reactive kaolinite composites

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Preparation and characterization of urea-formaldehyde resin/reactive kaolinite composites Shiwei Chen a,b , Xuchen Lu a,c,∗ , Tizhuang Wang a , Zhimin Zhang a a b c

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China United Research Center for Resources and Materials, Wuhai 016000, China

a r t i c l e

i n f o

Article history: Received 7 January 2015 Received in revised form 28 May 2015 Accepted 30 May 2015 Keywords: Composites Reactive kaolinite Urea-formaldehyde resin Morphology Thermal property

a b s t r a c t Novel urea-formaldehyde resin/reactive kaolinite composites containing 20–40 wt% kaolinite were prepared by in situ polymerization. The kaolinite was modified with tetraethoxysilane and a silane coupling agent to introduce reactive groups. Fourier-transform infrared spectroscopy and X-ray diffraction confirmed preparation of the urea-formaldehyde resin/reactive kaolinite composites. The composite morphology was investigated using scanning electron microscopy; the composites consisted of uniform spherical particles. The surface chemical components of the composites were determined using energy-dispersive X-ray spectroscopy. The spectra showed that the reactive kaolinite was encapsulated by the urea-formaldehyde resin. The thermal properties of the composites were examined using differential scanning calorimetry and thermogravimetric analysis. The results showed that their thermal stability was much better than that of pure urea-formaldehyde resin. Reactive kaolinite addition greatly decreased formaldehyde emissions and improved the water resistance of the composites. A mechanism for urea-formaldehyde resin/reactive kaolinite composite synthesis is proposed. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Introduction Much research on polymer/inorganic composites has been performed, to improve the properties of traditional polymers and reduce the cost of polymeric materials. Many new materials with excellent properties such as high strain, high conductivity, and high water absorption have been successfully prepared (Ammala, Hill, Lawrence, & Tran, 2007; Chen et al., 2006; Podsiadlo et al., 2007; S¸en, 2010; Tunc & Duman, 2010; Tunc, Duman, & Uysal, 2008; Villanueva, Cabedo, Lagaron, & Gimenez, 2010; Yoon, Hunter, & Paul, 2003; Zhang et al., 2014). However, some drawbacks, including weight increase, brittleness, and opacity, are also encountered (Fischer, 2003; Gorrasi, Tortora, Vittoria, Galli, & Chiellini, 2002; Varlot, Reynaud, Kloppfer, Vigier, & Varlet, 2001). Scientists have now begun to study composites that have superior properties but retain their original characteristics such as opacity (Chafidz, Ali,

∗ Corresponding author at: Chinese Academy of Sciences, Institute of Process Engineering, No. 1 Bei-er-tiao, Zhong-guan-cun, Beijing, China. Tel.: +86 10 82544889; fax: +86 10 62561822. E-mail address: [email protected] (X. Lu).

Mohsin, Elleithy, & Al-Zahrani, 2012; Misra, Raney, De Nardo, Craig, & Daraio, 2011; Nakane, Kurita, Ogihara, & Ogata, 2004; Wheeler, Wang, & Mathias, 2006; Zhang et al., 2011; Zhao, Zhang, Huang, & Wei, 2012). Urea-formaldehyde resins are widely used as adhesives, coating materials, synthetic woods, and mold powders, because of their panel properties, including fast adhesive curing, water solubility, and high reactivity. The main disadvantages of ureaformaldehyde resins are high formaldehyde emissions, low resistance to humid conditions, and poor thermal stability. Stricter control of the resin properties, especially in terms of thermal stability, formaldehyde emissions, and water resistance, is needed to enable expansion of their applications. Much research has been performed on enhancement of the thermal stability and mechanical properties. Effective methods for improving the properties of these resins include adding organic monomers such as melamine, trimethoxymethylmelamine, and dimethoxymethylmelamine, or inorganic additives such as carbon ´ nanotubes and silicates (Roumeli et al., 2012; Samarˇzija-Jovanovic, ´ Konstantinovic, ´ Markovic, ´ & Marinovic-Cincovi ´ ´ 2011; Jovanovic, c, Xu, Tang, Gu, Fang, & Tong, 2007; Zorba, Papadopoulou, Hatjiissaak, Paraskevopoulos, & Chrissafis, 2008). However, to the best of our

http://dx.doi.org/10.1016/j.partic.2015.05.007 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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knowledge, there are no reports of thermal stability enhancement, formaldehyde emission reduction, and water resistance improvement of urea-formaldehyde resins using kaolinites. Kaolinites are 1:1 layered silicates; they are readily available and widely used. The layers are held together via hydrogen bonds, dipolar interactions, and attractive van der Waals forces, resulting in small interlamellar spaces, in which it is hard for organic materials to intercalate. Although polymer/kaolinite composites have been prepared using ion-exchange reactions, industrialization is difficult and challenging (Chafidz, Ali, & Elleithy, 2011; Letaief & Detellier, 2007; Letaief, Diaco, Pell, Gorelsky, & Detellier, 2008; Tonle, Letaief, Ngameni, & Detellier, 2009). Another method for polymer/kaolinite composite synthesis is surface modification, in which a coupling agent is covalently bonded to the surface of the clay to make the kaolinite compatible with polymers. Typical examples are polyimide/kaolinite, poly(methyl methacrylate)/kaolinite, and polybutadiene/kaolinite (Piscitelli et al., 2010; Wheeler et al., 2006; Xu et al., 2007; Zhang, Liu, Zhang, Cheng, & Lu, 2012). Recently, many composites with outstanding properties have been synthesized; the amounts of clay in the composites were low, ranging from 1 wt% to 5 wt% (Khezri, Haddadi-Asl, Roghani-Mamaqani, & Salami-Kalajahi, 2012; Wang, Gao, Ma, & Agarwal, 2006). This work focuses on the preparation of urea-formaldehyde resin/reactive kaolinite composites by in situ polymerization. This is the first time that reactive kaolinites have been used to improve the properties of urea-formaldehyde resin; the urea formaldehyde resin/reactive kaolinite composites consisted of uniform spherical particles; the kaolinite content of the composite was up to 40 wt%. The aims of the research were to prepare urea-formaldehyde resin/reactive kaolinite composites with high thermal stabilities, strong water resistance, and low formaldehyde emissions. In addition, we expect that a high kaolinite content will reduce the cost of the composites.

Experimental Materials Kaolinite particles of thickness 20–50 nm, average diameter 400 nm, and specific surface area 32 m2 /g were supplied by the San Xing High-New Material Company (Zaozhuang, China). (3-Aminopropyl)triethoxysilane (KH550) and tetraethoxysilane (TEOS, chemical grade) were purchased from the Beijing Chemical Reagents Company (Beijing, China). Sodium hydroxide, urea, formalin (37 wt%), acetone, ammonia (28 wt%), and anhydrous ethanol (99.5%) were of analytical grade were purchased from Beijing Chemical Reagents Company (Beijing, China). Deionized water (18.2 M cm) was used in all experiments.

Preparation of reactive kaolinite The desired amount of dried kaolinite and anhydrous ethanol (100.0 mL) were placed in a ball grinding mill and ball-milled for 1 h. The mixture was transferred to a 250 mL three-necked roundbottomed flask containing ammonia (18.4 mL). After stirring at 60 ◦ C for 1 h, TEOS (10.0 mL) was added, and the mixture was stirred for 6 h. The slurry was filtered and washed several times with ethanol. The solid residue was dispersed in ethanol and hydrolyzed KH550 (5.0 mL) was added, with continuous stirring for 1 h. The slurry was filtered, washed, dried at 110 ◦ C for 12 h and crushed to a powder. The product yield was 96%. The product is denoted by KT-kaolinite. For comparison, pristine kaolinite (denoted by P-kaolinite) was directly modified with hydrolyzed KH550; the product is denoted

by K-kaolinite. The product yield (actual output/theoretical output) was 90%. Preparation of urea-formaldehyde resin/reactive kaolinite composites Urea (3.0 g), formaldehyde (3.6 mL), and deionized water (25.0 mL) were placed in a three-necked round-bottomed flask. The pH of the solution was adjusted to 8–9 with sodium carbonate, followed by continuous stirring for 1 h at 75 ◦ C. The product is denoted by pre-UF. The desired amount of KT-kaolinite, deionized water (100.0 mL), and sodium dodecylbenzenesulfonate (2.0 g) were placed in a three-necked round-bottomed flask and the mixture was stirred for 2 h at room temperature. The pre-UF was transferred to the flask at 60 ◦ C, followed by continuous stirring for 3 h. The slurry was filtered, washed several times with water, and dried at 120 ◦ C for 12 h. The product yield was 92%. The product is denoted by KT–UF. The products containing KT-kaolinite of 20, 30, and 40 wt% are denoted by KT–UF 20%, KT–UF 30%, and KT–UF 40%, respectively. Pure urea-formaldehyde resin and urea-formaldehyde resin/pristine kaolinite composite containing 30 wt% pristine kaolinite were also prepared; they are denoted by pure UF and P–UF 30%, respectively. Preparation and testing of plywood Three-layer plywood panels of dimensions 300 mm × 300 mm × 1.5 mm were prepared using eucalyptus veneers. The veneers were dried to a moisture content of approximately 3% before use. The urea-formaldehyde resin/reactive kaolinite composites were mixed with 5 wt% ammonium chloride solution and applied to both sides of the veneer at a spreading rate of 350 g/m2 (to form two glue lines). The veneers were aged for 15 min to allow the composites to penetrate the veneers. The veneers were pre-pressed at room temperature and 1.0 MPa for 1 h, and then hot pressed at 120 ◦ C and 1.5 MPa for 4.5 min (1 min/mm). After hot pressing, the panels were cooled and conditioned at 20 ◦ C and (65 ± 2)% relative humidity until the weight was constant, to test the formaldehyde emissions and water resistance. Characterization Fourier-transform infrared (FTIR) spectroscopy was performed (Spectrum 1000, Perkin-Elmer, USA) in the region 400–4000 cm−1 with a resolution of 4 cm−1 and 32 scans. Specimens were prepared by grinding the sample with KBr. The X-ray diffraction (XRD) patterns were acquired using a Siemens D-500 diffractometer operated at 40 kV and 30 mA (1200 W), with filtered Cu K␣ radiation, at 2 from 5◦ to 60◦ and a scanning speed of 20◦ /min. The microstructures of the samples were examined using scanning electron microscopy (SEM, XL 30, FEI, USA; accelerating voltage 20 kV). The specimens were coated with a thin carbon film prior to SEM observations. Energy-dispersive X-ray spectroscope (EDS) coupled with SEM, which enables qualitative analysis, was used to determine the elements present in the samples. The thermal stabilities of the samples were determined by thermogravimetric analysis (TGA) under nitrogen at a heating rate of 20 ◦ C/min from 25 to 700 ◦ C using a TG-differential thermal analysis (DTA) instrument (STA449, Netzsch, Germany). The sample (more than 8 mg) was placed in an alumina crucible. An empty alumina crucible was used as a reference. Three-layer plywood test specimens of dimensions 10 mm × 50 mm × 150 mm were placed in a 10 L glass desiccator together with a Petri dish filled with a specified amount of

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Fig. 1. IR spectra of (a) P-kaolinite, K-kaolinite, and KT-kaolinite, and (b) KT–UF 30%.

deionized water. The formaldehyde emission tests were performed for 24 h at 20 ◦ C. The quantity of formaldehyde emitted was determined spectroscopically from the concentration of formaldehyde absorbed by the water. The results were recorded as average of the values obtained from three samples for each composition. Three-layer plywood test specimens of dimensions 10 mm × 50 mm × 150 mm were placed in boiling water to determine water resistance. The cracking times were measured. Five specimens of each composition were used for the measurements and average values were reported. Results and discussion IR spectroscopic analysis Fig. 1(a) shows that there were three new vibration bands, at 2964.0, 2856.2, and 1261.3 cm−1 in the FTIR spectra of K-kaolinite and KT-kaolinite compared with that of P-kaolinite. The vibration bands at 2964.0 and 2856.2 cm−1 are attributed to the C H and CH2 stretching vibrations, respectively, from KH550. The peak at 1261.3 cm−1 for K-kaolinite and KT-kaolinite corresponds to the stretching vibration of Si C from KH550. The increased intensities of the peaks at 2964.0, 2856.2, and 1261.3 cm−1 increased progressively, suggesting that more KH550 covered the kaolinite. These results show that TEOS was beneficial in the modification of kaolinite compared with KH550 (Zhang, Liu, Mark, & Noda, 2009; Zhang et al., 2012). This may be because TEOS molecules are smaller than those of KH550 and become more accessible to attached to the kaolinite surface more easily. Some of the hydroxyl groups in the

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Fig. 2. Powder XRD patterns (2 = 5–42◦ ) of (a) P-kaolinite and KT-kaolinite, and (b) pure UF, P–UF 30%, and KT–UF 30%.

hydrolyzed TEOS could react with hydroxyl groups located present on the kaolinite surface, and others could interact with hydroxyl groups in the silane coupling agent, and form a large network (Qian, Hu, Zhang, & Yang, 2008). The bands at 3694.8 and 3621.4 cm−1 are ascribed to the outer hydroxyl groups and inner hydroxyl groups, respectively, of kaolinite. The peak at 3350.5 cm−1 in Fig. 1(b) is attributed to stretching vibrations of N H groups in the backbone or at the ends of the polymer chains. The peak at 1637.1 cm−1 corresponds to the C O stretching vibration. The peak at 1562.1 cm−1 is assigned to the N H bending vibration. The peak at 1030.7 cm−1 is characteristic of the stretching vibration of C N groups in the polymer backbone. The peak at 1248.0 cm−1 is ascribed to the C O stretching vibration. The peak at 927.9 cm−1 in the spectra of P-kaolinite and KT-kaolinite was also present in the composite spectra. The results confirm the preparation of urea-formaldehyde resin/reactive kaolinite composites. XRD pattern analysis The structural characterization of the kaolinite samples were conducted using XRD. The XRD patterns of P-kaolinite and KTkaolinite are shown in Fig. 2(a). P-kaolinite is a layered silicate with a d0 0 1 diffraction peak at 12.3◦ . According to the Bragg equation, the corresponding interlayer spacing is around 7.2 nm. KT-kaolinite has the same d0 0 1 diffraction peak at 12.3◦ and the same interlayer spacing of around 7.2 nm, showing that the layered structure of kaolinite was maintained after silanization, and no organic

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Fig. 3. SEM images of (a) P-kaolinite, (b) KT-kaolinite, (c) pure UF, (d) P–UF 30%, and (e) KT–UF 30%.

molecules intercalated into the kaolinite interlayers (Letaief et al., 2008). There are no broad peaks in the KT-kaolinite pattern, and other kaolinite sharp diffraction peaks are still present, suggesting that the modified kaolinite did not undergo structural degradation (Cheng, Liu, Zhang, Yang, & Frost, 2010). All these results show that KH550 and TEOS did not change the kaolinite structure. KH550 and TEOS modified the kaolinite surface or edges, but did not intercalate into the kaolinite layers. Fig. 2(b) shows that pure UF gave a sharp diffraction peak at 22.2◦ and two broad peaks at 2 24.6◦ and 31.5◦ ; these peaks are typical of the XRD pattern of pure urea-formaldehyde resin reported in the literature (Arafa, Fares, & Barham, 2004). The d0 0 1 diffraction peak of kaolinite present in the XRD patterns of both P-UF 30% and KT–UF 30% further confirms formation of the urea-formaldehyde resin/reactive kaolinite composite.

a few regular particles were observed. P–UF 30% contained both kaolinite particles and pure UF particles, indicating that pristine kaolinite was not encapsulated by the urea-formaldehyde resin. KT–UF 30% consisted of uniform round particles. The diameters of the particles were mainly 1 ␮m. The surface appearance and particle sizes show that the KT–UF 30% morphology is clearly different from those of P-kaolinite and pure UF. SEM results showed that the urea-formaldehyde resin/reactive kaolinite composites consisted of spherical particles. The principal components of the samples, i.e., Si, Al, N, and O, were detected at specific points in all the clay samples. As the data in Table 1 show, the contents of Si and O in the KTkaolinite were higher than that in the P-kaolinite, and N was present in KT-kaolinite, suggesting that KH550 covered the kaolinite surface. The amounts of N and O in pure UF were nearly equivalent to those in KT–UF 30%, showing that the composite shell

Sample morphology and elemental analysis Fig. 3(a) and (b) shows SEM images of P-kaolinite and KTkaolinite. The P-kaolinite particles were of different sizes and irregularly shapes. The KT-kaolinite surface was covered with many small particles. These small particles are organic materials such as TEOS or KH550. Fig. 3(c), (d) and (e) shows SEM images of pure UF, P–UF 30%, and KT–UF 30%. The pure UF particles varied in size and shape;

Table 1 Elemental contents (wt%) of samples. Sample

Al

Si

O

N

Total

P-kaolinite KT-kaolinite Pure UF KT–UF 30%

22.63 10.36 1.06

26.76 33.97 2.45

50.61 51.34 75.09 72.43

4.33 24.91 24.06

100 100 100 100

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Table 2 Formaldehyde emissions (mg/L) from plywood samples with different UF additives. Pure UF

KT–UF 20%

KT–UF 30%

KT–UF 40%

4.2

1.5

1.7

2.1

Table 3 Plywood sample cracking times (min).

Fig. 4. (a) TG and (b) DTA curves of pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40%, obtained under N2 at a heating rate of 20 ◦ C/min.

was mainly urea-formaldehyde resin. In addition, Al and Si were detected on the KT–UF 30% surface; the amounts were 1.06% and 2.45 wt%, respectively, compared with 10.36% and 33.97%, respectively, on KT-kaolinite. This shows that most of the KT-kaolinite was encapsulated by urea-formaldehyde resin, and again confirmed the preparation of urea-formaldehyde resin/reactive kaolinite composites.

Pure UF

KT–UF 20%

KT–UF 30%

KT–UF 40%

50

97

83

74

and 209.4 ◦ C, respectively. The decomposition temperatures of the composites for 5% weight loss were up to 53.5 ◦ C higher than that of pure UF. The decomposition temperatures for 10% weight loss were 218.0, 237.8, 244.4, and 246.0 ◦ C, i.e., the decomposition temperatures for KT–UF 20%, KT–UF 30%, and KT–UF 40% were 19.8, 26.4, and 28.0 ◦ C higher, respectively, than that of pure UF. These results clearly show that the degradation temperature at which a certain level of weight loss occurred for the ureaformaldehyde resin/reactive kaolinite composites increased with increasing amount of reactive kaolinite added. This shows that the thermal stabilities of the urea-formaldehyde resin/reactive kaolinite composites were better than that of pure UF, and the addition of KT-kaolinite improved the stability of the urea-formaldehyde resin. Possible reasons are as follows. First, because of the amino end groups, KT-kaolinite has better dispersion in, and compatibility with, the urea-formaldehyde resin. Second, KT-kaolinite can absorb heat or limit heat transfer. Third, KT-kaolinite and the network can prevent the release of molecules and escape of urea and formaldehyde from the composites, further decelerating polymer decomposition. The thermal stabilities of the urea-formaldehyde resin/reactive kaolinite composites clearly increased with increasing KT-kaolinite addition. This is because the presence of a larger number of reactive groups facilitates urea-formaldehyde resin interactions with KT-kaolinite, and enables construction of a composite network. Fig. 4(b) shows that when the temperature was above 200 ◦ C, the cured resin chains began to disassemble. The maximum endothermic peaks were attributed to the degradation of methylene ether bridges to methylene bridges and branching and cross-linking reactions in the resin network (Samarˇzija-Jovanovic´ et al., 2011; Siimer, Kaljuvee, & Christjanson, 2003). The peak temperatures for pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40% were 261.9, 276.6, 277.1, and 279.1 ◦ C, respectively, showing that the temperatures gradually increased and the thermal stability of the composites improved with increasing addition of KT-kaolinite; this is consistent with the TGA results.

Thermal analysis Improvement of composite stability and water resistance Fig. 4(a) shows the thermal curves of pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40% consisting of two main weight-loss processes. The first weight loss occurred at 50–200 ◦ C. The weight loss between 50 and 100 ◦ C is attributed to water evaporation. Low amounts of formaldehyde were released from pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40% in the temperature range 100–200 ◦ C, resulting in small weight loss from all the samples. The second mass loss for pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40% occurred above 200 ◦ C. The polymer chains were broken and the network was disassembled. The maximum degradation occurred when the stable methylene ether linkages were destroyed (Roumeli et al., 2012). As shown in Fig. 4(a), the 5% weight losses for pure UF, KT–UF 20%, KT–UF 30%, and KT–UF 40% occurred at 155.9, 195.5, 205.1,

Formaldehyde emission is an important parameter in practical applications of urea-formaldehyde resins. The formaldehyde emissions were measured, and the results are shown in Table 2. It is clear that the addition of reactive kaolinite reduced formaldehyde emissions. The reasons are as follows. First, the reactive kaolinite can absorb the formaldehyde released from the resin. Second, the network between the reactive kaolinite and the resin prevents formaldehyde from escaping. It is worth noting that the formaldehyde emission increased with increasing amount of reactive kaolinite. This may be because a larger amount of kaolinite would hinder the reaction between urea and formaldehyde, resulting in the presence of more formaldehyde molecules in the resins and increased formaldehyde emissions.

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Fig. 5. Scheme of preparation of urea-formaldehyde resin/reactive kaolinite composites. Preparation and characterization of urea-formaldehyde resin/reactive kaolinite composites.

The results of water resistance tests are shown in Table 3. The results show that reactive kaolinite improved the water resistance of the composites. This is because the addition of reactive kaolinite increases the chain stiffness, and the network between the reactive kaolinite and resin further strengthens the structural stability of the chains. The water resistance of the composites decreased with increasing addition of reactive kaolinite. This may be because of the formation of adhesion interfaces between the resin and the reactive kaolinite, and trapping of water at these interfaces. Furthermore, more aggregation occurred as the amount of reactive kaolinite increased, leading to formation of morphological defects, which could also trap water and reduce the water resistance. (SamarˇzijaJovanovic´ et al., 2011)

on the KT-kaolinite surface. Gradually, a urea-formaldehyde resin network is formed and encapsulates KT-kaolinite, giving regular urea-formaldehyde resin/reactive kaolinite composites; this was confirmed by FTIR and SEM/EDS. In addition, the amino groups on the KT-kaolinite surface can interact with the polymer matrix to make the network stronger and more stable. The network can obstruct escape of released formaldehyde from the composites and restrain resin decomposition, and therefore improve the thermal stabilities of the composites; this was confirmed by TG and DTA. The lower formaldehyde emissions and improved water resistance are also partly ascribed to absorption by the network.

Conclusions Potential mechanism of composite formation A possible mechanism for the formation of uniform spherical composite particles is depicted in Fig. 5. When TEOS is used to modify pristine kaolinite, the hydrolyzed TEOS can cover the kaolinite surface, and TEOS networks can form. The hydroxyl groups on TEOS make it is easier for the silane coupling agent to modify the kaolinite, and more KH550 can cover the kaolinite surface; this was confirmed by the FTIR and XRD results. Furthermore, organic molecules such as formaldehyde can attach to the KT-kaolinite surface via intermolecular interactions such as hydrogen bonding. Polymerization of the urea-formaldehyde resin then occurs

Urea-formaldehyde resin/reactive kaolinite composites containing 20–40 wt% kaolinite were successfully synthesized by in situ polymerization. The composites consisted of uniform spherical particles of average diameter ∼1 ␮m. The KT-kaolinite was almost completely encapsulated by the urea-formaldehyde resins. The initial decomposition temperature increased and the 5 wt% loss temperatures of the composites increased by up to 53.5 ◦ C compared to that of pure urea-formaldehyde resin. KT-kaolinite addition increased the thermal stabilities of the composites. Formaldehyde emissions from the composites were reduced and the water resistance was enhanced by addition of reactive kaolinite.

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Please cite this article in press as: Chen, S., et al. Preparation and characterization of urea-formaldehyde resin/reactive kaolinite composites. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.05.007