Introduction

Expanded polystyrene (EPS) and expanded polypropylene (EPP) foams are used in a wide range of applications due to their interesting properties such as low density, high thermal insulation, high mechanical absorption, and low cost; however, once their useful life is over, they become one of the main types of plastic waste, which causes a great negative impact on the environment [1]. Most EPS and PPE are used for single-use applications, including food, personal care, agricultural packaging, electronic equipment, among others. In addition, these polymers take hundreds of years to degrade and in the process release chemicals that are hazardous to the environment, affecting both human and animal health. It is estimated that the degradation of EPS is over 100 years; additionally, it is a material resistant to photooxidation. Although recycling is an option, the low density of these thermoplastic foams makes this operation very difficult [1, 2].

Faced with this problem, the need arises to develop foamed materials based on biodegradable and/or compostable polymers. One of the biopolymers with great potential for the development of environmentally friendly foamed materials is starch because it is a biodegradable, abundant polymer, has a relatively low cost, and is a renewable resource [1,2,3]. The rapid biodegradation of this carbohydrate is due to the fact that it is consumed by a wide variety of enzymes and microbes naturally present in the environment [1].

Starch is found in all organs of higher plants and is the main form of energy storage. It may be present in leaves, fruits, roots, stems, shoots, and pollen grains. The main botanical sources of this biopolymer are corn, cassava, wheat, and potato. Starch is composed of two types of molecules, linear amylose (20–25% w/w) and branched amylopectin (75–80% w/w) [1, 4, 5]. The proportion of these two polymers varies according to the source.

Starch in its native state is not a thermoplastic polymer; therefore, it must undergo a thermoplasticity process, which involves the addition of a plasticizer (such as water, glycerin, or sorbitol), temperature (> 100 °C), and shear forces, thus breaking the hydrogen bonds that hold the granule and its semi-crystalline structure together, giving way to an amorphous structure with a viscous behavior; this new substance is called thermoplastic starch (TPS) [4, 5].

Starch thermoplastic foams can be obtained mainly by four technologies: baking, molding, extrusion, and foaming by supercritical fluids [2]. Foam production by extrusion is the most versatile and economical and involves heating and mixing raw materials under pressure and then forcing them through a screw extruder to produce foam. When the pressure is released, the dissolved gases form bubbles that are trapped in the starch matrix [2, 6, 7]. In extrusion foaming, operating conditions (such as screw speed and temperature) and the amylose to amylopectin ratio of the starch affect the mechanical properties of the final product [6,7,8]. This extrusion process can be performed in single or twin-screw extruders. The single-screw extruder is the most commonly used machine type due to its relatively low operating cost; however, the twin-screw extruder has a higher throughput [6, 9].

In practice, although 100% starch-based thermoplastic foams have the advantage of being completely biodegradable, they nevertheless have significant disadvantages, such as high affinity for water, low mechanical properties, and poor dimensional stability, especially in the presence of water or in humid environments [6, 10]. Also, TPS in the molten state has high viscosity and high resistance to extension, which generates low bubble stability during the foam expansion process [11, 12]. One way to largely overcome these limitations is to blend TPS with other polymers, such as poly (butylene-co-adipate terephthalate) (PBAT), polylactic acid, and proteins such as wheat gluten [13,14,15]. PBAT is a biodegradable aliphatic–aromatic co-polyester with high elongation at break and good ductility [16, 17]. In the case of PLA, it is a renewable, biocompatible, and biodegradable polymer with high mechanical and optical properties. It is generally obtained by direct polycondensation of the lactic acid monomer [18].

In the literature, there are several reports of studies of thermoplastic starch and PBAT blends for the manufacture of foams [16, 19, 20]. In general, the inclusion of PBAT in the starch foams helped to decrease the bulk density of the material and increase its expansion rate, and the foams were less hydrophilic. Similarly, [21] studied mixtures of corn starch and polylactic acid in obtaining loose-fill foams, these researchers identified that the ratios starch90%/PLA10% and starch80%/PLA20% were the most suitable to obtain a packaging material with good cushioning properties. Likewise, [7] prepared foams by reactive extrusion from wheat starch, pea starch, and potato starch using different concentrations of starch/glycerol/gluten/sodium bicarbonate. The results showed that the addition of gluten increased foam elasticity and facilitated glycerol incorporation during extrusion.

The objective of the present research was to develop a flexible thermoplastic biodegradable foam with low water affinity from a blend of thermoplastic cassava starch, poly (butylene adipate-co-terephthalate), polylactic acid, and wheat gluten using a single screw extruder machine.

Materials and methods

To obtain the biodegradable thermoplastic foam, native cassava starch (12% moisture) supplied by POLTEC (Medellín-Colombia) was used. Commercial grade glycerin (99.7% purity), supplied by DISAN S.A. (Cali-Colombia), was used as a plasticizer. A commercial biodegradable polyester was handled under the Ecovio brand, in reference Ecovio F2332, composed of polylactic acid (PLA) and poly (butylene adipate-co-terephthalate) (PBAT), this resin was supplied by BASF (Ludwigshafen, Germany). Maleic anhydride (98% purity) was used as coupling agent and benzoyl peroxide (97% purity) as catalyst, both produced by Alfa Aesar Company (Tewksbury, USA). Wheat gluten (82%) distributed by Tecnas (Cali, Colombia) was added. Also, sodium sulfite anhydride analytical grade (98% purity), this helps to reduce chemical cross-linking during the protein plasticization process, this is produced by Loba Chemie PVT. LTD (Jehargir, India). Citric acid and sodium bicarbonate were used as a foaming agent. Citric acid (99.5% purity) was produced by Merck Millipore Company (Massachusetts, USA). Food grade sodium bicarbonate was manufactured by Laboratorios OSA S.A.S. (Yumbo, Colombia).

Experimental design

To determine the process conditions for obtaining the biodegradable thermoplastic foam, it was based on previous studies [22] and other literature reports [23, 24]. An extreme vertices designs was proposed, where the thermoplastic starch precursor mixture (TPS), premixed vegetable protein (glycerin plus wheat gluten) (GLU), and biodegradable polyester (ECO) (independent variables) were evaluated. The response variables were radial expansion index, bulk density, and compressibility. The mix design was created using the Minitab 18 test version program, restrictions were established for the components according to previous studies and a second-order statistical model (m: 2) was used. The statistical model resulted in 13 treatments corresponding to mixtures of the three components, each in triplicate, as shown in Table 1.

Table 1 Treatments and replications of the extreme vertices designs for the optimization of obtaining a biodegradable foam
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Production of biodegradable foam

To obtain the thermoplastic starch precursor mixture (TPS), native cassava starch with a moisture content of approximately 12% was taken and dried for a period of 16 h at a temperature of 73 °C, until reaching a final moisture content of less than 2%. The dried starch (moisture content 1.9%) was then mixed with plasticizer (glycerin) in a ratio of 70/30, placed in an airtight container and allowed to stand for a period of 24 h. The protein was dried at 50 °C for 24 h until a moisture content of 2%. This protein (gluten) was mixed with glycerin at a ratio of 75/25. Then, 2% sodium sulfite was added (in relation to the weight of the protein). Finally, biodegradable polyester (ECO), TPS precursor mixture, and plant protein premix (GLU) were mixed at the concentrations indicated in Table 1. A foaming agent (citric acid 60% and sodium bicarbonate 40%) and other additives (maleic anhydride and benzoyl peroxide) were also added. The blends were processed on a single-screw extruder at a speed of 160 rpm and a temperature profile of 145–150-140–135 °C, starting at the feed throat and ending at the die. The foamed material was obtained in cord form. Once the foamed materials were manufactured from the respective extrusions, they were conditioned in a climatic chamber at a constant temperature and relative humidity of 23 ± 2 °C and 50 ± 10%, respectively, for a period of 8 days.

Application of a rubber latex-based coating

A natural rubber latex-based coating was applied to the treatment with the most outstanding mechanical properties to create a moisture barrier. In this case, the methodology proposed by [25] was followed with some modifications. The coating was applied by spraying on the samples of the foamed material, and then left at room temperature (23 °C) for 10 h, to achieve solvent elimination and thus obtain a thin film on the surface of the thermoplastic foam.

Radial expansion index

This property was determined by the ratio between the cross-sectional area of the foaming (Rf) and the cross-sectional area of the die nozzle (Rd). The diameter of the foams was measured with a vernier caliper along a 10 cm section, an average of ten measurements was used, assuming a circular cross-sectional area of the foam obtained, three replicates were made per treatment. Equation 1 mathematically describes the relationship used to determine the radial expansion ratio (REI) [26].

$$REI={\left(\frac{{R}_{f}}{{R}_{d}}\right)}^{2}$$
(1)

Bulk density

It was determined by the mathematical relationship presented in Eq. 2, generally expressed as the mass per unit volume, in units of kilograms per cubic meter (kg/m3). This procedure is contained in ASTM D3748 [27], which sets forth the standard practice for the evaluation of high-density rigid cellular plastics, in addition to referencing test methods such as ASTM D1622 [28]. Using a vernier caliper, the diameter of the extruded materials was measured along a 10 cm section; an average of ten diameter measurements was used to determine the volume of each specimen (v), assuming a circular cross-sectional area of the foam (all measurements were performed in triplicate). The weight (Ws) was determined using an analytical balance [27, 28].

$$D=\frac{{W}_{S}}{V}$$
(2)

Compressibility

Compressibility is the force required to compress a foamed part divided by the cross-sectional area of the part [26]. To measure this property, five specimens were cut (in triplicate), maintaining a length/diameter ratio of 2:1 [29]. Then, the samples were stored for 8 days in a climate chamber at a temperature of 23 ± 2 °C and relative humidity of 50 ± 5%. Afterward, compressibility measurements were conducted using a universal testing machine equipped with a 100 N load cell and stationary (lower) and mobile (upper) plates. The specimen was placed on the fixed plate perfectly centered, then a first compression was performed at a loading rate of 30 mm/min, until 90% of the original length is reached. Once this stop was reached, the load was released. After 60 s, the procedure was repeated [26].

Morphology

The cross-sectional morphology of the foams was examined using a Hitachi UHR-FE-SEM scanning electron microscope model SU8010. For this purpose, pieces of the cords with a thickness of less than 1 mm were cut transversely and placed in a desiccator for a period of 24 h. Each specimen was coated with a gold layer (40–50 nm). Subsequently, with the help of a specimen holder, each specimen was introduced into the SEM chamber, with a vacuum of 30 Pa, when the vacuum stabilized, the field emitter (FE) was irradiated with an accelerating power of 3 kV and then the surface morphology was observed at different magnifications [10].

Moisture adsorption

The methodology used by [11] was followed with some modifications. Coated and uncoated samples were taken, each with a volume of 3000. The specimens were then dried at 105 °C for 24 hours, and the initial weight (m1) was determined. They were then placed in an airtight container with a relative humidity of 90%. Storage was carried out for 30 days, after which the final weight (m2) of the samples was taken. The amount of adsorbed water was calculated using Eq. 3. Each measurement was performed in triplicate.

$$ADA=\frac{{m}_{2}-{m}_{1}}{{m}_{1}} \times 100$$
(3)

Biodegradation test

It was carried out following the methodology established in ISO 14855 [30]. Three hundred grams of inoculum was mixed with 15 to 16 g of coated foamed material. Subsequently, the moisture content of the mixture was adjusted to between 45 and 75%, as indicated in the standard. The resulting mixture was deposited in glass vials, which were placed in forced convection incubators. Subsequently, the bottles were attached with plastic tubing to a respirometer configured in an open chamber system. The system was incubated at 58.0 °C ± 0.1 for 30 days, during which the CO2 concentrations generated during the biodegradation process were measured at regular intervals (6 h). At the end of the experiment, the percentage of mineralization understood as percentage of biodegradation (%B) was determined using Eq. 4, where M is the CO2 is the gaseous carbon of the sample measured in grams of CO2, B is the gaseous carbon of the blank measured in grams of CO2, and T is the maximum theoretical carbon dioxide that can be produced by the sample in grams.

$${\%}\text{B} = \left(\frac{{\text{CO}}_{2} \, {\text{M}} - {\text{CO}}_{2}\text{ g B}}{{\text{CO}}_{2} \, {\text{T}}}\right)\times {100}$$
(4)

Results and discussion

Experimental design

The statistical analysis of the data obtained (Table 2) began with a Shapiro–Wilk normality test, which showed that the data for expansion index, bulk density, damping index, and compressibility followed a normal distribution. Then, based on the p-value, the coefficient of determination R2 (adjusted) and the value of lack of fit, we proceeded to select the most appropriate model, among linear, quadratic, or special cubic, to describe the behavior of the variables studied. Using the p-value criterion, the most complicated model with a p-value of less than 0.05 is usually chosen. With this criterion, the special cubic model was selected for the expansion index, but the p-value of the other models was also small (practically less than 0.02), i.e., they can also be considered. In addition, the coefficient of determination criterion (R2ajus), which shows the percentage of the variation of y1 explained by the corresponding model, is used for model selection. In this case, the quadratic model is the most suitable, since its coefficients of determination were higher than those of the linear model. While the special cubic model, although it could be an alternative, managed to increase the coefficient of determination by a small amount, and it does not compensate for the additional complication of the model. Regarding bulk density and compressibility, the quadratic model was the most appropriate because it had a p-value < 0.05; it showed an R2 (fit) greater than 70% and had no lack of fit (p < 0.05) [31, 32].

Table 2 Results of the evaluation of the physical properties of the foamed material obtained in each treatment for the experimental design
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Radial expansion index and bulk density

The results of the analysis of variance for the expansion index data indicated that the TPS*ECO and TPS*GLU interactions had a significant influence. In the case of bulk density, the TPS*ECO, TPS*GLU, and GLU*ECO interactions influenced this response variable. In the case of the expansion index, Fig. 1a shows that the dark green zone had the highest expansion (< 25), the highest TPS concentration (70%), and the lowest inclusion of both biodegradable polyester (30%) and wheat gluten (0%).

Fig. 1
figure 1

Contour plot. a Radial expansion index. b Bulk density

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An inverse behavior was observed for bulk density (Fig. 1b), as the dark green zone (< 800 kg/m3) is where the lowest density was found, an area where the highest amount of TPS and the lowest concentrations of polyester and wheat gluten converge. A possible explanation for this result is the formation of an inhomogeneous mixture of separated phases (between biodegradable polyester, wheat gluten, and thermoplastic starch), in which the presence of strong covalent bonds was low, resulting in the polymeric mixture having a low radial expansion capacity and high bulk density [34]. In this case, the polyester interfered, preventing the glycerol and starch and/or protein from coming into contact, resulting in incomplete thermoplasticity, in which the starch granules were not destructured and, consequently, the adhesion between the phases was low [34]. Also, the composition of the biodegradable polyester could probably have an effect, since it consists of approximately 15% polylactic acid and 85% PBAT, so increasing the amount of this resin also increased the inclusion of PLA. Due to its linear molecular structure, this polymer has a low elasticity in the molten state, which hinders bubble growth. PLA also has low strength in the molten state, which is a drawback in the foaming process, as it prevents the cell structure from stabilizing before cooling and solidification, allowing bubbles to coalesce [35, 36].

In the case of wheat gluten, [37] found that increasing the inclusion of wheat gluten in starch extrudates generated the decrease in the radial expansion rate, they consider that the thermal denaturation of gluten generated changes in the rheological behavior of the melt, which negatively affected the expansion of the extrudates. Similarly, [38] found that inclusion of wheat gluten below 11% in starch extrudates led to a decrease in the expansion rate. In the same way, [39] reported that increasing protein inclusion reduces the expansion rate and increases the bulk density of extruded products; the opposite is true for starch, which tends to increase the expansion of the extruded material as its concentration increases.

Compressibility

Figure 2 shows that as the inclusion of TPS increased, the compressibility of the material decreased; the opposite behavior occurred when the addition of both biodegradable polyester and wheat gluten increased. This result can be explained on the basis of the foam bulk density results, since the higher the TPS concentrations and the lower the presence of polyester and protein, the lower the density of the thermoplastic foam, [19] consider that there is a power law relationship between compressibility and foam density. Less dense foams tend to have thinner cell walls and therefore resist less compressive strength than higher density foams with thicker cell walls. Additionally, with the decrease of polyester in the mixture, the presence of polylactic acid was reduced; therefore, the rigidity and low elasticity characteristic of this polymer decreased, and consequently, the resulting foamed material was less rigid and less mechanical force was needed to compress it [40].

Fig. 2
figure 2

Compressibility contour plot

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Morphological study

The micrographs show as decreasing the inclusion of biodegradable polyester and/or wheat gluten increased the bubble area of the foamed material. Thus, the thermoplastic foam containing 40% polyester had cells with an area between 0.18 and 1.6 mm2 (Fig. 3d1) compared to foams containing 30% polyester, whose cell area ranged from 1.7 to 11 mm2 (Fig. 3n1). As for the thickness, the increase in the addition of both polyester and protein generated an increase in the thickness of the cell walls, since the material containing 40 or 35% polyester had a thickness between 0.5 and 2.9 mm (Fig. 3d1, 3k1); however, by reducing to 30%, the thickness decreased and was located between 0.08 and 0.5 mm (Fig. 3n1).

Something similar happened with wheat gluten; with the addition of 5% of this protein, the thickness of the cells was between 0.359 and 2.093 mm (Fig. 3b1, 3j1); when its inclusion was reduced to 2.5 or 1.3%, the thickness was reduced, being between 0.17 and 0.7 mm (Fig. 3l1, 3m1); and even without the presence of gluten, the thickness reached its lowest values, between 0.08 and 0.5 mm (Fig. 3n1).

On the other hand, in Fig. 3k1 and 3n1, it can be observed that the cell structure presented rupture walls, which turned it into a group of bubbles fused into a single cell, a phenomenon that was possibly a consequence of the low elasticity and low melt strength (resistance to melt stretching) of both TPS and PLA [36, 41]. In this regard, other authors indicate that low melt strength can lead to cell wall rupture and coalescence, resulting in inhomogeneous cell morphologies with a high content of open cells [42].

In addition, the low elasticity and melt strength of the polymer blend could also be a consequence of the formation of an inhomogeneous mixture of separated phases, especially between the thermoplastic starch and the biodegradable polyester [33]. This class of mixtures is characterized by the formation of spherical particles (droplets) with sizes between 0.1 and 20 µm [17, 43]. The presence of droplets was evident in the TPS/polyester blends and can be clearly seen in the images of each of the treatments (Fig. 3). A possible explanation for this behavior is the interference of the biodegradable polyester in the thermoplasticization of the starch during the extrusion process, which was carried out in a single step. In this case, the polymer was able to prevent the glycerin and starch from coming into contact, the spherical particles present in the mixtures being unstructured and plasticized starch granules, a problem that was accentuated by increasing the inclusion of the biodegradable polyester. In this respect, [34], in studies performed with thermoplastic starch/PBAT blends, processed by extrusion, both in one and two steps, found that samples prepared in one step had a “marine island” structure, while treatments processed in two steps showed a continuous phase structure. During one-pass extrusion, starch is mixed with glycerol and PBAT, so the probability of contact with this plasticizer decreases, as a result, the degree of starch plasticization is low. Additionally, the use of a single screw extruder possibly played a role in the low efficiency in thermoplastification the starch and blending with the biodegradable polyester (high concentration of the polyester). Since this type of machine is less efficient in feeding, conveying, mixing and plasticizing in extrusion processes than twin screw extruders [44].

Fig. 3
figure 3

SEM images of starch-based thermoplastic foams

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Natural rubber latex coated foams

Optimization calculations indicated that with the addition of 70% TPS, 30% biodegradable polyester, and 0% vegetable protein, the highest radial expansion rate, lower bulk density, and high compressibility were obtained. Consequently, a thin layer of rubber latex was applied to this thermoplastic foam to protect it from moisture.

Moisture absorbing latex rubber coated foam

The moisture adsorption test was carried out to establish the behavior of starch foams (coated and uncoated with rubber latex) with ambient free water. The addition of a rubber latex coating on the foamed material had significant effect (p ≤ 0.05) on moisture adsorption. This can be evidenced in Fig. 4, where the foamed material was coated with rubber latex (CNR) had a minor adsorption of moisture with respect to the non-coated foamed material (SNR). Thermoplastic foams made from native starch have a high moisture adsorption capacity due to the large amount of hydroxyl groups with the possibility of forming hydrogen bonds with water molecules. By placing a thin layer of rubber latex (a hydrophobic natural polymer), it acted as a barrier against moisture, thus decreasing the number of water molecules interacting with the thermoplastic foam [11, 45]. In addition, the latex layer possibly covered the pore spaces of the foamed material making it less hygroscopic [46].

Fig. 4
figure 4

Moisture adsorption in coated and uncoated foams

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Spring index and compressibility of foams coated rubber latex

The addition of a rubber latex coating over the foamed material had no significant effect (p ≤ 0.05) on the spring index. In the case of compressibility, the coating did have a significant effect (p ≤ 0.05). As shown in Table 3, the latex layer on the surface of the foamed material generated an increase in compressibility, possibly the latex adhered to the surface of the foamed material, acting as reinforcement and increasing the resistance of the material to compressive forces. In this sense, [47], in studies carried out on wood coated with natural rubber latex, found that this coating increased the compressive strength of the wood. Also, [48] reported that materials used in filtration systems increased their compressive strength by applying a thin layer of rubber latex.

Table 3 Coated and uncoated foam results
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The spring index (SI) is an indirect measure of a material’s capacity to absorb energy and indicates its ability to recover its original shape after being deformed [49]. When a material has an SI equal to one, it means that it behaves as a perfect elastic body, which recovers 100% of its initial shape [50]. Compressibility is a direct measure of the ability of a material to deform under a force and is physically related to the relative hardness or softness of the product. Compressibility can be an indicator of the cushioning capacity of a thermoplastic foam [49].

Commercially, foams with low compressibility are preferred, as a high value may indicate that the material is rigid, brittle and easy to break into small pieces, which easily lose their cushioning capacity. Although, there is no general rule indicating a standard value for both compressibility and spring index of packaging foams, materials with an SI close to 1 and low compressibility are preferred [50]. In this regard, [51] report an SI of 0.97 and a compressibility of 97 kPa for polystyrene foams and 0.96 and 136 kPa for commercial starch foams. Comparing these values with the results obtained in the present study, it is possible to state that the addition of the thin rubber latex layer did not affect the cushioning capacity of the thermoplastic foam, since the coated foam presented a spring index of 0.98 ± 0.003 and a compressibility of 87.22 ± 5.89 kPa.

The apparent density of the foamed material (prior to coating) measured 375.894 ± 7.900 kg/m3, which is higher than the reported density for polystyrene-based foams (8.9 kg/m3). This particular property may be difficult for the utilization of this foam in packaging and cushioning applications. Nevertheless, there exists the potential to reduce this density. According to the literature, the nominal density of starch is 1500 kg/m3; however, through the extrusion process, it can be decreased by as much as 90%. One feasible approach the implementation of a twin-screw extruder and/or the use of low molecular weight plasticizers [41].

Evaluation of the biodegradability of thermoplastic foams

In biodegradation tests according to ISO 14855 [30], the suitability of the inoculum is demonstrated if the positive reference shows a mineralization rate of at least 75% during the first 45 days of the test and if the target CO2 production rates are between 50 and 150 mg CO2 per gram of volatile solids in the compost during the first 10 days. In this test, the cellulose after 30 days of exposure presented a mineralization percentage of 87.222 ± 4.82% and the respiration rates of the inoculum reached 50.55 mgCO2/g SV. For this reason, it can be assured that the inoculum presented an adequate microbial activity, since it meets the requirements of the standard.

Figure 5 shows the biodegradation curves for both the sample of interest (starch-based thermoplastic foam) and the positive reference (cellulose). The biodegradation rates of the thermoplastic foam exhibited an accelerated increase (exponential phase) during the first 5 days of the test; by day 13 these rates decreased, entering what is possibly the stationary period of the process. At day 30, the foam presented a mineralization percentage of 85.134 ± 6.074%, which basically suggests a complete degradation of the material. This, taking into account that some authors affirm that in the biodegradation processes, around 30% of the carbon in the samples becomes part of the biomass of the medium; therefore, conversion levels between 70 and 80% of the carbon contained in the samples to CO2 indicate total biodegradability [52].

Fig. 5
figure 5

Biodegradation curves of the foamed material

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According to EN 13432 [53], the biodegradability of a plastic material under composting conditions is verified if it meets the criteria of biodegradability, disintegrability, ecotoxicity, and heavy metal content. The biodegradability criterion indicates that a biodegradable material must show 90% mineralization percentages in relative or absolute terms in less than 180 days.

Conclusion

It was observed that an increase in the concentration of both biodegradable polyester (from 30 to 40% by weight) and vegetable protein (from 0 to 5% by weight) led to a reduction in the radial expansion index and an increase in the apparent density of the thermoplastic foam. This can be attributed to the potential interference of these two components, preventing glycerin and starch from coming into contact. Consequently, incomplete thermoplasticization occurred, where a significant amount of starch granules failed to plasticize, resulting in poor adhesion between phases. This led to the separation of phases within the mixture, comprising biodegradable polyester, wheat gluten, and thermoplastic starch. As a result, the mixtures gradually lost their radial expansion capacity, increased in apparent density, and became more compressible, ultimately yielding a rigid foam with low cushioning capacity. Additionally, the inclusion of polylactic acid, a biopolymer with a linear molecular structure and low elasticity in its molten state, increased as the polyester content increased. This inclusion further hindered expansion during the foam formation process. Concerning the incorporation of a thin layer of a natural rubber latex-based coating, this allowed the porous material to reduce moisture absorption. It acted as a barrier, blocking the hydroxyl groups of the starch from interacting with water molecules. In addition, this latex layer served as a reinforcement, increasing resistance to compressive forces. However, its impact on the cushioning capacity of the foam was minimal. In relation to the biodegradability assessment, the foamed material showed a relative mineralization of approximately 97.60%, meeting the biodegradability requirement stipulated by EN 13432. This remarkable degree of biodegradability is due to the use of biodegradable components in the development of the foamed material.