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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 696 - 708

DOI: https://doi.org/10.7536/PC230907

Review

Preparation and Application of Inherent Flame-Retardant Polyurethane Foams

  • Zhicheng Fu , * ,
  • Luping Feng ,
  • Wei Luo ,
  • Ting Wang ,
  • Jinni Deng ,
  • Wenli An ,
  • Mingjun Chen , *
Expand
  • Department of Chemistry, School of Science, Xihua University, Chengdu 610039, China
* e-mail: (Mingjun Chen);
(Zhicheng Fu)

Received date: 2023-09-14

  Revised date: 2024-01-14

  Online published: 2024-06-27

Supported by

National Natural Science Foundation of China(U22A20150)

National Natural Science Foundation of China(21975208)

Central Guide Local Science and Technology Development Special Project of Sichuan Province(2023ZYD0030)

Open and Innovative Fund of Hubei Three Gorges Laboratory(2022LF2021)

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Abstract

polyurethane foam,As one of the most important and widely used synthetic polymers since the 20th century,exhibits advantages such as low density,high strength,and excellent thermal insulation.it finds widespread applications in fields like aerospace noise reduction,railway track stabilization,and building insulation.However,its high flammability poses a serious threat to human life and property,limiting its further development.the addition or surface coating of flame retardants can indeed enhance the flame retardancy of polyurethane foam efficiently.However,these methods often result in the migration and precipitation of flame retardants,ultimately compromising its flame retardant properties and internal performance.Alternatively,the copolymerization of reactive flame retardant molecules into the polymer chain offers a more effective solution.This approach not only addresses the issues of flame retardant migration and precipitation but also minimizes the impact on the substrate’s properties.as a result,It is highly favored by the industry and holds immense potential for future engineering applications.This review aims to provide a comprehensive overview of various methods for the preparation of intrinsic flame-retardant polyurethane foams over the past decade,focusing on the perspective of monomer molecular design and synthesis.This includes polyol modifications,isocyanate modifications,additive modifications,and other modifications.Furthermore,the review will delve into the challenges that remain to be addressed and offer insights into potential future directions for application development。

Contents

1 Introduction

2 Polyol modifications

2.1 Petroleum-based polyol

2.2 Bio-based polyol

3 Isocyanate modifications

4 Additive modifications

5 Other modifications

6 Conclusion and outlook

Key words： polyurethane foam; inherent flame retardant; preparation; application progress

Cite this article

Zhicheng Fu , Luping Feng , Wei Luo , Ting Wang , Jinni Deng , Wenli An , Mingjun Chen . Preparation and Application of Inherent Flame-Retardant Polyurethane Foams[J]. Progress in Chemistry, 2024 , 36(5) : 696 -708 . DOI: 10.7536/PC230907

1 Introduction

polyurethane is a general term for macromolecular compounds with repeating urethane(-NHCOO-)units in the main chain,which is produced by the stepwise polymerization of polyols and isocyanates.in 1849,Wurtz first reported the synthesis of isocyanate,and in 1937,Bayer first prepared Polyurethane from polyester and isocyanate[1][2]。 since then,polyurethane has been used In all aspects of people's production and life,and has become one of the most important synthetic polymers Since the 20th century.in 2017,the annual production of polyurethane in the world has exceeded 20 million tons,and is still growing at an annual rate of 5%.In 2021,the consumption of polyurethane reached 79 billion US dollars,accounting for nearly 10%of global plastic consumption,and China has become the world's largest producer and consumer of polyurethane[3]。
polyurethane foam is one of the most important varieties of Polyurethane materials,with huge consumption,covering 29%of the material market.Its structure is porous,showing the advantages of low density,high strength and high thermal insulation,which is widely used in many fields such as earthquake prevention,shock absorption,soft gasket materials and building thermal insulation[4,5]。 polyurethane foam is mainly divided into Rigid polyurethane foam and flexible polyurethane foam according to the different raw materials of polyol and isocyanate and the preparation process.rigid polyurethane foam is a highly crosslinked polymer with very low thermal conductivity,which is an indispensable thermal insulation material for large-scale applications in the automotive,furniture and construction industries[6⇓~8]。 As a highly open-cell polymer,flexible polyurethane foam shows excellent resilience,sound absorption,shock resistance and other properties,and is widely used in railway solidified ballast bed,furniture mattress,automobile cushion and so on[9⇓⇓⇓~13]。
However,polyurethane foam has a large number of hydrocarbon chain segments,which makes it highly flammable,and its limiting oxygen index(LOI)is only 17%~19%.After ignition,it will burn rapidly and release a large amount of heat,accompanied by the release of smoke and toxic gases(CO,HCN,NO,etc.),which poses a serious threat to the safety of people's lives and property[14⇓~16]。 for example,according to the fire data in the first quarter of 2022,about 38%of the fires occurred in residential places,resulting in 503 deaths,accounting for 80%of the total deaths,and most of these fires are related to the flammability of Polyurethane foam,an external wall insulation material.polyurethane foam has a low initial decomposition temperature and a high heat release rate,and the conditions required for combustion include combustibles,oxygen,a fire source,and a chain radical reaction.Therefore,chemical or physical methods can be used to cut off combustion elements in the gas phase or condensed phase by diluting the concentration of combustible gas,terminating the chain reaction of free radicals,and blocking heat and mass transfer,so as to achieve the purpose of delaying the spread of fire and reducing heat release。
With the introduction of higher fire safety standards for polyurethane foam materials in high-tech industries,it is very urgent and necessary to study the preparation of high flame retardant polyurethane foam.According to the classification of action mode,surface flame retardant coating and adding flame retardant strategy are the two main schemes to improve the flame retardant performance of polyurethane foam.Although some progress has been made in the flame retardant coating strategy on the surface of polyurethane foam,including in-situ deposition,plasma technology,sol-gel process and layer-by-layer self-assembly technology,some major problems,such as coating durability,complex process and high cost,still need to be solved,and there are unpredictable challenges in the application prospect[17⇓⇓~20]。 In addition,the addition of flame retardants has the advantages of simple operation,low cost and high flame retardant efficiency,but the addition of flame retardants often has poor compatibility with the substrate,is easy to leach out,and is very easy to deteriorate the mechanical properties of the foam,especially under extreme natural conditions such as wet and hot erosion,day and night high and low temperature alternation,and high frequency compression recovery[21⇓~23]。 in contrast,reactive flame retardants can be used to prepare intrinsic polyurethane foams by introducing flame retardant segments into the polymer chain through copolymerization.This strategy can not only achieve its long-term flame retardancy,but also maintain or enhance the comprehensive performance,which is more popular In the future industrial application of polyurethane foam and has a bright future[24]。
According to the structure-activity relationship of structure control performance,polyurethane foam is a block polymer prepared by copolymerization of different monomers such as isocyanate,polyol,chain extender,crosslinking agent,foaming agent,catalyst and surfactant.the modification of monomer design has pointed out the direction for the preparation of high flame retardant polyurethane foam,and has achieved rapid development in recent years.Although the latest research and progress of flame retardancy of polyurethane foam were systematically reviewed by Professor Gupta's team of Pittsburgh State University and Professor Song Pingan of the University of Southern Queensland in 2022,they discussed more from the perspective of flame retardant elements,and less on intrinsic flame retardant polyurethane foam[25][26]。 the literature in the past 10 years is reviewed,and it is found that there are few review articles on intrinsic flame retardant polyurethane foam.Therefore,this paper will focus on the preparation methods of intrinsic flame retardant polyurethane foam in the past 10 years(2014-2023)from the perspective of monomer molecular flame retardant modification design.Including polyol modification,isocyanate modification,additives modification and other modifications,and their applications were summarized and prospected,hoping to provide some reference and thinking for scholars in this field to design intrinsic flame retardant polyurethane foam materials[27,28]。

2 Intrinsic Flame Retardancy of Polyurethane Foam by Polyol Modification and Its Application Progress

polyols are polymers containing two or more hydroxyl groups at the end of the molecule(pendent groups),which are usually divided into polyether polyols and polyester polyols.as one of the key raw materials of polyurethane foam,the hydroxyl functionality in the polymeric polyol molecule is Generally required to be≥2,and the number average molecular weight is generally in the range of hundreds to thousands.in the past 10 years,great progress has been made in the research of intrinsic flame retardancy of polyurethane foam by polyol modification.According to the source classification,there are two main forms of molecular structure design of flame retardant polyols:one is to add a certain proportion of reactive flame retardants to traditional petroleum-based polyols.generally,it contains small polyol molecules such as phosphorus,nitrogen,silicon and boron,and then directly embeds the flame retardant segments into the soft segments of polyurethane through chemical action,so as to realize the flame retardant functionalization of polyurethane foam.Secondly,the intrinsic flame retardant polyurethane foam is obtained by replacing the traditional polyol with the bio-based polyol modified by the flame retardant element,which has the advantage of environmental friendliness.the two strategies complement each other,which is the most common strategy to achieve intrinsic flame retardancy of polyurethane foam by polyol structure modification。

2.1 Preparation of Intrinsic Flame Retardant Polyurethane Foam from Petroleum Based Polyol

Reactive flame retardants are a class of molecules that participate in the polymerization process and bind to the backbone or branched chain of the polymer to function as flame retardants.It overcomes the shortcomings of additive flame retardant,such as easy migration,poor long-term effectiveness and damage to the physical properties of foam,and has the advantages of good stability and not easy to precipitate.Reactive flame retardants commonly used in polyurethane foam materials refer to polyols containing phosphorus,silicon,nitrogen,boron and other elements,which are often mixed with industrial polyols such as polyether polyol 4110 or polyethylene glycol 400 to prepare intrinsic flame retardant polyurethane foam。
halogen-free flame retardant polyols are the most kinds of halogen-free reactive flame retardants,among which phosphorus-containing or nitrogen-containing polyols are the most widely studied and applied.in particular,organophosphorus flame retardants meet the requirements of green environmental protection and high flame retardant efficiency,and have dual effects of flame retardant and plasticization,which can realize halogen-free flame retardants and improve the fluidity of polymer materials In the process of processing,so they have become a hot research topic.Fig.1 shows representative reactive flame retardant polyols[29⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~43]。
图1 石油基反应型多元醇制备本征阻燃聚氨酯泡沫[29⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~43]

Fig. 1 Preparation of inherent flame retardant polyurethane foam using petroleum-based reactive polyols[29⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~43]

in 2015,Li Jinchun et al.Of Changzhou University synthesized a bulky reactive flame retardant hexa-(phosphite-hydroxy-methyl-phenoxy)-cyclotriphosphazene(HPHPCP)In two steps using hexachlorocyclotriphosphazene,diethyl phosphite and 4-hydroxyphenol as raw materials[29]。 the flame retardant not only contains Two different types of phosphorus-containing flame retardant groups of phosphazene and phosphonate,but also contains six active secondary hydroxyl groups.when it is mixed with polyether polyol GR-4110 to prepare rigid polyurethane foam,the density,comprehensive strength and thermal stability of the rigid foam will be significantly improved due to the high crosslinking concentration caused by the multi-functional group of HPHPCP.In addition,when the content of HPHPCP was 10 wt%,the foam could pass the UL-94 HF-1 grade,and when the content of HPHPCP was 20 wt%,the LOI of the foam could reach 26%,which indicated that the addition of reactive polyol HPHPCP could obtain intrinsic flame retardant rigid polyurethane foam with excellent comprehensive properties.two years later,the team replaced diethyl phosphonite with 5,5-dimethyl-1,3,2-dioxaphosphahexanolactonate and synthesized the reactive flame retardant HDPCP through the addition reaction to aldehydes,which can give 25%LOI value to rigid foams when 25 wt%HDPCP is added[30]。 This design idea for the synthesis of bulky polyols through the"flame retardant element convergence"strategy provides an enlightening idea for the preparation of intrinsic flame retardant polyols。
Zhao Haibo of Sichuan University designed triol reactive flame retardant TDHTPP based on the structure of triazine and phosphate[31]。 the triol is chemically incorporated into the backbone of the material as a chain extender to produce rigid polyurethane foams that are inherently flame retardant.the results show that TDHTPP has good solubility in industrial alcohols such as polyether polyol 4110 and polyethylene glycol 400,which is beneficial to the processing of foaming process.the good compatibility between TDHTPP and polymer matrix makes the compressive strength of flame retardant rigid polyurethane foam much higher than that of pure rigid polyurethane foam,and the thermal conductivity is also lower.It is worth noting that only with the addition of 5 wt%TDHTPP,the prepared rigid foam showed UL-94 V-0 rating,and more importantly,showed long-term flame retardant performance in the thermal accelerated aging test at 140℃。
Yang Rongjie's team at Beijing Institute of Technology has been studying the cycloaddition reaction of isocyanates and epoxides for a long time,and believes that the isocyanate functional group at the end of polyurethane can be chemically transformed,which is expected to achieve the regulation of material properties[32]。 in 2019,they synthesized a reactive flame retardant triglycidyl phosphate(POG)from phosphorus oxychloride and glycidol.the epoxy group of the latter reacted with the isocyanate group At the end of the foam to introduce the oxazolone skeleton containing phosphate into the crosslinked network structure of the rigid foam.at the same time,they systematically studied the effects of POG on the mechanical properties,morphology,thermal stability and flame retardancy of rigid foam system.the results show that the compressive strength of RPUF is reduced by POG,but the thermal insulation ability is improved,the LOI value is increased to 22.3%,and the total heat release is reduced by 30.2%.in the vertical burning test,it is shown that POG flame retardant can effectively limit the spread of flame and eliminate the dripping of melt.Further mechanism study confirmed that the addition of POG was beneficial to inhibit the flame In the gas phase and inhibit the carbonization In the condensed phase,thus improving the flame retardancy。
the introduction of the third component additive not only contributes to the improvement of flame retardancy,but also maintains other properties.Expanded graphite(EG)has a flake graphite structure and is often used as an additive physically dispersed in the matrix of rigid polyurethane foam.it mainly plays a role in the condensed phase and effectively improves the flame retardancy of rigid foam.When compounded with a small amount of reactive flame retardant,It can achieve better flame retardancy and has less impact on the physical and mechanical properties of rigid foam。
In 2016,Hu yuan's team at the University of Science and Technology of China synthesized bis(4-hydroxybutyl)phenylphosphonate(BHPP)and melamine-derived polyol flame retardant(MADP)[33]。 in the presence of industrial polyol LY-4110,When the mass ratio of BHPP/MADP was 1∶1,the synergistic effect of the two flame retardants could significantly reduce the heat release rate of rigid polyurethane foam.when expanded EG was added,the LOI value of the syntactic foam increased from 27.0%to 33.5%,and the charring effect was also outstanding.Therefore,in this system,the expanded graphite plays a more critical role in improving the comprehensive properties of the material.In the second year,Professor Liu Yaqing of North University of China used reactive phosphoryl alcohol amine diethyl-N,N-bis(2-hydroxyethyl)phosphoramide(DEPA)to compound with expanded graphite.the rigid polyurethane foam is also endowed with a protective carbon layer with high yield,lower heat release and smoke yield,and a high LOI value of 30.4%,both of which significantly inhibit the generation of smoke in the combustion of the substrate through synergistic flame retardancy[34]。
In addition to inorganic additives such as metal hydroxides,phosphates,expandable graphite and inorganic particles,reactive flame retardants also contain additives such as silicon-containing organic clay。
In 2020,Chen Shanhua's team of Chengdu University of Technology used a reactive phosphorus-containing compound diethyl bis(2-hydroxyethyl)aminomethyl phosphonate(DBHP)to prepare flame-retardant rigid foam by combining it with organic clay(OMMT),and studied the modification effect of organic clay addition and the effect of OMMT/DBHP system on the physical and mechanical properties and flame retardancy of rigid foam[35]。 The results showed that the OMMT/DBHP system significantly improved the LOI value from 19.2%to 29.9%and the UL-94 test grade from no grade to V-0 grade in terms of flame retardancy.In the combustion process,DBHP can promote the formation of a strong carbon layer in the rigid foam matrix,and release PO·and PO2·active radicals into the gas phase to achieve gas phase flame retardancy,while nano-clay can also form a carbosilicate structure in the condensed phase to achieve condensed phase flame retardancy.Therefore,the OMMT/DBHP system achieves good flame retardancy of rigid foam by biphasic synergistic flame retardant mechanism 。
the molecular weight of the reactive flame retardant also affects the flame retardant effect of polyurethane foam.polymers,oligomers and hyperbranched Polymers have attracted wide attention because of their lower dosage and higher content of flame retardant elements。
In 2018,Wang Yuzhong's research group of Sichuan University designed a new liquid phosphorus-containing polyol oligomer dimethoxypropane(DMOP),which was blended with industrial polyol TMN3050 and applied to the regulation of flame retardancy,mechanics and smoke suppression of flexible polyurethane foam[36]。 In terms of mechanical properties,DMOP plays the role of soft segment In the main chain of polyurethane by chemical covalent bonding,which increases the flexibility and elongation at break of the foam.When 10 phr of DMOP is used,the synthesized flexible polyurethane foam has higher tensile strength and elongation at break.the results of thermogravimetric analysis(TGA)show that the thermal stability of these flexible foams is lower than that of ordinary polyurethane.the results of a series of combustion behavior tests showed that the addition of DMOP significantly improved the flame retardancy of the foam,and the LOI value could reach up to 22.7%.In addition,the accelerated aging test at 140℃for 64 H showed that the foam still maintained almost the same flame retardancy,indicating that the flexible foam with reactive DMOP had lasting flame retardancy.Therefore,the reactive polyol DMOP is added into the framework of polyurethane,so that the flexible polyurethane foam has durable and long-acting flame retardancy,and overcomes the disadvantages of poor compatibility and easy migration of additive flame retardants.In the same year,the team developed a new class of phosphorus-containing liquid polyether polyols PDEO[37]。 After doping the third component EG,the LOI of flexible polyurethane foams(FPUFs)was up to 25.5%.the reactive PDEO is also added to the FPUF chain in the form of covalent bonds,while the expandable graphite is mixed into the foam matrix through the foaming process.the physical charring effect of EG improves the compactness and high temperature thermal stability of the carbon layer,and plays an effective role in barrier protection.More importantly,the synergistic effect of PDEO and EG makes the flexible foam have good flame retardancy,droplet resistance and smoke suppression.This work provides a promising strategy for flexible polyurethane foams with flame retardancy,smoke suppression,and droplet resistance。
In 2019,Wu Ningjing et al.,Qingdao University of Science and Technology,synthesized a novel reactive phenyl phosphoryl glycol ether oligomer(PPGE)[38]。 the PPGE segment in the rigid foam simultaneously improves its compressive modulus and flame retardancy.When the phosphorus content was 2%,the limiting oxygen index value of 2%PRPUF was increased from 20.0%of pure RPUF to 24.5%.the results of cone calorimeter showed that the total heat release and heat release rate of 2%PRPUF were about 24%and 18%lower than those of pure RPUF,respectively.the combination of 2%PRPUF and expanded graphite effectively reduces the flame spread rate and smoke toxicity of the flame retardant RPUF.the flame retardant mechanism of 2%PRPUF/EG shows that the inhibition of phosphorus-containing radicals and the expansion into char synergistically achieve efficient flame retardancy.in particular,the active phosphorus-containing oligomer can obviously improve the compression modulus of a base material,and has wide application prospect in the aspect of developing high-efficiency flame-retardant thermal insulation rigid foam。
In 2023,Zhang Sheng et al.of Beijing University of Chemical Technology synthesized two reactive flame retardants FRC and FRD with multiple hydroxyl groups by solvent-free transesterification of erythritol and dimethyl methyl phosphonate(DMMP),and introduced them into rigid foam by reaction with isocyanate respectively[39]。 Flammability of the prepared RPUF samples was evaluated by LOI and cone calorimeter tests.In the presence of 10 phr of flame retardant FRD,the LOI value of RPUF increased from 18.3%to 29.8%,and the heat release rate(pHRR)and total heat release(THR)decreased by 31.4%and 47.8%,respectively,compared with the control RPUF.In addition,the compressive strength and thermal conductivity remained unchanged compared with the control RPUF。
In 2022,Yan Shouke et al.of Qingdao University of Science and Technology also synthesized an oligomer reactive diol(BEOPMS)through the esterification of 9,10-dihydro-10-(2,3-di(hydroxycarbonyl)propyl)10-phosphate phenanthrene-10-oxide(DDP)and diethylene glycol(DG)for the preparation of rigid polyurethane-polyisocyanurate foams(PIRs)[40]。 By studying the effect of the molar ratio of NCO/OH on the properties of PIRs,it was found that the LOI value of PIRs increased from 25.0%to 28.0%when the ratio of NCO/OH increased from 2.0 to 3.5,showing excellent flame retardancy.the thermal stability of the modified composite is significantly improved due to the presence of a large proportion of the trimeric ring isocyanurate structure。
In 2022,Luo Yumei et al.of Southwest Petroleum University designed and prepared high-performance rigid polyurethane foam(RPUF-HBPU)by using functional hyperbranched polyurethane polyol(HBPU)to replace traditional industrial polyol,with hydroxyl value of 399 mg KOH/G and functionality of 9.59[41]。 Compared with the rigid foam obtained from industrial polyether polyol,RPUF-HBPU showed better heat resistance,and the compressive strength,flexural strength and tensile strength were 4.5 times,4 times and 3.5 times higher than those of the traditional rigid foam,respectively,which provided a new idea for the preparation of intrinsic flame retardant rigid polyurethane foam.In the same year,Hu yuan's project combined a new type of hyperbranched polyol containing phosphine oxide(POCHP)[42]。 the POCHP and/or expandable graphite were then incorporated into the rigid polyurethane foam.the compressive strength of the rigid foam material ERPUF50 containing POCHP and EG is increased by 12.8%,and reaches the LOI value of 30.0%and the UL-94 V-0 standard value,and the rigid foam material ERPUF50 also has immediate self-extinguishing behavior after leaving the flame.In particular,the heat release rate,total heat release,and total smoke rate of ERPUF50 were reduced by 71.1%,52.2%,and 71.1%,respectively。
Zhang Long of Changchun University of Technology is committed to developing green reactive flame retardant polyether polyols to solve the flammable problem of rigid polyurethane foam[43]。 They synthesized green reaction flame retardant polyether polyol(GPP)with hydroxyl value of 390~420 mg KOH/G by ring-opening polymerization of propylene oxide with environmentally friendly melamine resin(EFMR).flame retardant rigid polyurethane foam was successfully prepared by using GPP as polyol.the results showed that GPP foam could greatly improve the thermal stability and flame retardancy of the prepared rigid foam.the LOI of the rigid foam prepared by the full substitution of GPP is 30.4%,and the compressive strength is 350 kPa,which is suitable for industrial production and has a good application prospect。
in addition to flame retardant polyols containing phosphorus or nitrogen,flame retardant polyols containing boron have also been studied,but few reports have been reported In The past 10 years.the synthesis of representative boron-containing flame retardant polyols is mainly based on boric acid.Sadowska and Czupry Czupryński prepared boron-containing flame retardant polyols with different structures by reacting boric acid with 1,3-propanediol,2,3-butanediol and 1,4-butanediol,respectively[44]。 the rigid polyurethane foam prepared by The three boron-containing flame retardant polyols not only has improved flame retardancy,but also has reduced brittleness and increased compressive strength。
to sum up,petroleum-based reactive polyol compounding or full substitution is a relatively mature method for the preparation of intrinsic flame retardant polyurethane foam.There are three main ideas:one is to functionalize the soft segment of polyurethane foam with reactive bulky or small molecular flame retardant polyols based on industrial polyols,but the overall performance is not ideal;Condly,in order to maintain the comprehensive performance,the industrial polyol,the reactive small molecule flame-retardant polyol and the inorganic and organic additive of the third component are compounded,and the flame-retardant performance is regulated and controlled while the comprehensive performance is ensured;the third is to change the molecular weight of the reactive flame retardant polyol,and to prepare the intrinsic flame retardant polyurethane foam by synthesizing the flame retardant polyol with oligomer and hyperbranched structure to partially or completely replace the industrial polyol.These three ideas complement each other and enrich the content of petroleum-based polyol modification。
However,the above strategies are widely used in rigid polyurethane foam flame retardant,and there are few examples in improving the fire safety performance of flexible polyurethane foam.the difficulty is that the hydroxyl value(generally 30~100 mg KOH/G),viscosity(the lower the viscosity,the better)and functionality(generally 2.5~4)of the synthesized flame retardant polyol need to meet the requirements of flexible foam polyol;the content of flame retardant elements should be as high as possible to achieve better flame retardant effect;the hydroxyl equivalent value of the flame retardant polyol should be as large as possible.the smaller the hydroxyl equivalent value is,the more isocyanate is consumed,the higher the hard segment content of the foam is,the more brittle the foam is,and the worse the mechanical properties are;If the hydroxyl activity in the flame retardant polyol is quite different from that in the polyether(ester)polyol,it is difficult to adjust the balance between the foaming speed and the crosslinking speed,and the foam is prone to collapse and shrinkage,resulting in foaming failure.Therefore,how to explore and develop reactive polyols with simple synthesis process,low cost,little effect on foaming and high flame retardant efficiency is an important trend in the development of flame retardants for polyurethane flexible foam。

2.2 Preparation of Intrinsic Flame Retardant Polyurethane Foam from Bio-based Polyol

with the world's attention to the ecological environment and under the background of the global"double carbon"policy,it is urgent to find renewable resources to replace fossil energy chemicals.In recent years,people have begun to replace fossil resources With biomass resources such as natural terpenoids,lignin,cellulose,vegetable oil,starch and protein[45][46,47][48][49⇓~51][52][53]。 in the field of polyurethane foam,the functional modification of bio-based polyols,such as castor oil,soybean oil and tung oil,to replace traditional petroleum-based polyols has become a research hotspot In this field.Fig.2 shows a representative bio-based reactive polyol[59⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~70]。
图2 生物基多元醇制备本征阻燃聚氨酯泡沫[59⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~70]

Fig. 2 Preparation of inherent flame retardant polyurethane foam using bio-based polyols[59⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~70]

In 2014,Zhou Yonghong et al.,Institute of Chemical Industry of Forest Products,Chinese Academy of Forestry,explored a new method to introduce castor oil-based flame retardant polyol(COFPL)into polyurethane foam[59]。 castor oil was first reacted with glycerol,hydrogen peroxide and diethyl phosphate to convert into flame retardant polyol,and a new type of castor oil-based rigid polyurethane foam was prepared by free foaming.the test results show that although the phosphorus content In the foam is only about 3%,the flame retardant fragments incorporated into the molecular chain of castor oil can significantly improve the thermal stability of the rigid foam.Without any other flame retardant,the LOI value can reach 24.3%.Therefore,rigid foams made from castor oil-based flame-retardant polyols have a wide range of applications.in 2017,based on this work,Li Mei of the same research group introduced the flame retardant fragment 9,10-dihydro-9-oxa-10-phosphatidyl-phenanthrene-10-oxide(DOPO)containing phosphine and phenanthrene into the modified castor oil chain,and then compounded with dihydroxy alcohol(BHAPE)as a crosslinking agent to improve the flame retardancy and mechanical properties of rigid foam[60]。
In 2019,Donempudi et al.,Indian Institute of Chemical Technology,innovatively synthesized a novel triazole-functionalized phosphate flame retardant monomer PTFM by using the click reaction of azide-alkynyl between triprop-2-ynyl phosphate and 2-azidoethanol[61]。 Triazole functionalized castor oil based polyurethane foam was prepared by the reaction of PTFM and hydroxylated castor oil polyol with hydroxyl value of 310 mg KOH/G.Compared with pure PUFs,when the mass ratio of PTFM to castor oil polyol was 1∶1,the prepared intrinsic polyurethane foam had a LOI value of 27%,a char yield of 30.82%at 700℃,a significant reduction in thermal release,and an obvious expansion char behavior。
In 2021,two novel biomass castor oil-based rigid polyurethane foams were prepared by Wang Deyi and others at the Institute of Advanced Materials,Madrid,Spain[62]。 the polyol used was diethanolamine transaminated castor oil(bio1),and then the epoxy fragment of bio1 was further modified by phenylphosphonic acid to obtain a novel biomass polyol BIO2.by doping carbon materials such as expanded graphite and graphene oxide,it is found that the prepared rigid foam has excellent thermal insulation,flame retardancy and mechanical properties,and that there is a synergistic effect between castor oil BIO-based rigid foam and carbon-based materials,which can also improve the insulation performance.This work provides a new strategy for the development of fire safety insulation materials for BIO-based flame retardant rigid polyurethane foam by doping inorganic additives。
In addition to castor oil,the modification Of soybean oil to achieve flame retardancy of polyurethane foam has also become a research direction.In 2018,Wang Niangui et al.of Hubei University synthesized phosphorus-containing polyether polyol(PTMA-PO)with trimethylol phosphine oxide(PTMA)and propylene oxide(PO)as raw materials,and then synthesized soybean oil-based polyol(SBP)with epoxidized soybean oil as raw material through lactic acid ring-opening reaction[63]。 SBP has good compatibility with PTMA-PO,and can be used as an intrinsic bio-based polyol when mixed.the results showed that the LOI value of PUFs increased to 27.5%when 40 wt%PTMA-PO and 60 wt%SBP were added,which indicated that the flame retardant polyurethane foam prepared by modified soybean oil had excellent fire resistance by adding reactive flame retardant PTMA-PO。
In the same year,Chen Mingjun et al.Of Xihua University also synthesized a new phosphorus-containing soybean-based polyol(PCSO)[64]。 Then,PCSO was used to completely replace the petroleum-based polyol to prepare biomass flame retardant polyisocyanurate foam(P-PIR).However,the porous structure of the prepared P-PIR foam was destroyed and the crosslinking density was reduced,resulting in a certain degree of decrease in compressive strength and thermal conductivity.When the commercial phosphorus-containing polyol(by30)and EG were added to the P-PIR system,the test results showed that the limiting oxygen index was significantly increased to 35%,the peak heat release rate and the total heat release rate were significantly reduced BY 65%and 87%,respectively,and the flame retardancy was improved.Especially after doping expanded graphite,the decomposition of PIR foam is obviously hindered,and the charring effect is obvious.the surface PIR foam is combined BY the gas phase effect of PCSO and the condensation phase effect of EG to achieve the synergistic flame retardant effect。
In addition to soybean oil,Zhang Meng et al.,Institute of Chemical Industry of Forest Products,Chinese Academy of Forestry,prepared tung oil-based flame retardant polyols[65]。 Under the action of sodium methoxide,they transesterified tung oil with glycerol to produce monoglycerides and diglycerides,and Then epoxidized them in situ with acetic acid and hydrogen peroxide in the presence of sulfuric acid catalyst.then,reactive flame retardant polyols TOFPs were prepared by ring-opening reaction of epoxidized alcoholyzed tung oil(EGTO)with DOPO,diethyl phosphate(DEP)and diethanolamine(DEA),respectively,and used as raw materials to prepare polyurethane foam TOPUFs.the results showed that the LOI value of TOPUFs could reach 26.2%when the content of DOPO-EGTO was 100 wt%。
In 2022,Prociak et al.,Krakow University of Technology,used modified cooking rapeseed oil to synthesize open-cell flexible polyurethane foam,and used water as a chemical foaming agent to study the effect of water on the physical and mechanical properties of the synthetic foam[66]。 the foam with 22%water content was determined to have the most favorable physical and mechanical properties,and an average increase in compressive strength of 10 kPa was observed when the isocyanate index of the selected foam was changed from 0.6 to 1.1.Unfortunately,neither the flame retardant properties nor the mechanical properties were dominant。
lignin,as an abundant natural organic carbon resource,contains a large number of aromatic rings and abundant hydroxyl functional groups.At present,the utilization efficiency of lignin is very low,and most of it is wasted as fuel.Wan Pingyu et al.,Beijing University of chemical Technology,prepared lignin-based phosphate melamine compound(LPMC)by Chemical grafting of lignin with compounds containing phosphorus and nitrogen groups through liquefaction-esterification-salinization process[67]。 Then,the LPMC containing the remaining hydroxyl groups was used to replace part of the polyether polyol and copolymerized with isocyanate to prepare lignin-modified polyurethane foam(PU-LPMC)with excellent flame retardancy.Due to the rigid aromatic structure of lignin and the covalent bond connection between LPMC and polymer matrix,the compressive strength of PU-LPMC was nearly doubled,and the PU-LPMC showed excellent thermal stability,char residue formation,self-extinguishing,and droplet and smoke inhibition.In addition,a large amount of non-combustible gas is released during the thermal degradation process,and a dense intumescent carbon layer is formed on the surface of the foam after combustion,thereby improving the flame retardancy of the polymer。
In 2022,Dongweifu of Jiangnan University prepared biomass polyols from dextrin and glycerol,and used them to prepare rigid polyurethane foam[68]。 bio-polyols with different dextrin contents were obtained by simple and environmentally friendly physical mixing,and the effects of different Bio-polyols on foaming process,combustion behavior,mechanical properties and biodegradability were studied.Compared with the conventional foam,the rigid foam based on dextrin and glycerol has higher compressive strength and biodegradability.in addition,when the ratio of dextrin to glycerol in the biopolyol is 1∶1,the self-extinguishing time of the foam is only 9 s,and the LOI value is 14.5%higher than that of the conventional foam.These results indicate that dextrin-glycerol polyols are expected to completely replace petroleum-based polyols to prepare rigid foams with light weight,high compressive strength,good flame retardancy and good degradability,which have potential application prospects in the field of wood-like and rapid packaging。
In 2018,Gupta et al.of Pittsburgh State University synthesized a flame-retardant polyol PPA-PO based on the reactivity of phenylphosphonic acid and propylene oxide,and compounded it with limonene-based polyol of biomass to prepare flame-retardant rigid polyurethane foams with different phosphorus contents by adjusting the addition of PPA-PO[69]。 When The phosphorus content is 2%,the rigid foam compressive strength is increased to 160%;the self-extinguishing time of the foam decreases from 81 s to 3.2 s;the PHRR and THR were reduced by 68.6%and 23.44%,respectively.PPA-PO is considered to be an ideal reactive flame retardant for high performance rigid polyurethane foam。
in 2022,Li Jinchun of Changzhou University and others also proposed a biomass flame retardant strategy for rigid polyurethane foam.They used dimethyl methylphosphonate,malic acid,citric acid and 1,6-hexanediol to synthesize bio-based phosphorus-containing polyol(PMCP)In one pot without solvent,and its hydroxyl value was as high as 485 mg KOH/G[70]。 The results showed that the LOI value of RPUF-PMCP40 increased from 18.3%to 24.5%after using PMCP polyol,and reached 27.3%when adding expanded graphite,and the heat release rate decreased from 264.2 kW·m-2to 129.6 kW·m-2 。
to sum up,using soybean oil,castor oil and other biomass as polyol raw materials to prepare polyurethane foam has become an important direction of green and sustainable development of materials In the future.In order to improve the flame retardancy of polyurethane foam,bio-based polyol mainly uses its own polyhydroxy unit structure to introduce phosphorus-containing flame retardant elements through phosphorylation reaction.In order to balance the flame retardant performance and other properties,it is mainly through the combination of small molecular flame retardants or inorganic substances.However,at present,bio-based flame retardant polyols can not completely replace petroleum-based polyols,and the flame retardant effect is not good because of the low content of flame retardant elements.it is worth noting that it is difficult to control the hydroxyl value of bio-based polyols by using this strategy,which is still limited to the application of polyurethane rigid foam。

3 Intrinsic Flame Retardancy of Polyurethane Foam by Modification of Isocyanate and Its Application

isocyanate is also one of the main raw materials for the preparation of polyurethane foam.there are many kinds of isocyanates,such as toluene diisocyanate(TDI),diphenylmethane diisocyanate(MDI)and polymethylene polyphenyl polyisocyanate(PAPI),which are widely used in polyurethane industry.TDI is mainly used in the manufacture of flexible polyurethane foam,polyurethane coatings,casting polyurethane elastomers,adhesives,paving materials and plastic runways;MDI is mostly used in the manufacture of thermoplastic polyurethane elastomers,synthetic leather resins,sole resins,spray polyurethane(urea)resins,one-component solvent-type adhesives,etc;PAPI is mainly used in the synthesis of rigid polyurethane foam,adhesives and so on.However,There are some challenges in the design and modification of isocyanates because of their high activity and toxicity.At present,the main design idea of isocyanate modification is to add flame retardant units to react with it to generate functional monoisocyanate in situ,and finally add polyols to react with the remaining isocyanate groups on the chain to connect the flame retardant functional groups into the polyurethane chain segment to obtain intrinsic flame retardant polyurethane foam。
In 2008,K König et al.Reported that melamine was used as a reactive flame retardant to react with TDI system to prepare flame-retardant flexible polyurethane foam,and the density and compressive strength of the foam increased with the increase of melamine[71]。 Through infrared spectrum analysis,the author believes that melamine first reacts with TDI to obtain disubstituted urea containing monoisocyanate,and then reacts with polyol to construct intrinsic flame retardant polyurethane foam。
Inspired by this,in 2019,Yang Rongjie and others also adopted this idea in the preparation of intrinsic flame retardant rigid polyurethane foam[72]。 As shown in Fig.3,they utilized 3,3′,4,4′-bisphenyltetracarboxylic dianhydride(PTDA)and DOPO derivatives as reactive flame retardants to introduce imide and oxazolone fragments into polyurethane chain segments to prepare intrinsic rigid polyurethane foams with preserved density and thermal stability,reduced thermal conductivity,and enhanced compressive strength.The foam combustion behavior was evaluated by LOI and cone calorimetry experiments,and the LOI of polyurethane foam containing PTDA and e-DOPO reached 22.4%,and the peak exothermic rate decreased from 281.28 kW·m-2to 227.50 kW·m-2,showing some flame retardant properties 。
图3 具有亚酰胺和噁唑酮交联结构的硬质聚氨酯泡沫[72]

Fig. 3 Cross linked structure of RPUF with imide and oxazolidinone[72]

in 2020,Zhang Chen of Beijing University of Chemical Technology reported that modified graphene oxide was used to modify isocyanate,so that isocyanate was functionalized to participate In rigid polyurethane foaming[73]。 the results show that the cell morphology and mechanical properties of polyurethane can be effectively improved by adding appropriate amount of modified graphene oxide.Isocyanate-modified graphene oxide can form stable chemical bonds With the matrix,which is more conducive to improving its mechanical properties.with the increase of modified graphene oxide,the thermal conductivity increases slightly,and the improvement of flame retardancy is limited。
to sum up,There are few studies on flame retardant isocyanates,but there are still good potential applications.there are few studies on flame retardant isocyanates,mainly because the modified isocyanates have high viscosity,great changes in reactivity,complex and difficult synthesis process,high price and difficult industrialization.How To control the activity of isocyanates is the key point for the wide application of this strategy。

4 Intrinsic Flame Retardancy of Polyurethane Foam by Additive Modification and Its Application

additives are essential components of polyurethane foam.Surfactants,crosslinking agents,foaming agents,etc.are added to polyurethane foam.Although the amount of these additives is small,They play a vital role in the overall performance.they are usually added to polyol premix or isocyanate.However,there are few reports on the intrinsic flame retardancy of polyurethane foam modified by additives due to the small amount of additives。
In 2014,Chen Mingjun et al.Synthesized a halogen-free trimethylolphosphine oxide(PTMA)[74]。 as shown in fig.4,the substance can be used As both a crosslinking agent and a reactive flame retardant.Based on this,they prepared flexible polyurethane foam(FPUF)with intrinsic flame retardancy.Because of its good charring property,the low content(7.8 wt%)of PTMA in the polyurethane chain can reduce the PHRR and THR by 27%and 56%,respectively,and increase the LOI value to 23%,which significantly improves the flame retardancy of FPUF.the results showed that more than 60%of the phosphorus-containing substances in the PTMA-crosslinked flexible foam were decomposed into polyphosphoric acid or its derivatives,which were retained in the carbon residue.Since then,the synthesis of flame retardants around the PTMA structure has become a worthy research direction[75⇓⇓⇓⇓⇓~81]。
图4 基于交联剂三元醇PTMA合成具有本征阻燃的软质聚氨酯泡沫[74]

Fig. 4 Synthesis of inherently flame-retardant FPUF using phosphorus-based reactive polyol (triol) PTMA[74]

In the same year,Schmidt et al.of the University Of Massachusetts reported a new bio-based crosslinking agent,3-hydroxy-N,N-bis(2-hydroxyethyl)butyramide(HBHBA),which was used to prepare flexible polyurethane foam[82]。 Compared with the conventional crosslinking agent diethanolamine(DEOA),the trifunctional crosslinking agent HBHBA can be used as a chain extender due to the presence of secondary hydroxyl groups with low reactivity,and tends to form larger cell size and more complete cell opening;the mechanical properties test showed that the elongation at break and tensile strength of HBHBA foam were 33%and 41%higher than those of DEOA foam,respectively;the HBHBA foam also exhibits good tear strength and higher compressive strength without any loss of elasticity.These results indicate that this bio-based crosslinker has significant potential in enhancing foam performance,but unfortunately,the flame retardant properties of this system foam have not been studied。
to sum up,due to the low addition of crosslinking agent or chain extender,It is difficult to prepare high flame retardant polyurethane foam only by using flame retardant crosslinking agent or chain extender,but there are many ways to modify this method.it can balance the flame retardancy and other properties such as hydrophobicity,especially in the application scenarios with low demand for flame retardancy,this strategy can be used as an important idea for the preparation of intrinsic flame retardant polyurethane foam。

5 Intrinsic Flame Retardancy of Polyurethane Foam by Other Modifications and Its Application

isocyanate monomers used in traditional polyurethane foams are toxic,and recently researchers have used non-isocyanates to prepare polyurethane foams.the main content is that polyhydroxy polyurethane foam(PHU)is obtained by The reaction of cyclic carbonate compound and primary amine.The non-Isocyanate polyurethane foam synthesized by this method has better hydrolysis resistance and mechanical properties than traditional polyurethane foam,but it also has the problem of flammability。
in 2022,Negrell et al.of the University Of Montpellier synthesized a new DOPO-cyclic carbonate monomer containing phosphorus to obtain an intrinsically flame-retardant PHU foam,as shown In Figure 5[83]。 According to the performance analysis,the more the aromatic rings In DOPO are introduced into the foam,the higher the hardness is.All the foams containing DOPO cyclic carbonates have relatively good flame retardancy.in the cyclic carbonate structure,the foam with 2 wt%phosphorus content and two aromatic rings has the best effect,and the total heat release is also significantly reduced。
图5 阻燃聚羟基脲泡沫的反应型含磷环状碳酸酯合成[83]

Fig. 5 Synthesis of reactive phosphorus-based carbonate for flame retardant polyhydroxyurethane foams[83]

to sum up,there are few reports on non-isocyanate intrinsic flame retardant polyurethane foam,but this strategy avoids the use of toxic isocyanates,and using carbon dioxide as raw material is conducive to"carbon reduction",which has the advantage of green environmental protection,but due to the difficulty of flame retardant modification design of cyclic carbonate monomer,there are few studies on flame retardant application.Under the international background of"double carbon",the research on intrinsic flame retardancy of polyhydroxyurea foam will inevitably become the trend of future research。

6 Conclusion and prospect

polyurethane foam materials are widely used in construction,rail transit,new energy vehicles,electronic equipment and other fields,but it contains a large number of hydrocarbon chains,which are very flammable and limit The scope of use to a great extent,so it is urgent to improve the flame retardancy of polyurethane foam.intrinsic flame retardant polyurethane foam has a bright application prospect in the future because of its long-term effectiveness and durability.in this paper,the preparation and application progress of intrinsic flame retardant polyurethane foam were reviewed.the preparation strategy and application of intrinsic flame retardant polyurethane foam were systematically summarized from the latest achievements of polyol modification,isocyanate modification,additive modification and other modifications.the advantages and disadvantages of various modification methods are summarized in Table 1。
表1 Summary of Advantages and Disadvantages of Modification Methods

Table 1 Summary of advantages and disadvantages of modification methods

Modification methods Advantages Disadvantages
Polyol modifications Petroleum-based polyol：Simple in process; efficient in flame retardancy; divergent in molecular desgin Small molecular weight; lacking green friendliness; big challenges in achieving a balanced comprehensive performance
Bio-based polyol：Larger molecular weight; green and sustainable modification Low flame retardant element content; difficult to control the hydroxyl value; Poor flame-retardant effect
Isocyanate modifications Good flame retardancy; huge potential in inherent flame-retardant PUF Difficult to control in reactivity; complex process; expensive; difficult to industrialize
Additive modifications Low addition; divergent ways; assignable
multifunctionality		 Limited effect in flame retardancy
Other modifications Environmental-friendliness and huge potential in fire-safety PUFs Difficult structural design in non-isocyanates
from the current progress,It is the most common strategy to achieve intrinsic flame retardancy of polyurethane foam by polyol modification.reactive polyols,ranging from small molecules and oligomers to hyperbranched polymers,have been reported.Among them,polymeric polyols with high phosphorus content have better overall flame retardant performance,which is an important research direction for the design of reactive flame retardants in the future.in order to maintain other properties such as mechanical properties,it is also a common complementary scheme to compound inorganic additives such as expanded graphite in reactive flame retardants.However,from the overall performance,this strategy is more suitable for rigid polyurethane foam,and it still faces great challenges to achieve the intrinsic flame retardancy of flexible polyurethane foam.it is a key scientific problem to be solved in the field of fire safety of polyurethane foam to explore and develop reactive polyols with simple synthesis process,low cost,little influence on foaming and high flame retardant efficiency。
in response to the environmental and energy crisis,bio-based flame retardant polyols have also begun to emerge.Structural modification of vegetable oils such as castor oil and soybean oil to replace petroleum-based polyols has also become a hot direction in recent years,but Because the hydroxyl value is difficult to control,it is mostly limited to the use of polyurethane rigid foam materials.Due to the high activity of isocyanate,it is difficult to modify the structure,and the foaming process is often controlled in situ.the existing reported methods are complex,expensive and difficult to industrialize.because of the small amount of additives,although they can play a role in charring,it is difficult to play a leading role in flame retardancy in foam materials.Among other modification methods,the preparation of non-isocyanate intrinsic flame retardant polyurethane foam is rarely reported,but its advantages of green and environmental protection will become the trend of future research。
the current high-end industry has higher and higher requirements for the flame retardant application of polyurethane foam.rigid foam is usually used as thermal insulation materials,and its flame retardant grade generally needs to meet B1 grade,in which LOI≥30%is the Rigid requirement,while flexible foam flame retardant can achieve LOI≥26%on the premise of maintaining high resilience[84]。 According to the reported performance results,there is still a long way to go for the intrinsic polyurethane foam prepared at present to meet the above requirements,especially the key and difficult points for the practical application of polyurethane foam are the maintenance of inherent characteristics and efficient flame retardancy.From the perspective of molecular structure design,researchers can develop new polymer synthesis methods such as photochemical reaction,control the structure and properties,and design and synthesize flame retardant polyol polymers with high flame retardant element content and controllable properties,which is expected to achieve the above goals[85]。 bio-based flame retardant polyurethane foam is the future development direction,and the functional design of Bio-based materials will also have great potential。

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