Process for producing granulated polytetrafluoroethylene powder

Details Process for producing granulated polytetrafluoroethylene powder Process for producing granulated polytetrafluoroethylene powder

1999-03-24

2001-03-20

Wilson, Donald R. (Department: 1713)

Plastic and nonmetallic article shaping or treating: processes

Forming articles by uniting randomly associated particles

Agitating to form larger particles

C528S494000

Reexamination Certificate

active

06203733

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method for preparing a granulated powder of polytetrafluoroethylene (hereinafter referred to simply as PTFE).
2. Background Art
PTFE is subjected to molding in a powder form, since it can not be melt-molded like a thermoplastic resin. Accordingly, the PTFE molding powder is required to have specific powder characteristics. One of the characteristics is that the powder flowability is good. It is also required that the apparent specific gravity is large, and it is hardly broken and is not so brittle.
A primary powder obtained by finely pulverizing a granular solid of PTFE obtained by suspension polymerization, usually has an average particle size of at most 200 &mgr;m, but it does not have the above mentioned powder characteristics, and accordingly, such a primary particle of PTFE is stirred and granulated in a solvent and used for molding. As a method for granulating PTFE, there is a method wherein only an water-insoluble organic liquid is used as a solvent, or a method wherein granulation is carried out in a two phase liquid medium comprising water and an organic liquid.
The present invention belongs to the later method. As the organic liquid to be used in this method, it is known to employ an organic liquid which is water-insoluble and which has a surface tension at 25° C. of at most 35 dyne/cm to wet PTFE and a solvent-recovery temperature, i.e. a boiling point, of from about 30 to 150° C. to obtain a PTFE granulated powder having a suitable softness (JP-B-44-22619, JP-B-54-40099 and JP-A-57-18730).
Specific examples of such an organic liquid include chlorofluorocarbons in addition to aliphatic hydrocarbons and aromatic hydrocarbons. Chlorofluorocarbons may, for example, be trichlorotrifluoroethane, trichlorofluoromethane difluorotetrachloroethane, Cl(CF
2
CFCl)
n
Cl (wherein n is integer of from 2 to 4), and trichloropentafluoropropane. Further, hydrochlorofluorocarbons such as 1,3-dichloro-1,1,2,2,3-pentafluoropropane and a hydrofluorocarbon such as C
6
F
13
H may also be mentioned, and these organic liquids are mainly used for practical granulation.
Meanwhile, in recent years, destruction of the ozone layer has been internationally taken up as an environmental problem of global scale, and use of certain chlorofluorocarbons which are considered to be causal substances, has been prohibited. Therefore, there is substantial demand for developing a substitute for chlorofluorocarbons to be used for preparing the granulated powder of PTFE.
DISCLOSURE OF THE INVENTION
The present invention provides a method for industrially advantageously preparing a granulated powder of PTFE having excellent powder flowability and a high apparent specific gravity, without using chlorofluorocarbons or perfluorocarbons which have large ozone layer destruction coefficients and which cause warming up of the earth.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is a method for preparing a granulated powder of polytetrafluoroethylene, which comprises stirring and granulating a polytetrafluoroethylene powder having an average particle size of at most 200 &mgr;m in a two phase liquid medium comprising water and a perfluoroalkyl alkyl ether (hereinafter referred to as PFAE) of the formula 1:
R—O—R′  Formula 1
wherein, R is a C
2-6
perfluoroalkyl group, and R′ is a C
1-2
alkyl group.
The PTFE primary powder to be used in the present invention, includes, for example, a homopolymer of tetrafluoroethylene (hereinafter referred to as TFE) and a polymer of TFE modified with not more than 2 wt % of a copolymerizable monomer. Examples of the above modifying agent include a C
3-6
perfluoroalkene (such as hexafluoropropylene) and a C
3-6
perfluoro(alkyl vinyl ether) (such as perfluoro(propyl vinyl ether)). The copolymer modified with such a modifying agent, has no melt-processability just like PTFE.
As the PTFE primary powder, such a polymer finely pulverized to an average particle size of at most 200 &mgr;m and non-baked, may be employed. If the particle size is large, the apparent specific gravity of the granulated powder tends to be low, voids are likely to remain in the molded product, and the uniformity of the molded product tends to be impaired. Accordingly, one having an average particle size of at most 70 &mgr;m, is preferably employed. In the present invention, the non-baked PTFE powder means a PTFE powder which has not been heated to a temperature of at least the melting point after the polymerization, and such a powder is suitable for granulation.
If the boiling point of PFAE to be used in the present invention, is too high, the temperature for recovery of PFAE tends to be high, whereby the granulated powder tends to be hard, and if the boiling point is too low, coagulation tends to be incomplete, and the granulated powder tends to be easily broken by a small external force. The range of the boiling point of PFAE is preferably from 25 to 80° C.
In the formula 1, R may be of a straight chain structure or a branched structure, and the polyfluoroalkyl group R is preferably a perfluoroalkyl group.
Specifically, PFAE is preferably CF
3
CF
2
CF
2
CF
2
OCH
3
, CF
3
CF
2
CF
2
CF
2
OCH
2
CH
3
, CF
3
CF
2
CF
2
OCH
3
or (CF
3
)
2
CFOCH
3
. These PFAE may be used alone or in combination as a mixture of two or more of them.
PFAE is free from the possibility of destroying the ozone layer and yet has a characteristic such that the coefficient for warming up the earth is lower than the perfluoro compound, since it has hydrogen atoms. For example, an estimated value of the life time as an index for the coefficient for warming up the earth, is about 100 years with C
6
F
14
and from 20 to 30 years with C
6
F
13
H, whereas it is as small as 4.1 years with F(CF
2
)
4
OCH
3
, 1.9 years with F(CF
2
)
3
OCH
3
and 1.2 years with F(CF
2
)
4
OC
2
H
5
.
Further, PFAE has a surface tension which is small, for example 13.6 dyne/cm of CF
3
CF
2
CF
2
CF
2
OCH
2
CH
3
as compared with at least 16 dyne/cm of chlorofluorocarbon. Accordingly, by using PFAE, the PTFE powder will be wettened in the medium and will readily be coagulated with stirring, to give a granulated powder, of which the interior is uniform with little voids and which has a high apparent specific gravity and improved flowability with a smooth surface.
Further, when PFAE is used alone as a medium, the interior of the PTFE granulated product tends to be hard, but by employing the two phase liquid medium comprising PFAE and water, the PTFE powder will be wetted to a proper degree and coagulation will not excessively proceed, whereby a granulated product having a proper softness can be obtained, and the physical properties of the molded product will be improved.
The ratio of the water/PFAE/PTFE powder to be used in the present invention, is usually preferably 2 to 20/0.2 to 2/1 (by weight ratio).
In the present invention, the temperature during the contact treatment with water is important, it is preferably at a temperature of not higher than the boiling point of PFAE, specifically from 10 to 50° C., particularly preferably from 20 to 40° C. Further, the treating time is usually from about 1 minute to 10 hours, within the above temperature range.
In the present invention, the contact treatment with water is carried out by stirring the mixture of PTFE and water, wetted with a granulation medium, under the above mentioned conditions in a treating apparatus equipped with stirring vanes. Further, in order to obtain a granulated powder having a uniform particle size distribution, stirring with a certain intensity is preferred. As a specific stirring condition, the peripheral speed of stirring vanes is usually from 1 to 50 m/sec.
It is possible to prepare a granulated powder of a PTFE composition having a filler such as glass fibers, carbon fibers, a powder of bronze or graphite, a melt-moldable other fluorine resin, or a heat resistant resin, incorporated to a PTFE primary powder. In a case where a filler is to be incorporated, it is possible to incorpo

Affiliated with

Also associated with

Rating

Process for producing granulated polytetrafluoroethylene powder does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for producing granulated polytetrafluoroethylene powder, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing granulated polytetrafluoroethylene powder will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2512294