Basic knowledge of tin surface of the hottest glas

Basic knowledge of tin surface of glass

flat glass is a general term for plate-shaped inorganic glass products. At present, the most advanced and widely used process for producing flat glass in the world is float process. As early as 1902, heal[1] and others had proposed the idea of forming glass floating on the surface of molten metal, but it could not be realized due to the limited process level at that time. Subsequently, Pilkington Glass Co., Ltd. in the UK began research in 1953. After seven years of unremitting efforts, it finally launched the float process, a revolutionary production process of flat glass [2]. Float glass is formed in a tin bath with metal tin as the floating liquid and full of protective reducing gas. The upper surface of the glass is in the atmosphere of protective reducing gas, and the lower surface is in contact with the tin liquid. The tin surface is the tin stained side of the glass in the production process. Why do we spend so much energy on studying the characteristics of tin surface? Why should the tin side of glass be accurately identified in the production process? I believe you will have a preliminary understanding after reading this article

1 characteristics of float glass preparation process

compared with other processes, float can give full play to the role of glass surface tension, produce better glass quality, easy to widen and produce extra thick and extra thin glass, better avoid the defects of glass crystallization, high output of single machine, and easy to realize mechanization of the whole line

of course, the float process also has some characteristic defects. When the float glass is formed, the high-temperature glass belt floats on the molten tin liquid, the lower surface of the glass (tin surface) contacts the tin liquid, and the metal tin and the glass are not soaked. However, at high temperature, the tin liquid is easy to be oxidized, and a series of complex physical and chemical reactions occur with the glass, entering the tin surface of the glass, and tin infiltration occurs. When the tin penetration on the lower surface of the glass reaches a certain amount, it will change the chemical composition and structure of the surface layer of the glass, and then change the physical and chemical properties of the float glass, which will affect the mechanical strength of the glass, and there will be toughened iridescence, fog point and other phenomena

2 what's the matter with tin seepage

due to the existence of tin infiltration, the physical and chemical properties of tin surface and non tin surface are very different. It is necessary to explore the mechanism of tin infiltration. Many people have studied the mechanism of tin infiltration many years ago. Now a widely recognized theory [3] is: as shown in Figure 1, although the tin bath is full of protective gas and reducing gas, a small amount of oxygen will enter the tin bath from the opening and gap of the tin bath. Under high temperature environment, liquid tin will be oxidized to sn2+ and sn4+, but due to the existence of a large amount of H2, sn4+ will be reduced to sn2+, Therefore, tin ions in liquid tin mainly exist in the form of sn2+. Alkali metal ions (mainly na+) on the glass surface will exchange with sn2+ in the tin solution, that is, sn2+ enters the glass surface in the form of ion exchange. There are fe3+ and other reducing ions in the glass, which will oxidize sn2+ entering the surface layer of the glass to sn4+. However, due to the influence of the reducing gas H2 in the tin bath, fe3+ will be reduced to fe2+ in the shallow layer, and the concentration of fe3+ in the shallow layer will become smaller, so there is a large amount of unoxidized sn2+ in the shallow layer. With the increase of sn2+ diffusion depth, the concentration of fe3+ gradually returns to the normal level, and more and more sn2+ is oxidized to sn4+. When it diffuses to the interface between deep layer and shallow layer (critical interface), most of sn2+ is oxidized to sn4+. Because sn2+ acts as complex regulating ions in glass, sn4+ acts as complex forming ions in glass, the diffusion rate of sn2+ is faster than that of sn4+, and sn2+ will continue to reach the critical interface and be oxidized to sn4+ at a faster diffusion rate, while the diffusion rate of sn4+ is small, so more sn4+ will "accumulate" at the critical interface, This is why the bearing ball is in point contact with the axis. In Figure 1, a small peak appears on the critical surface, and the tin in the deep layer exists as +4 valence sn4+

many experiments have confirmed this theory. Williams[4] et al. Studied the tin infiltration on the surface of glass tin surface and found that the depth is less than 3.5 μ M there is 60% Sn 2+ in the shallow layer, and a large amount of sn4+ exists in the deep layer of glass

note: in the silicon oxygen tetrahedral complex structure of glass, silicon is +4 valence, and sn4+ with the same +4 valence can replace si4+ and form complex, so sn4+ is called complex forming ion+ 2 valence sn2+ can not form complex, but can change the complex structure, thus changing the properties of glass, so sn2+ is called complex regulating ion

3 what is the effect of tin infiltration on the properties of the tin surface of glass

if there is a serious tin infiltration phenomenon, float glass will appear various optical and mechanical strength defects in the subsequent deep processing. What is the principle

at present, it is generally accepted that float glass has the function of reducing protective gas in the preparation process. Tin in the shallow surface layer of tin surface mainly exists in the form of SnO. During thermal processing, there is no reducing gas, and sn2+ in the shallow surface layer of tin surface of heated float glass is easy to be oxidized to sn4+ by oxygen in the air, That is, 1. The instrument panel currently used can be divided into hard instrument panel and semi-hard instrument panel. SnO on the surface layer oxidizes to form SnO2 [5]. Studies have shown that [6], the unit cell volume of SnO2 molecule is about 3% larger than that of SnO. The surface volume of the tin surface will expand, and the protrusions generated in the small areas of the surface will squeeze each other, which will increase the roughness of the glass surface and form defects such as microbubbles, folds or microcracks on the surface [5]. The microbubbles produced by the volume expansion of the tin surface will produce fog points during the use of glass; The wrinkles will interfere with visible light and produce toughened iridescence; The micro cracks will affect the mechanical strength of the glass

of course, some people believe that these defects on the tin surface are caused by the difference in thermal expansion coefficient between SnO and SnO2 after SnO on the glass surface is oxidized to form SnO2 during glass deep processing. There are different opinions about the mechanism of tin surface defects. Which one is correct still needs the continuous exploration of glass workers

there are great differences in various properties between the tin side and the non tin side of the glass, so the wire printing and coating should be carried out on the non tin side, otherwise the color will deviate. When hot bending and tempering, the tin side should be on the convex side, otherwise it is easy to appear iridescence. When producing hot-melt glass, if the tin side is reversed, the transparency of the produced hot-melt glass is poor. Seeing this, we should all realize that it is of great significance to identify the tin surface of glass efficiently and accurately in the production process of float glass and the subsequent deep processing process when the steel yield to a certain level