Page 92 - Handbook of Plastics Technologies

THERMOPLASTICS

2.32 CHAPTER 2

used in many food packaging applications. These films are soft and tacky and therefore
appropriate for cling-wrap applications (they are more thermally stable than the PVDC
films often used as cling wrap) as well as interlayers in coextruded and laminated films.
EVA copolymers with approximately 11 mole percent vinyl acetate are widely used in
the hot-melt coatings and adhesives arena, where the additional intermolecular bonding
promoted by the polarity of the vinyl acetate ether and carbonyl linkages enhances melt
strength while still enabling low melt-processing temperatures. At 15 mole percent vinyl
acetate, a copolymer with very similar mechanical properties to plasticized PVC is
formed. There are many advantages to an inherently flexible polymer for which there is no
risk of plasticizer migration, and PVC-alternatives is the area of largest growth opportu-
nity. These copolymers have higher moduli than standard elastomers and are preferable in
that they are more easily processed without concern for the need to vulcanize.

2.2.15.6 Ethylene-Vinyl Alcohol (EVOH) Copolymers. Poly(vinyl alcohol) is pre-
pared through alcoholysis of poly(vinyl acetate). PVOH is an atactic polymer but, since
the crystal lattice structure is not disrupted by hydroxyl groups, the presence of residual
acetate groups greatly diminishes the crystal formation and the degree of hydrogen bond-
ing. Polymers that are highly hydrolyzed (have low residual acetate content) have a high
tendency to crystallize and for hydrogen bonding to occur. As the degree of hydrolysis in-
creases, the molecules will very readily crystallize, and hydrogen bonds will keep them as-
sociated if they are not fully dispersed prior to dissolution. At degrees of hydrolysis above
98 percent, manufacturers recommend a minimum temperature of 96°C to ensure that the
highest molecular weight components have enough thermal energy to go into solution.
Polymers with low degrees of residual acetate have high humidity resistance.
2.2.15.7 Ethylene-Carbon Monoxide Copolymers (ECOs). These polymers are ran-
dom copolymers of ethylene and carbon monoxide, with properties similar to low-density
235
polyethylene. They are sold by Shell under the trade name Carilon. These polymers ex-
hibit low water absorption and good barrier properties, but they are susceptible to UV deg-
radation. They find application in packaging, fuel tanks, fuel lines, and in blends.

2.2.16 Modified Polyethylenes
The properties of PE can be tailored to meet the needs of a particular application by a vari-
ety of different methods. Chemical modification, copolymerization, and compounding can
all dramatically alter specific properties. The homopolymer itself has a range of properties
that depend on the molecular weight, the number and length of side branches, the degree
of crystallinity, and the presence of additives such as fillers or reinforcing agents. Further
modification is possible by chemical substitution of hydrogen atoms; this occurs preferen-
tially at the tertiary carbons of a branching point and primarily involves chlorination, sul-
phonation, phosphorylination, and intermediate combinations.

2.2.16.1 Chlorinated Polyethylene (CPE). The first patent on the chlorination of PE
was awarded to ICI in 1938. 236 CPE is polymerized by substituting select hydrogen atoms
on the backbone of either HDPE or LDPE with chlorine. Chlorination can occur in the
gaseous phase, in solution, or as an emulsion. In the solution phase, chlorination is ran-
dom, while the emulsion process can result in uneven chlorination due to the crystalline
regions. The chlorination process generally occurs by a free-radical mechanism, shown in
Fig. 2.25, where the chlorine free radical is catalyzed by ultraviolet light or initiators.
Interestingly, the properties of CPE can be adjusted to almost any intermediary posi-
tion between PE and PVC by varying the properties of the parent PE and the degree and
tacticity of chlorine substitution. Since the introduction of chlorine reduces the regularity

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