Reflective composite material having a varnished aluminum carrier having a silver reflection layer and method for production thereof

Abstract

The invention relates to a reflective composite material (V) having a carrier (1) consisting of aluminum, having an interlayer (2) which is present on a side (A) on the carrier (1) and is composed of a varnish, and having an optically active multilayer system (3) which has been applied atop the interlayer (2) and consists of at least three layers, wherein the upper layers (4, 5) are dielectric and/or oxidic layers, and the lowermost layer (6) is a metallic layer which consists of silver and forms a reflection layer (6). To increase the aging resistance, it is proposed that the interlayer (2) comprise an organic layer-forming varnish or be formed entirely from such a varnish that has been cured in an ionic photopolymerization and crosslinking or that has been cured after UV irradiation by free-radical photopolymerization and crosslinking.

Claims

1. A reflective composite material comprising, an aluminum substrate, with an intermediate layer made of a lacquer located on one side on the substrate, and an optically active multi-layer system applied above the intermediate layer; the multi-layer system includes at least three layers, wherein two upper layers comprise upper and lower dielectric or oxidic layers and a bottom layer is a metallic layer consisting essentially of silver, which forms a reflective layer, the intermediate layer includes an organic, layer-forming, silane-free, lacquer, which is hardened by ionic polymerization initiated by photo-chemical excitation and crosslinking, such that no monomers remain in the intermediate layer, wherein the upper dielectric or oxidic layer of the multi-layer optical system is a higher refractive layer than the lower dielectric or oxidic layer of the multi-layer optical system, wherein the upper dielectric or oxidic layer consists essentially of at least one of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, MoO.sub.3, TiO.sub.2 and ZrO.sub.2, and the lower dielectric or oxidic layer consists essentially of at least one of Al.sub.2O.sub.3 and SiO.sub.2, wherein the intermediate layer is formed from several partial layers, of which an upper partial layer consists essentially of the organic lacquer having a thickness in the range of 0.3 micrometers (μm) and 1.0 μm, and a lower partial layer consists essentially of anodic oxidized aluminum of the substrate, the lower partial layer having a thickness in the range of 0.010 μm to 10.0 μm, the upper partial layer penetrating into pores within the lower partial layer, wherein the reflective layer is bonded to the intermediate layer by an adhesion-promoting layer, the adhesion-promoting layer being an oxidic layer formed from at least one of Al.sub.2O.sub.3, TiO.sub.2 and CrO.sub.s having a thickness in the range from 0.1 nm to 50 nm, wherein s denotes a stoichiometric or non-stoichiometric ratio and is in the range of 0<s<1.5.

2. The reflective composite material according to claim 1, wherein the ionic polymerization and crosslinking of the organic lacquer of the intermediate layer is accomplished by cationic photo-polymerization under an effect of UV-radiation.

3. The reflective composite material according to claim 1 further comprising, the organic lacquer of the intermediate layer is formed from a solvent-free or a solvent-containing single-component system, or a single-component acrylate system with an alcohol solvent.

4. The reflective composite material according to claim 1 further comprising, the organic lacquer of the intermediate layer is one or more of a cyclo-aliphatic, an aromatic ether, or an epoxy lacquer which contains a photo-initiator.

5. The reflective composite material according to claim 4 further comprising, the photo-initiator is one or more of a sulfonium salt, an iodonium salt, a diaryl iodonium salt, a diaryl iodonium salt with an added sensitizer, or a diaryl iodonium salt with a sensitizer in the form of a thioxanthone derivate.

6. The reflective composite material according to claim 1 further comprising, the organic lacquer of the intermediate layer features a degassing rate after hardening of less than 10.sup.−4 mbar|s|m.sup.2.

7. The reflective composite material according to claim 1 further comprising, an additional adhesion-promoting layer arranged between the reflective layer and the lower dielectric or -oxidic layer of the multi-layer optical system.

8. The reflective composite material according to claim 1 further comprising, the thickness of the reflective layer is in the range from 30 nm to 200 nm.

9. The reflective composite material according to claim 1 further comprising, a silicon oxidic covering layer, a silicon nitridic covering layer, a silicon oxide-nitride covering layer, or a mixed covering layer having the chemical composition Si.sub.aC.sub.bO.sub.cN.sub.dH.sub.e with the indices a, b, c, d, and e denoting a stoichiometric or non-stoichiometric ratio applied onto the multi-layer optical system.

10. The reflective composite material according to claim 9, wherein the covering layer is the mixed layer having a chemical composition of Si.sub.aC.sub.bO.sub.cN.sub.dH.sub.e, wherein the indices a, b, c, d and e are adjusted such that fora selected layer thickness, the covering layer has a light absorption of less than 10 percent, and wherein the carbon content; relative to the total mass of the covering layer; is in the range of 0.2 atom-percent to 15.0 atom-percent.

11. The reflective composite material according to claim 1, further comprising, a covering layer applied onto the multi-layer optical system, the covering layer being an organic silicon lacquer layer based on a sol-gel layer, with a thickness in the range from 0.5 μm to 5 μm.

12. The reflective composite material according to claim 1 wherein the upper and lower dielectric or oxidic layers of the multi-layer optical system each have a thickness in the range from 30 nm to 200 nm.

13. The reflective composite material according to claim 1 wherein the upper and lower dielectric or oxidic layers of the multi-layer optical system each have a thickness which amounts to one-quarter of an average wavelength for a spectral range of electromagnetic radiation to be reflected by the reflective composite material.

14. The reflective composite material according to claim 1 further comprising, a surface of the intermediate layer upon which the optically active multi-layer system is applied has an arithmetic mean roughness in the range of less than 0.02 μm.

15. The reflective composite material according to claim 1 further comprising, the aluminum of the substrate has a greater purity than 97%.

16. The reflective composite material according to claim 1 further comprising, the reflective composite material is formed as a coil with a width up to 1600 mm, and with a thickness of about 0.1 to 1.5 mm.

17. The reflective composite material according to claim 1 further comprising, a total light reflectivity on a side of the multi-layer optical system determined according to DIN 5036, part 3, is greater than 97%.

18. A method for production of a reflective composite material, wherein an intermediate layer made of a lacquer with a multi-layer optical system located thereon is applied to one side of an aluminum substrate, such that the multi-layer optical system is applied above the intermediate layer and includes at least three layers, wherein two upper layers are upper and lower dielectric or oxidic layers, and a bottom layer is a metallic layer consisting essentially of silver, which forms a reflective layer, wherein the intermediate layer is formed from several partial layers, of which an upper partial layer of the intermediate layer is formed by application of an organic, layer-forming, silane-free lacquer to a lower partial layer, wherein after the application, the organic, layer-forming, silane-free lacquer is hardened by ionic polymerization initiated by photo-chemical excitation and crosslinking, such that no monomers remain in the intermediate layer, the upper partial layer consisting essentially of the organic, silane-free lacquer having a thickness in the range of 0.3 micrometers (μm) and 1.0 μm and penetrating into pores within the lower partial layer, and the lower partial layer consisting essentially of anodic oxidized aluminum of the substrate and having a thickness in the range of 0.010 μm to 10.0 μm; wherein the upper dielectric or oxidic layer of the multi-layer optical system is a higher refractive layer than the lower dielectric or oxidic layer of the multi-layer optical system, wherein the upper dielectric or oxidic layer consists essentially of at least one of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, MoO.sub.3, TiO.sub.2 and ZrO.sub.2, and the lower dielectric or oxidic layer consists essentially of at least one of Al.sub.2O.sub.3 and SiO.sub.2; wherein the reflective layer is bonded to the intermediate layer by an adhesion-promoting layer, the adhesion-promoting layer being an oxidic layer formed from at least one of A.sub.2O.sub.3, TiO.sub.2 and CrO.sub.s having a thickness in the range from 0.1 nm to 50 nm, wherein s denotes a stoichiometric or non-stoichiometric ratio and is in the range of 0<s<1.5.

19. The method according to claim 18, wherein the ionic photo-polymerization is a cationic photo-polymerization, which takes place through the use of an iodonium or sulfonium salt as photo-initiator, from a monomer which contains epoxy groups.

20. The method according to claim 18 further comprising, in that one or a plurality of the two upper layers or the bottom layer arranged over the intermediate layer is applied onto the intermediate layer by sputtering, by CVD or PE-CVD methods, or by vapor-coating.

21. The method according to claim 18 further comprising, in that at least two of the layers arranged over the intermediate layer are applied in a continuous process in a vacuum.

22. The method according to claim 18 further comprising, in that the lacquer is applied by immersion, by electro-immersion lacquering, by brushing on, by rolling on, by centrifuging, by spraying, or in a continuous belt cycle method.

23. The method according to claim 18 further comprising, in that the lacquer is hardened by the use of at least one of UV, IR, electron and laser radiation, at least until a hand-tack is attained, which is indicated by the appearance of a no-longer tacky surface of the intermediate layer, in a time period of 100 seconds to 300 seconds.

24. The method according to claim 23 further comprising, in that the lacquer contains an acrylate and is exposed to UV radiation at an energy dosage in the range of 1 J/cm.sup.2 to 3 J/cm.sup.2 by use of mercury medium-pressure lamps, for formation and hardening of a film.

25. The method according to claim 23 further comprising, in that until its final strength is attained, the lacquer is hardened over a period of time of 25 hours to 150 hours.

26. The method according to claim 25 further comprising, in that coating of the intermediate layer with the optically active multi-layer optical system takes place during hardening of the lacquer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in greater detail by means of the attached drawing which illustrates one exemplary embodiment.

(2) The sole illustration provided, FIG. 1, shows a fundamental cross-sectional representation of a composite material according to the invention, wherein the layer thicknesses indicated therein are purely schematic and are not illustrated true to scale.

DETAILED DESCRIPTION

(3) The described design relates to a composite material V according to the invention with a high reflectivity, in particular in the solar wavelength range. It can be used preferably for reflecting optical radiation; that is, electromagnetic radiation in the wavelength range of 100 nm up to 1 mm.

(4) The composite material V consists of a band-like; especially a deformable; substrate 1 of aluminum, an intermediate layer 2 located on one side A of the substrate 1, and an optically active multi-layer system 3 applied onto the intermediate layer 2.

(5) A total light reflectivity determined according to DIN 5036, part 3, amounts to more than 90% on side A of the optical multi-layer system 3, preferably at least 95%, most preferably at least 97%.

(6) The composite material V can be produced preferably as a coil with a width up to 1600 mm, preferably of 1400 mm, and; including any possible provided layers on its reverse side B; created with a thickness D of about 0.2 mm to 1.7 mm, preferably of about 0.3 to 0.8 mm, most preferably 0.5 mm to 0.7 mm. The substrate 1 alone can have preferably a thickness D1 in the range of 0.2 mm to 0.6 mm.

(7) The aluminum of the substrate 1 can have in particular a higher purity than 99.0%, so that its thermal conductivity is promoted. Thus the generation of thermal peaks is prevented and then the coefficient of diffusion can be kept small. For example, the substrate 1, but also the band-like aluminum sheet metal can be Al 98.3 (purity 98.3 percent) with a thickness D1 of 0.5 mm. The minimum thickness D1 of one such sheet metal can be at 100 μm, whereas the upper limit of thickness D1 can be at about 1.5 mm. It is also possible to use aluminum alloys as substrate 1, such as AlMg-alloys for example.

(8) The intermediate layer 2 in the illustrated case consists of two partial layers 8a, 8b. The upper partial layer 8a according to the invention consists entirely of an organic, layer-forming lacquer, which is hardened by means of an ionic photo-polymerization and crosslinking, and can have a maximum thickness D8a in the range of 1 μm to 15 μm. The lower partial layer 8b is made of anodic oxidized aluminum of the substrate material, and can have a thickness D8b in the range of 10 nm to 10.0 μm, preferably in the range of 500 nm up to 2.0 μm, most preferably in the range of 700 nm to 1.5 μm. It can be prepared by wet-chemical means wherein the pores of the aluminum oxide layer in the final phase of the process chain can be mostly sealed by a hot-compression.

(9) In this case it is preferable that the surface of the intermediate layer 2 (lacquer-partial layer 8a) has an average arithmetic roughness value R.sub.a in the range of less than 0.05 μm, in particular of less than 0.01 μm, most preferably of less than 0.005 μm. When the above-mentioned high total light reflectivity is present, this average roughness will aid in adjusting of a minimum diffuse light reflectivity defined according to DIN 5036. If a higher diffuse light reflectivity is required, then the roughness can be increased accordingly.

(10) The optical multi-layer system 3 includes at least three layers 4, 5, 6, wherein the upper layers 4, 5 are dielectric and/or oxidic layers, and the lowest layer 6 is a metallic layer consisting of silver, which forms a reflective layer 6. The particular optical thickness D4, D5 of the upper and of the middle layers 4, 5 of the optical layer system 3 should be dimensioned; in order that the layers 4 5 can act as reflection-elevating interference layers; so that they amount to about one-fourth of the average wavelength of the spectral range of the electromagnetic radiation to be reflected. The thickness D6 of the reflection layer 6 can be preferably in the range of 40 nm to 150 nm.

(11) In the illustrated embodiment, an optionally provided silicon oxidic covering layer 7 with a thickness D7 is applied onto the upper layer 4 of the optical multi-layer system 3. It is also possible to apply a silicon nitride or silicon oxide-nitride layer 7 onto the optical multi-layer system 3. The optical multi-layer system 3; including the covering layer 7; can be applied in a technologically favorable manner by application of a continuous vacuum band-coating process.

(12) Likewise, the optionally provided covering layer 7 can pertain to a mixed layer having the chemical composition Si.sub.aC.sub.bO.sub.cN.sub.dH.sub.e, wherein the indices a, b, c, d, e denote a stoichiometric or non-stoichiometric ratio, and are adjusted such that the covering layer 7 at a selected layer thickness D7 has only a small light absorption, in particular a light absorption of less than 10 percent, preferably of less than 5 percent, and most preferably of less than 1 percent, and wherein the carbon content; relative to the total mass of the covering layer 7; is in the range of 0 atom-percent, in particular of 0.2 atom-percent, up to 15.0 atom-percent, preferably in the range of 0.8 atom-percent to 4.0 atom-percent. For an index a=1, the other indices can be arranged in the following ranges: 0<b<2, 0<c<2, 0<d<4/3, 0<e<1. At least one of the indices b, c, d in this case should be different from zero.

(13) A layer of this kind can be applied in particular as a CVD-layer, preferably as a PE-CVD layer, wherein the advantage of its use consists in that it has a barrier effect against corrosive media, wherein especially due to a fraction of carbon, a greater flexibility and toughness of the layer can be adjusted than for fully ceramic SiO.sub.2 layers.

(14) Finally, a silicon-organic lacquer layer based on a sol-gel layer, in particular with a preferred layer thickness in the range of 0.5 μm to 5 μm, can be applied onto the optical multi-layer system 3 as covering layer 7.

(15) In order to achieve a reduced loss of the total light reflectivity over long-term use of the inventive composite material and/or use at elevated temperature, that is, in order to retard the ageing process, the intermediate layer 2 according to the invention includes the organic, layer-forming lacquer, or is formed entirely from one such lacquer, when no additional partial layer 8b is present in the intermediate layer 2. The organic lacquer, preferably a cyclo-aliphatic or cyclo-aromatic ether, in particular an epoxy lacquer, is hardened preferably by a cationic photo-polymerization and crosslinking, wherein the reaction proceeds according to the following mechanism: 1. Acid formation under UV radiation cationic photo-initiator+“acid anion”.fwdarw.acid+residue 2. Chain start reaction epoxy monomer+acid.fwdarw.“epoxy monomer-cation”+“acid anion” 3. Polymerization “epoxy monomer-cation”+epoxy monomer+epoxy monomer . . . .fwdarw.polymer

(16) As has been mentioned, sulfonium or iodonium salts can be used as photo-initiators here.

(17) A UV-hardening cyclo-aliphatic epoxy lacquer which forms a 100% system (1-K system) with the photo-initiator and which post-crosslinks after the UV exposure at room temperature, appears to be particularly suitable. Mercury medium-pressure emitters with a power output in the range of 150 to 250 W/cm were used for irradiation of this kind of lacquer, wherein the applied UVA-dosage was in the range of 500 to 1000 mJ/cm.sup.2. Using the resulting layer thickness D8a of the lacquer layer 8a in the range of maximum 1 μm to a maximum 15 μm, the intermediate product 1/2 formed from substrate 1 and intermediate layer 2, after the initial UV-hardening, could be rolled up into a coil without crack formation and/or delamination, and the post-crosslinking was carried out in this condition. The maximum, final hardness appeared after one week.

(18) The confectioning of the intermediate product 1/2 to the final composite material V, that is, the application of the optically active multi-layer optical system 3, took place during this time period after post-hardening by means of a PVD-coating, or specifically within 24 hours after the application of the lacquer and irradiation, so that a good adhesion strength between the intermediate layer 2 and the optically active multi-layer system 3 could be obtained. The reflection layer 6 was bonded to the intermediate layer 2 here by means of an adhesion-promoting layer 9.

(19) The adhesion-promoting layer 9 is provided so as to additionally increase the adhesion of the reflective layer 6 consisting of silver, onto the intermediate layer 2. Suitable layer materials herein are in particular, oxidic adhesion promoting agents, such as preferably Al.sub.2O.sub.3, TiO.sub.2 or CrO.sub.s, wherein s denotes a stoichiometric or non-stoichiometric ratio and should be in the range of 0≤s≤1.5. The layer thickness D9 of the adhesion-promoting layer herein can be in the range of 0.1 nm to 50 nm, preferably in the range of 0.5 nm to 25 nm. A range of between 10 nm to 20 nm is viewed as being particularly preferred.

(20) To improve the adhesion to and prevent any delamination of the lower refractive layer 5 from the silver layer 6, an additional adhesion-promoting layer 10 can be provided; as illustrated; which is likewise in particular oxidic and can consist of CrO.sub.s. The second adhesion-promoting layer 10, which is associated with the optical multi-layer system 3 due to its position, can have a thickness D10 here which resides in the same range as that of the first adhesion-promoting layer 9. A light-absorptive effect is known for the chrome-oxidic, in particular the sub-stoichiometric CrO.sub.s-compounds compared to tri-valent chromium. However, this effect reduces the high, total light reflectivity only marginally, especially when thickness D10 of the second adhesion-promoting layer 10 is in a preferred range of 0.5 nm to 10 nm.

(21) The adhesion-promoting layers 9, 10; like the layers 4, 5, 6 of the optical multi-layer system; can be sputter layers, in particular layers produced by reactive sputtering, CVD or PE-CVD layers or layers produced by vapor coating, especially by electron bombardment or generated from thermal sources, so that the entire multi-layer optical system present on the intermediate layer 2 consists of layers applied in a vacuum sequence, in particular in a continuous process.

(22) All layers and/or partial layers of the invented composite material V, as well as the good adhesion between them contribute to a high scratch resistance. In particular, in a synergistic cooperation, both the comparatively thicker and harder lacquer layer 8a; if used; the anodized layer 8b, and also the comparatively thinner and less hard dielectric layers located thereon, all make a contribution.

(23) The improvement in properties attainable according to the invention is expressed in particular in that the following stress tests were passed: 1008 h QUV-B-stress (8 h irradiation at 70° C./313 nm/0.48 W/m.sup.2; 4 h condensation at 50° C.) 72 h UV-A/B stress at 150° C., 1008 h temperature stress at 80° C. 72 h UV-A/B temperature stress at 150° C., 72 h UV-A/B temperature stress at 200° C., 72 h UV-A/B temperature stress at 250° C.,

(24) The criterion used to determine whether a test was passed, was in each case that the composite material V displayed no black spots, no cloudiness, and no delamination when peeled off with an adhesive tape (e.g. Scotch tape 600).

(25) The person skilled in the art can supplement the invention through additional favorable measures, without leaving the scope of the invention. For example; as is likewise indicated in the illustration; an additional decorative layer 12 can be applied onto the side B facing away from the optical multi-layer system 3, in particular on the substrate 1, which can also optionally have an anodic oxidic layer 11 on this side. This decorative layer 12 can be, for example, a metallic reflective layer or one made of titanium nitride or other suitable materials, which can lend the layer a sheen and also a certain coloration. This is an advantage in particular when reflector elements for lighting are to be produced from the composite material V according to the invention.

(26) Another preferred application is the placement of LED lighting sources, e.g. in the form of chips, onto the surface of the invented composite material V. With regard to the additional possible details, the reader is referred to the document DE 10 2012 108 719 A1 in its entirety, for example.

(27) Finally, owing to its long-term stability and high total light reflection, the composite material V according to the invention has outstanding suitability for use in solar facilities which are installed in greenhouses and concentrate sunlight into heat energy, as is described in U.S. Pat. No. 8,915,244 B2, for example. Here too, the reader is referred to the referenced document in its entirety for a description of additional possible details.

(28) Not only can a pair of upper dielectric and/or oxidic layers 4, 5 be disposed in the optical multi-layer system 3 over the reflective layer 6, but rather also several such pairs can be arranged so that the reflectivity of the invented composite material V can be even further enhanced. The optionally provided adhesion-promoting layer 10 can in this case be preferably a constituent on one such layer pair, wherein a layer located above it should display a correspondingly greater refractive index.

(29) The invention also pertains to an anionic photo-polymerization, by which in particular methacrylates and acrylates can be converted into an organic lacquer suitable for the formation of the intermediate layer.

(30) Now where Standards (DIN, ASTM etc.) are named in this present application, these each pertain to the version of the Standard applicable at the time of the application.

(31) While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.