Why do car windows shatter instead of cracking?
Automotive glass that is used today is designed to shatter into small, containable pieces for one reason – safety. Prior to the invention of tempered glass (constructed from multiple layers of glass with a polymer or plastic middle), glass would crack into multiple, large pieces when struck by an object. Anybody that has broken a house window knows how dangerous flying glass can be when it cracks into multiple pieces. In order to avoid additional injuries caused by flying glass, automotive manufacturers began to switch to tempered safety glass for windshields shortly after World War II.
Most people don’t realize this, but the modern automotive glass we use today was invented prior to the first Model-A rolled off of Henry Ford’s assembly plant in Michigan. In 1903 Edouard Benedictus, a French scientist, knocked a glass flask on the floor of the workshop he was working in at the time. Although he heard the glass shatter, when he looked down he noticed that the broken pieces were still fused together.
The reason the glass didn’t shatter into multiple, large chunks of sharp pieces was because the flask was filled with a mixture of cellulose nitrite (or a liquid plastic) that fused on the inside of the glass flask. This prevented the glass from shattering and lead to the invention of today’s safety glass. Benedictus then read a newspaper report on the increase in automotive accidents causing severe cuts and even eye trauma due to broken windshield glass.
Although it’s often suggested that the first practical use of tempered glass was automotive in nature, this isn’t true. The first use of this type of glass was used for the military (the United States and France) during World War I, when this material was made into lenses for gas masks. After seeing the success this material had protecting our troops, automotive manufacturers decided to spend the extra money to help save their consumers.
The windshields on today’s cars, trucks, and SUVs are made out of laminated safety glass. The glass is comprised of multiple layers of glass that is infused with UV-A and UV-B coating to protect the driver from ultraviolet rays. In the middle of these layers is a thin, polycarbonate material (plastic similar to the cellulose nitrite) that bonds glass particles together when the glass is hit with a blunt object. When it’s hit, the glass will break into very small particles that will hold together; as opposed to breaking in larger pieces comprised of sharper edges.
When a windshield cracks, the outer layer of safety glass is what is broken. While it is still possible to drive with a cracked windshield, it’s recommended to have the glass replaced or repaired as soon as possible in order to comply with the windshield rules and regulations for each US state.