Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
2000-10-18
2002-04-02
Acquah, Samuel A. (Department: 1711)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C528S287000, C528S295000, C528S298000, C528S300000, C528S302000, C528S307000, C528S308000, C528S308600, C528S491000, C528S503000, C524S081000, C524S474000, C524S801000, C524S845000, C524S904000, C524S908000
Reexamination Certificate
active
06365315
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to manufacturing processes used to make spherical polyester particles. The process can be exactly adjusted to pick a mean particle size anywhere in the range of from 1 to 200 &mgr;m, and can produce particles of very narrow particle size distribution. Particles made by this process have utility in a variety of applications, including applications as powder coatings and as particles suitable for application for toners and use in high resolution electrophotography.
Polymer particles are traditionally prepared by subjecting resin and additive components to intense mixing in an extruder at a temperature above the softening point of the film-forming polymer and then, by means of a milling process bringing the resulting extrudate to a particle form. For applications in powder coating, for example, irregularly shaped powders of particle sizes ranging from 20 to 100 &mgr;m are typically achieved. For applications as toners in electrophotography, powders with a mean particle size of from 7 to 20 &mgr;m are typically achieved. The milling process to make particles such as this has a number of deficiencies:
1. It leads to powders of irregular structure and broad particle size distribution,
2. It routinely produces significant amounts of oversized and undersized particles (fines), which can result in significant material loss and process expense due to sieving,
3. The irregular shape of the particles plus the broad particle size distribution can lead to less than ideal flow and charge behavior of the particles.
4. There is added expense associated with production of a finished resin and, in a separate step, conducting a milling or other particle generating operation.
To make toner particles suitable for high resolution laser printers and copy machines, for example, 1200 dot per inch resolution, it is necessary to have a particle with a size of 5 &mgr;m to meet these resolution requirements. Existing milling technology will make particles of this size only with considerable effort and waste, and the processing and economic problems mentioned above with irregular particle structure and broad particle size distribution are exacerbated as the size of the particle is reduced.
The breadth of a particle size distribution is characterized using not only the parameter d
50
, for which just 50% of the particles are greater than or smaller than the value d
50
, but also two further parameters: d
10
designates the particle size for which 10% of the particles are smaller than this value. Correspondingly, d
90
designates the particle size for which 90% of the particles are finer than the value d
90
. To characterize the breadth of a particle size distribution it is usual to form a quotient which is referred to as the span and is calculated in accordance with the following formula: span=(d
90
−d
10
)/d
50
. The relationship is thus: the smaller the span the narrower the particle size distribution. A powder comprising spheres identical in size would have a span of 0. For milled powders with a mean particle size d
50
of 50 &mgr;m, for example, a span of 3-4 is typically obtained.
It is also desirable, on the basis of economic considerations, to have processes for the manufacture of polymer particles which start with either monomeric components or oligomeric components, in which the polymer and the powder are produced in one process step. A process such as this which would produce a powder with the desired average particle size (d
50
) with a narrow particle size distribution would be of even greater advantage. Some of the advantages would be a reduction in manufacturing cost (via combination of the polymerization and powder production steps, a reduction in the amount of waste, improvement in process yield, reduction in process time and improvement in energy efficiency.
There have been no lack of efforts to develop alternative methods for powder production by means of new technologies without incurring the above mentioned disadvantages in proccessability. The aim is, in general, to prepare particles with a near-ideal spherical form, since such powders exhibit substantially more favorable flow behavior than the irregular milled powders. It has been attempted, for example, to prepare near-spherical particles by spraying polymer melts. The results presented in WO 92/00342 indicated, however, that this leads only to moderate success. The particles obtained by this technique, although having a smoother surface than milled powders, are still far removed from the ideal structure of the sphere.
Another method which has been investigated for the preparation of spherical particles is the spraying of polymers from a supercritical solution, as described, for example, in EP-A-0 661 091 or EP-A-0 792 999. This method too has substantial disadvantages. For example, in the cited applications it is stated that owing to the sudden evaporation of the supercritical “solvent”, a powder is obtained which has a porous structure. When these powders are employed to prepare films there is—in comparison with nonporous powders, an increased occurrence of bubble formation and thus of defects in the coating, since the porous structure means that a large amount of gas is trapped in the powder and must be removed in the process of film formation. The use of supercritical solvents, moreover is technically complex since. for example, it requires operation under high pressure.
A method of producing spherical particles which differs in its principle is to produce a dispersion. Physical laws dictate that, in a dispersion, the perfect spherical form is the preferred geometry of the particles obtained. There has therefore been no lack of attempts in the past to obtain polymer particles which can be used, for example, as binders in coating systems, by preparing them in dispersion. (Keith Barett, Dispersion Polymerization in Organic Media, John Wiley and Sons, London, 1975). GB-1 373 531, for example, describes the preparation of stable dispersions of polycondensation polymers, such as polyesters.
The possibility of using the polymer particles from nonaqueous dispersion processes, based in particular on polyesters, as a powder coating is addressed in DE-C-21 52 515. Here, an existing polymer is brought into dispersion at a temperature <200° C. and, by addition pigments, in some cases at room temperature, a coloration is achieved. However, the resulting particles are described as substantially spherical “aggregates” of primary polymer particles and pigment particles. The isolation of material by spray drying leads to apparently to relatively large structures which it was necessary to convert back into a fine powder by mechanical means. Following the breaking up of the initially formed agglomerates, the stated particle size range is from about 2 to 50 &mgr;m, although there is no information whatsoever about the mean particle size or the particle size distribution.
The use, as described in DE-C-21 52 515, of a polymer which has already been condensed to high molecular weights as a starting product for dispersion preparation, moreover, has the following disadvantages: the already considerable viscosity of the polymers makes it difficult to achieve good division of the melt and to obtain a homogeneous particle size distribution.
U.S. Pat. No. 5,312,704 describes a toner composition comprised of pigment particles, and a resin comprised of a monomodal polymer resin or blends. This still suffers from issues described above in the extrusion blending followed by milling process, plus the dispersity of pigment particles as opposed to dyes. U.S. Pat. Nos. 5,660,963, and 5,556,732 all describe polyester resins blended with colorants in a melt extruder, followed by milling.
U.S. Pat. No. 5,346,798 describes a suspension polymerization method to make toner particles. This is .an aqueous dispersion method used to make addition polymers, and is not applicable to the non-aqueous dispersion method described here to make condensation polymer particles.
U.S. Pat. No. 5,621,055 describes a
Jacobs Alexandra
Mörs Volker
Shiwaku Toshio
Ward Benett Clayton
Wulf Stefan
Acquah Samuel A.
Connolly Bove & Lodge & Hutz LLP
Ticona GmbH
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