**2. Formation of PVA/starch films**

*Fillers - Synthesis, Characterization and Industrial Application*

expose a serious ecological problem due to hydrophobic properties and very low water vapor transmission rate [2, 3]. The growing public interest on the environment is induced by a growing research on biohybrid films (i.e., biobased films) as alternatives to traditional nondegradable plastics due to the harmful effect of petroleum-based plastic packaging [4]. The eco-friendly polymeric resources can be categorized into three main groups depending on the raw material used: renewable natural, biodegradable synthetic polymers, and microbially produced biopolymers. The renewable natural polymers can be obtained from several sources such as starch, cellulose, chitosan, etc. [5], while biodegradable synthetic polymers such as polyvinyl alcohol, polycaprolactone, polylactic acid, polybutylene succinate, and copolymers are produced by using natural or petroleum-based monomers. On the other hand, microbially produced biobased polymers (e.g., polyhydroxybutyrate

and valerate copolymer) are manufactured via various microorganisms.

films because of changing amylose and amylopectin ratio [7–10].

tapioca have been studied in combination with PVA polymers [9, 10].

PVA is an important polymer having superior gas barrier properties along with higher strength, tear, and flexibility than those of natural biobased polymers. Nevertheless, it has weak dimensional stability owing to high water uptake. Furthermore, PVA has relatively high manufacturing cost in comparison with the other commercial polymers in the market. Thus, if PVA is blended with renewable and abundant natural sources like starch, the manufacturing costs can be reduced. This method also resulted in improved moisture resistance and rapid biodegradation [17, 18].

Over the last 10 years, hundreds of studies about PVA/starch biobased films have been carried out on the topic using various production techniques. The researchers

Starch among all natural biopolymers has been believed to be one of the most suitable biopolymer resources due to its biodegradable, regularly available, and inexpensive features. There are two major polymers of starch: amylose and amylopectin. Amylose is a linear molecule with a spiral structure unlike a branched structure of amylopectin. Moreover, the molecular weight of amylose is commonly a smaller molecule (1–1.5 million), while amylopectin is a large molecule (50–500 million) [6]. Various starches have been used in the biohybrid films because of changing amylose/ amylopectin ratio. And these are classified by amylopectin content [7]. In most studies, biobased films have been manufactured from starch of corn, wheat, rice, potato, tapioca, and cassava [8]. Various starch types have been also used to the biobased

However, the biobased films obtained from starch have demonstrated some disadvantages such as brittleness, low processability, high water sensitivity (i.e., low moisture resistance and hydrophilic character), and poor mechanical properties compared to petroleum-based conventional polymers [37]. Moreover, the starches are not thermoplastic biopolymers due to the intra- and intermolecular hydrogen bonds, because the degradation temperature of starch exceeds the melting temperature [11]. These make them inadequate for some packaging purposes limiting their widespread industrial applications [12–14]. Therefore, physicochemical and biological properties of the starch should be improved. Several modification techniques are needed to improve the mechanical and physical properties to overcome the inadequate features of the starch-based biobased films [15]. Thus, several efforts have been made to improve thermal properties of starch by blending with a plasticizer for its stability, elasticity, and edibility. Despite the above-mentioned modifications and applications, the biodegradable films produced from starch are still limited due to poor mechanical and hydrophilic properties along with susceptibility to the biological attacks. Accordingly, polyvinyl alcohol (PVA) known as synthetic biodegradable polymer and thermoplastic starch was utilized together to obtain excellent compatibility [16]. In several studies, starch was used with polyvinyl alcohol (PVA). As an example, various starches such as corn, potato, rice, and

**2**

PVA/starch-based biodegradable formulations are produced from polyvinyl alcohol (PVA) and starch known as main compositions with different additives such as plasticizer, cross-linkers, and filing materials. Until now, PVA/starch blend biobased films have been prepared using casting (sol-gel or mixing) and thermal (extruder or extrusion) methods by many researchers in the literature.

In casting method, PVA is dissolved in hot water with the gelatinized starch in order to form intermolecular interaction. The obtained mixture is then stirred in a mechanical high-speed mixer for homogenization [19] over 1000 rpm/min and at 85–95°C temperature, and then some of the additives are added into the mixture under continuous stirring. After removing the bubbles formed during the preparation of biobased films by an aspirator, it is dried at room temperature [20]. The hybrid films are generally heated in an oven at 80–95°C for 1 h to induce the crosslinking reactions [21]. Another approach for the fabrication of PVA/starch blend biobased films is using a single or twin screw extruders. Primarily, the plasticizing starch is mechanically mixed with PVA, and then, PVA/starch blend granules are obtained following the extrusion process under the optimum conditions at various temperature and screw speeds. After that, the biobased films are obtained from the prepared granules by using the blown film extrusion or hot-press molding [4, 22].

In casting method, considerable amounts of water are evaporated from aqueous solutions or suspensions with a high energy-consuming process to obtain PVA/ starch hybrid films [23]. The PVA/starch blends are generally produced via solution casting method. Nevertheless, this method has several deficiencies such as low solution density, low manufacturing yield, high energy consumption, etc., which limit its industrial practices [19]. On the other hand, the extrusion films have a great importance due to energy-efficiency process, high productivity, and continuous industrial production possibilities. However, the solution casting method has gained much more attention compared to extrusion process for the production of PVA/starch blend films in scientific publications due to its easy applicability in laboratories [4, 23, 24].
