The mechanism of fire retardancy by phosphonated polystyrene

Fire Safety Journal, 2 (1979/80) 257 - 263 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

257

The Mechanism of Fire Retardancy by Phosphonated Polystyrene

G. CAMINO*, M. BERT and A. GUYOT C N R S - Laboratoire des Matdriaux Organiques, 2, Avenue A. Einstein, 69626 Villeurbanne (France)

J. BROSSAS and G. CLOUET CNRS -- Centre de Recherches sur les Macromoldcules, 6, Rue Boussingault, 67083 Strasbourg (France)

(Received December 1, 1979)

SUMMARY

INTRODUCTION

Rates of volatilisation and styrene evolution from isothermal degradations (330 360 °C) of radical polystyrene (RPS), telechelic c h l o r o m e t h o x y p h o s p h o n a t e d polystyrene (TPPS) and their mixtures were measured by a coupled thermogravimetric (t.g.) gas-liquid chromatographic (g.l.c.) technique, and degradation products analysed. Phosphonated groups substituted at chain ends (TPPS), strongly reduce the rate of degradation of the polymer whereas the same groups are inactive when substituted on aromatic rings. The rate of volatilisation (T > 350 °C) of RPS is reduced by addition of TPPS (> 20%). At increasing temperature and a m o u n t of additive, the mixtures tend to behave like the pure additive. Analysis of degradation products shows that during volatilisation of TPPS and its mixtures with RPS, phosphorus tends to remain in the residue while chlorine is completely volatilised. The oxygen index (O.I.) of mixtures of TPPS (> 20%) and RPS increases regularly with the a m o u n t of TPPS in a similar way in 02/N 2 and N20/N2 atmospheres, showing that the flame retardancy mechanism of PPS is chiefly a solid phase action. This is confirmed by a correlation f o u n d between O.I. and the rate of volatilisation or styrene evolution from the polymers and mixtures investigated.

Polystyrene is most c o m m o n l y made fire retardant by means of low molecular weight additives [ 1 ]. Owing to their incompatibility with the polymer, they tend, however, to migrate to the surface of articles made from the mixture. The fire retardant characteristics of the material decrease, then, with time. This problem is overcome by using recently synthesised telechelic phosphonated polystyrenes which are highly effective fire retardant macromolecular additives [2, 3]. We have investigated the mechanism of flame retardancy of these compounds in order to establish whether they act in the solid or in the gas phase. Polystyrene is highly flammable because, on heating, it volatilises almost quantitatively producing a high proportion of m o n o m e r (ca. 50%) and decreasing amounts of dimer, trimer, etc .... [4]. Therefore, interactions occurring in the solid phase between polymer and additive have been studied by measuring the total rate of volatilisation and styrene formation during the thermal degradation of the additive, their mixtures with polystyrene, and by analysis of the degradation products. Gas phase reactions have been investigated by oxygen index measurements either in nitrogen/oxygen or in nitrogen/nitrous oxide atmosphere. EXPERIMENTAL Materials

*Present address: CNR -- Istituto di Chimica Macromolecolare, dell'UniversitY, V. G. Bidone 36, 10125 Torino (Italy).

Radical polystyrene (RPS)

Styrene was polymerized at 60 °C in aqueous suspension, using the system arabic g u m -

258

L y t r o n 810 (styrene copolymer, maleic anhydride, Monsanto) as suspending agent and 2,2'-azo-bis-isobutyronitrile (AIBN) as free radical initiator [ 5]. 100 g of commercial styrene and 0.25 g of AIBN were added under gentle stirring to a mixture of water (200 cm 3) and gum arabic (4 g) in a flask thermostatted at 60 °C. A few minutes later stirring was raised to 200 rpm and L y t r o n (5 cm 3 of a solution: 22.7 g of L y t r o n 810 and 12 cm 3 of 22% ammonia in 1 1 of water) introduced. After 20 h the polymer (70 g) was recovered by filtration and twice purified by dissolution in tetrahydrofuran (THF) and precipitation from methanol. The molecular weight was measured by_gel permeation_ chromatography (g.p.c.) : Mw 179 500, Mn 100 000.

Polychloromethoxyphosphonated polystyrene (PPPS) The polymer was prepared by direct metalation of polystyrene with the complex seebutyllithium (secBuLi) N, N, N',N' tetramethylenediamine (TMEDA) [6]: CH 2

-

CH

H

+ secBuLi - TMEOA by r~ac ~ior] with POCI 3

" F o l !o~,Jed

CH 2 - CH

CH30H J'~'-

CH 2

-~H

~

POCI 2

O92

~

CH 2

-BCHSLI

~

POCI3,

C1

C1

i 0

0

~

*

P

CH3CH

I -

CH

-

CH 2

--

CH 2

-

CH

-

P

-',-

Cl

CI

0CH 3 l

0CH 3 I

P

-

04

'O

Cl

-

CM?

~

CH 2

-

CH

-

®'

P

~

CH 2

-

CH

P -~ 0

OCH3

The polymer was prepared by quenching with POC13, the living polymerisation of styrene being initiated by lithium naphthalene in THF at --40 °C and the polymer obtained by reacting with methanol [3] : ~)@CH-

~

i

Telechelic chloromethoxyphosphonated polystyrene (TPPS)

Li @

(2)

POCI 3 LI-TMEOA 4 secBuH ~

,

(1)

-~ 0

CI

F r o m the elemental analysis (C, 90.79; H, 7.65; P, 0.49; CI, 0.67%) and molecular weight (Mn 8 000, g.p.e.) the average number of P atoms per polymer molecule can be calculated at 1.27. This means that reaction (I) did not occur quantitatively. The polymer is most likely a mixture of macromolecules with one, two, or no phosphonated end groups; the exact distribution was not investigated.

Anionic polystyrene (APS) By quenching with methanol a fraction of the carbanionic solution in which living polymerisation of styrene was carried o u t to prepare the telechelic phosphonated polymer, a polystyrene was obtained with the same molecular weight as the phosphonated polymer.

All reactions were carried out under argon. 10 g of polystyrene (Mn 6 500) prepared by anoinic mechanism in benzene at 35 °C using sec BuLi as initiator, were dissolved in 600 cm 3 of oxygen-free cyclohexame in a 2 1 flask. 19.2 × 10 -3 mole of sec BuLi and 13.4 × 10 -3 mole of TMEDA were added and the reaction mixture stirred for 3 h at room temperature. The multicarbanionic solution was then quenched by slow addition to a THF (600 cm 3) solution of POC13 (0.4 mole) in a flask cooled at --80 °C. The solution obtained was poured into cold methanol (--40 °C). The precipitated polymer was filtered and freeze-dried from a benzene solution. The molecular weight of the original polymer is not affected by the treatment. It has been found that by this m e t h o d phosphonation occurs almost exclusively on the meta (66%) and para (33%) positions of aromatic rings [ 7]. Elemental analysis: C, 85.96; H, 7.50; P, 1.66; C1, 2.26%.

Mixtures of radical polystyrene with telechelic chloromethoxyphosphonated polystyrene Mixtures containing 10(M10), 20(M20), 35(M35), 50(M50), 70(M70) per cent.w/w of phosphonated polymer were prepared by freeze-drying benzene solutions of both polymers, and dried at 50 °C under vacuum.

259

THERMAL DEGRADATION TECHNIQUE (t.g.g.l.c.)

Thermal degradations were carried out under nitrogen (60 cm 3/min) in a Ugine-Eyraud B60 thermobalance. Samples of the gas stream from the balance were injected in an Intersmat IGC 15 gas chromatograph by means of a slider type gas sampling valve (Fig.

1). A closed system was built at the exit of the thermobalance with a glass tube (1.30 cm), cooled by a water jacket, connected at one end to the quartz tube of the balance by a ground-glass socket, and to a stainless steel tube by a Sovirel fitting at the other end. A Swagelok fitting connects the steel tube to a teflon tubing of the valve (B, Fig. 1). Every ten minutes, a pneumatically driven lever, automatically operated by a timer, switches the slider from normal to injecting position (Fig. 1). The gaseous degradation products trapped in the sample loop (2 cm a) are swept by the carrier gas into the column of the gas chromatograph. The weight of the sample (100 mg) and its temperature, monitored by a thermocouple placed near the sample boat, were recorded as a function of time in isothermal or programmed heating (1 °C/min) experiments.

boiling products volatile at degradation temperature but condensable at room temperature. Owing to the dynamic conditions and complexity of the gaseous mixture evolved by degrading polystyrene [4], quantitative fractionation was possible only for styrene and lower boiling products (benzene, toluene, ethylbenzene) which are not condensed. Dimers, trimers- etc .... of styrene were found mostly in the condensed products or, to a lesser extent, in the gaseous mixture conveyed to the gas chromatograph.

High boiling fraction (HBF). Condensed products were recovered by washing the tubes connecting the balance to the gas sampling valve with chloroform and evaporating the solution obtained under nitrogen at room temperature. Infrared spectra were measured on films deposited on an NaC1 disc by evaporation of chloroform solutions. Gaseous fraction (GF). Some degradations were carried out under helium (60 cm3/min) at I °C/min up to 500 °C, and the watercooled gas mixture, condensed in a trap, allowed to warm up to room temperature and the liquid obtained analysed by gas chromatography-mass spectroscopy (g.c.-m.s., mass spectrometer Ribermag GC/MS R10-10, 70 eV).

ANALYSIS OF DEGRADATION PRODUCTS

Water cooling of the gas stream from the thermobaiance causes condensation of higher 1

3

:P0sToN 2

4 3

•

2

4

Fig. 1. L a y o u t o f the gas sampling valve. 1, CG, carrier gas; 2, GLC, gas-liquid chromatograph; 3, B, t i e r -

mobalance; 4, V, vent; 5, SL, sample loop; 6, L, pneumatically driven lever.

Residue (R). Residues at increasing degree of degradation were analysed by infrared as KBr pellets: occasionally the soluble fraction extracted by toluene was analysed as film evaporated on an NaC1 disc. Some residues of the order of a gram were prepared by degradation of larger samples (2 - 3 g) in a Heraeus furnace under nitrogen flow (90 cm3/ min). Determination o f styrene evolved. The instantaneous rate of styrene evolution from the degrading polymer is proportional to the surface of the peak of styrene obtained in t.g.-g.l.c, experiments. A calibration curve, peak area vs. rate of evolution was determined by isothermally (35 - 110 °C) evaporating styrene in the thermobalance at constant rates measured by weight loss. The total a m o u n t of styrene evolved during degradation of the polymers was calculated from the area below the curve obtained by

260

plotting instantaneous rates of evolution vs. time of degradation.

O X Y G E N I N D E X (O.I.)

Measurements were carried out on a Stanton-Redcroft instrument either in oxygen/ nitrogen or nitrous oxide/nitrogen atmosphere. We have used a modified technique in order to avoid dripping of the sample during combustion which, in the case of polystyrene, could lead to erroneous conclusions on the fire retardant efficiency of additives [ 8, 9]. The sample was placed in the glass cup of Fig. 2 whose stem was fitted to the sample holder of the instrument. Owing to the limited a m o u n t of the samples to be characterised, only about 200 mg/g of product was introduced in the cup. The O.I. was taken as the concentration of oxygen in the atmosphere surrounding the sample which sustains its combustion for 30 s after ignition with an external flame. This value, named by us oxygen index (O.I.), n o t to be confused with standard limiting oxygen index (L.O.I.), allowed comparisons good enough for the purpose of this work, although it is less precise (-+ 5%) than LOI (+ 1%). With larger samples in a larger cup the precision of the measurement could be increased.

RESULTS AND DISCUSSION

Thermal degradation of telechelic chloromethoxyphosphonated polystyrene The TG curves for complete degradation of TPPS and the unmodified polymer (APS) are compared in Fig. 3. Both polymers volatilise almost completely (ca. 95% between 250 and 430 °C. The introduction of the phosphonated groups at chain ends increases, however, the

thermal stability of the polymer, APS reaches 50% volatilisation at 380 °C, TPPS at 400 °C. The major degradation products {monomer, dimer, trimer, etc...) are identical in both polymers as shown by g.c.-m.s, analysis of the gas fraction (GF) as well as g.p.c, and infrared spectra of the high boiling fraction (HBF). In the case of PPS a trace a m o u n t of phosphorus-containing products was detected in the GF.

Rates of degradation Initial rates of total volatilisation and styrene evolution from RPS, TPPS, their mixtures, and from APS and PPPS were measured at 3 3 0 , 3 4 5 and 360 °C. The results are reported in Table I as weight per cent. for the original sample volatilised per minute. Over this range of temperature, rates are constant up to 50% degradation. A 10 - 15% volatilisation is observed during the time required by the thermobalance to reach thermal equilibrium after introduction of the sample. The values reported in Table 1 refer to the constant rates observed between 15 and 50% degradation. Precision in total volatilisation rate measurements was estimated to be about + 5% and + 15% in styrene evolution rates. The data of Table 1 show that at 330 °C TPPS volatilises at about half the rate of APS, whereas PPPS volatilises at the same rate. This indicates that phosphonated groups at chain ends are able to decrease the rate of degradation of polystyrene whereas the same groups on the aromatic rings are inactive.

Radical polystyrene (RPS) This is similar to commercial polystyrene and volatilises at a rate fifty per cent. higher Z

o= 50

0 30 m m r . . . .

! I~:

___~4mm

/\

~n

'llllLILl'JlllLIIlIJiLILLIJ,I, 150

200

250

300

i ~J,L!~l~l

550

400

450

TEMPERATURE ('C)

Fig. 2. Sample holder for m o d i f i e d o x y g e n index measurements.

Fig. 3. Thermogravimetry of APS (solid line) and TPPS (broken line). Heating rate: 1 °C/min, atmosphere: nitrogen, 60 cm3/min, sample size, 100 rag.

261 TABLE 1 Rates of volatilisation and styrene evolution in isothermal degradation Product

Rate of volatilisation (%/min) 330 °C E*

RPS APS TPPS M10 M20 M35 M50 M70 PPPS

345 °C C**

0.28 0.17 0.10 0.24 0.18 0.17 0.12 0.18

Rate of styrene evolution (%/min)

E

360 °C C

E

0.65

0.24 0.22 0.19 0.15

330 °C C

E

2.10

C

0.13 0.090 0.037

0.38 0.66 0.63

0.63 0.60

1.20 1.70 1.23

2.02 1.92

0.34

0.52

1.30

1.65

* : Experimental. **: Calculated.

345 °C E

360 °C C

0.43

0.120 0.096 0.064 0.10

0.097 0.085 0.065

E

C

0.82

0.23 0.41 0.37

0.41 0.39

0.40 0.65 0.60

0.78 0.74

0.19

0.33

0.40

0.61

TABLE 2 Elemental analysis of residues of isothermal degradations at 360 °C

2

Product TPPS M50

~0.5

330

,j

j/

1--

340 350 TEMPERATURE ('C)

360

Fig. 4. Rates of volatilisation and styrene evolution from isothermal degradations of RPS (©), TPPS (A), M10 (o) and M50 (A). Experimental (), calculated for M10 (- - - ), calculated for M50 ( . . . . ). t h a n t h a t o f anionic p o l y s t y r e n e (APS). This is a well k n o w n p h e n o m e n o n , p r o b a b l y t o be ascribed t o a higher c o n t e n t o f w e a k s t r u c t u r a l irregularities in radical p o l y s t y r e n e [ 1 0 ] , b u t it is still u n d e r investigation [11, 1 2 ] . A t l o w e r t e m p e r a t u r e ( 3 3 0 °C), m i x t u r e s o f TPPS a n d RPS volatilise at the e x p e c t e d rate (C values, Table 1), w h i c h can be calculated f r o m a linear c o m b i n a t i o n o f the degrad a t i o n rate o f t h e t w o p o l y m e r s . A t h i g h e r t e m p e r a t u r e ( 3 6 0 °C), h o w e v e r , t h e rate o f

Volatilisation (%)

C

H

P

(%)

(%)

(%)

C1 (%)

57 86 74

90.79 91.40 89.06 91.30

7.65 7.66 7.40 7.62

0.49 0.85 2.07 0.64

0.67 trace trace trace

volatilisation o f m i x t u r e s is l o w e r t h a n calcul a t e d (Fig. 4). A t increasing c o n t e n t o f TPPS (M50, Fig. 4) m i x t u r e s t e n d to behave as the p u r e p h o s p h o n a t e d p o l y m e r , even at l o w e r temperature. These results s h o w t h a t the t h e r m a l degrad a t i o n o f p o l y s t y r e n e is altered by the presence o f the p h o s p h o n a t e d additive TPPS.

Elemental analysis of degradation products D a t a referring t o residues (R) o f i s o t h e r m a l d e g r a d a t i o n s at 3 6 0 °C are r e p o r t e d in Table 2. I t is seen t h a t at 57% volatilisation o f TPPS, a b o u t 75% o f t h e original p h o s p h o r u s is still in t h e residue whereas c h l o r i n e is c o m p l e t e l y volatilised. W h e n volatilisation is a l m o s t c o m plete (87%), 60% o f p h o s p h o r u s r e m a i n s unvolatilised. These results indicate t h a t phosp h o r u s t e n d s t o a c c u m u l a t e in the residue d u r i n g t h e r m a l d e g r a d a t i o n o f TPPS whereas chlorine is readily volatilised. T h e same b e h a v i o u r is f o u n d in the case o f m i x t u r e s . F o r e x a m p l e , at 74% volatilisation, 70% o f the original p h o s p h o r u s is p r e s e n t in the residue o f M 5 0 d e g r a d a t i o n .

262 TABLE 3 Oxygen indices

sop

Product

O2/N 2

N20/N 2

RPS APS TPPS M10 M20 M35 M50 M70 PPPS

21 28 46 19 19 26 30 38 22

48

z

8O 48 48 54 59 70 -

30--

'~

loi L _ _ 01

J

~__

02

03

_

RATE OF VOL,~TILISATION, % /MIN

Fig. 6. Oxygen indices as a f u n c t i o n of rate of isothermal volatilisation at 330 °C. RPS (©), TPPS (e), M (*) and APS (m).

80~

× 50f 5C

• _e/

: o

50 20

\ o\

o

i

10

o --o"

[

~o Li 01

i

!

30~ I

02

03

04

~_l,lil,

RPS MIOM20 M35 M50

M70

05

06

ii

i

P,

TPPS

Fig. 5. O x y g e n indices in O2/N 2 and N 2 0 / N 2 of RMS, TPPS and mixtures (©), O2/N2; (0), N 2 0 / N 2 .

Oxygen index results In Table 3 are listed the results of O.I. measurements in O2/N 2 and N20/N2. These data show that phosphonated end groups increase the O.I. of polystyrene by fifty per cent. (APS 28, TPPS 46), whereas the same groups substituted on aromatic rings (PPPS) have a rather adverse effect. Above a minimum level (> 20%), addition of TPPS to RPS regularly increases the O.I. of mixtures with the a m o u n t of fire retardant additive {Fig. 5). Since this effect is independent of the nature of the oxidising gas, it is most likely that the contribution to the fire retardancy properties of the additive by flame inhibition in the gas phase is negligible [8, 1]. On the other hand, of the two atoms (P, C1) which are known to be able to play such an inhibiting role in oxidation reactions taking place in flames, only chlorine showed a marked tendency to volatilise during thermal degradation of the additive or mixtures. Owing

01 02 RATE OF STYRENE EVOLUTION "/o/MIN

Fig. 7. Oxygen indices as a f u n c t i o n of rate of styrene e v o l u t i o n from isothermal degradations at 330 °C. RPS (©), TPPS (o), M (*) and APS (I).

to its low concentration, the effect of chlorine could be overwhelmed by the interactions occurring in the solid phase. The correlations shown in Figs. 6 and 7 between O.I. and the rate of volatilisation or the rate of styrene evolution at 330 °C from RPS, APS, TPPS, PPPS and mixtures RPS-TPPS, confirm the importance of thermal degradation in determining the combustion behaviour of such products. A similar correlation was n o t found with rates of degradation measured at higher temperature. For example, TPPS and the mixture M50 volatilise at the same rate at 360 °C but the O.I. of PPS is 50% higher than that of M50. We believe that this is related to the m e t h o d by which the O.I. was measured and, in particular, with the temperature at which the sample was heated by the ignition flame. It is, however, most likely that the strong reduction in the rate of volatilisation of RPS by the addition of TPPS implies an equivalent

263

strong increase in its fire resistance when the material is heated at high temperature before being reached by flames, as may occur in a real fire.

CONCLUSIONS

The c h l o r o m e t h o x y p h o s p h o n a t e d group considerably increases the fire resistance properties of polystyrene by reducing the rate of thermal volatilisation of the polymer. This occurs when the group is a chain end substituent, but n o t when it is substituted directly on the aromatic rings. As a fire retardant additive for commercial polystyrene, the chain end phosphonated polymer is very efficient since it can interfere with the thermal degradation process of polystyrene by reducing its rate of volatilisation. At increasing temperature or additive concentration, the mixture tends to behave as the pure additive (synergistic effect). It is now well established t h a t the mechanism of fire retardance of the additive is a solid phase mechanism. Work is in progress in order to investigate in detail the mechanism of thermal degradation of the additive and its mixtures with polystyrene.

REFERENCES 1 R. F. Lindemann, in W. C. Kuryla and A. J. Papa (eds.), Flame Retardancy o f Polymeric Materials, Dekker, New York, 1973. 2 G. Clouet and M. Riou, Fr. Patent, demande 76.30. 002 (1976), Rh6ne-Poulenc. 3 G. Clouet and J. Brossas, ler Congr~s International des Composds phosphords, Rabat, 1 7 - 21 Oct. 1977, Preprints p. 241. J. Brossas and G. Clouet, C.R. Adac. Sci., 280, c, (1975) 1459. 4 S. L. Madorsky, Thermal Degradation of Organic Polymers, Interscience, New York, 1964. 5 H. Jacobelli, M. Bartholin and A. Guyot, J. Appl. Polym. Sci., 23 (1979) 1. 6 A. W. Langer, Trans. Acad. Sci. (NY), 27 (1965) 741. 7 G. Clouet and J. Brossas, Makromol. Chem., to be published. 8 C. P. Fenimore, in M. Lewin, S. M. Atlan and E. M. Pearce (eds.), Flame Retardant Polymeric Ma ~ terials, Plenum Press, New York, 1975. 9 E. D. Weil, inW. C. Kuryla and A. J. Papa (eds.), Flame Retardaney of Polymeric Materials, Dekker, New York, 1973. 10 G. G. Cameron and G. P. Kerr, Eur. Polym. J., 4 (1968) 709; 6 (1970) 423. 11 L. A. Wall, S. Straus and R. E. Florin, J. Res. Nat. Bur. Std. (U.S.), A, Phys. and Chem., 77A (1) Jam-Feb., (1973) 157. 12 G. G. Cameron, J. M. Meyer and J. T. McWatter, Macromolecules, 11 (1978)696. 13 P. C. Warren, in W. L. Hawkins (ed.), Polymer Stabilization, Wiley-Interscience, New York, 1972.