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Polypropylene (PP), also known as polypropene, is
a thermoplastic polymer, made by the chemical industry and used in a
wide variety of applications, including packaging, textiles (e.g.
ropes, thermal underwear and carpets), stationery, plastic parts and
reusable containers of various types, laboratory equipment,
loudspeakers, automotive components, and polymer banknotes. An
addition polymer made from the monomer propylene, it is rugged and
unusually resistant to many chemical solvents, bases and acids.
In 2007, the global market for polypropylene had a volume of 45.1
million tons, which led to a turnover of about 65 billion US-dollars
(47.4 billion Euro).
Most commercial polypropylene is isotactic and has an intermediate
level of crystallinity between that of low-density polyethylene
(LDPE) and high-density polyethylene (HDPE). Polypropylene is
normally tough and flexible, especially when copolymerized with
ethylene. This allows polypropylene to be used as an engineering
plastic, competing with materials such as ABS. Polypropylene is
reasonably economical, and can be made translucent when uncolored
but is not as readily made transparent as polystyrene, acrylic, or
certain other plastics. It is often opaque or colored using
pigments. Polypropylene has good resistance to fatigue.
The melting of polypropylene occurs as 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).[2]
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 used is typically
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 and makes the polymer more
transparent.
Degradation
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 aldehydes and 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.
History
Propylene was first polymerized to a crystalline isotactic polymer
by Giulio Natta and his coworkers in March 1954.This pioneering
discovery led to large-scale commercial production of isotactic
polypropylene from 1957 onwards.Syndiotactic polypropylene was also
first synthesized by Giulio Natta and his coworkers.
An important concept in understanding the link between the structure
of polypropylene and its properties is tacticity. The relative
orientation of each methyl group (CH3 in the figure) relative to the
methyl groups in neighboring monomer units has a strong effect on
the polymer's ability to form crystals.
A Ziegler-Natta catalyst is able to restrict linking of monomer
molecules to a specific regular orientation, either isotactic, when
all methyl groups are positioned at the same side with respect to
the backbone of the polymer chain, or syndiotactic, when the
positions of the methyl groups alternate. Commercially available
isotactic polypropylene is made with two types of Ziegler-Natta
catalysts. The first group of the catalysts encompases solid (mostly
supported) catalysts and certain types of soluble metallocene
catalysts. Such isotactic macromolecules coil into a helical shape;
these helices then line up next to one another to form the crystals
that give commercial isotactic polypropylene many of its desirable
properties.
Another type of metallocene catalysts produce syndiotactic
polypropylene. These macromolecules also coil into helices (of a
different type) and form crystalline materials.
When the methyl groups in a polypropylene chain exhibit no preferred
orientation, the polymers are called atactic. Atactic polypropylene
is an amorphous rubbery material. It can be produced commercially
either with a special type of supported Ziegler-Natta catalyst or
with some metallocene catalysts.
Modern supported Ziegler-Natta catalysts developed for the
polymerization of propylene and other 1-alkenes to isotactic
polymers usually use TiCl4 as an active ingredient and MgCl2 as a
support.,The catalysts also contain organic modifiers, either
aromatic acid esters and diesters or ethers. These catalysts are
activated with special cocatalysts containing an organoaluminum
compound such as Al(C2H5)3 and the second type of a modifier. The
catalysts are differentiated depending on the procedure used for
fashioning catalyst particles from MgCl2 and depending on the type
of organic modifiers employed during catalyst preparation and use in
polymerization reactions. Two most important technological
characteristics of all the supported catalysts are high productivity
and a high fraction of the crystalline isotactic polymer they
produce at 70-80°C under standard polymerization conditions.
Commercial synthesis of isotactic polypropylene is usually carried
out either in the medium of liquid propylene or in gas-phase
reactors.
Commercial synthesis of syndiotactic polypropylene is carried out
with the use of a special class of metallocene catalysts. They
employ bridged bis-metallocene complexes of the type
bridge-(Cp1)(Cp2)ZrCl2 where the first Cp ligand is the
cyclopentadienyl group, the second Cp ligand is the fluorenyl group,
and the bridge between the two Cp ligands is -CH2-CH2-, >SiMe2, or
>SiPh2.[8] These complexes are converted to polymerization catalysts
by activating them with a special organoaluminum cocatalyst,
methylalumoxane MAO
Manufacturing
Melt processing of polypropylene can be achieved via extrusion and
molding. Common extrusion methods include production of melt-blown
and spun-bond fibers to form long rolls for future conversion into a
wide range of useful products, such as face masks, filters, nappies
(diapers) and wipes.
The most common shaping technique is injection molding, which is
used for parts such as cups, cutlery, vials, caps, containers,
housewares, and automotive parts such as batteries. The related
techniques of blow molding and injection-stretch blow molding are
also used, which involve both extrusion and molding.
The large number of end-use applications for polypropylene are often
possible because of the ability to tailor grades with specific
molecular properties and additives during its manufacture. For
example, antistatic additives can be added to help polypropylene
surfaces resist dust and dirt. Many physical finishing techniques
can also be used on polypropylene, such as machining. Surface
treatments can be applied to polypropylene parts in order to promote
adhesion of printing ink and paints.
Applications
Since polypropylene is resistant to fatigue, most plastic living
hinges, such as those on flip-top bottles, are made from this
material. However, it is important to ensure that chain molecules
are oriented across the hinge to maximize strength.
Very thin sheets of polypropylene are used as a dielectric within
certain high-performance pulse and low-loss RF capacitors.
High-purity piping systems are built using polypropylene. Stronger,
more rigid piping systems, intended for use in potable plumbing,
hydronic heating and cooling, and reclaimed water applications, are
also manufactured using polypropylene.[10] This material is often
chosen for its resistance to corrosion and chemical leaching, its
resilience against most forms of physical damage, including impact
and freezing, its environmental benefits, and its ability to be
joined by heat fusion rather than gluing.
Many plastic items for medical or laboratory use can be made from
polypropylene because it can withstand the heat in an autoclave. Its
heat resistance also enables it to be used as the manufacturing
material of consumer-grade kettles. Food containers made from it
will not melt in the dishwasher, and do not melt during industrial
hot filling processes. For this reason, most plastic tubs for dairy
products are polypropylene sealed with aluminum foil (both
heat-resistant materials). After the product has cooled, the tubs
are often given lids made of a less heat-resistant material, such as
LDPE or polystyrene. Such containers provide a good hands-on example
of the difference in modulus, since the rubbery (softer, more
flexible) feeling of LDPE with respect to polypropylene of the same
thickness is readily apparent. Rugged, translucent, reusable plastic
containers made in a wide variety of shapes and sizes for consumers
from various companies such as Rubbermaid and Sterilite are commonly
made of polypropylene, although the lids are often made of somewhat
more flexible LDPE so they can snap on to the container to close it.
Polypropylene can also be made into disposable bottles to contain
liquid, powdered, or similar consumer products, although HDPE and
polyethylene terephthalate are commonly also used to make bottles.
Plastic pails, car batteries, wastebaskets, cooler containers,
dishes and pitchers are often made of polypropylene or HDPE, both of
which commonly have rather similar appearance, feel, and properties
at ambient temperature.
A common application for polypropylene is as biaxially oriented
polypropylene (BOPP). These BOPP sheets are used to make a wide
variety of materials including clear bags. When polypropylene is
biaxially oriented, it becomes crystal clear and serves as an
excellent packaging material for artistic and retail products.
Polypropylene, highly colorfast, is widely used in manufacturing
carpets, rugs and mats to be used at home.
Polypropylene is widely used in ropes, distinctive because they are
light enough to float in water.For equal mass and construction,
polypropylene rope is similar in strength to polyester rope.
Polypropylene costs less than most other synthetic fibers.
Polypropylene is also used as an alternative to polyvinyl chloride
(PVC) as insulation for electrical cables for LSZH cable in
low-ventilation environments, primarily tunnels. This is because it
emits less smoke and no toxic halogens, which may lead to production
of acid in high-temperature conditions.
Polypropylene is also used in particular roofing membranes as the
waterproofing top layer of single-ply systems as opposed to
modified-bit systems.
Polypropylene is most commonly used for plastic moldings, wherein it
is injected into a mold while molten, forming complex shapes at
relatively low cost and high volume; examples include bottle tops,
bottles, and fittings.