Summary of the hottest inorganic adhesives and the

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Review of inorganic adhesives and their applications (II)

1.2 Sodium silicate

commonly known as sodium silicate, also known as foamed alkali. It is a soluble silicate, which consists of a series of products with different proportions of alkali metals (sodium) and quartz sand (SiO2). Its typical molecular formula is Na2O · nsio2 · xH2O, and its simple formula is Na2SiO3. Generally, products contain water. Water glass has been used for hundreds of years because of its low price, good flame retardancy and convenient use. It is widely used in the bonding of paperboard and carton. The initial adhesion of sodium silicate adhesive is small, so proper force should be applied to fix it and make it stick firmly. The dried adhesive layer is brittle, easy to absorb moisture and alkaline, which will pollute objects. Recently, starch glue and synthetic resin glue have been used to replace some sodium silicate glue for gluing cartons and cartons, but the consumption of sodium silicate glue is still the first; In addition, the use of metal oxide and other modified water glass to produce heat-resistant adhesive with heat resistance of thousands of degrees, which is used for the bonding of metal, ceramic and quartz materials has been in development, and it will still be the leading role in inorganic adhesives

(1) preparation method. The production methods of sodium silicate mainly include alkali method and sodium sulfate method. Alkali method can be divided into wet method and dry method, or one-step method and two-step method. The performance of the product is related to the modulus (n), which refers to the weight percentage of silicon dioxide (SiO2) and sodium oxide (Na20). Products with different moduli can be obtained with different proportioning ratios, and the moduli of products are generally between 2.2 and 3.7. High modulus and strong bonding strength. The dry method (soda ash method) is easy to control the production of high modulus products, and the wet method (caustic soda method) is relatively simple for low modulus products

① wet method (caustic soda method). The proportion of raw materials is determined according to the product modulus. Put the quartz sand and liquid caustic soda into the stirred autoclave according to the required proportion, mix evenly, heat it with direct steam to the temperature of 175 ℃, control the pressure in the autoclave at 0.7 ~ 0.8MPa, and react for hours. After cooling the reactant, filter it, remove the unreacted quartz sand, and then concentrate it to obtain liquid sodium silicate (water glass). The reaction formula is as follows:

② dry method (soda ash method). Dry production needs to go through two steps: melting and dissolution, which is also called two-step process. In this method, quartz sand and soda ash are mixed evenly in a certain proportion, and the melting reaction is carried out at a high temperature of about 1400 ℃. After about 4 ~ 6 hours, molten materials are generated, which are glass frits after water quenching and cooling, that is, solid sodium silicate; Then put the solid into a pressurized drum or an open dissolution pot while it is hot, and heat it with steam to dissolve it. After sedimentation and concentration, a colorless and transparent viscous liquid, namely water glass, is obtained. The reaction formula is as follows:

③ sodium sulfate method. The dosage of sodium sulfate depends on the modulus of products with excellent production performance. First, it is evenly mixed with pulverized coal, then it is evenly mixed with quartz sand according to the specified ratio, and it is quickly put into the reverberatory furnace for reaction. After the reaction is completed, the materials are discharged for cooling, crushing, dissolution and separation. Not even many civil aircraft seats are made of soluble and concentrated fire-retardant materials to obtain water glass. The reaction formula is as follows:

among the above three methods, the production conditions of soda ash method are easy to control, easy to industrialized production, and easy to produce high modulus products. However, the operating environment is poor and energy consumption is high. The caustic soda method has simple production conditions, good operating environment and low energy consumption, but it is not suitable for the production of high modulus products. Sodium sulfate method is easy to produce strong corrosive materials and sulfur dioxide in the reaction, which pollutes the environment and is rarely used in industry

(2) application process. The adhesive force of sodium silicate depends on its modulus and concentration. The higher the modulus or concentration, the higher the adhesive force. Products with different moduli should be selected for different applications. As an adhesive, the modulus of choice is 2.9 ~ 3.4. Water glass can be used directly without coordination, and different additives such as clay, lime, casein, starch, etc. can also be added to increase the consistency, improve the water resistance and improve the bonding strength. After the water glass solution is coated on the adherend, after absorption and evaporation, it slowly loses water, and quickly gel until it hardens to have a certain bonding strength. It is advantageous to use hot water glass solution for bonding paper products. The bonding pressure is generally contact pressure. When the fiberboard is laminated by five 0.4mm fiber sheets, the pressure is as high as 5.5mpa

(3) performance. Water glass is a colorless, light yellow or cyan gray transparent viscous liquid. Soluble in water and alkaline. In case of acid, at present, the ocean freight of ultra flexible ships from Indonesia to southern China is US $70/ton (50.6 million tons), and the Panamax freight is 5.5 (6) 5 dollars/ton (70000 tons) is the colloidal precipitation of silicic acid due to the decomposition of carbon dioxide in the air. The relative density increases with the decrease of modulus. The national standard number of the product is gb4209-84, and the index values are shown in Table 1

Table 1 national standard of sodium silicate

index values of class II products are the same except that iron ≤ 0.05% and water insoluble ≤ 0.4 ~ 0.8%

water glass has the characteristics of convenient use, fast bonding, heat resistance and fire resistance; However, it has poor water resistance and will deliquesce and contaminate objects

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