TabLE 3 GLASS-FORMING ORGANIC MATERIALS Refractive index NAS Methylmethacrylate(70% Styrene(30%)Copolymer Polycarbonate 1.586 e-styrene *=depending on Copolymer 2.1.5 CRYSTALLINE AND AMORPHOUS BEHAVIOUR OF POLYMERS Generally the plastic materials can exist in an amorphous, in a crystalline or in a mixed state. The crystalline state is rare because the mobility of the large molecules that is required for the formation of a periodic arrangement is very low. The complex chemical bonds and the predominant homopolar character lead, in contrast to many inorganic materials, mostly to the formation of very complicated structures, e.g. mono- clinic, rhombic or triclinic crystals Many plastics exist in an amorphous state. The growth of macromolecules produces chains and three-dimensional networks. The disordered polymer structure of organic glass is very similar to the network structure of silicate glass. Typical organic glass hows, however, practically no tendency to crystallisation because of steric hindrance by sidegroups or other large substituents. Some plastics exist in a mixed structural type The crystalline areas in the amorphous matrix are often so extended that they can easily be detected by light scattering. In this case, the material cannot be used for optical applications 2.2 THERMAL BEHAVIOUR OF INORGANIC AND ORGANIC GLASSES Vitrification and softening are second-order phase transitions for both inorganic and organic glasses. To obtain a homogeneous melt with inorganic glass, a temperature is required where the viscosity n of the melt is about 10 Poise. The softening point is at n 10 Poise and the working point has a viscosity of n "10" Poise. The temperature T in the transformation interval for solidification corresponds to a viscosity range for glass of between n=10Poise, the annealing point, and n=10 Poise, the strain It was first clearly shown by Bartenev [21] that when the cooling rate is decreased the solidification te are of silicate glass decreases proportionally. This behaviour was confirmed for many types of silicate glass and organic polymers in subsequent papers by various authors [22]and [23]
12 TABLE 3 GLASS-FORMING ORGANIC MATERIALS Type Composition Refractive index Acrylic Polymethyl methacrylate 1.491 Styrene Polystyrene 1.590 NAS Methylmethacrylate (70%) 1.562' Styrene (30%) Copolymer Poly carbonate --- 1.586' CR 39 Allyldiglycolcarbonate 1.490 ABS Acrylonitril-butadiene-styrene Copolymer *=depending on composition. 2.1.5 CRYSTALLINE AND AMORPHOUS BEHAVIOUR OF POLYMERS Generally the plastic materials can exist in an amorphous, in a crystalline or in a mixed state. The crystalline state is rare because the mobility of the large molecules that is required for the formation of a periodic arrangement is very low. The complex chemical bonds and the predominant homeopolar character lead, in contrast to many inorganic materials, mostly to the formation of very complicated structures, e.g. monoclinic, rhombic or triclinic crystals. Many plastics exist in an amorphous state. The growth of macromolecules produces chains and three-dimensional networks. The disordered polymer structure of organic glass is very similar to the network structure of silicate glass. Typical organic glass shows, however, practically no tendency to crystallisation because of steric hindrance by sidegroups or other large substituents. Some plastics exist in a mixed structural type. The crystalline areas in the amorphous matrix are often so extended that they can easily be detected by light scattering. In this case, the material cannot be used for optical applications. 2.2 THERMAL BEHAVIOUR OF INORGANIC AND ORGANIC GLASSES Vitrification and softening are second-order phase transitions for both inorganic and organic glasses. To obtain a homogeneous melt with inorganic glass, a temperature is required where the viscosity r I of the melt is about 102 Poise. The softening point is at rl = 107.6 Poise and the working point has a viscosity of r I -104 Poise. The temperature Tg in the transformation interval for solidification corresponds to a viscosity range for glass of between 11 = 1023 Poise, the annealing point, and 1"1 = l0145 Poise, the strain point. It was first clearly shown by Bartenev [21] that when the cooling rate is decreased, the solidification temperature of silicate glass decreases proportionally. This behaviour was confirmed for many types of silicate glass and organic polymers in subsequent papers by various authors [22] and [23]
On the other hand, the softening temperature Tw is a function of the rate of heating This observation is of great practical importance in glass technology because during nnealing and tempering temperature changes may occur at very different speeds. Thus for different processes, differences in the vitrification temperature of up to 50 to 100C may result. It follows from experimental and mathematical treatments [7] that, at a standard glass transition temperature Tg or at a standard softening temperature Tw vitrification or softening occurs if the rate of cooling or heating is equal to 0.2 deg s for inorganic glass and to 0. 1 deg s" for organic polymers Some experimental data obtained from [7] and [ 18] are listed in Table 4. The values for the linear expansion coefficient a were taken near but clearly below the softening TABLE 4 SOFTENING TEMPERATURE AND LINEAR EXPANSION COEFFICIENT OF INORGANIC AND ORGANIC GLASSES Material 0056-0.08 440-480 0.70-1.10 Aluminium silicate glass 582-842 046-065 Soda lime borosilicate glass 708-815 032-0.52 Polystyrene It can be concluded from these investigations that the structure of glass dep its thermal history. Annealing always increases the density of the glass Different pieces of glass each with a different structure will have different softening temperatures Tw when heated at the same rate For technical applications, it is therefore Useful to choose maximum annealing temperatures below a temperature(T_200)oC to prevent unwanted deformations The expansion coefficient is a property of glass that is greatly affected by changes in composition. The linear expansion coefficient a in the glassy state does not, however, depend on the heating rate in the region below the softening point. It is assumed to be constant within this wide temperature range. As can be seen from Table 4, the thermal expansion of the plastics is much higher than that of inorganic glass. Generally, when glass is bonded with other materials having different rates of expansion, temperature hanges create mainly undesirable forces in the two materials. This affects many prop- erties such as adhesion in the case of deposited thin films. In many problems concerning heat transfer, the thermal conductivity n of the mate- als is an important factor. The rate at which heat is transmitted through glass by con- duction depends on size and shape, on the difference in temperature between the two faces and on the composition of the material. Thermal conductivity is commonly ex
13 On the other hand, the softening temperature Tw is a function of the rate of heating. This observation is of great practical importance in glass technology because during annealing and tempering temperature changes may occur at very different speeds. Thus for different processes, differences in the vitrification temperature of up to 50 to 100~ may result. It follows from experimental and mathematical treatments [7] that, at a standard glass transition temperature Tg sT or at a standard softening temperature Tw sT, vitrification or softening occurs if the rate of cooling or heating is equal to 0.2 deg s ~ for inorganic glass and to 0.1 deg s l for organic polymers. Some experimental data obtained from [7] and [18] are listed in Table 4. The values for the linear expansion coefficient ~t were taken near but clearly below the softening temperature. TABLE 4 SOFTENING TEMPERATURE AND LINEAR EXPANSION COEFFICIENT OF INORGANIC AND ORGANIC GLASSES Material T sT ctl0 -5 (~ (~ Fused silica 1580 0.056 - 0.08 Alkali silicate glass 536 - 696 1.15 - 0.96 Lead silicate glass 440 - 480 0.70 - 1.10 Aluminium silicate glass 582 - 842 0.46 - 0.65 Soda lime borosilicate glass 708 - 815 0.32 - 0.52 Acrylic 76 7-9 Polystyrene 72 6-8 It can be concluded from these investigations that the structure of glass depends on its thermal history. Annealing always increases the density of the glass. Different pieces of glass each with a different structure will have different softening temperatures Tw when heated at the same rate. For technical applications, it is therefore useful to choose maximum annealing temperatures below a temperature (Tg200)~ to prevent unwanted deformations. The expansion coefficient is a property of glass that is greatly affected by changes in composition. The linear expansion coefficient (x in the glassy state does not, however, depend on the heating rate in the region below the softening point. It is assumed to be constant within this wide temperature range. As can be seen from Table 4, the thermal expansion of the plastics is much higher than that of inorganic glass. Generally, when glass is bonded with other materials having different rates of expansion, temperature changes create mainly undesirable forces in the two materials. This affects many properties such as adhesion in the case of deposited thin films. In many problems concerning heat transfer, the thermal conductivity ~, of the materials is an important factor. The rate at which heat is transmitted through glass by conduction depends on size and shape, on the difference in temperature between the two faces and on the composition of the material. Thermal conductivity is commonly ex-
pressed in calories per centimetre per second per degree. Some data are listed in Tab.5 Compared with metals, the values for glass and plastic are low. Radiation is another heat-transfer process. It may be of greater importance than thermal conduction when the temperature is increased to higher ranges. Table 5 also contains some data on the specific heats of different types of glass and plastic [1, 18]. The specific heat cp of glass, which is important in determining its heat capacity, is a nearly additive quantity and can be calculated from the composition by using the factors for the various oxides. The factors and some experimental data are reviewed in refs. [I] and[4 TABLE 5 THERMAL CONDUCTIVITY AND SPECIFIC HEAT OF INORGANIC AND ORGANIC GLASSES (Typical mean values) 100° 100° 315 354 0.11 0.160.20 Soda-lime silicate glass 23 27 020 Soda-lime borosilicate glass 21 26 30 027 Lead silicate glass 17 0.21 Aluminium silicate glass 24 0.19 3.6 0.32 Thermal conductivity: A(cal cms deg)10 Specific heat: Cp(cal g deg) The ability to withstand thermal shock resulting from sudden changes in tempera ture is important for technical applications of glass. The thermal endurance of inorganic glass, studied mainly by Schott and Winkelmann [24](see also refs. [I] and [25]), is a very complex property. The investigations have led to the definition of a coefficient of thermal endurance F: aE in which P is the tensile strength, E is Young s modulus and p is the density It is interesting to know that most types of glass can withstand much greater tem- perature changes when suddenly heated than when rapidly cooled 2.3 MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES At normal temperatures, glass usually behaves as a solid material. The most ant properties of solids are elasticity, rheology and strength. These properties not only on the molecular mechanisms of the deformation process, but on the
14 pressed in calories per centimetre per second per degree. Some data are listed in Tab. 5. Compared with metals, the values for glass and plastic are low. Radiation is another heat-transfer process. It may be of greater importance than thermal conduction when the temperature is increased to higher ranges. Table 5 also contains some data on the specific heats of different types of glass and plastic [l,18]. The specific heat Cp of glass, which is important in determining its heat capacity, is a nearly additive quantity and can be calculated from the composition by using the factors for the various oxides. The factors and some experimental data are reviewed in refs. [l] and [4]. TABLE 5 THERMAL CONDUCTIVITY AND SPECIFIC HEAT OF INORGANIC AND ORGANIC GLASSES (Typical mean values) XI04 Cp Material - 100 ~ 0~ 100~ - 100 ~ 0~ 100 ~ Fused silicia 28 31.5 35.4 0.11 0.16 0.20 Soda-lime silicate glass 19 23 27 -- 0.20 -- Soda-lime borosilicate glass 21 26 30 -- 0.27 -- Lead silicate glass 11 14 17 -- 0.21 -- Aluminium silicate glass -- 22 24 -- 0.19 -- Acrylic -- 4.7 .... 0.35 -- Polystyrene -- 3.6 .... 0.32 -- Thermal conductivity: ~, (cal cm "1 s "l deg -l) 10 -4. Specific heat: Cp (cal g-1 deg-l). The ability to withstand thermal shock resulting from sudden changes in temperature is important for technical applications of glass. The thermal endurance of inorganic glass, studied mainly by Schott and Winkelmann [24] (see also refs. [l] and [25]), is a very complex property. The investigations have led to the definition of a coefficient of thermal endurance F: F= P / X (1) ctE ~/ p. Cp in which P is the tensile strength, E is Young's modulus and p is the density. It is interesting to know that most types of glass can withstand much greater temperature changes when suddenly heated than when rapidly cooled. 2.3 MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES At normal temperatures, glass usually behaves as a solid material. The most important properties of solids are elasticity, rheology and strength. These properties depend not only on the molecular mechanisms of the deformation process, but on the viscous
low and on the structural peculiarities of industrial glass samples. For the application of polymeric materials, it is important to know that they possess extraordinarily com- plex mechanical properties. The high deformability, the marked incompressibility and the high sensitivity to changes in temperature are typical Table 6, some mechanical data on various types of glass are listed [7, 18]. For grinding glass, the so-called grinding hardness is important. The value of this for glass is dependent on chemical composition. The silica base used in most inorganic glass compositions is essentially very hard, but additions of other materials to modify, for example, the optical properties in optical glass will reduce the hardness to a greater or lesser extent. The grinding hardness G H. is defined as a quotient: G H. = rate of re oval of standard glass /rate of removal of sample glass. Values are published in the glass catalogues. Unfortunately, there is no exact relationship between hardness and grinding hardnes 2.4 CHEMICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES Although many types of technical silicate glasses are highly resistant to chemical attack, inorganic glass cannot be treated as an inert material. Chemical reactions take place with water, acids, alkali, salt solutions and various vapours, e.g. SO2. Even appa ently chemically resistant glasses may be attacked locally, producing remarkable changes in the composition of their surfaces compared with the bulk. The effect is much stronger with some optical glasses. Generally, however, plastic materials are more resistant than inorganic glass The water attack starts with the diffusion of H into the glass. This is a rapid process at higher temperatures. The water uptake increases with increasing pressure and the glass begins to swell. The quantity of incorporated water usually amounts to a very small percentage of the weight of the sample. Its presence promotes the tendency to crystallisation. As can be seen in Table 7, silicate glasses are more strongly attacked in alkaline solutions than in neutral or acidic solutions because the alkali supplies hy droxyl ions, which react with the silica network. No protective layer forms during the corrosion The attack of acids differs from that of water because the dissolved alkali and basic xide components are neutralised by the acid. In this way, a silica-rich surface layer is formed which reduces the rate of attack with time. When major amounts of soluble oxides are present, which may occur with some types of highly refractive optical glass, the glass will disintegrate. The corrosion resistance is influenced by the glass composi tion. It increases with higher amounts of SiO2 or of Meo, e.g. CaO, MgO, ZnO and Pbo. Addition of even small amounts of Me2O3 impurities such as B2O3 and AlO ease the resistance. Proper tempering of leached silica-rich surface films on techni- cal glass decreases the porosity and increases the stability with regard to chemical attacks[,3,18,25]
15 flow and on the structural peculiarities of industrial glass samples. For the application of polymeric materials, it is important to know that they possess extraordinarily complex mechanical properties. The high deformability, the marked incompressibility and the high sensitivity to changes in temperature are typical. In Table 6, some mechanical data on various types of glass are listed [7,18]. For grinding glass, the so-called grinding hardness is important. The value of this for glass is dependent on chemical composition. The silica base used in most inorganic glass compositions is essentially very hard, but additions of other materials to modify, for example, the optical properties in optical glass will reduce the hardness to a greater or lesser extent. The grinding hardness G.H. is defined as a quotient: G.H. = rate of removal of standard glass / rate of removal of sample glass. Values are published in the glass catalogues. Unfortunately, there is no exact relationship between hardness and grinding hardness. 2.4 CHEMICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES Although many types of technical silicate glasses are highly resistant to chemical attack, inorganic glass cannot be treated as an inert material. Chemical reactions take place with water, acids, alkali, salt solutions and various vapours, e.g. SO2. Even apparently chemically resistant glasses may be attacked locally, producing remarkable changes in the composition of their surfaces compared with the bulk. The effect is much stronger with some optical glasses. Generally, however, plastic materials are more resistant than inorganic glass. The water attack starts with the diffusion of H + into the glass. This is a rapid process at higher temperatures. The water uptake increases with increasing pressure and the glass begins to swell. The quantity of incorporated water usually amounts to a very small percentage of the weight of the sample. Its presence promotes the tendency to crystallisation. As can be seen in Table 7, silicate glasses are more strongly attacked in alkaline solutions than in neutral or acidic solutions because the alkali supplies hydroxyl ions, which react with the silica network. No protective layer forms during the corrosion. The attack of acids differs from that of water because the dissolved alkali and basic oxide components are neutralised by the acid. In this way, a silica-rich surface layer is formed which reduces the rate of attack with time. When major amounts of soluble oxides are present, which may occur with some types of highly refractive optical glass, the glass will disintegrate. The corrosion resistance is influenced by the glass composition. It increases with higher amounts of SiO2 or of MeO, e.g. CaO, MgO, ZnO and PbO. Addition of even small amounts of Me203 impurities such as B203 and AI20 increase the resistance. Proper tempering of leached silica-rich surface films on technical glass decreases the porosity and increases the stability with regard to chemical attacks [1,3,18,25]
MECHANICAL PROPERTIES OF INORGANIC AND ORGANIC GLASSES modulus (HV, kp mm2)extension p mm )(kp mm )(kg mm) Fused silica 7220-8000 244-2.476900-7400 5,4-8.7 Soda-lime borosilicate glass 202-2276100-7310 65 Aluminosilicate glass 2.40-2.765700-6900 1.19-122300 2-8 6-8 11 yrene 300-400 15 2-6 7-10 TABLE CHEMICAL PROPERTIES Material Chemical attack by H2O NaoH (5%)HCI(5%) 100°C.24h or immersed * Fused silica 8x10 4x10 oda lime silicate glass 12x103 1 x 10 65-1.725°C,4h 26.550°C Soda-lime borosilicate glass 8x10 3.6 2x10 24x104 3.5x10 ractically no weight loss 0.3,25°C,24h* Polystyrene Practically no weight loss 0.2,25°C,24h**
16 9 r~ 9 z 9 r~ <z <z 0 r,D W,,-,,4 < ~ m L~ ! L~ K'~L~ 666 m m u m ~.~ ~ ~ "~ ..c~ ~ ~~ ~~ ! ! ! ! ! ! 0 [.- .~.-- r ~ ~.~ 2: o ~o ooX X Z X X r X X "~" o o 0 0 0 0 b~ N