Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-02-12
2004-11-23
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...
C526S078000, C526S081000, C526S234000, C526S915000, C526S919000
Reexamination Certificate
active
06822060
ABSTRACT:
The present invention relates to a process for producing a tetrafluoroethylene polymer (hereinafter referred to as PTFE) excellent in strength. Particularly, it relates to a process for producing PTFE excellent in strength, which is suitable for stretching after paste extrusion.
Heretofore, tetrafluoroethylene homopolymer (PTFE) has been obtained by polymerizing tetrafluoroethylene (hereinafter referred to as TEE) while tetrafluoroethylene copolymer has been obtained by polymerizing tetrafluoroethylene together with a copolymerizable modifying monomer, and both have been used for various purposes.
PTFE can be produced by an aqueous dispersion polymerization of TFE and can be obtained in the form of an aqueous dispersion having PTFE particles dispersed, or can be obtained in the form of PTFE fine powder by coagulating the aqueous dispersion polymerization solution, followed by drying.
A conventional PTFE fine powder has a high melt viscosity and does not readily flow at the melting temperature, and thus, it has non-melt processability. Therefore, a paste extrusion is carried out usually in such a manner that a PTFE fine powder is blended with a lubricant, and the lubricated PTFE is extruded. Then, an extruded product obtained by removing the lubricant, is usually fused (sintered) at a temperature higher than the melting of PTFE, to the shape of the final product.
On the other hand, other important products obtained from the PTFE fine powder may, for example, be air permeable fabrics for products such as clothing, tents and separation membranes. These products can be obtained by rapidly stretching in a non-sintered state an extruded product obtained by paste extrusion of a PTFE fine powder, to impart a nature such that steam can permeate, but condensed water can not permeate.
The physical properties required for stretched products obtained by stretching PTFE, have become increasingly high year after year, and even with stretched products obtained by such PTFE, the strength is not sufficient. To solve such problems, various studies have been made with respect to the polymerization method. For example, U.S. Pat. No. 4,016,345 discloses a method of continuously adding an inorganic persulfate initiator at a temperature of from 95 to 125° C., until from 50 to 80% of the total amount of TFE for polymerization has been polymerized. The obtained PTFE has a stretchable property whereby the stretch rate at a speed of 100%/sec., is at least 20 times.
U.S. Pat. No. 4,159,370 discloses a method of using a persulfate initiator and changing the polymerization conditions after the initiation of polymerization, in order to obtain a stretchable PTFE fine powder having an average molecular weight of at least 5,000,000. For example, the polymerization is carried out at a polymerization temperature of from 55 to 85° C., and the polymerization temperature is lowered by 5 to 30° C. during the polymerization. The obtained PTFE has a uniform stretchable property even when the stretch rate is 20 times at a speed of 100%/sec.
U.S. Pat. No. 4,363,900 discloses a dispersion polymerization method for producing a stretchable fine powder. In this method, polymerization is carried out at a temperature of from 55 to 120° C., and during the polymerization, a polymerization inhibitor such as hydroquinone is added, whereby the polymerization time is prolonged to a level of at least 130%. PTFE prepared by this method has a uniform stretchable property at a level of up to 30 times at a speed of 100%/sec. Further, this specification discloses that the PTFE fine powder disclosed in U.S. Pat. No. 4,159,370 has a good stretchability, but it is still difficult to accomplish uniform stretching.
U.S. Pat. No. 4,766,188 discloses a dispersion polymerization method of TFE, wherein ammonium sulfite is added after initiation of the polymerization. PTFE prepared by this method is stretched at a speed of 17%/sec, but the stretch rate is up to 7 times. The standard specific gravity value is as low as 2.149, but the cooling rate employed, is 1.5° C./min, not 1.0° C./min as stipulated in ASTM.
U.S. Pat. Nos. 4,576,869 and 4,654,406 disclose a method wherein addition of a permanganate initiator is stopped near the end of polymerization, whereby the termination time can be prolonged by at least 7% as compared with a case where addition of the initiator is continued to the end of the reaction. PTFE prepared by this method is a stretchable PTFE fine powder. With the obtained PTFE, a stretch uniformity of at least 75% (i.e. a good stretch uniformity) is accomplished by adding 17 mass % of a lubricant and stretching it at least 1,000% within a range of from 10%/sec to 100%/sec. This PTFE has uniform stretchability even though a very low speed of 10%/sec is employed for the evaluation of the stretchability.
JP-A-2000-143707 discloses a polymerization which is initiated at a temperature of not higher than 60° C. and terminated at a temperature higher than 55° C., and the termination temperature is made to be higher by at least 5° C. than the initiation temperature, and the polymerization is terminated in the presence of a liquid stabilizer.
The stretchability of PTFE obtained by the above prior art is practically not yet sufficient, and it is desired to develop PTFE whereby it is possible to obtain a stretched product having a more improved property (such as higher strength).
It is an object of the present invention to provide a process for producing PTFE which is PTFE having stretchability, fibrillation property and non-melt processability, whereby it is possible to obtain a stretched product having an improved property (such as higher strength).
In order to solve the above problems, the present inventors have conducted an extensive study and as a result, have found it possible to solve the above problems by using, as a polymerization initiator, a redox polymerization initiator of a halogen acid salt/a sulfite. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides a process for producing a tetrafluoroethylene polymer, which comprises polymerizing tetrafluoroethylene in an aqueous medium in the presence of a dispersant, a stabilizer and a polymerization initiator, wherein the polymerization initiator is a redox polymerization initiator comprising a halogen acid salt YXO
3
/a sulfite Z
2
SO
3
wherein X is a chlorine atom, a bromine atom or an iodine atom, Y is a hydrogen atom, ammonium, an alkali metal or an alkaline earth metal, and Z is ammonium, an alkali metal or an alkaline earth metal.
Further, the present invention provides the above process for producing PTFE, wherein both the halogen acid salt and the sulfite of the redox polymerization initiator are added to the polymerization system simultaneously, or either the halogen acid salt or the sulfite is added preliminarily and the other is added intermittently or continuously during the polymerization.
Now, the present invention will be described in detail with reference to the preferred embodiments.
In the polymerization process of TFE of the present invention, as the polymerization initiator, a redox polymerization initiator comprising a combination of a halogen acid salt and a sulfite, is used.
The halogen acid salt is one represented by YXO
3
, and the sulfite is one represented by Z
2
SO
3
. In the formulae, X is a chlorine atom, a bromine atom or an iodine atom, Y is a hydrogen atom, ammonium, an alkali metal or an alkaline earth metal, and Z is ammonium, an alkali metal or an alkaline earth metal.
Among such redox polymerization initiators, a redox polymerization initiator comprising a combination of a bromate and a sulfite, is preferred, and a redox polymerization initiator comprising a combination of potassium bromate and ammonium sulfite, is most preferred.
By means of the above redox polymerization initiator, it is possible to obtain PTFE having a low standard specific gravity (hereinafter referred to as SSG), a low extrusion pressure and a high break strength.
When the redox polymerization initiator is employed,
Hirai Hiroyuki
Hoshikawa Jun
Kamiya Hiroki
Kato Kazuo
Kobayashi Shigeki
Asahi Glass Company Limited
Hu Henry S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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