สำนักราชบัณฑิตยสภา

233 Suda Kiatkamjornwong et al วารสารราชบัณฑิตยสถาน ปีที่ ๓๗ ฉบับที่ ๑ ม.ค.-มี.ค. ๒๕๕๕ Table 2 Surface energies of PLA sheets at various exposure times to the oxygen plasma Treatment Contact angle (º) Surface energy (mN m -1 ) Power Exposure Deionized Methylene γ s γ s γ s (W) time Water iodide p d 0 0 68.1±1.6 47.8±2.6 14 26 40 30 30 s 61.6±2.6 41.7±1.6 25 26 51 30 10 min 54.7±3.0 44.2±1.3 25 23 48 30 15 min 57.3±0.9 48.1±1.1 23 22 45 30 20 min 50.3±1.3 46.3±1.3 28 22 50 30 25 min 50.8±0.9 45.5±0.7 27 23 50 30 30 min 55.1±0.7 46.1±0.9 26 23 49 30 40 min 51.9±0.9 45.2±0.6 26 23 49 Printing quality As shown in Figure 3, the quality of the printed sheets depends largely on the oxygen plasma treatment, i.e., the power and exposure. The higher power of the treater, 70 W and 100 W, produced low printing qualities [Figure 3 (b) and Figure 3 (c)] when compare with the lower treater power, 30 W, with the longer exposure time, 20 and 30 min [Figure 3 (d) and Figure 3 (e)]. More details are given below. Effect of plasma power Although the surface energy of the untreated PLA sheet is about 10 mN m -1 higher than those of the inks, the inks cannot spread evenly on the areas of high tonal density [Figure 3(a)]. The maximum tonal density to achieve the even ink spreading was found at 40% tone (Figure 4). Color bleeding was also found in the red to green and black colors [Figure 3(a)]. The better print qualities were found in the treated areas where increasing the treating power increased to give a higher tonal density up to 70% and with a little smooth leveling. When increasing the exposure times, all the color inks printed on the tone reproduction wedges became smoothly spread. The optimal exposure time for the good leveling quality was found with the exposure times of 20 to 25 min (Figures 3-5); afterwards, a slightly poor leveling or spreading was seen in the printed strips. Less color bleeding was observed when a longer exposure time was given to the PLA sheet [Figure 3(d) and 3(e)].