The term ‘glass’ is commonly used for amorphous materials formed as a result of rapid cooling of a liquid, bypassing the crystallization phase. From a structural point of view, glass is a solid with a non-periodic lattice of atoms. Silicon, boron and phosphorus oxides, i.e. SiO2, B2O3 and P4O10, and their alloys with other oxides, e.g. alkali metals and alkaline earth metals, have the ability to form glass (by solidification into an amorphous mass). As a primary element, selenium, sulphur, carbon, silicon, tellurium, arsenic, germanium, boron and phosphorus have glass-forming properties. In addition to the previously mentioned substances, the same properties are observed in some highly polymerized organic substances, such as polystyrene and compounds with a hydroxyl group, such as glycerine.
The properties of glass
Unlike anisotropic crystalline bodies, glasses have isotropic properties. As the material is heated, it gradually softens and continuously transitions from a solid-like state to a state where it can be described as a super-cooled, highly viscous liquid. The temperature range in which this transformation is observed is relatively narrow and is known as the glass transformation range. Several significant changes can be observed – there is a rapid change in specific heat, the refractive index, the thermal expansion coefficient and permittivity. At temperatures below the transformation range, glass is hard and brittle. As the temperature increases, it becomes more and more plastic until it turns into a more mobile liquid. The transformation range of quartz glass is around 1500 K, while for silicate glasses the temperatures are slightly lower, around 800-1000 K, depending on the exact material composition.
The properties of glass
Both quartz and silicate glass have a structure resembling crystalline silicates. It is composed of tetrahedral SiO4 groups that connect to form a rigid three-dimensional structure. The difference, however, is their arrangement, because unlike crystalline bodies with an ordered crystal lattice, the groups present in glass are interconnected in a disordered way. The glass system is described as apparently stable, which means that it does not reach an equilibrium, but tends towards the crystalline state.Under normal conditions, this process is so slow that it is imperceptible. It can only be observed on very old glasses. However, the speed of the process can be increased by higher temperatures of 1200-1400 K, depending on the grade of the glass. A noticeable post-crystallization change is a characteristic cloudiness and an increased brittleness of glass. The plasticity of the glass mass can be freely changed using the appropriate processing temperature, and it can be formed by blowing, pressing, etc.
Examples of glass materials
- Soda-lime glass 12.9% Na2O (soda), 11.6% CaO (lime, calcium carbonate), 75.5% SiO2 (glass sand).
- Potassium-calcium glass, where Na2O has been replaced by K2
- Soda-potassium-lime glass, which contains both sodium and potassium oxides.
- Jena glass 74.5% SiO2, 8.5% Al2O3, 4.6% B2O3, 7.7% Na2O, 3.9% BaO, 0.8% CaO, 0.1 % MgO.
Silicate glass
Silicate glass is the most commonly used type of glass, which is produced by fusing quartz sand with soda Na2CO3 and limestone CaCO3 at a temperature of approx. 1800 K. Thanks to such conditions, it is possible to introduce silicon, sodium and calcium oxides (SiO2, Na2O i CaO) into the mass. The basic glass-forming oxide in its composition is SiO2, and its lattice is the so-called silicon–oxygen bond that contains intermediate ion substitutions with intervening modifier ions. They come from additionally introduced oxides, which is to to change the properties of the glass.
Glass staining
Transition metal oxide additives are used to stain the glass.Cobalt oxides give a violet-blue colour, dichromium trioxide gives a green colour, and iron oxides, depending on the conditions in the furnace, give a green colour in a reducing atmosphere, and a brown colour in an oxidizing atmosphere. Staining the glass to a ruby red colour is done by using colloidally dispersed gold – the glass mass is melted, and during decomposition, gold in atomic fragmentation is released. It is initially colourless, but after reheating to a temperature of approx. 800-900 K and slow cooling, it becomes ruby red. A similar mechanism is used to produce yellow glass, but colloidal silver is used instead of gold.
Glass reinforcement
It is possible to improve the quality of the glass surface and to modify it so that there are no cracks or their displacement. There are three main types of glass reinforcement processes:
- Quenching, in which the material is heated to high temperatures and then cooled in air or oil. Since the surface cools faster than the inner layer, its dimensions cannot match. The interior is stretched by the surface and the surface is compressed by the interior.
- Chemical hardening allows to achieve similar effects as quenching. The glass is placed in a molten salt containing potassium cations, e.g. in KNO3 heated for 12 hours at 500 oC Diffusion causes the exchange of ions from Na+ to K+ as they stretch the outer surface.
- Glass lamination is a method of placing a layer of polymer between a minimum of two layers of glass. This is possible in two ways – the glass can be pressed with a polymer, or a liquid polymer can be poured onto glass layers.
Raw materials
Most of the substances needed in glass production are of mineral origin. These include: sand, limestone, dolomite, anhydrite.However, substances that are products of the chemical industry, such as soda, are also used.Currently, more and more importance is also given to secondary raw materials, i.e. cullet. Cullet is classified into two categories – cullet formed in the production process, which, after grinding. is suitable for reprocessing, and foreign cullet, i.e. post-consumer material that must be cleaned and refined in order to be reused.
Glass recycling
The key aspect is understanding that not all glass is recyclable. Cullet is a very important secondary raw material, but such materials as containers permanently connected with other raw materials, ceramics, lenses from glasses, heat-resistant glasses, light bulbs, syringes, etc. are not suitable for reprocessing. Glass recycling is a multi-stage process, and the first stage is proper waste sorting. At the recycling facility, waste is weighed and checked for suitability for reprocessing. The next stage is crushing and removing labels and minor dirt from previously separated materials. After initial cleaning, the waste is divided according to colour and transported to the glassworks. In such facilities glass waste is melted at 1200oC into a glass mass, from which new products are then formed. It is interesting that the processing of glass, unlike paper or plastic, is practically endless. After remelting, the properties of glass do not change.