Coated synthetic foil and synthetic laminate made from this foil.

p0001The invention relates to a coated plastic film of high transmittance for visible light and high reflectance for infrared radiation, with a carrier film of a transparent polymer and an embedded between two dielectric layers and a reflection metal layer made therefrom plastic laminate.

p0002Transparent coated plastic films serve as a transparent heat mirror, including optical filters are to be understood, the infrared or heat radiation and reflect visible light, ie radiation from the visible range of the electromagnetic spectrum, are permeable.

p0003Such heat filters to the largest possible amount of visible light let through and the greatest possible proportion of the incident reflect thermal radiation, to which in particular prevents the interior of a room heated by sunlight.

p0004The automotive industry has been working in recent years to help reduce the fuel consumption of cars. This is achieved primarily by a significant reduction in the Luftwiderstandbeiwerts cars as known to the vast majority of the time needed for the drive power to overcome air resistance is required. In the course of this development, the glass surfaces of the cars are getting bigger and thus the laminated glass tilted becoming stronger. As a result thereof is the famous "greenhouse effect" in the car interior, especially in the summer months, much stronger to bear. In cars that are at an outside temperature of eg 35 ° C in the sun four hours temperatures are measured inside the car up to 75 ° C. An overheated interior represents a huge physical and psychological strain for the car occupants, particularly for the driver, are thus contributing to a reduction in road safety.

p0005One way not to allow the temperature inside the car, even in summer, become too high, is to equip the glass panes of the car with a coating which has high transparency in the visible spectral range and strongly reflected in the infrared region.

p0006In DE-OS 33 07 661 a method for the manufacture of glass panes with high transmission in the visible spectrum and high reflection behavior is outlined against thermal radiation. According to this method flat glass sheets are coated directly by sputtering. In the flat glass panes, a first layer of the group In₂O₃, SnO₂ or mixed oxides is generated. a second layer of silver, a third layer of the group Al, Ti, Ta, Cr, Mn, Zr and a fourth layer of the group In₂O₃, SnO₂ or their mixed oxides are applied to this first layer. In this way, transparent heat reflectors can be produced, however, the method has drawbacks: to coat individual glass panes, is very complicated and expensive. Add to that the problem that for the production of windshields for cars the flat, coated glass panes an addition must undergo bending process what the quality of the coating is detrimental. Moreover, a coating that does not require the third layer, which has a purely protective function for the second layer of silver is desirable.

p0007From EU-A 0,077,672 is a transparent film consisting of a transparent polymer carrier film and a translucent metal layer of a metal selected from the group Ag, Au, Cu and their alloys known. A dielectric layer of titanium, bismuth, tungsten, indium oxide, zirconium oxide or silicon oxide is deposited on one or both sides of the metal layer. In addition, a protective layer made of Ti, Zr, Se, C on one or both sides of the metal layer may be formed. The transparent film is laminated on both sides with polyvinyl butyral to form a laminate, which is introduced, for example, between two sheets of glass and with this integrated into a laminated safety glass pane.

p0008The carrier film of the laminate is generally made of polyethylene terephthalate. The two dielectric layers, which are applied to both sides of the metal layer are deposited, for example by hydrolysis of tetrabutyl in alcohol on the metal layer.

p0009The object of the invention is to improve the coated plastic film of the type described above so that it forms a heat mirror having a high transmittance for visible light and high reflectance for infrared radiation, without a protective layer for the silver layer is required, and that it comprises a is an essential part of a laminate in a laminated safety glass, which can be processed as easily and by the same methods as a conventional polyvinyl butyral film.

p0010This object is achieved in that each of the two dielectric layers consisting of a metal nitride. In the invention, the reflective metal layer is made of a metal of the group silver, gold, copper. The dielectric layers are preferably made of aluminum or silicon nitride.

p0011In an embodiment of the invention, the reflective metal layer has a thickness between 5 and 40 nm and the thickness of each of the two dielectric layers 20 to 60 nm. The carrier film is, for example, polyethylene terephthalate having a thickness between 5 and 100 microns. The ray transmittance in the visible spectrum at a wavelength λ = 550 nm is greater for the coated plastic / equal to 73% and the reflectance of the coated plastic film in the near infrared region for a wavelength λ = 2500 nm is at least 80%.

p0012The layers are successively deposited by sputtering onto the carrier film. The coated plastic film is the starting material for a plastic laminate, and each of the two sides of the plastic film is laminated thereto, each with a sheet of polyvinyl butyral. The thickness of the individual polyvinylbutyral amounts to 0.2 to 0.4 mm. This plastic laminate embedded between two shaped glass plates together with this, forms a laminated safety glass pane having the structure glass / plastic laminate / glass. Such laminated safety glass panes are particularly incorporated as windshields in motor vehicles.

p0013High reflectivity in the infrared range is a characteristic property of metals having high electrical conductivity. The light transmittance in the visible spectrum is in turn typical of semiconductors with wide bandgap .DELTA.E ≧ 3 eV. Typically, blank transparent heat mirror on two types realize either by semiconductors with a large band gap as a layer material which have a non-stoichio metric composition and / or are suitable doped or by very thin metal layers having a thickness in the order of 10 nm or less, which are provided on both sides with a dielectric layer. The first type of implementation requires excessive film thicknesses of about 1 micron and more than that it would be suitable for use in windshields. Therefore, replacing the use of transparent heat mirrors in laminated safety glass panes for motor vehicles, only the embodiment into consideration, be used in the very thin metal layers, which are provided on both sides with a dielectric layer.

p0014Suitable metals for this metal layer are primarily of silver, gold and copper as well as alloys of these metals. At wavelengths less than 600 nm, ie, in a region in which the solar spectrum has its maximum, the absorbance of copper and gold is much greater than that of silver. The reduced light transmission of a silver layer in the visible spectrum is due to the higher reflectivity of silver compared to gold and copper. By embedding the silver layer between two dielectric anti-reflection layers, the high reflectivity of the silver layer is reduced in the visible spectral range. Possible dielectric materials for the antireflection layers are metal oxides or metal nitrides.

p0015If the carrier film, namely, a polyester film, first with a metal oxide layer and then a silver layer having a layer thickness smaller than 25 nm, coated so tune the measured optical properties such as the reflectance and transmittance, largely with the theoretical predictions match. If a further oxide layer is applied to the silver layer, so the coated film has a light transmittance of about 75%, and - contrary to expectation - a reflectance for radiant heat of a maximum of only 35%. Studies by scanning electron microscope and an electron spectroscopy chemical analysis apparatus of surfaces (ESCA device), that during application of the top metal oxide initially closed silver layer is torn under the influence of the sputtering process when applying the topmost metal oxide layer, that are formed in this case insulated from each drop-shaped "silver mountains" with a diameter of about 1 micron. Thereby, the reflectance of a thus coated polyester film is greatly reduced. This mechanism is also described in DE-OS 33 07 661st

p0016The tearing of the silver layer is prevented in that an additional metal layer is deposited between the silver layer and the uppermost metal oxide layer. In this way, transparent heat reflectors can be produced in which the individual layers are applied by sputtering, respectively.

p0017In contrast, the transparent heat mirror according to the invention do not require this additional metal layer.

p0018Embodiments of the invention will be explained in more detail below.

p0019Show it:<ul><li>1 shows a cross section through a coated plastic film according to the invention,</li><li>2 shows a cross section through a plastic laminate consisting of a coated plastic film and on each side of the plastic laminated film of polyvinyl butyral, and</li><li>3, the light transmittance and reflectance of a coated plastic with the structure of carrier film / aluminum / silver / aluminum, depending on the wavelength of the incident radiation.</li></ul>

p00201 shows a cross section through a coated plastic film 5 consisting of a carrier film 1 and a layer structure on the carrier film, which comprises a plurality of layers. On the carrier film 1, a first metal nitride layer, a metal layer and a second metal nitride layer are brought on by sputtering. The carrier film 1 is a polyester film or a polyethylene terephthalate film having a thickness between 10 and 50 microns. The two metal nitride layers 2 and 4 are dielectric layers. They form antireflection layers including the metal layer. 3 The thickness of each of the two metal nitride is 20 to 60 nm. The metal nitride of the two layers 2 and 4 is aluminum nitride. The metal layer 3 forms a reflection metal layer and consisting of a metal of the group silver, gold, copper, with the preference of silver. The metal layer 3 generally has a thickness between 5 and 40 nm.

p0021The two dielectric layers 2 and 4 can, in addition to aluminum, also consist of silicon nitride having a thickness of 20 to 60 nm.

p00222 shows a cross section through a plastic laminate 8, consisting of a coated plastic film 5, as shown in Figure 1, and two polyvinyl butyral films 6 and 7 that are laminated to each side of the plastic film. 5 Such polyvinyl butyral (PVB) are known in the art and are thus not described in detail. The connection between the PVB films 6 and 7 and the coated plastic film 5 can be effected by suitable transparent adhesives which are not shown in FIG. 2 The thickness of each PVB sheet 6 and 7 is from 0.2 to 0.4 mm, so that the total thickness of the plastic laminate 8 reaches a maximum of 0.85 mm.

p0023In the event that minor compromising the reflectivity of the coated plastic film 5 still satisfy the requirements, the first metal nitride layer 2 may be omitted.

p0024Figure 3 shows a transmission curve 9 in the wavelength range between 400 and 2500 nm of a coated plastic film which has a layer structure carrier film / aluminum / silver / aluminum. The maximum of the transmission curve is at a wavelength λ = 550 nm and amounts to 86%. From the reflection curve 10 it is seen that the maximum of the reflectance is at a wavelength λ = 2500 nm exceeding 90%. The thickness of the carrier film of polyethylene terephthalate (Hostaphan^R 4400 from Hoechst AG) is 36 microns.

example

p0025the three layers 2 to 4 were applied in accordance with the drawings in Figure 1 in a cathode sputtering on a polyethylene terephthalate film 1 of the thickness of 36 microns. The sputtering of aluminum took place in a nitrogen atmosphere, so that the layers 2 and 4 were deposited as aluminum nitride. The atomization of silver was carried out in non-reactive atmosphere to give the silver layer 3. The two aluminum nitride layers 2 and 4 and the silver layer 3 have a total layer thickness of 96 nm. The optical properties of the coated plastic film 5 are shown in Figure 3, wherein the transmittance at a wavelength λ = 550 nm is 86% and the reflectance at a wavelength λ = 2500 nm exceeding 90%. The spectral behavior of the coated plastic film 5 in the wavelength range from 400 to 2500 nm is from the transmission curve 9 and the reflection curve 10 of Figure 3 can be seen.

p0026The dielectric layers of silicon nitride, which is obtained by sputtering silicon in an atmosphere of nitrogen in the cathode sputtering, resulting similarly high values ​​for transmission and reflection.

p0027For larger thicknesses of the metal nitride and the metal layer, the transmission and the reflection will decrease, but the thicknesses of the layers are to be chosen in each case so that the ray transmittance of the coated plastic film in the visible spectrum at a wavelength λ = 550 nm is greater than / equal to 73 % and the reflectivity in the near infrared region is at least 80% for a wavelength λ = 2500 nm.

p0028As is known, nitrides form very smooth surfaces, resulting in so-coated plastic films a very high resistance to salt water and other aggressive atmosphere. In out of the water absorption of such films is very low for extended periods of time, so that the plastic films are storage stable.