Poly(l -lactic acid) nonwoven fabric prepared by carbon dioxide laser-thinning method

European Polymer Journal 45 (2009) 278–283

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Poly(L-lactic acid) nonwoven fabric prepared by carbon dioxide laser-thinning method Akihiro Suzuki *, Yu Akaoka Interdisciplinary Graduate of School of Medicine and Engineering, University of Yamanashi, Takeda-4, Kofu 400-8511, Japan

a r t i c l e

i n f o

Article history: Received 25 December 2007 Received in revised form 23 October 2008 Accepted 29 October 2008 Available online 5 November 2008

Keywords: Poly(L-lactic acid) Nonwoven fabric Carbon dioxide laser SEM

a b s t r a c t Poly(L-lactic acid) (PLLA) nonwoven fabric was obtained by using a carbon dioxide laserthinning method. The obtained PLLA nonwoven fabric was made of endless microfibers with a uniform diameter without droplets. The fiber diameter can be varied by controlling an airflow rate supplied to the air jet, a supplying speed of an original fiber into a laser-irradiating point, and laser intensity. When the microfiber prepared by irradiating the laser operated at a laser intensity of 66 W cm2 to the original fiber supplied at Ss = 0.1 m min1 was dragged at an airflow rate of 30 L min1, the thinnest microfiber with an average diameter of 3.4 lm was obtained. The obtained microfiber had a degree of crystallinity of 45%, and the degree of crystal orientation of 84%. The existence of highly oriented crystallites suggests that a flow-induced crystallization occurred during the laser-thinning. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Nonwoven fabrics are industrially produced by a meltblowing [1–3], spunbonding [4–6], flashspinning, and electrospinning [7–10] and are exploited in a large number of applications such as membrane [11–13], reinforcing fibers [14], biomedical devices [15], and scaffold for tissue engineering [16–20]. The spunbonding is the integrated process combining fiber spinning, nonwoven fabric formation, and bonding. The spunbonded fabrics are made of the continuous filament with a diameter of about 10 lm. The meltblown and flashspun fabrics are composed of staple fibers with an inhomogeneous diameter in the range of 1–10 lm, and the meltblown fabrics included droplets sometimes. A carbon dioxide (CO2) laser-thinning method developed by us could easily produce the many kinds of microfibers [22–26] without using a highly complex spinneret such as the conjugate spinning [21]. The CO2 laser-thinning apparatus preparing the microfiber can also produced the nonwoven fabric by using an air jet and a collecting net in place of a winder. The improved laser-thinning apparatus was applied to poly(ethylene terephthalate) (PET) in * Corresponding author. E-mail address: [email protected] (A. Suzuki). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.10.035

order to make its nonwoven fabric [27]. The obtained PET nonwoven fabric was made of the endless microfibers with a diameter of 3.6 lm without droplet. In this study, the improved CO2 laser-thinning apparatus was applied to PLLA in order to prepare PLLA nonwoven fabric. We present here the results pertaining to the PLLA nonwoven fabric obtained by CO2 laser-thinning method. The nonwoven fabrics prepared by varying experimental conditions were characterized by SEM, WAXD, and DSC measurements. 2. Experimental 2.1. Material The original fiber used in this study was an as-spun PLLA fiber with Mn = 80,000 and Mw = 140,000 supplied by Unitika Ltd. (Japan). The original fiber had a diameter of 75 lm and birefringence of 6.3  103 and was isotropic as shown in Fig. 1. 2.2. Measurements The morphology of microfiber was determined with scanning electron microscopy (SEM) (JSM-6060LV, Jeol

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Supplying spool Nipping rolls

Original fiber

Fiber guide CO 2 Laser emitter Microfiber Air jet Fig. 1. Wide-angle X-ray diffraction pattern of as-spun PLLA fiber.

Ltd., Japan). SEM micrographs of the fibers were observed with an accelerating voltage of 10 kV. Before the observation, the samples were coated with gold using a sputter coater. The average diameter and the diameter distribution were obtained by using imaging analyzer (SMile View Jeol Ltd., Japan). Birefringence was measured with a polarizing microscope equipped with a Berek compensator (Olympus Optical Co., Ltd., Japan). Wide-angle X-ray diffraction images of the nonwoven fabrics were taken with an imaging-plate (IP) film and an IP detector R-AXIS DS3C (Rigaku Co., Japan). The IP film was attached to the X-ray generator (Rigaku Co., Japan) operated at 40 kV and 200 mA. The radiation used was Ni-filtered CuKa. The sample-to-film distance was 40 mm. The fiber was exposed for 5 min to the X-ray beam from a pinhole collimator with a diameter of 0.4 mm. The degree of crystal orientation (p) estimated from the halfwidth (H) of the meridian reflection peak. The p value was estimated from WAXD pattern measured by the imaging-plate through the software for analyzing data. The p value is given by the equation:

p¼

180  H  100 180

ð1Þ

The DSC measurements were carried out using a THERM PLUS 2 DSC 8230C calorimeter (Rigaku Co.). The DSC scans were performed within the temperature range of 25–200 °C, using a heating rate of 10 °C min1. All DSC experiments were carried out under a nitrogen purge. The DSC instrument was calibrated with indium. The degree of crystallinity (Xc) was determined from heat of fusion (DHm) and enthalpy of cold crystallization (DHcc) as follows

X c ð%Þ ¼

DHm þ DHcc  100 93:0

Nonwoven

Dry air Collecting net

fabric

Fig. 2. CO2 laser-thinning apparatus used for nonwoven fabric formation.

2.3. CO2 laser-thinning apparatus used for nonwoven fabric formation The CO2 laser-thinning apparatus to continuously produce the nonwoven fabric consists of supplying motor with spool with a diameter of 90 mm, a continuous wave CO2 laser emitter, supplying system composed of a fiber guide and nipping rolls, an air jet, and a collecting net as shown in Fig. 2. The continuous wave CO2 laser emitted at 10.6 lm, and the laser beam was a 2.4 mm diameter spot. A laser intensity was measured by the power meter during the laser-irradiating. The laser intensity was estimated by dividing the measured laser power in the area of the laser spot. The laser power of more than 90% is obtained in the area of the laser spot. It is necessary to supply the original fiber to a laser-irradiating point at a constant speed in order to stably obtain the microfiber nonwoven fabric with a uniform diameter. The supplying system pulls out the original fiber of the supplying spool and supplies it to the laser-irradiating point at a constant speed. The supplying system plays an important role in the CO2 laser-thinning apparatus. The fiber attenuated at the laser-irradiating

ð2Þ

where 93.0 J g1 is used as the heat of fusion of the crystalline phase of PLLA [28]. A tensile testing machine (EZ Graph Shimadzu, CO.) was used to determine tensile modulus and tensile strength. A gauge length of 20 mm and elongation rate of 100 mm min1 were used. The experimental results are the average of 10 measurements. A test sample was the rectangular nonwoven fabric about 10 mm wide and 30 mm long cut from the nonwoven fabric collected on the collecting net.

Fig. 3. Airflow rate dependence of average fiber diameter of nonwoven fabrics obtained various supplying speeds (Ss), (d) Ss = 0.1 m min1; (s) Ss = 0.2 m min1; (j) Ss = 0.3 m min1; (h) Ss = 0.4 m min1; (N) Ss = 0.5 m min1.

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point is pulled out on the collecting net by the air jet, and the nonwoven fabrics with a uniform diameter are formed by reciprocating the collecting net at constant speed.

A preparatory experiment was carried out to determine the optimum laser intensity to prepare stably PLLA nonwoven fabric. In the laser-thinning performed in the supplying speed range of 0.1–0.5 m min1, the microfiber without droplets was stably obtained by irradiating the laser at a power intensity of 66 W cm2. Henceforth the laser-irradiation in this study was carried out at a power intensity of 66 W cm2. Fig. 3 shows the airflow rate dependence of the average fiber diameter for the PLLA nonwoven fabrics obtained at five different supplying speeds (Ss). The increase of airflow

3. Results and discussion In the preparation of the nonwoven fabric by the laserthinning apparatus, the diameter of microfiber composing nonwoven fabric was determined by the fiber supplying speed, the laser intensity, and airflow rate supplied to the air jet.

( a ) Diameter distribution 60

(b)

SEM

-1

q = 10 L min

Counts

a

40 20 10 µm

0 60

-1

q = 20 L min

Counts

a

40 20 10 µm

0 -1

q = 30 L min

Counts

60

a

40 20

10 µm

0 -1

Counts

60

q =40 L min a

40 20

10 µm

0 -1

Counts

60

q =50 L min a

40 20

10 µm

0 0

2

4

6

8

10

12

Diameter / µm Fig. 4. (a) Fiber diameter distributions and (b) SEM photographs of 1500 magnifications of nonwoven fabrics obtained at five different airflow rates (qa) (Supplying speed: 0.1 m min1).

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Table 1 Cold crystallization temperature (Tcc), melting temperature (Tm), enthalpy of cold crystallization (DHcc), heat of fusion (DHm), the degree of crystallinity (Xc) estimated from DHcc and DHm, and the degree of crystal orientation (p) for the original fiber and the nonwoven fabrics obtained at various airflow rates (qa) at Ss = 0.1 m min1.

Fig. 5. Airflow dependence of birefringence of nonwoven fabrics obtained various supplying speeds (Ss), (d) Ss = 0.1 m min1; (s) Ss = 0.2 m min1; (j) Ss = 0.3 m min1; (h) Ss = 0.4 m min1; (N) Ss = 0.5 m min1.

led to an increase in the air suction speed and drag force. The diameter of the nonwoven fabric obtained at each Ss increases with increasing the airflow after decreasing in the airflow rate range of 10–30 L min1. The decrease in throughputs led to an increase in the draw force and then the reduction in fiber diameter. However, the increase in the average diameter of microfiber obtained at above 30 L min1 is caused by swaying the fiber at laser-irradiation point at the higher airflow rate. The larger sway of fiber at the laser-irradiating point leads to disproportionation of fiber diameter, and the average diameter increases. When the fiber prepared by irradiating the laser to the original fiber supplied at Ss = 0.1 m min1 was dragged at an airflow rate of 30 L min1, the thinnest nonwoven fabric with an average diameter of 3.4 lm was obtained.

qa (L min1)

Tcc (°C)

Tm (°C)

DHcc (J g1)

D Hm (J g1)

Xc (%)

p

Original At qa = 10 =20 =30 =40 =50

91 73 73 74 73 74

167 166 167 167 166 167

11.59 7.18 9.55 7.04 11.04 11.24

45.26 46.66 47.78 48.45 47.03 46.41

36 42 41 45 39 38

– 80 84 84 83 82

(%)

Fig. 4(a) and (b) shows the diameter distributions and the SEM photographs of 1500 magnifications for the nonwoven fabrics obtained at five different airflow rates at a supplying speed of 0.1 m min1. As the airflow rate increases, the diameter distribution broadens and the diameter increases after the distribution becomes narrower, and the average fiber diameter decreases. As was stated previously, the diameter uniformity decreases and the average diameter increases when swaying of fiber at the laser-irradiating point is too large. The SEM photographs show that the nonwoven fabrics have a smooth surface without a surface roughened by a laser-ablation, and that there are not droplets contained in the nonwoven fabric. Fig. 5 shows the airflow rate dependence of the birefringence for the microfibers obtained at five different supplying speeds (Ss). As the airflow increases, the birefringence of the microfiber produced at each Ss decreases after increasing in the airflow rate range of 10–30 L min1. The highest birefringence of 0.021 was obtained when the fiber, obtained by irradiating the laser to the original fiber

Fig. 6. Wide-angle X-ray diffraction patterns for the bundled microfibers obtained at four different airflow rates (qa). (Supplying speed: 0.1 m min1).

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Fig. 8. Changes of tensile modulus and tensile strength of nonwoven fabric with airflow rates.

Fig. 7. DSC curves of original fiber and nonwoven fabrics at five different airflow rates (qa).

supplied at 0.1 m min1, was dragged at an airflow rate of 30 L min1. Its birefringence is almost the same as that (0.024) of the microfiber prepared by winding at 800 m min1 [26] and reaches 70% of the intrinsic crystallite birefringence (0.030) of the PLLA [29]. The observed birefringence is defined as the sum of birefringence values due to the crystalline and amorphous phases [30]. The increase of birefringence with increasing the airflow rate is attributed to the increases in the degree of the orientation for amorphous chains and the existence of the oriented crystallites formed by the flow-induced crystallization. To confirm the existence of the oriented crystallites formed by the flow-induced crystallization during the CO2 laser-thinning, the WAXD measurement was carried out for the microfiber collected in a bundle. The microfiber was caught by the rod placed under the air jet to arrange microfiber in direction of the fiber axis. Fig. 6 shows the wide-angle X-ray diffraction (WAXD) patterns of the bundled microfibers collected at five different airflow rates (qa) at Ss = 0.1 m min1. The WAXD patterns show the slight equatorial arc reflection due to the oriented crystallites, and those of the microfibers obtained at qa = 20 and 30 L min1 show the stronger reflection among them. The amount capable of collecting microfiber in the bundle is small because it is difficult to arrange microfiber in direction of the fiber axis and to bundle it. This is the reason why the WAXD images show the weak reflection.

The PLLA crystallizes in two polymorphic forms: a form (orthorhombic) and b form (trigonal) [31–33]. The a form has 10/7 helix and is can be obtained by crystallization from the melt or from solution. The reflections of the microfibers obtained are attributable to the (0010)a reflection of the a form crystal. To quantitatively determine the degree of crystal orientation (p), the p value was estimated from the half-width (H) of the meridian the (0010)a reflection peak. The p values estimated are shown in the last column of Table 1. The p values show the existence of highly oriented crystallites, and those of microfibers obtained at qa = 20 and 30 L min1 reach 84%, but are lower than those (p = 91–95%) of PLLA microfibers wound onto a reel in the speed of 200– 800 m min1 [26]. The results of the WAXD measurement suggest that the oriented crystallites formed by the flow-induced crystallization occurred during the CO2 laser-thinning process exist in the PLLA nonwoven fabric. Fig. 7 shows the DSC curves for the original fiber and the nonwoven fabrics obtained at various airflow rates (qa) at Ss = 0.1 m min1. Table 1 lists cold crystallization temperature (Tcc), melting temperature (Tm), enthalpy of cold crystallization (DHcc), heat of fusion (DHm), the degree of crystallinity (Xc) estimated from DHcc and DHm, and the degree of crystal orientation (p) for them. The DSC curve of the original fiber shows a change in slope in the specific heat at about 70 °C, which corresponds to the Tg; an exothermic transition at 91 °C caused by a cold crystallization; and a broad melting endotherm peaking at 167 °C. The obtained nonwoven fabrics have a cold crystallization temperature (Tcc) of about 73 °C and a melting temperature (Tm) of about 166 °C. Its Tcc is 18 °C lower than that of the original fiber. The decrease in the Tcc was caused by the increase in the degree of orientation of amorphous

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chains. As the airflow rate increases, the melting peak broadens after becoming sharp. The sharpening of the melting peak is caused by an increase in the degree of perfection of the crystallites. The nonwoven fabric obtained at qa = 30 L min1 is highest degree of perfection of the crystallites among them because its melting peak is sharpest. The Xc value decreases after increasing in the airflow rate range of 10–30 L min1, and that of the nonwoven fabric obtained at qa = 30 L min1 are 45%. This Xc value is almost the same as that of the PLLA microfiber prepared by winding at 800 m min1. Fig. 8 shows the tensile modulus and tensile strength of the nonwoven fabrics obtained at various airflow rates at Ss=0.1 m min-1. As the airflow rate increases, the tensile modulus and tensile strength increases. In general, the mechanical properties of fibers depend strongly on its birefringence and degree of crystallinity. However, there is no direct relationship between the birefringence of the microfiber (see Fig. 5) and the mechanical properties nonwoven fabric, and the mechanical properties depend on the airflow rate. The superstructure of the microfiber was not reflected directly in the mechanical properties of the nonwoven fabric although it seems that there is an obvious relationship between the two in the monofilament microfiber. This result suggests that the microfibers were packed closely by blowing them on the collecting net with the higher airflow rate, and that the increase in the packing density of the nonwoven fabric causes increase of its mechanical properties. 4. Conclusions The CO2 laser-thinning method was applied to the PLLA fiber to prepare the PLLA nonwoven fabric. It was found that the obtained nonwoven fabric was made of the microfibers with a uniform diameter without droplet, and that the microfiber composing the nonwoven fabric had the highly oriented crystallites formed by the flow-induced crystallization. Unlike meltblowing and flashspinning, if the laser-irradiation is stably performed, the obtained microfiber is basically endless monofilament. It is important to laser-irradiate the original fiber without swaying it at laser-irradiating point in order to prepare the thinner PLLA nonwoven fabric with uniform diameter.

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