Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-05-03
2002-09-17
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06451962
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of stabilizing a melt-processable fluorine-containing polymer, particularly to the method for efficiently stabilizing unstable end groups and/or unstable bonds being present in a trunk chain in a short period of time.
BACKGROUND ART
A lot of melt-processable fluorine-containing polymers are known, for example, a copolymer (FEP) of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), a copolymer (PFA) of TFE and perfluoro(alkyl vinyl ether) (PAVE), a copolymer (ETFE) of TFE and ethylene, and the like. Among such melt-processable fluorine-containing polymers, there are those which cause bubbles and cavity in a molded article produced by melt-processing. This is considered attributable to a volatile substance generated by decomposition of unstable end groups of the fluorine-containing polymer due to heat (U.S. Pat. No. 3,085,083).
The unstable end groups being present in the melt-processable fluorine-containing polymer depend on a polymerization method and kinds of a polymerization initiator, chain transfer agent, etc. For example, in case where a persulfate (ammonium persulfate, potassium persulfate or the like) which is usually used in emulsion polymerization is used as a polymerization initiator, a carboxylic acid end group is generated. The carboxylic acid end group changes to a vinyl end group (—CF═CF
2
) or an acid fluoride end group (—COF) by melt-kneading, though it depends on melting conditions. Those end groups are thermally unstable and generate a volatile substance which causes bubbles and cavity in a final product.
In U.S. Pat. No. 3,085,083, those unstable end groups are treated in the presence of water and heat and thus are converted to stable —CF
2
H groups. In a method described in JP-B-46-23245, unstable end groups are converted to stable end groups such as —CF
3
by reacting with a fluorinating agent such as fluorine gas.
Also there is a case where unstable bonds are generated in a trunk chain of a fluorine-containing polymer depending on a method of bonding of recurring units. For example, in case of a copolymer FEP of TFE-HFP, it is said that bonding of HFP with HFP is unstable and is cut by a mechanical force (shearing force) to be applied at melt-kneading, thus generating unstable vinyl end groups (U.S. Pat. No. 4,675,380).
In U.S. Pat. No. 4,675,380, a large shearing force is applied with a twin-screw extruder at melt-kneading to cut a HFP-HFP bond, thus giving an unstable end group. However in that method, since a kneading machine which is a twin-screw extruder being capable of applying a large shearing force for a short period of time is used, an object thereof is limited to cutting an unstable bond of a trunk chain and giving an unstable vinyl end, and it is not expected to treat the generated unstable end group in the twin-screw extruder. On the contrary, if oxygen is present, the vinyl end group is converted to acid fluoride end group. Therefore the fluorine-containing polymer is treated in an atmosphere being free from oxygen substantially and the fluorine-containing polymer having the vinyl end group is taken out of the twin-screw extruder as it is and then subjected to stabilizing treatment outside the extruder. Also the inside pressure of the extruder is reduced (less than 0.1 MPa in an absolute pressure) to exhaust volatile substances, etc. to be generated in the twin-screw extruder at the time of melt-kneading outside the extruder. However there arises coloration attributable to carbon generated by depolymerization of the vinyl end group.
In order to eliminate such a defect of the twin-screw extruder, in the method described in WO 98/09784, treatment for stabilizing an unstable group is carried out by using a so-called surface renewal type kneader having an effective volume ratio (effective volume in vessel/volume of vessel) larger than 0.3 at nearly an atmospheric pressure for a residence time of as long as 10 minutes or more under mild kneading condition of a power coefficient K of less than 8,000 which is represented by the equation: K=Pv/&mgr;
2
, wherein Pv is a power required per unit volume (W/m
3
), &mgr; is a melt viscosity (Pa·s) of a polymer at 372° C. and n is the number of rotations (rps).
However in the long-term melt-kneading by using a surface renewal type kneader , a problem with coloring is solved, but not only lowering of a treating efficiency cannot be avoided but also depolymerization of the vinyl end group advances and deterioration of the resin easily occurs. Further a size of the treating equipment becomes large and there arises a problem that it takes a long time for replacing a resin remained in the kneader when changing the resin to be treated.
An object of the present invention is to efficiently stabilize unstable groups of a melt-processable fluorine-containing polymer in a short period of time and to provide a molded article free from bubbles and coloration.
DISCLOSURE OF INVENTION
Namely the present invention relates to the method of stabilizing a melt-processable fluorine-containing polymer having unstable groups by melt-kneading the polymer in a kneader having a stabilization treatment zone satisfying any one of the following conditions.
Condition 1
(1) An oxygen-containing gas is present in the stabilization treatment zone, and
(2) water is present in the stabilization treatment zone.
Condition 2
(a) An oxygen-containing gas is present in the stabilization treatment zone in a sufficient amount that the fluorine-containing polymer after the stabilization treatment has the number of carbon radical spins measured by an electron spin resonance absorption analysis at a temperature of 77 K of 5×10
13
spins/g or less, preferably 1×10
13
spins/g or less, and
(b) water is present in the stabilization treatment zone.
The stabilization treatment zone may be under reduced pressure, under atmospheric pressure or under pressure.
In case where the stabilization treatment zone is made being in a pressurized state, an absolute pressure thereof is adjusted to not less than 0.2 MPa, preferably not less than 0.3 MPa.
The unstable group of the fluorine-containing polymer may be present at an end of polymer chain or at an unstable bond of a trunk chain.
With respect to the method of making an oxygen-containing gas and water being present in the stabilization treatment zone, the oxygen-containing gas and/or water may be mixed previously to the fluorine-containing polymer or may be introduced into the stabilization treatment zone for the first time. It is a matter of course that the oxygen-containing gas and/or water may be mixed previously to the fluorine-containing polymer and further supplied into the stabilization treatment zone. The oxygen-containing gas is preferably air.
It is preferable that a compound containing alkali metal, alkali earth metal or ammonium salt, an alcohol, an amine or a salt thereof or ammonia which accelerates the stabilization of unstable end groups of carboxylic acid or its derivative is present in the stabilization treatment zone. Those additives may be previously mixed to the polymer or may be added in the treatment zone.
For melt-kneading, a kneader like a screw type kneader giving a relatively large shearing force may be used. Preferable examples thereof are multi-screw type kneader, particularly a twin-screw kneader.
The residence time is less than 10 minutes, preferably less than eight minutes. When the residence time is too long, it becomes difficult to eliminate heat generated by shearing and there is a case where the polymer is deteriorated.
The stabilization method of the present invention can be applied to melt-processable fluorine-containing polymers having unstable groups. The stabilization method is effectively used particularly for stabilization treatment of unstable groups of a copolymer comprising at least two monomers selected from the group consisting of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) (PAVE), ethylene (ET), vinylidene fluoride (VdF) and chlorotrif
Hiraga Yoshiyuki
Komatsu Satoshi
Noda Tomohisa
Utsumi Yasuhiro
Boykin Terressa M.
Daikin Industries Ltd.
Sughrue & Mion, PLLC
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