| Microencapsulation technology is to use natural or synthetic polymer to encapsulate solids, liquids, gases or even micro-capsule of nuclear material, forming a 1—1000μm in diameter of a semi-permeable or closed membrane. Ammonium polyphosphate (APP) has flame retardance, high decomposition temperature, thermal stability; low solubility, small hygroscopicity. And it is widly used in paint, rubber and plastics, with little destruction of materials'physical properties. Since the restruction of APP's production conditions, general APP obtained hasn't good compatibility with materials, which can not meet the requirements of mechanical properties. So it needs surfacial modification to meet the thermal stability requirements of certain polymers. And there are still shortcomings in the larger moisture absorption of APP, which limits the application in materials.Firstly, this paper outlined the current methods of preparing microcapsulated flame retardant and the recent development. By comparing the relative merits of various methods, it found that in-situ polymerization was more appropriate to microcapsulate APP.Secondly, using of pentaerythritol and 2,4 - toluene diisocyanate as raw materials to prepare nuclear-shell type flame retardants by in-situ Polymerization. Microencapsulated APP's characteristic peaks disappear and / or weakened, and a noticeable characteristic peaks of polyurethane appeared; the surface of microcapsule has low phosphorus content, and the surface elemental composition was very close to experimental design elements of the product. the water solubility of MCAPP at different temperatures decreased at least 80%; the product of the T-5wt% is 316.1℃, And MCAPP can enhance its thermal stability of PU and compatibility, even in the damp environment, MCAPP could maintain its main flame resistance, PUMCAPP30 treated by water at 75℃for 7d, it can pass UL-94 V-0 testing. its PHRR and THR decreased 38.4% and 28.1%, due to the compact char forming in combustion process.Thirdly, useing PEG to improve the toughness of polyurethane microcapsule shell based the front work.when PEG-1000/PER=2:1, microcapsule has the most complete morphology, with the best water resistance. When introducting flexible PEG chain, the hydrogen bond decreased. And surfacal phosphorus content of MAPP decreased from 11.31% to 1.46%. The initial decomposition temperature (T-5wt%) of MAPP is 292.6℃, which is suit to flame -retardant major material. MAPP greatly improved the thermal stability, flame-retardant and compatibility with PU. At the same time, PUMAPP25 dealed in water with 7d, had only loss of 4.9% flame retardant, and can reach UL-94 V-0 test.Fourthly, using sol - gel method to prepare silica gel microcapsulated APP, to improve water solubility of the silicon-phosphorus flame-retardant system. Microcapsules appeared in Si-O-Si bond and Si-O-C bond stretching vibration peak in FTIR. The content of phosphorus N atoms of MCAPP decreased to 1.84 and 3.79%, the O1s peaks belonged to Si-O bond in XPS. The contact angle with water of MCAPP decreased, which means significant improvement of wettability. The SEM of MCAPP showed rough surface, with no sharp edges and corners of the crystal. T-5wt% of MCAPP is 284. 2℃, and it enhanced thermal stability and compatibility with PU. PU composite blending MCAPP decreased the PHRR, THR, as well as MLR, which indicated the synergistic effect of phosphorus - silicon flame-retardant.Finally, it studied the properties of PU composite flame-retarded by different APP and silica gel. Silica gel in PUAPP system can not change the thermal degradation behavior; but the degradation of functional groups (O=CN and NH) obvious inhibited in PUMCAPP. ATR-FTIR study showed that in the beginning of melt, the high melt viscosity resisted the migration of microcapsules, so the silicon content in the suface of PUMCAPP is relatively small. With the rise in temperature, the main chain of PU matrix beginned to decompose, microcapsules dramatically tended to migration by enthalpy driving. As a result, silicon content decreased in the surface of PUMCAPP. Then it formed a small amount of dispersed carbon near the surface, which impeded the migration of microcapsules. As the increase of temperature, the further decomposition of internal microcapsules released much gas, so that the melt viscosity decreased, the carbon layers rapid expanded and a lot of microcapsules migrated to the surface. Then the microcapsules formed a large number of nano-silica-IPN in high temperature, which can effectively prevent transmission of oxygen and heat to the matrix, and improve the dropping of melt.
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