Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – By extrusion spraying or gravity fall through orifice
Utility Patent
1999-08-24
2001-01-02
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
By extrusion spraying or gravity fall through orifice
Utility Patent
active
06168733
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for forming pellets from viscous materials including solutions and melts. More particularly, the present invention relates to a process for forming pellets from viscous materials using a step-wise pressure differential and increase in fluid velocity. The present process is particularly useful for making pellets of a substantially uniform size from viscous materials having a viscosity greater than about 40,000 centipoise.
2. Description of the Related Art
Polymer pellets are formed commercially by several different methods. The current methods for pelletizing viscous polymer solutions and melts are highly mechanical in nature. These devices involve blades, wires or pumps that are used to cut polymer strands into pellets, or dropping liquid polymer pellets onto moving trays to harden into pellets, or vibrating strands of liquid polymer into pellets. Because of the mechanical nature of these processes, maintenance, energy and labor costs can be a significant part of the overall cost of making the product.
Pelleting of viscous material solutions and melts is widely practiced on a commercial scale. Current practice includes many different variations of the following pelleting systems:
1. Under water pellet cutters. These devices use a series of blades or a pump to cut polymer strands into pellets. The cutting blades can be placed immediately after the polymer is extruded out of the die face, or some distance down stream from the die face. The process is carried out under water.
2. Pastilles (hemispherical pellets) can be formed using a process such as the process known under the trademark “SANDVIK ROTOFORM PROCESS”. In this process small blobs of liquid polymer are dropped onto a moving steel belt. The belt is cooled with water which serves to cool and solidify the polymer, forming the pellet.
3. Microsphere technology produces spherical beads ranging from 0.2 to 5 mm diameter. Liquid polymer is pumped through a vibrating nozzle, and the fluid stream breaks into uniform droplets. This process is currently limited to low viscosity solutions.
U.S. Pat. No. 3,414,640 discloses a method of precipitating and forming cellulose ester granules. The cellulose ester solution is extruded through die holes, and the polymer extruding out of the die holes is cut into granules with a blade periodically swept over the surface.
U.S. Pat. No. 3,213,170 discloses a method for making granulated materials by continuously extruding the material through a die opening and directing a stream of gas transversely to the direction of the extrusion to force the extruded material to break off from the opening.
U.S. Pat. No. 4,192,838 discloses a process for making fibers from cellulose acetate dopes. The process uses a capillary needle to introduce the polymer solution into the throat of a venturi orifice. This process produces cellulose ester fibrets that are similar to natural fibers in morphology and size.
U.S. Pat. No. 4,013,744 discloses a process for producing fibrids by extruding strands of molten thermoplastic polymer through orifices into a shear gradient zone created by a propulsive jet of liquid flowing at 10 to 100 meters/second.
In the production of organic derivatives of cellulose, and especially organic esters of cellulose, such as cellulose acetate, cellulose propionate and cellulose butyrate, the esterification of cellulose with an organic acid results in a solution of the derivative of cellulose in an acid solvent. For example, in making cellulose acetate, cellulose is acetylated by contacting a cellulosic material with acetic anhydride and a catalyst in the presence of acetic acid. The acetic acid dissolves the cellulose acetate that is formed to produce a very heavy and viscous solution, referred to herein as the “acid dope”. Usually after hydrolysis, this solution of cellulose acetate is precipitated by adding water until the concentration of the acid reaches a point below which the acid will not hold the cellulose acetate in solution.
The isolation of cellulose acetate, or secondary cellulose materials, from organic solvent solutions, referred to herein as “solvent dope”, has been extensively investigated. The known process for preparing cellulose acetate, i.e., a cellulose acetate with an approximate average degree of substitution of 2.5, with its acetylation and hydrolysis steps, results in a solution of the acetate in an acetic acid and water mixture. As noted above, one practice for precipitating cellulose acetate from the reaction mixtures obtained in the acetylation of cellulose, is to introduce the reaction mixture in pellet form into a greater volume of water.
Accordingly, there is a need for a process of forming discretely identifiable pellets using equipment having little or no moving parts. There is further a need for making pellets of a substantially uniform size from a viscous material.
SUMMARY OF THE INVENTION
The present invention provides a process for forming discrete pellets from viscous materials, i.e., discrete pellets from a viscous material such as polymer solutions and melts. Briefly, in the process, a strand of viscous material is extruded through a first zone of relatively slow moving pelleting fluid before entering a second zone of fast moving pelleting fluid. The first zone establishes the pellet length, and the second zone serves to form the pellet by separating the pellet from the extruding strand. An advantage of the present process is that it requires no moving parts to form pellets from a viscous material. The process includes the steps of extruding a viscous material from a first conduit into a pelleting fluid in a first zone; and passing the extruded viscous material and the pelleting fluid into a second zone having a relatively lower pressure than the first zone so that the velocity of the pelleting fluid and viscous material increases substantially to form discrete pellets.
In a preferred embodiment of the invention, the process includes providing a viscous material in a first conduit; providing a pelleting fluid in a second conduit which at least partially surrounds the first conduit; extruding the viscous material from the first conduit into the pelleting fluid stream in a first zone; and passing the pelleting fluid stream and the extruded viscous material through a second zone having a means for restricting or partially blocking the pelleting fluid stream so that the pelleting fluid stream velocity increases substantially and produces a step-wise reduction in the pelleting fluid pressure and forms a discrete pellet from the viscous material.
It is an object of the present invention to provide a process in which discrete pellets of substantially uniform dimensions can be formed from a viscous material and overcoming the drawback of requiring moving parts for forming the pellets.
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Naylor David Mark
Scherrer Paul Keith
Eastman Chemical Company
Gwinnell Harry
Theisen Mary Lynn
Tubach Cheryl
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