This invention relates to electrostatic printing methods and compositions and more particularly to an improved ink and liquid developer composition for converting a latent electrostatic image to a visible image, the ink being characterized by constant viscosity with changing temperatures.
Processes for forming latent electrostatic images, existing as electrostatic charge patterns upon a substrate, and for subsequently converting the latent electrostatic image into a visual pattern, are well known. Generally, such electrostatic techniques have been carried out by using toners which are dry powders. However, many techniques have been developed in which the toner particles are suspended in a liquid carrier, and in electrostatic printing wherein latent electrostatic images are formed on a photoconductive surface of a recording element by uniformly charging the surface thereof, as by a corona discharge device, followed by exposure to light in the desired image pattern, such images may also be developed by liquid developers.
Liquid developer compositions for developing electrostatic images generally comprise a dispersion of pigment or toner particles in a volatile, insulating liquid of high dielectric strength and high volume resistivity, generally in excess of 10.sup.9 ohm-cm. The dispersed particles may carry either a positive charge or a negative electrical charge, depending upon their chemical composition, the non-conductivity and the high dielectric strength of the volatile liquid in the liquid developing composition preserves the electrostatic image and permits the deposition of the dispersed toner particles to form a visible image. Generally these liquid developer compositions have been commercially available inks comprising water or oil bases. Other suitable insulating liquids have included aromatic hydrocarbons, such as, benzene, toluene, and xylene; aliphatic hydrocarbons, such as, hexane, cyclohexane, and heptane; freons and halogenated hydrocarbons; and silicone oils.
Liquid toners or developers are described in U.S. Pat. No. 2,907,674, U.S. Pat. No. 2,899,335, U.S. Pat. No. 2,890,911, U.S. Pat. No. 3,135,095, U.S. Pat. No. 3,115,546, and U.S. Pat. No. 3,535,244.
In one type of liquid development the suspended toner particles are electrostatically charged and develop the latent image by migration of the particles to the image surface under influence of the image charge. This is known as electrophoretic development and utilizes the developers having insulating liquids of relatively high volume resistivity.
In another type of electrostatic image development disclosed by Gundlach in U.S. Pat. No. 3,084,043 and U.S. Pat. No. 3,551,146, liquid developers having relatively low viscosity, low volatility, contrast in color in the usual case to the surface on which it will remain, and relatively high electrical conductivity (relatively low volume resistivity), are disclosed for converting the electrostatic latent image to a visible image. According to this method liquid developer from a reservoir is deposited on a gravure roller and fills the depression in the roller surface. Excess developer is removed from the lands between depressions, and as a receiving surface charged in image configuration passes against the gravure roller, the liquid developer is attracted from the depressions in image configuration by the charge. This method of development is referred to as polar liquid development. Typical liquid developers found useful in this mode of development are described by Amidon et al in U.S. Pat. No. 3,806,354 where typical developers comprise by weight from about 1 to about 20 percent dispersant, from about 15 to about 60 percent colorant and from about 40 to about 85 percent total vehicle. Although the liquid developers of Amidon et al are excellent for electrostatographic development of latent images, they encompass inks the viscosities of which have substantial variation with temperature changes. Liquid developer compositions comprising carrier liquid, colorant and polymeric material capable of imparting negative-working characteristics to the toner are described by Mutaffis in U.S. Pat. No. 3,542,681. However, there are no critical limitations in Mutaffis which would be indicative of liquid developer compositions having substantially constant viscosities when changes in temperature occur.