Fire research abstracts and reviews Volume 13, 1971 (1971) / Chapter Skim
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Pages 84-115
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From page 84... ...
THE CHEMISTRY OF FIRE RESISTANT MATERIALS AND SUPPRESSION I R V I N G N E I N H O R N College of Engineenng, University of Utah H I S T O R I C A L B A C K G R O U N D Durmg the past decade, we have witnessed a rapid and continued expansion of plastic products In this period the flammabihty characteristics of the predonunantly orgamc polymers have assumed great importance In the 1950's and 1960's plastics were considered for many applications due to theu- ease of fabrication, novelty, and other favorable physical properties in spite of undesirable flammabihty characteristics Nitrocellulose enjoyed a long era of extensive use m spite of its almost explosive character The gradual maturity of the plastics mdustry with a correspondmg mcrease m the variety and sophistication of its products resulted m a correspondmg demand by the consumer and by regulatory agencies for improved flammabihty characteristics At the present time, the lack of adequate fire retardance of most commercially available plastics appears to be one of the largest barriers to the opemng of extensive new markets for these materials m the buildmg and construction mdustry, in transportation apphcations, m household furmshmgs and furmture, m floor covermgs, and m clothmg The importance of fire-retardant chemicals and technology to the chemical and plastics mdustry has recently been the subject of several meetmgs whose proceedmgs add greatly to the techmcal literature I n addition to reviewmg the current technology in fire retardation of polymeric matenals, it was pomted out at these meetmgs that the consumption of fire-retardant chemicals has risen from approximately 65 million pounds m 1960 to greater than 200 milhon pounds m 1969 Figure 1 illustrates the growth m sales of fire retardants m the Umted States for the period 1960 through 1969 The importance of fire retardance to the overall growth of the plastics mdustry has led to the mtroduction of many new fire-retardant compositions with mcreasmg frequency durmg the past five years Although this expandmg research mto the act of fire-retardant technology has uncovered many new facts about the mechanisms of fire retardation of polymeric materials, the technology of fire retardancy m polymer compositions still retams a high degree of empirical character Of the many empirical facts accumulated m the technology of fire-retardant plastics over the years, the followmg have been shown to be the most useful and commercially practical 1. The mcorporation of halogen atoms into a polymeric composition, either as an additive or by chemical reaction, decreases the flammability or mcreases the fire retardance of the composition 2 A combmation of antimony oxide and halogen is more efficient than either of the mdividual materials at the same total concentration In other words, 236
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From page 85... ...
A B S T R A C T S A N D R E V I E W S 237 the combination of antimony and halogen displays an efficient s3Tiergism as a fire-retardant combination.' 3. The addition of some phosphorous compounds of specific structures retard the burning of many plastics.' 4.
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From page 86... ...
238 Radiant Heat F I R E B E S E A B C H Radiant Heat A i r Smoke Zone Flame Zone Carbonaceous Char Layer Pyrolys is Products Pyrolys is Zone Virgin Polymer Fia 2 Generalized model of burning polymer I [ Pyrolys i s Products tardants under burning conditions were summarized by Bel l ' ' as follows: 1 Gas theory large volumes of incombustible gases are produced, which dilute the oxygen supply.
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ABSTRACTS AND R E V I E W S 239 is shown in Eq.
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240 " ' ' F I H E R E S E A B C H below 200°C. The cyclization reaction of the nitrile groups is shown in Eq.
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ABSTRACTS AND R E V I E W S 241 Role of Halogen %n the Fire Retardation of Polymers Rosser, Wise, and Miller,^^ and Wilson, O'Donovan, and Fnstrom^' showed that HBr , HCl , and CI2 prenuxed in hydrocarbon-air flames were active flame-inhibiting agents. I t appears that the prmciple action is the modification of radical concentration and/or distribution m the reaction zone Thus, mterference to the propagatmg and branchmg steps is produced Halogen compounds appear to act by substitutmg a radical of low reactivity for the propagatmg radicals.
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242 shown below, to take place.
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ABSTRACTS AND R E V I E W S 243 ing diffusion flame than is required to pyrolyze the polymer. Recently, Stuetz"" reported on further modifications to the limiting-oxygen mdex test by usmg an upward burnmg flame as well as the downward flame to study fire retardancy mechanisms.
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From page 92... ...
244 F I B E R E S E A B C H the condensed phase of polyethylene. A fire retardant which is effective in the condensed phase should remain effective when the polymer bums m an oxidizer other than oxygen Fenimore and Jones" proved this by showing the effective chlorine inhibition of polyethylene burmng m N2O-N2 as well as i n O2-N2 This mechamsm suggests that fire retardants which work well m one polymer may not perform in the same manner in another material.
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From page 93... ...
ABSTRACTS AND R E V I E W S 245 in preparing unsaturated polyester resins A chlorine content of at least 25% was considered essential to obtam a desired level of flame retardancy. Al'Shits ' ' and coworkers used a mixture of pentaerythritol dichloride and t r i chloride to flame retard unsaturated polyester resins.
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From page 94... ...
246 F I R E R E S E A R C H raismg the halogen level m the polyester. These polyester resms were prepared usmg a two-stage condensation reaction I n the first stage, excess maleic and phthalic anhydride were reacted with pentaerythritol dichloride Sufficient pentaerythritol trichloride was then added so that the total hydroxyl groups m the system would predommate and the condensation reaction would be completed.
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From page 95... ...
ABSTRACTS AND R E V I E W S 247 TABLE 2 Formulations for polyester resins based on tetrabromophthahc anhydride Calculated % bromine on dilution with Moles Moles Moles TBPA PA MA 25% styrene 30% styrene 0 80 0 30 1 0 30 4 0 60 0 60 1 0 22 0 20 5 0 30 0 80 1 0 14 8 13 8 0 25 0 85 1 0 12 7 11 8 0 23 0 87 1 0 11 8 11 0 0 20 0 90 1 0 10 5 9 8 0 07 1 03 1 0 5 0 -- Figures 4 and 5 present the ideahzed structures of crosslinked polyester-based polymers contaimng the halogenated fire retardants The effect of fire retardants on the thermal characteristics of model polyester resm systems was deternuned usmg differential thermal analysis and thermogravimetric analysis Figures 6 and 7 illustrate the effect of fire-retardant concentration on the decomposition temperature for specimens fire retarded wi th the two anhydrides Relatively l i t t le difference was observed m the decomposition temperatures for the series contammg 25% styrene and the series contaimng 30% styrene The control samples contaimng only the halogenated fire retardant showed a shght lowermg of the decomposition temperature The mcorporation of triethylphosphate into these halogenated systems showed no marked synergistic effects Incorporation of antimony trioxide ( 2 % or 5% by weight) resulted m a substantial lowermg of the decomposition temperature wi th an observed minimum occurrmg at approximately 1 1 % bromme concentration or at approximately 21 7% chlorme content Two small-scale test methods were employed under controlled laboratory conditions to evaluate the relative effectiveness of the polyesters modified by the halogenated fire retardants.
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From page 96... ...
248 y H wr -- ^ HC-C-OH " HC-C-OH F I R E R E S E A R C H HC=CH2 o Propylene Glycol Maleic Acid Phthalic Acid Tetrachloro Styrene Phthalic Anhydride H H ,0 ^ CI H 6 CH^ CI CI, c H ^ H FiQ 4 Idealized structure of a crosslinked polyester-based polymer containing tetrachlorophthalic anhydride
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ABSTRACTS AND R E V I E W S H ^ HC-g-OH l^^^:;i^-OH ^""^W^g ^ 249 H5=CH2 -g-OH ^ Propylene Glycol Maleic Acid Phthalic Acid Tetrabromo Styrens Phthalic Anhydride H H HO^-C^Hj 0-C-CH C H , 0 6=0 O o Br, Q 0=0 H H H H Fia 5 Ideahzed structure of a crosslinked polyester-based polymer containmg tetrabromophthahc anhydnde
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From page 98... ...
250 F I B E R E S E A B C H No Additive 3 percent TEP 2 percent SbzOa 5 percent SbaOa 368 h / / 25% S T Y R E N E MONOMER 0 4 8 12 16 20 24 28 WEIGHT PER CENT BROMINE FiQ 6 Effect of tetrabromophthalic anhydnde on polymer decomposition 32 anhydride had lower flame-propagation values than were observed for similar specimens based on tetrachlorophthalic anhydride The incorporation of tr iethyl phosphate reduced the burmng shghtly The mcorporation of antimony trioxide exhibited a strong synergistic effect and markedly improved the flame propagation characteristics of the test specimens The synergistic type of reaction wi l l be discussed in greater detail later m this section. The HLT-15 -- Intermit tant Flame Test is more severe than the A S T M D-757 test because the specimen is suspended in a vertical position and heat f rom the flame is carried upward by convection along the length of the specimen I n addition, the specimen is igmted five times usmg mcreasmgly longer igmtion periods The brommated specimens obtamed a maximum ratmg without additional co-retardants Shght improvements were seen wi th the addition of t r iethyl phosphate.
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From page 99... ...
ABSTRACTS AND R E V I E W S 251 Fire Retardance Conferred by a Halogen m a Nonreachve Intermediate Nonreactive fire retardants are generally used wi th thermoplastic polymeric systems where mcorporation of a reactive material would affect crystallmity, heatdistortion temperature, and resistance to ultraviolet light or humidity. Lowmolecular-weight fire retardants have a tendency to migrate or outgas durmg processmg or while m actual use Thus, consideration should be given to those compounds having a molecular weight high enough to prevent subhmation, migration, or volatilization S Y N E R G I S M D U R I N G F I R E EXPOSURE Synergism is the term applied to the observed ability of two materials to influence the properties or response of a material to a greater degree than equal amounts of the two materials used separately Antimony-Halogen Synergism Various antimony compounds have been used as flame retardants for material and synthetic polymers Antimony trioxide (SbaOj)
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252 F I R E R E S E A R C H (SbOCl) are the most commonly used antimony compounds for modifying the flammabihty characteristics of polymenc materials Schnudt" and Drake" reported that antimony trioxide by itself was a very poor flame retardant Little^' observed that the optimum flame retardancy occurred when the mole ratio of antimony, chlorme is 1:1.
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ABSTRACTS AND R E V I E W S 253 60 Seconds b) - 5 Seconds (d)
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254 F I R E B E S E A R C H (a) - 1 Second (b)
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ABSTRACTS AND R E V I E W S 255 extension of the melting zone and further crack growth. Figure 8d shows char smtermg and flame propagation behmd the weak nonhomogeneous char front.
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From page 104... ...
256 F I B E R E S E A R C H The fluxing action of phosphorous is probably hmited to the interface of the combustion and pyrolysis regions so that any excess over the amount needed to stabihze the char is meffective Since phosphorous oxides can effect only so much dehydration while the continually regenerated bromides can repeat the dehydrohalogenation process phosphorous/bromme combmations would be more effective than phosphorous oxides m promoting char formation The mechamsm of the phosphorous-chlorine synergism is similar to that of the phosphorous-bromme synergism It is more widely utilized because of the greater availabihty of chlorme-contaimng compounds The phosphorous-chlorine synergism is less effective than the phosphorous-bromine synergism to the same degree and for the same reasons that chlorme is not as effective, on an equal weight basis, as broimne The chlorides of phosphorous are lower boilmg and lighter gases than the correspondmg phosphorous bromides, and can be expected to have a shorter residence time in the combustion zone PHOSPHOROUS-BASED F I R E R E T A R D A N T S Phosphorous has an atomic number of 15, and an atomic weight, for the single natural-occurrmg isotope, of 30 98 The electromc structure is W2s'2p^Ss'Sp^ for a total of 15. Bonding m phosphorous compounds can mclude simple sharmg of the 3p orbitals or various 3s-3p orbitals The 3d orbitals are readily available, thus allowmg for spd hybrids Most phosphorous-ligand smgle bonds are now thought to be hybrids The PO smgle bonds m P O 4 ' - are sp' hybrids, the Cl-p bonds m P O C I 3 are a mixture of p' and sp' character The pentavalent and hexavalent compounds are spd hybrids as in PClsCsp'd)
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From page 105... ...
ABSTRACTS AND R E V I E W S 257 Similar findings were observed by Einhom" in the formation of chars and the effect of reaction and additive-type phosphorous-based fire retardants on urethane polymers Phosphorous compounds that can decompose to acids are effective fire retardants I t IS currently believed that phosphorous compoimds are acid precursors and further that the acids perform the key role in char formation and in the inhibition of afterglow. Equation (30)
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From page 106... ...
258 F I R E R E S E A R C H CHpOH - C H 2 OH .C -- H I \ -- V 0 i n . Fia 10 Levoglucosan formation from cellulose Phosphoric acid is much more effective than sulfuric, sulfomc, and boric acids because it has sufficiently low volatility to remam m or near the site of combustion, is highly reactive, and is a mineral acid (pK, x ON)
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From page 107... ...
ABSTRACTS AND R E V I E W S 259 normal gaseous products from the decomposing fiber to give a less-flammable gas mixture Phosphorous contaimng fire retardants which generate phosphoric acid may be added to prevent the formation of levoglucosan and form instead a char percursor during combustion R E T A R D A T I O N O F A F T E R G L O W The glowmg reaction is a solid-phase oxidation of carbon to oxides, CO and CO2 Phosphorous compounds are widely used to elimmate afterglow, but little information IS known about this mechanism Lyons^' summarizes the possible role of the phosphorous compound in retardation of afterglow by stating that the polyacid, formed durmg the decomposition of the phosphorous compound, forms a physical barrier or m some way alters the oxidation process Polyphosphoric acid is a very VISCOUS, film-formmg substance, which could coat the surface and retard oxygen diffusion into the reaction zone durmg combustion The greater tendency for phosphorous compounds to form a gummy film as compared to sulfur or boron may explam the superiority of phosphorous compounds as glow-mhibitors Alternative mechamsms such as favormg the production of CO2 because of some as yet unknown catalysis effect attributable to the phosphorous acids and not from other acids are not as easily argued. B O R O N F I R E R E T A R D A N T S The largest use of boron compounds as fire retardants is in wood and wood products Eickner'^ published a comprehensive review covering the performance characteristics of fire-retardant wood Borax, boric acid, and sodium borate are the most common boron compounds used in the treatment of wood McCluer" tested a number of morgamc boron salts as fire retardants for fabrics The most effective of those compounds evaluated was sodium fluoborate In general, organoboron derivatives (esters)
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From page 108... ...
260 F I R E R E S E A R C H Rudner and Moores assumed that the methoxy groups were displaced by the hydroxyl groups of the cellulose to give an integrally bound boron-contaimng crosslinkage. The mechamstic details of how bone acid and borate salts retard the combustion of cellulose are only partially understood Schwenker and Beck*
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From page 109... ...
ABSTRACTS AND R E V I E W S 261 T A B L E 6 Polymethylene polyphenyl polyisocyanate boron-polyol foam formulations*
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262 F I R E R E S E A R C H T A B L E 7 Flammability characteristics of boron-contammg foams Foam Density Bum rate,*
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ABSTRACTS AND R E V I E W S 263 be stabilized has been reported by Einhorn.'' Simultaneous differential thermal analysis ( D T A ) , thermogravimetric analysis ( T G A )
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From page 112... ...
264 F I R E R E S E A R C H r 4 TGA T 1 1 -NCO NCO 312 M W Tr1ol -DIG . -- -- Pressure (Vacuum) J _ 30 20 10 Tine - f'lnutes F I G 13 Degradation nonfire-retarded urethane polymer The choice of fire retardants, is of course, dependent on the nature of the polymer, the method of processing, the proposed service conditions, and economic considerations Generally, reactive fire retardants are more stable than mert systems Numerous mvestigations of materials exposed m actual fires have mdicated that the nonreactive retardants may sublime out of the polymer when exposed to the hot gases which normally proceed the flame front Figure 12 illustrates weight losses equivalent to the weight of fire retardant in urethane polymers when the nonreactive |
From page 113... ...
ABSTRACTS AND R E V I E W S 265 7^ t ' -1 1 -- CH3 r NCO 312 H H Triol NCO 30% Fire Retardant Fire Retardant Sublimation (Vacuum) Pressure 30 20 10 0 Tine - 'linutes F I G 14 Degradation of fire-retarded urethane polymer during coloring, weavmg, or sizmg operations Nonreactive fire retardants may be added as an after treatment with only minor adverse effects to the process economics.
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From page 114... ...
266 F I R E R E S E A R C H 5. 'Tlammabihty Characteristics of Polymeric Materials," Proceedmgs of Polymer Conference Senas, Umversity of Utah, Salt Lake City, Utah (June 1970)
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