Studies on Mechanism of Degradation of Bisphosphates and Synthesis and Application of Phosphaphenanthrene Derivatives

Author:Liu Chuan Chuan

Supervisor:yao qiang


Degree Year:2018





With the improvement of environmental awareness and the development of relevant laws and regulations,the use of halogen-based flame retardants has been increasingly restricted and even banned,thus halogen-free flame retardants have been greatly developed.Phosphorus-based flame retardants(PFR)are one of the best alternatives due to their high flame-retardant efficiency,good compatibility with the matrix,and low to none toxicity.Moreover,polymeric materials blended with PFR release less smoke when burning.Althrough there are numerous studies on PFR,only some of PFR have been commercialized.The main reasons include that the flame retardant mechanism of PFR is complex and hasn’t been clearly uncovered and the relatively low thermal stability of PFR restrict their wide application.These problems severely restrict the development and commercial application of PFR.Focusing on these problems encountered in the development of PFR,we have performed systematical studies on the degradation and flame retardant mechanisms of PFR and the chemical structure factors that affect the flame-retardant efficiency of PFR in this paper.Some useful conclusions were obtained which have a positive role in the development of PFR.The detailed research and achievements are as follows:(1)The degradation mechanism and the flame retardant mode of bisphosphates(RDP and BDP)have been systematically illustrated.RDP and BDP were heat-treated in a TGA,in a tube furnace and on a flame respectively,and then the degradation products were analyzed by 31P NMR,Q-TOF,and GPC.The results shows that RDP or BDP generates arynes and acid phosphates directly without existence of water,and acid phosphates can condense to pyrophosphate and water.Thus,hydrolysis can happen to RDP with the formed water.In addition to acid phosphates,phenol and hydroxyphenyl diphenyl phosphate are also produced through hydrolysis.Phenol reacts with a second RDP to yield TPP and more hydroxyphenyl diphenyl phosphates,which are in turn able to transesterify another RDP to produce oligomeric aryl polyphosphatesandphenol.Throughtheintertwinedcondensations,transesterifications and elimination reactions,the reactive acid phosphates eventually convert the bisphosphates into volatile triphenyl phosphate,non-volatile oligomeric aryl/acid polyphosphates and polyphosphoric acid.It is these reactions and products that provide the bisphosphates effective pathways for entry into the vapor phase or action in the condensed phase.The facility of these reactions is critical for the high efficiency of flame retardancy displayed by the bisphosphates.In addition,aryl phosphonates containing P-C bonds were observed in NMR and Q-TOF mass spectra,suggesting that the RDP(or BDP)undergoes Melvin Rearrangement during extreme heating.That the degradation of aryl phosphates begins with the elimination reaction to generate arylyne has been proposed for the first time.Meanwhile,the electron-withdrawing groups favor the elimination of the reaction according to comparison of the electronic effects,which accelerates the production of phosphoric acid.These findings not only overcome the limitations of the traditional understanding that the formation of(poly)phosphoric acid requires an external water,but also provide a theoretical foundation for designing efficient PFR.In addition,the present study found that aryl phosphates can generateβ-hydroxy phosphonates through rearrangement reaction,which corrected the conventional misunderstanding that polymers(PC)need to degrade and rearrange before reacting with bisphosphates.(2)The effect of different bridges between P and N in P/N containing flame retardants on flame retardant efficiency has been sysmatically illustrated.Inspired by the base-catalyzed flame retardant mechanism of sulfonates in PC,an organic basic group and a phosphaphenanthrene group were introduced into the molecule of flame retardant from the view of relationship between flame retardancy and chemical structure.DFPZ(P-C-N)with alkalinity and DPZ(P-N)with no alkalinity were prepared according to the different bridging modes of P atom and N atom.In spite of the close similarities in the structures,DFPZ and DPZ show a significantly difference in flame retardant efficiency in PC.DFPZ exhibits excellent flame retardancy and enables PC to achieve a UL-94 V0 rating at only 3 wt%,DPZ is much less effective and it requires 10 wt%of DPZ for PC to gain a UL-94 V0 rating.The products from the reaction between DFPZ(or DPZ)and the model compound DPC were analyzed by GC-MS.The degradation behaviors of PC/FRs were studied by TGA and FTIR.The morphologies and chemical components of the combusted residues were analyzed through SEM、ICP and ATR-IR analyses.Finally,the flame retardant mechanism was given as follows:Alkaline DFPZ can catalyze the rearrangement of PC to generate ortho-substituted benzoic acid which further transesterifies PC,and thus the PC chains are broken to generate oligomers.However,DPZ cannot catalyze PC to perform the above reactions.Therefore,PC/3%DFPZ generates severe dripping when the fire source is applicated during UL-94 test,droplets produced take away heat,and thus the temperature of the combustion surface is reduced.All tested specimens of PC/3%DFPZ show―driping without burning‖.This study shows that the structural arrangement of the phosphorus-based flame retardants has an important influence on the flame-retardant efficiency.(3)Phosphorus-based flame retardant(PhDOPO)with high thermal stability was designed and synthesized,and its applications in bisphenol A type epoxy resin(EP)and Poly(lactic acid)(PLA)has been studied.In order to overcome the deficiency of the thermal resistance of traditional phosphorus-based flame retardants,phosphaphenanthrene derivatives PhDOPO was synthesized through introducing phthalimide structure,which exhibited high thermal stability with T5%>320°C in the air.PhDOPO shows good flame retardancy in EP.The LOI of EP is increased from 24%to 35%or more with only 5 wt%of PhDOPO.And V1 rating was received when 10wt%or 15 wt%of PhDOPO was added.By compounding with organically modified hydrotalcite(OLDH)and PhDOPO,EP can pass through the V0 rating with a total addition of 10%by weight.Further studies show that the addition of 4 wt%of PhDOPO and 1 wt%of organoclay(OMMT)allows the EP to pass through the V0rating.PhDOPO not only endows EP excellent flame retardancy but also improves the mechanical and electrical properties.The tensile strength and modulus of EP increase by 25.4%and 14.9%respectively,and the dielectric constant decrease by 9.2%when15 wt%of PhDOPO is added.In addition,PhDOPO also has good flame retardant effect in PLA.7.5 wt%of PhDOPO helps PLA pass V0 rating,also it has no adverse effect on the physical appearance,thermal stability,processing performance and mechanical properties of the material.