The invention relates to a mirror comprising a reflective surface formed from a composite of a reflective film and a substrate, wherein the reflective film is disposed on the substrate and comprises at least one polymer layer as well as a metallic layer disposed underneath the at least one polymer layer. In addition, the invention relates to a method for manufacturing such a mirror as well as a device for carrying out the method.
Regenerative energy is produced in solar power installations which encompass a large number of large-scale mirrors. Due to the immensely large total of reflective surfaces required for solar power installations, vaporised reflective surfaces made from glass do not appear to be economically viable. Therefore, in order to provide these mirrors in an economically viable manner, one has to revert to a particularly simple process. One measure for providing reflective surfaces consists in laminating a substrate normally comprised of a metal sheet such as of steel, aluminium or an aluminium alloy, onto a reflective film. The reflective film consists of a polymer layer which, for example, consists of a PMMA plastic and is characterised by its very high transparency. The reverse of the polymer layer may be a metal layer, preferably a silver layer sputtered onto it, which has a thickness in the nanometre range, for example. In addition, two metal layers can also be used, such as a first silver layer which has an additional copper layer sputtered onto it. The metal layer, in particular the silver layer, is characterised by extremely high reflectivity allowing reflective films with very high reflectivity to be manufactured. If these are laminated, for example, onto an aluminium sheet, steel sheet or steel band, low-cost reflective surfaces are obtained. The substrate may, of course, also be a composite material which again consists of a polymer. The mirrors of a solar power installation are very much exposed to the vagaries of weather. In particular the silver layer, which is important for reflectivity, tends to corrode with the result that the corroded areas loose much of their reflectivity and become “blind”, thereby loosing their usefulness as reflective surfaces.
Therefore, various methods have been employed in the past to protect the edges of such mirrors against corrosion. For example, an adhesive tape has been used as edge protection, which was applied to the edges subsequently or during manufacture of the mirror. This adhesive tape or another sealing material such as varnish around the edges or the use of a silicon sealing material, was meant to prevent moisture from penetrating between the polymer layer and the metal layer, in particular the silver layer. For it is well known that penetrating moisture causes corrosion in the metal layer resulting in the mirror becoming blind. In particular due to the capillary effect, corrosion continues to spread across the entire reflective surface so that the entire reflective surface deteriorates within a short space of time. Any previously taken anti-corrosion measures for the protection of mirrors have been expensive and at the same time, have given rise to problems during prolonged application.
It is therefore the requirement of the invention to propose a mirror which can be manufactured in a simple way and, at the same time, exhibits improved corrosion behaviour. In addition, a method for manufacturing such a mirror as well as a device for carrying out the method shall be proposed.
According to a first teaching of the present invention the stated requirement is met in that a sealing seam is provided for corrosion protection, at least in certain areas, which is formed by bonding the polymer layer to the substrate in a material-locking manner, wherein the metal layer is interrupted in the area of the sealing seam.
The sealing seam of the mirror according to the invention, therefore, consists of an area in which the metal layer, in particular the silver layer, is interrupted and the polymer layer is bonded to the substrate in a material-locking manner. The material-locked bond between the polymer layer disposed above the metal layer and the substrate makes it possible, for example, that the metal layer remaining in the reflective surface is hermetically separated from the edge regions of the mirror without any corrosion-promoting moisture being able to penetrate through the sealing seam. Salt-spray tests have shown that the reflective surfaces which were completely enclosed by the sealing seams constructed according to the invention did not tend to corrode or to become “blind”.
According to a first design of the mirror according to the invention the sealing seam is an ultrasonic weld. Ultrasonic welding makes it possible to carefully bond the polymer layer to the substrate whilst at the same time to cut through the silver layer disposed underneath the polymer layer. This is also true in the case where an additional copper layer is provided, which for example is disposed underneath the silver layer for its protection.
A particularly low-cost manufacture of large reflective surfaces permits a next design of the mirror according to the invention in that the mirror is shaped like a band and along its longitudinal edges comprises at least one sealing seam extending in longitudinal direction of the band. This permits a band-shaped mirror already sealed along its longitudinal edges to be provided, which can be cut to size on site, for example, and can then be sealed, for example manually along the additionally created edges. But it is also feasible for the band-shaped mirror with sealed longitudinal edges to be processed into blanks which are then automatically provided with one or more additional, for example transverse, sealing seams.
If according to a next embodiment of the mirror the reflective film comprises a polymer layer of PMMA, it is possible to use a plastic layer particularly suitable for ultrasonic welding and a highly transparent plastic as a carrier material for the reflecting metal or silver layer and to allow the manufactured mirror to be provided with sealing seams in a very easy way.
According to a further design of the mirror several parallel sealing seams are provided. Several parallel sealing seams, for example a dual seam, have, albeit, an increased areal requirement, but redundancy is also increased, if the metal layer in one sealing seam has not been completely cut through.
If according to a next embodiment of the mirror the substrate is a metal or a composite material of a hardness higher than the polymer layer, the sealing seam can be produced in a simply way, since ultrasonic welding does not affect the substrate and only the polymer layer is bonded to the substrate. Therefore, suitable substrates are steel, aluminium or aluminium alloys or plastic composite materials of increased hardness. Preferred aluminium alloys are AlMg1, AlMg1Mn1 or aluminium magnesium alloys with a higher alloy content, since thinner and lighter versions of these can already provide the required stability. The thicknesses of substrates consisting of an aluminium alloy lie, for example, between 0.4 mm and 1.5 mm. Typical low-cost bands of aluminium alloys with good processing and very good stability characteristics in relation to the use as mirrors comprise bands from aluminium alloys of type AA 3005 and AA 3105 for the thicknesses quoted.
According to a second teaching of the present invention the requirement stated for a method for manufacturing a mirror consisting of a composite of reflective film and substrate, in which the substrate is laminated to a reflective film which comprises at least one polymer layer and a metal layer disposed underneath the polymer layer, is met in that following laminating, a sealing seam is produced on the mirror, wherein production of the sealing seam is effected by laminating the polymer layer onto the substrate in a material-locking manner, wherein the metal layer is interrupted in the areas of the sealing seam.
As already explained above, the inventors have recognised that an interruption of the silver layer has the effect of preventing the corrosion processes, in an area of the silver layer, from invading other areas of the silver layer and thus from clouding the reflective surface. A particularly good sealing effect at the interruption point of the silver layer is achieved if the polymer layer is bonded to the substrate in a material-locking manner, thereby utilising this bond as a moisture barrier.
Preferably, the sealing seam is produced by ultrasonic welding, wherein on the one hand, the ultrasonic sonotrodes cause the polymer layer to become melted-on and on the other hand, the metal layer is undone in the area of the sealing seam by selective contact pressure and altogether interrupted. Ultrasonic welding is a very safe method of connecting the polymer layer with the substrate, whilst at the same time interrupting the metal layer. In addition, ultrasonic welding is extremely locally limited preventing any adjacent areas of the mirror from being affected.
According to a first design of the method according to the invention the substrate and the reflective film are shaped as a band, the reflective film is continuously laminated onto the band-shaped substrate, and the mirror manufactured in this way is wound onto a coil, wherein sealing of the longitudinal edges of the band-shaped mirror is effected either inline together with laminating the reflective film, or by renewed unwinding of the mirror from the coil in an additional manufacturing step. Preferably, the sealing seams are produced inline on the band-shaped mirror, since no subsequent process step is required for sealing the longitudinal edges of the band-shaped mirror. In this case, but also in the case that the coil with the band-shaped mirror is unwound once more, the longitudinal edges are sealed by continuously sealing the edge regions, for example by ultrasonic welding. The method used is called a “roll-to-roll” method.
A so-called “roll-to-sheet” method is provided according to a further design, in that the substrate is band-shaped and divided into areal substrate blanks, in that the reflective film is then bonded onto the substrate blanks and the substrate blanks with bonded-on reflective film are provided, at least on the longitudinal edges, with at least one sealing seam, respectively. In this way mirror blanks with already sealed longitudinal edges can be provided, which subsequently can be provided with a sealing seam transversely to the sealing seams in longitudinal direction. In particular this offers the possibility of immediately adapting the blanks to the respective mirror size of the mirror to be manufactured and of automatically providing them with transverse sealing seams. In this case completely sealed mirrors can be offered.
It has become evident that the sealing seams can be produced by ultrasonic welding with especially high speed, using ultrasonic roller sonotrodes and optional anvil rollers. Ultrasonic roller sonotrodes as well as the optional anvil rollers used as counter holders are employed in particular then, when linearly extending sealing seams are produced.
Preferably, the ultrasonic roller sonotrode geometry comprises a radius of 1.0 mm-3.00 mm, preferably 1.5 mm-2.5 mm. Due to the corresponding ultrasonic roller sonotrode geometry it is ensured that the polymer layer is bonded in a material-locking manner to the substrate with the metal layer being completely interrupted at the same time in the area of the sealing seam. For it has become evident that for larger radii the interruption of the metal layer was not sufficiently reliable. If radii smaller than 1.0 mm are used, there is a danger that with material-locked laminating the substrate becomes damaged.
In addition, it has become evident that sufficient process-reliable sealing can be achieved with a sealing seam produced at a contact pressure of 1-6 MPa, a frequency of 20-60 kHz and an amplitude of the ultrasonic sonotrode of between 25 and 70 μm. The said parameters have proved to be very process-reliable resulting in very tight sealing seams and thus in excellent protection against corrosion of the reflective surface. At the same time sufficiently high processing speeds could be achieved which permit economic production of the sealed mirrors consisting of a reflective film and a substrate.
Finally, the requirement stated above is met by a device for manufacturing a mirror comprising a decoiler for a band-shaped substrate, a laminating device for laminating the band-shaped reflective film onto a band-shaped substrate as well as at least two ultrasonic roller sonotrodes which are disposed, respectively, in the region of the opposing longitudinal edges of the band-shaped substrate and downstream of the laminating device and which are provided for sealing the band-shaped mirror along the longitudinal edges, wherein additionally a coiler is provided to winding the edge-sealed mirror. The device just described permits a roll-to-roll process for manufacturing an edge-sealed band-shaped mirror. This process is particularly economic. The band-shaped mirror can be cut to size as input material at the location of the solar power installation and be provided, as required, with a further sealing seam for enclosing the reflective surface as such. Moreover, it is feasible with the device according to the invention to provide anvil rollers as a means for producing counter pressure against the ultrasonic roller sonotrodes, in order to avoid any adverse mechanical impact upon the band-shaped substrate during sealing of the longitudinal edges.
According to a next design of the device a cutting device is provided for the substrate downstream of the decoiler which cuts the substrate into blanks before or after the laminating process, and instead of the coiler a depositing device is provided for the mirror blanks. Using the device just described and starting with a band-shaped substrate, for example an aluminium alloy band, cut-to-size mirrors with sealing seams along the longitudinal edges may be provided, which are already adapted for their intended purpose or cut-to-size and need to be provided merely with a transverse sealing seam, in order to completely protect the reflective surface from corrosive influences. The transverse sealing seams may, for example, be produced automatically in a subsequent process step. Using the device just described a so-called “roll-to-sheet” process can be realised.
Finally, the device can be further adapted in that instead of the decoiler, a receptacle for cut-to-size substrates and a feed for these to go the laminating and sealing stations is provided. This implementation of the device according to the invention permits a “sheet-to-sheet” process, i.e. the manufacture of a plate-shaped mirror with sealing seams along its longitudinal edges, starting with a plate-shaped substrate.
The invention will now be described in detail by way of embodiments in conjunction with the drawing, in which
a) and 1b) schematically show the construction of an exemplary embodiment of a mirror consisting of a composite of a reflective film and a substrate,
a) and 4b) show a top view of two further exemplary embodiments of a mirror,
a) shows the principal construction of a mirror consisting of a reflective film 1 and a substrate 2. The reflective film 1 preferably comprises a polymer layer 3 and a metal layer 4. Optionally, the reflective film 1, as shown in the present embodiment, comprises an adhesive layer 5 allowing the reflective film 1 to be laminated to the substrate 2. The substrate 2, as already explained, may consist of a metal such as steel, aluminium or an aluminium alloy or of a composite material which comprises a hardness higher than the polymer layer 3 of the reflective film. If, for example, an aluminium alloy, preferably an aluminium alloy of type AA3005 or AA3105 is used, the substrate normally has a thickness of 0.1-2 mm, preferably 0.4-1.3 mm. Optionally, the substrate may comprise a conversion layer not shown in
From the two individual materials of the reflective film 1 and the substrate 2, a mirror 7, in particular a composite mirror, is manufactured by bonding or laminating, such as shown in a schematic sectional view in
Preferably, both the anvil roller 9 and the ultrasonic roller sonotrode 8 are driven by a drive in such a way that these perform a rolling movement at the same speed as the band. In addition, both the ultrasonic roller sonotrode 8 and the anvil roller 9 must be pressed against each other at a certain contact pressure, in order to produce a sealing seam with an interruption of the metal layer 4. In the present embodiment with a PMMA polymer layer 3 and an aluminium alloy substrate 2 from an aluminium alloy of type AA3105, contact pressures of 2-4 bars were sufficient in order to produce a sealing seam in a process-reliable manner. During tests it also became evident that the chosen geometry of the ultrasonic sonotrode or the anvil roller can affect the quality of the sealing seam 10. Thus, the use of a radius of 8.5 mm independently of the set contact pressures or the set ultrasonic output, did not produce a sufficiently tight sealing seam 10. In the tests the ultrasonic frequency used was 20 kHz with an amplitude of 40 μm for the ultrasonic roller sonotrodes and an output of 500 W. The results of the corresponding tests are shown in table 1.
The tests V1 to V4 showed that in the salt spray test corrosion of the silver layer occurred below the polymer layer leading to poor marks (−−) being given to the result. Tests V5 and V6, by contrast, did not show any corrosion at all and, therefore, were given very good marks (++).
Corresponding mirror elements are illustrated in
If the shape of a double hump with two radii is chosen for the geometry of the ultrasonic roller sonotrode or the anvil roller, a double sealing seam 11′ or 12′ can be produced, for example in longitudinal direction or even in transverse direction to the longitudinal extent of the mirror, such as shown in
Corresponding constructions are also depicted in
Alternatively, the cutting process to produce the required reflective surfaces may be performed directly following production of the sealing seams 10 along the longitudinal edges 6 of the band-shaped mirror 7. An embodiment for performing a respective “roll-to-sheet” process is shown in
In
Number | Date | Country | Kind |
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102012100293.2 | Jan 2012 | DE | national |
This patent application is a continuation of PCT/EP2013/050114, filed Jan. 4, 2013, which claims priority to German Application No. 10 2012 100 293.2, filed Jan. 13, 2012, the entire teachings and disclosure of which are incorporated herein by reference thereto.
Number | Date | Country | |
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Parent | PCT/EP2013/050114 | Jan 2013 | US |
Child | 14327636 | US |