Fire and Polymers - American Chemical Society

Chapter 13

The Design of Flame Retardants

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch013

Charles A. Wilkie Department of Chemistry, Marquette University, Milwaukee, WI 53233

A detailed understanding of the course of a reaction between a polymer and an additive will permit one to use that information to design new flame retardants. The reaction between poly(methyl methacrylate), PMMA, and red phosphorus is described and that information used to determine that CIRh(PPh ) should be used as a flame retardant. The results of this investigation are then used to choose the next additive. A recurring theme is the efficacy of cross-linking as a means to impart an increased thermal stability. 3

3

The process of burning requires that large polymer chains be broken down to give smaller and smaller molecules. Eventually a stage is reached at which the molecules produced have sufficient volatility to escape the surface and enter the gas phase. One may choose to intervene either at an early stage in the decomposition process, and have a relatively small number of molecules present, or at some later stage when many molecules are present. It seems most advantageous to prevent the initial decomposition of the polymer and instead cause aggregation, and charring, of the polymer chains. Van Krevelen (i) has shown that there is a correlation between char residue and the oxygen index of a polymer, the higher the char, the lower the flammability. Parker (2) has noted that an increase in aromaticity yields high char residues which also correlate with the oxygen index. It has been pointed out (3) that additives to protect polymers from heat and light are developed largely on a empirical basis. The basic understanding of the chemical functions of additives in polymeric systems is frequently unclear. Several rather fundamental questions about the course of a reaction between an additive and a polymer need to be answered and that information used to design suitable additives for the stabilization of polymers. 0097-6156/90/0425-0178$06.00/0 © 1990 American Chemical Society

In Fire and Polymers; Nelson, Gordon L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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The Design oj Flame Retardants

We separate the process of polymer burning into two stages, a fuel generation stage and a stage where these fuels are consumed. The generation of fuel typically does not require the presence of oxygen while the burning stage, of course, does require oxygen. In this work only the decomposition, and hence fuel generation, of the polymer is investigated. If one can find methods to prevent fuel generation, then flame retardancy of the polymer is a natural consequence. The objectives of this work are to understand how a particular polymer and an additive interact, i.e., at what site in the polymer does reaction occur and what functionality of the additive causes this reaction. If enough information on reactions of additives and polymers becomes available then this will permit the rational design of a suitable flame retardant. There is a severe temperature requirement in these investigations. Since the polymer must be processed, it is necessary that no reaction between the additive and the polymer occur at or below the processing temperature. The reaction must occur at some temperature which lies between the processing temperature and the point at which decomposition of the polymer occurs. We note at the outset that our objective is not immediately to design flame retardants, rather it is to build up enough information that will permit the design of flame retardants. None of the materials that are described in this paper would be suitable additives, however the detailed reaction information that becomes available through this work will permit flame retardants to be designed. In this paper we describe the reaction of poly(methyl methacrylate), PMMA, and red phosphorus and use that information to predict that the reaction of Wilkinson's catalyst, CIRh(PPh ) , and PMMA may be a worthwhile investigation. Finally information from this reaction is utilized to identify other potential additives and the reaction of these cobalt compounds with PMMA is described. Part of the strategy that will be explored is a strategy of cross-linking to produce materials with greatly increased thermal stability. 3

3

Experimental Chemicals used in this study were obtained from Aldrich Chemical Company. H NMR spectra were obtained on a Varian EM-360, C NMR spectra were obtained on a JEOL-FX-60Q or a G E QE-300. An Analect FX-6200 FT-IR spectrometer was used for infrared spectroscopy and a Hewlett-Packard Model 5980 with a model 5970 mass-selective detector was used for GC-MS. TGA measurements were performed on a PerkinElmer TGA-7 instrument. HPLC measurements were obtained using a Waters Instrument. All reactions were performed in evacuated sealed Pyrex tubes of about 80 cm volume. Typically, a vessel is thoroughly evacuated on a high vacuum line (pressure < 1 x 10" Torr) for several hours, then it is sealed off from the line and placed in a muffle furnace preheated to the desired temperature. After two hours the tubes are removed and immediately cooled in liquid nitrogen. Care must be taken in these operations, tubes have been known to explode in the oven and upon removal. Typically, the 1

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material was dissolved in a solvent, separated by chromatography, and identified by spectroscopy. Some tubes were equipped with break-seals, upon removal from the oven these were resealed to the vacuum line and gasses were quantified by pressure-volume-temperature measurements and identified by infrared spectroscopy. PMMA and Model Reactions. Reactions between PMMA and red phosphorus were carried out with the ratio of PMMA to red phosphorus varying from 10:1 to 1:1, i.e., 1.0g PMMA and 0.1 g phosphorus to 1.0g PMMA and 1.0g phosphorus, at temperatures ranging from 300°C to 375°C. The PMMA-rhodium system used a 1.0g sample of PMMA and 0.5g of CIRh(PPh ) at 260°C for 2 hours. Reactions with the model compound, dimethylglutarate, DMG, utilized a 1:1 stoichiometry, 0.17g DMG and 1.0g CIRh(PPh ) (4-7). 3

3

3

3

Preparation of Cobalt Hvdroqenation Catalysts. K Co(CN) was prepared following Brauer (8), HCo[P(OPh) ]. was prepared following an Inorganic Synthesis procedure (9), and K Co(CN) was prepared following the procedure of Adamson (10). 3

6

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Reaction of K Co(CN) with PMMA. A 1.0 g sample of PMMA and 1.0g of the cobalt compound were combined in a standard vessel and pyrolyzed for 2 hrs at 375°C. The tube was removed from the oven and the contents of the tube were observed to be solid (PMMA is liquid at this temperature). The tube was reattached to the vacuum line via the break-seal and opened. Gases were determined by pressure-volume-temperature measurements on the vacuum line and identified by infrared spectroscopy. Recovered were 0.22g of methyl methacrylate and 0.11g of CO and C 0 . The tube was then removed from the vacuum line and acetone was added. Filtration gave two fractions, 1.27g of acetone insoluble material and 0.30g of acetone soluble (some soluble material is always lost in the recovery process). The acetone insoluble fraction was then slurried with water, 0.11g of material was insoluble in water. Infrared analysis of this insoluble material show both C-H and C-0 vibrations and are classified as char based upon infrared spectroscopy. Reactions were also performed at lower temperature, even at 260°C some char is evident in the insoluble fraction. 3

6

2

Reaction of HCofPfOPh)^ with PMMA. A 1.0g sample of PMMA and 1.0g of the cobalt compound were combined as above. After pyrolysis at 375°C for two hours the tube is noted to contain char extending over the length of the tube with a small amount of liquid present. The gases were found to contain CO, C 0 , hydrocarbon (probably methane), and 0.1 Og methyl methacrylate. Upon addition of acetone, 1.0g of soluble material and 0.19g of insoluble may be recovered. The infrared spectrum of the insoluble fraction is typical of char. 2

Reaction of K Co(CN) with PMMA. A 1.0g sample of PMMA and 1.0g of K Co(CN) were combined as above. After pyrolysis for two hours at 375°C the tube was noted to contain char. The gases consisted of 0.07g CO, 0.23g C 0 , and 0.11g methyl methacrylate, these were separated by 3

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standard vacuum line techniques and identified by infrared spectroscopy. The tube was then opened and chloroform and acetone added. The soluble fraction had a mass of 0.36g while the insolubles were 1.1 Og. The insoluble fraction was slurried with water and filtered, 0.22g was insoluble. The infrared spectrum was typical of char.


Oxygen-Index Measurements. The measurement of oxygen-index was performed by bottom ignition, as discussed by Stuetz (11). Results and Discussion The majority of investigations reported in the literature report on the efficacy of a material as a flame retardant but do not address the chemistry that gives rise to this retardant effect. The objective of this work is to come to an understanding of the course of the reaction between polymer and additive and to use that understanding of chemistry to choose new additives. Since the purpose of this work is to identify the chemistry that occurs between the additive and the polymer, unusually large amounts of the additive relative to the polymer are used. This permits one to more easily determine the course of the reaction. PMMA - Red Phosphorus System. The initial reaction that was investigated was that between PMMA and red phosphorus (4-5). Phosphorus was chosen since this material is known to function as a flame retardant for oxygen-containing polymers Q2). Two previous investigations of the reaction of PMMA with red phosphorus have been carried out and the results are conflicting. Raley has reported that the addition of organic halides and red phosphorus to PMMA caused moderate to severe deterioration in flammability characteristics. Other authors have reported that the addition of chlorine and phosphorus compounds are effective flame retardant additives (12). The initial observation is that PMMA is essentially completely degraded to monomer by heating to 375°C in a sealed tube while heating a mixture of red phosphorus and PMMA under identical conditions yields a solid, nondegraded, product as well as a lower yield of monomer. One may observe, by C NMR spectroscopy, that the methoxy resonance is greatly decreased in intensity and methyl, methoxy phosphonium ions are observed by P NMR. Additional carbonyl resonances are also seen in the carbon spectrum, this correlates with a new carbonyl vibration near 1800 cm" in the infrared spectrum and may be assigned to the formation of anhydride. The formation of anhydride was also confirmed by assignment of mass spectra obtained by laser desorption Fourier transform mass spectroscopy, LD-FT-MS. Mass spectra (5) were obtained by the addition of potassium bromide to the sample as an ion source. Since the molecular weight of a methyl methacrylate monomer is 100, one expects to see peaks in the mass spectrum at 100n + 39, where n is the number of monomer units and 39 is a potassium ion. In the low mass region, up to 1200, peaks are observed at 100n + 41, while in the high mass region, 2100-2200, the peaks occur at 100n + 37, in the intermediate mass region the peaks are 3 1

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at 100n + 39 as expected. This is clear evidence for hydrogen transfer reactions that occur between various ions. Indirect evidence for such hydrogen transfer has been reported (13). This investigation only detected PMMA oligomers up to 2200 molecular weight, there is no reason to think that higher oligomers may not be observed with improved instrumentation. The evidence presented above clearly indicates that red phosphorus attacks PMMA at a carbonyl site with the elimination of methyl and methoxy and the formation of anhydrides. The evidence indicates that anhydride formation occurs via an intra-molecular process, this second bond along the polymer chain does provide added stability that will render depolymerization more difficult and the polymer more thermally stable. The suggested course of the reaction between red phosphorus and PMMA is delineated in Scheme 1.

Scheme 1 The previous conflicting investigations may now be rationalized. Red phosphorus is known to thermally convert to white phosphorus, which will burn in air. If white phosphorus is formed, a fire is expected and no flame retardant activity will be observed. On the other hand, if the phosphorus reacts with the polymer as in Scheme 1_, then thermal stabilization is expected. The efficacy of red phosphorus seems to be closely related to the efficiency of mixing of the additive and the polymer, when they are wellmixed the phosphorus will react with the polymer and lead to flame retardant activity, if the mixing is poor then the phosphorus will be converted to the white allotrope and burning will result. Since all of the work reported herein was carried out in sealed tubes under vacuum, the phosphorus must react and lead to stabilization of the polymer against molecular weight loss and fuel production, i.e. thermal stabilization.


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The Design ofFlame Retardants

1S3


The Importance of Cross-Linking. The observation that intramolecular anhydride formation leads to an increase in stability with respect to molecular weight loss and fuel generation suggests that cross-linking is an advantageous process for thermal stabilization of polymers. One stage of the analysis of this work was to determine that cross-linking should be a goal of this work and to begin to identify reagents that will effectively convert thermoplastic materials into cross-linked thermosets during the burning process and produce an intractable char. The reaction is shown below as Scheme 2.

Scheme 2

On the left we show four polymer strands, all independent of each other, on the right these are now cross-linked. An input of thermal energy to a polymeric system (shown as cleavage in the figure) is sufficient to cause breakage of a bond to liberate a small molecule which is the source of fuel for polymer degradation. On the left side note that the cleavage of any bond will yield such small molecules. On the other hand, in the crosslinked structure (right-hand side), cleavage of any one bond is insufficient to produce small molecules, rather a second, and very specific, bond must also be broken. Since bond cleavage is a random process, it is unlikely that the specific bond will be cleaved without a great deal of thermal input, i.e., a very high temperature is required to produce small molecules. Thus the cross-linked structure is inherently less likely to produce fuel since much more heat is required to cause disruption of the molecule to produce the small molecules required for burning to occur. PMMA - Wilkinson's Catalyst System. The reaction with phosphorus identifies the carbonyl as the site of reactivity in PMMA, accordingly an additive that will react at the carbonyl site should be used. Since


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Wilkinson's catalyst, CIRh(PPh ) , is known to react with acid halides with the elimination of the carbonyl, this reagent was chosen (14). 3

CIRh(PPh ) 3

+ RCOCI

3

3

==>

3

2

The reaction between CIRh(PPh ) and PMMA produces both chloroform soluble and chloroform insoluble fractions (6-7). The soluble fraction contains a material which is very much like PMMA but also contains anhydride by FT-IR analysis. This polymeric fraction also contains rhodium, ligated by methyl and methoxy as well as triphenylphosphine. The chloroform insoluble fraction is about 25% of the total material and will not dissolve in any common laboratory solvent. Charring of PMMA under such conditions has not been previously seen. The char contains rhodium and is also found to retain chloroform, indicative of cross-linking. TGA analysis indicates that the chloroform may be driven off at about 150°C and the remainder is non-volatile at 600°C. Films containing about 10% CIRh(PPh ) in PMMA were prepared and subjected to oxygen index, TGA, and DSC measurements. The oxygen index, bottom ignition (1J_), increases from about 14 for pure PMMA to about 20 for the rhodium compound in PMMA. TGA analysis indicates that about 25% of the sample is non-volatile at 600°C and the glass transition temperature increases by about 15°C by DSC. In order to better understand the reaction of CIRh(PPh ) and PMMA model compound studies were begun. The model of choice is dimethylglutarate, DMG, since this provides a similar structure and a suitable boiling point for sealed tube reactions. 3


RCI + CIRh(CO)(PPh )

3

3

3

3

I I 0=1 c

C

I

H

3

=0

I

C 0

OCH

I

I

0= c

C

I

6

3

PMMA

=0

I

H CO

3

3

C

H

3

DMG

The reaction of DMG and CIRh(PPh ) leads to the formation of several rhodium compounds, six of which were isolaied and characterized. The compounds arise from oxidative insertion of the rhodium species into a carbon-oxygen bond of the DMG followed by ligand exchange reactions (7). We believe that a similar reaction occurs between PMMA and CIRh(PPh ) . A summary of the reaction is presented in Scheme 2. The initial step is an oxidative addition of RhCI(PPh ) to a C-0 bond of the ester moiety and produces rhodium-carbon and rhodium-oxygen bonds. Adjacent rhodium species can undergo further reaction with the formation of anhydride linkages. This anhydride formation may occur between adjacent pairs of reactants, between pairs in the same chain, or between pairs that are present in different chains. All of these reactions are observed, and in however the last reaction is the one of interest here since this leads to cross-linking and char formation. Rhodium is present in both the chary material and in the soluble fractions. From the reaction pathway in order for rhodium elimination to occur, two rhodium-inserted 3

3

3

3

3

3


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185

The Design of Flame Retardants HX W W

CH,

CHj-C

C H ^ C -WWW

+

CIRh(PPh ) ;5

3

Cz=0 H3CO

DCH

7


oxidative addition

H,C *

v A A A A

CH,

I

CH«— C w w w

W W W CH«

3

C 'WSAAA,

I

l

o=c

c=o

I

I

Rh

0 1

H ci 3

Rh

I

I

CH

3

loss of rhodium

CH,

H,C

J11

A/WNA C H j — C * W W W

CH

3

1w w w

•"^CHT-C—CH--C

I

C=0

0=-C

I c=o

I *sC — C H j w w v CH,

Scheme 3 moieties must be in close proximity. Since this cannot always occur, rhodium must be present both in the char as well as in the soluble fraction. It is unlikely that CIRh(PPh ) will ever be useful as a flame retardant due to its red color, expense, and the potential toxicity associated with a heavy metal. An additional disadvantage of the rhodium system is the fact that char formation occurs at a temperature of 250°C, since this is near the processing temperature of PMMA char formation may occur during processing rather than under fire conditions. This discovery is nonetheless 3

3


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important because it allows one to delineate a set of requirements for flame retardants that function by this mechanism. These requirements are: 1.

The compound must be able to effectively bind to carbonyl, thus it must be a transition metal compound. The compound should be coordinatively unsaturated - capable of undergoing oxidative addition to the carbonyl. Since Wilkinson's catalyst is a catalyst for a variety of reactions, a compound known to have catalytic activity may be desirable. This is a corollary of 2. The compound should be colorless and inexpensive.


2.

3.

4.

Extension to Cobalt Compounds. We believe that this set of requirements provide an excellent guide to the elucidation of compounds that will function as flame retardants for PMMA. Since Wilkinson's catalyst is best known as a hydrogenation catalyst and since a hydrogenation catalyst must possess all of the characteristics identified above, it seems reasonable to suggest that transition metal compounds that are known to function as hydrogenation catalysts may be useful flame retardants additives for PMMA. Cobalt compounds were selected for investigation since cobalt is in the same periodic family as rhodium and cobalt compounds may be expected to be inexpensive, possibly colorless, and possibly non-toxic. Two cobalt hydrogenation catalysts were identified, K Co(CN) (15-16) and C H Co[P(OMe) ] (17). Both the cobalt(ll) cyanide, K Co(CN) , and the cobalt(lll) analogue, K Co(CN) , were selected for further study. The allyl cobalt phosphite was very air-sensitive and thus not suitable for these investigations. A related hydridocobalt compound, tetrakistriphenylphosphitehydridocobalt, HCo[P(OPh) ] , was chosen for study. We have found that the oxygen index increases by about 5 or 6 points, from 14 for neat PMMA to about 19 or 20 for all three of these compounds, K Co(CN) , K Co(CN) , and HCo[P(OPh) ] . Char formation and reduced monomer production are observed for all of these additives upon reaction with PMMA. Char formation increases as a function of temperature, for the hydrido cobalt compound, there is 5% char at 262°, 8.5% at 322°, 15% at 338°, and 19% at 375°C; the cobalt(lll) cyanide produces 3% char at 338° and 11% at 375°C; the cobalt(ll) cyanide yields 11% char at 375°C. At the highest temperature, 375°C, the amount of monomer formation is 22% for K Co(CN) , 11% for K Co(CN) , and 10% for HCo[P(OPh) ] . Ideally one would hope to observe no monomer formation and complete char production. Such is not the case here, these materials probably have no utility as flame retardant additives for PMMA since monomer formation, even at a reduced level, will still permit a propagation of the burning process. While somewhat positive results for these three additives do not prove the validity of the hypothesis, we take this to be a starting point in our search for suitable additives, further work is underway to refine the hypothesis and to identify other potential hydrogenation catalysts and other additives that may prove useful as flame retardants for PMMA 3

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It should be noted that other schemes for the production of anhydrides are also possible. We should also note that anhydride formation is not the only means of cross-linking PMMA chains, other schemes for cross-linking are also under investigation.


Conclusion In this paper we have presented evidence to show that it is quite feasible to determine the detailed course of reaction between a polymer and an additive. Further, the understanding of this reaction pathway provides insight into new additives and schemes for the identification of efficacious flame retardant additives. Finally, we have elucidated schemes for the cross-linking of PMMA and have shown that the schemes do provide a route for flame retardation. It is imperative to realize that the purpose of this work is not to directly develop new flame retardants, rather the purpose is to expose the chemistry that occurs when a polymer and an additive react. This exposition of chemistry continually provides a new starting point for further investigations. The more that pathways for polymeric reactions are determined the more information is available to design suitable additives to prevent degradation of polymers. Literature Cited 1. 2.

3.

4. 5.

6. 7. 8.

9. 10.

Van Krevelen, D. W., Polymer, 1975, 16, 615. Parker, J. A.; Fohlen,G. M.; Sawko, P. M. cited in Pearce, E. M.; Khana, Y. P.; Raucher, D. "Thermal Characterization of Polymeric Materials," Turi, E. A., Ed., Academic Press, New York, 1981, p. 807. Starnes, Jr., W. H. in "Developments in Polymer Degradation, Vol. 3," Grassie, N., Ed., Applied Science Publ., London, 1981, p. 135. Wilkie, C. A.; Pettegrew, J. W.; Brown, C. E. J. Polym. Sci., Polym. Lett. Ed., 1981, 19, 409. Brown, C. E.; Wilkie, C. A.; Smukalla, J.; Cody, Jr., R. B.; Kinsinger, J. A. J. Polym. Sci., Polym. Chem. Ed., 1986, 24, 1297. Sirdesai, S.; Wilkie, C. A. J. Appl. Polym. Sci., 1989, 37, 863. Sirdesai, S.; Wilkie, C. A. J. Appl. Polym. Sci., 1989, 37, 1595. Brauer, G. "Handbook of Preparative Inorganic Chemistry," Academic Press. New York, 1965, p. 1541. Levison, J. J.; Robinson, S. D. Inorganic Synthesis, 1972, 13, 105. Adamson, A. W. J. Am. Chem. Soc., 1951, 73, 5710.


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11.

12. 13.


14. 15. 16. 17.

Stuetz, D. E.; Diedwardo, A. H.; Zitomer, F.; Barnes, B.P. J. Polym. Sci., Polym. Chem. Ed., 1973, 13, 585. Peters, E. N. Flame Retardancy of Polymeric Materials, 1979, 5, 113. Gritter, R. J.; Seeger, M.; Johnson, D. E. J. Polym. Sci., Polym. Chem. Ed., 1978, 16, 169. Tsuji, J. Org. Syn. via Metal Carbonyls, 1977, 2, 595. King, N. K.; Winfield, M. E. J. Am. Chem. Soc., 1961, 83, 3366. Kwiatek, J.; Mador, I.L; Seyler, J. K. J. Am. Chem. Soc., 1962, 84, 304. Bleeke, J. R.; Muetterties, E. L. J. Am. Chem. Soc., 1981, 103, 556.

RECEIVED January8,1990


Fire and Polymers - American Chemical Society

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