Theoretical analysis of the spunbond process and its applications for polypropylenes

Transcription

1 Received: 17 April 2017 Accepted: 18 June 2017 DOI: /adv RESEARCH ARTICLE Theoretical analysis of the spunbond process and its applications for polypropylenes Toshitaka Kanai 1 Youhei Kohri 2 Tomoaki Takebe 2 1 KT Polyme, Sodegaura, Chiba, Japan 2 Performance Materials Laboratories, Idemitsu Kosan Co., Ltd. Anesaki, Ichihara, Chiba, Japan Correspondence Toshitaka Kanai, KT Polyme, Sodegaura, Chiba, Japan. toshitaka.kanai@ktpolymer.com Abstract The theoretical analysis of the spunbond process was set up and applied to polypropylene (PP) and its blends. The effect of blending low tacticity polypropylene (LMPP) to high tacticity PP on the spinnability of the spunbond process was investigated. A small amount of LMPP was found to stabilize the high- speed spinning of PP, and very fine fibers were obtained. From the calculated results, it was speculated that the improvement of spinnability originated from the suppression of crystallization speed and strain rate in the spinning process. The physical properties of the spunbond nonwoven fabrics were also investigated. KEYWORDS fiber, low modulus PP, nonwoven fabrics, spunbond, tacticity, theoretical analysis 1 INTRODUCTION The spunbond nonwoven fabrics become very popular and are widely used for disposal diapers and masks. The softness of texture, low weight, and good spinnability are required. In particular, stable spinnability and fine denier at a high- speed condition are prospective. Many companies, which produce the spunbond nonwoven fabrics, set up process conditions and select materials with the help of their experiences without the theoretical prediction. There are many papers which reported the theoretical analyses of melt spinning. [1 12] This process sets up take- up speed which is already known as one of the process conditions, and it is easy to solve. But the take- up speed of spunbond process is unknown and the theoretical analyses of melt spinning cannot apply for the spunbond process. From these reasons, it sometimes takes much time to fix the process conditions when they want to produce new products which require to produce fine denier fabrics and to control physical properties or to use new resins, etc. If many trials using a production line under various process conditions are taken place and then the optimum condition is determined, much time, materials, and cost are required. Furthermore, as it is very difficult to observe the structure formation of fibers during the spunbond process, to clarify the mechanism in order to improve spinnability is also difficult. This research will play an important role to reduce the experimental time and research expenses and to improve the efficiency of trials. To solve these problems and find out the optical process conditions, the theoretical analysis is useful and could help us to know the mechanism of spunbond process. Furthermore, the nonwoven fabrics are sometimes required to be soft and composed of fine fibers. Our previous research found that the blending of a small amount of low tacticity polypropylene (LMPP) [13] to commercial high tacticity polypropylene (PP) could produce very fine fiber without breaking under the high- speed spinning conditions. [14 17] This blend was also applicable for the PP spunbond process and highly improved spinnability. It was surmised that these results came from the retardation of crystallization speed of PP and the prevention of quick deformation during the spinning process by adding LMPP. [16,17] The theoretical analysis during the spunbond process was formulated and applied for the blends of LMPP to PP in order to investigate the deformation behavior during the spunbond process. Adv Polym Technol. 2017;1 10. wileyonlinelibrary.com/journal/adv 2017 Wiley Periodicals, Inc. 1

2 2 KANAI et al 2 THEORETICAL ANALYSIS OF SPUNBOND PROCESS To set up the theoretical equations of the spunbond process shown in Figure 1, several assumptions were made as follows. The cross section of the fiber was round and axisymmetric Heat conduction along the axis was not considered because the spinning speed is very fast The spinning line was treated as being in the steady state The swell was assumed to be constant The elongational viscosity of the polymer is as a function of temperature and crystallinity. The temperature dependence of viscosity is followed by the Arrhenius type and the temperature is balanced between the crystallization heat and the air cooling during the region of crystallization. The crystallization temperature is assumed to be the same as the crystallization temperature obtained by DSC data. After the latent heat ΔH was consumed, the temperature decreases. The temperature distribution along the radius direction was neglected, and the heat transfer was only considered from the surface. The x-axis was taken in the center along the spinning axis. Figure 2 shows the flowchart of spunbond process simulation. Under the above conditions, momentum Equation (1), Schema c view of Spunbond Process Spunbond Melt Spinning Minute elements (Dividing) Die Output Rate W Posi on x 0 =0 Temp. T 0 Diameter R 0 Cooling Air Cooling Air x T D Dx Cabin System Uni-Axial Stretching Collector Thro le Plate L x FIGURE 1 Schematic view of spunbond process Flowchart of Spunbond Process Simulation F uaternionic Simultaneous Equations Runge Kutta Method Solution Spinning Speed Fiber Diameter D Stretching Force at the entrance of Throttle Plate F x=l Boundary Condition Stretching Force at the entrance of Throttle PlateF x=l FIGURE 2 Flowchart of spunbond process simulation Calculation Repeated till coincidence of Stretching Force Newton RaphsonMethod consistent Not consistent

3 KANAI et al 3 continuity Equation (2), energy Equation (3), and rheological Equation (4) with the help of boundary conditions were used to solve the spinning behavior using the Runge Kutta Verner method. The boundary conditions were fiber temperature, fiber speed, and cross- sectional area at the die exit. The spunbond process which was different from melt spinning did not have a winder, so the final spinning speed and take- up force at the entrance of channel in a chamber are unknown. Therefore, an arbitrary force was used at first and then the spinning force at the entrance of the channel in a chamber which was obtained by solving the above equations along the spinning line was compared with the friction force induced on the fiber surface in the channel obtained from Equation (5). Using the Newton Raphson method, an appropriate value was determined by obtaining the same values between the spinning force at the entrance of the channel and the friction force induced on the fiber surface in the channel. These procedures gave the diameter distribution, spinning speed distribution, deformation rate distribution, fiber temperature, and spin- line force. ( df dv dx = W dx g ) ( dv +πdτ v f = W dx g ) (Av 2 ) v v (1) FIGURE 3 Time variation of relative crystallinity dv dx = da dx = A v ( dt 2πA dx = CpW F 3η 0 A exp [ E R dv dx ) h(t T air ) ( 1 T 1 ) ] +GX T 0 F, spinning force; W, throughput per nozzle; g, gravity acceleration; τ f, air friction stress; A, cross section of fiber; v, spinning speed; D, fiber diameter; h, heat transfer coefficient; Cp, specific heat; η 0, zero- shear viscosity; E, activation energy of melt resin viscosity; T, fiber temperature; T air, cooling air temperature; T 0, resin temperature at the nozzle exit; G, viscosity rising index of crystallization (PP 100%;1.0, PP/LMPP10%;0.8, PP/ LMPP20%; 0.6); X, crystallinity [ ] (vair v)d γ ρ F x=l =α (v v air v)2 2 πdl (5) α, air friction coefficient; v air, cooling air speed at the narrow channel in the cabin; ρ*, air density; D, final fiber diameter at the narrow channel in the cabin; ν*, air kinematic viscosity; L, length of the narrow channel in the cabin. 3 RESULTS AND DISCUSSION 3.1 The blending effect of LMPP to PP for crystallinity and crystallization speed The high tacticity PP (PP Exxon3155 produced by Exxon Mobil, MFR = 36 g/10 min, T m = 160 C) 100% and (2) (3) (4) FIGURE 4 TABLE 1 process) Process conditions REICOFIL IV S Nozzle IPP contents (wt%) T c and ΔH as a function of IPP Experimental conditions of spunbond process (reicofil 5,800 holes/m, Die width 1,080 mm, ϕ 0.3 mm C Resin temperature Out-put rate/hole g/min/hole Air temperature 20 C Cabin pressure 3,000 8,000 Pa Calender temperature 137 C Weight 15 gsm Resins PP Exxon3155 MFR 36 g/10 min, T m 160 C LMPP-1 MFR 50 g/10 min, T m 75 C LMPP contents 0, 10, 20 (wt%/wt%)

4 4 KANAI et al and addition of LMPP to PP decreased the crystallization speed of PP. The crystallization temperature and the latent heat ΔH shown in Figure 4 were obtained by PerkinElmer DSC7. The spinnability was carefully observed for two hours under various conditions and whether fiber breaks occurred were confirmed and then stable conditions which had no breaks were determined to be the stable conditions. The trial conditions of the spunbond process using the Reicofil process are shown in Table Spinning behavior of spunbond process FIGURE 5 Comparison of predicted deniers with experimentally observed denier for the spunbond process the blends of wt% LMPP [13] (LMPP produced by Idemitsu Kosan) to high tacticity PP were used. LMPP (LMPP1, MFR = 50 g/10 min, T m = 75 C; LMPP2, MFR = 600 g/10 min, T m = 75 C) was polymerized using a single- site catalyst. The blends of LMPP 10 wt% or 15 wt% (LMPP1-10%,- 15%, LMPP2-15%) with high tacticity PP were used for the spunbond process. The crystallization speed was evaluated using Flash DSC (Mettler Toledo) at the high cooling speed 2,000 C/s shown in Figure 3. PP- 100% had a half crystallization time of s, and LMPP1-10% had a half crystallization time of s The comparison of the predicted results with experimentally observed denier for the spunbond process is shown in Figure 5. It shows the predicted denier agreed with experimentally observed denier. This means that the theoretical analysis of the spunbond process can predict the fiber diameter if the process conditions are input. Figure 6 shows deformation behaviors along the spinning line PP- 100 wt%, cabin pressure 4,500 Pa under various throughput conditions. Fiber diameter decreases with decreasing throughput. If throughput decreases, the solidification point moves upstream and maximum strain rate and spinning stress increase remarkably. Figures 7 shows the deformation behaviors such as fiber diameter, strain rates, and stresses were predicted along the spin- line at PP- 100 wt%, throughput 0.6 g/min/hole under the various cabin pressures. Spinning speed increases and fiber diameter decreases with increasing cabin pressure. The maximum strain rate and spinning stress increase with increasing cabin pressure. Maximum strain rate and spinning stress increase. FIGURE 6 Deformation behaviors along spinning line under various throughputs

5 KANAI et al 5 FIGURE 7 Fiber diameter, strain rate, and stress distribution under different cabin pressures FIGURE 8 Deformation behaviors along spinning line under various nozzle temperature Figure 8 shows deformation behaviors along spinning line at PP- 100 wt%, throughput 0.6 g/min/hole, cabin pressure 4,500 Pa under various nozzle temperatures. As the nozzle temperature increases, melt viscosity becomes low and fiber diameter decreases. Maximum strain rate increases with increasing nozzle temperature, and diameter changes gradually downstream. Stretching stress at the maximum strain rate decreases with increasing the nozzle temperature. The cabin pressure strongly influences the deformation behavior. The throughput and the resin temperature are also important to control the denier of each fiber. Fiber diameter decreases with decreasing throughput, increasing cabin pressure, and increasing nozzle temperature. To reduce maximum strain rate is effective to prevent the fiber break. The maximum strain rate should reduce, and moderate strain rate along the spinning line is preferable.

6 6 KANAI et al FIGURE 9 Fiber diameter profiles of IPP/low tacticity polypropylene blends FIGURE 10 Fiber diameter, spin- line stress, and strain rate of IPP/low tacticity polypropylene blends The stretching stress at the maximum strain rate should be reduced to prevent the fiber break. 3.3 The blending effect of LMPP to PP for spinnability of spunbond nonwoven process The effect of blending LMPP with PP on spinnability of the spunbond process was also investigated by solving the theoretical equations during the spunbond process. Figures 9 and 10 show the fiber diameter, strain rate, and stress profiles of PP and of the PP/LMPP 10% and 20% blends along the spin- line obtained by the theoretical predictions under the same conditions as throughput 0.6 g/min/hole and cabin pressure of 4,500 Pa. As the viscosity during the crystallization which was changed by G value of viscosity rising index of crystallization

7 KANAI et al FIGURE 11 Process window of functions of spin- line tension and maximum strain rate can be reduced by adding LMPP, the fiber diameter gradually decreases along the spin- line. The maximum strain rate and stress decrease with increasing the LMPP content. Furthermore, additional studies were carried out by changing LMPP content (0%, 10%, 15%) and LMPP molecular weight (MFR 50, 600). The maximum strain rate decreased by adding LMPP, and the strain rate gradually decreased in the region of the downstream area. The rapid 7 change of the strain rate during the spinning was suppressed by adding LMPP. The process window as functions of spin- line tension and maximum strain rate is shown in Figure 11. The spin- line tension was obtained at the maximum strain tension. Our previous research showed the film break occurred over the critical stress at the maximum strain rate. [18,19] This idea applied for the spinnability of the spunbond process. The process window exists under the maximum stress 6 MPa. The entanglement of polymer chain was not relaxed at the high- speed strain and high spin- line stress over the critical conditions, and then, the fiber break occurred. The solid line shows from PP 100 wt% to LMPP1 10 wt% blend, and the dotted line shows from LMPP1 10 wt% blend to LMPP1 20 wt% blend. The blend of LMPP into PP makes lower strain rate and stress during the spunbond process. Figure 12 shows spinnability window as functions of cabin pressure and throughput. It is easy to understand the process window in terms of process conditions of the spunbond nonwoven process. PP stable process condition area was expanded by blending LMPP into PP, compared with PP standard condition. By adding LMPP to PP, the stable spinning was possible without fiber break under high cabin pressure and low throughput/hole. Continuous and stable spinning could be achieved to produce 1.0 denier fiber by adding LMPP to PP. Table 2 shows some of the trial results using the Reicofil process. The process conditions such as throughput, cabin FIGURE 12 Spinnability windows as functions of cabin pressure and throughput TABLE 2 Fiber diameter and spinning velocity of various polypropylene (PP) blends under the limited conditions of stable spinning Sample code Throughput (g/min/hole) Cabin pressure (Pa) Fiber diameter (denier) IPP-100% , ,200 LMPP1-10% , , , , , ,300 LMPP1-15% , ,200 LMPP2-15% , ,800 Spinning velocity (m/min)

8 8 KANAI et al TABLE 3 Physical properties of various polypropylene (PP) blends spunbond nonwoven fabrics Sample code Throughput (g/min/hole) Cabin pressure (Pa) Fiber fineness (denier) Load at 5% strain (N/5 cm) Strain at break (%) Tensile strength (N/5 cm) MD CD MD CD MD CD IPP , LMPP , LMPP , LMPP , NonWoven Fabrics Black Paper A4 size use a scanner to capture images uniform the number of pixels A photography image is made a gray scale image and then a binary image. Counts Worse Uniformity Better Uniformity 0 Gray Scale 255 Black White The nonwoven fabrics having high gray scale peak and narrow distribution are good disperse uniformity. FIGURE 13 Evaluation method of nonwoven fabrics uniformity FIGURE 14 The influence of fiber denier on the nonwoven fabric uniformity

9 KANAI et al FIGURE 15 Load- strain curves in MD and CD for multilayer nonwoven fabrics (three layer spunbonds) pressure, spinning speed, and minimum fiber diameter give maximum stable conditions for blends IPP- 100% and LMPP- 10% which could produce fine denier fiber. 9 The stable conditions for PP- 100% was a throughput rate of 0.6 g/min/hole, a cabin pressure of 4,500 Pa, and a fiber diameter of 1.7 denier at the maximum spinning speed of 3,200 m/min. On the contrary, the blends of LMPP1 10% into IPP could expand the stable condition area and produce fine fiber diameters of 1.1 denier at the maximum spinning speed of 4,200 m/min and throughput 0.50 g/min/hole. The blends of LMPP2 15% into IPP could fine fiber diameters of 0.9 denier at the spinning speed of 3,800 m/min and throughput 0.36 g/min/hole, because of a lower maximum strain rate and less stress. These trial results could be predicted theoretically without experiment, and there was good agreement between the theoretical predictions and experimental results. As Table 3 shows, it was concluded that the increase of maximum spinning speed for PP and LMPP blends originated from the suppression of crystallization speed of PP and the reduction of maximum strain rate in the spinning process. There was big difference between PP only and PP/LMPP blend in terms of producing fine denier fiber. When the comparison between LMPP1-15% and LMPP2-15% which had different molecular weights was made, the lower viscosity of LMPP 2 could stably produce the finer fiber diameter under the higher cabin pressure and reach 0.9 denier. FIGURE 16 Photograph of polypropylene (PP) and PP/low tacticity polypropylene blend spunbond nonwoven fabrics with emboss FIGURE 17 Cantilever test of nonwoven fabrics (smaller bending length means nonwoven is softer)

10 10 KANAI et al Figure 13 shows the evaluation method of nonwoven fabric uniformity. The nonwoven fabrics were put on a A4 size black paper, and then, a scanner was used to capture images (uniformity was evaluated by the pixels number). A photography image was made a gray scale image and then a binary image. The nonwoven fabrics having high gray scale peak and narrow distribution are good disperse uniformity. Figure 14 shows the influence of fiber denier on the nonwoven fabric uniformity. It was found that the uniformity of nonwoven fabrics was improved by decreasing the fiber diameter and improving the dispersibility of fibers. 3.4 Blending effect of LMPP on physical properties of nonwoven fabrics Figure 15 shows the load strain curve of nonwoven fabrics of basis weight 15 gsm. When the throughput per hole decreased to produce fine fiber of 1.1 denier, the tensile strength and elongation at break on both MD and TD increased remarkably. Figure 16 shows photographs of PP and PP/LMPP blend spunbond nonwoven fabrics with emboss. It is considered that as the high- speed spinning was possible by blending LMPP with PP to produce fine fibers, the number of fibers with melt bonds at the emboss increased, and binding force between fibers in spunbond fabrics increased. Figure 17 shows the results of softness for nonwoven fabrics by the Cantilever test. Smaller bending length means nonwoven is softer. The nonwoven fabrics blending LMPP have low resistance force and softness. This result means the improved softness was obtained by thinning fibers and could produce nonwoven fabrics with both softness and high strength. 4 CONCLUSION The theoretical analysis of the spunbond process was set up and applied to PP and its blends. From the theoretical analysis predicted that the cabin pressure strongly influences the deformation behavior. The throughput and the resin temperature are also important to control the denier of each fiber. Fiber diameter decreases with decreasing throughput, increasing cabin pressure, and increasing nozzle temperature. To reduce maximum strain rate is effective to prevent the fiber break. Maximum strain rate should reduce, and moderate strain rate along the spinning line is preferable. Stretching stress at the maximum strain rate should be reduced. Addition of LMPP to PP made good spinnability and could produce nonwoven fabrics composed of fine denier for spunbond process. Adding LMPP to PP reduced the crystallization speed and maximum strain rate. Adding LMPP to PP reduced crystallinity and hardening of elongational viscosity after the beginning of crystallization and decreased stretching stress during the spinning process. In particular, adding high MFR (low molecular weight) LMPP to PP could reduce the maximum stretching stress. By blending a small amount of LMPP (especially low molecular weight LMPP) to PP, nonwoven fabrics having soft and fine and good uniform properties can be produced. REFERENCES [1] S. Kase, T. Matsuo, Sen-i Gakkaishi 1965, 18(3), 188. [2] T. Ishibashi, K. Aoki, J. Ishii, J. Appl. Polym. Sci. 1970, 14, [3] K. Katayama, Y. Moon-Guh, Sen-i Gakkaishi 1982, 38, 434. [4] K. Toriumi, A. Konda, K. Fujimoto, Sen-i Gakkaishi 1985, 41, T8. [5] S. Kawai, K. Ibata, J. Nakaho, Sen-IGakkaishi, 1980, 36, T116. [6] H. Yasuda, H. Sugiyama, H. Harikawa, Sen-i Gakkaishi 1979, 35, T370. [7] A. N. Beris, B. Liu, J. Non- Newtonian Fluid Mech. 1988, 26, 341. [8] R. R. Lamonte, C. D. Han, J. Appl. Polym. Sci. 1988, 16, [9] S. Kase, Sen-i Gakkaishi 1982, 38, P418. [10] A. Ziabicki, Fundamental of Fiber Formation, John Wiley & Sons, Chichester [11] Y. Sano, K. Orii, Sen-I Gakkaishi 1968, 24, 212. [12] J. Shimizu, K. Toriumi, Sen-IGakkaishi 1977, 33, 208. [13] T. Takebe, Y. Minami, T. Kanai, Seikei-Kakou 2009, 21(4), 202. [14] Y. Kohri, W. Takarada, H. Ito, T. Takebe, Y. Minami, T. Kanai, T. Kikutani, Seikei-Kakou 2008, 20, 831. [15] N. Okumura, H. Ito, T. Kikutani, T. Kanai, Polymer Processing Society Annual Meeting [16] Y. Kohri, T. Takebe, T. Kanai, T. Kikutani, Seikei-Kakou 14, Annual Meeting Proceedings 2014 [17] Y. Kohri, T. Takebe, Y. Minami, T. Kanai, W. Takarada, T. Kikutani, J. Polym. Eng. 2015, 35, 277. [18] T. Kanai, Intern. Polym. Process. 1987, 1(3), 137. [19] T. Kanai, Sen-i Gakkaishi 1986, 42, T-1. How to cite this article: Kanai T, Kohri Y, Takebe T. Theoretical analysis of the spunbond process and its applications for polypropylenes. Adv Polym Technol. 2017;00: