Process for preparing modified PTFE

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...

Reexamination Certificate

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C526S229000, C526S250000, C526S253000, C526S254000

Reexamination Certificate

active

06262209

ABSTRACT:

The present invention relates to a process for producing polytetrafluoroethylene (PTFE) modified by a suspension process which allows to obtain a product with improved application characteristichs and with high productivity.
It is known that the modified PTFE is a polymer based on tetrafluoroethylene (TFE) containing small amounts of comonomers, which, as well as the PTFE homopolymer, cannot be transformed as thermoprocessable.
The PTFE is produced with a polymerization process in which only a small amount or no amount of surfactant is used and a strong stirring is applied so as to obtain precipitated polymer flocks. This process is called polymerization in suspension.
Another TFE polymerization process is also known: the polymerization in dispersion. In this process a sufficiently high amount of surfactant is used so as to be able to stabilize the PTFE colloidal particles and a mild stirring is applied so as to avoid the polymer coagulation (precipitation). Successively the latex obtained from this process can be coagulated and a PTFE powder can be obtained. This kind of powder is called “fine powder”.
The two polymerization processes give two completely different products:
the suspension product can be molded or extruded by RAM extrusion; on the contrary the fine powder of the dispersion process can be transformed by paste extrusion;
viceversa the suspension powder cannot be transformed by paste extrusion; the “fine powder” cannot be transformed by molding or RAM extrusion.
It is also known that by modifying the suspension PTFE with small amounts of perfluorinated comonomers, in particular with perfluoroalkylvinylethers or perfluoro-dioxoles, an increase of the amorphous part in the sintered polymer is obtained which allows to obtain very good tensile characteristics in the polymer with a lower molecular weight in comparison with the PTFE homopolymer. This allows to obtain a product having improved dielectric and optical properties. It is also noticed an improvement of the seal propoerties, a decrease of the permeability to gases and of the creep of the modified PTFE in comparison with the homopolymer.
One of the main applications of the suspension modified PTFE powder is the production of the skived tape. Besides the above characteristics, the tape homogeneity characteristics are very important: pollutions, transparence variations, colours. In particular, the dishomogeneity of the skived tape of the suspension modified PTFE can appear under the form of transparent halos or opaque marbleizations.
It is known that because of the lower reactivity of the fluorinated comonomers compared with that of the TFE and because of the chain transfer reactions due both directly to the comonomers and to the impurities present in the same, the polymerization reactions of the suspension modified PTFE have a meaningfully lower productivity with respect to the suspension PTFE. This fact is the main reason of the cost difference between the modified PTFE and the suspension homopolymer, since the difference due to the cost of the comonomer is rather slight, due to the small amounts used.
In U.S. Pat. No. 3,855,193 it is described a process for producing the suspension modified PTFE, by using as comonomer a perfluoroalkylvinylether, preferably perfluoropropyl vinylether (PPVE), in amounts from 0.03 to 0.16% by weight, and by using from 3 to 200 ppm of fluorinated surfactant together with a buffering salt. The product is characterized by an amorphous material content lower than 8%, it has a good resistance to creep, a low surface roughness and an improved thermal stability in comparison with the product, modified with hexafluoropropene, described in U.S. Pat. No. 3,655,611. However by operating according to the examples described in the U.S. Pat. No. 3,855,191, polymers giving skived tapes having a low homogeneity degree and not high tensile characteristics, are obtained (see the comparative examples of the present invention).
In EP 649,863 a method to obtain the suspension modified PTFE by with very good tensile and dielectric characteristics is described, by using as initiators the permanganate salts with oxalates, which establish a combined redox/thermal system and carrying out the process at low tempratures. However the use of permanganate makes it possible the formation of the manganese oxide which with difficulty can be removed from the polymer, or makes it necessary to carry out the polymerization in a strongly acid ambient which gives rise to problems connected to the autoclave corrosion and to the polymer pollution from the corros ion products. moreover in this patent low reaction temperatures (lower than 60° C.) and low polymerization pressures from 5 to 15 bar are suggested. These two choices, since the PPVE reactivity strongly decreases with the decrease of the polymerization temperatures, lead to low polymerization yields which, indeed, are in the range of 90-130 g/(l·hr).
Moreover in EP 649,863 it is stated that the use of a fluorinated surfactant in the process described therein leads to the presence of agglomerates. These are undesired since they bring dishomogeneity of the product according to what described in the patent.
In U.S. Pat. No. 5,576,402 a process for producing the suspension modified PTFE is described, by using as initiators persulphates and polymerizing without introducing in the polymerization medium ammonium ions which give rise to coloured pollutions in the final product. However it is indicated, according to the process described therein, to polymerize in absence of perfluorinated surfactant, operating at a pressure between 5 and 15 bar. This has the disadvantage not to obtain a good productivity.
It is known that the use as comonomers of perfluoropropylvinylether (PPVE) or of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) or perfluoromethoxydioxole (MDO) allows to obtain two kinds of the suspension modified PTFE by which differ in some application characteristics. For example, the products modified with perfluoropropylvinylether usually have better tensile characteristics but poorer lower optical characteristics in comparison with the products modified with perfluoromethoxydioxole.
In EP 111,343 a process for producing the suspension modified PTFE is described, by using as comonomer a fluorinated dioxole having the formula:
wherein R
1
, R
2
can be F, perfluoroalkyl or hydroperfluoroalkyl, wherein the alkyl groups comprise from 1 to 6 carbon atoms, X
1
, X
2
can be F, Cl or H; the dioxole is preferably the perfluoro 2,2-dimethyl-1,3-dioxole (PDD) wherein R
1
═R
2
═CF
3
; X
1
═X
2
═F.
The obtained product has a lower surface roughness in comparison with a product modified with PPVE. In the same patent it is used the addition of the fluorinated surfactant, as initiator the ammonium persulphate (APS) which thermally disocciates, and the process is carried out at 17.5 bar with good yields (240-250 g/(l·hr)). It has been however found from tests carried out by the Applicant (see the examples) that, by operating according to the modalities described in EP 111,343, with monomers more reactive than PPVE, such as for example the perfluorodioxole (PD), the PDD, or the MDO, there is a lowering of the tensile characteristics and the presence of homogeneity defects of the skived tape.
It was therefore felt the need to produce the suspension modified PTFE having the good application properties of this kind of polymers combined with a good homogeneity and a low presence of defects in the skived tapes and with a process having a high productivity.
The Applicant has surprisingly and unexpectedly found that it is possible to obtain high tensile characteristics combined with very good homogeneity of the foiled tape together with a process for preparing modified PTFE allowing an high productivity.
An object of the present invention is a method for preparing the suspension modified PTFE, wherein, as comonomers, perfluoroalkylvinylethers in which alkyl has from 1 to 5 carbon atoms, preferably perfluoropropylvinylether; and/or a perfluorodioxole, preferab

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