« PreviousContinue »
i i i i i i i i i 1 1 r
380 420 460 500 540 580 620 660 700 740 780 820
BRIGHT METAL FLAKE BASED PIGMENTS
This application is a continuation of U.S. application Ser. No. 09/207,121 filed on Dec. 7, 1998, now U.S. Pat. No. 6,150,022. 5
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to metal flake 10 pigments. More specifically, the present invention relates to metal flake pigments having improved specular reflectance.
2. The Relevant Technology
Pigments are generally used to contribute to the optical and other properties of applications such as coatings, inks, :5 extrusions, paints, finishes, glass, ceramics and cosmetics. Many varieties of pigments exist, some of which are metal flake based. These metal flakes comprise a thin film metal layer for improving the lustre, sparkle, shine, absorption, hiding and/or reflective properties of the application. The 20 optical performance of the pigments, however, are duly constrained by the inherent limitations of each metal flake therein.
In general, it is known that for the application to achieve the greatest specular reflectance across visible wavelengths 25 (300-800 nm), metal flakes should individually lay as flat as possible. As a collection of numerous flakes, the greatest reflectance, and hence greatest brightness, occurs when the flakes are collectively planar oriented to expose the greatest amount of surface area of the metallic flakes to the incident 30 light and reflect as much of that light as possible.
A major factor, however, affecting those reflectance characteristics is the size or dimensions of the flake as the flake is used in a particular application. For example, if the flakes 3J are thick, a plurality of thick flakes combined together in an application are prevented from lying together in a generally flat or horizontal singular plane because adjacent flakes cannot easily overlap each other due to their thickness. As a result, many flakes are adversely caused to be oriented in a 4Q substantially vertical manner and the plurality of flakes are formed into a radically non-planar layer. Incident light then exposed upon the non-planar layer is subject to extreme scatter and non-specular reflection. Thus, the favorable reflective properties of the application are diminished by 4J thick flakes. To a lesser extent, thick flakes frequently cause other difficulties such as the clogging of automatic-spray paint guns during painting applications.
However, it is also well known that as the thicknesses of the flakes is reduced, the point is reached where the flakes 50 become so flimsy (non-rigid, flaccid) that they begin to curl and/or wrinkle. This decreases favorable planarity and reflective properties because incident light exposed upon the flakes is subject to scatter and non-specular reflection. Additionally, if the flakes are too thin when applied onto a 55 surface during applicational use, the flakes will assume any microscopic defects in the contour of that surface. For example, if that contour is rough, the flakes will correspondingly be rough or non-planar. Again, disfavored planarity and reflective properties result because incident light 60 exposed on the surface is subject to scatter and non-specular reflection.
Some manufacturing processes form flakes from a singular, larger sheet or film of metal which is "fractured" by well known means into smaller, flake-sized particles. Two 65 types of fracture may result, "ductile" or "brittle." Ductile fractures cause the metal to undergo substantial plastic
deformation near the vicinity of fracture before fracture occurs. This deformation causes numerous malformed regions having disfavorable planar characteristics to appear. As before, these malformed regions, such as regions having curled or wrinkled metal, disadvantageously tend to scatter and diffuse incident light exposed thereupon. Brittle fractures, on the other hand, tend to cause little or no plastic deformation of the metal before the fracture occurs which enables the produced metal flake to maintain, as much as possible, the original planarity of the larger metal sheet. Consequently, it is desirable that brittle fracture occur during manufacturing.
However, brittle fracture does not occur with most metals having high reflectivity. In fact, brittle fracture is only likely to occur with materials having a large compressive strength as compared to its corresponding tensile strength. This is because the internal bond strength distributed throughout a material is composed of tensile and compressive components. The tensile strength compensates for forces out of the plane of the material and the compressive strength is related to forces in the plane. Thus, similar compressive and tensile strengths will allow ductile deformations since the relative strength into and out of the plane is equivalent. In contrast, brittle deformation occurs when the compressive strength is greater than the tensile strength and the material strength is directed into the plane, not out of the plane. Consequently, a high compressive strength relative to tensile strength results in bond rupture and material cracking when a force is applied. Thus, aluminum, for example, which has a tensile strength of about 13-24 lb/in2 and a compressive strength of about 13-24 lb/in2, would most likely undergo a ductile fracture under a uniaxial stress which would cause the aluminum to exhibit disfavored reflective characteristics. Moreover, once the aluminum is bent or deformed, as would occur with ductile fracture, the aluminum remains deformed and the disfavored reflective characteristics would persist. Consequently, it is difficult to manufacture metal flakes, such as aluminum, without malformations that reduce reflectance.
As is well known, fracture mechanics are not only important for metal flakes during the manufacturing process, but are as equally important during use. For example, applicational processes, such as the drying of a paint or ink solvent, also induce stresses on the flake. These stresses, caused by surface tension, again cause the flake to undergo fracture or malformation. Since brittle fracture of the flake during the applicational process also tends to produce smaller flakes that maintain much of the original planarity of the larger flake, instead of curled or deformed flakes, flake planarity and reflective properties are improved. Thus, flake brittleness is a characteristic not only preferred during the manufacturing process but also preferred during the applicational use. Accordingly, the prior art has attempted to produce thin, rigid and brittle flakes by facilitating both the manufacturing thereof and the reflective properties of the application.
Yet all prior solutions have involved compromises. For example, in U.S. Pat. No. 5,198,042, entitled "Aluminum Alloy Powders for Coating Materials and Materials Containing the Alloy Powders," it is taught to alloy the metal flake with other materials and metals to reduce the adverse curling, wrinkling and malleability of thin flakes. Alloying, however, dilutes the reflectance properties of the flake.
In U.S. Pat. No. 4,213,886, entitled "Treatment of Aluminum Flake to Improve Appearance of Coating Compositions," a surface bound species that pulls the flake flat in a coating resin is disclosed. This method, however, requires chemical tailoring of the flake and the resin in order