Pyrolysis characteristics and thermal kinetics of expanded polystyrene (EPS) and styrene-methyl methacrylate (St-MMA) copolymer in LFC process

& Development CHINA FOUNDRY Research Vol.15 No.6 November 2018 https://doi.org/10.1007/s41230-018-8093-3 Pyrolysis characteristics and thermal kinetics of expanded polystyrene (EPS) and styrene-methyl methacrylate (St-MMA) copolymer in LFC process Xi Li1, Ya-jun Yin1, *Jian-xin Zhou1, Xu Shen1, Ming-guo Xie2, Wei Liu2 1. State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science & Technology, Wuhan 430074, China. 2. Hefei Casting and Forging Plant of Anhui Heli Co., Ltd., Hefei 230022, China. Abstract: The pyrolysis behaviors of foam patterns have critical influences on fluid morphology and defect formation in Lost Foam Casting (LFC). The pyrolysis behaviors of expanded polystyrene (EPS) and styrenemethyl methacrylate (St-MMA) foams were compared using synchronous thermal analysis (STA), which was performed under argon atmosphere at different heating rates (from 10 to 40 K•min-1). The degradation heat was calculated by integrating DSC curves. Results show that the calculated degradation heat of St-MMA (605.28 J•g-1) was significantly lower than that of EPS (706.71 J•g-1). Furthermore, the non-isothermal iso-conversional method was used to determine the pyrolysis apparent activation energies of EPS and St-MMA, and results show that the activation energy of St-MMA (200.36 kJ•mol-1) was apparently higher than that of EPS (167.92 kJ•mol-1). These calculated results indicate that the weight loss rate of EPS is greater than St-MMA in the pyrolysis process. In addition, the apparent activation energies at various pyrolysis stages demonstrate that the pyrolysis reactions of EPS and St-MMA may involve physical and chemical changes in the decomposition layer of the LFC process. Key words: lost foam casting; EPS, St-MMA; pyrolysis characteristics; pyrolysis kinetics CLC numbers: TG221+.1 Document code: A L ost foam casting, a near net shape and green casting technology, have various advantages, like high precision, high surface quality, environmental cleanliness, simple process and so on [1-4]. However, in ordinary sand casting, the foam would experience complex physical and chemical reactions with gasification and decalescence in the high temperature effect of molten metal during the filling process. These reactions will decrease temperature rapidly in the metal front and produce backpressure causing a huge impact on filling and even causing casting defects [5-8]. In order to further understand the filling mechanism of lost foam casting, a great number of studies have been made to explore the pyrolysis characteristics and thermal kinetics of different kinds of foam patterns. Shivkumar S. et al. [9, 10] have investigated foam degradation of EPS and EPMMA since the 1980s. They used scanning electron *Jian-xiu Zhou Male, born in 1975, Professor, Ph.D. His research interests mainly focus on the casting process, especially on lost foam casting and casting process simulation. E-mail: Received: 2018-04-23; Accepted: 2018-07-30 428 Article ID: 1672-6421(2018)06-428-08 microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to obtain the effect of foam density and foam bead structure in the pyrolysis process on metal filling. Dong et al. [11] applied gas chromatography to reveal the pyrolysis characteristics of three kinds of foams including EPS, EPMMA and St-MMA, finding that the main products of the three foams in pyrolysis processes were all polymer monomers, and the pyrolysis of EPMMA and St-MMA produced a great amount of small molecules reducing gas with a flame retardant effect on magnesium alloy. Zhou et al. [12] discovered that the primary products of polystyrene pyrolysis were styrene monomer, dimer and trimer, and raising temperature would promote monomer production. Jiao at el. [13] determined the thermal degradation kinetics and reactants of PU, XPS and EPS foams by using thermogravimetric (TG) and simultaneous thermal analysis (STA) coupled with mass spectrometry and Fourier transform infrared spectroscopy (TG-DSC-MS-FTIR). They found that the degradation processes of the three materials were different: EPS had a single weight loss process, XPS was divided into two stages, and the PU degradation process was more complex and composed of three Research & Development CHINA FOUNDRY Vol.15 No.6 November 2018 stages. Zeng et al. [14] used a non-linear iso-conversional method to attest a kinetic model for polystyrene pyrolysis. Pravin Kannan [15] designed an experimental device to testify a kinetics model for EPS pyrolysis, suggesting that the kinetics of EPS pyrolysis are controlled by reaction at low temperature and heat transfer at high temperature. A non-isothermal kinetic equation was used to study the pyrolysis kinetics of St-MMA pre-expansion beads under oxidative and non-oxidative conditions by Hamid Reza Azimi et al. [16-18], who found that the pyrolysis rate of St-MMA was lower than that of EPS. Pyrolysis characteristics include pyrolysis mass loss, volatiles, pyrolysis heat and so on. Nevertheless, most researchers focus on the mass loss and thermal kinetics of different foams, and little research has been carried out on degradation decalescence changes and speculating reactions. In this study, the pyrolysis characteristics of EPS and St-MMA foams were studied systematically by measuring weight loss and decalescence. Foam samples were treated specially, including compressing and cutting, to measure mass and heat loss accurately. The degradation heat of the two foams was calculated and compared, and the thermal kinetics of the two foams were determined using the iso-conversional method. Furthermore, the degradation reactions associated with weight loss were derived based on the activation energy at each stage. By investigating the pyrolysis characteristics of EPS and St-MMA foams, the research result lays the foundation for the accurate prediction of mold filling process as well as provides guidance for the improvement of casting quality. 1 Experimental EPS and St-MMA foam patterns were made with white closed honeycomb (Fig. 1). Thermal decomposition characteristics and kinetics of EPS and St-MMA were investigated with a Netzsch STA449F3 thermal analyser, including thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). Fig. 1: St-MMA and EPS foam patterns In order to mimic the vacuum condition and decrease the temperature gradient inside the foam during the pyrolysis process, the EPS and St-MMA foam patterns were compressed under a pressure of 40 MPa for 5 min, and the air inside the foam cell was discharged as much as possible. Then, the foam samples were tableted by a tableting machine before the foams were cut into crucible-sized round flakes (5 mm, Fig. 1) in order to make proper contact with the crucible. Each kind of foam was divided into four groups with different heating rates of 1040 K•min-1 in the synchronous thermal analyzer. Considering the foam decompo (...truncated)