UV Curing of a Cycloaliphatic Epoxy Resin using TOPEM and Conventional DSC - METTLER TOLEDO

UV Curing of a Cycloaliphatic Epoxy Resin using TOPEM and Conventional DSC

The curing of materials with (ultraviolet) light is often performed at relatively low temperatures, for example even at room temperature.

Under these conditions, the material can vitrify. Depending on the temperature, curing slowly continues in the glassy state.

The article shows how these processes can be investigated using TOPEM® and conventional DSC measurements.

 

 

Introduction

UV-curing paints and varnishes, coatings, and adhesives are nowadays widely used. The main practical advantages of such systems are that they cure within a few seconds and produce high quality coatings at relatively low temperatures (even at room temperature) so that the substrate is subjected to only low levels of thermal stress. In addition, there are important ecological advantages, for example no solvent emission and no drying processes with consequent energy consumption.

Difficulties arise only with the curing of pigmented paints and varnishes because the pigments also absorb UV light, which can lead to incomplete curing.

In this article, we show how isothermal curing processes of light-curing resins can be measured by DSC. We investigated the influence of temperature on the degree of cure determined from dynamic postcuring experiments. Furthermore, using TOPEM® , we describe the behavior of the resin immediately after exposure to the UV light.

 

Experimental Details

The resin chosen for this study was the aliphatic epoxy resin UVR-6107 (Dow) to which a cationic photoinitiator UVI- 6976 (Dow) was added to promote rapid crosslinking under UV light. This resin is mainly used for coating metals and as an overprint varnish. 

The measurements were performed using a METTLER TOLEDO DSC 1 equipped with a photocalorimeter accessory that allows a sample to be exposed to UV light for certain times [1]. The measurements were performed in open 40-µL aluminum crucibles. The sample mass was typically about 7 mg. The light intensity was about 400 mW/cm2 . Three different types of measurements were performed.

  1. Determination of the degree of cure at different temperatures: The sample was first heated to the desired curing temperature and allowed to stabilize for 3 minutes. The UV light source was then switched on and the sample exposed to UV light for 4 minutes. The sample was then held for a further 6 minutes at the curing temperature and again exposed to UV light for a further 4 minutes. Finally, the cured sample was heated twice from 30 °C at a heating rate of 10 K/min. 

  2.  Dynamic postcuring experiments using TOPEM® : The sample was first cured with UV light as described above under 1. The postcuring enthalpy was then measured in a TOPEM® experiment at a heating rate of 0.8 K/min.
  3.   Investigation of the curing behavior at different temperatures using TOPEM® : The isothermal “aging behavior” of the samples after exposure to UV light was investigated by increasing the measurement time to 4 hours after UV exposure.

 

Results

Determination of the Degree of Cure of the Sample After UV Exposure using Conventional DSC

Figures 1 and 2 summarize the measurement results. Figure 1 shows the results of isothermal measurements with UV light at different curing temperatures. The measurement curves shown are difference curves between the first and second exposures, shown in the example in the upper right diagram in Figure 1. The peak area corresponds to the reaction enthalpy (∆HUV) produced in the curing reaction under the action of the UV light. The curves indicate that the reaction is practically complete after about 0.5 minutes. The high rate of reaction results in large maximum heat flows, for example at 120 °C almost 400 mW. These high heat flows (and the reaction enthalpy) lead to a short-term increase in the temperature of the sample during the reaction. For example, in the experiment at 120 °C, the temperature increase was about 15 K. The curves show that the reaction enthalpy (∆HUV, peak area) becomes larger with increasing temperature.

Figure 2 shows the results of heating measurements performed on the UV-cured samples. The peak area corresponds to the reaction enthalpy of thermal postcuring, ∆HPC. One immediately sees that the postcuring enthalpy, ∆HPC, of the samples cured at low temperatures is significantly larger than that of the samples cured at higher temperatures. Furthermore, the curves differ in shape and in their location on the temperature axis. It is apparent that thermal postcuring begins at about the temperature at which the light curing was performed. This indicates that the material vitrifies during the light curing. The degree of cure, αc, after UV exposure can be calculated from the ratio of the reaction enthalpy produced during UV exposure (∆HUV) and the total enthalpy involved in the curing reaction (∆HPC + ∆HUV): 

Curing Kinetics of EVA using DSC, DMA and Model Free kinetics | Thermal Analysis Application No. UC 313 | Application published in METTLER TOLEDO Thermal Analysis UserCom 31