LOW TEMPERATURE-CURABLE ANTIREFLECTIVE COATINGS HAVING TUNABLE PROPERTIES INCLUDING OPTICAL, HYDROPHOBICITY AND ABRASION RESISTANCE

Abstract

Disclosed herein is an inventive low-temperature curable antireflective (AR) coating produced by a single layer sol gel deposition process comprising a low-temperature curing step, whereby temperatures well below 100 C. for under 8 hours result in highly robust AR coatings having excellent transmittance and abrasion resistance. Optical, mechanical and chemical properties may be tuned by adjustment of the formulation of the wet coating solution. In this way, the inventive AR coating is able to provide enhanced mechanical and moisture resistance, as well as superior optical performance that can be optimized to suit a particular environment. The innovation advantageously enables applying AR coatings to substrates installed in the field, allowing passive heating of the substrate by sun exposure to provide the heat for curing the inventive coatings outdoors.

Claims

1. A single layer energy transmission enhancement coating, comprising a composition of 60-100% silicate, 0-20% siloxane, and 0-20% solid silica nanoparticles having a size range of 5-200 nm, and exhibiting an abrasion test result of over 65% when said coating is subject to an abrasion test consisting of 2000 strokes with a 1 cm1 cm felt pad with 500 g of force over the coating and having a HAST result of over 95%.

2. A multiple layer energy transmission enhancement coating, comprising a composition of 60-100% silicate, 0-20% siloxane, and 0-20% hollow silica nanoparticles of a size range of 5-200 nm, and exhibiting an abrasion test result of over 85% when said coating is subject to an abrasion test consisting of 2000 strokes with a 1 cm1 cm felt pad with 500 g of force over the coating and having a HAST result of over 95%.

3. A single layer energy transmission enhancement coating produced by the process comprising the steps of: i) providing a substrate in an ambient; ii) providing a coating apparatus having a coating distribution means adapted to distribute a liquid energy transmission enhancement coating solution on a surface of the substrate; iii) engaging the coating apparatus with the substrate wherein the coating distribution means of the coating apparatus is in functional proximity of the substrate; iv) depositing the liquid energy transmission enhancement coating solution from the distribution means of the coating apparatus onto the substrate surface wherein the distribution means is adapted to the cover at least a portion of the substrate surface; and v) curing the deposited coating solution in the ambient at ambient temperatures less than or equal to 60 C. for a time period less than 24 hours, wherein the resulting cured single-layer has a composition of 60-100% silicate, 0-20% siloxane, and 0-20% solid silica nanoparticles having a size range of ( ) and exhibiting an abrasion test result of over 65% when said coating is subject to an abrasion test consisting of 200 strokes with a 1 cm1 cm felt pad with 400 g of force over the coating and having a HAST result of over 95%.

4. The method of claim 3, wherein the substrate is a photovoltaic panel.

5. The method of claim 4, where the substrate is a photovoltaic panel array.

6. The method of claim 3, wherein the substrate is a solar thermal panel.

7. The method of claim 3, wherein the substrate is a glass window pane.

8. The method of claim 3, wherein the ambient is out of doors.

9. The method of claim 3, wherein the ambient is indoors.

10. A method for depositing an energy transmission enhancement coating on a substrate, comprising: i) providing a substrate in an ambient; ii) providing a coating apparatus having a coating distribution means adapted to distribute a liquid energy transmission enhancement coating solution on a surface of the substrate; iii) engaging the coating apparatus with the substrate wherein the coating distribution means of the coating apparatus is in functional proximity of the substrate; and iv) depositing the liquid energy transmission enhancement coating solution from the distribution means of the coating apparatus onto the substrate surface wherein the distribution means is adapted to the cover at least a portion of the substrate surface.

11. The method of claim 10, further comprising the step of curing the deposited coating solution in the ambient at ambient temperatures less than or equal to 50 C. for a time period less than 24 hours.

12. The method of claim 10, wherein the substrate is a photovoltaic panel.

13. The method of claim 12, where the substrate is a photovoltaic panel array.

14. The method of claim 10, wherein the substrate is a solar thermal panel.

15. The method of claim 10, wherein the substrate is a glass window pane.

16. The method of claim 10, wherein the ambient is out of doors.

17. The method of claim 10, wherein the ambient is indoors.

18. The method of claim 10, wherein the step of curing the deposited coating solution comprises sun curing of the deposited film.

19. The method of claim 10, wherein the step of curing the deposited coating solution comprises curing the deposited film in a dark environment.

20. A method for depositing a uniform fluid film with a thickness of less than 20 microns on a substrate located outdoors, comprising: i) providing a substrate in an outdoor environment; ii) providing a coating apparatus having a coating distribution means adapted to distribute a liquid energy transmission enhancement coating solution on a surface of the substrate; and iii) depositing the liquid energy transmission enhancement coating solution from the distribution means of the coating apparatus onto the substrate surface wherein the distribution means is adapted to the cover at least a portion of the substrate surface.

21. The method of claim 20, further comprising the step of curing the deposited coating solution in the outdoor temperatures at ambient temperatures less than or equal to 50 C. for a time period less than 24 hours to yield a performance enhancement coating.

22. The method of claim 21, wherein the step of curing the deposited coating solution in the outdoor environment comprises sun-curing the deposited coating solution.

23. The method of claim 21, wherein the performance enhancement coating is an energy transmission enhancement coating.

24. The method of claim 22, where the performance enhancement coating is substantially transparent.

25. The method of claim 22, where the performance enhancement coating is abrasion resistant according to ASTM D 2486.

26. The method of claim 22, where the performance enhancement coating is humidity resistant according to JESD22-A102B.

27. The method of claim 20, wherein the substrate is a glass window pane.

28. The method of claim 20, wherein the ambient is a solar panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1. TEM micrograph showing and example of thin-shell hollow nanoparticles that may be used in the inventive energy transmission enhancement coating.

[0021] FIG. 2. TEM micrograph showing an example of thick-shell hollow nanoparticles that may be used in the inventive energy transmission enhancement coating.

[0022] FIG. 3. SEM cross-sectional view of the energy transmission enhancement coating deposited on a thin-film semiconductor layer on a glass substrate.

[0023] FIG. 4. Zoom of view shown in FIG. 3, revealing structural details of the inventive energy transmission enhancement coating.

[0024] FIG. 5. Transmission spectral comparison between coated (inventive AR coating) and uncoated smooth glass substrate.

[0025] FIG. 6. Transmission spectral comparison between coated and uncoated textured glass substrate.

[0026] FIG. 7. Transmission spectral comparison between coated and uncoated smooth acrylate (PMMA) substrate. Upper graph shows comparison of the transmission spectra of the uncoated and coated substrate, and lower graph shows % T.

[0027] FIG. 8. Results from abrasion scrub test. Comparison of transmission spectra of virgin single pass coating and same after abrasion scrub test. See text for test conditions.

[0028] FIG. 9. The change in transmittance (% T) of the innovative coating after the abrasion scrub test. Same substrate as in FIG. 8

[0029] FIG. 10. Results from abrasion scrub test on a double-pass coating. Comparison of transmission spectra of virgin double-pass coating and same after abrasion scrub test.

[0030] FIG. 11. Results of HAST testing. A single-pass coating subjected to HAST conditions. See text for test conditions.

DETAILED DESCRIPTION

[0031] The tunable coating property aspect of the instant innovation may be derived in part through variations in the deposition process. In some embodiments this may be accomplished by varying the thickness of the coating. The tunable coating property aspect of the instant innovation may be derived in part from variation of the final composition of the coating, which in turn is determined by the relative amounts of precursors in the wet coating precursor solution, or precursor ratio, can be selected from a continuum of precursor ratios disclosed herein to produce desired coating characteristics. In some embodiments, the concentration of the siloxane component is changed to yield desired properties. Siloxanes or hardcoats are available through a variety of manufacturers. An example of a hardcoat is poly dimethyl siloxane (PDMS) and derivatives.

Formulation

[0032] In some embodiments, the dry content composition of the inventive AR coating may comprise the following composition ranges in terms of solids content (dry weight percentages):

[0033] Matrix/Silicate: 60-100%

[0034] Siloxane: 0-20%

[0035] Nanoparticles (hollow and/or solid): 0-20%

[0036] In other embodiments, the dry composition may comprise the following ranges:

[0037] Matrix/Silicate76-90%

[0038] Siloxane5-12%

[0039] Hollow NP5-12%

General Coating Precursor Solution

[0040] The matrix sol gel precursor is derived from base-catalyzed hydrolysis of an organic orthosilicate, for example tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). Sol gel creation from organic orthosilicates such as TMOS and TEOS is well known in the art, and the exact concentrations and final pH adjustments of the acid and base catalysts can vary. Many examples of particular conditions can be found in both the patent and scientific literature.

[0041] One embodiment of the coating precursor solution is formulated as a mixture of the following at room temperature: [0042] Organic orthosiicate (TMOS) sol gel concentration can range up to 50% in alcohol-base-catalyzed [0043] Hardcoat siloxane concentration can range up to 50% in alcohol [0044] Hollow-spherical nanoparticle (HSNP) concentration can range up to 50% as an alcoholic suspension [0045] wherein the alcohol may comprise any one of C1 to C10 alcohols and mixtures thereof. Any suitable solvent known to those skilled in the art may be used.

[0046] The volumetric ratios of the individual coating precursor solutions may be adjusted to yield precursors having the following concentrations based on ratios of one to the other or final percentages in solution:

[0047] HSNP 0-20%

[0048] Hardcoat 0-20%

Example of Low Temperature Curing AR Coating Composition

[0049] A low temperature curing AR coating solution composition comprises the following components. Base-catalyzed orthosilicate tetramethyl orthosilicate (TMOS-b) system is prepared, by mixing TMOS, water, methanol or ethanol, and a base catalyst that may include any of the following basic compounds: ammonia, organic amines (RNH.sub.2, R.sub.2NH, R.sub.3N, where R=C.sub.1-3 alkanes), basic amino acids (arginine, lysine) and quartenary ammonium halides, where the quartenary ammonium ion has the formula RNMe.sub.3, where R=C.sub.6-12 alkanes.

[0050] A pre-mixture of 4:1 TMOS-b binding agent is prepared. TMOS-b tends to polymerize into long linear chains and does not extensively cross-link. A binding agent that undergoes hydrolysis during curing, forming linear or branched structures at low temperature, occurring readily under 100 C., is added as well. In the inventive energy transmission enhancement coating composition, the binding agent may be used as a minority reagent in combination with TMOS-b to provide for the cross-linking of the long linear silicate chains made by the polymerization of TMOS-b. The combination of the binding agent and TMOS-b in the composition disclosed advantageously cures to form hard scratch resistant coatings at substantially lower temperatures for less curing time than previously disclosed coatings of similar composition.

[0051] The novel AR coating composition further comprises organosilane additives for improvement of hydrophobicity, and any of the organosilanes having the structure (R.sub.1).sub.nSi(O R.sub.2).sub.4-n (R.sub.1, R.sub.2: C.sub.1-3 alkane, alkene, n=0-3) and RSiCl.sub.3 (R: C.sub.1-3 alkane, alkene) have been found to may be added in varying ratios to the binder: TMOS-b mixture. Optical properties of the coatings are controlled using both solid and hollow sphere silica nanoparticles, described below. In other embodiments, no nanoparticles are added to the mixture. Low temperature-curable coatings according to the innovation form high transmittance and excellent abrasion resistance (see FIGS. 8-10) when cured at, for example, 40 C. for 24 hours, 50 C. for 8 hours, 65 C. for 4 hours, 150 C. for 1 hour. This contrasts more typical curing regimes of curing temperatures ranging between 90 C.-700 C. for 10 minutes or less for the higher temperatures, up to five hours for the lower temperatures. Such a treatment may yield a film thickness ranging between 50-250 nm. Other thermal treatment regimes, as well as more exotic plasma and microwave methods are not excluded.

[0052] Formulations for the low temperature AR coating solution compositions may comprise the following ranges:

Without Nanoparticles

TMOS-b (Matrix/Silicate): 50-95%

Binder: 5-50%

With Nanoparticles

TMOS-b (Matrix/Silicate): 60-90%

Binder: 5-20%

[0053] Nanoparticles (hollow and/or solid): 5-20%

Nanoparticle Addition

[0054] It may be desired to incorporate added hollow silica nanoparticles to the precursor coating solution. Syntheses of hollow spherical silica nanoparticles are well known in the art. Many examples of silica HSNP can be found in both the patent and scientific literature. FIGS. 1 and 2 are transmission electron micrographs showing typical examples of silica HSNPs prepared and used in the inventive energy transmission enhancement coating solution.

[0055] FIG. 1 shows thin-shell hollow nanoparticles, whereas FIG. 2 depicts thick-shell hollow nanoparticles. Procedures to synthesize hollow nanoparticles are abundant in the patent and scientific literature. In terms of size, the hollow nanoparticles can range between 5 to 200 nm. In terms of distribution, the hollow nanoparticles can be within a narrow size range, or within in a bimodal size range, a trimodal size range, multimodal or completely random size distribution. In addition to hollow nanoparticles, solid nanoparticles may be incorporated into the film. One such method is to procure solid nanoparticles from a commercial source and incorporate them into the solution mix prior to making the film.

[0056] Incorporation of pre-synthesized nanoparticles creates additional costs to manufacture the innovative coating precursor solution. The instant coating precursor solution does not incorporate the addition of pre-synthesized nanoparticles, and instead produces a coating where nanoparticles may form spontaneously.

Example of Coating Panel Substrates on in the Field

[0057] The coating deposition comprises mixing the individual coating precursor components together to form the coating solution. The coating solution is then deposited on a substrate using a coating apparatus adapted to coat substrates such as photovoltaic panels and solar thermal panels already existing in a field installation. Such a coating apparatus is described in detail in co-pending U.S. Utility patent application Ser. No. 14/668,956, incorporated herein by reference in its entirety, but coating apparatuses for the purpose of this disclosure are not limited to any particular type, and in general comprise a coating distribution means.

[0058] The coating distribution means include, but are not limited to, spray coating nozzles, brushes and contact applicators of the like. This point is explained below. By virtue of the capability of the inventive coating precursor solution to cure at temperatures well under 100 C., the ability to retrofit or re-coat substrates in existing installations with an optical coating, such as an antireflective coating, is provided. This improvement eliminates the need to dismantle the substrate from the installation to send it to the factory of origin or to a special facility for coating, avoiding a costly and disruptive maintenance procedure.

[0059] The installations referred to in this disclosure comprise a single substrate, such as a single individual photovoltaic panel, or an array of multiple panels, as in a photovoltaic array. The term array is meant to be understood to consist of a single panel or multiple panels. Substrates may be extended to include solar thermal panels, regarded individually (single panel arrays) or in multi-panel arrays. In addition, glass window panes installed in residential and commercial buildings are included in the definition of substrate as well for the purposes of this disclosure.

[0060] A coating apparatus may be a standard one known in the art to make thin-film coatings, such as, by way of example, a roll coater, spin coater, dip coater and spray coater. The coating process may be carried out at ambient temperatures, but temperatures both above and below ambient are not excluded. Coating thickness may be controlled by certain coating parameters, such as the viscosity of the coating solution, speed of a moving substrate, and/or the curing process, as described below.

[0061] In some embodiments, the coating is applied to a substrate, such as a solar photovoltaic panel installed in an outdoor photovoltaic array, by use of the coating apparatus disclosed in co-pending U.S. Utility patent application Ser. No. 14/668,956, incorporated by reference herein in its entirety. The coating apparatus disclosed therein is adapted to deposit an optical thin-film coating layer of uniform thickness by use of innovative coating heads, or brushes, on substrates such as photovoltaic panels in both indoor and outdoor installations.

[0062] For the purposes of this disclosure, the substrate is disposed in an ambient, where an ambient can be defined either as an indoor or outdoor environment. Outdoors, or out of doors is defined as being outside, or disposed in the open environment, whereas indoors is defined as being inside, or disposed in the interior of an enclosed structure, such as a building. For purposes of this disclosure, field is used, such as field-coated, to mean the coating process takes place outside of a facility where the substrate would normally be manufactured, and rather the inventive coating process occurs in an individual or array installation of the substrate, typically out of doors.

[0063] An example coating procedure is the following:

[0064] A substrate is provided, where the substrate can be any one of the following: a photovoltaic panel, a solar thermal panel, a glass pane. In practical terms, the substrate may be referred to as a panel or pane, and may be part of an existing installation, either as a single panel or multi-panel array, for photovoltaic and solar thermal installations, or as glass windows installed in a structure. As discussed above, a coating apparatus is provided, comprising a coating distribution means.

[0065] Such a coating distribution means may be based on a brush methodology where the coating distribution means is an applicator head having one or more brushes in intimate contact with the substrate surface, applying a uniform layer of liquid coating precursor solution on the substrate, where the coating distribution means is capable of applying a liquid coating layer that may be less than or equal to 20 microns in thickness. Such a coating means is described in detail in co-pending U.S. Utility patent application Ser. No. 14/668,956, incorporated herein by reference in its entirety. Alternatively, the coating distribution means may be based on a spray methodology, where one or more spray nozzles are used to apply a uniform layer of optical-coating precursor solution to the substrate, where the nozzles are positioned at a distance above the substrate surface.

[0066] The coating apparatus may be positioned on the substrate surface, which for photovoltaic panels or solar thermal panels, may be inclined at an obtuse angle with respect to the vertical. As an example, the coating apparatus may be placed on the lower end of the panel. The coating apparatus may be hand-driven, in which case it may have an elongated handle attached to it. An operator may then move the coating apparatus along the substrate surface in an excursion from the initial position to the upper end of the substrate. For a brush applicator, the one or more applicator heads may be engaged on the surface during the excursion. Alternatively, the applicator heads may be engaged during the return excursion, or during both excursions. The coating apparatus may also be adapted to move in a grid pattern, being displaced laterally. The foregoing is also true for a coating apparatus having a spray distribution means.

[0067] A thin film layer of the inventive precursor solution is then applied to either the entire surface of the substrate, or a portion thereof, with a substantially uniform thickness. In some embodiments, the precursor layer is of such a thickness that a cured coating thickness of 50-250 nm will result. Moreover, the coating may be deposited in a single pass or by multiple passes, where the same or different coating precursor solution is deposited over a previous coating layer of the same composition.

[0068] In some embodiments, the innovative coating is prepared as a single-pass layer or a double-pass layer. In other embodiments, the coating apparatus is motorized, where a motor drive is engaged with the traction means of the coating apparatus, and provides a constant speed of translation of the apparatus. The constant speed is one form of operation, as the rate of deposition of the layer is a strong function of the speed of translation of the apparatus. By precise control of the speed of the coating apparatus during its coating excursions, the final thickness of the layer is well controlled and spatially uniform. This is best done by a motorized coating apparatus. In this manner, the thickness may readily be tuned to wavelengths of target portions of the solar spectrum or other ambient lighting.

[0069] The precursor layer may now undergo a curing step, wherein the substrate, as part of an outdoor installation, is passively cured out of doors in the sun at ambient temperatures. In some embodiments, the substrate surface temperatures range from 10 C. to over 100 C. Surface temperatures such as those figuring in the quoted range may be engendered by ambient sunlight, and related to air temperature, which is primarily dictated by weather conditions, season and geographic location. According to the innovation, the warmer the substrate surface temperature, the faster the curing process occurs.

[0070] Alternatively, the curing process may take place under conditions of low light levels, or in the dark entirely, as the curing chemistry is a thermal process. As an example, a coated substrate in an outdoor installation may be cured under cloud cover, or at night. Moreover, the substrate may be cured indoors, where the surface temperature is approximately the ambient temperature.

[0071] A cross sectional view of the innovative cured coating is shown in the SEM micrographs of FIGS. 3 and 4. FIG. 3 shows a cross-sectional view of the innovative coating on a thin-film photovoltaic device deposited on a glass substrate. FIG. 4 shows a zoom of the interfacial portion of the device, having the innovative coating applied at the surface of the device. The surface in the case of the photovoltaic film is uneven, and the innovative coating can be seen forming a smooth optical film above. The innovative coating shown in FIGS. 3 and 4 incorporate nanoparticles. The innovative coating is a single layer coating, as explained above, being substantially compositionally and structurally homogeneous across its thickness.

Optical Performance

[0072] The effect of using the inventive AR coating on glass and plastic substrates is shown in FIGS. 5-7. In FIG. 5, the visible wavelength transmission spectra are shown for a smooth flat window glass substrate. The upper curve represents the glass substrate coated with the inventive energy transmission enhancement coating, in this case intended as an AR coating, on one side. The data show an improvement of transmission (T) of up to 4% between 500 and 600 nm, and minimum 3% elsewhere, with an average gain in transmittance of 3.65%.

[0073] Direct reflectance measurements on textured glass are shown in FIG. 6. Here, the data show the decrease in reflected light (upper graph, dashed curve) across the visible spectrum due to the presence of the inventive energy transmission enhancement coating. The average decrease is 3.73%. In the lower graph of FIG. 6, the dashed curve represents the change in reflectance from the surface of the substrate coated with the innovative coating.

[0074] FIG. 7 shows the effect of the inventive energy transmission enhancement coating on both sides of an acrylic (PMMA) substrate. The comparison between the coated transmission spectra of a PMMA substrate with the innovative coating (dashed curve) to the same substrate uncoated (solid curve) is shown in the upper graph. The lower graph of FIG. 7 shows that the inventive coating resulted in an average increase of transmittance of 6.75% across the visible spectrum from 400 to 750 nm

[0075] Abrasion resistance of the low-temperature curable energy transmission enhancement coating is demonstrated in FIG. 8. The abrasion scrub test experiments were carried out with 2000 strokes of a brush meeting ASTM D2486 standards with 500 g of force over the coating. FIG. 8 shows the transmission spectrum of a single layer of the inventive AR coating over the wavelength range between 400-900 nm, before and after the abrasion test, where the solid red curve represents the spectrum of a virgin single-pass coating before the abrasion resistance test, and the broken curve was measured after the abrasion test.

[0076] FIG. 9 compares the change in transmittance over the indicated spectrum for the coating before and after the abrasion test. The data show only a 0.3% average decrease in the transmission of light after completion of the abrasion resistance test, indicating that over 90% of the virgin film was retained after the test, therefore demonstrating that the single-layer film has a high degree of scratch resistance. The lower solid curve represents the transmission spectrum of the bare glass substrate, showing that the AR coating provides for an average of a 3.5% increase in light transmission through the substrate, almost 100% suppression of reflection by the novel AR coating.

[0077] FIG. 10 shows results from the same abrasion scrub test applied to a double-pass energy transmission enhancement coating. Again, the solid red curve shows the transmission spectrum of the virgin double-layer coating, and the broken curve is the resulting transmission spectrum after the abrasion test. The data here show that the change in the optical characteristics is only about 0.09%, indicating over 97% of the coating was retained. The results here demonstrate that the double-layer coating exhibits a greater degree if robustness than the single-layer.

[0078] The moisture degradation performance of the inventive films is measured and shown in FIG. 11. The data in this figure are taken from subjecting the inventive AR coatings to conditions dictated by the industry-standard Highly Accelerated Stress Test (HAST). In this test, the coatings were subjected to high temperature of 140 C, 85% humidity at approximately 30 psi (2 atmospheres) of pressure. Under these conditions, the HAST test simulates humidity degradation over a 20 year period. The solid red curve of FIG. 11 is the optical transmission spectrum of the coating on a glass substrate before the test. The blue curve is the transmission spectrum of the coating after the test, whereas the lower solid curve is the transmission spectrum of the bare glass substrate. The results here show that the before and after change of transmission characteristics of the coating is about 0.06%, which indicates that over 98% of the coating was retained after the HAST process.

[0079] While the forgoing embodiments disclosed above describe the innovation in its various manifestations, the foregoing embodiments are to be understood by persons skilled in the art as exemplary in nature, and are in no way intended to be construed as the only embodiments possible for the innovation. Those skilled in the art will also understand that other embodiments and examples of deployment of the inventive AR coatings are conceivable and possible without departing from the scope and spirit of the innovation.