Why is laminated glass used for windshields

Classic car glass

Always safety glass in the car: When broken, regular "window glass" forms dangerously sharp and large pieces of broken glass which, in an emergency, pose an irresponsible risk of injury. Therefore, various types of safety glass are used in the automotive industry. A basic distinction is made between single-pane (ESG) and laminated safety glass (VSG).

Disc stamp gives information: Tempered safety glass can be identified in the window stamp, e.g. by the designation "tempered". Another clue is when the two parallel slashes (//), the character for laminated glass is missing.

ESG for side windows: While laminated safety glass is now used almost exclusively for windshields, side and rear windows are largely made of Toughened safety glass (English: tempered glass).

Patent from 1930: Regular glass panes were used for car glass until the 1920s. In accidents, the large pieces of broken glass led to serious injuries that were sometimes fatal. The Herzogenrath glass works near Aachen (today Saint Gobain) then applied for a patent for toughened safety glass as early as 1930. The brand name "Sekurit" has been synonymous with safety glass for many years.

ESG: Crumb jars bring security

Unlike a laminated glass pane, toughened safety glass can withstand even the heaviest loads, such as multiple hammer blows, mostly without damage.

Characteristic fracture pattern: Tempered safety glass does not form long cracks, as is typical in laminated glass, but rather shows a characteristic image with a dense network of fine cracks.

Crumb jar: In the event of breakage, toughened safety glass does not break into dangerously sharp shards, but rather crumbles explosively into small, relatively harmless glass crumbs. It is therefore also called the "crumb jar".

Toughened glass

Cooling down brings excitement: In order to obtain the desired properties of crumbling, the still hot flat glass is first cooled down quickly. In a so-called tempering furnace, the glass is heated to 620 ° C within a defined period of time. At this temperature the glass becomes soft as dough and can be easily shaped into the desired shape.

The formed glass is then shock-cooled with cold air. The glass surface cools down significantly faster than the glass core. This gives the surface compressive stress and the core tensile stress. This is why one speaks of toughened glass. When ESG breaks, this preload is suddenly “discharged” - the result is the thousands upon thousands of small fragments.

Tempered glass = robust glass

In addition to the increased security, single-pane safety glass is characterized by further advantages:

  • Increased flexural strength (120 N / mm² compared to 45 N / mm² for float glass)
  • Increased shock and impact resistance
  • Increased resistance to temperature changes across the pane surface (200 K instead of 40 K with normal float glass)

Relatively insensitive: In practice, the insensitivity of toughened safety glass becomes noticeable at the latest when it is removed or installed. The side and rear windows made of toughened safety glass are comparatively robust and, if necessary, can be pushed out from the inside with your feet in a rustic way. A windshield usually does not survive this type of treatment.

Limited design options

Compared to laminated glass, the design options for single-pane glass are severely limited. Here only the glass itself can be colored, while in the case of laminated glass panes, the glass itself and / or the intermediate film can be colored.

Limited editing options

After ESG has been pre-tensioned, i.e. it has cooled down, it cannot be shaped, cut or drilled any further. All shaping procedures must be completed before the cooling phase. As resistant as the toughened safety glass is against bending and impacts on the surface, the pane reacts sensitively if the balanced pre-tensioning is deliberately disturbed. Bumps against the side edge of the window, for example, quickly result in a total write-off.