Thermal Analysis UserCom 37

“Horst die Hüpfknete” is the German name for a make of bouncing modeling putty used as a toy for children. The material exhibits two quite different types of behavior at room temperature: On the one hand, it can be easily shaped and stretched and flows like a viscous liquid. In contrast, if it is rolled into a ball and allowed to fall to the ground, it behaves elastically and bounces back up like a tennis ball.

Introduction

How can this most unusual behavior be explained? – In this article, we will try to answer this question.

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This article describes how the curing behavior of a highly crosslinked epoxy resin system was investigated using conventional DSC and TOPEM^® measurements. The results showed that the curing behavior is much easier to study by TOPEM^® than by conventional DSC. Furthermore, the glass transition temperature of the completely cured material can only be determined by TOPEM^®.

Introduction

Epoxy resins are thermosetting polymers that are used for many different applications, for example, as adhesives, surface coatings, matrix materials for fiber-reinforced composites or for polymer-layered silicate nanocomposites (PLS).

One of the most frequently used epoxy resins for these applications is the bifunctional epoxy diglycidyl ether of bisphenol-A (DGEBA). However, many high-performance applications, for example in the aerospace industry, put greater demands on the properties of the resin and require very highly crosslinked composites that are stable up to temperatures of 250 °C.

One resin that is suitable for producing such materials is the trifunctional epoxy triglycidyl p-aminophenol (TGAP). The resin is cured with a diamine.

The resulting rigid, three-dimensional crosslinked polymer has a higher network density, a correspondingly high glass transition temperature, and better thermal stability compared with DGEBA. Conventional differential scanning calorimetry (DSC) is a technique widely used to investigate the curing behavior of crosslinking systems. Both isothermal and dynamic methods are employed.

Nevertheless, the technique has certain disadvantages when used to characterize the curing reaction of TGAP with a diamine:

• Due to the large reaction enthalpy involved in the curing reaction, the curing of TGAP is usually performed in several isothermal steps at temperatures below the final glass transition temperature. In this process, the material vitrifies during each individual curing step. To perform the multistep curing process as efficiently as possible, the vitrification time (the time taken by the material to vitrify at a certain temperature) has to be known. However, the determination of the vitrification time by conventional DSC requires considerable experimental time and work.
  
• The glass transition of the fully cured TGAP/DDS thermoset cannot be measured by conventional DSC. The network density is so high that the change in heat capacity at the final glass transition is too small to detect. The glass transition temperatures given in the literature for this system are therefore usually determined by DMA [1, 2].

In this article, we will show how these difficulties can be elegantly solved using TOPEM^®.

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References

[1] Becker, O.; Cheng, Y-B.; Varley, R.J.; Simon, G.P.: Layered silicate nanocomposites based on various high-functionality epoxy resins: The influence of cure temperature on morphology, mechanical properties, and free volume. Macromolecules 36 (2003) 1616 –1625.
[2] Frigione, M.; Calò, E.: Influence of an hyperbranched aliphatic polyester on the cure kinetic of a trifunctional epoxy resin. J. Appl. Polym. Sci. 107 (20 0 8) 174 4 –175 8

The level of thermal comfort nowadays needed in buildings has led to an increase in energy consumption in the residential and service sectors. Thermal energy storage (TES) is an important alternative for reducing high energy consumption. Phase change materials (PCMs) have been studied as suitable materials for storing thermal energy due to their high heat storage capacity.

Introduction

The authors of this article have developed and tested a phase change material (PCM) incorporated in a polymer matrix. Unexpected difficulties however arose in the measurement of the thermal properties of this material using differential scanning calorimetry (DSC). The thermal effects of the polymer matrix and the PCM overlapped.

A new method was therefore developed to overcome this problem. This consisted of using a reference crucible containing the polymer matrix as a blank instead of the usual empty crucible. This enabled a clear signal of the PCM to be obtained.

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One of the major problems of the plastics we use today is their great stability – they impact the environment for years without undergoing decomposition. Recently, a number of research projects have been started to develop plastics that degrade within a short time in the environment.

Introduction

Up until now, two different approaches have been followed for making biologically degradable polymers.

The first approach is to produce plastics from biomaterials such as maize (corn) or wheat, which rapidly decompose to smaller, biologically degradable compounds after use.

The other approach is to modify commonly used polymers such as polyethylene so that they degrade more quickly and can then be digested by microorganisms. This article deals with this second category of polymers, also referred to as oxo-biodegradable polymers.

The degradation of such polymers is accelerated by the addition of small amounts of metal salts that act as catalysts and promote oxidization of the polymer chains. Since oxidation can only take place when oxygen is present, this type of decomposition only works when the plastic is exposed to an open-air environment. If the plastic is disposed of in a landfill under anaerobic conditions, decomposition is no faster that with ordinary plastics.

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