Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-03-29
2002-03-19
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S333700, C525S333900, C525S386000, C525S387000
Reexamination Certificate
active
06359077
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved process for the manufacture of pellets of high melt flow polyolefin polymers, which process involves the incorporation of degradation or “cracking” aids (e.g., free radical generators) into pellets of alpha-olefin polymers. Particularly, this invention relates to a novel process for producing pellets of alpha-olefin polymers that include unreacted free radical generators which, when heated, undergo chemical degradation producing a low viscosity polymer that preferably can be processed into films or fiber.
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 is 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 has 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 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 often 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 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 an 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. The most common way of making ultra high melt flow polymers having such high melt flows is to resort to chemical reaction modification of reactor grade resins. In this case, reactor granular resin is admixed with a radical generating agent which is thermally activated. Provided a certain temperature and residence time standard is achieved, usually above the melting temperature of polypropylene, the polypropylene molecular chains undergo scission resulting in lower weight average molecular weight polymer. Owing to the random nature of the cleaving process, the molecular weight distribution becomes less narrow. Under the right conditions, the final product is amenable for spinning and meltblowing process applications. This would have been impossible to achieve using unreacted reactor resin alone.
In order to employ such 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 due to the inherent problems associated with granular feed stock. Pelletization of polymers using conventional pelletization systems is a well known method of providing a pellet feedstock. 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 making pellets of 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 making pellets of crystalline polymers, such as isotactic polypropylenes, that have a melt flow greater than 100 dg/min.
U.S. Pat. No. 5,198,506 assigned to Phillips Petroleum Company, describes a composition of matter consisting of 80-90 weight percent homogeneous free-flowing polypropylene fluff having a particle size of 100-500 microns and 10-20 percent liquid organic peroxide mixed in at high intensity and temperature below the decomposition temperature of said peroxide allowing the composition to set for 1 to 240 hours to form homogeneous, free-flowing composition of mater. There are a number of diff
Avgousti Marios
Kharazi Alex
Lipman Bernard
Union Carbide Chemicals & Plastics Technology Corporation
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