Plastic article or earthenware shaping or treating: apparatus – Stock pressurizing means operably associated with downstream... – Including auxiliary shaping means associated with molding...
Reexamination Certificate
2000-11-13
2003-01-14
Silbaugh, Jan H. (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Stock pressurizing means operably associated with downstream...
Including auxiliary shaping means associated with molding...
C425S382400, C425S467000
Reexamination Certificate
active
06506043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of paste extrusion of lubricated polytetrafluoroethylene fine powder.
2. Description of the Related Art
Polytetrafluoroethylene (PTFE) fine powder is a type of PTFE that is made by aqueous dispersion polymerization, followed by coagulation of the dispersion and drying of the resultant coagulum to obtain the fine powder. Because the PTFE fine powder does not flow in the melt condition, the powder has been fabricated into an article by an extrusion method which does not require melt flow. This extrusion method is known as paste extrusion and is described for example in U.S. Pat. No. 2,685,707. In paste extrusion, a paste extrusion composition is formed by mixing PTFE fine powder with an organic lubricant which has a viscosity of at least 0.45 centipoise at 25° C. and is liquid under the conditions of subsequent extrusion. The PTFE soaks up the lubricant, resulting in a dry, pressure coalescing paste extrusion composition that is also referred to as lubricated PTFE fine powder.
The process of paste extrusion may be understood with reference to
FIG. 1
wherein there is shown an extrusion device
1
having a barrel section
3
and a die section
5
. Inside device
1
there is defined a chamber
7
which terminates in an extrusion orifice
9
. Lubricated PTFE fine powder, usually as a charge shaped under pressure to conform to chamber
7
, is placed in chamber
7
and is then “paste extruded” by a ram
11
positioned in chamber
7
at the end opposite extrusion orifice
9
which moves toward extrusion orifice
9
to force the lubricated PTFE fine powder through extrusion orifice
9
. Typically, there is a mandrel
13
positioned in chamber
7
so that the lubricated PTFE fine powder is paste extruded in the form of a sheet, rod, tubing or coating. The lubricated extrudate is also known as a lubricated green extrudate.
In
FIG. 2
there is shown an extrusion device which has a mandrel with a configuration different from that in FIG.
1
. Specifically, there is shown an extrusion device
21
having a barrel
23
defining a chamber
25
. A ram
27
is positioned at one end of chamber
25
. A mandrel
29
is fixed within chamber
25
and has a cylinder
31
attached at its distal end. Cylinder
31
has a conical surface and a cylindrical surface. As lubricated PTFE fine powder is paste extruded, the powder is pushed by ram
27
through chamber
25
, flows around cylinder
31
and out of extrusion device
21
as extruded tube
32
.
The lubricated green extrudate produced by paste extrusion may also be formed into an article of a desired shape. The paste extrusion is usually carried out at a temperature of 20 to 50° C., though extrusion temperatures outside this range are known.
In most cases, the lubricated green extrudate is then heated, usually at a temperature of 100 to 250° C., to make volatile and drive off the lubricant from the extrudate, followed by the optional step of sintering the PTFE. Some articles such as thread sealant tape are calendered and cut after drying, and are not subjected to sintering.
Lubricated PTFE fine powder includes PTFE primary particles which, during paste extrusion, are deformed into small interconnected fiber-like clusters, called fibrils. In the devices in both
FIGS. 1 and 2
, the lubricated PTFE fine powder is shaped into fibrils as it is forced around the mandrel and through the end of the extrusion device. In
FIG. 1
, this fibrillation occurs in the area between mandrel
13
and the walls of chamber
7
in die section
5
. In
FIG. 2
, fibrillation occurs in the area between cylinder
31
and barrel
23
.
These fibrils are oriented in the direction of the paste extrusion which means that after extrusion in devices such as are shown in
FIGS. 1 and 2
, the lubricated extrudate and articles formed therefrom are strong in the extrusion direction, but particularly weak in the extrusion-cross direction. For example, the fibrils formed by paste extrusion in device
21
in
FIG. 2
are oriented in one direction, vertically, as the fibrils pass out of device
21
. This weakness in the extrusion cross direction means that special care has to be taken to handle the lubricated extrudate before subsequent drying and optional sintering.
The anisotropy in properties of lubricated extrudate may be corrected after extrusion by stretching the extrudate or articles formed therefrom in two or more different directions, thereby reorienting the fibrils. Techniques of fibril reorientation by multi-lateral stretching of unsintered PTFE extrudate are known in the art such as are described in U.S. Pat. No. 5,321,109. Reorientation of the fibrils formed in the extrusion direction may also be accomplished by reorientation within the extrusion process. In each case, the prior art teaches forming the fibrils in the direction of the extrusion and then subsequently disorienting the fibrils by various methods, such as with expansion chambers, as shown in U.S. Pat. No. 3,315,020, or by applying a spiral-type movement to the lubricated extrudate, as shown in U.S. Pat. No. 3,008,187 and U.S. Pat. No. 4,876,051. It is shown in the literature that the anisotropy in the unsintered lubricated green extrudate or articles made therefrom is generally extended to the sintered finished articles. This anisotropy may be compensated by subjecting the articles to long sintering cycles, but this is cumbersome and expensive. Increased isotropy achieved before sintering is usually also found in the sintered finished articles.
Further, even when the lubricated extrudate is subject to the prior art processes of stretching, the resultant articles are still difficult to handle and still have undesirable physical properties, including shrinkage in the extrusion direction.
SUMMARY OF THE INVENTION
The present invention relates to a process of paste extruding lubricated PTFE fine powder in an extrusion device by forcing the lubricated PTFE fine powder through the extrusion device and out of an orifice as a lubricated green extrudate, wherein before exiting through the orifice, the direction of the flow of the lubricated PTFE fine powder in the extrusion device is reoriented in a direction at least 60 degrees different from the initial direction of the flow of the lubricated PTFE fine powder through the extrusion device.
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Aubertin Fabrice
Levy Daniel N.
Del Sole Joseph S
E. I. du Pont de Nemours and Company
Silbaugh Jan H.
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