Abstract:

Three types of hyperbranched polyphosphoramide coated carbon nanotubes （CNTs/HBPPA） with different coating thicknesses were prepared through one-step polymerization （A₂+B₃） using carboxylic CNTs （CNT-COOH） as a core material， phosphorus oxychloride （POCl₃） and N， N?diaminodiphenylmethane （DDM） as monomers， and triethylamine as an acid binding and catalytic agent. A series of epoxy （EP） composites with different contents of CNT/HBPPA were then fabricated， and the reinforcing and flame?retarding effects were investigated. It was found that with increasing the mass ratio of CNT to （POCl₃+DDM） （1∶1.5~6）， the there is an increase in the coating thickness of CNT/HBPPA but an reduction in T_5% （the characteristic temperature at 95 wt.% mass loss） and residual char yield. The CNT/HBPPA cross-linked together when the mass ratio increased to 1：6. Among those composite samples， the CNT/HBPPA-3 exhibited optimum reinforcing and flame-retarding effects on the EP composites. The experimental results indicated that the EP composites containing 2 wt% CNT/HBPPA achieved the largest LOI of 28.2 %， highest tensile of 53.28 MPa， and highest impact strength of 11.19 kJ/m² as well as a UL 94 V-1 classification. There are a great number of active groups of -NH₂ dispersed on surface of CNT/HBPPA， leading to better interfacial compatibility with the EP matrix. Therefore， the additives could be distributed more homogeneously and act as the net-shaped skeleton in the EP composites to achieve better flame retardancy and mechanical properties. During the combustion process， the skeleton of CNT/HBPPA potentially cross-linked together to construct a more compact and continuous char layer， which could protect the underlying materials against the heat and combustible volatiles transmission efficiently.

Key words: hyperbranched polyphosphamide, carbon nanotubes, epoxy resin, flame retardancy