Reductive Dechlorination of a Cycloaliphatic Fire ... - ACS Publications

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Chapter 20

Reductive Dechlorination of a Cycloaliphatic Fire Retardant by Antimony Trioxide and Nylon 6,6: Implications for the Synergism of Antimony and Chlorine William H. Starnes, Jr., Yun M . Kang, and Lynda B. Payne Departments of Chemistry and Applied Science, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187-8795

In nylon 6,6 containing Sb O , the Diels-Alder adduct made from hexachlorocyclopentadiene (two equivalents) and 1,5cyclooctadiene experiences partial reductive dechlorination at 320-330 °C. This reaction is accompanied by a weight loss that apparently results, in part, from the volatilization of HC1 and/or the gas-phase fire retardant, SbCl . Thus, at higher temperatures, the reaction seems likely to play a major role in the antimony/chlorine synergism that suppresses flame in this system. Possible mechanisms for the reductive dechlorination are described. 2

3

3

Mixtures of chloro- or bromoorganics with antimony trioxide (Sb20, commonly called "antimony oxide") have been widely used for many years as fire retardants for polymers. The predominant feature of their performance is the synergism that arises from their generation in situ of volatile antimony halides that act as flame suppressants in the vapor phase (1-3). Surprisingly, 3

© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

253


254 the reactions that form those substances still are not well-understood. Condensed-phase interactions of antimony/halogen systems might also inhibit flame by reducing the formation of volatile fuel. However, this type of inhibition frequently seems to be much less important, as in the case of (antimony oxide)/poly(vinyl chloride) (PVC) blends (4-6). This chapter reports a preliminary investigation of the condensed-phase chemistry that is operative in heated mixtures of antimony oxide, nylon 6,6, and compound 1, a well-known fire retardant for polymers (7). This work has revealed the occurrence of an unexpected reaction that seems likely to play a major role in the generation of SbCl . 3

Background Vapor-Phase Fire Retardance by Antimony Trichloride In antimony/chlorine-containing polymer systems, S b C l ordinarily is regarded as the actual fire retardant (2,5,5). Yet in one recent study, Shah et al. (9) observed no SbCl in the volatile pyrolysate formed from antimony oxide and P V C at 1000 °C. They detected SbCl , instead, but pointed out that their experimental methodology (flash pyrolysis and analysis by chemical ionization mass spectrometry) might not have been representative of actual fire situations (P). Costa and co-workers (10) have provided evidence for the operation, in polypropylene, of a synergistic retardance process in which certain metal chlorides inhibit fuel volatilization by oxidizing macroradicals. Reoxidation of the resultant reduced-metal species by an admixed chloroparaflBn then recreates the metal chloride (10). However, this condensed-phase redox mechanism apparently is inoperative with S r ^ 0 (8 10), a result that can be 3

3

2

3

9


255

explained, at least in part, by the very rapid volatilization of antimony trichloride at the temperatures of burning polymers [typically, 400-700 °C (8)].

Possible Routes from Antimony Oxide to Antimony Trichloride The mechanism for the formation of SbCl is controversial. Several types of mechanisms have, in fact, been proposed. One of them pertains to the use of chloroorganics that can easily liberate HC1 at combustion temperatures. The HC1 may react with antimony oxide to form SbCl , either without the intervention of observable intermediates (eq 1) (77) or via an intermediate oxychloride such as SbOCl (eqs 2 and 3) (77).


3

3

Sb 0

3

+ 6HCH

•

2SbCI t

+

3H 0f

(1)

Sb 0

3

+ 2HCIt

•

2SbOCI

+

H 0| 2

( )

+ 2HCIt

•

SbCI t

+

H 0|

(3)

2

2

SbOCl

3

3

2

2

2

Another type of mechanism relates to chloroorganics such as 1 that cannot undergo the facile thermal loss of HC1. With these substances, the route to SbCl obviously must begin with the direct transfer of chlorine to antimony, and numerous ways of accomplishing such a transfer have been suggested. They include (a) the direct attack of a C - C l moiety by Sr>20 (72), (b) the reaction of Sl>20 with chlorine-containing species (other than HC1) that result from the thermolysis of the organic chloride (72), (c) oxidation of the polymer by Sb203 to form Sb(0), which then acquires chlorine from a C - C l bond (75) [cf. (70)], and (d) the abstraction of chlorine from such a bond by an (antimony oxide)/polymer complex (75). Subsequent transfers of chlorine can be envisaged to occur in similar ways. However, an alternative possibility (which also applies to systems where HC1 is formed) is that SbCl results from the thermal disproportionation of oxychloride intermediates. A disproportionation pathway that currently seems acceptable appears in eqs 4-6 (8). 3

3

3

3

5

S

b

0

C

|

2 7 0

"

2 7 5

° > C

Sb 0 CI 4

5

2

11Sb 0 Cl2

4

0

5

-

4

7

5

° »

ôSbeOnCk

SSbeO-nCfe

4

7

5

-

5

7

0

° >

11Sb 0

4

5

c

C

2

3

+

+

+

SbCI t

(4)

3

4SbCI t 3

(5)

2SbCI *

(6)

3


256 A third type of mechanism for antimony trihalide formation also involves certain additives that do not give hydrogen halide directly (75). They are proposed to undergo reactions with macroradicals to produce free halogen atoms that then abstract hydrogen from the polymer i n order to form the hydrogen halide that reacts with antimony oxide (75). The literature that relates to the formation of SbCl is extensive. Thus we will make no attempt to review it exhaustively here. Instead, we will simply call the reader's attention to some published observations that seem to relate rather closely to our present work. One of these is the demonstration by Gale (14) that antimony trihalide formation from Sb203 benefits greatly from the presence of labile hydrogen as well as organically bonded halogen. Another pertinent inquiry (75) used thermogravimetric analysis (TGA) to compare the pyrolysis characteristics of mixtures of antimony oxide with two organic chlorides at 200-800 °C. When HC1 could be formed directly from the chloride, the Sl>20 was converted into SbCl without the noticeable incursion of reactions o f intermediate oxychlorides (75). On the other hand, when the dehydrochlorination of the starting chloride was structurally unattainable, the weight loss profile was consistent with the initial formation of SbOCl and its later thermal conversion into SbCl and antimony oxide via a series of steps (75) (cf. reactions 4-6). In an especially relevant study, both SbCl and other chlorine-containing (presumably organic?) species were found to be evolved when S D 2 O 3 was heated with compound 1 at ca. 400-700 °C (8). However, the researchers (8) reported no experiments with ternary mixtures containing 1, Sr>20, and a combustible polymer.


3

3

3

3

3

3

Research Objectives The present work was concerned with the pyrolysis chemistry of mixtures of nylon 6,6 with compound 1 and antimony oxide. Weight losses from such mixtures were determined by T G A , and the pyrolysis products were identified primarily by (gas chromatography)/(mass spectrometry) (GC/MS). Quantitative product analysis was not attempted, because some of the products could not be characterized, and the G C sensitivity factors o f several others could not be measured, owing to the unavailability of the pure materials. One objective of the research was to understand the initial interaction of antimony oxide with the chlorinated fire retardant. For that reason, rather low pyrolysis temperatures were used in an attempt to minimize complications.


257

Experimental


Materials Antimony oxide, compound 1, nylon 6,6, and polymer/additive mixtures prepared by molding were supplied by the Occidental Chemical Corporation (OxyChem). The other chemicals were obtained from various commercial suppliers. They had the highest available purities and were used as received.

Instrumental Analysis Thermogravimetric analysis (TGA) was carried out with a Seiko SSC 5040 system incorporating a T G / D T A 200 analyzer with Version 2.0 software. The samples used (12-20 mg) were taken from the molded blends supplied by OxyChem or from mixtures prepared by dry blending with a mortar and pestle at liquid nitrogen temperature. Heating was conducted under nitrogen (50 mL/min) at the rate of 10 °C/min up to 320 °C, and that temperature then was maintained for 30 min prior to cooling. (Gas chromatography)/(mass spectrometry) (GC/MS) analyses were performed with a Hewlett-Packard apparatus (Model 5890/5971A) equipped with a fused-silica HP-1 capillary column [12 m χ 0.2 mm (i.d.)] and HewlettPackard G1034B software for the M S ChemStation (DOS Series). The carrier gas was helium, and the temperature of the injection port was 200 °C. Column temperature was increased from 50 to 300 °C at a programmed rate and then was held at 300 °C for 10 min. Where possible, products were identified by comparing their G C retention times and mass spectra with those of authentic specimens. Identification of the chlorinated products was aided considerably by comparisons of the relative abundances of their mass peaks with those expected for various ions whose chlorine isotopes differed (16). A General Electric QE-300 N M R spectrometer and a Perkin-Elmer 1600 Series FTIR instrument also were used for product characterizations. The FTIR samples were examined in K B r pellets.

Pyrolysis Product Identification Pyrolysis of the additives, the polymer, and their various mixtures were performed on a 2.0-g (total weight) scale for 30 min at 325±5 °C under flowing nitrogen. Volatile products were trapped at -85 °C and dissolved in


258


tetrahydrofuran (THF) before G C / M S analysis. Other G C / M S analyses were performed on T H F extracts of the pyrolysis residues that had not volatilized. Compounds (e. g., 1) derived from one molecule of 1,5-cyclooctadiene (2) and two molecules of chlorinated cyclopentadienes usually did not exhibit parention mass spectral peaks. These compounds were identified by their lengthy G C retention times and by their apparent partial conversion, in the mass spectrometer, into the cyclopentadienes that would have resulted from retroDiels-Alder reactions.

Reactions of Hexachlorocyclopentadiene (3) with Antimony Oxide These reactions were carried out in glass tubes that had been sealed under argon with rigorous exclusion of air and mosture. The tubes were heated at 250±10 °C for various lengths of time.

Preparation of the Diels-Alder Monoadduct, 4 In an adaptation of a published procedure (77), 2.68 g (24.8 mmol) of diene 2 and 1.70 g (6.23 mmol) of cyclopentadiene 3 were heated together under nitrogen for 4 h at 100±1 °C. Analysis of the resultant mixture by G C / M S revealed the presence of substantial amounts of unchanged 2 and 3, together with a major G C peak that was identified from its mass spectrum as monoadduct 4. In addition to weak parent ions at mle 378, 380, 382, and 384 ( C i H C l ) , the spectrum showed stronger fragments at mle 343, 345, 347, and 349(C H C1 ). 3

12

13

6

12

5

CI CI

4

Results Table I shows total weight losses, as determined by T G A , for nylon 6,6, the two additives, and several mixtures thereof. The tabulated values are the


259 averages of those from a number of replicate runs, and the data for mixtures refer to dry blends prepared by us, for which the deviations generally amounted to ± 5 - 1 5 % of the totals listed. The molded blends supplied by OxyChem showed equally good reproducibility but tended to give loss values that were higher by a few percent.


Table I. T G A Weight Loss Study wt% Run Nylon 6,6 1 100 2 3 4 90 5 90 6 7 88 8 84 9 76 10 72 a

1

wt loss, % Sb 0 2

3

Theoret

100 100 10 90 10 10 19 18

9

10 10 2 6 5 10

11 5 47 11 10 14 14

Found Dev. % 6 52 0 15 5 41 16 19 26 28

36 0 (13) 45 90 86 100

b

10 °C/min to 320 °C, 30 min hold. ^Negative value.

Pure nylon 6,6 (run 1) produced only a small weight loss that was shown to consist, at least partially, of cyclopentanone, as expected (18). N o attempts were made to identify the anticipated gaseous products ( N H , H 0 , and C 0 ) (18) having lower molecular weights. In contrast, compound 1 (run 2) lost a considerable amount of weight, a result that was ascribable primarily to sublimation, in that the volatile product was a white solid whose decomposition point (ca. 350 °C) and FTIR spectrum were essentially the same as those of the starting 1. Run 3 confirmed the expected nonvolatilization of antimony oxide at the temperatures used. For runs 4-10, the theoretical weight losses in the table are the values predicted from runs 1-3 by arithmetic additivity. The deviations in the last column represent the differences between the theoretical and actual losses (Found - Theoret), and they are reported as percentages of the theoretical values. Run 4 shows a mass loss increase which, though rather small, seems statistically valid. In the corresponding preparative pyrolysis, all of the volatiles could not be identified, but two of the major ones were shown to be diene 2 and 3

2


2

260


the monoadduct (4) that would have resulted from the retro-Diels-Alder sequence shown in eq 7. The G C retention time and mass spectrum of 4 were identical to those of an authentic specimen prepared directly from 2 and 3. Under the conditions of run 4, the retro-Diels-Alder reactions should have been favored by the dilution effect of the polymer, which would have retarded the back reactions in eq 7 by reducing the concentrations of compounds 2-4. Interestingly, 3 was not actually detected as a pyrolysis product, presumably because it was diverted into insoluble and nonvolatile (polymeric?) material.

The most interesting feature, however, of the preparative pyrolysis analogous to run 4 was the formation of two volatile products (probably stereoisomers) whose elemental composition was C H i C l . This result and their mass spectral cracking patterns indicated that these compounds were analogues of 4 in which one of the chlorines had been replaced by hydrogen. For purposes of comparison, pure 1 was heated in a sealed tube under argon for 22 h at 320-370 °C. Under those conditions, the isomer composition of the substance changed dramatically (from a ratio of -2:5 to a ratio of -4:1), and several lighter products were formed. They included 3 (in low yield), 4, pentachlorobenzene, tetrachloroethylene, and two polychlorides whose lowest parent-mass values were 364 and 376. These results cannot be interpreted fully at this time. Nevertheless, they reaffirm the occurrence of retro-Diels-Alder reactions in this system and provide further indirect evidence for the thermal conversion of compound 3 into insoluble product(s), in that the yield of 3 was much less than that of 4. The change observed in the isomer ratio of 1 could signify an approach to the equilibrium composition via reaction sequence 7. On the other hand, the change in ratio might have resulted, instead, from the selective and irreversible conversion of one isomer into non-Diels-Alder products. The most striking results in Table I are those for runs 7-10. These data show that ternary mixtures of the polymer, 1, and S b 0 underwent total weight losses that were much greater than those expected on the basis of additivity. The observed weight losses were larger when the Sr>20 :l weight ratio was increased. Moreover, plots (not shown) of weight loss vs time showed that antimony oxide also increased the initial weight-loss rate. In analogous preparative runs, cyclopentanone and diene 2 were identified as products. The 1 3

2

3

5

3

3


261 major products, however, were substances that would have resulted from the partial reductive dechlorination of compounds 1 and 4. Those substances are identified in Figures 1 and 2, which show portions of the gas chromatograms of the product fractions obtained from a ternary mixture similar to that in run 8. Remarkably, no unreduced 4 ( C H i C l ) appears in either chromatogram, and the amount of unchanged 1 (both isomers) is much less than the total yield of dechlorinated analogues, which have lost up to four chlorines. Thus it is clear that the reductive dechlorination is a facile process, even at the relatively low temperature of ca. 325 °C.


13

2

6

Discussion When antimony oxide is not present (as in run 4), the occurrence of reductive dechlorination can be explained by C - C l homolysis, followed by hydrogen abstraction from the polymer by the carbon-centered radicals that are formed. Indeed, no reasonable alternative rationalization is apparent at this time. The other homolysis product, CI", is an extremely reactive radical that also is likely to abstract hydrogen, perhaps to a major extent. Thus the formation of the gaseous substance, HC1, can account for some of the increased weight loss. The much larger losses in weight caused by antimony oxide could be due to the increased evolution of HC1 and organic products, and/or to the evolution of SbCl . Low solubility of the latter substance in the extraction solvent (THF) would have tended to prevent its detection. However, when mixtures of S b 0 and the retro-Diels-Alder product, 3, were heated at 250±10 °C under argon for various lengths of time, SbCl was formed as a crystalline sublimate and identified conclusively by its mass spectrum. Thus, in runs 7-10, SbCl formation does not seem unlikely. It could have resulted from various combinations of reactions 1-4. A t present, we do not know whether the reaction of S r ^ 0 with 1 and nylon 6,6 involves free radicals or ionic intermediates. Nevertheless, as noted above, a free-radical mechanism seems very probable for the small amount of reductive dechlorination that occurred under the conditions of run 4. Equations 8-11 represent a simple but speculative free-radical scheme that accounts for enhanced reductive dechlorination and greater weight loss when antimony oxide is present. According to this mechanism, the S b 0 acts as a catalyst for both processes. Some of the CI" radicals may attack the nylon, of course, but reaction 9 does not need to be quantitative in order to account for catalysis. The HC1 that results from reaction 11 may either volatilize as such or react with antimony oxide to initiate the formation of volatile SbCl . Partial reductive 3

2

3

3

3

3

2

3

3


262

C H CI


1 3

1 3

5

N y l o n 6,6(82)/CFR(10)/Sb O (8), 325 °C, 0.5 h, N Volatile Fraction 2

3

2

C H CI 1 3

1 3

C

5

13 14 H

C ,

4 C

18 15 H

C ,

9 Cl8 14 H

C Hi5Cl9

10

C I

18

'J ' ' 10X30

1

' I 1 1200 1

1

"I I

I ' 1 I j

14ÇQ

I ' •

1600

\

I 1M 1800

I I '

2000

.m

time, S

Figure I. Partial gas chromatogram of the THF-soluble volatile products of a pyrolysis described in the heading. Parenthesized numbers in the heading are percentages by weight; chlorinatedfire retardant 1 is designated as "CFR".



263

Nylon 6,6(82)/CFR(10)/Sb O (8), 325 °C, 0.5 h, N Residue 2

3

2

C H CI 1 8

1 4

1 0

C18H14CI10

C

C

13 14 U H

C

C H CI 1 3

1 3

1 8

il

ρ-*,

10.00

,

,

Cl

CFR (C H CI )

9

1 8

1 8

Ί

1 5

5

C H CI

—,

H

1 6

j

15.00

1 2

1 2

8

>

1

ι

1

1

j

1

1

—ι

20*00

\

—τ—

time, s

Figure 2. Partial gas chromatogram of the THF-soluble residuefroma pyrolysis described in the heading. Parenthesized numbers in the heading are percentages by weight; chlorinatedfireretardant 1 is designated as "CFR".


264 dechlorination will significantly enhance the vaporizabilities of several organic products. Alternatively, a direct attack of a C - C l group by antimony oxide could also lead to catalysis, and the continual repetition of the entire scheme will obviously give the products whose dechlorination is more extensive.


1

ο Η α^, + α · 18

Cl*

+

Sb 0 2

C H Chî 1 8

+

1 2

C!Sb 0|

^* ^

3

(β)

12

CISb 0 2

-NHCH CH 2

C H Cln

2

1 8

+ -NHCHCH —

+

1 3

- NHCH=CH- +

2

2

(9)

3

-NHCHCH 2

HCI + S b 0 2

3

(10)

(11)

Ionic mechanisms for the dechlorination can be written, as well. Moreover, consideration can be given to the possibility that the initial attack by antimony oxide occurs on compound 3 or on chlorine atoms that result from its homolysis. A n especially intriguing speculation is that antimony oxide may be sufficiently nucleophilic to abstract a chloronium cation from a bridge carbon of 1 or 4, as in eq 12. In that event, the retro-Diels-Alder reactions of the resultant anions (e. g., eq 13) would be exceptionally fast, because they would form an aromatic species, C C1 ". Numerous precedents for the great rapidity of similar reactions are available (19-22). 5

Sb 0 2

3

+

5

(Ci Hi Ciio)CCI - » C I S b 0 1 7

2

(Ci Hi Clio)CCr 7

2

2

-»

2

+ 3

+

(Ci Hi Clio)CCr 7

2

4 + C CI ~ 5

5

(12)

(13)

Summary and Conclusions The exploratory study described in this chapter has shown that nylon 6,6 can reductively dechlorinate compound 1 in a process that is strongly promoted by S b 0 . This process occurs at lower temperatures than those encountered in fires, and it leads to losses of mass that are likely to consist, in part, of HCI and/or the gas-phase fire retardant, SbCl . Hydrogen chloride, antimony trichloride, and antimony oxychlorides would seem to be the only reasonable 2

3

3


265 dechlorination should make a major contribution to the antimony/chlorine synergism that suppresses flame in this system. Further studies of the mechanism for the dechlorination clearly would be worthwhile.


Acknowledgment We thank R. L . Markezich for useful discussions and R. F. Mundhenke for the preparation of molded specimens. This research was supported in part by the Occidental Chemical Corporation.

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16.

Hastie, J. W. J. Res. Natl. Bur. Stand., Sect. A 1973, 77, 733-754. Cullis, C. F.; Hirschler, M. M. The Combustion of Organic Polymers; Oxford University Press: New York, 1981; pp 276-295. Camino, G.; Costa, L.; Luda di Cortemiglia, M. P. Polym. Degrad. Stab. 1991, 33, 131-154. Lum, R. M. J. Appl. Polym. Sci. 1979, 23, 1247-1263. Starnes, W. H . , Jr.; Edelson, D. Macromolecules 1979, 12, 797-802 (see also references therein). Lum, R. M.; Seibles, L.; Edelson, D.; Starnes, W. H . , Jr. Org. Coat. Plast. Chem. 1980, 43, 176-180. Markezich, R. L . ; Mundhenke, R. F. In Chemistry and Technology of Polymer Additives; Al-Malaika, S., Golovoy, Α., Wilkie, C. Α., Eds.; Blackwell Science: Malden, MA, 1999; pp 151-181. Costa, L . ; Goberti, P.; Paganetto, G.; Camino, G.; Sgarzi, P. Polym. Degrad. Stab. 1990, 30, 13-28. Shah, S.; Davé, V.; Israel, S. C. ACS Symp. Ser. 1995, No. 599, 536-549. Costa, L.; Luda, M. P.; Trossarelli, L. Polym. Degrad. Stab. 2000, 68, 6774. Lum, R. M. J. Polym.Sci.,Polym. Chem. Ed. 1977, 15, 489-497. Brauman, S. K . J. Fire Retard. Chem. 1976, 3, 117-137. Drews, M. J.; Jarvis, C. W.; Lickfield, G . C. ACS Symp. Ser. 1990, No. 425, 109-129 (see also references therein). Gale, P. J. Int. J. Mass Spectrom. Ion Processes 1990, 100, 313-322. Avento, J. M.; Touval, I. Kirk-Othmer Encycl. Chem. Technol., 3 Ed. 1980, 10, 355-372. Beynon, J. H. Mass Spectrometry and Its Applications to Organic Chemistry; Elsevier: New York, 1960; pp 298-299. rd


266


17. Ziegler, K.; Froitzheim-Kuhlhorn, Η. Justus Liebigs Ann. Chem. 1954, 589, 157-162. 18. Levchik, S. V . ; Weil, E . D . ; Lewin, M. Polym. Int. 1999, 48, 532-557 (see also references therein). 19. Finnegan, R. Α.; McNees, R. S. J. Org. Chem. 1964, 29, 3234-3241. 20. Bowman, E . S.; Hughes, G . B . ; Grutzner, J. B . J. Am. Chem. Soc. 1976, 98, 8273-8274. 21. Neukam, W.; Grimme, W . Tetrahedron Lett. 1978, 2201-2204. 22. Blümel, J.; Köhler, F. H. J. Organomet. Chem. 1988, 340, 303-315.



Errata Sheet ACS Symposium Series 797 Fire and Polymers: Materials and Solutions for Hazard Prevention

Gordon L. Nelson and Charles A. Wilkie

In Chapter 20 entitled "Reductive Dechlorination of a Cycloaliphatic Fire Retardant by Antimony Trioxide and Nylon 6,6: Implications for the Synergism of Antimony and Chlorine”by William H . Starnes, Jr., Yun M . Kang, and Lynda B . Payne, copy is missing between the bottom of page 264 and the top of page 265. The last sentence starting on page 264 should read as follows: Hydrogen chloride, antimony trichloride, and antimony oxychlorides would seem to be the only reasonable products that could arise from the chlorine atoms lost by 1, regardless of the mechanism by which the loss takes place (if the polymer became chlorinated, it would simply serve as a thermal source of HCI). A t higher temperatures, the HCI formation rate should increase, an effect that would tend to increase the rate of SbCl generation by reactions 1-4. Also, i f temperatures were sufficiently high, reactions 5 and 6 might raise the yield of SbCl . Thus, in actual fire situations, the reductive dechlorination should make a major contribution to the antimony/chlorine synergism that suppresses flame in this system. Further studies of the mechanism for the dechlorination clearly would be worthwhile. 3

3


Reductive Dechlorination of a Cycloaliphatic Fire ... - ACS Publications

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