Fragment Retention Film for Glass Vs. Tempering

Retention Film

• Fragment retention film increases the strength of window glass and keeps fragments together if the glass does fail. Fragment retention film has a big advantage in that it can be installed on existing windows. But its major drawback is relatively limited service life. Most films have to be replaced about every 10 years because of deterioration. They are made of layers of polyester film bonded together with a special laminating adhesive. Films are offered in thicknesses of 4/1,000th of an inch to 21/1,000th of an inch. The film is coated on one side with a mounting adhesive that absorbs ultraviolet light. It is available as clear film or in a variety of tints.

Mounting is Crucial

• The film is only part of the fragment retention system. The other part is the mounting. The film can just be stuck onto the glass, which would keep the glass fragments together but wouldn't do much to increase blast resistance. The window would still blow out of its frame. To keep the broken glass in the window frame, the film also needs to be attached to the frame. One attachment system uses metal battens and screws to secure the edges of the film to the window frame. Another system uses a silicone adhesive to glue the film edges to the window frame. The film can be attached to the frame on one, two or four sides. The four-sided attachment of heavy-grade film offers the most impact resistance and may be stronger than the window frame itself.

Tempered Glass

• Tempered glass, also known as safety glass, is ordinary glass that is made stronger through a process of heating followed by rapid cooling that compresses a thin layer of atoms on the surface and edges of the glass. To use tempered glass, though, you have to replace the entire window. This can be a problem with historic buildings. Also, tempered glass must be cut to exact size and specifications before tempering. Once the glass has been treated, any attempt to cut it will shatter it. Tempered glass, though, is roughly four times stronger than ordinary glass of the same size, type and thickness. But a sufficiently strong force will break it. When tempered glass fails, it breaks into hundreds of small blunt bits that vaguely resemble dice instead of the razor-sharp shards of ordinary glass.

Tempered Applications

• Tempered glass is used in automobile windows, patio doors, store countertops and other locations where strength and safety are important. But tempered glass must be properly installed. Tempered glass doesn't crack to relieve stresses from improper installation, but the stress still is there. A sharp contact or sudden temperature change can cause the glass to release the stress in a sudden shattering eruption of glass dice in a phenomenon known as spontaneous breakage.