Preparation Of Fast-crystallizing Waterborne Polyurethane And Its Application In Thermally Conductive Adhesives

Based on the fact that electronic products are easy to generate heat during the process of long-term use,resulting in thermal aging damage and other problems.We urgently need materials with good heat transfer performance to dissipate the heat generated under high-frequency working conditions.Water-borne polyurethane(WPU)is an environmentally friendly polymer material,the incompatibility between the soft and hard segments leads to the generation of a phase separation structure,the degree of phase separation depends on the magnitude of hydrogen bonding,which determines the thermal conductivity of WPU.In the other words,the short range,long range order and microcrystalline structure in the polyurethane are conducive to the heat transfer of WPU.However,the thermal conductivity of WPU is low,so we add fillers in WPU to form a thermal network through the connection between particles and broaden the phonon conduction pathway to improve the thermal conductivity of WPU furtherly.In this paper,a series of WPU were synthesized with PBA,HDI,IPDI,which were used as the main material,and the hydrogen bonding force between molecules was studied from the structure.Finally,the WPU with excellent crystallization rate,crystallinity and bonding strength was selected to prepare the thermal conductive adhesive.In the second chapter,a series of waterborne polyurethane emulsions were synthesized by changing the ratio of isocyanate and n_—NCO:n_—OH,while the structure and performance of WPU emulsion and film were characterized.As the proportion of HDI increases,the hydrogen bonding degree of the system increases,the hydrogen bonding force between the hard segment and the soft segment decreases,the water resistance of the film improved,the particle size of the emulsion becomes smaller,and the viscosity becomes higher.DSC,XRD test and crystallization kinetic analysis showed that the regular linear structure of isocyanate promoted the crystallization of PBA.The mechanical performance test results found that the initial adhesive strength of the film increased,the tensile strength and final peel strength first increased and then decreased.With the increase of the ratio of n_—NCO:n_—OH,the infrared peaks show that the hydrogen bonding force of the system decreases,the water absorption of WPU becomes larger,the particle size of the emulsion decreases,and the viscosity becomes higher.Through the test and analysis of the crystallization and mechanical properties,it is found that the increase of the proportion of the hard segment limits the growth of the soft segment crystals,which leads to the decrease in the degree of crystal perfection,and the initial bond strength and final peel strength are reduced gradually.In conclusion,when m_HDI:m_IPDI=2:1,n_—NCO:n_—OH=1.4,the crystallinity,mechanical properties and adhesion properties of WPU film are excellent.In the third chapter,a series of waterborne polyurethanes were synthesized by six chain extenders,and the effects of different types of glycol chain extenders on the crystallization and bonding properties of polyurethane films were investigated.Under the infrared peaks,the hydrogen bonding degree results of WPU films prepared with different chain extenders show that aliphatic chain extenders>aromatic chain extenders>alicyclic chain extenders.The lower the methylene content of aliphatic chain extender,the stronger the hydrogen bond force of WPU.Through the study of crystallization properties,it is found that the crystallinity and crystallization rate of WPU decrease as the steric hindrance of the chain extender increases,but the aromatic ring structure of HER was favorable to the crystallization of WPU due to its polarity.The mechanical test results show that the stretch and adhesion properties of the film prepared by the aromatic chain extender are stronger than those prepared by the aliphatic chain extender and the alicyclic chain extender.In the fourth chapter,graphene oxide(GO)was prepared first,and modified with 3-aminopropyltriethoxysilane(KH550),and then reduced under acidic conditions to obtain modified reduced graphite oxide(MRGO).After that,the modified alumina(MAl₂O₃)was loaded on MRGO and introduced into the waterborne polyurethane to get the modified alumina-reduced graphene oxide/water-based polyurethane adhesive(MRGO-MAl₂O₃/WPU)composite material.The effect of the ratio between fillers and the amount of mixed fillers on the adhesion and thermal conductivity of waterborne polyurethane was explored.The characterization of the composite filler indicates that MAl₂O₃is successfully loaded on MRGO.SEM shows that a small amount of filler disperses evenly in the matrix,while too much is easy to agglomerate.The Tg test shows that the addition of fillers improves the heat resistance of the film.The mechanical performance test shows that with the increase of the ratio of m_MRGO:m_MAl2O3,the tensile and adhesive properties of the film are improved.When the content of mixed filler was 0.2%,the bonding strength of the composite film reaches122.3N/25mm.The thermal conductivity test results show that the thermal conductivity of composite adhesive film is higher than that of the pure WPU matrix.It can be seen that the fast-crystallizing waterborne polyurethane can be potentially used in thermally conductive adhesives.