photo degradation

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Polymer Degradation and Stability 93 (2008) 561e584www.elsevier.com/locate/polydegstab

Review article

Mechanistic implications of plastic degradation

Baljit Singh*, Nisha Sharma

Department of Chemistry, Himachal Pradesh University, Shimla 171 005, India

Received 21 June 2007; received in revised form 6 November 2007; accepted 13 November 2007

Available online 19 November 2007

Abstract

Plastics have become an indispensable ingredient of human life. Their enormous use is a matter of great environmental and economicconcern, which has motivated the researchers and the technologists to induce different degrees of degradations in the plastic. These degradationscan be induced in a better way if their mechanistic implications are properly understood. A better understanding of the mechanism for thesedegradations is also advocated in order to facilitate the proper use of the alternative waste disposal strategies. In view of the facts concerningthe plastic degradation, in this review article, we have discussed various types of polymeric degradations along with their mechanisms, whichinclude photo-oxidative degradation, thermal degradation, ozone-induced degradation, mechanochemical degradation, catalytic degradation andbiodegradation. This article also discusses the different methods used to study these degradations and the factors that affect these degradations.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Photo-oxidative degradation; Thermal degradation; Ozone-induced degradation; Mechanochemical degradation; Catalytic degradation and

biodegradation

1. Introduction

Plastics due to their versatility become the essential ingre-dients to provide a quality to life. These are now rival metalsin breadth of use and in severity of applications because oftheir flexibility, toughness, excellent barrier and physicalproperties and ease of fabrication [1e5]. The accumulationof plastics in the environment is a matter of great concernleading to long-term environment, economic and wastemanagement problems. Degradation of waste plastics throughvarious means becomes one of the alternatives to deal withsuch problems [6,7]. A wide variety of synthetic polymersabsorb solar ultraviolet (UV) radiation and undergo photolytic,photo-oxidative, and thermo-oxidative reactions that result inthe degradation of these materials [8,9]. The propensity ofplastic products to undergo solar UV radiation induced degra-dation/ozone-induced degradation has been increased byaddition of some additives in these polymers [10,11]. Besidesthese degradations, biodegradation offers another most

* Corresponding author. Tel.: þ91 0177 2830944; fax: þ91 0177 2830775.

E-mail address: [email protected] (B. Singh).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.11.008

efficient and attractive route to environmental waste manage-ment. The mechanisms involved in the biodegradation arecomplex due to the interaction of different oxidative processeswhich are caused by the oxygen present in the air either by themicroorganisms or by the combination of the two [12].

On the other hand, sometime the product needs stabilityinstead of degradation. Therefore, depending upon the prod-uct applications plastic needs controlled stability. In orderto increase the outdoor uses of plastics, the developmentand production of polymers with increased weathering resis-tance is required similarly for the use in high temperatureapplications, polymers need thermal stability [13e15]. Tocontrol the stability, understanding of the mechanism is theprimary requisite. Keeping in view the facts for the plasticdegradation, in this review article, we have mainly discussedthe various types of polymeric degradations (photo-oxidativedegradation, thermal degradation, ozone-induced degradation,mechanochemical degradation, catalytic degradation and bio-degradation) and mechanism followed by these degradations.We have also discussed the various methods used to studythese degradations and various factors which affect thesedegradations.

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562 B. Singh, N. Sharma / Polymer Degradation and Stability 93 (2008) 561e584

2. Types of plastics

Plastics can be synthesized via the polymerization (polyaddi-tion or polycondensation) of small molecules and are in generalclassified into two groups i.e. thermoplastics and thermosetplastics [16]. Thermoplastics are linear chain macromoleculeswhere the atoms and molecules are joined end-to-end into a se-ries of long, sole carbon chains. The bi-functionality necessaryto form a linear macromolecule from vinyl monomers can beachieved by opening the double bond and reaction proceeds bya free radical mechanism. Such type of polymerization is knownas addition polymerization, polyethylene and polypropylene arethe examples [17e19]. On the other hand, thermoset plastics areformed by step-growth polymerization under suitable conditionsallowing bi-functional molecules to condense inter-molecularlywith the liberation of small by-products such as H2O, HCl, etc. ateach reaction step [18]. In this class, the monomers undergosome chemical changes (condensation) on heating and convertthemselves into an infusible mass irreversibly [20,21].

3. Types of plastic degradation

Changes in polymer properties due to chemical, physical orbiological reactions resulting in bond scissions and subsequentchemical transformations are categorized as polymer degrada-tion [22]. Degradation reflects changes in material propertiessuch as mechanical, optical or electrical characteristics in craz-ing, cracking, erosion, discoloration and phase separation [23].Depending upon the nature of the causing agents, polymer degra-dations have been classified as photo-oxidative degradation, ther-mal degradation, ozone-induced degradation, mechanochemicaldegradation, catalytic degradation and biodegradation [24].

3.1. Photo-oxidative degradation

Photo-oxidative degradation is the process of decompositionof the material by the action of light, which is considered as oneof the primary sources of damage exerted upon polymeric sub-strates at ambient conditions. Most of the synthetic polymersare susceptible to degradation initiated by UV and visible light.Normally the near-UV radiations (290e400 nm) in the sunlightdetermine the lifetime of polymeric materials in outdoorapplications [25e27]. Polymer degradation occurs mainly inthe ether parts of the soft-segments, where photo-irradiationgenerates ester, aldehyde, formate and propyl end groups[28]. UV radiations have sufficient energy to cleave CeCbond [29]. The most damaging UV wavelength for a specificplastic depends on the bonds present and the maximum degra-dation therefore occurs at different wavelengths for differenttypes of plastics, e.g. it is around 300 nm for polyethylene(PE) and around 370 nm for polypropylene (PP). Photodegra-dation changes the physical and optical properties of the plastic.The most damaging effects are the visual effect (yellowing), theloss of mechanical properties of the polymers, the changes inmolecular weight and the molecular weight distribution forthe same [30e40]. PE and PP films when exposed to solarUV radiation readily lose their extensibility, mechanical

integrity and strength along with decrease in their average mo-lecular weight [33e37]. Mechanical integrity of polystyrene(PS) is diminished through extensive chain scission during pho-todegradation [38,39]. Another important study carried out onlight-induced damage to PS plastics compounded with flameretardant additives has reduced the risk of fire when the plasticshave been used as building construction materials [40].

At any given temperature and moisture content, the rate ofweathering increases with an increase in UV flux. Tensilestressing of stabilized types of PP in thermo-oxidative andphoto-oxidative environment has accelerated embrittlement ofthe polymers [31]. In one study, tensile stress has been appliedat constant load (in the range of 0e6.86 MN/m2) to observe thebehavior of stabilized and unstabilized types of isotactic PP inthe course of thermo-oxidative aging at 80e130 �C, photo-oxidative aging at 45 �C and relative humidity of 65%. Fromkinetic evaluation of the temperature-dependence of weightchanges of unstabilized PP during thermo-oxidative aging, ithas been found that the weight losses of unstressed andtension-stressed specimens have obeyed the kinetic equationsfor a reaction of the first order [32]. Along with stress, introduc-tion of carbonyl group on polymeric backbone has also acceler-ated photochemical degradation. PS undergoes photochemicalring-opening reactions, which leads to backbone cleavage bysubsequent Norrish Type II reactivity [41,42].

3.1.1. Mechanism of photodegradationThe mechanisms of the degradation and oxidation reactions

are determined by the extraneous groups and/or impurities inthe polymer, which absorb light quanta and form excited states.Initially short-lived singlet state is transformed to long-livedtriplet state [43]. Excited triplet states may cleave the polymerchains and form radical pairs (Norrish Type I reaction) or formpairs of saturated and unsaturated chain ends by hydrogentransfer (Norrish Type II reaction) [44]. The polymer radicalsthus formed may add molecular oxygen (in triplet ground state)to peroxy radicals, which abstract hydrogen and form hydroper-oxide groups, which absorb UV light or become excited byenergy transfer, the weak OeO bonds break and pairs of alkoxyand hydroxyl radicals are formed which may react in variousways, e.g. by hydrogen abstraction, chain scission, rearrange-ment, etc. and accelerate photodegradation [45]. Double bondsmay add excited oxygen molecules in singlet state. In thisreaction, the double bond is shifted to an adjacent CeC bondand a hydroperoxide group is formed. Some synthetic poly-mers, e.g. aromatic polyesters and polyamides, have inherentabsorption of UV light, causing excitation, radical formation,oxygen addition, splitting off small molecules, chain scission,etc. Some of these polymers are auto-stabilized towards photo-degradation by formation of an oxidized surface layer with highabsorption of near UV and visible light of short wavelengths,preventing further penetration of light into deeper layers [46].In photo-oxidative degradation, mechanism involves auto-oxidation cycle comprising various steps shown in Scheme 1.

3.1.1.1. Initiation. The absorption of UV light that has suffi-cient energy to break the chemical bonds in the main polymer

jhonalyn

Highlight

Initiation

Propagation

Termination

R O2 ROO

ROOHR H

ROO ROOR

O2 RiOO.

Ri

.

.

R.

R.

R. .

R.

.

ROO.

Initiator Ri

.

R H RiOOH R.

RiOO.

R R

Scheme 1.

Z

Z Z

CCCC

hv *

Z*

+

Z*

+

Scheme 3.

563B. Singh, N. Sharma / Polymer Degradation and Stability 93 (2008) 561e584

chain leads to the initiation of mechanism responsible forpolymer degradation. It involves a radical chain mechanismfor the formation of initial radical. Different initiation stepsunder varied conditions have been undertaken in differentpolymers.

3.1.1.1.1. Direct UV initiated photolysis of CeC and CeHbond. Bond dissociation energy of CeC bond (375 kJ/mol)and CeH bond (420 kJ/mol) is equivalent to UV radiationof 320 nm and 290 nm. Thus, direct photolysis of CeC andCeH bond is possible and the radical formed in thesereactions become a source of initiation radicals as shown inScheme 2 [47].

3.1.1.1.2. Photosensitized cleavage. Photosensitizers arehighly photosensitive, readily get excited on exposure to lightand are generally employed to bring about effective homolysisof the polymeric chains, which otherwise do not undergosufficient photo-excitation at the frequency of light availableto the system (Scheme 3) [45,48].

3.1.1.1.3. Catalyst residues as source of generation ofradicals. Some metal salts and oxidation products of suchresidues when added to the polymers act as catalysts to generateinitiation radicals (Scheme 4) [48]. Many commercial polymerscontain metallic compounds as impurities or deliberately incor-porated additives. The bulk of the latter types are pigments,whereas the former include polymerization catalyst residuesor debris from processing equipment. Polymerization catalystssuch as transition metals (Ti) may remain in polyolefins at

C

C

CCCuv

C

H

uv

H

..

Scheme 2.

2e100 ppm, depending on workup and catalyst efficiency.These residues have been implicated in both photo- and thermalstability problems. For example, TiO2 is a well-known photo-sensitizer for polyamide and polyolefin degradation andabsorbs at 480 nm. Photosensitization involves the formationof highly reactive species including atomic oxygen,

�OH,

�OOH and O2

�. The primary process involves the promotion of

the Ti electron to the conduction band of the semiconductorto form an electron-positive hole pair. The relative proportionsof the reactive species depend on the presence of water. NoTiO2 sensitization will be observed unless both oxygen andwater are present [29].

3.1.1.1.4. Incorporation of carbonyl groups. Carbonylgroups formed by mild oxidation of polymer during synthesisor processing act as chromophores and become source of theinitiation radicals. Carbonyl chromophore absorbs near-UVradiations and subsequently forms radicals following NorrishType I, Norrish Type II and H-atom abstraction processes(Scheme 5) [49].

3.1.1.1.5. Introduction of peroxides or site of unsaturation.The peroxides or C]C sites become source of initiationradicals as shown in Scheme 6 [45]. The near UV componentof sunlight (280e390 nm) is energetic enough to cleave CeCbond and Ceheteroatom bonds provided that light of the ap-propriate wavelength is absorbed. Chain oxidation occurs inmost of the polymers because of the labile OeO bond presentin the macrohydroperoxide eCeOOH, the macroalkoxyl andhydroxyl radicals thus formed may abstract hydrogen fromthe surrounding polymer matrix to generate alcohol, waterand new macroalkyl radicals which can then take part inmany cycles of the chain-initiation reactions. In the case ofunsaturated polymers, light generated singlet oxygen 1O2

reacts with an unsaturated site by way of an ‘‘ene’’ reactionand starts chain oxidation (Scheme 6) [29].

3.1.1.1.6. Reactions of singlet and triplet stage. Reaction ofa ketone in a triplet-excited state with ground state oxygen isanother potential initiation reaction (Scheme 7). In these reac-tions, UV light is absorbed by carbonyl or other chromophoricgroups and the energy of the excited groups is transferred tooxygen molecules. The resulting excited oxygen species react

hvTiCl3 Cl

TiO2 TiO2 TiO2 HOO RRHO2 ,hv

TiCl4

Scheme 4.

jhonalyn

Highlight

H2

C

CH2

CCH2

OC

CH2

CH 2

H 2

C

CH2

H2

C

CH

2

CCH2

CH 2CH2

CH 2

O

CH 3

O

RH

OH

R

CH2

Type I

Type II

Scheme 5.


with vinyl or other unsaturated groups forming hydroperoxidewhich then initiate free radical reaction mechanism [43].

3.1.1.2. Propagation reaction. The propagating reactions ofauto-oxidation cycle are common to all carbon backbonepolymers. These reactions lead to generation of hydroperoxidespecies (Scheme 8) and are not directly led to backbone cleav-age but are the key intermediates to further reactions as shownin Scheme 1. Hydroperoxide species generated in propagatingstep lead to backbone degradation through cleavage ofhydroperoxide OeO bond followed by b-scission (Scheme 9).

Polymer backbone cleavage occurs through Norrish Type Iand II reactions of chromophores (carbonyl) following b-scission route, which is a prevailing route for photo-oxidativedegradation. In semi-crystalline polymers, this scission occursin the amorphous domains. The scission process generates twochain ends that are free to restructure, and can often lead toincrease in crystallinity as oxidative degradation proceeds[29,41]. PS undergoes photo-oxidative bulk degradation viaa free radical mechanism (Schemes 10 and 11). Oxidativeprocess involves two steps: (1) formation of hydroperoxideand (2) decomposition of these hydroperoxides. Such oxida-tive reactions are initiated when hydrogen is removed througha photoreaction. The free radical formed on polystyrene reactswith oxygen to form peroxy radical, which can abstract a pro-ton from some other labile positions, thereby forminghydroperoxides and a new radical site. The newly formed hy-droperoxide group is subjected to decomposition and resultingultimately in chain cleavage into ketones and olefins [46].

3.1.1.3. Termination reactions. The termination of photode-gradation is achieved by ‘mopping up’ the free radicals tocreate inert products. This occurs naturally by combiningfree radicals or assisted by using stabilizers in the plastic.

C

CH2

1O 2

C

CH2

OH

OH+

+CH

CH

OO

Scheme

Macroalkyl radicals may combine to give a crosslinked,branched or disproportionated product. Peroxyl radicals even-tually terminate by reaction with other radicals to give dialkylperoxides, carbonyl species or alcohols [29,30].

3.1.2. Methods for photodegradation

3.1.2.1. Natural weathering method. Outdoor exposure can beperformed on samples mounted on testing racks, oriented un-der standard conditions to expose the material to the fullradiation spectrum besides the temperature and humidity ofthat location [23]. In order to observe the aging of the material,it is characterized with respect to mechanical properties (elon-gation at break, tensile properties or impact strength) and vis-ible characteristics, such as crack formation, chalking, andchanges in color [5]. The alterations in the polymeric materialson exposure can be characterized with FTIR spectroscopy andultra violet/visible (UV/vis) spectroscopy [50,51].

3.1.2.2. Artificial weathering method/laboratory test. Purelaboratory testing involves using environmental chambersand artificial light sources to approximately replicate outdoorconditions but with a greatly reduced test time under highlycontrolled conditions. Laboratory testing can quickly assessthe relative stability of plastics but has the major disadvantagethat the quicker the test lower is the correlation to real behav-ior in the field [52]. Lundin et al. [53] have studied acceleratedweathering of fiber-filled PE composites.

3.2. Thermal degradation

Under normal conditions, photochemical and thermaldegradations are similar and are classified as oxidative degra-dation. The main difference between the two is the sequence

CH Radical formationvia above eq.

CH2

O OH

HC CH

6.

R

RCH2CH RCH

1O 2

1O2

C

O

CH2CHCH2OOH

CHCH2OOH

3R

+

++ O2R

O

C R

hv

RCH Radical Reactions

Scheme 7.


of initiation steps leading to auto-oxidation cycle. Otherdifference includes that thermal reactions occur throughoutthe bulk of the polymer sample, whereas photochemicalreactions occur only on the surface [41]. Thermal degradationof polymers occurs through random and chain degradation(depolymerization reaction) initiated by thermal and UV light[54]. The depolymerization reaction in thermal degradationneed not be initiated at terminal end of the macromolecule,instead, imperfections in the chain structure (initiator fragmentor a peroxide or ether link) form a weak link from wheredepolymerization starts. A large number of addition polymersdepolymerize at elevated temperature [55], for example,polymethylmethacrylate (PMMA) has been converted almostquantitatively back to the monomer and PE has been decom-posed into longer olefinic fragments and actually producinglittle monomer. Thermal degradation above 200 �C leads tochain scission and largely depends on impurities like unsatura-tion sites, head-to-head units, etc. [56]. Polyolefins are knownto be sensitive to thermal oxidation, due to the impuritiesgenerated during their manufacture at high temperatures[57]. Drago et al. [58] have studied thermally stimulatedoxidative degradation of high impact PS with nitric acid.Mechanism of oxidation and nitration depends upon tempera-ture and leads to the molecular and chemical heterogeneityand also lowering in mechanical properties of the polymer.PS thermally gets degraded into organic compounds such asphenol, quinine, naphthalene and diphenylamine at the exper-imental temperature of 350e450 �C [59e62]. Complexreactions occurring in thermal degradation of polymers dependon various factors like heating rate, pressure, reaction medium,and reactor geometry. Polymers have high viscosity, whichcomplicates the process by impeding heat and mass transfer[63,64]. Thermo-oxidative degradation of polyesters yieldsa variety of products such as formaldehyde, acetaldehyde,formic acid, acetic acid, CO2 and H2O. In addition to thesecompounds, various other compounds like hydroxyaldehydes,

CH3 CH3

C

C(O)OCH3

C

CH2

Scheme

hydroxyacids, aldehyde acids, etc. are also identified in smallquantities [65].

3.2.1. Mechanism of thermal degradationThe mechanism of thermal degradation of polymers is an

interesting subject, not only from fundamentals of polymer re-actions’ point of view but also in understanding heat-resistingcharacteristics, polymer processes such as extrusion or injec-tion molding, and the effective utilization of plastic wastes.The thermal degradation of polymers consists of two distinctreactions, which occur simultaneously in the reactor. One isa random scission of links, causing a molecular weight reduc-tion of the raw polymer, and the other is a chain-end scissionof CeC bonds, generating volatile products. The chain-endscission takes place at the gaseliquid interface in the workingreactor [66]. The type and composition of the pyrolysisproducts give useful information about mechanism of thermaldegradation [67,68]. Thermal dehydrochlorination of poly(vi-nyl chloride) (PVC) begins with internal allylic chloride andtertiary chloride structural defects formed during polymeriza-tion. During thermal degradation, ordinary monomer units areconverted into internal allylic chloride defects by a mechanismthat may include the abstraction of hydrogen by triplet cationdiradicals derived from polyene intermediates. Cyclizationreactions seem likely to contribute to the termination ofpolyene growth [69]. Thermal degradation of the polymersfollows either chain end degradation (also known as unzippingroute) (Eqs. (1 and 2)) or random degradation route (Eq. (3))as shown below:

M�n/M�n�1þM ð1Þ

M�n�1/M�n�2þM ð2Þ

Mn/Mx þMy ð3Þ

The chain end degradation starts from the end of the chain andsuccessively releases the monomer units. This type of degrada-tion route is also known as depolymerization reaction, whichinvolves successive release of monomer units from the chainends. Such reactions are the opposite of the propagation stepin addition polymerization and occur through free radicalmechanism. In this type of degradation, molecular weight ofthe polymer decreases slowly and large quantity of themonomer is liberated simultaneously. For example, poly a-methylestyrene synthesized through anionic mechanism uponheating from �70 �C to þ60 �C undergoes degradation fromthe chain end carrying the active carbanion in such a manner

CH3 CH3

CCCH2

C(O)OCH3

8.

C

OHO

O

OH

OH

C

HC

H

hv

Scheme 9.


that at 60 �C, the entire polymer gets converted into themonomer [20]. The major degradation products for fluorinatedpolyacrylate are monomer, dimer, saturated diester, trimer, andcorresponding methacrylate along with remarkable amounts ofalcohol [67,68]. In general, chain-end degradation occurswhen the backbone bonds are weaker than the bonds of theside groups and only with polymer molecules, carrying activechain ends with a free radical, cation, anion, etc. In general, a-substituted vinyl polymers degrade mostly through the processof depolymerization. PMMA, poly a-methyl styrene, PP,polytetrafluoroethylene and poly a-methyl acrylonitrile, onpyrolysis, have been converted to their respective monomersalmost quantitatively. The unzipping process involves a freeradical path [70e72].

Random degradation occurs at any random point along thepolymer chain. This is reverse to polycondensation processwhere the polymer degrades to lower molecular weight frag-ments but practically no monomer is liberated. For randomdegradation to occur, the polymer chain does not requirenecessarily to carry any active site [20]. Polyesters undergohydrolytic degradation resulting in chain scission. PE also un-dergoes random degradation through migration of a hydrogenatom from one carbon to another thus generating two frag-ments [72]. In general, vinyl polymers such as polystyrene,poly(acrylonitrile), etc. are degraded by the random chainscission process. The monomer yield from such polymers isusually low, and the pyrolyzed fragments are ordinarily largerthan corresponding monomer unit [71]. For example, poly(eth-ylene terephthalate) follows random thermal degradation route(Scheme 12); the initial step resulting in scissions leads tocarboxyl and vinyl groups [46]. In general, initiation reactionoccurs by random chain scission or chain end initiation. Thisstep is followed by de-propagation step forming monomer.

C

O

O2

CCH2 CH2 CH2

CH2

CH2

CH2CH2 CH2

H H H

hv

+

+

O

C C

H

C C

H

C C

H

Scheme 10.

Termination takes place through radical coupling and radicaldisproportionation (Scheme 13) [73].

Rodryguez-Vazquez et al. [74] while investigating thermaldegradation of LDPE at 400 �C observed that intra-molecularhydrogen abstraction followed by b-scission is a majordegradation route [15]. Radical termination occurred by com-bination and disproportionation. PS on thermal degradationyields 42% monomer as volatile product in dispropagationreaction (Scheme 14) [18]. Thermal degradation of saturatedpolymers such as PS follows a hydroperoxide mechanism[20]. Thermal oxidation of PP at 120e280 �C results inmultiple oxidative chain scission and around 40 volatile com-pounds are identified by GC/MS technique. Backbone scissionroute shown by polyolefins is b-scission of the macroalkoxylradicals [29].

3.2.2. Methods for thermal degradation

3.2.2.1. Batch reactor method. The thermal degradation ofwaste plastic can be carried out in a glass reactor underatmospheric pressure with definite weight of sample that isloaded into bottom of the reactor for thermal degradation.The purging of the reactor with nitrogen gas at a flow rateof 10 mL/min at 120 �C for 60 min is required to removethe physically adsorbed water from the plastic sample. Afterstopping the nitrogen flow, the reactor temperature is increasedto the degradation temperature (430 �C) at a heating rate of3 �C min�1 and the waste plastic bed temperature is taken asthe temperature of the degradation. The gaseous productscan be condensed (using a cold-water condenser) to liquidproducts and trapped in a measuring jar. The quantitative anal-ysis of the liquid products can be performed by using a gaschromatograph equipped with a flame ionization detector,atomic emission detector, an ion chromatograph and massselective detector. The solid residue can be identified byFTIR spectrometer [75].

3.2.2.2. Thermogravimetric analysis. Thermogravimetricanalysis (TGA) can be used for the investigation of oxidativeand thermal degradation of the polymers [76]. The rate of deg-radation in TGA (da/dt) is defined as the rate of change of thedegree of conversion. The degree of degradation or conversioncan be calculated in terms of mass as shown below:

a¼W0�W=W0�WN

where W0, W and WN are the initial weight, the actual weightat each point of the curve, and the final weight measured at theend of the degradation process, respectively [77]. In TGA

C

HO

+O

C C

HO

O

C C CH2CH2CH2 CH2

CH2

H

OH

H2C C+

Scheme 11.


instrument, sample is submitted to constant heating rate fromroom temperature to 600 �C under the nitrogen flow. Thereaction products can be analyzed by gas chromatography[78e84].

3.2.2.3. Pyrolysis GC/MS method. In this method, volatilepyrolysis products at different temperatures can be identifiedwith the help of GC/MS [73]. The pyrolyser is based on a tech-nique in which a small deactivated stainless steel cup loadedwith a sample is dropped into a small sized vertical furnace

OC

CH

O

O+OHC

O

CH2

CH2

CH2

CH2

C HCO

O

O

C

O CCH OC

OO

C

O

OH

CH3

CO

O

Scheme

by gravitational free-fall with push button mechanism. Thissystem is designed to provide precise temperature control.Once the system starts, the sample controller stops samplingafter 0.5 min and the pyrolysis gases from the samples directlypass into GC/MS analysis system [85].

3.3. Ozone-induced degradation

Atmospheric ozone usually causes the degradation ofpolymers under conditions that may be considered as normal;

C OO C

C

O O

OO

C O

CH2

CH2

CH

CH

CO

OH

CH

+

+

+

+

CH

C

O

O

O

CH

CO2

C

O

CH3

CH3 HC

O

12.

R R(R)n

(R)n

(R)n

(R)n-1

ROO

O2

ROOR

ROOR

ROOR

RO

O2ROO

ROO

ROR

ROR

R

R

RR

R R

RR

R

RO

RO

RO

ROO

R

R

R

+

+

+

+

+ +

+ +

+

Initiation

Propagation

Termination

R

Scheme 13.


when other oxidative aging processes are very slow and thepolymer retains its properties for a rather longer time[86e88]. The presence of ozone in the air, even in very smallconcentrations, markedly accelerates the aging of polymericmaterials [89]. This process in saturated polymers is accompa-nied by the intensive formation of oxygen-containingcompounds, by a change in the molecular weight and byimpairment of the mechanical and electrical properties ofthe specimens [5]. Exposure of polymers to ozone results inthe rapid and consistent formation of a variety of carbonyland unsaturated carbonyl products based on aliphatic esters,ketones, and lactones as well as aromatic carbonyl associatedwith the styrene phase. This follows by a more gradual forma-tion of ether, hydroxyl and terminal vinyl groups with timeand concentration [90]. These reactions of ozone withpolymers occur with main chains containing C]C bonds,aromatic rings or saturated hydrocarbon links. The reactionproceeds through unstable intermediates such as the bipolarion or peroxy radicals, which can isomerize or degrade, causedecomposition of macromolecules [91]. Under the influence ofexternal load, the rate of ozone oxidation and hydrogen atom

CH2 CH2 CH2 CH2CH2

CH2CH2CH2CH2CH2CH2

CH CH CHCHO2200º C

CH

CH CH C

O

CH C

Scheme

abstraction from the polymer chain increases. This is due tothe change in hybridization of the reaction centre from thestate sp3 to the state sp2 [92e95]. In poly(vinyl alcohol)(PVAL) the chain scission is based on the ozone oxidationof the alcoholic groups of PVAL with formation of ketonegroups which in turn are the source of a ketoeenol tautomer-ism which leads to random chain scission by further ozoneattack. PVAL forms a strong hydrogen-bond complex withozone for which the interaction energy is of the order of47.3 kJ/mol and it is slowly degraded by ozone. The analysiswith FTIR spectra indicates that the final product is a PVALoligomer with numerous ketone groups along the mainoligomer backbone and with carboxylic end groups [96].

PS is slowly attacked by ozone as compared to unsaturatedpolymers. The relative yield of products identified from thisattack is 35% peroxide, 18% ketone and 47% acid. Theseproducts are similar to those expected from free radical chainoxidation processes. Ozen et al. [97] have studied the effect onstructural and mechanical properties of packaging films byozone exposure and have reported that ozone treatment has af-fected the PE and polyamide films differently. It is responsiblefor formation of oxygen-containing functional groups,degradation of polymeric chains in PE films and increase inthe eCeNe stretching in polyamide. Clough and Gillen[98] have observed that the ozone generated by the action ofthe ionizing radiation in the air present in the atmospheresurrounding the samples affect the surface of the polymer[98]. This observation has been made during gamma-radiationdegradation studies of poly(butadiene-co-styrene) [SBRrubber] and poly(butadiene-co-acrylonitrile) [Buna-n rubber]in the presence of air. The oxides of nitrogen, sulfur, andozone promote UV-induced oxidative damage of commonplastics, particularly in rubber products [11]. Ozone mainlyaffects vulcanized rubbers with unsaturation in the main poly-mer chain and causes cracking in stretched form in rubber[17]. Exposure to ozone gas causes change in the mechanicalproperties of linear LDPE, oriented PP and biaxially orientednylon [99]. Gatenholm et al. [100] have studied the effect ofozone on the microporous structure of PP. Exposure of isotac-tic PP to ozone results in surface oxidation which furtherincreases when a microporous membrane has large surfacearea [100].

CH2 CH2 CH2

CH2 CH2 CH2CH2

CH CHC

OOH

CHCH CH C

O

CHO

HO

14.


3.3.1. Mechanism of ozone-induced degradationOzone normally attacks the unsaturation in unsaturated

polymers and this reaction generally occurs in three principalsteps (Schemes 15 and 16). The first step is a cycloaddition ofozone to the olefin double bond to form ozoneeolefin adductreferred to as the ‘‘primary ozonide’’ which is an unstablespecies because it contains two very weak OeO bonds. Thesecond step in the ozonolysis mechanism is the decompositionof the primary ozonide to carbonyl compounds and a carbonyloxide. The carbonyl oxide is considered to be the key interme-diate in the C]C bond ozonolysis mechanism. The third stepin the ozonolysis mechanism is the fate of the carbonyl oxide,which depends on its source, as well as on its environment.The carbonyl oxide flips over with the nucleophilic oxyanionattacking the carbon atom of the carbonyl group [101e104].The ozonization of conjugated polymers (polyacetylene,polyphenylacetylene) has showed that the conjugated doublebonds of the polyenes react more slowly with ozone than theisolated double bonds of the diene rubbers [86e88].

3.3.2. Method for ozone degradation

3.3.2.1. Ozone exposure test. Sample of the polymer can beexposed in a HAMPDEN (model 2000-AM ozone cabinet)at 40 �C in atmospheres of 50, 100 and 200 ppht (parts perhundred thousand) of ozone in air. Before exposure to ozoneatmosphere the specimens are subjected to a deformation of20% under stress and maintained this way for 72 h in theabsence of light. Those samples that in general terms exhibitany crack formation, minimal or even superficial, during theozone attack assay are termed ‘‘non-ozone resistant samples’’and those which do not exhibit any crack formation are termed‘‘ozone resistant samples’’. The samples can be analyzed withFTIR to study the formation of new functional moieties in thesample [90].

3.4. Mechanochemical degradation

Mechanochemical degradation of polymers involves thedegradation of polymer under mechanical stress and by strongultrasonic irradiations [105]. Breakdown of molecular chainsunder shear or mechanical force is often aided by a chemicalreaction and is known as mechanochemical degradation. Forexample, mastication of rubber can lead to chain breakageand development of plasticity under shear. In the atmosphereof nitrogen at ordinary temperature mastication of rubberdoes not change the plasticity and molecular weight apprecia-bly, but in the presence of oxygen, degradation occurs rapidly.

C

O

C

O-

O

O

O O

O3 +

+-

CC C C

Scheme

This is due to the reason that the rubber molecule breaks intoradicals, and oxygen as radical scavenger readily reacts withthem, leading to permanent chain breakage, whereas nitrogenis not a radical scavenger and thereby led to radical recombi-nation [18]. The degradation of high molecular weight PSoccurs under the turbulent flow and the drag reduction effi-ciency decreases with time due to the mechanical degradationof the polymer molecules [106,107]. In one study mechanicaldegradation of polymers during their mechanical mixing of bi-nary mixtures of cis-polyisoprene with stereo-regular poly(bu-tadiene), and with butadieneemethylstyrene or butyl rubbersleads to considerable changes in the molecular characteristicsof the components [108]. The mechano-degradation of poly-mers in melts occurs in terms of free radical processes. Inthis processing, there is a decrease in the width of the molec-ular weight distribution function, a quantitative correlationbetween ruptures and crosslinks on the one hand and changein double bond concentration on the other and an increase inlong-chain branching due to the reaction of chain-side radicals[109e111]. The mechanochemical degradation of PMMA inthe presence of nitroxides acting as chain terminating agentscreates macro-radicals, which are used in free radical polymer-ization reactions [112]. Mechanochemical dechlorination ofPVC with various oxide powders, i.e. CaO, Fe2O3, SiO2 andAl2O3, in air reduces the molecular weight of PVC [113].

3.4.1. Mechanism of mechanochemical degradationWhen excessive stress is applied, the molecular chain

breaks and produces a pair of free radicals, which can takepart in subsequent reactions. In the presence of oxygen, thefirst reaction results in the formation of peroxy radicals. Themost important application of this type of reaction is the mas-tication of natural rubber to make it processable. Mechanicallygenerated radicals are believed to result from the cleavage ofthe main backbone segments of polymer chains in the stressedamorphous regions connecting crystallites [114]. High inten-sity ultrasounds can induce mechanochemical degradation inpolymeric materials. Polymer in such a case is subjected tovery high vibrations, which are only mechanical forces. Asultrasonic waves pass through the solution, the localized sheargradient produces tear off molecules leading to chain scissionand decrease in molecular weight [20]. The relevant mecha-nism for the observed phenomena has been explained by theinteraction of ultrasound and component molecules. In a liquid,upon irradiation of ultrasound, molecules have been exposedto alternate compression and expansion modes, by which bub-bles are formed and eventually collapsed. On the molecularlevel, this implies a rapid motion of solvent molecules to

O

O-O

CC C+

+ OO

15.

C

CH2 CH2 CH2

CH2CH2CH2

CH2 CH2 CH2 CH2 H2O2

H2O

CH2

CH2

CH3

CH3

CH3

O3

CH3

CH3

C CHCH C

O O

CH

O

C

C CH

O

CH C

CH 3

CH

O

Hydrolysis

Scheme 16.


which the macromolecules embedded in the solvent cannot beadjusted. Thus, friction is generated which causes strain andeventually bond rupture in the macromolecules [115]. In theabsence of oxygen, various carbon radicals have been gener-ated from polyolefins, PMMA, etc. At low temperature, theradicals from main-chain scission have been detected; uponwarming, these attack the polymer matrix and lead to furtherscission reactions through radical-rearrangement reactions(Scheme 17) [29].

In the presence of oxygen above �113 �C, quantitativeconversion of macroalkyl to peroxy radicals occurs. Thusmechanical work generates the primary radicals and the oxida-tion can form the usual oxidation products. In the melt, it isdifficult to separate the degradative effect of heat and mechan-ical work [29]. Gel permeation chromatographic (GPC)studies of the degradation of LDPE and HDPE under highshear conditions have indicated that most of the changes inmolecular weight distribution and long-chain branching haveoccurred from thermal or thermo-oxidative degradation. Onthe other hand, the orientation of solid PP under high shear

CH3

77ºC

mill

C

CH2

CH2

C

CC

CH3

O

O

O

C

OCH 3OCH 3

OCH3

CH2 C CH2

+

CH3

CH2 C

C

O OCH 3

CH3

C

C

O OCH

C

H

CH3

Scheme

conditions (high draw rates and relatively low temperatures)has produced oxidation products directly as a result of theshear process [116]. Ultrasound is responsible for the breakageof macromolecular CeC bonds and termination reactions ofmechano-radicals occur as disproportionation and combina-tion reactions which suppress in the presence of radicalscavengers [112].

3.4.2. Method for mechanochemical degradationIn mechanochemical degradation polymer is subjected to

mechanical stresses through high speed stirring or milling.Agitation, grinding or extrusions are the methods of mechano-chemical degradation. The basic phenomenon involved issubjecting the polymer to very powerful shearing force thatwill break the molecule. Mechanical degradation reduces theaverage molecular weight of the polymer [117]. Electron-spin spectroscopy has graphically demonstrated that stretch-ing, grinding, milling and shearing process has producedfree radicals because of main chain fracture [29].

-scission

PMMA attack

CH2

C

CH3

C

C

O OCH3

CH3

C

C

O OCH3

+

CH3

CH2 C

CC

CH3

OO OCH3OCH3

C

H

3

17.


3.5. Catalytic degradation

Catalytic transformation of waste polymers into hydrocar-bons with higher commercial value is a field of great interest.Polyolefins are thermally or catalytically degraded into gasesand oils. The interest has been directed towards polyolefins(PE, PP, PS) which constitute an important part of industrialand domestic waste. The addition of a catalyst not onlyimproves the quality of products obtained from pyrolysis ofplastic wastes, lower the temperature of decomposition, butalso enable a given selectivity to a certain product to beachieved. Solid acid catalysts, such as zeolites, favor hydrogentransfer reactions due to the presence of many acid sites. Theaccess of molecules to catalyst reactive sites is limited tothe pore size as well as the growth of end products insidethe pores. Therefore, zeolite catalysts may produce molecularsieving and shape selectivity [118]. Garforth et al. [119] haveinvestigated catalytic degradation of polyolefins using TGA asa potential method for screening catalysts and have found thatthe presence of catalyst led to the decrease in the apparentactivation energy. For polymer degradation, different typesof catalysts have been reported in the literature whichinclude PteCo and PteMo supported over SiO2 [120], zeolitecatalysts and non-zeolite catalysts [121], transition metalcatalysts (Cr, Ni, Mo, Co, Fe) on the support (Al2O3, SiO2)[122], zeolite [123] and zirconium hydride [124].

3.5.1. Mechanism of catalytic degradationSekine and Fujimoto [125] have proposed a free radical

mechanism for the catalytic degradation of PP using Fe/activated carbon catalyst. The various steps in catalytic degra-dation are shown in Scheme 18. In initiation step, randombreakage of the CeC bond of the main chain occurs with

Initiation

Propagation

Termination

Catalysts

Fe/ Activated charcoal +H2

R1

R1

R2

R5 R5

R2R4 R4

R2 R2

H

H H

H

R1

R3 C2

R6

R7

R11 R11

R9

R10 R10

R9

R8 R7 R8

R6

Scheme 18.

heat to produce hydrocarbon radicals. In propagation, the hy-drocarbon radical decomposes to produce lower hydrocarbonssuch as propylene, followed by b-scission and abstraction of Hradicals from other hydrocarbons to produce a new hydrocar-bon radical. Disproportionation or recombination of two radi-cals is termination reaction. During catalytic degradation withFe/activated charcoal (AC) in a H2 atmosphere, hydrogenationof hydrocarbon radical (olefin) and the abstraction of the Hradical from hydrocarbon or hydrocarbon radical generateradicals, enhancing degradation rate. In a reaction temperaturelower than 400 �C or a reaction time shorter than 1.0 h, manymacromolecular hydrocarbon radicals exist in the reactor, andrecombination occurs readily because these radicals cannotmove fast. However, with Fe/AC in a H2 atmosphere, theseradicals are hydrogenated and therefore recombination maybe suppressed. Consequently, it seems as if the decompositionof the solid product is promoted, including low polymerswhose molecular diameter is larger than the pore size of thecatalysts. Wall et al. [126] have studied the catalytic degrada-tion of waste plastics and have found that when these polymershave been heated above 380 �C, they undergo depolymeriza-tion and degradation by a free radical chain reaction.

3.5.2. Methods for catalytic degradation

3.5.2.1. Batch autoclave method. The thermal and catalyticdegradation of plastic sample can be performed in a batchautoclave. Definite weight of polymer sample and definiteweight of catalyst are charged into the reactor. After purgingthe inner air of the reactor with a reaction gas (H2 or Ar),more reaction gas is added to 4.0 MPa and heated to the reac-tion temperature while being agitated at 70 times/min. At theend of the reaction, the heaters are removed and the reactor iscooled to ambient temperature in air. The gaseous productscan be analyzed through gas chromatography and liquid prod-ucts can be separated from solid products by suction filtrationwith an aspirator. The boiling-point distribution of the liquidproducts is obtained by simulated distillation. The residuefrom the suction filtration is then washed with toluene, driedin air, and weighed as a solid product. This solid productincludes both unreacted product (low polymer) and excessivecracked product (coke precursor or coke). However, the latteris produced on the surface of the catalyst and could not beseparated from the former when the reaction temperature islow or the reaction time is short [127]. PP has been pyrolyzedover various catalysts using a laboratory fluidized-bed reactoroperating isothermally at ambient pressure using zeolitecatalysts [121]. Solid phase microextraction has also beenused to analyze the products evolved in the thermal degrada-tion of PP [59].

3.6. Biodegradation

Biodegradation is a biochemical transformation of com-pounds in mineralization by microorganisms. Mineralizationof organic compounds yields carbon dioxide and water underaerobic conditions, and methane and carbon dioxide under


anaerobic conditions. Abiotic hydrolysis, photo-oxidation andphysical disintegration of polymers may enhance biodegrada-tion of polymers by increasing their surface area for microbialcolonization or by reducing molecular weight [128]. Biodegra-dation has been defined in various ways by different investiga-tors. It is defined as change in surface properties or loss ofmechanical strength [129], assimilation by microorganisms[130], degradation by enzymes [131], backbone chainbreakage and subsequent reduction in the average molecularweight of the polymers [132,133]. Degradation can occur byany of the above mechanisms alone or in combination withone another. According to ASTM standard D-5488-94dbiodegradation is defined as ‘‘process which is capable ofdecomposition of materials into carbon dioxide, methane,water, inorganic compounds, or biomass in which the predom-inant mechanism is the enzymatic action of microorganisms,that can be measured by standard tests, in a specified periodof time, reflecting available disposal conditions’’. Biodegrad-ability is also defined as the propensity of a material to getbreakdown into its constituent molecules by natural processes(often microbial digestion). The metabolites released bydegradation are also expected to be non-toxic to the environ-ment and redistributed through the carbon, nitrogen and sulfurcycles. Biological degradation is chemical in nature but thesource of the attacking chemicals is from microorganisms.These chemicals are of catalytic nature e.g. enzymes. Thesusceptibility of the polymers to microbial attack generallydepends on enzyme availability, availability of a site in thepolymers for enzyme attack, enzyme specificity for thatpolymer and the presence of coenzyme if required [134].

Biodegradation can occur at different structural levels, i.e.molecular, macromolecular, microscopic and macroscopic de-pending upon the mechanism [135]. It has been argued that thephenomenon of degradation in vivo might not be equated withthe term biodegradation, since biodegradation implies theactive participation of biological entities such as enzymes ororganisms in the degradation process [136]. It is, however,difficult to identify the involvement and the role of biologicalspecies in in vivo degradation. Both hydrolytic and enzymaticprocesses may contribute to the degradation to different extentduring the different stages of the degradation process. Degra-dation may begin by hydrolysis, but as the polymer breaks andsurface area and accessibility increases, enzymatic degrada-tion may dominate. Thereby biodegradation includes all typesof degradation occurring in vivo whether the degradation isdue to hydrolysis or metabolic processes. Therefore, biodegra-dation is also defined as the conversion of materials into lesscomplex intermediates or end products by solubilization,simple hydrolysis or the action of biologically formed entitiessuch as enzymes and other products of the organism. Polymermolecules may, but not necessarily breakdown to producefragments in this process, but the integrity of the materialdecreases in this type of process [137].

3.6.1. Mechanism of biodegradationBiodegradation of polymers occurs through four different

mechanisms: solubilization, charge formation followed by

dissolution, hydrolysis and enzyme-catalyzed degradation[138,139].

3.6.1.1. Solubilization. The hydration of polymers depends onthe hydrophilicity of the polymer. The hydration results fromdisruption of secondary and tertiary structure stabilized byvan der Waals forces and hydrogen bonds. During and afterhydration, the polymer chains may become water solubleand/or the polymer backbone may be cleaved by chemicalor enzyme-catalyzed hydrolysis to result in the loss of polymerstrength [140]. For crosslinked polymers, the polymer strengthmay be reduced by cleavage of either of the polymerbackbone, crosslinker or pendent chains. In non-swellablepolymer systems, the reduction in the polymer molecularweight may lead to the loss of coherence between polymerchains [141]. Barkay et al. [142] have studied biodegradationof polyaromatic hydrocarbons using a bio-surfactant whichhave enhanced the solubility and biodegradation of hydropho-bic hydrocarbons.

3.6.1.2. Ionization. Some polymers are initially waterinsoluble but become solubilized by ionization or protonationof a pendent group. Polyacids become soluble at high pH andbecome hydrophilic [143]. Cellulose acetate phthalatebecomes water soluble at a pH> 6, while poly(vinyl acetatephthalate) and hydroxyl propyl methyl cellulose phthalateare ionized at a lower pH [144].

3.6.1.3. Hydrolysis. Water insoluble polymers containingpendent anhydride or ester groups may be solubilized if anhy-dride or esters hydrolyze to form ionized acids on the polymerchain. For example, poly(methacrylate) and poly(methylmethacrylate) which are esters derived from poly(acrylicacid) and poly(methacrylic acid), respectively, are not watersoluble but become water soluble upon hydrolysis of thependent esters, and subsequent ionization of the carboxylicgroup [145]. Chain length of ester group and degree of substi-tution exert the effect on anaerobic biodegradation [146].Hydrolysis of the polymer backbone is most desirable sinceit produces low molecular weight by-products. Natural poly-mers undergo degradation by hydrolysis whereas syntheticpolymers are water insoluble. They tend to be more crystallineand this property account for their water-insolubility [147].For hydrolysis to occur, the polymer has to contain hydrolyt-ically unstable bonds, which should be reasonably hydrophilicfor the access of water. Biodegradable polymers include estersand ester derivative polymers [148e151]. Polyesters aredegraded mainly by simple hydrolysis. The first stage of thedegradation process involves non-enzymatic, random hydro-lytic ester cleavage and its duration is determined by the initialmolecular weight of the polymer as well as its chemicalstructure [152,153].

3.6.1.4. Enzyme-catalyzed hydrolysis. Enzymes function ascatalysts for a specific reaction or a series of reactions, suchas oxidation, reduction, hydrolysis, esterification, synthesisand molecular inter-conversions [154]. Hydrophobic polymers


with no hydrolyzable bonds such as PE, PP, PMMA and PS areexpected to be the most stable [21]. Wool et al. [155] havestudied degradation of PEestarch composites’ enzyme diffu-sion, which results in the back-diffusion of small moleculesto the surface for further assimilation by microorganisms[155]. The enzymatic degradation of certain natural polymersfollows the unzipping or chain-end degradation mechanism.For example, b-amylase degrades starch to maltose, beginningwith the chain ends [20]. Chattopadhyay and Madras [156]have studied the lipase catalyzed degradation of poly(vinyla-cetate) (PVAc) and observed that the ester bonds in the sidechains are broken specifically to yield oligomers with acidand alcohol groups. There are many side branches in PVAc be-cause chain transfer to the methyl groups in the acetate groupsof PVAc takes place frequently during the polymerization ofvinyl acetate. Enzymatic degradation of poly(3-caprolactone)has been successfully carried out in supercritical carbondioxide (scCO2). Candida antarctica lipase has smoothlycatalyzed the hydrolytic degradation in scCO2 to giveoligo(3-caprolactone) [157].

3.6.1.5. Microbial degradations. Microbial degradationresults from the action of naturally occurring microorganismssuch as bacteria, fungi, algae, etc. The product so formed isnot visible and need not be screened after composting [158].The production of biodegradable plastics that get decomposedcompletely in nature have received remarkable attention glob-ally as they are totally ecofriendly and helpful in waste landfillmanagement. Addition of natural polymers to thermoplasticshaving long-term potential is one of the approaches to enhancebiodegradability [159]. To prepare biodegradable plasticinvolves adding special additives to the synthetic polyolefins,which make it susceptible to microbial degradation, and theseadditives also disconnect the continuity of CeC chain ofpolyolefin. Few additives having hydrophilic groups makeplastic hydrophilic and susceptible for photo- and chemicaldegradation. Modification of backbone through copolymeriza-tion and through anchoring monosaccharide with polyolefin orblending with nutrients and biodegradable fillers makes theplastic degradable [160e163]. These micronutrients help thegrowth of specific microorganisms thus inducing biodegrad-ability and through incorporation of polar functional groupswhich can serve as point of microbial attack [164,165].

Generally, polyolefin constitutes a non-biodegradable classof plastic and in order to make it degradable, research has beencarried out on biological degradation of PE. Biodegradation ofPE occurs through two mechanisms: hydro-biodegradationand oxo-biodegradation [166]. These two mechanisms worksuccessfully on the modified PE (by additive starch) makingthe material hydrophilic. The starch present in the compositehas been catalyzed by amylase enzyme. Chiellini et al. [167]have reported the oxidative degradation of PE film containingpro-oxidant additives. Some strains of the bacteria such asPseudomonas aeruginosa, Pseudomonas fluorescens and fungiPenicillium simplicissimum have been reported as the mostcommonly used organisms for the plastic degradation[168,169]. Sielicki et al. [170] have studied the microbial

degradation of [b-14C] PS and 1,3-diphenylbutane in soiland liquid enrichment cultures, and it has been determinedby 14CO2 evolution from applied [14C] PS. Metabolism of1,3-diphenylbutane appeared to involve the attack by a mono-oxygenease to form 2-phenyl-4-hydroxyphenylbutanefollowed by a further oxidation and subsequent fission of thebenzene ring to yield 4-phenylvaleric acid and an unidentified5-carbon fragment via the classic meta-fission pathway. Thebasal salt medium (water containing trace elements) triggeredauto-oxidation of the pro-oxidant through decomposition oftrace hydroperoxides, which in synergistic combination withbiodegradation of the starch, eventually initiated auto-oxidation of the LDPE matrix [171]. Hadad et al. [172] haveisolated thermophilic bacterium strain of Brevibacillus borste-lensis for the degradation of LDPE and have studied the effectof UV photo-oxidation on the extent of biodegradation of PEwhich has enhanced the extent of biodegradation. Gilmoreet al. [173] have carried out degradation study of six typesof plastics and plastic blends in municipal wastewater. Sam-ples consist of 6% starch in PP, 12% starch in linear LDPE,30% polycaprolactone in LDPE, and poly(-hydroxybutyrate-co-hydroxyvalerate) (PHB/V), a microbially producedpolyester in activated sludge of 5 months and found no signof degradation of blended samples except PHB/V, which hasshowed a considerable loss of mass and a significant loss oftensile strength in municipal wastewater. Biodegradable poly-mers can be synthesized by the modification of natural poly-mers by blending and fermentation [174e180]. Nakajima-Kambe et al. [181] have isolated, purified and characterizedpolyurethane (PU) degrading enzymes from Comamonasacidovorans [182,183]. These enzymes are cell-associatedmembrane bound PU-esterase and an extracellular PU-ester-ase. The membrane bound enzyme provides cell mediatedaccess to the hydrophobic polyurethane and extracellularenzyme stick on surface.

3.6.2. Methods for biodegradationThe most important factor in determination of biodegrada-

tion is the proper selection of test procedure based on the natureof plastic and the climatic conditions of the study environment[184]. There are wide varieties of methods currently availablefor measuring the biodegradability of polymeric materials[185]. Several test methods to assess the potential biodegrad-ability of plastics have been developed by InternationalStandard Organization (ISO) [186] and American Society forTesting and Materials (ASTM) [187]. Biodegradation can becharacterized with loss of weight, change in tensile strength,change in dimensions, change in chemical and physical proper-ties, carbon dioxide production, bacterial activity in soil andchange in molecular weight distribution [188,189].

3.6.2.1. Soil burial method. Soil burial method is one of thefrequently used methods for the determination of biodegrad-ability of plastic [185,190]. In this method, biodegradationtest is performed under natural conditions or laboratory condi-tions. Sample with definite weight and dimension is buried inspecific depth in the soil for different time intervals. After


a specified time, sample is taken out of soil, thoroughly rinsedwith distilled water following immersion in distilled water andafter that dried at 50 �C for 24 h in a vacuum oven. Sample isallowed to equilibrate to ambient temperature and humidityfor at least 24 h before measurement. In one study starcheplastic films buried in a forest soil have developed rapidcolonization (15 days later) by fungal hyphae and showeddegradation of the starch granules on the films [191].

3.6.2.2. Pure culture method. In pure culture method, specificbacteria and fungi can be applied for degradation of polymers.In laboratory conditions, isolated microorganism strain hasbeen allowed for sufficient growth in different nutrient media[160,172,188]. In pure culture method, pre-weighed disin-fected films are aseptically added to sterilized culture mediumand films in culture medium are incubated with shaking for24 h before inoculation to ensure asepsis. Culture medium isinoculated with spores from a specific microorganism and isincubated with shaking at 125 rpm for 4 weeks at optimalgrowth temperature for the selected microorganism. Fourreplicates are prepared for each different pretreated film. Thesample is weighed after washing with 70% ethanol and dryingat 45 �C until equilibrated. Each of the different films is thencompared with the corresponding uncultured material[192,193]. The presence of microbes can be confirmed byusing a microscope [194].

3.6.2.3. Compost method. In this method, the definite weightof the dry plastic is subjected to the mixture of definite amountof mature compost and then incubated at 58 �C with main-tained moisture content at 65%. Biodegradation is measuredbased on the amount of material carbon converted to gaseouscarbon dioxide [195]. Nature of compost affects the degree ofdegradation [196e198]. Unexpected trends have been reportedfor the biodegradation of plastics in the compost stored at�20 �C, 4 �C and 20 �C for different periods. Viable cells inthe compost stored at �20 �C is expected to be fewer thanthose in the compost stored at 4 �C and 20 �C, becausemicrobes may be under stress or even be killed due to the for-mation of ice crystals at the subzero temperature. Mesophilicbacteria and mesophilic actinomycetes [the mesophilic micro-organism live at medium temperatures (20e45 �C)] are fewerin number in the compost stored at 20 �C than in the compoststored at the other two lower temperatures contrary toexpectation. In contrast, both thermophilic bacteria and ther-mophilic actinomycetes [thermophilic microorganism thriveabove 45 �C and some live at or even above the boiling pointof water] are fewest in the compost stored at �20 �C asexpected, indicating that thermophilic microbes are more sus-ceptible to stress in the freezing conditions than the mesophilicones. Activity of the extracellular enzymes plausibly excretedby the microbes in the compost decreased as a result of thestorage [196]. Shape of the plastic sample [185] and additivesin the compost [199] effect the plastic degradation in thecompost.

Vermiculite, a clay mineral, can be activated and used asa solid matrix in place of mature compost. The composting

test method based on activated vermiculite is a comprehensivesystem for the assessment of the environmental impact ofbiodegradable plastics. Activated vermiculite affects neitherthe biodegradation rate nor the final biodegradation level.On the other hand, possible metabolic intermediates and poly-meric residues left after biodegradation can be recovered moreeasily from activated vermiculite than from mature compost,a very complex organic matter. Therefore, at end of the testit has been possible to determine the carbon balance that isby taking into account both the evolved CO2 and a polymericresidue extracted from vermiculite [200].

3.6.2.4. An aerobic degradation in the presence of sewagesludge. Because of enriched environment of sewage sludge,the microbes present are more diverse in composition thanother disposed methods encountered [140]. In laboratoryconditions, samples have been inoculated with sewagemicrobes. Gaseous CO2 and CH4 are monitored by headspaceanalysis using GC/MS [201].

4. Factors affecting polymer degradation

Degradation is defined as a process which leads to a deteri-oration of any physical property of a polymer [17]. In general,the degradation process affects the thermal stability, mechan-ical properties, crystallinity and lamellar thickness distributionand begins in the amorphous/crystalline interface [56]. Degra-dations of plastic affected by various factors are given below:

4.1. Chemical composition

Chemical composition of the polymers plays a very impor-tant role in their degradation. Presence of only long carbonchains in the thermoplastic polyolefins makes these polymersnon-susceptible to degradation by microorganisms. By theincorporation of the heterogroups such as oxygen in polymerchain makes polymers labile for thermal degradation andbiodegradation [20]. Presence of heteroatom in the polymerchain affects the strength of neighboring CeH bonds of thepolymer and promotes carbanion formation in the presenceof bases. Linear saturated polyolefins are resistant to oxidativedegradation. Presence of unsaturation in the polymer chainmakes them susceptible to oxidation, for example natural rub-ber is more susceptible to degradation than PE [3]. Amorphousregions in the polymer have been reported to be more labile tothermal oxidation as compared to crystalline areas because oftheir high permeability to molecular oxygen [17]. Oxidationrates depend on the reactivity of the peroxy radicals formedand on the dissociation energies of available carbonehydrogenbonds in the polymer substrate. A polymer with no hydrogenat all or with unreactive methyl and phenyl groups showsresistance to oxidation [17].

4.2. Molecular weight

Increase in molecular weight of the plastic decreases therate of plastic degradation [202]. It has been reported that


some microorganisms utilize polyolefins with low molecularweight faster as compared to high molecular weight polyole-fins [203,204]. Linear polyolefins with molecular weight lowerthan 620 support microbial growth [205]. It is easy to form theozone adduct in low molecular weight olefins which rear-ranges to ozonide and during this operation, if the polymerchain is under tension during reaction, the broken chain endsseparate out before rearrangement to ozonide occurs [17].

4.3. Hydrophobic character

Petrochemical-based plastic materials are not easilydegraded in the environment because of their hydrophobiccharacter and three-dimensional structure [203]. Hydrophobic-ity of PE interferes with the formation of a microbial bio-film,thus decreases the extent of biodegradation [172].

4.4. Size of the molecules

Size of the molecules in the polymers affects theirmechanical degradation, thermal degradation and biodegrada-tion. These degradations increase as the size of the moleculedecreases [20].

4.5. Introduction of functionality

Introduction of carbonyl groups in polyolefins makes thesepolymers susceptible to photodegradation. As the number ofchromophores increases the rate of photodegradation increasesdue to the more sites, which are available to absorb morephotons and to initiate the reaction for degradation. Thecarbonyl chromophore absorbs near-UV radiation and formradicals by the Norrish Type I, II and H-abstraction processesfor photochemical degradation. Chromophores other thancarbonyl group pC]O such as metalemetal bond if incorpo-rated in polymer backbone induce photodegradability. In sucha case, metalemetal bond cleaves homolytically on irradiation[206]. Polyolefins undergo slow sulfonation and oxidativedegradation by reagents based on concentrated sulfuric acid.In chromic acid, sulfonation results in more rapid oxidativedegradation of PP with a slower attack on PE. Relative ratesof attack on CeH sites are 4600:75:1 for tertiary:secondary:primary (Scheme 19) [207].

To observe the effect of hydrophilic moieties on thedegradations of the polymers, Singh and Sharma [208] havecarried out biodegradation study of pure PS and grafted PS

H2Ochromic acid

80-100ºC

R

H

Cr

CCH2 CH2CH2 CH2

R

O

C

(OH)3

Scheme

that is [PS-g-poly(AAc)], [PS-g-starch] and [PS-g-poly(AAc)-co-starch] films by soil burial method. It has been observedthat 37% degradation has occurred in PS when modifiedwith starch after 160 days and no degradation has been seenin the case of PS grafted with poly(AAc). This is becausestarch is a natural hydrophilic polymer and has more suscep-tibility for bacterial and fungal degradations in soil as ambientenvironment available for their growth. This is possible due toscissoring caused by the microorganism in some chains at thesite of grafting and polymer chains may get breakdown anddecrease the number of carbon atom in the chains [208]. Ithas also been observed by Wool and co-workers that the fol-lowing exposure to soil test conditions, micrographs of thinfilms clearly showed the colonization of microorganismswithin channels of the matrix that has been initially occupiedby starch [155]. Apart from weight loss, change in color of thestarch modified PS films has also been observed. FTIR spectrahave shown that with passage of time, the soil microorganismshave consumed starch portion of the grafted product [208].

4.6. Additives

Non-polymeric impurities (such as residues of polymeriza-tion catalysts, transformation products of additives), fillers orpigments affect the resistance to degradation. Yang et al.[209] have reported that the thermal stability slightly de-creases and the ash content increases when the ligno-cellulosicfiller loading increases in the sample. The dispersion and inter-facial adhesion between the ligno-cellulosic filler and thermo-plastic polymer are the important factors affecting the thermalstability of the composite system [209]. Metals act as goodpro-oxidants in polyolefins making polymer susceptible forthermo-oxidative degradation. For example, manganese (Mn)is a suitable metal participating in metal combination forpro-oxidant activity. Upon activation by heat in the presenceof oxygen, pro-oxidants have produced free radicals on thepolyethylene chain which have undergone oxidation andhave changed the physical properties of the polymers [4]. Inaddition, the pro-oxidant catalyzes the reaction of chain scis-sion in the polymer, producing low molecular mass oxidationproducts, such as eCOOH, eOH, C]O group containingproducts [210]. Traces of transition metals have acceleratedthermal oxidative processes of polyolefins by inducing hydro-peroxide decomposition [21]. For example, TiO2 delustranthas made the polyamides susceptible for heat- and light-induced oxidation [29]. Oxidation of plastic (polyolefin) is

O

O O

OHH

CH2CCH2 CH2CH2

CC

R

OH

C

,

,

19.


influenced by the amount of pro-oxidant additives, chemicalstructure, morphology of plastic sample and the surface area[211]. Chain-breaking (hindered phenol) antioxidants inhibitthe oxo-biodegradation of the polyolefins under thermo-oxidative conditions during service [212]. Photo-stabilizersdecrease the chances of photodegradation of plastic materials.Effective light absorbers such as benzotriazoles, benzophe-nones, and phenyl esters, as well as hindered amine light sta-bilizers are presently used in plastic formulations intended foroutdoor use (usually at a 0.05e2.0 wt.% level) [213]. Manytransition metal ions are effective accelerators for peroxidationand hence for the biodegradation of the hydrocarbon poly-mers. Since the sequence of reactions shown in Scheme 20leads to the rapid buildup of hydroperoxides in the polymer,peroxidation cannot be controlled by chain-breaking antioxi-dants such as the hindered phenols since the latter are rapidlydestroyed by hydroxyl and peroxyl radicals (Scheme 20) [212].

As the antioxidants and the stabilizers make polyolefins re-sistant to degradation, the use of pro-oxidant additives cantherefore make them oxo-biodegradable by making polymerhydrophilic and also catalyze breakdown of high molecularweight polyolefin to lower molecular weight product [213].Susceptibility towards biodegradation of polyolefins enhancesby blending with starch or other polyesters and biodegradationdepends upon the type of polymer and the blend composition[214]. Along with this filler, pro-oxidants accelerate theprocess of biodegradation. Averous and co-workers [215]have studied the effect of ligno-cellulosic fractions from wheatstraw as natural fillers in composites of a polyolefin (a copol-ymer of PE and PP) and a biodegradable polyester [poly(buty-lenes adipate-co-terephthalate)] [216,217]. It has beenobserved from the study that amount of natural composites af-fects the degradation of the composites. In another case graft-ing of the wheat starch had affected the tensile strength of thecomposites [218].

4.7. Chemical bonding

Linkage affects the degree of degradation in plastic. Inthermoplastic, head-to-head addition of monomer units andtail-to-tail addition of monomer units during addition polymer-ization create weak points which make the plastic susceptiblefor degradation. Head-to-head linkage in PMMA enhances

Iron Catalysed Peroxidationof Hydrocarbon Polymers

PH

Fe2+

Fe2+

Fe3+

Fe3+

O2

+

+ +

++

+

PHPH

POO

POO

POOH

POOHPOOH

POOH

P

P

OHPO

PO POH

+

+ +

+

H+

/

O2 PH POOH/

Scheme 20.

thermal degradation of the polymer [219]. Branching in poly-mer chain increases thermal degradation [3,20]. Crosslinkingdecreases the rate of photodegradation by locking the polymerstructure and preventing lamellar unfolding because theseactions prevent the separation of photo-produced radicals andfavor the radicaleradical combination [41].

4.8. Methods of synthesis

Methods of synthesis show the noteworthy effect on thestability of the polymers [220]. For example, anionic polymer-ized PS showed more photo-stability than free radicallyformed polymer due to the presence of peroxide residue inthe latter, which is labile for photodegradation [221]. PPsynthesized by bulk polymerization and by ZieglereNattacatalyst is more susceptible towards photodegradation ascompared to copolymerized PP [222].

4.9. Effect of substituents

Substituents effect the degradation processes in manypolymers containing labile a-hydrogen in the repeating unitsand modify the reaction course profoundly even if the main-chain scission reactions prevail. For ethylene and 1-substitutedethylene, chain-end scission reactions are predominant,producing either little or up to 50% monomer as volatiles. In1,1-Disubstituted ethylene, repeated units apparently favored100% monomer yield, whereas chlorine as substituents did notfavor chain end degradation route [18]. Thermal stability ofa polymer decreases as the number of substituents on polymerbackbone increases. Phenyl group in styrene unit of PS makesCeC backbone labile for thermal degradation. Based on thesame fact PE shows more thermal stability as compared toPP and polyisobutylene. The effect of substituent groups onthe stability of the backbone CeC bond is apparent from thecomparison of the bond dissociation energies for CeC bond(in kcal/mol), CH3eCH3¼ 88, CH3CH2CH3¼ 85, (CH3)3

CCH3¼ 80 and C6H5CH2CH3¼ 70. In PVC, the presence ofallylic positioned chlorine atom favored dehydrochlorinationof the polymeric chain as double bond activated the releaseof chlorine radical as shown in Scheme 21.

The elimination of the first molecule of HCl and conse-quent formation of a double bond on PVC chain subsequentlyactivates the neighboring chlorine atom which has assumedthe nature of an allylic chlorine, causing elimination ofanother HCl molecule, with the process continuing to repeat

Cl

CH2 CH2

CH2CH2

CH2 HClCHCl

CHCl

CH

CHCHCl

CH

CHCl

CH CHClCHCHCl

+

+

CHCHCl

Scheme 21.


itself finally producing a conjugated polyene structure by anautocatalytic effect [18,29]. Contrary to this, all substituentsnot always reduce thermal stability of the polymeric systems.For example, the presence of electronegative groups asfluorine in teflon increases the thermal stability of the polymerdue to high dissociation energy (108 kcal/mol) of CeF bond[3,20]. Aromatic groups in a polymeric backbone increasethe thermal stability.

4.10. Effect of stress

Stress has significant effect on polymer degradation.Tensile stress increases the rate of photodegradation whereascompressive stress retards the photodegradation rate. Stresschanges the quantum yields of the reactions that lead tobond photolysis, the efficiency of radical recombination fol-lowing homolysis, and changes the rate of the radical trappingreaction (Scheme 22) [41,220].

The polymer morphology is one of the key factors tointerpret the effect of stress on degradation rates. Higher stresscauses significant morphological changes, including thestraightening of the polymer chains in the amorphous regions.When stretched bonds are cleaved by light, the probability ofradical recombination decreases as compared to non-stressedpolymer because entropic relaxation of the chain drives theradicals apart and prevents their efficient recombination be-cause of their increased separation [41]. Stress induces a chainorientation that is more favorable for degradation, apparentlydue to chain conformation that becomes more susceptible tohydrogen abstraction [223]. Stress also effects the thermo-oxidative degradation [32]. Degradation of stressed specimensoccurred with considerably higher rate as compared to theunstressed state and is marked by a decrease in apparentactivation energy as well as by an increased rate of crackdevelopment. The cracks developed in the direction perpendic-ular to the tensile stress and have the shape of hollow funnelswidening outward. An accelerated embrittlement of the poly-mer occurred due to the tensile stressing of stabilized types ofPP in thermo-oxidative and photo-oxidative environments,manifested particularly by a marked decrease of elongationat break [32].

4.11. Environmental conditions

Biodegradation of polymer depends upon environmentalconditions such as moisture, temperature, oxygen, and suitablepopulation of microorganisms [4]. In warm climates when therelative humidity exceeds above 70%, the rate of polymerdegradation by the microorganisms increases [17]. High

MM

k recombination

Mhomolysis

Scheme

temperature and high humidity enhance hydrolytic degrada-tion of the polymers. Temperature of the material and thepresence of moisture show a considerable synergistic effecton the photodegradation of the polymeric materials [11].Moisture promotes the light-induced degradation due to thefact that under high humidity soluble photo-stabilizers mightleach out of the plastic matrix, reducing the effectiveness ofthe light stabilizer and leading to degradation. The presenceof high humidity, particularly at the higher temperatures tendsto increase the photo-damage in polymers such as thermoplas-tic polyester elastomers [39]. Even a small increase in solarUV level dramatically accelerates the deterioration processesin plastics at high temperature [224]. It is the synergistic effectof high temperature and solar UV radiation that is responsiblefor the rapid degradation of the polyethylene films under theseconditions [224]. Weathering is a degradation process and assuch is temperature dependent, i.e. it will occur more rapidlyat higher temperatures. The general rule is that for every 10 �Cincrease in temperature the reaction rate will double. Tropicalareas therefore suffer not only from an increase in UVexposure but also faster reaction rates because of the increasedtemperatures. Humidity also affects the degradation processes;most weathering processes are considerably slower in hot dryclimates than in hot wet climates [30].

Oxygen affects the mechanochemical degradation of rubberat ambient temperature. For instance, degradation has beenfound to be almost absent when rubber is masticated in an at-mosphere of nitrogen. However, when the process has beenrepeated in the presence of a small amount of oxygen or air,degradation has been observed very quick and significant.This is because during degradation free radicals are producedat the end of the scissor fragments of the chain. In nitrogen,however, the primary radicals formed under shear immediatelyrecombine to give no effective chain breakage. Oxygen due toits radical scavenger nature readily reacts with the free radicalsformed at the fragmented ends of the polymer molecule andrenders the chain break permanent [65]. Thermal dehydrochlo-rination of PVC in the presence of oxygen occurs faster thanthe inert atmosphere [213]. During processing, carbon radicalsare formed from chain scission because of strong shear forces.These radicals then react with oxygen to form peroxy radicals,and finally, hydroperoxides [225].

5. Plastic materials: future perspectives,challenges and other issues

Various types of plastics may be of use in packages,biomedical products, electronic components, non-returnablegoods, construction materials, etc. However, an important

M MX = trap

2 X

22.


aspect of these materials for a sustainable society is to predicttheir lifespan. How a material affects its environment is alwaysimportant [226,227]. Sustainability, industrial ecology, eco-efficiency, and green chemistry are the new principles thatare guiding the development of the next generation of plasticproducts and processes. Ecofriendly, biodegradable polymersare the major breakthrough in plastic and polymer industries.Bio-based economy is challenging to agriculture, forestry,academic, society, and industry to develop and commercializeagricultural based new materials. A grand challenge facinggovernment, industry, and academia in the relationship ofour technological society to the environment is reinventingthe use of materials. To address this challenge, collaborationfrom an interdisciplinary group of stakeholders will be neces-sary. An emerging approach to this grand challenge seeks toembed the diverse set of environmental perspectives and inter-ests in the everyday practice of the people most responsible forusing and creating new materials chemists [212,228e230].

5.1. Sustainable polymers

Polymer material has brought not only tremendous wealthfor human being, but also unfortunately the serious environ-mental problems. We must solve these problems throughsustainable development which can be carried out in twoways: (1) recycling the waste plastic and (2) developing envi-ronmental friendly polymer materials [231]. The term‘‘sustainable’’ is often assumed to be synonymous with ‘‘re-newable’’. Sustainable polymers also have to be industriallyacceptable and biodegradable. The ideal biopolymer is ofrenewable biological origin and biodegradable at the end ofits life. Biopolymers include polysaccharides such as celluloseand starch, carbohydrate polymers produced by bacteria andfungi, protein polymer derived from soy protein and animalprotein based biopolymers such as wool, silk, gelatin, andcollagen. On the other hand, polylactic acid (PLA), PVA,and poly(butylenes succinate) (PBS) are examples of polymersthat have synthetic origin but are also biodegradable. Theextremely high price of petroleum and the growing environ-mental problems are some of the driving forces in lookingtowards renewable source based plastics to meet thechallenges [212,232,233].

5.2. Plastic recycling

Recycling is an expanding area of the chemical industry ingeneral, plastic recycling and waste management in particularbeing closely examined in academia, commercial organiza-tions and governmental institutions. Finding suitable solutionscan lead to the recycling of potentially hazardous materials onthe one hand and production of acceptable commercial prod-ucts with little or no outlay in raw materials on the other.The recycling of post-consumer plastics is of increasingimportance as land filling and incineration becomes moreexpensive and environmentally risky. In addition to reducingthe amount of plastic waste requiring disposal, recycling plas-tic can have several other advantages such as conservation of

non-renewable fossil fuels, reducing energy consumption,reducing carbon dioxide emissions, and many others [234].A fairly new utilization of plastic waste is conversion ofpost-consumer plastic waste into oil by two methods that is di-rect liquefaction and pyrolysis followed by hydroprocessing ofthe pyrolysis liquids [235]. Post-consumer waste plastics canbe used in cement-based composite. Also, mechanochemicaltreatment can be used to develop composite plastic materialsfrom post-consumer plastic waste [236]. Recycling plasticusually involves processes such as melting, shredding orgranulation of waste plastics. An option with great potentialis feedstock recycling, which includes a variety of processeslike pyrolysis, catalytic conversion, depolymerization and gas-ification, designed to transform plastic waste into hydrocarbonproducts for use in the preparation of recycled polymers,refined chemicals or fuels. Despite not being used extensively,feedstock recycling is attracting increasing scientific andcommercial attention as an alternative with the potential toabsorb very large amounts of plastic wastes. The applicationof catalysts is giving feedstock recycling a new impulse thatis expected to put this technology at the forefront of plasticmanagement [237e239].

5.3. Biodegradable plastics: as an alternativeto the existing petrochemical-based polymers

Advanced technology in petrochemical polymers hasbrought many benefits to mankind. Synthetic- and petro-leum-based polymers are extremely stable and are commonlyused in agriculture, food packaging, fast-food restaurants, andon military and commercial ships. However, their attractivestability is counterbalanced by two problems: polymerscontribute to the demand for expensive imported oil and theirgreat resistance to biodegradation. Petroleum reserves will beexhausted in less than a century. It is therefore necessary tofind another raw material for the fuel area, but also for plasticindustry. The shortage and high cost of fossil resources requirethat alternative resources and processes must take over in thenear future. Biodegradable polymers are now being consideredas an alternative to the existing petrochemical-based polymers[175,240].

New applications based on synthetic polyester-based biode-gradable polymers have been commercialized, especially inthe packaging industry, paper coating and garbage bags. Thesepolyesters are made using modified polyethylene polymeriza-tion processes and as blends. A second class of biodegradablepolyesters includes polylactic acid (PLA) and thermoplasticstarch-based polymers. These polyesters environmentallydegrade into water and carbon dioxide when exposed to micro-organisms and water in composting piles. A third type ofbiodegradable plastics includes aliphatic co-polyesters suchas polyhydroxyalkanoates or PHAs produced by bacteria.Potato, corn and wheat starch are renewable raw materialsfor starch-based plastics [241]. Fully biodegradable syntheticpolymers are commercially available since 1990. Among thesebiopolymers, PLA was extensively studied in medicalimplants, suture, and drug delivery systems since 1980s due


to its biodegradability. PLA has been attractive for disposableand biodegradable plastic substitutes due to its better mechan-ical properties. However, PLA is still more expensive thanconventional plastics. Also, the degradation rate is still slowas compared to the waste accumulation rate. The newlydeveloped plastics from this research could provide possiblesolutions to the problem. Also, the plastics from this proposedresearch could be used as biomedical and pesticide slow-releasing carrying matrix. The industrial-scale production ofPLA from lactic acid generated by fermentation now providesa renewable resources-based polyester as a commodity plasticfor the first time [242e246].

Starch may offer a substitute for petroleum-based plastics.Starch is a renewable, degradable carbohydrate biopolymerthat can be purified from various sources by environmentallysound processes. It is a relatively low-cost polymer availablefrom agricultural surplus with potential thermoplasticity.However, it needs water for processing and the water contentof the product influences its stability. Another widely availablecarbohydrate material is cellulose, which is biodegradable, butcannot be thermally processed, because it decomposes beforemelting. Chemical modifications can improve thermoplasticitybut decreases biodegradability. While numerous attempts havebeen made using starch and a few other carbohydrates ascomponents in various formulations for biodegradable plastics,generally the major technical problem is the need to strikea balance between stability and moldability. Water contentand molecular weight are two keys to this balance. The finalproducts should be biodegradable after fulfilling the require-ments of their use, however, they must be stable during the ap-plication. The stability requires a low water content andresistance to water. On the other hand, most agriculturalpolymers use water as a plasticizer during molding. Highmolecular weight provides better stability and physicalstrength but decreases the moldability [2,240,241,247,248].Synthetic hydrophobic biodegradable polymers are availablesince 1990. However, these synthetic polymers are usuallymore expensive than petroleum polymers and also havea slow degradability. Blending starch with these degradablesynthetic polymers has recently become the focus ofresearchers [2,56]. This will increase the cost and decreasethe mechanical strength properties of the blends. Potato,corn and wheat starch are renewable raw materials forstarch-based plastics [161e163,249].

5.4. Biodegradable plastics: limitations andobstacles for use

While a number of alternatives have emerged, one of themain limitations for synthetic biodegradable polymers suchas polyesters and starch-based blends is higher cost comparedto synthetic polymers. Another serious obstacle is a lack ofsuitable infrastructure for sorting, recycling and compostingsolid wastes. Development of process technology for com-pletely biodegradable polymers is generally costly. Eitherthe cost of monomers is prohibitive for its commercializationor the polymerization technology is not yet competitive with

the conventional processes. Further research and developmentefforts are necessary to make the biodegradable polymers costeffective. Another drawback in the development of completelybiodegradable polymers is the limited availability of mono-mers. Processability, performance limitations, cost, and rateof degradation present formidable obstacles to widespreadadoption. Bio-based polymers such as cellulose and starchare not so conducive to mechanical recycling, and they alsohave a lower calorific value on incineration [228,176,211,250e252].

5.5. Future directions

Any decision to use a degradable polymer should be basedon a good understanding of where and how the product willdegrade, recognize and minimize life cycle environmentalimpacts (not just end of life), and deliver real commercialbenefit [228,252].

6. Conclusion

It is concluded from the foregone discussion thatunderstanding about the mechanism can go a long way inhelping the researchers and the technologists to induce thedifferent types of degradation in the plastic. These degrada-tions can further be enhanced by the addition of the additivesin the plastic and by understanding the various factors respon-sible for these degradations. It is also concluded from thisdiscussion that plastic degradation could be enhanced by itsmodification with natural polymers and this acquaintancecan further be exploited for environmental waste management.

Acknowledgement

This article is prepared from the literature collected for theproject sponsored by the Ministry of Environment and Forest.The authors wish to thank the Ministry of Environment andForest, Government of India, for providing the financialassistance for the project.

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