Tetrafluoroethylene polymer for stretching

Details Tetrafluoroethylene polymer for stretching Tetrafluoroethylene polymer for stretching

2001-10-05

2003-02-11

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C521S050000, C428S304400, C526S242000, C526S250000

Reexamination Certificate

active

06518381

ABSTRACT:

The present invention relates to a tetrafluoroethylene polymer (hereinafter referred to as PTFE) for stretching. Particularly, it relates to PTFE which is useful suitably for stretching operation after paste extruded molding.
Heretofore, PTFE has been obtained by polymerizing tetrafluoroethylene (hereinafter referred to as TFE) alone, or together with a comonomer, as the case requires, and it has been used for various applications.
PTFE can be produced by an aqueous dispersion polymerization and can be obtained in the form of an aqueous dispersion wherein polymer particles are dispersed, or in the form of a fine powder prepared by coagulation and drying the aqueous polymer dispersion.
Conventional PTFE fine powder has a high melt viscosity and does not easily flow at the melting temperature, and thus, it has a non-melt fabricable quality. Accordingly, PTFE fine powder is usually subjected to paste extruded molding, wherein PTFE fine powder is blended with a lubricant, such lubricated PTFE is molded by extrusion, then the lubricant is removed, and the extruded product thereby obtained is usually fused (sintered) at a temperature higher than the melting point of PTFE and formed into the shape of a final product.
On the other hand, other important products obtainable from PTFE fine powder may be air permeable cloth materials for products such as clothes, tents, membranes for separation, etc. Such products can be obtained by rapidly stretching an extruded product obtained by paste extruded molding of PTFE fine powder in an unsintered state, to impart a nature to let steam permeate, but not to let condensed water permeate.
U.S. Pat. Nos. 4,654,406 and 4,576,869 discloses that at least 75% of stretching uniformity can be accomplished by an improvement in the technology of stretchable PTFE fine powder i.e. by stretching an extruded product obtained by an addition of 17 mass % of a lubricant, at a rate of 10 %/sec to 100 %/sec for at least 1,000%.
However, the level of physical properties required for stretched products obtained by stretching PTFE has become higher year after year, and even a stretched product obtained by this improved PTFE fine powder has a problem that the strength is not sufficient.
It is an object of the present invention to provide PTFE which has high stretchability, a fibrillatable character and a non-melt fabricable quality and has a small standard specific gravity, which is useful suitably for stretching operation after paste extruded molding, and which provides a porous product having high strength.
The present invention provides PTFE having high stretchability, a fibrillatable character and a non-melt fabricable quality, which has a standard specific gravity of at most 2.160 and a tensile break strength of from 32.0N (3.26 kgf) to 49.0N (5.0 kgf). Here, the standard specific gravity means a standard specific gravity as measured by JIS K6935-2.
Further, the present invention provides PTFE having high stretchability, fibrillatable character and a non-melt fabricable quality, which has a standard specific gravity of at most 2.160 and an endothermic ratio of at most 0.15 as calculated from the measurement by differential thermal analysis.
Further, the present invention provides the above PTFE, wherein PTFE is a fine powder.
Further, the present invention provides the above PTFE, wherein PTFE is a dispersed solid component of an aqueous dispersion.
Still further, the. present invention provides a porous material made of PTFE having the above properties, or its article.
Now, the present invention will be described in detail with reference to the preferred embodiments.
PTFE of the present invention may be a homopolymer of TFE, or a copolymer of TFE with a comonomer such as a fluorinated monomer having an ethylenically unsaturated group other than TFE. The fluorinated monomer having an ethylenically unsaturated group may, for example, be hexafluoropropylene, perfluorobutene-1, perfluorohexene-1, perfluorononene-1, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro(heptyl vinyl ether), (perfluoromethyl)ethylene, (perfluorobutyl)ethylene or chlorotrifluoroethylene. Such fluorinated monomers may Ibe used alone or may be used in combination as a mixture of two or more of them. The comonomer is usually preferably at most 1 mass %, more preferably at most 0.5 mass %.
PTFE of the present invention has high stretchability, a fibrillatable character and a non-melt fabricable quality. These properties are properties which are usually required for paste extruded molding.
Further, PTFE of the present invention has a standard specific gravity and a tensile break strength, or a standard specific gravity and an endothermic ratio, which are within specific ranges, and it is thereby characterized.
The standard specific gravity (hereinafter referred to as SSG) of PTFE of the present invention, is at most 2.160, preferably at most 2.157. SSG is an index of an average molecular weight. SSG of PTFE of the present invention has a very small value, thus indicating a high average molecular weight. SSG tends to decrease as the average molecular weight increases. Namely, with PTFE of the present invention, the value of SSG is small, and accordingly, it is expected that its average molecular weight is considerably high. PTFE having a SSG value of at most 2.160, will have a stretching ratio of its extruded product exceeding 3,000% and is excellent also in stretching uniformity.
The tensile break strength of a stretched product of PTFE of the present invention is within a range of from 32.0N (3.26 kgf) to 49.0N (5.0 kgf), preferably from 34.3N (3.5 kgf) to 49.0N (5.0 kgf). Surprisingly, this is higher than the tensile break strength of PTFE disclosed in JP-A-2000-143727. The higher the tensile break strength, the better the durability, etc., such being desirable. On the other hand, PTFE having a tensile break strength exceeding 49.0N (5.0 kgf), tends to be practically very difficult to produce.
The endothermic ratio of PTFE of the present invention is at most 0.15, preferably at most 0.13, more preferably at most 0.10. This endothermic ratio is defined by the measurement of an endothermic ratio, as will be described hereinafter. Usually, in the crystal fusion behavior by a differential thermal analysis of PTFE, a plurality of peaks are observed, which indicates that there are the corresponding number of differences in the crystal structure, etc. In the stretching operation, the structure should better be the same as much as possible, so that uniform stretching can be carried out, and a porous material thereby obtainable will have excellent properties. The endothermic ratio is an index to show the uniformity of the structure, and the smaller the endothermic ratio, the smaller the irregularity in the structure in PTFE. If the endothermic ratio exceeds 0.15, stretching at a high stretching ratio tends to be difficult, or the tensile break strength tends to be small.
In a case where the endothermic ratio of PTFE is at most 0.15, the tensile break strength of a stretched product of PTFE is preferably within a range of from 19.6N (2.0 kgf) to 49.0N (5.0 kgf), more preferably within a range of from 29.4N (3.0 kgf) to 49.0N (5.0 kgf), particularly preferably within a range of from 34.3N (3.5 kgf) to 49.0N (5.0 kgf).
Further, PTFE of the present invention is preferably one, of which the extrusion pressure is from 9.8 MPa (100 kgf/cm
2
) to 19.6 MPa (200 kgf/cm
2
), more preferably from 9.8 MPa (100 kgf/cm
2
) to 16.7 MPa (170 kgf/cm
2
), particularly preferably from 9.8 MPa (100 kgf/cm
2
) to 15.2 MPa (155 kgf/cm
2
).
Of PTFE of the present invention, the stress relaxation time is preferably at least 650 seconds, more preferably at least 700 seconds, particularly preferably at least 730 seconds.
PTFE of the present invention can be produced by aqueous dispersion polymerization.
The aqueous dispersion polymerization can be carried out by using TFE alone or TFE together with a comonomer, in an aqueous medium containing a dispersant and a polymerization i

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