Polymer Degradation and Stability 84 (2004) 451e458

www.elsevier.com/locate/polydegstab

The effect of melamine on the combustion and thermaldecomposition behaviour of poly(butylene terephthalate)

A.I. Balabanovich)

Research Institute for Physical Chemical Problems of the Belarussion State University, ul. Leningradskaya 14, 220050 Minsk, Belarus

Received 4 September 2003; received in revised form 1 December 2003; accepted 12 December 2003

Abstract

Melamine (MA) improves the fire retardant performance of poly(butylene terephthalate) (PBT) as measured by the limitingoxygen index test and the UL94 standard. Thermogravimetric and FT-IR studies revealed that MA promotes the formation of soliddecomposition products that mostly consist of the condensation products of MA. As shown by GC/MS, MA affects the

development of low and high boiling degradation products. As a result, aromatic nitriles, amides and alkenyl- andcycloalkylenemelamines were identified among the volatiles indicating the occurrence of an interaction between PBT and MA onheating. The fire retardant condensed-phase effect of MA is discussed.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: PBT; Melamine; Flame retardance; Thermogravimetry; FT-IR; GC/MS

1. Introduction

The utility of melamine as an additive flame retardanthas been widely recognized in coatings (usually witha charrable polyol and a phosphate charring catalyst toinduce an intumescent behaviour [1]). The recentdevelopments deal with the use of melamine as a flameretardant in thermoplastics.

As a finely divided material, melamine, a white solid,is suitable for dispersal in thermoplastics, remainingmostly undissolved. When heated under atmosphericpressure it does not melt but sublimes. The mode of thefire retardant action of melamine has been reviewedelsewhere [2] and appears to involve endothermicsublimation and vapour-phase dissociation. An addi-tional effect comes from its conversion to non-volatileproducts and ammonia [2].

The addition of melamine improves the fire retardantperformance of polyamide 6: the formulations self-extinguish very quickly in air and their limiting oxygen

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E-mail address: balabanovich@bsu.by

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doi:10.1016/j.polymdegradstab.2003.12.003

index increases with increasing concentration of theadditive [3]. The fire retardant effect has been attributedto the promotion of melt dripping of the polymer. Inaddition, mechanistic studies have showed that mela-mine mostly evaporates from the melamine-containingformulation, whereas other part condenses producingsolid residue [4].

The main goal of this paper was to study a fireretardant effect of melamine in poly(butylene terephthal-ate) (PBT). PBT is an important plastic in the electronicand electrical industries, where flame-retardant proper-ties are required to meet fire safety conditions.

The present paper continues the search [5e8] forhalogen-free fire retardants in PBT.

2. Experimental

2.1. Materials

Standard PBT (Ultradur� B4520, registered trade-mark of BASF AG, Germany) was used as a basepolymer in this study. Melamine (I) was from DSM. Thepolymer was dried before compounding at 120 (C for

452 A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

15 min, whereas the fire retardant additive was used asreceived.

PBT was mixed with the additive in a bowl mixer(60 rpm, 5 min) at 230e240 (C. Bar-shaped specimensfor the combustion tests were cut from slabs prepared bypressing at 250 (C. The neat PBT used as a standardwas treated in the same way.

2.2. Combustion

The combustion performance of the formulationswas studied by the oxygen index test (limiting oxygenindex, LOI) following the ASTM D 2863 standard andby the UL94 standard on 2.0 mm specimens in thevertical configuration [9]. The LOI test is appropriate asa research semi-qualitative indicator of additive effec-tiveness, whereas the UL94 test shows the tendency ofa material to spread a flame [10].

2.3. Thermogravimetric analysis

Thermogravimetric analysis was carried out usinga Mettler TA 3000 thermal analyser. Standard measure-ments were performed at a heating rate of 10 (C/min inan argon flow of 90 cm3/min.

2.4. Products of thermal degradation

For details of the thermal degradation procedure, thereader is referred to Ref. [6]. Briefly explaining, highboiling degradation products (HBPs) were collectedunder argon or helium in a degradation tube inisothermal experiments at 420 (C, using samples ofapproximately 100 mg. In addition, low boiling degra-dation products (LBPs) were trapped at liquid nitrogentemperature and subsequently analysed by GC/MS. The

N

N

N

NH2

NH2H2N

(I)

HBPs condensed in the upper part of the test tube wereanalysed by FT-IR on an ASI/Mettler-Toledo ReactIR-1000 FT-IR System with a multi-reflection diamondsensor. Additionally, they were washed out by acetone,and the acetone solution was subsequently subjected toa GC/MS analysis (HP6890/5972) using an HP-1 60 mcolumn which was temperature programmed from 50 (C(3 min) to 290 (C (20 min) at a heating rate of 10 (C/min. For analysis of LBPs, the 60 m HP-1 capillarycolumn was also found to be appropriate. The columnwas heated to 250 (C at a rate of 10 (C/min after aninitial 3 min period at �40 (C. The mass spectra wereobtained by electron ionisation at 70 eV keeping thesource at about 180 (C. A mass spectrometric identifi-cation was carried out using Wiley and NBS libraries. Inthe few cases, when compounds were not included in thelibraries, they were identified on the basis of both themolecular ion m/z value and the ion decompositionpattern constructed for the best fit with the massspectrum.

Solid residues collected at different steps of ther-mal decomposition in the degradation tube in isother-mal experiments at 420 (C were investigated by IRspectroscopy on an ASI/Mettler-Toledo ReactIR-1000FT-IR System with a multi-reflection diamond sensor.

3. Result and discussion

3.1. Combustion performance

Table 1 reports the fire retardant behaviour of PBTformulations with melamine. Pure PBT is a highlycombustible material that burns with flammable drip-ping. The LOI value of PBT grew with increasingcontent of melamine indicating flame-retardant activityof the additive in the polymer. The UL94 performanceimproved as well. At 20 wt.% melamine the specimensbecame self-extinguishing. The formulations did notdrip at the 30 or 40 wt.% additive after the firstapplication of the flame. At 40 wt.% melamine thespecimens might show dripping but not flammable one,and, therefore, passed the UL94 test with a V-0 rating.

Table 1

Fire retardant behaviour of PBT formulations with melamine

No. Melamine (wt.%) LOI UL94 Na

Ranking Dripping t1/t2 (s)b

1 e 22 Fail Yes >250 >25

2 10 24 Fail Yes >250 >25

3 20 25 V-2 Yes 55/9 2/2

4 30 25 V-2 No/yesc 12/15 0/1

5 40 26 V-0 No/yes 0/9 0/0.6

a Ndthe average number of drops after the first/second application of the flame ( five pieces).b t1/t2dtotal combustion time after the first/second application of the flame.c No/yes corresponds to the first/second flame application.

453A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

In the LOI apparatus the formulations showed a meltflow up to 30 wt.% melamine. However, at higherloading (40 wt.%), the formation of a carbonaceouslayer between the flame and the polymer was observed.

3.2. Thermogravimetric analysis

Melamine shows a one step thermal decompositionbehaviour mostly due to evaporation [11] leaving a littleresidue. In a combination with PBT, a two step thermaldecomposition behaviour is expected (Fig. 1, thecalculated curve that is a linear combination of thethermograms of the single components) and observed(the experimental curve). However, the mixture leavesa residue that is higher than that predicted from anindependent behaviour. The trend was also observed forPBTC 40 wt.% melamine, and the effect was evengreater. These facts indicate the occurrence of aninteraction between PBT and melamine on heating.

3.3. FT-IR study

As melamine mainly volatilises on dynamic heatingbefore PBT starts to decompose (Fig. 1), pyrolysisproducts of the fire retardant PBT were collected inisothermal experiments.

The IR Spectrum of the untreated formulation showsthe absorption bands at 3466, 3409, 3326, 3112, 1652,1540, 1464, 810 cm�1 due to melamine and at 2927,2852, 1707, 1407, 1242, 1097, 872 and 724 cm�1 due toPBT (Fig. 2, Spectrum a).

The spectral changes observed on isothermal heatingat 420 (C (Fig. 2, Spectra bed) involve a decrease andelimination of the absorption bands of C]O(1707e1713 cm�1), (O])CeO (1242e1265 cm�1) andOeCH2 (1097e1101 cm�1) indicating a degradation ofthe ester bond of PBT. The absorption band of thearomatic CeH at 724e727 cm�1 disappears as well(Spectrum d). These facts signify a complete destructionof the PBT macromolecules.

100 200 300 400 500 6000

20

40

60

80

100

PBT + 30% MA(experimental)

PBT + 30% MA(calculated)

MA

PBT

Wei

ght (

%)

Temperature (°C)

Fig. 1. Thermogravimetry of PBT, melamine (MA) and their mixture

(experimental and calculated). Inert gas flow. Heating rate 10 (C/min.

A peak at 872 cm�1 becomes broader (Spectra c andd), and this is likely to be due to the formation ofpolyaromatic structures [12]. The polyaromatic struc-tures exhibit a broad absorption around 1600 cm�1 [12]that is masked by a strong absorption band at1524 cm�1 characteristic of s-triazine structures (Spec-trum d).

The overall profile of CeH stretch spectral region(2927e2852 cm�1) did not change remarkably indicat-ing that some aliphatic structures are incorporated inchar (Spectrum d).

The 3466, 3409, 3112 and 1652 cm�1 bands ofmelamine are affected as well. Their elimination is con-sistent with a disappearance of the NH2-group due toprogressive condensation with evolution of ammonia toform polymeric products melem and melon [11]. In par-ticular, the strong absorption bands at 1524, 1450 and1335 cm�1 (Spectrum d) are very close to those reportedfor an IR spectrum of melon [11] indicating that thecondensation products of melamine are built-in in char.The broad absorption band at 3332 cm�1 is associatedwith the NH stretching vibration of secondary amines.

In addition, it is worth noting that the thermaldecomposition behaviour of the fire retardant PBTdramatically differs from that of the non fire retardantPBT. As shown in Fig. 3 (Spectra a and b), the pyrolysisof pure PBT resulted in the formation of aromatic acid(2657, 1687 and 935 cm�1) and anhydride (1789, 1715,

ec

na

br

os

bA

4000 3500 3000 2600 1850 1300 650

Wavenumber (cm–1

)

(a)

(b)

(c)

01

8

23

33

-1

53

3-

66

43

25

61

53

61

(d)

90

43

1707

24

92

62

33

21

13

72

92

25

82

04

51

-

46

41

-

70

41

1242 1097

724

27

8

44

13

-

95

92

01

71 6

06

1

13

51

45

41

18

31

-

56

21

10

11

71

01

16

8

76

77

27

09

6

55

33

-

26

92

31

71 7

06

1

13

51

55

41

87

31

- 06

8

76

7

26

82

91

71

42

51

05

41

53

31

-

45

88

08

Fig. 2. Infrared spectra of initial PBTC 30wt.%melamine (a) and solid

products of thermal decomposition of the mixture collected in the de-

gradation apparatus at different steps of weight loss in inert atmosphere

on isothermal heating at 420 (C: 30% (b); 50% (c); 70% (d).

454 A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

1207 cm�1) structures. The structures were not detectedon the pyrolysis of the flame-retardant formulation.

In conclusion, the FT-IR data support the thermo-gravimetric evidence that melamine changes the decom-position path of PBT.

3.4. GC/MS study

Condensable volatiles collected on the pyrolysis ofthe fire retardant PBT were subjected to the GC/MSanalysis. A corresponding total ion gas chromatogram ispresented in Fig. 4. The assignment of the peaks isshown in Table 2 . Products 8 and 13 arise from the puredecomposition of PBT [5], whereas the appearance ofthe aromatic nitriles and amides (1e4, 6, 7, 9, 11, 12 and17) are in agreement with the ammonolysis concept ofPBT [6,7,13]. In addition, new products (10, 15, 16, 18,19) were identified among the HBPs. The difference ofthe molecular ions of the products 10 and 5, or 15, 16and 10, or 18, 19 and 15 is equal to 54 (the molecularmass of butadiene) signifying that those are additionproducts of butadiene to melamine. The mass spectra of

Ab

so

rb

an

ce

4000 3500 3000 2600 1850 1300 650

Wavenumber (cm–1

)

(a)

(b)

50

61

45

92

69

01

98

71

78

61

04

21

27

8

61

01

51

71

94

82

60

71

70

41

722

70

41

1265

70

21

99

01

53

01

53

9

17

7

726

64

92

-

75

62

-

Fig. 3. Infrared spectra of initial PBT (a) and a solid product of thermal

decomposition of PBT collected in the degradation apparatus at 90%

weight loss in inert atmosphere (b) on isothermal heating at 420 (C.

the products are presented in Figs. 5e9. The twoisomers 15 and 16, or 18 and 19 were distinguished onthe basis of their mass spectra. The corresponding iondecomposition patterns for the products 15 and 16 areshown in Fig. 10.

Fig. 11 illustrates a total ion gas chromatogram of theLBPs of PBTC 30 wt.% melamine. Comparison of thegas chromatogram with that of pure PBT [5] reveals thatthe addition of PBT induces the formation of ammonia,pyrrole and significant amounts of 3-butene-1-ol.

From these data it is concluded that melamine affectsthe formation of not only solid products but alsovolatile ones.

4. Conclusion: fire retardant effect

When set on fire, PBT does not produce any char.The addition of melamine promotes the formation ofsolid decomposition products (Fig. 1) for expenses offuel, thus contributing to fire retardancy. An additionaleffect comes at 40 wt.% of the additive, as a protectivelayer (char) starts to form on the surface of the burningpolymer. Polymeric products arising from a condensa-tion of melamine and polyaromatic structures (Fig. 2)are very likely to contribute to the creation of the char.

The fact that the addition of melamine altersdegradation pathways of PBT can also be regarded asa condensed-phase effect of the additive. The primaryprocess accounting for the phenomenon is an evolutionof ammonia from the condensation of amino-groupsfollowed by an ammonolysis reaction of PBT, as shownin Fig. 12. The reaction sequence would explain theformation of aromatic nitriles and primary amides.Secondary amides might be formed from reaction ofaromatic nitriles with alcoholic groups.

The most striking point is the appearance ofsubstituted melamines. These compounds might begenerated from an addition reaction of melamine toa double bond formed on the pyrolysis of PBT. Asa result of the reaction, 1,3-butadiene is trapped onmelamine, such retarding the volatilisation of theflammable gas.

Fig. 4. Gas chromatogram of the soluble in acetone HBPs of PBTC 30 wt.% melamine collected in the degradation apparatus in inert atmosphere at

420 (C. Peak assignment in Table 2.

455A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

Table 2

HBPs emitted from pyrolysis of PBTC 30 wt.% melamine

1. 103

2. 117

3. 128

4. 147

5. 126

6. 201

7. 146

8. 220

9. 200

10. 180

11. 200

12. 219

13. 274

14. 265

15. 234

16. 234

17. 273

18. 288

19. 288

CN

CN

CNNC

COOHNC

NN

NNH2

NH2

H2N

COONC

CONH2NC

COOHOOC

CONHNC

NN

NN

NH2

H2NCONNC

CONHHOOC

COOOOC

C17H35CN

CONHOOC

NN

NN

NH2

HN

NN

NN

NH2

N

NN

NN

NH

N

NN

NN

N

N

PeakNo.

Molecularmass (m/z)

Compound

456 A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

Fig. 7. Mass spectrum of product 16 (Table 2).

Fig. 5. Mass spectrum of product 10 (Table 2).

Fig. 6. Mass spectrum of product 15 (Table 2).

Fig. 8. Mass spectrum of product 18 (Table 2).

457A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

NN

NN

NH2

HN

m/z = 234

-N

NN

N

NH2

NH

m/z =193

C2H4- NN

NN

NH2

NH

m/z = 165

NN

NN

NH2

N

m/z = 234

NN

NN

NH2

N- C2H4- C2H4 N

NN

N

NH2

N

m/z = 206 m/z =178

Fig. 10. Mass spectral fragmentation patterns of the molecular ions of products 15 and 16.

Fig. 11. Gas chromatogram of the LBPs of PBTC 30 wt.% melamine collected in the degradation apparatus in inert atmosphere at 420 (C. 1, CO2;

2, NH3; 3, propene; 4, 1-butene; 5, 1,3-butadiene; 6, H2O; 7, tetrahydrofuran; 8, 3-butene-1-ol, 9, benzene, 10, 3-methyltetrahydrofuran; 11, pyrrole;

12, toluene; 13, 4-ethenylcyclohexene.

Fig. 9. Mass spectrum of product 19 (Table 2).

458 A.I. Balabanovich / Polymer Degradation and Stability 84 (2004) 451e458

CO

CO

O(CH2)4O+ NH3

CO

CNH2

O+ HO(CH2)4 O

- H2O

CO

CN

+ NH3

H2NC CNO

HO(CH2)2CH=CH2

NC CN- H2O

Fig. 12. Ammonolysis of PBT and formation of primary amides, aromatic nitriles and 3-butene-1-ol.

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