Toughened and laminated glass are, of course, both forms of “safety” glass. Toughened, or “tempered”, glass is glass that has been heated to very high temperatures – in the region of 650°C – and cooled quickly. This process creates compression on the surface, and tension in the centre, of the glass, such that it is up to 400% or 500% more resistant to heat and physical shock than standard plate, or “annealed”, glass of the same thickness. Laminated glass, on the other hand, is composed of two, or more, pieces of glass laminated together and interleaved with a layer of PVB (“Polyvinyl Butyral”). PVB is a tough and flexible resin, and a combination of heat and pressure are used to bind the layers together.
Toughened versus Laminated Glass
Unlike standard annealed glass – which has little residual surface compression, and breaks into large, sharp shards when shattered – toughened glass is durable, and when it does break, breaks into small, blunt pieces. These pieces are relatively harmless, and unlikely to cause more than minor scratches or cuts, even in the worst cases. Toughened glass can also withstand changes in temperature of up to 135°C – some 60° greater than standard plate, or float, glass – and the fact that it smashes when hit with force can be an important safety consideration when choosing glass for domestic double glazing, for example. These desirable characteristics do come at a premium – typically 20%, or so – when compared to plate glass, but, as toughened glass is difficult (although not impossible) to break, replacement costs are likely to be greatly reduced.
Laminated glass, by contrast, tends to crack rather than smash, and this can present difficulties if a window needs to broken to provide a means of escape, from a domestic fire, for example. This characteristic does, however, have its benefits if laminated glass is employed in suitable surroundings. Laminated glass typically appears in shop windows, car windscreens and shower screens, and, if the glass breaks, the entire pane remains intact because of the PVB layer, so there is no danger of flying glass shards, or exposed sharp edges. If laminated glass is used externally, this behaviour can also allow it to provide an effective barrier against the elements, even if the glass is broken.
Laminated glass is typically constructed from two, 3mm plies of glass, giving it a total thickness of 6.4mm, including the PVB layer. This is thicker than toughened glass, and the insulation properties of laminated glass are correspondingly higher. This does, of course, also make laminated glass heavier than toughened glass, which may affect the longevity of doors and windows, which need to bear the extra weight when opened, or closed. Laminated glass is also more expensive than toughened glass, size for size.