Thermoprocessable copolymers of TFE

Details Thermoprocessable copolymers of TFE Thermoprocessable copolymers of TFE

2001-06-22

2003-04-08

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, C526S247000

Reexamination Certificate

active

06545111

ABSTRACT:

The present invention relates to thermoprocessable perfluorinated polymers for the preparation of manufactured articles used in the semicon industry, which show very low values of extractable F
−
, both on perfluoropolymer pellet and the manufactured articles therefrom.
Specifically, the present invention relates to thermoprocessable copolymers of tetrafluoroethylene (TFE) which show very low values of extractable F
−
combined with very good mechanical and elastomeric properties combine wherefore they result particularly suitable to be used in the semicon industry in the preparation of pipes, fittings and tanks for the storage and the transportation of chemical compounds and of ultrapure water. Besides, the thermoprocessable copolymers of tetrafluoroethylene (TFE) of the invention show very low values of extractable Cl
−
, lower than 0.2 ppm by weight with respect to the polymer weight, preferably lower than the analytical detection limits.
It is well known that in the semicon industry tanks and piping systems (pipes and fittings) are made of fluorinated polymer materials which during the use must release a minimum amount of F
−
, lower than or equal to 1 ppm, so as not to contaminate the transported fluids, thus avoiding the damaging of silicon-based wafers. In the paper “Fluoride Contamination from Fluoropolymers in Semiconductor Manufacture” published on “State Solid Technology” pages 65-68, July 1990, it is stated that for the semicon industry it would be desirable to obtain finished manufactured articles which during the use release a minimum amount of F
−
, in particular lower than or equal to 1 ppm.
The manufactured articles for the semicon industry are generally prepared with thermoprocessable copolymers of TFE and perfluoropropylvinylether (PPVE), belonging to the PFA class, wherein PPVE is about 3.5-4.5% by weight. Said copolymers are preferably obtained by a polymerization process in aqueous emulsion which allows to obtain a high productivity and the formation of polymer structures having a high molecular weight, therefore characterized by good mechanical properties and high ductility. However the TFE/PPVE copolymers contain a certain amount of chain end groups of ionic type —CF
2
COOH and of —COF type. The —COF end group, as a consequence of hydrolysis reactions which occur during the polymerization itself, can be transformed into ionic —CF
2
COOH end group. In the processing, for example for the preparation of pipes or fittings, said end groups can decompose producing hydrofluoric acid HF. The formed hydrofluoric acid is released during the time from the manufactured article during the use in the semicon production plant, causing an unacceptable corrosion of silicon-based wafers. To minimize the HF formation during the processing so that the finished manufactured article shows the minimum amount of extractable F
−
, the prior art uses a fluorination process to transform the end groups into stable perfluorinated groups. See for example U.S. Pat. No. 4,743,658, wherein the TFE/PPVE copolymer is subjected to fluorination with elemental fluorine for reducing the amount of ionic end groups so that the fluorine extractable from the finished manufactured article is lower than 3 ppm by weight with respect to the polymer. Said process requires an additional step and from the industrial point of view it is not easily feasible since it uses elemental fluorine, very aggressive agent which requires special equipments for making the treatment safe and reliable.
Other processes for reducing the ionic end group number in perfluorinated copolymes are known. See for example U.S. Pat No. 5,093,409 wherein the TFE/PPVE copolymer under the form of latex is treated with amines at 160°-400° C. for a sufficient time to convert the ionic —CF
2
COOH end groups into —CF
2
H groups. Also this post-treatment for stabilizing the fluoropolymer requires a specific unit and it results therefore expensive from the economic point of view.
A class of thermoprocessable copolymers of TFE having very high chemical inertia and thermal stability is described in U.S. Pat. No. 5,463,006, wherein terpolymers formed by TFE/PPVE/PMVE (perfluoromethylvinylether) are described. Tests carried out by the Applicant, see the comparative Examples, show that with these terpolymers release values of F
−
lower than 1 ppm are not obtained both on the polymer (pellet) and on the finished manufactured article. Therefore also in this case it is necessary to carry out one of the above post-treatments for reducing the values of extractable F
−
.
In conclusion, the thermoprocessable TFE copolymers of the prior art used for pipes in the semicon industry show a very good combination of properties, such high flex life, good mechanical properties at high and low temperature and release values of F
−
lower than 1 ppm. However the drawback of said products is that the combination of said properties is obtained with the proviso to subject them to a specific treatment of fluorination or conversion of the end groups of the previously described type. All this requires a further processing step and an additional specific unit, making therefore difficult the process from the industrial point of view and making it expensive from the economic point of view.
Besides values of extractable F
−
lower than or equal to 1 ppm, good mechanical properties both at low (23° C.) and high temperature (250° C.), good flex life, it would be desirable to carry out the processing of the manufactured articles, for example pipes, without compromising the productivity thereof. At present in the preparation of pipes having a low release of F
−
for the semicon industry, perfluoropolymers post-treated as above indicated are absolutely necessary and not high extrusion rates are used.
The need was therefore felt to have available in the semicon industry a thermoprocessable fluoropolymer able to give finished manufactured articles having the following combination of proprties:
release values of F
−
ions, both on the polymer pellet and on the finished manufactured article, lower than 1 ppm by weight with respect to the polymer weight;
said release values of F
−
ions lower than 1 ppm, in the case of extruded manufactured articles (pipes), are obtainable also at high extrusion rate;
very low values of extractable Cl
−
, lower than 0.2 ppm by weight with respect to the polymer weight, preferably lower than the analytical limits;
flex life values higher than 20,000 combined with a MFI range (measured at 372° C. with a 5 Kg load) comprised between 1 and 5.
very good mechanical properties both at low temperature (23° C.) and at high temperature (250° C.).
The Applicant has surprisingly and unexpectedly found that it is possible to obtain the combination of the above properties by using the specific monomeric composition of thermoprocessable copolymers of TFE as defined hereunder.
An object of the present invention is therefore a thermoprocessable copolymer of TFE consisting essentially of:
(a) from 4.5 to 8.5% by weight of perfluoromethylvinylether (PMVE);
(b) from 0.3 to 1.6% by weight of a perfluorodioxole of formula:
 wherein:
Y═F, OR
f
, R
f
wherein R
f
is a perfluoroalkyl having from 1 to 5 carbon atoms;
X
1
and X
2
, equal to or diferent from each other, are —F or —CF
3
;
(c) TFE, forming the remaining part to 100%; said copolymer having:
a number of ionic end groups lower than 4×10
−4
mol/Kg of polymer;
flex life values higher than 20,000 in a MFI range (measured at 372° C. with a 5 Kg load) comprised between 1 and 5;
release values of F
−
ions, both on the polymer in pellet and on the finished manufactured article, lower than 1 ppm by weight with respect to the polymer weight;
release values of F
−
ions lower than 1 ppm on extruded manufactured articles (pipes), obtained both at low and high extrusion rate of the polymer.
The ionic end groups are for example of —COOH and —COF type.
Among the comonomers (b) the one wherein Y═F, X

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