Study Of Polymer Foaming Process And Properties Regulated By Organic Cage Compounds

Microcellular foaming can not only reduce the weight of plastic products,but also endow them with the functions of heat and sound insulation.It has become one of the important means to achieve the requirements of energy saving and emission reduction in many industries,and has received extensive attention from many researchers at home and abroad.Previous studies had shown that adding nano-inorganic particles into the polymer as heterogeneous nucleating agent can effectively improve the quality of cells,thus enhancing the impact toughness and heat insulation performances of microfoam materials.However,the nucleation efficiency of nano-inorganic powders is unsatisfying because of their high surface compactness and agglomeration effects.In addition,the compatibility between inorganic nucleating agent and matrix resin is poor,which often disfavors the properties of foamed materials.Thus,the design and development of efficient organic nucleating agents with a specific structure is of great significance for improving the foaming behaviors and properties of foaming materials.Starting from the classical nucleation theory and molecular structure design,weselected three types of organic cage compounds(namely,cucurbit[6]uril(Q[6]),hemicucurbit[6]uril(HQ[6])as well as?-cyclodextrin(BC))with different cavity structures as heterogeneous nucleation agents for polypropylene(PP)foaming injection molding process.The experimental results showed that Q[6]with a“natural”cavity structure possessed the best nucleation efficiency among these three cage compounds.The nucleation mechanism of organic cage compounds was preliminarily explored by virtue of designing cavity structures of organic nucleating agents(“shielding”the cavity and“constructing”loose pore structures),analyzing its effects on the foaming behaviors of polypropylene,and combining in-situ visual injection molding analysis.The influences of different cage compounds on the crystalline,rheological&mechanical properties of polymer materials were carefully investigated and the mutual relationships between chemical structures and foaming properties of composite materials were discussed and established,laying a foundation for the potential application of organic cage compounds as nucleation agents in polymer foams.The following conclusions were drawn in this thesis:(1)The gas and melt interface energy(?_lgA_lg)formed by bubbles were key factorsdetermining the nucleation of bubbles,based on the analysis of heterogeneous nucleation mechanism.The nucleation efficiency was effectively improved,after decreasing the?_lgA_lg value during the nucleation of the bubbles.Organic cage compounds including Q[6],HQ[6]and BC with different cavity structures were prepared.Among them,Q[6]showed the optimal nucleation efficiency because it had a naturally existing cavity(that is,large-area“naturally existing”gas/solid interfaces),which could effectively reduce both the?_lgA_lg values and energy barrier for heterogeneous nucleation.(2)A supramolecular complex Q[6]@Zn was prepared and this complex could reach the purpose of“shielding”Q[6]hollow structures by X-ray single-crystal diffraction analysis.BC nano-sponges NJ and BL with different degrees of polymerization were designed and prepared.NJ was proved to have a three-dimensional hollow structure based on the SEM observation,meeting the requirements of building a cavity structure to improve nucleation effects of BC.(3)Cage nucleating agent/polypropylene composite foam materials were prepared.The addition of 1.0 wt%Q[6]reduced the pore diameter of PP foamed materials by up to 50%.Q[6]with a hollow cage structure had a better nucleation efficiency,as compared with HQ[6]and BC with incomplete cage structures.After“shielding”the cavity,the nucleation efficiency of Q[6]@Zn was significantly lowered relative to that of Q[6].The nucleation efficiency of NJ was remarkably improved after BC modification.The in-situ foaming behaviors of PP and PP composites were observed using visual injection molding devices,under the premise of the same quantity of nucleation agent.PP composites after the addition of Q[6]have the largest slope value and the fastest nucleation rate during the growth stage of bubble number.After the mold-opening time was extended,the PP composite material treated with Q[6]reduced the number of bubbles at the lowest rate.The growth rules of bubble hole observed from visual injection molding process was well consistent with the conclusions from the real injection molding process,further confirming the conclusion that nucleating agents with low surface densification properties could reduce the nucleation energy barrier and improve the nucleation efficiency.(4)Q[6],HQ[6]and BC were all?-nucleating agents of PP,which could raise the crystallization temperature and rate of PP,wherein Q[6]needed the lowest amount of addition(0.5 wt%).The maximum viscosity value of composite materials was obtained,with the addition amount of 1 wt%for the above three nucleation agents.Moreover,the PP composite materials after the addition of Q[6]had the highest viscosity and modulus values.Q[6]had more advantages in improving mechanical properties of PP foamed materials owing to its structural rigidity and good interface interactions with PP.The tensile,bending and impact resistance of PP-Q[6]composites were improved by 21.1%,6.8%and 10.2%,respectively,as compared with those of pure PP materials.The modified nucleating agents of NJ and BL of BC exhibited better nucleation effects of PP crystallization and improved the viscosity of composite materials at a lower content(0.15 wt%).Compared with pure PP,the tensile,bending and impact strengths of BC,NJ and BL composite foam materials were enhanced to different degrees.(5)In the presence of free radical initiator-diisopropyl peroxide,composite materials of PLA grafted BC were prepared by the reactive extrusion.A small amount of BC could considerably improve the melt strength of PLA,whereas a higher content of BC would reduce the melt strength.Improving melt strength was more effective than bubble nucleation in improving PLA foam behavior.