Page 90 - Handbook of Plastics Technologies

THERMOPLASTICS

2.30 CHAPTER 2

6 6
tween 3 × 10 and 6 × 10 . The high MW imparts outstanding abrasion resistance, high
toughness (even at cryogenic temperatures), and excellent stress cracking resistance, but it
does not generally allow the material to be processed conventionally. The polymer chains
are so entangled, due to their considerable length, that the conventionally considered melt
point doesn’t exist practically, as it is too close to the degradation temperature—although
an injection-molding grade is marketed by Hoechst. Hence, UHMWPE is often processed
as a fine powder that can be ram extruded or compression molded. Its properties are taken
advantage of in uses that include liners for chemical processing equipment, lubrication
coatings in railcar applications to protect metal surfaces, recreational equipment such as
ski bases, and medical devices. 225 A recent product has been developed by Allied Chemi-
cal that involves gel spinning UHMWPE into lightweight, very strong fibers that compete
with Kevlar in applications for protective clothing.

2.2.15 Polyethylene Copolymers
Ethylene is copolymerized with many nonolefinic monomers, particularly acrylic acid
variants and vinyl acetate, with EVA polymers being the most commercially significant.
All of the copolymers discussed in this section necessarily involve disruption of the regu-
lar, crystallizable PE homopolymer and as such feature reduced yield stresses and moduli,
with improved low-temperature flexibility.
2.2.15.1 Ethylene-Acrylic Acid (EAA) Copolymers. EAA copolymers, first identified
in the 1950s, have enjoyed a renewed interest since 1974, when Dow introduced new
226
grades characterized by outstanding adhesion to metallic and nonmetallic substrates.
The presence of the carboxyl and hydroxyl functionalities promotes hydrogen bonding,
and these strong intermolecular interactions are taken advantage of to bond aluminum foil
to polyethylene in multilayer extrusion-laminated toothpaste tubes and as tough coatings
for aluminum foil pouches.
2.2.15.2 Ethylene-Ethyl Acrylate (EEA) Copolymers. EEA copolymers typically con-
tain 15 to 30 percent by weight of ethyl acrylate (EA) and are flexible polymers of rela-
tively high molecular weight suitable for extrusion, injection molding, and blow molding.
Products made of EEA have high environmental stress cracking resistance, excellent resis-
tance to flexural fatigue, and low-temperature properties down to as low as –65°C. Appli-
cations include molded rubber-like parts, flexible film for disposable gloves and hospital
sheeting, extruded hoses, gaskets and bumpers. 227 Typical applications include polymer
modifications where EEA is blended with olefin polymers (since it is compatible with
VLDPE, LLDPE, LDPE, HDPE, and PP 228 ) to yield a blend with a specific modulus, yet
with the advantages inherent in EEA’s polarity. The EA presence promotes toughness,
flexibility, and greater adhesive properties. EEA blending can cost effectively improve the
impact resistance of polyamides and polyesters. 229
The similarity of ethyl acrylate monomer to vinyl acetate predicates that these copoly-
mers have very similar properties, although EEA is considered to have higher abrasion and
heat resistance, while EVA tends to be tougher and of greater clarity. 230 EEA copolymers
231
are FDA approved up to 8 percent EA content in food contact applications.
2.2.15.3 Ethylene-Methyl Acrylate (EMA) Copolymers. EMA copolymers are often
blown into film with very rubbery mechanical properties and outstanding dart-drop impact
strength. The latex-rubber-like properties of EMA film lend to its use in disposable gloves
and medical devices without the associated hazards to people with allergies to latex rub-
ber. Due to their adhesive properties, EMA copolymers, like their EAA and EEA counter-

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