Modification of Physico-Chemical Properties of Acryl-Coated Polypropylene Foils for Food Packaging by Reactive Particles from Oxygen Plasma

This investigation was focused on the influence of long-living neutral reactive oxygen species on the physico-chemical properties of acryl-coated polypropylene foils for food packaging. Reactive species were formed by passing molecular oxygen through a microwave discharge and leaking it to a processing chamber of a volume of 30 L, which was pumped by a rotary pump. The density of neutral O-atoms in the chamber was tuned by adjustment of both the effective pumping speed and the oxygen leak rate. The O-atom density was measured with a catalytic probe and was between 3 × 1018 and 5 × 1019 m−3. Commercial foils of biaxially oriented polypropylene (BOPP) coated with acrylic/ poly(vinylidene chloride) (AcPVDC) were mounted in the chamber and treated at room temperature by O atoms at various conditions, with the fluence between 1 × 1021 and 3 × 1024 m−2. The evolution of the surface wettability versus the fluence was determined by water contact angle (WCA) measurements, the formation of functional groups by X-ray photoelectron spectroscopy (XPS), and the morphology by atomic force microscopy (AFM). The WCA dropped from the initial 75° to approximately 40° after the fluence of a few 1022 m−2 and remained unchanged thereafter, except for fluences above 1024 m−2, where the WCA dropped to approximately 30°. XPS and AFM results allowed for drawing correlations between the wettability, surface composition, and morphology.


Introduction
Today, food technology is constantly evolving in response to different challenges. The changes in consumer demands and the necessity for the production of safe and high-quality foods are responsible for the innovation and improvement of already established food processes. In this sense, the introduction of new technologies could lead to a reduction of the processing time or an improvement in operating conditions, thereby decreasing both environmental and financial costs. Plasma treatments cause several chemical and physical changes on the plasma-polymer interface, which improve the surface properties [1][2][3][4][5][6]. Plasma-induced effects on the polymer surface are nowadays exploited in surface functionalization of the packaging polymers for promoting adhesion or sometimes anti-adhesion [7], enhanced printability [8], sealability [9], assuring anti-mist low-pressure plasma reactor. The vapor is the major constitute of the residual atmosphere and is also formed due to chemical etching of the polymer upon oxygen plasma treatment. The water molecules dissociate under plasma conditions and the resulting OH and H radicals are excited upon inelastic collisions with energetic electrons. The excited states de-excite to the corresponding ground states by ration in the UV range: Lyman hydrogen series in the vacuum UV range and OH band of bandhead at 309 nm. All this radiation causes bond scission in the polymer film of a thickness of the order of several µm. The reactive species interact with dangling bonds on the polymer surface, causing the formation of LMWOM that are often volatile. Therefore, rather extensive etching is observed upon the treatment of a polymer material with oxygen plasma [29]. In fact, precise measurements of the oxidation rate for the same polymer exposed to oxygen plasma and only neutral O-atoms at the same O-atom flux on the sample surface showed a two orders of magnitude higher etching rate for the case where synergistic effects of radiation and reactive species were effective [30,31]. Such synergies should therefore be avoided if functionalization of the polymer surface with oxygen functional groups is the goal.
The aim of this research was to examine the effect of surface oxidation of commercial PP foils used for food packaging. Such foils are covered with a very thin acrylic coating. Unlike other authors, neutral reactive particles from late afterglow were used instead of gaseous plasma, because glowing plasma always causes the etching of polymers and the acrylic coating could have been removed by direct exposure to oxygen plasma [30]. Furthermore, in afterglow, a density of oxygen species interacting with the polymer can also be precisely determined. This allowed determination of the minimal oxygen atom fluence necessary for saturation of the surface with polar functional groups and thus optimal wettability at a minimal treatment time.

Materials
Biaxially oriented polypropylene (PP) films (Bicor 32MB777, ExxonMobil, Antwerp, Belgium) were used in the experiments. One side of the film had an acrylic acid coating and the other side was coated with a thin film of poly(vinylidene chloride) (PVDC), which means that plasma interacted with the coating and not with the PP substrate. Only the acrylic side was treated with plasma. The thickness of the foil was 32 µm.

Plasma Afterglow Treatment
The polymer foil was cut into pieces of 2 × 2 cm 2 and treated with reactive neutral oxygen species created in the center of the processing chamber. Oxygen species which were created in the surfatron plasma were passed through the narrow glass tube to the processing chamber. The experimental system is shown schematically in Figure 1. The processing chamber was a pyrex cylinder with a diameter of 33 cm and a length of 40 cm. The chamber was pumped with a two-stage oil rotary pump of a nominal pumping speed of 40 m 3 ·h −1 and ultimate pressure well below 1 Pa. A zeolite trap was used to prevent back-diffusion of the oil vapor. The pump was mounted on the flange at the bottom of the processing chamber via bellows of a large conductivity at the pressure of 20 Pa and above, and a manually adjustable shutter valve which allowed for suppressing the effective pumping speed in a gradual manner from the maximal speed (40 m 3 ·h -1 ) down to zero. The upper flange of the Pyrex tube was equipped with a pressure gauge, a discharge tube, and a movable catalytic probe which was used for O-atom density measurements [36]. Oxygen of commercial purity 99.99% was leaked continuously in the discharge tube through a manually adjusted leak valve. A standard quartz tube with an inner diameter of 6 mm was used. The pressure was measured with an absolute gauge (baratron) calibrated for the pressure range 0.1-100 Pa. A microwave cavity of approximately 5 cm in length was mounted onto the discharge tube and connected to the microwave power supply. The configuration allowed for sustaining the gaseous plasma in the surfatron mode inside the discharge tube. The microwave power was set to 200 W. Continuous leakage of oxygen on one side and pumping of the processing chamber on the other side allowed for a drift of gas through the discharge into the processing chamber. Molecular oxygen from the flask partially ionized, dissociated, and excited in the plasma within the microwave cavity. Charged particles quickly neutralized and excited species relaxed on the way between the gaseous plasma and the processing chamber. Therefore, the only highly reactive oxygen species left for treatment of the polymer samples was neutral O atoms. The density of O atoms above the surface of the polymer samples was measured with a calibrated catalytic probe. The probe consists of a catalytic tip which is heated in the plasma because of the recombination of O atoms to O 2 molecules on the surface of the catalyst [36]. The temperature of the catalyst is measured by a thermocouple. The heating rate of the probe is proportional to the flux of oxygen atoms. The O-atom density (n) was calculated from the probe temperature derivate using the following equation [37]: where m is the mass of the probe tip, c p is its specific heat capacity, W D is the dissociation energy of an oxygen molecule, γ is the recombination coefficient for O atoms on the catalyst surface, A is the area of the catalyst, and dT/dt is the time derivative of the probe temperature just after turning off the discharge. More details regarding the O-atom density calculation are explained in the works [36,37].
We have used cobalt as the catalyst. This material is particularly suitable for the detection of atomic oxygen at a low density. The lower detection limit of the probe was approximately 2 × 10 18 at a pressure above 10 Pa, whereas the upper at 10 22 m −3 . The experiments presented here were performed at the pressure of 20 Pa. At these conditions, the O-atom density in the system was 5.3 × 10 19 m -3 when the shutter valve was fully open (the effective pumping speed was equal to the nominal pumping speed of the vacuum pump). By adjusting the shutter and leak valves simultaneously, it was possible to keep the pressure in the processing chamber constant but the O-atom density variable: less opened valves caused a lower atom density because the drift velocity of the gas through the discharge chamber was suppressed by closing valves. A detailed description of this effect was reported elsewhere [38]. Four adjustments of the O-atom density in the vicinity of the samples were chosen: 5.3 × 10 19 , 2.9 × 10 19 , 1.0 × 10 19 , 8.7 × 10 18 , and 3×10 18 m −3 . The corresponding fluxes of O-atoms onto the sample surface were calculated as: where n is the measured density of oxygen atoms and v is an average thermal velocity of O atoms at room temperature (v = 630 m·s -1 ). The fluence of O atoms to the surface of the sample was calculated as j × t, where j is the flux of oxygen atoms to the surface and t is the treatment time. Various treatment times were used for modification of the sample's surface. Such an experimental setup allowed for the treatment of samples in a broad range of fluences from 5 × 10 21 to 3 × 10 24 m −2 -almost three orders of magnitude. adjusting the shutter and leak valves simultaneously, it was possible to keep the pressure in the processing chamber constant but the O-atom density variable: less opened valves caused a lower atom density because the drift velocity of the gas through the discharge chamber was suppressed by closing valves. A detailed description of this effect was reported elsewhere [38]. Four adjustments of the O-atom density in the vicinity of the samples were chosen: 5.3 × 10 19 , 2.9 × 10 19 , 1.0 × 10 19 , 8.7 × 10 18 , and 3×10 18 m −3 . The corresponding fluxes of O-atoms onto the sample surface were calculated as: where n is the measured density of oxygen atoms and v is an average thermal velocity of O atoms at room temperature (v = 630 m·s -1 ). The fluence of O atoms to the surface of the sample was calculated as j × t, where j is the flux of oxygen atoms to the surface and t is the treatment time. Various treatment times were used for modification of the sample's surface. Such an experimental setup allowed for the treatment of samples in a broad range of fluences from 5 × 10 21 to 3 × 10 24 m −2 -almost three orders of magnitude.

X-ray Photoelectron Spectroscopy (XPS) Characterization
Chemical composition of the samples was determined with an XPS instrument model TFA XPS (Physical Electronics, Ismaning, Germany) from Physical Electronics. Analyses were performed 15 min after the plasma treatment. Monochromatic Al Kα1,2 radiation at 1486.6 eV was used for sample excitation. Photoelectrons were detected at an angle of 45° with respect to the normal of the sample surface. XPS survey spectra were measured at a pass-energy of 187 eV using an energy step of 0.4 eV. High-resolution spectra of carbon C1s were measured at a pass-energy of 23.5 eV using an energy step of 0.1 eV. Because the samples are insulators, an electron gun was used for the additional charge compensation. The spectra were analyzed using MultiPak v8.1c software (Ulvac-Phi Inc., Kanagawa, Japan, 2006) from Physical Electronics.

Atomic Force Microscopy (AFM) Measurements
The surface morphology of the samples was analyzed with an AFM (Solver PRO, NT-MDT, Moscow, Russia). Images were recorded in a tapping mode using ATEC-NC-20 tips (Nano And More GmbH, Germany). A resonance frequency of the tip and the force constant were 210-490 kHz and 12-110 Nm −1 , respectively. An average surface roughness of the samples (Ra) was determined by using the program Spip 5.1.3 (Image Metrology A/S). The average surface roughness was calculated from the images taken over an area of 5 × 5 µ m 2 .

X-ray Photoelectron Spectroscopy (XPS) Characterization
Chemical composition of the samples was determined with an XPS instrument model TFA XPS (Physical Electronics, Ismaning, Germany) from Physical Electronics. Analyses were performed 15 min after the plasma treatment. Monochromatic Al Kα 1,2 radiation at 1486.6 eV was used for sample excitation. Photoelectrons were detected at an angle of 45 • with respect to the normal of the sample surface. XPS survey spectra were measured at a pass-energy of 187 eV using an energy step of 0.4 eV. High-resolution spectra of carbon C1s were measured at a pass-energy of 23.5 eV using an energy step of 0.1 eV. Because the samples are insulators, an electron gun was used for the additional charge compensation. The spectra were analyzed using MultiPak v8.1c software (Ulvac-Phi Inc., Kanagawa, Japan, 2006) from Physical Electronics.

Atomic Force Microscopy (AFM) Measurements
The surface morphology of the samples was analyzed with an AFM (Solver PRO, NT-MDT, Moscow, Russia). Images were recorded in a tapping mode using ATEC-NC-20 tips (Nano And More GmbH, Germany). A resonance frequency of the tip and the force constant were 210-490 kHz and 12-110 Nm −1 , respectively. An average surface roughness of the samples (Ra) was determined by using the program Spip 5.1.3 (Image Metrology A/S). The average surface roughness was calculated from the images taken over an area of 5 × 5 µm 2 .

Contact Angle Measurements
Changes of the surface wettability of the plasma-treated samples were determined immediately after the plasma treatment. An instrument by See System (Advex Instruments, Brno, Czech Republic) was used. A demineralized water droplet of a volume of 3 µL was applied to the surface. The measured contact angles were analyzed by the software supplied by the producer. For each sample, three measurements were taken to minimize the statistical error. Figure 2 illustrates the variation of the water contact angle of the acrylic coating versus the fluence of oxygen atoms. As mentioned earlier, the treatment was performed at several different densities of O atoms in the vicinity of the sample and at various treatment times. It seems that the water contact angle only depends on the fluence and not on the O-atom density because all measured points in Figure 2 follow the same curve. The contact angle at first decreases rapidly with the increasing fluence, but later the decrease becomes less and less rapid until the water contact angle becomes constant at approximately 40 • . The particular measured points in Figure 2 are somehow scattered; however, the trend is obvious: no knee is observed in the curve which is only plotted for eye guidance. The contact angle becomes constant (approximately 40 • ) after the fluence of a few 10 22 m -2 is used. Further exposure to O-atoms does not influence the wettability of this particular material. The exemptions are both measured points at very large fluences where the contact angles are approximately 30 • . A feasible explanation for this effect will be presented and discussed later in this paper.

Contact Angle Measurements
Changes of the surface wettability of the plasma-treated samples were determined immediately after the plasma treatment. An instrument by See System (Advex Instruments, Brno, Czech Republic) was used. A demineralized water droplet of a volume of 3 μL was applied to the surface. The measured contact angles were analyzed by the software supplied by the producer. For each sample, three measurements were taken to minimize the statistical error. Figure 2 illustrates the variation of the water contact angle of the acrylic coating versus the fluence of oxygen atoms. As mentioned earlier, the treatment was performed at several different densities of O atoms in the vicinity of the sample and at various treatment times. It seems that the water contact angle only depends on the fluence and not on the O-atom density because all measured points in Figure 2 follow the same curve. The contact angle at first decreases rapidly with the increasing fluence, but later the decrease becomes less and less rapid until the water contact angle becomes constant at approximately 40°. The particular measured points in Figure 2 are somehow scattered; however, the trend is obvious: no knee is observed in the curve which is only plotted for eye guidance. The contact angle becomes constant (approximately 40°) after the fluence of a few 10 22 m -2 is used. Further exposure to O-atoms does not influence the wettability of this particular material. The exemptions are both measured points at very large fluences where the contact angles are approximately 30°. A feasible explanation for this effect will be presented and discussed later in this paper.  is only 6 ms at the atom density of 1 × 10 22 m -3 , which is typical for the extremely reactive oxygen plasma [39]. Such a short treatment time is achievable only when using pulsed discharges. Unfortunately, this experimental setup does not allow for verification of the calculated values presented in Figure 3. Furthermore, in practice, such small treatment times are not very suitable, because the treated surface may be contaminated with impurities. This means that at such a short treatment time, plasma radicals interact with the contaminants rather than with a pure polymer surface.  From this figure, one can conclude that the required treatment time for receiving the fluence of 1 × 10 22 m -2 is only 6 ms at the atom density of 1 × 10 22 m -3 , which is typical for the extremely reactive oxygen plasma [39]. Such a short treatment time is achievable only when using pulsed discharges. Unfortunately, this experimental setup does not allow for verification of the calculated values presented in Figure 3. Furthermore, in practice, such small treatment times are not very suitable, because the treated surface may be contaminated with impurities. This means that at such a short treatment time, plasma radicals interact with the contaminants rather than with a pure polymer surface.  The increase is at first rapid but then less pronounced; however, the x-axis in Figure 4 is plotted in the logarithmic scale and therefore the measured points appear in a line. The oxygen concentration thus increases as a logarithm of the fluence. It is interesting that the oxygen concentration keeps increasing after the fluence that corresponds to the saturation of the wettability. Numerous explanations can be stated for this observation. A trivial one is that already approximately 30 at % of oxygen is enough for the optimal wettability. The second possibility is that the surface (which influences the wettability) is already saturated with the polar functional groups at a moderate fluence and oxidation of the sub-surface layers occurs at higher fluences. Yet another explanation could be the formation of oxides on the surface-this effect will be discussed later. The high-resolution spectra of the carbon C1s peak for selected samples are presented in Figure  5. The spectra are normalized to the height of the main peak at 285 eV. The deconvolution of selected spectra is presented in Figure 6. The untreated sample (Figure 6a) contains three peaks: the main one at 285 eV corresponding to C-C, C-H bonds, and two small peaks at 286.5 and 289 eV corresponding to C-O and O=C-O groups, respectively. The spectrum in Figure 6a supports the information that the original sample has the acrylic coating. Figure 6b  The increase is at first rapid but then less pronounced; however, the x-axis in Figure 4 is plotted in the logarithmic scale and therefore the measured points appear in a line. The oxygen concentration thus increases as a logarithm of the fluence. It is interesting that the oxygen concentration keeps increasing after the fluence that corresponds to the saturation of the wettability. Numerous explanations can be stated for this observation. A trivial one is that already approximately 30 at % of oxygen is enough for the optimal wettability. The second possibility is that the surface (which influences the wettability) is already saturated with the polar functional groups at a moderate fluence and oxidation of the sub-surface layers occurs at higher fluences. Yet another explanation could be the formation of oxides on the surface-this effect will be discussed later.  The increase is at first rapid but then less pronounced; however, the x-axis in Figure 4 is plotted in the logarithmic scale and therefore the measured points appear in a line. The oxygen concentration thus increases as a logarithm of the fluence. It is interesting that the oxygen concentration keeps increasing after the fluence that corresponds to the saturation of the wettability. Numerous explanations can be stated for this observation. A trivial one is that already approximately 30 at % of oxygen is enough for the optimal wettability. The second possibility is that the surface (which influences the wettability) is already saturated with the polar functional groups at a moderate fluence and oxidation of the sub-surface layers occurs at higher fluences. Yet another explanation could be the formation of oxides on the surface-this effect will be discussed later. The high-resolution spectra of the carbon C1s peak for selected samples are presented in Figure  5. The spectra are normalized to the height of the main peak at 285 eV. The deconvolution of selected spectra is presented in Figure 6. The untreated sample (Figure 6a) contains three peaks: the main one at 285 eV corresponding to C-C, C-H bonds, and two small peaks at 286.5 and 289 eV corresponding to C-O and O=C-O groups, respectively. The spectrum in Figure 6a supports the information that the original sample has the acrylic coating. Figure 6b The high-resolution spectra of the carbon C1s peak for selected samples are presented in Figure 5. The spectra are normalized to the height of the main peak at 285 eV. The deconvolution of selected spectra is presented in Figure 6. The untreated sample (Figure 6a) contains three peaks: the main one at 285 eV corresponding to C-C, C-H bonds, and two small peaks at 286.5 and 289 eV corresponding to C-O and O=C-O groups, respectively. The spectrum in Figure 6a supports the information that the original sample has the acrylic coating. Figure 6b,c show an example of deconvolution of the sample treated at short (low oxygen fluence) and long (high oxygen fluence) treatment times. It can be observed that the intensity of C-O and O=C-O groups increased, especially for longer treatment times. It is difficult to judge about the formation of additional peaks corresponding to functional groups like C=O; however, if such groups develop upon treatment with the O atoms, their concentration on the polymer surface is much lower than the concentration of C-O and O=C-O groups. Figure 5 shows a gradual increase of the polar functional groups versus the fluence of the O-atoms, thus it is in good agreement with Figure 4. The increase is not equal for C-O and O=C-O groups, though. This can be seen from Figure 7, which shows the concentration of the functional groups versus the O-atom fluence. The highly polar O=C-O group increases somehow more intensively than the C-O group and actually prevails at the highest fluence. Interesting enough, this observation is not sound with the wettability presented in Figure 2. Namely, on the basis of the results presented in Figure 7, one would expect a monotonous decrease of the water contact angle with the increasing O-atom fluence. As mentioned above, this phenomenon could be related to surface saturation with the polar functional groups already at moderate fluences, and to oxidation of the sub-surface layers at higher fluences, or to the formation of Si oxides (discussed later).  Figure 2. Namely, on the basis of the results presented in Figure 7, one would expect a monotonous decrease of the water contact angle with the increasing O-atom fluence. As mentioned above, this phenomenon could be related to surface saturation with the polar functional groups already at moderate fluences, and to oxidation of the subsurface layers at higher fluences, or to the formation of Si oxides (discussed later).   Figure 2. Namely, on the basis of the results presented in Figure 7, one would expect a monotonous decrease of the water contact angle with the increasing O-atom fluence. As mentioned above, this phenomenon could be related to surface saturation with the polar functional groups already at moderate fluences, and to oxidation of the subsurface layers at higher fluences, or to the formation of Si oxides (discussed later).   Another observation about the surface composition is worth stressing and discussing. Figure 8 represents survey XPS spectra for selected samples. Apart from carbon and oxygen, one can observe tiny peaks at binding energies of approximately 102 and 153 eV. The peaks correspond to silicon levels of Si 2p and Si 2s, respectively. The peaks are easily overlooked for the untreated sample (lowest curve in Figure 8), but become more pronounced after the sample has received a large fluence (upper curve). Detailed spectrum in the range 88-188 eV is shown in the insert of Figure 8. Doubtlessly, silicon is presented in the as-received sample and its concentration as detected by XPS increases with the increasing O-atom fluence. Figure 9 represents the concentration of Si in the surface of selected samples. Although the initial concentration is at the limit of this experimental technique, the trend is well justified. The origin of Si in the untreated sample is known to polymer scientists: i.e., silicon is often added to polymers as an anti-block or slipping agent in order to improve their performance. When the polymers are exposed to oxygen atoms, etching occurs. The effect has been elaborated elsewhere [40]. The oxygen atoms at first cause surface functionalization, but as the polymer surface becomes saturated with the O-rich functional groups, they form unstable molecular fragments which desorb from the surface. The polymer is thus slowly etched, leaving on the surface compounds that do not form volatile oxides. The effect is sometimes called plasma ashing [41]. Here, the acryl coating is slowly degraded and thus etched, leaving oxidized silicon nanoparticles on the surface. This effect explains the increase of Si concentration versus the O-atom fluence presented in Figure 9. It may or may not be a coincidence that the Si concentration ( Figure 9) starts rising as the sample wettability becomes stable ( Figure 2).

Results and Discussion
The virtual discrepancy between Figures 2 and 4 can be attributed to the appearance of silicon on the polymer surface. As explained above, the wettability ( Figure 2) assumes a rather constant value after the fluence of about 3 × 10 22 m −2 , but the concentration of oxygen on the polymer surface still increases (Figure 4). Taking into account the measured values of Si ( Figure 9) and assuming that silicon is in the form of oxide (SiO2), one can replot Figure 4 by considering that a part of oxygen is bonded to silicon, i.e., subtracting 2 × [Si] oxygen from the curves. The new plot of O concentration and the O/C ratio by considering this effect is plotted in Figure 10. The behavior of the curve for oxygen in Figure 10 is now almost sound with the observations presented in Figure 2. Namely, the oxygen concentration as determined from XPS results also approaches a constant value for large fluences. Unfortunately, the saturation in Figure 10 does not appear at the same fluence as in Figure 2.
The role of silicon dioxide on the sample wettability is worth discussing. Figure 2 represents numerous measured data that fit the curve well, but the two points at the highest fluences definitely do not fit the general behavior. The decrease of the WCA for the highest fluences could be explained by oxidized silica nanoparticles on the sample surface, because well activated silicon oxide (treated by oxygen plasma) is hydrophilic [42]. The hydrophilicity is, however, lost soon after the plasma treatment because of the adsorption of organic impurities. That is one of the reasons why wettability Another observation about the surface composition is worth stressing and discussing. Figure 8 represents survey XPS spectra for selected samples. Apart from carbon and oxygen, one can observe tiny peaks at binding energies of approximately 102 and 153 eV. The peaks correspond to silicon levels of Si 2p and Si 2s, respectively. The peaks are easily overlooked for the untreated sample (lowest curve in Figure 8), but become more pronounced after the sample has received a large fluence (upper curve). Detailed spectrum in the range 88-188 eV is shown in the insert of Figure 8. Doubtlessly, silicon is presented in the as-received sample and its concentration as detected by XPS increases with the increasing O-atom fluence. Figure 9 represents the concentration of Si in the surface of selected samples. Although the initial concentration is at the limit of this experimental technique, the trend is well justified. The origin of Si in the untreated sample is known to polymer scientists: i.e., silicon is often added to polymers as an anti-block or slipping agent in order to improve their performance. When the polymers are exposed to oxygen atoms, etching occurs. The effect has been elaborated elsewhere [40]. The oxygen atoms at first cause surface functionalization, but as the polymer surface becomes saturated with the O-rich functional groups, they form unstable molecular fragments which desorb from the surface. The polymer is thus slowly etched, leaving on the surface compounds that do not form volatile oxides. The effect is sometimes called plasma ashing [41]. Here, the acryl coating is slowly degraded and thus etched, leaving oxidized silicon nanoparticles on the surface. This effect explains the increase of Si concentration versus the O-atom fluence presented in Figure 9. It may or may not be a coincidence that the Si concentration ( Figure 9) starts rising as the sample wettability becomes stable (Figure 2).
The virtual discrepancy between Figures 2 and 4 can be attributed to the appearance of silicon on the polymer surface. As explained above, the wettability ( Figure 2) assumes a rather constant value after the fluence of about 3 × 10 22 m −2 , but the concentration of oxygen on the polymer surface still increases (Figure 4). Taking into account the measured values of Si ( Figure 9) and assuming that silicon is in the form of oxide (SiO 2 ), one can replot Figure 4 by considering that a part of oxygen is bonded to silicon, i.e., subtracting 2 × [Si] oxygen from the curves. The new plot of O concentration and the O/C ratio by considering this effect is plotted in Figure 10. The behavior of the curve for oxygen in Figure 10 is now almost sound with the observations presented in Figure 2. Namely, the oxygen concentration as determined from XPS results also approaches a constant value for large fluences. Unfortunately, the saturation in Figure 10 does not appear at the same fluence as in Figure 2.
The role of silicon dioxide on the sample wettability is worth discussing. Figure 2 represents numerous measured data that fit the curve well, but the two points at the highest fluences definitely do not fit the general behavior. The decrease of the WCA for the highest fluences could be explained by oxidized silica nanoparticles on the sample surface, because well activated silicon oxide (treated by oxygen plasma) is hydrophilic [42]. The hydrophilicity is, however, lost soon after the plasma treatment because of the adsorption of organic impurities. That is one of the reasons why wettability tests were performed just after the treatment of samples with the O-atoms; however, hydrophobic recovery cannot be excluded completely.
tests were performed just after the treatment of samples with the O-atoms; however, hydrophobic recovery cannot be excluded completely.            In view of the upper discussion, let us also discuss the AFM images of selected samples. The images are shown in Figure 11. The images were taken over the area of 5 × 5 µm 2 . The untreated sample (Figure 11a) exhibits small un-evenly distributed particles of virtually the same lateral size protruding from the surface. The typical lateral dimension of the particles is almost 100 nm and the height as determined by AFM is several 10 nm. The origin of these particles is probably polymer additives containing silicon. According to the XPS results (Figure 9), the density of the particles fits the concentration of silicon on the surface of the untreated sample. Figure 11b is the image of the sample after receiving a small O-atom fluence. According to the upper results and discussion, the fluence received by this sample was too small to cause any detectable polymer etching. The image actually does not differ significantly from Figure 11a. Also, the surface roughness of the sample shown in Figure 11b did not change much (from the initial 5.8 nm it increased to 5.9 nm). One can qualitatively conclude that the concentration of the particles protruding from the sample surface is similar in Figure 11a,b, which is sound with the observations presented in Figure 9.
The AFM images in Figure 11c,d vary significantly from Figure 11a,b. The particles protruding from the surface are now much denser, which could be a consequence of the polymer etching. Moreover, the surface roughness increased to 6.8 nm. From Figure 11, one can therefore assume that the surface is enriched with silica nanoparticles, which has been proposed on the basis of the XPS results presented in Figure 9. In view of the upper discussion, let us also discuss the AFM images of selected samples. The images are shown in Figure 11. The images were taken over the area of 5 × 5 µ m 2 . The untreated sample (Figure 11a) exhibits small un-evenly distributed particles of virtually the same lateral size protruding from the surface. The typical lateral dimension of the particles is almost 100 nm and the height as determined by AFM is several 10 nm. The origin of these particles is probably polymer additives containing silicon. According to the XPS results (Figure 9), the density of the particles fits the concentration of silicon on the surface of the untreated sample. Figure 11b is the image of the sample after receiving a small O-atom fluence. According to the upper results and discussion, the fluence received by this sample was too small to cause any detectable polymer etching. The image actually does not differ significantly from Figure 11a. Also, the surface roughness of the sample shown in Figure 11b did not change much (from the initial 5.8 nm it increased to 5.9 nm). One can qualitatively conclude that the concentration of the particles protruding from the sample surface is similar in Figure 11 a,b, which is sound with the observations presented in Figure 9.
The AFM images in Figure 11 c,d vary significantly from Figure 11 a,b. The particles protruding from the surface are now much denser, which could be a consequence of the polymer etching. Moreover, the surface roughness increased to 6.8 nm. From Figure 11, one can therefore assume that the surface is enriched with silica nanoparticles, which has been proposed on the basis of the XPS results presented in Figure 9. In Figure 12, AFM topographic and phase images of the untreated sample and of one the selected treated sample recorded at a higher magnification of 2 × 2 µ m 2 are shown. The phase signal depends on the viscoelastic properties of the materials; therefore, the signal variation between the soft polymer surface and stiff silica particles can be observed. Figure 12 clearly shows a big difference in the variation of the phase signal for the treated sample in comparison to the untreated one. Many black spots with a big phase shift are observed on the treated sample, which confirms our conclusions about the presence of silica particles. In Figure 12, AFM topographic and phase images of the untreated sample and of one the selected treated sample recorded at a higher magnification of 2 × 2 µm 2 are shown. The phase signal depends on the viscoelastic properties of the materials; therefore, the signal variation between the soft polymer surface and stiff silica particles can be observed. Figure 12 clearly shows a big difference in the variation of the phase signal for the treated sample in comparison to the untreated one. Many black spots with a big phase shift are observed on the treated sample, which confirms our conclusions about the presence of silica particles.

Conclusions
An early stage of activation of commercial acrylic coated polypropylene foils for food packaging has been elaborated. The results clearly show that the maximum achievable surface wettability is already obtained at a rather low fluence of O-atoms of the order of a few 10 22 m -3 . This information is particularly useful for users who want to activate the material without losing the acrylic surface film. Namely, larger fluences (in practice it means prolonged treatment time) has little or no effect on the surface wettability but causes etching of the thin acrylic film and thus loss of the functional properties of such foils. As stated in the introduction to this paper, the acrylic coating protects the polypropylene foil from external influences, and should therefore remain on the PP foil after accomplishing the activation procedure.

Conclusions
An early stage of activation of commercial acrylic coated polypropylene foils for food packaging has been elaborated. The results clearly show that the maximum achievable surface wettability is already obtained at a rather low fluence of O-atoms of the order of a few 10 22 m -3 . This information is particularly useful for users who want to activate the material without losing the acrylic surface film. Namely, larger fluences (in practice it means prolonged treatment time) has little or no effect on the surface wettability but causes etching of the thin acrylic film and thus loss of the functional properties of such foils. As stated in the introduction to this paper, the acrylic coating protects the polypropylene foil from external influences, and should therefore remain on the PP foil after accomplishing the activation procedure.