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
1998-02-13
2004-02-10
Reddick, Judy M. (Department: 1713)
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...
C524S779000, C524S780000, C524S787000, C524S796000
Reexamination Certificate
active
06689833
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of fluoropolymer resins, and deals with the stabilization of such resins having unstable end groups.
BACKGROUND OF THE INVENTION
Melt-fabricable copolymers of tetrafluoroethylene (TFE) and various copolymerizable mononmers such as hexafluoropropylene (HFP) are well known, as are polymerization processes for making them. Bro & Sandt in often-cited U.S. Pat. No. 2,946,763 disclose an aqueous process for TFE/HFP copolymers using water-soluble free-radical initiator. As a consequence of the initiators used, TFE copolymers made by aqueous processes such as that of Bro & Sandt have unstable end groups, notably —COOH or salts thereof, that can decompose during subsequent processing and result in unacceptable bubbling in finished products.
Various processes for stabilizing the end groups of such polymers have been devised. Schreyer, for example, in U.S. Pat. No. 3,085,083 discloses a humid heat treatment process for improving the stability of such polymers by converting unstable carboxylate end groups to relatively stable —CF
2
H (hydride) end groups. Imbalzano & Kerbow in U.S. Pat. No. 4,743,658 disclose fluorine treatment of copolymers of TFE and perfluoro(alkyl vinyl ether) (PAVE) to reduce the population of unstable end groups to very low levels. Such polymer finishing steps are time-consuming and costly.
Morgan & Sloan in U.S. Pat. No. 4,626,587 disclose a high-shear thermo-mechanical process for reducing the backbone instability in TFE/HFP copolymers. It is disclosed that, if the polymer contains unstable end groups or has poor color after removal from the high-shear extruder, such problems can be eliminated by fluorination (contact with elemental fluorine). All of Examples 1-3 of Morgan & Sloan did in fact have poor color after backbone stabilization by extrusion, as indicated by low values (5-18) of the %G color parameter, and required fluorine treatment to improve %G to the 49-54 range. As mentioned above, fluorine treatment is costly.
Mallouk & Sandt in U.S. Pat. No. 2,955,099 disclose viscosity stabilized TFE/HFP interpolymers prepared according to the Bro & Sandt '763 patent for which the viscosity stability is achieved by incorporation of cationic metal compound. Compounds of cationic metals such as potassium, caesium and rubidium, particularly in the form of salts of labile anions such as iodide and bromide, are said to be most active. Potassium perchlorate is said to be used with particular advantage to improve color since it functions in the dual role of oxidant and viscosity stabilizer. However, Mallouk & Sandt also disclose that compounds of cationic metal with anions such as carbonate and nitrate tend to cause inhomogeneities in the interpolymer and are therefore not suitable for use as viscosity stabilizers. As shown by the Reference Example below, TFE/HFP interpolymers prepared according to Bro & Sandt, additionally, are not suitable for use in the process of the present invention.
Gibbard in U.S. Pat. No. 5,180,803 discloses a method for the production of a melt-fabricable fluoropolymer, which method comprises, after preparing an aqueous dispersion of the fluoropolymer using dispersion polymerization with an initiator that yields carboxylic acid groups on the fluoropolymer, converting the carboxylic acid groups on the fluoropolymer in the aqueous dispersion to carboxylate anion groups using a base and then heating the so-modified fluoropolymer dispersion to a temperature of 190°-240° C. to cause simultaneously (1) substantial removal of carboxylate anion groups, thereby to yield stable groups in their place, and (2) isolation of the fluoropolymer from the dispersion by coagulation of the fluoropolymer dispersion. Gibbard's EXAMPLE 1 discloses stabilization and isolation of TFE/HFP copolymer manufactured by the process of Bro & Sandt by addition of 1 wt % of potassium hydroxide, based on dry polymer solids, followed by neutralization with nitric acid which would correspond to formation of about 1.6 wt % of KNO
3
, again based on the weight of dry TFE/HFP copolymer solids. One skilled in the art would then expect about 1700-4000 ppm of KNO
3
to be in the copolymer resin after isolation and drying.
TFE/HFP copolymers of the prior art, such as mentioned above, prepared by the process of Bro & Sandt have a high degree of instability, with end group and backbone contributions. This instability may be characterized by measurement of a total unstable fraction (TUF), as described herein. Polymers of the Bro & Sandt type generally have TUF of at least 0.5% when melt viscosity is in the range of 1-10×10
3
Pa·s.
SUMMARY OF THE INVENTION
A new polymerization process for TFE/HFP/PAVE copolymers, disclosed in European Patent Application 96-112948 (Publication 0 789 038), yields copolymer having high stability, characterized by a total unstable fraction of no more than 0.3%. In this process, copolymerizing is carried out with chain transfer agent present and with initiator present in an amount effective to initiate no more than half of the copolymer molecules made. The combined use of increased CTA and reduced initiator results in a TFE/HFP copolymer having improved total stability, i.e., the combination of reduced backbone instability and a reduced —COOH end group population. The resultant TFE/HFP copolymer can be used for many purposes without special stabilization finishing steps. However, if very high stability or complete absence of discoloration is required, some additional treatment such as fluorination would be necessary.
The problem of obtaining TFE/HFP copolymer and other melt-fabricable fluoropolymer resins of even higher stability and/or good color to permit commercial use without a costly stabilization finishing procedure is solved by a convenient and inexpensive process. The process for stabilizing and whitening melt-fabricable fluoropolymer resin having total unstable fraction, as defined herein, of no more than 0.3%, comprises extruding said resin in the presence of alkali metal nitrate to obtain said fluoropolymer resin having improved color and/or stability. The nitrate can be introduced into aqueous polymerization of the fluoropolymer, before isolation of the fluoropolymer solids from the aqueous dispersion, or after isolation of the resin from the polymerization medium and prior to melt extrusion. Preferred fluoropolymers for treatment by this process are tetrafluoroethylene copolymers.
DETAILED DESCRIPTION
It has been discovered that a small amount of alkali metal nitrate present during melt extrusion of fluoropolymer resin having a low level of instability as delivered to the extruder results in improved stability and/or color.
The fluoropolymers for which the process of this invention is suitable are melt-fabricable. As such, they generally have melt viscosity (MV) in the range of 0.5-50×10
3
Pa·s, though MV outside this range is known. MV of 1-20×10
3
Pa·s is preferred.
Preferred fluoropolymers for the process of this invention include melt-fabricable copolymers of TFE with a sufficient amount of at least one fluorinated comonomer to reduce the melting point of the copolymer substantially below that of homopolymer polytetrafluoroethylene, e.g., to a melting point of no more than about 320° C. Comonomers that can be copolymerized with TFE include, for example, HFP and fluorinated vinyl ethers (FVE) of the formula CF
2
═CFOR or CF
2
═CF—OR′—OR wherein —R and —R′— are independently completely-fluorinated or partially-fluorinated linear or branched alkyl and alkylene groups containing 1-8 carbon atoms. Preferred —R groups contain 1-4 carbon atoms, while preferred —R′— groups contain 2-4 carbon atoms. FVE of the formula CF
2
═CFOR are preferred, particularly perfluoro(alkyl vinyl ether) (PAVE). Preferred TFE copolymers include TFE/PAVE and TFE/HFP/PAVE. Preferred PAVE include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE). As one skilled in the art will recognize, the amount
Bidstrup David Elton
Blair Leslie Mitchell
E. I. du Pont de Nemours and Company
Reddick Judy M.
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