Amorphous perfluorinated copolymers

Details Amorphous perfluorinated copolymers Amorphous perfluorinated copolymers

2002-05-07

2004-10-26

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S242000, C524S462000, C525S326200, C428S542800

Reexamination Certificate

active

06809166

ABSTRACT:

The present invention relates to amorphous perfluorinated polymers for optical applications, in particular optical fibers, the preforms obtained therefrom, and a process for preparing said polymers.
With the term “preform” it is generallty meant a compact and transparent solid formed by polymers having optical properties.
The polymer optical fibers (POF) have a core-sheath structure. Compared with the optical fibers wherein the core is based on quartz or multicomponent glass, POFs generally show the following advantages:
higher diameter,
improved flexibility (according to “bending test”)
improved number opening (difference of the refractive index between core and sheath),
improved connection easiness to the light source.
Polymer optical fibers having a core of polymethylmethacrylate, polystyrene or polycarbonate and a sheath of a perfluoroalkylmethacrylate polymer are at present on the market. At wave lengths of 400-600 nm polymethylmethacrylate acrylate POFs show a good attenuation, of the order of 150-400 dB/Km. At wave lengths higher than 650 nm the attenuation of said POFs becomes very high. This represents a drawback since on industrial scale sources of light at wave lengths higher than 700 nm, up to the near infrared, are used.
U.S. Pat. No. 4,966,435 describes POFs having a good transmission to the light both in the visible spectrum and in the near infrared and an excellent thermal resistance. In said fibers the core is a copolymer of a PD or PDD perfluorodioxole. Said copolymers are substantially amorphous, have a Tg higher than 100° C., a refractive index which can be controlled between 1.29 and 1.40 in function of the comonomer, and can easily be transformed into optical fibers having an uniform diameter. The thermal resistance of said substantially amorphous copolymers is high, the number of the unstable end groups being lower than 5 meq/Kg, preferably lower than 2 meq/Kg. This reduced number of end groups is obtained by treating the polymers with amines or alcohols and subsequently by reacting with fluorine at a temperature lower than the polymer Tg and comprised between 20° C. and 200° C. The sheath is formed by a PD/TFE copolymer. The core-sheath fibers obtained according to said patent have an attenuation of 270 dB/km at 650 nm, 330 dB/Km at 780 nm and 285 dB/km at 1,550 nm. Said values are too high for “last mile” or LAN (Local Area Network) applications, wherein an attenuation lower than 100 dB/km in the range 700-1,550 nm is required.
EP 710,855 describes graded refractive index POFs having an improved band amplitude, formed by an amorphous polymer and by at least another polymer (doping agent) having refractive index different of at least 0.001 units from that of the amorphous polymer. The doping agent is distributed in the fiber so as to have a concentration gradient from the centre to the cable periphery. The fluoropolymer contains cyclic structures; in particular when alicyclic polymers are used the attenuation is higher than 100 dB/km at a wave length between 700 and 1,550 nm. Said attenuation results too high for LAN applications.
EP 752,598 describes graded refractive index POFs similar to those of previous patent, wherein a third component, having a specular concentration gradient with respect to that of the doping agent, is used, to maintain contact the Tg along the fiber diameter. As third component a perfluoropolyether having the structure
—CF
2
—CF(CF
3
)O— or —(CF
2
)
n1
O—
is used, wherein n1 is an integer from 1 to 3. Preferably the number average molecular weight of the perfluoropolyether is from 300 to 10,000. In the Examples of said patent it is described that fibers having a Tg of 82° C. can have an attenuation of 100 dB/Km at 780 nm and 65 dB/Km at 1,550 nm. In the comparative Examples it is shown that in absence of PFPE the attenuation increases to 400 dB/Km at 780 nm, and to 280 dB/Km at 1,550 nm, while the Tg ranges from 82° C. in the core to 102° C. in the sheath respectively. In absence of PFPE microvacua are present in the fibers. The optical fibers obtainable according to said patent have the drawback to have a very low Tg for LAN applications, which require a fiber Tg higher than 100° C. (S. Ando et al., ChemTech. Dec.
1994 20-27).
The need was felt to have available amorphous perfluorinated polymers for optical applications in particular for optical fibers, having the following combination of properties:
attenuation values lower than 100 dB/km at wave lengths from 500 to 1,550 nm;
Tg higher than 120° C.;
high thermal resistance;
narrow distribution of the monomeric composition of the amorphous perfluorinated copolymer according to the following test: the polymer is dissolved at a concentration of 5% w/w in perfluoroheptane solvent, after filtration on PTFE filter having a maximum pore diameter of 0.2 micron, no polymer residue remains on the filter; the filter weight before and after filtration remains unchanged.
The Applicant has unexpectedly and surprisingly found perfluorinated amorphous polymers which solve this technical problem.
An object of the present invention are amorphous perfluorinated copolymers, comprising cyclic perfluorinated units deriving from at least two different perfluorinated comonomers, optionally with units deriving from a non cyclic perfluorinated monomer, or which does not cyclize during the polymerization, containing at least one olefinic unsaturation (perfluoroolefin), or comprising cyclic perfluorinated units and units deriving from a non cyclic perfluorinated monomer, or which does not cyclize during the polymerization, containing at least one olefinic unsaturation, said perfluorinated copolymers having the following combination of properties:
substantial absence of unstable polar end groups, in particular end groups as COF, COOH or their amidic derivatives, esters or salts; said end groups being not detectable in the copolymer, i.e. present in an amount lower than 0.05 mmoles/kg polymer when they are determined by Fourier transform IR spectroscopy by Nicolet® Nexus FT-IR equipment (256 scannings, resolution 2 cm
−1
), wherein:
on a sintered polymer powder pellet having 5 mm diameter and thickness from 50 to 300 micron (corresponding to a weight of 1.75 and 10.5 mg of polymer, respectively) a scanning between 4,000 cm
−1
and 400 cm
−1
is initially carried out, the pellet being then kept for 12 hours in an environment saturated with ammonia vapours;
the IR spectrum is finally recorded under the same conditions of the initial IR spectrum;
the two spectra are elaborated by subtracting from the signals of the spectrum relating to the untreated sample (initial spectrum) those corresponding to the specimen spectrum after exposure to ammonia vapours;
the “difference” spectrum is obtained, which is normalized by the following equation:
“
Difference
⁢
 
⁢
spectrum
”
[
pellet
⁢
 
⁢
weight
⁢
 
⁢
(
g
)
/
pellet
⁢
 
⁢
area
⁢
 
⁢
(
cm
2
)
]
 the optical densities related to the end groups which have reacted with the ammonia vapours are measured, which give rise to detectable peaks in the IR spectrum, said end groups being the COOH and COF groups;
the optical densities are converted in mmoles/kg polymer using the extinction coefficients reported in Table 1, page 73 of the paper by M. Pianca et Al. “End groups in fluoropolymers”, J. Fluorine Chem. 95 (1999), 71-84 (herein incorporated by reference); the so found values give the concentrations of the residual polar end groups as mmoles of polar end groups/kg polymer: in the spectrum of the amorphous (per)fluorinated polymers after fluorination bands related to COOH groups (3,600-3,500, 1,820-1,770 cm
−1
) and/or COF groups (1,900-1,830 cm
−1
) are not detectable, the method detectability limit being 0.05 mmoles/Kg polymer;
polymer Tg higher than 120° C., preferably 125° C.; narrow distribution of the monomeric composition of the amorphous perfluorinated copolymer measured according to the following test: the polymer is dissolved a

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