ta384 thermal analysis investigation of a polylactic acid biodegradable plastic

8/12/2019 TA384 Thermal Analysis Investigation of a Polylactic Acid Biodegradable Plastic

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ThermalAnalysisInvestigationofaPoly(LacticAcid)BiodegradablePlastic

ABSTRACT

With thepush in recentyears for industrytodevelopandmanufactureproductsona

more environmentally friendly basis, a variety of new biodegradable polymer systems have

beendevelopedtofitthatmold. Poly(lacticacid)(PLA) isabiodegradablealiphaticpolyester

which is produced from lactides and lactic acid monomers that are derived from the

fermentationofcornstarchandsugar feedstocks.Thepolymerizationof lacticacid intoPLA

producesabiodegradablethermoplasticpolyesterthathashighmechanicalstrength,similarto

PETE, with good biocompatibility. PLA has most commonly been used in biomedical

applications for drug delivery systems as well as sutures and stents, but the number of

applications

for

PLA

is

constantly

growing.

Understanding

the

properties

of

PLA

becomes

essentialtothefuturedevelopmentofnewapplications. Thisstudywillexamineavarietyof

thethermalandmechanicalpropertiesofaPLAdrinkingcupusingthermalanalysistechniques

suchasDSC,MDSC,RapidHeat/CoolDSC(RHC)andDMA.

INTRODUCTION

AgenericdrinkingcupmadeofPLAwasusedasthesolememberofthis investigation

andwasselectedsimplybecauseofthephysicalappearanceofthecup. Thetophalfofthecup

hadanopticallyclear,uniformappearancewhilethebottomhalfofthecuphadamuchmore

irregularlookthatgavetheappearanceofmaterialroughnesswithoutanyphysicaltexture. It

wasassumed that the tophalfof thecupwasmoreamorphouswhile thebottomhalfmore

crystalline. Inordertodeterminetheamorphousandcrystallinecomponentsinthetopofthe

cupversusthebottomofthecup,theinvestigationstartedwithstandardDSCmeasurements,

which is probably the simplest technique. More advanced DSC methods as well as DMA

measurements were incorporated into the investigation to refine and verify the results

obtainedonthePLAcup.

RESULTS&DISCUSSION

AstandardDSCheatcoolheatmethodwasinitiallyrunonasampletakenfromthetop

halfofthecup,whichwasassumedtobemoreamorphous. ThedataisshownbelowinFigure

1. Thecupsamplewasheatedat20deg/minandcooledatacontrolled rateof10deg/min

between25

and

170C.

All

of

the

DSC

and

MDSC

samples

were

tested

on

aQ2000

DSC

in

TZero

Aluminumpanswith a nitrogen purge. The first heat, in this instance, shows a small glass

transition (Tg)around58Cwith some coldcrystallizationbeginningaround90Candamelt

withapeaktemperatureofabout148C. Thesecondheatonthissampleafteritwascooledat

10deg/minshowsalargerTgaroundthesametemperatureasthefirstheat,butnoevidence

ofcoldcrystallizationormelting. Theresultssuggestthatthetopportionofthecuphadsome

amountofamorphousstructure,butbecausethesampleundergoessomecoldcrystallization

or crystalperfection immediatelybefore themelt, theamountofcrystalline structure in the


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TA384 3

-0.3

-0.2

-0.1

0.0

0.1

0.2

HeatFlow(W/g)

Figure2:MDSCresultsonasampletakenfromthetopofthePLAcup

AsampletakenfromthebottomofthePLAcupwastestedusingthesameModulated

conditionsasthesample fromthetopofthecup. Theresultsareshown inFigure3. While

therearesomesimilaritiesbetweentheMDSCresultsofsamplestakenfromthetopandthe

bottomof

the

cup,

there

is

one

major

difference

that

directly

points

to

amajor

difference

in

materialstructure. TheTgisapproximatelyatthesametemperatureasbeforeandthereisstill

some enthalpic recovery, crystal perfection and melting that show up in both the Non

Reversing and Reversing Heat Flows out beyond 120C. The major difference seen in the

bottom of the cup is a large cold crystallization peak that begins around 80C. This peak

suggeststhatsomeoftheamorphousstructureinthematerialisorderingandconvertingtoa

morestructured,crystallinestructurewhichwasaneventthatdidnotshowup inthesample

fromthetopofthecup. TheotherinterestingcharacteristicdeterminedfromtheMDSCresults

isthefactthattheareaoftheNonReversingHeatFlow,includingthecoldcrystallization,and

theReversingHeatFlowarenearlythesamevalue. Thisresultsuggeststhatthesampledidnot

haveanycrystallinitytobeginwithandallofthecrystallinematerialthatmeltedwasformed

duringthe

cold

crystallization,

which

is

counter

to

the

initial

assumption

based

off

of

the

optical

appearancethatthesamplewasmorecrystalline.

142.89C

136.98C24.22J/g

142.24C

136.38C61.25J/g

61.25 - 24.22 = 37.03 J/g

Heat Only MDSC80 sec periods3 deg/min ramp

-0.3

-0.2

-0.1

0.0

0.1

0.2

RevHeatFlow(W/g)

-0.2

-0.1

0.0

0.1

NonrevHeatFlow(W

/g)

20 40 60 80 100 120 140 160 180 200

Temperature (C)Exo Up Universal V4.7A TA Instruments


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-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

RevHeatFlow(W/g)

Figure3:MDSCresultsonasampletakenfromthebottomofthePLAcup

Inorder toverifythatallofthecrystallinitywas formedduring theheatingrampand

wasnot inthesample initially,aRapidHeatCoolDSC(RHC)experimentwasusedtoheatthe

sample at 1000 deg/min in an attempt to suppress the cold crystallization. For the RHC

experiment,aBeta

RHC

DSC

was

used

and

approximately

0.3mg

of

sample

was

tested

in

aluminumpanscustomdesigned fortheRHC. Thetesttemperature rangewaswidenedto

150Conthelowendand225Conthehighendinordertoaccommodatethehigherheating

rateof1000deg/min. TheHeatFlowresultsfortheRHCexperimentareshowninFigure4and

showonlyaTgaround76Candnothingelse. TheshiftupintemperatureoftheTginthiscase

issimplyaresultoftheincreasedheatingratewhencomparedtoMDSCandDSC,whichboth

have heating rates over an order of magnitude slower. There is no evidence of cold

crystallization upon heating and the fact that there isno evidenceofmelting suggests that

there is no crystallinematerial in the sample tomelt. For comparisonpurposes, the same

sample taken from thebottomof the cupwas allowed to crystallizeby slowingheating the

material toa temperaturearound125Cand then testedagain in theRHCat1000deg/min.

Theresults

for

the

material

that

was

allowed

to

crystallize,

shown

in

Figure

5,

prove

that

a

samplewithcrystallinematerialthatmeltswillshowthatmeltingendothermevenifheatedat

1000 deg/minwith very little shift in the onset ofmelting. If the sample initially had any

crystallinestructure,themeltshouldhaveshownupinthefirstRHCexperiment.

98.67C

91.09C48.56J/g

145.54C

139.28C51.20J/g

Heat Only MDSC80 sec periods3 deg/min ramp

51.20 - 48.56 = 2.64 J/g

-0.3

-0.2

-0.1

0.0

0.1

HeatFlow(W/g)

-0.3

-0.2

-0.1

0.0

0.1

NonrevHeatFlow(W

/g)

20 40 60 80 100 120 140 160 180 200



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TA384 5

75.97C(H)

0.296mg of sample1000 deg/min

0

500

1000

1500

Deriv.

Temperature(C/min)

-45

-25

-5

15

HeatFlow(W/g)

-150 -100 -50 0 50 100 150 200 250Temperature (C)Exo Up Universal V4.7A TA Instruments

Figure4: Thebottomofthecup,asreceived,testedintheRHCat1000deg/min

75.46C(H)

0.296mg of sample

1000 deg/min

0

500

1000

1500

Deriv.

Temperature(C/m

in)

-45

-25

-5

HeatFlow(W/g)

-100 -50 0 50 100 150 200 250


Figure5: ThebottomofthecuptestedintheRHCat1000deg/minafteritwasallowedtocrystallize

To verify the conclusionsmade about the topof the PLA cup that therewas indeed

some crystalline structure in thematerial initially, the same RHC experimentwas run on a


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TA384 6

sample taken from the top of the cup. The results shown in Figure 6 actually show two

endothermic peaks around 75 and 125C. Based on some of the previous experiments,

however,thefirstendothermaround75Cismostlikelytheresultofsomeenthalpicrecovery

thatovershadowstheTgsincetheotherRHCexperimentsat1000deg/minshowtheTgright

around75Caswell. The secondendotherm isassociatedwith initialcrystallinity thatmelts

around 125Cwhich compareswellwith theMDSC experiments of the samematerial that

beginstomeltaround120Cminusthecrystalperfection.

0.145mg of sample1000 deg/min

0

500

1000

1500

Deriv.

Temperature(C/min)

-65

-45

-25

-5

HeatFlow(W/g)

-100 -50 0 50 100 150 200 250Temperature (C)Exo Up Universal V4.7A TA Instruments

Figure6: Thetopofthecup,asreceived,testedintheRHCat1000deg/min

Anothertechniqueavailabletoexaminethecrystallinityorthechangeincrystallinityas

afunctionoftemperature istheDMA. Figures7and8areexperimentsrunonthesamePLA

cupsamplesusingtheRSADMA. Theresultscollectedonasamplefromthetopofthecupin

Figure7showthattheStorageModulusofthesampledropsaround60Cwherethematerial

goesthrough

the

Tg

and

then

plateaus

before

dropping

again

around

130C

where

the

material

melts. Bothof theseeventscorrelatequitewillwith the resultscollectedbyDSC. Figure8,

whicharetheresultscollectedfromasamplefromthebottomhalfofthecupshowadropin

StorageModulus around 60C for the Tg and then a significant increase inModulus around

100C,which isusually indicativeofan increase instiffness/mobilityandcorrelatesverywell

withthecoldcrystallizationshownintheMDSCresultsaroundthesametemperature. Figure9

showsanoverlaycomparisonoftheStorageModulusbetweenthetwosamples.


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TA384 7

20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

106

107

108

109

1010

0.0

0.2

0.4

0.6

0.8

1.0

Tem [C]

E'(

)

[Pa]

E"(

)

[Pa]

tan_delta(

)

[]

Figure7: DMAdatafromthetopportionofthecup

20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

105

106

107

108

109

1010

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Tem C

E'(

)

[Pa]

E"(

)

[Pa]

tan_delta(

)

[]

Figure8: DMAdatafromthebottomportionofthecup


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TA384 8

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

106

107

108

109

1010

Tem C

E'(

)

[Pa]

E'

Top

Bottom

Figure9: OverlayoftheEDMAdatafromboththetopandbottomsectionsofthecup

CONCLUSIONS

A plastic drinking cup made from the biodegradable plastic, poly(lactic acid) was

examinedusingDSC,MDSC,RapidHeat/CoolDSCandDMA. Theaimoftheinvestigationwas

tocomparetwoareasthatappearedopticallyandstructurallydifferent,withthesupposition

thatadifferenceincrystallinitymayberesponsible. Resultswereconsistentacrosstechniques

showingadifferenceinitialcrystallinitybetweenthetwosections.

REFERENCES

1.E1269,DeterminingSpecificHeatCapacitybyDifferentialScanningCalorimetry,ASTM

International,West

Conshohocken,

PA.

2.L.ThomasandS.Aubuchon,HeatCapacityMeasurementsUsingQuasiIsothermal

ModulatedDSC,TAInstrumentsApplication(TA230).

3.L.Thomas,ModulatedDSCPaper#9,MeasurementofAccurateHeatCapacityValues,TA

InstrumentsApplication(TP014).


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ta384 thermal analysis investigation of a polylactic acid biodegradable plastic

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