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On the other hand, ... Many process methods such as extrusion and injection moulding involve pumping molten ... possible as a result of poor moulding. – PowerPoint PPT presentation

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  • FKP

  • Chemical Degradation
  • Biological Degradation
  • Stabilizers

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.

5.4 Continue
  • 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

5.4 Continue
  • (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

5.4 Continue
  • 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
  • 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

5.4 Continue
  • (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.

5.4 Continue
  • 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

5.4 Continue
  • 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.

5.4 Continue
  • (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.

5.4 Continue
  • 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

5.4 Continue
  • 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.

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.

5.5 Continue
  • 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.

5.5 Continue
  • 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.

5.5 Continue
  • 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.

5.5 Continue
  • 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
  • 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.

5.5 Continue
  • 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

5.5 Continue
  • 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

5.5 Continue
  • 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

5.5 Continue
  • (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.

5.5 Continue
  • 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

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.

5.6 Continue
  • 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.

5.6 Continue
  • (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

5.5 Continue
  • 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
  • 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

5.5 Continue
  • 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

5.5 Continue
  • 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
  • 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

5.5 Continue
  • 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
  • Antioxidants inhibits the formation of the free
    radicals thereby enhancing the stability of
    polymers against light and heat.

5.5 Continue
  • (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'

5.5 Continue
  • B Although they are traditionally considered as
    light stabilizers, it can also stabilize thermal
  • 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

5.5 Continue
  • 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.

5.5 Continue
  • Strongly light-absorbing PPS is difficult to
  • Even antioxidants fail in this polymer since the
    polymer is electron-rich and behaves as
  • 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

5.5 Continue
  • Antiozonant
  • Antiozonants prevent or slow down the degradation
    of material caused by ozone gas in the air (ozone
  • (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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