EP0664353A1

EP0664353A1 - Reinforced particle-loaded fibrillated PTFE web

Info

Publication number: EP0664353A1
Application number: EP95100215A
Authority: EP
Inventors: William V. C/O Minnesota Mininig And Balsimo; Lloyd R. C/O Minnesota Mininig And White
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1994-01-10
Filing date: 1995-01-09
Publication date: 1995-07-26
Also published as: JPH07205321A; CA2138078A1

Abstract

A composite article including a particle-loaded fibrillated polytetrafluoroethylene web with a reinforcing screen or scrim partially embedded therein.

Description

This invention describes a particle-loaded fibrillated polytetrafluoroethylene web with a reinforcing screen or scrim at least partially embedded therein. Compared to an unreinforced web, this reinforced web can resist a much greater pressure drop across it without deforming and displays greater strength against various mechanical stresses.
Particle-loaded, non-woven, fibrous articles wherein the non-woven fibrous web can be compressed, fused, melt-extruded, air-laid, spunbonded, mechanically pressed, or derived from phase separation processes have been disclosed as useful in separation science. Web products of non-woven webs having dispersed therein sorbent particulate have been disclosed to be useful as, for example, respirators, protective garments, fluid-retaining articles, wipes for oil and/or water, and chromatographic and separation articles. Coated, inorganic oxide particles have also been enmeshed in such webs. Such webs with enmeshed particles which are covalently reactive with ligands (including biologically-active materials) have also been recently developed.
Numerous examples of PTFE filled with or entrapping particulate material are known in many fields. Many applications for PTFE filled with electroconductive materials are known. These include circuit boards, oil leak sensors, electrical insulators, semipermeable webs, and various types of electrodes. Other such combinations have been used as gasket or seating materials and wet friction materials. Still others have been used to produce high-strength PTFE films and webs with applications as structural elements and electronic components. Where the particulate has catalytic properties, this type of combination provides a form which can be conveniently handled. U.S. Patent No. 4,153,661 discloses various particulate distributed in a matrix of entangled PTFE fibrils as being useful in, among other things, electronic insulators and semipermeable webs.
Numerous combinations of PTFE and metals in which the metal is not entrapped within a PTFE matrix are also known. These include PTFE webs completely or partially coated with metal and metal matrices with a network of fibrillated PTFE in the pores thereof. PTFE powder with metal filler has been used (in paste form) as a battery electrode and as a self-lubricating layer coated on bronze bearings. PTFE films coated onto metal films and plates are also known.
Methods of preparing fibrillated PTFE webs have been described in, for example, U.S. Patent Nos. 4,153,661, 4,460,642, and 5,071,610.
The physical properties of such particle-loaded fibrillated PTFE webs are somewhat limited, however. They do not resist high pressure drops without deforming and have limited strength against mechanical stresses created by, for example, fluid flow, tensile force, mechanical impact, and abrasion.
In one aspect, the present invention provides a composite article comprising a fibrillated polytetrafluoroethylene (PTFE) web having particulate entrapped therein and, at least partially embedded in the web, means for reinforcing the web. Preferably, this reinforcing means is a screen or scrim.
In another aspect, the present invention provides a method for malting this article comprising the steps of providing a fibrillated PTFE web with particulate entrapped therein and pressure bonding to the web a means for reinforcing the web so that the reinforcing means is at least partially embedded in the web.
Unless otherwise indicated, the following definitions apply in this application:

"screen" means a reinforcing material with a regular geometric pattern of threads which can be polymeric, glass, metallic, etc.;
"scrim" means a non-woven web the fibers of which are not in a regular geometric pattern and which can be polymeric, glass, metallic, etc.; and
"partially embedded" (when used in connection with a reinforcing means) means the reinforcing means is (a) at least partially depressed in the web to which it has been pressure bonded so that the reinforced web, when viewed from an edge, shows only up to 95 %, preferably up to 90%, more preferably up to 75 %, and most preferably up to 50 %, of the reinforcing means and (b) at least partially mechanically entangled with the web.

Embedding a reinforcing means in a fibrillated PTFE web provides the web with mechanical strength and resistance to deformation when a pressure drop is applied across it. Use of such a reinforcing means also allows the web to be formed into configurations that were previously difficult, if not impossible to achieve.
The reinforcing means used in the composite article of the present invention is at least somewhat porous, preferably very porous (i.e., at least 50% voids), so as not to greatly interfere with the porosity of the fibrillated PTFE web. This reinforcing means is at least partially embedded in the fibrillated PTFE web. The PTFE fibrils appear to actually attach to or become mechanically entangled with the reinforcing means. This is further illustrated by reference to the drawings described below.

FIG. 1 is a scanning electron micrograph (SEM) of a fibrillated PTFE web reinforced with a polymeric screen.
FIG. 2 is an SEM of the reinforced web from FIG. 1 with the screen partially pulled away from the web.
FIG. 3 is an SEM providing a inore magnified view of the reinforced web from FIG. 2.
FIG. 4 is an SEM of one screen thread of the reinforced web from FIG. 3.
FIG. 5 is an SEM of a fibrillated PTFE web reinforced with a polymeric scrim.
FIG. 6 is an SEM of the reinforced web from FIG. 5 with the scrim partially pulled away from the web.
FIG. 7 is an SEM providing a more magnified view of the reinforced web from FIG. 6.
FIG. 8 is an SEM of one scrim fiber of the reinforced web from FIG. 7.

FIG. 1 shows an SEM (60x magnification) of reinforced web 10 comprising particle-loaded fibrillated PTFE web 12 with screen 14 partially embedded therein. In this particular embodiment, fibrillated PTFE web 12 entraps activated carbon particulate. Any particulate, regardless of shape, that can be entrapped in a nonwoven polymeric web can be used in the reinforced fibrillated PTFE web of the present invention. Representative examples of useful particulate include those listed in U.S. Patent Nos. 4,153,661, 4,460,642, 5,071,610, and 5,209,967 as well as U.S. Ser. No. 08/004,967,^* which lists are herein incorporated by reference. Particularly useful particulate materials include activated carbon, silica, derivatized silica, glass beads and bubbles, chitin, and the like. In this particular embodiment, screen 14 is Nitex-37 nylon (TETKO, Inc.; Rolling Meadows, IL). Any porous screen can be used as a reinforcement means, although those with a very fine mesh can interfere with the porosity of the web and cause undesirable resistance to flow and channeling. Screen 14 can be pressed into web 12 by standard pressure bonding techniques as are well known in the art, optionally under elevated temperature.
FIG. 2 shows an SEM (15 x magnification) of fibrillated PTFE web 12 with screen 14 partially pulled away therefrom. The screen pattern can be seen clearly in web 12.
FIG. 3 shows a more magnified close-up SEM (35 x magnification) of that seen in FIG. 2. Again, the pattem of screen 14 can be seen clearly in web 12.
FIG. 4 shows an SEM (500X magnification) of PTFE fibrils 16 from web 12 (of FIGS. 1-3) still attached to screen 14 after screen 14 has been partially pulled away from web 12. That fibrils 16 become mechanically entangled with or attached to screen 14 threads while screen 14 is embedded in web 12 is readily apparent.
FIG. 5 shows an SEM (250X magnification) of reinforced web 20 comprising particle-loaded fibrillated PTFE web 22 with scrim 24 partially embedded therein. In this embodiment, fibrillated PTFE web 22 entraps silica, although, as mentioned above, many other types of particulate can be used. In this embodiment, scrim 24 is a nonwoven polypropylene web available from, for example, AMOCO Fabrics & Fibers Co. (Atlanta, GA). Any porous scrim can be used as a reinforcement means, however.
FIG. 6 shows an SEM (35 x magnification) of fibrillated PTFE web 22 with scrim 24 partially pulled away therefrom. The scrim pattern can be seen clearly in web 22.
FIG. 7 shows a more magnified close-up SEM (150X magnification) of that seen in FIG. 6. PTFE fibrils 26 can be seen being pulled away from web 22 as scrim 24 is removed therefrom.
FIG. 8 shows a more magnified close-up SEM (330X magnification) of that seen in FIG. 7. Silica particles 28 entrapped within fibrils 26 are apparent. Once again, that fibrils 26 become mechanically entangled with or attached to scrim 24 fibers while scrim 24 is embedded in web 22 is readily apparent.
To make the web in which are entrapped the active particulate, one begins with an aqueous PTFE dispersion. This milky-white dispersion contains about 20% to 70% (by weight) of minute PTFE particles suspended in water. A major portion of these PTFE particles range in size from 0.05 to about 0.5 µm. Commercially available aqueous PTFE dispersions may contain other ingredients such as surfactants and stabilizers that promote continued suspension. Examples of such commercially available dispersions include Teflon Tm 30, Teflon ^Tm 30B, and Teflon ^Tm 42 (DuPont de Nemours Chemical Corp.; Wilmington, DE). Teflon TM 30 and Teflon™ 30B contain about 59 % to 61 % (by weight) PTFE solids and about 5.5 % to 6.5 % (by weight, based on the weight of PTFE resin) of a non-ionic wetting agent, typically octylphenyl polyoxyethylene or nonylphenyl polyoxyethylene. Te- flonTM42 contains about 32 % to 35 % (byweight) PTFE solids and no wetting agent (but does contain a surface layer of organic solvent to prevent evaporation).
The particle-loaded fibrillated PTFE web preferably is prepared as described in any of U.S. Patent Nos. 4,153,661,4,460,642, and 5,071,610, the processes of which are incorporated herein by reference, by blending the desired reactive particulate into the aqueous PTFE emulsion in the presence of sufficient lubricant to approach or, preferably, exceed the sorptive capacity of the solids yet maintain a putty-like consistency. This putty-like mass is then subjected to intensive mixing at a temperature preferably between 40° and 100°C to cause initial fibrillation of the PTFE particles. The resulting puttylike mass is then repeatedly and biaxially calendered, with a progressive narrowing of the gap between the rollers (while at least maintaining the water content), until the shear causes the PTFE to fibrillate and enmesh the particulate and a layer of desired thickness is obtained. Removal of any residual surfactant or wetting agent by organic solvent extraction or by washing with water after formation of the web article is generally desirable. The resultant web is then dried. Such webs preferably
^*U.S. Patent No. 5,354,603
have thicknesses in the range of 0.1 to 0.5 mm. Web articles with a thickness in the general range of 0.05 to 10 mm can be useful.
If a web article with multiple particulate layers is desired, the component layers themselves are stacked on each other and calendered until they form a composite where the PTFE fibrils of the separate layers are entwined at the interface of adjacent webs. Such multilayer webs demonstrate little boundary mixing between adjacent layers of particles. Multilayer articles preferably have thicknesses in the range of 0.1 to 10 mm.
The void size and volume within such a web article can be controlled by regulating the lubricant level during fabrication as described in U.S. Patent No. 5,071,610. Because both the size and the volume of the voids can vary directly with the amount of lubricant present during the fibrillation process, webs capable of entrapping particles of various sizes are possible. For instance, increasing the amount of lubricant to the point where it exceeds the lubricant sorptive capacity of the particulate by at least 3 % (by weight) and up to 200 % (by weight) can provide mean void sizes in the range of 0.3 f..lm to 5.0 f..lm with at least 90% of the voids having a size of less than 3.6 µm. This process can be used to create a web article with one or more kinds of reactive particulate enmeshed therein. The PTFE which forms the web within which particulate is to be trapped can be obtained in resin emulsion form wherein the PTFE and lubricant are already premixed (e.g., Teflon ^Tm 30 or 30B, DuPont de Nemours; Wilmington, DE). To this emulsion can be added additional lubricant in the form of water, water- based solvents such as a water-alcohol solution, or easily-removable organic solvents such as ketones, esters, and ethers, to obtain the aforementioned desired proportion of lubricant and particulate.
Active particulate (i.e., those which perform a funtion such as chemical reaction with or sorption of a solute or conduction) useful in the present invention includes any such particulate that can be immobilized in a non- woven, fibrous matrix. Representative sorptive particles include, but are not limited to, activated carbon, silica, derivatized silica, ion exchange resins, intercalated styrene divinylbenzene, and chitin. Conductive particles such as silver-coated glass spheres can also be used. Particulate material can be of regular (flat, spherical, cubic, rod- or fiber-like, etc.) or irregular shape. Average diameters of useful particles are within the range of 0.1 to 100 µm, more preferably within the range of 0.1 to 50 µm, and most preferably within the range of 1 to 10 µm. Such particulate can be incorporated directly into the web article.
Particulate is generally distributed uniformly in the web article, but matrices which include combinations of particulate can be prepared. Alternatively, layers containing different particulate can be calendered into a single matrix with distinct strata of particulate. Such multilayer composite articles show minimal boundary mixing (between the various particulate) and retain good uniformity throughout each layer. Whether in a heterogeneous or homogenous form, this type of article can selectively sorb or react with one or more chemical species to be removed from a fluid where these webs are to be used in chromatographic or separations applications.
Total particulate content of the web article can range up to about 97% (by weight), (although particulate amounts in the range of 80 to 95% (by weight) tend to produce more stable web articles). The enmeshing fibrils retain the enmeshed particulate, by entrapment or adhesion, within the matrix, and the enmeshed particles resist sloughing.
The web article of the present invention preferably comprises active particulate in an amount of at least 10% (by weight), more preferably comprises active particulate in an amount of at least 50% (by weight), and most preferably comprises active particulate in an amount of at least 80% (by weight). High active particulate loading is desirable to maximize the sorptive capacity or chemical activity of the substrate.
Non-active adjuvant particles with average diameters in the same ranges as listed previously with respect to active particulate can be included. Representative examples of useful adjuvants that can be incorporated in the web article include property modifiers such as glass beads and/or bubbles, glass particles other than beads or bubbles, energy-expandable hollow polymeric particles such as Expancel^TM microspheres (Nobel Industries; Sundsvall, Sweden) and mica. When present, such non-active particulate can comprise from more than 0 to 95 % (by weight), preferably from more than 0 to 50% (by weight), and most preferably from more than 0 to 10% (by weight) of the web article.
Particle-loaded fibrillated PTFE webs that have been reinforced in the above manner display improved resistance to ballooning and/or tearing and to shrinkage. This is very desirable in applications where the web must withstand a pressure drop caused by fluid flowing through it or must display dimensional stability. (Unreinforced fibrillated PTFE webs tend to shrink in the direction in which they were last machined.) Also, reinforced webs are easier to handle and less likely to be damaged during normal use.
If desired, a fibrillated PTFE web with particulate entrapped therein can also be reinforced on both sides. In other words, reinforcement means can be partially embedded in both sides of the web. This can increase the composite article's resistance to the aforementioned undesirable properties. Additionally, multilayer web - reinforcing means composite articles also can be made. This might be desirable where each web layer contains a different type of particulate.
The reinforced particle-loaded fibrillated PTFE webs of the present invention can be used wherever unreinforced particle-loaded fibrillated PTFE webs are useful, particularly in separation science (e.g., chromatographic and other separations as well as extractions).
Preferred embodiments of the present invention are composite articles according to any of claims 1 to 9 and the method according to claim 10.
Objects and advantages of this invention are further illustrated by the following examples.

EXAMPLES

Example I

The particle-loaded fibrillated PTFE web described herein was made essentially according to the procedure described in columns 3 to 6 of U.S. Patent No. 4,153,661.
The following materials were added together and the mixture was mixed in a Ross ™ mixer (Charles Ross & Son Co.; Hauppage, NY) at 30 rpm for 45 sec at 38°C:

400 g dry super-activated carbon with a surface area of 2000 to 3000 m²/g and an average particle size of 30 µm with a range of 3.9 to 200 µm (Kansai Coke and Chemicals Co. Ltd.; Amagasaki City, Japan)
312 g FLUON^TM PTFE emulsion, 22.6% PTFE in water (ICI Americas, Inc.; Wilmington, DE)
894 g deionized water

This dough-like mass was passed through a two-roll mill with an initial gap setting of 3.81 mm. The first few passes resulted in a web without enough strength to support its own weight; however, after a few more passes, the web was strong enough to maintain its integrity so that it could be folded into three layers and rotated 90° for its next pass through the mill. This biaxial calendering was followed for a total of ten passes. Thereafter, the gap was adjusted from 2.54 mm to 1.27 mm to 0.64 mm (with web passes through each gap) to produce a long web.
After the above three passes, the web was folded into eight layers and rotated 90°. The gap was adjusted from 2.54 mm to 1.90 mm to 1.27 mm to 0.76 mm (with web passes through each gap). This process yielded a fibrillated PTFE web that was 1.14 mm thick which was dried by passing through a belt oven.
This web was reinforced by placing it between two layers of NaltexTM LWS filtration netting (Nalle Plastics, Inc.; Austin, TX) and passing this composite article through a two-roll mill (gap = 0.89 mm, roll speed = 7.6 cm/sec). The tensile strength of the web-netting bond, measured by pulling the netting away from the web in a tensile tester (Thwing-Albert Instrument Co.; Philadelphia, PA), was 0.18 N/cm.

Example 2

A particle-loaded PTFE web was prepared as in Example 1 with the exception that final web thickness was 0.76 mm. This web was reinforced as in Example 1 with the exception that gap width during reinforcement was 0.51 mm. The tensile strength of this web-netting bond was 0.33 N/cm.

Example 3

A particle-loaded PTFE web was prepared as in Example 1 with the exception that final web thickness was 1.52 mm. This web was reinforced as in Example 1 with the exception that gap width during reinforcement was 1.14 mm. The tensile strength of this web-netting bond was 0.21 N/cm.

Example 4

A particle-loaded PTFE web was prepared as in Example 1 except that extra calendering produced a final web thickness of 0.38 mm. This web was sandwiched between two layers of 25 g/m² basis weight polyethylene non-woven web, and this composite article was passed through a two-roll mill with a gap of 0.25 mm. The tensile strength of this web - non-woven web bond was 0.035 N/cm.

Example 5

A particle-loaded reinforced PTFE web was prepared as in Example 4 except that a polyester non-woven web with a basis weight of 45 g/m² was used. The tensile strength of this web - non-woven web bond was 0.14 N/cm.

Example 6

A particle-loaded reinforced PTFE web was prepared as in Example 5 except that only one layer of reinforcement was used and the gap thickness during calendering was 0.64 mm.
This reinforced web and a similar unreinforced web were evaluated three ways: 1) use as an absorbing means on a generic base, 2) tensile strength, and 3) shrinkage.

Use with a generic base:

Generic bases for in-line filter holders are those designed for
paper or paper-like webs. They are available from a wide variety of commercial sources such as Gelman Co. (Ann Arbor, MI) and Nalgene Co. (Rochester, NY). Unreinforced webs were soft and conformed to the contours of the generic bases. Thus, when a blue food coloring solution was passed through an unreinforced web, flow was restricted to those areas where the web was not in direct contact with the base, and the dye was absorbed only in those areas (i.e., channeling occurred). When a similar dye solution was passed through a reinforced web supported on a generic base, no such conformation was observed, and the dye solution was evenly distributed throughout the web (i.e., channeling, leaks, and/or breakthrough were not observed).

Tensile strength:

The tensile strength of the reinforced webs were at least an order of
magnitude greater. The tensile strength of various reinforced webs is shown in Table I below.

Shrinkage:

Shrinkage induced by mechanical stimulation and by long-term exposure to heat were both tested.

Disks (with diameters of 47 mm) of reinforced and unreinforced webs were biaxially shaken on a sieve shaker (C.E. Tyler Co.; Mentor, OH) for approximately 60 minutes. The unreinforced disks shrunk into an elliptical shape with a minor axis of about 42 mm (i.e., 30 about 10% shrinkage). Essentially no shrinkage of the reinforced disks was observed.
Two disks (47 mm diameters) of reinforced web were placed in an oven at 71 °C for 51 days. No shrinkage was observed. Under similar conditions, unreinforced webs would have shrunk by at least 10%.

Example 7

A particle-loaded fibrillated PTFE web was prepared using the same procedure as that described in Example 1. Instead of activated carbon, however, the particulate was a C_1s-derivatized silica (J.T. Baker Co.; Phil- lipsburg, NJ). The final thickness of the web was 0.51 mm. The web was 90% (by weight) particulate.
A layer of NaltexTM LWS filtration netting was placed between two of these webs. The layered composite was passed through a two-roll mill (gap = 1.27 mm).
A 50 mm x 50 mm square sample of this reinforced web was placed in a 47 mm manifold vacuum system (Millipore Corp.; Bedford, MA) with the supporting screen removed therefrom. A liter of water with one drop of blue food coloring was pulled through the reinforced web. While removing dye from the solution, the reinforced web did not balloon or tear and was not pulled into the vacuum flask.

Example 8

A large web with C₁₈ as the entrapped particulate was prepared using the same procedure as in Example 1. Portions of this web were reinforced with various reinforcing materials, and these reinforced webs were tested against a comparative unreinforced web. The results are summarized in Table I below.
NITEX^TM nylon screens are made by TETKO, Inc. (Rolling Meadows, IL). MonodurTM Nylon 475 and 850 screens are made by Industrial Fabrics Corp. (Minneapolis, MN). NaltexTM filtration netting is made by Nalle Plastics, Inc. (Austin, TX). Celestra™ and PBN II™ non-woven webs are made by Fiberweb Inc. (Pensacola, FL). BrookjngsTM non-woven webs are made by Minnesota Mining and Manufacturing Co. (St. Paul, MN). Ty- parTM and 4dpf^TM Straight non-woven webs are made by Reemay, Inc. (Old Hickory, TN). Coverstock™ non- woven webs are made by Bonar Fabrics (Greenville, SC). RFX^TM non-woven webs are made byAMOCO Fibers and Fabrics, Inc. (Atlanta, GA).
All samples other than No. 1 were reinforced. Stress and strain values for reinforcement means only are not shown but are of the same order of magnitude as for the corresponding reinforced webs.
Because samples 2-4 were improperly cut, their stress and strain values could be off by up to 30%; however, these numbers help to prove the overall conclusion that can be drawn from Table I, i.e., that reinforced webs, regardless of the type of reinforcement, are at least an order of magnitude stronger in both the crossweb and downweb directions than an otherwise identical unreinforced web.

Claims

1. A composite article comprising:

a) a fibrillated polytetrafluoroethylene web having particulate entrapped therein; and

b) means for reinforcing said web;

said reinforcing means being at least partially embedded in said web.

2. The composite article of claim I wherein said reinforcing means is a screen.

3. The composite article of claim 2 wherein said screen is metallic.

4. The composite article of claim I wherein said reinforcing means is a scrim.

5. The composite article of claim 1 further comprising a second fibrillated polytetrafluoroethylene web.

6. The composite article of claim 1 further comprising a second reinforcing means.

7. The composite article of claim I wherein said particulate is capable of chemical reaction with or sorption of a solute passing through said web.

8. The composite article of claim 7 wherein said particulate is selected from the group consisting of activated carbon, silica, derivatized 30 silica, intercalated styrene divinylbenzene, ion exchange resins, and chitin.

9. The composite article of claim 1 wherein said particulate is conductive.

10. A method of making a reinforced particle-loaded fibrillated polytetrafluoroethylene web article comprising the steps:

a) providing a fibrillated polytetrafluoroethylene web having particulate entrapped therein; and

b) pressure bonding to said web a means for reinforcing said web so that said reinforcing means is at least partially embedded in said web.