Kursus Kaedah Penyelidikan, UTeM, Mei 2010
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CHAPTER 5
DEGDRADATION OF
POLYMERS
DR. MOHD WARIKH BIN ABD
RASHID
FKP
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Kursus Kaedah Penyelidikan, UTeM, Mei 2010
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OUTLINE CONTENT
• Chemical Degradation
• Biological Degradation
• Stabilizers
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Kursus Kaedah Penyelidikan, UTeM, Mei 2010
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5.4: Chemical Degradation
(i) Solvolysis
• Step-growth polymers like polyester, polyamides
and polycarbonates can be degraded by
solvolysis and mainly hydrolysis to give lower
molecular weight molecules;
• The hydrolysis takes place in the presence of
water containing an acid or a base as catalyst.
• Polyamide is sensitive to degradation by acids
and polyamide mouldings will crack when
attacked by strong acids.
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For example, the fracture surface of a fuel
connector showed the progressive growth of the
crack from acid attack (Ch) to the final cusp (C) of
polymer;
The problem is known as stress corrosion
cracking, and in this case was caused by hydrolysis
of the polymer. It was the reverse reaction of the
synthesis of the polymer:
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(ii)
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Ozonolysis
Cracks can be formed in many different elastomers by
ozone attack;
Tiny traces of the gas in the air will attack double
bonds in rubber chains;
Ozone cracks form in products under tension but the
critical strain is very small;
Cracks are always oriented at right angles to the strain
axis, so will form around the circumference in a rubber
tube bent over;
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Such cracks are dangerous when they occur in fuel
pipes because the cracks will grow from the outside
exposed surfaces into the bore of the pipe and fuel
leakage and fire may follow;
The problem of ozone cracking can be prevented by
adding anti-ozonants to the rubber before
vulcanization.
Ozone cracks were commonly seen in automobile
tire sidewalls, but are now seen rarely thanks to
these additives.
On the other hand, the problem does recur in
unprotected products such as rubber tubing and
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seals.
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(iii) Oxidation
• Polymers are susceptible to attack by atmospheric
oxygen, especially at elevated temperatures
encountered during processing to shape.
• Many process methods such as extrusion and
injection moulding involve pumping molten polymer
into tools, and the high temperatures needed for
melting may result in oxidation unless precautions
are taken;
• For example, a forearm crutch suddenly snapped
and the user was severely injured in the resulting
fall.
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The crutch had fractured across a polypropylene
insert within the aluminium tube of the device,
and infra-red spectroscopy of the material
showed that it had oxidised, possible as a result of
poor moulding.
Oxidation is usually relatively easy to detect
owing to the strong absorption by the carbonyl
group in the spectrum of polyolefins.
Polypropylene has a relatively simple spectrum
with few peaks at the carbonyl position like
polyethylene.
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Oxidation tends to start at tertiary carbon atoms
because the free radicals formed here are more
stable and longer lasting, making them more
susceptible to attack by oxygen.
The carbonyl group can be further oxidised to
break the chain, this weakens the material by
lowering its molecular weight, and cracks start to
grow in the regions affected.
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(iv) Chlorine-induced craking
• Another highly reactive gas is chlorine, which will
attack susceptible polymers such as acetal resin and
polybutylene pipework.
• There have been many examples of such pipes and
acetal fittings failing in properties in the US as a
result of chlorine-induced cracking.
• In essence, the gas attacks sensitive parts of the
chain molecules (especially secondary, tertiary, or
allylic carbon atoms), oxidizing the chains and
ultimately causing chain cleavage.
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• The root cause is traces of chlorine in the water
supply, added for its anti-bacterial action, attack
occurring even at parts per million traces of the
dissolved gas.
• The chlorine attacks weak parts of a product, and in
the case of an acetal resin junction in a water supply
system, it is the thread roots that were attacked
first, causing a brittle crack to grow.
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Discoloration on the fracture surface was caused by
deposition of carbonates from the hard water supply,
so the joint had been in a critical state for many
months.
The problems in the US also occurred to polybutylene
pipework, and led to the material being removed from
that market, although it is still used elsewhere in the
world.
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Kursus Kaedah Penyelidikan, UTeM, Mei 2010
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5.5: Biological Degradation
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Biodegradable plastics can be biologically
degraded by microorganisms to give lower
molecular weight molecules.
To degrade properly biodegradable polymers
need to be treated like compost and not just left
in a landfill site where degradation is very difficult
due to the lack of oxygen and moisture.
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• Many opportunities exist for the application of
synthetic biodegradable polymers in the biomedical
area particularly in the fields of tissue engineering
and controlled drug delivery.
• Degradation is important in biomedicine for many
reasons. Degradation of the polymeric implant
means surgical intervention may not required for
removal at the end of its functional life, eliminating
the need for a second surgery.
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• In tissue engineering, biodegradable polymers can
be designed such to approximate tissues, providing a
polymer scaffold that can withstand mechanical
stresses, provide a suitable surface for cell
attachment and growth, and degrade at a rate that
allows the load to be transferred to the new tissue.
• In the field of controlled drug delivery,
biodegradable polymers offer tremendous potential
either as a drug delivery system alone or in
conjunction to functioning as a medical device.
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• In the development of applications of
biodegradable polymers, the chemistry of some
polymers including synthesis and degradation
will discuss later.
• A description of how properties can be
controlled by proper synthetic controls such as
copolymer composition, special requirements
for processing and handling, and some of the
commercial devices based on these materials
are discussed.
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When investigating the selection of the polymer for
biomedical applications, important criteria to consider
are;
* The mechanical properties must match the
application and remain sufficiently strong until the
surrounding tissue has healed.
* The degradation time must match the time required.
* It does not invoke a toxic response.
* It is metabolized in the body after fulfilling its
purpose.
* It is easily processable in the final product form with
an acceptable shelf life and easily Sterilization
(microbiology) sterilized.
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• Mechanical performance of a biodegradable
polymer depends on various factors which include
monomer selection, initiator selection, process
conditions and the presence of additives.
• These factors influence the polymers crystallinity,
melt and glass transition temperatures and
molecular weight.
• Each of these factors needs to be assessed on how
they affect the biodegradation of the polymer;
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• Biodegradation can be accomplished by synthesizing
polymers with hydrolytically unstable linkages in the
backbone.
• This is commonly achieved by the use of chemical
functional groups such as esters, anhydrides,
orthoesters and amides.
• Most biodegradable polymers are synthesized by
ring opening polymerization
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• Once implanted, a biodegradable device should
maintain its mechanical properties until it is no
longer needed and then be absorbed by the body
leaving no trace.
• The backbone of the polymer is hydrolytically
unstable. That is, the polymer is unstable in a water
based environment.
• This is the prevailing mechanism for the polymers
degradation. This occurs in two stages
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(a) Water penetrates the bulk of the device, attacking the
chemical bonds in the amorphous phase and
converting long polymer chains into shorter watersoluble fragments. This causes a reduction in molecular
weight without the loss of physical properties as the
polymer is still held together by the crystalline regions.
Water penetrates the device leading to metabolization
of the fragments and bulk erosion.
(b) Surface erosion of the polymer occurs when the rate at
which the water penetrating the device is slower than
the rate of conversion of the polymer into water
soluble materials.
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Kursus Kaedah Penyelidikan, UTeM, Mei 2010
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• Biomedical engineers can tailor a polymer to slowly
degrade and transfer stress at the appropriate rate
to surrounding tissues as they heal by balancing the
chemical stability of the polymer backbone, the
geometry of the device, and the presence of
catalysts, additives or plasticisers.
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5.6: Stabilisers
• Hindered amine light stabilisers (HALS) stabilise against
weathering by scavenging free radicals that are
produced by photo-oxidation of the polymer matrix.
• UV-absorbers stabilises against weathering by
absorbing ultraviolet light and converting it into heat.
• Antioxidants stabilize the polymer by terminating the
chain reaction due to the absorption of UV light from
sunlight. The chain reaction initiated by photooxidation leads to cessation of crosslinking of the
polymers and degradation the property of polymers.
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• Stabilizers for polymers are used directly or by
combinations to prevent the various effects such as
oxidation, chain scission and uncontrolled
recombinations and cross-linking reactions that are
caused by photo-oxidation of polymers.
• Polymers are considered to get weathered due to the
direct or indirect impact of heat and ultraviolet light.
• The effectiveness of the stabilizers against weathering
depends on solubility, ability to stabilize in different
polymer matrix, the distribution in matrix, evaporation
loss during processing and use.
• The effect on the viscosity is also an important concern
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(i) Antioxidants
• Antioxidants are used to terminate the oxidation
reactions taking place due to different weathering
conditions and reduce the degradation of organic
materials. For example, synthetic polymers react with
atmospheric oxygen;
• Organic materials undergo auto-oxidizations due to
free radical chain reaction.
• Oxidatively sensitive substrates will react with
atmospheric oxygen directly and produce free radicals.
Free radicals are of different forms, consider organic
material RH
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• This material reacts with oxygen to give free radicals
such as R• , RO• , ROO• , HO•[1].
• These free radicals further react with atmospheric
oxygen to produce more and more free radicals.
• For example,
R• + O2 -> ROO• ROO• + RH -> ROOH + R•
• This can be terminated using the antioxidants. Then
this reaction comes to,
2R• -> R----R ROO• + R• -> ROOR 2ROO•
• Non-radical products[1] Weathering of polymers is
caused by absorption of UV lights, which results in,
radical initiated auto-oxidation
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• This produces cleavage of hydro peroxides and
carbonyl compounds.
• This is because of the weak bond in hydro peroxides
which is the main source for the free radicals to
initiate from.
• Homolytic decomposition of hydro peroxide
increases the rate of free radicals production.
Therefore it is important factor in determining
oxidative stability.
• The conversion of peroxy and alkyl radicals to nonradical species terminates the chain reaction,
thereby decreasing the kinetic chain length.
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• Hydrogen-donating antioxidants (AH), such as hindered
phenols and secondary aromatic amines, inhibit
oxidation by competing with organic substrate (RH) for
peroxy radicals, thereby terminating the chain reaction
and stabilizing the further oxidation reactions.
At K17, ROO• + AH -> ROOH + A•
At K6,
ROO• + RH -> ROOH + R•
• Here K17 is larger than K6, therefore AH can be at low
concentrations. At low concentrations AH are more
effective because the usual concentration in saturated
plastic polymer range from 0.01 to 0.05% based on the
weight of the polymer.
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• Benzofuranones is another most effective
antioxidant, which terminates the chain
reaction by donating weakly bonded benzylic
hydrogen atom and gets reduced to a stable
benzofuranyl (lactone).
• Antioxidants inhibits the formation of the free
radicals thereby enhancing the stability of
polymers against light and heat.
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(ii) Hindered Amine Light Stabiliser (HALS)
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A The ability of hindered amine light stabiliser
(HALS) to scavenge radicals which are produced by
weathering, may be explained by the formation of
nitroxyl radicals through a process known as the
Denisov Cycle.
The nitroxyl radical(R-O•) combines with free
radical in polymers:
R-O• + R'• -> R-O-R'
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• B Although they are traditionally considered as light
stabilizers, it can also stabilize thermal degradation.
• Even though HALS are extremely effective in
polyolefins, polyethylene and polyurethane, they are
ineffective in polyvinyl chloride (PVC).
• It is thought that their ability to form nitroxyl radical
is disrupted. HALS act as base and become
neutralized by hydrochloric acid (HCl) that is released
by photooxidation of PVC.
• The exception is the recently developed NOR HALS
which is not a strong base and is not deactivated by
HCl ;
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• Benzofurano The UV absorbers dissipate the
absorbed light energy from UV ray as heat by
reversible intramolecular proton transfer.
• This reduces the absorption of UV ray by
polymer matrix and hence reduces the rate of
weathering.
• Typical UV-absorbers are oxanilides for
polyamides, benzophenones for PVC,
benzotriazoles and hydroxyphenyltriazines for
polycarbonate.
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• Strongly light-absorbing PPS is difficult to
stabilize.
• Even antioxidants fail in this polymer since the
polymer is electron-rich and behaves as
antioxidant.
• The acids or bases in the PPS matrix can
disrupt the performance of the conventional
UV absorbers such as HPBT.
• PTHPBT which is modification of HPBT are
shown to be effective even in these
conditions;
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(iii) Antiozonant
• Antiozonants prevent or slow down the
degradation of material caused by ozone gas in
the air (ozone cracking).
(iv) Organosulfur compounds
• Organosulfur compounds are efficient
hydroperoxide decomposers, which thermally
stabilize the polymers. Sulfuric acids are
produced as the product of decomposition,
which catalyse further hydroperoxide
decomposition;
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