Chemical and physical properties
Polypropylene is in many aspects similar to
polyethylene, especially in
solution behaviour and electrical properties. The additionally present
methyl group improves mechanical properties and thermal resistance, while the chemical resistance decreases.
:19 The properties of polypropylene depend on the molecular weight and molecular weight distribution, crystallinity, type and proportion of comonomer (if used) and the iso
 In isotactic polypropylene, for example, the CH3 groups are oriented on one side of the carbon backbone. This creates a greater degree of crystallinity and results in a stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene.
The density of PP is between 0.895 and 0.92 g/cm³. Therefore, PP is the
commodity plastic with the lowest density. With lower density,
moldings parts with lower weight and more parts of a certain mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density. However, the density of polyethylene can significantly change with fillers.
Young's modulus of PP is between 1300 and 1800 N/mm².
Polypropylene is normally tough and flexible, especially when
ethylene. This allows polypropylene to be used as an
engineering plastic, competing with materials such as
acrylonitrile butadiene styrene (ABS). Polypropylene is reasonably economical.
Polypropylene has good resistance to
The melting point of polypropylene occurs at a range, so a melting point is determined by finding the highest temperature of a
differential scanning calorimetry chart. Perfectly isotactic PP has a melting point of 171 °C (340 °F). Commercial isotactic PP has a melting point that ranges from 160 to 166 °C (320 to 331 °F), depending on
atactic material and crystallinity.
Syndiotactic PP with a crystallinity of 30% has a melting point of 130 °C (266 °F).
 Below 0 °C, PP becomes brittle.
The thermal expansion of polypropylene is very large, but somewhat less than that of polyethylene.
Polypropylene is at room temperature resistant to fats and almost all organic
solvents, apart from strong oxidants. Non-
oxidizing acids and
bases can be stored in
containers made of PP. At elevated temperature, PP can be dissolved in nonpolarity solvents such as
decalin. Due to the tertiary carbon atom PP is chemically less resistant than PE (see
Most commercial polypropylene is
isotactic and has an intermediate level of
crystallinity between that of
low-density polyethylene (LDPE) and
high-density polyethylene (HDPE). Isotactic & atactic polypropylene is soluble in P-xylene at 140 °C. Isotactic precipitates when the solution is cooled to 25 °C and atactic portion remains soluble in P-xylene.
The melt flow rate (MFR) or
melt flow index (MFI) is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow-molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.
There are three general types of polypropylene:
homopolymer, random copolymer, and
block copolymer. The
comonomer is typically used with
ethylene. Ethylene-propylene rubber or
EPDM added to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity, lowers the melting point and makes the polymer more transparent.
Polypropylene is liable to chain degradation from exposure to heat and
UV radiation such as that present in
sunlight. Oxidation usually occurs at the
tertiary carbon atom present in every
repeat unit. A
free radical is formed here, and then reacts further with
oxygen, followed by chain scission to yield
carboxylic acids. In external applications, it shows up as a network of fine cracks and
crazes that become deeper and more severe with time of exposure. For external applications, UV-absorbing additives must be used.
Carbon black also provides some protection from UV attack. The polymer can also be oxidized at high temperatures, a common problem during molding operations.
Anti-oxidants are normally added to prevent
polymer degradation. Microbial communities isolated from soil samples mixed with starch have been shown to be capable of degrading polypropylene.
 Polypropylene has been reported to degrade while in human body as implantable mesh devices. The degraded material forms a tree bark-like layer at the surface of mesh fibers.
PP can be made
translucent when uncolored but is not as readily made transparent as
acrylic, or certain other plastics. It is often
opaque or colored using pigments.