APP(2013)机理[16]

Flame retardant mechanism of an efficientflame-retardant polymeric synergist with ammonium polyphosphate for polypropyleneZhao-Zan Xu,Jian-Qian Huang*,Ming-Jun Chen,Yi Tan,Yu-Zhong Wang*Center for Degradable and Flame-Retardant Polymeric Materials,National Engineering Laboratory of Eco-Friendly Polymeric Materials(Sichuan),State Key Laboratory of Polymer Materials Engineering,College of Chemistry,Sichuan University,Chengdu610064,Chinaa r t i c l e i n f oArticle history:Received4April2013 Received in revised form30May2013Accepted8July2013Available online18July2013Keywords:Flame retardancy PolypropyleneIntumescenceSynergism a b s t r a c tAn efficientflame retardant polymeric synergist poly[N4-bis(ethylenediamino)-phenyl phosphonic-N2, N6-bis(ethylenediamino)-1,3,5-triazine-N-phenyl phosphonate](PTPA)was designed and synthesized from cyanuric chloride,ethylenediamine and phenylphosphonic dichloride.It was characterized by Fourier Transform Infrared(FTIR),1H NMR and31P NMR,Elemental Analysis(EA)and Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES).Combined with ammonium polyphosphate (APP),a new intumescentflame retardant(IFR)was obtained.Theflammability behaviors of poly-propylene(PP)/IFR system were investigated by limiting oxygen index(LOI),vertical burning test(UL-94) and cone calorimetry.With25wt%of IFR(APP:PTPA¼2:1),the PP/IFR system could achieve a LOI value of34.0%and UL-94V-0rating,and the heat release rate(HRR),peak heat release rate(PHRR),total heat release(THR)and smoke production rate(SPR)were considerably reduced,especially HRR and SPR were decreased by85%and79%,respectively.The results indicate that there is an excellent synergism between APP and PTPA,which endows PP with both goodflame retardancy and good smoke suppression. Furthermore,the thermal degradation mechanism of IFR and theflame-retardant mechanism of PP/IFR system were investigated by thermogravimetric analysis(TGA),FT-IR,TG-FTIR and scanning electron microscope(SEM).The study on theflame-retardant mechanism of IFR indicated that a structure con-taining e CN was formed due to the reaction between APP and PTPA.Ó2013Elsevier Ltd.All rights reserved.1.IntroductionPolypropylene(PP)is widely used in apparel,floor covering, upholstery,medical,geotextile and automobile due to its low cost, light weight,good mechanical properties and excellent resistant to many chemicals.However,PP is a highly combustible material, which restricts its application in manyfields.Therefore,it is essential to endow PP with goodflame retardancy[1].In the research for environmentally friendlyflame retardants for PP,intumescentflame retardants(IFR)have attracted considerable attention[2e19].However,there are some shortcomings with traditional IFR,such as not high enoughflame-retardant efficiency, low thermal stability and low water resistance[20].In the efforts to improve theflame-retardant efficiency of IFR system,the synthesis and application of the phosphorus-nitrogen containing intumes-centflame retardants become a trend for the synergistic effect between phosphorus and nitrogen.Among them,triazine and its derivatives attracted the interest of scientists due to its high ther-mal stability and structure of tertiary nitrogen[4,17,21e24].Cya-nuric chloride is an excellent raw material to design and synthesize differentflame retardants because of its high selectivity[4,21,24]. Some phosphorus-nitrogen containing IFR systems with low mo-lecular weights have been synthesized from cyanuric chloride [25,26].They might endow polymer withflame retardancy alone or together with APP.Literature indicated that some reactive phos-phonic amides were used as the curing agents for epoxy resins and showed good thermal stability and charring ability[27,28].It was shown that phosphorus oxynitride structure was helpful for the formation of stable char layer[29].However,the research of the compound containing both triazine ring and phosphorus oxynitride structure is not reported. To further improve theflame-retardant efficiency of IFR,it deserves more efforts to design and prepare new IFRs.In the present article, we mainly aim to design and synthesize a novel polymericflame retardant named PTPA containing triazine ring,benzene ring and phosphorus oxynitride structure,which might exhibit better ther-mal stability and charring bining this polymericflame*Corresponding authors.Tel.:þ862885410755;fax:þ862885410259.E-mail addresses:aaa_16@(J.-Q.Huang),yzwang@ (Y.-Z.Wang).Contents lists available at ScienceDirectPolymer Degradation and Stability journal homepage:w ww.elsevi /locate/polydegstab0141-3910/$e see front matterÓ2013Elsevier Ltd.All rights reserved./10.1016/j.polymdegradstab.2013.07.010Polymer Degradation and Stability98(2013)2011e2020retardant with APP,a new high-efficient IFR for PP is obtained.The structure of PTPA is characterized by FT-IR,1H NMR,31P NMR,EA and ICP-AES.Theflame-retardant properties,thermal degradation mechanism andflame-retardant mechanism are investigated in detail.2.Experimental2.1.MaterialsCyanuric chloride was purchased from the Yingkou Sanzheng Organic Chemical Industry Co.,Ltd.,China.Ethylenediamine,so-dium hydroxide and chloroform were supplied by Changzheng Chemical Reagent Co.,China.Phenylphosphonic dichloride was provided by Weili Flame Retardant Chemicals Industry Co.,Ltd (Chengdu,China).Ammonium polyphosphate(APP)was supplied by Sichuan Shifang Chuanhong Phosphorus Chemical Co.,Ltd. China.Polypropylene(PP)resin(T30S)used in this work was pur-chased from Lanzhou Petroleum Chemical Co.,China.2.2.Preparation of poly[N4-bis(ethylenediamino)-phenyl phosphonic-N2,N6-bis(ethylenediamino)-1,3,5-triazine-N-phenyl phosphonate](PTPA)The synthesis route of PTPA is shown in Scheme1.N,N0-bis(2-aminoethyl)-phenyl phosphorodiamidate(BPPA) was synthesized according to literature[28].Ethylenediamine (126g)and chloroform(400ml)were added in a1000ml four-necked reaction vessel provided with a stirrer,a thermometer, and a constant pressure dropping funnel,followed by simulta-neously dropwise adding a solution of phenylphosphonic dichlor-ide(117g)dissolved in chloroform while keeping the temperature atÀ5 C to5 C with stirring.After the completion of the dropwise addition,the mixture was further agitated at the same temperature for6h,thenfiltered to remove ethylenediamine hydrochloride and got the solution of BPPA which would be used immediately in the following reaction.Cyanuric chloride(92g)and chloroform(500ml)were added into a1000ml four-necked reaction vessel with a stirrer,a ther-mometer and two constant pressure dropping funnels,followed by simultaneous dropwise addition of half of the BPPA solution and an aqueous solution of sodium hydroxide(20g),while keeping the temperature atÀ5 C to5 C with stirring.After completion of the dropwise addition,the mixture was further agitated at the same temperature for6h,followed by heating it up to room temperature, dropwise adding the other half of the BPPA solution and sodium hydroxide(20g)dissolved in water.After the completion of the reactant addition,the mixture was agitated under reflux for8h at 61 C.The intermediate wasfiltered and dispersed in the aqueous solution of BPPA and keeping pH at6to8by adding sodium hy-droxide for8h at100 C.The mixture was then cooled down to room temperature,followed byfiltration,washing the product with water,drying it to a constant weight in vacuum at80 C,and150g of yellowish solid was obtained.Its solubility in100g water at70 C was0.09Æ0.01g.2.3.Preparation offlame-retardant PP samplesAll samples with different compositions of PP/flame retardants (FR)were prepared in a parallel co-rotating twin-screw extruder with a length/diameter ratio of40and a screw diameter of25mm. The PP/FR samples were extruded at190 C at the screw speed of 80rpm.Then,the specimens for LOI and UL-94tests were compression-molded by means of a vulcanizing press at200 C under10MPa for3min to obtain3.2mm thick plaques.Finally,the compressed sheets were cut into standard testing bars.2.4.CharacterizationThe structures of BPPA and PTPA were characterized by NMR(1H NMR,31P NMR,Bruker FT-80A NMR),FT-IR(Nicolet FTIR170SX infrared spectrophotometer,KBr powder)and Elemental Analysis (EA).The phosphorus content of PTPA was analyzed using an IRIS ADV Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES).Thermogravimetry(TG)was carried out on an SDT Q600 thermal analyzer in a60ml/minflowing N2at a heating rate of 10 C/min.Theflame retardancy and burning behaviors were determined by LOI test(JF-3oxygen index meter,Jiangning,China; using sheet dimension of130mmÂ6.5mmÂ3mm according to ASTM D2863-97),UL-94measurement(CZF-2-type,Jiangning, China;using sheet dimension of130mmÂ13mmÂ3mm ac-cording to ASTM D3801)and cone calorimetry test(FTT cone calorimeter;square specimens,100mmÂ100mmÂ3mm were irradiated at a heatflux of50kW/m2according to ISO5660stan-dard procedures).3.Results and discussion3.1.Characterization of BPPA and PTPAFig.1shows the FT-IR spectra of BPPA(a)and PTPA(b).For the spectrum of BPPA,the absorptions of N e H(primary)at3440andScheme1.The synthesis route of PTPA.Z.-Z.Xu et al./Polymer Degradation and Stability98(2013)2011e202020121652cm À1,N e H (secondary)at 3264cm À1,phenyl (1600,1557and 1517cm À1),P-phenyl bond (1431cm À1)and P e N e C (1384，1044and 740cm À1)are in keeping with literature [28].For PTPA,the absorptions at 3416and 3256cm À1(N e H in e NH 2and e NH e group),1227cm À1(P ]O),2928and 2869cm À1(C e H bond in e CH 2e CH 2e )and 1421cm À1（P e Ph bond)are observed.The ab-sorptions of 1580cm À1(C ]N bond of triazine ring),1620and 1506cm À1(C ]C bond of benzene ring)and the appearance of 1336cm À1(C triazine e NH e )indicate the successful reaction between BPPA and cyanuric chloride.Fig.2shows the 1H (a)and 31P (b)NMR spectra of BPPA.The signals at 7.66e 7.39ppm correspond to five hydrogen protons of benzene ring and signals at 2.78e 2.67ppm corresponds to eight hydrogen protons of two e CH 2e CH 2e moieties [28].The peak area ratio 1:1.63is in keeping with the calculated value 1:1.60.No resonance of e NH e or e NH 2proton appears because of the addi-tion of D 2O.The signal of phosphate is observed at 17.49ppm.On the basis of the above results,it can be concluded that BPPA has been synthesized successfully.Fig.3shows the 1H NMR spectrum (600MHz,CF 3COOD)of PTPA.The multiplet between 7.52and 7.92ppm corresponds to the hydrogen protons of benzene ring.The signals at 3.87e 4.08ppm correspond to the hydrogen protons of e CH 2e in middle repeating units [28].The peaks at 3.60and 3.64ppm correspond to the hydrogen protons of e CH 2e in end group.With the peak area ratioof hydrogen protons of e CH 2e in end group and hydrogen protons of e CH 2e in middle repeating units,the n value we calculated is 22and the number-average molecular weight of PTPA is 1.26Â104g/mol.To further con firm the structure of the product,ICP-AES was used to measure the actual phosphorus content,and the contents of nitrogen,carbon,and hydrogen were measured by EA.Table 1shows the EA and ICP-AES data of PTPA.The actual content of carbon,hydrogen,nitrogen,and phosphorus is mainly in accord with their calculated content respectively.All the results above indicate that the object product has been synthesized successfully.3.2.Flammability of PP/IFR systemsPTPA was mixed with APP and PP to get a new intumescent flame-retardant PP.The results of the limiting oxygen index (LOI)and vertical burning test (UL-94)were given in Table 2.When the two components (APP and PTPA)act separately,the flame retard-ancy is not so good.However,when the additive amount of IFR which combines APP with PTPA (2:1by weight)is 25wt%,the LOI value of PP/APP/PTPA is increased to as high as 34.0%and the UL-94rating is V-0,showing an excellent synergy between APP and PTPA.When the additive amount of IFR (APP/PTPA ¼3:1)is decreased to 18wt%,the LOI value of PP/APP/PTPA is decreased to 32.1%and the UL-94rating is V-0,showing a high flame-retardant ef ficiency.The cone calorimeter is one of the most effective bench-scale methods to study the flammability of materials.Some of the flame-retardant PP samples were studied by cone calorimeter.All samples were repeated three times and the deviation of their data was within Æ10%.Time to ignition (TTI),Heat release rate (HRR),especially the peak of HRR (PHRR),total heat release (THR)and smoke production rate (SPR)are important parameters for evalu-ating flame retardancy and flammability of polymeric materials.Figs.4and 5and Table 3show the cone calorimetry tests data of pure PP and PP/IFR systems at an external heat flux of 50kW/m 2.Fig.4and Table 3show that TTI is shortened after addition of flame retardant.That is because APP,PTPA and IFR degrades at lower temperature than PP.HRR value of pure PP increases very fast after ignition,and a sharp HRR peak (809kW/m 2)appears at pared with pure PP,the PHRR is decreased by about a half with adding APP or PTPA separately.However,combining PTPA with APP together to form IFR,the PHRR of PP/IFR is decreased to 121kW/m 2at 330s,indicating that the flammability of the PP/IFR systems is obviously restrained and the rate of fire spreading isdecreasedFig.2.1H (a)and31P (b)NMR spectra ofBPPA.Fig.1.FT-IR spectra of BPPA (a)and PTPA (b).Z.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202013largely.It is noticeable that for PP/IFR,HRR increases very fast after ignition (up to 92kW/m 2),then dramatically decreased and grad-ually raised up again till the maximum.This phenomenon is closely related with the fast formation of compact char layer.When poly-propylene burns fast releasing a lot of heat,a compact intumescent char layer forms in a short time with the degradation of APP and PTPA.The char layer covers the sample and prevents the flammable gaseous products of PP to permeate.The flammable volatiles can almost only escape from the edge of char layer and burns slowly as shown in Fig.10f.Thus the release rate decreases.But due to the continuous thermal radiation of weak fire,PP under char layer degrades and produces lots of flammable volatiles.The gases escape from the edge of char layer and burn fast.It comes to be the main burning stage.Consequently the heat release rate increases and gradually rises up to its second maximum.On the other hand,comparing the time to PHRR (T PHRR )and PHRR of pure PP with flame-retardant PP,adding APP or PTPA separately cannot form compact char layer in time to inhibit the fire.As a result,the heat release rate rises up directly to the maximum.We can conclude that the speed of char layer formation and the compactness of char layer have a strong impact on the reduction of the heat release rate and the flame retardancy.From Fig.4and Table 3,it can be seen that the value of THR of PP/IFR is reduced considerably,and the total burning time is also obviously prolonged compared with that of pure PP.Fig.5(a)and Table 3indicate that the smoke emission is retarded and delayed during the whole emission process with PP/APP/PTPA system.The curves of mass loss rate (MLR)against combustion time of pure PP and PP/IFR are shown in Fig.5(b).It can be seen that the IFR composed of APP and PTPA could effectively prohibit the decomposition of PP and form an amount of char on the surface.The barrier effect of char layer reduces the smokeproduction rate and the heat release rate.The mass loss behavior of the composite is in accord with the heat release behavior and the smoke emission behavior.From the data above,it can be demonstrated that the heat transfer and flammable volatiles transfer between the flame and the polymer are decreased greatly.Thus the flame retardancy of the intumescent flame-retardant PP system is improved considerably with IFR composed of PTPA and APP.3.3.Flame-retardant mechanism3.3.1.Thermal degradation of flame retardantsFig.6shows TGA (a)and DTG (b)curves of PTPA,APP,IFR (APP:PTPA ¼2:1)and IFR calculation in N 2,respectively.The data of thermal degradation are shown in Table 4.The TGA and DTG curves show that the initial decomposition temperature of PTPA is 269 C based on 5wt%mass loss.It can be seen that PTPA undergoes two stages of thermal degradation process at 269e 359 C and 359e 700 C,respectively.The weight loss is 15wt%for the first stage and 47wt%for the second stage leaving 33.16wt %of char residue at 700 C.It can be seen that there are three degradation steps for IFR composed of APP and PTPA at 285e 310 C,310e 450 C and 450eFig.3.1H NMR spectrum (600MHz,CF 3COOD)of PTPA.Table 1EA and ICP-AES data of PTPA.Content (wt %)CH N P Calculated value 49.14 6.1628.6510.56Actual value42.425.9329.858.04Table 2The formulation and the flame properties of PP/IFR system.Sample PP/%APP/%PTPA/%LOI/%UL-9411000017.6No rating 27525020.6No rating 37502520.6No rating 4758.316.726.1No rating 57512.512.532.5V-067516.78.334.0V-077518.75 6.2532.5V-087520 5.033.0V-198013.33 6.6733.0V-010829.09.028.5No rating 118212 6.032.0No rating 128213.5 4.532.1V-0138412428.5No rating 148412.83.228.5No ratingZ.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202014700 C,respectively.The mass loss rate of IFR is much higher than the calculated value in the temperature range from 337 C to 433 C,but it is lower than the calculated value in the temperature range from 433 C to 700 C.The second maximum weight loss rate (À3.8%/min)is much quicker than the calculated value (À1.8%/min).However,the first and third maximum weight loss rates are smaller than the calculated values.It can be inferred that there is a reaction between the decomposition products of PTPA and APP which not only promotes their degradation and volatilization of their products,but also promotes the formation of more thermal stable char residue at lower temperature.Therefore,we can see that the mass loss rate of IFR is small at temperature above 450 C and the amount of IFR residue is more than the calculated value.Fig.7(a)shows the FT-IR spectra of volatile products of PTPA at different temperatures in N 2.During the first stage (from 269 C to 359 C),the gas released is ammonia whose absorptions are observed at 964and 930cm À1and the volume of ammonia is increased with the temperature increased.During the second stage (from 359 C to 604 C),the gas released is composed of ammonia,ole fin compounds (3080,1648and 1470cm À1),aromatic compounds (3058cm À1),nitrile com-pounds (2251and 2285cm À1),vaporing phosphonates (1244cm À1)and volatiles with P e NH e bond (3333,1391,1304,1220and 713cm À1).The gaseous production gradually increases with the increase of temperature,maximizing and then going down.During the third stage of degradation (from 604 C to 700 C),the unstable partial of the char residue is further decom-posed.Only non flammable gas composed of nitrile and vaporing compounds with P ]O bond and P e NH e bond is observed.Fig.7(b)shows the FT-IR spectra of condensed products of PTPA at different temperatures in N 2.At 25 C,the absorptions of original PTPA are observed at 3416and 3256cm À1(N e H in e NH 2and e NH e group),1269cm À1(P ]O),2928and 2869cm À1(C e H bond in e CH 2e CH 2e ),1580cm À1(C ]N bond of triazine ring),1620and 1506cm À1(C ]C of benzene ring)and 1336cm À1(C triazine e NH e bond).With the increase of temperature,the absorption at 3416cm À1becomes gradually weaker,meaning the chain scission of e NH 2,P e NH e and C e NH e .The same is observed with the ab-sorptions at 2928and 2869cm À1.Also,a new absorption at 2168cm À1(e CN)appears obviously in the spectrum of residue at 435 C (the second weight loss step)and disappears in the spec-trum of residue at 700 C,which means the new nitrile compound is generated during decomposition and then decomposed at higher temperature.The absorptions at 1620cm À1,1421cm À1,897cm À1become wider and stronger at 435 C,suggesting that the new ar-omatic compound and the new compound with P e N bond form during the combustion.Fig.8(a)shows the FT-IR spectra of volatile products of IFR at different temperatures in N 2obtained from TG-FTIR technology.IFR is the mixture of APP and PTPA with the weight ratio of 2:1.DuringFig.4.HRR (a)and THR (b)plots of pure PP and PP/IFR systems obtained from conecalorimetry.Fig.5.SPR (a)and Mass Loss (b)plots of pure PP and PP/IFR systems obtained from cone calorimetry.Z.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202015the first stage,ammonia from decomposition of PTPA and APP is evolved.During the second stage of weight loss from 337 C to 543 C,gaseous product of thermal decomposition of IFR is the mixture of carbon dioxide (2318and 2357cm À1),ammonia and a little amount of aromatic hydrocarbons (3060cm À1).At the tem-perature above 543 C,char residue further degrades and vaporing compound with P ]O bond and P e NH e bond evolves.To further investigate the thermal degradation behavior,the char residues of PTPA and IFR (APP:PTPA ¼2:1)burned at different temperatures are investigated by FT-IR.Fig.8(b)shows the FT-IR spectra of condensed products of IFR at different temperature in N 2.The FTIR spectrum of IFR at 25 C in Fig.8(b)shows that typical absorptions of APP are evident at 1252cm À1(P ]O)and 883cm À1(P e O e P),whereas those of PTPA are observed at 1632cm À1(C ]C of benzene ring)and 3000e 2800cm À1(e CH 2e ).With the increase of temperature,the ab-sorptions of e CH 2e groups at 3000e 2800cm À1,e NH 2at 3202cm À1and aromatic hydrocarbons (3050cm À1)are signi fi-cantly decreased,whereas there is a new absorption appearing at 2137cm À1(e CN).The exact association of these absorptions to speci fic bonds (P e N or P e O)is not easy by FTIR because of broad and overlapping peaks [30].Moreover,the absorptions at 3430and1655cm À1and the broad tail at 2800e 2400cm À1could indicate the presence of polyphosphoric acid [29].Based on the results above,possible decomposition mechanism of PTPA and IFR is shown in Scheme 2.The phosphorus oxynitride structure in PTPA is stable to high temperature.On one hand,the polymerization or aromatization of carbon e carbon double bonds formed from the degradation of PTPA might promote to produce stable char residue [29].On the other hand,the P e O e P bonds in the polyphosphate chain are broken down and probably form stable cross-linking structures.An alternative route for polyphosphate chain scission is due to the reaction between the decomposition products of PTPA and APP which involves the formation of non flammable vaporing compounds and some stable compounds containing P e N bond which is shown in Scheme 2.3.3.2.Flame-retardant mechanism of PP/IFR systemFig.9and Table 5present the TG (a)and DTG (b)curves and data of the flame-retardant PP with 25%PTPA,25%APP and 25%IFR,respectively.It can be seen that pure PP begins to degrade at 380.6 C and the maximum decomposition temperature is at 445.8 C with almost no char residue left at 700 C.However,the initial decomposition temperature of PP/IFR is 355.8 C which is related with the decomposition and cross-linking reactions,form-ing the char which can protect the matrix from heat and decomposition.When the temperature is increased,PP with IFR is further decomposed,further cross-linked and char is formed.The char residue of flame-retardant PP is much more than that of pure PP,PP/PTPA and PP/APP at 700 C.The maximum weight loss rate is smaller and appears at higher temperature than that of pure PP,which means that the matrix is protected during burning.Table 3Data obtained from cone calorimetry of pure PP and PP/IFR system.SampleTTI (s)PHRR (kW/m 2)T PHRR (s)THR (MJ/m 2)SPR (m 2/s)Residue (%)PP20809130960.09958.975%PP-25%APP 1139765870.064621.575%PP-25%PTPA 1252980880.10709.875%PP-25%IFR11121330810.021317.1Fig.6.TGA (a)and DTG (b)curves of PTPA,APP,IFR and IFR calculation in N 2.Table 4Thermogravimetric data of flame retardants in N 2.Samples T onset a ( C)T max1b ( C)R max1c (%min À1)T max2( C)R max2(%min À1)T max3( C)R max3(%min À1)Char residues (%)at 700 C PTPA 269288À1.4435À4.4No 33.16APP 297315À1.9353À1.3572À2.550.60IFR285296À1.5404À3.8513À0.947.13IFR-calculation286315À1.7435À1.8573À1.944.86a T onset :the initial temperature at 5wt%mass loss.b T max :the temperature at the maximum mass loss rate.cR max :the maximum mass loss rate value.Z.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202016Fig.10shows the digital photographs of residual char after cone calorimeter test for different systems.The thermal insulation and physical barrier to mass transfer through the intumescent char was previously shown to be of paramount importance in the fire retardance mechanism [31].The compact and firm carbon layer obviously reduces heat release rate,weight loss rate and smoke production release rate.We can see that the residual char of sample 75%PP-25%APP (Fig.10b)is continuous with lots of holes.The char surface of sample 75%PP-25%PTPA (Fig.10c)is fragmented by cracks orholes.Fig.7.FT-IR spectra of volatile products (a)and condensed products (b)of PTPA inN2.Fig.8.FT-IR spectra of volatile products (a)and condensed products (b)of IFR in N 2.Scheme 2.Possible pyrolysis mechanism of PTPA and IFR.Z.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202017Both of them cannot segregate oxygen and combustible gases effectively.The carbon layer of 75%PP-25%IFR (APP:PTPA ¼2:1)(Fig.10d)is continuous,compact and thick.The thick carbon layer means the intumescent ratio is very high,suggesting that the decomposed gases can hardly permeate through the char layer (Fig.10e).The combustible gases can only escape from the loweredge of char layer and keep burning very slowly just like showing in Fig.10f.In order to clarify the morphology of the char formed after combustion,the surfaces of char residues after cone calorimetry test for different systems were investigated by SEM,as shown in Fig.11.The char residue of PP/PTPA (Fig.11a-1and a-2)is coralloid network structure with lots of cavities.That is because PTPA is rich in carbon (charring agent)and nitrogen (blowing agent),but poor in phosphorus (acid source).Thus,the char layer becomes frag-mented when plenty of decomposed gases easily permeate through the char layer.The char residue of PP/APP (Fig.11b-1and b-2)is continuous with some holes and spherical shape tubers in the surface because the cross-linked polyphosphoric acid formed on heating could provide a stable softened glassy coating on the sur-face of the polymer,but it is not firm enough.It can be permeated through by decomposed gases,thus holes are formed.TheFig.9.TG (a)and DTG (b)curves of the flame-retardant PP.Table 5Thermal degradation data for PP with 25%PTPA,25%APP and 25%IFR in N 2.SampleT onset a ( C)T max b ( C)R max c (%min À1)The char residues (%)at 700 C Pure PP 380.6445.8À22.380.1875%PP-25%PTPA 341.9450.3À14.758.7175%PP-25%APP 366.0461.6À16.7414.9775%PP-25%IFR 355.8464.5À14.4912.24a T onset :the initial temperature at 5wt%mass loss.b T max :the temperature at the maximum mass loss rate.cR max :the maximum mass loss ratevalue.Fig.10.The digital photographs of residual char after cone calorimeter test for different systems,a:PP,b:75%PP-25%APP,c:75%PP-25%PTPA,d:75%PP-25%IFR (APP:PTPA ¼2:1),e:75%PP-25%IFR,f:the burning sample of 75%PP-25%IFR.Z.-Z.Xu et al./Polymer Degradation and Stability 98(2013)2011e 20202018。