Inducing one-way toughness in laminated polycarbonate glazing

To aid escape in an emergency from a vehicle fitted with seemingly indestructible glazing, a laminate system has been developed that permits directional impact failure to be induced in polycarbonate glazing components. Test components exhibit near-full impact resistance when impacted from one side, but fail at much lower energy levels when impacted from the other. Thus, any glazing component made from this material system will exhibit excellent intruder resistance from the outside, whilst emergency escape becomes much easier from the inside. The system comprises a combination of polymer layers with the polycarbonate on the inside of the vehicle and a second polymer laminate on the outside. Additional features can be incorporated into the polymer laminate such as extra UV and chemical protection, i.e. areas where polycarbonate is less resilient than other materials. The mechanism of directional failure depends on the outside laminate containing a series of small flaws, generated during the manufacturing process, that close up when impacted from the outside. When impacted from the inside, the outer laminate is placed into tension and these flaws initiate cracks, which propagate rapidly backwards through the component inducing premature failure. The level of inside impact resistance can be varied by altering the processing conditions under which a component is made so that the interfacial strength between the polycarbonate and laminate is weakened