4. Bond
Functional groups
Hydroxyl ,ester and amide group can increase the susceptibility of plastics to
chemical degredation.
Tertiary carbon atoms
Molecular weight distribution
Chain configuration
Crystallinity
Impurities
5. Example : polyvinyl chloride
Irregularity of repeating molecular subunits binding
~CH2CHCl-ClCHCH2~
Branching of the main chain ~CHCl-CH(CH2-ClCH~)-CHCl~
Final groups ~CCl=CH2 ~CH=CH2 ~CHCl-CH2Cl ~CH2-CHCl2
Impurities and oxygen groups Fe3+ ~CH=CH-CO~ ~CO~
6. Chemical Structure
Chemical composition
Molecular structure
Molecular weight
Materials components
Physical structure
Crystalline
structure
Crystallinity
Morphology
Free volume
Orientations
Residual stresses
A glassy solidified plastic is not in a state of equilibrium. If this material is exposed to higher
temperatures, yet still well below glass transition, the concentration of free volume is quickly
reduced and the state of the solid amorphous plastic approaches equilibrium. This also
Reduces the high molecular motion of the material, which is due to the free volume in the solid
state, and it losses some of its potential for absorbing energy under mechanical load.
Annealing leads to reordering among the molecular chains together with a reduction in free
volume. This is caused by releasing and rearranging physical bonds(such as hydrogen bridge
bonds and van-der Waals bonds) as well as by relaxing load bearing, entangled molecular
chains.
Metallic contaminations can accelerate chemical degradation
Metals introduced by contamination during reaction, in storage silos, or due to friction at
the walls of the processing machine during processing can act catalytically.
9. Chemical aging This process is not reversible.
The fundamental weaknesses of plastics are caused by the macromolecular structure and
relatively weak bonding forces.
Chemical aging processes in polymer materials cause changes on the molecularscale and lead to
chain cleavage but also to crosslinking and cyclization.
Physical aging
Physical aging processes are always the result of thermodynamically unstable states (residual
stresses, orientation, incomplete crystalline structure) caused by process-dependent cooling conditions
during the manufacture of plastic products.
10. Thermal destruction
Chemical Degradation
Cleavage of chemical bonds in the polymer backbone is characteristic of thermal degradation
in plastics.
The result is a reduction in molecular weight and the potential formation of volatile low-
molecular degradation by-products.
Chain cleavage of linear polymers may lead to crosslinking or cyclization- causes an increase
in molecular weight
thermally induced types of degradation:
• Statistical chain splitting with reduction in molecular weight without the
formation of low-molecular compounds,
• Depolymerization (also called unzipping or depropagation) with monomer
formation without a significant change inmolecular weight (e. g., PMMA,
PS, POM),
• Elimination with side group cleavage and formation of low-molecular
compounds (e. g., PVC with HCl formation)
11. Thermal Mechanical degradation
Mechanical degradation is defined as chain cleavage caused by external
mechanical load.
Eg; Strain
Temperature gradients cause stresses in molded parts made from
polymers.
Temperature thus influences physical aging processes by increasing both
the mobility of the molecular chains and the free volume. Both factors
promote relaxation or post-crystallization.
Thermal-Oxidative Degradation
Oxidative degradation is usually initiated when polymer chains form radicals .
This can occur during manufacture, processing or during service when exposing
the polymer to heat.
12. Oxidative Degradation
• Oxidants
• Oxygen , air
• Ozone
• Nitrogen oxides
Oxidation in plastics affects both their molecular structure
(chemical structure) and their micro-structure.
Mechanisms is dominant
• Chain cleavages
• Chain crosslinking
• Chain branching
OZONATION REACTION
Particularly polymers with high unsaturation (i.e. rubbers)
will suffer from ozone degradation, because the double bonds
in unsaturated polymers readily react with ozone.
13. Radiation
Light waves acting on plastics can
• Reflect on the surface
• Be dispersed in the mass
• Be transmitted and/or absorbed by plastics
Photochemical reactions result from the absorption of light energy
by chemical structures. The radiation in sunlight that is effective
on the Earth’s surface and is the main cause of degradation in
plastics lies in the ultraviolet range between 300 and 400 nm.
Photo Degradation
Photo degradation usually starts at the surface with visible cracks
and discoloration, which leads to rapid loss of mechanical
properties.
14. Ionizing Radiation
• Ionizing radiation here means atomic or nuclear particles, such as γ -
radiation, electrons, neutrons, and others.
• radiation is used for crosslinking or sterilization
• lower the carbon contents in the polymer, the stronger its radiation
sensitivity
Corrosive Gases
Nitrous acid and nitric acid can form from nitrogen monoxide and dioxide
in the presence of water and may lead to hydrolytic cleavage in plastics
with ester and amid bonds. The products created from sulfur dioxide,
sulfur trioxide and sulfuric acid cause hydrolytic cleavage in plastics with
susceptible bonds, such as the amide bonds in polyamide.
15. Hydrolysis
Water plays a special role:
• Water soluble degradation products capable of catalyzing further
degradation are washed off
• Water can act as a plasticizer and be responsible for swelling
and deterioration of plastics.
The chemical effect of water can be observed in particular in plastics
with hydrolyzable groups in the backbone.
The physical effect of water can be seen after water absorption by
polymers: in the accompanying plasticizing effect and swelling, in
changes in crystallinity, or in the extraction of additives
16. Corrosive Gases
Nitrous acid and nitric acid can form from nitrogen monoxide and dioxide in
the presence of water and may lead to hydrolytic cleavage in plastics with
ester and amid bonds.
Biological Influences
most important biological influencing factors are microorganisms- bacteria
and fungi. These organisms are extremely versatile and adaptive; they produce
an enormous variety of specific, degradation promoting enzymes
Mechanism of Biodegradable
Polymers
Biodegradation is a surface erosion process because enzymes are too
large to diffuse into the biodegradable polymer.The microbial
degradation is influenced by humidity, oxygen level, light, and
temperature
17. Mechanical Load
• Mechanical degradation is defined as deformation caused by external
mechanical loads resulting in changes in the bond angle and bond distance and
thus inmechanical failure in the polymer chain
• high mechanical loads can result in chain cleavage and accelerate thermal-
oxidative degradation (the formation of radicals. The alkyl radicals formed can
create disproportionation, leading to unsaturated compounds or reactions with
oxygen)
• mechanical loading can alter the physical structure in such a way as to retard
diffusion processes
PROTECTION
• Structural modification of polymers
• End-group blocking ( used with polyoxymethylene)
• Physical stabilization by polymer orientation (increasing crystallinity)
• Addition of stabilizers
Stabilizers are chemical substances that are added to the polymer in low
concentrations —to protect the polymer of temperature, light etc.
18. ANTIOXIDANTS
• Free-radical and peroxide scavengers
• Free-radical scavengers or primary antioxidants react with chain-propagating
radicals in a chain terminating reaction.
PRIMARY ANTIOXIDANTS
widely used are sterically hindered phenol
SECONDARY ANTIOXIDANTS
most common are trivalent phosphorus compounds (phosphites)
LIGHT STABILIZERS
• Using pigments such as carbon black or titanium dioxide
• UV absorbers, such as benzophenones or benzotriazoles
• Deactivation of excited molecules by UV quenchers
UV ABSORBERS
These compounds are capable of absorbing UV light in the
harmful wavelength range and transforming the absorbed
radiation energy into harmless energy, e. g., heat.
They must not absorb in the visible light range, because otherwise
they would cause undesirable discoloration
19. QUENCHERS
Quenchers are substances capable of absorbing energy from a fluorophore (such as a
fluorescent dye) and re-emitting much of that energy as either heat (in the case of dark
quenchers) or visible light. Dabcyl is an example of a dark quencher
In contrast to the UV absorbers, quenchers are not significantly dependent on layer
thickness.
RADICAL SCAVENGERS AND HYDROPEROXIDE
DECOMPOSERS
• hindered amine light stabilizers (HALS)
• This class of amine stabilizers is based on 2,2,6,6-tetramethyl-piperidine derivatives.
• high molecular weight HALS are more effective long-term heat stabilizers than low
molecular weight HALS
• HALS are not very effective processing stabilizers. For this reason, they are often used
in
combination with primary and secondary antioxidants
20. METAL DEACTIVATORS
• To prevent or reduce metal catalyzed degradation, metal deactivators ar often
combined with antioxidants.
• The function of a metal deactivator or metal-deactivating agent (MDA) is to
form an inactive complex with the catalytically active metal ion.
BIO-STABILIZERS
• Bio-stabilizers are natural or synthetic, mostly low-molecular substances that
are utilized to ensure the stability of a material against biological attack.
THERMOSTABILIZERS, PVC
STABILIZERSHeat stabilizers are used to prevent degradation of plastics by heat, especially
during processing, but also in applications. For example, they are widely used
in PVC compounds. Heat stabilizers act by stopping thermal oxidation or by
attacking the decomposed products of oxidation
