Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-08-28
2004-02-03
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S242000, C526S250000
Reexamination Certificate
active
06686426
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to perfluorinated copolymers of tetrafluoroethylene and perfluoro alkyl vinyl ethers. In particular, the present invention relates to copolymers that include a blend of a linear perfluoro alkyl vinyl ether and a branched perfluoro alkyl vinyl ether. The invention further relates to articles produced therefrom and a method of making molded articles using the perfluoro copolymers.
2. BACKGROUND OF THE INVENTION
Polytetrafluoroethylene (PTFE) is widely used and is well-known for its excellent mechanical properties combined with chemical inertness, heat resistance, non-flammability, anti-stick properties and exceptional dielectric properties. However, a well-known disadvantage of PTFE is also its extreme high melt viscosity making it unsuitable for melt-processing techniques. As a result, the processing of PTFE into desired shapes or articles requires cumbersome techniques such as sintering.
Accordingly, the art has developed melt-processible perfluoropolymers that can be extruded or molded into shapes and that approach the beneficial properties of PTFE as much as possible. For example, copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) were the first melt processible perfluoropolymers developed. These copolymers, which are known as FEP polymers, however do not have desired heat stability and high temperature properties as may be desired in some applications. A further known type of melt-processible perfluoropolymers is disclosed in U.S. Pat. No. 3,635,926 and includes copolymers of TFE and perfluoropropylvinyl ether (PVE). These copolymers are known as PFA (perfluoroalkoxy polymers) and have improved heat stability and high temperature performance.
WO 97/07147 discloses copolymers of perfluoro ethyl vinyl ether with TFE and U.S. Pat. No. 4,864,006 discloses copolymers of TFE with perfluoro methyl vinyl ether. Still further known copolymers of TFE with a perfluorovinyl ether monomer are disclosed in U.S. Pat. No. 4,029,868 disclosing a terpolymer of TFE, HFP and a perfluorovinyl ether and in U.S. Pat. No. 4,262,101 disclosing a copolymer of TFE and perfluoroalkylvinyl ether optionally in combination with HFP.
EP 75 312 discloses FEP type of polymers that have from 80 to 95.8% by weight of units derived from TFE, 4 to 14% of units derived from HFP and 0.2 to 6% by weight of units derived from a branched perfluoro alkoxy alkyl vinyl ether. The resulting polymers are taught to have a good moldability and good stress crack resistance. However, since such polymers are of the FEP type, they do not have the desired heat stability and performance at high temperature.
U.S. Pat. No. 4,381,387 discloses copolymers of TFE, ethylene, HFP and a bulky comonomer. These polymers are taught to be suitable for wire coatings and making of extruded articles. However, these copolymers are not fully fluorinated and are therefore do not have high chemical and heat resistance.
Copolymers of TFE and perfluoroalkyl vinyl ethers are also used to obtain fluoroelastomers. For example, U.S. Pat. No. 4,920,170 discloses a copolymer that consists of (a) 30 to 80 mol % of TFE, (b) 5 to 60 mol % of perfluoro(lower)alkyl vinyl ether and (c) a perfluoroether of the formula CF
2
═CFOCF
2
CFXOR
f
in which X is F or CF
3
and R
f
is a perfluoro alkyl group, and wherein the sum of (a), (b) and (c) is 100 mol %. Accordingly, the here disclosed copolymers would have at least 20 mol % of perfluorovinyl ether monomer which makes these polymers elastomeric as opposed to thermoplastic and semicrystalline as is the case for PFA type of perfluoropolymers.
The thermal degradation takes place mainly via the thermally unstable end groups formed during the polymerization, i.e. from the end of the chain. The mechanism of this degradation is described in more detail in “Modern Fluoropolymers”, John Wiley & sons, 1997, p. 223. Thermal degradation produces corrosive gases which considerably impair the quality of the final product by way of metal contamination or the formation of small bubbles, and they can corrode tooling and processing machinery. Naturally, the effect increases as molecular weight falls (lower melt viscosity). Degradation can be substantially suppressed by using postfluorination to convert the thermally unstable end groups into stable CF
3
end groups, for example as in U.S. Pat. No. 4,743,658 and DE-A-1 901 872.
Another problem associated with thermoplastic semicrystalline copolymers of TFE is the so called mechanical degradation which is believed to proceed through chain breakage during processing of the copolymer. As a result, the lower molecular weight fraction formed may negatively impact the mechanical properties, in particular flex life and stress crack resistance.
Similarly, low molecular weight fractions may also result from the polymerization method by which the copolymers are produced. For example, aqueous emulsion polymerization is often used to produce the aforementioned polymers which generally leads to the presence of a small fraction of low molecular weight polymer. As a result, these polymers may also have less than desirable mechanical properties such as pressure cycle resistance, flex life and creep rupture strength.
Further PFA type polymers are semicrystalline and include spherolites which reduce the transparency. The level of transparency depends on the size of the spherolites that are present in the polymer. Generally, these spherolites will have an average size of 5 &mgr;m to 10 &mgr;m or even more. Also, because of the presence of large sized spherolites, the surface of articles produced is somewhat rough and therefore picks up dust. It would thus be desirable to further reduce the dust pick-up.
3. SUMMARY OF THE INVENTION
Despite the many known thermoplastic perfluorocopolymers of TFE, there continues to exist a need to find further perfluorocopolymers of TFE that are melt-processible and that have desirable properties. It is desired to find perfluorocopolymers of TFE that have high temperature resistance, high chemical resistance, easy processing, good mold release and good mechanical properties in particular better flex life in combination with better transparency, and better surface smoothness of articles produced from the polymer. It is further desirable to find perfluoropolymers that are particularly suitable for making hoses and pipes particularly for use in heat exchange applications.
In accordance with the present invention, thermoplastic semicrystalline copolymers of TFE and a mixture of a first and second perfluoro vinyl ether and wherein the copolymers have a melting point of at least 285° C., preferably at least 300° C. are provided. In particular, the first perfluoro vinyl ether ether is a perfluoro n-alkylvinyl ether having 1 to 4 carbon atoms in the alkyl group and the second perfluoro vinyl ether corresponds to the formula:
wherein R represents a linear perfluoro alkyl group having 1 to 4 carbon atoms.
Such perfluoropolymer can be produced with a thermal conductivity at 23° C. of at least 0.19 W/mK and have particularly suitable properties for use in applications such as hoses and pipes, in particular for heat exchange applications. Beneficial properties of such polymers include a good flex life and good transparency.
The thermoplastic perfluorocopolymers of TFE and the mixture of the first and second perfluoro vinyl ether that have a melting point of at least 285° C., preferably at least 300° C.
generally have small sized spherolites when made into an article through for example molding or extrusion. Accordingly, in a further aspect, the invention relates to an article of a thermoplastic, semicrystalline fluoropolymer having a melting point of at least 285° C. and comprising repeating units derived from tetrafluoroethylene, repeating units derived from a first vinyl ether monomer and repeating units derived from a second vinyl ether monomer, wherein said first vinyl ether is a perfluoro n-alkylvinyl ether having 1 to 4 carbon atoms in the alkyl group and said second vinyl ether is a perfluoro v
Kaulbach Ralph
Traunspurger Helmut
3M Innovative Properties Company
Harts Dean M.
Zitomer Fred
LandOfFree
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