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
2001-12-10
2003-06-24
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S200000
Reexamination Certificate
active
06583226
ABSTRACT:
The invention relates to melt processible fluoropolymer compositions, in particular compositions that comprise a melt processible fluoropolymer comprising repeating units derived from tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). Such copolymers are called “FEP”.
FEP fluoropolymers have been known for a long time (U.S. Pat. No. 2,946,763), and are commercially available. FEP fluoropolymers are perfluorinated thermoplastic fluoropolymers that have excellent heat resistance and chemical resistance. FEP fluoropolymers also have a low dissipation factor (EP-A-423 995). Due to all of these properties FEP polymers are of interest for use as an insulating material for cable-wire insulation, in particular for what are known as plenum wire cables, used for example in LANs (local area networks). The processing speeds for producing insulating plenum cables are very high. FEP polymers that can be used in producing such plenum cables are therefore generally those which permit processing at high shear rates without loss of the necessary mechanical properties.
FEP polymers which have a broad molar mass distribution ensure relatively fast processing at relatively high shear rates (DE-A-26 13 795, DE-A-26 13 642, EP-A-88 414, EP-A-362 868). Modification with another comonomer (DE-A-27 10 501, EP-A-75 312), such as perfluoro vinyl ethers, yields retention of the necessary mechanical properties. To generate high extrusion speeds while retaining a smooth melt surface, nucleating agents are often added to the polymeric materials to suppress, and/or shift the occurrence of “shark skin” (melt surface instability, giving a rough surface) to higher shear rates(U.S. Pat. No. 5,688,457).
Besides the formation of “shark skin” at high shear rate, the tendency of perfluorinated thermoplastics to form die deposits has to be considered. These die deposits are processing condition dependent, and take effect in different ways. In fast extrusion procedures, such as cable-wire insulation, large accumulations of die deposits separate from the die and cause break-off of the melt cone and thus interruption of the production, and also interruption of the continuous cable. High processing temperatures promote die deposits, and at these temperatures the FEP products decompose more rapidly, as becomes apparent through discoloration and molecular degradation. This thermal instability is attributable to unstable end groups, HFP diads in the main polymer chain (EP-A-150 953) and metal contamination. The decomposition reaction of the thermally unstable end groups has been described in “Modern Fluoropolymers”, Ed. John Scheirs, Wiley & Sons 1997, page 228. For this reason thermally unstable end groups, including COOH, CONH
2
and COF groups, are preferably converted into thermally stable end groups by fluorination (GB-A-1 210 794, EP-A-150 953, EP-A-222 945) or by a stabilization process in the presence of water vapor (DE-A-26 13 795, DE-A-26 13 642). The amounts of die deposits can be minimized by preparing FEP materials with stable end groups, combined with high purity with respect to metal ions and narrow molecular weight distribution (German Patent Application 199 03 657.8 of Jan. 29, 1999, corresponding to PCT/EP00/00528 of Jan. 24, 2000). However, this high purity is accompanied with increased purification costs, and a narrow molecular weight distribution, which further promotes the onset of “shark skin”.
When FEP polymers are melt extruded, high extrusion speeds increase deposits on the die. These deposits, known as die deposits, accumulate as time passes and break away from the die when they reach a particular size. This results in damage to the final product or, in the case of break-off of the melt, to interruption of production with other serious consequences. After a break-off production has to be interrupted until a new cable has been threaded into the die. The break-off also limits the length of the cable, thus producing unnecessary waste material in the twisting of a number of cables of different length. Die deposits of this type are therefore regularly removed from the die during processing, but this removal is almost impossible during high-speed processing, such as cable-wire insulation, and particularly in this application it had to be accepted that frequent break-offs of the melt cone would occur.
The present inventors have found that it would thus be desirable to reduce the number of times a melt cone breaks off in the extrusion of FEP polymers, in particular at high speed. Preferably, this problem is solved without sacrificing the resulting mechanical properties. Desirably, the mechanical properties of the FEP polymers are further improved.
In accordance with the present invention there is provided a melt-processible perfluorinated polymer composition comprising
a) a melt-processible perfluoropolymer comprising
(i) from 80 to 98% by weight of repeating units derived from tetrafluoroethylene,
(ii) from 2 to 20% by weight of repeating units derived from hexafluoropropylene, and
(iii) from 0 to 5% by weight of repeating units derived from further comonomers other than tetrafluoroethylene and hexafluoropropylene, and wherein the proportion by weight of the repeating units derived from hexafluoropropylene units is greater than that of the repeating units of said further comonomers, and
b) from 0.01 to 5% by weight, based on perfluoropolymer a), of a high-molecular-weight perfluorinated polymer with a melting point at least 20° C. above that of the fluoropolymer a).
The invention further provides a method of producing the above melt-processible composition, the use thereof in melt-extrusion, in particular to extrude insulation around a wire to produce an electrical cable. The invention also relates to an electrical cable having the melt-processible composition as an insulation.
The inventors have recognized that the actual problem is not the die deposit itself but excessive accumulation of the same and the release of relatively large accumulations, which finally leads to break-off of the melt cone during wire coating.
A mixture of the aforementioned melt-processible perfluoropolymer with a small proportion of a high-melting, i.e. having a higher melting point than the melt-processible perfluoropolymer, and high-molecular-weight fluoropolymer, i.e. having a higher molecular weight than the melt-processible perfluoropolymer, performs quite differently than known FEP melt-processible perfluoropolymers. Under the same conditions, there is very little accumulation of the die deposits which form, since they regularly break away from the die at very short intervals. Without intending to be bound by any theory, it is believed that a possible reason for this is the presence of minor non-uniformities in the melt, which may be brought about by the high-molecular-weight, higher-melting fluoropolymer which entrains the die deposits. The use of the mixture of the melt-processible perfluoropolymer with the higher-melting, higher molecular weight perfluoropolymer for high-speed cable-wire sheathing, for example plenum wire production, may reduce by a factor of 5 the number of break-offs observed of the melt cone. This ensures continuous production with fewer interruptions to production and longer cables.
The melt-processible composition of the invention generally also permits an improvement in mechanical properties in comparison with prior art FEP polymers.
For example the flexural fatigue strength (“flex life”) of the melt-processible composition of this invention is often many times greater than that of known FEP copolymer products with the same melt flow index (MFI). In the art, high flexural fatigue strengths of FEP products have hitherto been achieved by modification with perfluoro alkyl vinyl ethers (PAVEs). However, the dipole moment of PAVEs and the high dissipation factor associated with this makes these materials disadvantageous, particularly for high-frequency cable applications.
The melt-processible perfluorinated polymer composition according to the invention, may achieve comparable flexural fatigue p
Kaulbach Ralph
Kloos Friedrich
3M Innovative Properties Company
Harts Dean M.
Nutter Nathan M.
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