UV Curing Measured by DSC - METTLER TOLEDO

UV Curing Measured by DSC

Purpose

The purpose of this application is to show how UV curing systems can be investigated by DSC. 
In UV curing, three main questions arise:

  1. How long does the sample have to be exposed to UV light to achieve an adequate degree of cure or crosslinking?
  2. What influence do the temperature and light intensity have on the curing process?
  3. What are the optimum parameters for the crosslinking reaction?

The following section describes the technique of photocalorimetry (here UV-DSC). The questions outlined above will be addressed in more detail Volume 2.

Sample

The sample investigated was a commerically available powder coating material that is used as a transparent primer especially on medium density fiberboard (MDF) and chipboard.

Conditions

Measuring cell: DSC with air cooling and photocalorimetry accessory

Pan: Aluminum 40 μL, without lid

Sample preparation: The powder sample (8.42 mg) was spread evenly over the bottom of the crucible, forming a layer about 0.8 mm thick.

DSC measurement: The sample was held isothermally for 25 minutes at the curing temperature. During this time interval, the sample was exposed UV light of a defined intensity for 15 minutes.

Atmosphere: Static Air

Design of the UV-DSC

The DSC was equipped with an accessory that allowed the sample to be exposed to UV light. The design and geometry of the photocalorimeter system is shown in Figure 1. The light source emits UV light mainly in the so-called UV-A region (315-400 nm). The UV light emitted in the 290-315 nm region is referred to as UV-B light. UV light of still shorter wavelengths is called UV-C light. Other light sources can be used for other applications, e.g. halogen light. 


Figure 1. Optical setup for projecting UV light into the DSC

Figure 2. Determination of the incident light intensily from DSC curves. The shutter of the light source was opened and closed manually.

The light intensity on the sample side was determined using carbon black to ensure that the light was 100% absorbed. The carbon black to ensure that the light was 100% absorbed. The carbon black must cover the bottom of the crucible as evenly as possible. The DSC cell is held isothermally at any desired temperature. After constant equilibrium heat flow has been reached, the light source is switched on for a few minutes while the reference sides remain covered. A step evaluation of this curve yields the heat flow generated through the UV light absorption by the sample (Fig. 2.)

Conclusions 

An accessory to the DSC enables a sample to be exposed to UV or visible light during a measurement (photocalorimetry). This allows the influence of exposure time, light intensity and temperature on the curing process to be investigated. The optimum parameters needed for a sample to achieve an adequate degree of crosslinking can then be determined. 

UV Curing Measured By DSC | Thermal Analysis Handbook No.HB14| Application published in METTLER TOLEDO TA Application Handbook Thermosets, Volume 1

 
 
 
 
 
 
 
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