Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 97

When it was invented in the 1950s, the float process turned out to be an epoch‐making method to produce flat glass with a smooth surface without any additional polishing. Its production cost was low enough to make possible an extensive use of the material in buildings and automobiles, which is one of the hallmarks of current civilization. The basic process of making flat glass on a molten metal was in fact patented in various ways as early as in 1848 in England by H. Bessemer, of steel‐converter fame, and then several times in the United States by W. Heal and J. H. Forrest (1902 and then in 1925), and by Halbert K. Hitchcock (1905 and 1925), but a great many technical problems had to be solved before the process could be made practical. Through a seven‐year expensive research program of Pilkington Brothers in the United Kingdom, which in its last stage included 13 months of production that had to be discarded, all these problems had eventually been overcome in July 1958 by a team led by L.A.B. Pilkington (no relationship with the Company's owners) and K. Bickerstaff. The following year it even became possible to produce with the same process the distortion‐free glasses needed for mirrors, thus abolishing the long‐standing distinction between window and polished‐plate glass (Chapter 10.9).

Figure 6 Sketch of single‐pass wire roll out process (upper part insertion process). The wire is inserted into the molten glass, which is pressed and cooled by rotating water‐cooled rolls [8].

Although the float process became continuously profitable only in 1963, the previous year it began to be licensed all over the world by Pilkington Brothers in rapidly expanding markets. Within a decade, the good optical quality of float glass resulted in a vanishing share for other sheet‐ and plate‐glass processes. As of 2015, more than 400 float plants are operated worldwide, units being up to about 500 m long (Figure 7). With a typical size of about 25 × 60 m, melting tanks are bigger than Olympic swimming pools to produce 600 metric tons (and even up to 1300 tons in the biggest plants) of flat glass per day with an investment cost ranging from 70 to 200 million dollars or euros, depending on actual size, location, and product complexity. As for the inflation‐adjusted production cost (cf. Chapter 9.6), it has been almost continuously decreasing by a factor of 4 from 1965 to reach today a range of 200–300 dollars or euros per ton (Chapter 9.6), raw materials and energy accounting both for about 20% of it.

When a molten glass is poured onto a clean molten metal bath, it floats, thanks to its much lower density, spreads out, and thins to the point where the gravitational forces and the surface tensions among the glass, molten metal, and atmosphere are in equilibrium to reach the so‐called equilibrium thickness. The lower and especially the upper surfaces of the molten glass are fire‐polished, perfectly flat, and parallel except at the edges. The float process is based on this principle. Among metals or alloys that are liquid between 600 and 1050 °C, the relevant temperature range for glass forming, pure tin was the obvious choice because of its low melting temperature of 232 °C, high density of about 6.5 g/cm³ at 1000 °C, low vapor pressure of about 10⁻⁷ atm at 1000 °C, high boiling point of 2602 °C, low reactivity with silicates in the metallic state, and not too high cost (about 20 dollars/kg as of 2015).

Figure 7 Overview of a float‐glass plant (scale not right: size of the right‐hand side, for instance, much exaggerated) http://www.glassforeurope.com/en/industry/float‐process.php

The aforementioned equilibrium thickness T_e is given by

(1)

where S_ga, S_gt, and S_ta are the surface tensions at the glass–atmosphere, glass–molten tin, and tin–atmosphere interfaces, respectively, g is the gravitational constant and ρ_t and ρ_g are the density of the molten tin and glass, respectively (Table 2). For soda‐lime silicate glass floating on clean molten tin under a nitrogen‐hydrogen atmosphere, T_e is 6.9 mm (Figure 8), a thickness that is actually insensitive to small changes in the chemical compositions of the atmosphere, metal bath, or glass [1, 3–9].