Preparation Of Phosphorus-containing MXene-based Hybrid Materials And Study Of Their Flame-retardant Thermoplastic Polyurethane Properties And Mechanism

Thermoplastic polyurethane（TPU）is one of commercial engineering elastomer material that is used in a wide variety of applications in fields such as coating materials,wires and cables,adhesives due to its excellent processing properties,good wear resistance,high mechanical properties,and good chemical stability.Nevertheless,serious flammability issues of TPU have severely restricted its applications.Thus,the development of TPU composites with both excellent flame retardancy and good mechanical properties are extremely encouraged.In this work,based on the physical and chemical characteristics of MXene,inorganic and organic phosphorus with different valences were used to modify the surface of MXene through covalent and non-covalent bonds.And then,a series of phosphorus-containing MXene-based flame retardants were successfully synthesized and used to flame retardant TPU.A series of problems of the agglomerate,flammability,and mechanical properties were solved.At the same time,to make MXene-based TPU composites be applied in energy storage,an MXene-based TPU composite phase change material with both energy storage and flame retardancy was fabricated,which improved its application field.The main research contents have been summarized as follows:（1）The easy agglomeration issue of MXene nanosheets limits the dispersion effect in polymers.Therefore,a bio-based phosphorus-containing MXene nanohybrid of phosphorylated chitosan decorated MXene（PCS-MXene）by covalent bonding was fabricated.Compared to the unmodified MXene,the introduction of PCS-MXene evidently enhanced the mechanical performance of the TPU nanocomposites,due to the excellent dispersibility of PCS-MXene in TPU matrix.And its tensile strength and elongation at break were increased by 54.0%and 24.5%,respectively.As only 3 wt%PCS-MXene was introduced,peak heat release rate（p HRR）,total heat release（THR）,and total smoke production of the composite decreased by 66.7%,21.0%,and 27.7%,respectively,compared to the pure TPU,indicating that PCS-MXene exhibited excellent flame retardancy and smoke suppression.（2）Based on the characteristics of the layer structure of black phosphorus（BP）,as well as the biocompatibility and the structural stability of polydopamine（PDA）,a BP-MXene@PDA nanohybrids were manufactured by hydrogen bonding,π-πstacking,and in-situ polymerization.Particularly,with an introduction of 2 wt%BP1-MXene2@PDA,the p HRR,THR,peak CO production rate,and peak CO₂ production rate of the sample were reduced by 64.6%,27.3%,43.4%,and 62.5%,respectively.The tensile strength and elongation at break increased from 36.3 MPa and 671.5%for pure TPU to 44.9 MPa and 779.4%for TBM-1.Therefore,the BP-MXene@PDA not only improved the thermal stability and flame retardancy of TPU composites,but also further improved the mechanical properties.（3）To overcome the low LOI and poor UL-94 issues of MXene-based flame-retardant polymers,Si-MXene was reacted with pentaerythritol octahydrogen tetraphosphate and phenylphosphonyl dichloride to synthesize an MXene-based intumescent phosphorus-containing flame retardant（PB-MXene）.The results demonstrated that the pure TPU was a highly flammable polymer with an LOI value of23.1%and a V-2 rating in the UL-94 test.By comparison,a sample with 8 wt%PB-MXene could reach the maximal LOI value of 32.2%,achieve the UL-94 V-0 rating.Furthermore,the p HRR and THR of the sample reduced by 72.6%and 25.8%,respectively.（4）Based on the good charring behavior of stearyl alcohol and its excellent energy storage and heat release capacity as a phase change material（PCM）.The phosphorus-modified stearyl alcohol@MXene（P-SAL@MXene）was used as a flame retardant and phase change filler.Subsequently,a flame-retardant TPU multifunctional composite PCM was prepared,where TPU was selected as the packaging substrate,and P-SAL@MXene was introduced into the TPU through solvent mixing.The research results showed that the enthalpy values of MXene-based PCMs were all higher than120 J/g,the phase change properties of the sample exhibited negligible change after200 thermal cycles,indicating superior thermal reliability and reusability of the MXene-based PCMs.The results of cone calorimetry demonstrated that the incorporation of phosphorus was effective for decreasing the heat release of TPU/SAL.After being replaced by P-SAL@MXene,the p HRR and THR values of the TPU/P-SAL@MXene were reduced by 68.2%and 41.5%,respectively.These results indicated that the presence of P-SAL@MXene could indeed improve the fire safety of TPU-based PCM.（5）A catalytic charring mechanism of Lewis acid site in closed char-cages with phosphorus-containing MXene-based flame retardant was proposed.The phosphorus derivatives could accelerate the formation of stable char of the TPU matrix by dehydration and carbonization during burning.On the one hand,MXene/Ti O₂ was protected and wrapped in the char-cages,thus effectively inhibiting the migration of Ti components.On the other hand,the pyrolysis products of TPU could be confined to the closed char-cages,which was beneficial for providing sufficient reaction space and time for the capture and catalytic charred by the MXene/TiO₂,forming the graphitized char residue.