Low solar absorptance coatings
Coating compositions include a binder that has a first polymeric material that does not undergo ultraviolet (UV) degradation and/or does not absorb UV radiation/light. The coating compositions may further include at least a first UV-reflective pigment dispersed throughout the binder. The coating compositions may be provided in a cured form as a solid coating layer that may overly at least a portion of a substrate.
1. A method of making a coating, the method comprising:
(i) providing or forming a flowable coating composition comprising:
(1) a binder comprising a first polymeric material comprising a first polysiloxane that (a) does not undergo ultraviolet (UV) degradation, (b) does not absorb UV radiation/light, or (c) both of (a) and (b); and
a second polymeric material comprising a second polysiloxane that (a) does not undergo ultraviolet (UV) degradation, (b) does not absorb UV radiation/light, or (c) both of (a) and (b),
where the first polysiloxane is different than the second polysiloxane; and
(2) at least a first UV-reflective pigment dispersed throughout the binder, wherein the first UV-reflective pigment has an average diameter from about 0.3 to about 1 μm;
(ii) coating a substrate with the flowable coating composition;
(iii) curing the flowable coating composition to form a cured coating layer attached to the substrate;
(iii) disposing an overcoat on top of the cured coating layer, the overcoat comprising calcium carbonate dispersed in a polysiloxane.
2. The method of claim 1 , wherein the first polymeric material is devoid of an aromatic functionality.
3. The method of claim 1 , wherein the first polymeric material comprises a methyl-polysiloxane.
4. The method of claim 1 , wherein the first polysiloxane has a first structure and the second polysiloxane has a second structure, and wherein the second structure is more rigid than the first structure.
5. The method of claim 1 , wherein the second polysiloxane is a polysilesquioxane having a cage structure, wherein the first polysiloxane imparts additional flexibility to the coating composition when the coating composition is provided in a cured form.
6. The method of claim 1 , wherein the cured coating layer comprises from about 25% to about 60% by volume of the first polymeric material.
7. The method of claim 1 , wherein the first polymeric material accounts for about 1% to about 100% of a total polymeric content of the cured coating layer.
8. The method of claim 1 , wherein the first UV-reflective pigment comprises an inorganic pigment.
9. The method of claim 1 , wherein the first UV-reflective pigment comprises ZnO, CaCO 3p , or a combination of ZnO and CaCO 3 in a ratio of x:y, where x may vary from 1 to 99, while y varies from 99 to 1.
10. The method of claim 1 , further comprising dispersing at least a second UV-reflective pigment throughout the binder.
11. The method of claim 10 , wherein the first UV-reflective pigment comprises ZnO and the second UV-reflective pigment comprises CaCO 3 .
12. The method of claim 1 , wherein the first UV-reflective pigment has an average diameter from about 0.3 to about 0.6 μm.
13. The method of claim 1 , wherein the cured coating layer comprises from about 40% to about 75% by volume of the first UV-reflective pigment.
14. The method of claim 1 , wherein the cured coating layer comprises a thickness from about 25 μm to about 250 μm.
15. The method of claim 1 , wherein the coating has an average reflectance from about 80% to about 100% across wavelengths from about 0.4 μm to about 3 μm.
16. The method of claim 1 , wherein the coating has an average reflectance from about 80% to about 100% across wavelengths from about 0.4 μm to about 2.5 μm.
17. The method of claim 1 , wherein the cured coating layer is a hydrolysis reaction product of the first polysiloxane and the second polysiloxane.
18. The method of claim 1 , wherein the first UV-reflective pigment comprises zinc oxide.