Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
1999-02-16
2001-12-11
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S211210, C264S211230, C264S349000, C425S208000
Reexamination Certificate
active
06328919
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatuses useful for the plastication and extrusion of low bulk density polycarbonate materials.
BACKGROUND OF THE INVENTION
Normal or conventional bulk density thermoplastic materials are commonly melt or heat plasticized and extruded as a molten stream through extrusion dies without considerable difficulty. The continuous single or twin screw extruders employed have rotary screws designed to plasticate the thermoplastic material being processed and to convey the molten polymer to and through a downstream extrusion die at a desired rate.
Conventional extruder screws have feeding, transition, and metering sections successively along their length. The feeding and transition sections are required to provide an over-supply of input molten polymer to the metering section which controls and limits the throughput rate of the over-all extruder. However, should it be desired to process low bulk density thermoplastic materials such as certain forms of polycarbonate or blends of such low bulk density polycarbonate materials with conventional bulk density thermoplastic materials problems are immediately encountered.
As employed herein, the term “conventional bulk density thermoplastic materials” is intended to denote thermoplastic materials having a bulk density in the normal bulk density range encountered with virgin or neat polymer in pellet or bead form. For example, virgin or regular thermoplastic material in pellet or bead form normally have a bulk density greater than about 0.5 grams per cubic centimeter (g/cm
3
) and less than about 1.2 g/cm
3
, for conventional bulk density polycarbonate materials this is typically about 0.5 g/cm
3
to about 0.75 g/cm
3
. As employed herein, the term “low bulk density polycarbonate materials” is intended to denote polycarbonate materials having a bulk density less than about 0.5 g/cm
3
. For example, virgin or neat polycarbonate can exist in a regularly shaped low bulk density granular form such as powder or as an irregularly shaped low bulk density form such as agglomerated granules having an appearance resembling “popcorn” and sometimes referred to as flake. Polycarbonate which has been previously processed then mechanically ground can exist in irregularly shaped low bulk density forms sometimes referred to as chip, scrap, regrind, or recycle.
During extrusion of regularly and especially irregularly shaped low bulk density polycarbonate materials or blends of such low bulk density polycarbonate materials with conventional bulk density thermoplastic materials, solids conveying in the screw feeding section is often inadequate to properly fill the screw and hence results in undersupplying the screw plasticating and metering sections. This results in polymer surging, lower throughput rates, high melt temperatures, and excessive polymer degradation. The term “extrudate quality” generally refers to the uniformity of the temperature, pressure, and composition of the extrudate at the dieface. If the extrudate is of low quality, temperature and pressure fluctuations are large and the flow of thermoplastic material through the die will be irregular and unsteady and the quality of the product degraded. An important determinant of an end product is how well its dimensions conform to specification. For example, in polycarbonate sheet variations in thickness greater than ±2 percent can degrade end product physical properties such as practical toughness and impact strength, end product aesthetics by causing such problems as undesirable roughness, variations in polish and/or appearance of wrinkles, and processability of the sheet in subsequent molding operations such as vacuum forming and thermoforming.
There have been attempts in the prior art to provide steady throughput rate from the metering section by modifying the feed section of the screw. For example, U.S. Pat. No. 4,461,734 reported a feed section with relatively deeper screw root depth and/or greater screw lead and/or greater screw length than normally employed in feeding sections of extruders handling virgin or high bulk density linear low density polyethylene (LLDPE). Although the modifications assisted in improving the throughput rate of the LLDPE flow throughout the process, a downstream melt pump, sometimes referred to as a rotary gear pump, was required to fully control the throughput rate of the over-all process. Further, the overall energy efficiency of a pump-extruder system may actually be lower than the extruder itself.
While melt pumps will make more uniform the volumetric flow of melted thermoplastic material through the die, experience has shown that passage through a melt pump will not significantly reduce temperature variations in the polymer. Hence a melt pump can only ensure that the volumetric flow rate of the extruder is uniform and if the extrudate has a large temperature variations in it, these will pass through the pump and appear at the die, where they can cause irregular flow and degrade product quality. Various techniques have been employed to minimize the temperature fluctuations such as providing internally heated extruder screws alone or in conjunction with motionless mixers after the melt pump as taught for styrene-butadiene elastomers in EP 0 144 932 B 1.
In view of the continuing need for high extrudate quality during heat plastication and extrusion of low bulk density polycarbonate materials, it would be desirable if there were provided methods and apparatuses which would reduce polymer surging, lower pressure and temperature fluctuations, and reduce polymer degradation while providing good overall throughput rates.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to methods and apparatuses that reduce polymer surging, lower pressure and temperature fluctuations, and reduce polymer degradation while providing good overall throughput rates when plasticating and extruding low bulk density polycarbonate materials.
In one aspect this invention involves a single screw extruder for plasticating and extruding sheet, profile, or film from a thermoplastic material comprising a low bulk density polycarbonate material, said polycarbonate having a bulk density in the range of about 0.15 g/cm
3
to about 0.5 g/cm
3
, whereby a constant throughput of plasticated material characterized by a steady head zone melt pressure maintained within about ±10 percent is provided to and through a die.
In another aspect this invention involves a single screw extruder for plasticating and extruding sheet, profile, or film from a thermoplastic material comprising a low bulk density polycarbonate material, said polycarbonate having a bulk density in the range of about 0.15 g/cm
3
to about 0.5 g/cm
3
, comprising:
(a) a two-stage extruder screw comprising:
(i) a feed section having a screw lead to diameter ratio greater than about 1 and a constant channel depth of at least about 12 percent of the screw diameter,
(ii) a transition section having a compression ratio of at least about 3:1 and a length of at least about 5 turns, and
(iii) a metering section which is at least about 3 turns long,
(b) a hopper having an included cone angle of at least about 30°,
(c) a feed throat opening through a barrel, said opening having an aspect ratio greater than about 1 and a width of at least about 1 times the inside diameter of the barrel, and
(d) an extrusion die,
whereby a constant throughput of plasticated material characterized by a steady head zone melt pressure maintained within about ±10 percent is provided to and through the die.
In a further aspect this invention involves a two-stage extruder screw comprising:
(a) a feed section having a screw lead to diameter ratio of greater than about 1 and a channel depth of at least about 12 percent of the screw diameter,
(b) a first transition section having a compression ratio of at least about 3:1 and a length of at least about 5 turns,
(c) a first metering section which is at least about 3 turns long,
(d) optionally, a mixing section,
(f) a venting
Haemhouts Paul D.
Kirk Richard O.
Pham Hoang T.
Eashoo Mark
The Dow Chemical Company
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