Interface-Morphology Controlling And Foaming Behavior Of Polyolefins/Hydrotalcite Composites

The highly heat resistance and easily recyclable characteristics of polyolefin-based foam materials make them have broad application prospects.Physical methods of preparing polyolefin microporous composites by supercritical carbon dioxide foaming has the advantage of low cost and no pollution compared to chemical foaming.However,due to the weak tendency of gas molecules to nucleate in the homogeneous systems,and the poor melt strength of polyolefin matrix,the quality of foam cells after supercritical foaming is not ideal.This thesis aims to improve the bubble quality of supercritical polyolefin foams by using hydrotalcite(LDHs),an inorganic material with a layered peelable structure,to improve the polyolefin matrix properties.The layered structure increases the interfacial sites for bubble heterogeneous nucleation in the melt.And it is facilitating the enhancement of the melt strength of polyolefin composites and adjusting the crystalline behavior of polyolefin molecular chains.The inorganic material dispersion in the polymer matrix has the inherent limitation of poor dispersion and compatibility.Therefore,APTES modified hydrotalcite was used to investigate the changes in the dispersion state and interfacial compatibility of LDHs in the matrix,which in turn adjusted the melt strength and crystallization behavior of the composites.The influence of LDHs/polyolefin interfacial compatibility and dispersion in the matrix on the melt strength and crystallization properties of the polyolefin material was further investigated.Thus,the relationship between the LDHs/polyolefin interfacial behavior and the evolution of the supercritical foaming bubble was further investigated,and the main findings obtained were as follows:(1)To address the problem of poor dispersion and compatibility of LDHs in the polymer matrix,the LDHs were modified with the silane coupling agent APTES,and the modified LDHs were only grafted with APTES on the surface without changing their own laminar structure.When the modified A-LDHs and unmodified LDHs were added to polypropylene(PP),the interface between the A-LDHs and the PP matrix was more blurred and more uniformly dispersed in the matrix than PP/LDHs,and the PP/A-LDHs composites had higher crystallinity and higher melt strength.According to the kettle-pressure foaming experiments,the curve of the number of bubble holes with time was fitted using a mathematical model equation(N-t curve equation)in combination with a CNC heating table-high-speed camera-video capture software visualization device.And the results showed that the better dispersed A-LDHs acted as excellent heterogeneous nucleating agents,which could reduce the Gibbs’nucleation barrier?G of the PP melt and increase the nucleation rate.The final result was an increase in the pore density of the foamed PP/A-LDHs composite from 1.33x10~7 cell/cm~3 to 4.43x10~7 cell/cm~3 and a reduction in the pore size from 43.88μm to 27.59μm.(2)In order to expand the scope of research on polyolefin/LDHs composites,A-LDHs and LDHs were added to low density polyethylene(LDPE)respectively.The introduction of A-LDHs reduces the loss factor of LDPE composites,improves the melt strength and facilitates the foaming of LDPE/A-LDHs composites.According to the kettle-pressure foaming experiments combined with the visualization of the supercritical foaming device,the effect of the dispersion of A-LDHs in the LDPE matrix on the foaming behaviour was investigated,and the results showed that after the introduction of A-LDHs,the final foamed LDPE/A-LDHs composite had an increase in pore density from 3.51x10~6 cell/cm~3 to 6.45x10~6 pcs/cm~3 due to the dual effect on the melt strength and crystallisation properties of the LDPE composite.increased to 6.45×10~6cell/cm~3 and the pore size decreased from 65.67μm to 49.74μm.(3)Using a mathematical model of bubble hole deformation,based on polyolefin visualisation foaming results and conventional foaming experiments.It was found that the introduction of A-LDHs into the system resulted in a faster blister nucleation rate and a higher number of blisters in the polyolefin/A-LDHs composites,due to the fact that A-LDHs act as a heterogeneous nucleating agent,lowering the barrier to blister nucleation,increasing the crystalline nucleation ability and providing more nucleation sites;also The introduction of A-LDHs was found to enhance the melt strength of the matrix material,and the polyolefin/A-LDHs composites showed a smaller degree of vesicle deformation.