Melt-Spun Hollow Fibers for Use in Nonwoven Structures - American

The polypropylene continuity equation can be combined with the nitrogen continuity equation to give excellent predictions of the inside diameter and t...
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Ind. Eng. Chem. Res. 2001, 40, 176-187

Melt-Spun Hollow Fibers for Use in Nonwoven Structures Anne de Rove` re and Robert L. Shambaugh* School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019

Polypropylene hollow fibers were produced via the spunbonding process. Nitrogen gas was used as the lumen fluid. The polypropylene continuity equation can be combined with the nitrogen continuity equation to give excellent predictions of the inside diameter and the outside diameter of the product fiber. Tests were developed to measure the compression-recovery behavior of hollow fiber mats (nonwoven structures) versus mats of solid fibers. Tests were also developed to measure the opacity of hollow fiber mats versus solid fiber mats. In terms of compressionrecovery and opacity, mats of hollow fibers are nearly identical to mats of solid fibers of the same outside diameter. Introduction Presently, hollow fibers are used as membranes in the separations business. However, solid fibers, not hollow fibers, are presently used in spunbonded nonwovens. The study described herein is the synthesis of hollow fiber technology with spunbonded nonwoven technology to produce an improved nonwoven material. Hollow Fiber Technology. Water desalination (reverse osmosis) is the biggest single use of hollow fibers; use in hemodialysis (artificial kidneys) is also large. Uses in gas separation, microfiltration, and pervaporation are growing.1 Typical hollow fibers have outside diameters of 20-250 µm and inner diameters of 2-50 µm.2 Hollow fibers used in separations are difficult to make and are expensive (>$20/kg) compared with ordinary solid filaments (e.g., ≈$2.50/kg for polypropylene fiber). Hollow fibers are prepared by spinning a polymer or polymer solution through an annular die or spinneret. The following three spinning methods can be used [see ref 3]: (1) wet spinning in which the polymer solution is spun from the spinneret and into a liquid coagulation bath; a distance (“air gap”) between the spinneret and coagulant is often used; (2) dry spinning in which the solvent in the polymer evaporates into a gas; no liquid bath is used; and (3) melt spinning in which the polymer is cooled in air without evaporation of any plasticizer or other material. Wet spinning is by far the most common method for producing hollow fibers.4 Spinning speeds are quite slow. For example, Kim et al.5 produced wet-spun fibers at speeds of only 10-35 m/min (speeds of 3000-6000 m/min are common in the melt spinning of conventional solid fibers); these low speeds translate into high fiber cost. The structure of the fiber wall can be varied by controlling (a) the concentration of the polymer in the feed solution, (b) the coagulant temperature, and (c) the coagulant concentration. Often the fiber wall is “asymmetric”, with an outer skin of solid polymer supported by a thicker inner layer of porous material. The thin skin permits a high permeation rate, whereas the porous layer provides mechanical support. A number of researchers have described methods for controlling the formation of the fiber wall: see Li et al.,4 Kim et al.,5 Doi and Hamanaka,6 Miao et al.,7 and Kesting.8 * To whom correspondence should be addressed.

Figure 1. Spunbonding process.

When melt spinning is used to produce hollow filaments, the polymer may or may not be combined with a second component. For example, a polypropylene/ soybean oil mixture can be melt-spun into a hollow fiber: see Kim et al.9 After spinning, the soybean oil can be leached from the fiber to create a porous membrane. If polypropylene is melt-spun without the soybean oil, the resulting fiber can be post-treated with several annealing and drawing steps. The result of this post-treatment will be the creation of pores in the fiber wall (see Kim et al.10). Production of Nonwovens via Spunbonding and Melt Blowing. Figure 1 shows a typical spunbonding process in which a molten polymer is pressurized and forced through a fine capillary. The polymer stream is then rapidly attenuated by the drawing force exerted by an air venturi. The fiber is collected and forms a mat

10.1021/ie000503d CCC: $20.00 © 2001 American Chemical Society Published on Web 12/08/2000

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Figure 3. Effect of the inner diameter on the cross-sectional area and hollowness ID2/OD2 with a constant OD ) 20 µm.

could lower the basis weight of a fabric by using hollow filaments. In this case, the cost of the fabric would be reduced because of the reduction in polymer cost (i.e., less polymer is needed). Let us define fiber hollowness (h) as

h ) ID2/OD2

Figure 2. Melt-blowing process.

on an open screen. Because weaving (or knitting) is not used to form the mat, the mat is referred to as a nonwoven. The spunbonding process uses only one quick step to go from polymer pellet to nonwoven mat. The melt-blowing process is similar to the spunbonding process.11 However, in the melt-blowing process the venturi is built right into the spinning die, and hot air is used in the die (versus cool air for spunbonding; see Figure 2). Spinning speeds are quite high in both processes: spunbonding is typically done at speeds of 2000-4000 m/min, whereas melt blowing is done at speeds of 2000-30 000 m/min. Worldwide, about 150 × 106 kg/year of melt-blown fabric is produced, and a similar amount of spunbonded material is made. Typical examples of these materials are Pall Corp.’s cartridge filters, Kimberly-Clark’s Spunguard surgical gowns, and 3M’s Thinsulate high-performance insulation for apparel. Synthesis: The Spunbonding of Hollow Filaments. This paper describes the rapid production of hollow filaments via spunbonding. (Because of the similarities between melt blowing and spunbonding, many of the results presented herein also apply to melt blowing.) Heretofore, hollow fibers have not been commonly produced via spunbonding. The commercially practical goal of this is to take a relatively low-cost material (e.g., a spunbonded fabric) and impart higher value to this material by making the fibers hollow. Furthermore, our study of the rapid production (via melt spinning, not wet or dry spinning) of hollow fibers could translate into cheaper production of hollow fibers for use in separations. In the fibers business, spinneret throughput (i.e., spinning speed) is generally the limiting factor in process economics. Because hollow fibers are lighter than solid fibers of the same outer diameter, a nonwovens manufacturer

(1)

where ID ) final (product) inside diameter and OD ) final (product) outside diameter. The value of h ranges from 0 for a solid fiber to 1 for a hollow fiber with an infinitely thin wall. Figure 3 shows the calculated effect of the inner diameter on the hollowness and the fiber cross-sectional area. The polymer cost is directly proportional to the crosssectional area. Also, cost is proportional to 1 - h. If solid filaments in nonwovens are replaced with hollow fibers, are the properties of the nonwovens changed? This question is addressed in this paper. Use of the Continuity Equation in Hollow Fiber Spinning. Figure 4 shows a diagram of the fiber spinline during the spunbonding process for hollow fibers. d2 is the outside diameter of the filament, whereas d1 is the inside diameter of the filament. Both d2 and d1 vary along the spinline. Of course, at the fiber collection screen (see Figure 1), the values of d2 and d1 reach their final product diameters (OD and ID). Because mass neither enters nor leaves the spinline during melt processing of the fibers, the mass at any point along the spinline is constant. For the polymer, the continuity equation (mass balance equation) for any z position along the spinline is

mp ) Fp(π/4)(d22 - d12)vf

(2)

where mp ) polymer mass rate, Fp ) polymer density, and vf ) fiber velocity. For the core fluid (nitrogen), the mass balance equation is

mn ) Fn(π/4)d12vf

(3)

where mn ) nitrogen mass rate and Fn ) nitrogen density. Implicit in eq 3 is that the nitrogen moves at the same velocity as the polymer. For typical core diameters (d1 values) on the order of 40 µm, this is probably a very good assumption: gas boundary layers are generally

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Figure 5. Equipment used for the spunbonding of hollow fibers. Table 1. Dimensions of Spinnerets Used To Produce Hollow Fibers

spinneret 1 spinneret 2

Figure 4. Fiber spinline during hollow fiber production.

>>40 µm in thickness. Of course, near the spinneret, the core diameter might be several hundred microns in diameter. However, even here the nitrogen is substantially dragged along with the polymer, and vf is a lot slower at positions near the spinneret. To use eq 3, information about the nitrogen density (Fn) along the spinline is needed. Fn is a function of pressure and temperature. The pressure in the spinline is greater than the ambient atmospheric pressure. This difference can be calculated from the fundamental equation of capillarity which is (see Adamson12)

∆P ) γ(1/R1 + 1/R2)

(4)

where ∆P ) pressure difference between the two phases, R1 and R2 ) radii of curvature of the interface between the two phases, and γ ) surface energy. For a cylindrical fiber, one of the radii is infinite. Thus, the capillarity equation becomes

∆P ) γ(1/R)

(5)

Bicerano13 gives data for the surface energy of polypropylene and polyethylene at temperatures of 20-180 °C. His data show that temperature has relatively little effect on γ. On the basis of Bicerano’s data, it can be assumed that γ ≈ 25 dyn/cm for the air-polypropylene interface and the nitrogen-polypropylene interface. Because the hollow fiber has both an inner and outer surface, eq 5 must be applied twice. Let us assume an inner diameter (d2) of 40 µm and an outer diameter (d1) of 70 µm. On the basis of these dimensions and assuming γ ) 25 dyn/cm, the pressure in the core nitrogen is only 1964 Pa higher than the outside pressure. On the basis of an outside pressure of 1.01 × 105 Pa, the core pressure is 1.0296 × 105 Pa. Because this core pressure is only 1.9% higher than the outside pressure and because the assumed fiber dimensions are typical of our minimum fiber sizes (the pressure difference would be

outer diameter of polymer annulus, mm (in.)

inner diameter of polymer annulus, mm (in.)

diameter of nitrogen capillary, mm (in.)

7.01 (0.276) 1.98 (0.078)

3.78 (0.149) 1.22 (0.048)

1.00 (0.039) 0.76 (0.030)

even less for larger fiber dimensions), it is safe to approximate the inner core pressure as atmospheric. Knowledge of the core pressure permits the determination of core density Fn (from tabulated or correlated data). Fn is very useful. For example, if the injection rate of nitrogen (mn) into a spinning system is known and if the spinning speed (vf) is known, then, from eq 3, the fiber inner diameter (d1) can be predicted (for the collected product material). Furthermore, if the mass rate of polymer (mp) is known and if the polymer density (Fp) at room temperature is known (and it is for all common polymers), then eq 2 will allow the calculation of the fiber outer diameter (d2). The continuity equations can also be used in other ways. For example, if off-line measurements of d1 and d2 (ID and OD) are performed on the collected fibers, then eqs 2 and 3 can be used to predict mn and mp. Experimental Equipment for Melt Spinning. The experimental equipment used is shown in Figure 5. Polypropylene pellets were melted and pressurized in a Brabender screw extruder. The barrel had a 19.0 mm diameter, a 381 mm length, a 20:1 L/D ratio, and a 3:1 compression ratio. The extruder temperature was set at 225 °C for all experiments. After exiting the extruder, the polymer was fed to a spin pack containing a modified Zenith pump, which in turn fed the spinneret assembly. The polymer exited from an annular ring in the spinneret, and a center hole allowed nitrogen to be injected into the core of the polymer stream (see Figure 4). Two different spinnerets were used in spinning; the dimensions of these spinnerets are given in Table 1. For spinneret 1, both the spinneret and the spin pack were set at 200 °C; for spinneret 2, both were set at 190 °C. For spunbonding, the fibers were drawn with an airpowered venturi; the fibers were collected on an open screen. For conventional melt spinning, the venturi was replaced with a mechanical roll and the fibers were collected on the roll. The polymer used in our studies was 88 MFR Dypro isotactic polypropylene donated by the Fina Co. This polypropylene has an Mw of 165 000 and an Mn of 41 500. Table 2 gives experimental conditions used to produce fibers for off-line analysis.

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 179 Table 2. Experimental Conditions Used To Produce Fibers for Off-Line Analysis parameters

values

polymer flow rate (g/min) fiber speed (m/min) nitrogen flow rate (mL/min)

0.33, 0.57, and 1.13 150, 260, and 460 0-4

Figure 7. Effect of speed on the outer diameter.

Figure 6. Effect of nitrogen flow rate on the outer diameter, inner diameter, and wall thickness.

Outside diameters and inside diameters (for hollow fibers) were determined by first microtoming the samples and then measuring with a Nikon Labophot2-Pol microscope at 100× magnification. Results and Discussion Effect of Operating Parameters on Fiber Dimensions. In the melt spinning of hollow fibers, an additional dimension must be considered. This dimension is the fiber inside diameter, or (equivalently) the fiber wall thickness. To gain a better understanding of how hollow fibers are formed, the effects of experimental parameters on the fiber OD and ID were examined. Fibers were spun using a hollow fiber spinneret 1 (see Table 1). Because analysis of individual fibers (rather than a fiber mat) was the goal, a windup roll was used instead of an air venturi (see Figure 5). The experimental parameters studied were polymer flow rate, fiber spinning speed, and nitrogen flow rate. The ranges of parameters studied are shown in Table 2. Figure 6 shows the effect of nitrogen flow rate on OD, ID, and wall thickness for a polymer flow rate of 0.57 g/min and a fiber speed of 263 m/min. Increased nitrogen flow rate results in a larger OD and a larger ID. However, the wall thins as the nitrogen rate increases. Results similar to Figure 6 were found for other combinations of polymer flow rates and nitrogen flow rates. For the OD and ID measurements, each data point in Figure 6 is the average of three diameter measurements; the standard deviation of each point is about 2 µm. Also shown in Figure 6 are model estimates of OD, ID, and thickness. These estimates were based on the continuity equations (eqs 3 and 4). Inputs to these equations were the polymer flow rate mp (based on fiber samples collected and weighed over time), nitrogen flow rate mn (based on the nitrogen flowmeter reading), and windup speed (based on a digital tachometer reading of the windup roll speed). As an examination of Figure 6 shows, the model fits the data quite well. The goodness of fit lends credibility to the aforementioned assump-

Figure 8. Effect of fiber speed on the inner diameter.

tions relating to the development of the continuity equations and the capillarity equation. Figure 7 shows the effect of the nitrogen flow rate on the outer diameter for three different fiber speeds. Also shown are the model predictions. The model does a very good job of predicting diameter behavior as a function of operating parameters. For the same conditions as those for Figure 7, Figure 8 shows the effect of the nitrogen flow rate on the inner diameter. As before, the model equations do a very good job of predicting diameters. In Figure 8, the largest difference between the model and the data occurs for the high fiber speed (466 m/min). However, even for this case, the model is not far off. Figure 9 shows how wall thickness is affected by the nitrogen flow rate and fiber speed. The thinnest walls occur at the highest fiber speeds and the highest nitrogen flow rates. The model equations fit the data very well for Figure 9. Figure 10 shows how the ratio ID/OD is affected by the nitrogen flow rate. As was done for previous plots, the continuity equations (eqs 3 and 4) can be used to produce a model equation. This model equation is

ID/OD ) {[mn/Fn]/[mp/Fp + mn/Fn]}0.5

(6)

What is interesting about this equation is that it is independent of fiber speed. Also, indeed, the data follow this prediction: there is no dependence on fiber speed. The ratio ID/OD exceeds 0.9 for nitrogen flow rates above 3 mL/min. This corresponds to a mass ratio (mn/

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Figure 9. Effect of fiber speed on the wall thickness.

Figure 12. Effect of polymer throughput on the inner diameter.

Figure 10. Effect of fiber speed on the ratio of the inner to outer diameter.

Figure 13. Effect of polymer throughput on the wall thickness.

Figure 11. Effect of polymer throughput on the outer diameter.

mp) above 0.0072. In other words, it takes relatively little gas to produce a large hole in the fiber. The effects of variable polymer flow rates are shown in Figures 11-14. Figure 11 shows the variation in the outer diameter, whereas Figures 12-14 show the variations in respectively the inner diameter, wall thickness, and ID/OD. The model equations are also included in these figures. The model equation for Figure 12 is

ID ) {4mn/πFnvf}0.5

(7)

Note that ID is independent of mp. This makes sense if

Figure 14. Effect of polymer throughput on the ratio of inner to outer diameter.

you consider that mn, not mp, determines how much the fiber is “blown up”. The mp only affects the wall thickness. Figure 13 shows how the wall thickness is affected by mp, whereas Figure 14 shows the variation of ID/ OD. Unlike the situation in Figure 10, the model equation for ID/OD is not a single curve in Figure 14. Polymer flow rate does affect ID/OD. Figure 15 shows actual photographs of cross sections of hollow fibers produced in our laboratory. Figure 15a shows a thick-walled fiber with a hollowness of less than 1% (0.21%). Figure 15b shows a medium-walled fiber

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Figure 15. (a) Cross-sectional view of a typical thick-walled PP hollow fiber: fiber ID ) 1.5 µm, fiber OD ) 32.5 µm, and fiber hollowness ) 0.21%. (b) Cross-sectional view of a typical medium-walled PP hollow fiber: fiber ID ) 33 µm, fiber OD ) 55 µm, and fiber hollowness ) 36%. (c) Cross-sectional view of a typical thin-walled PP hollow fiber: fiber ID ) 60 µm, fiber OD ) 72 µm, and fiber hollowness 69%.

with a hollowness of 36%. Figure 15c shows a very thinwalled fiber with a hollowness of 69%. This fiber is mostly nitrogen! These three photographs illustrate the wide range of hollowness that is possible with the spinning equipment described herein. Experimental Equipment for Compression Tests. For many end uses, nonwoven structures must be able to both resist compression and recover after compres-

sion. An Association of the Nonwoven Fabrics Industry (INDA) standard test [IST 120.4 (95); see ref 14] is available for determining compression and recovery. This test, which is a pending American Society for Testing and Materials test method, uses weights and plates to determine product properties (see Figure 16 for a diagram of the apparatus). In the INDA test, the nonwoven specimen is 200 × 200 mm, and a 7.26 kg

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For all disks, the final disk had a minimum thickness of 25 mm with the cover plate installed. With a polypropylene density of 0.895 g/cm3, the 9.0 g standard weight corresponds to a fiber volume of 10.05 cm3. To compare solid filaments with hollow filaments, this fiber volume was kept constant for all disk samples. Thus, the weight of hollow fiber in a nonwoven disk sample was

wh ) [(OD2 - ID2)/OD2]ws

Figure 16. Compression-recovery apparatus.

weight is used. Because our spinning system is of small capacity, for our tests, both the specimen size and the weight were scaled back to 25% of the INDA value. However, this scaling kept the pressure on the sample the same (1825 Pa). Thus, the results of our tests and the INDA tests are comparable. Our apparatus consisted of the following pieces: (a) The base plates were 150 × 150 × 6.4 mm (5.9 × 5.9 × 0.25 in.) plywood plates covered with aluminum foil. (b) The cover plates were 150 × 150 × 6.4 mm (5.9 × 5.9 × 0.25 in.) plywood plates covered with aluminum foil. The weight of these plates was 106 ( 3 g (0.23 ( 0.007 lb). (c) The weights were 1.80 kg (3.96 lb) stainless steel cylinders with a 9.00 cm (3.54 in.) diameter and a height of 3.61 cm (1.42 in.). (d) The rulers were 10 cm (4 in.) long and were placed at the midpoints of the sides of each base and cover plate combination (i.e., there were four rulers per test assembly). Four separate test assemblies (each similar to that shown in Figure 16) were constructed. These four assemblies permitted simultaneous testing of four samples. Because a single specimen test takes 25 h, this capability was quite useful for reducing experimental time. Solid and hollow fibers were produced using the spinning apparatus described previously (see Figure 5). The fibers were collected by placing a hand-held metal screen below the venturi. This screen was constructed of 0.2 mm diameter aluminum wire; the screen was 6 × 8 mesh/cm (48 holes/cm2). Fiber samples were collected for about 15 s while the screen was moved below the venturi. A number of these samples were carefully stacked on a 22 × 22 cm cutting board to produce a fiber mat (the number of samples used depended on the fiber hollowness, etc.). Next, 114 mm (4.49 in.) diameter disks were cut from the mat with an X-ACTO knife. An aluminum tube was used as a cutting guide. This tube had an ID of 102 mm, an OD of 114 mm, and a length of 54 mm, and the tube had a sharpened lip on the end that was placed against the mat. A standard weight of 9.0 ( 0.1 g was used for all compression tests on disks of solid filaments. Cut disks were stacked upon each other to produce this desired weight. The stacking took place on an electronic balance.

(8)

where wh ) weight of hollow fiber and ws ) weight of solid fiber ) 9.0 g. By keeping the fiber volume constant for the disk samples, the adverse effects (if any) of the presence of a hollow core could be quantified. The disk samples were placed in the compression test apparatus, and a procedure very similar to that described in IST 120.4 (95) was followed. At the prescribed times, the height of the web was determined from the average of four measurements taken at the midpoint of each side of the cover plate. The heights were taken at the following times and conditions: A. Initial height with cover plate on sample but no additional weight. B. Height with weight added. C. Height with weight added after 10 min. D. Height with weight removed after 10 min. E. Height with weight removed after 20 min. F. Height with weight added after 20 min. G. Height with weight added after 30 min. H. Height with weight removed after 30 min. I. Height with weight removed after 40 min. J. Height with weight added after 40 min. K. Height with weight added after 50 min. L. Height with weight removed after 50 min. M. Height with weight removed after 60 min. N. Height with weight removed after 8 h. O. Height with weight added after 8 h. P. Height with weight added after 24 h. Q. Height with weight removed after 24 h. R. Height with weight removed after 25 h. As given in IST 120.4 (95), compression and recovery parameters are defined as follows:

% compression resistance ) C/A × 100%

(9)

% elastic loss ) {(A - E)/A} × 100%

(10)

% immediate recovery ) N/E × 100%

(11)

% long-term recovery ) R/E × 100%

(12)

The letters in the above equations refer to the heights taken during the alphabetized steps of the aforementioned procedure. The test IST 120.4 (95) is suitable for highloft nonwovens. Highloft nonwovens have a low-density fiber structure with (a) no more than 10% solids by volume and (b) a thickness greater than 3 mm. For both the solid and the hollow fibers, the disk samples that we prepared had thicknesses of at least 25 mm, so criterion b was satisfied. To check if criterion a was satisfied, the volume density of the polymer was measured for disk samples with all fiber diameters and types (solid or hollow) that were included in our experimental program. The results of these measurements are shown in Tables 3 and 4. (For all thickness measurements and density measurements, the cover plate, but not the weight, was

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 183 Table 3. Percent Solids in the Nonwoven Samples Containing Solid Filaments fiber diameter (µm)

Vsolid/Vsamplea (%)

fiber diameter (µm)

Vsolid/Vsamplea (%)

17.0 21.0 24.5

3.07 3.07 2.73

32.0 39.0 48.0

3.07 3.07 3.40

aV solid ) volume occupied by polymer. Vsample ) volume occupied by nonwoven sample.

Table 4. Percent Solids in the Nonwoven Samples Containing Hollow Filamentsa OD (µm)

hollowness (%)

Vfiber/Vsample (%)

Vsolid/Vsample (%)

32 32 32 40 40 40 40 47 47 47 47 47

0 25 35.6 0 27 34 40 0 28 46 51 66

3.07 2.29 2.00 3.07 2.73 2.46 2.29 3.40 3.07 3.93 2.40 2.66

3.07 1.72 1.22 3.07 2.00 1.61 1.37 3.40 2.21 2.09 1.20 0.98

Figure 18. Compression resistance and elastic loss for solid fibers.

aV solid ) volume occupied by polymer. Vsample ) volume occupied by nonwoven sample. Vfiber ) volume occupied by fiber (i.e., polymer plus hollow core).

Figure 19. Immediate and long-term recovery for solid fibers.

Figure 17. Compression-recovery graph for solid filament, D ) 39 µm (sample weight ) 9 g), and hollow fiber, OD ) 37.75 µm and ID ) 20.75 µm (sample weight ) 6.28 g).

placed on the nonwoven.) Because the tables show a density range of 0.5-4%, all disk samples had densities well below the maximum of 10%. In particular, the hollow filaments had very low volume densities. Compression Tests. Fibers were spun with the hollow fiber spinneret 2 (see Table 1), and the fibers were collected on a screen below the air venturi (see Figure 5). Then, nonwoven disks were prepared as has just been described. Figure 17 shows typical results from two compression-recovery tests. The solid line represents the height of a disk sample from a 39 µm diameter solid fiber, whereas the dashed line represents the height of a disk sample from a hollow fiber with 38 µm OD and 21 µm ID. Both samples had an original thickness of about 32 mm. The letters on the solid line correspond to the height measurements (for the solid fiber) in the experimental procedure. The results indicate that compression performance is similar for the solid versus the hollow fibers (interestingly, for these

data, the hollow fibers are slightly more compression resistant than the hollow filaments). Figure 18 shows the compression resistance and elastic loss for solid fibers with diameters ranging from 17 to 48.5 µm. Figure 19 shows immediate recovery and long-term recovery for the same fibers. To estimate the reproducibility of the experimental measurements, eight replicate runs were made on samples of 32 µm fiber. From these tests, the average values and standard deviations were as follows: compression resistance, 39.16 ( 3.07; elastic loss, 35.18 ( 6.29; immediate recovery, 90.25 ( 3.97; long-term recovery, 70.01 ( 4.09. The error bars in Figures 18 and 19 are based on these standard deviations. From Figure 18, it appears that there is a slight loss of compression resistance as the diameter increases. Also, the elastic loss slightly increases. Perhaps the smaller fibers can integrate (function) together better than the larger fibers. In contrast, from Figure 19, it appears that the immediate recovery is slightly better as the fiber size increases. Long-term recovery also appears to slightly increase as the diameter increases. Now, immediate and long-term recovery involve measurements taken later on in the compression-recovery test (see eqs 9-12). Hence, the mat is more “tamped down” during these later measurements, and this may explain why the solid fiber diameter increase has a different effect for immediate and long-term recovery. In any case, the four resistance and recovery parameters do not change all that much over the range of diameters that we tested.

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Figure 20. Effect of hollowness on the compression resistance and the elastic loss.

Figure 22. Cross-sectional view of an opacity measurement device.

Figure 21. Effect of hollowness on the immediate and long-term recovery.

Figure 20 shows compression resistance and elastic loss for hollow fibers as a function of hollowness (which was previously defined as [ID/OD]2). Hollow fibers with three different OD values were used to produce this graph, and solid fiber data are also included along the ordinate. It appears that there is a small decrease in compression resistance and a small increase in elastic loss as hollowness increases. Perhaps it is most interesting that there is in fact so little change, because a substantial part of the hollow fibers is air (the fibers with the greatest hollowness are 63% air). Figure 21 shows immediate recovery and long-term recovery for hollow fibers. It appears that neither parameter shows any significant change as hollowness increases. Both Figures 20 and 21 imply that hollowness has little effect on compression-recovery properties. Thus, hollow fibers can be used to replace solid fibers in end uses where compression-recovery properties must be maintained. Experimental Equipment for Opacity Tests. The cover of a fabric represents the fabric’s ability to obscure or hide (see Hathorne15). We developed an opacity meter to measure the cover of our nonwoven fabrics. Of interest to us is whether hollow fibers have as much opacity (cover) as solid fibers. The body of our opacimeter was constructed from a cardboard mailing tube with a 5.5 cm OD and a 5.0 cm ID (see Figure 22). The upper section was 10.0 cm long, and the lower section was also 10.0 cm long. A 150 W

Nikon MKII fiber-optic light provided the illumination. The end of the flexible fiber-optic tube was placed at the top of the upper section of the opacimeter body. The top and bottom sections of the body were separated by a 2 cm gap. The nonwoven samples were placed on a clear polypropylene sheet (a transparency sheet), and then the samples were inserted into the gap. Only the sample’s center (a 5.0 cm diameter circle) was illuminated during the measurements. However, to avoid edge effects, each nonwoven sample had substantially more area than the area of the illuminated center. A photocell was placed at the base of the lower tube section. The photocell had a diameter of 4.2 cm and was 0.5 cm thick. The photocell was part of a complete photometer (LX-101 lux meter from Lutron, Coopersburg, PA). The photometer had three ranges: 0-1999, 2000-19 999, and 20 000-50 000 lux. The second range was used in our studies. The photometer supported the lower section of the opacimeter tube, whereas a ring stand and clamp held the upper section. When using the opacimeter, a measurement of the intensity of light (I0) was done with no sample inserted (only the polypropylene sheet). The intensity of the light source was adjusted until I0 equaled 8000 lux. A nonwoven sample was then placed on the polypropylene sheet, and the intensity of light (I) that went through the sample was measured. The illuminated portion of the sample (a 5 cm diameter portion) was then cut out from the rest of the sample by using an X-ACTO knife. A sharpened stainless steel tube (5.0 cm OD × 4.7 cm ID) was used as a cutting guide. Sample weights ranged from 0.01 to 1 g, and the sample weight was determined to within (0.1 mg. After taking the opacity measurements, a small quantity of fibers was removed from the web. These fibers were microtomed to allow crosssectional observation, and the fiber’s OD and ID were determined with a micrometer eyepiece on a Nikon microscope at 100× magnification. Opacity Tests. Solid Fibers. From the basic principles of light transmission and absorption,16 transmit-

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Figure 23. Effect of weight per area on the transmittance for solid fibers.

Figure 25. Effect of the projected area on the absorbance for solid fibers.

The lines in Figure 24 are least-squares fits of eq 13 to the data for the different diameter ranges. Interestingly, K ≈ 1 for all of the diameter ranges. As an alternative way of quantifying concentration, let us use the projected area Ps, which is defined as

Ps ) L × OD

Figure 24. Effect of weight per area on the absorbance for solid fibers.

tance T is defined as T ) I/I0, whereas absorbance A is defined as A ) -log(I/I0). Figure 23 shows the effect of fabric weight per area on transmittance for solid fibers. A range of fiber sizes from 15 to 80 µm in diameter is included in the figure. As expected, transmittance decreases as the weight per area increases. Figure 24 shows the effect of fabric weight per area on absorbance for the same fibers. Obviously, because transmittance increases as the weight per area increases, then absorbance decreases. Both figures show that the diameter influences the transmittance and absorbance. (Rigorously speaking, “absorption” describes only chemical absorption. However, in this paper, absorption also includes a complex process of reflection and refraction of light.) Figure 24 shows that, for a small weight per area, the absorbance can be approximated as linear (constant slope) with respect to the weight per area. However, the slope decreases at larger values of weight per area. This behavior is analogous to what happens when the BeerLambert law is applied to solutions of high concentration.16 The data of Figure 24 were empirically fit with the following equation:

A ) K[(ws/s)/(ws/s + K′)]

(13)

where ws ) weight of the solid fiber, s ) sample surface area, and K and K′ ) empirical constants.

(14)

where L ) total length of the fiber in the sample and OD ) outer fiber diameter. If fiber crossovers are neglected, the projected area Ps is a good quantitative measure of the fraction of a sheet which “covers” (obscures or hides). Figure 25 shows the data of Figure 24 replotted with the projected area on the abscissa. As can be seen, all of the data follow the same trend: the use of the projected area has minimized the effect of the fiber diameter. (The projected area Ps is the fiber area contained in a disk 5.0 cm in diameter.) Analogous to eq 13, the following equation can be used to fit the data of Figure 25:

A ) k[Ps/(Ps + k′)]

(15)

where k and k′ ) empirical constants. The lines in Figure 25 are least-squares fits of the data to eq 15. The data and the model fits are much more clustered together in Figure 25 than in Figure 24. Hence, the projected area is a better parameter to use than the weight per area. Hollow Fibers. Hollow fibers of various ODs and IDs were produced with spinneret 1. Our opacity meter was used to study mats of these fibers. Figure 26 shows the effect of weight per area on absorbance for hollow fibers of various inner diameters. The effect of the OD is the same as that shown in Figure 24 for solid fibers: a smaller OD produces a higher absorbance. Figure 27 is a plot of absorbance as a function of the projected area for several ranges of fiber IDs. As was the case for correlating solid fiber data, the projected area was selected as a better parameter to use than the weight per area. As was also done for the solid fiber data, eq 19 was used to fit the various data sets in Figure 27. The figure illustrates that absorbance is not affected by the presence of a hole in the fiber. Fiber ODs of 40-50 µm are included in Figure 27. Figures 28 and 29 consider respectively fiber ODs of 5060 and 60-70 µm. For Figure 28, the midrange IDs (24-

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Figure 26. Effect of weight per area on the absorbance for hollow fibers (with various inner diameters).

Figure 27. Effect of the total projected area on the absorbance for OD ) 40-50 µm.

Figure 29. Effect of the total projected area on the absorbance for OD ) 60-70 µm.

Figure 30. Effect of the total projected area on the absorbance for hollow and solid fibers.

(Figure 30 includes all of the data from Figures 27-29, plus additional data for fiber sizes beyond the range of Figures 27-29.) Although there is some scatter, the hollow fiber data can be approximated by a single empirical curve. Also included in Figure 30 are data for solid fibers with about the same OD range as the hollow fibers. These data can also be approximated by a single empirical curve. What is most interesting is that the two empirical curves are nearly coincident. In other words, the presence of a hole in a fiber does not affect its opacity. So, as far as opacity is concerned, less polymer is required to achieve the same function. Conclusions Figure 28. Effect of the total projected area on the absorbance for OD ) 50-60 µm.

29 µm) show absorbance values which are a little bit higher than the values for the other IDs. For Figure 29, the absorbance values are a little bit low for the largest ID range (45-55 µm). The k and k′ values are similar for Figures 27-29. In Figures 27-29, any particular data subset (a specific range of IDs and ODs) does not appear to diverge to any great extent from the overall data trend. To illustrate this more clearly, Figure 30 is an overall plot of absorbance versus projected area for hollow fibers with IDs from 20 to 78 µm and ODs from 35 to 105 µm.

1. In the production of polypropylene hollow fibers from the melt, the polymer and gas continuity equations can be used to predict the ID and OD of the product fiber. 2. The compression-recovery behavior of hollow polypropylene fibers is quite similar to that of solid polypropylene fibers. 3. The opacity behavior of hollow polypropylene fibers is nearly identical to that of solid polypropylene fibers. 4. For many nonwoven uses, hollow polypropylene fibers can be substituted for solid polypropylene fibers. When this substitution is made, the polymer cost savings can be substantial.

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Acknowledgment The authors are most grateful for the financial support provided by 3M, Du Pont, and Conoco. Nomenclature A ) absorbance d1 ) filament inside diameter, µm d2 ) filament outside diameter, µm h ) fiber hollowness I ) light intensity with the sample present, lux I0 ) light intensity without the sample, lux ID ) final product inside diameter, µm k ) empirical constant in eq 19 k′ ) empirical constant in eq 19, cm2 K ) empirical constant in eqs 16 and 17, cm2/g K′ ) empirical constant in eq 17, g/cm2 L ) fiber length, m mn ) nitrogen mass rate, g/min mp ) polymer mass rate, g/min OD ) final product outer diameter, µm Ps ) projected area of the solid filament, cm2 ∆P ) pressure difference between the two phases, Pa R1 and R2 ) radii of curvature of the interface between the two phases, m s ) sample surface area, cm2 T ) transmittance vf ) fiber velocity, m/s wh ) weight of the hollow fiber, g ws ) weight of the solid fiber, g Greek Symbols γ ) surface energy, N/m Fp ) polymer density, g/cm3 Fn ) nitrogen density, g/cm3 Subscripts f ) fiber h ) hollow n ) nitrogen p ) polymer s ) solid

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(3) Baum, B.; Holley, W.; White, R. A. Hollow Fibres in Reverse Osmosis, Dialysis, and Ultrafiltration. In Membrane Separation Processes; Meares, P., Ed.; Elsevier: New York, 1976; pp 187227. (4) Li, S.-G.; Koops, G. H.; Mulder, M. H. V.; van den Boomgaard, T.; Smolders, C. A. Wet Spinning of Integrally Skinned Hollow Fiber Membranes by a Modified Dual-bath Coagulation Method using a Triple Orifice Spinneret. J. Membr. Sci. 1994, 94, 329-340. (5) Kim, J. H.; Park, Y. I.; Jegal, J.; Lee, K. H. The Effects of Spinning Conditions on the Structure Formation and the Dimension of the Hollow Fiber Membranes and Their Relationship with the Permeability in Dry-Wet Spinning Technology. J. Appl. Polym. Sci. 1995, 57, 1637-1644. (6) Doi, S.; Hamanaka, K. Pore Size Control Technique in the Spinning of Polysulfone Hollow Fiber Ultrafiltration Membranes. Desalination 1991, 80, 167-180. (7) Miao, X.; Sourirajan, S.; Lau, W. W. Y. Production of Polyethersulfone Hollow Fiber Ultrafiltration Membranes, II. Effects of Fiber Extrusion Pressure (EP) and PVP Concentration in the Spinning Solution. Sep. Sci. Technol. 1996, 30 (3), 349363. (8) Kesting, R. E. Synthetic Polymeric Membranes: a Structural Prospective; Wiley-Interscience: New York, 1985; pp 10-15. (9) Kim, J. J.; Hwang, J. R.; Kim, U. Y.; Kim S. S. Operation Parameters of Melt Spinning of Polypropylene Hollow Fiber Membranes. J. Membr. Sci. 1995, 108, 25-36. (10) Kim, J. J.; Jang, T. S.; Kwon, Y. D.; Kim U. Y.; Kim, S. S. Structural Study of Microporous Polypropylene Hollow Fiber Membranes Made by the Melt-Spinning and Cold-Stretching Method. J. Membr. Sci. 1994, 93, 209-215. (11) Shambaugh, R. L. A Macroscopic View of the Melt Blowing Process for Producing Microfibers. Ind. Eng. Chem. Res. 1988, 27, 2363. (12) Adamson, A. W. Physical Chemistry of Surfaces, 2nd ed.; Interscience: New York, 1967; pp 3-6. (13) Bicerano, J. Prediction of Polymer Properties, 2nd ed.; Marcel Dekker: New York, 1996; p 195. (14) Standard Method for Determination of Compression and Recovery of Highloft Nonwoven Fabrics at Room-Temperature Using Weights and Plates; INDA IST 120.4 (95); Association of the Nonwoven Fabrics Industry: Cary, NC, 1995. (15) Hathorne, B. L. Woven Stretch and Textured Fibers; Interscience: New York, 1964; p 57. (16) Meyer-Arendt, J. R. Introduction to Classical and Modern Optics, 2nd ed.; Prentice-Hall: New York, 1984; p 481.

Received for review May 18, 2000 Revised manuscript received August 29, 2000 Accepted September 12, 2000 IE000503D