Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-09-21
2001-02-06
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
Reexamination Certificate
active
06184270
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing powder, and more particularly, to a process for producing powder formulations by creating a suspension in a compressible fluid.
BACKGROUND OF THE INVENTION
Powder coatings are promoted as environmentally benign coating systems in that no solvent is used during the application of the coating to the substrate. Powder coating formulations are quite expensive, however, primarily as a result of the extensive processing required to generate the powder. During such processing, resin, which is typically either an acrylate or polyester, is blended with a variety of additives (for example, pigments, stabilizers, fillers) and then extruded to generate an intimate mixture. The efflux from the extruder is pelletized, then ground, then sieved to recover only the smaller particles. The extensive processing of powder coatings and the relatively low yield (as a result of the sieving step) results in a per-pound cost typically in excess of $5. The expensive nature of powder coating has been a substantial factor in preventing powder coatings from capturing more of the overall coatings market.
Attempts to improve the processing of powder coatings have met with limited success. For example, U.S. Pat. Nos. 5,399,597 and 5,548,004 disclose a process purported to generate powder coating formulations via processing with supercritical carbon dioxide (CO
2
). In general, the process comprises the steps of (a) adding all of the mix components to a pressure vessel, (b) adding CO
2
at a supercritical pressure and temperature, (c) stirring the components within the pressure vessel; and (c) subsequently blowing the contents through a nozzle into a second vessel maintained at a lower pressure, thereby exploding the mixture into droplets.
There are a number of potential problems with the process of U.S. Pat. Nos. 5,399,597 and 5,548,004. For example, explosion of the contents of the pressure vessel in that process may result in an undesirably broad distribution of particle sized in the resulting powder coating. Moreover, control of the process via control of the aperture size of the nozzle is likely to be difficult. Furthermore, very high pressures are required.
European Patent Application No. 95308726.9 discloses a process in which supercritical CO
2
and typically a cosolvent are purportedly used to dissolve some components of a coating powder. The mixture is then flashed through a valve, causing rapid precipitation of a powder. A substantial problem exists with the process of European Patent Application No. 95308726.9 in that some of the starting components are partially soluble in CO
2
(in the case of resin, for example, only a part of the molecular weight distribution), while others are completely insoluble in CO
2
(for example, the pigments and other inorganic components). It is thus likely that fractionization of the resin will occur under the described process conditions when using only CO
2
as a solvent. Moreover, the use of cosolvents such as tetrahydrofuran (THF) to reduce fractionization through creation of a single phase can render the process environmentally unfriendly as a result of residual solvent in the powder coating. Residual solvent can also make particles stick together. Furthermore, dissolving the starting materials in a single phase is also likely results in submicron sized particles which are undesirably small for use in powder coatings.
Given the limitations of current processes for the generation of powder formulations, it is very desirable to develop improved processes for the generation of powder formulations.
SUMMARY OF THE INVENTION
The present invention provides generally a method for producing a powder formulation comprising the steps of:
a. charging a vessel with a mixture of at least two solid components, at least one of the solid components comprising a polymeric resin;
b. charging the vessel with a fluid in which the polymeric resin and, preferably, all other components of the mixture are not substantially soluble, the fluid selected such that the density of the fluid can be adjusted upon adjustment of temperature and pressure in the vessel to enable creation of a suspension of the mixture within the vessel upon agitation of the contents of the vessel;
c. maintaining or adjusting the mixture to a temperature above the glass transition temperature of the polymeric resin in the fluid;
d. adjusting the pressure of the fluid such that the density of the fluid enables creation of the suspension of the mixture within the vessel upon agitation of the contents of the vessel;
e. agitating the contents of the vessel; and
f. reducing the temperature to a temperature below the glass transition temperature of the polymeric resin component in the fluid.
The present invention also provides generally a method for producing a powder formulation comprising the steps of:
a. charging a vessel with a mixture of at least two solid components, at least one of the solid components comprising a polymeric resin;
b. charging the vessel with a fluid in which the polymeric resin and, preferably, all other components of the mixture are not substantially soluble, the fluid selected such that the density of the fluid can be adjusted upon adjustment of temperature and pressure in the vessel to be within approximately 50% of the average density of the mixture of components;
c. maintaining or adjusting the temperature of the mixture to a temperature above the glass transition temperature of the polymeric resin in the fluid;
d. adjusting the pressure of the fluid such that the density of the fluid is within approximately 50% of the average density of the mixture of components;
e. agitating the contents of the vessel; and
f. reducing the temperature to a temperature below the glass transition temperature of the polymeric resin component in the fluid.
In the methods of the present invention, upon removal of the fluid (for example, via a pressure quench) a powder product remains. Preferably, the temperature and pressure are decreased simultaneously in step f, for example, via flashing the contents of the vessel through a valve.
In general, the pressure in the vessel is preferably increased to within approximately 50% of the average density of the mixture of components and the mixture of components mixed sufficiently vigorously to create a generally uniform suspension of the components in the fluid. As used herein, the phrase “within approximately 50% of the average density of the mixture of component” means that the density of the fluid does not deviate from the average density of the mixture of components by more than approximately 50%. For example, if the average density of the mixture of components is 1 g/cc, the density of the fluid should be adjusted to a density in the range of approximately 0.5 g/cc to approximately 1.5 g/cc.
As clear to one skilled in the art, the more closely the density of the fluid is matched to the average density of the mixture of components, the easier it is to achieve a suspension of the mixture of components. Therefore, the density of the fluid is more preferably adjusted to within approximately 30% of the average density of the mixture. Even more preferably, the density of the fluid is adjusted to within approximately 15% of the average density of the mixture. Most preferably, the density of the fluid is adjusted to within approximately 5% of the average density of the mixture.
As used herein, the “average density of the mixture” is calculated using the following formula:
&rgr;
avg
=&Sgr;w
1
&rgr;
1
+w
2
&rgr;
2
+w
3
&rgr;
3
wherein &rgr;
avg
is the average density (g/cc) of the mixture; w
1
, w
2
, and w
3
are the weight percents of components
1
,
2
, and
3
, respectively, divided by 100; and &rgr;
1
, &rgr;
2
, and &rgr;
3
are the densities of components
1
,
2
and
3
, respectively. For example, in a two component mixture comprising 95 wt % of a resin having a density of 1.4 g/cc and 5 wt % of a curing agent having a density of 2.4 g/cc the average density of the mix
Beckman Eric J.
O'Niell Mark
Bartony & Hare
Michl Paul R.
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