Safety screening of chemicals.
Recent accidents in chemical plants involving compounds previously thought to be harmless demonstrate the importance of complete safety screening.
Modern DSC instruments using rugged sensors and CRT graphics allow fast investigations of substances and mixtures in question.
This article deals with measurements of selected inorganic and organic samples that undergo decomposition in the applied temperature range of room temperature up to 600 [degrees] C. The measurements are performed by the Mettler TA4000 system with the new computer graphics and evaluation software GraphWare TA72.
Exothermic chemical reactions which get out of control constitute various hazards: formation of toxic or flammable gases or vapours, deflagration or even detonation as well as setting inflammable goods on fire. Such accidents in chemical plants and warehouses are a result of several unfortunate circumstances. But often there is still a lack of knowledge regarding potential exothermic reaction of chemical compounds. Chemical engineers of course know the hazard potential of the most popular substances such as peroxides, organic nitro and diazo compounds and certain mixtures (ie. gun powder and propellants) but other compounds actually are in doubt.
Differential scanning calorimetry (DSC) allows the determination of any heat of reaction occurring in the covered temperature range of usually from room temperature up to approximately 600C. The small amount of sample required for such a screening test (approx. 1 mg) practically excludes any danger for the operating personel. The use of air (oxygen) or nitrogen as a purge gas enables comparison of heat evolved by combustion/oxidation with the heat of decomposition in inert atmosphere. Volatile samples should be analyzed in pressure-resistant sealed crucibles. Otherwise they can evaporate before the reaction starts.
In addition DSC supplies information concerning drying, melting, purity, polymorphic transitions, sublimation and vaporization. The specific heat, cp, is also obtained and can be used to calculate the maximum possible temperature increase, [Delta] T, caused by the exothermicity.
Here, it is assumed that the heat of reaction remains in the reaction material, namely that there is no heat exchange with the surroundings (adiabatic conditions): [Delta] T=[Delta] H/ cp The enthalpy change, [Delta] H, can range from a few joules per gram up to several kJ/g.
Usually an enthalpy change above 200 J/g (or an adiabatic temperature rise higher than 150 [degrees] C) is thought to be of potential danger. Thus further investigations are required.
The shape of the reaction peak allows an insight into the reaction kinetics. Consecutive isothermal measurements, if possible at several isothermal temperatures, provide additional kinetic information, ie. on autocatalytic behaviour which means that the onset of the chemical reaction occurs only at the end of an induction period. Another approach is the use of several dynamics measurements with different heating rates. Additional stages in the safety investigation concern non-thermo analytical methods; the measurements of the deflagration behaviour and the impact sensitivity of the reaction components and products. For scale up, that is the extrapolation to manufacturing conditions, fairly large batches (10 . . . 2000 g) are investigated in a reaction calorimeter under conditions reflecting actual practice.
In this work safety screening is performed with selected compounds that undergo decomposition in the applied temperature range of room temperature up to 600 [degrees] C. The measurements are performed by the Mettler TA4000 system with its DSC20 cell with silver furnace and standard DSC sensor. The applied purge gas flow is 50 ml/min. Some studies include thermogravimetric measurements in the TG50 cell too.
Data storage, processing, on-screen (CRT) curve comparison, numeric evaluation and recording the diagrams by a multi-pen plotter is performed with Mettler GraphWare TA72, the new computer graphics and evaluation software. The computer hardware consists of an IBM PC/AT03 with 1.2 MB floppy disk drive, 30 MB hard disk, coprocessor 80287, 1152 K RAM, EGA colour display. An Epson H180 with HP-GL ROM provides the diagrams.
Results and Discussion
A first measurement is performed at constant pressure using air as a purge gas (figures 1 and 2). Besides polymorphic transitions this treatment indicates vaporization and oxidative decomposition. An additional analysis under nitrogen often makes interpretation easier (figures 3 and 4). Reweighing the sample after the run allows the determination of the total weight loss. A TGA curve provides more information (figure 4).
To make sure the whole sample is exposed to the temperature programme and no volatiles can escape, a further measurement is performed in a high-pressure crucible at constant volume (figure 5). The Mettler high-pressure crucible is made of Nimonic 80A alloy with a volume of 270 [micro] 1. The burst disk withstands a maximum pressure of 10 MPa ( | 100 atm). The enclosed air (250 [micro] 1 | 60 [micro] g oxygen) is sufficient to oxidize approximately 30 [micro] g organic matter. With a heat of combustion of 25 kJ/g these 30 [micro] g can cause an exotherm of 750 mJ! When using small amounts of sample (1. . .2 mg) this oxidation reaction can simulate a result of 500 J/g! To avoid the unwanted reaction which can hide decompositions, it is necessary to flush the crucible with nitrogen before closing it (glove box or laminar flow device). It is obvious that the non-specific DSC method just measures the sum of all simultaneous thermal effects occurring at a time. In addition the respective chemical reactions are sometimes quite complex. A scientific investigation of thermally induced degradation reactions begins with DSC screening but other techniques, such as TGA and MS as well as chemical analysis of the obtained intermediate or end product, are required too.
DSC safety screening is a rapid means of testing the hazard potential of materials exposed to heat. Within a few hours a decision can be made whether the substance is harmless or may lead to exothermal reactions under certain conditions. A powerful graphics software package provides the necessary tool for efficient curve comparison and interpretation as well as quantitative curve evaluation. [Figures 1 to 5 Omitted]
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|Publication:||Canadian Chemical News|
|Date:||Feb 1, 1990|
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