The Use of Thermal Analysis to Characterize the Hardness of Pencil Leads

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.

Figure 1: DSC measurement curves of pencil lead 2H from manufacturer G. The diagram shows the peak integrals used in Figure 2.

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 finetunes 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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Conclusions

There is an almost linear correlation between the TGA residue and lead hardness. This relationship is based on the different contents of clay and on the increased ceramization of the leads due to the sintering conditions used with leads of increasing hardness. Standardization would be possible, at least with the leads of the two manufacturers investigated here if the raw mixture were standardized.

DSC analysis yields characteristic curves of the melting behavior of the oils and waxes used to fill the pores. The type and composition of the oils and waxes are specific to the manufacturer and allow a clear classification to be made. The enthalpies of melting correlate only to a limited extent with lead hardness. The melting curve however can be used to identify the manufacturer.

The use of Thermal Analysis to Characterize the Hardness of Pencil Leads | Thermal Analysis Application No. UC342 | Application published in METTLER TOLEDO Thermal Analysis UserCom 34