Starch-based Blends, Composites and Nanocomposites: Volume 37 (Green Chemistry Series) - Hardcover

Hardcover

ISBN 10: 184973979X ISBN 13: 9781849739795

Publisher: Royal Society of Chemistry, 2015

Synopsis

A comprehensive overview of recent developments in starch-based materials, ideal for students and researchers working in bio-based and biodegradable polymers and composites.

About this title

From the Back Cover

Starch is one of the most widely available natural biomaterials and is commonly used in biodegradable packaging. This book provides a comprehensive overview of recent developments in starch-based materials.

The book focuses on the types of starch available from different sources, in particular the various aspects of preparation, structure, processing, morphology, properties and applications of starch materials and their polymer blends, composites and nanocomposites. It is ideal for students and researchers in chemistry, polymer science, materials science, biotechnology and life sciences working in bio-based and biodegradable polymers and composites, well as those interested in its applications.

From the Inside Flap

Starch is one of the most widely available natural biomaterials and is commonly used in biodegradable packaging. This book provides a comprehensive overview of recent developments in starch-based materials.

The book focuses on the types of starch available from different sources, in particular the various aspects of preparation, structure, processing, morphology, properties and applications of starch materials and their polymer blends, composites and nanocomposites. It is ideal for students and researchers in chemistry, polymer science, materials science, biotechnology and life sciences working in bio-based and biodegradable polymers and composites, well as those interested in its applications.

Excerpt. © Reprinted by permission. All rights reserved.

Starch-based Blends, Composites and Nanocomposites

By P. M. Visakh, Long Yu

The Royal Society of Chemistry

Copyright © 2016 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-979-5

Contents

Chapter 1 Starch: State-of-the-Art, New Challenges and Opportunities Visakh P. M., 1, 
Chapter 2 Starch: Introduction and Structure-Property Relationships Khongsak Srikaeo, 17, 
Chapter 3 Preparation and Characterization of Starch Nanocrystals Mehran Ghasemlou, Seyed Mohammad Taghi Gharibzahedi and Marlene J. Cran, 60, 
Chapter 4 Natural Fibre-reinforced Thermoplastic Starch Composites J. Sahari, S. M. Sapuan, Y. A. El-Shekeil, M. R. Ishak and R. Akhtar, 109, 
Chapter 5 Applications of Starch Nanocrystal-based Blends, Composites and Nanocomposites Verônica M. A. Calado and Andresa Ramos, 143, 
Chapter 6 Chemical Modification of Thermoplastic Starch Antonio José Felix Carvalho, 217, 
Chapter 7 Outstanding Features of Starch-based Hydrogel Nanocomposites Antonio G. B. Pereira, André R. Fajardo, Artur J. M. Valente, Adley F. Rubira and Edvani C. Muniz, 236, 
Chapter 8 Starch-based Blends Jiwei Li and Xuegang Luo, 263, 
Chapter 9 Fracture and Failure of Starch-based Composites Celina R. Bernal, 326, 
Chapter 10 Application of Starch Nanocomposites in the Food Industry Khongsak Srikaeo, 352, 
Chapter 11 Effects of Additives on the Properties of Starch Wei Wang, Hong Yang and Min Cui, 403, 
Subject Index, 433, 

CHAPTER 1

Starch: State-of-the-Art, New Challenges and Opportunities

VISAKH P. M.

Tomsk Polytechnic University, Lenin Av. 30, 634050 Tomsk, Russia Email: visagam143@gmail.com

1.1 Starch: Introduction and Structure–Property Relationships

Starch is a polysaccharide consisting of D-glucose units, referred to as homoglucan or glucopyranose, and has two major biomacromolecules – amylose and amylopectin. Amylopectin is a much larger molecule than amylose, with a molecular weight of 1x107–1x109 and a heavily branched structure built from about 95% α-(1[right arrow]4) and 5% α-(1[right arrow]6) linkages. Amylopectin unit chains are relatively short compared with amylose molecules, with a broad distribution profile. Starch varieties contain primarily two different types of anhydroglucose polymers, amylase and amylopectin.

Both amylose chains and the exterior chains of amylopectin can form double helices, which in turn may associate to form crystalline domains. In most starches these are confined to the amylopectin component. Double helices form more or less ordered arrays where the ordered structures are crystalline entities. The starch granule is a very complex structure, the complexity being built around variations in the composition (α-glucan, moisture, lipid, protein and phosphorylated residues) and structure of the components. In wheat, the starch surface protein friabilin has attracted much attention because of its proposed association with grain hardness.

Integral proteins have a higher molecular weight than surface proteins (~50–150 and ~15–30 kDa, respectively) and include residues of enzymes involved in starch synthesis, especially starch synthase. Starches also contain relatively small quantities (<0.4%) of minerals (calcium, magnesium, phosphorus, potassium and sodium), which are, with the exception of phosphorus, of little functional significance. As the starch paste cools, the viscosity increases due to the formation of a gel held together by inter-molecular interactions involving amylose and amylopectin molecules. The retrogradation of amylose in processed foods is considered to be important for properties relating to stickiness, ability to absorb water and digestibility, whereas retrogradation of amylopectin is probably a more important determinant in the staling of bread and cakes.

Most starches contain a portion that digests rapidly [rapidly digesting starch (RDS)], a portion that digests slowly [slowly digesting starch (SDS)] and a portion that is resistant to digestion [resistant starch (RS)]. Starch modification not only decreases retrogradation, gelling tendencies of pastes and gel syneresis but also improves paste clarity and sheen, paste and gel texture, film formation and adhesion. These highly functional derivatives have been tailored to create competitive advantages in new products, improve product aesthetics, lower recipe/production costs, eliminate batch rejects, ensure product consistency and extend shelf-life while clearly making starch relevant in all stages of a food product's life cycle. Modification of starch is an ongoing process as there are numerous possibilities. There is a huge market for the many new functional and added-value properties resulting from these modifications.

1.2 Preparation and Characterization of Starch Nanocrystals

Acid hydrolysis is possibly the most common and optimized method to produce starch nanocrystals. Acid treatment dissolves the regions of low lateral order to reveal the concentric lamellar structure of starch granules. By this approach, water-insoluble and highly crystalline residues may be converted into stable suspensions by a subsequent vigorous mechanical shearing action. During acid hydrolysis, regions of low lateral order and also amorphous phases in the starch granules start to dissolve, while the highly crystalline water-insoluble lamellae remain undissolved. Le Corre et al. conducted an experiment to determine whether starch from many different sources could be used to prepare starch nanocrystals and if the amylose content and/or botanic origin of the starch influenced their final properties. Starch nanocrystals are reported to be derived from starch granule crystallites and result from the disruption of the semicrystalline structure of native starch granules at temperatures below the gelatinization temperature. Under these conditions, the amorphous regions in starch granules are hydrolysed, which allows the separation of nanoscale crystalline residues. Starch nanocrystals of different sizes and shapes can be obtained depending on the origin of the starch and the isolation process. Xu et al. prepared starch nanocrystals from corn, barley, potato, tapioca, chickpea and mung bean starches using an acid hydrolysis method.

Mélé et al. studied the processing of nanocomposite materials consisting of natural rubber filled with waxy maize starch nanocrystals. Angellier et al. employed starch nanocrystals in natural rubber composites and found a remarkable enhancing effect, but when the starch nanocrystal content exceeded 20%, the enhancement decreased. Bouthegourd et al. reported the extraction and characterization of potato starch nanocrystals and their nanocomposites with a natural rubber latex matrix with the preparation performed using sulfuric acid at 40 °C. Kim et al. claimed that obtaining individual nanoparticles from starch was almost impossible regardless of the origin of the starch. In another study by the same group, a hydrolysis process combined with a physical treatment such as ultra-sonication for the formation of a uniform dispersion of starch nanocrystal was investigated.

Li et al. reported three stages corresponding to the stepwise hydrolysis of the amorphous, semicrystalline and crystalline layers of the starch structure. Some authors have suggested that high-amylose starches are more susceptible to acid hydrolysis than those with lower amylose contents, which are more easily hydrolysed. This can be explained by either the greater extent of starch inter-chain associations in the amorphous regions, which are more compactly organized, or by the slower penetration of hydrogen ions into the granules due to the limited swelling of high-amylose starch.

Large-scale starch nanocrystals (10–50 nm) obtained from the acid hydrolysis of amylopectin-rich waxy maize starch have been employed to prepare nanobiocomposites with natural rubber using a mastication technique. Habibi and Dufresne found that the mechanical characteristics of nanocomposite materials were improved by using chemically modified starch nanocrystals, which resulted in better dispersion of the filler within the matrix. Chen et al. reported a reduction in the moisture uptake of a poly(vinyl alcohol) (PVA) matrix from 78 to 62% for the unfilled matrix and 40% w/w starch nanocrystal-reinforced composites, respectively. Angellier et al. modified starch nanocrystals with alkenyl succinic anhydride or isocyanates and observed that the toluene uptake of the composite was higher than that of unmodified starch nanocrystals.

Thielemans et al. observed that the thermal behaviour of starch nanocrystals was improved by grafting to alkyl polymer chains, which they suggested may be due to the protective crystalline layer formed by the oxygen-poor stearate surface. Namazi and Dadkhah found similar results in relation to hydrophobically modified starch nanocrystals using octanoyl, nonanoyl and decanoyl chloride in an aqueous medium under mild conditions. They evaluated the thermal properties of the starch nanocrystals using thermogravimetric analysis (TGA) and observed that the decomposition onset temperature increased for modified starch nanocrystals, revealing their higher thermal stability than the unmodified form. The synthesis of starch nanocrystals with various sizes and shapes has been widely reported, with the industrially important platelet starch nano-crystals obtained from hydrolysis of native starch granules being relatively easy to obtain with thicknesses of 6–8 nm, lengths of 40–60 nm and widths of 15–30 nm.

1.3 Natural Fibre-reinforced Thermoplastic Starch Composites

Starch can be processed into a mouldable thermoplastic known as thermoplastic starch (TPS). TPS is plasticized starch that has been processed (typically using heat and pressure) to destroy completely the crystalline structure of the starch to form an amorphous thermoplastic material. Water contained in the starch and the added plasticizers play an indispensable role because the plasticizers can form hydrogen bonds with the starch, replacing the strong interactions between the hydroxyl groups of the starch molecules, thus making the starch thermoplastic. McHugh et al. suggested that, owing to its small size, glycerol was more effective than sorbitol in plasticizing the starch. Many studies have been carried out on the preparation of TPS using glycerol as plasticizer. Park et al. developed biodegradable thermoplastic potato starch by using 30% glycerol as plasticizer. Averous and Boquillon studied the thermal and mechanical behaviour of composites made from TPS reinforced with agro-materials (cellulose and lignocellulose fibres). The TPS composite modulus displays a regular behaviour where the reinforcement effect increases with increase in the fibre length from short-length fibre (SF) to medium-length fibre (MF) and fibre content whereas the elongation at break decreases with increase in fibre content and length.

1.4 Applications of Starch Nanocrystal-based Blends, Composites and Nanocomposites

The main advantages of starch nanocrystals (SNCs) are their renewable nature, low cost, high barrier properties, availability, compatibilization with biopolymers, high specific strength, non-abrasive and non-toxic nature that allows easier processing even at high filling levels, biodegradability and a relatively reactive surface. They are edible, versatile and light weight and have a high aspect ratio, high specific strength and high modulus. Starch nanoparticles and nanocrystals have many potential applications, such as plastic fillers, food additives, drug carriers, implant materials, vehicles for carrying bioactive substances and nutraceuticals, fillers in biodegradable composites, coating binders, adhesives and a source of energy at the end of their life cycle. The starch nanocrystals can also be used in biomedical, biochemical and technological applications and as vehicles for carrying bioactive substances and nutraceuticals. However, they tend to aggregate and settle down in aqueous solutions, which is a limitation to their application in most biological and food systems. Wei et al. mentioned that SNCs are crystalline platelets originating from the breakdown of the semicrystalline structure of starch granules by acid hydrolysis of amorphous parts. Because of their unique properties, SNCs have been widely used as particle emulsifiers to prepare Pickering emulsions and as a reinforcement to prepare nanocomposites, such as biodegradable films and natural polymers.

1.5 Chemically Modified Thermoplastic Starches

Grafting with maleic anhydride (starch-g-MA) is one of the most traditional and relevant techniques used to compatibilize non-polar polymers, such as polyolefins [e.g. low- and high-density polyethylene (LDPE and HDPE) and polypropylene (PP)] and polar materials, such as starch. Diisocyanates, such as 4,4'-diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI), have been employed for the reactive compatibilization of starch with polyesters such as poly(lactic acid) (PLA).

Starch can be esterified by processing TPS with acid anhydrides and catalysts. The main anhydrides used are acetic, propionic, maleic and succinic anhydride. Other reagents such as formic acid, acid halides and vinyl acetate have also been reported. Other reactions may occur during the esterification process, such as hydrolysis and glycosylation with glycerol.

Propylene oxide is the most commonly used reagents in the preparation of hydroxypropyl starch. Reaction with 3-chloro-2-hydroxypropyltri-methylamine chloride with reactive extrusion (REX), in the presence of 15 wt% glycerol, has been used to prepare cationic starch. Reactions with vinyl acetate, styrene and acrylamide have been described in processes for the grafting of starch in the molten state. Reduction of the molar mass of starch by glycolysis, catalysed by inorganic or organic acids, has been used to prepare modified TPS with lower melt viscosity. Citric acid was used to improve the compatibility of TPS and other polymers, including LDPE. Blends with linear LDPE were described by Ning et al. In their studies, it was shown that citric acid can indeed improve the compatibility of the system. A recent study by our group also investigated blends of starch modified with citric acid and polyethylene in a two-stage process. Citric acid has also been used in nanocomposites of TPS with clays such as mont-morillonite (MMT), for the purpose of modifying MMT to increase its rate of exfoliation and to alter the properties of TPS so as to increase the wettability of the clay. The future of the reactive extrusion of TPS lies in its use in an intensive form so that starch can be radically modified, generating new materials that can be tuned to a wide assortment of uses. Reactive extrusion of TPS has proved to be a green process in that native starch is used without previous modification and the reactions take place at the same time as TPS is produced.

1.6 Outstanding Features of Starch-based Hydrogel Nanocomposites

The highly branched structure renders high molecular weight amylopectin that is in general 1000 times higher than that of amylose. Indeed, amylopectin is a "titan" natural molecule: one of the largest with a molecular weight of up to 400 x 106. The ratio of mylose to amylopectin in any native starch is dependent on the source. In] addition to these two main components, starch granules could also present other minor components in their composition such as some particulate material (e.g. cell-wall fragments) or surface and internal components (e.g. proteins, enzymes, lipids, amino and nucleic acids).

Another important feature of starch is the large number of hydroxyl groups in their backbone. These groups have great affinity for other hydroxyl groups (hydrogen bonding), which can act as a driving force to hold the starch chains together in a regular pattern. Where such ordering occurs, crystalline regions are deposited in the starch granules. Many research groups have explored in depth the potential of starch-based hydrogels and their derivatives (such as starch-based hydrogel composites) in a wide variety of technological and biological fields. As a result, outstanding advances in starch-based hydrogels have published in recent decades. The application of hydrogels is no longer focused only on liquid absorption/retaining and the advantages of this very promising class of materials are now being exploited in the most varied industrial, technological and biotechnological sectors.

Hydrogels prepared from biopolymers (mainly from polysaccharides) have found great applicability as biomaterials. The interesting properties of polysaccharides derive from their structure, which, in general, contains a large number of functional groups (–COOH, –OH, –NH2, –NHOCCH3 and –OSO3H) that can be crosslinked by reaction with a coupling agent or that allow the insertion of crosslinkable groups or polymeric chains on the polysaccharide backbone. Most of the hydrogels prepared by this methodology show semi-interpenetrating network (semi-IPN) characteristics. IUPAC defines a semi-IPN as a polymeric material comprising at least one network and at least one linear or branched polymer characterized by the penetration of both on a molecular scale. Starch-based hydrogels prepared from hydrophilic polymers/monomers, in a general way, have the capacity to absorb and retain large amounts of liquid, which classifies these hydrogels as superabsorbent. Raw starch is not so hydrophilic owing to its granular structure, and for this reason the association of starch with more hydrophilic polymers is required in order to prepare materials with a high liquid uptake capacity. In polymer science, hydrogels have evolved into materials with outstanding features and many potential applications, from soil conditioners and hygienic products to tissue engineering, drug delivery systems and imprinted polymers. Al et al.81 prepared superabsorbent hydrogel composites by grafting acrylic acid onto a starch backbone at a monomer to starch weight ratio of 1.5 (ca. 40 wt% maize starch) using N,N'-methylenebisacrylamide as crosslinker, cerium ammonium nitrate as initiator and Na/MMT as the reinforcing phase. Eid reported the preparation of starch polyelectrolyte hydrogels [starch/polyvinylpyrrolidone, starch/polyacrylamide and starch/poly(acrylic acid)] polymerized via gamma irradiation loaded with silver nanoparticles prepared by in situ reduction of silver nitrate with sodium borohydride, at room temperature.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2015
• Language: English
• ISBN 10: 184973979X
• ISBN 13: 9781849739795
• Binding: Hardcover
• Number of pages: 555
• Editor: Visakh P M