General knowledge of coating: application prospect of nano silica sol

The key points are as follows: in the development of waterborne wood coatings, the industrial production of waterborne wood coatings has been greatly hindered due to some defects of the resin itself, such as low hardness, slow drying speed and so on In the formulation design of waterborne wood coatings, the properties of the coatings can be improved by mixing or adding some film-forming materials or additives In the waterborne wood coatings, the addition of silica sol can improve the hardness, water resistance and aging resistance of the coatings, as well as the adhesion resistance of the coatings Therefore, it has been widely used Introduction of 1 nanometer silica sol Silica sol Shenyang steel structure fireproof coating construction expert punde Building Fire Engineering Co., Ltd Recommended introduction: since its establishment, Shenyang punde Construction Fire Engineering Co., Ltd has cultivated a group of professional construction teams with excellent equipment, rich experience and serious responsibility with advanced and scientific management methods and strict engineering quality assurance and after-sales service system It has undertaken a number of fire system installation projects, which has been praised by customers Every idea and special requirement of you will be fully considered by us and fully realized in the construction technology, which will become the cornerstone of our successful long-term development We are looking forward to your invitation, to provide you with quality services and high-quality technical support! All construction coatings have passed ISO9001 Paint Home News: in the development process of waterborne wood coatings, due to some defects of the resin itself, such as low hardness, slow drying speed, etc., the industrial production of waterborne wood coatings has been greatly hindered In the formulation design of waterborne wood coatings, the properties of the coatings can be improved by mixing or adding some film-forming materials or additives In the water-based wood coatings, the addition of silica sol can improve the hardness, water resistance and aging resistance of the coating film, as well as the adhesion resistance of the coating film Therefore, it has been widely used Nano silica sol: silica sol, also called colloid SiO2, is a kind of colloidal solution with slight blue milky light It has the characteristics of large specific surface area, strong adsorption capacity, negative charge and dense hydroxyl groups It is usually gathered in the form of siloxane, and it is easy to form a transparent layer that is insoluble in water It is due to the comprehensive effect of penetration, hardening and bonding of silica sol, The coating has excellent surface adhesion [1] The molecular formula can be expressed as msio2 · nH2O Nanoscale silica sol is a water dispersive solution [2] containing amorphous and dispersed nanoscale silica particles It is generally transparent, translucent or microemulsion liquid Because it is an aqueous solution of inorganic silica, it is a non-toxic and tasteless environmental protection product Because there are many kinds of nano silica sol, it can be divided into modified sodium type, modified ammonium type and ultrapure type according to the chemical composition; according to the different pH value, it can be divided into acidic, neutral and alkaline types of nano silica; according to the different charge, it can be divided into anion type, cation type and deionization type The particle size and specific surface area of silica particles in the dispersion colloid are generally 5-150 n m and 20-800 m2 / g for different types of nano silicon, the pH value is 2-4 or 9-11 In addition, the viscosity of nano silicon is relatively small, generally less than 5 MPa · s, in particular, the viscosity may be between 1-15 MPa · s 2 Application in Waterborne Wood Coatings 2.1 application in sealer primer in waterborne wood coatings, many coating problems are often caused due to poor back cover, such as poor sealing of base materials on fiberboard is not easy to fill, and fiberboard absorbs too much water, In the later stage of coating, it may be deformed and moldy; the back cover is not good or easy to be filled when the full sealing coating is done on the veneer boards such as maple and walnut; when the water-based sealing primer is used as the bottom coloring, if the permeability is not good, it is not easy to be colored, etc These problems are closely related to sealing primer Due to the good permeability of nano silica colloid, it is easy to form a transparent layer insoluble in water after film formation, which has the comprehensive effect of penetration, hardening and bonding, and it is easy to produce mechanical occlusion and chemical bonding with the substrate surface Therefore, adding a proper amount of water-based sealer can not only improve the sealability (water resistance, organic solvent resistance) and hardness of sealer, but also improve its sanding and filling properties This is because the particle size of nano silicon colloid is small, it can easily enter into the small holes of wood or fiberboard, fill and seal these holes, and after drying and film-forming, it can be bonded with the substrate to form a water-insoluble coating, which has a good sealing effect Because the pure nano silicon colloid can't form film after drying, it can only form powder particles with high hardness, so auxiliary film-forming materials must be added Generally, waterborne emulsion or dispersions with smaller particle size and better permeability are selected as auxiliary film forming materials However, the hardness of these resins with small particle size is often low, and the water resistance and solvent resistance of the coating film are relatively poor after drying After drying, it is necessary to use grinding aids to polish After mixing with nano silicon colloid in a certain proportion, the hardness of nano silicon colloid is improved, making the coating film soft and brittle, so that the polishing performance is also improved 2.2 in the application of primer in water-based wood coatings, due to the poor filling property of primer, the process will increase, the amount of coating per unit area will increase, thus increasing the cost of coating In particular, walnut and maple are the most commonly used materials in the whole sealing coating of varnish Walnut wood has shallow holes and is not easy to be filled, while maple wood has uneven hard and soft materials When the bottom is sealed with water-based paint, the softer part of the wood will collapse and be difficult to be filled Whether the former or the latter, in order to minimize the number of times of painting when using primer to fill these wooden holes, it is necessary to solve the problems of primer penetration between layers, swelling and residual moisture or organic solvent in the film This is one of the main reasons for the poor fullness of the film or the collapse of the film at the later stage of drying The same is true for solvent based wood coatings In the water-based wood varnish system, in order to maintain a good dry film transparency, only a small amount of powder or no powder is added to the primer Therefore, its filling property can only depend on the solid part of the resin However, the water-based resin is difficult to fully dry in a short time under normal temperature, which is the main reason for the later collapse In order to minimize the collapse of the full seal coating, it is necessary to find a fast drying, solvent (or water) releasing water-based resin as primer Generally speaking, the stronger the hydrophilicity of the resin, the worse the release of water or organic solvent; the larger the molecular weight or particle size of the resin, the better the release of solvent or water, and the faster the drying speed However, when the particle size or molecular weight of the resin is large, its stability, transparency and so on will be affected to some extent; at the same time, its low-temperature film-forming ability will also become worse, so that the amount of film-forming assistant will increase when it is low-temperature film-forming However, some film-forming auxiliaries with better film-forming performance are often high boiling alcohol ether solvents (boiling point is higher than or far higher than 100 ℃), and the residual of these organic solvents in turn affects the hardness of the film It can not only accelerate the release of water in the primer, improve the drying speed, but also improve the hardness of the film and reduce the cost of the primer 2.3 the application of water-based finish paint in the totally closed coating of water-based wood coatings, because the coating film is thick and the real drying of the finish paint is slow, the time to be packed after the coating is finished is often prolonged, which reduces the production efficiency; the adhesion at room temperature or high temperature makes the storage and transportation difficult Solid color coating has more powder, so it is better in drying and anti adhesion The main problem is the completely closed coating of water-based varnish Especially when water-based PU resin is used as finishing coat, more attention should be paid to drying and anti viscosity Generally, the water-based PU resin is better in adhesion, flexibility and weather resistance, but it is worse in water resistance, adhesion resistance and hardness The water resistance, adhesion resistance and hardness of the coating can be improved by adding nano silica sol into the coating The addition of nano silica sol to water-based a-c topcoat has similar effect, and the high-temperature adhesion of AC resin is improved obviously 3 Factors restricting the application of nano silica sol in Waterborne Wood Coatings 3.1 stability influencing factor 3.1.1 pH value nano silica sol has been limited in the application of waterborne wood coatings The main reason is that nano silica sol will break emulsion when mixed with waterborne resin, or the storage stability is not good after mixing After being placed at room temperature for a period of time, flocculation or gel will appear The influencing factors include the pH value of water-based resin and silica sol, the type of water-based resin, the particle size and the particle size of silica sol Through a large number of experiments, it can be concluded that the closer the particle size is, the better the stability will be This is a successful example in emulsion type resin or waterborne P U dispersion type resin In addition, nano silica sol was also treated by grafting, so that it has better compatibility with water-based resin This kind of products have appeared on the market and have been applied in a certain range, but the modified silica sol has lower solid content and higher price, which limits its application 3.1.2 particle size and solid content at present, the silica particle size of nano silica sol is generally 5-150 n m, with the increase of particles, its appearance state is also from transparent to translucent or even microemulsion, and the solid content is from 20% - 50% Generally, the higher the solid content is, the larger the SiO2 particles are, the less transparent they are, and the less compatible and stable they are As the addition of nano silica sol in the mixing process is generally 20% - 40% (resin content), it has an obvious effect Therefore, the solid content is too low, which affects the solid content of water-based wood coating products, and has no great practical significance The high solid content compatibility is not good, and the majority of nano silica sol with 30% - 35% solid content is on the market 3.2 easily adsorbed, flocculated and precipitated silica sol has the characteristics of large specific surface area, strong adsorption capacity and dense hydroxyl groups Therefore, it also has certain adsorption effect on other suspended solids in the suspension This is one of the reasons for flocculation of nano silica sol mixed with most of the waterborne resins or after several days of storage Especially in the matte varnish, the matte powder is suspended in the paint liquid by the dispersant or the resin wrapped itself The strong adsorption of nano silica sol greatly reduces the stability of the matte varnish The result of strong adsorption is that nano silica sol particles precipitate together with the dispersion particles or emulsion particles in the matte powder and resin, resulting in the complete separation of solid phase and liquid phase However, some people [3] grafted polyols on the surface of Nano-SiO2 by chemical synthesis method to make nano-sized particles uniformly dispersed in the resin system in the process of melt polymerization, and directly formed a polyester molecular coating on its surface to avoid the hydration of nano-sized composite polyester resin without the deposition of nano-sized particles When the amount of Nano-SiO2 reaches 7%, the hardness of coating film can reach 200% However, this kind of resin is rare in the market 3.3 easy to crack or whiten when the mixing method is used, the addition amount of nano silica sol can play an obvious role when it is relatively large, but if the addition amount is too large, the coating will show a more obvious effect when it is cured