Fabrication Of Polyurethane/black Phosphorous Nanocomposites:Investigation On Flame Retardancy And Solar Steam Conversion

Since it’s discovered,black phosphorus(BP)nanosheets has been regarded as an ideal candidate in many fields due to its high carrier mobility,layer-dependent bandgap structure,and rapid photo-thermal response.Meanwhile,as a high-efficiency flame retardant element,phosphorus(P)can react with polymer matrix to promote the formation of char layer,thus sippressing the delivery between heat and pyrolysis products.Therefore,compared to traditional and other 2D nano-flame retardants,BP nanosheets present hard-to-reach superiority in enhancing the mechanical properties,thermal stability and fire safety of polymer materials.However,the interfacial compatibility between BP nanosheets and polymers materials is poor which makes BP nanosheets are easy to accumulate and agglomerate.Besides,in high temperature environment,BP nanosheets will be thermally cracked and lose the two-dimensional layered structure,which can’t play the role of barrier in suppressing the exchange of heat and mass during combustion process.Furthermore,BP nano sheets are easily degraded to phosphoric acid under the action of water and oxygen molecules,which limits their application in the ambient/moist environment and affects the photo thermal response performance accompanying with limited application of BP nanosheets in solar steam conversion.Therefore,it is necessary to design functional BP nanosheets to regulate the interfacial interaction,improve flame retardancy efficiency,impart high environment stability to promote the fire safety performance of polymer materials with BP nanosheets and broad the solar photo thermal vapor conversion application of BP.To solve the meationed problems of BP,llots of efforts and works were done in this dissertation.Firstly,to improve the interfacial interaction of BP and polymer matrix,ionic liquid and cyanuric acid-derived crosslinked coating were used to conduct the surface modification of BP nanosheets to improve the interfacial interaction with polymer matrix.High polar modifier will interact with the polymer chains,thus significantly increasing the interfacial interaction Secondly,in order to obtain the layered barrier flame retardant effect of BP nano sheets under high temperature environment,a hierarchical nanohybrid of graphene and BP nano sheets was constructed to synergistically enhance the flame retardant efficiency of BP nanosheets.Finally,to overcome the problem of easy degradation of BP nanosheets,based on design philosophy of the elimination of superoxide radicals and water molecule isolation,the surface of BP nanosheets were modified with antioxidants and hydrophobic agents to endow itself with high environmental stability.Besides,with desirable environment stability,the photo-thermal conversion capacity of BP nanosheets was employed in solar steam generation.(1)For solving the problem of poor interfacial compatibility between BP nanosheets and polymer,the electrostatic self-assembly process was firstly used to attach 1-allyl-3-methylimidazolium chloride(AMIMCl)onto the surface of BP nanosheets.The free radical initiator Benzoyl peroxide(BPO)was then added to initiate the polymerization of vinyl functional groups to prepare polymeric ionic liquid functionalized BP nanosheets(IL-BP).Finally,polyurethane(TPU)/IL-BP composite(TPU/IL-BP)were prepared by solvent mixing.The interface interaction between IL-BP nanosheets and TPU was studied by scanning electron microscope(SEM)and torque rheometer.It’s shown in SEM images that compared with the smooth fracture surface of pure TPU,the fracture surface of TPU/IL-BP-2.0 presented a rougher undulation.At the same time,the IL-BP nanosheets increased the composite viscosity of TPU material in the range of 0.01 to 100 Hz,which was much higher than that of pure TPU resin.The mechanical test results showed that the breaking strength of TPU with the introduction of 1.0 wt%IL-BP increased by 50%.The thermal stability and flame retardant properties of TPU composites were invesitgated by thermogravimetric analysis(TGA)and cone calorimeter test(cone).It’s found in TGA results that the initial thermal decomposition temperature of TPU composites was significantly increased under nitrogen and much more char residue was generated.The peak heat release(PHRR)of TPU with 2.0 wt%IL-BP reduced by 38.3%.The flame retardant mechanism of TPU/IL-BP composites was investigated by Thermogravimetric-infrared spectroscopy(TGIR),Raman spectroscopy(Raman),infrared spectroscopy(F IR),X-ray elemental analysis(XPS)and other testing methods.These results showed that BP nanosheets would pyrolyze into phosphorus-containing free radicals,eliminating H· and OH·in flame,thus ending the combustion chain reaction.It’s found from FTIR and XPS spectra of char residue of TPU/IL-BP that phosphorus-oxygen bonds were existed in char layer.Meanwhile,it’s confirmed that the introduction of BP nanosheets would reduce the graphitization degree of char residue of TPU/IL-BP composites based on the analysis of Raman spectrum.Therefore,it can be concluded that BP nanosheets play an important role in both gas phase and condensed phase flame retardant of TPU composites.(2)Even though the encapsulation layer is needed for BP nanosheets,it is usually regarded as impurity.However,for BP nanosheets,not only the encapsulation layer is needed to overcome its shortcomings of degradation but also high crystal purity of BP is required to meet the practical application requirements.In response to this problem,a controllable recycling packaging-unsealing method has been developed.Through Diels-Alder reversible reaction,monoallyl of triallyl isocyanurate and conjugated olefin of 1-furfurylpyrrole reacted into ring-shaped cross-linked polymer,thus encapsulating BP nanosheets.The contact angle test results showed that BP nanosheets changed from super-hydrophilic to hydrophobic after encapsulation,accompanying with a water contact angle of 128°.Meanwhile,after being immersed in 30℃ water for 7 days,the phosphorus-oxygen bonds content of DA-BP slightly increased from 8.7%to 9.8%.After being heated at 130℃ under nitrogen,the cross-linked polymer coating was opened and unsealed.The pre-polymer was then removed by vacuum filtration to obtain purified BP nanosheets.Besides,the crystal structure of BP nanosheets was still ordered.Therefore,the feasibility of BP nanosheets packaging-unsealing strategy based on the Diels-Alder reversible reaction was confirmed by above results.Besides,TPU/DA-BP composites were prepared as well.It’s found that the char residue of TPU/DA-BP composites were significantly increased with the presence of isocyanate-based coating.Furthermore,it’s shown in the cone results that the PHRR of TPU/DA-BP-2.0 was decreased significantly by 61.6%.In addition,the total heat release(THR)and total CO2 release were also obviously suppressed.It’s found from Raman and FTIR spectra of char residue that,with the introduction of DA-BP,the char residue of TPU/DA-BP composites possessed phosphorus-oxygen bonds structure and reduced graphitization degree which confirmd that the isocyanurate-based cross-linked coating would promote BP nanosheets to participate in the formation process of char residue,thus suppressing the heat release behavior of TPU composites.(3)In view of the shortcomings of BP nanosheets that it will pyrolyze and lose the sheet structure at high temperatures,and cannot present the barrier effect of 2D nanosheet,graphene-melamine-black phosphorus(GNS/MA/BP)was prepared by means of electrostatic self-assembly using protonated melamine as the medium.The hierarchical structure can prevent the delivery of heat and(?)layer structure of graphene.The characteristic peaks of D and G bands in Raman specturm of GNS showed a weak scattering intensity,indicating that BP nanosheets will suppress the molecular scattering of GNS,confirming that BP nanosheets were covered onto the surface of GNS rather than physical mixing.TGA results showed that the introduction of graphene would significantly promote the initial degradation of TPU resin.However,the formation of hierarchical structure with BP nanosheets could increase the initial thermal decomposition temperature of TPU resin.Meanwhile,it’s found that the char residue of TPU-GNS/MA/BP at high temperature was significantly higher than that of TPU-GNS and TPU-BP.From cone results of TPU composites,the peak smoke generation rate(SPR)of TPU composite with the introduction of GNS/MA/BP reduced from 0.29 m2/s to 0.16 m2/s.The numerical calculation results confirmed that GNS and BP synergistically suppressed the smoke emission behavior during combustion.In addition,this research showed that the addition of pure BP did not have a significant promotion effect on the char formation of polyether TPU.And the char residue of TPU/BP presented a cracked morphology while TPU-GNS/MA/BP possessed a much denser and complete char layer.Interestingly,N element only existed in the char residue of TPU-BP and TPU-GNS/MA/BP.Meanwhile,with the addition of GNS,the graphitization degree of char residue of TPU composites was increased.Therefore,it’s concluded based on the metioned analysis that BP nanosheets would react with polymer pyrolysis products to form char residue and smoke particles.With the help of high thermally stable layer structure of graphene,the pyrolysis products of BP and polymer were prevented from entering the gas phase to form smoke particles and remained in the condensed phase to promote the formation of char residue.(4)In view of the poor environmental stability of BP nanosheets,based on the design philosophy of the elimination of superoxide free radicals and the isolation of water molecules,functionalized technologies of antioxidants and hydrophobic agents were developed to prepare tannic acid(TA)and cross-linked polydimethylsilicone(PDMS)modified BP nanosheets(TA-BP and PDMS-BP).After being exposed to the environment for 10 days and immersed in aqueous solution for 14 days,high-resolution XPS spectra and TEM lattice fringes together confirmed that the phosphorus-oxygen bond content and lattice structure of TA-BP and PDMS-BP nanosheets weren’t deteriorate significantly.Meanwhile,TGA results showed that the thermal oxidation temperature of TA-BP nanosheets was increased by 10.6℃.The synergistic flame retardant effect of TA and PDMS to BP nanosheets was studied utilizing a series of tests,including thermogravimetry,thermogravimetric infrared,and cone calorimeter.Compared to the PHRR of pure TPU,the PHRR of TPU composites with inclusion of 2.0 wt%TA-BP and PDMS-BP were reduced by 56.5%and 59.6%,respectively,which revealed that both TA and PDMS can promote the flame retardant efficiency of BP with different mechanism.Even though both TA and PDMS promoted the flame retardant efficiency of BP,their mechanism was diferent.In Raman spectrum of char residue,it’s found in Raman spectrum of char residue that TA-BP significantly increased the graphitization degree of char residue of TPU/TA-BP composites,while PDMS-BP decreased the graphitization degree.From the XPS spectrum of char residue of TPU/PDMS-BP-2.0,the existences of silicon-oxygen and silicon-carbon bonds were found,and complete and dense morphology was also presented.These results showed that TA as a polyphenol structure would pyrolyze into highly graph it ized char residue,preventing heat delivery.In addition,PDMS reacted with polymer matrix to enhance the stability and integrity of char residue by forming silicon-containing bonds.(5)In order to develop the application of BP nanosheets in solar steam conversion,layer-by-layer self-assembly and polymer bonding technology were integrated to prepare BP/PU composite foam.During layer-by-layer self-assembly process,positively charged protonated chitosan(CS)and negatively charged BP nanosheets were alternately deposited on PU foam surface.Through the polymer bonding technology,BP nanosheets and collagen prepolymers were firstly dispersed and mixed.PU foams were then immersed in above solution and cured with glutaraldehyde to form photo-thermal conversion coating.In the above two ways,BP nanosheets were attached to the surface of PU foam skeleton,forming photo-thermal conversion coating,combining with the foam opening structure to provide a transmission channel for steam rise.With simulated solar radiation intensity of 1kW/m2,the surface tenperatures of PU foams were increases by 40.9℃ and 26.6℃,respectively.The water evaporation rates were 1.082 kg/m2h and 0.8910 kg/m2h,respectively.Meanwhile,the chitosan and collagen effectively blocked water and oxygen molecules from contacting BP nanosheets,which imparted BP-based photo-thermal conversion coating with excellent durability.