Curing of Powder Coatings Using UV Light - METTLER TOLEDO

Curing of Powder Coatings Using UV Light

Introduction

Today, powder coating technology is applied to a wide range of different materials (wood, plastics, metals, etc.).

Besides the excellent properties of such coatings, their use also offers important ecological advantages. For example, unlike liquid paints, no solvents are used so that only negligible amounts of volatile organic compounds (VOCs) are released into the atmosphere.

The powder coating is usually sprayed onto the parts to be coated and then cured. The curing or cross-linking process is then performed either thermally in an oven (typically at about 180 °C) or by means of UV light. Curing with UV light has the great advantage that materials sensitive to temperature (such as wood or plastic products) can be coated.

In practice, a combined IR/UV processing line is used for the UV curing of powder coatings. In the IR zone, the powder «melts» under the influence of the infrared (IR) light and forms a homogeneous film on the substrate to be coated.
The liquid film so formed is then cured in the UV zone within seconds or minutes. This article describes how DSC can be used to investigate the UV curing behavior of powder coatings. 

 

Experimental Details

Design of the UV-DSC System

The measurements were performed with a METTLER TOLEDO DSC822e equipped with an accessory that allowed the sample to be exposed to UV light. The UV-DSC system is pictured in Figure 1 and a schematic diagram is shown in Figure 2. The light source used was a Hamamatsu «Lightningcure 200» system. The spectrum of the built-in mercury-xenon lamp is shown in Figure 3.

 

 

Sample Preparation and Evaluation of the Measurements

The sample investigated was a commercially available powder coating material used as a clear primer for wood fiberboard and chipboard surfaces. Typically 8.5 mg of the material was spread evenly over the bottom of the crucible forming a layer about 0.8 mm thick. In each case, the samples were held for 25 minutes at the temperature at which curing was to be performed. During this time interval, the sample was exposed to UV light for 15 minutes. The degree of cure was afterward checked by measuring the glass transition temperature of the sample in a separate heating experiment at 10 K/min.

The baseline during the curing process is of course required to determine the enthalpy of cure. To a good approximation, this unknown baseline can be determined by repeating the measurement with the cured sample under exactly the same conditions (including UV exposure). Another possibility is simply to integrate the exothermic curing peak during the UV exposure interval using a horizontal baseline. Figure 4 shows that the two methods in fact yield similar results. For our experiments, only the second method was used.

 

Measurements and Results

With UV curing, three basic questions arise:

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

The first two questions are investigated in more detail in the following sections. The latter point is discussed in the final conclusions.

 

Influence of Temperature and UV Light Intensity

To investigate the influence of temperature and UV light intensity on the curing reaction, isothermal DSC curves were measured at four different temperatures using three different UV light intensities. An example is shown in Figure 5. The figure displays curves of samples that were measured at 100, 110, 120 and 130 °C while being exposed to a UV light intensity of 1.12 mW/cm2 (falling on the sample). The results show that temperature does not have any appreciable influence on the enthalpy of cure at this level of light intensity. This also means that glass transition temperatures measured after curing are independent of temperature. At higher temperatures, however, the maximum heat flow increases and the peak width decreases, i.e. the curing or cross-linking reaction takes place more rapidly.

Conclusions 

Powder coating technology with UV light curing is used to apply high-quality coatings to the surfaces of temperature-sensitive materials. The influence of temperature, UV light intensity, and exposure time on the curing process can easily be determined using a DSC822e equipped with a UV accessory. The experiments clearly showed that temperature has a negligible influence on the actual curing or cross-linking process for the powder coating system studied here. Temperature does however have an important influence on the flow properties of the uncured coating material and hence on the homogeneity of the coating produced in the cross-linking reaction. A number of other studies have established that the flow behavior of this powder coating at 110 °C is optimal for the UV cross-linking process. UV-DSC experiments showed that about 5 minutes were needed to attain an adequate degree of cross-linking at this temperature and with a light intensity of 1.12 mW/cm2. If the UV light intensity is doubled, the exposure time is reduced to about 3 minutes. There is however a limit to the possible reduction in UV exposure time: light intensities that are too high lead to incomplete curing of the powder coating. On the other hand, UV exposure times that are too long lead to an increase in temperature on the surface of the substrate. This inevitably results in the coating having poorer mechanical properties.

Curing of Powder Coatings Using UV Light | Thermal Analysis Application No. UC 194 | Application published in METTLER TOLEDO Thermal Analysis UserCom 19