One common method for printing images on a receiver material is referred to as electrophotography. The production of black-and-white or color images using electrophotography generally includes the producing a latent electrostatic image by uniformly charging a dielectric member such as a photoconductive substance, and then discharging selected areas of the uniform charge to yield an imagewise electrostatic charge pattern. Such discharge is generally accomplished by exposing the uniformly charged dielectric member to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device directed at the dielectric member. After the imagewise charge pattern is formed, it is “developed” into a visible image using pigmented or non-pigmented marking particles (generally referred to as “toner particles”) by either using the charge area development (CAD) or the discharge area development (DAD) method that have an opposite charge to the dielectric member and are brought into the vicinity of the dielectric member so as to be attracted to the imagewise charge pattern.
Thereafter, a suitable receiver material (for example, a cut sheet of plain bond paper) is brought into juxtaposition with the toner image developed with the toner particles in accordance with the imagewise charge pattern on the dielectric member, either directly or using an intermediate transfer member. A suitable electric field is applied to transfer the toner particles to the receiver material in the imagewise pattern to form the desired print image on the receiver material. The receiver material is then removed from its operative association with the dielectric member and subjected to suitable heat or pressure or both heat and pressure to permanently fix (also known as fusing) the toner image (containing toner particles) to form the desired image on the receiver material.
Plural toner particle images of, for example, different color toner particles respectively, can be overlaid with multiple toner transfers to the receiver material, followed by fixing of all toner particles to form a multi-color image in the receiver material. Toners that are used in this fashion to prepare multi-color images are generally Cyan (C), Magenta (M), Yellow (Y), and Black (K) toners containing appropriate dyes or pigments to provide the desired colors or tones.
It is also known to use special spot toners to provide additional colors that cannot be obtained by simply mixing the four “primary” toners. An example is a specially designed toner that provides a color spot or pearlescent effect.
With the improved print image quality that is achieved with the more recent electrophotographic technology, print providers and customers alike have been looking for ways to expand the use of images prepared using electrophotography. Printing processes serve not only to reproduce and transmit objective information but also to convey esthetic impressions, for example, for glossy books or pictorial advertizing. A significant problem is posed in the production of metallic hues that are imperfectly reproducible by a color mixture formed from the primary colors and black (such as CMYK noted above). A gold tone is particularly difficult to reproduce by means of such a color mixture. Common metallic pigments are typically conductive and not readily incorporated into toner particles without adversely affecting magnetic, electrical, or electrostatic properties.
Nonetheless, there have been proposals for incorporating metallic components in toner compositions. For example, U.S. Pat. No. 5,180,650 (Sacripante et al.) describes toner compositions that contain lightly colored metallic components such as copper, silver, or gold for example that are provided with an overcoat comprising a metal halide. However, the appearance of images obtained using metal halides can be adversely affected by oxidation (for example tarnishing or toning of metals) promoted by those metal halides making the metallic quality to be unattractive or it disappear completely.
Further, when metallic components are incorporated into toner particles using known manufacturing procedures, the metallic flakes are generally randomly oriented within the particles. This random orientation leads to a loss of metallic hue and causes a dark appearance when such toner particles are fixed (fused) to a receiver material using heated rollers.
More recently, there have been proposals to modify the surface of metallic flakes such that becomes hydrophobic and non-conductive, as described in U.S. Pat. No. 7,326,507 (Schulze-Hagenest et al.). Printing compositions in this publication provide metallic effects in which a metallic pigment is provided with coatings of silicate, titanate, or aluminate and an organic layer, and is then combined with polymeric toner particles. Thus, the metallic pigments are outside the toner particles and can become detached from those toner particles during manufacture or mixing during development, resulting in non-homogeneity in the toner composition that can result in transfer and cleaning problems.
These problems were addressed with porous toner particles that are described in U.S. Patent Application Publication 2011/0262858 (Nair et al.), which porous toner particles comprise encapsulated metallic or metal oxide flakes. Porous toner particles provide certain advantages but may not be useful in every application due to their porosity. Further, such dry toner particles, when prepared by the method described by Nair et al. are formed by coalescence of very small particles. This method limits the largest size that can be achieved for the formation of toner particles containing metallic pigments. It is desirable to not be so limited and to be able to provide larger dry toner particles containing metallic pigments to produce metallic appearance or luster in printed images.
There is a need to further improve metallic toner particles that provide metallic effects in toner images. Bronze and aluminum powders have been used as pigments to provide metallic effects but they do not disperse well in polymeric toner particles. Such pigments are also very fragile and easily broken during extrusion processes used to form polymeric toner particles. These pigments are also generally conductive and can adversely affect the charging abilities of the polymeric toner particles.
Printing processes for providing one or more color toner images are known, but it is also desired that such color toner images, including four-color toner images, be modified with a metallic effect. However, this has not been readily achieved using known metallic toner particles because it has been difficult to introduce metallic particles into known dry toner particles. There are various problems with known processes. For example, it has been difficult to provide suitable metallic effects in toner images because in order to have high reflecting surface area. Many reflective metallic particles are too easily broken into smaller particles during handling or manufacture of toner particles. If the metallic particle size can be maintained, the size of the toner particles must be larger than is normally used in the industry, but larger toner particles are more difficult to fix (fuse) on receiver materials because of the low thermal conductivity associated with larger toner particles.
There is a need to design suitable dry toner particles and a method of using them to provide an unlimited number of metallic effects in four-color toner images.