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Effect of mechanical, barrier and adhesion properties on oxygen plasma surface modified PP
Innovative Food Science and Emerging Technologies 30 (2015) 119–126
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Effect of mechanical, barrier and adhesion properties on oxygen plasma surface modified PP M. Vishnuvarthanan ⁎, N. Rajeswari College of Engineering, Guindy, Anna University, Chennai – 600025, TamilNadu, India
a r t i c l e
i n f o
Article history: Received 17 December 2014 Received in revised form 30 March 2015 Accepted 18 May 2015 Available online 30 May 2015 Keywords: Polypropylene Oxygen plasma treatment Contact angle FTIR AFM XRD Tensile strength OTR WVTR
a b s t r a c t In this work, the Polypropylene (PP) film was surface modified by Oxygen plasma treatment and the effect of mechanical, barrier and adhesion properties was studied. The PP film was plasma treated with various RF power settings of 7.2 W, 10.2 W and 29.6 W in various time intervals of 60 s, 120 s, 180 s, 240 s and 300 s. To characterize the wettability, the contact angle was measured and the surface energy values were estimated with different test liquids. The generation of oxygen functional groups on the surface of plasma modified PP and the surface change characterization were observed by attenuated total reflection-Fourier transform infrared spectroscope (ATRFTIR) and they resulted in wettability improvement. The roughness of the PP film and the surface morphology were analyzed by Atomic Force Microscopy (AFM). It was found that the roughness value increased from 1.491 nm to 7.26 nm because of the increase of treatment time and RF power. The PP crystallinity structure of the untreated and treated PP was evaluated by X-ray diffraction analysis (XRD). The bond strength of the untreated and surface modified films were measured by T-peel test method. For the untreated and oxygen plasma treated sample, the mechanical properties like Tensile Strength and the barrier properties like oxygen transmission rate (OTR), Water vapor transmission rate (WVTR) were also calculated. From the results, the tensile strength reduced from 6 MPa to 1.350 MPa because of polypropylene etching and degradation. The OTR increased from 1851.2 to 2248.92 cc/m2/24 h and the Water vapor transmission rate increased from 9.6 to 14.24 g/m2/24 h. Industrial Relevance: Plasma technology applied to packaging and printing industry is a dry, environmentally- and worker-friendly method to achieve surface alteration without modifying the bulk properties of different materials. In particular, atmospheric non-thermal plasmas are suited because most are heat sensitive polymers and applicable in continuous process. In the last years plasma technology has become a very active, high growth research field, assuming a great importance among all available material surface modifications in packaging industry. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Polymers like PE, PP, PS and PET are the mostly used packaging materials because these are available in large quantities at low costs (Pankaj et al., 2013). In industry, PP films are used in many packaging applications. It has good mechanical, physical, barrier and chemical resistance properties. These polymers are having low surface energy and therefore it have poor adhesive properties (Shin et al., 2002). For a particular application, if the polymer surface does not have the desired properties, it leads to material failure and the system or device containing it (Hoffman, 1995). For good adhesion between polymer and coating the polymer surface must be activated (Vesel & Mozetic, 2012). This can be done by the plasma treatment. By this technique, the surface properties of the polymers can be modified without affecting the bulk properties. The lack of adhesion can be owed without surface modification (Shin et al., 2002). For modifying the surface properties of polymers, a ⁎ Corresponding author. Tel.: +91 9943769268. E-mail address: [email protected] (M. Vishnuvarthanan).
http://dx.doi.org/10.1016/j.ifset.2015.05.007 1466-8564/© 2015 Elsevier Ltd. All rights reserved.
number of techniques have been developed such as mechanical, thermal, chemical and plasma treatments. For packaging, the use of plasma treatment seems to be suitable and to attain important change in the properties governed by surface characteristics (Wei, Gao, Hou, & Wang, 2005). The plasma refers to a partly or entirely ionized gas that consisted essentially of ions, photons and free electrons. The atoms in the excited or fundamental states are holding a net neutral charge (Liu, Cui, Brown, & Meenan, 2005). To attain the surface modification characteristics of polymeric materials, the effective technology is the plasma treatment (Cireli, Kutlu, & Mutlu, 2007). The surface phenomena such as crosslinking, etching and activation are done by the interactions between surface molecules of polymers and plasma (Riccardi et al., 2003). Over the conventional process, these plasma treatments offer many benefits because these are normally a dry process and they cannot generate chemical waste (Yang, Chen, Guo, & Zhang, 2009). Dependent on the conditions and the plasma species, the polymer surface properties such as hydrophobicity, morphology and the adhesion can be altered (Kauling et al., 2009). They can give impact on the barrier, mechanical and adhesion properties of the polymers. The reactive
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functional group and the surface roughness can be introduced and also improved in the polymer surface by the plasma (Kirk et al., 2010). By this the mechanical performance and the adhesion property can also be improved. This was done due to the mechanical interlocking mechanism and also by the chemical interaction (Shin et al., 2002). In this present study, the surface of the polypropylene was modified by RF oxygen plasma. This oxygen plasma promoted the surface modification by surface activation and slight etching. The contact angle and the polypropylene surface morphology were analyzed by goniometer and AFM. The surface energy studies were carried out and the bonding strength was also calculated. The mechanical and the barrier properties of untreated and the oxygen plasma treated polypropylene samples were carried out and the influence of surface modification on these properties was analyzed. 2. Materials and methods 2.1. Materials In this study, the commercial polypropylene film was used and it was purchased from Jayanthi Plastics, Erode, India. It has the thickness of 0.048 ± 003 mm and density of 0.92 g/cm3. Double distilled water and Formamide were used for contact angle measurements as test liquids. Ethanol and acetone were obtained from Sigma Aldrich, India. 2.2. Preparation of sample For oxygen plasma treatment the samples were prepared in the dimension of 20 × 20 cm. The samples were cut into various dimensions for different studies. The films were washed with ethanol and then with acetone and they were kept dried under vacuum for 24 h. 2.3. Oxygen plasma treatment The Harrick plasma equipment was used to carry out the oxygen plasma treatment. In this equipment, the principle is under suitable low pressure when a gas is passed, it is imperiled to a high frequency oscillating magnetic field and with the gas molecules the accelerated ions in the gas strike with them and ionizing them resulting in forming plasma. In the reaction chamber, the sample should be placed. At a pressure of 200–600 mtorr and the flow rates of 5–10 SCFH the gas was processed. Within the chamber, the plasma was created at RF electromagnetic radiation at 8–12 MHz near the ambient temperature. The minimum pump speed of vacuum pump is 1.4 m3/h and the minimum total pressure is 200 mtorr. It was functioned at RF power settings of low, medium and high with 7.2 W, 10.2 W, 29.6 W.
Table 1 Polar, dispersion and surface energy of test liquids. Liquid
γd1 (mJ/m2)
γp1 (mJ/m2)
γ1 (mJ/m2)
Distilled water Formamide
21.7 39.5
51.0 18.6
72.7 58.1
By the Fowkes equation, the polar and dispersive components of the surface energy of the film surface were calculated. 1=2 1=2 γ1 ð1 þ cos θÞ ¼ 2 γd1 γds þ 2 γp1 γps
ð1Þ
In Eq. (1), θ is the measured contact angle of the liquid with the solid surface, γ1 is the surface tension of the liquid, γ1d and γp1 are the polar and dispersive components of test liquids. The total surface tension of the liquids γ1 and their polar and dispersion components were summarized in Table 1. Finally, the total surface energy γs was estimated by the following Eq. (2) γs¼ γps þ γds
ð2Þ
2.5. Surface morphology analysis By using the non-contact mode of AFM, a Dualscope–Rasterscope C26 (Denmark) was used to analyze the morphology and the surface roughness of the untreated and oxygen plasma treated polypropylene film in an ambient atmosphere at room temperature. For sample imaging, the tapping mode was used and the scanning range was 5 μm × 5 μm. The silicon tip probes were used with a spring constant of 20–80 N/m. The resonance frequencies were in the range of 250– 300 kHz and the Nanoscope image processing software was used to analyze the images. 2.6. Surface chemical composition analysis To obtain the surface chemical changes made by the oxygen plasma treatment on the untreated and oxygen treated polypropylene films were performed on total reflection – Fourier transform infrared spectroscope (Perkin-Elmer SL, Spain). The spectra were investigated in the wavenumber range of 4000 to 650 cm− 1 at a resolution of 4 cm− 1 with 16 scans. 2.7. X-ray diffraction analysis
2.4. Contact angle measurements and surface energy estimation The static contact angle measurements were carried out at room temperature (23 ± 2 °C) by sessile drop method on a KSV CAM 200 goniometer (KSV Instruments, Helsinki, Finland) equipped with the DIGIDROP Image analysis software. The samples were dried in a vacuum oven for 24 h at 50 °C before the measurement. The test liquids were double distilled water and formamide. 10 μL of MilliQ grade water drop and formamide were placed with a micro syringe on the sample and the contact angle was measured within 5 s. At six different locations, the contact angle measurements were obtained and the average values on each polymer film were calculated and the experimental uncertainty was within ±1°. From the contact angle values the surface energy was estimated. To calculate the surface energy of the untreated and oxygen plasma treated PP film, the test liquids water and the formamide with known dispersive component γd and the polar component γp were used and they were represented in Table 1.
Wide angle X-ray diffraction was obtained using Bruker AXS D8 advance X-ray diffractometer utilizing nickel filtered CuKα radiation having the wavelength of 1.54056 Å. The current and the voltage were 40 mA and 40 kV. The countings were carried out at 10 steps per degree. 2.8. Bonding strength analysis The standard T-peel test method of ASTM D 1876-72 was used to analyze the bonding strength of the untreated and the plasma treated PP samples. In this, the commercially available adhesive tape was used. The test was carried out at room temperature by Universal Testing Machine (UTM, H10KS, Tinius Olsen, UK) machine at a rate of 10 mm/min. The adhesive tape was pasted over a length of 15 cm with a width of 5 cm. The test was carried out by fixing the sample in one of the holder and the tape in turn adhered to the piece of paper is fixed in another holder. The peel strength was carried for three specimens for every treatment time and the mean value was calculated.
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2.9. Tensile property The tensile strength of the untreated and the oxygen plasma treated polypropylene films were investigated by Universal Testing Machine (UTM, H10KS, Tinius Olsen, UK) at 23 °C by the standard of ASTM D638. The samples with the dimensions of 150 mm × 25 mm × 0.048 mm with a gauge length of 25 mm at a cross head speed of 10 mm/min. The tensile strength was expressed in MPa. 2.10. Oxygen transmission rate (OTR) The OTR of the untreated and the treated PP films were calculated by OXTRAN Oxygen permeability Tester (MOCON, Minneapolis, MN) at 23 °C under the condition of 0% RH at 1 atm by the standard of ASTM D3985. The measurements were taken at three times at different places of the film and the average value was calculated. All specimens were conditioned at ambient conditions. The oxygen permeability can be calculated by the following mathematical formula. Oxygen permeability ¼ OTR=ðFilm thickness x O2 partial pressureÞ
2.11. Water vapor Transmission Rate (WVTR) The WVTR of the untreated and the oxygen plasma treated PP samples were calculated by Mocon Permatran, according to the standard of ASTM F 1249-90. Under the condition of 100% RH the tests were carried out at 35 °C at ambient conditions and the tests were repeated for three times and the average mean values were calculated. 3. Results and discussions 3.1. Surface morphology analysis For the untreated and the oxygen plasma treated PP samples, the surface morphology and the surface roughness were investigated by AFM. In Fig. 1, it represent the AFM images in a three dimensional view of untreated and plasma treated samples in a time of 60 s with various RF power rate. The scan size of all the AFM images were 5 × 5 μm2. Obviously, the surface morphology of the polypropylene films was modified by oxygen plasma treatment.
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Most of the surfaces were even and randomly dispersed all over the surface in the untreated polypropylene sample. After the oxygen plasma treatment, the surface of the film becomes rougher and the annular shape changes can be observed and by the oxygen plasma chemical reaction, the topographical changes can also occur. After the oxygen plasma treatment on polypropylene the peaks are within the area with average size. The cone like structures on the treated polypropylene samples for different treatment times with various power were due to the etching effect on PP and the oxidative reactions of plasma which were applied. Moreover, the change in surface topography and surface roughness can be computed by one of the surface parameter namely RMS (Root mean square) roughness value (Sanchis, Blanes, Blanes, Garcia, & Balart, 2006). Because of the polymer degradation and surface modification the surface roughness was increased and the crystalline regions will remain intact, while the amorphous regions are etched away after the plasma treatment (Jacobs et al., 2010). In Fig. 2, the roughness values of all the untreated and plasma treated samples with different parameters were summarized. The lower rms value indicated the smoother surface. For the untreated PP sample the rms value was 1.49 nm. After the oxygen plasma treatment of 60 s in 7.2 W, the roughness value was increased to 2.705 nm and it was still increased to 3.321 nm for 60 s in 10.2 W. In 300 s of 29.6 W the highest roughness value of 7.26 nm was observed. It clearly showed that the surface roughness of the plasma treated polypropylene increases with increasing power rate and also with the treatment time. Because of this, the surface morphologies became more complicated with the duration of oxygen plasma treatment (Zhang et al., 2008). The oxygen plasma treated polypropylene samples showed higher surface roughness value as compared to the untreated sample. These results indicated that the oxygen plasma gave great impact on the polypropylene surface by removing the top layer. It may be related with the chain scission, chemical or physical removal of molecules and the degradation process (Navaneetha Pandiyaraj, Selvarajan, Deshmukh, & Gao, 2009). By eliminating the few layers on the PP film surface by the oxygen plasma treatment, the surface roughness was increased and also the wettability and the bonding strength were increased. The bonding strength strongly depends on the polar functional groups on the polypropylene surface (Vijayalakshmi, Mekala, Yoganand, & Navaneetha Pandiyaraj, 2011). Due to the higher energy bonding, the oxygen plasma treatment mainly etched the amorphous than the crystalline region. Later, the amorphous regions were etched and it leads to rougher surface (Khaleel, Jasim,
Fig. 1. AFM images untreated and oxygen plasma treated PP films: (a) Untreated PP; (b) treated PP of 60s of 7.2 W.
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attributed. The CH2 symmetric and asymmetric stretching vibrations showed the peaks range at 2920 and 2842 cm−1. In the ATR-FTIR spectrum, the two peaks at 1459 and 1377 cm−1 were also shown. Due to the CH3 asymmetric vibrations or by the CH2 scissor vibrations the peak at 1459 cm−1 appeared. The untreated PP showed many small peaks in the range of 1200–750 cm−1. The C\\C asymmetric stretching can be attributed at the peak 1168 cm−1. Due to the CH3 asymmetric vibrations the peak at 996 cm−1 was seemed. The C\\C asymmetric stretching vibrations and the CH3 asymmetric rocking are caused by the peak at 972 cm− 1. The peak at 902 cm−1 was caused by C\\C symmetric and asymmetric stretching. The CH2 rocking vibrations were caused by the peaks at 842 and 809 cm−1 (Socrates, 2001). 3.3. Studies on contact angle and surface energy Fig. 2. Roughness value of oxygen plasma treated PP samples with various treatment time and plasma power.
Ahmed, Vijay, & Srivastava, 2012) by the bombardment of energetic particles such as ions, electrons, neutrals, radicals, UV vis radiations and excited atoms (Yang et al., 2009). Hence, the oxygen plasma treatment could physically and chemically alter the outermost surface of the polypropylene film which results in the increase of surface roughness and the changes in surface morphology.
3.2. ATR-FTIR analysis By the ATR-FTIR the surface chemical changes induced by the oxygen plasma treatment were characterized. In Fig. 3, it showed the ATR-FTIR spectra of the untreated and the plasma treated polypropylene films of 60 s for 7.2 W, 10.2 W and 29.6 W. It clearly showed the polar group evolution on the polypropylene surface. From the interaction between the oxygen plasma and the polypropylene surface, these polar groups occurred and they consist mainly of carbonyl, carboxyl and hydroxyl groups (Drnovska, Lapcik, Bursikova, Zemek, & BarrosTimmons, 2003; Lehocky et al., 2003). The hydrophilic nature on the polypropylene surface increased because of the strong influence of polar groups. The spectra were normalized by the height equalizing of the transmittance at 2922 cm−1 and it represented the CH2 group in the polypropylene chain. The FTIR spectrum showed the peaks in the range of 3000–2800 cm−1 and in the peaks 2956 and 2872 cm−1 the CH3 symmetric and the asymmetric stretching vibrations can be
Untreated PP
Transmittance (%)
7.2 W
4000
10.2 W
29.6 W
3500
3000
2500
2000
1500
1000
500
Wavelength (cm-1) Fig. 3. ATR-FTIR spectrum of untreated and oxygen plasma treated PP samples. (a) Water contact angle. (b) Formamide contact angle.
From the contact angle (°) the wettability of the surface was determined. In a smooth and flat surface the contact angle decreases if the affinity of the liquid drop increases. By the high contact angle the poor wetting was determined and the hydrophilic property was indicated by the water wettable surface. The contact angle was measured for both untreated and oxygen plasma treated polypropylene samples to understand the wetting properties (Theapsak, Watthanaphanit, & Rujiravanit, 2012). The hydrophobic surfaces were represented by the contact angle which was above 90° and lower the contact angle represents more the hydrophilic surface (Labay, Canal, & Garcia – Celma, 2010). After the oxygen plasma treatment on polypropylene with 7.2 W, 10.2 W and 29.6 W and the treatment time of 60 s–300 s the contact angles were measured immediately for double distilled water and formamide and it was represented in Fig. 4 and also represented in Tables 2 and 3. The water contact angle for the untreated PP film was 74.56° and for the formamide it was 70.54°. In 60 s of 7.2 W, the water contact angle was greatly decreased from 74.56° to 60° and for the formamide it decreased to 65.30°. A significant decrease in the contact angle for both the liquids was observed after the oxygen plasma treatment in polypropylene film. The value of the water contact angle was still decreased to 58.20° and 64.21° for the formamide when the treatment time was 120 s. This reported that, if the plasma treatment time increased, the value of contact angle was decreased (Theapsak et al., 2012). The contact angle was dependent on both the treatment time and power. This signified that, higher the treatment time and the plasma power more noticeable decrease of contact angle value. This may be due to the existence of oxygen functional groups on the surface of the oxygen plasma treated PP samples and it becomes hydrophobic to hydrophilic. The oxygen functional groups were nonpolar in nature and therefore it increased the surface energy of the polypropylene resulting in the decrease of contact angle value. The contact angle was further decreased with increase in treatment time. The samples treated at 10.2 W and 29.6 W at different intervals of time the contact angle values were decreased because of the degradation and etching happened in the polypropylene sample. When the oxygen plasma power increased, the contact angle value was greatly decreased (Paisoonsin, Pornsunthorntawee, & Rujiravanit, 2013). The decrease in the contact angle value proved that the oxygen plasma increased the hydrophilicity of the polypropylene film due to the existence of polar functional groups on the polypropylene film surface (Theapsak et al., 2012) and the crystallinity of the polymer may also be affected (Kim, Ryu, Park, Sur, & Park, 2003). Finally, it was observed that higher the plasma power and treatment time, lower the contact angle. It was observed that, after the surface modification of PP by Oxygen plasma, there was an increase in the surface energy when compared to the surface energy of the untreated PP and it was represented in Fig. 5. The surface energy of the untreated PP is 56.489 mJ/m2. The oxygen plasma increases the surface energy significantly to 69.425 mJ/m2 for
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a) Water Contact angle
b) Formamide contact angle
Fig. 4. Contact angle values for untreated and the plasma treated PP samples (a) Water contact angle; (b) Formamide contact angle.
To analyze the crystalline structure of the polymer films, XRD spectra were used. For the untreated and the oxygen plasma treated
PP films, the ratio of the diffraction peak areas was used for crystal structure analysis. It was shown in Fig. 6. It clearly indicates that, in the shape and position of the diffraction peaks there is no significant change that occurred. But for the treated polymer the peak is more intense. From the diffraction peak, the ratio of areas of the plasma treated to the untreated polymers is higher than one. The value 1.16 in the PP film indicates the higher degree of crystallinity after the oxygen plasma treatment. The crystal dimension (D100) of the oxygen plasma treated PP film surface increased and the lattice spacing (d) decreased. From the results, it confirms that the oxygen plasma treatment improved the degree of crystallinity of the PP film surface.
Table 2 Water contact angle for untreated and plasma treated PP samples.
Table 3 Formamide contact angle for untreated and plasma treated PP samples.
60 s of 7.2 W. The surface energy was increased constantly in various time intervals of 7.2 W. In 60 s of 10.2 W, the surface energy was 77.04 mJ/m2 and the highest value of surface energy 94.09 mJ/m2 was obtained at 300 s of 29.6 W. This indicates that, the surface energy increases in increase of treatment time and power. It was mainly due to the polar group's incorporation of OH, CO and COO.
3.4. XRD analysis
Sample
PP – 7.2 W PP – 10.2 W PP – 29.6 W
Water contact angle (°)
Sample
60 s
120 s
180 s
240 s
300 s
60 55.42 46.41
59.45 55.21 45.53
58.20 54.13 45.21
57.55 53.62 44.21
56.65 50.37 43.85
PP – 7.2 W PP – 10.2 W PP – 29.6 W
Formamide contact angle (°) 60 s
120 s
180 s
240 s
300 s
65.30 59.45 51.80
64.21 58.25 50.25
63.15 57.65 49.65
62.12 56.30 48.75
61.20 55.10 47.15
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Fig. 5. Surface energy of the untreated and plasma treated PP samples.
Fig. 7. Bonding strength of untreated and plasma treated PP samples.
3.5. Studies on bonding strength
3.6. Effect of oxygen plasma on tensile strength
The T-peel test was carried to understand the bonding strength of untreated and plasma treated PP samples because of hydrophilic groups. Fig. 7 represents the bonding strength of the untreated and the oxygen plasma treated PP film. For the untreated PP film, the peel strength was 2.94 N/cm and for the oxygen plasma treated sample of 60 s with 7.2 W, the value was 3.48 N/cm. For the time of 120 s the peel strength was increased to 3.54 N/cm. This indicates, due to the plasma treatment time the bonding strength increases. The 4.21 N/cm was obtained for 60s of 10.2 W. The highest value of 8.42 N/cm gained for 300 s of 29.6 W. If both the plasma treatment time and the plasma energy increases, the bonding strength increases. It totally depends on the treatment time and power (Chashmejahanbin, Salimi, & Ershad Langroudi, 2014). The increase of bonding strength of polypropylene by oxygen plasma treatment is by the generation of carboxyl, hydroxyl and carbonyl groups on the surface layer of the polypropylene and because of this the improved polar attraction between the PP and the tape and increase in the wettability (Yeh, Lai, Suen, & Chen, 2011) As a result, on the surface of PP, the adhesion layer spreads more easily. With the adhesive material, when these functionalities come in contact, it forms a bond due to Van der Waal's force (Vijayalakshmi et al., 2011). This force of attraction between the adhesive material and the plasma treated polymer surface gives the increase in bonding strength. It has also the mechanical anchoring of adhesive on the PP film surface and the increase in surface roughness was associated to a better adhesive strength and it can be revealed from the AFM images. Finally, the Oxygen plasma treatment gives more bonding strength mostly by oxygenated groups and the formation of holes and pores on the surface.
The tensile strength of the untreated and the oxygen plasma treated PP films with various treatment times and different power rates was carried out and it was represented in Fig. 8. The effect of plasma treatment on tensile strength was also discussed. The tensile strength of the polypropylene samples was totally dependent on the treatment time and plasma power. When compared to the untreated PP, the tensile strength of the plasma treated PP was decreased. Furthermore, the higher tensile strength drop was observed for longer treatment times (Kale & Desai, 2011). The tensile strength of the untreated polypropylene sample was 6 MPa. After the oxygen plasma treatment time of 60 s, the tensile strength of the polypropylene sample was 4.680 MPa. It was significantly reduced when compared to the untreated polypropylene sample. When the plasma treatment time increases, the tensile strength of the polypropylene film was greatly decreased. Completely the values of the plasma treated samples at 7.2 W were lesser than the untreated one. The tensile strength of the polymeric films were influenced mainly by the plasma power and also by the treatment time (Theapsak et al., 2012). Later the plasma power was increased to 7.2 W, the tensile strength was decreased greatly. In 60 s of 10.2 W the tensile strength of the PP sample was 3.480 MPa. It was observed that, when the time increases, the tensile strength decreases. In the plasma power of 29.6 W, the tensile strength for all PP samples was decreased rapidly and the least value of 2.2 MPa was noted at 29.6 W of 300 s. From these results, it seems that when there was an increase in plasma power and the treatment time, the rate of etching increases. Because of this, the tensile strength decreases. The tensile strength detraction was due to the plasma etching. Therefore the tensile strength of the polypropylene film
Fig. 6. XRD spectrum of untreated and plasma treated PP samples.
Fig. 8. Tensile strength of untreated and treated PP samples.
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Table 4 OTR for untreated and plasma treated PP samples. Samples
OTR (cc/m2/24 hrs) 60 s
120 s
180 s
240 s
300 s
PP – 7.2 W PP – 10.2 W PP – 29.6 W
1853.3 ± 1.0 1891.6 ± 3.1 2212.5 ± 3.2
1858.1 ± 1.3 1892.9 ± 2.4 2219.4 ± 3.1
1862.2 ± 1.5 1894.3 ± 2.4 2229.6 ± 2.5
1865.5 ± 1.2 1895.9 ± 2.2 2233.5 ± 2.2
1868.1 ± 1.3 1898.8 ± 3.2 2248.9 ± 3.3
could be altered by the oxygen plasma treatment time and plasma power.
3.7. Effect of oxygen plasma on OTR The oxygen permeability is one of the most important property in a packaging material to decide its suitability for different applications (Misra, Tiwari, Raghavarao, & Cullen, 2011). For both the untreated and the oxygen plasma treated polypropylene samples the OTR values were measured and it was represented in Table 4. In a packaging material, oxygen permeability is one of the most important properties for deciding its suitability for various applications. For the untreated polypropylene sample the average OTR value was 1851.2 cc/m 2 /24 h. For the treatment time of 60 s in 7.2 W, the OTR value was increased to 1858.1 ± 1.3 cc/m2/24 h. After the plasma power increased to 10.2 W of 60 s, there was a great variance in the OTR values and it was noted as 1891.65 cc/m 2 /24 h. When the treatment time increases, the value was further increased. The OTR value was increased rapidly in all the treatment time of 29.6 W The highest value of 2248.92 cc/m2/24 h was observed in 29.6 W of 300 s. It was noted that the oxygen transmission rate increased with increase of plasma power and the treatment time. These results were most important and showed that, if the plasma treatment time and plasma power increases, the value of oxygen transmission rate was also increased.
4. Conclusion In this study, the polypropylene films were surface modified by oxygen plasma treatment with different treatment times and with various plasma power. The oxygen plasma treatment created the oxygen containing polar functional groups on the polypropylene film surface and induced the chemical composition which was confirmed by FTIR. The surface degradation of the polypropylene film after the plasma treatment causes the change in surface topography, which results in the increase of film surface roughness and it can be noted by atomic force microscopy analysis. The oxygen plasma treatment increased the film surface wettability and enriched the hydrophilicity of the polypropylene film and it caused a decrease of contact angle values and the increase of surface energies that can be attributed to the various mechanisms. XRD characterization showed the increase of the polypropylene film crystallinity and the oxygen plasma treatment enhanced the bonding strength on the PP surface. The oxygen plasma treatment was assumed to provide structural changes in the outermost layer of the polypropylene which induced major changes in tensile strength and also in the barrier properties. When the oxygen plasma treatment time and the plasma power increase, the tensile strength was greatly reduced and the gas transmission and the water vapor transmission rate were significantly increased because of the etching or degradation of polymers. From the practical point of view, the plasma treatment time and the plasma power were found to have the most important key role to decide the property of the surface modified polymers.
Acknowledgments 3.8. Effect of oxygen plasma on WVTR The WVTR measurements were allowed to determine the impact of the surface treatment on the transport properties such as diffusivity of water through the film and permeability. The water vapor permeability of the polymer film was totally dependent on the solubility parameter and it was linked to the affinity between the permeant and on the water diffusivity in the polymer material. It was related primarily to the structure of the film and particularly to the density which was connected to the degree of crystallinity and also to the stiffness of the film (Tenn et al., 2012). In Fig. 9, the WVTR of untreated and the oxygen plasma treated polypropylene films were represented. The changes of WVTR value were noticed after the increase in plasma treatment time and plasma power. For the untreated PP sample, the WVTR was 9.6 g/m2/24 h and the value was increased slightly in other exposure times of 7.2 W. The oxygen plasma treatment greatly increases the water vapor transmission rate for 10.2 W and 29.6 W. The maximum transmission rate of 14.24 g/m2/24 h was obtained at 300 s of 29.6 W. The polymer water vapor permeations were governed by concentration grades and vapor pressures (Chaiwong, Rachtanapun, Wongchaiya, Auras, & Boonyawan, 2012). For the water vapor permeability and the water absorption, various processes were responsible even though it involves the water molecule penetration through the polymer. From these results, it was noted that the water vapor transmission rate was significantly increased, if the plasma power and the treatment time increased.
The authors would like to thank Dr. Ashis Kumar Sen and Mr. Sajeesh Parayil, Department of Mechanical Engineering, IIT Madras for providing the Harrick plasma facility to do the oxygen plasma treatment. The authors thank Dr. M.V. Panchagnula and Mr. Nachiketa Janardhan, IIT Madras for extending their support for contact angle measurements. We also acknowledge Mr. S.M. Suresh Kumar and Mr. P. Prabhunathan for FTIR characterization and also the Crystal growth Centre, Anna University for the AFM characterization.
Fig. 9. WVTR of untreated and plasma treated PP samples.
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Navaneetha Pandiyaraj, K., Selvarajan, V., Deshmukh, R.R., & Gao, C. (2009). Modification of surface properties of polypropylene (PP) film using DC glow discharge air plasma. Applied Surface Science, 255, 3965–3971. Paisoonsin, S., Pornsunthorntawee, O., & Rujiravanit, R. (2013). Preparation and characterization of ZnO-deposited DBD plasma-treated PP packaging film with antibacterial activities. Applied Surface Science, 273, 824–835. Pankaj, S.K., Bueno-Ferrer, C., Misra, N.N., O'Neill, L., Jiménez, A., Bourke, P., et al. (2013). Applications of cold plasma technology in food packaging. Innovative Food Science & Emerging Technologies, 21, 107–113. Riccardi, C., Barni, R., Selli, E., Mazzone, G., Massafra, M.R., Marcandalli, B., et al. (2003). Surface modification of poly (ethylene terephthalate) fibers induced by radio frequency air plasma treatment. Applied Surface Science, 211, 386–397. Sanchis, M.R., Blanes, V., Blanes, V., Garcia, D., & Balart, R. (2006). Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment. European Polymer Journal, 42, 1558–1568. Shin, G. h., Lee, Y.H., Lee, J. s., Kim, Y. s., Choi, W. s., & Park, H.J. (2002). Preparation of plastic and biopolymer multilayer films by plasma source ion implantation. Journal of Agricultural and Food Chemistry, 50, 4608–4614. Socrates, G. (2001). Infrared and Raman characteristic group frequencies — Tables and charts (3rd ed.). John Wiley & Sons. Tenn, N., Follain, N., Fatyeyeva, K., Valleton, J. -M., Poncin-Epaillard, F., Delpouve, N., et al. (2012). Improvement of water barrier properties of poly (ethylene-co-vinyl alcohol) films by hydrophobic plasma surface treatments. Journal of Physical Chemistry C, 116, 12599–12612. Theapsak, S., Watthanaphanit, A., & Rujiravanit, R. (2012). Preparation of chitosan-coated polyethylene packaging films by DBD plasma treatment. ACS Applied Materials & Interfaces, 4, 2474–2482. Vesel, A., & Mozetic, M. (2012). Surface modification and ageing of PMMA polymer by oxygen plasma treatment. Vacuum, 86, 634–637. Vijayalakshmi, K.A., Mekala, M., Yoganand, C.P., & Navaneetha Pandiyaraj, K. (2011). Studies on modification of surface properties in polycarbonate (PC) film induced by DC glow discharge plasma. International Journal of Polymer Science. http://dx.doi.org/10. 1155/2011/426057. Wei, Q.F., Gao, W.D., Hou, D.Y., & Wang, X.Q. (2005). Surface modification of polymer nanofibers by plasma treatment. Applied Surface Science, 245, 16–20. Yang, L., Chen, J., Guo, Y., & Zhang, Z. (2009). Surface modification of a biomedical polyethylene terephthalate (PET) by air plasma. Applied Surface Science, 255, 4446–4451. Yeh, J., Lai, Y., Suen, M., & Chen, C. (2011). An improvement on the adhesion-strength of laminated ultra HMWPE surface-etching modification using highly effective helium– oxygen–nitrogen plasma treatment. Polymers for Advanced Technologies, 22, 1971– 1981. Zhang, C., Chen, P., Sun, B., Li, W., Wang, B., & Wang, J. (2008). Surface analysis of oxygen plasma treated poly (p-phenylene benzobisoxazole) fibers. Applied Surface Science, 254, 5776–5780.
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Effect of mechanical, barrier and adhesion properties on oxygen plasma surface modified PP M. Vishnuvarthanan ⁎, N. Rajeswari College of Engineering, Guindy, Anna University, Chennai – 600025, TamilNadu, India
a r t i c l e
i n f o
Article history: Received 17 December 2014 Received in revised form 30 March 2015 Accepted 18 May 2015 Available online 30 May 2015 Keywords: Polypropylene Oxygen plasma treatment Contact angle FTIR AFM XRD Tensile strength OTR WVTR
a b s t r a c t In this work, the Polypropylene (PP) film was surface modified by Oxygen plasma treatment and the effect of mechanical, barrier and adhesion properties was studied. The PP film was plasma treated with various RF power settings of 7.2 W, 10.2 W and 29.6 W in various time intervals of 60 s, 120 s, 180 s, 240 s and 300 s. To characterize the wettability, the contact angle was measured and the surface energy values were estimated with different test liquids. The generation of oxygen functional groups on the surface of plasma modified PP and the surface change characterization were observed by attenuated total reflection-Fourier transform infrared spectroscope (ATRFTIR) and they resulted in wettability improvement. The roughness of the PP film and the surface morphology were analyzed by Atomic Force Microscopy (AFM). It was found that the roughness value increased from 1.491 nm to 7.26 nm because of the increase of treatment time and RF power. The PP crystallinity structure of the untreated and treated PP was evaluated by X-ray diffraction analysis (XRD). The bond strength of the untreated and surface modified films were measured by T-peel test method. For the untreated and oxygen plasma treated sample, the mechanical properties like Tensile Strength and the barrier properties like oxygen transmission rate (OTR), Water vapor transmission rate (WVTR) were also calculated. From the results, the tensile strength reduced from 6 MPa to 1.350 MPa because of polypropylene etching and degradation. The OTR increased from 1851.2 to 2248.92 cc/m2/24 h and the Water vapor transmission rate increased from 9.6 to 14.24 g/m2/24 h. Industrial Relevance: Plasma technology applied to packaging and printing industry is a dry, environmentally- and worker-friendly method to achieve surface alteration without modifying the bulk properties of different materials. In particular, atmospheric non-thermal plasmas are suited because most are heat sensitive polymers and applicable in continuous process. In the last years plasma technology has become a very active, high growth research field, assuming a great importance among all available material surface modifications in packaging industry. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Polymers like PE, PP, PS and PET are the mostly used packaging materials because these are available in large quantities at low costs (Pankaj et al., 2013). In industry, PP films are used in many packaging applications. It has good mechanical, physical, barrier and chemical resistance properties. These polymers are having low surface energy and therefore it have poor adhesive properties (Shin et al., 2002). For a particular application, if the polymer surface does not have the desired properties, it leads to material failure and the system or device containing it (Hoffman, 1995). For good adhesion between polymer and coating the polymer surface must be activated (Vesel & Mozetic, 2012). This can be done by the plasma treatment. By this technique, the surface properties of the polymers can be modified without affecting the bulk properties. The lack of adhesion can be owed without surface modification (Shin et al., 2002). For modifying the surface properties of polymers, a ⁎ Corresponding author. Tel.: +91 9943769268. E-mail address: [email protected] (M. Vishnuvarthanan).
http://dx.doi.org/10.1016/j.ifset.2015.05.007 1466-8564/© 2015 Elsevier Ltd. All rights reserved.
number of techniques have been developed such as mechanical, thermal, chemical and plasma treatments. For packaging, the use of plasma treatment seems to be suitable and to attain important change in the properties governed by surface characteristics (Wei, Gao, Hou, & Wang, 2005). The plasma refers to a partly or entirely ionized gas that consisted essentially of ions, photons and free electrons. The atoms in the excited or fundamental states are holding a net neutral charge (Liu, Cui, Brown, & Meenan, 2005). To attain the surface modification characteristics of polymeric materials, the effective technology is the plasma treatment (Cireli, Kutlu, & Mutlu, 2007). The surface phenomena such as crosslinking, etching and activation are done by the interactions between surface molecules of polymers and plasma (Riccardi et al., 2003). Over the conventional process, these plasma treatments offer many benefits because these are normally a dry process and they cannot generate chemical waste (Yang, Chen, Guo, & Zhang, 2009). Dependent on the conditions and the plasma species, the polymer surface properties such as hydrophobicity, morphology and the adhesion can be altered (Kauling et al., 2009). They can give impact on the barrier, mechanical and adhesion properties of the polymers. The reactive
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functional group and the surface roughness can be introduced and also improved in the polymer surface by the plasma (Kirk et al., 2010). By this the mechanical performance and the adhesion property can also be improved. This was done due to the mechanical interlocking mechanism and also by the chemical interaction (Shin et al., 2002). In this present study, the surface of the polypropylene was modified by RF oxygen plasma. This oxygen plasma promoted the surface modification by surface activation and slight etching. The contact angle and the polypropylene surface morphology were analyzed by goniometer and AFM. The surface energy studies were carried out and the bonding strength was also calculated. The mechanical and the barrier properties of untreated and the oxygen plasma treated polypropylene samples were carried out and the influence of surface modification on these properties was analyzed. 2. Materials and methods 2.1. Materials In this study, the commercial polypropylene film was used and it was purchased from Jayanthi Plastics, Erode, India. It has the thickness of 0.048 ± 003 mm and density of 0.92 g/cm3. Double distilled water and Formamide were used for contact angle measurements as test liquids. Ethanol and acetone were obtained from Sigma Aldrich, India. 2.2. Preparation of sample For oxygen plasma treatment the samples were prepared in the dimension of 20 × 20 cm. The samples were cut into various dimensions for different studies. The films were washed with ethanol and then with acetone and they were kept dried under vacuum for 24 h. 2.3. Oxygen plasma treatment The Harrick plasma equipment was used to carry out the oxygen plasma treatment. In this equipment, the principle is under suitable low pressure when a gas is passed, it is imperiled to a high frequency oscillating magnetic field and with the gas molecules the accelerated ions in the gas strike with them and ionizing them resulting in forming plasma. In the reaction chamber, the sample should be placed. At a pressure of 200–600 mtorr and the flow rates of 5–10 SCFH the gas was processed. Within the chamber, the plasma was created at RF electromagnetic radiation at 8–12 MHz near the ambient temperature. The minimum pump speed of vacuum pump is 1.4 m3/h and the minimum total pressure is 200 mtorr. It was functioned at RF power settings of low, medium and high with 7.2 W, 10.2 W, 29.6 W.
Table 1 Polar, dispersion and surface energy of test liquids. Liquid
γd1 (mJ/m2)
γp1 (mJ/m2)
γ1 (mJ/m2)
Distilled water Formamide
21.7 39.5
51.0 18.6
72.7 58.1
By the Fowkes equation, the polar and dispersive components of the surface energy of the film surface were calculated. 1=2 1=2 γ1 ð1 þ cos θÞ ¼ 2 γd1 γds þ 2 γp1 γps
ð1Þ
In Eq. (1), θ is the measured contact angle of the liquid with the solid surface, γ1 is the surface tension of the liquid, γ1d and γp1 are the polar and dispersive components of test liquids. The total surface tension of the liquids γ1 and their polar and dispersion components were summarized in Table 1. Finally, the total surface energy γs was estimated by the following Eq. (2) γs¼ γps þ γds
ð2Þ
2.5. Surface morphology analysis By using the non-contact mode of AFM, a Dualscope–Rasterscope C26 (Denmark) was used to analyze the morphology and the surface roughness of the untreated and oxygen plasma treated polypropylene film in an ambient atmosphere at room temperature. For sample imaging, the tapping mode was used and the scanning range was 5 μm × 5 μm. The silicon tip probes were used with a spring constant of 20–80 N/m. The resonance frequencies were in the range of 250– 300 kHz and the Nanoscope image processing software was used to analyze the images. 2.6. Surface chemical composition analysis To obtain the surface chemical changes made by the oxygen plasma treatment on the untreated and oxygen treated polypropylene films were performed on total reflection – Fourier transform infrared spectroscope (Perkin-Elmer SL, Spain). The spectra were investigated in the wavenumber range of 4000 to 650 cm− 1 at a resolution of 4 cm− 1 with 16 scans. 2.7. X-ray diffraction analysis
2.4. Contact angle measurements and surface energy estimation The static contact angle measurements were carried out at room temperature (23 ± 2 °C) by sessile drop method on a KSV CAM 200 goniometer (KSV Instruments, Helsinki, Finland) equipped with the DIGIDROP Image analysis software. The samples were dried in a vacuum oven for 24 h at 50 °C before the measurement. The test liquids were double distilled water and formamide. 10 μL of MilliQ grade water drop and formamide were placed with a micro syringe on the sample and the contact angle was measured within 5 s. At six different locations, the contact angle measurements were obtained and the average values on each polymer film were calculated and the experimental uncertainty was within ±1°. From the contact angle values the surface energy was estimated. To calculate the surface energy of the untreated and oxygen plasma treated PP film, the test liquids water and the formamide with known dispersive component γd and the polar component γp were used and they were represented in Table 1.
Wide angle X-ray diffraction was obtained using Bruker AXS D8 advance X-ray diffractometer utilizing nickel filtered CuKα radiation having the wavelength of 1.54056 Å. The current and the voltage were 40 mA and 40 kV. The countings were carried out at 10 steps per degree. 2.8. Bonding strength analysis The standard T-peel test method of ASTM D 1876-72 was used to analyze the bonding strength of the untreated and the plasma treated PP samples. In this, the commercially available adhesive tape was used. The test was carried out at room temperature by Universal Testing Machine (UTM, H10KS, Tinius Olsen, UK) machine at a rate of 10 mm/min. The adhesive tape was pasted over a length of 15 cm with a width of 5 cm. The test was carried out by fixing the sample in one of the holder and the tape in turn adhered to the piece of paper is fixed in another holder. The peel strength was carried for three specimens for every treatment time and the mean value was calculated.
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2.9. Tensile property The tensile strength of the untreated and the oxygen plasma treated polypropylene films were investigated by Universal Testing Machine (UTM, H10KS, Tinius Olsen, UK) at 23 °C by the standard of ASTM D638. The samples with the dimensions of 150 mm × 25 mm × 0.048 mm with a gauge length of 25 mm at a cross head speed of 10 mm/min. The tensile strength was expressed in MPa. 2.10. Oxygen transmission rate (OTR) The OTR of the untreated and the treated PP films were calculated by OXTRAN Oxygen permeability Tester (MOCON, Minneapolis, MN) at 23 °C under the condition of 0% RH at 1 atm by the standard of ASTM D3985. The measurements were taken at three times at different places of the film and the average value was calculated. All specimens were conditioned at ambient conditions. The oxygen permeability can be calculated by the following mathematical formula. Oxygen permeability ¼ OTR=ðFilm thickness x O2 partial pressureÞ
2.11. Water vapor Transmission Rate (WVTR) The WVTR of the untreated and the oxygen plasma treated PP samples were calculated by Mocon Permatran, according to the standard of ASTM F 1249-90. Under the condition of 100% RH the tests were carried out at 35 °C at ambient conditions and the tests were repeated for three times and the average mean values were calculated. 3. Results and discussions 3.1. Surface morphology analysis For the untreated and the oxygen plasma treated PP samples, the surface morphology and the surface roughness were investigated by AFM. In Fig. 1, it represent the AFM images in a three dimensional view of untreated and plasma treated samples in a time of 60 s with various RF power rate. The scan size of all the AFM images were 5 × 5 μm2. Obviously, the surface morphology of the polypropylene films was modified by oxygen plasma treatment.
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Most of the surfaces were even and randomly dispersed all over the surface in the untreated polypropylene sample. After the oxygen plasma treatment, the surface of the film becomes rougher and the annular shape changes can be observed and by the oxygen plasma chemical reaction, the topographical changes can also occur. After the oxygen plasma treatment on polypropylene the peaks are within the area with average size. The cone like structures on the treated polypropylene samples for different treatment times with various power were due to the etching effect on PP and the oxidative reactions of plasma which were applied. Moreover, the change in surface topography and surface roughness can be computed by one of the surface parameter namely RMS (Root mean square) roughness value (Sanchis, Blanes, Blanes, Garcia, & Balart, 2006). Because of the polymer degradation and surface modification the surface roughness was increased and the crystalline regions will remain intact, while the amorphous regions are etched away after the plasma treatment (Jacobs et al., 2010). In Fig. 2, the roughness values of all the untreated and plasma treated samples with different parameters were summarized. The lower rms value indicated the smoother surface. For the untreated PP sample the rms value was 1.49 nm. After the oxygen plasma treatment of 60 s in 7.2 W, the roughness value was increased to 2.705 nm and it was still increased to 3.321 nm for 60 s in 10.2 W. In 300 s of 29.6 W the highest roughness value of 7.26 nm was observed. It clearly showed that the surface roughness of the plasma treated polypropylene increases with increasing power rate and also with the treatment time. Because of this, the surface morphologies became more complicated with the duration of oxygen plasma treatment (Zhang et al., 2008). The oxygen plasma treated polypropylene samples showed higher surface roughness value as compared to the untreated sample. These results indicated that the oxygen plasma gave great impact on the polypropylene surface by removing the top layer. It may be related with the chain scission, chemical or physical removal of molecules and the degradation process (Navaneetha Pandiyaraj, Selvarajan, Deshmukh, & Gao, 2009). By eliminating the few layers on the PP film surface by the oxygen plasma treatment, the surface roughness was increased and also the wettability and the bonding strength were increased. The bonding strength strongly depends on the polar functional groups on the polypropylene surface (Vijayalakshmi, Mekala, Yoganand, & Navaneetha Pandiyaraj, 2011). Due to the higher energy bonding, the oxygen plasma treatment mainly etched the amorphous than the crystalline region. Later, the amorphous regions were etched and it leads to rougher surface (Khaleel, Jasim,
Fig. 1. AFM images untreated and oxygen plasma treated PP films: (a) Untreated PP; (b) treated PP of 60s of 7.2 W.
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attributed. The CH2 symmetric and asymmetric stretching vibrations showed the peaks range at 2920 and 2842 cm−1. In the ATR-FTIR spectrum, the two peaks at 1459 and 1377 cm−1 were also shown. Due to the CH3 asymmetric vibrations or by the CH2 scissor vibrations the peak at 1459 cm−1 appeared. The untreated PP showed many small peaks in the range of 1200–750 cm−1. The C\\C asymmetric stretching can be attributed at the peak 1168 cm−1. Due to the CH3 asymmetric vibrations the peak at 996 cm−1 was seemed. The C\\C asymmetric stretching vibrations and the CH3 asymmetric rocking are caused by the peak at 972 cm− 1. The peak at 902 cm−1 was caused by C\\C symmetric and asymmetric stretching. The CH2 rocking vibrations were caused by the peaks at 842 and 809 cm−1 (Socrates, 2001). 3.3. Studies on contact angle and surface energy Fig. 2. Roughness value of oxygen plasma treated PP samples with various treatment time and plasma power.
Ahmed, Vijay, & Srivastava, 2012) by the bombardment of energetic particles such as ions, electrons, neutrals, radicals, UV vis radiations and excited atoms (Yang et al., 2009). Hence, the oxygen plasma treatment could physically and chemically alter the outermost surface of the polypropylene film which results in the increase of surface roughness and the changes in surface morphology.
3.2. ATR-FTIR analysis By the ATR-FTIR the surface chemical changes induced by the oxygen plasma treatment were characterized. In Fig. 3, it showed the ATR-FTIR spectra of the untreated and the plasma treated polypropylene films of 60 s for 7.2 W, 10.2 W and 29.6 W. It clearly showed the polar group evolution on the polypropylene surface. From the interaction between the oxygen plasma and the polypropylene surface, these polar groups occurred and they consist mainly of carbonyl, carboxyl and hydroxyl groups (Drnovska, Lapcik, Bursikova, Zemek, & BarrosTimmons, 2003; Lehocky et al., 2003). The hydrophilic nature on the polypropylene surface increased because of the strong influence of polar groups. The spectra were normalized by the height equalizing of the transmittance at 2922 cm−1 and it represented the CH2 group in the polypropylene chain. The FTIR spectrum showed the peaks in the range of 3000–2800 cm−1 and in the peaks 2956 and 2872 cm−1 the CH3 symmetric and the asymmetric stretching vibrations can be
Untreated PP
Transmittance (%)
7.2 W
4000
10.2 W
29.6 W
3500
3000
2500
2000
1500
1000
500
Wavelength (cm-1) Fig. 3. ATR-FTIR spectrum of untreated and oxygen plasma treated PP samples. (a) Water contact angle. (b) Formamide contact angle.
From the contact angle (°) the wettability of the surface was determined. In a smooth and flat surface the contact angle decreases if the affinity of the liquid drop increases. By the high contact angle the poor wetting was determined and the hydrophilic property was indicated by the water wettable surface. The contact angle was measured for both untreated and oxygen plasma treated polypropylene samples to understand the wetting properties (Theapsak, Watthanaphanit, & Rujiravanit, 2012). The hydrophobic surfaces were represented by the contact angle which was above 90° and lower the contact angle represents more the hydrophilic surface (Labay, Canal, & Garcia – Celma, 2010). After the oxygen plasma treatment on polypropylene with 7.2 W, 10.2 W and 29.6 W and the treatment time of 60 s–300 s the contact angles were measured immediately for double distilled water and formamide and it was represented in Fig. 4 and also represented in Tables 2 and 3. The water contact angle for the untreated PP film was 74.56° and for the formamide it was 70.54°. In 60 s of 7.2 W, the water contact angle was greatly decreased from 74.56° to 60° and for the formamide it decreased to 65.30°. A significant decrease in the contact angle for both the liquids was observed after the oxygen plasma treatment in polypropylene film. The value of the water contact angle was still decreased to 58.20° and 64.21° for the formamide when the treatment time was 120 s. This reported that, if the plasma treatment time increased, the value of contact angle was decreased (Theapsak et al., 2012). The contact angle was dependent on both the treatment time and power. This signified that, higher the treatment time and the plasma power more noticeable decrease of contact angle value. This may be due to the existence of oxygen functional groups on the surface of the oxygen plasma treated PP samples and it becomes hydrophobic to hydrophilic. The oxygen functional groups were nonpolar in nature and therefore it increased the surface energy of the polypropylene resulting in the decrease of contact angle value. The contact angle was further decreased with increase in treatment time. The samples treated at 10.2 W and 29.6 W at different intervals of time the contact angle values were decreased because of the degradation and etching happened in the polypropylene sample. When the oxygen plasma power increased, the contact angle value was greatly decreased (Paisoonsin, Pornsunthorntawee, & Rujiravanit, 2013). The decrease in the contact angle value proved that the oxygen plasma increased the hydrophilicity of the polypropylene film due to the existence of polar functional groups on the polypropylene film surface (Theapsak et al., 2012) and the crystallinity of the polymer may also be affected (Kim, Ryu, Park, Sur, & Park, 2003). Finally, it was observed that higher the plasma power and treatment time, lower the contact angle. It was observed that, after the surface modification of PP by Oxygen plasma, there was an increase in the surface energy when compared to the surface energy of the untreated PP and it was represented in Fig. 5. The surface energy of the untreated PP is 56.489 mJ/m2. The oxygen plasma increases the surface energy significantly to 69.425 mJ/m2 for
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a) Water Contact angle
b) Formamide contact angle
Fig. 4. Contact angle values for untreated and the plasma treated PP samples (a) Water contact angle; (b) Formamide contact angle.
To analyze the crystalline structure of the polymer films, XRD spectra were used. For the untreated and the oxygen plasma treated
PP films, the ratio of the diffraction peak areas was used for crystal structure analysis. It was shown in Fig. 6. It clearly indicates that, in the shape and position of the diffraction peaks there is no significant change that occurred. But for the treated polymer the peak is more intense. From the diffraction peak, the ratio of areas of the plasma treated to the untreated polymers is higher than one. The value 1.16 in the PP film indicates the higher degree of crystallinity after the oxygen plasma treatment. The crystal dimension (D100) of the oxygen plasma treated PP film surface increased and the lattice spacing (d) decreased. From the results, it confirms that the oxygen plasma treatment improved the degree of crystallinity of the PP film surface.
Table 2 Water contact angle for untreated and plasma treated PP samples.
Table 3 Formamide contact angle for untreated and plasma treated PP samples.
60 s of 7.2 W. The surface energy was increased constantly in various time intervals of 7.2 W. In 60 s of 10.2 W, the surface energy was 77.04 mJ/m2 and the highest value of surface energy 94.09 mJ/m2 was obtained at 300 s of 29.6 W. This indicates that, the surface energy increases in increase of treatment time and power. It was mainly due to the polar group's incorporation of OH, CO and COO.
3.4. XRD analysis
Sample
PP – 7.2 W PP – 10.2 W PP – 29.6 W
Water contact angle (°)
Sample
60 s
120 s
180 s
240 s
300 s
60 55.42 46.41
59.45 55.21 45.53
58.20 54.13 45.21
57.55 53.62 44.21
56.65 50.37 43.85
PP – 7.2 W PP – 10.2 W PP – 29.6 W
Formamide contact angle (°) 60 s
120 s
180 s
240 s
300 s
65.30 59.45 51.80
64.21 58.25 50.25
63.15 57.65 49.65
62.12 56.30 48.75
61.20 55.10 47.15
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Fig. 5. Surface energy of the untreated and plasma treated PP samples.
Fig. 7. Bonding strength of untreated and plasma treated PP samples.
3.5. Studies on bonding strength
3.6. Effect of oxygen plasma on tensile strength
The T-peel test was carried to understand the bonding strength of untreated and plasma treated PP samples because of hydrophilic groups. Fig. 7 represents the bonding strength of the untreated and the oxygen plasma treated PP film. For the untreated PP film, the peel strength was 2.94 N/cm and for the oxygen plasma treated sample of 60 s with 7.2 W, the value was 3.48 N/cm. For the time of 120 s the peel strength was increased to 3.54 N/cm. This indicates, due to the plasma treatment time the bonding strength increases. The 4.21 N/cm was obtained for 60s of 10.2 W. The highest value of 8.42 N/cm gained for 300 s of 29.6 W. If both the plasma treatment time and the plasma energy increases, the bonding strength increases. It totally depends on the treatment time and power (Chashmejahanbin, Salimi, & Ershad Langroudi, 2014). The increase of bonding strength of polypropylene by oxygen plasma treatment is by the generation of carboxyl, hydroxyl and carbonyl groups on the surface layer of the polypropylene and because of this the improved polar attraction between the PP and the tape and increase in the wettability (Yeh, Lai, Suen, & Chen, 2011) As a result, on the surface of PP, the adhesion layer spreads more easily. With the adhesive material, when these functionalities come in contact, it forms a bond due to Van der Waal's force (Vijayalakshmi et al., 2011). This force of attraction between the adhesive material and the plasma treated polymer surface gives the increase in bonding strength. It has also the mechanical anchoring of adhesive on the PP film surface and the increase in surface roughness was associated to a better adhesive strength and it can be revealed from the AFM images. Finally, the Oxygen plasma treatment gives more bonding strength mostly by oxygenated groups and the formation of holes and pores on the surface.
The tensile strength of the untreated and the oxygen plasma treated PP films with various treatment times and different power rates was carried out and it was represented in Fig. 8. The effect of plasma treatment on tensile strength was also discussed. The tensile strength of the polypropylene samples was totally dependent on the treatment time and plasma power. When compared to the untreated PP, the tensile strength of the plasma treated PP was decreased. Furthermore, the higher tensile strength drop was observed for longer treatment times (Kale & Desai, 2011). The tensile strength of the untreated polypropylene sample was 6 MPa. After the oxygen plasma treatment time of 60 s, the tensile strength of the polypropylene sample was 4.680 MPa. It was significantly reduced when compared to the untreated polypropylene sample. When the plasma treatment time increases, the tensile strength of the polypropylene film was greatly decreased. Completely the values of the plasma treated samples at 7.2 W were lesser than the untreated one. The tensile strength of the polymeric films were influenced mainly by the plasma power and also by the treatment time (Theapsak et al., 2012). Later the plasma power was increased to 7.2 W, the tensile strength was decreased greatly. In 60 s of 10.2 W the tensile strength of the PP sample was 3.480 MPa. It was observed that, when the time increases, the tensile strength decreases. In the plasma power of 29.6 W, the tensile strength for all PP samples was decreased rapidly and the least value of 2.2 MPa was noted at 29.6 W of 300 s. From these results, it seems that when there was an increase in plasma power and the treatment time, the rate of etching increases. Because of this, the tensile strength decreases. The tensile strength detraction was due to the plasma etching. Therefore the tensile strength of the polypropylene film
Fig. 6. XRD spectrum of untreated and plasma treated PP samples.
Fig. 8. Tensile strength of untreated and treated PP samples.
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Table 4 OTR for untreated and plasma treated PP samples. Samples
OTR (cc/m2/24 hrs) 60 s
120 s
180 s
240 s
300 s
PP – 7.2 W PP – 10.2 W PP – 29.6 W
1853.3 ± 1.0 1891.6 ± 3.1 2212.5 ± 3.2
1858.1 ± 1.3 1892.9 ± 2.4 2219.4 ± 3.1
1862.2 ± 1.5 1894.3 ± 2.4 2229.6 ± 2.5
1865.5 ± 1.2 1895.9 ± 2.2 2233.5 ± 2.2
1868.1 ± 1.3 1898.8 ± 3.2 2248.9 ± 3.3
could be altered by the oxygen plasma treatment time and plasma power.
3.7. Effect of oxygen plasma on OTR The oxygen permeability is one of the most important property in a packaging material to decide its suitability for different applications (Misra, Tiwari, Raghavarao, & Cullen, 2011). For both the untreated and the oxygen plasma treated polypropylene samples the OTR values were measured and it was represented in Table 4. In a packaging material, oxygen permeability is one of the most important properties for deciding its suitability for various applications. For the untreated polypropylene sample the average OTR value was 1851.2 cc/m 2 /24 h. For the treatment time of 60 s in 7.2 W, the OTR value was increased to 1858.1 ± 1.3 cc/m2/24 h. After the plasma power increased to 10.2 W of 60 s, there was a great variance in the OTR values and it was noted as 1891.65 cc/m 2 /24 h. When the treatment time increases, the value was further increased. The OTR value was increased rapidly in all the treatment time of 29.6 W The highest value of 2248.92 cc/m2/24 h was observed in 29.6 W of 300 s. It was noted that the oxygen transmission rate increased with increase of plasma power and the treatment time. These results were most important and showed that, if the plasma treatment time and plasma power increases, the value of oxygen transmission rate was also increased.
4. Conclusion In this study, the polypropylene films were surface modified by oxygen plasma treatment with different treatment times and with various plasma power. The oxygen plasma treatment created the oxygen containing polar functional groups on the polypropylene film surface and induced the chemical composition which was confirmed by FTIR. The surface degradation of the polypropylene film after the plasma treatment causes the change in surface topography, which results in the increase of film surface roughness and it can be noted by atomic force microscopy analysis. The oxygen plasma treatment increased the film surface wettability and enriched the hydrophilicity of the polypropylene film and it caused a decrease of contact angle values and the increase of surface energies that can be attributed to the various mechanisms. XRD characterization showed the increase of the polypropylene film crystallinity and the oxygen plasma treatment enhanced the bonding strength on the PP surface. The oxygen plasma treatment was assumed to provide structural changes in the outermost layer of the polypropylene which induced major changes in tensile strength and also in the barrier properties. When the oxygen plasma treatment time and the plasma power increase, the tensile strength was greatly reduced and the gas transmission and the water vapor transmission rate were significantly increased because of the etching or degradation of polymers. From the practical point of view, the plasma treatment time and the plasma power were found to have the most important key role to decide the property of the surface modified polymers.
Acknowledgments 3.8. Effect of oxygen plasma on WVTR The WVTR measurements were allowed to determine the impact of the surface treatment on the transport properties such as diffusivity of water through the film and permeability. The water vapor permeability of the polymer film was totally dependent on the solubility parameter and it was linked to the affinity between the permeant and on the water diffusivity in the polymer material. It was related primarily to the structure of the film and particularly to the density which was connected to the degree of crystallinity and also to the stiffness of the film (Tenn et al., 2012). In Fig. 9, the WVTR of untreated and the oxygen plasma treated polypropylene films were represented. The changes of WVTR value were noticed after the increase in plasma treatment time and plasma power. For the untreated PP sample, the WVTR was 9.6 g/m2/24 h and the value was increased slightly in other exposure times of 7.2 W. The oxygen plasma treatment greatly increases the water vapor transmission rate for 10.2 W and 29.6 W. The maximum transmission rate of 14.24 g/m2/24 h was obtained at 300 s of 29.6 W. The polymer water vapor permeations were governed by concentration grades and vapor pressures (Chaiwong, Rachtanapun, Wongchaiya, Auras, & Boonyawan, 2012). For the water vapor permeability and the water absorption, various processes were responsible even though it involves the water molecule penetration through the polymer. From these results, it was noted that the water vapor transmission rate was significantly increased, if the plasma power and the treatment time increased.
The authors would like to thank Dr. Ashis Kumar Sen and Mr. Sajeesh Parayil, Department of Mechanical Engineering, IIT Madras for providing the Harrick plasma facility to do the oxygen plasma treatment. The authors thank Dr. M.V. Panchagnula and Mr. Nachiketa Janardhan, IIT Madras for extending their support for contact angle measurements. We also acknowledge Mr. S.M. Suresh Kumar and Mr. P. Prabhunathan for FTIR characterization and also the Crystal growth Centre, Anna University for the AFM characterization.
Fig. 9. WVTR of untreated and plasma treated PP samples.
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