Wettability monitoring by surface modification with femtosecond laser irradiation.

Coman, Diana ; Vrinceanu, Narcisa ; Grigoriu, Aurelia 等

1. INTRODUCTION

Laser irradiation of textiles can generate characteristic modifications of surface morfology. In many industrial applications there is a need to modify the polymer surface by keeping unchanged their bulk properties. Chemical activation of surfaces is the most commonly used method, however, the ecological requirements have forced the industry to search for alternative environmental safety methods. Surface modification of polymers by means of laser irradiation has also lead to a roughness alteration, achieving a ripple-like microstructure.

Due to low surface energy (approximately 20-25 mj/[m.sup.2]), viscose has very weak hydrophilic properties. The aim of the present work is to investigate the surface modification of viscose fabrics by means of laser treatment so as to improve their hydrophilic properties. Many laser applications in the surface modification field were made at reduced fluences (Zorba et al, 2006). Excimer UV laser impact on textile properties as a result of surface modifications has been researched on ( Kan 2008).

This work is focused on laser surface activation of viscose textiles and controlled observance of permeability properties. The novelty of this approach lies in the laser treatment application and the wettability monitoring on regenerated cellulose, so far such studies having been made mostly on synthetic fibres.

2. EXPERIMENTAL

2.1 Materials

In this study the laser activation of 100% viscose fabrics was investigated. The conditioned samples are employed before the preparation stage technology.

2.2. Laser treatment

The surface activation was provided by irradiation with a femtosecond (fs) LPX 200 Excimer KrF laser operating at 248 nm with the parameters shown in Table 1. In high-fluence laser irradiation, samples were irradiated directly from the laser beam without using either special photomask or focusing lens.

The laser energies in terms of applied fluence and number of pulses varied in different experiments so as to study their effects upon samples. Laser fluence was regulated in range from 29 to 43 mJ/[cm.sup.2] and the number of pulses varied between 0 and 4, with the pulse repetition constant at 1Hz to avoid any possible heat accumulation. At first, several attempts at optimizing the process of surface modification have been made.

Water contact angle was measured using a Sigma 700 computer-controlled research tensiometer. Water permeability of samples before and after laser treatment was determined by measuring the time necessary for the penetration of 5 ml of the testing liquid across the sample. The absorption properties of sample were measured as a difference of the weights of the sample before and after the dipping in the testing liquid. The contact angle was measured directly from the observation of the solid-liquid meniscus.

3. RESULTS AND DISCUSSION

Laser treatment of viscose proceeds by a free-radical mechanism that introduces a wide variety of oxidized functional groups onto the surface of the treated polymer. These oxidized functional groups may include C-OH, C=O, COOH, C-O-C, or hydroperoxide and they are responsible for the change in the polymer surface properties. The surface properties of treated and untreated samples were characterized by means of the contact angle measurement and the water permeability measurement (Xiaodong et al, 2004). The contact angle of liquid on solid is closely related to surface free energy and this parameter is useful in the discussion of hydrophilicity, absorbency of sample and adhesivity. For the determination of total surface energy from contact angle measurement Wu's equation (Tammar et al, 2004) and Owens-Wendt-Kaeble equation (Lam et al, 2001) were used for plots from diagrams presented below. Figure 1 shows the dependence of the water contact angle and water permeability on the laser treatment number of pulses. Fast increase in permeability was observed during the first 2 pulses, followed by a slow increase with increasing the number of pulses. However, the water contact angle linearly decreases with increasing of pulses. The effect of the fluences on the water contact angle and water permeability was tested and the results are shown in Figure 2. Strong influence of the fluence of hydrophilic properties of viscose textiles was observed. The permeability exponentially increases with increasing fluence, however, the water contact angle decreases linearly. The contact angle decreased linearly with increasing laser treatment time. Laser irradiation treatment longer than 4 pulses can cause hydrophilisation of textile surface. As a result, the optimal laser treatment time at this condition is 4 pulses. After 4 pulses one can perform to the burning of fabric surface.

The aim of our work has been to increase the absorption capacity of the material, as well.

For the laser surface activation, it is possible to reach 500% absorption improvement of viscose textiles as is shown on Figure 4.

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The water contact angle decreased to 70[degrees]. The laser treatment acted only on the surface of the viscose material i.e. the water contact angle decreased to 80[degrees], however the absorption increased only up to 90 % (see Figure 5).

4. CONCLUSION

The experimental part of this study has shown the utility of laser treatment on viscose textiles as well as highlighted the following problems:

* It is obvious that femtosecond laser assisted-modification of viscose surface induces remarkable change in wettability of the textile material resulting from contact angle modifications and water permeability.

* High water absorption due to variation of irradiation parameters (fluence, pulse width, pulse number) will lead to a controlled improvement of comfort properties.

* Changes in the observed wettability may be attributed to a sinergy of surface chemistry and roughness effects.

* Laser surface modification will constitute the most effective tool for a non-contact, ecological technique for such textile types.

The aim of further experiments will be the optimization of laser treatment conditions with the purpose of reaching the lowest laser exposure time and increasing the stability of induced properties.

5. ACKNOWLEDGEMENTS

The authors would like to thank to Prof. FOTAKIS Costas, Director of Institute of Electronics Structure and Lasers (IESL), Foundation of Research Technology Hellas (FORTH), Heraklion, Greece, for having facilitated access to the abovementioned institute's labooratories and also to the technical assistance offered by Dr. POULI and Ms. MELLESANAKI.

6. REFERENCES

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

Lam, C.N.C.; Ko, R.H.Y.(2001). Dynamic cycling contact angle measurements: study of advancing and receding contact angles, J.Colloid Interf.Sci., 243, 208

Tammar, S.; Meiron, A.; Sam Saguy, I. (2004). Contact angle measurement on rough surfaces, J.Colloid Interf.Sci.,274, 6

Xiaodong W.; Xiaofenand, P.; Buxuan W. (2004). Contact angle hysteresis and hysteresis tension on rough solid surface, Chinese J. Chem. Eng. 12 (5) 615

Zorba, V.; Persano, L.; Pisignano, D.; Athanassiou, A.; Stratakis, E.; Cingolani, R.; Tzanetakis, P.; Fotakis, C. (2006). Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation. Nanotechnology, Vol.17, (3234-3238).

Tab. 1. Table with some parameters
of laser irradiation treatment

E (mJ) S F Pulses
 /0,4 ([cm.sub.2]) (J/[cm.sub.2])

170.00 0.39 0.435897 4p
170.00 0.39 0.435897 2p
170.00 0.45 0.377778 1p
170.00 0.45 0.377778 4p
170.00 0.57 0.298246 2p
170.00 0.57 0.298246 1p