Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D1/00—Coating compositions, e.g. paints, varnishes or lacquers, based on inorganic substances
  • C09D1/02—Coating compositions, e.g. paints, varnishes or lacquers, based on inorganic substances alkali metal silicates
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D7/00—Features of coating compositions, not provided for in group C09D5/00; Processes for incorporating ingredients in coating compositions
  • C09D7/40—Additives
  • C09D7/60—Additives non-macromolecular
  • C09D7/61—Additives non-macromolecular inorganic
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D5/00—Coating compositions, e.g. paints, varnishes or lacquers, characterised by their physical nature or the effects produced; Filling pastes
  • C09D5/004—Reflecting paints; Signal paints
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D5/00—Coating compositions, e.g. paints, varnishes or lacquers, characterised by their physical nature or the effects produced; Filling pastes
  • C09D5/32—Radiation-absorbing paints
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D7/00—Features of coating compositions, not provided for in group C09D5/00; Processes for incorporating ingredients in coating compositions
  • C09D7/40—Additives
  • C09D7/66—Additives characterised by particle size
  • C09D7/68—Particle size between 100-1000 nm

Definitions

• This invention relates to curable coating compositions and cured coatings suitable for use as thermal control coatings for passive cooling applications.
• examples of such applications include electronics and radiators for terrestrial applications, as well as space borne systems, including spacecraft, satellites and components thereof.
• Spacecraft are subjected to a wide range of thermal environments during their service. For example, in use, one side of a spacecraft may face in a direction away from the sun, while another side faces towards the sun. Thermal control is desirable because heat is radiated into space, which cools the spacecraft, but the spacecraft can simultaneously be heated intensively in direct sunlight. Active and passive temperature control techniques are therefore generally used to maintain the interior temperature of the spacecraft, which generally contains persons or sensitive instruments, within acceptable operating limits. Active temperature control may involve machinery or electrical devices, such as electrical heaters and/or coolers. In contrast, passive temperature controls are techniques that do not involve machinery or electrical devices, but include thermal control coatings or structural designs.
• thermal control coatings typically termed thermal control coatings or thermal control paints
• a thermal control coating may be defined as a surface whose thermo-optical properties may be designed in order to achieve a desired surface temperature when subjected to a known solar flux or other source of radiation.
• a white thermal control paint for example, has a low solar absorbance, while a black paint has a high solar absorbance. Selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature. It is generally recognised that the temperature at the surface of a thermal control coating is dependent on the ratio of the coating's optical absorption to thermal emissivity, which is naturally greatly affected by the material(s) of the coating.
• BOL optical absorbance also known as solar absorbance or solar absorptance
• a s optical absorbance
• a thermal emissivity £N
• a coating applied to the surface of spacecraft to dissipate electrostatic charges (i.e. to be capable of electrostatic dissipation, ESD) that may develop along the external surface of the spacecraft. Otherwise, the electrostatic charges may accumulate and cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft.
• the coating In order to dissipate electrostatic charge, the coating must have at least some electrical conductivity. It is generally accepted that it is desirable that coatings capable of electrostatic dissipation (ESD) have volume and surface resistivities of less than about 10 9 Dm and 10 9 ⁇ /sq respectively.
• a coating suitable for use on spacecraft and spacecraft components should exhibit additional characteristics for spacecraft applications.
• the coating should be stable during long-term service in a space environment, including the ability to survive micrometeroid impacts and high levels of radiation exposure.
• the coating should be moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.
• a number of white, electrostatic-dissipative coatings are known for spacecraft use.
• One of the most well-known coatings is Z-93, developed by the Illinois Institute of Technology Research Institute (IITRI), 10 West 35th Street Chicago, IL 60616.
• NSA National Aeronautics and Space Administration
• Z-93 has been widely used to coat spacecraft in the past, Z-93 is not without its disadvantages.
• Z-93 has been reported as being porous, thermochromic, and having less than optimal electrostatic dissipation.
• the porosity of the Z-93 coating was discussed by William F. Carroll during the 1964 internal NASA conference on spacecraft developments (William F. Carroll. Coating development and environmental effects. In NASA Conference Proceedings on Spacecraft Coating Developments, pages 1-9, May 1964), wherein he stated with regard to Z-93 that "The ZnO-Potassium silicate coating is the most stable formulation developed, but, like all non-vitreous inorganic coatings, has adverse physical properties which limit its use.
• the coating is porous and therefore easily soiled and difficult to reclean. Therefore, use should be limited to applications where surfaces can be easily protected from contamination or where requirements for maximum stability justify extreme precautions for prevention of contamination".
• thermochromic nature of Z-93 is also well recognised, meaning that at high temperatures of greater than about 300°C, the colour of Z-93 changes from white to yellow, as is the case with all zinc oxide based surface treatments.
• Thermochromism in such coatings can be disadvantageous as an immediate increase in a s will be observed upon exposure to high temperature, which is undesirable. Additionally, temperature fluctuations in service will result in further instability of a s .
• the electrical resistivity (ESD) for Z-93 has been reported as 9.26 ⁇ 10 15 to 3.65 ⁇ 10 16 Qm (see e.g. page 2-14 of Deshpande & Harada.
• Variants of Z-93 also exist, for example the white thermal control paint AZ-93 developed by AZ Technology.
• the same disadvantages remain with the AZ- 93 coating as discussed above for Z-93 in terms of being porous, thermochromic, and having less than optimal ESD.
• It is also an object of embodiments of the invention to provide a coating, especially a white thermal control coating, which is operable and stable in a space environment. It is a further object of embodiments of the invention to provide white thermal control coatings which have an optical absorbance (a s ) of no greater than 0.20 at 100 ⁇ thickness and a thermal emissivity (£N) of no less than 0.80 at temperatures up to 500 °C, which are also capable of electrostatic dissipation, are less porous than conventional coatings, and which are non-thermochromic at temperatures up to or even greater than 500 °C.
• a curable coating composition comprising:
• the curable coating composition comprises: - from about 30% to about 90% by weight of the silicate;
• the percentages by weight being percentages by weight of the total curable coating composition.
• the silicate comprises a metal silicate.
• Suitable metal silicates include alkali metal silicates but are not limited thereto.
• the silicate optionally comprises an alkali metal silicate, further optionally, an alkali metal silicate selected from sodium silicate, potassium silicate and lithium silicate, or a combination thereof.
• the silicate is optionally present in an amount of from about 40% to about 80%, further optionally from about 40% to about 70%, still further optionally from about 50% to about 60%, still further optionally about 50% or about 51 % or about 52% or about 53% or about 54% or about 55% or about 56% or about 57% or about 58% or about 59% or about 60%, by weight of the curable coating composition.
• the silicate is present in an amount of about 60% by weight of the curable coating composition.
• the silicate comprises sodium silicate or potassium silicate in an amount of about 60% by weight of the curable coating composition.
• the silicate is present in an amount of about 50% by weight of the curable coating composition. In a certain embodiment, the silicate comprises lithium silicate in an amount of about 50% of the curable coating composition.
• the calcium phosphate comprises a tricalcium phosphate, a tetracalcium phosphate, hydroxyapatite or a derivative thereof, or a combination thereof.
• Suitable tricalcium phosphates include a-tricalcium phosphate (a-TCP) and ⁇ - tricalcium phosphate ( ⁇ -TCP), or a combination thereof.
• ⁇ -TCP is preferred.
• the calcium phosphate has a particle size in the range of from about 0.1 x 10 "6 m to about 5 x 10 "6 m. Further optionally the calcium phosphate has a particle size in the range of from about 0.1 x 10 "6 m to about 1 .0 x 10 "6 m.
• the calcium phosphate has a particle size in the range of from about 0.2 x 10 "6 m to about 0.5 x 10 "6 m.
• the term "derivative" of hydroxyapatite is intended to mean calcium phosphates which are non-stoichometric as opposed to hydroxyapatite having the formula Caio(PO 4 ) 4 (OH).
• the calcium phosphate may be doped with additional metal ions, such as zinc, magnesium, strontium, iron, aluminium, lanthanide elements or silicon.
• the calcium phosphate is optionally present in an amount of from about 5% to about 40%, further optionally from about 8% to about 20%, still further optionally from about 10% to about 15%, even further optionally from about 10% to about 12.5%, even further optionally about 10% or about 1 1 % or about 12% or about 12.5%, by weight of the curable coating composition.
• the calcium phosphate is present in an amount of about 10% by weight of the curable coating composition. In a certain embodiment, the calcium phosphate comprises ⁇ -TCP in an amount of about 10% by weight of the curable coating composition.
• the calcium phosphate is present in an amount of about 12.5% by weight of the curable coating composition.
• the calcium phosphate comprises ⁇ -TCP in an amount of about 12.5% by weight of the curable coating composition.
• the metal oxide comprises an oxide of magnesium, aluminium, scandium, yttrium, zirconium, lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium or lutetium, or a combination thereof.
• the metal oxide comprises magnesium oxide, aluminium (III) oxide, scandium (III) oxide, yttrium (III) oxide or zirconium oxide; even further optionally, scandium (III) oxide, yttrium (III) oxide or zirconium oxide; even further optionally scandium (III) oxide or yttrium (III) oxide; still further optionally yttrium (III) oxide.
• the metal oxide may comprise zinc oxide, ZnO (which, although thermochromic above around 300°C, is non-thermochromic at lower temperatures; coating compositions which include ZnO therefore have particular use in low temperature applications (below around 300°C), or in higher temperature applications for which thermochromism is not a concern).
• the metal oxide has a particle size in the range of from about 0.1 x 10 "6 m to about 5 x 10 "6 m.
• the metal oxide is optionally present in an amount of from about 10% to about 50%, further optionally from about 20% to about 40%, still further optionally from about 25% to about 40%, even further optionally from about 30% to about 40%, even further optionally from about 30% to about 37.5%, even further optionally about 30% or about 31 % or about 32% or about 33% or about 34% or about 35% or about 36% or about 37% or about 37.5%, by weight of the curable coating composition.
• the metal oxide is present in an amount of about 30% by weight of the curable coating composition.
• the metal oxide comprises yttrium (III) oxide in an amount of about 30% by weight of the curable coating composition.
• the metal oxide is present in an amount of about 37.5% by weight of the curable coating composition.
• the metal oxide comprises yttrium (III) oxide in an amount of about 37.5% by weight of the curable coating composition.
• the metal oxide is non-thermochromic at a temperature of greater than 300°C.
• the metal oxide is non-thermochromic at a temperature of up to or even greater than 500°C.
• thermochromic refers to a colour change upon increase of temperature. The colour change is generally reversible.
• the curable coating composition is a liquid curable coating composition.
• the curable coating composition is a curable thermal control coating composition.
• a curable coating composition comprising:
• a metal oxide examples and options for the silicate, phosphate and metal oxide species are as mentioned above in relation to the first aspect of the invention.
• the phosphate species may alternatively be another alkali phosphate - preferably one which is white in colour, such as magnesium phosphate or sodium phosphate.
• a curable coating composition comprising:
• the curable coating composition comprises:
• the metal sulphate comprises a barium sulphate.
• the amount of the metal sulphate present in the curable coating composition is as defined above for the metal oxide.
• a white thermal control paint comprising the curable coating composition according to the first, second or third aspects of the invention.
• a curable coating composition according to the first, second or third aspects of the invention as a non-thermochromic thermal control coating composition for application to spacecraft and components thereof.
• a cured coating produced by curing the curable coating composition according to the first, second or third aspects of the invention.
• a cured coating comprising:
• the cured coating comprises:
• the percentages by weight being percentages by weight of the total cured coating.
• the silicate is as defined above for the curable coating composition.
• the silicate is optionally present in an amount of from about 10% to about 70%, further optionally from about 10% to about 60%, still further optionally from about 15% to about 50%, still further optionally from about 20% to about 40%, even further optionally about 20% to about 37%, even further optionally about 20% or about 21 % or about 22% or about 23% or about 24% or about 25% or about 26% or about 27% or about 28% or about 29% or about 30% or about 31 % or about 32% or about 33% or about 34% or about 35% or about 36% or about 37%, by weight of the cured coating.
• the silicate is present in an amount of about 22% or about 36% or about 37% by weight of the cured coating.
• the silicate comprises sodium silicate in an amount of about 36% by weight of the cured coating.
• the silicate comprises potassium silicate in an amount of about 37% by weight of the cured coating.
• the silicate comprises lithium silicate in an amount of about 22% of the cured coating.
• the calcium phosphate is as defined above for the curable coating composition.
• the calcium phosphate is optionally present in an amount of from about 5% to about 60%, further optionally from about 10% to about 50%, still further optionally from about 10% to about 40%, still further optionally from about 10% to about 30%, still further optionally from about 12% to about 20%, still further optionally from about 15% to about 20%, even further optionally about 15% or about 16% or about 17% or about 18% or about 19%, by weight of the cured coating.
• the calcium phosphate is present in an amount of about 16% or about 19% by weight of the cured coating.
• the calcium phosphate comprises ⁇ -TCP in an amount of about 16% by weight of the cured coating.
• the calcium phosphate comprises ⁇ -TCP in an amount of about 16% by weight of the cured coating. In a still further embodiment, the calcium phosphate comprises ⁇ -TCP in an amount of about 19% of the cured coating.
• the metal oxide is as defined above for the curable coating composition.
• the metal oxide is optionally present in an amount of from about 10% to about 70%, further optionally from about 20% to about 65%, still further optionally from about 30% to about 60%, still further optionally from about 40% to about 60%, still further optionally from about 45% to about 60%, even further optionally from about 47% to about 59%, even further optionally about 47% or about 48% or about 49% or about 50% or about 51 % or about 52% or about 53% or about 54% or about 55% or about 56% or about 57% or about 58% or about 59%, by weight of the cured coating. In certain embodiments, the metal oxide is present in an amount of about 48% or about 47% or about 58% by weight of the cured coating.
• the metal oxide comprises yttrium (III) oxide in an amount of about 48% by weight of the cured coating. In a further embodiment, the metal oxide comprises yttrium (III) oxide in an amount of about 47% by weight of the cured coating. In a still further embodiment, the metal oxide comprises yttrium (III) oxide in an amount of about 58% of the cured coating.
• the phosphate species is present in an amount of from about 10% to about 25% by volume; further optionally from about 15% to about 20% by volume; further optionally about 18% by volume, or about 19% by volume. More particularly, a total volume fraction of the phosphate species within the coating composition of around 18-19 vol% has been determined as giving a good level of electrical conductivity without undue detriment to the optical properties of the coating.
• a cured coating comprising:
• the phosphate species may be an alkali phosphate other than calcium phosphate - for example magnesium phosphate or sodium phosphate.
• a cured coating comprising:
• the cured coating comprises:
• the percentages by weight being percentages by weight of the total cured coating.
• the cured coating may be formed from a curable coating composition according to the first, second and third aspects of the invention.
• the first, second and third aspects of the invention conveniently provide curable coating compositions for forming the cured coatings of the seventh, eighth, ninth and tenth aspects of the invention.
• the cured coating is a thermal control coating.
• a further aspect of the invention provides a thermal control coating comprising the cured coating according to the seventh, eighth, ninth or tenth aspects of the invention.
• a cured white thermal control paint comprising the cured coating according to the seventh, eighth, ninth or tenth aspects of the invention.
• a further aspect of the invention there is also provided the use of a cured coating according to the seventh, eighth, ninth or tenth aspects of the invention as a non-thermochronnic thermal control coating for spacecraft and components thereof.
• a coated substrate comprising a substrate and a curable coating composition according to the first, second or third aspects of the invention provided thereon.
• a coated substrate comprising a substrate and a cured coating according to the seventh, eighth, ninth or tenth aspects of the invention provided thereon.
• the substrate comprises an aluminium substrate, a magnesium substrate, a titanium substrate or a plastic substrate.
• the substrate may be a composite such as a carbon-fibre reinforced plastic (CFRP) substrate.
• CFRP carbon-fibre reinforced plastic
• the fibre reinforcement may consist of or comprise glass or basalt fibres.
• a process for making a coated substrate comprising a substrate and a curable coating composition according to the first, second or third aspects of the invention, comprising applying to a substrate a curable coating composition according to the first, second or third aspects of the invention.
• a process for making a coated substrate comprising a substrate and a cured coating according to the seventh, eighth, ninth or tenth aspects of the invention, comprising applying to a substrate a curable coating composition according to the first, second or third aspects of the invention, and curing the curable coating composition.
• the curable coating composition is applied to the substrate by spraying.
• the curable coating composition is dried.
• the silicate is produced using a sol-gel process.
• a sol-gel process enables the use of a lower curing temperature, as well as a longer duration room temperature cure. Furthermore, a sol-gel is more resistant to moisture and high humidity. Low moisture resistance is a known aspect of certain (alkali-based) silicates, so a sol-gel derived silicate is an advantageous alternative. It may be used as a top coat.
• the cured coatings of embodiments of the invention are opaque, white, reflective coatings which are suitable for use as white thermal control coatings for coating spacecraft and components thereof.
• the use of both yttrium (III) oxide and ⁇ -TCP in the coatings provides for opacity and for reflectance across the whole UV-Vis-NIR spectrum.
• the cured coatings of embodiments of the invention are less porous than conventional coatings. Lower porosity allows for greater robustness and resistance to contamination etc. It is proposed that the use of a calcium phosphate, optionally ⁇ -TCP, enables a cured thermal control coating to be produced which is less porous than conventional coatings.
• the reduced porosity of the coating is indicative of greater encapsulation of the pigment within the coating. This provides protection of the pigments (e.g. metal oxide and calcium phosphate) against atomic oxygen attack, x-ray and electron and proton degradation.
• the pigments e.g. metal oxide and calcium phosphate
• the cured coatings of embodiments of the invention are non-thermochromic to up to 1000°C.
• Z-93 exhibits a significant colour change at 300°C.
• Table 5 of Kamalisarvestani et al “Performance, materials and coating technologies of thermochromic thin films on smart windows", Renewable and Sustainable Energy Reviews 26 (2013) 353-364, shows that zinc oxide (contained within Z-93 and AZ-93), turns from white to yellow when heated.
• the electrical resistivity (ESD) for each of SWN79, SWK66 and SWL40 was determined to be 10 6 ⁇ , in contrast to the resistivity of the order of 10 15 to 10 16 Qm noted for Z-93 and AZ-93.
• the cured coatings of embodiments of the invention are UV reflective, whereas Z-93 and variants thereof absorb UV radiation.
• the cured coatings of embodiments of the invention can be carbon-free. This is advantageous since any residual carbon present in the coating can give rise to issues in space. Indeed, some space missions are simply too hot for organic molecules to survive. Under intense radiation bombardment, organic molecules can become fragmented and may vaporise and outgas. This process can alter the structure of the coating and represents a potential variation in the coating over time, which is generally to be avoided. For example, organic molecules will absorb strongly in the Vacuum Ultraviolet (VUV) and Ultraviolet (UV) wavelengths, accelerating the degradation of the material when exposed to solar radiation, and also giving rise to surface contamination. In particular, VUV radiation carries enough energy to sever H-C bonds, usually leaving carbon behind, which will ruin the optical properties of the coating. In addition, the outgassed material may also condense on nearby surfaces or components and may thus contaminate sensitive optical or other components and thereby damage the overall device.
• VUV Vacuum Ultraviolet
• UV Ultraviolet
• the outgassed material may also con
• Figure 1 shows scanning electron micrograph (SEM) images of a cured coating of Z-93 and a cured coating of our SWN79 composition, as described in more detail in Example 3(D) relating to porosity;
• Figure 2 shows a high magnification optical microscopy cross section image of SWN79 showing no evidence of porosity (top layer: mounting resin; middle layer: SWN79 coating; bottom layer: titanium substrate); and
• Figure 3 shows reflectance curves of SWN79 and Z-93 between 250 and 2500 nm wavelength. SWN79 is measured; Z-93 obtained from literature (L. Kauder, NASA/TP-2005-212792 entitled “Spacecraft Thermal Control Coatings References” and specifically figure 5.10).
• curable coating compositions each comprising: (a) a silicate, such as (but not limited to) sodium silicate, potassium silicate or lithium silicate; (b) a phosphate, such as (but not limited to) calcium phosphate, magnesium phosphate or sodium phosphate; and (c) a metal oxide, such as (but not limited to) magnesium oxide, aluminium (III) oxide, scandium (III) oxide, yttrium (III) oxide or zirconium oxide.
• a silicate such as (but not limited to) sodium silicate, potassium silicate or lithium silicate
• a phosphate such as (but not limited to) calcium phosphate, magnesium phosphate or sodium phosphate
• metal oxide such as (but not limited to) magnesium oxide, aluminium (III) oxide, scandium (III) oxide, yttrium (III) oxide or zirconium oxide.
• Such curable coating compositions may be used as non-thermochromic thermal control coating compositions for application to spacecraft and components thereof.
• such coating compositions provide thermal control from a 'passive cooling' standpoint, i.e. through low absorbance and high emissivity, as is particularly suited to space applications.
• the key heat transfer mechanism is via radiation.
• this radiative aspect renders the present coatings fundamentally different from flame-retardant or heat-resistant type coatings, which are based on the principle of preventing combustion (which is not an issue in the vacuum of space).
• the metal oxide species may be zinc oxide (zinc oxide being thermochromic at temperatures above around 300°C).
• curable coating compositions each comprising: (a) a silicate (e.g. as above); (b) a phosphate (e.g. as above); and (c) a metal sulphate, such as a barium sulphate.
• a silicate e.g. as above
• a phosphate e.g. as above
• a metal sulphate such as a barium sulphate.
• Curable coating compositions according to embodiments of the invention which we refer to as SWN79, SWK66 and SWL40, were prepared in accordance with the m (wet%) of ingredients shown under Formulation A of Table 1 .
• Y2O3 is Y2O3, i.e. yttrium (III) oxide, also known as yttria, available from HC Starck, Im Schleeke 91 , 38642 Goslar, Germany;
• ⁇ TCP is ⁇ -TCP i.e. ( -Ca 3 (PO 4 ) 2 ), available from Sigma Aldrich, St. Louis, MO, USA;
• N79 is sodium silicate, namely [3.22 SiO 2 /NaO] Na 2 SiO 3 *xH 2 O, available from PQ Corporation, 1700 Kansas Ave, Kansas City, KS 66105, USA;
• K66 is potassium silicate, namely [2.18 SiO 2 /KO] K 2 SiO 3 » xH 2 O, available from PQ Corporation, details provided above;
• L40 is lithium silicate, namely [8.20 SiO 2 /LiO] Li 2 SiO 3 » xH 2 O, available from PQ Corporation, details provided above.
• the x used in formulae of the silicates indicates the amount of water. It will also be appreciated by a skilled person that the silicates used are not limited to the exact formulae indicated above.
• a curable coating composition (SWN79 in Formulation A of Table 1 ) was prepared using sodium silicate as a liquid binder phase.
• the yttrium (III) oxide and ⁇ -TCP powders were mixed in a 3:1 ratio by weight to make up the powder component of the curable coating composition, in accordance with the m (wet%) shown in Table 1 for Formulation A.
• the silicate was then mixed with the yttrium (III) oxide and ⁇ - TCP powders in a 60:40 ratio by weight, again in accordance with the m (wet%) shown in Table 1 for Formulation A.
• the powder charge yttrium (III) oxide and ⁇ -TCP
• a curable coating composition (SWK66 in Formulation A of Table 1 ) was prepared using potassium silicate as a liquid binder phase. The process was carried out as described above for SWN79, except that potassium silicate was used instead of sodium silicate.
• a curable coating composition (SWL40 in Formulation A of Table 1 ) was prepared using lithium silicate as a liquid binder phase. The process was carried out as described above for SWN79, except that lithium silicate was used instead of sodium silicate, and the silicate was mixed with the yttrium (III) oxide and ⁇ -TCP powders in a 50:50 ratio by weight, in accordance with the m (wet%) shown in Table 1 for Formulation A. Each of SWN79, SWK66 and SWL40 was a viscous, aqueous solution.
• Curable coating compositions namely cured SWN79, SWK66 and SWL40, according to embodiments of the invention, were deposited onto metal samples and cured as follows. Cured SWN79
• the liquid SWN79 composition was sprayed onto the surface of a Grade V titanium (Ti6AI4V) substrate using a TrilogyTM AS spray gun, available from Nordson Corporation (Westlake, OH, USA). Spraying was carried out at a distance of 25 cm from the target surface until a coverage of 100 ⁇ thickness was achieved. After spraying, the sprayed surface was covered with, but not in contact with, aluminium foil and allowed set overnight, i.e. for 12 to 16 hours at a temperature of 20°C. At this stage, the surface was no longer glossy in appearance, and the sample was transferred to a conventional fan assisted oven. The sample was heated to 250°C in the oven as per the following thermal (cure) cycle:
• the cured coating was a hard, inorganic film, of 100 ⁇ thickness.
• the cured coating had a composition as indicated in m (dry%) for Formulation A in Table 1 .
• Cured SWK66 and Cured SWL40 were prepared by application to a Grade V titanium substrate, drying and curing, as described for SWN79 above.
• the cured coatings had respective composition as indicated in m (dry%) for Formulation A in Table 1 .
• Cured coatings were also prepared using the curable coating compositions formed using Formulations B - J.
• the diffuse reflectance R of the cured coatings SWN79 and SWL40 present on the titanium substrates prepared in Example 2 was measured in the UVA/is/NIR range (250 - 2500 nm) using a Perkin Elmer Lambda 9/19 double beam spectrophotometer with a 150 mm integrating sphere attachment. Diffuse reflectance was measured against a Spectralon® reference to produce a reflectance trace (not shown) which was weighted against the Air Mass Zero (ASTM E490) solar irradiance spectrum. As the coated substrates were opaque, solar absorbance a s is simply: 1 -R. The results are shown in Table 2.
• the solar absorbance of the cured coatings SWN79 and SWL40 was found to be excellent, having a s of no greater than 0.2 (i.e. a s (%) no greater than 20%).
• ECSS-Q-ST-70-09C Surface emissivity measurements of the coatings were carried out to ECSS Standard (ECSS-Q-ST-70-09C). Measurements were taken using a Jenoptik VarioCam IR camera. Samples were placed on a hot plate with known emissivity reference samples. The hot plate temperature was measured using a Type K thermocouple that was embedded in another sample that was on the hotplate. Thermocouple readings were used in conjunction with the infra-red camera software to obtain emissivity data for the coatings tested. The IR camera and the thermocouple measurements were subsequently recorded over a set duration. During post processing the most stable portion of the temperature measurement graph (not shown) was used to determine the max and min temperature variation. The mean temperature was used to calculate the emissivity for each particular sample through the IR camera software.
• the solar absorptance of the samples was measured subsequent to outgassing testing in ESA's XTES Facility (ESTEC, Noordwijk, The Netherlands).
• the samples were then added to ESA's Synergistic Temperature-Accelerated Radiation (STAR-II) Facility, where a s was remeasured at 500 °C under 10 "6 mbar atmospheric pressure. Any thermochromic effects would appear as a change in solar absorptance or ⁇ .
• BOT Synergistic Temperature-Accelerated Radiation
• thermochromic As a further test, separate samples of the components yttrium (III) oxide, ⁇ -TCP and zinc oxide (ZnO), all white powders, were heated to 300 °C in a Carbolite 1 100 electric furnace. The resultant samples were visually examined at 300 °C and the differences were clear to the naked eye (results not shown). The zinc oxide had become discoloured from white to a yellow colour upon being heated. In contrast, yttrium (III) oxide and ⁇ -TCP, being components of the cured coatings of certain embodiments of the invention, showed no discolouration and remained white. Accordingly, the components of the cured coatings of certain embodiments of the invention may advantageously be non-thermochromic. (D) POROSITY
• FIG. 2 A further representation of the SWN79 sample is shown in Figure 2.
• the sample was sectioned, mounted and polished and examined using optical microscopy.
• top layer mounting resin
• middle layer SWN79 coating
• bottom layer titanium substrate
• SWN79 shows 0% porosity when examined using optical microscopy.
• the SWN79 sample produced a solid and non-porous surface at all magnifications tested.
• each of the cured coatings namely cured SWN79, cured SWK66 and cured SWL40
• surface resistivity of each of the cured coatings was measured by applying the respective curable coating composition prepared in Example 1 to an insulating substrate (glass) and curing as detailed in Example 2.
• a concentric ring resistivity probe was placed on the coated surface and attached to an insulating Fluke 1507 test meter. The surface resistivity was recorded over a range of voltages from 50 to 1000 Volts.
• Example 3(A) the UV reflectance, and by association the UV absorption, was obtained by integrating the trace, with respect to the ASTM-G490 Air-mass Zero (AM0) solar irradiance standard spectrum, between wavelengths of 250 and 380 nm.
• NEO Near-Earth-Orbit
• the cured coatings of embodiments of the invention have excellent optical absorbance and thermal emissivity, and are less porous than the conventional white thermal control coatings. They are also non-thermochromic, have reduced electrical resistivity, and have increased UV reflectance, compared with conventional white thermal control coatings. Accordingly, the cured coatings of embodiments of the invention are excellent candidates for use as white thermal control coatings for coating spacecraft and components thereof, and for other passive temperature control applications.
• Increasing the amount of calcium phosphate in the coating composition results in greater electrical conductivity of the cured coating.
• increasing the amount of calcium phosphate in the coating composition can also be detrimental to the optical properties (in particular the reflectance) of the cured coating (i.e. it becomes less white). It is therefore preferable to use an optimum amount of calcium phosphate in the coating composition, such as to provide a good level of electrical conductivity in the cured coating, without undue detriment to its optical properties.
• the total volume fraction of the calcium phosphate species (e.g. ⁇ -TCP) within the coating composition (including binder phases) should be present in fractions greater than 0.183, or 18.3 vol%. Higher volume fractions of the calcium phosphate species will improve electrical conductivity further, but at the expense of an increase in solar absorptance (a s ).
• the packing density of rigid spheres cannot exceed the Kepler Conjecture, which defines cubic- or hexagonal-close packing as the densest possible sphere packing configurations, having a maximum density of ⁇ /(3 ⁇ /2) « 74.048 %. Therefore, in practice, 0.183 ⁇ ⁇ 0 ⁇ 0.740.
• a particle size of the calcium phosphate species in the range of 200 nm - 500 nm has been found to give optimal optical properties in the cured coating. More particularly, optimised scattering is achieved with particles in this size range. This distribution was determined from the Mie solution to Maxwell's equation. For example, a particle diameter of about 270 nm will maximise the scattering coefficient for tricalcium phosphate (TCP). (Y2O3, by way of comparison, requires a particle size of around 220 nm to achieve a similar effect.) It should also be noted that a small particle size will increase the surface area per g (cm 2 /g) and this is expected to improve the conductivity (if based on surface area) while minimising the total mass included.
• TCP tricalcium phosphate
• Y2O3 is primarily given as the metal oxide species in the curable coating composition
• other metal oxides may be used instead.
• zinc oxide (ZnO) could be used in place of Y2O3.
• Such a coating would be suitable for use in lower temperature environments where thermochromism is not expected to be an issue.
• ZnO Zinc Oxide
• ⁇ -TCP as the calcium phosphate species should allow this composition to retain the advantageous electrical properties of the above-described Y2O3-based coating (e.g. in respect of electrical conductivity), especially if care is taken to fulfil the criteria to achieve both the percolation threshold and critical volume fractions.
• the coating curable composition comprises a silicate, a calcium phosphate and a metal sulphate.
• a curable coating composition comprises from about 30% to about 90% by weight of the silicate, from about 2% to about 60% by weight of the calcium phosphate, and from about 2% to about 60% by weight of the metal sulphate, the percentages by weight being percentages by weight of the total curable coating composition.
• ⁇ -TCP is primarily given as the calcium phosphate species in the coating composition, in alternative embodiments other calcium phosphate species may be used instead.
• the phosphate species need not be a calcium phosphate, but may be a different phosphate species.
• the phosphate species could an alternative alkali phosphate - preferably one which is white in colour, such as magnesium phosphate or sodium phosphate.
• sodium silicate, potassium silicate and lithium silicate are primarily given as examples of the silicate species in the coating composition, in alternative embodiments other silicate species may be used instead.
• silicates e.g. sodium silicate
• an insoluble barrier layer may be applied to or incorporated in the cured coating, e.g. as a top coat or by cross-linking the outer surface of the silicate (via conversion (CaCI treatment for example) or via exposure to an oxygen-rich post treatment (low temperature plasma or laser treatment).
• the coating may be cured to a temperature equivalent to the intended operating temperature.
• the silicate species is made using a sol-gel process.
• a sol-gel can be made up of organic or inorganic silicon-oxygen backboned precursors (-Si-O-R where R can be organic or inorganic).
• a cured sol-gel coating will crosslink and form an inorganic crosslinked S1O 2 structure.

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