Page 106 - Handbook of Plastics Technologies

THERMOPLASTICS

2.46 CHAPTER 2

There are a variety of starting materials available for use in the preparation of polyure-
thanes, some of which are listed below.
Diisocyanates
• 4,4´-diphenylmethane diisocyanate (MDI)
• Hexamethylene diisocyanate (HDI)
• Hydrogenated 4,4´-diphenylmethane diisocyanate (HMDI)
Chain Extenders
• 1,4 butanediol
• Ethylene glycol
• 1,6 hexanediol
Polyols
• Polyesters
• Polyethers
Polyurethanes are generally classified by the type of polyol used—for example, poly-
ester polyurethane or polyether polyurethane. The type of polyol can affect certain proper-
ties. For example, polyether polyurethanes are more resistant to hydrolysis than polyester-
based urethanes, while the polyester polyurethanes have better fuel and oil resistance. 320
Low-temperature flexibility can be controlled by proper selection of the long-chain polyol.
Polyether polyurethanes generally have lower glass transition temperatures than polyester
polyurethanes. The heat resistance of the polyurethane is governed by the hard segments.
Polyurethanes are noted for their abrasion resistance, toughness, low-temperature impact
strength, cut resistance, weather resistance, and fungus resistance. 321 Specialty polyure-
thanes include glass-reinforced products, fire-retardant grades, and UV-stabilized grades.
Polyurethanes find application in many areas. They can be used as impact modifiers for
other plastics. Other applications include rollers or wheels, exterior body parts, drive belts,
and hydraulic seals. 322 Polyurethanes can be used in film applications such as textile lam-
inates for clothing and protective coatings for hospital beds. They are also used in tubing
and hose in both unreinforced and reinforced forms because of their low-temperature
properties and toughness. Their abrasion resistance allows them to be used in applications
such as athletic shoe soles and ski boots. Polyurethanes are also used as coatings for wire
and cable. 323
Polyurethanes can be processed by a variety of methods, including extrusion, blow
molding, and injection molding. They tend to pick up moisture and must be thoroughly
dried prior to use. The processing conditions vary with the type of polyurethane; higher
hardness grades usually require higher processing temperatures. Polyurethanes tend to ex-
hibit shear sensitivity at lower melt temperatures. Post-mold heating in an oven, shortly af-
ter processing, can often improve the properties of the finished product. A cure cycle of 16
to 24 hr at 100°C is typical. 324

2.2.26 Styrenics
The styrene family is well suited for applications where rigid, dimensionally stable
molded parts are required. PS is a transparent, brittle, high-modulus material with a multi-
tude of applications, primarily in packaging, disposable cups, and medical ware. When the
mechanical properties of the PS homopolymer are modified to produce a tougher, more

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