Study on the Fiber Diameter of Polyactic Melt Blown Nonwoven Fabrics

Lili Wu 1, a, Ting Chen1, 2, b and Jianyong Yu3, c

1College of Textile and Clothing Engineering, Soochow University, Suzhou 215006, China

2National Engineering Laboratory for Modern Silk, Soochow University, Suzhou 215123, China

3College of Textiles, Donghua University, Shanghai 201620, China

aliliwu@suda.edu.cn, btingchen@suda.edu.cn, cCorresponding author, yujy@dhu.edu.cn

Keywords: polyactic; melt blown; nonwoven fabrics; fiber diameter.

Abstract. Polyactic fibers have superior biodegradability, moisture absorption, resiliency and

processibility and can be used in various fields especially in blood filtration. Polyactic can be melt

blown into nonwoven fabrics. To predict the fiber diameter of the polyactic melt blown nonwoven

fabric, the air drawing model of polyactic was established. The predicted fiber diameter tallies well

with the measured fiber diameter. Computer simulations of the effects of the processing parameters

on the fiber diameter were performed with the help of the air drawing model. The simulation results

show that smaller polymer flow rate, larger initial air velocity and larger die-to-collector distance

can all produce finer fibers while too large initial air velocity and too large die-to-collector distance

contribute little to the air drawing of polyactic melts.

Introduction

In the melt blowing nonwoven process, polymer melts are drawn into microfibers by the air jets

with high velocity and high temperature. The polymer air drawing model for the melt blowing

process has been established in our previous research [1], which has shown that the predicted fiber

diameter of melt blown nonwoven fabrics tallies well with the measured fiber diameter with special

reference to polypropylene [1]. Besides polypropylene, many kinds of polymers can be

manufactured into nonwoven fabrics via the melt blowing process. For example, polyactic is mainly

made of corn and can be melting blown into nonwoven fabrics. Polyactic fibers have superior

biodegradability, moisture absorption, resiliency and processibility and can be used in various fields

especially in blood filtration. However, the flow behavior of polyactic is quite different from that of

polypropylene. Is our air drawing model able to predict the fiber diameter of polyactic melt blown

nonwoven fabrics? In this paper, the air drawing model of polyactic melt blown nonwoven fabrics

was established. And the predicted fiber diameter was compared with the measured fiber diameter.

Effects of some processing parameters were also simulated using the model.

Air Drawing Model of Polyactic

There are continuity equation, momentum equation, energy equation and constitutive equation in

the air drawing model of polyactic. The air jet flow field of the dual slot die is simulated

numerically and the air velocity and air temperature could be obtained [1].

Continuity equation:

ρπ

uDG 2

4= . (1)

where G is the polymer flow rate, D is fiber diameter, u is the fiber velocity and ρ is the polymer

Advanced Materials Research Vols. 175-176 (2011) pp 580-584Online available since 2011/Jan/20 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.175-176.580

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density. The polymer density of polyactic is 1.29 g/cm3.

Momentum equation:

gDuuDCjx

uG

x

Faaf

r ρπ

ρπ 22

4)(

2

1

d

d

d

d−−+= . (2)

where Fr is the rheological force, ρa is the air density, x is the axial position, ua is the x-component

of air velocity, Cf is the air drawing coefficient and g is the gravitational acceleration.

A sign flag, j, is defined, j is – 1 when ua > u and 1 when ua < u. It means that, near the

spinneret, the airflow acts a positive (downward) force on the polymer, but on the far side of the

spinneret the force is negative.

The rheological force is

( )yyxxr DF ττ

π−= 2

4. (3)

where τxx is the axial tensile stress of polymer, τyy is the transversal tensile stress of polymer.

The air drawing coefficient Cf was given by Matsui with the following correlation [2]

n

f ReC −⋅= β . (4)

where β, n are constants of Matsui’s correlation and Re is the Reynolds number.

The Reynolds number is defined by

a

a uuDRe

υ−

= . (5)

where υa is the air kinematic viscosity.

Majumdar and Shambaugh [3] found that β = 0.78 and n = 0.61 are appropriate for the use in Eq.

4. These values were used in our computations.

Energy equation:

( )p

ap

GC

Dh

x

θθπθ −−=

d

d . (6)

where Cp is the specific heat capacity at constant pressure of polymer, hp is the heat transfer

coefficient and θa is the air temperature. The specific heat capacity at constant pressure of

polyactic is 1.05 cal/(g ·°C).

A value for the heat transfer coefficient can be calculated from the following relation:

kReNu ⋅= γ . (7)

where Nu is the Nusselt number, γ, k are constants of this correlation. The assumed values of γ and k are 0.42 and 0.334, respectively

[4].

Constitutive equation:

As the polymer melt of polyactic is a kind of non-Newtonian fluid, the constitutive equation of

power-law fluid is introduced:

m

xxx

u

=d

d2ητ . (8)

Advanced Materials Research Vols. 175-176 581

m

yyx

u

−=d

dητ (9)

where η is the shear viscosity and m is the power-law exponent. The power-law exponent m of

polyactic is 0.32, which differs from m of 0.78 in our previous study on the polypropylene [1].

We defined the ‘freezing point’ boundary condition as the point where the rheological force

equaled the sum of the gravitational force and air drawing force acting upon the frozen part of the

polymer. Beyond this point, the fiber diameter remained constant until the fiber was laid on a

collection screen.

Experiments and Results

Experiments were performed on the melt blowing nonwoven equipment with a dual slot die. The

equipment parameters of the dual slot die were as follows: die width h = 0.7 mm, die length l = 200

mm, slot width e = 0.2 mm, head width f = 0.5 mm, angle between the slot and spinneret axis α =

30°, and spinneret diameter c = 0.3 mm. The initial air temperature was 220 °C and 320 °C,

respectively [5]. The polymer used was 12 MFI polyactic. Table 1 shows the experimental program

and results.

Several processing parameters were changed to study their effects on the fiber diameter. The

processing parameters changed were the polymer flow rate, initial air velocity and die-to-collector

distance. The image analysis method was utilized to measure the fiber diameter. The images of

nonwoven samples were acquired by the QUESTER three-dimensional video frequency microscope

with the enlargement factor of 600 and depth of focus of 1 mm and then processed by the image

analysis software named Image-Pro Plus to measure the fiber diameter. The image processing

included enhancement, smoothing, binary and filtering. The mean value of the diameters of two

hundred fibers was considered as the fiber diameter of the nonwoven sample.

Table 1 Experimental program and results

No

Polymer

flow

rate

(g/s)

Initial

polymer

temperature

(°C)

Initial

air

velocity

(m/s)

Initial air

temperature

(°C)

Die-to-

collector

distance

(mm)

Measured

fiber

diameter

(µm)

Predicted

fiber

diameter

(µm)

Error

(%)

1 0.510 220 247 320 90 3.70 3.46 6.49

2 0.510 220 260 320 100 3.20 2.97 7.19

3 0.510 220 273 320 110 2.71 2.53 6.64

4 0.538 220 260 320 90 4.13 3.82 7.51

5 0.538 220 273 320 100 3.83 3.51 8.36

6 0.538 220 247 320 110 4.21 3.96 5.94

7 0.567 220 273 320 90 5.09 4.65 8.64

8 0.567 220 247 320 100 6.07 5.73 5.60

9 0.567 220 260 320 110 5.83 5.41 7.20

Mean 7.06

The measured fiber diameters, predicted fiber diameters and prediction errors are shown in Table

1. From Table 1, it can be seen that the predicted fiber diameters tally well with the experimental

582 Silk

data. The mean prediction error is 7.06% and the maximum is only 8.64%. Because the melt

blowing process was very complicated and had many influencing factors with random pulsations,

such prediction errors as smaller than 10% can be tolerable as far as predictions of this kind of

complicated processing problems were concerned. The small prediction errors confirmed that the

air drawing model of polyactic established in this paper was effective. It can also be found that all

the predicted diameters were smaller than the measured diameters, which implied that there was a

systematic error and the model should be improved by considering more factors.

0.45 0.50 0.55 0.60 0.652.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

F

iber

dia

met

er (µm

)

Polymer flow rate (g/s)

Fig. 1 Effect of the polymer flow rate on fiber diameter

With the help of the air drawing model, effects of the processing parameters on the fiber

diameter were simulated. Fig. 1 shows the effect of the polymer flow rate on the fiber diameter. It

can be found that reducing the polymer flow rate gave a finer fiber diameter. For the conditions in

Fig. 1, the fiber diameter for G = 0.510 g/s was 23% smaller than the final fiber diameter for the

high polymer flow rate (G = 0.567 g/s).

220 240 260 280 300 3202.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Fib

er d

iam

eter

(µm

)

Initial air velocity (m/s)

Fig. 2 Effect of the initial air velocity on fiber diameter

Fig. 2 gives the effect of the initial air velocity on the fiber diameter. As can be seen, the larger

the initial air velocities, the finer the fibers will be. It also shows that the fiber diameter decays less

rapidly when the initial air velocity increases over 290 m/s, which can be concluded that too large

initial air velocity contributes little to the polymer drawing as far as the polymer melt of polyactic is

concerned. The result gave us valuable insights on reducing the energy consumption of melt

blowing processing of polyactic.

Advanced Materials Research Vols. 175-176 583

70 80 90 100 110 120 130 1402.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Fib

er d

iam

eter

(µm

)

Die-to-collector distance (mm)

Fig. 3 Effect of the die-to-collector distance on fiber diameter

Fig. 3 illustrates how changes in die-to-collector distance cause changes in the rate of fiber

attenuation. It could be observed that the larger distance would cause the fibers to be attenuated

much higher. When the distance increased to 120 mm, the final fiber diameter was 12% smaller

than that of the distance 90 mm. It also could be noted that the fiber diameter only decreased

slightly when the die-to-collector distance was larger than 120 mm as far as this research was

concerned. The reason might be the polymer drawing procedure completed within a certain distance,

and the fiber diameter would not change even when the die-to-collector distance still increased.

Summary

The air drawing model of polyactic in the melt blowing nonwoven process was established. The

predicted fiber diameter tallied well with the measured fiber diameter. Effects of the processing

parameters on the fiber diameter were simulated with the help of the air drawing model. The

simulation results showed that smaller polymer flow rate, larger initial air velocity and larger

die-to-collector distance can all produce finer fibers while too large initial air velocity and too large

die-to-collector distance contributed little to the air drawing of polyactic melts.

Acknowledgements

Financial support for this work was provided by the National Natural Science Foundation of China

(51076110), Foundation for the Author of National Excellent Doctoral Dissertation of China

(200761), Fok Ying Tung Education Foundation for University Youngsters (111076), and Natural

Science Foundation of Jiangsu province (BK2009123).

References

[1] T. Chen and X. Huang: Modelling Simul. Mater. Sci. Eng. Vol. 12 (2004), p. 381

[2] M. Matsui: Trans. Soc. Rheol. Vol. 20 (1976), p. 465

[3] B. Majumdar and R.L. Shambaugh: J. Rheol. Vol. 34 (1990), p. 591

[4] V. Bansal and R.L. Shambaugh: Ind. Eng. Chem. Res. Vol. 37 (1998), p. 1799

[5] X.Zhao: Study on the Meltblown Process of Biodegradable PLA. Master Thesis, Donghua

University, Shanghai (2005), p. 40

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Silk 10.4028/www.scientific.net/AMR.175-176 Study on the Fiber Diameter of Polyactic Melt Blown Nonwoven Fabrics 10.4028/www.scientific.net/AMR.175-176.580

DOI References

[2] M. Matsui: Trans. Soc. Rheol. Vol. 20 (1976), p. 465

doi:10.1122/1.549434 [3] B. Majumdar and R.L. Shambaugh: J. Rheol. Vol. 34 (1990), p. 591

doi:10.1122/1.550097 [4] V. Bansal and R.L. Shambaugh: Ind. Eng. Chem. Res. Vol. 37 (1998), p. 1799

doi:10.1021/ie9709042