Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
2000-01-11
2002-11-26
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S176100, C264S211230
Reexamination Certificate
active
06485664
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to melt processing of olefin polymers and more particularly relates to melt extrusion and pelletization of elastomeric propylene polymers.
Extrusion of thermoplastic crystalline olefin polymers is well known in the art. Extrusion of these polymers is widely practiced from laboratory-scale test units to full-size commercial equipment. Extruders typically are used to convert polymer powder formed in a polymerization reactor to melted strands that may be chopped into pellets. Extruders also are used to blend polymer with additive materials such as stabilizers, anti-oxidants, and acid scavengers such that additives are well dispersed into the polymer. In a typical extruder apparatus, normally-solid polymer is transported through a barrel by action of a rotating screw. The polymer typically is heated by mechanical action and externally-applied heat through zones in the extruder barrel. Additives may be blended with the polymer in the screw-agitated process, and melted polymer which incorporate such additives is extruded through an orifice or die into strands, fibers, or sheets.
In a conventional extrusion process for a crystalline thermoplastic polymer, such as isotactic polypropylene, the final extruder barrel zone temperature is maintained above the polymer melting point. Cyrstalline isotactic polypropylene has a narrow temperature melting range with rapid crystallization upon cooling. If this material flows through the last extruder zone and die and falls below the polymer crystallization temperature, the isotactic polypropylene would solidify in the barrel and at the extruder orifice. This would result in excessively high torque on the extruder and high barrel pressures and, ultimately, would shut down the extruder. Thus, in a conventional extrusion process for crystalline propylene polymer, the last extruder zone temperature must be high enough to prevent polymer freezing in the barrel.
There is now a class of partially crystalline olefin polymers with a broad melting range, which may exhibit elastomeric properties. In order to incorporate additives and form into pellets, these polymers should be processed in an extruder. If these partially-crystalline polymers are extruded in a conventional manner in which the barrel exit temperature is above the polymer melting temperature, the resulting extruded polymer strand has no rigidity and is very sticky. Such a sticky strand does not easily feed into a pelletizer and may wrap around the cutting blades of a pelletizer. Even when some polymer strands get to the cutting blades, the blades may not cut completely through the strand.
This invention permits melt processing and subsequent formation of polymer pellets of such a polymer with a broad melting range without these problems.
SUMMARY OF THE INVENTION
A method to melt process a thermoplastic, partially-crystalline, olefin polymer in a multi-temperature stage extruder wherein the polymer has a broad melting temperature range comprises setting the temperature profile of the extruder such that a portion of the polymer crystallizes in the extruder and passing the resulting partially-crystallized polymer through an extruder die.
DESCRIPTION OF THE INVENTION
The method of this invention permits melt processing of a thermoplastic, partially-crystalline, olefin polymer which has a broad melting temperature range. Melt processing such a polymer through an extruder using conventional techniques in which polymer exits the extruder in a fully-melted state, produces a polymer strand which has little if any rigidity and is very sticky. As a result the strand is difficult to feed into a pelletizer and would stick to and wrap around the take-up rollers instead of feeding to the cutting blades.
In the method of this invention, such a thermoplastic, partially-crystalline, olefin polymer preferably is melted in an initial stage of an extruder, but the extruder temperature profile is set such that the polymer partially crystallizes at least at the last extruder zone. A partially-crystallized polymer is extruded into a strand which has sufficient rigidity to feed satisfactorily into a pelletizer. An adequate polymer strand is firm, typically translucent, has low stickiness, and is readily pelletized. Such strands do not significantly stick together in a collection container.
A typical thermoplastic, partially-crystalline, olefin polymer useful in this invention has a broad melting temperature range of over 50° C. and up to about 200° C. A broad melting temperature typically indicates the compositions contain a minor amount of crystallizable material within a matrix of amorphous material.
In describing this invention, melting ranges and crystallization temperature are measured using Differential Scanning Calorimetry (DSC). Using DSC to measure melting characteristics of a polymer useful in this invention shows a range of melting in a polymer in which crystalline phase will be present in a melted phase. In contrast to a DSC measurement of an isotactic polypropylene which shows a narrow temperature range of melting, polymers useful in this invention will show a broad melting range of over 50° C. and up to about 200° C. Typical melting ranges are about 100 to 150° C. The melting range typically is measured as the width of the crystalline melting endotherm as observed in the DSC. The melting range is sufficiently broad to permit a minor amount of crystalline phase to be incorporated within a major amount of a flowable non-crystalline matrix phase at a temperature within the melting range. A flowable polymer will pass through an extruder without using significant pressure or torque. Although melted polymer exists throughout the melting range, typically the melting temperature as measured by DSC (T
m
) is the maximum peak (or inverse peak) of the DSC thermogram heating at 20° C./min. This should correspond to the temperature at which the largest portion of crystalline material melts.
Another temperature measurable by DSC is the crystallization temperature (T
c
) which is determined by cooling a totally melted polymer and determining the maximum peak (or inverse peak) in the DSC cooling at 10° C./min. As the polymer is cooled, it passes through a supercooled phase before crystallization occurs. Thus the T
c
will be lower than the T
m
. Polymers used in this invention may have a T
c
20 to 100° C. (typically 30 to 90° C.) lower than the T
m
. Typically, polymers useful in this invention will be sticky if rapidly cooled from a total melt phase because a solid supercooled phase is produced which does not include significant amounts of crystalline phase.
Olefin-based polymers useful in this invention include polymers of ethylene, propylene and C
4
-C
8
olefins having a broad melting range. Partially crystalline olefin polymers having a broad melting range include elastomeric propylene polymers and propylene-ethylene copolymers which may have up to 50 mole % of ethylene.
Propylene polymers useful in this invention should have about 10 to about 30 percent crystallinity which corresponds to m4 values (as measured by
13
C NMR) of about 25 to 55%. The isotactic pentad (m4) content is the percentage of isotactic stereosequences of five contiguous stereocenters as measured by
13
C NMR. The m4 of a statistically atactic polypropylene is about 6.25% while that of a highly isotactic polypropylene approaches 100%. Typical polymers useful in this invention have a crystallinity of 15 to 25% at room temperature (20° C.) which corresponds to m4 values of about 25 to 45%. Typical melting temperatures for useful propylene polymers are about 75 to about 155° C., preferably about 100 to about 150° C. Typical crystallization temperatures for useful propylene polymers are about 45 to about 120° C., preferably about 80 to about 110° C.
At the exit zone of an extruder in the process of this invention, crystallized polymer is incorporated into a matrix of flowable non-crystalline phase. Thus, after such polymer passes through the extruder exit zone, the polymer will solidify into a f
BP Corporation North America Inc.
Eashoo Mark
Oliver Wallace L.
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