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Chemical and Physical Mechanism and Method of Preparation of Thermoplastic Starch

Wen Zhou , Xin Zhang , Hongpeng Ma , Jie Xu , Bin Guo , Panxin Li

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 1972 -1982.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 1972-1982 DOI: 10.7536/PC200874
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Chemical and Physical Mechanism and Method of Preparation of Thermoplastic Starch
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Abstract

Starch, as a natural polymer, its attractiveness stems from the annual renewability, low cost and biodegradability in the areas of food, degradable plastics and packaging materials etc. However, the natural starch cannot be directly used in plastics application due to the strong intra- and inter-molecular hydrogen bonds, resulted in the lower thermal decomposition temperature than its melting point. It must be plasticized with low molecular weight compounds such as polyols to form thermoplastic starch(TPS) for processing on conventional processing equipment. Therefore, studying the mechanisms and methods in the processing of TPS is the key point to expand its application. Herein, based on the recent works reported, and considering different physical and chemical nature in mechanism, we summarized as 8 factors(hydrogen bond, chemical bond, stress, shear force, thermal conductivity, microwave, hot-air, γ-ray) in TPS formation, and discussed 8 preparation methods(melting, solution, compression molding, ball milling, spray drying, high hydrostatic pressure, chemical modification, high-energy radiation) of TPS, emphasizing on their relationship. This review is expected to be helpful in studying and developing various TPS products in future.

Contents

1 Introduction

2 Preparation methods

2.1 Melting method

2.2 Solution method

2.3 Compression molding

2.4 Ball milling

2.5 Spray drying

2.6 High hydrostatic pressure

3 Chemical modification

3.1 Oxidation

3.2 Esterification

3.3 Etherification

3.4 Cross-linking

3.5 Dual modification

4 High-energy radiation

5 Conclusion and outlook

Graphical abstract

Keywords

thermoplastic starch / preparation / mechanisms / methods

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Wen Zhou, Xin Zhang, Hongpeng Ma, Jie Xu, Bin Guo, Panxin Li. Chemical and Physical Mechanism and Method of Preparation of Thermoplastic Starch. Progress in Chemistry, 2021, 33(11): 1972-1982 DOI:10.7536/PC200874

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1 Introduction

Starch is composed of amylose (linear) and amylopectin (highly branched), which is widely found in cereals, fruits, tubers and roots of plants. It has the advantages of abundant sources, renewable, completely degradable and low price, so it has attracted wide attention[1,2][3⇓~5]. Due to the existence of crystalline structure, the melting temperature of starch is higher than its decomposition temperature, so it can not be processed thermoplastically, that is, it does not have thermoplasticity. Under the action of high temperature, shearing force and plasticizer, the starch granules with crystalline structure are destroyed, so it can be processed into thermoplastic starch (TPS)[6⇓~8]. The essence of TPS preparation process is that the hydrogen bonds within and between starch macromolecules are destroyed, and the crystalline starch macromolecules are finally transformed into a uniform amorphous state[9,10].
In the preparation process of thermoplastic starch, according to the different mechanism or principle, it can be divided into two categories from the chemical and physical point of view. The chemical aspect is mainly the formation of hydrogen bonds and chemical bonds[11][12]. The physical mechanism focuses on the influence of external conditions, such as force (pressure, shear force), heat (conduction, microwave and hot air) and high-energy radiation (gamma ray)[13][14][15]. At the same time, the specific preparation methods of thermoplastic starch in practice do not only involve a single chemical or physical mechanism, but sometimes involve a combination of multiple mechanisms. At present, according to the actual application status, the preparation methods of thermoplastic starch are mainly divided into 8 kinds: melting method (extrusion method and internal mixing method), solution method, molding method, ball milling method, spray drying, high hydrostatic pressure, chemical modification and high-energy radiation, as shown in Table 1. In this paper, the eight preparation methods in practice are taken as the main line, and the relationship between different chemical and physical mechanisms and methods involved is taken as the focus, and the research status and progress in this field are systematically introduced.

2 Method of preparation

2.1 Melting method

The mechanism of the melting method is to make the natural starch into a molten state. Natural starch can not be directly melted, because its melting temperature is greater than the decomposition temperature, so the essence of this method is to reduce the melting and glass transition temperature by destroying the strong hydrogen bonds in and between starch macromolecules, so that it can reach the melting state and form thermoplastic starch. In practice, according to the specific equipment, it is mainly divided into extrusion method and internal mixing method. The common point is that the melting process is realized by the synergistic effect of high temperature, shear force, plasticizer and other factors.

2.1.1 Extrude

Single (twin) screw extruder is mainly used for plastic processing, which has the advantages of continuous production, no organic solvent, simple and convenient, and is also one of the most commonly used methods to prepare thermoplastic starch[16]. The mechanism of preparing thermoplastic starch by extrusion mainly involves the formation of hydrogen bonds and/or chemical bonds, external pressure or shear force, heat conduction and other factors, which act synergistically in the whole preparation process.
(1) Conventional extrusion
In the traditional extrusion process, under the combined action of higher pressure, shearing force, heating and plasticizer, the plasticizer replaces the strong hydrogen bonding in and between starch macromolecules to form new hydrogen bonds without the action of chemical bonds, and at the same time, the crystalline structure is destroyed and starch granules disappear, thus forming thermoplastic starch[17⇓~19].
In conventional extrusion, the processing temperature of the extruder is a key parameter, which is mainly related to the added components. Compared with general plastics, the processing temperature range of thermoplastic starch with polyols such as glycerol, sorbitol or amines as plasticizers is lower, usually 90 ~ 125 ℃, and the screw speed is 60 ~ 170 rpm (revolutions per minute), while when some inorganic mineral fillers are added, the extrusion processing temperature will be significantly increased[20,21][22][20⇓~22]. For example, Zhang et al. mixed montmorillonite (MMT) with potato starch and glycerol and passed them through a micro twin-screw extruder. The temperature settings were generally high, 100, 160 and 135 ℃, respectively, with the highest being 160 ℃, which was related to the formation of nano-intercalation structure between filler and starch[23].
(2) Reactive extrusion
Reactive extrusion (REX) is a process in which the crystalline structure of starch granules is destroyed by the combined action of chemical reagents, high temperature and shear force in the extruder, and new molecular structures are formed after the reaction of starch macromolecules with chemical reagents[24,25]. The method is commonly used for chemical modification of natural or synthetic polymers, and in recent years, the method is also used for preparing thermoplastic starch and starch-based nanocomposites, and has the advantages that starch functionalization and continuous production can be simultaneously realized, the method is economical, and the operation is simple[26,27].
Oxidation and esterification have been reported as the main types of chemical modification of starch during reactive extrusion[28][29,30]. The method is characterized by high modification efficiency. Among them, the oxidants used by Kalambur et al were hydrogen peroxide, copper and ferrous sulfate catalysts, and the oxidation rate of starch reached 100% at 120 ℃ for 3 min in the extruder[28]. Maleic anhydride (MAH) is a commonly used esterifying agent. As shown in Figure 1, Zuo et al. prepared maleic anhydride esterified thermoplastic starch (MTPS) through a single-screw extruder, and found that the esterification reaction between corn starch and MAH was successful, which reduced the crystallinity, increased the contact angle, reduced the water absorption and improved the hydrophobicity[29]. Raquez et al. Further confirmed that the granular structure of starch was completely destroyed during reactive extrusion and that the C6 position in the starch backbone was preferentially esterified[30].

2.1.2 Banbury

Internal mixer (closed rubber mixer) is mainly used for plastication and mixing of rubber or plastics[31,32]. Compare with that extruder, the internal mix has the advantages of simple operation and the disadvantages of intermittent production, so it is widely used in the preparation of small batch of thermoplastic starch and is called internal mixing method. Consistent with the extrusion method, the preparation mechanism also involves the formation of hydrogen bonds or chemical bonds, pressure or shear force, heat conduction and other factors.
Due to the intermittent characteristics of internal mixer, the process of preparing thermoplastic starch and its composites is often carried out in multiple steps. For example, Olivato et al. prepared TPS/poly (butylene adipate)/poly (butylene terephthalate) (PBAT) nanobiocomposites with different sepiolite additions by using an internal mixer twice[33]; Jantanasakulwong et al. Obtained TPS/PE-MAH/CTS (chitosan) composites in three steps by using an internal mixer, a two-roll mill and an internal mixer, respectively. As shown in fig. 2, the amino (—NH2) of CTS reacted with MAH of PE-MAH, which increased the compatibility between TPS and CTS and improved the mechanical properties of the composites[34].
In addition, some processes use an internal mixer for the first step and a blown film machine or hot press for the second step[35][36]. This is because the banburying process mainly plays the role of uniform mixing of materials, and the preparation of products such as films or sheets also needs to be completed by other processing methods.

2.2 Solution Method

The mechanism of solution casting (SC) is that the intramolecular and intermolecular hydrogen bonds of starch are replaced by plasticizers (solvents) to obtain thermoplastic starch. The solvent used in the solution method is water, polyol or a mixture of water and polyol. Usually, a certain amount of starch solution is poured into a mold, and then dried at a constant temperature and demoulded to obtain a film. It has the characteristics of simple equipment, easy operation and suitable for small-scale preparation in the laboratory[37,38][39][40].
The key parameter of solution method is temperature. At present, there are two modes: normal temperature and normal pressure and high pressure. The preparation temperature at normal temperature and pressure is usually between 65 and 95 deg C, and the preparation process is slow due to the low temperature, and the film forming period is long[37,41]. In order to improve the efficiency, it can be prepared in high pressure mode, that is, in special equipment such as autoclave, which is characterized by high temperature (130 ~ 150 ℃) and short time (5 ~ 30 min), and is often used for high amylose starch. Koch et al. heated different proportions of high amylose (70%) corn starch (2.2 or 2.6 G) and water (100 mL) at 130, 140 and 150 ℃, respectively, and stirred vigorously for 5, 15 and 30 min, cooled to about 80 ℃ at room temperature, added glycerol, poured the solution into a polystyrene Petri dish, and evaporated at 23 ℃ to obtain a film[42]. Liu et al. First mixed high amylose (80%) starch/deionized water at 5% (w/w) and stirred at 140 ℃ and 600 rpm for 30 min in a high temperature and high pressure reactor.Thermoplastic starch solution was obtained, and then starch-chitosan-glycerol blend was prepared and film was prepared. The decrease of glycerol-related NMR peak intensity in the composite film proved that strong interaction occurred between starch, chitosan and glycerol[43].
In addition, the thermoplastic starch can also be prepared by heating in a microwave oven. Mohan et al. Mixed different concentrations of tamarind seed starch, okra mucopolysaccharide and bagasse cellulose, and stirred at 95 ℃ for 30 min to prepare the matrix solution[44]. 100 mL of the matrix solution was mixed with 1. 5 mL of glycerol and then cast onto a glass Petri dish, which was treated by a microwave oven at different power ranges (175, 350 and 525 W) for 15 to 20 min. It was found that the microwave oven method with low power (175 W) was the fastest method to prepare thermoplastic starch films, but it was only effective in the matrix solution with low concentration.

2.3 Molding method

compression molding is a method of molding by putting the material into the mold cavity at the molding temperature, closing the mold and pressurizing[45]. The mechanism of preparing thermoplastic starch by compression molding is that the plasticizer enters into the starch granules through high pressure at high temperature, thus destroying its crystalline structure and obtaining thermoplastic starch. Molding method has the advantages of high production efficiency and being able to form products with complex structure at one time, but it has the disadvantages of complex mold manufacturing and large investment[46].
The preparation of thermoplastic starch by compression molding was usually carried out at 135-140 ℃ for 10-20 min. For example, Perdomo et al. Slowly added water to tapioca starch with continuous stirring and allowed to equilibrate overnight[47]. Compression molding was performed using a hydraulic press at 3500 psig (pound per square inch, gauge) at 135 ° C for 20 min. The entire assembly was cooled (up to 40 ° C) before releasing the pressure to avoid the formation of blisters in the plate.
Zdanowicz et al. ground starch and deep eutectic solvents (DES) prepared from choline citrate (CCit), α-hydroxyl anion and glycerol in a mortar to obtain a uniform paste, which was then put into a PP bottle and sealed at room temperature for 24 H, and then hot-pressed at 140 ℃ under 12 t pressure for 10 min, and cooled to 85 ℃ under holding pressure to obtain the film material[48].
For thermoplastic starch foam materials, the molding temperature is set higher, 200 ~ 220 ℃, and the corresponding time is shorter, 4 ~ 8 min, due to the need of foam molding. For example, Sanhawong et al. First mixed starch, cotton fiber, natural rubber latex concentrate (CNRL), distilled water and all additives at room temperature for 5 min, and then poured into a mold inside a molding press at 220 ° C and hot-pressed at 1.01×108Pa pressure for 4 min to obtain cassava starch bio-foam[49]. In addition, Trongchuen et al. mixed cassava starch, extracted waste coffee grounds (ex-SCG) and oregano essential oil (OEO) for 3 min, poured them into a mold, and hot-pressed them at 200 ℃ and 7 MPa for 8 min to obtain starch foam composites[50].

2.4 Ball Milling

Ball milling is the use of external mechanical force, through the grinding medium (ball), the material is strongly impacted, stirred and milled, so that the material is repeatedly squeezed, deformed and broken[51]. The mechanism of preparing thermoplastic starch by ball milling is that the plasticizer enters into the starch granules through the repeated extrusion of grinding balls, thus destroying the crystalline structure of starch[52,53]. Its advantages are no need of heating and operation at ordinary temp. The disadvantage is that the cycle is longer[54⇓~56].
Prabhakar et al. First prepared a homogeneous solution by continuously stirring 45 mL of glycerol with 80 ℃ hot water for 10 min, then mixed it with 150 G of corn starch, and ball-milled it at 200 rpm for 2 H to obtain TPS, which was reinforced by flax fabric (FF) and added ammonium polyphosphate (APP), and then hot-pressed at 160 ℃ and 7 MPa for 30 min to prepare TPS/FF/APP composite (TFA) with flame retardant properties[52]. In addition, Prabhakar et al. Prepared TPS by the same ball milling process, and hot-pressed TPS with chitosan (CS) and FF to prepare composites[53].
The ball milling method usually uses a planetary ball mill, the rotating speed of which is generally 200 to 500 rpm, and the time is several hours to tens of hours. The structure and properties of thermoplastic starch prepared by different processes are obviously different. For example, Lv et al. milled a blend of potato starch (NPS), tea polyphenols (TPs) and distilled water in a planetary ball mill at a rate of 350 rpm for 7 H to obtain thermoplastic starch[54]. It was found that the crystalline structure of starch was destroyed after milling for 7 H, and the crystallinity decreased from 38. 1% to 8. 3%.
Liu et al. treated corn starch using a planetary ball mill at 500 rpm for 1, 2, and 3 H, respectively, to obtain thermoplastic starch[55]. It was found that the surface of starch granules changed obviously, and a cavity in the center of the deformed granules was observed when the alternating growth ring disappeared. Ball milling treatment significantly increased the strength at 3382 cm-1 and decreased the proportion of 1047/1018 cm-1 and 995/1018 cm-1, which corresponded to the fraction of crystalline/amorphous regions of the starch.
Ren et al. Dissolved dried tapioca starch in anhydrous ethanol[56]. The suspension was divided into four parts, placed in four vacuum tanks of a ball mill, and ball milled at 500 rpm for 6, 18, 30, 42, and 54 H, respectively, to obtain thermoplastic tapioca starch. It was found that the morphology of starch granules changed during the ball milling process: the glass transition temperature (Tg) and peak height index (PHI) decreased when the size of starch granules decreased; As the size of starch granules decreased to 9.11 μm, they had a tendency to agglomerate and their ΔH increased.

2.5 Spray drying

Spray drying is a very rapid drying process that converts liquid solutions or suspended droplets into amorphous or semi-crystalline particles[57]. The mechanism of preparing thermoplastic starch by spray drying is that the atomized starch solution is heated by hot air to change the crystalline structure of starch into amorphous state, and the average size, swelling force and other physical and chemical properties of starch granules are significantly changed.
The inlet temperature of the drying air is the key parameter in the spray drying method, generally 130 ~ 200 ℃, which ensures the completion of the destruction process of the crystalline structure of starch. For example, Ahmed et al. dissolved corn starch and hydrophilic plasticizers (malic acid) and amphiphilic plasticizers (isoleucine and butyric acid) in different proportions in water, stirred them at 95 ℃ and 600 rpm for 25 min, and then dried them with a micro spray dryer. The inlet and outlet temperatures were 140 ℃ and 80 ℃, respectively. The dried powder was hot-pressed to obtain samples[58]. In addition, Khan also prepared malic acid and citric acid plasticized thermoplastic starch by the same process, and prepared TPS dog-bone-shaped samples by hot pressing. It was found that the TPS powder and samples prepared by spray drying were amorphous regardless of the amount of plasticizer added, and there was no significant difference in morphology between the samples[59].
Before spray drying, most of them need a pretreatment process to improve the preparation efficiency. Fu et al. added the dried corn starch into deionized water at 24 ± 1 ℃, and stirred for 15 min at 300 rpm in a constant temperature water bath at 64 ~ 72 ℃, and then spray-dried the starch dispersion with a desktop spray dryer[60]. Niazi et al. conducted a comparative study on different pretreatment processes. High amylose starch and waxy corn starch were dispersed in distilled water, and maltodextrin (DE-16) was added under continuous stirring at normal temperature and pressure to obtain starch dispersion. At the same time, part of the starch dispersion was treated in an autoclave reactor at 140 ℃, nitrogen pressure of 10 bar and 600 rpm to obtain starch solution[61]. The starch dispersion and the solution were respectively injected into a Buchi micro spray dryer for drying. It was found that the powder obtained from the starch solution was completely amorphous, with small and highly irregular particles. The powder obtained from the starch dispersion retains more of the granular structure of the original components. Therefore, the spray drying effect of the pretreated starch solution is better.

2.6 High static pressure

The high hydrostatic pressure (HHP) is a non-thermal processing technique for food processing or sterilization at room temperature or mild conditions with a pressure of 100-2000 MPa in special equipment[62]. The mechanism of high static pressure method is that the plasticizer (solvent) enters the starch granules under high pressure, thus destroying its crystalline structure and obtaining thermoplastic starch. The specific process is that the amorphous phase of starch undergoes reversible hydration under HHP, followed by irreversible deformation of the crystalline region, the secondary and tertiary structures are destroyed, and then the overall structure of the particle is destroyed, but the covalent bonds remain intact[63].
At present, the highest pressure reported is 600 MPa. For example, Li Hongyun et al. treated sweet potato starch (SPS) suspensions with HHP at 0, 200, 300, 400, 500 and 600 MPa, respectively[64]. It was found that after 600 MPa HHP treatment, some particles lost their birefringence, and there were obvious cracks and some agglomerations on the surface of the particles.At the same time, the crystallinity decreased by 24. 83%. In addition, their research group also used the same method to treat and study 30% cassava starch suspension[65].
In addition to sweet potato and tapioca starch, wheat and corn starch and rice starch have also been reported to be treated under HHP[66][67]. In general, the birefringence of the starch treated by HHP disappeared completely, and the starch showed a gel-like appearance, and a fully plasticized thermoplastic starch was obtained.

3 Chemical modification

Chemical modification is a method that changes the macromolecular structure of starch by chemical reagents, directly destroys the intramolecular and intermolecular hydrogen bonds of starch, and then changes its crystalline structure. Different from the above methods, the chemical modification method emphasizes that the starch is modified in advance, then the plasticizer is added, and the thermoplastic starch is obtained by extrusion and other methods. The common chemical modifications include oxidation, esterification, etherification and crosslinking.

3.1 Oxidation

Oxidized starch may be prepared using sodium hypochlorite, potassium periodate and permanganate, H2O2, and the like[68][69][70]. The oxidation reaction converts part of the hydroxyl groups of α-D-glucose units into carbonyl groups, and the number of hydroxyl groups in the molecule will decrease, thus reducing the binding capacity of hydrogen bonds within and between starch molecules, which is conducive to thermoplastic processing[71][72].
Oxidized starch is usually mixed with unoxidized starch to improve the water resistance and mechanical properties of TPS. For example, Oluwasina et al. Used potassium periodate as an oxidant to prepare blend films with different amounts of oxidized starch. With the increase of the amount of oxidized starch, the moisture content of the film decreased from 7.93% to 5.36%, and the water resistance improved significantly[73]. Zhang et al. Used CuSO4 solution and H2O2 as oxidant to prepare thermoplastic starch with different oxidation degree and oxidized starch content, and the addition of oxidized starch improved its elongation at break[74]. In addition, thermoplastic oxidized starch (TPOS) films can also be prepared directly from oxidized starch[75].

3.2 Esterify

3.2.1 Acetylation

Starch acetylation is a common chemical modification method, which changes the molecular structure of starch by replacing part of the hydroxyl groups in the starch molecule with acetyl groups[76]. Acetylated starch has some unique properties, such as lower glass transition temperature (Tg) and gelatinization temperature, lower retrogradation tendency, and better thermoplasticity than starch[77]. Furthermore, the presence of a hydrophobic acetyl group decreases the hydrophilicity[78].
The introduction of pulsed electric field in the process of acetylation to improve the degree of substitution is a new development in recent years. In the presence of 10% (w/w) acetic anhydride, Hong et al. Treated tapioca starch with a high degree of substitution of 0.131 by pulsed electric field (PEF) at 2 kV/cm for 51.0 ms[79]. It was found that the crystalline regions of acetylated starch were destroyed and rearranged, and the method significantly improved the surface roughness of starch with outstanding freeze-thaw stability.
At present, acetic anhydride is mainly used for acetylation modification in the research carried out, and starch includes tapioca starch, potato starch, barley starch and rice starch[80][81][82][83]. Among them, potato starch showed higher acetyl percentage and substitution value than cassava starch. Acetylation decreases the tensile strength and increases the elongation of the film; Acetylated starch films have lower decomposition temperature, higher thermal stability and faster biodegradation behavior than starch films.

3.2.2 Other esterifications

Other types of esterification methods that have been reported are mainly maleic anhydride, oleic acid, and rosin acid. Zuo et al. synthesized maleic anhydride esterified starch from corn starch and maleic anhydride by dry method, as shown in Fig. 3. Quantitative analysis by Fourier transform infrared spectroscopy (FTIR) showed that the esterification rate of dry preparation reached 92.34%[84]. Corn starch/polylactic acid (NS/PLA) and esterified starch/polylactic acid (ES/PLA) composites were prepared by melt extrusion after corn starch and esterified starch were blended with PLA respectively. The results showed that the esterified starch increased the interfacial compatibility of the two phases, and improved the tensile strength and elongation at break of ES/PLA. The hydrophobic ester bond also enhanced the water resistance of ES/PLA.
Zarski et al. successfully synthesized starch oleate by esterification of potato starch with oleic acid using ionic liquid 1-butyl-3-methylimidazolium chloride as reaction medium and immobilized lipase from Thermomyces Lanuginosus as catalyst[85]. The results showed that the highest degree of substitution was 0. 22 when the reaction was carried out at 60 ℃ for 4 H. The crystallization and morphology of potato starch changed gradually during the preparation process, and the thermal stability of starch oleate decreased gradually.
Lin et al. Used rosin acid as raw material and lipase as catalyst to esterify cassava starch to obtain rosin acid esterified starch[86]. It was found that the morphology and crystallinity of cassava starch were greatly damaged, and the esterification reaction occurred in the amorphous and crystalline regions of starch. The thermal stability of rosin acid starch decreases, and it has higher viscosity and emulsifying properties, but its swelling power, solubility and transparency in water decrease.

3.3 Etherification

3.3.1 Hydroxypropylation

Hydroxypropylation is a commonly used method to chemically modify starch. The hydroxypropyl starch is generated by the etherification reaction of the starch and the propylene oxide in an alkaline environment, so that the crystalline structure of the starch is destroyed, the crystallinity is reduced, and the hydroxypropyl starch can be used for preparing thermoplastic starch[87]. In addition, hydroxypropyl starch belongs to nonionic starch ether, which has good acid resistance, alkali resistance, salt resistance and freeze-thaw stability[88].
At present, the research of hydroxypropyl starch focuses on its basic physical and chemical properties, such as crystal structure, swelling ability and so on. For example, Lawal et al. Hydroxypropylated starch isolated from white yam (Dioscorea Rotundata) and studied its crystalline structure and properties[89]. Lawal et al. Obtained hydroxypropyl starch by hydroxypropylation of purified pigeonpea starch (NPPS), and studied the crystalline structure, morphology, swelling ability and other properties of the starch and modified starch[90]. Chun et al. studied the effect of HHP treatment on the conventional hydroxypropylation of corn starch[91]. Overall, hydroxypropylation is an effective method to prepare thermoplastic starch.

3.3.2 Carboxymethylation

Carboxymethyl starch (CMS) is a water-soluble anionic etherified starch, which is obtained by etherifying starch with chloroacetic acid under alkaline conditions. The introduction of carboxyl can increase the hydrophilicity of starch molecules, provide good water solubility, and improve its mechanical properties, which is widely used in food, paper, textile, plastics, medicine and other industries[92][93].
Wilpiszewska et al. obtained CMS in isopropanol/water solvent by using potato starch as raw material and monochloroacetic acid (MCA) as etherifying agent, and then obtained a new CMS-based biodegradable film containing calcium montmorillonite (MMT-Ca) by casting method, using glycerol and citric acid as plasticizer and crosslinking agent, respectively[94]. In addition, starch/CMS films were prepared by the same method. The highest tensile strength and Young's modulus of the starch/CMS films with the lowest CMS content (10 wt%) were 1. 1 and 15 MPa, respectively. Dynamic mechanical thermal analysis (DMTA) confirmed that the system had reached the highest crosslinking density[95].
Xie et al. prepared CMS with ionic liquid 1-butyl-3-methylimidazolium chloride (BMIMCl) as reaction medium and sodium monochloroacetate (SMCA) as etherifying agent. As shown in Fig. 4, when the molar ratios of NaOH and SMCA to anhydroglucose unit (AGU) were 1.2 and 1.0, respectively, and the reaction was carried out at 90 ℃ for 3 H, the highest degree of substitution was 0.76. The hydroxyl group of starch was replaced by carboxymethyl, and the crystallinity disappeared, showing amorphous characteristics[96].

3.4 Cross-linking

Cross-linking treatment is usually used to modify starch to improve the plasticizing properties by forming new chemical bonds to destroy the crystalline structure[97]. In addition, cross-linking is a viable method to increase the affinity of starch for adsorbates[98]. As shown in fig. 5, commonly used crosslinking agents are phosphorus chloride (POCl3), sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), epichlorohydrin (EPI) and a mixture of adipic acid and acetic anhydride, which can form ether or ester bonds with the hydroxyl groups in starch[99,100].
Zhou et al. Crosslinked starch with epichlorohydrin (ECH), then plasticized with glycerol, and then melt compounded the thermoplastic crosslinked starch (TPCLS) with polylactic acid (PLA) and poly (ether-block-amide) (PEBA) to prepare a biodegradable PLA/PEBA/TPCLS blend with high impact resistance[101]. The results showed that the impact strength, tensile strength and elongation at break increased with the increase of ECH content, and the impact strength, tensile strength and elongation at break reached the maximum at 0. 5 wt% ECH content, and the mechanical properties were improved significantly.
Jose et al. prepared polyvinyl alcohol/starch/graphene nanocomposites with citric acid (CA) as crosslinking agent and glycerol as plasticizer, combined with solution casting process[102]. It was found that the cross-linking reaction enhanced the interaction between the hydroxyl groups on the PVA and/or starch and the oxygen-containing groups present on the graphene sheets, which improved the mechanical properties of the composites and significantly increased the thermal stability. CA is an effective crosslinking agent and/or compatibilizer for improving the performance of PVA/starch/graphene,

3.5 Double Modification

Since a single modification sometimes fails to impart the desired properties to the thermoplastic starch, further modifications are required. Therefore, "double modification" is a new trend in recent years, which mainly refers to two types of chemical modification (acetylation/oxidation, crosslinking/acetylation or crosslinking/hydroxypropylation)[103,104]. This method is widely used in food and non-food industries (emulsifiers, adhesives, thickeners, etc.).
In order to overcome the poor film-forming property of native starch (NS), Tanetrungroj et al. prepared double modified starch films, including cross-linked oxidized starch (C-OS) and oxidized cross-linked starch (O-CS) films, using hydrogen peroxide and boric acid, and compared them with single modified starch films such as cross-linked starch (CS) and oxidized starch (OS)[105]. It was found that compared with the single modified starch film, the crystallinity and swelling properties of the double modified starch film were reduced, and the mechanical properties, water resistance and thermal stability were significantly improved.
In addition, the double modification of starch can also increase its functionality compared with single modified starch. Gonz González-Soto et al. prepared films of dually modified potato starch (acetylated and crosslinked)[106]. It was found that the crystallinity of the film prepared by double modified starch was lower than that of the unmodified starch. The modified starch film had higher elongation (82.81%) but lower tensile strength (3.51 MPa for starch film and 2.17 MPa for double modified starch film) than the corresponding starch films. The solubility and the water vapor permeability are reduced by the double modification.
Hazarika et al. Used phosphorus oxychloride (POCl3) and propylene oxide as crosslinking agent and hydroxypropylation agent respectively to modify taro starch singly and doubly[107]. It was found that the degree of swelling, solubility and transparency increased with the increase of hydroxypropylation and decreased with the increase of crosslinking. The viscosity of cross-linked and double modified starch was higher than that of starch and hydroxypropylated starch. The hardness of double modified starch is higher than that of starch and single modified starch. Thus, dually modified starches have the advantages of both types of modification and can be used for specific purposes such as foods requiring high viscosity and freeze-thaw stability.

4 High Energy Radiation

High-energy radiation mainly includes gamma rays, X-rays and high-energy ions. Among them, 60Coγ irradiation has been widely used in the field of polymer modification and starch[108,109]. The high energy of γ-ray can destroy the crystal and non-crystal area inside the starch granules, resulting in the destruction of the order of starch macromolecules and the decrease of crystallinity, at the same time, the average molecular weight decreases, the viscosity decreases, the solubility increases, and the carboxyl content increases[110][111].
Bashir et al. First used 60Co as radiation source to modify chickpea flour with radiation doses of 0. 5, 1, 2. 5, 5 and 10 kGy, respectively, and then extracted starch from chickpea flour[112]. It was found that the swelling capacity, solubility index, oil absorption and water absorption capacity of starch increased significantly with the increase of radiation dose. In irradiated starch treated with 5 and 10 kGy, slight cracks on the surface of starch granules were observed by scanning electron microscopy (SEM); X-ray diffraction (XRD) showed a characteristic C-type pattern of the starch, and the crystallinity decreased with increasing dose.
Li et al. First put corn starch in polyethylene bags, and then used BFT-4 industrial 60Co radiator to modify it by irradiation at the doses of 0 kGy (non-irradiation) and 10-40 kGy, respectively, and finally obtained the irradiated starch film by solution casting method[113]. It was found that radiation had a slight effect on the molecular structure of corn starch, and the crystallinity decreased with the increase of radiation dose. The size of 59.1% of the corn starch was ≥ 8 μm, which decreased from 59.1% to 24.1% after 40 kGy irradiation. The tensile strength of the resulting films increased with increasing irradiation dose, but the water vapor permeability (WVP) decreased significantly.
Cie Cieśla et al. prepared starch-surfactant composite films containing sodium laurate (NaLau) and cetyltrimethylammonium bromide (CTAB) by a solution casting method using potato starch irradiated with 60Coγ rays at a dose of 30 kGy and adding 30 wt% glycerol as a plasticizer[114]. It was found that, compared with the unirradiated starch film, the irradiated starch film had smoother and more uniform surface, higher tensile strength and toughness, and better water resistance.
In our group, corn starch was irradiated by 60Coγ rays, and thermoplastic starch was prepared from the starch before and after irradiation[115]. The results showed that the crystalline structure of starch molecules was destroyed after irradiation. The peak temperature of the melting endothermic peak of TPS plasticized by glycerol decreased from 192.93 ℃ to 157.89 ℃, the tensile strength decreased, the elongation at break increased, and the plasticity increased significantly.

5 Conclusion and prospect

Due to the existence of intermolecular and intramolecular hydrogen bonds in crystalline and semicrystalline structure of starch, it can not be processed thermoplastically, which limits its application and development in various fields. Through various preparation methods, the crystalline and semi-crystalline structures of the starch are reduced or completely disappeared, and the starch becomes a thermoplastic amorphous material which can be further processed and applied. Therefore, it is of great significance to strengthen the research on the mechanism and method of the preparation process of thermoplastic starch to promote the application and development of starch. In the future, the research and development trends in this field may include the following four points: (1) continuously optimizing the existing methods and developing new efficient, green and economical preparation methods; (2) To explore the effects of different preparation mechanisms (physical and chemical methods) and method combinations on the properties of thermoplastic starch; (3) The relationship between the preparation process, the degree of plasticization and the properties of thermoplastic starch was studied quantitatively, so as to prepare thermoplastic starch with better comprehensive properties on the basis of the best degree of plasticization. (4) To study the migration and retrogradation of plasticizers after the preparation of thermoplastic starch, to inhibit the migration and retrogradation of plasticizers, and to promote its in-depth application in food, medicine and other fields.

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Funding

Jiangsu Government Scholarship for Overseas Studies

Nanjing Forestry University Yong Top Talent Program

PostGraduate Research & Practice Innovation Program of Jiangsu Province in 2019(SJKY19_0867)

Higher school in Jiangsu Province College Students’ Practice Innovation Training Programs in 2018(201810298078Y)

Natural Science Foundation of Jiangsu Province(BK20140967)

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