Study On Morphological Evolution And Toughening Mechanism Of Sandbag Microstructure-toughened PA6/EPDM-M/nano-CaCO₃ Ternary Composites

In this work, ethylene propylene diene terpolymer rubber (EPDM)/maleated ethylene propylene diene terpolymer rubber (EPDM-g-MAH) was employed as elastomer (EPDM-M), nano calcium carbonate (nano-CaCO₃) was employed as inorigid particles,"sandbag"microstructure particle embedded nano-CaCO₃ agglomerate into EPDM-M was constructed in PA6/EPDM-M/nano-CaCO₃ ternary composites by two-step compounding route. The"sandbag"microstructure could effectively improve the toughness of PA6. Morphology evolution of"sandbag"microstructure particles in PA6 matrix under external force was studied by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and Dynamic Mechanical Analysis (DMA). The toughening mechanism of"sandbag"microstructure particles was further discussed. The main works and conclusions were listed as follows:1,The micromorphologys of the tensile fracture surfaces and the fracture surfaces along the draw direction under different tensile rates and tensile strain were observed by SEM and TEM. The results showed the cavitation within the rubber phase and debonding of interphase abhesion between the rubber particles and PA6 matrix were happened in the PA6/EPDM-M binary blends (E1). Debonding of interphase abhesion between the nano-CaCO₃ agglomerates and PA6 matrix and the cavitation within the rubber phase were observed in the PA6/EPDM-M/nano-CaCO₃ ternary composites prepaired by one-step compounding route (E2). Cavitation within the sandbag particles and debonding of interphase abhesion between the EPDM-M and nano-CaCO₃ agglomerates in the sandbag particles were observed in the PA6/EPDM-M/nano-CaCO₃ ternary composites by two-step compounding route (E3). Whereas, there were hardly any voids in the PA6/nano-CaCO₃ binary composites (E4). In accord with the volume strain of the PA6 based composites testing by video-aid tensile tests was followed as: E3>E2>E1>E4. It was meanings that E3 with"sandbag"microstructure could induce more voids to make bigger volume strain than other composites.2,The effect of ambient humidity on the properties and morphology of the PA6 based composites were studied. The water absorption and toughness of PA6 based composites were increased by the raising of the relative humidity. Among which the PA6/EPDM-M/nano-CaCO₃ ternary composites by one-step compounding route (E2) had the markedly change, and the impact strength was enhanced from 31.2 kJ/m² under dry state to 48.6 kJ/m2 at relative humidity of 100 %, and for 55.6 % addition. DSC was used to analysis the crystallization and glass-transition temperature (T_g). The results showed that the T_g of PA6 decreased from 52.06℃of dry state to 2.39℃of relative humidity of 100 %. Therefore, PA6 was plasticized by water. But the change of Tg by the effect of humidity in the ternary composites by two-step compounding route (E3) was a little. It will be seen from this that the influence of moisture for water absorption and mechanical properties was reduced in the ternary composites with"sandbag"microstructure,and the environment resistance was improved. 3,The micromechanics of"sandbag"microstructure in the PA6/EPDM-M/nano-CaCO₃ ternary composites was used to discuss the toughening mechanism. Under the external force, the composites with"sandbag"microstructure particles was deformed untill destructed. Three aspects of external energy dissipation were listed: First, stress concentration. The"sandbag"particles become to stress concentration point. Second, tensile deformation and shear zone forming. EPDM-M and its internal nano-CaCO₃ agglomerates were synergy deformation, which caused the"sandbag"particles corresponding deformation and formed shear zone around the"sandbag"particles. Last, cavitation, forming shear yield and craze. Hence, nano-CaCO₃ agglomerates will be subject to a compressive stess along the equatorial direction and a tensile stress along two poles and the cavitation will happen between the interphase of nano-CaCO₃ and EPDM-M. The rubber phase exhibited fiber-forming and induced the matrix to form shear yield and craze to dissipate external energy. Furthermore,"sandbag"particles were arranged at the equatorial direction,which was beneficial for craze embranchment and shear yield, and forming dilatation bands to dissipate external energy. Consequently, the toughness of the ternary composites with"sandbag"microstructure was significantly improved.