In many applications for polymer particles it is very important for such particles to be reasonably small and have a narrow size distribution. Such polymer particles are very useful matting agents, opacifying agents and spacers, particularly in photographic and electrostatographic materials such as toners and carriers. For example, U.S. Pat. Nos. 4,994,312 and 5,055,371 describe the use of small polymer particles to provide voids in polymeric shaped articles such as polyester sheets. Such sheets exhibit unique properties, including texture, opacity and whiteness which make them particularly suitable photographic supports and receiver sheets for electrostatic toner images.
The preparation of polymeric particles having controlled average particle size and narrow size distribution by a technique that has become known as "limited coalescence" is described in several patents including, for example, the aforementioned U.S. Pat. Nos. 4,994,312 and 5,055,371 and U.S. Pat. Nos . 2,932,629; 2,934,530; 4,833,060; 4,835,084 and 4,965,131.
In the limited coalescence method, as illustrated by the aforementioned patents, polymer particles are prepared by forming an aqueous suspension of polymer droplets in a medium containing small particles of a solid stabilizer to form droplets having a layer of such solid stabilizer particles on their surfaces (often referred to as an oil phase) in the aqueous medium (aqueous phase), forming solid polymer particles from the droplets and recovering the polymer particles. During the course of the process, coalescence of the oil (discontinuous) phase takes place to form larger size droplets. These droplets are limited in size by the presence of the particles of the solid stabilizer (often referred to as a suspension stabilizer or suspension stabilizing agent) in the aqueous (continuous) phase. The solid stabilizer particles limit coalescence from taking place by a mechanism that is generally believed to be a physical phenomenon of preventing, by physical separation, one droplet from wetting another and thereby joining together to form a larger droplet. Solid polymer particles that are covered with a layer of smaller stabilizer particles are formed from the droplets and recovered. For some applications of the polymer particles such as in some electrostatographic toners the smaller stabilizer particles are removed from the surfaces of the polymer particles, as illustrated by U.S. Pat. Nos. 4,833,060 and 4,835,084. In other applications such as their use in photographic supports or in electrostatographic receivers, the presence of such stabilizer particles on the surfaces of the polymer particles is desirable. See, for example, U.S. Pat. Nos. 4,994,312 and 5,055,371.
A problem that can occur in a limited coalescence method of making polymer particles of controlled size and size distribution, and which is particularly troublesome with polymer particles having an average particle size of less than 5 .mu.m, is that such particles tend to agglomerate and form clumps of particles. The formation of such agglomerates defeats the purpose of utilizing the limited coalescence method in the first place to achieve small particles and a narrow particle size distribution,
In conventional prior art limited coalescence methods, it is accepted practice to separate the polymer particles from the aqueous reaction medium in which they are prepared, for example, by collection on a fine mesh screen or filter, and then wash the separated particles, i.e., the filter cake, with water prior to drying. Suitable washing techniques include rinsing or spraying the filter cake with water, which is commonly known as displacement washing, or dispersing the filter cake in water, which is commonly known as reslurry washing. Although such washing methods are usually acceptable for larger polymer particles, for example those having an average particle size of 5 .mu.m or more, for example, 5, 10 or 25 .mu.m, undesirable particle agglomerates form when these methods are used with smaller size polymer particles such as those having an average particle size of 5 .mu.m or less. In earlier work conducted prior to this invention, we found that using methanol as the wash liquid to displacement wash polymer particles prepared in a limited coalescence method improved particle separation, provided that polymer particles had an average particle size of 4 .mu.m or more. This improved method has been used successfully by us for several years to provide free-flowing polymer particles having an average particle size of 4 .mu.m or more. Unfortunately, even this improved method is not effective to avoid the formation of polymer particle agglomerates when the average particle size of the polymer particles is 3 .mu.m or less. Such small particle agglomerates require further processing such as milling and classification procedures that add to processing times and cost. Polymer particles having an average particle size of 3 .mu.m or less are useful for all of the purposes discussed previously herein, but they are particularly useful in photographic supports of the type described in U.S. Pat. No. 4,994,312 where they provide a relatively smooth surface which improves handling characteristics of the supports.
The problem of this invention is to provide a method of making polymer particles having an average particle size of 3 .mu.m or less using a limited coalescence method which avoids or substantially eliminates the formation of polymer particle agglomerates or clumps upon drying.