Study Of Lignin Based Intumescent Flame Retardants In Rigid Polyurethane Foam

Lignin,as the second largest renewable resource in nature after cellulose,is a complex polymer with aromatic properties cross-linked by phenylpropane units.It has active groups such as phenolic hydroxyl group,alcoholic hydroxyl group,and carbonyl group,which could be substituted part of diethylene glycol and copolymerized with isocyanate to produce lignin based rigid polyurethane foams.Meanwhile,lignin exhibits excellent thermal stability and higher carbon-forming ability due to its aromatic macromolecular properties and rich carbon content.In this paper,the flame retardant properties of rigid polyurethane foams are improved by using a lignin monomer phosphorus-nitrogen intumescent flame retardant,a"three-in-one"lignin-based phosphorus-nitrogen intumescent flame retardant,expandable graphite coupled lignin-based phosphorus-nitrogen intumescent flame retardant,and a hybrid intumescent flame retardant with lignin as carbon forming agent.Furthermore,Fourier transforms infrared analysis（FTIR）,X-ray photoelectron spectroscopy（XPS）,thermogravimetric analyzer（TG）,oxygen index（LOI）,cone calorimeter（CCT）,scanning electron microscope（SEM）,and other analytical methods are used to investigate the effects of the synthesized intumescent flame retardants on the flame retardant properties and thermal stability of rigid polyurethane foams.The main results of this thesis are listed as follows:（1）Three kinds of"three-in-one"DH,DV,and DS intumescent flame retardants containing phosphorus and nitrogen elements are successfully prepared using lignin monomers of p-hydroxybenzaldehyde（H）,vanillin（V）,and syringaldehyde（S）with different methoxyl groups on its aromatic ring as raw materials.Compared with RPU foam,the flame retardant index（FRI）values of RPU-DH,RPU-DV,and RPU-DS are 2.3,2.3 and 2.4,respectively,indicating that DH,DV,and DS are good flame retardants.The limiting oxygen index（LOI）and carbon yield at 600 s(CY₆₀₀)of RPU-DH,RPU-DV,and RPU-DS foams are significantly improved,while the heat release rate（HRR）,total heat release（THR）,flame time to extinguish（TOF）,fire growth index（FIGRA）,effective heat of combustion（EHC）,and carbon dioxide production（CO₂Y）are relatively reduced.Meanwhile,the time to peak carbon monoxide yield(t _PCO)of RPU-DH,RPU-DV and RPU-DS foams are widely delayed.The excellent flame retardant properties of RPU-DH,RPU-DV,and RPU-DS foams are found by SEM attributed to the denser and uniform expanded carbon layer formed during their combustion.The carbon layer can effectively reduce the heat exchange in the combustion process,dilute the oxygen and the non-combustible gas generated in the combustion process,and then endow the rigid polyurethane foam with good flame retardancy and thermal stability.In addition,because DS possesses better thermal stability and char-forming ability than DH and DV,RPU-DS foam exhibits better flame retardancy and thermal stability than RPU-DH and RPU-DV.（2）A"three-in-one"lignin-based intumescent flame retardant N/P-BKL is successfully prepared by grafting melamine and diethyl phosphite.The study finds that the addition of N/P-BKL can improve the carbon-forming ability and flame retardant performance of rigid polyurethane foam,so that the THR,HRR,EHC,and FIGRA values of the N/P-BKL RPU foam are much lower than that of BKL RPU foam.Compared with BKL RPU foam,the maximum weight loss temperature of N/P-BKL RPU foam is increased by 74℃,and the mass loss is reduced by 18.1 wt%,which further verified that N/P-BKL improved the thermal stability performance of N/P-BKL RPU foam.In addition,compared with the nitrogen-modified lignin-based flame retardant N-BKL,the nitrogen/phosphorus synergistically lignin-based intumescent flame retardant N/P-BKL has better flame retardant properties.For example,the FRI value of N/P-BKL RPU foam is 2.8,which is much higher than that of N-BKL RPU foam.The analysis shows that the lignin-based intumescent flame retardant N/P-BKL can make the foam generate a more continuous and dense intumescent carbon layer,which will greatly reduce the heat and mass transfer during the combustion of the material,and then resulting in good flame retardancy and thermal stability properties.（3）Firstly,a"triple-in-one"lignin-based intumescent flame retardant N/P-BKL-I is prepared by grafting melamine and aminotrimethylene phosphate onto bamboo lignin.Then,N/P-BKL-I and expandable graphite（EG）are compounded to synergistically produce flame-retardant rigid polyurethane foams.The results show that the THR,HRR,FIGRA,and EHC values of the compounded RPU-5IFR/5EG foam are superior to both RPU-10EG and RPU-10IFR foams by the single addition of N/P-BKL-I and EG intumescent flame retardants.Those results indicate that the synergistic compounding of N/P-BKL-I and EG can better improve the flame retardant properties of RPU foams,although the LOI values of RPU-10IFR and RPU-5IFR/5EG foam are comparable.In addition,compared with RPU-10EG and RPU-10IFR foams,RPU-5IFR/5EG foams exhibit less mass loss and higher char residues,demonstrating its better thermal stability performance.Studies have shown that N/P-BKL-I can not only inhibit the combustion of materials by releasing incombustible gas,but also restrain the combustion process by capturing the free radicals generated from combustion via thermally decomposed phosphoric acid or polyphosphate substances.Moreover,the introduction of EG leads to a denser"worm-like structure"expanded carbon layer,which can further improve the flame retardancy and thermal stability of the RPU-5IFR/5EG foam through condense phase and gas phase.（4）By replacing pentaerythritol（PER）with a more stable and higher carbon content lignin,a hybrid intumescent flame retardant is fabricated by composing lignin（carbon source）with ammonium polyphosphate（acid source）and melamine（gas source）.When lignin replaced 100%of PER,the LOI value of RPU/LS foam is higher than that of RPU/PER,proving that lignin can replace PER as a carbon source for intumescent flame retardants.Moreover,the mass loss of the RPU/LS foam in the main thermal weight-loss stage is lower,and the carbon residue is higher,indicating that the use of lignin as the carbon source can more effectively improve the flame retardancy and thermal stability of the rigid polyurethane foam.In addition,the residual carbon after the combustion of RPU/LS foam presents a continuous and relatively dense carbon layer,which can play the role of heat insulation and oxygen insulation,and can inhibit further combustion of polyurethane foam materials through gas-solid phase synergistic flame retardancy.Furthermore,studies have shown that when the acid source/gas source/carbon forming agent compound ratio in the intumescent flame retardant formulation is 3:1:1,the rigid polyurethane foam RPU/LS1 exhibits the best flame retardant and thermal stability properties.In conclusion,the flame retardant properties and thermal stability of rigid polyurethane foams are improved by using a lignin monomer phosphorus-nitrogen intumescent flame retardant DH,DV and DS,a"three-in-one"lignin-based phosphorus-nitrogen intumescent flame retardant N/P-BKL and N/P-BKL-I through condense phase and gas phase.Further,expandable graphite coupled lignin-based phosphorus-nitrogen intumescent flame retardant N/P-BKL-I have synergistic effect in the improvement of flame retardancy of rigid polyurethane foam.Lignin can 100%replace PER as carbon forming agent,combining with ammonium polyphosphate（acid source）and melamine（gas source）for hybrid intumescent flame retardant formulation for RPU foams.When the acid source/gas source/carbon forming agent ratio is 3:1:1,the rigid polyurethane foam RPU/LS1 has the best flame retardancy and thermal stability.