|Muhammad Zulkarnain||Fakulti Teknologi Kejuruteraan Mekanikal dan Pembuatan, Universiti Teknikal Malaysia Melaka (UTeM), 75450 Ayer Keroh, Malacca, Malaysia|
|Rahida Wati Sharudin||School of Chemical Engineering, College of Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Malaysia|
|Masahiro Ohshima||Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan|
Thermoplastic elastomer Polystyrene-b-polybutadiene-b-polystyrene (SEBS) foams are prepared by using carbon dioxide (CO2) as a blowing agent via a pressure quench method. During the foaming process, various pore shapes are developed inside the foam, which is influenced by several parameters such as rigidity, solubility, and diffusivity of CO2. A previous study revealed the theory of how SEBS foams may shrink due to low rigidity and high CO2 diffusivity, but empirical verification on how the final cell properties like cell shape, cell size, cell distribution, and percentage of porosity may affect the thermal conductivity of SEBS foam is challenging to represent experimentally. This is due to difficulty in preparing foam samples at different cell shapes for the same polymer, different percentages of porosity, and cell distribution while keeping the same cell size of the SEBS foam. This paper discussed how numerical analysis is employed to investigate various properties of pores such as cell shape, cell size, cell distribution, and percentage of porosity on the thermal conductivity. The simulation results are corroborated with experimental value where the reduction of thermal conductivity is observed with a higher percentage of porosity which is shown by all cell shapes foam such as spherical, ellipse, and irregular.
Numerical; Polystyrene-b-polybutadiene-b-polystyrene (SEBS); Pore; RVE.
Polymer foams are a class of lightweight
materials that possess unique properties and amazing versatility (Monie et al., 2021). They are found virtually
everywhere, either in liquid or solidified form. They are widely used for
packaging applications by their porous structure and superior properties of the
low density of foams. The advancement of foaming technology is still in
progress because the demand for foam products is widely expanded nowadays. The
cell properties and cellular structure are highly dependent on their
application. The essential concerns for these Polymer foams are their final
foam structures and cell properties to achieve a high functionality-to-weight
ratio for any polymer foam materials. To improve the thermal insulating
properties of foam materials the average cell size of foam must be reduced to
approach the mean free path of gas molecules in the air, as the thermal properties of air are comparable
to the thermal properties of foam in a vacum (Dai
et al., 2021). In biomedical applications, the requirement to be met as a
bioscaffold is higher porosity, adequate pore size, structural integrity, and
shape stability to the tissue defect to enable tissue regeneration during
implantation (Roedel et al., 2018).
The relationship between foam properties and its overall performance needs to be well understood by properly designing the polymer foam structure. Such an optimized cell structure design will provide superior properties by resulting in high strength, good heat transferability, and good performance to the targeted application foam. Therefore, it is vital to design the foam properties prior to preparing the foam material for any specific applications. Many studies are reported on different control strategies for controlling cell properties like selecting optimum foaming conditions (Mantaranon & Chirachanchai, 2016), utilizing nucleating agent (Chauvet et al., 2016), controlling the polymer rigidity (rheological properties) (Lee et al., 2016; Rainglet et al., 2021), performing a different method of foaming (Solbakken et al., 2021; Muayad et al., 2020), utilizing different blowing agent (Coste et al., 2020), selecting an appropriate pair of polymers in case of polymer blend foam is desired and many more methods. Zakyan et al. (2014), for example, controlled foam morphology of Polystyrene (PS) via surface chemistry, size, and concentration of nano-silica particles. They found that the size of nano-silica particles as well as silica loading affected the pore size and cell density. There are are also studies reported on enhancing the pore properties of polymer foams by modifying the surface of silica nanoparticles (Rende et al., 2013) and controlling cell size and number of cells by using surfactant (Eaves, 2004). It is found that surfactants can produce a higher miscibility rate of the polymer blends by reducing surface tension. On the other hand, cell openers can be utilized to improve the dimensional stability of the foam.
Generally, polymer foam can be prepared either by thermoplastic or a thermoset polymer. A thermoplastic-based foam usually has characteristics of stable sphere dimension if foaming conditions like temperature are adequately controlled at a temperature near either Tg or Tm of the polymer. On the other hand, stable dimension thermoset-based foam only can be successfully prepared when the appropriate degree of curing or cross-linking for foaming and cell stabilization are satisfied. In case of thermoplastic elastomer (TPE) that has combinational properties of thermoplastic polymer and rubber is more likely complicated to be foamed as it may shrink and less uniform if the cross-linked chains are reduced, low solubility, as well as high blowing agent diffusivity and low rigidity, is possesses by the TPE (Sharudin & Ohshima, 2012).
The reliability of the numerical method of interpreting the thermal conductivity-cell properties relationships was supported by the analytical model of Maxwell-Eucken and further validated with the experimental results. Using the cell properties in terms of cell size, cell shape, and percentage of porosity, numerical results were able to clarify the thermal behavior of different SEBS cell shapes by their heat distribution profile. As shown in this study, the cell having an ellipse shape has lower thermal conductivity than the irregular one. However, the effect of cell size seems insignificant to the thermal conductivity value for each RVE study. The insignificant differences between the numerical and experimental results have shown it successfully demonstrated. Since preparing foam with different cell sizes while maintaining the percentage of porosity is challenging to control, the computerized approach would be affording a favorable thermal conductivity estimation alternative to enable the designing thermal insulators of elastomer-based foam at various cell properties.
On behalf of all authors, the corresponding author states that they much appreciate Universiti Teknikal Malaysia Melaka (UTeM) supporting financially by Short term grant PJP/2020/FTKMP/PP/S01765, the Research Management Centre (RMC), Universiti Teknologi MARA (UiTM) for the financial support of the project under the grant 600-IRMI 5/3/LESTARI (042/2019) and Material Process Engineering Laboratory, Kyoto University for technical support during research completion.
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