silica nanocomposite resins

silica nanocomposite resins

Progress in Organic Coatings 54 (2005) 120–126 Preparation and characterization of polyester/silica nanocomposite resins Yongchun Chen, Shuxue Zhou, ...

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Progress in Organic Coatings 54 (2005) 120–126

Preparation and characterization of polyester/silica nanocomposite resins Yongchun Chen, Shuxue Zhou, Guodong Chen, Limin Wu ∗ Department of Materials Science and the Advanced Coatings Research Center of China Educational Ministry, Fudan University, Shanghai 200433, PR China Received 11 March 2004; accepted 11 March 2004

Abstract A series of silica particles with different size and surface groups were prepared through the sol–gel process of tetraethylorthosilicate, then directly introduced into polyester polyol resins via in situ (IS) polymerization or blending (BL) method and investigated by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), viscosity measurement, particle size analyzer and transmittance electron microscope (TEM), respectively. The results showed that polyester segments were chemically bonded onto silica particles for both IS and BL methods, but more polyester segments chemically bonded onto the surfaces of silica particles during IS polymerization than those during BL process, resulting in lower viscosity of nanocomposite resins from IS polymerization than their corresponding resins from BL method. TEM indicated that silica particles could be dispersed into polymer resins during IS polymerization while causing obvious aggregation during BL method. © 2005 Published by Elsevier B.V. Keywords: Polyester polyol; Silica; Nanocomposite resin; Viscosity; Morphology

1. Introduction A lot of studies show that introducing nanosilica into polymers can obviously improve some properties such as hardness, abrasion resistance, static and dynamic mechanical properties, scratch resistance, adhesive strength, UV protection, etc., and could be widely used in coatings, rubbers, plastics, sealants, fibers, etc. [1–6]. Up to now, however, almost all the silica modified polymers were obtained using silane coupling agents or some components with functional groups such as –COOH, –OH, –CHCH2 O or –NCO to modify silica or organic components to promote the miscibility and interaction between hydrophilic inorganic silica phase and hydrophobic organic phases, which are usually acrylic-based or siloxane-based polymers [7–18]; very few literature involved polyester/silica nanocomposite resins [19]. For example, Hu et al. [15] used commercially available hydropho∗

Corresponding author. Tel.: +86 21 65643795; fax: +86 21 65103056. E-mail address: [email protected] (L. Wu).

0300-9440/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2004.03.013

bic modified silica to obtain PMMA/silica nanocomposites with improved storage and loss modulus. Liu et al. [16] prepared PMMA/silica nanocomposite films from copolymerizing methylmethacrylate with allylglycidylether functionalized silica nanoparticles. Chan and Chu [17] prepared nanocomposite materials by radical polymerization of n-butyl methacrylate with methacryloxypropyl trimethoxysilane to obtain poly(n-butyl methacrylate-co-(3(methacryloxypropyl)) trimethoxysilane), then directly mixing it with silica sol to obtain a composite film. Transparent reinforced polyacrylates were prepared using nanosized silica particles with radiation-curable acrylates, and the particle surface was chemically modified by trimethoxysilanes [18]. In this study, a series of silica particles with different diameter and surface groups were prepared by sol–gel process of tetraethylorthosilicate (TEOS), then directly introduced into polyester polyol resins by in situ (IS) polymerization and blending (BL) method without any surface treatment. The obtained polyester/silica nanocomposite resins were investigated by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), viscosity mea-

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Table 1 The feeding composition and properties of the silica sols Silica sol no.

Molar feeding ratios

Particle size (nm), by N4 plus

Amount of silanol groups (×10−3 mol –OH/g)

H2 O/TEOS/EtOH

NH3 /TEOS

S1 S2 S3 S4 S5 S6

4/1/8

0.03 0.05 0.10 0.15 0.20 0.35

14 28 66 103 154 260

0.82 1.30 1.25 1.02 0.88 0.80

S7 S8 S9 S10 S11 S12

2/1/8

0.10 0.15 0.20 0.25 0.35 0.45

34 63 83 101 132 187

0.88 0.95 1.00 0.92 0.85 0.83

surement, particle size analyzer and transmittance electron microscope (TEM). The objective of this study is to investigate how the size and surface groups of silica particles influence the viscosity of polyester polyol resins and how these particles are dispersed into the resins, which obviously have very important impact on the succedent polyester-based coatings. 2. Experimental 2.1. Materials TEOS (SiO2 content, 28.5 wt%), absolute ethanol (99.7%) and ammonia solution (25–28%) were purchased from Shanghai Chemical Agent Company of China. Phthalic anhydride (98%), adipic acid (99%), 1,4-butanediol (98%), neopentyl glycol (98%) and dibutyltin dilaureate (99%) were supplied from Bayer Company. All the ingredients were used as received. 2.2. Preparation of silica sols Two series of silica sols were prepared based on the recipe in Table 1 with molar ratios of H2 O/TEOS/EtOH = 2/1/8 or 4/1/8 while changing the ammonia content via sol–gel process [20]. TEOS and 90% of absolute ethanol were charged into a 500 ml round-bottom flask equipped with a heating mantle, mechanical stirrer, thermometer with a temperature controller and heated to 50 ± 2 ◦ C, then charged by the solution of ammonia solution, water and the residual 10% of absolute ethanol and stirred at 50 ◦ C for 10 h to obtain silica sol. 2.3. Preparation of polyester/silica nanocomposite resins Polyester polyol was synthesized in a 500 ml roundbottomed flask equipped with mechanical stirrer, thermome-

ter with a temperature controller, N2 inlet, a Graham condenser and a heating mantle. Pathalic anhydride, adipic acid, neopentyl glocyl and 1,4-butanediol with molar ratio of 1:4.95:1.36:6.98 were charged into the flask and heated to around 120 ◦ C at a slow stream of N2 , then 0.05 wt% of dibutyltin dilaurate based on total weight of monomers was added as the catalyst; the reaction was carried out at 165 ◦ C for 4 h. Polyester/silica nanocomposite resins were prepared by two methods, namely IS and BL methods. For IS method, silica sol was firstly mixed with the monomers, then condensation polymerization was carried out according to the process as described above. For BL method, silica sol was directly mixed with polyester polyol resins at 165 ◦ C for 0.5 h by vigorous stirring. The time used is enough to thoroughly remove the solvent. 2.4. Characterization of silica particles and composite resins 2.4.1. Amount of silanol groups at the surfaces of silica particles The silica sols were air-dried at room temperature to get silica powders and then dried at 100 ◦ C under vacuum oven for 2 h. The amount of silanol groups at the surfaces of silica particles was determined by titration method. 2.4.2. FTIR spectra The silica powders from silica sols were obtained according to the same method for titration of surface silanol group. The silica particles from nanocomposite resins were obtained by dissolving the composites with acetone, then centrifuged, and washed with acetone for five times to remove the substances which physically adsorbed on the surface of silica particles, and then dried at 100 ◦ C for 2 h. The silica powders were characterized by MAGNA-IR® 550 FT-IR (Nicolet Instruments, Madison, WI) with 2 cm−1 resolution.

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2.4.3. TGA measurement The silica powders obtained using the same treatment method as in FTIR were analyzed using TGA (SDT 2960, TA Instrument, USA) method from ambient temperature to 600 ◦ C with a heating rate of 5 ◦ C/min in air atmosphere. 2.4.4. Viscosity measurement The viscosities of polyester polyol and its nanocomposite resins were determined using a NDJ-1A rheoviscometer (Shanghai Shengdi Technology Development Corporation, China) at 20 ± 1 ◦ C. 2.4.5. Particle size and its distribution The particle size of nanosilica was determined by N4 plus submicron particle size analyzer (Beckman Coulter Company, USA). The size distribution processor (SDP) model was used to analyze the data. The silica sol or the nanocomposite resin was firstly diluted in ethanol for matching the measured concentration. 2.4.6. TEM observation The morphology of silica particles in the sols as well as in the nanocomposite resins was obtained by a transmission electron microscope (Hitachi H-600, Hitachi Corporation, Japan). Samples of the silica were diluted with ethanol and then dried on copper grids for direct observation.

Fig. 1. FTIR spectra of silica particles obtained from silica sol (1) and corresponding nanocomposite resin obtained by BL method (2) and IS method (3).

from the resin by BL method, suggesting that more polyester segments have bonded with silica particles during IS polymerization than BL method. This is probably because the relatively long reaction time (4 h) and high concentration of –COOH and –OH groups allow more monomers or polyester

3. Results and discussion 3.1. Preparation and characterization of silica particles Silica particles were obtained by a sol–gel process of TEOS based on the recipes in Table 1. Their particle size and amount of silanol groups at surfaces are determined and also summarized in Table 1. The data indicate that the particle size and the amount of surface silanol groups depend not only on the content of water but also on the content of ammonia. The most amount of silanol groups occurred in the particles within the range of 28–66 nm at H2 O/TEOS = 4 and within the range of 63–83 nm at H2 O/TEOS = 2. 3.2. Interactions between silica particles and polyester polyol resins Fig. 1 displays representative FTIR spectra of the silica particles separated from the nanocomposite resins obtained by IS and BL methods. For the sake of comparison, the spectrum of the silica particles from the silica sol is also shown in Fig. 1. Comparing with the FTIR spectrum of the silica particles from silica sol, a new absorbing peak at 1744 cm−1 belonging to C O group is observed in the spectra of the silica particles separated from the resins by both IS and BL methods, indicating polyester polyol has reacted with silica particles. The relative intensity of this peak of the silica particles separated from the resin by IS method is higher than that

Fig. 2. TGA curves of silica particles obtained from silica sol and their corresponding nanocomposite resins with 4 wt% silica content by different methods: (a) S8 series and (b) S4 series. (1) Silica sol; (2) by BL method; (3) by IS method.

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Fig. 3. Effect of silica size on the viscosity of polyester/silica nanocomposite resins embedded by S1–S6 with 4 wt% silica content.

oligomers to chemically or hydrogen bond with silica particles during IS polymerization, while for blending method, only some macromolecules of polyester polyol with relatively less –OH groups bond with silica particles in relatively short mixing time (0.5 h). The absorbing peak at 950 cm−1 due to Si–OH group changes from a sharp peak for the particles from silica sol to a shoulder peak for the silica particles separated the resins by IS or BL method, further confirming that polyester segments are chemically bonded onto silica particles. Fig. 2 compares the typical TGA curves with the silica particles from silica sols S8 (obtained by H2 O/TEOS = 2/1), S4 (obtained by H2 O/TEOS = 4/1) and from their corresponding resins by IS and BL methods as examples. The weight loss before 200 ◦ C is due to the evaporation of the substances physically adsorbed on the surfaces of silica particles. The weight loss in the range of 220–600 ◦ C can be attributed to the thermal decomposition of chemically bonded groups such as hydroxyl, ethoxy and polyester segments on the surfaces. The weight losses of the silica particles from sol S4 and from the resins by BL or IS method are 4.9%, 17.7% and 22.0%, respectively, while the weight losses of the silica particles from sol S8 and from the resins by BL or IS method are

Fig. 4. Effect of silica size on the viscosity of polyester/silica nanocomposite resins embedded by S7–S12 with 4 wt% silica content.

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Fig. 5. Effect of silica content on the viscosity of nanocomposite resins embedded by silica sol S2 or S9.

Fig. 6. The size and its distribution of silica particles in silica sol S3 (1) and corresponding nanocomposite resins obtained by IS method with 2 wt% silica content (2) and 8 wt% silica content (3).

8.0%, 15.8% and 18.7%, respectively, also indicating that there are more polyester segments chemically bonded to silica particles by IS polymerization than by BL method just as discussed by FTIR. The weight loss of the silica particles from sol S4 is less than that from sol S8, while the weight

Fig. 7. The size and its distribution of silica particles in silica sol S3 (1) and corresponding nanocomposite resins obtained by BL method with 2 wt% silica content (2) and 8 wt% silica content (3).

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Fig. 8. TEM micrographs of the silica particles in silica sol S3 (a) and corresponding nanocomposite resins obtained by IS method with 2 wt% (b) and 8 wt% silica content (c), and by BL method with 2 wt% (d) and 8 wt% silica content (e).

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losses of the silica particles from the resins with S4 are higher than those from the resins with S8, which is because that more water is in favor of the hydrolysis of ethoxy groups to silanol groups, leaving much less ethoxy groups and more hydroxyl groups on the particle surfaces of S4 than S8, and resulting in more polyester segments chemically bonded with silanol groups of silica particles. 3.3. Change in viscosity of nanocomposite resins Figs. 3 and 4 illustrate the change in viscosity of nanocomposite resins with 4 wt% silica as a function of particle size and preparation method, respectively. It can be seen that the viscosity increases first, then decreases as the particle size of silica increases for all the cases. What is very interesting is that the viscosity of the nanocomposite resin prepared by BL method is even higher than that by IS method at the same particle size of silica. The possible reason is explained as follows: during IS method, more polyester segments covered the surfaces of silica particles due to chemical reaction between silanol groups on the silica particle surfaces with carboxyl groups or hydroxyl groups from monomers or oligomers, decreasing hydrogen bonding interaction between silica particles themselves and between polyester polyol molecules and silica particles. Fig. 5 further manifests the effect of silica content on the viscosity of nanocomposite resin. The viscosity of nanocomposite resin prepared by IS method slightly increases with increasing silica content, then sharply increases when silica content is over 10 wt%, while the sharp increase for the nanocomposite resins prepared by BL method occurs at silica content of around 6 wt%. The possible critical silica content in resins is probably attributed to forming some networks structure in nanocomposites since the polyester polyol segments attached to one silica particle can further react with another silica particle when the silica content is relatively high. The nanocomposite resin with 12 wt% silica content could not dissolve in any organic solvent, proving the formation of networks in nanocomposite resins. 3.4. Dispersion of silica particles in polyester polyol resin Two methods were adopted to characterize the dispersion of silica particles in polyester polyol resins. One is by particle size analyzer and another is by transmission electron microscope. Figs. 6 and 7 demonstrate the typical particle size and its distribution curves of silica sol S3 and its corresponding nanocomposite resins prepared by IS and BL method as examples, respectively. Both IS and BL methods cause one more peak and an increase in particle size due to some trivial agglomerations or aggregations even slight crosslinking. The difference is BL method results in an obvious increase in particle size while IS method causes a slight increase in particle size with increasing silica content, indicating that silica particles more easily agglomerate or aggregate even

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crosslink by BL method than by IS polymerization since the latter can form protecting polyester layer on silica surfaces just as discussed above. Fig. 8 presents the typical TEM micrographs of dispersion of silica particles in silica sol and corresponding nanocomposite resins prepared by IS polymerization and BL methods. For IS polymerization, the silica particles in polyester polyol resins nearly have the same dispersion of silica as in silica sol even at as high as 8 wt% silica content (see Fig. 8a and c), while for BL method, some agglomeration or aggregation of silica particles actually has happened even at 2 wt% silica content, as demonstrated in Fig. 8d. 4. Conclusions A series of polyester/silica nanocomposite resins were prepared by IS and BL methods. It was found that polyester segments were chemically bonded with silica particles for both methods, but IS polymerization caused more polyester segments chemically bonded onto the surfaces of silica particles than BL process. These polyester segments could weaken the interaction between silica particles themselves and between polyester with silica particles, causing lower viscosity of nanocomposite resins from IS polymerization than from BL process at other parameters being equal. The viscosity of nanocomposite resin first increased, then decreased as the particle size of silica increased. The critical silica content for sharp increase in viscosity was observed at around 6 wt% and 10 wt% for BL and IS methods, respectively. Morphology observation showed that silica particles could be individually dispersed during IS polymerization while some agglomeration or aggregation occurred during BL process.

Acknowledgments The authors would like to thank National “863” Foundation, Shanghai Special Nano Foundation, the Doctoral Foundation of University, Trans-Century Outstanding Talented Person Foundation of China Educational Ministry and Key Project of China Educational Ministry for financial support for this research.

References [1] Y.T. Wang, T.C. Chang, Y.S. Hong, H.B. Chen, Thermochim. Acta 397 (2003) 219. [2] R. Wu, C.S. Xie, H. Xia, J.H. Hu, A.H. Wang, J. Cryst. Growth 217 (2000) 274. [3] R.X. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Mater. Chem. Phys. 75 (2002) 39. [4] M.N. Xiong, L.M. Wu, S.X. Zhou, B. You, Polym. Int. 51 (2002) 693. [5] S.X. Zhou, L.M. Wu, J. Sun, W.D. Shen, Prog. Org. Coat. 45 (2002) 33.

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[6] S.X. Zhou, L.M. Wu, B. You, J. Appl. Polym. Sci. 88 (2003) 189. [7] T. Jesionowski, A. Krysztafskiewicz, J. Dispersion Sci. Technol. 20 (1999) 1609. [8] A. Krysztafkiewicz, B. Rager, J. Adhes. Sci. Technol. 13 (1999) 393. [9] N.R.E.N. Impens, K. Possemiers, N. Maes, K. Schrijnemakers, E.F. Vansant, J. Porus Mater. 4 (1997) 121. [10] T.I. Suratwala, M.L. Hanna, E.L. Miller, J. Non-Cryst. Solids 316 (2003) 349. [11] K.L. Mittal, Silane and Other Coupling Agents, VSP, Utrecht, 1992. [12] S.K. Young, G.C. Gemeinhardt, J.W. Sherman, R.F. Storey, K.A. Mauritz, D.A. Schiraldi, A. Polyakova, A. Hiltner, E. Baer, Polymer 23 (2002) 6101.

[13] S. Kang, C.R. Choe, M. Park, Polymer 42 (2001) 879. [14] L. Sartore, M. Penco, F. Bignotti, J. Appl. Polym. Sci. 85 (2002) 1287. [15] Y.H. Hu, C.Y. Chen, C.C. Wang, Polym. Degrad. Stab. 84 (2004) 545. [16] Y.L. Liu, C.Y. Hsu, K.Y. Hsu, Polymer 46 (2005) 1851. [17] C.K. Chan, I.M. Chu, Polymer 42 (2001) 6823. [18] F. Bauer, H.J. Gl¨asel, U. Decker, H. Ernst, A. Freyer, E. Hartmann, V. Sauerland, R. Mehnert, Prog. Org. Coat. 47 (2003) 147. [19] C. Gellermann, W. Storch, H. Wolter, J. Sol–Gel Sci. Technol. 8 (1997) 173. [20] B.M. Novak, Adv. Mater. 5 (1993) 422.