Thermoprocessable copolymers of TFE

Details Thermoprocessable copolymers of TFE Thermoprocessable copolymers of TFE

2002-12-19

2004-05-04

Lipman, Bernard (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S250000

Reexamination Certificate

active

06730761

ABSTRACT:

The present invention relates to thermoprocessable perfluorinated polymers for the preparation of manufactured articles used in the semicon industry, obtained for example by injection moulding and therefore characterized by the presence of welding lines.
In particular, the present invention relates to thermoprocessable copolymers of tetrafluoroethylene (TFE) showing very low values of extractable F
−
on pellets, not higher than 1 ppm, to which very good hot mechanical and elastomeric properties at temperatures up to 250° C. and elongation at break measured at 23° C. along the welding lines ≧130% are associated. Therefore said thermoprocessable polymers are particularly suitable to be used in the semicon industry in the preparation of fittings and tanks, wherein welding points or lines are present, used for the storage and the transportation of chemical products and ultrapure water. Besides, the thermoprocessble copolymers of tetrafluoroethylene (TFE) of the invention show very low values of extractable chlorides (Cl
−
), lower than 0.2 ppm by weight with respect to the polymer weight.
It is well known that in the semicon industry tanks and piping systems (pipes and fittings) formed by fluorinated polymer compounds, during the use can release F
−
ions. In order not to contaminate the transported fluids and avoid the damaging of the silicon-based wafers, it is required that the released F
−
ion amount be very low, not higher than 1 ppm. Indeed in the publication “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 F
−
amount lower than or equal to 1 ppm.
Said polymer compounds having a low F
−
release must have also good mechanical and flowing properties inside the mould, so as to guarantee a suitable mechanical resistance on the welding lines of the various fronts of the melted compound. An index of the suitable mechanical resistance on the welding lines is the elongation at break value on the welding lines which must be ≧130%.
The manufactured articles for the semicon industry are generally prepared with thermoprocessable copolymers of TFE and perfluoropropylvinylether (PPVE), belonging to the PFA class, wherein the 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 good ductility. However the so obtained TFE/PPVE copolymers contain a certain amount of chain end groups of ionic type —CF
2
COOH and —COF. In the processing, for example for the preparation of pipes or fittings, said end groups —COF and —CF
2
COOH can decompose producing hydrofluoric acid. The formed hydrofluoric acid during the time is released by the manufactured article. Therefore the use of said manufactured articles in semicon manufacture plants can lead to corrosion phenomena of the 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 perfluorinated stable groups. See for example U.S. Pat. No. 4,743,658, wherein the TFE/PPVE copolymer is subjected to fluorination with elementary fluorine to reduce the amount of ionic end groups so that the fluorine extractable from the finished manufactured article be 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 of easily feasible since it uses, as said, elementary fluorine which is a very aggressive agent. Therefore the fluorination step requires a special plant for its accomplishment on an industrial scale. Therefore said solution represents an additional cost for the manufacture plant of said polymers.
In the prior art other processes to reduce the number of ionic end groups in perfluorinated copolymers have been proposed. See for example U.S. Pat. No. 5,093,409 wherein the TFE/PPVE copolymer in the latex form is treated with amines at 160°-400° C. for a time sufficient to convert the ionic end groups —CF
2
COOH into —CF
2
H groups. Said post-treatment shows the same drawbacks mentioned above for fluorination.
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. Said polymers are commercially available under the name HYFLON®. Tests carried out by the Applicant, see the comparative Examples, show that with said terpolymers, F
−
release values lower than 1 ppm are not obtained on the polymer (pellet). Also in this case it is necessary to carry out one of the above post-treatments to reduce the values of extractable F
−
to the required levels.
Therefore the thermoprocessable copolymers of TFE of the prior art usable in the semicon industry show the following combination of properties: good mechanical properties at high and low temperature, release values of F
−
lower than 1 ppm. However the drawback of said copolymers is that the combination of said properties is obtained with the proviso to subject them to a specific fluorination or conversion treatment of the end groups, as previously described.
Patent publication EP 1,167,401 describes fluorinated polymers usable for applications in the semicon industry, in particular suitable to produce systems of pipes with low F
−
release. The process to obtain said fluorinated polymers with low F
−
release is such not to require any of the above additional steps. However the injection moulding of the manufactured articles with the respective mechanical properties measured along the welding lines is not mentioned. In fact application examples on fittings are not reported. Tests carried out by the Applicant have shown that by operating with the composition indicated in said patent application it is not possible to obtain an optimal combination of good mechanical properties at high temperature with good mechanical properties at room temperature along the welding lines, in particular elongations at break≧130%.
As known, the plant for the semicon manufacture, besides by the piping systems obtainable by extrusion, is formed also by valves, fittings and connectors, which are generally produced by injection moulding. The manufactured articles obtained by said process show one or more welding lines; said lines are the sites in the mould where different fronts of the same melted compound come into contact. They solidify in contact with the cooled mould wall. Said lines are weak points of the manufactured article if the different fronts of the melted material do not weld well each other, giving rise to low values of elongation at break, lower than 130%, measured along the welding lines, to which unsatisfactory mechanical properties are associated.
The need was felt to have available in the semicon industry a thermoprocessable fluoropolymer capable to allow the preparation of manufactured articles, in particular fittings, having the following combination of properties:
good mechanical properties measured at room temperature (23° C.) on the welding lines of injection moulded articles; in particular an elongation at break≧130%;
F
−
release values, on the polymer pellet not higher than 1 ppm by weight with respect to the polymer weight;
extractable Cl
−
values lower than 0.2 ppm by weight with respect to the polymer weight;
amount of ionic end groups not higher than 5.10-4 moles/Kg polymer;
good hot mechanical properties up to temperatures of 250° C., in particular for a polymer which is processable by injection moulding, the stress at break m

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