Research Of Flame Retardant Polyamide 6 Composites With Synergistic System Based On Aluminum Hypophosphite

Polyamide 6 (PA6) has been widely used in transport, electronics, building and other fields due to its excellent mechanical performance and chemical resistance. However, PA6 materials is flammable and produce a great deal of dripping during combustion, which easily cause blaze. Consequently, it is important of flame retaredant modification for the PA6 materials. It is demonstrated that aluminium hypophosphite (AHP) has excellent flame retardancy for PA6, but there is also some critical issues, such strong hygroscopicity, poor compatibility and flame retardant efficiency.In this paper, hexa-(4-aldehyde-phenoxy)-cyclotriphosphazene (HAPCP) was firstly synthesized and used in surface modification for AHP to prepare the modified AHP, the structure was characterized and confirmed and the wettability of AHP before and after modification was evaluated by water contact angle tests. The water contant angle of the modified AHP dramatically increased from 0° of pure AHP to 137° from, which demonstrate the hydrophobicity of AHP was greatly enhanced due to the surface modification. The modified AHP was incorporated into PA6 matrix to prepare flame retardant PA6 composites. The flame retardancy of materials was investigated by limiting oxygen index (LOI) and vertical burning (UL-94) tests, the mechanical properties of materials were measured by the tensile, flexural and impact strength tests, the thermal degradation behavior of materials were studied by thermogravimetric analysis (TGA) tests, the combustion behavior of materials was investigated by cone calorimeter (CONE) tests, the surface morphologies of the char layer and the compatibility between flame retardant and PA6 matrix were measured by scanning electron microscopy (SEM), the elemental composition on the char layer surface were tested by X-ray photo electron spectroscopy(XPS), the water resistant properties of-materials were evaluated by putting the samples into distilled water at 70℃ for 168 h. The results indicated that the samples passed the UL-94 V-0 flamability rating and the LOI value reached 25.6% when the loading amount of AHP was 24 wt%. However, the AHP was surface modified with HAPCP and the mass ratio of HAPCP to AHP was 19:1, the flame retardant PA6 materials passed UL-94 V-0 flamability rating and the LOI value enhanced to 27.6% when the loading amount of the modified AHP was 21wt%. Compared with the pure AHP, the modification of AHP improved the compatibility between the flame retardant and PA6 matrix, consequently the mechanical properties of materials were corresponding increased. The results of TGA and cone tests revealed that the surface modification of AHP was almost not change the thermal degradation behavior of materials, but the total smoke production (TSP) of materials was decreased. The incoroporation of AHP modified with HAPCP stimulated the formation of more compact and continuous char layer with rich phosphorus content after combustion. After water resistance test, the PA6/AHP composite can not pass UL-94 rating and the LOI value was 22.5%, meanwhile the weight loss rate of the composite was 1.03 wt%. However, the PA6/ modified AHP composite samples after water resistance tests still successfully passed UL-94 V-0 flamability rating with the LOI value of 26.2%, and the weight loss rate was only 0.49 wt%. Compared with the water treated PA6/AHP composite, the combustion parameters of THR and TSP obviously lower after water resistance test. The fact domonstrated that the surface modification of AHP improved the water resistant properties of flame retardant PA6 composites.In order to futher improve the efficiency of AHP and reduce the damage to mechanical performance for PA6 materials, epoxy-silicon resin (ESR) was selected and combined with AHP to flame retardant PA6 matrix. The results indicated that the PA6/AHP/ESR composite passed UL-94 V-0 rating and LOI value reached 25.8% when the loading amount of flame retardant was 18wt% with the mass ratio of ESR to AHP was 19:1. The fact revealed that ESR and AHP had excellent synergistic effect for flame retardant PA6.. The mechanical properties of the materials were increased due to the introduction of ESR compared with the same amount of AHP alone. Meanwhile, the introduction of ESR increased the char residue of flame retardant materials at 700℃, the combustion parameters of SPR, TSP were decreased and the formed char layer was more compact and continuous with more rich carbon content.When AHP alone was used to flame retardant PA6 with low additional amount, the composites showed the climbing pole phenomenon during combustion. Triphenyl phosphate (TPP) with low molecular weight was selected and combined with AHP to flame retardant PA6, and TPP was expected to migrate to the surface of materials during processing and combustion. The results indicated that the PA6/AHP/TPP composite passed UL-94 V-0 rating and LOI value reached 28.4% when the loading amount of flame retardant was 22wt% with the mass ratio of TPP to AHP was 98:2. Compared with PA6/AHP composite, the introduction of TPP enhanced the char residue at 700℃ and the combustion parameter of HRR, SPR, TSP of materials were corresponding decreased, and the phosphorus content in the char layer surface was also increased after combustion.