Introduction

In the last few decades, the demand for biobased coatings and films has increased drastically to address environmental problems from petroleum-based plastics. For this reason, and considering the importance of protecting natural resources, using renewable raw materials and developing natural materials are important in this respect. In particular, compounds such as vegetable oils, carbohydrates and biobased resins derived from lignin can replace many traditional materials [1,2,3]. Epoxy resins obtained from vegetable oils are synthesized with a completely environmentally friendly production. These resins with high transparency are biodegradable, recyclable, and well-processable due to their bio-origin and are considered promising raw materials for biobased film production [3]. Vegetable oil-based epoxy resins are widely used in various products such as films, composites, plasticizers, adhesives, surfactants, cosmetics, etc. [4]. In addition, various plant extract compounds are used to improve the functional properties of biofilms. These compounds also extend the shelf life of foods, reducing food waste and limiting the application of synthetic chemicals. In addition, they meet the goals of sustainable industrial and social development by adding antioxidant, antibacterial, etc., properties to these films or coatings [5, 6].

Vegetable oils are among the renewable resources from which some polymeric resins used in the production of biocomposites are obtained today [7, 8]. The use of polymer derivatives obtained from vegetable oils such as soybean and linseed oil together with both natural (fibers, cellulose, bamboo) and other fillers (such as carbon black, carbon nanotubes, etc.) creates a great advantage for many applications. Vegetable oils are triglycerides containing different fatty acids with different levels of polyunsaturation in chemical structure, but their high degree of polyunsaturation (–C=C–) results in poor thermal and oxidative stability. Apart from that, since triglycerides have aliphatic chains, the materials produced do not have the desired hardness and mechanical strength for some applications [9]. Unfortunately, double bonds in soybean oil are not highly reactive and therefore cannot be thermally and photocured by radical polymerization [10]; therefore, these bonds can often be used to functionalize the oil with different chemical groups. One of the most common reactions is the conversion of the C=C bond to the epoxy group followed by the synthesis of acrylated epoxidized soybean oil (AESO) to obtain a thin biofilm from its reaction with acrylic acid [3, 9, 10]. Although vegetable oil-based epoxy resins have many advantages, they also have several disadvantages such as low polymer crosslinking and low thermal and mechanical properties [11]. In the literature, studies with AESO were generally carried out only with one type of curing: UV curing [12] or thermal curing [11, 13] if pure AESO, and thermal curing if AESO-modified thermoset resin [14, 15]. From the literature results, no other studies were found on UV irradiation of AESO followed by thermal curing, except for the author's previous work [16].

Another issue is the use of natural additives in biofilms. Aloe vera gel (AVG) has many properties such as antibacterial, antiseptic, and anti-inflammatory. It has an important role in food preservation as edible coatings [17]. Aloe vera has also been reported to have the ability to reduce water vapor transmission and solubility [18]. Hydrogel films composed of alginate and aloe vera gel were prepared, and physical, morphological, and water absorption properties of biofilms depending on aloe vera content were characterized [19]. Yosboonruang et al. [20] developed a biocellulose sheet containing aloe vera gel extract (AE) for application in healing chronic wounds. In addition, aloe vera gel also was used as an additive to improve the film properties of edible glucomannan [18], thermoplastic starch [21], egg white [22], chitosan [23], etc.

Every year, large quantities of stems and pits are generated during sour cherry processing, without any substantial use. Kazempour Samak et al. [24] investigated the antimicrobial activity of sour cherry seed oil and explained that a solution could be provided to the problem of antibiotic resistance. Moreover, it was reported by Stryjecka et al. [25] that cherry seed oil has antioxidant properties. Studies in the literature on the sour cherry kernel (seed) are generally related to its composition [26, 27], and no evidence has been found regarding its use as an additive in any biofilm.

Due to the properties described above, sour cherry kernel (SCK) and aloe vera gel (AVG) were deemed appropriate to be used as additives in the formation of AESO biofilms. Although there is sufficient reported literature on biofilm formation based on epoxidized vegetable oleoresin and the use of AVG in biofilms, to the best of our knowledge, biobased composite film formation using SCK or AVG in AESO matrix was achieved for the first time in this study. In addition, some properties of the obtained biofilms were examined comparatively and evaluated by the ANOVA test.

Materials and method

Chemicals

Acrylated epoxidized soybean oil (AESO) (density: 1.04 g/cm3; viscosity: 18,000–32000 cps; acid value (mgKOH/g): ≤ 10) was obtained from Sigma-Aldrich. Two types of agents were used as curing agents. The first is the UV curing agent: Irgacure 184 (Sigma-Aldrich) is a radical photoinitiator. The second is IPOX EH 2041 (amine value: 305–335 mg KOH/g; viscosity: 125–225 MPa.s; density: 1.04 g/cm3), a modified cycloaliphatic amine-type curing agent purchased from Sar Chemical Co. (Turkey). In addition, 2,4,6-tris-(dimethyl aminomethyl)phenol (density: 0.969 g/cm3) was used as an epoxy cure accelerator. The formulas of the chemicals mentioned above are shown in Fig. 1.

Fig. 1
figure 1

Curing reactions of AESO with: a amine-type hardener from epoxide groups; b radical photoinitiator from acrylate groups (benefited and redrawn from [9, 12, 31,32,33,34]

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Natural additives

The cherry fruit was purchased from the local market. The fruit seed was crushed, and the seed was separated from the shell, dried, and ground. It was then sieved to a size of ≤ 63 microns. It was reported in the literature that as a result of the extraction process of sour cherry kernels, the oil ratio was 26% and the crude protein content was 25.3%. In addition, it contains 34.5% total carbohydrates, 9.5% fiber, 11.3% reducing sugars, 4.6% ash, and significant amounts of K, P, Mg, and Ca [28]. The aloe vera plant was obtained from the Selcuk University Faculty of Agriculture, and its gel was separated from its leaf. The composition of fresh aloe vera gel (AVG) by moisture and some compounds is reported by Soltanizadeh and Mousavinejad [29].

Preparation of films

Mixtures of AESO and AVG or SCK at 10, 20, 30, 40, and 50% by weight ratios were mixed at 2000 rpm and 25 °C for 15 min on a magnetic stirrer. Then, 4% by weight of Irgacure 184 [12] and 30 wt% IPOX were added and mixed. The samples were placed on Teflon surface material cut in the size of 10 × 10 × 0.5 cm by gel pouring method. In the UV curing device, both sides of the films were cured for 6 min following post-cured in an oven at 100 °C for 24 h.

Analyses and tests

UV curing was carried out using the UV device (emission wavelength: 246–365 nm; UV lamp power: 80 W/cm2; distance between sample and lamp 2.54 cm). The Shimadzu AGS − X (Japan) mechanical testing device was used to determine the mechanical properties of the films. The tests were carried out using film strips cut in 50 mm × 10 mm dimensions and drawing at a 15 mm/min speed. A digital micrometer (Mitutoyo, MDC-25SX, Japan) was used to calculate the thickness values of the films. TGA was performed on the Seteram Thermogravimetric Analyzer. Using the Philips XL30 SFEG SEM device, images of biocomposite samples were taken and their surface morphologies were examined. FTIR absorption spectra were obtained with a Thermo Scientific Nicolet i550 model spectrometer device at a spectral resolution of 4 cm−1 between 4000 and 400 cm−1 wavelength.

Apart from the analyses, the following tests were also carried out: water vapor permeability (WVP), mass loss, swelling, corrosion, biodegradation, and bacteriological. Detailed explanations about the tests are given in Supplementary Material 1. In addition, mechanical, bacteriological, swelling, mass loss in water, and water vapor permeability tests’ data were statistically analyzed using a one-way analysis of variance (ANOVA) test (MINITAB® release 16.0 program).

Results and discussion

Curing reactions of thermally and UV-cured AESO

The main purpose of producing acrylated epoxidized vegetable oils is to provide better properties by curing them with UV radiation [30]. The remarkable feature is the easy polymerization of the AESO molecule from acrylate unsaturated functional groups, both by thermal decomposition of radical initiators and under various initiator systems such as photoinitiators using UV or visible radiation [9]. In addition, the other advantage of AESO is that it contains epoxide groups as well as acrylate groups and is, therefore, suitable for two different curing processes. The thermal and UV curing reactions performed for AESO in this study are shown in Fig. 1.

In the case of using the amine-type hardener, the epoxide groups of AESO can react equivalently with the hardener amine groups via a polyaddition mechanism [13] (Fig. 1a). The density of crosslinking that occurs during the thermal curing process depends on the number of epoxy groups present in the acrylated epoxidized vegetable oil, whereas in UV curing the number of acrylate groups is important. A suitable photoinitiator for an acrylate-based composition absorbs light energy through UV radiation to form free radicals and can perform the curing reaction within seconds, rapidly initiating polymerization from the acrylate double bond (Fig. 1b).

Characterization of biobased films

FTIR

Figure 2 shows FTIR spectra of uncured and cured AESO, also biofilms. The strong C–H stretching band belonging to the triglyceride chain methyl group (CH3) and the methylene group (CH2) of the saturated fatty acid main chain was observed at 2925 cm−1 and 2858 cm−1, respectively [35], in the FTIR spectrum of uncured AESO (Fig. 2a1). The other detected bands were at 3456 cm−1 (–OH), 1726 and 1267 cm−1 (ester C=O), and 1639 cm−1 (acrylate group C=C) [36]. Moreover, the band in the range of 3460–3320 cm−1 may belong to the hydrogen bond stretching between AESO molecules containing –OH or C=O, –OH, and epoxy groups [37]. The decrease in the number of epoxy groups in the AESO structure compared to ESO is due to the reaction of acrylic acid with this group, and probably because of this, small and medium bands belonging to this group were detected at 925 and 810 cm−1 [33, 38], respectively. It has also been explained by Seabra et al. [33] that the chemical structure of AESO consists of long aliphatic chains of fatty acids with various functional groups such as epoxy, epoxy, and hydroxy. The disappearance of the peaks of the C=C bond and the epoxy group in the FTIR spectrum of AESO after (UV + thermal) curing (Fig. 2a2) indicate that the curing was successful from both the acrylate and the epoxy groups.

Fig. 2
figure 2

FTIR spectra of: a neat AESO (a1-uncured; a2-(UV + thermally) cured); b (UV + thermally) cured AESO/AVG biofilms (b1-10 wt% AVG; b2-50 wt% AVG); c (UV + thermally) cured AESO/SCK biocomposite films (c1-10 wt% SCK; b2-50 wt% SCK)

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In FTIR spectra of cured biofilms with 10 and 50 wt% AVG, except for the bands belonging to the acrylate C = C bond and the epoxy group, other bands of AESO continued to be seen (Fig. 2b1 and b2). This proves that AVG-containing biofilms also cure well. In the case of SCK biocomposite films, although curing is predicted to occur well in 10 wt% SCK film (Fig. 2c1), the detection of the small band at 1651 cm−1 belonging to C=C bond at 50 wt% SCK ratio (Fig. 2c2), it is considered that curing is largely achieved.

SEM

Figure 3 illustrates SEM images of neat AESO and biofilms. From the SEM analysis results, the fact that AESO and AESO/AVG biofilms have a very smooth surface without any defects and do not display an image specific to brittle epoxy resins which exhibit generally regular and parallel fold-like structures [39] suggests that crack propagation may be difficult in these films, which may lead to an increase in tensile strength. In contrast, rough surfaces and agglomerations were observed in AESO/SCK composites due to solid particle reinforcement. It is thought that this causes these composite films to exhibit lower mechanical properties than AVG films (see Fig. 4).

Fig. 3
figure 3

SEM images of neat AESO and biofilms

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Fig. 4
figure 4

Mechanical test results of neat AESO and biofilms: a tensile strength; b elongation at the break

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Mechanical properties

Although AVG is difficult to use as a material for films due to its low solids content, it can improve mechanical and functional properties when combined with other biomaterials [40]. The thickness and mechanical test results of biofilms are given in Table 1 and Fig. 4, respectively. As seen in Table 1, the thickness of the AESO/SCK composite films was slightly higher than that of AESO/AVG films, probably due to the solid reinforcement content. It was determined that AVG thinned the film thickness of AESO, while SCK increased it. Pereira et al. [19] stated that the thickness of the dry alginate/aloe vera hydrogel films showed similar thickness regardless of whether they were crosslinked or not. In addition, they explained that the wet film thickness was dependent on the aloe vera content only and that the increase in the AV content caused a significant increase in the thickness.

Table 1 Thickness of biofilms
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The average tensile strength of neat AESO film is 0.797 MPa and the tensile elongation is 8.15% (Fig. 4). We think that the low tensile strength is due to the very thin film thickness. The tensile strength of AESO/AVG biofilms increased significantly with increasing AVG ratio and had values in the range of 3.09–4.60 MPa. The reason for this can be explained as follows: It is estimated that the high amount of water molecules of the AVG in the film solution evaporate during curing at 100 °C. On the other hand, it can be said that AVG provides a good distribution within the AESO matrix and creates a homogeneous blend (see Fig. 3). AVG is approximately 99.5% water, and the remainder (0.5–1% solids content) is made up of various compounds such as water- and fat-soluble vitamins, enzymes, minerals, phenolic compounds, organic acids, etc. [41]. Moreover, Jung et al. [42] noted that polar functional groups enhance tensile strength.

In the case of AESO/SCK composites, according to the tensile strength data, the best ratio was found to be 30 wt%. Oral et al. [43] also stated that the inclusion of waste apricot kernel shells into the epoxy matrix is known as a fast and low-cost method to obtain the desired properties. These composites' tensile strength and tensile elongation values varied between 2 and 2.8 MPa and 12.55–15.6%, respectively. The tensile elongation values decreased as the SCK ratio increased. As reported by Kocaman and Ahmetli [44], tensile elongation showed a tendency to decrease with an increase in hazelnut shell filler ratio in bioepoxy composites. The tensile strength and tensile elongation values of AESO/AVG films were found to be higher than those of neat AESO and AESO/SCK films (Fig. 4). The tensile elongation percentage values of AESO/AVG biofilms changed in the range of 17.3–34.5% (Fig. 4b). According to the Japanese Industrial Standard, the standard tensile elongation % value of edible films is around 10–50%, so it can be concluded that the AESO-based biofilm samples with 10–50% AVG are the samples that are by the standard [18].

Thermal properties

The thermal stability of the AESO and biofilms was evaluated using TGA (Fig. 5). The thermal values are given in Table 2.

Fig. 5
figure 5

TGA results of neat AESO and composites: a AESO/AVG composites; b AESO/SCK composites

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Table 2 TGA data of biofilms
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The max. decomposition temperature for AESO was observed at 447 °C. This temperature was changed in the range of 456–462 °C and 452–463 °C for AESO/AVG and AESO/SCK biofilms, respectively. Moreover, the initial decomposition temperature of AESO was 196 °C, while it was observed at 202–212 °C and 183–217 °C for AVG- and SCK-containing biofilms, respectively. In addition, from the other thermal values of the biofilms (T5, T10, and T50 corresponding to 5, 10, and 50% decomposition temperatures) in Table 2 it is obvious that the thermal stability of the biofilms is slightly higher than that of AESO. The thermal stability of the biofilms was found to be close to each other.

Swelling, mass loss in water and water vapor permeability (WVP) properties

When films are developed for some applications, e.g., as wound dressings, evaluation of their capacity to absorb aqueous solutions is essential in determining their ability to maintain a moist environment. However, films are more susceptible to hydrolytic degradation due to water absorption, which leads to the cleavage of degradable linkages, increasing film degradation [19]. The effects of AVG and SCK content on the swelling capacity of biofilms, water vapor permeability, and mass loss in water were examined for 24 h, and the obtained data are summarized in Table 3. On average, the swelling percentage of AESO in water was recorded as 0.2437%. The results showed that an increase in AVG content slightly increased the swelling percentage of the films, whereas an increase in SCK ratio significantly increased it. The swelling percentage changed in the range of 0.2412–0.2555% and 1.2860–4.6343% for AVG- and SCK-containing biofilms.

Table 3 ANOVA test results using swelling, WVP, and mass loss in water data
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Polymers with hydrolytically labile aliphatic ester linkages undergo hydrolytic degradation [45]. Although AESO has ester bonds, it will probably be very difficult to degrade from these bonds due to its crosslinked nature. For this reason, the mass loss of AESO was as low as 0.1103%. When mass losses of biofilms are compared, there was an increase in mass loss for both additives, but the mass loss of films with AVG showed a greater increase of 9.4–11.35%. The fact that the mass loss values were determined as 0.1207–0.7380% with an increase of 0.09–5.69% for the films with SCK indicates that the hydrolytic degradation is very low.

Coatings and films used in food packaging should have low water vapor permeability (WVP) values to reduce water transfer between the food and the environment. AESO consists of water-insoluble triglyceride oil and can form a hydrophobic polymer barrier film against steam or water [46]. Ge et al. [47] reported a significant improvement in moisture permeability after coating the starch film with crosslinked AESO. In addition, replacing glycerine with partially epoxidized soybean oil in biofilm significantly reduced the permeability of water vapor [48]. The WVP of neat AESO was measured as 1.38191 × 10–7. With the addition of AVG, this value decreased to 2.261 × 10–10 and the WVP value of AESO significantly decreased, but the addition of more AVG did not change the water vapor permeability much. The reason for this can be considered as the fact that AVG has closed a small number of pores in the AESO structure that allow water vapor permeability. Studies also have shown that the presence of AVG in the film structures reduces WVP, which supports our results [40, 49]. The same was the case for AESO/SCK films, and values close to AVG-containing films were found.

Biodegradation

Biodegradable polymers are increasingly used for short-lived applications. Many studies have been conducted to improve the biodegradation and biocompatibility properties of polymers and their composites [50]. To examine the biodegradability of AESO/AVG and AESO/SCK films in the natural soil environment, the images before and 12 months after embedding, as well as weight loss, are given in Figs. 6 and 7, respectively.

Fig. 6
figure 6

Photographs of AESO/AVG biofilms before and after of biodegradation test in soil for 12 months

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Fig. 7
figure 7

Photographs of AESO/SCK biocomposite films before and after of biodegradation test in soil for 12 months

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It was observed that there was a change in color and some deterioration in the structure of AESO/AVG biofilms from the images 12 months after they were buried in the soil in Fig. 6. The mass loss of AESO was determined as 7.095%. The mass loss of the films increased with the increase of the AVG ratio and varied in the range of 5.7088–9.4768%. Despite the use of a biobased matrix and natural reinforcement, the main reason for the low mass loss is thought to be due to the strong crosslinking in the structure (see Fig. 2b1 and b2). It has been reported in the literature that the resistance of the structure to biodegradation increases as the crosslinking increases, i.e., biodegradation rate is generally reduced by crosslinking [51]. In addition, these values are considered to be low due to the ability of AVG to prevent microbial spoilage and reduce the incidence of decay [52].

The biodegradability of the biocomposite films obtained using SCK as reinforcement was quite high compared to AVG films, and the mass loss was observed at the most of 39.9514% for 50 wt% SCK film (Fig. 7). SCK contains bioactive compounds (tocopherols, carotenoids, polyphenols, etc.), as well as a significant amount of high oleic-linoleic oil [53]; therefore, its biodegradability can be considered to be much higher than AVG.

Corrosion resistance

It has been reported that films with improved adhesion and impact resistance can be obtained due to the flexible triglyceride structure of AESO. In addition, a visible improvement in coating adhesion was observed with the AESO content exceeding 40% in its blend with polyester acrylate, and excellent adhesion was observed at 70% AESO content [54]. Chu et al. [55] also reported that the cured AESO film has strong adhesion, low hardness, good thermal stability, and other properties and therefore can be widely used in coating manufacturing. AESO adheres well to most surfaces thanks to its ester and hydroxyl polar groups and shows high flexibility and good corrosion resistance against moisture and chemicals due to long hydrophobic chains [2]. Also, the crosslinking density greatly influences the final properties of thermoset coatings. The increase in crosslinking density increases the corrosion resistance of coatings [56].

Polarized microscope images of AESO/AVG and AESO/SCK coatings are illustrated in Figs. 8 and 9, respectively. It was observed that the AESO coating, which was kept in 5% HCl solution for 15 days, was in very good condition, but bubbles formed on the AESO/AVG coatings above the 10 wt% AVG ratio, and the coating detached from the surface in some areas (Fig. 8a). In a basic environment, AESO coating was still in very good condition, but as the AVG ratio increased, the films were more affected by the base, deteriorating their film structure and the coatings were separated from the surface (Fig. 8b). When the coatings kept in 3% NaCl solution were examined, it was determined that the neat AESO, AESO/10 wt% AVG, and AESO/30 wt% AVG coatings were not affected by the salty environment, only a decrease in adhesion between the AESO/50 wt% AVG film and the coated surface (Fig. 8c).

Fig. 8
figure 8

Microscope images of AESO/AVG coatings in: a 5% HCl; b 5% NaOH; c 3% NaCl

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Fig. 9
figure 9

Microscope images of AESO/SCK coatings in: a 5% HCl; b 5% NaOH; c 3% NaCl

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It has been observed that, unlike AVG-containing coatings, SCK biocomposite coatings are not affected at all by acid and salt (Figs. 9 a and c), only the coating containing 50% SCK is affected in the basic environment, and there are both some deteriorations in its structure and the separation of the coating from the metal (Fig. 9b).

Antibacterial activity

The authors' previous study reported the antibacterial activity properties of AESO/SCK biocomposite films against three Gram-negative (Escherichia coli DH5α, Pseudomonas aeruginosa, Klebsiella pneumoniae) bacteria and three Gram-positive (S. aureus, S. aureus MRSA, and E. faecalis) bacteria. An increase was observed in the zone diameters of the bacterial species examined in the biocomposite films, depending on the SCK ratio [16]. Being a naturally antimicrobial substance, AVG also has properties such as anticancer, anti-inflammatory, and antioxidant [49]. The efficacy of AVG as an antibacterial agent against a wide range of Gram-positive and Gram-negative bacteria is well established. For example, antimicrobial agents in the gel have been reported to effectively inhibit or eliminate the growth of many bacteria, such as Staphylococcus aureus, Klebsiella pneumoniae (K. pneumoniae), Streptococcus pyogenes, Pseudomonas aeruginosa (P. aeruginosa), Escherichia coli (E. coli) [16, 57, 58]. The zone inhibition diameters obtained also vary according to the bacteria and the chemicals from which AVG is extracted [58].

The antibacterial activity properties of AESO/AVG biopolymer films were evaluated against three Gram-positive (S. aureus, S. aureus MRSA, and E. faecalis) and three Gram-negative (K. pneumoniae, E. coli DH5a, P. aeruginosa,) bacteria. No zones of inhibition were formed in the control films without AVG. Zone images formed by the AVG ratio in the biopolymer film are given in Fig. 10, and the inhibition diameters measured accordingly are summarized in Table 5.

Fig. 10
figure 10

Zone images formed by the effect of AVG in the biofilms a Gram-positive, b Gram-negative bacteria

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For the tested bacterial species, a certain increase in zone diameters was observed as the amount of AVG in the polymer film increased. Among the bacteria tested for a 10 wt% AVG ratio, K. pneumoniae was found to be more resistant with a minimum inhibition zone of 10.102 mm, whereas inhibition diameters detected for all bacteria at a 50% AVG ratio were > 13 mm. This AVG-dependent activity is due to the presence of acemannan, anthraquinone, and salicylic acid, which adhere to the bacterial cell wall, disrupt cell membrane integrity, bind to proteins, and cause their death [49, 59].

ANOVA test

For statistical analysis, ANOVA was applied and the results obtained are summarized in Tables 3–5. p value < 0.05 indicates a significant difference between the average data of biofilms at a 95% confidence level [60]. As seen in Tables 3 and 4, two different variety additives (AVG and SCK) addition and four different weight percent have a significant effect on the biofilms’ mass loss in water, tensile strength, and tensile elongation. For example, the tensile strength of AESO films incorporated with AVG and SCK was higher than the neat AESO sample, so these films showed statistically significant differences to neat AESO film (p < 0.05). Moreover, Tukey analysis revealed that in AESO/AVG biofilms swelling and WVP, all biofilms with AESO fall under the same group A which indicates that they are not significantly different compared to each other. On the contrary, the swelling of AESO/SCK films is significantly different compared to each other. Also, two different variety additives (AVG and SCK) addition and four different weight percent have a significant effect on the biofilms’ antibacterial activity and the antibacterial activity of AESO/SCK films is significantly different compared to each other (Table 5). The p > 0.05 indicates that there is no statistically significant difference between the interaction effect of additive types and the weight percent on the biofilms’ water vapor permeability [61, 62]. In addition, the swelling values of neat AESO and AVG-containing films were close to each other so the difference was not significant (p > 0.05) (Table 3).

Table 4 ANOVA test results using mechanical data
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Table 5 ANOVA test results using antibacterial test data
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Comparison of results with literature

Biopolymers and biocomposites have long been applied in the food industry. Generally, all materials must be compatible with each other to develop composite films that offer improved biological and technological functionality. Apart from this, different types of interactions, covalent or non-covalent (electrostatic, hydrogen bonding, hydrophobic, and Van der Wall interactions), may also develop between different polymers [63].

The comparison was made to summarize the results in this article and to reveal the difference between some of the other studies. Since SCK is not used as an additive in biofilms, studies in the literature were selected from biofilms containing AVG. Table 6 summarizes these comparison data. It can be seen from Table 6 that aloe vera gel is compatible with various biopolymers commonly used from its application in many edible coatings and films [18,19,20,21,22, 64,65,66,67,68,69,70]. As presented in Table 6, several studies are showing that the addition of aloe vera gel improves the mechanical properties of composite films and reduces film thickness and WVP. It is also seen that in composite biofilms, the AVG ratio is generally preferred between 10–50% by mass. The properties of composites depend not only on the additive material but also largely on the matrix material. Therefore, although the mechanical properties of some composite biofilms in Table 6 appear to be higher, the important issue is to what extent the AVG additive improves the properties of the matrix. For example, in this study, AVG and SCK increased the tensile strength of the AESO matrix by 292.4–482.3% and 165.8–254.4%, respectively. It is seen that in only one of the studies summarized in Table 6, the AVG matrix used at 50% increased the tensile strength by 787% [21]. In other studies, AVG has been found to either reduce or slightly increase the tensile strength of the matrix. Moreover, since the standard tensile elongation percentage value of edible films is preferred to be around 10–50% [18], it is also seen that the tensile elongation of both biofilms in this study is in this range and at higher values than many studies.

Table 6 Comparison of film properties in the present and previous studies
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The film is expected not to easily transmit water vapor from the surroundings. Therefore, the low water vapor permeability of the films means an excellent water vapor barrier. That is, a high WVP value indicates that the film cannot create a sufficient barrier when used as a packaging material, because moisture can penetrate the film and cause damage to the product in the package [71]. WVP values of AESO/AVG and AESO/SCK biofilms were found to be close to or lower than literature values. This comment also applies specifically to AESO/AVG film thickness (Table 6).

Conclusion

In this study, various biofilms were created using AVG and fruit industrial waste SCK. Some properties of biofilms such as swelling, WVP, mass loss, corrosion, biodegradation, etc., were investigated comparatively. Due to the antibacterial properties of AVG, it was determined that AESO/AVG films also have antibacterial activity by testing against some Gram ( +) and Gram (-) bacteria. Both additives increased the biodegradability of AESO while decreasing its WVP. Biofilms with AVG exhibited better mechanical and antibacterial properties, while biofilms containing lower-cost SCK waste had higher biodegradability and anticorrosion properties against different corrosive media. Although both additives did not have a significant effect on the thermal properties, the thermal strength of AESO/SCK films was found to be slightly higher. In addition, the lower mass loss values for films with SCK indicate that hydrolytic degradation is lower in these films. In addition, the ANOVA test was applied for statistical analysis. As a result, two different variety additives (AVG and SCK) addition and four different weight percent have a significant effect on the biofilms’ swelling percentage, mass loss in water, tensile strength, and tensile elongation.