Page 94 - Handbook of Plastics Technologies

THERMOPLASTICS

2.34 CHAPTER 2

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The concentration of acrylic acid groups is generally in the range of 3 to 20 percent.
The acid groups are then reacted with a metal containing base, such as sodium methoxide
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or magnesium acetate, to form the metal salt as depicted in Fig. 2.26. The ionic groups
can associate with each other, forming a cross-link between chains. The resulting materi-
als are called ionomers in reference to the ionic bonds formed between chains. They were
originally developed by DuPont under the trade name of Surlyn.

FIGURE 2.26 Structure of an ionomer.

The association of the ionic groups forms a thermally reversible crosslink that can be
broken when exposed to heat and shear. This allows ionomers to be processed on conven-
tional thermoplastic processing equipment while still maintaining some of the behavior of
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a thermoset at room temperature. The association of ionic groups is generally believed
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to take two forms: multiplets and clusters. Multiplets are considered to be a small num-
ber of ionic groups dispersed in the matrix, whereas clusters are phase-separated regions
containing many ion pairs and also hydrocarbon backbone.
A wide range of properties can be obtained by varying the ethylene/methacrylic acid
ratios, molecular weight, and the amount and type of metal cation used. Most commercial
grades use either zinc or sodium for the cation. Materials using sodium as the cation gen-
erally have better optical properties and oil resistance, whereas those using zinc usually
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have better adhesive properties, lower water absorption, and better impact strength.
The presence of the comonomer breaks up the crystallinity of the polyethylene so that
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ionomer films have lower crystallinity and better clarity compared to polyethylene. Ion-
omers are known for their toughness and abrasion resistance, and the polar nature of the
polymer improves both its bondability and paintability. Ionomers have good low-tempera-
ture flexibility and resistance to oils and organic solvents. Ionomers show a yield point
with considerable cold drawing. In contrast to PE, the stress increases with strain during
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cold drawing, giving a very high energy to break.
Ionomers can be processed by most conventional extrusion and molding techniques us-
ing conditions similar to other olefin polymers. For injection molding, the melt tempera-
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tures are in the range 210 to 260°C. The melts are highly elastic due to the presence of
the metal ions. Increasing temperatures rapidly decreases the melt viscosity, with the so-
dium and zinc based ionomers showing similar rheological behavior. Typical commercial
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ionomers have melt index values between 0.5 and 15. Both unmodified and glass-filled
grades are available.
Ionomers are used in applications such as golf ball covers and bowling pin coatings,
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where their good abrasion resistance is important. The puncture resistance of films al-
lows these materials to be widely used in packaging applications. One of the early applica-
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tions was the packaging of fishhooks. They are often used in composite products as an
outer heat-seal layer. Their ability to bond to aluminum foil is also utilized in packaging
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applications. Ionomers also find application in footwear for shoe heels.

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