molybdenum oxidic systems as flame retardants and smoke suppressants for rigid PVC

Polymer Degradation and Stability 14 (1986) 307-318

A l l9mSn M6ssbauer Study of Tin/Molybdenum Oxidic Systems as Flame Retardants and Smoke Suppressants for Rigid PVC Paul A. Cusack, Peter J. Smith International Tin Research Institute, Perivale, Middlesex, Great Britain

& William J. Kroenke B. F. Goodrich Research and Development Center, Brecksville, Ohio, USA

(Received: 22 October, 1985)

ABSTRACT Tin(IV) oxide, either alone or in combination with melam&ium betaoctamolybdate or molybdenum(V1) oxide, has been jound to be an effective flame retardant/smoke suppressant jbr rigid PVC. llgmSn M6ssbauer spectroscopy has been used to stud), the chemical changes undergone by the tin species during." (a) thermal degradation and (b) combustion. Under these conditions, the SnO 2 is partially reduced to tin(II) species (SnCl 2 and SnO) and metallic fl-Sn. Elemental analysis of the char residues indicates that a proportion of the tin and molybdenum is lost from the polymer (possibly as volatile metallic chlorides or oxychlorides) and it is suggested that these metals can operate as fire retardants in both the condensed and vapour phases.

INTRODUCTION Poly(vinyl chloride) is one of the most important synthetic polymers in terms of its industrial usage, accounting for greater than 25 % of the total 307

Polymer Degradation and Stability 0141-3910/86/$03.50 (~ Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

308

Paul A. Cusack, Peter J. Smith, William J. Kroenke

thermoplastic market. 1 Because of its versatility, PVC finds use in a wide range of applications, 2 and there is currently much concern about its flammability and smoke production on burning) Although PVC is intrinsically flame retardant, large quantities of smoke and toxic gases are evolved when it is forced to burn. 4 In addition, improved flame retardancy of PVC is desirable for applications such as aircraft interiors. 5 Consequently, there is much interest in developing novel flame-retardant systems which incorporate smoke-suppressant properties. Tin chemicals have been shown, in laboratory studies, to be effective as flame retardants and/or smoke suppressants in a number of synthetic polymers, 6 including polyolefins, polystyrene, polyamides, polyesters and acrylonitrile-butadiene-styrene (ABS) plastic. Recent work by one of the authors 5'%8 has indicated that certain inorganic tin compounds exhibit smoke-suppressant behaviour in PVC and, when used in combination with melaminium beta-octamolybdate (K¢) or molybdenum(VI) oxide, they show a synergistic reduction in smoke evolution. In this paper, the results of a 119mSn M6ssbauer investigation on rigid PVC samples containing 2.5-10wt.% SnO 2, either alone or in combination with the above molybdenum compounds--both before and after thermal degradation or combustion--are reported. It is of interest to note that di- and mono-alkyltin(IV) compounds find extensive use as heat and light stabilisers for rigid PVC 9 but, in the current study, a non-tin-containing stabiliser system is employed to avoid unnecessary complications in interpreting the M6ssbauer spectra.

EXPERIMENTAL

Incorporation of tin and molybdenum compounds into PVC The model rigid PVC compound which is used throughout this study is a hundred parts Geon 103EP resin, two parts Microthene 510 polyethylene, two parts C-16 aminocrotonate and two parts Epon 828 epoxy resin. The Geon resin is a homopolymer with an inherent viscosity of 0.98-1.04 and an ASTM classification of GP-5-1443. 8 The Microthene 510 is a processing aid, whilst the organic stabiliser system consists of the beta-aminocrotonate and epoxy resin. The tin and molybdenum compounds were incorporated into the polymer at levels of 0-10% by

119,.Sn M6ssbauer study of Sn/Mo fire retardantsfor rigid PVC

309

weight of PVC, by milling at about 160 °C on a rolling rubber mill. Sheets of appropriate thickness were moulded under pressure at about 165 °C.

Determination of flammability and smoke evolution Flammability of the samples, in the form of thin strips of approximate dimensions of 150 mm x 7 m m x 3 mm, was determined by measurement of their limiting oxygen indices (LOI's), according to ASTM D2863. Smoke evolution was assessed by means of the NBS smoke chamber test. PVC samples measuring 73 mm x 73 mm x 0.6 mm were burned in the flaming mode in accordance with ASTM E662-79 and the NBS smoke number is expressed as Dm g-~ of sample burnt.

Thermal degradation studies These were carried out by heating c a . I g samples of the plastic in air for 60min at temperatures between 150°C and 350°C.

119mSn M6ssbauer investigations Samples for M6ssbauer studies were prepared by cutting small discs from the PVC sheets. The spectra were recorded using a constant acceleration microprocessor spectrometer, with a 512-channel data store. A 10 mCi Ca 119mSnO3 source was used at room temperature and the samples were cooled to 80K, using a liquid nitrogen cryostat. The M6ssbauer parameters were obtained from computer least-squares fits to the spectra, using Lorentzian line shapes. ~° 'Thin' M6ssbauer samples were used throughout to reduce saturation effects and to ensure justification in assuming proportionality between measured line areas and M6ssbauer thickness. ~

Elemental analyses Tin The weighed sample was 'ashed' at 900 °C in a muffle furnace and the residue was transferred to a Pyrex glass tube, mixed with a m m o n i u m iodide and heated. The sublimed SnI 4 was dissolved in 2M hydrochloric acid and the tin determined by atomic absorption spectrophotometry. ~2

310

Paul A. Cusack, Peter J. Smith, William J. Kroenke

Molybdenum The weighed sample was 'ashed' as described above, transferred to a nickel crucible and fused with sodium peroxide. The fusion product was dissolved in water and acidified with hydrochloric acid. Molybdenum was determined photometrically on a suitable aliquot as the thiocyanate complex, stabilised with diethyleneglycol monobutyl ether (butyldigol). 13 Since MoO 3 begins to sublime in the neighbourhood of its melting point, control experiments were run to establish that, in the presence of PVC, no molybdenum was lost during ashing at 900 °C. Apparently, the presence of the PVC acts to reduce the loss of Mo as M o O 3. This is consistent with the fate of MoO 3 in PVC combustion chars. One of the authors has shown ~4 that the molybdenum ends up as a mixture of MozC and MoO 2, n o t M o O 3.

Chlorine Analytical data for chlorine were obtained by the Microanalytical Laboratory, University College, London.

RESULTS AND DISCUSSION Flammability results and smoke emission data for the PVC samples are given in Table 1. Anhydrous tin(IV) oxide, melaminium betaoctamolybdate (K¢) and molybdenum(VI) oxide all appear to be effective flame retardants and smoke suppressants for PVC at a total additive loading of 5wt. ~ , as shown by the substantial increases in LOI and reductions in visible smoke, compared with the PVC containing no additives. In general, the incorporation of these additives at a higher level (i.e. 10wt. ~o) does not result in a significant elevation in LOI values, although a noticeable further decrease in smoke production is observed when the higher levels are employed (Table 1). In these systems, SnO 2 appears to be the better flame retardant (giving larger increases in LOI) and the molybdenum compounds are more effective in reducing smoke formation. In line with these findings, there have been several reports of the effectiveness of SnO 2 as a flame-re,tardant synergist with halogen, both in PVC 15 and other thermoplastic materials, 6,16. and of the use of molybdenum compounds (particularly MOO3) as smoke suppressants.7,8,17- 22

119rnSn M6ssbauer study of Sn/Mo fire retardants for rigid PVC

311

TABLE 1 Flammability and Smoke Evolution Data for PVC Samples

Additives ( °//o)a

Limiting o.vygen

Smoke evolution c

index ~ Sn02

Kcb

MoO3

--

Dm g - 1

Per cent reduction

--

--

39.9

67.9

5

--

62,5

40. l

40.9

I0

--

62.1

38.3

43.6

--

10

5 -

55.6

32-2

52.6

55.0

24-3

64,2

--

5

56-4

31,3

53,9

--

10

56-4

28.2

58.5 56.7

2.5

2.5

--

55-6

29.4

2.5

--

2.5

56.9

32-4

52.3

5

5

58.1

25-5

62.4

5

--

56' 1

34.7

48.9

5

Based on the concentration of PVC in the compound. b Melaminium beta-octamolybdate. c See 'Experimental' section.

The high chlorine content in PVC means that the lack of flameretardant synergism between the tin and molybdenum additives cannot be attributed to low halogen: metal ratios, as has been suggested in other systems, z3 Instead, it appears most likely that the reduction in smoke and the increase in LOI promoted by the Sn and Mo additives, result predominantly from their action in the condensed phase to catalytically change the normal thermal degradation pattern of the PVC. 8 Recent model compound studies have shown how MoO 3 and Cu20 act to convert the degrading PVC chains into a thermally stable char rather than highly combustible volatile aromatic pyrolysates, z4'25 SnO 2 presumably functions in a similar manner, although it is possible that some volatile tin chloro- species are formed. If so, this could account for SnO 2 being more effective than the molybdenum compounds in raising the LOI of the PVC.

Interestingly, the weak, but statistically significant, smoke-reducing synergism, previously observed for the 5 ~ equal weight SnO2-MoO 3 system in a simple organotin-stabilised PVC compound, s is not found in this study. Perhaps this reflects the impact of the all-organic stabiliser

Paul A. Cusack, Peter J. Smith, William J. Kroenke

312

TABLE

2

l x9mSn M6ssbauer Data for PVC Samples

Sample

Isomer shift (3) (rams-l) a'b

Full width at half height (F) (mm s- 1)b

Relative concentration of the components present

(%) (i) Unburnt polymer 5 ~o SnOz c 10~o SnO 2 2.5 ~o SnO/, 2.5 ~o Kca 2.5 ~o SnO2, 2.5 % MoO 3 5 ~ SnO 2, 5% K~ 5 % SnO2, 5 ~o MoO3

-0.01 -0.02 - 0.06 -0.01 0.00

1.70 2.20 1.57 1.56 1.75 1.75

5 % SnO 2

0.04 4.18 2-69

1.39 1.86 1.22

65 28 7

5% SnO 2, 5~o Kc

0-09 4-20 2.81

1.63 1.20 1.25

80 11 9

5%SnO 2,5%MOO 3

0.05 4.24 2.76

1.38 1.81 0.95

79 17 4

5~o SnO2

0.06 2.62

1.31 1.12

74 26

5% SnO2, 5% K c

0.05

1.47

100

5% SnO 2, 5~o MoO 3

0.12 4.19

1.46 0.75

95 5

-0.03 2.75 2.60 4.13

2.88 2.21 2.01 2-37

- 0.02

100 100 100

100 100

100

(ii) 350°C residue

(iii) Residue from LOI test

(iv) Pure compound SnO/ SnOe /#Sn SnC12

w

m

m

Relative to CaSnO 3. b Experimental error in isomer shift and full width at half height is +0.05 mm s -1. c Based on the concentration of PVC in the compound. a Melaminium beta-octamolybdate. e SnO gives a resonance with a small quadrupole splitting (AEo= 1.59mms-1). The quoted value of F is the average of the two linewidths observed.

119"Sn M6ssbauer study of Sn/Mo fire retardants for rigid PVC

313

system. There is some evidence of a weak smoke-suppressant synergism in the SnO2-K c formulations, where PVC containing 2.5 ~o of each additive shows a slightly lower smoke rating than the polymer containing 5 ~o of either SDO 2 or Kc alone. However, this effect is not observed at the higher total additive level of 10 ~o (Table 1). The measured l l9mSn M6ssbauer parameters for the PVC samples, both before and after thermal degradation (at 350 °C) or combustion in the LOI test, and for some relevant pure tin compounds, are given in Table 2. The isomer shift values for the unburnt samples are indicative of an octahedral oxidic tin(IV) site and, within experimental error, these do not differ significantly from the value for pure anhydrous SnO 2 itself. However, an interesting linewidth effect is observed for these samples: a decreasing SnO 2 concentration results in a reduction in the measured full width at half height (Table 2). This effect has been previously reported for SnO 2 deposits in cotton fabric, 26 and for thin SnO 2 films on glass. 27 It is not possible to differentiate between anhydrous tin(IV) oxide and an anhydrous 'tin(IV) molybdate', on the basis of this data alone, since the tin atom environments in these compounds are likely to be very similar. In line with this, recent studies of the SnO2/MoO 3 system at elevated temperatures, using M6ssbauer spectroscopy, X-ray diffraction and electron spin resonance, suggest that the phases formed correspond most closely to solid solutions of molybdenum(VI) in tin(IV) oxide, rather than stoichiometric tin(IV) molybdates. 2s The M6ssbauer spectra of SnO2-containing PVC samples, with or without K~ or M o O 3 , heated at temperatures between 150 °C and 300 °C, show little change from those of the corresponding initial polymer. Apparently, there is no significant interaction between S n O 2 and the PVC at these temperatures. However, the samples heated at 350°C show, in each case, two other resonances in their M6ssbauer spectra (Table 2 and Fig. 1), indicating that the SnO 2 has partially undergone chemical reaction, yielding at least two other tin species. The dominant peak is still from SnOz, whilst the additional peaks are assigned to SnC12 (major reaction product) and fl-Sn (metallic tin) and/or SnO, on the basis of their isomer shifts. Although pure SnO gives a M6ssbauer resonance with a small quadrupole splitting, it is not possible to differentiate between this species and fl-Sn (which gives a single resonance), at the low levels present in the PVC matrix. However, powder X-ray diffraction studies o n S n O 2c o n t a i n i n g PVC combustion residues have indicated that both elemental tin and tin(II) oxide are present, with Sn metal being the dominant

314

Paul A. Cusack, Peter J. Smith, William J. Kroenke •

+

*+<+~, :

• :

...

.+. •

::~:.++'+

:'+

II-III flip ImO

m t+

+.

: : +<+~<.

• • :+,."" •

+

"-:

,r<~ .

"

In¢l I

Q

~c

llmO|

i, I0

Fig. I.

'.

*I0

I

O0

'.

10 ¥1LOClTT

'

|.0 (III/I)

•

4 .' 0

I I'0

119mSnM6ssbauer spectrum of PVC containing 5 ~ SnO 2 and 5 ~o melaminium beta-octamolybdate, which has been thermally degraded at 350°C in air.

species. 29 As expected, there is no evidence of any anhydrous tin(IV) chloride, SnC14 (6 = 0 . 7 8 m m s - 1 ) 3° or tin(IV) oxychloride, SnOC12 (6 = 0.25 mms-1), a° in any of the 350°C residues. At this temperature these compounds would be expected to volatilise. 31,32 SnC12, however, is observed in the residues, and, in line with this, is known to have a relatively high boiling point (652 °C). 31 The ltamsn M6ssbauer spectra of the char residues from the LOI test indicate that the major tin compound present is SnO 2. In the char from the PVC containing SnO 2 only, i.e. no Mo additives, a second tin species is observed, and this js likely to be either fl-Sn or SnO (or a mixture of the two). Neither fl-Sn nor SnO is observed in the spectra of the Sn/Mocontaining chars, although a small amount of SnC12 is evident in the SnOE/MOO 3 PVC residue (Table 2). Elemental analyses (C1, Sn and Mo) for the PVC samples, before and after thermal degradation or combustion, are presented in Table 3. In each case, heating to 350 °C results in a loss of the majority of the chlorine from the polymer. The chlorine which remains may exist either as relatively involatile metallic chlorides or oxychlorides, or as rechlorinated partially hydrogenated segments of the char residues. In line with this presumption, M6ssbauer spectra confirm the presence of SnC12 in the

t19"Sn M6ssbauer study of Sn/Mo fire retardants for rigid PVC

315

TABLE 3 Analytical D a t a for P V C S a m p l e s

Sample

Elemental analyses (o/o)

Tin species present a

CI

Sn

Mo

52.4 50.2 47.8 47.8

-3.8 3.9 3-8

2.4 3.4

-SnO 2 SnO 2 SnO 2

2.4 4.3 6.1 3.6

~3.7 3.8 3.7

2.5 2.4

S n O 2, SnC1 z, fl-Sn, S n O S n O 2, SnC12, fl-Sn, S n O S n O 2. SnCI2,/~-Sn, S n O

0 0-5 2-6 3.4

. . . . 2.4 2.5 2.3 2.7 2.4

(i) Unburnt polymer N o additives 5~o SnO2 b 5 °/o S n O 2, 5 ~/o K~ c 5 o~ SnO2 ' 5 ~,~/oM o O 3

(ii) 350°C residue N o additives 5°~ S n O 2 5°~ SnO2, 50/o K~ 5 .... S n O 2, 5 ....o M ° O 3

(iii) Residue jrom LOI test N o additives 5 '~, S n O 2 5 ,°o S n O 2, 5 °/jo K¢ 5 o~ SnO2 ' 5 % M o O 3

S n O 2, 13-Sn, S n O SnO 2 S n O 2, SnCI 2 (trace)

a Identified by 119mSn M 6 s s b a u e r s p e c t r o s c o p y (Table 2). h Based o n the c o n c e n t r a t i o n o f P V C in the c o m p o u n d . c Melaminium beta-octamolybdate.

chars, and the metal-containing residues show the largest concentrations of chlorine. Analysis of the residues from the LOI test for PVC containing no additives or SnO2 alone shows an almost complete loss of chlorine (Table 3) and, accordingly, the M6ssbauer spectrum of the latter material contains no peaks due to tin chloro-species. A small amount of chlorine is retained in the combustion chars from each of the Sn/Mo-containing PVC samples, presumably mainly as relatively involatile molybdenum chlorides or oxychlorides, or attached to carbonaceous fragments as previously discussed. A trace quantity of SnC12 is observed in the M6ssbauer spectrum of the char residue from the SnO2/MoO 3containing PVC. Since thermal degradation (at 350°C) or combustion of the PVC samples results in a weight loss, it would be expected that, if the metals are wholly retained in the residues, their concentrations in these chars would

316

Paul A. Cusack, Peter J. Smith, William J. Kroenke

be significantly higher than in the corresponding unburnt polymers. This is clearly not the case (Table 3) and it may therefore be inferred that, in the thermal degradation experiments, where the temperature involved is considerably lower than in the analytical 'ashing' procedure (see 'Experimental' section), partial volatisation of the tin and molybdenum, as chlorides or oxychlorides, has occurred. In support of this idea, it has been found that both SnO 2 and MOO3, when heated with an excess of a chlorinated paraffin wax, are partially volatilised. 33 Furthermore, studies of the vapour phase species formed by heating a mixture of PVC and MoO 3, using a mass spectroscopic technique, have indicated that MoO2C12 or MoO2C12 . H 2 0 is present in the vapour. 34 However, in the LOI test, there is the additional possibility that the tin and molybdenum may be lost from the plastic as finely divided metal oxide particles, which are carried by convection currents in the flowing gas mixture.

CONCLUSIONS It has been shown that tin(IV) oxide, either alone or in combination with melaminium beta-octamolybdate or molybdenum(VI) oxide, is an effective flame retardant/smoke suppressant for rigid PVC, at total additive levels of 5-10wt. ~ . 119mSn M6ssbauer studies of char residues indicate that the strongly reducing conditions prevalent in the thermally degrading PVC matrix during combustion a5 give rise to a partial reduction of the S n O 2 t o tin(II) species (SnC12 and SnO) and metallic/3-Sn. The majority of the tin and molybdenum in the PVC is retained in the char residue, indicating that these metals act predominantly in the condensed phase, probably by a mechanism involving the promotion of char formation. 8'35 However, the observation that significant amounts of the metals appear to be lost from the polymer (possibly as volatile chlorides or oxychlorides) suggests that vapour phase flame inhibition also may be in operation.

ACKNOWLEDGEMENTS The International Tin Research Council, London, and the BF Goodrich Co., Brecksville, are gratefully acknowledged for supporting this research and for permission to publish this paper. The authors would also like to

ll9man M6ssbaaer study of Sn/Mo fire retardantsfor rigid PVC

317

thank the Analytical Department, International Tin Research Institute, for tin and molybdenum analyses. REFERENCES 1. Anon., British Plastics Federation (1984). 2. A. J. Ejk, in, Modern plastics encyclopedia, Vol. 57, No. 10A (Agranoff, J. (Ed)), McGraw-Hill, New York, 104 (1980/81). 3. J. M. Schwarcz, J. Fire Retard. Chem., 1, 78 (1974). 4. M. M. O'Mara, W. Ward, D. P. Knechtges and R. J. Meyer, in, Flame retardancy of polymeric materials. Vol. 1 (Kuryla, C. C. and Papa, A. J. (Eds)), Marcel Dekker, New York, 195 (1973). 5. W. J. Kroenke, US Pat. 3,883,480 (1975). 6. P.A. Cusack and P. J. Smith, Rer. Silicon. Germanium, Tin, Lead Compds., 7, 1(1983). 7. W. J. Kroenke, US Pat. 4,055,537 (1977). 8. W. J. Kroenke, J. Appl. Polym. Sci., 26, 1167 (1981). 9. A. G. Davies and P. J. Smith, in, Comprehensive organometallic chemistry (Wilkinson, G. (Ed)), Pergamon, Oxford, 519 (1982). 10. F. W. D. Woodhams, Software Microsyst., I, 108 (1982). 11. J. M. Williams and J. S. Brooks, Nucl. Inst. Methods, 128, 363 (1975). 12. J. A. Bowman, Anal. Chim. Acta, 42, 285 (1968). 13. W. Westwood and A. Mayer, Chemical analysis of cast iron and foundry materials, Allen and Unwin, London (1951). 14. R. P. Lattimer and W. J. Kroenke, in, Analyticalpyrolysis (Voorhees, K. J. (Ed)), Butterworths, Woburn, Massachusetts, 453 (1984). 15. I. Touval, J. Fire Flammability, 3, 130 (1972). 16. J. D. Donaldson, J. Donbavand and M. M. Hirschler, Eur. Polym. J., 19, 33 (1983). 17. D. A. Church and F. W. Moore, Plast. Eng., 31, 36 (1975). 18. F. W. Moore, Soc. Plast. Eng. Tech. Pap., 23, 414 (1977). 19. F. W. Moore and G. A. Tsigdinos, J. Less Common Metals, 54, 297 (1977). 20. R. M. Lum, J. Appl. Polym. Sci., 23, 1247 (1979). 21. W. H. Starnes and D. Edelson, Macromol., 12, 797 (1979). 22. D. Edelson, V. J. Kuck, R. M. Lum, E. Scalco, W. H. Starnes and S. Kaufman, Combust. Flame, 38, 271 (1980). 23. M. M. Hirschler, Polymer, 25, 405 (1984). 24. W. H. Starnes, L. D. Wescott, W. D. Reents, R. E. Cais, G. M. Villacorta, I. M. Plitz and L. J. Anthony, Org. Coatings Plast. Chem., 46, 556 (1982). 25. R. P. Lattimer, W. J. Kroenke and R. G. Getts, J. Appl. Polym. Sci., 29, 3783 (1984). 26. P.A. Cusack, L. A. Hobbs, P. J. Smith and J. S. Brooks, International Tin Research Institute Publ. No. 641 (1984). 27. C. D. Tkhiep, Ts. Bonchev, M. Mitrikov, St. Peneva, K. Topalova and E. Tsukeva, Bulg. J. Phys., 4, 130 (1977).

318 28. 29. 30. 31. 32. 33. 34. 35.

Paul A. Cusack, Peter J. Smith, William J. Kroenke F. J. Berry and C. Hallett, J. Chem. Soc. Dalton Trans., 451 (1985). W. J. Kroenke, unpublished work (1985). J. S. Brooks, D. W. Allen and J. Unwin, Polym. Degrad. Stab., 10, 1 (1985). P. A. Cusack and P. J. Smith, in, Speciality inorganic chemicals, (Thompson, R. (Ed)), Roy. Soc. Chem. Spec. Publ. No. 40, 285 (1981). K. Dehnicke, Z. Anorg. Allg. Chem., 308, 72 (1961). C. F. Cullis, Dev. Polym. Degrad., 3, 283 (1981). J. W. Hastie, unpublished work, quoted in: F. W. Moore, Soc. Plast. Eng. Tech. Pap., 23, 414 (1977). R. P. Lattimer and W. J. Kroenke, J. Appl. Polym. Sci., 26, 1191 (1981).