Float glass

Float glass is a sheet of glass made by floating molten glass on a bed of molten tin. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass. Most float glass is soda-lime glass, but relatively minor quantities of specialty borosilicate [ [http://www.schott.com/hometech/english/products/borofloat/index.html Schott Borofloat] ] and flat panel display glass are also produced using the float glass process. [Not all flat panel display glass is produced by the float glass process. The company Corning is using the overflow downdraw technique, while Schott uses the float glass technique (see [http://www.schott.com/fpd/english/lcd_glass/index.html Schott website] ).]

History

In earlier centuries, window glass or flat glass was made by blowing large cylinders or large disks. The cylinders were cut open and flattened, and then panes were cut from the sheets. Most glass for windows up to the early 19th century was made from such rondels, while most window glass during the 19th century was made using the cylinder method (these 'cylinders' were 6 to 8 feet (2 to 2.5 m) long and 10 to 14 inches (250 to 350 mm) in diameter).

The first advances in automating glass manufacturing were patented in 1848 by Henry Bessemer, an English engineer. His system produced a continuous ribbon of flat glass by forming the ribbon between rollers. This was an expensive process, as the surfaces of the glass needed polishing. If the glass could be set on a perfectly smooth body this would cut costs considerably. Attempts were made to form flat glass on a molten tin bath, notably in the US. Several patents were awarded, but this process was unworkable.

Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface, and then polishing both sides, a costly process. From the early 1920s, a continuous ribbon of plate glass was passed through a lengthy series of inline grinders and polishers, reducing glass losses and cost.

Glass of lower quality, sheet glass, was made by drawing upwards from a pool of molten glass a thin sheet, held at the edges by rollers. As it cooled the rising sheet stiffened and could then be cut. The two surfaces were less parallel and of lower quality than those of float glass. This process continued in use for many years after the development of float glass.

Between 1953 and 1957, Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers developed the first successful commercial application for forming a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight. [http://www.google.com/patents?id=Po1sAAAAEBAJ&dq=2911759 US patent 2911759 – Manufacture of flat glass] ] Full scale profitable sales of float glass were first achieved in 1960.

Manufacture

Float glass is made by melting raw materials, typically consisting of sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate). Other materials may be used as colorants, refining agents or to adjust the physical and chemical properties of the glass. The goal of the glass technologist is to use a glass recipe for maximum glass yield, efficient use of plant assets, at minimal costs to achieve the best quality product.

The raw materials are mixed and fed into a furnace that is natural gas or fuel oil fired at 1500°C. Common flat glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass. The raw materials, referred to as batch, blend together to form a large pool of molten glass. The molten glass is fed into a bath of molten tin (about 3-4 m wide, 50 m long, 6 cm deep) through a delivery canal.B. H. W. S. de Jong, "Glass"; in "Ullmann's Encyclopedia of Industrial Chemistry"; 5th edition, vol. A12, VCH Publishers, Weinheim, Germany, 1989, ISBN 3-527-20112-5, p 365-432.] The amount of glass allowed to pour onto the molten tin is controlled by a refractory gate called a tweel. The tin bath is provided with a protective atmosphere consisting of a mixture of nitrogen and hydrogen to prevent oxidation of the tin. The glass flows out onto the tin surface forming a floating ribbon with perfectly smooth glossy surface on both sides with an even thickness of approximately 7 mm. Thinner glass is made by stretching the glass ribbon to achieve the proper thickness. Thicker glass is made by not allowing the glass pool to flatten to 7 mm. Machines called attenuators are used in the tin bath to control both the thickness and the width of the glass ribbon.

As the glass flows along the tin bath, the temperature is gradually reduced from 1100°C until the sheet can be lifted from the tin onto rollers at approximately 600°C. It then passes through the lehr where it is further cooled gradually so that it anneals without strain and does not crack from the change in temperature. The glass travels down the rollers in the lehr for about 100 meters and comes out at the "cold end" where it is cut by machines.

Some tin is absorbed into the glass, and with a proper ultraviolet light a sheen can be seen which differentiates the tin from the non-tin side.

Market

As of 2007, the world float glass market is dominated by the four companies: Asahi Glass, NSG/Pilkington, Saint-Gobain, and Guardian Industries. Other companies include PPG, Central Glass, Hankuk, Visteon, Cardinal Glass Industries. [ [http://www.constructiondigital.com/NewsArticle.aspx?articleid=811 "Flat glass market to be 39 million tons by 2010"] ]

ee also

* Soda-lime glass, including composition and some properties of float glass
* Glass production
* Glass batch calculation

References and footnotes