Curing Behavior of Aqueous Resins Using High Pressure Crucibles - METTLER TOLEDO

Curing Behavior of Aqueous Resins Using High Pressure Crucibles

The curing behavior of an aqueous melamine-formaldehyde resin was investigated by DSC. Predictions were then made for the curing reaction at different isothermal temperatures using model free kinetics (MFK). The validity of the predictions was checked and confirmed by practical isothermal measurements.

Curing of Melamine-Formaldehyde

 

Introduction 

Melamine-formaldehyde resins are aqueous liquids that are used for making decorative surfaces on wood products and for coating laminates. The resins release water during the curing or so-called condensation reaction.

Optimum curing of the resins is crucial for ensuring that the final products exhibit perfect surface properties. This presents a challenging development task [1]. DSC investigations under optimized conditions followed by kinetic predictions can be very helpful for this work [2, 3].

 

Kinetics

Chemical kinetics, also known as reaction kinetics, is used to study the rate at which a chemical process occurs. This requires experimental data, mathematical relationships and verification of the results regarding the plausibility of predictions. The kinetics of a reaction describes how quickly the reaction takes place. The mathematical relationships allow the reaction behavior to be simulated so that the number of experiments can be reduced.

The most important application of kinetics in thermal analysis is the use of increased temperatures to accelerate reactions, reduce measurement times and from this predict reaction behavior - in other words, to determine how long a reaction takes to reach a desired conversion at a low process temperature [2, 3].

METTLER TOLEDO offers three software packages for kinetic studies:

  • so-called nth order kinetics
  • model free kinetitcs (MFK) and 
  • advanced model free kinetics (AMFK)

MFK is the most suitable for describing complex reactions. The aim is to predict reaction behavior at other temperatures and to optimize times for processes such as curing reactions, to assess storage properties, and to check thermal stability. The evaluations described in this article were performed using MFK [4]. At least three dynamic heating measurements are required to make predictions.

 

Experimental Details

Aqueous melamine formaldehyde resins are somewhat difficult to investigate because the water contained in a sample vaporizes before the actual curing reaction. In addition, water is also released during the curing process. If samples are measured in hermetically sealed 40-µL standard aluminum crucibles, the crucible lid ruptures above about 100 °C due to the vaporization of the water and the resulting increase in pressure.

The sudden rupture of the crucible of course causes an artifact in the DSC curve. The curing reaction begins at about 120 °C and releases more water. This water also vaporizes and gives rise to a vaporization peak in the DSC curve which overlaps the curve due to the curing reaction. Correct evaluation of the curing reaction is then no longer possible under these circumstances.

The problem was solved by performing the measurements in high-pressure crucibles. This suppresses the vaporization of the water so that a vaporization peak no longer occurs.

Sample specimens were measured between 5 and 280 °C at heating rates of 5, 10 and 15 K/min using a DSC 1. The sample container was shaken well each time to homogenize the sample before weighing about 10 mg into the crucible.

The following experiment was performed: The original aqueous sample was measured in 40-µL gold-plated high-pressure crucibles (maximum pressure 15 MPa, used only once) in the DSC 1 using a flow rate of 50 mL/min nitrogen.

The results were evaluated using Version 12 STARe software.

 

Results

Figure 1 shows the results for three fresh specimens of the original sample. The analysis process can be divided into four steps: measurements, conversion curves, MFK calculations, and predictions. In the first step, Figure 1 above left, the temperature range of the reaction and the baselines for the integration are defined. In the second step, above right, the conversion curves for the three different heating rates are determined by partial integration.

Conclusions

The curing behavior of an aqueous melamine resin sample can be easily characterized by DSC using high-pressure crucibles. The high-pressure crucibles allow the curing reaction to be separated from effects due to vaporization.

Predictions for the curing behavior can be made using MFK (model free kinetics). In particular, the time needed to reach a certain conversion at a particular temperature can be predicted. The results were checked using practical experiments at an isothermal temperature of 140 °C. The predictions agree well with the practical isothermal measurement.

 

Curing Behavior of Aqueous Resins using High Pressure Crucibles | Thermal Analysis Application No. UC 414 |  Application published in METTLER TOLEDO Thermal Analysis UserCom 41