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PROCESS FOR EXPANDING
PRODUCTS PRODUCED THEREBY
This application is a continuation of application Ser. No. 5 07/850,862, filed on Mar. 13, 1992.
BACKGROUND OF THE INVENTION
Many fluoropolymer materials, such as polytetrafluoroet- 1Q hylene (PTFE), are thermoplastic polymers. That is, they have the property of softening when heated and of hardening again when cooled. PTFE is generally produced in the form of white powder referred to as resin. It has a higher crystalline melting point (327° C.) and higher viscosity than ]5 other thermoplastic polymers, which makes it difficult to fabricate in the same manner as other plastics.
PTFE is a long chain polymer composed of CF2 groups. The chain length determines molecular weight, while chain orientation dictates crystallinity. The molecular weight and 20 crystallinity of a given resin prior to sintering are controlled by the polymerization process.
Currently, three different types of PTFE resins are available which are formed from two different polymerization processes. The three resins are granular polymer, aqueous 25 dispersions, and coagulated dispersion products.
In the coagulated dispersion of PTFE resin, small diameter (0.1-0.2 micrometer) particles are coagulated under controlled conditions to yield agglomerates ranging in size from 400 to 500 micrometers in diameter. The morphologi- 30 cal structure of these agglomerates can be considered as long chains of PTFE that are intermingled in a tangled network.
A known method of forming articles from fluoropolymer resins, such as PTFE, is to blend a resin with an organic lubricant and compress it under relatively low pressure into 35 a preformed billet. Using a ram type extruder, the billet is then extruded through a die in a desired cross-section. Next, the lubricant is removed from the extruded billet by drying or other extraction method. The dried extruded material (extrudate), is then rapidly stretched and/or expanded at 40 elevated temperatures below the crystalline melting point of the resin. In the case of PTFE, this results in the material taking on a microstructure characterized by elongated nodes interconnected by fibrils. Typically, the nodes are oriented with their elongated axis perpendicular to the direction of 45 stretch.
After stretching, the porous extrudate is sintered by heating it to a temperature above its crystalline melting point while it is maintained in its stretched condition. This can be 5Q considered as an amorphous locking process for permanently "locking-in" the microstructure in its expanded or stretched configuration.
It has been found that the effect caused by stretching PTFE is dependent on extrudate strength, stretch tempera- 55 ture, and stretch rate. Extrudate strength is a function of the molecular weight and degree of crystallinity of the starting resin and extrusion conditions such as extrusion pressure, lubricant level, and reduction ratio. These parameters also control the degree of alignment that results from extrusion, go As stated, the degree of alignment, in tum, affects one's ability to homogeneously stretch the extrudate.
Most known methods for processing PTFE describe unilateral stretching techniques and stress the importance of stretching the fluoropolymer at rapid rates. For example, 65 U.S. Pat. Nos. 3,953,566 and 4,187,390 to Gore state that while there is a maximum rate of expansion beyond which
fracture of the material occurs, the minimum rate of expansion is of much more practical significance. Indeed, the patents state that at high temperatures within the preferred range for stretching (35° C.-327" C.) only the lower limit of expansion rate has been detected. The patents estimate this rate to be ten percent of the initial length of the starting material per second. The patents go on to note that the lower limit of expansion rates interact with temperature in a roughly logarithmic fashion so that at higher temperatures within the preferred stretching range, higher minimum expansion rates are required.
U.S. Pat. No. 4,973,609 to Browne describes another method for producing porous PTFE products by stretching at a rate of 10% per second. The patent also states that a differential structure is obtained by using an alloy of two different fluoropolymer resins which are characterized by significantly different stretch characteristics. The resins typically have different molecular weights and/or crystallinities. Accordingly, the final physical properties, such as strength, of PTFE articles formed in such a way are affected by the different molecular weights and/or crystallinities of the starting resins.
U.S. Pat. Nos. 4,208,745 and 4,713,070 also describe methods for producing porous PTFE products having a variable structure. The processes utilize a sintering step having a differential sintering profile. That is, one surface of an expanded PTFE article is sintered at a temperature which is higher than the sintering temperature of another surface. This results in fibrils being broken and provides an inherently weak material.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a process for producing a shaped porous article which is more truly semi-permeable than known articles formed of fluoropolymer materials. It is another object of the invention to provide such a process in which a fluoropolymer extrudate can be homogeneously stretched independently of rate. Still another object is to provide a porous article. Yet another object of the invention is to provide a porous article having a porosity which is variable in the direction of the article's cross-section.
These and other objects are achieved by the present invention which in one aspect features a process for producing a porous article. The process includes the steps of providing an extrudate of a fluoropolymer material which is capable of being stretched and bilaterally stretching the extrudate along its longitudinal axis. Conditions are maintained during stretching sufficient to yield an article which is substantially uniformly stretched over a major portion of its length. These conditions include stretch rate, ratio, and temperature.
The stretched extrudate has a microstructure which is characterized by elongate nodes which are connected by fibrils. This microstructure is locked in by sintering the stretched extrudate while maintaining it in its stretched state.
An important feature of the invention is that the fluoropolymer extrudate is bilaterally stretched. That is, in accordance with the invention both ends of the extrudate are displaced along the extrudate's longitudinal axis away from a central portion of the extrudate. It has been found that this stretching method provides significant advantages over known stretching methods wherein one end of an extrudate is held stationary while only the other end is displaced.
In various embodiments of this aspect of the invention the
bilateral stretching is carried out at rates not greater than ten percent per second. Indeed, it has been found that stretching at rates slower than even one percent per second provides a material having an extremely desirable microstructure of nodes and fibrils, the nodes being significantly larger than 5 nodes resulting from known processes of rapidly stretching single-resin extrudates unilaterally.
In carrying out the stretching step in accordance with the process of the invention, the ends of the extrudate can be displaced either simultaneously or sequentially. For 10 example, in one embodiment of the invention, a first end of the extrudate is displaced to a stretch ratio of not greater than two to one. That first end is then held stationary while the second end of the extrudate is displaced in the opposite direction to again result in a stretch ratio of not greater than 15 two to one. Restricting the individual stretches to stretch ratios of not greater than two to one ensures a substantially homogeneous microstructure along a major portion the length of the extrudate.
In another aspect, the invention features a process for 20 producing a porous tube of polytetraftuoroethylene including the step of providing a preformed billet of a mixture of a polytetrefluoroethylene resin and a lubricant. As with the above-described aspect of the invention, the billet is extruded, the extrudate is then dried, and bilaterally 25 stretched along its longitudinal axis under conditions sufficient to yield a tube having a substantially homogenous microstructure over a major portion of its length. The stretched tube is then sintered while being maintained in its stretched state to produce the porous tube. 3°
In one embodiment of this aspect of the invention, the preformed billet is formed to have a lubricant level which selectively varies in the direction of the billet's crosssection. That is, for example, the billet might have a lubricant level of fifteen percent by weight at its inner and outer 35 surfaces and a lubricant level of approximately twenty percent at a radial position between its inner and outer surfaces. When extruded and stretched, such a billet results in a porous tube having a microstructure which varies in a controlled fashion in the direction of the tube's cross- 40 section. This phenomenon and its advantages are described below in greater detail.
Accordingly, in the various embodiments of this aspect of the invention, a porous article having a desired microstruc- 4J ture is provided by controlling the billet lubricant level, the billet reduction ratio, and bilateral stretching conditions such as stretch rate and ratio. This avoids the problems such as weak material which are associated with known resinblending and varied-profile sintering techniques. 50
In still another aspect, the invention features a tube formed of an expanded porous fluoropolymer material. The material has a microstructure characterized by ring shaped nodes interconnected by fibrils. An important feature of this aspect of the invention is that substantially all of the nodes 55 each circumscribes the longitudinal axis of the tube and extends from the inner to the outer surface of the tube wall, thereby creating between the nodes continuous throughpores from one surface to the opposite surface.
These and other features of the invention will be more 60 fully appreciated by reference to the following detailed description which is to be read in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a porous article formed in accordance with the teachings of the present
FIG. 2 is a scanning electron microscopic view of a longitudinal cross-section of a porous article in accordance with the invention,
FIG. 3 is a scanning electron microscopic view of a radial cross-section of a porous article in accordance with the invention,
FIG. 4 is a schematic depiction of a billet suitable for extrusion in accordance with the invention,
FIG. 5A is a scanning electron microscope longitudinal cross-section view of another porous article in accordance with the invention,
FIG. 5B is a scanning electron microscope view of the inner surface of the porous article shown in FIG. SA,
FIG. 5C is a scanning electron microscope view of the outer surface of the porous article shown in FIG. 5A, and
FIG. 6 is a schematic longitudinal cross-section view of still another porous article in accordance with the invention.
As stated above, in one aspect the invention features a process for producing a shaped porous article. A significant feature of the process is that an article having a homogeneous microstructure is formed independently of the rate at which it is stretched.
Various fluoropolymer resins are suitable for use in the present invention. For example, polytetrafluoroethylene or copolymers of tetrafluoroethylene with other monomers may be used. Such monomers may be ethylene, chlorotrifluoroethylene, perfluoroalkoxytetrafluoroethylene, or fluorinated propylenes such as hexafluoropropylene. In particular, however, polytetrafluoroethylene (PTFE) works well. Accordingly, while the inventive process can be utilized to produce porous articles formed of various fluoropolymer materials, the following description pertains specifically to the formation of an article from PTFE resin.
For purposes of the present invention, when PTFE is used, resin of a molecular weight between 10,000,000 and 70,000, 000 is suitable. Since, however, PTFE does not dissolve in any common solvent its molecular weight cannot be measured by the usual methods. According to the Encyclopedia of Polymer Science and Engineering (Wiley and Sons, 1989), though, the following relationship has been established between number-average molecular weight (Mn), for molecular weights between 5.2x10s and 4.5xl07, and the heat of crystallization (AHc) in Joules/gram (calories/gram).
Accordingly, by determining the heat of crystallization of a given PTFE resin, a number average molecular weight of the resin is determinable from this relationship.
As with known methods of processing PTFE, the invention utilizes a preformed billet which comprises a PTFE resin mixed with an organic lubricant. Various lubricants are suitable such as naphtha, ISOPAR-G and ISOPAR-H available from Exxon Corporation. Low odor paraffin solvents can be used as well. The blended resin is compressed at low pressure (less than 1000 PSI) into a tubular billet of approximately one third of the resin's original volume. Billet forming processes are generally known in the art.
As discussed above, extrusion conditions have a significant effect on the resulting extrudate's reaction to being stretched. In particular, once a resin of a given molecular weight and crystallinity has been selected, extrudate quali