Thermal Analysis UserCom 38

Polypropylene (PP), is a semicrystalline thermoplastic made by the polymerization of propane using a metal catalyst. This can result in isotactic PP (methyl groups lie on the same side of the carbon chain), syndiotactic PP (methyl groups alternate above and below the carbon chain) or atactic PP (methyl groups are randomly arranged). Isotactic PP is characterized by high crystalline content while atactic PP is more amorphous. The crystallites in isotactic PP can also be larger. In this study, two samples of polypropylene were compared that had been produced under different process conditions. The measurements were performed using DSC and Flash DSC [1]. The two polypropylene samples were supplied as granules. Both samples were unfilled.

Introduction

The structural formula of polypropylene is shown in Figure 1. Polypropylene is widely used as packaging material, as a basic material for a variety of consumer goods, for textiles, in the automobile industry, for medicinal technical products or for pipes.

To guarantee continued constant quality, we wanted to find out how the two different production methods influence the polypropylene samples and whether the products differed in their thermal properties. It turned out that the mechanical properties of the final products were in fact different.

The two different types of polypropylene were supplied as granules with a diameter of about 3 mm. The thermal properties of the materials were investigated in DSC experiments.

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The vapor pressure of a chemical compound is a measure of its volatility: the higher the vapor pressure at a given temperature, the greater the probability that the substance is in the gaseous state rather than the condensed phase (liquid or solid). The vapor pressure of a substance increases with increasing temperature. The boiling point is reached when the vapor pressure is equal to the total pressure of the surroundings.

Introduction

Knowledge of this thermophysical property and the enthalpy of the corresponding phase transition are of fundamental importance for areas such as process control, storage of materials and stability.

The data is used for establishing environmental guidelines and to define the maximum allowable limiting values and is also needed for preparing safety data sheets.

There is a need for a simple method to determine the vapor pressure of substances for industrial applications as well as for basic research. The method should be routinely applicable, reliable, rapid and straightforward to perform. In this article, we will show that thermogravimetric analysis (TGA) using a reference substance is a suitable technique.

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TOPEM^® measurements of semicrystalline polylactide (PLA) show how overlapping effects in the first heating run can be separated and interpreted. This allows the correct enthalpy of melting of the original sample to be determined.

Introduction

Several different thermal processes occur in the first DSC heating run of a material such as reorganization, the relaxation of internal stress, and changes in the thermal contact between the sample and crucible.

Conventional DSC curves are sometimes difficult to interpret if these processes produce effects that overlap thermal events in the sample such as the glass transition, cold crystallization, and melting. A second heating run after controlled cooling does not always help you to understand the effects observed in the first heating curve.

The problem can be solved by performing temperature-modulated measurements using TOPEM^®. This technique allows non-reversing effects such as reorganization and stress relaxation to be separated from reversing effects such as the glass transition.

The small temperature modulation hardly influences non-reversing effects, but with reversing effects gives rise to a corresponding modulation of the heat flow.

This work describes the analysis of biodegradable polylactide (PLA) disposable cups used for serving cold drinks [1]. The cups were manufactured using a deep drawing process at temperatures around the glass transition. In the production process, the PLA material experiences high and very localized cooling rates.

The article discusses the conclusions can be drawn about the material from the first heating run using TOPEM^®.

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The sorption behavior of a zeolite was investigated using a TGA / DSC 1 coupled to a humidity generator. The sample was first dried in the TGA / DSC 1 and then exposed isothermally to different relative humidities. The sorption capacity was measured as a change of sample mass. Furthermore, sorption enthalpies were also determined.

Introduction

The Swedish mineralogist Axel Fredrick von Cronstedt noted in 1756 that certain stones seemed to boil on heating and at the same time released considerable amounts of water vapor.

He called these « boiling stones » zeolites (a word whose roots are derived from the Greek words « zeein » for boiling and « lithos » for stone) [1, 2].

The phenomenon of the boiling stones can be understood through the special structure of the zeolites. Zeolites are crystalline materials whose crystal lattices contain extensive pores and channels in which specific foreign molecules (e.g. water, ethanol, and ammonia) can be reversibly stored.

Adsorbed molecules can be removed from a zeolite by heating without affecting its structure. Zeolites can be used for any number of adsorption / desorption cycles. Naturally occurring zeolites consist mainly of SiO₂- and AlO₄- tetrahedrons that are connected to one another via oxygen atoms, so that pores and channels are formed internally (see Figure 1) [3, 4].

The pores often contain free mobile cations (e.g. Na⁺), that can be exchanged for other ions (e.g. Ca²⁺) or for free water. Most zeolites used nowadays are made synthetically.

A wide range of zeolites is therefore available with very different chemical compositions and pore sizes optimized for specific applications.

Zeolites have numerous fields of application. Natrolite is from the technical point of view of great importance and is used for hydraulic cement, as a filler material in the manufacture of paper, as an adsorption agent, for water softening and as an ion exchange material [3].

Synthetic zeolites, such as molecular sieves ( Zeolite A, Na₁₂[(AlO₂)₁₂(SiO₂)₁₂]₂₇H₂O), which is available in different pore sizes, is used as drying agent in laboratories or as a catalyst [2].

Furthermore, zeolites are also employed in washing powder (as water softeners) or in cat litter in order to absorb both liquids and unpleasant odors.

This article describes the investigation of the sorption behavior of water in a zeolite.

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References

[1] Wikipedia, Zeolites, http://de.wikipedia.org/wiki/Zeolites_(Stoffgruppe), http://de.wikipedia.org/wiki/Zeolith_A
[2] Lehrbuch the anorganischen Chemie, Holleman-Wiberg, De Gruyter, 1985
[3] Mineralien and Gesteine, Walter Schuhmann, BLV Buchverlag GmbH & Co KG, 2009
[4] Anorganische Chemie, Riedel, De Gruyter, 1987

The wax content (paraffin wax) in petroleum oils is an important parameter because critical physical properties of the oil such as the viscosity change with the wax content. Furthermore, the wax present in an oil may solidify and precipitate out at low temperatures. Determination of the wax content is therefore an important analytical requirement in quality control.

Introduction

The wax most often present in petroleum oil is paraffin wax (C_nH_2n+2) with a chain length of about 18 to 36 carbon atoms. These substances melt above room temperature between 45 and 65 ° C.

Different methods are available for determining the wax content in oils, for example :

• Extraction of the wax
  
• Determination of the pour point of the oil
  
• Determination of the cloud point of the oil
  
• Optical measurements using polarization microscopy
  
• NMR

These methods all have various disadvantages such as long analysis time, the use of chemicals, large sample amounts or relatively poor detection limits.

DSC can be used to measure the melting behavior of the wax. The analysis of the wax peak in the DSC curve is a simple method that requires only a small amount of sample. Systematic studies of oils with different waxes can be found in the scientific literature [1, 2].

These publications describe methods in which liquid waxes are measured at lower temperatures and solid waxes such as paraffin wax are measured at temperatures above room temperature.

At high wax concentrations (>1%), the determination is very easy. This article presents and discusses measurements of samples with a very low wax content.

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References

[1] Jun Chen, Jinjun Zhang, Hongying Li, Thermochimica Acta 410 ( 2004 ) 23 – 26.
[2] Z. Jianga, J.M. Hutchinson, C.T. Imrie, Fuel 80 ( 2001 ) 367 – 371.