Study On Synergistic Reinforcement And Toughening Effects Of Nucleating Agent And Calcium Carbonate On Polypropylene

Polypropylene (PP) is one of the most popular thermoplastic which has been widely used, but its further application is restricted for its poor fracture toughness at relatively low temperature and high strain rate. Thus, the toughening of PP has been the main subject of many researches in the last decades. Using inorganic particles to fill PP can improve its fracture toughness. It has been reported that the factors which influence the toughening effect of inorganic particles include content of inorganic particles, particles size and its distribution, surface modification and the experiment conditions. In fact, the toughening effect of PP/inorganic particles composite is also greatly determined by the matrix parameters, e.g. crystalline morphology and structure. However, the effect of the two parameters on improving the toughness of PP/inorganic particles composite has been seldom investigated.In this work, two highly active nucleating agents (NAs), i.e. a-form NA 1,3:2,4-bis (3,4-dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) andβ-form NA aryl amides compounds (TMB-5), were selected to control the crystalline morphology and structure of PP matrix, and the effect of Calcium carbonate (CaCO3) on microstructure and physical properties of PP/CaCO3 and PP/CaCO3/DMDBS composites was studied systemically. And then, the reinforcement and toughening mechanisms were discussed. Furthermore, the microstructure and physical properties of PP/CaCO3/β-NA samples obtained before and after being annealed at different temperatures (90-150℃) and different annealing durations (3-12 h) have been comparatively investigated. And the toughening mechanism has been discussed. Moreover, the effect of mechanical pre-conditioning on fracture resistance of PP was investigated, too. In the last of the work, the synergistic toughening effect of POE/CaCO3 and NA on PP was discussed. The major results are listed as follows:(1) The addition of CaCO3 and DMDBS into PP induces synergistic effect on crystallization behavior of PP matrix. Enhanced crystallization temperature, reduced spherulites diameters, and improved spherulites distribution as well as the largely increased degree of crystallinity (Xc(%)) are achieved for PP/CaCO3/DMDBS sample. Furthermore, improved distribution of CaCO3 is achieved when DMDBS was added into PP. At the same time, the tensile modulus, tensile strength and heat distortion temperature (HDT) are greatly improved for PP/CaCO3/DMDBS.(2) The reinforcement mechanism of PP/CaCO3/DMDBS composite can be explained as follows. On the one hand, the plastic flow of PP matrix involving nucleation of voids is suppressed due to the presence of CaCO3 particles. On the other hand, the modulus and Xc(%) of PP/CaCO3/DMDBS composite are improved synergistically when CaCO3 and DMDBS are present simultaneously in PP. Thus, the ability of resistance to stress whitening of PP matrix is enhanced. As a consequence, CaCO3 and DMDBS exhibit synergistic effect in the reinforcement of PP to a certain degree. The toughening mechanism of PP/CaCO3/DMDBS composite can be attributed to the fact that DMDBS induces the increase of the spherulites numbers and simultaneously improves the interface adhesion among spherulites due to the enhancement of tied-molecule structure among the intra-and inter-spherulites.(3) After being annealed, the crystalline structure of PP matrix in CaCO3/β-NA is greatly improved. By the way, theβto a transition is observed during the annealing process, and the transition is proved to be dependent upon the annealing temperature. The fracture toughness is proved to be greatly dependent upon the Ta and ta, too. At low Ta (<110℃), the fracture toughness increases slightly; At moderate Ta (120-140℃), the fracture toughness improved significantly. At high Ta (>140℃), the fracture toughness showed a great deterioration. Further work proves that longer ta is beneficial to the improvement of fracture toughness. The toughening mechanism of PP/CaCO3/β-NA composite at moderate Ta (120-140℃) is ascribed to the decrease of the number of chain segments in the amorphous induced by the second crystallization during annealing, which makes the lamellae structure becomes loose and more available to slip and/or elongate along the impact direction.(4) The mechanical pre-conditioning samples exhibit higher fracture resistance compared with the conventional samples without any mechanical pre-conditioning. To a certain range, the fracture resistance of pre-impacted samples increases with the increasing of applied pre-impact energy. Further results based on the surface morphologies show that the improvement of the fracture resistance is determined by the stress whitening zone. The bigger the area of stress whitening zone, the higher the fracture resistance is.(5) The primary experimental result shows that the fracture toughness of PP/POE/CaCO3/NA composite is further improved compare with pure PP.