Melting Boundaries: How Heat Transforms Recycled Bottles into Chemical Time Bombs ^†

Abstract

Plastic recycling, especially of polyethylene terephthalate (PET), is essential for reducing plastic waste and promoting sustainability. This study examines the migration of phthalic acid esters (PAEs) from locally sourced recycled PET (rPET) bottles under high-temperature conditions (24 °C, 50 °C, and cyclic 70 °C) over a period of three weeks. High-Performance Liquid Chromatography (HPLC) analysis revealed increased PAE leaching at elevated temperatures, though levels remained below international safety limits. Thermo-Gravimetric Analyzer (TGA) confirmed that plastic caps exhibit higher thermal stability and decompose more completely than plastic bottles under various thermal conditions, highlighting the influence of material composition and thermal aging on degradation behavior. Findings highlight the importance of proper storage and ongoing monitoring to ensure consumer safety. Future research should investigate alternative plasticizers to improve the safety of PET recycling.

1. Introduction

Plastic recycling, particularly of polyethylene terephthalate (PET), has become increasingly crucial due to the escalating environmental concerns associated with plastic waste accumulation. PET, which is widely used in disposable packaging and products with short shelf-lives, contributes significantly to global solid waste [1,2]. The recycling of PET is essential for addressing environmental pollution, reducing dependence on petrochemical resources, and promoting a circular economy [2,3]. Chemical recycling methods, such as glycolysis, have emerged as promising solutions for PET waste management. These methods offer the potential to transform post-consumer PET into its building block chemicals, enabling closed-loop recycling [1,4]. Additionally, biotechnological recycling approaches involving enzymatic degradation and bioconversion of PET monomers into value-added chemicals present an environmentally friendly alternative to conventional mechanical and chemical processes.

Recycled polyethylene terephthalate (rPET) has found significant applications in the bottled water industry, contributing to sustainable packaging solutions and circular economy efforts. The primary application of rPET in bottled water is the production of new food-grade beverage bottles. Studies have shown that rPET can be safely reprocessed into new food contact packaging applications, including water bottles, for over two decades [5]. This recycling process involves the collection of post-consumer PET beverage bottles, which are then processed and transformed into rPET suitable for food contact materials (FCM) [6]. The recycling process can involve mechanical recycling, chemical recycling, or dissolution/precipitation methods [7]. Interestingly, the recycling process can safely accommodate a certain percentage of non-food PET containers that inevitably enter the PET recycling feed stream. Research indicates that a fraction of up to 5% non-food PET in the recycling feed stream, which is common in usual recollection systems, does not pose any risk to consumers. Even fractions up to 20%, which may occur in certain local recollection systems, have been found to be safe [5].

Phthalic acid esters (PAEs) are synthetic organic compounds that function primarily as plasticizers in the plastics industry, enhancing flexibility and durability during manufacture [8,9]. They are dialkyl or alkyl/aryl ester derivatives of phthalic acid and are characterized as colorless, odorless, and flavorless oily liquids [9]. PAEs are poorly soluble in water but dissolve readily in organic solvents. The most commonly used PAEs include di-(2-ethyl hexyl) phthalate (DEHP), the primary plasticizer in the polymer industry, while dimethyl phthalate (DMP) and diethyl phthalate (DEP) are mainly used as solvents or fixatives in cosmetics and personal care products [9]. Other notable PAEs include di-n-butyl phthalate, benzyl butyl phthalate, and di-n-octyl phthalate [10]. Despite their widespread use, PAEs pose significant environmental and health concerns due to their ability to leach from plastic products during production, use, degradation, packaging, and transportation [9]. This leaching is facilitated by the fact that PAEs are not chemically bound to the plastic polymer, making them prone to migration into surrounding environments [11,12]. As a result, PAEs have become ubiquitous contaminants found in soil, water, air, and biota [9,10]. The presence of PAEs in PET and rPET further complicates their environmental impact [12]. While PAEs improve the mechanical properties of these materials, they also leach easily, raising concerns about human health and ecological risks [12]. PAEs are classified as endocrine-disrupting chemicals and potential carcinogens, making them contaminants of emerging concern. Moreover, exposure to PAEs during prenatal and early developmental stages is particularly concerning, as it can disrupt normal development patterns and alter susceptibility to diseases later in life [13,14]. The public health implications of endocrine-disrupting chemicals (EDCs), including PAEs, are substantial due to their ubiquitous nature and potential long-term effects. These chemicals have been linked to various health issues, including obesity, diabetes, thyroid diseases, and certain cancers [15,16]. Additionally, PAEs may affect fertility, as they are associated with disturbances in steroidogenesis and ovarian follicle development [17]. Given the widespread exposure to these compounds, even small effects on human health could have significant public health consequences, emphasizing the need for further research and regulatory measures to mitigate their impact [18]. Consequently, there is growing interest in developing alternative plasticizers and exploring biodegradation strategies for both PAEs and PET/rPET.

rPET and virgin PET (vPET) exhibit some differences in their chemical stability and polymer properties, primarily due to the degradation processes that occur during recycling. The recycling process of PET can lead to polymer chain degradation, which affects the molecular structure and properties of the material [19]. This degradation is reflected in changes to thermal, mechanical, and crystalline properties. However, the extent of these changes varies, and the properties of rPET often fall within the broad range of vPET properties [19]. The recycling process typically results in a reduction in molecular weight, with weight-average molecular weight (Mw) dropping more noticeably compared to mechanical properties. Interestingly, while there may be changes in the crystallization process between vPET and rPET (such as differences in the size and number of spherulites), the overall crystallinity of the material may remain unchanged [20]. This suggests that the recycling process affects the microstructure of the material, which can impact its mechanical properties and processing behavior. Thermal degradation of PET and rPET involves the breakdown of polymer chains under elevated temperatures. Studies have shown that the thermal stability and degradation mechanisms of PET are influenced by various factors. The heating rate significantly affects the thermal decay curves of PET, with specific temperatures triggering heating and cooling cycles from room temperature to the molten state [21]. PET exhibits a relatively low activation energy for degradation at 93.5 kJ/mol and retains 14 wt% after thermal recycling, regardless of molecular weight [21].

This study aims to investigate the impact of high temperatures, mimicking Qatar’s summer conditions, on the migration of PAEs from locally sourced rPET bottles into water when stored under unknown storage conditions. It will assess how heat exposure affects the structural integrity of rPET bottles and quantify the concentration of PAEs leaching into water at varying temperature conditions. The findings will be compared with international safety limits to evaluate the potential health risks associated with this contamination. Additionally, the study will provide insights into the implications for food safety regulations, public health policies, and environmental risks, particularly with regard to the improper disposal of contaminated water. By addressing these concerns, this research aims to contribute to safer packaging practices and improved regulatory measures for rPET bottle storage and usage in hot climates.

2. Methodology

2.1. Sample Preparation and PAEs Extraction

A study was conducted using two locally recycled 500 mL PET mineral water bottles stored under three different temperature conditions: room temperature (24 °C), elevated temperature (50 °C), and cyclic heating at 70 °C for three weeks. To isolate PAEs that may have leached into the water, a Liquid–Liquid Extraction (LLE) method, based on the protocol outlined by the U.S. Environmental Protection Agency [22], was utilized. The extracted samples were subsequently analyzed using High-Performance Liquid Chromatography (HPLC) Agilent 6530C LC/QTOF-MS (Santa Clara, CA, USA) (system with a reversed-phase column was used under positive ESI mode, using a water–methanol mobile phase (each with 5 mM ammonium formate and 0.1% formic acid), a 10 µL injection volume, 0.6 mL/min flow rate, and a 50–1700 m/z scan range, with calibration prepared from a 1000 ppm methanolic stock diluted to 2.5 ppb., as illustrated in Figure 1.

2.2. Thermal Analysis for the Plastic

After three weeks of exposure to varying thermal stress conditions, the plastic samples were carefully cut and prepared for further analysis. These samples were then sent for Thermo-Gravimetric Analyzer (TGA) was performed using a PerkinElmer (Waltham, MA, USA) Pyris 6 TGA, with each sample analyzed over a temperature range of 25 °C to 650 °C at a heating rate of 20 °C/min under a nitrogen atmosphere; approximately 1 hour was allocated for instrument and sample preparation, and 32 minutes for each analysis run to assess their thermal stability, decomposition behavior, and changes in their thermal properties.

3. Results

3.1. Leaching Result

Figure 2 shows the quantified levels of four PAEs—dibutyl phthalate (DBP), DMP, DEP, and DEHP—in two brands of locally rPET water bottles (Brand A and Brand B), analyzed by HPLC. At 24 °C, Brand A exhibited higher concentrations of DMP, DEP, and DBP, indicating baseline leaching linked to plasticizer content and polymer degradation. Brand B, however, had elevated DEHP levels, likely due to differences in recycling feedstock or additive formulations. At 50 °C, phthalate concentrations generally decreased or stabilized, attributed to volatilization of lower molecular weight phthalates and polymer matrix reorganization. Brand A maintained higher overall phthalate levels, suggesting stronger additive retention or a higher initial load, while DEHP levels remained similar in both brands due to its lower volatility and stronger polymer binding. Under cyclic heating at 70 °C, Brand B showed a significant increase in phthalate release, with DBP rising over fourfold to 6.3 ng/mL, likely due to increased molecular mobility and polymer degradation. Brand B’s greater leaching suggests lower thermal stability or higher recycled content with contaminants. In contrast, Brand A exhibited minor changes, indicating a more thermally stable polymer matrix and stricter manufacturing control. These findings highlight the impact of thermal stress on phthalate migration and the need for quality control in rPET bottle.

3.2. Thermal Analysis Result

The TGA results in Figure 3 indicate distinct differences in the thermal degradation patterns of brand A plastic bottles and caps when exposed to various thermal conditions. Each sample showed a single-step degradation process within the temperature range of about 390 °C to 500 °C, typical of polymer breakdown. Interestingly, plastic caps consistently exhibited greater thermal stability compared to plastic bottles, with degradation starting at temperatures as high as 420 °C under cyclic thermal stress. Conversely, plastic bottles began to degrade at lower temperatures, approximately 390–405 °C, under all conditions. The impact of cyclic thermal stress was notably significant in improving the thermal resistance of the plastic caps, potentially due to structural reorganization or annealing effects. Additionally, residue analysis reveals that plastic bottles consistently left around 1 mg of residual mass, indicating the presence of non-volatile inorganic fillers. In contrast, caps decomposed more thoroughly, leaving little to no residue. These observations are consistent with the expected thermal behaviors of common materials used in bottles and caps, such as PET and polypropylene (PP), respectively. The differences in degradation temperatures and residue support the conclusion that thermal aging and material composition play a crucial role in determining the thermal stability of plastic packaging components.

4. Conclusions

This research confirms that the locally available bottled water brands are safe for human consumption, with detected phthalate levels well below the 6000 ng/mL limit set by the U.S. EPA, demonstrating the effectiveness of Qatar’s regulatory standards and quality control. However, findings also reveal that high ambient temperatures, common during Qatar’s summer, can accelerate the migration of PAEs. Analysis revealed brand-specific differences in the migration of four phthalates—DMP, DEP, DBP, and DEHP—with Brand B showing significantly higher DBP leaching under cyclic thermal stress (up to 6.3 ng/mL), indicating lower thermal stability and the influence of recycling feedstock and additive composition. Although levels remain low, repeated exposure may pose long-term health risks, particularly to vulnerable populations. TGA showed rPET degradation beginning at 390–405 °C, while polypropylene caps exhibited higher thermal stability (~420 °C), indicating material-specific responses. The presence of fillers may also influence leaching behaviour. These results underscore the importance of ongoing monitoring, enhanced material standards, updated regulatory frameworks, and the development of safer, more sustainable packaging alternatives.