WOOD PROCESSING IN PENCIL MANUFACTURING

N. P. Chashchin, V. V. Koyain, L. M. Kovaleva

(Submitted by the scientific seminar of the Department of General Chemical Technology)

For the production of pencils, wood is needed that would have the following properties: it should be light and soft; should have approximately the same resistance to cutting across the grain as along the grain; a shiny, non-fibrous fracture; a smooth cut surface; an attractive, non-marking color, etc. [1].

Among domestic raw materials for pencil production, linden, juniper, and Siberian cedar are used. But mainly the wood of Pinus sibirika—Siberian cedar pine—is used as the raw material, as being the most consistent with the requirements imposed, although it does not plane well enough. At the Tomsk Pencil Factory, which is the sole supplier of this raw material in the Union, the pencil slat undergoes preliminary thermo-ammonia treatment. This method of treating slats has been used from 1939 to the present.

The method of thermo-ammonia treatment of slats is imperfect, since with this method only 20% of the wood is used, while 80% goes to waste. In this connection it is necessary to search for other methods of upgrading the pencil slat. Thus, attempts are known to treat the slat by an acid method [2, 3, 4, 5].

In the present work, experiments were carried out with three types of wood: normal wood, wood with inadmissible heartwood, and wood with continuous heartwood.

The working procedure was as follows.

A pencil slat was placed in an aqueous solution containing 0.2% nitric and 0.5% tartaric acid, held (2–6 h) under a vacuum of 85–90%, heated in the indicated solution at atmospheric pressure for 4 h at \(t = 80^\circ\text{C}\), and subjected to heat treatment at a temperature of 150°C for 3 h. A total of 12 experiments were carried out.

The sharpening ability of the pencils was checked on a special sharpening machine. Czechoslovak pencils of the firm Toison D’ok were used as the standard; the number of revolutions of a Czechoslovak pencil on this sharpening machine is 8.5.

The best results with respect to sharpening ability are achieved when the slats are treated under vacuum for 4 h. However, under the indicated conditions uniform staining is not achieved, and the slats are subject to warping.

To eliminate warping, the next series of experiments was conducted with the addition to the indicated solution of various amounts of ethyl-

ethylene glycol (5%–30%), which promotes greater shrinkage of the wood without the formation of cracks. A total of 36 experiments were conducted.

The best result was obtained when the wood was treated for two hours under vacuum in the presence of 30% ethylene glycol. Pencils made from the slat did not warp, but the slat was dyed unevenly. In order to ensure uniform staining, in the subsequent experiments the slat was impregnated under pressure.

The experimental procedure was as follows: the slat was kept in a solution (0.2% nitric acid, 0.5% tartaric acid, and 15% ethylene glycol) under a vacuum of 85–90% for 0.5 h, then under a pressure of 10 atmospheres at a temperature of 80° C for 4 h, and heat treatment was carried out for 3 h at 150° C. Slats treated in this manner acquired a uniform pink color upon drying and did not warp.

The aim of the next series of experiments was to determine the heat-treatment regime. The impregnation procedure remained the same; only the time and temperature (90–120° C) of the heat treatment were varied.

The process of heat treatment of acid-impregnated slats is associated not only with the removal of moisture, but at the same time the acid acts on the wood, breaks it down, and at a certain stage gives it the necessary properties: a pink color and good sharpening ability.

To achieve these properties, it is necessary to select certain parameters: the temperature and duration of heat treatment.

A total of 50 experiments were conducted. The results of some of them are given in Table 1.

Table 1

From the data in Table 1 it may be concluded that the slat acquires a pink color, when the heat-treatment temperature is reduced, only with an increase in the holding time.

This is true only up to certain limits, since at a temperature of 30°–50° C no pink coloration forms at all. It was established experimentally under laboratory conditions that, for normal wood, unacceptable and solid compression wood, different amounts of time are required to acquire a pink color. With an increase in temperature to 120° C, 6–8 h are required to obtain the necessary pink color.

Conclusions

1. When a slat is impregnated with a solution under vacuum without the subsequent application of pressure, uniform coloration is not achieved and the slat is subject to warping.

2. Treating the pencil slat by the acid method in the presence of ethylene glycol, first under vacuum and then under pressure, promotes uniform pink coloration and eliminates the warping that occurs when cedar wood is dried.

Literature

1. D. N. Lektorskii. Impregnation of Wood. GLTI, p. 168, 1940.

2. D. N. Lektorskii, T. V. Gordon, V. A. Baum. Refinement of cedar for the purpose of its use in pencil manufacturing. Proceedings of TsNILKhI, issue 1, 1935.

3. E. Louis Wise, C. Edwin Jahn. Wood Chemistry. Goslesbumizdat, vol. 2, p. 55, 1960.

4. A. I. Stamm. Modern Plastic Encyclopedia, p. 725. New York, Ilustres Catalogue Corp, 1998.

5. A. A. Berlin. Research in the field of the chemistry and technology of refined wood and wood plastics. Goslesbumizdat, p. 93, 1950.