|
this is the page introduction of the PP( polypropylene) and
partly of the products made my virgin or recycle polypropylene like dvd cases,
poly cases,multiple DVD Cases. |
Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the
chemical industry and used in a wide variety of applications, including
packaging, textiles (e.g. ropes, Under Armour, 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.
Most commercial polypropylene is isotactic and has an intermediate level of
crystallinity between that of low density polyethylene (LDPE) and high density
polyethylene (HDPE); its Young's modulus is also intermediate. PP is normally
tough and flexible, especially when copolymerised 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 and/or coloured using pigments.
Polypropylene has good resistance to fatigue.
Polypropylene has a melting point of ~160°C (320°F), as determined by
Differential scanning calorimetry (DSC).
The MFR (Melt Flow Rate) or MFI (Melt Flow Index) is a measure of PP's molecular
weight. This helps to determine how easily the melted raw material will flow
during processing. Higher MFR PPs 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 PP: homopolymer, random copolymer and block
copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber
or EPDM added to PP homopolymer increases its low temperature impact strength.
Randomly polymerized ethylene monomer added to PP homopolymer decreases the
polymer crystallinity and makes the polymer more transparent.
Degradation
Polypropylene is liable to chain degradation from exposure to UV radiation such
as that present in sunlight. 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
moulding operations. Anti-oxidants are normally added to prevent polymer
degradation.
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 at left) relative to the methyl groups on
neighboring monomers has a strong effect on the finished polymer's ability to
form crystals, because each methyl group takes up space and constrains backbone
bending.
Like most other vinyl polymers, useful polypropylene cannot be made by radical
polymerization due to the higher reactivity of the allylic hydrogen (leading to
dimerization) during polymerization. Moreover, the material that would result
from such a process would have methyl groups arranged randomly, so called
atactic PP. The lack of long-range order prevents any crystallinity in such a
material, giving an amorphous material with very little strength and only
specialized qualities suitable for niche end uses.
A Ziegler-Natta catalyst is able to limit incoming monomers to a specific
orientation, only adding them to the polymer chain if they face the right
direction. Most commercially available polypropylene is made with such
Ziegler-Natta catalysts, which produce mostly isotactic polypropylene (the upper
chain in the figure above). With the methyl group consistently on one side, such
molecules tend to coil into a helical shape; these helices then line up next to
one another to form the crystals that give commercial polypropylene many of its
desirable properties.
ore precisely engineered Kaminsky catalysts have been made, which offer a much
greater level of control. Based on metallocene molecules, these catalysts use
organic groups to control the monomers being added, so that a proper choice of
catalyst can produce isotactic, syndiotactic, or atactic polypropylene, or even
a combination of these. Aside from this qualitative control, they allow better
quantitative control, with a much greater ratio of the desired tacticity than
previous Ziegler-Natta techniques. They also produce narrower molecular weight
distributions than traditional Ziegler-Natta catalysts, which can further
improve properties.
To produce a rubbery polypropylene, a catalyst can be made which yields
isotactic polypropylene, but with the organic groups that influence tacticity
held in place by a relatively weak bond. After the catalyst has produced a short
length of polymer which is capable of crystallization, light of the proper
frequency is used to break this weak bond, and remove the selectivity of the
catalyst so that the remaining length of the chain is atactic. The result is a
mostly amorphous material with small crystals embedded in it. Since each chain
has one end in a crystal but most of its length in the soft, amorphous bulk, the
crystalline regions serve the same purpose as vulcanization.
Mechanism of metallocene catalysts
The reaction of many metallocene catalysts requires a co catalyst for
activation. One of the most common co catalysts for this purpose is
Methylaluminoxane (MAO)[2]. Other co catalysts include, Al(C2H5)3[3].There are
numerous metallocene catalysts that can be used for propylene polymerization.
(Some metallocene catalysts are used for industrial process, while others are
not, due to their high cost.) One of the simplest is Cp2MCl2 (M = Zr, Hf).
Different catalyst can lead to polymers with different molecular weights and
properties. Active research is still being conducted on metallocene catalyst.
In the mechanism the metallocene catalyst first reacts with the co catalyst. If
MAO is the co catalyst, the first step is to replace one of the Cl atoms on the
catalyst with a methyl group from the MAO. The methyl group on is replaced by
the Cl from the catalyst. The MAO then removes another Cl from the catalyst.
This makes the catalyst positively charged and susceptible to attack from
propylene[4].
Once the catalyst is activated, the double bond on the propene coordinates with
the metal of the catalyst. The methyl group on the catalyst then migrates to the
propene, and the double bond is broken. This starts the polymerization. Once the
methyl migrates the positively charged catalyst is reformed and another propene
can coordinate to the metal. The second propene coordinates and the carbon chain
that was formed migrates to the propene. The process of coordination and
migration continues and a polymer chain is grown off of the metallocene
catalyst.
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 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 PP 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
PP surfaces resist dust and dirt. Many physical finishing techniques can also be
used on PP, such as machining. Surface treatments can be applied to PP parts in
order to promote adhesion of printing ink and paints.
|
|
|
|
