POLYLACTIC ACID: SYNTHESIS, PROPERTIES AND APPLICATIONS

POLYLACTIC ACID: SYNTHESIS, PROPERTIES AND APPLICATIONS

VINCENZO PIEMONTE

Nova Science Publishers, Inc. New York

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com

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Published by Nova Science Publishers, Inc. ; New York

CONTENTS

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Preface i Chapter 1 Properties of Plastics and Composites Prepared From Different

Sources/ Grades of PLA Error! Bookmark not defined. Chapter 2 Poly(lactic acid) Polymer Blends A Review of Recent WorksError! Bookmark not defined. Chapter 3 Melt Processing of Poly(Lactic Acid): Morphology, Mechanical

Behavior, Thermal Stability and Kinetics of the Thermal Decomposition Error! Bookmark not defined.

Chapter 4 Improvement of Mechanical Properties and Processability by Addition of Polyesters Error! Bookmark not defined.

Chapter 5 Filler-Matrix Compatibility of Poly(Lactic Acid) Based Composites Error! Bookmark not defined.

Chapter 6 Properties and Biodegradation of PLA and PLA/Nanocomposites Error! Bookmark not defined.

Chapter 7 Modeling Hydrolytic Degradation of PLA DevicesError! Bookmark not defined. Chapter 8 New Perspectives for Application of PLA in Cultural HeritageError! Bookmark not defined. Chapter 9 Flash Co-Pyrolysis of Biomass with Polylactic Acid – Statistical

and Comparative Analytical InvestigationError! Bookmark not defined. Chapter 10 Synthesis of Polylactic Acid for Biomedical, Food Packaging

and Structural Applications: A Review 5 Chapter 11 Engineering Polylactic Acid with Thermal-Responsive Shape

Memory Effect for Biomedical ApplicationsError! Bookmark not defined. Chapter 12 Preparation and Applications of Poly (lactide acid) Microsphere

and Microcapsules for Drug Delivery Error! Bookmark not defined. Chapter 13 Drug–Loaded Microspheres of Poly(D,L-Lactide) and Poly(D,L-

Lactide-Co-Glycolide) Polymers Prepared by Spray-Drying: Characterization and In Vivo AdministrationError! Bookmark not defined.

Chapter 14 Bioengineering Applications of Ultra-Thin Poly(Lactic Acid) Nanofilms Towards Cell-Based Smart BiomaterialsError! Bookmark not defined.

Index Error! Bookmark not defined.

PREFACE In past few years, due to the increase of environmental pollution and global worming, the

development of systems based on sustainable use of materials has accelerated. Particularly interesting is the effective usage of the limited carbon resources and the conservation of the energy resources. In this framework, bio-based polymers, made from biomass, can play an important role.

The reasons of the growing interest in the development of biodegradable plastics are environmental and strategic. As a matter of fact, in order to reduce the environmental impact of plastics (especially in terms of CO2eq released in the environment) some of the products obtained from agriculture (starch, cellulose, wood, sugar, etc.) are used as raw materials. This way the net balance of carbon dioxide is reduced since growing bioplastic feedstocks remove carbon dioxide from the atmosphere. Furthermore, petroleum, with a constantly rising price, is replaced by renewable raw materials obtained from agriculture.

In some countries, the adoption of bioplastics was also stimulated by other environmental reasons, related to the issue of waste disposal: in the 1990s, the municipal solid waste in Italy (with a fraction of plastics equal to 20–30%) was usually disposed in landfills. Traditional plastics undergo degradation phenomena with very slow kinetics; hence, the volume occupied by these materials in landfills is virtually stable over time. On the other hand, bioplastics show higher degradation rates in landfills and, therefore, the required volumes can be reduced.

It is worth highlight that the word ‘‘bioplastic’’ is actually ambiguous, because it can refer simultaneously to two different aspects, which are not necessary connected. In fact, it can refer to the source (renewable) used as raw material for bioplastics or to the biodegradability and/or compostability that foreshadows a specific final destination of the bioplastics. Furthermore, a plastic based on renewable sources (biobased) may not be biodegradable (i.e. polyethylene (PE) obtained from bioethanol) or may be biodegradable but obtained from nonrenewable sources (e.g., polycaprolactone). Finally, there are also bioplastics that are both biodegradable and bio-based such as polylactic acid (PLA), which represents a solution for the need of environmentally-friendly packaging materials (produced from renewable sources) and biodegradable plastics with a low environmental impact.

Although PLA has been produced for several decades, its use was limited to biomedical application (e.g. biocompatible sutures, prostheses, biologically active controlled release devices) because of the high cost. The breakthrough occurred in the early 1990s when Cargill

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Inc. succeeded in polymerizing high molecular weight PLA using a commercially viable lactide opening reaction. Since then, the scientific interest on the PLA has grown exponentially over the years, together with the importance that it has acquired on the market. Nowadays, thanks to its unique features in terms of versatility and reliability, PLA is the most important and most used bioplastic in the world.

This book will appeal to all the readers who not only want to have a reference book of consolidated notions on PLA, but also, and especially, to those who want to discover new potentials and new application fields of this unique biodegradable polymer.

The book describes the synthesis, properties and applications of PLA through 14 original and very interesting chapters that will guide the reader through a fascinating journey into the world of the PLA, providing interesting insights for those who intend to use this polymer for innovative applications, or simply those who want learn more about this very important biodegradable and bio-based plastic.

The book starts with an overview of the different types of PLA which are now available on the market and mechanical characteristics and properties, strengths and weaknesses of PLA are reported. Then the book continues with chapter 2, where the reader can find an useful review of the different production techniques of PLA-based copolymers, at the aim of extending the already broad range of applications of this biopolymer. Chapters 3 and 4 are intended to give a better insight of the PLA processability, through the analysis of mechanical behaviour, thermal stability and kinetics of thermal decomposition of PLA. Some possible techniques for improving the mechanical properties and processability of PLA by adding polyesters are also presented and discussed.

The creation of composite materials that can meet the most demanding users' expectations is certainly a very important field and therefore, in this book some chapters will be devoted to this subject. Through chapters 5 and 6 the reader will know the great potentials of the PLA in the construction of composites and nanocomposites, appreciating the great versatility of this polymer.

At the beginning of this preface it has been mentioned that the PLA is not only a bio-based plastic: its biodegradability introduces this polymer in a world of eco-friendly and human-friendly applications in several technological fields. Therefore, chapter 7 is dedicated to deepen the PLA degradation, delving into the hydrolysis mechanisms which are the basis of the PLA biodegradability.

After chapter 7, the reader is catapulted into the fascinating world of PLA applications, ranging from environment to the biomedical fields, from pharmaceutical to food industry, to go up to the conservation of cultural heritage.

Particularly, in chapter 8 a comprehensive survey of conservation materials and the most relevant achievements related to the application of PLA in conservation of cultural heritage are reported and discussed, while chapter 9 deals with the end of life of PLA by analysing the flash co-pyrolysis of biomass with polylactic acid through a statistical and comparative analytical investigation.

Chapters from 11 to 14 are dedicated to PLA applications in the pharmaceutical and biomedical fields, introduced by a review reported in chapter 10 on interesting applications of PLA in biomedical, food packaging and structural fields. Then, chapter 11 reviews recent progress in engineering PLA-based thermal-responsive shape memory polymers and discusses their potential and limitations for biomedical applications. Chapters 12 and 13 are

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devoted to the investigation of the production of PLA microcapsules for drug delivery with different techniques.

The fascinating journey on the PLA world concludes with chapter 14, which describes the preparation methods to obtain PLA structures with a nanometric thickness and a large surface area. The main features of such devices and the possibilities of functionalizing them by the inclusion of nanoparticles with targeted physical properties in the polymeric matrix are presented.

Finally, let me conclude this preface by thanking all the authors who have contributed to the realization of this book, without whom this project would have never been born. I thank them for their participation and patience during the preparation of this book and I am grateful that they have entrusted me to edit their contributions. I hope the readers will find this book useful. I am looking forward to receive comments and constructive feedbacks about contents.

Tom Cornelissen, Geert Molenberghs, Mark Stals, et al.

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Chapter 10

SYNTHESIS OF POLYLACTIC ACID FOR BIOMEDICAL, FOOD PACKAGING AND STRUCTURAL

APPLICATIONS: A REVIEW

Mohammad S. Islam Institute of Polymer and Composites, The University of Minho, Portugal

ABSTRACT

Interest in bio-polymers from natural resources such as plants, stems largely from increasing regulation and public concern on ecological issues and the need for sustainable alternatives to finite petrochemical products. Performance of bio-polymers can be obtained as good as synthetic polymers but with several environmental advantages: they create almost no toxic emissions; their raw materials are sustainable, renewable and widely grown. Over the last decade polylactic acid (PLA) has been one of the most promising biopolymers and better manufacturing practices have improved the economics of producing lactic acid monomer through fermentation of renewable resources like corn, sugar beet and sugarcane. In addition to being thermoplastic, biodegradable, biocompatible and compostable, PLA shows good mechanical and barrier behavior and it has the ability to reinforce fillers. This chapter gives a review of available literatures on the production sources of PLA, provides information on its synthesis routes and deals its interesting applications in biomedical, food packaging and structural fields.

1. INTRODUCTION Management of solid waste consists of polymeric materials from petrochemical resources

such as thermoplastics (e.g. products of polyethylene terephthalate, polypropylene and polyethylene) and thermosets (e.g. products of synthetic rubbers, phenolic and epoxy resins) is an important worldwide problem. There are high capital and operating costs as well as environmental pollution problems associated with incineration. Used dumped polymeric materials such as plastic bottles and containers, tyres and electric switches have slow or nominal degradation in the landfill. The landfills are limited and dumped polymeric materials

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contain toxic materials such as lead and cadmium which were used as colorants, stabilizers and plasticizers during their production. These toxic materials may leach to the soil and impose serious environmental risk [1]. Moreover, petrochemical resources are progressively diminishing and the oil prices are radically increasing. All of these issues add with increasing consumer interest in sustainable green products and have sparked research and development in favor of biodegradable polymeric materials from renewable resources. Polylactic acid or polylactide (PLA) belongs to the family of aliphatic polyesters is one of the most widely used biodegradable materials [2-6] manufactured from lactic acid that in turn is produced from renewable resources such as corn, sugarcane and rice [7]. Besides being biodegradable and having produced from renewable resources, PLA provides numerous other benefits including: fixation of carbon dioxide, a greenhouse gas [8]; significant amount of energy savings [9]; improvement of farm economies [10] and manipulation of physico-mechanical properties using polymer architecture such as orientation, blending, branching, cross-linking, or plasticization [11-15]. Figure 1 shows the novel ecologically friendly life cycle of PLA.

Figure 1. Novel ecologically friendly life cycle of PLA.

Although production processes for lactic acid and PLA are well known [16-21], very few processes have been commercialized [18] and still the cost of PLA is not very competitive with synthetic plastics [19, 22]. The core of PLA production technology is the fermentative production of lactic acid and its recovery [19, 22]. Many processes are reported in the literature for the recovery of lactic acid [23-26] and still offer an extensive scope for research and development. In addition to being biodegradable, PLA has many favorable properties such as good biocompatibility and bioresorbability [27-30], barrier properties [10, 18], mechanical strength [31, 32] and the ability to reinforce fillers [33, 34]. All of these properties make it a polymer of choice for biomedical (e.g. implants, sutures, drug encapsulation), food packaging (e.g. food wrap, container, drinking cups) and structural applications (e.g. civil structures, furniture, marine, automobiles). This chapter deals with production sources (i.e.

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raw materials), synthesis routes and its applications in biomedical, food packaging and structural fields.

2. POLYLACTIDE: SOURCES AND SYNTHESIS ROUTES OF PRODUCTION

Currently, PLA is synthesized by a two-step process consisting of (i) lactic acid

production by chemical synthesis from petroleum resources or mainly fermentation of environment friendly renewable resources or (ii) polymerization of lactic acid by direct condensation or solvent-based azeotropic dehydrative condensation and ring opening polymerization of lactide, a cyclic dimer resulting from the dehydration of lactic acid.

2.1. Sources of Production: Lactic Acid as a Precursor The industrial scale production of lactic acid (α-hydroxypropionic acid), started at the

end of nineteenth century to be used mainly in food industry as an acidity regulator although it had uses in pharmaceutical, cosmetic, leather and textile industries [18, 35]. Recently, there has been a renewed interest in lactic acid production because it is the monomeric precursor of PLA [36]. It is generally obtained either by chemical synthesis from petroleum feed stocks (could produce racemic DL-lactic acid only) or by renewable carbohydrate fermentation (could produce an optically pure L(+) or D (-) lactic acid stereoisomer) [37]. The physical properties of PLA depend on this isomeric composition [38, 39]. Therefore, for PLA production fermentative pathway is desirable not only as a renewable resource, but also to obtain desired physical properties.

Over the years several authors have studied a large number of carbohydrates for production of lactic acid [40-44]. Their investigations were on the basis of the increase of the yield of lactic acid, optimization of the production of biomass, minimization of the formation of by product, enhancement of the rate of fermentation, reduction of any pre-treatments involved, ease of availability and reduction of costs involved [45].

In the USA, corn is the cheapest starch-rich and most widely available annually renewable resource from which lactic acid is produced. In other parts of the world, locally available crops such as rice, cottonseed hull, grapefruit, sugar beets, sugarcane, wheat and sweet potatoes can be used as a starch/sugar feedstock [7, 45]. Conversion technologies to facilitate the use of lignocellulosic biomass feedstocks, such as corn stover (stover refers to stalks, leaves and cobs that remain in corn fields after the grain harvest to revitalize the soil), grasses, wheat and rice straws, and bagasse (the pulpy residue of sugarcane) have also been investigated [7]. Effort has been given to develop microbial processes for the production of high purity L (+) lactic acid at low cost from sago starch which is in abundance in Sarawak, Malaysia and Indonesia [45]. Lactic acid has also been produced by simultaneous saccharification and fermentation of pre-treated alpha fibre [45].

Although for the renewable sources to be converted to lactic acid, most investigations have paid attention to the carbohydrates, the finicky nature of lactic acid bacteria is still the main hurdle to the economical feasibility of the fermentation process. The fermentation

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processes to obtain lactic acid can be classified into two types namely, heterofermentative and homofermentative depending on the type of bacteria used. In the heterofermentative method less than 1.8 moles of lactic acid per mole of hexose is produced along with significant levels of other metabolites such as acetic acid, ethanol, glycerol, mannitol and carbon dioxide. In the homofermentative method an average of 1.8 moles of lactic acid per mole of hexose and minor levels of other metabolites are produced [10]. Most lactic acid bacteria require a wide range of growth factors including amino acids, vitamins, fatty acids, purines, and pyrimidines for their growth and biological activity [46, 47] and the more supplemented the medium, the higher the productivity of the lactic acid could be obtained. Among the various complex nitrogen sources for lactic acid fermentation (e.g. yeast extract, malt sprouts, peptones, grass extract, N-Z-case plus, corn steep liquor, N-Z-amine YT, casein hydrolysate, distiller’s waste, ammonium phosphates, beef extract, urea, whey permeate and soybean hydrolysate) [45, 48, 49] that have been studied thus far, yeast extract is the best choice for both microbial growth and lactic acid production [50-53]. However, for the production of lactic acid as a source for commodity chemicals, yeast extract is not cost effective [54, 55]. Therefore, efforts have been given to use other sources of growth factors from industrial byproducts to achieve a partial or total replacement of yeast extract [56-59].

2.2. Synthesis Routes of Production Synthesis of PLA can be obtained generally by two routes, namely (i) direct condensation

of lactic acid [16, 60] and (ii) ring-opening polymerization (ROP) of lactide (a cyclic dimmer of lactic acid) [39, 61] as shown in Figure 2. In ROP route, either D-lactic acid, L-lactic acid, or a mixture of the two are prepolymerized to obtain an intermediate low molecular weight poly(lactic acid), which is then depolymerized into a mixture of lactide stereoisomers. Lactide is generally formed by the condensation of two lactic acid molecules as follows: L-lactide from two L-lactic acid molecules, D-lactide from two D-lactic acid molecules, and meso-lactide from one L-lactic acid and one D-lactic acid molecule. After purification the lactides are polymerized into high molecular weight PLA with a constitutional unit of [OCH(CH3)CO]– [20, 62, 63]. Melting point, mechanical strength and crystallinity of PLA are determined by different proportions of L, D, or meso-lactide and the molecular weight are determined by the proper addition of hydroxylic compounds. PLA can be produced so that it is totally amorphous or up to 40% crystalline by the proportion of D and L-lactides. PLA containing more than 93% L-lactic acid are semicrystalline while PLA with 50–93% L-lactic acid are strictly amorphous [63]. The presence of both meso- and D-lactide forms in the PLA introduces sufficient irregularity to limit crystallinity. Moreover, the presence of meso-lactide in the PLA structure produces a depression of the melting temperature (Tm) as follows: Tm (oC) ≈ 175oC–300Wm, where Wm is the fraction of meso-lactide and 175oC is the melting temperature of PLA made of 100% L-lactide [63]. The glass transition temperature (Tg) of PLA is also determined by the proportional presence of different lactides in the PLA. This results in PLA polymers with a wide range of hardness and stiffness values. Generally, the Tg of PLA ranges from 50 to 80oC while the Tm ranges from 130 to 180oC [64]. Since direct condensation route is an equilibrium reaction, the achievable molecular weight using this route may become limited due to the difficulties of removing trace amounts of solvent such as water at the end of the polymerization process due to hydrolysis of ester bonds [19].

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Thus, although direct condensation is the least expensive route [18], other approaches such as azeotropic dehydration to remove most of the condensation water in the direct esterification process have been assessed [18, 65]. However, most work has been focused so far on the ring opening polymerization of lactide for the production of pure high molecular weight PLA [18, 19].

Figure 2. Synthesis routes of PLA [20].

2.2.1. Synthesis by Direct Condensation of Lactic Acid

PLA produced by direct condensation of lactic acid is a brittle polymer and generally unusable [18]. This polymerization technique has some other drawbacks including need for large reactor, evaporation, solvent recovery and increased racemization [66]. Although PLA can be synthesized with molecular weight of over 1 × 105 by direct condensation of lactic acid [30, 60]. The use of chain coupling agent or esterification promoting adjuvents to increase the molecular weight of PLA adds cost and complexity to the process [20, 67-70]. The function of chain coupling agent is to react with either the hydroxyl (OH) or the carboxyl (COOH) end-groups of the PLA [10, 20, 71]. The advantages of esterification promoting adjuvents are that the final product is highly purified and free from residual catalysts and oligomers [18]. To produce high molecular weight PLA without the use of chain coupling agents or adjuvents and the drawbacks associated with them, azeotropic dehydration or azeotropic condensation polymerization method is employed by Mitsui Chemicals, Japan [18, 72]. In this method, the monomer lactic acid and a catalyst are dehydrated azeotropically in a high boiling, refluxing and aprotic solvent under reduced pressure to obtain high molecular weight PLA (Mw ≥ 300,000) [20, 73]. This polymerization method gives considerable amount of catalyst residue which may cause degradation and hydrolysis during processing. For biomedical applications, this residual catalyst is deactivated or precipitated and filtered out [18]. The molecular weight of PLA is of critical importance for the type of biomedical

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application. Generally, the construction of screws and plates for use as orthopedic implants needs material with high Young’s modulus with molecular weight in the area of several hundreds of thousand [74]. On the other hand, mechanical strength is not of particular importance for the preparation of slabs for use in controlled release but requires proper biodegradation characteristics and low melting point for ease of the drug incorporation with molecular weights in the area of 2–3 × 103 [75]. Henton et al. reported that high molecular weight PLA is naturally resistant to supporting bacterial and fungal growth, which allows its safe use in food packaging and sanitation [19].

2.2.2. Ring Opening Polymerization of Lactide

Ring-opening polymerization (ROP) of lactide is the established and preferred route for the large scale production of high molecular weight PLA (Mw ≥ 100,000) especially in various commercial processes [10, 21, 71, 76]. For ROP of lactide, many catalyst systems have been assessed to increase the reaction productivity including metallic, organometallic, inorganic and organic complexes of aluminum, zinc, tin, and lanthanides [19, 20, 70, 77, 78]. Even strong bases such as metal alkoxides have also been used with success to some extent [19]. Depending on the catalyst system and reaction conditions, many mechanisms such as cationic [79], anionic [80] and coordination [81, 82] have been proposed to explain the kinetics, side reactions, and nature of the end groups observed in ROP of lactide [19, 83]. Among the studied catalyst systems, tin compounds, especially tin (II) bis-2-ethylhexanoic acid (stannous octoate, Sn(Oct)2) which is approved by the US Food and Drug Administration [84], are preferred for the bulk polymerization of lactide due to their solubility in molten lactide, high catalytic activity, low toxicity and ability to give high molecular weight with low rate of racemization of the polymer [19, 85]. The mechanism of ROP with Sn(Oct)2 has been studied by many authors [86-88] and a comprehensive discussion of all mechanism is available in [83]. For the polymers to be used for medical applications even traces of the catalyst that remain in the system must be removed [21].

In addition to producing PLA by two step process as discussed above, many researchers have also attempted to improve the material properties of PLA by copolymerization or blending with other polymers [89-92]. Despite these efforts, existing chemical methods are not effective considering the availability of lactonized monomers used in copolymerization and their cost. Yang et al. [93] have reported for the first time a one-step fermentative production process of PLA and its copolymers containing various lactate fractions from renewable resources. They have described that PLA and its copolymers containing various lactate fractions can be produced in recombinant Escherichia coli (E. coli) by establishing heterologous pathways employing evolved Clostridium propionicum propionate CoA transferase (PctCp) and Pseudomonas sp. MBEL 6-19 polyhydroxyalkanoate (PHA) synthase 1 (PhaC1Ps6-19). As E. coli cannot supply (D)-lactyl-CoA, which is a monomer required for the synthesis of PLA, evolved PctCp was introduced to generate (D)-lactyl-CoA more efficiently. PHA synthase was also evolved to accept (D)-lactyl-CoA more preferentially as a substrate by rational design and saturation mutagenesis. In the second consecutive study of Yang et al. [94], the metabolic pathways of E. coli were further engineered by knocking out the ackA, ppc, and adhE genes and by replacing the promoters of the ldhA and acs genes with the trc promoter based on in silico genome-scale metabolic flux analysis in addition to rational approach. Using this engineered strain, they have showed that from glucose, PLA homopolymer could be produced up to 11 wt% and P(3HB-co-LA) copolymers containing 70

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mol% lactate could be produced up to 46 wt%. From glucose and 3HB, P(3HB-co-LA) copolymers containing 55–86 mol% lactate could be produced up to 56 wt% by introducing the Cupriavidus necator b-ketothiolase and acetoacetyl-CoA reductase genes. Thus, the strategy of combined metabolic engineering and enzyme engineering has allowed efficient bio-based one-step production of PLA and its copolymers. They have expected that this strategy should be generally useful for developing other engineered organisms capable of producing various unnatural polymers by direct fermentation from renewable resources.

3. PLA IN BIOMEDICAL APPLICATIONS PLA has high mechanical properties and excellent shaping and molding properties [32].

It is biocompatible (which means it has good blood and tissue compatibility, and if slow degradation takes place, the degradation products are non-toxic) as well as bioresorbable (which means it can be broken down by the body and does not require mechanical removal) in the human body [18, 95]. Therefore, it is considered as one of the most important biodegradable polymers investigated for a wide range of biomedical and pharmaceutical applications such as controlled drug release, resorbable prostheses, biodegradable sutures, medical implants and scaffolds for tissue engineering [96-98].

The molecular weight of the polymer material directly influences the mechanical and sorptive properties of PLA and hence critical depending on the end use [99, 100]. High molecular weight polymer material is necessary for applications such as bone plates or temporary internal fixation of broken or damaged bones [21]. For being bioresorbable, PLA including other biodegradable polymers have been shown to be advantageous over the conventional polymers in case of parenteral controlled release systems [21]. The preparation of slabs for use in controlled release does not require high strength. Therefore, low molecular weight material can be used having the advantage of favorable degradation behavior in the human body. PLA has been used thus far as a drug carrier in the form of microcapsules [101, 102], microspheres [103], pellets [104] and tablets [105-108] for the administration of antimycobacterial drugs, quinolones, anti malarial drugs, antiinflammatory drugs, antitumor agents, hormones and fluoride containing tablets for oral use [21].

PLA can be reinforced with non-resorbable materials such as carbon fibres to achieve carbon fibre/PLA composites of high mechanical properties before the implantation [18]. The long term effects of resorbed materials in the living tissues are not completely known and are concerns yet to be resolved [109].

Materials from PLA and its copolymers have been designed to replace metal and other non-absorbable polymers as therapeutic aids in surgery, including pins [110], plates [111], screws [111, 112], suture anchors [113], and intravascular stents [114, 115]. Other medical applications currently being practiced include dressings for burn victims [116], substrates for skin grafts [110] and dental applications [114].

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4. PLA AS FOOD PACKAGING MATERIALS PLA has a great opportunity to displace less environmentally benign materials in the area

of packaging because it has favourable physical properties such as transparency, high elastic modulus and high melting temperature which often improves function as well as diminish environmental impact [113, 117]. It is also medically proven as a safe material for food as the level of lactic acid that migrates to food from packaging materials is much lower than the amount of lactic acid used in common food ingredients [118]. However, as a packaging material, initially PLA has found its application in high value films, rigid thermoforms, coated papers and food and beverage containers because of its higher cost. PLA may find its packaging applications for a broader array of products [119-122] as modern and emerging production technologies lower its production costs [36, 119, 120, 123]

By providing consumers with extra end-use benefits, such as meeting environmental regulations, PLA has become a growing alternative as a packaging material for demanding markets [10]. PLA can be used as a food packaging polymer for short shelf life products such as fruit and vegetables. PLA also has common packaging applications such as containers, drinking cups, sundae and salad cups, overwrap and blister packages [124, 125]. PLA can be processed in traditional polymer processing operations such as injection molding, blow molding, extrusion, and extrusion coating. As a result, lids, trays, and clamshells used in food handling can be thermoformed from extruded sheet PLA, even yielding products with higher flex-crack resistance in living hinges than those made of polystyrene [113]. The thermoformed PLA containers are also being used in retail markets for fresh fruit and vegetables [126, 127]. Disposable cutlery can also be produced using injection molding of PLA [113].

Films are one of the largest application areas for PLA. The ability to be stress and thermo crystallized and also the ability to modify the crystallization kinetics and physical properties for a wide variety of applications by incorporation of D-or meso-co-monomer, branching, and change of molecular weight makes PLA an extremely versatile commodity polymer. Films are transparent when stress crystallized and have acceptance by customers for food contact. PLA films can be prepared by blown double bubble technology or by cast-tentering. The films obtained from cast-tentering have many properties desirable for consumer food packaging such as very low haze, excellent gloss, and good gas (O2, CO2, and H2O) transmission rates. For twist wrap packaging, PLA films have superior dead fold or twist retention [19].

Thin sheets of many PLA variants possess high gloss, excellent heat sealability and clarity, allowing extruded thin sheets of PLA to replace cellophane and polyethylene terephthalate (PET) in transparent packaging [128]. Tough and puncture resistant bags for yard and/or food wastes form another set of applications in which not only the physical properties but also the biodegradable nature of PLA can be used to advantage. The biodegradable characteristic helps the bags to degrade within 4-6 weeks in a composting environment [128].

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5. STRUCTURAL APPLICATIONS/COMPOSITES PRODUCTION OF PLA The use of traditional composites made of glass, aramid or carbon fibre with synthetic

polymers of petrochemical origin have been used for aerospace, defense, marine, automotive, civil infrastructure and sporting goods for more than two decades for their high strength and stiffness, dimensional stability, and thermal properties [129, 130]. However, finite nature of the synthetic polymers and fibres and due to new imperatives on environmental pollution, the focus of the researchers is shifted to bio-composites based on natural resources with the application areas remaining the same [131, 132]. Bio-composites made of biopolymers (e.g. PLA, cellulose esters, polyhydoxyalkanoates and starch) and natural fibres (such as flax, hemp, kenaf, jute or cotton) are generally biodegradable and are important research and development achievements [133].

LA is one of the most important types of biopolymer (produced with a capacity of over 140,000 tons per year) which can be used in the biocomposites production because of its biodegradability and maintained mechanical properties without rapid hydrolysis during use. PLA is expensive in comparison to conventional thermoplastics and they are sometimes too weak for some applications especially the inherent brittleness of PLA has been the main obstacle to expanding its commercial use. To reduce the cost and also to improve the properties of the material PLA without interfering its ‘‘green polymer” image, many researchers [33, 134, 135] have investigated natural fibre reinforced PLA bio-composites for structural applications.

Graupner et al. [136] described the production and the mechanical characteristics of composites (which maybe used for different components in automobiles as per equipment) of different types of natural fibres like cotton, hemp, kenaf and man-made cellulose fibres like lyocell with PLA. They reported that kenaf and hemp fibre bundles, as well as their mixtures, could significantly increase the tensile strength and Young’s modulus of composites, but lower the impact strength of the pure matrix significantly. One the other hand, they reported that lyocell/PLA composites showed better tensile and impact characteristics and hence, could be used for interior parts of automobiles or safety helmets at the compromise of price. Bodros et al. [137] reported the possibility of substituting a frequently used polyester/glass fibre laminate by a PLA/flax fibre biocomposite for applications where the composite structure undergoes mechanical stress by tensile loading. Over the years, mechanical performance of natural fibre-reinforced PLA have been investigated with different types of fibres like kenaf, jute, flax abaca, hemp and cordenka rayon fibres with various types of short and long fibres, unprocessed and processed fibres, untreated and treated fibres, random and aligned fibres and non-woven and woven fibres for structural applications [33, 133-135, 138-140]. Other natural fillers such as rice straw fibres, wood flour, sugar beet pulp, rye husk and wheat husk have also been investigated with PLA for improved mechanical performance in order to be used in structural applications [34, 141-143].

6. CONCLUSION PLA is a widely used material for medical application for decades. Recently, it has

gained a huge credibility for food packaging application and a more recent progression

Mohammad S. Islam

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towards green products has led to its development as a replacement of synthetic polymers in the production of biocomposites for structural applications. However, there still are certain features which limit the commercialization of PLA such as thermal instability, moisture permeation, processing conditions and inefficient technology.

The greatest obstacle to greater market penetration of PLA is its high production cost. Indeed, reduction of production cost is a necessity for its further widespread applications. However, increasingly higher oil prices and progressive dwindling of fossil resources are enabling PLA to compete directly with the polymers from petroleum resources.

The use of PLA as a cost-effective alternative to commodity petrochemical-based polymers will increase demand for products from renewable resources such as corn and sugar beets, and is an advanced example of sustainable technology. PLA is finding its rapid growth and expansion in the areas including blends of PLA, copolymers, and impact-modified products which are creating opportunities in the invention of unlimited new products. As we move forward, like PLA, increased cost-effective utilization of renewable resources will be one of the strong drivers for sustainable products with emphasis on reduced energy consumption, waste generation, and emission of greenhouse gases.

REFERENCES

[1] G. Kale, T. Kijchavengkul, R. Auras, M. Rubino, S.E. Selke, and S.P. Singh, Macromolecular Bioscience 7(3), 255-277 (2007)

[2] Y. Ohya, T. Nakai, K. Nagahama, T. Ouchi, S. Tanaka, and K. Kato, Journal of Bioactive and Compatible Polymers 21(6), 557-577 (2006)

[3] K. Madhavan Nampoothiri, N.R. Nair, and R.P. John, Bioresource Technology 101(22), 8493 (2010)

[4] A. Yamada, F. Niikura, and K. Ikuta, Journal of Micromechanics and Microengineering 18(2), 25035 (2008)

[5] D.J. Darensbourg and O. Karroonnirun, Macromolecules 43, 8880-8886 (2010) [6] O. Dechy-Cabaret, B. Martin-Vaca, and D. Bourissou, Chem. Rev. 104, 6147-6176

(2004) [7] E.T.H. Vinka, K.R. Ra´bago, D.A. Glassnerb, and P.R. Gruberb, Polymer Degradation

and Stability 80, 403-419 (2003) [8] J.R. Dorgan, H.J. Lehermeier, L.-I. Palade, and J. Cicero, Macromol. Symp. 175, 55

(2001) [9] J.-C. Bogaert and P. Coszach, Macromol. Symp. 153, 287 (2000) [10] R. Auras, B. Harte, and S. Selke, Macromolecular Bioscience 4(9), 835-864 (2004) [11] R.G. Sinclair, Pure Appl. Chem. A33, 585 (1996) [12] A. Helminen, H. Korhonen, and J.V. Seppälä, Polymer 42, 3345 (2001) [13] A.O. Helminen, H. Korhonen, and J.V. Seppälä, Macromol. Chem. Phys. 203, 2630

(2002) [14] F. Jin, S.-H. Hyon, H. Iwata, and S. Tsutsumi, Macromol. Rapid. Commun. 23, 909

(2002) [15] K.A. Davis, J.A. Burdick, and K.S. Anseth, Biomaterials 24, 2485 (2003)

Synthesis of Polylactic Acid for Biomedical, Food Packaging, Structural Applications

15

[16] E.M. Filachione and C.H. Fisher, Industrial and Engineering Chemistry Research 36, 223-228 (1944)

[17] F. Achmad, K. Yamane, S. Quan, and T. Kokugan, Chemical Engineering Journal 151, 342-350 (2009)

[18] L. Avérous, Polylactic Acid: Synthesis, Properties and Applications, in Monomers, Polymers and Composites from Renewable Resources, L. Avérous, B. Mohamed Naceur, and G. Alessandro. Editor, Elsevier, Amsterdam (2008) p. 433.

[19] D.E. Henton, P. Gruber, J. Lunt, and J. Randall, Polylactic acid technology. 1st ed. Natural Fibers, Biopolymers, and Biocomposites, ed. A.K. Mohanty, M. Misra, and L.T. Drzal. CRC Press, New York (2005) Vol.527-577.

[20] H. Hartmann, High molecular weight polylactic acid polymers. 1st ed. Biopolymers from Renewable Resources, ed. D.L. Kaplan. Springer-Verlag, Berlin (1998) Vol.367-411.

[21] C.S. Proikakis, T.P. A., and A.G. Andreopoulos, Journal of Elastomers and Plastics 34, 49-63 (2002)

[22] A.N. Vaidya, R.A. Pandey, S. Mudliar, M. Suresh Kumar, T. Chakrabarti, and S. Devotta, Critical Reviews in Environmental Science and Technology 35(5), 429-467 (2005)

[23] K.L. Wasewar, A.A. Yawalkar, J.A. Moulijn, and V.G. Pangarkar, Industrial & Engineering Chemistry Research 43(19), 5969 (2004)

[24] M. Yagihashi and T. Funazukuri, Industrial & Engineering Chemistry Research 49(3), 1247 (2009)

[25] P.P. Barve, I. Rahman, and B.D. Kulkarni, Organic Process Research & Development 13(3), 573 (2009)

[26] A.A. Dietz, E.F. Degering, and H.H. Schopmeyer, Industrial & Engineering Chemistry Research 39(1), 82 (1947)

[27] M. Yin and G.L. Baker, Macromolecules 32(23), 7711-7718 (1999) [28] Y. Wan, H. Wu, A. Yu, and D. Wen, Macromolecules 7, 1362-1372 (2006) [29] C. Gottschalk and H. Frey, Macromolecules 39, 1719-1723 (2006) [30] W.-x. Zhang and Y.-z. Wang, Chinese Journal of Polymer Science 26(4), 425−432

(2008) [31] J. Guan and M.A. Hanna, Ind. Eng. Chem. Res. 44, 3106-3115 (2005) [32] T. Ouchi and Y. Ohya, Journal of Polymer Science: Part A: Polymer Chemistry 42,

453-462 (2004) [33] M.S. Islam, K.L. Pickering, and N.J. Foreman, Composites Part A: Applied Science and

Manufacturing 41(5), 596-603 (2010) [34] D. Kim, Y. Andou, Y. Shirai, and H. Nishida, ACS Appl. Mater. Interfaces 3, 385-391

(2011) [35] T. VickRoy, Lactic acid. ed. Comprehensive Biotechnology, ed. M. Moo-Young.

Pergamon Press, New York (1985) Vol. 3.761-776. [36] R. Datta, S.P. Tsai, P. Bonsignore, and S.H. Moon, FEMS Microbiology Reviews 16,

221-231 (1995) [37] J.-S. Yun, Y.-J. Wee, and H.-W. Ryu, Enzyme and Microbial Technology 33(4), 416

(2003) [38] J.H. Litchfield, Adv Appl Microbiol 42, 45–95 (1996) [39] J. Lunt, Polymer Degradation and Stability 59, 145-152 (1998)

Mohammad S. Islam

16

[40] G.C. Inskeep, G.G. Taylor, and W.C. Breitzke, Ind Eng Chem 44, 1955-1966 (1952) [41] P. Cheng, R.E. Mueller, S. Jaeger, R. Bajpai, and L. E.L., J Ind Microbiol 7, 27-34

(1991) [42] K. Hofvendahl and B. Hahn–Hagerdal, Enzyme Microb Technol 20, 301–307 (1997) [43] S.P. Tsai and S.-H. Moon, Appl Biochem Biotechnol 70, 417–428 (1998) [44] A. Aeschlimann and U. von Stockar, Enzyme Microb Technol 13, 811–816 (1991) [45] N. Narayanan, P.K. Roychoudhury, and A. Srivastava, Electronic Journal of

Biotechnology 7(2), (2004) [46] S.C. Prescott and C.G. Dunn, Industrial Microbiology. 3rd ed., ed. McGraw-Hill, New

York (1959) Vol. [47] A.G. Moat, Biology of the lactic, acetic, and propionic acid bacteria. ed. Biology of

Industrial Microorganisms, ed. A.L. Demain and N.A. Solomon. Butterworths, Boston (1985) Vol.174-180.

[48] M. Hujanen and Y.-Y. Linko, Appl Microbiol Biotechnol 45, 307-313 (1996) [49] S. Kwon, P.C. Lee, E.G. Lee, Y.K. Chang, and N. Chang, Enzyme and Microbial

Technology 26, 209–215 (2000) [50] A. Aeschlimann and U. von Stockar, Appl Microbiol Biotechnol 32, 398-402 (1990) [51] A. Olmos-Dichara, F. Ampe, J.-L. Uribelarrea, A. Pareilleux, and G. Goma, Biotechno

Lett 19, 709 -714 (1997) [52] A. Amrane and Y. Prigent, World J Microbiol Biotechnol 14, 529 -534 (1998) [53] T. Payot, Z. Chemaly, and M. Fick, Enzyme Microb Technol 24, 191–199 (1999) [54] C.N. Mulligan, B.F. Safi, and D. Groleau, Biotechnol Bioeng 38, 1173–1181 (1991) [55] S. Tejayadi and M. Cheryan, Appl Microbiol Biotechnol 43, 242- 248 (1995) [56] L. Chiarini, L. Mara, and D. Tabacchioni, Appl Microbiol Biotechnol 36, 461-464

(1992) [57] A. Demirci, A.L. Pometto, B. Lee, and P.N. Hinz, J Agric Food Chem 46, 4771–4774

(1998) [58] M.B. Leh and M. Charles, J Ind Microbiol 4, 71-75 (1989) [59] B. Lund, B. Norddahl, and B. Ahring, Biotechnol Lett 14, 851- 856 (1992) [60] S.I. Moon, C.W. Lee, I. Taniguchi, M. Miyamoto, and Y. Kimura, Polymer 42, 5059-

5062 (2001) [61] F.J. Van Natta, G.L. Dorough, and W.H. Carothers, Journal of the American Chemical

Society 54, 761-772 (1932) [62] G.B. Kharas, F. Sanchez-Riera, and D.K. Severson, Polymers of Lactic Acid. ed.

Plastics from Microbes, ed. D.P. Mobley. Hanser Publishers, New York (1994) Vol.92–238.

[63] D.R. Witzke, Introduction to Properties, Engineering, and Prospects of Polylactide Polymers, in Chemical Engineering. 1997, Michigan State University: East Lansing, Michigan. p. 32-72.

[64] R.A. Auras, B. Harte, S. Selke, and R. Hernandez, Journal of Plastic Film and Sheeting 19, 123 (2003)

[65] K. Enomoto, M. Ajioka, and A. Yamaguchi, in 5, 310, 865. 1995: U.S. Patent. [66] E.W. Fisher, H.J. Sterzel, and G. Wegner, Kolloid Z. Z. Polym. 251, 980 (1973) [67] R. Dunsing and H.R. Kricheldorf, Polym. Bull. 14, 491 (1985) [68] J.W. Leenslag and A.J. Pennings, Makromol. Chem. 188, 1809 (1987)

Synthesis of Polylactic Acid for Biomedical, Food Packaging, Structural Applications

17

[69] Y. Zhao, Z. Wang, J. Wang, H. Mai, B. Yan, and F. Yang, J. Appl. Polym. Sci. 91, 2143 (2004)

[70] S.H. Hyon, K. Jamshidi, and Y. Ikada, Biomaterials 18, 1503 (1997) [71] A. Sodergard and M. Stolt, Progress in Polymer Science 27, 1123-1163 (2002) [72] M. Ajioka, H. Suizu, C. Higuchi, and T. Kashima, Polym. Degrad. and Stab. 59(137),

(1998) [73] D. Garlotta, J. Polym. Environ. 9(2), 63-84 (2002) [74] M.J. Manninen, J. Mater. Sci.: Mater. Med. 4, 179 (1993) [75] A.G. Andreopoulos, E.C. Hatzi, and M. Doxastakis, J. Mater. Sci.: Mater. Med. 11, 393

(2000) [76] K.M. Stridsberg, M. Ryner, and A.C. Albertsson, Adv. Polym. Sci. 157, 41-65 (2001) [77] H.R. Kricheldorf, JMS Pure Appl. Chem. A30(6 and 7), 441 (1993) [78] H.R. Kricheldorf, Makromol. Chem. 194, 1653 (1993) [79] H.R. Kricheldorf and I. Krieser, Makromol. Chem. 187, 1861 (1986) [80] P. Kurcok, J. Penczek, J. Franek, and Z. Kedlinski, Macromolecules 25, 2285 (1992) [81] A.J. Nijenhuis, Y.J. Du, H.A.M. Van Aert, and C. Bastiaansen, Macromolecules 28,

2124 (1995) [82] H.R. Kricheldorf, I. Kreiser-Saunders, and C. Boettcher, Polymer 36, 1253 (1995) [83] A. Duda and S. Penczek, Biopolymers. ed. Polyesters II - Properties and Chemical

Synthesis, ed. Y. Doi and A. Steinbüchel. Wiley-VCH, Weinheim (2002) Vol. 3b.371-430.

[84] FDA, Indirect Food Additives: Adhesives and Components of Coatings., in Code of Federal Regulations 21. Editor, Department of Health and Human Services, F. D. A, 175.300, (2009) p. 159-176.

[85] H.R. Kricheldorf and M. Sumbel, Eur. Polym. J. 25(6), 585-591 (1989) [86] A. Kowalski, A. Duda, and S. Penczek, Macromolecules 33, 7359-7370 (2000) [87] H.R. Kricheldorf, I. Kreiser-Saunders, and A. Stricker, Macromolecules 33, 702-709

(2000) [88] G. Schwach, J. Coudane, R. Engel, and M. Vert, J Polym Sci Pol Chem 35, 3431-3440

(1997) [89] G.Q. Chen and Q. Wu, Biomaterials 26, 6565-6578 (2005) [90] D. Haynes, N.K. Abayasinghe, G.M. Harrison, K.J. Burg, and D.W. Smith,

Biomacromolecules 8, 1131-1137 (2007) [91] K.M. Schreck and M.A. Hillmyer, J Biotechnol 132, 287-295 (2007) [92] I. Noda, M.M. Satkowski, A.E. Dowrey, and C. Marcott, Macromol Biosci 4, 269-275

(2004) [93] T.H. Yang, T.W. Kim, H.O. Kang, S.-H. Lee, E.J. Lee, S.-C. Lim, S.O. Oh, A.-J. Song,

S.J. Park, and S.Y. Lee, Biotechnology and Bioengineering 105(1), 150-160 (2010) [94] Y.K. Jung, T.Y. Kim, S.J. Park, and S.Y. Lee, Biotechnol. Bioeng. 105, 161–171 (2010) [95] H.R. Kricheldorf, Chem. Rev. 109, 5579-5594 (2009) [96] H. Qian, B. J., and S. Wang, Polym. Degrad. Stab. 68, 423-429 (2000) [97] K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, and K.M. Shakesheff, Chem. Rev. 99,

3181-3198 (1999) [98] E. Chiellini and R. Solaro, Adv. Mater. 8, 305-313 (1996) [99] T.G. Park, J. Controlled Release 30(4), 161 (1994)

Mohammad S. Islam

18

[100] T. Nakamura, S. Hitomi, S. Watanabe, Y. Shimizu, K. Jamshidi, S.H. Hyon, and Y. Ikada, J. Biomed. Mater. Research 23, 1115 (1989)

[101] A. Kader and R. Jalil, Drug Develop. Industr. Pharm. 25(2), 141 (1999) [102] W.P. Yu, J.P. Wong, and T.M.S. Chang, Biotechnology 28(1), 39 (2000) [103] I. Soriano, M. Llabres, and C. Evora, Internat. J. Pharmaceutics 125(2), 223 (2000) [104] A. Kader and R. Jalil, Drug Develop. Industr. Pharm. 24(6), 535 (1998) [105] A.G. Andreopoulos, E.C. Hatzi, and M. Doxastakis, J. Mater. Sci.: Mater. in Medic. 12,

233 (2001) [106] R. Steendam and C.F. Lerk, Internat. J. Pharmaceutics 175, 33 (1998) [107] J. Jain, N.H. Shah, A.W. Malick, and C.T. Rhodes, Drug Develop. and Industr. Pharm.

24(8), 703 (1998) [108] M. Miyajima, A. Koshika, J. Okada, A. Kusai, and M. Ikeda, J. Controlled Release 56,

85 (1998) [109] S. Ramakrishna, J. Mayer, E. Wintermantel, and K.W. Leong, Composites Science and

Technology 61, 1189-1224 (2001) [110] S. Vainionpää, P. Rokkanen, and P. Törmala, Progress in Polymer Science 14, 679-708

(1989) [111] J. Eitenmüller, A. David, A. Pommer, and G. Muhr, Der Chirug 67, 413-418 (1996) [112] J.T. Viljanen, H.K. Pihlajamäki, P.O. Törmälä, and P.U. Rokkanen, Journal of

Orthopaedic Research 15, 398-407 (1997) [113] J.R. Dorgan, B. Braun, J.R. Wegner, and D.M. Knauss, Poly(lactic acids): A Brief

Review. ed. Degradable Polymers and Materials, ed. K.C. Khemani and C. Scholz. ACS Symposium Series, New York (2006) Vol. 939.102-125.

[114] J.C. Middleton and A.J. Tripton, in Medical Plastics and Biomaterials Magazine. 1998. [115] A. Pétas, M. Talja, T.L.J. Tammela, K. Taari, T. Valimaa, and P. Törmälä, British

Journal of Urology 80, 439-443 (1997) [116] H.R. Kircheldorf, I. Kreiser-Saunders, C. Jurgens, and D. Wolter, Macromolecular

Symposium 103, 85-102 (1996) [117] M. Ajioka, K. Enomoto, K. Suzuki, and A. Yamaguchi, Journal of Polymers and the

Environment 3(4), 225-234 (1995) [118] R.E. Conn, J.J. Kolstad, J.F. Borzelleca, D.S. Dixler, L.J.J. Filer, B.N. LaDu, and M.W.

Pariza, Fd. Chem. Toxic 33, 273 (1995) [119] P.R. Gruber and M. O’Brien, Polylactides. NatureWorksTM PLA. 1st ed. Biopolymers.

Polyesters III. Applications and Commercial Products, ed. Y. Doi and A. Steinbüchel. Wiley-VCH Verlag GmbH, Weinheim (2002) Vol.235–250.

[120] N. Kawahima, S. Ogawa, S. Obuchi, M. Matsuo, and T. Yagi, Polylactic acid ‘‘LACEA". 1st ed. Biopolymers. Polyesters III. Applications and Commercial Products, ed. Y. Doi and A. Steinbüchel. Wiley-VCH Verlag GmbH, Weinheim (2002) Vol.251–274.

[121] B. Eling, S. Gogolewski, and A.J. Pennings, Polymer 23, 1587 (1982) [122] H. Tsuji, R. Smith, W. Bonfield, and Y. Ikada, J. Appl. Polym. Sci. 75, 629 (2000) [123] R.E. Drumright, P.R. Gruber, and D.E. Henton, 12, 1841 (2000) [124] J. Baillie, Packag. Week 13, (1997) [125] N. Whiteman. "Packexpo". IL November 6. 2002 (Chicago) 37–42 [126] M. O’Brien. Ilip Launches New Fresh Food Packaging Solution. 2003 [cited

2/27/2003]; http://www.cargilldow.com/corporate/release.asp?id¼110].

Synthesis of Polylactic Acid for Biomedical, Food Packaging, Structural Applications

19

[127] NatureWorksTM PLA Makes North American Commercial Debut in Grocery Stores Taking a Fresh Approach to Deli Packaging. 2003 [cited 10/6/2003]; http://www.cargilldow.com/corporate/release.asp].

[128] R.A. Auras, B. Harte, S. Selke, and R.J. Hernandez, Worldpak-IAPRI, East Lansing, MI. 48824-1224. 2002: USA.

[129] A.K. Mohanty, M. Misra, and G. Hinrichsen, Macromolecular Materials and Engineering 1, 276-277 (2000)

[130] F.H. Chowdhury, M.V. Hosur, and S. Jeelani, Mater Sci Eng A 421, 298–306 (2006) [131] K.S. Ahmeda and S. Vijayarangan, J Mater Process Technol 207, 330-335 (2008) [132] S. Mecking, Angew Chem Int Ed 43, 1078–1085 (2004) [133] B. Bax and J. Müssig, Composites Science and Technology 68 (7-8), 1601 (2008) [134] K. Oksman, M. Skrifvars, and J.F. Selin, Compos Sci Technol 63, 1317-1324 (2003) [135] D. Plackett, T.L. Andersen, W.B. Pedersen, and L. Nielsen, Compos Sci Technol 63,

1287–1296 (2003) [136] N. Graupner, A.S. Herrmann, and J. Müssig, Composites Part A: Applied Science and

Manufacturing 40, 810 (2009) [137] E. Bodros, I. Pillin, N. Montrelay, and C. Baley, Composites Science and Technology

67(3-4), 462 (2007) [138] T. Nishino, K. Hirao, M. Kotera, K. Nakamae, and H. Inagaki, Composites Science and

Technology 63(9), 1281 (2003) [139] M. Shibata, K. Ozawa, N. Teramoto, R. Yosomiya, and H. Takeishi, Macromolecular

Materials and Engineering 288(1), 35 (2003) [140] B.-H. Lee, H.-S. Kim, S. Lee, H.-J. Kim, and J.R. Dorgan, Composites Science and

Technology 69, 2573–2579 (2009) [141] L.J. Qin, J.H. Qiu, M.Z. Liu, S.L. Ding, L. Shao, G.H. Zhang, and Y. Zhao, Materials

Science Forum 357, 675-677 (2011) [142] A.A. Mamun, A.K. Bledzki, A. Jaszkiewicz, and J. Volk. "PLA composites reinforced

by wheat and rye husk". ANTEC@NPE. June 22-26, 2009 (Chicago, USA) [143] L. Liu, M.L. Fishman, K.B. Hicks, and C.-K. Liu, J. Agric. Food Chem. 53, 9017-9022

(2005)