Preparation Of High-quality Chemical Foaming Polyolefin Materials And Research On The Fracture Mechanical Behavior

To obtain microcellular polymer material with better performance, to meet particular needs, to reduce production costs, the research on polymer micro-foaming technology is of great significance. Microcellular polyolefin materials with a large number of microcells dispersing uniformly, obtained through micro-foaming, are a series of new high-performance composite materials. These materials can maintain the original good mechanical performance and meanwhile low cost. The structure of microcellular polyolefin depends mostly on the processing technology in the process of molding, resin characteristics. Therefore, it's important theoretically and empirically for the control and optimization of material properties to study the molding process, resin characteristics and the structure of the relationship between morphology and properties of fracture and deformation mechanisms.In this thesis, the effect of the chemical foam injection molding processes, intrinsic properties of polyolefin (different gel content of polyethylene, polypropylene of different fluid flow rates), nano-particles of inorganic materials on foaming behavior and mechanism of polyolefin was studied systematically, which aim to prepare high-quality microcellular polyolefin . Deformation and fracture behavior of microcellular polyolefin was detailedly investigated to clarify the effect of various factors and toughening mechanism of the new opinion, and set up a mathematical model of physics. Research paper made the following main results:First, the effect of injection temperature on cell size, cell density and cell size distribution of foamed HDPE is foremost, which lead to different decomposing rate, gas amount of foaming agent, melt-viscosity of HDPE and injection pressure. The effect of injection pressure is related to balance pressure. The average cell diameter of 21μm, cell density of 3.2×107 (cells. cm-3), the cell size uniform distribution of high-quality microcellular polyolefin samples were produced By optimizing the process conditions.Second, the effect of intrinsic properties of polyolefin and processing parameters on the average cell diameter, cell size distribution, cell density and the melt strength of polyolefin was studied. In the same processing conditions, the high melt strength leaded to larger bubble nucleation radius of the critical hole, while the low-melt-strength enhanced bubbling. Thus to obtain high-quality polyolefin foam materials, the melt intensity should be within a certain range. The polyolefin foam materials with different melt strength can be obtained under different processing conditions. Furthermore, the effect of nano-inorganic particles on the average cell diameter, cell size distribution, cell density was studied. On the one hand, the addition of Nano-OMMT leaded to higher cell nucleation rate and melt viscosity. The microcellular polyolefin / Nano–OMMT composite foam material with average cell diameter of 19.49μm, cell size uniform distribution, cell density of 3.8×107 cell.cm-3 was obtained. On the other hand, the introduction of intercalated Nano-OMMT dispersing uniformly played the role of "pinning" and strain-hardening, resulting in foam with more stable injection molding process and a broader temperature range. The experiment showed that the injection temperature of 170℃~ 195℃range of polypropylene / nano- montmorillonite composite foam materials. The average cell diameter, cell size distribution, the cell density changed inconsiderably, while there could be a large change when injection temperature is higher than 195℃.Finally, the relationship between the impact fracture toughness of polyolefin material and intrinsic toughness of polyolefin was introduced for the first time. Mechanism of fraction and deformation of microcellular polyolefin material indicated that the existence of microcells leaded to a change in fracture mechanism. The existence of microcells may reduce the actual section area of samples under impact test, meanwhile, cause easily the cell sample polyolefin matrix material to relax the crack tip stress concentration to prevent crack propagation. The main cracks were broken down to secondary ones, so the direction of crack propagation changed. The resistance performance to crack growth is related with intrinsic impact strength (αk0). If the crack growth in the actual section area of polyolefin materials reduced, the decreased value of impact strength (αk1) was less than the existence of microcells relaxation of crack tip stress. And the microcells toughening property were enhanced provided that the value of impact strength (αk2) were improved, which arised from extended function of the secondary cracks in different directions by means of varying stress distribution. Otherwise, the existence of microcells reduced the toughness of polyolefin materials. And we drew a conclusion that the impact strength (αk) of these microcellular polyolefin materials can be expressed by the following physical mathematical expression: a k = a k 0 ? a k 1 + ak2.