Study On Design And Flame Retardant Properties Of Melamine Based Covalent Organic Framework/Polymer Composites

Covalent organic frameworks（COFs）with many advantages such as low density and large specific surface,etc are novel two-dimensional or three-dimensional organic crystalline porous materials that are connected by strong covalent bonds.However,the drawbacks of expensive raw materials and harsh experimental conditions have greatly limited their further development.The main research approaches of this paper are divided into the following sections.The first is to design and prepare the COFs with the advantages of low cost,easy to preparation,good thermal stability and good char forming.Secondly,for the problem of low thermal stability and low fire safety of polymer composites,we design and prepare high-safety COFs-based composite materials to further expand the application of COFs in engineering.In the second to fifth chapters of this thesis,we prepare COFs nanosheets-based polymer nanocomposites and study their pyrolysis behavior,combustion properties and flame-retardant mechanism.In the sixth chapter of this thesis,considering the shortcomings of poor stability and poor electrolyte wettability of commercial lithium battery polyolefin separators,we prepare COFs/polyolefin microporous composite membranes with high size stability,high flame retardancy and battery performance.The specific research works are as follows:1.Two type of COFs nanosheets were prepared by mechanical ball milling and n-butyllithium（n-BuLi）intercalation-exfoliation due to the graphite-like structure of two-dimensional COFs.And then thermoplastic polymer/COFs nanocomposites were prepared by solution blending.COFs nanosheets improve the fire safety and mechanical properties of polymer composites significantly.As a nano-material composed of all-organic component,COFs nanosheets have good dispersibility in polymer at a high addition of more than 5 wt%compared with traditional inorganic nanomaterials.The first is due to the better compatibility of the COFs nanosheets with the polymer matrix.The second is due to the hydrogen bonding effect,which results in good interfacial interaction between the COFs nanosheets and the polymer matrix.2.The sodium lignin sulfonate was used as a modifier to exfoliate and functionalize the COFs nanosheets（mCOFs）simultaneously by non-covalent interaction under the action of ultrasound.The chitosan/mCOFs nanocomposite film was prepared by solution blending.The results show that mCOFs nanosheets can significantly improve the flame retardancy,thermal stability and mechanical properties of chitosan.This is because the good compatibility and hydrogen bonding interaction between mCOFs nanosheets and chitosan.The mCOFs nanosheets not only act as a physical barrier to block heat radiation and retard thermal degradation of chitosan,but also improve the degree of graphitization of char residue,thereby improving the thermal stability and flame retardancy of chitosan.3.The COFs nanosheets were prepared by mechanical ball milling.The covalently bonded functionalized flame-retardant COFs（FCOFs）nanosheets were designed and prepared,and then flame-retardant epoxy resin/FCOFs nanocomposites were prepared.The mechanical properties of the epoxy resin are significantly improved.It has been proved that COFs nanosheets have good compatibility and dispersibility not only in thermoplastic polymers but also in thermosetting thermoplastic polymers.FCOFs nanosheets can significantly reduce the burning properties and the generation of smoke gas during combustion of epoxy resins.This is t because FCOFs nanosheets not only promote char formation,but are themselves an intumescent flame retardant that combines an acid source,a carbon source and a gas source.4.Based on the feature of rich surface functional groups of COFs,the cobalt trioxide（Co₃O₄）/COFs nanohybrids were prepared by electrostatic interaction and then added to polypropylene（PP）to prepare PP composites.Due to the catalytic action of Co₃O₄,PP showes good thermo-oxygen stability.Co₃O₄/COFS nanohybrids can significantly reduce the heat of combustion and the generation of smoke gas during combustion of PP composites.This is because the Co₃O₄/COFS nanohybrids prevent the release of thermal radiation and gaseous pyrolysis products during combustion,resulting in a decrease in the PHRR,THR,SPR and TSR values of PP.Moreover,Co₃O₄ catalyzes the oxidation of CO,which in turn reduces the release of toxic gases during the combustion of PP composites.5.Based on the good thermal stability and electrolyte wettability of COFs,COFs/polyolefin microporous composite membranes with different COFs content were prepared by coating.COFs-based composite membranes have good battery performance and safety performance.When the mass ratio of COFs:PVDF is 8:1.5,the composite separator has the outstanding dimensional stability,combustion properties and battery performance.Surface-grafted technology was used to grow hydroxyl-terminated phosphorus-containing hyperbranched flame retardants on the surface of COFs（PCOFs）and PCOFs/polyolefin microporous composite membrane was prepared.It was found through experiments that the battery performance and safety performance of the polyolefin microporous composite membrane were further improved.