Ordinary glass is widely used in our daily life, such as glass windows, mirrors, water glasses and bottles. The earliest glass was made of quartz sand by adding a co-solvent and melting at high temperature. Its main component is silicon dioxide, so it is called quartz glass or silicate glass. Because it needs to be heated to a high temperature, it consumes a lot of energy and costs a lot, and is gradually replaced by soda-lime glass.
The flat glass used to make glass doors and windows and the glass used to make glass bottles have slightly different proportions of minerals. In general, they contain 73% to 74% silicon dioxide and 13% to 14% sodium oxide. , 9% ~ 10.5% of calcium oxide, and the corresponding ratio of magnesium oxide, aluminum oxide, potassium oxide and other additives. Flat glass is easy to process, can be produced on a large scale, and is widely used. But its hardness is not enough, it is fragile, and it is easy to be injured due to irregular rupture. People hope to find a way to use the advantages of glass and make it not easy to break; even if it is broken, do not cause great harm to people, and tempered glass was born.
Speaking of tempered glass, it is necessary to mention the famous "Prince Rupert's Tears". In fact, it is a small glass ball, the shape of the head is like a water drop, and at the same time, it has a thin and long tail, like a long tear. Its head has incomparable strength, and even the impact of a bullet cannot break it. Its long, slender tail, you just have to gently break its tail, and the whole glass bead will instantly turn into powder.
When we drop a molten glass droplet into water, its surface will quickly condense and solidify. However, the internal temperature of the glass beads does not decrease so fast, so it only shrinks and hardens the surface, and the central part is still in a state of hot expansion, so the volume of the entire glass beads has not decreased much. When the glass in the middle of the droplet slowly cools, its outer shell is fixed, and the central glass shrinks but is pulled tightly by the outer shell so that it cannot shrink. As a result, a strong stress was formed between the center of the glass bead and the shell. This stress pulls the silica molecular network tightly together, making the already fragile glass very hard, which can withstand pressures up to 20 tons.
Production of tempered glass
Tempered glass is made by ordinary glass through heat treatment process, this process is called "tempering". Tempering generates strong stress that causes the surface of the glass to be compressed and internally pulled, so the rigidity and toughness of the glass are greatly enhanced. We can easily see the condition of the internal stress section of the tempered glass through the comparison of the fracture after the glass breaks.
After cleaning, the glass is heated to a high temperature above 650 ° C in a long heating furnace. At this time, the glass will soften, and the stress formed in the previous processing will be eliminated. When the glass is conveyed out of the furnace by the conveyor belt, the cooler placed on the outside cools the glass surface uniformly from all directions to make the outer layer harden quickly, and then the interior of the glass will cool down a little on the long conveyor belt.
Tempering causes a strong compressive stress on the glass surface, which is balanced with the tensile stress in the center of the glass. When the glass surface compressive stress is 3500 ~ 7500psi (pounds per square inch), we call it heat strengthened glass; when the surface compressive stress is greater than 1000psi, it will break into small pieces when it breaks, we call it tempered glass Only when the surface compressive stress is greater than 15000 psi (approximately 100 MPa) does the edge of the chip become less sharp. This glass is called safety glass.