Page 148 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 111

more effective Lewis base or nucleophile. Phosgene is added typically via a chlorinated solvent and
tertiary amine. The product is an unusually tough and transparent plastic available under the trade
names of Lexan (General Electric) and Merlon (Mobay).
Polycarbonates can also be made from the ester interchange between diphenyl carbonate and
o
o
BPA. The melting point of PCs is decreased from 225 C to 195 C when the methyl pendant groups
are replaced by propyl groups.
O CH 3
CH 3 R
HO OH + O O
(4.57)
Cl Cl O
CH 3
CH 3
R
Bisphenyl A Phosgene Polycarbonate
Polycarbonates and PC-polyester copolymers are used for glazing, sealed beam headlights, door
seals, popcorn cookers, solar heat collectors, and appliance housings.
Essentially all compact discs (CD), and related audio and video storing devices, have similar
components. Here, we will focus on the composition of CDs that are purchased already containing
the desired information and CDs that can be recorded on, CD/Rs. The major material of all of these
storing devices is a PC base. Thus, these devices are PC laid over with thin layers of other materials.
Of the less than 20 g CD weight, over 95% is the PC.
The aromatic rings contribute to PCs high glass-transition temperature and stiffness (Table 4.4).
The aliphatic groups temper this tendency giving PC a decent solubility. The two methyl groups also
contribute to the stiffness because they take up space somewhat hindering free rotation about the
aliphatic central carbon moiety. Factors contributing to PC chain association are interaction between
the aromatic rings of different parts of the same or different PC chain segments and the permanent
dipole present within the carbonyl group. The lack of “hydrogen-bonding” hydrogens on PC means
that this type of association is not present. The associations between PC segments contribute to a gen-
eral lack of mobility of individual chains. This results in PC having a relatively high viscosity, which

ultimately leads to a low melt flow during processing. The moderate inflexibility, lack of ready mobil-
ity, and nonlinear structure contribute to PC having a relatively long time constant for crystallization.
Cooling is allowed to be relatively rapid so that most PC products possess a large degree of amor-
phous nature and accounts for PC having a high impact strength that is important in its use to blunt
high impacts and important to CDs to provide a semirigid disc that can be dropped and not readily

shatter. Thus, control of the rate of flow and cooling is an important factor in producing CD-quality
PC material. A high degree of amorphous nature also contributes to the needed optical transparency
with amorphous PC having a transparency near that of window glass.

TABLE 4.4
General Physical Properties of a Polycarbonate

o
Heat-deflection temperature (1,820 kPa; C) 130
Maximum resistance to continuous heat ( C) 115
o
Crystalline melting point ( C) 225
o
o
Coefficient of linear expansion (cm/cm- C, 10 ) 6.8
–5

Compressive strength (kPa) 8.6 × 10 4
Flexural strength (kPa) 9.3 × 10 4
Impact strength (Izod: cm-N/cm of notch) 530
Tensile strength (kPa) 7.2 × 10 4
Ultimate elongation (%) 110
Density (g/mL) 1.2

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