Co-pyrolysis of Key Organic Components in End-of-life Photovoltaic Laminates: Effect of Proportional Variations on Pyrolysis Mechanisms and Characteristics

China is a large country in the photovoltaic (PV) industry, with a vast amount of end-of-life (EOL) PV modules requiring proper management. However, encapsulants (EVA) and fluorine-containing backsheet (TPT) in PV laminates, as high molecular weight organic polymers, impede subsequent PV recycling processes. Pyrolysis is an effective means for recycling end-of-life PV laminates by removing the EVA and TPT. However, in practical pyrolysis processes, EVA and TPT are inevitably co-pyrolysis together. Due to variations in manufacturing methods, the proportion of organic components differs among PV laminate models, potentially affecting their co-pyrolysis behavior—A topic that remains underexplored. This study focuses on investigating the co-pyrolysis characteristics and mechanisms of EVA and TPT at different mixing mass ratios. Kinetic modeling was employed to analyze the heat loss behavior and determine the co-pyrolysis mechanisms.

The EVA hot melt adhesive used in the experiment was sourced from Jiacheng Plastics Co., Ltd (China) with a particle size lower than 100 mesh. The TPT was obtained from Youdisheng Electronic Materials Co.,Ltd (China). The TPT was crushed, ground, and sieved to pass through a100 mesh standard sieve, and then dried in an oven at 60 ℃ for 12 h. The mixing mass ratios of EVA and TPT were determined to be 5 : 1, 3.4 : 1.0, and 2 : 1 through literature research and experimental validation.

A thermogravimetric analyzer (TG, STA 449 F5 Jupiter, NETZSCH) was used under an argon atmosphere with a flow rate of 50 mL/min. Samples of (10.0 ± 1.0) mg of EVA, TPT, and different ratios of their mixtures were placed in crucibles. The samples were heated from 50 ℃ to 800 ℃. Based on an extensive literature review, the heating rates of 10, 15, 20 ℃/min, and 25 ℃/min were selected. The synergistic effect was characterized using the difference between the theoretical and actual degrees of pyrolysis (ΔW) to investigate potential pyrolytic interactions during the co-pyrolysis process. The Flynn-Wall-Ozawa (FWO) method and the Kissinger-Akahira-Sunose (KAS) method (Model-free method) were employed to calculate the activation energy, and the Coats-Redfern (CR) method (Model-free method) was chosen to ascertain the pyrolysis reaction mechanism.

The co-pyrolysis weight loss was divided into two stages, the first stage was treated at 300–380 ℃, which was mainly due to the deacetylation of EVA to produce acetic acid. The second stage was at 380–510 ℃, corresponding to the degradation of EVA and TPT long chains. At 800 ℃, the co-pyrolysis of EVA and TPT with the ratio of 2 : 1 resulted in a total weight loss of approximately 94.5%, while ratios of 3.4 : 1.0 and 5 : 1 achieved higher total weight losses of 96.7% and 97.9%, respectively. As the proportion of TPT decreased, the total weight loss ratio of the mixture gradually increased, which may be due to the presence of PET in TPT. The presence of benzene rings in PET caused cross-linking of the product, generating more polyaromatic hydrocarbons and promoting coking. The overall synergistic interaction between EVA and TPT during co-pyrolysis was antagonistic effect. The antagonistic effect was most pronounced when the ratio of EVA to TPT was 2 : 1, probably due to the increase in TPT, which made the coking more pronounced. The average activation energy of co-pyrolysis was higher than that of EVA and TPT pyrolysis alone, suggesting that more energy was required for co-pyrolysis, further supporting the antagonistic effect of co-pyrolysis. The average activation energies were 244.7–245.5, 239.6–240.1 kJ/mol and 261.2–262.8 kJ/mol for EVA and TPT mass ratios of 2 : 1, 3.4 : 1.0 and 5 : 1, respectively. In order to further determine the reaction mechanism of the co-pyrolysis, the reaction mechanism of the co-pyrolysis process was explored using the CR method. The co-pyrolysis followed the diffusion controlled D5 model, D3 model, and D6 model for EVA and TPT mass ratios of 2 : 1, 3.4 : 1.0 and 5 : 1, respectively. Changes in the mechanism functions of different ratios of co-pyrolysis further indicate the influence of the mass ratio of EVA and TPT on the interactions.

In this paper, the pyrolysis behaviour, kinetics and reaction mechanisms involved in the co-pyrolysis of EOL PV laminates at three mass ratios (2 : 1, 3.4 : 1.0 and 5 : 1) of EVA and TPT were investigated, and key findings are as follows:

1) The main pyrolysis happens between 300 ℃ to 510 ℃. As the proportion of TPT decreased, total weight loss increased. Specifically, the weight loss rose from 94.5% to 97.9% as the EVA : TPT mass ratio increased from 2 : 1 to 5 : 1.

2) The degree of synergistic interaction varied with mixing ratio. TPT promoted char formation and reduced overall pyrolysis efficiency, particularly at the 2 : 1 ratio, where its proportion was highest. This antagonistic effect was reflected by an average activation energy of 245.1 kJ/mol.

3) Coats–Redfern (CR) model fitting revealed that the co-pyrolysis of EVA and TPT followed diffusion-controlled mechanisms. The reaction models corresponded to D5, D3, and D6 for the 2 : 1, 3.4 : 1.0, and 5 : 1 ratios, respectively.

This work provides mechanistic insights into the co-pyrolysis behavior of key organic components in PV laminates and offers theoretical support for the development of thermal recovery methods for waste PV modules.