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APPE N D I X F COMBUSTI ON PROD UCTS OF P OLY ME RS IN F I RE S
APP E N D I X F I n bui lding f i res one nearly always encou nters pyrolysis and comb ustion pro· ducts of cel l ulosic fuels along with var ious p l astics. F i res are class i fied accord i ng to N F PA categories into four general ty pes [4 ) : C lass A : F i res i nvo lving ord i nary combustible materials ( wood , cloth , paper, rub· ber, and many plastics) . Class B : F i res i nvo l v i ng flammable or combustible l i q u ids, flammable gases, a nd greases. C lass C : F i res i nvolv i ng electr ical eq u i pment. These are treated as C lass A or B f i res after the electricity is turned off. Class D : F i res i n vo l v i ng combustible metals . . . . S i nce most enclosure fi res are of the class A type, i nvo l v i ng ce l l u l osic fuels, i t i s i mportant t o consider t h e d i fferent temperature zones since th i s contro l s the f i re envi ron ment . B ased on a review by B rowne, ( 5 ) descri bed by B ea l l and E i chner, [6) four disti nct temperature zones are given for the Thermal Decomposition of Wood as fol lows : ° Zone A : Be l ow 200 C . Appearance o f noncombustible gases, prima r i ly H 2 0 va· por, traces of C02 , formic and acetic acids, and glyoxa l . D eh ydrat i on of sorbed wa ter is complete. ° ° Zone B : 200 to 2B0 C. Same gases as i n Zone A are prod uced along w i th greatly reduced quantities of water vapor and CO. R eact ions are endo· the r m i c and produ cts are al most ent i rely nonflammable. ° ° Zone C : 2B0 t o 500 C . Acti ve py rolysis takes pl ace u nder e xothe rmic cond i · tions lead i ng t o secondary reactions among the products. Largely com· bust ible products, CO, CH4 , etc. , and flammable tars in form of smoke particl es. ° Zone D : Above 500 C. R esidue consists primari ly o f charcoal , wh ich provides an extremely active s i te for secondary reactions. The ea rly co mbustion stages are s i m i lar to the pyrolysis stages, modi fied sl ightly by ox idat i on . These stages are categor i zed as fol l ows: Zone A: S i m i lar to Zone A above, but slightly affected by some ox idation pro· cesses. Zone B : Primary exothe r mi c react ion takes pl ace without ign ition. Zone C : Combustible gases that are ign i table are prod uced after secondary py r· olysis. F lam i ng combustion can occur in gas ph ase if the gases are ign i ted . If ign i t ion is not i nduced f l a m i ng may not occur u n t i l near the end of py ro lys is when the evoked gases cannot i nsu l ate the charcoal l ayer from 0 2 ⢠Spontaneous ignit ion of ch a rcoal takes place at tem pera· ture lower than any of the products evo lved. ° Zone D : G reate r than 500 C the charcoa l glows and is consumed ; greate r than ° 1 000 C non l um i nous fl ames are su pported by th e combustion of H 2 and CO .
M I N E S AND B U N K ER S These zones i ll u strate the com ple x i ty o f cel l u l os i c combustion processes . I n an enclosure one would expect an agglome ration of both py rolysis and combustion products. Th is is i l l ustrated in the fl ow diagram in F igure 1 . Cond uctive h eating w i l l i nduce pyrolysis. This wo uld be l i m i ted to the percolation of gases through materials that l eave porous char·l ike residues. Radiation and convect ive hea l transfer are pr i mar i l y respons ible for flame spread i ng and are longe r ra nge . The l iqu id and sol id ph ases are also present in vary ing degrees . Faa . .1. Flow diocram lor pyrolysio and a>mbuotiun of celluloooi e luet. io .., enelosure. An enclosure f i re of pl asti cs can be represented by the simpl ified flow d ia· gra m in F igure 2. Common plastics are designated by ·C·H·O·N · type structural YAâ¢tOUS vtCtU ,...... f O a iC I 0t'U 10UIIt(i Qif f't,. � f iC A.. O AOOt flYII Fao. 2. Flow diocram lor pyroty ... &ad eombusliOD of plutico io aa eoeloeuN. 1 76
APP E N D I X F a r ra ngements. Py rolysis ca n resu lt from the cel l u losic fuels and also f rom self· i nduced modes of heat i ng. I n add i t i on to the flame -spread mechanisms common to cel l u losic fuels a rheological f lame spread mechan ism occurs w i th thermosett i ng plastics. H ere mol ten or f l a m i ng d rops or e � en st reams of these f l u ids ca n d rast ica l l y a l te r f l ame spread mechan isms and req u i re evacuation a n d e x t i ngu ishment tech · niques. Both py rolysis and combust ion of p l ast ics must be conside red as eq ually i mportant i n l ight of 1 ece n t stud ies that i n d i cate many p l astics forme rly considered self -ex t ingui sh i ng can be burn t continuously from be low ( bottom b u r n i ng) by in· corporation of a noncombust ible wick . ( 7 ] W i c k i ng action is nearly always prov id· ed by the conten ts of a n enclosure. The chem ical and physica l mod i f ications of p last i cs. the i ncorporation of add i t i ves along wi th the thousands of trade names [ 8J make it exceed i ng l y d i ff i cu l t to genera l i ze the produ cts as wi th cel l u l ose fuels. A breakdown i nto groups such as char formers, vapor formers, a nd com b i ned effects such as char rers p l us vapor formers is h e l pfu l . . . . [Summary d i scussion of certa in pol y mers l ater i n article. I References 4. F i re Protect i on Handbook, National F ire Protect ion Association, 1 3t h ed . , 1 969. 5. F . l . B rowne , Theories on the Combustion of Wood and i ts Control , U . S. F orest Products Laboratory, Rept. 2 1 36, M ad i son , W iscons i n , 1 958 . 6. F . C. Bea l l and H . W . E ickner, Therma l Degradation of Wood C.:> mponents, F orest Products Lab. , R e po r t 1 30, F o rest Serv i ce, U. S. Dept. of Agr icul ture, May 1 970. 7 . D. E. Steutz, B asic Pri nciples i n Polymer Combust ion, Polymer Conference Ser· ies. U n iversity of Utah . 2 1 -26 J u ne 1 97 1 . 8 . Plastics Note 9A. T rade Design ations of Pl ast ics and R e l ated Mater ials, AD· 7 1 540 1 P i ca t i n n y Arsenal, Dover, New Jersey , M ay 1 970. 1 77 ...
