Page 99 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.39

FIGURE 2.30 Polymerization route for polymethylpentene.

hydrocarbons. Like polyethylene and polypropylene, it is susceptible to environmental
271
stress cracking and requires formulation with antioxidants. Its use is primarily in injec-
tion molding and thermoforming applications, where the additional cost incurred com-
pared to other polyolefins is justified by its high melt point (245°C), transparency, low
density, and good dielectric properties. The high degree of transparency of polymethyl-
pentene is attributed both to the similarities of the refractive indices of the amorphous and
crystalline regions, as well as to the large coil size of the polymer due to the bulky
branched four carbon side chain. The free-volume regions are large enough to allow light
of visible-region wavelengths to pass unimpeded. This degree of free volume is also re-
3
sponsible for the 0.83 g/cm low density. As typically cooled, the polymer achieves about
272
40 percent crystallinity, although with annealing can reach 65 percent crystallinity. The
structure of the polymer repeat unit is shown in Fig. 2.31.

FIGURE 2.31 Repeat structure of polymethyl-
pentene.

Voids are frequently formed at the crystalline/amorphous region interfaces during in-
jection molding, rendering an often undesirable lack of transparency. To counter this,
polymethylpentene is often copolymerized with hex-1-ene, oct-1-ene, dec-1-ene, and oc-
tadec-1-ene, which reduces the voids and concomitantly reduces the melting point and de-
gree of crystallinity. 273 Typical products made from polymethylpentene include
transparent pipes and other chemical plant applications, sterilizable medical equipment,
light fittings, and transparent housings.

2.2.21 Polyphenylene Oxide
The term polyphenylene oxide (PPO) is a misnomer for a polymer that is more accurately
named poly-(2,6-dimethyl-p-phenylene ether), and which in Europe is more commonly
known as a polymer covered by the more generic term polyphenyleneether (PPE). This en-
gineering polymer has high-temperature properties due to the large degree of aromaticity
on the backbone, with dimethyl-substituted benzene rings joined by an ether linkage, as
shown in Fig. 2.32.
The stiffness of this repeat unit results in a heat-resistant polymer with a T of 208°C
g
and a T of 257°C. The fact that these two thermal transitions occur within such a short
m

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