Page 149 - Carrahers_Polymer_Chemistry,_Eighth_Edition

112 Carraher’s Polymer Chemistry

While PC has the necessary desirable qualities as the basic material for information storage, it
also has some debits. First, PC is relatively expensive in comparison with many polymers. Its supe-
rior combination of properties and ability for a large cost markup allows it to be an economically
feasible material for specific commercial uses. Second, the polar backbone is susceptible to long-

term hydrolysis so that water must be ruthlessly purged. The drying process, generally 4 hours, is
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often achieved by the placement of PC chips in an oven at 120 C with a dewpoint of −18 C.
The PC utilized for information storage has strict requirements including a high purity, greater
than 87% spectral light transmission based on a 4 mm thick sample, a yellowness index less than
2
2, and light scattering less than 0.3 cd/(m -lx). Sources of PC are mainly two, virgin and recycled.
Virgin PC has an index of yellowing of 1.8 but the first reground PC has a yellow index to about 3.5.

Thus, CDs employ only virgin PC.
Requirements for CD-quality material requires PC with low levels of chemical impurities, low

particle levels, thermal stability, excellent mold release, excellent clarity, and constant flow and con-
stant mechanical behavior (for reproducibility). There exists a time/cost balance. High molecular

weight PC offers a little increase in physical property but the flow rate is slow making rapid produc-

tion of CDs difficult. The molecular weight where good mechanical strength and reasonable fl ow
occurs, and that allows for short cycles, is in the range of 16,000–28,000 Da.
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Injection molding requires the barrel temperature to be about 350 C with a barrel pressure in
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excess of 138 MPa. The mold is maintained at 110 C to insure uniform flow and high defi nition, and
to discourage an uneven index of refraction, birefringence. The CD is about four one-hundredths of
an inch (0.5 mm) thick. For prerecorded CDs, the PC is compression molded on a stamper imprinted
with the recorder information. This takes about 4 seconds. Once the clear piece of PC is formed, a

thin, reflective aluminum layer is sputtered onto the disc. Then, a thin acrylic layer is sprayed over
the aluminum to protect it. The label is then printed onto the acrylic surface and the CD is complete.
This process is described in greater detail shortly.
The construction of the recordable CD or CD/R is more complex. Standard CDs contain four or
five (a label layer) layers. These four (five) layers are:

• PC base
• Dye surface
• Refl ective layer
• Lacquer layer
• Label
We must remember that the information is closest to the label side of the CD not the clear plastic
side the data is read from. It is fairly easy to scratch the top surface of the CD rendering it unusable.
Some CDs have a special hard top coating that helps resist surface damage. Dust, minor scratches,

and finger prints are generally not harmful because the laser assemblies are designed to focus
beyond the disc surface.
The “play only” CD contains a series of pits and lands generated during the mastering process

based specifically on the data provided. The PC layer for the CD/R does not contain these pits and
lands but rather contains a shallow groove or pregroove used for timing and tracking. A CD/R
writes, records, information by using its lasers to physically “burn” pits into the organic dye layer.
When heated beyond a certain temperature, the area that was “burned” becomes opaque and refl ects
a different amount of light in comparison to areas that have not been “burned.” These “burned” and
“unburned” areas correspond to the pits and lands of the prerecorded CD allowing it to be “read”
by a regular CD player.
CD/Rs are Write Once/Read Many (WORM) storage discs. They cannot be erased once they
have been written on.
The dye coating is the most important, expensive, and complex part of the CD manufacturing
process. The dye layer serves to inhibit or permit laser radiation from piercing to the PC. There are

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