Degradation
&
Stabilization of Polymers
Resin Identification Code
The Society of the Plastics Industry, Inc. (SPI) introduced its resin
identification coding system in 1988 at the urging of recyclers around
the country
Plastics
A Plastic is...
.. a material that contains as an essential
ingredient, an organic high molecular
weight polymer, is solid and rigid in its
finished state, and at some stage in its
manufacture or its processing into a
finished article, can be shaped by flow.
Production of polymer-based
products
Primary
Resources
Basic
Petrochemical
Polymer
Materials
End
Products
Plastics
Crude
Oil
Ethylene
Propylene
Styrene
Vinyl Chloride
Butadiene
Cyclohexane
Acetylene
Natural
Gas
HDPE
LDPE
LLDPE
PP
PVC
ABS
PA
Acetal
PC
PUR
PBT
etc.
Elastomer
Fibers
Adhesives +
Coatings
Polymer Definition
A Chemical compound formed by
many monomers linking to form
larger molecules that contain
repeating structural units.
Mono-one
Mer-unit
Monomer
-----------------------------------------------
Polymer Molecule
Poly-many
Polymer Families
Materials
Plastics (Polymers)
Thermosets
Thermoplastics
Polymer Families
Thermoplastics
Plastics capable of softening and flowing
when heated, hardening when cooled, and
softening when reheated REVERSIBLE PROCESS
Thermosets
Plastics which become permanently rigid
when heated and cooled IRREVERSIBLE PROCESS
Polymer Families
Materials
Plastics (Polymers)
Thermosets
Thermoplastics
Engineering
Commodity
Polymer Families
Engineering and Commodity
Corrosion Resistance
 Thermal/Electrical Resistance
 Practical Toughness and
Stiffness
 Light Weight

Engineering
• High Temperature Resistance
• Flame Resistance
Polymer Families
Plastics
Thermoplastics
Thermosets
Commodity
Amorphous
PMMA
PVC
PS
Crystalline
PE
PP
High Performance
Engineering
Amorphous
ABS
PC/ABS
ASA
PC
MPPO
Blends
PC/PBT
PPO/PA
ABS/PA
Crystalline
PBT
PA
POM
Amorphous
PEI
PEEK
Crystalline
PPS
Engineering Plastics
Five EP
Polyamide - PA
Polycarbonate - PC
Polyoxymethylene - POM
Poly(butylene terephthalate) - PBT
modified Poly(1.4-phenylene oxide) –
mPPO
Poly(phenylene sulfide) – PPS
SIX…
Plastics Tree
HIGH PERFORMANCE PLASTICS
PEI
PSU
LCP
PPS
PA
PC
PA
blends
ENGINEERING PLASTICS
PPE / PS PC
blends blends
PMMA
PBT
blends
PBT
POM
ABS
PET
Polypropylene
COMMODITIES
PS
HIPS
PVC
Polyethylene
AMORPHOUS
SEMICRYSTALLINE
History of Major Plastics
PS
PMMA
PVC
LDPE
PA
Teflon
Silicone
ABS
PET
HDPE
PP
PC
1930
1934
1933
1939
1939
1943
1943
1952
1953
1955
1957
1959
Germany
UK
Germany/US
UK
US
US
US
US
US
Germany
Italy
Germany/US
Polymer Morphology
Refers to the Structure
of the Polymer Material
 Amorphous
 Crystalline
Polymer Morphology
Amorphous Resins
Polymer Morphology
Crystalline Resins
Polymer Morphology
Crystalline Polymers are Actually Semi - Crystalline
Regions of Crystallinity in an
Otherwise Amorphous Mass
Polymer Morphology
Amorphous
Broad
Softening Range
Crystalline
Sharp
Melting Point
Polymer Morphology
Amorphous Polymers:

Are Structural Below the
Glass Transition Temperature
(TG) and Rubbery Above It

Rely on Physical Entanglements
of the Molecular Chains for
Structural Properties Below TG
Polymer Morphology
Glass Transition Temperature (TG)
Glassy
TG
Rubbery
Raise Temperature of Polymer
Both amorphous and crystalline polymers
exhibit a glass transition temperature.
Polymer Morphology
Model of Amorphous Polymers
Locked
Entanglements
TG
Stiff Flow
Easier Flow
Raise
Raise
Temperature
Temperature
ofof
Polymer
Polymer
Adding Heat Increases Space
Between Molecular Chains
Polymer Morphology
Model of Crystalline Polymers
Rigid Solid
TG
Soft Solid
TM
Flows Easily
Raise Temperature of Polymer
Adding heat increases space between molecular chains
but crystalline structure prevents flow.
Polymer Morphology
Amorphous Polymer
Modulus
(Stiffness)
TG
Temperature
Polymer Morphology
Crystalline Polymer
Modulus
(Stiffness)
Glassy State
TG
TM
Glass Transition
Leathery Region
Rubbery Plateau
Liquid Flow
Temperature
Polymer Morphology
Amorphous vs. Crystalline
Amorphous
Modulus
(Stiffness)
Crystalline
Crystalline
TG
Amorphous
TG
Crystalline
TM
Temperature
Polymer Softening Range
Crystalline Materials Have
a Sharp Melting Point
TG
Solid
(glassy)
TM
Stiff Flow
(rubbery)
Temperature
Flows
Easily
Polymer Flow
Characteristics

Adding Heat to a Polymer Melt will
Increase Flow

Adding Too Much Heat or Heating for
Too Long May Cause Degradation

It is Important to Know the Processing
Temperature Range for Each
Plastic to
Make Good Parts
Polymer Amorphous
Processing Range
TG
Degradation
Processing
Temperature
Range
Raise Temperature of Polymer
Polymer Crystalline
Processing Range
TG
TM
Degradation
Processing
Temperature
Range
Raise Temperature of Polymer
Tg = Glass transition temperature
Tm = melting temperature
Each processing step causes degradation, a result of the
combined action of shear, heat and oxygen.
Modes of initiation
(Degradation)
Thermal*
 Photo (light induced)*
 Chemical
 Mechanical
 Biological
 High Energy Radiation
*Will be discussed

What is degradation
In practice, any change of the polymer
properties relative to the initial, desirable
properties is called degradation. In this
sense, "degradation" is a generic term
for any number of reactions which are
possible in a polymer.
 These reactions, in turn, lead to a change
in the physical and optical properties of
the polymer.

Some of these properties include:
Tensile Strength
Brittleness
Impact strength
Toughness
Drawability
Adhesive strength
Elastic modulus
Melt viscosity
Hardness
Softening temperature
Gloss
Tensile strength is important for a material that is going to
be stretched or under tension
TENSILE STRENGTH - Tensile strength is defined as the force
required to break the specimen or cause complete separation of
constituents in a linear direction.
ELONGATION - Elongation is defined as the distance (in percent) the
specimen will stretch from its original size to the point atwhich it breaks.
Calculation
1. Tensile Strength = Max Load / Cross-sectional area of test
specimen
2. The displacement (stretching) of a due to the imposed force
% Elongation = (DL / L ) x 100 (L = original length of test
specimen)
3. Modulus =The ratio of stress to strain in the elastic region
Modulus of Elasticity = Stress / Strain (Young Modulus)
The modulus is the slope of the stress-strain curve. If the modulus large
(corresponding to steep angle of the curve), the material resists
deformation strongly. Such materials are said to be Stiff.
The Number average molecular weight
Mn , Weight average molecular weight
Mw , and the most fundamental
characteristic of a polymer its molecular
weight distribution. MWD
These
values
are
important, since
molecular weight
and molecular
weight distribution affect many of the
characteristic physical properties of a
polymer.
Toughness
If one measures the area underneath the stress-strain curve, colored red
in the graph below, the number you get is something we call toughness.
Toughness is really a measure of the energy a sample can absorb before it
breaks. Think about it, if the height of the triangle in the plot is strength,
and the base of the triangle is strain, then the area is proportional to
strength times strain. Since strength is proportional to the force needed to
break the sample, and strain is measured in units of distance (the distance
the sample is stretched), then strength times strain is proportional is force
times distance, and as we remember from physics, force times distance is
energy.
In general a higher molecular weight increases all of these properties.
The reason is primarily explained by entanglement. Higher molecular
weights imply longer polymer chains and longer polymer chains imply
more entanglement.
*The impact toughness is reduced by a broad MWD.
*The impact toughness is generally increased by increasing molecular
weight up to the point where embrittlement becomes important.
In geneal the ultimate tensile strength and elongation, brittle
temperature, and softening point will be affected adversely by a
decrease in molecular weight.
The relative magnitude of the effect will depend on the initial molecular
weight. This is because most properties become independent of
molecular weight when the degree of polymerization is greater than
700-800.
Melt Flow (Index or Rate)(MFI)
MFI
Molecular Weight
Physical Properties
of Polymer
The Melt Flow
Rate (MFR) as
defined by ASTM
D-1238, defines a
polymers flow in
terms of the
number of grams
extruded in 10
minutes at
standard
conditions, using
specific geometric,
temperature, and
rate conditions. At
the end of the
specified time, the
melt strand is cut
off, weighted, and
the MFR is
calculated. The
material with high
viscosity will have a
low MFR, and viseversa.
Most commercial plastics are manufactured
by processes involving chain
polymerization, polyaddition, or
polycondensation reactions. These
processes are generally controlled to
produce individual polymer molecules with
defined Molecular weight (or molecular
weight distribution) Degree of branching,
and Composition
Once the initial product of these processes is exposed to further shear
stress, heat, light, air, water, radiation or mechanical loading, chemical
reactions start in the polymer which have the net result of changing
the chemical composition and the
polymer.
molecular weight of the
!! Goal: Keep molecular architecture intact !!
MECHANISTIC ASPECT OF POLYMER DEGRADATION
Mode of Initiation
Chain scission
Random Chain scission (Hydrocarbons)
Thermal
Norrish Type (I, II) chain scission (SSR)
Photochemical
Enzymatic attack of peptide and glucoside (SSR)
Chemical
Side Chain elimination
Depolymerization
Autooxidation
Cross-linking
Solvolysis of ester linkage (SSR)
Chemical
Elimination of HCl (PVC) (CR)
Thermal
Thermal
Thermal, Photochemical,
Mechanical, chemical
SSR= Single step reaction
CR = Chain reaction
Side Chain Elimination
H
Cl
Cl
H
Cl
H
H
Cl
H
PVC
Cl
Yellowness index
H
Cl
H
Cl
H
- HCl
Mn, Mw
Polyene
Aromatics
Cl
H
Cl
Random scisson
MFI
Viscosity
.+ .
+
+
Alkanes
M n, M w
Alkenes
Alkadienes
Depolymerization
COOR
COOR
COOR
COOR
COOR
COOR
COOR
.
COOR
COOR
.+
COO
COOR
COOR
COOR
COOR
Monomer
Typical for Poly Methyl methacrylate and mainly for other acrylate based polymers
Cross-linking
Heat
or
Light
In general, chain scisson will
cause an initial hardening and
rise in tensile strength.
.
.
.
.
.
.
Viscosity
MFI
.
cross-linking
Flexibility
Polymer network
With the influence of heat, shear, oxygen or light, the polymer
backbone can react via free radicals reactions. These
reactions are very complex and can lead to numerous species
depending on the nature of the radicals and the polymer
structure.
Polyethylene or Polypropylene can react very differently. In
presence of radical, Polyethylene generates macroradicals
having tendency to recombine generally - but not always - to
branching and even gelling. In film extrusion, where optical
properties are important, this phenomena is called "fish eyes"
or unmelts.
For polypropylene, the very unstable ternary macroradicals
generated have a tendency to stabilize through a
recombination reaction called beta-scission. This reaction
leads to chain breaking responsible for mechanical
performance drop. The figure below summarize two
mechanisms one with oxygen and one without.
PP without Oxygen
PP with Oxygen
Beta-scission of tertiary alkoxy radicals
Dissociation energies of bonds A-B in kJ.mol-1.
H
C
N
O
F
H
C
N
O
F
Cl
Br
I
S
436
413
391
463
563
432
366
299
399
348
292
352
441
329
276
240
259
160
222
270
200
139
185
203
153
254
Cl
243
Br
219
290
369
250
198
178
151
I
213
S
Si
Si
541
359
289
The chemical bonds in polymer structure are strong enough. At 486 oC, in
one mol of C-C bonds, only one bond exist having that energy corresponding
to its dissociation. Despite this fact, in the temperature interval 350-600 oC,
most polymers in an inert atmosphere decompose relatively rapidly into
low molecular fragments.
Wavelength λ
(nm)
Energy of quanta
Kcal/mol
200
kJ/mol
(eV Quanta –1)
143
597
6.19
95
395
4.11
400
71
299
3.10
400
71
299
3.10
48
199
2.06
36
149
1.55
300
600
800
Ultraviolete
Visible light
The bond strengths or
bond dissciation
energies for the
chemical bonds in the
most common
polymer materials are
expressed as kcal
mol-1, kJ mol-1 and
eV bond-1. These
different units are
recalculated using the
relations 1eV bond-1
is 23.1 kcal mol-1 or
96.5 kJ mol-1
The wave length of the radiation from the sun which reaches the earth’s
surface extends from infra-red (> 700 nm) through the visible spectrum
(approximately 400-700 nm) into ultraviolet (< 400 nm) with cut-off at
approximately 300 nm depending upon atmospheric conditions. The
energies of 700-400 nm and 300 nm photons are approximately 170, 300,
and 390 kJ mol-1 respectively.
1 kLangley = 1 kcal/cm2 = 41.84MJ/m2
1 kLangley/year = 1.33 W/m2
FACTS !!
Color Development of Mass Stabilized PC after
Natural Weathering
ISO 4607, Florida; 2 mm Injection Molded Plaques
Polymer Stabilizer Systems
Stabilizers are added to the polymer
to inhibit degradation caused by:
Oxygen
Oxidative Stability
Light Energy
Ultraviolet Stability
Heat Energy
Thermal Stability
Water
Hydrolytic Stability
Certain Stabilizers protect the polymer during processing
and others guard against the affects of weathering