Synthesis Of Polyhydroxy-substituted Aromatic Schiff Base And Its Application On Flame-retarded Nylon 6

Schiff base,as a new type of organic compound with C=N double bond in molecular structure,has been used in the fields of catalysis,antibacterial and pharmaceutical.The C=N double bond of polyhydroxy-substituted aromatic Schiff base,which directly attached to aryl groups,can form a stable polybenzoxazine and a hexatomic ring structure by self-crosslinking reaction at high temperature.Simultaneously,dehydration and cross-linking reaction also occur between the polyhydroxy groups on the benzene ring.Under the two combination effect,a stable and high residual char layer will be formed during combustion,which has a broad prospect as flame retardant.Two kinds of polyhydroxy-substituted aromatic Schiff base are synthesized firstly,then used as additive flame retardant by itself or in association with ammonium polyphosphate?APP?to modified polyamide-6.This work not only realizes the application of polyhydroxy-substituted aromatic Schiff base in flame-retarded polymer,but also extends the research scope of polyhydroxy-substituted aromatic Schiff base.Two isomeric polyhydroxy-substituted aromatic Schiff bases N,N'-bis?2,4-dihydroxybenzilidence? oxydianiline ?D₁? and N,N'-bis?2,5-dihydroxybenzilidence? oxydianiline ?D₂? are obtained by the reaction of 4,4'-oxydianiline with 2,4-dihydroxybenzaldehyde and 2,5-dihydroxybenzaldehyde respectively.FTIR and NMR results confirmed the successful synthesis of D₁ and D₂.Thermogravimetric?TG?analysis reveals that both D₁ and D₂ possess high thermal stability and rich char residue.Their initial decomposition temperatures(T_5%)are higher than the processing temperature of PA6,and the char residue rate at 700°C is 63%and 54%,respectively.At the same time,Differential Scanning Calorimetry?DSC?analysis indicates that D₁ has no melting point before its decomposition temperature but D₂,which play a positive action in anti-dripping.D₁ and D₂ with high thermal stability and char residue rate can be used to halogen-free flame retarded PA6 by melt blending.D₁ shows the better flame retardancy and anti-dripping behavior between the two Schiff bases.25 wt%D₁ upgrades the UL-94 ranking of PA6 to class V-1 without melt-dripping.On the contrary,droplet behavior still occurs in PA6/30D₂sample containing 30 wt%D₂.It is deduced that isomeric D₁ and D₂ perform the different function on droplet suppression in PA6,probably due to their different melting behavior and char cross-linking degree.Importantly,based on the analysis of test results,the char-forming flame retardant mechanism of D₁ is systematically discussed.In order to improve the flame retardant efficiency,D₁ and D₂ are respectively used as a synergist to prepare halogen-free flame-retarded PA6 with APP.The addition of 20 wt%APP alone hardly improves the flame retardancy of PA6,and there is still a behavior of melt-dripping during the combustion process.The LOI value of PA6 with 12 wt%APP and 8wt%D₁?PA6/12APP/8D₁?is significantly increased and passes UL-94 V-0@3.2 mm grade with an expanded char layer.However,the combination between D₂ and APP can not effectively improve the flame retardancy and anti-dripping behavior of PA6.From the cone calorimeter results,the total heat realease?THR?value and the total smoke realease?TSR?value of PA6/12APP/8D₁ are both lower than that of PA6/20APP.TG analysis and Raman Spectroscopy Analysis of char residue demonstrate that D₁+APP could promote the char formation,forming a compact intumescent char structure.It is proposed that the synergism between D₁ and APP might be responsible for the improvement of flame retardancy.In addition,the overall mechanical property of PA6/12APP/8D₁ is higher than that of PA6/20APP.