Page 110 - Handbook of Plastics Technologies

THERMOPLASTICS

2.50 CHAPTER 2

plastics. The polymers can be prepared as either a star block form or as a linear,
multiblock polymer. The butadiene exists as a separate dispersed phase in a continuous
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matrix of polystyrene. The size of the butadiene phase is controlled to be less than the
wavelength of light, resulting in clear materials. The resulting amorphous polymer is
tough with good flex life and low mold shrinkage. The copolymer can be ultrasonically
welded, solvent welded, or vibration welded. The copolymers are available in injection-
molding grades and thermoforming grades. The injection-molding grades generally con-
tain a higher styrene content in the block copolymer. Thermoforming grades are usually
mixed with pure polystyrene. Styrene-butadiene copolymers can be processed by injection
molding, extrusion, thermoforming, and blow molding. The polymer does not need to be
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dried prior to use.
Styrene-butadiene copolymers are used in toys, housewares, and medical applica-
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tions. Thermoformed products include disposable food packaging such as cups, bowls,
“clam shells,” deli containers, and lids. Blister packs and other display packaging also use
styrene-butadiene copolymers. Other packaging applications include shrink wrap and veg-
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etable wrap.
2.2.27 Sulfone-Based Resins
Sulfone resins refer to polymers containing SO groups
2
along the backbone as depicted in Fig. 2.38. The R groups are
generally aromatic. The polymers are usually yellowish, trans-
parent, amorphous materials and are known for their high stiff-
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ness, strength, and thermal stability. The polymers have low
creep over a large temperature range. Sulfones can compete
against some thermoset materials in performance, while their
FIGURE 2.38 General ability to be injection molded offers an advantage.
structure of a polysulfone. The first commercial polysulfone was Udel (Union Carbide,
now Amoco), followed by Astrel 360 (Minnesota Mining and
Manufacturing), which is termed a polyarylsulfone, and finally Victrex (ICI), a polyether-
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sulfone. The different polysulfones vary by the spacing between the aromatic groups,
which in turn affects their T s and their heat distortion temperatures. Commercial polysul-
g
fones are linear with high T values in the range of 180 to 250°C, allowing for continuous
g
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use from 150 to 200°C. As a result, the processing temperatures of polysulfones are
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above 300°C. Although the polymer is polar, it still has good electrical insulating prop-
erties. Polysulfones are resistant to high thermal and ionizing radiation. They are also re-
sistant to most aqueous acids and alkalis but may be attacked by concentrated sulfuric
acid. The polymers have good hydrolytic stability and can withstand hot water and
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steam. Polysulfones are tough materials, but they do exhibit notch sensitivity. The pres-
ence of the aromatic rings causes the polymer chain to be rigid. Polysulfones generally do
not require the addition of flame retardants and usually emit low levels of smoke.
The properties of the main polysulfones are generally similar, although polyethersul-
fones have better creep resistance at high temperatures and higher heat distortion tempera-
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ture, but more water absorption and higher density than the Udel-type materials. Glass-
fiber-filled grades of polysulfone are available, as are blends of polysulfone with ABS.
Polysulfones may absorb water, leading to potential processing problems such as
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streaks or bubbling. The processing temperatures are quite high, and the melt is very
viscous. Polysulfones show little change in melt viscosity with shear. Injection molding
melt temperatures are in the range of 335 to 400°C, and mold temperatures are in the
range of 100 to 160°C. The high viscosity necessitates the use of large cross-sectional run-
ners and gates. Purging should be done periodically, as a layer of black, degraded polymer

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