3.2. The effect of laser wavelength

3.2. The effect of laser wavelength and micro-hole density on theWVPFig. 4 shows the WVP value of the laser-drilled PU syntheticleather. The samples for WVP test were prepared according to theoptimized laser parameters listed in Table 2. It could be observedthat the WVP showed an upward trend with a decrease in laserwavelength.The WVP values of all laser-drilled samples weresignificantly influenced by laser beam wavelength at the fixedmicro-hole density. The shorter the laser wavelength, the higherthe WVP value was. Meanwhile, the greater the micro-hole density,the higher the WVP value was. The highest WVP growth ratio was38.4%, 46.8% and 53.5% achieved by the 1064 nm laser, the 532 nmlaser and the 355 nm laser, respectively. In comparison with oxy-gen plasma treating method (with the highest WVP growth ratio of40.2%) reported by Chen et al. [11], both 532 nm-laser and 355 nm-laser drilled PU synthetic leathers present higher WVP growth ratio.In addition, it takes 120 s for the oxygen plasma treatment to get thehighest WVP growth ratio. Meanwhile, laser micro-drilling processspends only about 10 s to get the maximum WVP value. It is obviousthat laser micro-drilling process is characterized by a much higherefficiency.