Field of the Invention
Pigments are widely used as colorants, for example, in paints, varnishes, polymers, toners, coatings, and inks. Such pigments generally have average particle sizes (diameters) in the range of 0.1 to 10 micrometers, more typically, 1 micrometer or greater. To achieve these particle sizes, mechanical devices are most often used to comminute relatively large solid particulate matter into smaller primary particles. The most common mechanical devices include ball mills, attritors, sand/bead mills, and roll mills. All of these devices require moving parts in order to generate the mechanical forces required to break up the pigment particles. Milling times of several hours are typical, with certain pigments requiring a day or longer in order to break up, or comminute, the particles. Moreover, comminution of the pigment by contact with the milling media results in pigment surfaces exhibiting a high number of surface asperities, a large variation in particle size, and significant variation in particles and their properties from batch to batch. Furthermore, contamination of the dispersions from the mechanical parts of the milling equipment can result due to the intimate contact of the pigment with the milling media. Silicon dioxide, a grinding medium, is a common contaminant found after sand milling, for example.
There is an inherent disadvantage of mechanical processing of pigments in the resulting fact of the large breadth of distribution of particle sizes resulting from mechanical processes. This results in the presence of particles having diameters of one micrometer or greater, even in dispersions where the average particle size is significantly less. For dispersions requiring transparency in the final article, these larger particles lead to unwanted light scattering and are detrimental. The presence of these micrometer sized particles also leads to an inherent instability, or tendency to flocculate, in the dispersions. For a more general description of present limitations in dispersing solids in liquids, see D. J. Walbridge, Solid/liquid Dispersions, Th. F. Tadros, Ed., Academic Press, 1987, p. 50. Additionally, the uncontrolled variation in particle sizing affects the performance of particles both within a single batch and between batches. As the specifications for ingredients becomes more stringent, the failure to control the parameters of particle products is a significant problem that may lead to waste and product refusal.
More stable pigment dispersions can be obtained by chemically altering the process for manufacturing pigment and the resulting pigments as described in EP 1,544,839. This type of process often results in smaller average particle diameters but has the disadvantages of requiring a chemical pretreatment of the pigment, still requires mechanical milling, and still provides a dispersion having a wide particle size distribution.
Dry organic pigments, such as for example members of the phthalocyanine family, have been generated by evaporative techniques. Wagner et al., J. Matls. Sci., 17, 2781 (1982), describes a train sublimator for purification of pigments in the phthalocyanine family with the main purpose of removing impurities present in the as-supplied pigments so as to allow preparation of ultrapure pigment thin films for photovoltaic cells. The only reference to particle size of the purified pigments is for vanadyl phthalocyanine, in which the particle size of the sublimed pigment, even after extensive milling (16 days), could not be reduced below 2 micrometers. A further reduction in particle size was obtained after acid pasting, a technique commonly used to generate pigment grade phthalocyanines (see R. Lambourne, “Paint and Surface Coatings”, John Wiley and Sons (1987) p. 159).
EPA 209403 (Toyatoma) describes a process for preparing dry ultrafine particles of organic compounds using a gas evaporation method. The ultrafine particles, having increased hydrophilicity, are taught to be dispersible in aqueous media. Particle sizes obtained are from 500 Angstroms to 4 micrometers. These particles are dispersed by ultrasound to provide mechanical energy that breaks up aggregates, a practice that in itself is known in the art. The resulting dispersions have improved stability towards flocculation.
Kimura and Bandow, Bull. Chem. Soc. Japan, 56, 3578 (1983) disclose the non-mechanical dispersing of fine metal particles. This method for prepares colloidal metal dispersions in nonaqueous media also uses a gas evaporation technique. General references by C. Hayashi on ultrafine metal particles and the gas evaporation technique can be found in Physics Today, December 1987, p. 44 and J. Vac. Sci. and Tech., A5, p. 1375 (1987).
Numerous references have appeared describing use of the gas evaporation technique to produce ultrafine metal powders, especially magnetic metal/metal oxide powders (often referred to as magnetic pigments). These appear to exclusively refer to a dry process and do not involve contact with liquids. Yatsuya et al., Jpn. J. Appl. Phys., 13, 749 (1974), involves evaporation of metals onto a thin film of a hydrocarbon oil (VEROS technique) and is similar to Kimura (supra). Nakatani et al., J. Magn. Magn. Mater., 65, 261 (1987), describe a process in which surface active agents stabilize a dispersion of a ferromagnetic metal (Fe, Co, or Ni) vaporized directly into a hydrocarbon oil to give a ferrofluid using a metal atom technique. The metal atom technique requires high vacuum (pressures less than 10−3 torr) such that discrete metal atoms impinge onto the surface of a dispersing medium before the metal atoms have a chance to contact a second species in the gas phase. In this metal atom process, nucleation and particle growth occur in the dispersing medium, not in the gas phase. Thus, particle size is dependent on the dispersing medium and is not easily controlled. Additionally, U.S. Pat. No. 4,576,725 describes a process for making magnetic fluids which involves vaporization of a ferromagnetic metal, adiabatic expansion of the metal vapor and an inert gas through a cooling nozzle to condense the metal and form small metal particles, and impingement of the particles at high velocity onto the surface of a base liquid.
Other references for dispersing materials that are delivered to a dispersing medium by means of a gas stream include U.S. Pat. No. 1,509,824, which describes introduction of a molecularly dispersed material, generated either by vaporization or atomization, from a pressurized gas stream into a liquid medium such that condensation of the dispersed material occurs in the liquid. Therefore, particle growth occurs in the dispersing medium, not in the gas phase, as described above. Furthermore, the examples given are all materials in their elemental form and all of which have appreciable vapor pressures at room temperature.
UK Patent 736,590 describes a process in which a finely divided pigment is carried by a gas stream and wetted by a liquid miscible with the final dispersing medium. The finely divided pigment is suspended in the gas stream during these steps, and then the wetted pigment is mixed with the dispersing medium. This method requires a pulverizer to first subject the pigment to mechanical forces prior to its introduction into the gas stream. Therefore, it suffers from many of the shortcomings cited above. Particle sizes on the order of 1 micrometer are obtained.
U.S. Pat. No. 4,104,276 discloses the conversion of crude copper phthalocyanine into a pigmentary form by introducing milled copper phthalocyanine into an organic or aqueous medium together with a basic copper phthalocyanine of specified formula.
Pigmented water-absorbable plastic materials, including contact lenses, are disclosed in U.S. Pat. No. 4,638,025 to contain an organic binder, a pigment, a hydrophilic polymerizable plastic material and a crosslinking agent. A binder such as cellulose acetate butyrate is required to keep the pigment in a dispersed form.
Contact lenses prepared from hydroxyethyl methacrylate containing copper phthalocyanine (commercially available and then milled) as a colorant are disclosed in U.S. Pat. No. 4,252,421.
U.S. Pat. No. 5,030,669 describes a method consisting essentially of the steps: (a) vaporizing a nonelemental pigment or precursor to a nonelemental pigment in the presence of a nonreactive gas stream to provide ultrafine nonelemental pigment particles or precursor to nonelemental pigment particles; (b) when precursor particles to a nonelemental pigment are present, providing a second gas capable of reacting with the ultrafine precursor particles to a nonelemental pigment and reacting the second gas with the ultrafine precursor particles to a nonelemental pigment to provide ultrafine nonelemental pigment particles; (c) transporting the ultrafine nonelemental pigment particles in said gas stream to a dispersing medium, to provide a dispersion of nonelemental pigment particles in the medium, the particles having an average diameter size of less than 0.1 micrometer; wherein the method takes place in a reactor under subatmospheric pressure in the range of 0.001 to 300 torr.
U.S. Pat. No. 5,106,533 provides a nonaqueous dispersion comprising pigment particles having an average size (diameter) of less than 0.1 micrometer dispersed in an organic medium. That invention provides an aqueous dispersion comprising certain classes of inorganic pigment particles having an average size (diameter) of less than 0.1 micrometer dispersed in a water or water-containing medium. The dispersions require less time for preparation, are more stable, have a more uniform size distribution, a smaller number average particle diameter, fewer surface asperities, and avoid contamination of dispersed material due to the presence of milling media and the wear of mechanical parts, these problems having been noted above for dispersions prepared by conventional methods employing mechanical grinding of particulates. Additionally, no chemical pretreatment of the pigment is required in order to achieve the fine particle sizes obtained in the final dispersion. The pigments of the dispersions are found to have narrower size distributions (standard deviations generally being in the range of ±0.5x, where x is the mean number average particle diameter), are more resistant to flocculation (i.e. the dispersions are stable, that is they are substantially free of settled particles, that is, no more than 10% of the particles settle out for at least 12 hours at 25° C.), and demonstrate superior overall stability and color as demonstrated by lack of turbidity, by increased transparency, and by greater tinctorial strength, compared to mechanically dispersed pigment dispersions. Furthermore, the method requires no mechanical energy, such as ultrasound, to break up aggregates. Aggregates do not form since there is no isolation of dry ultrafine pigment particles prior to contacting the dispersing medium. The dispersions of any organic or inorganic pigment or dispersion that can be generated from a pigment precursor, are prepared by a gas evaporation technique which generates ultrafine pigment particles. Bulk pigment is heated under reduced pressure until vaporization occurs. The pigment vaporizes in the presence of a gas stream wherein the gas preferably is inert (nonreactive), although any gas which does not react with the pigment may be used. The ultrafine pigment particles are transported to a liquid dispersing medium by the gas stream and deposited therein by bubbling the gas stream into or impinging the gas stream onto the dispersing medium.
U.S. Pat. No. 6,267,942 describes a process for manufacture of spherical silica particles. Silica gel particles to be dispersed in a mixed solution of an alkali silicate and an acid are required to have an average particle size of from 0.05 to 3.0 micrometers. In a case where the average particle size of the silica gel particles is smaller than 0.05 micrometers, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. Similarly, in a case where the average particle size of the silica gel particles is larger than 3.0 micrometers, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. The more preferred range of the average particle size of the silica gel particles is from 0.1 to 1.0 micrometers.
U.S. Pat. No. 5,958,329 describes a method and apparatus for producing nanoparticles (there defined as from 1 to 50 nano-meter diameter particles) at a high rate. Two chambers are separated by a narrow duct. A source material is provided from a lower chamber where the source material is heated (e.g., to vaporization and then continuously fed into an upper chamber. In the upper chamber, nanoparticles are nucleated, the nanoparticles being formed when the vapor fed from the lower chamber collides with a gas (inert or reactive) in the upper chamber. A cooled deposit site (e.g., defined as finger 107) collects the particles, which are then scraped from the collection site. The particles are said to move to the collection site in a natural connective flow stream.
U.S. Pat. No. 5,128,081 describes a method of preferential phase separation of aluminum oxide nanocrystalline ceramic material. The nanoparticles are collected on a cold surface (20). Following oxidation of the particles, a vacuum chamber (in which the particles were formed) is evacuated and the oxide particles are collected and consolidated under various atmospheric conditions, such as vacuum and selectively with oxygen and/or air.
The collection process in these particle manufacturing and particle treating processes is cumbersome, inefficient, costly, time-consuming and damaging to the particles. For the collection process, the chamber must be opened and particles scraped from the deposition surface. This requires a long term shut down of the system. Scraping of particles from the deposition surface will fracture some particles and leave others agglomerated. Scraping can also damage the deposition surface. The small elongate finger deposition surface allows for the production and collection of only small amounts of materials layering of collected particles reduces the efficiency of deposition onto the surface. Coating and surface treatment of the particles can be done, but only as re-dispersion of the dried and agglomerated particles.
An alternative method of particle collection is filtration. This is performed by placing in sequence a source of particles, a filtration medium and a vacuum source. The filter has two surfaces, one front surface facing the particle source and the other rear surface facing the vacuum source. The reduced pressure at the rear surface allows the higher pressure at the font surface to push gas and particles against the filter where the particles are entrapped. There are a number of problems in a filtration system, particularly when it is used with nanoparticles. For example, to collect nanoparticles having an average particles diameter of from 1 to 100 nanometers, the largest pore size in the filter must be less than about 1 nanometer. It is difficult to maintain an effective pressure across that filtration surface, even before particles start collecting. As nanoparticles collect on the filter surface, gas flow (and pressure driven movement) become more restricted, fewer particles can collect, and process efficiency diminishes. The particles clog pores rapidly and particles do not collect efficiently.
U.S. Pat. No. 5,857,840 describes a vacuum pump system for making a closed container vacuous, comprising a vacuum pump and a dust collector provided on a pipe connecting the closed container and the vacuum pump, the pipe including:                a main pipe having a first main pipe which connects the closed container and the collector and        a second main pipe which connects the centrifugal collector and the vacuum pump;                    a bifurcated pipe which is branched out from the first main pipe and connected to the vacuum pump;            a metal mesh dust collector disposed on the bifurcated pipe; and            pipe switching means for switching over between the main pipe and a bifurcated pipe.The dust collector is provided intermediate the source of dust and vacuum pump, which may include a dry pump.                        
U.S. Pat. No. 6,050,787 provides a dry pump comprising a magnetically responsive elastic tube stretched onto, thereby sealing to, a shaft with inlet and outlet ports at or adjacent to it's ends of the tube. Local to the inlet port a magnetic field is generated in the enclosing body. This field is substantially concentric to the tube, which then responds by expanding circumferentially towards the magnetic field. This creates a volume between the tube and shaft, the length of tube outside the influence of the magnetic field remains sealed upon the shaft. Subsequent movement of the magnetic field along the axis of the pump gives transport of this volume and any media now enclosed within it from the inlet port to the outlet port, whereupon reduction of the magnetic field results in exhaustion of the volume. This cycle results in pumping action.