Study Of Polyurethane/Silica In-Situ Composite Modified Bitumen

In this paper,modified bitumen was prepared by in situ reaction of polyurethane using 4,4-diphenylmethane diisocyanate（MDI）,polyethylene glycol（PEG）and silicon dioxide（SiO₂）as bitumen modifiers.The in-situ reaction of polyurethane induces the formation of networks of nanoparticles in the modified bitumen,which can positively affect the rutting resistance and fatigue life of the modified bitumen while maintaining its excellent properties.By varying the SiO₂ premixing method,SiO₂ addition and MDI addition,the correspondence between the structure and properties of the modified bitumen was investigated.（1）Effect of SiO₂ premixing method.Bitumen/PEG/MDI（100/4/12）was used as a blank control experiment,and the amount of SiO₂ addition was fixed at 0.08 wt.%of the base bitumen.Three different addition sequences were used:（i）SiO₂ was added directly to the matrix bitumen and PEG and MDI were added sequentially after mixing well;（ii）SiO₂ was pre-blended with PEG and added to the matrix bitumen,followed by MDI;（iii）SiO₂ was pre-blended with MDI and added to the matrix bitumen,followed by PEG,so as to prepare the modified bitumen.The rheological results showed that the modified bitumen prepared by methods（ii）and（iii）had better high temperature rutting resistance and elasticity than the modified bitumen prepared by method（i）.Bending beam rheometer（BBR）investigated the crack resistance of modified bitumen at low temperatures and found that the modified bitumen prepared by method（ii）could mitigate the effect of reduced low temperature crack resistance due to nanoparticles.（2）Effect of SiO₂ addition.In this part,bitumen/PEG/MDI（100/4/12）was used as the basic research object,and the SiO₂ addition was limited to 0～4 wt.%of the base bitumen.The high-temperature rutting resistance and medium-temperature fatigue resistance of the modified bitumen were investigated by multiple stress creep recovery（MSCR）and linear amplitude scanning（LAS）tests,respectively,and the low-temperature performance of the modified bitumen was evaluated by dynamic thermomechanical apparatus（DMA）.The experimental results show that the modified bitumen has the best rutting resistance,fatigue life and low temperature toughness at high temperature when the SiO₂ addition is about 1 wt.%of the base bitumen.When the SiO₂ addition is gradually increased in the range of 0～1 wt.%,the microstructure of modified bitumen will have a transformation from island structure to network structure;when the SiO₂addition is further increased in the range of 1～4 wt.%,nanoparticles can be observed to form agglomerates.The formation of network structure in modified bitumen was analyzed by infrared spectroscopy（FTIR）and rheological results,and it was found that the formation of network structure was related to the tight aggregation of SiO₂nanoparticles themselves,while the polyurethane reaction between MDI and PEG would have a certain promotion and stabilization effect on the particle networking process.（3）Effect of MDI addition.In this part,the effect of MDI addition on the structure and properties of modified bitumen was investigated by fixing the MDI addition amount to 0%,2%,4%and 8%of the mass fraction of the base bitumen,using bitumen/PEG/SiO₂（100/4/1）as the basic research object.The rheological results showed that with the increase of MDI addition,the composite modulus of the modified bitumen gradually increased and the phase angle gradually decreased,indicating that the rutting resistance and elasticity of the modified bitumen were improved.Comparing the microstructures of the modified bitumen film samples before and after etching,it was found that the higher MDI addition contributed to the stabilization of the SiO₂nanoparticle network structure.