POLY(LACTIC ACID)
Wiley Series on Polymer Engineering and Technology
Richard F. Grossman and Domasius Nwabunma, Series Editors
Polyolefin Ble...
POLY(LACTIC ACID)
Synthesis, Structures, Properties, Processing, and
Applications
Edited by
RAFAEL AURAS
LOONG-TAK LIM
S...
Copyright Ó 2010 John Wiley & Sons, Inc. All rights reserved.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Pu...
CONTENTS
PREFACE
xvii
CONTRIBUTORS
xxi
PART I
1
CHEMISTRY AND PRODUCTION OF LACTIC ACID, LACTIDE, AND
POLY(LACTIC A...
vi
CONTENTS
2.2.2 Temperature Dependence of Densities of Lactic Acid
2.2.3 Temperature Dependence of Viscosity of Lactic...
CONTENTS
5
Structure and Properties of Stereocomplex-Type Poly(lactic acid)
vii
59
Masayuki Hirata and Yoshiharu Kimu...
viii
CONTENTS
7.4
7.5
7.6
7.7
Polymer Compatibility
Environmental Stress Cracking
Rational Composite Design
Diffusion a...
CONTENTS
11
Mechanical Properties
ix
141
Gabriele Perego and Gian Domenico Cella
11.1 Introduction
11.2 General Mech...
x
13
CONTENTS
Migration
181
Herlinda Soto-Valdez
13.1 Migration Principles
13.2 Legislation
13.3 Migration and Toxic...
CONTENTS
xi
16.2.3 Elastomers and Rubbers
16.2.4 PMMA/PLA Blends
16.3 PLA/Biodegradable Polymer Blends
16.3.1 Polyanhydr...
xii
CONTENTS
18.4.1 Surface Modification
18.4.2 Compatibilizing Agent
18.5 PLA Nanocomposites
18.6 Processing
18.7 Proper...
CONTENTS
20.7.1 Wet Spinning
20.7.2 Dry Spinning
20.7.3 Factors Affecting Solution Spinning
20.7.4 Dry-Jet Wet Spinning
2...
xiv
CONTENTS
22.3.4 Enzymatic Degradation of LB Film
22.3.5 Application of Selective Enzymatic Degradation
22.4 Enzymati...
CONTENTS
xv
25.2.3 PLLA Degrading Microorganisms
25.3 Poly(L-Lactide) Degrading Enzymes
25.3.1 Proteinase K
25.3.2 PLLA-...
xvi
CONTENTS
28.2.2 Flexible Film
28.2.3 Shrink Labels and Film
28.2.4 Thermoforming
28.2.5 Lamination
28.3 Other Commer...
PREFACE
Poly(lactic acid) (PLA) cannot be considered as a new
polymer. As early as 1845, PLA was synthesized by
Thophile-...
xviii
PREFACE
FIGURE P.1 Number of works published since 1960 based on Scholars Portal Search (accessed on
28 Dec 09) us...
PREFACE
their help and contributions. We are indebted to them for their
participation and patience during the preparation...
CONTRIBUTORS
Steven Abbott, Steven Abbott TCNF Ltd., Ipswich,
England
Hideki Abe, Chemical Analysis Team, RIKEN Institute...
xxii
CONTRIBUTORS
Tadahisa Iwata, Department of Biomaterial Sciences,
Graduate School of Agricultural and Life Sciences,...
CONTRIBUTORS
Naoto Tsutsumi, Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto, Japan
Charles ...
PART I
CHEMISTRY AND PRODUCTION OF LACTIC ACID,
LACTIDE, AND POLY(LACTIC ACID)
1
PRODUCTION AND PURIFICATION OF LACTIC
ACID AND LACTIDE
WIM GROOT, JAN VAN KRIEKEN, OLAV SLIEKERSL,
1.1
AND
INTRODUCTI...
4
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
For decades, ROP has been the preferred route to PLA for
biomed...
LACTIC ACID
TABLE 1.1
Physical Properties of Lactic Acid [14]
Property
Value
CAS number
General: 50-21-5
(S)-Lactic ...
6
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
FIGURE 1.3 During conversion of glucose to pyruvate, chemical e...
LACTIC ACID
Several natural D-lactic acid producing bacterial species
exist; Sporolactobacillus inulinus, Sporolactobacil...
8
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
and pentoses. In all, lignocellulose contains roughly 80%
ferme...
LACTIC ACID
TABLE 1.2
9
Summary of Lactic Acid Purification Methods
Lactic Acid Purification Method
Advantages
Disadva...
10
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
.
separate from or integrated with fermentation. The use
of e...
LACTIDE
described in a patent by Gruter and Pohl in 1914 [72]. Lactic
acid was self-esterified at 120–135 C, and air was d...
12
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
equipment. For the present system, this means that one must
de...
LACTIDE
lactide from the liquid. At different residence times,
the conversion on the tray could be as high as 93% at
210–...
14
PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE
.
.
columns and is performed continuously [4]. Part of the
d...
LACTIDE
FIGURE 1.9 Equilibrium reaction of tin octoate with alcohol
initiator or impurities to form catalytically active ...
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Poly Lactic Acid E-book

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  • 1. POLY(LACTIC ACID)
  • 2. Wiley Series on Polymer Engineering and Technology Richard F. Grossman and Domasius Nwabunma, Series Editors Polyolefin Blends Edited by Domasius Nwabunma and Thein Kyu Polyolefin Composites Edited by Domasius Nwabunma and Thein Kyu Handbook of Vinyl Formulating, Second Edition Edited by Richard F. Grossman Total Quality Process Control for Injection Molding, Second Edition M. Joseph Gordon, Jr. Microcellular Injection Molding Jingyi Xu Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications Edited by Rafael Auras, Loong-Tak Lim, Susan E.M. Selke, and Hideto Tsuji
  • 3. POLY(LACTIC ACID) Synthesis, Structures, Properties, Processing, and Applications Edited by RAFAEL AURAS LOONG-TAK LIM SUSAN E. M. SELKE HIDETO TSUJI
  • 4. Copyright Ó 2010 John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data: Poly(lactic acid) : synthesis, structures, properties, processing, and applications/ edited by Rafael Auras ... [et al.]. p. cm. Includes index. ISBN 978-0-470-29366-9 (cloth) 1. Biodegradable plastics. 2. Lactic acid. 3. Polymers. I. Auras, Rafael. TP1180.B55P65 2010 620.1’92323–dc22 2010006494 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1
  • 5. CONTENTS PREFACE xvii CONTRIBUTORS xxi PART I 1 CHEMISTRY AND PRODUCTION OF LACTIC ACID, LACTIDE, AND POLY(LACTIC ACID) Production and Purification of Lactic Acid and Lactide 1 3 Wim Groot, Jan van Krieken, Olav Sliekersl, and Sicco de Vos 1.1 Introduction 1.2 Lactic Acid 1.2.1 History of Lactic Acid 1.2.2 Physical Properties of Lactic Acid 1.2.3 Chemistry of Lactic Acid 1.2.4 Production of Lactic Acid by Fermentation 1.2.5 Downstream Processing/Purification of Lactic Acid 1.2.6 Quality/Specifications of Lactic Acid 1.3 Lactide 1.3.1 Physical Properties of Lactide 1.3.2 Production of Lactide 1.3.3 Purification of Lactide 1.3.4 Quality and Specifications of Polymer-Grade Lactide 1.3.5 Concluding Remarks on Polymer-Grade Lactide References 2 3 4 4 4 4 5 8 10 10 10 11 13 14 16 16 Chemistry and Thermodynamic Properties of Lactic Acid and Lactide and Solvent Miscibility 19 Zhengyu Jin, Yaoqi Tian, and Jinpeng Wang 2.1 General Properties 2.1.1 Physical and Chemistry Properties of Lactic Acid 2.1.2 Physical and Chemical Properties of Lactide 2.2 Thermodynamic Properties 2.2.1 Vapor Pressures of Lactic Acids at Different Temperatures 19 19 19 19 19 v
  • 6. vi CONTENTS 2.2.2 Temperature Dependence of Densities of Lactic Acid 2.2.3 Temperature Dependence of Viscosity of Lactic Acid 2.2.4 Thermodynamic Properties 2.3 Miscibility Properties of Lactic Acid and Lactide 2.3.1 Miscibility of Lactic Acid with Different Solvents 2.3.2 Miscibility of Lactic Acid with Modifiers in Diluents 2.3.3 Physical and Chemical Equilibrium of Lactic Acid 2.3.4 Miscibility of Lactide with Solvents References 3 20 20 21 21 21 21 22 25 25 Industrial Production of High Molecular Weight Poly(Lactic Acid) 27 Anders S€dergard and Mikael Stolt o 3.1 Introduction 3.2 Lactic Acid Based Polymers by Polycondensation 3.2.1 Direct Condensation 3.2.2 Solid-State Polycondensation 3.2.3 Azeotropic Dehydration 3.3 Lactic Acid Based Polymers by Chain Extension 3.3.1 Chain Extension with Diisocyanates 3.3.2 Chain Extension with Bis-2-oxazoline 3.3.3 Dual Linking Processes 3.3.4 Chain Extension with Bis-epoxies 3.4 Lactic Acid Based Polymers by Ring-Opening Polymerization 3.4.1 Polycondensation Processes 3.4.2 Lactide Manufacturing 3.4.3 Ring-Opening Polymerization References 4 27 28 29 30 31 32 32 33 34 34 34 35 35 36 37 Design and Synthesis of Different Types of Poly(Lactic Acid) 43 Ann-Christine Albertsson, Indra Kumari Varma, Bimlesh Lochab, Anna Finne-Wistrand, and Kamlesh Kumar 4.1 Introduction 4.2 Copolymerization 4.2.1 Synthesis of Copolymers of Lactic Acid: Glycolic Acid 4.2.2 Synthesis of Copolymers of Lactic Acid: Poly(ethylene glycol) 4.2.3 Synthesis of Copolymers of Lactic Acid: d-Valerolactone and Lactic Acid: b-Butyrolactone 4.2.4 Synthesis of Copolymers of Lactic Acid: e-Caprolactone 4.2.5 Synthesis of Copolymers of Lactic Acid: 1,5-Dioxepan-2-one 4.2.6 Synthesis of Copolymers of Lactic Acid: Trimethylene Carbonate 4.2.7 Synthesis of Copolymers of Lactic Acid: Poly(N-isopropylacrylamide) 4.2.8 Synthesis of LA: Alkylthiophene (P3AT) Copolymers 4.2.9 Functional Poly(lactic acid) 4.2.10 Branched Copolymers 4.3 Properties of Copolymers 4.3.1 Degradation of Homo- and Copolymers 4.3.2 Drug Delivery from PLLA Copolymers 4.3.3 Radiation Effects References 43 43 44 44 45 46 46 46 47 47 47 48 53 54 54 55 55
  • 7. CONTENTS 5 Structure and Properties of Stereocomplex-Type Poly(lactic acid) vii 59 Masayuki Hirata and Yoshiharu Kimura 5.1 5.2 5.3 5.4 Introduction Formation of Stereocomplex Crystals Thermal Properties of sc-PLA Crystal Structure of sc-PLA 5.4.1 Unit Cell Parameters and Molecular Conformation of sc-PLA 5.4.2 Density and Heat of Fusion of Sccrystals 5.5 Formation of sb-PLA 5.5.1 ROP Routes to Diblock and Multiblock sb-PLA 5.5.2 SSP Routes to Multiblock sb-PLA 5.6 Applications of sc-PLA References PART II 6 PROPERTIES OF POLY(LACTIC ACID) Chemical Structure of Poly(lactic acid) 59 59 60 60 60 60 61 61 63 64 65 67 69 Xue Jiang, Yan Luo, Xiuzhi Tian, Dan Huang, Narendra Reddy, and Yiqi Yang 6.1 Introduction 6.2 Chain Structure and Configuration 6.2.1 Chain Structure 6.2.2 Configuration 6.2.3 Interlocked Structure, Polymer Blend, and Resistance to Hydrolysis 6.3 Syndiotactic Polymerization and Syndiotacticity 6.4 Conformation 6.5 Amorphous Structure and Thermal Properties 6.5.1 Amorphous and Three-Phase Models 6.5.2 Glass Transition 6.6 Orientation Structure of PLA 6.6.1 Mechanical Orientation by Stretching or Compression 6.6.2 Thermal Orientation (Phase Transition) 6.7 Semicrystalline Structure 6.7.1 General 6.7.2 Three Forms (a, b and c) of the Crystal Structure 6.8 Frustrated Structure 6.9 Molecular Weight 6.10 Summary References 7 69 69 69 70 71 72 74 74 74 75 77 77 77 78 78 78 79 80 80 80 Chemical Compatibility of Poly(lactic acid): A Practical Framework Using Hansen Solubility Parameters 83 Steven Abbott 7.1 A Practical Framework 7.1.1 Thermodynamics Versus Kinetics 7.1.2 Hansen Solubility Parameters 7.2 Solvent Compatibility 7.3 Plasticizers 83 83 83 84 86
  • 8. viii CONTENTS 7.4 7.5 7.6 7.7 Polymer Compatibility Environmental Stress Cracking Rational Composite Design Diffusion and Barrier Properties 7.7.1 Gases 7.7.2 Water 7.8 Pharmacological Transport 7.9 Summary References 8 87 89 89 90 91 91 93 93 94 Optical Properties 97 ˜ Carla M. B. Goncalves, Joao A. P. Coutinho, and Isabel M. Marrucho ¸ 8.1 8.2 8.3 8.4 8.5 Introduction Absorption and Transmission of UV-Vis Radiation Refractive Index Specific Optical Rotation Infrared and Raman Spectroscopy 8.5.1 Infrared Spectroscopy 8.5.2 Raman Spectroscopy 8.6 1H and 13C NMR Spectroscopy References 9 97 97 99 100 100 101 105 108 111 Crystallization and Thermal Properties 113 Luca Fambri and Claudio Migliaresi 9.1 Introduction 9.2 Crystallinity and Crystallization 9.3 Crystallization Regime 9.4 Fibers 9.5 Hydrolytic Degradation References 10 113 114 118 119 121 123 Rheology of Poly(lactic acid) 125 John R. Dorgan 10.1 Introduction 10.2 Fundamental Chain Properties from Dilute Solution Viscometry 10.2.1 Unperturbed Chain Dimensions 10.2.2 Real Chains 10.2.3 Solution Viscometry 10.2.4 Viscometry of PLA 10.3 Processing of PLA: General Considerations 10.4 Melt Rheology: An Overview 10.5 Processing of PLA: Rheological Properties 10.6 Conclusions Appendix 10.A Description of the Software References 125 126 126 126 127 128 130 131 132 137 138 138
  • 9. CONTENTS 11 Mechanical Properties ix 141 Gabriele Perego and Gian Domenico Cella 11.1 Introduction 11.2 General Mechanical Properties and Molecular Weight Effect 11.2.1 Tensile and Flexural Properties 11.2.2 Impact Resistance 11.2.3 Hardness 11.3 Temperature Effect 11.4 Annealing 11.5 Orientation 11.6 Stereoregularity 11.7 Plasticization 11.8 Relaxation and Aging 11.9 Conclusions References 12 141 141 141 143 143 143 144 147 148 149 151 152 152 Permeation, Sorption, and Diffusion in Poly(lactic acid) 155 Eva Almenar and Rafael Auras 12.1 Introduction 12.2 Factors Affecting Permeability, Sorption, and Diffusion in PLA 12.2.1 L-Lactide Unit Content 12.2.2 Plasticizers 12.2.3 Crystallization 12.2.4 Orientation 12.2.5 Free volume 12.2.6 Branching 12.2.7 Environmental Factors 12.3 Permeability, Sorption, and Diffusion of Pure PLA 12.3.1 Gases 12.3.2 Water Vapor 12.3.3 Organics 12.4 Copolymers 12.5 PLA Blends 12.5.1 PLA/Starch Blends 12.5.2 PLA/Poly(e-caprolactone) Blends 12.5.3 PLA/Chitosan Blends 12.5.4 PLA/Poly((R)-3-hydroxybutyrate) Blends 12.6 PLA Laminations 12.7 Coated PLA 12.8 PLA Composites and Fibers 12.8.1 PLA Composites 12.8.2 Fiber-Reinforced PLA 12.9 PLA Nanocomposites 12.10 Future of PLA Membranes References 155 157 157 158 159 160 160 160 161 163 163 165 167 169 169 169 170 171 171 171 171 171 172 172 172 176 176
  • 10. x 13 CONTENTS Migration 181 Herlinda Soto-Valdez 13.1 Migration Principles 13.2 Legislation 13.3 Migration and Toxicological Data of Lactic Acid, Lactide, Dimers and Oligomers 13.3.1 Lactic Acid 13.3.2 Lactide 13.3.3 Oligomers 13.4 EDI of Lactic Acid 13.5 Other Potential Migrants from PLA 13.6 Conclusions References PART III 14 PROCESSING AND CONVERSION OF POLY(LACTIC ACID) Processing of Poly(lactic acid) 181 182 182 182 186 186 187 187 187 188 189 191 Loong-Tak Lim, Kevin Cink, and Tim Vanyo 14.1 Introduction 14.2 Properties of PLA Relevant to Processing 14.3 Modification of PLA Properties by Process Aids and Other Additives 14.4 Drying 14.5 Extrusion 14.6 Injection Molding 14.7 Film and Sheet Casting 14.8 Stretch Blow Molding 14.9 Extrusion Blown Film 14.10 Thermoforming 14.11 Electrospinning 14.12 Conclusion: Prospects of PLA Polymers References 15 191 191 193 194 195 196 201 204 207 208 209 211 213 Poly(lactic acid)/Starch Blends 217 Long Yu, Eustathios Petinakis, Katherine Dean, and Hongshen Liu 15.1 Introduction 15.2 Blending Hydrophobic PLA with Hydrophilic Starch 15.3 Compatibilizers Used for Starch/PLA Blends 15.4 Enhancing Function of Compatibilizer by Controlling Compatibilizer Distribution 15.5 Reactive Blending 15.6 Summary References 16 217 218 219 220 223 225 225 Poly(lactic acid) Blends 227 Sukeewan Detyothin, Ajay Kathuria, Waree Jaruwattanayon, Susan E. M. Selke, and Rafael Auras 16.1 Introduction 16.2 PLA/Nonbiodegradable Polymer Blends 16.2.1 Polyolefins 16.2.2 Vinyl and Vinylidene Polymers and Copolymers 227 227 228 229
  • 11. CONTENTS xi 16.2.3 Elastomers and Rubbers 16.2.4 PMMA/PLA Blends 16.3 PLA/Biodegradable Polymer Blends 16.3.1 Polyanhydrides 16.3.2 Vinyl and Vinylidene Polymers and Copolymers 16.3.3 Aliphatic Polyesters and Copolyesters 16.3.4 Aliphatic-Aromatic Copolyester 16.3.5 Elastomers and Rubbers 16.3.6 Poly(ester amide) 16.3.7 Polyethers and Copolymer 16.3.8 Annually Renewable Biodegradable Materials 16.4 Plasticization of PLA 16.5 Conclusion References 17 238 239 240 240 242 244 255 257 258 258 261 264 266 266 Foaming 273 Laurent M. Matuana 17.1 Introduction 17.2 Plastic Foams 17.3 Foaming Agents 17.3.1 Physical Foaming Agents 17.3.2 Chemical Foaming Agents 17.4 Formation of Cellular Plastics 17.4.1 Dissolution of Blowing Agent in Polymer 17.4.2 Bubble Formation 17.4.3 Bubble Growth and Stabilization 17.5 Plastic Foams Expanded with Physical Foaming Agents 17.5.1 Microcellular Foamed Polymers 17.5.2 Solid State Batch Microcellular Foaming Process 17.5.3 Microcellular Foaming in a Continuous Process 17.6 PLA Foamed with Chemical Foaming Agents 17.6.1 Effect of CFA Content 17.6.2 Effect of Processing Conditions 17.7 Mechanical Properties of PLA Foams 17.7.1 Batch Microcellular Foamed PLA 17.7.2 Microcellular Extrusion of PLA 17.7.3 Microcellular Injection Molding of PLA 17.8 Foaming of PLA/starch Blends References 18 273 273 274 274 274 275 275 275 276 276 276 277 282 286 286 287 288 288 288 288 289 289 Composites 293 Subrata Bandhu Ghosh, Sanchita Bandyopadhyay-Ghosh, and Mohini Sain 18.1 Introduction 18.2 PLA Matrix 18.3 Reinforcements 18.3.1 Natural Fiber Reinforcement 18.3.2 Synthetic Fiber Reinforcement 18.3.3 Organic Filler Reinforcement 18.3.4 Inorganic Filler Reinforcement 18.4 Fiber/Matrix Adhesion 293 293 294 294 295 296 298 298
  • 12. xii CONTENTS 18.4.1 Surface Modification 18.4.2 Compatibilizing Agent 18.5 PLA Nanocomposites 18.6 Processing 18.7 Properties 18.7.1 Mechanical Properties 18.7.2 Thermal Properties 18.7.3 Degradation 18.8 Applications 18.8.1 Biomedical Applications 18.8.2 Packaging Applications 18.8.3 Automotive Applications 18.8.4 Electronic Applications 18.9 Future Developments and Concluding Remarks References 19 298 299 299 300 300 300 303 304 305 305 306 306 307 307 307 Nanocomposites 311 Suprakas Sinha Ray 19.1 Introduction 19.2 PLA Nanocomposites Based on Clay 19.2.1 Structure and Properties of Clay 19.2.2 Preparation and Characterization of PLA/Clay Nanocomposites 19.3 PLA Nanocomposites Based on Carbon Nanotubes 19.4 PLA Nanocomposites Based on Various Other Nanoparticles 19.5 Properties of PLA-Based Nanocomposites 19.6 Biodegradability 19.7 Melt Rheology 19.8 Foam Processing 19.9 Possible Applications and Future Prospects Acknowledgments References 20 311 312 312 312 314 315 316 317 318 319 320 321 321 Spinning of Poly(lactic acid) Fibers 323 Ashwini K. Agrawal 20.1 Defining Fiber And Fiber Spinning 20.2 Melt Spinning Line 20.3 Fluid Dynamics During Spinning 20.3.1 Instabilities During Flow Through Spinneret 20.3.2 Instabilities in the Spinning Zone: Draw Resonance 20.4 Structure Development During Melt Spinning 20.4.1 Spinning at Low Speeds 20.4.2 Spinning at High Speeds 20.5 Post-Spinning Operation 20.5.1 Drawing 20.5.2 Finish Application 20.5.3 Crimping 20.5.4 Heat Setting 20.6 Structure Development During Drawing 20.7 Solution Spinning of PLLA 323 323 326 326 327 328 328 328 330 330 330 330 330 331 333
  • 13. CONTENTS 20.7.1 Wet Spinning 20.7.2 Dry Spinning 20.7.3 Factors Affecting Solution Spinning 20.7.4 Dry-Jet Wet Spinning 20.7.5 Solution Spinning of Stereocomplex Fiber 20.8 Mechanical Properties 20.8.1 Melt-Spun PLLA 20.8.2 Solution-Spun PLLA 20.8.3 Stereocomplex Fiber References PART IV 21 DEGRADATION AND ENVIRONMENTAL ISSUES Hydrolytic Degradation xiii 334 335 335 337 337 338 338 339 341 341 343 345 Hideto Tsuji 21.1 Introduction 21.2 Degradation Mechanism 21.2.1 Molecular Degradation Mechanism 21.2.2 Material Degradation Mechanism 21.2.3 Degradation of Crystalline Residues 21.3 Parameters for Hydrolytic Degradation 21.3.1 Effects of Surrounding Media 21.3.2 Effects of Material Parameters 21.4 Structural and Property Changes During Hydrolytic Degradation 21.4.1 Fractions of Components 21.4.2 Crystallization 21.4.3 Mechanical Properties 21.4.4 Thermal Properties 21.4.5 Surface Properties 21.4.6 Morphology 21.5 Applications of Hydrolytic Degradation 21.5.1 Material Preparation 21.5.2 Recycling of PLA to Its Monomer 21.5.3 Miscellaneous Applications 21.6 Conclusions References 22 345 345 346 355 360 362 362 365 371 371 371 372 372 372 373 373 373 375 376 376 376 Enzymatic Degradation 383 Tadahisa Iwata, Hideki Abe, and Yoshihiro Kikkawa 22.1 Introduction 22.1.1 Definition of Biodegradable Plastics 22.1.2 Enzymatic Degradation 22.2 Enzymatic Degradation of PLA Films 22.2.1 Structure and Substrate Specificity of Proteinase K 22.2.2 Enzymatic Degradability of PLLA Films 22.2.3 Enzymatic Degradability of PLA Stereoisomers and Their Blends 22.2.4 Effects of Surface Properties on Enzymatic Degradability of PLLA Films 22.3 Enzymatic Degradation of Thin Films 22.3.1 Thin Films and Analytical Techniques 22.3.2 Crystalline Morphologies of Thin Films 22.3.3 Enzymatic Adsorption and Degradation Rate of Thin Films 383 383 383 384 385 385 386 388 390 390 391 391
  • 14. xiv CONTENTS 22.3.4 Enzymatic Degradation of LB Film 22.3.5 Application of Selective Enzymatic Degradation 22.4 Enzymatic Degradation of Lamellar Crystals 22.4.1 Enzymatic Degradation of PLLA Single Crystals 22.4.2 Thermal Treatment and Enzymatic Degradation of PLLA Single Crystals 22.4.3 Single Crystals of PLA Stereocomplex 22.5 Future Perspectives References 23 394 394 395 395 396 397 397 398 Thermal Degradation 401 Haruo Nishida 23.1 Introduction 23.2 Kinetic Analysis of Thermal Degradation 23.2.1 Changes in Molecular Weight 23.2.2 Thermogravimetry 23.3 Thermal Degradation Behavior of PLA Based on Molecular Weight Change 23.4 Thermal Degradation Behavior of PLA Based on Weight Loss 23.4.1 Diverse Mechanisms of PLA Pyrolysis 23.4.2 Effects of Polymerization Catalyst Residues 23.4.3 Effects of Chain-End Structures 23.4.4 Thermal Degradation Catalysts 23.4.5 Thermal Degradation Behavior of PLA Stereocomplex: sc-PLA 23.4.6 Control of Racemization 23.4.7 Selective Depolymerization of PLA in Blends 23.5 Conclusions References 24 401 401 401 402 403 403 403 404 406 406 408 409 409 410 410 Photodegradation and Radiation Degradation 413 Wataru Sakai and Naoto Tsutsumi 24.1 Introduction 24.2 Mechanisms of Photodegradation 24.2.1 Photon 24.2.2 Photon Absorption 24.2.3 Photochemical Reaction of Carbonyl Groups 24.3 Mechanism of Radiation Degradation 24.3.1 High Energy Radiation 24.3.2 Basic Mechanism of Radiation Degradation 24.4 Photodegradation of PLA 24.5 Photosensitized Degradation of PLA 24.6 Radiation Effects on PLA 24.7 Modification of PLA by Irradiation References 25 413 413 413 414 415 415 415 415 416 418 419 420 420 Biodegradation 423 Buenaventurada P. Calabia, Yutaka Tokiwa, Charles U. Ugwu, and Seiichi Aiba 25.1 Introduction 25.2 Microbial Degradation 25.2.1 Field Test 25.2.2 ISO and ASTM Standards 423 423 423 424
  • 15. CONTENTS xv 25.2.3 PLLA Degrading Microorganisms 25.3 Poly(L-Lactide) Degrading Enzymes 25.3.1 Proteinase K 25.3.2 PLLA-Degrading Enzyme of Amycolatopsis sp. 25.3.3 Induction of PLLA Degrading Enzymes with Natural Substrates 25.3.4 Other Serine Proteases 25.3.5 Lipase 25.3.6 Enzymatic Degradation in Organic Solvents 25.3.7 Evolution of PLA Degrading Enzymes 25.4 Conclusion and Future Prospects References 26 424 426 426 426 426 426 426 427 428 428 429 Cradle to Gate Environmental Footprint and Life Cycle Assessment of Poly(lactic acid) 431 Amy E. Landis 26.1 Introduction to LCA and Environmental Footprints 26.1.1 Life Cycle Assessment 26.1.2 Uncertainty in LCA 26.2 Life Cycle Considerations for PLA 26.2.1 The Life Cycle of PLA 26.2.2 Energy USE and Global Warming 26.2.3 Environmental Trade-Offs 26.3 Review of Biopolymer LCA Studies 26.4 Improving PLA’s Environmental Footprint 26.4.1 Agricultural Management 26.4.2 Feedstock Choice 26.4.3 Energy Further Reading on LCA References PART V 27 APPLICATIONS Medical Applications 431 431 432 432 432 433 434 434 438 438 439 439 440 440 443 445 Shuko Suzuki and Yoshito Ikada 27.1 Introduction 27.2 Minimal Requirements for Medical Devices 27.2.1 General 27.2.2 PLA as Medical Implants 27.3 Preclinical and Clinical Applications of PLA Devices 27.3.1 Fibers 27.3.2 Meshes 27.3.3 Bone Fixation Devices 27.3.4 Microspheres, Microcapsules, and Thin Coatings 27.4 Conclusions References 28 445 445 445 446 447 447 448 448 453 454 454 Packaging and Other Commercial Applications 457 Shoji Obuchi and Shinji Ogawa 28.1 Introduction 28.2 Applications in Packaging and Containers 28.2.1 Oriented Film 457 457 457
  • 16. xvi CONTENTS 28.2.2 Flexible Film 28.2.3 Shrink Labels and Film 28.2.4 Thermoforming 28.2.5 Lamination 28.3 Other Commercial Applications 28.3.1 Agricultural and Engineering Materials 28.3.2 Electrical Appliances 28.3.3 Automotive Materials 28.4 Conclusions References 29 458 461 462 462 462 462 463 466 467 467 Textile Applications 469 Masatsugu Mochizuki 29.1 Introduction 29.2 Manufacturing, Properties, and Structure of PLA Fibers 29.2.1 PLA Fiber Manufacture 29.2.2 PLA Fibers and Textile Properties 29.2.3 Effects of Structure on Properties 29.2.4 PLA Stereocomplex Fibers 29.3 Key Performance Features of PLA Fibers 29.3.1 Biodegradability and the Biodegradation Mechanism 29.3.2 Moisture Management 29.3.3 Antibacterial/Antifungal Properties 29.3.4 Low Flammability 29.3.5 Weathering Stability 29.4 Potential Applications 29.4.1 Geotextiles 29.4.2 Industrial Fabrics 29.4.3 Filters 29.4.4 Towels and Wipes 29.4.5 Home Furnishings 29.4.6 Clothing and Personal Belongings 29.5 Conclusions References 30 469 469 469 469 470 471 471 471 472 472 473 474 474 474 475 475 475 475 475 475 476 Environmental Applications 477 Akira Hiraishi 30.1 Introduction 30.2 Application to Water and Wastewater Treatment 30.2.1 Application as Sorbents 30.2.2 Application to Nitrogen Removal 30.3 Application to Bioremediation 30.3.1 Significance of PLA Use 30.3.2 Bioremediation of Organohalogen Pollution 30.3.3 Other Applications 30.4 Concluding Remarks and Prospects Acknowledgments References INDEX 477 477 477 479 482 482 482 483 484 484 484 487
  • 17. PREFACE Poly(lactic acid) (PLA) cannot be considered as a new polymer. As early as 1845, PLA was synthesized by Thophile-Jules Pelouze by the condensation of lactic acid e [1]. In 1932, Wallace Hume Carothers et al. developed a method to polymerize lactide to produce PLA that was later patented by DuPont in 1954 [2]. Although PLA existed for several decades, its use was limited to biomedical applications (e.g., biocompatible sutures, implants, biologically active controlled release devices) due to its high cost. The low molecular weight PLA polymers obtained also hampered their wide-ranging applications. The breakthrough occurred in the early 1990s when Cargill Inc. succeeded in polymerizing high molecular weight PLA using a commercially viable lactide ring opening reaction [3, 4]. In 1997, Cargill Dow LLC, a joint venture between Cargill Inc. and The Dow Chemical Company, was formed to begin truly commercially significant production of PLA resins under the trade name NatureWorksÔ. This is a major landmark in PLA’s history because it signifies the beginning of a large-scale use of this bio-based polymer, transforming PLA from a specialty material to a commodity thermoplastic. The increased availability of PLA stimulated an increased in its research and development activities. A survey of the literature revealed that the number of published articles related to PLA increased exponentially over the past decade, which can be also partly attributed to the escalating “green” movement that is stimulating the use of bio-based polymers. To date, the major PLA resin suppliers have been Cargill (in the United States known as IngeoÔ), Mitsui Chemicals, Inc. (in Japan known as LACEAÔ), Purac (The Netherlands), and Teijin Limited (in Japan known as BiofrontÒ ). Other important events that took place pertaining to PLA are summarized in Figure P.1 and Table P.1. While the information available in the literature is massive, at the inception of this volume, no reference book could be found that coherently assembled the scientific and technological knowledge about PLA. Our main motive for editing this book was to consolidate the most relevant information on PLA into a volume that serves as a one-source reference for readers who are keen on this unique biodegradable polymer. Organized in five parts, Part I of this book covers several important topics, including chemistry and production of lactic acid (Chapter 1) and lactide (Chapter 2), which are the essential building blocks of PLA. Different polymerization reactions for the production of PLA are covered in Chapter 3. In view of certain shortcomings of PLA, copolymerization of PLA with other monomers and stereocomplexation with optimal enantiomer lactide ratios are gaining increased popularity as ways to enhance the material properties of the resulting polymer. These topics are covered in detail in Chapters 4 and 5, respectively. These chapters set the stage for discussions in Part II of this book, in which different material properties of PLA are covered in eight separate chapters. Chain configuration, tacticity, and crystal structure are discussed in Chapter 6 to illuminate how chain structure affects the material properties of PLA and its copolymers. Chapter 7 investigates the compatibility of PLAwith solvents and other polymers, an important aspect that should be considered during end-use applications. The interaction of PLA with electromagnetic radiation for probing the molecular structure and interactions are discussed in Chapter 8. The essence of spectroscopy techniques for PLA analysis, including UV–VIS, FTIR, Raman, and NMR, is reviewed in this chapter. Crystallization, thermal, and rheological properties of PLA are discussed in Chapters 9 and 10, and these xvii
  • 18. xviii PREFACE FIGURE P.1 Number of works published since 1960 based on Scholars Portal Search (accessed on 28 Dec 09) using keywords “polylactide”, “poly(lactic acid)”, and “polylactic acid”. TABLE P.1 Significant Events Related to PLA Production that Occurred over the Past Few Decades 2010 2009 2009 2009 2008 2008 2007 2007 2005 2003 1997 1997 1996 1994 Jung et al. employed recombinant Escherichia coli to produce PLA [5] PURAC, Sulzer, and Synbra announced production of PLA from solid lactide for foamed products Galactic and Total Petrochemicals from Belgium created a joint venture, Futerro, to begin PLA production Cargill Inc. acquired full NatureWorks ownership from Teijin Limited Uhde Inventa Fischer and Pyramide Bioplastics announced large-scale production of PLA in Guben, Germany PURAC started to commercialize solid lactide monomers under PURALACTÔ Teijin launched heat-resistant stereocomplex PLA under BiofrontÔ NatureWorks LLC and Teijin Limited formed 50–50 joint venture to market IngeoÔ bio-based thermoplastic resins Cargill Inc. acquired The Dow Chemical Company’s share in Cargill Dow LLC 50–50 joint venture Toyota produced and developed PLA for automotive applications Formation of Cargill Dow LLC, a 50–50 joint venture of Cargill Inc. and The Dow Chemical Company, to commercialize PLA under the trade name NatureWorksÔ Fiberweb (now BBA, France) introduced melt-blown and spun-laid PLA fabrics under DeposaÔ brand name Mitsui Chemicals, Inc. commercialized PLA produced by polycondensation route Kanebo Ltd. introduced LactronÒ PLLA fiber and spunlaid nonwovens are important to elucidate the melt processing phenomena of PLA. In the remainder of Part II, Chapters 11, 12, and 13 deal with the mechanical, permeability, and migration behaviors of PLA, respectively, and will serve as handy references for designing and engineering PLA products for various end-use applications. In Part III, seven chapters are devoted to summarizing the state of the art of processing and conversion technologies for PLA, covering topics such as extrusion and molding (Chapter 14), polymer blending (Chapters 15 and 16), foaming (Chapter 17), preparation of micro- and nanocomposites (Chapters 18 and 19), and fiber spinning (Chapter 20). One of the hallmarks of PLA polymers is that they are degradable, which has been viewed as an attractive feature for certain applications. In Part IV, six chapters are included to discuss in great detail the various degradation modes of PLA, including hydrolytic degradation (Chapter 21), enzymatic degradation (Chapter 22), thermal degradation (Chapter 23), photodegradation (Chapter 25), and biodegradation (Chapter 25). This part ends with Chapter 26 in which the life cycle assessment and the environmental footprint of PLA are objectively discussed. Finally, in Part V, various applications for PLA are discussed, including medical items (Chapter 27), packaging (Chapter 28), textiles (Chapter 29), and environment-related applications (Chapter 30). Rather than eliminating all duplicate materials between chapters, we deliberately allowed some overlap in discussions to enable the chapters to stand alone to some extent. This volume skillfully brings together the work of many contributors who are experts in their respective research areas. This volume would not have been possible without
  • 19. PREFACE their help and contributions. We are indebted to them for their participation and patience during the preparation of this book and are grateful that they have entrusted us to edit their contributions as per the requirements of each chapter. We hope that readers will find this book useful. We are looking forward to receiving comments and constructive feedback regarding the content of this book [5]. Finally, we are indebted to our three academic institutions, Michigan State University, University of Guelph, and Toyohashi University of Technology, for allowing us to dedicate our effort and time to the completion of this edited book. Our most grateful thanks are to our colleagues for providing a sounding board to discuss ideas and explore new concepts about biodegradable polymers and materials in general; to our editor at John Wiley Sons, Inc., Jonathan T. Rose, for supporting this proposal, and walking us through its completion; to Lisa Van Horn for coordinating the production of the book; and to Sanchari Sil, our project manager at Thomson Digital, for her invaluable patience to in answering our endless questions about the final proofing of the book. Overall, we could not put our effort into this task without the unconditional support of our families, so that our most special thanks go to all of them. xix REFERENCES 1. H. Benninga, A History of Lactic Acid Making, Springer, New York, 1990. 2. W. H. Carothers, G. L. Dorough, F. J. van Natta. J. Am. Chem. Soc.1932, 54, 761–772. 3. P. R. Gruber, E. S. Hall, J. J. Kolstad, M. L. Iwen, R. D. Benson, R. L. Borchardt, U.S. Patent 6,326,458, 2001. 4. P. R. Gruber, E. S. Hall, J. J. Kolstad, M. L. Iwen, R. D. Benson, R. L. Borchardt, U.S. Patent 5,357,035, 1994. 5. Y. K. Jung, T. Y. Kim, S. J. Park, S. Y. Lee, Biotechnol. Bioeng. 2010, 105, 161–171. RAFAEL AURAS East Lansing, Michigan LOONG-TAK LIM Guelph, Ontario, Canada SUSAN E. M. SELKE East Lansing, Michigan HIDETO TSUJI Toyohashi, Aichi, Japan May 2010
  • 20. CONTRIBUTORS Steven Abbott, Steven Abbott TCNF Ltd., Ipswich, England Hideki Abe, Chemical Analysis Team, RIKEN Institute, Saitama, Japan Seiichi Aiba, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Ashwini K. Agrawal, SMITA Labs, Department of Textile Technology, Indian Institute of Technology, New Delhi, India Ann-Christine Albertsson, Department of Fiber and Polymer Technology, Royal Institute of Technology, Stockholm, Sweden Sukeewan Detyothin, School of Packaging, Michigan State University, East Lansing, Michigan; Department of Agroindustry, Faculty of Agriculture, Natural Resources and Environment, Naresuan University, Phitsanulok, Thailand John R. Dorgan, Colorado School of Mines, Golden, Colorado Luca Fambri, Department of Materials Engineering and Industrial Technologies and BIOtech Research Center, University of Trento, Trento, Italy Anna Finne-Wistrand, Department of Fiber and Polymer Technology, Royal Institute of Technology, Stockholm, Sweden Eva Almenar, School of Packaging, Michigan State University, East Lansing, Michigan Subrata Bandhu Ghosh, Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Ontario, Canada Rafael Auras, School of Packaging, Michigan State University, East Lansing, Michigan Carla M. B. Gon¸ alves, CICECO, Chemistry Department, c University of Aveiro, Aveiro, Portugal Sanchita Bandyopadhyay-Ghosh, Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Ontario, Canada Buenaventurada P. Calabia, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Gian Domenico Cella, Novamont SpA, Novara, Italy Kevin Cink, NatureWorks LLC, Minnetonka, Minnesota Wim Groot, PURAC, Gorinchem, The Netherlands Akira Hiraishi, Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, Aichi, Japan Masayuki Hirata, Department of Biomolecular Engineering, Center for Fiber and Textile Science, Kyoto Institute of Technology, Kyoto, Japan Jo~o A. P. Coutinho, CICECO, Chemistry Department, a University of Aveiro, Aveiro, Portugal Dan Huang, Key Laboratory of Eco-textiles of Ministry of Education, College of Textile and Clothing, Jiangnan University, Wuxi, China Katherine Dean, CSIRO, Materials Science and Engineering, Melbourne, Australia Yoshito Ikada, Faculty of Medicine, Nara Medical University, Nara, Japan xxi
  • 21. xxii CONTRIBUTORS Tadahisa Iwata, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan Haruo Nishida, Eco-Town Collaborative RD Center for the Environment and Recycling, Kyushu Institute of Technology, Fukuoka, Japan Waree Jaruwattanayon, School of Packaging, Michigan State University, East Lansing, Michigan; Thai Packaging Center, Thailand Institute of Scientific and Technological Research, Bangkok, Thailand Shoji Obuchi, Materials Laboratory, Mitsui Chemicals, Inc., Chiba, Japan Xue Jiang, Key Laboratory of Eco-textiles of Ministry of Education, College of Textile and Clothing, Jiangnan University, Wuxi, China Zhengyu Jin, School of Food Science and Technology, Jiangnan University, Wuxi, China Ajay Kathuria, School of Packaging, Michigan State University, East Lansing, Michigan Yoshihiro Kikkawa, Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Shinji Ogawa, Process Technology Center, Mitsui Chemicals, Inc., Chiba, Japan Gabriele Perego, Vanadis Srl, Colleretto Giacosa, Italy Eustathios Petinakis, CSIRO, Materials Science and Engineering, Melbourne, Australia Narendra Reddy, Department of Textiles, Clothing and Design, University of Nebraska-Lincoln, Lincoln, Nebraska Mohini Sain, Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Ontario, Canada Yoshiharu Kimura, Department of Biobased Materials Science, Center for Fiber and Textile Science, Kyoto Institute of Technology, Kyoto, Japan Wataru Sakai, Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto, Japan Kamlesh Kumar, Center for Polymer Science and Engineering, Indian Institute of Technology, New Delhi, India Susan E. M. Selke, School of Packaging, Michigan State University, East Lansing, Michigan Amy E. Landis, Department of Civil and Environmental Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania Suprakas Sinha Ray, DST/CSIR Nanotechnology Innovation Center, National Center for Nano-Structured Materials, Council for Scientific and Industrial Research, Pretoria, Gauteng, Republic of South Africa Loong-Tak Lim, Department of Food Science, University of Guelph, Guelph, Ontario, Canada Hongshen Liu, CSIRO, Materials Science and Engineering, Melbourne, Australia; Center for Polymer from Renewable Resources, SCUT, Guangzhou, China Bimlesh Lochab, Center for Polymer Science and Engineering, Indian Institute of Technology, New Delhi, India Yan Luo, Department of Applied Chemistry, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China Isabel M. Marrucho, CICECO, Chemistry Department, University of Aveiro, Aveiro, Portugal; Instituto de Tec´ nologia Qu ımica e Biologica, ITQB2, Universidade Nova de Lisboa, Oeiras, Portugal Laurent M. Matuana, School of Packaging, Michigan State University, East Lansing, Michigan Claudio Migliaresi, Department of Materials Engineering and Industrial Technologies and BIOtech Research Center, University of Trento, Trento, Italy Masatsugu Mochizuki, Center for Fiber and Textile Science, Kyoto Institute of Technology, Kyoto, Japan Olav Sliekers, PURAC, Gorinchem, The Netherlands ´ Herlinda Soto-Valdez, Centro de Investigacion en Alimen´ tacion y Desarrollo A.C., Hermosillo, Sonora, Mxico e Anders S€dergard, Laboratory of Polymer Technology, o Abo Akademi University, Turku, Finland Mikael Stolt, Laboratory of Polymer Technology, Abo Akademi University, Turku, Finland Shuko Suzuki, Faculty of Medicine, Nara Medical University, Nara, Japan Yaoqi Tian, School of Food Science and Technology, Jiangnan University, Wuxi, China Xiuzhi Tian, Key Laboratory of Eco-textiles of Ministry of Education, College of Textile and Clothing, Jiangnan University, Wuxi, China Yutaka Tokiwa, Okinawa Industrial Technology Center, Uruma, Okinawa, Japan Hideto Tsuji, Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, Aichi, Japan
  • 22. CONTRIBUTORS Naoto Tsutsumi, Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto, Japan Charles U. Ugwu, Okinawa Industrial Technology Center, Uruma, Okinawa, Japan Jan van Krieken, PURAC, Gorinchem, The Netherlands Tim Vanyo, NatureWorks LLC, Minnetonka, Minnesota Indra Kumari Varma, Center for Polymer Science and Engineering, Indian Institute of Technology, New Delhi, India xxiii Sicco de Vos, PURAC, Gorinchem, The Netherlands Jinpeng Wang, School of Food Science and Technology, Jiangnan University, Wuxi, China Yiqi Yang, Department of Textiles, Clothing and Design and Department of Biological Systems Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska Long Yu, CSIRO, Materials Science and Engineering, Melbourne, Australia; Center for Polymer from Renewable Resources, SCUT, Guangzhou, China
  • 23. PART I CHEMISTRY AND PRODUCTION OF LACTIC ACID, LACTIDE, AND POLY(LACTIC ACID)
  • 24. 1 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE WIM GROOT, JAN VAN KRIEKEN, OLAV SLIEKERSL, 1.1 AND INTRODUCTION Natural polymers, biopolymers, and synthetic polymers based on annually renewable resources are the basis for the twenty-first-century portfolio of sustainable, eco-efficient plastics [1]. These biosourced materials will gradually replace the currently existing family of oil-based polymers as they become cost- and performance-wise competitive. Polylactide or poly(lactic acid) (PLA) is the front runner in the emerging bioplastics market with the best availability and the most attractive cost structure. The production of the aliphatic polyester from lactic acid, a naturally occurring acid and bulk produced food additive, is relatively straightforward. PLA is a thermoplastic material with rigidity and clarity similar to polystyrene (PS) or poly(ethylene terephthalate) (PET). End uses of PLA are in rigid packaging, flexible film packaging, cold drink cups, cutlery, apparel and staple fiber, bottles, injection molded products, extrusion coating, and so on [2]. PLA is bio-based, resorbable, and biodegradable under industrial composting conditions [1, 3, 4]. PLA can be produced by condensation polymerization directly from its basic building block lactic acid, which is derived by fermentation of sugars from carbohydrate sources such as corn, sugarcane, or tapioca, as will be discussed later in this chapter. Most commercial routes, however, utilize the more efficient conversion of lactide—the cyclic dimer of lactic acid—to PLA via ring-opening polymerization (ROP) catalyzed by a Sn(II)-based catalyst rather than polycondensation [2–6]. Both polymerization concepts rely on highly concentrated polymer-grade lactic acid of excellent quality SICCO DE VOS for the production of high molecular weight polymers in high yield [2–4, 7]. Purification of lactic acid produced by industrial bacterial fermentation is therefore of decisive importance because crude lactic acid contains many impurities such as acids, alcohols, esters, metals, and traces of sugars and nutrients [4]. The lactide monomer for PLA is obtained from catalytic depolymerization of short PLA chains under reduced pressure [4]. This prepolymer is produced by dehydration and polycondensation of lactic acid under vacuum at high temperature. After purification, lactide is used for the production of PLA and lactide copolymers by ROP, which is conducted in bulk at temperatures above the melting point of the lactides and below temperatures that cause degradation of the formed PLA [4]. Processing, crystallization, and degradation behavior of PLA all depend on the structure and composition of the polymer chains, in particular the ratio of the L- to the D-isomer of lactic acid [2, 4, 6, 8, 9]. This stereochemical structure of PLA can be modified by copolymerization of mixtures of L-lactide and meso-, D-, or rac-lactide resulting in high molecular weight amorphous or semicrystalline polymers with a melting point in the range from 130 to 185 C [3, 4, 6–10]. Isotactic PLLA homopolymer—comprising L-lactide only—is a semicrystalline material with the highest melting point, while PLA copolymers with higher D-isomer content exhibit lower melting points and dramatically slower crystallization behavior, until they finally become amorphous at D-contents higher than 12–15% [8–10]. Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications, edited by R. Auras, L.-T. Lim, S. E. M. Selke, and H. Tsuji Copyright Ó 2010 John Wiley Sons, Inc. 3
  • 25. 4 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE For decades, ROP has been the preferred route to PLA for biomedical applications with small production volumes. PLLA and copolymers with rac-lactide, glycolide, and ecaprolactone for resorbable biomedical applications have been produced by, for example, PURAC, previously known as CCA, since the 1970s [5]. Since the 1990s, the ROP concept is also used for high-volume production of PLA grades for other end uses. Large-scale production of PLA, copolymers of L- and meso-lactide, was started in 2002 by a joint venture of Cargill and Dow under the name NatureWorks LLC. Nowadays, since July 1, 2009, NatureWorks LLC is again wholly owned by Cargill and has a production capacity of 140 ktpa for its Ingeo PLA grades in Blair, Nebraska [11]. The attractive price and commercial availability of lactic acid were important reasons why PLA became the first massproduced bio-based polyester. The critical success factor for a final breakthrough of all green chemicals and plastics based on annually renewable materials is economic sustainability. Thus, the very basis of cost-competitive PLA is an industrial fermentative production process for lactic acid with efficient use of carbohydrates followed by excellent purification technology with minimum generation of by-products. An important impulse for the expanding bioplastics market is the commercialization of lactide monomers for PLA by PURAC in 2008. Solid D- and L-lactides are now available in bulk quantities and can be polymerized into a whole range of tailor-made polylactides by continuous melt polymerization processes, like the technology based on static mixing reactors that was jointly developed by Sulzer and PURAC. PLA offers an unprecedented market potential to lactic acid producers all over the world, but not all potential players can succeed, because PLA production poses stringent demands to lactic acid quality and price. The chemistry and physics of today’s fermentative production and industrialscale purification of lactic acid and lactide are the subject of this chapter. 1.2 1.2.1 LACTIC ACID History of Lactic Acid Lactic acid was discovered in 1780 by the experimental chemist Carl Wilhelm Scheele, who isolated ‘‘acid of milk’’ from sour whey [12, 13]. A further description of the history of lactic acid by Holten and Benninga shows that industrial production of lactic acid started in the United States in the 1880s [14, 15]. Avery patented and applied a process of fermentation of vegetable sugars [16]. The actual application was the use of a mixture of calcium lactate and lactic acid as baking powder. Unfortunately, this application was not a big success, but other applications in food and textile dyeing were developed. FIGURE 1.1 Two enantiomeric forms of lactic acid: (S)- and (R)2-hydroxypropionic acid. In 1950, the first commercial production of synthetic lactic acid started in Japan [15]. Lactonitrile was produced from acetaldehyde and hydrogen cyanide and hydrolyzed in the second stage to lactic acid. For some decades, synthetic lactic acid competed with lactic acid obtained by fermentation, but currently almost all lactic acid is produced by fermentation. 1.2.2 Physical Properties of Lactic Acid Lactic acid (2-hydroxypropanoic acid) is the simplest 2-hydroxycarboxylic acid (or a-hydroxy acid) with a chiral carbon atom and exists in two enantiomeric forms (Figure 1.1). The chirality of lactic acid often results in confusion regarding nomenclature. A number of different names are used in the literature. This confusion is the result of mixing the molecular structure and a physical property (optical rotation). (S)-Lactic acid (or L-lactic acid) has a slightly positive specific optical rotation and is frequently named L-( þ )-lactic acid [14]. However, a concentrated solution of (S)-lactic acid at equilibrium contains lactic acid oligomers, which results in an overall negative optical rotation. Therefore, it is advised to use the structural R/S notation or the older notation of L and D and avoid the þ and À of the optical rotation (Table 1.1). 1.2.3 Chemistry of Lactic Acid The lactic acid molecule has a hydroxyl and an acid functional group, which may result in intermolecular and intramolecular esterification reactions. The first step is the formation of a linear dimer (lactoyl lactic acid). This condensation reaction can proceed to higher oligomers and is promoted by removal of water. Also a cyclic dimer, lactide, is formed in small amounts. Lactide can be formed by intramolecular esterification of lactoyl lactic acid or by breakdown of higher oligomers. All reactions are equilibrium reactions (Figure 1.2). Due to these reactions, a solution of lactic acid at equilibrium consists of monomeric lactic acid, dimeric lactic acid or lactoyl lactic acid, higher oligomers of lactic acid, and
  • 26. LACTIC ACID TABLE 1.1 Physical Properties of Lactic Acid [14] Property Value CAS number General: 50-21-5 (S)-Lactic acid: 79-33-4 (R)-Lactic acid: 10326-41-7 90.08 Molecular weight (g/mol) Formula Melting point ( C) Crystal structure Solid density (g/mL) Solubility in water (wt%) Heat of fusion (kJ/mol) Boiling point ( C) Liquid density (g/mL, 20 C) Viscosity (mPa s) pKa Specific heat (J/(g K) at 25 C) C3H6O3 18 (racemic) 53 (chiral pure) (S)-Lactic acid: orthorhombic, space group P212121 1.33 (solid, 20 C) 86 (20 C, monomeric (S)-lactic acid) (S)-Lactic acid: 16.8 122 (at 14 mmHg) 1.224 (100% undercooled liquid) 1.186 (80.8% solution in water) 28.5 (85.3% solution in water, 25 C) 3.86 Crystalline (S)-lactic acid: 1.41 Liquid lactic acid: 2.34 Reference [20] [20] [21] [22] [22] [23] [23] [24] [25] [26] lactide. The ratios between all substances depend on the amount of water present; for example, a 90.1% lactic acid solution (total acidity) contains about 59.3% of monomeric lactic acid and 27.3% of lactoyl lactic acid and higher oligomers [14]. FIGURE 1.2 The condensation reactions are also the reason that it is quite difficult to obtain pure, solid, and enantiopure lactic acid. This can only be achieved by crystallization [27, 28]. The kinetics of the condensation reactions determine the stability of a solution of monomeric lactic acid and have a large influence on the stability of solid lactic acid. 1.2.4 [17] [18] [19] 5 Production of Lactic Acid by Fermentation Almost all lactic acid available on the market is produced by fermentation. During fermentation, a suitable carbohydrate is converted to lactic acid by microorganisms. Although some of the microorganisms used, such as the mold Rhizopus, need oxygen for growth, the actual conversion of sugars to lactic acid is carried out without oxygen. As a matter of fact, the complete oxidation of a sugar to carbon dioxide and water is energetically much more favorable, so lactic acid is mainly formed under anaerobic conditions. Indeed, most lactic acid producing microorganisms are inactive when oxygen is continuously present in high amounts [29]. Upon entering the cell, the sugar is first converted to pyruvate by several enzymatic steps. This conversion yields chemical energy in the form of ATP (adenosine triphosphate) and reducing equivalents (NADH); see the reaction in Figure 1.3. In order to recycle these reducing equivalents, microorganisms convert the pyruvate into the more reduced lactic acid; see the reaction in Figure 1.4. In other words, lactic acid is mainly produced to keep the cellular processes going [30]. The chemical energy obtained is used by several processes elsewhere in the cell, for example, cell growth, maintenance, and sometimes even motility. The reaction in Figure 1.3 takes place in the so-called homofermentative lactic acid bacteria (LAB). Homofermentative bacteria have almost exclusively lactic acid as a Lactic acid condensation reactions: interchange between lactide, oligomers, and poly(lactic acid).
  • 27. 6 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE FIGURE 1.3 During conversion of glucose to pyruvate, chemical energy (ATP) is generated as well as reducing equivalents (NADH). fermentation product, in contrast to heterofermentative bacteria that produce a mixture of lactic acid, acetate, CO2, and acetate or ethanol [31]. Heterofermentative bacteria were believed to use exclusively the so-called phosphoketolase pathway, and homofermentative bacteria were believed to use exclusively the glycolysis (Figure 1.3) that splits C6 into two C3 molecules [31]. The phosphoketolase pathway is a route where a C6 is transformed to a C5 sugar (and CO2) and split into a C2 and a C3 molecule. The C3 molecule is then converted to lactic acid whereas the C2 molecule is converted to acetate or ethanol. In the same traditional view, C5 sugars were regarded as leading to this heterofermentative metabolism, which is less interesting from the point of view of industrial production as a lot of acetic acid or ethanol is produced simultaneously. Although some bacteria seem to fit well in this paradigm, more recent literature has shown that this view is oversimplified and somewhat obsolete for a number of reasons. . . . Some heterofermentative bacteria are shown to have both pathways active at the same moment and produce mostly lactic acid under certain circumstances [32, 33]. Pentoses can lead exclusively to lactic acid as a fermentation product [34]. Lactic acid producing organisms that do not have a phosphoketolase pathway can still produce acetate or ethanol, formed by the usual mixed acid fermentation, via pyruvate. This is the case for, for example, Lactococcus lactis [35]. FIGURE 1.4 Lactic acid formation from pyruvate: reoxidation of NADH and NAD takes place; NAD can be used again in the reaction of Figure 1.3. The reason why even heterofermentative bacteria prefer to produce mostly lactic acid is related to the fast generation of chemical energy and thus fast growth and acidification of the environment [32, 36, 37]. The fast growth and acidification gives lactic acid bacteria a competitive advantage and that is exactly why lactic acid bacteria are so troublesome in ethanol fermentations [38]. Lactic acid production is certainly not restricted to bacteria or fungi. Higher organisms, including humans, also use lactic acid formation for fast supply of energy in muscles when needed [39]. The uniformity in this biochemistry is in sharp contrast with the degrees of freedom one has in choosing the microbes, the acid-neutralizing agent, nutrients, and carbohydrates needed for industrial lactic acid fermentation. Only delicate weighing of the pros and cons of every possibility leads to an economically feasible fermentation. 1.2.4.1 The Microbes There are several important features a microorganism used for the production of lactic acid must have in order to be industrially attractive: . . . . . . high productivity to reduce fermentation time, high conversion yield to reduce carbohydrate costs, ability to use cheap sources of nutrients to reduce nutrient costs, high end concentration to reduce evaporation costs, low amount of by-products to increase purification yield, and, of course, the organisms must be robust with regard to contamination and infections. Every microorganism has its own benefits and drawbacks, but lactobacilli (present in many food fermentations) and Rhizopus (a fungus) are the most reported [40]. Besides lactobacilli and Rhizopus, Streptococcus, Pediococcus, Sporolactobacillus inulinus, Bacillus coagulans, and several yeasts are mentioned in the excellent overview by Vaidya et al. [41]. Lactobacilli generally have high productivity, but special and often expensive nutrient requirements. Rhizopus needs much less nutrients, but has a lower yield, needs oxygen, and its morphology is sometimes difficult to handle. Of course, via genetic manipulation, researchers have tried to make an ideal lactic acid producing microorganism. 1.2.4.2 Stereochemical Purity In order to make semicrystalline, high-melting PLA, stereochemically pure lactic acid is needed. Not all microorganisms yield such stereochemically pure lactic acid and some even produce a racemic mixture [29]. Therefore, a strain must be chosen that meets the quality demands. Finding such a strain that produces L-lactic acid in an economically feasible manner is relatively easy. Producing D-lactic acid by bacterial fermentation on an industrial scale is far more difficult.
  • 28. LACTIC ACID Several natural D-lactic acid producing bacterial species exist; Sporolactobacillus inulinus, Sporolactobacillus laevolacticus (previously Bacillus laevolacticus), and Lactobacillus delbrueckii are among these bacteria [29, 42, 43]. Also, patents have been filed claiming the production of D-lactic acid by a genetically modified microorganism. Several different species such as Kluyveromyces and Escherichia coli have been claimed so far [44, 45]. 1.2.4.3 Nutrients The most well-known lactic acid producing organisms, such as Lactobacillus and Lactococcus species, are members of the taxonomic order of Lactobacillales, also commonly referred to as lactic acid bacteria. These lactic acid bacteria have their really complex nutrient need in common [29]. Vitamins and peptides need to be added to the medium to enable growth. This can be done by adding peptones, yeast extract, or corn steep liquor, but this is expensive. Nutrients for lactic acid production can also be derived from nutrient-rich waste streams such as rice bran, fish waste, or vinification lees [46–48]. 1.2.4.4 Neutralization Lactic acid fermentation inevitably leads to a drop in pH, and without neutralization the microorganism is quickly unable to continue the fermentation, as the environment becomes too acidic. Several bases can be used to neutralize the acidity during fermentation, and the choice of the base will determine the nature of the downstream processing (DSP). Most industrial lactic acid plants use Ca(OH)2 or CaCO3, which results in the production of a large amount of gypsum as a by-product. A major challenge in lactic acid production is to find or construct an efficient microorganism that can produce at such a low pH that the fermentation does not require neutralization. Lactic acid bacteria are usually able to grow at low pH, but it is difficult to find an organism capable of producing lactic acid in reasonable amounts at pH close to the pKa of lactic acid [49]. Another solution is to construct a lactic acid producing yeast but organisms like this still suffer from low productivities (amount of lactic acid produced per hour) and low final concentrations, leading to the requirement for large fermenter volumes and high amounts of water evaporation [50]. Some basic hurdles have to be overcome in order to improve the low-pH fermentation by yeasts. Although yeasts are very resistant to low pH, the export of lactate from the yeast cell to the outside medium costs them as much energy as they get from lactic acid production by fermentation. For this reason, lactic acid producing yeasts need reasonable amounts of oxygen in order to generate enough energy to survive [51]. In contrast, traditional lactic acid bacteria use another way to transport lactic acid across the membrane and even gain extra energy by exporting lactic acid to the medium [52]. 7 1.2.4.5 Carbohydrates for Lactic Acid Production In principle, any carbohydrate source containing pentoses (C5 sugars) or hexoses (C6 sugars) can be used for the production of lactic acid, although it is very rare that any particular microorganism is able to use all possible and available C5 and C6 sugars. Pure sucrose from sugarcane or sugar beets and glucose from starch are available in large amounts and readily fermentable. Polysaccharides such as cellulose or starch are more complex and need special pretreatment. When using less pure sources such as raw sugar beet juice, the impurities must be removed somewhere in the total lactic acid production process [53]. This can be done before, during, or after the fermentation. This often leads to special adaptations in the production plant. Last but not least, the local price and availability of the carbohydrate source determine the raw material of choice for industrial fermentation. Another usable disaccharide is lactose present in whey, as was used by Scheele when he discovered lactic acid in 1780 [12]. 1.2.4.6 Starch Starch occurs in discrete granules and is usually a mixture of two homopolymers of glucose, amylopectin and amylose. Starch can be derived from corn, wheat, potato, or tapioca [54]. Although some microorganisms are able to degrade and ferment starch directly to lactic acid, most lactic acid producing microorganisms cannot hydrolyze starch themselves. A solution is to hydrolyze the starch to glucose prior to fermentation with the commercially available enzymes, a-amylase and glucoamylase. This can be done in a separate process, so no incompatibilities are present between the optimal pH and temperatures of the enzymes on one hand and the optimal pH and temperature of the microbes on the other. However, if the right combination of enzymes, microorganisms, pH, and temperature is carefully chosen, the hydrolysis and fermentation can be carried out in one reactor. This process is generally called SSF (simultaneous saccharification and fermentation) [55]. Prior to SSF, the starch granules usually must be gelatinized at high temperature by cooking. However, even a cooker is optional nowadays as commercial enzymes are becoming available that are able to attack and hydrolyze the granules efficiently and fast enough at relatively low temperatures. 1.2.4.7 Lignocellulose Sucrose and starch have in common that they are used for food and nowadays, with oil wells drying out and prices rising, also for biofuels. A decrease in the availability of fossil fuels is envisaged for the future, and with increasing population, more food is needed at reasonable prices. Therefore, the ideal raw material for biofuels and bioplastics is carbohydrates that are not edible. Such material is abundantly available around the globe as lignocellulose, like in corn stover or wheat straw. Lignocellulose consists of the glucose homopolymer cellulose, the heteropolymer hemicellulose, and lignin. Hemicellulose consists of hexoses
  • 29. 8 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE and pentoses. In all, lignocellulose contains roughly 80% fermentable sugars, but this largely depends on the source [54]. The remainder, lignin, is a phenolic polymer that is difficult to degrade and is not directly usable for lactic acid production. It may be used for energy production though, which can be returned to the lactic acid plant. A purer source of cellulose without lignin is waste paper that can be used for lactic acid production at lab scale [56]. Thus, even this book can eventually be converted into PLA! Complete utilization of cellulose and hemicellulose requires selection or genetic modification of an organism that is able to ferment pentoses. In order to obtain monosaccharides from the raw material, several pretreatments and/or separations are required. First, the lignocellulosic material is mechanically treated and then delignified (pulped) by strong alkali or acid treatment. The (hemi)cellulose part becomes more accessible for enzymes at the same time. Subsequent enzymatic treatment mainly yields glucose and xylose and some arabinose. The enzymatic treatment and subsequent fermentation can be done in separate reactors or in one fermenter, in an SSF concept similar to starch SSF [57]. 1.2.4.8 Batch versus Continuous Fermentation A process can be run in batch or continuous mode. In continuous mode, there is a constant flow of fermented sugar out of the reactor that is equal to a continuous flow of fermentation medium into the reactor. During batch fermentation, there can be an inflow of medium, but there is no outflow [58]. Batch fermentation needs to be inoculated with a starter culture every time, whereas this is not needed in a continuous fermentation setup. However, in case of problems, the continuous fermentation needs to be restarted, so an infrastructure for starter cultures is needed anyway. A high volumetric production rate can be achieved when combining continuous FIGURE 1.5 fermentation with biomass retention, leading to smaller fermenter size [59]. It must be stated that the lactic acid concentration is lower compared to batch culture [58]. The concentration of lactic acid influences the water balance in the production plant. In all scenarios, microorganisms produce an aqueous lactic acid solution, comprising mainly lactate and counterions from the base, impurities from raw materials or fermentation by-products, residual sugars and polysaccharides, and the microorganism itself. 1.2.5 Downstream Processing/Purification of Lactic Acid When Scheele discovered lactic acid, he recovered and purified the lactic acid from sour whey by saturation with lime, filtering off the crude calcium lactate, acidifying the crystal mass with ‘‘acid of sugar’’ (oxalic acid), filtering off the calcium oxalate, and evaporating to obtain a crude viscous lactic acid [12, 13]. Basically, this process with a calcium-based neutralized fermentation and sulfuric acid instead of oxalic acid is the same process used in industry today for the production of crude lactic acid. Drawbacks are the continuously rising costs of lime/chalk, sulfuric acid, and other chemicals and the disposal of large quantities of gypsum (CaSO4Á2H2O), as an unavoidable side product of this technology. In such a process also the first down stream processing (DSP) step, biomass removal by filtration, can be accomplished relatively easily in a (mild) liming step, in essence quite similar to the traditional liming step to remove protein in sugar beet or sugarcane processing in sugar mills. A simplified block scheme of the traditional lactic acid production process including fermentation is shown in Figure 1.5. Simplified block scheme of traditional lactic acid production process.
  • 30. LACTIC ACID TABLE 1.2 9 Summary of Lactic Acid Purification Methods Lactic Acid Purification Method Advantages Disadvantages Crystallization [27, 28] Esterification/distillation [52] Lactic acid distillation [27, 28, 53] Highly pure lactic acid product Highly pure acid, scale-up Good splitting for heavy compounds Potentially high yield Amount of mother liquor by-product, scalability Relatively high utility cost, amount of residue as by-product Amount of residue as by-product Extraction [54, 55] 1.2.5.1 Purification Methods for Lactic Acid Crude lactic acid, which may be upgraded by simple active carbon treatment and/or ion exchange to remove impurities and salts, can be directly used in a large number of food applications. Traditionally, taste, smell, and heat stability for color formation have been used to express lactic acid quality. The presence of acids (e.g., acetic acid and pyruvic acid), alcohols (e.g., methanol and ethanol), and esters can directly influence taste and smell [4]. The presence of residual sugar and nitrogen compounds greatly influences heated color, that is, browning of the liquid upon heating. The formation of color upon heating prohibits the use of crude acid in foods that need to undergo pasteurization/sterilization. Over the decades, the demand for purer lactic acid with improved color stability upon heating has increased, as exemplified by the need for ultrapure lactic acid as a sodium lactate base in pharmaceutical infusion products. At present, a chemical engineer can choose from a number of mature industrial methods to purify lactic acid. Table 1.2 lists their relative advantages and disadvantages. Choices in an overall process are governed by raw material costs, utility costs, and, last but not least, outlets for by-products. The purification methods described above each involve considerable technological know-how: . . . Esterification/Saponification. Esterification of lactic acid with methanol/ethanol yields systems with good separation characteristics to separate many impurities with different boiling points [60]. However, the energy demand of a full reaction/distillation route from crude acid to pure acid is high. Crystallization. Crystallization can yield an excellent lactic acid grade, but the yield is low. Lactic Acid Distillation. Industrial equipment is available to distill lactic acid at low vacuum. Higher molecular weight components such as sugar and protein will leave the system as a residue. Heat-stable lactic acid is obtained as the top product. In the stages of dewatering the crude lactic acid prior to distillation, the formation of oligomers will limit an overall high distillation yield. Complex (e.g., for emulsion, entrainment issues), extractant cost . Extraction. An extraction/back-extraction process, for example, with the well-described tertiary amine systems, is a suitable way to purify lactic acid [61, 62]. The possible combination of extraction with low-pH fermentation yields an elegant concept to arrive at a gypsum-free process. For future large-scale, low-cost lactide/PLA production, lactic acid DSP will need to meet new challenges: . . Use of Low-Cost and Nonedible Substrates. Whereas production of lactic acid from sucrose or glucose syrup is well established, crude sources (starches, sugars, or future lignocellulose hydrolysates) will form the next hurdle as they contain much more impurities and possible fermentation inhibitors. Gypsum-Free Processing. For large-scale, sustainable PLA production, a fermentation process that does not coproduce a mineral salt is a must. 1.2.5.2 Gypsum-Free Lactic Acid Production Gypsumfree lactic acid production can be briefly categorized as follows: . . Low-pH Fermentations Coupled to In Situ Product Removal. As discussed in Section 1.2.4, fermentations can be carried out without neutralization at pH 2–3 with genetically modified yeast or at pH 4 with LAB with partial neutralization [50]. When a separation method to recover the undissociated acid is integrated with fermentation, a process route can be designed in which no gypsum is produced. In the literature, a number of separation methods are described with an emphasis on extraction [63]. Cost efficiency in the fermentation (e.g., nutrients, yield) and the practical processing of large dilute streams need breakthroughs for economical processing. Electrochemical Splitting of a Neutral Lactate Salt. Numerous articles have described the splitting of a lactate salt, notably sodium lactate, into lactic acid and the original base [64]. With this principle, a gypsumfree process can be designed, with electrodialysis
  • 31. 10 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE . separate from or integrated with fermentation. The use of electrodialysis with new bipolar membranes is straightforward, but a large-scale commercial breakthrough as in the 1980s and 1990s with monopolar membranes for the chloro-alkali process is still pending. Electrodialysis involves relatively high electricity costs and a huge membrane area, but these costs may be managed in biorefinery concepts with integrated energy production. Chemical Salt Splitting of a Lactate Salt. Lactate salts can be split with the help of auxiliary chemicals and the regeneration of these chemicals. A patent by Baniel et al., for example, describes a method in which a sodium lactate solution is acidified with CO2 under pressure, and simultaneously undissociated lactic acid is extracted and insoluble sodium bicarbonate (NaHCO3) is formed [65]. Another patent describes the splitting of ammonium lactate by esterification with butanol while liberating ammonia [66]. In the distillation process, the butyl lactate can be hydrolyzed with water to liberate lactic acid. This is an interesting option, but the energy consumption and side reactions such as the formation of lactamide and racemization require attention. Chemical salt splitting processes with the recycle of chemicals can be complex, but it is a challenge to develop a system with straightforward chemistry, high yield, low energy consumption, and good scaleability. 1.2.5.3 Modern Industrial Methods In overall process development, knowledge about dealing with impurities will be important. Residual sugar in the broth and sugar degradation products play a role throughout the process at the various levels of temperature and acidity. Color may be formed at any step from low- to high-boiling color precursors. Volatile acids such as acetic acid and formic acid will partition throughout DSP and their concentration in recycle streams must be prevented. In the design of a modern lactic acid plant, mathematical models are indispensable. For example, the kinetic model of oligomerization of lactic acid and the right thermodynamic model for the gas/liquid equilibria are important in design for the concentration of lactic acid by evaporation as well as for prepolymerization in the lactide route. Lactic acid solutions and vapors are quite corrosive and knowledge of the material of construction is a must for a lowmaintenance plant. Also, wastewater treatment is an integral part of a lactic acid plant. Aerobic systems are state of the art, but anaerobic systems are increasingly used to treat acidcontaining wastewater streams. The biogas can then be profitably used for steam production. While a plant using sucrose has a net intake and net purge of water, future plants using crude, low-cost, water-rich substrates will need to pay more attention to the water balance and wastewater treatment. Although the fermentation industry can be considered traditional, new technologies may quickly find uses. The rapid commercial application of filtration techniques such as in membrane bioreactors in wastewater treatment and the fast introduction of nanofiltration for making process water from river water are examples. The discovery of ionic liquids with high distribution coefficients for lactic acid in dilute solutions may lead to breakthroughs [67]. New steam boiler concepts that can handle residues can drastically change DSP layout in energy-efficient integrated biorefineries. 1.2.6 Quality/Specifications of Lactic Acid The dehydration of lactic acid to make the prepolymer should start with an ÀOH to ÀCOOH ratio of 1:1. All other components with ÀOH and ÀCOOH functionality disrupt the stoichiometric balance and may be incorporated as comonomers during prepolymerization, which limits the final lactide production yield from lactic acid. Little public information is available on the technical and economic relationship between lactic acid quality and lactide synthesis. Only a few patents mention the effect of metal impurities on racemization [68, 69]. Stereochemical purity is one of the key parameters determining lactic acid purity. Lactic acid purified by crystallization may be taken as the benchmark in lactide manufacture, but the expected unfavorableeconomicsofmakingcrystalline acid inrelationto mother liquor processing may prevent its commercial use for lactide/ PLA. The next level of quality with the right commercial relevance is heat-stable lactic acid. Heat stability puts constraints on the content of sugar, and thus on the DSP method used in the process. It is unlikely that suitable acid for making lactide will contain sugar because of the high temperatures involved (see the next section) and the well-known practical decomposition problems when sugars are cracked. In practice, this means that color, or actually heated color (color after heating of the acid), is an important indicator for the suitability of the acid for lactide/PLA production [6, 70]. The appeal for lactic acid with little or no sugar and the DSP methods mentioned in practice lead to demands for separation methods that are similar for sugar and other heavy components such as proteins, amino acids, and polysaccharides. It is expected that the desired quality of lactic acid for making lactide/PLA will evolve, with overall process yields and economics as the criteria. 1.3 1.3.1 LACTIDE Physical Properties of Lactide The dehydrated, cyclic dimer of lactic acid is commonly called lactide (3,6-dimethyl-1,4-dioxane-2,5-dione). Due to
  • 32. LACTIDE described in a patent by Gruter and Pohl in 1914 [72]. Lactic acid was self-esterified at 120–135 C, and air was drawn in to remove the water. Next, zinc oxide was added as a catalyst and lactide was distilled off under vacuum at 200 C. In practice, modern industry cannot dispense with this concept of thermal catalytic depolymerization for lactide production. A major step forward was the use of a tin catalyst, a frequently used coordinating catalyst in polymerizations, in the process. The general scheme of lactide manufacture including the purification is shown in Figure 1.7. In the past two decades, several papers have appeared on lactide manufacture [73, 74]. A main underlying problem in understanding all information is that the reaction from oligomer to lactide is an equilibrium reaction. In order to pull the reaction toward the right, lactide must be withdrawn from the system. In reaction engineering terms, this means that the chemical kinetics of the reaction cannot be understood without consideration of the method and efficiency of lactide removal. In terms of know-how described in patents, this means that reported lactide production rates depend to a large extent on the geometry of the equipment in which lactide synthesis is performed and that provides for removal of lactide vapor from the reaction zone. In modern chemical technology, one of the goals is to fully understand a given system, capture the knowledge in models to describe experimental work, and ultimately use these models to design, optimize, and debottleneck large-scale FIGURE 1.6 The three diastereomeric structures of lactide (3,6dimethyl-1,4-dioxane-2,5-dione). the two asymmetric carbon atoms in the molecule, lactide exists in three different forms (Figure 1.6). In addition to the three diastereomeric structures mentioned above, also a racemate of D-lactide and L-lactide exists: rac-lactide or DL-lactide (Table 1.3). 1.3.2 Production of Lactide The synthesis of lactide was first described by Pelouze in 1845 [71]. He investigated the self-esterification of lactic acid by heating and driving off water and obtained a prepolymer that was no longer fully miscible with water. Upon continued heating of the prepolymer, he noticed that in a certain distillate fraction nice crystals were formed. He was able to deduce the chemical formula and gave the name ‘‘lactid’’ to the substance. An improved procedure was TABLE 1.3 11 Physical Properties of the Lactides Unit CAS number Molecular weight Melting point Boiling point Heat of fusion Heat of vaporization Solid density Liquid viscosity FIGURE 1.7 g/mol C C J/g kJ/mol g/mL mPa s D-Lactide 13076-17-0 144.12 96––97 L-Lactide [6] meso-Lactide 4511-42-6 144.12 96 13076-19-2 144.12 53 [64] 146 63 1.32–1.38 2.71 (110 C); 2.23 (120 C); 1.88 (130 C) 128 [64]; 118 [6] rac-Lactide 116559-43-4 125 [6] 142 (20 mbar) [64] 185 [6] 1.32–1.38 [6] Schematic illustration of lactide manufacture by thermal catalytic depolymerization of lactic acid oligomers.
  • 33. 12 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE equipment. For the present system, this means that one must develop process know-how on chemical kinetics and thermodynamics of lactide and HL oligomers, and on physical phenomena related to equipment design. These aspects will be relevant for both the prepolymerization and the synthesis of lactide, as these chemical systems are highly similar. In practice, however, lactide synthesis is more complex as chemistry, recovery and type of equipment are intertwined, and the viscous nature of reaction mixtures requires special attention. With these aspects in mind, the information on the lactide synthesis that can be found in the literature is summarized below. 1.3.2.1 Prepolymerization A general procedure for batch prepolymerization is described in a patent by O’Brien et al. [75]. Typically, vacuum pressures of 70–250 mbar and temperatures up to 190 C are used to dewater lactic acid to a prepolymer with an average degree of polymerization (DP) of around 10 in a batch process time of 6 h. For lab-scale equipment, it was also found that thin film and rotating flask vacuum equipment showed faster reaction times than a stirred tank, indicating the importance of mass transfer of water in the already viscous prepolymer. Continuous prepolymerization has also been described in a number of patents, for example, in stirred tanks in series or in evaporator-type equipment [68, 76, 77]. Usually patents describe prepolymers with a DP of 7–20 as feed to the lactide synthesis. Using modern HPLC methods, it has been shown that in oligomeric systems up to DP 10, an equilibrium is present with constant equilibrium constants between the oligomers [6, 72]. 1.3.2.2 Lactide Synthesis During Prepolymerization Because the composition of a mixture comprising lactic acid oligomers and lactide is governed by chemical equilibria, a prepolymerization exhibits relatively high concentrations of lactide (HL2–H2O–L2 equilibrium) around DP 2. Sinclair et al. distilled these fractions to recover lactide, but the crude lactide was quite impure, which may prevent economical processing [73]. In hindsight, the patent describes trials to optimize Pelouze’s original lactide synthesis without catalyst [71]. (stannous 2-ethylhexanoate) is a liquid catalyst that can be handled easily, is food grade, and is widely available. Thinking in terms of mechanisms, the equilibrium concentration of lactide in an oligomer mixture is 5% or less, and it will boil off at low vacuum [6, 68]. The catalyst increases the rate of lactide formation by facilitating lactide formation by backbiting from hydroxyl chain ends of oligomers [4, 74]. In a batch experiment, the rate is initially constant, but during synthesis esterification also occurs, and the DP of the polyester rises concomitantly. The melt viscosity of the reaction mixture increases accordingly and at the end of a batch process, mixing the highly viscous residue becomes very difficult, which limits the extent to which the residue can be depleted of lactide. In engineering terms, this means that mass transfer of lactide from the liquid to the gas phase decreases as viscosity increases. The balance between lactide production and lactide removal plays a role in all experiments that one might want to investigate on lab scale. For example, catalyst concentrations of 0.05–0.2 wt% tin(II) octoate are mentioned in the literature, but traditional experiments to verify the order of the reaction for the catalyst are difficult because of the influence of mass transfer limitations. 1.3.2.4 Continuous Synthesis In 1992, Gruber et al. [68] described a continuous lactide synthesis in which prepolymer is fed continuously to a reactor, crude lactide is evaporated under vacuum, and residue is removed. Typical operating conditions for the reactor were residence time around 1 h, vacuum pressure 4 mbar, temperature 213 C, and catalyst amount 0.05 wt% tin(II) octoate on feed. The conversion per pass was around 70%, and the overall yield was increased by recycling the residue to the lactic acid section of the process, where the oligomers are hydrolyzed again. Especially in the patent literature, several different reactor types are described for continuous lactide synthesis: . . . . 1.3.2.3 Basic Research on Batch Lactide Synthesis and the Catalysts Used Noda and Okuyama reported on the batch synthesis of lactide from DP 15 prepolymer with various catalysts at 4–5 mbar and 190–245 C [74]. In a batch synthesis with 50 g of oligomer in a stirred flask, the evolution rate of crude lactide is rather constant and then starts to decline and the conversion levels off at 80–90%. The tin catalyst performed best compared to other catalysts and showed the lowest levels of racemization. Tin octoate . Stirred tank reactor with different stirrer types [76]. On a bench scale, the reactor is jacketed for heating. Stirred reactor with a distillation section on top of the reactor to fractionate the product [50]. Thin film evaporator with a typical conversion of 80% on pilot scale [70]. Horizontal wiped film evaporator. In a patent by Kamikawa et al. [77], the use of horizontal wiped film is described. In the horizontal mode, the residence time of the reaction mixture can be controlled and a conical form is used in which wipers transport the viscous residue. Distillation column. In a patent by O’Brien et al. [75], a distillation column with perforated plates and optional use of packing material and heating on the stage are described. In an experiment with a single tray, a DP 10 feed was fed to the top, and N2 was used to strip the
  • 34. LACTIDE lactide from the liquid. At different residence times, the conversion on the tray could be as high as 93% at 210–215 C. In other patents, the use of N2 gas as a stripping agent is mentioned, but it is to be expected that in large-scale equipment the processing of large amounts of inert gases will be less economical compared to the use of vacuum systems. Some metal cations such as sodium and potassium in the feed increase racemization risk, while other metals (Al, Fe) are catalytically active in transesterification, resulting in competitive polylactide formation [68, 69]. Through corrosion, metals may be released in the residue and will build up there [6, 75]. Some patents discuss the presence of acid impurities in the process [6, 7, 67, 78]. Mono- and dicarboxylic fermentation acids are responsible for stoichiometric imbalance in the lactic acid polycondensation reaction. Consequently, the composition of the obtained lactic acid oligomer chains can differ from pure PLA, resulting in impeded and incomplete catalytic depolymerization of the oligomers into lactide. In PLA manufacture, degradation reactions play a role, mainly via intramolecular chain scission, and this may also affect lactide synthesis. Reviewing the literature provides a list of process aspects that need consideration in the design of a solventless synthesis operated with vacuum equipment. . . . . . . Temperature. Intrinsic reaction rates increase with temperature. At higher temperature also, the vapor pressure of lactide above the reaction mixtures increases. The reaction rate of racemization will also increase with temperature. In Witzke’s Ph.D. study, information on activation energies can be found [6]. Pressure. Pressures of 10 mbar or less are used. At higher pressures, the driving force for lactide evaporation will be lower, and the overall reaction rate will be lower. Low pressures will require detailed considerations of equipment size, vacuum systems, condensers, and so on. Feed DP. The feed DP has two effects. First, a low DP feed will contain more monomer lactic acid that boils at a lower temperature than lactide, and this will contaminate the crude lactide distilled off from the reactor. Also, monomer lactic acid can be released from DP 3 with the catalyst, leading to more acidity in the crude lactide. Second, it is to be expected that at a higher feed DP the residue in the reactor will have a higher DP and viscosity with consequences for equipment design. The influence of prepolymer DP on the meso-lactide level formed during lactide synthesis was discussed by Gruber et al. [69]. Increasing feed DP clearly resulted in a decrease in the lactic acid concentration in the crude lactide. A drawback is that the meso-lactide concentration also increased significantly. Catalyst Concentration. More catalyst will increase the overall reaction rate. In practice, this effect may not be linear, since next to kinetics mass transfer in the equipment will play a role. Racemization. In the production of stereochemically pure lactide, formation of the other lactic acid enantiomer and meso-lactide is unwanted. Higher temperatures, longer reaction times, and increased catalyst levels result in increased rates of racemization [4, 6, 69]. Since temperature and catalyst influence the rate of lactide formation as well, controlling the racemization rate can become quite complex. Impurities. Data in the literature on the role and fate of impurities from the feed in the synthesis are scarce. 13 On the one hand, it can be concluded that the lactide synthesis is straightforward in the sense of making a prepolymer and releasing lactide by thermal catalytic depolymerization at low pressure. On the other hand, it can be concluded that the scale-up from a lab-scale process to an economical, large-scale process with high yield and no compromises on stereochemical purity is a complex multifaceted task. 1.3.3 Purification of Lactide A lactide synthesis reactor invariably produces a crude lactide stream that contains lactic acid, lactic acid oligomers, water, meso-lactide, and further impurities. The specifications for lactide are stringent mainly for free acid content, water, and stereochemical purity. Basically, two main separation methods, distillation and crystallization, are currently employed for lactide purification: . Distillation. Splitting the multicomponent mixture consisting of lactide, water, lactic acid, and its oligomers into pure fractions requires considerable knowhow on kinetics and operation of vacuum equipment. Distillates and bottoms may be recycled, but the accumulation of impurities from the feed or the production of meso-lactide during the process requires careful finetuning of temperatures and residence times. Distillation is well described in the patent by Gruber et al. in 1993 [68]. The crude lactide from the synthesis is distilled in the first column to remove the acids and water, and then meso-lactide is separated from lactide in the second column. As the boiling points of all compounds are in the range of 200–300 C, low pressures are used. Since the difference in boiling temperature of lactide and meso-lactide is quite small, this distillation requires a lot of theoretical stages (30). The Cargill/ NatureWorks distillation uses a series of distillation
  • 35. 14 PRODUCTION AND PURIFICATION OF LACTIC ACID AND LACTIDE . . columns and is performed continuously [4]. Part of the distillation can also be integrated with the reaction [79]. Solvent Crystallization. A commonly used laboratory method for lactide purification is recrystallization from mixtures of toluene and ethyl acetate [4]. Lactide of extremely high purity can be obtained by repeated crystallization with different toluene/ethyl acetate ratios. Several patents also mention the use of solvents for the crystallization of lactide, but for large scale, melt crystallization without the use of solvents is preferred. Melt Crystallization. Lactide crystallizes easily and several patents describe how crystallization can yield lactide with required specifications regarding lactic acid content, oligomers, meso-lactide, and water. An early patent describes such a crystallization method and includes some information on the thermodynamic equilibria (eutectica) of the lactide/lactic and the lactide–meso-lactide system, which define the maximum yield as a function of these impurities in the feed [80]. In patents, the use of different types of equipment is mentioned: static equipment, falling film crystallizers, vertical column with scraper to remove crystal mass from the cooled wall, and scraped heat exchanger coupled to a wash column [70, 80, 81]. For large scale, it is a challenge to design and scale-up the crystallization equipment with respect to the needed heat transfer areas and hydrodynamics, and the possible increase of viscosity of mother liquor by oligomerization of lactide and residual acid. The choice between distillation, crystallization, or novel separation methods such as absorption or membrane separation is determined by the desired stereochemical purity of the product. Crystallization yields highly pure lactide, suitable, for example, for high-melting PLLA homopolymer of high molecular weight. Affordable distillation equipment does not fully remove all meso-lactide, and consequently, a lactide monomer mixture for PLA copolymers with other thermal properties is obtained upon ring-opening polymerization. The design of the separation system relies on detailed knowledge of the thermodynamic properties of the compounds and the kinetics of the reactive system. Obtaining FIGURE 1.8 such know-how requires sophisticated analytical methods for lactic acid and its oligomers, lactides, and residues. Impurities can also be formed in lactide synthesis, similar to PLA degradation reactions, and gas chromatography (GC) methods are needed to identify these compounds and determine their fate in the process. 1.3.4 Quality and Specifications of Polymer-Grade Lactide The specifications and allowed impurity levels of lactide monomer for PLA are defined by the polymerization mechanism and the applied catalyst. PLA is commercially produced by ROP of lactides in bulk. The tin(II)-catalyzed process offers good control over molecular weight and reaction rate provided that it is performed in the absence of impurities such as water, metal ions, lactic acid, or other organic acids. Purification of crude lactides is therefore indispensable for the industrial manufacture of high molecular weight PLA (Mw 100 kg/mol). In fact, lactide is the ultimate form of lactic acid, in its dehydrated and purest form. 1.3.4.1 Role of the Catalyst and Initiator in Lactide Polymerization The theoretical description of the Sn(Oct)2-catalyzed ROP of cyclic esters has been studied by many authors, but there does not appear to be a theory that consistently explains all experimental results of the coordination–insertion polymerization [3, 4, 82–84]. Different polymerization mechanisms may dominate, depending on polymerization conditions, catalyst and initiator concentration, and the presence of a solvent. Here it is assumed that lactide is polymerized in bulk with Sn(Oct)2—a Lewis acid—and that the mechanism follows the model proposed by Kowalski et al. [84]. Since lactide is a cyclic ester, its ring can be opened by nucleophilic attack on the ester bond to start polymerization. Suitable initiators (nucleophiles) are water and alcohols, including the hydroxyl group of lactic acid. One ester linkage of a lactide ring is cleaved by reaction of the OH group of the initiator R-OH, creating a new R-O-C(O)- ester end group and an OH end group (Figure 1.8). Every initiating molecule is covalently bonded as an end group to each polymer chain [84]. Via transesterification Ring-opening polymerization of lactide to PLA initiated by an alcohol.
  • 36. LACTIDE FIGURE 1.9 Equilibrium reaction of tin octoate with alcohol initiator or impurities to form catalytically active tin alkoxide bonds Sn-O-R [76]. reactions, the 2-ethylhexanoate ligands of the SnOct2 catalyst will also end up as octanoic ester groups in the polymer. In some papers, the Sn(II) catalyst is indicated as the initiator, presumably because lactide also polymerizes upon addition of that substance, and the effect of impurities is overlooked. An initiator—or coinitiator—is a substance that can start polymerization, in the case of lactide by opening the lactide ring, and thus offers control over molecular weight. This has to be a nucleophile and cannot be the Sn catalyst itself, as supported by the excellent work of Kowalski et al. who proved that SnOct2 needs activation with R-OH (Figure 1.9) [84]. In a nutshell, the total hydroxyl content, including R-OH initiator and lactic acid impurities, determines the maximum attainable Mn (number-average molecular weight) [4, 6]. The rate of polymerization is controlled by factors such as temperature and catalyst content, with the remark that a tin (II) octoate catalyst requires traces of the initiator to become active. 1.3.4.2 Alcohols If water is the initiator, R equals H and hydrolysis of lactide produces lactoyl lactic acid (HL2). Propagation with lactide in the presence of a polymerization catalyst produces PLA with a hydroxyl and one carboxylic acid end group, as if the PLA was obtained by polycondensation of lactic acid. If the hydroxyl group of lactic acid acts as an initiator, PLA with one hydroxyl end group and a lactic acid end group (HOOC-CH(CH3)-O-C(O)-) is obtained. If the initiator itself is polymeric in nature, for example, polyethylene glycol (PEG), lactide can polymerize from the hydroxyl end group(s) of PEG resulting in PEG–PLLA diblock or triblock copolymers. The molar ratio of monomer to initiator (M/I)—where initiator can also be read as total hydroxyl content—basically controls the final, average molecular weight (Mn) of the PLA. A high amount of initiator produces short polymer chains, and a low amount of initiator produces high molecular weight polymer. The lower the amount of potentially initiating hydroxyls in the lactide monomer, the higher the maximum attainable degree of polymerization [69]. Since water and lactic acid can both cause ring scission of the lactide and initiate polymerization, their amounts in the lactide must be low and should be specified. 1.3.4.3 Carboxylic Acids Carboxylic acids are poor initiators, but they are believed to interfere with the commonly used Sn(II) polymerization catalyst. According to Kowalski, 15 carboxylic acids may suppress the rate of polymerization by shifting the equilibrium between ROH and Sn(Oct)2 to the inactive Sn(Oct)2 side [83, 84]. Consequently, longer polymerization times are needed to achieve the desired molecular weight, accompanied by unavoidable degradation caused by the extra residence time at high temperature in the presence of a catalyst [84]. The effect of carboxylic acids on lactide polymerization rate was published in 1993 in patents by Ford and O’Brien [78, 85]. The results clearly show the dramatic rate-decreasing effect of organic acids: according to O’Brien, melt polymerization slows down by a factor of 2 upon increasing free acidity from less than 2 to between 2 and 4 meq/kg [85]. Witzke, however, states that the presence of lactic acid did not negatively influence polymerization rate [4, 6]. Lactic acid is therefore a practically used initiator that is already present in lactide as an impurity. Lactic acid and its oligomers have a hydroxyl group and a carboxylic acid group. Consequently, a free acidity of 10 meq/kg—that is, 900 ppm expressed as lactic acid equivalents—in lactide corresponds to a hydroxyl concentration that limits Mn to 100 kg/mol. Free acidity of 4 meq/kg sets a theoretical limit of 250 kg/mol to Mn. Free acid and water content specifications are essential for any lactide grade; the lower the amount of hydroxyl impurities, the better the storage stability and product properties of the lactide. 1.3.4.4 Metals Metal cations such as Sn, Zn, Fe, Al, and Ti not only accelerate polymerization, but can also affect hydrolysis, oxidation, racemization, or other degradation mechanisms of PLA and lactides [4, 6]. Consequently, the lactic acid used for lactide preparation should be very low (ppm) in metal cations in order to avoid considerable racemization during lactide synthesis. O’Brien has shown that the formation of dark color of lactide was a direct function of the iron content of the material in which the lactide was in contact [86]. Other examples in the patent (Examples 7 and 8) demonstrate the desirability of having low alkali (e.g., sodium) content and minimizing the depolymerization temperature. Cationic impurities such as sodium ions have no direct effect on lactide production rate, but the sodium content has a direct correlation with the meso-lactide content in the crude lactide [67, 87]. 1.3.4.5 Stereochemical Purity The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity and crystallization rate, and mechanical strength [8, 9, 88]. The strong dependence on D-isomer content presents an opportunity to control polymer properties. NatureWorks

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