Fire and Polymers IV - American Chemical Society

Chapter 22

Recent Progress in Flame Retardancy of Polyurethane and Polyisocyanurate Foams 1

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Sergei V. Levchik a n d E d w a r d D. Weil Downloaded by COLUMBIA UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch022

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Supresta U. S. LLC, 1 Livingstone Avenue, Dobbs Ferry, N Y 10522 Polytechnic University, Six Metrotech Center, Brooklyn, N Y 11201

The review covers developments since the previous Fire and Polymers symposium which are o f commercial significance. The polyurethane foam industry is currently undergoing serious changes because of the banning of some chlorofluorocarbon blowing agents. Moreover, the banning of pentabromodiphenyl ether as a flame retardant requires finding alternative means for avoiding scorch (thermal damage during production exotherms). We discuss recent insights into the scorch mechanism in relation to the choice of flame retardant.

We recently published a review of flame retardants for polyurethanes which are in commercial use (1) and a more comprehensive review covering the literature and patents (2). In the present paper, we will discuss developments of the last four years, since the previous A C S Fire and Polymers symposium in 2000.

Additives in Rigid Foams The leading method for flame retarding rigid foam at present is to use additives, although reactive diols are occasionally employed where there is some 280

© 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

Downloaded by COLUMBIA UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch022

281 speciaJLrequirement. The well-established additives are still tris(2-chloroethyI) phosphate and tris(l-chloro-2-propyl) phosphate. The principal recent change here is the need for higher percentages of these additives due to the use of hydrocarbon blowing agents in place of the ozone-depleting chlorofluorocarbons. The roofing test and some other rigid foam flame retardancy tests can be passed with relatively high loadings of chlorinated phosphates alone. Recently, Weil and Levchik (3) reported that the B-2 rating in the German DIN 4102 test can be achieved by the combination of a chlorinated phosphate and pigment grade iron(III) oxide. A significant decrease of smoke was another advantage of that particular combination. Although the chloroalkyl phosphates continue to dominate, there is an interest especially in Europe in non-halogenated flame retardants. A nonhalogenated phosphorus additive, which has found usage in rigid polyurethane foam for several decades is dimethyl methylphosphonate (DMMP). This compound contains 25% phosphorus, the basis of its high flame retardant activity, and only about 8 phr is required in a sucrose-amine based rigid foam. DMMP now has a "R46" (mutagen) labeling in Europe so it is not used much there. Diethyl ethylphosphonate or triethyl phosphate are also effective for the same purpose, and have better label status in Europe. Bayer has recently introduced dimethyl propylphosphonate (LEVAGARD™ DMPP or LEVAGARD™ VP SP 51009) which they advocate as a replacement for the halogen-containing flame retardants in rigid foams (4). It is a low viscosity liquid, having over 20% P content. Also, for non-halogen applications of importance in Europe, high molecular weight finely-divided ammonium polyphosphate (APP) has been found effective in pentane-blown polyurethane or polyisocyanurate foams (5). Ammonium polyphosphate combined with a char former and blowing agent to make a complete intumescent system is also useful in both rigid and some flexible foams. Stabilized red phosphorus, which we recently reviewed (6), also has found usage in Europe in rigid polyurethane foams. It is highly efficient on a weight basis, and can be used at rather low loadings to meet stringent flammability standards. Dispersions in polyol, castor oil or tris(chloroisopropyl) phosphate are variously available from Clariant or Italmatch. Effective combinations of red phosphorus with melamine compounds have recently been patented as flame retardants for polyurethanes in rail vehicles (7). Expandable graphite can be used as an efficient flame retardant in rigid PU foams in combination with triethyl phosphate, which helps to decrease the heat conductivity and the deleterious physical effect of the graphite (8,9). Compared to pentane-blown polyisocyanurate-polyurethane foams which have an oxygen


282 index (OI) of about 20-25, an OI of 35 was found for a similar foam with 15 wt. % expandable graphite and 3 wt. % triethyl phosphate.


Reactive Non-halogen Flame Retardants in Rigid Polyurethane Foams Reactive diols such as VIRCOL™ 82, now Albemarle's ANTIBLAZE™ 82, and Akzo Nobel's FYROL™ 6 or Bayer's LEVAGARD™ 4090N are old products which continue to be used, and perhaps the fact that they are nonhalogen reactives will get them more attention. We notice some new studies of FYROL™ 6 in China (70) which confirm that this reactive diol has a charenhancing mode of action.

Impact of Blowing Agent on Flame Retardancy of Rigid Foams As mentioned, the chlorofluorocarbon blowing agents are being (or have been) phased out to avoid their upper-atmosphere ozone-depleting action. The use of pentanes (cyclopentane, isopentane, n-pentane or mixture) for blowing of foams in place of chlorofluorocarbon blowing, imposes a need for more flame retardants to counteract the flammability of the blowing agent. The usual flame retardants such as FYROL™ CEF and PCF can still be used but the level will usually have to be raised. It is well known that by raising the isocyanate index to get more isocyanurate structure, a lower level of flame retardant can be used to meet a standard. A review of blowing agents is available (77). New non-ozone-depleting blowing agents containing fluorine can allow for less flame retardant, or with a high enough isocyanurate content, no flame retardant at all. An example of a non-ozone-depleting blowing agent in commercial development is 1,1,1,3,3pentafluoropropane or Honeywells' ENOVATE™ 3000 (HFC-245fa) or Solvay's HFC-365mfc. This compound does have a flash point, but can be made less flammable by blending with tetrafluoroethane (HFC-134a) (12,13,14,15). There is also a possible trade-off in the use of a pentane with HFC-245fa. The pentane lowers the cost of the blowing agent but may require an increase in the flame retardant. A typical flame retardant such as tris(chloroisopropyl) phosphate can be then elevated to compensate for the pentane.


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Brominated Diols as Reactive Flame Retardants Brominated diols have been used in rigid urethane foams for many years, specifically Great Lakes PHT-4™ diol or Albemarle SAYTEX™ RB-79 to meet ASTM E-84 Class I or II ratings. The original product has one primary hydroxyl group (faster reacting) and one secondary hydroxyl group (slower reacting). In order to pass the E-84 tunnel test with a class I rating, DeLeon et al.(16) recommended the use of a combination of this brominated diol and phosphorus from three different fire retardants, the lowest possible amount of polyether polyol, and the highest possible isocyanate index. Based on a small scale test simulating performance of the FM 4450 calorimeter (used for roofing insulation testing), blends of the brominated phthalate diol with chlorinated phosphate flame retardants have been recommended (17) as an effective FR combination for this application. Recently, a related tetrabromophthalate diol with both hydroxyl groups primary, thus faster-reacting, has been introduced by Great Lakes. Dead Sea Bromine Group has introduced a series of proprietary blends of a brominated neopentyl alcohol, a tetrabromobisphenol-based polyether polyol (probably) and a chlorinated phosphate, as SaFRon 6601 and 6700, aimed at rigid foams for the building industry (18).

Additives in Flexible Foams A major fraction of the flexible polyurethane foams used in furniture is flame retarded. Additives, such as tris(2-chloroisopropyl) phosphate and tris(l,3-dichloro-2propyl) phosphate are still dominant, although much research has been expended on reactives. Furniture manufacturers in the U.S. usually try to have their foam cushions comply with the CAL 117 tests, which is currently under revision. When and if the California requirements are stiffened, the expected result is more likely to affect the choice of upholstery or force the use of a flame-resistant interliner (19,20). Typical interliners are polyimides, aramides, polybenzimidazoles, woven glass fabric and a melamine-based fiber (BASOFIL™).

Halogen-free Additives for Flexible Foams An oligomeric ethyl phosphate additive containing 19% phosphorus has been introduced by Akzo Nobel as FYROL™ PNX. It is an oligomeric additive (21,22) with a repeating unit of the structure:


284 O II

0 - P - 0 - C H — CH?


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Because PNX is halogen-free it is especially of interest in Europe, particularly with respect to the automotive industry and their low fogging/VOC emission requirements. In the MVSS 302 test, this oligomer is on average 4050% more efficient than the chloroalkyl phosphates. In terms of volatile organic content (VOC), PNX compares well with other flame retardants used in the automotive industry, but it becomes especially advantageous because the low use level also helps to maintain low VOC. PNX was suggested for use in automotive applications in combination with alkylated phenyl phosphates, which not only improve the fire retardant performance of PNX, but also decrease its viscosity (23). PNX also outperforms traditional products in Cal 117A and D test which is a small-scale ignition tests for upholstered furniture. PNX is 50 - 60 % more effective than traditional chlorinated alkyl phosphates. This oligomer was found to be synergistic with tris(dichloroisopropyl phosphate) (24). One advantage of the use of an additive instead of a reactive is that little change needs to be made in the foam formulation. Oligomeric alkyl phosphonates of the following structure:

were synthesized in the Albright & Wilson laboratories, later acquired by Rhodia, and evaluated in flexible PU foams (25). It was found that 10 php of the oligomeric phosphonate makes foam self-extinguishing. Triaryl phosphates, such as isopropylphenyl diphenyl phosphate, are now finding use in flexible foam formulations sometimes in combination with a bromine-containing additive (see Scorch discussion). A recent introduction by Great Lakes, REOFOS™ NHP, a low viscosity aryl phosphate, is designed to meet MVSS 302 for hot-molded automotive seating, and to be non-fogging (26).


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Understanding and Overcoming the "Scorch" Problem in Flexible Foams In the manufacture of flexible polyurethane foams, allowing the foam to reach an excessively high temperature during the latter stages of foaming, after the addition of the water, can lead to "scorch." This is, minimally, a discoloration of the interior of the foam slab or bun, more seriously a loss of mechanical properties indicative of structural degradation. It has always been thought that scorch involved some kind of free radical and acid generation chemistry (27), and improved products such as tris(dichloroisopropyl) phosphate with various antioxidants and acid acceptors have been successfully marketed (28,29). Experiments with indicators however show little or no acid in the scorched region of a foam bun. Infrared spectroscopic studies, done either by diffuse reflectance FTIR (E. D. Weil, unpublished) or transmission FTIR, show that the scorched part of a foam bun has a lower unreacted isocyanate group content than the unscorched part. More insight into the chemistry of scorch comes from a study done at University of Turin (30) sponsored by Akzo Nobel. These isocyanate groups are present in the foam because of entrapment in the crosslinked network where they cannot come into proximity with reactive groups. When they are hydrolyzed by water, added to make C 0 for blowing and urea linkages, these isolated isocyanate groups cannot find an OH or NH to react with, so they generate free amino groups. Aminophenylamido structures, are known to be readily oxidized to quinoneimine structures, which are extremely chromophoric with very strong visible light absorption bands. Spectroscopic evidence for such structures was found in the Turin study. The question as to why the flame retardants such as the chloroalkyl phosphates aggravate scorch was then addressed. It is known that these structures can alkylate an aminoaryl group, by nucleophilic displacement of the chloride or phosphate anion or both (31). It is also likely that the more electron-rich alkylaminophenyl groups are more readily oxidized than the unalkylated aminophenyl groups. The formation of the chromophoric groups is aggravated, amongst other factors, by the presence of flame retardants with alkylating capabilities such as the chloroalkyl or alkyl phosphates, and this alkylation reaction also adds to the exotherm. Those flame retardants which are most reactive towards the hypothesized arylamino groups tend to be those which aggravate scorch. Many such conjugated structures are likely. 2


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Conversely, phosphorus compounds that cannot alkylate amino groups are those which do not aggravate scorch; examples are aryl phosphates.

Low-Scorch Flame Retardants The polybromoaromatic compounds are relatively unreactive and act as inert additives under the conditions of scorch, thus foams containing them tend to be scorch-resistant. Consequently, a widely used low scorch flame retardant additive has been a liquid blend of pentabromodiphenyl ether and isopropylphenyl diphenyl phosphate/triphenyl phosphate, both quite stable liquids which perform well together as Akzo Nobel's FYROL™ PBR or Great Lakes* DE-61™ (52). In the early 2000 era, environmental concerns regarding pentabromodiphenyl ether have led to a sharp decline in its usage, regulatory actions in California and in Europe have the effect of a ban, and manufacturing in the U.S. is being discontinued in 2004 (33). Bromine-containing alternatives introduced into the market include a tetrabromobenzoate ester, Great Lakes FIREMASTER™ BZ-54 (CN 2065) (34) which can be used alone or blended with an alkylphenyl diphenyl phosphate. The stated 54% Br fits an octyl tetrabromophthalate. When used alone, this additive has a favorable effect on flame lamination, and also unlike the pentabromodiphenyl oxide formulations, it does not cause center softening in high resilience foam formulations. Its blend with an alkylphenyl phosphate is Great Lakes new FIREMASTER™ 550, used in both high-resilience and conventional foams (35). A blend of tribromoneopentyl alcohol and triaryl phosphate has been offered by Dead Sea Bromine for use in non-scorching flexible foam (18). It has high efficiency and provides good thermal and UV stability. Akzo Nobel Chemicals has developed new highly stable phosphate esters and their blends known as AC003 and AC007 (36,37). AC003 is a non-halogen material of low viscosity with a phosphorus content of 10.9%. AC007 contains 9% phosphorus and 24.5% chlorine. In the MVSS 302 test, the flame retardant efficiency of AC003 is comparable to chlorinated alkyl phosphates and/or pentabromodiphenyl ether at 1.5 pcf but at a density of 1.8 pcf AC003 is less efficient than these conventional additives. The blended product AC007 is more


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efficient than chlorinated alkyl phosphates or pentabromodiphenyl ether at 1.5 pcf and comparable at 1.8 pcf. In terms of volatile organic content (VOC), both AC003 and AC007 are superior to pentabromodiphenyl ether and comparable or better than chlorinated alkyl phosphates. AC003 is actually very close to the fogging performance of non-flame-retarded foam. In the CAL 117 A and D tests, AC003 compares favorably with chlorinated alkyl phosphates. AC007 is significantly more efficient than any tested additive at 1.5 pcf. In order to further improve efficiency of AC003 and AC007 these products can be combined with synergists.

Reactive Flame Retardants for Flexible Foams A newer halogen-free phosphorus-containing diol was developed by Hoechst for rigid or flexible foams and has been marketed for several years as EXOLIT™ OP 550 by Clariant (38). The product is an ethyl phosphate oligomer diol of the following probable structure: O

HOC H —o-fp2

4

The primary hydroxyl groups are reactive with TDI and this diol can be used as a partial replacement of the polyol. This diol containing about 17% P can be used for rigid or flexible foams, especially for molded and high density slabstock flexible foams. At relatively low loadings of 4 - 7.5 phr it allows the foam to pass the MVSS 302 automotive test, with superior performance in the fogging test. Monofunctional phosphates were made in the Daihachi laboratories by reacting cyclic acid phosphates with propylene oxide or ethylene oxide (39) to obtain monohydric alcohols of the structure:

This additive is said to be efficient at 8.5 parts per hundred of polyol in the MVSS 302 automotive test.


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Melamine in Flexible Foams


Especially in Europe, melamine has been used for some years as a part of the flame retardant system in flexible foams. It is often used in combination with a haloalkyl phosphate, such as tris(l,3-dichloro-2-propyl) phosphate. A synergistic interaction noted with melamine and TDCPP (but not with the more volatile tris(l-chloro-2-propyl) phosphate TCPP) in combustion-modified highresilience foam has been recently explained by Bastin et al. (40,41) on the basis of a chemical interaction producing char or a difficultly ignitable low-melting semi-solid. The more volatile TCPP escapes before this interaction can take place.

Conclusion At this time, the older chloroalkyl phosphate additives continue to be predominantly used in both rigid and flexible foams. A number of new candidates have made their appearance, competing for market niches opened up by changes in blowing agent, by the demise of pentabromodiphenyl ether and by the interest in halogen-free, low VOC and reacted-in flame retardants.

References 1. 2. 3.

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289 12. Williams, D. In Polyurethanes Expo '02, Salt Lake City, UT, 2002, pp 135143. 13. Bogdan, M.; Williams, D.; Verbiest P. J. Cel. Plast. 2001, 37, 58. 14. Wu, J.; Dillon, D.; Crooker, R. In Polyurethanes Expo '02, Salt Lake City, UT, 2002, pp 144-149. 15. Zipfel, L.; Dournel, P. J. Cell. Plast. 2002, 38, 51. 16. DeLeon, A.; Shieh, D.; Feske F.F. In Polyurethanes Expo '01, Columbus, OH, 2001, pp 125-128. 17. Feske, E. F.; Brown, W. R. In Polyurethanes Expo '02; Salt Lake City, UT, 2002, pp 32-40. 18. Borms, R.; Wilmer, R.; Bron, S.; Zilberman, Y.; Georlette, P. In Flame Retardants 2004; Intersience Communications, London, 2004, pp 73-84. 19. Hirschler, M.M. In Proc. Fall FRCA Conference; Cleveland, OH, 2002, pp 1-26. 20. Damant, G. In Flame Retardants 2004; Interscience Communications: London, 2004, pp 221-232. 21. Bradford, L.L.; Pinzoni E.; Wuestenenk J. In Polyurethanes Expo '96; Las Vegas, NE, 1996, pp 358-361. 22. Blundell, C.; Bright, D.A.; Halchak, T. In Proc. UTECH 2003, The Hague, NL, 2003. 23. Bradford, L.L.; Pinzoni E.; Williams, B.; Halchak T. to Akzo Nobel, European Patent 1,218,433, 2003. 24. Bradford, L.L.; Pinzoni E.; Williams, B.; Halchak T. to Akzo Nobel, US Patent 6,262,135, 2001. 25. Harris, C. J.; Woodward, G.; Taylor, A.J.; Manku, J.S. to Albright & Wilson, British Patent 2,319,251, 1998. 26. Rose, R.S.; Buszard D.L.; Philips M.D. and Liu F.J. to Pabu Services, US Patent 6,667,355, 2003. 27. Gray, R.L.; Lee, R.L. In Plastics Additives, Pritchard, G. Ed.; Chapman Hall: London, 1998, pp 567-575. 28. Andrews, S. In 60 Years of Polyurethanes, Kresta, J.E.; Elfred E.W. Eds.; Technomic: Lancaster, 1998, pp 101-119. 29. Williams, B.A.; De Kleine, L.A. to Akzo Nobel, PCT Patent Application 200279315, 2002. 30. Luda, M.P.; Bracco, P.; Costa, L.; Levchik, S.V. Polym. Degrad. Stab. 2004, 83, 215. 31. Bissell, E.R. J. Heterocyclic Chem. 1977, 14, 535. 32. Darnerud, P.A. Environ. Int. 2003, 29, 841. 33. Hardy, M.L.; Biesemeier, J.; Manor, O.; Gentit, W. Environ. Int. 2003, 29, 793. 34. Rose, R.S.; Likens, L.J.; Elliott, J.L. In Proc. Polyurethane Foam Association, 1966, http://www.pfa.org/abstracts/ab96,html



290 35. Jacobs, P.; Rose, R.; Likens, J.; Elliott, J. In Proc. Polyurethanes World Congress '97, Amsterdam, NL, 1997, pp 215-219. 36. Bradford, L.; Williams, B.; Pinzoni, E. In Proc. Spring FRCA Conference, Washington, DC, 2000. 37. Bradford, L.; Pinzoni, M.; Halchak, T. In Polyurethanes Expo '02, Salt Lake City, UT, 2002, p 647. 38. Sicken, M.; Staendeke, H. to Clariant, US Patent 5,985,965, 1999. 39. Tokuyasu, N.; Matsumura, T. to Daihachi, US Patent 6,127,464, 2000. 40. Bastin, B.; Paleja, R.; LeFebvre, J. In Polyurethanes Expo '02, Salt Lake City, UT, 2002, pp 244-254. 41. LeFebvre, J.; Bastin, B.; Le Bras, M.; Paleja, R.; Delobel, R. J. Fire Sci. 2003, 21, 343.


Fire and Polymers IV - American Chemical Society

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