Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-11-27
2004-05-18
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S250000, C526S253000
Reexamination Certificate
active
06737490
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a tetrafluoroethylene/perfluoro (alkyl vinyl ether) copolymer (hereinafter referred to as PFA) excellent in injection molding properties.
BACKGROUND ART
PFA is used in various fields, since it is excellent in heat resistance, chemical resistance, solvent resistance, etc. However, when PFA is molded by means of injection molding to obtain a molded product with a fine shape or a complex shape, there is a problem that the molded product is apt to have a roughened surface, leading to a low yield of the product.
In order to solve such a problem in the injection molding, a method has been considered wherein the molecular weight of PFA is lowered to reduce its melt viscosity. However, the mechanical properties of the product become deteriorated, while the molding properties of the PFA are improved. Under these situations, development of PFA has been required which has an excellent molding properties, mechanical properties, and less possibility of causing the surface roughness of the molded product.
DISCLOSURE OF THE INVENTION
The present invention provides PFA comprising polymerized units (A) based on tetrafluoroethylene and polymerized units (B) based on a perfluoro (alkyl vinyl ether), wherein the molar ratio of (A)/(B) is in the range of from 98.1/1.9 to 95.0/5.0, and its melt flow rate (hereinafter referred to as MFR) at 372° C. of from 35 to 60 g/10 min., and M
W
/M
n
of from 1 to 1.7 (M
w
is a weight-average molecular weight and M
n
is a number-average molecular weight.).
Further, the present invention provides a method for producing the PFA, which comprises copolymerizing, in the presence of a polymerization initiator, tetrafluoroethylene (a), and a perfluoro (alkyl vinyl ether)(b) in a molar ratio (a)/(b) in the range of from 90/10 to 55/45, wherein the fluctuation range of the (a)/(b) in the copolymerization is controlled to be within the range of ±20% in the presence of a chain transfer agent.
BEST MODE FOR CARRYING OUT THE INVENTION
The PFA of the present invention comprises (A) and (B) and the molar ratio of (A)/(B) is in the range of from 98.1/1.9 to 95.0/5.0. When the content of (B) is too small, the mechanical properties of PFA will be deteriorated. When the content of (B) is too large, the thermal stability will be lowered. When the content of (A) is too small, the production of the copolymer will be adversely affected. The molar ratio of (A)/(B) is preferably in the range of from 98.1/1.9 to 97.0/3.0, since the mold-ability of the copolymer and the durability in the mechanical properties of the resulting molded product are excellent.
The perfluoro (alkyl vinyl ether) (b) forming (B) is preferably a perfluorovinyl ether having a perfluoroalkyl group which has a straight-chain structure, a branched-chain structure or a cyclic structure. As its specific example, perfluoro (methyl vinyl ether), perfluoro (ethyl vinyl ether), perfluoro (propyl vinyl ether), perfluoro (hexyl vinyl ether) and perfluoro (octyl vinyl ether) can be mentioned. Particularly preferred is (b) having a straight-chain perfluoroalkyl group with a carbon number of from 1 to 8.
MFR of the PFA in the present invention measured in accordance with ASTM D-3307, is in the range of from 35 to 60 g/10 min. When MFR is too small, the molded product will have roughened surface, when the molded product is molded in a precise shape or a complex shape. When MFR is too large, the thermal resistance and the mechanical properties of the product will be deteriorated. MFR is preferably in the range of from 40 to 55 g/10 min., more preferably from 40 to 50 g/10 min.
The PFA of the present invention has excellent durability in mechanical properties with a MIT flex life of at least 6500 times, particularly 8000 times, in spite of having a relatively high MFR value. Among the PFA of the present invention, it has been found that there is the following relation between MFR (x g/10 min.) and MIT flex life (y time):
Y>−
875
x+
40000
In the present invention, the M
W
/M
n
of the PFA is in the range of from 1 to 1.7. The lower limit of M
W
/M
n
is 1. It means that PFA having a M
W
/M
n
of less than 1 cannot be produced. When M
W
/M
n
is too large, the mechanical properties of the PFA will be practically insufficient. M
W
/M
n
is preferably in the range of from 1.2 to 1.7.
When the PFA of the present invention is produced, a polymerization method is not limited and may be employed various polymerization methods such as a suspension polymerization method, an emulsion polymerization method, a solution polymerization method and a bulk polymerization method. As a polymerization initiator, a free radical type initiator, an oxidation-reduction type initiator, an ionizing radiation, heat, light, etc. may be employed. A free radical type initiator may be a bis (fluoroacyl) peroxide such as (C
3
F
7
COO)
2
, a bis (chlorofluoroacyl) peroxide such as (ClC
3
F
6
COO)
2
, a diacyl peroxide such as diisobutyryl peroxide, a dialkylperoxydicarbonate such as diisopropyl peroxydicarbonate, a peroxyester such as tert-butylperoxyisobutylate and tert-butylperoxypivalate, a peroxosulfate such as ammonium persulfate, an azobis-type initiator such as azobisisobutyronitrile, etc.
As a polymerization medium to be used for the various polymerization methods mentioned above may be mentioned hereinafter. In a solution polymerization, a hydrochlorofluorocarbon such as CClF
2
CF
2
CClFH (hereinafter referred to as HCFC225 cb), a hydrofluorocarbon such as CF
3
CF
2
CF
2
CF
2
CH
2
CH
3
, CF
3
CF(CF
3
)CFHCFHCF
3
, a chlorofluorocarbon such as CCl
3
F and C
2
Cl
3
F
3
, an alcohol having a carbon number of from 4 to 6 such as sec-butyl alcohol and tert-butyl alcohol may be employed. In a suspension polymerization and an emulsion polymerization, water or a mixed medium of water and other medium may be employed. Such an other medium is preferably the one similar to the above-mentioned medium in a solution polymerization.
The preferred method for producing PFA of the present invention comprises copolymerizing tetrafluoroethylene (a), and a perfluoro (alkyl vinyl ether)(b) in a molar ratio (a)/(b) of from 90/10 to 55/45, in the presence of a polymerization initiator, wherein the fluctuation of the molar ratio of (a)/(b) in the copolymerization is controlled to be in the range of ±20% in the presence of a chain transfer agent.
In the preferred production method mentioned above, a molar ratio of (a)/(b) in the copolymerization is from 90/10 to 55/45, more preferably from 85/15 to 60/40 (molar ratio). If the molar ratio in copolymerization is not within the above range, the PFA of the present invention cannot be readily produced.
In the preferred production method mentioned above, it is important to control the fluctuation of the molar ratio of (b)/(a) in the copolymerization within the range of ±20%, more preferably within the range of ±15%. If the fluctuation exceeds ±20%, PFA having a M
W
/M
n
of more than 1.7 is more likely to be produced.
In order to produce PFA having a M
W
/M
n
of from 1 to 1.7, it is effective to maintain the monomer composition ratio of (a) to (b) in the polymerization system at a constant value as much as possible. When the monomer composition ratio fluctuates during the copolymerization, the molecular weight of the PFA obtained will vary to result in a large M
W
/M
n
, since (a) and (b) are different in their monomer reactivity. Accordingly, it is important to employ a polymerization operation which can maintain the monomer composition ratio at a constant value.
As such a specific polymerization operation, may be mentioned a method wherein the monomers having an almost same composition as the one in PFA to be produced in the copolymerization are supplied continuously to a polymerization tank. This method may be carried out in a continuous type polymerization system and a batch type polymerization system. However, this method causes such a problems described as follows. When the viscosity of polymerization system containing the result
Funaki Atsushi
Sumi Naoko
Umino Masao
Asahi Glass Company Limited
Lipman Bernard
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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