Page 103 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.43

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thus, it behaves as a soft flexible material. It is used in applications such as sealing
strips, paper laminating, and adhesives.
Unlike polyethylene, which crystallizes in the planar zigzag form, isotactic polypropy-
lene crystallizes in a helical form because of the presence of the methyl groups on the
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chain. Commercial polymers are about 90 to 95 percent isotactic. The amount of isotac-
ticity present in the chain will influence the properties. As the amount of isotactic material
(often quantified by an isotactic index) increases, the amount of crystallinity will also in-
crease, resulting in increased modulus, softening point, and hardness.
Although in many respects polypropylene is similar to polyethylene, both being satu-
rated hydrocarbon polymers, they differ in some significant properties. Isotactic polypro-
pylene is harder and has a higher softening point than polyethylene, so it is used where
higher stiffness materials are required. Polypropylene is less resistant to degradation, par-
ticularly high-temperature oxidation, than polyethylene, but it has better environmental
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stress cracking resistance. The decreased degradation resistance of PP is due to the
presence of a tertiary carbon in PP, allowing for easier hydrogen abstraction compared to
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PE. As a result, antioxidants are added to polypropylene to improve the oxidation resis-
tance. The degradation mechanisms of the two polymers are also different. PE cross-links
on oxidation, while PP undergoes chain scission. This is also true of the polymers when
exposed to high-energy radiation, a method commonly used to cross-link PE.
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Polypropylene is one of the lightest plastics, with a density of 0.905. The nonpolar
nature of the polymer gives PP low water absorption. Polypropylene has good chemical
resistance, but liquids such as chlorinated solvents, gasoline, and xylene can affect the ma-
terial. Polypropylene has a low dielectric constant and is a good insulator. Difficulty in
bonding to polypropylene can be overcome by the use of surface treatments to improve the
adhesion characteristics.
With the exception of UHMWPE, polypropylene has a higher T and melting point
g
than polyethylene. Service temperature is increased, but PP needs to be processed at
higher temperatures. Because of the higher softening, PP can withstand boiling water and
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can be used in applications requiring steam sterilization. Polypropylene is also more re-
sistant to cracking in bending than PE and is preferred in applications that require toler-
ance to bending. This includes applications such as ropes, tapes, carpet fibers, and parts re-
quiring a living hinge. Living hinges are integral parts of a molded piece that are thinner
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and allow for bending. One weakness of polypropylene is its low-temperature brittle-
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ness behavior, with the polymer becoming brittle near 0°C. This can be improved
through copolymerization with other polymers such as ethylene.
Comparing the processing behavior of PP to PE, it is found that polypropylene is more
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non-Newtonian than PE and that the specific heat of PP is lower than polyethylene. The
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melt viscosity of PE is less temperature sensitive than PP. Mold shrinkage is generally
less than for PE but is dependent on the actual processing conditions.
Unlike many other polymers, an increase in molecular weight of polypropylene does
not always translate into improved properties. The melt viscosity and impact strength will
increase with molecular weight but often with a decrease in hardness and softening point.
A decrease in the ability of the polymer to crystallize as molecular weight increases is of-
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ten offered as an explanation for this behavior.
The molecular weight distribution (MWD) has important implications for processing.
A PP grade with a broad MWD is more shear sensitive than a grade with a narrow MWD.
Broad MWD materials will generally process better in injection molding applications. In
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contrast, a narrow MWD may be preferred for fiber formation. Various grades of
polypropylene are available tailored to particular application. These grades can be classi-
fied by flow rate, which depends on both average molecular weight and MWD. Lower-
flow-rate materials are used in extrusion applications. In injection molding applications,

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