Bioplastic and its morphological characteristics
Pectin extract yielding was 75% with respect to dry mass. The films were flexible, with a smooth texture, a pleasant smell, and an opaque yellow color whose tone visibly decreases with increasing glycerol content, as previously reported by Ramos-Alvarado (Ramos-Alvarado et al., 2020). The thickness was ~ 0.3 mm. The films did not present cracks and were homogeneous in their conformation, which indicates that the drying temperature was optimal. They presented sufficient mechanical integrity to easily detach from the aluminum foil. Figure 1 shows the surfaces of the bioplastic films. Both samples have a heterogeneous surface, with agglomerates and striations that may be caused by fibrous material. It is evident that the PG3 sample has a greater number of striations and present agglomerates above 2 mm, on the contrary, the PG5 sample is seen to be smoother and more uniform. It is possible that the proportion of 3% of plasticizer in the PG3 sample was not enough to completely disintegrate the pectin domains, creating agglomerates that could be caused by its interaction with the residual fibrous content generated during the extraction process. It is known that pectin films are characterized by the absence of a homogeneous structure, which is apparently due to the formation of packed pectin agglomerates (Giancone et al., 2011).
Mechanical analysis
The mechanical behavior of the bioplastic films was evaluated by the stress-strain curve. Elastic modulus, tensile strength, and strain at break, were obtained. Figure 2 presents the curve of the tensile test of both samples. It is observed that they possess a characteristic behavior of a plastic material, since no change in the slope of the curve is observed.
The mechanical properties of bioplastic are presented in Fig. 3. It is observed that the proportion of plasticizer modifies the mechanical properties of the film. The elastic modulus is reduced by 67.86% when going from 23.976 ± 3.993 MPa, to 7.705 ± 0.470 MPa, for PG3 and PG5, respectively. Regarding the tensile strength, the PG5 sample presented a reduction of 57.41%, when changing from 4.560 ± 2.588 MPa, to 1.942 ± 1.550 MPa. Finally, the strain at break was improved with the increase in plasticizer, since the PG5 sample presented a value of 43.716 ± 4.435%, against 31.370 ± 4.765% for PG3. Bioplastic films based on pectin with a high content of methoxy groups have a high degree of esterification, which gives them the ability to form networks with a high degree of gelation at room temperature. This type of film has high values of tensile strength ~ 21 MPa, which makes it rigid and with little deformation capacity (~ 1–3.6%), making it unfeasible to produce films for food packaging (Bátori et al., 2019; Giancone et al., 2011). To improve the performance of these films, the use of a plasticizer is required. Glycerol is one of the most widely used plasticizers in the food and pharmaceutical industries. Due to its low molecular weight (92.09 g/mol), glycerol can permeate the inter-molecular spaces of the pectin main chains, hindering the formation of hydrogen bonds and reducing polymer-polymer interactions, promoting an increase in the mobility of the polysaccharide chains. The plasticizing effect of glycerol can be explained in two ways; the first is that the hydroxyl groups of glycerol can interact with the OH and COOH groups of pectin, to form covalent ester bonds of the hydroxyl-hydroxyl or hydroxyl-carbonyl type, through condensation reactions; the second way of glycerol-pectin interaction is through the solvation of the polar sites in polysaccharide main chain, which causes a masking of the hydrophilic sites. This produces a reduction in pectin-pectin interactions, increasing the mobility of the chains to produce films with a lower elastic modulus and tensile strength, but with greater deformation capacity (Costanza et al., 2019; Darni et al., 2017).
There is an adjacent mechanism that helps explain the plasticization of the material induced by pectin-glycerol interactions. This mechanism is related to the amount of water admitted to the pectin network, which increases with increasing proportion of glycerol in the film. With the increase in glycerol, pectin-glycerol interactions increase, which favors the availability of hydrophilic regions along the main chain of pectin, causing an increase in hydrophilicity and mobility of the chains.
This triggers more water molecules to be linked to the interior of the polysaccharide matrix, promoting its plasticizing effect (Costanza et al., 2019). As shown below in the thermogravimetric analysis, the water content in the bioplastic films increased with the glycerol content. It is possible that this same mechanism explains the difference observed in the surface of the films, in which a higher proportion of plasticizer in PG5 has served to promote a better distribution of the pectin domains, obtaining a smoother surface, since in the dehydrated state and without glycerol, the hydrophilic regions of the pectin form aggregates causing hydrogen bonds and reducing the amount of water admitted.
One way to infer the level of pectin-glycerol interaction in the film is by monitoring the peaks at 925 cm− 1 and 850 cm− 1 of the FTIR spectrum, corresponding to the vibration of the C-C bonds. These peaks will be reduced if this interaction is increased, which can be achieved by increasing the proportion of glycerol in the film. On our orange peel bioplastic, this behavior is observable since these peaks reduce their intensity by changing the proportion of glycerol from 3–5%, as reported in the Ramos-Alvarado FTIR analysis (Ramos-Alvarado et al., 2020).
Thermogravimetric analysis (TGA)
Five events of thermal decomposition were observed in the TGA curve of the bioplastic (Fig. 4). The behavior depends on the proportion of glycerol in the film. The first two events correspond to the evaporation of water and the thermal degradation of pectin, respectively. There is a clear difference in PG3 and PG5, which corresponds to the plasticizing effect of glycerol, as mentioned above. The greater proportion of water in PG5 originates from the fact that a greater number of glycerol molecules favors the formation of more intermolecular spaces to admit water. On the other hand, the thermal degradation of pectin occurs differently for both samples. The pectin in PG5 suffers an alteration in its thermal stability due to the plasticizing effect of glycerol increasing the mobility of the pectin chains, reducing thermal stability, and showing a shift to the left with the degradation peak at 184.6°C. It has been observed that pectin films can thermally decompose in an interval from 150°C to 580°C, with a maximum decomposition peak between 201°C and 260°C. The thermal degradation of pectin begins with the depolymerization and degradation of the galacturonic acid chains, followed by secondary decompositions related to the breaking of bonds and functional groups, which gives rise to a gas phase; subsequently, chemical reactions of the gaseous phase and oxidation of volatile organic compounds take place to form CO, CO2 and H2O, even under inert atmosphere conditions (Aburto et al., 2015; Al-Amoudi et al., 2019; Espitia et al., 2014; Giancone et al., 2011).
The DTG curve (Fig. 5) represents the kinetics of thermal degradation. It is observed that pectin in PG3 decomposes in two events, the first event with a peak at 156.8°C and the second at 220.1°C. As can be seen, the first presents a shift to the left, while the second is within the common interval of decomposition. This degradation behavior may be due to structural differences within the polymeric matrix, caused by the heterogeneous distribution of the hydrogen bonds of the pectin-pectin interactions, which may benefit from the heterogeneous dispersion of the glycerol molecules, possibly caused by the high fibrous content of the sample (Costanza et al., 2019; Darni et al., 2017).
The third and fourth events of thermal decomposition correspond to glycerol and fiber content, respectively. The PG3 sample has a higher content of fibrous material, and it is possible that this has hindered the dispersion of the glycerol molecules in the film, perhaps that agglomerations of this material have formed, which is inferred from the characteristics reviewed in the morphological analysis of the surface. This fiber content may also be related to the better thermal stability of PG3. The insoluble fiber content in orange peel is between 42.7% and 48.3%, mainly composed of cellulose and lignin (Garcia-amezquita et al., 2019; Rincón et al., 2005). Finally, the last event corresponds to the gasification of carbonaceous residues. The results of the quantitative analysis of the thermal decomposition are presented in Table 1.
Table 1
Thermal events of the orange peel bioplastic
PG3
|
PG5
|
Events
|
TOn
|
TP
|
TOff
|
\(\varDelta\)m
|
TOn
|
TP
|
TOff
|
\(\varDelta\)m
|
|
(°C)
|
(°C)
|
(°C)
|
(%)
|
(°C)
|
(°C)
|
(°C)
|
(%)
|
|
33
|
63.9
|
98.5
|
7.55
|
33
|
55.3
|
96.1
|
8.13
|
Water evaporation
|
117.4
|
-
|
280.2
|
53.03
|
101.9
|
184.6
|
277.5
|
72.33
|
Pectin decomposition
|
328.1
|
340.6
|
392.0
|
15.25
|
329.2
|
338.8
|
409.2
|
5.68
|
Glycerol decomposition
|
425.8
|
454.9
|
475.5
|
15.96
|
418.6
|
455.7
|
489.5
|
9.09
|
Cellulose-lignin breakdown
|
476.0
|
-
|
559.3
|
6.71
|
490
|
-
|
565
|
3.1
|
Carbonaceous gasification
|
1.25%
|
1.43%
|
Ashes
|
Ton Onset Temperature, TOff Offset Temperature, TP Peak Temperature, \(\varDelta\)m mass loss during TGA. |
The water in in the polymeric matrix of pectin exist as free water and as bound water. The bound water consists of all the water molecules that are joined by hydrogen bonds to the different components of the polymeric matrix, while the free water is admitted within the free volume caused by the plasticizer, without forming hydrogen bonds. This phenomenon can be observed in the curve of the second derivative of the mass loss (2ndDTG). During the evaporation of the water, a peak will appear in the 2ndDTG indicating the change in the kinetics of mass loss, which means that the free water finishes evaporating to give way to the evaporation of bound water. The matching of maximum of that peak with the TGA curve indicates the mass of free water evaporated. The occurrence of this peak depends on factors such as the rate of heating and the velocity of the gas flow during the TGA; it has been observed that the peak of this event appears shifted to the left with the increase in the heating ramp (Aburto et al., 2015; Costanza et al., 2019; Wang et al., 2018). In our bioplastic films, the proportion of water evaporated from the sample corresponds to the proportion of glycerol, as can be seen in Fig. 6, since a higher content of plasticizer molecules increases the admission of water. The amount of free water in the samples is presented in Table 2.
The difference in free water content is an indicator of the type of pectin structural arrangements in the film matrix. It could also be an indicator that the greater the amount of free water, the greater the amount of energy required to convert the water molecules to the gaseous state.
Table 2
Quantification of the free water contained in the bioplastic films
Parameter
|
PG3
|
PG5
|
TP (°C)
|
35.98
|
38.15
|
Mass loss (%)
|
0.18
|
0.40
|
Free water ratio (%)
|
2.38
|
4.92
|
TP, Peak temperature indicates the change in kinetics. |