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Surface modification of biaxially oriented polypropylene (BOPP) film using acrylic acid-corona treatment: Part II. Long term aging surface properties
Surface & Coatings Technology 234 (2013) 67–75
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Surface modification of biaxially oriented polypropylene (BOPP) film using acrylic acid-corona treatment: Part II. Long term aging surface properties Nanticha Kalapat a, Taweechai Amornsakchai b,c,⁎, Toemsak Srikhirin a a b c
Materials Science and Engineering Program, Faculty of Science, Mahidol University, Phyathai, Bangkok 10400, Thailand Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand Center for Alternative Energy, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand
a r t i c l e
i n f o
Available online 20 May 2013 Keywords: Acrylic acid Corona treatment BOPP film Wettability
a b s t r a c t In this work particular attention has been paid to the aging behavior of biaxially oriented polypropylene (BOPP) film surfaces modified with the acrylic acid (AAc) corona discharge treatment previously reported. Three different corona energies of 15.3, 38.2 and 76.4 kJ/m2 were studied. The surface properties of treated films during 90 days of aging were compared with those of normal air-corona treated films prepared with the same corona energies. The surface chemical compositions of aged films were analyzed by curve-fitting of the ATR-FTIR spectra. The wettabilities of all aged films were monitored by water contact angle and surface free energy measurements. The change of surface topology of air- and AAc-corona treated films was investigated at 1 day, 7 days and 90 days of aging using the AFM technique. In addition, the surface adhesions of aged films were determined with the T-peeling test. The results showed that the amount of polar functional groups on the surface of aged films had changed. However, the aged films of the AAc-corona treated films still showed greater wettability than did the air-corona treated films and could retain high surface hydrophilicity for more than 90 days of aging under ambient condition. The surface topology of both types of aged films changed after aging from a globular structure to a flatter surface, due to mobility of the deposited polymer layer. The AAc-corona treated films showed rougher surfaces due to the influence of poly(acrylic acid) deposition and they could retain the improved surface wettability despite the change in surface topography. The adhesion peel forces of aged films decreased slightly due to the topological changes. A mechanism for the change in surface topography and in chemical functionality of each type of aged film is proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Generally a biaxially oriented polypropylene (BOPP) film has low hydrophilicity and does not accept printing ink or adhesives very well. Therefore, the corona discharge technique is used to introduce polar functional groups such as hydroxyl, carbonyl and carboxylic groups onto the film surfaces. Strictly speaking, the technique should be referred to as an ac barrier corona, because it combines the features of the familiar barrier and corona discharges (between an edge and a dielectric-coated roller) [1]. Since the term is so widely used and accepted in plastic industries, corona discharge will be used. The presence of these functional groups raises the film surface free energy resulting in an increase of wettability and adhesion. However, the improved surface energy decays over time due to the mobility and reorientation of polymer chains on the surface [2]. Novak et al. [3] have studied the chemical change, on a polyolefin ⁎ Corresponding author at: Center for Alternative Energy, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand. Tel.: +66 2 441 9817x1161; fax: +66 2 441 0511. E-mail address: [email protected] (T. Amornsakchai). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.05.012
surface modified by corona discharge, during long-term aging. Their results showed that the improved surface energy decayed after 1 month of aging and that the surface properties were mainly affected by the polymer chain mobility on the surface. In order to eliminate these time-dependent effects after corona treatment, the polymer chain mobility of the treated surface should be obstructed with a layer of some functional groups. This can be achieved without altering the film property by grafting of large chain segment onto the surface. Attempts have been made to produce AAc layer onto polymer films. Lei and Liao [4] used the corona discharge technique to introduce radical on a low density polyethylene (LDPE) film surface before grafting with AAc solution. Besides the use of grafting solution, the introduction of AAc vapor into a plasma reactor also produced a grafted polymer film through plasma polymerization. Jafari and coworkers [5] used plasma polymerization at low pressure to deposit plasma polymerized AAc on PE films. Sciarratta and coworkers [6] used radio frequency plasma technique for functionalizing and coating PP with AAc vapor. Their processes involved activation of PP film with hydrogen plasma before grafting with AAc vapor, thereby requiring more steps to produce the polymer film. Morent and coworkers [7–9] have studied the deposition of plasma-polymerized
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AAc coating on PP substrates using atmospheric pressure dielectric barrier discharge (DBD). They could obtain coatings with high carboxylic acid densities. However, their process was performed by using helium as carrier gas in the chamber. In our previous paper [10], we described the surface modification of BOPP films by introducing AAc vapor into the corona region of a normal corona treater without carrier gas. The surface properties of AAc-corona treated BOPP films were compared with those of normal air-corona treated films. The surface wettability of both types of treated films increased with the change of chemical composition and topography of treated surface. The degree of wettability and surface adhesion depended on the corona energy. It is known that factors affecting the change of surface wettability during storage time are chemical and physical changes on the treated surface [11–14]. As to the chemical change, when exposed to air, the polar groups on the corona treated surface tend to overturn or migrate below the film surface. In the case of physical change, the treated film surface after a high dose of corona treatment shows the formation of bumps. This surface topology will continuously change to a new equilibrium of lower surface energy [15]. So both types of change occur during long-term aging because of the thermodynamic tendency to minimize the interface energy. In this study, the surface properties and durability of both types of corona films treated with different corona energies have been monitored during aging. The difference between the surface behavior of air-corona and that of AAc-corona treated films is discussed. From our findings, schematic models are proposed to describe the relationship between the chemical and physical changes and the corresponding decrease of surface wettability. 2. Experimental methods
and ‘AAc-corona’, respectively. For ‘AAc-corona’, the monomer vapor was generated using an air-bubble method. AAc aqueous solution was placed in a reservoir which had an air inlet. The setup is shown in our previous paper [10]. The monomer outlet tubing was squeezed with a clamp to form a long, thin outlet and placed as close as possible to the corona area. The average AAc feed rate was approximately 0.6 mmol/min. The AAc-corona treated area was about 25 mm wide. Unreacted AAc vapor and ozone generated during the treatment were ventilated through a fume hood. AAc is highly reactive and that released to the atmosphere through the corona zone will react with ozone and photochemically produce hydroxyl radicals, resulting in a half-life of the AAc of 6–14 h [18]. Since a fair amount of ozone is generally generated during the corona discharge, a minimal amount of AAc is left for release to the atmosphere. In these experiments it was not detectable.
2.3. Contact angle and surface free energy measurements The contact angles of the film surfaces were determined with a Kruss, G-1 contact angle goniometer at the ambient temperature. All measurements were made by 10 μl of D.I. water with the sessile drop method for ten points on the sample surfaces. The surface energy was calculated from the additional measurement with diiodomethane on the sample surface [19–21]. The contact angle of each solvent was measured for ten points with a drop volume of 10 μl at the immediate time of drop insertion. The average contact angles were used for calculating total surface free energy with the geometric mean approach of the Owens–Wendt method [22,23].
2.4. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy
2.1. Materials Biaxially oriented polypropylene (BOPP) films used in this study were manufactured and kindly provided by the Thai Film Industries Public Co., Ltd. The average film thickness was 20 ± 4 μm. The melting point and percentage of crystallinity of BOPP films, as measured by differential scanning calorimetry (DSC) at a heating rate of 20 °C/min (Q200-TA DSC), were 167.8 °C and 46.8%, respectively. Acrylic acid, AAc (98 wt.% aqueous solution) was purchased from Fluka. Diiodomethane, CH2I2 (99% stabilized with copper) from Sigma-Aldrich and D.I. water were used for surface free energy measurement. BOPP films and all chemicals were used as-received.
The modified surfaces were studied with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). ATR-FTIR spectra were taken using a 45° germanium crystal (Ge) for single reflection mode on a Nicolet 6400 FT-IR spectrophotometer. The measurements were carried out with 128 scans at a resolution of 4 cm−1 over a range of 4000 to 600 cm−1. Furthermore, the curve-fitting of carbonyl group absorptions was performed on the original spectra with Gaussian band shapes in the range of 1850–1550 cm−1. The peak areas were calculated with OMNIC software for FTIR spectroscopy [24].
2.2. Corona treatment
2.5. Atomic force microscopy (AFM)
Corona treatment was carried out on a locally made corona treater (RUNGTHAI Plastic Corona, Bangkok, Thailand). The treatment was carried out with film traveling at a fixed linear speed of approximately 1.44 m/min and the air-gap between the electrode and the sample film was set at 3 mm. The unit energy of the corona treatment (E) was determined from the following equation [16,17];
Surface topographies of the films before and after aging were obtained with an atomic force microscope (AFM), Nanoscope IIIa multimode (Digital Instruments (DI), Santa Barbara) in tapping mode. The sample films were probed with standard silicon cantilevers/tips. AFM images were acquired on at least three different areas for each sample surface. The arithmetic mean surface roughness (Ra) of the film was calculated directly from the AFM signal using the instrument software.
E¼
P uL
ð1Þ
where P is the corona discharge power (kJ/s), calculated from the input line voltage, V, and the current drawn by the machine, I u is the linear speed of the film in the gap (m/s) L is the length of the discharge electrode (m). Treatment was carried out with three corona energies of 15.3, 38.2 and 76.4 kJ/m2 with and without the vapor of AAc monomer being blown into the corona region. The samples are coded as ‘air-corona’
2.6. T-peel testing Adhesion properties of the aged films to 3M-810 tape were determined by T-shape peeling with a tensile tester INSTRON 5566. The test specimen was made by bonding one end of treated BOPP film (25 mm × 70 mm) and one end of 3M-810 tape (25 mm × 70 mm). This was then peeled apart with a constant rate of 20 mm/min. An average value from at least three measurements was taken as the average peel force (N).
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2.7. Aging of treated films
3.2. ATR-FTIR results
The stability of all treated films was evaluated by using water contact angle measurement for 90 days of aging. All treated films were kept at ambient temperature prior to measuring water contact angle.
In this work, the change in the chemical composition on the aged films can be examined with ATR-FTIR. Generally, the changes on air-corona treated film are difficult or even impossible to detect with this technique because the changes only occurred on the very top layer of a few nanometers. However, with the high dose of corona discharge in this work, the low molecular weight oxidized materials (LMWOM) might be generated on air-corona treated film surface. It resulted in an increase of ATR-FTIR signal. Furthermore, the depth of analysis in this work is calculated to be less than 0.5 μm and this allowed the ATR-FTIR signal of air-corona treated film to be detected. Figs. 3 and 4 display the spectra of air-corona and AAc-corona films treated with 76.4 kJ/m2 of corona energy during aging. The main changes are in the regions between 1500 and 1750 cm−1 and between 3000 and 3700 cm−1 corresponding to carbonyl and hydroxyl stretching, respectively. The intensities of these peaks decreased over time due to the changes of the surface characteristics. In the case of AAc-corona treated films at 90 days of aging, the ATR-FTIR spectrum shows a clear regular sine wave pattern. These are known as interference fringes that occur due to the thin polymer layer [25–28]. This suggests that the film surface had changed to a more uniform top poly(AAc) layer. Such a thin layer would cause the IR beam to reflect many times within the top poly(AAc) layer. As was described in the previous study [10], curve-fitting of the carbonyl stretching region was used to determine the amount of different types of carbonyl functional groups. Four peaks at 1640, 1715, 1725 and 1740 cm−1, corresponding to C_C stretching of unsaturated compound, C_O stretching of ketone, C_O stretching of saturated carboxylic acid and C_O stretching of saturated ester, respectively, were fitted to the region. The amount of carbonyl groups for all aged films was determined from the ratio of the peak area of each
3. Results and discussion In this work, the surface behavior of all treated films during aging was monitored. Both the chemical and topological changes on the surface were studied and are reported as follows.
3.1. Water contact angle and surface free energy results The surface stabilities of both types of treated films were monitored under ambient condition by measuring the water and diiodomethane contact angles. Fig. 1(a) shows the water contact angle results of air-corona treated films which clearly change during aging time. In contrast to the result of air-corona treated films, the water contact angle of AAc-corona treated film in Fig. 1(b) changes only slightly and can be retained until 90 days of aging. Diiodomethane contact angle for air- and AAc-corona treated films is shown in Fig. 1(c) and (d), respectively. All films exhibit very similar behavior of monotonous increase in diiodomethane contact angle with increasing aging time. These results relate to the surface free energy results in Fig. 2. Although the surface energy of both types of aged films decreases, the AAc treated film still displays higher wettability than does the untreated BOPP film. Moreover, the aged AAc-corona treated films show better wetting than do the air-corona treated films because of the polarity of poly(AAc) on the surface.
120
a
110
Water Contact Angle (degree)
Water Contact Angle (degree)
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Untreated BOPP 15.3 kJ/m 2 38.2 kJ/m 2 76.4 kJ/m 2
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Aging Period (days) CH2I2 contact angle (degree)
CH2I2 contact angle (degree)
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Untreated BOPP 15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m2
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Aging Period (days)
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Untreated BOPP 15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m2
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Aging Period (days)
80
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0
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Aging Period (days)
Fig. 1. Water contact angle of air-corona treated BOPP (a) and AAc-corona treated BOPP films (b) and CH2I2 contact angle of air-corona treated BOPP (c) and AAc-corona treated BOPP films (d) at different aging periods.
N. Kalapat et al. / Surface & Coatings Technology 234 (2013) 67–75
70
70
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Surface energy (mJ/m2)
Surface energy (mJ/m2)
70
50 40 30 20
15.8 kJ/m2
Untreated Air-corona AAc-corona
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Untreated Air-corona AAc-corona
38.2 kJ/m2
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Aging period (days)
Aging period (days)
Surface energy (mJ/m2)
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Untreated Air-corona AAc-corona
76.4 kJ/m2
10 0 0
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Aging period (days) Fig. 2. Surface free energy of aged BOPP films treated with corona energies of 15.8, 38.2 and 76.4 kJ/m2 at ambient temperature.
assigned peak to that of PP at 1456 cm−1. Table 1 compares the peak area ratios for aged films of air-corona and AAc-corona treated films. All peak area ratios increase with increasing corona energy for both types of treatment. After aging, the air-corona treated films show a decrease in the amount of ketone functional groups on the film surface when compared with those of the just treated films. For AAc-corona treated films, the amount of carboxylic acid does not decrease over the aging period. 3.3. Surface topographical results The changes in surface topology of all treated films were monitored by AFM measurement [10]. The surface morphology of BOPP films changed after corona treatment into a globular structure. The
AAc-corona treated films showed rougher surfaces due to surface oxidation and polymer formation, whereas, air-corona treated films displayed a similar structure but of smaller size. The depth of surface treatment of air-corona and AAc-corona treated films was determined using AFM section software analysis. To determine the mechanism of structure formation on the film surface, part of the BOPP films were masked with adhesive tape before corona treatments with the highest corona energy and then, the mask was removed from the treated surface before AFM measurement. The section analysis of air-corona treated films is displayed in Fig. 5(a). It is clearly seen that corona discharge is physically aggressive to the polymer surface. When compared with the smooth untreated area (on the right side of the image), the surface of the treated area was etched into the bulk of the film. The depth between the untreated area and
1725
1715 3400
1640
Fig. 3. ATR-FTIR spectra of untreated BOPP compared with that of air-corona treated with 76.4 kJ/m2 of corona energy during aging.
3400
1640
Fig. 4. ATR-FTIR spectra of untreated BOPP compared with that of AAc-corona treated with 76.4 kJ/m2 of corona energy during aging.
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Table 1 Comparison of ATR-FTIR peak area ratio of air-corona and AAc-corona treated BOPP films during aging by curve-fitting. Samples
Area ratio 1740/1456 1 day
1725/1456
1715/1456
1640/1456
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
0.39 0.49 0.72
0.06 0.21 0.38
0.00 0.01 0.01
0.03 0.04 0.04
0.01 0.07 0.05
0.15 0.45 0.60
0.18 0.28 0.26
0.01 0.03 0.09
0.19 0.39 0.86
0.24 0.34 0.52
0.28 0.32 0.33
AAc-corona treated BOPP film 0.17 0.43 15.3 kJ/m2 0.38 0.35 38.2 kJ/m2 2 0.59 0.52 76.4 kJ/m
0.27 0.26 0.49
0.19 0.80 1.47
0.66 0.69 1.00
0.94 1.45 1.34
0.05 0.15 0.71
0.29 0.05 0.07
0.24 0.25 0.25
0.58 0.85 1.05
0.46 0.52 0.91
0.49 0.69 0.71
Air-corona treated BOPP film 0.21 15.3 kJ/m2 0.72 38.2 kJ/m2 76.4 kJ/m2 1.68
the treated area of film as seen between the marked points is approximately 44 nm. Presumably, the etched depth decreases with decreasing the corona power. In the case of AAc-corona treated films, the section analysis is shown in Fig. 5(b). The difference between the two marked points is approximately 130 nm. It is clear that there is the deposition of poly(AAc) on the treated surface which differs markedly from the surface of air-corona treated film. The comparison of AFM 3D images of air-corona treated BOPP films with corona energies of 15.3, 38.2 and 76.4 kJ/m2 after aging for 1, 7 and 90 days, respectively is displayed in Fig. 6. The globule size increases with increasing corona energy. After aging for 7 days, the globular features disappear. At 90 days of aging, all air-corona treated surfaces become smoother than those of the freshly treated films. This is consistent with a decrease in surface roughness shown in Table 2. It is thought to be due to the migration of the polymer at the interface. This would cause the surface topology to continuously change into a new, smoother equilibrium shape that has lower surface area and so lower surface energy [21].
For AAc-corona treated films, the surface topography and surface roughness after treatment tend to increase with increasing corona energy as observed from AFM images at 1 day in Fig. 7. With the same AAc concentration and the same flow rate, the higher corona energy employed shows larger poly(AAc) droplets on the film surface. When compared with air corona treatment, the films show a rougher surface due to the deposition of polymerized AAc, consistent with the calculated surface roughness shown in Table 2. From the AFM images of 7 day aged films, it seems that some poly(AAc) droplets had moved to combine together to form larger droplets while some remained unchanged. As a result, a wider distribution in droplet size was obtained and this led to an increase in the roughness value. However, after 90 days of aging, these poly(AAc) droplets seem to spread out on the film surface and this results in a decrease in surface roughness. With this topological change, the aged film surface would become covered with a poly(AAc) layer on top of the BOPP. This agrees with the increased amount of carboxylic acid on the film surface observed in the ATR-FTIR results.
Fig. 5. Section analysis of AFM height image of air-corona (a) and AAc-corona (b) treated BOPP films.
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a
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
b
c
Fig. 6. AFM 3D imaging of aged air-corona treated films: corona energy of 15.8 kJ/m2 (a), 38.2 kJ/m2 (b) and 76.4 kJ/m2 (c) at different aging times.
3.4. Adhesion results Fig. 8 displays the representative peel force curves of air-corona and AAc-corona treated films. The result shows very low variation of force during each peeling test. The average peel force of air-corona and
AAc-corona treated BOPP films immediately after treatment and after 90 days of aging was compared and summarized in Table 3. After treatment, all surfaces show high average peel forces when compared with those of the untreated BOPP. After aging, the average peel force of all aged films decreases due to the topological and chemical changes on
Table 2 Surface roughness of air-corona and AAc-corona treated BOPP films with different aging times. Sample
Surface roughness (nm)
Untreated film
5.4
Corona energy (kJ/m2)
After treatment Air-corona
AAc-corona
Air-corona
AAc-corona
Air-corona
AAc-corona
15.3 38.2 76.4
11.1 16.8 24.5
16.4 28.0 34.7
2.3 12.2 11.6
20.6 41.9 57.3
2.5 11.1 8.8
6.9 12.5 17.7
Aging 7 days
Aging 90 days
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a
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
b
c
Fig. 7. AFM 3D imaging of aged AAc-corona treated films: corona energy of 15.8 kJ/m2 (a), 38.2 kJ/m2 (b) and 76.4 kJ/m2 (c) at different aging times.
the film surface. However, the average peel forces of all aged film surfaces are still higher than that of the untreated film. In the case of aged AAc-corona films, the decrease in the average peel force after aging must be mainly due to the topographical changes of film surface. This is because the amount of polarity on aged AAc-corona film surface is quite stable whereas its surface roughness decreases, resulting in a decrease of surface area for making contact. Despite such topological changes, these surfaces still show a greater surface adhesion than do the aged air-corona films. Combining the observations during aging of the air-corona treated film surface the following conclusions can be summarized. A rough surface and polar functional groups are generated on the BOPP surface by the corona discharge and both the surface topography and surface chemistry undergo changes on storage. Although the corona causes the degradation of the polymer surface and etches away the
oxidized polymer, the film thickness is not significantly changed. The polar functionalities that exist on the surface after air-corona treatment agree well with the appearance of C_O and C_C groups on the surface and cause a decrease in the water contact angle. After one week of aging, the topography becomes flatter. This may be due to the rearrangement of the polymer surface or the loss of degradation products originating from the polymer surface [29]. Since the water angles increase rapidly, it is postulated that, in addition to the decrease in the polar area of the surface [30], the reorientation of polar groups from the surface into the bulk, or the non-polar groups from the bulk to the surface, occurs simultaneously [31]. This phenomenon occurs within the detection volume of ATR-FTIR. After 90 days of aging, low molecular weight molecules with polar groups migrate further into the bulk and thus less ATR-FTIR signal is observed. A mechanism for the changes in surface topography and
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4.5
a
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Extension (mm) 4.5
b
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seen in the AFM images of aged films. The ATR-FTIR and peel adhesion results show that, although the deposited poly(AAc) can move and affect the topographical characteristic of film surface, it can retain film wettability and show a long lasting hydrophilic stability until 90 days. These processes can be illustrated by the model in Fig. 10.
3.5
Load (N)
3.0 2.5 2.0
Sample 1 Sample 2 Sample 3
1.5
4. Conclusion
1.0 0.5 0.0 0
Fig. 9. Schematic drawing of the proposed surface topography and chemical change of air-corona treated BOPP film during aging.
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Extension (mm) Fig. 8. Representative peel force curves of air-corona (a) and AAc-corona treated films (b) with the same aging period and corona energy employed.
chemical functionality of aged film can be illustrated as the model in Fig. 9. In the case of AAc-corona treatment, a rough surface and polar functional groups are generated on the BOPP surface. Poly(AAc) is deposited as droplets on the BOPP surface as observed from AFM analysis and ATR-FTIR results. After aging for 1 week, the surface topography changes due to the mobility of deposited AAc. Presumably due to the low molecular weight of poly(AAc), the droplets can flow to combine together resulting in greater surface roughness. With the increase of surface roughness in this case, the entrapment of air into the depressions of the rough surface produces a less hydrophilic surface. Therefore, the surface wettability seems to be decreased in this aging time. This agreed with the change of water contact angle value over this aging period. Then during 90 days of aging, the poly(AAc) droplets spread out on the BOPP surface as
In this work, surface topology of AAc-corona treated films changed by deposition of poly(AAc) on the treated surface. These films differed from the surface of air-corona treated films, on which the change was etched into the bulk of the film. These poly(AAc) droplets spread out on the film surface on storage. Although the topographical characteristics changed to reach a new equilibrium, the surface can retain film wettability and provide hydrophilic characteristics longer-lasting than those of the normal air-corona treatment. The average peel force of the aged AAc-corona treated films decreased with decreasing surface area of the polar part. However, they still showed a greater surface adhesion than did the aged air-corona films.
Acknowledgments This research work is partially supported by a RA scholarship for NK from the Faculty of Graduate Studies, Mahidol University (Academic Year 2012). TA is supported by the Center of Excellence for Innovation in Chemistry (PERCH-CIC).
Table 3 An average peel force of air-corona and AAc-corona treated BOPP films before and after aging. Sample
Average peel force (N)
Untreated BOPP film
2.55 ± 0.03
Corona treated film
Air-corona
15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m2
AAc-corona
After treatment
After 90 days
After treatment
After 90 days
3.09 ± 0.02 3.35 ± 0.04 3.63 ± 0.04
2.89 ± 0.05 2.98 ± 0.06 3.37 ± 0.14
3.58 ± 0.02 3.73 ± 0.02 3.92 ± 0.04
3.00 ± 0.06 3.29 ± 0.11 3.69 ± 0.10
Fig. 10. Schematic drawing of surface topography and chemical change on the AAc-corona treated BOPP film during aging.
N. Kalapat et al. / Surface & Coatings Technology 234 (2013) 67–75
References [1] Y.S. Akishev, A.V. Dem'yanov, V.B. Karal'nik, A.E. Monich, N.I. Trushkin, Plasma Phys. Rep. 29 (2003) 82. [2] C.-M. Chan, T.-M. Ko, Surf. Sci. Rep. 24 (1996) 1. [3] I. Novak, V. Pollak, I. Chodak, Plasma Process. Polym. 3 (2006) 355. [4] J. Lei, X. Liao, Eur. Polym. J. 37 (2001) 771. [5] R. Jafari, M. Tatoulian, W. Morscheidt, F. Arefi-Khonsari, React. Funct. Polym. 66 (2006) 1757. [6] V. Sciarratta, U. Vohrer, D. Hegemann, M. Muller, C. Oehr, Surf. Coat. Technol. 174 (2003) 805. [7] R. Morent, N.D. Geyter, S.V. Vlierberghe, E. Vanderleyden, P. Dubruel, C. Leys, E. Schacht, Plasma Chem. Plasma Process. 29 (2009) 103. [8] R. Morent, N.D. Geyter, M. Trentesaux, L. Gengembre, P. Dubruel, C. Leys, E. Payen, Appl. Surf. Sci. 257 (2010) 372. [9] R. Morent, N.D. Geyter, S.V. Vlierberghe, A. Beaurain, P. Dubruel, E. Payen, Prog. Org. Coat. 70 (2011) 336. [10] N. Kalapat, T. Amornsakchai, Surf. Coat. Technol. 207 (2012) 594. [11] S. Guimond, M.R. Wertheimer, J. Appl. Polym. Sci. 94 (2004) 1291. [12] C.-M. Chan, Polymer Surface Modification and Characterization, Carl Hanser Verlag, New York, 1994. 63. [13] I. Novak, S. Florian, J. Mater. Sci. 39 (2004) 2033.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
75
C.Y. Kim, J. Evans, D.A.I. Goring, J. Appl. Polym. Sci. 15 (1971) 1357. C. Sun, D. Zhang, L.C. Wadsworth, Adv. Polym. Technol. 18 (1999) 171. O. Lesly-Ann, L. Stuart, P. Bhukan, Surf. Interface Anal. 33 (2002) 335. E. Vassiliadi, P.A. Tarantili, J. Appl. Polym. Sci. 105 (2007) 1713. Acrylic Acid, Product Safety Summary Sheet, BASF, 2007. R.N. Shimizu, N.R. Demarquette, J. Appl. Polym. Sci. 76 (2000) 1831. O.C. Van, J. Adhes. Sci. Technol. 16 (2002) 669. A. Salimi, S.M. Mirabedini, M. Atai, M. Mohsei, M.R. Naimi-Jamal, Int. J. Adhes. Adhes. 31 (2011) 220. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. J. Schultz, K. Tsutsumi, J.-B. Donnet, J. Colloid Interface Sci. 59 (1977) 272. Z. Siqiang, E.H. Douglas, J. Vinyl Addit. Technol. 13 (2007) 57. A. Larena, G. Pinto, F. Millan, Appl. Surf. Sci. 84 (1995) 407. A. Larena, F. Millan, G. Perez, G. Pinto, Appl. Surf. Sci. 187 (2002) 339. A. Larena, F. Millan, M. Verdu, G. Pinto, Appl. Surf. Sci. 174 (2001) 217. N. Leopold, S. Busche, G. Gauglitz, B. Lendl, Spectrochim. Acta A 72 (2009) 994. J. Song, U. Gunst, H.F. Arlinghaus, G.J. Vancso, Appl. Surf. Sci. 253 (2007) 9489. R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. F. Garbassi, M. Morra, E. Occhiello, Polymer Surface: From Physics to Technology, John-Wiley & Sons Ltd., West Sussex, 1998.
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Surface modification of biaxially oriented polypropylene (BOPP) film using acrylic acid-corona treatment: Part II. Long term aging surface properties Nanticha Kalapat a, Taweechai Amornsakchai b,c,⁎, Toemsak Srikhirin a a b c
Materials Science and Engineering Program, Faculty of Science, Mahidol University, Phyathai, Bangkok 10400, Thailand Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand Center for Alternative Energy, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand
a r t i c l e
i n f o
Available online 20 May 2013 Keywords: Acrylic acid Corona treatment BOPP film Wettability
a b s t r a c t In this work particular attention has been paid to the aging behavior of biaxially oriented polypropylene (BOPP) film surfaces modified with the acrylic acid (AAc) corona discharge treatment previously reported. Three different corona energies of 15.3, 38.2 and 76.4 kJ/m2 were studied. The surface properties of treated films during 90 days of aging were compared with those of normal air-corona treated films prepared with the same corona energies. The surface chemical compositions of aged films were analyzed by curve-fitting of the ATR-FTIR spectra. The wettabilities of all aged films were monitored by water contact angle and surface free energy measurements. The change of surface topology of air- and AAc-corona treated films was investigated at 1 day, 7 days and 90 days of aging using the AFM technique. In addition, the surface adhesions of aged films were determined with the T-peeling test. The results showed that the amount of polar functional groups on the surface of aged films had changed. However, the aged films of the AAc-corona treated films still showed greater wettability than did the air-corona treated films and could retain high surface hydrophilicity for more than 90 days of aging under ambient condition. The surface topology of both types of aged films changed after aging from a globular structure to a flatter surface, due to mobility of the deposited polymer layer. The AAc-corona treated films showed rougher surfaces due to the influence of poly(acrylic acid) deposition and they could retain the improved surface wettability despite the change in surface topography. The adhesion peel forces of aged films decreased slightly due to the topological changes. A mechanism for the change in surface topography and in chemical functionality of each type of aged film is proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Generally a biaxially oriented polypropylene (BOPP) film has low hydrophilicity and does not accept printing ink or adhesives very well. Therefore, the corona discharge technique is used to introduce polar functional groups such as hydroxyl, carbonyl and carboxylic groups onto the film surfaces. Strictly speaking, the technique should be referred to as an ac barrier corona, because it combines the features of the familiar barrier and corona discharges (between an edge and a dielectric-coated roller) [1]. Since the term is so widely used and accepted in plastic industries, corona discharge will be used. The presence of these functional groups raises the film surface free energy resulting in an increase of wettability and adhesion. However, the improved surface energy decays over time due to the mobility and reorientation of polymer chains on the surface [2]. Novak et al. [3] have studied the chemical change, on a polyolefin ⁎ Corresponding author at: Center for Alternative Energy, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand. Tel.: +66 2 441 9817x1161; fax: +66 2 441 0511. E-mail address: [email protected] (T. Amornsakchai). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.05.012
surface modified by corona discharge, during long-term aging. Their results showed that the improved surface energy decayed after 1 month of aging and that the surface properties were mainly affected by the polymer chain mobility on the surface. In order to eliminate these time-dependent effects after corona treatment, the polymer chain mobility of the treated surface should be obstructed with a layer of some functional groups. This can be achieved without altering the film property by grafting of large chain segment onto the surface. Attempts have been made to produce AAc layer onto polymer films. Lei and Liao [4] used the corona discharge technique to introduce radical on a low density polyethylene (LDPE) film surface before grafting with AAc solution. Besides the use of grafting solution, the introduction of AAc vapor into a plasma reactor also produced a grafted polymer film through plasma polymerization. Jafari and coworkers [5] used plasma polymerization at low pressure to deposit plasma polymerized AAc on PE films. Sciarratta and coworkers [6] used radio frequency plasma technique for functionalizing and coating PP with AAc vapor. Their processes involved activation of PP film with hydrogen plasma before grafting with AAc vapor, thereby requiring more steps to produce the polymer film. Morent and coworkers [7–9] have studied the deposition of plasma-polymerized
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AAc coating on PP substrates using atmospheric pressure dielectric barrier discharge (DBD). They could obtain coatings with high carboxylic acid densities. However, their process was performed by using helium as carrier gas in the chamber. In our previous paper [10], we described the surface modification of BOPP films by introducing AAc vapor into the corona region of a normal corona treater without carrier gas. The surface properties of AAc-corona treated BOPP films were compared with those of normal air-corona treated films. The surface wettability of both types of treated films increased with the change of chemical composition and topography of treated surface. The degree of wettability and surface adhesion depended on the corona energy. It is known that factors affecting the change of surface wettability during storage time are chemical and physical changes on the treated surface [11–14]. As to the chemical change, when exposed to air, the polar groups on the corona treated surface tend to overturn or migrate below the film surface. In the case of physical change, the treated film surface after a high dose of corona treatment shows the formation of bumps. This surface topology will continuously change to a new equilibrium of lower surface energy [15]. So both types of change occur during long-term aging because of the thermodynamic tendency to minimize the interface energy. In this study, the surface properties and durability of both types of corona films treated with different corona energies have been monitored during aging. The difference between the surface behavior of air-corona and that of AAc-corona treated films is discussed. From our findings, schematic models are proposed to describe the relationship between the chemical and physical changes and the corresponding decrease of surface wettability. 2. Experimental methods
and ‘AAc-corona’, respectively. For ‘AAc-corona’, the monomer vapor was generated using an air-bubble method. AAc aqueous solution was placed in a reservoir which had an air inlet. The setup is shown in our previous paper [10]. The monomer outlet tubing was squeezed with a clamp to form a long, thin outlet and placed as close as possible to the corona area. The average AAc feed rate was approximately 0.6 mmol/min. The AAc-corona treated area was about 25 mm wide. Unreacted AAc vapor and ozone generated during the treatment were ventilated through a fume hood. AAc is highly reactive and that released to the atmosphere through the corona zone will react with ozone and photochemically produce hydroxyl radicals, resulting in a half-life of the AAc of 6–14 h [18]. Since a fair amount of ozone is generally generated during the corona discharge, a minimal amount of AAc is left for release to the atmosphere. In these experiments it was not detectable.
2.3. Contact angle and surface free energy measurements The contact angles of the film surfaces were determined with a Kruss, G-1 contact angle goniometer at the ambient temperature. All measurements were made by 10 μl of D.I. water with the sessile drop method for ten points on the sample surfaces. The surface energy was calculated from the additional measurement with diiodomethane on the sample surface [19–21]. The contact angle of each solvent was measured for ten points with a drop volume of 10 μl at the immediate time of drop insertion. The average contact angles were used for calculating total surface free energy with the geometric mean approach of the Owens–Wendt method [22,23].
2.4. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy
2.1. Materials Biaxially oriented polypropylene (BOPP) films used in this study were manufactured and kindly provided by the Thai Film Industries Public Co., Ltd. The average film thickness was 20 ± 4 μm. The melting point and percentage of crystallinity of BOPP films, as measured by differential scanning calorimetry (DSC) at a heating rate of 20 °C/min (Q200-TA DSC), were 167.8 °C and 46.8%, respectively. Acrylic acid, AAc (98 wt.% aqueous solution) was purchased from Fluka. Diiodomethane, CH2I2 (99% stabilized with copper) from Sigma-Aldrich and D.I. water were used for surface free energy measurement. BOPP films and all chemicals were used as-received.
The modified surfaces were studied with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). ATR-FTIR spectra were taken using a 45° germanium crystal (Ge) for single reflection mode on a Nicolet 6400 FT-IR spectrophotometer. The measurements were carried out with 128 scans at a resolution of 4 cm−1 over a range of 4000 to 600 cm−1. Furthermore, the curve-fitting of carbonyl group absorptions was performed on the original spectra with Gaussian band shapes in the range of 1850–1550 cm−1. The peak areas were calculated with OMNIC software for FTIR spectroscopy [24].
2.2. Corona treatment
2.5. Atomic force microscopy (AFM)
Corona treatment was carried out on a locally made corona treater (RUNGTHAI Plastic Corona, Bangkok, Thailand). The treatment was carried out with film traveling at a fixed linear speed of approximately 1.44 m/min and the air-gap between the electrode and the sample film was set at 3 mm. The unit energy of the corona treatment (E) was determined from the following equation [16,17];
Surface topographies of the films before and after aging were obtained with an atomic force microscope (AFM), Nanoscope IIIa multimode (Digital Instruments (DI), Santa Barbara) in tapping mode. The sample films were probed with standard silicon cantilevers/tips. AFM images were acquired on at least three different areas for each sample surface. The arithmetic mean surface roughness (Ra) of the film was calculated directly from the AFM signal using the instrument software.
E¼
P uL
ð1Þ
where P is the corona discharge power (kJ/s), calculated from the input line voltage, V, and the current drawn by the machine, I u is the linear speed of the film in the gap (m/s) L is the length of the discharge electrode (m). Treatment was carried out with three corona energies of 15.3, 38.2 and 76.4 kJ/m2 with and without the vapor of AAc monomer being blown into the corona region. The samples are coded as ‘air-corona’
2.6. T-peel testing Adhesion properties of the aged films to 3M-810 tape were determined by T-shape peeling with a tensile tester INSTRON 5566. The test specimen was made by bonding one end of treated BOPP film (25 mm × 70 mm) and one end of 3M-810 tape (25 mm × 70 mm). This was then peeled apart with a constant rate of 20 mm/min. An average value from at least three measurements was taken as the average peel force (N).
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2.7. Aging of treated films
3.2. ATR-FTIR results
The stability of all treated films was evaluated by using water contact angle measurement for 90 days of aging. All treated films were kept at ambient temperature prior to measuring water contact angle.
In this work, the change in the chemical composition on the aged films can be examined with ATR-FTIR. Generally, the changes on air-corona treated film are difficult or even impossible to detect with this technique because the changes only occurred on the very top layer of a few nanometers. However, with the high dose of corona discharge in this work, the low molecular weight oxidized materials (LMWOM) might be generated on air-corona treated film surface. It resulted in an increase of ATR-FTIR signal. Furthermore, the depth of analysis in this work is calculated to be less than 0.5 μm and this allowed the ATR-FTIR signal of air-corona treated film to be detected. Figs. 3 and 4 display the spectra of air-corona and AAc-corona films treated with 76.4 kJ/m2 of corona energy during aging. The main changes are in the regions between 1500 and 1750 cm−1 and between 3000 and 3700 cm−1 corresponding to carbonyl and hydroxyl stretching, respectively. The intensities of these peaks decreased over time due to the changes of the surface characteristics. In the case of AAc-corona treated films at 90 days of aging, the ATR-FTIR spectrum shows a clear regular sine wave pattern. These are known as interference fringes that occur due to the thin polymer layer [25–28]. This suggests that the film surface had changed to a more uniform top poly(AAc) layer. Such a thin layer would cause the IR beam to reflect many times within the top poly(AAc) layer. As was described in the previous study [10], curve-fitting of the carbonyl stretching region was used to determine the amount of different types of carbonyl functional groups. Four peaks at 1640, 1715, 1725 and 1740 cm−1, corresponding to C_C stretching of unsaturated compound, C_O stretching of ketone, C_O stretching of saturated carboxylic acid and C_O stretching of saturated ester, respectively, were fitted to the region. The amount of carbonyl groups for all aged films was determined from the ratio of the peak area of each
3. Results and discussion In this work, the surface behavior of all treated films during aging was monitored. Both the chemical and topological changes on the surface were studied and are reported as follows.
3.1. Water contact angle and surface free energy results The surface stabilities of both types of treated films were monitored under ambient condition by measuring the water and diiodomethane contact angles. Fig. 1(a) shows the water contact angle results of air-corona treated films which clearly change during aging time. In contrast to the result of air-corona treated films, the water contact angle of AAc-corona treated film in Fig. 1(b) changes only slightly and can be retained until 90 days of aging. Diiodomethane contact angle for air- and AAc-corona treated films is shown in Fig. 1(c) and (d), respectively. All films exhibit very similar behavior of monotonous increase in diiodomethane contact angle with increasing aging time. These results relate to the surface free energy results in Fig. 2. Although the surface energy of both types of aged films decreases, the AAc treated film still displays higher wettability than does the untreated BOPP film. Moreover, the aged AAc-corona treated films show better wetting than do the air-corona treated films because of the polarity of poly(AAc) on the surface.
120
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80
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Aging Period (days)
Fig. 1. Water contact angle of air-corona treated BOPP (a) and AAc-corona treated BOPP films (b) and CH2I2 contact angle of air-corona treated BOPP (c) and AAc-corona treated BOPP films (d) at different aging periods.
N. Kalapat et al. / Surface & Coatings Technology 234 (2013) 67–75
70
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Surface energy (mJ/m2)
Surface energy (mJ/m2)
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Aging period (days) Fig. 2. Surface free energy of aged BOPP films treated with corona energies of 15.8, 38.2 and 76.4 kJ/m2 at ambient temperature.
assigned peak to that of PP at 1456 cm−1. Table 1 compares the peak area ratios for aged films of air-corona and AAc-corona treated films. All peak area ratios increase with increasing corona energy for both types of treatment. After aging, the air-corona treated films show a decrease in the amount of ketone functional groups on the film surface when compared with those of the just treated films. For AAc-corona treated films, the amount of carboxylic acid does not decrease over the aging period. 3.3. Surface topographical results The changes in surface topology of all treated films were monitored by AFM measurement [10]. The surface morphology of BOPP films changed after corona treatment into a globular structure. The
AAc-corona treated films showed rougher surfaces due to surface oxidation and polymer formation, whereas, air-corona treated films displayed a similar structure but of smaller size. The depth of surface treatment of air-corona and AAc-corona treated films was determined using AFM section software analysis. To determine the mechanism of structure formation on the film surface, part of the BOPP films were masked with adhesive tape before corona treatments with the highest corona energy and then, the mask was removed from the treated surface before AFM measurement. The section analysis of air-corona treated films is displayed in Fig. 5(a). It is clearly seen that corona discharge is physically aggressive to the polymer surface. When compared with the smooth untreated area (on the right side of the image), the surface of the treated area was etched into the bulk of the film. The depth between the untreated area and
1725
1715 3400
1640
Fig. 3. ATR-FTIR spectra of untreated BOPP compared with that of air-corona treated with 76.4 kJ/m2 of corona energy during aging.
3400
1640
Fig. 4. ATR-FTIR spectra of untreated BOPP compared with that of AAc-corona treated with 76.4 kJ/m2 of corona energy during aging.
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Table 1 Comparison of ATR-FTIR peak area ratio of air-corona and AAc-corona treated BOPP films during aging by curve-fitting. Samples
Area ratio 1740/1456 1 day
1725/1456
1715/1456
1640/1456
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
0.39 0.49 0.72
0.06 0.21 0.38
0.00 0.01 0.01
0.03 0.04 0.04
0.01 0.07 0.05
0.15 0.45 0.60
0.18 0.28 0.26
0.01 0.03 0.09
0.19 0.39 0.86
0.24 0.34 0.52
0.28 0.32 0.33
AAc-corona treated BOPP film 0.17 0.43 15.3 kJ/m2 0.38 0.35 38.2 kJ/m2 2 0.59 0.52 76.4 kJ/m
0.27 0.26 0.49
0.19 0.80 1.47
0.66 0.69 1.00
0.94 1.45 1.34
0.05 0.15 0.71
0.29 0.05 0.07
0.24 0.25 0.25
0.58 0.85 1.05
0.46 0.52 0.91
0.49 0.69 0.71
Air-corona treated BOPP film 0.21 15.3 kJ/m2 0.72 38.2 kJ/m2 76.4 kJ/m2 1.68
the treated area of film as seen between the marked points is approximately 44 nm. Presumably, the etched depth decreases with decreasing the corona power. In the case of AAc-corona treated films, the section analysis is shown in Fig. 5(b). The difference between the two marked points is approximately 130 nm. It is clear that there is the deposition of poly(AAc) on the treated surface which differs markedly from the surface of air-corona treated film. The comparison of AFM 3D images of air-corona treated BOPP films with corona energies of 15.3, 38.2 and 76.4 kJ/m2 after aging for 1, 7 and 90 days, respectively is displayed in Fig. 6. The globule size increases with increasing corona energy. After aging for 7 days, the globular features disappear. At 90 days of aging, all air-corona treated surfaces become smoother than those of the freshly treated films. This is consistent with a decrease in surface roughness shown in Table 2. It is thought to be due to the migration of the polymer at the interface. This would cause the surface topology to continuously change into a new, smoother equilibrium shape that has lower surface area and so lower surface energy [21].
For AAc-corona treated films, the surface topography and surface roughness after treatment tend to increase with increasing corona energy as observed from AFM images at 1 day in Fig. 7. With the same AAc concentration and the same flow rate, the higher corona energy employed shows larger poly(AAc) droplets on the film surface. When compared with air corona treatment, the films show a rougher surface due to the deposition of polymerized AAc, consistent with the calculated surface roughness shown in Table 2. From the AFM images of 7 day aged films, it seems that some poly(AAc) droplets had moved to combine together to form larger droplets while some remained unchanged. As a result, a wider distribution in droplet size was obtained and this led to an increase in the roughness value. However, after 90 days of aging, these poly(AAc) droplets seem to spread out on the film surface and this results in a decrease in surface roughness. With this topological change, the aged film surface would become covered with a poly(AAc) layer on top of the BOPP. This agrees with the increased amount of carboxylic acid on the film surface observed in the ATR-FTIR results.
Fig. 5. Section analysis of AFM height image of air-corona (a) and AAc-corona (b) treated BOPP films.
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a
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
b
c
Fig. 6. AFM 3D imaging of aged air-corona treated films: corona energy of 15.8 kJ/m2 (a), 38.2 kJ/m2 (b) and 76.4 kJ/m2 (c) at different aging times.
3.4. Adhesion results Fig. 8 displays the representative peel force curves of air-corona and AAc-corona treated films. The result shows very low variation of force during each peeling test. The average peel force of air-corona and
AAc-corona treated BOPP films immediately after treatment and after 90 days of aging was compared and summarized in Table 3. After treatment, all surfaces show high average peel forces when compared with those of the untreated BOPP. After aging, the average peel force of all aged films decreases due to the topological and chemical changes on
Table 2 Surface roughness of air-corona and AAc-corona treated BOPP films with different aging times. Sample
Surface roughness (nm)
Untreated film
5.4
Corona energy (kJ/m2)
After treatment Air-corona
AAc-corona
Air-corona
AAc-corona
Air-corona
AAc-corona
15.3 38.2 76.4
11.1 16.8 24.5
16.4 28.0 34.7
2.3 12.2 11.6
20.6 41.9 57.3
2.5 11.1 8.8
6.9 12.5 17.7
Aging 7 days
Aging 90 days
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a
1 day
7 days
90 days
1 day
7 days
90 days
1 day
7 days
90 days
b
c
Fig. 7. AFM 3D imaging of aged AAc-corona treated films: corona energy of 15.8 kJ/m2 (a), 38.2 kJ/m2 (b) and 76.4 kJ/m2 (c) at different aging times.
the film surface. However, the average peel forces of all aged film surfaces are still higher than that of the untreated film. In the case of aged AAc-corona films, the decrease in the average peel force after aging must be mainly due to the topographical changes of film surface. This is because the amount of polarity on aged AAc-corona film surface is quite stable whereas its surface roughness decreases, resulting in a decrease of surface area for making contact. Despite such topological changes, these surfaces still show a greater surface adhesion than do the aged air-corona films. Combining the observations during aging of the air-corona treated film surface the following conclusions can be summarized. A rough surface and polar functional groups are generated on the BOPP surface by the corona discharge and both the surface topography and surface chemistry undergo changes on storage. Although the corona causes the degradation of the polymer surface and etches away the
oxidized polymer, the film thickness is not significantly changed. The polar functionalities that exist on the surface after air-corona treatment agree well with the appearance of C_O and C_C groups on the surface and cause a decrease in the water contact angle. After one week of aging, the topography becomes flatter. This may be due to the rearrangement of the polymer surface or the loss of degradation products originating from the polymer surface [29]. Since the water angles increase rapidly, it is postulated that, in addition to the decrease in the polar area of the surface [30], the reorientation of polar groups from the surface into the bulk, or the non-polar groups from the bulk to the surface, occurs simultaneously [31]. This phenomenon occurs within the detection volume of ATR-FTIR. After 90 days of aging, low molecular weight molecules with polar groups migrate further into the bulk and thus less ATR-FTIR signal is observed. A mechanism for the changes in surface topography and
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Extension (mm) 4.5
b
4.0
seen in the AFM images of aged films. The ATR-FTIR and peel adhesion results show that, although the deposited poly(AAc) can move and affect the topographical characteristic of film surface, it can retain film wettability and show a long lasting hydrophilic stability until 90 days. These processes can be illustrated by the model in Fig. 10.
3.5
Load (N)
3.0 2.5 2.0
Sample 1 Sample 2 Sample 3
1.5
4. Conclusion
1.0 0.5 0.0 0
Fig. 9. Schematic drawing of the proposed surface topography and chemical change of air-corona treated BOPP film during aging.
5
10
15
20
25
30
35
40
45
50
55
Extension (mm) Fig. 8. Representative peel force curves of air-corona (a) and AAc-corona treated films (b) with the same aging period and corona energy employed.
chemical functionality of aged film can be illustrated as the model in Fig. 9. In the case of AAc-corona treatment, a rough surface and polar functional groups are generated on the BOPP surface. Poly(AAc) is deposited as droplets on the BOPP surface as observed from AFM analysis and ATR-FTIR results. After aging for 1 week, the surface topography changes due to the mobility of deposited AAc. Presumably due to the low molecular weight of poly(AAc), the droplets can flow to combine together resulting in greater surface roughness. With the increase of surface roughness in this case, the entrapment of air into the depressions of the rough surface produces a less hydrophilic surface. Therefore, the surface wettability seems to be decreased in this aging time. This agreed with the change of water contact angle value over this aging period. Then during 90 days of aging, the poly(AAc) droplets spread out on the BOPP surface as
In this work, surface topology of AAc-corona treated films changed by deposition of poly(AAc) on the treated surface. These films differed from the surface of air-corona treated films, on which the change was etched into the bulk of the film. These poly(AAc) droplets spread out on the film surface on storage. Although the topographical characteristics changed to reach a new equilibrium, the surface can retain film wettability and provide hydrophilic characteristics longer-lasting than those of the normal air-corona treatment. The average peel force of the aged AAc-corona treated films decreased with decreasing surface area of the polar part. However, they still showed a greater surface adhesion than did the aged air-corona films.
Acknowledgments This research work is partially supported by a RA scholarship for NK from the Faculty of Graduate Studies, Mahidol University (Academic Year 2012). TA is supported by the Center of Excellence for Innovation in Chemistry (PERCH-CIC).
Table 3 An average peel force of air-corona and AAc-corona treated BOPP films before and after aging. Sample
Average peel force (N)
Untreated BOPP film
2.55 ± 0.03
Corona treated film
Air-corona
15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m2
AAc-corona
After treatment
After 90 days
After treatment
After 90 days
3.09 ± 0.02 3.35 ± 0.04 3.63 ± 0.04
2.89 ± 0.05 2.98 ± 0.06 3.37 ± 0.14
3.58 ± 0.02 3.73 ± 0.02 3.92 ± 0.04
3.00 ± 0.06 3.29 ± 0.11 3.69 ± 0.10
Fig. 10. Schematic drawing of surface topography and chemical change on the AAc-corona treated BOPP film during aging.
N. Kalapat et al. / Surface & Coatings Technology 234 (2013) 67–75
References [1] Y.S. Akishev, A.V. Dem'yanov, V.B. Karal'nik, A.E. Monich, N.I. Trushkin, Plasma Phys. Rep. 29 (2003) 82. [2] C.-M. Chan, T.-M. Ko, Surf. Sci. Rep. 24 (1996) 1. [3] I. Novak, V. Pollak, I. Chodak, Plasma Process. Polym. 3 (2006) 355. [4] J. Lei, X. Liao, Eur. Polym. J. 37 (2001) 771. [5] R. Jafari, M. Tatoulian, W. Morscheidt, F. Arefi-Khonsari, React. Funct. Polym. 66 (2006) 1757. [6] V. Sciarratta, U. Vohrer, D. Hegemann, M. Muller, C. Oehr, Surf. Coat. Technol. 174 (2003) 805. [7] R. Morent, N.D. Geyter, S.V. Vlierberghe, E. Vanderleyden, P. Dubruel, C. Leys, E. Schacht, Plasma Chem. Plasma Process. 29 (2009) 103. [8] R. Morent, N.D. Geyter, M. Trentesaux, L. Gengembre, P. Dubruel, C. Leys, E. Payen, Appl. Surf. Sci. 257 (2010) 372. [9] R. Morent, N.D. Geyter, S.V. Vlierberghe, A. Beaurain, P. Dubruel, E. Payen, Prog. Org. Coat. 70 (2011) 336. [10] N. Kalapat, T. Amornsakchai, Surf. Coat. Technol. 207 (2012) 594. [11] S. Guimond, M.R. Wertheimer, J. Appl. Polym. Sci. 94 (2004) 1291. [12] C.-M. Chan, Polymer Surface Modification and Characterization, Carl Hanser Verlag, New York, 1994. 63. [13] I. Novak, S. Florian, J. Mater. Sci. 39 (2004) 2033.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
75
C.Y. Kim, J. Evans, D.A.I. Goring, J. Appl. Polym. Sci. 15 (1971) 1357. C. Sun, D. Zhang, L.C. Wadsworth, Adv. Polym. Technol. 18 (1999) 171. O. Lesly-Ann, L. Stuart, P. Bhukan, Surf. Interface Anal. 33 (2002) 335. E. Vassiliadi, P.A. Tarantili, J. Appl. Polym. Sci. 105 (2007) 1713. Acrylic Acid, Product Safety Summary Sheet, BASF, 2007. R.N. Shimizu, N.R. Demarquette, J. Appl. Polym. Sci. 76 (2000) 1831. O.C. Van, J. Adhes. Sci. Technol. 16 (2002) 669. A. Salimi, S.M. Mirabedini, M. Atai, M. Mohsei, M.R. Naimi-Jamal, Int. J. Adhes. Adhes. 31 (2011) 220. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. J. Schultz, K. Tsutsumi, J.-B. Donnet, J. Colloid Interface Sci. 59 (1977) 272. Z. Siqiang, E.H. Douglas, J. Vinyl Addit. Technol. 13 (2007) 57. A. Larena, G. Pinto, F. Millan, Appl. Surf. Sci. 84 (1995) 407. A. Larena, F. Millan, G. Perez, G. Pinto, Appl. Surf. Sci. 187 (2002) 339. A. Larena, F. Millan, M. Verdu, G. Pinto, Appl. Surf. Sci. 174 (2001) 217. N. Leopold, S. Busche, G. Gauglitz, B. Lendl, Spectrochim. Acta A 72 (2009) 994. J. Song, U. Gunst, H.F. Arlinghaus, G.J. Vancso, Appl. Surf. Sci. 253 (2007) 9489. R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. F. Garbassi, M. Morra, E. Occhiello, Polymer Surface: From Physics to Technology, John-Wiley & Sons Ltd., West Sussex, 1998.