Synthesis Of Bio-based Halogen-free Flame Retardants And Flame-retarded Epoxy Prepolymers And Properties Of Epoxy Thermosets

Epoxy resins have been widely used in various industrial fields due to their good mechanical properties,thermal and chemical resistance.However,flammability is one of the major disadvantages of epoxy resins,and the flame-retardant property of epoxy resins is generally improved by adding flame retardants.With the increasing attention to environmental protection,various laws and regulations have been issued all over the world to restrict the use of halogen-containing flame retardants.Thus,green and practical flame retardants have become the trend of development.In addition,with the shortage of non-renewable resources such as petroleum,the demand of renewable biomass resources in industrial applications is increasing.Bio-based flame retardants and flame-retardant epoxy prepolymers are able to be synthesized by suitable chemical reactions from bio-based raw materials.Based on the above research background,in this paper,bio-based flame retardants and flame-retardant epoxy prepolymers were prepared by molecular design and chemical reactions using bio-based raw materials,and then the corresponding thermosets were compared with petroleum-based epoxy resins from several aspects,such as environmental friendliness,flame retardancy and mechanical properties.（1）A new 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide（DOPO）derivative6,6’,6’’-（1,3,5-triazine-2,4,6-triyl）tris（oxy）)tris（3-methoxybenzene-4,1-diyl）)tris（hydroxymeth ylene）)tris（dibenzo[c,e][1,2]oxaphosphinine 6-oxide）（Trif-D）was prepared from bio-based vanillin and cyanuric chloride.The structure of Trif-D was confirmed by ¹H nuclear magnetic resonance（NMR）and Fourier transform infrared spectroscopy（FT-IR）.Trif-D was employed as the flame retardant for commercial bisphenol A epoxy resin/4,4′-diaminodiphenyl methane（DGEBA/DDM）epoxy curing system to prepare the flame-retarded epoxy resin DGEBA/DDM/Trif-D.For the cured epoxy resin containing 5.0 wt%Trif-D（Trif-D-5）,the limiting oxygen index（LOI）reached 33.1%（with the V-0 rating in UL-94 test）,and peak heat release rate（PHRR）,total heat release（THR）and total smoke production（TSP）values were decreased by 66.3%,62.9%and 51.3%,respectively,compared with DGEBA/DDM（Trif-D-0）.The excellent flame retardancy of Trif-D-5 was attributed to the excellent charring property of Trif-D.Compared with Trif-D-0,Trif-D-5 had the lower degradation rate and higher char residue although the slightly lower initial decomposition temperature(T_{d 5%}).（2）Abio-basedDOPO-containingflameretardant6,6’-（（（methylenebis（4,1-phenylene））bis（azanediyl））bis（furan-2-ylmethylene））bis（dibenzo[c,e][1,2]oxaphosphinine 6-oxide）（MBF-DOPO）was prepared.The intermediate was firstly synthesized by the reaction of furfural with DDM,and then C=N bond of intermediate reacted with P-H bond of DOPO to prepare the bio-based flame retardant.MBF-DOPO had good thermal stability and charring properties.T_{d 5%}of MBF-DOPO was 312.0 ^oC in nitrogen atmosphere,and its char residue was 38.9%（at 700 ^oC）.MBF-DOPO was used as the co-curing agent of DDM to prepare flame-retardant epoxy resins DGEBA/DDM.For the cured epoxy resin containing 4.0 wt%MBF-DOPO,the LOI reached 31.5%（with the V-0rating in UL-94 test）,and the PHRR and THR values were decreased by 29.3%and 12.6%,respectively,compared with DGEBA/DDM（MBF-DOPO-0）.The results showed that the addition of MBF-DOPO obviously improved the flame retardancy of epoxy resin.（3）ArenewableSchiffbaseflameretardant4,4’-（（1E,1’E）-（（oxybis（4,1-phenylene））bis（azanylylidene））bis（methanylylidene））bis（benzene-1,2-diol）（PH-ODA）was prepared by the reaction of protocatechualdehyde with4,4’-diaminodiphenyl ether（ODA）.T_{d 5%}of PH-ODA was 256.5°C,and it presented very high residue（60.1%）at 700?°C,suggesting the good thermal stability and excellent charring ability of PH-ODA.PH-ODA（as carbonization agent）combined with ammonium polyphosphate（APP）（PH-ODA/APP=1/3 by weight）was used as intumescent flame retardants for DGEBA/DDM curing system.For the cured epoxy resin containing 10 wt%PH-ODA/APP blend（7.5 wt%APP and 2.5 wt%PH-ODA）,the limiting oxygen index（LOI）reached 29.9%（with the V-0 rating in UL-94 test）,and the PHRR and THR values were decreased by 87.1%and 61.6%,respectively,compared with DGEBA/DDM（PH-ODA-0）.The enhancement of fire-safety performance was due to that PH-ODA/APP promoted the formation of compact intumescent char structure.It was also found that the synergistic effect between PH-ODA and APP was helpful to enhance the flame retardancy of epoxy matrix.（4）A novel difunctional epoxy monomer bis（2-methoxy-4-（oxiran-2-ylmethyl）phenyl）phenyl phosphate（BEU-EP）was synthesized from eugenol and the compound containing phosphate（phenyl phosphorodichloridate）in two steps.In the first step,bis（4-allyl-2-methoxyphenyl）phenyl phosphate（BEU）was synthesized by eugenol and phenyl phosphorodichloridate,and then the allylic groups of BEU were epoxidized with m-CPBA to obtain BEU-EP.BEU-EP was cured by 4,4’-diaminodiphenyl methane（DDM）and DGEBA/DDM was used as a reference.T_{d 5%}of BEU-EP/DDM was 300.5°C,which was lower than that of DGEBA/DDM.However,the char residue of BEU-EP/DDM reached23.4%,which was much higher than that（14.8%）of DGEBA/DDM.BEU-EP/DDM passed V-0 rating（in UL-94 testing）with limiting oxygen index（LOI）of 38.4%and greatly decreased the PHRR and THR by 85.1%and 82.8%as compared to that of DGEBA/DDM,respectively.A new and efficient method was developed for the determination of epoxy groups in epoxy prepolymers and amine groups in aliphatic amine hardeners based on headspace gas chromatography（HS-GC）.Firstly,the epoxy groups in epoxy prepolymers（or amine groups in aliphatic amine hardeners）reacted with excessive hydrochloric acid（HCl）,and then the remaining HCl reacted with sodium bicarbonate solution to generate carbon dioxide.Since the reaction of HCl with sodium bicarbonate was carried out in a sealed headspace vial,the epoxy groups in epoxy prepolymers（or amine groups in aliphatic amine hardeners）can be indirectly determined by measuring the carbon dioxide from the reaction of remaining HCl and bicarbonate.（5）Abio-basedflame-retardantepoxyprepolymer（1E,1’E）-N,N’-（oxybis（4,1-phenylene））bis（1-（3,4-bis（oxiran-2-ylmethoxy）phenyl）methanimin e）（PH-ODA-EP）containing Schiff base structure was synthesized from PH-ODA,epichlorohydrin and sodium hydroxide（Na OH）.After curing PH-ODA-EP with DDM,the cured resin PH-ODA-EP/DDM network possessed a high T_gof 204.9?℃,which showed42.7°C higher than DGEBA/DDM（162.2°C）.The char residue of PH-ODA-EP/DDM reached 41.7%at 700°C,which was nearly 3 times of that（14.8%）of DGEBA/DDM.PH-ODA-EP/DDM possessed excellent fire-safety properties（limited oxygen index（LOI）value=40.5%,passed the UL-94 V-0）due to the introduction of Schiff base structure,showing a 90.9%and 71.9%reduction（compared with DGEBA/DDM）in PHRR and THR values.Compared with DGEBA/DDM,the tensile modulus and flexural modulus of PH-ODA-EP/DDM were increased by 87.0%and 84.6%,respectively,and the tensile strength and flexural strength of PH-ODA-EP/DDM were increased by 42.3%and 45.9%,respectively.（6）Afullybio-basedSchiffbaseintermediate4,4’-（（1E,1’E）-（butane-1,4-diylbis（azanylylidene））bis（methanylylidene））bis（2,6-dimethoxyphe nol）（SH-BDA）was synthesized by the reaction of vanillin with 1,4-butanediamine（BDA）.Then,abio-basedflame-retardantepoxyprepolymer（1E,1’E）-N,N’-（butane-1,4-diyl）bis（1-（3,5-dimethoxy-4-（oxiran-2-ylmethoxy）phenyl）methana zomethine）（SH-BDA-EP）containing Schiff base structure was synthesized by reacting SH-BDA with epichlorohydrin.After curing SH-BDA-EP with DDM,the bio-based flame-retardant epoxy resin SH-BDA-EP/DDM network was obtained.SH-BDA-EP/DDM possessed the high char residue of 39.1%at 700°C,which was much higher than that of DGEBA/DDM（14.8%）.SH-BDA-EP/DDM possessed excellent fire-safety properties（limited oxygen index（LOI）value=39.6%,passed the UL-94 V-0）,showing a 82.2%and52.3%reduction（compared with DGEBA/DDM）in PHRR and THR values.Compared with DGEBA/DDM,the tensile modulus and flexural modulus of SH-BDA-EP/DDM were increased by 27.1%and 30.3%,respectively,and the tensile strength and flexural strength of SH-BDA-EP/DDM were increased by 18.1%and 13.2%,respectively.The results showed that SH-BDA-EP/DDM had excellent flame retardancy,mechanical properties and heat resistance.The above results indicate the excellent flame retardancy and thermal stability.