Adv Polym Technol. 2017;1–10. wileyonlinelibrary.com/journal/adv | 1© 2017 Wiley Periodicals, Inc.

Received: 17 April 2017 | Accepted: 18 June 2017DOI: 10.1002/adv.21866

R E S E A R C H A R T I C L E

Theoretical analysis of the spunbond process and its applications for polypropylenes

Toshitaka Kanai1 | Youhei Kohri2 | Tomoaki Takebe2

1KT Polyme, Sodegaura, Chiba, Japan2Performance Materials Laboratories, Idemitsu Kosan Co., Ltd. Anesaki, Ichihara, Chiba, Japan

CorrespondenceToshitaka Kanai, KT Polyme, Sodegaura, Chiba, Japan.Email: toshitaka.kanai@ktpolymer.com

AbstractThe theoretical analysis of the spunbond process was set up and applied to polypro-pylene (PP) and its blends. The effect of blending low tacticity polypropylene (LMPP) to high tacticity PP on the spinnability of the spunbond process was investi-gated. 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 crystalliza-tion speed and strain rate in the spinning process. The physical properties of the spunbond nonwoven fabrics were also investigated.

K E Y W O R D Sfiber, low modulus PP, nonwoven fabrics, spunbond, tacticity, theoretical analysis

1 | INTRODUCTIONThe 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 nonwo-ven 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 the-oretical 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 prod-ucts which require to produce fine denier fabrics and to con-trol 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 use-ful and could help us to know the mechanism of spunbond process.

Furthermore, the nonwoven fabrics are sometimes re-quired 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 tac-ticity polypropylene (PP) could produce very fine fiber with-out 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 spin-ning 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.

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2 | KAKAI etKal

2 | THEORETICAL ANALYSIS OF SPUNBOND PROCESS

To set up the theoretical equations of the spunbond pro-cess 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 func-

tion 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 crystalliza-tion. 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 tempera-ture 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 simula-tion. Under the above conditions, momentum Equation (1),

F I G U R E 1 Schematic view of spunbond process

Die

Collector

Cabin System

Thro�le Plate

Cooling Air

Spunbond

x0=0

x

Cooling Air

Output Rate W

Uni-Axial Stretching

Posi�on

Melt Spinning

T0T

Dx

D

R0

DiameterTemp.

x

Minute elements(Dividing)

L

Schema�c view of Spunbond Process

F I G U R E 2 Flowchart of spunbond process simulation

F

Calculation Repeated till coincidence of Stretching ForceNewton Raphson Method

uaternionic Simultaneous Equations Runge Kutta Method

Stretching Force at the entrance of Throttle Plate Fx=L

Boundary ConditionStretching Force at the entrance of Throttle PlateFx=L

Solution Spinning Speed Fiber Diameter D

consistentNot consistent

Flowchart of Spunbond Process Simulation

| 3KAKAI etKalcontinuity 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 spun-bond 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 ob-taining 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 diame-ter distribution, spinning speed distribution, deformation rate distribution, fiber temperature, and spin- line force.

F, spinning force; W, throughput per nozzle; g, gravity acceler-ation; τf, air friction stress; A, cross section of fiber; v, spinning speed; D, fiber diameter; h, heat transfer coefficient; Cp, spe-cific heat; η0, zero- shear viscosity; E, activation energy of melt resin viscosity; T, fiber temperature; Tair, cooling air tempera-ture; T0, 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

α, air friction coefficient; vair, 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 DISCUSSION3.1 | The blending effect of LMPP to PP for crystallinity and crystallization speedThe high tacticity PP (PP Exxon3155 produced by Exxon Mobil, MFR = 36 g/10 min, Tm = 160°C) 100% and

(1)

dF

dx=W

(

dv

dx−

g

v

)

+�Dτf =W(

dv

dx−

g

v

)

+0.0023(Av2)0.195v

(2)dA

dx=−

A

v

dv

dx

(3)dTdx

=

(

2�A

CpW

)

h(T −Tair)

(4)dvdx

=F

3η0Aexp

[

−E

R

(

1

T−

1

T0

)

+GX

]

(5)Fx=L =α[

(vair−v)D

v∗

]γ

(vair−v)2 ρ

∗

2�DL

F I G U R E 3 Time variation of relative crystallinity

F I G U R E 4 Tc and ΔH as a function of IPP

IPP contents (wt%)IPP contents (wt%)

T A B L E 1 Experimental conditions of spunbond process (reicofil process)

Process conditions

REICOFIL IV S

Nozzle 5,800 holes/m, Die width 1,080 mm, ϕ 0.3 mm

Resin temperature 200–250°C

Out- put rate/hole 0.2–0.6 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, Tm 160°C

LMPP- 1 MFR 50 g/10 min, Tm 75°C

LMPP contents 0, 10, 20 (wt%/wt%)

4 | KAKAI etKal

the blends of 10–15 wt% LMPP[13] (LMPP produced by Idemitsu Kosan) to high tacticity PP were used. LMPP (LMPP1, MFR = 50 g/10 min, Tm = 75°C; LMPP2, MFR = 600 g/10 min, Tm = 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 0.066 s, and LMPP1- 10% had a half crystallization time of 0.094 s

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 pro-cess are shown in Table 1.

3.2 | Spinning behavior of spunbond processThe comparison of the predicted results with experimen-tally observed denier for the spunbond process is shown in Figure 5. It shows the predicted denier agreed with experi-mentally observed denier. This means that the theoretical analysis of the spunbond process can predict the fiber diam-eter if the process conditions are input.

Figure 6 shows deformation behaviors along the spin-ning line PP- 100 wt%, cabin pressure 4,500 Pa under vari-ous throughput conditions. Fiber diameter decreases with decreasing throughput. If throughput decreases, the solidifi-cation 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.

F I G U R E 5 Comparison of predicted deniers with experimentally observed denier for the spunbond process

F I G U R E 6 Deformation behaviors along spinning line under various throughputs

| 5KAKAI etKal

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 in-creasing nozzle temperature, and diameter changes gradually downstream. Stretching stress at the maximum strain rate de-creases 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 tempera-ture. 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.

F I G U R E 7 Fiber diameter, strain rate, and stress distribution under different cabin pressures

F I G U R E 8 Deformation behaviors along spinning line under various nozzle temperature

6 | KAKAI etKal

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 processThe 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 pro-files 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 pres-sure of 4,500 Pa.

As the viscosity during the crystallization which was changed by G value of viscosity rising index of crystallization

F I G U R E 9 Fiber diameter profiles of IPP/low tacticity polypropylene blends

F I G U R E 1 0 Fiber diameter, spin- line stress, and strain rate of IPP/low tacticity polypropylene blends

| 7KAKAI etKal

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 mo-lecular 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

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 pre-vious research showed the film break occurred over the crit-ical 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 entan-glement 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 spun-bond 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 possi-ble 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

F I G U R E 1 1 Process window of functions of spin- line tension and maximum strain rate

F I G U R E 1 2 Spinnability windows as functions of cabin pressure and throughput

T A B L E 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)Spinning velocity (m/min)

IPP- 100% 0.60 4,500 1.7 3,200

LMPP1- 10% 0.60 6,500 1.4 3,900

0.50 6,500 1.1 4,200

0.40 5,500 1.1 3,300

LMPP1- 15% 0.36 6,500 1.0 3,200

LMPP2- 15% 0.36 6,500 0.9 3,800

8 | KAKAI etKalT A B L E 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- 100 0.6 4,500 1.7 9.3 2.3 43 50 32 15

LMPP- 10- 1 0.6 6,500 1.4 6.2 1.2 60 69 32 17

LMPP- 10- 2 0.5 6,500 1.1 8.7 1.5 72 79 45 20

LMPP- 10- 3 0.4 5,500 1.1 8.8 1.4 67 87 45 21

F I G U R E 1 3 Evaluation method of nonwoven fabrics uniformity

Black Paper

NonWoven Fabrics 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.

Cou

nts

WhiteBlackGray Scale0 255

Better UniformityWorse Uniformity The nonwoven fabrics having

high gray scale peak and narrow distribution are good disperse uniformity.

F I G U R E 1 4 The influence of fiber denier on the nonwoven fabric uniformity

| 9KAKAI etKal

pressure, spinning speed, and minimum fiber diameter give maximum stable conditions for blends IPP- 100% and LMPP- 10% which could produce fine denier fiber.

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 origi-nated 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 com-parison 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.

F I G U R E 1 5 Load- strain curves in MD and CD for multilayer nonwoven fabrics (three layer spunbonds)

F I G U R E 1 6 Photograph of polypropylene (PP) and PP/low tacticity polypropylene blend spunbond nonwoven fabrics with emboss

F I G U R E 1 7 Cantilever test of nonwoven fabrics (smaller bending length means nonwoven is softer)

10 | KAKAI etKalFigure 13 shows the evaluation method of nonwoven fab-

ric 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 photog-raphy 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 non-woven fabric uniformity. It was found that the uniformity of nonwoven fabrics was improved by decreasing the fiber di-ameter and improving the dispersibility of fibers.

3.4 | Blending effect of LMPP on physical properties of nonwoven fabricsFigure 15 shows the load–strain curve of nonwoven fab-rics 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 in-creased 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 fab-rics by the Cantilever test. Smaller bending length means non-woven is softer. The nonwoven fabrics blending LMPP have low resistance force and softness. This result means the im-proved softness was obtained by thinning fibers and could pro-duce nonwoven fabrics with both softness and high strength.

4 | CONCLUSIONThe 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 in-creasing 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 crystal-lization 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 mo-lecular weight LMPP) to PP, nonwoven fabrics having soft and fine and good uniform properties can be produced.

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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;00:1–10. https://doi.org/10.1002/adv.21866

https://doi.org/10.1002/adv.21866