18
An A u t o m a t e d Thermal Reaction
Analysis System f o r
Kinetics 1
A.F.KAH, M. E. KOEHLER, T. H. GRENTZER , T.F.NIEMANN ,and T. PROVDER Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch018
1
SCM Corporation, Glidden Coatings and ResinsDivision,Strongsville,OH44136 A DuPont Model 990 Thermal Analysis Console with Model 910 DSC accessory was interfaced to a minicomputer system by means of a microcomputer for automated data c o l l e c t i o n . A program to provide the analysis of reaction kinetics data by the single dynamic scan method for DSC kinetics was developed. Features of this program include a fit of the data to a single equation by multiple regression techniques to y i e l d the reaction order, the energy of a c t i v a t i o n and the Arrhenius frequency factor. The rate constant k(T) is then calculated and conversion data as a function of time and temperature can be generated at the operator's option. The automation of the single dynamic DSC scan approach has provided an accurate, time e f f i c i e n t , routine method for obtaining quantitative reaction kinetics information for decomposition, polymerization and curing reactions. The majority of work reported in the l i t e r a t u r e for obtaining reaction kinetics information from differential scanning c a l o r i metry (DSC) has been devoted to isothermal studies which require a number of the thermograms over a range of reaction temperatures. In our previous work (_1,2J, we described the methodology involved in using a single dynamic DSC scan (one thermogram) to obtain reaction k i n e t i c s information on model systems such as the decomposition of 2,2-azo-bis-isobutyronitrile, the decomposition of calcium oxalate, and the reaction of phenylglycidyl ether with 2-ethyl-4-methylimidazole, as well as practical coatings systems 1
Current address: Owens Corning Fiberglas Corp., Technical Center, Route 16, P.O. Box 415, Granville, OH 43023 0097-6156/82/0197-0297$06.00/0 © 1982 American Chemical Society
Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE
such as high s o l i d s c o a t i n g s , gel coat r e s i n s and powder c o a t i n g s . In t h i s work, we discuss the methodology used to automate the s i n g l e dynamic scan method t o p r o v i d e , r o u t i n e l y , reaction kinetics information.
Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch018
Data A q u i s i t i o n System Automation of the DSC was achieved by i n t e r f a c i n g a DuPont Model 990 Thermal A n a l y s i s Console with Model 910 DSC accessory t o a micro-mini computer system. Analog and d i g i t a l s i g n a l s from the instrument are i n t e r f a c e d t o t h e m i c r o c o m p u t e r which is r e s p o n s i b l e f o r data c o l l e c t i o n and a l l r e a l - t i m e a c t i v i t i e s . At the completion of the experiment, data are t r a n s f e r r e d via a s e r i a l l i n e to the minicomputer f o r s t o r a g e , a n a l y s i s , report generation and p l o t t i n g . D e t a i l s of the mini-microcomputer system and i t s o r g a n i z a t i o n and operation have been reported elsewhere (3,4). The analog s i g n a l s are taken from the recorder p o r t i o n of the DuPont 990 console at points labeled as 'test point 1' on the a m p l i f i e r p r i n t e d c i r c u i t board f o r each channel. This y i e l d s a voltage proportional to the pen displacement and that i s a f f e c t e d by the p o s i t i o n of the s e n s i t i v i t y (attenuation) d i a l on the c o n s o l e , but i s not a f f e c t e d by the zero o f f s e t c o n t r o l s . The actual gain and o f f s e t of the s i g n a l at t h i s point are measured for each channel. Analog s i g n a l s from the instrument are then f u r t h e r conditioned and scaled before passing them to the A/D converter. This i s accomplished by means of instrumentation a m p l i f i e r s f o l l o w e d by low pass a c t i v e f i l t e r s which are implemented on p r i n t e d c i r c u i t boards designed i n our l a b o r a t o r y . Automated Instrument A n a l y s i s Process There a r e f o u r s t a g e s i n an automated i n s t r u m e n t a n a l y s i s . These a r e shown s c h e m a t i c a l l y i n F i g u r e 1 o f C h a p t e r 13. I n the f i r s t s t a g e t h e i n s t r u m e n t o p e r a t o r i n i t i a t e s t h e a n a l y s i s b y means of a d i a l o g program on t h e minicomputer. An example o f t h e d i a l o g f o r t h e DSC o p e r a t i o n i s shown i n F i g u r e 1. The d i a l o g a s k s a s e r i e s of q u e s t i o n s about t h e sample i d e n t i f i c a t i o n and parameters r e q u i r e d f o r instrument o p e r a t i o n and data a n a l y s i s . When t h e d i a l o g i s
complete the minicomputer sends a l l of the input data to the microcomputer. The microcomputer acknowledges that i t received the data and turns on a status l i g h t at the instrument i n d i c a t i n g that i t i s ready. The second s t a g e i s d a t a a c q u i s i t i o n . This stage i s i n i t i a t e d when the operator s t a r t s the instrument. For the DSC o p e r a t i o n , d a t a are c o l l e c t e d on t h r e e analog channels corresponding to the two pens (Y and Y') on the DuPont 990 console and to the temperature a x i s . Data are c o l l e c t e d on a time base using the microcomputer's c r y s t a l based r e a l - t i m e programmable clock.
Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
18.
KAH ET AL.
Automated
Thermal Analysis for Reaction
299
Kinetics
During the t h i r d stage the microcomputer t r a n s m i t s the data i t has c o l l e c t e d and stored i n i t s memory t o the minicomputer. The minicomputer stores these data i n data f i l e s on one of the disks. The fourth stage i s the data a n a l y s i s and takes place i n the minicomputer. This data reduction i s done by FORTRAN programs. Reports and p l o t s are generated at t h i s t i m e .
Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch018
Data A n a l y s i s Methods The basic assumption of the DSC k i n e t i c method i s that the rate of heat e v o l u t i o n from a r e a c t i o n i s p r o p o r t i o n a l t o the rate of the chemical r e a c t i o n and hence, the t o t a l heat evolved up to any point during the r e a c t i o n i s p r o p o r t i o n a l t o the amount of reactants consumed. Assuming t h a t r e a g e n t s a r e p r e s e n t in stoichiometric proportion and that thepp i s only one slow step i n the r e a c t i o n mechanism, the general n order rate expression can be w r i t t e n i n terms of concentration i n l o g a r i t h m i c form y i e l d i n g the f o l l o w i n g expression. In k (T) = In
(1)
where C i s the i n i t i a l c o n c e n t r a t i o n , C i s the amount reacted at t i m e , t , dC/dt i s the rate of disappearance of r e a c t a n t s , n i s the r e a c t i o n o r d e r and k(T) i s t h e t e m p e r a t u r e dependent rate constant. Rewriting equation (1) in terms of the observable v a r i a b l e s obtained from the DSC experiment, we obtain /AH -H(t,T)\
1 /dH(t,T) i n k (T) = i n < j p —
n
(2)
AH
where AHo i s the t o t a l heat of r e a c t i o n , H(t,T) i s the heat evolved up t o t i m e , t , and temperature, T, and dH(t,T)/dt i s the time and temperature dependent heat flow shown d i a g r a m a t i c a l l y i n Figure 2. By s u b s t i t u t i n g the Arrhenius expression In k (T) = In A into equation.
equation 1
l
n
AH,
(2)
(3)
and r e a r r a n g i n g , ^
our
working
AH -H(t,T)
/dH(t,T) dt
we obtain
Q
In A
RT + n
ln
AH,
Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
(4)
300
COMPUTER APPLICATIONS I N APPLIED POLYMER SCIENCE
DIA 30 Instrument
Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch018
JOB
5432
No*•*23 DSC R E A C T I O N
KINETICS:23
Initials,**MEK 1 S a m p l e I D * • • A I B N IN DNBTH 2 H e a t i n g r a t e * * * 15 3 A i r o r N i t r o g e n
