Construction Of Novel Phosphorus-nitrogen/Metal Organic Framework Flame-retardant Systems And Their Effect Mechanism On Flammability Of Poly (Lactic Acid)

Poly(lactic acid)(PLA)with bio-based and biodegradable nature receives popular favor in the decade year,and becomes the most widely used bio-based products in commercial applications.However,its inherent flammability is a serious fire hazard in the application,it is imperative to develop PLA products with flame retardancy.Unfortunately,flame-retardant PLA products exist many problems,such as the low flame-retardant efficiency,single function and severe deterioration of mechanical properties,which limits the popularization and application of PLA products.Based on the combustion characteristics of PLA and the flame-retardant mechanism of different systems,this work attempts to solve the existing problems of flame-retardant PLA plastics and non-woven fabrics from both theoretical and technical levels.Novel flame retardants(FRs)were designed and synthesized,containing novel P-N FRs,lamellar carbonizing agent with polyhydroxy structure,metal-organic frameworks(MOFs)molecular encapsulation FRs,polyhydroxy polyelectrolyte FRs,and two synergistic systems were constructed separately.This work aimed to enhance the flame retardancy of PLA plastics and non-woven fabrics,explore flame-retardant mechanism of that.Meanwhile,this work gives more attention to balance the mechanical properties of PLA composites and rich the application functions of PLA non-woven fabrics.The main research results are as following:1.Accounting to the reaction mechanism of Kabachnik-Fields,the novel P-N flame retardant SHPE was synthesized using salicylaldehyde,ethylenediamine,and hypophosphate with high phosphorus contenting,and its structure and composition were confirmed by nuclear magnetic resonance(NMR),fourier transform infrared(FTIR),and organic element analysis(OEA).PLA/SHPE composites were prepared,and the thermal stability,flame retardancy,and mechanical properties of that were characterized.SHPE was superior to the traditional flame retardant ammonium polyphosphate(APP),PLA/10%SHPE could pass V-0 level in vertical burning test(UL-94),and improve LOI value to 28.3%;in comparison to PLA,the total heat release(THR)and peak heat release rate(pHRR)of PLA/10%SHPE decreased by 15.3%and 30.1%.The mechanism analysis presented SHPE exerted flame-retardant effect in both gas and condensed phase,the acid catalysis of SHPE could promote chain scission of PLA,and accelerate the formation flameless molten dripping to take away heat.Besides,the elongation at break of PLA/SHPE was increased to 5.7%,and the mechanical strength showed some deterioration to 50.5 MPa.2.Based on the strong designability of MOFs structure,the organic ligand containing polyhydroxy and Schiff base groups was designed and synthesized using tris(hydroxymethyl)aminomethane and salicylaldehyde,and then reacted with Ni2+ to form Ni-MOF nanosheet.The structure,composition,micromorphology,and thermal stability of Ni-MOF were confirmed by NMR,FTIR,OEA,scanning electron microscope(SEM),thermo-gravimetric analysis(TGA)and differential scanning calorimeter(DSC).After that,Ni-MOF was incorporated into PLA matrix together with APP,which showed excellent synergistic flame-retardant effect.For PLA-3.3%APP/1.7%Ni-MOF,the UL94 test passed V-0 level with a LOI about 31.0%,the pHRR,THR and total smoking production(TSP)decreased by 26.9%,19.3%and 50.0%compared with PLA,respectively,the introduction of Ni-MOF inhibited the emission of smoke.The flame-retardant mechanism in condensed phase played a major role,the polyhydroxy structure of Ni-MOF could form crosslink network,the Ni oxide generated by pyrolysis improved thermal stability of char layer.Besides,the organic lamellar structure of Ni-MOF had good compatibility with PLA matrix,and alleviated mechanical properties deterioration of PLA composites caused by APP introduction,the tensile strength of PLA-3.3%APP/1.7%NiMOF composites was kept at 57.5 MPa,and the impact strength was increased to 12.9 kJ/m2.3.Based on the high pore volume and high specific surface areas of MOFs,triethyl phosphate(TEP)was encapsulated into Zr-based metal organic framework(UiO66-NH2)via in situ encapsulation strategy,and UiO@TEPx was prepared(X is the volume fraction of TEP in the solution).The influence of TEP loading on the micromorphology,crystal structure,pore structure and thermal stability of UiO66-NH2 was studied in detailly.UiO@TEP50 was chosen and added into PLA plastic,the flame retardancy,ultraviolet resistance,and mechanical properties of.composites were studied.The encapsulation strategy enhanced the flame-retardant efficiency of UiO66-NH2,PLA/2.0%UiO@TEP50 passed V-0 level in UL-94 test,and the pHRR,THR,TSP,and total CO production(TCOP)were decreased by 14.2%,9.8%,29.5%,and 24.2%.The mechanism analysis indicated TEP was released from UiO@TEP50 during combustion,and played a part in gas and condensed phase.Its porous structure and pyrolysis products were able to adsorb and catalyze smoke.Besides,the mechanical strength of PLA/2.0%UiO@TEP50 was increased to 59.7 MPa due to the good compatibility between UiO@TEP50 and PLA;UV resistance of that was improved due to the scattering and absorption effect of UiO@TEP50.4.A novel polyhydroxy polyelectrolyte flame retardant APTris was synthesized via ion exchange reaction,the flammability and serious molten dripping of PLA non-woven fabrics were solved effectively.The structure and composition of APTris were confirmed by FTIR,OEA and X-ray photoelectron spectroscopy(XPS).Compared with PLA,the LOI of sample PLA/25%APTris increased to 30.0%,the damage length and damage areas in UL-94 test decreased by 87.1%and 44.8%,respectively,as well as self-extinguishing after removing igniter,pHRR and THR decreased by 41.0%and 28.2%,char residue in TGA test increased to 12.3%from 1.7%.Mechanism analysis showed that APTris had excellent rapid thermal expansion performance,the dehydration of polyhydroxy played a diluting and cooling role at the initial stage of combustion,and then cross-linked and carbonized to form dense char layer to delay the exchange of heat and mass at high temperature,which played a role in condensed phase.Compared with previous reports about flame retardant PLA non-woven fabrics,APTris was higher efficient,lower cost,and more ecofriendly.5.The imidazolyl metal organic framework-67(ZIF-67)was grew in-situ on the surface of PLA/APTris non-woven fabrics,the integration of APTris and ZIF-67 endowed PLA non-woven fabrics flame retardancy,antibacterial,and UV block properties.The loading of ZIF-67 was confirmed by SEM,FTIR and X-ray powder diffraction(XRD).The synergistic carbonization of APTris and ZIF-67 significantly improved the char residue of PLA non-woven fabrics to 18.1%,LOI value increased to 27.5%;compared with pure PLA,the pHRR and THR of PLA/APTris/ZIF67 were decreased by 33.1%and 26.4%,the damage length and damage areas in UL-94 test were decreased by 83.0%and 47.2%,respectively.Mechanism analysis showed that APTris rapidly constructed the intumescent carbon layer,and the maintained porous structure of ZIF-67 could suppress smoke release,the cobalt oxide generated by pyrolysis attached on the surface of char layer as clusters,and more P,N and non-oxidation state C were remained in condensed phase to build a physical barrier for thermo-oxygen stability.Besides,the metal ions released by ZIF-67 hydrolysis improved the antibacterial performance of non-wovens against E.coli and S.aureus,the reflection and blocking effect of that improved UV resistance of non-wovens.