Thermal Analysis UserCom 11

Introduction

In the first part of this work (UserCom 10), the basic principles of the glass transition as well as its measurement and evaluation were discussed. This second part describes a number of practical aspects.

A glass transition always requires the presence of a certain degree of disorder in the molecular structure of the material under investigation (e.g. amorphous regions). It is very sensitive to changes in molecular interactions. Measurement of the glass transition can therefore be used to deter- mine and characterize structural differences between samples or changes in materials. The following article presents a number of examples to illustrate the type of information that can be obtained from an analysis of the glass transition.

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Many of the pure starting materials used in the pharmaceutical industry and in food technology can be routinely analyzed and characterized with the help of melting point determination. The situation is quite different, however, for edible oils, fats, and waxes.

Thermal values

The variable composition and different crystal modifications of such products mean that they cannot effectively be characterized by one single thermal value, e.g. the melting point.

Nevertheless, at least for comparison purposes, a number of different procedures have been developed to obtain thermal values that can be easily measured in routine analysis, e.g. softening points, dropping points, slip melting points , melting point according to Wiley and Ubbelohde, etc.

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Introduction

In cement chemistry the following symbols are used for simplicity:

A for Al₂O₃ , C for CaO, H for H₂O, S for SiO₂ and S̅ for SO₃ . For example, tricalcium aluminate, 3CaO.Al₂O₃ becomes C₃A and gypsum, CaSO₄ .2H₂O, becomes CS̅H₂ .

The addition of water to Portland cement initiates the setting or hardening reaction, which binds the whole mass together. The hydration of Portland cement leads to the formation of different hydrates and is a very complicated process:

• Portland cement contains various components that take up water of crystallization at different rates.
• Many different hydrates, some of which are not stoichiometric, are formed.
• The degree of crystallinity of the hydrates is low.
  

In the first few hours after mixing water with Portland cement, C₃A reacts rapidly with the formation of a number of different calcium aluminum hydrates:

• 3CaO.Al₂O₃.6H₂O (C₃AH₆),
• 2CaO.Al₂O₃.8H₂O (C₂AH₈) and
• 4CaO.Al₂O₃.19H₂O (C₄AH₁₉)

The presence of calcium and sulfate in the aqueous phase (dissolved gypsum) causes C₃A to hydrate to ettringite (C₆AS̅₃H₃₂):

3CaO.Al₂O₃ + 3CaSO₄.2H₂O + 26H₂O ⇒ 6CO.Al₂O₃.3SO₃.32H₂O

C_3A + 3CS̅H₂ + 26H ⇒ C₆AS̅₃H₃₂

At the same time, a small amount of colloidal calcium silicate gel (CSH) is formed from the C₃S.

C₃S + nH₂O ⇒ C₃S.nH₂O (gel)

The interpretation of the thermogravimetric curves in the early stages of this hydration is made more difficult because the decomposition temperatures of CSH, ettringite and calcium sufate dihydrate lie close together.

The thermogravimetric measurements were performed with a METTLER TOLEDO TGA/ SDTA850. The adaptive event-controlled heating rate option (MaxRes [3 - 5]) was used to improve the separation of the dehydration processes.

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Introduction

Alternating differential scanning calorimetry (ADSC) is a DSC technique in which a periodically varying temperature is superimposed on a linear heating rate. In the case of a sinusoidal modulation of amplitude A_T and frequency ω, the heating rate, β, is described by the equation:

β = βo + AT cos (ωt)         (1)

In conventional DSC, the temperature program is defined by the initial and final temperatures and the heating rate. In ADCS, however, in addition to the underlying heating rate βo, there are two addi- tional parameters, namely the modulation amplitude A_T and the modulation frequency ω. These parameters must be carefully chosen in order to obtain meaningful information from the experiment (see also the article in USER COM 6).

The modulation of the heating rate results in a modulated heat flow signal, Φ. This modulated signal is subjected to Fourier analysis and separated into different components. One of these components is the total heat flow, which corresponds closely to the signal obtained from a conventional DSC measurement at a heating rate of βo. In addition, the curve of the complex heat capacity |Cp∗| is calculated according to the equation:

|Cp∗| =  AΦ / Aβ                 (2)

where AΦ and Aβ are the amplitudes of the heat flow and the heating rate respectively. The phase angle between the modulated heating rate and the modulated heat flow is also calculated. This allows certain assertions to be made about relaxation processes in the sample.

The use of ADSC allows the isothermal curing of epoxy resins to be investigated. Of particular interest in this respect are vitrification and the determination of the temperature-time-transformation diagram [2, 3]). This article describes how the ADSC technique can be used to investigate dynamic curing. Vitrification (liquid → solid transition) followed by devitrification (solid → liquid transition) can be observed on the heat capacity and the phase angle curves if the heating rate is sufficiently slow. The corresponding temperatures are determined from the |Cp*| signal and entered in the continuous heating cure diagram (CHT diagram). The CHT diagram shows the temperatures and times that are required to reach these transitions at various different constant heating rates (4). Analogous to the isothermal TTT diagram, the CHT diagram is used to investigate the properties and the influence of curing conditions on such resins.

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Literature

[1] C. T. Imrie, Z. Jiang, J. M. Hutchinson, Phase correction in ADSC measurements in glass transition, USER COM No.6, December 97, p.20-21
[2] S. Montserrat, Vitrification in the isothermal curing of epoxy resins by ADSC, USER COM No.8, December 98, p.11-12
[3] S. Montserrat, I. Cima, Thermochim. Acta, 330 (1999) 189

Introduction

Fibers are produced worldwide in enormous quantities. More than 20 million tons of synthetic fibers and 20 million tons of natural fibers are manufactured each year. The total length of these fibers corresponds to about 10 000 times the distance from the earth to the sun.

A characteristic feature of a fiber is that its length is much greater than its diameter. The great anisotropy of the microstructure and the physical properties originating from spinning and stretching processes are two of the main reasons for the special properties and peculiarities of fibers [1, 2]. Spinning, stretching and annealing are in fact the most important steps in the manufacture of fibers. These processes determine properties such as the modulus of elasticity (Young’s modulus, E) and toughness that are required for the application envisaged. Coloring properties, shrinkage (contraction of fibers) and thermal stability are determined by the size, number and orientation of the crystallites, as well as the molecular structure in the amorphous regions. Thermomechanical analysis (TMA) in particular, as well as DMA, DSC, TGA and TOA are all excellent techniques for the investigation of the effects of temperature and mechanical loading on fibers and yarns. They allow the relationship between structure, properties and the manufacturing process [3] to be investigated. Very often comparative measurements under identical conditions are sufficient to characterize transition temperatures, expansion and shrinking behavior. TMA measurements also yield numerical values such as the coefficient of linear expansion, Young’s modulus, E, and the force of contraction as a function of temperature.

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Literature

[1] L.H. Sperling, Introduction to physical polymer science, 2nd ed., Wiley- Interscience, New York (1992), p. 263.
[2] M. Jaffe, J. D. Menczel, W. E. Bessey, Chapter 7 in Thermal Characterization of Polymeric Materials, 2 nd ed. (E. A. Turi, Ed.), Academic Press, New York (1997) 1767 - 1954.
[3] ibid., Seite 1785.