Fine powders of polytetrafluoroethylene

Details Fine powders of polytetrafluoroethylene Fine powders of polytetrafluoroethylene

2001-07-19

2002-11-12

Seidleck, James J. (Department: 1711)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S200000, C526S247000, C526S244000, C526S250000

Reexamination Certificate

active

06479591

ABSTRACT:

The present invention relates to fine powders of PTFE having a very good thermal stability, particularly suitable for lubricated extrusion processes with high reduction ratios.
Specifically, the invention relates to fine powders of modified PTFE having a core-shell structure wherein the core is formed by PTFE modified by a perfluorodioxole monomer or which cyclizes during the polymerization to give a ring containing at least one oxygen atom, and wherein the shell is formed by PTFE modified by perfluoropropene. The fine powders of the modified PTFE of the present invention show very good thermal stability and are processable by lubricated extrusion at high reduction ratios at low extrusion pressure. This treatment allows to obtain manufactured articles without surface defects, i.e. without fractures and roughnesses.
It is known that the modified PTFE is a polymer based on tetrafluoroethylene (TFE) which contains small amounts of comonomers and which, as well as the PTFE homopolymer, cannot be transformed as thermoprocessable.
The fine PTFE powders are obtained by the polymerization in dispersion (emulsion). In said process a sufficiently high amount of surfactant is used so to stabilize the colloidal PTFE particles and a mild stirring is applied to avoid the polymer coagulation (precipitation). Subsequently the latex obtained from said process is coagulated, and the powder obtained from the coagulation is called “fine poder”.
Another TFE polymerization process is known: the suspension polymerization, wherein only a small or no amount of surfactant is used and a strong stirring is applied to obtain the precipitated polymer flocks. With this process in suspension fine powders are not obtained.
The PTFE fine powders are transformed by the lubricated extrusion process for obtaining the desired manufactured articles. To obtain a productivity increase and manufactured articles having thin wall, for example used in the cable insulation, it is important to be able to extrude at high reduction ratios. With the term reduction rate (RR) the A2:A1 ratio is meant, wherein:
A2 is the cyclinder section surface in which the PTFE preform is introduced before the extrusion,
A1 is the outlet section surface of the extruder nozzle.
Generally, by increasing the reduction ratio the extrusion pressure increases and therefore the defects of the extruded manufactured article increase. Therefore it is important to obtain a fine PTFE powder which is suitable to be extruded at high reduction ratios with a sufficiently low pressure obtaining extruded manufactured articles which do not show surface defects, such as for example fractures, roughnesses.
The PTFE manufactured articles are often used under extreme conditions, including applications at high temperature, wherein it is important to have a material having the highest thermal stability as possible. Furthermore, a high thermal stability of the PTFE is required since the PTFE has not to undergo thermal degradation during the transformation process.
In U.S. Pat. No. 4,036,802 it is described how to obtain fine PTFE powders suitable for the transformation by lubricated extrusion at high reduction ratios (RR), obtaining in the polymerization process a core-shell structure, wherein both the core and the shell of the particle are formed by PTFE modified with the same comonomer, and the content of the modifying agent comonomer in the core is higher than that of the shell. Preferably perfluoroalkylvinylethers are used as modifying comonomers. The process described in this patent requires, after the polymerization step of the core, a partial removal of the TFE and of the comonomer from the reactor, the stop of the polymerization, the subsequent repressurization by using only TFE, and the subsequent restarting of tile polymerization. Such process, called “vent-repressure”, has the purpose to reduce the comonomer amount in the particle shell with respect to the core. However, the reaction stop and restarting very often cause latex destabilization phenomena which cause the coagulum formation both in the reactor and during the subsequent latex processing (transfers, storage, filtering, concentration, etc.). Besides such process complicates the productive process and it reduces the productivity of the polymerization reactor.
In U.S. Pat. No. 5,731,394 it is described how to obtain modified fine powders suitable for the extrusion at high reduction ratios without the vent-pressure procedure, using the combination of perfluoroalkylethylene and perfluoropropylvinylether for the core modification. In this patent the obtained fine powders show a low extrusion pressure and a good adhesion to the cable. The low extrusion pressure is obtained by using a shell based on PTFE having a low molecular weight. However, the use of PTFE having a low molecular weight implies a low thermal stability of the obtained fine powder. Tests carried out by the Applicant, see the Examples, show that the presence of perfluorobutylethylene as modifying agent in the core decreases the thermal stability of the obtained fine powder.
In EP 764,668 it is described how to obtain modified fine powders suitable for high reduction ratios, preparing the core with PTFE modified by the perfluorobutylethylene (PFBE), while the shell is modified by perfluoropropene and applying the vent-repressure procedure for a complet removal of PFBE and TFE after having polymerized the core. Tests carried out by the Applicant, see the Examples, wherein perfluorobutylethylene is used for the core modification, give a polymer having a low thermal stability. Besides, the vent-repressure procedure used in EP 764,668 has an impact on the stability of the obtained latex and on the reactor productivity as above said.
In U.S. Pat. No. 4,391,940 modified fine powders suitable for the extrusion at high reduction ratios are obtained, making a three-layer core-shell structure of the particle: the core is formed by PTFE modified with at least a comonomer, the intermediate layer is PTFE homopolymer, and the particle shell is PTFE modified by fluoroolefins; the ratio by weight between the intermediate layer and the shell being in the range 75:25-99.5:0.5, the core being from 5 to 20% by weight of the total particle. To obtain said structure two techniques are used: vent-repressure and seed. The use of these techniques complicates the process, lowers the stability of the latex and reduces the reactor productivity.
In EP 380,120 fine powders having a core-shell structure are obtained, wherein the core is modified by a perfluorodioxole, the shell being formed by PTFE homopolymer or PTFE modified with a perfluoroalkylvinylether. The fine powders obtained according to this patent are not suitable for the extrusion at high reduction ratios. In fact, the manufactured articles obtained by extrusion at high reduction ratios of fine powders having a core modified by a perfluorodioxole and the shell modified by perfluoropropylvinylether show a rough surface (see the Examples).
The need was therefore felt to have available fine powders of modified PTFE having an improved thermal stability, suitable to be transformed by lubricated extrusion at high reduction ratios with a low extrusion pressure for obtaining extruded manufactured articles not showing surface defects, for example fractures, roughnesses.
An object of the present invention are therefore fine powders of modified polytetrafluoroethylene (PTFE) having a core-shell structure; the fine powder being formed by:
1) a “core” of tetrafluoroethylene (TFE) modified by one or more monomers selected from:
a) dioxoles of formula:
wherein W
1
and W
2
, equal to or different from each other, represent F or CF
3
, W
4
═F, R
f
, OR
f
, with R
f
equal to a perfluoroalkyl radical containing from 1 to 5 carbon atoms;
b) one or more perfluorinated monomers which cyclize during the polymerization;
2) a “shell” of TFE modified by perfluoropropene (PFP); the amount, referred to the core+shell total particle, of comonomers of 1) being in the range 0.01-0.06% by weight; the amount, referred to the c

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