Biopolymers E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 622

Veröffentlichungsjahr: 2011

Beschreibung

As an area of high topical interest, Biopolymers – New materials for Sustainable Films and Coatings covers the development and utilization of polymers derived from bioresources, with a particular focus on film and coating applications.

With growing concern for the environment and the rising price of crude oil, there is increasing demand for non-petroleum-based polymers from renewable resources. Leading research groups worldwide in industry and academe are working on such technology with the objective of applying the latest advances in the field.

Written by well-respected experts, this text systematically covers the extraction and production of selected biopolymers as well as their properties and application as films or coatings in a variety of uses. The areas addressed include food packaging, edible coatings, paper coatings and agricultural films.

Intended for researchers and students, this book will also be of interest to industry, especially in terms of the practical applications.

Leseprobe

Contents

Cover

Title Page

Copyright

Preface

About the Editor

List of Contributors

Part I

Chapter 1: Introductory Overview

1.1 Introduction

1.2 Worldwide Markets for Films and Coatings

1.3 Sustainability

1.4 Bio-Derived Polymers

1.5 Other Topics

References

Chapter 2: Production, Chemistry and Degradation of Starch-Based Polymers

2.1 Introduction

2.2 Gelatinization

2.3 Effect of Gelatinization Process and Plasticizer on Starch Properties

2.4 Retrogradation

2.5 Production of Starch–Polymer Blends

2.6 Biodegradation of Starch-based Polymers

2.7 Concluding Remarks

2.8 Acknowledgement

References

Chapter 3: Production, Chemistry and Properties of Polylactides

3.1 Introduction

3.2 Production of Polylactides

3.3 Polylactide Chemistry

3.4 Properties of Polylactides

3.5 Concluding Remarks

References

Chapter 4: Production, Chemistry and Properties of Polyhydroxyalkanoates

4.1 Introduction

4.2 Polyhydroxyalkanoate Synthesis

4.3 Properties of Polyhydroxyalkanoates

4.4 Polyhydroxyalkanoate Degradation

4.5 PHA-Based Multiphase Materials

4.6 Production and Commercial Products

References

Chapter 5: Chitosan for Film and Coating Applications

5.1 Introduction

5.2 Physical and Chemical Characterization of Chitosan

5.3 Properties and Applications of Chitosan

5.4 Processing of Chitosan

5.5 Concluding Remarks

References

Chapter 6: Production, Chemistry and Properties of Proteins

6.1 Introduction

6.2 Plant-Based Proteins

6.3 Animal-Based Proteins

6.4 Solution Casting of Proteins – an Overview

6.5 Dry Forming of Protein Films

6.6 Concluding Remarks

References

Chapter 7: Synthesis, Chemistry and Properties of Hemicelluloses

7.1 Introduction

7.2 Structure

7.3 Sources

7.4 Extraction Methodology

7.5 Modifications

7.6 Applications

7.7 Concluding Remarks

References

Chapter 8: Production, Chemistry and Properties of Cellulose-Based Materials

8.1 Introduction

8.2 Pristine Cellulose as a Source of New Materials

8.3 Novel Cellulose Solvents

8.4 Cellulose-based Composites and Superficial Fiber Modification

8.5 Cellulose Coupled with Nanoparticles

8.6 Electronic Applications

8.7 Biomedical Applications

8.8 Cellulose Derivatives

8.9 Concluding Remarks

References

Chapter 9: Furan Monomers and their Polymers: Synthesis, Properties and Applications

9.1 Introduction

9.2 Precursors and Monomers

9.3 Polymers

9.4 Biodegradability of Furan Polymers

9.5 Concluding Remarks

References

Part II

Chapter 10: Food Packaging Applications of Biopolymer-Based Films

10.1 Introduction

10.2 Food Packaging Material Specifications

10.3 Examples of Biopolymer Applications for Food Packaging Materials

10.4 Research Directions and Perspectives

10.5 Concluding Remarks

References

Chapter 11: Biopolymers for Edible Films and Coatings in Food Applications

11.1 Introduction

11.2 Materials for Edible Films and Coatings

11.3 Edible Films and Coatings for Food Applications

11.4 Concluding Remarks

References

Chapter 12: Biopolymer Coatings for Paper and Paperboard

12.1 Introduction

12.2 Biopolymer Films and Coatings

12.3 Bio-nanocomposite Films and Coatings

12.4 Concluding Remarks

12.5 Acknowledgement

References

Chapter 13: Agronomic Potential of Biopolymer Films

13.1 Introduction

13.2 The Potential Role of Biodegradable Materials in Agricultural Films

13.3 Presently Available Biopolymers and Biocomposites

13.4 Past and Current International Projects on Biodegradable Agricultural Films

13.5 Present Applications of Biopolymer Films in Agriculture

13.6 Potential Uses: Current Limitations and Future Applications

13.7 Concluding Remarks

13.8 Acknowledgements

References

Chapter 14: Functionalized Biopolymer Films and Coatings for Advanced Applications

14.1 Introduction

14.2 Optoelectronics

14.3 Sensors

14.4 Miscellaneous Applications

14.5 Concluding Remarks

References

Chapter 15: Summary and Future Perspectives

15.1 Introduction

15.2 Bioplastics

15.3 Bio-Thermoset Resins

15.4 Nanocomposites Based on Inorganic Nanofillers

15.5 Nanocomposites Based on Cellulose Nanofillers

15.6 Concluding Remarks

References

Index

This edition first published 2011

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Library of Congress Cataloging-in-Publication Data

A catalogue record for this book is available from the British Library.

Print ISBN: 9780470683415

ePDF ISBN 9781119994329

oBook ISBN: 9781119994312

ePub ISBN: 9781119995791

eMobi ISBN: 9781119995807

Preface

Since you have picked up this book and are now reading the preface, chances are that you have a special interest in the science and technology of new materials from bioresources. Perhaps you are a university student, research scientist, university professor or are involved in technology development in industry. Whatever the case, I hope that this volume will have some content of real value to you. As a reader of this book, you are likely to be keenly interested in the future prospects for a world in which we can become less dependent on fossil fuels for energy and materials, take steps to dramatically reduce greenhouse gas emissions and efficiently address the significant challenges associated with plastic waste in the global environment. The bio-derived polymers discussed in this book and their applications in packaging products and beyond can, as suggested by the various authors, provide part of the solution to these problems.

The invitation to prepare this book came from the publisher after a colleague and I had produced a journal article reviewing research on films and coatings from hemicelluloses, a widely available but relatively underutilized component of most biomass. After accepting the offer to edit this book, I decided to aim for chapters combining state-of-the-art summaries of our knowledge about individual biopolymers and complementary chapters written from the perspective of key applications in films and coatings. With the help of all the chapter authors, that structure is basically what you find here. There were a few challenges along the way, but not too many! At least one author remarked that various chapters might overlap and there is some truth to this comment; however, I think this has not turned out to be a significant problem. Where overlapping comments do occur, this is mostly related to the topic of packaging, which is easily the largest commodity market for bio-derived plastics. Of the original 13 authors invited to contribute, I am pleased to say that 12 of them stayed with the process to the very end and were relatively prompt in sending me their input, for which I am indeed grateful. One of the authors did thank me for my “infinite patience” – an indicator that scientists do naturally have other priorities as well as writing book chapters!

There are already a number of fine texts that comprehensively cover the subject of polymers from renewable resources in great detail, but it is hoped that this update covering key bio-derived polymers, combined in a unique way with a discussion on their use in films and coatings, will also make a contribution to this rapidly growing scientific field. It has been a privilege to work with the other chapter authors on this book and a real pleasure to read and edit their contributions, which I think have captured the essence of each topic. As a final tribute, I would like to express my sincere gratitude to Sarah Tilley and her colleagues at Wiley as well as Sarah's predecessors in managing this book.

David Plackett Ris DTU Roskilde Denmark 13 December 2010

About the Editor

David Plackett holds a PhD in Chemistry from the University of British Columbia in Canada and has held research and research management positions in various companies, research institutes and universities in the UK, Canada, New Zealand and Denmark. He has a career background in bio-based materials research and since 2002 he has been Senior Scientist and Biopolymers group leader at RisØ National Laboratory for Sustainable Energy, part of the Technical University of Denmark (DTU) located near Roskilde. Dr Plackett has more than 60 peer-reviewed publications and his research interests currently include the production and characterization of bioderived polymers and their property enhancement through the use of nanotechnology.

List of Contributors

Ann-Christine Albertsson, Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden.

H. Angellier-Coussy, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

Christian Aulin, Innventia AB, Stockholm, Sweden, and Wallenberg Wood Science Center, Royal Institute of Technology, Stockholm, Sweden.

Luc AvÉrous, LIPHT-ECPM, UniversitÉ de Strasbourg, Strasbourg, France.

Mohamed Naceur Belgacem, Grenoble INP-Pagora, St. Martin d'Ères, France.

Juan Ignacio MatÉ Caballero, Department of Food Technology, College of Agricultural Engineering, Universidad PÚblica de Navarra, Pamplona, Spain.

P. Chalier, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

Viviana Cyras, Research Institute of Materials Science and Technology, Faculty of Engineering, National University of Mar del Plata, Argentina.

Ulrica Edlund, Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden.

Idoya FernÁndez-Pan, Department of Food Technology, College of Agricultural Engineering, Universidad PÚblica de Navarra, Pamplona, Spain.

Patricia FernÁndez-Saiz, Novel Materials and Nanotechnology Group, IATA-CSIC, Paterna, Spain.

MarÍa Laura Foresti, Institute of Engineering in Technology and Science, Faculty of Engineering, University of Buenos Aires, Argentina.

Mikael GÄllstedt, Innventia AB, Packaging Solutions, Stockholm, Sweden.

Alessandro Gandini, CICECO and Chemistry Department, University of Aveiro, Aveiro, Portugal.

E. Gastaldi, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

N. Gontard, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

V. Guillard, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

C. Guillaume, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

Mikael S. Hedenqvist, Royal Institute of Technology, Department of Fiber and Polymer Technology, Stockholm, Sweden.

Saara Inkinen, Laboratory of Polymer Technology, Åbo Akademi University, Turku, Finland.

Vimal Katiyar, RisØ National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmark.

JosÉM. Lagaron, Novel Materials and Nanotechnology Group, IATA-CSIC, Paterna, Spain.

Tom LindstrÖm, Innventia AB, Stockholm, Sweden, and Wallenberg Wood Science Center, Royal Institute of Technology, Stockholm, Sweden.

LluÍs MartÍn-Closas, Department of Horticulture, Botany and Gardening, University of Lleida, Lleida, Spain.

Ana M. Pelacho, Department of Horticulture, Botany and Gardening, University of Lleida, Lleida, Spain.

S. Peyron, Joint Research Unit Agro-polymers Engineering and Emerging Technologies, UniversitÉ Montpellier II, Montpellier, France.

David Plackett, RisØ National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmark.

Eric Pollet, LIPHT-ECPM, UniversitÉ de Strasbourg, Strasbourg, France.

Anders SÖdergÅrd, Laboratory of Polymer Technology, Åbo Akademi University, Turku, Finland.

Hasan Ture, Royal Institute of Technology, Department of Fiber and Polymer Technology, Stockholm, Sweden.

Indra K. Varma, Centre for Polymer Science and Engineering, Indian Institute of Technology, Delhi, India.

AnalÍa VÁzquez, Institute of Engineering in Technology and Science, Faculty of Engineering, University of Buenos Aires, Argentina.

Part I

Chapter 2

Production, Chemistry and Degradation of Starch-Based Polymers

Analía Vázquez and María Laura Foresti

Institute of Engineering in Technology and Science, Faculty of Engineering, University of Buenos Aires, Argentina

Viviana Cyras

Research Institute of Material Science and Technology, Faculty of Engineering, National University of Mar del Plata, Argentina

2.1 Introduction

Starch is the most abundant reserve polysaccharide in plants and as such is a renewable resource with many practical uses. Starch is biodegradable, produced in abundance at low cost and can exhibit thermoplastic behavior. The botanical sources of starch are seeds, roots and tubers, in which starch occurs as an organized structure called granules. Starch granules are insoluble in cold water. The main commercial sources of starch are maize, potato and tapioca; however, there are a significant number of species that have high starch contents, including legumes, grain (e.g., amaranth) and various nuts. Starch properties depend on the source, granule size distribution and morphology, genotype, amylose/amylopectin ratio and other factors such as composition, pH, and any chemical modifications. In starch-based products, gelatinization and reorganization behavior control the texture and stability of the final product [1–6].

Starch granules have two components, amylose and amylopectin, consisting of α(1→4) linked D-glucose units. Amylose is linear, whereas amylopectin is a highly branched polymer with 5–6% of α(1→6) linkages. A schematic representation of structural levels of the starch granule and the involvement of amylose and amylopectin is shown in Figures 2.1a and Figure 2.1b. The ratio between amylose and amylopectin varies depending on the starch source. In many starches, amylose constitutes about 15–30% by weight (Table 2.1).

Figure 2.1a (a) Schematic representation of structural levels in the starch granule [3]. Reprinted from A. Buléon, P. Colonna, V. Planchot, et al., Starch granules: structure and biosynthesis, Int. J. Biol. Macromol., 23, 85–112. Copyright (1998) with permission from Elsevier

Figure 2.1b Starch granule structure showing organization in semi-crystalline and crystalline shells, blocklet structure in association with amorphous channels, internal blocklet structure and the crystal structures of starch [7]. Reprinted from D. Gallant, B. Bouchet and P. Baldwin, Microscopy of starch: evidence of a new level of granule organization, Carbohydr. Polym., 32, 177–191. Copyright (1997) with permission from Elsevier

Starch granules exhibit a crystalline/amorphous structure, in which amylopectin is the major crystalline constituent, while amylose and the amylopectin branches form the amorphous part. The native structure of starch is made of helices that are more or less organized radially forming a granule, with concentric rings representing semi-crystalline shells separated by essentially amorphous regions. The x-ray scattering study of crystallites shows that helices are organized according to two main crystalline lattices: an A-type allomorph mainly found in cereal starches and a B-type allomorph found in tubers and amylose-rich starches. A C-type structure consisting of a mixed organization of A and B crystalline forms also occurs naturally, such as in pea and bean starches [3, 9–11]. There is also a V-type conformation which is a result of amylose being complexed with substances such as fatty acids or emulsifiers. The main difference between A and B types is that the former adopt a close-packed arrangement with water molecules between each double helical structure while the B-type is more open with more water molecules. . The various starch types can be distinguished by their different x-ray diffraction (XRD) patterns (Figure 2.2).

Figure 2.2 X-ray diffractograms for starches of different types [12]. Reprinted from J.J.G. van Soest and J.F.G. Viegenthart, Crystallinity in starch plastics: consequences for material properties, Trends Biotechnol., 15, 208–213. Copyright (1997) with permission from Elsevier

Table 2.1 Amylose and amylopectin content, granule size and crystallinity for different starches [14]. Reprinted from L. Avérous, Biodegradable multiphase systems based on plasticized starch: A review, J. Macromol. Sci. Polym. Rev. C44, 231–274. Copyright (2004) with permission from Taylor & Francis.

Starch granules contain minor components, of which lipids are the most important fraction. The main surface constituents are proteins, enzymes, amino acids and nucleic acids. Some components can be extracted without granule disruption, for example approximately 10% of proteins and 10–15% of lipids. Triglycerides represent a major fraction of the surface lipids of maize and wheat. The presence of internal lipids is a characteristic of cereal starches. In contrast to tubers and legume starches, cereal starches are characterized by the presence of monoacyllipids and lysophospholipids in amounts positively correlated to amylose content [12].

2.2 Gelatinization

Gelatinization is an irreversible order–disorder transition, which occurs when starch is heated in the presence of water over a temperature range characteristic of the particular starch source. The gelatinization process in excess water involves primary hydration of amorphous regions. This in turn facilitates molecular mobility in the amorphous regions (swelling) which then provokes an irreversible molecular transition. This irreversible step involves dissociation of double helices and a radial expansion of granules, loss of birefringence, uptake of heat, loss of crystalline order, uncoiling and dissociation of double helices and amylose leaching. Heat is taken up, according to the characteristic gelatinization endotherm and can be measured using differential scanning calorimetry (DSC). The gelatinization transition temperatures, To (onset), Tp (mid-point), Tc (conclusion) and the enthalpy of gelatinization (ΔH) are influenced by the molecular architecture of the crystalline region. The temperature at which the gelatinization process starts depends on the starch concentration.

Various techniques are used to study starch gelatinization. Birefringence is employed to follow the size of the granule during the gelatinization process. XRD can be used to study the crystallinity of starch and Fourier transform infrared spectroscopy (FTIR) has been used to describe the organization and structure of starch at various moisture contents. Amorphous starch can be characterized by an IR absorbance band at ∼ 1022 cm−1 and the crystalline state identified by the development of a band at 1047 cm−1 [15]. DSC measurements can also be used to study the loss of order that takes place during gelatinization. Amylose chains are released during gelatinization and these chains can be determined colorimetrically through formation of a blue complex with iodine [16]. Gelatinization affects the dielectric properties of the starch–water system and therefore conductance measurements can also be used to monitor the process [17]. Starch gelatinization can be followed by viscosity measurements because the viscosity of the starch–water mixture changes during the process due to swelling of the granules.

2.3 Effect of Gelatinization Process and Plasticizer on Starch Properties

Thermoplastic starch can be obtained from native starch by disruption of starch granules and plasticization. This process occurs through the transformation of granules into a homogeneous material with the destruction of hydrogen bonds between the starch molecules and with the formation of hydrogen bonds between added plasticizer and starch molecules. Disruption can be achieved in the presence of an appropriate plasticizer by applying heat and shearing in a continuous process such as extrusion to obtain a homogeneous molten phase. The degree of gelatinization depends on the content and type of plasticizer and on processing parameters such as shear stress, melt viscosity, time and temperature [12].

Plasticizers such as glycerol, glycerol monostearate, glycol, xylitol, sorbitol, polyethylene glycol, sugars or oligosaccharides, fatty acids, lipids and derivatives are used to overcome film brittleness and to improve flexibility and extensibility. Small molecules containing the –CO–NH– functional group such as urea and formamide or formamide/urea mixtures can also plasticize native starch. The water resistance of formamide-plasticized thermoplastic starch is slightly improved relative to traditional glycerol-plasticized starch [1, 18–22].

The glass transition temperature (Tg) of starch–plasticizer systems is a function of plasticizer content. As an example, Figure 2.3 shows the effect of glycerol and water on starch Tg. Glycerol addition increases the toughness and strength of the materials due to strong hydrogen bonding with starch molecules [12].

Figure 2.3 Starch glass transition temperatures as a function of glycerol and water content [12]. Reprinted from J.J.G. van Soest and J.F.G. Viegenthart, Crystallinity in starch plastics: consequences for material properties, Trends Biotechnol. 15, 208–213. Copyright (1997) with permission from Elsevier

Famá and co-workers (2006) observed that introducing antimicrobial potassium sorbate in edible films of cassava starch and glycerol resulted in a decrease in the degree of crystallinity and an increase in moisture content (Table 2.2) [23]. The increase in sorbate content displaced the Tg of a glycerol-rich phase toward lower temperatures. The decrease in crystalline fraction with antimicrobial increase is associated with a corresponding increase in moisture content.

Table 2.2 Moisture content and crystallinity of glycerol (33%)/cassava starch films two weeks after gelatinization when equilibrated at 57.5% humidity and 25 °C [23]. Reprinted from L. Famá, S. Flores, L. Gerschenson, et al., Physical characterization of cassava starch biofilms with special reference to dynamic mechanical properties at low temperatures, Carbohydr. Polym. 66, 8–15. Copyright (2006) with permission from Elsevier.

Sorbate content (g/100 g starch)Crystallinity (%)Moisture content (g/100 g starch)033.4251.2824.222.32.1922.2382.6917.944.9

Da Róz et al (2006) studied the effect of different additives on corn starch containing 28% amylose [24]. Ethylene glycol, propylene glycol, 1,4-butanediol, diethylene oxide glycol and sorbitol (shorter glycols) were effective in destructuring and plasticizing starch. Ethylene glycol was the most effective of these compounds. On the other hand, addition of 1-hexanol, 1-octanol, 1-dodecanol, 1-octadecanol, 1,6-hexanediol, 2,5-hexanediol, ethylene glycol monomethyl ether, polyethylene oxide glycol, polypropylene oxide glycol (high-molecular-weight glycols) did not plasticize starch. The stress–strain curves obtained by Da Róz et al. (2006) are shown in Figure 2.4. The plots are linear at low strains, showing high modulus values, thus displaying the typical behavior of semi-crystalline materials. With ethylene glycol (EG) and propylene glycol (PG) as plasticizers, the modulus increased with increasing plasticizer content, except for ethylene glycol contents above 30 wt %, for which the modulus was affected in two opposite ways by the plasticizer, involving a decrease induced by plasticization and an increase caused by a corresponding increase in crystallinity. With 1,4-butanediol (BUT), diethylene oxide glycol (DEG) and D-sorbitol (SOR), the modulus decreased as the plasticizer content increased, which suggests that for these additives the first effect prevailed.

Figure 2.4 Stress–strain curves of plasticized starch [24]. Reprinted from A.L. Da Róz, A.J.F. Carvalho, A. Gandini, et al., The effect of plasticizers on thermoplastic starch compositions obtained by melt processing, Carbohydr. Polym. 63, 417–424. Copyright (2006) with permission from Elsevier

2.4 Retrogradation

Gelatinized starches suffer ageing during storage and cooling and there is a tendency for amylose and amylopectin to interact forming a more ordered structure. These molecular interactions are called retrogradation [25]. The retrogradation properties of starches are indirectly influenced by the structural arrangement of starch chains in the amorphous and crystalline regions of the non-gelatinized granule, which influences the degree of granule breakdown during gelatinization and the interactions that occur between the starch chains during gel storage [26]. In the work of Karim et al. (2000), the transition temperatures of retrogradation were found to be lower than the gelatinization temperatures. This could be because recrystallization causes a lower order in amylopectin chains than is present in native starch [27].

Starch retrogradation is accompanied by increases in the degree of crystallinity and gel firmness, exudation of water and the appearance of a ‘B-type’ x-ray pattern [25, 28]. Since starch retrogradation is a kinetically controlled process, the alteration of time, temperature, and water content during processing can produce a variety of end-products [29]. After processing, the properties of the metastable starch–water system can also be influenced by moisture content, the botanical source of starch, storage time, and storage temperature. At lower concentrations, water acting as a plasticizer is well known to affect the Tg of semi-crystalline polymers. Water acts as a plasticizer of amorphous and partially crystalline starch systems and, as a result, water content influences the Tg and therefore also the properties, processing and stability of many starch-based foods [32]. When a stored starch gel is reheated in a DSC experiment, an endothermic transition occurs that is not present in the DSC scan of a recently gelatinized sample. This transition is generally attributed to the melting of recrystallized amylopectin.

Many research groups have studied the use of polysaccharides to control the retrogradation rate of starch-based products, including mixtures of amylose and dextran [33], amylose and galactomannan [34], potato starch and maltodextrin, and potato starch and xanthan gum [35]. These formulations enhance the interaction between starch granules, promoting retrogradation. Lii et al. (1998) reported that rice starch (33% wt/wt) in the presence of maltodextrins (5–20%) with a high average degree of polymerization strongly promoted retrogradation [36]. The retrogradation of wheat starch (30% wt/wt) was advanced by higher molecular weight chains of amylopectin [37]. The retrogradation rate of tapioca starch increased in the presence of xyloglucan [38].

Many methods including rheological techniques, sensory evaluation of texture, DSC, turbidimetry, XRD, nuclear magnetic resonance spectroscopy (NMR), Raman spectroscopy and FTIR spectroscopy have been used to study retrogradation [27]. Syneresis data and turbidity measurements yield information on both amylose and amylopectin crystallization, whereas DSC and NMR can provide information on amylopectin crystallization and changes in water mobility during retrogradation respectively. Sometimes, annealing-type processes are confused with retrogradation. However, annealing of starch granules is a process that retains granular structure and original order. Retrogradation occurs as amorphous α-glucan chains form double helices and, perhaps eventually, align themselves in crystallites.

2.5 Production of Starch–Polymer Blends

Starch-based plastics have some negative aspects, including limited stability caused by water absorption, ageing-induced retrogradation, inferior mechanical properties and poor processability. To overcome these limitations, the use of plasticized starch with other biodegradable polymers has been explored as a way to obtain low-cost, compostable materials.

Blending thermoplastic starch and other biodegradable polymers permits: (i) reduction in production costs; (ii) adjustable rates of degradation; (iii) a combination of properties derived from those of the individual polymers; and (iv) improved mixing during processing. Mixing is the process by which a combination of materials is made homogeneous. The quality of mixing can be defined by the intensity of segregation and the scale of segregation of the components. The scale of segregation can be related to the interfacial area and the intensity of segregation to the local concentration gradients as illustrated schematically in Figure 2.5.

Figure 2.5 Changes in the quality of mixing of two components by decreasing the intensity and scale of the segregation

A homogeneous mixture is obtained when the physical or chemical properties do not vary within the mixture. The rate of change of the intensity of segregation as depicted in Figure 2.5 will depend on diffusion coefficients. It is also necessary to know the characteristic length scales in the mixing field (i.e., the scale of segregation). Some authors have defined other variables including exposure or the potential to reduce the segregation, a nonlinear function of the scale of segregation [39]. The decrease in the scale of segregation occurs when a break point occurs, which is dependent on the viscous forces acting on a drop surface overcoming the stabilizing surface tension [40].

Homogeneity also depends on the level at which it is considered. When the dimensions of any inhomogeneity increase, the blend approaches a composite material in which the phases have clearly different properties [41]. Starch can be modified by mixing with other polymers, by adding reinforcements or by forming multilayer structures (Figure 2.6). However, in this chapter we will only discuss blends of starch with different polymers.

Figure 2.6 Scheme showing various starch-based materials [14]. Reprinted from L. Avérous, Biodegradable multiphase systems based on plasticized starch: A review, J. Macromol. Sci. Polym. Rev. C44, 231–274. Copyright (2006) with permission from Taylor & Francis

When two polymers are dissolved in a suitable solvent to produce a low viscosity solution, mixing can be done by means of an agitator, turbine, or mixer with blades or helix, which promote convective mixing. Films can then be obtained from solution by casting or solvent evaporation. However, when two polymers are mixed with zero or low solvent content, viscosity increases and an intensive mixer (laboratory scale), twin-screw extruder or co-kneader (industrial scale) must be used. These types of equipment can develop the high shear forces which are necessary for full mixing. As indicated, a break point occurs when shear force reaches a particular value and the final material shape will depend on interfacial tension values. Figure 2.7