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clay nanocomposites
G Model APSUSC-26935; No. of Pages 6
ARTICLE IN PRESS Applied Surface Science xxx (2014) xxx–xxx
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Investigation of thermal properties of PUF/clay nanocomposites C¸etin Do˘gar a , Ahmet Gürses b,∗ , Semra Karaca c , Sevda Köktepe a , Ferda Mindivan b , Kübra Günes¸ b a b c
Department of Science Education, Erzincan University, Erzincan 24030, Turkey Department of Chemistry Education, Ataturk University, Erzurum 25240, Turkey Department of Chemistry, Ataturk University, Erzurum 25240, Turkey
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 18 December 2013 Accepted 19 December 2013 Available online xxx Keywords: Clay Organoclay PUF/organoclay nanocomposite Thermal properties
a b s t r a c t This study aims to investigate the thermal properties of phenol–urea–formaldehyde (PUF)/organoclay nanocomposites synthesized at various organoclay ratios by using their DSC thermograms, FT-IR spectra, and HRTEM images. From these analyses, it has been concluded that at low organoclay ratios, the platelets exfoliated of organoclay, which dispersed homogenously in the polymer matrix, created a positive effect in conformational arrangement as suitable to crystallization of PUF chains. DSC analyses revealed that two endotherms in all thermograms imply the presence of linear UF, PF, and PUF chains together. Low organoclay content may also lead to a cross-linked lattice, being more predominant than the linear chain arrangement. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Phenolic resins have been under continuous development as an important thermosetting resin material after the first successful trial production of the synthetic resin. Phenolic resin molding compounds, which have been among the major applications of phenolic resins since their inception, exhibit highly favorable characteristics in terms of strength, heat resistance, long-term reliability, cost, superior mechanical strength, and dimensional stability, as well as, high resistance against various solvents, acids, water and therefore have been used in a wide range of applications [1–4]. However, a major drawback that has prevented wider use of phenol formaldehyde (PF) resins in the manufacture of impregnating resins and adhesives is their relatively low cure rate compared to other thermosetting adhesives such as urea–formaldehyde (UF) [4]. The use of urea–formaldehyde resin as a major adhesive by the forest products industry is due to low cost, low cure temperature, water solubility, and lack of color, high reactivity, good performance in the panel and ease of use under a wide variety of curing conditions [5–7]. Recently, urea component has been introduced into PF resins to form phenol–urea–formaldehyde (PUF) co-condensed resins for reducing the cost and improving the curing behavior of the resins [8]. One of the most used procedures for PUF synthesis is
∗ Corresponding author. Tel.: +90 442 2314004; fax: +90 442 2314004. E-mail address: [email protected] (A. Gürses).
the two-step polymerization process, by which the trimethylolphenol (TMeP) is formed in the first step (Scheme 1) [9]. In the second step of PUF synthesis, urea bonds are formed by the reaction of urea with TMeP (Scheme 2) [10]. The cross-linked structure of PUF resin is shown in Scheme 3 [11]. Although there are lot of studies on the PF and UF resins, the number of publications limited related to PUF resins. Traditionally, polymeric materials have been filled with synthetic or natural inorganic compounds in order to improve their properties (improved tensile strength and moduli, decreased thermal expansion coefficient, decreased gas permeability, increased swelling resistance, enhanced ionic conductivity, flammability, and other physical or mechanical properties) or simply to reduce cost. Conventional fillers are materials in the form of particles (e.g. calcium carbonate), fibers (e.g. glass fibers) or plate-shaped particles (e.g. mica). However, although conventionally filled or reinforced polymeric materials are widely used in various fields, it is often reported that the additions of these fillers have revealed some disadvantages such as weight increase, brittleness, and opacity. [12–18]. Clays have long been used as fillers in polymer systems because of low cost and improved mechanical properties of resulting polymer composites [19]. In recent years there has been a significant amount of work looking at using layered clay silicate (montmorillonite) fillers to improve thermal stability and mechanical properties of a wide variety of thermoset and thermoplastic resin systems [20]. Also among many nanocomposites studied and applied in industry very important are polymer nanocomposites with clay platelet nanofillers,
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Please cite this article in press as: C¸. Do˘gar, et al., Investigation of thermal properties of PUF/clay nanocomposites, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2013.12.110
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OH
OH
OH
CH2OH
CH2OH
CH 2OH
O +
H
+
C H
CH 2OH
OH
CH2OH
CH 2OH
Scheme 1. The formation of trimethylolphenol (TMeP) [9].
OH
O
CH2OH
O
O +
H 2N
C
NH2
HO3HC
NH
C
NH2
HO3HC HN
+
C
NH
HO3HC
Scheme 2. The reaction of urea with TMeP [10]. CH 2OH
NHCHONHCH
CH 2
OH
2
CH 2
OH HOH 2C CH 2
NHCONH
CH 2
CH 2
CH 2OH
CH 2OH
OH CH2
OH
NCONH
OH
CH2
CH 2
CH2
Scheme 3. The crosslinked structure of the PUF resin [11].
most frequently with montmorillonite (MMT) [21]. MMT is a multilayer silicate mineral that naturally possesses inorganic cations within its galleries to balance the charge of the oxide layers in a hydrophilic environment. The ion exchange of these cations with organic ammonium ions improves the compatibility of the modified montmorillonite with polymers and the dispersion of the layers into the matrix. Moreover the modified clay can react or interact with the monomer or the polymer to reduce the interfacial strength between clay nanolayers and the polymer matrix [22–26]. The aim of this study is to investigate the thermal properties of PUF/organoclay nanocomposites as function of ratio of organoclay by using various techniques such as DSC, FT-IR, and HRTEM. 2. Experimental 2.1. Materials Solid urea, phenol, and aqueous solution of formaldehyde (37%), NaOH, and HCl which were used in the synthesis of PUF composites,
supplied from Merck. Also, commercial organoclay was purchased from Sigma–Aldrich (Cat. No: 682624). XRD diffractogram for the commercial organoclay is given in Fig. 1. 2.2. Preparation of phenol–urea–formaldehyde (PUF) composites In this study, resin/clay composites were prepared by in situ polymerization of phenol (18. 2%)–urea (36.3%)–formaldehyde (45.5%) (phenol-urea-formaldehyde, PUF) and using the various ratios of organoclay (7.69–45.45%) that is commercial. Phenol–urea–formaldehyde composites are coded as OPUF 2-11. The preparation procedure was as follows: phenol was mixed with formaldehyde (as a 37% formalin solution) and urea in a glass reactor equipped with a mechanical stirrer, a thermometer, and a reflux condenser (see Fig. 2) and then commercial organoclay in a given ratio was added to the mixture at 70 ◦ C. Finally, the resultant mixture was stirred for 1 h and the viscous mixture following after adding a few drops of concentrated HCl was pelleted mechanically in 2.0 tons/cm2 . Their compositions are given in Table 1.
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nitrogen atmosphere. A typical sample weight was about 10 mg and the scan speed was 20 ◦ C/min.
3. Results and discussion 3.1. FT-IR analysis
Fig. 1. XRD diffractogram for the commercial organoclay.
Fig. 2. The experimental set-up.
2.3. Characterization of OPUF composites Many techniques, such as FT-IR, HRTEM, and DSC have been incorporated to characterize the composites and to evaulate their thermal stabilities in this study. FT-IR spectra for the commercial organoclay and the composites were taken on by using a Perkin–Elmer Spectrum-One and KBr pelleting method for a range of 4000–400 cm−1 at a scanning rate of 2◦ /min. All measurements were obtained by an average of 100 scans and a resolution of 1 cm−1 . In order to estimate the framework rearrangement of composites, the specimens from the samples were examined by using a JEOL 2100 high resolution transmission electron microscope (LaB6 filament) operated at 200 Kv. The curing behaviors of PUF resin and OPUF composites were observed with a differential scanning calorimeter (DSC7020) under
The analysis results for FT-IR spectra for virgin resin, commercial organoclays and the composite in various contents are examined (see Table 1). Fig. 3 represents FT-IR spectra at various ratios of commercial organoclay. The characteristic absorbencies of the phenolic resin and urea– formaldehyde resin are assigned taking account the data in literature [27–30]. The peaks at 880–760 cm−1 are attributed to NH out-of-plane; para substituted, CH out of plane; ortho-substituted, respectively. These peaks show that out of phase stretching vibration of NH on the clay. The peaks at 1110 and 1050 cm−1 are attributed to asymmetric stretching vibration of C O C aliphatic ether and the single bond C O stretching vibrations of CH2 OH group respectively. In Fig. 3, the peaks of C O C and C O bonds shifted from 1110 to 1120 cm−1 and from 1050 to 1080 cm−1 , respectively. The reason of these shifts is ortho-para linkages for C O vibrations. The peak at 1375–1350 cm–1 is associated with bending vibration in plane CH in CH2 /CH2 OH/N CH2 N. These vibrations decreased with increase in the amount of organoclay, that is, with energy increased corresponding to this vibration. The peak at 1610–1594 cm−1 corresponding to the C C aromatic ring peak shifted to higher energy bands with increase in the amount of organoclay. This indicates that the linkages on phenol rings may occur in three different edges, leading to the formation of more rigid texture. Zhou et al. [4] studied high-performance thermoset polymer composites are synthesized by using both long fibers and nanoclays. Mechanical and thermal properties of synthesized composites are compared with both long-fiberreinforced composites and polymer layered silicate composites. Every phenol ring can be a crosslinking site because it is a trifunctional molecule. Therefore, a large number of crosslinks exist in the phenolic resin, resulting in a rigid structure which is agreed with DSC results of composites. Even though, the presence of nanoclays may affect some crosslinks in the phenolic resin. The peak (1680 cm−1 ) appeared from C O vibration in CONH2 in the higher ratio of organoclay can be correlated with the presence of a few monomeric urea. The peaks at 2970–2874 cm−1 are associated with in phase stretching vibration of CH2 -alkane, out of phase stretching vibration of CH2 -alkane, respectively. The peak intensities of all composites, except for OPUF6 increase with increased organoclays content. This situation can be explained to the aggregation and not better dispersion of clay particles in the matrix.
Table 1 The compositions of PUF composites including commercial organoclay. Samples
Phenol (wt.%)
Urea (wt.%)
Formaldehyde (wt.%)
Organoclay (wt.%)
PUF1 OPUF2 OPUF3 OPUF4 OPUF5 OPUF6 OPUF7 OPUF8 OPUF9 OPUF10 OPUF11
18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2
36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3
45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5
– 7.69 14.3 20.0 25.0 29.4 33.3 36.8 40.0 42.85 45.45
Fig. 3. FT-IR spectra for composite samples coded as OPUF, and PUF1 resin.
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Fig. 4. Thermograms for composite samples coded as OPUF, and PUF1 resin.
3.2. DSC analysis The characteristic temperature range of the DSC curves gives valuable information on the cure reaction of resins [5]. DSC curves for PUF resin without and with addition of organoclay are shown in Fig. 4. Thermogram for PUF1 consisted of two peaks indicating curing reactions in the temperature range from 20 to 280 ◦ C. It is known that there were two dominant reactions involved, the first being the addition reaction and then the condensation reaction. The first peak corresponded to the addition reaction between formaldehyde and the aromatic ring or urea, while the second peak corresponded to the co-condensation and condensation reactions between methylolphenol and methylolurea groups [12]. The first peak is almost absent in the DSC curves of PUF resin, suggesting that only condensation reactions occur in the curing process of this PUF resin. From this it can be concluded that urea successfully reacted with free formaldehyde indicating that the addition reactions almost completed and that the condensation reactions are the main reactions during the curing of PUF1 [12]. The composites coded as OPUF2, OPUF3, OPUF4, OPUF5, and OPUF6 exhibits the two well-separated endothermic peaks, crystallization, and melting peaks at their thermograms. From the thermograms belonging to OPUF9, OPUF10, and OPUF11, it can be seen that broad endotherms indicating the degradation of polymer matrix have emerged. Some thermodynamic parameters (Tm and H) from DSC scans for PUF1 resin and OPUF composites samples were shown in Table 2. First peak shows the reaction which formed by urea–formaldehyde linear polymer chains. This reaction is dominant at composites of OPUF3, OPUF5, OPUF6, OPUF9, Table 2 Some parameters from DSC scans for composite samples coded as OPUF, and PUF1 resin. Samples
Tm1 (◦ C)
H1 (mJ/mg)
Tm2 (◦ C)
H2 (mJ/mg)
PUF1 OPUF2 OPUF3 OPUF4 OPUF5 OPUF6 OPUF7 OPUF8 OPUF9 OPUF10 OPUF11
112.9 130.7 128.3 181.1 127.8 126.4 108.7 125.0 129.5 129.8 99.9
229.0 163.0 175.0 18.7 161.0 159.0 104.0 152.0 280.0 158.0 39.3
250.0 250.3 248.4 242.3 248.6 258.9 242.8 240.5 241.9 262.6
31.0 22.2 12.4 13.1 7.08 256.0 8.54 17.3 11.9 207.0
OPUF10, and OPUF11. Second one is attributed to formation of cross-linked structures. PUF1 resin exhibits such a peak between 250 and 260 ◦ C and it shifts to about 230 ◦ C in presence of organoclay. As can be seen from Fig. 4 the peak temperature decreased with increase in the amount of organoclay, that is, this reaction occurred in the lower temperature compared to the polymerization reaction which will cause to more heterogeneous dispersion of clay platelets. Xu et al. investigated the curing reactions between epoxy groups and the curing agent, 4, 4-diaminodiphenylmethane (DDM) for DGEBA/PSAA-clay (diglycidyl ether of bisphenol A / polystyreneco-acrylic acid modified clay) composites and according to DSC data, one could have a conclusion that the exothermic transition of the small shoulder peak region in DGEBA/DDM/PSAA-clay composite is due to the reaction happening between the carboxylic acid groups in PSAA-clay and epoxy groups in DGEBA. In addition, this reaction occurred in the lower temperature compared to the polymerization reaction between the epoxy matrix and DDM, which will improve the dispersion of clay in the DGEBA In the case of DGEBA/pure clay, the slight decrease in the onset temperature compared to the pure epoxy has been observed which is attributed to the lowered crosslink density due to the addition of clay [31]. Fig. 4 shows thermograms for composite samples coded as OPUF, and PUF1 resin. It can be seen that the composites containing the higher amount of clay have the higher values of Tg, because of the confinement of the silicate layers and their ability to hinder the motion of the molecular chains and network junctions [32]. A slight decrease in Tg value for OPUF7, compared with that of OPUF9, OPUF10, and OPUF11 implies that cross-linking density in the resin matrix increased. The rigidity of polymer backbone is increased with increasing cross-linking density [32]. The FT-IR analysis results also indicate that the network arisen may be a rigid structure Also, it is known that the Tg values changed with various parameters such as the molecular weight, the stiffness and the free volume entrapped in the network [33]. The results show that a slight decrease of Tg for OPUF7 may due to the lower molecular weight, more flexible chain, and the large free volume. Zhou et al. [4] reported that for phenolic resins, the dramatic increase of the Tg is due to the strong interfacial interaction between the silicate nanolayers and phenolic matrix, which reduces the molecular mobility of the matrix molecules. The Tg may be unaffected significantly with a slight decrease of crosslinks. Crystallization and melting peaks for the composites OPUF2, OPUF3, OPUF4 appeared at almost same temperatures. The intensity of crystallization peaks for OPUF5 and OPUF6 decreased, compared to the others ones and their melting peaks shifted to the lower temperatures. The high melting peaks without considering the contribution of the lower temperature melting peaks (OPUF5 and OPUF6), which is due to the formation of thinner and less perfect crystals [34]. 3.3. HRTEM analysis Generally, the structure of nanocomposites has typically been established using transmission electron micrographic (TEM) observation. The nanocomposite structure (intercalated or exfoliated) may be identified. On the other hand, TEM allows a qualitative understanding of the internal structure, spatial distribution of the various phases, and views of the defect structure through direct visualization [14]. The microstructures of the OPUF composites were studied by HRTEM analysis. In the micrographs at Fig. 5(a–j), dark lines and gray clouds represent the clay layers, and the resin matrix, respectively [36]. HRTEM images of OPUF composites are shown in Fig. 5(a–j). From Fig. 5(a–e), it can be seen that the clay in the composites with low organoclay content exhibited a homogenous dispersion i.e. partially exfoliated dispersion. HRTEM images for the OPUF7-11(f–j)
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Fig. 5. HRTEM images for OPUF2 (a), OPUF3 (b), OPUF4(c), OPUF5 (d), OPUF6 (e), OPUF7 (f), OPUF8 (g), OPUF9 (h), OPUF10 (i), and OPUF11 (j).
show that intercalated and tactoidal structures heterogeneously distributed in the matrix. It also can be thought that the formation of large polymeric chain positively affected tactoidal dispersion of clay layers. HRTEM images (f–j) show clearly that the narrow and wide tactoids emerged. From Fig. 5, it can be seen that low ratio of organoclay affected positively the crystallization of polymer chains and the exfoliated platelets of the organoclay homogenously dispersed in the polymeric matrix [35]. The decrease in crystallinity with increase in the amount of organoclay is consistent with DSC and FT-IR results. 4. Conclusions This study investigates thermal properties of phenol–urea– formaldehyde (PUF)/organoclay nanocomposites synthesized at various organoclay ratios by using their DSC thermograms, FT-IR spectra and HRTEM images. The main findings of the study could be summarized as follows: At low organoclay ratios, the platelets exfoliated of organoclay, which dispersed homogenously in the polymer matrix, have created a positive effect in conformational arrangement as suitable to crystallization of PUF chains. The C C aromatic ring peak shifted to higher energy bands with increase in the amount of organoclay. The peak intensities of all composites, except for OPUF6 increased with increased organoclays content. DSC analyses revealed that two endotherms emerged in all thermograms imply the presence of linear UF, PF, and PUF chains together. Also,it can be said that low organoclay content may lead to a cross-linked lattice, being more predominant than the linear chain arrangement. HRTEM images show that at low organoclay content, partially exfoliated structure but at high organoclay content, heterogeneously distributed intercalated and tactoidal structures emerged. References [1] K. Hirano, M. Asami, Phenolic resins—100 years of progress and their future, React. Funct. Polym. 73 (2013) 256–269. [2] C.P.R. Nair, Thermal characteristics of addition-cure phenolic resins, Prog. Polym. Sci. 29 (2004) 401–498.
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Investigation of thermal properties of PUF/clay nanocomposites C¸etin Do˘gar a , Ahmet Gürses b,∗ , Semra Karaca c , Sevda Köktepe a , Ferda Mindivan b , Kübra Günes¸ b a b c
Department of Science Education, Erzincan University, Erzincan 24030, Turkey Department of Chemistry Education, Ataturk University, Erzurum 25240, Turkey Department of Chemistry, Ataturk University, Erzurum 25240, Turkey
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 18 December 2013 Accepted 19 December 2013 Available online xxx Keywords: Clay Organoclay PUF/organoclay nanocomposite Thermal properties
a b s t r a c t This study aims to investigate the thermal properties of phenol–urea–formaldehyde (PUF)/organoclay nanocomposites synthesized at various organoclay ratios by using their DSC thermograms, FT-IR spectra, and HRTEM images. From these analyses, it has been concluded that at low organoclay ratios, the platelets exfoliated of organoclay, which dispersed homogenously in the polymer matrix, created a positive effect in conformational arrangement as suitable to crystallization of PUF chains. DSC analyses revealed that two endotherms in all thermograms imply the presence of linear UF, PF, and PUF chains together. Low organoclay content may also lead to a cross-linked lattice, being more predominant than the linear chain arrangement. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Phenolic resins have been under continuous development as an important thermosetting resin material after the first successful trial production of the synthetic resin. Phenolic resin molding compounds, which have been among the major applications of phenolic resins since their inception, exhibit highly favorable characteristics in terms of strength, heat resistance, long-term reliability, cost, superior mechanical strength, and dimensional stability, as well as, high resistance against various solvents, acids, water and therefore have been used in a wide range of applications [1–4]. However, a major drawback that has prevented wider use of phenol formaldehyde (PF) resins in the manufacture of impregnating resins and adhesives is their relatively low cure rate compared to other thermosetting adhesives such as urea–formaldehyde (UF) [4]. The use of urea–formaldehyde resin as a major adhesive by the forest products industry is due to low cost, low cure temperature, water solubility, and lack of color, high reactivity, good performance in the panel and ease of use under a wide variety of curing conditions [5–7]. Recently, urea component has been introduced into PF resins to form phenol–urea–formaldehyde (PUF) co-condensed resins for reducing the cost and improving the curing behavior of the resins [8]. One of the most used procedures for PUF synthesis is
∗ Corresponding author. Tel.: +90 442 2314004; fax: +90 442 2314004. E-mail address: [email protected] (A. Gürses).
the two-step polymerization process, by which the trimethylolphenol (TMeP) is formed in the first step (Scheme 1) [9]. In the second step of PUF synthesis, urea bonds are formed by the reaction of urea with TMeP (Scheme 2) [10]. The cross-linked structure of PUF resin is shown in Scheme 3 [11]. Although there are lot of studies on the PF and UF resins, the number of publications limited related to PUF resins. Traditionally, polymeric materials have been filled with synthetic or natural inorganic compounds in order to improve their properties (improved tensile strength and moduli, decreased thermal expansion coefficient, decreased gas permeability, increased swelling resistance, enhanced ionic conductivity, flammability, and other physical or mechanical properties) or simply to reduce cost. Conventional fillers are materials in the form of particles (e.g. calcium carbonate), fibers (e.g. glass fibers) or plate-shaped particles (e.g. mica). However, although conventionally filled or reinforced polymeric materials are widely used in various fields, it is often reported that the additions of these fillers have revealed some disadvantages such as weight increase, brittleness, and opacity. [12–18]. Clays have long been used as fillers in polymer systems because of low cost and improved mechanical properties of resulting polymer composites [19]. In recent years there has been a significant amount of work looking at using layered clay silicate (montmorillonite) fillers to improve thermal stability and mechanical properties of a wide variety of thermoset and thermoplastic resin systems [20]. Also among many nanocomposites studied and applied in industry very important are polymer nanocomposites with clay platelet nanofillers,
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OH
OH
OH
CH2OH
CH2OH
CH 2OH
O +
H
+
C H
CH 2OH
OH
CH2OH
CH 2OH
Scheme 1. The formation of trimethylolphenol (TMeP) [9].
OH
O
CH2OH
O
O +
H 2N
C
NH2
HO3HC
NH
C
NH2
HO3HC HN
+
C
NH
HO3HC
Scheme 2. The reaction of urea with TMeP [10]. CH 2OH
NHCHONHCH
CH 2
OH
2
CH 2
OH HOH 2C CH 2
NHCONH
CH 2
CH 2
CH 2OH
CH 2OH
OH CH2
OH
NCONH
OH
CH2
CH 2
CH2
Scheme 3. The crosslinked structure of the PUF resin [11].
most frequently with montmorillonite (MMT) [21]. MMT is a multilayer silicate mineral that naturally possesses inorganic cations within its galleries to balance the charge of the oxide layers in a hydrophilic environment. The ion exchange of these cations with organic ammonium ions improves the compatibility of the modified montmorillonite with polymers and the dispersion of the layers into the matrix. Moreover the modified clay can react or interact with the monomer or the polymer to reduce the interfacial strength between clay nanolayers and the polymer matrix [22–26]. The aim of this study is to investigate the thermal properties of PUF/organoclay nanocomposites as function of ratio of organoclay by using various techniques such as DSC, FT-IR, and HRTEM. 2. Experimental 2.1. Materials Solid urea, phenol, and aqueous solution of formaldehyde (37%), NaOH, and HCl which were used in the synthesis of PUF composites,
supplied from Merck. Also, commercial organoclay was purchased from Sigma–Aldrich (Cat. No: 682624). XRD diffractogram for the commercial organoclay is given in Fig. 1. 2.2. Preparation of phenol–urea–formaldehyde (PUF) composites In this study, resin/clay composites were prepared by in situ polymerization of phenol (18. 2%)–urea (36.3%)–formaldehyde (45.5%) (phenol-urea-formaldehyde, PUF) and using the various ratios of organoclay (7.69–45.45%) that is commercial. Phenol–urea–formaldehyde composites are coded as OPUF 2-11. The preparation procedure was as follows: phenol was mixed with formaldehyde (as a 37% formalin solution) and urea in a glass reactor equipped with a mechanical stirrer, a thermometer, and a reflux condenser (see Fig. 2) and then commercial organoclay in a given ratio was added to the mixture at 70 ◦ C. Finally, the resultant mixture was stirred for 1 h and the viscous mixture following after adding a few drops of concentrated HCl was pelleted mechanically in 2.0 tons/cm2 . Their compositions are given in Table 1.
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nitrogen atmosphere. A typical sample weight was about 10 mg and the scan speed was 20 ◦ C/min.
3. Results and discussion 3.1. FT-IR analysis
Fig. 1. XRD diffractogram for the commercial organoclay.
Fig. 2. The experimental set-up.
2.3. Characterization of OPUF composites Many techniques, such as FT-IR, HRTEM, and DSC have been incorporated to characterize the composites and to evaulate their thermal stabilities in this study. FT-IR spectra for the commercial organoclay and the composites were taken on by using a Perkin–Elmer Spectrum-One and KBr pelleting method for a range of 4000–400 cm−1 at a scanning rate of 2◦ /min. All measurements were obtained by an average of 100 scans and a resolution of 1 cm−1 . In order to estimate the framework rearrangement of composites, the specimens from the samples were examined by using a JEOL 2100 high resolution transmission electron microscope (LaB6 filament) operated at 200 Kv. The curing behaviors of PUF resin and OPUF composites were observed with a differential scanning calorimeter (DSC7020) under
The analysis results for FT-IR spectra for virgin resin, commercial organoclays and the composite in various contents are examined (see Table 1). Fig. 3 represents FT-IR spectra at various ratios of commercial organoclay. The characteristic absorbencies of the phenolic resin and urea– formaldehyde resin are assigned taking account the data in literature [27–30]. The peaks at 880–760 cm−1 are attributed to NH out-of-plane; para substituted, CH out of plane; ortho-substituted, respectively. These peaks show that out of phase stretching vibration of NH on the clay. The peaks at 1110 and 1050 cm−1 are attributed to asymmetric stretching vibration of C O C aliphatic ether and the single bond C O stretching vibrations of CH2 OH group respectively. In Fig. 3, the peaks of C O C and C O bonds shifted from 1110 to 1120 cm−1 and from 1050 to 1080 cm−1 , respectively. The reason of these shifts is ortho-para linkages for C O vibrations. The peak at 1375–1350 cm–1 is associated with bending vibration in plane CH in CH2 /CH2 OH/N CH2 N. These vibrations decreased with increase in the amount of organoclay, that is, with energy increased corresponding to this vibration. The peak at 1610–1594 cm−1 corresponding to the C C aromatic ring peak shifted to higher energy bands with increase in the amount of organoclay. This indicates that the linkages on phenol rings may occur in three different edges, leading to the formation of more rigid texture. Zhou et al. [4] studied high-performance thermoset polymer composites are synthesized by using both long fibers and nanoclays. Mechanical and thermal properties of synthesized composites are compared with both long-fiberreinforced composites and polymer layered silicate composites. Every phenol ring can be a crosslinking site because it is a trifunctional molecule. Therefore, a large number of crosslinks exist in the phenolic resin, resulting in a rigid structure which is agreed with DSC results of composites. Even though, the presence of nanoclays may affect some crosslinks in the phenolic resin. The peak (1680 cm−1 ) appeared from C O vibration in CONH2 in the higher ratio of organoclay can be correlated with the presence of a few monomeric urea. The peaks at 2970–2874 cm−1 are associated with in phase stretching vibration of CH2 -alkane, out of phase stretching vibration of CH2 -alkane, respectively. The peak intensities of all composites, except for OPUF6 increase with increased organoclays content. This situation can be explained to the aggregation and not better dispersion of clay particles in the matrix.
Table 1 The compositions of PUF composites including commercial organoclay. Samples
Phenol (wt.%)
Urea (wt.%)
Formaldehyde (wt.%)
Organoclay (wt.%)
PUF1 OPUF2 OPUF3 OPUF4 OPUF5 OPUF6 OPUF7 OPUF8 OPUF9 OPUF10 OPUF11
18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2
36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3 36.3
45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5
– 7.69 14.3 20.0 25.0 29.4 33.3 36.8 40.0 42.85 45.45
Fig. 3. FT-IR spectra for composite samples coded as OPUF, and PUF1 resin.
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Fig. 4. Thermograms for composite samples coded as OPUF, and PUF1 resin.
3.2. DSC analysis The characteristic temperature range of the DSC curves gives valuable information on the cure reaction of resins [5]. DSC curves for PUF resin without and with addition of organoclay are shown in Fig. 4. Thermogram for PUF1 consisted of two peaks indicating curing reactions in the temperature range from 20 to 280 ◦ C. It is known that there were two dominant reactions involved, the first being the addition reaction and then the condensation reaction. The first peak corresponded to the addition reaction between formaldehyde and the aromatic ring or urea, while the second peak corresponded to the co-condensation and condensation reactions between methylolphenol and methylolurea groups [12]. The first peak is almost absent in the DSC curves of PUF resin, suggesting that only condensation reactions occur in the curing process of this PUF resin. From this it can be concluded that urea successfully reacted with free formaldehyde indicating that the addition reactions almost completed and that the condensation reactions are the main reactions during the curing of PUF1 [12]. The composites coded as OPUF2, OPUF3, OPUF4, OPUF5, and OPUF6 exhibits the two well-separated endothermic peaks, crystallization, and melting peaks at their thermograms. From the thermograms belonging to OPUF9, OPUF10, and OPUF11, it can be seen that broad endotherms indicating the degradation of polymer matrix have emerged. Some thermodynamic parameters (Tm and H) from DSC scans for PUF1 resin and OPUF composites samples were shown in Table 2. First peak shows the reaction which formed by urea–formaldehyde linear polymer chains. This reaction is dominant at composites of OPUF3, OPUF5, OPUF6, OPUF9, Table 2 Some parameters from DSC scans for composite samples coded as OPUF, and PUF1 resin. Samples
Tm1 (◦ C)
H1 (mJ/mg)
Tm2 (◦ C)
H2 (mJ/mg)
PUF1 OPUF2 OPUF3 OPUF4 OPUF5 OPUF6 OPUF7 OPUF8 OPUF9 OPUF10 OPUF11
112.9 130.7 128.3 181.1 127.8 126.4 108.7 125.0 129.5 129.8 99.9
229.0 163.0 175.0 18.7 161.0 159.0 104.0 152.0 280.0 158.0 39.3
250.0 250.3 248.4 242.3 248.6 258.9 242.8 240.5 241.9 262.6
31.0 22.2 12.4 13.1 7.08 256.0 8.54 17.3 11.9 207.0
OPUF10, and OPUF11. Second one is attributed to formation of cross-linked structures. PUF1 resin exhibits such a peak between 250 and 260 ◦ C and it shifts to about 230 ◦ C in presence of organoclay. As can be seen from Fig. 4 the peak temperature decreased with increase in the amount of organoclay, that is, this reaction occurred in the lower temperature compared to the polymerization reaction which will cause to more heterogeneous dispersion of clay platelets. Xu et al. investigated the curing reactions between epoxy groups and the curing agent, 4, 4-diaminodiphenylmethane (DDM) for DGEBA/PSAA-clay (diglycidyl ether of bisphenol A / polystyreneco-acrylic acid modified clay) composites and according to DSC data, one could have a conclusion that the exothermic transition of the small shoulder peak region in DGEBA/DDM/PSAA-clay composite is due to the reaction happening between the carboxylic acid groups in PSAA-clay and epoxy groups in DGEBA. In addition, this reaction occurred in the lower temperature compared to the polymerization reaction between the epoxy matrix and DDM, which will improve the dispersion of clay in the DGEBA In the case of DGEBA/pure clay, the slight decrease in the onset temperature compared to the pure epoxy has been observed which is attributed to the lowered crosslink density due to the addition of clay [31]. Fig. 4 shows thermograms for composite samples coded as OPUF, and PUF1 resin. It can be seen that the composites containing the higher amount of clay have the higher values of Tg, because of the confinement of the silicate layers and their ability to hinder the motion of the molecular chains and network junctions [32]. A slight decrease in Tg value for OPUF7, compared with that of OPUF9, OPUF10, and OPUF11 implies that cross-linking density in the resin matrix increased. The rigidity of polymer backbone is increased with increasing cross-linking density [32]. The FT-IR analysis results also indicate that the network arisen may be a rigid structure Also, it is known that the Tg values changed with various parameters such as the molecular weight, the stiffness and the free volume entrapped in the network [33]. The results show that a slight decrease of Tg for OPUF7 may due to the lower molecular weight, more flexible chain, and the large free volume. Zhou et al. [4] reported that for phenolic resins, the dramatic increase of the Tg is due to the strong interfacial interaction between the silicate nanolayers and phenolic matrix, which reduces the molecular mobility of the matrix molecules. The Tg may be unaffected significantly with a slight decrease of crosslinks. Crystallization and melting peaks for the composites OPUF2, OPUF3, OPUF4 appeared at almost same temperatures. The intensity of crystallization peaks for OPUF5 and OPUF6 decreased, compared to the others ones and their melting peaks shifted to the lower temperatures. The high melting peaks without considering the contribution of the lower temperature melting peaks (OPUF5 and OPUF6), which is due to the formation of thinner and less perfect crystals [34]. 3.3. HRTEM analysis Generally, the structure of nanocomposites has typically been established using transmission electron micrographic (TEM) observation. The nanocomposite structure (intercalated or exfoliated) may be identified. On the other hand, TEM allows a qualitative understanding of the internal structure, spatial distribution of the various phases, and views of the defect structure through direct visualization [14]. The microstructures of the OPUF composites were studied by HRTEM analysis. In the micrographs at Fig. 5(a–j), dark lines and gray clouds represent the clay layers, and the resin matrix, respectively [36]. HRTEM images of OPUF composites are shown in Fig. 5(a–j). From Fig. 5(a–e), it can be seen that the clay in the composites with low organoclay content exhibited a homogenous dispersion i.e. partially exfoliated dispersion. HRTEM images for the OPUF7-11(f–j)
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Fig. 5. HRTEM images for OPUF2 (a), OPUF3 (b), OPUF4(c), OPUF5 (d), OPUF6 (e), OPUF7 (f), OPUF8 (g), OPUF9 (h), OPUF10 (i), and OPUF11 (j).
show that intercalated and tactoidal structures heterogeneously distributed in the matrix. It also can be thought that the formation of large polymeric chain positively affected tactoidal dispersion of clay layers. HRTEM images (f–j) show clearly that the narrow and wide tactoids emerged. From Fig. 5, it can be seen that low ratio of organoclay affected positively the crystallization of polymer chains and the exfoliated platelets of the organoclay homogenously dispersed in the polymeric matrix [35]. The decrease in crystallinity with increase in the amount of organoclay is consistent with DSC and FT-IR results. 4. Conclusions This study investigates thermal properties of phenol–urea– formaldehyde (PUF)/organoclay nanocomposites synthesized at various organoclay ratios by using their DSC thermograms, FT-IR spectra and HRTEM images. The main findings of the study could be summarized as follows: At low organoclay ratios, the platelets exfoliated of organoclay, which dispersed homogenously in the polymer matrix, have created a positive effect in conformational arrangement as suitable to crystallization of PUF chains. The C C aromatic ring peak shifted to higher energy bands with increase in the amount of organoclay. The peak intensities of all composites, except for OPUF6 increased with increased organoclays content. DSC analyses revealed that two endotherms emerged in all thermograms imply the presence of linear UF, PF, and PUF chains together. Also,it can be said that low organoclay content may lead to a cross-linked lattice, being more predominant than the linear chain arrangement. HRTEM images show that at low organoclay content, partially exfoliated structure but at high organoclay content, heterogeneously distributed intercalated and tactoidal structures emerged. References [1] K. Hirano, M. Asami, Phenolic resins—100 years of progress and their future, React. Funct. Polym. 73 (2013) 256–269. [2] C.P.R. Nair, Thermal characteristics of addition-cure phenolic resins, Prog. Polym. Sci. 29 (2004) 401–498.
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