Thermal investigation of unstable substances including a comparion of different thermal analytical techniques

THERMAL INVESTIGATION OF

UNSTABLE SUBSTANCES,

INCLUDING A COMPARISON

OF DIFFERENT

THERMAL ANALYTICAL

TECHNIQUES

R.B. Barendregt

P 1 6 4 4

4 0 6 8

C10Q47

8 9 4 2 4

BIBLIOTHEEK TU Delft

P 1644 4068

C 478942

t

THERMAL INVESTIGATION OF UNSTABLE SUBSTANCES, INCLUDING A COMPARISON OF DIFFERENT THERMAL ANALYTICAL TECHNIQUES

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR

IN DE TECHNISCHE WETENSCHAPPEN AAN DE

TECHNISCHE HOGESCHOOL DELFT,OP GEZAG VAN DE

RECTOR MAGNIFICUS PROF.IR.B.P.TH.VELTMAN,

VOOR EEN COMMISSIE AANGEWEZEN DOOR

HET COLLEGE VAN DEKANEN TE

VERDEDIGEN OP WOENSDAG 11 FEBRUARI 1981

TE 14.00 UUR DOOR \ ( o U PROEFSCHRIFT

ROBERT BERNARD BARENDREGT

SCHEIKUNDIG INGENIEUR

GEBOREN TE DEN HAAG.

T H E R M A L I N V E S T I G A T I O N O F U N S T A B L E S U B S T A N C E S , I N C L U D I N G A C O M P A R I S O N O F D I F F E R E N T T H E R M A L A N A L Y T I C A L T E C H N I Q U E S

DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE

PROMOTOR :

To Patrio

THE INVESTIGATIONS DESCRIBED IN

THIS THESIS HAVE BEEN CARRIED OUT

IN THE PRINS MAURITS LABORATORY

C o n t e n t s Summary 1 S a m e n v a t t i n g 3 L i s t o f Symbols 4 C h a p t e r I INTRODUCTION 7 1. D e g r a d a t i o n o f U n s t a b l e S u b s t a n c e s 7 2. Methods o f M e a s u r i n g C h e m i c a l I n s t a b i l i t y 8 3. Arrangement o f t h i s T h e s i s 10 C h a p t e r I I METHODS USED 11 1. Survey 1 1 2. D i f f e r e n t i a l Thermal A n a l y s i s (DTA) 12 3. T h e r m o g r a v i m e t r y (TG) 15 4. I s o t h e r m a l S t o r a g e T e s t (1ST) 17 5. A d i a b a t i c S t o r a g e T e s t (AST) 20

C h a p t e r I I I EVALUATION OF THE METHODS USED 23

1. I n t r o d u c t i o n 23 2. D i f f e r e n t i a l Thermal A n a l y s i s 24 A. S i g n a l 24 B. I s o t h e r m a l DTA 27 a. Heat G e n e r a t i o n 27 b. K i n e t i c P a r a m e t e r s 30 C. N o n - i s o t h e r m a l DTA 35 a. Heat G e n e r a t i o n 35 b. E v a l u a t i o n o f some Methods 36 c. E x p e r i m e n t s 44 d. R e s u l t s o f t h e E x p e r i m e n t s w i t h TBPP C25 45 e. D i s c u s s i o n and C o n c l u s i o n s 47 3. T h e r m o g r a v i m e t r y 51 4. I s o t h e r m a l S t o r a g e T e s t 53 A. S i g n a l 53 B. Heat G e n e r a t i o n 54 C. K i n e t i c P a r a m e t e r s 58 D. S a f e D i a m e t e r 59 5. A d i a b a t i c S t o r a g e T e s t 61 A. S i g n a l 61 B. Heat G e n e r a t i o n 61 C. K i n e t i c P a r a m e t e r s 62 6. C o n c l u s i o n s 65

C h a p t e r IV SOME ASPECTS CONCERNING THE

DEGRADATION OF SMOKELESS POWDER 67

1. I n t r o d u c t i o n 67 2. E x p e r i m e n t a l P a r t 72 3. T h e o r e t i c a l A s p e c t s 73 4. R e s u l t s 75 A. I s o t h e r m a l DTA and TG E x p e r i m e n t s 75 B. N o n - i s o t h e r m a l DTA and TG E x p e r i m e n t s 79 C. 1ST E x p e r i m e n t s 85 5. D i s c u s s i o n 87 6. C o n c l u s i o n s 91 C h a p t e r V COMPATIBILITY OF THERMITE AND POLYURETHANE 9 3 1. I n t r o d u c t i o n 93 2. M a t e r i a l s 96 3. E x p e r i m e n t a l P a r t 97 4. R e s u l t s 99 A. D e g r a d a t i o n o f P o l y u r e t h a n e 99 B. D e g r a d a t i o n o f T h e r m i t e and T h e r m i t e / B i n d e r 107 5. D i s c u s s i o n 1 16 6. C o n c l u s i o n s 123 C h a p t e r V I FINAL CONCLUSIONS 125 Acknowledgements 127 R e f e r e n c e s 129

Summary

To t e s t t h e s t a b i l i t y o f u n s t a b l e s u b s t a n c e s f o u r t h e r m a l a n a l y t i c a l t e c h n i q u e s a r e d e s c r i b e d . T h e y a r e d i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA), t h e r m o g r a v i m e t r y ( T G ) , i s o t h e r m a l s t o r a g e t e s t (1ST) and a d i a b a t i c s t o r a g e t e s t ( A S T ) . I n t h e case o f i s o t h e r m a l DTA and 1ST a number o f e x p e r i m e n t s w i t h i n e r t s u b s t a n c e s were done so as t o c a l c u l a t e t h e e r r o r s . I n t h e c a s e o f TG and AST the e r r o r s were e v a l u a t e d t h e o r e t i c a l l y and e m p i r i c a l l y r e s p e c t i v e l y .

From t h e s e e v a l u a t i o n s i t has become a p p a r e n t how t o work o u t and i n t e r p r e t the e x p e r i m e n t a l r e s u l t s , w h i l e a l s o c o n c l u s i o n s can be drawn c o n c e r n i n g the a c c u r a c y and r e p r o d u c i b i l i t y o f t h e measurements.

I n t h e c a s e o f n o n - i s o t h e r m a l DTA s e v e r a l w e l l - k n o w n e v a l u a t i o n t e c h n i q u e s a r e compared on t h e b a s i s o f e x p e r i m e n t s w i t h an o r g a n i c p e r o x i d e as t e s t s a m p l e . C o n c l u s i o n s can be drawn about the v a l i d i t y o f the d i f f e r e n t methods. Two methods a r e t o be p r e f e r r e d .

The t h e o r e t i c a l c o n s i d e r a t i o n s were t e s t e d and a p p l i e d i n t h e s t u d y o f two problems c o n c e r n i n g u n s t a b l e s u b s t a n c e s , v i z . t h e d e g r a d a t i o n o f a gunpowder and t h e c o m p a t i b i l i t y o f a p y r o t e c h n i c m i x t u r e and a p o l y m e r .

The main c o n c l u s i o n t h a t c an be drawn i s t h a t t h e r m a l a n a l y t i c a l t e c h n i q u e s a r e v e r y u s a b l e t o o l s i n the h a n d l i n g o f t h e s e problems.However,the

t r a n s l a t i o n o f h i g h t e m p e r a t u r e r e s u l t s to n o r m a l t e m p e r a t u r e s t o r a g e c o n d i t i o n s i s t o be t e s t e d e x p e r i m e n t a l l y . I n c a s e s where i t i s t h e p u r p o s e t o s o l v e t h e r e a c t i o n mechanism i t i s n e c e s s a r y t o combine t h e s e methods w i t h o t h e r a n a l y t i c a l t e c h n i q u e s .

S a m e n v a t t i n g

Om de s t a b i l i t e i t v a n i n s t a b i e l e s t o f f e n t e onderzoeken.worden v i e r t h e r m i s c h e - a n a l y s e t e c h n i e k e n b e s c h r e v e n . H e t z i j n differentiële t h e r m i s c h e a n a l y s e ( D T A ) , t h e r m o g r a v i m e t r i e ( T G ) , i s o t h e r m e b e w a a r p r o e f (IBP) en

a d i a b a t i s c h e b e w a a r p r o e f (ABP).In h e t g e v a l van i s o t h e r m e DTA en IBP werden e r een a a n t a l e x p e r i m e n t e n met; i n e r t e s t o f f e n v e r r i c h t om de f o u t e n t e b e r e k e n e n . I n h e t g e v a l v a n TG en ABP werden de f p u t e n t h e o r e t i s c h b e r e k e n d , r e s p e k t i e v e l i j k e m p i r i s c h v e r k r e g e n .

U i t deze b e s c h r i j v i n g e n komt n a a r v o r e n hoe de e x p e r i m e n t e l e r e s u l t a t e n v e r d e r v e r w e r k t en geïnterpreteerd moeten w o r d e n , t e r w i j l tevens c o n c l u s i e s g e t r o k k e n kunnen worden v o o r wat b e t r e f t de n a u w k e u r i g h e i d en r e p r o d u c e e r -b a a r h e i d .

In h e t g e v a l van n i e t - i s o t h e r m e DTA werden v e r s c h e i d e n e bekende e v a l u a t i e - r t e c h n i e k e n v e r g e l e k e n op b a s i s v a n e x p e r i m e n t e n met een o r g a n i s c h p e r o x i d e a l s t e s t m o n s t e r . C o n c l u s i e s o m t r e n t de waarde v a n de v e r s c h i l l e n d e methoden kunnen worden g e t r o k k e n . A a n twee methoden wordt de v o o r k e u r gegeven.

De t h e o r e t i s c h e o v e r w e g i n g e n werden g e t e s t en t o e g e p a s t i n de s t u d i e v a n twee problemen b e t r e f f e n d e i n s t a b i e l e s t o f f e n . n . l . de d e g r a d a t i e van een rookzwak b u s k r u i t en de v e r e n i g b a a r h e i d v a n een p y r o t e c h n i s c h mengsel en een p o l y m e e r . De b e l a n g r i j k s t e c o n c l u s i e d i e g e t r o k k e n kan worden i s d a t t h e r m i s c h e

-a n -a l y s e t e c h n i e k e n een z e e r b r u i k b -a -a r h u l p m i d d e l z i j n b i j h e t o p l o s s e n v -a n deze p r o b l e m e n . E c h t e r de v e r t a l i n g van r e s u l t a t e n , v e r k r e g e n b i j hpge tempera-t u r e n , n a a r o p s l a g o m s tempera-t a n d i g h e d e n moetempera-t e x p e r i m e n tempera-t e e l g e tempera-t e s tempera-t worden.In g e v a l l e n waar h e t de b e d o e l i n g i s om h e t r e a k t i e m e c h a n i s m e op l^e l o s s e n i s h e t nood-z a k e l i j k om denood-ze methoden t e kombineren met a n d e r e a n a l y t i s c h e t e c h n i e k e n .

L i s t o f Symbols AST a d i a b a t i c s t o r a g e t e s t DPA d i p h e n y l a m i n e DNT d i n i t r o t o l u e n e DTA d i f f e r e n t i a l t h e r m a l a n a l y s i s D j ^ s a f e d i a m e t e r a t 344 K (m) E a c t i v a t i o n e n e r g y ( J mole ') EC e t h y l c e n t r a l i t e ( d i e t h y l d i p h e n y l u r e a ) F ( Q ) h e a t g e n e r a t i o n f a c t o r ( W kg ') G s e n s i t i v i t y o f t h e a m p l i f i e r (uV) I e x p o n e n t i a l i n t e g r a l X e x p ( - ^ ) ^ /*T E I' e x p o n e n t i a l i n t e g r a l X e x p ( - - ) dT U KT 1ST i s o t h e r m a l s t o r a g e t e s t K c a l i b r a t i o n f a c t o r i n DTA o r 1ST (W V~') MDI 4 , 4 ' - b i s - ( 4 - i s o c y a n o p h e n y l ) m e t h a n e M sample mass (g) Mg,M^ s t a r t i n g and f i n a l mass r e s p e c t i v e l y i n TG-measurements (g) NMR n u c l e a r m a g n e t i c r e s o n a n c e N s i g n a l o f t h e z e r o l i n e i n i s o t h e r m a l

DTA and 1ST (uV) Qt t o t a l h e a t g e n e r a t e d from the s t a r t of

an e x p e r i m e n t d u r i n g a c e r t a i n time t

i n i s o t h e r m a l DTA,1ST and AST ( J kg"')

Qq t o t a l heat generated d u r i n g the

e x p e r i m e n t ( h e a t o f r e a c t i o n ) ( J kg ') R gas c o n s t a n t (=8.31432) ( J m o l e "1^ ' ) R^c r e s i s t a n c e o f the h e a t i n g c o i l i n an 1ST c a l i b r a t i o n v e s s e l (Q) S shape i n d e x o f a n o n - i s o t h e r m a l DTA c u r v e s' s t e e p n e s s o f a DTA s i g n a l (yV K ') T t e m p e r a t u r e (K) T ,T ,T ,T (T +AT) t e m p e r a t u r e i n DTA measurements o f the

ov t h r s r

oven,the t h e r m o c o u p l e s , t h e r e f e r e n c e

v e s s e l and the sample v e s s e l (K)

T. 1 TA ' VTM AT êT(T-T^) Ô T ( T j - T2) TG TDI TLC U V c ? s c ,c p p A B c , c p p cp( T ) 0 c f ( x ) g ( x )

hj.hj

maximum) o f a n o n - i s o t h e r m a l DTA c u r v e (K) DTA s i g n a l (K) s m a l l t e m p e r a t u r e i n t e r v a l i n n o n - i s o t h e r m a l DTA (K) t e m p e r a t u r e v a r i a t i o n o f t h e m e t e r i n t h e 1ST (K) t h e r m o g r a v i m e t r y 2 , 5 - d i i s o c y a n o t o l u e n e t h i n l a y e r chromatography m e a s u r i n g s i g n a l i n DTA and 1ST volume o f v e s s e l h e a t c a p a c i t y o f t h e sample h e a t c a p a c i t y o f r e f e r e n c e and sample v e s s e l r e s p e c t i v e l y i n DTA measurements ( J g h e a t c a p a c i t y o f t h e sample s u b s t a n c e i n n o n - i s o t h e r m a l DTA i n p o i n t A and B ( J g h e a t c a p a c i t y as a f u n c t i o n o f t h e t e m p e r a t u r e i n n o n - i s o t h e r m a l DTA ( J g h e a t c a p a c i t y o f t h e sample v e s s e l i n AST ( J K" s t a r t i n g c o n c e n t r a t i o n (mole m J) c o n s t a n t a n a l y t i c a l r a t e e x p r e s s i o n ( d i f f e r e n t i a t e d form) a n a l y t i c a l r a t e e x p r e s s i o n ( i n t e g r a t e d form) h e a t exchange c o e f f i c i e n t h e a t exchange c o e f f i c i e n t f o r a DTA v e s s e l by c o n v e c t i o n between t h e r e f e r e n c e and sample v e s s e l r e s p e c t i v e l y K ) K ') K ') -3, (W

m

V

eje(0„) ^ ( N , ) r a t i o o f volume o f a i r , o x y g e n and n i t r o g e n , r e s p e c t i v e l y and volume o f s u b s t a n c e i n t h e sample v e s s e l A h e a t c o n d u c t i v i t y c o e f f i c i e n t (W m 'K ') _3 b u l k d e n s i t y o f a s u b s t a n c e (kg m ) —3 p s p e c i f i c d e n s i t y o f a s u b s t a n c e (kg m ) x time c o n s t a n t ( s ) <|> h e a t i n g r a t e i n DTA (K s~') h e a t i n g power i n t h e AST (W) 6

C h a p t e r I I N T R O D U C T I O N

1. D e g r a d a t i o n o f U n s t a b l e S u b s t a n c e s

T h i s t h e s i s d e a l s w i t h t h e c h e m i c a l i n s t a b i l i t y o f s u b s t a n c e s , i . e . t h e change i n c h e m i c a l c o m p o s i t i o n under s t o r a g e c o n d i t i o n s . T h i s p r o c e s s i s o f t e n c a l l e d c h e m i c a l a g i n g .

One l a r g e group o f u n s t a b l e s u b s t a n c e s i s f o u n d i n the f i e l d o f e n e r g e t i c m a t e r i a l s such as e x p l o s i v e s and p r o p e l l a n t s . T h e y a r e used f o r many c i v i l and m i l i t a r y p u r p o s e s , f o r i n s t a n c e m i n i n g , r o a d b u i l d i n g , f i r e weapons, r o c k e t s and so on.A second group c o n s i s t s o f p l a s t i c s , w h i c h o f t e n show v e r y s l o w a g i n g a t s t o r a g e c o n d i t i o n s . P o l y u r e t h a n e (PU) and p o l y v i n y l c h l o r i d e (PVC) a r e c a s e s i n p o i n t . I n t h e s e c a s e s t h e a g i n g may be p h y s i c a l as w e l l as c h e m i c a l . b e c a u s e the change i n p r o p e r t i e s i n time a r e b a s e d on m e c h a n i c a l , p h y s i c a l and c h e m i c a l f a c t o r s . T h e s e a g i n g p r o c e s s e s a r e

a c c e l e r a t e d by h e a t , r a d i a t i o n and the p r e s e n c e o f oxygen and o f t e n w a t e r ' . I n t h e group o f h i g h e n e r g e t i c s u b s t a n c e s a g i n g phenomena i n c l u d e n o t o n l y t h e change i n m a t e r i a l p r o p e r t i e s i n time and hence the q u a l i t y , b u t a l s o c h e m i c a l changes r e l a t e d t o the i n h e r e n t h a z a r d s o f t h e s e m a t e r i a l s . These s u b s t a n c e s r e l e a s e t h e i r e n e r g y i n a s h o r t time and n o r m a l l y " b u r n " w i t h o u t t h e s u p p o r t o f oxygen from the s u r r o u d i n g s . T h e y c o n t a i n p r o d u c t s t h a t a r e t h e r m o d y n a m i c a l l y u n s t a b l e and hence s e n s i t i v e t o d e g r a d a t i o n p r o c e s s e s . S p e c i a l l y i f the a c t i v a t i o n energy o f the a g i n g p r o c e s s e s i s low d e g r a d a t i o n i s f a s t . D e g r a d a t i o n may have d i r e c t i n f l u e n c e on the amount o f energy r e l e a s e d a t o p e r a t i n g c o n d i t i o n s . l t i s a l s o p o s s i b l e t h a t t h e r e a c t i o n h e a t o f t h e d e g r a d a t i o n cannot be r e a d i l y t r a n s f e r r e d t o t h e s u r r o u d i n g s , w h i c h c a u s e s s e l f - h e a t i n g o f the s u b s t a n c e . T h i s w i l l a c c e l e r a t e the d e g r a d a t i o n p r o c e s s . f i n a l l y r e s u l t i n g i n s e l f - i g n i t i o n or e x p l o s i o n . A s p e c i a l p r o b l e m a r i s e s i f a h i g h e n e r g e t i c m a t e r i a l i s i n c o n t a c t w i t h a c o n s t r u c t i o n m a t e r i a l s u c h as a p l a s t i c . T h e s e c o n s t r u c t i o n m a t e r i a l s may take p a r t i n the p r o c e s s a t o p e r a t i n g c o n d i t i o n s , t o o . H o w e v e r , i t o f t e n a p p e a r s t h a t the h i g h e n e r g e t i c s u b s t a n c e and the c o n s t r u c t i o n m a t e r i a l i n t e r a c t , e v e n a t s t o r a g e c o n d i t i o n s r e s u l t i n g i n an a d d i t i o n a l d e g r a d a t i o n .

. . . 2

T h i s phenomenon i s c a l l e d i n c o m p a t i b i l i t y .Sometimes i n c o m p a t i b i l i t y has l e d t o r a t h e r dangerous s i t u a t i o n s w i t h e x p l o s i v e s w h i c h were c o m p l e t e l y s a f e when s t o r e d i n a p u r e s t a t e .

2 , Methods o f M e a s u r i n g C h e m i c a l I n s t a b i l i t y

To d e t e r m i n e t h e i n s t a b i l i t y o f p r o d u c t s a number o f methods t o c h a r a c t e r i z e 3

h i g h e n e r g e t i c s u b s t a n c e s have been d e v e l o p e d .They a r e d i v i d e d i n t o c h e m i c a l and p h y s i c a l methods.depending on the c h e m i c a l c o m p o s i t i o n o r the t y p e o f phenomenon on w h i c h t h e y a r e b a s e d .

To d e t e r m i n e the q u a l i t y the p h y s i c a l methods a r e g a i n i n g i n t e r e s t l a t e l y . Because the a g i n g p r o c e s s e s a t s t o r a g e c o n d i t i o n s a r e n o r m a l l y e x t r e m e l y slow, i t would t a k e f a r too much time t o t e s t a t t h e s e c o n d i t i o n s . T h e t e s t can be a c c e l e r a t e d by i n c r e a s i n g the t e m p e r a t u r e . H o w e v e r , t h e n the p o s s i b i l i t y o f a change i n the n a t u r e o f the p r o c e s s o c c u r s . I n t h a t case t h e r e s u l t s l e a d t o an e r r o n e o u s p r e d i c t i o n about the b e h a v i o u r under s t o r a g e c o n d i t i o n s .Because most t e s t s commonly use a c c e l e r a t e d a g i n g a t t e n t i o n s h o u l d be p a i d t o t h i s i n h e r e n t drawback.

Nowadays.products a r e o f t e n d e t e r m i n e d by t h e r m a l a n a l y z i n g t e c h n i q u e s . I n t h e s e t e c h n i q u e s a sample o f the p r o d u c t i s h e a t e d f o l l o w i n g a c e r t a i n

t e m p e r a t u r e program i n time,and some c h e m i c a l o r p h y s i c a l p r o p e r t y i s measured as a f u n c t i o n o f the t e m p e r a t u r e o r t i m e .

T h e r e a r e a l a r g e number o f t h e r m a l a n a l y z i n g t e c h n i q u e s ,some o f them h a v i n g g e n e r a l a p p l i c a b i l i t y , b u t the m a j o r i t y o f them a r e f o r s p e c i a l p u r p o s e s . A l l t e c h n i q u e s a t the moment have been c l a s s i f i e d and d e f i n e d by the N o m e n c l a t u r e Committee o f the I n t e r n a t i o n a l C o n f e d e r a t i o n f o r Thermal A n a l y s i s (ICTA)"*. Some methods which have g e n e r a l a p p l i c a b i l i t y and a r e w i d e l y used a r e d i f f e r e n t i a l t h e r m a l a n a l y s i s ( D T A ) , t h e r m o g r a v i m e t r y (TG) and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC).In v i e w o f the r e l a t i v e l y low s e n s i t i v i t y o f t h e s e t e c h n i q u e s and the q u a n t i t y o f sample used measurements a r e o n l y p o s s i b l e a t h i g h e r t e m p e r a t u r e s i f c h e m i c a l r e a c t i o n s p l a y a r o l e .

To measure s m a l l h e a t e f f e c t s m i c r o c a l o r i m e t e r s have been d e v e l o p e d , f o r i n s t a n c e the c o m p e n s a t i o n m i c r o c a l o r i m e t e r , i n v e n t e d by A . T i a n ^ and l a t e r improved by E . C a l v e t ^ . A t t e m p t s to measure s m a l l h e a t e f f e c t s have a l s o been p e r f o r m e d by o t h e r i n v e s t i g a t o r s ^ ' ^ ' . A t the P r i n s M a u r i t s L a b o r a t o r y the

11 12 13

i s o t h e r m a l h e a t g e n e r a t i o n meter was d e v e l o p e d by Van G e e l ' ' and the 13 14

a d i a b a t i c h e a t g e n e r a t i o n meter by G r o o t h u i z e n * .These i n s t r u m e n t s were s p e c i a l l y d e s i g n e d f o r t e s t i n g a t n e a r s t o r a g e c o n d i t i o n s . T h e t e s t s p e r f o r m e d w i t h t h e s e i n s t r u m e n t s a r e c a l l e d i s o t h e r m a l s t o r a g e t e s t (1ST) and a d i a b a t i c s t o r a g e t e s t (AST) r e s p e c t i v e l y . T h e measurements o f i n s t a b i l i t y a t r e l a t i v e l y low t e m p e r a t u r e s was made p o s s i b l e by the i n c r e a s e d s e n s i t i v i t y of the m e a s u r i n g d e v i c e s and the l a r g e amount o f sample used.The time r e q u i r e d to

c a r r y o u t t h e measurements i s r e l a t i v e l y l o n g .

I t i s i n t e r e s t i n g t h e n t o compare t h e methods and to draw c o n c l u s i o n s about the t r a n s l a t i o n o f t h e r e s u l t s o f a c c e l e r a t i n g t e s t s i n t o s t o r a g e c o n d i t i o n s . The main q u e s t i o n s t o be answered a r e :

1. I s i t p o s s i b l e i n p r i n c i p l e t o use t h e r m a l a n a l y z i n g methods a t an e l e v a t e d t e m p e r a t u r e t o d e t e r m i n e i n s t a b i l i t y a t lower t e m p e r a t u r e s ?

2. I f so,what a r e t h e o p t i m a l c o n d i t i o n s f o r t h e s e t e s t s ? 3. What o f t h e r e l i a b i l i t y and t h e r e p r o d u c i b i l i t y o f t h e t e s t s ?

To answer t h e s e q u e s t i o n s DTA and TG were r u n a t h i g h e r t e m p e r a t u r e s . w h i l e the 1ST and AST were p e r f o r m e d a t r e l a t i v e l y low t e m p e r a t u r e s . T h e s e methods were e v a l u a t e d from a t h e o r e t i c a l and p r a c t i c a l p o i n t o f v i e w and the r e p r o d u c i b i l i t y o f t h e e x p e r i m e n t s was d e t e r m i n e d . A g a i n s t t h i s b a c k g r o u n d a s t a b i l i t y and a c o m p a t i b i l i t y p r o b l e m were i n v e s t i g a t e d and the v a l i d i t y o f the d i f f e r e n t methods was d e t e r m i n e d .

3 . Arrangement o f t h i s T h e s i s

The methods used a r e d e s c r i b e d and e v a l u a t e d i n c h a p t e r I I and I I I . A

c o m p a r i s o n o f DTA,TG,IST and AST i s made i n c h a p t e r IV and V . I n c h a p t e r IV a gunpowder i s i n v e s t i g a t e d w i t h DTA and T G , w h i l e the r e s u l t s a r e compared w i t h those o f the 1ST;the i n f l u e n c e o f the s u r r o u n d i n g atmosphere i s l o o k e d i n t o as w e l l . I n c h a p t e r V DTA,1ST and AST a r e used t o i n v e s t i g a t e the a g i n g o f p o l y u r e t h a n e and the c o m p a t i b i l i t y p r o b l e m w h i c h c o u l d a r i s e i f t h i s polymer i s i n c o n t a c t w i t h a h i g h e n e r g e t i c s u b s t a n c e .

C h a p t e r I I METHODS USED 1 . S u r v e y In t h i s c h a p t e r a c h a r a c t e r i z a t i o n w i l l be g i v e n c o n c e r n i n g the methods used.They a r e d i f f e r e n t i a l t h e r m a l a n a l y s i s ( D T A ) , t h e r m o g r a v i m e t r y ( T G ) , i s o t h e r m a l s t o r a g e t e s t (1ST) and a d i a b a t i c s t o r a g e t e s t (AST) r e s p e c t i v e l y . E s p e c i a l l y the e x e c u t i o n o f the e x p e r i m e n t s i s d e s c r i b e d , w h i l e o n l y a s h o r t d e s c r i p t i o n o f the a p p a r a t u s i s g i v e n . F o r a d e t a i l e d d e s c r i p t i o n t h e r e f e r e n c e s a r e t o be c o n s u l t e d .

2 . D i f f e r e n t i a l T h e r m a l A n a l y s i s (DTA)

I n DTA the t e m p e r a t u r e d i f f e r e n c e between a sample and a r e f e r e n c e h o l d e r i s r e c o r d e d as a f u n c t i o n of time o r t e m p e r a t u r e ' ^ . T h e r e a r e many

2

DTA systems .The system t h a t was u s e d i n the e x p e r i m e n t s o f t h i s t h e s i s i s a M e t t l e r TA-2000 a p p a r a t u s .

A s c h e m a t i c r e p r e s e n t a t i o n o f the a p p a r a t u s i s shown i n F i g u r e l . T h e a p p a r a t u s c o n s i s t s o f a s i l v e r oven t h a t i s h e a t e d l i n e a r l y i n time a t d i f f e r e n t h e a t i n g r a t e s , o r i s k e p t a t a c o n s t a n t t e m p e r a t u r e . l t c o v e r s a t e m p e r a t u r e range f r o m 100 t o 770 K.The h e a t i n g r a t e s v a r y from 1.67 x 1 0 ~3 K s_ 1 t o 0.498 K s_ 1.

I n the c e n t r e o f the oven t h e r e i s a c y l i n d r i c a l room accommodating the m e a s u r i n g c e l l . T h i s c e l l c o n s i s t s o f a p l a s t i c f o i l o n t o w h i c h the t h e r m o c o u p l e s a r e s u b l i m a t e d .

T h e r e a r e two s e t s o f t h e r m o c o u p l e s , o n e f o r m e a s u r i n g the t e m p e r a t u r e o f the sample h o l d e r and one f o r m e a s u r i n g t h e t e m p e r a t u r e o f the r e f e r e n c e h o l d e r . T h e y are c o n n e c t e d i n o p p o s i t e p h a s e . I n t h i s way the r e s u l t i n g e l e c t r i c o u t p u t i n d i c a t e s t h e t e m p e r a t u r e d i f f e r e n c e between sample and r e f e r e n c e h o l d e r .

The oven space can be c o n t i n u o u s l y p u r g e d w i t h gas so as t o a l t e r the e x p e r i m e n t a l c o n d i t i o n s .

T h e r e a r e two t y p e s o f v e s s e l s i n w h i c h the t e s t s can be c a r r i e d o u t , v i z . • . . -9 3

an a l u m i n i u m v e s s e l w i t h a volume o f 40 x 10 m and a maximum s t a t i c o v e r p r e s s u r e of about 0.1 MPa.and a n i c k e l / c h r o m i u m p r e s s u r e v e s s e l w i t h a

—6 3

volume of 0.5 x 10 m and a maximum s t a t i c o v e r p r e s s u r e o f about 10 MPa. The e x p e r i m e n t s may be c o n d u c t e d i n open as w e l l as i n c l o s e d v e s s e l s . T h e r e a r e two t y p e s o f e x p e r i m e n t s . n a m e l y n o n - i s o t h e r m a l (dynamic) and i s o t h e r m a l ( s t a t i c ) e x p e r i m e n t s .

In dynamic e x p e r i m e n t s the oven i s h e a t e d l i n e a r l y i n t i m e . T h i s r e s u l t s i n a t e m p e r a t u r e d i f f e r e n c e between t h e r e f e r e n c e and sample h o l d e r . b e c a u s e o f the h e a t c a p a c i t y o f the sample mass.The s i g n a l does n o t change anymore i f the t e m p e r a t u r e d i f f e r e n c e i s c o n s t a n t ; t h e r e s u l t i n g s t r a i g h t l i n e i s c a l l e d the b a s e l i n e . T h e heat c a p a c i t y changes w i t h any c h e m i c a l o r p h y s i c a l c h a n g e , c a u s i n g the b a s e l i n e t o s h i f t . I f t h e r e i s a l s o a h e a t e f f e c t t h e s i g n a l change w i l l r e s u l t i n a peak.

In s t a t i c e x p e r i m e n t s the oven i s h e a t e d f a s t t o the d e s i r e d t e m p e r a t u r e . The e x p e r i m e n t i s s t a r t e d when the sample h o l d e r i s put i n t o the oven.When the a p p a r a t u s has come t o t h e r m a l e q u i l i b r i u m t h e s i g n a l i s r e c o r d e d as a

Figure 1. Diagram of the DTA apparatus.

2. aover measuring celt

2. silver oven

3. DTA measuring cell

4. sublimated thermocouples

5. sample vessel

6. reference vessel

7. electric heating

8. platinum temperature sensor

f u n c t i o n o f t i m e . B e f o r e an i s o t h e r m a l e x p e r i m e n t i s p e r f o r m e d one o r more e x p e r i m e n t s have t o be done w i t h a sample v e s s e l w h i c h i s empty o r f i l l e d w i t h an i n e r t s u b s t a n c e . T h i s r e s u l t s i n a s t r a i g h t l i n e , t h e z e r o l i n e , w h i c h i s t h e c o l l e c t i o n o f s i g n a l s r e p r e s e n t i n g e q u a l t e m p e r a t u r e s o f sample and r e f e r e n c e h o l d e r . I n an i s o t h e r m a l e x p e r i m e n t t h e t e m p e r a t u r e d i f f e r e n c e between t h e sample and t h e r e f e r e n c e h o l d e r i s o b t a i n e d by s u b t r a c t i n g t h e z e r o l i n e from the measured l i n e .

W i t h some a p p r o x i m a t i o n s (see c h a p t e r I I I ) t h e DTA s i g n a l may d i r e c t l y be t r a n s l a t e d i n t o a h e a t g e n e r a t i o n X h i s r e l a t i o n i s o b t a i n e d by c a l i b r a t i o n w i t h s t a n d a r d s u b s t a n c e s whose h e a t o f t r a n s i t i o n i s p r e c i s e l y known. Depending on t h e s e n s i t i v i t y o f the c e l l t h i s c a l i b r a t i o n c o n s t a n t i s about 100 pWuV ' f o r t h e h i g h p r e s s u r e v e s s e l s and about 70 uWyV ' f o r t h e a l u m i n i u m v e s s e l s . A n a l t e r n a t i v e way o f c a l i b r a t i o n i s t h e use o f an e l e c t r i c power s o u r c e t o h e a t a c o i l i n s e r t e d i n t h e v e s s e l . T h i s method o f f e r s a b e t t e r o p p o r t u n i t y o f e s t a b l i s h i n g t h e c a l i b r a t i o n c o n s t a n t as a f u n c t i o n o f a l l s o r t s o f p a r a m e t e r s such as t e m p e r a t u r e , m a s s . h e a t i n g r a t e and so on.Attempts t o do t h i s a r e b e i n g made'^''^.

Owing t o the h e a t i n g r a t e t h e r e w i l l be a t e m p e r a t u r e l a g between t h e sample v e s s e l and the oven and a l s o between the r e f e r e n c e v e s s e l and the o v e n . T h i s l a g , t h e so c a l l e d time c o n s t a n t , i s dependent on the r a t e o f the h e a t exchange mechanism:for h i g h p r e s s u r e v e s s e l s the time c o n s t a n t (t ) i s about 108 s and f o r a l u m i n i u m v e s s e l s about 25 s.

Because o f the t r a n s i t i o n i n the sample mass and the r e s u l t i n g h e a t e f f e c t s the sample t e m p e r a t u r e w i l l be d i f f e r e n t from t h e oven t e m p e r a t u r e . T h i s i s c a l l e d the s t e e p n e s s (S) o f the DTA c e l l and amounts t o about 100 yVK I t i s c l e a r t h a t n o n - i s o t h e r m a l e x p e r i m e n t s have to be c o r r e c t e d f o r b o t h time c o n s t a n t and s t e e p n e s s , a n d i s o t h e r m a l e x p e r i m e n t s o n l y f o r s t e e p n e s s . C o r r e c t i o n s o f t h i s t y p e a r e made by r e c o r d i n g the r i g h t t e m p e r a t u r e (see

c h a p t e r I I I , s e c t i o n 2 . C . c . e q u a t i o n 64) o r by e x t r a p o l a t i n g the h e a t g e n e r a t i o n and the time t o the oven t e m p e r a t u r e (see c h a p t e r I I I , s e c t i o n 4 B , e q u a t i o n 71).

A s i g n a l o f 5 mm may be r e g a r d e d as d e t e c t a b l e . M e a s u r i n g w i t h the h i g h e s t s e n s i t i v i t y then r e s u l t s i n a s i g n a l of 0.5 y V . T h i s c o r r e s p o n d s w i t h about 50 yW i f the h i g h p r e s s u r e v e s s e l s a r e used and w i t h about 35 yW i n c a s e of the aluminium v e s s e l s . H e n c e t h e s e v a l u e s may be r e g a r d e d as the d e t e c t i o n l i m i t s o f the a p p a r a t u s .

3 . T h e r m o g r a v i m e t r y (TG)

Figure 2. Diagram of the TG apparatus.

1. beam of the balance 4. reflector

2. DTA measuring cell S. gas inlet

3. oven 6. gas outlet

TG i s a t e c h n i q u e i n which the mass of a s u b s t a n c e i s r e c o r d e d as a f u n c t i o n of t e m p e r a t u r e o r t i m e ' ,The system used i n the e x p e r i m e n t s i n s u p p o r t of t h i s t h e s i s i s a M e t t l e r TA-2 Thermal A n a l y z e r .

A s c h e m a t i c r e p r e s e n t a t i o n o f the a p p a r a t u s i s shown i n F i g u r e 2.With t h i s equipment a DTA s i g n a l may be r e c o r d e d b e s i d e s the TG s i g n a l .

A r e f e r e n c e v e s s e l and a sample v e s s e l a r e p l a c e d on the beam.The v e s s e l s are i n c o n t a c t w i t h the DTA c e l l , s i t u a t e d on the beam,to measure the DTA s i g n a l . D u r i n g the e x p e r i m e n t s the DTA c e l l i s s u r r o u n d e d by an oven,made of s i n t e r e d aluminium o x i d e . T h i s oven i s h e a t e d l i n e a r l y i n time w i t h h e a t i n g

-3 -1 -1

r a t e s from 3.33 x 10 Ks to 1.67 Ks ,or can be k e p t at a c o n s t a n t t e m p e r a t u r e . l t c o v e r s a t e m p e r a t u r e range from 300 K t o 1300 K.Temperature

changes from the s u r r o u n d i n g s a r e r e d u c e d by a m e t a l r e f l e c t o r e n c l o s i n g t h e oven.

The b a l a n c e c a s e w i t h the beam and w e i g h t s i s s e p a r a t e d from the oven by a w a t e r - c o o l e d s h i e l d and a few r a d i a t i o n s h i e l d s . P e n e t r a t i o n o f gaseous p r o d u c t s from the r e a c t i o n i n t h e sample mass i s p r e v e n t e d by a s l i g h t o v e r p r e s s u r e o f i n e r t g a s e s , f o r i n s t a n c e d r i e d a i r o r n i t r o g e n t o w h i c h the sample i s a l s o e x p o s e d . I n t h i s way i t i s p o s s i b l e t o work under a c o n t r o l l e d atmosphere and i n h i g h vacuum.

The v e s s e l s a r e made o f p l a t i n u m o r aluminium.They a r e as l a r g e as t h e aluminium v e s s e l s f o r t h e TA-2000 system.Of c o u r s e i f a l u m i n i u m v e s s e l s a r e used the t e m p e r a t u r e must n o t exceed 823 K.

D u r i n g an e x p e r i m e n t the mass o f the sample (TG s i g n a l ) , t h e t e m p e r a t u r e d i f f e r e n c e between sample and r e f e r e n c e h o l d e r (DTA s i g n a l ) and t h e sample t e m p e r a t u r e a r e r e c o r d e d . H e r e t o o , n o n - i s o t h e r m a l as w e l l as i s o t h e r m a l e x p e r i m e n t s a r e p o s s i b l e .

With n o n - i s o t h e r m a l e x p e r i m e n t s t h e l o s s o f mass w i l l r e s u l t i n an S-shaped c u r v e . F o r the TG measurements i t i s n o t n e c e s s a r y to know e x a c t l y t h e i n i t i a l mass because the p r o p o r t i o n a l l o s s o f mass can v e r y e a s i l y be o b t a i n e d by d i v i d i n g t h e d i f f e r e n c e between t h e s i g n a l and i n i t i a l s i g n a l by t h e d i f f e r e n c e between the f i n a l and i n i t i a l s i g n a l .

With i s o t h e r m a l e x p e r i m e n t s t h e sample v e s s e l has to be p u t on t h e beam b e f o r e the e x p e r i m e n t s t a r t s . T h e n t h e oven i s h e a t e d as f a s t as p o s s i b l e

to t h e d e s i r e d t e m p e r a t u r e ( f o r the a l u m i n i u m o x i d e t h e h e a t i n g r a t e s h o u l d not exceed 0.25 Ks ') a f t e r which t h e e x p e r i m e n t i s c o n t i n u e d i s o t h e r m a l l y . I f o n l y one r e a c t i o n p r o c e e d s t h e p r o p o r t i o n a l mass l o s s w i l l be i d e n t i c a l to the d e g r e e o f c o n v e r s i o n . I f a s e r i e s o f c o n s e c u t i v e r e a c t i o n s o c c u r , t h e s t a r t i n g and end p o i n t s o f t h e i n d i v i d u a l r e a c t i o n s have t o be c o n s i d e r e d . Assuming t h a t a s i g n a l o f 5 mm i s d e t e c t a b l e and r e g a r d e d as s i g n i f i c a n t the d e t e c t i o n l i m i t o f the TG a p p a r a t u s i s about 0.02 mg.

For the DTA s i g n a l t h e same c o n s i d e r a t i o n s as s t a t e d i n t h e p r e v i o u s

p a r a g r a p h s a r e v a l i d . O n l y the time c o n s t a n t i s o f no i m p o r t a n c e h e r e , b e c a u s e the t e m p e r a t u r e o f t h e sample v e s s e l i s measured d i r e c t l y . T h e c a l i b r a t i o n c o n s t a n t f o r the c e l l i s about 20% o f t h a t o f the c e l l i n the TA-2000 system, so t h a t DTA measurements w i l l be much l e s s s e n s i t i v e . B e s i d e s . i t i s v e r y d i f f i c u l t to p u t the c e l l i n the c e n t r e o f the oven,so t h a t the c o n d i t i o n s of symmetry mentioned i n s e c t i o n 2A o f c h a p t e r I I I a r e o f l i t t l e v a l i d i t y f o r t h i s a p p a r a t u s . T h i s r e s u l t s i n poor r e p r o d u c i b i l i t y o f t h e measurements, r e n d e r i n g them l e s s s u i t a b l e f o r q u a n t i t a t i v e p u r p o s e s .

4 . I s o t h e r m a l S t o r a g e T e s t (1ST)

Figure 3. Diagram of the isothermal heat generation meter. 1. sample 10. heating wires

2. sample vessel 11. glass wool

3. cylindrical holder 12. platinum resistance sensor

4. air spaces temperature control

5. Peltier elements 13. platinum resistance sensor

5. electrical circuit for safety control

7. aluminium block 14. platinum resistance

S. inert material thermometer

fl.

The h e a t g e n e r a t i o n measurements a t d i f f e r e n t t e m p e r a t u r e s and o v e r d i f f e r e n t p e r i o d e s a r e c a r r i e d out i n the i s o t h e r m a l h e a t g e n e r a t i o n meter (see F i g u r e 3 ) . T h i s i n s t r u m e n t e n a b l e s the measurement of the h e a t g e n e r a t i o n

- . 11,12,13 i n a sample as a f u n c t i o n of time a t a c o n s t a n t t e m p e r a t u r e

Measurements c a r r i e d out i n the h e a t g e n e r a t i o n meter a r e c a l l e d i s o t h e r m a l s t o r a g e t e s t s . T h e sample v e s s e l i s s i t u a t e d - i n a c y l i n d r i c a l h o l d e r which i s p l a c e d on a P e l t i e r element.The major p a r t of the h e a t g e n e r a t e d by the sample p a s s e s t h r o u g h the P e l t i e r element.The e l e c t r i c o u t p u t of t h i s element i s a m p l i f i e d and r e c o r d e d .

The t h e r m o s t a t i c m e a s u r i n g chamber i s e n c l o s e d i n an a l u m i n i u m b l o c k . T h i s b l o c k i s i n s u l a t e d by a l a y e r o f Kaowool ( c e r a m i c f i b r e ) , w h i c h i s a g a i n e n c l o s e d i n a h e a t i n g j a c k e t . T h e t e m p e r a t u r e of t h i s j a c k e t i s k e p t c o n s t a n t by an e l e c t r i c h e a t i n g system.The r a p i d temperature f l u c t u a t i o n s of the

h e a t i n g j a c k e t c a u s e d by the t e m p e r a t u r e c o n t r o l a r e e f f e c t i v e l y a t t e n u a t e d i n the m e a s u r i n g chambers by the h i g h h e a t c a p a c i t y o f the a l u m i n i u m b l o c k and the low h e a t c o n d u c t i v i t y o f the Kaowool l a y e r .

I n o r d e r t o r e d u c e the i n f l u e n c e o f the slow t e m p e r a t u r e v a r i a t i o n s o f the s u r r o u n d i n g s on the m e a s u r i n g system the h e a t g e n e r a t i o n m e t e r i s i n s u l a t e d w i t h g l a s s w o o l . H o w e v e r , i t s h o u l d be n o t e d t h a t the slow v a r i a t i o n s i n

ambient t e m p e r a t u r e a r e n o t e f f e c t i v e l y a t t e n u a t e d i n the m e a s u r i n g chambers, b e c a u s e t h e y a r e too slow w i t h r e s p e c t t o t h e time c o n s t a n t of the m e t e r . They r e s u l t i n h e a t f l u x e s t h r o u g h the P e l t i e r element o f the m e a s u r i n g u n i t i n w h i c h the sample i s l o c a t e d .

To compensate f o r t h i s u n d e s i r e d e f f e c t two i d e n t i c a l m e a s u r i n g u n i t s o f e q u a l s e n s i t i v i t y have been i n c l u d e d s y m m e t r i c a l l y i n the h e a t g e n e r a t i o n m e t e r . E l e c t r i c a l l y they a r e c o n n e c t e d i n o p p o s i t e p h a s e s . T o a c h i e v e e q u a l

s e n s i t i v i t y a v a r i a b l e e l e c t r i c r e s i s t a n c e i s i n s t a l l e d a c r o s s the most s e n s i t i v e P e l t i e r element.One m e a s u r i n g chamber c o n t a i n s the sample,the o t h e r an i n e r t m a t e r i a l w i t h a c o n d u c t i v i t y and t o t a l h e a t c a p a c i t y as c l o s e as p o s s i b l e t o the sample s u b s t a n c e . U s u a l l y g l a s s p e a r l s a r e used f o r t h i s p u r p o s e . I n t h i s way the h e a t f l o w f r o m the sample s u b s t a n c e i s measured w i t h r e g a r d to the h e a t f l o w from t h e r e f e r e n c e s u b s t a n c e . H o w e v e r , i n s p i t e of t h e s e measures the v a r i a t i o n s i n the ambient t e m p e r a t u r e a r e n o t c o m p l e t e l y compensated.There s t i l l remains a f l u c t u a t i o n i n the s i g n a l of 0.5 uV c o r r e s p o n d i n g w i t h a h e a t g e n e r a t i o n o f about 12 uW.Hence,this v a l u e may be r e g a r d e d as the d e t e c t i o n l i m i t o f the a p p a r a t u s .

The v e s s e l s used i n the e x p e r i m e n t s a r e made o f s t a i n l e s s s t e e l . T h e y have a volume o f about 75 m l . D u r i n g the e x p e r i m e n t s the sample as w e l l as t h e r e f e r e n c e v e s s e l a r e c l o s e d g a s t i g h t .

The h e a t g e n e r a t i o n m e t e r i s c a l i b r a t e d e l e c t r i c a l l y by p u t t i n g a r e f e r e n c e v e s s e l w i t h an e l e c t r i c h e a t i n g c o i l on t h e sample s i d e . B y means o f a s t a b i l i z e d power s u p p l y a c o n s t a n t h e a t f l u x i s p r o d u c e d and the r a t i o between the heat g e n e r a t i o n and the e l e c t r i c o u t p u t i s c a l c u l a t e d . T h i s r a t i o (the c a l i b r a t i o n c o n s t a n t ) v a r i e s between 10 and 20 uWyV-' f o r the

d i f f e r e n t m e t e r s .

B e f o r e p e r f o r m i n g the experiment the z e r o l i n e has t o be r e c o r d e d by

p u t t i n g a r e f e r e n c e v e s s e l on the sample s i d e . T h e z e r o l i n e may be r e g a r d e d as a measurement w i t h o u t any h e a t g e n e r a t i o n . T h e heat g e n e r a t i o n t h e n i s o b t a i n e d by s u b t r a c t i n g the z e r o l i n e from the m e a s u r i n g l i n e and

t r a n s l a t i n g the e l e c t r i c o u t p u t w i t h the h e l p of the c a l i b r a t i o n c o n s t a n t and the sample mass.

The e x p e r i m e n t s t a r t s a t the moment o f p l a c i n g t h e sample v e s s e l i n t o t h e meter ( t h e r e f e r e n c e v e s s e l always s t a y s i n t h e meter).When the meter has a t t a i n e d t h e r m a l e q u i l i b r i u m t h e f i r s t r e l i a b l e m e a s u r i n g p o i n t s may be o b t a i n e d . T h e sample v e s s e l i s c l o s e d g a s t i g h t by means o f a s t a i n l e s s s t e e l membrane t o p r e v e n t v o l a t i l e p r o d u c t s t o e s c a p e . D u r i n g t h e measurements t h e t e m p e r a t u r e o f t h e a l u m i n i u m b l o c k i s measured w i t h a p l a t i n u m r e s i s t a n c e t h e r m o m e t e r . T h i s t e m p e r a t u r e may v a r y ±Q,5 K. As m e a s u r i n g t e m p e r a t u r e the mean v a l u e i s t a k e n .

5. A d i a b a t i c S t o r a g e T e s t (AST)

The measurement o f the t e m p e r a t u r e as a f u n c t i o n of time may be c a r r i e d out i n t h e a d i a b a t i c h e a t g e n e r a t i o n meter ( s e e F i g u r e 4).These measurements

. . 13,14 are c a l l e d a d i a b a t i c s t o r a g e t e s t s

The sample,which may be a s o l i d or a l i q u i d , i s i n t r o d u c e d i n t o a Dewar -3 3

v e s s e l w i t h a c a p a c i t y o f 1.5 x 10 m .To o b t a i n a r e l i a b l e measurement t h i s v e s s e l has t o be f i l l e d f o r a t l e a s t 65%.The Dewar v e s s e l i s t h e n c l o s e d by means o f a s t a i n l e s s s t e e l c o v e r w h i c h i s p r o v i d e d w i t h i n s u l a t i n g m a t e r i a l a t the i n s i d e . I n s i d e the c o v e r t h e r e i s a l s o a PTFE

-3

tube ( l e n g t h 2 m , i n n e r d i a m e t e r 1.35 x 10 m) which forms the o n l y open c o n n e c t i o n between the c o n t e n t s o f the Dewar v e s s e l and the s u r r o u n d i n g s . T h i s tube s e r v e s t o p r e v e n t p r e s s u r e b u i l d - u p i n s i d e the Dewar v e s s e l

d u r i n g the experiment.whereas i t i n h i b i t s e v a p o r a t i o n o f v o l a t i l e components B e s i d e s , i t i s p o s s i b l e t o purge the sample w i t h a i r o r o t h e r gases d u r i n g the e x p e r i m e n t .

The Dewar v e s s e l i s p l a c e d i n an oven i n which the t e m p e r a t u r e i s k e p t e q u a l t o the t e m p e r a t u r e of the sample w i t h t h e a i d o f t h e r m o c o u p l e s i n the sample and the oven.The v o l t a g e d i f f e r e n c e i s u s e d t o c o n t r o l the oven t e m p e r a t u r e .

The a p p a r a t u s i s c a l i b r a t e d by means of an e l e c t r i c h e a t i n g c o i l i n the sample v e s s e l , w h i c h i s f i l l e d w i t h sodium c h l o r i d e f o r t h i s p u r p o s e . T h i s c a l i b r a t i o n i s a d e t e r m i n a t i o n o f t h e h e a t l o s s e s and the h e a t c a p a c i t y of the empty v e s s e l , b y means of a s t a b i l i z e d power s u p p l y . T h e sodium c h l o r i d e o n l y s e r v e s as h e a t c o n d u c t i n g m a t e r i a l w i t h h e a t exchange p r o p e r t i e s which a r e thought t o be r e p r e s e n t a t i v e o f a sample.The h e a t l o s s e s amount t o 0.02 W and the h e a t c a p a c i t y o f the empty v e s s e l i s 500 JK Thermocouples i n the sample made o f c o p p e r - c o n s t a n t a n a r e used t o measure the t e m p e r a t u r e - t i m e c o u r s e . B a s e d on e x p e r i e n c e a s i g n a l r i s e of 10 mm ( e q u i v a l e n t t o 1 K) o v e r 24 h may s t i l l be d e t e c t e d . I f the sample mass i s 1 kg and the h e a t c a p a c i t y i s 3 k J k g 'K ' i t f o l l o w s t h a t the d e t e c t i o n l i m i t i s 0.02 W.

B e f o r e the s t a r t o f an e x p e r i m e n t the s u b s t a n c e i n the Dewar v e s s e l i s s l o w l y p r e h e a t e d by means o f an e l e c t r i c h e a t i n g s p i r a l e n c l o s e d i n a s t a i n l e s s s t e e l tube type 321,which i s submerged i n t h e s u b s t a n c e . T h i s p e r i o d i s used to d e t e r m i n e the h e a t c a p a c i t y o f the substance.When the d e t e c t i o n l i m i t o f s e l f - h e a t i n g o f the s u b s t a n c e i s r e a c h e d the e x t e r n a l h e a t i n g i s s t o p p e d and the e x p e r i m e n t s t a r t s .

Figure 4. Diagram of the adiabatio heat 1. oover of the meter

2. space inside the meter 3. oover of the oven 4. space inside the oven 5. Dewar vessel

generation meter. 6. PTFE tube

7. V-shaped stainless steel cooling tube

8. electric heating spiral 9. sample

I f t h e h e a t g e n e r a t i o n r e a c h e s 10 W t h e e x p e r i m e n t i s s t o p p e d and t h e sample i s c o o l e d by means o f an a i r f l o w t h r o u g h a U-shaped s t a i n l e s s s t e e l tube type 3 2 1 . I f t h e c o o l i n g i s c a r r i e d out by means o f w a t e r t h e h e a t g e n e r a t i o n may r e a c h 400 W.These v a l u e s may n o t be e x c e e d e d , b e c a u s e t h e n t h e c o o l i n g would be i n s u f f i c i e n t and the sample would c o n t i n u o u s l y h e a t i t s e l f c a u s i n g

damage t o t h e a p p a r a t u s .

A f t e r c o m p l e t i o n o f the c o o l i n g i t i s p o s s i b l e t o s t a r t t h e e x p e r i m e n t a g a i n w i t h t h e same s a m p l e . R e p e a t i n g t h i s f i n a l l y completes the r e a c t i o n .

C h a p t e r I I I EVALUATION OF THE METHODS USED

1. I n t r o d u c t i o n

The methods used f o r t h e i n v e s t i g a t i o n s a r e e v a l u a t e d . E s p e c i a l l y i t i s c l a r i f i e d how t h e p r a c t i c a l r e s u l t s have t o be worked o u t f u r t h e r , a n d how the e x p e r i m e n t a l e r r o r s i n f l u e n c e the a c c u r a c y o f t h e f i n a l r e s u l t s . F o r i s o t h e r m a l DTA and the 1ST t h e v a l u e o f the e x p e r i m e n t a l e r r o r s i s b a s e d on measurements w i t h i n e r t s u b s t a n c e s and p a r t l y on e m p i r i c a l f a c t s . F o r n o n - i s o t h e r m a l DTA s e v e r a l methods o f a q u a n t i t a t i v e e v a l u a t i o n o f t h e c u r v e s were compared.The b a s i s f o r c o m p a r i s o n were t h e e r r o r s t o be e x p e c t e d , the a p p l i c a b i l i t y , t h e amount o f time i n v o l v e d , t h e dependence o f t h e r e s u l t s on the h e a t i n g r a t e and the agreement w i t h i s o t h e r m a l r e s u l t s . I n t h i s case a t e s t s u b s t a n c e was used.

F o r TG t h e v a l u e of t h e e x p e r i m e n t a l e r r o r was o n l y o b t a i n e d by t h e o r e t i c a l c o n s i d e r a t i o n s . w h e r e a s f o r the AST t h i s v a l u e i s c o m p l e t e l y b a s e d on e x p e r i m e n t a l e x p e r i e n c e .

2. D i f f e r e n t i a l T h e r m a l A n a l y s i s

A. S i g n a l

As s t a t e d e a r l i e r the DTA s i g n a l i s a r e s u l t o f a t e m p e r a t u r e d i f f e r e n c e between a r e f e r e n c e and a sample h o l d e r . T h e o r e t i c a l l y t h e f o l l o w i n g e q u a t i o n s a r e v a l i d : R e f e r e n c e h o l d e r : M.cr.-^ = h ! . ( T - T ) + h ^ . ( T 4 - T4) + h , . ( T - T ) p d t 1 ov r 2 ov

x '

3 -

t h

"

V

The l e f t member o f t h e s e e q u a t i o n s r e p r e s e n t s t h e h e a t i n g by the oven and i n case o f the sample h o l d e r the h e a t g e n e r a t i o n i n the sample as a r e s u l t o f d e g r a d a t i o n . T h e r i g h t members o f t h e s e e q u a t i o n s r e p r e s e n t t h e r e s p e c t i v e h e a t exchange t e r m s , b e i n g the c o n v e c t i o n between t h e oven and the v e s s e l , t h e r a d i a t i o n between t h e oven and t h e v e s s e l and the c o n d u c t i o n between t h e t h e r m o c o u p l e s and t h e v e s s e l . T h e h's d e n o t e the r e s p e c t i v e h e a t exchange c o e f f i c i e n t s , T t h e t e m p e r a t u r e , A T t h e t e m p e r a t u r e d i f f e r e n c e between sample and r e f e r e n c e h o l d e r and c t h e h e a t c a p a c i t y o f the h o l d e r and t h e s u b s t a n c e

P

i n i t . T h e s u p e r s c r i p t s r and s d e n o t e the r e f e r e n c e and sample h o l d e r

r e s p e c t i v e l y ; d Qt/ d t i s t h e h e a t g e n e r a t i o n i n the sample s u b s t a n c e a t time t .

By c o m b i n a t i o n o f t h e s e two e q u a t i o n s i t f o l l o w s t h a t : cS- cr dT cS d(AT) M d Qt Ah(T) AT = M . {h (T ) • j£ + j ^ y • d t > + 1000.h(T) ' " d T " ~Hff (2) w i t h : Ah(T) = (h»- ^ ) ( TO V- TR ) + ( h * - h^) ( TQ4- t / ) - <h|- h*) ( T ^ T ^ ) and : h ( T ) = h^ + ^h^T^.3 - h® E q u a t i o n (2) can be s i m p l i f i e d i f the f o l l o w i n g c o n d i t i o n s a r e v a l i d : l.The sample and r e f e r e n c e h o l d e r a r e i d e n t i c a l and t h e i r p l a c e s i n the

oven a r e e x a c t l y s y m m e t r i c a l . W i t h t h i s c o n d i t i o n the h e a t exchange between sample and r e f e r e n c e h o l d e r i s e q u a l , s o h ( T ) = 0 .

2.The h e a t can o n l y be exchanged i n t h e t h r e e ways m e n t i o n e d e a r l i e r . 3 . I n s i d e t h e c e l l t h e t e m p e r a t u r e i s u n i f o r m . H e n c e t h e h e a t exchange

i n s i d e the c e l l may be n e g l e c t e d . T h i s c o n d i t i o n w i l l be a p p r o x i m a t e d b e t t e r i f the sample and t h e h e a t i n g r a t e a r e s m a l l e r .

cS. M d(AT)

4. The h e a t i n g term r - ~ - .— - i s small,compared w i t h the heat g e n e r a t i o n

"S

t e m d t - 1000.h(T)

5. D u r i n g the p r o c e s s t h e r e i s a s m a l l change i n c - v a l u e ; c r e m a i n s about r P P

e q u a l t o c .Hence t h e r e f e r e n c e and sample v e s s e l have about t h e same h e a t c a p a c i t y . 6. T h e r e i s o n l y one r e a c t i o n a t t h e t i m e , f o r w h i c h t h e f o l l o w i n g e q u a t i o n i s v a l i d : W i t h t h e s e c o n d i t i o n s e q u a t i o n (2) may be s i m p l i f i e d t o : M M d Qt AT = irinn W_{1000.h(T) ' 1000.h(T) ' d t} t " i • 9 _ Q .M dx AT = . — û h ( T ) d t (4)

In o t h e r words the t e m p e r a t u r e d i f f e r e n c e between t h e sample and r e f e r e n c e h o l d e r i s d i r e c t l y p r o p o r t i o n a l t o t h e h e a t g e n e r a t i o n and t h e r a t e o f r e a c t i o n . I n t h e e v a l u a t i o n o f t h i s method e q u a t i o n (4) w i l l be assumed t o be v a l i d . W i t h DTA n o n - i s o t h e r m a l as w e l l as i s o t h e r m a l measurements a r e p o s s i b l e . I n i s o t h e r m a l measurements e q u a t i o n . ( 4 ) i s more r e a l b e c a u s e c o n d i t i o n 4 i s s t r i c t l y v a l i d = 0 and t h e time i s t h e o n l y v a r i a b l e p a r a m e t e r ) . I n t h i s r e s p e c t the i s o t h e r m a l method would l e a d t o more a c c u r a t e r e s u l t s . A d i s a d v a n t a g e o f the i s o t h e r m a l method i s t h e need t o do more e x p e r i m e n t s a t d i f f e r e n t t e m p e r a t u r e s ( a t l e a s t t h r e e ) . A n o t h e r drawback i s t h e f a c t t h a t t h e r e i s a c e r t a i n t i m e f o r the a p p a r a t u s and t h e sample t o r e a c h t h e r m a l e q u i l i b r i u m . H e n c e the p r o c e s s cannot be r e c o r d e d from t h e s t a r t . E s p e c i a l l y w i t h r a p i d p r o c e s s e s t h i s may be a s e r i o u s drawback.

In n o n - i s o t h e r m a l e x p e r i m e n t s t h e r e a r e two v a r i a b l e s . n a m e l y time and t e m p e r a t u r e . I n t h a t c a s e f o r the degree o f c o n v e r s i o n (x) the f o l l o w i n g

19 20 21

e q u a t i o n ' ' i s v a l i d :

dx ,3x. ,3x. dT rc

_s

2

(Et/RT ) c o u l d be s m a l l . N e v e r t h e l e s s , t h e i n f l u e n c e o f t h e h e a t i n g r a t e can 22

be q u i t e l a r g e . T h i s appears from e x p e r i m e n t s w i t h t h e same s u b s t a n c e . F o r i n s t a n c e the d e c o m p o s i t i o n o f c a l c i u m c a r b o n a t e has been w i d e l y

i n v e s t i g a t e d w i t h v e r y d i v e r g e n t r e s u l t s . w h i c h was a t t r i b u t e d t o t h e 23

p r o c e s s i t s e l f and the w o r k i n g c o n d i t i o n s .Yet e q u a t i o n (10) i s u s u a l l y s i m p l i f i e d t o :

ft = ko -e xP(" ^ - f ( x )

dx kg E

or: dT = T 'eXp(" R T}-f ( x )

( i oa)

The n e x t s e c t i o n s d e s c r i b e the d e r i v a t i o n o f the h e a t g e n e r a t i o n and the k i n e t i c p a r a m e t e r s from t h e measurements.For i s o t h e r m a l DTA an e s t i m a t i o n of the t h e o r e t i c a l l y based e r r o r i s i n c l u d e d . F o r n o n - i s o t h e r m a l DTA a few

well-known e v a l u a t i o n methods w i l l be t r e a t e d and m u t u a l l y compared. E x p e r i m e n t a l l y t h e r e s u l t s w i l l a l s o be compared w i t h t h o s e o f i s o t h e r m a l measurements. B. I s o t h e r m a l DTA a -

§ £

.

§

i 2

M = + _ £ ! L _ + AK AM ,

q (U-N) (U-N) K M v '

I f t h e e r r o r i n the weighed mass i s n e g l e c t e d t h e n i t appears t h a t t h e e r r o r c o n s i s t s o f t h r e e p a r t s : t h e e r r o r i n the m e a s u r i n g s i g n a l , t h e e r r o r i n the z e r o l i n e and t h e e r r o r i n the c a l i b r a t i o n c o n s t a n t .

T h i s e r r o r c o n s i s t s o f two t e r m s , v i z . t h e e r r o r i n t h e r e c o r d e r and the r e a d i n g e r r o r . I n f o r m u l a :

_ |m+l1 • 0.5 _G_

AU too • G 200 (13 )

where m i s the p o s i t i o n o f t h e s u p p r e s s o r and G i s t h e s e n s i t i v i t y ( y V ) . I t f o l l o w s t h a t :

A U = ( f ^ ) . G (14)

e r r o r (AU) v a r i e s between 0.2 uV (n=0 , G=20 uV) and 40 uV (n=6 , G=1000 uV).which i s 1% a t t h e most ( w i t h n=0 and r e c o r d e r o u t p u t m a x i m a l ) .

a2. The e r r o r i n t h e z e r o l i n e

T h i s e r r o r c o n s i s t s o f the same terms as t h e m e a s u r i n g s i g n a l . A s the e l e c t r i c o u t p u t o f t h e z e r o l i n e n e v e r exceeds 20 uV the e r r o r i n t h i s r e s p e c t w i l l always be 0.2 uV.due t o the r e c o r d e r and r e a d i n g e r r o r s . T h e r e p r o d u c i b i l i t y of the z e r o l i n e was checked by m e a s u r i n g i t t e n t i m e s w i t h open s t a i n l e s s s t e e l v e s s e l s f i l l e d w i t h g l a s s p e a r l s . B e t w e e n the s u c c e s s i v e e x p e r i m e n t s the v e s s e l s were removed, c o o l e d and p l a c e d i n t o the oven a g a i n . T h e s e e x p e r i m e n t s were made a t two d i f f e r e n t t e m p e r a t u r e s (373 K and 473 K) w i t h t h r e e d i f f e r e n t masses ( v e s s e l empty,170 mg and 700 mg o f g l a s s p e a r l s ) . I f the e l e c t r i c o u t p u t i s f i x e d a t 0 uV w i t h t h e r m a l e q u i l i b r i u m , t h e c o u r s e o f the z e r o l i n e w i l l be as drawn i n F i g u r e 5.Changing mass and t e m p e r a t u r e d i d not i n t r o d u c e l a r g e d i f f e r e n c e s . I n t h e most u n f a v o u r a b l e c a s e the time f o r the system t o a t t a i n t h e r m a l

e q u i l i b r i u m was 15 m i n u t e s . w h i l e i n t h i s c a s e the e r r o r b a s e d on two times the s t a n d a r d d e v i a t i o n was n o t g r e a t e r t h a n 4 uV w i t h r e s p e c t t o the mean v a l u e .

A l t h o u g h such e x p e r i m e n t s have n o t been p e r f o r m e d w i t h the s m a l l e r a l u m i n i u m v e s s e l s the t i m e t o a t t a i n t h e r m a l e q u i l i b r i u m may be e x p e c t e d t o be s h o r t e r and the s t a n d a r d d e v i a t i o n o f the measurements t o be s m a l l e r i n t h o s e c a s e s .

C a l i b r a t i o n s o f the m e a s u r i n g c e l l a r e p e r f o r m e d w i t h s t a n d a r d s u b s t a n c e s . U s u a l l y more t h a n one s u b s t a n c e i s needed,because the s e n s i t i v i t y o f the m e a s u r i n g c e l l i s a f u n c t i o n of the t e m p e r a t u r e , e s p e c i a l l y a t h i g h t e m p e r a t u r e s . H o w e v e r . i t would be a time consuming p r o c e d u r e t o c a l i b r a t e e v e r y m e a s u r i n g c e l l w i t h a l l s o r t s of

s t a n d a r d s u b s t a n c e s . T h e r e f o r e t h i s was done once by M e t t l e r

I n s t r u m e n t s t o f i x the r e l a t i o n between the c a l i b r a t i o n c o n s t a n t and the temperature.Now o n l y one s t a n d a r d s u b s t a n c e i s s u f f i c i e n t t o d e t e r m i n e the c a l i b r a t i o n c o n s t a n t w h i c h can be e x t r a p o l a t e d to e v e r y d e s i r e d t e m p e r a t u r e w i t h the a f o r e m e n t i o n e d r e l a t i o n . F o l l o w i n g the recommendation o f M e t t l e r I n s t r u m e n t s i n d i u m was used,because i t s m e l t i n g h e a t i s a c c u r a t e l y known (28.5 k J k g ') and o c c u r s a t 429 K. T h i s i s about i n the m i d d l e of the range o f i n t e r e s t i n t h i s t h e s i s