Davan Mulligan, Year 2 Research
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Plastic pollution is a growing problem the world faces, as it litters our world and natural environment. Plastic is used in many people’s daily lives and has many uses and applications. In this project, starch bases were compared in production of eco-friendly biodegradable plastic substitutes. Cornstarch, wheat starch, potato starch, and tapioca starch were used as bases for the bioplastics in this experiment, and the products were tested for strength. It was found that the bioplastic made from potato starch was the strongest, while the wheat starch bioplastic was the weakest viable plastic.
Traditional (petroleum-based) plastics have a wide variety of applications, and play a significant role in people’s day to day lives (Marichelvam et al. 2019). As a country, Canadians produce 3 million tonnes of plastic waste annually (Government of Canada 2020). According to the Canadian government, only 9% of plastic in Canada is recycled, with 29,000 tonnes finding its way into our natural environment every year (Government of Canada 2020). On a global scale, about 359 million metric tonnes of plastic was produced in 2018 (Narancic et al. 2020), and although large-scale production of plastic only began in the 1950s, an estimated 8300 million metric tonnes of new plastic has been produced from the mid twentieth century to date (Geyer et al. 2017). Of the estimated 8300 million metric tonnes of plastic produced since about 1950, only 6% was recycled (Ritchie and Roser 2018). If the world continues to produce and manage plastic as it currently does, by the year 2050, approximately 12,000 Metric tonnes of plastic waste will occupy landfills as well as our natural environment (Geyer et al. 2017).
Starches are abundantly available, biodegradable, cheap, and renewable (Narancic et al. 2020), and they may have the potential to yield an environmentally-friendly plastic alternative. The purpose of this project is to determine a starch base that can produce a strong, biodegradable plastic alternative. In this project, various types of starch was used to determine the most efficient biodegradable plastic alternative, alongside vegetable glycerin, a natural plasticizer (Sen et al. 2017).
Amylose and Amylopectin are the two primary molecules present in starch, with Amylose being straight-chained, and Amylopectin being branch-chained (Sen et al. 2017). As Amylose’s straight-chain structure contributes to its gelling strength, starches high in Amylose will provide gelling properties, whereas Amylopectin-dense starches will provide more viscosity, as the large molecular size of Amylopectin due to its branch-chained structure yields this trait (Hegenbart 1996). The Amylose-Amylopectin ratio of the starches used in this project are as follows: potato starch, 20:80; cornstarch, 28:72; wheat starch, 25:75 (Jha et al. 2019); and tapioca starch, 15:85-18:82 (Hegenbart 1996). In order to isolate starch as the only variable, the same measurements of glycerin and water were used in experimentation of all starch bases. It was hypothesized if the starch was higher in Amylose, then it would be more successful as a bioplastic, due to the gelling properties of the molecule potentially causing the product to be more durable.
Creating the Bioplastics
4 ½ tablespoons of one starch was mixed into 400 mL of water and stirred until it appeared completely dissolved. The mixture was then heated on a stovetop, and once boiling, 70 mL of vegetable glycerin was added. The glycerin was stirred into the water and starch, and the mixture was left at a simmer for 5 minutes following the addition of glycerin. It was then spread onto a tray lined with parchment paper and left to cool and dry for a week at room temperature.
Tensile Strength Tests
Two strength tests were conducted, both measuring different aspects of tensile strength. For the first strength test (Strength Test A), each bioplastic was secured at each opposing ends to two tabletops as shown in figure 1. Weights were then placed in the centre of the bioplastic and its ultimate strength was measured. Ultimate strength is defined by Collins Dictionary as “the maximum tensile stress that a material can withstand before rupture.” Tensile strength in both tests were calculated by the equation:
Figure 1: Diagram of tensile strength test A
For the second strength test (Strength Test B), each bioplastic was secured at one end by a weight, and, at the opposing end, attached to clips and pulled by a spring balance shown in figure 2. The ultimate strength of the bioplastic was then measured and calculated by the aforementioned equation.
Figure 2: Diagram of Tensile Strength Test B
Strength Test A
Table 1: Force in Newtons divided by thickness in millimetres
The bioplastic with a potato starch base was calculated to have the highest strength, at 3.27 N/mm, having 0.98 N/mm of strength more than that of cornstarch, and 2.18 N/mm more than that of wheat starch.
Strength Test B
Table 2: Force in Newtons divided by thickness in millimetres
The bioplastic with a potato starch base was again calculated to have the highest strength, at 2.45 N/mm, with 0.65 N/mm of strength more than that of cornstarch, and 1.58 N/mm more than that of wheat starch.
Results from both strength tests suggested the bioplastic made from potato starch had the most tensile strength, with cornstarch second, followed by wheat starch. It is important to note that data could not be obtained from the bioplastic made from tapioca starch, as the starch failed to produce a testable result, which is why it was absent from the data tables. The bioplastic made with a tapioca base broke before experimentation could be conducted. It was hypothesized that the starch highest in Amylose would present the best results, however, both cornstarch and wheat starch were higher in Amylose than potato starch, yet potato starch was found to have the highest strength. As the tapioca starch broke before experimentation could occur, it may be inferred that there was not enough Amylose to bind the plastic. Similarly, the wheat starch was the most qualitatively thick and sticky, possibly due to the high ratio of Amylopectin present in the starch, however, further experimentation is necessary before drawing these conclusions.
In future experiments, temperature when producing the bioplastic should be measured and recorded, as different starches may have different temperatures necessary for gelatinization (Abe et al. 2021). Given more time, it would have been beneficial to experiment on the concentration and type of plasticizer used. In this project, 70 mL of vegetable glycerin was used for each bioplastic, although using different ratios and other plasticizers, such as sorbitol, may have provided more insight. Future experiments should include more trials to improve accuracy of data, as only one trial was used for each strength test due to limited time and resources. More variables would also be beneficial to finding a successful plastic alternative, as, plastic has a plethora of uses that were not covered in this experiment. Further studies on different bioplastics should explore the wide variety of uses traditional plastic has, and attempt to create eco-friendly plastic that suits all roles traditional plastic is currently used for.
References
Abe et al. “Advantages and Disadvantages of Bioplastics Production from Starch and Lignocellulosic Components.” Polymers, vol 13, no. 15, July 2021, https://doi.org/10.3390/polym13152484
Geyer et al. “Production, Use, and Fate of All Plastics Ever Made.” Science Advances, vol. 3, no. 7, July 2017, https://doi.org/10.1126/sciadv.1700782.
Government of Canada. “Canada one-step closer to zero plastic waste by 2030.” Environment and Climate Change Canada, Oct. 2020, https://www.canada.ca/en/environment-climate-change/news/2020/10/canada-one-step-closer-to-zero-plastic-waste-by-2030.html.
Hegenbart, Scott. “Understanding Starch Functionality.” Natural Products INSIDER, 1 Jan. 1996, https://www.naturalproductsinsider.com/foods/understanding-starch-functionality.
Jha et al. “Effect of Amylose–Amylopectin Ratios on Physical, Mechanical, and Thermal Properties of Starch-Based Bionanocomposite Films Incorporated with CMC and Nanoclay.” Starch, vol 72, no. 1-2, Jan. 2020, https://onlinelibrary.wiley.com/doi/10.1002/star.201900121.
Marichelvam, et al. “Corn and Rice Starch-Based Bio-Plastics as Alternative Packaging Materials.” Fibers, vol. 7, no. 4, Apr. 2019, p. 32. Crossref, https://doi.org/10.3390/fib7040032.
Narancic et al. “Recent Advances in Bioplastics: Application and Biodegradation.” Polymers, vol. 12, no. 4, Apr. 2020, p. 920. Crossref, https://doi.org/10.3390/polym12040920.
Ritchie and Roser. “Plastic Pollution.” Our World in Data, University of Oxford, 1 Sept. 2018, https://ourworldindata.org/plastic-pollution#citation. Sen et al. “Synthesis and Testing of Corn Starch Based Biodegradable Plastic and Composite.” 8th International Science, Social Science, Engineering and Energy Conference, Pattaya Beach, Thailand, 15-17 March 2017, https://www.researchgate.net/publication/330845696_Synthesis_and_Testing_of_Corn_Starch_Based_Biodegradable_Plastic_and_Composite
In this study, 30.08% of the amylose was found in the starch, which can be considered high amylose content. The amylose content in starch is an important characteristic for bioplastics production as it is responsible for gelatinization and retrogradation, which are required during film formation. Amylose is responsible for hydrogen bonding between hydroxyl groups of polymers forming junction zones between molecules and leading to film formation (Shimazu et al. 2007 ). Chemical, physical and functional properties of edible films and coatings depend on the amylose and amylopectin ratio (Xie et al. 2012 ). Joshi et al. ( 2013 ) noted that compared to corn and potato starches, lentil starch (30% amylose) possesses a strong gel-forming tendency at a relatively low concentration. Whatever the botanical origin, starch exhibits several disadvantages such as strong hydrophilic character (water sensitivity), which make it unsatisfactory for some applications.
The lipid content found in the jackfruit seed starch was low (0.3%). High lipid content in starch may cause the fixation of the color of the gel and consequently interfere in the color of the bioplastics and cause alterations of aroma and formation of complexes. In addition, lipids may negatively influence the swelling of the starch granule, change the gelatinization temperature, and limit amylose retrogradation, giving rise to brittle bioplastics (Singh and Nath 2013 ).
The percentage of ash was low as indicated by low concentrations of minerals in the sample. Low concentrations of minerals favor film formation. Large concentrations of minerals may hamper the formation of bioplastics due to a possible interaction between these compounds with amylose, amylopectin and plasticizer.
The centesimal composition analysis detected the presence of 1.17% ± 0.03 of total protein, 0.3% ± 0.01 of lipids, 0.15% ± 0.02 of fibers, 0.21% ± 0.03 of ash, 7.16% ± 0.04 of moisture and 30.08% ± 0.1 of amylose. In the current study, it is important to consider the amounts of ash, lipids, and amylose found since these are the components that influenced the formation of bioplastics.
Thickness of bioplastics made with different concentrations of starch and glycerol ranged from 0.099 to 0.1599 mm. A linear effect of the starch on the thickness was found, in which thickness increased with increasing starch concentration (Eq. 11). The increase in starch concentration is associated with higher concentrations of amylose and amylopectin which results in higher solids content in the film-forming solution and, consequently, the formation of a more viscous paste giving rise to thicker films. Similar behavior was reported for bioplastics by Zavareze et al. (2012). These authors reported that the film made with 3% oxidised potato starch had thickness of 0.073 mm and 5% native potato starch had thickness of 0.168 mm. According to López and García (2012) the comparison of film thickness with those reported in the literature for hydrocolloid-based films is difficult since this property depends on the casting relation, as data rarely mentioned.
Esp = 0.0699 + 0.0075X1
11
where, Esp is the film thickness in mm and X 1 is the starch concentration in % w/w.
Morphology and structure were analyzed only for bioplastics made with 2 and 3% starch with 30 and 60% glycerol. The morphology of the starch-based bioplastic of jackfruit seed plasticized with glycerol is shown in Fig. a, b.
Open in a separate windowSEM images shows that the film surfaces exposed to air are rough with some grooves.
The micrographs show some intact starch granules, which means that the starch was not fully gelatinized during the film forming process. Analyses of bioplastics cross sections reveal that the films have irregular structure, with ridges and grooves. The cross-sectioning of the films also showed the presence of non-gelatinized starch granules. The occurrence of grooves may be explained by the presence of microbubbles formed during the gelatinization process, which was also verified in the study of Wu et al. (2009).
In addition, cross-sectional images reveal phase separation due to empty spaces inside the polymer matrix. These spaces may be formed by the volatilization of the plasticizer under the high vacuum of SEM. It is important to emphasize that starch gelatinization process occurs in a temperature range. This was resulted from gelatinization kinetic of each starch granule. It is possible that in a starch mass there are non-gelatinated granules even though the ideal conditions of this process have been used. Another factor that may influence to jackfruit starch no-gelatinized granules in the film is its high gelatinization temperature (75–85 °C), displaying a loss of molecular structure in the granules of 40–86%, respectively. This show that even above the gelatinization temperature 14% of jackfruit starch granules did not got gelatinized. Singh et al. (2009) studied the effect of iodine on the pasting properties of kidney bean starch. They reported that the swelling and gelatinization of starch was retarded by presence of glycerol.
The presence of granules with superstructure in starch (Fig. ) don't indicate that they are intact/native. The granular structure is maintained even if the granules had lost the birefringence. According to Debet and Gidley (2007), starch dissolution is incomplete since amylopectin-rich fractions may remain insoluble and undamaged. In this case, as the starch has 70% amylopectin, this molecule can be responsible for insoluble material points in the film. Similar result was found by Garcia-Hernandez et al. (2017) for aqueous corn starch dispersion (5% w/w), with glycerol added as plasticizer (1.5% w/w).
The bioplastics produced had different surfaces on each of their sides. The surface in contact with the acrylic tray is highly glossy and smooth, while the interface exposed to the air is rough and opaque. The hydrophobicity analysis was performed only for the rough surface because, in case of bioplastics application, this is the surface that will be in direct contact with the food.
Analyzing the global free energy ΔGsasTOT (mJ m−1) (Table ), it found that the bioplastics made of starch and glycerol had hydrophilic behavior. The bioplastics with 6% starch and 40% glycerol were the most hydrophilic (ΔGsasTOT= 41.35 mJ m−1). This result is explained by the high hydrophilicity of the starch, due amylose concentration (> 30%) and glycerol. So these bioplastics have a high number of hydroxyl groups. According to Awadhiya et al. (2017) due the large difference in electro negativities of hydrogen and oxygen, the –OH group becomes highly polar. This polarity gives the compound a strongly hydrophilic nature. Similar results were obtained by Faradilla et al. (2017) working with film produced from banana pseudo-stem nanocellulose.
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ΔsasTOT
23024.442240− 2.87625028.84926010.32533016.23034027.14535025.39936014.74543036.44944021.84245031.37646015.30153039.29554039.84955035.9565605.80563032.58464041.35965034.98266030.853Open in a separate windowColor parameters of bioplastics were influenced by the variation in the starch concentrations of the film formulations. The different concentrations of glycerol caused no changes. There was a decrease in the color of the bioplastics, observed for the parameters of tonality (H) and saturation (C) with the increase in the starch concentration. The variation in the film color parameters with the increase in the starch concentration may be associated with the centesimal composition of the starch. It is known that among the film constituents, lipids are the compounds that interfere with color, and the increase in the starch concentration to make the film results in an increase in the concentration of lipids. These lipid particles scatter visible light through the film (Yoshida and Antunes 2009).
For the opacity (Y) and the color difference properties it was verified an increase of their values with the increase in the starch concentration (Eqs. 12–15). According to Basiak et al. (2017) these properties are dependent on both starch origin (amylose/amylopectin ratio, size and shape of starch granules) and thickness. Matrices which contain more amylose are thicker and thus more opaque. In the present study this result is associated with the increase in amylose amount which is directly proportional to starch concentration in the film. According to Zavareze et al. (2012) the film opacity is a critical property if the film is used as a food surface coating. Transparent films are characterized by low values of opacity. In this study, the obtained opacity values showed that films with higher jackfruit starch content were less transparent.
H=-1.0719-0.0593X1+0.0025X12
12
C=-1.8200+0.6000X1+0.0299X12
13
ΔE=53.4600-0.7200X1+0.0310X12
14
Y=13.2900+0.4490X1+0.0208X12
15
where, H is the Tonality, C is the saturation, ΔE is color difference, Y is the opacity and X 1 is the starch concentration.
The results for solubility of the bioplastics ranged from 16.42 to 23.26%. There was a continuous increase in solubility with increasing starch concentration in the film-forming solution, while the glycerol concentration had no effect on the solubility of the films (Eq. 16). This result can be explained by the higher concentrations of starch compared to the plasticizer used to produce the bioplastics. The glycerol molecule has only 3 free hydroxyls, while the starch molecule has several, thus, some hydroxyls in the starch are still available to interact with water making the bioplastic more soluble.
S = 12.99 + 0.856X1
16
where, S is solubility and X 1 is the starch concentration.
The highest water vapor permeability was found for bioplastics made up of 6% starch and 60% glycerol (0.001374 g m/day m2) and the lowest WVP was for bioplastics with 2% starch and 30% glycerol (0.000307 g m/day m2). The influence of the concentration of starch and glycerol was observed by means of multiple regression analysis. Thus, as the concentration of glycerol and starch increases, the increase in WVP occurs. Equation (17) presents the statistical model that best fit WVP.
WVP = - 0.00036 + 0.0001X1 + 0.000085X2
17
where, X 1 is the starch concentration and X 2 is the glycerol concentration.
There was influence of the starch and glycerol concentrations on the permeability of the films produced, with the results showing an increase in WVP with the increase in the starch and glycerol concentrations (Fig. ). Adsorption, diffusion and desorption are the main mechanisms involved in the transfer of water molecules through the starch films. These mechanisms are affected by the microstructure as determined by chemical composition, crystallinity and plasticizer (Garcia-Hernandez et al. 2017). Therefore, the observed increase in WVP could be attributed to the hydrophilicity of the bioplastics constituents. In addition, the plasticizer interacts with the starch chains, resulting in a material with greater mobility, which increases the permeability of water through the bioplastic. According to López and García (2012), glycerol interferes with polymeric chain association decreasing the rigidity of the network; thus a less ordered and compact film structure is developed, decreasing WVP as observed in this work.
Open in a separate windowThe tensile strength in the bioplastics was only affected by the plasticizer, showing a linear decreasing effect, with reduction in the tensile strength with increasing glycerol concentration (Eq. 18). The lowest tensile strength (1.65 MPa) was recorded for bioplastics with the highest glycerol concentration (60%), whereas the highest resistance (3.12 MPa) was recorded for bioplastics with 30% glycerol. When glycerol was incorporated into the polymer matrix, there was a reduction in the direct interactions resulting in a reduction in the proximity between the starch chains, thus movements of starch chains were facilitated, decreasing the bioplastics tensile strength. Tensile strength values were within the range of those recorded by López and García (2012) and Basiak et al. (2017) for ahipa and cassava and weath and cassava, respectively, starch films plasticized with a similar concentration of glycerol with those used in the present work.
Ts = 4.6082 - 0.0493X2
18
where, Ts is the tensile strength and X 2 is the glycerol concentration.
In the determination of the percentage of elongation of the plasticized bioplastics with glycerol, the influence of starch and glycerol was evaluated by regression analysis. By Eq. (19) the percentage of elongation of the biopolymers was determined.
Elogation%=29.5500+5.3200X1-0.5280X2-0.3181X12
19
where, X 1 is the starch concentration and X 2 is the glycerol concentration.
The percent elongation of the bioplastics was influenced by both the starch and glycerol. The percent elongation increased with increasing starch concentration, but this increase was observed up to 5% of starch, while there was a decrease in this property for the bioplastic containing the concentration of 6% starch. On the effect of glycerol, this study found that the lower the concentration, the greater the elongation of the bioplastics (Fig. ).
Open in a separate windowThe film stiffness (Young’s Modulus) was influenced by the concentrations of starch and glycerol and the interaction between them (Eq. 20 and Fig. ). The higher the starch concentration and the lower the plasticizer concentration, the higher was the stiffness of the material. Ramaraj (2007) attributed the increase of Young’s modulus to a filler effect promoted by the addition of starch in the blends, with the starch granules acting as reinforcement. The bioplastics produced with 20% glycerol showed an antiplasticizer effect, which was characterized by ruptures and the non-formation of a continuous matrix, and for this reason they were not described in this study. The films obtained in this study were stiffer than corn and potato starch films studied by López and García (2012) and Basiak et al. (2017), respectively.
Young's Modulus=106.87-0.0348X22+0.1564X1X2
20
where, X 1 is the starch concentration and X 2 is the glycerol concentration.
Open in a separate windowFrom the results of Tensile strength, percent elongation and Young’s Modulus can be verified that the jackfruit starch and glycerol provided a film with good mechanical properties. This fact is due to the formation of inter-molecular hydrogen bonding between OH groups of starch–glycerol. It is observed that the glycerol plasticizing effect in starch films, i.e., with increasing glycerol content, the films exhibited a decrease in tensile strength and Young’s Modulus. The same behavior were obtained by Zanela et al. (2015), Thakur et al. (2017) working with cassava starch–PVA–glycerol and pean starch–chitosan–glycerol film, respectively.
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