The present invention concerns new solid solutions which are useful as inorganic color pigments. More particularly, the present invention concerns new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments, some of which exhibit low Y CIE tri-stimulus values and high reflectivity in the near infrared portion of the electromagnetic spectrum.
Chromium green-black hematite (basic chemical formula: Cr2O3) is an inorganic color pigment, C.l. Pigment Green 17, having a corundum-hematite crystalline structure. It is commonly used to impart a green color to ceramics, paints, polymers, and other materials. The DCMA Classification and Chemical Description of the Complex Inorganic Color Pigments, Third Addition (1991), published by the Dry Color Manufacturer""s Association, states that its composition may include any one or a combination of the modifiers Al2O3 (alumina), Fe2O3(iron oxide), or Mn2O3(manganese oxide).
Iron brown hematite (basic chemical formula: Fe2O3), is an inorganic pigment, C.l. Pigment Red 101 and 102, having a dark brown to black color and a corundum-hematite crystalline structure. See DCMA Classification and Chemical Description of the Complex Inorganic Color Pigments, Third Addition (1991). It is commonly used to impart dark brown to black color to ceramics, paints, plastics, and other material. Its composition may include any one or a combination of the modifiers Cr2O3(chrome oxide), Fe2O3(iron oxide), Mn2O3(manganese oxide), or NiO (nickel oxide).
Chromium green-black hematite is one of the principle pigments used in the manufacture of green shade military camouflage paint and netting. In such applications, chromium green-black hematite is combined with cobalt bearing mixed metal oxides, such as cobalt containing spinel pigment V12600 available from Ferro Corporation of Cleveland, Ohio. This combination of pigments is effective in simulating the reflectivity of chlorophyl in the visible portion of the electromagnetic spectrum, being that portion of the spectrum which is viewable by the naked eye with wavelengths ranging from approximately 0.40 xcexcm to 0.70 xcexcm.
Chlorophyl, which is an organic pigment, generally exhibits a relatively uniform high degree of reflectivity in the near infrared, being that portion of the electromagnetic spectrum with wavelengths ranging from approximately 0.7 xcexcm, to 2.5 xcexcm. Cobalt, however, exhibits a strong absorption band (i.e., low reflectivity) in a portion of the near infrared with wavelengths ranging from approximately 1.2 xcexcm to 1.6 xcexcm. In recent years, advancements in imaging technology have made it possible to contrast known military green shade camouflage painted or covered objects from the background foliage in that portion of the near infrared. A substitute military green shade camouflage pigment which contains no cobalt and which closely matches the reflectivity of chlorophyl in the visible and near infrared is therefore highly desired.
In order to satisfy military specifications, a substitute green shade camouflage pigment would have to exhibit a dark drab green appearance in the visible portion of the spectrum and would also have to simulate the reflectance curve for chlorophyl in the near infrared. Generally speaking, known inorganic pigments which exhibit a low degree of reflectivity in the visible portion of the light spectrum (i.e., dark drab colored pigments) also tend to exhibit a correspondingly low degree of reflectivity (i.e., high absorption) in other portions of the light spectrum, including the near infrared. A chromium green-black hematite pigment manufactured by Bayer Corporation of Germany, product number AC 5303, was observed to exhibit a higher near infrared reflectance than other chromium green-black hematite sources (this pigment, however, does not have the desired dark drab appearance in the visible spectrum required for military green shade camouflage paint applications). It was found by chemical analysis that this pigment contained both alumina and titania (basic chemical formula: TiO2) as minor additives. A search failed to disclose any references teaching the use of alumina and titania to improve the near infrared reflectance of chromium green-black hematite pigments.
The present invention provides new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments. Solid solutions according to the present invention are comprised of a host component having a corundum-hematite crystalline structure which contains as a guest component one or more elements from the group consisting of aluminum, antimony, bismuth, boron, chrome, cobalt, gallium, indium, iron, lanthanum, lithium, magnesium, manganese, molybdenum, neodymium, nickel, niobium, silicon, tin, titanium, vanadium, and zinc. Solid solutions according to the present invention are formed by thoroughly mixing compounds, usually metal oxides or precursors thereof, which contain the host and guest components and then calcining the compounds to form the solid solutions having the corundum-hematite crystalline structure.
Some of the new solid solutions according to the present invention, such as for example chrome oxide as a host component containing the elements iron, boron, and titanium as guest components, exhibit dark drab colors in the visible and high reflectivity in the near infrared portions of the electromagnetic spectrum. One of the primary uses for new solid solutions having these properties would be as inorganic color pigments in military camouflage paint or netting applications, which would permit the radiation signature of a painted or covered object to be tailored to match the reflectance curve of the background foliage in the visible and near infrared portions of the electromagnetic spectrum. Because many of these new solid solutions exhibit relatively high near infrared reflectance, they would also be suitable for use in the general paint and polymer markets, most specifically for architectural applications where increased near infrared reflectance would result in lower heat build-up and thus lower energy costs.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.