Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
2002-11-12
2004-09-28
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S211210, C264S920000
Reexamination Certificate
active
06797216
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to solid state shear pulverization of polymeric material, which may include thermodynamically incompatible polymers, to form without compatibilizing agents pulverized particulates that are directly melt processable as powder feedstock to shaped articles of manufacture by conventional blow molding, rotational molding, extrusion, and spray coating techniques without color streaking in the resulting articles of manufacture.
Decreasing landfill space and rapidly rising disposal costs have forced many municipalities to begin curbside recycling of post-consumer plastic (polymeric) waste. In general, plastic materials comprise approximately 20% by volume of the municipal waste stream. For example, Chem Systems, 1992, reports that municipal solid waste comprises, by weight, 48% polyethylene (PE) (27% being low density PE and 21% being high density PE), 16% polypropylene (PP), 16% polystyrene (PS), 6.5% polyvinyl chloride (PVC), 5% polyethylene terephthalate (PET), 5% polyurethane, and 3.5% other plastics.
Post-consumer polymeric waste, as opposed to industrial plastic waste, typically includes substantial quantities of plastic bottles, containers and packaging materials. Plastic bottles are molded of different polymeric materials depending upon the product they are to contain. For example, plastic bottles for water, milk, and household chemicals typically are made of high density polyethylene (HDPE), while soft drink bottles are typically made of polyethylene terephthalate (PET) with or without base caps made from high density polyethylene (HDPE). Generally, HDPE bottles account for approximately 50-60% and PET bottles account for approximately 20-30% of the bottles used by consumers. The balance of bottles, bottle caps and other containers used by consumers comprises other polymeric materials, such as low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and other resins and multi-layered materials.
Plastic packaging materials also are made of a wide variety of polymers. For example, according to Plastics Compounding, Nov/Dec, 1992, the following polymers were used in packaging material in the %'s set forth: 27% LDPE, 21% HDPE, 16% PS, 16% PP, and 5% PET.
Post-industrial plastic waste can comprise polyolefins, PS, PET and other polymeric materials used for plastic packaging. Currently, collection of plastic waste material exceeds the market demand for recycled plastic products as a result of the dearth of viable recycling technologies that are low cost and produce high quality recycled plastic products. One recycling approach has involved the high energy consuming batch grinding of commingled, unsorted mixed color plastic waste to form flake scrap material, melt processing and pelletizing the melt processed material to pellets, and extruding the pelletized plastic waste to form recycled plastic products. However, recycled plastic products made in this manner suffer from severe deficiencies that render the products unsatisfactory for many purposes and are of inferior, low value compared to products made of virgin polymeric materials. For example, these recycled plastic products exhibit inferior mechanical properties (e.g. tensile, flexural and impact strength) and inferior appearance in terms of color (dark brown or gray color) with streaking of colors within the molded product as a result of the chemical incompatibility of the different polymers present in the initial plastic waste stream and variations in the plastic waste stream composition over time.
A typical example of a low value, recycled plastic product is recycled plastic lumber having a dark brown or gray color with noticeable color streaking and inferior mechanical properties compared to components molded of virgin materials. As a result of the less than pleasing appearance, recycled plastic lumber is oftentimes painted to improve its appeal to the customer, or expensive pigments and other additives are added to the feedstock during the manufacturing process to this end. However, the cost of the recycled product is increased thereby.
Furthermore, certain melt processing techniques, such as blow molding, rotational molding, extrusion (e.g. extruded PVC pipe and profiles), and spray coating, require a plastic powder feedstock. That is, the flake scrap material is not directly melt processable to articles of manufacture by such powder feedstock-requiring melt processing techniques. To be useful as feedstock in such melt processing techniques, sorted or unsorted flake scrap material produced by batch grinding must be pelletized and then ground to powder form. The need to pelletize and grind sorted or unsorted flake scrap polymeric material prior to such melt processing adds considerably to the cost and complexity of recycling scrap plastics as well as the capital equipment expenditures required.
Conventional injection molding techniques require plastic pellets for high speed production of molded parts. Although unsorted, commingled flake scrap materials could be pelletized to provide feedstock for injection molding, the resultant molded products would suffer from the types of deficiencies discussed above attributable to polymer incompatibility.
So-called compatibilizing agents and/or reinforcing agents can be added to flake plastic scrap material comprising chemically incompatible polymers in attempts to produce a recycled plastic product exhibiting more desirable characteristics. However, addition of these agents to the plastic scrap material makes recycling more difficult and adds considerably to its cost. The Mavel et at. U.S. Pat. No. 4,250,222 relates to this type of recycling approach and is representative of the disadvantages associated with such an approach to plastic recycling.
Attempts have been made to sort commingled, post-consumer plastic scrap to overcome the polymer incompatibility problems associated with the recycling of commingled plastic scrap. To-date, HDPE and PET are recovered from plastic waste streams by recycling technologies requiring sorting of the commingled plastic materials. Sorting, however, can require use of costly techniques, such as video cameras, electronic devices, infrared detectors, and organic “markers”, to provide effective segregation of like plastics.
The high cost of sorting has greatly limited widespread use of recycling approaches that require a sorting step. In particular, collected and sorted post-consumer plastic materials are usually more expensive than the corresponding virgin plastic materials. Thus, users of plastic materials are discouraged from using sorted, recycled plastic materials.
Further, sorted plastic scrap must be subjected to batch grinding to produce flake scrap material that then must be pelletized and ground again to provide powder feedstock for blow molding, rotational molding, some extruding, spray coating and other melt processing techniques that require powder feedstock.
Even sorted plastic waste, however, can present problems in processing as a result of density and chemical differences among polymers falling in the same general class and made by different plastics manufacturers. The same polymer, for example, may have different viscosities in different products. Such differences in viscosity tend to make melt mixing of the same polymer from different products difficult and time consuming.
A study of the effect of viscosity differences on the ability to melt mix polymers was conducted by Chris E. Scott and Sandra K. Joung at the Massachusetts Institute of Technology, Department of Materials Science and Engineering. The results of this study appear in Scott & Joung,
Viscosity Ratio Effects in the Compounding of Low Viscosity, Immiscible Fluids into Polymeric Matrices
, Polymer Engineering and Science, Vol. 36, No. 12, June 1996 (hereinafter “Scott & Joung”), the contents of which are incorporated herein by reference.
According to Scott & Joung, many low viscosity, immiscible fluids are difficult to incorporate into polymer matrices because of thermod
Furgiuele Naomi
Khait Klementina
Torkelson John M.
Eashoo Mark
Liniak, Berenato & White, LLC
Northwestern University
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