The Impact of Input Energy On the Performance Of Hydroentangled Nonwoven Fabrics

Transcription

1 ORIGINAL PAPER/PEER-REVIEWED The Impact of Input Energy On the Performance Of Hydroentangled Nonwoven Fabrics By Huabing Zheng, Abdelfattah M. Seyam, and Donald Shiffler, Nonwovens Cooperative Research Center, College of Textiles, NC State University, Raleigh, NC, US ABSTRACT The main goal of this study is to investigate the performance behavior of hydroentangled nonwoven fabrics in terms of input water jet energy and fabric structural parameters (basis weight and fiber orientation). The input jet energy was varied by altering jet pressure, number of passes, process speed and fabric basis weight. To achieve the goal, two sets of trials, which had different jet pressures, process speeds, basis weights and number of passes, were executed to reveal the significance of these parameters in achieving high fabric tensile strength. Image analysis was used to determine the fiber orientation of the hydroentangled nonwoven fabrics to aid in understanding fabric performance behavior. KEY WORDS Hydroentanglement, Fiber Orientation Distribution Function (ODF), Specific Energy, Jet Pressure, Tensile Strength INTRODUCTION Hydroentanglement is a process of entangling a web of loose fibers on a porous forming surface (endless belt or perforated drum) to form a sheet structure nonwoven fabric by subjecting the fibers to multiple rows of fine jets of highly pressurized water. The fibers are pushed down by the water jets from the top of the cross-over points (knuckles) to the interstices of the forming wire, conformed to the forming wire, and these results in the rearrangement and intermingling of fibers. It is basically an energy transfer process that strengthens the array of loose fibers, imparting the desired physical, mechanical, texture and aesthetics properties. Hydroentangled fabrics rely primarily on fiber-to-fiber friction and fiber bending properties to achieve physical integrity and are characterized by relatively high strength, softness, drape and conformability. Many factors may affect the ease of fiber entanglement. The level of specific energy is a major parameter to the properties of the final product. Also, the process requires some special fiber properties that fibers can bend easily around small radii while possessing some degree of mobility. The important properties include those related to polymer composition and fiber properties, such as bending modulus, linear density, fiber length, fiber wettability, fiber cross-section shape [2] and fiber-to-fiber friction. The forming surface provides several functions: (1) support of the fiberweb, (2) increasing entangling efficiency, (3) creating desired texture (non-aperture or aperture, for example). The constituents and the geometry of the forming surface, which is usually fine-mesh stainless steel, bronze, or polymeric screen, must be fine enough to minimize fiber losses and maximize water reflection back into the web to increase entanglement while, at the same time, be open enough to adequately drain the water from the jets, and provide the required fabric aesthetics. Various steps are important in the hydroentanglement process. The steps for hydroentangled nonwoven fabrics include fiber selection, web formation, web entanglement, fabric drying and optional finishing treatments. While some of these steps are typical in a nonwoven process, some of them are unique to the process of hydroentanglement. Fiber selection is important to satisfy certain end use due to the role of fiber properties in fabric formation and the fabric structure which determine the final fabric properties [2, 7]. 34 INJ Summer 2003

2 Various web formation systems produce different fiber orientation. Cross-lapped web are characterized by their bimodal fiber orientation (expressed in orientation distribution function or ODF) with significant fiber percentage oriented in the cross machine direction (or CD) and low orientation in machine direction (or MD). The two domain angles can be controlled by the carding speed and the crosslapper conveyor belt speed. The ODF of the final hydroentangled fabric is, however, the deciding factor in determining the fabric performance. The basis weight also has relationship with specific energy needed to form hydroentangled fabrics. The step of web entanglement is the major process cost because of energy consumption in providing highly pressurized water jets. To achieve cost reduction, the energy transfer from the high pressure water jet to the fabric need be understood and such transfer maximized. This includes: good jet orifice design forming uniform jet flow with minimum friction loss, optimization of the energy distribution, and tailoring of jet orifice and forming wire to the fabric application [5]. The principle of bonding fibrous webs with water energy was established back in 1968 by researchers at Du Pont [4]. Since then, the hydroentanglement process has been a rapidly growing segment of the nonwoven technology complex because of its ability to achieve excellent fabric properties at high processing speeds. While there are many articles published in trade journals that describe the process and products of hydroentanglement, very few articles have been published in the scientific literature [1-6, 8-10] in the area of the impact of processing and fiber parameters and their interactions on the aesthetics and performance of the spunlace fabrics. This may be attributed to the fact that spunlace fabric producers consider the information highly proprietary. OBJECTIVE The objective of this paper is to study the performance behavior of hydroentangled fabrics in terms of input water jet energy and fabric structural parameters. The input jet energy was varied by altering jet pressure, number of passes, process speed and fabric basis weight. EXPERIMENTAL Material and Fiberweb Formation Polyester fibers of fineness of 1.63 dtex and 38 mm length were provided in bale form. The fibers were processed in the carding and crosslapping line at the College of Textiles, NC State University. This line is comprised of an opener, garnet, roller-top card, flat-top card, crosslapper, tacking, and fiberweb take-up. The fibers were passed through the opener, the flat-top card, tacking, and finally to the take-up to form the required fiberwebs. Two fiberwebs with required quantities of nominal weights of 50 g/m 2 and 100 g/m 2 were produced. A SIMPLE SKETCH OF A HONEYCOMB HYDROENTANGLING SYSTEM Filter Tank City Water Fine filter Main Pump Air Exhaust Vacuum Pump Circulation Water Jets Max pressure = 1500 psi (100 bar) Suction Water Water Tank Belt Speed: feet/min. (0-35 m/min) Drain 35 INJ Summer 2003

3 Table 1 CONSTANT VARIABLES Variable Value Jet Density, jets/cm 15.8 Jet Diameter, mm Discharge Coefficient 0.7 Number of Manifolds 3 Mesh of Forming Screen 100 Table 2 INDEPENDENT VARIABLES Variable Trial 1 Trial 2 Fiberweb Weight, g/m Process Speed, m/min Jet Pressure, bar 27.6, 27.6, 55.2, , 82.7 * First manifold pressure was fixed at 13.8 bar for the first pass. Table 3 PRESSURE LEVELS AND NUMBER OF PASSES OF TRIAL 1 Fabric Passes Pressure, bar Manifold 1 Manifold 2 Manifold three manifolds (see sketch on previous page). Fabric samples were produced using different energy levels by varying manifold pressure, number of passes, process speed and fiberweb basis weight. The pressure of the first manifold was kept constant at 13.8 bar for the first pass only to prewet the samples. The fiberweb was placed manually on the forming wire. After each pass the sample was removed manually and then the sample was fed again with its side reversed. This was repeated until the required number of passes is achieved. The hydroentanglement unit has several basic elements: Hydraulic Entanglement Stand: This stand provides support to transport the web under the three entanglement headers so as to expose the web to a series of high pressure water jets that will entangle the fibers of web. Water and air are transported from this stand to the air-water separator system. High Pressure Water Pumping System: This regulates the volume and pressure of the water delivered to the hydraulic entanglement stand. Air-Water Separator System: This separates the water from the air so that the water may be reused for closed loop operation or sent to drain for open loop operation. Water Storage System: This system supplies water in open loop operation that is employed in our study. Water Filtration System: This system filters unwanted particles from the city water. Experimental Design Two sets of trials (referred at as Trial 1 and Trial 2 throughout the paper) were performed to achieve the purpose of the study. The variables that were kept constant are summarized in Table 1 and the independent variables investigated are summarized in Table 2. As shown in Table 2, the measured basis weight of the carded/crosslapped fiberwebs of Trials 1 and 2 are different from the nominal. Tables 3 and 4 show the levels of pressures for each manifold and pass of Trials 1 and 2 respectively. As mentioned before, the pressure of the first manifold was kept constant at 13.8 bar for the first pass. Fabric sample 1 (Table 3), for example, was produced using two passes. The pressures of the manifolds were 13.8 bar,, and respectively in the first pass. The pressures of the three manifolds were,, and in the second pass. Fabric sample 7 (Table 3) was processed using 6 passes. The pressures of the manifolds were 13.8 bar,, and respectively in the first pass. The pressures of the three manifolds were,, and in the passes 2-6. Hydroentanglement The fiberwebs were entangled using a 50.8-cm (20-inch) wide honeycomb model hydroentanglement machine with Testing and Evaluation Fabric properties reported here are tensile strength, basis weight and fiber orientation. The fabric tensile and basis 36 INJ Summer 2003

4 Table 4 PRESSURE AND NUMBER OF PASSES OF TRIAL 2 Fabric Passes Pressure, bar Manifold 1 Manifold 2 Manifold weight testing and evaluation were performed according to ASTM. Tensile testing was performed on an MTS 30/G tensile tester using the ASTM D (strip method). Five samples in the machine direction and five samples in the cross direction were tested. Fabric basis weight was determined using the ASTM D Five samples were tested. NCRC image capturing and processing (software and hardware) systems were used to measure fiber orientation of the carded/crosslapped fiberwebs and hydroentangled fabrics [8]. RESULTS AND DISCUSSION The objective of trials 1 and 2 was to determine whether energy delivered or jet force is more important in developing fabric properties. Additionally, the trials were designed to reveal the nature of the specific energy/fabric performance relationship when varying basis weight and process speed. The two trials were designed so that the throughput or mass per unit time (process speed multiplied by basis weight) is about constant. As shown in Table 2 and 3, three different jet pressures (,, and ) and four different numbers of passes (2, 4, 6 and 8) were used to process the hydroentangled fabrics of trial 1. The results are shown in Figure 1 (a)-(f). The mean tensile strength shown in Figure 1 (d) is the average of tensile strength in MD and CD. The specific energies (x-axis) were calculated using the formula shown in the Appendix. The fabric basis weight {Figure 1 (a)} decreases with increasing specific energy and number of passes because of fabric stretching caused by water jets impact and peeling the fabric from the forming wire after each pass. The change of basis weight is significant so normalized tensile strength was calculated and reported. The tensile strength results of Figure 1 (b)-(d) show that tensile strength increases with specific energy until a critical energy (threshold energy). After reaching the threshold energy, increasing energy has no significant effect on hydroentangled fabric tensile strength. With the exception of one data point, the elongation at peak load in MD and CD {Figure 1 (e) and 1(f)} decreases slightly with increasing specific energy. Additionally, the results of Figure 1 show that the jet pressure has no or little effect on the hydroentangled fabric tensile strength and elongation. The results of trial 2 are shown in Figure 2. Figure 2 (a) shows that fabric basis weight decreases with increasing specific energy and hence normalized tensile strength was used to nullify the effect of fabric weight. The weights reported in Table 2 were used as the normalized weights. Tensile strength increases in MD and decreases in CD with specific energy for all jet pressure levels used as it can be seen from Figures 2 (b) and 2 (c). For fabrics produced at jet pressure of and, the mean tensile strengths show clearly threshold energies {Figure 2 (d)}. For fabrics processed at, however, specific energy shows little effect on the mean tensile strength and it is almost constant at all levels of specific energy {Figure 2 (d)} indication of reaching the threshold at low specific energy. The elongation at peak load decreases in MD and increases in CD with specific energy at all levels of jet pressures {Figure 2 (e) and (f)}. Contrary to the results of Figure 1, the results of Figure 2 indicate that both specific energy and jet pressure play an important role in deciding fabric tensile strength and elongation when dealing with lighter basis weight. 37 INJ Summer 2003

5 Fabric Basis weight (g/m 2 ) Tensile Strength in MD (N/cm) Figure 1 (a) Fabric Weight Figure 1 (b) Tensile Strength in MD Tensile Strength in CD (N/cm) Mean Tensile strength (N/cm) Figure 1 (c) Tensile Strength in CD Figure 1 (d) Mean Tensile Strength Strain at Peak Load in MD (%) Strain at Peak Load in CD (%) Figure 1 (e) Elongation at Peak Load in MD Figure 1 (f) Elongation at Peak Load in CD Figure 1 EFFECT OF JET PRESSURE AND SPECIFIC ENERGY ON FABRIC PROPERTIES OF FABRICS OF NORMALIZED WEIGHT OF 99.6 g/m 2 Results of Figures 1 and 2 show that: (i) lighter fabrics tensile strength is reduced with jet pressure while heavier fabrics tensile strength did not get affected by jet pressure and (ii) the MD tensile strength is higher than the CD tensile strength. The tensile behavior of fabrics in MD and CD with jet pressure and specific energy indicates that there must be changes in the 38 INJ Spring 2003

6 Figure 2 EFFECT OF JET PRESSURE AND SPECIFIC ENERGY ON FABRIC PROPERTIES FABRICS OF NORMALIZED WEIGHT OF 56.9 G/M 2 39 INJ Summer 2003

7 (a), 2 passes, 56.9g/m 2 (SE: 1.77x103kJ/kg) (b), 12 passes, 56.9g/m 2 (SE: 11.66x103kJ/kg) (c), 2 passes, 56.9g/ m 2 (SE: 1.98x103kJ/kg) (d), 6 passes, 56.9g/m 2 (SE: 15.98x103kJ/kg) e), 2 passes, 56.9g/ m 2 (SE: 3.54x103kJ/kg) (f), 8 passes, 56.9g/m 2 (SE: 39.53x103kJ/kg) (g), 2 passes, 99.6g/ m 2 (SE:9.92x103kJ/kg) fiber orientation and the structure of the resultant hydroentangled fabrics. To find out whether such change actually took place, fiber orientation was evaluated using image analysis to determine the ODF (Fiber Orientation Distribution Function) of cross-lapped fiberwebs and hydroentangled fabrics. Figures 3 (a)-(h) show the images of hydroentangled fabrics processed at different jet pressures and specific energies. The effect of jet pressure on the texture of the hydroentangled fabric at constant number of passes (two passes) can be noticed from Figures 3 (a), (c) and (e). At jet pressure the fabric exhibits only small holes. At jet pressure the texture of the fabric shows clear streaks and medium size holes. The jet pressure caused the fabric to form the largest size round holes and a clear streak pattern. This explains why high jet pressure lowered the tensile strength of lighter fabrics {Figure 2 (b) and (d)}. In such fabrics there are fewer fibers in the fabric cross-section and the high jet pressure caused the fibers to be displaced apart and form larger holes or weak spots. The fiber displacement was not noticed at high pressure when dealing with heavier fabrics {Figure 3 (g)} due to the high number of fibers in the fabric cross-section thus weak spots were not formed. This is the reason why the tensile strength of heavier fabrics is not influenced by jet pressure {Figure 1 (b)-(d)}. The effect of the number of passes on fabric structure at low jet pressure of can be seen by comparing Figures 3 (a) (h), 8 passes, 99.6g/m 2 (SE: 45.15x103kJ/kg) Figure 3 IMAGES OF FABRICS WITH DIFFERENT PRESSURE, PASSES, AND BASIS WEIGHT 40 INJ Summer 2003

8 Frequency (%) Orientation Angle (degree) Figure 4 ODF OF WEB Left Central Right and (b). Increasing the number of passes at such low pressure caused the formation of large diameter holes in the fabric. The number of passes at jet pressure did not show similar effect {Figures 3 (c) and (d)}. The clear streaks of the fabric of Figure 3 (c) disappeared and hole size is reduced by increasing the number of passes to 6 {Figure 3 (d)}. For the fabrics processed at jet pressure of, the streaks did not disappear when the number of passes is increased. Additionally, the texture was changed from round-shaped holes to ellipseshape with the major diameter in the machine direction. Figure 4 shows the ODF of the fiberweb. The bi-modal fiber orientation distribution of the crosslapped web is obvious. Figures 5 (a)-(d) show the ODF of hydroentangled fabrics processed at different jet pressures and specific energies. The results of Figures 5 (a) and (b) indicate that the fiber orientation increases in MD with increasing the number of passes. Although fabrics processed with different jet pressures have Figure 5 ODF OF FABRICS WITH DIFFERENT PRESSURES AND NUMBER OF PASSES 41 INJ Summer 2003

9 Normalized Mean Tensile Strength (N/cm) 26.6 bar 99.6 gsm 56.9 gsm Normalized Mean Tensile Strength (N/cm) 99.6 gsm 56.9 gsm (a) Mean Tensile Strength () (b) Mean Tensile Strength () Normalized Mean Tensile Strength (N/cm) 99.6 gsm 56.9 gsm MD/CD Ratio of Tensile Strength 99.6 gsm, 4.57 m/min 56.9 gsm, 9.14 m/min (c) Mean Tensile Strength () (d) MD/CD Ratio of Tensile Strength () gsm MD/CD Ratio of Tensile Strength 99.6 gsm 56.9 gsm MD/CD Ratio of Tensile Strength 56.9 gsm (e) MD/CD Ratio of Tensile Strength (55.2bar) (f) MD/CD Ratio of Tensile Strength () Figure 6 EFFECT OF BASIS WEIGHT AND CONVEYOR SPEED ON MEAN TENSILE STRENGTH AND MD/CD RATIO OF TENSILE STRENGTH different fiber orientation {Figure 5 (c)}, the fiber orientation become quite similar at high number of passes regardless of jet pressure level {Figure 5 (d)}. The results of the ODF explain why the fabric tensile strength in MD is higher than that of the fabric in CD {Figures 2 (b) and (c) and Figures 1 (b) and (c)}. The behavior of fabric elongation at peak load of Figures 2 (e) and (f) can be also explained in terms of the ODF. With increase in fiber orientation in the MD the fab- 42 INJ Summer 2003

10 Load (N) ric strength in MD increases and in the CD decreases. This behavior is reversed for the elongation at peak load. The results of load-strain of Figure 7 support this explanation. As discussed above, the average tensile strength in MD and CD of fabrics processed at jet pressure of did not change significantly with increasing number of passes (specific energy) as it can be seen from Figure 2 (d). It seems that the specific energy of two passes spent to form the fabric was high enough to provide the maximum entanglement for the given experimental parameters employed. Further energy (or passes) did not cause significant change in the degree of entanglement. Additionally, the first two passes caused the fibers to totally conform to the forming surface. It seems that at low jet pressure of, 2 passes were not enough to provide the highest degree of fiber entanglement. This is why further passes under such low pressure causes additional entanglement and produced higher strength fabric than those produced at higher jet pressures. The increase in number of passes causes the tensile strength to increase the degree of entanglement until the threshold is reached. Further increase in number of passes beyond the threshold did not cause any increase in tensile strength of the fabric. The tensile behavior of the fabrics of trial 1 is significantly different from that of trial 2 as it can be seen from Figures 1 (b), (c), and (d) and Figures 2 (b), (c) and (d). The difference is more pronounced for those fabrics processed at high jet pressure of. Fabrics of trial 1 produced at jet pressure of exhibited tensile strength values comparable to those produced at jet pressure of and {Figure 1 (d)}. Fabrics of trial 2 produced at jet pressure of are significantly weaker as compared to those produced at jet pressure of and {Figure 2 (d)}. The reason behind such behavior in tensile can be understood by observing the 43 INJ Summer 2003 Strain (%) images of Figures 3 (e)-(h). It is obvious that the jet pressure of caused the fibers in light weight fabric {Figures 3 (e) and (f)} to disintegrate thus causing large holes and extremely localized light weight areas with few fibers holding the structure together. This is not the case for fabrics of higher basis weight {Figures 3 (g) and (h)}. For fair comparison of tensile strength data of fabrics of trial 1 and trial 2, the values of mean tensile strength were normalized based on the their basis weights and the results are shown in Figure 6 (a), (b) and (c). Fabric basis weight has significant effect on the fabric tensile strength. Further, there is a significant interaction of fabric weight and jet pressure on fabric strength. For the fabrics produced at, fabrics with lower basis weight are stronger than fabrics of higher basis weight {Figure 6 (a)}. This is reversed for fabrics produced at jet pressures of 55.2 bar and {Figure 6 (b) and (c)}. Figures 6 (d), (e), and (f) show the effect of fabric basis weight and specific energy on the MD/CD ratio of tensile strength. The increase in the MD/CD with specific energy is obvious due to increase in fiber orientation in MD with specific energy. 2 passes, MD 2 passes, CD 8 passes, MD 8 passes, CD Figure 7 LOAD-STRAIN CURVES OF FABRICS (82.7 BAR) CONCLUSIONS 1) While crosslapped web has a bi-modal ODF, hydroentangled fabrics have a uni-modal ODF due to fiber rearrangement caused by peeling the fabrics, which caused fabric stretch in MD, after each pass and the jet pressure. Increasing the number of passes caused more fiber orientation in the machine direction. 2) There exists threshold energy, after which, increasing pressure and number of passes can t improve the fabric strength and may damage the fabric strength due to introducing weak links. 3) Basis weight and specific energy affect MD/CD tensile strength ratio. Higher specific energy causes higher MD/CD strength ratio. 4) There is a significant effect of jet pressure and fabric basis weight interaction on fabric strength. Our results indicate that tensile behavior of lighter fabrics is influenced by jet pressure and specific energy. The tensile properties of heavier fabric, however, are significantly affected by specific energy with no or little jet pressure effect. At low jet pressure, fabrics with lower basis weight are stronger than fabrics of higher basis weight. This behavior is reversed for fabrics produced at higher jet pressures. Determination of minimum jet pressure that provides the highest strength for a given basis weight and the critical values of basis weight/jet pressure at which the reverse takes place would be beneficial to industry to minimize the energy cost of producing hydroentangled nonwoven fabrics.

11 ACKNOWLEDGEMENT This work was supported by a grant from the Nonwovens Cooperative Research Center, NC State University. Their generous support of this project is gratefully acknowledged. REFERENCES 1. Berkalp, O.B., Pourdeyhimi, B., and Seyam, A.M., Texture Evolution in Hydroentanglement, International Nonwoven Journal, Vol 12, No. 1, pp 28-35, Bertram, D., Cellulosic Fibers in Hydroentanglement, INDA Journal of Nonwoven Research, Vol 5, No 2, pp 34-41, Connolly, T., and Parent, L., Influence of Specific Energy on the Properties of Hydroentangled Nonwoven Fabrics, Tappi Journal, Vol 76, No 8, pp , Gilmore, T., Timble, N., and Morton, G., Hydroentangled Nonwovens Made from Unbleached Cotton, Tappi Journal, Vol 80, No 3, pp , Ghassemieh, E., Acar, M., and Versteeg, H.K., Improvement of the Efficiency of Energy Transfer in the Hydroentanglement Process, Composites Science and Technology, Vol 61, pp , Hwo, C., and Shiffler, D., Nonwoven from Poly(trimethylene-terephthalate) Staples, The proceedings of the 10th Annual TANDEC Conference, November Morton, W.E. and Hearle, J.W.S., Physical Properties of Textile Fibers, John Wiley, New York, Pourdeyhimi, B., Dent, R., and Davis, H., Measuring Fiber Orientation in Nonwovens, Part III: Fourier Transform, Textile Research Journal, 67, , Timble N, and Gilmore, T, Spunlaced Fabric Performance for Unbleached, Bleached and Low Micronaire Unbleached Cotton at Different Specific Energy Levels of Water, The Proceedings of INDA-TEC, Crystal City, Virginia, September Timble, N., and Allen, C., Hydroentangled Fabric Performance for Polyester/Unbleached Cotton Blends at Various Specific Energy Levels, The Proceedings of INDA- TEC, Crystal City, Virginia, September, APPENDIX Calculation of Input Energy Specific energies required to form the fabrics of both trials were calculated from: Where C = coefficient of discharge, dimensionless d 1 = inlet diameter, mm P g = gauge pressure, bar N = number of jets per inch of manifold Passes = number of passes acted on fabrics W = basis weight, g/m 2 S = line speed, m/min INJ 44 INJ Summer 2003