Melt-processable perfluoropolymers having improved thermal and mechanical properties after heating treatment

Abstract

The invention mainly pertains to a method for heat treating a composition [composition (C)] which contains at least one melt-processible perfluoropolymer [polymer (F)] formed of tetrafluoroethylene (TFE) copolymer with one or more perfluorinated comonomers [comonomer (F)] containing at least one unsaturation of ethylene type in amounts from 0.5% to 13% by weight, preferably from 0.6% to 11% by weight, and more preferably from 0.8% to 9% by weight; said polymer (F) possessing reactive end groups comprising at least one of the group consisting of hydrogen atoms, oxygen atoms and ethylenically unsaturated double bonds in an amount of at least 4.5 mmol/kg, the process comprising at least the step of heat-treating the composition (C) at a temperature of at least 260 C.

Claims

1. A method for heat treating a composition (C) which contains at least one melt-processible perfluoropolymer [polymer (F)], the process comprising at least the step of heat-treating the composition (C) at a temperature of at least 260 C., wherein polymer (F) is formed of tetrafluoroethylene (TFE) copolymer with one or more perfluorinated comonomers [comonomer (F)] containing at least one ethylene unsaturation in amounts from 0.5% to 13% by weight, and wherein polymer (F) possesses reactive end groups comprising at least one of the group consisting of hydrogen atoms, oxygen atoms and ethylenically unsaturated double bonds in an amount of at least 4.5 mmol/kg.

2. The method according to claim 1, wherein said comonomer (F) is selected from: C.sub.3-C.sub.8 perfluoroolefins; CF.sub.2CFOR.sub.f perfluoroalkylvinylethers (PAVE), wherein R.sub.f is a C.sub.1-C.sub.6 perfluoroalkyl; CF.sub.2CFOX perfluorooxyalkylvinylethers wherein X is a C.sub.1-C.sub.12 perfluorooxyalkyl having one or more ether groups; or perfluorodioxoles.

3. The method according to claim 2, wherein said comonomer (F) is selected from the following comonomers: PAVEs selected from perfluoromethylvinylether (PMVE of formula CF.sub.2CFOCF.sub.3), perfluoroethylvinylether (PEVE of formula CF.sub.2CFOC.sub.2F.sub.5), perfluoropropylvinylether (PPVE of formula CF.sub.2CFOC.sub.3F.sub.7), or mixtures thereof; perfluoromethoxy vinyl ether (MOVE) of general formula CF.sub.2CFOCF.sub.2OR.sub.f2, wherein R.sub.f2 is a linear or branched C.sub.1-C.sub.6 perfluoroalkyl group, cyclic C.sub.5-C.sub.6 perfluoroalkyl group, a linear or branched C.sub.2-C.sub.6 perfluoroxyalkyl group; and perfluorodioxoles having the following formula: ##STR00002## wherein X.sub.1 and X.sub.2, equal to or different from each other, are selected from F or CF.sub.3.

4. The method according to claim 3, wherein said comonomer (F) is selected from PMVE, PEVE, PPVE, MOVE, or combinations thereof.

5. The method according to claim 4, wherein polymer (F) is a TFE copolymer consisting essentially of: (a) from 0.5 to 8% by weight of recurring units derived from PPVE; (b) recurring units derived from TFE, in such an amount that the sum of the percentages of the recurring units (a) and (b) is equal to 100% by weight.

6. The method according to claim 1, wherein polymer (F) comprises recurring units derived from at least one C.sub.3-C.sub.8 perfluoroolefin.

7. The method according to claim 6, wherein polymer (F) is a TFE copolymer consisting essentially of: (a) from 0 to 6% by weight of recurring units derived from PMVE; (b) from 0.4 to 5% by weight of recurring units derived from one or more than one fluorinated PAVE comonomer different from PMVE; (c) from 0 to 6% by weight of recurring units derived from at least one C.sub.3-C.sub.8 perfluoroolefin; and (d) recurring units derived from TFE, in such an amount that the sum of the percentages of the recurring units (a), (b), (c) and (d) is equal to 100% by weight.

8. The method according to claim 1, wherein composition (C) contains polymer (F) as the sole component.

9. The method according to claim 1, wherein the temperature of heat treatment for composition (C) is at least 270 C.

10. The method according to claim 1, said process further comprising a step of processing in the molten state the composition (C), so as to provide a solid shaped article, and consequently heat treating said composition (C) under the form of said solid shaped article.

11. The method according to claim 1, wherein polymer (F) is formed of tetrafluoroethylene (TFE) copolymer with one or more comonomer (F) containing at least one unsaturation of ethylene type in amounts from 0.6% to 11% by weight.

12. The method according to claim 1, wherein polymer (F) is formed of tetrafluoroethylene (TFE) copolymer with one or more comonomer (F) containing at least one unsaturation of ethylene type in amounts from 0.8% to 9% by weight.

13. The method according to claim 3, wherein R.sub.f2 is CF.sub.2CF.sub.3, CF.sub.2CF.sub.2OCF.sub.3, or CF.sub.3.

14. The method according to claim 3, wherein X.sub.1 and X.sub.2 are each F.

15. The method according to claim 5, wherein polymer (F) is a TFE copolymer consisting essentially of: (a) from 0.7 to 6% by weight of recurring units derived from PPVE; (b) recurring units derived from TFE, in such an amount that the sum of the percentages of the recurring units (a) and (b) is equal to 100% by weight.

16. The method according to claim 7, wherein polymer (F) is a TFE copolymer consisting essentially of : (a) from 0 to 6% by weight of recurring units derived from PMVE; (b) from 0.4 to 5% by weight of recurring units derived from PEVE and/or PPVE; (c) from 0 to 6% by weight of recurring units derived from hexafluoropropylene (HFP); and (d) recurring units derived from TFE, in such an amount that the sum of the percentages of the recurring units (a), (b), (c) and (d) is equal to 100% by weight.

17. The method according to claim 9, wherein the temperature of heat treatment for composition (C) is at least 310 C.

Description

DESCRIPTION OF EMBODIMENTS

(1) The invention will now be explained in more detail with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

EXAMPLE

(2) Measurement of the Melt Flow Index (MFI)

(3) The determination of the MFI was carried out according to the ASTM D1238 standard test method at 372 C. under a load of 5 Kg.

(4) Measurement of the Second Melting Temperature, Tm(II)

(5) The second melting temperatures of the test samples were determined based on the ASTM D4591 standard test protocol, using a heating rate of 10 C. per minute. For all samples, the second melting temperature [Tm(II)] was evaluated after a first melting cycle up to 350 C. and cooling from 350 C. using a cooling rate of 10 C. per minute.

(6) Measurement of the Mechanical Properties

(7) MIT flex life of the polymer was measured using the standard equipment described in ASTM D-2176-82T, on a moulded film having a thickness of approximately 0.3 mm.

(8) Measurement of the Weight Percentage of the Perfluorinated Comonomer in the Polymer

(9) Determination of the perfluorinated comonomer content was carried out by FT-IR analysis and expressed as percent by weight.

(10) The perfluorinated comonomer (F) content was determined under the following conditions: the band optical density (OD) at 994 cm.sup.1 was normalized with the band optical density (OD) at 2365 cm.sup.1 by means of the following formula:
Comonomer (F) [% wt]=(OD at 994 cm.sup.1)/(OD at 2365 cm.sup.1)0.99.

(11) Specifically, the PEVE or HFP content was determined by FTIR spectroscopy according to the method described in U.S. Pat. No. 5,703,185 (column 4, lines 9-16), using the multiplying coefficient 1, 3 as described therein. In order to carry out the IR spectrum subtraction, a TFE/MVE copolymer was used.

(12) The PMVE content in a polymer was determined under the following conditions: a band optical density (OD) at 889 cm.sup.1 was normalized with the band optical density (OD) at 2365 cm.sup.1 by means of the following formula:
PMVE [% wt]=(OD at 889 cm.sup.1)/(OD at 2365 cm.sup.1)11.8

(13) The PPVE content was determined under the following conditions: a band optical density (OD) at 994 cm.sup.1 is normalized with the band optical density (OD) at 2365 cm.sup.1 by means of the following formula:
PPVE [% wt]=(OD at 994 cm.sup.1)/(OD at 2365 cm.sup.1)0.99

(14) Materials

(15) Polymer samples named are TFE/PAVE copolymers commercially available from Solvay Specialty Polymers Italy SpA, under the trade name of Hyflon MFA P125X (PVE content: 1.7% wt), HYFLON PFA P450 (PVE content: 4.2% wt) and HYFLON PFA P420 (PVE content: 4% wt).

(16) Fluorination

(17) Specimens of certain of aforementioned TFE/PAVE copolymers were preliminarily submitted to extensive fluorination so as to fluorinate all terminal groups and reduce until undetectable amounts groups comprising hydrogen, oxygen and ethylenically unsaturated double bonds. Fluorinated counterparts are designated below with (F.sub.2) suffix.

(18) Annealing Procedure

(19) Polymer samples as received was melt and subjected to MFI determination, shown as original MFI value in Table 1. The MFI value was re-measured for each treated polymer, as a function of duration of heat treatment at 300 C. as indicated in Table 2.

(20) As shown in Table 2, the MFI of each tested PFA polymer significantly decreased after receiving the heat treatment.

(21) TABLE-US-00001 TABLE 1 Time of heat treatment at 300 C. (h) MFI (g/10) MFI/MFI P125X 0 1.8 1 24 1.5 0.83 48 1.27 0.7 168 0.76 0.42 336 0.65 0.36 1056 0.482 0.27 P125X-(F.sub.2) 0 2.4 1 3 2.4 1 6 2.4 1 24 2.4 1 48 2.4 1 168 2.4 1

(22) TABLE-US-00002 TABLE 2 Time of heat treatment at 300 C. (h) MFI (g/10) MFI/MFI P420 0 1.9 1 24 1.5 0.78 528 1.0 0.52 P420-(F.sub.2) 0 1.9 1 24 1.9 1 168 1.9 1

(23) TABLE-US-00003 TABLE 3 Time of heat treatment at 300 C. Flex Life (h) (cycles) MFI (g/10) MFI/MFI P450 0 2660 15.4 1 3 13.7 0.89 6 12 0.78 24 22090 8.5 0.55 48 126985 5.45 0.36 168 4.07 0.26 336 3.63 0.23

(24) Determination of End Groups

(25) End groups were determined by Fourier Transform IR spectroscopy according to the method described in PIANCA, M., et al. J. Fluor. Chem. 1999, p. 95-71. on samples of cold pressed specimens having a 5 mm diameter and thickness from 50 to 300 microns, using Nicolet Nexus FT-IR equipment (256 scans, spectral range 4000-400 cm.sup.1, resolution 2 cm.sup.1). Results determined on samples before and after annealing are detailed in the following table:

(26) TABLE-US-00004 TABLE 4 End groups COOH COF CONH.sub.2 CFCF.sub.2 CH.sub.2CH.sub.3 CF.sub.2H TOT (mmol/kg) P125X 2.1 n.d. 0.4 n.d. 2.6 0.4 5.5 P125X-HT* 0.1 3.3 n.d. n.d. n.d. n.d. 3.4 P125X-(F.sub.2) n.d. n.d. n.d. n.d. n.d. n.d. n.d. P420 2.6 0.5 0.06 0.5 2 0.8 6.46 P420-(F.sub.2) n.d. n.d. n.d. n.d. n.d. n.d. n.d. P450 8 0.6 n.d. 1.5 3.5 n.d. 13.6 P450-HT** n.d. 2.6 n.d. n.d. n.d. n.d. 2.6 n.d.: not detectable; *P125X-HT is P125X after 1056 hours of heat treatment at 300 C.; **P450-HT is P450 after 336 hours of heat treatment.