The Research On The Mechanism And Synthesis Of Inherently Flame Retarded Polyurethane

In the past several decades, the flame retardant industry experienced rapiddevelopment. It is more and more comprehensive and in-depth in researching about flameretardant, flame retardant methods and flame retardant mechanism. In general, there are twomethods used for achieving flame retardancy in polymers, namely,“additive” and“reactive” types. Nevertheless, a variety of problems from “additive” type, such as poorcompatibility, leaching, and deterioration in the mechanical properties of composites,diminish its appeal. Therefore, there has been an increasing interest in the use of “reactive”type imparting permanent flame retardancy, and retaining the mechanical properties of theoriginal polymers. This study has opened up a new direction for the research of flameretardant polyurethanes.The topic of the paper is to prepare a series of intrinsically flame-retardantthermoplastic poly(imide-urethane)s (TPIUs) by incorporating PMDA as a reactive flameretardant in the main chains. The structure and properties of TPIUs were characterized, andthe flame-retardant mechanism was also discussed. Furthermore, other kinds of reactiveflame retardant had been incorporated into the TPIUs in order to improve the flameretardancy of TPIUs. The main works are described as following:A series of TPIUs, based on4,4′-diphenylmethane diisocyanate (MDI) andpyromellitic anhydride (PMDA) as hard segments and poly(tetrahydrofuran)(PTMG) assoft segments, was been prepared by a two-step polymerization process. The thermalbehavior and flame retardancies of the TPIUs had been characterized by thermalgravimetric (TG) analysis and limiting oxygen index (LOI), UL-94vertical burning, conecalorimeter tests. The results indicate that the TPIUs show good thermal stability, flameretardancy, and exhibit excellent anti-dripping effects. Compared to the conventionalthermoplastic polyurethane TPU211(molar ratio: MDI: PTMG:1,4-butanediol=2:1:1),TPIU211(molar ratio: MDI: PTMG: PMDA=2:1:1) with the same formulation exhibitbetter thermal stability, the T5%and Tmaxof TPIU211is33.2°C and18.7°C higher thanthose of TPU211, respectively. Moreover, THR and p-HRR of TPIU211are also15%and64%lower than the respective parameters of TPU211. These results indicate that imides group exhibit good thermal resistance and carbon formation, and suppress the combustionof TPIUs effectively. Furthermore, time to ignition (TTI) and LOI value of TPIUs increasewith the hard-segment contents, while the total heat released (THR) and peak of heatrelease rate (p-HRR) show the opposite trend. The tensile strengths of the TPIUs reached23.1-37.6MPa with increasing hard segment contents, which meets the requirement ofmechanical properties with regard to practical use.Alkynyl groups can fully cross-link to form more stable network structures in heatingconditions, contributing to the flame retardancy of polymers with alkynyl groups.Pyromellitic dianhydride (PMAD) and1,4-butynediol (BTO) were used as chain extenderto synthesis two kinds of TPIUs:(1) PBDCB/TPIU prepared by MDI, PTMG, and PBDCBsynthesized by PMAD and BTO;(2) PMDA/BTO/TPIU prepared by MDI, PTMG, PMDAand BTO. It is showed that the tensile strength and number-average molecular (Mn) ofPMDA/BTO/TPIU are much higher than those of PBDCB/TPIU. This may be attributed tothe higher reactive activities of PMDA and BTO than that of PBDCB, contributing tolonger main chains, and then improving the mechanical properties. Subject to the sameformulation, TG analysis shows that PBDCB/TPIU shows better thermal stability than thatof PMDA/BTO/TPIU. Not only PBDCB, but PMDA and BTO also served as an effectiveflame retardant for TPIU. PBDCB/TPIU and PMDA/BTO/TPIU with thicknesses3.2mmboth can attain ratings of V-2. Furthermore, the flame quickly extinguished after thePBDCB/TPIU was taken away from the fire, however, PMDA/BTO/TPIU did not show thisphenomenon. The LOI value of PBDCB/TPIU increases with the hard segments, subject tothe same formulation, the LOI value of PBDCB/TPIU is higher than that ofPMDA/BTO/TPIU. These results indicate that PBDCB exhibits more excellent flameretardnacy than that of PMDAand BTO.Poly(dimethylsiloxane)(PDMS), as an attractive flame retardant, was used as a partof soft segments to synthesis a series of novel intrinsically flame-retardant TPIUs in thischapter. Compared to TPU211, the obtained TPIU/PDMS exhibited better tensile strengthand thermal stability. Based on cone calorimeter testing, PDMS improves the flameretardancy of TPIU. The condensed phase of TPIU/PDMS had been characterized byscanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDXS). The results indicate that the good flameretardancy of TPIU/PDMS can be attributed to thermally stable compounds with Si-Ostructures on the surface of its char layers.9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is a kind of cyclicphosphate, its derivatives have been widely incorporated into polymers for improving theirflame-retardant properties and thermal stabilities. In this chapter, DOPO derivatives,DOPO-naphthoquinone (DOPO-NQ), were used alone or companied with PMDA as chainextenders to prepared flame retarded DOPO-NQ/TPU or DOPO-NQ/TPIU, respectively.TG analysis shows that the T5%and Tmaxof DOPO-NQ/TPU and DOPO-NQ/TPIU have thesimilar values. DOPO-NQ served as an effective flame retardant for TPU. DOPO-NQ/TPUpolymers with thicknesses3.2mm attain ratings of V-2and the flame quickly extinguishedafter the samples were taken away from the fire. At1.08%wt of P elements, the LOI ofDOPO-NQ/TPIU211(molar ratio: MDI: PTMG: PMDA: DOPO-NQ=2:1:0.5:0.5) is28.2. When incorporates PMDA into DOPO-NQ/TPU, DOPO-NQ/TPIU321(molar ratio:MDI: PTMG: PMDA: DOPO-NQ=3:2:0.5:0.5) and DOPO-NQ/TPIU211withthicknesses3.2mm also attain ratings of V-2, but DOPO-NQ/TPIU312with40.2%of hardsegment contents shows no rating class, furthermore, LOI values of DOPO-NQ/TPIUdecrease with the hard segment contents, indicating that DOPO-NQ alone show moreexcellent flame retardancy than that of DOPO-NQ companied with PMDA.In order to study the flame retardant effect of Intumescent flame retardants (IFRs) onTPIU, a series of flame-retarded TPIU with either ammonium polyphosphate (APP) aloneor APP and dipentaerythritol (DPER) had been prepared. The fracture strength and strain ofthe TPIU composites decrease with increasing amount of IFRs, as shown by tensile tests.The thermal degradation behaviors of TPIU composites with different IFR loadings hadbeen investigated by thermogravimetry (TG). The results show that the initialdecomposition temperature (T5%) and the amount of residue decrease with increasing IFRloading, this is because APP can accerelate the decomoposition of TPIU. The flameretardant properties of these TPIU composites have been tested according to the LOI andUL-94standards, and the results have indicated that the LOI value of TPIU/APP is a littlehigher than that of TPIU/APP/DPER with the same loading. Furthermore,50%APP alone or APP/DPER incorporation into TPIU allowed attainment of the UL-94V-0standard.Details of flame retardancy had been tested by means of a cone calorimeter. For the TPIUcomposites, observation of the chars suggested that the esterification between APP andDPER can help the TPIU to form stronger and denser chars more effectively, inducing abetter flame-retardancy capability. To further investigate the flame retardant mechanism,the char residues of TPIU composites had been investigated in detail by SEM, FTIR, andEDXR.