A Recycling Alternative for Expanded Polystyrene Residues Using Natural Esters

The objective of this research is to provide a new recycling method for one of the most consumed plastics today, since it is used for the manufacture of a wide variety of industrial products, which leads to an environmental problem caused by incorrect handling and final disposal. The dissolution of expanded polystyrene waste was evaluated by using natural esters for its post treatment and recovery. The use of omega-3 as a natural solvent creates an opportunity to take advantage of natural biomass, since it can be obtained from the residues from fishing activities, this being an economic advantage for obtaining raw material and also friendly with the environment. For the development of this research, expanded polystyrene containers were dissolved in omega-3, glyceryl tributyrate, and ethyl butyrate; methanol and isopropanol were used for its recovery. The characterization of the recovered material was carried out with thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, infrared spectroscopy and X-ray powder diffraction techniques. The experimental data obtained indicated that the use of these esters is a good alternative for the recycling of expanded polystyrene.


Introduction
In the last century, petroleum-based polymers gained importance for human's lives covering different needs of end-consumer products that are essential to modern life [1]. The use of plastic is growing rapidly, in 1950 1.65 million tons were produced worldwide and in 2018 global plastics production almost reached 360 million tons [2]. At current growth rates, the accumulation of plastics waste in landfills and/or in the natural environment is projected to reach nearly 12,000 Mt globally by 2050 [3]. Plastics exhibit many outstanding features such as low density, durability, low cost and excellent moldability [4] and are utilized primarily in packing (39.9%), building and construction (19.8%), the automotive industry (9.9%) with the remainder for inclusion with electronics, furniture, etc. [2]. The excessive use of plastic products has generated enormous quantities of waste, which has become a global issue [5].
Current methods of managing WEP include incineration, landfill, mechanical, chemical, and dissolution recycling. Globally, only 18% of polymer waste is recycled and 24% is incinerated. The incineration of these residues contributes to the generation of considerable amounts of toxic pollutants such as CO, NO x , SO x and polycyclic aromatic hydrocarbons [6]. The remaining 58% is deposited in landfills or enters the natural environment, where plastics accumulate and persist for a long period [3].
Mechanical recycling is employed to reprocess polymeric waste to form a product with the same inherent characteristics, this method has a lower environmental impact, and chemical recycling converts polymeric waste into fuels and chemical feedstocks. Finally, dissolution, despite requiring multiple steps including removal of impurities, dissolution of the polymer, and ultimately re-precipitation, has been identified as the most environmentally friendly and profitable avenue to address the vast amount of plastic waste [7].
Expanded polystyrene is made from small spheres of polystyrene (from crude oil) containing an expansion agent (e.g., pentane C 6 H 12 ) that expands by heating with water vapor and encompasses 98% air and 2% polystyrene [8]. Due to chemical inertness, thermal resistance and insulation, mechanical strength and low manufacturing-cost, it is one of the most widespread polymers around the world. However, its non-degradability and low-density lead to vast volumes of white pollution, which consists of plastic materials, such as foam boxes, bags, and plastic cups. In 2015, 6200 Kt of expanded polystyrene was produced, and just around 40% was reused. Expanded polystyrene waste (WEPW) recycling is managed by two main approaches, incineration and mechanical recycling. Incineration of polystyrene can produce hazardous chemicals while mechanical recycling, despite being more environmentally friendly, has high costs associated with it due to the need for transportation to recycling sites [7]. Another method is dissolving the WEP in organic solvents, such as toluene, xylene, benzene, chloroform, acetone, cyclohexane, butyl acetate, ethyl acetate, and methyl ethyl ketone, reducing its volume ~ 100 times [9]. The use of these organic solvents, while assisting in the reduction of the WEP waste volume, is toxic and produces additional noxious waste [10].
An environmentally friendly alternative to the use of organic solvents involves dissolution of WEP in d-limonene. d-limonene (4-isopropenyl-1-methylcyclohexene) is a biodegradable natural terpene found in citrus peels that dissolves the same amounts of WEP as some organic solvents, but suffers from both low yield and high cost of extraction. Natural essential oils with short dissolution times and good recovery performance of WEP include anise, chamomile, thyme and eucalyptus [11]. On the other hand, fatty acids or also called "omega" are hydrocarbon chains with a carboxylic group and a methyl group at each end of the chain [12]. The omega-3 fatty acids are polyunsaturated and they can be found in food in three main compounds as eicosapentaenoic acid (C20:5n − 3; EPA), docosapentaenoic acid (C22:5n − 3, DPA) and docosahexaenoic acid (C22:6n − 3, DHA) [13]. EPA and DHA are in marine fish that lives in cold water such as tuna, salmon, sardines among other varieties. The α-linolenic acid (ALA) can be obtained from vegetable oils, chia, walnuts, and olives.
It was observed that some commercial fish oils dissolved polystyrene, this effect is attributed to those fatty acids remaining as ethyl esters. In this work, a fish oil from a local drug store was tested. Glyceryl tributyrate and ethyl butyrate were also used to compare both synthetic and natural esters for WEP dissolution performance. Our outlined method represents a low cost and environmentally friendly alternative.

Materials
Ethyl ester fish oil commercial brand PharmaLife Natura ® Fish Oil 1000 mg, was used without additional treatment. Methanol, isopropanol, glyceryl tributyrate and ethyl butyrate were purchased from Sigma Aldrich Company and all of them were used without further purification. The WEPW collected was waste packaging.

Dissolution Process
Weight percent solutions of 10-100% of WEPW with respect to 1 g of ester were used. As soon as the ester was poured into the WEPW, it was constantly stirred. Until complete dissolution, a clear solution was obtained, and the dissolution time was recorded. The experiments were carried out at room temperature.

Recovery of Polystyrene
The clear solution was dissolved in methanol and the polystyrene precipitated. Then the precipitate was filtered and dried on a filter paper. The precipitate was washed four times with isopropanol. To remove ester residues from the recovered WEP, it was left in isopropanol overnight on a mechanical shaker. Finally, the WEP was allowed to dry and stored.

Calculation of Apparent Activation Energy E a
The apparent activation energy of degradation can be determined by methods or kinetic models based on the analysis of the data obtained from the thermograms from the thermogravimetric analysis (TGA) analysis. In this case, the Friedman differential method was applied to calculate the apparent activation energies of degradation of untreated WEP and recovered WEP. The calculations were carried out with the help of Origin 8 software.

Friedman's Kinetic Model
The results obtained from the TGA analysis can be expressed as a function of the conversion or degree of advance alpha (α), according to the Eq. 1: where W 0 corresponds to the initial mass of the sample, W is the mass of the sample at time t and W a is the final mass of the sample.
Kinetic studies use the basic equation of velocity (Eq. 2): The independent temperature conversion function f(α) is represented by Eq. 3: The Arrhenius equation is expressed in Eq. 4: Substituting Eqs. 3 and 4 in Eq. 2: The heating rate β = T/t (K/min) is introduced to Eq. 5, obtaining Eq. 6: Substituting in Eq. 5, the following separable differential equation is obtained: Applying a linearization to Eq. 7 we obtain the equation that allows us to calculate the kinetic parameters based on the data from the thermogravimetric analysis (see Eq. 8). Its graphical representation is shown in Eq. 9 [14].
The ln (dα/dT) was plotted against the inverse of the temperature (1/T), with the slope m = − E a /RT, where m is the slope, R is the constant of ideal gases (R = 8.314 J/ Kmol), T is the temperature and E a is the apparent activation energy of degradation. To obtain a good linear fit, it is necessary to select an appropriate advance degree range α, this is defined as the range where the degradation reaction of the material begins [14], for this is considered dα/dT is a function of α, that is, the graph of (dα/dT) vs α is analyzed. The analysis of these curves will show a maximum in the range of α = 0.4-0.6 (maximum point of the hood where the behavior is linear) [15].

Calculation of the Glass Transition Temperature T g
The calculation of the glass transition temperature was obtained by means of differential scanning calorimetry (DSC) curves of untreated polystyrene and recovered polystyrene, this with the help of Proteus software version 5.2.1.

Characterization
Thermal gravimetric analysis and DSC was carried out with a simultaneous analysis equipment (Netzsch STA 449 F3 Jupiter) and was carried out in the range of 25-540 °C with a ramp of 20 °C/min, a nitrogen atmosphere of 99.999% purity for WEP and WEP recovery. Natural and synthetic esters were analyzed by infrared (FTIR). Data were collected using a Shimadzu IRTacer-100, with attenuated total reflection (ATR) accessory (35 scans, 4 resolutions, 4000-400/cm). Scanning electron microscopy (SEM, JEOL JSM-6510lV) was used to analyze the surface morphology. Untreated and recovered WEPs were characterized using a Rigaku Ultima IV X-ray powder diffractometer equipped with CuKα radiation. The conditions used were a scanning speed of 2°/min, step width of 0.5°, incident slit of 2/3°, with a continuous scanning mode.

Dissolution Curves
The methodology proposed in this work is two steps process, the dissolution of WEP (Fig. 1) and its recovery by precipitation. The curves of dissolution time show how long it takes for WEP to dissolve, starting from an initial concentration of 10% to a concentration of 100% WEP (%w/w) using 1 g of ester (Fig. 2). The results indicate that glyceryl tributyrate takes 130 min to dissolve 100% by weight of WEP (1 g of WEP), while omega-3 and ethyl butyrate take 31 min and 7 min, respectively, indicating that the best solvent is ethyl butyrate, since it dissolves WEP in a shorter time.
Noguchi and co-workers reported that d-limonene essential oil dissolves 30% wt of WEP in 53 min [16]. In a more recent report, Gil-Jasso, used essential oils for the dissolution of WEP, reporting that star anise is the most efficient of the series, and for dissolution of 30% wt of WEP it requires 5 min [11]. In this work, the use of omega-3 under the same conditions required 6 min, glyceryl tributyrate needed 11 min, and lastly ethyl butyrate needed 2 min to fully dissolved WEP. These results confirm that esters are a suitable and competitive alternative to dissolve expanded polystyrene. In all three cases, the curves of the times required to dissolve a certain amount of expanded polystyrene increase exponentially as the concentration of expanded polystyrene C o (%w/w) in relation to the ester increases.
The recovery of the expanded polystyrene consisted of washing with sufficient methanol and stirring until a white solid was obtained, then rinsing with isopropyl alcohol to remove any remaining solvent residue. The WEP was allowed to dry at room temperature and characterized by SEM.

SEM
WEP samples prior to and after dissolution and recovery were analyzed by SEM. Figure 3a shows the WEP prior to treatment, in which a closed cell structure filled with air is observed [17], with cells or holes characteristic of the expanded polystyrene [18]. After treatment, this feature is no longer present; micrographs show a rough, airless surface with irregular stacked layers or sheets (Fig. 3b, c), indicating reduced WEP volume.

FTIR
The characterization of the esters used for the dissolution of WEP by the IR technique is shown in Fig. 4 all the characteristic bands of polystyrene are present in each of the samples indicating a good recovery of the WEP. However, in spectrum (b) a band around 1740/cm is observed, this band is due to the ester still present in the sample, so the result suggests an extra treatment to be able to eliminate traces of the ester used.

X-Ray Powder Diffraction (XRD)
The recovered materials and the untreated WEP were subjected to powder X-ray diffraction analysis. Figure 5 shows the comparison of the diffraction patterns for each sample.
The powder X-ray diffraction patterns indicate the presence of an amorphous material within both the untreated and recovered WEP samples. The results indicate that the recovery process does not cause the material to undergo a phase change.

TGA
In Fig. 6, the results of the Thermogravimetric Analyses representing % mass loss as a function of temperature from samples of untreated WEP and as well as WEP recovered from omega-3, glyceryl tributyrate, and ethyl butyrate are shown. Untreated WEP shows only a 99.29% mass loss, with a degradation temperature of ~ 360 °C [19,20]. For WEP recovered from omega-3, two mass losses were observed: the first mass loss of 3.97% in the temperature range 110-285 °C corresponds to persistent omega-3 remaining in the sample while the second loss of 94.6% at ~ 414 °C is associated with the degradation temperature of the recovered WEP.
In the case of WEP treated in glyceryl tributyrate, two slopes are similarly observed; the first mass loss of 19.29% at 120 °C is due to the presence of the glyceryl tributyrate remaining in the sample. This can be corroborated since the boiling point of glyceryl tributyrate is approximately 174 °C. The degradation temperature of the recovered material is 408 °C, at which point the second slope correspond to the loss of mass of 68.39%, which belongs to the amount of recovery of the expanded polystyrene in the sample.
Finally, for the WEP treated in ethyl butyrate, two slopes are observed: the first mass loss of 2.66% in the temperature range 110-225 °C, due to the presence of residual ethyl butyrate in the sample (bp = 120 °C). The degradation temperature is 364 °C, in which the second mass loss of 96.89% is observed, a mass percentage that is related to the amount of WEP recovered.

DSC y T g
In Fig. 7 the curves for untreated WEP as well as WEP samples recovered after dissolution in omega-3, glyceryl tributyrate, and ethyl butyrate as determined by Differential Scanning Calorimetry are presented, showing an exothermic transition in all cases. Through the thermogram obtained from the DSC analysis, the glass transition temperatures T g of the four samples were calculated (see Table 1), the value obtained from the T g of the WEP untreated is 106.7 °C, a temperature similar to that already reported (101-102 °C) [21]. Despite the recovered WEP samples having glass transition temperatures different from that of untreated WEP, the presence of a measurable T g implies that the amorphous nature of the recovered polymeric material was conserved.

Apparent Activation Energy of Degradation E a
Activation energy was calculated with Friedman's differential method. This method compares the rates of weight loss (dα/dT) for a fractional weight loss with a certain rate of heating β (K/min) [22].
In this work, the analysis was done with a heating rate β of 20 °C/min (293.15 K/min) for the four samples. According to the results, the value E a of the WEP without treatment was 268.5 kJ/mol, this value enters the range of 240-275 kJ/ mol that is reported in other work for experimental conditions under nitrogen flow, with an average of 245 kJ/mol [23]. WEP values recovered show variations to the approximate value of the WEP untreated, this could be due to the remnants of esters employed for each test, as these are still present in the polymer. The results are shown in Table 2.
On the other hand, different ranges and values of the WEP E a have been reported, such as values of 125-147 kJ/mol with an average of 138.39 kJ/mol [24], 100-107 kJ/mol with an average of 104.31 kJ/mol and 126.52 kJ/mol [25], 46-170 kJ/ mol with an average of 89.62 kJ/mol [26] and 60-100 kJ/mol [27], these variations are due to the experimental conditions, such as, nitrogen or air flow, the temperature and the heating rate, as well as the kinetic model with which the values were obtained of the E a . However, studies show that the degradation reaction of WEP in a nitrogen or air environment is carried out in a single reaction step [23,24].

Conclusions
Herein we outlined that the esters omega-3, glyceryl tributyrate, and ethyl butyrate can satisfactorily dissolve WEP residues as evidenced by their times to effect complete WEP dissolution and ultimate recovery. The various analytical techniques, among them FTIR, PXRD, and SEM, demonstrated that the WEP recovered after dissolution in the esters did not undergo significant structural changes during the process as characteristic bands and structural features remained present in the material upon recovery. While reagent grade solvents demonstrated faster dissolution times, the use of natural esters derived from biomass from fishing offers a new green and environmentally-benign option for the mass recycling of WEP without generating toxic residues linked with organic solvents. With the continued expansion of WEP use globally, new and novel methods to reduce and reuse this material will be an ongoing field of research for many years to come.