Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2000-08-10
2004-11-16
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
C264S310000, C264S328170
Reexamination Certificate
active
06818173
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to improving the melt properties of polymers by forming polymeric blends using solid state shear pulverization. In particular, it has been found that polymeric blends can be formed by solid state shear pulverization, to yield moldable materials which have excellent molding processability without sacrificing and often improving melt strength. The formed polymeric blends have particular use in blow molding such as in forming blow molded films as well as in other mold processing, such as injection, extrusion, and rotational molding.
BACKGROUND OF THE INVENTION
It is well-known to shape thermoplastic polymers by melt processing. Among the most common melt processing techniques are injection and extrusion molding of objects including those of relatively complex shape and blow molding 3-dimensional objects such as bottles as well as forming films. During injection molding, a polymer, usually in the form of pellets is melted in a screw extruder and the melted polymer is pushed through one or more gates into a mold cavity having a configuration of the article to be formed.
The mold gates are hollow passages in the mold which communicate with the mold cavity and are provided in numbers sufficient to allow the molten polymer to fill every crevice of the mold cavity. An injection mold cycle, thus, involves the injection of the molten polymer through the gates and into the mold cavity, hardening of the molten polymer sufficient to be free-standing once the mold is removed and final removal of the mold and cooling. Obviously, on a commercial scale, it is preferred to reduce the injection mold cycle time and increase production of molded objects. An important component of the cycle time is the flow of the molten polymer through the mold gates and into the mold cavity to completely fill same. Thus, it would be desirable to have a polymer which has a high melt flow rate to reduce the cycle time and further to insure that all crevices or complex portions of the mold cavity are properly filled less the molded article which is formed be incomplete. Molded articles of increasing complexity can be made if the polymer can readily fill the mold cavity. Unfortunately, simply using polymers which have high melt flow rates most often results in the sacrifice in tensile strength, impact strength, and other physical properties of the molded product as polymers which have a high melt flow rate typically have a low molecular weight and consequent lower physical properties. In extrusion molding, the polymer is melted and conveyed by a rotating screw through a die at the end of the extruder barrel which shapes the molten polymer into the desired object. Again, polymers with high melt flow rates are desired to reduce production time. However, polymers with high melt flow rates are not necessarily advantageous since these lower molecular weight materials may not be self-supporting after being shaped at the extrusion die face and often provide the molded object with less than desirable physical properties.
The compromise between melt flow rate and physical properties is no less critical, and most likely more critical in melt shaping a polymeric material by the blow molding process. In blow molding, a molten resin is either extruded or injected into the form of a tube which is expanded by air or other gas into a parison or bubble which is shaped by clamping a mold around the molten parison if an object such as a bottle is to be formed or the parison can be cut or passed between the nip of rollers to form thin films. In order to take advantage of blow molding techniques, a polymer must have sufficient melt strength to be blow moldable into an object. Not only must the polymer have the physical, tensile and thermal properties necessary for specific end use applications, the polymer must have a sufficient melt strength such that the polymer can support its own weight in the molten state after being extruded or injected into a parison. Thus, polymers have sufficient melt strength when the polymers can be extruded downward into the shape of the desired parison without the molten parison breaking. It is desirable to use polymers which are viscous, have a relatively low melt flow rate and which can be self-supporting in the form of the molten parison. On the other hand, polymers which have a low melt flow rate are not readily injected or extruded into the parison, and in the case of blown films, the thickness of the film may be excessive, degrading not only the overall properties of the film but degrading the economics of the blow molding process by increased material costs. Thus, as in injection and extrusion molding, it would be desirable to increase the melt flow rate of the molten polymer so as to improve productivity and form thinner films. At the same time, the melt strength of the polymer must be maintained so as to sustain the parison.
A relatively new type of molding process being increasingly used is rotational molding. In this molding process, a moldable polymer powder is placed within a hollow mold. The hollow mold is simultaneously heated and rotated to melt the powder and spread and evenly coat the inner mold surface with the molten polymer. In this molding process, it would be preferred that the molten polymer have a sufficient melt flow rate to evenly cover the inner mold surface. At the same time, the molded article must have the necessary physical properties for the end use of the rotational molded article.
Attempts have been made to alter the properties of one polymer by mixing or incorporating a second polymer with the first to form a polymeric blend. For example, secondary polymers have been added to primary polymer compositions to improve impact strength, elongation, tensile properties and even melt flow. Unfortunately, in most of these attempts, the whole blend which is formed is less than the sum of its parts. In other words, often, the property sought by adding the secondary polymer is realized at the expense of one or more other properties or, in fact, the properties sought to be achieved are not attainable. There are many, many patents directed to polymeric blends, but only a handful, at most, of polymeric blends are in the commercial market place.
Among the reasons that polymer blends do not often realize the gain in properties which is sought is due to the incompatibility, both chemically and thermodynamically, between the individual resins which make up the blend and, the inability to provide a uniform mix of the two polymers especially if significant viscosity differences are involved. With regard to compatibility, it has been found that blended polymers which are chemically different, e.g. polar and non-polar, or have different thermodynamic properties such as melting point, viscosity, T
g
, etc., tend to segregate in the blend during and after melt blending. Accordingly, the blend is not molecularly uniform and as such articles molded from the polymeric blends do not realize the advantages in physical and/or chemical properties sought or certain properties are sacrificed at the expense of others. It is known to incorporate a compatibilizing agent, specifically synthesized for particular binary polymer blends, to compatibilize the blend. However, such agents are not universal and are not useful outside the scope of the blend for which they were synthesized. Thus, such compatibilizers are expensive and since they represent a third component to a blend, such agents can actually degrade optical, thermal and other physical properties desired for the end use of the molded objects or continuous films.
If it is desired to increase the melt flow rate of a molding resin, it would appear that this could be readily accomplished by simply mixing in a low viscosity material with the higher viscosity molding resin. Unfortunately, differences in the viscosity between polymers renders it extremely difficult to form a uniform mixture. It has been found that mixing polymers of unmatched viscosity also results in a segregation of the polymers, or la
Liniak, Berenato & White, LLC
McDowell Suzanne E.
Northwestern University
LandOfFree
Polymeric blends formed by solid state shear pulverization... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polymeric blends formed by solid state shear pulverization..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polymeric blends formed by solid state shear pulverization... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3320288