Method of making carriers having coatings with fillers

#This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions with coated carrier particles prepared by dry powder processes, and wherein supercritical fluids such as carbon dioxide is selected. In embodiments of the present invention, the carrier particles are comprised of a core with coating thereover, and which coating contains a surfactant to, for example, provide contrasting triboelectric carrier charging. In embodiments, the present invention relates to carrier particles and processes thereof, which processes comprise the preparation of polymer like poly(methacrylate) particles by supercritical polymerization in a medium, such as carbon dioxide, and wherein a surfactant, such as substituted polyacrylates, is selected for the reaction mixture. More specifically, in embodiments the present invention relates to the polymerization of a monomer like methyl methacrylate in supercritical carbon dioxide in the presence of a surfactant, and wherein small size carrier coatings, such as submicron to micron polymethylmethacrylate (PMMA) carrier coatings with a size, for example, of from about 0.05 to about 5 microns, and more specifically, from about 0.05 to about 1 micron. The surfactant selected for the aforementioned polymerization is believed to stabilize the particles during polymerization, and such surfactant enables the preselection of the triboelectric charge on the carrier particles comprised of a core and the polymers obtained with the invention processes. The carrier polymers thus can be comprised of a polymer like PMMA, and thereover a controlled amount of contrasting triboelectric surfactant. Moreover, in another aspect of the present invention the carrier particles are prepared by a dry coating process wherein a mixture polymer obtained with covered surfactant is applied to a carrier core enabling insulating particles with relatively constant conductivity parameters; and also wherein the triboelectric charge on the carrier can be varied depending on the coating selected. Specifically, therefore, with the carrier compositions and process of the present invention there can be formulated developers with selected triboelectric charging characteristics and/or conductivity values in a number of different combinations. Developer compositions comprised of the carrier particles prepared by the dry coating process of the present invention are useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging and printing processes. Additionally, developer compositions comprised of substantially insulating carrier particles prepared in accordance with the process of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core. With the processes of the present invention, costly washing and drying steps can be avoided or minimized, environmental concerns such as the discarding of waste solvent can be eliminated or minimized, carrier morphologies can be controlled, and the carrier coating can include conductive components, such as carbon black, metal oxides like tin oxide, and the like therein in an amount, for example, of from about 20 to about 45 weight percent to obtain carrier particles with certain conductivities.

#The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that enable continued constant developed images of high quality and excellent resolution.

#Additionally, carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may be comprised of various cores, including steel, with a coating thereover of fluoropolymers; and terpolymers of styrene, methacrylate, and silane compounds. Many of the commercial carrier coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.

#A high pressure steel reactor charged with monomer was added under nonsupercritical conditions, and then the reactor was pressurized with carbon dioxide to supercritical conditions; the monomer was methyl methacrylate, 25 w/v percent, (the expression "w/v percent" is known and refers to the equivalent of kilograms/Literx 100.TM., so for example, 0.50 kilogram of material in a 1 liter reactor would be a loading of 50 w/v percent; in supercritical reactions the density of the supercritical fluid varies considerably with pressure, unlike water or organic solvents under non-supercritical conditions. This means that the total weight loading of the reactor, which includes the carbon dioxide, is not usually constant but rather depends on the pressure. However, the reactor volume is known, thus w/v percent is selected), VAZO-64.TM. initiator obtained from E. I. DuPont (0.375 w/v percent) and a poly(1,1-dihydrofluorooctylmethacrylate) surfactant (6.25 w/v percent). The mixture was pressurized to approximately 200 (1 bar=1 atmosphere) bar with carbon dioxide and the reactor was heated to raise the temperature to 75.degree. C. These conditions were maintained for 10 hours, after which the reactor was cooled to 25.degree. C. and vented. During the aforementioned polymerization, stirring was maintained at 500 rpm. The product resulting was comprised of PMMA (polymethylmethacrylate) particles, approximately 1 micron in volume average diameter covered with a layer of poly(1,1-dihydrofluorooctylmethacrylate) approximately 0.2 micron thick. Yield of product was 85 to 90 percent. Optionally, the layer of poly(1,1-dihydrofluorooctylmethacrylate) can be removed by repeated flushing of the reactor 4 times with carbon dioxide after the polymerization, and prior to cooling and venting. Removal of the surfactant provides 1 micron diameter PMMA particles without a fluoropolymer layer on the surface. The above prepared product particles are suitable for use as carrier powder coatings in two component developers comprised of toner and carrier for xerographic imaging methods.

#The carrier particles can be prepared by coating about 68,000 grams of a Toniolo atomized steel core, 120 microns in diameter, with 680 grams of the above prepared PMMA/poly(1,1-dihydrofluorooctylmethacrylate), and by mixing these components for 60 minutes in a Munson MX-1 Minimixer, rotating at 27.5 RPM. There resulted uniformly distributed and electrostatically attached, as determined by visual observation, on the carrier core the PMMA/poly(1,1-dihydrofluorooctylmethacrylate). Thereafter, the resulting carrier particles were metered into a rotating tube furnace at a rate of 105 grams/minute. This furnace was maintained at a temperature of 503.degree. F. thereby causing the polymer to melt and fuse to the core.

#A developer composition was then prepared by mixing 97.5 grams of the above prepared carrier particles with 2.5 grams of a toner composition comprised of 92 percent by weight of a styrene n-butylmethacrylate copolymer resin, 58 percent by weight of styrene, 42 percent by weight of n-butylmethacrylate, 10 percent by weight of carbon black, and 2 percent by weight of the charge additive cetyl pyridinium chloride. Thereafter, the triboelectric charge on the carrier particles was determined by the known Faraday Cage process, and there was measured on the carrier a charge of -68.3 microcoulombs per gram. Further, the conductivity of the carrier as determined by forming a 0.1 inch long magnetic brush of the carrier particles, and measuring the conductivity by imposing a 10 volt potential across the brush, was 10.sup.-15 mho-cm.sup.-1. Therefore, these carrier particles were considered insulating.