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Journal of Oleo ScienceCopyright ©2015 by Japan Oil Chemists’ Societydoi : 10.5650/jos.ess15033J. Oleo Sci. 64, (8) 817-824 (2015)

Introduction of Atmospheric Pressure Plasma to Aqueous Detergent ProcessesKeiko Gotoh1＊ , Yu Kanasaki2 and Haruka Uchinomaru1

1 Faculty of Human Life and Environment, Nara Women’s University (Kita-uoya-nishi-machi, Nara 630-8506, JAPAN)2 School of Natural Science and Ecological Awareness, Nara Women’s University (Kita-uoya-nishi-machi, Nara 630-8506, JAPAN)

1 INTRODUCTIONIn recent years, organic solvents as cleaning media have

been regulated from the viewpoints of environmental and human health issues. Water, as a sustainable cleaning medium, has high polarity and hence cannot remove oily contaminants by dissolution. To promote soil removal in aqueous detergent systems, hydrophilization of the soil and the substrate is expected to be effective because it en-hances liquid wetting in the first stage of cleaning action1, 2）.

Exposure to plasma prior to aqueous cleaning is a tech-nique for hydrophilization of polymer substrates soiled with organic species in which oxidation of the exposed surface can be expected. In the plasma state, which is gen-erated by electric discharge, gaseous molecules are dissoci-ated to first form a gas of atoms and then a gas of freely moving electrons and positive ions3－5）. Oxygen and nitro-gen radicals in plasmas can bind to substrate and soil sur-faces to produce functional groups capable of hydrogen bonding. Although applications of plasma treatment to

＊Correspondence to: Keiko Gotoh, Faculty of Human Life and Environment, Nara Women’s University, Kita-uoya-nishi-machi, Nara 630-8506, JAPANE-mail: [email protected] April 24, 2015 (received for review February 11, 2015)Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 onlinehttp://www.jstage.jst.go.jp/browse/jos/　　http://mc.manusriptcentral.com/jjocs

textile processes in the fields of scouring and bleaching have been proposed6, 7）, no work, to our knowledge, has been conducted on plasma pretreatment for laundering. In addition, processing with low-pressure plasmas is expen-sive, requires maintenance for vacuum systems, and is re-stricted in the size of the object that can be treated8）.

The atmospheric pressure plasma（APP）jet, which can generate plasma plumes containing a number of active species at high concentration in open space and achieve continuous in-line material processing at high speed, has been recently developed9, 10）. The authors11－13） have inves-tigated the effect of APP treatment on the contact angle of water on a PET film and fiber using the sessile drop and Wilhelmy techniques. After exposure to the APP jet, the wettability of these surfaces was found to increase remark-ably due to surface oxidation. Based on these experimental results, various fabrics soiled with carbon black or oleic acid were exposed to the APP before cleaning in aqueous detergent solutions in the previous studies14, 15）. The deter-

Abstract: The effects of exposure of polymer surfaces to atmospheric pressure plasma (APP) on detergency were investigated from the viewpoint of pretreatment to cleaning in aqueous systems using three PET substrates: film, mesh, and fabric. The PET substrates were soiled with stearic acid as a model oily contaminant, and were treated with the APP jet immediately before cleaning. Stir washing in aqueous solutions with and without alkali or anionic surfactant was performed, and then the detergency was evaluated from the microscopic image analysis or surface reflectance measurement. For all PET samples and detergent solutions, APP exposure was found to promote the removal of stearic acid. Contact angle measurements showed that APP exposure enhanced the hydrophilicity of PET and stearic acid. The increase in the surface oxygen concentration on PET and stearic acid due to the APP exposure was also observed by XPS analysis. The simultaneous oxidation of the PET substrate and stearic acid soil by the APP pretreatment resulted in detergency improvement via surface hydrophilization. Furthermore, microscopic observations suggested that the collapse of crystallized stearic acid deposited on the PET substrate by APP heating facilitated its removal. In situ detergency evaluation by a quartz crystal microbalance technique confirmed that the removal of stearic acid from the PET substrate was promoted by the APP exposure. The experimental findings of this study demonstrate the effectiveness of the APP exposure before cleaning in aqueous solutions.

Key words: atmospheric pressure plasma, PET substrate, stearic acid, detergency, quartz crystal microbalance

K. Gotoh, Y. Kanasaki and H. Uchinomaru

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gency evaluated by the surface reflectance method indicat-ed that soil removal from the fabrics was promoted and soil redeposition onto the fabrics was prevented by the APP pretreatment.

In the present study, three PET substrates with different geometry̶film, mesh, and fabric̶were used in detergen-cy experiments in order to obtain further experimental evi-dence that the APP pretreatment is applicable to aqueous cleaning processes for various PET products. The removal efficiency of stearic acid soil from the PET substrates was evaluated from microscopic binary images and surface re-flectance, and the effect of the APP pretreatment on the soil removal was discussed. Moreover, in situ detergency evaluation using a quartz crystal microbalance1, 16） coated with the PET film was performed to check the effective-ness of the APP pretreatment.

2 EXPERIMENTAL PROCEDURES2.1 Materials

A biaxially oriented PET film with 100 μm thickness（EMBLET SA-100, Unitika, Japan）, a PET mesh with a thread diameter of 242 μm and a mesh opening of 114 μm（T-N0150T, NBC Meshtec Inc.）, and a PET filament fabric（JIS Test Fabric, Japanese Standards Association）were used. Before use, the PET substrates were ultrasonically cleaned twice in water at 40℃ and were cut into 10×40 mm2 swatches.

The AT-cut, 9 MHz quartz crystal microbalance（QCM, QA-A9M-AU（E）, SEIKO EG&G, Japan）was used for deter-gency evaluation. To prepare the PET film on the Au elec-trodes of the QCM, 0.5 g PET pellets（Scientific Polymer Products Inc., USA）were dissolved in 5×10－2 dm3 hexa-fluoroisopropanol（Central Glass Co., Ltd., Japan）. The PET solution（5×10－5 dm3）was placed on the center of the Au electrode, which was spinning at 6400 rpm, for 3 s. The procedure was repeated twice.

Stearic acid（99％, MP Biomedicals, LLC, USA）was used as a model oily soil. Sudan III（CI 26100）, an oil-soluble dye, was used as an indicator of stearic acid. Acetone（99.5％, Wako Pure Chemical Industries, Ltd., Japan）was used as a solvent for stearic acid.

To prepare detergent solutions, 1 mol/dm3 aqueous sodium chloride solution（analytical grade, Wako Pure Chemical Industries, Ltd., Japan）, 1 mol/dm3 aqueous sodium hydroxide solution（analytical grade, Wako Pure Chemical Industries, Ltd., Japan）, and sodium dodecyl sulfate（SDS, critical micelle concentration: 8 mmol/dm3, Wako Pure Chemical Industries, Ltd.）were used. The water was purified（resistivity: 18 MΩ cm）using a Direct-Q UV apparatus（Millipore, Massachusetts, USA）.

2.2 Deposition of stearic acid onto PET substratesPrior to the soil removal experiment, stearic acid was

deposited on the PET substrates. The PET film was sprayed three times with a solution of stearic acid in acetone（0.7 g/dm3）, and then air-dried. The PET mesh and fabric were immersed at 60℃ in a soil bath containing stearic acid（4.5 g）, Sudan III（0.02 g）, and acetone（0.2 dm3）for 3 min under sonication. After soiling, the mesh and fabric were air-dried.

The soiled PET substrates were stored in a room main-tained at 20℃ and 65％RH for 24 h prior to the APP expo-sure.

2.3 APP exposureThe APP exposure was performed using a plasma pre-

treatment equipment（Plasmatreat GmbH, Germany）con-sisting of a plasma generator（FG1001）, a high-voltage transformer（HTR1001）, and a rotating nozzle jet（RD1004）11）. The reactive gas used was N2, which was regulated at a pressure and flow rate of 0.3 MPa and 20 L/min, respec-tively, at room temperature. The plasma nozzle jet was set vertically, with the soiled PET samples horizontally dis-placed on the platform just below the nozzle and recipro-cated in the horizontal direction at 0.16 m/s during the ex-posure. The nozzle diameter was 20 mm or 40 mm, the separation distance from the nozzle was 7 mm or 5 mm, and the number of treatment cycles was 2 or 4 for the PET film or the PET mesh and fabric, respectively. These condi-tions were determined in preliminary studies to be optimal for obtaining high hydrophilicity without thermal damage12）. In the cases of the PET mesh and fabric, both sides were exposed to the APP.

Stearic acid deposited on the PET film before and after the APP exposure was observed by a reflected light micro-scope system（BX51, Olympus Corporation, Japan）and a scanning electron microscope（SEM, SU1510, Hitachi High-Technologies Corporation, Japan）.

2.4 Detergency evaluationImmediately after the APP jet treatment, the PET

sample was cleaned by immersing it perpendicularly into 0.02 dm3 of an aqueous solution containing NaCl（1 mmol/dm3）, NaOH and NaCl（1 mmol/dm3 each）, or SDS and NaCl（8 mmol/dm3 and 1 mmol/dm3, respectively）. As a mechani-cal action for soil removal, stirring with a rotational speed of 600 rpm was applied with a magnetic stirrer bar（diame-ter 1.5 mm×10 mm）17, 18）. The cleaning time and tempera-ture were 5 min and 25±1℃, respectively. After the clean-ing, the PET samples were dried in air.

The efficiency of soil removal from the PET film was de-termined by the binary processing of microscopic images before and after the cleaning17, 18）. The system used con-sisted of a biological microscope（CKX41, Olympus, Japan）, a complementary metal-oxide-semiconductor（CMOS）

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camera（Lu275, Lumenera, Canada）, image tiling software（e-Tiling, Mitani Corporation, Japan）, and 2D image analy-sis software（WinRoof Ver. 6, Mitani Corporation, Japan）. Microscopic images within the same field of view before and after the cleaning were obtained over five different areas（0.90 mm2 per area）located 10 mm from the lower edge of the PET film. The microscopic image of the soil de-posited on the PET film was converted from an original gray-scale digital image with 256 possible intensity values to a binary image using a suitable threshold level. The removal efficiency was calculated from the changes in the total area of stearic acid deposited on the PET film due to the cleaning.

To determine the soil removal efficiency from PET mesh and fabric, the surface reflectance of the original and soiled fabrics was measured using a spectrophotometer（NF333, NIPPONN DENSHOKU, Japan）, stacking the same kinds of sample in fours. The wavelength used was 500 nm（maximal absorption wavelength of Sudan III）. Each side of the PET sample was read in two different spots using a measuring aperture of diameter 8 mm for reflectance; the average of the four readings was calculated to be the final value. The value of the Kubelka–Munk function, K/S, was obtained from each surface reflectance measurement19）. The removal efficiency was calculated from the K/S values of the original and soiled fabrics before and after the washing20）.

Removal（％）＝（K/S）s－（K/S）w

（K/S）s－（K/S）o

×100 （1）

where the subscripts s and w refer to the soiled fabrics before and after the washing, respectively, and the sub-script o refers to the original fabrics.

2.5 In situ detergency evaluation by QCM techniqueFor the PET film prepared on the QCM, the mass of the

materials deposited on the Au electrodes was calculated from the QCM frequency change using the Sauerbrey equa-tion21）. QCM frequency measurements were performed using a frequency counter（QCA922, SEIKO EG&G, Japan）. In air, 1 Hz frequency shift of the QCM used in this study corresponded to 1.07 ng of deposited mass. However, in the liquid environment, the calculated equivalent Sauer-brey mass is smaller than the true deposited mass of the stearic acid film because of the missing mass effect22, 23）. Therefore, the frequency of the QCM before the deposition of stearic acid, Fo, was measured in the aqueous solution. After drying in air, stearic acid was deposited onto the QCM by spraying its solution, as mentioned above. After storage for 24 h, the QCM was exposed to the APP, and then horizontally set in a polytetrafluoroethylene（PTFE）cell24）. At this moment, the frequency recording of the QCM was started. The cell was then filled with the aqueous detergent solution, which was stirred with rotational speed of 600 rpm using a PTFE mixing impeller（diameter 25

mm）. The removal efficiency after washing for 5 min was calculated by the following equation.

Removal（％）＝ Fw－Fs

Fo－Fs

×100 （2）

where Fs and Fw are the frequency immediately and 5 min after the washing, respectively.

2.6 Surface characterization of PET and stearic acidPET and stearic acid films were used for surface charac-

terization. The stearic acid film was prepared on the PET film as base material by repeating dropping of a solution of stearic acid in acetone（0.05 g/dm3）and evaporation of acetone three times.

Contact angles of water on PET and stearic acid were measured by the sessile drop method using a video contact angle system equipped with a CCD camera（VCA-2500, AST Products, USA）. The contact angles measured 1 s after placing a 2–3 μdm3 water drop on the PET film were deter-mined as an approximate value of the advancing angle12）. The measurements were repeated at 10–20 different posi-tions on the same sample.

X-ray photoelectron spectroscopy（XPS）analysis of the PET and stearic acid was performed using an Axis-Ultra DLD spectrometer（Kratos, UK）with monochromated Al Kα radiation at 1486.7 eV（75 W）under charge neutraliza-tion12）. Wide scan spectra were acquired with a pass energy of 80 eV and a slot aperture of 0.8×2.0 mm. The spectra were collected at a photoelectron take-off angle of 90°.

Atomic force microscopy（AFM）images of PET were col-lected using Nanoscope IIIa（Digital Instruments, USA）in tapping mode12）. The cantilever used（RSTEP, Bruker, Japan）was high-sensitivity silicon probe（no coating）with an average spring constant of 40 N/m and a resonant fre-quency of about 300 kHz. The surface roughness parame-ters, root mean square roughness, average roughness, and maximum roughness depth were determined from the images obtained from a 1×1 μm area.

3 RESULTS AND DISCUSSION3.1 Change in surface properties of PET and stearic acid

due to APP exposureThe contact angles of water on PET and stearic acid are

shown in Fig. 1. In both cases, the contact angle was found to decrease considerably after the APP exposure.

Figure 2 illustrates the XPS survey spectra of PET and stearic acid. After the APP treatment, the atomic ratio of oxygen to carbon（the area of the O1s peak divided by the area of the C1s peak）increased and the N1s peak appeared. Surface elemental compositions obtained from the spectra are inserted in Fig. 2. For the PET surface, the oxygen and nitrogen content increased by 5.4％ and 2.0％, respective-ly, after the APP exposure. Similarly, in the case of stearic


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acid, the APP exposure resulted in a slight increase of the oxygen concentration. The XPS results show that oxygen and nitrogen radicals in plasmas bind to PET and stearic acid, which may cause the surface hydrophilization shown

in Fig. 1.The AFM images of the PET film, and the surface rough-

ness parameters obtained from them, are presented in Fig. 3. Although five images of the APP-treated film lacked re-producibility（±40％ variation of surface roughness param-eters）, they were clearly different from the relatively smooth untreated film. In a previous study13）, we observed a considerable increase in the roughness of the single fiber surface after the APP treatment of PET fabrics. Such topo-graphical changes can arise from thermal effects of the plasma or from chemical reactions and sputtering25）.

Figure 4 shows optical microscopic images of stearic acid deposited on the PET film. Both images indicate that stearic acid was crystallized before the APP exposure because of the evaporative deposition from the acetone so-lution26）. The collapse of crystal grain was observed after the APP exposure, which was caused by partial melting of stearic acid（melting point ca ～70℃）by plasma-induced heating27）.

3.2 Effect of APP exposure on removal of stearic acidThe efficiencies of stearic acid removal from the PET

film, mesh, and fabric are shown in Fig. 5. The cleaning

Fig. 1　 Effects of plasma exposure on water contact angle on PET and stearic acid surfaces.

Fig. 2　 XPS survey spectra of PET and stearic acid films before and after the APP exposure and obtained surface atomic concentrations.


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tests were repeated 3–6 times under the same conditions; the experimental errors（bars in Fig. 5）were relatively small. For all substrates and detergent solutions, the removal of stearic acid by aqueous cleaning was found to be enhanced by the APP pretreatment. The soil removal after the APP treatment was not as large in the NaCl solu-tion with less detergency power. In the SDS solution, soil removal was moderately promoted after the APP exposure. In the NaOH solution, the APP exposure effect was ex-tremely large and the removal efficiencies from the APP-exposed samples were 3–5 times greater than those from

the unexposed samples.The attachment area of solid stearic acid to the films

could be reduced by nanoscale topographical changes（Fig. 3）, resulting in the detergency improvement seen in Fig. 5. However, it seems likely that under the APP exposure, stearic acid is not in the solid state but it rather shows flu-idity. In this case, the attachment area to the PET film would increase. Conversely, the hydrophilic nature of the substrate and the soil（Figs. 1 and 2）and the collapse of the crystalline structure of the soil（Fig. 4）can promote soil removal. A chemical transformation of stearic acid by oxi-

Fig. 3　AFM images and obtained surface roughness parameters of the PET films before and after the APP exposure.

Fig. 4　 Optical microscopic (upper) and SEM (lower) images of stearic acid deposited on the PET film before and after the APP exposure.


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dation and nitrogen attachment can also cause structural disorder to inhibit recrystallization after the APP treat-ment. Such changes in the chemical and crystalline struc-tures of stearic acid may overcome the topographical dis-advantage to allow the promotion of soil removal.

The main soil removal mechanisms in NaOH and SDS so-lutions are, respectively, saponification of stearic acid and liquid penetration at the interface between stearic acid and the PET film. If the attachment area of stearic acid to the film surface is increased by the APP exposure, saponifica-tion rather than penetration will be the favored mecha-nism. This accounts for the extremely large APP effect ob-served in Fig. 5 for cleaning in the NaOH solution.

In the present study, the removal efficiency was deter-mined from the deposited area of stearic acid for the PET film or the surface reflectance for the PET mesh and fabric. The results obtained by the two different evaluation methods showed similar trends. This indicates that Sudan III is a suitable indicator of stearic acid in the detergency experiment.

3.3 QCM evidence for APP effect on removal of stearic acid

Figures 6–8 show the change in the QCM frequency with time during cleaning in the aqueous detergent solu-tions. Immediately after immersion of the QCM in the de-tergent solutions, the abrupt decrease in the frequency due to the displacement of the medium from air to the de-tergent solution was observed. The frequency was then found to increase due to the removal of stearic acid from the PET film on the QCM. By substituting the minimum frequency and the frequency obtained after 5 min in equa-tion 1 as FS and FW, respectively, the removal efficiencies after 5 min were calculated and inserted in Figs. 6–8. It was found that the removal of stearic acid was promoted by the APP pretreatment, especially in the NaOH solution. The removal efficiencies obtained by the QCM technique

showed similar trends to the results obtained for the three PET substrates discussed above（Fig. 5）.

4 CONCLUSIONSThe effects of APP exposure on the removal of stearic

acid soil from three PET substrates̶film, mesh, and fabric̶in aqueous detergent solutions were investigated. For each PET substrate, soil removal during cleaning in aqueous solutions is considerably increased by the APP pretreatment. The surface characterization and observa-tions showed that hydrophilization of the substrate and the soil and collapse of the crystalline structure of the soil were the effects of APP exposure that promoted soil removal. In situ detergency evaluation using the QCM technique confirmed the effectiveness of the APP exposure as a pretreatment method for aqueous detergent process-es.

ACKNOWLEDGEMENTSWe wish to express our gratitude to Dr. Yasuyuki Ko-

bayashi of Osaka Municipal Technical Research Institute, Japan, for helping in surface characterization. Gratitude is expressed to the Ministry of Education, Sports, Culture, Science and Technology, Japan for a Grant-in-Aid for Sci-entific Research to carry out this work.

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Fig. 5　Effects of plasma exposure on removal of stearic acid from three PET substrates.


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Fig. 8　Change in QCM frequency due to removal of stearic acid in the aqueous SDS solution with time.


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