A study of plasma treatment of clay-coated paper is presented. The wettability of the paper surface before and after the plasma treatment was determined by measuring the water contact angle and surface free energy. The appearance of the surface-functional groups was determined by using high-resolution X-ray photoelectron spectroscopy, while changes in the surface morphology were monitored by scanning electron microscopy. Already after 0.25 s of the plasma treatment, the paper surface improved its wettability. The X-ray photoelectron spectroscopy analyses showed a gradual increase of oxygen-rich functional groups on the surface while scanning electron microscopy analyses did not show a significant modification of the morphology and no alternation of the surface structure after the plasma treatment.

Non-thermal atmospheric pressure plasma sources often provide a cheaper and convenient alternative to low-pressure plasma applications.1 

One of the most significant areas of atmospheric-pressure plasma applications is surface activation and modification of various materials.2,3 During the plasma treatment variety of active species generated by plasma react with the surface of the treated material. These species can, for example, introduce new functional groups and/or remove the contaminants from the surface. The mentioned processes can be the reason for the improvement of the surface properties such as wettability, adhesion and printability.4 

The dielectric barrier discharge (DBD) is a non-thermal equilibrium alternating current discharge. Nowadays exists a large number of different electrode geometries and setups of DBD. Depending on the application, it is possible to use planar or coplanar arrays with different geometries such as curved, coaxial or twisted electrodes. Another advantage is the manifold adaptability due to the various electrode geometries. Hence, a macroscopically/visually homogeneous discharge can be ignited over several square meters with nearly no limitations.

Diffuse Coplanar Surface Barrier Discharge (DCSBD) as a special type of “cold” non-thermal plasma source that has found a wide range of applications in the treatment of metals,5 textiles,6 polymers, etc., in common low-added-value materials.7 The advantages of curved DCSBD in a roll-to-roll arrangement, such as the speed of the treatment, the precisely controlled effective distance of treated material from the electrode as well as the capability of working at atmospheric pressure are encouraging results proving its suitability for in-line industrial processing. In some cases, the homogeneity of plasma treatment that can be obtained using DCSBD is a crucial requirement, especially for large-area flat surfaces.8 

Paper is a unique material that is used for a variety of purposes. Though, changes in functional properties of paper are still needed to satisfy quality demands at the present time as well as in the future. As an example, adhesion properties between paper and polymer or barrier properties for packaging purposes could be taken into account. So it could be seen that changing surface properties of paper is still interesting and perspective topic for research that can transform paper into a value-added product.9 

During last years low-temperature plasma treatment has been confirmed to be an effective technique for modification of natural polymers like cellulose-based materials because of the possibility of a predictable change in their physical and chemical properties for various practical applications.10–12 In particular, plasma processing of cellulose-based paper changes its layer properties.13 Different types of the discharge, as well as treatment conditions on various paper types, have been reported already in case of atmospheric9,14,15 low pressure.13,16–18 Surface modification by “cold” plasma allows the change in the surface chemistry of paper without altering the bulk material properties.

In presented work, the X-ray photoelectron spectroscopy (XPS) was used to study chemical changes taking place at the surface of paper samples treated in air and nitrogen DCSBD plasma in the time scale from 0.25 to 5 seconds. Correlations between chemistry and surface properties, such as wettability, are presented and discussed.

The paper tasted for DCSBD plasma surface modification was one-side “Si-base”, single layer, clay (Al2O3·2SiO2·2H2O)-coated paper, used for release-liner production. The base paper was produced with virgin fibres, a combination of hardwood and softwood, and used filler was calcium carbonate. Sizing of base paper was done with alkenyl succinic anhydride (ASA). The basis weight of the paper was 55 g/m2, and the moisture was 4.5%. The samples were stored under the laboratory conditions (40% humidity and 23°C temperature).

Diffuse Coplanar Surface Barrier Discharge is a special type of surface dielectric barrier discharge, generating visually diffuse plasma layer at atmospheric pressure. The electrode system of DCSBD consists of 32 parallel strip-line silver electrodes (220 mm long, 1.5 mm wide and 1 mm strip to strip) embedded in 96% alumina ceramics and is powered by AC with 15 kHz frequency with high voltage of 20 kV (peak-to-peak). DCSBD unit is cooled by the oil system because the device is heated by collisions between the charged particles and ceramic, as well as by dielectric dissipation in the whole device. The active area of DCSBD plasma layer is 80 × 200 mm.7 

During these experiments, the input power was 400 W and the plasma exposure time was in the range of 0.25-5 s that resulted in treatment speed of 19.2 m/min. The photo of the experimental setup is shown in Fig. 1.

FIG. 1.

Photograph (left) and the scheme (right) of the DCSBD in the roll-to-roll arrangement. 1 -power meter, 2-AC-HV generator, 3-rotor-speed control, 4-cooling oil input and output, 5-reactor chamber for DCSBD plasma, 6-sample of paper, 7-rotor, 8-adjustable holder of DCSBD plasma system, 9-DCSBD plasma system, 10-HV-power inlet.

FIG. 1.

Photograph (left) and the scheme (right) of the DCSBD in the roll-to-roll arrangement. 1 -power meter, 2-AC-HV generator, 3-rotor-speed control, 4-cooling oil input and output, 5-reactor chamber for DCSBD plasma, 6-sample of paper, 7-rotor, 8-adjustable holder of DCSBD plasma system, 9-DCSBD plasma system, 10-HV-power inlet.

Close modal

Since many applications of plasma surface modification are intended to improve the wettability or adhesion properties, measurement of contact angle provides a useful technique for quantification of interfacial intermolecular forces.

The water contact angle (WCA) and surface free energy (SFE) have been studied using Surface Energy Evaluation system (SEEsystem) from Advex Instruments (Brno, Czech Republic). The SEEsystem is an instrument for the contact angle measurement and the calculation of surface free energy (SFE). We investigated the wettability change of the paper before and after the plasma treatment. The volume of the droplet was 1 μl. At least ten measurements for each parameter took place.

The surface morphology was studied by scanning electron microscope MIRA3 from TESCAN (Brno, Czech Republic). The accelerating voltage was 5 kV, and the samples were coated by 20 nm of Au/Pd composite layer.

The XPS measurements were done on an ESCALAB 250Xi (Thermo Fisher Scientific, East Grinstead, United Kingdom). An X-ray beam with a power of 200 W (650 microns spot size) was used. The survey spectra were acquired with a pass energy of 50 eV and a resolution of 1 eV. High-resolution scans were acquired with a pass energy of 20 eV and a resolution of 0.1 eV. In order to compensate the charges on the surface, an electron flood gun was used. Spectra were referenced to the hydrocarbon type C 1s component set at a binding energy of 284.8 eV. Spectra calibration, processing and fitting routines were done using Avantage software.

To study the effect of plasma treatment on the clay-coated paper, the wettability of the samples before and after the plasma treatment was determined by measuring the WCA and SFE.

The images of the water droplet on the paper before and after the plasma treatment are presented in Fig. 2. As it can be observed, water droplet shape shows a hydrophilic property of the substrate surface (Fig. 2a). Meanwhile, the droplet starts losing its spherical shape (Fig. 2b) when the paper is plasma treated under different conditions. This result indicates that the substrate surface is modified, and the hydrophilic properties are achieved in a different manner depending on the treatment conditions. A considerable decrease in the contact angle of water droplets was detected, going from 74.1° for the untreated paper to around half of the value for plasma-treated samples. This decrease is mainly ascribed to the incorporation of polar groups (i.e. carboxyl groups) which favoured the water-wettability of the paper surface.

FIG. 2.

The images of the water droplet on the paper before (a) and after (b) the plasma treatment.

FIG. 2.

The images of the water droplet on the paper before (a) and after (b) the plasma treatment.

Close modal

As far as the gap between the DCSBD ceramics and the treated sample is an essential parameter during the treatment, a set of experiments was done and the change of water contact angle with time was studied. The summarised results are shown in Table I, II and III, respectively.

TABLE I.

Ageing of water contact angle on paper treated in the air at 0.3 mm above ceramics.

Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  42.2 ± 6.8  56.0 ± 2.0  62.1 ± 1.7  60.7 ± 2.2 
0.50  39.7 ± 5.1  46.3 ± 2.3  52.6 ± 1.8  58.4 ± 4.2 
37.9 ± 3.8  45.5 ± 2.0  52.1 ± 1.8  50.0 ± 1.8 
40.2 ± 1.7  50.1 ± 3.9  52.1 ± 4.6  51.7 ± 2.5 
39.0 ± 2.0  47.4 ± 1.5  54.4 ± 3.4  51.8 ± 1.6 
Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  42.2 ± 6.8  56.0 ± 2.0  62.1 ± 1.7  60.7 ± 2.2 
0.50  39.7 ± 5.1  46.3 ± 2.3  52.6 ± 1.8  58.4 ± 4.2 
37.9 ± 3.8  45.5 ± 2.0  52.1 ± 1.8  50.0 ± 1.8 
40.2 ± 1.7  50.1 ± 3.9  52.1 ± 4.6  51.7 ± 2.5 
39.0 ± 2.0  47.4 ± 1.5  54.4 ± 3.4  51.8 ± 1.6 
TABLE II.

Ageing of water contact angle on paper treated in the air at 0.4 mm above ceramics.

Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  48.2 ± 2.3  61.3 ± 2.5  64.5 ± 1.4  62.3 ± 1.7 
0.50  41.9 ± 2.8  56.1 ± 2.2  56.9 ± 1.5  59.3 ± 1.9 
40.3 ± 2.9  49.1 ± 3.4  55.2 ± 3.1  53.9 ± 1.1 
39.6 ± 1.3  51.1 ± 3.5  55.1 ± 3.3  52.2 ± 1.8 
40.2 ± 1.2  48.6 ± 1.3  53.4 ± 2.4  52.0 ± 0.7 
Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  48.2 ± 2.3  61.3 ± 2.5  64.5 ± 1.4  62.3 ± 1.7 
0.50  41.9 ± 2.8  56.1 ± 2.2  56.9 ± 1.5  59.3 ± 1.9 
40.3 ± 2.9  49.1 ± 3.4  55.2 ± 3.1  53.9 ± 1.1 
39.6 ± 1.3  51.1 ± 3.5  55.1 ± 3.3  52.2 ± 1.8 
40.2 ± 1.2  48.6 ± 1.3  53.4 ± 2.4  52.0 ± 0.7 
TABLE III.

Ageing of water contact angle on paper treated in the air at 0.5 mm above ceramics.

Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  56.8 ± 2.4  66.1 ± 1.8  71.0 ± 1.9  67.7 ± 1.1 
0.50  45.6 ± 2.6  57.5 ± 1.8  63.6 ± 3.6  61.6 ± 1.3 
36.1 ± 1.1  46.8 ± 1.3  57.5 ± 4.6  52.1 ± 1.9 
41.3 ± 4.4  47.3 ± 2.9  54.5 ± 2.9  51.2 ± 2.2 
41.6 ± 4.6  47.7 ± 2.1  53.7 ± 3.8  50.3 ± 0.9 
Treatment [s] Right after 1 week 1 month 3 months
Reference  74.1 ± 2.5       
0.25  56.8 ± 2.4  66.1 ± 1.8  71.0 ± 1.9  67.7 ± 1.1 
0.50  45.6 ± 2.6  57.5 ± 1.8  63.6 ± 3.6  61.6 ± 1.3 
36.1 ± 1.1  46.8 ± 1.3  57.5 ± 4.6  52.1 ± 1.9 
41.3 ± 4.4  47.3 ± 2.9  54.5 ± 2.9  51.2 ± 2.2 
41.6 ± 4.6  47.7 ± 2.1  53.7 ± 3.8  50.3 ± 0.9 

Based on the results from the tables above, several parameters were chosen for further analysis. At the present study, the samples that were plasma treated 0.3 mm above the DCSBD ceramics for 0.5 s and 5 s in air and nitrogen are discussed in more details.

The variation of contact angle with time was also studied in order to detect any time-dependent process occurring after plasma treatment (i.e. ageing). Obtained results are presented in Fig. 3 and Fig. 4 for WCA and SFE, respectively. All results presented at 0 value of the storage time are corresponding to WCA and SFE measurements right after the plasma treatment.

FIG. 3.

The ageing of the water contact angle.

FIG. 3.

The ageing of the water contact angle.

Close modal
FIG. 4.

The ageing of the surface free energy.

FIG. 4.

The ageing of the surface free energy.

Close modal

As it can be observed in Figure 3, the water contact angle on the paper is lower for the longer treatment time, which could be attributed to a higher amount of carboxylic acid groups present onto the surface. It is interesting to remark at this point, that a significant decrease in the WCA was achieved despite the short treatment time, which is interesting for industrial applications where high treatment rates are required.

It is important to highlight that the lowest WCA (23.7°) observed after the 5 s plasma treatment in the nitrogen atmosphere in the freshly treated sample increased to 48° in 3 months. In this sense, the tested DCSBD plasma technology not only allows obtaining paper substrates with low contact angle but also treated substrates are able to keep the improved properties throughout time. These results are very positive from an industrial point of view, because of its effective and stable in time treatment of paper substrate in ambient air and N2 at atmospheric pressure. Thus, at the same time avoiding the need for using expensive vacuum equipment.

It is well-known that the surface energy decays after the treatment, and this behaviour is referred to as ageing or hydrophobic recovery. As expected, the surface energy decreased as a function of time. In all cases, the decay in SFE was faster during the first week of storage, and then the SFE decreased its decay rate to a minimum (Fig. 4). Even after 3 months, the SFE value remained significantly higher than that of the untreated sample.

The influence of the working gas on the wettability of the treated paper was also analysed (Fig. 5). It is known that dielectric barrier discharges in nitrogen give more diffuse and glow-like plasma than the one in air.19 This can explain better homogeneity of the plasma treatment, thus an increase in surface free energy in comparison with the samples treated in air.

FIG. 5.

The surface free energy in dependence on gas and treatment time.

FIG. 5.

The surface free energy in dependence on gas and treatment time.

Close modal

Another important parameter that affected the wettability is the time of the plasma exposure of the treated samples. Figure 5 shows how the increase of treatment time in different working gases can influence the components of total surface free energy. Both polar and dispersion interactions were found to change, although the increase of the polar interactions was considerable due to induced oxygen-containing molecular groups shown in chapter 3.2 below.

The presented data are in good agreement with the tendencies described in the work of other researchers. For example, Lahti20 studied the ageing of extrusion-coated paper for several months after corona plasma treatment. The value of surface free energy was decreasing faster right after the treatment while later, this decrease slowed down. Though, even after a longer time, the value of surface free energy remained higher in comparison to untreated samples. Carlsson21 investigated the ageing of pulp treated with oxygen plasma. Authors suggested the reason for a reduction of surface energy is that the hydrophobic, low molecular weight compounds migrated to the surface. Activation by plasma in air and nitrogen atmosphere was also done by Pykönen.9 It was observed that the nitrogen plasma activation of dispersion-coated paper board resulted in more stable changes of surface free energy.

In the case of presented experiments, the data show that there is practically no difference between air and nitrogen plasma treatment in case of low treatment times. But higher treatment time using nitrogen atmosphere shows better results. The ageing effect on the surface of paper shows the same tendencies but remains higher in comparison to air.

To analyse the change of chemical composition and the chemical binding state on the paper surface, XPS was used. Table IV shows elemental composition including O/C ratios of differently treated paper samples. Table V shows the percentage and the ratio of the atomic concentrations of selected treated paper surfaces. As expected, based on the paper composition and plasma gases, the XPS survey spectra revealed the presence of carbon, oxygen, nitrogen, silicon, sodium and aluminium. From the chemical formula of the clay (Al2O3·2SiO2·2H2O), it is seen that Al and Si are already oxidized and should not be oxidized any further. The presence of bound oxygen after treatments in a nitrogen atmosphere may result from a residual concentration of O2 in the discharge gas or chamber. Though the chamber was flushed with nitrogen prior to the treatment, it is possible that some minor oxygen was still present in the chamber. Also, the post-treatment reactions of long-lived free radicals on the sample surface, which occur when the sample is exposed to the ambient atmosphere after the treatment.

TABLE IV.

Elemental composition of differently treated paper.

Treatment
Gas time O C Si Al Na N O/C
ref      34  47  0.7 
Right after  air  0.5s  40  41 
5s  43  37  1.2 
nitrogen  0.5s  41  39  1.1 
5s  42  35  11  1.2 
1 week  air  0.5s  39  42  0.9 
5s  42  38  1.1 
nitrogen  0.5s  39  40  10 
5s  42  36  11  1.2 
1 month  air  0.5s  39  42  0.9 
5s  41  38  10  1.1 
nitrogen  0.5s  39  42  10  0.9 
5s  40  38  11  1.1 
3 months  air  0.5s  39  43  10  0.9 
5s  42  41 
nitrogen  0.5s  44  42 
5s  43  42 
Treatment
Gas time O C Si Al Na N O/C
ref      34  47  0.7 
Right after  air  0.5s  40  41 
5s  43  37  1.2 
nitrogen  0.5s  41  39  1.1 
5s  42  35  11  1.2 
1 week  air  0.5s  39  42  0.9 
5s  42  38  1.1 
nitrogen  0.5s  39  40  10 
5s  42  36  11  1.2 
1 month  air  0.5s  39  42  0.9 
5s  41  38  10  1.1 
nitrogen  0.5s  39  42  10  0.9 
5s  40  38  11  1.1 
3 months  air  0.5s  39  43  10  0.9 
5s  42  41 
nitrogen  0.5s  44  42 
5s  43  42 
TABLE V.

Decomposition of C1s peak for differently treated paper, at%.

C-C/C-H C-O C=O COO carbonate
ref  81  16 
DCSBD air 0.25s  75  14 
DCSBD air 0.5s  72  15 
DCSBD air 5s  74  13 
DCSBD N2 0.5s  75  13 
DCSBD N2 5s  76  15 
C-C/C-H C-O C=O COO carbonate
ref  81  16 
DCSBD air 0.25s  75  14 
DCSBD air 0.5s  72  15 
DCSBD air 5s  74  13 
DCSBD N2 0.5s  75  13 
DCSBD N2 5s  76  15 

In general, plasma treatment led to the incorporation of oxygen functionalities. Chemical composition changes were more significant with increasing time of plasma treatment. The atomic concentration of carbon on the surface of the reference paper was 47%. After 5s air and nitrogen DCSBD plasma treatment, the total amount of C reached 37% and 35%, respectively. The lower concentration of the carbon is accompanied by an increase in the concentration of the oxygen.

The O/C ratio at the plasma-treated layers of the paper was higher than that of the reference paper. The reference showed the O/C ratio of approx. 0.7 whereas after treatment with 5 s air and nitrogen DCSBD plasma O/C ratio reached in both cases 1.2. The atomic concentration of Al and Si showed a slight change in the surface composition. Their concentration levels stayed within 6-9% and 7-11%, respectively. The results demonstrate that although DCSBD plasma treatment led to oxidation of the surface, thus the chemical mixture or ratio between Al and Si remained almost unaffected under plasma modification. The concentrations of Na and N were not influenced by plasma treatment and varied in the range of 0-2%.

Since the samples were exposed to plasma in ambient air and nitrogen, a small amount of the nitrogen appeared in the spectra. A trace level of sodium was detected for all samples, and its concentration remained unaffected by plasma treatment.

Chemical composition of the plasma-treated paper was investigated immediately after, 1 week, 1 month, and 3 months after the plasma treatment. Ageing had only a minor effect on the total concentration of oxygen. Even after 3 months, the O/C ratio for air DCSBD plasma treatment remained at level 0.8-1. A similar observation of O/C ratio was found for nitrogen plasma treatment. From the long-term point of view, the ageing of surface modification of clay-coated paper seems to be very promising.

Decay in surface energy and wettability occurred during the first weeks of storage after plasma activation, after which it levelled off. However, the O/C elemental ratio did not decrease significantly as a function of time, indicating that ageing could be accompanied by a re-orientation of polar groups or by contamination of the surface.

To gain deeper insights into processes on the paper surface modification, further investigation of the carbon chemical bonds was carried out. The chemical composition and the binding state of the differently treated paper were compared using the curve fitting of the XPS C1s core level. The high-resolution C1s peak was fitted with 5 principal components: C-C/C-H (binding energy at 284.8 eV), C-O (286.1 eV), C=O (287.6 eV), O-C=O (289.3 eV), and carbonate (291.3 eV). The expected position of the C-N (285.8 eV) bond is between C-C and C-O peaks, but it is not shown in the figure since it is difficult to distinguish from the Cl peak. The results from the deconvolution analysis of the C 1s peak are presented in Table V. The penetration depth of the XPS technique is very low, that is why on the surface of reference samples the C-C and C-H components of C1s peak that originate from the clay layer and alkenyl succinic anhydride are dominant.

It is apparent that the intensity of the hydrocarbon peak decreased with increasing oxygen content. XPS spectra in Fig. 6 indicates that the plasma process provides surface modification by incorporation of additional functional groups (C=O, O-C=O) into the upper atomic layer of the paper. Best results were observed with 0.5 s air DCSBD plasma modification, where the amount of hydrocarbon bond decreased, and oxygen-containing groups increased significantly. An increase in the polar moieties on the surface could increase the surface free energy, thereby making the surface hydrophilic. This effect can also be considered to be responsible for the decrease in WCA after plasma treatment.

FIG. 6.

The high-resolution of C1s peak for reference sample (left) and samples treated for 0.5 s in air (centre) and nitrogen (right), respectively.

FIG. 6.

The high-resolution of C1s peak for reference sample (left) and samples treated for 0.5 s in air (centre) and nitrogen (right), respectively.

Close modal

Samples that were treated in air and nitrogen (Fig. 7) were studied using the scanning electron microscopy in order to compare their morphology with the reference sample. The measurements did not show the significant modification of the morphology and no alternation of the surface structure after the plasma treatment.

FIG. 7.

SEM micrographs of untreated (left), 5 s in air (centre) and 5 s in nitrogen (right) treated paper samples. Scale bar 5 μm.

FIG. 7.

SEM micrographs of untreated (left), 5 s in air (centre) and 5 s in nitrogen (right) treated paper samples. Scale bar 5 μm.

Close modal

Several parameters such as the speed of the treatment, the precisely controlled distance of treated material from the electrode as well as working in air and nitrogen atmosphere were tested in order to see the effect of DCSBD plasma treatment on the surface properties of clay-coated paper. To determine the plasma treatment effectiveness, the time scale from 0.25 to 5 seconds was used, which correspond to the in-line treatment requirements.

The obtained results showed that the water contact angle was changed significantly after the plasma treatment in nitrogen as well as in ambient air. The significant change of polar interaction of surface energy was found assumedly due to induced oxygen-containing molecular groups. This was supported by the XPS analyses that confirmed a gradual increase of oxygen-rich functional groups on the surface. SEM analyses did not show the modification of the morphology after the plasma treatment.

The presented results show that advantages of DCSBD in roll-to-roll arrangement encourage its suitability for industrial processing of such kind of paper.

This research has been supported by the project 7D16003 funded by EUROSTARS 2 and projects LO1411 (NPU I) and LM2018097 funded by Ministry of Education Youth and Sports of the Czech Republic. The authors would like to thank Crown Van Gelder B.V. (CVG), Netherlands, for the “Si-base” paper samples and to Coating Plasma Innovation (CPI), France, for collaboration on the above mentioned EUROSTAR 2 project.

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