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7/23/2019 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
here.
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.
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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 '
4
W kg '
1
, 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
1
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
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I CHEM E SYMPOSIUM SERIES No. 134
1 - 3, in w hic h p ow er outpu ts (P W kg
-1
) 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
- 1
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
3
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
-1
),
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
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I CHEM E SYMPOSIUM SERIES No. 134
2 Calcu la te apparent act iva t ion energy, E, f rom
3 Calculate t imes to m axim um rate at T
0
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
coo
of the package
and read-of f the temperature o f no re turn , T
NR
. The fo l lo win g va lues o f t
con
were
used:
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
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I CHEM E SYMPOSIUM SERIES No. 134
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
1
, 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.
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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
1 'Recom me nda t ions on the T ranspor t o f Dangerous Go ods ' , 7 th Edn . Un i ted
Na t i o n s , Ne w Yo rk , 1 9 9 1 , p . 2 3 5 .
2 'Rec om me nda t ions on the Tran spo r t o f Dangerous Go ods, Tes ts and Cr i ter ia ' , 2nd
Edn, Un i ted Na t ions, New York , 1990 .
3 lbid p . 18 9.
4 lbid p . 2 0 0 .
5 i b j d , p . 19 4.
6 i b j d , p . 2 0 5 .
7 Grew er , T . and K la is , 0 . , 1988 , 'Exo therm eZe rse tz ung ' , VD I -Ver lag , Dusse ldo rf ,
p .36 .
8 Roge rs, R.L . and W righ t , T .K . , 198 6 'EFCE Publ ica t ion Ser ies No. 5 0 ' , Pergamon
Press, Oxfo rd , p . 121 .
9 Ko to y o r i , T . a n d Ma ru ta , M . , 1 9 8 3 , Th e rm o ch i m. Ac ta . 6 7 , 3 5 .
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.
14 Cro n in , J.L . and Nolan P.F. 19 87 , P lant /Ope rat ions Prog. . 6(2) . 89.
15 Cr on in , J.L . , 19 87 , PhD Thes is, Sou th Bank Polytechn ic (n ow South Bank
Un ive rs i t y ) , London .
16 Ho fe l ich , T .C . and Tho ma s, R.C., 19 89 , ' In ternat ion a l Sy mp osium on Runaway
React ions, March 1989 , Boston Massachusse t ts ' , A lChE, New York , p .74 .
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I CHEM E SYMPOSIUM SERIES No. 134
17 W i lbe r fo rc e , J .K . , 1 9 8 1 , 'The Use o f the Acce le ra t ing Ra te Ca lo rime te r to
Determine the SADT of Organic Peroxides' , Co lumbia Scient i f ic Indust r ies, Mi l ton
Keynes.
18 F isher, H.G. and Goe tz, D.D. , 1 9 9 1 , J . Loss Prev. Process Ind. . 4 , 3 05 .
19 W h i tm ore , M.W . , 19 92 , J . Loss Prev. Process Ind . . 5 (5 ) . 322 .
20 W h i tm ore , M.W. and W i lbe r fo rce , J .K . , 19 93 , J . Loss Prev. Process Ind . , 6 (2 ) ,
95 .
21 Tha rma l ingum , S . , 19 89 , ' In te rna t iona l Sym pos ium on Runa way React ions, March
1 9 8 9 Bo s to n , Ma ssa ch u sse t t s ' , A l Ch E , Ne w Yo rk , p . 2 9 3 .
22 F ischer , H.G. and Goetz, D.D. , 1 99 3, J . Loss Prev. Process Ind . . 6(3) . 18 3.
23 Ca ulcu t t , R., 19 83 , 'Sta t is t ics in Research and De velo pm ent ' , Chapm an and Hall,
London , p .140 .
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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
Kev
1 t -Buty lperoctoate (100%) 25l
2
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
2
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
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