Abstract : Nanomaterials have attracted wide attention due to its four major effects. The agglomeration of nanopowders has become a bottleneck in its industrial production and application. The agglomeration, dispersion mechanism and dispersion method of nanopowders were introduced. The application of inorganic nano-pigments in paper coating is described, and the existing problems and application potentials are discussed.
Keywords: Inorganic nanopigment; Dispersion; Papermaking coating
In recent years, preliminary studies on the application of inorganic nanopigments in paper coatings have been conducted at home and abroad, and inorganic nanopigments such as nano-SiO2, nano-TiO2, and nano-CaCO3 have been found to have greater rheological and coating properties for paper coatings. The influence provides the basis for the initial application of inorganic nanopigments in paper coatings.
1 Aggregation of nanometer powders Aggregation of nanometer powders refers to the phenomenon that primary nanometer powder particles are connected to each other during preparation, separation, processing, and storage to form larger particles.
The agglomeration of nanopowders has a very serious effect on their properties. First, the presence of agglomeration not only reduces the activity of the nanoparticles but also affects the performance of the nanoparticles in all aspects. Secondly, the agglomeration of nanomaterials brings great inconvenience to the mixing, homogenization and packaging of nanomaterials, and it becomes very difficult in practical production and application.
Through long-term in-depth research, the cause of the agglomeration of nano-powders can be summarized as the following four main aspects [1]: 1) During the nano-crystallization of materials, a large amount of positive charges accumulate on the surface of newly-born nanoparticles. Or negatively charged, these charged particles are extremely unstable. In order to stabilize, they attract each other and cause particles to agglomerate. The main force of this process is electrostatic Coulomb force. 2) In the process of nanocrystallization, the material absorbs a large amount of mechanical or thermal energy, so that the surface of the newly born nanoparticles has a relatively high surface energy. In order to reduce the surface energy, the particles often reach a stable state through mutual aggregation, thus causing the particles to agglomerate. 3) When the material is nanosized below a certain particle size, the distance between the particles is very short and the van der Waals force between the particles is much larger than the gravity of the particles themselves, and the particles tend to attract and agglomerate with each other. 4) Due to hydrogen bonding on the surface of nanoparticles, adsorption of wet bridges and other chemical bonds, it is easy to cause mutual adhesion and aggregation of particles.
2 Dispersion of nano-powders At present, the dispersion methods of nano-powders mainly include medium dispersion, mechanical stirring and dispersion, ultrasonic dispersion, electrostatic dispersion, dispersant dispersion, and surface treatment.
1) Dispersion of the medium According to the surface properties of the nanoparticles, an appropriate dispersion medium is selected to obtain a well-dispersed suspension. The basic principle for selecting a dispersion medium is that the non-polar particles are easily dispersed in a non-polar liquid (medium), and the polar particles are easily dispersed in a polar liquid, for example, silica is easily dispersed in water, and the graphite particles are easily in kerosene dispersion. Of course, the dispersion behavior of the particles in the medium is also affected by the particle surface forces and other physicochemical factors in the solution.
2) Mechanical agitation dispersion This dispersion method is used in almost all industrial processes. Strong mechanical agitation is a simple and easy method of breaking up flocs. Industrial mechanical dispersion of powder slurry mainly uses various types of mixers, sand mills, and colloid mills.
3) Ultrasonic Dispersion Ultrasonics applied to disperse nano-powder belongs to power ultrasound—acoustic cavitation, that is, the formation of liquid hollow cavity, oscillation, growth shrinkage and even collapse. The process of liquid acoustic cavitation is a process that concentrates the energy of the sound field and releases it rapidly. When the cavitation bubbles collapse, an extremely short intense pressure pulse occurs, and a short-lived local hot spot is formed at the collapse point. In an extremely small space around the internal cavitation bubble, a temperature of 5,000 K or more and a high pressure of about 50 MPa are generated, and a strong shock and an instantaneous flow of electro-luminescence with a speed of 400 km per hour are accompanied. This is sufficient to open the van der Waals forces between the small particles in the agglomerate so that the particles are uniformly dispersed in the dispersant.
4) Electrostatic Dispersion Electrostatic Dispersion is the addition of charges of the same polarity to the particles, and the particles are dispersed by the Coulomb repulsion between the charged particles. Therefore, maximizing the charge of particles is the key to electrostatic dispersion. Ren Jun et al [2] electrophoresis charge on the nano-scale ultrafine calcium carbonate electrostatic resistance anti-agglomeration, it has a significant anti-aggregation and dispersion of ultra-fine particles.
5) Dispersant Dispersion Dispersion is one of the commonly used dispersion methods for nanoparticles. He Xiaogu et al [3] used a hyperdispersant to modify the surface of the nano-TiO2 powder, and successfully coated the hyperdispersant molecule on the surface of TiO2. The non-polar modification of the surface of TiO2 made the surface polar. Hydrophilicity becomes non-polar lipophilicity.
6) Surface treatment Appropriate surface treatment can improve the dispersibility of nanoparticles in dry, polar and non-polar solutions as well as polymer binders or inorganic composite powders, and can also improve other properties of nanopowders or Features. Liang Shaojun [4] used titanium titanate coupling agent to surface-modify nanometer calcium carbonate. After titanate coupling agent treatment, the surface of calcium carbonate is covered with a layer of molecular film, which fundamentally changes the surface properties of calcium carbonate. Zheng Yongjun et al [5] found that the surface treatment mechanism of aluminate coupling agent is basically similar to the titanate coupling agent, and its modification effect is better than titanate coupling agent.
3 Inorganic nano pigments in paper coating
3. 1 Effect of Inorganic Nanopigment on Rheological Properties of Coatings The paper coating structure system is a kind of fluid with shear thinning and viscoelastic properties. The addition of inorganic nanoparticles can improve the rheological properties of coatings. Zhang Heng studied the effect of nano-SiO2 on the rheological properties of paper coatings. The results showed that under the low shear force field, after adding nano-SiO2, the yield stress of paper coatings increased while the Casson viscosity decreased; under high shear field After adding nano-SiO2, the high shear viscosity of paper coatings did not change significantly. The mechanism study shows that this is because the nano-SiO2 mainly forms hydrogen bonds with the fine particles in the coating through the unsaturated hydroxyl groups on the surface. These hydrogen bonds are not easily broken under the action of low shear forces, and therefore have a higher yield stress. However, it is easy to break under the action of high shear force, and the effect on high shear viscosity is not obvious. In addition, the viscoelasticity experiment of the coating shows that the addition of nano-SiO2 significantly increases the viscoelasticity of the coating. With the same coating solid content, the dynamic elastic modulus of the coating increases by a factor of two, while the viscous modulus slightly increases after adding nano-SiO2, which indicates that the interaction between the particles in the coating system is enhanced after the addition of nano-SiO2. This type of interaction contributes more to the elasticity of the system. However, experiments have shown that compared with traditional thickeners such as CMC and PVA in coatings, nano-SiO2 is far inferior to the former in improving the rheological properties of paper coatings. Because of its extremely small particle size, nanoparticles can only be used in coatings with neighboring Particles have a force, and many of them are associated with latex particles and very fine pigment particles of which the particle size is equivalent. The strength of the network structure is inferior to the former, and the increase of viscoelasticity is limited.
Wang Jin et al. studied the rheological properties of nano-SiO2 dispersions and color inkjet paper coatings. The results show that the concentration has a great influence on the rheological properties of nano-SiO2 dispersions. Increasing the nano-SiO2 dispersion concentration, the apparent viscosity and elastic modulus have greatly improved. The rheological properties of the nano-SiO2 dispersion also determine that it has a lower dispersion concentration when dispersed, and when it is higher than this dispersion concentration, it will inevitably cause the viscosity of the nano-SiO2 dispersion to rise. In addition, in the color ink-jet printing paper coatings, polyvinyl alcohol is added to the nano-SiO2 dispersion liquid, the elastic modulus of the coating is improved to some extent; the fixing agent and the nano-SiO2 in the coating, the adhesive and the auxiliary additive and the like chemically bond The effect causes micro-flocculation of the coating, forming an irreversible, high-strength, smaller three-dimensional network structure, increasing the initial shear viscosity and initial elastic modulus of the coating, and improving the shear resistance performance of the coating structure.
3.2 Effect of inorganic nano-pigments on coating printing performance China's application of inorganic nanopigments in paper coatings has just started. Wang Bao et al [6] added nano-SiO2 to the paper coating and found that the surface strength, smoothness, and gloss of the coated paper increased, while the surface absorption weight, ink absorption, and color change value decreased. Xiao Xianying et al [7] added nano-CaCO3 to the paper coating and found that the viscosity of the coating increased, the surface strength and ink absorption increased, but the whiteness did not change significantly, and there was an optimum value for the amount of nano-calcium carbonate. Wang Ge [8] explored the application of nano-CaCO3 in coated paper. Two nanometer calcium carbonate samples with different crystal forms and particle sizes were used in the coating formulation, and it was found that the ink absorption speed of the coating was greatly improved, but the glossiness and the printing gloss of the coated paper were greatly affected. Zhang Heng et al studied the effect of nano-SiO2, nano-TiO2 and nano-CaCO3 nano pigments on the coating performance. It was found that the addition of nano-SiO2 and nano-TiO2 in the coating respectively increased the surface strength of the coating and decreased the ink absorption. The surface absorption quality is reduced; and the appropriate amount of nano-CaCO3 can improve the surface strength and ink absorption of the coated paper, but the amount is too large to be dispersed, and the particles tend to agglomerate, resulting in a large drop in coating performance.
The Bohr Institute of Solid State Physics at Tongji University in Shanghai and the Shanghai Institute of Papermaking have conducted experiments on the application of inorganic nanomaterials as coating pigments [9]. The applied nanomaterials are nanoscale SiO2 and TiO2 in "nano-ethyl ethyl ester" or " Dispersions in butyl titanate, these are all lower density dispersions than the same type of material. More than one year of coating operation tests proved that the coating machine's production performance is in line with the design requirements. In addition, compared with the coating performance of non-nano pigments, the printing performance of self-made nano pigment coated papers has been significantly improved.
Wang Jin and others used silica sol, nano-SiO2 dispersion, and vapor phase SiO2 three kinds of nanotope paint to conduct comparative experiments. It was found that when nano-SiO2 was used as a topcoat pigment, the whiteness and smoothness of the paper sheet were greatly improved. However, the degree of improvement of different SiO2 is different. The ink absorption of different types of SiO2 coatings is quite different, and the ink absorption (K&N value) of vapor phase SiO2 is the largest. Considering ink absorption, color density, and print image quality, vapor-phase SiO2 is considered to be more suitable for topcoat pigments and can provide relatively high smoothness, absorption performance, color density, and image quality for paper sheets [10].
Several domestic research institutes for color ink-jet printing papers are under research and development, and Hunan Paper Research Institute took the lead in domestic research [11]. The technology adopts nanometer-scale SiO2 and nano-encapsulation technology of 5 to 12 nm, so that the fixing agent and the nano-SiO2 are fully mixed, and nano-sized fine particles of SiO2 and fixing agent are formed on the coating surface, so that the inkjet particles are The nano-particles are combined, and then the absorption and diffusion speed of the ink is adjusted by adjusting the absorption value of the nano-materials to achieve the high-definition image quality requirements. Replacing current photographic prints with image processing with a high-resolution color inkjet printer. In this area, foreign companies such as Epson, Fuji, Kodak and others are only in the R&D stage.
3. 3 Inorganic nano-particles enhance the anti-aging properties of coated paper Nano-pigment has a strong shielding effect against ultraviolet rays due to its specific surface effect. The use of nanopigments in paper coatings will not only improve the whiteness of the paper, but also improve the heat resistance and oxidation resistance of the paper coatings, making the paper more stable and less prone to ageing.
The application of anti-aging features is promising. The world papermaking industry has so far conducted tireless research on milled wood pulps and heat-refined products with high lignin content, which are yellow and brittle due to UV aging. For example, nanometer solution coating methods can avoid complex organic compounds. Sun protection process.
Ma et al [12] added the surface-modified nano-calcium carbonate to the paper coating latex, dispersed it by high-speed dispersion, and then applied it as a coating material to the surface of the base paper as a coated paper primer by a paper mill. After the calendering, the coated paper is obtained.