Poly lactic Acid Biodegradation
- 1. Seminar Group A Subgroup 8 Submitted to: Dr Sadia Mehmood Submitted by: 1) Arslan Ahmed 2) Asmaa Riaz 3) Sabahat Ali Topic: How to enhance biodegradation of PLA Modifications in PLA physical properties and amendment of the environment with various factors like stimulants will be critically analyzed and discussed.
- 2. Arslan Ahmed (16-Arid-2558) Biodegradation of Polylactic acid (PLA) Fromphysical angle,polymerdegradationisdividedintoheterogeneous And homogeneous degradation, which also called surface and intramolecular degradation of polymer. Polymerdegradationcanexistinthree differentwaysfromchemical angle: (A) scissionof the mainchains, (b) scissionof the side chains (c) Scissionof the intersectional chains(Amassetal.,1998; Müller, 2005). PLA degradationoccursmainlythroughscissionof ester bonds. Polymer degradation is induced by a range of factors in nature, such as oxidation, photo degradation, hemolysis, hydrolysis, biodegradation, or enzymolysis (Nampoothiri et al., 2010). PLA not only has complete biodegradability,butalsonot polluteenvironmentafterbiodegradation. 1 PLA Biodegradation Processes Biochemical processes of PLA degradation mainly include chemical hydrolysis and biodegradation in the natural soil microcosm. The ester bonds of PLA fragments into carboxylic acid and alcohol by chemical hydrolysis due to hydrion, but the progress of complete hydrolysis has to consume plenty of energy and time. Therefore, analysis of biochemical processes of PLA biodegradationisakeyfactor forexploringthe high efficientmethodsof PLA biodegradation. PLA-degrading microorganisms first excrete extracellular depolymerize of PLA. The rapid production of extracellular depolymerize need usually to be stimulated by some inducers such as silk fibroin, elastin, gelatin,and X.Qi etal. / International Biodeterioration&Biodegradation117 (2017) 215e223
- 3. Fig. 1. Synthesis of PLA. (a) Three stereo chemical forms of PLA. (b) Direct and ring-opening polymerizationmethodof PLA. Some peptides and amino acids (Jarret et al., 2004; Tonkawa and Calabria, 2006; Thanasi et al., 2015). Most of the inducers have L-alanine units, which is similar to L-lactic acid units of PLA in the stereL- alanineunits have same stereochemistry as L-lactic acid of PLA. The depolymerize attacks on intramolecular ester bonds of PLA and converts it into the oligomers, dimmers and monomers. The low molecular weight compounds get entered into the microbial membranes and are decomposed into corbondioxide, water or methane by intracellular enzymes (Go, 2003; Lips et al., 2016; Maharani et al., 2009; Tonkawaand Jarrett,2004). Required components: Two components, namely the appropriate microbes and a favorable environment, are required for the successful biodegradation of contaminants inwater or in a terrestrial environment.The
- 4. Degradation rate of PLA also depends on environmental factors, such as PH, humidity, temperature, oxygen and so on (Park and Xanthus, 2009). Therefore, the future studies on biochemical processes of PLA degradationshouldconsidermore environmental factors. Limitations of PLA Biodegradation Processes 1. Despite the fact that PLA is biodegradable, PLA takes several years to completely decomposed (Kimura et al., 2000). 2. Proper environmental factors such as ph, humidity, temperature and oxygen are necessary for PLA biodegradation (Park and Xanthus, 2009). 3. According to the soil burial experiments PLA material takes a long time to start decomposition and biodegradation process is very slow (Chita and Lee, 2006). 2 PLA Biodegradation by Microbes Integrated process coupling physicochemical and microbial degradation. However, PLA materials are less susceptible to microbial attack in the natural environment compared to otherdegraders of biodegradablepolyesters(TonkawaandCalabria,2006). Soil burial experiments show that PLA materials will take a long time for degradation to start and the degradation rate is quite slow (Chita and Lee, 2006). Therefore, the researches on pure isolation of PLA- degradingmicroorganismshave beenraisedinrecentyears. Currently, multiple types of microbes that are able to degrade PLA have been isolated from soil or water.But theymostlyare actinomycetes, afractionof thembelongtobacteriaandfungus. 2.1 Actinomycetes Actinomycetes are filamentous bacteria, mainly found in the soil, that are well-known for being antibiotic producers. Some actinomycetes efficiently degrade polyesters, including PLA, PCL, PHB and poly (ethylene succinct) (Hoang et al., 2007; Tseng et al., 2007). The first reported PLA degrader was Amycolatopsis HT-32, which was successfully isolated from soil by applying the plate count and clear- zone methods.
- 5. Fig.2. Schematicdiagramof biochemical processesinPLA degradation. In some researches, PLA degraders, Amycolatopsis strain No. 3118 and K104-1, also was obtained from various environment by selection and cultivation (Kura and Kudos, 1999; Nakamura et al., 2001). But it is quite difficult to isolate PLA-degrading microorganisms in these reports due to the lack of the related biotechnology. According to Jarret et al. (2002), analyzing 41 genera (105 strains) of actinomycetes based on 16SrRNA sequences, PLA degraders are found to be limited to members of the family Pseudonocardiaceae and related genera. They include Amycolatopsis, Saccharothrix, Lentzea, Kibdelosporangium, and Streptoalloteichus (Jarret et al., 2002), which plays a key role in microbial degradation of PLA. After that, with the development of the related molecular biological techniques, some researchers had found vast PLAdegradingactinomycetes(Table 1). 2.2 Bacterial PLA Biodegradation
- 6. Compared to actinomycetes, the bacterial degradation of PLA was reported rarely in the literature. As summarized by Jeon et al. and Penkhrue et al. (Jean and Kim, 2013; Penkhrue et al., 2015), the PLA- degrading bacteriainclude Bacillus, Pseudomonas, Stenotrophomonas, etc. Some studies show that PLA- degrading bacteria are not distributed widely in the natural environment. The first reported PLA- degrading bacteria was Bacillus brevis. After that, some PLA-degrading bacteria were successfully isolatedusinganenrichmentculture medium (Table1). 2.3 Fungal PLA Biodegradation Fungal degradation of PLLA had also been studied by using Tritirachium album (ATCC 22563) in a liquid culture (Jarrett and Tonkawa, 2001). It shows that most of PLA film is degraded after 14 days of cultivationbythe additionof gelatin. Recently, some researchers have explored the fungal degradation of PLA in soil and compost (Karamanlioglu et al., 2014; Saudi et al., 2012). It shows that temperature is a key parameter governing the fungal degradationof PLA. In addition, the biodegradation of PLA and some plasticized PLA with Trichoderma varied fungus, which have beenassessedinliquidmediumand controlledlaboratoryconditions(Lipsaetal.,2016). Limitations 1: fungal plabiodegradationistemperaturedependent. 2: Most of studies on PLA biodegradation were just conducted to find some new microorganisms in the lastdecades,andmainly focusonlaboratoryconditions 3 Enzymetic Biodegradation of PLA PLA-degrading enzymes from microorganisms play a vital role in biodegradation of PLA. Proteases: PLA-degrading activity of 56 commercially available proteases was tested by Ode et al. (2000). It shows that acid and neutral proteases have a little or no activity but some alkaline proteases, whichformappreciable numbersof lacticacidfrom PLA. Serine proteases as the main member of PLA-degrading proteases (such as proteinase K, a- chymotrypsin, subtilisin, trypsin and elastase) show vast potential for the future exploration of PLA degradation(Limetal.,2005). Serine (Ser) residueplayakeyrole in buildingof the active site. From the analysis of catalytic mechanism of proteases K against PLLA, the catalytic process takes place in fourstages:(a) substrate binding,(b) nucleophilicattack,(c) protonation,(d) esterhydrolysis.
- 7. 1. Serine residue is activated by the nearby serine residues chain, afterwards the activated carbonyl reactswithesterbondby the nucleophiliccatalysis. 2. Scission of ester, the carbon from PLLA_ is esterified by a serine, and the O-terminus of the cleavedPlanwill become the Free State. 3. In a final reaction, water comes in to the reaction, and it replaces the O-terminus of the cleaved Plan. This forms the second product lactic acid (LA) with a normal carboxyl group, and regenerates the serine hydroxyl. The second LA then dissociates from the enzyme to allow anothercatalyticcycle to begin (Edstrom, 2002; Kawai etal., 2011). Challenges: 1: Commercial proteases can't be applied to PLA plastics of real environment due to the limited conditions 2: The maximum activity of these enzymes depends on the optimum pH, temperature and PLA property (chainstereochemistryandmaterial crystallinity). Table 1: The main microorganisms and enzymes capable of PLA degradation as reported in the literature. Strains Substrate Sample source Type of enzyme pH g T(○C) h Reference Actinomycete Amycolatopsis strain HT-32, No.3118, KT-s-9 PLLA Soil Protease – – (Kawai, 2010) Amycolatopsis strain K104-1 PLLA Soil Protease 9.5 55-60 (Nakamura et al., 2001) Amycolatopsis strain 41 PLLA Soil Protease 6.0 37-45 (Pranamuda et al., 2001) Amycolatopsis orientalism PLLA IFO12362 a Protease 7.0 30 (Jarret) Amycolatopsis thailandensis PLA07 PLLA Soil Protease – – (Chomchoei et al., 2011) Amycolatopsis strain SCM_MK2-4 PLLA Soil Protease – – (Penkhrue et al., 2015) Saccharothrix waywayandensis PLLA JCM 9114 b Protease – – (Jarret) Kibdelosporangiu m radium PLLA JCM 7912 b Protease – – (Jarret) Actinomadura strain T16-1 PLLA Soil Protease 10.0 70 (Sukkhum et al., 2009) Lacey Ella saccharin LP175 PLLA Soil Protease 9.0 60 (Hanphakphoom et al., 2014) Pseudonocardia alni AS4.1531(T) PLLA Soil Protease – – (Konkit et al., 2012) Pseudonocardia sp. RM423 PLLA KU c Protease – – (Apinya et al., 2015) Bacteria Bacillus brevis PLLA Soil – – – (Tomita et al., 1999) Bacillus stearothermophilu s PDLA Soil – – – (Tomita et al., 2003) Bacillus smithii strain PL21 PLLA Garbage Esterase 5.5 60 (Sakai et al., 2001) Bacillus licheniformis PLLA Compost – – – (Arena et al., 2011) Paenibacillus amylolyticus strain TB- 13 PDLLA Soil Lipase 10.0 50 (Akutsu-Shigeno et al., 2003) Alcaligenes sp. PLLA MS d Lipase 8.5 55 (Hoshino and Isonomy, 2002) Geobacillu s thermocatenulatus PLLA Soil – – – (Tomita et al., 2004) Pseudomonas sp. strain DS04-T PLLA Activated sludge Lipase 8.0 50 (Wang et al., 2011) Stenotrophomonas maltophilia LB 2-3 PLLA Soil – – – (Jeon and Kim, 2013) Pseudomonas tamsuii TKU015 PLA Soil Lipase 10 60 (Liang et al., 2016) Alcanivorax borkumensis ABO2449 PDLLA – Esterase 9.5e10 30-37 (Hajighasemi et al., 2016) Rhodopseudomonas palestras RPA1511 PDLLA – Esterase 9.5e10 55-60 (Hajighasemi et al., 2016) Fungus Tritirachium album ATCC 22563 PLLA SCC e Protease 8.6 37 (Jarret) Cryptococcus sp. strain S-2 PDLA Wastewater Cutinize 7.0 37 (Masaki et al., 2005) Aspergillus orate RIB40 PDLLA Fermentation Cutinize – – (Maeda et al., 2005)
- 8. Trichoderma varied PLLA SIHB f – – – (Lipsa et al., 2016) 4. Note: a) Institute for Fermentation, Osaka; b) Japan Collection of Microorga nisms; c) Kasetsart University; d) Meito Sangyo (Tokyo, Japan); e ) Sigma Chemical Co.; f) Scientific Institute of Health Belgium; g),h) the optimum power of hydrogen and temperature of enzyme activity; “–” Unknown. Limitation: They mainly work at moderate temperatures and a neutral pH level and they can increase reaction rates by 108 to 1020 times. (Haider et al., 2019). 4 The simulated system – challenges Currently, plastic waste biodegradation has been advanced via methane oxidation in the simulated lysimeters (Mentee et al., 2015, 2016). Studies were conducted to establish the simulated system of PLA biodegradation due to the lack of available information on process parameters. How to establish the simulated system? Synthetic microbial communities were establishedinrecentyears(Großkopf andSoyer,2014). 1. As PLA degrading microorganisms are aerobic so simulated system should also in aerobic mode. In orderto accelerate degradationrate of PLA,variousfactors shouldbe consideredfully. 2. In addition to the direct power source, the oxygen supply of soil microbial communities may be considered inthe simulatedsystem. 3. Study on the PLA biodegradation in aerobic compost had been conducted (Karamanlioglu and Robson, 2013; Sikorska et al., 2015). Nutritional stress induces exchange of cell material such as metabolites (Benicar et al., 2015). The interaction with other species not only allows microorganisms to occupy environmental niches, but also can show different properties and functions. 4. Therefore, if the simulated system of aerobic biodegradation of PLA is established, it will provide more informationforexploringthe stresseffectsand acceleratingPLA biodegradation. Limitation It’s very difficult to establish the simulated system of aerobic biodegradation due to lack of proper information and technology. WAYS TO ENHANCE DEGRADATION OF PLA PHOTOGRAFTING:
- 9. Figure:Reaction scheme for photoinducedgraft polymerizationofacrylic acid onto a polymerfilm surface Photopolymerizationof poly(acrylicacid) andpoly(acrylamide) toPLA filmsurface usinga “nonspecific” plasmatreatmentora specificphotoinitiator,viz.,benzophenone.Duringplasmatreatment,carboxyl and hydroxyl groupsare producedonthe surface of PLA and esterbond iscleaveduponexposuretothe air,whichresultsinfree radicals.Inbenzophenonegrafting,benzophenone abstractsHfromthe substrate togenerate free surface radicalsandsemipinacol radicals,whichcombinetoform surface initiatorsin the absence of monomersolutionsinstep1.While inthe secondstep,the monomersare graftedto the substrate bya livingpolymerizationinitiatedbythe surface photoinitiator.Thenanalyzing the graftedfilmsby ATR-FTIR spectroscopy andcontact angle goniometry indicatedthatthe “optimum” UV irradiationtime requiredforthe PAA graftingstepwas 3 h whenplasma activation wasusedas step 1 and 2 h whenbenzophenone graftingwasusedasstep1. Similarly,The “optimum”time forthe PAAmgraftingstepwas 5 h whenplasma activation wasusedas step1 and was 3 h whenbenzophenonegraftingwasusedasstep1. The Set Of Parameters forthese ‘optimum’timesare: Energysetting(power) onplasmatreatmentunit(~18W),plasmatreatmenttime (2min),vessel pressure inplasmatreatmentunit( ~0.3- 0.5 Torr), conc.Of benzophenone solutionusedfordipcoating ( 5% w/winethanol),UV exposure time requiredforbenzophenone graftingstep( 10 min),conc.Of monomersolution( 10%v/v) & solventformonomergraftingstep(ethanol).Changinganyof parameterschanges UVexposure time requiredforPAA orPAAmgrafting& the amount of PAA or PAAmgraftedonPLA film. Here mild conditions(lowpowerUV lamps(100 W) at roomtemperature ) are usedas comparedto the literature where highergraftingtemperature>40 C or highpowerUV lamps(800-2000 W) were used. However,these conditionsare notwell forPLA surface graftingbecause PLA degradesathigh temperature &underhighpowerUV radiation.
- 10. Basedon ATR-FTIRresults,the PAA andPAAm graftingdensityobtainedusingbenzophenone graftingwashigherthanthat obtainedusingplasmaactivation.Whenincubatedinvarious buffer solutionsat differentpH(4,7, 10), the PLA-g-PAA filmsshowed fasterdegradationcomparedtoneat, unmodifiedPLA andPLA-g-PAAmfilms.The fasterrate of degradationisattributedto entangledPAA chains resultingfromthe acrylicacidmonomermigratedintothe film bulkandnotto the surface- graftedlayers.These monomersdisperse intothe PLA andenhance waterabsorptionwhichincreasesits degradation (Janorkar,Mettersetal.2004). The major objective of thisresearchwasto modifyPLA filmsurfacesbygraftinghydrophilic polymersfromthe surface withanultimate aimof makingthe surface bioactive while atthe same time enhancingthe degradationrate of the bulkpolymer (Janorkar,Mettersetal.2004). Advantages: By increasingthe temperature,degradationof pure PLA occursbut onlyat the surface.But by grafting,degradationoccursfrombulk. Low cost of operation. Mildreactionconditions. Selectivityof UV lightabsorptionwithoutaffectingthe bulkpolymer. Permanentalterationof the surface chemistry. LIMITATION: The surface propertiesof PLA are not easilyalteredbecause itisahighlycrystalline polyesterwithnochemicallymodifiable side-chaingroups,limitingitsuse inmanyapplications(Janorkar et al.,2004). GAP: The polymercanbecome brittle byUV irradiation. Most of these studiesare conductedinthe labandnot appliedtoanynatural environment.The conditionsdescribedinthe literature are hardto achieve. PURPOSEOF GRAFTING: Graftingor blendsof PLA are made to increase itsbiodegradation.Itsdegradationoccursundernormal conditionsbutisveryslowandrequires a hightemperature andpH,& thisdegradationoccursonlyat the surface.So,by makingblends,we modifythe surface of PLA byattachingdifferentmonomersor moleculestothe surface of PLA,whichthendisperse intoitsbulk,where theyincrease water absorptionbythe copolymer&therefore increase itsbiodegradation.
- 11. CHITOSAN/PLA COMPOSITES As alkalescence of Cscan effectivelyneutralize the acidicproductof PLA,the PLA/chitosan composite are more biocompatiblewiththeircomplementary characteristics. Advantage: The acidic degradation products generatedbyPLA can be neutralizedbyCs,while the mechanical properties (especiallythe brittlenessof the highcrystallinity) of Cscanbe improvedbythe presence of PLA. Limitations: 1. An effective blending isstill achallengeinthe followingtworespects: i. First,due to a highglasstransitiontemperature,Csstartto decompose before melting. ii.Moreover,co-solventthatisgoodforboth Cs andPLA has notyet beenfound. 2. Cs can onlybe dissolvedinafewtypesof dilute acidicaqueoussolutions,while PLAcantypicallybe dissolvedinorganicsolvents.Manyeffortshave beenmade toblendPLA withCs,howeveronlyafewof these have beensuccessful. Due to these difficultiesin suchsynthesis,some modificationsare made toincorporate two materialsintwodifferentforms(one innanoparticle formandthe otherinthe polymermatrix).Some chemical reactions have beenproposedasa suitable solutionforthisproblem, e.g.,different PLA grafting techniques withchitosan,includingadirectgraftingmethod(DG) anda ring-opening polymerizationmethod(ROP). Advantages of DG:The copolymerobtainedby DG ismore thermostable thanthe one obtainedbythe ROP method.Economically, directgraftingismore suitable fora large industrial scale becauseD,L-lactic acid isfar lessexpensivethanL-lactide. Drawback of Grafting: Unfortunately, the graftedchitosanis notsolubleinthe organicsolventsof PLA orin dilute acid,which isa goodsolventof pure chitosan. Zhang and Cui investigatedhowgraftinglacticacid(LC) ontothe aminogroupsinCs (PCLA) affected enzymaticdegradationandhydrolyticdegradationinabuffersolution. Advantages of Grafting:The degradationof graftedcompoundsof PCLA was faster and more significant than those of pure Cs and PLA.All of the PCLA copolymersamplesquickly lostmassbecause of the hydrolysisof the graftedlacticacid.Hydrolyzed lacticacidpromotedthe degradationofCs,whichalso ledto an increase inthe pH of PCLA.Meanwhile,the masslossratio became faster.However, PLA showedverylittle masslossorpHchange,whichmay be linkedtoitshighermolecularweight(as comparedto the graftedportioninPCLA compounds) (Elsawy,Kimetal.2017).
- 12. GAP: MixingwithCshas beenshowntoinfluencethe degradationof PLA.However,the mutual miscibilityof CsandPLA still needtobe improvedtoobtainefficientPLA/Csblends.PlacingaPLA filmin a basic solutionatan elevatedtemperature couldcatalyze the degradationof the filmduringthe modificationprocessitself. DIFFERENT DEGRADATION RATES IN DIFFERENT ENVIRONMENTS: The biodegradabilityof PLA dependsonthe environmenttowhichitisexposed..The rate of biodegradationisdependentonthe concentrationof enzymes,microorganisms,temperature,pHvalue, humidity,oxygensupplyandlight.PLA initiallydegradesviahydrolysisinhumanoranimal bodieswhich producesSoluble oligomersthatcanbe metabolizedbycells. Role of Microorganisms: PLA hydrolyzesintolow-molecular-weightoligomerswhendisposedinthe environment,whichare then mineralizedintoCO2andH2O bythe microorganisms presentinthe environment. Limitation: The degradationof PLA inthe soil isveryslow because of small numberof M.Othat mineralizePLA. In a compostingenvironment(45–60 daysat 50 ◦C),PLA hydrolyzesintosmallmoleculessuchas oligomers,dimers,andmonomers,whichcanbe mineralizedintoCO2 andH2O bythe microorganismin the compostin a much shorter time frame (Standau,Zhaoetal. 2019). GOAL: PLA has beenappliedtosome fields,butthe fate of PLA inapplicationsshouldbe directed towardsbiodegradation&recyclinginthe future (Qi,Renetal.2017). Aquaticenvironment: There are veryfew studiesondegradationof PLA inaquaticenvironment..Martinetal.observedno weightlossafter45 days in seawater.Inaddition,during asimultaneousdegradationtestof a PLA and PHBV bottle bythe CaliforniaDepartmentof ResourcesRecyclingand Recovery,the PLA bottle showed no disintegrationinmarine waterafterone yearat 25 C, while the PHBV bottle partiallydisintegrated. Bagheri etal. comparedthe biodegradationof aPLA filminartificial seawaterandfreshwaterwithfilms made of PLGA,PCL, PHB, PET, andpoly(butyleneadipate terephthalate) (PBAT,EcoflexU).PLGA fully degradedafter280 days,whereasthe PHBfoil lostabout10% of its initial weightafter380 days.All otherpolymers(includingPLA) showed nooronly minimal weightloss duringthistime frame (Haider, Völkeretal.2018). Role of Enzymes:
- 13. In additiontomicroorganisms, enzymesalsoplayasignificantrole indegradationprocesses.The enzymaticdegradationviahydrolysisisatwo-stepprocess.Firstly,the enzymesadsorbontothe surface of the polymer;then,the hydrolysisof the esterbondstakesplace (Standau,Zhaoetal.2019). The maximum activity of these enzymesdependsonthe optimumpH,temperature andPLA property (Qi, Renet al.2017). Theymainlyworkat moderate temperaturesandaneutral pH level andthey canincrease reactionrates by 108 to 1020 times(Haider,Völkeretal.2018). HOW TRANSFERABLE ARE LABORATORY TESTS: To be able totransfertestresultsfromenzymaticbiodegradationinthe labtobiodegradationinnatural environments,the microorganismsusedinalaboratorytesthave to be presentinthe environment where a plasticitemcouldpossiblyendup.Thus,the originof a microorganismisakeyfactor in whetheranenzymaticdegradationtestcancontribute tothe assessmentof the biodegradabilityof a certainpolymerinreal environments.These microorganismsare capable of degradingpolymers,butthe testsappliedinthe laboratoryare limitedtospecificmicroorganismsandthe testedpolymeristhe sole carbon source for the microorganism.Inanatural environment,the polymermaynotbe the preferred substrate inthe presence of alternate nutrients.Moreover,the microorganismsmightnotbe predominantinthe complex biotamix inthe environmentinquestion,and theirabilitytocompete, survive andthrive inthisspecificenvironmentisessential.Asaresult,moststandardizedbiodegradation testsby the AmericanNormative Reference (ASTM) orthe OrganizationforEconomicCo-operationand Development(OECD) prescribe the use of simulatedorreal environmentstoprovide amore realistic evaluation.Abioticfactorslike UV irradiation,grindingprocessesorthe influence of macro-organisms are all lackinginlaboratorytests.These factorsgenerallypromote the biodeteriorationstepandthus the overall biodegradationprocess.Giventheseconsiderations,the necessityof fieldtestsisobvious. Immersingthe polymerinsoil orcompost,or incubationinalake,a river,or the ocean,providesa realisticenvironment whereplasticlittercouldendup.However,fieldtestsare hamperedby environmental conditionslikepHvalue,temperature orhumidity,all of whichcannotbe well of weight lossisproblematicsince all small fragmentshave tobe collectedandrecovered once the material disintegrates.Simple physical disintegration/fragmentationof apolymermaterial isnotregardedas biodegradationandcouldproduce long-lastingmicroplastics.However,itisalmostimpossible todetect primarydegradationproductsin suchcomplex matriceslike soil orcompost.Also,full mineralization cannot be monitoredsince the ongoingmetabolismof differentorganismsinthe environmentcould influencethe analysis.The use of enzymatictestsandlaboratorytestsinsimulatedenvironmentscan actuallysupportbiodegradationfield-testing.Theyallow foracomprehensive understandingof the workingprinciplesof certainenzymesandthe ongoingprocessesinpolymerbiodegradationingeneral. Yet,as proof of biodegradabilityalone,laboratorytestsare notenoughandshouldalwaysbe conducted
- 14. simultaneouslytogetherwithbiodegradationfieldtests.Incombination,thesetestsminimize problems relatedtothe difficultanalysisof the biodegradationstatusandenhance the assessment of polymer biodegradation. (Haider,Völkeretal.2018). Role of temperature: PLA hasa glass- transitiontemperature Tg of about60o above whichthe degradationof polymertakes place.Makingcompositesof PLA withothermaterials, mildtemperature conditions canbe usedwith faster degradationrates. Limitation: However,thisdegradationisslow andoccursonlyat the surface.Such temperature ishard to achieve innatural conditions. In additiontothe environmental conditions,there are several otherfactorsthat can affectthe biodegradabilityof PLA,suchas molecularweight(distribution),crystallinity,andsurface properties.In general,the biodegradationisslowerwith increasingmolecularweight.Anincreasedcrystallizationismore resistanttodegradation (Standau, Zhao etal. 2019). GAP:The complete disappearance of PLA ina natural environmentmaytake several yearsbecauseof small no.of M.O present.Moststudieshave beendone underlaboratorialconditionsascomparedto the natural environment.There islittle researchinvolvedthe quorumsensingof M.O (Qi,Renet al. 2017). More studiesneedtobe carriedout regardingdegradationof PLA inaquaticenvironment. CHEMICAL DEGRADATION: Hydrolysis: A majorpathwayof chemical degradationforpolymerscontainingheteroatomslike esters,anhydrides, amides,orurethanesishydrolysis.The hydrolysisof the material proceedseitherviaa bulk or surface erosionmechanism. Bulk erosiondescribesdegradationthatoccursuniformlythroughthe thicknessof apolymericitem.It occurs whenthe rate of diffusionof waterexceedsthe rate of the hydrolysisreaction. Surface erosion describesadecrease inthe surface thickness. Surface erosion takesplace whenthe rate of hydrolysisexceeds the rate of diffusionof waterintothe bulk,or whenacatalyst (e.g.enzymes) cannotpenetratethe bulkpolymer.Surface erosionisthe predominatingmechanismforhydrophobicandsemi-crystallinepolymersandforpolymersshowinga veryrapidhydrolysis rate.
- 15. A material canchange its hydrolysismechanismfromsurface tobulkerosionwhenthe sample thicknessfallsbelowacritical value,the so-calledcritical samplethicknessLcrit. In general,the shape ofa material playsan importantrole as a larger surface area will promote degradation.The hydrolysisrate isinfluencedbyseveral externalfactors:anincrease inthe temperature promotesthe hydrolysisrate,andsowill achange inthe pH value (Haider,Völkeretal. 2018). Elsawy,M., Kim,K.-H.,Park,J.-W.,&Deep,A.(2017). Hydrolyticdegradationof polylacticacid(PLA) and itscomposites. Renewableand SustainableEnergy Reviews,79,1346-1352. doi:10.1016/j.rser.2017.05.143 Haider,T.,Völker,C.,Kramm, J.,Landfester,K.,&Wurm, F.(2018). Plasticsof the Future?The Impact of Biodegradable Polymersonthe EnvironmentandonSociety. AngewandteChemieInternational Edition,58. doi:10.1002/anie.201805766 Janorkar,A.,Metters,a., & Hirt, D. (2004). Modificationof Poly(lacticacid) Films:EnhancedWettability fromSurface-ConfinedPhotograftingandIncreasedDegradationRate Due toan Artifactof the PhotograftingProcess. Macromolecules,37.doi:10.1021/ma049056u Qi,X., Ren,Y.,& Wang,X. (2017). Newadvancesinthe biodegradationof Poly(lactic) acid. International Biodeterioration & Biodegradation,117,215-223. doi:https://doi.org/10.1016/j.ibiod.2017.01.010 Standau,T., Zhao,C.,MurilloCastellón,S.,Bonten,C.,&Altstädt,V.(2019). Chemical Modificationand Foam Processingof Polylactide(PLA). Polymers,11(2),306. doi:10.3390/polym11020306 Sabahat Ali (16-Arid-2569) Enhancement of biodegradationof polylactic acid by certain additive/changing its physical properties: Rate of biodegradation of PLA can be enhanced by
- 16. 1) Blending with starch and wood flour: Our focus is on the biodegradation of PLA materials in natural environment because of many more development of PLA based materials especially in soil and compost conditions. The biodegradation rate of PLA is so lower that the solid waste of PLA materials is delayed to recycle into the ecosystem, so we blend PLA with natural products. PLA biodegradation is influenced by its purity, chemical structure, molecular weight, as well as the environment conditions (such as temperature, humidity, and microorganism) Starch provides a biological fuel for the growth of microorganisms in the soil by enzymatic action causes change in structure and produce H2O, CO2, biomass and mineral salts without emitting toxic components. Starch was more susceptible to the natural soil environment than wood flour, wood flour is difficult to hydrolysis due to the high crystallinity of cellulose and the association between cellulose, hemicellulose and lignin. Lignin and hemicellulose are bound by covalent bond, forming a natural barrier for the cellulose to resist the attack of water or microorganism, resulting in the bio-refractory of wood flour. The biodegradation rate of starch/wood flour/PLA blends can be controlled by adjusting the ratio of starch/wood flour. The starch/PLA composite showed the fastest degradation rate; it was degraded within only 75 days. The PLA biodegradation follows two-step mechanism, at first stage, abiotic process in which the water molecule diffuse and attack the ester linkages in PLA molecular structure, results in chemical hydrolysis of PLA, followed by the biotic assimilation of polymer break-down products by the macro-organisms, generating water, carbon dioxide and biomass. The rate of hydrolysis degradation increases with temperature and catalyzed by increased free carboxyl groups of the hydrolyzed PLA terminal groups. Other factors, such as morphology, purity, chemical structure, shape and its molar mass may affect this mechanism.
- 17. In simulated soil, the growth of microorganism was increased with the presence of glucose provided by the hydrolysis of thermoplastic starch (TPS), which had a significant contribution to the degradation of PLA. The presence of mandelic acid promotes and the introduction of dicumyl peroxide slows down the PLA biodegradation rate. Grafting vinylic and acrylic chains onto the hydroxyl groups of starch and cellulose could reduce the enzymes concentration, resulting in less biodegradation of the material. The objective is to investigate the biodegradation behavior of the starch/wood flour/PLA blends under soil burial experiment. Biodegradation pattern in soil can be evaluated through mechanical strength, mass loss, morphologies, to assess the degradation of the starch/wood flour/PLA blends. Morphology is changed and weight loss is increased by increasing starch content. Surface of PLA was smooth before degradation & rough after degradation. Cavities spread on the surface of blends, and the roughness of composite surface increases. Degrading the blends for 75 days, with the increase of starch content, the cavities of the blends surface became larger, as well as the roughness of the surfaces. During the degradation process, interfacial adhesion between wood flour, starch and PLA was damaged more or less, which resulted in a decrease in mechanical properties. The mechanical strengths followed a first-order exponential decay model. In hydrolysis of PLA, polymer chain backbone randomly cleaved when the blends were buried in outdoor soil. Absorption of water during soil burial weakens the filler/PLA adhesion, which results in a reduction in mechanical strength of the blends, but addition of starch to PLA decreases the mechanical properties of PLA hence PHEE should also be added to improve its degradation without affecting mechanical properties. Weight loss is used as a qualitative tool to evaluate the effect of soil environment on the degradation behavior of the materials. Weight loss and macroscopic changes of the specimens serves as an indicator of degradation.
- 18. The pure PLA did not show significant macroscopic alternations on the surface at the early degradation period, which is of gray and homogeneous color. After 60 days, some bleached spots were observed on the surface of pure PLA which results in increase in weight loss. The insignificant changes of weight loss and appearance indicate low degradation rate of PLA, which shows hydrophobic performance of PLA. The goals are to increase the degradation rate and cut the cost of PLA as a promising biodegradable plastic. The starch and wood flour are cheaper than PLA, thus blending PLA with these natural fillers can reduce the cost of PLA-based materials. This method can also facilitate the biodegradation of the PLA-based materials. Gaps: 1. PLA shows poor resistance to moisture which is attributed to the presence of carbonyl groups in the backbone chain of PLA. Hence, most studies of the abiotic degradation of PLA have focused on the hydrolysis of pure PLA. 2. The simulation of the degradation model of PLA based composites in soil was not considered. 3. Although Extensive studies has been made about starch and polylactic acid composite but unfortunately behavior of this composite during the degradation of Polylactic acid is not clear yet. 4. Therefore, to improve the degradation of polylactic acid in future it is recommended to study more degradable and renewable polymers that can be blended with PLA to improve its degradability. 5. Lifetime and degradation behavior of PLA in the natural environment is unclear due to lack of knowledge. It needs further study and will be the subject of future work.(Lv, Zhang et al. 2017) 2) Cold Plasma Surface Modification:
- 19. Wet-chemical processing is used to chemically functionalize polymer surfaces. However, this method can result in a loss of mechanical properties, generation of hazardous chemical waste, and/or irregular etching of the polymer surface. Solvent-free techniques for polymer surface modification include treatments with flame, ultraviolet light, gamma-rays, ion-beams, lasers, and cold plasma. In particular, CP treatment (CPT) is currently under intense investigation. What is Cold plasma? CP is a partially or fully ionized gaseous system composed of free electrons, ions, radicals, ultraviolet(UV)photons, and excited or nonexcited molecules and atoms. CPT modifies the chemical structure at the surface of PLA films. It is likely that UV radiation, free electrons, and surplus microwaves in the plasma break chemical bonds in the polymer chains at the PLA surface, thereby introducing functional groups at the polymer surface. The degradation of PLA in compost increases if PLA films are treated with Cold plasma (CP) due to increased etching and surface functional groups. Photosensitive carbonyl groups are present on the surface of Polylactic acid which capture light and convert it into chemical energy and forms free radicals due to oxidation reactions and results in its photodegradation. When PLA is treated with cold plasma photosensitivity is much increased as compared to pure PLA and degradation increases. Advantage: When Polylactic acid is treated with cold plasma free volume of polymer increases and increased action of molds enzymes causes microbial degradation. PLA that is treated with cold plasma degrades completely after 35 days of its burial in compost. While pure PLA cannot be degraded completely.
- 20. Dynamic mechanical thermal properties: CP-treated films exhibited lower elastic moduli than the untreated films, thus CPT increases flexibility of the film. Exposure to CP may induce depolymerization in the amorphous and/or crystalline regions, resulting in free volumes within the film structure. Limitations: Potential CPT induced chain scission is likely to be limited to the surface of the films. Plasma treatment affects only the top layer of a material and unlikely to interfere with bulk properties.
- 21. Surface morphology: The surface of untreated PLA is homogeneous, flat, and uniform. The surface of the CP treated film was still relatively homogeneous, but rougher than the untreated film due to etching effect. Etching by CP occurs through chemical processes, including the breakage of chemical bonds, chain scission, and chemical degradation, and physical processes, including the physical removal of low-molecular-weight fragments The degree of etching by CP is dependent on the nature of the plasma-forming gas. Water contact angle: Water contact angles on the PLA films increased from 74.4 ± 0.4° to 88.0 ± 0.2° after CPT. This increase may be due to the observed changes in morphology (increase in roughness) opposed to an increase in the hydrophobicity of the film surface. The water contact angle of untreated PLA films (75.4±0.8°) did not change significantly during the 56-d storage period (76.4 ± 1.0°) whereas the contact angle of CP-treated PLA films (88.0 ± 0.2°) increased significantly to 94.3 ± 0.2°. This is an effect of increased surface roughness following CPT. (Song, Oh et al. 2016) Our Goal is to enhance biodegradation through induction of more functional groups or link some useful compounds to these functional groups that leads to biodegradation of bulk material. 3) Blending Polylactic AcidwithPoly hydroxybutyrate: Pure PLA does not biodegrade at room temperature whereas pure PHB shows rapid biodegradation at room temperature. The biodegradability of blends is increased by adding PHB. Adding PLA can be used to improve the mechanical properties of PHB. The PLA/PHB 75/25 blend shows improved tensile properties compared with pure PLA due to the reinforcement effect of the small finely dispersed PHB crystals. The water absorption by PHB is greater than that by PLA, and the addition of PHB increases the water absorption.
- 22. The weight change value of PLA is almost constant after the first 3 weeks, indicating that the PLA samples may not biodegrade at room temperature. The biodegradation of Pure PHB increases after 8 weeks. In PLA/PHB blend, weight loss is greater with greater PHB content in the blend. However, signs of patchy degradation and discoloration were observed from all tested samples except PLA, and the discoloration increased with increasing time and the PHB content. This is due to the increased water uptake and thus enhanced hydrolysis of PLA. PHB and PLA exhibit different degradation mechanisms: PHB is eroded from the sample surface, whereas degradation of PLA takes place throughout the whole of the sample. PHB is mainly degraded by attacked various enzymes at the surface, whereas degradation of PLA is mainly started with a non-enzymatically hydrolysis, which is strongly temperature dependent. Gap: 1. Recrystallization of the PLA component occurs in PLA/PHB blends during the heating process. PHB component in PLA/PHB blends shows stable melting temperatures, indicating the improved thermal stability caused by the interaction between PLA and PHB. The DMA results show the improved modulus of PLA/PHB blends above the Tg of PLA, due to the recrystallization of the PLA component. The annealing process can be used to produce a thermally stable PLA/PHB 75/25 blend ,which shows improved mechanical properties and thermal distortion temperature. Hence biodegradation decreased, so we need to work on blends in which PHB content is high. (Zhang and Thomas 2011) Elsawy,M., K.-H.Kim,J.-W.Parkand A.Deep(2017). "Hydrolyticdegradationof polylacticacid(PLA) and itscomposites."Renewable andSustainable EnergyReviews 79:1346-1352.
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