Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – To form particulate product
Reexamination Certificate
1999-12-28
2002-07-30
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
To form particulate product
C264S180000, C264S211000, C264S211130
Reexamination Certificate
active
06426026
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of pelletizing high melt flow polymers. More particularly, the invention relates to the production of high melt flow polyolefin polymer pellets.
2. Description of Related Art
Almost all of the plastic resin sold in the market today is in the form of pellets. Plastic resins are sold in the form of pellets to improve transportation, handling, safety and end-user material processability characteristics. Reactor granular resin is thus melted and extruded and made to flow through dies before being cut into pellets. The extrusion process also serves as a step for the addition of performance additives for the required stability and material properties. The size, shape and uniformity of the pellets are important and measures of these pellet characteristics are standard quality assurance/quality control (QA/QC) tests to be met during production. The pelletizing step is important from an operational standpoint. Any upset or malfunction of the pelletizer can result in process shutdown and halt manufacturing with serious financial consequences, especially for large extrusion lines. Therefore, the pelletizing step becomes an important component of the production line of any polyolefin production facility, and it is not to be taken lightly in cases where the polymer renders difficult cut.
Many fiber and film applications of polypropylene resins require that the polymer have high melt flow properties, usually 100 MFI and higher. In particular, the production of non-woven fabrics by melt blown fiber processes calls for polypropylene grades in the range of 500-2000 MFI. The current practice follows two paths. The first one is the coating of granular resin with peroxide and the second is the production of pellets having unreacted peroxide via a process of partial peroxide degradation. Both processes have the disadvantage that the processor has to perform further chemical modification during the production of the final article leading to process complications, product quality control problems and higher cost.
Other disadvantages are that the peroxide-doped granular or pelleted resin systems are not homogeneous blends of the polymer with peroxide resulting in a polymer product having non-uniform final melt flow property. Also, if the peroxide/polymer system is in masterbatch form, as is often the case, there may be problems with bleed out during storage and transportation to the customer. The granular system also has the additional disadvantage that the processor has to process granules and not pellets leading to bulk density variations of the feedstock, bridging of granules in feed hoppers, poor conveying, feeding and melting, excessive amounts of fines, and overall safety and housekeeping difficulties as compared to pelleted form. Similarly, the peroxide-laden pellets, besides problems with inhomogeneities in melt flow and peroxide concentrations, suffer from the limitation that their melt flow cannot exceed the level above which pellets can be produced by means known in the art, usually 100 MFI.
This limitation can lead to a too narrow molecular weight distribution of the final article produced using the processor's equipment because the melt flow control over and above the 100 level is governed only by peroxide degradation (which causes the narrowing of the molecular weight distribution). If, for example, the optimum polymer system calls for a reactor grade (starting) melt flow of 450 as is usually preferred, there is no means known in the art to produce a polymer pellet system of 450 MFI with unreacted residual peroxide.
An ultra high melt flow grade crystalline polymer typically has a melt flow (MF) of about 50 dg/min or greater. The MF of a ultra high melt flow crystalline polymer can be as high as 15,000 or greater. Ultra high melt flow polymers in the range of about 1000-2000 are particularly useful for the production of non-woven fabrics by melt blown fiber processes. In order to employ ultra high melt flow polymers in commercial processing equipment, it is desirable to utilize the ultra high melt flow polymer as a pellet feed stock.
Pelletization of polymers using conventional pelletization systems is a well known method of providing a pellet feedstock. There are many types of conventional pelletizers, depending on the material made, application, rates and user preferences. The most common types of pelletizers fall under the following categories: underwater pelletizers, water ring pelletizers and strand pelletizers. An example of a particular type of strand pelletizer system is the water slide pelletizer supplied by Rieter Corp. and Conair. This is well suited for hard to cool, sticky, low viscosity plastics. The strands leaving the die are directed to a declined water trough. Shallow water flow aids the strands down the water path into a cutting chamber. Along the trough, a number of water jet sprays cool the strands. The particulars of equipment design and operation can be found in a number of prior disclosures, examples of which are U.S. Pat. Nos. 5,441,394, 5,313,864, 5,242,289, 5,118,270, 5,310,515, and 4,528,157. The disclosure of each of these patents is incorporated by reference herein in its entirety.
Polypropylene homopolymer and copolymer high melt flow resins have been notoriously difficult to pelletize. Due to low melt strength associated with high melt flow polypropylene resins, reliable and robust underwater pelletizing operations can handle up to 100 MF, perhaps a little higher for lower rate pelleting lines. Due to the low melt strength of such ultra high melt flow crystalline polymers, attempts to pelletize ultra high melt flow polypropylenes with conventional pelletization systems, including underwater pelletization systems, result in an excess amount of non-uniform pellets, malformed pellets, pellet trash and high levels of “fines”. Deformation of the polymer pellet is caused by water currents created by rotating knives of the underwater pelletization system. Malformed and non-uniform pellets are undesirable since they tend to bridge in pellet feed hoppers and convey poorly (e.g., plug conveying filters). Further, significant amounts of malformed pellets alter the bulk density of the pellet stock may result in feeding problems in the extrusion line and voids in the final product. In addition to malformed pellets, “trashouts” occur frequently during the production of ultra high melt flow crystalline polymers. Trashouts are extruder shutdowns resulting from polymer buildup on the rotating knives. Such trashouts not only necessitate the consumption of enormous labor and time but induce deterioration of the quality of polyolefin polymer pellets being produced.
Therefore, there is a need to produce pellets of any desired melt flow and molecular weight distribution without any limitations on the reactor grade melt flow that produced them. It has long been desired to find a continuous process for pelletizing ultra high melt flow crystalline polymers to produce uniform, dust-free crystalline polymer pellets having narrow molecular weight distribution. In particular, it is desired to find a high speed continuous process for pelletizing crystalline polymers, such as isotactic polypropylenes, that have a melt flow greater than 100 dg/min.
Further, it is desired to find a process for pelletizing ultra high melt flow crystalline polymers that contain a uniform dispersement of the desired additives and are substantially cracked to produce uniformly compounded pellets having high bulk density.
U.S. Pat. No. 5,340,509, the disclosure of which is incorporated by reference herein in its entirety, which is assigned to Shell Oil Company, describes a process for pelletizing ultra high melt flow crystalline polymers to produce pellet products. It essentially makes use of commercially known technology for droplet forming. Although this disclosure would produce high melt flow pellets, it is limited to extremely low production rates as evidenced by the claims of the manufacturers of the dropformer
Avgousti Marios
Kharazi Alex
Leach Edward Allen
Eashoo Mark
Union Carbide Chemicals & Plastics Technology Corporation
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