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...
Utility Patent
1999-02-26
2001-01-02
Michl, Paul R. (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...
Utility Patent
active
06169139
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fluoroelastomer latices capable of producing films having reduced water sensitivity and to a process for preparation of such latices.
BACKGROUND OF THE INVENTION
Elastomeric fluoropolymers (i.e. fluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. As a consequence, compositions that contain fluoroelastomers as major components have found utility as protective coatings for substrates such as fabrics, fibers, metals, and plastics. In some applications, the fluoroelastomer coating compositions are applied to substrates in the form of thin surface veneers, while in other applications the substrates are impregnated to a significant depth. For example, the use of fluoroelastomer coatings in protecting and consolidating lapideous compositions is disclosed in U.S. Pat. No. 5,219,927. Protective coatings for textiles are disclosed in U.S. Pat. No. 4,654,235.
Fluoroelastomer coating compositions are generally of two types, organic solvent-based systems or water-based systems. The former can be prepared by a process wherein a substantially dry fluoroelastomer gum is dissolved in a solvent such as methyl ethyl ketone or methyl isobutyl ketone. In contrast, water-based coating compositions, i.e. latices, are not usually prepared directly from fluoroelastomer gums because of the difficulty of forming a stable aqueous composition from the substantially dry gum. Instead, fluoroelastomer latices are generally prepared by either of two multistep methods. According to the first method, a fluoroelastomer gum is dissolved in a suitable organic solvent. The resulting solution is then mixed with an aqueous solution containing stabilizers, such as surfactants, and the organic solvent is then removed. Such a process is described in U.S. Pat. No. 3,886,108. In the second method, the latex is prepared directly by polymerizing fluorinated monomers in an aqueous emulsion or suspension process. Such a process is disclosed in U.S. Pat. No. 3,962,169.
Although preparation of organic solvent-based coating and adhesive compositions is relatively simple and inexpensive, organic solvent solutions present significant safety and environmental hazards due to the flammability and volatility of the solvents employed. Preparative methods for fluoroelastomer latices that rely on dissolution in organic solvents as a first step suffer from these same disadvantages.
The preferred method for preparation of fluoroelastomer latices is by direct polymerization using a batch, semi-batch or continuous process. Regardless of the method used, the goal is to produce a latex which forms a continuous, pinhole-free film when applied to a substrate. Further, the film will preferably contain the minimum quantity of non-polymeric species. Impurities, such as traces of surfactants and inorganic salts, often remain in films formed from latices and such impurities can cause poor water resistance of the films. For example, films formed at room temperature can absorb over 3 wt. % of water upon immersion. The presence of impurities is a particular problem in situations where the final product cannot be heated to high temperatures to drive off volatile, water sensitive species. There is thus a clear need for an improved fluoroelastomer latex product having lower levels of impurities and consequently less water sensitivity.
SUMMARY OF THE INVENTION
The present invention is directed to a process for preparation of a fluoroelastomer latex which is useful for forming films having improved water resistance and adhesion to a coated substrate.
In particular, the present invention is directed to a process for preparation of a fluoroelastomer latex which comprises the steps of:
A) forming an aqueous emulsion of monomers capable of being copolymerized to form a fluoroelastomer, said emulsion comprising
1) a first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene,
2) at least one other fluorine-containing monomer,
3) a free radical initiator in an amount which catalyzes the free radical emulsion polymerization of said monomers,
4) from about 0.05-3.0 parts by weight per hundred parts monomer of a fluorinated surfactant, and
5) sufficient base to adjust the pH of said emulsion to between 3 to 8;
B) polymerizing said emulsion at a temperature of about 40-130° C. and a pressure of about 2-9 MPa to form a fluoroelastomer emulsion composition;
C) adding a sufficient amount of base to adjust the pH of said fluoroelastomer emulsion composition to between 5-9; and
D) concentrating said fluoroelastomer emulsion composition which has a pH of 5-9 to form a solids-rich fluoroclastomer latex.
The invention is further directed to a fluoroelastomer latex produced by a process which comprises the steps of
A) forming an aqueous emulsion of monomers capable of being copolymerized into a fluoroelastomer, said emulsion comprising
1) a first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene,
2) at least one other fluorine-containing monomer,
3) a free radical initiator in an amount which catalyzes the free radical emulsion polymerization of said monomers,
4) from about 0.05-3.0 parts by weight per hundred parts monomer of a fluorinated surfactant, and
5) sufficient base to adjust the pH of said emulsion to between 3-8;
B) polymerizing said aqueous emulsion at a temperature of about 40°-130° C. and a pressure of about 2-9 MPa to form a fluoroelastomer emulsion composition,
C) adding a sufficient amount of base to adjust the pH of said fluoroelastomer emulsion composition to between 5-9; and
D) concentrating said fluoroelastomer emulsion composition which has a pH of 5-9 to form a solids-rich fluoroelastomer latex.
In preferred embodiments of the invention a second surfactant is added to the fluoroelastomer emulsion composition to further stabilize the composition. This addition may take place prior to addition of base to adjust the pH of the fluoroelastomer emulsion composition (Step C) above). Alternatively, the additional surfactant may be added after adjustment of pH as well as after concentrating the fluoroelastomer emulsion.
In other preferred embodiments of the invention the concentration step is carried out by use of a creaming agent.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, solids-rich fluoroelastomer latices that can be used to form coating compositions of excellent quality, in particular having reduced water sensitivity, are prepared via a process which comprises polymerization, pH adjustment of the thus-prepared fluoroelastomer emulsion, and concentration of the emulsion to form a solids-rich latex composition. By “reduced water sensitivity” and “water resistance” is meant that films of 21 mils thickness or less made from the latices of this invention absorb less than 2 wt. % water when said films are immersed in water for 48 hours at 25° C.
In the first step of the process of an embodiment of the present invention an aqueous emulsion of at least two monomers is formed. The first monomer is selected from the group consisting of vinylidene fluoride and tetrafluoroethylene. At least one other fluorinated monomer is also present in the emulsion. The emulsion is generally formed by introduction of gaseous monomers and water into a reaction vessel.
Examples of common fluorinated monomers which are copolymerizable with vinylidene fluoride to form fluoroelastomers and which are useful in the practice of the invention include hexafluoropropylene, chlorotrifluoroethylene, 2-hydropentafluoropropene, 1-hydropentafluoropropene, dichlorodifluoroethylene, tetrafluoroethylene and perfluorinated alkyl vinyl ethers, for example trifluoromethyl trifluorovinyl ether, pentafluoroethyl trifluorovinyl ether, and heptafluoropropyl trifluorovinyl ether. It is preferable that the fluorinated monomer capable of copolymerization with vinylidene fluoride contain at least as many fluorine atoms as carbon atoms. Elastomeric copolymers of vinylidene fluoride and hexafluoropropylen
DuPont Dow Elastomers LLC
Michl Paul R.
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