Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-07-12
2004-12-21
Jackson, Monique R. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S328000, C428S330000, C428S402000, C424S400000, C424S489000
Reexamination Certificate
active
06833185
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fluidization additives to fine powders, and more particularly the present invention relates to fluidization additives to fine paint powders.
BACKGROUND OF THE INVENTION
Fluidization occurs when particulate materials having sizes ranging from sub-micrometers to several millimeters are suspended by up-flowing gas in a vessel or column which results in a gas-solid suspension, more commonly referred to as a fluidized bed. The fluidized beds formed with the gas-solid suspension are specifically referred to as gas-solid fluidized beds. The term “fluidized bed” applies because the gas-solid suspension formed by the solid particles and the upflowing gas behaves like a fluid. A gas-solid fluidized bed can operate in several fluidization regimes: particulate, bubbling, slugging and turbulent fluidization regimes (collectively called conventional fluidized beds), and fast fluidization and pneumatic transport regimes (collectively called high-velocity fluidized beds). There is a minimum gas velocity, called minimum fluidization velocity, below which the bed is not fluidized.
Key characteristics of fluidized beds include easy handling of particles, excellent contact between gas and solids, excellent heat and mass transfer between gas and solids and between gas-solid suspensions and the column wall, and good mixing of gas and solids to mention a few. These and other useful characteristics have led to the wide application of fluidized beds in powder processing and related industries. The “easy handling of particles” in fluidized beds is due to the uniform solids suspension inside the bed and the relatively free movement of the particles within the gas-solids suspension and of the suspension itself.
Powders may be classified into four groups in gas-solid fluidized systems, according to Geldart's classifications (Geldart, “The Effect of Particle Size and Size Distribution on the Behavior of Gas Fluidized Beds”, Powder Technology, Volume 6, 210, 1972 and Geldart, “Types of Gas Fluidization”, Powder Technology, Volume 7, 285, 1973). Groups B and D powders comprise large particles that typically result in large bubbles when fluidized. Group A powders comprise particles that first experience a significant expansion of the powder bed when fluidized before bubbles begin to appear. Both Group B and Group A powders can be well fluidized. Group C powders comprise very small (fine) particles for which interparticle forces significantly affect the fluidization behaviour in such a way that fluidization of these powders is very difficult. Typically, as the particle size reduces, interparticle forces increase significantly causing the fine particles to agglomerate since they become very cohesive. Typical Group C powders comprise particles under 25-35 &mgr;m in average size, although powders larger than these sizes which are very cohesive may also belong to the Group C powders. Thus, due to strong interparticle forces, Group C powders are either very difficult to fluidize (with channeling and/or very poor fluidization) or they primarily fluidize with the large agglomerates as pseudo-particles rather than as individual particles. In either case, fluidization of individual particles cannot be achieved easily which makes handling of Group C powders problematic. Group C powders also tend to clog up in certain areas of the fluidized bed, powder transport lines and powder processing equipment, such as above the gas distributor and around internals and at exit port(s), and tend to stick to the internal wall, corners or the ceiling of the bed, transport lines and other powder processing equipment.
There are many processes or uses for which Group C fine powders need to be handled. To enhance their flowabilities, different measures have been taken to assist the fluidization and transportation of these Group C powders. Those methods are usually referred to as fluidization aids. Fluidization aids include mechanical stirring, acoustic, mechanical or ultrasonic vibration, addition of much larger particles to provide extra stirring, pulsation of fluidization gas, to mention just a few. Some of these measures are more effective than others for a given Group C powder, but the effectiveness of almost all of these measures tends to diminish as the powder becomes finer or smaller in size. As used herein, the terms “fluidization aids”, “flow aids” and “transportation aids” are referring to the additional measures or methods applied to the fluidized bed and/or powder to enhance the fluidization and handlability of fine powders, while the terms “fluidizability”, “flowability”, “handlability” and “transportability” are referring to the same general concept, that is, the ability of a powder to flow better and therefore to be handled and transported more easily.
Another method of increasing fluidizability of powders involves the addition of some silica or alumina finer particles (additives). For example, it has been known that adding a small fraction of extremely fine silica powder improves the fluidization of Group C powder. On the other hand, addition of many other finer particles has been observed not to help in the fluidization of fine powders. Therefore, the mechanism is not yet clearly understood, although some have speculated a “lubricant” effect. As used herein, the terms “lubricant”, “lubricating agent” and “additive” are all referring to solid additives that are added to the finer powder, aiming at enhancing its fluidization.
An example where it is extremely important to maintain good fluidization and transportation of fine powders is powder coating. Powder coating is a process superior to the traditional liquid coating process. A traditional paint application technique, referred to as “wet coating”, involves the application of a liquid paint where solid paint components are first dissolved into or suspended in a solvent which is then applied to the surface of the part being painted. Polymerization and/or other reactions of the paint components then occurs in the wet paint layer on the surface, leading to the hardening of the paint coat while the solvent evaporates and is released to the atmosphere. Any over-sprayed paint and solvent are essentially wasted due to the non-recyclability. Since most of the liquid solvents are organic compounds, they cause serious environmental problems. Legislation and environmental concerns have led to the development of a new alternative coating procedure, which is called the “powder coating process”.
In contrast to the traditional wet coating techniques, the powder coating process involves directly applying a powder paint onto the surface of the part being painted using a carrier gas where the powder is “held” by electrostatic forces. The parts are then put through a curing oven where the powder paint melts and hardens through a series of chemical reactions. Most of the over-sprayed powder paint is recycled. Therefore powder coating is an environmentally friendly technology because it eliminates any organic or inorganic solvents and makes it possible to reuse the over-sprayed paint.
More particularly, a typical powder coating production line consists of a washer, a pre-dryer, a paint booth, a curing oven and a loading/downloading section, as shown in FIG.
1
. Parts loaded on the conveyer are first washed to remove dirt, soil and oil. The parts are then dried in a pre-dryer to remove the residual water after which they are ready for powder coating. A paint powder stored in a powder hopper is fluidized by a gas (normally air) and pneumatically transported to a spray gun (either a corona or tribo gun) where it is sprayed onto the surface of the part. Due to the fact that the powder is electrically charged before it reaches the part, the powder will be attracted to the surfaces of the parts that are electrically grounded. When the desired thickness of powder layer has been deposited, the parts are transferred to the curing oven where the paint is melted and hardened. This process has been widely used and it is known that only relati
Zhang Hui
Zhu Jesse
Hill & Schumacher
Jackson Monique R.
Schumacher Lynn C.
The University of Western Ontario
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