When Glass Breaks
Conventional window glass was not designed to resist wind-blown debris, earthquakes, explosions and terrorist attacks. When subjected to such stresses, existing glass often breaks into lethal shards that are hurled from the window frame, endangering building occupants and passers-by. Broken glass also causes additional property damage.
Security window film can improve the ability of existing glass to mitigate the impact of explosive force and wind-blown debris. The primary function of security film is to hold glass intact if it is broken.
A hurricane can hurl an object through a window, thus causing dagger-like glass shards to strike occupants. An explosion of a bomb creates a shock wave that breaks glass into lethal projectiles. A sufficient explosion may cause glass to become atomized. When an explosive shock wave causes victims to gasp, they breath in atomized glass particles and often die.
Typical window performance problems include unacceptable air infiltration, poor insulating capability, inability to block solar heat, transmission of ultraviolet radiation and noise, and vulnerability to electronic eavesdropping. Security enhancements to glass become more economically feasible if they also improve window performance capabilities.
Existing glass can be replaced with laminated glass, which is two or more pieces of glass bonded by a polyvinyl butyral plastic interlayer. Compared to conventional glass, laminated glass can provide increased resistance to wind-blown debris and seismic and explosive force.
Security window film is an alternative to replacing existing glass with laminated glass. Window film can be optically clear, tinted or reflective, and it is applied to the interior surface of existing glass. Typical film installations cover the visible portion of the glass surface to the edge of the frame, but do not extend to the glass edge inside the frame.
Film can be applied to either single pane or many types of insulating glass. When appropriate film is properly applied to insulating glass, it does not impact the integrity of an insulating glass sealant or generate thermal stress to glass from uneven heat absorption. Applied security window film is available with or without solar control capabilities.
Because security window film can stretch without tearing, it can absorb a significant degree of the shock wave of an explosion. As this explosive force moves toward the glass and pushes it inward, the glass eventually cracks and breaks. However, the security film applied to the rear of the glass continues to absorb the shock wave, stretching until it can no longer bear the pressure, at which time it bursts.
The shock wave, when great enough to break the glass, is not enough to shear the film. This results in the glass being broken but held intact by the film.
Both laminated glass and security window film can mitigate the impact of explosions, wind-blown debris and earthquakes. The performance of both depends on the relationship of each to the existing window frames.
In the case of laminated glass, the window frame must support the weight and thickness of the glass for the total glass and window system to resist stress. Installing laminated glass in existing window frames that are not designed to support the weight of laminated glass may not prevent the glass separating from the frames when the glass is stressed.
Similarly, the ability of security window film to resist force may increase if the film is not only applied to the glass but attached to the frame. Many film manufacturers market attachment mechanisms to secure film to the window frame.
Tests verify that many security window films provide equivalent, or in some cases, superior ability to withstand stress compared to laminated glass.
Also, laminated glass is not as energy-efficient as other glass options, resulting in a trade off between energy and safety/security performance. Its composition and resistance to force impede the ability of laminated glass to be broken for emergency entrance or egress.
The optimum security window film not only provides increased protection from stress, it may reduce a building’s energy consumption by blocking solar heat. The cost of disruptions to building occupants in removing and replacing existing glass compared to applying security window film to existing glass is also a factor when comparing laminated glass to security film.
Marty Watts is president and CEO of V-Kool Inc., a Houston-based North American distributor of energy efficient and security applied window films. Prior to founding V-Kool in 1997, Mr. Watts worked for more than 20 years in both window film and in the design and manufacturing of in-store retail signage systems.