Fluoropolymers resistant to stress cracking

Details Fluoropolymers resistant to stress cracking Fluoropolymers resistant to stress cracking

2001-07-12

2003-11-25

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S544000, C524S545000, C524S805000, C526S089000, C526S206000, C526S247000

Reexamination Certificate

active

06653379

ABSTRACT:

TECHNICAL FIELD
This invention relates to melt-processable fluoropolymers having improved stress crack resistance and smooth surfaces after extrusion.
BACKGROUND
Copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are commonly referred to as fluorinated ethylene-propylene (FEP) resins. The HFP content usually ranges between about 10 and 20 weight percent (wt %). The FEP resins are melt-processable, having a melt viscosity adequate for conventional melt processing. However, FEP does not exhibit the high temperature properties and thermal stability of PTFE. Depending on their HFP content, these resins have a crystalline melting point from about 250 to 270° C., and have a maximum use temperature of around 200° C. Typical applications include wire insulation and molded parts.
Copolymers of TFE with perfluoro (propyl vinyl ether) (PPVE) are commonly called perfluoroalkoxy (PFA) resins. The PPVE content usually ranges between about 2 and 10 wt %. The PFA resins generally have better thermal stability and better mechanical properties at high temperatures when compared to FEP resins. The PFA resins are also melt-processable with a crystalline melting point from about 300 to 310° C., and they have properties similar to PTFE. Typical applications include expansion joints and liners for pipes and fittings, tubing, and film. Equipment made from PFA is used extensively in the semiconductor industry for demanding chemical handling applications.
SUMMARY
Briefly, the present invention provides a fluorothermoplastic composition comprising interpolymerized units derived from about 94 to about 97.5 mole percent (mol %) tetrafluoroethylene (TFE), about 2 to about 3 mol % perfluoro (propyl vinyl ether) (PPVE), and about 0.5 to about 3 mol % hexafluoropropylene (HFP). The fluorothermoplastic composition has a number of double flexure cycles to failure (flex life cycles) that fits the equation:
log
(flex life cycles)≧0.71+4.0*(
MFI
(−0.294)
),
wherein MFI is melt flow index in grams per 10 minutes and is described below.
In another aspect, the present invention provides a fluorothermoplastic composition comprising interpolymerized units derived from about 94 to about 97 mol % tetrafluoroethylene, about 0.75 to about 3 mol % perfluoro (propyl vinyl ether), and about 1.5 to about 3.5 mol % hexafluoropropylene, which also has a minimum flex life according to the equation above.
The present invention also provides a method of improving stress crack resistance. The invention further provides a method of making a fluorothermoplastic composition comprising the steps of (a) providing comonomers in proportions selected from (i) from about 94 to about 97.5 mol % TFE, from about 2 to about 3 mol % PPVE, and from about 0.5 to about 3 mol % HFP, or (ii) from about 94 to about 97 mol % TFE, from about 0.75 to about 3 mol % PPVE, and from about 1.5 to about 3.5 mol % HFP, and (b) polymerizing the comonomers to produce a fluorothermoplastic, then (c) shaping an article from the fluorothermoplastic, wherein the fluoropolymer has a melt flow index of less than about 25 g/10 min and a Stress Crack Resistance of greater than 24 hours and/or the fluoropolymer has a number of double flexure cycles to failure that fits the equation: log(flex life cycles)>0.71+4.0*(MFI
(−0.294)
).
The present invention also provides fluorothermoplastic articles, such as tubing, tube fittings, film, and coatings.
It is an advantage of the present invention to provide a melt-processable fluorothermoplastic composition useful in applications requiring high chemical resistance and high stress crack resistance.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The following description more particularly exemplifies certain preferred embodiments utilizing the principles disclosed herein.
DETAILED DESCRIPTION
The present inventors have recognized that the stress crack resistance property has particular importance. Prior methods of improving the stress crack resistance of tetrafluoroethylene-based plastics involved the addition of perfluoropropylvinyl ether (PPVE) with the problems associated with this method, e.g., difficulty of incorporation during polymerization, multiple melting points over a broad temperature range in the resultant fluoropolymer, higher cost fluoropolymer. The present invention solves these problems and improves the stress crack resistance property of fluoropolymers.
The present invention provides a fluorothermoplastic composition. This composition can be represented as a modification of polymers classified in the art as PFA fluorothermoplastics, which are generally copolymers of TFE with a small amount of PPVE. The fluorothermoplastic composition of the present invention comprises particular ratios of interpolymerized units derived from a combination of TFE, PPVE, and HFP.
Surprisingly, the fluorothermoplastic compositions of the present invention have higher performance at lower levels of PPVE, as compared to known materials. Such higher performance is shown in test results, such as stress crack resistance and flex life.
In the present invention, at least three fluorinated monomers are copolymerized. The primary component is tetrafluoroethylene (TFE), which comprises at least about 90 mole percent (mol %), more preferably at least 94 mol %, of the interpolymerized units in the inventive fluorothermoplastic composition.
At least two other fluorinated monomers are interpolymerized with the TFE. These include perfluoro (propyl vinyl ether) (PPVE), and hexafluoropropylene (HFP). The level of PPVE is from about 0.5 to about 5 mol %, more preferably from about 0.75 to about 3 mol %. The level of HFP is from about 0.25 to about 5 mol %, more preferably from about 0.5 to about 3.5 mol %.
As used herein, the sum of the mol % of each of the first three components described totals 100. When another material is interpolymerized with these three comonomers, it is preferably present at levels below about 10 mol %, more preferably below about 5 mol %, and even more preferably below about 2 mol %, of the total composition including the first three comonomers and any additional material. In such an embodiment, the mol % levels of TFE, PPVE, and HFP described herein are relative to each other and do not include a fourth or other material.
In a particular embodiment, the interpolymerized units are comprised of about 94 to about 97.5 mol % TFE, about 2 to about 3 mol % PPVE, and about 0.5 to about 3 mol % HFP. In another particular embodiment, the interpolymerized units are comprised of about 94 to about 97 mol % TFE, about 0.75 to about 3 mol % PPVE, and about 1.5 to about 3.5 mol % HFP.
In another particular embodiment, the interpolymerized units are comprised of from about 94 to about 97.5 mol % TFE, above about 2 (more preferably above about 2.1 and in some instances above 2.5) to about 3 mol % PPVE, and from about 0.5 to about 3 mol % HFP. In another particular embodiment, the interpolymerized units are comprised of from about 94 to about 97 mol % TFE, from about 0.75 to about 3 mol % PPVE, and from about 1.5 (more preferably from about 1.9, and even more preferably from about 2.2) to about 3.5 mol % HFP. Minor amounts of additional fluorinated monomers and/or non-fluorinated monomers may be interpolymerized with the compositions described above without departing from the scope of the invention. Other monomers include, for example, perfluorovinyl ethers such as:
CF
2
═CFOCF
3
, CF
2
═CFOCF
2
CF
3
, CF
2
═CFOCF
2
CF(CF
3
)OCF
2
CF
2
CF
3
,
CF
2
═CFOCF
2
CF
2
CF
2
OCF
3
, CF
2
═CFOCF
2
CF(CF
3
)OCF
2
CF(CF
3
)OCF
2
CF
2
CF
3
, and
CF
2
═CFOCF
2
CF
2
OCF
3
.
For some fluoropolymer applications, other monomers particularly those not fully fluorinated are avoided, such as in semiconductor applications where high purity and/or

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