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Polymer Degradation and Stability 93 (2008) 1308–1315

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

A flame-retardant epoxy resin based on a reactive phosphorus-containingmonomer of DODPP and its thermal and flame-retardant properties

Li-Ping Gao a, De-Yi Wang a, Yu-Zhong Wang a,b,*, Jun-Sheng Wang a, Bing Yang a

a Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Key Lab of Green Chemistry and Technology,Ministry of Education, Sichuan University, 29 Wangjiang Road, Chengdu 610064, Sichuan Province, Chinab State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

Article history:Received 9 February 2008Received in revised form 3 April 2008Accepted 6 April 2008Available online 18 April 2008

Keywords:Flame retardantPhosphorusEpoxy resinCone calorimeter

* Corresponding author. Center for Degradable andMaterials, College of Chemistry, Key Lab of GreenMinistry of Education, Sichuan University, 29 WangjSichuan Province, China. Tel./fax: þ86 28 85410259.

E-mail address: yzwang@email.scu.edu.cn (Y.-Z. W

0141-3910/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2008.04.004

a b s t r a c t

A flame-retardant epoxy resin (EP) was synthesized based on a novel reactive phosphorus-containingmonomer, 4-[(5,5-dimethyl-2-oxide-1,3,2-dioxaphosphorinan-4-yl)oxy]-phenol (DODPP), and its struc-tures were characterized by FTIR, 1H NMR and 31P NMR spectra. The DODPP–EP3/LWPA (low molecularweight polyamide), which contains 2.5% phosphorus, can reach UL-94 V-0 rating and a limiting oxygenindex (LOI) value of 30.2%. The thermal properties and burning behaviours of cured epoxy resins wereinvestigated by differential scanning calorimeter (DSC), thermogravimetry (TG), LOI, UL-94 tests andcone calorimetry. The morphologies of residues of cured epoxy resins were investigated by scanningelectron microscopy (SEM). DSC shows that the glass-transition temperatures of cured epoxy resinsdecrease with increasing phosphorus content. TGA shows that the onset decomposition temperaturesand the maximum-rate decomposition temperatures decrease, while char yields increase, with the in-crease of phosphorus content. The data from the cone calorimeter tests give the evidence that heatrelease rate (HRR), peak heat release rate (PHRR), average heat release rate (Av-HRR), average mass lossrate (Av-MLR) and the fire growth rate index (FIGRA) decrease significantly for DODPP–EP3/LWPA. SEMshows that the DODPP–EP3/LWPA forms lacunaris and compact charred layers which inhibit the trans-mission of heat during combustion.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Epoxy resins are widely used in various industrial fields such asadhesive, surface coating, painting materials, laminates, semi-conductor encapsulation, and insulating material for electric de-vices due to their advantageous properties of adhesion, excellentcharacteristics of solvent, chemical and moisture resistance, goodtoughness, low shrinkage on cure, and superior electrical and me-chanical properties. However, conventional epoxy resins are veryflammable and they cannot satisfy some applications which requirehigh flame retardancy [1–3]. Epoxy resins can be imparted flameretardance either by adding flame retardants or by incorporatingreactive flame retardants. Recently, the researches mainly focus onthe latter because the flame retardants are chemically incorporatedinto the polymer structure, resulting in some advantages such asthe more stable flame retardance effect and the less gas emission

Flame-Retardant PolymericChemistry and Technology,

iang Road, Chengdu 610064,

ang).

All rights reserved.

during either high temperature processing or burning. Simulta-neously, the reactive approach can minimize negative impact uponphysical and mechanical properties of the polymer. Phosphorus-containing flame retardants are more environmentally benign thanhalogen-containing ones. Among phosphorus-containing flameretardants, organophosphorus compounds are particularly effec-tive for epoxy resin because P tends to react with OH groups ofcured epoxy resins [4–10]. Many epoxy resins [11–18] and curingagents [19–25] based on organophosphorus compounds have beendeveloped over the past few decades.

In this work, a novel reactive phosphorus-containing monomer,DODPP, was synthesized and then used to prepare novel flame-retardant epoxy resin. Low molecular weight polyamide (LWPA) wasselected as a curing agent for epoxy resins because of its relativelylow toxicity, volatility and thrill. On the other hand, cured epoxyresins with LWPA are imparted with good toughness and high im-pulsion strength, which are widely used in many areas such as dope,adhesion between metal and nonmetal materials, anticorrosivecoatings, encapsulating electronic components and chemical grout-ing, etc. [3,26,27]. The performances of the corresponding curedepoxy resins, such as thermal properties and burning behaviours,were studied by DSC, TGA, LOI, the UL-94 test and cone calorimetry.

Scheme 2. Synthesis of DODPP.

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–1315 1309

2. Experimental

2.1. Materials

2,2-Dimethyl-1,3-propanediol (neopentyl glycol), phosphorustrichloride (PCl3) and anhydrous ethanol (C2H5OH) were purchasedfrom Changzheng Chemical Reagent Corp. (Chengdu, China). p-Benzoquinone (BQ) was prepared in our laboratory. DGEBA wasobtained from Wuxi Resin Factory of Xingchen New ChemicalMaterial Co. Ltd (Wuxi, China). Low molecular weight polyamide(LWPA) was purchased from Haitian Chemical Reagent Co. Ltd(Hunan, China). Triphenylphosphine (Ph3P) was used as curingaccelerator. All the chemicals used were of reagent grade, except forDGEBA and LWPA.

2.2. Synthesis of 5,5-dimethyl-2-oxide-1,3,2-dioxaphosphorinane (DODP)

5,5-Dimethyl-2-oxide-1,3,2-dioxaphosphorinane was synthe-sized according to the reported literature [28,29]. 2,2-Dimethyl-1,3-propanediol (52 g, 0.5 mol) and 150 ml 1,2-dichloroethane were fedinto a 500-ml four-necked round-bottomed flask equipped witha thermometer, magnetic stirrer, an addition funnel and refluxcondenser with exhaust pipe. The reaction mixture was cooledwith an ice-water bath and phosphorus trichloride (68.5 g, 0.5 mol)was added dropwise for about 3 h. About 4 h later, the reactionmixture was maintained at room temperature and anhydrousethanol (23 g, 0.5 mol) was added dropwise over a period of 3 hwhile passing a stream of nitrogen gas through. And then, themixture was heated to 80 �C on an oil bath and kept under refluxinguntil no more HCl was emitted. The clear solution was distilledunder vacuum to obtain colorless solid. The synthetic route isshown in Scheme 1.

2.3. Synthesis of 4-[(5,5-dimethyl-2-oxide-1,3,2-dioxaphosphorinan-4-yl)oxy]-phenol (DODPP)

DODP (129 g, 0.50 mol) and 1,2-dichloroethane (150 ml) werefed into a 500-ml four-necked round-bottomed flask equipped witha thermometer, magnetic stirrer, an addition funnel and a circum-ference condenser. The reaction mixture was stirred at roomtemperature and p-benzoquinone (37.8 g, 0.35 mol) was addedincrementally. At the same time, triethylamine was added asa catalyst in a dropwise manner for about 3 h. The reaction mixturewas subsequently maintained at room temperature for an addi-tional 8 h. The precipitant was filtered and washed thoroughly withwater. The white powder was dried under vacuum at 80 �C for 8 h.The yield of white solid product was 75% and the melting point ofthe purified product was 170–172 �C. The synthetic route is shownin Scheme 2.

2.4. Synthesis of phosphorus-containing epoxy resins

DGEBA was reacted with various molar ratios of DODPP at160 �C for 6 h (Scheme 3) in the presence of 0.1 wt% Ph3P. A series ofepoxy resins with various phosphorus contents and epoxy

Scheme 1. Synthesis of DODP.

equivalent weights were synthesized. A typical experimental pro-cedure for DODPP–EP2 is shown as follows.

DGEBA (100 g, 0.44 mol) and DODPP (31 g, 0.12 mol) werecharged into a 500-ml three-necked round-bottomed flask equip-ped with a magnetic stirrer, thermometer and nitrogen inlet. Themixture was heated in an oil bath to 125 �C under nitrogen gas toachieve a transparent solution, and then 0.1 g Ph3P was added asa catalyst. The reaction mixture was gradually heated in an oil bathto 160 �C and maintained at that temperature for 6 h. A brownphosphorus-containing epoxy was obtained.

2.5. Curing procedure for phosphorus-containing epoxy resins

DGEBA and various phosphorus-containing epoxy resins wereseparately cured with low molecular weight polyamide (LWPA) indeterminate mass ratio as shown in Table 3. The reactants weremixed at 120 �C and 0.2 wt% Ph3P was added as a curing accel-erator. The mixture was subsequently stirred constantly untila homogeneous solution was obtained. The mixture was thenpoured into a hot iron mold, which had been preheated to 120 �C,and cured in a preheated oven at 120 �C for 6 h and then post-cured at 130 �C for 2 h. The samples were cooled slowly in theoven to room temperature to prevent cracking.

2.6. Characterization

The structure of DODPP was determined by FTIR, 1H NMR and31P NMR spectra, which were performed on a Nicolet FTIR 170SXinfrared spectrophotometer and an FT-80ANMR spectrum,respectively.

The structure of phosphorus-containing epoxy resin wasrecorded by using a Nicolet FTIR 170SX infrared spectrophotometer.

Differential scanning calorimetry (DSC) thermograms wererecorded with a Q100 V9.4 Build 287 DSC at a heating rate of10 �C/min under nitrogen atmosphere at a flow rate of 40 ml/min.

Thermogravimetric analysis (TGA) was performed on a Q500V6.4 Build 193 thermal analyzer at a heating rate of 10 �C/min.Samples of 7 mg were heated from room temperature to 700 �Cunder air or nitrogen atmosphere at a flow rate of 60 ml/min.

Epoxy equivalent weights (EEWs) of the epoxy resins weredetermined by the HCl/acetone chemical titration method.

The limiting oxygen index (LOI) values were measured on anHC-2C oxygen index meter (Jiangning, China) according to ASTMD2863-97, with sheet dimensions of 130� 6.5� 3.2 mm3.

The vertical burning tests (UL-94) were conducted on a CZF-2instrument (Jiangning, China) according to ASTM D 3801 testingprocedure, with sheet dimensions of 125�12.7� 3.2 mm3.

The cone calorimeter was used to test the fire performance onan FTT cone calorimeter according to ISO 5660 under an externalheat flux of 35 kW/m2. The dimension of the samples was100�100� 6 mm3. Heat release rate (HRR), time to ignition (TTI),mass loss rate (MLR) and other quantifiable parameters wererecorded simultaneously.

Scanning electron microscopy (SEM) observed on a JEOL JSM-5900LV was used to investigate the surface of residues of DGEBA/LWPA and DODPP–EP3/LWPA. The residues of samples for SEM

Scheme 3. Synthesis of phosphorus-containing epoxy resin.

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–13151310

were obtained after combustion in their limiting oxygenconcentration.

3. Results and discussion

3.1. DODPP synthesis and characterizations

DODPP was synthesized through the reaction of DODP and p-benzoquinone (BQ) according to Scheme 2. The structure ofDODPP was determined by FTIR, 1H NMR and 31P NMR spectra.

Fig. 1 shows the FTIR spectrum of DODPP. The peak at 3335 cm�1

indicates the formation of Ph–OH. However, no characteristicabsorption peak for P–H is observed at around 2400 cm�1, whichdemonstrates that, the reaction of DODP and p-benzoquinoneproceeded completely [18]. Simultaneously, the absorption of –CH2

and –CH3 from neopentyl glycol is observed at about 2893–2976 cm�1. The peaks at 1193 and 956 cm�1 can be assigned to P–O–C absorption. The peak at 1298 cm�1 is attributed to P]Oabsorption [30].

Fig. 2 shows the 1H NMR spectrum of DODPP. The peaks at about0.85 and 1.18 ppm are assigned to methyl protons (a). The multipletbetween 3.97 and 4.29 ppm corresponds to the methylene protons(b). The multiplet between 6.75 and 7.10 ppm can be assigned to thephenol ring protons (c). The peak at about 9.48 ppm should beassigned to the active –OH (d) of DODPP.

Fig. 1. FTIR spectrum of DODPP.

Fig. 3 shows the 31P NMR spectrum of DODPP. A single peak atabout �13.19 ppm indicates the phosphorus atom in cyclic DODPstructure of DODPP [31].

Above characterizations confirm that the target product wassynthesized successfully.

3.2. Synthesis and characterizations of phosphorus-containingepoxy resins

Fig. 4 shows the FTIR spectrum of the phosphorus-containingepoxy resin DODPP–EP2. The absorption of –CH2 and –CH3 fromneopentyl glycol is observed at about 2875–2966 cm�1. The peakat 1296 cm�1 can be assigned to P]O absorption. The peaks at1185 and 946 cm�1 can be assigned to P–O–C absorption. Thepeaks at 1246, 917 and 831 cm�1 can be assigned to character-istic absorption of oxirane ring. The peaks at 3421 and1109 cm�1 can be assigned to O–H stretching and O–H bending,respectively [30].

3.3. Epoxy equivalent weight (EEW)

The epoxy equivalent weights of DGEBA and various phospho-rus-containing epoxy resins were determined by the HCl/acetonechemical titration method and the results are presented in Table 1.It is noteworthy that every EEW of epoxy resins is very close to theirtheoretical values, indicating that the reaction between DGEBA andDODPP proceeded completely.

Fig. 2. 1H NMR spectrum of DODPP.

Fig. 3. 31P NMR spectrum of DODPP.

Table 1Epoxy equivalent weights of theoretical and experimental values of epoxy resins

Epoxyresins

Epoxy equivalent weight(theoretical value)(EEW, g/equiv)

Epoxy equivalent weight(experimental value)(EEW, g/equiv)

P% beforecuring (wt%)

DGEBA 225 227 0DODPP–EP1 370 366 2.5DODPP–EP2 417 400 2.8DODPP–EP3 454 460 3.0

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–1315 1311

3.4. Thermal properties of cured epoxy resins

3.4.1. Differential scanning calorimetry (DSC)DSC curves of DGEABA/LWPA and DODPP–EP/LWPA systems are

shown in Fig. 5, which were operated at 10 �C/min heating rateunder nitrogen atmosphere at a flow rate of 40 ml/min. As shown inFig. 5 and Table 2, the glass-transition temperatures (Tgs) of curedepoxy resins decrease with increasing phosphorus content. Thereason may be that the incorporation of DODPP into the epoxynetwork will reduce its crosslink density and rigidity.

3.4.2. Thermogravimetric analysis (TGA)TGA is widely used to evaluate the thermal stability and

thermal degradation behaviour of polymers [32]. TG and DTGcurves of all the cured epoxy resins under nitrogen and airatmospheres are presented in Figs. 6 and 7, and the test data aresummarized in Table 2, respectively. As shown in Figs. 6 and 7,the onset degradation temperatures of cured epoxy resins de-crease with increasing phosphorus content under nitrogen andair atmospheres, which is attributed to the poorer stability ofO]P–O than –C–C– bond and the decrease of crosslink density ofphosphorus-containing cured epoxy resins [6]. Although thethermal stabilities decrease with increasing phosphorus content,the cured epoxy resins exhibit high thermal stability. Further-more, it is obvious that all the cured epoxy resins under nitrogenatmosphere show one-stage decomposition processes, whilethose under air atmosphere show two-stage degradation pro-cesses. The maximum-rate degradation temperatures of firststage decrease with increasing phosphorus content under nitro-gen and air atmospheres. However, the maximum-rate

Fig. 4. FTIR spectrum of phosphorus-containing epoxy resin (DODPP–EP2).

degradation temperatures of second stage under air atmospheredecrease with increasing phosphorus content inconspicuously.This behaviour may be attributed to phosphorus-containinggroups that decompose at earlier stages and forms a phospho-rous-rich char layer. The thick layer prevents furtherdecomposition of the epoxy resin by raising the maximum-ratedegradation temperature of second stage and leads to a high charyield [16,33].

On the other hand, the char yield of DGEBA/LWPA undernitrogen atmosphere is 11.2% at 700 �C, while that of DODPP–EP/LWPA systems increase to 15.5–21.4% with increasing phosphoruscontent. Simultaneously, DGEBA/LWPA under air atmosphere de-grades completely and only about 0.1% char yield is found at700 �C. However, DODPP–EP/LWPA systems under air atmospheredegrade significantly differently from DGEBA/LWPA, especially inthe range of 450–700 �C. The char yields of DODPP–EP/LWPA in-crease to 4.0–5.3% with increasing phosphorus content at 700 �C.The char layer plays an important role in improving the flameretardancy of phosphorus-containing cured epoxy resins [34,35].Furthermore, the char yields increase with increasing phosphoruscontent, which is consistent with the flame retardancy of thesystems characterized by LOI measurement and UL-94 testdiscussed below.

3.5. Flammability of cured epoxy resins

3.5.1. LOI measurement and UL-94 testThe limiting oxygen index (LOI) measures the minimum oxygen

concentration (in an oxygen–nitrogen flowing mixture) required tosupport downward flame combustion, which can be used as anindicator to evaluate the flame retardancy of polymers. The LOI

Fig. 5. DSC curves of DGEBA/LWPA and DODPP–EP/LWPA systems.

Table 2Thermal properties of cured epoxy resins

Compositions Tg (�C) N2 atmosphere Air atmosphere

Tona (�C) Tmax

b (�C) Char yieldat 700 �C (%)

Tona (�C) Tmax,1

c (�C) Tmax,2d (�C) Char yield

at 700 �C (%)

DGEBA/LWPA 84 324 420 11.2 311 412 541 0.1DODPP – 233 286 12.6 255 301 – 19.2DODPP–EP1/LWPA 79 307 360 15.5 284 320 539 4.0DODPP–EP2/LWPA 57 302 355 18.4 283 318 537 4.5DODPP–EP3/LWPA 58 302 350 21.4 281 318 534 5.3

a Ton is the onset decomposition temperature at which the weight loss is 5%.b Tmax is the maximum-rate degradation temperature.c Tmax,1 is the maximum-rate degradation temperature of first stage.d Tmax,2 is the maximum-rate degradation temperature of second stage.

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–13151312

values of all the cured epoxy resins are presented in Table 3. It canbe seen that the LOI values significantly increase from 19.6 to 30.2when DODPP was incorporated into epoxy network. This indicatesthat incorporating phosphorus-containing group is very effective inimproving the flame retardancy of epoxy resins. Incorporation of2.5 wt% phosphorus into the thermosetting epoxy resins will makethem become flame-retardant polymers.

The vertical burning test (UL-94) determines the upward-burning characteristics of polymers. Test results of all the cured

Fig. 6. TG (a) and DTG (b) curves under N2 atmosphere of DGEBA/LWPA, DODPP andDODPP–EP/LWPA systems.

epoxy resins are also presented in Table 3. It is noteworthy that thecombustion time of DODPP–EP1/LWPA is shorter than that ofDGEBA/LWPA, although they cannot pass the UL-94 V-0 rating. It isclear that the flame retardance of cured epoxy resins increases withincreasing phosphorus content and a UL-94 V-0 rating can beobtained for DODPP–EP3/LWPA. This result is also consistent withthe LOI measurement.

3.5.2. Cone calorimeterCone calorimetry is used to evaluate the flammability and

potential fire safety of polymer materials under well-ventilated

Fig. 7. TG (a) and DTG (b) curves under air atmosphere of DGEBA/LWPA, DODPP andDODPP–EP/LWPA systems.

Table 3LOI measurement and UL-94 test of cured epoxy resins

Compositions Epoxyresin/LWPA(mass ratio)

P% aftercuring(wt%)

LOI values(%)

UL-94rating

Drip

DGEBA/LWPA 100:40 0 19.8 Fail YesDODPP–EP1/LWPA 100:40 1.7 27.8 Fail NoDODPP–EP2/LWPA 100:25 2.0 28.0 V-1 NoDODPP–EP3/LWPA 100:20 2.5 30.2 V-0 No

Fig. 9. Mass curves for DGEBA/LWPA and DODPP–EP3/LWPA.

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–1315 1313

conditions, which remains one of the most effective bench-scaletests that is used to predict the combustion behaviours of materialsin a real fire [36,37]. The material burns under homogeneous forcedflaming conditions in the cone calorimeter. Furthermore, the conecalorimeter brings quantitative analysis to materials flammabilityresearch by investigating parameters such as time to ignition (TTI),heat release rate (HRR), peak heat release rate (PHRR), time to peakheat release (TTPH) and mass loss rate (MLR) and so on. The resultsof all the cured epoxy resins from cone calorimeter investigationare shown in Figs. 8 and 9, and the test data are summarized inTable 4, respectively.

The heat release rates (HRRs) vs. time curves for DGEBA/LWPAand DODPP–EP3/LWPA are presented in Fig. 8 and the results arelisted in Table 4. It is noteworthy that DGEBA/LWPA burns veryrapidly after ignition and the peak heat release rate (PHRR) is946 kW/m2. As expected, DODPP–EP3/LWPA exhibits a consider-able reduction in HRR and the peak heat release rate (PHRR) de-creases to 183 kW/m2. Simultaneously, Table 4 reveals that Av-HRRdecreases significantly for DODPP–EP3/LWPA. The Av-HRR ofDGEBA/LWPA is 379 kW/m2, while that of DODPP–EP3/LWPAdecreases to 76 kW/m2, indicating that both PHRR and Av-HRR ofDODPP–EP3/LWPA decrease five times than those of DGEBA/LWPA.It may be due to the fact that DODPP–EP3/LWPA forms a stable andcompact char layer, which retards the transfer of heat inside thespecimen during the ignition process, thus improving the flameproperties.

Time to ignition (TTI) is used to determine the influence onignitability, which can be measured from the onset of an HRR curve.As shown in Fig. 8 and listed in Table 4, the TTI of DGEBA/LWPA is61 s, while that of DODPP–EP3/LWPA decreases to 17 s, decreasingby 28%. The reason for this may be due to the fact that the DODPPdecomposes at relatively low temperature, releasing small volatilemolecules, which causes the samples to burn more easily under theradiation of thermal flux.

Fig. 8. Comparison of the heat release rate (HRR) plots for DGEBA/LWPA and DODPP–EP3/LWPA.

The Fire Growth Rate Index (FIGRA) is calculated by dividingPHRR by TTPH, giving a unit of kW/m2 s, which can estimateboth the predicted fire spread rate and the size of a fire. Thehigher the FIGRA, the faster the flame spread and flame growthare assumed to be [38,39]. The FIGRA of DGEBA/LWPA is 3.2 kW/m2 s, while that of DODPP–EP3/LWPA decreases to 0.3 kW/m2 s,meaning that DODPP–EP3/LWPA performs much better thanDGEBA/LWPA.

Mass curves of DGEBA/LWPA and DODPP–EP3/LWPA are shownin Fig. 9. It is noteworthy that DGEBA/LWPA burns almost com-pletely, losing about 93% mass, while DODPP–EP3/LWPA showsabout 54% mass loss. Char formation during burning is desirablebecause char can form a protective isolation layer which retards theheat transmission during the ignition process. Thus, incorporatingDODPP into epoxy network can improve flame retardanceeffectively.

3.6. Morphology of the residue

The morphologies of residues after LOI test were investigated bySEM. Fig. 10(a) displays the morphologies of residues from DGEBA/LWPA and Fig. 10(b–e) displays the morphologies of residues fromDODPP–EP3/LWPA. It is interesting to note that the difference be-tween the morphologies of residues is very obvious. From Fig. 10(a),we can observe nothing on the surface of the residues. However,from Fig. 10(b and c), lacunaris and compact charred layers areobserved on the outer surface of the residues, which effectivelyprotect the internal structures and inhibit the heat transmissionand reduce the fuel gases when the fire contacts them. Meanwhile,from Fig. 10(d and e), we can observe some big holes and bubbleson the inner surface of residues. Therefore, DODPP–EP3/LWPA ex-hibits good flame retardancy, which agrees with the results of TGA,cone calorimeter, LOI and UL-94 testing.

Table 4Combustion parameters obtained from cone calorimeter

Sample TTI (s) Av-RHR(kW/m2)

PHRR(kW/m2)

TTPH(s)

FIGRA(kW/m2 s)

Av-MLR(g/s)

DGEBA/LWPA 61 379 946 295 3.2 0.12DODPP–EP3/LWPA 17 76 183 615 0.3 0.04

TTI: time to ignition; TTPH: time to peak heat release; Av: average.

Fig. 10. SEM morphology of the residues of samples, DGEBA/LWPA (a), outer surface (b, c) and inner surface (d, e) of residue of DODPP–EP3/LWPA.

L.-P. Gao et al. / Polymer Degradation and Stability 93 (2008) 1308–13151314

4. Conclusion

A novel flame-retardant epoxy resin was successfully synthe-sized based on a novel reactive phosphorus-containing monomerDODPP, which was synthesized from 5,5-dimethyl-2-oxide-1,3,2-dioxaphosphorinane (DODP) and p-benzoquinone (BQ). TheDODPP–EP3/LWPA can reach a V-0 rating of UL-94 and an LOIvalue of 30.2. The glass-transition temperatures and the thermalstabilities of cured epoxy resins decrease, while char yields in-crease with increasing phosphorus content. The DODPP–EP3/LWPA starts to degrade at lower temperature than DGEBA/LWPA,and there is a significant reduction in TTI due to releasing smallvolatile molecules. However, it has considerable decreases inHRR, PHRR, Av-HRR, Av-MLR and FIGRA compared to DGEBA/LWPA. The char layer of DODPP–EP3/LWPA is more stable ata much high temperature than that of DGEBA/LWPA. This stablechar layer will form the barrier which will inhibit the heattransmission and slow down the burning rate in the conecalorimeter.

Acknowledgements

The support from the National Science Foundation of China(20674053, 50525309) is gratefully acknowledged.

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