Page 78 - Handbook of Plastics Technologies

THERMOPLASTICS

2.18 CHAPTER 2

insulation properties. Pipes prepared from polybutylene can be solvent welded, yet the
160
polymer still exhibits good environmental stress cracking resistance. The chemical re-
sistance is quite good below 90°C but, at elevated temperatures, the polymer may dissolve
161
in solvents such as toluene, decalin, chloroform, and strong oxidizing acids.
Polybutylene is a crystalline polymer with three crystalline forms. The first crystalline
form is obtained when the polymer is cooled from the melt. This first form is unstable and
will change to a second crystalline form upon standing over a period of 3 to 10 days. The
third crystalline form is obtained when polybutylene is crystallized from solution. The
3
melting point and density of the first crystalline form are 124°C and 0.89 g/cm , respec-
tively. 162 On transformation to the second crystalline form, the melting point increases to
3
135°C, and the density is increased to 0.95 g/cm . The transformation to the second crys-
talline form increases the polymer’s hardness, stiffness, and yield strength.
Polybutylene can be processed on equipment similar to that used for low-density poly-
ethylene. Polybutylene can be extruded and injection molded. Film samples can be blown
or cast. The slow transformation from one crystalline form to another allows polybutylene
to undergo post forming techniques, such as cold forming of molded parts or sheeting. 163
A range of 160 to 240°C is typically used to process polybutylene. 164 The die swell and
shrinkage are generally greater for polybutylene than for polyethylene. Because of the
crystalline transformation, initially molded samples should be handled with care.
An important application for polybutylene is plumbing pipe for both commercial and
residential use. The excellent creep resistance of polybutylene allows for the manufacture
of thinner wall pipes compared to pipes made from polyethylene or polypropylene. Poly-
butylene pipe can also be used for the transport of abrasive fluids. Other applications for
polybutylene include hot melt adhesives and additives for other plastics. The addition of
polybutylene improves the environmental stress cracking resistance of polyethylene and
165
the impact and weld line strength of polypropylene. Polybutylene is also used in pack-
166
aging applications.
2.2.11 Polycarbonate
Polycarbonate (PC) is often viewed as the quintessential engineering thermoplastic due to
its combination of toughness, high strength, high heat-deflection temperatures, and trans-
parency. The world wide growth rate, predicted in 1999 to be between eight and ten per-
cent, is hampered only by the resin cost and is paced by applications where PC can replace
ferrous or glass products. The polymer was discovered in 1898, and by 1958, both Bayer
in Germany and General Electric in the United States had commenced production. Two
current synthesis processes are commercialized, with the economically most successful
one said to be the “interface” process, which involves the dissolution of bisphenol A in
aqueous caustic soda and the introduction of phosgene in the presence of an inert solvent
such as pyridine. The bisphenol A monomer is dissolved in the aqueous caustic soda then
stirred with the solvent for phosgene. The water and solvent remain in separate phases.
Upon phosgene introduction, the reaction occurs at the interface, with the ionic ends of the
growing molecule being soluble in the catalytic caustic soda solution and the remainder of
the molecule soluble in the organic solvent. 167 An alternative method involves transesteri-
fication of bisphenol A with diphenyl carbonate at elevated temperatures. 168 Both reac-
tions are shown in Fig. 2.14. Molecular weights of between 30,000 and 50,000 g/mol can
be obtained by the second route, while the phosgenation route results in higher-molecular-
weight product.
The structure of PC, with its carbonate and bisphenolic structures, has many character-
istics that promote its distinguished properties. The para-substitution on the phenyl rings
results in a symmetry and lack of stereospecificity. The phenyl and methyl groups on the

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