Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1997-11-14
2003-01-14
Wilson, D. R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S192000, C525S211000, C525S232000, C525S240000, C524S423000, C524S424000, C524S425000, C524S426000, C524S427000, C524S445000, C524S451000, C524S518000, C524S525000, C524S528000
Reexamination Certificate
active
06506842
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to rheology-modified thermoplastic elastomer (TPE) compositions that comprise an elastomeric ethylene/alpha (&agr;)-olefin (EAO) polymer or EAO polymer blend and a high melting propylene polymer, preparation of the compositions, use of such compositions in processes such as profile extrusion and injection molding to make articles of manufacture and the resulting articles of manufacture. This invention particularly relates to such compositions wherein both components are peroxide-modified, methods for preparing the compositions, such as by peroxide modifying a physical blend of the components, and use of such compositions to produce fabricated articles, some of which are thin-walled parts such as work boot shafts via injection molding, refrigerator gaskets via profile extrusion and automotive skins via sheet extrusion and or calendering and then thermoforming.
BACKGROUND OF THE INVENTION
Manufacturers of elastomeric parts engage in an ongoing search for elastomers with processing characteristics that allow them to attain any or all of higher rates of productivity, improved quality and broader markets. Conventional processes used to make such parts include, without limitation, injection molding, profile extrusion, film extrusion, calendering, thermoforming, blown film, sheet extrusion processes. Four useful measures of how a formulation or composition will process are: shear thinning index (STI), melt strength: (MS), solidification temperature (ST) and upper service temperature (UST). Improvements in these properties have,a direct affect upon productivity, quality and market breadth relative to such elastomeric parts.
STI, as used herein, is a ratio of polymer viscosity at a specified low shear rate divided by polymer viscosity at a specified high shear rate. For ethylene/alpha-olefin (EAO) polymers, a conventional STI test temperature is 190° centigrade (° C). Polymer viscosity is conveniently measured in poise (dyne-second/square centimeter (cm
2
)) at shear rates within a range of from 0.1 radian per second (rad/sec) to 100 rad/sec and at 190° C. under a nitrogen atmosphere using a dynamic mechanical spectrometer such as an RMS-800 or ARES from Rheometrics.
MS, as used herein, is a maximum tensile force, in centiNewtons (cN), measured on a molten filament of a polymer melt extruded from a capillary rheometer die at a constant shear rate of 33 reciprocal seconds (sec
−1
) while the filament is being stretched by a pair of nip rollers that are accelerating the filament at a rate of 0.24 centimeters per second per second (cm/sec
2
) from an initial speed of 1 cm/sec. The molten filament is preferably generated by heating 10 grams (g) of a polymer that is packed into a barrel of an Instron capillary rheometer, equilibrating the polymer at 190° C. for five minutes (min) and then extruding the polymer at a piston speed of 2.54 cm/min through a capillary die with a diameter of 0.21 cm and a length of 4.19 cm. The tensile force is preferably measured with a Goettfert Rheotens that is located so that the nip rollers are 10 cm directly below a point at which the filament exits the capillary die.
ST, as used herein, is the temperature of the highest temperature peak endotherm measured during cooling (in ° C) with a differential scanning calorimeter (DSC), such as that sold by TA Instruments, Inc., as the polymer is first heated at a rate of 10° C./minute (min) from ambient temperature to a temperature of 200° C., then cooled at a rate of 10° C./min to a temperature of −30° C. and then typically reheated at a rate of 10° C./min to a temperature of 200° C.
UST, as used herein, is that temperature (° C) at which a thermomechanical analyzer (TMA) penetration probe penetrates a specimen having a thickness of two to three millimeters (mm) to a depth of 900 micrometers (&mgr;m). A suitable TMA is produced by TA Instruments, Inc. A one Newton (N) force is applied to the penetration probe as it rests on a surface of the specimen that is in a chamber where temperature is ramped at a rate of 5° C./min.
When using a profile extrusion process, a manufacturer usually desires an elastomer that “shear thins” or decreases in viscosity with applied shear forces. Because pressure drop across an extruder die and amperage required to turn an extruder screw are directly related to elastomer viscosity, a reduction in elastomer viscosity due to shear thinning necessarily leads to a lower pressure drop and a lower amperage requirement. The manufacturer can then increase extruder screw speed until reaching a limit imposed by amperage or pressure drop. The increased screw speed translates to an increase in extruder output. An increase in shear thinning also delays onset of surface melt fracture (OSMF), a phenomenon that otherwise limits extruder output. Surface melt fracture is usually considered a quality defect and manufacturers typically limit extruder output and suffer a productivity loss to reach a rate of production that substantially eliminates surface melt fracture.
When producing profile extrusions with thin walls and a complex geometry, a manufacturer looks for an elastomer with high MS and rapid solidification upon cooling in addition to good shear thinning behavior. A combination of a high MS and rapid solidification upon cooling (high ST) allows a part to be extruded hot and cooled below the elastomer's solidification temperature before gravity and extrusion forces lead to shape distortion. Ultimately, for broad market acceptance, a finished part should also retain its shape despite short term exposure to an elevated temperature during processing, shipping or eventual use.
The characteristics of high STI, high MS, rapid solidification (high ST) and increased UST are also important to manufacturers who produce elastomeric parts via injection molding. Resin pressure during injection is directly related to viscosity of the resin under specific shear conditions. A viscosity reduction due to shear thinning lowers resin pressure and reduces clamp tonnage requirements. A high MS helps eliminate part distortion during removal of a non-molten, freshly molded part from a mold. In addition, rapid solidification and increased UST allow a second polymer to be injection molded over the part without that part being deformed or melted during the second injection. Rapid solidification leads to shorter cycle times. Finally, a part must retain its strength at service temperatures and an increase in UST opens up additional markets for elastomeric parts.
Elastomeric part manufacturers who fabricate thin-walled (e.g. 2.5 mm thick), injection molded parts such as shafts (with a height of, for example, 18 inches: (45.7 cm) for industrial work boots have additional requirements beyond those specified for injection molding. They require a Shore A hardness of 30-60, preferably 35-50, for comfort around a wearer's calf. For articles of manufacture like work boots, they also seek a lower density material to make the resulting article lighter, good low temperature flexibility and improved resistance to chemicals, solvents or both. As an illustration, flexible polyvinylchloride (PVC) has a density of about 1.33 g per cubic centimeter (g/cc) and less than desirable cold temperature flexibility. Oil extended styrene block polymers such as styrene/butadiene/styrene (SBS) polymers have a density of about 1.05 g/cc and undesirable chemical resistance, solvent resistance or both.
Manufacturers who prepare elastomeric extruded and blown films and calendered sheets seek the same characteristics as those who use injection molding. An improved or increased shear thinning rheology leads to higher production rates before OSMF with its attendant variability in film or sheet thickness. A high MS promotes bubble stability in a blown film operation and provides a wide window of operations for further processing of such films via thermoforming. A high MS also promotes roll release during calendering. Rapid solidification or solidification at a higher temperature keeps a
Bethea James Robert
Heck Henry George
Martin Michael Francis
Meiske Larry Alan
Parikh Deepak Rasiklal
DuPont Dow Elastomers L.L.C.
Wilson D. R.
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