Thermal Analysis UserCom 34

The hardness of pencils is still in many respects a company-specific value, that is, the hardness grade declared varies from manufacturer to manufacturer depending on the quality of the pencil lead. This article describes the use of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to characterize the hardness of pencil leads. The relationship between lead hardness and the residue in the TGA measurement is reproducible. DSC curves can be used to classify pencil leads with regard to their manufacturer.

Introduction

Pencil leads consist of clay and graphite to which other components such as cellulose are added. The clay and graphite are responsible for the blackness of the writing. Other components such as modified cellulose are mainly responsible for the hardness of the pencil lead.

Pencil leads are made by first mixing the finely ground individual components to form a paste. This is followed by an extrusion process that defines the geometry of the lead. Depending on the desired hardness, there then follows a sintering process (ceramization). This changes the cellulose chemically and creates a hard structure in which the clay and graphite particles are embedded.

The structure and clay-graphite ratio are responsible for the hardness of the lead. Finally, the leads are dipped in wax or oil. This fills the holes produced in the sintering process and furthermore fine-tunes the hardness of the lead.

European pencil makers use a combination letter-number system to define a wide range of grades of hardness consisting of the series 9H, 8H, to 2H, H, F, HB, B, 2B, to 9B. The letter H stands for hard, F for firm, and B for black. The maximum hardness grade is 9H and softest grade 9B. A number-only system is used in the USA by American manufacturers.

The hardness of pencils is frequently measured by means of writing tests performed by members of staff of the particular manufacturer. Up until now, no official standardization for pencil grades has been adopted. Furthermore, the writing tests are performed on devices that are specific to the manufacturer and are themselves not standardized.

There is neither an absolute scale for the hardness of pencils nor a standard test method. A committee of technical experts under the auspices of the International Organization for Standardization (ISO) has tried for the last 15 years to work out a reliable and reproducible standard test method for the determination of pencil hardness but no agreement has yet been reached.

Research into ways to characterize pencil leads are based on methods used for highly filled plastics. This analogy lends itself because the pencil leads consist of organic thermoplastic matrix material (cellulose) plus mineral fillers (clay, graphite) as well as waxes and oils. The small amounts of processing additives can be neglected.

In recent years, there have also been moves to replace the cellulose by thermoplastic materials. This would of course facilitate characterization by thermoanalytical methods. Leads of this type are being increasingly used in mechanical (propelling) pencils, colored pencils and crayons.

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Cobalt chloride is a blue, hygroscopic substance that turns red when it absorbs moisture. The characteristic color change makes it very useful as a moisture indicator for drying agents such as silica gel. This article describes how we investigated the drying behavior of cobalt chloride hexahydrate using DSC microscopy and TGA.

Introduction

Cobalt chloride, or more precisely cobalt chloride hexahydrate (CoCl₂ · 6H₂O) is a ruby red, poisonous salt. In aqueous solution, it can be used as ink that is almost invisible on paper. When the paper is warmed, for example over a flame, the written text becomes visible and is colored blue but mysteriously disappears again shortly after.

These properties made the CoCl₂ · 6H₂O solution popular as a secret ink, especially for writing love letters. Nowadays, cobalt chloride (CoCl₂ ) is important technically as a moisture indicator, for example in silica gel.

Silica gel is colorless amorphous silicon dioxide and has a gel-like to solid consistency. It is strongly hygroscopic and is often used as a drying agent. For this application, the silica gel is usually impregnated with cobalt chloride, which gives it a blue color when it is dry. If the impregnated silica gel absorbs moisture, it becomes red. This is due to the fact that anhydrous cobalt chloride absorbs water and forms the ruby-red cobalt chloride hexahydrate. The silica gel can be regenerated by heating to about 160 °C to remove the water.

This allows it to be reused as a drying agent. This article describes how we investigated the drying process of CoCl₂ · 6H₂O using DSC microscopy and TGA.

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Careful sample preparation is crucial for good quality DMA shear measurements. The most important influence factors are discussed with the aid of practical examples.

Introduction

In the DMA shear mode, two identical samples are clamped between the three clamping plates of the shear sample holder. The two outer plates are fixed in the clamping assembly while the oscillating force acts on the center plate.

The shear mode is widely used for materials with very different physical properties and geometries such as soft elastomers, hard composites, viscous liquids, powders, and plastic films.

The following section presents some general tips on sample preparation concerning

• the geometry of samples
  
• tools for preparing samples
  
• clamping and predeformation
  
• installation in the clamping assembly
  
• measurements below room temperature.

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How can air- and moisture sensitive substances be analyzed by thermal analysis without the substances decomposing beforehand? The solution is to install the TGA in a glove box. In this article, we show how this was done and describe some results from our current research.

Introduction

An important part of our research is concerned with the synthesis of suitable precursors for the deposition of thin films or nanostructured 1D and 2D materials by gas phase deposition, the so-called Metal Organic Chemical Vapor Deposition (MOCVD) process.

This technique was first developed in the 1970s. Since then it has become an important method for producing many kinds of films ranging from metals and semiconductors through to ceramic oxides and nitrides and hard materials. We are mainly interested in the preparation of binary semiconductor material films (aluminum antimonide, AlSb; gallium antimonide, GaSb; zinc nitride Zn₃N₂ ) and carbide- and nitride-based hard materials such as aluminum nitride (AlN) or titanium carbide (TiC).

To prepare them, we use special liquid and solid precursors whose elementary composition at the molecular level corresponds to that of the desired material. The precursor thus contains the elements of the film in a single compound in which the elements are bound together via stable chemical bonds.

In addition, the compounds contain organic ligands that kinetically stabilize the metal organic precursor. In this case, one refers to single molecule or single source precursors. In the MOCVD process, the precursor is first vaporized, typically in the pressure range 10^-3 to 1013 mbar, and then thermally decomposed on a suitable substrate.

An important requirement for a precursor is that it can be vaporized without decomposition. Information about its thermochemical properties such as vaporization temperature, vapor pressure, and stability in the gas phase is therefore essential.

Thermogravimetric analysis (TGA; possibly in combination with DSC) is a technique in which the change in mass of a sample is measured while the sample is heated according to a defined temperature program in a defined (inert or reactive gas) atmosphere. It is therefore a very important method for the identification of suitable precursors.

The metal organic precursors we use are however highly sensitive to the presence of air and moisture. This makes them more difficult to measure in a TGA instrument. A particular difficulty is for example how to fill the crucible and how to insert it into the measuring cell under oxygen- and moisture-free conditions.

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