Page 150 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 113

three competing organic dye polymers used to manufacture CD/Rs. The original dye polymer used
was the cyanine (and metal-stabilized cyanine) materials identified by a greenish tint with a gold

reflective layer or blue when a silver reflective layer is present. The employed cyanine dye is itself

blue. The suggested age for storage on cyanine dye discs is about 20 years and for metal-stabilized
cyanine it is about 100 years.
The second dye system is based on an almost clear yellow–green phthalocyanine dye. The esti-
mated lifetime of a disk based on the phthalocyanine dye is 100 years. The third dye system is based
on azo dyes. The azo dye used in the process has a deep blue color partially caused by its unique

silver alloy reflective layer. Again, a projected lifetime of 100 is sited.
The two most widely employed reflective layers are 24K gold and a silver alloy. The layers are

thin enough, about 50–100 nm thick, to allow us to see through them.
The first step in the manufacturing of a blank CD is the creation of a glass master with a laser

beam recorder (LBR). For the prerecorded CDs, pits and lands are etched into the photoresist or
nonphotoresist on the glass. For the CD/R, the LBR records a shallow groove as a continuous spiral
called the pregroove. This pregroove is not a perfect spiral, but it is “wobbled.” On the recorded
disc, the timing information necessary to control the disc’s rate of spin is included as data. But for
the CD/R disc, the CD recorder needs to have a way to guide the recording laser and the speed of the
blank disc. The wobbled pregroove provides the tracking and timing information for the recording
laser. The wobble is a slight sinusoidal wave that is about 0.03 mm from the center of the track path.
This wobble corresponds to about one-thousandth of the length of one complete waveform so it is
so small that it is not seen by the naked eye, but it is “seen” by the recoding laser. This slight diver-
sion, from a completely spiral pathway provides the timing information and guides the recorder.
This information is called the absolute time in pregroove or ATIP. It ensures that data is recorded
at a constant rate. The resulting data trail obliterates the wobble pregroove, leaving recorded data
in its place.
Once the glass master has been written, it is tested, silvered, and ready to be electroplated. It is
electrically conductive and is placed in a reservoir with an electrolyte solution containing a nickel
salt. Current is applied to the glass master eventually resulting in a metal layer on the glass after
about 2 hours. The nickel copy of the glass master is called the metal “father” and is used as a
stamper to produce CD/R discs. If the replication run is large, the metal “father” is returned to the
electroplating process for creation of metal “mothers” that are used to make metal “sons” identical
to the fathers. These “sons” are then employed in the manufacture of “blank” CD/R discs.
Injection molding is employed to stamp out copies of the master discs from PC. The molded PC
discs are cooled and hardened quickly, about 4–6 seconds, and evenly. The dye layer is then applied.

The applied dyes are often proprietary and continually modified in an attempt to get a “better” dye.
The dye must be compatible with the system and adhere to the PC base. It is applied by spin coating;
that is, the disc is spun and the dye is sprayed onto the surface. The dye is then dried and cured.
A metal layer is now added since the disc surface must reflect laser light. Aluminum is the

most cost effective and widely used for prerecorded discs but most CD/R discs use gold or silver

because of their greater reflectivity. The gold or silver is applied using sputtering or vacuum depo-
sition. A thin acrylic plastic layer is then applied by spin coating and cured in ultraviolet (UV)
light to aid in the protection of the disc. An optional label layer can now be added. I find that CD/

Rs with this additional layer are stronger and appear to record better. The CD/Rs are now pack-
aged, generally in packages of 10, 25, 50, and 100. The packaging is also plastic. My latest group
of CD/Rs has a black polystyrene base and a clear polypropylene cone. The bulk purchase price is
about 5–10 cents per disc, on sale. In a glass case, the individual CD/Rs often cost about a dollar.
Thus, for all the high technology required to produce a CD/R, it is inexpensive.
A laser is used to encode information through creation of physical features sometimes referred
to as “pits and lands” of different reflectivity at the PC-metal interface. As noted above, recordable

CDs contain an organic dye between the PC and metal film. A laser creates areas of differing refl ec-

tiveness in the dye layer through photochemical reactions.

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