STUDY ON THE EFFECTS OF TLCP ON PYROLYSIS OF PET AND ITS RETARDANT MECHANISM

Chinese Jourmal of Polymer Science Vol 26, No. I, (2008), 1116Chinese Journal ofPolymer Science02008 World ScientificNoteSTUDY ON THE EFFECTS OF TLCP ON PYROLYSIS OF PET AND ITSRETARDANT MECHANISMYi Deng2 , Yu-zhong Wang', Zhi-jing Zong, Xiu-hua Liu2 and Xiao-hua Du5China Academy of Engineering and Physics, Mianyang 621900, China"Center For Degradable and Flame reardant polymeric materials, Sichuan Universiy, Chengdu 610064, ChinaAbstract The fame retardancy of aromatic thermotropic liquid crystal phosphorus-containing copolyester, TLCP, on PETwas investigated. The results show that the presence of TLCP promotes char formation of the substrate and enhances thermalstability of char, hence delay its decomposition. SEM pictures show that the char forned from PETTLCP is more compact,therefore is more resitant to fire and heat than that from pure PET. Evolved gas analysis by L.R measurements indicates thatTLCP would decompose to produce phosphorus containing small molecular compounds in pyrolysis process. Phosphorus-containing volatile compounds are detected in gas pyrolytical products. It is suggested that TLCP could play an impotantrole of flame retardancy in vapor phase. TLCP could inhibit the generation of combustible volatile in the pyrolytical processof PET, and therefore prevent the fire propagation during the combustion since combustible volatile is necessary for thegeneration of fire.Keywords: Thermotropic liquid crystal copolymer, Flame retardant; PET.INTRODUCTIONLiquid crystal polymer (TLCP), in particular thermotropic liquid crystal polymers (TLCP)I-3), have atractedextensive attentions because of their superior integrated characteristics. Phophorus element is excellent in itsperformance on flame retardancy. Therefore, with the existence of phosphorus in the molecules of TLCP,additional perfect fire retardant performance is obtained by phosphorus containing TLCP on the basis of keepingits primary special properties.A kind of phosphorus containing thermotropic liquid crystal copolyesters (TLCPs) derived from 2-(6- oxide-6H-dibenz (<1, 2> oxa phosphorin 6-y)-1,4-dihydroxyphenylene, 4-hydroxybenzoic acid, terephthalic acidand isophthalic acid, was synthesized by melting polycondensation in our laboratoryl4.5, its chemical structure isas follows:0=p- 0tho--0HScheme 1 Molecular structure of TLCP中国煤化工* Corresponding author: Yi Deng (邓义)。E-mail: mayid0719@ 163.con.JYHCNMHGReceived June 4, 2007; Revised July 11, 2007; Accepted July 19, 2007112Y. Deng et al.In our previous study' , it was found that the composites of PET and the TLCP had good flame-retardantperformance, an oxygen index (LOI) of 31.4 and a V-0 rate of UL94 could be achieved when the content ofTLCP is 3% and 8% respectively. Further investigation indicated that the addition of TLCP could also restrainthe melt dripping in the combustion of PET. Moreover, the additive has lttle or slight effects on mechanicalproperties of the polymer.To make sure the mechanism of flame retardancy of the TLCP, we have analyzed the pyrolytical productsof PET, TLCP and PETTLCP using PY-GC-MSI8) method. In this paper, more tests were performed and furtherstudies about the mechanism were carried out.EXPERIMENTALMaterialsSamples of PET were obtained from Urumqi petrochemical complex. TLCP was synthesized from the meltingpolycondensations of 2-(6 oxide-6H-dibenz oxa phosphorin-6-y)-1,4-dihydroxyphenylene,4-hydroxybenzoic acid, terephthalic acid and isophthalic acid45. Then the TLCP and PET were mixed by themethod of melting blending. PET and TLCP are both dried at 100°C for 24 h in vacuum condition. TLCP wasadded to PET using SLJ-2 twin-screw extruder with a TLCP content of 20 wt% to get a composite. Temperaturesof zone 1 to4 are 255, 260, 270 and 275°C in sequence and that of die zone is 2559Cl81.TG-FTIR StudyA 2050 TG analyzer of PA company was connected to a NICOLET 710 FTIR spectrometer to perform the TG-FTIR measurements. The sample (about 7 mg) is heated from the room temperature to 800°C with a heating rateof 20 K/min, then kept 30 min at that temperature. The experiments were conducted under an oxygen flow of10 mL/min. The operation conditions of FTIR were as follows: frequency range: 4000- 400 cm~'; resolution: 8 cm~'.SEM TestPET and the mixture of PET/TLCP with mass ratio 80:20 were heated on elecric cooker of 1 kW in air for10 min to get the samples for SEM tests. The surface of the sample was coated with carbon. Tests wereperformed with a KYKY-1010B scanning electric microscope of Beijing Scientific Instrument Factory.Analysis of Phosphorus in Pyrolysed ProductsThe phosphorus distribution was investigated by comparing the relative phosphorus contents in gas products,liquid products and residues after the pyrolysis of flame retardant and flame-retardant PET at 550°C for 25 minrespectively in an in-house designed batch reactor (Fig. 1). The contents of phosphorus in the residues and liquidproducts that were collected in the U-tube immersed in iced water, were analyzed via NHMoO4-SnCl2spectrophotometry. Their contents in gas products were calculated from the difference between the originalphosphorus contents and those of liquid and residues.Air-=1口TFig. 1 Pyrolysis apparatus1) Temperature controlling equipment; 2) Sample chamber3) Heating coil; 4) Cooling tank; 5) Absorbing tube中国煤化工MYHCNMHGStudy on the Effects of TLCP on Pyrolysis of PET and Its Retardant Mechanism113RESULTS AND DISCUSSIONTG-FTIR AnalysisThermnogravimetric analysisFigures 2 and 3 were TG curves of PET and PETTLCP respectively. There were two thermal weight loss stagesfor both systems. At the first stage, the initial decomposition temperature (T) and the maximum decompositionrate temperature (Tmx) of PET were 421.10°C and 453.449C, respectively, and those of FR-PET were 422.16°Cnd 456.83°C, slight differences existed between the two systems. That signified TLCP did not acceleratedegradation of PET in lower temperature when heated. But the weight loss decreased considerably, from81.94% to 70.23% after the addition of TLCP, suggesting that the existence of TLCP could promote theformation of char in the combustion and as a result, the residues of thermal oxidative degradation increasedsignificantly in the flame -retarded system. At the second stage, the T; and Tmax of PET/TLCP were 593.26°Cand 624.13°C, respectively, both much higher than those of PET, which were 554.09°C and 581.18°C,respectively. Since the weight loss in this stage was related to oxidation of the unsaturated char residue (formedat the first stage) under air, it could be indicated that the existence of TLCP retarded the decomposition of thechar formed in the bumning process and enhances the capability of char to resist fire.53.44C420.10*C 32.47%/min30f81.949%(6.448 mg)581.18C10f3.648%/min470.4rCA 564.09C96929%596.321607627m8100 300400500 600 700Temperature (C)Fig.2 Thermogravimetric analysis curve for PET under air42216C A1.29%/C100F70.23%I (5.000 mg)60-624.13"C4Of”474.13C" 593.26C20叶645.01C0100 200 300 400 500600 70Temperature rcFig. 3 Thermogravimetric analysis curve for PET/TLCP (80:20) under air中国煤化工YHCNMHGY. Deng et al.FTIR analysis of evolved gasesResults of TG analysis displayed two weight loss stages in the thermal degradation of PET, and in situ FTIRmeasurements showed the volatile generated in the two stages were similar (Fig. 4), including CO2 (bands at 2357and 675 cm^"), CO (bands at 2179 and 2112 cmi*), RCHO (1757 and 2740 cm |) and some compoundsconsisting of aromatic ring (1514 cm ).Y400020001000Wavenumber (cm-)Fig. 4 Vapor phase FTIR spectrum for the volaile compounds during the thermnal degradation of PETIn the infrared spectra of PET/TLCP, the preceding compounds were all identified during the wholeprocess of pytolysis, and meanwhile some obvious variances were observed at different stages of thermaldegradation. As can be seen from Fig. 5, spectrum of the first stage was similar with that of PET, the mainproducts still involved CO2, CO, RCHO and so on. Then at the second stage, it can be seen that the peaks of CO2,co, RCHO became weaker, and that of aromatic structure stronger, indicating at higher temperatures thepresence of TLCP inhibited the production of the small molecular volatile products. Moreover, some new peaksappeared in the spectrum of this stage, including bands at 2313 and 1264 cm , assigned respectively to thestretching of P=O and P- H, suggesting some phosphorus containing volatile compounds were produced,which were not found at the first stage of the degradation. It signified that the phosphorus element of TLCPmainly remained in the condensed phase at lower temperatures, then with the increment of temperature,phosphorus element began to form small products and volatized to the vapor phase.ξ曾|3000”10003000”Wavenumber (cm)， Wavenumher (cm-1)中国煤化工Fig. 5 Vapor phase FTIR spectrum of PETTLCP for thea) First stage; b) Second stage.MYHCNM H G5SStudy on the Effects of TLCP on Pyrolysis of PET and Its Retardant Mechanism115SEM Analysis of the CharThe SEM images of the char from PET and PET/TLCP are shown in Fig. 6. It can be seen that the structure ofchar from PET is quite incompact. First, there were large numbers of holes dstributed over the surface of char.Then, the sizes of the holes were all relatively large, with diameters in the range of 200- -800 um. Last, as can beseen from picture (a), some holes were observed penetrating deeply into underlying layer of the char surface. Asgenerally known, the resistance of generated char to thermal oxidation depends on its structure, consequently,the char from PET was hard to resist fire atacks at high temperatures. Compared with that from PET, the charfrom PET/TLCP displayed relatively more compact structure. From picture (b), it can be found that the arearatio of holes on the surface of char has remarkably reduced, and the sizes of holes are also much smaller withdiameter not larger than 127 um. Furthermore, the depth of holes was significantly shallower than that fromPET. Therefore, the char from PET/TLCP is more efficient to barrier heat and air from surface of unburmedpolymer and hence inhibit flame spread.1000 um250 pumFig. 6 SEM images of char: (a) from PET and (b) from PET/TLCPDistribution of Phosphorus between Different PhasesTable 1 listed distributions of phosphorus in pyrolytic products of TLCP and PET/TLCP. As can be seen,phosphorus was detected mainly existing in liquid products and residues for both TLCP and PET/TLCP, andonly a small quantity of phosphorus volatized to form gas products. On the other band, the PET samplecontaining the flame retardant showed an increased phosphorus content in residues, whilst a decreased one inliquid products and gas products compared with the neat flame retardant, indicating an interaction existedbetween PET and TLCP especially in condensed phases.Table 1. Phosphorus distribution for pyrolytic products of TLCP and PETTLCPContents (%)SampleCharCondensateGasLCP32.143.524.4PET + 20%LCP58.427.14.4DiscussionFrom analysis given above, it can be seen that the flame retardant TLCP works in several ways. First, TLCP didnot accelerate degradation of PET in lower temperatures when heated as general phosphorus containing flameretardants behaved, but the tendency of PET to form char when burmed increased after adding TLCP, which wasproved by TG tests. It has been demonstrated that the inherent flammability of many polymers reduces as theirchar-forming tendency increases'-i. The char not only reduces the rates at which pyrolytic formation ofvolatile fuels and subsequent combustion occur, but also offer a barrier to heat and fame penetration tounderlying materials of surfaces, consequently reducing flameeasy to oxidize in air when they are heated above 400°C or中国煤化工polymeric char tothermal oxidation strongly affects polymer flammability. The rTH.CNMH GE char from fiameretarded PET was more efficient to resist fire attack since which naulvlc ouupaul bu ultuic uan that from PET.116Y. Deng etal .The results were in accordance with those of TG-FTIR measurements, which indicated that the presence ofTLCP delayed the thermal oxidation of char. Last, analysis of phosphorus in pyrolytic products indicatedalthough phosphorus mainly distributed in liquid products and residues, there were still a small quantity ofphosphorus detected in the gas pyrolysates at 550°C. The results of in situ FTIR measurements also showed inthe thermal degradation of PET/TLCP, phosphorus of TLCP volatized in the form of small molecular compoundsconsisting of P-H, P-0 and P=0 at higher temperatures. Phosphorus containing compounds in the vaporphase are reporteds, 14 to scavenge carrier species, such as H. and HO. radicals, in burming process which arerequired during the oxidation of volatile pyrolysis products in the flame. Thus it can be seen TLCP stillfunctioned as radical captor to retard flame in the vapor phase. Moreover, according to PY-GC-MSmeasurements in our previous study", an interaction between PET and TLCP was found in the pyrolysis ofPET/TLCP blends which led to the fact that the concentrations of small volatile molecular pyrolytic products ofblends were distinctly lower than those of PET and TLCP. It has been reported that volatile pyrolytical productsof PET mainly involve carbon monoxide, carbon dioxide, acetylene, acetaldehye, etc. which are flammableexcept carbon dioxide. Therefore, the reduction of flammable volatile products is in favor of retardancy of flameextension. Furthermore, PY-GC-MS tests still manifested that TLCP strongly inhibited generation of some smallmolecular combustible pyrolysis products, such as benzene, toluene, styrene and so on, which is necessary forfire generatedls, so the presence of TLCP could restrain propagation of flame in this way.CONCLUSIONSThe effects of thermotropic liquid crystal polymer, TLCP, on the flame retardancy of PET were invetigatedusing TG-FTIR and SEM methods. The results showed the fire retardant mainly functioned in condensed phase.The char residue increased from 8.5% of PET to 18.0% of PET/TLCP, indicating that the introduction of TLCPpromoted the formation of char in the buming process. The char became more compact and eficient to preventheat and oxygen transfer after adding TLCP, and decomposition of char was also retarded by addition of TLCP.It is suggested that phosphorus mainly exists in liquid products and residues in pyrolysis of both TLCP andPET/TLCP, which means actions in condensed phase are the main way of TLCP to resist fire. Furthermore,phosphorus containing small molecular volatile products formed due to the presence of TLCP played the role offire retardant in vapor phase in combustion. This could be verified by results of analysis of phosphorus inpyrolystical products and in situ FTIR. The first showed the existence of phosphorus in the gas products ofpyrolysis, and the latter signified that these phosphorus containing compounds involved P=O and P- Hstructures. 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