1. Introduction
Globally, the current generation rate of municipal solid waste (MSW) has been reported as 2.01 Bt/year and it is estimated to reach 2.59 Bt/year by 2030 [
1]. However, in Saudi Arabia, the reported generation rate of MSW in 2014 was is 15.3 Mt/year and it is expected to be double by 2033 [
2]. Plastic wastes, as one of the major ingredients, represent 5.2% of the MSW in Saudi Arabia [
3]. While incineration and landfill of waste plastics have some environmental issues, including harmful immissions and products [
4], pyrolysis products are more promising with a low volume of produced gases [
5]. However, to produce the desired type of oil, pyrolytic catalytic cracking is employed [
6,
7,
8].
Extensive research on the pyrolysis of a single type of plastic waste such as high-density polyethylene (HDPE) [
8,
9], low-density polyethylene (LDPE) [
9,
10], polyvinyl chloride (PVC) [
9,
11,
12,
13], polypropylene (PP) [
9,
12,
14], polycarbonate (PC) [
15], and polystyrene (PS) [
9,
16,
17] has been conducted. Although mixed plastic waste is the representative type of plastic waste worldwide, a limited number of works on the pyrolysis of mixed plastic waste plastic has been performed. In Saudi Arabia, plastic wastes contain mainly LDPE, HDPE, PS, PVC, and PP [
18].
Although plastic recycling has attracted the attention of many researchers, it is economically and technically challenging to recycle mixed plastic wastes [
19]. In addition, they cannot be easily separated from each other [
20]. However, the reaction mechanism of the pyrolysis of mixed plastic may be different due to the interaction between the mixture constituents [
19].
Chowlu et al. (2009) [
17] studied the pyrolysis behavior of a mixture of two polymers PP and LDPE with five different mixture compositions and heating rates. The Vyazovkin model, a model-free technique, was used to investigate the effect of conversion on the thermal decomposition activation energy. The relation passed through three different zones: slow at low conversions, slightly high at the middle-range of conversions, and strongly high till the end of the decomposition. Low activation energy, which is preferred for any reaction, was reported for a mixture of 65 wt% PP/35 wt% LDPE.
Aboulkas et al. (2010) [
21] studied the conversion model of the thermal decomposition of HDPE, LDPE, and PP by Coats–Redfern and Criado methods. It was reported that the Criado model described the “Contracting Sphere” model for HDPE and LDPE, and the “Contracting Cylinder” model for PP.
Silvarrey and Phan (2016) [
4] developed a kinetic model to find the real reaction mechanism of the decomposition of five different polymers: HDPE, LDPE, PP, PS, and polyethylene terephthalate (PET) using TGA and MATLAB. Kissinger–Akahira–Sunose (KAS), Malek, and linear model fitting methods were used to predict the thermogravimetric analysis (TGA) data of the pyrolytic process of these five polymers. TGA data at four heating rates (5, 10, 20, and 40 K/min) covering the temperature range of 30–700 °C were reported. It was reported that all polymer wastes showed a similar thermogravimetric (TG) shape with one stage of thermal decomposition. However, the characteristics of TG temperatures (Tonset, Tpeak, and Tendset) were slightly different for the five tested plastic samples. The average values of the obtained activation energy were as follows: PS = 192.6 kJ/mol, PET = 197.6 kJ/mol, PP = 261.2 kJ/mol, LDPE = 267.6 kJ/mol, and HDPE = 202.4 kJ/mol.
Yu et al. (2016) [
19] reviewed some of the published works investigating the pyrolysis of polyvinyl chloride (PVC) mixed with PP, polyethylene (PE), or PS. The effect of the added polymer on the mixture onset temperature, peak decomposition temperature, endset temperature, residue weight, and quantity, was reported. Different effects by different polymers were attributed to the nature of the added polymer.
Anene et al. (2018) [
22] studied the thermal degradation of a mixture of LDPE and PP at different compositions. It was reported that pyrolysis started at lower temperatures for the LDPE/PP mixture when compared with pure LDPE, proving the interaction between the mixed polymers. However, the pyrolysis of pure PP started earlier than that of pure LDPE.
Mumbacha et al. (2019) [
23] investigated the decomposition of plastic solid waste (PSW) by TG under inert conditions from 25 °C to 1000 °C with four heating rates (5, 10, 20, and 30 K/min). It was reported that the feedstock of PSW includes: 51.85 wt% PP, 17.28 wt% LDPE, 7.41 wt% HDPE, 17.28 wt% plastics with PVC, PET, and PS, and 6.18 wt% lignocellulosic. The kinetic parameters were obtained using four isoconversional methods (FWO, KAS, STK, and VYA models). However, the reaction model was identified by master plots. Three main reaction stages were observed. While the first decomposition reaction represented the main decomposition reaction of holocellulose and minor decomposition of the first degradation stage of PVC (dichlorination), the second decomposition reaction stands for the decomposition of a mixture of polymers, such as PS and some adhesive acrylic-based resins, and PVC (dichlorination), and the last decomposition reaction was mainly for the thermal decomposition of PP, LDPE and HDPE.
In addition, thermal degradation of PP with PE (LDPE and HDPE) blends were investigated to obtain the induction time [
24]. However, the catalytic thermal degradation of the ternary mixture of PP/LDPE/HDPE was early studied by Himmelblau [
25] in 1968. Furthermore, pyrolysis of PP with PE and PS was investigated [
26] and the pyrolysis products were targeted.
As discussed earlier, there is a research gap on the pyrolysis of a representative mixed plastic waste needed to be further investigated. In addition, most of the available studies of the pyrolysis of the mixed polymers assume the first-order reaction mechanism which may not be the accurate model. Therefore, this work aims to build knowledge on the thermal decomposition of mixed polymers where two model-fitting techniques, namely Criado and Coats–Redfern Models, are used to obtain the best reaction mechanisms and kinetic parameters of the pyrolysis process using TGA data.
4. Conclusions
The TG and DTG thermograms obtained from the TGA study showed similar shapes and trends for all pure (LDPE, HDPE, PP, and PS) and mixed polymer samples with different compositions. From the TGA data, it was observed that the data conform only to a single thermal degradation step and pyrolysis of the pure polymer samples occurs at different temperature ranges in the following order: PS < PP < LDPE < HDPE. However, the co-pyrolysis of all-polymer mixtures was delayed when it was compared with the pyrolysis of pure polymer samples. In addition, the synergistic effect of the co-pyrolysis of some polymer blends was observed. However, further investigation is highly recommended to explore more the synergistic phenomenon at a low heating rate.
Furthermore, two different model-fitting methods were used to determine the most suitable reaction mechanism for each test and then to obtain the kinetic parameters for each reaction. The obtained values of activation energy were ranging from 104 to 260 kJ/mol depending on the properly selected model reaction mechanism. The most controlling reaction mechanisms were in the following order: First-order reaction (F1), Contracting sphere (R3), and then Contracting cylinder (R2).