processing technologies for poly lactic acid

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Progress in Polymer Science 33 (2008) 820852

Contents lists available atScienceDirect

Progress in Polymer Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p p o l y s c i

Processing technologies for poly(lactic acid)

L.-T. Lim a,, R. Auras b, M. Rubino b

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 2007

Received in revised form 6 May 2008

Accepted 7 May 2008

Available online 19 June 2008

Keywords:

Polylactide

Poly(lactic acid)

PLA

Processing

Converting

Review

a b s t r a c t

Poly(lactic acid) (PLA) is an aliphatic polyester made up of lactic acid (2-hydroxy propionicacid)buildingblocks.It is alsoa biodegradableand compostable thermoplasticderived from

renewable plant sources, such as starch and sugar. Historically, the uses of PLA have been

mainly limited to biomedical areas due to its bioabsorbable characteristics. Over the past

decade, the discovery of new polymerization routes which allow the economical produc-

tion of high molecularweight PLA,along with theelevated environmental awareness of the

general public, have resulted in an expanded use of PLA for consumer goods and packaging

applications. Because PLA is compostable and derived from renewable sources, it has been

considered as one of the solutions to alleviate solid waste disposal problems and to lessen

the dependence on petroleum-based plastics for packaging materials. Although PLA can

be processed on standard converting equipment with minimal modifications, its unique

material properties must be taken into consideration in order to optimize the conversion of

PLA 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, and

compounding.

2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821

2. Structural composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821

3. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823

4. Crystallization behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824

5. Rheological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826

6. 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, endothermic

enthalpy relaxation; Hc, heat of crystallization; Hm , heat of fusion; HDPE, high density polyethylene; HIPS, high impactpolystyrene; HMDI, hexamethy-

lene diisocyanate;ISBM, injection stretch blowmolding; LDPE,low density polyethylene; MD, machine direction;MDO, machine directionorientation; MFI,

melt flow index; MMT, montmorillonite;Mn, numb er-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, pressurevolumetemperature; 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).

0079-6700/$ see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.progpolymsci.2008.05.004
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L.-T. Lim et al. / Progress in Polymer Science 33 (2008) 820852 821

7.1. Drying and extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828

7.2. Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830

7.3. Stretch blow molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833

7.4. Cast film and sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835

7.5. Extrusion blown film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836

7.6. Thermoforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837

7.7. Foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 838

7.8. Fiber spinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 840

7.9. Electrospinning of ultrafine fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8417.10. PLA blends with other polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844

7.11. Compounding of PLA composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 846

7.12. PLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847

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

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849

1. Introduction

Thermoplastic polymers exhibit many properties ideal

for use in packaging and other consumer products, such

as light weight, low process temperature (compared to

metal and glass), variable barrier properties to match end-

use applications, good printability, heat sealable, and ease

of conversion into different forms. Today, most plastics

are derived from non-renewable crude oil and natural

gas resources. While some plastics are being recycled and

reused, themajority aredisposed in landfills dueto end-use

contamination. In 2005, plastics were recovered at a rate

lower than 10% in the USA[1].Over the past decade, there

has been a sustained research interest on compostable

polymers derived from renewable sources as one of the

solutions to alleviate solid waste disposal problems and to

lessen the dependence on petroleum-based plastics.

Poly(lactic acid) (PLA) is a compostable polymer derived

from renewable sources (mainly starch and sugar). Until

the last decade, the main uses of PLA have been limited to

medical applications such as implant devices, tissue scaf-

folds, and internal sutures, because of its high cost, low

availability and limited molecular weight. Recently, new

techniques which allow economical production of high

molecular weight PLA polymer have broadened its uses

[2]. Since PLA is compostable and derived from sustain-

able sources, it has been viewed as a promising material

to reduce the societal solid waste disposal problem[3,4].

Its low toxicity[5],along with its environmentally benign

characteristics, has made PLA an ideal material for food

packaging and for other consumer products[6].

PLA belongs to thefamily of aliphatic polyesters derived

from -hydroxy acids. The building block of PLA, lacticacid (2-hydroxy propionicacid), can exist in optically active

d- or l-enantiomers. Depending on the proportion of the

enantiomers, PLA of variable material properties can be

derived. This allows the production of a wide spectrum of

PLA polymers to match performance requirements. PLA has

reasonably good optical, physical, mechanical, and barrier

properties compared to existing petroleum-based poly-

mers [7]. For instance, the permeability coefficients of CO2,

O2, N2,andH2O for PLA arelower than for polystyrene (PS),

but higher than poly(ethylene terephthalate) (PET)[810].

The barrier properties of PLA against organic permeants,

such as ethyl acetate and d-limonene, are comparable to

PET[11].Mechanically, unoriented PLA is quite brittle, but

possesses good strength and stiffness. Oriented PLA pro-

vides better performance than oriented PS, but comparable

toPET [9]. Tensile andflexuralmoduli of PLA arehigher than

high density polyethylene (HDPE), polypropylene (PP) and

PS,buttheIzodimpactstrengthandelongationatbreakval-

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 with

existing petroleum-based thermoplastics.

Today, the main conversion methods for PLA are based

on melt processing. This approach involves heating the

polymer above its melting point, shaping it to the desired

forms, and cooling to stabilize its dimensions. Thus, under-

standing of thermal, crystallization, and melt rheological

behaviors of the polymer is critical in order to optimize

the process and part quality. Some of the examples of

melt processed PLA are injection molded disposable cut-

lery, thermoformed containers and cups, injection stretch

blown 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

[1417].Recently, PLA has also been processed in conjunc-

tion with other filler materials to form composites which

possess various unique properties, including those based

on nanoclays[1823],biofibers[16,24,25],glass fibers[26]

and cellulose[27,28].The aim of this review is to discuss

the key process technologies for PLA and summarize the

properties of PLA related to the processing techniques used.

2. Structural composition

The basic building block of PLA, lactic acid, can be pro-

ducedby carbohydrate fermentation or chemical synthesis.

Currently, the majority of lactic acid production is based

on 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 drivers

for the recent expanded use of PLA is attributable to the

economical production of high molecular weight PLA poly-

mers (greater than

100,000 Da). These polymers can beproduced using several techniques, including azeotropic


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822 L.-T. Lim et al. / Progress in Polymer Science 33 (2008) 820852

Fig. 1. Synthesis of PLA from l- andd-lactic acids. Adapted from Auras et al. [3]by permission of WileyVCH Verlag GmbH & Co. KGaA.

dehydrative condensation, direct condensation polymer-

ization, and/or polymerization through lactide formation

(Fig.1). By andlarge, commerciallyavailable highmolecular

weight PLA resins areproducedvia the lactide ring-opening

polymerization 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]. The

l-isomer constitutes the main fraction of PLA derived from

renewable sources since the majority of lactic acid from

biological sources exists in this form. Depending on the

composition of the optically activel- andd,l-enantiomers,

PLA can crystallize in three forms (, and ). The -structure is more stable and has a melting temperatureTmof 185 C compared to the -structure, with aTmof 175 C[3]. The optical purity of PLA has many profound effects

on the structural, thermal, barrier and mechanical proper-

ties of the polymer[3036].PLA polymers with l-content

greater than 90% tend to be crystalline while those with

lower optical purity are amorphous. Moreover, Tm, glass

transition temperatureTg, and crystallinity decrease with

decreasing l-isomer content [30,34,37]. Tsuji et al. reported

that the optical impurity of PLLA films ranging from 050%

was insignificant in affecting the water vapor transmis-

sion rate (WVTR) of the polymer; nevertheless, the WVTR

values decreased with increasing film crystallinity in the

020% range[31].Thus, judicious selection of appropriate

PLA resin grade is important to match the conversion pro-

cess conditions used. Usually, PLA articles which require

heat-resistant properties can be injection molded using

PLA resins of less than 1% d-isomer. Alternatively, nucle-

ating agents may be added to promote the development of

crystallinity under relatively short molding cycles. In con-

trast, PLA resins of higher d-isomer contents (48%) would

be more suitable for thermoformed, extruded, and blow

molded (e.g., injection molded preform for blow molding)

products, since they are more easily processed when the

crystallinity is low[38].

When exposed to elevated temperatures, PLA is known

to undergo thermal degradation, leading to the formation

of lactide monomers (Section 3). It has been suggested

that this property may be leveraged for the feedstock

recycling of PLA[39,40]. However, the propensity for the

lactide monomer to undergo racemization to form meso-

lactide can impact the optical purity and thus the material

properties of the resulting PLA polymer[3943].Recently,

Tsukegi et al. reported that at temperature less than

200 C, conversion of PLLA into meso-lactide and oligomers

was minimal. However, above this temperature, the for-

mation ofmeso-lactide became quite significant (4.5 wt%

at 200 C and 38.7wt% at 300 C for 120 min heating).

Oligomers were reported to form at temperatures higher

than 230C [39]. These authors also reported that the

oligomerization proceeded rapidly in the presence of MgO,

to reach an equilibriumbetween monomers and oligomers;

the l,l:meso:d,d lactide composition ratio converged to

1:1.22:0.99 (w/w/w) after 120 min heating at 300 C[39].

Fan et al. reported that the racemization at 250300 C


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Fig. 2. Comparison of glass transition and melting temperatures of PLA

with other thermoplastics.

can be controlled by adding calcium oxide to PLLA, which

reduces the pyrolysis temperature, and more importantly,

leads to predominantl,l-lactide formation[40].

3. Thermal properties

Similar to many thermoplastic polymers, semicrys-

talline PLA exhibits Tg and Tm. Above Tg (58C) PLA is

rubbery, while below Tg, it becomes a glass which is still

capable to creep until it is cooled to its transition tem-perature at approximately 45 C, below which it behaves

as a brittle polymer[44].Fig. 2compares PLAsTg and Tmvalues with other polymers. As shown, PLA has relatively

highTgand lowTmas compared to other thermoplastics.

The Tg of PLA is dependent on both the molecular

weight and the optical purity of the polymer (Fig. 3).The

Tg increases with molecular weight to maximum valuesat infinite molecular weight of 60.2, 56.4 and 54.6 C for

PLA consisting of 100, 80, and 50% l-stereoisomer contents,

respectively. Furthermore, PLA with higher content of l-

lactide has higher Tg values than the same polymer with

the same amount of d-lactide[37]. Similar relationships

were reported by Tsuji and Ikada [34].Table 1shows the

Fig. 3. Glass transition temperatures for PLAs of differentl-contents as a

function of molecularweight.Curvesare created basedon theoriginaldatapublished by Dorgan et al.[37]by permission of The Society of Rheology.

Table 1

Primary transition temperatures of selected PLA copolymers

Copolymer ratio Glass transition

temperature (C)

Melting temperature (C)

100/0 (l/d,l)-PLA 63 178

95/5 (l/d,l)-PLA 59 164

90/10 (l/d,l)-PLA 56 150

85/15 (l/d,l)-PLA 56 140

80/20 (l/d,l)-PLA 56 125

Adapted from Bigg[33].

glass transition and melting temperatures of different PLA

polymers produced with different ratios of copolymer.

In general, the relationship betweenTg and molecular

weight can be represented by the FloryFox equation:

Tg =Tg K

Mn(1)

whereTg is theTg at the infinite molecular weight, Kis aconstant representing the excess free volume of the end

groups for polymer chains, and Mn is the number aver-age molecular weight. The values ofTg and K are around

5758 C and (5.57.3)104 as reported in the literature

for PLLA and PDLLA, respectively[45].

The glass transition behavior of PLA is also dependent

on the thermal history of the polymer. Quenching the poly-

mer from the melt at a high cooling rate(>500 C/min, such

as during injection molding) will result in a highly amor-

phous polymer. PLA polymers with low crystallinity have a

tendency to undergo rapid aging in a matter of days under

ambient conditions[46,47].The phenomenon is an impor-

tant contributor to theembrittlement of PLA. This topicwill

be discussed in greater details in Section7.2.

The Tm of PLA is also a function of its optical purity.The maximum practicalobtainable Tmfor stereochemically

pure PLA (either l or d) is around 180 C with an enthalpy

of 4050 J/g. The presence ofmeso-lactide in the PLA struc-

ture can depress the Tm by as much as 50C, depending

on the amount of d-lactide incorporated to the polymer.

Fig. 4 shows the variation of the Tmas 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).


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[48]and Hartmann[49].The relationship ofTmand meso-

lactide content can be approximated reasonably well by the

following expression[48]:

Tm (C) 175 C 300 Wm (2)

whereWmis the fraction ofmeso-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 of130160 C. The Tm depression effect ofmeso-lactide has

several 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 solid

and liquid states ranging from 5 to 600 K[36]. The heat

capacity (Cp-liquid , J K1 mol1) can be represented in a sim-

ple form: Cp-liquid = 120.17 + 0.076T, whereTis in Kelvin (K).

4. Crystallization behavior

The physical, mechanical and barrier properties of

PLA are dependent on the solid-state morphology andits crystallinity. Accordingly, the crystallization behaviors

of PLA have been studied in detail by many researchers

[4,32,5055]. PLA can be either amorphous or semicrys-

talline depending on its stereochemistry and thermal

history. The crystallinity of PLA is most commonly deter-

mined using the differential scanning calorimetry (DSC)

technique. By measuring the heat of fusion Hmand heatof crystallizationHc, 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% crystalline

PLLA or PDLA homopolymers.

On quenching the optically pure PLA polymer from the

melt phase (e.g., during injection molding process), the

resulting polymer will become quite amorphous. As shown

in Fig. 5, quenching thepolymer from melt at a high cooling

rate resulted in an exothermic crystallization peak on the

DSC thermogram during the subsequent reheat, while slow

cooling produced a polymer with higher crystallinity with

much lower enthalpy of crystallization. The tendency for

PLAto crystallizeuponreheat also dependedon theheating

rate (Fig. 6), as well as the optical purity of the PLA polymer

(Fig. 7).As shown inFig. 7,PLA polymers with greater than

8% d-isomer level remained amorphous even after 15 h of

isothermal treatment at 145 C. Incontrast, at 1.5%d-isomer

level, although the quenched sample (Quenched PLA-l)

has a minimal crystallinity, the isothermal treatment at

145C resulted in a largeendothermic melting peak around

450K (Fig. 7). In general, the crystallization half-time of

PLA increases about 40% for every 1% (w/w) meso-lactide

in the polymerization mixture, which is mainly driven by

the reduction of the melting point for the copolymer [56].

Nucleation parameters for PLLA crystallization under

isothermal and nonisothermalconditions were determined

by 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. 5. DSC thermograms of water quenched, air-annealed (cooled from

220 C to ambient temperature in 5 min), and full-annealed (cooled from

220 C to ambient temperature in 105min) PLLA samples. DSCscans were

performed at a heating rate of 10 C/min. Adapted from Sarasua et al. [32]

by permission of John Wiley & Sons, Inc.

Fig. 6. DSC scans for 1.5% d-lactide PLA samples cooled from the melt at

10 K/minand then reheatedat differentheatingratesfrom30 to0.3 K/min.

Adapted from Pyda et al.[36]by permission of Elsevier B.V.

related in the following form[54,57]:

Kg =4beTm

Hfk (4)

where Kgis 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 2shows

the nucleation parameters from isothermal and non-

Table 2

Nucleation parameters from isothermal and nonisothermal kinetic anal-

yses for PLLA

Parameter Isothermal Non-isothermal

Nucleation parameter,Kg (105) 2.44 2.69

Lateral surface energy,(103J/m2) 12.0 13.6

e (106J2/m4) 753 830

Adapted from Kishore and Vasanthakumari[54].


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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 the

melt and isothermally crystallized at 145C for 15 h. The quenched PLA-L

sample was cooled similarly from the melt but did not undergo the 15 h

isothermal crystallization. Thermograms are recreated based on the dataoriginally published by Pyda et al.[36]by permission of Elsevier B.V.

isothermal kinetic analysis of PLLA. Solving Eq. (4)with

Tm =480K, Hf= 111.083 106J/m3; b = 5.17108 cm,

12.03103J/m2, and e = 6.089104J/m2, Kg can bedetermined. This value can be used to evaluate the transi-

tion between two types of crystallization behavior in PLA.

In the first type of crystallization, the nucleation rate is

low and axialite morphology in the films is prevalent. In

the second type, the nucleation rate is high, so multinu-

cleation occurs and spherulitic morphology in the films

is observed[57]. For PLLA, both crystallization processes

have been observed depending on the molecular weight

of the samples. The infinite dissolution temperature T0d

(determined by the extrapolation of dissolution tempera-

ture Td versus crystallization temperature Tc plots to the

intersection where Td = Tc) for PLLA in p-xylene solution

was determined by Kalb and Pennings to be 126.5 C[58].

This temperature is relevant for fiber formation processes,

since fibers prepared from solution near this temperature

have ultra-high strength properties[58].

The formation of crystallinity may or may not be favor-

able depending on the end-use requirements of the PLA

articles. For instance, high crystallinity will not be opti-

mal for injection molded preforms which are intended

for further blow molding since rapid crystallization of the

polymer would hamper the stretching of the preform and

optical clarity of the resulting bottle. In contrast, increased

crystallinity will be desirable for injection molded articles

for which good thermal stability is important. Crystal-

lization of PLA articles can be initiated by annealing at

temperatures higher thanTg and below the melting point

to improve their thermal stability. For instance, Perego et

al. showed that crystallization of injection molded PLLA

parts by annealing at 105C for 90 min increased tensional

and flexural elasticity, Izod impact strength, and heat resis-

tance[59].After annealing PLA copolymers, the presence

of two melting peaks in a DSC scan is quite common, as

previously observed by Yasuniwa et al. [60].They reportedthat the low temperature Tm peak height increased with

Fig. 8. Development of crystallinity in biaxially stretched PLA at 80 C

using 100% s1 strain rate. Data are adapted from Drumright et al. [38]

by permission of WileyVCH Verlag GmbH & Co. KGaA.

increasing heating rate, whereas the high temperatureTmdecreased. In contrast, increasing the cooling rate reduced

the low Tm peak, while the high Tm peak increased. The

double-melting peak behavior was explained based on

melt-recrystallization model, in which small and imper-

fect crystals changed successively into more stable crystals

through the melting and recrystallization[60].

Another strategy to increase the crystallinity of PLA is

by incorporating nucleating agent in the polymer during

extrusion. This lowers the surface free energy barrier for

nucleation and enables crystallization at higher tempera-

ture to take place upon cooling. Kolstad showed that talc

can 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 from

3 minat 110 C toapproximately25 s. Atthe same percent of

talc, for 3% mesolactide copolymerized with the l-lactide,

thehalf-time reduced from about 7 minto about 1 min[56].

Li and Huneault compared the crystallization kinetics of

talc and montmorillonite (MMT, Cloisite Na+) for 4.5% d-

PLA. They reportedthat the lowest crystallization induction

period and maximum crystallization speeds were observed

around 100 C. By adding 1% (w/w) of talc, the crystalliza-

tion half time of PLA was decreased from a few hours to

8 min. In contrast, the MMT tested was less effective as a

nucleating agent; the lowesthalf-time achieved was30 min

[61].

Unlike quiescent crystallization discussed above, strain-

induced crystallization occurs when the polymer is

mechanically orientated. This phenomenon is prevalent

during the production of oriented PLA films, stretch blow

molding of bottles, thermoforming of containers, and fiber

spinning. As expected, the proportion ofd- and l-isomers

has an effect on the strain-induced crystallinity during

the mechanical orientation. As shown in Fig. 8, the per-

cent crystallinity of amorphous PLA sheet increases with

increasing draw ratio. Moreover, the crystallinity decreases

as the stereoisomeric purity of the polymer decreases [38].

The amount of crystallinity attained through orientationalso depends on the mode of stretching (sequential ver-


7/33


Fig. 9. Comparisonof zero-shear viscosityvaluesversusmolecular weight

for 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 annealing

conditions[38,62,63].More discussions on this topic will

be presented in Section7.2.

5. Rheological properties

Melt rheological properties of PLA have a profound

effect on how the polymer flows during the conver-

sion process. Since the PLA rheological properties are

highly dependent on temperature, molecular weight and

shear rate, they must be taken into consideration during

tooling design, process optimization, and process model-

ing/simulation. Melt viscosities of high-molecular-weight

PLA are in the order of 500010,000P (5001000 Pa s)

at shear rates of 1050 s1. These polymer grades are

equivalent to Mw 100,000Da for injection molding to

300,000 Da for film cast extrusion applications[4]. The

melts of high molecular weight PLA behave like a pseu-

doplastic, non-Newtonian fluid. In contrast, low molecular

weight PLA (40,000 Da) shows Newtonian-like behavior

at shear rates typical of film extrusion [64]. Under iden-

tical processing conditions, semicrystalline PLA tends to

possess higher shear viscosity than its amorphous counter-

part.Moreover, as shear rates increase, the viscosities of the

melt decrease considerably, i.e., the polymer melt exhibits

shear-thinning behavior[65].

Viscoelastic properties of polymer melts can be charac-

terized by zero-shear viscosity, 0, and recoverable shearcomplianceJOe. Both of these parameters can be obtainedfrom dynamic experiments by determining the dynamic

moduli at the limit of low frequency[48].The product of

these two values (0 JOe) gives the average relaxation time

required for final stress equilibration time in the liquid 0.Thevalue of0is stronglyaffectedby 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 onMw to the 4.0 power

instead of the theoretical value of 3.4[64]. In comparison,

Dorganet al.reported a power indexof 4.6[66]. Fig.9 showsthe relationship between 0 and Mw for PLLA (100:0) at

180C[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

EaR

1

T(K)

1

373

(5)

where a =3.380.13, the activation energy of flow

Ea = 190 kJ/mol, 0,ref=89,4009300Pas, R is the gasconstant 8.314 J/Kmol, and T is the temperature in K.Witzke further showed that 0 can be correlated withthe isomer composition by fitting to the well-known

WilliamsLandelFerry 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 =15.61.6,

and C2 = 11011 C; a1, a2, a3, and C1 do not have units;and T(C) is the testing temperature in C. Eq. (6) can

be used to predict 0 of amorphous polylactides withl-monomer composition higher than 50% between Tgand Tg +100 C. The equation predicts that 0 increaseswith increasing l-monomer and decrease as meso-lactide

content increases[48].

Therheological properties of PLAcan be modified by the

introduction of branching into the polymer chain architec-

ture. Many routes, such as multifunctional polymerization

initiators, hydroxycyclic ester initiators, multicyclic ester,

and crosslinking via free radical addition have been used

to introduce branching in PLA [12,6769]. Lehermeier

and Dorgan blended PLA with 5% d-isomer with varyingproportions of branched PLA produced through peroxide

initiated 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 to

the effect of free volume. Lehermeier and Dorgan showed

that tris(nonylphenyl) phosphite was effective for stabi-

lizing the viscosity of PLA during the thermorheological

time sweep experiment of branched PLA polymers[67].In

another study from the same research group, the stabiliz-

ing effect of tris(nonylphenyl) phosphate was elucidated

by using the time-temperature superposition technique,

showing that this compound greatly facilitated the ther-

morheological experiments by prevented the confounding

effect from degradation reactions[69,70].

CarreauYasuda model (Eq. (7)) has been used to model

the viscosity and shear rate relationship of linear PLA and

linear-branched PLA blends[69]:

= C1[1+ (C2 )C3 ](C41/C3) (7)

where is the viscosity, is the shear rate, and C1,C2,C3andC4 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 the

reciprocal of frequency for the onset of shear thinning.C3 determined the shear thinning which increased with


8/33


Table 3

CarreauYasuda model parameters for Eq. (7)

Blend, % CarreauYasuda parameters

C1 (Pas) C2 (s) C3 C4

0 10,303 0.01022 0.3572 0.0340

20 8,418 0.00664 0.3612 0.0731

40 6,409 0.01364 0.4523 0.0523

60 5,647 0.00513 0.4356 0.1002

80 4,683 0.00450 0.4754 0.1108

100 3,824 0.01122 0.7283 0.0889

Adapted from Lehermeir and Dorgan[69].

increasing linear content, i.e., branched PLA shear thinned

stronger than the linear material[69].The increase of both

0 and shear thinning with the addition of branching isalso reported by other studies on PLA polymers with star

polymer chain architectures[12,66].

Palade et al. studied the extensional viscosities of high l-

content PLA (100,000120,000Mw). They showed that PLA

can be drawn to large Hencky strains without breaking. The

polymer also exhibited strain-hardening behaviors duringthe deformation[70], which is an important characteris-

tic for processing operations, such as fiber spinning, film

casting, and film blowing. Yamane et al. reported that the

addition of PDLA to PLLA enhanced the strain hardening

properties of the resulting blends even at very low PDLA

contents (


9/33


Fig. 10. Thermal degradation of PLA. Adapted from McNeill and Leiper [74]by permission of Elsevier B.V.

the chain reaction[74].Although acetaldehyde is consid-

ered to be non-toxic and it is naturally present in many

foods, the acetaldehyde generated during melt processing

of PLA must be minimized, especially if the converted PLA

(e.g., container, bottle, and films) are to be used for food

packaging. The migration of acetaldehyde into the con-

tained food can result in off-flavor which may impact the

organoleptic properties and consumer acceptance of the

product[7577].

From the production point of view, the formation of

lactide due to depolymerization is undesirable. Besides

reducing PLA melt viscosity and elasticity, the volatile lac-

tide formed can result in fuming and/or fouling of the

processing equipment such as chilled rollers, molds and

tooling surfaces [78]. The latter is characterized by the

gradual building up of a layer of lactide on the equipment

surfaces, commonly known as plate out. To overcome this

problem, the temperature of the equipment is generally

elevated to reduce the tendency of condensation of lactide.

Taubner and Shishoo showed that the moisture content

of resin, temperature, and residence time of PLA melt

during extrusion are important contributors to molecular

weight drop of the polymer during extrusion [72]. Pro-

cessing of dried PLLA with initial Mn of 40,000 g/mol in

a twin-screw extruder at 210 C caused the Mn to drop to

33,600 and 30,200 g/mol, when screw rotation speeds of

120 and 20 rpm were used, respectively. Using the same

120 and 20 rpm screw speeds but processing at 240C,

the Mn values decreased dramatically to 25,600 and

13,600 g/mol, respectively. In contrast, Mn for extruded

articles produced from wet resins (equilibrated at 20 C

65% RH to give 0.3%, w/w, moisture content) were 18,400

and 12,000 g/mol, respectively. These results highlighted

the importance of minimizing the residence time and

process temperature during PLA extrusion. From a resin

formulation point of view, the residual polymerizing

catalysts present in the resin are also known to catalyze

the reverse depolymerization and hydrolysis reactions

[48,79]. This may partially explain the large variation of

molecular weight drop for melt processed PLA reported

in the literature. For instance, Witzke, Gogolewski et al.

and Perego et al. reported molecular weight losses for

injection molded PLA parts of 552%, 5088% and 1440%,

respectively [48,59,80]. To stabilize the polymer during

melt processing, the removal or deactivation of the residual

catalyst is important to minimize the molecular weight

loss which will impact the mechanical properties of the

PLA parts. Strategies to improve the melt stability of PLA

can be found in patent publications [79,81,82]. Due to

the different processes and technologies used, the melt

stability of PLA polymer may be different from supplier to

supplier. Injection molded PLA made from properly dried

good quality PLA resins and optimal processes should

exhibit 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 be

dried sufficiently to prevent excessive hydrolysis (molec-

ular weight drop) which can compromise the physical

properties of the polymer. Typically the polymer is dried to

less than 100 ppm (0.01%, w/w). Natureworks LLC, one of

the main suppliers for PLA polymers, recommended that

resins should be dried to 250ppm (0.025%, w/w) mois-

ture content or below before extrusion. Processes that

have longresidence times or hightemperature approaching

240

C should dry resins below 50 ppm to achieve maxi-mum retention of molecular weight[84,85].Drying of PLA


10/33


Table 5

Drying half times for PLA pellets under40 C dew point and air flow rate

of 0.016 m3/(minkg)[108]

Drying temperature (C) Drying half time (h)

Amorphous pellets

40 4.0

Crystalline pellets

40 4.3

50 3.9

60 3.3

70 2.1

80 1.3

100 0.6

takes place in the temperature range of 80100 C. The

required drying time is dependent on the drying temper-

ature (Table 5). Commercial grade PLA resin pellets are

usually crystallized, which permits drying at higher tem-

peratures to reduce the required drying time. In contrast,

amorphous pellets must be dried below the Tg(60C) to

prevent the resin pellets from sticking together, which can

bridge and plug the dryer. It is noteworthy that because

PLA degrades at elevated temperatures and high relative

humidity, the resins should be protected from hot and

humid environments. Henton et al. reported that amor-

phous PLA can dramatically reduce its Mw in less than a

month when exposed to 60 C and 80% RH (Fig. 11)[44].

To achieve an effective drying, the dew point of the

drying air should be40 C or lower. Drying of PLA is com-

monly achieved using a closed loop dual-bed regenerative

desiccant-type dryer. In this type of dryer, the resin pellets

are contained in a hopper that is purged with dry air at

elevated temperature. The dry air is generated by the des-

iccant bed. During the operation, one desiccant bed is in

the process air stream which removes moisture from the

resin, while the other stand-by bed is being regenerated

(Fig. 12).The hot air from the process stream removes the

moisture from the resin in the hopper. The air is then circu-

lated back to the dryer where it is cooled and the moisture

is stripped by the desiccant. The air is reheated before it is

channeled back to the hopper. When the dew point of the

Fig. 11. Plots of molecular weight loss of PLA versus time under different

environment conditions. Curves are based on the original data publishedby 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 goes

into the regeneration cycle where the desiccant is heated

to desorb the moisture from the desiccant and vent it to the

atmosphere. Meanwhile, the process air is directed to the

stand-by desiccant which was previously dried.

Extrusion is the most important technique for contin-

uously melt processing of PLA. The plasticizing extruder

can be part of the forming machine systems for injection

molding, blow molding, film blowing and melt spinning.

Fig. 13 shows a schematic representation of themajor com-

ponents of an extruder for an injection molding machine. A

typical screw consists of three sections: (1) feed section

acts as an auger which receives the polymer pellets and

conveys 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. In

order to segregate the molten polymer pool from the pellet

unmelted pellets, various barrier flight designs have been

adopted; (3) meteringsection characterized by a constant

and shallow flight depth, which acts as a pump to meter

accurately the required quantity of molten polymer. The

l/d ratio, which is the ratio of flight length of the screw

to its outer diameter, determines the shear and residence

time of themelt. Screws with large l/d ratio provide greater

shear heating, better mixing, and longer melt residence

time in the extruder. Commercial grade PLA resins can typ-

ically be processed using a conventional extruderequipped

with a generalpurposescrew ofl/d ratio of 2430. Extruder

screws for processing PET, which are typically low-shear for

gentle 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 feed

section to the flight depth in the metering section. The

greaterthe compression ratio a screw possesses, the greater

the shear heating it provides. The recommended compres-sion ratio for PLA processing is in the range of 23[86].


11/33


Fig. 13. Typical geometries of a screw for single-screw extruder.

During the plasticizing process, PLA resin pellets are

fed 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. The

heat required for melting is provided by the heater bands

wrapped around thebarrel. As the screw rotates, the flights

shear and push the polymer against the wall of the barrel

which also provides frictional heat for melting the poly-

mer. The combined thermal energy from the heater and

frictional heat due to friction between the plastic and thescrew and barrel, provide sufficient heat to raise the PLA

polymer above its melting point (170180 C) by the time

it reaches the end of the barrel. To ensure that all the crys-

talline phases are melted and to achieve an optimal melt

viscosity for processing, the heater set point is usually set

at 200210 C.

7.2. Injection molding

Injection molding is the most widely used converting

process for thermoplastic articles, especially for those that

are 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 extruderunit for injectionmolding machine is designed

such the screw can reciprocate within the barrel to pro-

vide enough injection pressure to deliver the polymer melt

into the mold cavities (Fig. 14). Most injection molding

machines for PLA are based on the reciprocating screw

extruder, although two-stage systems, which integrate a

shooting pot and extruder in a single machine, have also

been deployed for injection molding of preforms for PLA

bottles. The two-stage system consists an in-line extruder

integrated to a shooting pot. The extruder plasticizes and

feeds the melt into the shooting pot under relatively low

injection pressure, from which the melt is injected into the

hotrunner under high pressure by a plunger in theshooting

pot. While the reciprocating machine must stop the screw

during the injection and packing phases, the screw for the

two-stage machine can rotate during the majority of thecycle. The two-stage system presentssome advantages over

its reciprocating counterpart, including shorter cycle time,

small screw motor drive, more consistent melt quality, and

more consistent shot size[87].

A typical cycle for an injection molding machine is pre-

sented inFig. 15.The beginning of mold close is usually

taken as the start of an injection molding cycle. Immedi-

ately after the molds clamp up, the nozzle opens and the

screw moves forward, injecting the polymer melt into the

mold cavity. To compensate for the material shrinkage dur-

ing cooling in the mold, the screw is maintained in the

forward position by a holding pressure. At the end of the

holding phase, thenozzle is shut off andthe screwbegins 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 barrel

against a controlled back pressure exerted on the screw

by a hydraulic cylinder. To ensure that the part is dimen-

sionally stable enough to withstand the opening stroke the

Fig. 14. Major components of an injection molding machine showing the extruder (reciprocal screw) and clamp units.


12/33


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

molds, sufficient cooling time must be given. In the mold-

ing cycle, heat removal takes place predominantly during

thefill, hold and cool phases, although mold opening phase

also contributes to partial cooling since one side of the part

(core-contacting side) is still being cooled prior to ejection.

Cycle time is an important process parameter which is

often minimized to maximize the production throughput.

To reduce the cycle time, it is quite common to transfer

the partially cooled injection molded article to a post-mold

cooling device, to provide an extended cooling of the part

outside the molds, either by direct contact on a chilled sur-

face and/or by forced air. From Fig. 15, it is also evident

that minimizing the duration for non-process events, suchas mold opening, part ejection and mold closing is also

important for reducing the cycle time. Lowering mold tem-

perature can also increase the heat extraction rate from

the polymer. Nevertheless, the propensity of lactide con-

densation on the cold tooling surfaces, which can affect

the surface finish and weight of the molded articles, limits

the minimal temperature that can be used during injec-

tion molding of PLA to 2530 C. The use of molds with

polished surfaces, in conjunction with an increased injec-

tion speed during fill, can also reduce the deposition of the

lactide layers.

The fill, hold and cool events that take place during

injection molding have an important implication on the

shrinkage of the injection molded articles. This effect can

be best elucidated using a pressurevolumetemperature

(PVT) diagram.Fig.16 showsPVT diagramsfor PLAfromtwo

references [88,89]. The different profiles shown here are

likely due to the different grades of PLA used. During injec-

tion molding, the polymer is first subjected to isothermal

injection of the polymer melt into the mold cavity, dur-

ing which the pressure increases as the polymer is being

injected and packed to the holding pressure (trace ab in

Fig. 16). The polymer then undergoes isobaric cooling in

the holding phase (trace bc), followed by isochoric cool-

ing. When the polymer cools below the freezing point, the

gate freezes and the pressure in the mold cavities drops

to one atmospheric pressure (trace cd). In the last cool-

ing phase, the article continues to cool isobarically to room

temperature (trace de). The change in specific volume dur-

ing the final isobaric cooling (trace de) dictates the extent

of part shrinkage. The hold pressure and temperature play

an important role in determining how much the molded

article shrinks.

The PVT relationship can be modeled mathematically,

such as by using the modified two-domain Tait model

[9092]. This model is often used for numerical simula-

tion of injectionmolding processes involving finite element

analysis for predicting the shrinkage behavior of injection

Fig. 16. PVT plots for PLA based on the data from Sato et al. and Natureworks LLC [88,89].The continuous lines represent the fitted results based on thetwo-domain modified Tait model (Eq.(8)).


13/33


Fig. 17. Effects of temperature and time on the aging of injection molded 4% d-lactide PDLA specimens. (A) DSC curves of PLA aged at room temperature

for various aging times. (B) DSC curves of PLA annealed for 24 h at different temperatures. Plots are created based on the data from Cai et al. [47].

molded articles. The modified two-domain Tait PVT model

takes the form:

v(T, p) = V0(T)

1 Cln

1+

p

B(T)

+ Vt(T, p) (9)

where v(T, p) is specific volume at temperature Tand pres-sure P; V0 is specific volume at zero gauge pressure and Cis

a constant, 0.0894. When the temperature of the materialis greater than the transition temperature, V0(T) andB(T)

are determined byb1m,b2m,b3m,b4m andb5as follows:

V0 = b1m + b2m(T b5) (10)

B(T) = b3m exp[b4m(T b5)] (11)

In contrast, when the material temperature is lower than

the transition temperature,V0(T) andB(T) are determined

byb1s,b2s,b3s,b4sandb5as 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 and

the transition temperature at zero gauge pressure (b5) as

follows:

Ttrans(P) = b5 + b6p (14)

For non-amorphous materials, an additional transition

function is required:

Vt(T, p) = b7 exp[b8(T b5) b9p] (15)

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

rapid physical aging of the polymer since ambient tem-

perature is only about 25 C below the Tg [37,46,48]. The

aging of PLA can be evaluated by studying the Tgregion of

a 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 aging

also became faster. However, when the aging temperature

went above the Tg (60C), the excess enthalpy relaxation

was reduced, indicating that physical aging was no longer

taking place whenthe aging temperature wasabove Tg [47].

Celli and Scandola observed a similar aging trend for PLLA

using DSC and a dynamic mechanical analyzer [46].They

observed that the extent of aging increased with decreas-

ing molecular weight (i.e., Hrelincreased with decreasingmolecular weight), which was attributed to the increased

chain terminals that possess higher motional freedom than

the internal chain segments[46].The physical implication

of aging was elucidated by Witzke, who reportedthat injec-tion molded articles tested immediately after quenching

to very cold temperatures exhibited a much larger exten-

sion to break. However, when the molded specimens were

aged at room temperature for 38 h, they became very

brittle [48]. This phenomenon was attributed to the reduc-

tion of free volume of the polymer due to rapid relaxation

towardsthe equilibriumamorphousstate. Aging below Tg is

exclusively related to the amorphous phase of the polymer;

accordingly, increasing the crystallinityof the polymer (e.g.,

by adjusting d-isomer composition or the use of nucleat-

ing agents) will reduce the aging effect. Furthermore, the

crystallites formed also act likephysical crosslinks to retard

the polymer chain mobility. However, amorphous injection


14/33


molded articles which are intended for further process-

ing (e.g., preforms for stretch blow molding), the storage

conditions prior to subsequent processing may need to

be controlled. Moreover, process parameters such as mold

temperature, packing pressure, cooling rate, and post-mold

cooling treatment are expected to influence the PLA aging

behavior as well.

7.3. Stretch blow molding

Due to the recent consumers heightened environmen-

tal awareness, there is a sustained interest from the food

industry to replace the existing non-biodegradable ther-

moplastics with PLA for certain beverage products. To date,

PLA bottlesare predominantlyused for beverages which are

not sensitive to oxygen (e.g., flat water beverages, pasteur-

ized milk). While barrier properties of PLA bottles may be

improved by various technologies (multilayer structures,

external coating, internal plasma deposition, oxygen scav-

enger), their implementation is currently limited due to

higher production costs.The production of PLA bottles is based on injection

stretch blow molding (ISBM) technique. This process pro-

duces biaxial orientated PLA bottle with much improved

physical and barrier properties compared to injec-

tion molded amorphous PLA. The molecular orientation

induced during the ISBM process decreases the effect of

aging by stabilizing the polymer free volume [48]. The

crystallites produced during strain-induced crystallization

also reduce the aging effect since they can act as physical

crosslinksto stabilize the amorphousphase, thereby reduc-

ing its brittleness. Similar effects have been reported for

semicrystalline PET[93].The ISBM process for PLA bottles

is depicted inFig. 18.It involves first the formation of pre-

form (also known as parison) using an injection molding

machine. The preform is then transferred to a blow mold-

ing machine where it is stretched in the axial direction and

blown in the hoop direction to achieve biaxial orientation

of the polymer. In the blow molding machine, the preform

is heated in front of several banks of infrared heater to tem-

peratures (85110 C) suitable for blow molding (Fig. 18a).

Different power settings are usually applied to the infrared

heaters to give a temperature profile optimal for stretch-

ing the preform into bottle with uniform wall thickness

distribution. Frequently, reheat additives, such as carbon

black dispersed in a liquid carrier, are added to the resin

in the extruder to increase its infrared energy absorption.

PLA preforms have a tendency to shrink after reheat, espe-

Fig. 18. Injection stretch blow molding (ISBM) of PLA bottle.


15/33


cially regions near the neck and the end cap where the

residual injection molding stresses are the greatest. This

may be moderated through proper preform design, with

gradual transition regions. When the preform has attained

the optimal temperature, it is transferred to the blow mold

(Fig. 18b). The blow nozzle is lowered to seal the preform

finish, while the stretch rod travels towards the preform,

at a typical speed of 11.5m/s, and stretches the preform

to the base cup (Fig. 18ce). During the preblow phase

(Fig. 18dand e), compressed air of 0.52.0MPa is admitted

to the preform through the blow nozzle to partially inflate

thepreform to preventit from touching thestretchrod dur-

ing the axial stretching. When the stretch rod arrives at the

basecupandpinsthepreformtothemoldbase,theairpres-

sure ramps up to 3.84.0 MPa to fully inflate the preform.

This forces the inflated preform to take the shape of the

blow mold and to imprint the surface details of the bottles

(Fig. 18fand g). The high blow pressure is maintained for

several seconds to allowthe bottle to cool down sufficiently

before discharging the bottle.

The aforementioned process is known as the two-stage

process. In contrast, the one-stage process entails the injec-

tion and blow molding of the preform within the same

machine equipped with both injection and blow mold-

ing units. In this process, the injection molded preform

is partially cooled down to 100120 C and then stretch

blown in the blow molding station.Fig. 19summarizes the

thermal history of PLA from resin pellets to bottle for the

two processes. As shown, PLA preform made in the one-

stage process does not go through the aging process during

which the polymer tends to embrittle. Thus, PLA preforms

intended for one- and two-stage processes may need to be

designed and processed differently. The neck finish of the

preform is highly amorphous and is quite brittle. Therefore,

the neck finish must be designed such that the side wall is

thickenough topreventthe neck from blowing outor crack-

ing due to the compression load from the blow nozzle. The

blow mold temperature for PLA is typically set at around

35 C. Because the base of the bottle tends to be quite thick,

the residual heat can cause the base to roll out after the

bottle is ejected from the blow mold. This problem can be

overcome by incorporating radial ribs to reinforce the base

and/or chilling thebase mold insert to a temperaturelower

than the mold halves[94].

Similarly to PET, PLA exhibits strain-hardening when

stretched to high strain. This self-leveling phenomenon is

desirable for blow molding of preforms to achieve optimal

bottle side wall orientation and minimize wall thickness

variation. Since strain-hardening occurs only when the PLA

is stretched beyond its natural stretch ratio, the preform

must be designed to match thetarget bottle size and shape,

such that optimal stretch ratios are achieved during blow

molding (Fig. 20).Preforms that are under-stretched will

result in bottles with excessive wall thickness variation,

weak mechanical properties and pooraesthetic appeal (e.g.,

lens defect below the support ledge region). In contrast,

overstretched bottles can also result in stress whitening

due to the formation of micro-cracks on the bottle surfaces

that diffract light. Typical commercial grade PLA resins for

bottle applications require preform axial stretch ratios of

2.83.2 and hoop stretch ratios of 23, with the desirable

planar stretch ratio of 811 [94,95]. It is noteworthythat the

ultimate amount of crystallinity after stretching decreases

with the decreasing stereoisomeric purity of the polymer

[38].Accordingly, the optimal stretch ratios depend on the

grade of PLA used.

Preform designs are often proprietary, and therefore

there is a lack of information in theopen literature. An opti-

mal preform design should meet the minimum required

stretching which is above the natural stretch ratio, by vary-

ing the shape, diameter, length, blend radius, and transition

features, to meet the part weight requirement. Depending

Fig. 19. Thermal history of PLA polymer during one- and two-stage PLA bottle manufacturing.


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Fig. 20. Schematic representation of PLA preform (left) and bottle (right), showing their key features and main stretch ratios used for preform design.

on the shape of the bottle, subtle but critical features such

as transition shape (reverse versus standard taper), step

changes, and pinch points on the core and cavity may also

be incorporated in the preform design. Since the stretching

behavior of PLA is similar to PET but not entirely the same,

conversion of materials using existing PET preform designs

may be feasible, although design modifications are often

required to achieve an optimized bottle.

7.4. Cast film and sheet

PLA with l-lactide contents of 9298% have been

successfully extruded using conventional extruders. The

production of PLAfilm andsheet is practically identical; the

main difference between them is their stiffness and flexi-

bility due to the difference in their thicknesses. Typically,

films are 0.076 mm (0.003 in.) in thickness, while sheets

are typically0.25mm (0.01 in.). In cast film extrusion, the

molten PLA is extruded through a sheet die and quenched

on polished chrome rollers that are cooled with circulating

water.DuetothethermalsensitivityofPLA,theuseofexter-

naldeckles on thedie shouldbe avoided since thedegraded

resin behind the deckles can leadto edgeinstability. Usually

thediegapissetto10%or2550m (12 mils)greater thanthe target sheet thickness[84]. Ljungberg et al. extruded

neat PLA in a HaakeRheomex 254 extruder with a Rheo-

cord 90 drive unit (Karlsruhe, Germany)[96].The 19.3 mm

diameter screw has a compression ratio of 2:1 and l/d ratio

of 25. In this study, the temperatures for the feeding zone,

the barrel and the die were 160, 180, and 175 C, respec-

tively [96]. Similar extruder temperatureprofileswere used

by Gruber et al.[79].

Sheet and film forming can be achieved on a three-roll

stack. Because of the low melt strength of PLA, horizontal

roll stacks configuration is preferred. To avoid the con-

densation of lactide monomers and slippage of web onthe rollers, relatively high roller temperatures (2550 C)

are usually used. Lactide monomer buildup around the

die could be further prevented by using an exhaust sys-

tem. Nevertheless, extreme high temperatures should be

avoided as the web will stick to the rollers, resulting in

poor quality sheet. To reduce the chance of trapping air

and reduce film or sheet defects, one resin supplier recom-

mended that the die be positioned as close as possible to

the 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 fishtail

die with a 300400m split gap and a casting air gap of15 mm[96]. Generally, hydraulic rolls stands, capable of

producingpressure around 800900 lbs/linear inchof die is

required to prevent floating of the rolls which would result

in uneven PLA surfaces, edge instability, and neck-in [84].

Good contact between the web and rolls is also important

to minimize lactide buildup. Casting of PLA film usually

requires edge pinning (electrostatic or low pressure air) to

eliminate streaking, reduce neck-in, and improve edge sta-

bility[97]. Slitting and web handling of PLA is similar to

PS. Edge trimming of PLA should be carried out with rotary

shear knives since razor knives may yield rough edges and

web breaks. Winding of the PLA web should be done with

good tension control in order to obtain a consistent gauge.

Similar to PP, PET and PS films, the physical properties

of PLA films can be enhanced through orientation. Uniaxial

orientation of PLA is achieved in conventional machine-

directionorientation (MDO) rolls. Since PLA tends to neck in

during drawing, nipped rolls are usually required. Through

mechanical drawing, it is possible to improve thermal and

impact 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 obtained

by stretching it to two to ten times its original length at

6080 C [51], which is much lower compared with OPP

and PET. Typical drawing temperatures for PLA films in the

machine (MD)and transverse directions (TD)are presented


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Table 6

Recommended drawing conditions in the machine and transverse direc-

tion for PLA[97]

Section Temperature range (C)

Machine direction preheat 4565

Slow draw 5570

Fast draw 7075

Annealing 4555

Transverse direction

Preheat 6570

Draw 7085

Annealing 125140

in Table 6. In generally, for 98% l-lactide PLA, machine

direction orientation of 23 is expected, while transverse

stretch ratios of 24 may be used. At higher d-lactide

contents, the machine and transverse stretch ratio can be

increased.Fig. 21shows a typical extrusion cast line for

producing biaxially oriented PLA film.

The orientation in PLLA films depends on the draw

rate, temperature and ratio. High strain rate, low tem-perature and high stretch ratio favor strain-induced

crystallization during orientation. Taking the competitive

crystallization and relaxation effects into consideration,

Lee at al. concluded that the optimal drawing tem-

perature to obtain highly oriented PLLA films (Mw of

190,000 g/mol) is about 80 C [63]. In contrast, Gruber et al.

used somewhat lower temperatures for biaxial orientation

of 100,000150,000Mn PLA polymer with 1020% meso-

lactide content (6572 and 20 C for preheat and cooling

rolls, respectively, for MD stretching; 6370 C and circu-

lated ambient air cooling for TD drawing) [79]. Ou and

Cakmak prepared biaxially oriented PLA films by stretching

cast PLA in both MD and TD to different ratios, followed byannealing these films at elevated temperatures to induce

crystallinity and dimensional stability [62]. Their wide

angle X-ray (WAXS) results showed that the development

of crystalline order and orientation were dependent on

the mode of orientation. They observed that simultaneous

biaxial stretching of PLA film resulted in poor crystalline

order, while sequential stretching promoted a greater crys-

talline order[62]. Hence, the properties of PLA films are

expected to change depending on the stretching sequence

used during the orientation process.

PLA has excellent optical properties and high modulus.

However, it has low elongation, tear and burst strengths.

To overcome these shortcomings, PLA is often coextruded

with other polymers to form multilayer structures to

enhance its properties. For instance, to reduce electrostatic

buildup, Rosenbaum et al. disclosed methods for forming

biaxially oriented multilayer films made of one PLA-based

layer and two outer layers consist of PLA and glycerol fatty

acidesters to achievefilms withantistatic surfaces[98]. The

extruder temperatures usedranged from170 to 200 C with

the take off roll set at 60 C. The biaxial orientation took

place sequentially, first at 68 C in the machine direction

by 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 transverse

direction stretching, respectively. To impart dimensional

stability 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. By

preventing the web from sticking to itself or the processing

equipment, the speed of production and product quality

can be improved[99].

PLA films tend to have higher surface energy than

untreated polyolefin films. Gruber et al. reported surface

energy 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 3438dynes/cm,

respectively [100]. Higher surface energy will provide more

satisfactory printing properties without surface treatment.

If higher surface energyis 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 extruded

to form a tube using an annular die. By blowing air through

the die head, the tube is inflated into a thin tubular bub-

ble and cooled. The tube is then flattened in the nip rolls

and taken up by the winder (Fig. 22).The ratio of bubble

diameter to the die diameter is called the blow-up-ratio

(BUR). BUR ratios of 2:14:1 with the die temperature

of 190200 C have been used for extrusion blowing of

PLA films[101,102]. By varying the BUR, screw speed, air

pressure, and winder speed, films of different thicknesses

(10150m) and degree of orientation can be achieved.

Fig. 21. Biaxial oriented extrusion cast film machine.


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Fig. 22. Extrusion blown film line.

PLA has a specific density of about 1.24g/cm3 which is

muchhigher than polyolefins (0.910.96 g/cm3). WhilePLA

may be processed in extruders designed for polyolefins,

if the extruder is already operating at close to maximum

power of the screw drive, theextruder maynot have enough

power to process PLA due to the substantial higher den-

sity for PLA [103]. Compared to polyolefins, PLA has weaker

melt strength, and therefore, the formation of a stable bub-

ble during extrusion blowing is more difficult. As a result,

extrusion blowing of PLAfilm often requires theuse of addi-

tives, such as viscosity enhancers to strengthen its melt

strength. These additives protect the polymer from degra-

dation and/or couple polymer chains to attenuate overall

loss of molecular weight and viscosity of the polymer melt.

Theformulation of couplingagentsis oftenproprietary. One

commercially available coupling agent for PLA is made up

of copolymer of styrene, methyl methacrylate and glycidyl

methacrylate[102].Sodergard et al. disclosed a method to

stabilize PLA and enhance its melt strength by adding an

organic peroxy compound (e.g., tert-butylperoxybenzoate,

dibenzoylperoxide, tert-butylperoxyacetate) during melt

processing, wherein the peroxide is addedin about0.013%

by weight of PLA[101].

Since PLAfilms arequite stiff andhavemuchlower elon-

gation than polyolefins, collapsing of bubble in the nips

rolls tends to produce wrinkles which tend to permanently

remain in the film due to the high dead-fold properties of

PLA. This problem can be overcome by incorporating fillers

into PLA during extrusion. To reduce the adhesion between

films, Hiltunen et al. blended PLA with triacetin plasticizer

(glycerol triacetate), together with various anti-adhesion

agents, such as talc, TiO2and CaCO3. They claimed that the

bursting strengths of the resulting blown films were better

than typical polyethylene and PP films[104].Slip additives

(e.g., oleamide, stearamide, N,N-ethylene bisstearamide,

oleyl palmitamide) have also been added to reduce the

coefficientof friction betweenoverlapping films[102]. Typ-

ically, slip additive of less than 0.51.0% by polymer weight

is used, as excessive amounts will compromise the abil-

ity of print inks, stickers to adhere to the film surface. To

avoid the use of copolymerization techniques, blending, or

plasticizers, Tweed et al. developed a method to obtain PLA

blown films by elevating the viscosity of PLA through suc-

cessive steps in a polymercooling unitor by internalcooling

of the die mandrel using air or liquid fluid to control the

temperatureof the die [102]. Mitsui Chemicals successfully

developed PLA-based films by copolymerization technol-

ogy, and it is commercializing it as one of the LACEA brand

resins[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 into

disposable cups, single-use foodtrays, lids,and blisterpack-

aging.

Fig. 23 shows the typical steps for thermoforming of

PLA container. In this process, PLA sheet is heated to soften

the polymer, forced either pneumatic and/or mechanically

against 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 IR

absorbancefrequency in the IR region.Therefore, the heater

element should be set at the temperature at which the

majority of energy is absorbed by the polymer. For PS, theideal wavelength is 3.23.7m[105].Values for PLA have


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Fig. 23. Main steps for thermoforming process.

not been reported in the literature. In general, the thermo-

forming temperatures for PLA are much lower than other

conventional thermoformed plastics (e.g., PET, PS, and PP)in the range of 80110 C when the sheet enters the mold

[106,107].

Typically, aluminum molds are used for thermoform-

ing PLA containers. Molds, trim tools and ovens designed

for 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 considerably

than PLA during cooling. For a given part thickness, cool-

ingtimesrequired for PLAcontainers in themold tend to be

higherthan PET andPS containers dueto thelower thermal

conductivity and Tgfor PLA polymers. Table 7 compares the

thermal 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 minimal

orientation. Extruded sheet prior to thermoforming is rela-

tively brittle at room temperature. To ensure smooth travel

of the web and to prevent web breakage, a tight radius

should be avoided in the unwind stations and skeleton

rewind stations. A minimum rewind radius of 25cm is

recommended [108]. If PLA sheet needs to be trimmed

before thermoforming, it should be heated to tempera-

tures near 90 C to prevent cracking. Storage conditions

for the sheet stock need to be controlled as well. As a

guide, PLA should not be exposed to temperature above40 C or to RH above 50% as the sheet will block and resist

unwinding due to its low heatdeflection temperature. After

thermoforming, precaution should be taken to store PLA

below 40 C since Mw breakdown can accelerate when it

is exposed to elevated temperature (Fig. 11). A compari-

Table 7

Thermal properties of PLA, PS, and PET[106]

PLA PS PET

Thermal conductivity (104 cal.cm1 s1 C1) 2.9 4.3 5.7

Heat capacity (cal. g1 C1) 0.39 0.54 0.44

Glass transition temperature (C) 55 105 75

Thermal expansion coefficient (106 C1) 70 70 70

son of the mechanical, physical and barrier properties of

thermoformed PLA, PS, and PET containers showed that

PLA containers outperform PET and PS at lower temper-atures[109].Moreover, the use of 4050% PLA regrind did

not 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 medical

implant applications [110112]. Foamingof PLAis generally

carried out by dissolving a blowing agentin thePLA matrix.

The solubility of the blowing agent is then reduced rapidly

by producing thermodynamic instability in the structure

(e.g., temperature increase or pressure decrease), to induce

nucleation of the bubbles.To stabilize the bubbles, the foam

cells are vitrified when the temperature is reduced below

theTgof the polymer[113,114].

Various foaming strategies have been adopted to reduce

PLA density and improve foam mechanical properties. Di

et al. used 1,4-butanediol (BD) and 1,4-butane diisocyanate

(BDI) as chain extenders to increasethe molecularweightof

PLA so that its viscoelastic properties are more optimal for

foaming. They produced modified PLA samples by sequen-

tially adding different ratios of BD and BDI in a Haake melt

mixer operating at 170 C and mixer speed of 60rpm under

a nitrogen atmosphere. Tin(II) 2-ethylhexanoate was added

as a catalyst at 0.05wt% of PLA. They found that the chain-

extender modified PLA produced foams with reduced cell

size, increased cell density and lowered bulk foam density

as compared to the neat PLA foam control[113].Mikos et

al. prepared PLLA membranes with and without sodium

chloride, sodium tartrate, and sodium citrate by solvent-

casting techniques [114]. The PLLA and PLLA/salt composite

membranes were foamed by heating them at 195 C (15 C

higher than Tm) for 90min and then quenched in liquid

nitrogen for 15 min. They were able to produce membranes

with porosity as high as 93% with a desired surface/volume

ratio depending on the salt used. Ajioka et al. disclosed in

a patent the method of manufacturing PLA foams suitable

for use as disposable food trays, cups, thermal insulators,and cushioning materials [115]. Their approach involves


20/33


mixing various proportions of PLLA and PDLA together

with 0.5% talc (w/w) in an extruder at 200 C. An expand-

ing agent, either dichlorodifluoromethane or butane was

charged under pressure into the extruder. The mixture was

cooledto140 Candextrudedthroughaslitdietogivesheet

foam. An alternate method adopted by these inventors

involved mixing and heating azodicarbonamides powder

(a food additive) with PLA resins using an extruder, in

which the azodicarbonamide decomposed, thereby releas-

ing nitrogen gas to induce the formation of bubbles [115].

Another patent described a method for injection molding

of PLA foams by adding 1525 wt% of solvent to PLA dur-

ing extrusion[116]. Solvents reported to be suitable here

were methyl formate, ethyl formate, methyl acetate, propyl

acetate, dioxane and methyl ethyl ketone.

Loose-fill packaging materials provide cushioning, pro-

tection, and stabilization of packaged goods during

shipping. Over the past decade, the use of expanded PS

foams for loose-fill packaging has declined due to the

processing technologies for poly lactic acid

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