聚合物燃烧与成炭.ppt

,北京理工大学,第2章 聚合物燃烧与成炭,2.1 聚合物热降解过程及机理2.2 聚合物燃烧化学反应及控制2.3 聚合物燃烧模式和特点2.4 聚合物热分解成炭2.5 聚合物燃烧性能分类2.6 促进聚合物成炭的途径2.7 热行为研究及炭的表征2.8 烟的形成及抑制,北京理工大学,为什么要了解聚合物的热降解过程与机理？,The Importance of Thermal Degradation of Polymers Polymer degradation during heating or combustion is of considerable importance on understanding and possibly prediction fuel production and fire behavior.The efficiency of matching the decomposition temperature of the flammable substrate and the FRs is one of the main keys to good flame retardancy.The changes in degradation pathway of the polymers in the presence of FRs is a important aspect of the modification in the fire retardancy.,2.1 聚合物热降解过程及机理,了解聚合物燃烧行为及产物；阻燃剂的选择及配方设计；指导新型阻燃材料的研究.,北京理工大学,2.1 聚合物热降解过程及机理,参考文献：Jang B N,Costache M,Wilkie C A.The relationship between thermal degradation behavior of polymer and the fire retardancy of polymer/clay naocomposites J.Polymer,2005,46:10678-10687.The changes in degradation pathway of these polymers have elucidated and the common factor is that inter-chain reactions,such as intermolecular aminolysis/acidolysis,radical recombination and hydrogen abstraction,become significant in the presence of clay.It was suggested that the number of degradation pathways of the polymer is one of the factor that can affect the fire retardancy of polymer/clay nanocomposites.,Thermal decomposition：a process of extensive chemical species change caused by heatThermal degradation：a process whereby the action of heat or elevated temperature on a material,product,or assembly causes a loss of physical,mechanical,or electrical properties,2.1 聚合物热分解过程及机理,北京理工大学,Classification of Thermal Degradation Reactions Depolymerisation Reactions(radical and non-radical)Fragmentation(monomer,chain fragments)Substituent Reactions Elimination,Cyclization,北京理工大学,2.1 聚合物热分解过程及机理,Structural defectsReactive impurities“Special”structures(e.g.chain end,etc.）“Weak links”,Depolymerisation Reactions(radical and non-radical)breaking of the main polymer chain backbone at any intermediate stage the products are similar to the parent material(the monomer units)the ultimate product may be monomer or volatile chain fragments,北京理工大学,2.1 聚合物热分解过程及机理,Substituent Reactions the substituents attached to the backbone of the polymer are involved the chemical nature of the repeat unit is changed although the chain structure may remain intact volatile products,if they are produced,will be quite unlike monomer,北京理工大学,2.1 聚合物热分解过程及机理,（1）Polyolefins Of the polyolefins,low-density polyethylene,high-density polyethylene,and polypropylene are of the greatest commercial importance because of their volume.Their uses are too numerous to mention,but fire properties are critical when used for electrical wire and cable-coating applications.Upon thermal decomposition,very little monomer formation is observed for any of these polymers;they form a large number of different small molecules,mostly hydrocarbons.Thermal stability of polyolefins is strongly affected by branching,with linear polyethylene most stable and polymers with branching less stable.,Thermal Degradation Behavior of Some Polymers,北京理工大学,2.1 聚合物热分解过程及机理,Polyethylene（Random-chain scission）PE begins to decompose at 292(inert atmosphere,reductions in molecular weight),although extensive weight loss is not observed below 372.Piloted ignition of PE due to radiative heating has been observed at a surface temperature of 367.Products:propane,propene,ethane,ethene,butane,butene-1 and hexane-1.,北京理工大学,2.1 聚合物热分解过程及机理,Fig.Intramolecular transfer processes,北京理工大学,2.1 聚合物热分解过程及机理,Polypropylene（Random-chain scission）PP：reductions in molecular weight are first observed at 227 to 247 and volatilization becomes significant above 302.Piloted ignition of PP due to radiative heating has been observed at a surface temperature of 337.Products:pentane(24%),2methyl-1-pentene(15%),and 2-4 dimethyl-1-heptene(19%).,北京理工大学,2.1 聚合物热分解过程及机理,Polystyrene（End-chain scission,R）Monomer(approximately 40%)is the principal volatile product with very much smaller amounts of benzene and toluene.,北京理工大学,2.1 聚合物热分解过程及机理,PS shows no appreciable weight loss below 302 although there is a decrease in molecular weight due to scission of“weak”links.Above this temperature,the products are principally monomer with decreasing amounts of dimmer,trimer,tetramer and pentamer.These oligomers are formed in intramolecular transfer reactions,which are in direct competition with the monomer-producing depolymerisation process.,北京理工大学,2.1 聚合物热分解过程及机理,Polymethyl methacrylate（End-chain scission）PMMA is a favorite material for use in fire research because it decomposes almost solely to monomer and burns at a very steady rate.Methyl groups effectively block intramolecular hydrogen transfer,leading to a high monomer yield.It has been shown that depolymerisation is a radical chain reaction and that degradation in the region of 300-400 is associated with initiation by both end-chain and random-chain initiation processes.,北京理工大学,2.1 聚合物热分解过程及机理,Fig.Thermal degradation mechanism of PMMA,北京理工大学,2.1 聚合物热分解过程及机理,Poly(vinyl chloride)（chain stripping,substituent elimination）The radical mechanisms:Initiation involves liberation of chorine atom from a labile centre.,北京理工大学,2.1 聚合物热分解过程及机理,Molecular elimination of HCl is proceeded by a cyclic transition state,北京理工大学,2.1 聚合物热分解过程及机理,The thermal degradation of PVC involves the elimination of HCl and leads to the formation of macromolecular residues with polyene sequences.The latter then rearrange and decompose to yield sizeable amounts of aromatic hydrocarbons(toluene,methylnaphthalene,etc.)Aromatic compounds(benzene,naphthalene,anthracene)are produced simultaneously with HCl in the first decomposition stage.,北京理工大学,2.1 聚合物热分解过程及机理,引发聚合物燃烧有三个必要环节：加热 分解/裂解 被点燃,北京理工大学,2.2 聚合物燃烧化学反应及控制,北京理工大学,2.2 聚合物燃烧化学反应及控制,发,By preventing fire outbreak or By delaying ignition and flame spread in the developing fire By interrupting the combustion process,北京理工大学,2.2 聚合物燃烧化学反应及控制,Ignition temperature:The lowest temperature of a combustible material at which sustained combustion can be initiated,i.e.heat lost heat gainedFlashover:The rapid transition to a state of total surface involvement in a fire of combustible materials within an enclosure,北京理工大学,2.2 聚合物燃烧化学反应及控制,What Happens When Something Burns?The simplest example of a combustion process is the burning of methane.CH4+O2 CO2+H2OBut even this simple combustion involves many free-radical production steps:,北京理工大学,2.2 聚合物燃烧化学反应及控制,Propagation steps that produce free radicals keep the burning going;CH4+HO CH3+H2O CH4+H CH3+H2 CH3+O CH2O+H CH2O+HO CHO+H2O CH2O+H CHO+H2 CHO CO+H CO+HO CO2+H,北京理工大学,2.2 聚合物燃烧化学反应及控制,Chain-branching steps produce two free radicals,accelerating the reaction with explosive force;H+O2 HO+O Termination steps occur when radicals are removed,quenching combustion.H+R+M RH+MWhere R is any organic radical and M is any surface.Heat is transferred to the surface,producing higher energy M.The flame-carrying radicals H,HO,O,occur in all flames,whether in methane or in polymers.,北京理工大学,2.2 聚合物燃烧化学反应及控制,Two ways to interrupt the cycle:Solid phase inhibition involves changes in the polymer substrate.Systems that promote extensive cross-linking at the surface,form a carbonaceous char upon heating.Char insulate the underlying polymer from the heat of the flame,preventing production of new fuel and further burning.Other systems evolve water during heating,cooling the surface and increasing the amount of energy needed to maintain the flame.,北京理工大学,2.2 聚合物燃烧化学反应及控制,Vapor gas inhibition involves changes in the concentration of free radical HOand H.Reactive species are built into the polymer which are transformed into volatile free-radical inhibitors during burning.These materials diffuse into the flame and inhibit the branching radical reaction.As a result,increased energy is required to maintain the flame and the cycle is interrupted.Of course,for many materials both solid and vapor phase inhibition are involved.,北京理工大学,2.2 聚合物燃烧化学反应及控制,Combustion Modes Flaming Mode Non-flaming Mode Chemical reaction Chemical reactionin gases phase in condensed phase Smouldering combustion and Glowing combustion,北京理工大学,2.3 聚合物燃烧模式和特点,Smouldering combustion statistics in real fire about 75%,lethal rate 50%;being able to cause burning through smoke;can occur at the beginning of burning;a big volume of CO produced converted further to burning.,北京理工大学,2.3 聚合物燃烧模式和特点,Glowing combustion taking place after the formation of char;taking place at the surface/interface;accompanied by light colour;in the presence of metallic,non-metallic elements(Mg,Al,Li,Na,K,Ti,Zr,U,Pu,Zn,P,B,etc.)The combustion temperature displays higher than the combustible gases and liquid.,北京理工大学,2.3 聚合物燃烧模式和特点,Table 2-1 Polymers and Flammability Classification,北京理工大学,2.4 聚合物燃烧性能分类,2.4 聚合物燃烧性能分类,The first group consists of relatively flame retardant structures containing either high halogen,or aromatic groups that confer high thermal stability as well as the ability to form char on burning.Second are the less flame retardant materials,many of which can be made more flame retardant by appropriate chemistry.The third class consists of quite flammable polymers which are more difficult to make flame retardant because they decompose readily,forming large quantities of fuel,but these can be made appropriately flame retardant for particular applications by the addition of additives.,北京理工大学,High temperature materials are intrinsically flame retardant;There are three general types of structures:(1)linear single-strand polymer;(2)ladder polymers consisting of an uninterrupted sequence of cyclic aromatic or heterocyclic structures;(3)and spiro polymers in which one carbon is common to two rings.Polymer with high aromatic character and very strong connecting linkages between rings produce more char on heating,retaining most of the potential fuel of the original polymer as residue;In practice,the choice of a polymer depends on cost and on the importance of flame retardancy in relation to its final use.,北京理工大学,2.4 聚合物燃烧性能分类,2.5 聚合物分解成炭,Formation of Char 燃烧的聚合物表面温度接近300-600时，强烈的热降解发生。该温度也是炭层底部靠近聚合物表面的温度。而炭层的外表面与火焰接触，其温度可达1500。因此，炭形成的温度是在300-1500之间。,北京理工大学,A polymer passes through several steps in the formation of char:交联（cross-linking）芳香化（aromatization）稠环芳香化（fusion of aromatics）涡流层状炭（turbostratic char formation）石墨化（graphitization）,北京理工大学,2.5 聚合物分解成炭,The Role of Char Char formation may limit the amount of fuel available.When an adherent and insulating layer of char is built up on the surface of a polymer,this layer may insulate the underlying polymer from the flame and,thus,make further degradation more difficult.,北京理工大学,2.5 聚合物分解成炭,什么是炭？聚合物燃烧表面所产生的炭，其化学及物理结构决定了炭的阻燃作用。炭是复杂的不溶混合物，是由许多芳香-脂肪化合物及杂原子（O、N、P）组成。从微观形貌看，炭有结晶和非晶区。炭具有一些物理性质，如热传导、导电性、对辐射的反射和吸收等；炭也有一些机械性能，如完整性、硬度、塑性等。,北京理工大学,2.5 聚合物分解成炭,Non-charable or charablepolymers that undergo chain scission and volatilize with,at most,a negligible amount of the char formation,e.g.,PE,PP,PS,PMMA;polymers that undergo chain stripping reactions,producing unsaturation in the main chain with loss of hydrogen atoms and the pendant groups and give a moderate amount of char,e.g.PVC,PVA,PAN;polymers that contain aromatic rings that can cross-link simultaneously with chain scission reactions and produce moderate to high amounts of char,e.g.,aromatic polyamides,polyesters,polycarbonates,polyimides.,北京理工大学,2.5 聚合物分解成炭,Char-forming polymers Bisphenol A-polycarbonate,北京理工大学,2.5 聚合物分解成炭,北京理工大学,2.5 聚合物分解成炭,CO2,Bisphenol A-polycarbonate 450开始热降解；25%残炭稳定到650；TGA曲线形状由以下过程构成：polymervolatiles polymercross-linkingchar charvolatilesMain Products of Degradation:35%carbon dioxide,bisphenol A and phenol,北京理工大学,2.5 聚合物分解成炭,PA6、PA66（Do yourself）Pieter Gijsman et al.Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66 J.Polymer Degradation and Stability，2002,78:219-224.,北京理工大学,2.5 聚合物分解成炭,Polymers that naturally Produce Char 在一定条件下，甚至脂肪烃的碳氢聚合物都能够产生一些炭。碳氢聚合物中C-C键离解能（dissociation energy）为65-90 kcal/mol，而C-H键的离解能为90-105 kcal/mol。两者能量接近，因此高温下随着链的断裂可以脱氢：,北京理工大学,2.6 促进聚合物成炭的途径,烯丙基链端上的H是活泼的，其C-H键离解能为85 kcal/mol，因此可以导致脱氢，形成共轭双烯（conjugated dienes）。双烯可与活泼双键反应并进一步形成芳环：,北京理工大学,2.6 促进聚合物成炭的途径,双烯还可进一步脱氢形成三烯进而环化：,北京理工大学,2.6 促进聚合物成炭的途径,Polyacrylonitrile PAN在迅速加热的条件下，如燃烧条件下，产生碎片及挥发产物。放慢加热速度，250-350下侧基分子内反应发生，导致环状多胺（polyamine）结构生成。这种环状多胺结构对于较高温度下的成炭是十分有利的。,北京理工大学,2.6 促进聚合物成炭的途径,接枝促进聚合物热降解成炭 参考文献：电子束辐射接枝EVA共聚物薄膜阻燃性能的研究与表征 J，科学通报,1997,42（20):2167-2171 氧化还原反应促进成炭 Cu2O和MoO3对PVC阻燃抑烟的作用,北京理工大学,2.6 促进聚合物成炭的途径,添加剂促进成炭（Additives that promote char formation）APB(Ammonium pentaborate NH4B5O84H2O)is an effective char promoter in PU thermoplastics.As in the case of cellulose,both physical and chemical factors help to increase char in polyurethanes(e.g.,boric acid,which evolves upon thermal decomposition of APB,react with alcohols producing borate esters or with isocyanurates producing borate amides).Both the esters and amides serve as highly cross-linked char precursors.APB is also found to be somewhat effective in natural rubber,polyester resin,PVC,and polyamide-66.The following figure illustrates the char-promoting tendency of APB in PA-66.The addition of 25 parts per hundred of PA-66 leads to the conversion of 28%of PA-66 into nonvolatile residue.,北京理工大学,2.6 促进聚合物成炭的途径,北京理工大学,2.6 促进聚合物成炭的途径,热行为研究手段：热失重分析（TGA，Thermogravimetric Analysis）微分热失重分析（DTG，Differential Thermogravimetric Analysis）差热分析（DTA，Differential Thermal Analysis）差热扫描量热分析（DSC，Differential Scanning Calorimetry）热机械分析（TMA，Thermomechanical Analysis）热挥发分析（TVA，Thermal Volatilization Analysis）裂解气相色谱（PGC，Pyrolysis Gas Chromatography）色质联用（GC-MS，Gas Chromatography-Mass Spectrometry）,北京理工大学,2.7 热行为研究及炭的表征,炭的结构表征 傅立叶红外光谱法 拉曼光谱法 核磁共振波谱法 X-射线光电子能谱法 元素分析 扫描电镜等,北京理工大学,2.7 热行为研究及炭的表征,（1）一些聚合物燃烧发烟的数据 聚合物燃烧过程中生烟量是不同的 聚合物发烟来自于不完全燃烧；聚合物的产烟量除与结构因素有关外，还与燃烧模式（无焰燃烧还是有焰燃烧）样品尺寸、样品位置、通风与否、热通量等有关。文献报导的烟的数据应客观看待，因数据出自不同的标准和不同的测试设备。,北京理工大学,2.8 烟的形成及抑制,Table.2-2 Smoke Chamber Results for Various Materials（Rohm&Haas XP-2 Chamber（ASTM D-2843）,北京理工大学,2.8 烟的形成及抑制,Table 2-3 Some examples taken from Hilado book（ASTM E-662）,北京理工大学,2.8 烟的形成及抑制,（2）烟的组成Consisting of a dispersion of solid(carbon flakes,soot beads,ash,sublimed pyrolysis products,oxides of inorganic compounds)or liquid(sublimed pyrolysis products)particles in a carrier gas(combustion gas and air).Varying in diameter from 0.002-0.5m.,北京理工大学,2.8 烟的形成及抑制,Fig.Polymer pyrolysis,burning and smoke evolution,北京理工大学,2.8 烟的形成及抑制,（3）烟的产生 Pasternak et al(Combustion Science and Technology,1982,28:263-270)consider smoke generation during the combustion of polypropylene to follow a stepwise ring-growth mechanism consists of three phases:,北京理工大学,2.8 烟的形成及抑制,thermalPhase 1:polymer simple fuel(polymer fragments and degradation pyrolysis products)Phase 2:simple fuel reactive aromatic smoke nuclei intermediates stable polycyclic aromatic hydrocarbons particulate growthPhase 3:smoke nuclei smoke particles coagulation and agglomeration,北京理工大学,2.8