Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-08-12
2002-04-30
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C430S110100, C430S111400, C430S137170, C524S315000, C524S501000, C524S500000, C524S502000, C524S560000, C524S561000, C524S576000, C524S577000, C524S579000, C524S580000, C524S581000, C524S583000, C524S585000, C524S602000, C524S650000, C524S546000
Reexamination Certificate
active
06380297
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for the preparation of polymeric powders suitable for use as electrostatographic toner, and more particularly, to a method for the preparation of polymer particles of controlled shape in which surface active materials are employed for controlling shape of the polymer particles.
BACKGROUND OF THE INVENTION
Polymer particles can be prepared by a process frequently referred to as “limited coalescence”. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al., incorporated herein by reference for all that they contain.
U.S. Pat. No. 5,283,151 is representative of earlier work in this field and describes the use of carnauba wax to achieve non-spherical toner shape. The method comprises the steps of dissolving carnauba wax in ethyl acetate heated to a temperature of at least 75° C. and cooling the solution, so resulting in the precipitation of the wax in the form of very fine needles a few microns in length; recovering the wax needles and mixing them with a polymer material, a solvent and optionally a pigment and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent and washing and drying the resultant product.
However, this technique requires the use of elevated temperature to dissolve the wax in the solvent and cooling the solution to precipitate the wax. The wax does not stay in solution of ethyl acetate at ambient temperature and as a result it is very difficult to scale up using this methodology.
The shapes of the toner particles have a bearing on the electrostatic toner transfer and cleaning properties. Thus, for example, the transfer and cleaning efficiency of toner particles have been found to improve as the sphericity of the particles are reduced. Thus far, workers in the art have long sought to modify the shape of the evaporative limited coalescence type toner particles by means other than the choice of pigment, binder, or charge agent. The shape of the toner particles is modified to enhance the cleaning and transfer properties of the toner. For example, U.S. Ser. No. 09/265,750 now U.S. Pat. No. 6,207,338 seeks to modify the shape of the evaporative limited coalescence type particles by a process in which aluminum or gallium salts are introduced into the aqueous phase of the limited coalescence process in a limited amount prior to homogenization. However, U.S. Ser. No. 09/265,750 now U.S. Pat. No. 6,207,338 shows that when aluminum or gallium salts are added to the aqueous phase of a formulation normally yielding spherical particles, the resulting particle diameter decreases anywhere from 10-30%. For example, Comparative Example IV in U.S. Ser. No. 09/265,750 now U.S. Pat. No. 6,207,388 has a size of 6.7 &mgr;m without the aluminum or gallium salts, while their Inventive Example 7, consisting of the same formulation as Comparative Example IV but with the addition of an aluminum salt, has a size of 4.5 &mgr;m. There is, therefore, need in the art for a method of modifying polymer particle shape without affecting the resulting particle size and size distribution.
SUMMARY OF THE INVENTION
In accordance with the present invention, the prior art limitations are effectively obviated by a novel process in which surface active materials are introduced into the aqueous phase of a limited coalescence process after droplets of solvent with dissolved polymer have been dispersed to predetermined and narrow size distributions in an aqueous phase. The use of this novel process results in the formation of non-spherical polymer particles after the solvent is removed. The particle shape is controlled independently of the particle composition (resin, binder matrix, pigment, charge control agent, etc.) and without effecting the particle size or size distribution as pre-established by limited coalescence droplet formation. The degree of non-sphericity is directly related to the surface active material type and concentration.
Thus, viewed from one aspect, the present invention is directed to a method for the preparation of polymer particles comprising the steps of:
a) forming an organic phase by dissolving a polymer material in a solvent;
b) dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the resultant dispersion;
c) adding a surface active material, and
d) removing the solvent.
Viewed from another aspect, the present invention is directed to a process for preparing electrophotographic toner by dispersing an organic phase in an aqueous phase to yield a layer of particulate suspension stabilizer on the surface of the polymer. The improvement in the process comprises adding the surface active material to the dispersed organic phase droplets, in the aforementioned limited coalescence process, after the step of homogenization, thus modifying the toner's shape, while not affecting the final toner size and size distribution.
It is an advantage of the present invention that elevated temperatures are not needed. It is also an advantage that the toner particle size and size distribution are not adversely affected as they are in the prior art discussed above. This makes it relatively easy to scale up from laboratory flasks to large production equipment.
These and other features and advantages of the present invention will be better understood taken in conjunction with the following detailed description and claims.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, a polymer material, a solvent, and optionally, a pigment dispersion and/or charge control agent are combined to form an organic phase in which the solids consist of between 1 and 50% by weight of the total mixture and the pigment concentration can range from about 4% to 20% by weight of total solids. The pigment dispersion is prepared by conventional techniques as, for example, by media milling, melt dispersion and the like. The charge control agent can be employed in an amount ranging from 0 to 10 parts per hundred, based on the total weight of solids, with a preferred range from 0.2 to 3.0 parts per hundred. This mixture is permitted to stir overnight and then dispersed in an aqueous phase comprising a particulate stabilizer and optionally a promoter.
The solvents chosen for use in the organic phase steps may be selected from among any of the well-known solvents capable of dissolving polymers. Typical of the solvents chosen for this purpose are chloromethane, dichloromethane, ethyl acetate, propyl acetate, vinyl chloride, methylethylketone and the like. Ethyl acetate and propyl acetate are preferred.
The particulate stabilizer selected for use herein may be selected from amon
Ezenyilimba Matthew C.
Smith Dennis E.
Yoon Hichang
Zion Todd C.
Kessler Lawrence P.
Lee Rip A
NexPress Solutions LLC
Wu David W.
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