Sepiolite Nanocomposites

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ScienceDirect Procedia - Social and Behavioral Sciences 195 (2015) 2067 – 2075

World Conference on Technology, Innovation and Entrepreneurship

In Situ Preparation of Resol/Sepiolite Nanocomposites Ümran Burcu Alkana, Nilgün KÕzÕlcanb* a

Istanbul Technical University, Graduate School of Science Engineering and Technology, Polymer Science and Technology Department, Istanbul, 34469, Turkey b Istanbul Technical University, Department of Chemistry, Faculty of Science, Istanbul, 34469, Turkey

Abstract In this study, in situ modified phenol formaldehyde resins were prepared from clay (sepiolite) in the presence of base catalyst. Different clay contents (3 wt%, 5wt%, 8 wt%, 10 wt%, 15wt%, 20wt%, 30wt% and 50 wt%) were used to produce sepiolite modified resol nanocomposite resins (SEP-PFNCRs). SEP-PFNCRs were partially cured by heat and the effects of the curing process and the clay content in the resol resin were determined on the spectroscopic, thermal, and microscopic properties of the final products. The structures of the specimens were characterized by means of Fourier Transform Infrared (FTIR-ATR) spectroscopy. Thermal properties of the samples were determined with Thermogravimetric Analyzer (TGA). The dispersion of sepiolite in the resol nanocomposites were examined by X-ray Diffraction (XRD). The obtained samples were also characterized morphologically by Scanning Electron Microscope (SEM).

© 2015 byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Istanbul University. Peer-review under responsibility of Istanbul Univeristy.

Keywords: Resol; sepiolite; nanocomposites; clay; resin.

1. Introduction Phenol formaldehyde resins (PFRs) which are known as the oldest thermosetting polymers have been irreplaceable materials for industrial applications, such as wood products, coatings, laminates, structural adhesives, moulding compounds, thermal insulation materials and composites. Most of these applications are due to the fact that their low-cost, easily processability, high chemical resistance, fine dimensional stability, good temperature resistance and superior mechanical strength characteristics (Hsief & Beeson, 1997; Knop & Scheib, 1979;

* Corresponding author. Tel.:+90 212 285 3242; fax:+90 212 285 6386. E-mail address: [email protected]

1877-0428 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Istanbul Univeristy. doi:10.1016/j.sbspro.2015.06.232

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Gardziella, Pilato, & Knop, 2000; Astarloa-Aierbe, Echeverria, Martin, Etxeberria & Mondragon, 2002; AstarloaAierbe, Echeverria, Vazquez & Mondragon, 2000; Higuchi, 1999; Zmihorsa-Gotfryd, 2004; Fan, Qin, Wang & Chu, 2010). The common phenol-formaldehyde resin (PFR) formed from phenol and formaldehyde in the presence of alkali, that named as resol. The structure of resol is given in Figure 1 (Pilato, 2010).

Figure 1. Structure of resol.

On the other hand, resol resins have three dimensional (3D) molecular structure even before curing, therefore the synthesis of phenolic resin nanocomposites is rather difficult. This network structure is the result of polyfunctionality of phenol having more than one reactive site for aromatic substitution reaction and excess of formaldehyde (Özkaraman & KÕzÕlcan, 2013). So that, number of studies that deals with phenolic resins are limited, compared to other thermosetting resins such as epoxy and polyester (Özkaraman & KÕzÕlcan, 2013; Lopez, Blanco, Vazquez, Gabilondo, Arbelaiz, Echeverria & Mondragon, 2008; Lopez, Blanco, Martin & Mondragon, 2012; Kaynak & Tasan, 2006). Nevertheless, phenolic resin nanocomposites have attracted considerable attention in both scientific and industrial fields on account of their unexpected hybrid properties (Özkaraman & KÕzÕlcan, 2013).

Figure 2. Structure of sepiolite.

Recently, special attention is being paid to the application of clay minerals in the formulation of clay–polymer nanocomposites (Bergaya & Lagaly, 2007; de Paiva, Morales & Valenzuela-Diaz, 2008). Clay minerals have been

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adapted to the field of organic-inorganic nanocomposites because of their small particle size and intercalation properties, especially in the application of reinforcement materials with polymers (Lan & Pinnavaia, 1994; Kawasumi, Hasegawa, Koto, Usuki & Okada, 1997; Vaia, Sauer, Tse & Giannelis, 1997). Sepiolite clay is a hydrated magnesium silicate which has fibrous morphology resulting as an ideal unit formula: Si12O30Mg8(OH)4 .(H2O)4 . 8H2O, shown in Figure 2. Sepiolite structure is established by a magnesium octahedral sheet in between two layers of silica tetrahedrons. This continues with an inversion of the apical ends every six units. Therefore, sepiolite has discontinuous octahedral sheet that provides rectangular tunnel, along the fibre axis. In cross section, these nanostructured tunnels have approximately 3.5 x 10.6 AÛ2 area. Also, these tunnels completely filled by zeolitic water [H2O]zeol under ambient conditions. The Mg2+ cations placed at the edges of the octahedral sheets complete their coordination, bound with two molecules of structural water [H2O]coord (Tartaglione, Tabuani, Camino & Moisio, 2008; Tartaglione, Tabuani & Camino, 2008). Sepiolite exhibits nano-needle morphology with particle size over a wide range, however the dimensions of the fibres are generally 100–5000 nm length, 10–30 nm width, and 5–10 nm thickness. Furthermore, sepiolite has high surface area at the range of 200-300 m2/g due to the presence of blocks and tunnels along the needle direction (Duquesne, Moins, Alexandre & Dubois, 2007; Kuang, Facey & Detellier, 2004). In sepiolite structure blocks are linked together along their longitudinal edges by Si-O-Si bonds only in one direction. It is assumed that needle like clays can be more easily dispersed in polymeric matrices because it has lower specific surface area compared with platelet-like clays with the same aspect ratio. The relatively small contact surface thus the reduced tendencies to agglomerate, leads to better mechanical reinforcement of needle-like clays (Bilotti, Fischer & Pejis, 2008; Yu, Qi, Zhan, Wu, Yang & Wu, 2011). The discontinuity of the silica sheets provides increasing of the silanol groups (SiOH) at external surface of the sepiolite particles which improves the interfacial interaction of sepiolite with organic solvent and polymers. As the presence of Si-OH, sepiolite can dispersed well in polymer matrix and thereby improve the mechanical properties, thermal stability, flame retardancy and barrier properties of polymers (Yu, Qi, Zhan, Wu, Yang & Wu, 2011; Lopez, Fernandez, Merino, Santaren & Pastor 2010; Chen, Zheng, Sun & Jia, 2007). Recently, sepiolite has been used for the preparation of nanocomposites using different polymers as matrix such as epoxy (Zotti, Borriello, Martone, Antonucci, Giordano & Zarrelli, 2014; Zheng & Zheng, 2006), polypropylene (Bilotti, Fischer & Pejis, 2008; Morales, Ojeda, Linares & Acosta, 1992; Acosta, Ojeda, Morales & Linares, 1986), polyimide (Yu, Qi, Zhan, Wu, Yang & Wu, 2011), polyethylene (Arroyo, Perez & Wigo, 1986; Alonso, Martini, Iannoni, Terenzi, Kenny & Barbosa, 2015; Carrero, Grieken, Suarez & Paredes, 2012), ethylene vinyl acetate (Huang, Chen, Yi & Wang, 2010; Bidsorkhi, Soheilmoghaddam, Pour, Adelnia & Mohamad, 2014), chitosan (Darder, Lopez-Blanco, Aranda, Aznar, Bravo & Ruiz-Hitzky, 2006), polyvinylalcohol (Alkan & Benlikaya, 2009; Killeen, Frydrych & Chen, 2012; Wicklein, Aranda, Ruiz-Hitzky & Darder, 2013), polyurethane (Chen, Zheng, Sun & Jia, 2007; Chen, Zeng, Xiao, Zheng, Ke & Li, 2011) and polyamid 6 (Lopez, Fernandez, Merino, Santaren & Pastor, 2010; Bilotti, Duquesne, Deng, Zhang, Quero, Georgiades, Fischer, Dubois & Pejis, 2014). However, sepiolite effects of resol resins has not been studied before. In this study, sepiolite effects of resole nanocomposite resins were investigated by in situ method. Spectroscopic, thermal, and microscopic properties of the final nanocomposite samples have been searched and determined by Fourier Transform Infrared (FTIR-ATR) spectroscopy, Thermogravimetric Analyzer (TGA), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). 2. Literature Review And Hypotheses 2.1. Materials Phenol and formaldehyde solution (37%) were purchased from Carlo Erba Reagents and MERCK, respectively for the synthesis of resol resins. Sodium hydroxide pellets were used from Carlo Erba Reagents. The sepiolite clay

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that has the commercial name of Pangel S9 with 99% purity, was kindly supplied from Tolsa Group (Spain). All chemicals were used without further purification. 2.2. Synthesis of Phenol Formaldehyde Resol Resins (PFRs) Into a three-necked flask equipped with a mechanical stirrer, 18.8 g (0.2 mol) of phenol and 33 mL (12 g - 0.4 mol) of formaldehyde solution were added. To prevent the evaporation of mixture reflux condenser was set to the system. The mixture was stirred with mechanical stirrer for 4 hours. Throughout the reaction, pH was adjusted to 910 using the 20 wt% NaOH solution and the temperature of oil bath was set to 85oC. After 4 hours, the liquid resin was transferred in a rotary evaporator under vacuum at 85oC for 15 minutes to remove water which occured during the reaction. At the end of rotary evaporation process, like a gel very viscous resin was obtained. For curing, the resin was dried in an aluminum utensil under vacuum oven for 2 hours at 100oC hence solid resin was synthesized. Finally, the solid resin was pulverized in a mortar to obtain powder form.

Figure 3. Sepiolite Modified Resol Nanocomposite Resins

2.3. Synthesis of Sepiolite Modified Resol Nanocomposite Resins(SEP-PFNCRs) SEP-PFNCRs were synthesized in eight different initial feed sepiolite content by phenol weight (3 wt%, 5wt%, 8wt%, 10wt%, 15wt%, 20wt%,30wt% and 50wt%). At each polymerization, 0.2 mol (18.8 g) of phenol , 0.4 mol (33 mL) of formaldehyde solution were added into a three-necked flask. Sepiolite reinforced resol resins were synthesized by the same way and by the same experiment setup. pH was adjusted to 10 using the 20 wt% NaOH solution and the temperature of oil bath was set to 85oC. The mixture was stirred with mechanical stirrer for 4 hours. For 3% and 5% sepiolite contents, the resins were liquid therefore rotary evaporator used for removing water to obtain viscous resin for 15 minutes at 85oC. The other sepiolite samples were very viscous thus rotary evaporation process was not applied. The viscous resin was dried in an aluminum utensil under vacuum oven at 100oC until the resin solidified. To make powder form, obtained reddish resin product was pulverized in the mortar . 2.4. Characterization of PFR and SEP-PFNCRs The structure of the specimens were studied using Fourier Transform Infrared (FTIR-ATR) spectroscopy. FTIR analyses of samples were carried out with Nicolet 6700 FTIR-ATR reflectance spectrophotometer using OMNIC as software.

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In order to investigate the morphology of resin samples, ESEM XL30 ESEM-FEG scanning electron microscope (SEM) was used and the samples for the SEM measurement were prepared by gold coating. Thermogravimetric analysis was performed in nitrogen atmosphere at a heating rate of 10 oC/min up 30 oC to 700 oC temperature by TG/DTA 7200 Exstar. X-ray diffraction patterns were recorded on a Rigaku D/Max-Ultima+/PC XRD instrument. Cu K Į radiation with a wavelength of Ȝ= 0.15406 nm was applied with the scanning rate in the range of 10o to 70o. Interlayer distance (d001) of resins calculated by Bragg equation. (n . Ȝ = 2 . d . sinș) 3. Results and Discussion 3.1. Morphological Characterization SEM analysis was performed to understand the distribution of clay particles in polymeric resol resin matrix and in order to gain an idea about the degree of intercalation and/or exfoliation of the clay layers. The SEM crosssectional images of PFR-SEP10% nanocomposite are illustrated in Figure 4. It is obvious that the sepiolite has a fibrous and needle like morphology and sepiolite nanofillers are uniformly dispersed in resol matrix. Furthermore, the resol covered the surface of sepiolite (Fig. 4) suggesting interaction between the surface of the sepiolite and resol matrix. This situation supports to the bonding between the resol and silanol groups of sepiolite.

Figure 4. SEM images of PFR-SEP10% at different magnifications.

3.2. X-ray Diffraction Analysis The structures and the variations of spacing of the sepiolite clay and modified resol resins were detected by X-ray diffraction. In Table 1. d-spacings (d001), which was calculated by Bragg’s Equation (n.Ȝ=2.d.sinș), and 2ș angles were shown. 2ș measurements and d001 spacings (nm) (where d001 is interlayer distance, d-spacing in the direction of lattice plane diffraction peak (001)) of the final samples are given in Table 1. The d001 spacings (nm) were calculated with the Bragg equation with the help of the obtained 2ș measurement values. The 2ș value of the neat sepiolite clay used was found to be 1.184nm. The relative intensity of the d001 peak increased with the percent of sepiolite loading (Fig.5). In addition, dispersion of sepiolite in resol was influenced by the amount of sepiolite; the more sepiolite in composites, the poorer dispersion. As observed in SEP-PFNCRs graph the nanocomposites have intercalated structure due to the peak of sepiolite.

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Ümran Burcu Alkan and Nilgün Kızılcan / Procedia - Social and Behavioral Sciences 195 (2015) 2067 – 2075 Table 1. Diffraction characteristics of SEP and SEP-PFNCRs. Sample

2ș (deg)

d001(nm)

SEP

7.46

1.184

PFR-SEP3%

7.50

1.177

PFR-SEP5%

7.38

1.196

PFR-SEP8%

7.56

1.168

PFR-SEP10%

7.42

1.190

PFR-SEP15%

7.42

1.190

PFR-SEP20%

7.42

1.190

PFR-SEP30%

7.56

1.168

PFR-SEP50%

7.62

1.159

Figure 5. XRD patterns of specimens.

3.3. Thermogravimetric Analysis The TGA results of PFNCRs with different amounts of sepiolite are shown in Fig. 6, and the values of weight loss at different temperatures are summarized in Table 2. The different stages of degradation could be obtained in the TGA thermogram: the first stage (up to 350oC) and the second stage (350–700oC). In the first stage, formaldehyde release caused by breaking of ether bridges, and the presence of phenol and water, and in the second second stage the oxidation of the network was observed. In this study, the first stage continued up to 300oC and second stage was observed in the temperature range 300–700oC. At 700oC the all nanocomposite residues are upper the 50% , so that the thermal stability of specimens were enhanced. Futhermore, increasing clay content supplies higher residues of nanocomposites.

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Sample

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Table 2. TGA results of SEP-PFNCRs Residue (%) at 700 (oC)

PFR-SEP3%

55

PFR-SEP5%

56

PFR-SEP8%

57

PFR-SEP10%

57

PFR-SEP15%

58

PFR-SEP20%

58

PFR-SEP30%

59

PFR-SEP50%

64

Figure 6. TGA graphs of PFNCRs

3.4. FTIR Analysis To characterize the chemical structure of PFR and SEP-PFNCRs, the samples were subjected to FTIR analyses.

Figure 7. FTIR of PFR and SEP-PFRNCRs

The spectra were presented in Figures 7-8 and prominent peaks are identified. The FTIR spectrum of the PFR was given in Figure 8, and was recorded in the transmittance mode. The characteristic peaks of PFR were observed

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at 3284 cm-1, 2880 cm-1, 1610 cm-1, 1478 cm-1, 1208 cm-1, 1014 cm-1 corresponding to –OH groups, aliphatic C-H stretch, C=C stretch of phenolic ring, CH2 bending and phenolic C-O stretch respectively. In the spectrum of sepiolite, the absorption peaks at 1010 cm-1, 780 cm-1 are ascribed to the Si-O vibrating and bending of sepiolite, and the biggest peak at 974 cm-1 is the Si-O-Si vibrating characteristics of sepiolite. As shown in figure 7 in addition to the characteristic peaks of PFR, Si-O-Si vibrating peak of sepiolite was also seen in nanocomposites. The intensity of sepiolite peaks strengthened with increasing amount of SEP in the nanocomposites and the intensity of resol peaks decreased.

Figure 8. FTIR of sepiolite clay and resol resin.

4. Conclusion This is the first study that investigated and discussed the influence of the sepiolite clay in the resol resin. In FTIR analysis the intensities of resol peaks were decreased and the intensities of sepiolite peaks due to sepiolite loading. SEP-PFNCRs were synthesized with high clay percents and the problem of clay agglomeration did not occur due to needle like structure of sepiolite. SEM measurements confirmed that the nanocomposite resins with the clay content of 10wt% had a good dispersion of sepiolite clay. The XRD results showed the intercalated structure of SEPPFNCRs. Considerable thermal stability enhancement of resol matrix has been found by incorporation of sepiolite due to the good dispersion and strong interfacial interaction between silanol groups (Si–OH) of sepiolite and methylol groups of resol. The thermal stability of SEP-PFNCRs increased with the percent of sepiolite clay in matrix. With this study, the thermal resistant SEP-PFNCRs were synthesized. References Hsief, F., and Beeson, H.D. (1997). Flammability Testing of Flame Retarded Epoxy Composites and Phenolic Composites, Fire Mater, 21, 4149. Knop, A., and Scheib, W. (1979). Chemistry and Application of Phenolic Resins, Springer, New York. Gardziella, A., Pilato, L.A., and Knop, A. (2000). Phenolic Resins: Chemistry, Applications, Standardization, Safety and Ecology, 2nd Edition, Springer-Verlag, Berlin. Astarloa-Aierbe, G., Echeverria, J.M., Martin, M.D. Etxeberria, A.M., and Mondragon, I. (2002). Influence of the Temperature on the Formation of a Phenolic Resol Resin Catalyzed with Amine, Polymer, 43:8, 2239-2243. Astarloa-Aierbe, G., Echeverria, J.M., Vazquez, A., and Mondragon, I. (2000). Influence of the Amount of Catalyst and Initial pH on the Phenolic Resol Resin Formation, Polymer, 41:9, 3311-3315. Higuchi, M. (1999). Phenolic Resin: Alkali–Catalyzed Phenol–Formaldehyde Reactions, Journal of the Japan Wood Research Society, 45, 425– 433. Zmihorska-Gotfryd, A. (2004). Coating Compositions Based on Modified Phenol–Formaldehyde Resin and Urethane Prepolymers, Progress in Organic Coatings, 49, 109–114. Fan, D., Qin, T., Wang, C., and Chu, F. (2010). Evaluation of Forestry Residue-Source Oil-Tea Cake as an Extender for Phenol–Formaldehyde Plywood Adhesive, Forest Products Journal, 60, 610–614. Pilato, L. (2010). Phenolic Resins: A Century of Progress, Springer-Verlag Berlin Heidelberg.

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