Study On Polypropylene/M-EPDM And Mechanisms Of Toughening And Reinforcing

A series of methods were employed to modify EPDM before it was blended with PP. These methods, including filling with CaCO3, plasticizing with oil and grafting with maleic anhydride, aim at: 1. enlarging the volume of EPDM through the EPDM shell- CaCO3 core structure; 2. softening EPDM; 3. forming chemical bond between CaCO3 and EPDM to prevent them from being pulled apart during blending with PP. The mechanical properties of PP/modified EPDM (M-EPDM) were carefully studied to determine the effects of the dispersed phases with shell-core structure on toughness and hardness of the composites. Special tensile tests were carried out on specimens which have several holes drilled and the mechanisms of toughening and reinforcing were discussed. The main works and conclusions were listed as following:1. Six M-EPDMs of different EPDM/CaCO3 ratios were prepared and used as impact modifier for three PP resins. As the EPDM/CaCOs ratio decreases, the brittle-ductile transition of PP/M-EPDM moves to high M-EPDM content. However, the net content of EPDM is reduced a lot, meaning the CaCO3 and the EPDM cooperate to toughen the PP. Moreover, M-EPDM of smaller EPDM/CaCO3 ratio lead to higher yield strength as well as flexural modulus when blended with PP. Compared with PP/EPDM/CaCO3 that were directly mixed together, PP/M-EPDM have a better impact strength. SEM photos demonstrated that the CaCO3 particles in PP/M-EPDM are well capsulated by EPDM, while in PP/EPDM/CaCO3 more are dispersed in PP matrix.2. Tensile tests were carried out on specimens of PP and PP/EPDM (65/35) after 1~7 holes were drilled on them. It was discovered whether the micro-deformation could spread or not is the key factor for the enhancing of the yield strength of materials. For composites of PP/CaCO3 or PP/M-EPDM, when the density of dispersed phase is high, the stress field in the matrix will be strongly affected and micro-deformation could spread to the whole materials. As a result, the composites have lower yield strength. On the contrary, if the density of dispersed phase is very low, the micro-deformation will be restrained. Since the micro-deformation makes the matrix stronger, the materials could have larger yield strength than PP.3. The dynamic processes in the impact tests were analysed. It was found that the brittle material is torn open during the crack, while the ductile material is pulled open during the crack. Since the pulling process causes a lot of matrix to deform and more energy is consumed, the material will get high impact strength. The pulling open mechanism of ductile material also lead to understandable explanation to the cooperative toughening effects of EPDM with CaCO3 and the limited toughening ability of hard particles such as CaCO34. Two kinds of plasticizer were added into M-EPDM before they are used to toughen PP. The measure of density of M-EPDM reveals that the plasticizer is absorbed by EPDM and makes its volume larger. Comparing the PP/M-EPDM with or without plasticizer, the results showed that the plasticizer nearly acts as EPDM which has the same volume as the plasticizer. If the plasticizer reduces the hardness of M-EPDM, the impact strength will be higher, though the flexural modulus will reduce to some extent.5. Maleic anhydride grafted EPDM was used to replace EPDM in M-EPDM partly or totally. After the maleic anhydride grafted EPDM was mixed with CaCO3, the M-EPDM can't dissolve in solvents any longer, demonstrating that chemical bond are formed between CaCO3 and EPDM. SEM photos show clearly that the chemical bond maintains the EPDM shell- CaCO3 core structure very well, which resulting even more higher impact strength than those PP/M-EPDMcomposites where the EPDM and the CaCO3 could be pulled apart during the blending with PP.