OECD GUIDELINE FOR TESTING OF CHEMICALS
Adopted:12 May 1981
\"Screening Test for Thermal Stability and Stability
in Air\"
(Accelerated Storage Test [CIPAC-Test] - Thermal AnalysisMethods, including differential thermal analysis [DTA] and
thermogravimetric analysis [TGA])
1.I N T R O D U C T O R Y I N F O R M A T I O N•
G u i d a n c e i n f o r m a t i o n– Structural formula– Vapour pressure curve– Melting point– Boiling point
•Q u a l i f y i n g s t a t e m e n tThe test methods can be applied to pure and commercial grade substances. The potentialeffects of impurities on results must be considered.
•S t a n d a r d d o c u m e n t sThis Test Guideline is based on the
–CIPAC-recommendations (4) for stability testing of pesticides (short time storage test); andon
–consensus methods of thermal analysis (DTA, TGA).
2.M E T H O DA.INTRODUCTION, PURPOSE, SCOPE, RELEVANCE,APPLICATION AND LIMITS OF TESTThe purpose of the methods is to obtain a preliminary judgement of the stability of asubstance with respect to heat and air in order to provide guidance in the performance of othertests.
The methods for determining storage stability discussed in this Test Guideline areapplicable to homogeneous solid and liquid substances and to mixtures of these.
Exothermic decomposition processes can be determined by differential thermal analysis(DTA). In order to determine endothermic effects it must be confirmed that it is adecomposition and no phase transition.
Users of this Test Guideline should consult the Preface, in particular paragraphs 3, 4, 7 and 8.113
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The thermogravimetric analysis (TGA) determination gives information aboutdecomposition reactions involving the elimination of volatile decomposition products.With a TGA kinetic evaluation and extrapolation to lower temperatures may be easierthan with DTA.
•D e f i n i t i o n s a n d u n i t s (1,2,3)–Thermal analysis (TA): General term describing analytical methods in which the changesin the physical parameters of a substance as a function of temperature are measured.–Differential thermal analysis (DTA): Measurement of the temperature difference between asample and the reference material as a function of time or temperature.–Thermogravimetric analysis (TGA): Measurement of the weight change of a substance usingan isothermal or anisothermal procedure as a function of time or temperature.–Peak: The term \"peak\" describes the upward or downward deviation of the recording curvefrom the base line.–Peak temperature: Temperature at the peak maximum.•R e f e r e n c e s u b s t a n c e sSuitable reference substances are urea, 4-nitrosophenol, α-naphthylamine andnaphthalene. These substances need not be employed in all cases when investigating a newsubstance. They are provided primarily so that calibration of the method may be performedfrom time to time and to offer the chance to compare the results when another method isapplied.
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•P r i n c i p l e o f t h e t e s t m e t h o d sAccelerated storage test (CIPAC)A long duration storage instability can be simulated by applying a higher temperatureduring a short test. Such a \"screening test\" is described in the CIPAC - recommendations (4)for stability testing of pesticides. This test calls for the controlled storage at 54°C to 55°C for14 days and subsequent analysis. In simple cases it will be enough to determine a characteristicproperty (e.g. melting point) before and after storage.
Thermal analysis methodsThe sample and the standard reference material are heated up to the final temperatureat a constant rate in a defined test atmosphere, either separately in a TGA or DTA apparatusor in a combined system, and the weight change of the sample or the quantities of heat absorbedor given off are measured and recorded. If, in the temperature range investigated, peaks areobserved from which a chemical reaction of the sample can be deduced, the thermal analysisshould be repeated in the immediate vicinity of the peak temperature.
•Q u a l i t y c r i t e r i aRepeatabilityDTA and TGA are well-known methods for determining thermal stability of chemicalcompounds. (See Section on accuracy, below).
SensitivityThe sensitivity of the method is determined by the sensitivity of the measurementapparatus (equipment type) and the test conditions.
Possibility of standardisation
Standardisation of the test conditions for thermal analysis is described by McAdie (5).Possibility of automation
There is some possibility for automation.
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B.DESCRIPTION OF THE TEST PROCEDURES•
A c c e l e r a t e d s t o r a g e t e s t(based on CIPAC - Test)
Apparatus
–Oven, thermostatically controlled
–Sample containers (250-ml beakers, glass bottles, sealable ampoules).
Procedure
Place 20 g of the sample in a bottle. If the substance is volatile place in a sealableampoule. Water saturated air should be used as test atmosphere. Seal the sample containerhermetically and keep it in the oven at 55 ± 2°C for 14 days. Remove the sample containerfrom the oven, cool down to room temperature, and determine by a suitable method (e.g.determination of melting point) whether decomposition or other chemical transformation hasoccurred.
•
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) a n dd 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)Apparatus
DTA or DSC apparatus of commercially available type. (Block diagram of DTAapparatus: Figure 1).
Heat flow and energy compensated methods may both be applied. For volatile substancesan apparatus should be available which allows measurements to be performed with closedsample containers or under elevated pressure.
Test conditions
Sample containers of different kinds may be used: open or sealed glass tubes, metal pans,or pressure resistant crucibles. For measurements under oxygen-containing atmosphere onlyopen sample containers are appropriate.
The test atmosphere is (a) nitrogen and (b) air. When air is used the sample is put in anopen pan.
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Figure 1: Block diagram of a DTA apparatus
12345678Two-channel recorderD.C. amplifierFurnace
Heater windings
Temperature regulator
Thermocouple with crucible P for sample substanceThermocouple with crucible V for reference substanceReference junction thermocouple
An inert reference substance is selected which undergoes no changes in the temperatureregion employed. Thermal conductivity and heat capacity of the inert reference sample shouldbe nearly equal to those of the sample to be investigated. In many cases aluminium oxide is auseful inert substance.
Procedure
Samples of about 5 to 50 mg are weighed and closed in the sample container. Theheating rate should be in the range of 2 to 20 K/min. At first a DTA diagram (for example seeFigure 2) of the substance at normal pressure is recorded.
If a thermal effect (a peak) is found between room temperature and 150°C, one proceedsas follows:(a)(b)
When the peak is due to an exothermic effect, it is assumed to be a decomposition.When the peak is due to an endothermic effect, the temperature at which it occurs shouldbe compared to the melting point of the substance.
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Figure 2: DTA curve
T∆T: temperature of the reference substance.
: temperature difference between sample and reference substance.
(c)
If the peak is due to an endothermic effect which is not related to the melting of thesubstance, the DTA should be repeated at a higher pressure (10-50 bar) or in a closedsample container. If the peak is shifted to a higher temperature, it comes from avaporisation process.
If the endothermic effect is due neither to melting nor to vaporisation, repeated heatingcycles are carried out around the peak temperature. If the peak does not persist achemical transformation has occurred.
(d)
•T h e r m o g r a v i m e t r i c a n a l y s i s (TGA)Apparatus
TGA Apparatus of common design, e.g. of commercially available type, allowing forheating the substance in air and in an inert atmosphere. (Diagrammatic sketch of the apparatus:Figure 3.)
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Figure 3: Diagrammatic structure of a TGA apparatus
WPOSBalance beamSampleFurnaceRecorderTest conditions
NDMSSGTPZero detector
Magnet coil (weight compensation)Control unit
Temperature programming
The test atmosphere is normally (a) nitrogen and (b) air. For testing oxidation stabilityair is used as atmosphere.
Procedure
A sample of about 10 to 500 mg is heated in (a) nitrogen and (b) air, and the weight lossis recorded. The heating rate should be in the range of 2 to 20 K/min. A weight loss which doesnot originate from volatilisation of the substance is considered as a decomposition.If a decomposition is observed at temperatures below 150°C the rate of decompositioncan be determined by isothermic measurements.
3.D A T A A N D R E P O R T I N G•
I n t e r p r e t a t i o n o f r e s u l t sThe substance is considered to be stable at room temperature if either
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a)
in the \"Accelerated Storage Test (CIPAC-Test)\" the melting point (or anothercharacteristic property) has remained constant or if the content of original substance asdetermined by analysis has decreased by not more than 5 per cent
or
b)in DTA or TGA no decomposition or chemical transformation is found below 150°C.
•T e s t r e p o r tThe test report should contain the following information:Accelerated storage test
–Type of sample container
–Method of determination of a chemical transformation
–Change of a typical property or percentage of decomposition after 14 days' storage.
Thermal analysis
–Type of apparatus employed
–Preliminary treatment and form of the sample–Precise information on reference and test substances
–Temperature range investigated, rate of temperature increase, temperature specifications forisothermal procedure–Quantity of substance
–Composition and purity of the test atmosphere–Type of sample container
–Changes observed on the treated sample during and after testing–Temperature of beginning chemical transformation–Conditions deviating from this method
–Where possible, report nature of decomposition products.
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4.L I T E R A T U R E01.H.J. Kretzschmar, Bundesanstalt für Material-prüfung (Federal Institute of Materials
Testing) personal communication.02.ASTM E 472 - 7303.ASTM E 473 - 73
04.CIPAC Handbook, Volume I. Analysis of Technical and Formulated Pesticides, G.R. Raw,
ed., Collaborative International Pesticides Analytical Council Ltd., p. 951 f., 1970.05.H.G. McAdie, Anal. Chem. 39, 543 (1967) and H.G. McAdie, Z. anal. Chem. 231, 36
(1967).06.DIN 53387
07.P.E. Slade and L.T. Jenkins, \"Techniques and Methods of Polymer Evolution, Vol. 1:
Thermal Analysis, 1966, Vol. 2: Thermal Characterization Techniques, Marcel Decker,New York, 1970.08.W.W. Wendlandt, Thermal Methods of Analysis, 2. Edition, J. Wiley & Sons, New York,
1974.09.Hoffman et al. Polymeranalytik II, Thieme Taschenbuch der organischen Chemie B 5,
Georg Thieme Verlag, Stuttgart 1977.10.D. Schultze, Differenzialthermoanalyse, 2 Ed., Verlag Chemie, Weinheim/Bergstr. 1971.11.B. Carroll and W.P. Manche, Thermochim. Acta 3, 449 (1972).12.H.A. Papazian, J. appl. Polymer Sci. 16, 2503 (1972).
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