Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1999-04-30
2001-06-26
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S050000, C264S053000, C264S146000, C264SDIG005
Reexamination Certificate
active
06251319
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to extruded thermoplastic foams.
Rigid foam sheets have a variety of applications, including feedstock for thermoforming applications, such as cups, bowls, non-flexible applications where stiffness is needed to span large distances, as in automobile headliner substrates or packaging decks, trays, etc. However, certain applications require higher heat resistance than are typically seen with foams of polymers such as polyethylene and polystyrene. Polyurethane foam is a foam typically employed in such applications, but may not be as readily thermoformable as a foam of a thermoplastic polymer. Foam sheet made from foamable polypropylene beads are a means to obtain a thermoformable foam having a relatively high heat resistance, but the foam may not have impact resistance or flex and tensile strength properties required for certain applications. Also, polypropylene bead foams are often made from polypropylene copolymers which are more flexible than polypropylene homopolymer foams and will have a lower heat resistance property as compared to foams made from homopolymer polypropylene.
U.S. Pat. No. 3,637,458 discloses microcellular foam sheet made by an extrusion process from polypropylene polymer, but the use of relatively large amount of blowing agent in such processes produces a foam which may not have the desired density, thickness, or stiffness needed for certain applications.
SUMMARY OF THE INVENTION
In one aspect, this invention is a process for making a polymer foam sheet, which comprises (i) extruding through an annular extrusion die a foamable composition comprising at least one polypropylene polymer and at least one blowing agent, at a pressure, temperature, and extrusion rate sufficient to prevent foaming of the composition prior to exiting the die, under conditions sufficient to form a foam tube having a density of at least 16 kg/m
3
and no greater than 200 kg/m
3
, an open cell content of less than 70 percent, a circumference of at least 1 meter, and a foam thickness of at least 2 mm and no greater than 25 mm, wherein the blowing agent is used in an amount of from 0.2 to 4.5 moles per kg of polymer, (ii) allowing the foam to cool to a temperature below the melt temperature of the polypropylene, and (iii) cutting the tube of foam along its radius to form a foam sheet, wherein at least 70 percent by weight of the polymer in such sheet is a polypropylene having a melt strength in the range of from 30 to 60 cN when measured at 190° C.
It has been discovered that the invention provides a means to prepare relatively dense, closed-cell or partially open cell, thermoformable foam sheets from polypropylene polymers. These and other advantages of the invention will be apparent from the description that follows.
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Christenson Christopher P.
Cornell Martin C.
Park Chung P.
Suh Kyung W.
Tusim Martin H.
Kuhns Allan R.
The Dow Chemical Company
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