Plasticizers on Alginate-Based Films: Comparison
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There has been a growing attempt to manipulate various properties of biodegradable materials to use them as alternatives to their synthetic plastic counterparts. Alginate is a polysaccharide extracted from seaweed or soil bacteria that is considered one of the most promising materials for numerous applications. 

  • alginate
  • hydrophilic and hydrophobic plasticizers
  • plasticization mechanisms

1. Plasticization Mechanisms

The plasticization mechanism can be explained by several theories. During the 1940s, the “lubricity theory” and the “gel theory” were developed. In these theories, scientists attempted to explain critical phenomena observed in plasticized polymers [38][1]. According to the lubricity theory, plasticizers reduce intermolecular friction between the polymer chains. Increasing the flexibility of the plastic part leads to the polymer molecules slipping over each other. Accordingly, plasticizers lubricate the movement of the polymer molecules by reducing their resistance to sliding. However, in the gel theory, polymers are considered as three-dimensional honeycomb structures in which the resistance of these 3D structures rather than the internal frictions (as the lubricity theory states) is primarily responsible for the rigidity of the polymers. In this theory, the plasticizer reduces the number of attachment points from chain to chain, letting the polymer be deformed without breaking by reducing the aggregation of the polymer molecules.
Later, during the 1950s, some other models were suggested; however, among them, the free volume theory gives more precise explanations of the plasticizing effect [37,39,40][2][3][4]. This theory attempts to explain the glass transition temperature (Tg) reduction which occurs with increasing plasticizer content. This theory originated when scientists attempted to explain various concepts related to materials such as thermal expansion coefficients, specific volume, viscosity, and the relationships between these properties and some variables related to polymer structure, such as molecular weight. This is why the free volume theory is still used for explanation of the plasticization mechanism. Although different authors have contributed to developing this theory, Fox and Flory were the first to postulate the idea [41][5].

Free Volume Theory

The internal space between polymer chains is called the free volume. In rigid polymers, the molecules are very compact, so there is very little free volume between the polymer’s chains. The main function of plasticizers is to enhance the free volume within the polymer by decreasing the intermolecular forces between chains, increasing chain mobility, and making the polymer rubbery and flexible [40,42][4][6]. Since the increase in free volume leads to increased motion of polymer molecules, to study the plasticization effect it is vitally important to study different ways of increasing free volume.
There are some models correlating free volume fraction to the glass transition temperature for polymers. For example, according to the empirical Boyer–Simha rule, the free volume fraction (fexp) is related to the cubic expansion coefficient (β) of liquid (L), and glass-like (G) polymers as well as the glass transition temperature (Tg) [38][1]. The fractional free volume of a liquid (f) is defined as f = v1/v0, where v1 is the free volume and v0 is the occupied volume.
𝑓𝑒𝑥𝑝≈(𝛽𝐿−𝛽𝐺)𝑇𝑔≈0.11±0.02
Moreover, according to the Williams–Landel–Ferry (WLF) approach, the fractional free volume and temperature have a linear correlation [43][7].
𝑓=𝑓0+𝛽𝑓(𝑇−𝑇0)
According to this theory, the free volume of a glassy polymer at Tg is 2.5% of the total volume. The WLF equation is one of the most successful theories of free volume.
Principally, the free volume in the polymer structure comes from three main motions: the motion of chain ends, side chains, and the main chain. These motions, and therefore, the free volume of a polymer can be increased by different methods. Firstly, the increase in the number of end chains achieved by decreasing the molecular weight of the polymer can promote the free volume between chains. Secondly, both internal (increasing the number or length of side chains) and external plasticization (incorporation of a lower molecular weight material that acts as a plasticizer) can induce this effect, and lastly, elevating the temperature also can increase the internal space of the polymer.
To experimentally measure the Tg of polymers, DMA (dynamical-mechanical analysis) is commonly used to determine the parameter known as tan δ. The value of tan δ provides information about changes associated with the movement of the polymer chains and their viscoelastic behavior [3][8]. Generally, in the tan δ profile of alginate-based films, two main transitions can be identified. There is a transition around 100 °C due to the glass transition (α-transition) and another at low temperatures (β-transition) related to the motion of hydroxyl groups connected to water molecules [44,45][9][10]. For example, Chen et al. [44][9] found that the Tg of the polymer decreased from 110 °C for the unplasticized polymer to about 60 °C for the glycerol-plasticized alginate film. Avella et al. [17][11] also evaluated the changes in the Tg of alginate by the incorporation of glycerol as a plasticizer. They added 33, 45, and 50%wt glycerol to the polymer. According to the DMA results, the addition of 33%wt glycerol did not affect the Tg of the polymer; however, introducing 45 and 50%wt glycerol led to a 30 °C decrease in Tg from 120 °C for the neat alginate to 90 °C for the plasticized polymer.

2. Classification of the Plasticizers

2.1. Water-Soluble Plasticizers

  • Water
Water can be considered a natural, low-molecular-weight but volatile plasticizer for most biopolymers. The water content of biopolymers readily changes with the environment’s relative humidity through sorption/desorption phenomena [22][12]. Water molecules can markedly reduce the glass transition temperature of the biopolymer by increasing the free volume between the polymer chains. Moreover, it is the main solvent of alginate, and there is a strong chemical interaction between alginate and water. As a result, water can be considered the most powerful natural plasticizer [40,52][4][13]. Treenate et al. [54][14] investigated the effect of water as a co-plasticizer (glycerol/water or sorbitol/water) on the Tg of chitosan/alginate films plasticized with 25, 40, and 50% w/w plasticizer. The DMA results showed that there was a dramatic decrease in the Tg of the plasticized films, especially for the film plasticized with glycerol. They claimed that the increase in water content led to the solvating of the plasticizer molecules in the water; as a result, the plasticizing ability of those plasticizers was enhanced. Accordingly, a more flexible film could be obtained by using water as a co-plasticizer [54][14].
For a better understanding of water permeation through edible films when water is either in its vapor or its liquid state, Hambleton et al. [79][15] studied the physicochemical properties of films based on iota-carrageenan and sodium alginate at three different relative humilities, 0%, 43%, and 84%. According to the Schroeder paradox, for the same activity differential, the mass transfer of permeate depends on its physical state. Water as a permeate plays a key role in the diffusion mechanism for most food systems, and it can modify the matrix structure and induce the diffusion of hydrophilic molecules into the polymer. The water vapor transfer rate (WVTR) results, in their study, showed that increasing the humidity differential led to increased WVTR. As water acts as a plasticizer in hydrophilic films, it can decrease the density or local viscosity by promoting the diffusion of molecules. This has been shown by other authors who indicated that the increase in relative humidity led to the increased permeability of the polysaccharide-based films due to swelling or plasticization [31,51][16][17]. Nevertheless, the liquid water transfer rate (LWTR) showed a completely different behavior than WVTR for both biopolymers, as the Schroeder paradox suggests. Since the transfer equilibrium was reached very quickly, Hambleton et al. concluded that the LWTR of both films was several orders of magnitude higher than the WVTR because there was no transfer resistance caused by the stagnant layer as it was supposed in WVTR. However, they suggested that Iota-carrageenan-based films had a different swelling behavior compared with sodium alginate-based films. Alginate-based films swelled very quickly, before penetrating the liquid water inside the films. The cross-linked chains of the alginate-based films known as the ‘‘egg-box’’ structure limited the available sites for water absorption and promoted the partial solubility of film components.
In the study performed by Olivas and Barbosa-Cánovas [31][16], the effect of RH on WVP (water vapor permeability) of alginate-based films was studied. According to their results, the WVP of films investigated under a RH differential of 100–0% was higher than that of films conditioned at 76–0% RH. This indicates that the WVP of films can be affected by the water activity of products as well as the RH of the environment. The decrease in the capacity of films to function as barriers to water vapor and gases with increasing RH has been reported in other studies [80[18][19],81], in which it was concluded that water may play the role of plasticizer in hydrophilic polymers such as alginate.
The plasticizing efficiency of water on the mechanical, thermal, and physical properties of alginate has been investigated widely in other studies [50,53,55][20][21][22]. Barbut and Harper [55][22] studied the effect of different relative humidity on the mechanical and physical properties of “dried” alginate films. According to their results, the 57% RH film was less transparent than its 100% RH counterpart because of the presence of salt crystals on the film conditioned at 57% RH. In addition, films conditioned at 57% and 100% RH had different mechanical properties: all of the films conditioned at 100% RH had higher elongation at break and lower tensile strength and Young’s modulus than their corresponding films conditioned at 57% RH, and the alginate films with glycerol had remarkably lower tensile strength when conditioned at 100% compared with those conditioned at 57% RH. They concluded that these differences in mechanical properties of films conditioned at 57% and 100% RH can be attributed to the plasticizing effect of water in the films. The mechanical properties of alginate-based films at different RH were also investigated by Hambleton et al. [79][15]. They showed that the elastic modulus and tensile strength decreased with increasing relative humidity, but the relative humidity affected elongation at break oppositely. This means that water has a plasticizing effect on alginate. According to the DSC results, increasing the moisture content led to a decreased glass transition of the plasticizer-enriched phase, which confirms the plasticizing effect of water on the biopolymer. Olivas and Barbosa-Cánovas [31][16] proved that the mechanical properties of alginate-based films are strongly affected by RH. By increasing RH, there was a decrease in tensile strength and an increase in the elongation of all films, indicating the plasticizing efficiency of water. In addition, when adding a plasticizer, the mechanical properties of films were more affected by changes in RH.
The effect of water as a plasticizer in “wet” alginate films has been investigated in a few studies [55,56,82,83][22][23][24][25]. Although “wet” films have been used commercially, there are few published scientific studies about them, and most of the papers have investigated the properties of ‘dried’ biopolymer films to be used after casting. “Wet” films are films that contain approximately 90% to 95% water, and are used mostly in the sausage industry. For the preparation of these films, the co-extrusion method has been used in recent years, and the films are not dried before being applied to the product. Co-extruded alginate casings are a type of edible biopolymer film that is mostly used for sausage casings [56][23]. Harper et al. [83][25] studied the mechanical properties of “wet” alginate-based films. According to the results, the elongation at break values was much higher than the values reported in the literature for “dried” alginate-based films. They concluded that the water in the ‘wet’ films acts as a plasticizing agent since plasticizers are known to increase the elongation at the break of films.
In addition, water can also be used as a “destructuring” agent in the thermo-mechanical mixing method to reduce the intermolecular bonds of the polymer and reach a molten state and homogenous phase [19,47,57][26][27][28]. To decrease the processing temperature, water and other plasticizers are added because polysaccharides such as alginate usually melt after degradation temperature, and this can lead to degraded polymeric material with diminished properties [47][27]. Gao et al. studied the effects of water as a destructuring agent and glycerol as a nonvolatile plasticizer on the thermal and mechanical properties of thermo-mechanically produced alginate films [19][26]. They showed that under the thermo-mechanical mixing method, the alginate particles were significantly destructured as demonstrated by SEM and XRD, and the mechanical properties. According to the XRD results, the unplasticized alginate film showed sharper peaks at 2Ɵ = 13.5° and 21.6° compared with the neat alginate powder. The reason might be the rearrangement of polymer chains and macromolecular networks when adding water during the film-forming process, which can improve the mobility of alginate chains. They also measured the water content of the alginate-based films after equilibration at 25 °C and 57% relative humidity since water also acts as a plasticizer in the system and then affects the thermal and mechanical properties of samples. According to the results, the water content of the films increased remarkably with increasing plasticizer (glycerol) concentration because glycerol has three hydroxyl groups that can bond to water molecules through hydrogen bonding and increase the water content of the films. In addition, the water content in plasticized alginate was higher than that of starch and chitosan-based film [84[29][30],85], indicating the fact that alginate possesses higher hydrophilic characteristics compared with other polysaccharides. However, according to the results obtained by Jost et al. [15][31], moisture content decreased with the addition of plasticizers such as glycerol and sorbitol because of their lower water-binding capability and less hydrophilic nature compared with alginate. The plasticizing efficiency and hygroscopic effect of glycerol have been reported widely in various studies and will be discussed in further detail in the following sections.
In another study on the effects of two plasticizers (glycerol and sorbitol) and their mixtures on alginate properties, the effects of water as both a plasticizer and destructuring agent in the preparation of alginate-based films by thermo-mechanical mixing method were highlighted [47][27]. It was reported that the water content of the glycerol-plasticized film was higher than that of the sorbitol-plasticized film because of the higher hydrophilicity of glycerol. Since water can also act as a plasticizer, the glycerol-plasticized films had higher plasticity than their sorbitol-plasticized counterparts. However, when the glycerol concentration exceeded 40 wt.%, the glycerol-plasticized alginate showed lower elongation at break than the sorbitol-plasticized alginate. This behavior has been reported in other studies on starch-based film [86][32] and is due to a segregation phenomenon caused by glycerol accumulation in different parts of the sample. This phenomenon has been also seen when high amounts of water (high relative humidity) are incorporated in other polysaccharides like starch, even with the addition of a low amount of glycerol [87][33]. These findings indicate that a high amount of water in the system can destroy the strong hydrogen bonds between starch–glycerol and starch–starch species, instead, the weaker hydrogen bonds between starch–water and glycerol–water species have been created, resulting in a decreased elongation at the break of the films.
Souza et al. [57][28] prepared CS (corn starch) and CS–SA (corn starch–sodium alginate) films by using single- and twin-screw extruders and a hot press, with 15% glycerol and different amounts of water. The mechanical properties of CS–SA blends were affected by the SA composition and water content. To control the water content in the blends, the samples were conditioned at different levels of relative humidity (RH). At low RH, 5%, the films were fragile and difficult to test. For all samples, the increase in water content to 9 and 15% led to a change in the state of the films from glassy to rubbery materials, a decrease in Young’s modulus and tensile strength, and an increase in the elongation at the break of the samples. Hence, they concluded that the macromolecules comprising the system can form a stronger entangled network at higher water contents.
However, water has a permanent equilibrium with the environment and is volatile at low temperatures. As a result, nonvolatile plasticizers are also added to the polymer to increase the flexibility of the final product. It is worthwhile mentioning that more stable properties can be obtained by the incorporation of nonvolatile plasticizers since the evaporation and migration of the plasticizer to the surface of the product can be prevented significantly during storage [19][26].
2.
Polyols
Polyols such as glycerol, ethylene glycol, diethylene glycol, tri-ethylene glycol, polyethylene glycol, xylitol, sorbitol, and mannitol are considered effective plasticizers to improve the properties of biopolymer films. In the case of alginate-based films, it has been reported that those plasticizers can improve the microstructure, ductility, and flexibility of the resulting plasticized alginate films [13,17][11][34].
  • Glycerol
Glycerol is a triol (a polyol with three hydroxyl groups) that is highly soluble in water. It can be recovered from the trans-esterification of fat and oils in biodiesel plants as well as by saponification and hydrolysis reactions in oleo-chemical plants. Glycerol is a by-product of those reactions, and it may contain impurities such as water, soap, remaining catalysts, salt, and free fatty acids [88][35]. It is extensively used in either pure or mixed form in the food industry as a food ingredient, as well as in cosmetic products and pharmaceutical formulations, thanks to its superior properties and low toxicity. In addition, glycerol is commonly used as a plasticizer for edible films since it has been approved by the FDA as a safe food additive [40][4]. Glycerol is a polar and nonvolatile material (boiling point of 290 °C) with high availability [77][36]. These characteristics combined with the acceptable mechanical and thermal properties of final glycerol-plasticized biopolymer films make glycerol an effective and one of the most widely used plasticizers in the industry. Glycerol has three hydroxyl groups which allow the creation of strong hydrogen bonds with the hydrophilic polymer and increase intermolecular spacing by reducing internal hydrogen bonding. The addition of glycerol reduces the brittle nature of alginate and provides the desired extent of flexibility [13,15,17,55][11][22][31][34]. As a result, glycerol-plasticized alginate has been extensively used for making composites or films [1][37].
In the study performed by Avella et al. [17][11], the influence of increasing amounts of glycerol (33, 45, and 50%) on the chemical and physical properties of sodium alginate-based films was investigated. They analyzed two types of alginates: one of them was high in guluronic acid content with a higher molecular weight (with the identification code “Ap”) and the other was low in guluronic acid content with a lower molecular weight (with the identification code “Ar”). The DSC results for the plasticized Ap samples showed that the incorporation of 33% glycerol decreased the water-releasing temperature (endothermic peak) from 130 to 110 °C, but the increase in glycerol content to 45 and 50% led to an increased water-releasing temperature. Because of the interaction between alginate and glycerol, water is expelled from the alginate structure, leading to a lower water-releasing temperature. However, for the samples with higher amounts of glycerol, there was an excess amount of glycerol that interacted with water, and as a result, the release of the bound water occurred at a higher temperature. The analysis of the DSC curves of the plasticized Ar system showed that the endothermic peak of samples containing 33 and 45% glycerol increased compared with the neat alginate, but there was a decrease in the endothermic peak of samples containing 50% glycerol, indicating a clear plasticizing effect of glycerol in the blends containing high amounts of glycerol. This means that the introduction of glycerol in the first two blends (33% and 45% of plasticizer) did not appreciably change the molecular network of the polymer; however, only in a blend containing 50% glycerol could an early plasticizing effect be seen, which was confirmed by the DMTA results, indicating there was a lower glass transition temperature of the glycerol in a blend containing 50% glycerol compared with the other samples. To interpret these results, they suggested that the higher molecular weight of the Ap system is an indicator of longer average chain length and the higher fraction of guluronic acid is responsible for a buckled structure of the polymer, which results in the plasticizer being entrapped quite firmly in the structure. Therefore, the addition of only 33% of glycerol caused a clear plasticizing effect on the polymer. In contrast, the lower molecular weight of the Ar polymer means a shorter average chain length of the polymer, and the higher M/G ratio corresponds to a higher fraction of mannuronic acid in the Ar polymer. As a result, the Ar polymer was more flexible than the Ap system, and water could not be strongly retained in the structure and was released at lower temperatures compared with Ap polymer, according to the DSC results.
Considering the mechanical properties, the Young’s modulus of both plasticized systems was lower than that of the neat polymer. In the Ap polymer, noticeable drops in the elastic modulus and stress at the break of the blends plasticized with 45 and 50% glycerol were detected. It was suggested that the excessive amount of glycerol is responsible for this behavior since it changes the chemical interactions among the polymer, glycerol, and water. This kind of behavior is also highlighted in other studies, confirming that the increase in glycerol content beyond a certain limit can lower the film strength [13,47,58][27][34][38]. In the Ar system, there were slight decreases in the modulus and stress at the break of blends containing 33 and 45% plasticizer in comparison with the neat polymer, and only in the blend with 50% glycerol was a considerable plasticizing effect observed.
In the work performed by Giz et al. [13][34], the synergistic effects of various concentrations of glycerol (0–30%) and calcium chloride (0 to 2%) on the thicknesses, mechanical and thermal properties, transmittance, water vapor permeability, and swelling properties of alginate-based films were investigated. The analysis of the stress–strain behavior of the films showed that the fracture strain values of all plasticized films increased with the incorporation of glycerol since it provides more mobility in polymer structure by replacing hydrogen bonds. Nevertheless, calcium chloride incorporation decreased the fracture strain but increased the tensile strength of the films. Generally, with high amounts of calcium chloride (1 or >1%), only small amounts of glycerol lead to an increase in tensile strength but larger amounts of glycerol have a deteriorating effect. They suggested that the increase in either glycerol or calcium chloride beyond certain limits leads to a weaker film. The investigation of the effects of various concentrations of calcium chloride and glycerol showed that their effects on the mechanical properties are nonlinear and synergic. It was also shown that crosslinking with calcium chloride had a negligible effect on tensile behavior without glycerol, but the incorporation of glycerol resulted in cross-linked films with better mechanical properties. By optimizing the mechanical properties of the samples containing various amounts of glycerol as a plasticizer and calcium chloride as a crosslinking agent, they concluded that films with 10% glycerol had the best mechanical properties. However, they suggested that wide parametric studies are needed to accurately predict the properties. In addition, the swelling behavior of the films was investigated by performing the test in three solutions: water, acetic acid, and citric acid. It was shown that swelling of the films decreased in all three solvents with increasing calcium chloride concentration, but swelling increased with increasing glycerol content in the water and citric acid.
Gao et al. [19][26] investigated the effects of glycerol as a plasticizer on the thermal and mechanical properties of alginate-based films. The produced films were transparent, flexible, and homogeneous, but the surface of the films containing more than 30 wt% glycerol was sticky, indicating that glycerol migrates to the surface of the film at higher contents. In addition, they reported that with rising plasticizer content, the film absorbed more water, since glycerol contains three hydroxyl groups which react with water molecules through hydrogen bonding and increase water content in the system. Olivas and Barbosa-Canovas [31][16] reported the same behavior in glycerol-plasticized alginate with increasing glycerol content, showing that the films plasticized with glycerol exhibited higher moisture content than pure alginate, especially at higher RH. In addition, the thermal stability of plasticized and pure alginate was assessed by thermogravimetric analysis (TGA) in two different environments (helium and air). It was found that the degradation temperature of the plasticized films was lower than that of neat alginate, indicating that the addition of glycerol reduced the thermal stability of alginate-based films, which is in agreement with the work on plasticized chitosan [89][39]. Moreover, the mechanical properties of the films were investigated using a uniaxial tensile testing machine. The stress–strain curves of the films showed that without a plasticizer, the material was brittle, but the incorporation of 10 and 20 wt% glycerol made the films more ductile, with plastic deformation. However, with high amounts of plasticizer (more than 30%), the modulus of the films reduced considerably with a decreased elongation at break as a result of the segregation phenomenon. Such behaviors were also reported for plasticized starch [90][40] and amylose amylopectin films plasticized with 30 wt% of glycerol [91][41].
Although glycerol is considered a nonvolatile plasticizer, it can evaporate even at low temperatures (about 10% loss at 25 °C) [77][36]. The large drying surface area in the solvent casting method is mainly responsible for this plasticizer loss. However, there are very few research works investigating the dry matter of glycerol in the film after drying. Bagheri et al. [77][36] found that the films dried at higher temperatures contained lower amounts of glycerol because of evaporation loss. Because of this plasticizer loss during drying at higher temperatures, the amount of film-forming solution decreased and the produced films possessed lower thickness compared with the films dried at lower temperatures. Silva et al. [59][42] also obtained the same result regarding the thickness of films. They determined the glycerol content in alginate film after drying at 30–60 °C and reported that at temperatures above 40 °C, a significant amount of glycerol was lost during drying. Moreover, the films dried at 60 °C were less plasticized due to glycerol loss during drying at higher temperatures.
  • Ethylene glycol
Ethylene glycol is an important organic material that is used extensively in industrial processes. It has unique properties and applications and has been synthesized through different methods such as reaction processes derived from fossil fuels, as well as biomass-based resources. Ethylene glycol is the simplest diol with a molecular structure that contains two hydroxyl groups [92][43]. Ethylene glycol can change the physical, thermal, and mechanical properties, as well as the water vapor permeability of polysaccharide-based films such as starch and alginate. Ethylene glycol, diethylene glycol, tri-ethylene glycol, and polyethylene glycol with various molecular weights are commonly utilized for plasticizing films.
In the study by Pongjanyakul and Puttipipatkhachorn, the effects of glycerin and polyethylene glycol 400 (PEG400) as plasticizers on the physicochemical properties of sodium alginate magnesium aluminum silicate (SA-MAS) micro-composite films were investigated [30][44]. The objective of their work was to use SA-MAS as a coating material for modifying drug release from tablets such as acetaminophen (ACT) and study the effects of plasticizers on the efficiency of the coatings. According to the findings, both plasticizers (glycerin or PEG400) could be used to improve the physicochemical properties of the SA-MAS films to control the drug release from tablets. As shown by FTIR spectroscopy, there is a possibility that the plasticizers interact with alginate and MAS through hydrogen bonding. Moreover, powder X-ray diffraction (PXRD) results revealed the possibility of intercalating glycerin and PEG400 into the silicate layers of MAS. Compared with glycerin-plasticized film, the film with PEG400 had higher crystallinity, which was confirmed by the higher intensity of the peak in the PXRD graph of the PEG400-plasticized film. This can lead to different thermal behavior of the films with PEG400, which was investigated by DSC. It was shown that glycerin and PEG400 had different thermal behavior than the neat alginate. The intensity of the exothermic and the endothermic peaks decreased with increasing glycerin or PEG400 content, which could be related to the recrystallization and phase transition of the films after heating. In addition, the degradation temperature (exothermic peak) of the films decreased by 34 °C with the incorporation of 50% glycerin, but only by 4 °C with the incorporation of 50% PEG400, indicating a different crystal structure of the films plasticized with PEG400, as confirmed by PXRD results. Considering the mechanical properties, glycerin was a better plasticizer than PEG400 and gave more flexibility to the films. This is in agreement with another study on the effects of both plasticizers on alginate films [93][45]. The main reason for the better plasticizing efficiency of glycerin is that it is a smaller molecule than PEG400. In addition, the higher crystallinity of PEG400-plasticized film increased the rigidity of the film compared with that with glycerin. The results for water vapor permeability (WVP) revealed that both plasticizers decreased the WVP of alginate when the concentration was 10–30%. They explained that intermolecular hydrogen bonds were formed by penetrating the plasticizers between alginate chains and silicate layers of MAS. This can increase the tortuosity of the pore channels in the films and reduce the WVP of the film. However, the increase in plasticizer amounts by up to 50% led to an increase in the WVP of the films because of the hygroscopic nature of both plasticizers. In addition, the films plasticized with PEG400 had a lower WVP than that of the films with glycerin, because of the higher crystallinity of the films with PEG400, as shown by the PXRD results. The lower WVP of PEG-plasticized films was also highlighted in other studies. For example, El Miri et al. [42][6] investigated the effects of various polyol-based plasticizers, such as glycerol, diethylene glycol (DEG), and polyethylene glycol (PEG), on the properties of alginate-based films filled with cellulose nanocrystals (CNC), and concluded that the WVP of films plasticized with PEG and DEG was lower than that of GLY-plasticized film. They concluded that this can be explained by the degree of hygroscopicity and the chemical structure of different plasticizers. Although GLY contains three hydroxyl groups, PEG and DEG have one and two hydroxyl groups, respectively, in their structures. As a result, GLY can hinder intermolecular and intramolecular bonding in the network, increasing the free volume and penetration of water molecules into the film and increasing the WVP of the film, but this phenomenon did not occur in the films plasticized by PEG and DEG, thus the WVP was lower. The lower viscosity of PEG- and DEG-plasticized alginate compared with glycerol-plasticized solutions can be explained similarly since the glycerol molecule with three OH groups suppressed the polymer chain motion and increased the viscosity of the film-forming solution. They also reported that PEG- and DEG-plasticized films had better mechanical properties than glycerol-plasticized films. Because of the lower molecular weight of glycerol compared with the two other plasticizers, the glycerol-plasticized film was more flexible. However, it had a lower tensile strength and Young’s modulus than the PEG- and DEG-plasticized films.
Olivas and Barbosa-Cánovas (2008) [31][16] developed alginate-based films with two different M/G ratios (0.45 and 1.5) with the incorporation at a 40% level of four different plasticizers: PEG-8000, glycerol, fructose, or sorbitol. It was observed that after drying, all the plasticized films were transparent except the film plasticized with PEG-8000. The PEG-plasticized film was opaque and white, which could be related to the phase separation between alginate and PEG. In other studies on edible films plasticized with PEG, the white residue on the surface of the film was observed at a high molecular weight of PEG (higher than 1450) [94,95][46][47]. Olivas and Barbosa-Cánovas analyzed the films by moisture sorption isotherms, and all plasticized films, including the PEG-8000-plasticized film, adsorbed more water in comparison with the non-plasticized films. However, the results obtained from mechanical testing showed that the incorporation of PEG-8000 diminished both tensile strength and elongation at the break of the alginate-based films in contrast with glycerol-plasticized film. It is also worth noting that although all plasticized films including PEG showed similar sorption isotherm curves, even at high RH, PEG did not increase the elongation at the break of the film. No difference between the elongation of PEG-8000-plasticized and unplasticized films was recorded, probably because of the phase separation between alginate and PEG, as indicated by the white surface of the PEG-plasticized film.
  • Sugar alcohols
Sugar alcohols are another group of polyols that are produced from the reduction of sugars. These polyols can be produced from the hydrogenation of three main sources: monosaccharides, disaccharides, and a mixture of saccharides and polysaccharides. For example, sorbitol (glucitol) is a polyol with six carbons that is used extensively for plasticizing polysaccharide-based films. It is produced by the hydrogenation of glucose (a kind of monosaccharide with six carbons) [96][48]. Apart from sorbitol, mannitol, maltitol, and xylitol [60,61][49][50] are examples of sugar alcohols used for plasticizing alginate.
Jost et al., (2014) [15][31] investigated the effect of two plasticizers (glycerol and sorbitol) on the microstructure, WVP, and mechanical properties of alginate-based films. The concentrations of glycerol and sorbitol were set at 20–40%, and 30–50%, respectively, because of sufficient flexibility at these ranges. Color measurement of the samples showed that the plasticized films were more transparent than pure alginate. Moreover, the sorbitol-plasticized films were more transparent than the glycerol-plasticized films since sorbitol molecules have a better steric fit to the alginate structure than glycerol molecules. SEM micrographs also showed that sorbitol-plasticized films were smoother than both neat alginate and glycerol-plasticized films. Considering the mechanical properties, both plasticizers showed a plasticizing effect on alginate-based film by decreasing the tensile strength and increasing the elongation at break. However, glycerol seemed to be more effective since the incorporation of 30% glycerol was as effective as the incorporation of 50% sorbitol. They explained that because the molecular weight of glycerol is almost half that of sorbitol, the amount of glycerol needed is more than half the amount of sorbitol needed to achieve the same mechanical properties. This shows that although glycerol is a more effective plasticizer based on mass content, the plasticizing efficiency of sorbitol was higher at the molecular basis. However, in another study, Tong et al. [36][51] concluded that glycerol had the greatest plasticization effect on pullulan–alginate–carboxymethylcellulose blend films compared with three other plasticizers (sorbitol, xylitol, and fructose) because lower amounts of glycerol were needed to increase the elongation at break.
In addition, Jost et al. [15][31] found that water vapor and oxygen permeability of alginate increased with increasing glycerol content, but not with increasing sorbitol. They explained that the different behavior of glycerol and sorbitol could be related to different plasticizing mechanisms. It has been proven by water vapor and oxygen permeability, as well as water content results, that the plasticizing behavior of glycerol can be explained by the free volume theory. However, the sorbitol behavior did not obey this theory but the gel theory. The sorbitol molecules had a good fit in the alginate macromolecule network, which not only reduces the intermolecular interactions between alginate chains but they can also bond chemically to the polymer through six hydroxyl groups. As a result of this interaction between alginate and sorbitol, the flexibility of the film increases and at the same time, water molecules will interact with alginate. Accordingly, the barrier properties can be controlled, even at higher sorbitol concentrations. Lower WVP levels of sorbitol-plasticized films were also highlighted in other studies [31,97,98][16][52][53]. Parris et al. [99][54] demonstrated that, unlike glycerol, the presence of sorbitol in alginate-based films can decrease the WVP of the films. However, they concluded that this difference is due to the lower efficiency of sorbitol compared with glycerol in reducing intermolecular interaction between alginate chains rather than their different plasticizing mechanisms.
Santana and Kieckbusch [60][49] investigated the effects of polyols (glycerol (G), xylitol (X), and mannitol (M)) as plasticizers on WVP and the physical and mechanical properties of alginate-based films. They observed that the incorporation of glycerol and xylitol led to more transparent and uniform films with higher hydrophilicity compared with mannitol-plasticized film, which was whitish before crosslinking. According to the SEM results, this whitish appearance is related to crystal formation in the film, which disappeared after immersion in CaCl2 solution. The plasticized films were thicker than the neat alginate films, with the M-film being even thicker. They explained that the high thickness of the G- and X-films could be attributed to the increase in the free volume between chains, making the films less dense, but the high thickness of the M-film is highly likely due to the phase separation between alginate and mannitol. However, the mannitol-plasticized film had the lowest WVP among the samples because it is less hygroscopic. Moreover, considering the mechanical properties, they concluded that glycerol and xylitol were more efficient plasticizers than mannitol.
3.
Sugars
Monosaccharides, disaccharides, and oligosaccharides are the sugars most commonly used as plasticizers in polysaccharide-based polymers, such as starch and alginate, because of the similarity between the molecular structures of the plasticizers and polymers. Monosaccharides are the simplest carbohydrates, and they cannot be hydrolyzed into smaller carbohydrates. Glucose, fructose, sucrose, and lactose are some examples of sugars that come from different sources such as fruits or milk.
The plasticizing efficiency of fructose was investigated by Olivasa and Barbosa-Canovas [31][16]. According to their results, because of the similarity in the sorbitol and fructose molecular weights (182.17 and 180.16, respectively), the plasticized films with these plasticizers possessed similar properties. At higher relative humidity (76% and 100%), the WVP of the films containing fructose was the lowest among the plasticizers. They reported that the low WVP of fructose-plasticized alginate films is comparable to that of lipid-plasticized whey protein emulsion film [100][55]. However, there was no considerable difference between the WVP of fructose and sorbitol-plasticized films at other RHs. They also showed that the films containing fructose and sorbitol have similar tensile strength and elastic modulus. The similar behavior of sorbitol and fructose is also highlighted in another study. According to Tong et al. [36][51], sorbitol- and fructose-plasticized blend films exhibited the lowest and similar elongation at break values at any given plasticizer concentration compared with glycerol and xylitol-plasticized films, with the fructose-plasticized film being even more brittle with higher tensile strength and lower elongation at break. However, the moisture contents of films containing fructose were lower than those of films containing sorbitol, maybe because of their different molecular structure. As a result of this behavior, the WVP of fructose-plasticized films was less affected by the plasticizer concentration. Zhang and Han [101][56] also reported that monosaccharide-plasticized pea starch films were more resistant to water vapor permeation than polyol-plasticized films, although their mechanical properties were comparable.
Fruit and vegetable purees have a good potential to be used in making edible films. Since they are high in sugars such as glucose, fructose, and saccharose, they can have a plasticizing effect on the produced film [102][57]. In the study by Kadzińska et al. [63][58], the physical properties of apple puree–sodium alginate edible films incorporated with vegetable oils were investigated. They found that in the samples containing apple puree, the glass transition temperature was lower, indicating the plasticizing effect of natural plasticizers such as fructose and glucose present in apple puree.

2.2. Water-Insoluble Plasticizers

All the plasticizers introduced above, such as sorbitol, glycerol, and polyethylene glycol, have hydrophilic characteristics. This hydrophilicity can lead to an increase in WVP and a tendency of the polymer to absorb more water from the environment. Hydrophobic plasticizers, such as vegetable oils, triacetin, tributyl citrate, triethyl citrate, and fatty acids are good alternatives to solve this problem.
  • Oils
Oils are of special interest for use as plasticizers in polysaccharide-based films because they are natural and hydrophobic; in addition, they are safe for food packaging applications since they have low toxicity and a low tendency to migrate to the surface as a result of their high molecular weight. In some research works, oils such as castor oil [64][59], olive oil [61][50], soybean oil [65][60], coconut oil [66][61], canola oil [67][62], cinnamon oil [68[63][64],69], oregano essential oil [51][17], and garlic oil [70][65] have been used as both antibacterial agents and plasticizers in alginate-based films. However, in these research studies, glycerol or other low molecular weight plasticizers were the main plasticizers in most cases, and oils were introduced for controlling the hydrophobicity and barrier properties, as well as bacterial and microbial activity of the films, but they had also a plasticizing effect on the films, usually decreasing tensile strength and increasing elongation at break of the polymer [51,64,65,67,69][17][59][60][62][64].
Benavides et al. [51][17] investigated the effect of oregano essential oil (OEO) on the antibacterial, optical, mechanical, and WVP of an alginate-based film. Before being applied to the film-forming solution, OEO was mixed with Tween 80 to create a uniform and stable dispersion in the alginate matrix. Tween and Span are the most common surfactants used for obtaining a uniform solution in different research works when adding hydrophobic plasticizers, such as oils to hydrophilic polymers like alginate [51,61,65,67,68,69,71][17][50][60][62][63][64][66]. In some other studies, however, a diluted solution of oil from ethanol was used to decrease the surface tension of coating-forming emulsions [64,66,70,103][59][61][65][67].
Benavides et al. [51][17] showed that the incorporation of OEO affected the appearance and physical properties of alginate film, making it less transparent and more flexible, with a lower WVP compared with neat alginate. They reported that elongation at the break of the films increased significantly with the addition of OEO, ranging from 2.2% for neat alginate to 3.7% for the film containing 1.5% of OEO, while the tensile strength of the OEO-alginate films decreased by 56% compared with the control sample. These results were consistent with those obtained by other researchers [64,70,72][59][65][68]. They explained that the incorporation of oils usually reduces the TS of the hydrophilic polymer because oils weaken the films by reducing the intermolecular interaction between polymer chains. On the other hand, at room temperature, oil in the liquid state is in the form of small droplets dispersed through the polymer structure that can be easily deformed, which enhances the film’s flexibility. As a result, oils can act as plasticizers by reducing the TS and increasing the EB of the films. However, in some studies, different results have been obtained. For example, in the study by Frank et al. [68][63], no significant differences between the tensile strength of the cinnamon essential oil/alginate films and control alginate films were observed, and the EB of cinnamon essential oil/alginate films decreased significantly with increases in the concentration of oil. They suggested that at high essential oil concentrations, stressed regions in the film may be created as a result of the formation of discontinuities between two phases, which can lead to rupture and decreased EB. In another study by Gutierrez-Jara et al. [65][60] about the effect of soybean oil and degree of crosslinking on the physical and mechanical properties of alginate-based films, it was found that at high calcium chloride concentrations, the EB of alginate decreased with increasing oil concentrations. They explained that the rigid surface layer created by calcium chloride as a crosslinking agent predominated over the plasticizing effect of the oil, making the film even more rigid with low EB.
It has been reported in many research works that the WVP of alginate-based films is generally decreased with the incorporation of oil [51,64,65,67][17][59][60][62]. It has been suggested that the hydrophobicity of oil is the main reason for this behavior, which makes the film more hydrophobic and as a result, reduces the absorption of water molecules. They also suggested that the lower porosity and higher tortuosity in the polymer microstructure brought by oils are other important reasons for the improvements in the WVP of films. In contrast, some studies reported an increased WVP of films with increasing oil content [69,70][64][65] because of the formation of pores via changes in the internal structure of the matrix.
  • Other water-insoluble plasticizers
Apart from oils, other hydrophobic plasticizers, such as fatty acids and citrates, have also been used for plasticizing alginate-based films.
Fatty acids are part of the structure of fats, oils, and waxes. Fatty acids can be defined as any long chain of hydrocarbon, with a single carboxylic group and aliphatic tail. Fatty acids, such as stearic acid, lauric acid [50][20], and oleic acid [3][8], have been used as plasticizers in a few studies. In the study conducted by Chen et al. [50][20], increases in the opacity of soy protein isolate/sodium alginate films as well as changes in the homogeneous structure of the films with the incorporation of both stearic acid and lauric acid were reported. As a result of the heterogeneous structure, the intermolecular interactions of the matrix might have been weakened, which led to the decreased tensile strength of the films. In addition, the elongation at the break of samples decreased with the addition of fatty acids because of the interaction of the polymer with polar plasticizers rather than fatty acids, which led to the deterioration in the flexibility of the films. However, at higher concentrations of lauric acid, elongation at the break of the film increased considerably since lauric acid has a shorter carbon chain compared with stearic acid, and hence, had a better plasticizing effect due to the higher mobility of the molecules. The water vapor permeability results showed that although the barrier properties of the films improved at lower concentrations, they diminished at higher concentrations. The researchers explained that the excessive amounts of fatty acids created a discontinuity on the film surface, which led to increased WVP values. Azucena Castro-Yobal et al. also reported that by adding oleic acid to alginate film, the WVP value slightly decreased compared with the control sample as a result of the hydrophobic nature of the fatty acid.
Citrate plasticizers are another type of hydrophobic plasticizer whose effect on alginate films has been investigated in some studies [33,34,44,93][9][45][69][70]. These plasticizers are tri- or tetra-esters that are produced by the esterification of citric acid. Citric acid is obtained from citrus fruits, sugar cane, and beetroots, and it is also used as a plasticizer [40,104][4][71]. Sharmin et al. [34][70] investigated the effect of citric acid (CA) as a crosslinking agent on the physical, mechanical, and barrier properties of alginate film and found that CA can act as a plasticizer at higher concentrations. CA was added at 0.5, 1, and 2 w/v% to alginate. At low concentrations (0.5 and 1 w/v%), the water vapor transition rate (WVTR) of the films decreased, indicating the creation of hydrophobic ester groups between citric acid and alginate chains. However, at higher concentrations, the WVTR increased because of the plasticizing effect of CA at higher amounts. On the other hand, according to mechanical testing, the plasticization effect of CA was seen only at high concentrations, with an increase of 475% in the elongation at a break of the 2% CA-alginate film compared with the control sample. These researchers explained that the presence of unreacted CA in the structure led to reduced intermolecular bonding between alginate chains; hence, CA acts as a plasticizer when added in excess amounts.

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