fire retardant and charring effect of poly(sulfonyldiphenylene phenylphosphonate) in poly(butylene...

Fire retardant and charring effect of poly(sulfonyldiphenylenephenylphosphonate) in poly(butylene terephthalate)

A.I. Balabanovicha,*, J. Engelmannb

aResearch Institute for Physical Chemical Problems, Belarussian State University, Leningradskaya 14, 220050 Minsk, BelarusbBASF AG, Kunststofflaboratorium, D-67056 Ludwigshafen, Germany

Received 13 June 2002; received in revised form 9 July 2002; accepted 23 July 2002

Abstract

Poly(sulfonyldiphenylene phenylphosphonate) (PSPPP) alone or together with polyphenylene oxide PPO, triphenylphosphateTPP or 2-methyl-1,2-oxaphospholan-5-one 2-oxide (OP) was mixed with poly(butylene terephthalate) (PBT). The combustion

behavior of the formulations was studied by limiting oxygen index and Underwriters Laboratories UL94 test. The mechanism ofthe fire-retardant action of PSPPP was elucidated by thermogravimetry, IR characterization of solid residues and GC/MS char-acterization of the gaseous and high-boiling products. The UL94 test V-0 rating was achieved by addition of 10 wt.% PSPPP, 10wt.% PPO and 10 wt.% OP. The fire retardant effect was attributed to promoting the char yield by involving the polymer in

charring. PSPPP was shown to induce the formation of thermally stable polyarylates and phenolic functionalities in PBT. Theadditive is likely to react with these functionalities and crosslink macromolecules.# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: PBT; 2-methyl-1,2-oxaphospholan-5-one 2-oxide (OP); Polyphenylene oxide (PPO); Triphenylphosphate (TPP); V-0 UL94; Combustion

performance; Thermogravimetry; FT-IR; GC/MS

1. Introduction

Poly(butylene terephthalate) (PBT) is widely used forelectrical mouldings (lamp holders, switches, circuitbreakers, motor casings), where flame retardant prop-erties are required. Highly combustible PBT is fireretarded with brominated plus antimony trioxide sys-tems. Brominated oligomeric polycarbonate or poly-styrene as well as other low molecular weightbrominated fire retardant additives are used. Despitehaving a number of technical advantages in applica-tions, the use of brominated compounds in polymersalso has drawbacks, e.g. they tend to produce corrosiveand obscuring smoke if they are burned.Possible alternatives are fire retardant systems

based, for example, on phosphorus, which are cur-rently being developed to substitute halogen flameretardants.

The aim of this work was to test a phosphorus-con-taining oligomer poly(sulfonyldiphenylene phenylphos-phonate) as a fire retardant additive in PBT. Thisproject continues our search for halogen-free fire retar-dants in this polymer [1–3].

2. Experimental

Standard PBT (Ultradur1 B4520, BASF AG) wasused as a base polymer in this study. It was fire retardedby poly(sulfonyldiphenylene phenylphosphonate)(PSPPP, formula I), a commercial product of WeiliFlame Retardant Chemicals Ind Co., China. 2-methyl-1,2-oxaphospholan-5-one 2-oxide (OP, formula II) alsoknown as 2-methyl-2,5-dioxo-1,2-oxaphospholane, is acommercial product of Fa. Clariant (Germany), which isavailable under the trade name Exolit PE 110. Poly(2,6-dimethylphenylene oxide) (PPO) was supplied by CheilIndustries Inc. Triphenyl phosphate (TPP, PhosflexTPP) was received from Akzo Nobel. The polymer wasdried before compounding at 120 �C for 15 min, whereasthe fire retardant additives were used as received.

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00258-6

Polymer Degradation and Stability 79 (2003) 85–92

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +375-17-2264698; fax: +375-17-

2264696.

E-mail address: [email protected] (A.I. Balabanovich).

PBT was mixed with the additives in a bowl mixer (60rpm, 5 min) at 230–240 �C. Bar-shaped specimens forthe combustion tests were cut from slabs prepared bypressing at 250 �C. The neat PBT used as a standardwas treated in the same way. The compositions of theformulations are shown in Table 1.The combustion performance of the formulations was

studied by the oxygen index test (limiting oxygen index,LOI) following the ASTM D 2863 standard and by theUL94 standard on 2.0 mm specimens in the verticalconfiguration [4]. According to the ASTM D 2863standard, the lowest limit of oxygen concentration innitrogen/oxygen mixtures capable of sustaining candle-like combustion of vertically positioned polymer speci-mens is determined. According to the UL94 test, thesample rod is placed in a holder in a vertical positionand the lower end of the rod is contacted by a methaneflame for 10 s thus initiating burning. A second ignitionis made after self-extinguishing of the flame at the sam-ple. The burning process is characterized by the times t1and t2 pertaining to the two ignitions. The times t1 andt2 denote the time between removing the methane flameand self-extinguishing of the sample. Moreover, it is

always noted whether flaming drips are released fromthe sample during the times t1 and t2. The observationsmade during the tests serve to classify the samples intothree groups as is illustrated in Table 2.Thermogravimetric analysis was carried out using a

Mettler TA 3000 thermal analyser. Standard measure-ments were performed at a heating rate of 10 �C/min inan argon or air flow of 60 cm3/min.Solid residues collected at different steps of thermal

decomposition in thermogravimetry were investigatedby IR spectroscopy on a Perkin Elmer ‘‘Spectrum 1000’’FT-IR spectrometer using KBr pellets. The high boilingdegradation products (HBPs) of the thermal decom-position were collected under argon in test tubes forthermal volumetric analysis. The test tube was placedvertically in an oven. While the bottom part of the testtube was heated up at 10 �C/min to 450 �C, the upperpart was cooled by running water. The gas flow fromthe tube was directed into a cell where volatile products(low boiling degradation products, LBPs) were trappedat liquid nitrogen temperature and subsequently ana-lysed by GC/MS. The HBPs condensed in the form ofrings in the upper part of the test tube were extracted byacetone. Insoluble products were analysed by FT-IRusing KBr pellets. The acetone extract was analysed byGC/MS (HP6890/5970) using a HP-5 30 m column orHP-1 60 m column which was temperature programmedfrom 40 �C (2 min) to 280 �C (10 min) at a heating rate of10 �C/min. For analysis of LBPs, the 30 mHP-5 capillarycolumn was also found to be excellent. The column wastemperature programmed from�10 �C (2 min) to 250 �Cat 10 �C/min. The mass spectra were obtained by elec-tron ionisation at 70 eV keeping the source at about180 �C.Mass spectrometric identification was carried outusing WILEY and NBS libraries. In the few cases, whencompounds were not included in the libraries, they wereidentified on the basis of both the molecular ion m/zvalue and of the ion decomposition pattern constructedfor the best fit with the mass spectrum.

3. Results and discussion

3.1. Combustion performance

Table 1 reports the results of the combustion tests ofPBT with the added additive PSPPP and co-additives

Table 1

LOI and UL94 combustion tests for PBT formulations

No Additive LOI UL94

Name/formula Wt.% Ranking Dripping t1/t2 a

1 – – 21.9 NCb Yes >302 PSPPP 5 23.0 NC Yes >30

3 PSPPP 10 22.2 NC Yes >30

4 PSPPP 20 22.7 V-2 Yes 4/4

5 PSPPP 30 25.7 V-2 Yes 2/6

6 PSPPP+PPO=2:1 30 25.2 V-1/V-2 No/yesc 5.5/3

7 PSPPP+PPO+TPP=3:2:1 30 25.7 V-1/V-2 No/yes 3.5/2

8 PSPPP+PPO+OP=3:2:1 30 26.6 V-0/V-2 No/yes 2/1

9 PSPPP+PPO+OP=1:1:1 30 28.0 V-0 No/yes 1/1

10 PSPPP+PPO+OP=1:1:1 25 26.4 V-0/V-2 No/yes 1/1

a t1/t2—average combustion time after the first and the second

application of the flame.b NC—not classified.c No/yes corresponds to the first/second flame applications.

Table 2

Classification of samples tested according to the UL94 protocol

Rating t1 (s) t2 (s) �(t1+t2)a (s) Dripping

V-0 <10 <10 <50 No

V-1 <30 <30 <250 No

V-2 <30 <30 <250 Yes

a Sum of t1 and t2 values recorded in 10 tests.

86 A.I. Balabanovich, J. Engelmann / Polymer Degradation and Stability 79 (2003) 85–92

PPO, OP and TPP. Highly combustible pure PBT showsLOI=21.9, burns with flammable dripping and is notself-extinguishing. PSPPP at the addition level up to10% is not active in PBT (formulations 2 and 3). At 20wt.% level of PSPPP the formulation reaches V-2 rank-ing in UL94 test showing short combustion time anddripping at both the first and the second flame applica-tion. A significant increase of LOI (from 21.9 to 25.7) isobserved on addition of 30 wt.% PSPPP (formulation5). However, the formulation reaches only a V-2 rank-ing due to flammable dripping. Partial substitution ofPSPPP by the char former PPO (formulation 6) is ben-eficial for fire retardancy. The formulation does not dripon the first application of flame, but some drops occurat the second application. Therefore we classified thisformulation as a transitory V-1/V-2. Further partialsubstitution of PSPPP by TPP (formulation 7) does notreally change LOI but decreases the average burningtime. Flammable dripping was sometimes observed onthe second flame application. Formulation 8 shows thatOP is a better fire retardant co-additive than TPPbecause of an increase in LOI and a decrease in theaverage burning time. Decreasing the PSPPP contentand simultaneously increasing the OP concentrationallows the highest level of fire retardancy to be reached(formulation 9). The formulation shows non-flammabledripping at the second application of the flame. Adecrease in total content of the additives from 30 to25% deteriorates the fire retardancy as can be seen fromformulation 10.

3.2. Thermal analysis

PSPPP exhibits a small weight decrease at 190–200 �Cfollowed by the main stage of weight loss starting at290 �C. There is finally about 25% of solid residue at600 �C in inert atmosphere (Fig. 1). Thermogravimetry

provides evidence that PSPPP interacts with PBTbecause of the increase in solid residue and the decreasein the onset of weight loss of the formulation containingPSPPP. This is well demonstrated by comparing experi-mental and calculated thermograms (Fig. 1). The calcu-lated curve is a linear combination of the thermogramsof the single components of the mixture, therefore it isrepresentative of a non-interacting behaviour. Thisresult also shows that PSPPP is a good char former: theamount of solid residue obtained at 600 �C is more thantwice as much as expected from their independent ther-mal decomposition behaviour. Addition of PSPPP alsoimproves the ‘‘quality’’ of the char. This is inferred fromits higher thermal stability in air compared to that frompure PBT (Fig. 2). Oxygen starts to oxidize it just above480 �C.Figs. 2 and 3 show the effect of co-additives on the

thermal decomposition of the fire retardant PBT. Co-addition of TPP improves charring in both atmospheresinert (Fig. 3) and air (Fig. 2). This points to a condensed-

Fig. 1. Thermogravimetry data for PBT, PSPPP (experimental curves)

and PBT+ 20 wt.% PSPPP (experimental and calculated curves).

Inert gas flow. Heating rate 10 �C/min.

Fig. 2. Thermogravimetry data for PBT, PBT with added PSPPP and

PBT with added PSPPP+co-additives. Air flow. Heating rate 10 �C/

min.

Fig. 3. Thermogravimetry data for PBT and PBT with added

PSPPP+co-additives. Inert gas flow. Heating rate 10 �C/min.

A.I. Balabanovich, J. Engelmann / Polymer Degradation and Stability 79 (2003) 85–92 87

phase flame retardant mode of action of TPP in theformulation and correlates with its fire retardant per-formance. In contrast co-addition of OP leads to adecrease in the amount of solid residue in both atmo-spheres (Figs. 2 and 3), indicating a gas phase mode ofaction of OP, which combined with the condensed fireretardant mode of PSPPP and PPO improves fire retar-dancy of PBT.

3.3. GC/MS study

In order to evaluate the influence of PSPPP on thethermal decomposition behaviour of PBT, a GC/MSstudy of the pyrolysis products of the pure additive wasfirst undertaken. A typical chromatogram is shown inFig. 4 and reports the formation of benzene, phenol,4,40-dihydroxydiphenylsulphone and diphenyl phenyl-phosphonate.The main LBPs from neat PBT are carbon dioxide,

1,3-butadiene, tetrahydrofuran and 4-ethenylcyclohex-ene (Fig. 5). According to McNeil and Bounekhel [5],

CO2 can be released after chain homolysis at the alkyl-oxygen link before the radicals initially formed abstracthydrogen. It is also possible [5] that formation of buta-diene is a homolytic process, in which cage dis-proportionation of the radicals occurs.4-ethenylcyclohexene is a [4+2] cycloaddition productof butadiene.The presence of PSPPP increases the amount of tet-

rahydrofuran generated, probably at the expense ofbutadiene and 4-ethenylcyclohexene (Fig. 6). A newproduct evolves, benzene, which results from PSPPP(Fig. 4). This implies a scission of the P–Ph bond, whichis in agreement with thermodynamic data: the energy ofthe P–C bond is lower than that of the P–O–(C) bond[6]. The formation of benzene was also observed duringthe pyrolysis of 1,2-oxaphospholane-5-one-2-phenyl-2-oxide [7]. Furthermore, scission at the P-C bond is pro-posed on thermal decomposition of 1,4-diisobutyl-2,3,5,6-tetrahydroxy-1,4-diphosphorinane 1,4-dioxide[8].A gas chromatogram of the soluble HBPs of pure

PBT is shown in Fig. 7. The peak assignments areshown in Table 3. The main pyrolysis products aremono-3-butenyl and di-3-butenyl terephthalate. Theycan also originate from a homolytic process followed bya cage disproportionation of the macroradicals [5]. Thepyrolysis of PBT containing PSPPP yields new pro-ducts: phenol, substituted phenols and a cyclic ether(Fig. 8, Table 3). The source of the first product isPSPPP (Fig. 4). Substituted phenols and the cyclic ethermay originate from PBT itself or from an interactionbetween PBT and PSPPP.In conclusion, the GC/MS data support the thermo-

gravimetric evidence that PSPPP changes the degrada-tion path of PBT as the tetrahydrofuran yield increasesin the LBPs and new products are formed in the HBPs.In addition, PBT influences PSPPP degradation as the

Fig. 4. Gas chromatogram of the HBPs of PSPPP obtained on heating

at 30–450 �C (10 �C/min). HP-1 60 m chromatographic column. 1—

benzene; 2—phenol; 3—4,40-dihydroxydiphenylsolphone; 4—diphenyl

phenylphosphonate.

Fig. 5. Gas chromatogram of the gaseous pyrolysis products of PBT

obtained on heating at 30–415 �C (10 �C/min), 415 �C (15 min). HP-5 30

m chromatographic column. 1—CO2; 2—propene; 4—1,3-butadiene; 5—

cyclobutane; 6—H20; 8—2,3-dihydrofuran; 9—tetrahydrofuran; 10—3-

butene-1-ol; 12—4-ethenylcyclohexene; 13—bicyclo[3.2.1.]oct-2-en.

Fig. 6. Gas chromatogram of the gaseous pyrolysis products of

PBT+30 wt.% PSPPP obtained on heating at 30–415 �C (10 �C/min),

415 �C (15 min). HP-5 30 m chromatographic column. 1—CO2; 3—

SO2; 4—1,3-butadiene; 5—cyclobutane; 7—furan; 9—tetra-

hydrofuran; 11—benzene; 12—4-ethenylcyclohexene; 14—ethylben-

zene.


formation of 4,400 - dihydroxydiphenylsulphone anddiphenyl phenylphosphonate is suppressed.

3.4. Infrared study

The spectrum of PSPPP (Fig. 9, spectrum a) is char-acterized by the weak �(CH aromatic) at 3098 and 3065cm�1, strong �(aromatic ring) at 1587 and 1488 cm�1,�as(SO2) at 1290 cm

�1, �s(SO2) at 1152 cm�1, �((P)–O–

Ph) at 1199 cm�1, �(P–O–(Ph)) at 922 cm�1, �(CH aro-matic) at 841 cm�1 and �(P=O) at 578 cm�1. Uponpyrolysis, the bands at 1204 and 920 cm�1 (Fig. 9,spectra b and c) become dominant indicating the accu-mulation of phosphorus in the solid residue [althoughsome amount of diphenyl phenylphosphonate con-tributes to the HBPs (Fig. 4)]. New absorption bands at2680, 2326, 1632, 513 cm�1 (spectrum c) are indicativeof appearance of the P–OH functionalities [9]. Thebands at 823 and 759 cm�1 (spectrum c) can be attrib-uted to the C–H vibrations of substituted poly-condensed aromatic ring [10].Most PSPPP absorption bands are recognizable in the

IR spectrum of the virgin formulation PBT+30 wt.%PSPPP (Fig. 10, spectrum a). In addition, the spectrumshows bands due to �((O–)CH2) at 2961, �((CH2–)CH2)at 2925 and 2854 cm�1, �(C=O) at 1716 cm�1,�((C=O)–O) at 1270 cm�1, �(O–CH2) at 1102 cm

�1,�(ring) at 1017 cm�1 and �(CH aromatic) at 728 cm�1

due to PBT.As studied by infrared (Fig. 10), the intensity of ali-

phatic functionalities decreases upon heating. Thosebonded to the methylene group are still detected in thesolid residue after the main stage of weight loss, whereasthose bonded to oxygen are likely to disappear.At 25% weight loss new absorptions are found at

2661, 2545 cm�1 and a shoulder at 1690 cm�1 which canbe attributed to aromatic acids. They disappear on fur-ther heating probably due to the evaporation of aro-matic acid derivatives.

At 25% weight loss the absorption band of �(C=O)shows a shoulder at 1745 cm�1. It increases in intensityat further heating whereas the intensity of �(C=O)decreases and disappears after the main stage of weightloss. A shift of �(C=O) to the violet is likely to becaused by an aromatic ring attached to the ester oxygen.A support for this assumption can be found at 1203cm�1 attributed to �(O–Cring). Moreover, the IR spec-trum of poly(p-hydroxybenzoate) (formula III) showsthe same absorption bands at 1740, 1200 cm�1 [11].

Fig. 7. Gas chromatogram of the HBPs of PBT obtained on heating

at 30–415 �C (10 �C/min), 415 �C (15 min). HP-5 30 m chromato-

graphic column. Peaks assignment in Table 3.

Table 3

HBPs of pyrolysis of PBT and PBT+30 wt.% PSPPP

Peak

No.

Molecular

mass (m/z)

Compound

1 94

2 148 ?

3 148

4 136

5 150

6 160

7 150

8 154

9 122

10 176

11 234

12 220

13 256 C15H31COOH

14 274

15 284 C17H35COOH


A shoulder at 1650–1630 cm�1 (Fig. 10, spectrum d) isattributable to n(C=O) of associated diarylketones [12].The broad absorption at 3250–3200 cm�1 could be

attributed to phenolic OH. The appearance of the phe-nol bands between 1260 and 1180 cm�1 is overlappedwith the strong absorption bands of PSPPP. Its presencemay be supported by the broad absorption around 700cm�1 (Fig. 10).The main absorption bands of PSPPP are still recog-

nizable after the main stage of weight loss. The sulfonyl

group is still present as detected at 1290 and 1152 cm�1.However, the degradation of the S–Cring occurs to someextent, as far as SO2 was found in the LBPs (Fig. 6). Alsophosphorus remains in the solid residue as observed at914 cm�1. It should be noted that the stretching vibrationof P–O moves from 931 to 914 cm�1 on pyrolysis.An essential feature of the pyrolysis of PBT is the

formation of anhydride structures in the solid residue[13]. The addition of PSPPP apparently changes thedegradation pathways of PBT resulting in the genera-tion of polyarylates. Their formation is not enough clearat this stage of the investigation but may involverecombination of carboxyphenylene and phenyleneradicals. In this connection the data of McNeil andBounekhel on homolytic route for the decomposition ofPBT [5] are of importance.According to McNeil and Bounekhel [5], one of the

homolysis routes results from scission at the next alkyloxygen bond as indicated in Fig. 11. PSPPP is likely topromote the cyclisation of alkylradicals to THF fol-lowed by the release of CO. This is supported by thepromoted release of THF at the expense of 1,3-buta-diene (Fig. 6). Subsequently, a recombination of the tworadicals produces polyarylates.It is known that arylates and organic carbonates are

capable of a Fries-type rearrangement induced byultraviolet irradiation [14]. In the literature [15] it isindicated that this rearrangement can also occur onpyrolysis of polycarbonates leading to the formation ofxanthone units (VI) as shown in Fig. 12. Actually, this

Fig. 10. IR spectra (a) of PBT with 30 wt.% of added PSPPP, solid

residues of the mixture (b) at 25% weight loss, (c) 60% weight loss and

(d) after the main stage of weight loss.

Fig. 8. Gas chromatogram of the HBPs of PBT+30 wt.% PSPPP

obtained on heating at 30–415 �C (10 �C/min), 415 �C (15 min). HP-5

30 m chromatographic column. Peaks assignment in Table 3.

Fig. 9. IR spectra (a) of original PSPPP, solid residues of PSPPP (b)

at 25% weight loss, (c) at 550 �C. Pellets in KBr.


scheme takes into consideration an isomerization ofpolyarylate (IV) to an aromatic ketone (V). Green [16]and Murashko [17] suggested that the isomerization ofpolycarbonates is catalyzed by aromatic sulfonate saltsand phosphorus compounds respectively. These datagive us reason to assume that the isomerization mightpartially take place on pyrolysis of polyarylates mod-ified by PSPPP as indicated in Fig. 13.The appearance of phenolic species and associated

aromatic ketones is in good agreement with this iso-merization. In the presence of PSPPP, phenolic groupsmay react with the P–O–C bond by a transesterification

mechanism, which is depicted in Fig. 14. The evolutionof phenol (Fig. 6) is in agreement with this Scheme.Finally, the formation of benzene should be discussed.

It can be generated by scission of the P–C bond fol-lowed by H-abstraction by the phenyl radical as illu-strated in Fig. 15. The formed phosphorus radical mayundergo a reaction with a phenolic hydroxyl group giv-ing rise to a new P–O–C bond. We can not find an IRspectroscopic evidence of it because of the presence ofthe P–O–C bond in the initial PSPPP. A support for therearrangement of a P–C bond to P–O–C was observed onpyrolysis of 1,4-diisobutyl-2,3,5,6-tetrahydroxy-1,4-diphos-

Fig. 12. Formation of xanthone units on the pyrolysis of polycarbonates.

Fig. 11. Formation of polyarylates on the pyrolysis of PBT modified by PSPPP.

Fig. 13. Isomerization of polyarylates.

Fig. 14. Formation of phenol on the pyrolysis of PBT modified by PSPPP.


phorinane 1,4-dioxide [8]. From a thermodynamic point ofview, formation of a P–O–C bond is favoured over a P–Cbond because of the higher bond strength.

4. Conclusion: fire retardant effect

Addition of PSPPP apparently changes the pyrolysispathways of PBT by formation of thermally stablepolyarylates and phenolic groups. PSPPP is likely toreact with the phenolic groups by its reactive bondsP–O–Ph and P–Ph which are low in thermal stability.This could be a reason for the increase in the char yieldof PBT on addition of PSPPP (Fig. 1). In connectionwith this, PPO is an effective co-additive for PSPPPbecause of producing phenolic structures upon pyrolysis[18]. The phenolic moieties generated may react withthe P–O–C of TPP and therefore this additive showsa condensed phase activity in PBT modified withPSPPP and PPO (Fig. 3). The present study showsthe usefulness of combining a condensed-phase-activephosphorus flame retardant (PSPPP) with a vapour-phase-active flame retardant (OP). By the combina-tion of these two modes of action it is possible toachieve the demanding UL94 V-0 ranking for PBTwith a moderate loading of completely halogen andantimony-free substances.

Acknowledgements

We wish to thank Dr. G.F. Levchik for her contribu-tion to this work. AIB is grateful to BASF AG for per-mission to publish these results.

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Fig. 15. Formation of benzene on the pyrolysis of PBT modified by PSPPP.


fire retardant and charring effect of poly(sulfonyldiphenylene phenylphosphonate) in poly(butylene...

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