WO2016027269A1

WO2016027269A1 - Coating compositions for solar applications

Info

Publication number: WO2016027269A1
Application number: PCT/IL2015/050832
Authority: WO
Inventors: Jacob Hormadaly; Noelia Martinez Sanz; Azucena Bello Fernandez
Original assignee: B.G. Negev Technologies And Applications Ltd., At Ben-Gurion University; Abengoa Solar New Technologies, S.A.
Priority date: 2014-08-18
Filing date: 2015-08-17
Publication date: 2016-02-25

Abstract

A paint composition for producing a solar absorber coating, comprising a silicone resin formulated with: (i) black spinel; (ii) a glass powder; and (iii) at least one auxiliary paint additive.

Description

COATING COMPOSI TIONS FOR SOLAR APPLICATIONS

An ideal solar absorber is a coating with very high absorptance in the solar portion of the spectrum (UV, VIS and near IR) , low emissivity at the working temperature, high chemical durability, good mechanical stability and low cost. Most of the materials in use today have some drawbacks such as thermal degradation at high temperature, instability on exposure to UV radiation, oxidation, and some materials do not protect the steel pipes used in the solar collector from corrosion.

For solar tower applications and other applications which are based on absorption of the solar energy and its conversion to heat, there is a need for high temperature black material as solar absorber which is compatible with steel such as carbon steel, stainless steel and Inconel™. The solar absorber should also have high absorption in the solar portion of the spectrum (UV, VIS and near IR) , exhibit stability at high temperatures (e.g. 400-700°C), withstand thermal cycling between room temperature to temperatures in the range of 400-700°C and be easily applied onto metal substrates.

In WO 2012/127468, we reported the preparation of black coatings for solar applications, based on black ruthenium compounds and a glass powder, formulated in an organic vehicle, in the presence of additional metal oxide (s) . The resultant formulation with paste consistency was applied onto Inconel™ substrates and on curing/ firing, coatings were formed displaying very high solar absorptivity.

It has now been found that silicone resins can be formulated with a mixture of solids comprising a glass powder and at least one compound selected from the group consisting of black ruthenium oxides and black spinel, in the presence of one or more paint additives (e.g., organic solvents, surfactant ( s ) , surface-wetting agents) to form heat-curable formulations with paint consistency. Application of the paint formulation onto a stainless steel substrate followed by heat-curing, e.g., at temperature not higher than 400°C, results in the cross-linking of the silicone, to give a black coating with very strong adhesion to the metal substrate. It has been also observed that on exposure to temperature higher than the curing temperature, the silicone resin transforms into a white powder which is amorphous Si02. In fact, the transformation to S1O2 probably starts during the curing stage but the transformation rate is slow; fast and almost complete transformation to S1O2 occurs when a cured paint formulation is subjected to 1 hour at 650°C in air atmosphere. X-ray diffraction (XRD) study reported below suggests that the amorphous S1O2 which originated from the silicone resin interacts with the glass component of the coating, to produce a modified glass. Without wishing to be bound by theory, it is assumed that after the curing, the cross-linked silicone resin accounts for the good adhesion of the coating. On exposure to high temperatures, the silicone resin transforms to S1O2, which in turn combines with the softened glass component to form a Si02-rich glass which acts as an inorganic "glue" of the coating. Hereinafter, the term "indigenously-formed S1O2" is sometimes used to describe the S1O2 which comes from the silicone resin on exposing same to high temperatures.

Accordingly, one aspect of the invention relates to a composition for producing a solar absorber coating, comprising a silicone resin formulated with:

(i) black spinel, and optionally at least one compound selected from the group consisting of black ruthenium oxides;

(ii) a glass powder; and

(iii) one or more auxiliary paint additives. Silicone resins suitable for use in the invention are silicone resins for high temperature paints, with aryl (e.g., phenyl) groups attached to the silicone backbone, namely, poly ( alkylarylsiloxane ) , e.g., poly (methylphenylsiloxane ) or poly (diarylsiloxane) , for example, poly (diphenylsiloxane ) . The siloxane may be functionalized with chemical groups, for example, hydroxyl . Workable silicone resins are commercially available as solutions of polysiloxane in solvents or as neat resins, from various manufacturers such as Dow Corning and Wacker Chemie AG (the Silres® series) . The experimental work reported below indicates that a commercially available solution of poly (methylphenylsiloxane ) resin in organic solvent (s) comprising aromatic hydrocarbon, e.g., xylene, where the concentration of the resin in the solution is not less than 35 wt%, e.g., not less than 45 wt%, is especially suitable for use in the invention. Such a silicone resin solution is available from Wacker Chemie AG as Silres® 50 or Silres® 60.

The term 'black ruthenium oxide' is used herein to indicate all oxide compounds of ruthenium, which are black. The term is not limited to ruthenium dioxide alone; it encompasses also ruthenium-containing mixed metals oxides. Thus, black ruthenium oxide for use in the invention is selected from the group consisting of:

ruthenium dioxide R.UO2;

mixed oxides, including a ruthenium-containing spinel, for example, C02RUO4 , Co2+xRui-_XC>4 with x= 0 to 0.5; Co2+x-_yM_yRui-_x04 with M= transition metals such as Cu, Zn, Fe, Mn, Cr and y= 0 to 0.75; a ruthenium-containing pervoskite, such as M'Ru03 where M'= Ca, Sr, Ba;

mixed oxides such as S^RUC ;

alkali ruthenates; pyrochlores and substituted pyrochlores of Ru, such as the compounds described in US 3, 583, 931 and US 3,681,262 and those reported by Longo et al . [Materials Research Bulletin 4, p. 191 (1969)]; Lead free pyroclores such aS Ndl. 75CU0 . 25RU2O6+5 ;

any precursor compound of ruthenium, which converts to an oxidized form of ruthenium at high temperatures, for example, the chloride of ruthenium.

A black spinel pigment suitable for use in the invention is a mixed oxide of the general formula AB2O4. In an ideal normal spinel the A site of the tetrahedral site is usually occupied by a divalent cation and the B site of the octahedral site is occupied by a trivalent cation. However, metals with higher and lower oxidations states such as A¹⁺, B⁴⁺, B⁵⁺ can also be accommodated in the spinel structure. Black transition metal spinels are mixed oxides of the general formula AB2O4 wherein the divalent and trivalent cations are preferably selected from the group consisting of Cu²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Fe³⁺, Cr³⁺ and Mn³⁺. It should be understood that the A site may be occupied by more than one type of a divalent metal and trivalent metal, and likewise, the B site may be occupied by more than one type of a trivalent metal and divalent metal. Preferred black inorganic pigments for the purpose of this invention are based on cobalt- iron-chromium mixed oxide with spinel structure, i.e., (Co,Fe) (Fe,Cr)04. In the experimental work illustrated below, a commercially available pigment, Ferro F6333 having the chemical formula (Co,Fe) (Fe,Cr)04 (chemically named cobalt iron chromite black spinel) has been found suitable for coloring the silicone resins-based formulations. Black pigments based on manganese- iron, copper-chromium or copper-manganese-iron have also demonstrated good coloring ability to achieve black coatings based on silicone resins. Such pigments are commercially available, typically with an average particle size in the range from a few tenths of a micron to a few microns, density from 3 to 6 g/cm³ and specific surface area from 1 m²/g to 50 m²/g. It should be noted that some commercial black spinel pigments (for example, Ferro F6333) also contain quartz which stabilizes the spinel. At high temperatures >600°C the quartz dissolves in the glass component of the composition of the invention.

Glass compositions suitable for use in the invention contain silicon dioxide, titanium dioxide and a mixture of alkali metal oxides as main components, with mole percentage in the ranges from 25.0-50.0%, 5.5-35.0% and 19.0-45.0%, respectively. Additional glass components are selected from the group consisting of boric oxide B2O3 , bismuth oxide B12O3 , aluminum oxide AI2O3 , tin dioxide Sn02, zirconium dioxide Zr02 and niobium oxide Nb205. Workable compositions are identified in Table 1 below (in terms of the glass ingredients and their respective molar concentrations) . It should be noted that the glass compositions set out in Table 1 refer to glasses useful as starting materials for preparing the coating paint formulations of the invention. On exposing the coating to temperatures higher than the curing temperature, the composition of the glasses undergoes a significant change due to the incorporation of the indigenously-formed S 1O2 into the glass.

Table 1 (glass useful as starting materials)

Ingredients Range in mole % Preferred range in mole %

Si0₂ 25.0-50.0 30-45

Ti0₂ 10.0-35.0 15-20

M₂0 (M=Na, K, Li or 20.0-45.0 35-45

their mixture)

B2O3 0-10.0 3-7 (e.g. 4-6)

Bi₂0₃ 0-15.0 0-2

Zr0₂ 0-5.0 0-2

AI2O3 0-7.0 0.5-4 (e.g., 1-4)

Sn0₂ 0-15.0 0.5-4 (e.g., 1-4)

Nb₂0₅ 0-7.0 0-3

CoO 0-5.0 0-5

NiO 0-5.0 0-5

MnO 0-5.0 0-5

CuO 0-5.0 0-5

FeO 0-5.0 0-5 An especially preferred glass starting material to be formulated with the silicone resin solution comprises 30 to 40 mole % Si0₂ (e.g., 35-40%); 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; and one or more of the oxides: 0.5 to 4 mole % A1₂0₃; 0 to 2 mole % Bi₂0₃ and 0 to 2 mole % Zr0₂.

It should be noted that part of the alkali oxides indicated in Table 1 may be substituted by the corresponding fluorides, for example, NaF may be used instead of Na₂0, LiF may be a substitute for Li₂0 and KF may be a substitute for K₂0.

Glass powders with the compositions set forth in Table 1 are prepared by weighing the individual metal compound powders according to the recipe (sometimes precursors of the oxide may be used, e.g. NaHC03, Na₂C03 and NaNC>3 can be used instead of Na₂0) and thoroughly mixing same to give a uniform blend, following which the blend is melted in a crucible (e.g., platinum crucible) in a furnace. The melt is maintained at a peak temperature of 1100°C-1400°C for at least one hour, followed by melt quenching in water, collecting the crude frit and milling same, as described in more detail below.

In addition to the silicone resin and the inorganic components set forth above, the coating composition of the invention further comprises one or more organic constituent ( s ) which are capable of modifying the properties of the paint and/or increase substrate wetting, reduce surface tension and improve other surface properties. These organic constituents used as auxiliary paint additives according to the invention may be divided into two groups: auxiliary solvents and auxiliary surface additives. It should be noted that commercial silicone resins are available in the form of solutions of polysiloxane in solvents; the term 'auxiliary' is used to indicate additives which are further incorporated into such commercial solutions.

Regarding the auxiliary solvents, it is preferred to add to the composition of the invention aromatic solvents such as alkyl- substituted benzene, e. g., toluene or xylene (either m-, o, p- xylene) . Alkyl-substituted benzene wherein the alkyl group is substituted with hydroxyl group, for example, benzyl alcohol, is also useful. These solvents have shown adequate miscibility with commercially available silicone resin solutions, such as silicone resin solutions in xylene (e.g., Silres® 60) . The added solvents serve two useful purposes: first, dilution of the silicone-containing paint formulation and second, control of the evaporation rate of the paint formulation upon drying. For example, Silres® 60 exhibits high evaporation rate which needs to be slowed down.

Regarding the auxiliary surface additives (surfactants), different types of commercially available paint additives may be used. One type of additives consists of silicone-based surface additives for increasing substrate wetting, e.g., modified polydimethylsiloxane, such as polyether modified polydimethylsiloxane recommended for solvent-borne systems [available from BYK (Atlanta Group) under the BYK®-300 series, e.g., BYK-330, BYK-331, BYK-332, BYK-333, BYK-336 or BYK-337]. Another type of auxiliary surface additives are surface-wetting additives, which act also as dispersants, consists of acid addition salts (e.g., polycarboxylic acid salts) of polyamine amides or alkylammonium salts of a polycarboxylic acid. Such salts are commercially available in the form of a solution in a mixture of solvents consisting alkylbenzene and polar solvents such as methoxypropanol ( (available from BYK (Atlanta Group) under the ANTI-TERRA®-200 series, e.g., ANTI-TERRA-203 , ANTI- TERRA-204 , ANTI-TERRA-205 or ANTI-TERRA-206 ) . The coating compositions of the invention are prepared by combining together the liquid silicone resin and the solids consisting of the glass constituent and the black pigment (s), i.e., either black ruthenium oxide, black spinel or a combination thereof, and also the auxiliary paint additives, to form a uniform formulation with paint consistency, such that it can be applied onto a substrate by conventional painting techniques such as brushing, dipping and spraying.

The weight concentration of the liquid silicone resin in the coating composition of the invention is adjusted to produce the desired consistency. The surface area of the solids incorporated in the composition affects the liquid to solid weight ratio. In general, the higher the surface area of the particles, the larger the amount of silicone resin used. Preferably, the weight ratio between the liquid and solid components of the formulation is in the range from 1:4 to 3:1, more preferably from 2:1 to 1:2. The presence of surfactants in the formulation may also affect the liquid/solid ratio. In some embodiments, the concentration of the silicone resin in the paint composition may be from 20 to 30% w/w, e.g., from 25 to 30% w/w.

The concentration of the ruthenium compound (s) in the solids can range from 0 to 100 wt%, preferably from 0.5 to 50 wt%, more preferably from 2 to 40 wt%, and even more preferably from 5 to 20 wt%. The concentration of the spinel pigment is from 0 to 100 wt%, preferably from 5 to 80 wt%, more preferably from 10 to 70 wt%. When the spinel pigment and the black ruthenium oxide are both present, then the weight ratio between the spinel pigment and the black ruthenium oxide is, for example, from 100:1 to 1:100. Preferably, the predominant pigment is the spinel, and the weight ratio between the spinel and the ruthenium oxide is from 20:1 to 1:1, e.g., 15:1 to 1:1. The glass powder concentration in the solids may range from 0 to 50 wt%; for formulations for high temperatures the desired range is from 5 to 50 wt% of the solids. Especially preferred are paint formulations comprising solid mixtures consisting essentially of R.UO2 from 7.5 to 12.5 wt% (e.g., around 10 wt%); glass powder from 30 to 40 wt% (e.g., around 35 wt%) and black spinel from 50 to 60 wt% (e.g., around 55 wt%) . The foregoing concentrations are relative to the total weight of the solids.

The concentration of the siloxane in the paint composition is preferably from 20 to 30% w/w, more preferably from 25 to 30% w/w. The concentration of the black spinel in the paint composition is preferably from 12 to 40% w/w, more preferably from 20 to 40% w/w, e.g., from 20 to 30% w/w and specifically from 25 to 30% w/w. The concentration of ruthenium oxide (e.g., R.UO2) in the paint composition is preferably from 1 to 20% w/w, more preferably from 3 to 15% w/w, e.g., from 3 to 6% w/w. The concentration of the glass powder in the paint composition is preferably from 5 to 25% w/w, more preferably from 10 to 15% w/w. The concentration of the additives is from 25 to 35% w/w, e.g., from 10 to 20% w/w.

The silicone-based paint of the invention can be used for coating iron, steel substrates and special alloys for absorption of solar energy, especially solar applications such as generation of electricity in solar towers and Stirling engines and solar heaters for domestic uses. The coating is dried and then heat-cured at a temperature of not more than 400 °C for several hours. Typical preparation of coated stainless steel substrate is as follows: the paint is applied onto stainless steel substrate using a brush and the fresh paint is left for about 15 minutes in the hood to level. The coated substrate is then dried by placing same in an oven at Ti, wherein 100<Ti<150°C, for example, Ti=120°C, for a period of time ti which is not less than 10 minutes, e.g., 20 minutes, to evaporate the volatiles (solvents) . The dried coated substrate is transferred to an electric furnace where it undergoes heat- curing. To this end, the coated substrate is kept in at least two different temperatures in the range from 200 to 400 °C for a period of time of not less than 30 minutes at each of said temperatures. More specifically, the coated substrate is gradually brought to a temperature T2, wherein 150≤T2≤300°C, for example T2=250°C, and kept at T2 for a period of time t2 of not less than 45 minutes, for example 1 hour. Then the temperature of the furnace is increased to T3, wherein 200≤T3≤400°C; preferably T3=375-400°C, and the coated substrate is kept at T3 for a period of time t3 of not less than 45 minutes, e.g. 1 hour. Temperature variation Ti → T2 → T3 is carried out at constant rate of not more than 30 deg/min, for example, about 20 deg/min.

The coated substrate is slowly cooled to room temperature. In the curing stage traces of the solvents evaporate and the silicone resin undergoes cross-linking forming a polymer which bonds to the metallic substrate and to the inorganic phases. During the curing stage some of the organic constituents probably oxidize to water and carbon dioxide.

Accordingly, another aspect of the invention is a method comprising applying the paint formulation set out above onto a metal substrate in a solar collector, drying the paint and heat-curing the dried coating (including on-site heat-curing), and optionally firing same. The substrate to be coated is preferably steel, such as carbon steel, stainless steel, Inconel™ (nickel-chromium based alloys) and other alloys which are useful at high temperatures. The experimental results reported below indicate that a coating composition comprising a silicone resin solution formulated either with black ruthenium compound(s), black spinel or their combination, and a glass comprising 35 to 45 mole % S1O2, 15 to 20 mole % T1O2; 8 to 15 mole % K₂0; 10 to 22 mole % Na₂0; 3 to 11 mole % Li₂0; 3 to 7 mole % B2O3; 1 to 3 mole % Sn02; and one or more of the following oxides: 0.5 to 2 mole % AI2O3; 0.5 to 2 mole % B12O3 and 0.5 to 2 mole % r02;

forms on curing a solar absorber coating with good color properties and high adhesive strength.

More specifically, we have found that a coating composition comprising a silicone [i.e., poly (methylphenylsiloxane) ] resin formulated with:

(i) a mixture of a black spinel and ruthenium dioxide at a weight ratio in the range from 7:1 to 3:1, preferably from 6:1 to 4:1;

(ii) glass comprising 35 to 45 mole % S1O2 (e.g., 35 to 39 mole %, or 35 to-40 mole % S1O2) , 15 to 20 mole % T1O2; 8 to 15 mole % K₂0; 10 to 22 mole % Na₂0; 3 to 11 mole % Li₂0; 3 to 7 mole % B2O3; 1 to 3 mole % Sn02," and one or more of the following oxides: 0.5 to 2 mole % AI2O3; 0.5 to 2 mole % B12O3 and 0.5 to 2 mole % r02," and

(iii) one or more auxiliary paint additives;

can be easily applied onto a metal substrate, to form upon curing a solar absorber coating with good color properties and high adhesive strength. The weight concentration of the silicon resin in the paint is from 20 to 30 %; the weight concentration of (i) the mixture of black spinel and ruthenium dioxide in the paint is from 25 to 30%; the weight concentration of (ii) the glass in the paint is from 20 to 25% and the weight concentration of (iii) the paint additives (solvents and others) in the paint is from 10 to 20%. The experimental work conducted in support of this invention also shows that silicone resins can be formulated with black spinel and a glass powder in the absence of ruthenium oxide to give Ru-free coatings suitable for solar collectors. Another aspect of the invention is therefore ruthenium-free coating composition comprising a silicone [i.e., poly (methylphenylsiloxane ) ] resin formulated with:

(i) at least one black spinel;

(ii) glass comprising 35 to 40 mole % S 1O2 , 15 to 20 mole % T 1O2 ; 8 to 15 mole % K₂0; 10 to 22 mole % Na₂0; 3 to 11 mole % L12O ; 3 to 7 mole % B2O3; 1 to 3 mole % Sn02 ; and one or more of the following oxides: 0.5 to 2 mole % AI2O3; 0.5 to 2 mole % B12O3 and 0.5 to 2 mole % r02 ; and

(iii) one or more auxiliary paint additives.

In the ruthenium-free compositions of the invention, the weight concentration of the silicon resin in the paint is from 20 to 30 %; the weight concentration of (i) the black spinel in the paint is from 12 to 35%; the weight concentration of (ii) the glass in the paint is from 16 to 25% and the weight concentration of (iii) the paint additives in the paint (solvents and others) is from 25 to 35%.

Upon exposing the cured paints to temperatures higher than 400°C, e.g. in the 500-700°C range, a ceramic coating is formed, displaying very good optical properties, e.g., high absorptivity. It should be noted that the silicone resin in the cured paint transforms into indigenously-formed S 1O2 which combines with the softened original glass to produce S i02-rich modified glass, i.e., with higher S 1O2 content than the starting glass composition, e.g., of not less than 60 mole%, preferably above 65 mole %, e.g., from 65 to 75 mole %. Some preferred compositions of this rich- S i02 glass component of the ceramic coating are set out in Table 2. Table 2 (composition of the rich-Si02 glass component of the high temperature (500-700°C) ceramic coatings)

EXAMPLE S

Materials

Some of the materials used for preparing the compositions illustrated in the experimental work reported below are set out in Table 2:

Table 2

TRADE NAME GENERAL DESCRIPTION FUNCTION

(Manufacturer)

Silicone resins

Silres 60 60 wt% of methyl phenyl Silicone

(Wacker Chemie AG) siloxane resin dissolved in a vehicle

solvent mixture consisting of

xylene and butanol

Black pigments

F6333 Black (Co, Fe) (Fe, Cr) 0₄ spinel Black pigment ( Ferro )

24-3060 Black spinel, Mn-Fe spinel,

( Ferro ) SA 32.7 (m²/g) Black pigment

24-3061 Black spinel Cu-Mn-Fe spinel,

Black pigment ( Ferro ) SA 5.2 (m²/g)

24-3095 Black spinel, Cu-Cr spinel,

( Ferro ) SA 2.7 (m²/g) Black pigment

Ruthenium compounds

Ru0₂ Ruthenium dioxide black pigment

Paint additives

BYK-333 Polyether modified Surface

(BYK, Atlanta Group) polydimethylsiloxane additive

Anti Terra 204 Solution of polycarboxylic acid Wetting and (BYK, Atlanta Group) salt of polyamine amides in dispersing methoxypropanol/alkylbenzene additive 3 : 2 mixture

Metal substrates

304 Stainless steel

316 Stainless steel

347 Stainless steel

Hayness 230

Hayness International,

Inc Methods

In the adhesion strength test, the adhesion of the cured paint was tested by applying an adhesive tape on the cured paint.

Optical properties (absorptance in the solar range from 300 nm to 2500 nm & emittance at room temperature and calculated emittance at 700°C) were measured in the University of Zaragoza. The absorptance is calculated measuring the near UV, visible and IR till 2500 nm with a spectrophotometer equipped with:

Monochromator CM110 and optical filters;

Xenon and halogen lamps;

Si and AsGalnSb detectors;

Lock-in detection system;

Quasi-normal incidence (8°), with collimated beam for

transmission and reflection;

Integrating sphere of 150 mm of internal diameter;

Sample ports of 25 mm diameter;

The standard values are in accordance with by the National Physics Laboratory at the United Kingdom.

The emittance was calculated from the IR measurements done in a system consisting of:

Spectrum 100 FT-IR spectrometer of Perkin Elmer;

Gold-coated integrating sphere Mid-IR IntegratIR from Pike;

inches sphere, 8- degree hemispherical reflectance measurement;

MCT detector;

Sample port of 23.5 mm;

Measuring range: 1.5-28 μ Preparation 1

Glass Powder Compositions

Glass compositions (coded herein Glass Xi, X2 and X3) were prepared in a platinum crucible at 1100°C-1400 °C . The compositions of the glasses are set out in Table 3 below in terms of mole% of each ingredient present in the glass.

Table 3

To prepare the glasses, the metal oxide 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 1.5-3.0 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 2

Silicone-containing additive

A solution of 10 wt% ethyl cellulose in a mixture of terpineol and dibutylcarbitol (1:1 weight ratio) was prepared. The solution (2g) was combined with terpinol (0.5g), dibutylcarbitol (0.5g) and Silres® 60 (3g) . The components were mixed together in a container to form an additive with 30 wt% silicone content.

EXAMPLES 1-2

Preparation and properties of a cured coating obtained from a silicone resin formulated with pigments and a glass

Formulations were prepared by grinding in an agate mortar (or on a Muller) solids consisting of the glass powder Xi of Preparation 1, ruthenium dioxide R.UO2 and the black spinel (Co, Fe) (Fe, Cr) 0₄, F6333), with a silicone resin Silres® 60 to form homogeneous mixtures. The so-formed formulations were brushed on stainless steel 304 substrates, each with a uniform coating applied onto 5x5 cm² surface area of the metal substrate. All samples were then heat-cured. The heat-curing process consists of drying the substrates in an oven at 150°C for 30 minutes, and then placing the dried substrate in a furnace and exposing same to the following temperature profile :

heating to 250°C at a constant heating rate of 20°C/min; keeping the substrate in 250°C for one hour;

heating to 375°C at a constant heating rate of 20°C/min; keeping the substrate in 375°C for one hour.

The cured paints were then examined to determine their optical and adhesion properties. The composition of each of the paint formulations prepared and the properties of the paints obtained on applying and curing the formulations are tabulated below .

Table 4

EXAMPLES 3-5

Formulations were prepared by grinding in an agate mortar solids consisting of the glass powder Xi of Preparation 1, ruthenium dioxide R.UO2 and the black spinel (Co,Fe) (Fe,Cr)04, F6333), with a silicone resin Silres® 60 to form homogeneous mixtures. The mixture was left in a hood to dry. On removal of the volatiles, the mixture was weighed; (6.9 g were recovered, slightly above the 6.8 calculated weight of solids) . To the almost dried mixture was added the silicon-containing additive of Preparation 2. The constituents were mixed with a spatula to give a paintable formulation. The so-formed formulation was brushed on three different types of stainless steel substrates: 304, 316 and 347. All samples were then heat- cured, using the curing protocol described in Examples 1-2. The cured samples were the subjected to a color test and adhesion strength test.

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. The adhesion strength test was described above .

Table 5

EXAMPLES 6-11

Formulations were prepared by grinding in an agate mortar (or on a Muller) solids consisting of the glass powder Xi of Preparation 1, ruthenium dioxide R.UO2 and the black spinel (Co,Fe) (Fe,Cr)04, F6333), with a silicone resin Silres® 60 and the silicone-additive of Preparation 2, according to the recipes set out in Table 6 below, to form homogeneous mixtures .

The so-formed formulations were applied onto different types of stainless steel (304, 316 and 347 steel), cured and tested to determine their adhesion strength and their absorptance . The compositions and the measured properties are tabulated in Table 6. Table 6

* on 304, 316 and 347 steel.

** on 304 steel.

*** on 347 steel.

EXAMPLES 12-16

Formulations were prepared by grinding in an agate mortar (or on a Muller) solids consisting of the glass powder Xi of Preparation 1, a black spinel ((Co,Fe) (Fe,Cr)04, F6333) and optionally also ruthenium dioxide R.UO2, with a silicone resin Silres® 60 and the silicone-additive of Preparation 2, and also in the presence of surfactants, according to the recipes set out in Table 7, to form homogeneous mixtures.

The so-formed formulations were applied onto various metal alloys, cured and tested to assess their optical properties and adhesion strength as set forth in Examples 1-2. The compositions and their properties are tabulated in Table 7. Table 7

Example Example Example Example Example 12 13 14 15 16

Composition

Silicone vehicle 3. OOg 3. OOg 3. OOg 3. OOg 3. OOg Silres® 60

Glass XI of 1.75g 2. OOg 1.75g 1.75g 1.75g Preparation 1

Black pigment 2.75g 3. OOg 2.25g 3. OOg 2.50g

Ru0₂ 0.5g 1. OOg 0.25g 0.75g

Silicon-containing 4. OOg 4. OOg 4. OOg 4. OOg 4. OOg additive

Surfactant BYK-333 2 drops 2 drops 2 drops 2 drops 2 drops

Surfactant Anti 2 drops 2 drops 2 drops 2 drops 2 drops terra 204

Properties (application onto 347 stainless steel)

adhesion test after Very Very Very Very Very curing at 375°C good good good good good

Absorptance after 94.7 95.4

curing at 375°C (%)

Calculated emittance 96.8 96.6

at 700°C after

curing at 375°C (%)

Properties (application onto 304 stainless steel)

Absorptance after 95 ND ND 93.3 94.2 curing at 375°C (%)

Calculated emittance 95.3 ND ND 94.5 94.9 at 700°C after

curing at 375°C (%)

Properties (application onto 316 stainless steel)

Absorptance after 95.96 90.98 96.07 ND ND curing at 375°C and

ten cycles at 650°C

(%)

Calculated emittance 95.48 80.20 95.04 ND ND at 700°C after

curing at 375°C and

ten cycles at 650°C

(%)

Properties (application onto Haynes 230)

Absorptance after 96.02 90.86 96.19 ND ND curing at 375°C and

ten cycles at 650°C

(%)

Calculated emittance 95.72 76.59 94.91 ND ND at 700°C after

curing at 375°C and

ten cycles at 650°C

(%) EXAMPLES 17-20

Formulations were prepared by grinding in an agate mortar (or on a Muller) solids consisting of the glass powder Xi of Preparation 1, a black spinel ((Co,Fe) (Fe,Cr)04, F6333) and ruthenium dioxide R.UO2, with a silicone resin Silres® 60 and the silicone-additive of Preparation 2 according to the recipes set out in Table 8, to form homogeneous mixtures. The weight concentration of the glass constituent was kept constant (35 wt% of the total solid content of the formulation) , whereas the weight ratio between the pigment and R.UO2 was varied across the range from 12:1 to ~ 2:1. The formulations were applied on stainless steel 304 substrate and heat-cured as described in Example 1-2. The adhesion strength and the optical properties of the cured paints were determined. The compositions of the formulations and the properties of the cured paints are tabulated in Table 8.

Table 8

Example 17 Example 18 Example 19 Example 20

Composition

Silicone vehicle 3. Og 3. Og 3. Og 3. Og

Silres® 60

Glass XI of 1.75g 1.75g 1.75g 1.75g

Preparation 1

Black pigment 3. OOg 2.75g 2.50g 2.25g

F6333

Ru0₂ 0.25g 0.50g 0.75g 1. OOg

Silicon-containing 3. Og 3. Og 3. Og 3. Og

additive

Properties

Adhesion test after Very good Very good Very good Very good curing at 375°C

Absorptance after 93.35 94.26 94.88 95.08

curing at 375°C (%)

Emittance at 700°C 93.83 95.00 95.13 95.25

after curing (%) EXAMPLES 21-24

Ru02~free formulations were prepared by grinding in an agate mortar (or on a Muller) solids consisting of the glass powder Xi of Preparation 1 and a black spinel, with a silicone resin Silres® 60 and the silicone-additive of Preparation 2, and optionally additional solvents, according to the recipes set out in Table 9, to form homogeneous mixtures. Different types of black spinels were tested. The formulations were applied onto stainless steel 304 substrate and heat-cured as described in Example 1-2. The adhesion strength and the optical properties of the cured paints were determined. The compositions of the formulations and the properties of the cured paints are tabulated in Table 9.

Table 9

Example 21 Example 22 Example 23 Example 24

Composition

Silicone vehicle 3. Og 3. Og 3. Og 3. Og

Silres® 60

Glass XI of 2. OOg 2. OOg 2. OOg 2. OOg

Preparation 1

Black pigment 3. OOg

F6333

Black pigment 3. OOg

24-3060

Black pigment 3. OOg

24-3061

Black pigment 3. OOg

24-3095

Silicon-containing 3. Og 3. Og 3. Og 3. Og

additive

solvents Xylene,

0.5g

Properties

Adhesion test after Very good Very good Very good Very good curing at 375°C

Absorptance after 87.51 97.24 95.51 95.39

curing at 375°C (%)

Emittance at 700°C 82.21 89.46 91.26 93.91

after curing (%) EXAMPLES 25-27

Formulations were prepared by grinding in an agate mortar solids consisting of the glass powder Xi of Preparation 1, a black spinel and ruthenium dioxide, with a silicone resin Silres® 60 and the silicone-additive of Preparation 2, and optionally additional solvents, according to the recipes set out in Table 10, to form homogeneous mixtures. Different types of black spinel were tested, this time in combination with ruthenium dioxide. The formulations were applied onto stainless steel 304 substrate and heat-cured as described in Example 1-2. The adhesion strength and the optical properties of the cured paints were determined. The compositions of the formulations and the properties of the cured paints are tabulated in Table 10.

Table 10

Example 25 Example 26 Example 27

Composition

Silicone vehicle Silres® 3. Og 3. Og 3. Og

60

Glass XI of Preparation 1 1.75g 1.75g 1.75g

Ru0₂ 0.25g 0.25g 0.25g

Black pigment 3. OOg

24-3060

Black pigment 3. OOg

24-3061

Black pigment 3. OOg

24-3095

Silicon-containing 3. Og 3. Og 3. Og

additive

Properties

Adhesion test after Very good Very good Very good curing at 375°C

Absorptance after curing 97.46 96.33 96.47

at 375°C (%)

Emittance at 700°C after 94.37 93.88 95.41

curing (%) EXAMPLES 28-31

Ru02-free Formulations were prepared by grinding in an agate mortar solids consisting of the glass powder X i of Preparation 1 and a black spinel, or a combination of two black spinel compounds, with a silicone resin Silres® 60 and the silicone- additive of Preparation 2, and optionally additional solvents, according to the recipes set out in Table 11, to form homogeneous mixtures. Different types of black spinel were tested. The formulations were applied onto stainless steel 304 substrate and heat-cured as described in Example 1-2. The adhesion strength and the optical properties of the cured paints were determined. The compositions of the formulations and the properties of the cured paints are tabulated in Table 11.

Table 11

Example 28 Example 29 Example 30 Example 31

Composition

Silicone vehicle 3. Og 3. Og 3. Og 3. Og

Silres® 60

Glass XI of 2.50g 2. OOg 2. OOg 2. OOg

Preparation 1

Black pigment 1.50g 2. OOg 1. OOg 1. OOg

24-3060

Black pigment 1.50g 2. OOg

24-3061

Silicon-containing 3. OOg 3. OOg 3. OOg 3. OOg

additive

solvents Xylene

0.5g

Properties

Adhesion test after Very good Very good Very good Very good curing at 375°C

Absorptance after 97.25 97.19 96.78 96.65

curing at 375°C (%)

Emittance at 700°C 91.46 91.77 90.91 90.08

after curing (%) EXAMPLES 32-34

Preparation and properties of a cured coating obtained from a silicone resin formulated with pigment (s) and glass

Formulations were prepared by grinding in an agate mortar solids consisting of the glass powder Xi of Preparation 1, a black spinel and ruthenium dioxide, with a silicone resin Silres® 60 and the silicone-additive of Preparation 2, and optionally additional solvents, according to the recipes set out in Table 12, to form homogeneous mixtures. This time the black spinel tested was Mn-Fe spinel, optionally in combination with ruthenium dioxide. The formulations were applied onto stainless steel 304 substrate and heat-cured as described in Example 1-2. The adhesion strength and the optical properties of the cured paints were determined. The compositions of the formulations and the properties of the cured paints are tabulated in Table 12.

Table 12

Example 32 Example 33 Example 34

Composition

Silicone vehicle Silres® 60 3. Og 3. Og 3. Og

Glass XI of Preparation 1 2. OOg 1.75g 2.50g

Ru0₂ 0.25g 0.25g

Black pigment 3. OOg 3. OOg 1.50g

24-3060

Silicon-containing additive 4. OOg 4. OOg 4. OOg

Surfactant BYK-333 2 drops 2 drops 2 drops

Surfactant Anti Terra 204 2 drops 2 drops 2 drops

Solvents (xylene) 0.5g 0.5g

Properties

Adhesion test after curing Very good Very good Very good at 375°C and one hour hold

at 600 °C

Absorptance after curing at 96.5 96.7 95.8

375°C and and one hour hold

at 600 °C (%)

Calculated emittance at 90.3 94.6 94.9

700°C after curing at 375°C

and one hour hold at 600 °C

(%) EXAMPLE 35

XRD study

A mixture consisting of a siloxane resin solution, Silres® 60 from Wacker Chemie AG (0.5980g) and glass Xi of Preparation 1

(1.0030g) was prepared. X-ray diffraction was used to identify and characterize the formation of a silica solid phase and its interaction with the glass, on subjecting the mixture to drying

(Ti=150 °C) , followed by curing (heating at 20°C/min rate to T₂=250°C → one hour hold at T₂=250°C → heating at 20°C/min rate to T3=375°C → one hour hold at T2=375°C) and further exposure to higher temperature at T3=650°C for one hour.

As reference examples, the siloxane and the glass were separately subjected to the same temperature variation profile set out above. Figure 1 shows the XRD patterns of the fired (650°C) samples: the siloxane alone (A), the glass alone (B) and the siloxane/glass mixture (C) . As expected, the organic groups of the siloxane are burned off to give an amorphous silica, indicated by the broad hump centered at 26-23° in (A) . Glass Xi undergoes transformation from an amorphous phase to crystalliazble glass, seeing that the XRD pattern (B) displays diffraction lines. However, it appears that when the two components (glass and siloxane) are mixed together, the silica which originated from the siloxane and the glass component interact with one another to produce a different amorphous phase consisting of a modified glass form, as suggested by the shifted hump now centered at 26-30° (C) . It should be noted that XRD pattern (C) exhibits diffraction lines at positions 26-27.5, 26-38° and 26-39.5° assigned to Bi<°> and diffraction lines at positions 26-11° and 26-29° assigned to bismuth silicate. Thus, the results suggest that under the application of the heating regimen set out above, the combination of a siloxane and glass Xi results in the formation of a modified glass . EXAMPLES 36-38

In the next set of examples, different silicone resins were formulated with a glass and a black spinel and the formulation was applied onto 347 stainless steel substrate, followed by heating and curing as previously described (up to 375°C) . The coatings were then tested to determine their color and adhesion strength .

Table 13

The results indicate the advantage of poly (methylphenylsiloxane ) , e.g., Silres® 50 and Silres® 60, over silicone resins devoid of phenyl group (e.g., Silres® HK 46) .

Claims

1) A paint composition for producing a solar absorber coating, comprising a silicone resin formulated with:

(i) black spinel;

(ii) a glass powder; and

(iii) at least one auxiliary paint additive.

2) A paint composition according to claim 1, wherein the auxiliary paint additive is selected from the group consisting of auxiliary organic solvents and auxiliary surface additives.

3) A paint composition according to any one of claims 1 and 2, wherein the silicone resin has aryl groups attached to the silicone backbone.

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

5) A paint composition according to claim 4, wherein the silicon resin is poly (methylphenylsiloxane ) resin provided in the form of a solution in an organic solvent or in a mixture of organic solvents.

6) A paint composition according to claim 5, wherein the poly (methylphenylsiloxane ) resin is provided in the form of a solution in one or more organic solvent (s) comprising aromatic hydrocarbon ( s ) , with the poly (methylphenylsiloxane ) concentration in said solution being not less than 45 % w/w.

7) The paint composition of claims 5 or 6, wherein the weight concentration of the silicon resin in the paint is from 20 to 30% w/w and the weight concentration of (i) the black spinel in the paint is from 12 to 40% w/w.

8) The paint composition of claim 7, wherein the weight concentration of the (ii) glass powder in the paint is from 5% w/w to 25% w/w.

9) The paint composition of claim 8, wherein the at least one auxiliary paint additive is selected from the group consisting of auxiliary organic solvents or one or more auxiliary surface additives, wherein the weight concentration of said auxiliary paint additive in the paint is from 25% w/w to 35% w/w.

10) The paint composition of any one of claims 1 to 9, wherein the silicone resin is further formulated with black ruthenium oxide .

11) A paint composition according to claim 10, comprising:

black spinel of the formula AB2O4 wherein the divalent and trivalent cations A and B are selected from the group consisting of Cu²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Fe³⁺, Cr³⁺ and Mn³⁺; and ruthenium dioxide (R.UO2) .

12) A paint composition according to claim 11, wherein the black spinel is selected from the group consisting of:

cobalt-iron-chromium mixed oxide with spinel structure;

manganese-iron mixed oxide with spinel structure;

copper-chromium mixed oxide with spinel structure; and

copper-manganese-iron mixed oxide with spinel structure.

13) A paint composition according to claim 12, wherein the black spinel is cobalt-iron-chromium black spinel. 14) A paint composition according to any one of claims 10 to

13, where the concentration of black spinel in the paint is from 20% w/w to 40% w/w.

15) A paint composition according to any one of claims 10 to

14, where the concentration of black ruthenium oxide in the paint is from 1% w/w to 20% w/w.

16) A paint composition according to claims 10 to 15, where the total concentration of the black ruthenium oxide and black spinel in the paint composition is from 25% w/w to 30% w/w.

17) A paint composition according to claim 10 or 16, where the concentration of the silicone resin in the paint composition is from 20% w/w to 30%w/w, the concentration of black ruthenium oxides is from 3% w/w to 15% w/w and the concentration of black spinel is from 20% w/w to 30% w/w.

18) A paint composition according to claim 17, where the concentration of the silicone resin in the paint composition is from 25%w/w to 30%w/w, the concentration of black ruthenium oxides in the paint is from 3%w/w to 6%w/w and the concentration of black spinel in the paint is from 25%w/w to 30%w/w.

19) A paint composition according to claims 10 to 18, comprising glass powder in a concentration from 5%w/w to 25%w/w.

20) A paint composition according to claim 19, wherein the concentration of the glass powder in the paint is from 10%w/w to 15%w/w. 21) A paint composition according to any one of claims 10 to 20, 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.

22) A paint composition according to claim 21, wherein the glass powder further comprises one or more additional glass components selected from the group consisting of boric oxide B2O3, bismuth oxide B12O3, aluminum oxide AI2O3, tin dioxide Sn02 , zirconium dioxide Z r02 and niobium oxide Nb20s .

23) A paint composition according to claim 22, wherein the glass powder comprises:

30 to 45 mole % S 1O2 ;

15 to 20 mole % T 1O2 ;

35 to 45 mole % M2O wherein M is Na, K, Li or their mixture;

3 to 7 mole % B₂0₃;

0.5 to 4 mole % Sn02 ," and one or more of the following oxides: 0.5 to 4 mole % A1₂0₃; 0 to 2 mole % Bi₂0₃ and 0 to 2 mole % Z r0₂ .

24) A paint composition according to claim 23, wherein the glass powder comprises:

35 to 40 mole % S 1O2 ;

15 to 20 mole % T 1O2 ;

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 % Bi20₃; and

0.5 to 2 mole % Z r0₂ . 25) A paint composition according to any one of claims 10 to 24, comprising at least one auxiliary paint additive which is an organic solvent.

26) A paint composition according to claim 25, wherein the organic solvent is alkyl-substituted benzene, with said alkyl group being optionally substituted.

27) A paint composition according to claim 26, wherein the alkyl-substituted benzene is selected from the group consisting of toluene and xylene.

28) A paint composition according to any one of claims 10 to 27, comprising at least one auxiliary paint additive which is a auxiliary surface additive.

29) A paint composition according to claim 28, wherein the auxiliary surface additive is modified polydimethylsiloxane.

30) A paint composition according to claim 28, wherein the auxiliary surface additive is selected from the group consisting of acid addition salts of polyamine amide and alkylammonium salts of a polycarboxylic acid.

31) A paint composition according to claim 10, comprising poly (methylphenylsiloxane ) resin formulated with:

(i) a mixture of a black spinel and ruthenium dioxide at a weight ratio in the range from 7:1 to 3:1; and

(ii) an additive which is a glass powder comprising 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; and one or more of the following oxides: 0.5 to 2 mole % A1₂0₃; 0.5 to 2 mole % Bi₂0₃ and 0.5 to 2 mole % Zr0₂; and (iii) one or more auxiliary paint additives selected from the group consisting of auxiliary organic solvents and auxiliary surface additives.

32) A paint composition according to claim 31, wherein the weight concentration of the auxiliary paint additive (s) in the paint is from 10 to 20%.

33) A method for preparing a coating, comprising applying the paint composition of any one of claims 1 to 32 onto a substrate in a solar collector, drying, heat-curing and optionally firing said composition.

34) A method according to claim 33, wherein the substrate to be coated is made of carbon steel, stainless steel or Inconel™ (nickel-chromium based alloys) .

35) A method according to claim 33 or 34, comprising heat- curing the coating at the temperature range 25°C-375°C and exposing the cured coating to a temperature in the range from 400°C-700°C.