COATINGS FOR SOLAR APPLICATIONS

Abstract

The invention relates to a composition for producing a solar absorber coating, comprising a silicone resin formulated with: (i) at least one compound selected from the group consisting of black ruthenium oxides and black spinel; and (ii) a glass powder. A method of applying the composition and coatings formed are also provided.

Claims

1. A composition for producing a solar absorber coating, comprising a silicone resin formulated with: (i) at least one compound selected from the group consisting of black ruthenium oxides and black spinel; and (ii) a glass powder.

2. A composition according to claim 1, wherein the silicone resin has aryl groups attached to the silicone backbone.

3. A composition according to claim 2, wherein the silicone resin is selected from the group consisting of poly (methylphenylsiloxane) and poly (diphenylsiloxane).

4. A composition according to claim 1, comprising black ruthenium oxide selected from the group consisting of: ruthenium dioxide (RUO.sub.2); ruthenium-containing pervoskite of the formula M′RuO.sub.3, wherein M′ is either Ca, Sr or Ba; and ruthenium-containing lead-free pyroclores.

5. A composition according to claim 4, wherein the black ruthenium oxide is selected from the group consisting of: Ru0.sub.2; ruthenium-containing pervoskite which is BaRuO.sub.3; and ruthenium-containing lead-free pyroclores which is Nd.sub.1.75CU.sub.0.25RU.sub.2O.sub.6+δ.

6. A composition according to claim 1, comprising black spinel of the formula AB.sub.2O.sub.4 wherein the divalent and trivalent cations are selected from the group consisting of Cu.sup.2+, Co.sup.2+, Fe.sup.2+, Mn.sup.2+, Ni.sup.2+, Fe.sup.3+, Cr.sup.3+ and Mn.sup.3+.

7. A composition according to claim 6, wherein the black spinel is cobalt-iron-chromium mixed oxide with spinel structure.

8. A composition according to claim 1, comprising a combination of ruthenium oxide and black spinel.

9. A composition according to claim 8, comprising ruthenium oxide selected from the group consisting of RUO.sub.2, BaRuO.sub.3 and Nd.sub.1.75Cu.sub.0.25Ru.sub.2O.sub.6+δ and black spinel which is cobalt-iron-chromium mixed oxide with spinel structure.

10. A paint composition according to claim 1, wherein the glass powder comprises 25.0 to 50.0 mole % silicon dioxide, 5.5 to 35.0% mole % titanium dioxide and 19.0 to 45.0 mole % alkali metal oxides.

11. A paint composition according to claim 10, wherein the glass powder further comprises one or more additional glass components selected from the group consisting of boric oxide B.sub.2O.sub.3, bismuth oxide Bi.sub.2O.sub.3, aluminum oxide Al.sub.2O.sub.3, tin dioxide SnO.sub.2, zirconium dioxide ZrO.sub.2 and niobium oxide Nb.sub.2O.sub.5.

12. A paint composition according to claim 10, wherein the glass powder comprises: 30 to 45 mole % S.sub.1O.sub.2; 15 to 20 mole % TiO.sub.2; 35 to 45 mole % M.sub.2O wherein M is Na, K, Li or their mixture; 3 to 7 mole % B.sub.20.sub.3; 0.5 to 4 mole % SnO.sub.2; and one or more of the following oxides: 0.5 to 4 mole % A1.sub.20.sub.3; 0 to 2 mole % Bi.sub.20.sub.3 and 0 to 2 mole % Zr0.sub.2.

13. A paint composition according to claim 12, wherein the glass powder comprises: 35 to 40 mole % Si02; 15 to 20 mole % Ti02; 8 to 15 mole % K20, 10 to 22 mole % Na20, 3 to 11 mole % Li20; 3 to 7 mole % B203; 1 to 3 mole % Sn02; 0.5 to 4 mole % A1203; 0.5 to 2 mole % Bi2O3; and 0.5 to 2 mole % Zr0.sub.2.

14. A method comprising applying the coating composition of claim 1 onto a substrate in a solar collector, drying and heat-curing said composition and optionally firing the same.

15. A solar collector having a fired coating applied thereon, said coating comprising a compound selected from the group consisting of black ruthenium oxides, black spinel and their mixture and either non-crystallizable or crystallized Si0.sub.2-rich glass, with Si0.sub.2 concentration in the glass being higher than 65 mole %.

16. A composition according to claim 1, wherein the concentration of the black ruthenium oxide (s) is the mixture of solids consisting of said black ruthenium oxide (s), the black spinel and the glass powder is from 0 to 80 wt %.

17. A composition according to claim 1, wherein the concentration of the black spinel in the mixture of solids consisting of the black ruthenium oxides, said black spinel and the glass powder is from 0 to 80 wt %.

18. A composition according to claim 1, wherein the concentration of the glass powder in the mixture of solids consisting of the black ruthenium oxide (s), black spinel and said glass powder is from 5 to 60 wt %.

Description

EXAMPLES

Preparation 1

Silicone-Based Vehicle

[0030] A commercially available silicone-based heat resistant paint (Pyromark® 2500) was passed through a filter paper. The filtrate which consists of solvents and siloxane was slowly dried to remove most of the solvents, resulting in a resinous material. The resinous material was dissolved is a solvent and was subjected to nuclear magnetic resonance (NMR) analysis to determine the structure of its main component, i.e., the silicone resin. The NMR data indicated the presence of phenyl and methyl groups in the silicone resin. The filtrate (which in addition to the silicone resin contains also organic solvents and surfactants) was used in the experimental work described below as a vehicle to form coatings (hereinafter the silicone vehicle).

Preparation 2

Glass Powder Compositions

[0031] Glass compositions (coded herein Glass X.sub.1, X.sub.2 and X.sub.3) were prepared in a platinum crucible at 1100° C.-1400° C. The compositions of the glasses are specified below in terms of mole % of each ingredient present in the glass:

TABLE-US-00003 Ingredients Glass X.sub.1 Glass X.sub.2 Glass X.sub.3 SiO.sub.2 38.71 33.0 34.02 TiO.sub.2 16.13 17.0 17.53 K.sub.2O 12.92 8.0 8.25 Na.sub.2O 17.20 20.0 20.62 Li.sub.2O 5.38 10.0 10.31 B.sub.2O.sub.3 4.30 5.0 5.15 Bi.sub.2O.sub.3 1.08 1.0 — ZrO.sub.2 1.08 — — Al.sub.2O.sub.3 1.08 1.0 1.03 SnO.sub.2 2.15 2.0 2.06 Nb.sub.2O.sub.5 — 1.0 1.03 CoO — 2.0 —

[0032] To prepare the glasses, the metal compounds powders are premixed by shaking in a polyethylene jar with plastic balls, and are then melted in a platinum crucible. The melt is maintained at a peak temperature of 1100° C.-1400° C. for a period of 1.5 to 3.0 hours. The melt is then poured into cold water. The maximum temperature of the water during quenching is kept as low as possible by increasing the volume of water to melt ratio. The crude frit after separation from water is freed from residual water by drying in air, or by displacing the water by rinsing with methanol. The crude frit is then ball milled for 3-24 hours in alumina containers using alumina balls. Alumina picked up by the materials, if any, is not within the observable limit as measured by X-ray diffraction analysis. After discharging the milled slurry from the frit, the powder is air-dried at room temperature. The dried powder is then screened through a 325 mesh screen to remove any large particles.

Preparation 3

Spinel Pigment

[0033] One mole of CuO and two moles of MnO were ball milled with 225 g of water for 3 hrs, slip separated and dried at 150° C. The dried mixture was heated in Pt crucible at 900° C. for 64 hours and further heated at 950° C. for 15 hrs, and then ball milled to obtain a fine powder. XRD shows that the product consists mainly of spinel CuMn.sub.2O.sub.4, but that a phase of Mn.sub.3O.sub.4 is also present. The color of the material is dark brown.

Preparation 4

Ruthenium Compounds

[0034] Nd.sub.1.75Cu.sub.0.25Ru.sub.2O.sub.6+δ was prepared as described in U.S. Pat. No. 6,989,111 (J. Hormadaly), entitled “Thick Film Compositions Containing Pyrochlore-Related Compounds”.

[0035] BaRuO.sub.3 was prepared as described by Donohue et al. [The Crystal Structure of Barium Ruthenium Oxide and Related Compounds, Inorg. Chem., 1965, 4 (3), pp 306-310].

Examples 1-3

Preparation and Properties of a Cured Coating Obtained from a Silicone Vehicle Formulated with Pigments

[0036] Three inorganic compounds were tested to assess their ability to function as pigments in a silicone-based heat resistant paint. The inorganic compounds are:

[0037] A) black spinel of the formula (Co,Fe) (Fe,Cr)O.sub.4, commercially available from Ferro Corporation (F6333).

[0038] B) dark brown spinel of the formula CuMn.sub.2O.sub.4, synthesized as described in Preparation 3.

[0039] C) RuO.sub.2, commercially available.

[0040] Each of the inorganic compounds A, B and C was ground with the silicone vehicle of Preparation 1 in an agate mortar. The compositions are tabulated in Examples 1-3, respectively.

[0041] The compositions of Examples 1-3 were brushed on stainless steel 304 substrates, to form a set of samples, each with a uniform coating applied onto 5×5 cm.sup.2 surface area of the metal substrate. The substrates were dried in an oven at 150° C. for minutes. All samples were then heat-cured; the dried substrates were placed in a furnace and were exposed to the following temperature profile:

[0042] heating to 250° C. at a constant heating rate of 20° C./min;

[0043] keeping the substrate in 250° C. for one hour;

[0044] heating to 400° C. at a constant heating rate of 20° C./min;

[0045] keeping the substrate in 400° C. for one hour.

[0046] Some samples were held for an additional hour in the furnace at temperature of 500° C.

[0047] The samples were then subjected to a color test and an adhesion strength test, to determine if the compound is suitable for pigmenting the silicone resin and to assess the effect of holding the coated substrate at 500° C. for an additional hour.

[0048] In the color test, the samples were visually inspected to determine whether the original black color of the formulation remained unchanged after curing, or if color transition or hue changes occurred.

[0049] In the adhesion strength test, the adhesion of the cured paint was tested by applying an adhesive tape on the cured paint. The paints were rated “marginal”, “not good” or “good”.

TABLE-US-00004 TABLE 3 Example 1 Example 2 Example 3 Composition Silicone vehicle 5.34 g 3.3 g 3.59 g Compound A  5.0 g Compound B 5.0 g Compound C 2.50 g Properties color test after gray Brown black curing at 400° C. adhesion test after marginal good not good curing at 400° C. color test after black brown black curing at 400° C. and 1 hour holding at 500° C. adhesion test after good good good curing at 400° C. and 1 hour holding at 500° C.

[0050] The results reported above indicate that methyl phenyl silicone resin can be formulated with a black spinel of the formula (Co,Fe) (Fe,Cr)O.sub.4 and/or with RuO.sub.2, to form black coatings with high adhesive strength.

Examples 4-5

Preparation and Properties of a Cured Coating Obtained from a Silicone Resin Formulated with a Pigment and a Glass

[0051] The black pigment ((Co,Fe) (Fe,Cr)O.sub.4; F6333) and the glass powder X1 of Preparation 2 were ground with the silicone-containing vehicle of Preparation 1 in an agate mortar. Two different formulations were prepared, as set out in Table 4 below. The formulations were applied onto a stainless steel 304 substrate, and the coated metal pieces were placed in the furnace, to be heat-cured according to the temperature regimen described in the foregoing set of examples. However, due to a power problem, the temperature in the furnace rose to 670° C., following which the furnace was turned off. The accidentally-made samples were removed from the furnace and were tested to assess the color and adhesion strength of the so-formed paints. The formulations of both Examples 4 and 5 lead to the formation of black coatings displaying strong adhesion to the metal surface. Visual inspection of the coatings indicates a formation of a glaze-like layer in the case of Example 4 and darker matt black color in the case of Example 5. The results indicate that the formulated compositions with the tested pigment and glass can survive high temperatures, at least 670° C., to form durable black films. At temperatures higher than 400° C. the silicone resin converts to a white powder which was identified (XRD) as amorphous SiO.sub.2. The stability at high temperatures (670° C.) is probably due to a glass component which softens, dissolves some or all of the amorphous SiO.sub.2, bonds to the metallic substrate and the inorganic ingredients, forming either a modified non-crystallizable glass form, or a partially crystallizable glass during the hold-up at the high temperature range (>600° C.)

[0052] To test the properties of paints cured in accordance with the planned curing process described in Examples 1 to 3, the two formulations were again brushed on stainless steel substrates to form samples, which were subjected to the normal curing process. The formulations and test results are tabulated below.

TABLE-US-00005 TABLE 4 Example 4 Example 5 Composition Silicone vehicle 4.0 g 4.5 g black pigment F6333 2.5 g 3.0 g Glass X1 of Preparation 1 2.5 g 2.0 g Properties Adhesion test after curing at 400° C. good good Color test after curing at 400° C. black black

Examples 6-11

Preparation and Properties of a Cured Coating Obtained from a Silicone Resin Formulated with Pigment(s) and a Glass

[0053] Additional formulations were prepared by grinding solids consisting of the glass powder X.sub.1 of Preparation 2 and one or two black compounds [either an oxide of ruthenium alone, or in combination with the black spinel (Co,Fe) (Fe,Cr)O.sub.4, F6333)], with the silicone resin of Preparation 1 in an agate mortar to form homogeneous mixtures. The so-formed formulations were then applied onto the stainless steel substrates, and the coated metal pieces were heat-cured according to the curing protocol set forth in Examples 1-3. The cured paints were then examined to determine their color and adhesion properties. The composition of each of the paint formulations prepared and the characteristics of the paints obtained on applying and curing the formulations are tabulated below.

TABLE-US-00006 TABLE 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Composition Silicone vehicle 4.0 g 4.0 g 4.0 g 4.0 g 3.5 g 3.0 g Glass X.sub.1 2.5 g 2.5 2.5 g 2.5 g 2.5 g 2.5 g RuO.sub.2 2.5 g — — 1.0 — — Nd.sub.1.75Cu.sub.0.25Ru.sub.2O.sub.6+δ — 2.5 g — — 1.0 g — BaRuO.sub.3 — — 2.5 — — 1.0 g Black pigment F6333 — — — 1.5 g 1.5 g 1.5 g Properties color test after black Black Black Black Black black curing at 400° C. adhesion test after Not good Not good good good curing at 400° C. good good color test after Black Black Black Black Black black curing at 400° C. and 1 hour holding at 500° C. adhesion test after Good Good Good Good Good good curing at 400° C. and 1 hour holding at 500° C.

[0054] The results shown in Table 5 indicate that the silicone resin can be formulated with black oxides of ruthenium and glass to form coatings displaying strong adhesion to stainless steel and acceptable black color, provided that the cured paints are further exposed to a temperature above 400° C. (e.g., 500° C.) for a sufficient time. Notably, in the case of Nd.sub.1.75Cu.sub.0.25Ru.sub.2O.sub.6+δ, the desired properties of the film are achieved following curing at a temperature not higher than 400° C.

[0055] The results also illustrate that the silicone resin can be formulated with glass powder and a combination consisting of ruthenium oxide and a black spinel, which on curing at a temperature not higher than 400° C., produces a coating with particularly intense black color which strongly adheres to the metal surface.

Example 12

[0056] The solution of Preparation 1 was subjected to several heating schedules to estimate weight loss and finally heated for 1 hour at 700° C. in air, to achieve conversion to SiO.sub.2. Three samples underwent the treatment, and the results reported below are the averaged weight loss for these three samples.

[0057] After 0.5 hour at 150° C., the solution of Preparation 1 shows a weight loss of 62.34%. At 150° C., most of the volatile solvents evaporate.

[0058] The same three samples were then subjected to a heating profile consisting of 1 hour at 250° C. and 1 hour at 400° C., ramps up and down were 20 C/min. Average weight loss between 150° C. and the two ramps profile was 32.25%. After the heating at 400° C., the samples consist only of a resinous material, which was partially cross-linked.

[0059] The same three samples were then subjected to 1 hour at 700° C. A white powder was formed. The X-ray powder diffraction of the so-formed white powder is shown in FIG. 1, and is typical of an amorphous material.

[0060] The average weight loss of the three samples between 400° C. to 700° C. was 47.0%, meaning that the resinous material formed after 400° C. was converted to SiO.sub.2 and the conversion was 53.0%.

[0061] This experimental result is in agreement with the following considerations. Assuming that the repeating unit(mer) in the polysiloxane is Si—O(CH.sub.3) (C.sub.6H.sub.5) and the polymer consists of a linear long chain, then it could be estimated that the repeating unit (formula weight of 136.086) converts to one unit of SiO.sub.2 (formula weight of 60.086) after exposure to high temperature in the range of 500-700 C; i.e. 44.2% of the resin will convert to SiO.sub.2. Commercial silicones used for high temperature applications have short branched chains which contain OH groups. Therefore, a higher conversion to SiO.sub.2 is expected, probably around 50%.

Example 13

[0062] This example illustrates the marked change which occurs in the composition of a glass combined with a silicone resin, following the transformation of the resin into SiO.sub.2 at high temperatures, leading the incorporation of this indigenously-formed SiO.sub.2 into the glass to produce a modified glass.

[0063] A typical paint formulation containing ruthenium dioxide, black spinel (Co,Fe) (Fe,Cr)O.sub.4, glass X.sub.1, a commercial silicone resin and solvents was prepared. The total weight of the solids used was 52.5 g, with the concentration of the glass in the solids being 50 wt % (i.e., 26.25 g glass X.sub.1 in 52.5 g solids). The concentration of the solids in the paint formulation was 42.82 wt %. The weight of commercial silicone resin solution used to prepare the formulation was 70.10 g (57.18% of the paint formulation). The so-formed formulation is used to illustrate the composition of the modified glass, taking into account that the commercial silicone resin solution contains 50 wt % resin and the resin has 53% conversion to SiO.sub.2 as described in Example 12, i.e., the 70.1 g commercial silicone resin solution will convert to 18.6 g of silica powder (70.1 g×0.5×0.53=18.6 g) after one hour at 600-700° C. The indigenously-formed SiO.sub.2 (18.6 g) combines with the softened glass in the temperature range of 600-700° C. The dissolution of the indigenously-formed SiO.sub.2 into glass X.sub.1 will be a function of time in the temperature range of 600-700° C. The compositions of the original glass component (Glass X.sub.1) of the formulation and the modified glass are set out in Table 6.

TABLE-US-00007 TABLE 6 Glass component of heat- Glass X.sub.1 treated mixture of Batch Glass X.sub.1 siloxane and glass X.sub.1 Ingredients (g) wt % mol % wt % mol % SiO.sub.2 86.52 31.33 38.71 59.82 68.41 TiO.sub.2 47.94 17.36 16.13 10.16 8.73 K.sub.2O 45.22 16.38 12.90 9.58 6.98 Na.sub.2O 39.68 14.37 17.21 8.41 9.31 Li.sub.2O 5.98 2.17 5.38 1.27 2.92 B.sub.2O.sub.3 11.11 4.02 4.29 2.35 2.32 Bi.sub.2O.sub.3 18.64 6.75 1.08 3.95 0.33 ZrO.sub.2 4.93 1.78 1.08 1.04 0.30 Al.sub.2O3 4.08 1.48 1.08 0.87 0.59 SnO.sub.2 12.05 4.36 2.14 2.55 1.16

[0064] As shown in Table 6, due to the incorporation of the indigenously-formed SiO.sub.2 into the glass component, rich-SiO.sub.2 glass is generated, with SiO.sub.2 content above 65 mole %.