Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ
Patent
1995-01-03
1999-06-15
McCamish, Marion E.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
264119, 264127, 2642888, 264340, 264345, 264DIG68, 264DIG73, B27J 500
Patent
active
059119268
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 application of PCT/JP94/00735 filed May 2, 1994.
TECHNICAL FIELD
The present invention relates to a porous material of a polytetrafluoroethylene and a production process thereof, and more particularly to a porous material of a polytetrafluoroethylene, which has micropores, and is high in porosity and excellent in permeability, and a production process thereof.
BACKGROUND ART
Porous materials comprising a polytetrafluoroethylene (hereinafter abbreviated as "PTFE") as a material are used in a wide variety of fields such as separator for cells, membrane filters, electric wires, analytical instruments and artificial blood vessels. Now, in recent years, porous materials of PTFE having a minute pore diameter and excellent permeability have been required of application fields such as precision filters, high-performance separation membranes and artificial lung septa. Therefore, porous materials of PTFE having micropores and a high porosity have been required. However, it has been very difficult to produce a porous material of PTFE, which combines micropores with a high porosity and has excellent permeability.
As a process for producing a porous material of PTFE, it has heretofore been known to stretch an unsintered molded article obtained by paste extrusion of PTFE at a temperature not higher than the melting point of PTFE and then sinter the molded article (Japanese Patent Publication No. 13560/1967). According to this process of stretching the unsintered molded article, porous material of PTFE having various porosities can be obtained. However, the pore diameter becomes greater as the draw ratio is increased to enhance the porosity. Therefore, there has been a limit in the production of porous material of PTFE combining micropores and a high porosity.
As another process for producing a porous material of PTFE, it has also been proposed to heat a molded article of PTFE to a temperature not lower than 327.degree. C., slowly cool the sintered molded article so as to heat-treat it to give a crystallinity of 80%, and then uniaxially stretch the sintered molded article at a draw ratio of 1.5-4 times at a temperature of 25-260.degree. C. (Japanese Patent Publication No. 42794/1978). According to this process (hereinafter abbreviated as "the slow cooling process"), a porous material of PTFE in which micropores have been formed can be obtained. In the slow cooling process, however, crystallization is not allowed to fully progress if the cooling rate is too fast in the step of slowly cooling the sintered molded article of PTFE. It is thus necessary to decelerate the cooling rate. Accordingly, this process has involved a problem that precise temperature control and large equipment are required.
More specifically, it is said in the slow cooling process that it is preferable to cool the sintered molded article generally at a rate slower than about 0.5.degree. C./min for enhancing the crystallinity of the resulting sintered molded article of PTFE. In Examples of this publication, cooling rates of 0.25.degree. C./min, 0.1.degree. C./min and 0.05.degree. C./min are used. In order to perform slow cooling at such a low cooling rate, it is necessary to conduct temperature control with extremely high precision. In addition, porous materials of PTFE are generally formed as continuous molded articles such as rods, tubes, strips and sheets by paste extrusion of fine powder of PTFE. The PTFE porous material is formed through a heat-treating step, a stretching step and the like. It is however difficult and impracticable to apply the slow cooling process to these continuous sintered molded articles. For example, in order to cool a sintered molded article in the form of a continuous sheet from 350.degree. C. to 290.degree. C. at a cooling rate of 0.5.degree. C./min by means of an oven 3 m long, it is necessary to pass the sheet through the oven over 2 hours. The transit time in the oven is 1.5 m/hr in terms of linear velocity. Therefore, in the case where the length of the continuous sheet is 100 m, it take
REFERENCES:
patent: 5234751 (1993-08-01), Harada et al.
Harada Akira
Nishimura Akira
Uno Atsushi
Yamanouchi Shousuke
Cole Elizabeth M.
McCamish Marion E.
Sumitomo Electric Industries Ltd.
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