Mechanism of Thermal Degradation of Fire-Retardant Melamine Salts

Chapter 15

Mechanism of Thermal Degradation of Fire-Retardant Melamine Salts L. Costa, G. Camino, and M. P. Luda di Cortemiglia

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Dipartimento di ChimicaInorganica,Chimica Fisica e Chimica dei Materiali, Via Pietro Giuria 7, 10125 Torino, Italy

A thorough study of the mechanism of degradation of several fire retardant melamine salts is carried out using thermogravimetry, evolved gas analysis and spectroscopic characterisation of the products of degradation. The salts can be classified in three main classes: salts which undergo thermal dissociation to acid and melamine; salts of strong acids which catalyse melamine condensation; salts of acids which react with melamine condensation products. Implications of the thermal behaviour of the salts in the mechanism of f i r e retardance is briefly discussed. The i n c r e a s i n g u n d e r s t a n d i n g o f f i r e h a z a r d i n t h e c a s e o f o r g a n i c p o l y m e r i c m a t e r i a l s has r e c e n t l y brought t o general attention the importance o f such f a c t o r s as f o r m a t i o n o f dense smoke and of corrosive or toxic products in f i r e scenarios (1-3). Thus, the emphasis i n t h e e v a l u a t i o n o f f i r e r e t a r d a n t s y s t e m s i s now s h i f t i n g from pure flame e x t i n g u i s h i n g p a r a m e t e r s t o a more comprehensive a p p r a i s a l of hazard i n f i r e retarded m a t e r i a l s i n which production of s m o k e , c o r r o s i v e a n d t o x i c p r o d u c t s i s i n c l u d e d . I n t h i s r e s p e c t , some w e l l e s t a b l i s h e d c l a s s e s o f f i r e r e t a r d a n t s , as f o r example halogenbased systems, seem t o be r a t h e r u n s a t i s f a c t o r y . Most current developments i n t h i s f i e l d i n v o l v e indeed h a l o g e n - f r e e systems such as char forming or intumescent a d d i t i v e s (4-7). These act in the c o n d e n s e d phase i n d u c i n g t h e f o r m a t i o n o f c h a r on t h e s u r f a c e o f t h e polymeric m a t e r i a l which p r o t e c t s i t from t h e a c t i o n o f t h e flame. Melamine and i t s s a l t s a r e w i d e l y used i n f o r m u l a t i o n s o f f i r e r e t a r d a n t a d d i t i v e s , p a r t i c u l a r l y o f t h e i n t u m e s c e n t t y p e ( 4 - 7 ) . The r o l e p l a y e d by m e l a m i n e s t r u c t u r e s i n t h e s e a d d i t i v e s i s h o w e v e r n o t y e t u n d e r s t o o d . The t h e r m a l b e h a v i o u r i s o f paramount i m p o r t a n c e i n studies of the f i r e r e t a r d a n c e m e c h a n i s m . I t i s known t h a t m e l a m i n e undergoes progressive condensation on h e a t i n g w i t h e l i m i n a t i o n o f ammonia a n d f o r m a t i o n o f p o l y m e r i c p r o d u c t s named " m e l a m " , "melem", " m e l o n " (8,9). The f o l l o w i n g s c h e m a t i c r e a c t i o n i s r e p o r t e d i n t h e l i t e r a t u r e (10-12): 0097-6156/90/0425-0211$08.00/0

© 1990 American Chemical Society

In Fire and Polymers; Nelson, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

212

FIRE AND POLYMERS -NHo 2 C H N melamine 3

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W h i l e t h e r e i s c l e a r e v i d e n c e t h a t melam i s t h e d i m e r o f m e l a m i n e i n w h i c h t w o s - t r i a z i n e r i n g s a r e c o n n e c t e d by a NH b r i d g e ( d i 6 , [ 2 , 4 diamino-1,3,5-triazine]amine) (10):

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f o r w h i c h t h e r e i s not y e t a f i n a l p r o o f s u p p o r t i n g one o f them. As f a r a s m e l o n i s c o n c e r n e d , i t i s g e n e r a l l y a g r e e d t h a t i t i s f o r m e d by e l i m i n a t i o n o f ammonia b e t w e e n a m i n o g r o u p s o f m e l e m , w h i c h e v e r its s t r u c t u r e ( 8 - 1 3 ) . The d i f f i c u l t i e s e n c o u n t e r e d i n t h e a s s i g n m e n t of s t r u c t u r e s t o t h e s e p r o d u c t s d e p e n d s on t h e f a c t t h a t t h e y a r e e i t h e r s p a r i n g l y s o l u b l e (melamine, melam, melem) or i n s o l u b l e (melon). T h e r e f o r e c h a r a c t e r i s a t i o n i s p r a c t i c a l l y r e s t r i c t e d t o t h e u s e o f UV and s o l i d s t a t e IR s p e c t r o p h o t o m e t r y . ( 1 5 - 1 6 ) I n p a r t i c u l a r , i n s p i t e of t h e u n c e r t a i n t y of t h e s t r u c t u r e o f melem and c o n s e q u e n t l y of m e l o n , t h e r e i s a f a i r l y g o o d a g r e e m e n t i n t h e l i t e r a t u r e on t h e i r IR spectrum ( 1 6 ) ( F i g u r e 1 ) . I t c a n be s e e n t h a t me 1 a m i n e , _ m e l a m , melem a n d m e l o n a l l show t h e t y p i c a l a b s o r p t i o n a t 7 9 5 - 8 1 5 cm due t o o u t o f p l a n e d e f o r m a t i o n o f t h e s - t r i a z i n e r i n g w h e r e a s t h e y show c l e a r l y d i s t i n g u i s h a b l e f e a t u r e s i n t h e c o m p l e x r e g i o n b e t w e e n 1 0 0 0 - 1 7 0 0 cm i n v o l v i n g a b s o r p t i o n s d u e t o N H , NH, CN a n d s - t r i a z i n e r i n g (16-17). A s f a r a s f i r e r e t a r d a n c e i s c o n c e r n e d , i t i s t o be n o t e d t h a t 2


15.

COSTA E T A L .

Thermal Degradation of Fire-Retardant Melamine Salts

m e l a m i n e h e a t e d i n an open s y s t e m ( F i g u r e 2) m o s t l y evaporates u n a l t e r a t e d above 250*C l e a v i n g a l i t t l e residue (ca. 7%,) of c o n d e n s a t i o n p r o d u c t s w h i c h a r e more t h e r m a l l y s t a b l e t h a n m e l a m i n e i t s e l f : melam t o a b o u t 3 5 0 C , melem 4 5 0 C , m e l o n 6 0 0 * C ( 1 8 ) . T h e y i e l d o f c o n d e n s a t i o n p r o d u c t s c a n be i n c r e a s e d by h e a t i n g m e l a m i n e i n a c l o s e d system w i t h however l i m i t a t i o n by e q u i l i b r i u m w i t h ammonia e v o l v e d . ( 1 1 . 1 8 ) . T h e r e f o r e , when m e l a m i n e i s i n c o r p o r a t e d i n a polymer, in the absence of r e a c t i o n s w i t h the m a t r i x , i t should leave t h e m a t e r i a l when t h e r m a l d e g r a d a t i o n occurs, with negligible c o n t r i b u t i o n t o t h e c h a r r e d r e s i d u e p o s s i b l y formed. In t h i s case the c o n t r i b u t i o n o f m e l a m i n e t o f i r e r e t a r d a n c e c o u l d be s o u g h t i n a blowing a c t i o n during the c h a r r i n g step of intumescent systems, if t e m p e r a t u r e o f e v a p o r a t i o n and c h a r r i n g c o n v e n i e n t l y m a t c h . O t h e r w i s e , t y p i c a l g a s p h a s e a c t i o n s c o u l d be p e r f o r m e d by v o l a t i l i s e d m e l a m i n e such as d i l u t i o n o f flammable g a s e s , e t c . #


213

#

A c o m p l e t e l y d i f f e r e n t s i t u a t i o n may a r i s e when melamine d e r i v a t i v e s , e.g. melamine s a l t s a r e i n t r o d u c e d i n the p o l y m e r i c m a t e r i a l , o w i n g t o t h e t h e r m a l b e h a v i o u r o f t h e s a l t s w h i c h d e p e n d s on t h e t y p e o f a c i d c o m b i n e d w i t h m e l a m i n e . We h a v e c h a r a c t e r i s e d t h e t h e r m a l b e h a v i o u r o f a w i d e number o f m e l a m i n e s a l t s a n d f o u n d t h a t t h e y may be c l a s s i f i e d i n t o t h r e e m a i n c l a s s e s : 1. S a l t s w h i c h u n d e r g o t h e r m a l d i s s o c i a t i o n t o a c i d a n d m e l a m i n e 2. S a l t s o f s t r o n g a c i d s w h i c h c a t a l y s e m e l a m i n e c o n d e n s a t i o n 3. S a l t s o f a c i d s w h i c h r e a c t w i t h m e l a m i n e c o n d e n s a t i o n p r o d u c t s . Examples of the t h r e e c l a s s e s , i n c l u d i n g s a l t s used in fire r e t a r d a n t a p p l i c a t i o n s , a r e shown a n d d i s c u s s e d h e r e . The results o b t a i n e d on h e a t i n g i n n i t r o g e n a r e r e p o r t e d s i n c e t h e y are i d e n t i c a l to those obtained i n a i r . On t h e o t h e r hand, the supply of combustible v o l a t i l e products t o the flame i n polymer combustion is e s s e n t i a l l y due t o t h e t h e r m a l d e g r a d a t i o n p r o c e s s . The r o l e o f o x y g e n i n t h i s s t e p i s s t i l l m a t t e r o f d e b a t e and i t m i g h t depend on t h e t y p e o f p o l y m e r and c o n d i t i o n s o f c o m b u s t i o n s u c h as p r e s e n c e o r absence of flame. For example, i n the presence o f the f l a m e , oxygen is present at the material s u r f a c e i n low c o n c e n t r a t i o n and m i g h t s i m p l y c a t a l y s e t h e t h e r m a l d e g r a d a t i o n p r o c e s s by h e a t r e s u l t i n g f r o m l i m i t e d thermal o x i d a t i o n of the polymeric m a t e r i a l . Experimental. Materials. M e l a m i n e and m e l a m i n e s a l t s were s u p p l i e d by SKW, T r o s t b e r g , FRG. M e l a m was p r e p a r e d by h e a t i n g e q u i m o l a r a m o u n t s of m e l a m i n e and 2,4-diamino-6-chloro-s-triazine (19). Reference melam h y d r o b r o m i d e a n d n i t r a t e w e r e p r e p a r e d by r e a c t i n g melam w i t h 20% s o l u t i o n s o f HBr and H N 0 r e s p e c t i v e l y (20). T h e r m a l d e g r a d a t i o n . A Du P o n t 951 t h e r m o b a l a n c e - 1 0 9 0 t h e r m a l a n a l y s e r s y s t e m was u s e d c o n n e c t e d t o a U t r a p p r o v i d e d w i t h e n t r a n c e a n d e x i t stopcocks. S t a n d a r d c o n d i t i o n s w e r e : 10 mg s a m p l e ; h e a t i n g rate, 1 0 C / m i n ; n i t r o g e n f l o w , 60 cm / m i n . V o l a t i l e p r o d u c t s , s w e p t b y t h e p u r g i n g n i t r o g e n , c o n d e n s e o u t e i t h e r on t h e g l a s s e n v e l o p e o f t h e thermobalance emerging from the furnace (high b o i l i n g products) or i n the U t r a p cooled at l i q u i d n i t r o g e n temperature (gaseous p r o d u c t s ) . D e g r a d a t i o n p r o d u c t s w e r e g e n e r a l l y i d e n t i f i e d by F o u r i e r Transform I n f r a r e d ( F T I R , P e r k i n E l m e r 1710) e i t h e r i n KBr p e l l e t s ( r e s i d u e s and 3

#


FIRE AND POLYMERS


214

Figure 2. Thermogravimetric and d i f f e r e n t i a l thermogravimetric c u r v e s (TG a n d DTG) o f m e l a m i n e , h e a t i n g r a t e 1 0 C / m i n ; nitrogen, 60 c r r r / m i n . #


15.

COSTA E T A L .


215


h i g h b o i l i n g components) o r i n a gas c e l l a f t e r t r a n s f e r f r o m t h e U t r a p t h r o u g h a vacuum l i n e . 31P s o l i d s t a t e NMR was r e c o r d e d on a J e o l JX-270 instrument equipped w i t h magic angle s p i n n i n g accessory u s i n g CP-Mass c r o s s p o l a r i s a t i o n magic a n g l e s p i n n i n g w i t h r o t a t i o n speed o f the sample (U^) between 3.5 - 4.2 kHz. T h e r m o g r a v i m e t r y - e v o l v e d gas a n a l y s i s (TG-EGA). D e t e r m i n a t i o n o f w a t e r and ammonia e v o l v e d on h e a t i n g was c a r r i e d out by continuous m o n i t o r i n g o f t h e i r c o n t e n t i n t h e gas p u r g i n g s t r e a m e x i t i n g t h e t h e r m o b a l a n c e . W a t e r was m e a s u r e d by u s i n g a s p e c i f i c probe (Hygrometer III, P a n a m e t r i c s ) . Ammonia was m e a s u r e d by b u b b l i n g t h e g a s s t r e a m i n t o 0 . 1 N s u l p h u r i c a c i d s o l u t i o n w h o s e c o n d u c t i v i t y was c o n t i n u o u s l y r e c o r d e d . C o n d u c t i v i t y v a r i a t i o n was c o n v e r t e d i n t o ammonia e v o l v e d by means o f c a l i b r a t i o n w i t h t i t r a t e d NH^OH s o l u t i o n s . Results Salts

and

Discussion.

which Undergo

Dissociation

t o A c i d and

Melamine.

Cyanurate. T h e TG o f d i m e l a m i n e c y a n u r a t e ( F i g u r e 3 ) i s c h a r a c t e r i s e d by t h r e e d e g r a d a t i o n s t e p s a s shown by t h e DTG c u r v e . I n t h e first ( 2 5 0 - 3 5 0 * 0 , o n l y m e l a m i n e i s v o l a t i l i s e d a s shown by I R . T h e w e i g h t l o s s i n t h i s s t e p (30%) c o r r e s p o n d s t o e l i m i n a t i o n o f one m o l e c u l e o f m e l a m i n e p e r m o l e c u l e o f s a l t ( c a l c u l a t e d 3 3 % ) . I n d e e d t h e IR o f t h e r e s i d u e a f t e r melamine l o s s ( f i g u r e 4 ) , is identical with that of r e f e r e n c e m e l a m i n e c y a n u r a t e . T h i s b e h a v i o u r was f o u n d t o be t y p i c a l of s a l t s of p o l y f u n c t i o n a l a c i d s owing t o decrease of a c i d i t y w i t h i n c r e a s i n g number o f a c i d i c f u n c t i o n s . T h u s , t h e s e c o n d m o l e c u l e o f melamine b e i n g l e s s t i g h t l y b o u n d , m o s t l y t e n d s t o e v a p o r a t e on h e a t i n g ( s i m i l a r l y t o melamine heated alone) g i v i n g o n l y a n e g l i g i b l e amount o f c o n d e n s a t i o n . Above i t s m e l t i n g p o i n t ( 3 6 0 C ) a b o u t 90% o f m e l a m i n e c y a n u r a t e i s c o n v e r t e d t o v o l a t i l e p r o d u c t s ( 3 6 0 - 4 5 0 C , 2nd s t e p ) i n w h i c h f r e e m e l a m i n e a n d m e l a m i n e c y a n u r a t e w e r e r e c o g n i s e d by I R . T h i s i n d i c a t e s t h a t a c o m p e t i t i o n t a k e s p l a c e between e v a p o r a t i o n o f t h e u n a l t e r a t e d s a l t and i t s t h e r m a l d i s s o c i a t i o n t o m e l a m i n e and c y a n u r i c acid. Melamine behaves t h e n as d e s c r i b e d above w h i l e c y a n u r i c a c i d which, h e a t e d a l o n e i n TG v o l a t i l i s e s c o m p l e t e l y a b o v e 3 0 0 * C , i s known t o decompose t o c y a n i c a c i d ( 8 ) . T h e IR o f t h e r e s i d u e l e f t a t 4 5 0 C i s s i m i l a r t o t h a t of c o n d e n s a t i o n p r o d u c t s o b t a i n e d f r o m TG o f m e l a m i n e a n d t h e y i e l d ( 7 % ) i s t h a t e x p e c t e d on t h e b a s i s o f m e l a m i n e c o n t e n t o f t h e s a l t . T h e c o n d e n s a t e f r o m m e l a m i n e c y a n u r a t e seems however t o c o r r e s p o n d t o a somewhat l a r g e r d e g r e e o f c o n d e n s a t i o n , s i n c e i t s h o w s by IR a s t r u c t u r e c l o s e t o melon which i s o b t a i n e d from melamine a t higher t e m p e r a t u r e ( 5 0 0 C ) i n T G . T h i s s u g g e s t s t h a t c y a n u r i c a c i d f r e e d by h e a t i n g may d i s p l a y a c a t a l y t i c a c t i o n o n t h e c o n d e n s a t i o n o f m e l a m i n e o f t h e t y p e d i s c u s s e d b e l o w , w h o s e e f f i c i e n c y i s h o w e v e r l i m i t e d by decomposition to v o l a t i l e cyanic a c i d . The m e l a m i n e condensate undergoes then complete f r a g m e n t a t i o n t o v o l a t i l e p r o d u c t s (HCN, ( C N ) , NH CN, h i g h b o i l i n g f r a g m e n t s ) on h e a t i n g t o 6 5 0 C ( 3 r d s t e p , F i g u r e 3) a s i n t h e c a s e o f m e l a m i n e ( F i g u r e 2 ) . S u m m a r i s i n g , the f o l l o w i n g scheme c a n a c c o u n t f o r t h e thermal behaviour of dimelamine cyanurate: #

#

#

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2

2


216

FIRE AND POLYMERS


•>

1

700

r

800

Temperature,°C Figure Figure

3. 2.

4000

T G , DTG c u r v e s

3000

of dimelamine cyanurate.

2000

1500

Conditions

as

1000 500 wavenumber, cm"

F i g u r e 4 . IR o f r e s i d u e o f d i m e l a m i n e c y a n u r a t e h e a t e d a t melamine c y a n u r a t e (by comparison w i t h r e f e r e n c e compound).


350*C:

15.

COSTA E T A L .


dimelamine cyanurate 1 i 250-350 C / melamine c y a n u r a t e + melamine step 2 J 360-450 C melamine + c y a n u r i c a c i d

217

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step

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A behaviour

s i m i l a r to that of cyanurate

i s observed

f o r the oxalate.

o-Phtalate. In t h e f i r s t s t e p o f v o l a t i l i s a t i o n o f m e l a m i n e o p h t a l a t e ( 2 2 0 - 2 8 C T C , F i g u r e 5) a m i x t u r e o f o - p h t a l i c a n h y d r i d e and o p h t a l i c a c i d i s e l i m i n a t e d , a s i d e n t i f i e d by IR. T h e r e s i d u e (60%) s h o w s t h e IR s p e c t r u m o f f r e e m e l a m i n e . I n t h i s c a s e , u p o n m e l t i n g o f t h e s a l t ( 2 4 0 - 2 8 0 C ) , t h e a c i d r e s i d u e b e h a v e s a s when h e a t e d a l o n e where i t d i s t i l l s as a m i x t u r e o f o - p h t a l i c a c i d and a n h y d r i d e ( 2 1 ) . T h e c a l c u l a t e d amount o f o - p h t a l i c a c i d i n t h e s a l t i s 5 7 % t o be c o m p a r e d w i t h 4 0 % w e i g h t l o s s t o 2 8 0 * C . H o w e v e r t h e IR s p e c t r u m o f t h e c o m m e r c i a l o - p h t a l a t e we h a v e u s e d s h o w s t h e p r e s e n c e o f f r e e m e l a m i n e i n d i c a t i n g t h a t t h e a c i d was i n d e f e c t . On t h e o t h e r h a n d , a f e w p e r c e n t o f t h e a c i d o r o f i t s d e g r a d a t i o n p r o d u c t s may be l e f t i n t h e r e s i d u e at 280*C s i n c e i t s s u b s e q u e n t t h e r m a l b e h a v i o u r i s not i d e n t i c a l t o t h a t o f f r e e m e l a m i n e . I n d e e d t h e DTG c u r v e o f t h e s e c o n d step of degradation i n which evaporation-condensation o f melamine takes place (280-400*0, Figure 5) i s more c o m p l e x t h a n t h e c o r r e s p o n d i n g s t e p o f F i g u r e 2 . M o r e o v e r , a l a r g e r amount o f m e l a m i n e condensate i s l e f t from melamine o - p h t a l a t e a t 4 0 0 ' C (13%) as c o m p a r e d t o t h a t c a l c u l a t e d ( c a . 6%) o n t h e b a s i s o f m e l a m i n e c o n t e n t of the s a l t . F i n a l l y , the condensate obtained at 400 C is c h a r a c t e r i s e d b y a n IR s p e c t r u m s i m i l a r t o t h a t f o r m e d b y h e a t i n g m e l a m i n e c y a n u r a t e b u t i t seems m o r e s t a b l e b y a b o u t 1 0 0 * C , d e g r a d i n g between 6 0 0 - 7 5 0 C . #

#

#

The t h e r m a l degradation of s c h e m a t i c a l l y r e p r e s e n t e d as f o l l o w s :

\ o-phtalic

melamine

melamine o - p h t a l a t e step 1 \ | 220-280*0/ a c i d + o - p h t a l i c a n h y d r i d e + H 0 + melamine step 2 ^ ^ ^ J ^ 2 8 0 j 4 0 0

can

of Strong Acids

which C a t a l y s e Melamine

be

(4)

2

evaporation step 3

Salts

o-phtalate

C

#

^

melon + NH ) >600*C decomposition

3

Condensation.

Hydrobromide. A complex process t a k e s p l a c e i n t h e f i r s t s t e p o f w e i g h t l o s s o f m e l a m i n e h y d r o b r o m i d e ( 2 5 0 - 4 0 0 C ) a s shown b y t h e DTG c u r v e o f F i g u r e 6. T h e p r o d u c t s e v o l v e d i n t h i s s t e p w e r e s h o w n t o b e a m i x t u r e o f m e l a m i n e h y d r o b r o m i d e ( I R ) a n d ammonium b r o m i d e ( I R a n d X - r a y ) . The IR s p e c t r u m o f t h e r e s i d u e (55% at 400 C) which is compared w i t h t h e o r i g i n a l s a l t i n F i g u r e 7, i s i d e n t i c a l w i t h t h a t o f r e f e r e n c e melam h y d r o b r o m i d e . F u r t h e r m o r e , t h e IR o f t h e r e s i d u e #

#



218

FIRE AND POLYMERS

Temperature,°C Figure Figure

5. T G , DTG c u r v e s 2.

of

melamine

o-phtalate.

T

r

T

!

Conditions

as

I T T

80 _

j> 60

"\

o

ii «>— i iI 1 2X 1 1 / 1 / . M 1

f

4

° DTG

20

3

—

\

/

.' 1 0

•

\

I 1 —I

XX) Figure units)

\

.X1

1I

.

1I

1I

1I

J

I

.

I 1

V _ ,

.

1

I

.

•1

I1

I

I

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 700800 Temperature ,°C

6. T G , DTG c u r v e s a n d r a t e o f ammonia e v o l u t i o n ( a r b i t r a r y f o r m e l a m i n e h y d r o b r o m i d e . C o n d i t i o n s a s F i g u r e 2.


15.

COSTA E T A L .


t r e a t e d w i t h 1% NaOH i s t h a t o f e v a p o r a t i o n of melamine hydrobromide melamine t o t h e d i m e r melam: NH I 2

N

NH I

0

N

>

N

II

I H N-C N

N

H N-C

2

melam. These data show that competes w i t h t h e c o n d e n s a t i o n o f

NH I

0

N

I

C-NH

2


. HBr

2

219

0

N

N

II

I

II

C

C

C-NH

NH

X

• HBr + NH.Br

(5)

2

N

T h e b y - p r o d u c t ammonium b r o m i d e i s shown t o v o l a t i l i s e i n similar c o n d i t i o n s i n TG. T h i s i s a d i f f e r e n t b e h a v i o u r a l t o g e t h e r f r o m t h a t o f m e l a m i n e i n w h o s e t h e r m a l c o n d e n s a t i o n melam c a n be i s o l a t e d w i t h d i f f i c u l t y only in c a r e f u l l y c o n t r o l l e d experimental conditions ( 1 1 , 1 7 ) . I n d e e d melem i s t h e c o n d e n s a t i o n p r o d u c t m o s t e a s i l y i s o l a t e d a n d t h i s l e d some a u t h o r s t o d o u b t t h a t melam c o u l d be an i n t e r m e d i a t e p r o d u c t o f t h e c o n d e n s a t i o n o f m e l a m i n e t o melem a n d m e l o n ( 1 9 , 2 2 ) , T h e h i g h y i e l d o f c o n d e n s a t i o n c o u l d be d u e t o favoured n u c l e o p h i l i c a t t a c k when t h e a m i n o g r o u p o f m e l a m i n e i s p r o t o n a t e d : NH

3

Br

NH :

\

3

/

Br

>

\

/

^ C — N H — +

/

NH Br

(6)

4

NH Q u a n t i t a t i v e c o n d e n s a t i o n o f m e l a m i n e t o melam on h e a t i n g a t 2 9 0 C i n t h e p r e s e n c e o f z i n c c h l o r i d e a n d a c i d s was p r e v i o u s l y r e p o r t e d i n t h e literature ( 2 0 , 2 3 ) . The r e a s o n f o r l i m i t a t i o n o f t h e condensation p r o c e s s t o t h e d i m e r has however not been d i s c u s s e d . Melam h y d r o b r o m i d e undergoes thermal degradation between 4505 5 0 * 0 (2nd s t e p , F i g u r e 6) w i t h c o m p l e t e e l i m i n a t i o n o f b r o m i n e e i t h e r t h r o u g h e v o l u t i o n o f ammonium b r o m i d e , w h i c h i s t h e m a j o r p r o d u c t , o r m e l a m i n e h y d r o b r o m i d e a s shown by I R . A l s o a s m a l l amount o f f r e e ammonia i s e v o l v e d i n t h i s s t e p i n w h i c h m e l o n i s f o r m e d a s shown by t h e IR o f t h e r e s i d u e a t 5 5 0 * 0 ( c a . 35% o f o r i g i n a l melamine hydrobromide). T h e s e d a t a s u g g e s t t h a t , on h e a t i n g , c o n d e n s a t i o n o f melam t o melon o c c u r s e i t h e r t h r o u g h a r e a c t i o n s i m i l a r t o R e a c t i o n 5 w i t h e l i m i n a t i o n o f ammonium b r o m i d e o r t h r o u g h r e a c t i o n b e t w e e n f r e e a m i n o g r o u p s g i v i n g ammonia. F o r m a t i o n o f melamine h y d r o b r o m i d e , in this p r o c e s s , i s i n agreement w i t h r e p o r t e d p r o d u c t i o n o f melamine d u r i n g t h e r m a l c o n d e n s a t i o n o f melam ( 1 2 ) . D e g r a d a t i o n o f m e l o n t a k e s p l a c e as e x p e c t e d (18) above 6 0 0 * 0 ( 3 r d s t e p ) . The t h e r m a l b e h a v i o u r of m e l a m i n e h y d r o b r o m i d e c a n be d e s c r i b e d a s f o l l o w s : #

melamine step 1 evaporation step

hydrobromide I 250-400*0

melam h y d r o b r o m i d e + N H B r ^ \ | 450-550*0 / melon + NH + melamine hydrobromide 3 | >600*C decomposition 4

2

3

step

^ / + NH^Br


(7)


220

FIRE AND POLYMERS

Nitrate. The t h e r m a l b e h a v i o u r o f t h e n i t r a t e ( F i g u r e 8 ) , i s q u a l i t a t i v e l y s i m i l a r t o t h a t o f t h e h y d r o b r o m i d e ( F i g u r e 6 ) . Above i t s m e l t i n g point (225*C), melamine n i t r a t e undergoes competing e v a p o r a t i o n and c o n d e n s a t i o n t o melam n i t r a t e w i t h e v o l u t i o n o f ammonium n i t r a t e ( 1 s t s t e p , 2 5 0 - 3 7 0 * 0 . M e l a m i n e n i t r a t e a n d ammonium n i t r a t e , which v o l a t i l i s e s between 230-320*0 i n TG, were indeed i d e n t i f i e d by IR i n t h e v o l a t i l i s e d m a t e r i a l . M o r e o v e r , t h e IR s p e c t r u m o f t h e r e s i d u e o f t h i s s t e p o f d e g r a d a t i o n ( 4 2 % , F i g u r e 9B) i s i d e n t i c a l t o t h a t o f r e f e r e n c e melam n i t r a t e . T h e r e f o r e i n t h i s step of degradation a reaction s i m i l a r t o Reaction 5 takes place i n w h i c h HNOo i s s u b s t i t u t e d f o r H B r . S i m i l a r l y t o melamine hydrobromide, i n t h e second s t e p o f weight l o s s ( 3 7 0 - 4 5 0 * 0 , F i g u r e 8 ) , IR d a t a show t h a t melam n i t r a t e e v o l v e s f u r t h e r ammonium n i t r a t e , a m m o n i a a n d m e l a m i n e n i t r a t e g i v i n g melon w h i c h decomposes completely t o v o l a t i l e products above 500*C. The d e g r a d a t i o n scheme i s t h e n : melamine

step 1

nitrate 250-370*0

evaporation

/

melam n i t r a t e + N H N 0 step 2 \ I 370-450*0 \ m e l o n + NH + 4 3 melamine n i t r a t e step 3 j >500*C decomposition 4

N H

N 0

+

3

/

3

(8)

Sulphate. Melamine s u l p h a t e e l i m i n a t e s t h e water o f c r y s t a l l i s a t i o n o n h e a t i n g b e t w e e n 100 - 2 3 0 * C a s shown b y T G , DTG a n d w a t e r e v o l u t i o n curves of Figure 10 (1st step). The amount o f w a t e r evolved corresponds t o one m o l e c u l e p e r m o l e c u l e o f s a l t ( c a l c u l a t e d weight l o s s : 7 . 4 % e x p e r i m e n t a l : 8 % ) . T h e IR o f t h e a n h y d r o u s s a l t i s shown i n F i g u r e 11A i n w h i c h t h e t y p i c a l s t r o n g b a n d o f s u l p h a t e a n i o n g r o u p a t 1095 cm is evident. In t h e range 3 0 0 - 4 0 0 * 0 ( 2 n d s t e p , F i g u r e 10), the v o l a t i l e p r o d u c t s w e r e i d e n t i f i e d a s ammonia a n d w a t e r b y u s i n g t h e s p e c i f i c d e t e c t o r s a n d m e l a m i n e b y I R . From t h e w e i g h t l o s s a n d t h e m e a s u r e d amount o f w a t e r a n d ammonia ( 0 . 6 6 a n d 0 . 3 5 m o l e p e r m o l e o f s a l t respectively) i t c a n be c a l c u l a t e d t h a t a t t h e u t m o s t 15% o f t h e melamine p r e s e n t i n t h e s u l p h a t e s h o u l d be v o l a t i l i s e d . T h i s shows t h a t , i n c o n t r a s t t o t h e above s a l t s o f s t r o n g a c i d s (hydrobromide, n i t r a t e ) which tend t o e v a p o r a t e on h e a t i n g ( > 2 5 0 * C ) , melamine sulphate reaches decomposition temperature (>300*O w i t h l i b e r a t i o n o f melamine b e f o r e vapor p r e s s u r e a l l o w s s i g n i f i c a n t v o l a t i l i s a t i o n o f the s a l t . A t t h i s temperature t h e f r e e d melamine w i l l then v o l a t i l i s e . However, s i m i l a r l y t o t h e p r e v i o u s s a l t s , a l s o i n t h e case o f t h e s u l p h a t e , t h e l o s s o f s - t r i a z i n e s t r u c t u r e s i s l i m i t e d by t h e v e r y efficient promotion o f melamine condensation demonstrated by s i m u l t a n e o u s e v o l u t i o n o f ammonia. I n d e e d , i t c a n be c a l c u l a t e d t h a t a b o u t 0 . 4 m o l e c u l e s o f ammonia i s e l i m i n a t e d p e r m e l a m i n e m o l e c u l e l e f t i n t h e c o n d e n s e d p h a s e w h i c h i s c l o s e t o t h e amount c o r r e s p o n d i n g t o c o m p l e t e c o n v e r s i o n t o melam ( 0 . 5 ) . Limited decomposition of the sulphate and e x t e n s i v e melamine condensation overlap i n t h i s second step with condensation of s u l p h u r i c a c i d r e s i d u e s t o p y r o s u l p h u r i c s t r u c t u r e s . T h i s i s shown b y water e v o l u t i o n and appearance o f t y p i c a l a b s o r p t i o n s o f p y r o s u l p h a t e



15.

COSTA E T A L .

4000


3000

2000

1500

111

1000 500 wavenumber, env

F i g u r e 7 . IR o f m e l a m i n e h y d r o b r o m i d e ( A ) a n d o f r e s i d u e a t 4 0 0 C ( B ) : melam h y d r o b r o m i d e ( b y c o m p a r i s o n w i t h r e f e r e n c e c o m p o u n d ) . #

100 Figure units)

200

300

400

500 600 700 Temperature ,°C

8 . T G , DTG c u r v e s a n d r a t e o f ammonia e v o l u t i o n f o r melamine n i t r a t e . C o n d i t i o n s as F i g u r e 2.

800 (arbitrary


222

FIRE AND POLYMERS

••• r

i A

o

c

ro

n


i_ o

n

i 3000

4000

i 2000

i 1500

J_

1 —

1000 500 wavenumber, cm"

F i g u r e 9 . IR o f m e l a m i n e n i t r a t e ( A ) a n d o f r e s i d u e a t 3 7 0 ' C melam n i t r a t e ( b y c o m p a r i s o n w i t h r e f e r e n c e c o m p o u n d ) .

1

(B):

100i

» /| 3

80 ^ 60|cn 5

\

DTG

' 1 / l / I

1 40I

- y

i

I ,

w

y :

1 | 1

\

1

V

/»

-

/'\'

V

-

20 H

1 \ NH

2°

\

3

! \ v_

J 1

100

1

1

200

1

1

300

1

1

400

1

i . i i i 500 600 700 Temperature,°C

F i g u r e 1 0 . T G , DTG a n d r a t e o f w a t e r a n d ( a r b i t r a r y u n i t s ) . C o n d i t i o n s a s F i g u r e 2.

ammonia

800 evolution


15.

COSTA E T A L .


223

anion i n t h e IR s p e c t r u m o f t h e r e s i d u e a t 4 0 0 C ( e . g . : 1 0 2 0 , 1 0 5 0 , 1 2 2 5 cm , F i g u r e 1 1 B ) w h i c h w e r e a s s i g n e d b y c o m p a r i s o n with r e f e r e n c e ammonium p y r o s u l p h a t e . T h e r e a c t i o n s o c c u r r i n g i n t h i s s t e p w o u l d t h u s i n v o l v e a c o m b i n e d c o n d e n s a t i o n o f m e l a m i n e t o melam a n d o f sulphuric acid to pyrosulphuric acid. For example, condensation o f m e l a m i n e c o u l d t a k e p l a c e a s i n R e a c t i o n 5, f o l l o w e d by c o n d e n s a t i o n t o melam ammonium p y r o s u l p h a t e : #

r—

NH

tfH»

2

-I •H S 0 2


HLN-C

i_

X

C

j (NH ) S 0 4

NH.

0

2

I

2

NH

C

yC N ^

N

H N-C

• H S0 2

>

X

V

2

NH

NH

2

C-NH

/

+ H 0

7

2

+ NH HS0 4

4

9

V (9)

0 0 II M • H0-S-0-S-0NH II H 0 0

4

C-NH

0

i

4

0

N

H N-C

2

C

/\ N

N • H S0

y

N

2

2x | ( 9 b )

N ^

\

2

0

C

M - C

C-NHo

2

4

2

(9a)

2x NH

+ H 0

7

C-NH.

N / N o / S '

2

2

+ H 0 2

0

Condensation of sulphuric acid residues could also i n v o l v e two m o l e c u l e s o f melam s u l p h a t e g i v i n g d i m e l a m p y r o s u l p h a t e ( 9 a ) . On t h e o t h e r h a n d , ammonium s u l p h a t e g i v e s d i a m m o n i u m p y r o s u l p h a t e i n TG between 270-350*C ( 9 b ) . The o v e r a l l r e s u l t o f t h e combined c o n d e n s a t i o n p r o c e s s would n o t change i f condensation of sulphuric acid residues preceeds that of melamine. In t h i s case dimelamine p y r o s u l p h a t e w o u l d be t h e f i r s t p r o d u c t o f r e a c t i o n . The f o l l o w i n g c o n d e n s a t i o n p r o c e s s l e a d i n g t o melam c o u l d t h e n o c c u r w i t h f o r m a t i o n of t h e t h r e e types o f p y r o s u l p h a t e s d i s c u s s e d above. Formation o f t h e melam s a l t o f p y r o s u l p h u r i c a c i d o n h e a t i n g melam s u l p h a t e w a s previously reported (20). Ammonia i s e v o l v e d h e r e i n t h e c o n d e n s a t i o n p r o c e s s o f m e l a m i n e t o melam w h e r e a s t h e r e s p e c t i v e ammonium s a l t s a r e e v o l v e d i n t h e c a s e o f t h e h y d r o b r o m i d e a n d n i t r a t e . R e a c t i o n s 9 a n d 9b show t h a t ammonia s h o u l d be n e u t r a l i s e d b y p y r o s u l p h u r i c a c i d g r o u p s . I n TG we have found t h a t diammonium p y r o s u l p h a t e e l i m i n a t e s ammonia above 350*C, w i t h however c o m p l e t e d e c o m p o s i t i o n : (NH ) S 0 4

2

2

7

>

2NH

3

+ 2S0

3

+ H 0 2


(10)


224 FIRE AND POLYMERS


15.

COSTA E T A L .


225

Nevertheless, t h i s r e a c t i o n alone cannot account f o r the behaviour of m e l a m i n e s u l p h a t e s i n c e i t i m p l i e s t h e e l i m i n a t i o n o f o n e m o l e o f SOo p e r m o l e o f ammonia w h i c h w o u l d e x c e e d by a b o u t 70% t h e e x p e r i m e n t a l w e i g h t l o s s i n t h i s s t e p o f d e g r a d a t i o n . M o r e o v e r , we d i d n o t h a v e evidence of relevant e l i m i n a t i o n of S0 . On t h e o t h e r h a n d , water e v o l v e d ( 0 . 6 6 m o l e / g atom S) i s l a r g e r t h a n t h a t c a l c u l a t e d f o r R e a c t i o n 9 o r 9 a , 9b ( 0 . 5 0 m o l e / g a t o m S ) . T h e r e f o r e i t seems t h a t ammonium p y r o s u l p h a t e g r o u p s u n d e r g o t h e r m a l s c i s s i o n in t h i s step w i t h e v o l u t i o n o f ammonia w h i l e p y r o s u 1 p h u r i c acid is p a r t i a l l y s t a b i l i s e d by i n t e r a c t i o n w i t h melam w h i c h i s a t h e r m a l l y s t a b l e b a s e . T h e SO3 p o s s i b l y e v o l v e d by p a r t i a l decomposition of pyrosulphuric a c i d w o u l d d e c r e a s e t h e amount o f f r e e m e l a m i n e e v o l v e d c a l c u l a t e d on the basis of T G . T h e c a l c u l a t e d m o l e s o f ammonia e l i m i n a t e d p e r m o l e o f c o n d e n s i n g melamine would c o n s e q u e n t l y d e c r e a s e . However, s i n c e i t was c a l c u l a t e d t h a t o n l y 15% o f o r i g i n a l m e l a m i n e w o u l d be t h e maximum a m o u n t v o l a t i l i s e d , t h e r a t i o ammonia t o m e l a m i n e w o u l d f a l l in the range 0 . 3 5 - 0 . 4 0 which i s i n agreement w i t h t h e above d i s c u s s i o n once the experimental e r r o r i s taken into account.


3

I n t h e f o l l o w i n g t h i r d s t e p ( 4 5 0 - 5 2 0 C ) , ( F i g u r e 10) t h e melam p y r o s u l p h a t e i s f u r t h e r condensed t o melon w i t h d e c o m p o s i t i o n o f the pyrosulphuric s t r u c t u r e as shown by disappearance of typical a b s o r p t i o n s o f p y r o s u l p h a t e o r s u l p h a t e i n t h e IR o f t h e m e l o n l e f t a s a r e s i d u e . T h i s l a s t decomposes t h e n above 5 2 0 * 0 ( 4 t h s t e p ) . The p r o p o s e d d e g r a d a t i o n scheme f o r t h e s u l p h a t e i s : #

melamine s u l p h a t e * H 0 step 1 1 100 - 2 3 0 * C anhydrous melamine s u l p h a t e + H 0 step 2 4 300 - 4 0 0 * C > /* / melam p y r o s u l p h a t e + m e l a m i n e + NH3 + H 0 step 3 I 450 - 5 2 0 * 0 m e l o n + v o l a t i l e p r o d u c t s ( l i k e l y : NHo, H 0 , step 4 \ > 5 2 0 * 0 decomposition 2

2

(11)

2

2

Salts

of Acids

which React w i t h Melamine

SO3)

Condensation

Products.

P h o s p h a t e . The p h o s p h a t e , w h i c h i s a most w i d e l y u s e d f i r e r e t a r d a n t m e l a m i n e s a l t , e l i m i n a t e s w a t e r i n t w o s u c c e s s i v e s t e p s w i t h maximum r a t e a t 280 a n d 3 2 0 * C r e s p e c t i v e l y ( F i g u r e 1 2 ) . T h e s o l i d s t a t e 31P NMR o f m e l a m i n e p h o s p h a t e a n d o f t h e r e s i d u e o f t h e t w o d e h y d r a t i o n s t e p s ( 3 0 0 a n d 3 3 0 * C ) a r e shown i n F i g u r e 1 3 . I n t h e c a s e o f t h e o r i g i n a l s a l t ( F i g u r e 13A) t h e c h e m i c a l s h i f t a n i s o t r o p y i s a x i a l l y s y m m e t r i c and t h e i s o t r o p i c peak i s a t 2.5 5 t h a t i s i n t h e t y p i c a l region of o-phosphates (24). The r e s i d u e o f t h e f i r s t step of d e g r a d a t i o n shows t h e i s o t r o p i c peak c e n t e r e d a t - 8 . 6 $ ( F i g u r e 13B) that is at the chemical s h i f t t y p i c a l of pyrophosphates (24). S p l i t t i n g i n t o t w o p e a k s a t - 7 . 6 a n d 9 . 5 & i s l i k e l y t o be d u e t o a s y m m e t r y o f t h e m o l e c u l e o w i n g t o non e q u i v a l e n c e o f t h e m e l a m i n e m o l e c u l e s i n t h e pyrophosphate s a l t . In t h i s case t h e c h e m i c a l s h i f t anisotropy i s a x i a l l y s l i g h t l y a s y m m e t r i c a s shown by slightly d i f f e r e n t h e i g h t o f t h e s p i n n i n g s i d e b a n d s . A s m a l l amount o f t h e o r i g i n a l o - p h o s p h a t e i s s t i l l p r e s e n t i n t h i s r e s i d u e a s shown by t h e s m a l l peak a t 2.5 4 . The r e s i d u e o f t h e s e c o n d d e g r a d a t i o n s t e p shows t h e i s o t r o p i c p e a k a t - 2 1 . 5 b ( F i g u r e 13C) t y p i c a l o f m i d d l e g r o u p s o f



F i g u r e 1 2 . T G , DTG a n d r a t e o f w a t e r ( a r b i t r a r y u n i t s ) f o r melamine phosphate.

and ammonia C o n d i t i o n s as

evolution F i g u r e 2.


15.

COSTA E T A L .



A

1 120

80

40

1

1

1 0

-40

c

-80

-120 -160 6,ppm

F i g u r e 13. S o l i d s t a t e 31P NMR o f m e l a m i n e p h o s p h a t e ( A , i =3.5 K H z ) , o f r e s i d u e a t 300*C ( B , U ^ = 3 . 6 K H z : p y r o p h o s p h a t e ) and 330*C (C, =4.0 KHz p o l y p h o s p h a t e ) .


227

228

FIRE AND POLYMERS

linear polyphosphates (24) and t h e c h e m i c a l s h i f t a n i s o t r o p y is s t r o n g l y a x i a l l y a s y m m e t r i c . A d e g r e e o f p o l y m e r i z a t i o n a b o v e 60 c a n be c a l c u l a t e d on t h e b a s i s o f e x p e r i m e n t a l s e n s i t i v i t y a n d a b s e n c e o f s i g n a l s around - 8 S a r i s i n g f r o m end g r o u p s o f p o l y p h o s p h a t e s (24). Thus, the d e h y d r a t i o n processes i n v o l v e c o n d e n s a t i o n of melamine p h o s p h a t e t o p y r o p h o s p h a t e and p o l y p h o s p h a t e :

NH

0

II

2

HO-

P

-H 0 2

.H P0 3

P

—

*H

4


2

N

_

C

\

/

C-NH

1st

2

1

step

NH

U

C

HO-

—

P

2nd

A

N 2~\

-(n-1)

2

H 0 2

2

A H

x n

OH

NH, H

OH

-

I

N

N I"

C

N H

2

H

2

N

_

I\ (

step

N C

II"

N H

2

—

I

OH

I N H N-C 2

(12)

2

N l C-NH,

n/2

The e x p e r i m e n t a l w e i g h t l o s s i n t h e f i r s t and s e c o n d s t e p ( 4 . 0 and 4.5% respectively) is in agreement with that corresponding to condensation to pyrophosphate ( 4 . 0 % ) and p o l y p h o s p h a t e (4.2%, n 1). F u r t h e r m o r e , t h e IR s p e c t r a o f m e l a m i n e p h o s p h a t e a n d o f t h e r e s i d u e s a t 300 a n d 3 3 0 C ( F i g u r e 14 s p e c t r a A , B a n d C r e s p e c t i v e l yf)) show t h a t b e s i d e s t h e t y p i c a l b a n d s o f p h o s p h a t e s a l t s ( 9 5 0 - 1 3 0 0 cm ' ')) w h i c h a r e p r e s e n t i n t h e t h r e e s p e c t r a , a new a b s o r p t i o n d u e t o P - O - P b o n d s ( c a . 890 cm" ) a p p e a r s i n t h e s p e c t r a o f t h e r e s i d u e s . The a b s o r p t i o n s due to melamine s a l t s t r u c t u r e s (e.g. 7 8 0 - 7 9 0 a n d 1 4 5 0 - 1 7 5 0 cm ) are c l o s e l y s i m i l a r i n t h e t h r e e s p e c t r a o f F i g u r e 14. F i r e retardants b a s e d on m e l a m i n e p y r o p h o s p h a t e a n d p o l y p h o s p h a t e a r e r e p o r t e d i n t h e l i t e r a t u r e 151 as w e l l as methods f o r p r e p a r a t i o n o f t h e s e s a l t s (25#

m. T h e TG a n d DTG c u r v e s of Figure 12 s h o w that melamine polyphosphate undergoes a complex d e g r a d a t i o n process between 3306 5 0 C . I n s t e p 3 o f t h e DTG c u r v e (max. r a t e 3 9 0 * 0 ) , w a t e r , ammonia and melamine a r e e v o l v e d . In t h i s s t e p t h e t h e r m a l b e h a v i o u r of p o l y p h o s p h a t e i s somewhat s i m i l a r t o t h a t o f t h e s u l p h a t e i n t h e same range of t e m p e r a t u r e ( 3 0 0 - 4 0 0 * 0 ) . Indeed e v o l u t i o n of melamine indicates that thermal d i s s o c i a t i o n of polyphosphate g i v i n g free melamine t a k e s p l a c e above 330*0. However, e v a p o r a t i o n o f melamine c o m p e t e s w i t h i t s c o n d e n s a t i o n as shown by e v o l u t i o n o f a m m o n i a . #


15.

COSTA E T A L .


229

Moreover, simultaneous e v o l u t i o n of water i m p l i e s t h a t condensation of melamine i s combined w i t h t h a t o f p o l y p h o s p h o r i c s t r u c t u r e s . Assuming t h e r e a c t i o n s c h e m e p r o p o s e d a b o v e , melam p o l y p h o s p h a t e a n d ammonium p o l y p h o s p h a t e g r o u p s s h o u l d be f o r m e d f r o m m e l a m i n e polyphosphate:


0

I t i s k n o w n t h a t ammonium p o l y p h o s p h a t e t e n d s t o d i s s o c i a t e l i b e r a t i n g ammonia a b o v e 3 0 0 C a n d t h e r e s u l t i n g f r e e h y d r o x y g r o u p s condense giving crosslinked structures (ultraphosphate) with e l i m i n a t i o n of water (28.29): #

\ 2 0=P-0NH

4

>

0 = P - 0 - P = 0 + NHo + i l 3

(14)

H0 2 o

Owing t o o v e r l a p p i n g o f d i f f e r e n t p r o c e s s e s in t h i s range of t e m p e r a t u r e , q u a n t i t a t i v e c o n s i d e r a t i o n s b a s e d on d a t a o b t a i n e d in p r o g r a m m e d t e m p e r a t u r e TG c a n n o t be m a d e . H o w e v e r , i t c a n r o u g h l y be e s t i m a t e d t h a t up t o 4 1 0 C , t h a t i s t o t h e e n d o f t h e process c h a r a c t e r i s e d by t h e n a r r o w DTG p e a k 3 i n F i g u r e 1 2 , a b o u t o n e m o l e o f ammonia i s e v o l v e d p e r mole o f m e l a m i n e . T h i s w o u l d s u p p o r t the f o r m a t i o n o f melam by R e a c t i o n 1 3 , f o l l o w e d by e l i m i n a t i o n o f ammonia ( r e a c t i o n 14). The r e s i d u e o f t h e t h i r d s t e p o f d e g r a d a t i o n o f m e l a m i n e p h o s p h a t e ( 3 0 0 - 4 1 0 C ) s h o u l d t h e n be t h e melam s a l t o f u l t r a p h o s p h o r i c a c i d . The IR s p e c t r u m o f s u c h a r e s i d u e ( F i g u r e 1 5 A ) s h o w s b r o a d , poorly r e s o l v e d a b s o r p t i o n bands w i t h however c l e a r e v i d e n c e o f s - t r i a z i n e r i n g a b s o r p t i o n a t 7 9 0 - 8 1 0 cm . C r o s s l i n k i n g P-O-P bonds ("branching points") hydrolyse very fast in water to give phosphoric or polyphosphoric acid (30). Upon t r e a t m e n t w i t h w a t e r o f the u l t r a p h o s p h a t e r e s i d u e , a s m a l l f r a c t i o n d i s s o l v e d p r o b a b l y due t o phosphoric or polyphosphoric acid molecules r e s u l t i n g from random h y d r o l y s i s of the branching p o i n t s , whereas the s a l t s of products of c o n d e n s a t i o n o f m e l a m i n e a r e i n s o l u b l e . T h e IR s p e c t r u m o f t h e w a t e r extracted residue (Figure 15B) shows s l i g h t l y b e t t e r resolved a b s o r p t i o n bands than t h e o r i g i n a l r e s i d u e _ ( F i g u r e 15A). T y p i c a l a b s o r p t i o n s o f s - t r i a z i n e r i n g ( 7 9 0 - 8 1 5 cm ) a n d o f P=0 groups ( 1 0 8 0 , 1250 cm ) a r e e v i d e n t w h e r e a s i d e n t i f i c a t i o n o f t h e melam s t r u c t u r e i s not s t r a i g h t f o r w a r d . N e v e r t h e l e s s , the a t t r i b u t i o n of the IR s p e c t r u m o f F i g u r e 15B t o melam p h o s p h a t e o r p o l y p h o s p h a t e d e r i v i n g f r o m h y d r o l y s i s o f melam u l t r a p h o s p h a t e seems n o t u n l i k e l y if c o m p a r i s o n i s made w i t h IR a b s o r p t i o n s o f melam ( F i g u r e 1B) a n d i t s s a l t s ( F i g u r e 7 , 9 , 1 1 ) p a r t i c u l a r l y i n t h e r e g i o n s 1500-1700 and 3 0 0 0 3 8 0 0 cm . M e l a m p o l y p h o s p h a t e i s r e p o r t e d t o be o b t a i n e d o n h e a t i n g melam p h o s p h a t e ( 2 0 ) . A b o v e 410 *C t h e u l t r a p h o s p h a t e u n d e r g o e s a r e l a t i v e l y slow d e g r a d a t i o n process which i s completed a t about 650 C w i t h f o r m a t i o n o f a r e s i d u e (35% o f o r i g i n a l p h o s p h a t e ) m o s t l y s t a b l e t o 9 5 0 C #

#

#

#


230

FIRE AND POLYMERS T

1

J

1

1

r


A

I 4000

i 3000

i 2000

i 1500

i 1—» 1000 500 wavenumber, cm"

1

F i g u r e 1 4 . IR s p e c t r u m o f m e l a m i n e p h o s p h a t e ( A ) a n d o f r e s i d u e 3 0 0 C (B: p y r o p h o s p h a t e ) and a t 3 3 0 C ( C : p o l y p h o s p h a t e ) . #

I 4000

at

#

I

3000

I

2000

I

1500

L.

1—I

1000 500. wavenumber, c m "

F i g u r e 1 5 . IR s p e c t r u m o f r e s i d u e o f m e l a m i n e p h o s p h a t e h e a t e d 3 8 0 C (A) and o f t h i s r e s i d u e a f t e r w a t e r e x t r a c t i o n ( B ) . #


to

15.

COSTA E T A L .


231

( F i g u r e 1 2 ) . I n t h i s s t e p t h e DTG c u r v e s h o w s a v e r y b r o a d p e a k w i t h a n a r r o w e r maximum s u p e r i m p o s e d (550 C) i n d i c a t i n g the occurrence of different overlapping processes. This thermal behaviour cannot be e x p l a i n e d on t h e b a s i s o f t h a t o f m e l a m i n e c o n d e n s a t i o n p r o d u c t s o r o f u l t r a p h o s p h a t e s ( e . g . ammonium s a l t ) . I n d e e d m e l a m i n e condensate undergoes complete f r a g m e n t a t i o n t o v o l a t i l e p r o d u c t s below 750*0 (18) w h i l e ammonium u l t r a p h o s p h a t e d o e s s o m o s t l y b e l o w 7 0 0 C ( 2 9 ) , i n TG a t 1 0 * C / m i n . The p r e s e n c e o f P i n t h e m a t e r i a l o b t a i n e d a t 6 5 0 * 0 i s shown by t h e s o l i d s t a t e 31P NMR w h i c h h o w e v e r g i v e s b r o a d c o m p l e x signals t h a t c o u l d n o t be a s s i g n e d to s p e c i f i c structures at this stage. The i n f r a r e d s p e c t r u m ( F i g u r e 16A) shows b r o a d absorptions among w h i c h t h o s e t y p i c a l o f s - t r i a z i n e r i n g ( 8 1 0 cm ^) a n d o f N H groups of melamine or i t s condensates (1650 and 3000-3600 c m j l ) a r e r e c o g n i s a b l e . A n o t i c e a b l e a b s o r p t i o n b e g i n s t o show a t 2 2 0 0 cm . F u r t h e r m o d i f i c a t i o n o f t h e r e s i d u e t a k e s p l a c e on r a i s i n g the t e m p e r a t u r e a b o v e 6 5 0 * 0 a s shown by t h e l o w s t e a d y r a t e o f w e i g h t l o s s (8% t o 9 0 0 * 0 , F i g u r e 1 2 ) . I n d e e d , t h e t y p i c a l a b s o r p t i o n o f s - t r i a z i n e r i n g a n d o f N H g r o u p s , s t i l l e v i d e n t i n F i g u r e 16A, h a v e d i s a p p e a r e d i n t h e IR s p e c t r u m o f t h e m a t e r i a l o b t a i n e d a t 9 0 0 C ( F i g u r e 16B). H e r e , a few b r o a d a b s o r p t i o n bands a r e p r e s e n t c e n t e r e d a t 500, 950, 1280 a n d 2 2 0 0 cm . T h i s m a t e r i a l w h i c h i s a w h i t e powder, undergoes a v e r y r a p i d d e c o m p o s i t i o n t o v o l a t i l e p r o d u c t s w i t h maximum r a t e a t 9 6 0 C ( s t e p 5, F i g u r e 1 2 ) . W h i l e f u r t h e r c h a r a c t e r i s a t i o n i s under w a y , i t c o u l d be s u g g e s t e d t h a t t h e p r o d u c t o b t a i n e d a t 9 0 0 * 0 m i g h t be a h i g h l y t h e r m a l l y s t a b l e (PN) compound o f t h e t y p e o b t a i n e d on h e a t i n g phospham (NPNH) o r p h o s p h o r y l n i t r i d e (0PN) (31.32). Sumarising, the o v e r a l l degradation process of melamine phosphate f o l l o w s t h e scheme: #

#


2

2

#

#

n

melamine phosphate 1 I 250-300*0 + H 0 melamine pyrophosphate step 2 | 300-330*0 / melamine polyphosphate + H 0 (15) step 3 | 330-410*0 / / melam u l t r a p h o s p h a t e + NHo + H 0 + m e l a m i n e step 4 t 410-650*0 (unidentified phosphate-ultraphosphate-melamine type structures) + v o l a t i l e products | 650-940*0 l o s s o f m e l a m i n e s t r u c t u r e s ( l i k e l y f o r m a t i o n o f PN) step 5 J 940-970*0 decomposition step

2

2

2

Borate. D e h y d r a t i o n i s t h e f i r s t r e a c t i o n o c c u r r i n g on h e a t i n g t h e b o r a t e a s i n t h e c a s e o f t h e p h o s p h a t e b u t a t a much l o w e r t e m p e r a t u r e ( 1 3 0 - 2 7 0 * 0 , s t e p 1 F i g u r e 1 7 ) . B o r i c a c i d h e a t e d a l o n e i n t h e same c o n d i t i o n s i n TG, e l i m i n a t e s w a t e r b e t w e e n 8 0 - 3 0 0 * 0 i n two m a i n o v e r l a p p i n g s t e p s w i t h maximum r a t e a t 170 a n d 1 9 0 * 0 c o r r e s p o n d i n g t o f o r m a t i o n o f m e t a b o r i c a c i d and b o r i c a n h y d r i d e r e s p e c t i v e l y : -H 0

x 2

2

HoBOo 3

3

>

HBO2

> -H 0

B 0o 9

2

3

2


(16)


232

FIRE AND POLYMERS

I— 4000

i 2000

1

3000

i 1500

i i—l 1000 500 wavenumber, cm" 1

F i g u r e 1 6 . IR s p e c t r u m o f r e s i d u e 650 C (A) and t o 900 C ( B ) . #

o f melamine phosphate

heated t o

#

F i g u r e 1 7 . T G , DTG a n d r a t e o f w a t e r a n d a m m o n i a e v o l u t i o n ( a r b i t r a r y u n i t s ) f o r melamine b o r a t e . C o n d i t i o n s as F i g u r e 2.


15.

COSTA E T A L .


233

A p h y s i c a l m i x t u r e o f b o r i c a c i d and melamine shows t h i s two s t e p d e h y d r a t i o n p r o c e s s e s i n TG w h e r e a s i n t h e s a l t t h e t w o s t e p s seem t o merge. Indeed t h e d e h y d r a t i o n p r o c e s s b e g i n s a t h i g h e r t e m p e r a t u r e i n the s a l t (130*0 i n s t e a d o f 80*0) p o s s i b l y because i n t e r a c t i o n s of hydroxy g r o u p s w i t h melamine m o l e c u l e s have f i r s t t o be t h e r m a l l y b r o k e n . The o v e r a l l w e i g h t l o s s a t t h e end o f t h e f i r s t step of degradation of the borate ( 1 5 % , 2 7 0 * 0 F i g u r e 17) i s i n a g r e e m e n t w i t h occurrence of Reaction 16 ( c a l c u l a t e d : 1 4 . 4 % ) . The IR s p e c t r u m o f t h e p r o d u c t o f d e h y d r a t i o n ( F i g u r e 18B) s h o w s t h e t y p i c a l a b s o r p t i o n s of m e l a m i n e w i t h m i n o r d i f f e r e n c e s i n r e l a t i v e i n t e n s i t y o f some b a n d s ( s e e F i g u r e 1 A ) . W h e r e a s t h e a b s o r p t i o n s o f OH g r o u p s p r e s e n t i n t h e s p e c t r u m o f t h e o r i g i n a l s a l t ( e . g . 2 4 0 0 , 2 9 0 0 cm , F i g u r e 18A), have disappeared as e x p e c t e d f r o m o c c u r r e n c e of Reaction 1_6. The absorptions of boric anhydride ( 8 0 0 ( m ) , 1 2 0 0 ( m ) , 1 4 7 0 ( s ) cm ) are d i f f i c u l t to recognise i n F i g u r e 18B o w i n g t o o v e r l a p p i n g w i t h t h e r e l a t i v e l y s t r o n g a b s o r p t i o n s o f m e l a m i n e . The d e h y d r a t i o n p r o c e s s o f melamine borate should then r e s u l t i n a p h y s i c a l m i x t u r e or adduct of b o r i c anhydride w i t h melamine.


1

I n t h e r a n g e 2 7 0 - 3 5 0 * 0 ( 2 n d s t e p , F i g u r e 17) m e l a m i n e v o l a t i l i s e s as i n t h e c a s e o f p u r e m e l a m i n e ( F i g u r e 2) o r o f s a l t s w h i c h f r e e melamine at these t e m p e r a t u r e s , e . g . dimelamine cyanurate (1st step, F i g u r e 3 ) , melamine o - p h t a l a t e (2nd s t e p , F i g u r e 5 ) . However, i n t h e case of b o r i c a n h y d r i d e - m e l a m i n e o n l y a b o u t 50% o f t h e m e l a m i n e v o l a t i l i s e s a s i t c a n be c a l c u l a t e d f r o m t h e w e i g h t l o s s i n t h i s s t e p ( 3 5 % , F i g u r e 17) s i n c e b o r i c a n h y d r i d e i s s t a b l e t o >800*C i n T G . W h e r e a s more t h a n 90% o f f r e e m e l a m i n e c a n v o l a t i l i s e i n t h e same r a n g e o f t e m p e r a t u r e i n t h e e x a m p l e s m e n t i o n e d a b o v e . On t h e o t h e r hand f r e e melamine i s a b s e n t from t h e r e s i d u e o b t a i n e d when v o l a t i l i s a t i o n s t o p s a s shown by t h e IR s p e c t r u m o f F i g u r e 19A in w h i c h , however, t y p i c a l a b s o r p t i o n s o f t h e s - t r i a z i n e r i n g (790-815 cm ) a n d o f N H , NH g r o u p s ( 1 6 0 0 - 1 7 0 0 a n d 3 0 0 0 - 3 6 0 0 cm ) are r e c o g n i s a b l e i n the o t h e r w i s e complex a b s o r p t i o n p a t t e r n . T h i s r e s i d u e was b o i l e d w i t h 5N HC1 t o e l i m i n a t e s o l u b l e f r a c t i o n s s u c h a s b o r i c a n h y d r i d e . The e x t r a c t e d m a t e r i a l s h o w s an IR s p e c t r u m ( F i g u r e 19B) w h i c h c o u l d be c o n s i s t e n t w i t h a c o m p l e x p r o d u c t o f c o n d e n s a t i o n of m e l a m i n e . I n d e e d , t h e s p e c t r u m o f F i g u r e 19B i s v e r y c l o s e t o t h a t o f t h e w a t e r e x t r a c t e d p r o d u c t o b t a i n e d a t t h e same t e m p e r a t u r e from m e l a m i n e p h o s p h a t e ( F i g u r e 15B) a p a r t f r o m t h e p r e s e n c e o f m i n o r a b s o r p t i o n s b e t w e e n 9 0 0 - 1 3 0 0 cm ( e . g . 1080 cm , P=0) i n t h i s l a s t . However, i n t h e case o f t h e phosphate, t h e f o r m a t i o n o f t h i s residue o c c u r r e d w i t h e v o l u t i o n o f ammonia i n d i c a t i n g t h a t i t was i s s u e d f r o m a c o n d e n s a t i o n p r o c e s s i n v o l v i n g NH? g r o u p s o f m e l a m i n e . W h e r e a s , in t h e c a s e o f t h e b o r a t e , e v o l u t i o n o f ammonia o n l y b e g i n s a t 3 5 0 * 0 a n d takes place in the following t h i r d step (Figure 17). Although this p o i n t i s t o be f u r t h e r i n v e s t i g a t e d i t m i g h t be t h a t c o n d e n s a t i o n o f melamine t a k e s p l a c e i n the b o r i c anhydride-melamine system with h o w e v e r r e t e n t i o n o f ammonia by i n t e r a c t i o n w i t h b o r i c anhydride. A m m o n i a w o u l d t h e n be r e l e a s e d a t h i g h e r t e m p e r a t u r e when such i n t e r a c t i o n s a r e b r o k e n o n h e a t i n g . F o r e x a m p l e , ammonium p e n t a b o r a t e i s shown t o e l i m i n a t e ammonia b e t w e e n 3 3 0 - 4 0 0 * 0 i n TG ( 3 3 ) . T h e amount o f ammonia r e l e a s e d a b o v e 3 5 0 * 0 c o r r e s p o n d s t o 0 . 6 m o l e p e r m o l e o f melamine u n v o l a t i 1 i s e d i n t h e p r e v i o u s s t e p between 2 7 0 - 3 5 0 * 0 . T h i s i s c l o s e t o ammonia e v o l v e d when t h e e q u i v a l e n t r e s i d u e o f F i g u r e 15B i s f o r m e d f r o m t h e p h o s p h a t e ( 0 . 4 0 ) a n d t o t h e amount c o r r e s p o n d i n g to 1

1

0



234

FIRE AND POLYMERS

i 4000

F i g u r e 18. IR 280 C (B).

1

i

i

i

3000

2000

1500

1000

spectrum of

melamine

borate

i I 500

wavenumber, cm" (A)

and o f

1

residue

at

#

4000

3000

2000

F i g u r e 1 9 . IR s p e c t r u m o f r e s i d u e 380*C (A) and o f t h i s r e s i d u e a f t e r

1500

1000

500

wavenumber, c m

o f melamine b o r a t e heated acid e x t r a c t i o n (B).


-1

to

15.

COSTA E T A L .


235


melam f o r m a t i o n ( 0 . 5 0 ) . A physical mixture o f boric anhydride and m e l a m i n e s h o w s a TG i d e n t i c a l t o t h a t o f F i g u r e 1 7 , a p a r t f r o m t h e dehydration step. Although a d e t a i l e d i d e n t i f i c a t i o n o f t h e residue o b t a i n e d from the second step o f degradation o f melamine borate i s not yet a v a i l a b l e , i t i s c l e a r f r o m F i g u r e 19 t h a t i t c o n t a i n s s - t r i a z i n e r i n g and amino g r o u p s b e s i d e s b o r i c anhydride. This material undergoes f u r t h e r m o d i f i c a t i o n on h e a t i n g above 350*0 w i t h a r e l a t i v e l y slow w e i g h t l o s s t o 950*0 ( 3 r d s t e p , F i g u r e 17) t o g i v e a w h i t e p r o d u c t s t a b l e a t l e a s t t o 1100*0 (17% o f t h e o r i g i n a l melamine b o r a t e ) . T h i s s t e p o f d e g r a d a t i o n o c c u r s w i t h 32% w e i g h t l o s s o f w h i c h o n l y 2 . 6 % i s a c c o u n t e d f o r by ammonia. The c o n t e n t o f b o r o n in the material o b t a i n e d a t 950*0 i s 28% as d e t e r m i n e d by a method p r o p o s e d f o r r e f r a c t o r y boron n i t r i d e ( 3 4 ) . The IR s p e c t r u m o f t h i s r e s i d u e ( F i g u r e 20) s h o w s a r e l a t i v e l y s h a r p maximum a t 1 4 0 5 cm w h i c h c o u l d be a t t r i b u t e d t o B-N b o n d s ( 1 7 ) . a n d t w o o v e r l a p p i n g b r o a d m a x i m a i n t h e r e g i o n o f N H , NH s t r e t c h i n g f r e q u e n c y ( 3 0 0 0 - 3 8 0 0 cm ) w h e r e a s a b s o r p t i o n s d u e t o t h e s - t r i a z i n e r i n g ( e . g . c a 8 0 0 cm ) a r e a b s e n t . F u r t h e r b r o a d a b s o r p t i o n s a r e shown i n F i g u r e 2 0 i n t h e r e g i o n s 8 0 0 1300 a n d 1 5 0 0 - 1 7 0 0 cm . Many p a p e r s a n d p a t e n t s d e a l w i t h p r e p a r a t i o n of r e f r a c t o r y boron n i t r i d e ( s t a b l e i n i n e r t atmosphere t o 2700*0 2

(35) ) by h e a t i n g b o r o n o x y compounds w i t h m e l a m i n e g e n e r a l l y above 900*0 ( e . g . r e f . 3 6 - 4 1 ) . The IR s p e c t r a r e p o r t e d i n t h e l i t e r a t u r e f o r boron n i t r i d e ( 4 2 . 4 3 ) s h o w a weak a b s o r p t i o n a t 8 0 9 - 8 1 3 cm and a s t r o n g o n e a t 1 3 7 4 - 1 3 8 9 cm . T h e s p e c t r u m o f F i g u r e 2 0 c o u l d b e p a r t i a l l y c o n s i s t e n t w i t h t h a t o f a boron n i t r i d e s i n c e the s t r o n g e s t of t h eabove a b s o r p t i o n s i s c l o s e t o t h e s t r o n g e s t a b s o r p t i o n i n F i g u r e 2 0 . However, t h e IR s p e c t r u m o f F i g u r e 20 shows a l s o r e l e v a n t a b s o r p t i o n s w h i c h a r e n o t a t t r i b u t a b l e t o b o r o n n i t r i d e . On t h e o t h e r h a n d i t was s u g g e s t e d ( 3 6 ) t h a t o n h e a t i n g m i x t u r e s o f o r t h o b o r i c a c i d and m e l a m i n e o r o t h e r n i t r o g e n c o n t a i n i n g c o m p o u n d s ( e . g . : urea, c y a n a m i d e e t c . ) a BNO ( B = 3 1 . 6 % , N = 2 0 . 5 % ) r e s i d u e i s o b t a i n e d a t 600*0 w h i c h c o u l d n o t be t r a n s f o r m e d i n t o pure boron n i t r i d e on f u r t h e r h e a t i n g t o 1 3 0 0 * 0 ; w h e r e a s t h e BNO r e s i d u e g a v e b o r o n n i t r i d e o n heating i n a stream o f ammonia a t 500-900*0. Although IR c h a r a c t e r i s a t i o n o f t h e BNO w a s n o t s h o w n , t h e B c o n t e n t o f t h e r e s i d u e o f F i g u r e 20 (28%) i s c l o s e t o t h a t r e p o r t e d i n t h e l i t e r a t u r e ( 3 6 ) f o r BNO ( 3 1 . 6 % ) . T h e r e f o r e , t h e r e s i d u e o b t a i n e d a t 9 5 0 * 0 f r o m

4000

3000

2000

1500

1000 500 wavenumber, cm" 1

Figure 950*0.

2 0 . IR s p e c t r u m

o f residue

o f melamine

borate

heated t o


236

FIRE AND POLYMERS

m e l a m i n e b o r a t e c o u l d b e s i m i l a r t o t h i s BNO m a t e r i a l r a t h e r t h a n t o pure boron n i t r i d e . While f u r t h e r c h a r a c t e r i s a t i o n work is in progress, i t must be m e n t i o n e d t h a t a l s o p o l y a m i n o - b o r a z i n e s (35,44) are b o r o n - n i t r o g e n t h e r m a l l y s t a b l e p r o d u c t s which might be formed i n t h e a b o v e c o n d i t i o n s a n d w o u l d a c c o u n t f o r t h e N-H a b s o r p t i o n i n t h e IR s p e c t r u m o f F i g u r e 2 0 . Summarising t h e data available a t t h i s stage, t h e thermal d e g r a d a t i o n o f melamine b o r a t e can be r e p r e s e n t e d by t h e scheme: melamine b o r a t e I 130-270*0 / melamine-boric anhydride + H 0 (17) \ step 2 I 270-350*0 melamine + u n i d e n t i f i e d ( l i k e l y melamine c o n d e n s a t e - b o r i c anhydride) step 3 i 350-950 C l o s s o f m e l a m i n e s t r u c t u r e s ( l i k e l y f o r m a t i o n o f BN o r BNO) step

1


2

m

Conclusion T h i s s u r v e y shows t h a t melamine salts can display widely different thermal behaviour depending on t h e type o f a c i d residue. As f a r a s f i r e retardance i s concerned t h e f o r m a t i o n o f v o l a t i l e products and o f t h e r m a l l y s t a b l e r e s i d u e ( " c h a r " ) i n t h e range 300-500*C i n which most p o l y m e r s u n d e r g o t h e r m a l d e g r a d a t i o n , i s o f paramount i m p o r t a n c e . F i r s t o f a l l i t i s shown t h a t i n d i m e l a m i n e s a l t s o n e m o l e c u l e o f melamine behaves a s f r e e melamine v o l a t i l i s i n g almost q u a n t i t a t i v e l y above 2 5 0 * C . A s i m i l a r b e h a v i o u r i s a l s o shown by monomelamine salts o f a c i d s which tend t o decompose on h e a t i n g a t r e l a t i v e l y l o w temperature (200-400*C, e . g . o - p h t a l a t e , cyanurate, o x a l a t e ) . These s a l t s l e a v e o n d e g r a d a t i o n a s m a l l amount o f r e s i d u a l c h a r s t a b l e t o a b o u t 5 0 0 * C (

Mechanism of Thermal Degradation of Fire-Retardant Melamine Salts

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