Estimation of Stability Temperatures From Differential Thermal

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ESTIMATION OF STABILITY TEMPERATURES FROM DIFFERENTIAL THERMAL

ANALYSIS AND THERMAL ACTIVITY MONITOR DATA IN COMBINATION

M W W h i tmo re

AgrEvo Ltd , Chester ford Park, Saf f ron Walden, Essex CB10 1XL, UK

Many methods for determin ing stab i l i ty temperatures exist but

mo st have pract ica l d isadvantage s ar is ing f rom sca le , cos t or

t i m e .

Data is presented wh ich sho ws that reasonable est imates

of stab i l i ty temperatures can be obta ined by combin ing

d i f ferent ia l thermal ana lysis and thermal act iv i ty moni tor data .

Th is o f fers a qu ick and inexpensive smal l -sca le me thod w hic h c an

be useful in overal l strategies for determining stabi l i ty

tempera tu res .

Key W or ds : d i f fe rent ia l therm al ana lysis: d i f fe rent ia l sca nning ca lor im etry : therm al

ac t i v i t y mon i to r : se l f -acce le ra t ing decompos i t i on tempera tu re .

INTRODUCTION

A knowledge of stab i l i ty temperatures is necessary for the safe handl ing o f

exo therm ica l ly unstab le ma ter ia ls. W i th respect to t ran sp or t , the cr i t ica l parameter

def ined by the U ni ted Nat ions (UN) is the 7-day se l f -acce lera t ing dec om pos i t ion

tem pera ture (SAD T)(1) . W hi lst SAD T is not un iversa l ly the most usefu l measure,

SAD Ts co mp r ise m uch of the stab i l i ty temp erature data foun d in the l i te ra ture and the

principle of its es t im at io n is gen eral . I t is ta ke n, the ref ore , as the basis of d isc uss ion

UN Recommendat ions(2) descr ibe four methods for SADT: the Uni ted Sta tes (US)

SA DT te st (3 ) ; the iso therm al storage test ( IST)(4) ; the ad iabat ic storag e test (AST)(5)

and the heat acc um ulat ion storage tes t (6) . In ad d i t ion , ad iabat ic Dewar tests are

ava i lable(7 ,8) and a g-scale adiabat ic storag e de vic e, the sel f- ig ni t ion teste r (SIT), has

been descr ibed(9) which appears to g ive reasonable agreement wi th o ther

m e t h o d s ( 1 0 , 1 1 ) .

There is no shor tage o f methods, therefore , but those ment ioned suf fer one or

more o f the fo l lowing d isadvantages: la rge samples requ i red; lengthy and expensive.

Because of these drawbacks they are not idea l for use ear ly in deve lopment when

mater ia l def in i t ion and quant i t ies are l imi ted, or when h igh va lue mater ia ls are be ing

con side red. Con seque nt ly there is in terest in re la t ive ly qu ick and inexpens ive sm al l -

sca le methods for est imat ing SADT.

I CHEM E SYMPOSIUM SERIES No. 134

One approach is to est imate stab i l i ty temperatures f rom the corre la t ion o f smal l -

sca le exo therm onse t tempera tu res w i th la rge -sca le s tab i l i t y tempera tu res .

Di f ferent ia l scan ning ca lor ime try (DSC) and d i f ferent ia l therm al ana lysis (DTA) are

probably the techn iques most wide ly used in th is way(12) but a number o f

com me rc ia l o r i n -house g-sca le tes ts a re a lso em p loy ed (13 ,14 ,15 ) . The use fu lness

of th is approach is l imi ted because, s ince i t re l ies on genera l ised ext rapo la t ion, wide

safe ty marg ins are requ i red. W i th DSC or DT A, for exa m ple, the ' 1 0 0 degree ru le '

is o f ten app l ied and even th is wide marg in can be quest ioned(16) .

The use o f acce lera t ing ra te ca lor imeter (ARC) data to est imate SADT(17,18)

of fers bet ter precis ion because the ext rapo la t ion is speci f ic ra ther than genera l and

is over a narro we r tempe rature range. Even so, it can be argued that the necessary

safe ty marg ins are wide in re la t ion to the precis ion requ i red(19) .

INTERPOLATIVE ESTIMAT ION OF STABILITY TEM PERATURES

Genera l ly , in terpo la t ion is pre ferab le to ext rapo la t ion because the scope for er ror due

to inval id i ty of the model being used is reduced.

In est imat ing stab i l i ty temperatures, the possib i l i ty o f in terpo la t ion is a f forded by

the therm al ac t iv i ty m oni tor (T AM ). Th is is a h igh sen si t iv i ty iso therm al ca lor imeter

w hic h is descr ibed in deta i l e lsewhere (20) . Essent ia l ly , it consists o f m easur ing

vesse ls enclosed in meta l heat s inks which are kept a t a h igh ly stab le temperature by

being imm ersed in a large vo lum e of wa ter (2 5 l ) under the rm osta t ic con t ro l . The

heat f low f rom the sample to the heat s inks is moni tored by Pel t ie r ar rays.

W i th a sens i t i v i t y do w n to 1 0 '

W kg '

, the TA M is easi ly capab le o f measu r ing

hea t ou tpu ts we l l be low the hea t l oss fac to rs o f commerc ia l packs ( t yp ica l l y , 0 .05 -

0 .10 W kg

for 25 - 50 kg) . Inde ed, i t can mea sure direct ly heat gen erat ion rates at

the temperatures o f in terest for la rge masses o f unstab le mater ia ls(21) .

In com bina t ion w i th the AR C, T A M has been used to provide in terpo la t ive

est im ates o f SA DT (22 ) . App l ied to a set o f mater ia ls exh ib i t ing d iverse d eco m pos i t ion

behaviours, a s ing le TAM measurement combined wi th onset data f rom an ARC run

gave est imates o f SADT which were in bet ter overa l l agreement wi th US SADT

resu l ts than those obta ined by ext rapo la t ing ARC data .

In pr incip le , SADT cou ld be est imated in terpo la t ive ly by combin ing ra te data f rom

a number o f tes ts . For exam p le , in the ARC /TAM m etho d(20 ) , T A M da ta cou ld be

rep laced by data f rom the UN IST. In pract ice , how eve r , th is is not a t t rac t ive because

the IST requires more sample, is probably more expensive and is less readi ly avai lable.

On the o ther ha nd, the com binat ion o f DT A (or DSC) and T A M data cou ld have

economic and p rac t i ca l advan tages.

The com par iso n o f DTA o nse t , ARC onset and TA M data is exem pl i f ied in F igures

1 - 3, in w hic h p ow er outpu ts (P W kg

) have been p lo t ted in Arrhe n ius form . Overa l l

the corresp onde nce be twe en the metho ds is reasonable . In par t icu lar , it is not e vident

that in terpo la t ing f rom DTA po in ts ra ther than ARC po in ts would in t roduce a

consistent b ias in to SADT est imates.

EST IMAT ION OF SADT FROM DTA AND TA M DATA

DTA measurements

These were p er form ed using a Me t t le r TA 3 00 0 inst rum ent under se l f -pressur is ing

condi t ions (stain less steel pressure crucible or g lass ampoule) at a scanning rate of

2 K min

w i th a de tec t ion th resho ld o f 0 .2m W .

TAM me a su re me n ts

Glass sample conta ine rs o f 2 cm

vo lume w i th samp les o f approx imate ly

g were

e mp l o ye d .

Co m m on ly, T A M t rac es have the pro f i le i l lust ra ted in F igure 4A. Th us, the power

output stab i l ises and there is l i t t le doubt as to the va lue to be taken for ca lcu la t ion.

However , th is is not invar iab ly the case.

Figure 4B , for example, represents the case where there is a rapid in i t ia l react ion

in add i t ion to the react ion represented by the subsequent steady power output .

Provided the energy associated with the in i t ia l react ion is smal l (a few J g

ca lcu la t ion as out l ined be low is probably st i l l va l id . A l te rna t i ve ly , bo th react ions may

be taken in to account as previously descr ibed(21) .

In some cases a cont inua l ly increasing power output (Figure 4C) is ob se rve d. In

these instances, the ca lcu la t ion method g iven be low cannot be app l ied and i t i s

necessary to extend the t ime per iod o f the measurement and conduct measurements

a t l ower tempera tu res .

Calcu la t ion

2 Calcu la te apparent act iva t ion energy, E, f rom

3 Calculate t imes to m axim um rate at T

and T, accord ing to

where C = speci f ic heat (avai lable from DTA run)

4 Plot In t ag ains t 11 1 + 273) , in terpo la te to the t ime constant , t

of the package

and read-of f the temperature o f no re turn , T

. The fo l lo win g va lues o f t

25kg 8 hour , 50kg 10 hours; 135kg 16 hours.

5 Calcu la te SAD T

Results

SADT est imates were made by the above method for the set o f compounds,

embracing a range of decomposi t ion behaviour , previously used to examine the use

of ARC and TAM data in combinat ion. The resu l ts, together wi th ARC and ARC/TAM

est imates o f SADT and US SADT test resu l ts, are g iven in Table 1 .

DISCUSSION

The method used for ca lcu la t ing SADT, previously employed e lsewhered 7 ,20) , is an

approximate zero order Seminov- type so lu t ion to the prob lem of heat accumulat ion.

The oret ica l ly , i ts app l ica t ion to so lids can be st rong ly cha l lenge d. How eve r , for

masses in the range 10-150kg, in our exper ience SADTs obta ined in th is way for

so l ids agree c lose ly wi th e va luat ions based on Frank-Kam enetski i cr i t ica l pa rame ters.

Addi t ional ly, as t ime constants are simi lar for sol ids and l iquids in smal l commercial

packs (17), standardised values can be used and no addi t ional data are required.

Therefore, for est imat ions based on l imi ted kinet ic data, such a quick and simple

procedure is considered appropr ia te for smal l commercia l packs.

For other si tuat ions, more r igorous evaluat ion methods, such as those based on Frank-

Kam enets ki i cr i t ical parameters or t rans ient m ode ls(21 ), are avai lable. The Frank-

Kam enets ki i app roac h was usefu l ly sum ma rised rec ent ly , a lbei t in th is case appl ied

to ext rapo la ted k inet ic data(22) .

The data of Table 1 show that the DTA/TAM est imates o f SADT are in as good

agreement wi th US SADT test resu l ts as the ARC/TAM est imates: there is no

signi f icant increase in bias or loss of precision associated with the use of DTA as

opposed to ARC onset data .

On the basis that a t DTA onset the zero order k inet ics assumpt ion invo lved in

ca lcu la t ing SADT may be v io la ted and that DTA onsets are ra ther imprecise, th is

con clusion cou ld be regarded as surpr is ing . Ho we ver , it has a l ready been sh ow n

(Figures 1-3) tha t the co r respondence o f DTA, ARC and TAM da ta shows no

sys tem at ic b ias betw een the tech n ique s. A ls o, as the est ima t ion is in terpo la t ive , the

unc er ta in ty in the es t imated param eter may be less tha n in the me asure me nt .

The robustness o f in terpo la t ion wi th respect to precis ion can be i l lust ra ted by

consider ing the case o f sod ium d ich loro isocyanurate d ihydrate (Figure 5 ). The error

bar for the DTA po int represe nts ± 3 stand ard dev iat ions (5 °C ) or ± 1 5 °C .

For a cr i t ica l pow er outp ut o f 0 .13 5 W kg

, th is var ia t ion cou pled w i th that in the

TA M poin ts leads to a var ia t ion o f on ly ± 7 °C in the cr i t ica l tem pera ture e st ima te.

This trea tm en t is not r igorou s but nei ther is i t m islead ing. Sta t ist ica l ly derived

conf idence in terva ls are curved, be ing narrowest a t the cent ro id o f the data set (22) .

As a resu l t , in terpo la ted est imates may be more precise than the measurements upon

which they are based.

Add i t iona l unce r ta in ty can be caused by bias whic h in the present con tex t wo uld

ar ise f rom a change in react ion mec han ism . For instance , it has been sh ow n tha t

azodi isobutyroni tr i le probably starts to melt in the region of the ARC onset

tem per ature (20) . The refore , the data o f Figure 2 are probably better represented as

two l ines, one for the sol id and one for the l iquid, as shown in Figure 6 . Even so , as

has already been s ho w n, interpo lat ion resul ts in negl ig ib le bias. On the other hand ,

the potent ia l b ias f rom ext rapo la t ion is obvious.

The accurac y requ i red for stab i l i ty tempe rature est imates dep ends upon ho w close

the y are to the temp eratu res a ma ter ia l is likely to expe rienc e. W here th is gap is

w i d e ,

the ind icat ions o f DTA or DSC a lone can be su f f ic ient . Freque nt ly, tho ug h,

add i t ional data is required and in our experience a T A M m eas urem ent at th is stage can

be very he lp fu l . I f fu r ther re f inemen t is necessa ry, add i t iona l T A M data can be

obta ined or another techn iq ue, such as AR C, emplo yed . In cr i t ica l cases , resor t to

large-scale tests may be necessary.

CONCLUSION

The method of est imat ing stab i l i ty temperatures descr ibed re l ies on approximat ions

and assum pt ions w hic h may be cha l lenged but has been tested by app l ica t ion to a set

o f co m pou nds wi th d i ffe r ing dec om pos i t ion behav iours. I t p rodu ced est imates o f

SA DT w hic h were in reasonable w i th US SADT test resu l ts. I t , there fore , a f fords a

rea l t ive ly qu ick and inexpensive smal l-sca le means o f est im at ing sta b i l i ty tem peratu res

which can be usefu l in overa l l s t ra teg ies for determin ing safe handl ing temperatures

for exothermica l ly unstab le mater ia ls.

I CHEM E SYMPOSIUM SERIES N o. 134

ACKNOW L EDGEMENT

Tha nks are extended to Dr F Lopez o f Sc her ing AG for he lp fu l d iscus sions.

REFERENCES

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Na t i o n s , Ne w Yo rk , 1 9 9 1 , p . 2 3 5 .

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3 lbid p . 18 9.

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10 Yosh ida , T . , 19 87 , 'Sa fe ty o f React ive Chemica ls ' , E lsev ie r , Am ste rda m , p .200 .

11 K oto yo r i , T . , 19 89 , J. Loss Prev. Process Ind. . 2 , 16 .

12 Reference 10 , p .20 1 - 2 and 23 7.

13 Cr on in , J.L . and Nolan P.F. , 198 7, J. Haz. Ma ter . . 14 , 29 3.

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Un ive rs i t y ) , London .

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React ions, March 1989 , Boston Massachusse t ts ' , A lChE, New York , p .74 .

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TABLE 1 COM PA RI SON OF SAD T EST IM ATES W I TH US SADT TEST RESULTS

a From Reference 20

1 t -Buty lperoctoate (100%) 25l

Ace ty lace to ne perox ide (SA 3) 25 kg

3 Laury l perox ide 25 kg

4 t -Bu ty lperbenzoate 25 kg

5 Di - t -bu ty lperox ide 25k g

6 Azod icarbonam ide 50k g

7 Sod ium d i ch lo r o isocyanu r a t e .2H

0 135kg

8 Azo d i isobuty ron i t r i le 25k g

9 2 ,2 ' -Azo b is (2 -m ethy lbu ty ron i t r i le ) 25kg

CHEM E SYMPOS IUM SERIES No 134

I CHEM

SYMPO SIUM SERIES No. 134

I CHEM E SYMPOSIUM SERIES No 134

Estimation of Stability Temperatures From Differential Thermal

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