Ethylene fluoro-containing copolymers

Details Ethylene fluoro-containing copolymers Ethylene fluoro-containing copolymers

1999-07-16

2001-12-11

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...

C526S247000, C526S250000, C526S255000, C526S348000

Reexamination Certificate

active

06329481

ABSTRACT:

The present invention relates to ethylene fluoro-containing copolymers having a good processability and high mechanical properties.
More particularly the fluoro-containing copolymers are ethylene copolymers with TFE (tetrafluoroethylene) (ETFE) or ethylene copolymers with CTFE (chlorotrifluoroethylene) (ECTFE). The commercial name of ECTFE is Halar®.
It is known that modified and non modified fluoro-containing copolymers of ECTFE (ethylene-chlorotrifluoroethylene), and of modified ETFE (see the modifiers mentioned hereinafter), have good mechanical properties which however dramatically decay with the decrease of the melt viscosity (low molecular weights). Unfortunately the use of these products having higher molecular weights than those used at present, for pipes, sheets, and cables, is not possible owing to the impossibility to process them in the conventional machines currently used, as it is explained in detail later on.
As known, said machines work with a shear-rate from 1 to 10000 sec
−1
of the melt, preferably from 1 to 1000 sec
−1
.
For instance the Halar products available in the market are 900, 300 and 500 type, which differ in melt viscosity (the Melt Flow Index (MFI)). The 900 type is characterized by having a very high molecular weight (low MFI), and therefore good mechanical characteristics, however it has the drawback that it cannot be processed with shear-rates higher than about 40 sec
−1
, it is generally utilized from 1 to 20 sec
−1
. Indeed if one exceeds this shear-rate, the manufactured article shows a certain roughness and/or the melt shows instability phenomena in processing which does not allow the obtainment of the manufactured article. Therefore the productivity of these processes is very low.
Therefore it is defined window or processability zone the shear-rate range of the melt rheological curve (shear viscosity/shear-rate) wherein surface roughness phenomena and/or flow instability do not appear. Generally the viscosity measurement is carried out at temperatures of about 30° C. -50° C. higher than the second melting temperature of the copolymer.
Moreover if cables are desired to be prepared, it is not possible to obtain them with the 900 grade because the cables lines work at with higher shear-rates, generally higher than 100 sec
−1
.
The present available alternative was to use the 500 grade for cables. The 500 grade has a very low viscosity compared with the 900, and has a processability window between 100-500 sec
−1
. However the mechanical properties are much lower with respect to the 900 grade, especially at high temperatures, wherefore the cables do not show good mechanical properties.
The 300 grade has an intermediate viscosity between 900 and 500, therefore it has superior mechanical properties in comparison with the 500, however not yet sufficient to give an optimal properties profile. Besides the processability window is not yet sufficiently wide, indeed one can process up to a maximum of 60 sec
−1
. Therefore it cannot be used for cables.
Therefore it would be desirable to have available a grade similar to the 900 one (good mechanical properties) but with a processability window in the zone of the 500 type. There is no product in the market capable of meeting these requirements.
On the other hand if superior mechanical properties than those of the 900 grade are desired, this is not possible. This is due to the narrow processability window of the product, which would have a higher molecular weight than the 900 grade, bringing down the productivity, since it is necessary to shift towards the lower limit of the processability range in which the machines work. An example of this application consists in preparing pipes with very high performances, such as good mechanical resistance, even when subjected to thermal cycles at high temperature; and sheets in thermoformable processes.
The Applicant has surprisingly found that it is possible to synthesize ECTFE and ETFE ethylene fluoro-containing copolymers under such conditions as to obtain products having a good processability combined with excellent mechanical properties, especially at high temperatures, generally from 100 to 200° C., preferably from 140 to 180° C.
The processability of the copolymers of the invention is possible at shear-rates higher than 100 sec
−1
as the 500 grade but with much higher mechanical properties, similar or even superior to the 900 grade. In particular to make Halar cables, the copolymers of the invention having a melt viscosity lower than 10
4
Pa.sec at shear-rate of 1 sec
−1
, preferably higher than 10
2
, measured at the temperature of 275° C., are used. The viscosity measurement is generally carried out at temperatures of about 30° C.-50° C. higher than the second melting temperature of the copolymer.
In the case of pipes and sheets, for instance, the processability of the copolymers is possible at shear-rates lower than 60 sec
−1
as the 300 grade but with mechanical properties clearly superior to the 900. In particular to make these Halar articles, the copolymers of the invention having a melt viscosity lower than 10
5
Pa.sec with shear-rates of 1 sec
−1
, and preferably higher than 10
3
, measured at the temperature of 275° C., are used. The measurement of the viscosities is generally carried out at temperatures of about 30°-50° C. higher than the second melting temperature of the copolymer.
The copolymers of the invention, optionally in the presence of one or more comonomers, the total amount of which is comprised between 0.1 and 10% by moles, show a viscosity between 10
2
and 10
5
Pa.sec with shear rates of 1 sec
−1
measured at the temperature of about 30°-50° C. higher than the second melting temperature, and show a rheologic curve in the diagram shear viscosity/shear-rate having a gradient higher than 200, calculated as ratio between the viscosity in Pa.sec and the shear-rate in sec
−1
in the shear-rate range comprised between 1 and 10 sec
−1
, said curve having a processability zone with shear-rate values higher than 10 s
−1
, preferably higher than 50 sec
−1
.
The copolymers of the invention generally show a molecular weight distribution of bimodal type, i.e. they contain a polymeric fraction of low molecular weight having a viscosity comprised between 1 and 5.10
3
Pa.sec measured as above. Said low molecular weight fraction being generally comprised between 5 and 60% by weight, preferably between 30 and 55% by weight; the remaining part having a higher molecular weight and being in amounts corresponding to the complement to 100% by weight.
The high and low molecular weight distributions can be more or less broad, and/or more or less close, which show the contemporaneous presence of polymers having different molecular weight, one with a prevalence of low molecular weights and the other with a prevalence of high molecular weights.
Generally the measurement of the molecular weights of the polymers of the invention, as known, is hardly feasible because of the poor solubility of the fluoro-containing ethylene copolymers in solvents, for instance methylethylketone (MEK), at room temperature. Therefore the gel permeation chromatography (GPC) for determining the molecular weights cannot be utilized.
The flow curve, indicated above, shear viscosity/shear-rate, is measured according to ASTM D3835.
A process utilizable for the synthesis of the copolymers of the invention consists in modifying the processes for preparing Halar, see for instance the European patents EP 673 952 and 673 951 in the name of the Applicant, that is by polymerizing the comonomers for 40-95% by weight on the total of the obtained polymer, then by adding the transfer agent for preparing the low molecular weight fraction as defined above. In the polymerization phase without transfer agent high molecular weights are produced.
During the preparation phase of the high molecular weights, an amount of chain transfer agent, generally lower than 20% of the total utilized in preparing the final c

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