CHAPTER 5 DEGDRADATION OF POLYMERS

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CHAPTER 5DEGDRADATION OF POLYMERS

• DR. MOHD WARIKH BIN ABD RASHID
• FKP

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OUTLINE CONTENT

• Chemical Degradation
• Biological Degradation
• Stabilizers

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5.4 Chemical Degradation

• 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) 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
  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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5.5 Biological Degradation

• 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 water-soluble 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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• 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 photo-oxidation 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 for processing.

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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 , HO1.
• These free radicals further react with
  atmospheric oxygen to produce more and more free
  radicals.
• For example,
• R O2 -gt ROO ROO RH -gt ROOH R
• This can be terminated using the antioxidants.
  Then this reaction comes to,
• 2R -gt R----R ROO R -gt ROOR 2ROO
• Non-radical products1 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
  non-radical 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 -gt ROOH A
• At K6, ROO RH -gt 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)
• 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' -gt 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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• 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