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Cold plasma-assisted paper recycling
Industrial Crops and Products 43 (2013) 114–118
Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Cold plasma-assisted paper recycling Carla Gaiolas a , Ana Paula Costa a , Manuel Santos Silva a , Wim Thielemans b,c , Maria Emília Amaral a,∗ a
Research Unit of Textile and Paper Materials, University of Beira Interior, 6201-001 Covilhã, Portugal School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK c Process and Environmental Research Division, Faculty of Engineering, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK b
a r t i c l e
i n f o
Article history: Received 5 July 2012 Accepted 10 July 2012 Keywords: Paper recycling Plasma treatment XPS Statistical analysis Contact angle
a b s t r a c t The aim of this study is to modify the surface of a commercial paper, using the cold plasma treatment in order to increase its hydrophilic character, thus minimizing the disintegration time and/or the energy consumption, needed to recycle cellulose fibers and obtain a homogeneous suspension. Plasma treatment was applied for different exposure times, under optimal experimental conditions of ground pressure and power. The treated paper samples were characterized by contact angle measurements. The plasma treated samples were disintegrated using a series of different number of rotations (rpm) and the resulting fiber suspensions were used to prepare laboratory hand sheets using a conventional sheet forming. Before paper making, the morphological characteristics of the fibers were evaluated by a MorFi analyser. The effect of plasma treatment on the quality of recycling was evaluated measuring the first-order entropy of the sheet formation. These results show that for similar entropy values, disintegration time for the reference samples is longer than for the treated samples. X-ray photoelectron spectroscopy (XPS) showed that the surface of the treated samples underwent strong oxidation, which is probably responsible for easy recycling of the paper samples. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cold plasma treatment is an environmentally safe process, which offers many advantages, namely: (i) it is a solvent free process (no chemical pollution); (ii) it can be applied as a continuous process and (iii) it can be carried out under different controlled atmospheres, thus inducing a wide variety of chemical changes to yield materials with various properties (Cheng et al., 2006; Gaiolas et al., 2008; Popescu et al., 2011). Plasma can be defined as a partially ionized gas, which generates energetic species present in the discharge, such as electrons, ions, free radicals and photons, which possess sufficiently high energies to modify only the uppermost atomic layers of a material surface, without altering bulk characteristics (Abidi and Hequet, 2004; Yuan et al., 2004; Chen et al., 2011). Abidi and Hequet (2004) referred to the three different and independent processes related to plasma technology, namely: (1) modification of surface structure of the material itself, under the influence of glow discharge, (2) plasma polymerization and (3) grafting of molecules on the material surface after plasma activation. In the last decade, plasma has received
∗ Corresponding author at: Research Unit of Textile and Paper Materials, University of Beira Interior, Rua Marquês d’Avila e Bolama, 6201-001 Covilhã, Portugal. Tel.: +351 275314740; fax: +351 275314740. E-mail address: [email protected] (M.E. Amaral). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.07.016
considerable attention in the realm of the surface modification of various natural materials, such as wood (Carlsson and Ström, 1991; Verreault et al., 1990; Westerlind et al., 1987), cellulose fibers from different origins (Popescu et al., 2011; Vander-Wielen and Ragauskas, 2004; Vander Wielen et al., 2006; Vaswani et al., 2005), paper restoration (Vohrer et al., 2001) and also in textile fibers (Abidi and Hequet, 2004; Chen et al., 2011; Höcker, 2002). To the best of our knowledge, there are no studies on plasma treatment to assist paper recycling. This process represents a section of the paper industry that is oriented toward re-use and sustainability, which implies the preservation of essential woodland resources (Pèlach et al., 2003). The use of recovered paper for papermaking requires the re-wetting and its reduction to a fibrous suspension, a process known as disintegration. At a non-integrated paper industry, disintegration takes place inside pulper equipment. Savolainen et al. (1991) stated that a pulper consumes about 5–15% of the total energy used to obtain the final product. Therefore, an eco-friendly technique contributing to minimize this parameter can be very useful and interesting. This article reports preliminary results obtained from the use of plasma discharge to increase the hydrophilic character of a commercial paper. The treated and untreated samples were characterized by contact-angle measurements and XPS analyses, before being submitted to recycling operation, in order to produce a homogeneous fibrous suspension. The produced fibers were
C. Gaiolas et al. / Industrial Crops and Products 43 (2013) 114–118
2. Materials and methods 2.1. Raw material and disintegration A commercial paper (reference paper) with a basis weight of 80 g m−2 was used as raw material. The reference samples and those treated with plasma were disintegrated in a laboratory disintegrator (ISO 5263-1) varying the number of rotations, as follows: 2500, 5000, 7500, 10,000 and 20,000 rpm. This equipment consists of an integrator motor, an impeller, a removable cup impact device and a pre-selection to adjust the stirring speed (Amaral et al., 2000). These resulting suspensions were used to produce laboratory hand sheets (with a basis weight of 60 g m−2 ) using conventional sheet forming process, according ISO 5269-1 standard. 2.2. Plasma treatment The used radio frequency plasma generator was a EUROPLASMA apparatus equipped with a microcontroller, a vacuum system and a 2.54 GHz microwave generator. The treatment power was 200 W at a constant pressure of 700 mTorr, as already established in our previous studies (Gaiolas et al., 2008, 2009). The treatment time varied from 5, 10, 15, 30, 60, 120, 180 and 300 s, in the presence of air. The vacuum level was measured by a Pirani-type pressure gauge and the calibration was done using nitrogen. The reactor electrodes consisted of a cylindrical Pyrex glass tube with a diameter and a length of 60 mm. The energy input frequency was 13.56 MHz. The chamber was an aluminum made cylinder having a wall thickness of 2 cm. The useful dimensions of this cylindrical chamber are 200 and 150 mm, for the diameter and the length, respectively. 2.3. Morphological analysis Morphological properties of fibers were determined with a MorFi LB-01 fiber analyzer, produced by TECHPAP, France (Passas et al., 2004). The analysis is done on a fibrous suspension, so that the measurement occurs in the natural unrestrained environment of fibers. This approach allows reliable statistical measurement of thousands of fibers at high speed and accurate determination of important characteristics of their shape. This image analysis was done for both samples (reference and treated with plasma), after disintegrating for 20,000 rpm. 2.4. Samples characterization 2.4.1. Contact angle measurements A dataphysics OCA (optical contact angle) Absorption Tester was used in order to select the optimum time for plasma treatment application. In this standard method a droplet of liquid is dispensed onto a solid surface and the sessile drop is illuminated from one side using a diffuse light source and viewed from the other side, in such a way that the contour of the drop is observed. Contact angle measurements were used to characterize the reference and treated samples as described by (Gaiolas et al., 2008, 2009). Smaller contact angles, formed by a drop of water at the surface of investigated samples, correspond to an increase in its hydrophilic character and wettability. 2.4.2. Surface characterization by X-ray photoelectron spectroscopy (XPS) XPS data were obtained using a XR3E2 apparatus (Vacuum Generators, UK) with a mono-chromated MgK␣ X-ray source
(1253.6 eV) and operated at 150 kV under a current of 20 mA. Samples were placed in an ultrahigh vacuum chamber (10−8 mbar) with electron collection by a hemispherical analyzer at an angle of 90◦ . Signal decomposition was done using Spectrum NT and the C 1s signal was shifted to ensure that the C–H signal of the decomposition occurred at 285.0 kV. Spectra were analyzed using Spectrum software. 2.4.3. Image analysis Homogeneity of the distribution of fibers in the hand sheets was evaluated using a technique based on image analysis by light transmission, which consist in taking images of paper samples. The collected images are treated by a statistical approach (firstorder entropy), quantifying the homogeneity of the fibrous network (Cresson and Luner, 1990). The first-order entropy is calculated from the array of gray levels in the image. This parameter represents a more or less uniform distribution of the fibers in the plane of the paper sheet. Low values of entropy correspond to a more uniform paper appearance, which means less contrast of the image gray levels (Costa, 2001). 3. Results and discussion 3.1. Contact angle results The first focus of our research was select the optimum time for plasma application, in order to obtain a paper with the most hydrophilic character. Therefore several trials were carried out using different plasma times and the ensuing treated surfaces were characterized by contact angle measurements. The water contact angle for the samples versus the treatment time is shown in Fig. 1. The treatment of the paper sample by RF-plasma induced a decrease in contact angle of a drop of water from 110◦ to 28◦ , for a treatment time of 60 s. Longer treatment times did not yield any further improvement. The rest of experiments were carried out at this optimum time, because the extension of treatment time did not induce any significant gain, which does not justify spending further energy consumption, associated with prolonged treatment duration. The initial paper is surely sized by hydrophobic coupling agents, as commonly practiced is this writing paper grade (Roberts, 1996). In fact, lignocellulosic fibers are intrinsically hydrophilic and they usually give low contact angles with water (Gandini and Belgacem, 2011). After plasma treatment, the surface shifted from hydrophobic (water contact angle of 110◦ ) to hydrophilic character (water contact angle of 28◦ ). In fact, the work of adhesion of a droplet of water
120
100
Contact angle (θ°)
characterized in terms of morphological properties using a MorFi fiber analyzer.
115
80
60
40
20
0 0
50
100
150
200
250
300
Time (s)
Fig. 1. Contact angle versus plasma treatment time, under: 200 W and 700 mTorr.
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C6
OH
C5
C1
O
C4
O
OH OH
C3
C2
Scheme 1.
and the surface can be calculated according to Young–Duprès Eq. (1): WAT = LT (1 + cos w/pap )
(1)
WAT
3.2. XPS The most relevant works dealing with the use of X-ray photoelectron spectroscopy to characterize cellulose substrates were recently reviewed by Gandini and Belgacem (2011). These works showed that the low resolution C 1s spectra of cellulose revealed the presence of two main atoms situated at 285 and 531 eV and attributed to carbon and oxygen atoms, respectively. The deconvolution of C 1s peak shows that three entities are associated with carbon signal and centered at 285.0, 286.7 and 288.3 eV. These moieties were attributed to C1 (C–H), C2 (C–O) and C3 (O–C–O and/or C O), respectively. The following summary and scheme can give a useful picture of XPS considerations, as applied to cellulose (Gandini and Belgacem, 2011). In theory, pure cellulose exhibits two peaks in its deconvoluted C 1s XPS spectra, namely: (i) C–O at 286.7 eV and associated to alcohols and ether groups. This peak is noted as C2 and corresponds to 5 carbon atoms (C2 –C6 in Scheme 1), and (ii) O–C–O at 288.3 attributed to acetal moieties. This signal corresponds to one carbon atom (C1 in Scheme 1). The surface O/C ratio for pure cellulose (theoretical formula) is 0.83. For the majority of virgin cellulose (avicel, wood pulps, annual plants, etc.), this ratio is systematically lower, because of the presence C-rich molecular segments at the surface of the solids under study (Gandini and Belgacem, 2011). In the present work the low resolution spectra of the samples, before and after Table 1 Atomic surface composition of paper samples, as deduced from low resolution XPS spectra.
Virgin Plasma-treated Virgin and recycled Plasma-treated and recycled
R
LT
where is the work of adhesion of the liquid to a solid. is the surface tension of the liquid (water in our case, i.e., 72.8 mJ/m2 at 20 ◦ C). w/pap is the contact angle formed by water on the paper surface. Before treatment, WAT was close to 48 mJ/m2 , whereas after plasma discharge, WAT is practically three times higher, i.e., 137 mJ/m2 . This result suggests that the occurrence of chemical changes induced by plasma treatment on the paper surface; In fact, it is known that electrical discharge treatments (plasma or corona) introduces reactive polar groups and/or increases the surface roughness of paper (Belgacem et al., 1995; Cheng et al., 2006; Chen et al., 2011). In order to confirm these hypotheses XPS analyses were carried out.
Paper samples
modification, as well as those prepared from recycled fibers show that the main peaks detected originate from two main elements situated at 285 and 531 eV and attributed to carbon (C 1s) and oxygen (O 1s) atoms, respectively. The quantitative data associated with these spectra are summarized in Table 1. For the untreated sized paper, the O/C ratio is much lower than that theoretically calculated (about 0.61 instead of 0.83), which suggests that the sizing agent used to make the surface of cellulose hydrophobic is a carbon-rich molecule. In fact, the most common molecules used for sizing writing paper are, alkylketene dimmer, AKD, and alkenylsuccinic anhydride, ASA (see the chemical structure below).
Surface composition (%) C
O
60.92 58.04 60.44 59.27
37.04 39.55 38.63 39.56
O/C
0.61 0.68 0.64 0.67
O
O
( )n
( )m
R'
O
AKD
O
O
ASA
where R and R are C11 –C17 aliphatic sequences and m and n have a value of around 10. The presence of these long aliphatic chains explains the low O/C ratio, since in these molecules the O/C ratio is between 0.05 and 0.15. After plasma treatment, the O/C ratio increased to reach a value of 0.68, indicating that the sizing agent was oxidized by plasma treatment, which corroborates the contact angle measurements study. The wide spectra of the recycled virgin samples and the plasma-treated and recycled papers displayed O/C ratios of 0.64 and 0.67, respectively. For virgin samples, the deconvolution of C 1s peak shows that the dominant contribution is associated with C1 signals (amounting to ca. 26%), attributed to C–C and/or C–H links, as shown in Fig. 2 and summarized in Table 2. These moieties are most probably aliphatic and/or aromatic oxygen-free sequences, associated with the presence of sizing agents. In fact, theoretically speaking, pure cellulose does possess neither C1 nor C4 atoms. In practice, this signal is present, even for cellulose of high purity, such as microcrystalline cellulose (avicel) (Gandini and Belgacem, 2011). After plasma treatment, the amounts of C1 decreased drastically (from more than 26 to less than 15%) and the signal associated with C4 increased from around 1 to more than 6%. The evolution of these two signals indicates that the plasma ionized atmosphere reacted with the paper surface and oxidized the major part of aliphatic sequences. These results agree with those established by contact angle measurements. The C 1s deconvoluted spectra of the recycled initial paper and the plasma-treated and recycled samples displayed very close properties. In fact, the intensity of the four peaks is roughly the same for the two recycled samples. This indicates that no differences are observed between the content of each type of carbon atoms in these to samples. As mentioned before, this is most probably due to the limited effect of plasma treatment.
Table 2 C 1s deconvolution of carbon atoms present at the surface of cotton yarn surface, as deduced from high resolution XPS spectra. C 1s deconvoluted surface composition (%) Carbon type
C1
C2
C3
C4
Energy (eV) Virgin paper samples Plasma-treated Virgin and recycled Plasma-treated and recycled
285 26.8 14.5 25.1 21.1
286.5 57.1 60.5 58.9 60.3
288.1 14.8 18.7 15.0 16.2
289.8 1.3 6.3 1.0 2.4
C. Gaiolas et al. / Industrial Crops and Products 43 (2013) 114–118
C1s
292
C1s
A
B
C-C or C-H
C-C or C-H
C-O
C-O
O-C-O or C=O
O-C-O or C=O
O-C=O
O-C=O
290
288
286
284
282
117
292
290
288
286
284
282
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. C 1s deconvoluted spectra of (A) virgin, and (B) plasma-treated paper samples.
Table 3 Morphological properties of fibers, before and after plasma treatment.
3.4. Image analysis
Plasma treatment
Length weighted in length (mm) Width (m) Fines content (% in length) Kink angles (◦ )
Before
After
0.831 23.8 80.2 126
0.845 24.4 79.2 127
3.3. MorFI analysis The morphological properties of the fibers were established, as summarized in Table 3. These data shows that no significant changes in morphological characteristics of the fibers were observed between the treated and untreated samples. Thus, the length-weighted fiber length was 0.831 mm for the reference fibers and 0.845 mm for the treated ones. The width of the fiber shows the same tendency, i.e., 23.8 and 24.4 m, for treated and untreated fibers. The slight increase of the fiber dimensions could result from their higher ability to swell because of the oxidation reactions induced by plasma treatment. The other parameters (kink angles and fine contents) remained practically unchanged, indicating that they did not suffer any significant damage.
The reference and plasma-treated samples were disintegrated and the ensuing pulp suspensions were used to produce several hand sheets. The formation quality (homogeneity) of these samples was evaluated by analyzing the images shown in Fig. 3. These images show that samples arising from plasma-treated fibers seem to have a more homogeneous fibers distribution. Moreover, fibers recycled in the range of 2500–5000 rpm gave paper with increased uniformity, as confirmed by quantitative values of entropy.Thus, the image treatment of the obtained photographs gave rise to a way of quantifying the effect of plasma treatment on the quality of disintegrated paper. Fig. 4 illustrates the obtained results and shows that the homogeneity of the prepared samples is systematically better for the paper made from plasma-treated samples. Now, if one were to fix a target value of entropy (equal to 5.6 and suitable for writing paper grade), then, this value is obtained after 10,000 rpm for untreated samples and at 5000 rpm for the plasma treated counterpart. Therefore, the discharge treatment permitted performing disintegration with a substantial energy and time saving. As discussed before, this is probably due to the removal and/or oxidation of the hydrophobic layers from the paper surface, which enhance the water penetration into the fiber mat during the recycling and higher swelling ability of the fibers after plasma treatment.
Fig. 3. Images of hand sheets of recycled paper, using laboratory disintegrator with different number of rotations. Reference paper (R) and paper treated with plasma (PT).
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Fig. 4. Evolution of index of the first order entropy as a function of the number of rotations applied in disintegration process, for the reference and plasma treated papers.
4. Conclusions Plasma treatment can be applied usefully in the field of paper recycling and permit reducing the time and the energy associated with these operations. The surface changes observed in the treated plasma indicates that the sized initial paper undergoes oxidation reaction, which shifts the surface from hydrophobic to hydrophilic. Thus, contact angle measurement and XPS analyses revealed that the quantity of carboxyl functions significantly increased, whereas that of aliphatic sequences decreased. The plasma-assisted recycled fibers did not suffer any morphological damage. They gave recycled fiber with very good uniformity and fiber distribution homogeneity. Acknowledgements The authors thank FCT (Fundac¸ão para a Ciência e Tecnologia) for the awarding of post-doc grant to Carla Gaiolas, within the Community Support Framework III. References Abidi, N., Hequet, E., 2004. Cotton fabric graft copolymerization using microwave plasma I universal attenuated total reflectance-FTIR study. J. Appl. Polym. Sci. 93, 145–154. Amaral, M.E., Renaud, M., Roux, J.-C., 2000. Cinétique de désintégration des pâtes à papier: modélisation du phénomène. Revue ATIP 54 (3–4), 76–84.
Belgacem, M.N., Czeremuszkin, G., Sapieha, S., Gandini, A., 1995. Surface characterization of cellulose fibers by XPS and IGC. Cellulose 2, 145–157. Carlsson, C.M.G., Ström, G., 1991. Reduction and oxidation of cellulose surfaces by means of cold plasma. Langmuir 7, 2492–2497. Chen, G., Zhou, M., Zhang, Z., Lv, G., Massey, S., Smith, W., Tatoulian, M., 2011. Acrylic acid coatings on silk fibers by room-temperature APGD plasma jets. Plasma Process. Polym. 8, 701–708. Cheng, C., Liye, Z., Zhan, R.-J., 2006. Surface modification of polymer fibre by the new atmospheric pressure cold plasma jet. Surf. Coat. Technol. 200, 6659–6665. Cresson, T., Luner, P., 1990. The characterization of paper formation. Part 2. The texture analysis of paper formation. Tappi J. 73, 175–185. Costa, A.P., 2001. PhD Thesis, A Contribution to the Study of Paper Sheet Formation Factor. University of Beira Interior, Portugal. Gaiolas, C., Costa, A.P., Nunes, M., Santos Silva, M.J., Belgacem, M.N., 2008. Grafting of paper by silane coupling agentes using cold-plasma discharges. Plasma Process. Polym. 5, 444–452. Gaiolas, C., Belgacem, M.N., Silva, L., Thielemans, W., Costa, A.P., Nunes, M., Santos Silva, M.J., 2009. Green chemicals and process to graft cellulose fibers. J. Colloid Interface Sci. 330, 298–302. Gandini, A., Belgacem, M.N., 2011. Physical & chemical methods of fiber surface modification. In: Zafeiropoulos, E. (Ed.), Interface Engineering in Natural Fibre Composites for Maximum Performance. Woodhead Publishing Limited, Cambridge, UK, pp. 3–42 (Chapter 1). Höcker, H., 2002. Plasma treatment of textile fibers. Pure Appl. Chem. 74 (3), 423–427. Pèlach, M.A., Pastor, F.J., Puig, J., Vilaseca, F., Mutjé, P., 2003. Enzymatic deinking of old newspapers with cellulase. Process Biochem. 38, 1063–1067. Passas, R., Lecourt, M., Nougier, P., Minko, W., Khelifi, B., 2004. Effet de la remise en suspension des pâtes sur leur caractérisation mophologique. ATIP 58 (4), 6–13. Popescu, M.-C., Totolin, M., Tibirna, C.M., Sdrobis, A., Stevanovic, T., Vasile, C., 2011. Grafting of softwood kraft pulps fibers with fatty acids under cold plasma conditions. Int. J. Biol. Macromol. 48, 326–335. Roberts, J.C., 1996. In: Roberts, J.C. (Ed.), Paper Chemistry. Blackie Academic & Professional, Chapman & Hall, London, UK, p. 267. Savolainen, A., Jussila, T., Nikula, S., 1991. Defibering and specific energy consumption in bale pulpers. Tappi J., 147–153. Vander-Wielen, L.C., Ragauskas, A.J., 2004. Grafting of acrylamide onto lignocellulosic fibers via dielectric-barrier discharge. Eur. Polym. J. 40, 477–482. ˝ Vander Wielen, L.C., Ostenson, M., Gatenholm, P., Ragauskas, A.J., 2006. Surface modification of cellulosic fibers using dielectric-barrier discharge. Carbohydr. Polym. 65, 179–184. Vaswani, S., Koskinen, J., Hess, D.W., 2005. Surface modification of paper and cellulose by plasma-assisted deposition of fluorocarbon films. Surf. Coat. Technol. 195, 121–129. Verreault, M., Klemberg-Sapieha, J.E., Sacher, E., Wertheimer, A.M., 1990. X-ray photoelectron study of the plasma fluorination of lignocelluloses. Appl. Surf. Sci. 44, 165–169. Vohrer, U., Trick, I., Berhardt, J., Oehr, C., Brunner, H., 2001. Plasma treatment—an increasing technology for paper restoration? Surf. Coat. Technol. 142-144, 1069–1073. Westerlind, B., Larsson, A., Rigdahl, M., 1987. Determination of the degree of adhesion in plasma-treated polyethylene/paper laminates. Int. J. Adhes. Adhes. 7, 141–146. Yuan, X., Jayaramam, K., Bhattacharyya, D., 2004. Effects of plasma treatment in enhancing the performance of woodfibre–polypropylene composites. Compos. A: Appl. Sci. Manuf. 35, 1363–1374.
Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Cold plasma-assisted paper recycling Carla Gaiolas a , Ana Paula Costa a , Manuel Santos Silva a , Wim Thielemans b,c , Maria Emília Amaral a,∗ a
Research Unit of Textile and Paper Materials, University of Beira Interior, 6201-001 Covilhã, Portugal School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK c Process and Environmental Research Division, Faculty of Engineering, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK b
a r t i c l e
i n f o
Article history: Received 5 July 2012 Accepted 10 July 2012 Keywords: Paper recycling Plasma treatment XPS Statistical analysis Contact angle
a b s t r a c t The aim of this study is to modify the surface of a commercial paper, using the cold plasma treatment in order to increase its hydrophilic character, thus minimizing the disintegration time and/or the energy consumption, needed to recycle cellulose fibers and obtain a homogeneous suspension. Plasma treatment was applied for different exposure times, under optimal experimental conditions of ground pressure and power. The treated paper samples were characterized by contact angle measurements. The plasma treated samples were disintegrated using a series of different number of rotations (rpm) and the resulting fiber suspensions were used to prepare laboratory hand sheets using a conventional sheet forming. Before paper making, the morphological characteristics of the fibers were evaluated by a MorFi analyser. The effect of plasma treatment on the quality of recycling was evaluated measuring the first-order entropy of the sheet formation. These results show that for similar entropy values, disintegration time for the reference samples is longer than for the treated samples. X-ray photoelectron spectroscopy (XPS) showed that the surface of the treated samples underwent strong oxidation, which is probably responsible for easy recycling of the paper samples. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cold plasma treatment is an environmentally safe process, which offers many advantages, namely: (i) it is a solvent free process (no chemical pollution); (ii) it can be applied as a continuous process and (iii) it can be carried out under different controlled atmospheres, thus inducing a wide variety of chemical changes to yield materials with various properties (Cheng et al., 2006; Gaiolas et al., 2008; Popescu et al., 2011). Plasma can be defined as a partially ionized gas, which generates energetic species present in the discharge, such as electrons, ions, free radicals and photons, which possess sufficiently high energies to modify only the uppermost atomic layers of a material surface, without altering bulk characteristics (Abidi and Hequet, 2004; Yuan et al., 2004; Chen et al., 2011). Abidi and Hequet (2004) referred to the three different and independent processes related to plasma technology, namely: (1) modification of surface structure of the material itself, under the influence of glow discharge, (2) plasma polymerization and (3) grafting of molecules on the material surface after plasma activation. In the last decade, plasma has received
∗ Corresponding author at: Research Unit of Textile and Paper Materials, University of Beira Interior, Rua Marquês d’Avila e Bolama, 6201-001 Covilhã, Portugal. Tel.: +351 275314740; fax: +351 275314740. E-mail address: [email protected] (M.E. Amaral). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.07.016
considerable attention in the realm of the surface modification of various natural materials, such as wood (Carlsson and Ström, 1991; Verreault et al., 1990; Westerlind et al., 1987), cellulose fibers from different origins (Popescu et al., 2011; Vander-Wielen and Ragauskas, 2004; Vander Wielen et al., 2006; Vaswani et al., 2005), paper restoration (Vohrer et al., 2001) and also in textile fibers (Abidi and Hequet, 2004; Chen et al., 2011; Höcker, 2002). To the best of our knowledge, there are no studies on plasma treatment to assist paper recycling. This process represents a section of the paper industry that is oriented toward re-use and sustainability, which implies the preservation of essential woodland resources (Pèlach et al., 2003). The use of recovered paper for papermaking requires the re-wetting and its reduction to a fibrous suspension, a process known as disintegration. At a non-integrated paper industry, disintegration takes place inside pulper equipment. Savolainen et al. (1991) stated that a pulper consumes about 5–15% of the total energy used to obtain the final product. Therefore, an eco-friendly technique contributing to minimize this parameter can be very useful and interesting. This article reports preliminary results obtained from the use of plasma discharge to increase the hydrophilic character of a commercial paper. The treated and untreated samples were characterized by contact-angle measurements and XPS analyses, before being submitted to recycling operation, in order to produce a homogeneous fibrous suspension. The produced fibers were
C. Gaiolas et al. / Industrial Crops and Products 43 (2013) 114–118
2. Materials and methods 2.1. Raw material and disintegration A commercial paper (reference paper) with a basis weight of 80 g m−2 was used as raw material. The reference samples and those treated with plasma were disintegrated in a laboratory disintegrator (ISO 5263-1) varying the number of rotations, as follows: 2500, 5000, 7500, 10,000 and 20,000 rpm. This equipment consists of an integrator motor, an impeller, a removable cup impact device and a pre-selection to adjust the stirring speed (Amaral et al., 2000). These resulting suspensions were used to produce laboratory hand sheets (with a basis weight of 60 g m−2 ) using conventional sheet forming process, according ISO 5269-1 standard. 2.2. Plasma treatment The used radio frequency plasma generator was a EUROPLASMA apparatus equipped with a microcontroller, a vacuum system and a 2.54 GHz microwave generator. The treatment power was 200 W at a constant pressure of 700 mTorr, as already established in our previous studies (Gaiolas et al., 2008, 2009). The treatment time varied from 5, 10, 15, 30, 60, 120, 180 and 300 s, in the presence of air. The vacuum level was measured by a Pirani-type pressure gauge and the calibration was done using nitrogen. The reactor electrodes consisted of a cylindrical Pyrex glass tube with a diameter and a length of 60 mm. The energy input frequency was 13.56 MHz. The chamber was an aluminum made cylinder having a wall thickness of 2 cm. The useful dimensions of this cylindrical chamber are 200 and 150 mm, for the diameter and the length, respectively. 2.3. Morphological analysis Morphological properties of fibers were determined with a MorFi LB-01 fiber analyzer, produced by TECHPAP, France (Passas et al., 2004). The analysis is done on a fibrous suspension, so that the measurement occurs in the natural unrestrained environment of fibers. This approach allows reliable statistical measurement of thousands of fibers at high speed and accurate determination of important characteristics of their shape. This image analysis was done for both samples (reference and treated with plasma), after disintegrating for 20,000 rpm. 2.4. Samples characterization 2.4.1. Contact angle measurements A dataphysics OCA (optical contact angle) Absorption Tester was used in order to select the optimum time for plasma treatment application. In this standard method a droplet of liquid is dispensed onto a solid surface and the sessile drop is illuminated from one side using a diffuse light source and viewed from the other side, in such a way that the contour of the drop is observed. Contact angle measurements were used to characterize the reference and treated samples as described by (Gaiolas et al., 2008, 2009). Smaller contact angles, formed by a drop of water at the surface of investigated samples, correspond to an increase in its hydrophilic character and wettability. 2.4.2. Surface characterization by X-ray photoelectron spectroscopy (XPS) XPS data were obtained using a XR3E2 apparatus (Vacuum Generators, UK) with a mono-chromated MgK␣ X-ray source
(1253.6 eV) and operated at 150 kV under a current of 20 mA. Samples were placed in an ultrahigh vacuum chamber (10−8 mbar) with electron collection by a hemispherical analyzer at an angle of 90◦ . Signal decomposition was done using Spectrum NT and the C 1s signal was shifted to ensure that the C–H signal of the decomposition occurred at 285.0 kV. Spectra were analyzed using Spectrum software. 2.4.3. Image analysis Homogeneity of the distribution of fibers in the hand sheets was evaluated using a technique based on image analysis by light transmission, which consist in taking images of paper samples. The collected images are treated by a statistical approach (firstorder entropy), quantifying the homogeneity of the fibrous network (Cresson and Luner, 1990). The first-order entropy is calculated from the array of gray levels in the image. This parameter represents a more or less uniform distribution of the fibers in the plane of the paper sheet. Low values of entropy correspond to a more uniform paper appearance, which means less contrast of the image gray levels (Costa, 2001). 3. Results and discussion 3.1. Contact angle results The first focus of our research was select the optimum time for plasma application, in order to obtain a paper with the most hydrophilic character. Therefore several trials were carried out using different plasma times and the ensuing treated surfaces were characterized by contact angle measurements. The water contact angle for the samples versus the treatment time is shown in Fig. 1. The treatment of the paper sample by RF-plasma induced a decrease in contact angle of a drop of water from 110◦ to 28◦ , for a treatment time of 60 s. Longer treatment times did not yield any further improvement. The rest of experiments were carried out at this optimum time, because the extension of treatment time did not induce any significant gain, which does not justify spending further energy consumption, associated with prolonged treatment duration. The initial paper is surely sized by hydrophobic coupling agents, as commonly practiced is this writing paper grade (Roberts, 1996). In fact, lignocellulosic fibers are intrinsically hydrophilic and they usually give low contact angles with water (Gandini and Belgacem, 2011). After plasma treatment, the surface shifted from hydrophobic (water contact angle of 110◦ ) to hydrophilic character (water contact angle of 28◦ ). In fact, the work of adhesion of a droplet of water
120
100
Contact angle (θ°)
characterized in terms of morphological properties using a MorFi fiber analyzer.
115
80
60
40
20
0 0
50
100
150
200
250
300
Time (s)
Fig. 1. Contact angle versus plasma treatment time, under: 200 W and 700 mTorr.
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C6
OH
C5
C1
O
C4
O
OH OH
C3
C2
Scheme 1.
and the surface can be calculated according to Young–Duprès Eq. (1): WAT = LT (1 + cos w/pap )
(1)
WAT
3.2. XPS The most relevant works dealing with the use of X-ray photoelectron spectroscopy to characterize cellulose substrates were recently reviewed by Gandini and Belgacem (2011). These works showed that the low resolution C 1s spectra of cellulose revealed the presence of two main atoms situated at 285 and 531 eV and attributed to carbon and oxygen atoms, respectively. The deconvolution of C 1s peak shows that three entities are associated with carbon signal and centered at 285.0, 286.7 and 288.3 eV. These moieties were attributed to C1 (C–H), C2 (C–O) and C3 (O–C–O and/or C O), respectively. The following summary and scheme can give a useful picture of XPS considerations, as applied to cellulose (Gandini and Belgacem, 2011). In theory, pure cellulose exhibits two peaks in its deconvoluted C 1s XPS spectra, namely: (i) C–O at 286.7 eV and associated to alcohols and ether groups. This peak is noted as C2 and corresponds to 5 carbon atoms (C2 –C6 in Scheme 1), and (ii) O–C–O at 288.3 attributed to acetal moieties. This signal corresponds to one carbon atom (C1 in Scheme 1). The surface O/C ratio for pure cellulose (theoretical formula) is 0.83. For the majority of virgin cellulose (avicel, wood pulps, annual plants, etc.), this ratio is systematically lower, because of the presence C-rich molecular segments at the surface of the solids under study (Gandini and Belgacem, 2011). In the present work the low resolution spectra of the samples, before and after Table 1 Atomic surface composition of paper samples, as deduced from low resolution XPS spectra.
Virgin Plasma-treated Virgin and recycled Plasma-treated and recycled
R
LT
where is the work of adhesion of the liquid to a solid. is the surface tension of the liquid (water in our case, i.e., 72.8 mJ/m2 at 20 ◦ C). w/pap is the contact angle formed by water on the paper surface. Before treatment, WAT was close to 48 mJ/m2 , whereas after plasma discharge, WAT is practically three times higher, i.e., 137 mJ/m2 . This result suggests that the occurrence of chemical changes induced by plasma treatment on the paper surface; In fact, it is known that electrical discharge treatments (plasma or corona) introduces reactive polar groups and/or increases the surface roughness of paper (Belgacem et al., 1995; Cheng et al., 2006; Chen et al., 2011). In order to confirm these hypotheses XPS analyses were carried out.
Paper samples
modification, as well as those prepared from recycled fibers show that the main peaks detected originate from two main elements situated at 285 and 531 eV and attributed to carbon (C 1s) and oxygen (O 1s) atoms, respectively. The quantitative data associated with these spectra are summarized in Table 1. For the untreated sized paper, the O/C ratio is much lower than that theoretically calculated (about 0.61 instead of 0.83), which suggests that the sizing agent used to make the surface of cellulose hydrophobic is a carbon-rich molecule. In fact, the most common molecules used for sizing writing paper are, alkylketene dimmer, AKD, and alkenylsuccinic anhydride, ASA (see the chemical structure below).
Surface composition (%) C
O
60.92 58.04 60.44 59.27
37.04 39.55 38.63 39.56
O/C
0.61 0.68 0.64 0.67
O
O
( )n
( )m
R'
O
AKD
O
O
ASA
where R and R are C11 –C17 aliphatic sequences and m and n have a value of around 10. The presence of these long aliphatic chains explains the low O/C ratio, since in these molecules the O/C ratio is between 0.05 and 0.15. After plasma treatment, the O/C ratio increased to reach a value of 0.68, indicating that the sizing agent was oxidized by plasma treatment, which corroborates the contact angle measurements study. The wide spectra of the recycled virgin samples and the plasma-treated and recycled papers displayed O/C ratios of 0.64 and 0.67, respectively. For virgin samples, the deconvolution of C 1s peak shows that the dominant contribution is associated with C1 signals (amounting to ca. 26%), attributed to C–C and/or C–H links, as shown in Fig. 2 and summarized in Table 2. These moieties are most probably aliphatic and/or aromatic oxygen-free sequences, associated with the presence of sizing agents. In fact, theoretically speaking, pure cellulose does possess neither C1 nor C4 atoms. In practice, this signal is present, even for cellulose of high purity, such as microcrystalline cellulose (avicel) (Gandini and Belgacem, 2011). After plasma treatment, the amounts of C1 decreased drastically (from more than 26 to less than 15%) and the signal associated with C4 increased from around 1 to more than 6%. The evolution of these two signals indicates that the plasma ionized atmosphere reacted with the paper surface and oxidized the major part of aliphatic sequences. These results agree with those established by contact angle measurements. The C 1s deconvoluted spectra of the recycled initial paper and the plasma-treated and recycled samples displayed very close properties. In fact, the intensity of the four peaks is roughly the same for the two recycled samples. This indicates that no differences are observed between the content of each type of carbon atoms in these to samples. As mentioned before, this is most probably due to the limited effect of plasma treatment.
Table 2 C 1s deconvolution of carbon atoms present at the surface of cotton yarn surface, as deduced from high resolution XPS spectra. C 1s deconvoluted surface composition (%) Carbon type
C1
C2
C3
C4
Energy (eV) Virgin paper samples Plasma-treated Virgin and recycled Plasma-treated and recycled
285 26.8 14.5 25.1 21.1
286.5 57.1 60.5 58.9 60.3
288.1 14.8 18.7 15.0 16.2
289.8 1.3 6.3 1.0 2.4
C. Gaiolas et al. / Industrial Crops and Products 43 (2013) 114–118
C1s
292
C1s
A
B
C-C or C-H
C-C or C-H
C-O
C-O
O-C-O or C=O
O-C-O or C=O
O-C=O
O-C=O
290
288
286
284
282
117
292
290
288
286
284
282
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. C 1s deconvoluted spectra of (A) virgin, and (B) plasma-treated paper samples.
Table 3 Morphological properties of fibers, before and after plasma treatment.
3.4. Image analysis
Plasma treatment
Length weighted in length (mm) Width (m) Fines content (% in length) Kink angles (◦ )
Before
After
0.831 23.8 80.2 126
0.845 24.4 79.2 127
3.3. MorFI analysis The morphological properties of the fibers were established, as summarized in Table 3. These data shows that no significant changes in morphological characteristics of the fibers were observed between the treated and untreated samples. Thus, the length-weighted fiber length was 0.831 mm for the reference fibers and 0.845 mm for the treated ones. The width of the fiber shows the same tendency, i.e., 23.8 and 24.4 m, for treated and untreated fibers. The slight increase of the fiber dimensions could result from their higher ability to swell because of the oxidation reactions induced by plasma treatment. The other parameters (kink angles and fine contents) remained practically unchanged, indicating that they did not suffer any significant damage.
The reference and plasma-treated samples were disintegrated and the ensuing pulp suspensions were used to produce several hand sheets. The formation quality (homogeneity) of these samples was evaluated by analyzing the images shown in Fig. 3. These images show that samples arising from plasma-treated fibers seem to have a more homogeneous fibers distribution. Moreover, fibers recycled in the range of 2500–5000 rpm gave paper with increased uniformity, as confirmed by quantitative values of entropy.Thus, the image treatment of the obtained photographs gave rise to a way of quantifying the effect of plasma treatment on the quality of disintegrated paper. Fig. 4 illustrates the obtained results and shows that the homogeneity of the prepared samples is systematically better for the paper made from plasma-treated samples. Now, if one were to fix a target value of entropy (equal to 5.6 and suitable for writing paper grade), then, this value is obtained after 10,000 rpm for untreated samples and at 5000 rpm for the plasma treated counterpart. Therefore, the discharge treatment permitted performing disintegration with a substantial energy and time saving. As discussed before, this is probably due to the removal and/or oxidation of the hydrophobic layers from the paper surface, which enhance the water penetration into the fiber mat during the recycling and higher swelling ability of the fibers after plasma treatment.
Fig. 3. Images of hand sheets of recycled paper, using laboratory disintegrator with different number of rotations. Reference paper (R) and paper treated with plasma (PT).
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Fig. 4. Evolution of index of the first order entropy as a function of the number of rotations applied in disintegration process, for the reference and plasma treated papers.
4. Conclusions Plasma treatment can be applied usefully in the field of paper recycling and permit reducing the time and the energy associated with these operations. The surface changes observed in the treated plasma indicates that the sized initial paper undergoes oxidation reaction, which shifts the surface from hydrophobic to hydrophilic. Thus, contact angle measurement and XPS analyses revealed that the quantity of carboxyl functions significantly increased, whereas that of aliphatic sequences decreased. The plasma-assisted recycled fibers did not suffer any morphological damage. They gave recycled fiber with very good uniformity and fiber distribution homogeneity. Acknowledgements The authors thank FCT (Fundac¸ão para a Ciência e Tecnologia) for the awarding of post-doc grant to Carla Gaiolas, within the Community Support Framework III. References Abidi, N., Hequet, E., 2004. Cotton fabric graft copolymerization using microwave plasma I universal attenuated total reflectance-FTIR study. J. Appl. Polym. Sci. 93, 145–154. Amaral, M.E., Renaud, M., Roux, J.-C., 2000. Cinétique de désintégration des pâtes à papier: modélisation du phénomène. Revue ATIP 54 (3–4), 76–84.
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