Thermal Analysis UserCom 19
Thermal Analysis UserCom 19; Table of Contents:
- DSC measurements at high heating rates - advantages and limitations
New in our sales program
- Kinetic studies of complex reactions. Part 2: description of diffusion control
- Curing Kinetics of phenol-formaldehyde resins
- Curing of powder coatings using UV light
- Quality assurance of polymeric molded parts by DSC. Part 1: Incoming materials
Kinetic studies of complex reactions Part 2: Description of diffusion control
Cross-linking reactions of «hot curing systems» are complex reactions. Depending on the reaction conditions, an initially chemically controlled reaction can change and become diffusion controlled. The reaction rate thereby decreases rapidly. The reaction almost stops. The reason for this is chemically induced vitrification as a result of which the material changes from a liquid to a glassy state.
In the first part of this study , the curing reaction of the a two-component epoxy resin consisting of the diglycidylether of bisphenol A (DGEBA) and diaminodiphenylmethane (DDM) as hardener or curing agent was investigated by DSC and evaluated using the Model Free Kinetics (MFK) software. Here it was shown that curing at heating rates greater than 1 K/min is chemically controlled over the entire reaction range.
By means of MFK, it is possible to correctly predict the course of an isothermal reaction right up to vitrification. If the glass transition temperature is known as a function of conversion, MFK can be used to estimate the time at which the material vitrifies in an isothermal reaction. MFK must be extended in order to describe the complete course of a curing reaction that includes the transition from chemically to diffusion-controlled kinetics [2-4]. In this article, MFK is enhanced sufficiently for it to take the effect of diffusion control on the reaction kinetics into account.
 J. Schawe, UserCom 18 (2003) 13.
 S. Vyazovkin, New J. Chem., 24 (2000) 913.
 S. Vyazovkin N. and Sbirrazzuoli, Macromol. Rapid Commun, 21 (2000) 85.
 J.E.K. Schawe, J. Thermal. Anal. Cal., 64 (2001) 599
Curing kinetics of phenol-formaldehyde resins
DSC and TGA are well-known techniques commonly used to perform kinetic studies of materials in thermal analysis (TA). Knowledge of the influence of temperature on the reaction rate of phenol-formaldehyde resins (PF resols) allows the polymerization rate to be predicted. This is of great practical importance for example in the manufacture of wood composite materials or for the storage of the resins.
Most of the resol cure kinetics reported in the literature have been obtained using the Borchardt-Daniels method. Here only one run at one heating rate (dynamic measurement) is necessary to obtain the desired kinetic parameters. The kinetic parameters are, however, usually heating rate dependent and care must be taken when evaluating and interpreting the results. Likewise, the activation energy is often assumed to be constant throughout the reaction. This is, however, not true for complex curing reactions such as occur in phenolic resins. A phenol-formaldehyde (PF) resin may be of low viscosity at the start of the run and fit the assumptions for dilute solutions. But as the curing reaction proceeds, the material undergoes gelation, i.e. changes from a liquid to a rubbery state, and possibly vitrifies (transition from a rubbery to a glassy state). The cross-linking reduces molecular mobility and results in the process changing from being kinetically controlled to diffusion-controlled .
The model-free kinetics (MFK) method, developed by Vyazovkin [3-5] is based on the assumption that the activation energy, E a , is dependent on the conversion (α). At a particular conversion, the activation energy, Ea, is independent of the heating rate. Evaluation by MFK requires at least three dynamic measurements performed at different heating rates. Since the reaction of phenol and formaldehyde under alkaline conditions to a PF resin is complex, MFK evaluation seems a suitable method to describe the curing behavior.
 He, G., Riedl B., Aït-Kadi A., J. Applied Polymer Science, 87 (2003), p. 433.
 Prime, R.B., in: Thermal Characterization of Polymeric Materials. Academic Press, San Diego, (1997) p. 1636-1646.
 Vyazovkin, S., Lesnikovich, A. 165 Thermochimca Acta (1990), p. 273.
 Vyazovkin, S. Thermochimca Acta 194 (1992), p. 221.
 Vyazovkin, S., N. Sbirrazzuoli. Macro- molecules (1996) 29, p. 1867.
Curing of powder coatings using UV light
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.
Quality assurance of plastic molded parts by DSC Part 1: Incoming materials
The article describes several practical applications that illustrate the use of DSC for the quality control of plastic molded parts. It explains how DSC can be used to identify materials and detect differences between batches. The influence of additives such as colorants and stabilizers is also discussed.
The quality of incoming plastic molding materials is often only assessed by characterizing their flow properties in the melt or in solution, in particular by measuring the melt flow index (MFI) or the viscosity index (VI) in solution. These rheological methods do not however identify the material or allow information to be obtained about melting and solidification behavior, although the latter is directly relevant to the properties of the molded parts produced. This article discusses different applications of DSC for the quality control of incoming materials using practical examples.
Part 2 of this series will deal with the use of DSC for process and production control.