Improving the Dyeing of Polypropylene by Surface Fluorination

: The surface of polypropylene (PP) was modiﬁed with ﬂuorine gas at 25 ◦ C and 10–380 Torr for 1 h. The surface roughness of the ﬂuorinated PP samples was approximately two times larger than that (5 nm) of the untreated sample. The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy showed that the bonds (e.g., -C-C- and -CH x ) derived from raw PP decreased and were converted into ﬂuorinated bonds (e.g., -CF x ) after surface ﬂuorination. These ﬂuorinated bonds showed higher electronegativity according to the zeta potential results. Fluorinated PP could be stained with the methylene blue basic dye because of the increased surface roughness and the negatively charged surface. surface of the than of the untreated sample. Fine irregularities due to surface ﬂuorination formed on the sample surface at the nanoscale level, but not at the microscale level. The water contact angle of the untreated PP sample was approximately 94 ◦ . The water contact angles of the PP samples ﬂuorinated at 10–380 Torr to of the untreated sample. 2p3/2 1s O2 ﬂuorine atoms internal PC formation of HF in the staining process. F atomic percentages below 10% after staining with O2 dye, Table Although the PP resin is difﬁcult to dye, staining with MB solution was achieved after surface ﬂuorination owing to the formation of a dyeable layer. The formed ﬂuoride layer had a high surface roughness and negative surface charge, which allowed the retention of MB molecules. In addition, the staining capacity of the PP surface can be controlled by adjusting the ﬂuorination conditions.


Introduction
Polymers are some of the most used materials in daily life, because of their light weight, affordability, and useful mechanical and chemical properties. Many types of polymers with desirable properties have been developed and produced [1][2][3]. Among them, polypropylene (PP) is an olefin polymer of great commercial importance because of its low cost and many attractive properties [4,5]. As a saturated hydrocarbon, PP has a low surface energy and is classified as a nonspecific adsorbent, indicating its poor wettability by water and very low adsorptive and adhesive properties in relation to polar liquid [6]. PP cannot be dyed by conventional methods because of its high crystallinity and its nonpolar aliphatic structure that does not contain reactive sites [7]. However, the main difficulty arises from the absence of sites in which hydrogen bonding or electrostatic attraction can occur. The surface of commercial PP plastic can be modified to solve this problem. For example, hydrophilic modifications are beneficial for dyeing, metal plating, and other coating needs [8][9][10]. Surface modification methods for hydrophilizing and roughening plastic surfaces include physical treatments, such as plasma [11], ozone [12], UV light [13], corona discharge [14], and wet chemical treatments [15], and ion irradiation [16]. However, such treatments are expensive and cannot be applied to complex geometries. In addition to these conventional chemical and physical methods, direct fluorination has been investigated [17,18]. This process involves a gas-phase chemical reaction between fluorine gas (F 2 ) and the polymer surface. It is an effective chemical method for modifying and controlling the physicochemical surface properties of polymers. In a previous study, the surfaces of polyethylene terephthalate and polycarbonate (PC) were modified by direct fluorination to achieve high adhesion with plating and good dyability [19,20]. F 2 has strong electronegativity and high chemical reaction capability, which enables the introduction of polar groups (e.g., -C-F covalent bonds) onto the polymer surface via free-radical substitution reactions [20]. In the pigment coloration process, a colorant, usually a pigment, is added to the molten polymer before the actual spinning process [21]. This method is called "solution dyeing" or "mass coloration." This spin coloration of PP has been widely

Material Characterization
The chemical compositions of the untreated and modified PP samples were determined by Fourier-transform infrared (FTIR) absorption spectroscopy (Nicolet 6700; Thermo Electron Scientific, Waltham, MA, USA). The analysis was conducted in transmittance mode at 650-4000 cm −1 , from which 32 scans were acquired and air background removal was performed. The surface chemical states of untreated and modified PP samples were determined by X-ray photoelectron spectroscopy (XPS; JPS-9010, JOEL). All the binding energies were referenced to a carbon peak at 285.0 eV. The surface morphologies of untreated and modified PP samples were determined by a confocal laser scanning microscope (OLS5000; Olympus, Waltham, MA, USA), while the surface topography was evaluated by atomic force microscopy (AFM; Nanoscope IIIa, Digital Instruments, Tokyo, Japan). Scanning was conducted in the tapping mode within an area of 10 µm × 10 µm. The arithmetic mean surface roughness (Ra) was determined from the AFM roughness profile. The static water contact angles of the untreated and modified PP samples were measured at 25 • C using the sessile drop method. A 10 µL water droplet was used in a telescopic goniometer with a magnification power of 23× and a protractor with a graduation of 1 • (Krüss G10, Hamburg, Germany). Five measurements were acquired at different surface locations on each sample to determine the average value (±2 • ). The zeta potential profiles of the PP samples were measured using a solid sample cell unit with a zeta potential device (Otsuka Electronics Co., Ltd., ElSZ-2, Osaka, Japan).

Dye Staining of Polypropylene
Methylene blue (MB; Fujifilm Wako Chemical Corp., Hirono, Fukushima, Japan) and acid orange 7 (O2: OrangeII; Nacalai Tesque, Inc., Kyoto, Japan), shown in Figure 1, were the representative basic and acidic dyes, respectively. Staining solutions of 0.4 g/L dye in ultrapure water were placed in a water bath at 80 • C, and the PP samples were immersed for 30 min. The PP samples were then washed with ultrapure water and air-dried. The surface staining of each sample was evaluated based on the S and N content determined by XPS.
Colorants 2022, 1,3 ultrapure water were placed in a water bath at 80 °C, and the PP samples were immersed for 30 min. The PP samples were then washed with ultrapure water and air-dried. The surface staining of each sample was evaluated based on the S and N content determined by XPS.
(a) (b) Figure 1. Dye molecular structures of (a) methylene blue and (b) orange 2. Figure 2 shows the surface morphology and hydrophilicity of the untreated and fluorinated PP samples measured using confocal laser scanning microscopy (CLSM), AFM, and water contact angle test. Compared with the CLSM image of the untreated sample, the surfaces of the fluorinated samples did not change. The AFM image of the untreated sample showed a relatively flat and smooth surface with a low surface roughness of ~5.002 nm. However, the surface roughness of the fluorinated PP samples increased with increasing F2 gas pressure. The surface roughness of the F-380 sample was approximately 2.3 times higher than that of the untreated sample. Fine irregularities due to surface fluorination formed on the sample surface at the nanoscale level, but not at the microscale level. The water contact angle of the untreated PP sample was approximately 94°. The water contact angles of the PP samples fluorinated at 10-380 Torr were similar to that of the untreated sample.   Figure 2 shows the surface morphology and hydrophilicity of the untreated and fluorinated PP samples measured using confocal laser scanning microscopy (CLSM), AFM, and water contact angle test. Compared with the CLSM image of the untreated sample, the surfaces of the fluorinated samples did not change. The AFM image of the untreated sample showed a relatively flat and smooth surface with a low surface roughness of~5.002 nm. However, the surface roughness of the fluorinated PP samples increased with increasing F 2 gas pressure. The surface roughness of the F-380 sample was approximately 2.3 times higher than that of the untreated sample. Fine irregularities due to surface fluorination formed on the sample surface at the nanoscale level, but not at the microscale level. The water contact angle of the untreated PP sample was approximately 94 • . The water contact angles of the PP samples fluorinated at 10-380 Torr were similar to that of the untreated sample.

Effects of Fluorination on the Surface Morphology
for 30 min. The PP samples were then washed with ultrapure water and air-dr surface staining of each sample was evaluated based on the S and N content det by XPS.  Figure 2 shows the surface morphology and hydrophilicity of the untrea fluorinated PP samples measured using confocal laser scanning microscopy AFM, and water contact angle test. Compared with the CLSM image of the untrea ple, the surfaces of the fluorinated samples did not change. The AFM image o treated sample showed a relatively flat and smooth surface with a low surface ro of ~5.002 nm. However, the surface roughness of the fluorinated PP samples in with increasing F2 gas pressure. The surface roughness of the F-380 sample was mately 2.3 times higher than that of the untreated sample. Fine irregularities du face fluorination formed on the sample surface at the nanoscale level, but not a croscale level. The water contact angle of the untreated PP sample was approxima The water contact angles of the PP samples fluorinated at 10-380 Torr were simila of the untreated sample.   Figure 3 shows the FTIR spectra of untreated and fluorinated PP samples. The spectrum of the untreated PP sample exhibited absorption bands at 2960 and 2950 cm −1 (-CH 3 , asymmetry), 2919 cm −1 (-CH 2 -, asymmetry), 2867 cm −1 (-CH 3 , symmetry), 2839 cm −1 (-CH 2 -, symmetry), 1458 cm −1 (-CH 2 -, bending), and 1376 cm −1 (-CH 3 , Wagging). After fluorination, absorption bands appeared at 700-770 cm −1 (-CF 3 ) and 1000-1200 cm −1 (-CF, -CF 2 -). In contrast, the absorption intensity of the bonds derived from PP (e.g., -CH 3 , CH 2 ) decreased. Fluorination at higher F 2 gas pressures enhanced the peaks associated with fluorinated bonds (-CF, -CF 2 , -CF 3 ), and weakened the peaks associated with PP. As the peaks of the -CH 2 -and -CH 3 bonds almost disappeared in the fluorinated sample with a F 2 gas pressure of 380 Torr (F-380), we considered that the fluorinated layer was formed up to the detection limit depth of several micrometers by FTIR. In other words, fluorination can decrease the crystallinity of the PP surface. As a result, as the F 2 gas pressure increased, the formation rate of the fluorinated layer per unit time increased as the crystallinity of PP decreased.  Figure 3 shows the FTIR spectra of untreated and fluorinated PP samples. The spectrum of the untreated PP sample exhibited absorption bands at 2960 and 2950 cm −1 (-CH3, asymmetry), 2919 cm −1 (-CH2-, asymmetry), 2867 cm −1 (-CH3, symmetry), 2839 cm −1 (-CH2-, symmetry), 1458 cm −1 (-CH2-, bending), and 1376 cm −1 (-CH3, Wagging). After fluorination, absorption bands appeared at 700-770 cm −1 (-CF3) and 1000-1200 cm −1 (-CF, -CF2-). In contrast, the absorption intensity of the bonds derived from PP (e.g., -CH3, CH2) decreased. Fluorination at higher F2 gas pressures enhanced the peaks associated with fluorinated bonds (-CF, -CF2, -CF3), and weakened the peaks associated with PP. As the peaks of the -CH2-and -CH3 bonds almost disappeared in the fluorinated sample with a F2 gas pressure of 380 Torr (F-380), we considered that the fluorinated layer was formed up to the detection limit depth of several micrometers by FTIR. In other words, fluorination can decrease the crystallinity of the PP surface. As a result, as the F2 gas pressure increased, the formation rate of the fluorinated layer per unit time increased as the crystallinity of PP decreased.   Compared with the F-10 sample, the -CF 3 peak increased in the F-100 and F-380 samples. The F 1s peaks were detected in all fluorinated samples. Similarly to C 1s peaks, the intensity of F 1s peaks increased in the F-100 and F-380 samples. To confirm the changes in the temporal composition content of each sample, the elemental composition contents (Table 2) of C, O, and F were evaluated according to the XPS results ( Figure 4). After fluorination, the F content significantly increased with decreasing C content, which may lead to the formation of -C-Fx bonds with CF 4 gasification.    Figure 5 shows the peak-fitting results for the C1s peak of fluorinated samp ure 4). The portion ratio (%) of each bond derived from Figure 5 is shown in F After fluorination, the C-C bonds at 285 eV disappeared from all fluorinated sam the case of the F-10 sample, the C-O bond, -CHF-, -CF2-, and -CF3 were approximat 36%, 32%, and 9%, respectively. For the F-100 and F-380 samples, the percentages bonds decreased, and those of -CF3 bonds both increased to approximately 20% were no significant differences in the ratios of the peaks (CO, -CHF-, -CF2-, and the surfaces of the F-100 and F-380 samples. However, the AFM ( Figure 2) and FT ure 3) results showed that the fluorination of the F380 sample may diffuse into the samples. Consequently, the C-C bonds on the untreated surface were converted in in the form of -C(=O)OH groups with moisture in air. The content of polar group that induced hydrogen bond adsorption with water also increased after fluorinat   Figure 5 shows the peak-fitting results for the C1s peak of fluorinated samples (Figure 4). The portion ratio (%) of each bond derived from Figure 5 is shown in Figure 6. After fluorination, the C-C bonds at 285 eV disappeared from all fluorinated samples. In the case of the F-10 sample, the C-O bond, -CHF-, -CF 2 -, and -CF 3 were approximately 23%, 36%, 32%, and 9%, respectively. For the F-100 and F-380 samples, the percentages of C-O bonds decreased, and those of -CF 3 bonds both increased to approximately 20%. There were no significant differences in the ratios of the peaks (CO, -CHF-, -CF 2 -, and -CF 3 ) on the surfaces of the F-100 and F-380 samples. However, the AFM ( Figure 2) and FTIR (Figure 3) results showed that the fluorination of the F380 sample may diffuse into the internal samples. Consequently, the C-C bonds on the untreated surface were converted into -C=O in the form of -C(=O)OH groups with moisture in air. The content of polar groups (-C-F x ) that induced hydrogen bond adsorption with water also increased after fluorination.  After surface fluorination, the number of bonds derived from PP decreased and were converted into fluorinated bonds (e.g., -CFx). The fluorinated bonds formed on the PP surface had higher electronegativity according to the zeta potential results (Figure 7). The zeta potential on the surface of the untreated sample was weakly negatively charged. With increasing F2 pressure, the zeta potential of the fluorinated samples tended to increase negatively, as shown in Figure 7. In particular, the zeta potential (−36.09 mV) of the F-380 sample was approximately 2.6 times higher than that of the untreated sample (−13.8 mV). This may be attributed to the increase in the number of polar groups (e.g., -CHF-and -C-   After surface fluorination, the number of bonds derived from PP decreased and were converted into fluorinated bonds (e.g., -CFx). The fluorinated bonds formed on the PP surface had higher electronegativity according to the zeta potential results (Figure 7). The zeta potential on the surface of the untreated sample was weakly negatively charged. With increasing F2 pressure, the zeta potential of the fluorinated samples tended to increase negatively, as shown in Figure 7. In particular, the zeta potential (−36.09 mV) of the F-380 sample was approximately 2.6 times higher than that of the untreated sample (−13.8 mV). This may be attributed to the increase in the number of polar groups (e.g., -CHF-and -C- After surface fluorination, the number of bonds derived from PP decreased and were converted into fluorinated bonds (e.g., -CF x ). The fluorinated bonds formed on the PP surface had higher electronegativity according to the zeta potential results (Figure 7). The zeta potential on the surface of the untreated sample was weakly negatively charged. With increasing F 2 pressure, the zeta potential of the fluorinated samples tended to increase negatively, as shown in Figure 7. In particular, the zeta potential (−36.09 mV) of the F-380 sample was approximately 2.6 times higher than that of the untreated sample (−13.8 mV). This may be attributed to the increase in the number of polar groups (e.g., -CHF-and -C-F x ) on fluorinated samples. The negatively enhanced surface after fluorination corresponds to the results of a previous paper [20].

Effects of Fluorination on the Surface Composition and Structure
Colorants 2022, 1,7 Fx) on fluorinated samples. The negatively enhanced surface after fluorination corresponds to the results of a previous paper [20].  Figure 8 shows the results of the dyeing tests on untreated and fluorinated PP samples. Dyeing tests were performed using MB and O2 solutions as the representative basic and acidic dyes, respectively. In the case of staining with (c) O2 acidic dye, no staining of fluorinated PP samples occurred. In contrast, staining occurred in untreated and fluorinated PP samples with (a) MB basic dye. In particular, the visible degree of deep coloring increased in the fluorinated samples. This was attributed to the surface state of fluorinated PP, in which F on the PP surface allowed for high electronegativity and acidity (Figure 7). Unlike the O2 dye, MB has cationic properties that allow easy adsorption on the negative surface of fluorinated PP samples via Coulomb attraction [24]. Thus, fluorinated PP samples can be effectively stained using basic dyes but not acid dyes. In the case of (b) using rhodamine B as the basic dye, staining was also achieved on fluorinated PP, similarly to that with MB.   Figure 8 shows the results of the dyeing tests on untreated and fluorinated PP samples. Dyeing tests were performed using MB and O2 solutions as the representative basic and acidic dyes, respectively. In the case of staining with (c) O2 acidic dye, no staining of fluorinated PP samples occurred. In contrast, staining occurred in untreated and fluorinated PP samples with (a) MB basic dye. In particular, the visible degree of deep coloring increased in the fluorinated samples. This was attributed to the surface state of fluorinated PP, in which F on the PP surface allowed for high electronegativity and acidity (Figure 7). Unlike the O2 dye, MB has cationic properties that allow easy adsorption on the negative surface of fluorinated PP samples via Coulomb attraction [24]. Thus, fluorinated PP samples can be effectively stained using basic dyes but not acid dyes. In the case of (b) using rhodamine B as the basic dye, staining was also achieved on fluorinated PP, similarly to that with MB.

Dyeing of Surface-Modified PP Plates
Colorants 2022, 1,7 Fx) on fluorinated samples. The negatively enhanced surface after fluorination corresponds to the results of a previous paper [20].  Figure 8 shows the results of the dyeing tests on untreated and fluorinated PP samples. Dyeing tests were performed using MB and O2 solutions as the representative basic and acidic dyes, respectively. In the case of staining with (c) O2 acidic dye, no staining of fluorinated PP samples occurred. In contrast, staining occurred in untreated and fluorinated PP samples with (a) MB basic dye. In particular, the visible degree of deep coloring increased in the fluorinated samples. This was attributed to the surface state of fluorinated PP, in which F on the PP surface allowed for high electronegativity and acidity (Figure 7). Unlike the O2 dye, MB has cationic properties that allow easy adsorption on the negative surface of fluorinated PP samples via Coulomb attraction [24]. Thus, fluorinated PP samples can be effectively stained using basic dyes but not acid dyes. In the case of (b) using rhodamine B as the basic dye, staining was also achieved on fluorinated PP, similarly to that with MB.  The exhaustion of MB and O2 dyes after surface staining of the PP samples was evaluated by XPS analysis (Figures 9 and 10). The S content of the adsorbed MB and O2 dyes were determined according to the S 2p3/2 XPS spectra. As shown in Figure 9, the S 2p3/2 peak was detected in all samples. The intensity of the S 2p3/2 peak of the fluorinated PP samples was much higher than that of the untreated PP sample. As shown in Table 3, the S atomic percentages of the fluorinated samples were approximately 2.6 times higher than that of the untreated sample. From the results of the C 1s peak before and after staining with an MB solution, the intensity of carbon-fluorine bonds (e.g., -CF x ) significantly decreased after staining with MB solution. The F atomic percentages were reduced by half, as indicated in Table 3. It may be considered that a substitution reaction between the MB dye and fluorine occurred in the fluorinated PP [25]. This enhanced the degree of dyeing of the PP samples by Coulomb attraction. In the case of the untreated PP sample, however, it may be attributed to physical absorption via Coulomb attraction between the positively charged dyes and the negatively charged PP surface. Comparing the dyeability with the fluorinated PP samples, the S content derived from MB in F-10 samples was higher than those of other fluorinated samples, as indicated in Table 3. The dyeability of F-10 samples seemed to be better than those of other things in this study. It may be reasoned for good wetting ability of F-10 samples having a lower contact angle with water ( Figure 2). If there are changes in the staining conditions, such as staining time and temperature of the MB solution, however, the dyeability of F-380 samples may become superior to the dyeability others because of their higher surface roughness ( Figure 2) and negative surface charge (Figure 7). In the case of staining with the O2 dye, no S 2p3/2 peak was observed in any of the samples, as shown in Figure 10. Table 4 indicates that there were almost no S 2p3/2 peaks in any of the samples. The C 1s peak after staining with O2 dye is similar to that before staining, as shown in Figure 10. Some fluorine atoms physically adsorbed on the internal PC decreased, presumably by the formation of HF in the staining process. The F atomic percentages decreased to below 10% after staining with O2 dye, as shown in Table 4. Although the PP resin is difficult to dye, staining with MB solution was achieved after surface fluorination owing to the formation of a dyeable layer. The formed fluoride layer had a high surface roughness and negative surface charge, which allowed the retention of MB molecules. In addition, the staining capacity of the PP surface can be controlled by adjusting the fluorination conditions. The exhaustion of MB and O2 dyes after surface staining of the PP samples w uated by XPS analysis (Figures 9 and 10). The S content of the adsorbed MB and were determined according to the S 2p3/2 XPS spectra. As shown in Figure 9, th peak was detected in all samples. The intensity of the S 2p3/2 peak of the fluori samples was much higher than that of the untreated PP sample. As shown in Ta S atomic percentages of the fluorinated samples were approximately 2.6 times hig that of the untreated sample. From the results of the C 1s peak before and after with an MB solution, the intensity of carbon-fluorine bonds (e.g., -CFx) signific creased after staining with MB solution. The F atomic percentages were reduced as indicated in Table 3. It may be considered that a substitution reaction betwee dye and fluorine occurred in the fluorinated PP [25]. This enhanced the degree o of the PP samples by Coulomb attraction. In the case of the untreated PP sample, it may be attributed to physical absorption via Coulomb attraction between the p charged dyes and the negatively charged PP surface. Comparing the dyeability fluorinated PP samples, the S content derived from MB in F-10 samples was hig those of other fluorinated samples, as indicated in Table 3. The dyeability of F-10 seemed to be better than those of other things in this study. It may be reasoned wetting ability of F-10 samples having a lower contact angle with water ( Figure 2 are changes in the staining conditions, such as staining time and temperature o solution, however, the dyeability of F-380 samples may become superior to the d others because of their higher surface roughness ( Figure 2) and negative surfac ( Figure 7). In the case of staining with the O2 dye, no S 2p3/2 peak was observed the samples, as shown in Figure 10. Table 4 indicates that there were almost n peaks in any of the samples. The C 1s peak after staining with O2 dye is simil before staining, as shown in Figure 10. Some fluorine atoms physically adsorbe internal PC decreased, presumably by the formation of HF in the staining proce atomic percentages decreased to below 10% after staining with O2 dye, as shown 4. Although the PP resin is difficult to dye, staining with MB solution was achie surface fluorination owing to the formation of a dyeable layer. The formed fluor had a high surface roughness and negative surface charge, which allowed the ret MB molecules. In addition, the staining capacity of the PP surface can be cont adjusting the fluorination conditions.    Table 4. Atomic percentages of PP samples evaluated from XPS results ( Figure 10).

Conclusions
The surfaces of PP plates were successfully modified via fluorination treat F2 gas. Increasing the F2 pressure increased the surface roughness of the PP pla to the release of CF4 gas. Fluorination at higher F2 gas pressures enhanced the p ciated with fluorinated bonds and weakened the peaks associated with PP. Aft fluorination, the bonds derived from PP were converted into polar groups (e which increased the electronegativity on the surface. In the dyeing test, the fluor samples stained with the MB basic dye exhibited deep coloring. They can be e stained using basic dyes but not acid dyes. It may be considered that a substitu tion occurred between the MB dye and fluorine, which enhanced the degree of PP samples by Coulomb attraction. Consequently, the dyeable surface of the PP enhanced by surface fluorination owing to the increased surface roughness and tively charged surface.    Table 4. Atomic percentages of PP samples evaluated from XPS results ( Figure 10).

Conclusions
The surfaces of PP plates were successfully modified via fluorination treatment with F 2 gas. Increasing the F 2 pressure increased the surface roughness of the PP plates owing to the release of CF 4 gas. Fluorination at higher F 2 gas pressures enhanced the peaks associated with fluorinated bonds and weakened the peaks associated with PP. After surface fluorination, the bonds derived from PP were converted into polar groups (e.g., -C-F x ), which increased the electronegativity on the surface. In the dyeing test, the fluorinated PP samples stained with the MB basic dye exhibited deep coloring. They can be effectively stained using basic dyes but not acid dyes. It may be considered that a substitution reaction occurred between the MB dye and fluorine, which enhanced the degree of dyeing of PP samples by Coulomb attraction. Consequently, the dyeable surface of the PP resin was enhanced by surface fluorination owing to the increased surface roughness and the negatively charged surface.