Tempered glass, such as that used in car rear windows, demonstrates variations in stress that can be observed through a polarizing filter. Tempered glass is approximately four times stronger than annealed glass. During its manufacturing process, the rapid cooling of the outer layer creates compressive stresses on the surface, balanced by tensile stresses within the glass body. Fully tempered glass with a thickness of 6 mm must have a minimum surface compression of 69 MPa (10,000 psi) or an edge compression of at least 67 MPa (9,700 psi). To qualify as safety glass, the surface compressive stress must exceed 100 MPa (15,000 psi). These surface stresses ensure that when tempered glass breaks, it fractures into small, rounded pieces rather than sharp, jagged shards.
The compressive stresses on the surface of tempered glass enhance its strength. In contrast, annealed glass has minimal internal stress and often develops microscopic surface cracks. When tension is applied, these cracks propagate rapidly, concentrating stress at the crack tip and causing the glass to break into sharp, irregular fragments. Tempered glass, however, resists crack propagation due to its surface compressive stresses, which contain and prevent flaws from spreading.
It is important to note that cutting or grinding must be completed before the tempering process. Any such actions or sharp impacts after tempering will cause the glass to shatter. The strain patterns created during tempering can be observed using an optical polarizer, such as polarized sunglasses.
Uses of Tempered Glass
Tempered glass is widely used in applications where strength, thermal resistance, and safety are critical. For instance, passenger vehicles require all three properties. Since vehicles are exposed to outdoor conditions, they must endure constant heating, cooling, and significant temperature
fluctuations throughout the year. Additionally, they must resist minor impacts from road debris like stones and provide safety during accidents. To minimize the risk of injury, tempered glass is used for side and rear windows, as it breaks into blunt, harmless pieces. However, windshields are typically made of laminated glass, which does not shatter into fragments. Some modern luxury vehicles now use laminated side windows for enhanced occupant retention, anti-theft measures, and noise reduction.
Other Common Applications
Tempered glass is also used in a variety of other settings, including:
- Balcony doors
- Athletic facilities
- Swimming pools
- Building façades
- Shower doors and bathroom enclosures
- Exhibition displays
- Computer cases (e.g., for enthusiast computing and case modding)
- Mobile phone screen protectors
Buildings and Structures
In construction, tempered glass is used for frameless assemblies (like frameless glass doors), structural applications, and other scenarios where human impact could pose a danger. Building codes in the United States mandate the use of tempered or laminated glass in specific situations, such as skylights, glass near doorways and stairways, large windows, windows close to the floor, sliding doors, elevators, fire department access panels, and glass near swimming pools.
Household Applications
Tempered glass is also common in household furniture and appliances. Examples include frameless shower doors, glass tabletops, glass shelves, cabinet glass, and fireplace glass.
Food Service
In the food service industry, “rim-tempered” glassware is popular, where only the rim of the glass or plate is tempered. Fully tempered glass is also used for its strength and resistance to thermal shock. Some countries have specific regulations for tempered glass used in food service. Additionally, tempered glass has become more common in bars and pubs, particularly in the United Kingdom, Australia, and the United States, to prevent broken glass from being used as a weapon.
Cooking and Baking
Certain types of tempered glass are designed for cooking and baking. Brands like Pyrex, Corelle, Glasslock, and Arc International manufacture tempered glass products for these purposes. This type of glass is also used for oven doors.
Touchscreen Devices
Most touchscreen devices incorporate toughened glass, such as Corning’s Gorilla Glass, for durability. Additionally, separate tempered glass screen protectors are available as accessories for touchscreen devices.
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Conclusion
Tempered glass is a versatile material valued for its strength, safety, and thermal resistance. Its applications span from vehicles and buildings to household items, food service, and technology. By understanding its properties and proper usage, tempered glass continues to play a vital role in ensuring safety and functionality across various industries.
FAQs About Tempered Glass
1. What makes tempered glass stronger than regular glass?
Tempered glass undergoes a special heat treatment process that creates compressive stresses on its surface and tensile stresses inside. This structure enhances its strength, making it about four times stronger than annealed glass.
2. Why does tempered glass break into small pieces instead of sharp shards?
The compressive stresses on the surface of tempered glass ensure that when it breaks, it fractures into small, rounded pieces rather than dangerous, jagged shards, reducing the risk of injury.
3. Can tempered glass be cut or modified after manufacturing?
No, tempered glass cannot be cut, drilled, or modified after the tempering process. Any attempt to do so will cause the glass to shatter due to the internal stresses created during manufacturing.
4. Where is tempered glass commonly used?
Tempered glass is widely used in applications requiring strength and safety, such as car windows, shower doors, building façades, touchscreen devices, and household furniture like glass tabletops and shelves.
5. How can you identify tempered glass?
You can identify tempered glass using a polarizing filter, such as polarized sunglasses, which reveal the strain patterns caused by the tempering process. Some tempered glass also has an etched logo or stamp indicating it meets safety standards.