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Tempered glass breaks into small, blunt granular pieces (instead of long sharp shards) because the rapid-cooling step in its manufacture locks the outer surfaces in compression and the core in tension. When a crack finally beats the surface compression, the stored tensile energy in the core makes the entire pane crumble at once into tiny pebbles. Car side windows use tempered glass; windshields use laminated glass (two glass plies bonded with a PVB plastic interlayer), which holds the broken pieces in place after impact.
From coffee tables and skyscrapers to house windows and car windshields, we are surrounded by glass. It is a terrific and highly useful material – transparent, reflective and reasonably strong (depending on the type of glass).
However, strength is not the only thing that varies depending on how the glass is made; there’s another rather unusual trait that separates certain glass types from others – the way the glass breaks.
You have surely seen movie scenes where automobiles (like cars or trucks) blow up or get smashed against something. During any such scene, have you noticed that the glass in car windows doesn’t break like regular glass (that we use in our daily lives)? Instead, it breaks up into very tiny, granular pieces.
The question is, why does that happen?
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What Is Tempered Glass?
Judging by its appearance and how it feels to the touch, it’s easy to tell the glass they use in automobiles apart from regular glass. The former is called tempered or toughened glass, and is used in shower doors, refrigerator trays, architectural glass doors and tables, oven doors, and smartphone screen protectors. It is typically 4-5 times stronger than regular annealed glass and tolerates sustained temperatures up to roughly 250 °C (480 °F), with thermal-shock resistance of about 150-200 °C, far better than regular glass.
Apart from its strength, tempered glass is also known for the characteristic way it breaks. Unlike regular glass, which shatters into sharp shards that can potentially cause injuries, tempered glass breaks into smaller pieces that interlock with neighboring pieces and therefore don’t fall readily.
Why Does Tempered Glass Break Into Tiny Pieces?
As mentioned earlier, the characteristics of any glass type depend on the materials it is made with and the chemical and mechanical processes it undergoes during its manufacture. The way that tempered glass breaks into much smaller pieces is specifically attributed to the manufacturing process. The glass is heated to around 620 °C (above its softening point) and then rapidly cooled (quenched) with jets of cold air on both sides.
The cooling process occurs more quickly on the outer surface of the glass than at its center. As a result, the center attempts to retract from the outer surfaces as it cools. This causes the outer surfaces to experience compressive stress while the center is under tension.
The compressive stress gives toughened glass its increased strength; the higher the stress, the smaller pieces that the glass breaks into. The benefit of this differential heating comes in the form of increased endurance of the glass; it means that such glass can be stretched or pulled to a certain limit without breaking, which is a good thing to have in many forms of glass.
As an optional finishing step, some panels are etched with hydrofluoric acid to blunt microscopic surface cracks and squeeze out a bit more practical strength. One quirky failure mode worth knowing about: trace nickel sulphide (NiS) impurities, accidentally introduced from furnace refractories or raw materials, can become trapped in the glass. Over months or years, these inclusions slowly transform from their high-temperature alpha phase back to the beta phase, expanding by 2-4% and sometimes causing apparently random spontaneous shattering. To weed out at-risk panels, manufacturers run a “heat-soak test,” reheating finished glass to deliberately trigger failures before installation.
What Is Laminated Glass?
Note that toughened glass, i.e. the glass used in side windows, is different from the glass used in the windshield. The type used in the latter is called laminated glass, which is made by sandwiching a laminate (usually polyvinyl butyral or PVB plastic) between two (or more) layers of glasses and bonding them using high pressure and heat.
How Laminated Glass Is Different From Other Glasses?
In addition to being very strong, this also offers a unique advantage. Upon breaking, the broken pieces of glass don’t usually fall out; in fact, they stick to the laminate and tend to stay in the frame. This is why applying significant force on the windshield will make the whole thing pop out of its frame in a single piece.
Due to their strength and the ability to withstand a lot of pressure, laminated panels are widely used in office complexes, public buildings, and most bullet-resistant glazing (which is typically multi-layer laminated glass, sometimes combined with polycarbonate, rather than tempered glass on its own). Laminated glass also offers decent soundproofing. All in all, both types of glass are strong and have their own characteristic properties, so choosing the ‘right’ type of glass depends entirely on your requirements.
