Ptfe powder and method of producing ptfe powder for molding

Abstract

This invention provides a polytetrafluoroethylene powder capable of giving moldings which are improved in surface roughness Ra, tensile strength and / or tensile elongation as compared with the prior art ones and can be excellent in dielectric breakdown strength and, further, can be excellent, if desired, in apparent density and / or powder flowability as well, and a method of producing a polytetrafluoroethylene molding powder. The present invention is a PTFE powder wherein a surface roughness Ra of molded articles a1 for measurement is less than 0.92 μm.

Description

TECHNICAL FIELD The present invention relates to a polytetrafluoroethylene powder capable of giving moldings which are improved in surface smoothness, tensile strength and / or tensile elongation as compared with the prior art ones and can be excellent in dielectric breakdown strength and, further, can be excellent, if desired, in apparent density and / or powder flowability as well, and to a method of producing polytetrafluoroethylene (PTFE) molding powders. BACKGROUND ART Polytetrafluoroethylene (PTFE) is a thermoplastic resin excellent in heat resistance, chemical resistance, weather resistance, electrical insulation properties and nontackiness, among others, and is useful in a wide range of fields of application. However, it is very high in melt viscosity, so that it has been regarded as impossible to mold the resin by applying ordinary plastics molding processes used in molding general thermoplastic resins low in melt viscosity, such as conventional extrusion molding and injectio...

AI Technical Summary

Benefits of technology

The mechanisms of the PTFE powder of the present invention producing such advantageous effects are not clear but supposed to be as follows.

Thus, the PTFE powder of the present invention is not particularly restricted but may be any powder solid having such physical characteristics of moldings or such molding characteristics and powder characteristics as mentioned above. When the above-described granulation is not carried out, the physical characteristics of moldings are excellent and, in particular, in the case of the above-mentioned PTFE powder (I), PTFE powder (II) or PTFE powder (III), very good physical characteristics of moldings can be obtained.

Therefore, the excellent characteristics of the PTFE powder of the present invention are presumably due to the fact that the above-mentioned unpulverized PTFE powder of a not-modified PTFE having an AI of not less than 0.25 or a modified PTFE with a heat of crystallization of 18 to 25 J / g has been subjected to the above-described pulverization, as described hereinabove or to the fact that the above powder comprises a modified PTFE and has been pulverized to an average particle diameter of not greater than 100 μm. In particular when the pulverization is carried out by the above-described fibril-formation-free method of pulverization, substantial fibril-formation hardly occurs and it is easy to improve the physical characteristics of moldings.

When the PTFE powder of the present invention is prepared by the above-described granulation, the powder characteristics thereof can also be improved, as described hereinabove. When prepared by the above granulation, the PTFE powder (I), PTFE powder (II) or PTFE powder (III) can have both good powder characteristics and good physical characteristics of moldings.

The PTFE powder of the present invention has the following specific characteristics.

Problems solved by technology

However, it is very high in melt viscosity, so that it has been regarded as impossible to mold the resin by applying ordinary plastics molding processes used in molding general thermoplastic resins low in melt viscosity, such as conventional extrusion molding and injection molding techniques.

On the other hand, the PTFE powder ground just after polymerization is generally inferior in such powder characteristics as apparent density and powder flowability and, therefore, the molding powder tends to aggregate in the hopper or small-diameter cylinder of the molding machine, causing bridging or uneven filling in the mold or cylinder, hence the handleability tends to become unsatisfactory.

Further, the PTFE powder ground just after polymerization is low in apparent density and bulky per unit weight, so that it is not easy to reduce the size of the mold or cylinder.

It is thus difficult to increase the productivity per mold or cylinder; this is disadvantageous.

However, although the granules obtained are improved in apparent density and powder flowability, shearing forces have been applied thereon in the step of granulation, diminishing the physical properties of the final product moldings; another problem thus arises.

Thus, PTFE molding powders having good powder characteristics tend to give moldings poor in physical characteristics, while PTFE molding powders capable of giving moldings having good physical characteristics tend to have poor powder characteristics.

However, it has no description of such characteristics as surface roughness Ra, tensile strength, tensile elongation, and powder flowability.

However, because of their large size, there is a limit to the pressure in the step of preforming.

Further, any procedure of cutting thereof is not desirable for the prevention of product contamination but, on the other hand, they are required to show surface smoothness.

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