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Flammability of PolymersC. P , F ~mMORE a nd F . J . M~ TXN

Ge ne ra l E le c t r i c R e s e a r c h L a b o r a to r y , S c h e n e c ta d y , Ne w Y o r k(Re c e iv e d Julr 1965)

T h e / ta mm a b i l i t y o f p o ly me r s h a s b e e n me a s u r e d b y d e te rmin in g th e o x y g e ~ c o n te n t o f t h e ,a tmo s p h e r e s ju s t c a p a ld e o / b u r n in g th e m. T h e e f f e c t s o f t h e ] o llo w in g th r e e c o mm o n ly u s e dal ,, ents for reduc ing l~amma bil i ty have been com pared in po lye thy lene . ( I ) A mod erate degre,~o ] in h ib i tio n i s c on /e rr ed b y a n t im o n y t r io xid e p lu s s u ~c ie n t c h lor in e: th e ma x im u m e f f e ctrequires abou t 0 .01 ant im ony a tom s per Ce group in the po lye thy lene , and deve lops jus t aswe l l wh e n c h lo r in e ~a n t imo n y i s s i x a s ~h en i t i s twe n ty . T h e lu l l e f f e c t o ] th e a n t im o n y d o esnot deve lop when ch lor ine~ ant im ony is two . (2) Various phosphorus com pound s are a l lo / s imilar e f fec tiveness per a to m o] phosphorus added to po lye thyh;ne . (3) Wi th su ~c ien tsubs t i tu t ion o / hydrogen b y ha togen in po lye th y lene , a grea ter inh ib i t ion is ach ieved than~s poss ible by ( t ) or (2). W e have no ev idence how, (1) and (2) inh ib i t burn ing , ba t presen tev idence is tha t in (3) a large subs t i tu t io n o f ch lor ine works mo s t ly by a f fec t ing thedegradation o f the po lym er ra ther than by in ter /er ing w ith gas phase f lame reac t ions .IntroductionH . G. WOLFHARD and co-w orkers1 ,2 comparedthe f lamm abi l i t ie s of some l ight fue l~ l :y f inding

t he l i m i t i ng mi x t u re o f a i r a nd n i t roge n in wh i c he a c h c o u l d j u s t h u m . T h e y d e s c r i b e d t h e m i x -t a re s b y t he i r oxyg e n i nd ic e s ,Ox yge n i nde x = n = [Oz ] 7 ( [O~ ] + IN2] )F o r a i r , n = 0 ' 2 1 .

The sma l l e r t he l i m i t i ng n , t he more f l a mma b l et he fue l s i nc e t he more i t s f l a me su rv i ve d c oo l -i i Jg by n i t rogen. Some of the i r l imi t ing indicesfo r d i f fu s i on f l a me s a t a t mosphe r i c p re s su rew e r e :n = 0 .054 fo r hyd ro ge n= 0 .056 fo r a n e qu i mol e c u l a r mi x t u re o f hyd ro -

g e n p l u s c a rb o n m o n o x i d e---=0 .071 fo r mon ome r i c fo rma l de hyde= 0 . 1 8 3 t o 0 " 1 3 4 f o r b e n z en e a n d n o r m a lpa ra f fi n s c on t a in i ng 6 t o 10 c a rbon a t oms .W e ha v e c o r re l a t e d t he e f fe c ts o f subs t i t ue n tsa nd a dd i t i ve s i n po l yme rs , pa r t i c u l a r l y i n po l y -e t hy l e ne , by t he i r oxy ge n i nd ic e s . S o -c a l le d' ~ l f - e x t i ngu i sh i ng ' ma t e r i a l s , wh i c h do no tc on t i nue bu rn i ng i n a i r a f t e r be i ng i gn i t e d ,e v i de n t l y ha ve n g re a t e r t ha n 0 ,21 ; a nd t he a i rmus t be e n r i c he d wi t h oxyge n t o i nve s t i ga t et h e m . E x p e r i m e n t a lK n o w n a t m o s p h e r e s w e r e p r e p a r e d b y m e t e r in ga i r a nd e i t he r n i t roge n o r ox yge n t h rough c r it i ca l

f low or i f ices , mix ing the gases , an d feed ingt he m t o t he bo t t om o f a ve r t i c a l g l a s s t ube 60c m l ong by 8 .4 c m d i a me t e r . The ga se s pa s se dt h rough a be d o f g l a s s be a ds a t t he bo t t om o ft he t ube t o smoo t h t he i r f l ow, a nd t he n up t het u b e a n d o u t o f th e o p e n t o p . A s a m p l e o fpo l ym e r , u sua l l y a s t r i p 0 -6 c m x 0 .3 c rn x 8 c m,wa s c l a mpe d wi t h i t s l ong d i me ns i on ve r t i c a lin the cent re of the tube ; i t was igni ted a t theuppe r e nd wi t h a hyd roge n f l a me ; t he i gn i t e rf l am e w a s w i t h d ra w n a n d t h e a t m o s p h e r e f o u n di n wh i c h t he s a mpl e j u s t b u rne d fo r it s e n t i r elength .P o l y e t h y l e n e , p o l y m e t h y l m e t h a e r y l a t e ,Te f l on , o r o t he r subs t a nc e s wh i c h bu rn t wi t hou ta ny re s i due , ha d s t e a dy f l a me s wh i c h g r i ~ wsma l l e r un t i l t he y e x t i ngu i she d i n a t mosphe re sc o n t a i n i n g n o t q u i t e e n o u g h o x y g e n f o r c o n -t inuous combustion. In contrast , s i l ica-fi l led si l lcones had l i t t l e v i s ib le f lame; a zone of g lowings i li c a worke d t h rough t he spe c i me n , o r c e a se d t op ropa ga t e a nd c oo l e d a t e x t i nc t i on . Ot he rma t e r i a l s we n t ou t l e s s smoo t h l y . P o l yc a rbon-a t e s a nd po l yv i ny l e h l o r i de c ha r re d a s t he yburn t a nd s e e me d t o smot he r t he i r f l a me s i nc h a r . P o l y e t h y l e n e c o n t a in i n g a n t i m o n y o x i d ep l us c h l o r i ne fo rme d a mob i l e l i qu i d runn i ngov er the surface as it burnt ; the f lame f l ickeredba d l y i n t he ne i ghbourhood o f t he l i m i t i ng oxy-gen ind ex and or tega rem ained la rge , i f uns teady ,un t i l i t e x t i ngu i she d a b rup t l y . P o l ye t hy l e ne

185

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136 . P. Fenimorec o n t a i n i n g p h o s p h o r u s c o m p o u n d s h a d as i mi l a r f l i c ke r i ng f l a me a nd t he a p~ a ra nc e ofbe i ng b l own ou t a t e x t i nc t ion , t hou ga i t wa s no tre a l l y b l own o u t b y t he f low o f ox i da n t be c a uset he n va l ue wa s i nde pe nde n t o f t hL , f l ow.A fe w i nd i c e s a re r e po r t e d a t r e ]uc e d p re s -s u r es , T h e y w e r e o b t a i n e d b y b u r n n g s a m p l esin a be l l j a r in which cons tant p : ' e ssure wasma i n t a i ne d .Mos t s a mpl e s t r i p s we re c u t f rom ma t e r i a l smi l l e d t oge t he r a nd p re s se d by Ra l ph Be e rs o ft h i s l a bo ra t o ry . O ur i n t e re s t wa s i n a mm a b i l i tya nd t he o t he r p rope r t i e s o f t he s a mpl e s we rei gnore d .I n d i c es w e r e c o m p u t e d o n t h e a s s u m p ti o n t h a ta i r c on t a i n s 0 .21 oxyge n a nd 0 .7~ n i t roge n .Th e n va l ue s we re u sua l l y r e p roduc i ~ le t o a bou tone pe r c e n t , i nd e pe nde n t o f t he ~ ;a s ve l oc i t yi n t he r a nge 3 t o 12 c m se c -~, a n d i nde pe nde n to f s t r i p t h i c kne s s i n t he r a nge 0 .1 t o 0 .6 c m.T h e o n l y n o t e w o r t h y l i m i ta t io n t o t h e e x p e r i -me n t a l me t hod wa s t ha t some ma t e : : i a l s me l t e dt oo e a s i ly a n d a bs t ra c t e d mo re he ~- .t f rom t hesu r round i ng d i f fu s i on f l a me t ha n wa s r e qu i re dt o p y ro l ys e the po l ym e r . Th e n ob tr~i ne d whe nthis h app en ed was too la rge , a s c~ .n be i l lus-t r a t e d b y t he fo l l owi ng e xa m pl e .A s t r i p o f l o w m o l e c u l a r w e i g h t p o l y p r o p y -l e ne (Epo l e ne NIS ) bu rn t wi t h a l i gh t b l uef l a me wh i c h rode down t he spe c i me n on a wa veof mo l t e n po l yme r a nd l e f t t he c e : a t r e o f t hes t r i p unc onsume d . The n - -_ -0 .23 r ; qu i re d fo rsuc h bu rn i ng wa s h i gh be c a use he a t wa s l o s t t ogive the mo l ten w ave . Th e sam e ~ ~ , te rial cas ta bou t a c e ra mi c wi c k a nd bu rn t ~ s a c a nd l eg a v e n = 0 . 1 7 7 + 0 " 0 0 5 . T h a t t h e lo w e r in d e x w asc ha ra c t e r is t i c fo r po l yp rop y l e ne wa s show n byburning a s t r ip of h igher molecula r v . ' e ight poly-me r (H e rc u l e s P ro fa x 6505) wh i c h d id no tunde rgc e xc e s s i ve me l t i ng a nd ga ve n=0 .174_+0 .002 . W e a vo i de d e xc e s s i ve me l t ing wi t h o t he rma t e r i a l s by us i ng on l y h i gh mol e c u l a r we i gh tpo l ye t hy l e ne , po l y me t h y l me t ha c rs ' la t e , e t c . ,a nd by e xc l ud i ng po l yme rs f rom c ons i de ra t i on

w h i c h w o u l d n o t b u r n d o w n p r o p e r l y f r o m t h ei gn i t e d e nd . F o r e xa mpl e , we d i d no t f i nd aproper end-burning spec imen of poly-cx-methyl -s t y re ne .A re la ted ins tance of excess ive he~t loss f roma diffusio n flame wa s de scribe d by l?J. B. SPA~,-OING ~ wh o bu rn t ke ros ine on a spec ia l burner .

and F. J. blsrtln Vol. 10T h e b u r n e r m a i n t a i n e d a s p h e r e o f l i q u i d f u e l3 . 8 c m i n d i a m e t e r t h r o u g h w h i c h k e r o s i n ec o u l d b e p u m p e d i n d e p e n d e n t l y o f t h e a m o u n tv a p o r i z e d a n d b u r n t . W h e n m o r e f u e l w a sp u m p e d t h r o u g h t h e s p h e r e t h a n w a s b u r n t i nt he su r round i ng f l a me , e xc es s i ve he a t wa sd r a i n e d f r o m t h e f l am e w h i c h b e c a m e s m a l l a n db l ue i n s t e a d o f ye l l ow a nd smoky .0 .6 . . . . , - - '

z

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I,~ s for Yef lon.~ CIL sub stituted , /in potyethytene /

F subst i tu ted/ / ~Bu rn t a t 14 cm , // / 6 o f m e r c u r y p r e s s u r e / /

0'3 , / / // / ~ / / 2 a d d e d to / '/ po lyethylen e ,s/

0"2 ~ t , . - o - ~ p ~, co n ta in inge .~ HCt o r e l=

C2H4,C~H3C( mixturesO . t L ~ s .._0 1 2H a l o g e n o r s u l p h u r a t o m s p e r t w o c a r b o n a t o m s

F i g u r e 1. L i m i t i n g o x y g e n i n d i c e s ] o r m a t e r i a l s c o n .t a i n i n g h a l o g e n s ( o r s u l p h u r ) . A l l s a m p l e s b u r n t a ta t m o s p h e r ic p r es s u re e x c e p t / o r t h e t h r e e p o i n t s i n d i -c a t e d a t 1 4 c m o f m e r c u r y . " ./'h e F s u b s t i t u t e d s a m p l e s~ ,J ere : p o l y v i n y l [ l u or i d e, D u P o n t T e d l a r 2 0 0 S G 4 0T R ; p o l y v i n y l i d e n e f l u o r i d e , K y n a r , " T e f l o n / t o mstocl. The CI s u b s t i t u t e d s a m p l e s w ~ r e : c h l o r i n a t e dM a d e x - 5 0 , o r m i x t u r e s o f M a r l e x - 5 0 0 2 w i t h p o l y v i n y lc h l or i de ; p o l y v i n y l c h l o ri d e , G e o n - I O I o r - 4 0 4 ; p o l y -v i n y l i d e n e c h l o r i d e, D o w S a r a n 2 8 1 S 9 0 5 , T h e, t w os u l p h u r p o i n t s w e r e o b t a i n e d b y m i l l i n g e l e m e n t a r ys u l p h u r i n to M a r l e x - 5 0 0 2 a n d p r e ss i n g . T h e t w o l o w e rc u r v es a r e f o r g a se o u s m i x t u r e s o / e t h y l e n e a n di :h lo r in e c o m p o u n d s b u r n t a s d i / / u s i o n / l a m e sThe m e a su re me n t s a re g i ve n i n t he t a b l e s a ndgraphs . F i g u r e 1 a lso shows indices for a few

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June 1966 Flam mab i l i ty of polymers 13 7g a s e o u s f u e l s . T h e s e w e r e d e t e r m i n e d b y f e e d -i n g t h e f u e l s t h r o u g h a n 0 . 7 e r a d i a m e t e r t u b ec o u n t e r c u r r e n t t o t h e a s c e n d i n g f l o w o f o x i d a n t .T h e f l a m e s o f t h e g a s e o u s f u e l s s u r r o u n d e d t h et i p o f th e f u e l d e l i v e r y t u b e a t a d i s t a n c e w h i c hd e p e n d e d o n t h e f u e l f l o w , u s u a l l y a b o u t I c m .

D i s c u s s i o nW h e n a p o l y m e r b u r n s , t h e o x y g e n m a y b em o s t l y c o n s u m e d i n a n e a r b y c o m b u s t i o n r eg i o nw h i c h g i v e s h e a t t o p y r o l y s e t h e p o l y m e r a n di s f e d b y t h e p y r o l y s i s p r o d u c t s . T h e n e a r b yc o m b u s t i o n r e g i o n m a y b e a d i f fu s i o n f l am e s u r -r o u n d i n g t h e p o l y m e r , a s w h e n p o l y e t h y l e n eb u r n s , o r a r e g i o n o f s u r f a c e c o m b u s t i o n , a sw h e n s i l i c a - f i l l e d s i l i c o n e s b u r n a n d a z o n e o fg l o w i n g s i l i c a w o r k s t h r o u g h t h e s a m p l e .F o r s u c h a m o d e l , t h e o x y g e n i n d e x m i g h tre f l ec t t he ease o r d i f fcu l ty o f pyro lys i s and a l sot h e e a s e o r d i f f i c u l t y o f b u r n i n g t h e p y r o l y s i sp r o d u c t s i n t h e n e a r b y c o m b u s t i o n r e g i o n . T h en = 0 ' 9 5 i n d i c a te d i n Figure I f o r T e f lo n m a y b el a r g e b o t h b e c a u s e T e f l o n i s d i f f i c u l t t o p y r o -l y s e a n d b e c a u s e t h e m o n o m e r , w h i c h i s p r e -s u m a b l y t h e m a i n p y r o l y s i s p r o d u c t , i s d i f f i c u l tt o b u r n . T h e n = 0 . 6 0 i n d i c a te d i n Figure i f o rp o l y v i n y l i d e n e c h l o r i d e i s a b o u t t h e i u d e x r e -q u i r e d t o b u r n c a r b o n , l i s t e d i n Table I; a n ds in c e m u c h t r an s i e n t c h a r i s fo r m e d f r o m p o l y -v i n y l i d e n e c h l o r i d e a n d t h e m a t e r i a l g o e s o u tu n l e s s t h e c h a r i s c o n s u m e d , i t m a y b e t h a tp o l y v i n y l i d e n e c h l o r i d e b u r n i n g c a n b e r e p r e -s e n t e d t h u s :

(C2Ho.CI.~)~ ---+ c ha r + HC1 (py roly sis)C h a r + O .., - -- >- c a r b o n o x i d e s ( c o m b u s t i o n )a n d t h a t t h e l i m i t i n g n m o s t l y r e f l e c t s t h e d i f f i -c u l t y o f b u r n i n g t h e p y r o l y s i s p r o d u c t .P o l y v i n y l i d e n e c h l o r i d e i s ex c e p t i o n a l b e c a u s ei t d o e s n o t g i v e m u c h f l a m m a h l e g a s o n p y r o -l y s i s . M o s t p o l y m e r s g i v e o f f g a s e s w h i c h a r em o r e c o m b u s t i b l e t h a n t h e o r i g i n a l m a t e r i a l .T h u s , t h e n = - - 0 . 1 5 r e q u i r e d t o b u r n p o l y o x y -m e t h y l e n e i s l a r g e r t h a n t h e n q u o t e d i n t h ei n t r o d u c t i b n a s n e c e s s a r y t o b u r n t h e p r o b a b l ep y r o l y s i s p r o d u c t s , m o n o m e r i c f o r m a l d e h y d e o rh y d r o g e n a n d c a r b o n m o n o x i d e , w h e n t h e l a t -t e r a r e s u p p l i e d i n d e p e n d e n t l y t o a d i f f u s i o nf l a m e . I t i s r e a s o n a b l e t o a t t r i b u t e t h e l a r g e r nf o r p o l y o x y m e t h y l e n e t o t h e n e e d t o d e c o m p o s et h e p o l y m e r i n o r d e r t o g e t t he f l a m m a b l e g a s e s .

Table !. Limiting oxygen indices o/vavio u, materials(at atmospheric pressure unless o~,huwise stated)Polyoxymethylene nDelrin 0.150 4-0.003Celcon, O. 1 cm thick 0.148 4- 0.001

0"3 cm thick 0 .1 4 9Celcon at 14 cm H g pressure 0'190Polyethylene oxide,Polyox WSR-35, Union Carbide 0.150Kitchen candle 0.16Polymethyl methacrylate,Rohm and ltaas Plexiglas 0.173 + 0.001Polypropylene , Hercules Pro[ax 6505 0.174 + 0"002PolyethyleneAllied Chem-1220 0.174+0.001Phillips Marlex-5002 0.175 + 0.001Marlex at 14 cm Hg 0"210Polystyrene0.1 cm thick ]rom stock 0.1510"6 cm thick, Westlatce hightemperature sheet 0"183Polybutadiene, erosslinked withdicumyl peroxide . 0"183Polyvinyl alcohol, DuPont Elvanol70-0.5 0-225 4- 0.004Chlorinated polyether,Penton, Hercules[.C H,,---C (CH CI) ,---CH.,---C~ ]- 0'2 32

Polycarbonates, various clear Le~:anresins, General Electric[C, H.t--C(CHz).f.--C Ha---OC(O )O] 0.26 to 0.28Polyphenylene oxide, General Electric[C6H.,(CH3).,--O 0"28 to 0"29GraphitePorous carbon, PC-25, NationalCarbon 0.559 4-0.003Carbon electrode, L 8109, NationalCarbon 0"635Carbon electrode at 50 cm Hg 0.80Carbon electrode at 24 cm Hg 1"00Asbestos if[led polypropyletce, "slowburning" ]MDC-4400, Union Carbide 0.205+_0.005"Sell-extinguishing' polypropylene,]MDA-9490, Union Carbide, probably contains chlorine and antim ony oxide 0.282+0.003Silica-filled silicone rubber, GeneralElectricSE 9029 0'3C~3+0"005SE 9036 0"326+0"005SE 9036at 14 cm fig 0'391

I n t h e s e r i e s [ C H . , C H X - ] , i t c a n b e s e e n f r o mTable I a n d Figure I t h a t n i n c r e a s e s a s X i sc h a n g e d f r o m C H 3 o r H , t o C c H a , t .3 O H o r F ,t o C I . T h i s m i g h t b e t h e o r d e r o f i n c r e a s i n gs t a b i l i t y a g a i n s t p y r o l y s i s a s i t o c c u r s i n b u r n -i n g , a l t h o u g h i t i s n o t t h e o r d e r a c c o r d i n g t oS . L . M A D O R S K Y f o r t h e r m a l d e c o m p o s i t i o n a tr a t e s t h o u s a n d s o f t i m e s s l o w e r t h a n t h e d e c o m -p o s i t i o n r a t e s o f b u r n i n g p o l y m e r s . M a d o r s k yg i v e s C ,~ H ~, t o C H a o r F , t o H .W e i n v e s t i g a t e d h o w c h l o r i n e i n h i b i t s t h e

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1 38 C . P . F e n i m o r e a n d F . J . M a r t i n Vol. 10b u r n i n g o f .[ C H , C H X ~ a s X i s c l ta n g e d f r o mH t o e l ; w h e t h e r m o s t l y b y a f fe c ti n g t h e p y r o -l y si s, o r m o s t l y b y m a k i n g t h e g a s p h a s e f l a m ere a c t i ons more d i f f i c u l t . I f t he l a t : e r we re t hecase , n s ho uld " t ,~ a ffec ted abou t as s t rong ly bymi x i ng e qu i va i e ~ t c h l o r i ne i n t o t he a t mosphe rea s by subs t i t u t i ng c h l o r i ne i n t he po l yme r . Thee qu i va l e nc e o f c h l o r i ne i n t he a t t ro sphe re c a nbe ca lcula ted b .~ i, suppo s ing t ha t the ra t io ofc h l o r i ne t o oxy g t n e n t e r i ng t he r a m ~ i s the s a m ea s t he r a t i o mi xe d i n t he a t mosphe re , a nd bya s sumi ng some ove ra l l s t o i c h i ome t ry , s a y[CHoCH 2-]-+ 30= ~ 2CO~. + 2H~O. On thesea s sumpt i ons , t he n va l ue s we re de : e rmi ne d fo rt h e c u r v e i n F i g u r e I l a be l le d ' po l y e t hy l e ne bu rn ti n a t mosp he re s c on t a i n i ng HC1 o r C I= '. B y c om-pa r i son wi t h t he c u rve l a be l l e d ' C . subs t i t u t e di n po l ye t hy l e ne ' , i t i s c l e a r t ha t c h o r ine a dde dt o t he a t mosphe re i s i ne f fe c t i ve .T h e t w o l o w e z c u r v e s i n F i g u r e I r e p re se n te xpe r i me n t s i n wh i c h ga se ous mi x t u re s o f e t hy -l e ne a nd c h l o r i ne c ompounds we : ' e bu rn t a sd i f fu s i on f l a me s . The y show t ha t c h l o r i ne doe sno t i nh i b i t ga s -pha se f l a me re a c t i ons o f hyd ro -c a rbons a s s t rong l y a s i t i nh i b i t s po l ye t hy l e neburn i ng whe n i t i s subs t i t u t e d i n t he po l yme r .The re fo re , subs t i t u t e d c h l o r i ne p roba b l y i n -c re a se s n b y a f fe c ti ng t he py ro l ys i s .I t i s o f t en sugge s te d t ha t hyd ro ge n c h l o r i de issp l it ou t a n d a dd i t iona l c a rbon-c ~ , rbon bondsa re fo rme d whe n po l yv i ny l c h l o r i de be g i ns t ode c ompose . An a na l ogous sp l i t t i ng rmt o f wa t e ra n d f o r m a t i o n o f ~ d d i t i o n a l c a : b o n - c a r b o nbon ds i s poss ib le in poly viny l a lco hd , n--=0.225,bu t n o t i n t he i some r i c po l ye t hy l e ne ox i de ,n = 0 .15 . I f t he a dd i t iona l bond i ng de c re a se d t hege ne ra t i on o f f la mm a b l e gases , t he da m ma b i l i tyo f t he po l y me r wou l d be de c re a se d .Al l the p olym ers which cont~ ; ined muchc h l o r i ne c ha r re d a s t he y bu rn t ; a s d i d po l yv i ny la l c oho l, t houg h t o a s , ~ t ll er e x t e w t ha n po l y -v i ny l c h l o r i de , I t wa s s t a t e d i n t he c xpe r i me n t a ls e c t i on t ha t po l yme rs wh i c h c ha r t e d he a v i l yse e me d t o e x t i ngu i sh by smot he r i ng t he i r ownb u r n i n g ; a n d o n e m i g h t w o n d e r i t t h e f o r m a -t ion of much char is a suffic ient exp'~anationfor the decrease in flam~aabil i ty in such cases.W e t h i n k n o t . S i m i l a r h e a v y c h a r s ,t o n o t m a k ea l l po l yme ~ e qua l l y no : n - f l a mma bh ; . P o l yv i ny lc h l o ri d e h a s n = 0 . 4 5 a c c o rd i n g t o F i g u re ! ;P e n t o n [ - C H . , - -C ( C H : C I ) . , - -C H ~ - - O - ] w h id ~

c on t a i ns al mos t a s muc h c h l o r i ne a nd a l so c ha r sh e a v i ly h a s n = 0 . 2 3 a c c o r d in g t o T a b ! e 1 . T h ee a s ie r b re a k -up o f t he po l ye t he r c ha i n s t il l te l lse ve n i n s i mi l a r l y c ha r r i ng ma t e r i a l s ; a nd t hed e c o m p o s in g p o l y v i n y l ch l o r id e m o r e t h a n t h ede c ompos i ng c h l o r i na t e d po l ye t he r mus t got h rough a r e l a t i ve l y i ne r t , pe rh a ps c ros s l i nke d ,s t a ge wh i ch doe s no t r e a d i l y ge ne ra t e f l a mma b l egases.O t h e r k n o w n w a y s o f m a k i n g p o l y m e r s l e s sf l a mma b l e a re i l l u s t r a t e d by t he r e ma i n i ngf igures and tables . F i g u r e 2 shows t ha t a n t i -

0"30 Sol id c i r c le s , SbzO in C2H3. Br0 0 e l( Po l y e thy l ene p l us t e t r ab r omo benzene)

0 - 2 5 ' f f t g ? o p . . . . i rc ie s , S b z O 3 i n C z H 3t C |o .2 o ~/ f . _ \Open t ri ang l es , i n C 2H 394 C to .o t4 )/ / ' 7 - " ~ . . . .

Squar es , Sb l O3 in " ~ , ,C z H 3 n C I0 0 20 ~ . ~ .0"20, ~ . o In C 2H 4 (p0 1ye thy Len e)

' I0 " 150 0 " 5 1"0M o l e s S b 2 0 3 p e r 1 0 0 C 2 u n i t s

Figure 2. Inhibition by antim ony trioxide in poly-ethylenes containing chlorine or brominemony t r ioxide (Sb. ,Os) i s not an inhibi tor inpo l ye t hy l e ne i n t he a bse nc e o f ha l oge n , bu t i ti s i n t he p re se nc e o f c h l o r i ne o r b romi ne . Theamount of chlor ine present i s not c r i t i ca l a slong as i t i s enough; a chlor ina ted polye t i ly leneof com po sition C2H~.o.tClo.o,4h a s a b o u t t h e sa m eoxy gen index as one of com po si t ion C~H~.~CIo.=whe n 0 .5 mo l e o f S b20 ~ i s a dde d pe r 100 C2units . Th e co m pos it ion C2H~.98Clo.o2 con tainst oo l i t t l e c h l o r i ne t o de ve l op t he i nh i b i t o rya c ti on o f a n t i mo ny fu l l y . I t i s c om mo nl y be -l ieved tha t the inhibi t ion involves some ant i -mon y-c h l o r i ne c om pound , a nd suc h a v i e w iscons is tent wi th F i g u r e 2 a l t h o u g h w e d o n o tknow why t he i nh i b i t i on shou l d s a t u ra t e a ta r o u n d n = 0 " 2 6 . T a b l e 2 r e c o rds t he r e su l t s o fa dd i ng some o t he r c ompounds t o c h l o r i na t e d

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June 1966 Flammabil ity of polymers 139Polyethylene, none of th em being as effective asantimo ny trioxide.T a b le 2 . L im i t in g o x y g e n in d ic e s / o r m ix tu r e s o ] tO0C~H,.sCIo.a uni ts (ch lor ina ted po lye thy lene) p lus 0"5or 1 mo le o f var ious add i t ives

0 .5 mote o n0.211o l l eB i 2 0 a 0 . 2 1 6A s . ~ O s 0 . 2 3 9S b ~ , O 0 . 2 5 7

! m o l e o ft r icresy l phosphate 0'235Sb20s 0"257S n O ~ O . 2 2 7P b O 0 . 20 8

0'25

l Of~n circles; phosphonitr i l lc ehlor ide,PNCl2~CV~w d c i rc les ; t ric resy t phoc~pi'mte ]~ .o

0"2[ / r r iph,, ~yt phospl '~,teA f A S 2 03/ / $ b 2 0 3h J B i2 O 3 I2 '5 5 !0 7"5M o l e s a d d i t i v e p e r 100C 2 H 4 u n i t s

F ig u r e 3 . L im i t in g o x y g e n ind ic es fo r p o ly e th y le n e ,Mar lex -5002 , con ta in ing var ious addi t ivesF i g u r e 3 illustrates the action of phosphorus

compounds in Polyethylene, and add itional datain T a b l e 3 show that triphenyl phosphite andtrip henyl phosph ine are as effective as ~ne phos-phate. The similar effects of different phosph oruscompounds prompt the question whether theymight all form the same species at the surfaceof the bur nin g Polymer. For example, couldreduced phosphorus species be oxidized at thesurface? This is not impossible; some prelimin-ary analyses of gases collected within the diffu-sion flame, 0.1 cm from the suface of burningpolyethylene, show them to he pretty well oxi-dized although the concentration of molecularoxygen is very small at this point. The carbon

T a b le 3 . L im i t in g o x y g e n in d ic e s fo r m ix tu r e s o lpolye thy lene (100 CsH uni ts ) p lus tw o mo le s o ta d d i t i v e ( e x c e p t w h e r e n o te d o th e rw is e )A d d i t i v e s nN o n e 0.175T r ip h e n y l p h o s p h a te 0.194 t: 0.001T r ip h e n y l p h o s p h i t e 0.195T r ip h e n y l p h o s p Mn e 0.193T r ip h e n y l me th a n e 0.177Dip h e n y l s u lp h o n e 0.177Dip h e n y l c a r b o n a te o.177Dicumyl perox ide* 0.172L e a d d io x id e O. 178Bisphe nol A O"177T e t r a b r o mo b i s p h e n o l A t 0.203Sodium ch lor ide* 0.179B o r ic o x id e , BoO3 0"175B o r a x , NaaBdO 10H20 0" i80

*1 mole dtc-myl peroxide tmr 100 C,2H units. Two sampleswerep r e p a r e d ; one ~ncsscd at 125~C (uncured), the other cured 0 '5 hat 1550C. Both gave he ~ntc n.*Compare withFlsure 1. Br added as tetrabromo bisphenolA d o e snot increase n m uch re..re *hall that added as tetrabromobenzeae./;Twentymoles NaCI Ixr tOO C2H4 units. N~21ha s liUleffect com-pared to equivalentC1 substituted n polyethylene.1 mole Na2BdO 10H20per 100 C2H units,

in the gas phase is present in about equalamounts as unsaturated hydrocarbons, as car-bon monoxide, and as carbon dioxide; thehydrogen in the gas phase is about a thirdpresent in unsaturated hydrocarbons, a sixth inmolecular hydrogen, and a half in stemla.From these experiments, the oxygen indexseems a useful measure of pol ymer flammability.It is easy, reproducible, and numerical. Flame-resistant plastics are usually called 'slow burn-ing', 'self-extinguishing', etc., bu t these quali-tative terms could be recast numerically. Forexample, T a b l e ! shows that 'self-extinguishing'plastics can have n as small as 0.282 ('self-extinguishing' polypropylene) or 4).26 to 0.28(polycarbone.tes), while 'slow burning' plasticscan have n as large as 0.225 (polyvinylalcohol).Perhaps any substance for which n is largerthan about 0.27 can be considered 'self-extin-guishing'.

R e f e r e n c e st St,~tMc,~s, R. F. and WOLFHARD, H. G. C o mb u s -t io n ~ F la me , 1957, 1, 1552 thLL, A. R., McCoUBREY, J. c. and WOLFHARD,H. G . C o mb u s t io n "6, F la me , 1957; !, 533 Si'ALDtXG, D. B. S o m e F u n d a m e n t a l s o f C o m b u s -t ion , p 126. Batterworths: London; AcademicPress: New York, 1955 MADOJ:SKV, S. L. Therm al Degradat ion o[ OrganicPoly;,,ers, p 293, Interscienee: New Yorh, 1964