Polymerization of fluoromonomers

Details Polymerization of fluoromonomers Polymerization of fluoromonomers

2000-11-02

2002-05-28

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...

C526S209000, C526S250000, C526S253000, C526S254000, C526S255000, C526S910000, C526S911000, C523S203000, C524S316000, C524S805000

Reexamination Certificate

active

06395848

ABSTRACT:

FIELD OF THE INVENTION
This invention is in the field of processes for polymerizing fluorinated monomers, in particular aqueous dispersion polymerization processes.
BACKGROUND OF THE INVENTION
Dispersion processes for polymerizing fluorinated monomers (fluoromonomers) in aqueous media are well known. Such processes employ a surfactant, i.e., a dispersant, to provide stability and permit the polymerization to be carried to commercially acceptable solids concentrations.
Dispersants that have been used in dispersion polymerization processes include the dispersants containing fluoroalkyl groups, such as perfluoroalkyl carboxylic acids and salts, disclosed by Berry in U.S. Pat. No. 2,559,752; the perfluoroalkyl ethane sulfonic acids and salts disclosed by Khan & Morgan in U.S. Pat. No. 4,380,618, Blaise & Grimaud in U.S. Pat. No. 4,025,709 and Baker & Zipfel in U.S. Pat. Nos. 5,688,884 and 5,789,508; the perfluoroalkoxy benzene sulfonic acids and salts disclosed by Morgan in U.S. Pat. No. 4,621,116; the partially-fluorinated carboxylic acids and salts disclosed by Feiring et al. in U.S. Pat. No. 5,763,552; and the perfluoropolyether carboxylic acids and salts disclosed by Garrison in U.S. Pat. No. 3,271,341, Giannetti & Visca in U.S. Pat. No. 4,864,006 and Abusleme & Maccone in European Patent Application Publication 0 625 526. Different dispersants are chosen for use in dispersion polymerization because of their influence on reaction rate, dispersed fluoropolymer particle size, dispersion stability, color and the like. The examples of the '341 patent reveal that the use of perfluoropolyether carboxylic acids/salt yielded polytetrafluoroethylene dispersions having particle size in the range of 152-299 nm.
Perfluoropolyethers having neutral end groups have been added to dispersion polymerizations as disclosed, for example, in the '006 patent mentioned above and by Giannetti et al. in U.S. Pat. No. 4,789,717.
Mayer in U.S. Pat. No. 5,563,213 discloses aqueous dispersions of melt-processible dipolymer of tetrafluoroethylene (TFE) and fluoroalkyl perfluorovinyl ether having number-average particle size of at most 50 nm. Mayer states that the fluorinated emulsifier is advantageously added in an amount somewhat higher than customary, and the example uses an emulsifier concentration of 0.5 wt % based on the water charge. Mayer indicates that a customary amount is, for example, 0.1-0.15% based on the mass of polymerization liquor. Morgan in PCT Publication WO96/24625 (U.S. Pat. No. 5,708,131) discloses an aqueous process for polymerizing at least two fluoromonomers using a surfactant concentration that is at least 1.2× the critical micelle concentration for the surfactant. The disclosed process yields dispersions having small particle size. Example 1 uses a fluorosurfactant concentration of 0.4 wt % based on the water charge (a concentration greater than the critical micelle concentration) to obtain a dispersion of a copolymer of TFE and hexafluoropropylene (HFP) having average particle size of 29 nm.
The '752 Berry patent cited above also discloses the formation of elongated or ribbon shaped particles of polytetrafluoroethylene (PTFE). Example III for which substantially all of the polymer particles were elongated ribbons used a fluorosurfactant concentration of more than 2 wt % based on the water charge. Berry does not describe the molecular weight of the PTFE resins that he produced. Folda et al. in European Patent Application Publication 0 248 446 disclose a process for producing an anisotropic, liquid crystalline dispersion of TFE polymer by polymerizing in the presence of fluorosurfactant at a concentration in the range extending from the critical micelle concentration to the total solubility of the surfactant. The anisotropic dispersion of Folda et al. contains a high proportion of rod-like particles. The only molecular weight reported by Folda for his rod-shaped resin was 25,000. Seguchi, et al. (J. Polym. Sci., Polymer Phys. Ed., 12, 2567-2576 (1974)) state that higher surfactant levels in emulsion polymerization of PTFE afford rod-like particles but that molecular weight decreases. It is stated that rod-like particles are obtained when the resin molecular weight is between 10
5
and 5.5-10
5
and, furthermore, that granular particles are obtained with molecular weights above 10
6
.
Copolymers of the ethylene/tetrafluoroethylene (ETFE) type are known from, for example U.S. Pat. Nos. 3,624,250, 3,859,262, 3,960,825, 4,020,253, 4,123,602, and 4,381,387. In particular, U.S. Pat. Nos. 3,859,262 and 4,338,237 show aqueous dispersion polymerization of such polymers. The addition of small amounts of a third monomer which affords a pendant side chain to the resin has been found to be useful to improve the resin mechanical properties. The stability of aqueous ETFE dispersions has been a problem which restricted solids level to less than 15% (U.S. Pat. No. 3,859,262). This problem was addressed in U.S. Pat. No. 4,338,237 which reported that a combination of a specific type of chain-transfer-agent (which also afforded low MW) and a particular dispersion-stabilizing compound afforded stable dispersions of higher dispersion solids contents. Dispersions (0.4 wt % perfluorooctanoic acid) which contained 19-27% solids with dispersion particle sizes of 120 to 270 nm were reported in U.S. Pat. No. 4,338,237. Dispersions (0.31-0.32 wt % perfluarooctanoic acid dispersing agent) with a small dispersion particle size (60-120 nm) were reported in U.S. Pat. No. 5,506,281 but the dispersion solids levels were low (7.8-12.0%).
Improved dispersion polymerization processes are desired. Areas for improvement include increased polymerization rate, enhanced incorporation of comonomers having relatively low reactivity, reduced spherical dispersion particle size, production of rod-shaped dispersion particles, more stable dispersions, and reduced coagulum formation, especially with reduced fluorosurfactant concentration.
SUMMARY OF THE INVENTION
This invention provides a process comprising polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and dispersing agent to obtain an aqueous dispersion of particles of fluoropolymer, wherein said dispersing agent is a combination of at least two fluorosurfactants, at least one of said fluorosurfactants being perfluoropolyether carboxylic acid or sulfonic acid, or salt thereof, and at least one of said fluorosurfactants being fluoroalkyl carboxylic or sulfonic acid or salt thereof, or fluoroalkoxy aryl sulfonic acid or salt thereof.
The invention further provides a dispersion of substantially-spherical fluoropolymer, preferably perfluoropolymer, particles in an aqueous medium containing fluorosurfactant, said dispersion containing at least 20% solids by weight based on total weight of dispersion, said particles having average diameter of no more than 150 nm, and wherein the concentration of said fluorosurfactant is less than the critical micelle concentration for said fluorosurfactant based on the weight of water in said dispersion, and preferably is no more than 0.35% by weight based on the weight of water in said dispersion.
The invention additionally provides a dispersion of substantially rod-shaped fluoropolymer particles in an aqueous medium containing fluorosurfactant, wherein the concentration of said fluorosurfactant is less than the critical micelle concentration for said fluorosurfactant based on the weight of water in said dispersion, and preferably is no more than 0.35% by weight based on the weight of water in said dispersion.
The invention additionally provides a dispersion comprising fluoropolymer particles in an aqueous medium wherein said fluoropolymer particles have a number average molecular weight of at least about 1·10
6
preferably at least about 3·10
6
, and at least about 20%, preferably at least about 30%, of said particles have a length to diameter ratio of greater than 3.
DETAILED DESCRIPTION
It has been discovered that aqueous dispersion polymerization of fluoromonomers using a dispersing ag

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