Surface microstructuring of polyester fabrics by UV excimer laser irradiation.

Vrinceanu, Narcisa ; Coman, Diana ; Sandu, Ion 等

1. INTRODUCTION

Today, pulsed UV laser is one of the most commonly used noncontact treatment techniques in modifying surface properties of polymers physically as well as chemically (Knittel et al, 1998). It is regarded as an environmentally friendly process since no chemicals are involved (Wong et al, 2000). The surfaces of the irradiated fibers develop ripple-like structures that result in an enhancement of fiber surface area. Surface structures are important for the physical and chemical properties of textile fibers since wetting, adhesion and optical properties are strongly dependent on them. These properties are also affected after laser irradiation (Bahners et al, 1993). Chemical surface treatment has been traditionally used to modify fibre materials but it has some disadvantages, such as influence on bulk properties and environmental pollution. Therefore, to improve the hydrophilicity of polyester without affecting its bulk properties, a lot of studies have been performed on the laser treatment for improving surface properties (Kan, 2007); The laser irradiation of highly absorbing polymers such as polyester can generate characteristic modifications of the surface morphology (Bahners et al, 1990). Hence, it is reasonable to believe that such surface modification of a polymer may have an important impact on its textile properties (Wong et al, 2001).

2. EXPERIMENTAL

2.1. Materials and laser treatment

In this study, a 100% polyester fabric was used in all experiments. White plain weaves samples were conditioned at 20 [+ or -] 2[degrees]C and 65 [+ or -] 2% relative humidity before laser irradiation. Irradiation was performed using a a LPX 200 Excimer 248 nm KrF. In high-fluence laser irradiation, samples were irradiated directly from the laser beam without using either special photomask or focusing lens. Laser energies like fluence and number of pulses vary from experiments in order to study their effects upon samples. The laser fluence was regulated in the range from 29 to 43 mJ/[cm.sup.2] and the number of pulses varied between 0 and 4, the pulse repetition was kept constant at 1Hz to avoid any possible heat accumulation. During the laser treatment, the control of treatment was very important, considering the low melting point (260[degrees]C) of the fibres.

2.2. Morphological study

The morphology of samples was investigated by:

Scanning Electron Microscopy (SEM) (TESCAN). All samples were gold coated prior to SEM examination.

--Atomic Force Microscopy (AFM). AFM is the one of the effective tools to examine the microstructures of fibres. It is able to scan materials without any special preparation at normal temperature and pressure. The AFM used in this study was CSPM3300 produced by Benyuan Company. The vertical resolution of the machine is 0.1 nm, while the horizontal solution is 0.2 nm. The scanning mode used was contact mode in this study, and the scanning range was set at a size of 5.0 umX5.0 um.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

3. RESULTS AND DISCUSSION

Fig. 2, 3 show SEM micrographs of untreated and laser treated PES samples. Before the treatment, the sample had a smooth surface. After the laser treatment the surface developed a certain roughness or periodic roll, the so called ripple like or roll like structure, as shown in Fig. 3. The orientation of this type of structure is perpendicular to the fibrilar orientation of the fibres. Despite the fact that PES fibres without laser treatment appear to be smooth, in the AFM image, the lines and the roughness of the surface can be clearly shown.

[FIGURE 4 OMITTED]

The height of the lines ranges from 30 to 353 nm (Fig. 4a). Laser treatment with 37 mJ/cm2 and one laser pulse, etches the surface of PES fibres, forming aggregates on the surface as illustrated in Fig. 4(b). The surface topography has changed from an original smoothness to a fibrous one characterized by hills and groves. The AFM image also reveals the size of aggregates in the range between 10 and 314 nm. The longer exposure to the laser, the rougher the surface becomes. It can be seen from Fig. 4(c) that almost all the surface is etched and the etching effect makes the height of the surface fall by 10-716 nm. It can be noticed that the increasing the number of pulses, the surface becomes coarser and the height of the surface is now 716 nm. More fluence would give a larger distance between hils. The more number of pulses, the more the ripple will approach saturation. The surface roughness computed from AFM images are summarized in fig. 5 and 6.

In figures 5 and 6 both Ra and ripple spacing show almost linearity (linear relation) with laser number of pulses. Ra and ripple spacing increase with the number of laser pulses. For the roughness (Ra), the value increased dramatically after the second pulse. An increase in Ra represents an increase in ripple size.

4. CONCLUSION

The microscopic observations have revealed that the use of UV excimer laser upon PES textile fabrics is a valuable tool to change the surface morpfology and consequently, the properties of the textile materials will be modified, even improved. The laser irradiation of highly absorbing polymers such as polyester can generate characteristic modifications of the surface morphology. In this case, some well-oriented structure of grooves or ripple structures with dimensions in the range of micrometre are developed on surface with irradiation fluence above the so-called ablation threshold. The study has proven the etching effect on the surface roughness of the polyester fabrics. SEM and AFM have been proven to be powerful tools in the examination of surface morphology. The surface roughness and the ripple spacing increased with laser pulses and fluences. It has been concluded that a specific modification of polyester fibers may be perfomed by UV excimer laser irradiation, resulting in a roughness at the surface textiles. The surface roughness depends on laser treatment parameters: wavelength, pulse duration, fluence and number of pulses. The ablation threshold values depend upon both the UV radiation wavelength and the polymer absorptivity. We have reason to believe that after this number of pulses the topography has not been changed too much.

Acknowledgement

The authors would like to thank to Prof. FOTAKIS Costas, the Director of Institute of Electronics Structure and Lasers (IESL), Foundation of Research Technology Hellas (FORTH), Heraklion, Greece, for infrastructure access.

5. REFERENCES

Bahners, T.; Kesting, W.; Schollmeyer, E. (1993) Designing surface properties of textile fibers by UV-laser irradiation. Appl SurfSci, 69(1-4):12-5.

Kan, C.W. (2008) Impact on textile properties of polyester with laser, Optics & Laser Technology, 40, 113-119.

Knittel, D.; Schollmeyer, E. (1998) Surface Structuring of Synthetic Polymers by UV-Laser Irradiation. Part IV. Applications of Excimer Laser Induced Surface Modification of Textile Materials, Polymer International 45, 103-117.

Wong, W.; Chan, K.; Yuen, KW.; Lau, KS (2000) Morphological modification of poly(ethyleneterephthalate)Surface after Excimer Laser Irradiation, Journal of Textile Engineering,Vol. 46 No. 2 pp.25-31.

Wong, W.; Chan, K.; Yeung, KW; Lau, KS. (2001) Chemical modification of poly(ethylene terephthalate) induced by laser treatment, Textile Res J, 71(2):117-20,