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Article

Upcycling PVC and PET as Volume-Enhancing Functional Fillers for the Development of High-Performance Bio-Based Rigid Polyurethane Foams

by
Princess Claire D. Ochigue
1,2,
Roger G. Dingcong, Jr.
1,
John Christian S. Bondaug
1,2,3,
Brian Christian G. Magalong
4,
Gerard G. Dumancas
5,6,
Carlo S. Gutierrez
7,
Arnold C. Alguno
8,
Roberto M. Malaluan
1,9,
Arnold A. Lubguban
1,9,* and
Hernando P. Bacosa
1,2
1
Center for Sustainable Polymers, Mindanao State University–Iligan Institute of Technology, Iligan City 9200, Philippines
2
Environmental Science Graduate Program, Department of Biological Sciences, Mindanao State University–Iligan Institute of Technology, Iligan City 9200, Philippines
3
Main Campus, Mindanao State University, Marawi City 9700, Philippines
4
Sustainable Resource Engineering Research on Construction Technologies, Mindanao State University–Iligan Institute of Technology, Iligan City 9200, Philippines
5
Honors College, North Carolina A&T State University, 1601 East Market Street, Greensboro, NC 27411, USA
6
Department of Chemistry, New Science Building, 1601 E. Market Street, Greensboro, NC 27411, USA
7
Comparative Asian Studies, National University of Singapore, Singapore 11926, Singapore
8
Department of Physics, Mindanao State University–Iligan Institute of Technology, Iligan City 9200, Philippines
9
Department of Chemical Engineering and Technology, Mindanao State University–Iligan Institute of Technology, Iligan City 9200, Philippines
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(19), 8540; https://doi.org/10.3390/su16198540
Submission received: 21 August 2024 / Revised: 14 September 2024 / Accepted: 19 September 2024 / Published: 30 September 2024
Browse Figures
Versions Notes

Abstract

:
Polyvinyl chloride (PVC) and polyethylene terephthalate (PET) contribute significantly to global plastic waste, with only 9% recycled in recent years. In this work, these plastic wastes were upcycled as functional fillers to improve the rigid polyurethane foam (RPUF) properties. To attain this target, we leveraged the intrinsic polarity of the C=O and C-Cl groups of PVC and PET to induce intermolecular attractions with the N-H groups of the polyurethane matrix, evidenced by the observed IR peak shifts. This enhanced the nucleating effect during foaming, increasing the foams’ compressive strengths by 77% and 22% with the addition of 10% PVC and 5% PET filler, respectively. Furthermore, the addition of PVC and PET fillers increased the foam volume. Thus, the collective utilization of PPW and its corresponding impact on the CO-based RPUF’s properties signifies a reduction in carbon dioxide emissions by 14.15% and 17.52% for PVC and PET, respectively. Moreover, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) revealed improved thermal stability and degradation profiles of the produced RPUFs. Overall, this work highlights potential advancement in environmentally responsible upcycling strategies for common end-of-life plastic wastes, while enhancing rigid foam properties.

1. Introduction

Plastics have become integral components of modern society, playing crucial roles in diverse sectors, such as packaging, consumer goods, medical devices, and construction materials [1,2,3]. This surge is attributed to several factors, including the exponential rise in global plastic production, inadequate waste management, and societal preferences prioritizing convenience over ecological sustainability [4,5,6]. The annual global plastic manufacturing, exceeding 300 million metric tons, has resulted in a significant proportion ending up in landfills, thereby complicating waste management practices [7]. Furthermore, the enduring resistance of plastics to microbial breakdown raises alarming concerns about long-term ecological harms, as they degrade into microplastics [8]. Coupled with the imminent threats to soil and water ecosystems, this also amplifies the risk of food contamination [9]. On top of that, releasing harmful compounds, such as bisphenol A, phthalates, polychlorinated biphenyls, and dioxins from plastics, further contributes to environmental hazards [10]. Recognizing the environmental consequences, recycling has emerged as a crucial waste management strategy [11].
Modern recycling methods beyond conventional mechanical processes now incorporate advanced chemical and pyrolysis technologies [12]. However, challenges arise from recycling contaminated or mixed plastic waste streams, limiting the types of plastics and mixed-material products that can be effectively processed [13]. In addition, only approximately 9% of plastics are recycled [14]. Moreover, material downcycling during recycling procedures may also result in lower-quality or less functional products than their original form [15].
Polyvinyl chloride (PVC) and polyethylene terephthalate (PET) are common plastics facing alarming waste accumulation and recycling challenges [16,17]. Recycling PVC poses significant difficulties, primarily due to contamination caused by plasticizers and stabilizers [18]. Furthermore, recycling can generate toxic byproducts such as chlorinated chemicals [19], dioxins [20], furans [21], and heavy metals [22], particularly during high-temperature stages. Similarly, during the sorting processes of PET recycling, challenges arise due to impurities [23] and color variations [24], resulting in diminished mechanical properties of the recycled materials [25].
A recent study highlighted microbial and enzymatic degradation mechanisms, which offer viable approaches to plastic waste management, as certain microbes and enzymes can degrade complex polymers into simpler biodegradable components [26]. However, one major concern with this approach is its economic viability in large-scale operations [27]. While current solutions for addressing environmental issues related to PVC and PET have limitations, upcycling initiatives stand as a promising avenue for plastic waste management by transforming these wastes into high-value products [28,29,30,31]. Cao et al. developed a catalytic process for co-upcycling PVC and PET using a chlorine-containing ionic liquid and a ZnCl2 catalyst [32]. However, notwithstanding the successful conversion of PET into terephthalic acid and 1,2-dichloroethane with high yields, the reliance on chlorine-containing ionic liquid and ZnCl2 may pose drawbacks. The ZnCl2 catalysts are expensive and prone to deactivation due to coke formation; this requires their utilization at elevated temperatures [33]. Hence, it can lead to disadvantages such as prolonged reaction times [34] and laborious methodologies [35], limiting their practical application in large-scale processes.
In this work, we present a simple approach to upcycle end-of-life pulverized PVC and PET wastes, collectively referred to as pulverized plastic wastes (PPWs), as functional fillers for coconut oil (CO)-based rigid polyurethane foam (RPUF). Specifically, this study highlights the influence of PPW fillers on enhancing the RPUF’s physico-mechanical and thermal properties, such as volume increase, morphological features, compressive strength, thermal conductivity, and thermal stability. Moreover, beyond the conventional method of landfilling and energy recovery by incineration, this work demonstrates that upcycling PVC and PET wastes significantly lowers the carbon dioxide emission of the produced RPUFs. Thus, this work promotes more sustainable engineering practices for plastic waste management while offering a potential circular economic advantage in large-scale RPUF production.

2. Materials and Methods

2.1. Materials

In this study, CO was sourced from a local coconut processing facility and served as a key starting material for polyol synthesis. The polymeric methylene diphenyl diisocyanate (MDI, PAPI 27, Dow Chemical, Manila, Philippines) with an NCO functionality and content of 2.7 and 31.4 wt%, respectively, along with calcium oxide (CaO), zinc oxide (ZnO), silicone surfactants (INV 690), and catalyst (Polycat 8), all provided by The Dow Chemical Company, were utilized in the synthesis process. Additionally, refined glycerol from Chemrez Technologies Inc. and reagent-grade diethanolamine (DEA) from Ajax Finechem played essential roles in the experimental setup.

2.2. Collection and Preparation of PVC and PET Wastes

Samples of PVC and PET were identified and carefully collected from household waste and subjected to thorough cleaning before being mechanically crushed into a fine powder. The pulverized plastic materials passed through an 850-µm sieve and were retained in a 425-µm sieve screen for uniformity. Fillers with sizes ranging from 425 µm to 850 µm were selected for RPUF application based on prior research, suggesting that large filler sizes could impede polyurethane (PU) matrix formation, potentially diminishing their efficacy as fillers [36]. On the other hand, further size reduction would demand significant energy input [37]. Moreover, the heat generated from the friction between smaller plastic particles could potentially cause them to melt and agglomerate, compromising their workability.

2.3. Preparation of Coconut Oil-Based Polyol

The polyol employed in this study was synthesized through sequential glycerolysis and amidation processes involving CO triglycerides, as illustrated in Figure 1a,b. The glycerolysis of CO was performed with the addition of 0.05 wt% CaO catalyst in a closed Parr reactor, maintaining a temperature of 220 °C for 2 h with continuous and highspeed stirring at around 800 rpm [38]. This process converts CO triglycerides to coconut monoglycerides (CMGs) by reacting with glycerol, as shown in Figure 1a. Subsequently, the produced CMG was reacted with diethanolamine using a 0.15 wt% ZnO catalyst while maintaining 140 °C temperature for 4 h with constant agitation at a similar speed, producing a dark yellow CO-based polyol Figure 1b.

2.4. Foam Formulation

The CO-based polyol was blended with PPW fillers at different weight percent loadings (0, 5, 10, 15, 20) along with the catalyst, surfactant, and blowing agent, forming the B-side component of the RPUF formulation, while the A-side consisted of an MDI. The A-side and B-side components were reacted according to the modified formulation of Dingcong et al., (2023) [39], as detailed in Table 1. The RPUF formation generally involves the reaction of the hydroxyl (OH) groups and -N=C=O present in the polyol and MDI, respectively, forming the urethane segments as shown in Figure 1c,d [38,39]. Specifically, the PPW was blended with CO-based polyol using a mechanical stirrer at 1000 rpm for 10 min. Subsequently, a catalyst and surfactant were added dropwise to the mixture while maintaining a 2500 rpm stirrer speed for another 5 min [40]. The MDI was then introduced and vigorously stirred at 4000 rpm for 10 s [39]. Finally, the resulting mixture was poured completely into a mold and cured in ambient air for at least seven days before being cut into the desired dimensions for characterization. The PPW-filled RPUF samples were labeled according to the PPW/polyol mixture and its corresponding ratio in terms of weight percent (e.g., PU/PVC 5 wt% is an RPUF with 5% PVC filler).

2.5. Polyol Analyses

The OH value of the synthesized CO-based polyol was determined using the ASTM D4274 test method D [41]. The rotational viscosity of polyol samples was assessed using an AMETEK Brookfield DV3T rheometer (Middleborough, MA, USA) following the ASTM D4878 guidelines [42]. The torque was set between 30% and 40%, and the temperature was maintained at 25 ± 0.1 °C. Furthermore, the 1H-NMR spectra of the produced CO-based polyol were obtained using a Bruker Avance 600 MHz cryoprobe NMR spectrometer, with dimethyl sulfoxide (DMSO-d6) (0.4 mL) serving as the solvent for the CO-based polyol sample (150 mg).

2.6. Characterization of Rigid Polyurethane Foam

In this study, the functional group analysis of the RPUF was performed using an IR Tracer-100 spectrometer (Shimadzu, Kyoto, Japan) equipped with a diamond crystal ATR unit (QATR 10, Shimadzu, Kyoto, Japan). Thin foam samples were subjected to data collection for 40 scans, with wavenumbers from 400–4000 cm−1, at a 2 cm−1 resolution. The spectra for all foams were baseline-corrected, normalized (δN−H vibration at 1470 cm−1), and plotted using Origin 2019 software. The apparent densities of the RPUF specimens were calculated using ASTM D1622 [43]. The close cell content was calculated from the pycnometric density, which was obtained using a pycnometer (Quanta chrome ULTRAPYC 1200e, FL, USA) following ASTM 6226 [44]. The percent volume increase was determined according to the uncut RPUF’s apparent density at different PPW filler concentrations relative to the pristine RPUF. The reduced CO2 production emissions were derived from the percent volume increase of the RPUF according to the method established in the literature [45]. This reduction is calculated by assessing the decreased raw material requirements for producing the same volume of rigid foam with PPW compared to pristine RPUF.
The cellular morphology of the RPUF samples was examined through scanning electron microscopy (SEM) using JEOL JSM-6510LA (JEOL, Ltd., Tokyo, Japan). The analysis of each SEM image, containing a scale bar for reference, involved the examination of cells using ImageJ to calculate the average cell diameter and area.
The mechanical properties of prepared RPUF samples were characterized using a Shimadzu universal testing machine AGS-X Series in accordance with the ASTM D1621-04a [46]. Specifically, a specimen was cut into cubes (50 mm × 50 mm × 50 mm) for each sample. Additionally, the thermal conductivity of each RPUF sample (sample size 150 × 150 × 20 mm) was measured with a FOX 200 heat flow (Laser-179 Comp, Wakefield, MA, USA) following the ASTM C518-2017 standard [47]. Lastly, the thermogravimetric analysis (TGA) was carried out using a Shimadzu DTG 60H (Shimadzu Corp., Kyoto, Japan) in a nitrogen atmosphere. This thermal analysis used 5 to 10 mg samples, with heating rates set at 10 °C/minute from 45 °C to 800 °C.

3. Results and Discussion

3.1. Preparation of Polyol

The chemical properties of the polyol crucial for its application in RPUF production were investigated, as detailed in Table 2. The results revealed an OH number of 332 mg KOH/g, indicating the polyol’s classification for RPUF production [48]. This OH number is vital for calculating the required amounts of isocyanate in the RPUF formulation [49], emphasizing its role in the synthesis process. Additionally, the minimal acid value of 1.5 mg KOH/g suggests a tolerable presence of acidic impurities within the polyol [50], reducing the potential for competition with isocyanate during urethane formation. Moreover, the reported viscosity of 1750 mPa·s implies the polyol’s sufficient workability [51], which is essential for practical handling and processing in RPUF manufacturing. Collectively, these findings emphasize the suitability of the polyol for RPUF production, highlighting fundamental properties essential for its successful application in polyurethane foam synthesis.

3.2. Viscosities of Polyol and PPW Blends

The influence of the PPW filler concentrations on the rheological properties of B-side components was investigated, and PVC and PET were compared to assess the workability of the mixture under different weight percent loadings. The results indicate an increase in viscosity as the PPW concentration rises, particularly evident at a weight ratio of 10%, where the viscosity of CO-based polyol with PPW increased significantly from 252 mPa·s to 910 mPa·s and 907 mPa·s for PET and PVC, respectively, in comparison to pure CO-based polyol, as shown in Figure 2. This observed behavior can be attributed to the distinctive chemical features of PPW. Specifically, the presence of C-Cl branches in the PVC structure promotes chain entanglement within the B-side component, thus increasing the mixture’s viscosity [52]. On the other hand, the intrinsically longer polymer chain of PET compared to PVC results in a comparatively higher viscosity when blended with CO-based polyol [53]. Overall, these findings suggest that the optimal PPW loading should not exceed 20% by weight of polyol to maintain the desired workability of the B-side mixture, offering practical information for the preparation of PPW-filled RPUFs.

3.3. Chemical Structure of RPUF

The interactions between PU and PPW fillers are illustrated in Figure 3. These interactions are governed by the inherent polar nature of both materials, promoting various intermolecular attractions, including dipole–dipole and hydrogen bonding [54]. These phenomena are consistent with the previous reports on the interaction of PU/PVC and PU/PET blends [55,56]. Specifically, the intrinsic polarity of C=O in both PVC and PET induces a hydrogen-bonding type of intermolecular attraction toward the N-H group of the urethane segments in the PU matrix, as shown in Figure 3a,b [20]. Moreover, the presence of C-Cl groups in PVC promotes dipole–dipole attraction with the free hydrogen in the PU matrix (Figure 3a), thereby enhancing the overall intermolecular attraction between PU and PVC [57].
Figure 4 presents the FTIR spectra of the prepared RPUF samples, revealing the presence of their characteristic functional groups. The absorption bands at 3319 cm−1 (N-H and O-H stretching), 2926 cm−1 (C-H asymmetric stretching), and 2860 cm−1 (C-H symmetric stretching) can be found for both PU/PVC and PU/PET in Figure 4a,d, respectively. The observed reduction of these absorption peak intensities for both samples is attributed to the increased concentration of PPW fillers [58]. Moreover, the intermolecular attraction between the PPWs and RPUF matrix can be found through the distinct peak shifts of the IR spectra from higher to lower wavenumbers, as shown in Figure 4b,c,e for both PU/PVC and PU/PET samples. Specifically, Figure 4b reveals the peak shifts of C-Cl groups at 900–850 cm−1, signifying the presence of dipole–dipole intermolecular attractions, attributed to the C-Cl groups of PVC [55]. In addition, Figure 4c,e,f correspond to the IR spectra of C=O at 1750–1675 cm−1 for both PU/PVC and PU/PET. The observed peak shifts of C=O at increasing PPW filler concentration are associated with the presence of hydrogen bonding between C=O and N-H groups for both samples [59]. Noticeably, a greater shift in wavenumber is observed in Figure 4b,c relative to Figure 4e. This provides insights into the stronger intermolecular attractions in PU/PVC (both hydrogen bonding and dipole–dipole) compared to PU/PET (hydrogen bonding only) [60]. Moreover, the enhanced peak intensities associated with the C=O and C-O stretching vibrations at 1700–1780 cm−1 and 1590–1650 cm−1, respectively, imply the increasing concentration of urethane linkages with the addition of PPW fillers. These spectral features provide insights into the increased crosslinking densities, which can significantly enhance the physico-mechanical properties of the produced RPUF samples [61].

3.4. Morphological Structure of RPUF

Figure 5 shows representative SEM images of the RPUF samples with different PPW filler concentrations, revealing their corresponding microphotographs and cell size distributions. Noticeably, the average cell diameter decreases with increasing concentration of PPW fillers, as depicted in cell size distribution in Figure 5c,f,i,l,o. This phenomenon is attributed to the enhanced nucleating effect of PPW fillers, which are firmly attached to the foam cell walls, as shown in Figure 5b,e,h,k,n. This interaction promotes the formation of smaller cell sizes [62], as illustrated in Figure 5g,m as compared to Figure 5a,d,j having 0 wt% and 5 wt% filler, respectively [63].
During the foaming process of RPUFs, the intermolecular attraction exhibited by PVC and PET plays a significant role in enhancing cell nucleation, which has been known to regulate the dispersion of polymer chain networks [64,65,66,67,68]. This nucleation phenomenon reduces the surface energy of PU chains, thereby lowering the energy requirement for both chain growth and formation [69,70]. Consequently, more crosslinks are formed, as evidenced by the increasing peak intensity of C=O and C-O (Figure 4f), leading to an increased number of cells compared to pristine RPUFs, as shown in Figure 6. This enhanced crosslinking activity consequently promotes cell size reduction [71], which is known to influence the foam’s physico-mechanical properties [36] directly. However, the interference of PPW fillers in the nucleation and expansion process leads to incomplete cell closure [72], potentially resulting in an increase in open cell content at higher PPW filler concentrations, evidenced by the SEM images shown in Figure 5.

3.5. Physico-Mechanical and Thermal Properties of RPUF

The evaluation of RPUF’s physico-mechanical and thermal properties is essential, as it determines its suitability for various applications. Figure 7 illustrates the effect of increasing PPW fillers on the percent closed cell content, apparent density, and thermal conductivity of the prepared RPUF samples. Specifically, the decrease in closed cell content (as shown in Figure 6 and Figure 8) facilitates convective heat transfer, increasing thermal conductivity, as depicted in Figure 7 [73]. It is worth noting that despite these increases in thermal conductivities, the PPW-filled RPUF’s insulative performance remains comparable, if not better, with that of commonly used thermal insulation materials, such as mineral wool and wood fibers [74,75].
Moreover, variations in PPW filler concentration significantly affect the apparent density. For instance, while the apparent density of the pristine RPUF is 41 kg/m3, the addition of 20% PVC and PET fillers resulted in corresponding densities of 53 kg/m3 and 60 kg/m3, respectively. This observation is attributed to the higher density of PPW fillers compared to the pristine RPUF, highlighting the direct effect of PPW filler concentration on the RPUF’s apparent density [74]. These findings are consistent with previous literature, where authors reported an increase in the apparent density of RPUFs due to the addition of various types and concentrations of high-density fillers [36,76,77,78].
Generally, the RPUF’s compressive strength is directly related to its physical properties, such as its percent closed cell content, density, cell sizes, and filler contents [79,80,81,82,83]. This study achieved the highest compressive strengths by PU/PVC 10% and PU/PET 5% with 77% and 22% enhancement, respectively, compared with pristine RPUF (Figure 8a). This can be attributed to the enhanced foam density, reduced cell sizes, and enhanced gap-fillings [84,85,86] (Figure 5) with increasing PPW filler concentration. However, beyond these optimal PPW concentrations, the significant reduction of percent closed cell content compromises not just the thermal conductivity (Figure 7) but also the compressive strength [87]. It is worth noting that although there is a decrease in mechanical strength beyond the optimal concentrations, the compressive strengths of RPUFs with filler concentrations of up to 20% for PVC and 5% for PET still surpass that of the pristine RPUF. This suggests that a cost-effective formulation can be achieved at these filler concentrations, while maintaining desirable RPUF mechanical properties. Notably, beyond 10 wt%, RPUF with PVC fillers shows higher compressive strength compared to PET fillers. This is due to stronger intermolecular interactions in PU/PVC, as shown in the FTIR (Figure 4). The additional dipole–dipole interactions in PU/PVC result in more sites for attraction for filler–matrix adhesion, leading to higher compressive strength compared to PU/PET, which does not possess dipole–dipole interactions [58].
Moreover, a significant increase in foam volume with increasing PPW filler concentrations can be observed in Figure 8b. This phenomenon is rarely documented in the literature dealing with filler-enhanced RPUF properties. For instance, the inclusion of silica powder [76] and calcium carbonate [88] leads to a reduction in volume expansion, as these fillers alter polymerization kinetics and present compatibility concerns with other components in the formulation. Similarly, Ruamcharoen et al.’s study on sago starch fillers found a decrease in foam volume expansion due to increased viscosity in the polyurethane mixture, which slowed down the foaming process [89]. In contrast to these previous works, higher concentrations of PPW in this study have been shown to enhance foam volume significantly—achieving a 16% and 9% increase with 20% PVC and 5% PET-filled RPUFs, respectively, while improving their physico-mechanical and thermal properties. This observation is attributed to the reduced energy requirement induced by the nucleating effect of PPW fillers during chain growth, ultimately resulting in an increase in the volume of the entire PU matrix [90,91]. Ultimately, this significant volume increase suggests a promising economic advantage for large-scale production, as it boosts production volume without raising costs.

3.6. Thermal Stability of RPUF

The TGA was performed to evaluate the effect of PPW fillers on the thermal stability of the RPUF samples presented in Figure 9. It can be observed in the TGA curve that the pristine RPUF gradually loses its elastic modulus at temperatures between 70–220 °C, while the PPW-filled RPUFs exhibit excellent thermal stability, with negligible thermo-rheologic effects at this range of temperature. This suggests that the PPW-filled RPUFs can extend their utility at higher temperatures compared to the pristine RPUFs. Moreover, the initial stage of weight loss primarily involves the degradation of urethane hard segments present in PU/PVC and PU/PET similarly at 220 °C. Interestingly, it is observed that the PPW-filled RPUFs reveal a slower rate of weight loss compared to the pristine RPUFs at increasing temperatures [92]. This observation is expected, since PVC and PET inherently exhibits higher thermal stability compared to pristine RPUF, as reported in previous literature [32,93,94].
Additionally, the DSC analysis in Figure 10 demonstrates that adding PPW fillers to RPUF increases the corresponding glass transition (Tg) of PU/PVC and PU/PET RPUFs compared to the pristine PU, which has a Tg of around 50 °C. This enhancement is due to the rigid and thermally stable nature of PVC and PET [95], which naturally exhibit higher Tg at 65 °C and 70 °C, respectively, restricting the mobility of the PU polymer chains, resulting in stronger interactions between the fillers and the urethane segments [95]. These lead to improved hydrogen bonding, thus requiring more energy to overcome the glass transition, as observed by the increasing shift in Tg. Consequently, the increase in Tg indicates a higher energy requirement for molecular motion, leading to a more rigid polymer RPUF matrix. Hence, the addition of fillers contributes to a more thermally stable foam, with better structural integrity at elevated temperatures.

3.7. Implications to Sustainability

This study’s experiments and trials yielded highly promising results in incorporating end-of-life PVC and PET waste into coconut oil-based products. The findings showcase significant improvements in morphological properties, structural rigidity, and thermal stability, particularly in foam production, offering a viable alternative to the petroleum-derived materials prevalent in today’s market. This innovation holds potential across a spectrum of industrial applications, from insulation panels [96] to structural support [97] and beyond. It is commonly used in appliances such as refrigerators [98], transportation for lighter vehicle parts [99], and even medical equipment [100]. RPUF is also used in renewable energy applications, such as wind turbines and solar panels [101]. Notably, this study explored different weight percentages of filler in rigid foams, indicating versatility for tailored use in different industrial applications, thereby contributing to the ongoing pursuit of sustainable materials and practices.
On the other hand, this study tackles the pressing issue of PVC and PET waste accumulation, a longstanding environmental concern. Addressing this problem aligns with the urgent need for widespread societal action facilitated through institutional initiatives. Exploring potential policy measures such as monitoring and limited bans on plastic manufacturing underscores a commitment to combatting global plastic pollution [102,103]. Furthermore, the integration of pulverized PVC and PET waste into emerging technologies, without resorting to incineration, represents a significant reduction in resource strain across various domains, including thermal, chemical, and labor aspects [104,105].
Moreover, the utilization of coconut oil-based foam technology as a means of repurposing end-of-life plastic waste not only addresses environmental challenges but also establishes a sustainable value chain. By minimizing reliance on non-renewable resources, this approach exemplifies a truly green technology with far-reaching environmental, economic, and societal benefits. The result correlating ‘PVC and PET addition’ to foam volume is likewise promising for sustainable industrial usage. In the short term, a higher volume of foam achieved through cheaper alternatives, such as recycled plastic (infusion), allows for economies of scale in compliance with a green and sustainable market. Economically, the incorporation of PVC and PET fillers into RPUF greatly optimizes raw material use in large-scale production.
From an ecological point of view, integrating these alternative raw materials into RPUF production is highly relevant. To demonstrate the positive environmental effects, CO2 equivalent emissions were calculated, comparing conventional PU raw materials with PPW-filled RPUFs.
Converting GHG emissions to CO2 equivalents (CO2-eq) enables them to be added to provide a cumulative total, which amounts to the embodied carbon or the total amount of equivalent carbon emissions released due to the manufacturing of polyurethane foam raw materials [106]. Petroleum-based RPUF has a carbon footprint of approximately 2860 kg CO2-eq per ton [107]. Replacing the petroleum-based polyol with coconut oil and incorporating PVC and PET fillers reduces this footprint to 2360 kg CO2-eq and 2050 kg CO2-eq per ton, respectively, as shown in Figure 11. Producing one ton of PPW-filled RPUF using 15 wt.% PVC and PET fillers reduces emissions by 14.15% and 17.52%. Hence, the PPW-filled CO-based RPUF produced from this study overcomes the conventional polyurethane foam raw materials when considering the environmental impact. This approach contributes to addressing the issue of plastic waste management and achieving a significant reduction in carbon dioxide emissions, highlighting a cleaner and more sustainable method for RPUF production.
In adopting this innovative approach, certain concerns must be considered. First, careful monitoring is essential to ensure that the grinding process for PVC and PET waste does not inadvertently release harmful byproducts [108,109] into the environment. Additionally, questions regarding the projected lifespan of CO-based RPUFs and the feasibility of accelerating the degradation of end-of-life RPUF waste warrant further exploration [108]. Furthermore, the recyclability of PVC waste integrated into this work requires nuanced evaluation, acknowledging potential contaminants that may impede recyclability [108]. This study offers compelling evidence of the feasibility and sustainability of integrating PVC and PET waste into CO-based RPUF products. By addressing key environmental challenges and leveraging innovative technologies, this approach holds significant promise in advancing the circular economy and mitigating plastic pollution. The fact that existing plastic “wastes” were utilized and integrated into the RPUF production will make the production practical in terms of costs, access to supply, and raw materials, possibly allowing for a cheaper product to be introduced in the market. Thus, this bio-based rigid polyurethane foam has ecological and market potential.

4. Conclusions

This study evaluates the upcycling of end-of-life PPW, specifically PVC and PET wastes, as functional fillers for RPUF production. It was found that the intrinsic polarity of PPW fillers induces a nucleating effect during the PU cell formation, which significantly enhances the physico-mechanical and thermal properties of the produced RPUFs. The characterization results reveal that the highest compressive strengths can be achieved by PU/PVC 10% and PU/PET 5%. Notably, the highest allowable filler concentration can be attained using 20% for PVC and 5% for PET, while maintaining the desirable mechanical properties. In addition to this mechanical enhancement, PPW promotes a significant volume increase by 16% and a 9% increase, with 20% PVC and 5% PET-filled RPUFs, respectively, suggesting a promising economic advantage for large-scale production. Furthermore, there is a significant reduction in carbon dioxide emission by 14.15% and 17.52% at, respectively, 15 wt.% of PVC and PET, compared to the production of conventional PU foam materials and the disposal of these plastic wastes. Moreover, the TGA and DSC results revealed enhanced RPUF thermal stability by incorporating PPW fillers, thereby extending its utility at higher temperatures compared to pristine RPUFs. Overall, this research underscores the potential valorization of end-of-life PVC and PET wastes in material upcycling and advancing the circular economy of plastic waste while providing essential insights into the enhancement of RPUF properties for diverse engineering applications.

Author Contributions

Conceptualization, P.C.D.O. and R.G.D.J.; methodology, P.C.D.O. and R.G.D.J.; software, B.C.G.M.; validation, P.C.D.O., R.G.D.J., A.A.L., H.P.B. and R.M.M.; formal analysis, P.C.D.O., R.G.D.J., B.C.G.M., J.C.S.B., A.A.L. and R.M.M.; investigation, P.C.D.O., R.G.D.J., B.C.G.M. and J.C.S.B.; resources, A.A.L., R.M.M., H.P.B., A.C.A. and G.G.D.; data curation, P.C.D.O.; writing—original draft preparation, P.C.D.O.; writing—review and editing, P.C.D.O., R.G.D.J., A.A.L., R.M.M., H.P.B., G.G.D. and C.S.G.; visualization, P.C.D.O. and R.G.D.J.; supervision, A.A.L., R.M.M., H.P.B., G.G.D. and A.C.A.; project administration, A.A.L. and R.M.M.; funding acquisition, A.A.L. and R.M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding from the Philippine Department of Science and Technology (DOST) through the Niche Centers in the Region (NICER)–R&D Center for Sustainable Polymers, grant number 101-02-0194-2022, and the DOST-SEI Accelerated Science and Technology Human Resource Development Program (ASTHRDP) scholarship.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the Department of Science and Technology for its financial support through the Niche Centers in the Region (NICER)–R&D Center for Sustainable Polymers, as well as the United States Agency for International Development (USAID).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Ideal mechanism for the synthesis of coconut oil-based polyol comprising coconut diethanolamide and glycerol via sequential steps: (a) glycerolysis, (b) amidation of coconut triglycerides, and (c,d) the general mechanism for the formation of polyurethane from the reaction of the hydroxyl group (OH) of polyol and the NCO functional group of isocyanates.
Figure 1. Ideal mechanism for the synthesis of coconut oil-based polyol comprising coconut diethanolamide and glycerol via sequential steps: (a) glycerolysis, (b) amidation of coconut triglycerides, and (c,d) the general mechanism for the formation of polyurethane from the reaction of the hydroxyl group (OH) of polyol and the NCO functional group of isocyanates.
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Figure 2. Viscosities of polyol with increasing weight percentages of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) pulverized fillers, measured at a constant temperature of 25 ± 0.5 °C.
Figure 2. Viscosities of polyol with increasing weight percentages of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) pulverized fillers, measured at a constant temperature of 25 ± 0.5 °C.
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Figure 3. The intermolecular forces of attraction between rigid polyurethane foam (RPUF) and (a) polyvinyl chloride (PVC), which involved both hydrogen bonding (between C=O and N-H) and dipole–dipole interactions (between C-Cl and N-H), while with (b) polyethylene terephthalate (PET) pulverized fillers, only hydrogen bonding (between C=O and N-H) occurred.
Figure 3. The intermolecular forces of attraction between rigid polyurethane foam (RPUF) and (a) polyvinyl chloride (PVC), which involved both hydrogen bonding (between C=O and N-H) and dipole–dipole interactions (between C-Cl and N-H), while with (b) polyethylene terephthalate (PET) pulverized fillers, only hydrogen bonding (between C=O and N-H) occurred.
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Figure 4. FTIR spectra of pure polyvinyl chloride (PVC), pure polyethylene terephthalate (PET), and polyurethane (PU) with different weight percentages of PVC and PET pulverized plastic waste (PPW) fillers show the following: (a) an increase in intensity in N-H, O-H, and C-H bands at different PVC filler concentrations; (b,c) a notable shift in peaks in C-Cl and C=O band, respectively, at different PVC filler concentrations; (d) an increase in intensity in N-H, O-H, and C-H bands at different PET filler concentrations; (e) a shift in peaks at the C=O band at different PET filler concentrations; and (f) C=O and C-O stretching vibrations, indicative of the urethane linkages.
Figure 4. FTIR spectra of pure polyvinyl chloride (PVC), pure polyethylene terephthalate (PET), and polyurethane (PU) with different weight percentages of PVC and PET pulverized plastic waste (PPW) fillers show the following: (a) an increase in intensity in N-H, O-H, and C-H bands at different PVC filler concentrations; (b,c) a notable shift in peaks in C-Cl and C=O band, respectively, at different PVC filler concentrations; (d) an increase in intensity in N-H, O-H, and C-H bands at different PET filler concentrations; (e) a shift in peaks at the C=O band at different PET filler concentrations; and (f) C=O and C-O stretching vibrations, indicative of the urethane linkages.
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Figure 5. SEM micrographs (left and center with 45× and 150× magnification, respectively) and cell size distribution (right) of representative rigid polyurethane foam (RPUF) with (a–c) 0 wt % filler, (d–f) 5 wt% PVC filler, (g–i) 20 wt% PVC filler, (j–l) 5 wt% PET filler, and (m–o) 20 wt% PET filler. The cell size distribution (right) of foam samples was measured using ImageJ, acquired from individual SEM images.
Figure 5. SEM micrographs (left and center with 45× and 150× magnification, respectively) and cell size distribution (right) of representative rigid polyurethane foam (RPUF) with (a–c) 0 wt % filler, (d–f) 5 wt% PVC filler, (g–i) 20 wt% PVC filler, (j–l) 5 wt% PET filler, and (m–o) 20 wt% PET filler. The cell size distribution (right) of foam samples was measured using ImageJ, acquired from individual SEM images.
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Figure 6. Schematic illustration of mechanism induced by the nucleating effect of pulverized plastic wastes (PPWs) on rigid polyurethane foam (RPUF).
Figure 6. Schematic illustration of mechanism induced by the nucleating effect of pulverized plastic wastes (PPWs) on rigid polyurethane foam (RPUF).
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Figure 7. Closed cell content (%), apparent density (kg/m3), and thermal conductivity (W/m-K) of rigid polyurethane foam (RPUF), with increasing weight percentages of (a) polyvinyl chloride (PVC) and (b) polyethylene terephthalate (PET) pulverized fillers.
Figure 7. Closed cell content (%), apparent density (kg/m3), and thermal conductivity (W/m-K) of rigid polyurethane foam (RPUF), with increasing weight percentages of (a) polyvinyl chloride (PVC) and (b) polyethylene terephthalate (PET) pulverized fillers.
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Figure 8. (a) Compressive strength and (b) volume increase of rigid polyurethane foam (RPUF), with various weight percentages of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) pulverized fillers.
Figure 8. (a) Compressive strength and (b) volume increase of rigid polyurethane foam (RPUF), with various weight percentages of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) pulverized fillers.
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Figure 9. TG curves of rigid polyurethane foam (RPUF) with various content of (a) polyvinyl chloride (PVC) and (b) polyethylene terephthalate (PET) pulverized fillers, showing the thermal stability.
Figure 9. TG curves of rigid polyurethane foam (RPUF) with various content of (a) polyvinyl chloride (PVC) and (b) polyethylene terephthalate (PET) pulverized fillers, showing the thermal stability.
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Figure 10. DSC plot of rigid polyurethane foam (RPUF) showing glass transition temperature changes at optimized levels of pulverized plastic waste (PPW) fillers.
Figure 10. DSC plot of rigid polyurethane foam (RPUF) showing glass transition temperature changes at optimized levels of pulverized plastic waste (PPW) fillers.
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Figure 11. Reduction of greenhouse gas emissions of one ton of polymer from cradle to factory gate influenced by the upcycling of end-of-life PVC and PET fillers as foam fillers.
Figure 11. Reduction of greenhouse gas emissions of one ton of polymer from cradle to factory gate influenced by the upcycling of end-of-life PVC and PET fillers as foam fillers.
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Table 1. Formulation of polyurethane foam with PPW.
Table 1. Formulation of polyurethane foam with PPW.
Foam FormulationComponentsConcentration, php a
0% Filler5% Filler10% Filler15% Filler20% Filler0% Filler
B-Side Components
PolyolCoco Diethanolamide100100100100100100
PPW fillerPVC or PET02.557.5100
CatalystPolycat 80.50.50.50.50.50.5
SurfactantINV 690111111
Blowing agentWater222222
A-Side Component
Isocyanate index b of PAPI 27110110110110110110
a Concentrations of each ingredient are expressed in parts per hundred parts (php) of polyol, adhering to the convention where the cumulative total of all polyols amounts to 100 parts. b The isocyanate index denotes the ratio of the utilized isocyanate quantity to the theoretically required amount, multiplied by 100.
Table 2. Properties of the prepared coconut oil-based polyol.
Table 2. Properties of the prepared coconut oil-based polyol.
ParameterValue
OH number332 ± 3 mg KOH/g
Acid Number1.5 ± 0.2 mg KOH/g
Viscosity1750 ± 25 mPa·s
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MDPI and ACS Style

Ochigue, P.C.D.; Dingcong, R.G., Jr.; Bondaug, J.C.S.; Magalong, B.C.G.; Dumancas, G.G.; Gutierrez, C.S.; Alguno, A.C.; Malaluan, R.M.; Lubguban, A.A.; Bacosa, H.P. Upcycling PVC and PET as Volume-Enhancing Functional Fillers for the Development of High-Performance Bio-Based Rigid Polyurethane Foams. Sustainability 2024, 16, 8540. https://doi.org/10.3390/su16198540

AMA Style

Ochigue PCD, Dingcong RG Jr., Bondaug JCS, Magalong BCG, Dumancas GG, Gutierrez CS, Alguno AC, Malaluan RM, Lubguban AA, Bacosa HP. Upcycling PVC and PET as Volume-Enhancing Functional Fillers for the Development of High-Performance Bio-Based Rigid Polyurethane Foams. Sustainability. 2024; 16(19):8540. https://doi.org/10.3390/su16198540

Chicago/Turabian Style

Ochigue, Princess Claire D., Roger G. Dingcong, Jr., John Christian S. Bondaug, Brian Christian G. Magalong, Gerard G. Dumancas, Carlo S. Gutierrez, Arnold C. Alguno, Roberto M. Malaluan, Arnold A. Lubguban, and Hernando P. Bacosa. 2024. "Upcycling PVC and PET as Volume-Enhancing Functional Fillers for the Development of High-Performance Bio-Based Rigid Polyurethane Foams" Sustainability 16, no. 19: 8540. https://doi.org/10.3390/su16198540

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