Navigation and entertainment screens in cars are becoming ever-larger and, in some cases, the central control point for drivers and passengers. Many manufacturers consider using glass to cover the displays. It gives the same pleasing surface feel and high-quality look that users of tablets and smartphones have come to expect. However, glass also bears the additional risk that in the event of a crash it may splinter and injure the occupants.

Demonstration showing that Xensation Cover glass does not break even under great tension. (SCHOTT)
Unbelted passengers in the backseat are at particular risk as they might be catapulted forward through the gap between the front seats without obstacles. Data from the World Health Organization (WHO) shows that seat belt use in the backseat is compulsory in only 111 countries, excluding the United Arab Emirates or the US states of New York and Michigan. What is more important than the legal provisions is the acceptance of belts. Germany, with a use rate of 97 percent in the backseat, is a positive exception. The USA reaches a quota of 70 percent. Russia (8 percent), the Emirates (5 percent) and Mexico (4 percent) trail far behind. Unbreakable displays can protect both backseat passengers and front seat occupants against severe cutting injuries.

The Right Combination

DELO Industrial Adhesives and SCHOTT, a manufacturer of special glass, recently connected high-tech adhesives and specialized glass into an unbreakable material for automotive displays. In a series of head-on crash tests conducted by the two partners, it was demonstrated that a combination of high-strength aluminosilicate glass and special adhesives survives internationally standardized crash tests.

For the tests, the floated aluminosilicate glass Xensation® Cover made by SCHOTT was used. This special glass type is especially suitable for displays thanks to its resistance to scratches and breakage. It is currently the hardest glass available on the market. According to the tests conducted, it achieves compression stress values of 850 MPa, which is equal to a 20 percent higher bending tensile strength than that of comparable materials.

Comparison of a non-bonded display half (right) and a half that’s been bonded with DELO adhesive (left). (©DELO)
These strength values are achieved through a special procedure known as chemical hardening. The glass plate is deposited in a 100% potassium nitrate salt bath at a temperature of +390°C to +430°C for a period of 2 to 8 hours. The small ions of the glass plate are interchanged by large ions from the salt bath. The depth of layer (DOL) of at least 40 μm describes how deep the interchange of ions was.

The DELO adhesives used have been specifically optimized for the requirements of the automotive industry. They are highly flexible and optically completely transparent. Their index of refraction is adapted accordingly. Therefore, they reduce disturbing reflections caused, for example, by sunlight by two thirds and provide good color contrasts. Besides improved readability, bonding has further advantages. Contamination by dust between the LCD and glass cover are prevented. In addition, no humidity can condense behind the glass cover.

The adhesives also have a unique curing mechanism. First, they are light-cured within seconds to achieve a stable joint. Afterwards, the adhesive in shadowed areas is reliably cured through a reaction with air humidity. This mechanism even works at a very low relative air humidity of 20 percent. Further curing steps are not required, which results in short cycle times in production.

Head-On Crash Test

In a head-on crash test, the possible head impact in a car crash is simulated by a steel ball. DELO and SCHOTT analyzed how completely bonded displays behave in such a crash compared to conventional displays with an air gap between the protecting glass cover and the LCD. The head-on crash test was conducted in accordance with the criteria of the international ECE R21 standard, in which a steel ball with a weight of 6.12 kg impacts the displays with a speed of 25.3 km/h. The effects of the impact on the display or glass cover were analyzed and documented accordingly. The steel ball has an impact energy of 152J.