Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
1998-01-21
2001-05-15
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C239S033000, C428S036920, C428S315700, C521S079000, C521S142000, C521S143000
Reexamination Certificate
active
06231942
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to polymeric foam processing, and more particularly to a continuous microcellular polymer extrusion system and method that allows extrusion of microcellular, standard-grade polypropylene. Polymer foam straws also are produced.
BACKGROUND OF THE INVENTION
Foamed polymeric materials are well known, and typically are produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Under some conditions the cells can be made to remain isolated, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes chemical decomposition in the polymer material causing formation of a gas.
Foamed polyolefins are known. Of these, polyethylene is preferred because of ease of foaming control. While foams including polypropylene components are known, in most cases such foams include significant proportion of additives that add controlability to the foaming process.
U.S. Pat. No. 4,940,736 (Alteeping) describes a foamed product made by foaming a composition including a major proportion of a low viscosity polypropylene having a melt viscosity of less than 2×10
3
poise and a minor proportion of a high viscosity polypropylene having a melt viscosity of greater than 2.5×10
3
poise. Alteeping mentions that previously-proposed procedures for foaming polypropylene had suffered from severe disadvantages limiting their commercial application, noting specifically the following: U.S. Pat. No. 4,352,892 (Firma Carl Freudenberg), which discloses foaming a composition including crystalline polypropylene and a further component selected from polybutadiene, ethylene vinyl acetate copolymer, and ethylene-propylene terpolymer rubbers; U.S. Pat. No. 4,442,232 (Firma Carl Freudenberg) which discloses foams comprising crystalline polypropylene and polybutadiene that are cross-linked; U.S. Pat. No. 4,298,706 (Karengafuchi Dagaku Koguyo KK) which discloses foams of compositions comprising of polypropylene and polybutadiene kneaded together; U.S. Pat. No. 3,846,349 (Sumitomo Chemical Co.) which describes foam produced from a three-component mixture of crystalline polypropylene, non-crystalline propylene, and low density polyethylene; and U.S. Pat. No. 3,607,796 (Grunzweig and Hartmann AG) which describes a process for producing foam from a composition comprising high and low molecular weight polypropylene.
U.S. Pat. No. 5,180,751 (Park) describe polypropylene foam made of polypropylene resins having a z-average molecular weight above 1×10
6
and a z-average molecular weight/weight average molecular weight ratio above 3.0. Park states that unacceptable foam sheets show a unimodal molecular weight distribution, while resins which yield acceptable foam sheets show a bimodal molecular weight distribution.
U.S. Pat. No. 4,832,770 (Nojiri) describes a method of manufacturing a foamed polypropylene resin from a mixture of 80 to 20 weight percent of a crystalline polypropylene-ethylene block copolymer containing 20 weight percent or less of ethylene and having a melt index of two or less and 20 to 80 weight percent of a crystalline polypropylene-ethylene block or random copolymer containing 5 weight percent or less of ethylene and having a melt index of 6 to 20 or a polypropylene homopolymer having a melt index of 6 to 20.
One class polymer foams that can offer a variety of advantageous characteristics such as uniform cell size and structure, the appearance of solid plastic, etc. are microcellular foams. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In the technique of Martini-Vvedensky, et al., a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged at pressure in supercritical fluid for a period of time, and then quickly returned to ambient conditions creating a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded, then run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
While polymer foams containing polypropylene exist, it would be advantageous, in terms of added simplicity and reduced cost, to be able to produce high-quality foams including polypropylene without the need for significant amounts of foam-controlability additives or other co-polymerized or blended polymer components. It is an object of the present invention to provide such articles.
SUMMARY OF THE INVENTION
The present invention provides methods and systems for producing polymeric polypropylene foam which can be microcellular material, and articles produced thereby.
In one aspect the invention provides extrusion systems. Specifically, an extruder is provided that has an inlet for receiving a precursor of a foamed polypropylene material at an inlet end thereof, and an outlet at an outlet end thereof for releasing foamed polypropylene material from the extruder. An enclosed passageway connects the inlet with the outlet. The passageway is constructed and arranged to contain a product of the mixture of a blowing agent which can be a supercritical fluid, in particular supercritical carbon dioxide, with molten polypropylene material to be foamed within the passageway and to maintain the product within the passageway. The product can be maintained within the passageway above the critical temperature and pressure of the supercritical fluid. A nucleator is associated with the passageway and is capable of nucleating the product in the passageway in the absence of auxiliary nucleating agent, although nucleating agent can be used. An orifice is provided between the inlet and the outlet and is fluidly connectable to a source of blowing agent which can be supercritical fluid. The system receives polypropylene having a unimodal molecular weight distribution or other polypropylene described below in connection with the articles of the invention.
In another aspect the invention provides a method. One method involves providing a polypropylene material to be foamed, selected among polypropylene described below with respect to articles of the invention, and mixing a blowing agent into the material to create a mixture. A homogeneous single-phase solution is created from the mixture that has a uniform concentration of blowing agent distributed therein. The homogeneous single-phase solution is nucleated, and then essentially immediately thereafter shaped to create a shaped extrudate. The shaped extrudate can be released into ambient conditions essentially immediately after shaping. Nucleation can take place by passing the single-phase solution through a constriction creating a rapid pressure drop due to friction.
In another aspect the invention provides articles. One art
Blizard Kent
Malavich William
Tupil Srinath
Copenheaver Blaine
Trexel, Inc.
Wolf Greenfield & Sacks P.C.
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