Composite material and method for producing a composite material

Abstract

The invention relates to an optically transparent composite material consisting of: a polymer film (5) transparent to IR radiation, the polymer film (5) transparent to IR radiation having a mean spectral transmission of at least 40% in a wavelength range of 5 µm to 30 µm; and an IR radiation-reflecting coating (2) applied onto the polymer film, the coating comprising at least one electrically conductive material.

Description

[0001] Composite material and method for producing a composite material

[0002] Technical area

[0003] The invention relates to an optically transparent composite material with thermal radiation reflecting properties and to a method for producing such a composite material.

[0004] State of the art

[0005] According to the common definition, light visible to the human eye is electromagnetic radiation in the wavelength range from 380 nm to 780 nm. Infrared radiation is divided into three ranges: near IR from 690 nm to 3.0 pm, mid IR from 3.0 pm to 50 pm and far IR in the wavelength range from 50 pm to 1 mm.

[0006] Materials with a sufficiently high transmittance are described as "transparent" in the visible wavelength range. The transmittance is defined as the quotient of the light intensity behind an obstacle divided by the light intensity in front of the obstacle. The transmittance therefore ranges between 0 and 1, or between 0% and 100%.

[0007] An example of a material with a very high light transmittance is Plexiglas, which has a transmittance of 92%. Other types of glass have lower transmittances, such as thermal insulation glass with a transmittance of 73% to 80%. In comparison, heavily tinted sunglasses have a transmittance of around 18%.

[0008] Common thermal insulation methods are based on the absorption of radiation by appropriate dyes or pigments and are particularly suitable for shielding thermal radiation in the wavelength range between 0.8 pm and 2 pm. The absorbed energy is largely transferred to the object or room to be insulated by heat conduction. Particular problems arise in the case of glazed surfaces of buildings. These must generally be highly transparent to visible light and, at the same time, ensure good thermal insulation. In modern thermal insulation glazing (e.g. double and triple glazing), the space between the glass panes is filled with gas. Air was used in the past, but argon is now predominantly used. The side of the pane of effective thermal insulation glazing facing the gas side is provided with a thin, transparent, heat-reflecting layer.This type of layer is usually sputtered on and has very good heat reflection properties.

[0009] However, these very thin, sputtered layers are fragile to mechanical influences and cleaning. An important advantage of this type of heat-insulating window is that the window panes can be cleaned and polished without losing their heat-reflecting properties. This is ensured because the sputtered layer is protected from mechanical influences due to its positioning on the gas-exposed side of the window glass.

[0010] Retrofitting such thermal insulation in existing buildings is therefore only possible by completely replacing the windows, since subsequent sputtering of an installed window can only be carried out on its surfaces facing away from the gas, which means that the sputtered layer is subsequently not protected against external influences.

[0011] Various approaches to solving the problem of retrofitting and improving the thermal insulation of existing windows are known in the prior art. For example, US Pat. No. 6,830,713 B2 describes processes for producing coextruded polymer multilayer films. These films are used to produce films that effectively reflect UV, visible, and IR radiation.

[0012] DE 699 21 053 T2 describes a coated glass that can be used in windows for homes and vehicles. This type of coated glass offers effective solar protection with minimal emissions from the glass. The coatings contain tin oxide with various dopants, but the described method for applying the coating is not suitable for retrofitting existing glazing.

[0013] EP 1 025 057 B1 describes a thermal insulation coating that is almost completely transparent in the visible range of the electromagnetic spectrum and exhibits only low absorption in the near-IR range. The coating comprises several cross-linked or polymerized IR-reflecting layers. Conductive layers represent another option for reflecting electromagnetic radiation. For example, pigments such as graphite, silver, or gold can be used. Such layers are used, for example, as insulating or rescue films, which reflect heat radiation emitted by the body, thus keeping a person warm or minimizing heat loss. However, this type of film is not transparent.

[0014] Finally, transparent conductive layers with silver nanowires are also known from the state of the art (Julia Graubmann et al., "Silver nanowires: a new nanomaterial with advances for electrical, optical and IR systems", Proc. SPIE 11159, Electro-Optical and Infrared Systems: Technology and Applications XVI, 1115903 (9 October 2019); doi: 10.1117 / 12.2532245). Such conductive layers have the property of reflecting thermal radiation, particularly in the range between 3 pm and 50 pm, and also exhibit good transparency in the visible wavelength range. However, the layers are not stable against external influences such as mechanical impacts or scratching and can also be easily washed off with cleaning agents or water.

[0015] If a binder is added to such a transparent conductive layer in a typical concentration to improve the stability of the layer, this leads to a loss of the heat-reflecting effect, especially if the transparency of the layer in the visible wavelength range is to be maintained at the same time.

[0016] There is therefore still a need for a coating that can be subsequently applied to glazing, which on the one hand has a high optical transparency and on the other hand ensures good thermal insulation by reflecting infrared radiation, in particular by reflecting infrared radiation in the mid-IR range.

[0017] Description of the invention

[0018] The object of the present invention is therefore to provide a coating that exhibits high transparency in the visible wavelength range, ensures good thermal insulation by reflecting infrared radiation, can be easily applied to permanently installed surfaces, and exhibits good resistance to mechanical influences. This object is achieved according to the invention by the composite material according to independent claim 1. Further advantageous aspects, details, and embodiments of the invention emerge from the dependent claims, the description, and the drawings.

[0019] The present invention provides an optically transparent composite material consisting of a polymer film transparent to IR radiation, wherein the polymer film transparent to IR radiation has an average spectral transmission of at least 40% in the wavelength range from 5 pm to 30 pm, and an IR radiation-reflecting coating applied to the polymer film, wherein the coating comprises at least one electrically conductive material.

[0020] Surprisingly, it has been shown that the combination of a polymer film transparent to IR radiation with a coating applied to this polymer film, which comprises at least one electrically conductive material and reflects IR radiation, exhibits excellent properties with regard to the desired optical transparency. Furthermore, the composite material ensures good thermal insulation by reflecting infrared radiation.

[0021] Radiation in the visible wavelength range can penetrate glazed surfaces equipped with the optically transparent composite material according to the invention into the space behind the glazed surface with virtually no loss. The good thermal insulation properties of the composite material are based on the fact that the polymer film serving as the carrier material for the coating is transparent to IR radiation. Therefore, infrared radiation can penetrate the polymer film and is reflected back through the coating, which has a high reflectivity for IR radiation. Thermal radiation is reflected back into the space behind the glazed surface and does not penetrate to the outside. At the same time, the polymer film ensures reliable and durable protection of the coating against mechanical damage or abrasion.

[0022] The term "polymer film transparent to IR radiation" is understood in the context of the present invention to mean that the polymer film has an average spectral transmission of > 40% in the wavelength range from 5 pm to 30 pm. To determine the average spectral transmission, the transmission of the polymer film in the wavelength range from 5 pm to 30 pm is determined in wavelength steps of, for example, 5 nm, the measured transmission values ​​are added together, and the sum of the transmission values ​​is divided by the number of measured values. The term "IR radiation-reflecting coating" is understood in the context of the present invention to mean that the coating reflects at least 10% of the IR radiation averaged over the wavelength range in the wavelength range from 3 pm to 50 pm.To determine the reflected IR radiation, the IR reflection of the coating is determined in the wavelength range from 3 m to 50 pm in wavelength steps of, for example, 5 nm, the measured reflection values ​​are added together and the sum of the reflection values ​​is divided by the number of measured values.

[0023] According to a preferred embodiment, the electrically conductive material contained in the IR-reflecting coating comprises electrically conductive metal nanoparticles, in particular electrically conductive silver nanoparticles, electrically conductive metal nanowires, in particular electrically conductive silver nanowires, electrically conductive carbon, electrically conductive carbon nanotubes, graphene, electrically conductive polymers, or mixtures thereof. The aforementioned electrically conductive materials achieve particularly good coating properties with regard to their IR reflectivity.

[0024] According to a general definition, "nanoparticles" refers to particles with a size in the range of less than 100 nm. The use of the prefix "nano" thus represents a distinction from particles in the sub-micrometer range (> 100 nm), according to the official definition of ISO TC 229.

[0025] In this text, the term "metal nanowire" and in particular "silver nanowire" refers to all materials that

[0026] - consist predominantly of particles with a metal content and in particular metallic silver content of > 90 wt.%,

[0027] - have a "one-dimensional" geometry like a rod or a hair with a long axis (length) and a short axis (diameter),

[0028] - have an aspect ratio (length / diameter) of at least 5 and

[0029] - whose diameter is in the range between 1 nm and 1000 nm.

[0030] The term "nanotubes" (NT), as used here, describes structures that have similar dimensions in the range of 1 nm to 1000 nm in at least two spatial directions and have an extension in the third spatial direction of at least 5 times the other two dimensions and are at least predominantly hollow.

[0031] The production of the metal nanoparticles, in particular silver nanoparticles, and metal nanowires, in particular silver nanowires, used as electrically conductive material in the IR radiation-reflecting coating is described in detail in WO 2016 / 166074 A1, to which reference is hereby made and the content of which with regard to the production of metal nanoparticles, in particular silver nanoparticles, and metal nanowires, in particular silver nanowires, is made part of the present text.

[0032] According to a particularly preferred embodiment of the present invention, the IR-transparent polymer film consists of saturated or unsaturated hydrocarbon polymers, saturated or unsaturated halogenated hydrocarbon polymers, in particular saturated or unsaturated fluorinated hydrocarbon polymers, or mixtures thereof. In all cases, these can also be copolymers composed of different monomers.

[0033] The films formed from the aforementioned polymers exhibit particularly high transparency for IR radiation in the wavelength range from 5 pm to 30 pm and are therefore particularly well suited for the composite material of the present invention. Any type of polymer formed from monomers without functional groups is particularly suitable. These polymers are therefore particularly preferred within the scope of the present invention. The only exception to this general statement are halogenated and, in particular, fluorinated polymers, which also possess very good properties.

[0034] According to a further, particularly preferred embodiment of the present invention, the polymer film of the optically transparent composite material which is transparent to IR radiation consists of perfluorinated polymers, fluorinated polymers, polyolefins, polyisobutenes, polypropylene, polyethylene, ethylene-tetrafluoroethylene or mixtures thereof.

[0035] Most preferably, the polymer film transparent to IR radiation is a polypropylene film, a polyethylene film or an ethylene-tetrafluoroethylene film.

[0036] According to a preferred embodiment, the optically transparent composite material has an average transmission of at least 30% in the wavelength range from 400 nm to 800 nm. For the purposes of this text, a composite material that has an average transmission of at least 30% in the wavelength range from 400 nm to 800 nm is referred to as "transparent."

[0037] An optically transparent composite material can also be a translucent composite material. Generally, materials that allow light to pass through but through which objects behind the material cannot be seen, such as frosted glass, are referred to as translucent or translucent. However, a translucent composite material can certainly meet the average transmission required for an optically transparent composite material of at least 30% in the wavelength range from 400 nm to 800 nm.

[0038] The IR radiation-reflecting coating of the optically transparent composite material preferably has a transmission of less than 30% in the wavelength range from 5 pm to 30 pm. The wavelength range from 5 pm to 30 pm is mid-infrared light. Low transmission in this wavelength range, i.e. low permeability for radiation in this wavelength range, goes hand in hand with high reflectivity. The coating of the optically transparent composite material therefore has a high reflectivity in the wavelength range from 5 pm to 30 pm. The IR radiation-reflecting coating of the optically transparent composite material preferably has a reflection of more than 20%, preferably more than 30%, particularly preferably more than 40% in the wavelength range from 5 pm to 30 pm.

[0039] The polymer film transparent to IR radiation preferably has a transmission of at least 50%, preferably at least 60%, and particularly preferably at least 70% in the wavelength range from 5 pm to 30 pm. For the purposes of the present text, a polymer film having a transmission of at least 40% in the wavelength range from 5 pm to 30 pm is referred to as "IR-transparent."

[0040] According to preferred embodiments of the invention, the IR radiation-reflecting coating of the optically transparent composite material has a layer thickness of at most 5 pm, preferably at most 2 pm and particularly preferably at most 1 pm.

[0041] The IR-reflecting coating of the optically transparent composite material particularly preferably comprises one or more additives, wherein the additives are in particular surfactants, thickeners, crosslinkers, color pigments, flame retardants, or UV-absorbing substances. The color pigments can absorb predetermined portions of electromagnetic radiation in the wavelength range from 400 nm to 800 nm. In this way, predetermined portions of visible light are absorbed, and only those wavelengths that can be used, for example, for applications in the field of plant growth are transmitted. UV-absorbing additives serve to protect against UV radiation and absorb predetermined portions of electromagnetic radiation in the wavelength range from 100 nm to 400 nm.It is clear to the person skilled in the art that the transmittance of a polymer film does not depend exclusively on the type of polymer from which the film is made, but also on the film's thickness. The term "film" itself implies for the person skilled in the art a thickness that, on the one hand, should not fall below a certain minimum value, since otherwise the film lacks dimensional stability and is no longer handleable, and, on the other hand, should not exceed a certain maximum value, since otherwise it would no longer be called a "film" but rather a "sheet" or similar. It is not difficult for the person skilled in the art to select a suitable thickness for the corresponding polymer film given a polymer as the film material.IR-transparent polymer films with a thickness of 20 pm to 175 pm, particularly preferably with a thickness of 20 pm to 125 pm, and especially preferably with a thickness of 20 pm to 100 pm, have proven particularly suitable and therefore preferred within the scope of the present invention. These preferred thicknesses apply to any type of the preferred materials discussed above from which the IR-transparent polymer films can be made.

[0042] The present invention also encompasses a process for producing one of the optically transparent composite materials described above, the process comprising the following steps: a) providing a polymer film transparent to IR radiation, b) providing a dispersion containing an electrically conductive material, c) applying the dispersion provided in step b) to the polymer film provided in step a), d) drying the product obtained in step c) to form an optically transparent composite material consisting of a polymer film transparent to IR radiation and an IR radiation-reflecting coating made of an electrically conductive material applied to the polymer film.

[0043] In this way, the composite material according to the invention can be produced particularly easily and cost-effectively in good quality.

[0044] Preferably, the dispersion provided in step b) is prepared by a process comprising the following steps:

[0045] - Providing a concentrate of an electrically conductive material,

[0046] - Addition of solvent to the concentrate,

[0047] - Shake the mixture to obtain a homogeneous dispersion,

[0048] - Addition of a mixture of 1 wt% to 10 wt% ethanolamine and up to 2.0 wt% hydroxypropylmethylcellulose in water.

[0049] In the process for producing an optically transparent

[0050] Composite material, a mixture obtained from a polyol process is used as a concentrate of an electrically conductive material, wherein the mixture comprises at least one polyol, polyvinylpyrrolidone and metal nanoparticles or metal nanowires and an adsorptive, wherein the adsorptive is selected from the group consisting of ammonia, primary, secondary and tertiary amines, primary, secondary and tertiary thiols, amino alcohols, hydroxides, carboxylic acids, carboxylic acid esters and amino acids, wherein the adsorptive is present in a proportion of 0.1 wt.% to 5 wt.%, based on the weight of the total concentrate.

[0051] The production of metal nanowires in a polyol process is described in detail in DE 10 2010 017 706 B4, to which reference is hereby made and the content of which with regard to the production of metal nanowires in a polyol process is made part of the present text.

[0052] The present invention also encompasses the use of one of the optically transparent composite materials described above as a retrofit film for application to window glass, interior walls, exterior walls, and greenhouses. The retrofit film can be attached to the window glass, interior walls, exterior walls, or greenhouses, for example, using an adhesive, whereby the adhesive is applied first and the retrofit film is subsequently applied. The film can also be attached by electrostatic attraction.

[0053] Ethylene tetrafluoroethylene film is particularly suitable as a retrofit film for greenhouses. This retrofit film, in the form of an optically transparent composite material, can also contain one or more additives in addition to at least one electrically conductive material. These additives can include, in particular, surfactants, thickeners, cross-linkers, color pigments, flame retardants, or UV-absorbing substances. The color pigments can be selected to absorb predetermined portions of electromagnetic radiation in the wavelength range from 400 nm to 800 nm. UV-absorbing additives serve to protect against UV radiation and absorb predetermined portions of electromagnetic radiation in the wavelength range from 100 nm to 400 nm.

[0054] A retrofit film for interior walls and in particular a retrofit film for application to wallpaper can be particularly advantageously provided with a fire retardant as an additive.

[0055] Retrofit films for interior and exterior walls are particularly preferably designed to be permeable to diffusion. This property can be achieved, for example, by perforating the film. The present invention also encompasses a coated substrate, wherein the coating is an optically transparent composite material as described above.

[0056] In principle, any transparent or translucent material is suitable as a substrate, and the substrates can be synthetic or natural. The substrate is preferably a film, glass, or a transparent plastic sheet.

[0057] The optically transparent composite can be attached to the substrate using adhesive, for example. The composite can also be attached using electrostatic attraction.

[0058] In particularly preferred embodiments, the substrate to be coated and the IR-transparent polymer film are made of the same polymer material. In this case, the composite material can be bonded to the substrate particularly easily and permanently.

[0059] Embodiments with a sandwich structure consisting of multiple films are also conceivable. The IR-reflecting layer is located between a film that is non-transparent in the IR range and a film that is transparent in the IR range. Such a sandwich structure can also be attached to the substrate, for example, using adhesive or by electrostatic attraction. The IR-transparent side of the sandwich structure is arranged on the side facing away from the substrate.

[0060] If the polymer film transparent to IR radiation has hydrophobic properties, pretreatment of the substrate may be required before applying the optically transparent composite material, for example, plasma pretreatment or corona pretreatment.

[0061] The retrofitting of interior and exterior walls, and in particular wallpaper, can be carried out in two different ways within the scope of the present invention. Firstly, an optically transparent composite material according to the present invention can be produced by coating a polymer film transparent to IR radiation with a dispersion containing an electrically conductive material and applied in the form of a retrofitting film to interior and exterior walls, and in particular to wallpaper. Secondly, however, the substrate, i.e. the interior or exterior wall and in particular the wallpaper, can also be coated with a dispersion containing an electrically conductive material and subsequently a polymer film transparent to IR radiation can be applied. After drying, an optically transparent composite material within the meaning of the present invention is also formed in this case.A retrofitted substrate produced in one of the two described ways cannot be distinguished from a retrofitted substrate produced in the other described way.

[0062] For all embodiments of the present invention, the optically transparent composite material is applied to any substrate in such a way that the polymer film transparent to IR radiation faces away from the substrate surface. Only in this way can the desired protection of the IR-reflecting coating against mechanical influences be ensured.

[0063] Short description of the drawings

[0064] The invention will be explained in more detail below using exemplary embodiments in conjunction with the drawings.

[0065] Fig. 1A Representation of the measured transmittances of a PET film with a thickness of 100 pm (comparative example);

[0066] Fig. 1B Representation of the measured transmittances of a PP film with a thickness of 100 pm;

[0067] Fig. 2 schematic representation of a composite material applied to a window glass (comparative example);

[0068] Fig. 3 schematic representation of a composite material according to the invention applied to a window glass.

[0069] Ways to implement the invention

[0070] Determination of the transmittance of polymer films

[0071] To determine the transmittance of polymer films, the spectral, directed transmittance Tgg of the polymer films is measured at room temperature using a Bruker Vertex 70v Fourier transform infrared (FTIR) spectrometer. The film is aligned and fixed in the spectrometer's sample chamber perpendicular to the IR beam. The measurement is performed in a wavelength range from 2 pm to 50 pm with a step size of 5 nm, i.e., the transmittance measurements are taken at 5 nm intervals.

[0072] Please note that different IR spectrometers can produce reproducibly almost identical results. Therefore, any commercially available infrared spectrometer familiar to the expert can be used to perform the measurements.

[0073] Figure 1A shows the transmittance plotted against wavelength for a PET film with a thickness of approximately 100 pm.

[0074] Figure 1B shows the transmittance plotted against wavelength for a PP film with a thickness of approximately 100 pm.

[0075] As already explained, the term "polymer film transparent to IR radiation" in the context of the present invention means that the polymer film has an average spectral transmission of > 40% in the wavelength range from 5 pm to 30 pm. The transmittances shown in Figures 1A and 1B can be converted into the corresponding transmission values ​​[%] by multiplying by a factor of 100.

[0076] To determine the mean spectral transmission, the transmission values ​​determined in wavelength steps of 5 nm are added together and the sum of the transmission values ​​is divided by the number of measured values.

[0077] The PET film (Fig. 1A) has an average spectral transmission of 32% in the wavelength range from 5 pm to 30 pm and is therefore not suitable as an IR-radiation-transparent polymer film according to the present invention.

[0078] The PP film (Fig. 1B) has an average spectral transmission of 75% in the wavelength range from 5 pm to 30 pm and is therefore suitable as an IR-transparent polymer film according to the present invention.

[0079] Example 1: Coating a PET film with silver nanowires

[0080] Silver nanowires were produced in a polyol process according to the method described in DE 10 2010 017 706 B4. Aqueous formulations containing 0.3 wt.% silver were prepared from the resulting silver nanowire concentrate containing 4.0 wt.% silver. For this purpose, 3.75 g of silver nanowire concentrate were mixed with 39.75 g of water in a plastic screw-cap container and homogeneously dispersed by shaking. 6.5 g of SURFLINK were then added to the dispersion. SURFLINK is an additive commercially available from HeiQ RAS for activating Ag nanowire networks. It is a mixture of 1 wt.% to 10 wt.% ethanolamine and up to 2.0 wt.% hydroxypropylmethylcellulose in water.

[0081] The resulting aqueous formulation was applied to approximately 10 x 10 cm PET films using stainless steel doctor blades to form wet film thicknesses of 12 μm, 24 μm, and 40 μm, respectively. The coated substrates were oven-dried for 3 minutes at 150°C. The electrical conductivity of the substrates was measured using a four-point meter (RCHEK 4 Point Meter, manufacturer EDTM, model #RC2175). Within the device's measuring range (1-19990 ohms / sq.), conductivities in the range of 10-100 ohms / sq. were detected, depending on the wet film thickness.

[0082] Scanning electron microscope (SEM) images of representative areas clearly show the presence of a percolated network of silver nanowires in all samples. Measurements of the thermal emissivity of the coated PET films in an integrating sphere showed that increasing the wet film thickness, corresponding to an increase in the silver nanowire content in the coating, had a positive effect on the resulting IR reflection. An uncoated PET film was measured as a reference.

[0083] The detected thermal emissivity and IR reflection of the various coated and uncoated films are shown in the table below. The thermal emissivity of a body indicates how much radiation it emits compared to an ideal heat radiator, i.e., a blackbody. The thermal emissivity value therefore always lies between 0 (no absorption) and 1 (100% absorption). Example 2 (comparative example): Coating a PET film with silver nanowires and a protective layer

[0084] PET films coated with silver nanowires were produced and quality tested according to Example 1. A sol-gel-based protective layer was then applied directly to the silver nanowire coating to protect the silver nanowire layer from chemical and mechanical influences.

[0085] Measurements in the integrating sphere regarding the thermal emissivity of these coated PET films showed that the IR reflection is significantly reduced and in the order of magnitude of the reflection of the uncoated reference film (e n = 0.92).

[0086] Example 3 (comparative example): Silver nanowire-coated PET film on glass

[0087] PET films coated with silver nanowires were produced according to Example 1 and subjected to quality testing. As shown in Figure 2, the coated PET films were bonded to glass substrates 1. The silver nanowire coating 2 was in direct contact with the surface of the glass substrate 1 and was protected from chemical and mechanical influences by the uncoated backside of the PET film 3.

[0088] Subsequently, heat radiation 4 was radiated onto the structure using an IR heater. Measurements with a thermal imaging camera showed no visible reflection in the thermal image compared to the reference (adhesive film without coating).

[0089] Example 4: Silver nanowire-coated PP film on glass

[0090] A silver nanowire concentrate containing 4.0 wt.% silver was prepared as described in Example 1. Alcoholic formulations containing 0.3 wt.% silver nanowires were prepared from this concentrate. For this purpose, 3.75 g of silver nanowire concentrate were mixed with 37.76 g of isopropyl alcohol and 1.99 g of water in a plastic screw-cap container and homogeneously dispersed by shaking. 6.5 g of SURFLINK were then added to the dispersion.

[0091] The resulting alcoholic formulation was applied to approximately 10 x 10 cm plasma-treated PP films using stainless steel doctor blades to achieve wet film thicknesses of 12 μm, 24 μm, and 40 μm. The coated substrates were oven-dried for 3 minutes at 100°C. The electrical conductivity of the substrates was measured using a four-point meter (RCHEK 4 Point Meter, manufacturer EDTM, model #RC2175). Within the device's measuring range (1–19,990 ohms / sq.), conductivities in the range of 10–100 ohms / sq. were detected, depending on the wet film thickness.

[0092] Scanning electron microscope (SEM) images of representative locations clearly show that a percolated network of silver nanowires is present in all samples.

[0093] As shown in Figure 3, the coated PP films were bonded to glass substrates 1. The silver nanowire coating 2 was in direct contact with the surface of the glass substrate 1 and was to be protected from chemical and mechanical influences by the uncoated backside of the plasma-treated PP film 5.

[0094] Subsequently, thermal radiation 4 was radiated onto the structure using an IR heater. Measurements with a thermal imaging camera showed a significantly increased reflection of thermal radiation 6 compared to the reference (adhesive film without coating). In the wavelength range from 3 pm to 50 pm, a reflection of more than 45% of the IR radiation averaged over the wavelength range was measured.

[0095] The plasma-treated PP films coated with silver nanowires can be bonded directly to window glass, with the silver nanowire coating in direct contact with the surface of the window glass, thus protecting it from chemical (e.g., cleaning agents) and mechanical influences. The films can therefore be used to retrofit existing windows with IR-reflective properties, thereby saving energy and heating costs.

[0096] Example 5: Silver nanowire-coated PP film

[0097] A silver nanowire concentrate containing 4.0 wt.% silver, prepared according to Example 1, was used. A formulation containing 0.3 wt.% or 0.1 wt.% ethylcellulose and 5 wt.% terpeneol in 2-propanol was prepared from the concentrate.

[0098] Subsequently, PP films were coated using a doctor blade and a wet film thickness of 24 μm. The PP films were positioned next to a reference (PP film without coating) in front of an object at body temperature. Measurements with a thermal imaging camera showed a significantly increased reflection of thermal radiation compared to the reference (PP film without coating). In the wavelength range from 3 μm to 50 μm, a reflection of more than 45% of the IR radiation averaged over the wavelength range was measured. Example 6: Silver nanowire-coated wallpaper with laminated PP film

[0099] A silver nanowire concentrate containing 4.0 wt.% silver, prepared as described in Example 1, was used. Alcoholic formulations containing 0.3 wt.% silver nanowires were prepared from this concentrate. For this purpose, 3.75 g of silver nanowire concentrate were mixed with 39.75 g of isopropyl alcohol in a plastic screw-cap container and homogeneously dispersed by shaking. 6.5 g of SURFLINK were then added to the dispersion.

[0100] Wallpaper cuts were coated using a dip bath, left to drain briefly at room temperature and then dried in an oven at 150°C for 3 minutes.

[0101] The electrical conductivity of the substrates was measured using a high-ohmmeter (Metriso 2000). Within the device's measuring range (1 kOhm - 999 GOhm), conductivities in the range of 10-20 kOhm were detected.

[0102] Scanning electron microscope (SEM) images of representative areas clearly show the presence of a percolated network of silver nanowires.

[0103] PP film blanks were laminated to the coated wallpapers, creating a composite material consisting of an IR-transparent PP polymer film and an IR-reflecting coating made of electrically conductive silver nanowires applied to the polymer film. The PP layer protects the silver nanowire coating from chemical and mechanical influences.

[0104] The coated wallpapers were then adhered to a surface. Heat radiation was then radiated onto the structure using an IR heater. Measurements using a thermal imaging camera showed a significantly increased reflection of heat radiation compared to the reference (PP laminated wallpaper without coating). In the wavelength range from 3 pm to 50 pm, a reflection of more than 45% of the IR radiation averaged over the wavelength range was measured. Reference symbols: Window glass, layer with silver nanowires, PET film, heat radiation, PP film, reflected heat radiation.

Claims

Patent claims Optically transparent composite material consisting of an IR- A radiation-transparent polymer film (5), wherein the IR-transparent polymer film has an average spectral transmission of at least 40% in the wavelength range from 5 pm to 30 pm, and an IR-radiation-reflecting coating (2) applied to the polymer film, wherein the coating comprises at least one electrically conductive material. The optically transparent composite material according to claim 1, characterized in that the electrically conductive material comprises electrically conductive metal nanoparticles, in particular electrically conductive silver nanoparticles, electrically conductive metal nanowires, in particular electrically conductive silver nanowires, electrically conductive carbon, electrically conductive carbon nanotubes, graphene, electrically conductive polymers, or mixtures thereof.An optically transparent composite material according to claim 1 or 2, characterized in that the IR-radiation-transparent polymer film (5) consists of saturated or unsaturated hydrocarbon polymers, saturated or unsaturated halogenated hydrocarbon polymers, in particular saturated or unsaturated fluorinated hydrocarbon polymers, or mixtures thereof. An optically transparent composite material according to claim 1 or 2, characterized in that the IR-radiation-transparent polymer film (5) consists of perfluorinated polymers, fluorinated polymers, polyolefins, polyisobutene, polypropylene, polyethylene, ethylene-tetrafluoroethylene, or mixtures thereof. An optically transparent composite material according to one of claims 1 to 4, characterized in that the composite material has an average transmission of at least 30% in the wavelength range from 400 nm to 800 nm.An optically transparent composite material according to one of claims 1 to 5, characterized in that the IR-reflecting coating (2) has an average spectral transmission of less than 30% in the wavelength range from 5 pm to 30 pm. An optically transparent composite material according to one of claims 1 to 6, characterized in that the IR-transparent polymer film (5) has an average spectral transmission of less than 30% in the wavelength range from 5 pm to 30 pm. at least 50%, preferably at least 60%, particularly preferably at least 70%. Optically transparent composite material according to one of claims 1 to 7, characterized in that the IR radiation-reflecting coating (2) has a layer thickness of at most 5 pm, preferably of at most 2 pm, particularly preferably of at most 1 pm. Optically transparent composite material according to one of claims 1 to 8, characterized in that the IR radiation-reflecting coating (2) has one or more additives, wherein the additives are preferably surfactants, thickeners, cross-linkers, flame retardants, UV-absorbing substances or color pigments, wherein the color pigments absorb predetermined proportions of the electromagnetic radiation in the wavelength range from 400 nm to 800 nm.Optically transparent composite material according to one of claims 1 to 9, characterized in that the polymer film (5) transparent to IR radiation has a thickness of 20 pm to 175 pm, preferably a thickness of 20 pm to 125 pm, particularly preferably a thickness of 20 pm to 100 pm.A method for producing an optically transparent composite material according to one of claims 1 to 10, comprising the steps of a) providing a polymer film (5) transparent to IR radiation, b) providing a dispersion containing an electrically conductive material, c) applying the dispersion provided in step b) to the polymer film (5) provided in step a), d) drying the product obtained in step c) to form an optically transparent composite material consisting of a polymer film (5) transparent to IR radiation and an IR-reflecting coating (2) made of an electrically conductive material applied to the polymer film. Method according to claim 11, characterized in that the dispersion provided in step b) is produced by the steps. - Providing a concentrate of an electrically conductive material, - Addition of solvent to the concentrate, - Shake the mixture to obtain a homogeneous dispersion, - Addition of a mixture of 1 wt% to 10 wt% ethanolamine and up to 2.0 wt% hydroxypropylmethylcellulose in water.

13. The method according to claim 12, characterized in that the concentrate of an electrically conductive material is a mixture obtained from a polyol process, the mixture comprising at least one polyol, polyvinylpyrrolidone and metal nanoparticles or metal nanowires and an adsorptive, the adsorptive being selected from the group consisting of ammonia, primary, secondary and tertiary amines, primary, secondary and tertiary thiols, amino alcohols, hydroxides, carboxylic acids, carboxylic acid esters and amino acids, the adsorptive being present in a proportion of 0.1 wt% to 5 wt%, based on the weight of the total concentrate.

14. Use of an optically transparent composite material according to one of claims 1 to 10 as a retrofit film for application to window glass, interior walls, exterior walls and greenhouses.

15. Coated substrate, characterized in that the coating is an optically transparent composite material according to one of claims 1 to 10.

16. Coated substrate according to claim 15, characterized in that the substrate (1) is a film, glass or a transparent plastic plate.

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