Processes for forming fluoropolymer articles

Abstract

The present invention relates to a process for producing a fluoropolymer article having a high surface roughness and high coarseness which comprises the following steps: a) forming a paste comprising a fluoropolymer into a paste-formed fluoropolymer product at a temperature lower than 50° C., b) densifying the paste-formed product, and c) stretching the densified paste-formed fluoropolymer product in at least one direction. The present invention further relates to a fluoropolymer article obtainable by a process according to the invention. The present invention furthermore relates to a fiber comprising, or consisting of, a fluoropolymer having a surface roughness expressed as a peak to valley distance (Rt) greater than 10 micrometer and/or an average surface roughness (Ra) greater than 1.5 micrometer. The present invention furthermore relates to a membrane comprising, or consisting of, a fluoropolymer having a coarseness index ρ/EBP of at least 0.3, an air permeability of 15 ft.sup.3/ft.sup.2/min or higher and a node aspect ratio of below 25.

Claims

1. A process comprising: a) forming a paste comprising a fluoropolymer into a paste-formed fluoropolymer product at a temperature lower than 50° C., b) densifying the paste-formed fluoropolymer product, and c) stretching the densified paste-formed fluoropolymer product in at least one direction to form a fluoropolymer article, wherein the fluoropolymer article has an average distance between nodes of greater than 50 microns; wherein the fluoropolymer article comprises: a fluoropolymer fiber comprising: a surface roughness expressed as a peak-to-valley distance of greater than 10 micrometers, an average surface roughness of greater than 3 micrometers, and a coarseness index of at least 0.25 g/cm.sup.3/psi.

2. The process of claim 1, wherein the forming step is conducted at a temperature equal to or lower than 45° C.

3. The process of claim 1, wherein the paste comprising the fluoropolymer further comprises a lubricant.

4. The process of claim 3, wherein the lubricant is removed before the paste-formed fluoropolymer product is densified.

5. The process of claim 1, wherein the stretching step comprises at least one orientation step, wherein the at least one orientation step is performed at a temperature of 250° C. to 370° C.

6. The process of claim 1, wherein the stretching step is performed at a stretch ratio of 5 to 500.

7. The process of claim 1, wherein the stretching step comprises at least one orientation step, wherein the at least one orientation step is performed at an average stretch rate of 10% per second to 500% per second.

8. The process of claim 1, wherein the paste-formed fluoropolymer product has a porosity of less than 30%.

9. The process of claim 1, wherein the stretching step comprises two orientation steps carried out in the same direction.

10. The process of claim 1, wherein the fiber has an extension in one dimension that is greater than the extension in other dimensions.

11. The process of claim 10, wherein the fluoropolymer fiber has a tenacity of 2.0 gf/denier or more.

12. The process of claim 10, wherein the fluoropolymer fiber has a titer of 700 denier or more.

13. The process of claim 10, wherein the fluoropolymer fiber has a wicking height of at least 35 mm after 30 minutes.

14. The process of claim 10, wherein the fluoropolymer fiber is a dental floss.

15. The process of claim 10, wherein the fluoropolymer fiber has a porosity of less than 30%.

16. The process of claim 1, wherein the fluoropolymer fiber has a porosity of less than 30%.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the scanning electron micrograph of the surface of fiber F3. The machine direction is from the bottom of the figure to the top.

(2) FIG. 2 shows the scanning electron micrograph (SEM top view) of the surface of fiber F1. The machine direction is from the bottom of the figure to the top.

(3) FIG. 3 shows the scanning electron micrograph (SEM top view) of the surface of Tape T5. The machine direction is from the bottom of the figure to the top.

(4) FIG. 4 shows the scanning electron micrograph (SEM cross section) of Tape T5. The machine direction is from the left of the figure to the right.

(5) FIG. 5 shows the scanning electron micrograph (SEM cross section) of Tape T6. The machine direction is from the left of the figure to the right.

(6) FIG. 6 shows the scanning electron micrograph (SEM cross section) of membrane M2. The machine direction is from the left of the figure to the right.

(7) FIG. 7 shows the scanning electron micrograph (top view) of membrane M2. The machine direction is from the bottom of the figure to the top.

(8) FIG. 8 shows the scanning electron micrograph (top view) of membrane M3. The machine direction is from the bottom of the figure to the top.

(9) The present invention will be further illustrated by the examples described below.

EXAMPLES

(10) 1) Measurement Methods

(11) a) Surface Topography

(12) The surface topography of the examples was characterized by the height of peaks generated by nodal structures, the peak to valley distance, and the average distance between them preferably projected onto the direction of the first stretching step (machine direction). The data were generated from scanning electron micrographs of the surface and the cross-section parallel to the machine direction.

(13) In addition, the surface roughness and peak to valley distance (Rt) of fibers and tapes were characterized using a Zygo NewView™ 7200 3D optical surface profiler. A cylinder background form removal was applied to all samples to correct for the curvature. Subsequently, a high FFT frequency filter at 20 micrometer wavelength was applied to minimize noise. No filter trim was used to preserve edge data. Data analysis was conducted using MetroPro 8.3.5 from Zygo.

(14) The surface roughness and peak to valley distance are defined as follows:

(15) Ra: Arithmetical mean deviation. The average roughness or deviation of all points from a plane fit to the test part surface.

(16) Rq: Rq is the root mean square parameter corresponding to Ra.

(17) Rt: Maximum peak-to-valley height. The absolute value between the highest and lowest peaks.

(18) b) Microstructure

(19) The aspect ratio of the surface of the nodal areas was determined from scanning electron micrographs. At least five such measurements were taken of representative nodes.

(20) The average distance between nodes in machine direction (MD) has been determined from the average length of lines oriented in machine direction and connecting nodes. At least ten such measurements were taken of representative nodes.

(21) c) Air permeability was measured according to ASTM D 737 of at least three samples. At least five such measurements were taken.

(22) d) Ethanol Bubble Point (EBP) was determined according to ASTM F360-80. At least three such measurements were taken.

(23) e) Mechanical Testing

(24) Tenacity was determined according to EN ISO 2062.

(25) Ball burst was measured using a Chatillon TCD200 digital force tester. Burst strength measures the relative strength of a sample by determining the maximum load at break. A single layer of the sample is challenged with a 25 mm diameter ball while being clamped and restrained in a ring of xmm inside diameter. The sample is placed taut in the ring and pressure applied against it by the steel ball of the ball burst probe approaching the center of the sample at a constant speed of 10 inch/minute. Maximum load is recorded as “ball burst” in pounds. At least three such measurements were taken.

(26) f) Vertical Wicking Test

(27) The ability for the present invention to move liquid moisture was measured using the following test method. Two hundred ml of isopropanol alcohol (IPA) USP HPLC grade was placed in a clean and dry 500 ml Erlenmeyer flask. The Erlenmeyer flask rested on top a level lab bench surface such that the inside of the flask is easily observed. A piece of black construction paper the size of 8½×11 inch (216×279 mm) was placed behind the flask to aid in the observation of the wicking IPA media advancing up the test filament. A 250 mm long stainless steel ruler having the precision of 0.5 mm was affixed vertically against the back inside wall of the Erlenmeyer flask with double-sided adhesive tape such that the distal end starting at 0 mm rested on the floor of the flask. A length of dry filament approximately 147 mm was cut randomly from a spool of test filament candidate. A 1.67 gram Rubber-grip™ lead fishing (sinker) weight was affixed to one distal end of the filament and the second distal end was affixed to a wooden dowel/stick. The wooden dowel has a round cross-section approximately 2 mm diameter by 100 mm long. The overall length of the secured test filament is such that when the distal end containing the fishing weight is lowered inside the Erlenmeyer flask, at least 1 mm of filament and the fishing weight are totally submerged in the IPA with no slack in the filament as the wooden dowel/stick rest on top of the upper lip of the Erlenmeyer flask. Erlenmeyer contains 250 ml IPA before the filament is lowered inside.

(28) Once the dry test filament having the fishing weight affixed, it is lowered inside the Erlenmeyer flask, is submerged and the dowel stick is resting on the top lip of the flask, an electronic stopwatch (precision±0.1 seconds) was started. Observations of the IPA media wicking up the filament are made at certain time intervals, 1, 6, and 16 minutes.

(29) At least 5 wicking tests are performed for each test filament candidate. The graph below shows the wicking height vs. time for three examples of the present invention compared to commercially available Comfort Plus Dental Floss from the Procter and Gamble Company which had its waxed coating removed using five 5-minute rinses of hexane at 40° C. followed by three rinses of de-ionized water at ambient temperature and then dried at ambient temperature. No wicking was observed to occur within the 16 minute test duration using another commercially available PTFE dental floss and is thus not plotted in the graph. This floss is Original GLIDE® Dental Floss from the Procter and Gamble Company. The Original GLIDE® Dental Floss had its waxed coating removed prior to testing using five 5-minute rinses of hexane at 40° C. followed by three rinses of de-ionized water at ambient temperature and then dried at ambient temperature. After the test is completed, the filament is removed and the IPA is drained from the flask. The flask is cleaned and dried. At least three such measurements were taken.

(30) f) Coarseness Index

(31) The coarseness index is defined herein as the bulk density of the stretched porous article divided by the ethanol bubble point of that article.

(32) $\mathrm{Coarseness}\mathrm{index}=\frac{\mathrm{bulk}\mathrm{density}\left[g\text{/}{\mathrm{cm}}^3\right]}{\mathrm{ethanol}\mathrm{bubble}\mathrm{point}\left[\mathrm{PSI}\right]}$

(33) g) Conversion Factors to SI Units:

(34) 1 lbf=4.4482 N

(35) 1 denier=1 g per 9000 m length=0.1111 tex

(36) 1 tex=1 g per 1000 m length

(37) 1 gF/denier=0.8829 N/tex

(38) 1 ft.sup.3/ft.sup.2/min=0.00508 m.sup.3/m.sup.2/s

(39) 1 PSI=6894.757 Pa

(40) h) Bulk Density

(41) The bulk density is the ratio between the mass of the example and its volume as determined by the measured dimensions.

(42) i) Porosity

(43) Porosity was determined from the ratio between the actual bulk density ρ.sub.actual of the porous material and the highest density ρ.sub.max of the non-porous material according to

(44) $\mathrm{Porosity}=1-\frac{{\rho }_{\mathrm{actual}}}{{\rho }_{\mathrm{ma}x}}$

(45) 2) Preparation of Fluoropolymer Articles

(46) Following the procedures disclosed in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,064,214 a precursor tape was prepared in the following manner:

(47) A fine powder PTFE resin was mixed with mineral spirit (24.3 wt %) to form a paste and extruded through a die to form a wet tape of 0.775 mm thickness at 25° C. Subsequently, the wet tape was rolled down at 25° C., and than dried at 185° C. to remove the mineral spirit. The dry tape had a final thickness of 0.152 mm.

(48) To demonstrate the effect of the low process temperature when making the paste-formed intermediate product, another comparative dry precursor tape was made using the same recipe as above, but increasing the extrusion and calendaring temperatures to 50° C. The final dimensions of the comparative dry tape were similar to the dimensions of the tape described in the section above, which precursor has been used for which example is indicated below.

(49) Subsequently the dry tapes were densified to a bulk density of 2.2 g/cm.sup.3, i.e. porosity of <5% assuming ρ.sub.max=2.3 g/cm.sup.3, by passing them between two hard steel rolls at a line speed of 10 m/min and a line pressure of 25 kN.

(50) The densified precursor tapes can be cut and/or stretched into any desired shape according to the inventive process as follows.

(51) Fibers

(52) Prior to any stretching step the densified precursor tape described above was slit to 17.75 mm widths by passing it between a set of gapped blades to serve as precursor fibers.

(53) The precursor fibers were stretched over hot plates at 300° C. to 320° C. in a first pass, at 360° C. in a second pass, and finally heated to 425° C. without stretching for at least 5 seconds to form a fiber. The total stretch ratio was 50:1. The stretch ratios, average stretch rates, and temperatures of the individual passes were varied as shown in Table 1 to produce fibers (inventive examples ID F1-F3) with different degrees of coarseness and surface roughness from low to high. Comparative samples F4 and F5 denote two commercially available floss products with trade names Glide® original floss and Glide® comfort plus, respectively.

(54) The fibers were measured to characterize the mechanical properties, surface structure, and wicking behavior by the methods described hereinabove. The results are shown in Table 2.

(55) TABLE-US-00001 TABLE 1 Process parameters - fibers Pass 1 Pass 2 Pass 1 Average Pass 1 Pass 2 Average Pass 2 Sample Stretch stretch rate Temperature Stretch stretch rate Temperature ID ratio [%/s] [° C.] ratio [%/s] [° C.] F1 25 195.1 320 2 13.1 360 F2 15 78.9 310 3.34 24.6 360 F3 15 39.5 300 3.34 12.3 360

(56) TABLE-US-00002 TABLE 2 Characterization - fibers Average Wicking Average Root Mean distance height after surface Square surface Peak to between Sample Titer Tenacity 30 minutes roughness (Ra) roughness (Rq) valley (Rt) nodes in MD ID [denier] [gF/denier] [mm] [micrometer] [micrometer] [micrometer] [micrometer] F1 803 3.18 41 0.88 1.12 23.15 70 F2 1131 3.09 44 1.53 1.95 31.36 117 F3 797 2.23 71 6.17 7.58 78.80 454 F4 1247 4.04 0 n.a. n.a. n.a. n.a. F5 1000 2.86 30 0.77 0.96 15.37 n.a.

(57) Tapes

(58) The precursor tape as described above was stretched over hot plates in a single pass at 300° C. (inventive examples ID T1, T4 and T5). According to the procedure described in U.S. Pat. No. 3,953,566, the stretched tape was subject to an additional heat treatment or sintering step by passing it over hot rolls at 360° C. for 5 seconds making example T2.

(59) The stretch ratios, stretch rates, and temperatures were varied as shown in Table 3 to produce tapes with different degrees of surface roughness.

(60) A comparative precursor tape, which has been extruded and calendared at 50° C., was stretched over hot plates in a single pass at 300° C. From this comparative precursor tape examples T3, and T6 were produced using the same stretch ratios, stretch rates, and temperatures as used for making examples T1 and T5, respectively. Accordingly, the coarseness and surface roughness of samples T3, and T6 drops significantly as shown in FIGS. 4 and 5.

(61) The tapes were measured to characterize the mechanical properties, air permeability, bubble point, water entry pressure and surface structure by the methods described hereinabove. The results are shown in Table 4 and 5. Please note, that peak-to-valley values marked by a star were estimated from the largest peak-to-valley distance determined from SEM cross-sections along a single cut in machine direction. Due to the limited amount of available data, these values present only a lower limit.

(62) TABLE-US-00003 TABLE 3 Process parameter - tapes Estimated Stretch average stretch Sample ID ratio rate [%/s] Temperature [° C.] Sintering T1 10 17.8 300 no T2 10 17.8 300 yes T3 10 17.8 300 no T4 20 49.8 300 no T5 30 90.4 300 no T6 30 90.4 300 no

(63) TABLE-US-00004 TABLE 4 Characterization - tapes Ballburst Ballburst * Sample Area weight Thickness strength Airflow Airflow EBP ID [g/m.sup.2] [10.sup.−6 m] [lbs] [ft.sup.3/ft.sup.2/min] [lbs * ft.sup.3/ft.sup.2/min] [PSI] T1 22.8 88 5.1 1.8 9.05 0.87 T2 24.8 91 8.5 4.0 33.88 1.25 T3 23.1 87 12.2 0.7 8.07 1.95 T4 11.5 81 3.9 9.0 34.73 0.22 T5 7.8 53 3.2 13.3 42.37 0.05 T6 9.0 35 7.9 0.9 7.24 1.55

(64) TABLE-US-00005 TABLE 5 Characterization - tapes Root Mean Average Average Square distance surface surface Peak to between Coarseness roughness roughness valley nodes in index (Ra) (Rq) (Rt) MD Sample [g/cm.sup.3/ [microm- [microm- [microm- [microm- ID PSI] eter] eter] eter] eter] T1 0.30 3.22 4.30 53.59 130 T2 0.27 n.a. n.a. n.a. 144 T3 0.14 n.a. n.a. >4.9* 67 T4 0.64 7.12 9.37 111.91 222 T5 2.79 5.72 7.40 73.7 309 T6 0.16 n.a. n.a. >5.3* 149

(65) Membranes

(66) The densified precursor tape as described above was stretched over hot plates in a single pass in one machine direction (designated as x) at 300° C., and stretched along a direction perpendicular (transverse direction) to the first pass (designated y) at 300° C. in a second pass. According to the procedure described in U.S. Pat. No. 3,953,566, one sample of each biaxially stretched membrane was subject to an additional heat treatment or sintering step by subjecting to sample to hot circulated air at 375° C. for 5 seconds.

(67) The stretch ratios, average engineering stretch rates, and temperatures of the individual passes were varied as shown in Table 6 to produce membranes (inventive examples ID M1-M3) with different degrees of coarseness, surface roughness, and air permeability

(68) The membranes were measured to characterize the mechanical properties, air permeability, bubble point, water entry pressure and surface structure by the methods described hereinabove. The results are shown in Table 7 and 8. Please note, that peak-to-valley values marked by a star were estimated from the largest peak-to-valley distance determined from SEM cross-sections along a single cut in machine direction. Due to the limited amount of available data, these values present only a lower limit.

(69) TABLE-US-00006 TABLE 6 Process parameter - membranes Pass 1 -x Pass 2 -y Estimated average Pass 1 - x average Pass 2 - y Sample Pass 1 - x stretch rate Temperature Pass 2 - y stretch rate Temperature ID Stretch ratio [%/s] [° C.] Stretch ratio [%/s] [° C.] Sintering M1 16 36.8 300 8 700 300 no M2 16 36.8 300 8 700 300 yes M3 8 21.4 300 8 700 300 no

(70) TABLE-US-00007 TABLE 7 Characterization - membranes Ballburst * Sample Area weight Thickness Ballburst Airflow Airflow EBP ID [g/m.sup.2] [micrometer] [lbs] [ft.sup.3/ft.sup.2/min] [lbs * ft.sup.3/ft.sup.2/min] [PSI] M1 0.18 6.1 1,517 67.1 101,79 0.5 M2 0.19 5.2 1,645 83.7 137,69 0.36 M3 0.43 6.3 2,671 15.3 40,87 1.08

(71) TABLE-US-00008 TABLE 8 Characterization - membranes Average distance between Average Coarseness nodes Average Node Peak to index in MD node area valley Sample [g/cm.sup.3/ [microm- aspect [microm- [microm- ID PSI] eter] ratio eter.sup.2] eter] M1 0.59 156.45 1.53 67 >27.81* M2 1.01 148.38 1.22 90 >19.5* M3 0.64 86.91 1.54 124 >13.7*