Compatibilizer efficiency in enhancing marine plastic waste valorization through simulated recycled plastic blends

Citation: Piedade Cestari, S.; Rodrigues, P.V.; Ribeiro, A.C.; Castro, M.C.R.; Cruz, V.; Torres, A.R.; Ramos, N.; Machado, A.V. Compatibilizer Efficiency in Enhancing Marine Plastic Waste Valorization Through Simulated Recycled Plastic Blends. Polymers 2024,16, 3441. https:// doi.org/10.3390/polym16233441 Academic Editor: Naozumi Teramoto Received: 28 October 2024 Revised: 2 December 2024 Accepted: 5 December 2024 Published: 8 December 2024 Copyright: © 2024 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 Compatibilizer Efficiency in Enhancing Marine Plastic Waste Valorization Through Simulated Recycled Plastic Blends Sibele Piedade Cestari, Pedro Veiga Rodrigues , Ana Cristina Ribeiro , Maria Cidália Rodrigues Castro , Vasco Cruz , Ana Rita Torres, Nuno Ramos and Ana Vera Machado * Department of Polymer Engineering, Institute for Polymers and Composites (IPC), Campus de Azurém, University of Minho, 4804-533 Guimarães, Portugal; [email protected] (S.P.C.); pedro.r[email protected] (P.V.R.); [email protected] (A.C.R.); [email protected] (M.C.R.C.); [email protected] (V.C.); [email protected] (A.R.T.); [email protected] (N.R.) *Correspondence: [email protected] Abstract: This study investigated the optimal combination of compatibilizers and stabilizers to enhance the value of marine environment plastic (MEP). The composition of the plastics was analysed, and a simulated recycled plastic blend (sMEP) was prepared based on a simplified composition of actual MEP. Different concentrations of three commercial compatibilizers (C1, C2 and C3) were tested to improve tensile strength. The tensile tests indicated that the blend compatibilized with 10 wt. % C3 (polypropylene grafted with maleic anhydride) exhibited the highest increase in tensile strength. This optimal compatibilization was then combined with two commercial stabilizers and applied to a simulated MEP blend. Scanning electron microscopy images showed that all blends had a continuous polyethylene phase with dispersed poly(ethylene terephthalate) (PET) and polypropylene (PP) droplets. The simulated blend with 10 wt.% C3 exhibited a reduced PET droplet size in the dispersed phase. Differential scanning calorimetry results revealed a decrease in polyethylene crystallinity and an increase in PP crystallinity. The improved properties of the blend were attributed to the effectiveness of the C3 compatibilizer in enhancing the interface between the PP and PET phases. An effective formulation was developed to valorise marine-sourced plastics by leveraging existing scientific knowledge and accessible commercial additives. Applying this enhanced formulation to real MEP not only demonstrated its effectiveness, but also highlighted a practical approach for reducing plastic pollution and supporting circular economy principles, contributing to environmental conservation efforts. Keywords: recycling; marine environment plastics; polymer blends; blend properties 1. Introduction The most recent research initiatives focus on the sustainability issue and the role of plastic waste within it. This has resulted in significant funding being allocated to specific topics related to this theme [ 1 , 2 ]. However, instead of addressing the underlying cause of the problem, much of the research primarily focuses on treating the symptoms, particularly the presence of plastic in the environment. The irresponsible disposal of plastic by society is what ultimately leads to the pollution of the environment and harm to living beings. Scientific studies have shown that the issue lies not with plastic as a material itself. When it comes to marine environments, the damage occurs because plastic and other items are in places where they should not be [ 3 ]. Recent estimates indicate that nearly 70% of all produced plastic waste (more than 252 million tonnes in 2021) is not treated and continues to end up in the environment with negative impacts [ 4 ]. The packaging industry is undoubtedly responsible for 50%, followed by the construction, industrial, and agricultural sectors at 30%. To address this issue, several strategies have been adopted to reduce plastic pollution’s effect on the environment. Recovery and mechanical Polymers 2024,16, 3441. https://doi.org/10.3390/polym16233441 https://www.mdpi.com/journal/polymers Polymers 2024,16, 3441 2 of 15 recycling have been used for several years, often focusing on downcycling due to the reduced properties of the resultant plastic compared to the original material, making it unsuitable for food contact applications. Conversely, research has developed alternative polymers with biodegradable capabilities to lessen environmental harm [ 5 ]. Upcycling strategies have been employed to provide high-value solutions, such as improved plastic properties, pure H2, carbon-based nanomaterials and depolymerization, among others [ 6 ]. The attention surrounding polymer waste found on beaches is mainly due to its visibility. In reality, the number of animals entangled in plastic nets has decreased since 2000 [ 7 ]. Even the well-known Pacific Garbage Patch has been found to be different from what was initially believed. Contrary to common claims, it cannot be seen from space and mainly consists of fishing industry nets (46%) and debris from the Japanese tsunami of 2011 (20%) [ 8 ]. Interestingly, it has been discovered that construction materials and cigarette butts/filters make up the majority of beach litter in Europe [ 9 ], and the responsibility for the generation of anthropogenic litter on beaches primarily lies with beach users [ 10 ]. However, no one has called for a ban on building materials, smoking or using beaches. Under environmental conditions, polymers can undergo several degradation mechanisms, including physical (mechanical forces, such as abrasion), chemical (photochemical, thermosoxidative, and hydrolytic degradation) and biological (microorganisms) degradation [ 11 ]. In all cases, the integrity of the polymer’s chemical structure is disrupted. The main degradation mechanism of polyethylene and polypropylene is oxidative reactions, initiated by exposure to UV radiation or heat, leading to chain scission and the formation of smaller hydrocarbons and carbonyl groups [ 12 , 13 ]. Additionally, poly(ethylene terephthalate) is highly susceptible to hydrolysis and photo-oxidation, resulting in the cleavage of its ester bonds [ 14 ]. From these degradation mechanisms, smaller plastic particles with a lower molecular weight (shorter polymeric chains) are formed, leading to the emergence of microplastics. Compared to neat polymer, degraded plastic exhibits lower properties, affecting its application and valorisation. According to Galgani (2015), plastics are often the main component of marine litter, sometimes comprising all of the floating waste. Studies have shown that the density of debris on beaches is around 1 item per square meter [ 15 ]. Data from the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) in 2022 indicated an upward trend in marine litter on beaches in North Portugal. The most common types of litter found on beaches in the Northeast Atlantic marine region are fishing nets, large and small pieces of polystyrene, caps and lids [ 16 ]. However, collecting and sorting postconsumer plastic waste from any environment is challenging and expensive. Therefore, the most efficient recycling method for plastics in this marine environment skips the sorting process, mixes all the materials, and produces a mechanically recycled polymer blend [ 17 ]. The “Evaluation and valorisation of plastics and microplastics in marine environment—MarPlas ” project found that the plastic waste on beaches in North Portugal mainly consists of polyolefins and poly(ethylene terephthalate) from post-consumer plastics such as bottles, bags, and caps/lids. When these materials are mixed together, they create an incompatible blend with weak interfacial bonds and poor mechanical and physical properties [18]. The use of compatibilizers is widespread in polymer recycling; they can address contaminants in immiscible polymer elements when they exceed achievable thermodynamic miscibility or compatibility limits. The compatibilization process can be achieved by modifying the predominant polymer to provide reactive sites for covalent bonding with minor polymer elements. New synthetic approaches have produced highly effective block copolymer compatibilizers that incorporate the morphological characteristics of the polymers present in the blend [ 19 ]. In general, compatibilizers based on polyethylene or polypropylene grafted with maleic anhydride (PE-g-MA and PP-g-MA, respectively), styrene–ethylene-co-butene–styrene (SEBS), ethylene–vinyl acetate (EVA) and ethylene– methacrylic acid copolymer (EMA) are employed in the compatibilization of poly(ethylene terephthalate) (PET)/polyolefins [ 20 – 24 ]. Fasce and co-workers studied PET/PE 50/50 Polymers 2024,16, 3441 3 of 15 blends compatibilized with varying amounts of EMA. They reported increased adhesion at the interface with 7 wt. % EMA, attributing the overall improvement in mechanical properties to fibrillation in the PET phase [ 25 ]. Nomura and co-workers synthesized PET/PE multiblock copolymers to compatibilize a PET/PE 80/20 blend. They observed a reduction in PE droplet size and an increase in strain at break with the addition of 0.5 wt.% compatibilizer, which they linked to the localization of the multiblock copolymer at the blend’s interface [ 26 ]. Tang et al. tested three commercial compatibilizers based on ethylene acrylate copolymer (ELVALOY™ AC 2016 Acrylate Copolymer (EAA), ELVALOY™ PTW Copolymer (PTW), and SURLYN™ 1802 Ionomer (Surlyn)) in a 50/50 PET/HDPE blend [ 27 ]. They concluded that EAA did not improve the interface or mechanical properties of the blend, but PTW and Surlyn enhanced elongation at break and toughness. These authors emphasized the importance of understanding the molecular arrangement of the blend by considering the combination of mechanical properties, crystallinity and morphology data, as well as estimating the location (interface vs. bulk) of the compatibilizer molecules. Stabilisers have been essential for protecting and extending the lifespan of recovered polymers since the late 1980s [ 28 ]. Polyolefin degradation during processing is well understood and documented in the literature. To limit the radical chain reactions occurring during the production and use of the polymer, stabilising chemicals (antioxidants) must be added to polyolefins [ 29 ]. These processes, known as autoxidation reactions, begin with the production of free radicals through light exposure, heat, or shear. They then progress to a series of autocatalytic radical chain reactions that result in the formation of peroxy-, hydroperoxy-, alkoxyand hydroxyl-radicals, as well as additional carboncentred radicals through hydrogen abstraction mechanisms. Radical chain reactions are mitigated by phosphite and phenolic antioxidants working together. Phosphites inactivate hydroperoxides, while phenolic antioxidants inactivate oxygen-cantered radicals. The processing and stability of the material can be enhanced by using UV absorbers and other stabilisers. The amounts of active species (such as phosphite) present in recyclates are often insufficient to protect the material from degradation during reprocessing. Contaminants and impurities affect stability and the choice of stabilisation technique for the polymers. It is expected that recycling materials, such as marine plastic waste, which has been subjected to extreme environmental deterioration and contamination, will require the addition of one or more stabilisers. When collected from the environment, plastic material already exhibits some degree of degradation, which are unsuitable to be blended with virgin polymer. Therefore, this study investigates the optimal combination of compatibilizers and stabilizers to valorise marine environment plastic (MEP), searching for new strategies to upcycle the marine plastic waste collected. Thus, first the composition of the collected MEP was assessed, and then a recycled plastic blend was prepared to simulate this combination. Various amounts of commercial compatibilizers were tested to achieve the best improvement in mechanical properties. The optimal compatibilization was achieved using two commercial additives, and the enhanced formulation could be applied to the real MEP. 2. Materials and Methods The MEP was collected at the sandbank and mouth of the Cávado River in Esposende city (Braga, Portugal) in 2022. The recycled PE and PP were generously donated by Recuplás—Reciclagem de Plásticos and R3Natura (Braga, Portugal). The recycled PET was donated by Ecoibéria (Braga, Portugal). The compatibilizers studied were as follows: polyethylene grafted with maleic anhydride (C1) Fusabond ® E226 (supplied by Dow Chemical, Aveiro, Portugal), containing 0.5–1 wt.% of grafted maleic anhydride; styrene–butadiene block copolymer (C2) Styroflex ® 2G66 (donated by INEOS Styrolution), Barcelona, Spain, a thermoplastic elastomer with a hard–soft–hard block sequence; and polypropylene grafted with maleic anhydride (C3) Polybond ® 3200 (donated by AddivantTM, Basel, Switzerland), containing 0.8–1.2 wt.% of grafted maleic anhydride. The stabilisers were IrgaCycleTM PS 030 (S1), which improves the durability of rigid articles Polymers 2024,16, 3441 4 of 15 made with polyolefin recyclates, and IrgaCycleTM XT 034 (S2), for polyolefin recyclates with significant levels of impurities, fillers, or pigments, both donated by Colorstar/BASFSE (Porto, Portugal). 2.1. MEP Preparation About 4 kg of marine environment plastics (MEPs) were hand-washed with neutral soap, rinsed with running water and dried overnight in an oven at 60 ◦ C. The sand from the washed and dried MEPs was measured. We removed the labels, retaining the plastic ones and discarding the paper labels. After separation and identification, the films were pressed in a heated press and hand-cut. All materials (solids and compressed films) were ground in a knife mill to a flake size of approximately 5 mm. 2.2. Blend and Specimen Preparation Since the MEP amount was small, a simulated MEP (sMEP) of recycled plastics was prepared based on the simplified composition of the real MEP: 42 wt.% HDPE, 13 wt.% LDPE, 25 wt.% PET, and 20 wt.% PP. About 2 kg of sMEP were processed as received (chunks and flakes) in a Leistritz extruder model LSM co-rotating twin-screw configuration, with a temperature profile of 180 ◦ C (feeding zone)/240 ◦ C/240 ◦ C/240 ◦ C/240 ◦ C/ 250 ◦ C/260 ◦ C (die) at 100 rpm. The compatibilizers and stabilizers were cryogenically ground in a Retsch mill model ZM 100 and added in three different contents (2, 5 and 10 wt.%) to the sMEP (Table 1). Table 1. Composition of the sMEP blends and adjustment factor for degree of crystallinity. Blend Weight Percentage (wt.%) Adjustment Factor sMEP C1 C2 C3 S1 S2 PE PP PET sMEP 100.0 0.550 0.200 0.250 sMEP2%C1 98.0 2.0 0.559 0.196 0.245 sMEP2%C2 98.0 2.0 0.539 0.196 0.245 sMEP2%C3 98.0 2.0 0.539 0.216 0.245 sMEP5%C1 95.0 5.0 0.550 0.200 0.245 sMEP5%C2 95.0 5.0 0.571 0.190 0.238 sMEP5%C3 95.0 5.0 0.524 0.190 0.238 sMEP10%C1 90.0 10.0 0.524 0.238 0.238 sMEP10%C2 90.0 10.0 0.550 0.200 0.238 sMEP10%C3 90.0 10.0 0.591 0.182 0.227 sMEP10%C3S1 90.5 9.0 0.5 0.500 0.182 0.227 sMEP10%C3S2 89.0 9.0 2.0 0.500 0.273 0.227 sMEP10%C3S12 89.0 9.0 0.5 2.0 0.550 0.200 0.227 The sMEP blends were compression-moulded into laminates with a thickness of approximately 0.7 mm in a heated press at 270 ◦ C, 7.7 MPa, for 5 min, and then cooled in a press at 25 ◦ C, 4.8 MPa, for 5 min. After characterisation, to the blend that exhibited best tensile resistance (sMEP10 wt.% C3) was added with 0.5 wt.% stabiliser S1, 2 wt.% stabiliser S2 and a combination of both stabilisers (0.5 wt.% S1 + 2 wt.% S2). 2.3. Characterisation 2.3.1. Density The density tests were performed according to ASTM D-792 [ 30 ], in isopropyl alcohol at 25 ◦C (ρ= 785 kg·m−3), using five specimens of each sample. 2.3.2. Mechanical Properties Mechanical tests were performed in Zwick/Roell universal testing machine Z005 model (Ulm, Germany), using a load cell of 5 kN. The tests of sMEP were conducted according to the ASTM D882 [ 31 ], test speed of 5 mm/min and specimens of 100 ×10 ×0.7 mm . Polymers 2024,16, 3441 5 of 15 The percentage variation of Young’s modulus was determined, as the secant modulus at 2% of deformation. The results consider the mean of the five specimens for each sample. 2.3.3. Morphology Spetroscopy Morphological analysis was performed using ultra-high resolution field emission gun scanning electron microscopy (FEG-SEM) and a NOVA 200 Nano SEM (FEI, Amsterdam, Netherlands). Samples were fractured in liquid nitrogen and coated with a thin film ( 2 nm ) of Au-Pd (80–20 wt. %), using a high-resolution sputter coater (208HR Cressington Company, Watford, UK). To evaluate the dispersion state of the different polymers in the incompatible blend, the PET droplet size was assessed by adapting and simplifying the methods of Novais, Jamali, and Carson. As it was not possible to discern the PET phase from the PP phase in SEM, the samples sMEP, sMEP2 C3, sMEP5 C3, and sMEP10 wt.% C3 were extracted with hexafluoroisopropanol (HFIP) at room temperature to remove the PET phase, and then analysed in two different regions using SEM images at 1000×magnification . The area of the holes with a maximum dimension of at least 5 µ m was assessed using ImageJ software (available at https://imagej.net/, accessed on 1 October 2024). 2.3.4. Fourier Transform Infrared Spectroscopy (FTIR) The spectra were acquired at room temperature in an FTIR 4100 Jasco spectrometer apparatus (Tokyo, Japan), in attenuated total reflectance (ATR) mode, between 4000 and 600 cm −1 wavelength range, using 64 scans.min −1 and 8 cm −1 resolutions. Thin films of all samples were prepared by compression moulding in a hot press at 270 ◦ C under a pressure of 7.7 MPa. To evaluate the effect of the compatibilizers in the scission of the ester bond of rPET chains, the ratio between the absorption of both the variable carbonyl band at 1716 cm−1 and the invariable methylene band at 2914 cm −1 was determined; this rate was called the carbonyl index (CI) [32,33]. 2.3.5. Differential Scanning Calorimetry (DSC) The samples were analysed using Netzsch DSC 200 F3 Maya equipment (Selb, Germany) under a nitrogen atmosphere, following ASTM D3418-A [ 34 ]. In the first cycle, the sample was heated from 30 to 270 ◦ C at a rate of 10 ◦ C · min −1 , and held at 270 ◦ C for 1 min. The second cycle involved cooling at a rate of 10 ◦ C · min −1 until reaching 30 ◦ C. In the third cycle, the same temperature range and heating rate as the first cycle were applied. The crystallization temperature (T c ) was determined from the second cycle, while the polymer crystalline melting temperature (Tm) and degree of crystallinity (X c ) were obtained from the third cycle. The Xcwas calculated according to Equation (1): XcPolymer = HmPolymer H0 mPolymer ·a f , (1) where Hm is the melting enthalpy of the polymer, H0 m is the theoretical melting enthalpy of 100% crystalline polymer (293 J · g −1 for PE, 209 J · g −1 for PP and 140 J · g −1 for PET) and af is the adjustment factor (Table 1) adopted to estimate the polymer percentage being melted in a given temperature range based on the percentage of polymer type present in the blend [ 35 – 37 ]. As C1 is PE and C3 is PP, they were added to their polymer type in the af. 3. Results and Discussion 3.1. MEP Composition The quantitative and qualitative evaluation MEP characterisation is listed in Table 2, which also presents the amounts and percentages of solids and films. The MEP artefacts were categorised by solids, films and polymer type, based on their conventional identification symbols. Among the unidentified objects, some had no symbol (e.g., straws, bottle caps, bags) and were identified using information from public sources or literature. Others (monoand multilayer films, labels, ropes, fishing nets, and strings) were identified Polymers 2024,16, 3441 6 of 15 through FTIR analysis. A coffee pod, a syringe plunger and a toy could only be identified through density and DSC tests. The density/T m results were: 900 kg · cm −3 /161 ◦ C for the coffee pod, 950 kg · m −3 /126 ◦ C for the syringe plunger, and 0.92 g/cm 3 /166 ◦ C for the toy. According to the literature, the density and Tm obtained for the samples are consistent with results found for PP (coffee pod), LDPE (syringe) and PP (toy) [ 38 – 42 ]. From the quantitative analysis, the MEP sample was mainly composed of polyolefins (48 wt.% PE and 17 wt.% PP) and polyesters (21 wt.% PET), which was used as a basis to produce the simulated MEP (sMEP). Most of the collected samples were derived from packaging products, like films and bottles for food packaging. Table 2. Type and percentage of materials in the MEP sample. Material Subtotal Weight Weight (g) Total Weight Percentage (wt.%) (g) (wt.%) Solids Films Solids Films PE 1934.3 48.4 1464.5 469.8 48.9 48.4 PET 850.3 21.3 850.3 - 28.4 - PP 671.7 16.8 351.0 320.7 11.7 33.1 PVC 48.2 1.2 48.2 - 1.6 - PLA 18.8 0.5 18.8 - 0.6 - Metallised film 6.8 0.2 - 6.8 - 0.7 Multilayer PET/PE 6.7 0.2 - 6.7 - 0.7 Unidentified 429.5 10.7 263.6 165.9 8.8 17.1 Sand 32.0 0.8 - - - - Total 3998.3 100.0 2996.4 969.9 75.5 24.5 3.2. Tensile Properties As the sMEP blends’ tensile curves did not exhibit a linear region ( Figures S1 and S2 ), the secant moduli of each blend were calculated (Figure 1). The only blend showing an increase in tensile strength (10% ± 0.21) was the one compatibilized with 10 wt.% C3. Since the study aimed to add value to the recycled plastic, the focus was solely on C3compatibilized blends. The S12 combination in the sMEP had the smallest loss in tensile strength, at − 7%. This result aligns with the findings of Ahmadlouydarab and colleagues, who observed similar behaviour in PP/PET blends compatibilized with PP-g-MA [18]. Polymers 2024, 16, x FOR PEER REVIEW 6 of 16 The quantitative and qualitative evaluation MEP characterisation is listed in Table 2, which also presents the amounts and percentages of solids and films. The MEP artefacts were categorised by solids, films and polymer type, based on their conventional identification symbols. Among the unidentified objects, some had no symbol (e.g., straws, bottle caps, bags) and were identified using information from public sources or literature. Others (monoand multilayer films, labels, ropes, fishing nets, and strings) were identified through FTIR analysis. A coffee pod, a syringe plunger and a toy could only be identified through density and DSC tests. The density/Tm results were: 900 kg·cm−3/161 °C for the coffee pod, 950 kg·m−3/126 °C for the syringe plunger, and 0.92 g/cm3/166 °C for the toy. According to the literature, the density and Tm obtained for the samples are consistent with results found for PP (coffee pod), LDPE (syringe) and PP (toy) [38–42]. From the quantitative analysis, the MEP sample was mainly composed of polyolefins (48 wt.% PE and 17 wt.% PP) and polyesters (21 wt.% PET), which was used as a basis to produce the simulated MEP (sMEP). Most of the collected samples were derived from packaging products, like films and bottles for food packaging. Table 2. Type and percentage of materials in the MEP sample. Material Subtotal Weight Weight (g) Total Weight Percentage (wt.%) (g) (wt.%) Solids Films Solids Films PE 1 934.3 48.4 1 464.5 469.8 48.9 48.4 PET 850.3 21.3 850.3 - 28.4 - PP 671.7 16.8 351.0 320.7 11.7 33.1 PVC 48.2 1.2 48.2 - 1.6 - PLA 18.8 0.5 18.8 - 0.6 - Metallised film 6.8 0.2 - 6.8 - 0.7 Multilayer PET/PE 6.7 0.2 - 6.7 - 0.7 Unidentified 429.5 10.7 263.6 165.9 8.8 17.1 Sand 32.0 0.8 - - - - Total 3998.3 100.0 2996.4 969.9 75.5 24.5 3.2. Tensile Properties As the sMEP blends’ tensile curves did not exhibit a linear region (Figures S1 and S2), the secant moduli of each blend were calculated (Figure 1). The only blend showing an increase in tensile strength (10% ± 0.21) was the one compatibilized with 10 wt.% C3. Since the study aimed to add value to the recycled plastic, the focus was solely on C3compatibilized blends. The S12 combination in the sMEP had the smallest loss in tensile strength, at −7%. This result aligns with the findings of Ahmadlouydarab and colleagues, who observed similar behaviour in PP/PET blends compatibilized with PP-g-MA [18]. sMEP 2%C1 5%C1 10%C1 2%C2 5%C2 10%C2 2%C3 5%C3 10%C3 10%C3S1 10%C3S2 10%C3S1S2 0 100 200 300 400 500 sMEP blend Secant modulus (MPa) Figure 1. Secant moduli of the sMEP blends. Polymers 2024,16, 3441 7 of 15 3.3. Morphology All blends exhibited a continuous PE phase with dispersed PET and PP droplets. Figure 2compares the images of sMEP and the C3-compatibilized blends magnified 5000 × . The heterogeneity of the fractured section appears to decrease with increasing C3 content. Regions and large droplets of the dispersed phase tend to disappear from the 5 wt.% C3 content onward. The PP and PET dispersed phases are not easily distinguishable in these images. After extracting the PET phase, the dispersed phases of the sMEP could be accurately assessed. Polymers 2024, 16, x FOR PEER REVIEW 7 of 16 Figure 1. Secant moduli of the sMEP blends. 3.3. Morphology All blends exhibited a continuous PE phase with dispersed PET and PP droplets. Figure 2 compares the images of sMEP and the C3-compatibilized blends magnified 5000×. The heterogeneity of the fractured section appears to decrease with increasing C3 content. Regions and large droplets of the dispersed phase tend to disappear from the 5 wt.% C3 content onward. The PP and PET dispersed phases are not easily distinguishable in these images. After extracting the PET phase, the dispersed phases of the sMEP could be accurately assessed. Figure 2. SEM images of sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 (×5000). The SEM images of the extracted samples (Figure 3) showed a decrease in PET droplet size as the C3 content increased. These smaller droplets provided a better interface between the PET fraction and the polyolefin of the blend. The droplet size results were presented as the average droplet size per material. The uncompatibilized blend showed an average droplet area of 70 µm2, while the 2 wt.% C3 had 64 µm2, the 5 wt.% C3 had 48 µm2, and the 10 wt.% C3 showed the smallest droplet size of 32 µm2, equivalent to a 54% reduction in the average droplet size of the PET dispersed phase. A histogram was prepared to show the distribution of droplet sizes by area range (Figure 4, Table 3). Areas above 200 µm2 correspond more to regions than droplets and tend to disappear in the 10 wt.% C3 blend. In the ×5000 magnification images (Figure 5), PP crystals formed at the interface between PE and PET can be observed. The sMEP exhibited a partial wetting morphology regarding the phase size and shape [43]. Figure 2. SEM images of sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 (×5000). The SEM images of the extracted samples (Figure 3) showed a decrease in PET droplet size as the C3 content increased. These smaller droplets provided a better interface between the PET fraction and the polyolefin of the blend. The droplet size results were presented as the average droplet size per material. The uncompatibilized blend showed an average droplet area of 70 µ m 2 , while the 2 wt.% C3 had 64 µ m 2 , the 5 wt.% C3 had 48 µ m 2 , and the 10 wt.% C3 showed the smallest droplet size of 32 µ m 2 , equivalent to a 54% reduction in the average droplet size of the PET dispersed phase. A histogram was prepared to show the distribution of droplet sizes by area range (Figure 4, Table 3). Areas above 200 µ m 2 correspond more to regions than droplets and tend to disappear in the 10 wt.% C3 blend. In the × 5000 magnification images (Figure 5), PP crystals formed at the interface between PE and PET can be observed. The sMEP exhibited a partial wetting morphology regarding the phase size and shape [43]. Polymers 2024,16, 3441 8 of 15 Polymers 2024, 16, x FOR PEER REVIEW 8 of 16 Figure 3. SEM images of PET-extracted sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 (×1000). Figure 4. Histogram of PET droplet size distribution by area range. Figure 3. SEM images of PET-extracted sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.% C3 (×1000). Polymers 2024, 16, x FOR PEER REVIEW 8 of 16 Figure 3. SEM images of PET-extracted sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 (×1000). Figure 4. Histogram of PET droplet size distribution by area range. Figure 4. Histogram of PET droplet size distribution by area range. Polymers 2024,16, 3441 9 of 15 Table 3. Distribution of PET droplet size (%). Area (µm2) C3 (wt.%) 0 2 5 10 ≤25 41 40 35 55 25–50 27 29 34 31 50–75 10 7 17 9 75–100 5 7 7 1 100–125 4 6 1 2 125–150 3 3 2 2 150–175 2 4 1 0 175–200 1 1 0 0 200–225 1 1 0 0 225–250 0 1 1 0 ≥250 6 2 1 0 Average area (µm2)70 64 48 32 Polymers 2024, 16, x FOR PEER REVIEW 9 of 16 Table 3. Distribution of PET droplet size (%). Area (µm 2 ) C3 (wt.%) 0 2 5 10 ≤25 41 40 35 55 25–50 27 29 34 31 50–75 10 7 17 9 75–100 5 7 7 1 100–125 4 6 1 2 125–150 3 3 2 2 150–175 2 4 1 0 175–200 1 1 0 0 200–225 1 1 0 0 225–250 0 1 1 0 ≥250 6 2 1 0 Average area (µm 2 ) 70 64 48 32 Figure 5. SEM images of PET-extracted sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 showing PP crystals formed at the interface of PE/PET (×5000). Yellow arrows show PP crystals developed at the interface between PE and PET. The compatibilization mechanism is illustrated in Figure 6, adding compatibilizers to immiscible blends decreases the interfacial tension between incompatible polymers. Due to the non-polar nature of polyolefins (PE and PP) and the polar nature of PET, it is crucial to select the appropriate compatibilizer for each system. Thus, compatibilizers Figure 5. SEM images of PET-extracted sMEP, sMEP2C3, sMEP5C3 and sMEP10 wt.%C3 showing PP crystals formed at the interface of PE/PET ( × 5000). Yellow arrows show PP crystals developed at the interface between PE and PET. The compatibilization mechanism is illustrated in Figure 6, adding compatibilizers to immiscible blends decreases the interfacial tension between incompatible polymers. Due to the non-polar nature of polyolefins (PE and PP) and the polar nature of PET, it is crucial to select the appropriate compatibilizer for each system. Thus, compatibilizers with reactive functional groups, such as maleic anhydride (MA), are preferred to connect the polar (PET) and non-polar (PE and PP) phases. As depicted in Figure 6, the hydroxyl end groups of