Biannual Thermal Analysis Application Magazine, Volume 10
用户通讯

Thermal Analysis UserCom 10

用户通讯

UserComs are biannual application journals intended for all users of thermal analysis

Thermal Analysis UserCom 10
Thermal Analysis UserCom 10

Thermal Analysis UserCom 10; Table of Contents:

TA Tip

  • DSC purity determination

New in our sales program

  • TGA-FTIR interface

Applications

  • TGA-FTIR combination for the investigation of sealing rings
  • Oxidative stability of petroleum oil fractions
  • What can model free kinetics tell us about reaction mechanisms?
  • Safety investigations with model free kinetics
  • The glass transition from the point of view of DSC measurements: basic principles
  • Determination of the expansion coefficients of an injection molded machine part
  • Two-component phase diagram
  • Crosslinking and degree of cure of thermosetting materials

TGA-FTIR combination for the investigation of sealing rings

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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Oxidative stability of petroleum oil fractions

IntroductionThe determination of the oxidative induction temperature is a rapid method for assessing the stability of petroleum oil fractions. The same method can be used to measure the effectiveness of stabilizers. Besides this, aging processes can also be investigated. Standardized isothermal methods are often used instead of dynamic methods (e.g. ASTM D 5483 and ASTM E 1858). Analysis under high pres- sures of oxygen prevents the vaporization of volatile components and increases the rate of oxidation, thereby shortening the measurement time [1, 2].
SamplesDiesel oils of the following petroleum fractions: Light (LGO), Light Cycle (LCGO), Light Vacuum (LVGO), Vacuum1 (VGO1), Vacuum2 (VGO2), Vacuum3 (VGO3) and Kerosine.
Information expectedComparison of the products with respect to their stability in oxygen.
Measurement parametersMeasuring cellDSC27HP / TC15
 CrucibleAluminum 40 μl, with pierced lid (1 mm hole)
 DSC measurementPut the measuring cell under an atmosphere of oxygen and heat from 40 °C to 150 °C at 20 K/min (to save time), then continue up to 350 °C at 5 K/min. The measurement is automatically terminated when the value of the exothermic DSC signal reaches 10 mW (the combustion peak is not of interest).
 AtmosphereOxygen at 3 MPa, no purge gas.

 

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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

What can model free kinetics tell us about reaction mechanisms?

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

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 tmr ) 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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The glass transition from the point of view of DSC measurements; Part 1: basic principles

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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Determination of the expansion coefficients of an injection molded machine part

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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Two-component phase diagram

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.

Figure 1: Below the solidus line both substances exist in separate crystalline forms in the solid state. If the temperature of a mixture of the two components is slowly raised, a portion of the sam- ple melts at the melting point of the eutectic. In this liquid mixture phase, either pure A or pure B in the solid state is also present depending on whether one is on the left or the right side of the eu- tectic composition. On further heating, the remainder of the solid phase melts until finally the whole sample has completely melted at a temperature corresponding to the initial composition of the mixture x B . Above the liquidus line there is only one phase (homogeneous melt).
Figure 1: Below the solidus line both substances exist in separate crystalline forms in the solid state. If the temperature of a mixture of the two components is slowly raised, a portion of the sam- ple melts at the melting point of the eutectic. In this liquid mixture phase, either pure A or pure B in the solid state is also present depending on whether one is on the left or the right side of the eu- tectic composition. On further heating, the remainder of the solid phase melts until finally the whole sample has completely melted at a temperature corresponding to the initial composition of the mixture x B . Above the liquidus line there is only one phase (homogeneous melt).

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References

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

Crosslinking and degree of cure of thermosetting materials

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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