evaluation of oriented poly(lactide) polymers vs. existing pet and oriented ps for fresh food...

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2005; 18: 207–216Published online 25 May 2005 in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/pts.692

Evaluation of Oriented Poly(lactide) Polymersvs. Existing PET and Oriented PS for FreshFood Service Containers

By Rafael A. Auras,1* S. Paul Singh1 and Jagjit J. Singh2

1 School of Packaging, MSU, East Lansing, MI 48824-1223, USA2 California Polytechnic University, San Luis Obispo, CA, USA

Poly(lactide) (PLA) polymers have garnered increasing attention in the last fewyears as food packaging materials because they can be obtained from renewableresources; their production consumes quantities of carbon dioxide; they can berecycled and composted; and their physical and mechanical properties can betailored through polymer architecture. As a consequence, PLA is becoming agrowing alternative as a ‘green’ food packaging material. PLA’s optical, physicaland mechanical properties have been compared to those of polystyrene (PS) andpolyethylene terephthalate (PET), although studies comparing and showing theactual performance of PLA, PS and PET plastics containers are scarce. The purposeof this study was to investigate and compare the role of PLA in packagesustainability for the food service industry. Two of the commonly used materialsto make containers to package fresh food, PET and oriented polystyrene (OPS),were compared with oriented PLA (OPLA) and OPLA with 40% recycled contentfrom the industrial trimming process. The recycled OPLA provides an opportunityfor full material utilization and lower costs. This study involved a number of teststo quantify the physical, mechanical, barrier and compatibility properties thatwould affect the selection criteria for containers to be used for food serviceapplications. Based on the data collected, OPLA, OPLA + 40% regrind, OPS andPET performances were evaluated. Exposure of the four materials to vegetable oiland weak and strong acids show a minimal reduction in the performance of thesepolymers. At ambient temperature, PET has the highest impact resistance, followedby OPLA, OPS and OPLA + 40% regrind. In terms of barrier properties, PET showsthe highest oxygen barrier, followed by OPLA, OPLA 40% recycled content, andOPS. Thus, OPLA and OPLA with 40% recycled content can be used for fresh foodapplications as well as OPS and PET, and in many situations it performs betterthan OPS and PET. Copyright © 2005 John Wiley & Sons, Ltd.Received 30 November 2004; Revised 25 February 2005; Accepted 11 March 2005

KEY WORDS: poly(lactide); poly(ethylene terephthalate); polystyrene; biodegradable;packaging.

* Correspondence to: R. A. Auras, School of Packaging, MSU, East Lansing, MI 48824-1223, USAEmail: [email protected]

Copyright © 2005 John Wiley & Sons, Ltd.

INTRODUCTIONAs consumers are becoming more aware of munic-ipal waste problems and demand the use of bio-friendly packaging materials, the selection of foodpackaging polymers not only depends on the eval-uation of polymer standard properties, but also onthe recyclability and sustainability of these poly-mers. As a consequence, poly(lactide) (PLA) poly-mers have garnered increasing attention in the lastfew years as food packaging materials becausethey can be obtained from renewable resources;1

their production consumes quantities of carbondioxide;2 they can be recycled and composted;3 andtheir physical and mechanical properties can betailored through polymer architecture.4

High molecular weight PLA can be obtained bydifferent routes: (a) direct condensation–polymer-ization; (b) azeotropic dehydrative condensation;and (c) polymerization through lactide formation.Polymerization through lactide formation is, byand large, the current method used for producinghigher quantities of PLA for commercial packag-ing applications. Dextrose, which is obtained fromthe starch fermentation from corn, is further fermented to obtain either d-lactic acid (DLA), l-lactic acid (LLA) or a mixture of the two (DLLA).A mixture of LA is then further polymerized toobtain an intermediate low molecular masspoly(lactic acid), which is catalytically convertedunder lower pressure into a mixture of lactidestereoisomers.5 Lactide, the cyclic dimer of lacticacid, is formed by the condensation of two lactic acid molecules as follows: l-lactide (two l-lactic acid molecules), d-lactide (two d-lactic acidmolecules) and meso-lactide (a l-lactic acid and ad-lactic acid molecule). After vacuum distillationof the lactide, high molecular mass PLA with con-trolled optical purity and a constitutional unit of –[OCH(CH3)CO–O–CH(CH3)–CO] – is formed byring-opening polymerization of the lactides.4,6

Properties of high molecular weight PLA aredetermined by the polymer architecture (i.e. thestereochemical make-up of the backbone) and themolecular mass, which is controlled by the addi-tion of hydroxylic compounds. The ability tocontrol the stereochemical architecture in PLApolymers permits precise control over the speedand final degree of crystallinity, the mechanicalproperties and the processing temperatures of thematerial. In addition, the degradation behaviour

strongly depends on the stereochemical composi-tion, molecular weight and crystallinity of thesamples.7

PLA’s optical, physical and mechanical proper-ties have been compared to polystyrene (PS) andpolyethylene terephthalate (PET).8,9 Nonetheless,PLA polymers have the advantage of easily tailor-ing their physical properties by changing thechemical composition (amount of l- and d-isomer)and the processing conditions. PLA is approved bythe Food and Drug Administration for its intendeduse in fabricating articles for contact with food.4

PLA polymers show good barrier to aroma, andmedium barrier to gases and vapors. Currently,PLA is being used as a food packaging polymer forshort shelf-life products with common applica-tions such as containers, drinking cups, sundaeand salad cups, overwrap and lamination films,and blister packages.10,11 PLA is a growing alterna-tive as a ‘green’ food packaging material. Newapplications have been claimed in the arena offresh products, where thermoformed PLA con-tainers are used in retail markets for fruit and veg-etables. In the next years, PLA production andconsumption is expected to increase. Therefore,there is a need to better understand and describeits properties as a packaging material, especiallyfor food packaging applications. The purpose ofthis study was to investigate the role of packagesustainability for plastics made from renewablesources for the food service industry. The packag-ing used by this industry, which represents restau-rants, delicatessens and dispensing machines,among others, currently relies on the use of plas-tics made from petroleum-based resources. Two ofthe commonly used materials to make containersto package fresh food are polyethylene terephtha-late (PET) and oriented polystyrene (OPS). The aimof this study was to determine whether PLA canbe used to manufacture packaging from renewablesources for the food service sector by benchmark-ing its properties against commonly used petro-chemical-based packaging.This study specificallyaims to compare the performance of thermo-formed food service containers made from theexisting petroleum-based plastics (PET and OPS)to similar containers made from OPLA and OPLA+ 40% recycled content from the industrial trim-ming process. The recycled PLA provides anopportunity for full material utilization and lowerscosts. This study involved a number of tests to

R. A. AURAS, S. P. SINGH AND J. J. SINGHPackaging Technologyand Science

Copyright © 2005 John Wiley & Sons, Ltd. 208 Packag. Technol. Sci. 2005; 18: 207–216

quantify the physical, mechanical, barrier andcompatibility properties that would affect theselection criteria for containers to be used for foodservice applications.

EXPERIMENTAL

Materials

OPLA, OPLA with 40% recycled content from theindustrial trimming process, PET, and OPS rollstock sheet were provided by Wilkinson Manufac-turing Company, (Fort Calhoun, NE). OPLA wasmade with nominally 94% l-lactide with a densityof 1243 ± 2kg/m3.

Physical properties

Thickness. Thickness of films was determinedusing a TMI 549M micrometer (Testing Machines,Inc., Amityville, New York), according to ASTM D374–99.

Glass transition (Tg) and melting temperature(Tm). Samples of OPLA, OPLA + 40% regrind,PET and PS were used to determine the Tg and Tm,using a differential scanning calorimeter (DSC2010) made by TA Instruments (New Castle, DE),in accordance with ASTM D 3418-97. The DSCstandard calibration procedures were performedaccording to ASTM E 967-97 and E 968-99. Furthersamples were used to determine the enthalpy offusion and to calculate the percentage crystallinityof PET, OPS, OPLA and OPLA + 40% regrind byDSC, according to ASTM D 3417-97 (equation 1).

(1)

where DHm is the enthalpy of fusion, DHc is theenthalpy of cold crystallization, and DHm

c is theheat of melting of purely crystalline poly(l-lactide)or PET. The crystallinity amounts were calculatedaccording to equation 1.

Barrier properties

Water vapour transmission rate. Water vapourtransmission rate was tested using a Permatran

xH

Hcc m

mc%( ) = ¥

+100

D DHD

W3/31 from Modern Controls Inc. (Minneapolis,MN). Prior to testing the sample, a reference mate-rial with known transmission rate was tested toverify that the instrument was working properly.The temperature and relative humidity(rh) were37.8°C and 100% rh, respectively. The testing wasperformed until 10 values of constant transmissionrate (small fluctuation without upward/down-ward pattern) were obtained. The final transmis-sion rate was calculated by averaging the last 10constant values for duplicate samples. Watervapour permeability coefficients (WVPC) were cal-culated according to equation 2:

(2)

where WVTR is the water vapour transmissionrate (kg/m2/s), l is the thickness (m), and DP is thedifference in water vapour partial pressure acrossthe film (Pascal). DP = p1 - p2 where p1 is the partialpressure of the water at the temperature tested onthe test side, and p2 is equal to zero on the detec-tor side.

Oxygen transmission rate. Oxygen transmissionrate was tested using an Oxtran 2/21 made byModern Controls Inc. (Minneapolis, MN), in accor-dance with ASTM D 3985-02 and F 1927-98e1. Cal-ibration of the instrument was performed with astandard film provided by Modern Controls Inc.(Minneapolis, MN). The temperature and relativehumidity of the test conditions were 23°C and 0%rh, respectively. The samples were conditioned for4h prior to testing. The testing was performeduntil 10 values of constant transmission rate (smallfluctuation without upward/downward pattern)were obtained. The final transmission rate was cal-culated by averaging the last 10 constant values forduplicate samples.

Mechanical properties

Impact. Impact tests were conducted according toASTM D 1709-91. This standard describes themethod to determine the energy that causes theplastic film/sheet to fail under specified conditionsof impact of a free-falling dart. The energy isexpressed in terms of the weight (mass) of themissile falling from a specified height, whichwould result in a 50% failure of all specimens

WVPCWVTR l

P=

*

D


OPLA VS. PET AND OPS FOR FOOD CONTAINERS Packaging Technologyand Science

tested. The tests were conducted on the materialprovided in roll stock. The tests were conducted atambient 72°F (22=C) and frozen food storage tem-peratures of 0°F (-18°C), -10°F (-23°C), and -20°F(-29°C).

Tensile properties. The tensile properties of thesefour materials were tested using the roll stocksample materials. The tests were conducted inaccordance with ASTM D 882-97 in a Unitedtensile test machine (United Calibration Corpora-tion, Huntington Beach, CA). Tests were per-formed on three samples of each material at thefollowing temperatures: 72°F (23°C) ambient tem-perature; 0°F (-18°C); -10°F (-23°C); and -20°F (-29°C).

Chemical resistance

Acid and vegetable oil resistance test. Roll stocksamples were subjected to a chemical resistancetest. Samples (6 ¥ 1 in) were exposed to a weak acid(pH 6), strong acid (pH 2) for periods of 0, 1, 3, 5and 7 days, and OPLA + 40% regrind was alsoexposed to a vegetable oil for periods of 0, 1, 3, 5and 7 days. The weak acid solution was preparedusing acetic acid, and the strong acid solution wasprepared using hydrochloric acid. Three replicateswere tested after exposure at each condition forchanges in tensile properties, as compared to testsat ambient conditions (72°F).

RESULTS

Physical properties

PLA polymers pass through an endothermic eventwhich is superimposed on the Tg and is observedduring the first heating on the DSC.8 Thisendothermic relaxation, with an enthalpy around1.4 J/g, results from secondary molecular reorder-ing undergone in the amorphous phase of semi-crystalline polymers.12 The endothermic peak iseliminated as the sample is heated above Tg. Thesecond heating curve shows the glass transitiontemperature. Table 1 presents the glass transitionand melting temperatures and the enthalpy of

fusion of the samples. The percentage crystallinity,cC, was calculated from equation 1.

For PLA, DHm = 135J/g,13 and for PET, DHm =125.6 J/g.14 OPS used in packaging is atactic, so itcannot crystallize. Its lack of crystallinity makes ithighly transparent; it does not have a definedmelting point and so it gradually softens througha wide range of temperatures.

Barrier properties

While a package serves as a barrier between theproduct and the external environment, the degreeof protection varies. This variation is particularlyimportant in connection with the transport ofgases, vapours, water vapour, or other low molec-ular weight compounds between the externalenvironment and the internal package environ-ment.15 Unlike glass, metals and ceramics, plasticsare relatively permeable to small molecules such asgases, water vapour, organic vapours and liquids.Therefore, in terms of their properties, plasticmaterials provide a broad range of mass transfercharacteristics, ranging from excellent to lowbarrier values. The specific barrier requirements ofthe package system depend upon the product’scharacteristics and the intended end-use applica-tion. Water vapour and oxygen are two of the mainpermeants studied in packaging applications,because they may transfer from either the internalor external environment through the polymerpackage wall, resulting in a continuous change inproduct quality and shelf-life. Therefore, the deter-mination of water vapour and oxygen permeabil-ity of a polymer is crucial to estimate and predict



Table 1. Glass transition, melting temperature,and percentage crystallinity of OPLA, OPLA +

40% regrind, PET and OPS

Sample Tg (°C) Tm (°C) cc (%)

OPLA 62 ± 1 150 ± 0.5 29 ± 0.5OPLA + 40% 58 ± 0.5 150 ± 0.5 25 ± 3

regrindPET 80 ± 0.5 246 ± 0.5 27 ± 2OPS* 106 ± 0.5 N/A N/A

* Atactic.

the product-package shelf-life. The water vapourtransmission rate and the oxygen transmission rateof PET, OPS, OPLA and OPLA + 40% regrind arepresented in Tables 2 and 3, respectively.

Water vapour transmission. In the case of a pack-aged product whose physical or chemical deterio-ration is related to its equilibrium moisturecontent, the barrier properties of the package relat-ing to water vapour will be of major importance inmaintaining or extending shelf-life. For fresh foodproducts, the main concern is to avoid dehydra-tion, and in the case of bakery or delicatessen theconcern is to avoid the water permeation. Table 2shows the water vapour transmission rate andwater permeability coefficients of PET, OPS andOPLA. The water vapour permeability coefficients(WVPC) of OPS and PET are one order of magni-tude lower than that of OPLA. Similar values ofWVPC are reported in the literature for PET, PSand PLA.8 Previous studies show that the valuesof permeability coefficients of PLA are practically

constant as the relative humidity changes, despitePLA being a polar polymer.8 Activation energy forthe water vapour permeation process around -10kJ/mol has been reported.8

Oxygen transmission rate. Fresh food packagingcontainers are generally used for oxygen-sensitiveproducts such as fruits, salad and ready-to-eatmeals. Therefore, the oxygen barrier property of apolymer plays an important role in protecting afresh product in a package. Oxygen barrier isquantified by the oxygen permeability coefficients,which indicate the amount of oxygen that perme-ates per unit of area and time in a packaging mate-rial. When a packaging film with low oxygenpermeability coefficients is used, the oxygen pres-sure inside the package drops to the point whereoxidation is retarded, and the shelf-life of theproduct may be extended. The values of theoxygen transmission rate and oxygen permeabilitycoefficients of PET, OPS, OPLA and OPLA + 40%regrind are shown in Table 3. If we compare theoxygen permeability coefficients, the OPS oxygenpermeability coefficient is one order of magnitudehigher than that of OPLA, and two orders of mag-nitude higher than that of PET. A higher oxygenpermeability coefficient was observed for OPLA +40% regrind than for the OPLA without regrind.

Similar PLA, PET and OPS oxygen permeabilitycoefficients are reported in the literature.9,16,17

Lehermeier et al.16 report oxygen permeabilitycoefficients around 3.3 ¥ 10-17 kg/m/(m2/s/Pa) at25°C and activation energy of 11.1kJ/mol. Auraset al.9 determined and compared the diffusion, sol-ubility and permeability coefficients of PET andtwo PLA films by using an isostatic technique at 1and 0.21 atmosphere pressure at 5°C, 23°C and40°C and 0–90% rh. They reported an increase ofthe oxygen permeability coefficient from 3.5 ¥10-18 kg/m/(m2/s/Pa) at 5°C to 11 ¥ 10-18 kg/m/(m2/s/Pa) in the absence of moisture. However,when the temperature increased from 5°C to 40°Cand the humidity to 90%, the oxygen permeabilitycoefficients of PLA decreased to 8.5 ¥ 10-18 kg/m/(m2/s/Pa).

Mechanical properties

Impact. Impact properties of PET, OPS, OPLA andOPLA + 40% regrind were determined by the



Table 2.Water vapour transmission rate andpermeability coefficients of PET, OPS, OPLA

at 37.8°C (22°C) and 100% rh

Thickness WVTR PermeabilitySample (mm) [g/(m2/day)] [kg/m/(m2/s/Pa)]

PET 18 3.48 ± 0.02 2.82E-15 ± 1.50E-17OPS 18 5.18 ± 0.03 4.18E-15 ± 2.07E-17OPLA 20 15.30 ± 0.04 1.34E-14 ± 3.61E-17

Table 3. Oxygen transmission rate (OTR) andpermeability coefficients of OPLA, OPLA +40% regrind, PET and OPS. Results at 72°F

(22°C) and 0% relative humidity

Thickness OTR PermeabilitySample (mm) [cc/(m2/day)] [kg/m/(m2/s/Pa)]

PET 18 9.44 ± 0.06 6.95E-19 ± 1.90E-20OPS 18 531.58 ± 0.67 3.91E-17 ± 8.72E-19OPLA 20 56.33 ± 0.12 4.33E-18 ± 1.00E-19OPLA 18.0 106.3 ± 9.5 8.17E-18 ± 8.99E-19

+ 40%

falling dart method at -20°F, -10°F, 0°F and 72°F.Figure 1 shows the impact failure weight of thesefour polymers. It is possible to observe that PETfailed at a weight that is almost double that of OPS,OPLA and OPLA + 40% regrind. However, if wecompare OPS with OPLA, the average failureweight is in the same range. Moreover, OPLA +40% presents a higher average impact weight thanOPLA. The four polymers show a tendency oflower impact failures as the temperature increases.Since the test is done below the glass transitiontemperature and the amorphous region is in the glassy state, it is expected that an increase inthe temperature is translated into a decrease in theimpact failure.

Tensile. Tensile test results for PET, OPS, OPLAand OPLA + 40% regrind are presented in Figure2a, b and c. Figure 2a presents the values of tensilestress for the four different polymers at -20°F, -10°F, 0°F and 72°F. Although OPLA shows thebiggest tensile stress variation between -20°F and72°F, it presents higher tensile stress values at -20°F, -10°F and 0°F. PET shows tensile stressvalues similar to OPS but lower than OPLA. Thetensile stress, elongation at break and modulus ofelasticity of OPLA + 40% regrind at -20°F and -10°F are not presented because the sheet slippedfrom the jaws. This could be due to a change in thefriction coefficient and the surface tension of theOPLA film with 40% regrind. Figure 2b shows the elongation at break of the four polymers. PET

shows the higher elongation at break at -20°F;however, OPLA + 40% regrind shows the higherelongation at break at room temperature. Themodulus of elasticity show the same trend astensile stress, which is a higher modulus for OPLA,with higher values at 0°F.

Chemical resistance

Acid and vegetable oil resistance test. The inter-action of chemical compounds with a polymer is aunique characteristic between each chemical com-pound and polymer. The absorption of these chem-ical compounds may affect the final mechanicalproperties of a polymer. Therefore, the perfor-mance of polymers stored with common foodpackaging solution as a function of time is neces-sary to assess the suitability of the package.Figures 3 and 4 show the tensile stress, elongationat break, and modulus of elasticity of samples sub-merged in weak and strong acid as a function oftime. These simulant solutions represent the spec-trum of most products that could be packaged inthese polymers.

For samples submerged in weak acid (Figure 3),PET shows an increase of tensile stress of around30% at day 7. OPLA + 40% regrind and OPLAshow an increase of tensile stress of 19% and 4%,respectively. On the other hand, OPS samplesshow a reduction of the tensile stress of almost40%. Figure 3b shows that PET and OPS became



0

200

400

600

800

1000

1200

PET OPS OPLA OPLA+40%

Imp

act

Fai

lure

Wei

gh

t (g

)

–20°F –10°F 0°F 72°F

Figure 1. Impact failure weight (g) for PET, OPS, OPLA, and OPLA +40% regrind.

more brittle at day 7, with a reduction of the elon-gation at break of 12% for PET and 21% for OPS.OPLA become 10% more ductile at day 7 than day1, and OPLA + 40% regrind is 40% more ductile atday 7 that it was at day 1. As PET and OPS arebecoming more brittle, the modulus of elasticity isincreasing (53% for PET and 5% for OPS). In thecase of OPLA, the modulus of elasticity is reducedby 25%; however, in the case of OPLA + 40%regrind, the modulus of elasticity does not changeas a function of time.

For samples submerged in strong acid (Figure 4),PET and OPS do not show significant variation oftensile stress at day 7. OPLA + 40% regrind andOPLA show an increase of tensile stress of 11% and10%, respectively. Figure 4b shows no major vari-ation of the elongation at break for PET, OPS, and



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of

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kps

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–20ºF –10ºF 0ºF 72ºF

(a)

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(c)

Figure 2.Tensile strength, elongation at break, andmodulus of elasticity of PET, OPS, OPLA and OPLA + 40%

regrind.

0 1 2 3 4 5 6 7

Tens

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tres

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psi

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(a)

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Figure 3.Tensile stress, elongation at break and modulusof elasticity of PET, OPS, OPLA and OPLA + 40% regrind

polymer samples submerged in week acid simulantsolutions.

OPLA although it shows a net variation of 45% ofelongation at break of OPLA + 40% regrind. In allthe cases, the four polymers became more brittlewith an increase in the modulus of elasticity from4.6% for OPLA + 40% regrind to 50% for OPS. Theincrease in the modulus of elasticity in every

sample is an indication of the brittleness of thesample as a function of time.

OPLA + 40% samples submerged in vegetableoil (Figure 5a) show a decrease of the tensile stressof 8.5% in day 7 compared to day 1. Also, Figure5b shows a reduction of the elongation at break at



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Time, daysFigure 4.Tensile stress, elongation at break and modulusof elasticity of PET, OPS, OPLA and OPLA + 40% regrind

polymer samples submerged in strong acid simulantsolution.

Figure 5.Tensile stress, elongation at break and modulusof elasticity of OPLA + 40% regrind polymer samples

submerged in vegetal oil simulant solution.

day 7 of 14% compared to day 1. Samples testingat days 3 and 5 show a bigger variation of the elon-gation at break compared with the samples testedin other times. Finally, the modulus of elasticityincreased by 4.5% at day 7 compared with day 1.

CONCLUSIONS

PLA polymers are biodegradable polymers readyto be used as packaging materials for fresh foodcontainers. The new, commercially available PLApolymers promise to reduce the cost and toencourage the use of PLA. OPLA and OPLA + 40%regrind mechanical, physical and barrier proper-ties are comparable and, in some cases, better thanstandard OPS and PET containers. Based on thedata collected on the four materials tested in thiswork, the following findings can be highlighted:

• OPLA + 40% regrind showed the lower glasstransition temperature. There is no major varia-tion of Tg between OPLA and OPLA + 40%regrind. OPLA and OPLA + 40% regrindshowed a lower melting temperature than PET.

• In terms of water vapour barrier, PET showedthe best performance, followed by OPS andOPLA. In the case of oxygen barrier properties,PET showed the lowest oxygen permeabilitycoefficients, followed by OPLA and OPLA +40% regrind. OPS showed very poor oxygenbarrier.

• At ambient conditions (72°F), PET had thehighest impact resistance, followed by OPLA,OPS and OPLA + 40% regrind. The impact prop-erties of PET, OPS and OPLA + 40% regrindimproved at lower temperatures (below freez-ing), while OPLA showed a lowering of dartimpact resistance at these temperatures.

• OPLA and OPLA + 40% regrind materialshowed the higher modulus of elasticity at roomtemperatures, followed by PET and OPS. Theseproperties were also improved at lower tem-perature conditions.

• The results of chemical resistance tests showedthat exposure to acids (pH 6 and 2) and veg-etable oils resulted in minimal strength degra-dation for OPLA, PET and OPS, while OPLA +40% regrind actually showed an improvement.

OPLA and OPLA + 40% regrind containers canprovide better mechanical properties than OPS

and comparable to those of PET. OPLA polymershave barrier properties comparable to OPS poly-mers, and can easily replace this polymer. Finally,OPLA + 40% regrind material can be used for freshfood containers, due to their good physical,mechanical and barrier performance.

ACKNOWLEDGEMENTS

The authors thank Wilkinson Manufacturing Company(Fort Calhoun, NE) for PET, OPLA, OPLA + 40% regrindand OPS samples.

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