Polypropylene
Degradation
Control
1
Contents:
Introduction: ............................................................................................................................................ 3
Structures of polypropylene: ...................................................................................................................... 4
Properties of Polypropylene:...................................................................................................................... 6
Polypropylene classified in relation to Polyethylene: .................................................................................... 9
Structures and Synthesis of polypropylene: ............................................................................................... 11
Polypropylene Manufacturing Process: ..................................................................................................... 15
Modifications: ........................................................................................................................................ 17
Effect of Degradation on Molecular Structure and Properties of PP: ............................................................. 20
In general - the relationship between Polymer Degradation and Properties: ................................................. 24
Stabilization of PP by Additives: ................................................................................................................ 25
Protective Effects of Stabilizer Additives: ................................................................................................... 28
Reduction of Polymer Degradation during Processing: ................................................................................ 32
Influence of multiple reprocessing cycles on the rheological and mechanical properties of PP: ........................ 35
Summary: .............................................................................................................................................. 38
Bibliography: .......................................................................................................................................... 39
2
Introduction:
Polypropylene (PP) is the fourth main bulk plastic produced in the world after polyethene, poly (viny1 chloride),
and polystyrene.
After polymerization, polyolefins are subjected to one or more processing steps involving, molding, and
extrusion. Combinations of high pressure and shear provide the elevated temperature necessary for each of
these steps. Once, polyolefin are exposed to conditions that tend to degrade their polymer chains and change
their properties. The heat and shear of the processing environment can initiate degradation and oxidation
processes in the polymer well before the finished product has been packed for shipment.
Degradation can be initiated by heat shear, oxygen, catalyst residues or any combination of them. The thermal
oxidation of polyolefins includes initiation, propagation, and chain branching and termination steps.
Compared with PE, unsterilized PP is especially susceptible to oxidative degradation because of the high
reactivity of hydrogen atoms attached to tertiary carbon atoms.
The degradation of PP, initiated either by UV irradiation or through thermal activation, causes change in
crystallization and melting behaviors of PP. Degradation also leads to chain scission or cleavage, leading to a
decrease in the durability of the product. Loss in molecular weight (molar mass) also occurs. There are several
types of chain scission in PP. The most common is a unimolecular scission of carbon- and oxygen-centred
radicals.
The reduction in the molecular weight of the PP polymer leads to a change in many of its corresponding
properties. One of the most detrimental is the loss of durability and ductility, thus a drastic decrease in
toughness of the polymer. In addition, the chain scission will produce products that will tend to cause an
increase in the color of the polymer and the generation of oxygenated compounds, which will adversely affect
the durability, strength and physical properties of the final PP products.
Unsterilized PP is very prone to oxidation and degradation in the presence of air. Further thermal and oxygen
exposure in the application itself gradually leads to degradation unless adequate antioxidants (AOs) and heat
stabilizers have been added. To maintain a resin’s original molecular weight and mechanical properties
throughout its planned processing and designed lifetime, AOs and stabilizers are essential ingredients.
The aim of this paper is to understand why are antioxidants and heat
stabilizers needed for PP, and how do they work?
3
Structures of polypropylene:
PP as a commercially used material and in its most widely used form is made with catalysts that produce
crystallisable polymer chains. These give rise to a product that is a semi crystalline solid with good physical,
mechanical and thermal properties.
There are three polypropylene structures produced during polymerization: isotactic, syndiotactic, and atactic:
Isotactic polypropylene exhibits four crystalline structures:
 Alpha
 Beta
 Gamma
 Mesomorphic
Each of these structures forms under specific processing conditions and defines the properties of the
polypropylene. In polypropylene containing the alpha, beta, and gamma structures, the final material is typically
opaque due to the scattering of light by spherulites with sizes similar to the wavelength of visible light.
1. The most common polypropylene crystalline form is the alpha structure, in which the polypropylene
chain exhibits either a left or right handed helical conformation. Crystallites grow to form sheet-like
lamellae due to the preferential growth of the crystalline regions perpendicular to the principle axis
of the helices.
Secondary lamellae grow tangentially from pre-existing lamellae, resulting in a characteristic crosshatched
microstructure. These tangential growth structures can interact with neighboring lamellae to create a farspreading supermolecular structure.
Molecular helix of alpha crystalline form of isotactic
polypropylene.
Crosshatched microstructure of alpha crystalline
isotactic polypropylene
2. Beta Crystallinity - The beta crystalline form of isotactic polypropylene differs from the alpha form
by having a lower crystalline density and lower melting point. The beta form is metastable to the
alpha form and will rearrange to the alpha structure when heated to approximately 100 °C or placed
under strain. Beta lamellae form parallel stacks.
4
3. Gamma Crystallinity - Isotactic polypropylene’s gamma structure rarely forms under standard
processing conditions. It is believed that this form arises when an alpha crystalline material is
sheared in the growth direction as crystals form. Gamma crystallites form a crosshatched structure
that is similar to that of the alpha form. It differs, though, in that the crystallites grow in two
directions simultaneously. The resulting structure is more uniform than that seen in alpha crystalline
isotactic polypropylene. The density of the gamma crystallites exceeds that of both the alpha and
beta crystallites.
4. Smectic or Mesomorphic Crystallinity - structure in rapidly quenched isotactic polypropylene
products. In these materials, the polymer chains do not have the necessary time to orient,
preventing them from forming the large crystalline domains seen in the other three forms. This
means that the crystalline regions are small and that there is poor alignment among the individual
crystallites. The small crystallites are surrounded by amorphous regions. The behavior of smectic
isotactic polypropylene falls between that of alpha crystalline polypropylene and the atactic form.
The poorly organized crystallites do not effectively scatter light resulting in a transparent material.
This form of isotactic polypropylene is metastable
Syndiotactic polypropylene: is produced by the monomer units inserted alternately head-to-tail. This structure
is more flexible with better impact resistance and clarity than the isotactic structure.
Atactic Polypropylene –Atactic polypropylene’s irregular structure inhibits the regular packing required to
create crystallites. The polymer exhibits poor strength, high tackiness and is, in general, not terribly useful as a
thermoplastic resin in its pure state. It does, though, find a wide range of applications as a component of some
adhesives and as filler in asphalt, for some of the same reasons that make it ineffective in polymer applications.
Impact Modified Isotactic Polypropylene - Even though isotactic polypropylene has poor impact strength, its
other properties and low costs attract engineers. For this reason, impact modifying agents have become a
common ingredient in polypropylene products that require high impact strength, especially at low
temperatures.
Soft rubbers that are incompatible with the polymer matrix are effective impact modifiers. In the solid state, a
two-phase structure forms. Small discrete rubber particles are surrounded by a continuous matrix of
polypropylene. The rubber particles enhance the material’s impact strength by blunting the cracks that
propagate during the impact event. The rubber also absorbs and helps dissipate the impact energy.
5
Properties of Polypropylene:
The properties of polypropylene depend on the molecular weight and molecular weight distribution,
crystallinity, type and proportion of comonomer (if used) and the isotacticity.
Polypropylene is sold commercially as homopolymers, random copolymers, or impact resistance copolymers.
Physical properties range from high strength, stiffness, to a flexible polymer with lower strength but greater
toughness.
Properties of iPP, sPP and aPP:
polypropylene
structures
isotactic
degree of crystallinity
melting point
density [g/cm3]
40 to 70%
Has the highest melting
point, greatest crstallinity,
superior mechanical
properties.
160 to 180 °C
Syndiotactic
Atactic
˜30% crystallites have a
much more complex
structure than the
isotactic form
polypropylene
Comparisons of
stereoisomer
0.855,amorphous
approximately
138 °C
0.946, crystalline
crystallizes very slowly
Atactic polypropylene has no commercial
application because it's pretty much a
gooey, messy blob.
Have better impact strength
than isotactic.
Soft and rubbery, amorphous
and relatively weak. Low
density and low tensile
strength but high degree of
flexibility.
Some of the most significant properties of polypropylene are:
1. Isotactic homopolymer polypropylene - has a high degree of crystallinity thereby creating a material
that is strong, with low permeability to vapor or solvents, and high chemical resistance.
 With the exception of the mesomorphic crystalline form – is typically opaque, due to the high
concentrations of crystalline regions.
2. Syndiotactic polypropylene, which has lower crystallinity, is transparent and not as strong
3. Atactic form, due to its lack of crystallinity, has poor physical strength with lower resistance to
dissolution in solvents and greater permeability to low molecular weight gases such as oxygen and
water vapor.
Polypropylene’s melting temperature is high, useful material for many high-temperature applications. It also has
a high glass transition temperature. The result of this is that polypropylene is prone to brittle failure on impact,
which is especially noticeable at low temperatures. To address the engineering issues created by the high
propensity for brittle failure, polypropylene is often compounded with an impact modifying agent.
6
Polypropylene is produced as homopolymers and copolymers. The physical properties range from good tensile
strength and stiffness to a tough, flexible, and low strength polymer:
1. Polypropylene copolymers incorporate small amounts of ethylene which lower the crystallinity rate,
producing higher impact strength even at low temperatures, and more flexibility, but a lower melting
point and low melt mass-flow rate properties.
 Polypropylene Random Copolymer is produced by polymerizing together ethene and propene. It
features Ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene
chains. These polymers are flexible and optically clear making them suitable of applications
requiring transparency and for products requiring an excellent appearance
 Polypropylene Block Copolymer, ethene content is larger (between 5 and 15%). It has comonomer units arranged in regular pattern (or blocks). The regular pattern hence makes
thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable
for applications requiring high strength, such as industrial usages.
2. Polypropylene homopolymer has the highest melting point with a wide range of melt flow rates and
stiffness.
PP Homopolymer vs. Copolymer:
PP Homopolymer
a) High strength to weight ratio and stiffer &
stronger than copolymer
b) Good chemical resistance and weldability
c) Good process ability
d) Good impact resistance
e) Good stiffness
f) Food contact acceptable
g) Suitable for corrosion resistant structures
h) Melting Point - 160 - 165°C
i) Density - 0.904 – 0.908 g/cm3
PP Copolymer
a) Bit softer but has better impact strength;
tougher and more durable than homopolymer
b) Better stress crack resistance and low
temperature toughness
c) High process ability
d) High impact resistance
e) High toughness
f) Not preferable for food contact applications
g) Melting Point - 135 - 159°C
h) Density (Random Copolymer)- 0.904 – 0.908
g/cm3
In addition, other types of Polypropylene:
Material
PP impact copolymer
PP terpolymer
PP high melt strength (HMS PP)
Expanded polypropylene (EPP)
Description and benefits
It is a PP homopolymer with a co-mixed PP random copolymer phase, containing
between 45 – 65% ethylene. With its high impact resistance, it is suitable for
packaging, pipe and automotive applications.
It is a combination of propylene segments and randomly positioned monomers of
ethylene and butane. It has high optical transparency and low crystalline
uniformity and is a suitable material for sealing film applications.
A long chain branched PP that has both high melt strength and stretchability. This
polymer has a wide range of mechanical properties and high thermal and
chemical properties, making it suitable to be used as low-density foams for
various applications.
It is a greatly versatile closed-cell bead foam with low density. It exhibits
distinctive properties, such as high impact resistance, energy absorption, thermal
insulation, and high strength-to-weight ratio. It is also used in many industries,
such as automobiles, construction, and packaging.
7
Advantages and Disadvantages of Polypropylene:
Advantages














Low Cost
Excellent flexural strength
Good impact strength
Processable by all thermoplastic equipment
Low coefficient of friction
Excellent electrical insulation
Good fatigue resistance
Excellent moisture resistance
Service Temperature to 126 C
Very good chemical resistance



Disadvantages
High thermal expansion
UV degradation
Poor weathering resistance
Subject to attack by chlorinated solvents and
aromatics
Difficulty to bond or paint
Oxidizes readily
flammable
Typical Applications:
a) Flexible packaging films
b) Biaxially oriented packaging films
c) Stretched and oriented monofilament, tapes for textiles, carpeting, insulated medical fabrics and woven
carpet backing
d) Automotive interiors, bumpers, spoilers, air vent systems, under the hood components, internal wheel
guards, and bellows
e) Hygiene products, household goods and medical application trays, strainers, and containers
f) Consumer products, such as closures, over caps, trigger sprayers, rigid and semi-rigid packaging, video
cassette cases, toys, and electrical hardware
g) Appliance housings and components, outdoor furniture, and luggage
h) Injection blow molded stretch bottles with excellent stiffness, impact resistance, and clarity
Automobile injection
molded dash panel
Hot water dispenser
bottles
Electric rice cooker
8
Compact disk jewel
boxes
Variety of injection
molded products
Polypropylene classified in relation to Polyethylene:
Polyolefin
LDPE (Lowdensity
Polyethylene)
HDPE (Highdensity
)Polyethylene
LLDPE (linear
low-density
)polyethylene
PP Homopolymer
Molecular structure
Preparation method
Radical, with autoclave or tubular
reactor
Produced by polymerization of
ethylene using Ziegler-Natta or
supported chromium ("Phillips")
catalysts.
Small amounts (4%of) a-olefin
comonomers are used in many of the
commodity grades to introduce low
concentrations of short chain
branching, primarily to enhance
processability but also to improve
toughness and environmental stress
crack resistance
produced by copolymerization of
ethylene with a-olefins using:
 Ziegler-Natta
 supported chromium catalysts
(Phillips catalysts)
 single site catalysts
Mechanical properties
Non-Newtonian melt rheology,
good impact strength
High tensile strength, low
impact strength
None - Small amount of aolefin incorporated to
improve polymer
properties
Intermediate strength with
elasticity, melt rheology more
Newtonian than LDPE
Butene-1
hexene-1
octene-1
4-methyl- 1 - pentene
No branches - High tensile
strength, brittle, temperature
,resistance Transparent
high-strength and
temperature-resistant
glossy films
Ziegler-Natta catalysis
Polypropylene
copolymer
with
ethylene, block
or random
Ziegler-Natta catalysis
Polypropylene
and
copolymers
with ethylene
Single-site or metallocene-catalysed
Tough with high melting
Temperature (block) or softer
with lower melting
temperature(random)
Tough films, with
more milky colour
Narrow molar
mass distribution, random
comonomer distribution and
high isotacticity Flexible, elastic
transparent films
9
Comonomer
Is often blended with
linear low density
polyethylene and high
density polyethylene to
improve processability.
No branches
Ethylene
Ethylene
Comparison of mechanical and physical properties between PP and PE:
Polyolefin
LDPE
HDPE
LLDPE
Polypropylene
homopolymer, PP-H
Polypropylene random
polymer, PP-R
Polypropylene block
copolymer, PP-B
Polypropylene glass-fiber
reinforced polymer,
PPGF30
Polypropylene, PP +
Ethylene-propylene-(diene)
copolymer, EPDM
thermal
coefficient of
expansion in
longitudinal at
23 - 55 °C
23 - 25
14 - 18
18–20
density
(g/cm3 )
Tm [C°]
yield
strain
)%(
Tensile
modulus
(MPa)
HDT at 1.8 MPa
(°C)
0.915 - 0.930
0.940 - 0.970
0.915 - 0.940
105–115
130–138
120–130
ca. 20
8 - 12
20–30
200 - 400
600 - 1400
300–700
38 - 50
ca. 40
12 - 15
0.90 - 0.915
162 - 168
8 - 18
1300 - 1800
55 - 65
12 - 15
0.895 - 0.90
135 - 155
10 - 18
600 - 1200
45 - 55
12 - 15
0.895 - 0.905
160 - 168
10 - 20
800 - 1300
45 - 55
6
1.21 - 1.14
162 - 168
-
5200 - 6000
90 - 115
15 - 18
0.89 - 0.92
160 - 168
10 - 35
500 - 1200
40 - 55
Though Polyethylene and Polypropylene are similar in physical properties but here are key points to consider
selecting the polymer suitable to needs:
a)
b)
c)
d)
e)
f)
g)
Polypropylene (PP)
It can be produced optically clear
It is lighter in weight
PP exhibits a high resistance to cracking, acids,
organic solvents and electrolytes
It has high melting point and good dielectric
properties
PP is non-toxic
It is stiffer and resistant to chemicals and
organic solvents compared to polyethylene
PP is more rigid than polyethylene
a)
b)
c)
d)
e)
10
Polyethylene (PE)
Polyethylene can only be made translucent
like a milk jug
Its physical properties allow it to stand up
better in cold temperatures, particularly when
using it as signs
It is a good electrical insulator
PE offers good tracking resistance
Polyethylene is sturdy as compared to
Polypropylene
Structures and Synthesis of polypropylene:
The propylene molecule is the monomer unit of polypropylene. There are a number of different ways to link the
monomer together, depending on the stereo arrangement. Three major factors control the stereo regularity of
PP:
The first factor is the degree of branching. The molecular chain of PP will be straight (or linear) if the next
monomer unit always attaches to the chain end. If the next monomer may add on to the backbone, this results
in the formation of branches, as seen in.
The pendant methyl sequence can also change the stereo regularity of PP. The addition of propylene to the
growing PP chain can be regiospecific or non-regiospecific. The addition of monomer can be in a head-to-tail
manner or in other ways such as head-to-head or tail-to-tail.
Still another way to control the stereo regularity is the position of the tertiary hydrogen. There exist two
possibilities for the arrangement of the tertiary hydrogen. If the propylene monomer is always added in the
same stereo arrangement, the alignment of the tertiary hydrogen will be in a same hand way, either righthanded or left-handed. Any change in the stereo arrangement of the adding monomer can result in an opposite
hand distribution of the tertiary hydrogen.
Synthesis of polypropylene:
Monomers are produced by the cracking of petroleum products such as natural gas or light oils. For the
preparation of polypropylene the C3 fraction (propylene and propane) is the basic intermediate and this may be
separated from the other gases without undue difficulty by fractional distillation.
Polymerization of polypropylene is a coordination polymerization of monomer propylene. The monomer
propylene undergoes polymerization in the presence of catalyst such as Ziegler-Natta catalyst (or) metallocene
catalyst.





homopolymers produced by Ziegler-Natta catalysis
block copolymers produced by Ziegler-Natta catalysis
random copolymers produced by Ziegler-Natta catalysis
rubber-modified blends of the above
homopolymers and copolymers produced by metallocene catalysis
11
Ziegler-Natta is chain-growth polymerization is an alternative method that does not involve radicals.
Ziegler-Natta and metallocene catalysts are used commercially, the choice between the two depends largely on
the desired molecular weight distribution:
1. Ziegler-Natta catalysts are predominantly isotactic, with a comparatively broad molecular weight
distribution (Mw/Mn ≈ 3.5)
2. Metallocene catalysts have only one active polymerization site, which results in a narrower molecular
weight distribution (Mw/Mn ≈ 2.0).
Some characteristics of Metallocene catalysts and Ziegler-Natta catalysts polypropylene:
Characteristic
Melt flow (g/10 min)
molecular weight (g/mole)
Tm [C°]
Polydispersity
mi-PP
75
148000
150
̴2
ZN-PP
15.4
430000
165
̴9.2
The conditions of polymerization such as pressure, temperature, and concentrations of the reactants are given
by the grade of the polymer which is to be produced.
Using Ziegler Natta catalyst, there are two major processes for synthesizing of polypropylene. The properties of
polypropylene are affected by tacticity of polypropylene, the orientation of the methyl group. Based on the type
of catalyst that is used for the polymerization, the tacticity of the propylene could be selected.
The tacticity of a polymer affects the packing between molecules (crystallinity) and thus affects its physical
properties such as the melting temperature, mechanical strength and elasticity.
Structure regularity: Isotactic > Syndiotactic > Atactic
12
Zeigler Natta catalyst could form isotactic polypropylene (or) syndiotactic polypropylene.
One type of metallocene catalyst could form isotactic polypropylene and another type of metallocene catalyst
forms syndiotactic polypropylene.
An amorphous rubbery material is atactic polypropylene. It is synthesized with the help of special kind of
Zeigler-Natta catalyst (or) with certain type of metallocene catalyst.
Two types of coordination polymerization are:
1. Heterogeneous Ziegler-Natta polymerization: Coordination polymerization is carried out in the presence
of heterogeneous Ziegler Natta catalysts that are based on oregano aluminum co-catalyst and titanium
tetrachloride.
2. Homogeneous Ziegler-Natta polymerization: Certain homogeneous Ziegler-Natta polymerization such as
Kaminsky catalyst is used as catalyst in coordination polymerization. Nonpolar ethene and nonpolar
propene are the monomers that are used along with homogeneous Ziegler-Natta polymerization.
Heterogeneous Ziegler-Natta polymerization:
The monomer propylene would be polymerized at high temperatures and high pressures. The polymerization
could be effective only in the presence of inert solvent carried moderate pressure in presence of stereospecific
catalyst like TiCl4. In the Zeigler Natta polymerization of polypropylene, TiCl4 is the catalyst and Al(Et)3 is the cocatalyst. The polymer formed by the method of Ziegler Natta polymerization could have high molecular weights.
The polymerization reaction could be given as
Homogeneous Zeigler Natta polymerization of propylene:
Atactic polypropylene are formed when metallocene are reacted with methylaluminoxane. Here, the metals
could be titanium, zirconium, and hafnium. Methylaluminoxane in short is known as MAO. The reaction of
metallocene with methylaluminoxane to give atactic polypropylene is written as,
13
Example for Isotactic Polymerization by Ziegler-Natta catalyst:
Ziegler-Natta catalyst is a mixture of Titanium Tetrachloride and Triethyl Aluminium. The mechanism begins with
the formation of a complex between Titanium and Aluminium.
Isotactic Polymerization:
1.
Formed bond between the CH2 group of propylene and the Titanium and giving second carbon of the
propylene a positive charge.
2. Migration - The alkyl group of the aluminium transfers to the carbocation. This transfer happens so
rapidly that the carbocation formed in the previous step doesn't have time to rearrange. Thus, the
titanium stereo specifically inserts the propylene group between itself and the ethyl group.
3. The titanium complex continues to stereo specifically insert additional propylene monomers between
itself and last ethyl group added to the chain.
4. To end the reaction, chemists destroy the complex by treating it with methanol.
14
Polypropylene Manufacturing Process:
The PP manufacturing process can be divided into three generations:
1. the first generation (deashing and AP removal)
2. second-generation (non-deashing or non-solvent) and
3. third-generation (non-deashing and non-AP removal)
AP (atactic Polymers): non crystalline polymers where the methyl groups of propylene units are arranged irregularly on the chain
The operation for eliminating the catalyst is known as deashing.
Solvent polymerization process
Solvent polymerization process (Nondeashing)
Bulk polymerization process (Non-solvent)
Vaper phase polymerization process (Nondeashing, Non-AP)
Chronology of development in Ziegler-Natta catalysis:
Generation
first generation
second-generation
third-generation
Fourth generation
Catalyst
TiCl4/AlEt2Cl
TiCl4/ether/AlEt2Cl/ester
TiCl4
Al-oxame activated
metallocene complexes
Support
Activated MgCl2
Silica gel
15
Process steps
Remove catalyst residues
Remove catalyst residues
No purification
No purification No palletizing
extrusion
In addition, classification can be done according to the polymerization method into:
1. Slurry Process Polymerization of Polypropylene - In the slurry process, propylene monomer is dissolved
in a hydrocarbon diluent in which the polymerization process occurs. The polymerization products are
either soluble (the highly atactic components) or insoluble. Both the insoluble and soluble components
are collected and form separate product streams.
 Insoluble species form slurry in the solvent, from which they are removed by centrifugation.
 Soluble, atactic component is removed with the solvent as another product stream.
To separate the atactic polymer from the solvent, the solution is heated allowing the solvent to flash off, leaving
the atactic polymer behind. Any unreacted monomer is degassed from the solution and recycled to the start of
the polymerization process.
2. Liquid Propylene Polymerization of Polypropylene - employs the liquid monomer as the polymerization
solvent. This process, known as the liquid propylene or bulk-phase process has a major advantage over
the slurry method in that the concentration of the monomer is extremely high. The high concentration
increases the rate of the reaction relative to that seen in the slurry process. In addition, the heat of
polymerization can be removed from the process by the vaporization of the monomer. The gaseous
monomer is then recycled to the reactor, after liquefaction, as condensed monomer. Just as in the slurry
process, the polypropylene forms an insoluble phase in the propylene diluent. The insoluble phase is
isolated from the liquid propylene. Unlike the slurry process, the insoluble phase contains both the
atactic and isotactic fractions. Separation of these two components requires an additional step in which
the soluble portion is dissolved into an organic solvent.
3. Solution Polymerization of Polypropylene - During solution polymerization the monomer, catalyst, and
diluent are introduced to a reactor maintained at a temperature between 175 and 250 °C. The resulting
polymer forms a viscous solution in the solvent, which is pumped out of the reactor. If necessary, the
solution can be filtered to remove the catalyst residue. Solvent is removed from the solution and
recycled, leaving behind a mixture of isotactic and a small amount of atactic polypropylene.
4. Gas Phase Polymerization of Polypropylene - In gas phase reactors, the monomer is introduced to the
bottom of reactor where it percolates up through a fluidized bed of polymer granules and inert-media
supported catalyst. A fraction of the monomer reacts to form more polymer granules, the remaining
monomer being drawn from the top of the reactor, cooled, and recycled. Polymer granules are
continuously withdrawn from the bottom of the fluidized bed and the catalyst is replenished.
16
Modifications:
The property ranges of all reactor-made homo- and copolymers so far described can further be modified by
nonreactive or reactive compounding. Three types of modification are most relevant in practice:
1. Increasing stiffness and heat resistance by addition of mineral fillers or reinforcing fibers
2. Improving toughness and flexibility by addition of external elastomers or PE plastomers
3. Modifying the process ability by radical-based reactive modification
Fillers:
Using inorganic fillers in PP helps both in expanding the property profile of the materials, but also in reducing
the formulation cost. The most used inorganic fillers for PP are calcium carbonate (CaCO3), talc, and kaolin,
while wollastonite is used if a high dimensional stability of the compound is required. The end-use properties of
a compound are determined not only by the chemical type or composition of the filler, but also by the particle
shape and surface area which especially define the reinforcement quality.
The highest modulus and heat deflection temperature (HDT) are reached for anisotropic particles of fine particle
size equally distributed in the polymer. This property together with its capacity for crystal nucleation makes talc
the most commonly used inorganic filler for PP impact copolymers. The compounds exhibit a balanced property
profile including increased modulus, higher HDT, and better dimensional stability
.
PP-talc compounds may contain up to 50 wt% of the mineral, but its absorption can cause surface appearance
problems. Additionally, the impact strength decreases with increasing filler content due to the anisotropic
nature of the filler.
PP homo- and copolymers reinforced with glass fibers (GF) or carbon fibers (CF) are industrially used in the
automotive and other technically challenging areas to replace polycarbonate, polyamide, or
acrylonitrileebutadieneestyrene (ABS) terpolymers. Coupling agents to increase the adhesion between fiber and
polymer are needed, but also fiber coating (sizing) to avoid fiber breakage and further promote the adhesion.
Positioning:
17
Elastomers:
Blending PP with various impact modifiers such as EPR, EPDM, or polystyreneblock-poly(ethylene-co-but-1-ene)block-polystyrene (SEBS) was the earliest route for producing impact-modified PP. In blends of heterophasic PP
with polymeric modifiers the end-use properties such as mechanical performance and optical properties
strongly depend on the phase structure formed; this depends on structural factors (as for reactor-based
systems), but also on the compounding and even the processing step.
Commonly used factors to describe these parameters are the viscosity ratio l between dispersed phase and
matrix, the phase compatibility, and the deformation history in processing; most of these are relevant for both
reactor-based and compounded systems.
For example PP+EPDM Elastomer Blends:
These structures do not necessarily have to be connected to each other in a chain, but can be achieved by
blending PP with EPDM. These products exhibit high stiffness and softening temperatures, are easily modified
by copolymerization of PP, and are compatible with EPDM.
The properties of PP + EPDM elastomers depend on their blend ratio:
1. 90% PP results in the properties of conventional PP with slightly lower stiffness and softening
temperature, but also with increased impact resistance at −40 °C.
2. Blends with 40% PP exhibit the typical properties of thermoplastic rubbers.
Other determining factors are crystallinity, molecular weight, and molecular weight distribution of the PP. In
addition, it is important whether a homo- or a copolymer, random or sequential PP is used. It is also possible to
create blends with PE.
Reactive Processing:
Finally, structural modification is also possible by reactive post reactor processes. The earliest version is targeted
at modifying the MMD, where controlled radical degradation allows a tailored production of narrow MMDs. This
process known as visbreaking or controlled rheology process can be applied best in case of PP homopolymers or
random copolymers, leading to higher MFR and a broader Newtonian region of the viscosity curve.
Mostly liquid peroxides are used which are sprayed under nitrogen onto the warm reactor powder before it is
fed to the pelletization extruder. In large-scale plants, visbreaking also allows the production of a multiplicity of
grades from one reactor setting.
Transparency and surface gloss are improved and these grades have significant advantages in fiber spinning or
cast film processes. Further reactive modification steps for PP are also mostly based on radical-initiated grafting
reactions, applications ranging from the production of long-chain branched PP with high melt strength (strain
hardening) (offering advantages, e.g., in foaming, via stabilization) or partial cross-linking of phase structures (to
the production of thermoplastic vulcanizates) to polar modifiers (e.g., by grafting with maleic anhydride).
Blends of PP-I and, e. g., methyl methacrylate or styrene are produced by reactive blending. They have the
following advantages: low density (0.91–0.96 g/cm3), weather resistance, scratch resistance, little processing
shrinkage (little warpage) and moisture absorption.
18
Compounds with 3–6% hydrocarbon resins, for example, hydrogenated dicyclopentadiene (DCPD), increase the
glass transition temperature of PP films by up to 25 K and thus the modulus of elasticity by up to 50%, while
reducing water vapor permeability by up to 30%.
Properties for number modified PP Grades:
Property
Density
Tensile modulus of
elasticity
Yield stress
Elongation at yield
Elongation at
break
Melting
temperature
Heat deflection
temperature
HDT/A 1.8 MPa
Unit
PP +
EPDM
PP-T 20
Talcum
PP-T40
Talcum
PP-GF30
Glass fiber
g/cm3
0.89–0.92
1.21–1.24
1.12–1.14
MPa
500–1200
3500–4500
5200–6000
5500–6000
1850
MPa
%
10–25
10–35
1.04–1.06
2200–
2800
32–38
5–7
PP-GFC30 Glass
fiber, chem.
coupled
1.12–1.14
30–35
3
-
-
26
-
%
-
15–20
3–15
3–5
3–5
-
°C
160–168
162–168
162–168
162–168
162–168
-
°C
40–55
60–80
70–90
90–115
120–140
53
19
PP-B25
Barium
1.13
Effect of Degradation on Molecular Structure and Properties of PP:
The terms “durability” and “degradation”, as well as “long-term properties” and “end-of-life” of polymers, are
related to one another. Degradation refers to a process which results in the deterioration of any physical
property of a polymer. In general, the said process starts at the amorphous/crystalline interface, and affects the
mechanical properties, thermal stability, distribution of lamellar thickness, as well as crystallinity.
Each aforementioned component is subject to environmental effects individually or in combination. Accordingly,
various factors in a typical habitat – sun-based radiation, temperature, moisture, oxidative conditions, and
mechanical toxins – can act intelligently via a debasement procedure. Therefore, the general impact of
degradation can be greatly complex. With reference to the basis above, polymer degradation is categorized into:
1.
2.
3.
4.
5.
6.
7.
Thermal debasement (heat)
Thermo-oxidative debasement (heat and oxygen)
Thermomechanical debasement (heat and stress)
Photo degradation (light)
Photo-oxidative corruption (light and oxygen)
Biodegradation (natural operators)
Mechanical debasement (mechanical stress)
Virgin PP (due to the presence of a tertiary hydrogen) obtained directly from a commercial process is very
susceptible to UV irradiation and air oxidation. If stored un-stabilized at room temperature, the durability,
strength and physical properties of the PP product deteriorate rapidly over a period of weeks or months
depending on the physical form, temperature, available oxygen, intensity of UV radiation and other conditions.
At elevated temperatures, such as during summer storage, the degradation process can be accelerated.
When organic materials such as polyolefins are exposed to conditions such as heat, ultraviolet light, or
mechanical stress in the presence of atmospheric oxygen, free radicals are formed which initiate the oxidation
process.
Degradation of Thermoplastics:
20
This process is characterized degradation of PP can be divided into three steps:
1. Initiation
2. Propagation
3. Termination
Importance of AOs and Stabilizers for Polyolefins - Polymers are heat-, shear-, and oxygen-sensitive materials,
and exposure to the heat and shear of processing alone can cause chain scission—the breaking and shortening
of molecular chains. Upon exposure to oxygen, auto-oxidation, or autoxidation, is initiated by the creation of
free radicals (reactive molecular species with unpaired electrons). Autoxidation is a circular, self-propagating
process that, unless interfered with by AOs, gradually leads to increasing degradation of the polymer.
Polyolefins are susceptible to degradation by free radicals via breaking or cleavage of their polymeric chains (i.e.,
chain scission) or by cross-linking between chains. These reactions lead to changes in molecular weight,
molecular weight distribution, mechanical properties, and appearance.
Because of molecular structure differences, the tendency toward chain scission and reduced molecular weight is
more pronounced in polypropylene (PP) than in polyethylene (PE), while cross-linking tends to predominate
especially in unbranched types of PE.
Degradation cycle by Thermo-oxidative:
1. Heat, shear, and catalyst residues tend to strip hydrogen from the polymer chain (RH) to form alkyl free
radicals (R•)
2. Oxygen combines with the free-radical species to create new reactive species, including peroxy radicals
and hydroperoxides (O2 + R•  ROO• + RH  ROOH + R•) and other fragment species (H2O, H2, H2O2)
3. The hydroperoxides (ROOH) in turn are themselves reactive, creating new free-radical species, such as
hydroxy and alkoxy radicals (ROOH  •OH + RO•).
UV Degradation of Polyolefins – Because of the chemical structure of polypropylene, it has a high degradation
rate when exposed to UV light like the Sun. The light causes the bonds holding the polymer together to break
which weakens the plastic. This makes polypropylene unsuitable for uses that require longterm exposure to
sunlight.
The initiation of polymer degradation via UV light depends mainly on the presence of UV-absorbing species
(chromophores) mixed in with the polymer. Since saturated polyolefin molecules do not themselves absorb
much UV light directly, the most damaging UV effects are from absorption by chromophores such as catalyst
residues, pigments, processing aids, flame retardants, or generally any organic molecules containing double
bonds. These molecules release some of the UV energy they absorb by breaking bonds and releasing free
radicals, which begin a degradation cycle similar to the autoxidation processes.
Degradation cycle by Photo-oxidative:
1. UV energy is absorbed by chromophores, creating broken bonds and free radicals (R•) in the polymer.
2. Oxygen combines with free-radical species to create new species, including peroxy radicals and
hydroperoxides, which are reactive with the polyolefin chain (RH) itself (O2 + R• ROO• + RH-ROOH +
R•), as well as other fragment species (H2O, H2, H2O2).
3. The hydroperoxides (ROOH) in turn are themselves reactive and create new free-radical species such as
hydroxy and alkoxy radicals (ROOH•OH + RO•).
21
Photo-oxidation mainly takes place at the surface of an unprotected resin, but it can occur deep within the solid
material if the UV energy is not blocked by elements within the resin.
Either way, the resin changes induced by this degradation can range from minor to catastrophic; for example, it
has been proposed that chain scission leads to increased crystallization in the polymer and creates internal and
surface voids, resulting in cracks, embrittlement, and loss of gloss.
Different polyolefin structural forms react differently to UV. Overall, degradation takes place more easily within
the amorphous phase of a polymer than in the crystalline phase. Moreover, PE and PP photo-oxidation
behaviors are different enough that the same additives approach for protecting PE may not work the same in
PP, even in the same applications. And even different forms of PP—homopolymer and copolymer—can differ in
their sensitivity to UV when using the same stabilizer.
Example of Oxidation:
Polymer oxidation occurs through a free radical chain reaction. Mechanical stress, heat, or the presence of
oxygen or metal catalyst residues results in homolytic cleavage of the carbon-hydrogen or carbon-carbon
covalent bond in the polypropylene chain; each atom receives one electron from the two-electron covalent
bond, producing two free radicals, each with an unpaired electron.
An example of a chain initiation reaction in the presence of oxygen is given below:
The chain reaction is propagated through the
formation of a hydroperoxide, accompanied
by the formation of another free radical
The oxidation rate is determined by the rate of the slow step in the chain propagation reactions. Due to the
presence of the pendant methyl group, polypropylene contains tertiary (3°) hydrogen atoms, in which the
carbon atom covalently bonded to the hydrogen is also bonded to three other carbon atoms.
The free radical (PP·) formed from abstraction of a tertiary hydrogen is more stable than those formed from
abstraction of a primary (1 °  carbon atom attached to one other carbon) or secondary (2°  carbon atom
attached to two other carbons) hydrogen, due to the tendency of carbon atoms along the chain to electronically
donate electrons to the electron-deficient radical.
22
The higher probability of reaction with the tertiary hydrogen considerably increases the susceptibility of
polypropylene to oxidation.
In further reactions (chain branching reactions that increase the amount of free radicals), the hydroperoxide
decomposes in the presence of heat or metal catalyst residues to form an alkoxy radical. Oxidative chain
scission is believed to occur through disintegration of this alkoxy radical:
The decrease in molecular weight resulting from chain scission produces a gradual loss in mechanical properties.
One of the most detrimental is the loss of durability and ductility, thus a drastic decrease in toughness of the
polymer. In addition, the chain scission will produce compounds such as carboxylic acids, lactones, aldehydes,
and esters are also produced during oxidation reactions, resulting in chemical modifications such as yellowing.
Products that will tend to cause an increase in the colour of the polymer and the generation of oxygenated
compounds, which will adversely affect the durability, strength and physical properties of the final PP products.
Chain reactions are terminated when two radicals combine to form an inactive species.
In the solid form, PP is a semi crystalline polymer with a crystalline content that is normally between 40% and
60%. The crystalline regions are essentially impervious to oxygen, so the oxidation only occurs in the amorphous
region. It has been reported by that the diffusion rate of oxygen is much slower than the reaction rate, so that
the oxidation process is basically a surface phenomenon. In most cases, the surface can become dull, crazed, or
even powdery. Obviously, unstabilised PP is very prone to oxidation and degradation in the presence of air.
Therefore, adding appropriate stabilizer's is necessary to convert PP into a durable, useful material.
23
In general - the relationship between Polymer Degradation and Properties:
Changes of the
molecular structure of
polymers
Degradation reactions
can cause molecular
weight reduction, a
change of molar mass
distribution or the
formation of chain
branches or crosslinks.
These molecular
parameters can
influence rheological
properties of the
polymer melt (like
viscosity) and the
mechanical or optical
properties of the
processed granules or
finished parts.
Changes of mechanical
properties
Possible influence on some key polymer
properties.
The molecular composition
and homogeneity of the
mixture after compounding
of polymers with fillers or
after mixing of different
polymers can have an
influence on mechanical
properties, like tensile
strength or bending
strength, as well as on
brittleness or impact
strength.
Change of flow behavior
(rheological properties)
In addition to mechanical
properties, molecular weight
degradation may also change
rheological properties of polymer
melts. This changed flow behavior
might result in changed processing
behavior within the extruder. In
addition to changes of the mean
molecular weight, the width of the
molecular weight distribution can
be narrowed through degradation
or widened through crosslinking
reactions. Other parameters
related to molecular weight
distribution are the zero shear
viscosity, solution viscosity or melt
flow index (MFI) and melt volume
index (MVI), respectively.
Already small changes of mean
molecular weight via degradation
can lead to a significant change of
viscosity: with decrease of mean
molecular weight by a factor of 2,
the zero shear viscosity is
diminished by a factor of 10.
Rheological methods are therefore
a very sensitive indicator for
changes of the molecular structure
of polymers.
Molecular weight
degradation can influence
mechanical properties
significantly: below a critical
molecular weight the
tensile strength and impact
resistance are drastically
reduced. An example of the
critical molecular weight is
for polypropylene around
170,000 g/mol.
24
Optical properties
Other properties
Optical properties
comprise color changes
(e. g. yellowing,
discoloration) that
might be caused by
oxygen-containing
functional or
chromophoric groups
(conjugated double
bonds). Color changes
can be determined and
quantified via optical or
spectroscopic methods.
Crosslinks might lead to
gels or spots
(punctiform impurities)
that can influence
optical properties in
addition.
Depolymerization leads to
detachment of monomers,
which can lead to odor
formation, and related
molecular weight changes
can lead to mechanical
property changes. Side
group elimination can lead
to emissions of undesired or
toxic substances, e. g.
emission of hydrochloric
acid from polyvinyl chloride
(PVC). Aside from
mechanical properties,
polymer degradation may
also cause changes of
dielectric properties or
electrical conductivity
Stabilization of PP by Additives:
Plastics additives can be classified into three main groups:
1. Additives that stabilize plastics against degradation and aging during processing or in use: Degradation
usually involves chain cleavage of the macromolecules and can proceed through the addition of energy
(e.g., shear forces, heat, UV light) or chemical attack (e.g., oxidation, hydrolysis). These additives are
called antioxidants, light stabilizers, or heat stabilizers.
2. Additives that facilitate or control processing (e.g., lubricants, mold-release agents, or blowing agents)
3. Additives that impart new, desirable qualities to plastics, such as resistance to burning, transparency or
color, improved mechanical or electrical properties, dimensional stability, and degradability. Such
additives include flame retardants, fillers, dyes, pigments, antistatic agents, nucleating agents, optical
brighteners, impact modifiers, and plasticizers.
A variety of additives are used to overcome some of the limitations in plastic materials. Some of the common
additives used in compounding and their functions are given below:
1. Antioxidants: Prevent oxidative degradation of the polymers.
2. Heat stabilizers: Prevent the polymers from heat-induced breakdown, and degradation, when the plastic
product is exposed to high temperatures.
3. UV stabilizers: Prevent polymer deterioration when the plastic product is exposed to UV light.
4. Internal lubricants: Improve the processing ability of the polymer by lowering melt viscosity.
5. Plasticizers: Enhance the polymer flexibility, resiliency, and melt flow.
6. Antifogging agents: Prevent fogging that obscures viewing through clear plastic films or sheets.
Protective additives are characterized by their:


Chemistry (e.g. hindered phenols, phosphites) and/or
Function (e.g. radical scavengers, peroxide decomposers, UV absorbers).
Different families of additives are reactive with different intermediates in the oxidation process (e.g. free
radicals vs. hydroperoxides), present different levels of reactivity (e.g. at different temperatures), and have
different degrees of stability during exposure to environmental conditions such as high temperatures or UV
light.
As a result of their inherent reactivity and stability, some additives function primarily as melt processing
stabilizers at high temperatures, some as thermal stabilizers but at lower temperatures (i.e. in the solid state),
and still others as light stabilizers.
Some common families of stabilizer additives used in polypropylene:
a)
b)
c)
d)
e)
f)
g)
Hindered phenols
Phosphites and phosphonites
Thioethers
Hydroxylamines
Hindered amine light stabilizers (HALS)
Benzoates, and
Ultraviolet (UV) absorbers
25
Additives that stabilize plastics against from degradation how they work?
Organic stabilizers can react with molecular oxygen in a process called autoxidation, initiated by heat, light
(high-energy radiation), mechanical stress, catalyst residues, or reaction with impurities, to form alkyl radicals.
The free radicals can, in turn, react to cause the polymer to degrade, causing embrittlement, melt flow
instability, loss of tensile properties, and discolouration. Oxidation can be slowed by chain-breaking antioxidants, to reduce rate of propagation, or preventative anti-oxidants, which prevent initial formation of free
radicals. Antioxidants deactivate the sites by decomposing the hydroperoxide or by terminating the free radical
reaction.
There are many different stabilizer systems for plastics, depending on the type and products of oxidation.
Traditional stabilizer systems for polyolefins are based on a combination of a phenolic anti-oxidant and a
phosphorus-based melt processing stabilizer, the phenolic providing melt processing stability as a donor of
hydrogen atoms and a scavenger of free radicals, and a level of thermal stability. The phosphorus-based additive
functions as a hydroperoxide decomposer during the melt compounding stage.
Three forms of stabilization are used: prestabilization, stabilization during processing, and long-term
stabilization.
Numbers of requirements apply to antioxidants:
1.
2.
3.
4.
5.
6.
7.
They must be thermally stable and nonvolatile at processing temperatures
They must be soluble in polymers and no chalking should occur at service temperatures
They must not have an intrinsic color, and their oxidation products must have minimal color
Any acidic hydrolysis products must not corrode machinery
They must resist extraction
They must be odorless and tasteless
They must not create toxicity problems (many must be approved as indirect food additives)
Chemical Classes of antioxidants:
Antioxidants are divided into two classes on the basis of their mode of action:
1. Primary antioxidants : sterically hindered phenols, secondary aromatic amines, and sterically hindered
amines (HALS)
2. Secondary antioxidants: phosphites, phosphonites, thioethers (sulfides), and metal salts
Primary antioxidants can be used by themselves for prestabilization and long-term stabilization.
Secondary antioxidants are used in combination with primary antioxidants, especially for stabilization during
processing and for long-term stabilization under severe thermal conditions.
Synergistic effects often occur in such systems (e.g., phenol and phosph (on)ites or phenol and thioethers).
Antagonistic effects are also observed, however (e.g., between thioethers and HALS). The stabilizer system often
includes other components that complex or neutralize degradation-promoting catalysts or acid traces.
26
Mechanism of Action:
1. Primary antioxidants (free-radical scavengers) have reactive hydrogens which react with free radicals
(examples: hindered phenols, sterically hindered amines, and aromatic amines) - They interrupt the
primary oxidation cycle by removing the propagating radical. Hindered phenols donate hydrogen to
chain-propagating radicals such as alkoxy, peroxy, and hydroxy radicals.
2. Secondary antioxidants (peroxide scavengers) decompose hydroperoxides and prevent chain branching
of photochemical reactions (examples: phosphites, phosphonites, and thioesters).
 Another class of secondary antioxidants is thioethers or organic sulfides. They decompose two
molecules of hydroperoxide into the corresponding alcohols and are transformed to sulfoxides
and sulfones. These are very effective peroxide decomposers during long-term thermal aging.
These are also referred to as Thiosynergists.
Both functions are in some cases combined into one compound. Antioxidants are usually added in
concentrations from 0.03 to 0.3%.
Demonstrate the role of primary and secondary antioxidants in the degradation process. Each antioxidant plays
an important role as described in the following section:
This reaction converts
the chain propagating
radical into inert alcohol
and water, respectively.
Hindered phenols are
very effective radical
scavengers both during
processing and longterm thermal aging.
Secondary antioxidants
decompose
hydroperoxides (ROOH)
into nonreactive and
thermally stable
products before they
decompose into alkoxy
and hydroxy radicals.
27
Protective Effects of Stabilizer Additives:
Order to provide the most effective protection against oxidation it is beneficial to combine additives acting by
different mechanisms, i.e. by attacking the oxidation process at multiple steps.
These combinations often exhibit a level of performance far superior to that of their individual components
used alone.
Polymers and parts produced from them can be oxidized under a wide range of conditions encountered in their
manufacturing and end use. Combinations of additives (often referred to as systems) are usually needed to
provide effective protection.
Hindered phenol antioxidants are highly effective to prevent oxidation in polyolefins both during processing in
the melt and during long-term heat aging in the solid phase. These additives also have an influence on the light
stability of polypropylene.
Hindered phenols function by reaction with peroxy radicals (formed in of the oxidation process) to give
hydroperoxides, thus terminating the oxidation chains - Hindered phenols are radical- trapping antioxidants for
oxy and especially peroxy radicals:
Such compounds are also called chain breaking antioxidants and examples include the hindered phenols and
aromatic amines.
Combinations of hindered phenols with peroxide decomposers (e.g. phosphites or thioethers) are widely
employed and often provide enhanced protection during melt processing or long term heat aging.
The chemical structures of some common phenolic antioxidants:
phenolic antioxidan - Chemical structure of ethylenebis(oxyethylene)-bis(3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate)
phenol phenolic antioxidant - Chemical structure of
2,6-di-tert-butyl4-(4,6-bis(octylthio)-1,3,5-triazine-2-ylamino)
28
Phosphites and Phosphonites - Phosphites and phosphonites are used as melt processing stabilizers in
polyolefins, usually in combinations with hindered phenols or hydroxylamines.
Phosphite antioxidants function by decomposing peroxides (see green and blue arrow in the picture) and
provide protection to adhesives, plastics, and coatings during high-temperature processing but much less so in
the solid phase at lower temperatures.
The most important preventive mechanism is the hydroperoxide decomposition where the hydroperoxides are
transformed into non-radical, nonreactive, and thermally stable products as shown in the reaction schematic
below:
The chemical structures of some common phosphite antioxidants:
trinonylphenol phosphite
pentaerythritol diphosphite - Chemical structure of bis-(2,4-di-tbutylphenol)
29
Hindered Amine Light Stabilizers (HALS) - light stabilization of polyolefins. HALS also contributes to the thermal
oxidative stability of polyolefins in the solid phase, especially at lower temperatures ( ≤ 120 [℃])
HALS is conveniently classified as:


Low- to medium-molecular weight (HALS-1 through HALS-3) or
High molecular weight types (HALS-4 and HALS-5).
While HALS of all molecular weights is effective as light stabilizers, only the medium and high molecular weight
types are effective to improve thermal oxidative stability.
hindered amines and their transformation products (i.e. nitroxyl radicals and iminoether or “NOR” derivatives)
are capable of stabilizing polyolefins through multiple mechanisms involving the scavenging of free radical and
peroxide intermediates in the oxidation process.
An important feature differentiating hindered amines from other classes of stabilizers is their ability to act by a
regenerative mechanism - single hindered amine is capable of deactivating multiple oxidation chains. However,
regeneration mechanism is less efficient in polypropylene than in other polyolefins.
Regenerative mechanism for Hindered Amine Light Stabilizers:
The chemical structures of some common HALS:
Chemical structure of - 1,3,5-triazine-2,4,6- triamine, N,N0
v[1,2-ethane-diyl-bis[[[4,6-bis[butyl- (1,2,6,6-pentamethyl-4piperidinyl)amino]-1,3,5-triazine2-yl]imino]-3,1-propanediyl]
bis[N0 ,Nv-dibutyl-N0 ,Nv-bis (1,2,2,6,6-pentamethyl-4piperidinyl)-1,3,5-triazine-2,4, 6-triamine
Chemical structure of bis(2,2,6,6-tetramethyl-4-piperidyl)
sebacate
30
Hydroxylamines (Distearyl hydroxylamine) – Recently, distearyl hydroxylamine has been commercialized as a
new antioxidant (Irgastab FS-042) by Ciba Specialties Inc.
Distearyl hydroxylamine is melt processing stabilizers, acts by scavenging free radicals as well as decomposing
hydroperoxides:
Scavenging of Oxygen-centered Radical by Distearyl
Hydroxylamine
Scavenging of Carbon-centered Radical by Distearyl
Hydroxylamine
Distearyl hydroxylamine is basic, so can be used as a radical scavenger in place of a phenol additive with
potential antagonism to HALS, and does not color the stabilized material, although amine-type additives are
generally not used for polymer materials except for rubbers because of the tendency to color the polymer
materials.
In addition, not effective to prevent oxidation in the solid phase and must be used in combination with either a
hindered amine light stabilizer (HALS) or a hindered phenol.
Summary of the function of different families of stabilizer additives:
Stabilizer Class
Reactivity
·
Hindered phenols
POO
Phosphites
POOH
HALS
P· / POO·
Hydroxylamines
POO· / POOH
Thioethers
POOH
Benzoates
POO·
UV absorbers
hν
 Primary effect
 Secondary effect
High Temperature
Melt Processing


Heat Stability (Solid
Phase)


UV / Light Stability






31


Reduction of Polymer Degradation during Processing:
PP is a very useful material for various applications because of its good properties and process ability in largescale production by extrusion, injection molding and casting:
Various products can be manufactured from several types of PP, including:
a) isotactic
b) crystalline PP homopolymers
c) random copolymers and
d) Impact or heterophasic copolymers
PP can be processed in practically all known conversion types for thermoplastic polymers, even if it may require
specific selection or adaptation of the selected type or grade.
Roughly speaking a positive correlation between the MFR and the processing speed (or, more specifically, the
applied shear or extension rate) exists. This implies that the PP types with highest molar mass will be applied in
“slow” processes such as compression molding or pipe extrusion, while the grades with lowest molar mass find
their use in “fast” processes such as thin-wall injection molding or fiber spinning.
Antioxidants are used to prevent or inhibit thermal and/or photooxidation degradation process both during
processing, where melting temperatures are experienced by the resin, and during lifetime when exposed to its
upper service temperatures and radiations. In order to choose the most effective stabilizer package, it is
important to know what temperature range the polymer will be exposed to during processing and application.
The effectiveness of antioxidant depends on its concentration in the polymer, which decreases in processing and
during long-term use.
Morphology of a Melt:
Morphology is the type of arrangement of the molecules. Polypropylenes (PP) are crystalline materials. If a piece
of PP is melted, the molecules move away from each other and all the crystallites disappear. There is no
systematic arrangement of the molecules placing them in a random arrangement. Since they are random, the PP
in its melt form is in an amorphous state. The melt of a crystalline polymer is therefore always in an amorphous
state.
Crystallites are formed because of high molecular attraction and because of the possibility of the chains being
unhindered to form the bond. Sometimes just the presence of another molecule or a side chain prevents
crystallization. For melt processing, the crystallites must be dissolved and the chains separated from each other
in order to reduce the viscosity. It is this basic nature of forming and dissolving of the crystallites that dictates
various differences in processing and melt behavior.
32
Extruder Screw Design or Processing Parameters:
a) Extruder length and cylinder/screw geometry:
During processing of polymers a reduction of degradation is possible via suitable setup of the processing
zones and screw design. The aim is to design the melting zone in a way that the polymer granules are
processed to allow gentle fusion.
In the following mixing zone processing length and energy input should be as high as required, but as
low as possible. A higher mass temperature leads to higher degradation compared to a longer residence
time*. Therefore, if technically possible, the choice of lower temperatures is favorable with the
compromise of having possibly a longer processing zone and longer residence times. The dispersion
result and granule homogeneity definitely need to be assessed as there is no decoupling possible of
mixing efficacy, thermal stress, and polymer degradation.
Residence Time and Maximum Residence Time of a Plastic* - The residence time is the time each particle
spends in the extruder, from when it enters the first barrel section until it exits the die. Each screw configuration
is associated with a residence time distribution depending on number of parameters. At a given melt
temperature, a polymer can be held without degradation only for a finite amount of time. If the polymer is
subjected to longer lengths of time, the polymer chains break and start to degrade. This time is defined as the
maximum residence time.
b) Selection of mixing elements:
With a careful selection of gentle mixing elements a compromise can usually be realized ensuring
sufficient mixing efficacy and reducing mechanical as well as thermal stress during compounding.
Although not necessarily accessible via usual temperature sensors, high shear rates can cause a local
overheating of the melt, which can exceed the mean processing temperature by more than 50 °C.
Especially the area between mixing elements and their geometry influence this effect. The influence of
these temperature spots on polymer degradation depends on thermal sensitivity and stability of the
polymer or polymeric compound. In general, selection of gentle mixing elements can reduce damaging
influence as long as the mixing result is reached.
Distributive mixing elements:
Pin mixing section
Dulmage mixing section
Saxton mixing section
Pineapple mixing section
c) Screw speed and throughput:
Mechanical energy input in the melt is defined via specific energy input. Energy input can be minimized
via low screw speed and high throughput resulting in lower energy input per mass and higher filling
degree. This may result in higher residence time of the melt within the hot processing zones but allows
lower mean mass temperatures in comparison to processing with high screw speed and lower
throughput.
33
d) Recipe changes:
As polymer degradation is influenced by residence time from melting up to solidifying as well as by
possible impurities, a thorough cleaning of the extruder before or after changes to new recipes or
compounds is recommended. Long times of heated equipment without screw movement (e. g.
overnight) should also be avoided due to accelerated degradation at higher temperatures.
Changes of Melt Flow Behavior via Molecular Weight and Flow Modifiers:
a) Flow modifiers:
In order to improve incorporation of solids into melts their viscosity can be reduced via addition of lowmolecular waxes*. Due to their low melting point they can cause better wetting of added solids and
they also lead to a reduction of melt viscosity. These additives allow in many cases a reduction of
processing temperatures and as a result also a reduction of thermally induced degradation reactions.
Polyolefin waxes* can be introduced into the polymerization process during the preparation of a plastic.
Furthermore, they can be added to a polymer before a processing step. Various methods are possible for this:
polyolefin waxes can be dusted onto a polymer, compounded in, or introduced into a plastic in a cold or hot
powder mixing process.
b) Use of different polymer grades (molecular weight):
Reduction of melt viscosity during processing can also be achieved via use of polymers with lower
average molecular weight (or higher melt flow index, respectively), e. g. via use of easy-flowing injection
molding grades as mixing partner. This allows reduction of the temperature profile with similar
processing conditions compared to more viscous polymer grades. The following mixing and
compounding of lower-viscous grades of the same polymer is unproblematic due to miscibility of
polymers with the same chemical structure but different molecular weight.
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Influence of multiple reprocessing cycles on the rheological and
mechanical properties of PP:
PP, a material widely used for packaging and containers, causes the biggest portion of plastic waste (22.8% in
2013). Therefore, it has a high potential for recycling.
Unlike metals which are easily recoverable and recyclable, plastics wastes increasing day by day and forcing
governments to legislate for the limitation of such waste by introducing the concept of isofunctional recycling
which means he recycled plastics can be used in same application as pure plastics.
Thermoplastics Market Share 2015 (Plastics Europe Market Research Group, 2015):
The four key areas of recycling that are used today are primary, secondary, tertiary and quaternary:
1. Primary recycling (called closed-loop Recycling) involves the use of scrap plastics to make products
whose performance characteristics are equivalent to the one obtained with virgin products.
Usually, injection molding, extrusion, rotational molding, and compression molding represent the
mechanical processes that are used. Only thermoplastic polymers can be recycled with this process.
2. In secondary recycling, end-of-life waste products are transformed by mechanical means, and it leads to
applications where the demand of properties is lower than that required for the original article; it can be
applied to thermoplastics, or slightly cross-linked polymers (e.g., rubbers that are able to decross-link by
the combination of heat and high shearing). In this case, before the processing step is done, separation,
washing, and preparation of the polymer to produce homogenous end-products with
reasonably good properties is required.
3. In tertiary recycling (chemical recycling), polymer wastes are transformed into smaller molecules
(Liquids or gases). In the case of polyolefins they are converted into its oil/hydrocarbon component,
and into monomers in the case of polyesters and polyamides. These molecules can be used as raw
materials for new polymer production. Chemical recycling process includes hydrolysis, pyrolysis,
glycolysis, gasification, liquid gas hydrogenation, viscosity breaking, steam or catalytic cracking.
4. Quaternary recycling (incineration) known as EFW (Energy from Waste) - is commonly used when the
plastics are mixed and/or heavily contaminated and cannot be economically recycled by another
method. It involves burning the material and losing the material properties. Plastic materials are a
convenient energy source because of their high calorific value. Incineration results in a volume reduction
of 90-99%.
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The use of polypropylene has highly increased due to its good material properties such as better stiffness,
ductility, an acceptable elastic limit and an excellent chemical resistance at a relatively lower price. Due to the
longer exposure to high temperatures, intensive shearing and the presence of moisture, reprocessing may give
rise to thermal, thermo-oxidative or mechanical degradation that may affect the structure of both the matrix
and the fillers and as a consequence the properties of the composites may decrease. Thus, the development of
reprocessing operations needs knowledge of their effects on degradation, in order to assure a minimum
decrease in property values.
Effect of number recycling cycles on PP property:
Rheological characterization - MFI value:
MFI* value increased linearly with recycling. The viscosity of neat polypropylene melt under standard conditions
dropped through reprocessing and kept dropping with increase in recycling times. Lower viscosity of melt
usually means that the molecular weight of polymer decreased after recycling.
The absence of changes in the chemical structure (ATR test) and the increase in the MFI suggest that chain
scission, not oxidation, is the dominant degradation mechanism under the experimental conditions.
Change in viscosity with melt flow index (MFI)*, which indicates how much material in grams flows through a
small capillary within 10 min. The larger the MFI values, the lower the viscosity. The MFI increases slightly with
an increasing number of reprocessing steps. The increase in MFI during this continued reprocessing without
refreshing is caused by chain scission degradation due to thermal and mechanical degradation.
36
Continuation of - Effect of number recycling cycles on PP property:
Test 
The Young’s modulus as a function of
the number of recycling cycles:
The yield stress as a function of the
number of recycling cycles:
The modulus increased slightly with
increase in number of recycling up to
10 cycles, then decreased slightly till
20 cycles. Results for PP recycling and
that this could be due to the higher
crystallinity of the recycled PP. The
flexibility of a plastic depends basically
on the ability of its segments to
rotate. Crystalline structures hinder
such rotations; therefore, a crystalline
material is significantly stiffer than the
equivalent plastic in its amorphous
condition.
The difference of yield strain between the
20 cycles and first cycle is no more than
4%, which suggested that there is not
much effect of recycling on yield strain.
Breaking strain as a function of the
number of recycling cycles:
Impact resistance as a function of the
number of recycling cycles:
Elongation at break for polypropylene
dropped by almost 50% after 20 cycles
which was related to the reduction of
the molecular weight with repeated
processing cycles
Impact resistance of recycled
polypropylene reduced which indicated
that ductility of polypropylene
decreased as a result of molecule chain
breaking by shearing during
reprocessing.
Results
Analysis
of results
37
Summary:
The loss in properties and the financial cost associated with the degradation of polymers leads to a need to
understand the processes by which these chemical reactions occur and how to prevent them. As we have seen,
degradation can be affected by numerous factors at all stages in a product’s lifetime.
Degradation reactions induced during processing can lead to a reduced life cycle time of final parts. In order to
reduce degradation processes, processing conditions during extrusion are recommended to be chosen allowing
low thermal stress.
Therefore, careful engineering and thought must to go into developing polymer products. This is especially
important with regards to the processing conditions, the types of stabilizers we use and the expected lifetime of
the final product.
Recyclability of plastic waste depends on the origin of the waste as well as the sensitivity of the polymer(s) to
degradation. In most cases, preconsumer waste (manufacturing scrap) can be reprocessed with little
deterioration of properties. The property changes can even be minimized or extended by refreshing the regrind
waste with virgin plastic. Although the processing properties such as viscosity are affected, the changes in
service-life properties such as mechanical performance are often negligible.
Reprocessing of PP gives rise to an overall decrease in the melt viscosity due to degradation. The chemical
structure was unaffected by reprocessing. The mechanical properties were affected to a different extent as a
consequence of reprocessing. Small strain properties were slightly affected. However, after harsh reprocessing,
PP showed a general decrease in break strain in tensile and breaking energy in impact resistance. The main and
overall reason for this mechanical response is the chain scission because of the high shear rate and temperature
in injection molding.
38
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