Chemical Composition and Physical Characteristics of Glass

of disconnection caused by sodium oxide. Due to this reaction the network is closed and stabilized.

Figure 14: Reaction of Al₂O₃ to point of disconnection: Network is closed and stabilized [78]

In the case that the concentration of Al₂O₃ is higher than the concentration of the modifier the points of disconnection will be formed as well (see also Figure 13).

A compilation of the different substances and their network functions is displayed in Table 3.

Table 3: Substances classified as network former, network modifier and stabilizer, which can act as network former and modifier

Network former [3]

Network modifier [3], [78]

Stabilizer [78]

SiO2 CaO Al2O3

B2O3 K2O B2O3

P2O5 PbO

GeO2 Na2O

As2O3 Li2O

As2O5 Rb2O

Al2O3 CsO2

BaO

components leads to a lower melting temperature and usually to advantages during the following processing since lower temperature levels are required. A reduction of energy demand is the consequence during all production steps.

A large variety of substances which can be added to quartz and a high number of possible resulting mixtures exists. Each mixture with its specific composition has its specific properties. The melting temperature T_m, the transformation temperature T_g and other temperatures, different for each glass mixture and important for the manufacturing and forming process, are some of those properties. With regard to the manufacturing process, a unification based on the viscosity was correlated to defined characteristic points respective of temperatures. The viscosity was chosen because that property has the same value at the characteristic points for every single glass composition while changing temperatures. The determination of the viscosimetric fixed points is standardized in different procedures [80], [86]. A summary of the characteristic points is given in Table 4.

Table 4: Viscosities of different characteristic points during manufacturing process of glass [78]

Viscosity η [dPa s] Characteristic points

10^2.0 Melting temperature to reach a homogeneous molten mass

10^4.0 Working point

10^7.6 Softening point

10^13.0 Annealing point

10^14.5 Strain point

The technical most relevant glasses are quartz glass, soda-lime glass, borosilicate glass and aluminosilicate glass. Table 5 gives the composition of the main substances of these glasses.

Each of those four glass types is classified as silicate glass due to the fact that the essential element is SiO2. The opportunity of variation the percentage of components leads to different chemical compositions and glasses with different characteristics and properties even in those four types. The result is a multitude of glasses which can be classified in one of the mentioned glass types.

Table 5: Summary of components of technical most relevant glasses

Component Percentage [mass-%]

Quartz glass [1] Soda-lime glass [1]

Borosilicate glass [1], [87]

Aluminosilicate glass [4], [88]

SiO2 ≥ 99 69 - 74 70 - 87 53 - 60

CaO 5 - 12 - 0 - 7

B2O3 - 7 - 15 0 -8

Na2O 12 - 16 0 - 9 0 - 1

K2O 12 - 16 0 - 9 ≤ 0.5

MgO 0 - 6 - 0 - 3

Al2O3 0 - 3 0 - 8 14 - 18

BaO 0 - 2 0 - 3 0 - 19

Pure quartz glass is mostly used as refractory material, because of the high melting temperature. The very low coefficient of thermal expansion is one of the main advantages of using glass. It has a very high chemical resistance [4], low electrical conductivity and a high UV transparency. The high costs of manufacturing, a result of the high pureness and the necessity of high temperature, limits the use of quartz glass mostly to astronomical, optical or high temperature applications with maximal operating temperature of T_S ~ 1000 °C [4].

Soda-lime glass is the most used type of commercial glasses. The mixture of that glass leads to a decrease of the melting temperature in a range of T_m = 1400 °C to T_m = 1500 °C and results in large scale continuous melting and high-speed production compared to other glasses. Substances used in the mixture are low cost products, like Na2CO3 or CaCO3, so the price of soda-lime can be kept low as well. The main use is in window glasses, beverage containers or even as thermal insulation wool [4]. Soda-lime shows a good chemical durability, but the high coefficient of thermal expansion (CTE) and the resulting possibility of thermal shock make this a bad option for a wide range of applications.

Characterized by high durability against aggressive chemicals, in addition to low CTE, borosilicate is an excellent glass for laboratory and pharmaceutical glassware. Other applications of that glass are kitchen implements or headlamps in the automotive sector.

The hazard of failure caused by thermal shock is very small, as a result of a low CTE.

Temperature differences create only small displacements and stresses and do not lead to breakage. Moreover the B₂O₃ acts as a network former in concentrations existing in borosilicate glass. The network is stabilized and the chemical resistance is increased.

The manufacturing process is comparable to soda-lime, but the differences in mixture of basic materials leads to higher melting temperatures in the range of T_m = 1550 °C to T_m = 1600 °C [4].

The special feature of aluminum oxide as a component of glass allows it to act as both a network former and a network modifier. This along with a low amount of alkalis leads to the effect of very high chemical resistance while the coefficient of thermal expansion ranges between soda-lime glass and borosilicate. Aluminosilicate manufactured as glass fibers is used as a component in fiber reinforced plastics, especially E-glass and S-glass. E-glass is a representative of a mixed type of glasses - the alumo-borosilicate glass. This results from the percentage of aluminum oxide of 12 mass-percent to 15 percent and the percentage of boron oxide between 5 percent to 8 mass-percent [88]. S-glass is a special development for the application as a reinforcing material and features a higher fraction of aluminum oxide compared to E-glass and is free of alkaline or boron components [88]. The density is decreased while the hardness and tensile strength of the material are increased. In addition to the application in composite materials, aluminosilicate is also used as flat glass sheet for multimedia displays. The chemical composition gives the potential of hardening the surface and resulting in higher resistance against scratches.

The described types of glasses were used in tests in the context of that thesis and represent the most used and manufactured glasses of industrial interest.

Corresponding to the individual use, glass can be classified as one of three types depending on the application and manufacturing process. This classification is given in Table 6 with possible fields of applications. The most common type is flat glass, resulting in its use as architectural material for windows or glazing of buildings. In the past the float process came to be the most important in manufacturing flat glass. The molten mass is lead over a molten bath of tin. Because of lower density the glass floats on the surface of the tin bath. Smoother surfaces and thinner glass thicknesses can be realized with this manufacturing process. Older production techniques are the milling or the stretching and drawing of molten glass. These are still applied but features lower production capacities and rougher surfaces of the glass. Furthermore, irregularities in thickness can occur.

Table 6: Classification of glass by type of applications and manufacture

Glass type Application

Flat glass Windows; architectural glass, automotive

applications; mirrors; heat protection

Container glass and glass tubes Laboratory glassware; pharmaceutics; lighting technology;

Optical glass Lenses; prism; mirrors;

Containment glass is mostly produced from molten mass by glassblowing process. A special species of that type of glass are glass tubes, which were formed in a manufacturing process by drawing tubes from the melting in horizontal or vertical direction dependent on procedure used [89] - [91]. The melting flows through a circular slot and is blown up by compressed air to prevent the collapse of round shape of drawn tube [91], [92]. The flow of material and drawing speed defines the wall thickness and the diameter of the glass tube. Due to the production from glass tubes thin glass fibers can be classified as containment glass as well. Containment glass has to comply with special requirements which are contrasted to flat glass or optical glass. So the resistance against internal pressure and axial strain is more important than optical criterions.

Optical glass may have the same chemical composition as flat glass or containment glass. An important characteristic of optical glass is the index of refraction, whose change depending on the temperature and the machinability of the glass. Lenses or prisms are mostly formed from other shapes like round slices or blocks of glass by grinding and cutting.