Thermal Analysis UserCom 10

Thermogravimetric Analysis (TGA) is a well-established method that is used in quality assurance and quality control for the characterization of the thermal behavior of a very wide range of substances. With TGA alone, however, it is not possible to learn anything about the composition of the volatile substances evolved from a sample. The online combination of a TGA with a Fourier Transform Infrared Spectrometer (FTIR), however, enables both quantitative (TGA) and qualitative (FTIR) analysis to be performed simultaneously. The technique allows the substances evolved to be identified and correlated with the weight- loss steps detected by the TGA.

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Literature

[1] A .T. Riga and G. H. Patterson, Eds., Oxidative Behavior of Materials by Thermoanalytical Techniques, ASTM STP 1326, American Society for Testing and Materials, 1997
[2] H. Kopsch, Thermal Methods in Petroleum Analysis, VCH Verlagsgesellschaft, Weinheim,1995

Model free kinetics is based on an isoconversional computational technique that calculates the effective activation en- ergy (E) as a function of the conversion (α) of a chemical reaction, E = f(α). The variation of E = f(α), is not only of importance for reliable predictions, but also allows one to draw important mecha- nistic conclusions [1, 2]. For instance, the shape of the activation energy curve indi- cates directly whether a reaction is simple or more complex. For simple processes, E = f(α) is practically constant (horizon- tal line). Model free kinetics of thermoanalytically measured reactions, however, rarely gives a constant activation energy. The fact that a reaction is governed by a constant activation energy does not neces- sarily mean, however, that it is a single step reaction. Most probably it is a multi-step process that is controlled by the rate of the slowest step.

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Literature

[1] S.Vyazovkin, Int. J. Chem. Kinet., 28, 95 (1996).
[2] S.Vyazovkin, C. A. Wight, Annu. Rev. Phys. Chem., 48, 125 (1997)

Safety investigations with model free kinetics Hexel Composites is a company that manu- factures epoxy resin formulations at its production site in Duxford, Cambridge, UK. In order to assure safety in the chemical plant, a thorough understanding of the po- tential thermal hazards of these materials is essential. Recently, we started a program to assess the contribution that DSC kinetic data can make. In particular, we wanted to compare the results based on conventional nth order kinetics and the new model free kinetics (MFK) with the data obtained by direct measurement of the adiabatic behavior in hot storage tests (SPS6).

Introduction

Safety investigations frequently assume adiabatic conditions (worst case), in which an initial low level of exothermic energy slowly warms the reaction mass thereby causing the reaction rate to increase until the reaction finally "runs away". The temperature increases by an amount corresponding to the heat of reaction divided by the average specific heat of the reaction mass. At this temperature, organic substances can decompose and give rise to gases and vapors. The time taken to reach the maximum reaction rate (TMR, or as a symbol t_mr ) is of great importance. This is also sometimes called the intervention time because up until this time an intervention (e.g. cooling) can still successfully prevent an uncontrolled reaction. The TMR can be read off from the curve of the adiabatic temperature increase (point of inflection). The higher the adiabatic starting temperature chosen for the measurement, the shorter the TMR.

Adiabatic hot storage testing is more difficult and expensive to conduct than DSC measurements because it requires both large amounts of sample (up to 1 kg) and specially equipped laboratories to cope ecologically with the fumes that are given off. An alternative to hot storage testing could save much time and expense.

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Introduction

The glass transition is a phenomenon that can in principle occur in all noncrystalline or semicrystalline materials. The requirement is a sufficiently large degree of molecular disorder at least in one direction. To explain the processes that take place during a glass transition we assume for simplicity that we are dealing with a homogeneous liquid.
In a liquid, in addition to the molecular vibrations and rotations (of atoms or groups of atoms) that also occur in solids, there are cooperative movements or rearrangements in which several molecules or segments of molecular chains participate. The cooperative units can be regarded as tempory clusters that fluctuate with regard to both space and time. The size of these cooperative units is typically a few nanometers. This characteristic length decreases with increasing temperature. Another characteristic quantity is the time required for the cooperative rearrangements to take place. It can be described by an internal relaxation time τ.
The glass transition is very sensitive to changes in molecular interactions. Measurement of the glass transition can be used to determine and characterize structural differences between samples or changes in a sample. The glass transition is therefore an important source of information that can be obtained from the thermal analysis of materials. This first article discusses a number of basic principles that aid the interpretation of results. Practical aspects of the glass transition will be dealt with in Part 2 in the next edition of UserCom.

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Introduction

Parts that are made by the injection molding of fiberglass reinforced polymers usually have expansion coefficients that are direction dependent. The sample investigated here was a machine component (shaft) made of fiberglass filled polyphenylene sulfide (PPS). For the construction of the machine, it was important to know the the expansion coefficients of the shaft in the axial and radial directions. The information was quickly and easily obtained by TMA measurements.

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Introduction

The regions in which the various phases of a binary mixture are in equilibrium can be described by a so-called two-component phase diagram in which the temperature is plotted as a function of composition. The term eutectic melting diagram is also used if we are dealing with solid-liquid transitions. There are in fact 12 different basic types of two-component melting diagrams (see for example [1]). In practice, however, we of- ten encounter euctectic systems whose two- component phase diagrams are of the type shown schematically in Figure 1.

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References

[1] Landolt-Börnstein, Schmelzgleichgewichte , 6th Edition, Volume II/ Part 3, page 1

Thermosets (or thermosetting plastics) is the term used to describe a group of hard and amorphous plastics that remain rigid up until their degradation temperatures. They consist of close-meshed cross-linked macromolecules and cannot therefore melt or be dissolved.

Phenolics and aminoplastics, epoxy resins (EP resins) and unsaturated polyester resins (UP resins) are examples of thermosetting materials. The former are polycondensation products whereas the latter are made by polyaddition and polymerization. The precursor products are known as thermosetting resins. Following the addition of additives (hardeners, accelerators, fillers, etc.), they react or cure to form thermosets.

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