Page 104 - Handbook of Plastics Technologies

THERMOPLASTICS

2.44 CHAPTER 2

low-flow-rate materials are used for thick parts, and high-flow-rate materials are used for
thin-wall molding.
Polypropylene can be processed by methods similar to those used for PE. The melt
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temperatures are generally in the range of 210 to 250°C. Heating times should be mini-
mized to reduce the possibility of oxidation. Blow molding of PP requires the use of
higher melt temperatures and shear, but these conditions tend to accelerate the degradation
of PP. Because of this, blow molding of PP is more difficult than for PE. The screw meter-
ing zone should not be too shallow so as to avoid excessive shear. For a 60-mm screw, the
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flights depths are typically about 2.25 mm, and they are 3.0 mm for a 90-mm screw.
In film applications, film clarity requires careful control of the crystallization process
to ensure that small crystallites are formed. This is accomplished in blown film by extrud-
ing downwards into two converging boards. In the Shell TQ process, the boards are cov-
ered with a film of flowing, cooling water. Oriented films of PP are manufactured by
passing the PP film into a heated area and stretching the film both transversely and longi-
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tudinally. To reduce shrinkage, the film may be annealed at 100°C while under tension.
Highly oriented films may show low transverse strength and a tendency to fibrillate. Other
manufacturing methods for polypropylene include extruded sheet for thermoforming ap-
plications and extruded profiles.
If higher stiffness is required, short glass reinforcement can be added. The use of a cou-
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pling agent can dramatically improve the properties of glass filled PP. Other fillers for
polypropylene include calcium carbonate and talc, which can also improve the stiffness of
PP.
Other additives such as pigments, antioxidants, and nucleating agents can be blended
into polypropylene to give the desired properties. Carbon black is often added to polypro-
pylene to impart UV resistance in outdoor applications. Antiblocking and slip agents may
be added for film applications to decrease friction and prevent sticking. In packaging ap-
plications, antistatic agents may be incorporated.
The addition of rubber to polypropylene can lead to improvements in impact resis-
tance. One of the most commonly added elastomers is ethylene-propylene rubber. The
elastomer is blended with polypropylene, forming a separate elastomer phase. Rubber can
be added in excess of 50 percent to give elastomeric compositions. Compounds with less
than 50 percent added rubber are of considerable interest as modified thermoplastics. Im-
pact grades of PP can be formed into films with good puncture resistance.
Copolymers of polypropylene with other monomers are also available, the most com-
mon monomer being ethylene. Copolymers usually contain between 1 and 7 weight per-
cent of ethylene randomly placed in the polypropylene backbone. This disrupts the ability
of the polymer chain to crystallize, giving more flexible products. This improves the im-
pact resistance of the polymer, decreases the melting point, and increases flexibility. The
degree of flexibility increases with ethylene content, eventually turning the polymer into
an elastomer (ethylene propylene rubber). The copolymers also exhibit increased clarity
and are used in blow molding, injection molding, and extrusion.
Polypropylene has many applications. Injection molding applications cover a broad
range from automotive uses such as dome lights, kick panels, and car battery cases to lug-
gage and washing machine parts. Filled PP can be used in automotive applications such as
mounts and engine covers. Elastomer-modified PP is used in the automotive area for
bumpers, fascia panels, and radiator grills. Ski boots are another application for these ma-
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terials. Structural foams, prepared with glass-filled PP, are used in the outer tank of
washing machines. New grades of high-flow PPs are allowing manufacturers to mold
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high-performance housewares. Polypropylene films are used in a variety of packaging
applications. Both oriented and nonoriented films are used. Film tapes are used for carpet
backing and sacks. Foamed sheet is used in a variety of applications including thermo-

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