# **Bio-Based Materials Contribution to Advancing Circular Economy**

Edited by Maya Jacob John and Sabu Thomas

Printed Edition of the Special Issue Published in *Polymers*

www.mdpi.com/journal/polymers

## **Bio-Based Materials: Contribution to Advancing Circular Economy**

## **Bio-Based Materials: Contribution to Advancing Circular Economy**

Editors

**Maya Jacob John Sabu Thomas**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Maya Jacob John Council for Scientific and Industrial Research South Africa

Sabu Thomas Mahatma Gandhi University India

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Polymers* (ISSN 2073-4360) (available at: https://www.mdpi.com/journal/polymers/special issues/ Bio Based Mater).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-6047-2 (Hbk) ISBN 978-3-0365-6048-9 (PDF)**

© 2023 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND.

## **Contents**

## **About the Editors**

#### **Maya Jacob John**

Dr. Maya Jacob John is Principal Researcher and leader of the Bioplastics and Biocomposites Platform at the Council for Scientific and Industrial Research (CSIR) in South Africa. Dr. John's research tackles the crucial problems of waste plastic pollution and focuses on creating a bio-based feedstock to replace materials derived from petroleum reserves. Dr. Maya has published more than 90 peer-reviewed research articles, was granted two patents and presented at a number of international conferences which includes several plenary and key-note lectures. The impact of her research is evident in her overall citation record with a total of more than 10,000 citations and current h-index of 37. Dr. Maya Jacob John has been ranked among the World's Top 100,000 scientists in a recent list prepared by Stanford University and is globally ranked at position 973 within the Polymer field. She has also received several awards which include a CSIR award for research excellence (Young Researcher category) and a Best Patent award from CSIR in the year 2011. She was the 1st runner-up in the Physical and Engineering Sciences category at DST's Women in Science Awards (Young Woman Scientist) and her work on bio-based composite panels for transport sector received Special Award under Import Replacement category at the National Advanced Manufacturing Innovation Awards in 2018. In 2019, Dr. John was a finalist in the materials for inclusive economic and sustainable development category at the National Science and Technology Foundation (NSTF) awards.

#### **Sabu Thomas**

Prof. Sabu Thomas is currently the Vice-chancellor of Mahatma Gandhi University, Kottayam, Kerala, India, Director of Energy Materials, former Professor of Polymer Science and Engineering at the School of Chemical Sciences, and the former Director of the International and Inter-university Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, India. In 1987, he received his PhD in Polymer Engineering from the Indian Institute of Technology (IIT), Kharagpur, India. He is a fellow of the Royal Society of Chemistry, London and a fellow of the European Academy of Sciences. Prof. Thomas has been ranked No.2 in India by Stanford University, USA and ranked number 114 globally in the area of polymer science. He has published over 1200 peer-reviewed international journal articles and book chapters, holds 15 patents and has edited more than 150 books. Prof. Thomas has a h-index of 111, total citations of 58,000 and has supervised more than 115 PhD students and 30 post docs.

## *Editorial* **Special Issue—"Bio-Based Materials: Contribution to Advancing Circular Economy"**

**Maya Jacob John 1,2,\* and Sabu Thomas 3,4,\***

Bio-based materials have a significant role to play in the implementation of a functional circular economy. The circular economy stresses the reduced use of raw materials, the reuse of products, waste streams (where waste is converted to a valuable resource), and the recycling of products, the combined effect of which will ensure that materials are retained in the loop as opposed to a linear economy which is based on a manufacture, use, and disposal model. In a circular society, bio-based materials are reused, repaired, recycled, and remanufactured. A circular approach for plastics will address the issue of plastic waste pollution on land and in oceans and the adverse health effects that microplastics have on marine and human life. This can be achieved by methods to improve recycling and developing bio-based materials as an alternative to petroleum-based feedstock. The concepts of the bio-based economy and the circular economy are similar in that they reduce the demand for fossil carbon and enhance the use of waste and side streams. Hence, it is quite clear that bio-based materials (bioplastics and biocomposites) can make a critical contribution to the implementation of the circular economy.

In this Special Issue, leading researchers from academia and industry were invited to submit reviews or their latest research on topics aligned to the development of sustainable materials from renewable resources. Sustainable materials include waste-derived, recyclable, and biodegradable materials. Studies dealing with recycling, waste conversion to bio-based products, the development of bio-based composites, and surface treatments on cellulose fibres have been included in this issue. The manuscripts were subjected to a rigorous review process, after which there was a compilation of thirteen research articles, reflecting the latest trends in bio-based and compostable materials. This issue consists of three review articles and ten research articles. The review articles include an account on the plastic circular economy and applications of chitosan-based materials.

As guest editors of this Special Issue, we acknowledge all the authors and reviewers who have contributed to its publication. We would also like to thank the technical support team at MDPI for their assistance in preparing this Special Issue.

**Author Contributions:** Conceptualization, S.T. and M.J.J.; writing—original draft preparation, M.J.J.; writing—review and editing, S.T. All authors have read and agreed to the published version of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Citation:** John, M.J.; Thomas, S. Special Issue—"Bio-Based Materials: Contribution to Advancing Circular Economy". *Polymers* **2022**, *14*, 3887. https://doi.org/10.3390/ polym14183887

Received: 13 September 2022 Accepted: 14 September 2022 Published: 17 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

## *Article* **3D Printing Parameter Optimization Using Taguchi Approach to Examine Acrylonitrile Styrene Acrylate (ASA) Mechanical Properties**

**Abdul Zubar Hameed 1,\*, Sakthivel Aravind Raj 2,\*, Jayakrishna Kandasamy 2, Muhammad Atif Shahzad 1 and Majed Abubakr Baghdadi 1**

**Abstract:** Polymer composites with different reinforcements have many applications. By adjusting process settings and adding fibers and fillers, composite properties can be improved. Additive manufacturing is popular in the polymer industry because it can manufacture intricately designed parts with fewer defects and greater strength with less material consumption. Composites use thermoplastics and thermosetting polymers. Thermoset plastics cannot be reused or recycled; therefore, they are disposed in landfills, creating pollution and environmental harm. In this work, thermoplastic ASA (Acrylonitrile Styrene Acrylate) polymer filament is used for FDM 3D printing. The specimens are made by varying five process parameters that affected the materials' mechanical properties. The tensile, flexural and impact specimens are made using MINITAB software and ASTM requirements. The L18 orthogonal array experimental design, specimens and results were optimized. Infill density and layer height were most influential. Maximum tensile strength of 51.86 MPa, flexural strength of 82.56 MPa and impact strength of 0.180 J/mm2 were obtained by following the software-suggested input factors and compared with the predicted values. Final error percentage was obtained between the predicted and the experimental results and it was found to be under 3%, which is acceptable.

**Keywords:** ASA polymer; fused deposition modelling; infill density; L18 orthogonal array; 3D printing

#### **1. Introduction**

As early as the Paleolithic period, humans were able to produce things for their everyday needs; they had a material sense and used it to create tools for their own gain. However, the manufacturing process for the tools was based on subtractive techniques, in which the original components are turned into useful goods by removing surplus bulk material until the final object is achieved. The subtractive method of production has progressed substantially and new techniques continue to emerge. In the early 1980s, a paradigm known as additive manufacturing was established. It has proven to be an excellent technique for creating any geometry [1].

In the FDM process, different thermoplastic materials are utilized to print different complex shaped objects. Different materials used in the FDM process include Polylactic acid, PLA-based graphene, Acrylonitrile butadiene styrene, Polycarbonate, NinjaFlex 85 A, Polycarbonate–ABS, Polyurethane elastomer, thermoplastic Polyurethane and Carbon-Silicone rubber [2]. PLA is derived from renewable resources and is compostable. It will not produce toxic fumes during printing or when incinerated. It is basically derived from crops. ABS is usually used for processing foods and has improved resistance against impact, abrasion and chemicals. It has better machinability, it can be thermoformed and it is flexible enough to print complex shaped jobs. It has better electrical properties and

**Citation:** Hameed, A.Z.; Aravind Raj, S.; Kandasamy, J.; Shahzad, M.A.; Baghdadi, M.A. 3D Printing Parameter Optimization Using Taguchi Approach to Examine Acrylonitrile Styrene Acrylate (ASA) Mechanical Properties. *Polymers* **2022**, *14*, 3256. https://doi.org/10.3390/ polym14163256

Academic Editors: Maya Jacob John and Sabu Thomas

Received: 16 July 2022 Accepted: 4 August 2022 Published: 10 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

dimensional stability also. Polycarbonate has better thermal insulation when compared to other thermoplastic polymers and is light in weight. It has UV protection properties. NinjaFlex 85A is used in making sporting goods, in healthcare and in industrial manufacturing. It features good flexibility, excellent elongation properties, low friction and abrasion resistance. Thermoplastic Polyurethane has good load bearing capability, resistance to oil and ozone, and has good abrasion resistance also. Very few materials are used in making 3D-printed membranes, and for printing the membranes only a few additive manufacturing processes are used. Processes like electrospinning, sintering, stretching, track etching and phase inversion are used in printing membranes [3].

3D printing has introduced a new approach for layer-by-layer creation of items from the bottom up. By adding successive layers of material in response to computer-controlled instructions, additive manufacturing makes parts. People have grasped the significance and benefits of additive manufacturing as its popularity has increased relative to conventional techniques. Compared to conventional or subtractive manufacturing techniques, additive manufacturing has progressed and produced more promising results. It can be employed in small-scale production for customization and personalization, whereas conventional methods are limited to mass production.

To improve ASA's thermal insulation and weather resistance, chemicals have been added to ASA resin used as a construction material. The modified ASA has been subjected to an infrared radiation test, which revealed that the surface temperature of the specimen decreased by 6.4 ◦C. In addition, pigments with NIR reflectance enhanced the weather resistance of ASA. The setup for the radiation test to measure weather resistance with the help of a near infrared lamp is shown in Figure 1a, and the corresponding heat curves for pure and modified ASA are shown in Figure 1b [4].

**Figure 1.** (**a**). NIR lamp radiation test setup, (**b**). Heating curves of pure ASA and modified ASA [4] Copyright 2021 with permission from Elsevier.

The difference in interfacial tension between polymer pairs allows for the incorporation of two different elastomers, CPE and BR, into the core of ASA. Higher impact strengths were demonstrated by the mixes as a result of their testing [5]. After reinforcing ASA with carbon fiber and measuring its mechanical and thermal properties, the materials had superior tensile and flexural strength, and the thermal conductivity increased by 500 percent. Figure 2a depicts SEM images captured after a tensile test conducted at liquid nitrogen temperatures, whereas Figure 2b depicts the interaction between fiber and resin following flexural tests. The presence of a gap between the fiber and resin shows a lack of compatibility and interface strength, which results in fiber pullouts and degraded mechanical qualities [6].

**Figure 2.** (**a**). SEM images showing flexural fracture at lower temperatures, (**b**). Tensile tested specimen, (**c**). Details of interface between carbon fiber and ASA [4] Copyright 2020 with permission from Elsevier.

ASA is combined with barium titanate and evaluated as a roof-cooling and solarreflecting material. The introduction of barium titanate increased the NIR reflectance by two to three times, while the interior temperature dropped by 10 ◦C. Figure 3 depicts the increase in temperature of the specimen's inner surface, which is used to measure solar reflectance. ASA with barium titanate (1068 nm) exhibited superior solar reflectance compared to other combinations, including glass [7].

**Figure 3.** Inner temperature difference with respect to time [7] Copyright 2018 with permission from Elsevier.

The addition of antimony trioxide to ASA resin enhanced the cooling effect. Antimony trioxide exhibits solar-reflective qualities. When antimony trioxide is added to ASA, the water contact angle increases, indicating that the material can be used for roof cooling and composited as cool materials [8]. The use of 3D printing in the production of biological components is rising fast due to demand. PLA was discovered to be a biodegradable material suitable for use in the FDM production of bio-implants. The biodegradation test findings reveal good results throughout 30 to 60 days, and the weight loss was found to be satisfactory [9]. Stamp sand deposited as mine waste is combined with recycled ASA to create composites whose mechanical properties have been evaluated. It was noted that the tensile strength was reduced by half compared to recycled ASA in its pure form, and that stamp sand in conjunction with ABS produced positive results [10]. Metamaterials such as chiral, Re-entrant, and ASA are printed using FDM, and mechanical and ex situ tests revealed that structural alterations improved the materials' properties, although interlaminar fracture was seen during a compression test [11]. To determine the relationship

between the morphologies of ASA composites and their mechanical properties, RAFT emulsion polymerization is used to generate block copolymers from ASA. By increasing the spherical particle size, the inter-particle distance can be decreased and the tensile characteristics can be improved. Additionally, the impact resistance was enhanced in comparison to triblock copolymer materials [12]. The technique of melt blending is used to create composites composed of polycarbonate, ASA and acrylic resin. It was noticed that the addition of acrylic resin boosted the impact strength and abrasion resistance. Additionally, it was discovered that the addition of acrylic resin reduced the tensile, hardness and melt flow rates marginally. The crack path in the composite was tracked with SEM and LSCM images as shown in Figure 4a,b [13].

**Figure 4.** (**a**). Laser scanning confocal microscopy (Right). (**b**). SEM images of impact tested specimen showing the crack path [13] Reproduced with permission from MDPI.

Analyses of the effect of ASA addition to eucalyptus/PVC composites revealed that adding less than 15 percent ASA improves impact, tensile and flexural strengths. Adding 15 to 20 percent ASA to composites increased their thermal stability in the early stages but weakened them later due to ASA's thermal degradation [14]. Thus, in this study, the ASA polymer is 3D printed utilizing the fused deposition modelling technique by altering five process parameters at three distinct levels while keeping other parameters constant. In earlier 3D printing research work, a maximum of three process parameters were taken into account and varied. In this work, however, a total of five process parameters were taken into consideration and their influence on mechanical properties was studied. The curiosity to understand the combined influence of the input factors motivated this work and makes it novel when compared with previous work carried out in this field. By evaluating the mechanical characteristics of the manufactured specimens, the influence of the parameters is examined and the results are optimized based on the analysis.

#### **2. Materials and Manufacturing Process**

#### *2.1. Materials*

River Polymer Industries of Gujarat, India, provided ASA polymer (acrylonitrile-costyrene-co-acrylate). According to temperature tests, ASA's strong solar reflectance prevents heat conduction and offers a more effective cooling effect in sunshine than glass. ASA features a high service temperature, low thermal conductivity and resistance to the elements. ASA has a density of 1.06–1.1 g/cm3 at 20 ◦C and a softening point of 85 to 100 ◦C, making

it suitable for outdoor applications because it ignites at 400 ◦C. Low melting temperatures, which are 200–230 ◦C, 200–280 ◦C, and 200–250 ◦C, make it suitable for pipe extrusion, injection molding and thermoforming. Solvents including chloroform, dichlorobenzene, diethyl ether, ethyl benzoate, ethyl chloride, mesityl oxide, methyl chloride, methyl propyl ketone and xylene, dissolve ASA well. ASA is suited for outdoor sheet furniture, exterior cable enclosures and wide screen displays. ASA is a scratch-resistant polymer that can be utilized in automobile interiors and electrical devices. ASA has superior solar reflectivity and can be used in roof cooling materials [15]. The mechanical and physical properties of ASA are listed in Table 1.

**Table 1.** Properties of ASA [6]. Copyright 2020 with permission from Elsevier.

#### *2.2. Fabrication Method and Machinery Details*

Using additive manufacturing, Acrylonitrile Styrene Acrylate (ASA) specimens for experimental work were printed on a "Creality Ender—3" FDM-based 3D printing system. The printer has a printing size of 220 × 220 × 250 mm and is based on the Cartesian system, with all three axes moving independently utilizing stepper motors. The ASA filament used in the experiment was a 1.75 mm-diameter wire that was kept on its mounting spool. All three samples were modelled in the CAD software Autodesk Fusion 360 and produced in the stl file format [16]. Then, the file was sliced in Ultimaker Cura slicing software according to the parameters, its G-code (machine code for the printer) was generated, and this was inserted into the 3D printer machine. Figure 5 depicts the 3D printer with which the specimens were printed.

**Figure 5.** "Creality Ender—3" printer.

*2.3. Selected Process Parameters and Their Levels*

The mechanical properties of components are greatly affected by printing parameters. Printing temperature, infill density, layer height, raster angle and printing orientation

were chosen as experimental design factors to determine their effect on the specimen's mechanical properties. The remaining printing parameters were adjusted to the same value for each sample. The printing temperatures for the setup were determined to be 245 ◦C, 255 ◦C and 265 ◦C. The infill density ranged from 30%, 70% and 100%. The height of the print layer ranged between 0.08 mm, 0.12 mm and 0.16 mm. The raster angle values for the experiment were 15◦, 30◦ and 45◦, and two print orientations, X-90 and Y-90, were obtained [17–20]. An experiment was designed by systematically altering each of these values, and samples were subjected to mechanical testing to determine the optimal printing conditions for achieving maximum strength. All other printing parameters, including printing speed of 60 mm/s, number of perimeters at 4, print bed temperature of 90 ◦C, 40 percent print cooling and the retraction settings, were held constant for all samples. The selected process parameters and their respective levels are listed in Table 2.

Using MINITAB, the experimental design for five components and three levels was prepared according to Taguchi's experimental design. To avoid confusion and elicit precise results from a minimum number of trials, experimental designs were developed. By utilizing good experimental design, time, energy and money can be saved as fewer experiments will be required to attain the desired findings. The extra benefit of MINITAB software is that manual programming is not required, as the programs are already inbuilt into the software. A mixed-level design with two levels for the first factor (part orientation), and three levels for the remaining four factors, was selected in the software and an experimental design was created. The experimental design for the L18 orthogonal array is illustrated in Table 3. In accordance with the experimental design, the parameters were selected and the specimens were 3D-printed.

**Table 3.** L18 orthogonal array experimental design.

#### *2.4. Mechanical Properties Evaluation Methods*

For evaluating the mechanical properties of 3D-printed ASA, the components for the tensile test, flexural test and impact test were selected in accordance with ASTM standards. For tensile testing, an ASTM D638 sample of 165 mm in length, 19 mm in width and 3 mm in thickness was selected. Figure 6 depicts the dumbbell-shaped specimen's dimensions, whereas Figure 7 depicts the printed tensile test specimens. Figure 8 depicts an Instron 8801 UTM, the device utilized for tensile and flexural testing. The specimen was created in accordance with the experimental design by altering the process parameters. The printed samples were tidily packaged to prevent environmental contact and moisture contamination. The tensile test was conducted using an Instron 8801 model UTM machine with a 100 kN load capacity, two columns equipped with M30 load cells and actuators and a load capacity of 100 kN. The specimen was grasped in the grippers and testing commenced. During the test, the cross-head velocity and strain rate were both maintained at 5 mm/min. The tensile strength of the printed specimen can be found using Equation (1) [21].

Tensile strength (MPa) = Maximum load during breaking (Newton)/Cross-sectional area of the specimen (mm2) (1)

**Figure 6.** ASTM D638 specimen for tensile testing (all dimensions are in mm) [22].

**Figure 7.** Printed specimens for tensile test as per the experimental design.

For impact testing, an ASTM D6110 sample with dimensions of 110 mm in length, 10 mm in width and thickness, and a 45◦ notch angle at the specimen's midpoint was selected. The impact test was conducted using FIT-300-D model pendulum-type impact testing equipment with a capacity of 300 J and a minimum count of 0.1 J. Chennai's Herenba Instruments and Engineers supplied and calibrated the equipment. The test was conducted at 37 ◦C and normal atmospheric pressure. Initially, the striker was released without the location of the specimen and its error was evaluated. Figure 9 depicts the specimen dimensions according to ASTM standards, and Figure 10a,b depict the printed specimens prior to the impact test. Figure 11 depicts the machine configuration, and the specimen was

placed in the horizontal work holding fixture with the notch facing away from the striker. The striker was unleashed and caused to strike the specimen in its center. After breaking, the energy absorbed by the specimen was recorded and the specimen's impact strength was calculated using Equation (2).

**Figure 8.** Instron 8801 UTM for tensile and bending test.

Impact strength (Charpy test) (J/mm2) = Energy absorbed (Joules)/Cross-sectional area (mm2) (2)

**Figure 9.** ASTM D6110 specimen for impact testing.

For flexural testing, an ASTM D790 sample of 130 mm in length, 12.7 mm in width and 3 mm in thickness was chosen. The flexural test was conducted on the same Instron 8801 model UTM equipped with a unique attachment for three-point bending testing. Figure 12 depicts the dimensions of the specimen, whereas Figure 13a,b depict the printed specimens for the three-point bending test according to the experimental design. Using Equation (3), the specimen's flexural strength was determined.

Flexural strength (MPa) = 3 × Load (N) × Span of support (mm)/2 × Specimen width (mm) × (specimen height)2 (mm) (3)

**Figure 10.** (**a**) Printed specimens for impact test as per the experimental design. (**b**). Impact specimen with scale.

**Figure 11.** FIT-300-D model impact tester.

**Figure 13.** (**a**). Printed specimens for flexural test as per the experimental design. (**b**). Printed flexural specimen with scale.

#### **3. Results and Discussion**

Table 4 contains the experimental design that varies five process parameters and the results of the tests that were conducted using that design. When compared to others, specimen 7 demonstrated greater values for all the test findings and had the greatest tensile strength rating, which was 50.57 MPa. It demonstrated the second greatest output in the flexural test, which shows that this particular design parametric combination produces superior mechanical qualities.

**Table 4.** Experimental design with output responses.

**Table 4.** *Cont.*

#### *3.1. Tensile Strength*

The experimental data are provided as input to the MINITAB software for regression analysis in order to identify the most influential parameter which serves as the determining factor in producing output results. The tensile strength was determined to range between 39.97 MPa and 50.57 MPa. The majority of specimens subjected to tensile loads exhibited superior extension properties, as shown by the lengthening of the specimens prior to failure [24]. The majority of specimens exhibited plastic deformation prior to failure or cracking. The failure of the specimens happened approximately at the specimen's center, indicating that the specimens are defect free and that the load acts at the proper place. It also demonstrated that the tests yield precise findings. The experimental values were then inputted into the software for regression analysis in order to determine the most influential parameter affecting the results. The coefficient of regression is 93.82%, which is closer to the acceptable value. The most influential criteria for tensile strength were found to be layer height and infill density. The specimens subjected to tensile testing demonstrated an extended fracture failure mechanism rather than a brittle fracture. Figure 14 depicts the deformed portion following a tensile test, which has an extended section due to plastic deformation. Due to enhanced tensile characteristics, the specimen did not fail completely while undergoing deformation. Table 5 represents the elongation of the specimen at break and yield point values. The ANOVA findings are presented in Table 6, where the Fisher value of each selected parameter reflects the extent of its influence.

**Figure 14.** Tensile tested specimen.

**Table 5.** Elongation at break and yield point values for tensile test results.

**Table 6.** Tensile test response ANOVA.

Figure 15a–d depict the main effects, interaction, contour and contribution plots for tensile tested results. The larger is better concept was followed when plotting the S/N ratio's main effects. It was projected that 'X' part orientation, printing temperature of 265 C, layer height of 0.08 mm, 100 percent infill density and a raster angle of 15 (A1, B3, C1, D3, E1) would result in the highest tensile strength. The interaction plot reveals that there is no interaction between printing temperature and part orientation, other than the fact that all components are dependent on one another. The contour plot was drawn between the layer height and the infill density, as these were the most influential parameters among the others. In order to achieve higher tensile strengths, the contour plot indicates that the layer height and infill density must be 0.08 mm and 100 percent, respectively.

#### *3.2. Flexural Strength*

The specimen with the highest tensile strength also possessed superior flexural properties. Specimen number 16, having a 'Y' axis part orientation printed at 265 ◦C, with a layer height of 0.08 mm, a 100 percent infill density and a raster angle of 30 degrees, exhibited a maximum flexural strength of 82.87 MPa. Figure 16 depicts the specimen following the flexural test, which demonstrates that even after failure the cracks are not visible to the naked eye. Due to the formation of microscopic fractures that do not propagate further, the specimen did not fail completely or shatter into two pieces. It demonstrates the flexural stiffness of the specimen and the behavior of ASA prior to failure. The flexural strength ANOVA findings are presented in Table 7, and according to the Fisher value, infill density

was the most influential parameter, followed by layer height. In addition, the regression coefficient was determined to be greater than 90 percent, demonstrating the significance of the model.

**Figure 15.** *Cont*.

**Figure 15.** (**a**). Main effects plot for tensile test responses. (**b**). Interaction plot for tensile test responses. (**c**). Contour plot for maximum tensile strength. (**d**). Contribution plot of parameters for tensile strength.

**Figure 16.** Deformed specimen after flexural test.

**Table 7.** Flexural test response ANOVA.

A1, B4, C1, D3 and E1 were the anticipated optimal values for maximal flexural strength, based on the main effects figure. The same predictions were made regarding the tensile property. It illustrates the relationship between tensile and flexural properties, which are directly proportional. The interaction plot provides the same information as tensile responses. The orientation of the part and the printing temperature are unrelated. Figure 17d depicts the effect of each parameter on the flexural properties as a percentage. Figure 17a–c depict the main effects, interaction and contour plots for the flexural strength results.

#### *3.3. Impact Strength*

The specimens with the highest impact resistance to sudden loads were numbers 7 and 16. The maximum impact strength observed was 0.17 J/mm2, whereas the minimum was 0.09 J/mm2. Figure 18 depicts the front view of a specimen that was fractured due to impact stresses with the effect of layer height and infill density evident. The crack began at the location of the notch and the layers impeded its progression. There are irregularities in the fracture propagation as the specimen was not shattered in a straight line. The specimen remained intact during the impact test, demonstrating its resistance to impact loads. The layer height appears to be the most influential factor in impact test analysis, as shown in Table 8 of the ANOVA results. With 93.01 percent, the regression coefficient is adequate.

**Figure 17.** *Cont*.

**Figure 17.** (**a**). Main effects plot for bending test responses. (**b**). Interaction plot for bending test responses. (**c**). Contour plot for maximum flexural strength. (**d**). Contribution of each factor for maximum flexural strength.

**Figure 18.** Optical microscopic view of the deformed specimen after impact test, side view.

**Table 8.** Impact test response ANOVA.

Figure 19a–d represent the main effects plot, the interaction plot and the contour plots, respectively. According to the forecast of the main effects plot, A1, B3, C1, D3 and E1 are the optimal values that result in greater impact strengths. It is apparent from the interaction plot that all the components are interrelated. Contour plots were created for infill density and layer height, as well as for layer height and raster angle, since these are the most influential elements in terms of impact strength [25]. The contribution and effect of each factor can be understood from Figure 19e.

**Figure 19.** *Cont*.

**Figure 19.** *Cont*.

**Figure 19.** (**a**). Main effects plot for impact test responses. (**b**). Interaction plot for impact test responses. (**c**). Contour plot between layer height and infill density for maximum impact strength. (**d**). Contour plot between layer height and raster angle for maximum impact strength. (**e**). Contribution plot of various factors for maximum impact strength.

#### **4. Validation Test Details**

From the experimental data that were fed into the software, optimization outcomes and the factors that influence outputs were determined. The software predicted a result for each test and provided the optimal process parameters for the predicted result. Figure 20a–c depict the optimized parameters and the expected output for the optimal values. The comparison of anticipated and experimental values, together with the percentage of error, is displayed in Table 9. Specimens were prepared for each test using the optimum process parameters. For each test, three specimens were manufactured with identical process settings and their mean values were used to determine the final output. Validation was conducted by comparing the outcomes of each test, and the percentage of error was determined by computing the difference between the expected and experimental outcomes [26]. It was noticed that the error rate was less than 3 percent, which is within acceptable parameters.

**Figure 20.** *Cont*.

**Figure 20.** (**a**–**c**). Predicted results and optimum process parameters.

**Table 9.** Predicted and experimental results with error percentage.

#### **5. Conclusions**

#### **6. Future Scopes of Inquiry**

Already, elements and things that have been created using a 3D printer are being used in every industry. The advantages of 3D printing include the ease with which difficult profiles can be manufactured, less material waste, shortened production times and lower overall energy requirements. In the not-too-distant future, virtually all items and parts will be produced using 3D printing. A pie chart has been used in Figure 21 to show the many different applications for 3D printing technology. The quality of the items produced by additive manufacturing is also being improved through the utilization of a great number of recently developed methods and procedures. To further extend the scope of this work, the mechanical properties of the printed specimens could be improved by applying ceramic or metal coatings to them. The ASA polymer can be mixed with several fillers, each of which has the potential to contribute to an improvement in the polymer's fundamental properties. Wear, water absorption, abrasion resistance, conductivity and flammability are some additional tests that can be carried out in addition to the ones already mentioned. It is possible to reinforce ASA with high performance fibers in order to determine the extent of the improvement in all qualities necessary for specific applications.

**Figure 21.** Contribution of 3D printing in various applications [27] Copyright 2018 with permission from Elsevier.

**Author Contributions:** Conceptualization, A.Z.H., M.A.S. and M.A.B.; Funding acquisition, A.Z.H., M.A.S. and M.A.B.; Methodology, S.A.R. and J.K.; Validation, S.A.R. and J.K.; Writing—original draft, S.A.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to the Deanship for Research and Innovation, Ministry of Education in Saudi Arabia, for funding this research work through the project number IFPRC-022-135-2020 and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**

#### **References**

## *Review* **Towards a Circular Economy of Plastics: An Evaluation of the Systematic Transition to a New Generation of Bioplastics**

**Elda M. Melchor-Martínez 1, Rodrigo Macías-Garbett 1, Lynette Alvarado-Ramírez 1, Rafael G. Araújo 1, Juan Eduardo Sosa-Hernández 1, Diana Ramírez-Gamboa 1, Lizeth Parra-Arroyo 1, Abraham Garza Alvarez 2, Rosina Paola Benavides Monteverde 2, Karen Aleida Salazar Cazares 2, Adriana Reyes-Mayer 3, Mauricio Yáñez Lino 4, Hafiz M. N. Iqbal 1,\* and Roberto Parra-Saldívar 1,\***

**Abstract:** Plastics have become an essential part of the modern world thanks to their appealing physical and chemical properties as well as their low production cost. The most common type of polymers used for plastic account for 90% of the total production and are made from petroleum-based nonrenewable resources. Concerns over the sustainability of the current production model and the environmental implications of traditional plastics have fueled the demand for greener formulations and alternatives. In the last decade, new plastics manufactured from renewable sources and biological processes have emerged from research and have been established as a commercially viable solution with less adverse effects. Nevertheless, economic and legislative challenges for biobased plastics hinder their widespread implementation. This review summarizes the history of plastics over the last century, including the most relevant bioplastics and production methods, the environmental impact and mitigation of the adverse effects of conventional and emerging plastics, and the regulatory landscape that renewable and recyclable bioplastics face to reach a sustainable future.

**Keywords:** bioplastics; biobased plastics; plastic degradation; bioplastic sources; plastic business case; bioplastic legislation

#### **1. Introduction**

Since the creation of the first synthetic polymers in the 20th century, plastics have become an essential material in human activities due to their valuable characteristics, such as their low cost of production and broad potential applications. It is estimated that 370 billion tons of plastic materials are produced each year, a trend that is not expected to diminish in upcoming years [1]. Plastics are highly praised due to the ease of their production and diversity of applications thanks to their properties of durability, sturdiness, and lightweightedness, which are unmatched by other material categories and the current proposed alternatives [2].

Although there are an estimated 60,000 different plastic formulations, only six polymers account for 90% of the total plastic production: polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), poly(ethylene terephthalate) (PET), polyvinyl chloride (PVC), polystyrene (PS), and polyurethane (PU) [3]. The most

Macías-Garbett, R.; Alvarado-Ramírez, L.; Araújo, R.G.; Sosa-Hernández, J.E.; Ramírez-Gamboa, D.; Parra-Arroyo, L.; Alvarez, A.G.; Monteverde, R.P.B.; Cazares, K.A.S.; et al. Towards a Circular Economy of Plastics: An Evaluation of the Systematic Transition to a New Generation of Bioplastics. *Polymers* **2022**, *14*, 1203. https://doi.org/10.3390/ polym14061203

**Citation:** Melchor-Martínez, E.M.;

Academic Editor: Maya Jacob John

Received: 5 February 2022 Accepted: 10 March 2022 Published: 17 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

common synthetic polymers are manufactured from petroleum-derived compounds, a nonrenewable resource that has raised concerns over its long-term sustainability, as its fabrication model uses declining resources at increasing prices [4]. Moreover, these resources are not being used efficiently. Single-use plastics consume 1.6 billion liters of oil and are disposed of immediately after use, with a limited possibility of recycling [5].

The lack of management strategies for plastics once they reach the end of their lives, coupled with the inherent durability of polymeric materials, has given rise to the improper disposal of plastic products and the accumulation of plastic-related particles. This issue has been exacerbated by single-use plastics and an increase in the production of personal protection equipment brought on by the COVID-19 pandemic [6]. Plastic is the predominant type of human waste found in freshwater reservoirs, accounting for 57% of the total debris by weight [7]. Discarded plastics subjected to natural degradation turn into secondary microplastics, an emerging contaminant category of concern that often appears in modern research due to its potential effects on the environment and wildlife, as well as its status as a vector for adjunct pollutive particles such as heavy metals and hormone disruptors [8].

The critical impact of plastics on the environment as well as on human health, coupled with the need for sustainable methods to produce synthetic plastic analogues to alleviate the global demand, has been considered by policymakers and scientists in recent years. In the UN Global Sustainable Development goals, Goal #14 specifically mentions the need to mitigate the impact of plastic (Indicator 14.1.1 (b)—extraordinary efforts are required to reduce microplastic discharge into marine and freshwater ecosystems [9]), and another 12 goals relate to the alleviation of the problems created by the plastic industry in the human health and environmental spheres [10]. Nevertheless, the plastic alternatives that are currently presented lack widespread adoption. This is mainly due to their perceived inferiority, the lack of large-scale production processes to sustain their demand, and the higher costs associated with their production [11]. In recent years, bioplastics have been proposed as a promising alternative to conventional plastics because of their similar properties. The fabrication of this branch of polymeric materials has certain advantages over petroleum-based resins, such as their renewable nature and lower costs of industrial-grade production [12]. Bioplastics have recently been adopted for a few applications, such as single-use plastics and 3D-printing materials; however, their sustainability and ecocompatibility have remained a topic of discussion. Their widespread usage has been hindered by technical limitations in the production processes, as they offer lower yields than their petroleum-based parallels [13].

This article summarizes the historical relevance of plastic materials, their adoption and evolution, their current shortcomings, and the emerging trends in biobased plastic manufacturing and implementation. The environmental effects of conventional and emerging plastics are approached, as well as their mitigation strategies and the main applications of recent alternative materials. The challenges and opportunities of these biological plastics are discussed, as well as the relevant market in the context of the modern demands for plastic materials. The relevant legislation that concerns plastic production, certification, and the regulation of its disposal is also briefly discussed.

#### **2. Historical Development of Plastic Materials: A Brief Timeline**

The field of plastics is currently extremely prevalent in research and innovation, particularly with relation to new polymeric products, with a focus on their application and performance. Nevertheless, some concerns remain regarding their production, commercialization, and final disposal. The synergy between biotechnology, bioeconomy, and the chemical industry provides possible solutions to these problems [14].

Records of the use of plastics date from as early as a few centuries ago. Ancient civilizations around the world used materials such as resins, insoluble oils, and amber similarly to how we use plastics today. The first reference to rubber concerned cultures native to Central America who congealed latex to create waterproof shoes. In 1839, Charles Goodyear, an American inventor, discovered the elasticity and resistance of rubber heated with sulfur (Figure 1). This process was termed vulcanization and patented in 1844. Subsequently, ebonite was created and became relevant because it was thermosetting and was prepared from a natural material, rubber, though with larger quantities of sulfur. In the 1850s, the conditions for controlling the nitration of cellulose were optimized in Europe. A solid residue was produced from solvent evaporation, which demonstrated elastic and waterproofing properties. In 1862, Parkesine was prepared by the dissolution of cellulose nitrate in minimal solvent [15]. In 1863, two materials that had similar characteristics to Parkesine were developed: xylonite and ivoride. In 1869, billiard balls were being made out of cloth, ivory dust, and shellac in the US. In 1872, the term celluloid was first used to describe a material obtained from cellulose nitrate and camphor. The first protein-based member of the natural polymer family was developed in Germany in 1897 out of casein that was reacted with formaldehyde. Casein was separated from milk by coagulation. The formalized casein was used in buttons, dress ornaments, necklaces, manicure sets, pens, and other decorations. In 1899, ebonite was patented in the UK. It was made by reacting phenol and aldehyde resins and was used as an electrical insulation. In 1927, a nonflammable replacement for celluloid was created, called cellulose acetate, which was extensively used for artificial fibers [16].

**Figure 1.** Timeline of plastics from 19th to 21st century. Designed online at Freepik.com [Last accessed 23 February 2022].

Between 1930 and 1940, the current major industrial thermoplastics derived from ethylene were created: polystyrene (PS); poly(vinyl chloride) (PVC); and polyolefins, such as polyethylene (PE), polypropylene (PP), and polymethylmethacrylate (PMMA). During World War II (WWII), PMMA, a rigid, transparent plastic, was used for aircraft glazing and, to a lesser extent, in denture manufacture. The first polyurethane was synthesized by Otto Bayer in Leverkusen, Germany, in 1937 [17]. The diversity of raw materials that could act as sources for polyurethane production, as well as its wide range of uses, enabled the wide application of plastics in settings such as buildings, automobiles, coatings, and sealants [18].

More materials were designed before World War II. One was nylon, a sticky, bendable material, first formulated in 1933. Afterwards, during WWII, polyamide 66 and Teflon were discovered in 1941. During the mid-1950s, high-density polyethylene (PE) was produced, followed by polypropylene (PP). In 1956, polycarbonates were developed in the United States and Germany simultaneously. A variety of copolymers and blends were produced during the period 1960–2000. In the 1960s, there was a growth in the variety of synthetic fibers. Two important fibers to note were Nomex®® and Kevlar®®, the first meta-aramid and para-aramid fibers created by DuPont™. Nomex®® had a higher melting temperature [19], while Kevlar®®. had a structure that allowed it to create composites [20], replacing steel fibers in racing tires and later being used for consumer products and human armor [21].

Polybutylene terephthalate was introduced in 1969, and later, polycyclohexylenedimet hylene terephthalate, which was a plastic polyester with a higher melting temperature. High-performance thermoplastics were introduced during the 1970s and 1980s and could withstand temperatures of above 200 ◦C. In 1977, polyetherether ketone (PEEK) was invented, followed by polyether sulfone (PES) in 1983. In 1990, polyhydroxybutyrate

was commercially produced under name "Biopol". In the late 1990s, ethylene, propylene, and styrene were first introduced [22]. In the 2000s, the field of plastics was focused on materials produced from vegetable sources. Considerable research was carried out in bioplastics such as polyhydroxyalkanoate and poly-lactic acid (PLA) extracted from sugarcane, corn, and rice [23]. They were considered alternatives to petroleum-based plastics and could be biodegraded. Biobased polymers such as bio-polyethylene (bio-PE) and bio-poly(ethyleneterephthalate) (bio-PET) have been produced for its functionality and considerable capacity in packaging application. In 2019, biobased polypropylene was produced at a commercial scale, and its production capacity is expected to quadruple by 2025 [12,24].

#### **3. Environmental Impact and Health Effects of Synthetic Plastics and Bioplastics**

#### *3.1. Plastic Degradation and Insertion into the Environment: An Overview*

The affordability and desirability of plastic have enabled its ubiquity in all aspects of human development. Plastic production reached 368 million metric tons in 2019 and is projected to reach a total of 1.1 billion metric tons by 2050 [1,25]. Despite recent efforts, local and global policies for plastic disposal and handling after products have reached the end of their lifetimes have lagged behind production and consumption patterns, promoting the unregulated discharge of plastic materials into the ecosystem [26]. The stability conferred by the polymeric nature of plastics hinders their degradation in the environment by natural means. This results in the accumulation of plastics in the ecosystem as well as the macroscopic fragmentation of plastic particles through mechanical erosion, ultraviolet weathering, and biological assimilation, meaning that extended periods are needed for their complete breakdown beyond just their chemical degradation [27].

Once a plastic product is disposed of, its final fate can vary according to its geographical location; the available waste management infrastructure; the economy; and the intrinsic properties of the discarded product, such as its morphology and composition. Plastic waste can either be primed for reuse or recycling, managed through landfilling and incineration, or directly disposed of into the environment [1]. Unlawful means of plastic disposal such as littering and unregulated landfills also represent a significant source of plastic entry into ecosystem elements such as rivers and soil [28]. The plastics' morphology and chemical nature also determine the polluting potential of discarded products. The diversity and complexity of the interactions between plastic sources, compositions, morphologies, entry pathways, and degradative mechanisms hamper the development of a holistic understanding of the global plastic pollution issue, preventing practical and integral actions to mitigate its environmental impact (Figure 2) [29].

**Figure 2.** Insertion pathways of plastic-related pollutants into the environment. Designed online at flaticon.com [Last accessed: 23 February 2022].

As mentioned previously, the chemical nature of plastic hinders its quick breakdown in the environment. It tends to accumulate in soil, freshwater reservoirs, and oceans. Plastic permanence is estimated to last hundreds or even thousands of years, depending on factors that facilitate passive ageing and degradation. The onset of polymer decay is triggered by ultraviolet degradation and thermo-oxidative reactions promoted by the sun and environmental components such as beaches and pavements [30]. Plastics may also

degrade mechanically through erosion caused by continuous contact with rocks, wind, and water, which shear and tear plastic particles [31]. Finally, plastics may also degrade through biological means, either by the chewing and digestion of macroscopic animals or by the microscopic biodegradation mediated by bacteria, fungi, and other biologic actors (Figure 3) [32].

**Figure 3.** Size, features, and related effects of plastic-derived pollutants in the trophic chain. Designed online at flaticon.com [Last accessed: 23 February 2022].

The natural routes of watersheds transfer plastic waste from soil-based landfills into water bodies. Although pollution models have traditionally considered oceans as the final destination of plastic waste, freshwater networks have recently been recognized as both an active transport vector and a retention platform for polymeric waste [33]. The size and composition distribution of plastic pollutants in rivers is heterogeneous and characterized by retention in riverbeds, sediments, and vegetation reported throughout the world in landmarks such as the Great Lakes [34], the Danube River [35], the Ganges River [36], and the Thames River [37], to cite some examples. Polymer particles may then be ingested or absorbed by organisms, inserting plastics into the ecosystem's trophic chain [38]. The presence of plastic particles in freshwater also poses a threat to human health, as it is the primary source of drinking water worldwide, and the presence of microplastics has been reported in local water-supply networks [39,40]. Oceans represent the major worldwide sink for discarded plastic through natural transport pathways, and this is exacerbated by the indiscriminate product disposal caused by human activities such as fishing, commerce, industry, and tourism. Oceans are thus cited as the ecosystem most affected by human plastic pollution, which impacts the marine biosphere in all degrees and interactions [41]. Marine plastic debris floats and is diffused by global currents, acting as a carrier and a passive modifier of local ecosystems, introducing foreign particles and organisms, and potentially disrupting local environment interactions. Marine plastic is also deposited on beaches and shores. The persistence of marine plastic also favors material inflow as opposed to outflow mechanisms such as degradation, leading to product accumulation, with the Great Pacific Patch being the most well-known marine plastic reservoir in the

world [42]. The uptake of plastic particles by living organisms has also been studied and reported at macroscopic as well as microscopic trophic levels; nonetheless, the fate of this uptaken plastic, whether retained in tissues or excreted, is poorly understood [43].

Plastic fragments in soil are accumulated and distributed according to their particle size and the local environment dynamics, becoming a feature that alters soil properties and health. Microplastics can hinder soil density and bulk volume, affecting water retention and promoting the diffusion of other pollutants by creating surfaces with high adsorption coefficients [44]. The presence of plastic particles in terrestrial ecosystems negatively impacts biological developments at macroscopic and microscopic levels. Plastic debris may be ingested by soil-based organisms such as earthworms that act as microplastic concentrators, accumulating in their tissues [45]. Microplastics can thus be found in the terrestrial food supply chain through the ingestion of such organisms by poultry, later leading to negative implications for human health and posing a severe risk of microplastic exposure and ingestion [46]. An investigation conducted by Sun et al. [47] demostrated that polymer particles can also impact soil bacterial consortia, changing the composition of bacterial communities directly related to microplastic concentration and morphology.

The pathways through which plastic and related contaminants enter the atmosphere have been proposed, yet there still remains a knowledge gap. The incineration of plastic products is a common waste-management strategy for energy recovery, along with recycling and landfilling, as these are proposed as low-cost solutions that require minimal space [48]. Nevertheless, plastic incineration leads to the emission of hazardous contaminants and poses a risk to human and animal health while disrupting the environment through the release of carcinogenic and mutagenic compounds [49]. Plastic debris may also enter the atmosphere by mechanisms of natural erosion of predominantly synthetic textiles in the form of microfibres [50]. The fate of airborne plastic particles may vary according to the local ecosystem conditions, and they can be deposited into water ecosystems and soil, where they can be become airborne again in a dynamic cycle [51]. Plastic particles can also be inhaled and deposited in animal respiratory tracts, providing another pathway for trophic accumulation and transport [52].

#### *3.2. Macroplastics*

Macroplastics constitute a significant component of human litter and are commonly defined as plastic pieces over 25 mm in size [53]. This class of plastic debris impacts landscapes and ecosystem biology and serves as a source of secondary microplastics [54]. It is estimated that macroplastics kill 1 million marine animals through ingestion and entanglement [55], and the recent onset of the COVID-19 pandemic has exacerbated the disposal of single-use plastic products in the form of personal protection equipment, packaging, disposable cutlery, and containers [56,57].

Macroplastics have a negative impact on the environment due to their several hazardous effects. Depending on their nature, discarded plastic products have the potential to entrap organisms, hindering their mobility and visual capacity [58]. Macroplastics also act as vectors for the conveyance of undesirable ecosystem elements and pollutants through agglomeration and passive transport through water and air currents, exacerbated through climatic events such as rain and hurricanes [59], and may introduce nonindigenous species to foreign biospheres, such as mollusk stocks and algae dispersal through a mechanism known as rafting [60].

Animals may actively ingest macroscopic products which can be regurgitated, excreted, or retained for extensive periods [61]. In the latter case, plastic pieces large enough may cause gut blockage, starvation, or reduced nutrient absorption, leading to animal mortality [62]. Over time, plastic ingestion may act as an evolutionary trap, affecting the viability of animal populations and overall species through the persistent risk of prey resemblance and ever-increasing animal–plastic encounters [63].

Sizeable plastic debris is also likely to accumulate at drainage basin constrictions, causing blockages and leading to increased risks of flooding and raised water levels [64]. In

urban settings, the blockage of drainage and sewer systems causes undesirable effects such as bad smells in streets and propitiates the spread of diseases such as cholera and typhoid through contaminated drinking water [65]. In natural ecosystems, water obstructions promoted by plastic accumulation may promote changes in the hydrodynamics of rivers and lakes, altering the immediate landscape by the accidental creation of ponds during rainfall, leading to biofouling [66]. Floods caused by plastic blockages also provide a breeding opportunity for vector arthropods, increasing the risk of infectious viral and parasitic diseases in prolonged inundated zones [67].

Macroplastics can have adverse effects on soil, as they disrupt its physical and chemical properties. Plastic particles on land can alter the floor salinization and the root insertion of neighboring plants, with potential effects on the emergence of crops of agricultural interest [68]. Macroscopic plastic debris has also been evidenced to directly block photosynthesis and entangle plant seedlings, having a considerable impact in plant survivability in ecosystems deeply affected by littering [69]. Soil can also preserve plastic through earth-borne biota, such as earthworms, which are reported to burrow plastic particles that hinder solar-dependent degradation through UV radiation and lead to slow plastic decomposition [70].

#### *3.3. Microplastics*

The term microplastic has been commonly used in literature to describe a broad assortment of plastic-derived particles classified according to their size. Although the term is widespread and standard, the complexity and variety of microplastic compositions and morphologies are often overlooked in categorizing these contaminants [71,72]. This simplification has led to the inadequacy of studies and protocols that assess the environmental effect of microplastics, the irreproducibility of strategies for microplastic detection, and the ineffectiveness of pollutant-mitigation governance strategies [73].

Microplastic particles have become ubiquitous in all environments, and their presence has been reported in air, water, and soil ecosystems, as well as in food [74]. The primary sources of entry into the environment are either the release of synthetically designed plastic microparticles included in functional products such as cosmetics, drugs, and pellets (primary microplastics) or by the natural degradation of larger plastic debris through the previously discussed processes (secondary microplastics) [75]. The composition of the plastic in addition to its physical properties, environmental effects, and exposure time define a microplastic particle [30,76]. Its interaction with environmental elements, accumulation, and association with other plastic and pollutive particles, given by its physical properties, define its effects on the ecosystem [77]. Microplastic particles have a variety of shapes, including spheres, fibers, fragments, and films, each related to their source and composition with a varying pollutive potential through their macroscopic and microscopic biological effects [78].

The emission of plastic residues into aquatic environments is estimated to be between 4.8 and 12.7 million metric tons annually [79]. A fraction of such debris is degraded to create microparticles, along with the direct discarding of primary microplastics through water drainages. Once in oceans and freshwater reservoirs, microplastic can be ingested by fish and plankton and be inserted into food chains through trophic exchange into larger predators and humans, ultimately affecting these species through the consumption of contaminated prey [52]. Microplastics can float and drift in the water surface of oceans and rivers, interfering with surface microbiota and inhibiting the photosynthetic efficiency of algal plants, diminishing available nutrients in the marine ecosystem [80]. Degraded microplastic particles can also be deposited in the marine environment. Once there, they act as pollutant liberators, slowly releasing adjunct chemical pollutants such as endocrinal disruptors (BPA), while their further degradation is limited due to the inhibition of ultraviolet degradation that converts river and ocean beds to central microplastic repositories [81,82].

 In the case of heavy metals, PVC and PS microparticles have been reported to have a solid interaction with copper and zinc, common elements found in chemical additives such as paint [84,85]. Bioactive molecules such as pesticides and antibiotics have also been reported to have sorption activity through microplastic molecules, making them a bigger hazard in estuaries compared to oceans and freshwater due to a higher concentration of contaminant particles [86].

Human life is threatened by microplastics through several means. The ingestion of plastic particles through contaminated food is considered the main route of human exposure and can have molecular effects in cancer, obesity, and other oxidative-stress-related conditions. Additives found in microplastics, such as bisphenol A, are also endocrine disruptors that hinder proper development and have adverse reproductive effects [87]. Microplastic particles can also be inhaled, accumulating in lung tissue and causing cytotoxic effects in pulmonary cells, resulting in bronchial inflammation, fibrosis, allergic reactions, and interalveolar lesions [88]. Human exposure to airborne microplastics is estimated at 272 particles per day and is influenced by the material's nature, the performed activity, the quality of ventilation, and the particular season [89]. It is estimated that the primary microplastic morphology in the atmosphere is microfibers composed of nylon, polyester, and acrylic [90].

#### *3.4. Other Plastic-Derived Pollutants*

Besides the polymeric backbone, other chemicals are frequently added to synthetic plastics to heighten the qualities of the final product, such as the color, mechanical features, and stability. The classification of additives is as diverse as that of plastic polymers and includes plasticizers, antioxidants, dyes, lubricants, and fillers [87]. The release of chemical pollutants in conjunction with plastic debris particles may occur at all steps of the plastic lifecycle. The release of such compounds is mediated by the composition of the plastic and additive blend, the dynamic of the plastic particle with the additive, and the interaction of the compound with the environmental features (e.g., leachate solubility) [91].

Plasticizers are frequently employed to improve polymeric mechanical features, such as the flexibility and processability of polymer resins [84]. Over 90% of produced plasticizers are used to enhance PVC properties, expected to reach 59 million tons in 2020 [92,93]. Phthalic acid esters are the most used plasticizers globally, di(2-ethylhexyl) phthalate (DEHP) being the most pervasive plasticizer added to plastic [94]. Phthalates are not covalently associated with the polymeric matrix, and the mechanical wearing of the plastic product allows its migration to food and the environment [93]. Phthalate esters are recognized as a predominant category with contaminant effects, and their occurrence in all aspects of the environment has been comprehensively reported. This category of plastic additives has been recognized as potentially hazardous to the environment and human health through endocrine disruption [95]. DEHP also has a low toxicity threshold (LC50 of 0.50 ppm), causing embryonic mortality and necrosis in potentially exposed animals such as zebrafish [96,97].

The deleterious effect of phthalates on human health is documented and includes the alteration of estrogenic levels, infertility, asthma, diabetes, hyperglycemia, and endometriosis [98]. Phthalate exposure has also been associated with an increased BMI and the diagnosis of obesity in children [99]. The effects of DEHP exposure may also increase multigenerational hepatic fibrosis risk through the alteration of regular DNA methylation patterns [100]. Microplastics are considered a source of phthalate emission in the environment, and the high hydrophobicity and surface area of plastic particles enable the adsorption and transport of phthalates into water, soil, and sediment deposition [101]. The most frequent human entry pathway for such a family of compounds is ingesting contaminated foods that have come into contact with phthalate-supplemented packaging [102], followed by microplastic consumption through the drinking of contaminated water sources [103].

Different kinds of technologies can also promote the oxidative degradation of synthetic plastic based on fossil resources. This approach is frequently proposed as a pathway to minimize the environmental impact through the enhanced breakup of plastic resins without compromising their functionality [104]. Pro-oxidative additives are based on the salts of transition metal ions such as Co2+, Fe2+, or Ni2+, which stimulate the degradation of polymeric chains in the presence of oxygen and heat or radiation [105]. The most widely used commercial oxodegradable additives are d2w and TDPAs (totally degradable plastic additives); both are based on a combination of manganese, iron, cobalt, and nickel salts [106]. Oxodegradable plastics have enjoyed widespread adoption recently due to the technical feasibility of their implementation in already established manufacturing processes [107].

Concern has been raised over using transition metal compounds as additives designed to degrade environmental elements. Oxoplastics may then contribute and accelerate the production and accumulation of microplastics worldwide, and legislative regulations have recently been imposed due to the ambiguous nature of the conditions needed to degrade the modified resins completely [108]. The incomplete fragmentation of oxodegradable plastics also challenges their proenvironmental viability, as studies conflict with the true degradative capabilities of discarded oxoplastics. The usage of pro-oxidative additives does not ensure the complete degradation of plastics and may require special conditions nonreplicable in the environment [109]. However, transition metal salts have been regarded as safe due to their design as fatty acid salts of potential nutrient microelements [110].

Research has argued that the degradation pathway of oxoplastics may be further assisted by biological mechanisms. The term oxo-biodegradable plastics refers to the twostage degradation of additive-enhanced polymers. The first stage of degradation occurs by physical phenomena such as ultraviolet light and erosion, and the resulting plastic particles are then degraded by microorganisms through enzymatic pathways [111]. The addition of an oxodegradable additive has been found to enhance complete biodegradation with no toxicological effects on the obtained compost [107]. The resulting oligomers from photooxidized plastics have also been used as a carbon source for bacterial β-oxidation. Such a phenonmenon was observed when Rhodoccocus rhodochrous was observed to assimilate polyethylene film oligomers, leading to their biodegradation [112]. Both degradation mechanisms are considered biologically compatible, and the degradation of oxoenabled waste can be accelerated by metabolic action through compatible enzymes [113]. The findings of such reports show that the use of oxidant additives to promote plastic degradation may be justified from a combined physical and biological mechanism standpoint.

The Parliament of the European Union has carried out a ban on oxodegradable plastics under the "lack of evidence that oxo-degradable plastics are fully degraded in a reasonable time, are not appropriate for recycling or composting, and there is a risk that small plastic pieces will not completely biodegrade" [104]. Bioplastics are a better alternative in terms of complete product degradation versus oxo-bioplastics [114].

#### **4. New Generations of Plastics**

Bioplastics are polymers that can be biodegradable or not. Additionally, they can be synthesized from renewable biological sources such as bacterial, plant, and algal sources. However, their biodegradability depends on the chemical structure, but not on the sources used [115,116]. Nowadays, bioplastics represent only around one percent of the total plastic produced annually. However, it is expected that the demand for bioplastics will increase [117]. Figure 4 shows the current stages of bioplastic production, from the research and development stages to the commercial stage.

**Figure 4.** Stages of bioplastic production. PLA—polylactic acid; PBAT—poly(1,4-butylene adipate-co-1,4-butylene terephthalate); Bio-PET—bio-poly(ethylene terephthalate); PHAs—polyhydroxyalkanoates); Bio-PE—bio-polyethylene; Bio-PBS—bio-polybutylene succinate; Bio-PP—bio-polypropylene.

Bioplastics are generally reported in three categories (Figure 5): (i) fossil-resourcederived but biodegradable, (ii) partially biobased or biobased and non-biodegradable, and (iii) biobased and biodegradable [118,119].

**Figure 5.** Scheme of bioplastic classification. PLA—polylactic acid; PHAs—polyhydroxyalkanoates; Bio-PBS—bio-polybutylene succinate; Bio-PE—bio-polyethylene; Bio-PP—bio-polypropylene; Bio-PA—bio-polyamide; Bio-PET—bio-poly(ethylene terephthalate); PET—poly(ethylene terephthalate); HDPE—high-density polyethylene; PVC—polyvinyl chloride; LDPE—low-density polyethylene; PP—polypropylene; PS—polystyrene; PBAT—poly(1,4-butylene adipate-co-1,4-butylene terephthalate); PCL—polycaprolactone; PBS—polybutylene succinate.

#### *4.1. Biodegradable Plastics from Fossil Resources*

Generally, synthetic polymers are obtained from crude oil or natural gas. Most of these polymers are not biodegradable. Nevertheless, degradability can be achieved by integrating unstable bonds (e.g., amide, ester, or ether) [120]. Examples of biodegradable fossil-based plastics are poly(1,4-butylene adipate-co-1,4-butylene terephthalate) (PBAT) and polycaprolactone (PCL). PBAT is a polyester that is synthesized by polycondensation from the combination of dicarboxylic acids and diols. PBAT, due to its aliphatic unit in its molecule chain, has good biodegradability and excellent mechanical properties [121]. PCL is a hydrophobic and biodegradable polymer with sufficient mechanical strength and flexibility [122]. It has excellent biocompatible properties that enable its use for medical applications. To enhance their properties, PCL and PBAT can be used in blends with biobased and biodegradable materials such as polylactic acid (PLA) and polyhydroxyalkanoates (PHAs), and polybutylene succinate (PBS) [120].

Oxo-biodegradable plastics are composed of petroleum-based polymers such as polyethylene (PE) with pro-oxidant additives that promote the degradation process [14]. The materials used as additives are generally transition metals such as iron, manganese, cobalt, and nickel. The additive's function is to break down the large molecular polymer chain into smaller fragments that microorganisms can process and convert into biomass and carbon dioxide (CO2) [104,123]. The degradation of these plastics can be initiated by UV light, moisture, heat, and microorganisms. Usually, the degradation can be evaluated by measuring the changes in physical properties such as the loss of molecular weight, the amount of CO2 evolved, and the microbial growth on the polymer surface. The degradation of oxo-biodegradable plastics generally takes months to years; however, this length of time is occasionally unpredictable, because it depends on climate factors such as temperature and the intensity of solar radiation. The most commercially successful additives used as pro-oxidants are totally degradable plastic additives (TDPAs), Renatura, AddiFlex, d2W, and Reverte. AddiFlex is employed to produce single-use plastic bags. Reverte is used for bottle production, and different companies use d2W for their consumables, such as Pizza Hut, Walmart, and KFC [104].

#### *4.2. Biobased Non-Biodegradable Plastics*

Biobased plastics that are not degradable represent a group of biopolymers such as bio-polyethylene (bio-PE), bio-poly(ethylene terephthalate) (bio-PET), bio-polyamides (bio-PAs), and bio-polypropylene (bio-PP) [115]. They can be synthesized from biobased sources and offer a nearly equal chemical structure and properties to their fossil equivalents. Among them, bio-PET is the most widely produced bioplastic [124]. Since 2010, the precursors of PET, ethylene glycol and terephthalic acid (TPA), have been obtained from biological sources. Nowadays, the Toyota Tsusho Corporation, Japan and Futura Polyesters, the Coca-Cola–Gevo Venture, and the PepsiCo–Virent Venture are the principal producers of bio-PET [115]. Bio-PE is obtained from biological resources by the dehydration of bioethanol, obtained from glucose. Different natural feedstocks can be used to obtain glucose, such as maize, wheat, sugar cane, and sugar beet. The biopolymer is identical in its chemical, mechanical, and physical properties to fossil-based PE [115]. According to European Bioplastics [117], biobased but non-biodegradable plastics represent almost 40% of the global bioplastic production capacity; nevertheless, it is predicted that in the following years their participation will be lower.

### *4.3. Biobased and Biodegradable Plastics*

#### 4.3.1. Starch

Starch is an essential polysaccharide that plants synthesize and store in their structure as an energy reserve. It is considered one of the best biopolymers with extraordinary potential, because it is biodegradable, renewable, and available in huge quantities at a low cost. Starch could be extracted from different sources such as corn, wheat, potato, rice, tapioca, tam, and barley. However, most starch is produced from maize [104,125]. Starch

polymers are divided into two principal forms, amylose and amylopectin; the composition of these components can affect the properties of starch-based films [126]. Native starches have limits in their mechanical properties, thermal stability, and brittleness; that is why plasticizers (sorbitol, glycol, and glycerol) are necessary to improve the bioplastic functionality, transforming starch into thermoplastic starch [104]. Additionally, to enhance starch's characteristics, attempts have been made to mix it with synthetic polymers, lignocellulosic biomass, or agricultural waste (Table 1) [125]. Different studies demonstrate the applicability of starch bioplastics in shopping bags, food packaging, agriculture, and medicine.

#### 4.3.2. Cellulose

Cellulose is a polymer which is renewable and is present in large quantities. It is widely used because it is inexpensive and biodegradable. Cellulose is a polysaccharide chain that contains D-glucopyranose units joined by β-1,4-glycoside linkages. Cellulose from plants can be found from wood and non-wood plant lignocellulosic biomasses. It is the main constituent of plant fiber [139]. Cellulose is a potential film material with a highly crystalline structure; however, it is not soluble in water or common organic solvents. For this reason, it needs to be modified to transform it into water-soluble materials [139]. Cellulose acetate is a derivative of cellulose obtained from a chemical modification of cellulose. It is extensively used in the production of membranes, cigarettes, and food packaging [138].

Although cellulose is a plant material, some bacteria can produce cellulose, which is known as bacterial cellulose (BC) [139]. This is an interesting material due to its unique properties, making it an ideal candidate for industrial-scale production [140]. It has excellent mechanical strength and degradability. Additionally, it is purer than natural cellulose, and it has a higher water-holding capacity. These characteristics make it a promising natural polymer with multiple applications in medicine, food, electronics, and other fields [141]. Few companies produce this polymer on a large scale, such as Bowil Biotech (Poland) [140]. However, the high-cost production has limited its applications. Bacterial cellulose can be synthesized by multiple bacteria species, such as *Acetobacter* sp. and *Gluconacetobacter* sp. [141]. Using by-products and wastewater as raw materials for the synthesis of biopolymer is a strategy for large-scale production at a low cost (Table 2) [142]. The production of BC from agricultural and industrial waste uses food wastes [141,143,144], wastewater, and crude effluents [142,145–147]. An important advantage of using these residues is the reduction in both the cost of production and human waste [146].

#### 4.3.3. Polybutylene Succinate

PBS is an aliphatic polyester that is synthesized by the condensation of succinic acid and 1,4-butanediol. Traditionally, the monomers were synthesized from fossil resources; however, butanediol can be prepared from renewable sources such as renewable biomass resources and succinic acid from sugar fermentation (Table 2) [148]. Furthermore, PBS has good mechanical properties comparable to PE and PP, resulting in multiple applications for food packaging, shopping bags, agriculture mulch film, and hygiene products [120]. Nevertheless, the production cost is high, so future developments are needed to make it more economically viable [51].

#### 4.3.4. Polylactic Acid

Polylactic acid (PLA) is an aliphatic polyester produced by chemical or biological methods from cellulosic biomass [120]. It is biodegradable, and its mechanical properties are equivalent to those of petroleum-based plastics such as PET and PS, in addition to properties such as biocompatibility, innocuity, and compostability. The Food and Drug Administration (FDA) allowed the direct contact of the material with food. PLA is produced from lactic acid through the fermentation of renewable resources such as rice, corn, and potatoes. The fermentation is performed principally by lactic acid bacteria; however, different microorganisms have also been used (Table 2) [104]. PLA can be composed of D- or L-lactic acid isoforms or by both. It is necessary to obtain both isomers in separate processes [149], as the crystallinity and thermal stability of the final product depends on the isomer proportion.

PLA is probably the most well-known bioplastic and can be synthesized from renewable and natural resources. Its high manufacturing cost must be reduced in order to produce more bioplastics. Several works in the literature have reported the use of lignocellulosic wastes and food-derived wastes, such as spent coffee grounds [150], fruit wastes [149,151,152], alfalfa silages [153], and corn cobs [154] (Table 2). Despite PLA being extensively adopted, it still has drawbacks such as its cost and its structural breakability and fragility. Different approaches have been studied to improve its properties, such as the incorporation of nanoparticles [155].

#### 4.3.5. Polyhydroxyalkanoates

PHAs are natural aliphatic polyesters that consist of R-3-hydroxyalkanoate groups. There are synthesized by fungal and bacterial strains [156] from different substrates such as industrial by-products, oils, fats, lignocellulosic raw materials, agroindustrial waste materials, sugars, and wastewater (Table 2) [157]. PHAs are less porous and therefore trap less O2, CO2, and H2O, making them suitable for the production of packing materials such as films, coatings, bottles, and bags [156]. Additionally, PHA has good thermal-mechanical characteristics similar to synthetic polymers; therefore, it can substitute for PE and PP [118]. PHBs are one of the most studied polyhydroxybutyrates [158]. PHAs are commercialized by different companies, for example, Metaboli (Woburn, MA, USA); Procter & Gamble Co., Ltd. (Cincinnati, OH, USA); Tianjin Green Bioscience Co., Ltd. (Tianjin, China); Bio-on (Emilia Romagna, Italy); Biocycle PHB Industrial S.A. (Serrano, SP, Brazil); and Goodfellow Cambridge, Ltd. (Huntingdon, UK). The commercialization of this biopolymer has been increased due to the progress in purification technologies. However, in 2020, PHA biopolymers represented only 1.70% of the worldwide production of bioplastics [117]; the high production costs represent the principal reason for the limited production. To make the production of PHA competitive, it is necessary to use low-value waste materials as a substrate [159].

**Table 2.** Biopolymer production by microorganisms using agroindustrial residues as a substrate.

1 PHA—polyhydroxyalkanoates; 2 bmcl-PHA—medium-chain-length polyhydroxyalkanoate; 3 PHB—polyhydro xybutyrates; 4 BB—batch bioreactor; 5 SF—shake flask.

#### 4.3.6. Other Natural Sources

Seaweed is an excellent alternative for bioplastic production because seaweeds can grow fast and are easy to harvest and cheap. To improve their properties, they can be mixed with other species or materials [172]. Additionally, the use of seaweeds for bioplastics can reduce the impact on the food chain [123]. Standard methods include washing, milling, drying, alkalinization, acidification, neutralization, filtration, and precipitation to produce seaweed bioplastics. Nevertheless, the traditional methods are expensive and have a low yield. For these reasons, green production methods have gained attention due to

their high potential and viability [172]. Seaweed polysaccharides can be used for food industry applications, packaging materials, and coatings [123]. Microalgae also represent a promising feedstock for bioplastic production due to their fast growth rates. Numerous researchers have implemented the use of whole microalgae cells such as *Spirulina* sp. [135], *Chlamydomonas* sp. [135], and *Scenedesmus* sp. [173].

Similarly, fungal mycelia from seed peels and corn stalks were used to produce biodegradable packaging and tilling; the mycelia were composed of polysaccharides, chitin, proteins, and lipids. A New York company named Evocative and the Swedish company IKEA have used this bioplastic [123,174]. Chitin and cellulose from crab shells and fiber trees, respectively, have been used to contain liquids and foods [175].

#### **5. Green Industry of Plastics**

#### *5.1. Global Market, Business Cases, and Applications*

Currently, plastic products are the material of choice for different industrial, medical, and personal applications, among others. Most plastic products are derived from petrochemicals and are single-use, representing losses of 95% of the material value, which is reflected in annual economic losses between USD 80 and 120 billion [117]. To ensure greater environmental sustainability, it is necessary to exchange non-biodegradable plastics of petrochemical origin for biodegradable and compostable biobased materials. Bioplastics are attractive alternatives due to their rapid biodegradation in the environment into CO2 and H2O, causing fewer negative effects on the environment [176]. Environmental awareness about the impact of the use of plastics of petrochemical origin has increased both at the business and societal level; however, for economic reasons, bioplastics only represent about 1% of the 360 million tons that are produced annually. The advancement of knowledge and the development or improvement of biopolymers allows the reduction in production costs, and in turn the increased demand for bioplastics is generating an annual growth of 20–30% [177].

Currently, the production of bioplastics uses mainly carbohydrates from plants such as sugar cane or corn as a raw material. Nevertheless, the use of these crops, called primary crops, for bioplastics is in competition with the safety and economic stability of the food industry. Another strategy to produce bioplastics is the use of non-food raw materials, known as second-generation, such as cellulose from crop residues such as corn and wheat stubble, and third-generation raw materials such as micro- and macroalgae. The current production of bioplastics, 2.11 million tons, translates into approximately 0.7 million hectares of arable land, which represents 0.02% of the world's agricultural area. However, the growing demand for bioplastics will require a greater cultivation area, which will generate greater food competitiveness and is why it is important to implement the sustainable production of bioplastics through effective biotechnological processes from food waste, non-food crops, and cellulosic or algal biomass [117].

The production of bioplastics began due to the need to replace the non-biodegradable plastic materials of single-use products, such as bags, plates, bottles, plastic films, packaging, containers, and cutlery. However, only in recent years has this transition increased due to the decrease in the production costs of biodegradable biopolymers derived from new technological advances and new renewable sources for the production of biopolymers, in parallel with public policies and social conscientization, which have made it possible to produce economically viable bioplastics. Table 3 summarizes the properties of biopolymers in a relation to their applications. Biopolymers on their own do not provide the properties demanded by the market. The costs per kilogram were estimated according to the literature [176].

**Table 3.** Commercial applications of biobased polymers and their properties.

1 PLA—polylactic acid; 2 PHA—polyhydroxyalkanoates 3 Bio-PE—biobased polyethylene; 4 PBS—polybutylene succinate; 5 PCL—polycaprolactone; 6 PBAT—polybutylene adipate terephthalate.

Throughout the world, various companies have been established that develop and produce biodegradable biopolymers/bioplastics for different applications, which in turn has created greater competitiveness in the bioplastics market to reduce the costs of biodegradable biopolymers (Table 4). The most widely produced biopolymers are mainly PLA, PHA, and starch, which have been used in various applications from packaging to bioplastic materials for toys, parts for the automotive industry, construction, electronics, and agriculture, among others.

The Qmilk®® company has developed one of the most innovative biopolymers, managing to produce a 100% natural polymer from whey with antimicrobial, biodegradable, and compostable properties; it is also a flame-retardant with a low density and high hydrophobicity. This product is created by casein and forms an excellent barrier to gases such as oxygen and CO2 and aromas. All these properties give it a great advantage against other biopolymers for food and grocery packaging and textile-fiber production with silklike characteristics [179].

#### **Table 4.** Commercial bioplastics and business cases.

1 Website: https://plantic.com.au/ (accessed on 3 February 2022); 2 Website: https://materbi.com/ (accessed on 3 February 2022); 3 Website: https://unitedbiopolymers.com/ (accessed on 3 February 2022); 4 Website: https://biofase.com.mx/ (accessed on 3 February 2022); 5 Website: https://solanylbiopolymers.com/ (accessed on 3 February 2022); 6 Website: https://www.sdk.co.jp/ (accessed on 3 February 2022); 7 Website: https:// www.teijin.co.jp/ (accessed on 03 February 2022); 8 Website: https://www.natureworksllc.com/ (accessed on, 3 February 2022); 9 Website: https://weforyou.pro/ (accessed on 3 February 2022); 10 Website: https: //www.total-corbion.com/ (accessed on, 3 February 2022); 11 Website: https://danimerscientific.com/ (accessed on 3 February 2022); 12 Website: https://www.qmilkfiber.eu/ (accessed on 3 February 2022). 13 PLA—Polylactic acid; 14 PHA—Polyhydroxyalkanoates; 15 USA—United States of America.

Biofase®® is a Mexican company that has also stood out for the development of biobased biopolymers with avocado seed starch. This company uses an agroindustrial waste widely produced in Mexico to manufacture single-use bioplastics for applications such as cutlery, straws, and food containers [180].

Ingeo®® is a biopolymer developed by NatureWorks®® based on environmental sustainability and a circular economy, using greenhouse gases such as CO2 as a raw material to generate a polymer that is biodegradable and compostable. Ingeo®® polymers are currently obtained from sugars produced by plants such as cassava, sugar cane, corn, and beets, which function as CO2-sequestering plants, making them sustainable. These polymers are produced by a chain of technological bioprocesses that convert sugars into PLA and are used to create different products such as coffee capsules and electronic components [181].

The development and production of different polymers from biological origins with biodegradable and compostable characteristics have been carried out worldwide, but with greater impact and production in developed countries such as the US and European and Asian nations. Mexico is an example of a developing country where there are still a few limitations to the use of petrochemical plastics. However, the change of some single-use products (bags and packaging) to biobased or oxo-biodegradable products that have a lesser impact on the environment is the way towards a sustainable use of plastics.

Advances in the production chains of biopolymers and the environmental and social commitments of companies have been the result of public policies that limit the use of fossil-derived plastics to encourage a switch to the new generation of plastics. However, with these great changes come new challenges and a need to enhance the production of biopolymers.

#### *5.2. Strategies for Plastic Reinsertion and Environmental Impact Mitigation*

The recycling and reuse of plastic waste creates new challenges for strategies of collecting, separating, and treating materials of different chemical natures in mixed-waste streams [182]. The diversity of discarded plastic materials and the loss of mechanical properties in recycled polymers have given raise to alternative treatment strategies, such as chemical recycling, which turns plastic waste into chemical products of high value [183]. Biological routes of plastic waste degradation allow its reinsertion into natural carbon cycles, mitigating its environmental impact [184]. Current trends in modern plastic material design, such as biobased plastics, integrate recycling and degradation pathways in the polymer composition [185]. Recent chemical and biological strategies for assimilation are discussed in Table 5.

#### **Table 5.** Chemical and biological strategies for degradation.

**Table 5.** *Cont.*

1 PLA—polylactic acid, 2 PHA—polyhydroxyalkanoate, 3 HDPE—high-density polyethylene; 4 LDPE—low-density polyethylene; 5 PP—polypropylene; 6 PS—polystyrene; 7 PHB—poly(3-hydroxybutyrate-co-3-hydroxyvalerate); 8 PHBV—poly(butylene succinate); 9 PBS—poly(butylene adipate-co-terephthalate); 10 PCL—poly(ε-caprolactone); 11 PET—polyethylene terephthalate.

#### **6. Challenges and Opportunities for Biodegradable Plastics from Production to Degradation, and Further Perspectives under Circular Economy**

The world demand for plastics has grown rapidly in recent years due to their economic advantages, properties, and applications, with the packaging sector representing about 40% of the demand. As a result of this demand, the production of plastics is growing, while recycling systems are limited [198]. For a long time, there was no need to recover used plastic, since there was not a large demand for these raw materials, the costs were very low, and the impact of the presence of plastics in the environment had not been determined or recognized [199].

In recent years, challenges have arisen regarding the scale of production, the cost of production, industrial implementation, recycling, and the effects of degradation and contaminants derived from plastics, as well as issues relating to their regulation. The scale of the production of biodegradable plastics is small compared to plastics from fossil sources. The processes for the production of plastics from agroindustrial wastes such as corn crops and sugarcane bagasse are limited due to issues relating to the logistics of collecting, drying, and storing the raw materials. For example, in PLA production, optimization requires the selection of a microorganism that maximizes the yield [200]. Additionally, the microbial fermentation of lactic acid is associated with high costs derived from the pretreatment of the agroindustrial wastes and their conversion into fermentable sugars for the downstream biotechnological process. The optimization of cost–performance sustainability for bioplastics produced from organic wastes is needed before there can feasibly be a switch from fossil-based plastics to bioplastics [201].

The issues in the industrial implementation of biodegradables plastics are very clear in the food sector. For food packaging, biodegradable plastics should be a barrier against the transfer of gases and water vapor, preferably be transparent for the visual attraction of the customers, and form films with proper tensile and mechanical strength. Currently, the majority of these plastics are opaque, with limited tensile properties and a susceptibility to UV-triggered oxidation [202].

Biodegradable plastics are more susceptible to hydrolysis than petroleum-based plastics, depending on their composition. In the process of degradation, they exhibit a reduction in their molecular weight, leading to a low recycling quality. Their physical and chemical characteristics limit the possibility of mechanically recycling conventional and biobased plastics. The technologies and equipment used and the specific plastic waste influences the recyclability [120].

Additionally, the contamination of plastic waste with organic materials leads to the formation of thin films that often limit the recyclability of petro- and biobased plastics [203]. Of all the plastic produced, only 7% is recycled, about 8% is incinerated, and the rest reaches landfills or the environment [123]. Recyclability is a key element of the circular economy and is of great importance in biodegradable materials such as bioplastics. In the case of plastics of petrochemical origin, the recycling process is already well-established, while for bioplastics, it is still under development, since a fully effective and viable process has not yet been achieved [204]. Most bioplastics of biological origin (PLA, PHA, starch, etc.) may require separation based on their chemical composition; otherwise, they may suffer a loss in their quality and physical integrity due to contamination with other polymers. Currently, the most effective process for the treatment of bioplastics of biological origin is biodegradation [205]. The recycling of bioplastics for food-grade applications is a process that requires a higher degree of processing for the removal of contaminants, which is sometimes not economically viable when compared to the production of enriched compost, which is a more profitable process [176]. Biobased plastics, such as bio-PE, can be recycled by the same processes as their fossil-fuel counterparts, as well as with other plastics of petrochemical origin that are structurally similar [206].

The main concerns for the use of petrochemical plastic materials are their low rate of decomposition in the environment, as well as the release of microplastics and various toxic organic compounds that are known to be harmful to the environment and human health. Bioplastics, on the other hand, have a low environmental impact, since their biodegradation does not lead to the release of highly toxic compounds [207]. Different biopolymers obtained from natural resources are easily converted into bioplastics with biodegradable and compostable properties; however, these new bioplastics need regulations that verify and certify these properties to guarantee their safety of use and the lack of harm they pose to the environment [208]. On the other hand, the oxidation of biopolymers during burning releases the same amount of CO2 into the atmosphere as petroleum-based polymers. In this sense, strategies to control plastic use and its release are still needed to solve this environmental problem.

Despite the many efforts made in the design of standards to evaluate the compostability and biodegradability of bioplastics, at the industrial level, they have still not been extensively explored and remain unregulated [209].

From the perspective of a circular economy, which aims to properly use renewable natural resources without affecting the environment, biobased bioplastics are the most studied plastic materials, since they present the most advantages in their disposal after use [176]. Adopting a circular economy incorporating biobased resources hopes to reduce greenhouse-effect emissions while increasing the ecoefficiency of resources, valorizing waste and by-products, and thus reducing dependence on nonrenewable resources to reduce the impact of climate change and carbon footprints [210].

The European Commission (EC) implemented an action plan for the circular economy of plastics in 2015. The EC published two legal documents for the packaging industry and its waste management: Directive (EC) 2018/851 of the European Parliament and of the Council of May 30, 2018 which modifies Directive 2008/98/EC on waste, and Directive (EC) 2018/852 of the European Parliament and of the Council of 30 May 2018, which modifies Directive 94/62/EC on packaging and packaging waste. These regulations, based on a circular economy, focused on the reuse and recovery of plastic containers to allow a longer life for the products while minimizing waste and reducing the environmental impact; however, the full recycling potential of the European Union remains untapped, since only 30% of plastics are recycled, with hopes to reach 55% by 2025 [211]. The goal of the "Plastics 2030 Voluntary Commitment" is to recycle 60% of all plastic used and reduce the production of petroleum-based plastic by 100% by 2040 [212].

Biorefineries focus on the comprehensive use of different organic wastes such as food, agroindustrial waste, forestry, and algae biomass, which are of great interest for generating new precursors of bioplastics. Through different strategies, such as bioprocesses with

microorganisms, the reduction in the use of nonrenewable products has been notable. Different natural resources can be used or explored to obtain biopolymers for bioplastic fabrication, such as PLA, PHA, and PBS. These approaches advance society towards a circular economy and produce different single-use bioplastic products that are transformed into natural compostable compounds without any negative effects on the environment [210].

Biodegradable plastics are not the only solution to the problem of garbage accumulation in landfills and the environment. However, the use of biodegradable bioplastics that are degraded by biological agents under certain conditions and within certain time-frames allows a reduction in their environmental impact, which will leave a significant ecological footprint in the future [114].

#### **7. Legislation and Certifications for Biodegradable Plastics**

Due to the evolution of bioplastics over the last few decades, their regulation has become imperative. Experts from different fields have been working with organisms across the world to provide standards to analyze the properties, composition, and composability of different bioplastics using specific techniques and conditions that can be replicated. The major organizations creating these standards are the International Organization for Standardization (ISO), the European Committee for Standardization (CEN), and the American Society for Testing and Materials (ASTM). Table 6 is a compilation of the most used and cited standards from different organizations regarding plastics. The United States of America and the European Union currently have many regulations in place, and these are regarded with significant concern, since they have a great impact at the international level. In comparison, in Mexico, there are few reports of standards for bioplastics; meanwhile, the key standardization bodies have a wide variety of standards covering most of the bioplastic family known to this date.

Even when standards are in place, they are not mandatory but are a set of rules and tests to analyze the bioplastic. The portion of the industry sector that analyzes their bioplastic is motivated to obtain a certification for customer approval, particularly when their customers prefer brands with sustainable commitments [213]. The process to obtain the certification is to test the product by an independent certification laboratory that determines whether it complies with the standards and is deserving of a certification. Multiple organizations and associations offer these services and guide the business to obtain certifications that are recognized by governments and society in general. The Biodegradable Product Institute, Tüv Austria (formerly Vinçotte), DIN Certco, the Australasian Bioplastics Association, and the Japan BioPlastics Association are the most in-demand organizations that offer

this service (Figure 6). For instance, the Tüv Austria group offers diverse certification logos, which include the Seedling logo, OK compost INDUSTRIAL, and OK compost HOME, all of which are related to the compostability of organic material; the first two logos are acquired when products are in compliance with the standard EN 13432, and the later was developed by the Tüv Austria group without referring to a specific standard, but it contains all the technical requirements that a product must meet to serve as a basis for the creation of other standards across Europe and Australia [214]. Tüv Austria also offers logos that describe the biodegradation of plastic materials depending on the environment in which this process occurs, the logos being OK biodegradable MARINE, OK biodegradable SOIL, and OK biodegradable WATER. All of these are based on the idea of stopping littering and giving information as to the right environment for the biodegradation of a particular material to occur [215]. OK biobased is another logo that rewards companies that manufacture their products with an alternative to fossil-based raw materials. This certification presents the biobased percentage of renewable raw materials used as a base in the product, and the product can be rated up to four stars [216]. Lastly, the logo NEN BIO-BASED CONTENT refers to the biomass content in materials and products in general; this certification is based on the European standard EN 16785-1 and was recently added to the list of the TÜV AUSTRIA group [217].

**Figure 6.** Bioplastic certification world map. (**A**) Biodegradable Product Institute; (**B**) Tüv Austria; (**C**) DIN Certco; (**D**) Australasian Bioplastics Association; (**E**) Japan BioPlastics Association.

Notwithstanding the tendency to use bioplastics for their wide-ranging properties, functionalities, and applications, the scheme of regulations and certifications is changing towards more sustainable approaches. Alliances to join the governmental, industrial, and academic sectors to establish new regulations and improve the existing ones are needed. In

this sense, it should be a reality in the near future that there are more initiatives to certify the characteristics of industrial products which are standardized worldwide and that there are key standardization bodies to legislate and certify products with logos that describe the properties of the family of bioplastics.

#### **8. Conclusions**

The field of plastics has been changing over the years, with biobased molecules originally made to be synthetic, complex, and nonbiodegradable. The stability conferred by the polymeric nature of traditional plastics hinders their degradation in the environment, resulting in the accumulation of plastics in the ecosystem instead of their chemical degradation, macroscopic fragmentation, and biological assimilation. Research in the plastic field is overwhelmingly focused on tackling its environmental and economic problems. Trends in the use of oxodegradable additives and their potential degradation by microbial and physical means are an insight into these topics. Nevertheless, the expectation to use different raw materials derived from natural sources and create a circular economy to make biopolymers that substitute for fossil resources to produce plastics is on the rise. Although the physicochemical properties of the materials could be exceptional in relation to biodegradability, challenges derived from their production, implementation, and recyclability have been identified. The relationship between the activities of (1) governments with public policy, (2) academics with research on new materials and their properties, (3) the industry with the implementation and products, (4) international organizations and nonprofit organizations that supply regulations and certifications, and (5) the social culture regarding the use of plastics and recyclability will bring better initiatives for the future of plastics.

**Author Contributions:** Conceptualization, E.M.M.-M. and R.P.-S.; software, R.M.-G.; investigation, A.G.A., R.P.B.M. and K.A.S.C.; writing—original draft preparation, E.M.M.-M., R.M.-G., L.A.-R., D.R.-G. and L.P.-A. writing—review and editing, R.G.A., J.E.S.-H. and A.R.-M.; visualization, M.Y.L.; supervision, H.M.N.I.; funding acquisition, R.P.-S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Cadena Comercial OXXO, S.A de C.V, grant number DJ-PI-JMC-DO-ITESM-EIC-2021/12.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thankfully acknowledge Altera (Ricardo Alberto Ruiz Tapia and Cassandra Aguilar Figueroa); Qualyplast (Lorenia Miranda López, José Puig Escamilla, and Leobardo Burboa Maldonado); and Kristafilms for their support in this work. Consejo Nacional de Ciencia y Tecnología (CONACYT) is thankfully acknowledged for partially supporting this work under the Sistema Nacional de Investigadores (SNI) program awarded to Elda M. Melchor-Martínez (CVU: 230784), Juan Eduardo Sosa-Hernández (CVU: 375202), Rafael Gomes Araújo (CVU:714118), Adriana Reyes-Mayer (CVU: 288227), Hafiz M.N. Iqbal (CVU:735340), and Roberto Parra-Saldívar (CVU: 35753), and the PhD scholarship awarded to Lynette Alvarado Ramírez (CVU: 486638). The authors thank Tecnologico de Monterrey for providing access to scientific literature.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Effect of Prosopis Juliflora Thorns on Mechanical Properties of Plastic Waste Reinforced Epoxy Composites**

**Sakthi Balan Ganapathy 1, Aravind Raj Sakthivel 1,\*, Mohamed Thariq Hameed Sultan 2,3,4,\*, Farah Syazwani Shahar 2, Ain Umaira Md Shah 2, Tabrej Khan 5 and Tamer A. Sebaey 5,6,\***

**Abstract:** Plastics are unavoidable at this times, putting our planet in danger. The Prosopis juliflora (PJ) thorns are collected, processed, and powdered. The mechanical characteristics of these powders are examined when combined with polymer composites. Pores are the main cause of moisture input, hence using powder filler materials reduces the number of pores in the composite, increasing water resistance. The composites are made by altering three parameters: waste plastic content, filler powder composition, and chemical treatment. It was discovered that the integration of thorn powder increased the wear resistance. The composites were tested in accordance with ASTM standards, and the results were optimized. Based on the results, composite specimens were created and tested for validation.

**Keywords:** waste plastics; fillers; Prosopis juliflora; thorn powder; abrasive wear resistance

#### **1. Introduction**

Due to the plastic thrash, the land, water, and air are polluted [1]. If the plastics are dumped in the landscapes, they will block the percolation of rainwater into the land which leads to a decrease in the groundwater levels. If the plastics are thrown into water bodies, it leads to leaching, and the water gets polluted. We cannot burn the plastics, as it would release harmful gases into the atmosphere. To avoid the degradation of the earth, the used plastics have to be reused in some way. In the growing world, the need for alternate materials is increasing. At the same time, lots of landscapes are occupied by Prosopis juliflora plants, which makes it difficult to cultivate crops and also it absorbs the water and nutrients from the land and makes it uncultivable [2,3]. A lot of research works are undergoing to use these plants effectively for various applications. Kailappan et al. produced the activated carbon from Prosopis juliflora using a chemical method and proved it can be used in oil, food, and pharma industries [4]. As the plastics are used in this composite, it can be used in automobile, marine applications where lightweight and less corrosive materials are required. Plastics have moisture repelling property and the Prosopis juliflora thorn powder provides hardness to the composites and also it fills the minute pores produced during the production of composites.

**Citation:** Ganapathy, S.B.; Sakthivel, A.R.; Sultan, M.T.H.; Shahar, F.S.; Shah, A.U.M.; Khan, T.; Sebaey, T.A. Effect of Prosopis Juliflora Thorns on Mechanical Properties of Plastic Waste Reinforced Epoxy Composites. *Polymers* **2022**, *14*, 1278. https:// doi.org/10.3390/polym14071278

Academic Editors: Maya Jacob John and Sabu Thomas

Received: 1 March 2022 Accepted: 16 March 2022 Published: 22 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

When Saravanakumar and his colleagues analysed the parameters of the fibres, they found that they had an average microfibril angle of 10.64 and utmost elongation strength of 558 MPa. It was found that the aspect ratio of the PJ fibres had a significant impact on the mechanical qualities of the finished product. A fibre aspect ratio of 136 and fibre loading of 23.53 wt. percent were found to yield the best mechanical properties. [5]. The tensile and thermal properties of treated PJ fibres were shown to be superior to those of untreated fibres in a study by Madhu et al. [6].

Structural applications may benefit from the properties of PJ fiber- and glass-fiber reinforced polymer composites, according to Manoj Kumar et al. [7]. Due to the presence of lignin, researchers led by Luis Valencia found functional nanomaterials with a mean radius of 10 nm and a length of 150 nm in the PJ [8]. Filler-reinforced epoxy composites can improve mechanical properties by as much as 45 percent, according to research by Santhosh et al., who studied the morphology and properties of PJ and RH—reinforced composites [9]. PJ ash powder can be used as a substitute for cement up to 20%, according to Parthiban Kathirvel et al., and they were able to attain the same strength for their newly designed concrete [10]. An onion-like porous carbon made from the PJ has been advocated as an efficient electrode material by Sathyanarayanan Shanmugapriya and his colleagues [11].

The novel composites' mechanical, tribological, and water absorption properties must be thoroughly investigated. It was discovered by Sakthi Balan et al. that adding 30 wt percent of waste plastic particle to jute fibre and waste plastic-filled composites resulted in high resistance to water absorption [12]. Water absorption and tribological properties were increased by the inclusion of filler and fibres to the composite, which was made of plastic waste, fiber glass, and silica sand fillers. It has been reported that the thermal and mechanical characteristics of epoxy composites manufactured with 20% fibres and NaOH treatment have been improved [13] by Arthanarieswaran and colleagues. Using dates palm seeds and glass fibre reinforced polymer composite, Heba I. Elkhouly et al. demonstrated an increase in the composite's wear resistance and toughness. Tapas by Priyadarshi Tapas Ranjan Swain et al. made a composite out of jute fibres and studied how it wore. The wear resistance has been modified by the chemical treatment [14]. They observed that the abrading distance was the most important element in determining the wear of waste silk fiber-reinforced epoxy composites [15], followed by the loading of fibers.

Taguchi is an effective strategy for designing experiments. Materials scientists use it to their advantage when examining the effects of various process variables. Polymer-based composites are being made using the Taguchi method. Boron nitride reinforcement of Nylon composite was studied by Shiva Kumar and Chennakesava Reddy using the Taguchi approach and it was found that the composite's wear resistance was improved by the addition of boron nitride [16]. It was proposed by Wahid Ferdous et al. that the bond length and thickness for higher strength be studied using Taguchi design in polymer-based composites [17]. Polymer composites with various reinforcements such kevlar, carbon, and glass fibers were subjected to the Taguchi technique by Karthik et al. in order to optimize the wear parameters [18]. They found that the hybrid composites had improved wear behaviors. As a result of the Taguchi approach and ANOVA, Siva Prasad and Chaitanya were able to optimize the drilling parameters for the GFRP composites [19].

Natural fibers have gained popularity and are beginning to supplant synthetic fibers, owing to their contribution to sustainable practices. As a result of environmental, social, and economic development, numerous industries have altered their manufacturing processes, materials, and procedures in order to ensure a sustainable future. While natural fibers have significant disadvantages, they can be overcome with appropriate chemical treatments and fiber processing. Numerous goods composed of natural fibers are developed and used in sports, electronics, and musical instrument manufacturing [20]. Natural fibers exhibit comparable wear resistance to synthetic fibers. The wear resistance of natural fibers can be increased by reinforcing them with synthetic fibers [21]. Recent research has concentrated on green fillers such as date seed powders, coconut and cashew nut shell powders, and

rice and maize husks. Natural fibers and fillers are used in composites because they are inexpensive, widely available, biodegradable, recyclable, and lightweight [22]. Researchers are becoming more interested in starch-based bioplastics due to their environmentally beneficial characteristics. Starches have been isolated from a variety of plants, including PJ plants, and bio composites have been constructed and tested for mechanical properties and biodegradability. The results indicated that composites might be used as a substitute material in the packaging industry [1].

Plastic trash and PJ thorn granules are both included into the polymer matrix in this study. The motive of this work is to investigate the influence of chemical treatment, the amount of waste plastics, and PJ thorn powders on the composite's hardness and moisture absorption and wear capabilities using the Taguchi technique.

#### **2. Experimental Details**

#### *2.1. Materials*

The scraped compact discs are employed as reinforcement in this composite fabrication, and most of them are constructed of polycarbonate. The decomposition of polycarbonate can take hundreds of years, yet it is possible. The reinforcements are placed in a matrix made of epoxy resin. Araldite AW106 grade resin and HV 953U hardener are utilized in this. Epoxies have features that distinguish them from other resins, and adjustments can be made to meet our specifications. Epoxies have superior mechanical qualities, including increased thermal stability, wear and chemical resistance, and resistance to ageing caused by environmental factors [22]. In order to improve a composite's specific characteristics and traits, fillers are incorporated into the material. Fillers in this work include PJ thorn powders. In general, PJ plants cause a wide range of issues for both humans and the environment. As a result, the PJ plant's thorns are harvested, dried for a period of time, and ground into a powder. The waste compact discs were collected from Vellore, Tamilnadu, India, the resin and hardener were purchased from Ayishwarya polymers, Coimbatore, Tamilnadu, India, and the PJ thorn powders were made manually by collecting the PJ thorns from nearby areas in and around Vellore, Tamilnadu, India, drying them in the sun, and then powdering them using a mixer grinder. Finally, before use, the PJ powder is sieved and processed. Once these powders have been chemically treated, they can be used as reinforcements and the matrix, as they have been neutralized of their characteristics.

#### *2.2. Fabrication Method*

Initially, the raw materials are collected like waste used plastics. The collected plastics are then separated according to their grades, cleaned, dried, and then they are crushed into particulates. Then the filler powder is made by collecting the PJ thorns, drying and then they are chemically treated with alkalis and silanes for one hour. The NaOH solution (5% *w*/*v*) is used for alkali treatment and Triethoxy vinyl silane (5% *v*/*v*) is used for silane treatment [23]. In both cases, the thorn powders are dipped into the chemicals for an immersion time of one hour. After the alkali treatment, the thorns are washed with HCL solution to make their pH value-neutral and the silane treated thorns are washed with de-ionized water and then dried. Then they are crushed and made into powders. The resin and the hardener are mixed in the proper ratios and the composite is made by the spray layup method. Spray layup has some added advantages to the hand layup method and the defects are low when compared to other manufacturing techniques. As the plastic particulates and PJ thorn powder can be dispersed through air medium, spray layup is preferred. Initially, the resin was applied and then the reinforcement and the fillers are loaded in the spray gun, and with the help of compressed air as a medium, the particles are sprayed evenly on the resin surface. This ensures the even spread of the fillers.

#### *2.3. Mechanical Properties Examination Methods*

The Vickers micro hardness tester manufactured by Fuel Instruments & Engineers Pvt. Ltd., Ichalkaranji, Maharashtra, India, is used to measure micro hardness, and the ASTM E10 standards are followed for the method. Test specimens are imprinted in Vickers hardness testers with 2.5-mm diameter balls of 10 kg of force. The specimen was indented in numerous places, and the average value is used to get the end result. To compute the average values, the diagonals of the cavity generated by pressing a diamond pyramid, are used.

The elongation of specimens on a UTM was measured using the ASTM D 3039-76 standard testing procedure. In this experiment, we used a UK-made H10KS model from Tinius Olsen, Redhill, UK. Crosshead speed and strain rate are maintained at 5 mm/min. Water jet cutting minimizes the formation of micro cracks compared to other cutting procedures, the specimens are cut using this method.

ASTM D570 standards were used to conduct the water intake examination for composites. As a precautionary measure, sticky tape is used to cover the specimen's sides. A scale is used to weigh the samples in advance of testing, and the results are recorded. Test pieces are then submerged in water for 24 h for examination. Afterward, the test pieces are taken out, patted dry, and weighed to determine the weight gain that occurred during the testing period. The percentage of water consumed will be determined by comparing the pre- and post-testing results. The moisture percentage is calculated using the following equation [12].

$$\text{MA}\% = (\text{W}\_2 - \text{W}\_1) / \text{W}\_1 \times 100,\tag{1}$$

where,

MA% = Moisture absorption percentage W1 = Sample weight before the experiment in gms W2 = Sample weight after the experiment in gms

The abrasive wear of the composite is measured using a pin on a disc wear testing equipment. The specimen was cut in accordance with the specifications of ASTM G99-05. Wear testing equipment supplied by DUCOM instruments, Peenya Industrial Area, Bengaluru, Karnataka, India, has a maximum wear track diameter of 135 mm and a maximum disc speed of 2000 rpm, which was used for the testing.  An abrading distance of 420 m and a load of 10 Newtons are applied to the specimen, which measures 5930 mm in size. With the use of a cantilever unit, the specimen is put into the holder and held there by the disc. Before loading the specimen, it is weighed, and then the test is performed. The samples will be weighed before and after the tests. The density is used to compute the wear volume. In order to calculate the wear rates, Equation (2) is used [24].

Wear rate (Ks) = Differences in wear volumes in mm3/(Normal load in N) × (Abrading distance in m) (2)

#### *2.4. Optimization Technique*