Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-07-24
2003-03-04
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S082000, C526S231000, C526S247000, C526S250000
Reexamination Certificate
active
06528600
ABSTRACT:
The present invention relates to a method for producing a tetrafluoroethylene/perfluoro(alkyl vinyl ether) type copolymer excellent in stability.
A tetrafluoroethylene (hereinafter referred to as TFE)/perfluoro(alkyl vinyl ether) (hereinafter referred to as PAVE) type copolymer (hereinafter referred to as PFA) is known as a melt processable fluororesin and is widely used as a material for formed products such as tubes, pipes, joints and containers, for wire coatings, for coating, for lining, etc.
When PFA is used for containers, pipings or joints for a liquid reagent for semiconductors, there has been a problem that fluorine ions tend to elute from PFA, and cracking is likely to form by the liquid reagent. In such an application, it is common to use PFA stabilized by treatment with fluorine gas (hereinafter referred to fluorination) to convert unstable terminal groups in the molecule to stable terminal groups in order to reduce elution of fluorine ions or to improve the cracking resistance against a liquid reagent. PFA stabilized by fluorination is free from a problem such that the unstable terminal groups are decomposed during injection molding to form hydrofluoric acid which in turn corrodes a mold and has merits for molding, as compared with conventional PFA having unstable terminal groups.
However, for the production of PFA stabilized by fluorination, a treatment installation employing a fluorine gas is required, and there has been a problem that the production process tends to be cumbersome.
As the terminal groups in PFA, the following terminal groups are conceivable.
When PFA is produced by solution polymerization or suspension polymerization, terminal groups derived from a polymerization initiator tend to be substantial. In the production of PFA, as the polymerization initiator, a fluorine type polymerization initiator of e.g. (X(CF
2
)
n
COO)
2
(wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of from 1 to 10) is preferred. As a result, the terminal groups derived from the polymerization initiator will be stable terminal groups such as X(CF
2
)
n
groups.
On the other hand, the following three types are conceivable as unstable terminal groups in PFA which cause elution of fluorine ions or cracking by a liquid reagent.
(1) Terminal groups derived from a chain transfer agent.
(2) —COF terminal groups formed by a transition reaction of radicals formed when PAVE is added to growing chain radicals in the copolymerization process of TFE and PAVE as shown by the formula 1 and disclosed in JP-B-4-83:
wherein R
f
is a perfluoroalkyl group.
(3) —COOH terminal groups formed by hydrolysis of —COF groups.
Among them, the number of unstable terminal groups derived from a chain transfer agent is large. Such terminal groups derived from a chain transfer agent are different depending upon the type of the chain transfer agent to be employed.
U.S. Pat. No. 3,642,742 discloses a case where cyclohexane and methanol are used as chain transfer agents. With cyclohexane, cyclohexyl groups will be formed as terminal groups, and with methanol, —CH
2
OH groups are formed. When either one of the chain transfer agents was used, the thermal stability of the formed PFA was low.
When methane is used as a chain transfer agent, —CH
3
groups will be formed as terminal groups. As compared with —CH
2
OH groups, —COF groups and —COOH groups, the stability of PFA will be improved with —CH
3
groups. However, even then, the stability is not sufficient for an application to semiconductors, and it has been necessary to change —CH
3
groups to —CF
3
groups by fluorination for the purpose of stabilization.
Accordingly, it has been desired to develop a method for producing stable PFA, which does not require a treatment process employing a fluorine gas.
It is an object of the present invention to provide a method for producing PFA having a small content of unstable terminal groups contained in the molecule.
The present invention provides a method for producing PFA which comprises carrying out polymerization in a polymerization medium in the presence of a chain transfer agent by means of a polymerization initiator, wherein the chain transfer agent is a C
1 or 2
hydrofluorocarbon.
In the method for producing PFA according to the present invention, as the polymerization method, a known polymerization method such as suspension polymerization, solution polymerization, emulsion polymerization or bulk polymerization may be employed without any particular limitation. Particularly preferred is solution polymerization or suspension polymerization.
In the method for producing PFA according to the present invention, TFE and PAVE which are monomers, are co-polymerized.
As PAVE, a monomer represented by CF
2
═CFOR
f
is employed. R
f
is a perfluoroalkyl group, preferably a C
1-6
linear, branched or cyclic perfluoroalkyl group, more preferably a linear perfluoroalkyl group, still further preferably, a perfluoromethyl group, a perfluoroethyl group or a perfluoro n-propyl group.
In the present invention, in addition to TFE and PAVE, other monomers may be copolymerized in a small amount. As such other monomers, fluorine-containing olefins other than TFE, such as trichlorofluoroethylene and hexafluoropropylene, may be mentioned.
In the present invention, the ratio of polymerized units of TFE to polymerized units of PAVE in PFA is preferably such that polymerized units of TFE/polymerized units of PAVE are equal to from 99.5/0.5 to 95/5 (molar ratio). If the ratio is smaller than this range, the processability and stress cracking resistance tend to deteriorate, and if it is larger than this range, the mechanical properties tend to deteriorate. More preferably, polymerized units of TFE/polymerized units of PAVE are equal to from 99/1 to 97/3 (molar ratio).
The content of polymerized units of other monomers is preferably from 0 to 10 mol %, more preferably from 1 to 8 mol %, based on the total mol amount of polymerized units of TFE and polymerized units of PAVE.
As the polymerization medium in the present invention, a fluorine-containing organic medium having a small chain transfer coefficient is preferred. Particularly preferred is at least one polymerization medium selected from the group consisting of a C
3-10
perfluorocarbon, a C
3-10
hydrofluorocarbon and a C
3-10
hydrochlorofluorocarbon.
The perfluorocarbon is preferably a saturated perfluorocarbon having a linear, branched or cyclic structure (provided that it may contain an etheric oxygen atom in its molecule). Specific examples include perfluorocyclobutane, perfluorohexane, perfluoro(dipropyl ether), perfluorocyclohexane and perfluoro(2-butyltetrahydrofuran).
The hydrofluorocarbon is preferably a saturated hydrofluorocarbon having a linear, branched or cyclic structure, wherein the number of fluorine atoms in the molecule is larger than the number of hydrogen atoms (provided that it may contain an etheric oxygen atom in the molecule). Specific examples include CH
3
OC
2
F
5
, CH
3
OC
3
F
7
, C
5
F
10
H
2
, C
6
F
13
H and C
6
F
12
H
2
.
The hydrochlorofluorocarbon is preferably a hydrochlorofluorocarbon having a linear, branched or cyclic structure, wherein the number of hydrogen atoms is at most 3 (provided that it may contain an etheric oxygen atom in the molecule). Specific examples include CHClFCF
2
CF
2
Cl.
The amount of the polymerization medium to be used is preferably from 10 to 90%, more preferably from 30 to 70%, by volume %, based on the volume of the autoclave.
As the chain transfer agent in the present invention, a C
1 or 2
hydrofluorocarbon is used. Further, a hydrofluorocarbon wherein the molar ratio of number of hydrogen atoms
umber of fluorine atoms in its molecule is from 1/2 to 5/1, is preferred. Specific examples include CF
2
H
2
, CFH
3
, CFH
2
CH
3
, CF
2
HCH
3
, CFH
2
CFH
2
, CF
2
HCFH
2
, CF
3
CH
3
and CF
3
CFH
2
. Further, from the viewpoint of the chain transfer property, a hydrofluorocarbon having a larger number of hydrogen atoms than the number of fluorine atoms in its molecule, is preferred. By
Funaki Atsushi
Sumi Naoko
Asahi Glass Company Limited
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Zitomer Fred
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