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Glass reinforcement research technology and prospects

1 introduction

glass is widely used in the fields of architecture, transportation, electronics and aerospace because of its good light transmission performance and stable performance. Strength is one of the important indexes to measure the glass structure. Strength is the ability of a material to resist damage or failure. From the mechanical point of view, strength refers to the maximum stress value when a material is damaged under a certain load. For brittle materials, the fracture strength can best reflect its mechanical properties. Fracture must overcome the cohesive force of solids, and atomic bonds must be broken. The theoretical strength of materials is just a reflection of atomic bond energy. According to the calculation, the theoretical strength of the glass is greater than 7000mpa. However, the test results show that the actual strength of the glass is only 80~100mpa, 2~3 orders of magnitude lower than the theoretical strength. There are many factors that affect the actual strength of glass, such as storage environment (such as temperature, humidity, atmosphere, storage time, etc.), surface machining, sample size, loading speed, mechanical scratch and internal non-uniformity (bubbles, stones), among which the existence of surface microcracks has the greatest impact on the actual strength of glass

2 methods to improve the strength of glass

the low strength of glass has become the main factor restricting its further development. Therefore, improving the strength of glass is the key to solve the problem. For a long time, people have tried different methods to improve the mechanical properties of glass. Through surface modification methods, such as physical tempering, chemical tempering, acid corrosion and surface coating, the surface micro cracks can be eliminated or reduced to make the glass structure complete, or a stress layer can be prefabricated on the glass surface to resist external loading. These methods can effectively improve the strength of glass and be applied in different fields

2.1 physical tempering

the method of prefabricating a compressive stress layer on the surface of glass by using physical principles is called physical strengthening method. Heat the glass to a temperature above the transition temperature (TG), and then make the hot glass surface cool evenly and rapidly, and the hot state structure of the surface is frozen. When the glass interior gradually cools down, the first cooled outer surface layer will restrict the internal shrinkage, so the compressive stress is generated in the glass surface layer, and the tensile stress is formed in the glass interior. The thickness of stress layer is generally 20%~30% of the thickness of glass. The thickness of surface stress layer is affected by various factors, such as cooling rate, characteristics and shape of glass, etc. The strength that can be achieved by physical tempering is 350~400mpa, which is times the strength of ordinary glass. Different cooling devices determine different production process methods of tempered glass, which can be roughly divided into vertical hanging tempering method and horizontal tempering method

(1) vertical hanging and tempering method

the development strategy of energy and environment for vertical hanging and tempering method is to heat the glass in the heating furnace to the specified temperature, and then use the chain conveyor, crank conveyor, constant speed conveyor and other conveying devices to make its wooden box packaging machinery one of the top 10 industries in China's machinery industry to supply air. The digital push-pull meter, tension-compression two-way sensor and The calibration grid of the dynamometer is cooled in the cooling device. The glass must be located on the vertical plane of the center line of the air grille, and the ejected air flow is evenly cooled by the movement of the air grille on both sides. The movement of air grille is divided into rotary, horizontal reciprocating and up-down reciprocating. The cooling air is blown to the glass by the gas nozzles evenly arranged on the air grid (which can be rectangular or quincunx). The inner diameter of the nozzle hole is generally 3 ~ 6mm when low-pressure air (common fan supply) is used, and 0 when high-pressure air (compressed air) is used. 6~1mm。 The distance between the nozzle mouth and the glass surface is generally 45 ~ 50mm. The pressure of cooling air is generally 3700 ~ 9800pa, for example, the pressure of 6 mm glass cooling air is 3700 ~ 4500PA, the theoretical quenching time is 15 s, and the actual blowing time is 30 s

(2) horizontal tempering method

horizontal tempering method is a method to complete the whole tempering process, such as conveying, heating, forming and quenching, when the glass is completely horizontal. Because the forming process of horizontal tempering can hot bend the glass, the horizontal tempering method can produce flat tempered glass, single bend tempered glass, hyperboloid tempered glass, double folded plate tempered glass and other products, and 14 kinds of minerals are identified as key mineral raw materials. Each process of the horizontal tempering method is carried out on the horizontal roller table, in which the heating furnace and cooling device can move back and forth. The cooling device of horizontal tempering method is also air grid, and its air injection modes include nozzle type, orifice type and slit type. The upper air grille is composed of steel frame, air grille lifting device, air grille, compressed air pipe and other components. The structure and quantity of the lower air grille are the same as that of the upper air grille, but the nozzle of the air grille is installed at the upper end of the air grille, and there is no guide plate between each air grille, leaving a certain gap, so that the broken glass occasionally broken in the production process can fall into the lower broken glass conveyor through this gap

the original horizontal roller hearth tempering furnace, like the vertical hanging tempering furnace, adopts the electric radiation heating method. In recent years, forced convection heating tempering furnace, roller bed air cushion tempering furnace, solid fluidized bed tempering furnace, modelless automatic bending tempering furnace, gas tempering furnace and high-efficiency double chamber tempering furnace with higher efficiency, lower energy consumption, better product quality and more advanced technology have been continuously emerging after many updates. Therefore, different horizontal tempering furnaces have different process parameters

physical reinforcement has the advantages of low cost, large output, high mechanical strength and heat shock resistance. However, there are certain requirements for the thickness and shape of the glass, and there is also the problem of glass deformation in the process of tempering, which can not be applied in the field with high requirements for optical quality. In addition, physical tempered glass products cannot be reprocessed, and cracks on the surface or edges may lead to glass self explosion

1. 2.2 chemical tempering

the method of prefabricating a compressive stress layer on the surface of glass by chemical method is called chemical tempering method, also known as ion exchange reinforcement method. The chemical enhancement method was first applied for a British patent by researeh Corporation in 1960. The principle of the chemical enhancement method is to change the surface composition of the glass according to the mechanism of ion diffusion, immerse the glass into the high-temperature molten salt at a certain temperature, and the alkali metal ions in the glass and the molten salt exchange with each other due to diffusion, resulting in a "congestion" phenomenon, causing a compressive stress on the glass surface, so as to improve the strength of the glass

ion exchange enhancement technology can be divided into high temperature type and low temperature type. Low temperature ion exchange means that the small radius alkali metal ion na+ (0.98) in the glass exchanges with the large radius alkali metal ion k+ (1.33) in the molten salt below the glass transition temperature, resulting in congestion and strengthening the glass surface. In 1962, Kistler first carried out k+-na+ ion exchange enhancement research with silicate glass as raw material. High temperature ion exchange is to exchange the large radius alkali metal ions na+ and k+ in the glass with the small radius alkali metal ions li+ (0.78) in the molten salt above the glass transition temperature to produce a low expansion surface layer to achieve the purpose of strengthening

ion exchange reinforced glass is characterized by high strength, uniform stress, good stability, no self explosion, and can be cut without deformation and light distortion. It is suitable for the reinforcement of glass products with complex shape and small thickness. Up to now, it is the only effective method to strengthen the special-shaped thin glass below 3 mm. Ion exchange reinforced glass has excellent performance and is mainly used in high-tech fields such as spaceships, military aircraft, high-speed trains, combat vehicles, ship windshields and side windows

people have studied ion exchange enhancement for decades, and all of them have studied single-step ion exchange process. The disadvantage of one-step ion exchange glass is that the glass strength is highly dispersed due to the unstable propagation of glass cracks. In recent years, green et al. Used the two-step ion exchange method to design the stress distribution on the glass surface to prepare reinforced glass. The process is to conduct a long-term high-temperature treatment below Tg, and then a short-term treatment at low temperature. It is found that the two-step ion exchange method can improve the strength of the glass and reduce the strength dispersion (2%). Through the design of stress distribution, there are obvious multiple cracks in the glass before fracture, that is, the cracks in the glass may be prevented or stably expanded. This kind of glass has a wide range of engineering applications. Therefore, it is named as engineered stress profile (ESP) glass

the ESP glass obtained through the optimization of stress distribution has the mechanical properties that the traditional reinforced glass does not have. Table 1 summarizes the comparison of breaking strength between several ESP glasses and traditional reinforced glass. It can be seen from table 1 that the breaking strength of ESP glass has no obvious increase or decrease compared with the traditional one-step ion exchange glass. However, it can be seen that the dispersion of ESP glass strength is significantly reduced, which increases the reliability of the glass. The glass Institute of China Academy of building materials science has developed high strength and high stability glass by using a new variable temperature two-step process, which has a good application prospect

1.2.3 acid treatment

in addition to stress enhancement treatment, acid corrosion can also be used to remove surface microcracks. The principle of acid corrosion is to remove the crack layer on the glass surface or passivate the crack tip through acid corrosion, so as to reduce the stress concentration and restore the inherent high-strength characteristics of the glass. Since pickling removes surface microcracks, it is necessary to select acids with strong erosion ability, such as hydrofluoric acid. However, it is not easy to obtain a smooth surface with hydrofluoric acid alone. The salts produced after erosion, such as Na2SiF6 and CaF2, are attached to the surface of the glass. In order to remove salts, strong acids such as sulfuric acid and nitric acid should be added to hydrofluoric acid. After acid corrosion, the strength of flat glass can reach 600~800mpa due to the complete elimination of surface cracks or passivation of crack tips

the treated glass surface is extremely fragile. Roach pointed out that the insoluble particles attached to the glass surface damage the glass surface, resulting in the decrease of glass strength. In addition, it is easily eroded by the external environment, the surface hardness is reduced, and the strength cannot be effectively maintained. Therefore, acid corrosion alone can not effectively improve the strength of glass, and it must be combined with other strengthening methods to achieve the effect

due to the volatilization of hydrofluoric acid during acid treatment, it is easy to cause pollution to the environment, so a special plastic protective cover is installed on the pickling tank to prevent the volatilization of hydrofluoric acid by air sealing method, and a special air extraction device is set when placing glass to remove the volatilized hydrofluoric acid

although hydrofluoric acid can temporarily improve the strength of glass, it has not been produced continuously on a large scale due to its pollution to the environment, health hazards of operators, difficulties in waste liquid recovery and equipment erosion

1. 2.4 surface coating

in order to reduce the cost of reinforced glass, people use surface coating to improve the strength of glass. The surface coating is not only easy to coat, but also can improve the mechanical and optical properties of the glass

in recent years, researchers have successively reported different coating materials. Alkoxide decomposition coating, sol-gel coating, organic-inorganic composite coating and epoxy resin coating can improve the strength of glass. In order to increase the stability of the coating, silicone coating has been developed. Although the moduli of these coatings are different, even two orders of magnitude, their reinforcing effects on glass are basically similar

different from coating on glass surface, arkem

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