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
1999-05-13
2001-08-21
Zitomer, Fred (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...
C525S239000, C525S240000
Reexamination Certificate
active
06277919
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a polymer processing additive that employs a multimodal fluoropolymer; to a melt processable thermoplastic polymer composition that utilizes this polymer processing additive; and to a method of improving the melt processability of a melt processable thermoplastic polymer.
BACKGROUND
For any melt processable thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. See, for example, R. F. Westover,
Melt Extrusion
, Encyclopedia of Polymer Science and Technology, Vol. 8, pp 573-81 (John Wiley & Sons 1968). The desire for a smooth extrudate surface competes, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (i.e. at high shear rates).
Some of the various types of extrudate roughness and distortion observed in high and low density polyethylenes are described by A. Rudin, et al.,
Fluorocarbon Elastomer Aids Polyolefin Extrusion
, Plastics Engineering, Mar. 1986, at 63-66. The authors state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer melt defects. At low shear rates, defects may take the form of “sharkskin”, a loss of surface gloss, that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction. At higher rates, the extrudate can undergo “continuous melt fracture” becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from “cyclic melt fracture”, in which the extrudate surface varies from smooth to rough. The authors state further that lowering the shear stress by adjusting the processing conditions or changing the die configuration can avoid these defects to a limited extent, but not without creating an entirely new set of problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation.
There are other problems often encountered during the extrusion of thermoplastic polymers. They include a build up of the polymer at the orifice of the die (known as die build up or die drool), increase in back pressure during extrusion runs, and excessive degradation or low melt strength of the polymer due to high extrusion temperatures. These problems slow the extrusion process either because the process must be stopped to clean the equipment or because the process must be run at a lower speed.
Certain fluorocarbon processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. U.S. Pat. No. 3,125,547 to Blatz, for example, first described the use of fluorocarbon polymer process aids with melt-extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1:2 and wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers.
U.S. Pat. No. 4,904,735 (Chapman, Jr. et al.) describes a fluorinated processing aid for use with a difficultly melt-processable polymer comprising (1) a fluorocarbon copolymer which at the melt-processing temperature of the difficultly melt-processable polymer is either in a melted form if crystalline, or is above its glass transition temperature if amorphous, and (2) at least one tetrafluoroethylene homopolymer or copolymer of tetrafluoroethylene and at least one monomer copolymerizable therewith wherein the mole ratio is at least 1:1, and which is solid at the melt-processable temperature of the difficultly melt-processable polymer.
U.S. Pat. No. 5,397,897 to Morgan et al. Describes the use of copolymers of tetrafluoroethylene and hexafluoropropylene having high hexafluoropropylene content as processing aids in polyolefins.
U.S. Pat. Nos. 5,064,594 to Priester et al., and U.S. Pat. No. 5,132,368 to Chapman, Jr. et al. describe the use of certain fluoropolymer process aids containing functional polymer chain end groups including —COF, —SO
2
F, —SO
2
Cl, SO
3
M, —OSO
3
M, and —COOM, wherein M is hydrogen, a metal cation, or a quaternary ammonium cation for use with a difficultly melt-processable polymer.
U.S. Pat. No. 5,464,904 to Chapman et al. discloses the use of unimodal semicrystalline fluoroplastics such as copolymers of tetrafluoroethylene and hexafluoropropylene and terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride with a polyolefin. The only enhancement of melt-processability described in this patent is shown in Example 25 where a concentration of 1000 ppm of the fluoropolymer in linear low density polyethylene is said to reduce the extrusion pressure of the extrudable composition. There is no showing of a reduction in melt defects.
U.S. Pat. Nos. 5,015,693 and 4,855,013 to Duchesne and Johnson disclose the use of a combination of a poly(oxyalkylene) polymer and a fluorocarbon polymer as a processing additive for thermoplastic hydrocarbon polymers. The poly(oxyalkylene) polymer and the fluorocarbon polymer are used at such relative concentrations and proportions as to reduce the occurrence of melt defects during extrusion. Generally the concentration of the fluoropolymer is present at a level of from 0.005 to 0.2 weight percent of the final extrudate and the poly(oxyalkylene) polymer is present at a level of from 0.01 to 0.8 weight percent of the final extrudate. Preferably, the weight of the fluorocarbon polymer in the extrudate and the weight of the poly(oxyalkylene) polymer in the extrudate are in a ratio of 1:1 to 1:10.
U.S. Pat. No. 5,710,217 to Blong at al. Discloses an extrudable thermoplastic hydrocarbon composition that comprises an admixture of a melt processable hydrocarbon polymer as the major component and an effective amount of a chemically-resistant fluoropolymer process aid. The fluoropolymer contains at least 50% by weight of fluorine and comprises one or more fluoropolymers that are essentially completely ethylenically unsaturated.
SUMMARY
It has been discovered that a multimodal fluoropolymer is surprisingly effective in reducing melt defects such as sharkskin in thermoplastic polymers, or in postponing these defects to higher extrusion rates than can be typically achieved without the use of such a multimodal fluoropolymer, even when the melt flow index of the multimodal fluoropolymer as a whole is the same as the melt flow index of a unimodal fluoropolymer. Additionally, it has been discovered that multimodal fluoropolymers are also surprisingly effective in reducing the occurrence of die build up, reducing the amount of back pressure during extrusion of non-hydrocarbon polymers, and permitting the use of lower extrusion temperatures to achieve an equivalent throughput.
Briefly, in one aspect, the present invention provides a polymer processing additive composition based on a multimodal fluoropolymer. As used herein, the term multimodal means that the fluoropolymer has at least two components of discrete and different molecular weights. The first, or low molecular weight component (referred to hereinafter as Component A) has a relatively high melt flow index (MFI
A
). The second, or high molecular weight component (referred to hereinafter as Component B) has a relatively low MFI
B
. Preferably the multimodal fluoropolymer contains no more than two or three of the discrete molecular weight components.
The exact MFI value that defines Component A and Component B is determined by how the multimodal polymer is to be used. For example, if the multimodal polymer is to be used an additive to enhance the processability of a hydrocarbon polymer, MFI
A
is preferably greater than 50 while MFI
B
is preferably 50 or less, more prefe
Amos Stephen E.
Dillon Maria P.
Fronek Kirsten J.
Hintzer Klaus
Hirsch Bernhard
Dyneon LLC
Harts Dean M.
Lilly James V.
Zitomer Fred
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