Emulsion Aggregation Toner Technology

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1 Emulsion Aggregation Toner Technology In my article in this publication in a previous issue, I described the first commercial process for manufacturing chemically prepared toner. In this article I will describe another major manufacturing method, the Emulsion Aggregation method. This is a method of manufacture that has been adopted by several different companies all with slightly different techniques. All methods are patented but have the same fundamental principles. The first company to commercialize such products was Nippon Carbide, manufacturing toner for Samsung/Xerox and Fujitsu. The first production was in 1994 of black toner for monochrome printing. The companies that have followed this first step into commercial production are Konica/Minolta Fuji Xerox/Xerox and Avecia. The following is a description of the process in general. The methods used by individual manufacturers vary. Emulsion Polymerization and Aggregation Emulsion polymerization is a technique used in forming polymers in which monomers are diffused into a micelle where free radical polymerization proceeds with the resulting formation of polymer particles. In emulsion polymerization toner manufacture, other necessary components of toner pigment, charge control agent, wax, etc. cannot internalized into the polymer particles because such materials cannot diffuse into the micelle. If toner particle formation is attempted by direct combination of these components at the emulsion polymerization step, then they will reside on the polymer particle surface where they will affect the dispersion stability of the emulsion polymerization and cause erratic coagulation. At first this might seem to be a disadvantage. However, this method does of course offer the opportunity to separate the pigmentation and polymerization steps. The advantage is that there is no interference by the other toner ingredients with the polymerization process. The ability to adjust and finely control the chemistry of the polymer and the other materials in the toner particle formation step in emulsion aggregation means that the particle size and particle size distribution is more controllable. By contrast in suspension polymerization mechanical forces in mixing during polymerization dominate toner particle formation and hence particle size and distribution.

2 The process outline is shown in Figure 1. The manufacturing method for toner is from two phases. Monomers and surfactants Other Polymerization Ingredients Emulsion Polymerization/Latex Formation Pigment dispersion and other components in aqueous medium Aggregation Coalescence Wash and Dry Additive Blending Figure 1 Latex Formation Firstly the latexes of styrene acrylic are manufactured. A monomer mixture, typically styrene, acrylic ester and acrylic acid are blended in a low speed mixer for an appropriate amount of time to ensure homogeneity. The aqueous medium phase is also prepared in a mixer and contains hot deionized water with an anionic surfactant, initiator and chain transfer agent. In the manufacture of the latex the two phases are then mixed at elevated temperature in a low intensity mixer for a given number of hours. In this process the Primary particles are formed. These are in the form of a latex of non-pigmented emulsion polymerized particles between microns. Dispersion of Colorant and Internal Additives Pigment dispersion of the pigment is prepared by it s dispersion in deionized water with small amounts of dispersing aid, anionic surfactant. Similarly the wax component is dispersed by high shear mixing and applied heating of this material in deionized water with small amounts of dispersing aid, anionic surfactant. Aggregation The mixture of primary particles in their aqueous medium is transferred to a high-speed jacketed mixer and the dispersions colorant and wax and CCA are added. A metal compound coagulant is added to the mixture. The mixture is cooled somewhat and then dispersed for a further time. Secondary particles are formed by the agglomeration of the solids in the aqueous medium containing primary particles, the pigment, wax and CCA. The particle size at this stage of production is microns, typically about 2.5 microns.

3 Coalescence The next step is the further enlargement of the toner particle aggregation. This step is one of further stirring at elevated temperature for a set number of hours. The homogenized mixture is heated with continuous mixing in the reactor and with gradual ramping up of the temperature until it reaches about 90ºC and this is then mixed and held at this temperature for about 4 hours. The particles continue to grow under these conditions. The process of toner particle formation is complete at this point. However, if the toner is to be encapsulated, a shell latex can be added at this stage. This would be when the toner particles are slightly smaller than the desired finished size. When the particles reach the desired size the ph of this aqueous mixture is adjusted to stop the process. Shape adjustment is conducted at this stage by adjustment of the temperature and other conditions. This is illustrated in Figure 2. Increasing the temperature to above the Tg controls the viscosity of the heated polymer and allows interfacial interactions and surface tension to be used to change the particle shape. The particle shape may be changed from irregular to spherical by altering the conditions and stopping the process when the desired shape is achieved. Pigment, Wax CCA, dispersions Aggregate T>Tg Stabilized Styrene Acrylic Latex Irregular Particle T>>Tg Spherical Particle Figure 2 Washing, Filtration, Drying and Dry blending additives The mix is then filtered, washed and dried yielding a pre-toner ready for blending with silica as a flow and charging additive. A variety of methods of washing and drying are used. These are predominantly batch processes with a re-slurry step after each wash and

4 use of copious amounts of deionized water. After drying, surface additives such as fumed silicas are blended in similar fashion to conventional toner preparation. The Advantages of this Technology Emulsion/aggregation is claimed by those companies that developed and use this method of toner preparation to be directed toward achievement of the requirements for future generations of toner because it is said that it enables the maximum flexibility in the design of the toner particles. The demands for the future generations of toner are seen to be enabling high print quality and low contribution of toner cost to the total cost of ownership per page of print applications. High Print Quality Among the major factors affecting print quality in any toner based printing system are the narrowness of the toner particle size distribution, the mean particle size, particle shape, particle surface morphology and toner charge distribution. Particle Size Distribution The narrowness of the toner particle size distribution has an affect on the toner charge distribution and this has an affect on the variation in performance particle to particle in the development process. Hence, the narrower the toner particle size distribution, the more consistent the toner imaging performance will be in image development. With emulsion /aggregation it is possible to achieve a better GSD (pop) than conventionally prepared toner which is significant in terms of imaging performance. The narrowness of particle size distribution, combined with the evenness of shape and homogeneity of emulsion /aggregation toners helps to create a narrow charge distribution. Small Mean Particle Size Small mean particle size toners tend to be more expensive to produce than larger ones when using conventional toner preparation methods. This cost progression tends to be geometric with the reduction in particle size in attrition grinding and conventional classification but in emulsion /aggregation manufacturing there is essentially no relationship between mean particle size and cost. Under the right conditions, there is the potential for an EA toner to provide better print quality and be more competitive in cost than a conventional toner.

5 Mass/Area vs. Particle Diameter mg/sq.cm Particle Diameter (microns) In addition, the enablement of small mean particle size leads to the possibility of reduction in developed toner mass per unit area. This means that the amount of toner used per page is able to be decreased with consequent cost savings in total cost of ownership per page. Toner Particle Shape The shape of the toner affects the toner behaviorally in flowability, charging and adhesion force. The combination of improvements in these attributes determines important performance factors such as transfer efficiency, developability and photoconductor surface cleanability. There are a few methods proposed for the determination and metrication of shape. One such method is to describe the Shape Factor. The shape factor of a toner particle is measured by comparing the square of the maximum length of a particle (ML) to the maximum projected area (A). The formula for shape factor is: Shape Factor (SF) = ((ML) 2 / A) x (π/4) x 100 Thus with highly rounded toners the shape factor is close to 100. With such toners it is possible to achieve very high transfer efficiency rates, in excess of 99%. The adhesion force between a toner and surfaces in the engine, such as photoconductor and intermediate transfer belt, is minimized by the uniform shape and surface of the toner. These properties not only lower adhesion force but also help to create uniform charging properties particle to particle. High levels of transfer efficiency mean that the consumption of toner and level of waste can be minimized. This type of performance translates into high yield and helps to lower the toner element in the cost of printing. However, residual toner after transfer with shape factors of 100 that remains on the photoconductor surface is more difficult to clean using the common blade cleaning technique. In practice the toner manufacturer is able to optimize the toner shape with emulsion/aggregation according to the application.

6 Some SEMs of toner products prepared by Fuji Xerox are shown below in Figure 3. Preferred Shape Factor Range Shape Factor Spherical Potato Popcorn/Bunch of Grapes Irregular Shape Factor Range for Good Transfer Efficiency and Cleanability Improving Blade Cleanability Improving Transfer Efficiency Figure 3 Using their EA (Emulsion Aggregation) technology the pictures from left to right show toners of progressively increasing shape factor from a smooth surfaced spherical toner through to irregular conventionally produced toner. The first four pictures from left to right are emulsion/aggregation toners prepared under varying conditions to adjust the shape. Fuji Xerox preferred shape factor range is 120 to 140 characterized as potato shaped, as this represents the shape toner that has optimum transfer efficiency and cleanability using conventional blade cleaning technology. Toner manufacturing in future will embrace the concepts and methods of manufacturing which best enable the control and manipulation of particle shape and chemically prepared toner manufacturing methods will increase in popularity for this reason.