| The existing flame retardant technology generally has the problems of complex process,low efficiency,large addition amount,poor sustainability,etc.,and conventional physical blending can easily lead to significant reduction of mechanical properties of materials,so it is urgent to develop new flame retardants.In this study,the surface modification-chemical grafting modification was proposed to improve the flame retardancy of bamboo charcoal(BC),and the expanded carbon-based synergistic system with high flame retardancy and environmental protection was constructed by compounding with biological carbon source.The mechanical and flame retardant properties were taken into account,the preparation process of flame retardant BC was simplified,and the bonding mechanism and internal mechanism of flame retardant grafting were discussed,which provided experimental basis for the research and development of green and efficient flame retardant system and new high flame retardant and environmental protection carbon plastic board.The main results are as follows:(1)Flame retardant BC was successfully prepared,and the best flame retardant loading was 17.76%(BC-m).Surface modification makes the surface of BC form acidic oxygen-containing functional groups——phenolic hydroxyl and carboxyl,especially hydrogen peroxide under low-power microwave oxidation treatment.The chemical structure of the surface of BC changed and became amorphous,and the contents of N,O and P elements increased significantly,with new infrared peaks appearing at 3 370 cm-1,790 cm-1,at al.The aromatic spectrum shows a slight chemical shift to the high field,and many new crystal structures are formed at 2θ=15.92°,23.94°,et al.The average particle size of pure BC slightly increases from 50.93 μm to 54.20 μm.(2)BC-m significantly improves the flame retardant performance of PLA and the stiffness of composite materials.The peak heat release rate(pHRR)and total heat release(THR)of BC-m/PLA composites dropped by more than 50%compared with pure PLA.The filler caused only a small reduction in strength properties,but a slight increase in modulus of elasticity of PLA composites.The limiting oxygen index(LOI)was 28.0 vol%when 10 wt.%of BC-m was added,and 32.1 vol%for 30 wt.%addition,which was much greater than the value of 22.5 vol%for 30 wt.%pure BC.Unlike pure BC,adding BC-m at 20 wt.%or more gave a UL-94 vertical flame test rating of V-0 with significantly reduced melt dripping.The grafted biochar-based system provides an effective flame-retardancy effect by condensed phase protective barrier through the rapid formation of a dense,honeycomb-like cross-linked carbonized char layer.(3)The flame retardant synergistic system was successfully constructed and introduced into PLA,which achieved both mechanical and flame retardant properties.The chitosan(CS)as an adhesion promoter improved the interfacial compatibility between BC-m and PLA as well as the presence of CS promoted the reorganization of the internal crystal structure.When 3 wt.%CS and 30 wt.%BC-m were used,the crystallinity of FPLA-C33 increased to 38.92%,which is 43 times that of pure PLA(0.90%),and a maximum increase of tensile strength and modulus with the addition of CS were obtained by up to 11.11%and 8.42%,respectively.The residue(29.42%)was 55.09%higher than the theoretical value(18.97%)as consequence of cooperation of 5 wt.%CS and 30 wt.%BC-m.The results of LOI,UL-94 tests and the corresponding cooperative effectiveness(Ce)show that the synergistic effect of CS and BC-m is remarkable.Moreover,the pHRR and THR of FPLA composites for 3 wt.%CS added to 20 wt.%BC-m were reduced by 26.89%and 30.50%,respectively.The analysis of the morphology,chemical elements and structure of the carbon residue reveals that CS and BC-m can form more stable intumescent carbon layers containing pyrophosphate by cooperative catalysis.This surface char plays a good role in heat insulation to inhibit polymer pyrolysis,resulting in improved flame retardancy of PLA composites.A mix of 20%BC-m+3%CS is recommended. |
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