Page 88 - Handbook of Plastics Technologies

THERMOPLASTICS

2.28 CHAPTER 2

FIGURE 2.21 Chain configurations of polyethylene.

ally, yield strength and melt temperature increase with density, while elongation decreases
with increased density.
2.2.14.1 Very-Low-Density Polyethylene (VLDPE). This material was introduced in
1985 by Union Carbide, is very similar to LLDPE, and is principally used in film applica-
3 217
tions. VLDPE grades vary in density from 0.880 to 0.912 g/cm . Its properties are
marked by high elongation, good environmental stress cracking resistance, and excellent
low-temperature properties, and it competes most frequently as an alternative to plasti-
cized polyvinyl chloride (PVC) or ethylene-vinyl acetate (EVA). The inherent flexibility in
the backbone of VLDPE circumvents plasticizer stability problems that can plague PVC,
and it avoids odor and stability problems that are often associated with molding EVAs. 218

2.2.14.2 Low-Density Polyethylene (LDPE). LDPE combines high impact strength,
toughness, and ductility to make it the material of choice for packaging films, which is one
of its largest applications. Films range from shrink film, thin film for automatic packaging,
heavy sacking, and multilayer films (both laminated and coextruded), where LDPE acts as
a seal layer or a water vapor barrier. 219 It has found stiff competition from LLDPE in these
film applications due to LLDPE’s higher melt strength. LDPE is still very widely used,
however, and is formed via free radical polymerization, with alkyl branch groups (given
by the structure -(CH ) CH ) of two to eight carbon atom lengths. The most common
3
2 x
branch length is four carbons long. High reaction pressures encourage crystalline regions.
The reaction to form LDPE is shown in Fig. 2.22, where “n” approximately varies in com-
mercial grades between 400 to 50,000. 220

FIGURE 2.22 Polymerization of PE.

Medium-density PE is produced via the reaction above, carried out at lower polymer-
ization temperatures. 221 The reduced temperatures are postulated to reduce the randomiz-
ing Brownian motion of the molecules, and this reduced thermal energy allows crystalline
formation more readily at these lowered temperatures.
2.2.14.3 Linear Low-Density Polyethylene (LLDPE). This product revolutionized the
plastics industry with its enhanced tensile strength for the same density compared to

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.