Onset temperatures with calorimeters

Figure 19. Phase diagram of a narrow fraction of HPC (molar substitution MS = 5.0, M = 209x 10 g/mol) in H2O. Onset of phase separation by tur-bidimetry , transition temperatures with scanning Calvet calorimeter O (adapted from [2] and [72]).

To run the water sample, place an empty uncrimped sample pan on the reference side of the calorimeter. This is always the right side of the analyzer. Using a pipette or an eyedropper, fill another sample pan halfway with distilled water and place it on the sample side of the analyzer. Weighing the water sample is not necessary because we are only using the melting onset temperature, which is unaffected by sample weight. Install the furnace... 

The Accelerating Rate Calorimeter (ARC ) is another adiabatic test instrument that can be used to test small samples. The ARC with the clamshell containment design can handle explosive compounds. It is a sensitive instrument that can indicate the onset of exothermicity where the reaction mixture can be accurately simulated (HSE 2000). ARC testing results can be used in determining a time to maximum rate of decomposition, as well as in calculating a temperature of no return for a container or vessel with specific heat removal characteristics. Further information and references related to the ARC are given in CCPS (1995a) and Urben (1999). 

A brief overview of this test is that the sample is prepared and installed in a calorimeter that has been purged with oxygen. The sample is heated to 200°C and held isothermally at that temperature. The time it takes the material to decompose is measured. Decomposition is indicated by an exothermic downturn of the baseline. The extrapolated onset is calculated to determine the amount of time it took for decomposition. The longer the time, the more effective is the antioxidant (Fig. 10). 

The thermogram obtained by differential scanning calorimetry (DSC) shows a melting peak at 82.4 °C and an exothermal decomposition peak starting at 160 °C (AHaec = 200 J g ). Tests on an accelerating rate calorimeter (ARC) showed the onset of decomposition at 130 °C, with AHdec = 278 J g", and a final temperature of 179 °C. The accumulated data do not support the originally claimed stability... 

Thermal analysis is another technique which can be used to determine the antioxidant concentration in a polymer sample. The measurement of the oxidative induction time (OIT) of a sample determined with a differential scanning calorimeter (DSC) is a popular method. The OIT of a plastic material is determined by the thermo-analytical measurement of the time interval to the onset of exothermic oxidation of the sample at the specified temperature in an aerobic atmosphere (either air or... 

For experiments conducted at slower heating rates under adiabatic conditions, the accelerating-rate calorimeter (ARC) is the instrument of choice [13]. The ARC allows precise control of temperature and exposes the cell to more uniform conditions over longer periods. A typical experiment requires a few days rather than a few hours, as in the case of the heating block. Because of the adiabatic environment, the onset of self-heating due to chemical reactions in the interior of the cell can be detected with greater sensitivity. 

Differential scanning calorimeky (DSC) measurements were performed for the experimental determination of the glass kansition temperature T. The mean value of the onset and endset temperatures of the kansition was calculated, according to DIN 53 765 A20. A Mettler TA 8000 calorimeter with a TAS 811 system and a DSC... 

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