Thermal Analysis UserCom 20

The change in mechanical behavior of more than 10 orders of magnitude is discussed using an isothermal curing reaction as an example. The material passes through several different states (the low viscosity Newtonian liquid, non-Newtonian liquid, gel and glassy states). Such changes can now be measured over the complete range using a DMA/SDTA861^e and a Haake RheoStress 600.

Introduction

The mechanical properties of materials can change by many decades as a function of temperature or chemical structure. For example, when ice melts it changes from an elastic solid to a liquid with a viscosity of about 2 mPa·s. This represents a change of more than 12 decades in the mechanical properties. Similar changes of mechanical behavior occur with polymers. The two extremes in this case are the glassy state and the melt at high temperature. But reactive materials such as adhesives and polymeric coating materials can also exhibit changes in mechanical behavior of the same order during the reaction.

Knowledge of the mechanical properties provides information about structure and molecular interactions. In practical applications, for example adhesives or the production of composites, it is very important to have information about flow behavior, gelation and vitrification. To obtain this data, the mechanical quantities have to be determined over a range that is as wide as possible. From the point of view of measurement, this presents a problem because the change in material properties occurs on a logarithmic scale, but the sensors used in measuring instruments have a linear resolution. The classical solution to this problem is to use several different instruments and different sample geometries, whereby one instrument typically covers 3 to 5 decades with one sample geometry. If measurements are required that cover a large range, several measurement curves have to be recorded that correspond to one another reasonably well in the range of overlap. This problem was addressed in the development of the METTLER TOLEDO DMA/SDTA861^e and particular attention was paid to expanding the measurement range. In the shear mode, the DMA can determine modulus changes of up to 8 decades in one measurement using the same sample geometry. This makes it possible to determine the mechanical behavior of a material from the glassy state to below the gel point in a single measurement. To obtain measurement data in the range from the gel point through to the low viscosity liquid state, the DMA measurements can be combined with rheometric data (measured with the Haake RheoStress 600). In this case, the large overlap range of the two instruments is very advantageous.

This study shows how the results from a METTLER TOLEDO DMA/SDTA861^e and the Thermo Electron Haake RheoStress 600 can be combined to determine the modulus change over 10 decades using the isothermal curing of an epoxy resin as an example.

As model substance, a two-component stoichiometric epoxy resin mixture consisting of the diglycidylether of bisphenol A (DGEBA) and diaminodiphenylmethane (DDM) as hardener or curing agent was used.

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This article describes several applications that illustrate the use of DSC for the quality assurance of plastic molded parts in process control. It shows how process parameters such as the temperature of the polymer mass (melt temperature), the mold temperature, and the time the polymer spends in the melt (dwell time in the melt) can be optimized and controlled by DSC. Furthermore it describes how the use of recycled material and the quality of molded parts can be evaluated using DSC measurements. Part 1 was published in UserCom 19.

Introduction

Although the quality of technical plastic products is primarily influenced by their morphology (microstructure of the materials used), in practice, quality assessment very often only consists of determining the dimensional accuracy of the parts. In other words, a plastic part meets the quality requirements if its dimensions are within the specified tolerances. However, to satisfy the quality profile for plastic parts more closely, it is often necessary in quality assurance to include parameters that characterize the morphology of the parts. A relatively simple and rapid way to determine these parameters is to use differential scanning calorimetry (DSC). In the first part of this article series, we discussed applications of DSC for the incoming goods control of polymers. This second part covers practical examples of the use of DSC in process control.

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Introduction

Chemiluminescence (CL) is the term used to describe the emission of (usually) visible light as a result of a chemical reaction. In the 1960s, different polymers were also studied with respect to this phenomenon. Under natural conditions, most polymers degrade or decompose as a result of oxida- tion due to atmospheric oxygen. In a first step, unstable alkyl radicals are formed through the action of heat, mechanical stress or the effect of light. The radicals then react with oxygen to form peroxide radicals. Peroxide radicals can also be produced during the production of plastics and be present in the plastic as an undesirable secondary product. In the presence of oxygen, the peroxide radicals accelerate the decomposition of the polymer through a chain reaction mechanism.

The step of the reaction in which chemiluminescence occurs has not been fully clarified. A mechanism often described in the scientific literature assumes that it takes place when two peroxide radicals recombine, whereby, besides oxygen, an excited carbonyl radical is formed (Russell mechanism [2]). Chemiluminescence measurements allow specific studies to be made on the oxidation of polymers and hence on the influence of stabilizers. This article shows how this can be done using an HP DSC827^e .

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Literature

[2] G. A. Russell, J. Am. Chem. Soc. 79, 3871, 1956

Introduction

DSC is an excellent tool for the analysis of phase transitions in materials. It can quickly and easily measure the temperatures at which melting and crystallization processes, and solid-solid and liquid crystal transitions occur. The results are displayed as exothermic or endothermic peaks in so-called DSC curves or thermograms. On the basis of the DSC curves alone, one does not of course know how the thermal events relate to the structure of the material. To obtain this information requires the use of an optical display system.

Hot-stage microscopy is a technique that is widely used for the characterization of thermal transitions. The possibility of directly observing the morphological change of the sample as it is heated makes it much easier to interpret the DSC curve in question. Changes in the shape and structure of crystals are seen as well as their size and number. Hot-stage microscopy also has the advantage that the sensitivity of the system is not influenced by different heating rates.

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Sample changers are often used in thermal analysis when large series of samples have to be measured. The advantage is that the samples can be prepared all together at the same time and the analyses performed overnight or even over the weekend. Especially in lengthy thermogravimetric experiments, the samples ready for measurement on the turntable are however exposed to atmospheric conditions for longer periods. During this time the samples may lose volatile components such as moisture, or conversely, if hygroscopic, absorb moisture. Both processes have a direct influence on the accuracy of the analytical results. There are two ways to prevent such effects from occurring with samples stored on the turntable of the sample robot:

1. The samples are hermetically sealed in aluminum crucibles and the lid is not pierced until just before the measurement. To do this, the sample robot must be equipped with the crucible lid piercing kit (see Fig. 1). This method is particularly suitable for the determination of moisture, for example in pharmaceutical substances.
   
2. The samples are stored in ceramic crucibles with special aluminum lids. The sample robot removes the lid during the measurement.   

The following experiments performed with polyamide 6 illustrate the effect of sample storage in an open crucible and demonstrate the importance of the crucible lid piercing kit.

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