processing technologies for poly(lactic acid)

Progress in Polymer Science 33 (2008) 820–852

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Progress in Polymer Science

journa l homepage: www.e lsev ier .com/ locate /ppolysc i

Processing technologies for poly(lactic acid)

L.-T. Lima,∗, R. Aurasb, M. Rubinob

a Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canadab School of Packaging, Michigan State University, East Lansing, MI 48824-1223, USA

a r t i c l e i n f o

Article history:Received 6 June 2007Received in revised form 6 May 2008Accepted 7 May 2008Available online 19 June 2008

Keywords:PolylactidePoly(lactic acid)PLAProcessingConvertingReview

a b s t r a c t

Poly(lactic acid) (PLA) is an aliphatic polyester made up of lactic acid (2-hydroxy propionicacid) building blocks. It is also a biodegradable and compostable thermoplastic derived fromrenewable plant sources, such as starch and sugar. Historically, the uses of PLA have beenmainly limited to biomedical areas due to its bioabsorbable characteristics. Over the pastdecade, the discovery of new polymerization routes which allow the economical produc-tion of high molecular weight PLA, along with the elevated environmental awareness of thegeneral public, have resulted in an expanded use of PLA for consumer goods and packagingapplications. Because PLA is compostable and derived from renewable sources, it has beenconsidered as one of the solutions to alleviate solid waste disposal problems and to lessenthe dependence on petroleum-based plastics for packaging materials. Although PLA canbe processed on standard converting equipment with minimal modifications, its uniquematerial properties must be taken into consideration in order to optimize the conversion ofPLA to molded parts, films, foams, and fibers. In this article, structural, thermal, crystalliza-tion, and rheological properties of PLA are reviewed in relation to its converting processes.Specific process technologies discussed are extrusion, injection molding, injection stretchblow molding, casting, blown film, thermoforming, foaming, blending, fiber spinning, andcompounding.

© 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8212. Structural composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8213. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8234. Crystallization behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8245. Rheological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8266. Thermal degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8277. Processing of PLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828

Abbreviations: BD, 1,4-butanedial; BDI, 1,4-butane diisocyanate; DSC, differential scanning calorimetry; BUR, blow-up-ratio; �Hrel, endothermicenthalpy relaxation; �Hc, heat of crystallization; �Hm, heat of fusion; HDPE, high density polyethylene; HIPS, high impact polystyrene; HMDI, hexamethy-lene diisocyanate; ISBM, injection stretch blow molding; LDPE, low density polyethylene; MD, machine direction; MDO, machine direction orientation; MFI,melt flow index; MMT, montmorillonite; Mn, number-average molecular weight; Mw, weight-average molecular weight; OPLA, oriented poly (lactic acid);OPP, oriented polypropylene; OPS, oriented polystyrene; PEG, poly(ethylene glycol); PET, poly(ethylene terephthalate); PDI, polydispersity index; PDLLA,
poly(d,l-lactic acid); PHA, polyhydroxyalkanoate; PHO, poly(3-hydroxyloctanoate); PLA, poly(lactic acid); PLLA, poly(l-lactic acid); PP, polypropylene; PS,polystyrene; PVT, pressure–volume–temperature; TD, transverse direction; TDO, transverse direction orientation; Tg, glass transition temperature; Tm,melting temperature; WAXS, wide angle X-ray scattering; WVTR, water vapor transmission rate; �0, zero-shear viscosity.
∗ Corresponding author. Tel.: +1 519 824 4120x56586; fax: +1 519 824 6631.E-mail address: [email protected] (L.-T. Lim).

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L.-T. Lim et al. / Progress in Polymer Science 33 (2008) 820–852 821

7.1. Drying and extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8287.2. Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8307.3. Stretch blow molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8337.4. Cast film and sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8357.5. Extrusion blown film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8367.6. Thermoforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8377.7. Foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8387.8. Fiber spinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8407.9. Electrospinning of ultrafine fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8417.10. PLA blends with other polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8447.11. Compounding of PLA composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8467.12. PLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847

8. Conclusion: prospects of PLA polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849. . . . . . . .. . . . . . . .

1

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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Thermoplastic polymers exhibit many properties idealor use in packaging and other consumer products, suchs light weight, low process temperature (compared toetal and glass), variable barrier properties to match end-

se applications, good printability, heat sealable, and easef conversion into different forms. Today, most plasticsre derived from non-renewable crude oil and naturalas resources. While some plastics are being recycled andeused, the majority are disposed in landfills due to end-useontamination. In 2005, plastics were recovered at a rateower than 10% in the USA [1]. Over the past decade, thereas been a sustained research interest on compostableolymers derived from renewable sources as one of theolutions to alleviate solid waste disposal problems and toessen the dependence on petroleum-based plastics.

Poly(lactic acid) (PLA) is a compostable polymer derivedrom renewable sources (mainly starch and sugar). Untilhe last decade, the main uses of PLA have been limited to

edical applications such as implant devices, tissue scaf-olds, and internal sutures, because of its high cost, lowvailability and limited molecular weight. Recently, newechniques which allow economical production of high

olecular weight PLA polymer have broadened its uses2]. Since PLA is compostable and derived from sustain-ble sources, it has been viewed as a promising materialo reduce the societal solid waste disposal problem [3,4].ts low toxicity [5], along with its environmentally benignharacteristics, has made PLA an ideal material for foodackaging and for other consumer products [6].

PLA belongs to the family of aliphatic polyesters derivedrom �-hydroxy acids. The building block of PLA, lacticcid (2-hydroxy propionic acid), can exist in optically active- or l-enantiomers. Depending on the proportion of thenantiomers, PLA of variable material properties can beerived. This allows the production of a wide spectrum ofLA polymers to match performance requirements. PLA haseasonably good optical, physical, mechanical, and barrier
roperties compared to existing petroleum-based poly-ers [7]. For instance, the permeability coefficients of CO2,
2, N2, and H2O for PLA are lower than for polystyrene (PS),ut higher than poly(ethylene terephthalate) (PET) [8–10].he barrier properties of PLA against organic permeants,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849

such as ethyl acetate and d-limonene, are comparable toPET [11]. Mechanically, unoriented PLA is quite brittle, butpossesses good strength and stiffness. Oriented PLA pro-vides better performance than oriented PS, but comparableto PET [9]. Tensile and flexural moduli of PLA are higher thanhigh density polyethylene (HDPE), polypropylene (PP) andPS, but the Izod impact strength and elongation at break val-ues are smaller than those for these polymers [12]. Overall,PLA possesses the required mechanical and barrier proper-ties desirable for a number of applications to compete withexisting petroleum-based thermoplastics.

Today, the main conversion methods for PLA are basedon melt processing. This approach involves heating thepolymer above its melting point, shaping it to the desiredforms, and cooling to stabilize its dimensions. Thus, under-standing of thermal, crystallization, and melt rheologicalbehaviors of the polymer is critical in order to optimizethe process and part quality. Some of the examples ofmelt processed PLA are injection molded disposable cut-lery, thermoformed containers and cups, injection stretchblown bottles, extruded cast and oriented films, and melt-spun fibers for nonwovens, textiles and carpets [6,13,14].PLA also finds uses in other less conventional applications,such as for the housing for laptop computers electronics[14–17]. Recently, PLA has also been processed in conjunc-tion with other filler materials to form composites whichpossess various unique properties, including those basedon nanoclays [18–23], biofibers [16,24,25], glass fibers [26]and cellulose [27,28]. The aim of this review is to discussthe key process technologies for PLA and summarize theproperties of PLA related to the processing techniques used.

2. Structural composition

The basic building block of PLA, lactic acid, can be pro-duced by carbohydrate fermentation or chemical synthesis.Currently, the majority of lactic acid production is basedon the fermentation route. Various purification technolo-gies for lactic acid and lactide can be found in a recent
review by Datta and Henry [2]. One of the main driversfor the recent expanded use of PLA is attributable to theeconomical production of high molecular weight PLA poly-mers (greater than ∼100,000 Da). These polymers can beproduced using several techniques, including azeotropic

822 L.-T. Lim et al. / Progress in Polymer Science 33 (2008) 820–852

Auras e
Fig. 1. Synthesis of PLA from l- and d-lactic acids. Adapted from
dehydrative condensation, direct condensation polymer-ization, and/or polymerization through lactide formation(Fig. 1). By and large, commercially available high molecularweight PLA resins are produced via the lactide ring-openingpolymerization route [3,4,29].

Commercial PLA are copolymers of poly(l-lactic acid)(PLLA) and poly(d,l-lactic acid) (PDLLA), which are pro-duced from l-lactides and d,l-lactides, respectively [3]. Thel-isomer constitutes the main fraction of PLA derived fromrenewable sources since the majority of lactic acid frombiological sources exists in this form. Depending on thecomposition of the optically active l- and d,l-enantiomers,PLA can crystallize in three forms (�, � and �). The �-structure is more stable and has a melting temperature Tm

of 185 ◦C compared to the �-structure, with a Tm of 175 ◦C[3]. The optical purity of PLA has many profound effectson the structural, thermal, barrier and mechanical proper-ties of the polymer [30–36]. PLA polymers with l-contentgreater than ∼90% tend to be crystalline while those withlower optical purity are amorphous. Moreover, Tm, glasstransition temperature Tg, and crystallinity decrease withdecreasing l-isomer content [30,34,37]. Tsuji et al. reportedthat the optical impurity of PLLA films ranging from 0–50%was insignificant in affecting the water vapor transmis-
sion rate (WVTR) of the polymer; nevertheless, the WVTRvalues decreased with increasing film crystallinity in the0–20% range [31]. Thus, judicious selection of appropriatePLA resin grade is important to match the conversion pro-cess conditions used. Usually, PLA articles which require
t al. [3] by permission of Wiley–VCH Verlag GmbH & Co. KGaA.

heat-resistant properties can be injection molded usingPLA resins of less than 1% d-isomer. Alternatively, nucle-ating agents may be added to promote the development ofcrystallinity under relatively short molding cycles. In con-trast, PLA resins of higher d-isomer contents (4–8%) wouldbe more suitable for thermoformed, extruded, and blowmolded (e.g., injection molded preform for blow molding)products, since they are more easily processed when thecrystallinity is low [38].

When exposed to elevated temperatures, PLA is knownto undergo thermal degradation, leading to the formationof lactide monomers (Section 3). It has been suggestedthat this property may be leveraged for the feedstockrecycling of PLA [39,40]. However, the propensity for thelactide monomer to undergo racemization to form meso-lactide can impact the optical purity and thus the materialproperties of the resulting PLA polymer [39–43]. Recently,Tsukegi et al. reported that at temperature less than200 ◦C, conversion of PLLA into meso-lactide and oligomerswas minimal. However, above this temperature, the for-mation of meso-lactide became quite significant (4.5 wt%at 200 ◦C and 38.7 wt% at 300 ◦C for 120 min heating).Oligomers were reported to form at temperatures higherthan 230 ◦C [39]. These authors also reported that the
oligomerization proceeded rapidly in the presence of MgO,to reach an equilibrium between monomers and oligomers;the l,l:meso:d,d lactide composition ratio converged to1:1.22:0.99 (w/w/w) after 120 min heating at 300 ◦C [39].Fan et al. reported that the racemization at 250–300 ◦C


Fw

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Table 1Primary transition temperatures of selected PLA copolymers

Copolymer ratio Glass transitiontemperature (◦C)

Melting temperature (◦C)

100/0 (l/d,l)-PLA 63 17895/5 (l/d,l)-PLA 59 16490/10 (l/d,l)-PLA 56 150

ig. 2. Comparison of glass transition and melting temperatures of PLAith other thermoplastics.

an be controlled by adding calcium oxide to PLLA, whicheduces the pyrolysis temperature, and more importantly,eads to predominant l,l-lactide formation [40].

. Thermal properties

Similar to many thermoplastic polymers, semicrys-alline PLA exhibits Tg and Tm. Above Tg (∼58 ◦C) PLA isubbery, while below Tg, it becomes a glass which is stillapable to creep until it is cooled to its � transition tem-erature at approximately −45 ◦C, below which it behavess a brittle polymer [44]. Fig. 2 compares PLA’s Tg and Tm

alues with other polymers. As shown, PLA has relativelyigh Tg and low Tm as compared to other thermoplastics.

The Tg of PLA is dependent on both the moleculareight and the optical purity of the polymer (Fig. 3). The

g increases with molecular weight to maximum valuest infinite molecular weight of 60.2, 56.4 and 54.6 ◦C forLA consisting of 100, 80, and 50% l-stereoisomer contents,
espectively. Furthermore, PLA with higher content of l-actide has higher Tg values than the same polymer withhe same amount of d-lactide [37]. Similar relationshipsere reported by Tsuji and Ikada [34]. Table 1 shows the
ig. 3. Glass transition temperatures for PLAs of different l-contents as aunction of molecular weight. Curves are created based on the original dataublished by Dorgan et al. [37] by permission of The Society of Rheology.

85/15 (l/d,l)-PLA 56 14080/20 (l/d,l)-PLA 56 125

Adapted from Bigg [33].

glass transition and melting temperatures of different PLApolymers produced with different ratios of copolymer.

In general, the relationship between Tg and molecularweight can be represented by the Flory–Fox equation:

Tg = T∞g − K

M̄n(1)

where T∞g is the Tg at the infinite molecular weight, K is a

constant representing the excess free volume of the endgroups for polymer chains, and M̄n is the number aver-age molecular weight. The values of T∞

g and K are around57–58 ◦C and (5.5–7.3) × 104 as reported in the literaturefor PLLA and PDLLA, respectively [45].

The glass transition behavior of PLA is also dependenton the thermal history of the polymer. Quenching the poly-mer from the melt at a high cooling rate (>500 ◦C/min, suchas during injection molding) will result in a highly amor-phous polymer. PLA polymers with low crystallinity have atendency to undergo rapid aging in a matter of days underambient conditions [46,47]. The phenomenon is an impor-tant contributor to the embrittlement of PLA. This topic willbe discussed in greater details in Section 7.2.

The Tm of PLA is also a function of its optical purity.The maximum practical obtainable Tm for stereochemicallypure PLA (either l or d) is around 180 ◦C with an enthalpyof 40–50 J/g. The presence of meso-lactide in the PLA struc-
ture can depress the Tm by as much as 50 ◦C, dependingon the amount of d-lactide incorporated to the polymer.Fig. 4 shows the variation of the Tm as a function of % meso-lactide introduced in the PLA based on data from Witzke
Fig. 4. Peak melting temperature of PLA as a function of % meso-lactide.(©) Represents values reported by Witzke [48]; (�) represents valuesreported by Hartmann [49]; solid line is calculated based on Eq. (2).

olymer Science 33 (2008) 820–852

Fig. 5. DSC thermograms of water quenched, air-annealed (cooled from220 ◦C to ambient temperature in 5 min), and full-annealed (cooled from220 ◦C to ambient temperature in 105 min) PLLA samples. DSC scans wereperformed at a heating rate of 10 ◦C/min. Adapted from Sarasua et al. [32]by permission of John Wiley & Sons, Inc.

where Kg is the nucleation constant, b is the layer thicknessof the crystal, � is the lateral surface energy, �e is thefold surface energy, �Hf is the heat of fusion per unitvolume, and k is the Boltzmann constant. Table 2 showsthe nucleation parameters from isothermal and non-

Table 2Nucleation parameters from isothermal and nonisothermal kinetic anal-yses for PLLA

824 L.-T. Lim et al. / Progress in P

[48] and Hartmann [49]. The relationship of Tm and meso-lactide content can be approximated reasonably well by thefollowing expression [48]:

Tm (◦C) ≈ 175 ◦C − 300 Wm (2)

where Wm is the fraction of meso-lactide below 0.18 level,and 175 ◦C is the melting temperature of PLA made of 100%l-lactide. Typical Tm values for PLA are in the range of130–160 ◦C. The Tm depression effect of meso-lactide hasseveral important implications as it helps expand the pro-cess windows, reduce thermal and hydrolytic degradation,and decrease lactide formation.

Pyda et al. determined the heat capacity of PLA in solidand liquid states ranging from 5 to 600 K [36]. The heatcapacity (Cp-liquid, J K−1 mol−1) can be represented in a sim-ple form: Cp-liquid = 120.17 + 0.076T, where T is in Kelvin (K).

4. Crystallization behavior

The physical, mechanical and barrier properties ofPLA are dependent on the solid-state morphology andits crystallinity. Accordingly, the crystallization behaviorsof PLA have been studied in detail by many researchers[4,32,50–55]. PLA can be either amorphous or semicrys-talline depending on its stereochemistry and thermalhistory. The crystallinity of PLA is most commonly deter-mined using the differential scanning calorimetry (DSC)technique. By measuring the heat of fusion �Hm and heatof crystallization �Hc, the crystallinity can be determinedbased on the following equation:

crystallinity (%) = �Hm − �Hc

93.1× 100 (3)

where the constant 93.1 J/g is the �Hm for 100% crystallinePLLA or PDLA homopolymers.

On quenching the optically pure PLA polymer from themelt phase (e.g., during injection molding process), theresulting polymer will become quite amorphous. As shownin Fig. 5, quenching the polymer from melt at a high coolingrate resulted in an exothermic crystallization peak on theDSC thermogram during the subsequent reheat, while slowcooling produced a polymer with higher crystallinity withmuch lower enthalpy of crystallization. The tendency forPLA to crystallize upon reheat also depended on the heatingrate (Fig. 6), as well as the optical purity of the PLA polymer(Fig. 7). As shown in Fig. 7, PLA polymers with greater than∼8% d-isomer level remained amorphous even after 15 h ofisothermal treatment at 145 ◦C. In contrast, at 1.5%d-isomerlevel, although the quenched sample (“Quenched PLA-l”)has a minimal crystallinity, the isothermal treatment at145 ◦C resulted in a large endothermic melting peak around450 K (Fig. 7). In general, the crystallization half-time ofPLA increases about 40% for every 1% (w/w) meso-lactidein the polymerization mixture, which is mainly driven bythe reduction of the melting point for the copolymer [56].

Nucleation parameters for PLLA crystallization under
isothermal and nonisothermal conditions were determinedby Kishore and Vasanthakumari using DSC and microscopy[54]. They reported that the radius growth rate of the crys-tals decreased as molecular weight increased, as observedin many other polymers. The nucleation parameters are
Fig. 6. DSC scans for 1.5% d-lactide PLA samples cooled from the melt at10 K/min and then reheated at different heating rates from 30 to 0.3 K/min.Adapted from Pyda et al. [36] by permission of Elsevier B.V.

related in the following form [54,57]:

Kg = 4b��eTm

�Hfk(4)

Parameter Isothermal Non-isothermal

Nucleation parameter, Kg (×105) 2.44 2.69Lateral surface energy, � (×103 J/m2) 12.0 13.6� × �e (×106 J2/m4) 753 830

Adapted from Kishore and Vasanthakumari [54].


Fig. 7. DSC scans at 20 K/min for PLA with 1.5% (PLA-L), 8.1% (PLA-M),and 16.4% (PLA-H) d-isomers. All samples were cooled quickly from themsio

iT1dtIltciho(tiwTsh

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elt and isothermally crystallized at 145 ◦C for 15 h. The quenched PLA-Lample was cooled similarly from the melt but did not undergo the 15 hsothermal crystallization. Thermograms are recreated based on the datariginally published by Pyda et al. [36] by permission of Elsevier B.V.

sothermal kinetic analysis of PLLA. Solving Eq. (4) withm = 480 K, �Hf = 111.083 × 106 J/m3; b = 5.17 × 10−8 cm,2.03 × 10−3 J/m2, and �e = 6.089 × 10−4 J/m2, Kg can beetermined. This value can be used to evaluate the transi-ion between two types of crystallization behavior in PLA.n the first type of crystallization, the nucleation rate isow and axialite morphology in the films is prevalent. Inhe second type, the nucleation rate is high, so multinu-leation occurs and spherulitic morphology in the filmss observed [57]. For PLLA, both crystallization processesave been observed depending on the molecular weightf the samples. The infinite dissolution temperature T0

ddetermined by the extrapolation of dissolution tempera-ure Td versus crystallization temperature Tc plots to thentersection where Td = Tc) for PLLA in p-xylene solutionas determined by Kalb and Pennings to be 126.5 ◦C [58].

his temperature is relevant for fiber formation processes,ince fibers prepared from solution near this temperatureave ultra-high strength properties [58].

The formation of crystallinity may or may not be favor-ble depending on the end-use requirements of the PLArticles. For instance, high crystallinity will not be opti-al for injection molded preforms which are intended

or further blow molding since rapid crystallization of theolymer would hamper the stretching of the preform andptical clarity of the resulting bottle. In contrast, increasedrystallinity will be desirable for injection molded articlesor which good thermal stability is important. Crystal-ization of PLA articles can be initiated by annealing atemperatures higher than Tg and below the melting pointo improve their thermal stability. For instance, Perego etl. showed that crystallization of injection molded PLLAarts by annealing at 105 ◦C for 90 min increased tensional
nd flexural elasticity, Izod impact strength, and heat resis-ance [59]. After annealing PLA copolymers, the presencef two melting peaks in a DSC scan is quite common, asreviously observed by Yasuniwa et al. [60]. They reportedhat the low temperature Tm peak height increased with
Fig. 8. Development of crystallinity in biaxially stretched PLA at 80 ◦Cusing 100% s−1 strain rate. Data are adapted from Drumright et al. [38]by permission of Wiley–VCH Verlag GmbH & Co. KGaA.

increasing heating rate, whereas the high temperature Tm

decreased. In contrast, increasing the cooling rate reducedthe low Tm peak, while the high Tm peak increased. Thedouble-melting peak behavior was explained based onmelt-recrystallization model, in which small and imper-fect crystals changed successively into more stable crystalsthrough the melting and recrystallization [60].

Another strategy to increase the crystallinity of PLA isby incorporating nucleating agent in the polymer duringextrusion. This lowers the surface free energy barrier fornucleation and enables crystallization at higher tempera-ture to take place upon cooling. Kolstad showed that talccan be added to PLLA to effectively modify the crystalliza-tion rate of the polymer [56]. With 6% talc added to PLLA,the crystallization half-time of the polymer reduced from3 min at 110 ◦C to approximately 25 s. At the same percent oftalc, for 3% mesolactide copolymerized with the l-lactide,the half-time reduced from about 7 min to about 1 min [56].Li and Huneault compared the crystallization kinetics oftalc and montmorillonite (MMT, Cloisite® Na+) for 4.5% d-PLA. They reported that the lowest crystallization inductionperiod and maximum crystallization speeds were observedaround 100 ◦C. By adding 1% (w/w) of talc, the crystalliza-tion half time of PLA was decreased from a few hours to8 min. In contrast, the MMT tested was less effective as anucleating agent; the lowest half-time achieved was 30 min[61].

Unlike quiescent crystallization discussed above, strain-induced crystallization occurs when the polymer ismechanically orientated. This phenomenon is prevalentduring the production of oriented PLA films, stretch blowmolding of bottles, thermoforming of containers, and fiberspinning. As expected, the proportion of d- and l-isomershas an effect on the strain-induced crystallinity duringthe mechanical orientation. As shown in Fig. 8, the per-cent crystallinity of amorphous PLA sheet increases with
increasing draw ratio. Moreover, the crystallinity decreasesas the stereoisomeric purity of the polymer decreases [38].The amount of crystallinity attained through orientationalso depends on the mode of stretching (sequential ver-

826 L.-T. Lim et al. / Progress in Polymer

Fig. 9. Comparison of zero-shear viscosity values versus molecular weightfor poly(85% l-co-15% d-lactide) at 85 and 100 ◦C as reported by Witkze[48], and PLLA at 180 ◦C as reported by Dorgan et al.[12].

sus simultaneous), strain rate, temperature, and annealingconditions [38,62,63]. More discussions on this topic willbe presented in Section 7.2.

5. Rheological properties

Melt rheological properties of PLA have a profoundeffect on how the polymer flows during the conver-sion process. Since the PLA rheological properties arehighly dependent on temperature, molecular weight andshear rate, they must be taken into consideration duringtooling design, process optimization, and process model-ing/simulation. Melt viscosities of high-molecular-weightPLA are in the order of 5000–10,000 P (500–1000 Pa s)at shear rates of 10–50 s−1. These polymer grades areequivalent to Mw ∼100,000 Da for injection molding to∼300,000 Da for film cast extrusion applications [4]. Themelts of high molecular weight PLA behave like a pseu-doplastic, non-Newtonian fluid. In contrast, low molecularweight PLA (∼40,000 Da) shows Newtonian-like behaviorat shear rates typical of film extrusion [64]. Under iden-tical processing conditions, semicrystalline PLA tends topossess higher shear viscosity than its amorphous counter-part. Moreover, as shear rates increase, the viscosities of themelt decrease considerably, i.e., the polymer melt exhibitsshear-thinning behavior [65].

Viscoelastic properties of polymer melts can be charac-terized by zero-shear viscosity, �0, and recoverable shearcompliance JO

e . Both of these parameters can be obtainedfrom dynamic experiments by determining the dynamicmoduli at the limit of low frequency [48]. The product ofthese two values (�0 × JO

e ) gives the average relaxation timerequired for final stress equilibration time in the liquid �0.The value of �0 is strongly affected by the molecular weight,which is typically described empirically by the power law
equation. Cooper-White and Mackay reported that the �0of PLLA melt showed dependence on Mw to the 4.0 powerinstead of the theoretical value of 3.4 [64]. In comparison,Dorgan et al. reported a power index of 4.6 [66]. Fig. 9 showsthe relationship between �0 and Mw for PLLA (100:0) at
Science 33 (2008) 820–852

180 ◦C [12], and 15% d-lactide PDLA at 85 and 100 ◦C [48].Witkze showed that the temperature effect on �0 for 15%d-lactide PLA can be described by [48]:

�0 = n0,ref

(Mw

100, 000

)aexp

(Ea

R

(1

T(K)− 1

373

))(5)

where a = 3.38 ± 0.13, the activation energy of flowEa = 190 kJ/mol, �0,ref = 89,400 ± 9300 Pa s, R is the gasconstant 8.314 J/K mol, and T is the temperature in K.Witzke further showed that �0 can be correlated withthe isomer composition by fitting to the well-knownWilliams–Landel–Ferry equation (WLF) [48]:

�0 = (a1 + a2Wmeso + a3Wl-mer)(

Mw

100, 000

)3.38

×exp( −C1(T(C) − 100)

C2 + (T(C) − 100)

)(6)

where Wmeso and Wl-mer are the initial weight frac-tions for meso-lactide and l-lactide, respectively,a1 = −13,000, a2 = −142,000, a3 = 112,000, C1 = 15.6 ± 1.6,and C2 = 110 ± 11 ◦C; a1, a2, a3, and C1 do not have units;and T(C) is the testing temperature in ◦C. Eq. (6) canbe used to predict �0 of amorphous polylactides withl-monomer composition higher than 50% between Tg

and Tg + 100 ◦C. The equation predicts that �0 increaseswith increasing l-monomer and decrease as meso-lactidecontent increases [48].

The rheological properties of PLA can be modified by theintroduction of branching into the polymer chain architec-ture. Many routes, such as multifunctional polymerizationinitiators, hydroxycyclic ester initiators, multicyclic ester,and crosslinking via free radical addition have been usedto introduce branching in PLA [12,67–69]. Lehermeierand Dorgan blended PLA with 5% d-isomer with varyingproportions of branched PLA produced through peroxideinitiated crosslinking of linear PLA by reactive extrusion[67]. They observed that �0 of the blends deviated con-siderably from the log additive rule and attributed this tothe effect of free volume. Lehermeier and Dorgan showedthat tris(nonylphenyl) phosphite was effective for stabi-lizing the viscosity of PLA during the thermorheologicaltime sweep experiment of branched PLA polymers [67]. Inanother study from the same research group, the stabiliz-ing effect of tris(nonylphenyl) phosphate was elucidatedby using the time-temperature superposition technique,showing that this compound greatly facilitated the ther-morheological experiments by prevented the confoundingeffect from degradation reactions [69,70].

Carreau–Yasuda model (Eq. (7)) has been used to modelthe viscosity and shear rate relationship of linear PLA andlinear-branched PLA blends [69]:

� = C1[1 + (C2�̇)C3 ](C4−1/C3) (7)

where � is the viscosity, � is the shear rate, and C1, C2, C3and C4 are material dependent parameters. The constants
for the model are summarized in Table 3. C1 determines�0 which decreases with increasing linear content. C2 isthe relaxation time approximately corresponded to thereciprocal of frequency for the onset of shear thinning.C3 determined the shear thinning which increased with

L.-T. Lim et al. / Progress in Polymer

Table 3Carreau–Yasuda model parameters for Eq. (7)

Blend, % Carreau–Yasuda parameters

C1 (Pa s) C2 (s) C3 C4

0 10,303 0.01022 0.3572 −0.034020 8,418 0.00664 0.3612 −0.073140 6,409 0.01364 0.4523 0.052360 5,647 0.00513 0.4356 −0.1002

A

is�ap

ccpttcapcaneP

ppmtTou

[

wa

TM

P

((((((((((((

A

80 4,683 0.00450 0.4754 −0.1108100 3,824 0.01122 0.7283 0.0889

dapted from Lehermeir and Dorgan [69].

ncreasing linear content, i.e., branched PLA shear thinnedtronger than the linear material [69]. The increase of both0 and shear thinning with the addition of branching islso reported by other studies on PLA polymers with starolymer chain architectures [12,66].

Palade et al. studied the extensional viscosities of high l-ontent PLA (100,000–120,000Mw). They showed that PLAan be drawn to large Hencky strains without breaking. Theolymer also exhibited strain-hardening behaviors duringhe deformation [70], which is an important characteris-ic for processing operations, such as fiber spinning, filmasting, and film blowing. Yamane et al. reported that theddition of PDLA to PLLA enhanced the strain hardeningroperties of the resulting blends even at very low PDLAontents (<5 wt%). They also reported that low Mw PDLAffected the shear rheology of the blends much more sig-ificantly than high Mw PDLA [71]. This may provide anffective avenue for modifying the spinning behavior of theLA.

Although solution viscosity of PLA in solvent does notrovide direct relevance to the processing of molten PLAolymer, this property is frequently evaluated to deter-ine the molecular weight of resins and processed parts

o ensure that they are within the required specifications.he relationship between viscosity and molecular weightf PLA dissolved in dilute solution is commonly modeledsing the Mark–Houwink equation:

�] = K × Mav (8)

here [�] is the intrinsic viscosity, K and a are constants,nd Mv is the experimental viscosity average molecular

able 4ark–Houwink constants PLA in selected solvents

olymer types Equations

1) PLLA [�] = 5.45 × 10−4 M0.73v

2) PDLLA [�] = 1.29 × 10−5 M0.82v

3) PDLLA [�] = 2.21 × 10−4 M0.77v

4) Linear PLLA [�] = 4.41 × 10−4 M0.72v

5) “Star” PLLA (six arms) [�] = 2.04 × 10−4 M0.77v

6) PDLLA [�]¦ = 2.59 × 10−4 M0.689v

7) PDLLA [�] = 5.50 × 10−4 M0.639v

8) PLLA (amorphous) [�] = 6.40 × 10−4 M0.68v

9) PLLA (amorphous/semi-crystalline) [�]¦ = 8.50 × 10−4 M0.66v

10) PLLA (semi-crystalline) [�] = 1.00 × 10−3 M0.65v

11) PDLLA [�] = 2.27 × 10−4 M0.75v (one p

12) PDLLA [�] = 1.58 × 10−4 M0.78v

dapted from Garlotta [4].a THF: tetrahydrofuran.

Science 33 (2008) 820–852 827

weight. The Mark–Houwink equation is dependent on thetype of PLA, the solvent used, and the temperature of thesolution. Table 4 summarizes the Mark–Houwink parame-ters for different compositions of PLA polymers in differentsolvent solutions.

6. Thermal degradation

One of the drawbacks of processing PLA in the moltenstate is its tendency to undergo thermal degradation, whichis related both to the process temperature and the resi-dence time in the extruder and hot runner [72]. By andlarge, thermal degradation of PLA can be attributed to:(a) hydrolysis by trace amounts of water, (b) zipper-likedepolymerization, (c) oxidative, random main-chain scis-sion, (d) intermolecular transesterification to monomerand oligomeric esters, and (e) intramolecular transesteri-fication resulting in formation of monomer and oligomerlactides of low Mw [73]. Kopinke et al. proposed that above200 ◦C, PLA can degrade through intra- and intermolecu-lar ester exchange, cis-elimination, radical and concertednon-radical reactions [41], resulting in the formation of CO,CO2, acetaldehyde and methylketene. In contrast, McNeilland Leiper proposed that thermal degradation of PLA is anon-radical, “backbiting” ester interchange reaction involv-ing the -OH chain ends [74]. Depending on the point inthe backbone at which the reaction occurs, the productcan be a lactide molecule, an oligomeric ring, or acetalde-hyde plus carbon monoxide (Fig. 10). Similar degradationmechanisms were reported by Kopinke et al. [41]. At tem-peratures in excess of 270 ◦C, homolysis of the polymerbackbone can occur. The formation of acetaldehyde isexpected to increase with increasing process temperaturedue to the increased rate of the degradation reactions. Inthe 230–440 ◦C temperature range explored by McNeill andLeiper [74], acetaldehyde is formed in highest proportion at230 ◦C and a marked decrease is observed at 440 ◦C, whichis believed to be caused by the thermal degradation of
acetaldehyde, involving a complex chain reaction to formmethane and carbon monoxide at the elevated tempera-ture. McNeill and Leiper also proposed that the formationof butane-2,3-dione, another byproduct detected, is likelycaused by the radical combination of acetyl radicals from
Conditions

25 ◦C in chloroform [59,189]25 ◦C in chloroform [190]25 ◦C in chlofoform [59,189]25 ◦C in chloroform [190]25 ◦C in chloroform [190]35 ◦C in THFa [191]31.15 ◦C in THF [191]30 ◦C in THF [192]30 ◦C in THF [192]30 ◦C in THF [192]

oint method) 30 ◦C in benzene [193], Tuan–Fuoss viscometer25 ◦C in ethyl acetate [194]


m McNe
Fig. 10. Thermal degradation of PLA. Adapted fro
the chain reaction [74]. Although acetaldehyde is consid-ered to be non-toxic and it is naturally present in manyfoods, the acetaldehyde generated during melt processingof PLA must be minimized, especially if the converted PLA(e.g., container, bottle, and films) are to be used for foodpackaging. The migration of acetaldehyde into the con-tained food can result in off-flavor which may impact theorganoleptic properties and consumer acceptance of theproduct [75–77].

From the production point of view, the formation oflactide due to depolymerization is undesirable. Besidesreducing PLA melt viscosity and elasticity, the volatile lac-tide formed can result in fuming and/or fouling of theprocessing equipment such as chilled rollers, molds andtooling surfaces [78]. The latter is characterized by thegradual building up of a layer of lactide on the equipmentsurfaces, commonly known as plate out. To overcome thisproblem, the temperature of the equipment is generallyelevated to reduce the tendency of condensation of lactide.

Taubner and Shishoo showed that the moisture contentof resin, temperature, and residence time of PLA meltduring extrusion are important contributors to molecularweight drop of the polymer during extrusion [72]. Pro-cessing of dried PLLA with initial Mn of 40,000 g/mol ina twin-screw extruder at 210 ◦C caused the Mn to drop to33,600 and 30,200 g/mol, when screw rotation speeds of120 and 20 rpm were used, respectively. Using the same120 and 20 rpm screw speeds but processing at 240 ◦C,the Mn values decreased dramatically to 25,600 and13,600 g/mol, respectively. In contrast, Mn for extruded
articles produced from wet resins (equilibrated at 20 ◦C65% RH to give 0.3%, w/w, moisture content) were 18,400and 12,000 g/mol, respectively. These results highlightedthe importance of minimizing the residence time andprocess temperature during PLA extrusion. From a resin
ill and Leiper [74] by permission of Elsevier B.V.

formulation point of view, the residual polymerizingcatalysts present in the resin are also known to catalyzethe reverse depolymerization and hydrolysis reactions[48,79]. This may partially explain the large variation ofmolecular weight drop for melt processed PLA reportedin the literature. For instance, Witzke, Gogolewski et al.and Perego et al. reported molecular weight losses forinjection molded PLA parts of 5–52%, 50–88% and 14–40%,respectively [48,59,80]. To stabilize the polymer duringmelt processing, the removal or deactivation of the residualcatalyst is important to minimize the molecular weightloss which will impact the mechanical properties of thePLA parts. Strategies to improve the melt stability of PLAcan be found in patent publications [79,81,82]. Due tothe different processes and technologies used, the meltstability of PLA polymer may be different from supplier tosupplier. Injection molded PLA made from properly driedgood quality PLA resins and optimal processes shouldexhibit molecular weight loss of 10% or less [83].

7. Processing of PLA

7.1. Drying and extrusion

Prior to melting processing of PLA, the polymer must bedried sufficiently to prevent excessive hydrolysis (molec-ular weight drop) which can compromise the physicalproperties of the polymer. Typically the polymer is dried toless than 100 ppm (0.01%, w/w). Natureworks LLC, one ofthe main suppliers for PLA polymers, recommended that
resins should be dried to 250 ppm (0.025%, w/w) mois-ture content or below before extrusion. Processes thathave long residence times or high temperature approaching240 ◦C should dry resins below 50 ppm to achieve maxi-mum retention of molecular weight [84,85]. Drying of PLA


Table 5Drying half times for PLA pellets under −40 ◦C dew point and air flow rateof 0.016 m3/(min kg) [108]

Drying temperature (◦C) Drying half time (h)

Amorphous pellets40 4.0

Crystalline pellets40 4.350 3.960 3.3

traupapbPhhpm

dmdaeitr(mlic

Feb

70 2.180 1.3

100 0.6

akes place in the temperature range of 80–100 ◦C. Theequired drying time is dependent on the drying temper-ture (Table 5). Commercial grade PLA resin pellets aresually crystallized, which permits drying at higher tem-eratures to reduce the required drying time. In contrast,morphous pellets must be dried below the Tg (∼60 ◦C) torevent the resin pellets from sticking together, which canridge and plug the dryer. It is noteworthy that becauseLA degrades at elevated temperatures and high relativeumidity, the resins should be protected from hot andumid environments. Henton et al. reported that amor-hous PLA can dramatically reduce its Mw in less than aonth when exposed to 60 ◦C and 80% RH (Fig. 11) [44].To achieve an effective drying, the dew point of the

rying air should be −40 ◦C or lower. Drying of PLA is com-only achieved using a closed loop dual-bed regenerative

esiccant-type dryer. In this type of dryer, the resin pelletsre contained in a hopper that is purged with dry air atlevated temperature. The dry air is generated by the des-ccant bed. During the operation, one desiccant bed is inhe process air stream which removes moisture from theesin, while the other stand-by bed is being regeneratedFig. 12). The hot air from the process stream removes the

oisture from the resin in the hopper. The air is then circu-ated back to the dryer where it is cooled and the moistures stripped by the desiccant. The air is reheated before it ishanneled back to the hopper. When the dew point of the

ig. 11. Plots of molecular weight loss of PLA versus time under differentnvironment conditions. Curves are based on the original data publishedy Henton et al. [44].

Fig. 12. Typical closed loop dual-bed regenerative desiccant-type dryerfor drying PLA before extrusion.

process air is greater than the set point, the desiccant goesinto the regeneration cycle where the desiccant is heatedto desorb the moisture from the desiccant and vent it to theatmosphere. Meanwhile, the process air is directed to thestand-by desiccant which was previously dried.

Extrusion is the most important technique for contin-uously melt processing of PLA. The plasticizing extrudercan be part of the forming machine systems for injectionmolding, blow molding, film blowing and melt spinning.Fig. 13 shows a schematic representation of the major com-ponents of an extruder for an injection molding machine. Atypical screw consists of three sections: (1) feed section –acts as an auger which receives the polymer pellets andconveys the polymer into the screw; (2) transition sec-tion (also known as compression or melting sections) –flight depth decreases gradually, which compresses the pel-lets to enhance the friction and contact with the barrel. Inorder to segregate the molten polymer pool from the pelletunmelted pellets, various barrier flight designs have beenadopted; (3) metering section – characterized by a constantand shallow flight depth, which acts as a pump to meteraccurately the required quantity of molten polymer. Thel/d ratio, which is the ratio of flight length of the screwto its outer diameter, determines the shear and residencetime of the melt. Screws with large l/d ratio provide greatershear heating, better mixing, and longer melt residencetime in the extruder. Commercial grade PLA resins can typ-ically be processed using a conventional extruder equippedwith a general purpose screw of l/d ratio of 24–30. Extruderscrews for processing PET, which are typically low-shear forgentle mixing to minimize resin degradation and acetalde-hyde generation, are also suitable for processing PLA resin[14]. Another important screw parameter is the compres-
sion ratio, which is the ratio of the flight depth in the feedsection to the flight depth in the metering section. Thegreater the compression ratio a screw possesses, the greaterthe shear heating it provides. The recommended compres-sion ratio for PLA processing is in the range of 2–3 [86].


f a scre
Fig. 13. Typical geometries o
During the plasticizing process, PLA resin pellets arefed from a hopper near the end of a barrel. The screw,driven by an electric or hydraulic motor, rotates and trans-ports the material towards the other end of the barrel. Theheat required for melting is provided by the heater bandswrapped around the barrel. As the screw rotates, the flightsshear and push the polymer against the wall of the barrelwhich also provides frictional heat for melting the poly-mer. The combined thermal energy from the heater andfrictional heat due to friction between the plastic and thescrew and barrel, provide sufficient heat to raise the PLApolymer above its melting point (170–180 ◦C) by the timeit reaches the end of the barrel. To ensure that all the crys-talline phases are melted and to achieve an optimal meltviscosity for processing, the heater set point is usually setat 200–210 ◦C.

7.2. Injection molding

Injection molding is the most widely used convertingprocess for thermoplastic articles, especially for those thatare complex in shape and require high dimensional preci-sion. All injection molding machines have an extruder forplasticizing the polymer melt. Unlike a standard extruder,the extruder unit for injection molding machine is designed
such the screw can reciprocate within the barrel to pro-vide enough injection pressure to deliver the polymer meltinto the mold cavities (Fig. 14). Most injection moldingmachines for PLA are based on the reciprocating screwextruder, although two-stage systems, which integrate a
Fig. 14. Major components of an injection molding machine sh

w for single-screw extruder.

shooting pot and extruder in a single machine, have alsobeen deployed for injection molding of preforms for PLAbottles. The two-stage system consists an in-line extruderintegrated to a shooting pot. The extruder plasticizes andfeeds the melt into the shooting pot under relatively lowinjection pressure, from which the melt is injected into thehot runner under high pressure by a plunger in the shootingpot. While the reciprocating machine must stop the screwduring the injection and packing phases, the screw for thetwo-stage machine can rotate during the majority of thecycle. The two-stage system presents some advantages overits reciprocating counterpart, including shorter cycle time,small screw motor drive, more consistent melt quality, andmore consistent shot size [87].

A typical cycle for an injection molding machine is pre-sented in Fig. 15. The beginning of mold close is usuallytaken as the start of an injection molding cycle. Immedi-ately after the molds clamp up, the nozzle opens and thescrew moves forward, injecting the polymer melt into themold cavity. To compensate for the material shrinkage dur-ing cooling in the mold, the screw is maintained in theforward position by a holding pressure. At the end of theholding phase, the nozzle is shut off and the screw begins torecover, while the part continues to be cooled in the mold.During the recovery phase, the screw rotates and conveys
the polymer forward along the screw. At the same time,the screw is allowed to slide backward within the barrelagainst a controlled back pressure exerted on the screwby a hydraulic cylinder. To ensure that the part is dimen-sionally stable enough to withstand the opening stroke the
owing the extruder (reciprocal screw) and clamp units.


mita(

oTtcoftai

The PVT relationship can be modeled mathematically,

Ft

Fig. 15. Typical cycle for an injection molding process.

olds, sufficient cooling time must be given. In the mold-ng cycle, heat removal takes place predominantly duringhe fill, hold and cool phases, although mold opening phaselso contributes to partial cooling since one side of the partcore-contacting side) is still being cooled prior to ejection.

Cycle time is an important process parameter which isften minimized to maximize the production throughput.o reduce the cycle time, it is quite common to transferhe partially cooled injection molded article to a post-moldooling device, to provide an extended cooling of the partutside the molds, either by direct contact on a chilled sur-
ace and/or by forced air. From Fig. 15, it is also evidenthat minimizing the duration for non-process events, suchs mold opening, part ejection and mold closing is alsomportant for reducing the cycle time. Lowering mold tem-
ig. 16. PVT plots for PLA based on the data from Sato et al. and Natureworks LLwo-domain modified Tait model (Eq. (8)).

Science 33 (2008) 820–852 831

perature can also increase the heat extraction rate fromthe polymer. Nevertheless, the propensity of lactide con-densation on the cold tooling surfaces, which can affectthe surface finish and weight of the molded articles, limitsthe minimal temperature that can be used during injec-tion molding of PLA to 25–30 ◦C. The use of molds withpolished surfaces, in conjunction with an increased injec-tion speed during fill, can also reduce the deposition of thelactide layers.

The fill, hold and cool events that take place duringinjection molding have an important implication on theshrinkage of the injection molded articles. This effect canbe best elucidated using a pressure–volume–temperature(PVT) diagram. Fig. 16 shows PVT diagrams for PLA from tworeferences [88,89]. The different profiles shown here arelikely due to the different grades of PLA used. During injec-tion molding, the polymer is first subjected to isothermalinjection of the polymer melt into the mold cavity, dur-ing which the pressure increases as the polymer is beinginjected and packed to the holding pressure (trace ab inFig. 16). The polymer then undergoes isobaric cooling inthe holding phase (trace bc), followed by isochoric cool-ing. When the polymer cools below the freezing point, thegate freezes and the pressure in the mold cavities dropsto one atmospheric pressure (trace cd). In the last cool-ing phase, the article continues to cool isobarically to roomtemperature (trace de). The change in specific volume dur-ing the final isobaric cooling (trace de) dictates the extentof part shrinkage. The hold pressure and temperature playan important role in determining how much the moldedarticle shrinks.

such as by using the modified two-domain Tait model[90–92]. This model is often used for numerical simula-tion of injection molding processes involving finite elementanalysis for predicting the shrinkage behavior of injection

C [88,89]. The continuous lines represent the fitted results based on the


4% d-lant temp

Fig. 17. Effects of temperature and time on the aging of injection moldedfor various aging times. (B) DSC curves of PLA annealed for 24 h at differe

molded articles. The modified two-domain Tait PVT modeltakes the form:

v(T, p) = V0(T)[

1 − C ln(

1 + p

B(T)

)]+ Vt(T, p) (9)

where v(T, p) is specific volume at temperature T and pres-sure P; V0 is specific volume at zero gauge pressure and C isa constant, 0.0894. When the temperature of the materialis greater than the transition temperature, V0(T) and B(T)are determined by b1m, b2m, b3m, b4m and b5 as follows:

V0 = b1m + b2m(T − b5) (10)

B(T) = b3m exp[−b4m(T − b5)] (11)

In contrast, when the material temperature is lower thanthe transition temperature, V0(T) and B(T) are determinedby b1s, b2s, b3s, b4s and b5 as follows:

V0 = b1s + b2s(T − b5) (12)

B(T) = b3s exp[−b4s(T − b5)] (13)

Because the transition temperature, Ttrans(P), is often pres-sure dependent, it is often correlated with pressure andthe transition temperature at zero gauge pressure (b5) asfollows:

Ttrans(P) = b5 + b6p (14)

For non-amorphous materials, an additional transitionfunction is required:

V (T, p) = b exp[b (T − b ) − b p] (15)
t 7 8 5 9
The estimated parameter values for the modified two-domain Tait model are shown in the inserts in Fig. 16.

In general, injection molded PLA articles are relativelybrittle. The brittleness of PLA has been attributed to the

ctide PDLA specimens. (A) DSC curves of PLA aged at room temperatureeratures. Plots are created based on the data from Cai et al. [47].

rapid physical aging of the polymer since ambient tem-perature is only about 25 ◦C below the Tg [37,46,48]. Theaging of PLA can be evaluated by studying the Tg region ofa DSC scan. By measuring the development of endother-mic enthalpy relaxation �Hrel using DSC on injectionmolded samples made from PLA (96% l-lactide), Cai et al.showed that �Hrel increased with increasing aging time[47] (Fig. 17). They also showed that as the aging temper-ature increased towards the Tg, the rate of physical agingalso became faster. However, when the aging temperaturewent above the Tg (60 ◦C), the excess enthalpy relaxationwas reduced, indicating that physical aging was no longertaking place when the aging temperature was above Tg [47].Celli and Scandola observed a similar aging trend for PLLAusing DSC and a dynamic mechanical analyzer [46]. Theyobserved that the extent of aging increased with decreas-ing molecular weight (i.e., �Hrel increased with decreasingmolecular weight), which was attributed to the increasedchain terminals that possess higher motional freedom thanthe internal chain segments [46]. The physical implicationof aging was elucidated by Witzke, who reported that injec-tion molded articles tested immediately after quenchingto very cold temperatures exhibited a much larger exten-sion to break. However, when the molded specimens wereaged at room temperature for 3–8 h, they became verybrittle [48]. This phenomenon was attributed to the reduc-tion of free volume of the polymer due to rapid relaxationtowards the equilibrium amorphous state. Aging below Tg isexclusively related to the amorphous phase of the polymer;
accordingly, increasing the crystallinity of the polymer (e.g.,by adjusting d-isomer composition or the use of nucleat-ing agents) will reduce the aging effect. Furthermore, thecrystallites formed also act like physical crosslinks to retardthe polymer chain mobility. However, amorphous injection

olymer

micbtcb

7

timPniieeh

sd

L.-T. Lim et al. / Progress in P

olded articles which are intended for further process-ng (e.g., preforms for stretch blow molding), the storageonditions prior to subsequent processing may need toe controlled. Moreover, process parameters such as moldemperature, packing pressure, cooling rate, and post-moldooling treatment are expected to influence the PLA agingehavior as well.

.3. Stretch blow molding

Due to the recent consumers’ heightened environmen-al awareness, there is a sustained interest from the foodndustry to replace the existing non-biodegradable ther-

oplastics with PLA for certain beverage products. To date,LA bottles are predominantly used for beverages which areot sensitive to oxygen (e.g., flat water beverages, pasteur-

zed milk). While barrier properties of PLA bottles may bemproved by various technologies (multilayer structures,xternal coating, internal plasma deposition, oxygen scav-
nger), their implementation is currently limited due toigher production costs.
The production of PLA bottles is based on injectiontretch blow molding (ISBM) technique. This process pro-uces biaxial orientated PLA bottle with much improved

Fig. 18. Injection stretch blow mol

Science 33 (2008) 820–852 833

physical and barrier properties compared to injec-tion molded amorphous PLA. The molecular orientationinduced during the ISBM process decreases the effect ofaging by stabilizing the polymer free volume [48]. Thecrystallites produced during strain-induced crystallizationalso reduce the aging effect since they can act as physicalcrosslinks to stabilize the amorphous phase, thereby reduc-ing its brittleness. Similar effects have been reported forsemicrystalline PET [93]. The ISBM process for PLA bottlesis depicted in Fig. 18. It involves first the formation of pre-form (also known as parison) using an injection moldingmachine. The preform is then transferred to a blow mold-ing machine where it is stretched in the axial direction andblown in the hoop direction to achieve biaxial orientationof the polymer. In the blow molding machine, the preformis heated in front of several banks of infrared heater to tem-peratures (85–110 ◦C) suitable for blow molding (Fig. 18a).Different power settings are usually applied to the infraredheaters to give a temperature profile optimal for stretch-
ing the preform into bottle with uniform wall thicknessdistribution. Frequently, reheat additives, such as carbonblack dispersed in a liquid carrier, are added to the resinin the extruder to increase its infrared energy absorption.PLA preforms have a tendency to shrink after reheat, espe-
ding (ISBM) of PLA bottle.

olymer
cially regions near the neck and the end cap where theresidual injection molding stresses are the greatest. Thismay be moderated through proper preform design, withgradual transition regions. When the preform has attainedthe optimal temperature, it is transferred to the blow mold(Fig. 18b). The blow nozzle is lowered to seal the preformfinish, while the stretch rod travels towards the preform,at a typical speed of 1–1.5 m/s, and stretches the preformto the base cup (Fig. 18c–e). During the preblow phase(Fig. 18d and e), compressed air of 0.5–2.0 MPa is admittedto the preform through the blow nozzle to partially inflatethe preform to prevent it from touching the stretch rod dur-ing the axial stretching. When the stretch rod arrives at thebase cup and pins the preform to the mold base, the air pres-sure ramps up to 3.8–4.0 MPa to fully inflate the preform.This forces the inflated preform to take the shape of theblow mold and to imprint the surface details of the bottles(Fig. 18f and g). The high blow pressure is maintained forseveral seconds to allow the bottle to cool down sufficientlybefore discharging the bottle.

The aforementioned process is known as the two-stageprocess. In contrast, the one-stage process entails the injec-tion and blow molding of the preform within the samemachine equipped with both injection and blow mold-ing units. In this process, the injection molded preformis partially cooled down to 100–120 ◦C and then stretchblown in the blow molding station. Fig. 19 summarizes thethermal history of PLA from resin pellets to bottle for thetwo processes. As shown, PLA preform made in the one-stage process does not go through the aging process duringwhich the polymer tends to embrittle. Thus, PLA preformsintended for one- and two-stage processes may need to be
designed and processed differently. The neck finish of thepreform is highly amorphous and is quite brittle. Therefore,the neck finish must be designed such that the side wall isthick enough to prevent the neck from blowing out or crack-
Fig. 19. Thermal history of PLA polymer during one

Science 33 (2008) 820–852

ing due to the compression load from the blow nozzle. Theblow mold temperature for PLA is typically set at around35 ◦C. Because the base of the bottle tends to be quite thick,the residual heat can cause the base to roll out after thebottle is ejected from the blow mold. This problem can beovercome by incorporating radial ribs to reinforce the baseand/or chilling the base mold insert to a temperature lowerthan the mold halves [94].

Similarly to PET, PLA exhibits strain-hardening whenstretched to high strain. This self-leveling phenomenon isdesirable for blow molding of preforms to achieve optimalbottle side wall orientation and minimize wall thicknessvariation. Since strain-hardening occurs only when the PLAis stretched beyond its natural stretch ratio, the preformmust be designed to match the target bottle size and shape,such that optimal stretch ratios are achieved during blowmolding (Fig. 20). Preforms that are under-stretched willresult in bottles with excessive wall thickness variation,weak mechanical properties and poor aesthetic appeal (e.g.,lens defect below the support ledge region). In contrast,overstretched bottles can also result in stress whiteningdue to the formation of micro-cracks on the bottle surfacesthat diffract light. Typical commercial grade PLA resins forbottle applications require preform axial stretch ratios of2.8–3.2 and hoop stretch ratios of 2–3, with the desirableplanar stretch ratio of 8–11 [94,95]. It is noteworthy that theultimate amount of crystallinity after stretching decreaseswith the decreasing stereoisomeric purity of the polymer[38]. Accordingly, the optimal stretch ratios depend on thegrade of PLA used.

Preform designs are often proprietary, and thereforethere is a lack of information in the open literature. An opti-
mal preform design should meet the minimum requiredstretching which is above the natural stretch ratio, by vary-ing the shape, diameter, length, blend radius, and transitionfeatures, to meet the part weight requirement. Depending
- and two-stage PLA bottle manufacturing.


F , showin

oacbbcmr

7

spmbfiamownrttncdottb

srdt

ig. 20. Schematic representation of PLA preform (left) and bottle (right)

n the shape of the bottle, subtle but critical features suchs transition shape (reverse versus standard taper), stephanges, and pinch points on the core and cavity may alsoe incorporated in the preform design. Since the stretchingehavior of PLA is similar to PET but not entirely the same,onversion of materials using existing PET preform designsay be feasible, although design modifications are often

equired to achieve an optimized bottle.

.4. Cast film and sheet

PLA with l-lactide contents of 92–98% have beenuccessfully extruded using conventional extruders. Theroduction of PLA film and sheet is practically identical; theain difference between them is their stiffness and flexi-

ility due to the difference in their thicknesses. Typically,lms are ≤0.076 mm (0.003 in.) in thickness, while sheetsre typically ≥0.25 mm (0.01 in.). In cast film extrusion, theolten PLA is extruded through a sheet die and quenched

n polished chrome rollers that are cooled with circulatingater. Due to the thermal sensitivity of PLA, the use of exter-al deckles on the die should be avoided since the degradedesin behind the deckles can lead to edge instability. Usuallyhe die gap is set to 10% or 25–50 �m (1–2 mils) greater thanhe target sheet thickness [84]. Ljungberg et al. extrudedeat PLA in a Haake Rheomex 254 extruder with a Rheo-ord 90 drive unit (Karlsruhe, Germany) [96]. The 19.3 mmiameter screw has a compression ratio of 2:1 and l/d ratiof 25. In this study, the temperatures for the feeding zone,he barrel and the die were 160, 180, and 175 ◦C, respec-ively [96]. Similar extruder temperature profiles were usedy Gruber et al. [79].

Sheet and film forming can be achieved on a three-rolltack. Because of the low melt strength of PLA, horizontaloll stacks configuration is preferred. To avoid the con-ensation of lactide monomers and slippage of web onhe rollers, relatively high roller temperatures (25–50 ◦C)

g their key features and main stretch ratios used for preform design.

are usually used. Lactide monomer buildup around thedie could be further prevented by using an exhaust sys-tem. Nevertheless, extreme high temperatures should beavoided as the web will stick to the rollers, resulting inpoor quality sheet. To reduce the chance of trapping airand reduce film or sheet defects, one resin supplier recom-mended that the die be positioned as close as possible tothe entrance nip and slightly higher than the nip to accom-modate the slight drooping of the molten PLA web [84].To cast PLA film, Ljungberg et al. used a 200-mm fishtaildie with a 300–400 �m split gap and a casting air gap of15 mm [96]. Generally, hydraulic rolls stands, capable ofproducing pressure around 800–900 lbs/linear inch of die isrequired to prevent floating of the rolls which would resultin uneven PLA surfaces, edge instability, and neck-in [84].Good contact between the web and rolls is also importantto minimize lactide buildup. Casting of PLA film usuallyrequires edge pinning (electrostatic or low pressure air) toeliminate streaking, reduce neck-in, and improve edge sta-bility [97]. Slitting and web handling of PLA is similar toPS. Edge trimming of PLA should be carried out with rotaryshear knives since razor knives may yield rough edges andweb breaks. Winding of the PLA web should be done withgood tension control in order to obtain a consistent gauge.

Similar to PP, PET and PS films, the physical propertiesof PLA films can be enhanced through orientation. Uniaxialorientation of PLA is achieved in conventional machine-direction orientation (MDO) rolls. Since PLA tends to neck induring drawing, nipped rolls are usually required. Throughmechanical drawing, it is possible to improve thermal andimpact resistance of the PLA film or sheet to a level simi-lar as oriented polystyrene (OPS), oriented polypropylene
(OPP) or polyester. An oriented PLA film can be obtainedby stretching it to two to ten times its original length at60–80 ◦C [51], which is much lower compared with OPPand PET. Typical drawing temperatures for PLA films in themachine (MD) and transverse directions (TD) are presented


Table 6Recommended drawing conditions in the machine and transverse direc-tion for PLA [97]

Section Temperature range (◦C)

Machine direction preheat 45–65Slow draw 55–70Fast draw 70–75Annealing 45–55

Transverse directionPreheat 65–70Draw 70–85Annealing 125–140

in Table 6. In generally, for 98% l-lactide PLA, machinedirection orientation of 2–3× is expected, while transversestretch ratios of 2–4× may be used. At higher d-lactidecontents, the machine and transverse stretch ratio can beincreased. Fig. 21 shows a typical extrusion cast line forproducing biaxially oriented PLA film.

The orientation in PLLA films depends on the drawrate, temperature and ratio. High strain rate, low tem-perature and high stretch ratio favor strain-inducedcrystallization during orientation. Taking the competitivecrystallization and relaxation effects into consideration,Lee at al. concluded that the optimal drawing tem-perature to obtain highly oriented PLLA films (Mw of190,000 g/mol) is about 80 ◦C [63]. In contrast, Gruber et al.used somewhat lower temperatures for biaxial orientationof 100,000–150,000Mn PLA polymer with 10–20% meso-lactide content (65–72 and 20 ◦C for preheat and coolingrolls, respectively, for MD stretching; 63–70 ◦C and circu-lated ambient air cooling for TD drawing) [79]. Ou andCakmak prepared biaxially oriented PLA films by stretchingcast PLA in both MD and TD to different ratios, followed byannealing these films at elevated temperatures to inducecrystallinity and dimensional stability [62]. Their wideangle X-ray (WAXS) results showed that the developmentof crystalline order and orientation were dependent onthe mode of orientation. They observed that simultaneousbiaxial stretching of PLA film resulted in poor crystallineorder, while sequential stretching promoted a greater crys-
talline order [62]. Hence, the properties of PLA films areexpected to change depending on the stretching sequenceused during the orientation process.
PLA has excellent optical properties and high modulus.However, it has low elongation, tear and burst strengths.

Fig. 21. Biaxial oriented extrus

Science 33 (2008) 820–852

To overcome these shortcomings, PLA is often coextrudedwith other polymers to form multilayer structures toenhance its properties. For instance, to reduce electrostaticbuildup, Rosenbaum et al. disclosed methods for formingbiaxially oriented multilayer films made of one PLA-basedlayer and two outer layers consist of PLA and glycerol fattyacid esters to achieve films with antistatic surfaces [98]. Theextruder temperatures used ranged from 170 to 200 ◦C withthe take off roll set at 60 ◦C. The biaxial orientation tookplace sequentially, first at 68 ◦C in the machine directionby rollers running at different speeds, followed by trans-verse direction stretching using a tenter frame at 88 ◦C.Stretch ratios were 2.0 and 5.5 for machine and transversedirection stretching, respectively. To impart dimensionalstability to the film, heat-setting was conducted at 75 ◦C.Noda et al. disclosed a method of coextruding multi-layer laminate film consisting of polyhydroxyalkanoate(PHA) copolymer (copolymer of 3-hydroxybutyrate with 3-hydroxyhexanoate) and PLA to impart softness to the PLA,and at the same time reduce the tackiness of the PHA. Bypreventing the web from sticking to itself or the processingequipment, the speed of production and product qualitycan be improved [99].

PLA films tend to have higher surface energy thanuntreated polyolefin films. Gruber et al. reported surfaceenergy of about 44 dynes/cm for pure PLA films [79].The surface energy values for 98% l-lactide and 94% l-lactide films were reported as 42 and 34–38 dynes/cm,respectively [100]. Higher surface energy will provide moresatisfactory printing properties without surface treatment.If higher surface energy is needed for downstream process-ing, the surface can be treated by corona discharge.

7.5. Extrusion blown film

In extrusion blow film process, molten PLA is extrudedto form a tube using an annular die. By blowing air throughthe die head, the tube is inflated into a thin tubular bub-ble and cooled. The tube is then flattened in the nip rollsand taken up by the winder (Fig. 22). The ratio of bubblediameter to the die diameter is called the blow-up-ratio
(BUR). BUR ratios of 2:1–4:1 with the die temperatureof 190–200 ◦C have been used for extrusion blowing ofPLA films [101,102]. By varying the BUR, screw speed, airpressure, and winder speed, films of different thicknesses(∼10–150 �m) and degree of orientation can be achieved.
ion cast film machine.


sion blo

mmippsmbetsdlTcomsodpb

grrPifi(abt

Fig. 22. Extru

PLA has a specific density of about 1.24 g/cm3 which isuch higher than polyolefins (0.91–0.96 g/cm3). While PLAay be processed in extruders designed for polyolefins,

f the extruder is already operating at close to maximumower of the screw drive, the extruder may not have enoughower to process PLA due to the substantial higher den-ity for PLA [103]. Compared to polyolefins, PLA has weakerelt strength, and therefore, the formation of a stable bub-

le during extrusion blowing is more difficult. As a result,xtrusion blowing of PLA film often requires the use of addi-ives, such as viscosity enhancers to strengthen its melttrength. These additives protect the polymer from degra-ation and/or couple polymer chains to attenuate overall

oss of molecular weight and viscosity of the polymer melt.he formulation of coupling agents is often proprietary. Oneommercially available coupling agent for PLA is made upf copolymer of styrene, methyl methacrylate and glycidylethacrylate [102]. Sodergard et al. disclosed a method to

tabilize PLA and enhance its melt strength by adding anrganic peroxy compound (e.g., tert-butylperoxybenzoate,ibenzoylperoxide, tert-butylperoxyacetate) during meltrocessing, wherein the peroxide is added in about 0.01–3%y weight of PLA [101].

Since PLA films are quite stiff and have much lower elon-ation than polyolefins, collapsing of bubble in the nipsolls tends to produce wrinkles which tend to permanentlyemain in the film due to the high dead-fold properties ofLA. This problem can be overcome by incorporating fillersnto PLA during extrusion. To reduce the adhesion between
lms, Hiltunen et al. blended PLA with triacetin plasticizerglycerol triacetate), together with various anti-adhesiongents, such as talc, TiO2 and CaCO3. They claimed that theursting strengths of the resulting blown films were betterhan typical polyethylene and PP films [104]. Slip additives
wn film line.

(e.g., oleamide, stearamide, N,N′-ethylene bisstearamide,oleyl palmitamide) have also been added to reduce thecoefficient of friction between overlapping films [102]. Typ-ically, slip additive of less than 0.5–1.0% by polymer weightis used, as excessive amounts will compromise the abil-ity of print inks, stickers to adhere to the film surface. Toavoid the use of copolymerization techniques, blending, orplasticizers, Tweed et al. developed a method to obtain PLAblown films by elevating the viscosity of PLA through suc-cessive steps in a polymer cooling unit or by internal coolingof the die mandrel using air or liquid fluid to control thetemperature of the die [102]. Mitsui Chemicals successfullydeveloped PLA-based films by copolymerization technol-ogy, and it is commercializing it as one of the LACEA brandresins [51].

7.6. Thermoforming

Thermoforming is commonly used for forming pack-aging containers that do not have complicated features.PLA polymers have been successfully thermoformed intodisposable cups, single-use food trays, lids, and blister pack-aging.

Fig. 23 shows the typical steps for thermoforming ofPLA container. In this process, PLA sheet is heated to softenthe polymer, forced either pneumatic and/or mechanicallyagainst the mold, allowed to cool, removed from the mold,and then trimmed. Heating of PLA sheet for thermoform-ing is generally achieved by infrared red (IR) radiation
from heater elements. Each polymer has an optimum IRabsorbance frequency in the IR region. Therefore, the heaterelement should be set at the temperature at which themajority of energy is absorbed by the polymer. For PS, theideal wavelength is 3.2–3.7 �m [105]. Values for PLA have


for therm
Fig. 23. Main steps
not been reported in the literature. In general, the thermo-forming temperatures for PLA are much lower than otherconventional thermoformed plastics (e.g., PET, PS, and PP)in the range of 80–110 ◦C when the sheet enters the mold[106,107].

Typically, aluminum molds are used for thermoform-ing PLA containers. Molds, trim tools and ovens designedfor thermoforming of PET, high impact polystyrene (HIPS)and OPS can be used for forming PLA containers. However,molds for thermoforming of PP may not be used inter-changeably for PLA, since PP shrinks more considerablythan PLA during cooling. For a given part thickness, cool-ing times required for PLA containers in the mold tend to behigher than PET and PS containers due to the lower thermalconductivity and Tg for PLA polymers. Table 7 compares thethermal properties of PLA, PS and PET.

Orientation increases toughness of PLA containers.Regions of PLA articles that are highly drawn are less brit-tle as compared to flanges and lips that received minimalorientation. Extruded sheet prior to thermoforming is rela-tively brittle at room temperature. To ensure smooth travelof the web and to prevent web breakage, a tight radiusshould be avoided in the unwind stations and skeletonrewind stations. A minimum rewind radius of 25 cm isrecommended [108]. If PLA sheet needs to be trimmedbefore thermoforming, it should be heated to tempera-tures near 90 ◦C to prevent cracking. Storage conditionsfor the sheet stock need to be controlled as well. As aguide, PLA should not be exposed to temperature above

◦
40 C or to RH above 50% as the sheet will block and resistunwinding due to its low heat deflection temperature. Afterthermoforming, precaution should be taken to store PLAbelow 40 ◦C since Mw breakdown can accelerate when itis exposed to elevated temperature (Fig. 11). A compari-
Table 7Thermal properties of PLA, PS, and PET [106]

PLA PS PET

Thermal conductivity (×10−4 cal. cm−1 s−1 ◦C−1) 2.9 4.3 5.7Heat capacity (cal. g−1 ◦C−1) 0.39 0.54 0.44Glass transition temperature (◦C) 55 105 75Thermal expansion coefficient (×10−6 ◦C−1) 70 70 70

oforming process.

son of the mechanical, physical and barrier properties ofthermoformed PLA, PS, and PET containers showed thatPLA containers outperform PET and PS at lower temper-atures [109]. Moreover, the use of 40–50% PLA regrind didnot significantly change in the container performance [9].

7.7. Foaming

Due to their biocompatibility and large surface area,PLA foams have a niche in tissue engineering and medicalimplant applications [110–112]. Foaming of PLA is generallycarried out by dissolving a blowing agent in the PLA matrix.The solubility of the blowing agent is then reduced rapidlyby producing thermodynamic instability in the structure(e.g., temperature increase or pressure decrease), to inducenucleation of the bubbles. To stabilize the bubbles, the foamcells are vitrified when the temperature is reduced belowthe Tg of the polymer [113,114].

Various foaming strategies have been adopted to reducePLA density and improve foam mechanical properties. Diet al. used 1,4-butanediol (BD) and 1,4-butane diisocyanate(BDI) as chain extenders to increase the molecular weight ofPLA so that its viscoelastic properties are more optimal forfoaming. They produced modified PLA samples by sequen-tially adding different ratios of BD and BDI in a Haake meltmixer operating at 170 ◦C and mixer speed of 60 rpm undera nitrogen atmosphere. Tin(II) 2-ethylhexanoate was addedas a catalyst at 0.05 wt% of PLA. They found that the chain-extender modified PLA produced foams with reduced cellsize, increased cell density and lowered bulk foam densityas compared to the neat PLA foam control [113]. Mikos etal. prepared PLLA membranes with and without sodiumchloride, sodium tartrate, and sodium citrate by solvent-casting techniques [114]. The PLLA and PLLA/salt compositemembranes were foamed by heating them at 195 ◦C (15 ◦Chigher than Tm) for 90 min and then quenched in liquidnitrogen for 15 min. They were able to produce membranes
with porosity as high as 93% with a desired surface/volumeratio depending on the salt used. Ajioka et al. disclosed ina patent the method of manufacturing PLA foams suitablefor use as disposable food trays, cups, thermal insulators,and cushioning materials [115]. Their approach involves

olymer

mwiccfi(wiAoiwa

tsfrbopiPataftedTittsiaea

tcmid[pbwsosceiTian


ixing various proportions of PLLA and PDLA togetherith 0.5% talc (w/w) in an extruder at 200 ◦C. An expand-

ng agent, either dichlorodifluoromethane or butane washarged under pressure into the extruder. The mixture wasooled to 140 ◦C and extruded through a slit die to give sheetoam. An alternate method adopted by these inventorsnvolved mixing and heating azodicarbonamides powdera food additive) with PLA resins using an extruder, inhich the azodicarbonamide decomposed, thereby releas-

ng nitrogen gas to induce the formation of bubbles [115].nother patent described a method for injection moldingf PLA foams by adding 15–25 wt% of solvent to PLA dur-ng extrusion [116]. Solvents reported to be suitable here

ere methyl formate, ethyl formate, methyl acetate, propylcetate, dioxane and methyl ethyl ketone.

Loose-fill packaging materials provide cushioning, pro-ection, and stabilization of packaged goods duringhipping. Over the past decade, the use of expanded PSoams for loose-fill packaging has declined due to theeplacement with the environmentally more benign starch-ased expanded foams. To overcome the hydrophilic naturef starch, these biobased foams are often blended withetroleum polymers. Recently, PLA has been use for blend-

ng with starch during foaming. Guan et al. extrudedLA and acetylated starches, along with 5% talc (w/w)nd various amounts of ethanol in a co-rotating conicalwin-screw extruder (180 rpm screw speed; 160 ◦C barrelnd die temperatures) [117]. The blends were conditionedor 24 h at 25 ◦C before extrusion. The authors observedhat ethanol functioned as an effective blowing agent toxpand the foams, and it acted as a solubilizing agent toepolymerize PLA and starch to form homogenous dough.hey observed that increasing the PLA content caused anncreased foam expansion [117]. In an attempt to improvehe physical/mechanical properties, and moisture resis-ance of starch-based foams, Preechawong et al. preparedtarch–PLLA hybrid foams by baking PLA–starch mixturesn a hot mold at 220 ◦C for 2 min [118]. They found that theddition of PLA improved the ultimate tensile strength andlongation at break, as compared to the starch foam, withconcomitant increased resistance to water absorption.

Besides adjusting the cellular morphology of PLAhrough the use of different blowing agents and pro-ess optimization (pressure and temperature adjustments),uch recent research activity has been directed at affect-

ng the size and formation of the cellular structure byispersing MMT nanoclay particles in the bulk material119]. Ray and Okamoto foamed pure PLA and PLA com-osite with organically modified MMT in a batch systemelow Tm. These authors found that foamed PLA reinforcedith MMT produced homogeneous cells with closed cell

tructure with a diameter of 2.6 �m; however, in the casef neat PLA the cells were non-uniform with average cellize of 230 �m [23]. They concluded that the MMT parti-les acted as nucleating agents for the cell formation. Emat al. also incorporated nanoclay in PLA for batch foam-
ng using supercritical carbon dioxide as a foaming agent.hese researchers noted that cells of ∼200 nm were local-zed along the surfaces of the dispersed nanoclay and drew
similar conclusion that the dispersed nanoclay acted asucleating sites for the cell formation [120]. Di et al. foamed

Science 33 (2008) 820–852 839

neat PLA and PLA reinforced with two commercial organ-ically treated MMT samples. They found that the foam ofPLA/MMT exhibited a nicely interconnected, energeticallystable closed-cell structure with pentagonal and hexagonalfaces [121]. In contrast, pure PLA foam showed a relativelylarge cell size (∼230 �m). The size of the cells decreased asthe amount of MMT increased, and it leveled off at higherMMT concentrations. Di et al. concluded that it is possibleto foam PLA with different cellular structures by controllingthe amount of organoclay content [121].

Recently, supercritical CO2 has attracted consider-able research attention as an environmentally friendlysolvent for many processes because it is inexpensive, non-flammable and can be easily removed from the products.A liquid becomes a supercritical fluid when both pressureand temperature are above the critical point. In this state,the substance possesses a combined gas-like viscosity andliquid-like density, making it an excellent solvent for var-ious applications. For CO2, its solubility and diffusivity inmany polymers tend to increase considerably when it existsin the critical state, thereby facilitating the plasticizationof many polymers and enabling the forming process to beconducted at lower temperatures [122]. Moreover, the crit-ical conditions of CO2 at 31.1 ◦C and 7.38 MPa are readilyattainable within the safety of commercial and laboratorysetups [123,124]. The supercritical CO2 approach is basedon the larger Tg depression effect of supercritical CO2 onpolymers, which keeps the polymer in the liquid state at rel-atively low temperatures. The sudden reduction in pressureleads to the formation of CO2 nuclei which grow sponta-neously. Meanwhile, as the pressure decreases, the Tg forthe polymer also elevates and eventually rises above thefoaming temperature, at which point the cellular structureis locked in place to produce a cellular network. Fujiwaraet al. investigated the effect of d- and l-lactide composi-tions (1.0–28.5% d-lactide contents) on PLA foaming usingsupercritical CO2 [125]. The samples were heated to 50 ◦C inthe reactor chamber followed by adding CO2. The pressureof the vessel was ramped to 69 and 414 bar, respectively,for amorphous and crystalline samples. The temperaturewas increased stepwise while the pressure was kept con-stant. The expansion of the polymer was monitored duringheating using a linear variable differential transducer. Fuji-wara et al. reported that the average pore diameters for theporous structures were 5.4 and 3.3 �m for 1 and 4.2% d-lactide polymers, respectively, suggesting that the cellularmorphology was crystallinity dependent. In contrast, underthe same conditions employed, porous structures were notdetected for amorphous PLA samples with 10 and 28.5% d-lactide content. Moreover, they also reported an increasedlinear swelling of the polymer with decreasing crystallinityand the porous supercritical CO2 treated PLA sampleshave higher crystallinity than the as received polymers[125]. Porosity of supercritical CO2 treated foams not onlydepends on the pressure and temperature, but also on therate of its release from the polymer (Fig. 24) [123]. Mooney
et al. showed that the formation of PLA foams can also takeplace below the critical conditions by exposing the poly-mer to a pressure of 55 bar at 20–23 ◦C for 72 h, followed byrapid depressurization to atmospheric pressure in 10–15 s[126]. Pores of 10–100 �m were observed, although solid


al CO2 a
Fig. 24. SEM micrographs of PDLLA polymer after processing in supercritic[123]. Reproduced from Quirk et al. by permission from Elsevier B.V.
skin layers were present on the sample surface due to therapid diffusion of the dissolved gas from the surfaces [126].Matuana investigated the effect of microcellular structureson the mechanical properties of foamed PLA by using atwo-stage CO2 foaming process. The process first involvedsaturating PLA samples with CO2 in a pressure chamberat 5.5 MPa and room temperatures for 2 days. The CO2-saturated samples were then removed from the chamberand heated above Tg in a glycerin bath [127]. Compared withthe unfoamed PLA, Matuana reported a twofold increase inimpact resistance, up to twofold increase in strain at break,and up to fourfold increase in toughness for the foamedsamples. The increased impact strength was attributed tothe presence of small bubbles which inhibited the crackpropagation by blunting the crack tip and increasing theamount of energy needed to propagate the crack [127].The low temperature supercritical CO2 processes reviewedabove are expected to find their place in the manufactur-ing of structural foams for which mechanical properties arecritical, since thermal and hydrolytic degradation encoun-tered in the typical thermal processes can be avoided.

7.8. Fiber spinning

High water vapor transmission rate of PLA often pre-cludes its use in applications where moisture barrieris critical. However, this property can be leveraged forfabricating fibers used in garments (e.g., shirts, dresses,underwear, shoes, etc.) to improve their “breathability”.
While PLA fibers are not as wettable as cotton, they exhibitmuch greater water vapor transmission than polyester ornylon fibers.
The manufacturing of PLA fiber is carried out either bydry or melt spinning processes. Commercially PLA fibers

t 240 bar, 35 ◦C with (a and b) 12 min venting and (c and d) 60 min venting

are generally produced using the melt spinning tech-nique [128–136]. Here, PLA fibers are typically melt-spunat approximately 185–240 ◦C through spinnerets with l/dratios of 2–10. The processing temperature is compara-ble to polyolefins [137,138]. The melting temperature usedalso depends on the optical purity of the polymer used.Lower processing temperatures can be used for lower opti-cal purity polymer (i.e., greater d-isomer content), whichcan help reduce the thermal and hydrolytic degradation.Similar to the injection molding process, fiber-grade PLAneeds to be dried to less than 0.005% (w/w) moisture beforemelting in the extruder to minimize molecular weightdrop.

In a two-stage melt spinning process, the polymer is firstheated above its melting point and extruded through thespinneret. The solidification of the extrudate is achievedby cooling in the air and the take up roller. In the secondstage, the fiber undergoes hot drawing, where the filamentis pulled down by a take-up roll with a specific speed toachieve fiber orientation, which is important to increase thetenacity and stiffness of the fibers. PLA can be melt spun in ahigh-speed spinning process with take-up velocity of up to5000 m/min and a draw ratio of up to six [130]. The degreeof crystallinity of the fiber increases with spinning veloc-ity due to stress-induced crystallization. By adjusting thedraw ratio, a wide range of mechanical properties can beachieved for the fiber (Fig. 25). While high speed spinningprovides high fiber manufacturing throughput, manufac-turing of fiber of uniform diameter is not a trivial task under
high speed condition due to process instability caused bythe draw resonance known as spinline neck-like deforma-tion, which is characterized by an abrupt attenuation of thespin-line cross-sectional area. The phenomenon is relatedto the spinline flow-induced crystallization [131,139].


F 8% d-ise of spinnfi seconda blished

sitofinttitipfiplsrihdtfii0Pse

pohcsati[acr

ig. 25. Stress–strain curves for high-speed spun PLA fiber (l-PLA withxtruder used in both processes was 18 mm screw at 185 ◦C. The diameterlaments were taken up at 200 m/min by the first godet and drawn by thet 65 and 110 ◦C, respectively. Curves are reproduced based on the data pu

Another approach to form PLA fiber is based on the drypinning process, which involves dissolving the polymern a solvent (typically chloroform, toluene or a mixture ofhe two solvents) and extruding the polymer solution in airr inert gas. Evaporation of the solvent causes the extrudedlaments to solidify [136,140–143]. Although the melt spin-ing process is relatively straightforward, the process tendso induce molecular weight drop of the PLA polymer due tohermal-induced hydrolytic degradation during the melt-ng step in the presence of residual water. In contrast,he dry spinning technique is quite effective in preserv-ng the molecular weight of the polymer due to the lowerrocessing temperature used. In general, solution-spunbers are superior to melt-spun fibers from the stand-oint of mechanical properties. This is attributed to the

ower chain entanglement of polymer molecules in theolution state as compared to the melt state. By transfer-ing this dilute entanglement network to the solid staten the spinning process, the as-spun fibers tend to exhibitigh draw ratios. For instance, Penning et al. reported thatry spinning followed by hot drawing resulted in low crys-allinity fibers having a tensile strength of 1 GPa, whereasbers prepared from melt spinning followed by hot draw-

ng have much considerably lower strengths, ranging from.19 for completely amorphous copolymer to 0.53 GPa forLA homopolymer [141]. The major drawback to solutionpinning is the use of organic solvents, which can posenvironmental problems.

One important parameter that affects the fiber mor-hology during dry solution-spinning is the compositionf the solvent. Instead of using one solvent, several studiesave successfully manipulated the morphology and physi-al properties of PLA fibers by using multi or binary solventystems. Postema et al. showed that PLA fibers spun fromsolvent made up of 40/60 chloroform/toluene exhibited

he highest tensile strength of 2.3 GPa after hot draw-
ng among other chloroform/toluene proportions tested142,143]. Similar observations were reported by Leenslagnd Pennings [140]. They postulated that under the 40/60hloroform/toluene condition, the PLA adopted an inter-upted helical conformation. Solidification of PLA from
omer), subjected to various (a) drawing speed and (b) draw ratios. Theeret hole was 0.3 mm with a length of 0.6 mm. For the process in (b), thegodet spun at 800 and 1200 m/min. The first and second godets were setby Schmack et al. [130] by permission of John Wiley & Sons, Inc.

the solution during solvent evaporation caused the heli-cal aggregates to form crystalline junctions that hamperedre-entangling of polymer chains, leading to crystallinepolymer with good draw-ability [140]. In this binary sys-tem which contained a good PLA solvent (chloroform) anda poor PLA solvent (toluene), the greater tendency for chlo-roform to evaporate during spinning causes an increasedPLA concentration with a concomitant decrease in sol-vent power, resulting in the formation of polymer-poorand polymer-rich phases. The polymer-rich phase under-went rapid solidification, thereby generating porous fiberstructures [142,143].

The wet process is similar to the dry process exceptthat the polymer solution is spun into a bath containingcoagulating solution which causes the polymer filamentto solidify. With this approach, PLA is normally dissolvedin chloroform which is then extruded into a toluene ormethanol bath [136]. Tables 8 and 9 compare the pro-cess parameters between the melt and solution spinning.Compared to polyester, Lyocell, cotton, and viscose fibers,PLA fiber tends to be more extensible. When PLA stretchesbeyond several percent of extension, it yields, stretchesquite easily, and then ruptures at relatively high elonga-tion values. As shown in Fig. 26, the tenacity behavior ofPLA is akin to wool.

One variant of dry spinning is known as electrospinningwhich utilizes an electrostatic force to draw fiber. Recently,several research groups have successfully spun PLA fibers ofsubmicron diameter using this approach. The electrospin-ning process is discussed in the next section.

7.9. Electrospinning of ultrafine fibers

Electrospinning is a technique for producing fibersthat are much smaller in diameter than those producedusing the conventional techniques. Electrospun fibers typi-
cally have diameters range from micrometer to nanometer(Fig. 27). Similar to the conventional dry spinning process,electrospinning requires the solubilization of the polymerin a solvent. However, unlike drying spinning which relieson mechanical extrusion, the electrospinning process uses

842L.-T.Lim

etal./Progress

inPolym

erScience

33(2008)

820–852

Table 8Properties and conditions for melt-spun PLA fibers

References d-Content(%)

Initial MW(×103)

Final MW(×103)

Collection speed(m/min)

Spinneret d/l(mm)

Die temperature(◦C)

Draw ratio Draw temperature(◦C)

Crystallinity(%)

Tensile strenth(MPa)

Modulus(GPa)

Fiber diameter(�m)

Penning [141] 0–15 280–600 – 1 0.25/– 150–210 7–9 60–100 – 185–530 0.5–0.93 –Cicero [129] 4 111–131 67–79 – – – 4–8 – <35 60–400 0.5–3.1Fambri [128] 0 330 110 1.8–10 1.0/– 240 7–21 160 30–38 870 9.2 48–106Eling [136] 0 180–260 – 0.25–0.35 1/10 185 8–25 110 – 480–500 6–7 –Schmak et al. [130] 4 207 180 800–5000 0.3/0.6 185 4–6 65–110 0–24 198–450 3.1–6.3 <500Cicero [132] 2 99–109 62–71 – 2.16 – 1–8 – 43–50 100–350 1–3 300Mezghani [135] 0.7 212 163 1000 0.76/4 233 – – <43 90–380 3.6–6.0 12–73Yuan [134] 0 263–495 105–217 3.2 1.0/– 210–240 4.7–5.9 120 17–23 42–103 1.2–5.4 110–360

Table 9Properties of solution-spun PLA fibers

References d-Content(%)

Initial MW(×103)

Solvent Collection speed(m/min)

Spinneret d/l(mm)

Solutiontemperature (◦C)

Draw ratio Drawtemperature (◦C)

Crystallinity(%)

Tensile strength(MPa)

Modulus(GPa)

Fiber diameter(�m)

Postema [142] 0 910 4%, w/v, chloroform/toluene (40/60) 10–182 0.25/23 60 10–18 190 – 900–1400 – –Postema [143] 0 910 4%, w/v, chloroform/toluene (40/60) 3 0.25/23 9–60 12–14 190 – 1100–2200 – 17–28Penning [141] 5 – 5–6%, w/v, chloroform/toluene 3 0.25/– 60 – 110 – 950 9.2 –Eling [136] 0 300–500 6–12%, w/w, toluene 0.025–0.035 1/– 110 4–26 180–201 – 280–1000 7–10 –Leenslag [140] 0 900 4%, w/v, 40/60, v/v, chloroform/toluene 0.012–0.017 0.25/23 60 20 204 53 2100 9–16 6.6


Fmb

etampn

mnpnc

FA

ig. 26. Tenacity–stretch ratio curves of PLA as compared to other com-on textile fibers. The curves are based on the data originally published

y Farrington et al. [137] and Rajkhowa et al. [187].

lectrostatic force to spin the solution into fibers. Due toheir small diameter, electrospun fibers possess very largerea, making them an ideal material for applications such asedical tissue scaffold, wound dressing, carrier for drugs,

rotective fabrics, high performance filter media, filler foranocomposite materials, etc. [144].

A typical laboratory electrospinner is made up of fourain components: (1) a high voltage DC supply; (2) a spin-

eret, charged by a DC power supply; (3) an infusion oreristaltic pump to deliver polymer solution to the spin-eret; and (4) a metal fiber collector which also acts as aounter electrode (Fig. 28). To increase throughput, mul-

ig. 27. Comparison of surface area and diameter for various fibers.dapted from Ko [188].

Science 33 (2008) 820–852 843

tiple spinnerets have been used in conjunction with aconveyor belt to achieve a continuous process [145–147].Most of the setups reported in the literature involve apply-ing a positive electrode to the spinneret and grounding thecounter electrode, although it is also possible to spin fiberby reversing the polarity. The basis of electrospinning isto charge the polymer solution in the spinneret tip witha high voltage such that the induced charges cause thepolymer solution to eject and travel towards the ground(or oppositely charged) collector. When the polymer solu-tion is charged, the induced electrostatic repulsion worksagainst the surface tension of the solution, causing the poly-mer solution to elongate and form a characteristic featureknown as a Taylor cone (Fig. 28). When the voltage reachesa critical level (typically in the order of 10–20 kV), the elec-trostatic repulsion overcomes the surface tension of thesolution, causing the polymer to eject towards the collector.As the polymer jet takes flight in the air, the solvent vapor-izes rapidly, producing a continuous fiber which depositson the collector [144,148,149]. By allowing the fiber to spinfor some time, a nonwoven fibrous mat is formed on thecollector. The morphology of the fiber can be affected byusing collector of different configurations. For instance, bytranslating the collector plate in the X–Y plane, a large areaof fibrous mat consists of randomly laid fiber can be formed.To achieve a fibrous membrane with oriented fibers, a rotat-ing drum, disc, or parallel electrodes may be used (Fig. 28)[150].

PLA has been successfully electrospun into fibers,primarily for tissue engineering and biomedical appli-cations. For instance, a number of studies showed thatscaffolds for the regeneration of cardiac, neural, boneand blood vessel tissues can be fabricated from electro-spun PLA fiber through post-spinning orientation and/orusing rotating target collectors [151–154]. PLA has beenelectrospun into different forms of ultrafine fiber andused as carriers for bioactive agents, including antibiotics[155], anticancer drugs [156,157], and antibacterial silvernanoparticles [158]. Other composite PLA fibers contain-ing nano-components such as nanoclay (montmorillonite,MMT) and TiO2 nanoparticles have also been successfullyproduced using the electrospinning technique [157,159].

By and large, PLA solutions with lower polymer con-centrations favor the formation of small diameter fibers.But these fibers are less consistent in morphology andtend coexhibit beads along their length (Fig. 29A and B).These defects can be overcome by incorporating an organicor inorganic salt, such as pyridinium formiate, Kh2PO4,NaH2PO4, or NaCl, in the fiber-forming solution to enhanceits electrical conductivity [160,161] (Fig. 29C and D). Thesolvent used will also affect the surface morphology of thefiber. Using dichloromethane as a solvent and a PLLA poly-mer solution of 5%, Bognitzki et al. observed that the surfaceof the PLLA fibers exhibited regular pore structures, whichwere attributed to rapid phase separation of the solvent[162]. Typical solvents used for PLA, along with the pro-
cess conditions used for electrospinning, are summarizedin Table 10.
Recently, an innovative technique was reported for elec-trospinning PLA in the melt phase using a CO2-laser [163].This technique relies on using a laser beam to melt the PLA


ts. Sele
Fig. 28. Typical setup for electrospinning, showing the main componendrum/disc, and parallel electrodes.
polymer locally, thus minimizing thermal degradation dueto prolonged heat exposure, which has been observed pre-viously for melt-electrospinning for other polymers basedon the heat conduction melting approach. Moreover, thelaser technique also eliminates the use of solvent, makingthe process more environmentally benign. This techniqueis capable of producing fibers with diameters smaller than1 �m [163].

While research on electrospinning has exploded in thepast decade for fibers spun from various polymers, thecommercial production of ultrafine PLA fibers using thistechnique has not been forthcoming due to the low pro-duction throughput, the requirement for the use of specificsolvents, and variation in fiber diameter. Based on thepresent state-the-art of electrospinning, this technique islikely to find uses for products containing PLA-nanofibersin pharmaceutical and biomedical applications.

7.10. PLA blends with other polymers

In many film applications, such as grocery and garbagebags, bursting strength, elongation and tear strength areimportant properties. As discussed in Sections 7.4 and 7.3,these properties can be improved to a certain extent bymechanical drawing, such as biaxial orientation and stretchblow molding. However, for other PLA parts where theuse of mechanical orientation is not feasible (e.g., injectionmolded articles), blending of PLA with other polymers is auseful strategy to impart flexibility and toughness. Another
motive for blending PLA with other polymers is to reducethe material cost since the cost of PLA is relatively highercompared to other petroleum plastics.
Various polymers have been used for improving theproperties of PLA, including elastomers [164], thermoplas-

cted collector configurations are shown here: stationary plate, rotating

tic starch [165], poly(ethylene glycol) (PEG) [104,165,166],triacetin and tributyl citrate [96,104,167], and PHA [99,168].Lee and McCarthy modified the toughness of 4% d-PLAwith poly(3-hydroxyloctanoate) (PHO). In order to over-come the process difficulties due to the large differencein melt viscosity between the two polymers, they modi-fied the PHO with hexamethylene diisocyanate (HMDI) byreacting the hydroxyl group of PHO to form urethane link-ages. HMDI of 2–5.5% (w/w) was reacted with PHO usinga counter-rotating screw at 40 rpm for 2 min at 100 ◦C.The modified PHO was melt-blended with PLA at 40 rpmfor 3 min at 175 ◦C and then compression molded [168].Noda et al. prepared plastic films by melt compoundingvarious proportions of PLA, PHA copolymer (copolymer of3-hydroxybutyrate with 21 mol% of 3-hydroxyhexanoate),and poly(ethylene oxide). These authors claimed that theinherent tackiness of PHA polymer and brittleness of PLAcan be overcome by this approach [99]. Recently, basedon viscosity and gel permeation chromatography mea-surements, Conrad et al. reported that PLA/PHA blendsdegraded more rapidly as compare to the neat PLA [169]. Inanother patent, Randall et al. blended PLA with epoxidizednatural rubber in the presence of a compatibilizing agent(maleic anhydride/polybutadiene copolymer or maleicanhydride/polybutadiene/PS copolymer at 1–2 wt% relativeof the epoxidized rubber). They observed an increased ulti-mate elongation from 72.6 to 295%, and an increase of Izodimpact value from 2.08 to 3.8 ft lbs/in. for injection moldedtest bars, as compared to the non-compatibilized control
[164]. The blends were produced by extruding the poly-mers through a twin-screw extruder (150–175 ◦C extruderzone temperatures; 250 rpm), cooling in a water trough,chopping the extruded strand into pellets, drying to removewater, and then injection molding into test bars. Randell et


F d fromi thout ad( d from

aita

ge[tstpptPawoTaoa

ig. 29. Scanning electron micrographs of electrospun PLA fibers preparencreases the conductivity of the polymer solution: (A) 5% (w/w) PLA wiw/w) PF; (D) 0.1% (w/w) PLA with 0.8% (w/w) PF. Micrographs are adapte

l. noted that reprocessing the polymer blends, i.e., pass-ng them through the extruder more than once, tendedo increase the impact resistance of the resulting moldedrticles [164].

Averous attempted to blend thermoplastic starch, PEG,lycerol and oligomeric lactic acid by using a single-screwxtruder equipped with a conical-shaped shear element165]. Glycerol was found to be the least efficient plas-icizer, while oligomeric lactic acid and PEG providedubstantial increases in elongation. Affinity of PLA andhermoplastic starch was poor, leading to blends thatossessed much weaker mechanical properties as com-ared to the individual polymers [165]. Moura reportedhat the tensile strength, elongation, and damping forLA blended with starch particles increased with aver-ge particle size of the starch granules, but declinedhen the granules were greater than 45 �m [170]. More-

ver, crystallinity increased as the particle size decreased.he use of methylenediphenyl diisocyanate as a couplinggent dramatically improved the mechanical propertiesf the composite [170]. Sheth et al. melt-blended PLAnd PEG using a counter-rotating twin-screw extruder at

dichloromethane with and without pyridinium formiate (PF), a salt thatditive; (B) 1% (w/w) PLA without additive; (C) 5% (w/w) PLA with 0.8%

[160] with permission from e-Polymers Foundation.

120–180 ◦C. They reported that PEG can form miscible topartially miscible blends with PLA, depending on the blendconcentration. Below 50% PEG, the plasticized PLA sam-ples have high elongation with a concomitant reductionin modulus values. However, above 50% PEG content, theblend crystallinity increases, resulting in an increased mod-ulus and a considerable decrease in the elongation at break[166]. Although PEG is effective in decreasing the stiffnessof PLA, the use of low molecular weight plasticizer has adisadvantage in that it has a tendency to migrate in the PLAmatrix. For instance, Hiltunen et al. reported that althoughPEG plasticized PLA can be extrusion blown to form filmwith a reasonable tensile strength, PEG tended to migrateout of the film after a few days or weeks [104]. Ljungbergand Wesslen reported that the Tg of PLA can be effectivelydepressed by blending with triacetine and tributyl citrateup to about 25% (w/w), above which phase separation of
plasticizer tended to occur. They also observed that phaseseparation of plasticizer accelerated at elevated tempera-ture (50 ◦C) due to the increased crystallinity of the PLAphase [167]. During storage of triacetine and tributyl cit-rate plasticized PLA film, the migration of the plasticizer


Tab

le10

Typ

ical

pro

cess

con

dit

ion

san

dso

lven

tu

sed

for

elec

tros

pin

nin

gof

PLA

Ref

eren

ces

PLA

grad

ePo

lym

erso

luti

onSp

inn

eret

dia

met

er(m

m)

Spin

ner

et-t

arge

tdi

stan

ce(c

m)

Vol

tage

(kV

)So

luti

onfe

edra

te(m

L/h

)C

olle

ctor

typ

eFi

ber

dia

met

er(�

m)

Kim

etal

.[15

4]PL

A20

%(w

/v)

inch

loro

form

–15

250.

1R

otat

ing

man

dre

l1–

2V

azet

al.[

153]

PLA

14%

(w/v

)in

15:3

mix

ture

ofC

HC

l 3:d

imet

hylf

orm

amid

e–

2013

1.5

Rot

atin

gm

and

rel

0.8–

3

Zon

get

al.[

151]

5%d

-lac

tid

e10

%(w

/w)

in1,

1,1,

3,3,

3-h

exafl

uor

o-2-

pro

pan

ol–

1530

6St

atio

nar

yp

late

0.9–

1

Zon

get

al.[

161]

5%d

-lac

tid

e20

–35%

(w/w

)in

1.5:

1(w

/w)

met

hyle

ne

chlo

rid

e:d

imet

hylf

orm

amid

ebl

end

0.7

1520

-30

1.2

Rot

atin

gd

rum

0.2–

1

Yan

get

al.[

152]

PLLA

1–5%

(w/w

)in

70:3

0d

ich

loro

met

han

e/N

,N-

dim

ethy

lfor

mam

ide

blen

d

0.7–

1.2

1012

1.0

Stat

ion

ary

alu

min

um

pla

tean

dro

tati

ng

dis

c0.

1–3

Ken

awy

etal

.[15

5]PL

LA14

%(w

/v)

inch

loro

form

–30

1518

–21

Stai

nle

ssst

eels

hee

ton

rota

tin

gdr

um

3–6

Xu

etal

.[15

6]PL

LA5.

5–6%

(w/w

)in

chlo

rofo

rm0.

418

45–5

0.4

3.0–

4.2

Stat

ion

ary

0.3–

1Ju

net

al.[

160]

PLLA

1–5%

(w/w

)d

ich

loro

met

han

e–

1440

1.3

Stat

ion

ary

0.3–

2.4

Bog

nit

zkie

tal

.[16

2]PL

LA5%

(w/w

)in

dic

hlo

rom

eth

ane

0.5

2035

–45

–St

atio

nar

y∼1

Oga

taet

al.[

163]

1.4–

1.8%d

-lac

tid

eM

elt-

elec

tros

pin

nin

g—n

oso

lven

tu

sed

–1–

516

–41

–R

otat

ing

dis

k0.

7–2

Science 33 (2008) 820–852

to the film surfaces and the concomitant increased crys-tallinity of the bulk material have been reported to causedifficulty in subsequent heat welding [96].

7.11. Compounding of PLA composites

While PLA has modulus and tensile strength compara-ble to petroleum-based polymers, PLA is brittle and hasrelatively much lower thermal and impact resistance prop-erties. Moreover, the cost of PLA is higher than that forconventional thermoplastics. To overcome these limita-tions, other polymers, minerals and biobased materialshave been incorporated in PLA to produce compositeswith enhanced properties [171,172]. In particular, bio-fillersderived from renewable resources (e.g., natural fibers,starches, proteins) have attracted a great deal of interestfor the reinforcement of PLA due to their sustainable supplyand environmentally benign production.

The extent of reinforcement in PLA composites is largelydependent on how well the filler material disperses inthe PLA matrix, and the nature of interfacial interactionbetween the filler and the PLA phase. Typically, a conven-tional single screw extruder does not provide sufficientdispersive mixing to break up the additive agglomerates.Typically, a twin-screw compounder is needed to achievea better dispersion of filler particles. Ouchi et al. discloseda method of forming PLA nanocomposites which providesa rapid crystallization rate to improve heat resistance,moldability and mold release properties. The approachinvolves dispersing organically modified layered clay min-eral, along with a low molecular weight compound thathas one or more amide groups. One of the examples giveninvolves compounding 3 wt% MMT and 1 wt% ethylene-bis-12-hydroxystearic acid amide with PLA in a twin-screwcompounder at 220 ◦C at 300 rpm [173]. Mathew et al.investigated the mechanical properties of PLA compos-ites containing microcrystalline cellulose, wood flour andwood pulp. A twin-screw compounder was used to dispersethe biomaterials in PLA at up to 25 wt% cellulose load-ing [27]. The compounding conditions used were: screwspeed 250 rpm; heater temperature 170–200 ◦C; and vac-uum vented extruder. The injection molding was conductedat 200 ◦C at an injection rate of 60 mm/s, with a pack pres-sure of 400 bars. The mold temperature used was 50 ◦C,and the cooling time was 15 s. The studies showed that themicrocrystalline cellulose, which exists as aggregates didnot separate into nano-whiskers during the compoundingprocess. While good dispersion of cellulose reinforcementsin the PLA matrix was observed, these authors attributedthe reduced tensile strength and elongation at break tothe poor interfacial adhesion between the phases [27].Using the same compounding technique, Oksman et al.compared the compression of molded PLA and PP compos-ites containing flax fibers at 30 and 40 wt% loading. Theseauthors reported that the tensile strength for PLA-flaxfiber composites is about 50% better than PP-flax com-
posites, which are used today in many automotive panels[24].
Huda et al. evaluated the mechanical and thermo-physical properties of PLA composites containing choppedglass fiber or recycled newspaper cellulose fiber producing

olymer

b[aetTmaisefghw[

Pau1samwwrrceect

fii(a

Ffi1b


y a twin-screw extruder and an injection molding system26]. The dried cellulose fibers and PLA resin, mixed at 70nd 30% (w/w) ratios were compounded in the twin-screwxtruder with an l/d ratio of 30. The extruder tempera-ure was set at 183 ◦C and the screw speed was 100 rpm.he extrudate was palletized and dried prior to injectionolding. The extruder temperature was 183–185 ◦C. A rel-

tively long cooling time of 50 s was used to bring thenjection molded part to 65 ◦C in the mold. The studyhowed that both mechanical and thermo-physical prop-rties of the recycled newspaper composite comparedavorably with glass fiber reinforced PLA composite, sug-esting that these cellulose fiber reinforced compositesave a potential to replace glass composite in applicationshere very high load bearing capabilities are not needed

26].The effects of cooling on the crystallization behavior of

LLA-cellulose composites were investigated by Mathew etl. [28]. The 25% (w/w) composites were prepared in a vac-um vented twin-screw extruder operated at 250 rpm and70–200 ◦C. The resulting composites were pelletized andubsequently injection molded at 200 ◦C into a 50 ◦C moldt the injection speed of 60 mm/s. The Tg values for theolded PLLA, microcrystalline cellulose, cellulose fiber andood flour composites, were 54.1, 56.6, 57.5 and 58.3 ◦C,hile the crystallinity values were 19, 45, 35 and 45%,

espectively. These authors attributed the delayed polymerelaxation for the composites to the restriction of polymerhain mobility due to the increased crystallinity in the pres-nce of fillers. The presence of cellulose fillers has a similarffect of inducing crystallinity in PLLA polymer during slowooling (2 ◦C/min) from the melt (absence of glass transi-ion around 54–58 ◦C regions for traces III and V in Fig. 30).

Shibata et al. evaluated the effect of dispersing abacaber (Manila hemp) in several biodegradable polyesters,

ncluding PLA. The biofiber was incorporated up to 20%w/w) in a twin-screw compounder at 190 ◦C for 5 mint a screw speed of 50 rpm. These authors observed an

ig. 30. DSC curves of PLA composite materials obtained from different heatingbers, and (d) wood flour. (I) Heating from 30 to 200 ◦C at 10 ◦C/min; (II) cooling0 ◦C/min, after fast cooling; (IV) cooling from melt at 2 ◦C/min (slow cooling); (V)ased on the original data published by Mathew et al. [28] by permission from El

Science 33 (2008) 820–852 847

increased flexural moduli for poly(butylenes succinate)and polyestercarbonate/PLA blends as the fiber contentincreased. In contrast, for the PLA-abaca fiber composite, aminimal increase in flexural strength was observed whichis attributed to inherent high flexural strength of neat PLA[25].

7.12. PLA nanocomposites

Inorganic or organic nanoparticles have been incor-porated to enhance the mechanical, barrier and thermalproperties of PLA. Unlike micro- and macro-scaled particles(e.g., talc, glass fiber, carbon particles, etc.), nanopar-ticles can improve material properties at much loweradded quantities (2–8%, w/w). Over the past few years,various nanomaterials have been investigated for rein-forcing PLA, including layered silicates, carbon nanotube,hydroxyapaite, layered titanate, aluminum hydroxide, etc.[21–23,119,154,174–179]. Among the nanomaterials inves-tigated, layered silicate clays have been studied in thegreatest detail by researchers from both academia andindustry. The heightened interest for these nanofillerscan be attributed to their ability to dramatically improvematerial properties of the nanocomposite structures ascompared with the pure PLA, including improved mechan-ical and flexural properties, elevated heat distortiontemperature, enhanced barrier properties and acceleratedbiodegradation [21,171,175,180]. This section will focusmainly on reviewing the literature on the processingaspects and material properties of MMT nanocomposites.

MMT belongs to a family of clays known as smectitewith crystal structure made up of two fused silica tetra-hedral sheets sandwiched with an edge-shared octahedral
sheet of either aluminum or magnesium hydroxide. Thethickness of a single layer is about 1 nm, while the lat-eral dimension of the crystals can range from 30 nm toseveral microns or greater. The crystal layers are stackedregularly to provide Van der Waals gaps, known as gal-
-cooling cycles: (a) neat PLA, (b) microcrystalline cellulose, (c) cellulosefrom melt at 20 ◦C/min (fast cooling); (III) heating from 30 to 200 ◦C atheating from 30 to 200 ◦C at 10 ◦C/min, after slow cooling. The curves are

sevier B.V.

olymer
leries [119,181]. The silicate surface of MMT is relativelymore hydrophilic than PLA. Therefore, it must be organi-cally modified to compatibilize and facilitate its dispersionin PLA. One useful characteristic of MMT is that the cationsin the galleries, typically Na+, Li+, Ca2+, Fe2+ and Mg2+ canbe substituted readily through ion exchange with organiccations, by treating the clay with surfactants includingprimary, secondary, tertiary or quaternary alkylammo-nium or alkylphosphonium cations [119]. Another uniqueproperty of MMT is related to its ability to delaminateand disperse in a polymer to give individual imperme-able platelets of about 1 nm thick. By virtue of their veryhigh aspect ratio, large interfacial area and nano-thickness,the dispersed platelets lead to many characteristics thatare important for enhancing the performance of poly-mers.

Pluta evaluated the effect of organoclay and compound-ing conditions on polylactide/organoclay nanocomposites(4.1% d-lactide PLLA) [20]. The PLA pellets were melt-blended at 180 ◦C with Cloisite® 30B (3%, w/w; modifiedwith methyl-bis(2-hydroxyethyl) tallowalkyl ammoniumcations) in a counter-rotating mixer at 50 rpm screw speedfor 6–30 min. The study showed that compounding ofPLA/organoclay composite resulted in 14–32% molecularweight drop, when the composite mix was compoundedfor 6–30 min, even if a dry nitrogen atmosphere wasused during the melt processing. Similar reduction inPLA molecular weight was reported by Ray and Okamotoon their study of various organically modified layeredsilicates, which was attributed to the elevated shearmixing of PLA in the presence of the silicate, and thepresence of hydroxyl groups in the modified salt, bothof which are capable of causing hydrolysis or transes-terification reactions in PLA when exposed to elevatedprocessing temperature [182]. In this study, the nanocom-posites were first prepared by dry-mixing of PLA resinwith organoclay and extruding in twin-screw extruder(screw speed = 100 rpm, feed rate = 120 g/min) to formnanocomposite strands. The strand were pelletized anddried under vacuum at 60 ◦C for 48 h, pressed into0.7–2 mm sheet with 1.5 MPa at 190 ◦C for 3 min, and thenquenched and annealed at 110 ◦C for 1.5 h before testing[182].

The properties of MMT nanocomposites are highlydependent on how well the clay disperses in the polymermatrix. In general, clay dispersion can be distinguished inthree modes: (1) when the clay particles are not delam-inated, the resulting materials tend to exhibit similarproperties as conventional microcomposites. The unsep-arated MMT layers surrounded by the polymer are oftenreferred to as tactoids; (2) when the polymer chains areinserted into the galleries of the swollen silicate layers,the clay is known as intercalated, leading to decreasedpolymer chain mobility and resulting in material reinforce-ment; and (3) when the clay is completely delaminatedand homogeneously dispersed in the continuous polymer
matrix, the layered silicates are termed exfoliated nanocom-posites, giving rise to the maximal potential for physicalproperties enhancement. The exfoliation of MMT is largelydependent on the chemical compatibility of the clay andpolymer matrix, as well as the process conditions used to
Science 33 (2008) 820–852

disperse the silicate layers of the clay. By far, exfoliationof MMT in PLA can be best achieved by using a twin-screw compounder. Dennis et al. reported that in orderto exfoliate MMT effectively, the clay particles need tobe sheared and fractured to form smaller stacks of tac-toid platelets (∼100 nm in thickness). Once platelets ofshorter stack heights are formed, further delamination ofthe platelets was primarily driven by the diffusion of thepolymer chains into the clay galleries, which is highlydependent on the chemical compatibility of the poly-mer and the organoclay surface [183]. Therefore, unlessthe compatibility of the clay and polymer is improvedthrough chemical treatment, increasing the shear inten-sity alone will only improve the distributive homogeneityof the particles in the polymer, but not the delamina-tion of the clay particles. These authors concluded thatthe residence time under low and mild shearing condi-tions is required to allow polymer chains to penetrate theclay galleries and peel the platelets apart [183]. Unfortu-nately, under the typical extrusion conditions, increasingthe residence time will also cause unwanted thermaldegradation, leading to molecular weight drop whichdiminishes the performance-enhancing effect of nanopar-ticles [184].

Various forms of modified MMT have been used toenhance the material properties of PLA. Chang et al.incorporated hexadecylamine, dodecyltrimethyl ammo-nium bromide, and quaternary ammonium salt modifiedMMT clays in PLA films at 0–10% (w/w) MMT levels, basedon wet casting method, using N,N′-dimethylacetamide asa solvent [185]. These authors observed that there is anoptimal loading of organoclay (∼3–4%, w/w) for achiev-ing the greatest improvement in mechanical properties.At higher MMT contents, the nanoparticles tended toagglomerate, leading to mechanical weakening. In contrast,within the 0–10% MMT loading levels, oxygen transmis-sion rates decreased with increasing clay content whichwas attributed to the increased tortuous diffusion pathof oxygen molecules through the impermeable crystallineplatelets [185]. Ogata et al. used a similar film castingmethodology to evaluate the impact of organophilic MMT(0–10%, w/w) on PLA, except that chloroform was usedas a solvent. These authors reported that only microcom-posites were resulted and intercalated structures werenot achieved [18]. It is unknown whether the formationof microcomposites was due to incomplete exfoliationof MMT in the chloroform solution or caused by theagglomeration of MMT during the film drying step. Nev-ertheless, these studies do highlight the importance ofsolvent selection which can affect the exfoliation of thelayered silicates.

Thellen et al. studied acetyltriethyl citrate plasticizedMMT-PLA composite films formed using compounding andextrusion blowing processes [186]. They observed about50% improvement in oxygen and water barrier properties,and 20% increase in modulus as compared to the neat
PLA films. Although thermal properties were not affectedby the addition of MMT, the thermal stability improvedmarginally.
In summary, MMT exhibits a strong potential forenhancing the material properties of PLA. The promising

olymer

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citpiunutfefdtncaitfputipappapiRnb

oipccrucwtssv

A

ma


utlook for MMT is mainly lies in its abundant sources,enign characteristics to the environment, and low cost.

. Conclusion: prospects of PLA polymers

PLA is a highly versatile biodegradable polymer whichan be tailor-made into different resin grades for process-ng into a wide spectrum of products. More importantly,he polymer can be processed using the conventionalroduction infrastructure with minimal equipment mod-

fication. New technologies for processing PLA, such assing supercritical processes for foaming and electrospin-ing for producing nanofibers, will further expand these of this polymer. From the environmental viewpoint,he compostable characteristics of PLA are well suitedor many applications where recycling, reuse and recov-ry of products are not feasible. Since the raw materialsor PLA is based on agricultural feedstock, the increasedemand for PLA resins will create a positive impact onhe global agricultural economy. Nevertheless, there are aumber of areas which still need to be improved, espe-ially in applications where PLA is intended to be useds a substitution for existing thermoplastics. For instance,n food products where high barrier protection is impor-ant, replacement of PET by PLA packaging may not beeasible, since the barrier properties of PLA are not inar with PET. The brittleness of PLA may also prevent itsse in applications where toughness and impact resis-ance are critical. The fact that PLA is biodegradable mayn some cases result in unpredicted performance if theolymer is exposed to uncontrollable abusive temperaturend humidity conditions. Aging studies which take a widererspective covering different environmental exposure androcess conditions will be useful. Some of these challengesre expected to overcome through blending PLA with otherolymers, formation of micro- and nanocomposites, coat-

ng with high barrier materials, and polymer modification.esearch and development in these areas may open upew opportunities for PLA for use as high performanceiodegradable materials.

To date, PLA is relatively more expensive than mostf the petroleum based polymers. Nonetheless, increas-ng oil prices and the implementation of environmentalolicies from the government, such as “green taxes” inountries like Germany or Japan, and mandatory use ofompostable polymers for packaging by some large corpo-ations, will create a push to expand the use of PLA. As theses for PLA continue to increase, the demand for the agri-ultural feedstock for PLA production (mainly corn today),ill increase as well. To overcome the potential competi-

ive issues of raw materials with human and livestock foodupply chains, innovations involving the use of alternativetarch and sugar sources, including biomass and other lowalue byproduct wastes, are expected to take place.

cknowledgements

The authors are grateful to Susan E. Selke for value com-ents and suggestions, and Ana Cristina Vega Lugo for her

ssistance in preparing the manuscript.

Science 33 (2008) 820–852 849

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