Preparation Of Polyurethane-based Nanocomposite And Its Thermal Conductivity And Mechanical Property

Polyurethane is widely used as an engineering polymer material in various industries due to its excellent mechanical properties and low cost.Especially,casting polyurethane is often used to fabricate large mechanical components,such as transfer rubber rollers and lining materials for grinding equipment,etc..However,polyurethane as a mechanical support material for various devices will inevitably withstand various frequent stress loads in different operating environments,causing it to be heated.It is difficult to diffuse heat from polyurethane material surface to the environment due to its poor thermal conductivity,thus increasing the temperature in polyurethane material and leading to polyurethane material to soften and creep.As a result,the mechanical properties of polyurethane material will be reduced when it is suffered in a high-temperature operating environment for a long time,affecting the working efficiency and service life of equipment.Therefore,development of a polyurethane-based nanocomposite with excellent thermal conductivity and mechanical properties becomes popular for its application.As is well known,ultra-fine graphite,graphene oxide,and hexagonal boron nitride nanoparticles are all inorganic fillers with a high thermal conductivity and an excellent mechanical property.However,the main problem in the preparation of polyurethane-based nanocomposites by casting is to be rather difficult to effectively incorporate and disperse the inorganic particles,especially the nano-sized particles,in a high-viscosity polyurethane prepolymer without or with inappropriate modification and dispersion methods.Therefore,preparing polyurethane-based nanocomposites with superior thermal conductivity and mechanical property becomes a difficulty.In addition,polyurethane-based nanocomposites with inorganic fillers are mainly incorporated with one kind of inorganic nanoparticles to improve their thermal conductivity and mechanical property.Little work on polyurethane-based composites prepared by co-incorporating two kinds or more inorganic nanoparticles has been reported yet.In this dissertation,ultra-fine graphite,graphene oxide?GO?and hexagonal boron nitride?h-BN?nanoparticles were selected as fillers for polyurethane-based composites,and the surface of the filler particles was modified with a silane coupling agent named KH-550.The size distribution and surface properties of the filler particles before and after modification with the silane coupling agent were characterized by scanning electron microscope?SEM?,laser particle size analyzer and Fourier transform infrared spectrometer?FTIR?,respectively.Based on the results by particle size analysis and SEM,the surface modified particles have a better dispersibility and little agglomeration.This is attributed to the coupling of saline coupling agent molecules on the particles surface,thus improving the compatibility with polyurethane matrix and reducing the particles agglomeration in polyurethane matrix.The FTIR analysis show that,compared to the unmodified filler particles,the modified filler particles have some absorption peaks at 900-1100 cm^-1 and 2850-3000 cm^-1,indicating that the surface of inorganic filler particles can be effectively adsorbed and coated by the molecules of silane coupling agent.Also,modified ultra-fine graphite,graphene oxide,hexagonal boron nitride nanoparticles,and hexagonal boron nitride nanoparticles/graphene oxide mixture were dispersed and mixed in a cast polyurethane prepolymer in a vertical type and high energy-density stirred beads mill in the presence of a non-ionic organic dispersant Trition X-100,respectively.Subsequenty,a chain extender was added for cross-linking to prepare a cast polyurethane-based nanocomposite with different modified filler particles.The dispersibility,thermal conductivity and mechanical property of the samples were characterized by SEM,laser thermal conductivity analyzer,abrasion tester and Shore hardness tester.The effects of dispersive method?i.e.,mechanical agitation and stirred beads mill?,parameters of stirred beads mill?i.e.,rotational speed of agitator,bead size and dispersion time?and type and amount of dispersants on the thermal diffusivity,thermal conductivity,wear rate and the Shore hardness of the prepared polyurethane-based composites were investigated.The results show that,compared to the dispersion by mechanical agitation,the inorganic filler nanoparticles can be effectively mixed and dispersed in cast polyurethane prepolymer by stirred beads mill with 1.0-1.2 mm yttrium-stabilized zirconia beads at a rotational speed of 3000 rpm and dispersion time of 60 min,leading to the polyurethane-based nanocomposite with superior thermal conductivity and mechanical properties.This can be attributed to the appropriate addition of non-ionic organic dispersant Trition X-100 as well as the shear,compress and impact stress effects of beads in their intensive three-dimensional movement in the mill,which can promote the modified inorganic filler particles to be uniformly dispersed into polyurethane matrix and have a good compatibility with polyurethane matrix.The effects of incorporating one kind of inorganic particles?i.e.,modified ultra-fine graphite,modified graphene oxide or modified hexagonal nitridation nanoparticles?or co-incorporating modified hexagonal boron nitride nanoparticles/graphene oxide mixture and their respective content on the thermal conductivity and mechanical property of polyurethane-based nanocomposites were investigated.The thermal conductivity and mechanical property of sample were characterized by laser thermal conductivity analyzer,abrasion tester,Shore hardness tester and infrared thermal camera.The results show that the thermal conductivity of polyurethane-based composites can be improved by incorporating one kind of modified particles alone or co-incorporating two kinds of modified particles mixture.The thermal conductivity of polyurethane composite incorporating 15.00 wt.%ultra-fine graphite,1.50 wt.%graphene oxide,or 10.00 wt.%hexagonal boron nitride nanoparticles alone is 0.481 m W K^-1,0.489 m W K^-1 or 0.433 m W K^-1,respectively.However,the mechanical property?i.e.,wear rate and the Shore hardness?of polyurethane-based composites cannot be enhanced by the incorporation of one kind of modified particles.Co-incorporating modified hexagonal boron nitride nanoparticles/graphene oxide mixture can improve the thermal conductivity of nanocomposites,i.e.,the thermal conductivity of polyurethane-based nanocomposite with10.00 wt.%modified hexagonal boron nitride nanoparticles and 2.00 wt.%modified graphene oxide is 0.671 m W K^-1,which is increased by 187.98%,compared to that of neat polyurethane.The wear rate and the Shore hardness of the nanocomposites are 2.33%and 90,which are 7.91%lower and 3.45%higher than those of neat polyurethane,respectively.This is attributed to the inclusion of the inorganic filler particles with great thermal conductivities,which can build a"thermally conductive rapid path and network"in polyurethane matrix and improve the thermal conductivity of polyurethane-based nanocomposite.Also,the co-incorporation of modified hexagonal boron nitride nanoparticles/graphene oxide mixture into polyurethane matrix can prevent the micro-cracks in the matrix from expanding to synergistically reinforce the mechanical property of polyurethane-based nanocomposites.In addition,the effect media approximation?EMA?was also used to calculate and analyze the Kapitza thermal resistivity of the interface between unmodified or modified ultra-fine graphite,graphene oxide,or hexagonal boron nitride nanoparticles and polyurethane matrix.The analysis shows that the thermal resistance of the interface between modified inorganic particles and polyurethane matrix can be effectively reduced.Finally,this dissertation gives the conclusions obtained based on the experimental results and the corresponding analysis,and some future prospects.