1 Introduction

The environmental burden of plastics is boundless; from 1950 to 2018, nearly 6.3 billion tons of plastics have been generated worldwide, with 9% and 12% reused and combusted, respectively. The growing human population and consistent demand for plastics and plastic products are to blame for the continuous increase in plastic production, generation of waste, and accompanying environmental pollution [1]. Plastics can be categorized as degradable and non-degradable polymers based on their chemical properties [2]. Bioplastics are a source of hope because they are biodegradable polymers made from natural components, and researchers have spent a lot of time and effort trying to discover a way to replace petroleum-based plastics [3].

Plastics and bioplastics can degrade through abiotic processes such as photodegradation, oxidation, and hydrolysis [4]. Also, microbial degradation has arisen as one of the potential alternative methods of plastic breakdown, rather than the potential degrading procedures of plastic synthetic polymers due to their effectiveness, affordability, environmental friendliness, and sustainability [5]. Understanding the relationship between microorganisms and polymers is crucial for tackling environmental issues related to plastics since many living entities, particularly microorganisms, have evolved techniques for surviving and decomposing plastics. These microorganisms including bacteria [2] and fungi [6] are useful for green chemistry because of their capacity for bioremediation and plastic breakdown, which will help remove toxic polymers from the environment [7].

Polysaccharides are biopolymers that come in a variety of types, and at reasonable prices, and are spread throughout the world. Nowadays, starch is the most widely used polysaccharide in industries (food, cosmetics, paper, textiles, and pharmaceuticals), but its native form has functional properties and processability limitations; thus, modification of starch by physical, chemical, and physio-chemical modifications has been the suitable solution [8]. Starch is made up of granules that are arranged in a specific pattern and have a particular crystalline structure depending on its source. Starch granules are made up of linear amylose and branched amylopectin macromolecules with different ratios relied on their origin and type. Normal corn starch (25 percent amylose, 75% amylopectin), waxy corn starch (0–2% amylose, 98% amylopectin), and amylose-starch (up to 70% amylose) are examples of different types of starch from corn with different amylose and amylopectin ratios [9]. When starch dispersion is cooked at gelatinization temperature with water and a plasticizer such as glycerol, the resulting films are brittle and have poor mechanical and barrier characteristics [10]. To overcome these limitations various approaches have been researched. Blending thermoplastic starch with another synthetic polymer such as (polyvinyl alcohol, polypropylene, poly acrylic acid, poly lactic acid, and polyethylene) [10,11,12,13,14] is being examined to enhance the mechanical and barrier properties of thermoplastic starch.

PVA (polyvinyl alcohol) is a synthetic polymer with a wide range of applications and a strong affinity for starch due to its abundance of hydroxyl groups. The mechanical qualities of polyvinyl alcohol film are good, such as tensile strength and elongation [10, 15, 16]. Thermoplastic starch mixed with polyvinyl alcohol and various plasticizers such as polyol (glycerol, sorbitol, mannitol, polyethylene glycols, and so on…) [17, 18], inorganic salts (CaCl2, AlCl3, MgCl2) [19], and clays (Na-montmorillonite, hallow) [20, 21], as well as metal oxides (such as TiO2, ZnO2) [22, 23], nanoparticles ( such as graphene nanotubes, cellulose nanofibril, and starch nanoparticles) [24, 25], and oils (olive oil, corn oil, and eugenol oil) [26, 27] all have considerable considerations.

Cardanol oil is one of the most promising natural materials in the oleo chemistry and coating industry due to its chemical structure and properties. It is extracted from cashew nutshell after conversion to Anacardic acid. It is composed of a phenol ring linked aliphatic chain of 15 carbon atoms and has more than the unsaturated position at C1 and C3 as shown in Scheme 1 [28].

Scheme 1
scheme 1

Chemical structure of main components of cashew nutshell liquid

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However, there is little debate about the effect of the pre-gelatinization of starch on the film made from it. Also, the use of blending plasticizers such as sorbitol with hydrophobic oil such as cardanol oil to examine the implications on the physicochemical properties and microbial biodegradability of the film fabricated from modified waxy corn starch blended with polyvinyl alcohol.

This research aims to study the effect of physicochemical modifications on waxy maize starch when blending with polyvinyl alcohol. Also, study the effects of sorbitol-plasticized TPS/PVA and sorbitol-cardanol mixes with varying ratios of cardanol on the film performance and microbial degradation.

2 Materials and methods

2.1 Materials

Cargill provided acetylated di-starch adipate (12,605) and pregelatinized acetylated di-starch adipate (12,616) from an importer (AWA Food Solution, Cairo, Egypt), sorbitol syrup 70% dry substance was supplied from (Egyptian Starch and Glucose Company, Cairo, Egypt), cardanol oil (K2P chemicals, Mumbai, India), commercial polyvinyl alcohol from (M.wt 125,000, viscosity 4% at 20 °C 35–50 cS) (Egyptian British Company for chemicals, Cairo, Egypt), alpha-amylase from Bacillus licheniformis (E.C.3.2.1.1, 1000 RAU (reference amylase unit), where RAU is the amount of enzyme that will convert 1 mg of soluble starch per min. into soluble dextrin and oligosaccharide), and glucoamylase from Aspergillus Niger (E.C.3.2.1.3, 570 GAU/g, 1.16 g/ml, where GAU is the amount of enzyme that will liberate 1 g of glucose per hour from starch), both acquired from (GENEN CORE DePunt, China) and laboratory grade absolute ethanol were used.

2.2 Methods

To create thermoplastic starch and PVA films, 4 g of waxy corn starch powder was first gelatinized in 100 ml of distilled water for 30 min at 95 °C, then preparing a 10% solution of commercial PVA by dissolving 10 g of PVA granules was in 90 ml distilled water and stirred at 200 rpm during heating at 80 °C for 30 min.

Starch, polyvinyl alcohol (60:40) % content, and sorbitol by 25% of the whole mixture were blended to make the films as seemed in Table 1. For 30 min, the entire mixture was brought to a boil in a water bath to complete the starch gelatinization and homogenize the mixture.

Table 1 Different concentrations of cardanol oil during film processing
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Cardanol oil of 0.5% was prepared by dissolving the oil in absolute ethyl alcohol, which was then mixed with various percentages (0.2%, 0.4%, 0.6%). Finally, as indicated in Scheme 2, the final slurry was poured onto a petri dish and dried for 72 h at 50 °C in the oven.

Scheme 2
scheme 2

Preparation of TPS of modified waxy maize starch alone and TPS of modified waxy maize starch/PVA film

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In the synthesis of thermoplastic waxy maize acetylated distarch adipate film (WADA) and waxy maize pregelatinized acetylated distarch adipate film (PWADA), all these steps were completed, and various formulations were chosen for further characterization and evaluations, as indicated in Table 1.

2.3 Characterization of prepared films

2.3.1 Biodegradation

Enzymatic biodegradation

The plastic films were cut into 2 cm × 2 cm squares and placed in a tiny beaker containing 100 ml of 1:1 (amylase and glucoamylase) mixing enzymes, 25 ml of distilled water, and control samples without TPS foils, and incubated in 37 °C water bath. The hydrolysate was removed from the beaker at regular intervals and the solid content was adjusted to 1% using a refractometer (Atago a70000, Japan) and an osmometer (Model 3250 Osmometer, advanced instrument, USA) as a function of dextrose equivalent, which is used in the starch hydrolysis industry as an indicator of full starch hydrolysis [29]. The contents of the beaker were filtered through 0.45 filter paper (Mixed Cellulose Nitrate with Cellulose Ester, Sartorius, Germany) after some time, and the filtrate was injected into HPLC (Agilent 1260 infinity) using a metacarpi 87H column (300 × 7.8 mm) (Varian Inc., Palo Alto, USA) at a flow rate of 0.6 l/min [30]. The dextrose equivalent can be calculated from HPLC according to Delheye G. and Moreels E., 1988 [31]. The degradation percentage calculated was from Eq. (1).

$$Degradation\; percentage=\frac{DE}{W0}\times 100$$
(1)

W0 is the weight of starch present in the film before degradation, and DE is the dextrose equivalent acquired by HPLC spectra.

For weight loss estimates, the residue was dried in an air oven at 105°C12 h until it attained a constant weight Eq. (2).

$$Weight\; loss\; percentage=\frac{(W0-Wd)}{W0}\times 100$$
(2)

W0 is the weight of the dried plastic film before biodegradation, while Wd is the weight of the residue after biodegradation.

Soil burial degradation

Soil burial tests were done according to the approach reported by Hasan et al. with some modifications [32]. Waxy maize acetylated di-starch adipate (WADA) and waxy maize pregelatinized acetylated di-starch adipate (PWADA) thermoplastic starch (TPS) films were cut into 2 × 2 cm squares and put in the soil at a depth of 10 cm, irrigated at intervals to keep the soil moist. Examine the physical look of the films every 5 days and photograph them with a mobile camera until they have completely degraded. The experiment was set up as a three-replication randomized design. The research was carried out in botany and microbiology department, Faculty of Science, Al-Azhar University Cairo Governorate Egypt during the spring season (April 2021). Vicia faba (Fava bean) seeds were grown in 10 × 40 cm cylindrical plastic pots filled with 250 g of standard soil produced as recorded in Table 2 and replenished according to ISO1755: 2012-soil quality. To allow seeds to germinate, each container was filled with ordinary soil (2/3 by volume). The soil for this investigation came from the herbaceous garden of the Botany and Microbiology Department, Faculty of Science, Al-Azhar University, Cairo Egypt, Table 2. Plastic films containing cross-linked waxy maize starch (WADA) and pregelatinized, stabilized, and cross-linked waxy maize starch (PWADA) combined with polyvinyl alcohol polymer and plasticizers sorbitol and sorbitol-cardanol oil were compared to control films and placed in the experiment soil (free from bio-plastic films). Fava bean seeds were germinated in an incubator for 5 days at 25 °C on moist Whatman filter paper. In each pot, two radicle-emerged seeds were sowed at a depth of 3 cm. The plants were reduced to one plant per pot after seven days. During the experiment, plants were permitted to grow freely in pots for 3 weeks under natural growth conditions. At the end of the study, when the Fava bean plants were 21 days old, both pots were displaying strong growth, and data gathering on plant growth began.

Table 2 Normal soil preparation for 1 kg (dry)
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2.3.2 Scanning electron microscope (SEM)

A scanning electron microscope (Philips, model Quanta FEg250, England) was used for observing the fracture surface of the films. The samples for SEM analysis were firstly coated with a thin layer of gold and subsequently were imaged at an operating voltage of 20 kV under a vacuum.

2.3.3 XRD analysis

The X-ray diffractometer used was (LABX XRD-6000, Shimadzu, Japan). X-rays of 1.5410 Å wavelength were generated by the Cu Kα source. The angle of diffraction, 2θ was varied from 4 to 80° to identify any changes in the crystal structure and intermolecular distances between the inter-segmental chains.

2.3.4 Fourier transform infrared spectroscopy (FTIR)

A spectrum 100 spectrophotometer (PerkinElmer Inc., USA) was used to measure the starch/PVA films in total reflection mode. For FTIR measurement, the samples were mixed with anhydrous KBr and then compressed into thin disk-shaped pellets. The spectra were obtained with a resolution of 4 cm−1, and the tests were run from 4000 to 700 cm−1.

2.3.5 Water uptake pattern

The performance of water uptake was evaluated as mentioned by Tian H et al. with some adjustments [33]. The prepared films were divided into 2 cm × 2 cm. Then, they were dried in an oven at 50 °C for 24 h and weighed immediately and recorded as W0. The water uptake of the films was reported after storage in desiccator chambers over the salt solutions at a constant 75% RH for 10 days at 25 °C. Constant 75% RH was achieved with saturated salt solutions of NaCl (∼75% relative humidity) [33].

The water uptake (WU) of TPS of WADA and PWADA starch/PVA composite films was evaluated as (equal 3):

$$WU=((Wt-W0))/W0$$
(3)

where W0 is the weight of the dried sample and Wt is the weight of the sample after time t.

The second Fick’s law of diffusion can theoretically be used to show the adsorption of moisture into TPS of WADA and PWADA starch/PVA films (Eq. 4):

$$\frac{\partial C}{\partial t}=D\left(\frac{\partial 2C}{\partial X2}\right)$$
(4)

where C, t, D, and X are the concentration at time t, adsorption time, diffusion coefficient, and film thickness, respectively. The initial adsorption kinetic in a short time can be calculated as in (Eq. 5):

$$\frac{Mt}{M\infty }=\frac{4}{L} \frac{\sqrt{DXt}}{\sqrt{\pi }}$$
(5)

where Mt, M∞, and L are the mass of absorbed water at time t, the mass of absorbed water at thermodynamic equilibrium, and film thickness respectively [34].

The diffusion coefficient D can be obtained from a slope of fitting Mt/M∞ versus √t of R2 over 99% or the straight-line portion.

2.3.6 Water vapor permeability (WVP)

Water vapor permeability (WVP) was assessed according to the method carried out by Zheng et al. with some changes [27]. The films were divided into segments measuring 60 mm in diameter, mounted in permeation tubes with inner diameters of 15 mm and height of 30 mm, filled with granular anhydrous calcium chloride (0% RH), and lined with melt paraffin to ensure closure. After that, a desiccator containing a saturated potassium sulfate solution (95% RH) was used to hold the permeation tubes at this condition for a while.

The weight change of the permeation tubes was measured at different intervals of 24 h and 216 h. The WVP and WVR of the film were then calculated as Eqs. (6, 7) respectively:

$$WVP=\frac{\Delta m L}{\mathrm{A}. \Delta \mathrm{t}\Delta \mathrm{P}}$$
(6)
$$WVR=\frac{\Delta m}{A\Delta t}$$
(7)

where △m was the weight of the permeation tube acquired by desiccant (g), L was the average film thickness (m), A was the film area exposed to moisture transfer (m2), △t was the time of permeation (h), and △P was the water vapor pressure difference between the two sides of the film (Pa).

2.3.7 Thermogravimetric analyses (TGA)

Using 4 mg of material in aluminum pans, the thermal gravimetric analyses (TGA) were performed using a Shimadzu TGA-50 instrument (Japan) at a heating rate of 30 °C/min in a nitrogen environment at 30 ml/min. The onset degradation temperature, mass loss at the onset degradation temperature, second degradation temperatures, and the remaining mass at 650 °C were used to properly express thermal stability.

2.3.8 Mechanical properties

A universal testing machine (LR10K; Lloyd Instruments, Fareham, UK) was used to determine the mechanical properties of the films. Tests were conducted by ASTM D882-09. Strips of samples measuring 10 mm × 60 mm and having a thickness of nearly 0.2 mm were cut off and introduced into the device. Initial grip separation was set to 30 mm and crosshead speed was set to 20 mm/min. Calculations were performed for both the % elongation at break (EB) and the tensile strength (TS).

2.3.9 Transparency

The transparency of the films was measured according to previous research by Dai et al. [35]. Each film sample was sliced into a rectangle portion of 10 mm × 30 mm, which was then inserted into the cuvette’s side. A UV–Vis spectrophotometer (Jenway 6705, UK) was used to measure the absorbance of each film, with a blank cuvette serving as the reference. The number produced by dividing the absorbance at 600 nm by the film thickness served as a measure of the transparency (mm−1) of the film (mm). The results of each measurement were performed three times.

3 Result and discussion

3.1 Film biodegradation

3.1.1 Enzymatic biodegradation

Enzymes secreted by microorganisms degrade the starch plastic polymer into its molecular constituents, which are used as a carbon source for microbial growth [36]. Starch biodegradation has mostly been associated with the activity of alpha-amylase and glucoamylase, a particular enzyme for starch hydrolysis that is regarded as an important contributor to degradation [37, 38]. Hence, alpha amylase is an endo-amylase that hydrolyzes the α-l,4-linkages in starch (amylose and amylopectin) almost at random, while glucoamylase is an exo-amylase that hydrolyzes the α-1,4-linkages as well as the α-1,6-linkages in liquefied starch (amylose and amylopectin) almost at random [39]. The enzyme mixture degraded TPS/PVA films in a short amount of time, and the degradable materials were detected by raising the osmolality, as shown in Fig. 1, which correlates with the degree of starch hydrolysis by a linear relationship, as reported by Kearsley 1976 [29], who establish a linear relationship between the degree of starch hydrolysis and osmolality as feasible, and affordable methods to produce specialty sugars seem to be reverse osmosis and ultrafiltration of glucose syrups. Also, our results agree with contemporary research by Karimi and Biria (2016) [37]; they indicated that amylase may be effectively able to break down saturated linear hydrocarbons. Furthermore, pre-gelatinization slightly increasing the hydrolysis rate than unpregelatinized waxy starch, this might be due to the pre-gelatinization destroying the starch granules leading to ease of hydrolysis. Therefore, as seen in Fig. 2, the percent of weight loss during the experiment reached 96% indicating superior biodegradation of TPS prepared from modified waxy maize starch blended with PVA [40, 41]. Moreover, after complete hydrolysis happened to TPS/PVA films, the filtrate was analyzed by high-performance liquid chromatography, and the peaks appeared specified to saccharides produced from starch hydrolysis as seemed in Fig. 3. However, the addition of cardanol oil very slightly affect the rate of waxy starch hydrolysis; the degradation percentage of films reached 95.5% and decreased to 91.2% and from 90.5 to 89.6% in PWADA and WADA films respectively when cardanol oil increased to 0.6%, and this might be because of oil decrease the rate of starch hydrolysis as shown in Fig. 4 [30, 31, 42].

Fig. 1
figure 1

Osmotic pressure of control, TPS of PWADA/PVA, and TPS of WADA/PVA enzymatic degraded sample during different time

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Fig. 2
figure 2

Weight loss% after enzymatic degradation of the samples with different concentration of cardanol oil

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Fig. 3
figure 3figure 3

HPLC peaks of TPS /PVA of PWADA, WADA before enzymatic degradation and after. (a, a-) PWADA0, (b, b-) PWADA0.2, (c, c-) PWADA0.6, (d, d-) WADA0, (e, e-) WADA0.2. (f, f-) WADA0.6

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Fig. 4
figure 4

Degradation % after enzymatic degradation of the samples with different concentration of cardanol oil

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3.1.2 Soil burial test

Amylose and amylopectin make up the complex polymeric substance known as starch [43]. Numerous organisms including bacteria and fungi found in the soil environment can break down the main components of starch. Because microorganisms have extremely particular enzymes that can be used to hydrolyze the carbohydrate polymers into digestible units, weakening the structure and decreasing the strength of the starch polymer [44, 45]. The biodegradability of our waxy starch films combined with PVA was carried out on a laboratory scale following the soil burial test. The physical appearance results of the test indicate a visible total breakdown in all synthesized waxy starch films after 21-day completion. This occurs due to microbial feeding and biodegradation that cause the mixes’ structural properties to be lost, causing starch sites in the blends to be occupied by water or bacteria, which causes the combinations to break down. Also, the mixes swelled during the experiment due to the water absorption, which accelerated the biodegradation process, as shown in Fig. 5. Our results are in good agreement with Obasi et al. [46]; they synthesize polypropylene (PP)/plasticized from cassava starch (PCS) blended with and without compatibilizer (polypropylene-graft-maleic anhydride (PP-g-MA)) and found matched with the enzymatic degradation and variation occurs in tensile strength, elongation at break, and Young’s modulus with different levels of starch content in the blends.

Fig. 5
figure 5

Pictures Soil buiral degradation at different intervals of PWADA and WADA film and effect of bioplastic films on Faba beans growth parameters

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Furthermore, using synthesized waxy starch films combined with PVA as soil carbon and energy sources has a good impact on the growth parameter of Fava plants as shown in Figs. 5 and 6. Hence, plant height is a primary indicator of plant growth rate among plant health parameters; plant height was compared to the control to see if bio-plastic consumption had any negative effects on fava bean plant development. Data collected at the end of the experiment from the early vegetative stage, when fava bean plants were at the three-leaf growth stage, showed that plant leaves and roots exposed to bio-plastic films (WADA) and (PWADA) grew faster than the (no film) control, while plant stem exposed to bio-plastic film (PWADA) grew slower than the (no film) control and (WADA) as shown in Fig. 6. Overall, the findings demonstrate that using biodegradable materials on fava bean plants is not only safe but also encourages plant growth due to their 100% biodegradability. Our findings are consistent with those of Sforzini et al. [47], who found no major ecotoxic effects for Mater-Bi® (PBAT-corn starch) on Vibrio fischeri bacteria, slime mold, protozoa, green algae, Sorghum and cress plants, and Daphnia and earthworm invertebrates, indicating that it meets an acceptable level of environmental protection. Because of their biodegradability, they are also a feasible option for environmental preservation [48, 49].

Fig. 6
figure 6

Growth parameter, i.e., leaves, stems, and roots length (cm) of Fava beans planted with different bioplastic

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3.2 Film morphology

In polymer blends, it is critical to examine the morphology of the finished goods to keep track of the homogeneity of the product components, which influences other product qualities and the preparation process’s performance. Because of the action of physical modification (pre-gelatinization) in destroying starch granules, it leads to the enhancement of the thermoplasticization process of pregelatinized waxy modified starch than the unpregelatinized waxy modified starch. So, SEM images of TPS of PWADA/PVA plasticized with sorbitol are only more homogeneous than the image of WADA/PVA as shown in Fig. 7a, d [50]. Also, the addition of cardanol oil to PWADA film more is homogenous than WADA film due to PWADA films having linear chains after the a pre-gelatinization process which tends to form a complex with cardanol oil leading to compatibility between the matrix. In addition, after the plasticization process, the TPS of PWADA/PVA film has uniform morphology than WADA/PVA, as shown in images b, c and e, f, respectively, Moreover, the phenolic chemical structure of cardanol oil promotes the hydrogen bond formation in the matrix and increases the compatibility of the components of the film generally [18].

Fig. 7
figure 7

SEM images of TPS of PWADA/PVA film a sorbitol. b, c Sorbitol + cardanol/and TPS of WADA/PVA film d sorbitol, e, f sorbitol + cardanol

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3.3 XRD analysis

Starch granules are made up of linear amylose and branched amylopectin macromolecules. They are grouped in a laminar pattern and create growth rings, resulting in a semi-crystalline pattern in starch granules. XRD was carried out to show up variations in the crystalline structures of starch after thermoplasticization and to demonstrate that complete gelatinization was attained at the end of the blending process. Waxy maize starch has a type-A crystalline structure with peaks at 2θ 15°, 17°, and 23° that are characterized by more packed granules, and the disappearance of the later peaks indicated the effect of the thermoplasticization and blending process that altered the crystallinity of waxy corn starch [51]. New peaks appeared at 2θ of 13.2°, 19.5°, and 20.8°, which indicated a V-type crystal pattern. A new structure was formed due to new interactions between the plasticizers, amylose, and amylopectin macromolecules. On the other hand, the crystallinity of TPS of PWADA/PVA is lower than WADA/PVA one as the pre-gelatinization process partially damaged the granule crystalline structure, so as shown in Fig. 8, the peaks of PWADA are more unresolved with low intensity than WADA one and the peaks at 2θ 10° and 20° may ascribe to PVA and sorbitol [50]. The addition of cardanol oil enhances the plasticization of two types of TPS compared with a diffractogram of sorbitol only, as seen in Fig. 8.

Fig. 8
figure 8

X-ray diffraction pattern of TPS of WADA/PVA/sorbitol-cardanol and PWADA /PVA/sorbitol-cardanol

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3.4 Fourier transform infrared spectroscopy (FTIR)

Infrared spectroscopy is a potent tool for determining changes in chemical structure caused by mixing different molecules [52]. As demonstrated in Fig. 9, the TPS of PWADA and WADA/PVA films with sorbitol alone and sorbitol-cardanol mixture plasticizers can be distinguished. The specific band at 3560 and 3520 cm−1 depicts (O–H stretching) which is a numerous functional group in these formulations PWADA/PVA, WADA/PVA, and sorbitol-cardanol plasticizers which can boost the formation of more hydrogen linkage between the polymeric matrix leading to enhanced compatibility between them [52]. The asymmetric CH2 methyl group is also a distinct band at 2920 cm−1 in all films containing cardanol oil, and the skeletal vibration of the aromatic (–C = C–) linkages is a characteristic band at 1590 cm−1 in all films containing cardanol oil [28]. The presence of a C–O stretching aromatic ring is linked to the band at 1267 cm−1. The side chain (–C = C–) double bonds were found between 991 and 869 cm−1, 695 cm−1, and 781 cm−1, matching the vinyl vibrations [31]. At 1260, 1080, and 1019 cm−1, there are notable band peaks attributable to –CH2 stretching, –CH2OH (side chain) related mode, C = O stretching, and C–O–C stretching, respectively. The presence of the C = O feature band at 1710 cm−1 in all spectrums could be attributed to residual acetate groups in PVA and acetylated and cross-linking bonds in PWADA and WADA [53]. Anhydro glucose ring stretching is defined by the absorbance bands at 992, 929, 861, 765, and 575 cm−1 [54]. The microstructure images of the films under scanning electron microscope endorse the infra-red spectra, strong hydrogen bond formed between modified waxy maize starch with PVA and sorbitol-cardanol mixture which promote the compatibility of the film component.

Fig. 9
figure 9

FTIR of TPS of WADA and PWADA starch/PVA/sorbitol-cardanol

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3.5 Water uptake pattern of films

PWADA/PVA and WADA/PVA films have multifunctional hydroxyl groups and absorb moisture from the environment. As a result, the pregelatinization process during starch modification increases starch swelling power and solubility, causing PWADA/PVA films to uptake more moisture than WADA/PVA films when sorbitol is used as a plasticizer alone, as shown in Fig. 10a [55]. As shown in Fig. 10b, c, d, the addition of cardanol oil slightly decreased the rate of water uptake of TPS of WDADA/PVA and PWADA/PVA films. This could be because of the hydrophobic character of cardanol oil; the presence of an aliphatic group restricts the number of hydroxyl groups exposed to interaction with the hydroxyl group of moisture. Also, the reduction of water uptake by cardanol oil on PWADA films is more than on WADA films; this could be due to the formed curling path for the water molecule to penetrate the PWADA/PVA polymer matrix. The values of WU and diffusion coefficient, which were less than those found in the starch-chitosan matrix made by Hasan et al. and normal corn/PVA matrix made by the author, could be owing to the sorbitol-cardanol mixture forming a wrinkled channel for water molecules to penetrate the starch/PVA matrix [34, 56].

Fig. 10
figure 10

Water Uptake behavior of films a WADA and PWADA starch/PVA/sorbitol b WADA and PWADA starch/PVA/sorbitol-0.2cardanol. c WADA and PWADA starch/PVA/sorbitol- 0.4cardanol. d WADA and PWADA starch/PVA/sorbitol- 0.6cardanol

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3.6 Water vapor permeability

Because of the pregelatinization procedure, which increases the swelling and solubility of starch, PWADA/PVA films have somewhat higher water vapor permeability than WADA/PVA films, as shown in Table 3. WVP of PWADA/PVA films is unaffected by the addition of cardanol oil; however, WVP of WADA/PVA films is slightly reduced. As a result, when oil is added, the WVP decreases by 0.6%, from 2.95 to 2.7 (10−10 g.Pa−1 S−1 m−1) as shown in Table 3 and this might be due to the hydrophobicity nature of cardanol oil as alkyl chain in its structure. The addition of cardanol oil was believed to create hydrogen bonding between the starch network and cardanol compound, limiting the availability of hydroxyl groups in establishing interactions with water to form a strong structure, lowering free volume within the matrix, and restricting intermolecular hydrogen bonding [26]. However, the WVP of this study is less than that of Hasan et al. who investigated that the composite of starch-chitosan-palm oil has WVP values in the range of 3.51–4.53 × 10−9, and Zheng et al., who added eugenol oil to the starch chitosan composite [27, 34].

Table 3 Water vapor permeability (WVP), water vapor transmission rate (WVTR), diffusion coefficient (D), and mass absorbed (M∞) at the thermodynamic equilibrium of TPS of PWADA/PVA and WADA/PVA films
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3.7 Thermogravimetric analyses (TGA)

Thermal stability is an important property to consider when developing new film formulations, and it is measured by thermogravimetric analysis as a function of weight change as temperature rises. The thermogram can be broken down into three stages: first, the temperature range (0–200 °C), when weight loss is caused by evaporation of water and smaller molecular weight plasticizer; second, the temperature at which the first and second stages of deterioration occur (200–450 °C); finally, the weighted residual (450–600 °C) at the end of the experiment [31]. As shown in Fig. 11, the TPS of PWADA/PVA films has the lowest onset degradation temperature of 272 °C when plasticized with sorbitol alone, but it increased to 318 °C when cardanol oil was added by 0.2%, and the residual from 0 to 26.2%; indicating that cardanol oil increases the thermal stability of PWADA/PVA films. This could be due to the strong hydrogen bond formed in this formulation. Moreover, the cardanol oil slightly increases the onset degradation temperature of WADA/PVA films from 317 to 320 °C [57, 58].

Fig. 11
figure 11

a TGA of TPS of WADA and PWADA starch/PVA/sorbitol and TPS of WADA and PWADA starch/PVA/sorbitol-0.2cardanol. b dTG TPS of WADA and PWADA starch/PVA/sorbitol and TPS of WADA and PWADA starch/PVA/sorbitol-0.2 cardanol

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3.8 Mechanical properties analysis

The mechanical properties of thermoplastic PWADA/PVA and WADA/PVA films are shown in Table 4; the pre-gelatinization process slightly enhances the tensile strength by 11.6% of acetylated distarch adipate WADA which consider improvement in thermo-plasticization after physio-chemical modification when compared with chemical modification alone [59]. The addition of cardanol oil by 0.2% has no significant effect on the mechanical properties, but the increase in the oil percent decreases the tensile strength of the films by 17.5 and 55.6 in the case of PWADA/PVA and WADA/PVA films respectively. While the addition of 0.2% cardanol oil mixed with sorbitol plasticizer increases the elongation by 30.6 in the case of PWADA films whereas decreases WADA films by 33.8% indicating the sorbitol-cardanol mixture at cardanol portion 0.2% is a good plasticizer to PWADA films as proved in the water uptake of the films [60].

Table 4 Mechanical properties of TPS of PWADA/PVA and WADA/PVA films with different concentrations of cardanol oil
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3.9 Transparency

The safety of products that are susceptible to UV rays is an important need in the packaging sector. As indicated in Table 5, adding cardanol oil to the thermoplastic PWADA and WADA starch films reduces transmission by 69 and 63.4% in the UV area at 250 nm wavelength, showing that cardanol oil is an effective UV absorber. On the other hand, the addition of cardanol oil, on the other hand, reduces the transparency of PWADA/PVA and WADA/PVA films by 5.5 and 28.8%, respectively [61, 62].

Table 5 Transmittance at different wavelengths and transparency at 600 nm wavelength of TPS PWADA and WADA/PVA films with sorbitol only and sorbitol-cardanol mixture
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3.9.1 Comparison of different parameters of prepared films with similar studies

The prepared films from pre-geltinized waxy corn starch/PVA with sorbitol-cardanol oil 0.2 showed superior tensile strength compared with other work as shown in Table 6. Additionally, introducing cardanol oil to modified starch film promotes thermal stability as residual at 600 °C which is a large percentage as seen in Table 6.

Table 6 Comparative studies of TPS blend with various polymer and plasticizers
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4 Conclusion

Biodegradable bioplastic films made from waxy maize starch that had been physio-chemically and chemically modified, blended with polyvinyl alcohol, and plasticized with sorbitol and a sorbitol-cardanol mixture were tested. The pregelatinizing process increases the homogeneity and thermoplasticization of the film matrix. The pre-gelatinization process increased water uptake and WVP of films, whereas cardanol oil slightly decreased the water uptake rate of PWADA, possibly due to a stronger hydrogen bond between the oil and the polymer matrix in PWADA. Furthermore, the addition of cardanol oil improves the thermal stability of the films as well as their ability to absorb ultraviolet light. The pre-gelatinization phase increases tensile strength by 11%, and cardanol oil at 0.2% increases elongation by 30% in the film of PWADA but decreases in the film of WADA by 33.8%. As a result, modified waxy maize starch is a promising biopolymer for making bioplastic films for biodegradable packaging materials.