Table Of ContentPolypropylene
The Definitive User’s Guide and Databook
Clive Maier
Teresa Calafut Plastics Design Library
Copyright 0 1998, Plastics Design Library. All rights reserved,
ISBN 1-884207-58-8
Library of Congress Card Number 97-076233
Published in the United States of America, Norwich, NY by Plastics Design Library a division of
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Foreword and Acknowledgements
The creation of Polypropylene — The Definitive Credit for the layout and typesetting go to Jon
User’s Guide was a pursuit to assemble all the lat- Phipps. He accomplished the seeming impossible
est practical knowledge a technologist may need in task of creating one fully interactive electronic
using this versatile material. The book examines version of the manuscript. Future electronic edi-
every aspect — science, technology, engineering, tions will be easier due to his hard work. Jeri
properties, design, processing, applications — of Wachter is commended for her critique and input
the continuing development and use of polypro- into the design of the book and for her never-
pylene. The unique treatment provided by this ending support. In assembling the data collections,
book means that specialists cannot only find what Harold Fennel’s expertise helped make the process
they want but can understand the needs and re- easier. Also deserving of special acknowledgement
quirements of others in the product development is Rapra Technology Ltd. and their allowing the
chain. The entire work is underpinned by very ex- use of Rapra Abstracts and the Plastics Knowledge
tensive collections of data that allow the reader to Base System (KBS). For comprehensive informa-
put the information to real industrial and commer- tion on the world of plastics and rubbers, these
cial use. products are invaluable.
As evidenced by the extensive list of sources The entire Plastics Design Library staff also
consulted to compile this volume, the information deserves special recognition. Their continued ef-
reflects a comprehensive review of the results of forts to keep our publishing activities running
current research and practical knowledge about smoothly and profitably help ensure that volumes
polypropylene. The translation of the knowledge such as this will continue to be produced.
of many into a single, accessible source was ac- In reviewing the manuscript, I learned many
complished by the diligent and clearheaded work new and interesting things about polypropylene
of Teresa Calafut and Clive Maier. The culmina- and the reasons why its use is so widespread. I
tion of their pursuits of excellence is evidenced in also felt that I was being given the practical
these pages. Teresa is a staff technical writer for knowledge and rules of thumb that I wish I had
Plastics Design Library and Clive Maier is a available when I was using this material in previ-
highly respected writer with an extensive back- ous design work. This volume is unique, is cer-
ground in plastics and is based in London, Eng- tainly a complement to previous work on the sub-
land. My gratitude for making this project happen ject and is sure to provide its user years of help in
is extended to them. making decisions and solving problems.
William Woishnis
Editor in Chief
Plastics Design Library
© Plastics Design Library
1 Chemistry
1.1 Polymerization reaction
Polypropylene is prepared by polymerizing pro-
pylene, a gaseous byproduct of petroleum refining,
in the presence of a catalyst under carefully con-
trolled heat and pressure. [773] Propylene is an
unsaturated hydrocarbon, containing only carbon
and hydrogen atoms:
CH = CH
2
CH
3
Figure 1.1 Molecules of propylene and polypropyl-
Propylene
ene. In the polymerization reaction, propylene mono-
In the polymerization reaction, many propylene mers (top) are added sequentially to the growing poly-
molecules (monomers) are joined together to form mer chain (bottom), to form a long, linear polymer chain
composed of thousands of propylene monomers. The
one large molecule of polypropylene. Propylene is
portion of the chain shown in parentheses is repeated n
reacted with an organometallic, transition metal number of times to form the polymer. [642]
catalyst (see 1.4 Catalysts for a description of cata-
cal and crystal structure of the catalyst, and a regu-
lysts used in the reaction) to provide a site for the re-
lar, repeating three-dimensional structure is pro-
action to occur, and propylene molecules are added
duced in the polymer chain [763]. Propylene
sequentially through a reaction between the metallic
molecules are added to the main polymer chain, in-
functional group on the growing polymer chain and
creasing the chain length, and not to one of the
the unsaturated bond of the propylene monomer:
methyl groups attached to alternating carbon atoms
M* + CH = CH → (the pendant methyl groups), which would result in
2
branching. Propylene molecules are usually added
CH
3 head-to-tail and not tail-to-tail or head-to-head.
M − CHCH + CH = CH → Head-to-tail addition results in a polypropylene
2 2 2
chain with pendant methyl groups attached to alter-
CH CH nating carbons; in tail-to-tail or head-to head addi-
3 3
tion, this alternating arrangement is disrupted. [771]
M − CHCHCHCH → etc.
2 2 2
CH =CH* + − CH −CH − CH −CH − →
2 2 2
CH CH
3 3
CH CH CH
One of the double-bonded carbon atoms of the in- 3 3 3
coming propylene molecule inserts itself into the − CH −CH − CH −CH − CH −CH −
2 2 2
bond between the metal catalyst (M in the above
reaction) and the last carbon atom of the polypro- CH CH CH
3 3 3
pylene chain. A long, linear polymer chain of car-
Head-to-tail addition of propylene to the growing
bon atoms is formed, with methyl (CH) groups
polypropylene chain
3
attached to every other carbon atom of the chain
(Figure 1.1). Thousands of propylene molecules CH = CH* + − CH −CH − CH −CH − →
2 2 2
can be added sequentially until the chain reaction
is terminated. [764, 768] CH CH CH
3 3 3
−CH − CH − CH −CH − CH −CH −
2 2 2
1.2 Stereospecificity
CH CH CH
With Ziegler-Natta or metallocene catalysts, the 3 3 3
polymerization reaction is highly stereospecific. Tail-to-tail addition of propylene to the growing
polypropylene chain
Propylene molecules add to the polymer chain only
in a particular orientation, depending on the chemi-
© Plastics Design Library Chemistry
4
same side of the polypropylene chain, as in isotac-
tic polypropylene; however, other methyl groups
are inserted at regular intervals on the opposite
side of the chain. [794, 695, 810]
1.3 Effect on characteristics of
polypropylene
The structure and stereochemistry of polypropyl-
ene affect its properties.
1.3.1 Stereochemistry
Because of its structure, isotactic polypropylene
Figure 1.2 Stereochemical configurations of poly-
propylene. In isotactic polypropylene, top, the pendant has the highest crystallinity, resulting in good me-
methyl groups branching off from the polymer backbone chanical properties such as stiffness and tensile
are all on the same side of the polymer backbone, with
strength. Syndiotactic polypropylene is less stiff
identical configurations relative to the main chain. In syn-
than isotactic but has better impact strength and
diotactic polypropylene, middle, consecutive pendant
methyl groups are on opposite sides of the polymer clarity. Due to its irregular structure, the atactic
backbone chain. In atactic polypropylene, bottom, pen- form has low crystallinity, resulting in a sticky,
dant methyl groups are oriented randomly with respect
amorphous material used mainly for adhesives and
to the polymer backbone. The portion of the chain shown
is repeated n number of times to form the polymer. [642] roofing tars. [794, 691] Increasing the amount of
atactic polypropylene in a predominantly isotactic
Occasional tail-to-tail or head-to-tail additions of
formulation increases the room temperature im-
polypropylene to the growing polymer chain disrupt
pact resistance and stretchability but decreases the
the crystalline structure and lower the melting point
stiffness, haze, and color quality. [695] The
of the polymer; formulations in which this occurs
amount of atactic polypropylene in a polypropyl-
are used in thermoforming or blow molding. [694]
ene formulation is indicated by the level of room
Polypropylene can be isotactic, syndiotactic,
temperature xylene solubles; levels range from
or atactic, depending on the orientation of the pen-
about 1–20%. [771] Polypropylenes generally
dant methyl groups attached to alternate carbon
have higher tensile, flexural, and compressive
atoms. In isotactic polypropylene (Figure 1.2), the
strength and higher moduli than polyethylenes due
most common commercial form, pendant methyl
to the steric interaction of the pendant methyl
groups are all in the same configuration and are on
groups, which result in a more rigid and stiff
the same side of the polymer chain. Due to this
polymer chain than in polyethylene. [693] General
regular, repeating arrangement, isotactic polypro-
effects of atactic level on the properties of poly-
pylene has a high degree of crystallinity. In syndi-
propylene are listed in Table 1.1. [695, 642, 693]
otactic polypropylene, alternate pendant methyl
groups are on opposite sides of the polymer back-
1.3.2 Molecular weight and melt flow index
bone, with exactly opposite configurations relative
Longer polypropylene chain lengths result in a
to the polymer chain. Syndiotactic polypropylene
higher molecular weight for the polymer. The
is now being produced commercially using metal-
weight-average molecular weight of polypropyl-
locene catalysts. In atactic polypropylene, pendant
ene generally ranges from 220,000–700,000
methyl groups have a random orientation with re-
g/mol, with melt flow indices from less than 0.3
spect to the polymer backbone. Amounts of iso-
g/10 min. to over 1000 g/10 min. The melt flow
tactic, atactic, and syndiotactic segments in a for-
index (MFI) provides an estimate of the average
mulation are determined by the catalyst used and
molecular weight of the polymer, in an inverse re-
the polymerization conditions. Most polymers are
lationship; high melt flow indicates a lower mo-
predominantly isotactic, with small amounts of
lecular weight. [693, 642, 696, 797]
atactic polymer. New metallocene catalysts make
Viscous materials with low MFI values (<2) are
possible other stereochemical configurations, such
used in extrusion processes, such as sheet and blow
as hemi-isotactic polypropylene. In this config-
molding, that require high melt strength. Resins
uration, most pendant methyl groups are on the
with MFI values of 2–8 are used in film and fiber
Chemistry © Plastics Design Library
5
Table 1.1 Effect of Atacticity on Polypropylene Table 1.2 Effect of Increasing Molecular Weight on
Properties Properties of Polypropylene
With Increasing With Increasing
Property Atacticity Property Molecular Weight
Stiffness Decreases Impact Resistance Increases
Moduli Decrease Elongation Increases
Strength Decreases Moduli Decrease
Room Temperature Impact Increases Strength Decreases
Resistance
Die Swell Increases
Stretchability Increases
Shear Rheology Increases
Elongation Increases
Melt Strength Increases
Shear Rheology Increases
Heat Seal Strength Increases
Long Term Heat Aging Decreases
Heat Distortion Temperature Decreases
(LTHA) Resistance
Irradiation Tolerance Decreases
Heat Distortion Temperature Decreases
Haze Decreases
Heat Seal Strength Increases
Extractables (solubility) Decreases
Haze in Films Decreases
Crystallization Temperature Decreases
Blocking in Films Increases
Irradiation Tolerance Increases Molecular weight distribution, measured as the ratio
Extractables (solubility) Increase of weight-average molecular weight to number-
average molecular weight (Mw/Mn) can vary from
Smoke and Fume Generation Increases
2.1 to over 11.0. The number-average molecular
Color Quality Decreases
weight is related to the number of polymer chain
General Optical Properties Increase molecules at a particular molecular weight, while the
weight-average molecular weight relates to the mass
Melting Temperature Decreases
(or weight) of the polymer chain molecules at a par-
Heat of Fusion Decreases
ticular molecular weight. [642, 691, 795]
Crystallization Temperature Decreases The MWD influences the processability of a
resin due to the shear sensitivity of molten polypro-
applications, and materials with MFI values of 8–35
or more are used in extrusion coating, injection
molding of thin-walled parts that requires rapid
mold filling, and fiber spinning. [642]
The toughness of a grade of polypropylene is
directly related to molecular weight: higher mo-
lecular weights provide greater toughness. As a re-
sult, higher molecular weight polypropylenes have
greater impact resistance and elongation and less
brittleness. [693, 642, 696] General effects of in-
creasing molecular weight on polypropylene prop-
erties are summarized in Table 1.2. [693]
Figure 1.3 Graph of broad and narrow molecular
1.3.3 Molecular weight distribution
weight distributions in polypropylene. In a resin with a
A polypropylene resin is composed of numerous narrow molecular weight distribution, polymer chains have
approximately the same length and therefore the same
chains of varying lengths, with varying molecular
molecular weight. The frequency of occurrence of these
weights. The molecular weight distribution (MWD) molecular weight chains is high, resulting in a narrow, high
indicates the variation of molecular weight in a par- peak. A resin with a broad molecular weight distribution
consists of polymer chains of varying lengths and mo-
ticular formulation; the MWD is narrow if most mo-
lecular weights, resulting in a broad molecular weight dis-
lecular chains are approximately the same length and
tribution. The frequency of occurrence of any particular
broad if the chains vary widely in length (Figure 1.3). molecular weight is low, producing a low, broad peak.
© Plastics Design Library Chemistry
6
electron. An example of a chain initiation reaction
in the presence of oxygen is given below:
CH CH
3 3
− CH −C − CH −C − CH − + O →
2 2 2 2
H H
Polypropylene (PP)
CH CH
3 3
− CH −C − CH −C − CH − + (cid:127)O H
2 2 2 2
(cid:127)
H
Polypropylene free radical (PP)(cid:127)
Figure 1.4 Influence of the molecular weight distri-
The chain reaction is propagated through the for-
bution of a polypropylene resin on shear sensitivity.
In a Newtonian fluid, such as water, the viscosity of the mation of a hydroperoxide, accompanied by the
fluid is constant with varying shear strain. In molten formation of another free radical:
polypropylene, a shear sensitive material, the viscosity
fast
varies with the rate of shearing strain. A polypropylene
resin with a broad molecular weight distribution, A, is PP(cid:127) + O →
2
more shear sensitive than a resin with a narrow mo-
lecular weight distribution, B. [642]
CH CH
3 3
pylene — the apparent viscosity decreases as the slow
− CH −C − CH −C − CH − + PP →
applied pressure increases. Because a polypropylene 2 2 2
resin with a broad MWD is more shear sensitive
H O − O(cid:127)
than a narrow MWD formulation (Figure 1.4), ma-
terials with broad MWD’s are processed more eas- Peroxide free radical
ily in applications such as injection molding. Poly-
CH CH
propylene resins with narrow MWD’s are used in 3 3
extrusion, in which a narrower MWD generally re- PP(cid:127) + − CH −C − CH −C − CH
2 2 2
sults in a higher achievable extrusion output rate
[694], or in applications such as fibers. [642] H OOH
Hydroperoxide (HP)
1.3.4 Oxidation
The oxidation rate is determined by the rate of the
Polypropylene is highly susceptible to oxidation
slow step in the chain propagation reactions. Due
due to the presence of the tertiary hydrogen on the
to the presence of the pendant methyl group, poly-
carbon atom bonded to the pendant methyl group.
propylene contains tertiary (3°) hydrogen atoms,
Polypropylene undergoes oxidation more readily
in which the carbon atom covalently bonded to the
than polyethylene, and oxidative chain scission,
hydrogen is also bonded to three other carbon at-
which reduces the molecular weight, occurs under
oms. The free radical (PP(cid:127)) formed from abstrac-
normal processing conditions if the resin is not
tion of a tertiary hydrogen is more stable than
stabilized. [794, 795]
those formed from abstraction of a primary (1°;
Polymer oxidation occurs through a free radi-
carbon atom attached to one other carbon) or sec-
cal chain reaction. Mechanical stress, heat, or the
ondary (2°; carbon atom attached to two other car-
presence of oxygen or metal catalyst residues re-
bons) hydrogen, due to the tendency of carbon at-
sults in homolytic cleavage of the carbon-
oms along the chain to electronically donate
hydrogen or carbon-carbon covalent bond in the
electrons to the electron-deficient radical. The
polypropylene chain; each atom receives one elec-
higher probability of reaction with the tertiary hy-
tron from the two-electron covalent bond, pro-
drogen considerably increases the susceptibility of
ducing two free radicals, each with an unpaired
polypropylene to oxidation. [768, 817]
Chemistry © Plastics Design Library
7
1° 1.3.6 Chemical resistance
CH CH Because it is composed of only carbon and hydro-
3 3
gen atoms, and not polar atoms such as oxygen or
− CH −C − CH −C − CH − nitrogen, polypropylene is nonpolar. Nonpolar
2 2 2
molecules are generally soluble in nonpolar sol-
2° H H 3° vents, while polar molecules are more soluble in
polar solvents (“like dissolves like”); as a result,
In further reactions (chain branching reactions that
nonpolar molecules are more easily absorbed by
increase the amount of free radicals), the hydro-
polypropylene than polar molecules. Polypropyl-
peroxide decomposes in the presence of heat or
ene is resistant to attack by polar chemicals such
metal catalyst residues to form an alkoxy radical.
as soaps, wetting agents, and alcohols but can
Oxidative chain scission is believed to occur
swell, soften, or undergo surface crazing in the
through disintegration of this alkoxy radical:
presence of liquid hydrocarbons or chlorinated
CH CH solvents. Strong oxidizing agents such as fuming
3 3
nitric acid or hot, concentrated sulfuric acid can
HP → HO (cid:127) + − CH −C − CH −C − CH − → cause swelling and polypropylene degradation. A
2 2 2
large degree of absorption can cause a loss of
H O(cid:127) physical properties. [642, 795]
− CH −C =O + −CH − CH
2 3
1.4 Catalysts
CH3 (cid:127)CH 2 The development of catalysts for polypropylene
polymerization in the 1950’s made the production
The decrease in molecular weight resulting from
of stereospecific polypropylene possible and led to
chain scission produces a gradual loss in mechani-
the rapid growth rate of polypropylene that is still
cal properties. Crosslinking, which is common in
occurring today. Catalysts are substances that in-
polyethylene oxidation, producing an increase in
crease the rate of a reaction but undergo no perma-
viscosity, does not occur frequently in polypropyl-
nent chemical change themselves. In polypropylene
ene due to preferential oxidative attack at the terti-
polymerization, catalysts are organometallic transi-
ary hydrogen, which leads to chain scission. Com-
tion metal complexes. They provide active sites or
pounds such as carboxylic acids, lactones, alde-
polymerization sites where the polymerization re-
hydes, and esters are also produced during oxi-
action occurs, by holding the growing polymer
dation reactions, resulting in chemical modifica-
chain and the propylene monomer in close proxim-
tions such as yellowing. Chain reactions are termi-
ity to each other so that they can react. With com-
nated when two radicals combine to form an inac-
mercial catalysts, a high yield of stereospecific
tive species. [817, 818]
polypropylene is produced.
1.3.5 Electrical conductivity
1.4.1 Ziegler-Natta catalysts
Electrically conductive materials, such as metals,
Ziegler-Natta catalysts are the most common com-
have delocalized electrons that can easily move
mercial catalysts. Karl Ziegler and Guilio Natta
along a potential gradient. Electrons in the cova-
jointly received the Nobel Prize in 1963 for the
lent bonds of organic molecules such as polypro-
development of polyolefin polymerization cata-
pylene must remain near their host atoms and are
lysts with high yield and a high degree of stereo-
not free to move through the material; as a result,
specificity. The original Ziegler-Natta catalysts
they are poor conductors of electricity. [782] The
were a complex of transition metal halides, usually
high dielectric strength and low dielectric constant
titanium trichloride (TiCl), with an organometallic
and dissipation factor of polypropylene make it
3
compound, typically triethylaluminum, as co-
useful as an insulating material. [783, 642] Con-
catalyst to initiate the polymerization. Yield of
ductive materials such as carbon black can be
isotactic polypropylene in these original catalysts
added to a polypropylene formulation for applica-
was low, 30–40%, but was rapidly increased to
tions requiring electrical conductivity. [698]
over 80% with further development. [768, 788]
Due to the low isotacticity, postreactor treatment
© Plastics Design Library Chemistry
8
was necessary in order to remove catalyst residues Chemical breakdown of the polymer chains is
and atactic material. [695] accomplished by oxidative chain degradation initi-
Catalyst improvements have led to increased ated by a peroxide, a process called controlled
stereospecificity and productivity. The low surface rheology (CR) or visbreaking. This process short-
areas of early TiCl catalysts resulted in low cata- ens the average length of the polymer chains, low-
3
lyst activity; since only titanium atoms on the ers the molecular weight, and narrows the mo-
catalyst surface are accessible to the organo- lecular weight distribution, resulting in lower melt
metallic compound, few active sites were formed, viscosity, increased flow rates, and slightly en-
and the amount of polypropylene produced per hanced impact strength. Molding cycles can be up
gram of catalyst used was low. TiCl catalysts with to 15% faster than with conventional grades, and
3
increased surface areas resulted in increased pro- warpage and shrinkage are reduced. [794, 696,
ductivity and isotacticity (~95%). [764] 691, 693, 765]
Supported heterogeneous Ziegler-Natta cata- Metal catalyst residues that remain in the
lysts were developed in the 1960’s, with magne- polypropylene resin may affect the opacity of the
sium chloride (MgCl) used as the inert support resin, and resins made using different catalysts
2
material. Heterogeneous catalysts are present in a may have different levels of clarity. In addition,
different phase (solid, liquid, gas) from the reac- additives can interact with catalyst residues to pro-
tion mixture; they are fixed onto the surface of a duce yellowness. [692]
support material for feeding into the reactor during
processing and for control of polymer growth. Ad- 1.4.3 Metallocene catalysts
dition of a Lewis base, typically a benzoic acid Metallocene catalysts have recently been devel-
ester, as an electron donor (internal donor) and a oped for industrial use, and metallocene-produced
second Lewis base (methyl-p-toluate) as an exter- polypropylene is now available. In contrast to Zei-
nal donor to the MgCl-supported catalyst in- ger-Natta catalysts, metallocene catalysts are sin-
2
creased catalyst activity and stereospecificity and gle-sited — they have identical active sites — and
eliminated the necessity of post reactor removal of properties such as molecular weight and stere-
catalyst residues. [604, 764, 758] ostructure can be tailored to meet the needs of the
Catalyst systems using newer Lewis bases (al- application. [694, 758, 781] Syndiotactic polypro-
kylphthalates and alkoxysilanes as internal and pylene is now being produced commercially using
external donors, respectively) further increased metallocenes; commercial production was not pos-
isotacticity and activity and are currently used in sible with Ziegler-Natta catalysts. [794]
the industrial production of polypropylene. Cata- Metallocenes are organometallic compounds
lyst systems using new internal electron donors, with a sandwich-like spatial arrangement, consist-
developed in the latter part of the 1980’s, result in ing of a transition metal (iron, titanium, zirconi-
very high activity and isotacticity without use of um) situated between two cyclic organic com-
an external electron donor. They are not yet in in- pounds (Figure 1.5). [767] Geoffrey Wilkinson
dustrial use. [764, 758] and Ernst O. Fischer received the Nobel prize in
chemistry for elucidation of the structure of ferro-
1.4.2 Characteristics of polypropylene cene, one of the first metallocenes discovered.
produced using Ziegler-Natta catalysts [654] The first metallocenes used for polymeriza-
Zeigler-Natta catalysts are multi-sited catalysts, tion, titanocenedichloride and an aluminum alkyl
containing several reactive sites. As a result, the such as trimethylaluminum, showed poor activity
polypropylene produced can include polymer mole- and were used only in scientific studies. [758, 654]
cules with a broad range of molecular weights and However, in 1975, accidental introduction of water
some branching off from the main polymer chain. into a test tube containing a metallocene catalyst
[759] For film and fiber applications and for injec- system and ethylene increased the polymerization
tion molding of thin walls or parts with intricate rate 1000 times and led to the development of
structures, a narrower molecular weight distribution methylalumoxane (MAO), a product of the partial
and increased melt flow rate may be required. For hydrolysis of trimethylaluminum, as a catalyst ac-
these applications, the polypropylene produced tivator or cocatalyst. [758, 654]
must be chemically or thermally broken down in
post-reactor extrusion. [694, 794]
Chemistry © Plastics Design Library
9
structures, molecular weights, and other properties
can be produced by varying the transition metal and
organic compound used. [764, 654]
Metallocene polymerization in the laboratory
makes use of homogeneous catalysis; catalysts and
reacting materials are in solution. For large-scale
industrial processes, metallocenes must be fixed or
supported on powdery, insoluble substrates; SiO,
2
AlO, or MgCl, are generally used. A polypropyl-
2 3 2
ene chain is synthesized on each grain of powder,
and because active sites on each grain are identical,
the chains grow to a uniform length. [692, 654]
1.4.4 Characteristics of polypropylene
produced using metallocene catalysts
Figure 1.5 Structure of one type of metallocene Polypropylenes made using metallocene catalysts
catalyst. A zirconium atom is bound to two chlorine atoms exhibit increased rigidity and transparency, higher
and to a bridged alkyl group. The ZrCl2 complex is located
heat distortion temperatures, improved impact
in a cleft formed by the alkyl group; the polymerization re-
action occurs in the cleft. The molecule is represented in strength and toughness even at subambient tem-
three dimensions — the dotted line indicates that one peratures, and low extractables. [760, 654] Due to
chlorine is located behind the plane of the paper, while the
the uniformity of the polypropylene chains, met-
heavy bold line to the other chlorine indicates that it is lo-
cated in front of the plane of the paper. [182] allocene-catalyzed propylene has a very narrow
molecular weight distribution (Mw/Mn of 2.0)
The introduction of chiral, bridged metallo-
compared to conventional polypropylene (mini-
cenes using first titanium, then zirconium, in the
mum Mw/Mn of 3–6). The narrow MWD results
1980’s allowed the stereoselective polymerization
in lower shear sensitivity of the polypropylene
of propylene to isotactic polypropylene. In bridged
resin and provides low melt elasticity and elonga-
metallocenes, a molecular “bridge” connects the
tional viscosity in extrusion processes. [694]
two organic compounds of the metallocene “sand- The melting point (147–158°C; 297–316°F) of
wich”. A chiral molecule is one that, in its three
metallocene polypropylene currently produced is
dimensional configuration, cannot be superim-
generally lower than that of conventional polypro-
posed on its mirror image. In 1988, syndiotactic pylene (160–170°C; 320–338°F) and can be tai-
polypropylene was synthesized using zirconium-
lored to a specific application by using the appro-
containing metallocenes. [654, 767]
priate metallocene as catalyst. [654, 694] As with
Current metallocene catalyst systems commonly
Ziegler-Natta catalysts, resin color is affected by the
use zirconium chloride (ZrCl) as the transition metal
2 type and amount of catalyst residue present, and in-
complex, with a cyclopendadiene as the organic
teraction with additives may cause yellowing. [692]
compound and an aluminoxane such as MAO as co-
catalyst. Polypropylene resins with varying micro-
© Plastics Design Library Chemistry
