The present invention relates to thermal insulation, more specifically to a coating composition for forming a thermal insulating layer on a substrate, a method for forming such a thermal insulating layer, and a method for producing such a composition.
Unintended heat transfer causes inefficiencies in energy use by consumers as well as in industrial settings, with the attending financial costs. Examples of such undesired heat transfer are: heat gain by a building structure from absorption of solar radiation; heat loss from inadequately insulated pipes and other structures, etc. Heat transfer over a prolonged period may cause deterioration of the material to which the heat is transferred. Another impact of unintended heat transfer is the loss of worker productivity due to burn or freeze injuries caused by contact with uninsulated or insufficiently insulated equipment. At the very least, heat stress and cold stress in the workplace reduce worker productivity.
Methods of mitigating unintended heat transfer are known under the rubric of “insulation”, and are commonly identified by the composition that constitutes the solid portions of the insulation material. Examples include: polyurethane foam, fiberglass batting, rock wool, loose fill vermiculite or perlite, blown-in cellulose, polystyrene foam, polyisocyanurate foam, acrylic coatings, coatings containing ceramic particles or microspheres, etc.
Each of the insulation methods described above suffers from one or more of the following problems: loss of insulation properties after installation due to the infiltration of dust, moisture, mold, and mildew; corrosion when these materials are used on metal surfaces, due to the accumulation of moisture under the insulation, in particular condensation resulting from the temperature differential created at the interface between the insulation and the substrate; and insufficient insulating properties, particularly in the case of surface coatings.
A goal of the present invention is to provide insulation in the form of a thin layer or coating, which is a protective layer characterized by low thermal conductivity and exhibiting excellent and uniform adhesion to the substrate, thus protecting metal substrates against corrosion and protecting all substrates from mold and mildew. The layer is formed by the application of a liquid coating composition characterized by good storage stability. The inventors' work has resulted in the coating composition of the invention that meets the above stated goal.
The coating composition of the invention contains highly porous particles of a material obtained by drying a wet sol-gel. Such material includes, but is not limited to, materials known as aerogels and xerogels. In its conventional meaning, the term “aerogel” is used to describe a material obtained by drying a wet sol-gel at temperatures above the critical temperature and at pressures above the critical pressure. Under such conditions, the removal of the gel liquid, for example, water, from the sol-gel results in a porous structure without damaging the structure of the gel, so that a high porosity is obtained. Traditionally, the product obtained by drying at conditions below supercritical conditions is known as a “xerogel”, which has a lower porosity, with at least some of the pore structure being damaged during the drying process. Since the process of drying under supercritical conditions is very energy intensive and costly, attempts have been made to produce xerogels which approximate the properties of aerogels. Such xerogels are suitable for use in the composition of the invention. For example, U.S. Pat. No. 5,565,142, describes “an extremely porous xerogel dried at vacuum-to-below supercritical pressures but having the properties of aerogels which are typically dried at supercritical pressures. This is done by reacting the internal pore surface of the wet gel with organic substances in order to change the contact angle of the fluid meniscus in the pores during drying.”
Silica aerogels were the first extensively studied aerogels. However, aerogels and xerogels may be made with a wide range of chemical compositions. Other inorganic aerogels may be used in the present invention, as well as aerogels prepared from organic polymers, sometimes called “carbon aerogels.” Inorganic xerogels and organic xerogels are suitable for the composition of the invention, provided that they have properties similar to aerogels.
Aerogels and xerogels can also be surface treated to alter their properties. For example silica aerogel can be made less hydrophilic by converting the surface —OH groups into —OR groups (wherein R is an aliphatic group). U.S. Pat. No. 6,806,299, the content of which is incorporated herein by reference in its entirety, discloses the preparation of hydrophobic organic aerogels. These chemically modified aerogels are also suitable for use in the composition of the invention, as well as chemically modified xerogels that have properties similar to those of aerogels.
Aerogels are known to have excellent thermal insulation properties, and xerogels having a porosity and pore structure approximating those of aerogels are also good insulators. In known uses for insulation, aerogel particles have been compressed into panels or compacted in an enclosed container or in a flexible bag, optionally with the aid of a binder. In another use, U.S. patent publication 2003-0215640 describes “a heat resistant aerogel insulation composite comprising an insulation base layer comprising hydrophobic aerogel particles and an aqueous binder, and a thermally reflective top layer comprising a protective binder and an infrared reflecting agent.” The insulation layer “preferably comprises a foaming agent” and “it is desirable to use as little of the aqueous binder as needed to attain a desired amount of mechanical strength.”
U.S. patent publication 2004-0077738 A1 describes “an aerogel-hollow particle binder composition comprising an aqueous binder, hydrophobic aerogel particles, and hollow, non-porous particles, as well as an insulation composite comprising the aerogel-hollow particle binder composition, and a methods (sic) of preparing the aerogel-hollow particle binder composition and insulation composite.” The composition “preferably comprises a foaming agent” and “it is desirable to use as little of the aqueous binder as needed to attain a desired amount of mechanical strength.”
Prior to the work performed by the present inventors, which forms the subject of this patent application, aerogel and xerogel particles have not been successfully used as the primary insulating agent embedded in a thin layer or coating that is firmly adhered to a substrate, the thin layer or coating being formed by applying a film-forming liquid composition which has good storage stability and is not subject to excessive thickening upon storage. Furthermore, in the present invention, the thermal insulating properties of the aerogel and xerogel particles are not degraded by the other components of the coating composition, for example as a result of damage caused to the delicate pore structure of the particles and/or invasion and saturation of the pores. Also, the aerogel and xerogel particles in the coating formed from the composition of the invention are protected by the inherent physical properties of the coating from damage and disintegration induced by environmental conditions.