Page 455 - Carrahers_Polymer_Chemistry,_Eighth_Edition

418 Carraher’s Polymer Chemistry

Armed glass is most commonly used as roofing on factory buildings. It is glass that has a built-in
metal grill that strengthens the glass. The metal grill is often like chicken wire in appearance. The
glass breaks similar to regular glass, but the wire mesh helps hold it into place. The visibility is
reduced because of the presence of the metal mesh.
Laminated glass is used in automobiles and often used for added protection in windows, balconies,
or sloping glass roofs. The laminated glass resists breakage in comparison to ordinary glass because
the PVB inner layer(s) helps dampen sharp blows. Even so, laminated glass does crack and it is more
easily cracked than tempered glass. But, it is harder to pierce than tempered glass because of the PVB
inner layer(s). Another difference is that laminated glass can be cut, sawn, or drilled whereas tempered

glass cannot. The PVB film also has ultraviolet-screening properties reducing discoloration of objects
placed behind the safety glass. It also acts to dampen sound for additional soundproofi ng.
Laminated safety glass is available in different thicknesses depending on the number of PVB-glass
layers. Bullet-resistant glass is one use for thick, multilayer laminated safety glass. The laminated

construction allows the multilayer assembly to have some additional flexibility with the multiple lay-
ers yielding, allowing the PVB layers to absorb some of the energy of the bullet. The plastic layers
help hold the shattered glass fragments together aiding in retaining a restraining barrier. Such glass
is used in bank teller windows, and in windshields for aircraft, tanks, and special automobiles and
trucks.
The ability to resist bullets and blasts is increased by increasing the number of layers of lami-
nated safety glass. Such increases give an increased glass thickness and weight. So there is a trade-
off between expected abuse and practicality.

12.8 LENSES

About 1,000 AD, reading stones started being used. These were segments of a glass sphere that were
used to magnify letters. The reading stones were placed directly on the letters. By 1,350 AD, reading
stones were hung with ribbons and strings near the eyes allowing the “wearer” to magnify objects
within their “sight.” In 1730, a London optician, Edward Scarlett, developed rigid sidepieces that
rested on the ears of the user. These were the first somewhat modern reading and seeing glasses. The

first bifocals were developed by Benjamin Franklin by cutting two lenses in half and placing one above

the other. Until relatively recently, glasses fitting was largely trial-and-error with the person wearing
the glasses responsible for determining the correct lenses. Even today, we can go to certain stores and
select a pair of inexpensive glasses from a group of general eye glasses through this same process.
Below is a general menu between properties and type of reading glasses lenses that best demon-
strate this property:

• Strongest (hardest to break)—polycarbonate
• Lightest—polycarbonate and plastic
• Greatest resistance to scratching—glass
• Clearest vision—glass and plastic
• Most responsive photochromic lens—glass
• Lightest photochromic lens—plastic
• Most resistant to heat and common household chemicals—glass
• Most resistant to fl ying objects—polycarbonate

In general, polycarbonate lenses are best for people

• Active in sports
• That use power tools

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