| Building materials, indoor decoration materials and packaging materials with plant fibers as the main raw materials are a kind of flammable materials which is easy to cause fire. The research of the flame-retardant mechanism of plant fiber paper and the development of flame-retardant technology are required to reduce the fire hazard and cut down the casualties and property losses in the fire. Adding inorganic flame retardants into paper stock is an effective way to prepare flame-retardant paper. In this paper, in situ polymerization was used to prepare bi-component and tri-component composite flame-retardant fillers with ammonium polyphosphate(APP) and diatomite as the main raw materials. The flame retardancy, smoke suppressing and the mechanism of filled paper with composite fillers were studied systematically, and the effects of surface modification on the flame retardancy, smoke suppressing of composite fillers were investigated. The research provides an effective way for the preparation of inorganic composite flame-retardant fillers applied in papermaking, and is of great significance in the enrichment of flame retardant and smoke suppression theory of plant fiber paper.Bi-component APP-diatomite composite fillers of different compositions were prepared by in situ polymerization, and tri-component APP-diatomite/ synergist composite fillers were prepared with SiO2, aluminium hydroxide(ATH) and TiO2 as the main flame-retardant synergists, then silane coupling agent KH550 was used to modify the prepared composite fillers, finally, composite fillers before and after surface modification were added into paper stock to prepare flame-retardant paper. X-ray diffraction(XRD), fourier transform infrared spectroscopy(FTIR) and thermogravimetric analysis(TGA) were used to characterize the composite fillers, and limit oxygen index, cone calorimetry and scanning electron microscope(SEM) were applied to analyze and evaluate the flame retardancy and smoke suppressing of the filled paper, combined with energy dispersive spectrometer and pyrolysis-gas chromatography-mass spectrometry(PY-GC-MS), the flame-retardant and smoke- suppression mechanism of filled paper were discussed.The results show that APP-diatomite composite fillers have two weigh loss stages, the first decomposition stage is completed at about 550 ℃, and the second decomposition stage occurs at the temperature range of 550~750℃. When APP-10% diatomite composite filler loading in paper is 20%, the filled paper can reach the standard of flame retardancy, and its peak heat release rate(PHRR), effective combustion heat(ECH) and mean mass loss rate(mean MLR) are 41.01 k W /m2, 11.2 MJ/kg and 0.00396 g/s, respectively, which are the lowest among paper filled with other bi-component composite fillers. The filled paper has the maximum time to ignition(TTI) and fire performance index(FPI), which means that it has better safety. After combustion, the filled paper forms complete char layer with clear fibrous network, strong structure and little deformation, that is, APP and diatomite have synergistic charring effect in condensed phase. APP in composite filler increases the smoke release rate(SRR) and CO production rate, and diatomite in composite filler effectively inhibits the release of smoke and CO gas.After modification of APP-10% diatomite composite filler with silane coupling agent KH550, the LOI value of the filled paper increase by 2%, and the PHRR and mean MLR decrease by 17.34% and 3.79% respectively. Modification also results in the increase of the smoke release rate(SRR) and CO production rate of the filled paper. After combustion, the element C, Si and P in the residue char increase, and part of the element C exist in the form of C=C、C≡C and C≡N bonds, which increases the strength of the residual char and makes it more denser.Among the tri-component composite fillers, APP-10% diatomite/2%SiO2, APP-10% diatomite/4%TiO2 and APP-10% diatomite/4%ATH have better flame-retardant effect. [SiO(PO3)2]n、TiP2O7 and [Al(PO3)3]n are the reaction productions derived from SiO2, ATH, TiO2 and APP, which restrain the volatilization of APP in the second weight loss stage and decrease the weight loss of the system at high temperature. They produce obvious synergy effect on char formation and smoke suppression, and decrease the heat release rate, smoke release rate and smoke toxicity of the filled paper, thus reduce their risk in fire. Among the above three tri-component composite fillers, APP-10% diatomite/4%TiO2 has the best flame-retardant effect. After combustion, the residual char of paper filled with APP-10% diatomite/4%TiO2 and APP-10% diatomite /4%ATH have higher C, P and Si content than that of paper filled with APP-10% diatomite/2% SiO2, and the element P is higher in the residual char of paper filled with APP-10% diatomite/4%TiO2 than that of paper filled with APP-10% diatomite /4%ATH. The addition of TiO2 results in more C≡C group in char layer. The O-P-O stretching vibration can be observed in all of the FTIR spectra of the residual char, which indicates that more phosphate is formed. TiO2 and APP show better synergistic flame-retardant effect on filled paper which makes it form stronger residual char with no crack and bulge, and keep a complete reticular structure. Composite fillers influence the pyrolysis reaction of cellulose and hemicellulose during combustion, reduce the rupture of cellulose glycosidic bond, inhibit the production of dehydrated saccharides, and reduce the generation of the flammable volatile substances. Therefore, composite fillers protect cellulose and hemicellulose, and reduce smoke release rate and smoke toxicity of the filled paper.Surface modification of tri-component composite fillers improves the LOI values of the filled paper to some extent, further decreases the heat release rate and mass loss of the filled paper, and the filled paper forms stronger and denser residual char after combustion. However, surface modification of tri-component composite fillers also results in the rise of the smoke release rate and CO production rate. |
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