Thermal Analysis UserCom 27

Introduction

In injection molding, the liquid polymer mass is forced into the mold and held under pressure at a particular temperature (the dwell or mold temperature) for a certain period (the dwell or molding time) before cooling. The dwell temperature is usually appreciably higher than the glass transition temperature of the material. The question therefore arises as to what extent the raw material used changes during the injection molding process, and what influence the process parameters (dwell time and dwell temperature) have on possible changes in the final product. This was investigated using a blend of ABS (acrylonitrile-butadiene-styrene) and PC (polycarbonate) as an example. The study shows how a simple procedure can be used to characterize the individual and conjugated influence of the process conditions on the glass transitions of this material. The samples available were prepared at dwell temperatures of 260 °C and 300 °C with dwell times of 0 min and 5 min.

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Introduction

Polymerization is a very important process in the chemical industry. The products formed exhibit desirable properties such as durability, inertness toward many chemical substances, elasticity, transparency, as well as electrical and thermal resistance.

Polymers are produced in many different forms, for example as fibers, films, pipes, coatings and injection molding components.

An important metal-catalyzed polymerization is initiated by Ziegler-Natta catalysts [1]. The Ziegler-Natta catalyst is synthesized by treating crystalline α-TiCl₃ with [AlCl(C₂H₅)₂ ]₂ . An alternative route to the catalyst entails the reaction of titanium or zirconium tetrachloride with a trialkylaluminum reagent such as triethylaluminum, AlR₃ or Al(CH₂CH₃)₃. This catalyst system polymerizes alkenes, particularly ethylene, at relatively low pressures with remarkable ease and efficiency.

The advantage of Ziegler-Natta polymerization is the regularity with which substituted alkane chains are formed from substituted alkenes such as propylene, and the high linearity of the chains. The resulting polymers are of higher density and much stronger compared with polymers obtained by radical polymerization [2].

This article describes how the polymerization of ethylene and propylene was investigated in the laboratory using a METTLER TOLEDO Automated Lab Reactor (ALR).

To investigate the kinetics of the polymerization of ethylene and the copolymerization of ethylene and propylene on a small scale, the ALR was equipped with 50-mL reactors and a gas supply system. The gas uptake was measured by monitoring the pressure drop in the gas res - ervoir [3].

The reaction was also studied by monitoring the difference between the temperature of the reactor contents, T_r , and the temperature of the reactor jacket, T_j , i.e. T_r – T_j. The difference is a measure of the heat flowing into or out of the reactor.
The synthesized products were characterized by differential scanning calorimetry (DSC) because an ALR cannot be used for this purpose.
This article shows how reaction calorimetry and thermal analysis ideally complement one another for the synthesis and analysis of polymers.

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Literature

[1] Takahashi, T. «Titanium(IV) Chloride-Triethylaluminum»: Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2001.
[2] Alt, H.G.; Koppl, A.; «Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization» Chemical Reviews. 2000, 100, 1205–1221
[3] Visentin, F.; Graeme, P.; Kut, O. M.; Hungerbühler, K. Study of the Hydrogenation of Selected Nitro Copounds by Simultaneous Measurements of Calorimetric, FT-IR, and Gas-Uptake Signals. Ind. Eng. Chem. Res. 2006, 45, 4544–4553.

Introduction

In polymer nanocomposites, inorganic particles are so finely distributed in the polymer matrix that at least in one dimension structures smaller than 100 nm can occur [1].
Well-known, classical nanofillers such as carbon black are isotropic. In contrast, sheet silicates such as montmorillonite are anisotropic and have a high length-to-thickness ratio (aspect ratio) of about 1:100. Besides the conventional composite materials, one distinguishes between two types of lamellar nanocomposites. In intercalated structures, the polymer chains alternate with the silicate layers in a fixed compositional ratio and have a well defined number of polymer layers in the intralamellar space. In exfoliated nanocomposites, the number of polymer chains between the layers is almost continuously variable and the layers are largely delaminated.
The explanation of structure-property- relationships based on the underlying morphology and filler structure is an important aspect in the development of nanocomposites.
Established techniques for structure determination include transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The spatial distribution of the exfoliated silicate layers can only be studied by TEM although SA XS can measure the separation of layers down to 1–4 nm. TEM is a rather complex method that allows qualitative information to be obtained about the internal structures and the spatial distribution of platelets. SA XS can only be used for intercalated structures, but not for exfoliated.

To investigate orientation effects of the local filer-filler-network, the materials used in this study were measured in different spatial directions by Dynamic Mechanical Analysis (DMA). Classical methods such as TEM and SAXS support the results of the DMA analysis. The main advantage of DMA analysis is the easy preparation and handling of samples.

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Literature

[1] Menges G., Haberstroh E., Michaeli W., Schmachtenberg E.: Werkstoffkunde Kunststoffe, Carl Hanser Verlag München Wien 2000

Introduction

The combination of a thermobalance with a mass spectrometer (MS) measures the change in mass of the sample under investigation and at the same time yields qualitative information on the evolved decomposition or vaporization products. The quality of the mass spectra depends on a number of different experimental parameters. The most important of these are the sample mass, the gas flow, the accelerating voltage of the secondary electron multiplier (SEM) and the position of the tip of the connecting capillary in the furnace of the thermobalance. The following article discusses how these four parameters influence the mass spectrometric measurement results.
The experiments were performed with a TGA /SDTA851^e (large furnace) coupled to an Inficon Thermostar QMS300 mass spectrometer (mass range 1–300). The decomposition of calcium oxalate monohydrate (CaC₂O₄ ·H₂O) was chosen as the reaction model.

CaC₂O₄ ·H₂O loses its water of crystallization above about 120 °C. The anhydrous oxalate then decomposes in two separate reaction steps as follows:

CaC₂O₄ ⇒ CaCO₃ + CO    (I)

CaCO₃ ⇔ CaO + CO2        (II)

Associated with the above dehydration and two decomposition reactions are stoichiometric mass losses of 12.3%, 19.2% and 30.1% respectively. One mole each of H₂O, CO and CO₂ is evolved per mole of CaC₂O₄ ·H₂O in the different reaction steps.

Figure 1 summarizes the result s of the TG-MS measurements on calcium oxalate monohydrate. The mass spectrometer detected the loss of H₂O, CO and CO₂ corresponding to the three separate mass loss steps. Furthermore, the SDTA signal shows that the three reaction steps are endothermic.

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Introduction

People who eat candies (sweets) are usually interested in their taste and consistency rather than in their chemical-physical properties. The consistency of a candy is however strongly dependent on structural aspects.

This article illustrates how certain properties of candies can be investigated using DSC and microscopy, in particular softening, melting and crystallization behavior. Detailed knowledge of these properties is important for defining proper production and storage conditions.

The term candy includes a large number of very different products (bonbons, milk and crème caramels, gelatinized sugar products such as jelly beans, etc.). In this article, we have investigated a commercially available three-layer candy.

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