Thermal Analysis UserCom 43

The glass transition of semicrystalline polymers is often weak and difficult to measure by DSC. In this article, we show how a glass transition step of less than 0.1 J/g·K can be reproducibly determined using the DSC. The sample investigated was isotactic polypropylene (iPP) with a degree of crystallinity of 50%.

Introduction

Semicrystalline polymers have crystalline and amorphous regions. The glass transition takes place only in the amorphous region. The glass transition step of semicrystalline polymers is therefore appreciably smaller than that of 100% amorphous polymers.

In practice, this makes it more difficult to determine the glass transition of highly crystalline polymers. Whether a weak glass transition step can still be measured and evaluated by DSC also depends on the width of the glass transition. This becomes larger with increasing crystallinity.

Normally, glass transitions in polymers are measured at heating rates of 10 K/min using samples of about 10 mg. In the following experiments, iPP was employed as a test substance. The results show that the DSC 1 is capable of measuring weak glass transitions even with small samples weighing less than 5 mg. This improves the reproducibility of the analysis.

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The TGA-GC/MS system can be used to investigate the composition of unknown samples. This is done by installing the IST16 storage interface between the TGA and the GC/MS. The interface allows up to 16 evolved gas samples to be stored at different furnace temperatures during the TGA measurement. The gas samples are analyzed and identified by GC/MS when the TGA analysis is finished. This article describes how a black polymer granule was characterized using this technique.

Introduction

Unfortunately, TGA measurements do not provide any specific information about the nature of decomposition products. For this reason, TGA instruments are often coupled to instruments that allow decomposition products to be identified.

This includes coupling a TGA instrument to an FTIR or MS spectrometer. Both techniques have the disadvantage that products simultaneously released can only be distinguished from one another and identified with considerable difficulty. This is often the case with the pyrolysis of polymers.

The problem can be solved by separating the decomposition products before the identification step. This is possible using a TGA-GC/MS combination [1, 2].

In this example, the IST16 heated storage interface (Figures 1 and 2) was used. This storage unit allows up to 16 gas samples to be collected and stored at specific TGA furnace temperatures during the TGA analysis. The samples are then injected into a gas chromatograph and identified using a mass spectrometer. The sample investigated by TGA-GC/MS in this article was an unknown black polymer.

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Kinetic calculations based on TGA measurements of PA 6.6 compounds were performed to assess the influence of additives on the course of thermal decomposition.

Introduction

The flow behavior and flame protection properties of flame resistant, polyamide 6.6 compounds containing fillers was investigated as part of a research project at the Kunststoff Zentrum in Leipzig GmbH (www.kuz-leipzig.de).

The thermal properties of the materials were characterized by performing TGA measurements at different heating rates. The decomposition kinetics were evaluated from the measurement curves using model free kinetics (MFK).

The flame retardant used was melamine cyanurate. This compound contains nitrogen and acts mainly in the gas phase. The cooling effect produced during the combustion process is due to the strongly endothermic decomposition reaction of the additive. In addition, the gaseous non-combustible decomposition products reduce the oxygen concentration at the surface of the polymer.

Classification in the flammability rating UL-94 V-0 can be achieved for polyamide 6.6 (PA 6.6) by adding 10 mass % of this compound. The addition of this amount of melamine cyanurate reduces the mechanical and electrical properties of PA 6.6 only to a relatively small extent. Fillers are often added to composites of PA 6.6 and flame retardants in order to improve their mechanical properties. The research project referred to above examines the question of how the addition of an inert, silica filler influences combustion behavior.

The analysis of reaction kinetics is used to describe the course of the decomposition reaction and to simulate TGA measurement curves. In principle, two different approaches are possible, namely

• model-based kinetics, and
  
• model free kinetics based on the iso-conversion method.
  

In model-based methods, suitable reaction models are first chosen for the type of reaction involved; the activation energy for each reaction step is constant [1]. This type of approach is however not suitable for calculating the kinetics of complex reactions, to which polymer reactions belong. The reason for this is the large number of secondary reactions in which intermediate products participate. This causes the activation energy of the total reaction to change as the reaction proceeds.

For practical kinetic analysis, model free kinetics (MFK) is therefore advantageous, in which the activation energy is taken into account as a function of conversion [2]. The STAR^e software model free kinetics option also allows curves to be simulated. This enables TGA curves to be obtained that cannot be directly measured, for example due to technical reasons (heating rate is too high) or because of time limitations (heating rate is too low) [2].

Furthermore, isothermal data can be calculated from non-isothermal measurement data. The course of a reaction can then be estimated as a function of time at different temperatures. The calculations are based on measurement curves of the reaction recorded at three or more different heating rates.

This article shows examples that explain how the evaluation of the TGA data by MFK can be used to investigate the questions raised above and determine the limitations that arise.

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References
[1] S. Vyazovkin and C. A. Wight, International Reviews in Physical Chemistry 17 (1998) 407–433.
[2] S. Vyazovkin and N. Sbirrazzuoli, Macromolecular Rapid Communications 27 (2006) 1515–1532.

The mechanical properties of polymer-metal adhesive joints were studied as a function of the thickness of the adhesive layer using DMA. The glass transition temperature and the effective crosslinking density were evaluated from the shear modulus measurement curves. The results show that both quantities are strongly dependent on the thickness of the polymer layer. This is due to the formation of an interphase in the contact region of polymer and metal. The properties of the interphase depend on the metal used.

Introduction

The mechanical properties of adhesive joints and composite materials are largely determined by the viscoelastic behavior of the polymer used. The viscoelastic properties of the polymers depend on the temperature and deformation conditions in a complex way.

With metal-polymer adhesive joints, the mechanical properties of the adhesive joint are strongly influenced by interactions in the region between the surface of the metal and the polymer.

An interphase is formed, whose influence on the adhesive joint will be discussed in this article. The interphase is responsible for the adhesion between the polymer and the substrate. In composite materials, the interphase determines the interactions between the matrix polymer and the filler.

In general, any adhesive bonding mechanism immobilizes the polymer molecules in contact with the metal surface. The adhesive interactions lead to preferential orientation of the adhesive molecules close to the metal contact surface and trigger a tendency for polymer components to separate. The influence of the metal surface on the polymer structure and dynamics acts over a relatively large range.

Several articles describe the formation of such interphases in adhesive joints and report concentration gradients in the chemical composition of the adhesive in the contact region with the metal substrate [1].

These primary effects can lead to changes in other interphase properties such as mechanical properties and the distribution of internal mechanical stresses. This means that the mechanical properties of an adhesive-substrate composite also depend on the thickness of the adhesive layer.

In a joint with a thin adhesive layer, the interphase plays a much greater role than in a thick adhesive joint. In practice, the mechanical behavior of adhesives and of composite materials is often characterized by tension or bending experiments. These measurements are however not sufficient to completely describe the mechanical properties of adhesive joints because they do not take the influence of the substrate into account.

In this article, we investigate the suitability of dynamic mechanical analysis (DMA) for characterizing the relationship between the thickness of the layer and the effective mechanical behavior using adhesive joints of different thickness.

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References
[1] L. Krogh, J. E. K. Schawe, W. Possart, Dynamic mechanical properties of very thin adhesive joints, J. Applied Polymer Science, 132 (2015) 42058.