Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-06-07
2003-12-09
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06660798
ABSTRACT:
The present invention relates to aqueous fluoropolymer dispersions to be used in coating for surfaces, preferably metal and ceramic surfaces, and in the textile-impregnation and cast film manufacture.
Specifically, the present invention relates to aqueous fluoropolymer dispersions able to give films having an high critical thickness and an acceptable shelf life for industrial cycles, not lower than 6 months. The films obtained from these dispersions show good mechanical properties also at high temperatures, good optical properties, as well as reduced porosity and roughness of the film surface.
With critical thickness the maximum thickness obtainable without cracks and surface defects in the film, is meant.
It is very important from the application point of view to increase the limit of the critical thickness so as to have films free from cracks in correspondence of an higher thickness. A higher critical thickness allows greater freedom degrees in formulating and applying formulations with the various technologies and a higher reliability of the product in the transformation process. An higher critical thickness means high productivity in coating industrial cycles for metal surfaces, impregnation and cast films. This feature however must not decrease the mechanical and optical properties.
It is known in the prior art that dispersions of polymers with broadened particle size distribution (PSD) show improved Theological characteristics, in particular a lower viscosity, the solid fraction by volume in the dispersion being equal, from the point of view of their application in various industrial fields. See for example J. of Applied Polymer Sci., 15, 2007-2021 (1971) and Polymer, 33 (22), 4832-4837 (1992). This concept has been applied for obtaining dispersions with bimodal or multimodal PSDs. For example for styrene-butadiene rubber latexes (SBR) see U.S. Pat. No. 4,657,966; for acrylonitrile-butadiene-styrene (ABS) latexes see U.S. Pat. No. 4,385,157; for rubber latex blends see U.S. Pat. No. 4,334,039. As prior art scientific papers, J. of Applied Polymer Sci., 70, 2667-2677 (1998), Colloid Polymer Sci. 276, 305-312 (1998), Colloid Polymer Sci. 275, 986-991 (1997), J. of Rheology 32, 751-771 (1988), can be mentioned.
The same concept applied to the fluoropolymer aqueous dispersions is reported in EP 657,514. This patent describes the use of fluoropolymer dispersion mixtures, expressly excluding thermoplastic polymers, obtained by emulsion polymerization and mixed for obtaining bimodal particle size distributions in order to optimize, specifically in the textile impregnation field, the polymer amount applied in each passage without crack formation. The ratio between the particle sizes having a smaller diameter and the particle sizes having a greater diameter is in the range 0.3-0.7. The first dispersion has a particle number average diameter in the range 180-440 nm, while the second fluoropolymer dispersion has an average diameter in the range 50-150 nm. The amount of the component having the smaller diameter compared with the component with the greater diameter is in the range 5-50% by weight, preferably 5-20% by weight. The examples reported in this patent substantially relate to the textile impregnation and show that, by using an amount of 10% and 18% by weight of the component having a smaller size, cracks are eliminated and the fluoropolymer amount applied in each passage increases. If amounts other than these two values are used, cracks are noticed. In the Examples, the polymer having a smaller size, has particle sizes in the range 100-110 nm and the above mentioned ratio ranges from 0.45 to 0.5. The only example given on the metal coating shows that with an amount equal to 10% of the fluoropolymer having smaller size particles, the film hardness increases compared with the case where the film is obtained by solely using the fluoropolymer having the high sizes as above defined.
The Applicant has tried to reproduce the Examples by using the small particle sizes towards the lowest value mentioned in the patent, also by using for example a larger amount of surfactant with respect to the patent teaching and to the amounts mentioned in the patent Examples. The Applicant has also made other attempts to reproduce the patent by extrapolating the teachings beyond the lowest ratio limit between small and great particles indicated in the patent (lower than 0.3). All these attempts of the Applicant have caused instability phenomena of the latex, both as such and stabilized with non ionic surfactants, as well as a global property worsening, such as for example critical thickness, mechanical properties, film gloss, after a storage of even 1-2 days or 1-2 weeks. This represents a limit from the application point of view since the dispersions should be prepared and used in a short period of time, incompatible with the industrial cycles. In any case said latexes after 2-3 months ageing do not allow to maintain the initial properties as above indicated. From the industrial point of view said latexes have a poor utility.
The same PSD concept has been applied for bimodal distributions in the patent WO 98/58984 wherein, in comparison with EP 657,514, a fluorinated thermoplastic polymer as one of the mixture components is used. The teaching of this patent allow to materialize the favourable bimodality effects, extended towards small particle populations under 80 nm, but it is limited and specific for thermoprocessable polymers, therefore with the drawback to worsen the mechanical properties at high temperatures.
Processes of the prior art to obtain small fluoropolymer particles under 100 nm use high surfactant amounts. See for example EP 248,446 and EP 369,466 relating to polytetrafluoroethylene (PTFE) particles. The obtained particles are anisotropic with a length-diameter L/D ratio higher than 5: these particles are commonly called fibrils.
The need was felt to obtain aqueous fluoropolymer dispersions capable to give films having an high critical thickness combined with good optical and mechanical properties also at high temperatures, reduced porosity and roughness and having an industrially acceptable shelf life, i.e., such as to maintain said properties after a prolonged dispersion storage of at least 6 months.
The Applicant has unexpectedly and surprisingly found that the instability (shelf-life) problems and the non obtainment of the desirable film performances (see above) are correlated to the presence of fibrils in the small size particle population. The Applicant has found that it is possible to solve the above mentioned technical problem by using the dispersions as defined hereinunder.
It is therefore an object of the present invention fluoropolymer dispersion mixtures comprising:
a) one or more dispersions formed by tetrafluoroethylene (TFE) homopolymers or by its copolymers with one or more monomers containing at least one unsaturation of ethylene type in amounts from 0 to 8% by weight, preferably from 0.01 to 3% by weight; the average particle sizes range from 180 to 400 nm, preferably from 200 to 300 nm;
b) one or more tetrafluoroethylene (TFE) dispersions with one or more monomers containing at least one ethylene unsaturation, the comonomer amount being such that the dispersion contains a fibril number lower than 10% of the particle total number, preferably lower than 5%, still more preferably lower than 1%, said fibrils being polymer particles having a length/diameter (L/D) ratio higher than 5; the fibrile number determination is carried out with an atomic force microscope (AFM); the dispersion particle average sizes are smaller than about 90 nm, preferably in the range 10-80 nm, more preferably 20-60 nm; component b) contains a comonomer amount such that the resulting polymer is not elastomeric and furthermore it shows composition and viscosity of the melt such as to be non thermoprocessable.
The dispersion mixture formed by a) and b) generally shows composition and viscosity of the melt such as to be non thermoprocessable.
As said the dispersion b) contains a fibril number lowe
Lenti Daria
Marchese Enrico
Visca Mario
Arent Fox Kintner Plotkin & Kahn
Ausimont S.p.A.
Cain Edward J.
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