Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2002-05-14
2004-03-16
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S417000, C106S418000, C106S430000, C106S436000, C106S439000
Reexamination Certificate
active
06706109
ABSTRACT:
BACKGROUND OF THE INVENTION
Nacreous pigments, also known as pearlescent or effect pigments, exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depends upon optical interference phenomena as more fully described, for example, in “The Properties of Nacreous Pigments”, Greenstein and Miller, Technical Papers, Vol. XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Numerous patents and publications have described effect pigments based on titanium dioxide coatings on various substrates. Early examples include Linton U.S. Pat. No. 3,087,828 and 3,087,829 which describe the preparation of titanium dioxide and other metal oxide coated mica effect pigments, which optionally can be topped with a layer of another material such as, inter alia, iron. Since that time, numerous patents and publications have described the overcoating of titanium dioxide-coated mica to form a variety of effect pigments.
Recently, there has been renewed interest in a type of effect pigments known as “optically variable” because they exhibit different colors at different viewing angles, i.e., they exhibit color travel or “flop” as the angle of viewing changes. Such optically variable pigments have been described in the patent literature since the 1960s. For instance, Hanke in U.S. Pat. No. 3,438,796 describes the pigment as being “thin, adherent, translucent, light transmitting films or layers of metallic aluminum, each separated by a thin, translucent film of silica, which are successively deposited under controlled conditions in controlled, selective thicknesses on central aluminum film or substrate”. These materials are recognized as providing unique color travel and decorative optical color effects.
The recent approaches to optically variable pigments have generally adopted one of two techniques, both of which are designed to position a low refractive index layer such as silica (Rf 1.5) between reflective layer. In the first, a stack of layers is provided on a temporary substrate which is often a flexible web. The layers are generally made up of aluminum, chromium, magnesium fluoride and silicon dioxide. The stack of film is separated from the substrate and subdivided into appropriately dimensioned flakes. The pigments are produced by physical techniques such as physical vapor deposition onto the substrate, separation from the substrate and subsequent comminution or by other deposition techniques (plasma, sputtering etc.), subsequent deflaking of the decomposition product, etc. In the pigments obtained in this way, the central layer and all other layers in the stack are not completely enclosed by the other layers. The layered structure is visible at the faces formed by the process of comminution.
In the other approach, a platelet shaped opaque metallic substrate is coated or encapsulated with successive layers of selectively absorbing metal oxides and non-selectively absorbing layers of carbon, metal sulfide, metal and/or metal oxide. To obtain satisfactory materials using this approach, the layers are applied by multiple techniques such as chemical vapor deposition and/or sol-gel processes. A major shortcoming of this is that traditional metal flakes usually have structural integrity problems, hydrogen outgassing problems and other pyrophoric concerns.
The prior art approaches suffer from additional disadvantages. For instance, certain metals or metal flakes such as chromium, aluminum, copper, brass and bronze may have perceived health and environmental impacts associated with their use.
New optically variable effect pigment which do not suffer from the disadvantages of the prior art are clearly desirable and it is the object of the present invention to provide the same.
SUMMARY OF THE INVENTION
This invention is related to new articles exhibiting optically variable color and high reflectivity and their preparation. More particularly, the invention relates to a platelet pearlescent pigment having an oxide coating on titanium dioxide-coated substrate platelets from which a portion of the substrate has been eliminated.
DETAILED DESCRIPTION OF THE INVENTION
The optically variable effect pigments of the present invention are oxide-coated, for instance, iron oxide-coated, titanium dioxide platelet pigments from which a portion of the interior of the titanium dioxide has been removed. These titanium dioxide platelet pigments are derived from titanium dioxide-coated siliceous (e.g., mica) substrates from which a portion of the substrate has been removed. The formulation of coating and other compositions containing the resulting pigments and the coating of substrates is known.
Appropriately sized titanium dioxide platelets commonly referred to as “platy TiO
2
” or “self supporting TiO
2
” are described, for instance, in U. S. Pat. No. 4,192,691 and 5,611,851. Such platelets are substantially substrate free, generally containing less than about 20% of substrate based on the total weight of the product. U.S. Pat. No. 4,192,691 employs an aqueous solution of hydrofluoric acid and a mineral acid such as sulfuric acid to dissolve the mica from the pigment. It also discloses and illustrates the use of this dissolving agent to remove the mica from a titanium dioxide-coated mica having a surface layer of either iron or chromium oxide. U.S. Pat. No. 5,611,851 employs a combination of a mineral acid and phosphoric acid followed by an extractive dissolution using an alkali. Although the procedure of U.S. Pat. No. 5,611,851 is preferred, other procedures can be employed to obtain the titanium dioxide platelets used in the present invention. Titanium dioxide platelet types suitable for use in this invention can be prepared by removing gypsum from TiO
2
coated gypsum or by burning off graphite from TiO
2
coated graphite. Dissolving glass from a TiO
2
coated glass base also provides a substrate useful in this invention. Although there are several avenues for preparing the TiO
2
platelets which then can be coated further, the TiO
2
substrate of U.S. Pat. No. 5,611,851 is still preferred in order to obtain maximum reflectivity and color purity. Initially using a substrate aids in producing the relatively smooth and regular titanium dioxide surfaces needed to achieve high quality effect pigments, and the subsequent removal of the mica (refractive index 1.5) or other substrate and its replacement with air (Rf 1.0), allows the benefit of the refractive index of TiO
2
(2.6-2.9) to be more fully realized.
The platy TiO
2
pigments exhibit little, if any, color travel and cannot be considered optically variable. Surprisingly, however, it was discovered that when the average particle size (longest dimension measured by SEM) was limited, oxide overcoating produced a sharp and distinct color travel.
Accordingly, the platelets of titanium dioxide used in the present invention generally have an average longest dimension of about 1-25 &mgr;m, preferably about 2-15 &mgr;m and more preferably about 5-8 &mgr;m. The platelets can have a thickness of about 5-600 nm, and is more preferably about 20-400 nm. The TiO
2
is preferably in the rutile crystalline form but can also be in the anatase form.
While the use of platelets which are substantially substrate free, i.e., generally containing less than about 20% of substrate based on the total weight of the product, provides optically variable effect pigments, the need to eliminate so much of the substrate adds to the manufacturing cost. Also, because the center of the platy TiO
2
is essentially hollow, the pigment tends to be more fragile which, in turn, tends to complicate its use in applications where the pigment is subjected to more rigorous conditions. However, it has been found that the titanium dioxide need not be substantially substrate free, and platelets where only a part of the core substrate has been eliminated gives similar results while reducing manufacturing cost and making the pigment more sturdy, allowing it to be employed in more rigo
Brown Melanie L.
Engelhard Corporation
Koslow C. Melissa
Manlove Shalie
LandOfFree
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