Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-02-02
2001-03-06
Pezzuto, Helen L. (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, C526S253000
Reexamination Certificate
active
06197904
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of melt-fabricable copolymers of tetrafluoroethylene, specifically copolymers of tetrafluoroethylene and ethylene.
BACKGROUND OF THE INVENTION
Crystalline dipolymers of tetrafluoroethylene (TFE) and ethylene (E) can be produced over the entire composition range from polyethylene to polytetrafluoroethylene, creating polymers which melt between about 110° C. and 327° C. At about 50/50 molar ratio, where TFE/E copolymers are typically produced, a local maximum in the melting point curve is encountered. At this 50/50 ratio, a number of other properties also reach a local maximum (or minimum), such as modulus and crystallinity. This phenomenon was recognized by Carlson in U.S. Pat. No. 3,624,250. He defined a range of 60/40 to 40/60 which encompasses most of this local maximum. At the maximum, melting points of 270°-285° C. can be achieved.
Dipolymers of ethylene and tetrafluoroethylene have poor resistance to stress cracking, particularly at elevated temperature. Incorporation of a termonomer (Carlson, U.S. Pat. No. 3,624,250) was found to improve stress crack resistance and provide polymers which have found wide application in wire and cable coatings, films, and injection moldings. Many termonomers have been found to provide the desired cracking improvement in TFE/E polymers, including most common fluorocarbon and hydrocarbon vinyl compounds that introduce a bulky side group into the polymer. However, a limited number of such termonomers have been used in the commercial manufacture of TFE/E resins, including perfluorobutyl ethylene (PFBE), hexafluoropropylene (HFP), perfluoro(propyl vinyl ether) (PPVE), hexafluoroisobutylene (HFIB) and CH
2
=CF(CF
2
)
3
H. Known TFE/E copolymers that are terpolymers are described, for example, in U.S. Pat. Nos. 3,960,825; 4,123,602; 4,513,129 and 4,677,175; and in Published Japanese Patent Application (Kokai) 07(1995)-041522.
Sulzbach in U.S. Pat. No. 4,381,387 discloses a quaterpolymer having a melting point of 245°-280° C. and consisting essentially of 55-30 mol % TFE, 60-40 mol % ethylene, 10-1.5 mol % HFP, and 2.5-0.05 mol % of a bulky vinyl monomer selected from one of a number of classes, including a class of perfluoro(alkyl vinyl ethers) (PAVE). This patent is said to provide TFE/E copolymers which provide a satisfactory compromise between thermal and chemical stability on the one hand and the tensile and elongation behavior on the other hand, with the use of a smaller quantity of a bulky vinyl compound. However, melting points achieved in the '387 patent are not low despite substantial incorporation of HFP. Sulzbach's Example 6 quaterpolymer has TFE/E/HFP/PPVE molar composition of 47.0/44.3/8.4/0.4 and a melting point of 247° C. The Example 14 quaterpolymer uses perfluorohexyl ethylene (PFHE) instead of PPVE and has TFE/E/HFP/PFHE molar composition of 45.4146.5/3.7/0.2 and a melting point of 272° C. Furthermore, it is well-known that the reactivity of HFP is low, so the use of a substantial quantity of HFP adversely affects polymerization rate. Sulzbach & Hartwimmer in U.S. Pat. No. 4,338,237 disclose a polymerization process for preparing TFE/E copolymers, including the quaterpolymers disclosed in the '387 patent.
In EXAMPLE I of U.S. Pat. No. 3,624,250, Carlson discloses a TFE/E/PPVE copolymer having molar composition of 48.8/48.8/2.4, respectively, and a melting point of 255° C., and in EXAMPLE III the TFE/E/PEVE counterpart having a melting point of 262° C., PEVE being perfluoro(ethyl vinyl ether).
TFE/E copolymers that have low stiffness and have good flex life at elevated temperature are needed for such applications as flexible, thin-wall appliance wiring and in coatings on large-diameter cables. Particularly desired are TFE/E copolymers having sufficiently low melting temperatures that the copolymers can be melt processed together with thermally less stable polymers such as fluorine-free polymers, i.e., co-processed such as by co-extrusion. Such composites are currently sought, for example, for fuel hose applications which require a combination of fuel resistance, ruggedness, and flexibility.
SUMMARY OF THE INVENTION
This invention provides a partially-crystalline melt-fabricable copolymer comprising major fractions of tetrafluoroethylene and ethylene, and minor fractions of at least one perfluoro(alkyl vinyl ether), said alkyl having 1-5 carbon atoms, and optionally fluoroalkyl ethylene, said fluoroalkyl having 2-10 carbon atoms. The minor fractions are present in an amount effective to yield copolymer having a melting point of no more than 220° C. It has been discovered that, when fluoroalkyl ethylene is not present, this low melting point can be obtained when the total concentration of perfluoro(alkyl vinyl ether) in the copolymer is at least 3 mol % based on total monomer units in the copolymer. It has also been discovered that, when fluoroalkyl ethylene is present, the combined concentration of minor fractions in the copolymer is at least 2 mol %, preferably at least 3 mol %. More preferably, the total concentration of minor fractions in the copolymer, with fluoroalkyl ethylene present or not, is at least 4 mol %. Preferred perfluoro(alkyl vinyl ethers) include perfluoro(ethyl vinyl ether), and preferred fluoroalkyl ethylenes include perfluorobutyl ethylene.
The molar ratio of tetrafluoroethylene to ethylene in the copolymers of the invention is in the range of from 73/27 to 40/60. In the most preferred embodiment of the invention, this molar ratio is greater than 60/40.
In a preferred embodiment, the invention provides low-melting copolymers of tetrafluoroethylene and ethylene having low flex modulus.
As a result of low melting temperature, the copolymers can advantageously be co-processed with less thermally stable polymers at temperatures at which such polymers are essentially thermally stable.
DETAILED DESCRIPTION
Copolymers of the present invention comprise tetrafluoroethylene (TFE), ethylene (E), and perfluoro(alkyl vinyl ether) (PAVE). TFE and ethylene are present in major amounts, consistent with the general characterization as a TFE/E copolymer. PAVE is present in minor amount. Optionally, fluoroalkyl ethylene (FAE) can also be present in minor amount in addition to PAVE. These modifiers provide the desirable results of effective reduction of melting point, good flexibility (low flex modulus) and flex resistance, and high polymerization rate.
The TFE/E copolymers of the present invention are partially-crystalline, by which is meant that the copolymers exhibit a crystalline melting point by differential scanning calorimetry (DSC). Preferably, the melting endotherm has a heat of fusion of at least 3 J/g, more preferably at least 10 J/g.
The crystalline melting points of the copolymers of the present invention are surprisingly low, no more than 220° C., preferably no more than 215° C., and more preferably no more than 210° C. The amount of PAVE, and optionally FAE, present in the copolymer, is effective to yield such melting temperatures. As illustrated by examples to follow, exceptionally low melting temperatures have been obtained for TFE/E copolymer compositions for which the concentration of TFE exceeds the concentration of ethylene.
Preferably, in addition to low melting temperature, the TFE/E copolymers of the invention have low flex modulus. Flex modulus is preferably no more than 100,000 psi (690 MPa), more preferably no more than 70,000 psi (483 MPa), and most preferably no more than 65,000 psi (448 MPa).
The TFE/E copolymers of the invention contain units derived from TFE and ethylene in major amount, and units derived from PAVE and optionally FAE in minor amounts. “Major” and “minor” as used herein are relative to 20 mol %. That is, “major amount” or “major fraction” means that TFE and ethylene are each present in the copolymer in the amount of at least 20 mol % based on total copolymer, while “minor amount” or “minor fraction” means that PAVE and FAE, if present, are each present in an amount of less than 20 mol %.
For the TFE/E copo
Brown Robert G.
Gangal Subash V.
Kerbow Dewey L.
Lindner Patrick E.
Pedersen Scot David
E. I. Du Pont de Nemours and Company
Pezzuto Helen L.
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