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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 Basic Polymer End
Resources Petrochemical Materials Products
Plastics
Crude HDPE
Oil LDPE
Ethylene LLDPE
Propylene PP Elastomer
Styrene PVC
Vinyl Chloride ABS
Butadiene PA
Cyclohexane Acetal
Acetylene PC
Fibers
PUR
Natural PBT
Gas etc.
Adhesives +
Coatings
Polymer Definition
A Chemical compound formed by
many monomers linking to form
larger molecules that contain
repeating structural units.
Mono-one
Monomer Mer-unit
------------------------
------------------------
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
Thermosets Thermoplastics
Commodity Engineering High Performance
Amorphous Crystalline Amorphous Crystalline Amorphous Crystalline
Blends
PMMA PE ABS PBT PEI
PC/PBT PPS
PVC PP PC/ABS PA PEEK
PPO/PA
PS ASA ABS/PA POM
PC
MPPO
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
LCP
PPS
PSU
PA
PC PA
ENGINEERING PLASTICS blends
PPE / PS PC PBT
blends blends blends PBT
POM
PMMA ABS
PET
Polypropylene
COMMODITIES PS
PVC
HIPS
Polyethylene
AMORPHOUS SEMICRYSTALLINE
History of Major Plastics
PS 1930 Germany
PMMA 1934 UK
PVC 1933 Germany/US
LDPE 1939 UK
PA 1939 US
Teflon 1943 US
Silicone 1943 US
ABS 1952 US
PET 1953 US
HDPE 1955 Germany
PP 1957 Italy
PC 1959 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 Crystalline
Broad Sharp
Softening Range 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)
TG
Glassy Rubbery
Raise Temperature of Polymer
Both amorphous and crystalline polymers
exhibit a glass transition temperature.
Polymer Morphology
Model of Amorphous Polymers
Locked Stiff Flow Easier Flow
Entanglements TG
Raise Temperature Polymer
Raise Temperature ofof 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 TG
(Stiffness)
Temperature
Polymer Morphology
Crystalline Polymer
TG TM
Modulus Glassy State
(Stiffness) Glass Transition
Leathery Region
Rubbery Plateau
Liquid Flow
Temperature
Polymer Morphology
Amorphous vs. Crystalline
Amorphous
Modulus
Crystalline
(Stiffness)
Amorphous
TG
Crystalline
TG Crystalline
TM
Temperature
Polymer Softening Range
Crystalline Materials Have
a Sharp Melting Point
TG TM
Solid Stiff Flow Flows
(glassy) (rubbery) Easily
Temperature
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) 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
Physical Properties calculated. The
MFI Molecular Weight material with high
of Polymer
viscosity will have a
low MFR, and vise-
versa.
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 molecular weight of the
polymer.
!! Goal: Keep molecular architecture intact !!
MECHANISTIC ASPECT OF POLYMER DEGRADATION
Chain scission Mode of Initiation
Random Chain scission (Hydrocarbons) Thermal
Norrish Type (I, II) chain scission (SSR) Photochemical
Enzymatic attack of peptide and glucoside (SSR) Chemical
Side Chain elimination
Solvolysis of ester linkage (SSR) Chemical
Elimination of HCl (PVC) (CR) Thermal
Thermal
Depolymerization
Autooxidation Thermal, Photochemical,
Mechanical, chemical
Cross-linking
SSR= Single step reaction
CR = Chain reaction
Side Chain Elimination
H Cl H Cl H Cl H Cl H
PVC
Cl H Cl H Cl H Cl H Cl
Yellowness index - HCl
Mn, Mw
Polyene
Aromatics
Random scisson
MFI
Viscosity
.+ . M n, M w
+ +
Alkanes Alkenes Alkadienes
Depolymerization
COOR COOR COOR COOR
.
COOR COOR COOR COOR
COOR COOR
.+ COOR
COO COOR COOR
Monomer
Typical for Poly Methyl methacrylate and mainly for other acrylate based polymers
Cross-linking
Heat
or
In general, chain scisson will Light
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 Cl Br I S Si
H 436 413 391 463 563 432 366 299 399
C 348 292 352 441 329 276 240 259 290
N 160 222 270 200
O 139 185 203 369
F 153 254
Cl 243 250
Br 219 198 178
I 151
S 213
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 Energy of quanta
Kcal/mol kJ/mol (eV Quanta –1)
(nm)
200 143 597 6.19
300 Ultraviolete 95 395 4.11
400 71 299 3.10
400 71 299 3.10
600 Visible light 48 199 2.06
800 36 149 1.55
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
