Oxidative pyrolyisis of polymers before flaming ignition

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Oxidative pyrolyisis of polymers before flaming ignition

National Research

Conseil national

N21d

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Council Canada

de recherches Canada

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no.

1375

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BLDG

Institute for

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lnstitut de

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Construction

construction

Oxidative Pyrolysis of Polymers

Before Flaming Ignition

by A. Baignee and F.R.S. Clark

Reprinted from

Fire Safety Science

-

Proceedings of the

First International Symposium

Gaithersburg, MD, 9-1 1 October 1985

p. 381 -390

(IRC Paper No. 1375)

Price $2.00

NRCC 25922

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Oxidative Pyrolysis of Polymers

.

i

before Flaming Ignition

ALISON BAIGN~E

Department of Chemical Engineering University of Ottawa, Canada

1 FERRERS R. S. CLARK

Division of Building Research National Research Council Canada Ottawa, Canada, KIA OR6

ABSTRACT

Thin f i l m s of p o l y e t h y l e n e , p o l y s t y r e n e and poly(methy1 m e t h a c r y l a t e )

were exposed t o t h e wake of a l e a n hydrogen-oxygen f l a t flame. Exposure was

t e r m i n a t e d a t t i m e s r a n g i n g up t o t h e p o i n t when f l a m i n g combustion began. The f i l m s were t h e n a n a l y z e d by i n f r a r e d s p e c t r o s c o p y .

O x i d a t i o n of p o l y e t h y l e n e and p o l y s t y r e n e o c c u r r e d a f t e r a l a t e n c y p e r i o d but b e f o r e i g n i t i o n . The o x i d a t i o n r a t e remained c o n s t a n t d e s p i t e changes i n flame-polymer s e p a r a t i o n and f l a m e g a s e q u i v a l e n c e r a t i o . The l a t e n c y p e r i o d b e f o r e o x i d a t i o n observed f o r p o l y e t h y l e n e depended on e q u i v a l e n c e r a t i o and s e p a r a t i o n . No o x i d a t i o n was observed w i t h poly(methy1 m e t h a c r y l a t e ) .

A comparison of i n f r a r e d s p e c t r a o b t a i n e d by t r a n s m i s s i o n through f i l m s w i t h s p e c t r a o b t a i n e d by i n t e r n a l r e f l e c t i o n s p e c t r o s c o p y , demonstrated t h a t o x i d a t i o n of p o l y e t h y l e n e and p o l y s t y r e n e b e f o r e i g n i t i o n i s c o n f i n e d t o w i t h i n a p p r o x i m a t e l y 5 u m of t h e exposed s u r f a c e . KEYWORDS P o l y e t h y l e n e , p o l y s t y r e n e , poly(methy1 m e t h a c r y l a t e ) , hydrogen-oxygen, f l a m e , i n f r a r e d s p e c t r o s c o p y , o x i d a t i v e p y r o l y s i s , i g n i t i o n . INTRODUCTION

Study of chemical changes t h a t occur d u r i n g polymer combustion h a s been p r i m a r i l y c o n f i n e d t o i n d i r e c t methods s u c h a s t h e r m a l a n a l y s i s and t o a n a l y s i s of g a s e s e v o l v e d d u r i n g p y r o l y s i s o r formed i n t h e g a s phase d u r i n g b u r n i n g ( 1 ) . S t u d i e s of t h e chemical changes i n t h e condensed phase of polymer combustion systems have u s u a l l y been made d u r i n g s t e a d y - s t a t e c a n d l e - l i k e combustion of p o l y o l e f i n s ( 2 , 3 ) , by n e u t r o n a c t i v a t i o n a n a l y s i s f o r oxygen. This work h a s r e s u l t e d i n a c o n t r o v e r s i a l t h e o r y t h a t o x i d a t i v e p y r o l y s i s can be an i m p o r t a n t c o n t r i b u t o r t o t h e energy b a l a n c e a t t h e s u r f a c e of a b u r n i n g polymer ( 4 ) .

T r a n s i e n t phenomena t h a t o c c u r on b u r n i n g polymer s u r f a c e s have n o t been e x t e n s i v e l y s t u d i e d . J a k e s and Drews ( 5 ) r e p o r t e d o x i d a t i o n of p o l y p r o p y l e n e i n samples removed from a s l a b a l o n g which a f l a m e had been allowed t o burn;

samples were a n a l y z e d by iodometry f o r p e r o x i d e and h y d r o p e r o x i d e and by

hydrazine-hydrazone d e r i v a t i z a t i o n f o r c a r b o n y l f u n c t i o n s .

I I .

Nothing a p p e a r s i n t h e l i t e r a t u r e on condensed phase c h e m i c a l changes

d u r i n g t h e i g n i t i o n d e l a y of polymers exposed t o f l a m e s . I n p r e v i o u s

p u b l i c a t i o n s (6-8) C l a r k r e p o r t e d d i f f e r e n c e s between t h e g a s phase e v e n t s

h

s u r r o u n d i n g s u c h i g n i t i o n of p o l y s t y r e n e , p o l y e t h y l e n e and poly(methy1

m e t h a c r y l a t e ) . S p e c t r o m e t r i c measurements of o x i d a t i o n on t h e s u r f a c e of

t h o s e polymers, made d u r i n g t h e i g n i t i o n sequence u s i n g t h e same a p p a r a t u s and

-

e x p o s u r e c o n d i t i o n s , a r e r e p o r t e d i n t h i s paper.

EXPERIMENTAL DESCRIPTION L

M a t e r i a l s

Low m o l e c u l a r w e i g h t poly(methy1 m e t h a c r y l a t e ) powder ( A l d r i c h 18, 223-0) was h o t p r e s s e d o n t o aluminum f o i l (221°C, 8.6 m a ) t o form f i l m s of 40-100 pm t h i c k n e s s . High d e n s i t y p o l y e t h y l e n e p e l l e t s ( A l d r i c h 181, 190-0) were h o t p r e s s e d o n t o aluminum f o i l (154'C, 17 MPa) t o form f i l m s of 60-70 pm t h i c k n e s s . P o l y s t y r e n e p e l l e t s ( A l d r i c h 18242-7) were h o t p r e s s e d o n t o aluminum f o i l (204OC, 17 MPa) t o form f i l m s of 50-80 pm t h i c k n e s s . Exposure

The b u r n e r u s e d ( F i g . 1) was a f t e r a d e s i g n of Botha and S p a l d i n g ( 9 ) , u s i n g p r i n c i p l e s d e s c r i b e d by Hunter and H o s h a l l (10). A l e a n hydrogen-oxygen f l a t f l a m e was s u p p o r t e d above t h e polymer f i l m sample, which was sandwiched

l H E R M O C O U P L E

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1

C O O L I N G W A T E R C H A M B E R \ I - I I 0 8 ! , / J A C K E T ( B R A S S ) I B R O N Z E S I N T E R S A M P L E PLATE ( B R A S S 1 W A ~ E R W A ~ E R ~ O U T I N . g u r e 1. The b u r n e r

between a m e t a l p l a t e w i t h a c e n t r a l c i r c u l a r o r i f i c e of 4.0 cm d i a m e t e r and

;

_{an a s b e s t o s mat 2 rmn t h i c k on t h e w a t e r c o o l e d b a s e .}

The f l a m e was i n t h e form of an i n v i s i b l e f l a t c i r c u l a r d i s k 25 mm a c r o s s

4

and i t s wake e x t e n d e d t o t h e polymer s u r f a c e . I g n i t i o n of a polymer f i l m was s i g n a l l e d by t h e f i r s t a p p e a r a n c e of a v e r y b r i g h t d i s k of l i g h t j u s t above t h e polymer s u r f a c e . Heat t r a n s f e r was p r i m a r i l y by c o n v e c t i o n ; a t an e q u i v a l e n c e r a t i o of 0.1 t h e t o t a l f l u x a t t h e polymer s u r f a c e had a maximum

-

_{measured v a l u e of 19.7 k ~ . m - ~ , of which no more t h a n 3 k w ~ m - ~ was by}

r a d i a t i o n . ( E q u i v a l e n c e r a t i o i s d e f i n e d a s t h e volume of oxygen r e q u i r e d f o r complete c o n v e r s i o n of t h e hydrogen t o w a t e r d i v i d e d by t h e a c t u a l v a l u e s of oxygen s u p p l i e d . ) The b u r n e r g a s e s moved w i t h a v e l o c i t y t h a t e n s u r e d l a m i n a r

'

_{f l o w ; t h e Reynolds number was about 558.}

I n f r a r e d S p e c t r o s c o p y

A N i c o l e t Model 6000 F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o m e t e r was employed, f i t t e d w i t h a broad bandwidth mercury-cadmium t e l l u r i d e (MCT) d e t e c t o r . Each s p e c t r u m was t h e a v e r a g e of 200 s c a n s c o l l e c t e d a t 4.0 cm-l r e s o l u t i o n w i t h Happ-Genzel a p o d i z a t i o n and s i n g l e o r d e r z e r o f i l l i n g .

S p e c t r a l e x a m i n a t i o n of t h e f i l m s was conducted i n two ways. The o b j e c t of t h e f i r s t , termed t h e r e f l e c t i o n - t r a n s m i s s i o n t e c h n i q u e , was t o d i r e c t t h e c o l l i m a t e d beam of t h e s p e c t r o m e t e r t h r o u g h t h e e n t i r e a r e a of exposed f i l m , t o r e f l e c t o f f t h e aluminum f o i l , and t h r o u g h t h e f i l m a second time. T h i s was a c h i e v e d by r e p l a c i n g an e x i s t i n g p l a n e m i r r o r n e a r t h e d e t e c t o r w i t h t h e f i l m - f o i l l a m i n a t e , complete w i t h t h e metal p l a t e w i t h t h e 4 cm o r i f i c e u s e d i n t h e b u r n e r exposure. The c o l l i m a t e d beam of t h e s p e c t r o m e t e r i s

a p p r o x i m a t e l y 5 cm i n d i a m e t e r , and t h u s a b s o r p t i o n s over t h e e n t i r e flame- exposed f i l m were observed. Commercial a c c e s s o r i e s f o r t h e s p e c t r o m e t r i c e x a m i n a t i o n of l a r g e a r e a s p r o v i d e d c o v e r a g e of l e s s t h a n

117 of t h e a r e a

covered by t h i s t e c h n i q u e .

The second method of s p e c t r o s c o p i c i n s p e c t i o n i n v o l v e d t h e u s e of a c o n v e n t i o n a l i n t e r n a l r e f l e c t i o n a p p a r a t u s ( a Wilks Model 9 ) f i t t e d i n t o t h e normal sample p o r t of t h e N i c o l e t s p e c t r o m e t e r . A nominal 30 r e f l e c t i o n KRS-5

(45' b e v e l ) element was employed t h r o u g h o u t . S p e c t r a l d e c o n v o l u t i o n

I n an a t t e m p t t o d e t e r m i n e t h e mode of o x i d a t i o n of polymer f i l m s , F o u r i e r s e l f - d e c o n v o l u t i o n t e c h n i q u e s ( 1 1 ) were a p p l i e d t o bands o b s e r v e d i n t h e c a r b o n y l r e g i o n of i n f r a r e d s p e c t r a of sample f i l m s of e a c h polymer a f t e r f l a m e exposure. The i n t r i n s i c bandwidths of i n f r a r e d a b s o r p t i o n s of t h i s c l a s s a r e o f t e n g r e a t e r t h a n t h e s e p a r a t i o n between bands. The d e c o n v o l u t i o n t e c h n i q u e computes t h e bandshapes a t reduced bandwidths, a l l o w i n g r e s o l u t i o n of o t h e r w i s e h o p e l e s s l y o v e r l a p p i n g bands. The t e c h n i q u e was found most v a l u a b l e i n t h i s s t u d y i n d e m o n s t r a t i n g t h a t o x i d a t i o n had n o t o c c u r r e d ; i n t h i s c a s e d e c o n v o l u t i o n of s p e c t r a of f i l m s b e f o r e and a f t e r f l a m e e x p o s u r e

.

showed t h a t no new bands had been formed. The t e c h n i q u e a l s o allowed t h e

complexity of t h e o x i d a t i o n p r o c e s s f o r p o l y e t h y l e n e and p o l y s t y r e n e t o b e observed ( F i g s . 2 , 3).

Data h a n d l i n g

S p e c t r a o b t a i n e d o f exposed polymer f i l m s by t h e r e f l e c t i o n t r a n s m i s s i o n t e c h n i q u e were s t o r e d f o r l a t e r p r o c e s s i n g . The t r a n s m i s s i o n s p e c t r a were c o n v e r t e d t o a b s o r b a n c e s p e c t r a . S p u r i o u s bands due t o a d v e n t i t i o u s w a t e r i n

-

0 . 3 5 0 a t 0 . 2 8 3 Y

I

g

0 . 2 1 6 a m w 0 VI ca 0 . 1 4 9 a 0 . 0 8 2 L 0 . 0 1 5 1 96 W A V E N U M B E R S

F i g u r e 2. Deconvolved s p e c t r a of p o l y e t h y l e n e b e f o r e and a f t e r exposure

0 . 5 9 0 . 4 8 u

g

0 . 3 7 a m w 0 VI m 0 . 2 6 0 . 1 5 0 . 0 4 1 7 8 1 1757 1733 1 7 0 9 1685 1 6 6 1 1 6 3 1 W A V E N U M B E R S F i g u r e 3. Deconvolved s p e c t r a of p o l y s t y r e n e b e f o r e and a f t e r e x p o s u r e

t h e system were removed by automated s u b t r a c t i o n . Next a s m a l l p o s i t i v e y

a x i s s h i f t was added by coadding a s t r a i g h t l i n e t o each spectrum. A spectrum

of unexposed polymer was t h e n s u b t r a c t e d from t h a t of t h e exposed polymer. A t

t h i s s t a g e t h e e f f e c t s of o x i d a t i o n were c l e a r l y a p p a r e n t , e s p e c i a l l y i n t h e

c a r b o n y l r e g i o n of t h e spectrum. With s u i t a b l e b a s e l i n e c o r r e c t i o n , t h e a r e a

under t h e c a r b o n y l peak was t h e n c a l c u l a t e d (1770-1635 cm-l f o r : = l y s t y r e n e ,

1770-1683 cm-' f o r p o l y e t h y l e n e ) . At v e r y low l e v e l s of o x i d a t i o n , n o i s e i n

t h e spectrum o c c a s i o n a l l y l e d t o n e g a t i v e a r e a s b e i n g r e p o r t e d ; t h i s problem

'?

_{was avoided by t h e y a x i s d i s p l a c e m e n t mentioned above.}

1

RESULTS

I

I g n i t i o n d e l a y s

I

-

_{t h i c k s l a b s .} P r e v i o u s work w i t h t h i s equipment (6-8) was conducted w i t h t h e r m a l l y - _{I t was of i n t e r e s t t h e r e f o r e t o check t h a t i g n i t i o n d e l a y t i m e s}

I

w i t h polymer f i l m s were a s r e p r o d u c i b l e a s t h o s e r e c o r d e d w i t h polymer s l a b s . I Table 1 shows t h e r a n g e of v a l u e s o b t a i n e d i n r u n s of 10 e x p o s u r e s t o i g n i t i o n

I

i n v a r i o u s c o n d i t i o n s ; t h e r e p r o d u c i b i l i t y r e c o r d e d gave c o n f i d e n c e t h a t d i f f e r e n t f i l m s exposed f o r v a r y i n g t i m e s be£ o r e s p e c t r o s c o p i c a n a l y s i s could be meaningf u l l y compared. TABLE 1. I g n i t i o n d e l a y t i m e s a t e q u i v a l e n c e r a t i o 0.1; f i g u r e s a r e a v e r a g e s of 5 d e t e r m i n a t i o n s P o l y e t h y l e n e P o l y s t y r e n e Poly(methy1 m e t h a c r y l a t e ) S e p a r a t i o n (mm) 20 6.1

*

0.2 13.0 t 2.0 8.5

*

0.5 15 5.3 t 0.1 8.5 t 0.5 10 4.4 t 0.2 4.7 t 0.2 C o n s i s t e n t w i t h t h e i n c r e a s e i n g a s t e m p e r a t u r e ( 6 ) a t t h e polymer s u r f a c e a s burner-polymer s e p a r a t i o n d e c r e a s e d , t h e i g n i t i o n d e l a y s i n c r e a s e d w i t h s e p a r a t i o n . Homogeneity

O x i d a t i o n b e f o r e i g n i t i o n was observed f o r p o l y e t h y l e n e and p o l y s t y r e n e , but n o t f o r poly(methy1 m e t h a c r y l a t e ) ( F i g s . 2, 3). For t h o s e f i l m s t h a t o x i d i z e d , s u b s t a n t i a l v a r i a t i o n i n c a r b o n y l c o n c e n t r a t i o n was observed a c r o s s t h e exposed s u r f a c e , r a n g i n g from n o o x i d a t i o n t o a maximum, even a t t h e end of t h e i g n i t i o n d e l a y . The r e a s o n f o r t h i s i s n o t c l e a r b u t i t c o u l d have been due t o i r r e g u l a r i t i e s i n t h e f l a t flame. However, s p e c t r o s c o p i c examination by t h e r e f l e c t i o n - t r a n s m i s s i o n t e c h n i q u e a l l o w e d measurement of t h e a v e r a g e d e g r e e of o x i d a t i o n of t h e whole exposed f i l m f o r a given exposure time.

L o c a t i o n of O x i d a t i o n

Films of p o l y s t y r e n e and p o l y e t h y l e n e were exposed t o t h e f l a m e u n t i l o x i d a t i o n was a p p a r e n t i n t h e r e f l e c t i o n - t r a n s m i s s i o n spectrum. The exposed and unexposed f a c e s of e a c h f i l m were then examined by i n t e r n a l r e f l e c t i o n

- s p e c t r o s c o p y . I n t h i s t e c h n i q u e , t h e d e p t h of p e n e t r a t i o n of t h e a n a l y z i n g i n f r a r e d beam i s about 1 pm. The exposed f a c e s of f i l m s of b o t h polymers showed s u b s t a n t i a l o x i d a t i o n p r o d u c t s i n b o t h c a r b o n y l ( a p p r o x i m a t e l y 1700 cm-l) and hydroxyl ( a p p r o x i m a t e l y 3200 cm-l) r e g i o n s . By c o n t r a s t , t h e unexposed f a c e s of both polymers showed n o s i g n of o x i d a t i o n . These

e x p e r i m e n t s d e m o n s t r a t e t h a t o x i d a t i o n upon exposure t o f l a m e d i d n o t p e n e t r a t e r i g h t t h r o u g h t h e polymer f i l m s .

I

I G N I T I O N

I

I G N I T I O N I 1 0 m m l

I

TIME, s F i g u r e 4. O x i d a t i o n of p o l y s t y r e n e b e f o r e i g n i t i o n , r e l a t i v e a r e a of carbonyl band; 0.1 e q u i v a l e n c e r a t i o : 0 10 m s e p a r a t i o n , 15 mm s e p a r a t i o n , o 20 mm s e p a r a t i o n ; n 20 mm s e p a r a t i o n , 0.13 e q u i v a l e n c e r a t i o , A 20 mm s e p a r a t i o n , 0.15 e q u i v a l e n c e r a t i o

The r e l a t i v e s i z e s of t h e 0-H s t r e t c h and t h e a d j a c e n t C-H s t r e t c h e s were a p p r o x i m a t e l y t h e same i n t h e s u r f a c e and by r e f l e c t i o n - t r a n s m i s s i o n ,

i n d i c a t i n g t h a t t h e b u l k of t h e o x i d a t i o n was v e r y c l o s e t o t h e exposed s u r f ace.

V a r i a t i o n of Flame-Polymer S e p a r a t i o n and Flame Gas E q u i v a l e n c e R a t i o

F i g u r e s 4 t o 6 show t h e changes i n c a r b o n y l c o n c e n t r a t i o n i n p o l y s t y r e n e and p o l y e t h y l e n e f i l m s f o r v a r i o u s e x p o s u r e t i m e s a t t h r e e d i f f e r e n t burner-polymer s e p a r a t i o n s and t h r e e d i f f e r e n t f l a m e g a s e q u i v a l e n c e r a t i o s . Each p o i n t r e p r e s e n t s a n a l y s i s of t h e c a r b o n y l r e g i o n of t h e s p e c t r u m of a d i f f e r e n t , b u t v e r y s i m i l a r , f i l m exposed t o t h e f l a m e and a n a l y z e d by t h e r e f l e c t i o n t r a n s m i s s i o n t e c h n i q u e .

D e s p i t e changes i n flame-polymer s e p a r a t i o n and f l a m e g a s e q u i v a l e n c e r a t i o , t h e l o c u s of a l l p o i n t s on t h e o x i d a t i o n - t i m e p l o t f o r p o l y s t y r e n e was c o n s t a n t t o t h e l i m i t s of e r r o r ( F i g u r e 4). O x i d a t i o n began a f t e r a v e r y s i m i l a r p e r i o d of e x p o s u r e and o x i d a t i o n t o produce c a r b o n y l f u n c t i o n s o c c u r r e d a t t h e same r a t e u n t i l i g n i t i o n . Time t o i g n i t i o n , a s n o t e d a b o v e , was r e l a t e d t o t h e flame-polymer s e p a r a t i o n .

The o n s e t of o x i d a t i o n of p o l y e t h y l e n e was more s e n s i t i v e t o flame- polymer s e p a r a t i o n , b u t once o x i d a t i o n had begun, t h e r a t e of o x i d a t i o n was s i m i l a r a t t h e two s e p a r a t i o n s a t which t h e d a t a s c a t t e r was s m a l l enough t o a l l o w meaningful comparison ( F i g u r e 5 ) . Changing t h e e q u i v a l e n c e r a t i o of t h e f l a m e a l s o i n f l u e n c e d t h e t i m e of o n s e t of o x i d a t i o n ; t h e r i c h e r t h e f l a m e , t h e s h o r t e r t h e i n i t i a t i o n p e r i o d . A f t e r i n i t i a t i o n t h e r a t e of o x i d a t i o n was h i g h and s i m i l a r f o r e a c h f u e l r a t i o ( F i g u r e 6 ) .

4 _I 3 u

E

2 u 1 D

% Z U > -

0 L. 5 3.0 6 . 5 b . 0 T I M E , 5 Figure 5. Oxidation of p o l y e t h y l e n e b e f o r e i g n i t i o n a t 0.1 e q u i v a l e n c e r a t i o , r e l a t i v e a r e a of carbonyl band: 0 10 mm s e p a r a t i o n , 15 mm s e p a r a t i o n , 0 ₂₀ mm s e p a r a t i o n

o

7 4 6 a TIME, s F i g u r e 6. Oxidation of p o l y e t h y l e n e b e f o r e i g n i t i o n a t 20 mm s e p a r a t i o n , r e l a t i v e a r e a of carbonyl band: 0 _{0.1 e q u i v a l e n c e r a t i o , a 0.13} e q u i v a l e n c e r a t i o , A 0.15 e q u i v a l e n c e r a t i o Poly(methy1 m e t h a c r y l a t e )

The a n a l y s i s of t h i s polymer was complicated by t h e p r e s e n c e of s t r o n g carbonyl a b s o r p t i o n s i n t h e i n f r a r e d spectrum of t h e unexposed polymer. The

W A V E N U M B E R S F i g u r e 7. ~ o l y ( m e t h y 1 m e t h a c r y l a t e ) j u s t b e f o r e i g n i t i o n ( 9 s e x p o s u r e ) and b e f o r e e x p o s u r e a p p e a r a n c e of s p e c t r a of f i l m s b e f o r e and af

t e r

f l a m e e x p o s u r e was i d e n t i c a l and t h i s s i m i l a r i t y p e r s i s t e d a f t e r implementation of t h e s p e c t r a l d e c o n v o l u t i o n t e c h n i q u e d e s c r i b e d above.

The s p e c t r u m of poly(methy1 m e t h a c r y l a t e ) above 3000 cm-l i s l e s s c l u t t e r e d . However, h y d r o x y l a b s o r p t i o n i s g e n e r a l l y b r o a d e r and l e s s e a s i l y d i s t i n g u i s h a b l e f r o m t h e b a s e l i n e t h a n a r e c a r b o n y l a b s o r p t i o n s . N o n e t h e l e s s , u n l i k e p o l y e t h y l e n e and p o l y s t y r e n e , f l a m e exposed poly(methy1 m e t h a c r y l a t e ) showed n o e v i d e n c e of h y d r o x y l bands ( F i g u r e 7). Poly(methy1 m e t h a c r y l a t e ) i s

n o t o x i d i z e d i n t h e condensed p h a s e under t h e s e c o n d i t i o n s .

DISCUSSION

H i r s c h l e r r e c e n t l y q u e r i e d t h e v a l u e of t h e s t u d y of chemical changes i n polymers i n f i r e c o n d i t i o n s , a r g u i n g t h a t much had a l r e a d y been d e t e r m i n e d about polymer p y r o l y s i s (12). I n r e p l y , Kashiwagi and O h l e m i l l e r p o i n t e d o u t t h a t slow p y r o l y s i s i s n o t n e c e s s a r i l y a good p r o c e s s t o s t u d y i n o r d e r t o a s c e r t a i n performance i n f i r e c o n d i t i o n s , s i n c e p y r o l y s i s r a t e s i n f i r e c o n d i t i o n s a r e a t l e a s t two o r d e r s of magnitude f a s t e r .

The p r e s e n t s t u d y p r o v i d e s s u p p o r t t o t h e l a t t e r view; t h e phenomena o b s e r v e d would have been d i f f i c u l t t o p r e d i c t from p u b l i s h e d r e p o r t s on t h e

( s l o w ) p y r o l y s i s of t h e polymers burned. Varying t h e c o n d i t i o n s of f i r e e x p o s u r e changed t h e mode of p y r o l y s i s observed. Thus, i n t h e r a d i a n t e x p o s u r e c o n d i t i o n s u s e d by Kashiwagi and O h l e m i l l e r ( 1 2 ) , poly(methy1 m e t h a c r y l a t e ) was r e s p o n s i v e t o oxygen c o n c e n t r a t i o n , w h i l e under t h e

c o n v e c t i v e h e a t t r a n s f e r c o n d i t i o n s used i n t h e p r e s e n t work, condensed p h a s e o x i d a t i o n was n o t observed.

P o l y s t y r e n e and p o l y e t h y l e n e show s i m i l a r i t i e s and d i f f e r e n c e s i n t h e i r p r e - i g n i t i o n behaviour. Both e x h i b i t r a t h e r c o n s t a n t r a t e s of o x i d a t i o n , o v e r t h e r a n g e of c o n d i t i o n s used. However, f o r p o l y e t h y l e n e t h e o x i d a t i o n p r o c e s s

,

i s i n i t i a t e d a f t e r a t i m e which depends on e q u i v a l e n c e r a t i o and f l a m e

'

p r o x i m i t y ; t h e s e f a c t o r s a r e l e s s n o t i c e a b l e f o r p o l y s t y r e n e .

# The two polymers undergo f u n d a m e n t a l l y d i f f e r e n t p y r o l y s i s r e a c t i o n s ;

p o l y s t y r e n e decomposes by random- and end-chain s c i s s i o n i n t o a m i x t u r e of monomer and o l i g o m e r i c f r a g m e n t s , w h i l e p o l y e t h y l e n e , i n a random s c i s s i o n

p r o c e s s ( I ) , produces l i t t l e monomer. The behaviour observed i n t h i s s t u d y

can be a c c o u n t e d f o r by assuming t h a t t h e s u r f a c e of p o l y e t h y l e n e i s more

-

s e n s i t i v e t o oxygen t h a n t h a t of p o l y s t y r e n e , because of a g r e a t e r

c o n c e n t r a t i o n of u n s a t u r a t e d s i t e s .

One f i n d i n g of p o s s i b l e i n d u s t r i a l i m p o r t a n c e i s t h a t o x i d a t i o n i n t h e s e

' c o n d i t i o n s was c o n f i n e d t o v e r y n e a r t h e exposed s u r f a c e . To t h e e x t e n t t h a t

p r e - i g n i t i o n o x i d a t i o n s h o u l d b e r e s t r i c t e d , t h e p r o v i s i o n of f i r e r e t a r d a n t c h e m i c a l s t h r o u g h o u t a polymer may be an u n n e c e s s a r y and i n e f f e c t i v e mode of

p r o t e c t i o n . The r e s u l t s s u g g e s t s u r f a c e t r e a t m e n t w i t h f i r e r e t a r d a n t s may b e a l l t h a t i s r e q u i r e d . I f r e a c t i v e f i r e r e t a r d a n t s a r e r e q u i r e d , s u r f a c e g r a f t i n g may be t h e method of c h o i c e . However, t h e f i n d i n g t h a t o x i d a t i o n o c c u r s b e f o r e i g n i t i o n d o e s n o t n e c e s s a r i l y mean t h a t c o n t r o l of t h i s o x i d a t i o n w i l l r e s u l t i n i n c r e a s e d r e s i s t a n c e t o i g n i t i o n . Such an e x t r a p o l a t i o n must a w a i t t h e r e s u l t s of f u r t h e r s t u d y .

The a u t h o r s acknowledge t h e a s s i s t a n c e of Mr. Raymond F l a v i a n i i n

c o n d u c t i n g some of t h e e x p e r i m e n t s r e p o r t e d h e r e . T h i s a r t i c l e i s a

c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research C o u n c i l of Canada.

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T h i s p a p e r i s b e i n g d i s t r i b u t e d i n r e p r i n t f o r m by t h e I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t o f b u i l d i n g p r a c t i c e a n d r e s e a r c h p u b l i c a t i o n s a v a i l a b l e f r o m t h e I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , K I A 0R6. Ce document e s t d i s t r i b u 6 s o u s f o r m e d e t i r C - % p a r t p a r l ' l n s t i t u t d e r e c h e r c h e e n c o n s t r u c t i o n . O n p e u t o b t e n i r u n e l i s t e d e s p u b l i c a t i o n s d e 1 ' I n s t i t u t p o r t a n t s u r l e s t e c h n i q u e s ou l e s r e c h e r c h e s e n m a t i s r e d e b t t i m e n t e n g c r i v a n t

B

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