3.1 Mechanical Properties
The results of the mechanical properties of biodegradable materials are shown in Table 2.
Table 2
Mechanical properties of biodegradable materials produced by thermoplastic injection
Formulation | Tensile Strength (MPa) | Elongation at Break (%) | Young's Modulus (MPa) |
PBS | 12.5 ± 0.2a | 16.1 ± 0.5b | - |
F0 | 1.8 ± 0.1b | 20.2 ± 1.3a | 13.8 ± 2.8c |
F20 | 1.6 ± 0.1bc | 13.3 ± 0.9c | 15.9 ± 6.6c |
F40 | 1.4 ± 0.1cd | 11.1 ± 1.3d | 22.6 ± 8.2bc |
F60 | 1.2 ± 0.1de | 8.3 ± 2.2 e | 24.5 ± 10b |
F80 | 1.1 ± 0.1ef | 7.4 ± 0.6of | 17.9 ± 5.2c |
F100 | 0.8 ± 0.1f | 6.5 ± 0.4 f | 29.6 ± 6.6a |
a,b,c,d,eAverages with different letters represent a significant difference (p ≤ 0.05) between the formulations, according to Tukey's test.
With the exception of PBS, all materials had tensile strength [TS] values below 2 MPa. In contrast, PBS demonstrated significantly higher TS, approximately ten times greater than the others [12.5 MPa]. The higher the oat hull concentration, the lower the TS, probably due to the limited interfacial adhesion between the oat hulls with the polymeric matrix. The generation of interactions among the polymer matrix [PBS] and oat hulls can be a complex process, primarily due to the hydrophilic and more polar nature of these natural fibers, which have hydroxyl groups [14]. According to Mochane et al. [4], the hydrophilic characteristics of natural fibers may adversely affect interfacial adhesion, particulary when the matrix biopolymer [PBS] is hydrophobic in nature. This is a great challenge for natural fibers application in blends, as they present poor bonding with hydrophobic matrices, resulting in poor mechanical properties [15, 16].
It was expected the materials would gain strength as the oat hulls' concentration increased, but the opposite occurred. The utilization of natural fiber-reinforced composites presents many advantages, mainly great mechanical properties, as they act as reinforcement for many plastic matrices, however, other factors such as fiber length, fiber type, extraction process, and moisture absorption may have caused material strength to decrease [16]. Agglomeration and filling of natural fibers within the polymeric matrix also can contribute to the loss of strength. This was noted in blends of polyhydroxybutyrate and polylactic acid reinforced with cellulose nanofibrils produced by Aydemir and Gardner [17]. When the fiber concentration was increased from 1 wt. % to 4 wt. %, there was no improvement in the mechanical properties, while in the SEM a greater agglomeration and clogging of the fibers in the matrix structure was revealed. This may have caused higher stiffness and ductility of the blend. Ayu et al. [18] conducted a study where sheets of PBS, starch, and empty fruit buncher fiber [EFB] were successfully produced. However, the addition of the fiber fillers up to 8 wt% resulted in a decrease in both tensile and flexural strength. The authors suggested that this reduction was due to a lack of interfacial adhesion and poor dispersion of the fibers in PBS matrix. Similarly, Peixoto et al. [19] produced extruded composites using poly lactic acid [PLA], oat hull fiber, and sodium trimetaphosphate as a compatibilizer. The tensile strength of this composite material was found to be lower [4.65 MPa] than that of pure PLA [~ 60 MPa] [20].
In summary, the incorporation of natural fibers as indicated in the literature can be influenced by numerous factors [poor dispersion of fibers, polar and non-polar phases, fiber concentration, extrusion process] that can contribute or degrade the strength of the material [18, 19, 21].
The F0 material exhibited the highest flexibility, possibly due to its high thermoplastic starch content, which contributes to increase in the material elasticity. Thermoplastic starch [TPS], when in the presence of a plasticizer such as glycerol, has its flexibility increased by lowering the glass transition temperature [Tg] which can be beneficial for some applications such as food packaging and films [22]. On the other hand, the material with 20% oat hull content [F20] had approximately 7% lower elongation than F0. By increasing the concentration of oat instead of starch, the elongation at break decreased, indicating that the material had become more ductile. Natural fibers can act as fillers, reducing the flexibility and becoming more rigid. Calabia et al. [23] produced sheets of PBS and cotton fiber, and observed a decrease in elongation at break of approximately 8% in formulations containing fiber. This decrease in elongation was attributed to the reduction of PBS chains movement, leading to higher stiffness in the materials. Similarly, Yang et al. [24] produced injected materials using bamboo fiber and polypropylene, and they found that as the fiber concentration increased, the elongation at break [%] decreased. This decrease was attributed to the difficulty in maintaining fiber dispersion in the blend, which negatively impacted the material’s flexibility.
3.2 Scan Electronic Microscopy
The PBS material [Figure 1a, Fig. 1b] presented a smooth surface. This behavior can be explained by the absence of other components like fibers and starch in the polymeric matrix [18, 25]. In contrast, the images of the F0 [Figure 2a, Fig. 2b] material showed starch granules [circular shape] and a plasticized superficial area, characteristic of blends containing thermoplastic starch [25, 26]. Cracks were also observed, likely resulting from the incompatibility between PBS and starch, leading to the formation of a heterogeneous phase. Successful interfacial adhesion relies on the proper dispersion of materials in the matrix and is mainly affected by the hydrophilic or hydrophobic characteristics of the components. When similar, these characteristics can create a strong bond, resulting in dimensional stability and improved mechanical and barrier properties [27, 28].
The F20 [Figure 3a, Fig. 3b], F40 [Figure 4a, Fig. 4b], F60 [Figure 5a, Fig. 5b], F80 [Figure 6a, Fig. 6b], and F100 [Figure 7a, Fig. 7b] materials' surface and fracture images showed oat hulls fiber [cylindrical shape], most of which were agglomerated and aligned, possibly due to extrusion orientation. Many cavities and pores were also observed in the formulations containing oat hulls, which might explain the decrease in the mechanical properties of these materials. Similar observations were made by Calabia et al. [23] with PBS and cotton fiber composites, as well as Ayu et al. [18] with sheets containing empty fruit bunch, PBS, and modified tapioca starch. In both cases it was possible to observe the presence of long fibers and voids in the structure, indicative of weak interfacial adhesion.
Fibers in polymeric blends can act as fillers, improving the mechanical properties, but they can pose challenges at high concentrations, and the mechanical and barrier properties are highly dependent on the morphology status [4, 18]. The lack of interfacial adhesion is one of the challenges in producing natural fiber-reinforced materials, particularly due to the hydrophilic nature of the fiber and the hydrophobic biopolymer [PBS], leading to poor fiber distribution, consequently, reduced mechanical properties and increased hydrophilicity [4, 28].
3.3 X-ray diffraction [X-RD]
The X-ray diffractograms and the respective relative crystallinity index [RCI] of the biodegradable materials are presented in Fig. 8. Two peaks [19.9° and 22.3°] were identified in all blend formulations.
The oat hull concentration increase did not influence the materials' crystallinity. Hu et al. [29] produced blends containing PBS and different types of cellulose, and noted that all diffractograms showed reduced peaks, suggesting that the natural fibers used were amorphous and presented low crystallinity. Liu et al. [26] produced materials with starch, PBS, and ionic liquid; varying starch content did not modify the PBS crystalline phase also.
PBS showed peaks around 19.6°, 22.3°, and 28.8°. Peaks near 19.5° and 22.5° are characteristics of the crystalline phase of PBS [30, 31]. PBS also showed a larger peak [22.3°] when compared to the other formulations. Probably the addition of components such as starch, oat hulls, and glycerol has impaired the crystallinity properties of the PBS during the production/extrusion process of the materials. The better crystallinity [RCI 23.11%] was reflected in the mechanical properties, where pure PBS showed higher tensile strength [Table 2] and a crystalline surface [Figure 01a] compared to the other formulations containing starch or oat hull. This same behavior was observed in blends produced by Xu et al. [31] with PBS and corn starch also, possibly as starch particles obstruct PBS segments. The low relative crystallinity indexes [RCI] of the biodegradable materials containing starch, ranging from 10.57 to 15.24%, can be attributed to the destruction of the semi-crystalline structure of starch during the extrusion process, leading to the formation of higher amorphous zones [2, 32].
3.4 Linear Contraction Index [LCI]
The Linear Contraction Indexes [LCI] of the biodegradable materials are presented in Table 3. PBS was the material with the highest LCI [1.75%], followed by F0 [1.56%] and F20 [1.07%]. The LCI of the other materials had no statistical difference between them and ranged from 0.70 to 0.78%. Materials produced by injection molding can contract as they change from melting to solid under atmospheric pressure [33].
Table 3
Linear Contraction Index (LCI) of the biodegradable materials
Formulation | LCI (%) |
F0 | 1.56 ± 0.06b |
F20 | 1.07 ± 0.06c |
F40 | 0.78 ± 0.04d |
F60 | 0.75 ± 0.05d |
F80 | 0.74 ± 0.03d |
F100 | 0.70 ± 0.04d |
PBS | 1.75 ± 0.08a |
a,b,c,d Averages with different letters represent a significant difference (p ≤ 0.05) between the formulations, according to Tukey's test.
Increasing fiber concentration resulted in a lower contraction of the injected material because the fibers can act as fillers, i.e., occupying spaces in the blend structure and making it less capable of shrinking or expanding. The MEV analysis of the materials’ with oat hulls showed them oriented and agglomerated in a cylindrical shape, possibly acting as fillers in the blend. This suggests that these natural fibers, by being aligned and clogging the polymeric structure, may have caused the decrease in LCI values. The LCI also can be influenced by various factors, such as temperature, pressure, injection flow rate, design of the equipment, and material composition [33, 34].
3.5 Density
For the density of the materials, it can be observed that The PBS formulation had the lowest values, followed by F100, and the other formulations did not show significant differences (Table 4). This result can be explained by the inherent lower density of the PBS polymer compared to TPS. Additionally, the high oat hull content on the F100 formulation contributed to the decrease in density, as fibers typically have lower densities compared to TPS. Moreover, the presence of fibers in the blend can enhance the material's porosity, leading to reduction in its overall density. One of the main advantages of adding natural fibers in polymeric structures is to reduce material weight and density [4, 35, 36]. Aslan, Tufan, and Küçükömeroğlu [37] produced composites of polypropylene and sisal fiber, fiberglass and carbon fiber. With increasing concentration of sisal fiber it was evaluated that the material presented lower density also. The density of the material decreases with the addition of natural fibers in the blend in most cases. Despite not having a lower density than the pure polymer [PBS], the fiber still proved to be less dense and lighter than the starch-containing formulations.
Table 4
Density of the biodegradable materials produced by injection extrusion
Formulation | Density (g 100 g-1) |
F0 | 1.34 ± 0.02c |
F20 | 1.32 ± 0.04c |
F40 | 1.35 ± 0.04c |
F60 | 1.32 ± 0.04c |
F80 | 1.35 ± 0.03c |
F100 | 1.28 ± 0.02b |
PBS | 1.22 ± 0.03a |
a,b,cAverages with different letters represent a significant difference (p ≤ 0.05) between the formulations, according to Tukey's test.
3.6 Mass Loss in Water [MLW]
The results of mass loss in water [MLW] of the biodegradable materials are presented in Table 5.
Table 5
Mass loss in water (MLW) of the biodegradable materials
Formulation | MLW (%) |
F0 | 28.47 ± 0.05b |
F20 | 28.42 ± 0.12b |
F40 | 27.34 ± 1.07b |
F60 | 27.70 ± 0.28b |
F80 | 28.34 ± 0.58b |
F100 | 28.01 ± 0.02b |
PBS | 0.59 ± 0.10a |
a,b Averages with different letters represent a significant difference (p ≤ 0.05) between the formulations, according to Tukey's test.
All the formulations were statistically similar, with MLW around 28%, except for PBS material [0.59%]. This lower interaction with water was due to the hydrophobic structure of PBS compared to other blend components [starch, oat hulls, and glycerol]. Similar findings were reported by Yin et al. [38], who produced blends with cassava starch and PBS with maleic anhydride as a compatibilizer.
The authors observed that increasing the concentration of PBS in the blend resulted in lower water absorption by the starch and reduced water sensitivity of the blend, compared to other formulations. In addition, the amount of compatibilizer did not influence the water equilibrium, only varying PBS concentration did. This effect of reducing water absorption with increasing PBS content was also observed by Zeng et al. [39] in corn starch and PBS blends. As the concentration of PBS increased in the sample, the water absorption in the blend decreased. These studies demonstrate that PBS is highly hydrophobic and optimizing its concentration on the blend can be helpful to avoid water absorption and improve material stability. Also the high moisture content can be detrimental for the mechanical properties or the blend, and is one of the factors that hinder natural fibers from being applied on a large scale [15].
Starch-based biopolymers have drawbacks such as hydrophilicity, semi-crystalline structure, brittleness, and low barrier properties [2]. A highly hydrophilic material can have defects such as the creation of bubbles, change in mechanical properties, variation in fluidity, and especially the exchange of moisture with food, in packaging applications. Good water resistance properties are essential for materials to be used in food applications, as the water can be detrimental to the shelf life, like microorganism growth and texture changes [40]. To address this limitation, increasing PBS concentration or adding another hydrophobic component can be helpful for material water equilibrium. Increasing oat hull concentration did not influence in MLW values.
Despite the hydrophilic nature of oat hulls, it appears that their presence did not substantially affect the materials MLW. In films, fibers can decrease the materials water vapor permeability [WVP] because of the "tortuosity" effect, where water molecules must navigate a convoluted path through the material, resulting in lower WVP values [40]. However, most natural fibers are composed of cellulose which is very hydrophobic and polar, worsening moisture absorption. Other parameters such as fiber content and size, crystallinity, plasticization, and compatibility between fiber and matrix can influence materials interactions with water [41].
3.7 Moisture Sorption Isotherms
The Guggenheim-Anderson-de-Boer [GAB] parameter values of the biodegradable materials' moisture sorption isotherms are presented in Table 6.
Table 6
GAB model parameters of the biodegradable materials' moisture sorption isotherms
Formulation | GAB Model Parameter | R² |
m0(g 100 g-1) | k | C |
F0 | 16.11 | 0.78 | 10.000 | 0.97 |
F20 | 4.66 | 1.01 | 10.000 | 0.99 |
F40 | 4.56 | 1.01 | 10.000 | 0.98 |
F60 | 2.43 | 1.06 | 10.000 | 0.99 |
F80 | 2.31 | 1.06 | 10.000 | 0.99 |
F100 | 4.03 | 1.02 | 10.000 | 0.99 |
Formulations containing F - Oat Hulls in different concentrations
The m0 of the F0 material [16.11 g 100 g-1] was the highest among all formulations, mainly due to its higher proportion of TPS and the absence of oat hull. TPS is known for its high hydrophilicity [2]. As the oat hull content increased in the formulation containing oat hulls [F20, F40, F60, F80, F100], the m0 values decreased in comparison with F0 [only TPS and PBS]. This trend is attributed to the reduction of TPS proportion. The F100 material has no starch and has an intermediary m0 value between the F0 and F80 because the oat hull is less hydrophobic than PBS but more than TPS. The k and C values were similar for all formulations.
3.8 Color
The CIELabcolor parameters L*, a*, and b* of the biodegradable materials are presented in Table 7.
Table 7
– CIELabcolor parameters of the biodegradable materials
Formulation | Color Parameter |
L* | a* | b* |
F0 | 62.69 ± 0.27 | -2.71 ± 0.13 | 9.79 ± 0.26 |
F20 | 42.82 ± 0.47 | 4.09 ± 0.25 | 2.57 ± 0.31 |
F40 | 41.54 ± 0.74 | 3.17 ± 0.09 | 0.54 ± 0.29 |
F60 | 43.49 ± 1.91 | 2.62 ± 0.22 | 1.05 ± 0.92 |
F80 | 45.29 ± 3.50 | 2.47 ± 0.29 | 2.09 ± 1.97 |
F100 | 42.69 ± 2.03 | 2.84 ± 0.22 | 1.66 ± 1.29 |
PBS | 81.03 ± 0.41 | -2.26 ± 0.09 | 3.40 ± 0.24 |
The L* parameter is luminosity; a* red/green coordinate; b* yellow/blue coordinate
Color is an essential parameter in the commercial use of materials, as it can influence consumer acceptance based on the product’s transparency and the visual presentation [41]. For the L* parameter, PBS material [81.03] had the highest luminosity, followed by F0 [62.69]. In contrast, the other formulations containing oat hulls exhibited lower luminosity, ranging between 41 and 45. The decrease in luminosity can be attributed to the opaque nature of oat hulls, which can influence greatly in the color and luminosity parameters. The a* parameter of the PBS and F0 materials were similar [-2.26 and − 2.72, respectively], as F20 [4.09] presented the highest value. On the other hand, The b* parameter of the F0 material [9.79] was higher than the others because it did not contain an oat hull in its formulation, only PBS and starch, resulting in a yellowish color.
In this study, all the materials with oat hulls had a brownish color. The oat hulls are composed of pentosans [30–35%] and protein [4%] [42], which are components that could contribute to Maillard reaction during the extrusion process at high temperatures [90–110°C]. This Maillard reaction could explain the color alteration observed in the materials with oat hulls.