**3.1 Plasticizers**

Plasticizers are additives that increase the elasticity. These are the ingredients for nonthermoplastic starch, which are added in order to alter their physical properties. Plasticization takes place in the amorphous zone, which has a higher molecular dynamism. The type and the amount of plasticizer have an important influence on the ability to hinder hydrogen bonding along the polymer chains. The major gain obtained from utilization of plasticizers is that the tensile strength (TS) is decreased, while the elongation at break (E%) increases as well as they become more flexible [26]. One of the most important properties of an efficient plasticizer is to be compatible with the polymer matrix. The plasticizers such as glycerol, polyethylene glycol, urea, ascorbic acid, sorbitol, citric acid, and tartaric acid are usually used [19]. However, the plasticizers in the biobased films cause an increase in permeability to moisture, oxygen, and aromatic compounds [27]. **Table 1** presents the various plasticizers and used methods along with the effects of plasticization on some properties of PVA/starch films.

Water is accepted as a basic plasticizer for PVA/starch biofilms. Physicochemical properties of films could differ based on the changing water content. At the same time, the water is also compatible with other plasticizers. When the plasticizers are added into biofilm formulations, the physical properties are affected due to the increasing relative moisture because of compatibility with water. Possibly, water is absorbed because of polarity compliance to the solubility of other plasticizers added as additives. The plasticizer effect of water could usually be effective when it is used also together or not with an above-mentioned plasticizer. Furthermore, glycerol, sorbitol, and citric acid are generally favored as an efficient plasticizer for PVA/starch films. According to previous studies, the E% increased while TS decreased because of increasing glycerol, sorbitol, and citric acid ratio from 10 to 50% in the biobased films. It was also reported that E% and TS of sorbitol or citric acid–added films were higher than those of the glycerol-added films. However, the water absorption property of the biobased film decreased with the increasing glycerol ratio due to its hydrophobicity [29]. Moreover, the swelling behavior of the film containing glycerol was the lowest compared to the sorbitol- and citric acid-added films due to weaker hydrogen bonding capabilities, unlike the solubility value due to weaker hydrogen bonding capable [8].


**Table 1.**

*Effect of plasticization on some characteristics of PVA/starch films along with obtained improvements.*

**5**

*The Effects of Novel Additives Used in PVA/Starch Biohybrid Films*

ability of both urea and formamide as a plasticizer [19].

**of biobased films**

The homogeneity of PVA/starch biobased films could also be enhanced with the addition of urea, like glycerol. However, urea as a plasticizer showed stronger interactions with starch and PVA in biofilms than those of glycerol and sorbitol [11, 28]. Consequently, urea was considered a better plasticizer to improve the flexibility of PVA/starch films [19]. Furthermore, the crystallinity of biobased films was also decreased by the addition of urea and formamide. These agents could penetrate into the crystallization zone of PVA/starch biobased films during the process forming new hydrogen bonds with starch and PVA molecules, which damage the crystal region of PVA in the biobased films. However, formamide is not a good plasticizer and could not develop the compatibility and flexibility of the biobased films, while the compatibility was improved when it combined with urea. When the additives containing both urea and formamide are simultaneously used, their synergistic effects and the compatibility could occur in the blend. Besides, while TS and young modulus of biobased films were significantly decreased, the E% was substantially improved. With the increasing amount of urea in the biobased films, the sensitivity to water increased, while the melting point of blends decreased. It was likely due to the facilitation of molecular

**3.2 Effects of functional group type and number of plasticizers on the properties** 

Physicochemical properties of blend films are substantially affected by the functional groups of plasticizers used in PVA/starch biobased films. The total number of both carboxyl and hydroxyl groups in plasticizers were given in **Table 2** along with their behaviors in films. For instance, as regards the hydroxyl and carboxyl groups of glycerol (H.3, C.0) and succinic acid (H.0, C.2), the E% of the glycerol-added film has shown a high enhancement than that of the films containing succinic acid, contrarily to the TS behavior. However, when malic acid (H.1, C.2) in the same carboxyl number with succinic acid (H.0, C.2) was added to the film, the TS and E% were improved compared to glycerol (H.3, C.0) and sorbitol (H.6, C.0) because of the presence of two functional groups. Depending on the increasing functional groups of plasticizer, TS and E% of tartaric acid (H.2, C.2) added biobased films with two same functional groups were greater than those of malic acid, glycerol, and sorbitol [6, 30]. Furthermore, the biobased films containing citric acid (H.1, C.3) were stronger and more flexible than that of containing glycerol [7, 30] and xylitol (H.5, C.0) [31]. On the other hand, when the glycerol and xylitol added films were compared, it was found that xylitol-added biobased films had a higher strength and more elasticity than glycerol-added biobased films due to its 5 hydroxyl groups [31]. Even a few xylitol molecules can play an extra role in plasticizer than others [32]. Similarly, the comparison of glycerol- and sorbitoladded films showed that TS and E% of sorbitol-added film were greater than glycerol [7, 30]. Consequently, E% value increases while TS decreases with an increase in the

total functional groups and the amount of these plasticizers in blend films.

The concept of plasticization could be understood with the analysis of different properties such as elongation at break (mentioned above) or glass transition point (Tg). For instance, Aydin et al. reported that the addition of plasticizers reduced the Tg point clearly and the change of plasticizing performances could be observed by increasing Tg point. Apart from the above-mentioned plasticizers, 1,4-Butanediol (H.2, C.0), 1,2,6-Hexanetriol (H.3, C.0), pentaerythritol (H.4, C.0), xylitol (H.5, C.0), and mannitol (H.6, C.0) from 2 to 6 hydroxyl groups have also been investigated based on the changes in Tg point. Among the investigated plasticizers, 1,4-butanediol demonstrated the maximum plasticizing effect for starch and PVA due to small molecular size and geometry [32]. **Table 2** shows the effects of the various plasticizers with different functional groups and number on the properties of PVA/starch films.

*DOI: http://dx.doi.org/10.5772/intechopen.81727*
