Thermal Analysis UserCom 29
Table of Contents:
- Analytical measurement terminology in the laboratory. Part 1: Trueness, precision and accuracy
New in our sales program
- New Excellence Melting Point Systems
- Better results in thermal analysis thanks to validation
- Determination of phase transitions with simultaneous video observation
- Measurement of thin films in shear by DMA
- Vitrification during the isothermal cure of a thermoset studied by TOPEM®
- Analysis of the foaming behavior of a fire retardant by TMA and TGA
Determination of phase transitions with simultaneous video observation
The melting point is without doubt the thermal value most frequently used to characterize materials. This fact together with ever-increasing requirements for melting point determination were the two main reasons why METTLER TOLEDO decided to develop a completely new series of instruments. The new Excellence Melting Point Systems allow substances to be analyzed that could previously not be measured by conventional melting point instruments. The following article presents a number of different examples.
The melting point
The melting point is a characteristic property of a substance. It is the temperature at which the crystalline phase changes to the liquid state. A pure substance normally has a sharp melting point, wherea s an impure substance melts over a temperature range that is lower than the melting point of the pure substance. This effect is well known and called the melting point depression. Some organic compounds melt and decompose simultaneously. This makes it difficult to determine an exact melting point. Melting can also occur over a relatively wide temperature range. One then refers to a melting range rather than a melting point. This effect is especially observed with polymers.
In general, melting point determination is used in research and development as well as in quality control to identify and check the purity of substances.
Measurement of thin films in shear by DMA
Thin films with a thickness of 50 to 200 μm are usually measured in tension in the DMA. They can, however, also be measured in shear if proper attention is paid to sample preparation and other factors. In this article, we present two examples to show how this is done.
In the DMA, a sample undergoes periodic deformation. However, the force necessary to deform the sample acts not only on the sample but also on the sample holder. This means that the measured displacement amplitude is the sum of the deformation of the sample and the deformation of the sample holder. Ideally, the deformation of the sample holder should be negligible compared with the deformation of the sample.
When thin samples (thickness < 0.2 mm) are loaded in the shear sample holder, the danger is that the shear clamping plates tilt slightly and touch each other.
The results from a DMA measurement performed under these conditions are then completely wrong. To make sure the plates do not touch each other, one intuitively tends to measure thin samples with large diameters. The stiffness of such samples might then be greater than the stiffness of the sample holder. In such cases, the deformation of the sample holder contributes more to the total deformation than the deformation of the actual sample. When the modulus is calculated, the measured total deformation or stiffness must be corrected. This is done using the so-called stiffness correction.
Vitrification during the isothermal cure of a thermoset studied by TOPEM ®
Vitrification occurs during the isothermal cure of a crosslinking system if the cure temperature is below the glass transition temperature of the fully cured resin. The process of vitrification was studied using TOPEM® , a new temperature-modulated DSC technique. The results are compared with those obtained from conventional temperature-modulated DSC (ADSC). The comparison showed that TOPEM® offers several important advantages.
When an epoxy resin reacts with a hardener (crosslinking agent), the system changes from a viscous liquid (with an initial degree of conversion, α = 0) to a highly crosslinked molecular network (with a final degree conversion, α ≤ 1).
The reaction rate or rate of cure is initially controlled by chemical kinetics. The unreacted mixture has a glass transition temperature, Tg0 . As the curing reaction proceeds, the glass transition temperature, Tg , of the system increases parallel to the increase in the degree of crosslinking. If the curing temperature is sufficiently high, the crosslinking reaction proceeds to its limit, α = 1, and the final glass transition temperature, Tg∞ , is that of the fully cured thermoset.
On the other hand, if the curing process takes place at a temperature below Tg∞ , at some point the system changes to a glassy state and vitrifies. In fact, vitrification occurs when Tg reaches the isothermal cure temperature, Tc . Under these circumstances, molecular mobility is greatly reduced and the reaction becomes diffusion controlled. The reaction rate slows and the degree of conversion, α , reaches a final value that is less than 1. The time at which the reaction kinetics change from being mainly chemically controlled to diffusion controlled is known as the vitrification time.
 Schawe, J.E.K., Hütter, T., Heitz, C., Alig, I., Lellinger, D.: Stochastic temperature modulation: A new technique in temperature-modulated DSC, Thermochim. Acta, 446 (2006) 147–155.
Analysis of the foaming behavior of a fire retardant by TMA and TGA
What are intumescent fire-retardant coatings?
Intumescent fire-retardant paints or coatings are designed to protect the surfaces of materials from the effects of fire and heat. On exposure, intumescent systems first swell to a thick, robust, carbonaceous foam and then form a char. The closed foam structure inhibits the transport of oxygen to unburned regions beneath the char and provides a physical barrier to heat and mass transfer. Intumescent fire-retardant coating systems usually consist of a polymeric binder, polyphosphororic acid compounds (as an acid donor), carbon-rich polyhydric compounds (as a carbon donor) and a blowing agent (for gas formation). They react under the action of heat and at a certain temperature begin to foam  .
The basic mechanism can be described as follows: Decomposition of the acid donor supplies acid, which catalyzes the dehydration of the carbon donor. The mixture melts and the blower decomposes, producing gases and causing the liquid to foam. The foam simultaneously solidifies through crosslinking into a three-dimensional carbon structure and forms a char under the action of heat. The formulation is carefully designed to ensure that the different events occur in the proper order and so ultimately produce an effective char. The amount of foam produced during the intumescence reaction depends on the number of carbon atoms while the number of hydroxy-groups determines the reaction rate of the dehydration process and thereby controls the rate of foam formation 
 (WO/1993/011196) COMPOSITION WITH INTEGRATED INTUMESCENT PROPERTIES
 Troitzsch, Jürgen; International plastics flammability handbook: principles, regulations, testing, 2 nd. Ed. München [u.a.]: Hanser, 1990
 F. Laoutid, L. Bonnaud, M. Alexandre, J.-M. Lopez-Cuesta, Ph. Dubois; New prospects in flame retardant polymer materials: From fundamentals to nanocomposites; Materials Science Engineering R, 2008