The sample sheets (2 cm×1 cm) of plastic PP, PET, and PC were cut from transparent plastic cups bought from commercial markets using stainless steel scissors, followed by a gentle rinse with ultrapure water and a thorough drying at room temperature. The PE films were obtained by peeling them off from the PE-coated paper cups following a 3-hour immersion in ultrapure water at room temperature. Subsequently, they were trimmed into sample sheets, as previously described. All the chemicals used in the experiment were chemical pure reagents.

To mimic the usage of plastic containers, the plastic sheets were soaked in ultrapure water at temperatures of 20, 50, 70, and 95 °C, respectively, in glass containers covered with stainless steel lids. These containers then stood in a thermostatic box to maintain the initial temperature for 3 hours. After that, the plastic sheets were gently rinsed with ultrapure water and dried at room temperature. To observe the effect of chemical compositions, solutions with various pH conditions were prepared with hydrochloric acid (HCl) or sodium hydroxide (NaOH) as necessary. Glucose and sodium chloride (NaCl) were selected as typical chemical compositions in food. The plastic incubation experiment with various solution contents was carried out at 20 °C.

The surface morphography of the plastic sheets was examined using a Scanning Electron Microscope (SEM, Sigma 300, Zeiss, Germany). A gold sputtering process was carried out for 60 seconds at a current of 10 mA, then the images were collected at an acceleration voltage of 3 kV. The infrared spectra of plastic sheets were obtained using a Fourier-transform infrared spectrometer (FTIR, Nicolet iS20, Thermo, US) equipped with an ATR accessor. The analysis was performed at a resolution of 4 cm⁻¹, with a total of 32 scans across a wavenumber range from 600 to 4000 cm⁻¹. The surface Zeta potential of plastic sheets was measured using an electrokinetic solid surface analyzer (SurPASS 3, Anton Paar, Austria). The sample sheets were precisely affixed to the upper and lower walls of the sample cell. An Ag/AgCl electrode was connected, with a flushing pressure set at 200 Pa and a test pressure at 600 Pa. A 0.001 mol/L potassium chloride (KCl) solution serves as the testing medium. The solution’s pH within the sample cell was adjusted using a 0.05 mol/L NaOH solution or HCl as necessary. A comprehensive scan was conducted across a pH range of 3 to 10, and each data point was measured four times. The glass transition and melting temperatures of plastic were determined using a Thermogravimetric Analyzer (TGA, TG 209 F3 Tarsus, Netzsch, Germany) and a Differential Scanning Calorimeter (DSC, DSC250, TA, US). The TGA procedure involved heating the sample under a nitrogen atmosphere, commencing at an initial equilibrium temperature of 30 °C and increasing the temperature at a rate of 10 °C per minute up to 600 °C. The DSC procedure also took place under a nitrogen atmosphere, starting at an initial equilibrium temperature of 30 °C, with the temperature increased at a rate of 10 °C per minute to 300 °C, where it was held for 5 minutes, followed by cooling back to 30 °C at the same rate of 10 °C per minute. The crystallinity of plastic was determined using an X-ray diffractometer (XRD, X’Pert3 Powder, Malvern Panalytical, Netherlands) with a Cu target, at a scanning speed of 5° per minute over a range of 5–90°. The XRD pattern of the sample was then fitted using the Jade analysis software to get both the ordered and disordered peaks. The area under the ordered peaks (W_C) and the area under the disordered scattered peaks (W_A) were calculated. The crystallinity of the sample was subsequently determined using the following Eq. (1):

Temperature plays a pivotal role in influencing chemical reactions on plastic surfaces. When plastics are exposed to water, degradation and hydrolysis of plastic tend to occur at high temperatures [22], leading to surface deformations and fragments. Fig. 1 presents the selected SEM images of the studied plastic sheets after undergoing heat treatment. A noticeable increase in surface roughness is evident for both PP and PET sheets after incubating at 95 °C, as compared to those at room temperature. The treated surfaces exhibit a widespread distribution of numerous protrusions, indicating a pronounced effect of elevated temperatures on the plastic’s surface morphology.

The Zeta potentials of PP sheets post-heat treatment are illustrated in Fig. 2(a). At a neutral pH of 7, the PP sheets consistently display a negative surface charge, with Zeta potentials ranging from −51.9 mV to −72.1 mV as the heat treatment temperature is increased from 20 °C to 95 °C. It is easily concluded that more negatively charged functional groups are produced on the surface during the heat treatment at higher temperatures. Previous studies have indicated that plastics are susceptible to oxidative degradation when they absorb sufficient thermal energy, a reaction mechanism that is graphically represented in Fig. 3 [22–24]. The C-C or C-H bonds within polymer chains can be cleaved by the adsorbed energy, resulting in the formation of alkyl radicals. These radicals then react with oxygen, triggering an auto-accelerating cycle of free radical oxidation. This oxidative process leads to the random scission of polymer chains and the creation of oxygen-containing functional groups, such as aldehyde, carboxyl, and hydroxyl groups. Two-dimensional correlation spectroscopy (2D-COS), a versatile technique to extract subtle variations and reveal additional structural changes [25], has been employed here and the synchronous 2D-COS FTIR maps of PP and PET sheets post-heat treatment are presented in Fig. 1(c) and (f). In the case of PP, the sensitive autopeaks at 1104 cm⁻¹ and 1620 cm⁻¹ are identified which are attributed to the stretching vibration of C-OH and asymmetric stretching vibration of COO⁻ [26], verifying the occurrence of oxidation degradation. For PET sheets, the autopeaks at 1706 cm⁻¹, 1256 cm⁻¹, and 1096 cm⁻¹ corresponding to the C=O, C-O-C, and C-OH [26] are found sensitive to the temperature, probably being contributed by the oxidation and hydrolysis of polyester. The potential presence of catalyst residues and unreacted monomers within the plastic matrix may act as catalysts, initiating these degradation processes. This can lead to a heterogeneous degradation pattern across the polymer [27], culminating in the fragmentation and disintegration of the plastic material.

The Zeta potential of the PP sheets remains relatively stable under alkaline conditions, ranging from pH 7 to 10, as depicted in Fig. 2(a). However, the negative charge on the PP surface significantly diminishes with an increase in the acidity of the testing solution. The surface functional groups of the plastic bind with protons in the aqueous environment, thereby neutralizing the negative charges and even resulting in a positively charged surface. The newly formed oxygen-containing groups during the oxidation and hydrolysis processes are likely contributors to this phenomenon. In comparison, the pH-dependent variation in Zeta potential shows a significant dependence on the treatment temperature. PP sheets that have been treated at higher temperatures exhibit an enhanced presence of negative charges and protonatable groups on their surface, indicating the more severe degradation experienced at elevated temperatures by the plastic.

Fig. 2(b) exhibits the Zeta potential of the studied four types of plastic after heat treatment. Similar to the PP sheet, PE and PET show significantly increased presence of negative charges on their surface as temperature increases from 20 °C to 95 °C. Elevated temperatures increase the likelihood of polymer chain oxidative degradation in these plastics, which in turn raises the potential for the release of microplastic particles during everyday use. In contrast, the negative charges of PC sheets vary slightly at the studied temperature range.

To understand the effect of the temperature, the DSC curves of the studied plastics are depicted in Fig. 4(a). The endothermic peaks contributed by crystal melting can be observed from the DSC profiles of PP, PE, and PET, with the corresponding temperature (T_m) of 155 °C, 100 °C, and 240 °C, respectively. While there is no melting peak in the curve of polymer PC, indicating its amorphous structure. The crystallinity data calculated from XRD analysis (Fig. 4(b)) confirms that PC is an amorphous polymer with nearly negligible crystallinity at room temperature. PP, PE, and PET are verified as semi-crystallized polymers with coexisting amorphous regions and crystalline regions in their structure. An amorphous region in the polymer is assumed more labile to oxidative degradation as compared to the crystalline area because of its high permeability to molecular oxygen [22]. The amorphous regions of plastics typically exhibit two forms: the hard, glassy state and the more flexible, rubbery state. These states can interconvert at the glass transition temperature (T_g). The subtle peaks corresponding to T_g are observed at 80 °C and 150 °C in the DSC curves of PET and PC (Fig. 4(a)), respectively. The T_g of PP and PE is below 0 °C as reported [28]. In this experiment, the temperatures of thermal treatment are much lower than the T_g of PC, which may explain the insensitive variation in Zeta potential due to the limited activity of polymer chains in the glassy state. In contrast, PP, PE, and PET exhibit sensitive responses to thermal treatment. As the temperature rises, the flexible polymer chains in the amorphous regions of the rubbery state gain mobility [29], enabling them to move, oxidize, and potentially release small clusters. As a result, the polymer crystallinities of all the semi-crystallized plastics studied increased after heat treatment, as shown in Fig. 4(b). The crystallinity of PP shows a significant increase from the original 70% at room temperature to 83% at 75 °C. This profile parallels the pattern of MPs release from PP-based cups versus temperature, as observed in our previous study [21].

Plastic food containers usually withstand acidic or alkaline conditions, as well as chemical compositions, during their daily use. Although they typically exhibit good stability against chemicals, polymer oxidation and hydrolysis can still occur on the surface. The pH value and salinity of the solution are thought as most important chemical factors influencing plastic degradation [23]. The SEM images of PP sheets, as shown in Fig. 5, exhibit obviously rougher surfaces after subjecting acidic or alkaline solutions than the origin. With regards to the coexistence of pH and sugar/salt conditions, as commonly occurs in food, significantly increased roughness of the PP surface with cracks and debris was observed. These chemical conditions of contacting solution facilitate the destruction of plastic surface. Fig. 6 shows the FTIR spectrum of PP sheets, and the absorption bands of oxygen-containing carbonyl groups in the region of 1630 cm⁻¹ −1720 cm⁻¹ are clearly observed after soaking in various solutions, which confirms the occurrence of polymer degradation. The carbonyl index (CI), calculated from the spectra by dividing the area under the band A_{1630–1720 cm⁻¹} by the area under the band A_{1425–1500 cm⁻¹}, serves as a quantitative indicator of this change. It was found that the CI value increased from 0.0206 for control PP to 0.0598 and 0.0220 for PP samples dealing with HCl (pH 1) and NaOH (pH 10), respectively.

Surface Zeta potentials of PP, PE, PET, and PC sheets after incubating in solutions with various pH levels are summarized in Fig. 7(a). All the plastics studied show enhanced surface negative charges as the pH values move away from neutral conditions, either towards acidic or basic conditions. For example, the surface Zeta potential of PP dramatically decreased from −37.58 mV at pH 7 to −62.17 mV at pH 5 and −53.19 mV at pH 10, respectively. It is identified that the more ionizable functional groups present on the plastic surface after treatment at tougher pH conditions due to accelerated oxidation/hydrolysis reaction. From this point of view, natural conditions may be relatively safer for the use of plastic containing. High concentrations of H⁺ or OH⁻ are capable to catalyze the degradation of plastics [23]. Previous research has observed similar phenomena. Ariza-Tarazona et al (2020) found that an acidic condition (pH 3) favored hydroperoxide formation during photooxidation and accelerated the hydrolytic breakdown of PE polymer [30]. Rosmaninho et al found that acidic conditions significantly improve the rate of surface hydrolysis of PET [31]. In the present work, both acidic and alkaline circumstances are found to stimulate the surface degradation of these food-contact plastics, thus intensifying the release potential of MPs.

Salt and glucose are selected as typical inorganic and organic chemical compositions in food, and the Zeta potential of PP sheets after incubation is shown in Fig. 7(b). The appearance of salt in contacting solutions shows various influences on the PP surface charges depending on the solution pH value. In neutral conditions, a slight change in the Zeta potential is induced; a reduction in the increase in negative charge is attained in acidic solution, while an obviously enhanced negative charge is obtained in alkaline condition with a Zeta potential of −91.32 mV. The role of inorganic ions in plastic degradation is still a matter of dispute. Studies indicated that Cl⁻¹ ions can effectively capture OH· radicals and inhibit the formation of O₂^˙− radical, subsequently reducing the oxidation of plastic [32,33]. However, other publication found that Cl⁻¹ can accelerate plastic degradation by generating highly reactive Cl· and OH· radicals [34]. Taking the results of the present study, the distinct effects of NaCl on the degradation of plastic may be related to the pH condition, and a significant synergistic effect of alkaline and salt is indicated. A variety of DOMs, such as oxalate, citrate, and humid acid, have been proven to promote the environmental aging process of plastics by enhancing to generate hydroxyl radicals [35,36]. Glucose is revealed here to exhibit an acceleration in the increasing negative charge on PP sheets at food-contact conditions at all the pH levels studied, as observed in Fig. 7(b). It is noticed that glucose exhibits a more significant synergistic effect with acidic solution condition probably due to the stronger conversion ability to free radicals, showing a remarkably improved Zeta potential of −85.27 mV.