Simultaneous measurement of light and heat flow
Chemiluminescence (CL) is the term used to describe the emission of light as a direct result of a chemical reaction. CL originating from oxidative degradation processes in polymers was reported in the early 1960s. Technological developments have resulted in CL becoming a sensitive and well-established technique that is used for prescreening tests.
- CL is very selective – CL occurs only as a result of the oxidative decomposition reaction of the sample
- CL is extremely sensitive – measurements performed at lower temperatures are of greater practical relevance
- Unique imaging possibilities – allow the initiation and propagation of oxidative degradation to be studied
A good example is the testing of polymer additives, for instance to investigate the effect of stabilizers on the oxidative degradation of polymers. In the food and pharmaceutical industries, CL is mostly used to obtain information about the stability of various products such as oils or fats. Such studies are of importance for the conservation of food and pharmaceutical products.
Further features and benefits
- Simultaneous CL and DSC measurements – DSC provides complementary information on thermal effects such as glass transitions and melting
- Modularity – an HP DSC 2+, HP DSC 1 or an HP DSC827e can easily be upgraded to include CL capability
- High-pressure CL – high pressures dramatically accelerate oxidative degradation and suppress vaporization
| ||Isothermal OIT of a synthetic motor oil by chemiluminescence|
The determination of the oxidation stability of oils is an important test used in the petrochemical industry. It enables the behavior of an oil to be predicted under actual operating conditions such as in a motor vehicle engine. The diagram shows the DSC and CL measurement curves of a synthetic motor oil recorded simultaneously at a temperature of 200 °C at a pressure of 10 bar (1 MPa). Both curves show a steep increase after an induction time of about 66 minutes due to oxidation of the oil. A DSC signal could not be detected prior to the induction time. However, the CL data shows that weak CL does in fact occur before the induction time, and can be easily detected from the beginning of the measurement. The presence of weak CL most likely indicates that the stabilizers are oxidatively degraded. Once the stabilizers have been fully consumed (after 66 min), the CL curve begins to increase in the same way as the DSC curve. Due to surface tension effects, the oil and hence the CL are concentrated around the circumference of the crucible.
|Dynamic OIT by chemiluminescence and DSC of PET|
PET measured under nitrogen exhibits a glass transition, cold crystallization and finally melting. In oxygen, oxidation of the sample also occurs. The precise onset of oxidation is however difficult to determine because the DSC curve is the sum of several different effects. CL is however only observed when the sample oxidizes. The CL intensity curve shows that the onset of oxidation is at about 234 °C, which is well before the sample has melted. The shoulder at about 250 °C in the CL curve is caused by the sample melting and “flowing”, which affects the CL emission. This can be clearly seen in the CL images taken during the experiment. Before “flowing” occurs, the CL emission originates from all over the base area of the crucible. After melting, the sample, and hence the CL emission, are concentrated around the circumference of the crucible.