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  • Chapter 10

    Preparation, Structure and Properties of Organic-Inorganic Nanocomposite Coatings

    Shuxue Zhou, Limin Wu*, Bo You, and Guangxin Gu

    Department of Materials Science, The Advanced Coatings Research Center of China Education Ministry, Fudan University, Shanghai 200433,

    People's Republic of China

    Organic-inorganic(O/I) nanocomposite coatings have been developed very quickly in recent years and are gradually becoming one of the most active areas in coatings' research. This review classifies the preparation methods into three categories: embedding with nanoparticles, sol-gel process and self-assembly method. A l l these three routes are systematically discussed including the factors influencing the structure and properties of nanocomposite coatings. Some evaluations and suggestions are given for each method.

    2009 American Chemical Society 193

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    In Smart Coatings II; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

  • 194

    Organic-inorganic(0/I) nanocomposite coatings contain at least one phase with at least one dimensional size less than lOOnm. Compared to the traditional coatings obtained from resins, micro-fillers and additives, O/I nancomposite coatings can more efficiently combine the advantages of rigidity of the inorganic phase and softness of the organic phase, or endow the coatings with new functionality resulting from nano-effects or synergistic effects. Therefore, more and more papers and patents involving with nanocomposite coatings have been published. At the very beginning, however, the concept of "nanocomposite coatings" was ever controversial, some people even doubted the value of nanomaterials used in coatings and the competitive power of nanocomposite coatings in market. Nowadays, more and more studies indicate that nanocomposite coatings can find potential uses in abrasion and scratch resistant coatings, corrosion resistant coatings, transparent coatings, superhardness coatings, functional coatings such as antibacterial coatings, UV-shielding coatings, conductive coatings, self-cleaning coatings, photocatalytic coatings and etc., some of them have even been commercialized. For example, B A S F recently announced the use of scratch resistant automotive coatings containing nano ceramic particles (/). NEI (Nano Engineered Innovation) Corporation developed hard ceramic-like nanocomposite coating with trademark of NANOMYTE for the plastic substrate such as poly(methyl methacrylate) (PMMA) (2). Obviously, nanocomposite coatings are gradually becoming one of the most quickly developing areas in coatings.

    The related development of nanocomposite coatings was recently reviewed by Fernando (3) and Baer. et al. (4), respectively, but Fernando's paper was only a simple introduction of nanocomposite coatings while Baer et al.'s review mainly focused on inorganic nanostructure coatings. The objective of this review paper is focused on the preparation, structure and properties of O/I nanocomposite coatings and attempts to academically elucidate their relationships. Based on the literatures, we divide the preparation of O/I nanocomposite coatings into three methods: embedding with nanoparticles, sol-gel process, and self-assembly method. A l l the following discussions are carried out respectively based on these three methods. Meanwhile, some evaluations and suggestions are also given for each preparation method.

    Nanocomposite Coatings Prepared Through Embedding with Nanoparticles

    There are a lot of nanoparticles which could be used for improving the performance of coatings, different kind of nanoparticles may have various properties, even for the same kind of nanoparticles, the different source of nanoparticles (e.g., from powders or nanoparticles in sols), particle size, content and preparation method have also various influneces on the properties of coatings.

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    In Smart Coatings II; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

  • 195

    The Types of Nanoparticles and Their Purposes in Coatings

    The frequently studied nanoparticles in coatings include nano-silica (nano-Si0 2 ) , nano-tiantia (nano-Ti02), nano-zinc oxide (nano-ZnO), nano- calcium carbonate (nano-CaC03), nano-alumina (nano-Al 20 3), nano-zirconia (Zr0 2 ) and so on, which have been commercially available in the state of nanopowders or nanoparticle sols. Table I summarizes the physical properties and potential uses of these nanoparticles.

    Table I. Summary of the physical properties and potential uses of commercially available nanoparticles.

    Nanoparticles Mohn's hardness

    Refractive index Potential uses in coatings

    Nano-Si0 2 1 1.47 Scratch and abrasion resistance, UV-shielding, hardness

    Nano-Al 2 0 3 9 1.72 Scratch and abrasion resistance, UV-shielding, hardness

    Nano-Zr0 2 1 2.17 Scratch and abrasion resistance, UV-shielding, hardness

    Nano-ZnO 4.5 2.01 UV-shielding, fungus resistance, other functionality

    Nano-Ti0 2 6.0-6.5 2.70 UV-shielding, fungus resistance, other functionality

    Nano-CaC0 3 3 1.49 and 1.66

    Mechanical property, whitening

    The inherent rigid nanoparticles such as nano-Si0 2, nano-Al 2 0 3 , nano-Zr0 2 particles and etc. are adopted to mainly improve the mechanical properties of traditional coatings. Among of them, nano-Si0 2 particles were the first nanoparticles produced. It was found that nano-Si0 2 particles could more efficiently increase the macro hardness, scratch resistance, elastic modulus of acrylic based polyurethane coatings than micro silica particles (5) or enhance the microhardness and abrasion resistance of polyester based polyurethane coatings (6). Zhang et al. (7) found that nanosilica could reduce the wear rate and frictional coefficient of the epoxy coatings even at low filler loading (

  • 196

    of the acrylic or polyester based polyurethane coatings (10,11). Colloidal silica, another type of nano-silica particles, is also often used to improve the hardness, abrasion and scratch resistance of coatings. For example, Soloukhin et al. (12) found that nano colloidal silica particles could enhance the elastic modulus and hardness of acrylic-based UV-curable coatings on polycarbonate substrate. G. Chen et al. (13) indicated that nano colloidal silica could increase the storage modulus and abrasion resistance of acrylic base polyurethane coatings. Y . Chen et al. (14) presented increases in hardness, abrasion resistance and Tg for polyester-based polyurethane coats. Besides mechanical properties, heat resistance and barrier properties can also be improved by addition of nano-Si0 2 particles, which was observed in its application in nylon 11 coatings (8) and acrylate radiation-curable coatings (15-16). Mennig et al. even found that nano- S i 0 2 could provide both high scratch resistance and sufficient free volume for fast switching dyes of 3-glycidoxypropyltrimethoxysilane (GPTMS) pre-hydrolysate based coatings and thus developed a novel switching photochromic coatings for transparent plastics and glass (17). Nano-Al 2 0 3 and nano-Zr0 2 can also improve the hardness, abrasion and scratch resistance of coatings and even better than nano-Si0 2 (18), but their refractive indexes (1.72 for nano-Al 2 0 3 and 2.17 for nano-Zr02) are higher than that of nano-Si0 2 (1.46). Therefore, the coatings embedded with nano-Al 2 0 3 and nano-Zr0 2 powder are usually semitransparent if these particles are not completely dispersed into nanoscale level (150) while pure P V D F coatings had only 108of water contact angle. However, the poor acid fastness of nano-CaC0 3 and the low transparency of nano-CaC0 3 -containing coatings considerably confine its application in coatings. Hitherto, nano-CaC0 3 is mainly used in polyvinyl chloride (PVC) anti-chipping coatings for automobile and paper coatings.

    Nano-ZnO and nano-Ti0 2 seem to have similar properties because of their semiconductive characteristic. Differing from nano-Si0 2, nano-Al 2 0 3 and nano-Z r 0 2 , however, nano-ZnO and nano-Ti0 2 are preferentially employed to improve the optical property or to offer some functional properties of the coatings. For example, Xiong et al. (22) found that nano-ZnO particles had U V -shielding property for acrylic latex coatings. L i et al. (23) indicated that nanosized ZnO particle could be acted as U V absorbent for the inorganic-organic hybrid coatings derived from tetraethoxyasilane (TEOS) and GPTMS on P M M A substrate. X u et al. (24) found that tetrapod-like nano-particle ZnO had both electrostatic and antibacterial property and could be used to prepare multi-functional acrylic coatings. Nano-Ti0 2 , mainly in rutile, also has UV-shielding property (25-27), but it seems that nano-ZnO powder has higher UV-shielding

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