Abstract:
A pigment dispersion process includes providing a solution in a dispersion apparatus tha includes a container, a rotor-stator type homogenizer and a recirculation device. The process includes adding pigment particles to the solution to form a dispersion, flowing at least a portion of the dispersion to the homogenizer via the recirculation device, reducing a size of the pigment particles using the homogenizer, and recirculating the dispersion between the container and the homogenizer via the recirculation device. The dispersion system may include a container containing a dispersion that may contain an amount of deionized water, an amount of surfactant and an amount of pigment, and a recirculation device coupled to the container and to a rotor-stator homogenizer that allows the dispersion to flow between the container and the homogenizer.

Description:
BACKGROUND  
       [0001]     Toners, for use in printers, copiers, and the like, may be prepared by existing mechanical reduction processes, such as a conventional styrene acrylate copolymer based toner process. In such a process, the copolymer resin is melt kneaded or extruded with a pigment, pulverized and classified to provide toner particles of the desired volume average particle diameter and size distribution.  
         [0002]     As an improvement to the foregoing mechanical reduction processes, other processes are known in which toner is achieved via aggregation as opposed to particle size reduction. For example, in chemical aggregation processes, toner may be formed chemically in situ and does not require known pulverization methods. Chemical aggregation processes typically involve the formation of an emulsion latex of the resin particles, in which particles have a small size of, for example, from about 5 nanometers to about 500 nanometers in diameter, by heating the resin in water, or by making a latex in water using an emulsion polymerization method. A colorant dispersion of a pigment dispersed in water may also be separately formed. The colorant dispersion is added to the emulsion latex mixture, and an aggregating or complexing agent is then added to form aggregated toner particles. The aggregated toner particles are then heated to enable coalescence or fusing, thereby achieving aggregated, fused toner particles.  
         [0003]     The pigment dispersion is an important component in the preparation of chemically aggregated toner. In order for the pigment particles to form aggregates with the latex particles, the pigment particles should have size smaller or at least a size comparable to the latex particles, preferably between about 5 and 300 nanometers in diameter, and more preferably between about 5 and 150 nanometers in diameter. There are several well-known methods to prepare pigment dispersions with a particle size less than about 150 nanometers in diameter. For example, U.S. Pat. No. 5,026,427 teaches the use of a liquid jet interaction apparatus, such as a microfluidizer, to prepare pigment dispersion for use in ink jet inks. U.S. Pat. No. 5,085,698 teaches the preparation of pigment dispersions using a media mill, a ball mill, an attritor, or a liquid jet interaction apparatus. However, media mill and ball mill are known to generate contaminants from the media-media impaction, and the liquid jet interaction apparatus is prone to mechanical breakdown as a result of high application pressure.  
         [0004]     Rotor-stator type homogenizers have been widely used to prepare emulsions and dispersions. However, the particle size achievable with rotor-stator homogenizers is not as small as those with media mills or high pressure homogenizers equipped with homogenizing valves or liquid jet interaction chambers.  
         [0005]     A recirculation mode of operating a rotor-stator type of homogenizer can be illustrated by  FIG. 1 . In  FIG. 1 , a vessel  2  is operatively coupled to a mixer  6  via a liquid circulation network  8  that may include a mechanical pump to facilitate liquid circulation. The mixer  6  is generally a container that contains a rotating stirring device. The vessel  2  initially contains a mixture of pigment particles dispersed in a solvent such as, for example, water, and possibly a surfactant is equipped with a stirring device  10  (depicted as agitator blades) and a valve  12 . During operation, the valve  12  is opened to allow a portion of the mixture to flow from the vessel  2  to the mixer  6 . The post-mixer circulating composition returns to the vessel  2  via the liquid circulation network  8 . The mixer may be an in-line rotor-stator type homogenizer with a set of rotor-stator elements that may be arranged to provide more intensive mixing. The rotor-stator elements may be provided as supercoarse, coarse, medium, fine, superfine and neutral. The rotor-stator elements may provide more intensive mixing as they become finer. A combination of rotor-stator elements may be used. For instance, in the order of entrance into the homogenizer, the combination may be coarse, fine and superfine rotor-stator elements.  
       SUMMARY  
       [0006]     Particle reduction processes are currently available, such as, for example, media mills, micro-fluidization or high-pressure homogenization. However, these processes have limitations in capacity, can introduce unnecessary contaminants, have generally a high capital cost, or may be prone to mechanical breakdown.  
         [0007]     Exemplary embodiments of methods provide a solution in a dispersion apparatus that includes a container and a rotor-stator type homogenizer that are operatively coupled via a recirculation device. The process may include adding pigment particles to the solution to form a dispersion, flowing the dispersion to the rotor-stator type homogenizer via the recirculation device, reducing a pigment particle size of the dispersion using the homogenizer, and recirculating the dispersion having reduced pigment particle size via the recirculation device.  
         [0008]     Also, various exemplary embodiments of systems provide a solution that includes a container containing an amount of deionized water and an amount of surfactant stirred by a stirrer, an amount of pigment added to the water and the surfactant to form a dispersion, and a recirculation device operatively coupled to the container and to a rotor-stator homogenizer that allows the dispersion to flow between the container and the homogenizer repeatedly.  
         [0009]     Finally, various exemplary embodiments provide a device for preparing a pigment dispersion system that includes a device for providing a solution in a dispersion apparatus that comprises a container, a rotor-stator type homogenizer and a recirculation device, a device for adding a pigment to the solution to form a dispersion, a device for flowing at least a portion of the dispersion to the homogenizer via the recirculation device, a device for reducing a size of the pigment particles using the homogenizer, and a device for recirculating the dispersion between the container and the homogenizer via the recirculation device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Various exemplary embodiments of the systems and methods of this invention will be described in detail, with reference to the following figures, wherein:  
         [0011]      FIG. 1  illustrates a related art equipment for manufacturing a pigment dispersion;  
         [0012]      FIG. 2  is an illustration of an exemplary system for preparing a pigment dispersion;  
         [0013]      FIG. 3  is a plot illustrating the particle size as a function of the solid pigment concentration, produced with an in-line homogenizer;  
         [0014]      FIG. 4  is a flow chart illustrating an exemplary method of preparing a pigment dispersion; and  
         [0015]      FIG. 5  is a plot illustrating the particle size obtained for a dispersion produced with an in-line homogenizer and with a batch process. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0016]     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments.  
         [0017]      FIG. 2  is an illustration of an exemplary system  100  for preparing a pigment dispersion. In  FIG. 2 , a first container  110  can contain a solution that may be stirred by a stirring device  120  such as, for example, a stirring blade. The first container  110  is connected to a second container  140  via a recirculation device  130  such as, for example, a tube. The second container  140  contains a rotor-stator type homogenizer  170  that is composed of a rotor  160  and a stator  150 .  
         [0018]     During operation, a solution containing a solvent such as, for example, water, and a surfactant, may be mixed in the first container  110  using the stirring device  120 . Also, a pigment may be added to the first container  110  while the solution is being stirred to form a dispersion. As shown, the first container  110  may be operatively coupled to a second container  140  via a recirculation device  130 . A valve may be located at a portion of the liquid circulation network  130  to regulate the flow of the dispersion to the second container  140  or through the recirculation device  130 . The liquid circulation network  130  may include a tube that allows the dispersion to flow through the network  130 . The second container  140  may contain a rotor-stator homogenizer  170  composed of a stator  150  and a rotor  160 , and the dispersion may flow through the liquid circulation network  130  to the second container  140 .  
         [0019]     According to various exemplary embodiments, the dispersion is flowed to the rotor-stator homogenizer  170 . For example, the dispersion may be flowed in the space between the rotor  160  and the stator  150 , where the dispersion may be further subjected to the action of the rotor-stator homogenizer  170 . When in the rotor-stator homogenizer  170 , the pigment particles in the dispersion may be further reduced in size because of the action of the rotor-stator homogenizer  170 . According to various exemplary embodiments, the pigment particle size may be reduced to below 150 nm.  
         [0020]     After the dispersion passes through the rotor-stator type homogenizer  170 , the dispersion may be flowed back to the first container  110  via the liquid circulation network  130 . Accordingly, the dispersion may be further stirred by the stirring device  120 , and may repetitively be flowed back via the liquid circulation network  130  to the second container  140  and the rotor-stator homogenizer  170 , where the dispersion may be further subjected to the action of the rotor-stator homogenizer  170 . Also, the dispersion coming out of the rotor-stator type homogenizer  170  may be flowed to another container for further processing that is different from the first container  110 , before being recirculated back again into the second container  140  to be subjected further to the action of the rotor-stator type homogenizer  170 .  
         [0021]     During operation, a solution that may contain water and a surfactant may be provided in a first container  110 . The solution may include deionized water, and the surfactant may be, for example, Neogen R-K. The solution may be mixed in the first container  110 . For example, 776 g of deionized water and 24 g of Neogen R-K may be mixed until Neogen R-K is completely dissolved. A pigment that includes pigment particles such as, for example, carbon black Regal 330, may be added to the solution to form a dispersion. Also, the dispersion may then be flowed to the rotor-stator homogenizer  170  enclosed in the second container  140  via the recirculation device  130  where it may be subjected to the action of the rotor-stator type homogenizer  170 . The dispersion may be subjected to the action of the rotor-stator homogenizer  170  for a period of up to about 75 minutes. Also, the rotor-stator homogenizer  170  may be rotating at a speed of about 7000 revolutions per minute (rpm) to reduce pigment particle size.  
         [0022]     Furthermore, the dispersion that has been subjected to the action of the rotor-stator homogenizer may be flowed back to the first container  110  or to another container different than the first container  110 , and then flowed back again to the rotor-stator homogenizer  170  via the recirculation device  130 . According to various exemplary embodiments, a recirculation loop may be set up by having a discharge outlet in the second container  140  that contains the rotor-stator homogenizer  170 . Pipes may be connected between the discharge outlet of the second container  140  and the first container  110  via the recirculation device  130 . The first container  110  may be connected to the second container  140  in such a way that the dispersion in the homogenizer  170  may flow to the first container  110  and back to the homogenizer  170  in a substantially continuous manner. Pipes may also be connected between the discharge outlet of the second container  140  and another container different than the first container  110  via the recirculation device  130 . The recirculation of the dispersion back to the rotor-stator type homogenizer  170  allows the homogenizer to further reduce the size of the pigment particles dispersed in the dispersion each time the dispersion is recirculated in the homogenizer  170 . According to various exemplary embodiments, the size of the pigment particles may be reduced to below 150 nm.  
         [0023]     According to various exemplary embodiments, the amount of solid pigment mixed in the solution to form the dispersion may be 20% or more based on the combined total weight. A smaller particle size of the solid pigment is generally obtained with higher concentrations of solid pigment in the dispersion as illustrated in  FIG. 3 .  FIG. 3  is a plot illustrating the particle size as a function of the solid pigment concentration, produced with an in-line homogenizer. Therefore, a method is provided here where the final pigment particle size can be controlled by the pigment concentration. For the smallest possible pigment particle size, pigment concentration greater than 35% by weight is preferred. Very small pigment particles may be obtained by operating the rotor-stator type homogenizer in a recirculation mode, and by formulating the pigment dispersion with high pigment concentrations.  
         [0024]     Because there may be some heating in the second container  140  caused by the homogenizer, and the water temperature may be increased by more than 5° C. in the second container  140  when the homogenizer is on. Cascade control may be used to control the reactor temperature and prevent any excessive heating.  
         [0025]     According to various exemplary embodiments, various suitable pigments may be employed in dispersions of the present invention, including, but not limited to, carbon black, such as REGAL 330 carbon black, acetylene black, lamp black, aniline black, Chrome Yellow, Zinc Yellow, SICOFAST Yellow, SUNBRITE Yellow, NOVAPERM Yellow, Chrome Orange, Cadmium Red, LITHOL Scarlet, HOSTAPERM Red, FANAL PINK, HOSTAPERM Pink, LITHOL Red, RHODAMINE Lake B, Brilliant Carmine, HELIOGEN Blue, HOSTAPERM Blue, PV Fast Blue, CINQUASIA Green, HOSTAPERM Green, and mixtures thereof.  
         [0026]     Illustrative examples of suitable known surfactants or stabilizers selected for the process of the present invention include alkyl sulphates such as sodium dodecyl sulphate and sodium laural sulphate, alkyl benzene sulphonates such as sodium dodecylbenzene sulphonate, commercially known as Neogen R-K, Rhodacal DS-10 and Taycapower BN2060, etc., alkyl phenyloxide sulphonates such as sodium dodecylphenyloxide sulphonate, and the like. The concentration of surfactant in the aqueous phase depends on the type of surfactant and the pigment. A typical surfactant to pigment weight ratio may range from about 3% to 30%, although ratios outside of this range are also possible.  
         [0027]      FIG. 4  is a flow chart illustrating an exemplary method of preparing a pigment dispersion. In  FIG. 4 , the method starts in step S 100 , and continues to step S 110 , in which a solution, for example, surfactant dissolved in water, may be provided. According to various exemplary embodiments, the solution may be stirred. Next, control continues to step S 120 , in which a pigment may be added to the solution to form a dispersion. Next, control continues to step S 130 , in which the dispersion may be flowed to a rotor-stator homogenizer. The solution may be flowed to the rotor-stator homogenizer before the pigment is added to the solution to form the dispersion. Next, control continues to step S 140 , in which the action of the rotor-stator homogenizer on the dispersion results in a reduction in the size of the pigment particles. Next, control continues to step S 150 , in which the dispersion, after being subjected to the action of the homogenizer and after coming out of the homogenizer, is recirculated back into the homogenizer to be further subjected to the action of the homogenizer. Next, control continues to step S 160 , where the method ends.  
         [0028]      FIG. 5  is a plot that compares the particle size obtained for a dispersion produced by an in-line homogenizer versus a dispersion produced by a batch process. In  FIG. 5 , the measured pigment particle size, in terms of the volume median diameter d 3,50 , is plotted as a function of dispersion time, for both the batch process and the inline process. The batch process was performed using a two-liter beaker with 776 g of deionized water and 24 g of surfactant Neogen R-K. The content of the beaker was stirred until the surfactant was completely dissolved in the water to give a clear solution. Then 200 g of carbon black Regal 330 was added to the beaker and a spatula was used to incorporate the carbon black powder to the solution. The beaker was then placed under a batch rotor-stator type homogenizer at a rotational speed of 7000 revolutions per minute (rpm) for 75 minutes. Samples were drawn from the dispersion every 15 minutes. The plot in  FIG. 5  shows the measured particle diameter obtained by batch homogenization, plotted as squares, as a function of homogenization time.  
         [0029]     For the inline homogenization process, a recirculation loop was added to an in-line homogenizer, and a two-liter beaker with a bottom discharge outlet. Plastic tubings were connected in such a fashion that liquid in the beaker would pass through the inline homogenizer repeatedly. Using another two-liter beaker, 776 g of deionized water and 24 g of Neogen R-K surfactant were mixed until the surfactant completely dissolved. The solution was then added to the beaker and the inline homogenizer turned on. Then, 200 g of carbon black Regal 330 was slowly added to the beaker, and the rotational speed of the homogenizer slowly increased to 7000 rpm for 75 minutes. samples were drawn every 15 minutes. It is clear from  FIG. 5  that the in-line homogenizer produces smaller pigment particles compared to the batch process.  
         [0030]     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.