

- providing particles, wherein the particles: have a size of 5-500 nm, substantially absorb radiation in the near infrared range between wavelengths of 800 nm - 2500 nm, while being substantially transparent to visible radiation between wavelengths of 400 nm - 800 nm,

- dispersing the particles in the carrier matrix to a concentration of 0.05 - 50 wt.%, the particles transferring absorbed near infrared radiation as heat to the carrier matrix, thereby increasing the temperature of the carrier matrix, b) providing a substrate, onto a surface or a portion of a surface of which the coating is to be applied, c) applying the self-heating coating formulation on said substrate surface or portion of said substrate surface, d) drying the self-heating coating, e) optionally, repeating steps c) and d) one or more times.

21. The method of claim 20, wherein the self-heating coating formulation is applied on the substrate surface or to a portion of the substrate surface to a coating thickness of 0.001