The influence of UV light absorption and oxygen inhibition on photo DSC results

A wide variety of analytical techniques have been used over the years to aid research and development work in the field of UV curing. Many have come and gone but today two techniques have stood the test of time and are extensively used; real-time infrared spectroscopy 1 and photo 2 (Differential Scanning Calorimetry). The advantages and limitations of RTIR are now well understood and many groups have adapted the technique further, for example coupling to an ATR (attenuated total reflectance) IR accessory to study surface . In contrast, photo DSC, which is the subject of this paper, is used as a relatively routine tool to study the reactivity of new , but with many of the sample preparation and experimental procedures not having been closely studied since its first use in the late 1970's. The use of photo DSC has been popular because of the simplicity of the technique and the very short run times used, allowing a great deal of work to be done in a short space of time. In addition, the flexibility of the technique has shown itself to be a major advantage, allowing the investigation of variables such as irradiation wavelength, temperature, formulation and atmosphere. Although photo DSC is widely used there are relatively few commercially available instruments, with those from Perkin Elmer (used throughout the work reported here) and TA Instruments being the only notable examples. Most are in fact "home-made", being engineered from an existing DSC/DTA by fitting quartz windows and a UV light source of some description directed through them at the sample. The design of the systems varies widely in terms of light source and transfer optics, with some systems being based on UV
a monochromator or band pass/neutral density filters. Photo DSC is clearly a valuable tool that if used correctly gives outstanding results. However, it has now been observed that the technique is very sensitive to a number of sample preparation and experimental variables. As such, if experiments are designed to suit the research programme or sample type rather than to fit in with the strengths of the instrument/technique then potentially misleading results can be obtained with little obvious evidence that this is the case. It is this aspect of the technique which will be explored in this paper by concentrating on the two most significant factors influencing results; UV light absorption and oxygen inhibition.