Surface & Coatings Technology 207 (2012) 594–601

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Surface & Coatings Technology

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Surface modification of biaxially oriented polypropylene (BOPP) film using acrylicacid-corona treatment: Part I. Properties and characterization of treated films

Nanticha Kalapat a, Taweechai Amornsakchai b,c,⁎a Materials Science and Engineering Program, Faculty of Science, Mahidol University, Phyathai, Bangkok 10400, Thailandb Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailandc Center for Alternative Energy, Faculty of Science, Mahidol University, Phuttamonton 4 Road, Salaya, Nakhon Pathom 73170, Thailand

⁎ Corresponding author at: Department of Chemistry anovation in Chemistry, Faculty of Science, Mahidol UnivSalaya, Nakhon Pathom 73170, Thailand. Tel.: +66 2 440511.

E-mail address: taweechai.amo@mahidol.ac.th (T. A

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.07.081

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 April 2012Accepted in revised form 30 July 2012Available online 7 August 2012

Keywords:Acrylic acidCorona treatmentBOPP filmWettability

In this work, the acrylic acid (AAc)-corona discharge was carried out on biaxially oriented polypropylene(BOPP) films by introducing AAc vapor into the corona region of a normal corona treater. Three different co-rona energies of 15.3, 38.2 and 76.4 kJ/m2 were studied. Surface properties of treated films were comparedwith those of air-corona treated films prepared with the same corona energies. The change in chemical com-position on the film surface was characterized by curve-fitting of the ATR-FTIR spectra. The wettability oftreated films, before and after aging in different environments, was observed by water contact angle and sur-face free energy. The surface morphology of air- and AAc-corona treated films was investigated using SEMand AFM techniques. Adhesion of the treated films to some other substrate was determined with theT-peeling test. It was found that the hydrophilicity of all treated films increased with increasing corona ener-gy. AAc-corona treated films showed greater wettability than did the air-corona treated films and could re-tain the surface hydrophilicity for more than 90 days of aging under ambient conditions. The surfacemorphology of BOPP films changed after corona treatment into a globular structure. The AAc-corona treatedfilms showed rougher surfaces due to surface oxidation and polymer formation, whereas, air-corona treatedfilms displayed a similar structure but of smaller size due to the formation of low molecular weight oxidizedmaterials (LMWOM) arising from the degradation of BOPP films. AAc-corona treated films showed greaterpeel strength than did the air-corona treated films.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

One of the most important commercial polyolefin films is biaxiallyoriented polypropylene (BOPP)film. It is used in awide variety of appli-cations including food packaging, multi-purpose packaging andadhesive-tape packing [1–3]. Generally BOPP film has low hydrophilic-ity and does not accept printing ink or adhesives very well. So it is nor-mally surface modified for improved wettability and adhesion. Themost common methods for surface modification are corona dischargeand plasma treatments [4]. It is known that the best method is a lowpressure plasma technique which provides a uniformly treated surface.However, plasma treatment requires a special low pressure reactor,which is expensive and difficult for a continuous production. Less effec-tivemethods that can be operated under ambient conditions are one at-mosphere uniform glow discharge (OAUGD) and corona discharge. Thecorona discharge technique, due to its simplicity, its suitability for

nd Center of Excellence for In-ersity, Phuttamonthon 4 Road,1 9817x1161; fax: +66 2 441

mornsakchai).

rights reserved.

continuous and inline operation and its cost effectiveness, has gainedpopularity in the film industry. This technique can introduce polar func-tional groups such as hydroxyl, carbonyl and carboxylic groups onto thefilm surfaces. The presence of these functional groups on the film sur-face raises the film surface free energy. However, due to the thermody-namic driving force and their small sizes, these polar functional groupstend to overturn and bury themselves, or migrate, below the film sur-face [5,6]. Novak et al., [7] showed that a corona treatment increasedthe surface free energy of PP film from 34 to 39.2 mN/m, but this treat-ment was not stable because the surface free energy went down from39.2 to 37 mN/m after 30 days due to this reorientation. This is alsoknown as hydrophobic recovery.

In order to eliminate these time-dependent effects after coronatreatment, the polymer chain mobility of the treated surface should beobstructed with a layer of some functional groups [8]. This can beachieved without altering the film property by either grafting of largechain segment or crosslinking the structure onto the surface. Liao andcoworkers [9] used the corona discharge technique to explore graftingon active sites on a low density polyethylene (LDPE) film surface.After introducing radicals onto the LDPE surface, acrylic acid (AAc) inaqueous solution was grafted on. Their results showed a good degreeof grafting and improved hydrophilicity.

Fig. 1. Schematic of process for AAc-corona treatment.

595N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

However, the use of grafting solution for post-corona surfacegrafting means additional steps for the treatment, which significantlylengthens the process. Attempts have been made to introduce AAcvapor into a plasma reactor to produce a grafted polymer film throughplasma polymerization [10,11]. Sciarratta and coworkers [12] usedsuch a plasma technique for functionalizing and coating PP with AAcvapor. Their process involved activation of PP film with hydrogen plas-mabefore graftingwith AAc vapor, thereby requiringmore steps to pro-duce the polymerfilm. If similar surface structures can be obtainedwitha corona technique running under ambient condition, the methodwould be very useful and more attractive for industrial applications.

In this research, we present a new approach for the surface treat-ment of BOPP films in a single step and under ambient conditions.AAc vapor is blown into the corona region during the treatment. Themonomer polymerizes and forms a thin film with high polarity on theBOPP surface. The effectiveness of this AAc-corona treatment is com-pared with the conventional method, air-corona treatment. The surfaceproperties of all films treated with different corona energies are com-pared and discussed.

2. Experimental methods

2.1. Materials

Biaxially Oriented Polypropylene (BOPP) Films used in thisstudy were manufactured and kindly provided by Thai Film Indus-tries Public Co., Ltd. The average film thickness was 20±4 μm. Themelting point and percentage of crystallinity of BOPPfilms, asmeasuredbydifferential scanning calorimetry (DSC) at a heating rate of 20 °C/min(Q200-TADSC), 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. waterwere used for surface free energy measurement. BOPP films and allchemicals were used as-received.

2.2. Corona treatment

Corona treatment was carried out on a locally made corona treater(RUNGTHAI Plastic Corona, Bangkok, Thailand). The treatmentwas car-ried out with film traveling at a fixed linear speed of approximately1.44 m/min and the air-gap between the electrode and the samplefilm was set at 3 mm. The unit energy of the corona treatment (E)was determined from the following equation [13,14];

E ¼ PuL

ð1Þ

Where P is the corona discharge power (kJ/s).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, 38.2and 76.4 kJ/m2. Treatment was performed without and with thevapor of AAc monomer blown into the corona region. The samples arecoded as ‘Air-corona’ and ‘AAc-corona’, respectively. For ‘AAc-corona’,the monomer vapor was generated using an air-bubble method. AAcaqueous solution was added into a reservoir which had an air inlet.The set up is shown in Fig. 1. The monomer outlet tubing was squeezedwith a clamp to form a long, thin outlet and placed as close as possible tothe corona area. The average AAc feed rate was approximately0.6 mmol/min. The AAc-corona treated area was about 25 mm wide.Unreacted AAc vapor and ozone generated during the treatment wereventilated through a fume hood. AAc is highly reactive and that releasedto the atmosphere through the corona zone will react with ozone andphotochemically produce hydroxyl radicals, resulting in a half-life ofthe AAc of six to fourteen hours [15]. Since a fair amount of ozone is

generally generated during the corona discharge, a minimal amountof AAc is left for release to the atmosphere. In these experiments itwas not detectable.

2.3. Contact angle and surface free energy measurements

The contact angles of the film surfaces were determined with aKruss, G-1 contact angle goniometer at the ambient temperature. Allmeasurements were made by 10 μl of D.I.water with the sessile dropmethod. This method was used to estimate the wetting properties of alocalized region on the film surface. Each contact angle reported inthis work was an average of the values obtained for ten points on thesample surfaces.

The surface energy (γs) and its polar (γsp) and dispersive (γs

d) com-ponents were calculated from the contact angles of sessile drops of se-lected liquids deposited on the sample surface. In this experiment,water and diiodomethane were used with the geometric mean ap-proach of the Owens–Wendt method [16]. The surface tension (γL) ofwater and diiodomethane are 72.8 and 50.8 mJ/m2, respectively. Thedispersive component (γL

d) and the polar component (γLp) are 21.8

and 51 mJ/m2 for water and 50.8 and 0 mJ/m2 for diiodomethane,respectively [17–19].

The contact angle of each solvent was measured for ten pointswith a drop volume of 10 μl at the immediate time of drop insertion.The average contact angles were used for calculating polar and dis-persive components from the following equation [20]:

γL 1þ cosθð Þ ¼ 2ffiffiffiffiffiffiffiffiffiffiffiγdSγ

dL

qþ 2

ffiffiffiffiffiffiffiffiffiffiffiγpSγ

pL

qð2Þ

Where; the subscripts S and L represent solid and liquid phases,respectively. The total surface free energy γs is a summation of itspolar and dispersive components.

γs ¼ γds þ γp

s ð3Þ

2.4. Attenuated total reflectance Fourier transform infrared (ATR-FTIR)spectroscopy

The modified surfaces were studied with attenuated total reflectionFourier transform infrared spectroscopy (ATR-FTIR). ATR-FTIR spectrawere taken using a 45° germanium crystal (Ge) for single reflectionmode on a Nicolet 6400 FT-IR spectrophotometer. The measurementswere carried out with 128 scans at a resolution of 4 cm−1 over arange of 4000 to 600 cm−1.

596 N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

Furthermore, the curve-fitting of carbonyl groups was performedon the original spectra with Gaussian band shapes in range of1850–1550 cm−1. The peak areas were calculated with OMNIC soft-ware for FTIR spectroscopy [21].

2.5. Atomic force microscopy (AFM)

Surface topographies of the films before and after corona treat-ment were obtained with an AFM, Nanoscope IIIa multimode (DigitalInstruments (DI), Santa Barbara) in tapping mode. The sample filmswere probed with standard silicon cantilevers/tips. AFM imageswere acquired on at least three different areas for each sample sur-face. The arithmetic mean surface roughness (Ra) of the film was cal-culated directly from the AFM signal using the instrument software.

2.6. Scanning electron microscopy (SEM)

The surface morphologies of untreated and treated films wereexamined with SEM (Hitachi S2500, Japan) operated at 15 kV. Pt–Pdwas coated on the samples prior to the observation.

2.7. T-peel testing

Adhesion properties of the treated films to 3M-810 tape were de-termined by T-shape peeling with a tensile tester INSTRON 5566. Thetest 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. Anaverage value from at least three measurements was taken as theaverage peel force (N).

1725

16403400

a)

b)

1715

16403400

4000 3500 3000 2500Wavenumbers (cm-1)

Abs

orba

nce

Abs

orba

nce

2000 1500 1000

4000 3500 3000 2500Wavenumbers (cm-1)

2000 1500 1000

Fig. 2. ATR-FTIR spectra of untreatedBOPPwith air-corona (a) andAAc-corona (b) treatedBOPP using different corona energies.

2.8. Aging of treated films

The stability of all treated films was evaluated by using water con-tact angle measurement after different aging times. All treated filmswere kept in different storage conditions (at ambient temperatureand in water) and storage times (0–7 days) prior to measuring watercontact angle. Then, the result of hydrodynamic recovery of the treatedsurface was compared with that of the untreated film.

3. Results and discussion

In this work, BOPP film surface was modified with air-corona andAAc-corona. The effectiveness of the treatment was studied by varyingthe corona energy at three levels of 15.3, 38.2 and 76.4 kJ/m2. Surfaceproperties of the film after treatment were studied and reported asfollows.

3.1. ATR-FTIR results

ATR-FTIRwas used to examine the change in functional groupon thefilm surface. Fig. 2 displays the ATR-FTIR spectra of films treated underdifferent conditions. The main changes are in the regions between1500–1750 cm−1 and 3000–3700 cm−1 which correspond to carbonyland hydroxyl stretchings, respectively. The intensities of the changes

Fig. 3. Curve-fitting of air-corona treated BOPP with corona energies of 76.4 kJ/m2 (a),38.2 kJ/m2 (b) and 15.8 kJ/m2 (c).

Fig. 4. Curve-fitting of AAc-corona treated BOPP with corona energies of 76.4 kJ/m2 (a),38.2 kJ/m2 (b) and 15.8 kJ/m2 (c).

Untreated 15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m20

20

40

60

80

100

120a)

b)

Wat

er C

on

tact

An

gle

(d

egre

e)

Corona Energy

Air-corona AAc-corona

Untreated 15.3 kJ/m2 38.2 kJ/m2 76.4 kJ/m20

10

20

30

40

50

60

70

80

Su

rfac

e F

ree

En

erg

y (m

J/m

2 )

Corona Energy

Air-corona AAc-corona

Fig. 5. Water contact angle (a) and surface free energy (b) of air-corona and AAc-coronatreated BOPP films after treatment with different corona energies.

597N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

increase with increasing the corona power. When the carbonylstretching region is considered, it was found that in the case ofair-corona (Fig. 2(a)), the vibration peaks at 1715 cm−1 and1640 cm−1 correspond to the C_O and C_C stretching, respectively.These groups were formed as a result of surface oxidation and theirnumbers increased with increasing the corona energy. For AAc-coronatreated films illustrated in Fig. 2(b), the peaks are seen at 1725 and1640 cm−1 for C_O and C_C stretching, respectively. The former

Table 1Comparison of ATR-FTIR peak area ratio of air-corona and AAc-corona treated BOPPfilms by curve-fitting.

Samples Area ratio

1740/1456 1725/1456 1715/1456 1640/1456

Untreated BOPP film – – – –

Air-corona treated BOPP film15.3 kJ/m2 0.21 0.00 0.15 0.1938.2 kJ/m2 0.72 0.01 0.45 0.3976.4 kJ/m2 1.68 0.01 0.60 0.86

AAc-corona treated BOPP film15.3 kJ/m2 0.17 0.19 0.05 0.5838.2 kJ/m2 0.38 0.80 0.15 0.8576.4 kJ/m2 0.59 1.47 0.71 1.05

shifts approximately 10 cm−1 to higher wavenumber or frequency in-dicating different types of carbonyl functional group.

The change in the 3400–3600 cm−1 region will now be considered.The peaks are relatively broad indicating the existence of OHgroupwithintermolecular H-bonding. Both air-corona and AAc-corona providesimilar change in shape and position to the spectrum but AAc-coronaprovides greater intensity. The presence of these peaks and carbonylpeak discussed above suggest the formation of carboxylic acid function-al groups on the film surface [22,23]. This can be attributed to the pres-ence of AAc vapor in the corona regionwhich can polymerize to providea greater number of oxygen containing functional groups.

To verify the different types of carbonyl functional groups thatoccurred on these surfaces, the curve-fitting was used to quantitativelyestimate the peak area of each component representing various carbon-yl derivatives. The experimental spectra werefittedwith Gaussian bandshapes from the range of 1850–1550 cm−1 by an iterative curve fitting

Table 2Surface free energy of aged BOPP films at ambient temperature.

Corona energy(kJ/m2)

Surface free energy (mJ/m2)

Beforetreatment

After treatment After aging (90 days)

Air-corona AAc-corona Air-corona AAc-corona

15.3 34.8 48.0 54.1 42.4 51.038.2 34.8 49.6 58.4 44.3 53.576.4 34.8 55.7 64.0 47.0 56.8

Table 3Surface stability of treated BOPPs with corona energy of 76.4 kJ/m2 in different agingenvironments.

Aging period Water contact angle (degree)

Air-corona BOPP AAc-corona BOPP

At ambienttemperature

In water At ambienttemperature

In water

Untreated BOPP 105±3 105±3 105±3 105±35 min 55±1 66±3 44±2 65±12 h 58±2 68±4 46±1 68±21 day 67±2 67±4. 49±3 72±42 days 65±3 70±1 53±2 73±73 days 63±2 72±3 48±2 79±24 days 60±2 71±4 59±3 76±45 days 58±3 72±4 60±4 78±36 days 60±5 77±2 52±2 81±27 days 61±4 77±4 52±5 79±4

598 N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

procedure. Four peaks at 1640, 1715, 1725 and 1740 cm−1, corres-ponding to C_C stretching of unsaturated compound, C_O stretchingof ketone, C_O stretching of saturated carboxylic acid and C_Ostretching of saturated ester, respectively, were used to fit the curves[24,25]. The curve-fitting of air-corona and AAc-corona treated BOPPwith different corona energies are shown in Figs. 3 and 4, respectively.The amount of carbonyl groups were quantified by using the peakarea ratio of the assigned four peaks to that of the asymmetric C-Hstretching of CH3 peak at 1456 cm−1. Table 1 compares the ATR-FTIR

Fig. 6. SEM images of untreated BOPP film (a), AAc-corona treated BOPP

peak area ratio of air-corona and AAc-corona treated BOPP films. Bothtreated conditions show the existence of ketone, saturated ester andunsaturated functional groups on the film surface. These peak area ra-tios increase with increasing corona energy. However, the main differ-ence between air-corona and AAc-corona treated film is the amountof carboxylic acid and hydroxyl groups which is clearly observed inAAc-corona treated film.

3.2. Water contact angle and surface free energy

Although ATR-FTIR technique can detect the difference in chemicalspecies on treated surface, it is not considered a truly surface-sensitivetechnique like water contact angle measurement. The water contactangle is a good indicator of the polar nature of the outermost atomiclayers of the surface [6,20]. Therefore, this technique was used to eval-uate the change in hydrophilicity and the wettability of the polymersurface after surface treatments and storing in different environments.

Fig. 5(a) compares water contact angle of air-corona and AAc-corona treated surfaces after treating with different corona energies.When compared with the untreated film, the water contact angle ofboth types of treated film significantly decreased. The water contactangle of air-corona treated films after treatment with corona energiesof 15.3, 38.2 and 76.4 kJ/m2 were 67°, 64° and 54°, respectively, where-as, that of AAc-corona treated films were 54°, 46° and 39°, respectively.Surface free energy for these treated films was determined with addi-tional measurements with diiodomethane and the results are displayedin Fig. 5(b). The surface free energy after air-corona exposure increases

film using corona energies of 15.8 (b), 38.2 (c) and 76.4 kJ/m2 (d).

Fig. 7. AFM height images of untreated BOPP film (a) and AAc-corona treated BOPPfilm using a 76.4 kJ/m2 of corona energy (b).

599N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

with increasing corona energy. These surface free energies agree withthat reported by other researchers that studied the improvement of sur-face free energy on BOPP film by using air-corona treatment [5,13,14].In the case of AAc-corona, the surface free energy of treated film ishigher than that of air-corona due to the influence of AAc polarity.Its surface free energy shows a maximum value of 64 mJ/m2 which iscomparable to the reported value of 68 mJ/m2 for poly (acrylic acid)(PAA) [26].

Surface free energies of aged films are reported in Table 2. The surfacefree energies of both types of treatedfilms decrease after 90 days of aging.Surface free energies of air-corona treatedfilm surfaceswith corona ener-gies of 15.3, 38.2 and 76.4 kJ/m2 were 42.4, 44.3, 47.0 mJ/m2 comparedwith 51.0, 53.5, 56.8 mJ/m2, respectively, for AAc-corona treated films.All aged films can retain a hydrophilic surface, but to different degrees.

It is known that not only the aging time that affects the hydrophobicrecovery of the treated surface, but also the aging environment. There-fore, the surface stabilities of both types of treatedfilmsweremonitoredunder ambient conditions and in water. The results are shown inTable 3. Under ambient conditions, the water contact angles of bothtypes of film slightly increase with increasing aging time. In water, thecontact angles increase after 5 min of aging. Both types of treatedfilms show very similar water contact angle after aging in water. Thesurfaces of both air-corona and AAc-corona treated films retain goodwetting when compared with the untreated BOPP film. The increasein contact angle in the case of air-corona treated films may be due tothe washing off of the low molecular weight oxidized materials(LMWOM) from the film surface. In the case of AAc-corona treatedfilms, the surface is also washed. Since PAA is a water-soluble polymer,the PAA layer on AAc-corona treated films can easily be dissolved off, sothat the surface hydrophilicity from the acrylic acid disappears. In orderto extend the shelf-life of the treated films, suitable storage conditionsare needed and a wet environment should be avoided.

3.3. Surface topography analysis

The topographical characteristics of treated films also affect the wet-ting properties [27,28]. Microscopic morphology of AAc-corona treatedfilms after treatment with corona energy of 15.8, 38.2 and 76.4 kJ/m2 isshown by SEM images in Fig. 6. The surfaces of all AAc-corona treatedfilms become much rougher than those of untreated BOPP films. This isconsistent with the deposition of polymerized AAc monomer on theBOPP surface. When the untreated film in Fig. 6(a) and AAc-corona treat-edwith 76.4 kJ/m2 in Fig. 6(d) are compared, the variously sized dropletsformed onAAc-coronafilmare clearly observed. These results are also ob-servedwith theAFMheight images in Fig. 7. Generally, the SEMtechniqueprovides a large area view and has the ability to image the rough surface.However, with this technique it is difficult to determine features of thesurface such as roughness and height. Therefore the AFM technique wasused to verify the details of the treated surface. Fig. 8 displays AFM3D im-ages of untreated and treated BOPP films. It can be seen that untreatedBOPP film has a rather smooth and featureless surface while both typesof corona treated BOPP films exhibit significant topographical changes.The untreated BOPP shows a topographical appearance with a fibrillarstructure having a surface roughness (Ra) of 5.44 nm. After air-coronatreatment, this fibrillar structure is lost and replaced by globular featureswith an average diameter of approximately 240 nm. It shows rough sur-face with Ra of 24.49 nm. It is known that the original surface structure ofthe polymer is broken down under high doses of plasma treatment bysome kind of chain scission [29]. Many researchers have observed thatthis globular structure occurred due to the formation of LMWOM that de-graded on the surface of the treated BOPP film [5,7,30,31]. In other wordsthe use of high energies results in the degradation of the BOPP surface. Inthe case of AAc-corona treatment, similar but coarser globular featuresare seen. The average diameter of each droplet is about 340 nm, givinga rougher surface with a Ra of 34.68 nm. The change observed in

AAc-corona treated film can be attributed to the deposition of polyme-rized AAc on the film surface.

3.4. Adhesion properties

Table 4 shows the average peel force of air-corona and AAc-coronatreated BOPP films. All treated surfaces show high average peel forceswhen compared with those of the untreated BOPP. AAc-corona treatedfilms display greater average peel forces than do the air-corona treatedfilms. Films treated with air-corona at the highest energy of 76.4 kJ/m2

show the same average peel forces as those films treated withAAc-corona having the lowest energy of 15.3 kJ/m2. This means thatAAc-corona treated films have better peel adhesion than those treatedwith average energy air-coronas. Due to the polarity of AAc on the sur-face, AAc-corona films treated with a corona energy of 76.4 kJ/m2 dis-play the highest force of 3.92 N. In addition to the presence of the PAAlayer on the treated film surface, surface topography also influencesthe adhesion. Films treated with higher energy have high surfaceroughness and show higher peel forces. This is because the rough sur-face has a form of mechanical interlocking to increase the surface adhe-sion. Therefore, the AAc-corona treated films, which had roughersurface, showed better adhesion than did the air-corona treated films.

According to these observations, both air-corona and AAc-coronatreatment can improve the wettability of BOPP films. ATR-FTIR re-sults show that all treated films have oxygen-containing functionalgroups and unsaturated compounds on the surface. These functionalgroups arose from oxidation on the film surface. The films treatedwith the highest corona energy exhibit the highest carbonyl intensity,due to greater surface oxidation. This, in turn, results in an increase inthe surface free energy of the BOPP films. When the wettability resultsof air-corona and AAc-corona treated film are compared, AAc-coronatreated films exhibit greater wettability with lower water contact

Untreated BOPPRa= 5.44 nm

Air-corona treated BOPPRa= 24.49 nm

AAc-corona treated BOPPRa= 34.68 nm

2

300 nm 300 nm

300 nm

4

6

8

µm

2

4

6

8

µm

2

4

6

8

µm

Fig. 8. AFM 3D images and surface roughness (Ra) of untreated BOPP film, air-corona treated and AAc-corona treated BOPP using 76.4 kJ/m2 of corona energy.

600 N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

angle and higher surface free energy than do the air-corona treatedfilms at all corona energies, due of course to the AAc polarity. These re-sults agreewithATR-FTIR results showing that the amount of carboxylicacid groups, quantified by curve-fitting, on AAc-corona treated films isgreater than that on the air-corona treated films. Also, the AAc-coronatreated surfaces contain the same functional groups, ketone, ester andunsaturated compounds, as are observed in the air-corona treatedfilms. It can benoted that both surface oxidation and polymer formationoccurred on AAc-corona treated films. For this reason, the AFM imagesof AAc-corona treated films show rougher surfaces with various sizesof droplets. From all the results, it can be deduced that AAc-corona con-ditions can be used to enhance the wettability of BOPP surfaces betterthan can air-corona conditions. It is known that the air-corona treatedsurfaces have limited shelf-life. The polar functional groups on

Table 4Average peel force of air-corona and AAc-corona treated BOPP films from 3M-810 tape.

Sample Average peel force (N)

Untreated BOPP film 2.55±0.03Air-corona treated BOPP film

15.3 kJ/m2 3.09±0.0238.2 kJ/m2 3.35±0.0476.4 kJ/m2 3.63±0.40

AAc-corona treated BOPP film15.3 kJ/m2 3.58±0.0238.2 kJ/m2 3.73±0.0276.4 kJ/m2 3.92±0.04

air-corona treated surfaces tend to overturn themselves or migratebelow the film surface causing a decay over time of the surface energyand water contact angle. The presence of AAc on the BOPP surface canbe employed to retard the overturn of these polar functional groupsmore than 90 days after aging under ambient conditions. Additionally,it is also possible to prevent the formation of LMWOM on treated sur-faces because the film surface is not directly charged with the coronaas occurs under the normal air-corona conditions. Examination of thestability of all treated surface films after different environmental agingshowed that the AAc-corona treated films were not stable in waterdue to their water-soluble property. Therefore, care should be taken inthe storing and handling of these films, just as it should be for otherwater soluble polymers. Under dry storage conditions, these filmswould have a longer shelf-life.

4. Conclusion

In this work, vapor of AAc monomer was successfully introducedinto the corona region to form a polymeric thin film on BOPP sur-faces. The surface wettability of BOPP films can be much improvedby comparison with the normal air-corona treatment and can retainthe hydrophilicity more than 90 days of aging. Different levels of hy-drophilicity can be obtained, depending on the corona energy. Inaddition, the peel adhesion can be improved with AAc-corona treat-ment. The average peel forces of AAc-corona treated films are signif-icantly higher than those of untreated and air-corona treated films. Itis anticipated that this approach will be highly beneficial for the filmmaking industries.

601N. Kalapat, T. Amornsakchai / Surface & Coatings Technology 207 (2012) 594–601

Acknowledgement

TA is supported by the Center of Excellence for Innovation in Chemis-try (PERCH-CIC).

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