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
2001-12-11
2004-09-14
Nolan, Sandra M. (Department: 1772)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S199000
Reexamination Certificate
active
06790912
ABSTRACT:
TECHNICAL FIELD
This invention relates to melt processable fluorothermoplastic compositions comprising a blend of different fluoropolymers, as well as methods of improving extrusion properties in extruded fluorothermoplastics.
BACKGROUND
Fluoropolymers have found wide utility in a vast array of applications. However, they are often beset by processing difficulties. Specific difficulties include surface roughness such as melt fracture and other problems such as die drooling.
Higher processing temperatures can reduce some of these problems, but may involve other problems. One such problem is the molecular weight degradation of the fluoropolymer. This can result in reduced physical properties, can contribute to the die drool, and the corrosive by-products can lead to premature wear of the processing equipment.
Another alternative to reduce the surface roughness of extrudates is to reduce the processing rate. This increases the residence time of the fluoropolymer in an extruder, which also contributes to degradation. Reducing production rates is also economically undesirable.
Decreasing the molecular weight of the input fluoropolymer can allow for limited improvements in output, but this also decreases the mechanical properties of the polymer. Such a mechanical property detriment may then be partially offset by the addition of costly comonomers, but this modification can add production complications and detract from other physical properties.
Another approach toward reducing surface defects in fluoropolymers has been to create a mixture of several fluoropolymers having similar composition yet of significantly different molecular weights in attempt to balance the polymer properties with the processing parameters. In theory, a lower molecular weight portion allows for higher output rate with the blend, while a higher molecular weight portion improves the mechanical properties of the blend. This compromise achieves limited success and increases the complexity required to produce such a material.
Yet another approach involved adding a polyolefin to specific fluoropolymers. However, the temperatures necessary for processing fluoropolymers are usually too high for this approach. In addition, such a material can negatively affect properties of the fluoropolymer, such as color, permeation rate, and chemical resistance.
JP 60-23701 describes a blend of a fluorinated elastomer and a copolymer of tetrafluoroethylene and hexafluoropropene (FEP) to achieve heat stress-crack resistance. U.S. Pat. No. 5,051,479 describes a melt-processable thermoplastic consisting essentially of a blend of a fluoropolymer and an elastomeric tetratluoroethylene-perfluoro(alkyl vinyl) ether copolymer.
SUMMARY
Despite these attempts, there is still a great need to be able to process fluoropolymer materials at higher output rates while maintaining a quality finish on the part and without sacrificing mechanical properties.
Accordingly, the present invention provides a melt processable fluorothermoplastic composition comprising a major amount of a first semi-crystalline fluorinated copolymer and a minor amount of a second fluoropolymer effective to reduce melt defects in the composition. Each fluoropolymer is selected from four classes. The first class, (a), includes a semi-crystalline perfluorinated copolymer. The second class, (b), includes a fluoropolymer derived from interpolymerized units of at least one essentially perfluorinated monomer and at least one non-fluorinated hydrogen-containing monomer. The third class, (c), includes a fluoropolymer derived from interpolymerized units of at least one partially-fluorinated monomer, and optionally at least one essentially perfluorinated monomer. The fourth class, (d), includes an amorphous copolymer of tetrafluoroethylene and hexafluoropropylene. When the first fluoropolymer is selected from (a), the second fluoropolymer is a semi-crystalline fluoropolymer selected from (b) and/or (c). When the first fluoropolymer is selected from (b), the second fluoropolymer is selected from (a), (c), and/or (d). When the first fluoropolymer is a copolymer selected from (c), the second fluoropolymer is selected from (a), (b), and/or (d).
In another aspect, the present invention provides a melt processable fluorothermoplastic composition comprising a major amount of a semi-crystalline fluorinated copolymer and a minor amount of a fluoropolymer effective to reduce melt defects (such as melt fracture or surface roughness) in the composition. The second fluoropolymer (minor amount) is selected from an amorphous fluorinated copolymer derived from interpolymerized units of a perfluoro (alkoxy vinyl) ether and a comonomer which may be partially or fully fluorinated, and/or an amorphous fluorinated copolymer derived from interpolymerized units of at least 3 mole percent (mol %) of an hydrogen containing comonomer, and a perfluoro (alkoxy vinyl) ether and/or a perfluoro (alkyl vinyl) ether. This group of materials may also be used in conjunction with the other minor component fluoropolymers as defined herein.
In another aspect, the present invention provides a method of improving extrusion properties in an extrudate. This method comprises blending a major amount of a first semi-crystalline fluorinated copolymer and a minor amount of a second fluoropolymer effective to improve extrusion properties in the composition, and melt processing the blend to form the extrudate. In this method, each fluoropolymer is selected from one of four classes:(i) a semi-crystalline perfluorinated copolymer; (ii) a fluoropolymer derived from interpolymerized units of at least one essentially perfluorinated monomer and at least one non-fluorinated hydrogen-containing monomer; (iii) a fluoropolymer derived from interpolymerized units of at least one partially-fluorinated monomer, and optionally at least one essentially perfluorinated monomer; and/or (iv) an amorphous fluorinated copolymer derived from interpolymerized units of a perfluoro (alkoxy vinyl) ether and a comonomer which may be partially or fully fluorinated, and/or an amorphous fluorinated copolymer derived from interpolymerized units of at least 3 mole percent (mol %) of an hydrogen containing comonomer, and a perfluoro (alkoxy vinyl) ether and/or a perfluoro (alkyl vinyl) ether. Also in this method, when the first fluoropolymer is selected from (i), the second fluoropolymer is a fluoropolymer selected from at least one material of class (ii), a semi-crystalline material of class (iii), and/or a material from class (iv); when the first fluoropolymer is selected from (ii), the second fluoropolymer is selected from (i), (iii), and/or (iv); and when the first fluoropolymer is a copolymer selected from (iii), the second fluoropolymer is selected from (i), (ii), and/or (iv).
When a fluoropolymer is melt-processed, issues of surface defects, output rates, and mechanical properties must be addressed. Surprisingly, when these same fluoropolymers are used in blends of the present invention, remarkably improved processing flexibility results. The extrudable compositions of the invention are not as prone to the thermal instability issues of known blends. In addition, the end-use properties of articles made from the major component fluoropolymer, such as permeation, extraction, and chemical resistance, can be preserved in the fluoropolymer blends of the present invention.
DETAILED DESCRIPTION
In one aspect of the present invention, a fluoropolymer blend is comprised of combinations of fluoropolymers selected from four distinct classes. Materials are selected from at least two different classes to form the fluoropolymer blend of the invention.
The fluoropolymers and fluoropolymer blends of the invention are melt processable. As used herein, “melt processable” means that the material can be repeatedly processed with the assistance of heat. That is, the fluoropolymers and fluoropolymer blends can be extruded without thermosetting or cross-linking.
The fluoropolymers of the invention preferably have a melt flow index (MFI) of about 1 g per 10 min or greater, under a lo
3M Innovative Properties Company
Harts Dean M.
Nolan Sandra M.
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