Application of the hottest nano material technolog

The application of nano material technology in coating industry

China's coating industry, especially architectural coating industry, has developed rapidly in recent years, but compared with the world's advanced countries, the overall level of coating products still has a big gap. Using some high and new technologies to transform the traditional coating industry is a shortcut to rapidly improve the level of China's coating industry. Nano materials have attracted more and more attention because of their special physical and chemical properties. Using nano materials to modify and improve coating products is an active direction in the field of coating research. According to the research progress of key scientific and technological innovation projects in Beijing undertaken by us, this paper introduces the application of nano materials in coatings

1. About nano materials

nano materials refer to materials with special physical and chemical properties with particle sizes between L and 100 nm. In a broad sense, nano materials also include materials with one-dimensional length between 1-100 nm or nanostructures in three-dimensional structures. It is generally believed that nano materials have four effects, namely interface effect, small size effect, macro quantum tunneling effect and so on. From the perspective of physics, there are some important problems in nano materials:

(1) nano materials will directly integrate the European mature research and development achievements of carbon fiber composites, the strong correlation or coherence of electrons in materials

(2) changes in energy level splitting and electron population in nano materials

(3) excited states and exciton processes in nano materials

(4) surface structure and surface state: the surface of nano materials accounts for a considerable proportion, When the size of the material is reduced to about 10nm, the number ratio of surface atoms to body atoms is almost 50%. The chemical environment of surface atoms is very different from that of internal atoms, so they may form a surface phase. In the study of bulk materials, people have long recognized that the surface has an important impact on many physical processes. However, due to the small proportion of surface phase, the study of surface phase is greatly limited. For nano materials, when the surface phase has reached a similar proportion to the bulk phase, it not only makes it easy to study its properties, but also plays a significant role in many physical and chemical processes. For example, the change and role of surface structure in the catalytic process, the role of surface states in the process of luminescence and transport, as well as surface adsorption and surface diffusion. In particular, when the material is in the nano scale, the changes of the surface shape and structure of the material will affect the properties of the material

(5) localization and quantum transport

(6) coupling of quantum tunneling and nano scale

at present, nano materials used in modified coatings are generally nano semiconductor materials, such as nano SiO2, TiO2, ZnO, etc. These nano materials have some characteristics: optical characteristics: semiconducting nanoparticles (1-100nm) due to the significant quantum size effect, Therefore, their photophysical and photochemical properties have rapidly become one of the most active research fields. Among them, the ultra fast optical nonlinear response and (room temperature) photoluminescence of nano semiconductor particles have attracted much attention. Usually, when the size of the conductor particle is close to its exciton Bohr radius, with the decrease of the particle size, the effective band gap of the semiconductor particle increases, and the corresponding absorption spectrum and fluorescence spectrum blue shift, thus forming a series of discrete energy levels in the energy band

photocatalysis characteristics: Ueda et al. Studied the micro heterogeneous photocatalysis reaction carried out by nano semiconductors from the perspective of solar energy earlier. These reactions mainly focus on photolysis of water, immobilization of C02 and N2, photocatalytic degradation of pollutants and photocatalytic organic synthesis

peculiar selectivity: ① different particle sizes lead to different selectivity of the reaction. Anpo et al. Have studied the photocatalytic reaction of propargyne with water vapor with platinum TiO2 particles. The results showed that the reaction products were methane, ethane and propane, and the selectivity of the reaction (defined as the molar ratio of propane to ethane) decreased with the decrease of particle size. When the particle size was reduced from 200nm to 5.5nm, the selectivity of the reaction was reduced by 10 times. ② The selectivity of photocatalytic reaction of nano semiconductor particles is different from that in PEC cells with electrode separation. This is due to the small particle size and the close distance between the oxidation and reduction positions on the surface. For example, in the battery composed of Ti02 and platinum electrode, acetic acid is photodegradated to produce ethane and CO2, while Pt/TiO2 nanoparticles are photodegradated to produce methane and C02. ③ Different kinds of nano semiconductor particles have different selectivity in catalytic reactions. Using nano TiO2 and ZnS semiconductor particles as photocatalysts, the former product is H2, while the latter product is glycerol and H2

absorption characteristics: for nano semiconductor suspension system, the particle size dispersed in the solution is very small, the number of particles per unit mass is large, and the absorption efficiency is high, so it is not easy to reach the degree of light absorption saturation; On the other hand, the specific surface of the reaction system is very large, which is also conducive to the adsorption of reactants. The results show that the adsorption of reactants on the surface of catalyst is a pre step of photocatalytic reaction. The rate of catalytic reaction is related to the amount of the substance adsorbed on the catalyst. The strong adsorption effect of nano semiconductor particles even allows photogenerated carriers to react preferentially with adsorbed substances, regardless of the redox unit order of other substances in the solution

photoelectric conversion characteristics: in recent years, porous large specific surface FEC cells composed of nano semiconductor particles have attracted much attention because of their excellent photoelectric conversion characteristics. In 1991, Gratzel et al reported the excellent performance of nano TiO2 PEC cells sensitized by tripyridine. Under the irradiation of simulated solar light source, its photoelectric conversion efficiency can reach 12%, and the photocurrent density is greater than 12mA · cm-2. This is because the number of dye molecules adsorbed on the surface of nano-TiO2 multi space electrode is more than 50 times more than that adsorbed on the surface of ordinary electrode, and almost every dye molecule is in direct contact with TiO2 molecules, so the interface electrons of photogenerated carriers transfer quickly, so it has excellent characteristics of light absorption and photoelectric conversion. Following this work, many scientists have conducted a lot of research on Nanocrystalline photovoltaic cells and found that ZnO, CdSe, WO3, Fe2O3, SnO2, nb2o2 and other nanocrystalline photovoltaic cells have excellent photoelectric conversion performance. Nevertheless, expensive dye sensitization is still necessary. In addition, the spectral response and photostability of dye-sensitized nanocrystalline photovoltaic cells need to be further studied

II. Modification effect of nano materials on UV aging resistance of coatings

1. Effect test of nano materials on white coatings

conduct appropriate surface treatment on nano materials. Make 6 paint samples containing nano-TiO2, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and white paint respectively, and make 6 artificial aging sample plates × 3 pieces, 18 pieces in total, take 6 × 2 a total of 12 plates are subject to 500h artificial aging test, and 6 are reserved × 1 a total of 6 samples were used as comparison samples, and the color changes before and after aging were measured with Nikon spectrophotometer; The samples of nano sio20.0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% white paint were made respectively, and the test was the same as above

2. Test results and analysis

in styrene acrylic coating, the amount of nano TiO2 (or SiO2) added accounts for 0.5%, 1.0%, 1.5%, 2.0%, 2.5% of the coating amount (mass) and without nano comparison. The discoloration size (△ e value) before and after 500 hours of artificial aging (mainly UV aging) is compared as follows

color changing styrene acrylic paint before and after artificial aging of paint

blank styrene acrylic paint plus 0.5% plus 1.0% plus 1.5% plus 2. For example, more than 50% of the fuselages of Boeing B787 and Airbus A350XWB are made of carbon fiber composites 0% plus 2.5% plus 5.0%

TiO2 TiO2 TiO2 TiO2 3225

color difference △ e 1.65 0.42 0.33 0.66 0.48 1.02 0.50

styrene acrylic paint blank plus 0.5% plus 1.5% plus 2.0% plus 2.5%

SiO2 SiO2 SiO2

color difference △ e 1.65 0.46 0.45 0.46 0.48 0.62

through the artificial aging test, it can be concluded that when nano SiO2 or nano TiO2 is added to styrene acrylic paint, when the addition amount is 0.5% - 2%, the Δ e is smaller than the blank, The aging of the coating film (lotion resin) is significantly slowed down, indicating that the coating film is protected due to the shielding effect of nano SiO2 or nano TiO2 on ultraviolet light

in addition, as a comparison, the discoloration difference △ before and after aging of s-3225d (made in Switzerland) coating with emulsion paint anti UV anti-aging dispersion is 0.5, which is similar to the result of adding nano SiO2 or nano TiO2. It shows that nano-SiO2 or nano-TiO2 really plays the role of UV absorber

3. mechanism analysis (taking nano-TiO2 as an example):

aging resistance is an important property of coatings, and UV is an important factor causing coating aging

ultraviolet is an electromagnetic wave longer than visible light, and its wavelength is between 200~400nm. According to the wavelength, it can be divided into short wave UVC (200~280nm), medium wave UVB (280~320) and long wave UVA (320~400nm). The shorter the wavelength of ultraviolet, the stronger the energy, and the greater the harm to people. This can be seen from the following formula and table:

e=nhc/λ

where: energy of e-1mol light quantum

n - Avogadro constant

h--- Planck constant

c--- speed of light

λ--- Speed of light wavelength

calculate the energy of different wavelengths from the above formula, as shown in the following table

wavelength and energy of light

wavelength/nm 200300400500700800

optical energy/kj · mol-1 598397301 238171 146

key name O-H C-H C-C c-c1 N-N O-O

covalent bond energy/kj · mol-1 46241347326158 138

the shielding of nano TiO2 against UV is mainly based on scattering, and the particle size is one of the important factors affecting the radiation ability. There are many calculation formulas for the optimal particle size. Different researchers have different calculation formulas, and the results are also different. The most commonly used ones are as follows:

D is the best= λ/2.1 (NP NB)

in the formula, D is the best -- the particle size with the largest scattering efficiency, μ m;

λ-- Incident light wavelength, nm

np - refractive index of dispersion, anatase TiO2 is 2.52, rutile TiO2 is 2.7

nb - refractive index of dispersion amputation

according to the above formula, the optimal particle size of anatase TiO2 with different wavelengths scattering ultraviolet rays in water is calculated in the following table

optimal particle size of TiO2 dispersed in water by ultraviolet light of different wavelengths

wavelength/nm 200 290 380500

optimal particle size μ M 0.077 0.111 0.115 0.191

III. photocatalytic coating

nano photocatalytic coating can be developed by using the photocatalytic properties of some nano materials. In our experiments, we used imported and domestic nano-TiO2, which was prepared with pure acrylic resin after special surface treatment. The tests of relevant units showed that it had obvious catalytic degradation effect on nitrogen oxides, oils, formaldehyde and other substances. The degradation efficiency of nitrogen oxides can reach 80%

at present, it is generally believed that the photocatalytic degradation mechanism is as follows:

titanium dioxide has three different crystal phase structures: anatase, brookite and rutile. Among them, anatase titanium dioxide has high photocatalytic oxidation ability, and its band gap width is eg = 3.2eV, which is equivalent to the energy of 387nrn light, which is just in the ultraviolet region. Therefore, taking TiO2 as the photocatalytic oxidation reaction requires ultraviolet light sources, such as sunlight, halogen tungsten Ting, mercury lamp. Under the action of ultraviolet light, the electrons in its valence band are excited to the conduction band, and holes are generated in the valence band. The process is: TiO2 generates free electron hole pairs under the irradiation of ultraviolet light. They make