Preparation Of Microencapsulated LDHs With Melamine-formaldehyde Resin And Its Flame Retardant Application In Epoxy Resin

Epoxy resin?EP?,one of the most important thermosetting materials,has been widely applied in many high-tech fields due to its excellent comprehensive properties.However,just as most of the polymer materials,the inherent flammability of conventional EP restricts its application in many areas where fire safety is paramount.Recently,layered double hydroxides?LDHs?,as a new class of halogen-free and inorganic flame retardant,had also been paid considerable attention to be potential fillers for improving flame retardancy and smoke suppression.However,-OH group on the surface of LDHs can promote the formation of the hydrogen bond resulting in agglomeration.Meanwhile,its strong hydrophilicity prevents good compatibility within hydrophobic polymer matrices,which is an important factor that influences the mechanical and flame retardant properties of composites.Hence,it is inevitable to transform the hydrophilic nature of LDHs into hydrophobic one to improve its dispersion and compatibility.Microencapsulation technology had been widely used in magnesium hydroxide,red phosphorus and ammonium polyphosphate,which was meaningful to our study.Firstly,LDHs were synthesized by coprecipitation.Secondly,the microencapsulated LDHs with melamine-formaldehyde?MF?resin were prepared by the in-situ polymerization.During this process,the optimal preparation process of LDHs emulsion,MF prepolymerization and polycondensation were determined by single factor method.Finally,the MCLDHs were added into the epoxy resin in order to investigate the influence of their amount on the mechanical and flame retardant properties of composites and then determined the optimal amount of MCLDHs.The mechanism of mechanical increasing and flame retardant of MCLDHs in epoxy resin were investigated by DMA,TG-FTIR,CONE and SEM.The main results were as follows:?1?The sample LDHs synthesized by co-precipitation method possessed the main characteristic peaks of Mg/Al hydrotalcite,which was in agreement with the theoretical results.At the same time,LDHs had a good layered structure and a degree of crystallinity greater than50%.They can play a role of flame retardancy by releasing water vapor and CO₂ to absorb heat,dilute and isolate flammable gases.Their grain sizes in the a and c directions were all less than100 nm,suggestting LDHs reached the nano-scale.Comparing the effect of emulsifier type,amount and LDHs content on the properties of the emulsion,it was found that the stability and dispersion of LDHs emulsion were best when the emulsifier was SDBS,the emulsifier content was 10 g/L and the LDHs content was 30 g/L.Comparing the effect of M/F molar ratio,prepolymerization time,core/wall ratio,pH and temperature on the overall performance of MCLDHs,it was found that when M/F molar ratio was 1:3,prepolymerization time was 1.5 h,and the core wall ratio was 1:6,pH was 5.5 and polycondensation temperature was 70°C,the surface of MCLDHs was uniformly coated by MF resin,resin coverage rate was 80.56%,the contact angle was 122.1°and activation index was 97.25%.Under this condition,overall performance of MCLDHs was best.In terms of the results of FTIR,XPS,XRD,TEM,and DLS,the cross-linked MF resin existed in the MCLDHs.After microencapsulation,the lamellar structure of MCLDHs was not destroyed under the acid of pH=5.5.Results showed that the water contact angle?WCA?of MCLDHs increased from 8.9oto 122.1o,which indicated good compatibility.The particle size of MCLDHs decreased sharply and 37%were up to sub-micron scale,which can be conducive to dispersion.Below 400oC,MCLDHs was more stable than LDHs.With the increasing of temperature,MCLDHs degraded rapidly.MCLDHs released more small molecules such as CO₂ and NH₃,which can dilute flammable gases to protect the polymer matrix.?2?Comparing the effect of different flame retardant additions on the mechanical and flame retardant properties of epoxy resin,it was found that when the addition amount of MCLDHs is 20%,the overall performance of EP/MCLDHs composites was best:the LOI value was 29.0,the UL-94 rating was V-1,the tensile strength was 28.3 MPa,and the elongation at break was 6.68%.In terms of DMA and SEM,MCLDHs had smaller particle size and similar polarity with epoxy resin,so that it had better dispersion and compatibility in the substrate,which can greatly improve the mechanical properties of EP/MCLDHs.According to the results of CONE,when MF resin and LDHs were separately added to epoxy resin,the flame-retardant effects of the two were not as good as those of MCLDHs,indicating that the synergistic flame-retardant effect existed between MF resin and LDHs.During combustion,MCLDHs not only released H₂O,CO₂,and NH₃,but also formed a stable and continuous protective layer covering the surface of substrate,which can prevent heat transfer and oxygen exchange,thereby inhibiting further burning of substrate.