Page 87 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.27

FIGURE 2.20 General structure of polyetherimide.

The polymer should be dried before processing, and typical melt temperatures are 340
214
to 425°C. Polyetherimides can be processed by injection molding and extrusion. In ad-
dition, the high melt strength of the polymer allows it to be thermoformed and blow
molded. Annealing of the parts is not required.
Polyetherimide is used in a variety of applications. Electrical applications include
printed circuit substrates and burn-in sockets. In the automotive industry, PEI is used for
under-the-hood temperature sensors and lamp sockets. PEI sheet has also been used to
form an aircraft cargo vent. 215 The dimensional stability of this polymer allows its use for
large flat parts such in hard disks for computers.

2.2.14 Polyethylene (PE)
Polyethylene (PE) is the highest-volume polymer in the world. Its high toughness, ductil-
ity, excellent chemical resistance, low water vapor permeability, and very low water ab-
sorption, combined the ease with which it can be processed, make PE of all different
density grades an attractive choice for a variety of goods. PE is limited by its relatively low
modulus, yield stress, and melting point. PE is used to make containers, bottles, film, and
pipes, among other things. It is an incredibly versatile polymer with almost limitless vari-
ety due to copolymerization potential, a wide density range, a MW that ranges from very
low (waxes have a MW of a few hundred) to very high (6 × 106), and the ability to vary
MWD.
Its repeat structure is (-CH CH -) , which is written as polyethylene rather than poly-
2
2 x
methylene (-CH ) , in deference to the various ethylene polymerization mechanisms. PE
2 x
has a deceptive simplicity. PE homopolymers are made up exclusively of carbon and hy-
drogen atoms and, just as the properties of diamond and graphite (which are also materials
made up entirely of carbon and hydrogen atoms) vary tremendously, different grades of
PE have markedly different thermal and mechanical properties. While PE is generally a
whitish, translucent polymer, it is available in grades of density that range from 0.91 to
3
0.97 g/cm . The density of a particular grade is governed by the morphology of the back-
bone: long, linear chains with very few side branches can assume a much more three-di-
mensionally compact, regular, crystalline structure. Commercially available grades are
low-density PE (LDPE), linear low-density PE (LLDPE), high-density PE (HDPE), and
ultra-high-molecular-weight PE (UHMWPE). Figure 2.21 demonstrates figurative differ-
ences in chain configuration that govern the degree of crystallinity, which, along with
MW, determines final thermomechanical properties.
Four established production methods are (1) a gas phase method known as the Unipol
process practiced by Union Carbide, (2) a solution method used by Dow and DuPont, (3) a
216
slurry emulsion method practiced by Phillips, and (4) a high-pressure method. Gener-

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