Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2003-06-02
2004-11-02
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...
C526S247000, C526S249000, C526S250000, C526S255000
Reexamination Certificate
active
06812310
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing a fluoropolymer that has a reduced amount of polar end groups, in particular ionized or ionizable end groups such as sulfonic acid groups or carbonyl containing end groups such as carboxylic acid groups and carboxylate groups. In particular, the invention relates to the use of a chloride salt in the initiation of the free radical polymerization of fluorinated monomers to produce a fluoropolymer that has an improved processibility and/or heat resistance. The invention further relates to fluoropolymers that can be obtained with the process.
BACKGROUND OF THE INVENTION
Fluoropolymers have, depending on the monomers used and the applied initiator systems during the radical polymerization, different polar end groups; for example polar end groups quite often present in fluoropolymers are: —CH
2
OH, —COO
−
, —CH
2
—O—SO
3
−
, —SO
3
−
. These polar end groups are not desired, because they can undergo further reactions (e.g. decarboxylation) during processing or heat treatment and are a cause for processing difficulties and finally also for discolorations.
For example, U.S. Pat. No. 6,211,319 describes perfluoroelastomers that have carbonyl containing end groups. It is disclosed in this patent that the presence of these carbonyl containing end groups, while providing desirable cure properties, may present problems in the processing of these polymers. In particular, the presence of these groups may cause the polymer to become too viscous. Accordingly, U.S. Pat. No. 6,211,319 proposes to reduce the number of carbonyl containing end groups by a decarboxylation which involves a heat treatment. However, this process has the disadvantage that an additional process step is needed which additionally involves supplying energy. Accordingly, such a process increases the cost of manufacturing and causes inconveniences in the manufacturing.
Additionally, the presence of carboxylic or carboxylate containing end groups generally diminishes beneficial properties of the fluoropolymer including thermal properties such as heat resistance. Methods have therefore been developed in the prior art to reduce the amount of these groups. Such methods include for example post fluorination as well as the use of special initiators such as sulfinates as disclosed in U.S. Pat. No. 5,285,002. However, these methods have disadvantages such as increasing cost and reducing convenience of manufacturing.
It is therefore desired to find a further method of reducing the content of polar end groups and to produce fluoropolymers that have improved processibility and/or thermal properties including improved heat resistance. Preferably, the method of making these polymers allows for a cost effective and convenient manufacturing.
SUMMARY IN THE INVENTION
In accordance with the present invention, it was found that when the fluoropolymer is produced by free radical polymerization of fluorinated monomers and the initiation of the polymerization is carried out in the presence of a chloride salt, a fluoropolymer results that has a reduced amount of polar end groups. Polar end groups include hydroxy groups and ionized or ionizable end groups. Examples of ionized or ionizable end groups include acid groups and salts thereof such as sulfonic acid and salts thereof, —CH
2
—OSO
3
H groups, sulfates and carbonyl containing end groups such as carboxylic and carboxylate groups. Due to the presence of chloride salt at the initiation of the free radical polymerization, CF
2
Cl end groups are formed instead of the polar end groups. As a result, a fluoropolymer is obtained that has improved processibility and heat resistance. By the term “fluoropolymer” in connection with this invention is meant a polymer that has a fluorinated backbone, i.e. a partially or fully fluorinated backbone.
In accordance with one aspect of the present invention, the ratio of the amount of chloride salt to initiator is selected such that the amount of polar end groups in the resulting fluoropolymer is at least 10% less, preferably at least 15% less and most preferably at least 20% less compared to a similar fluoropolymer produced without the presence of a chloride salt. By the term “similar fluoropolymer” in this connection is meant that the fluoropolymer has approximately, i.e. within experimental error, the same chemical constitution and about the same molecular weight. The amount of polar end groups and the reduction thereof can be measured with common analytical methods including for example fourier transform infrared spectroscopy, NMR and titration methods.
An additional advantageous property of the obtained fluoropolymer is that due to the presence of the CF
2
Cl end groups, the fluoropolymer has improved bonding properties to other substrates including elastomeric layers such as silicone rubbers when bonded to these substrates in the presence of an organic compound having one or more hydride functions MH wherein M is selected from the group consisting of Si, Ge, Sn and Pb. This compound can be present in the fluoropolymer composition or can be present in a layer of the substrate. Also, by including the organic compound in the fluoropolymer that can be obtained with the process of this invention, the fluoropolymer can be cured using a peroxide cure system.
The use of sodium chloride or ammonium chloride in the free radical polymerization is disclosed in EP 320 940. However, the latter publication does not appreciate that the amount of polar end groups can be reduced by initiating the polymerization in the presence of chloride salt, nor does the publication appreciate the improved processibility and/or heat resistance of the fluoropolymer that results. Rather, this EP patent application teaches the use of a certain amount of cations, which may derive from a chloride salt but which may also derive from another salt, to improve the emulsion copolymerization of certain liquid vinyl ether monomers.
According to a further aspect of the present invention, there is also provided a fluoropolymer comprising CF
2
Cl end groups and that is free of acid groups and salts thereof or that contains said acid groups and their salts in an amount of not more than 0.1 milli equivalents NaOH per gram of fluoropolymer, with the proviso that when said fluoropolymer is a terpolymer of tetrafluoroethylene, hexafluoropropylene and C
3
F
7
[(CF
2
)
3
O]
2
CF═CF
2
, the fluoropolymer is free of iodide containing end groups.
Still further, the present invention also provides a fluoropolymer composition for making a fluoroelastomer, comprising the above fluoropolymer and a cure composition.
The invention also provides the use of a chloride salt in the initiation of a free radical polymerization of one or more fluorinated monomers to improve the processibility and/or heat resistance of the fluoropolymer resulting from said free radical polymerization.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the process of the invention, a chloride salt is used in the initiation of the free radical polymerization of fluorinated monomers to obtain a fluoropolymer with a reduced amount of polar end groups. Suitable chloride salts for use in the invention include those of the formula:
M Cl
n
(I)
wherein M represents a mono- or multi-valent cation and n corresponds to the valence of the cation. Suitable cations M include organic and inorganic cations. Particularly useful cations are ammonium and metal cations including mono-valent cations such as sodium and potassium as well as divalent cations such as calcium and magnesium. Examples of ammonium chloride salts include tetraalkyl ammonium chlorides such as tetrabutyl ammonium chloride.
The amount of chloride salt is selected relative to the amount of the polymerization initiator so that the amount of polar end groups is reduced by at least 10%, preferably at least 15% and most preferably at least 20%. In a particular preferred embodiment of the invention, the total amount acid groups and their salts in the fluoropolymer is not
Grootaert Werner M. A.
Hintzer Klaus
Hirsch Bernhard
Kaspar Harald
Löhr Gernot
3M Innovative Properties Company
Hu Henry S
Szymanski Brian E.
Wu David W.
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