ACID METHOD FOR PROCESSING PENCIL SLATS

V. V. Koyan, I. P. Chashchin, S. N. Karbainova

(Presented by the final conference of scientific-research work for 1965 of the Department of OHT)

According to existing standards, wood used for the manufacture of pencils must possess certain properties, of which the principal ones are the following:

1. Sufficient lightness and softness;
2. Equal, or almost equal, resistance to cutting both along and across the grain;
3. A shiny, non-fibrous fracture and cut surface;
4. A dark, non-marking coloration.

The aim of the present work was to study the effectiveness of the acid method for upgrading pencil slats made from Siberian cedar.

As early as 1909, Kroker [1] proposed boiling wood in a 1% solution of sulfuric acid, followed by impregnation with molten wax at a temperature exceeding the boiling point of the acid. Under these conditions, simultaneously with impregnation by the fattening substance, the wood was dried.

In 1931, engineer Timofeev [2] proposed a method of two-stage treatment of cedar slats, consisting in the following: the wood was first impregnated for 4 hours at 90–100°C and atmospheric pressure with a 1% solution of hydrochloric acid, and then, without preliminary drying, was treated in an autoclave under increased pressure with various fatty emulsions.

In 1933, the Central Laboratory of Moskhimtrest [2] studied the possibility of treating pencil slats with a hot 1% solution of hydrochloric acid at atmospheric pressure, followed by firing them with paraffin and staining.

However, all these methods, for various reasons, did not yield positive results.

In 1933, the impregnation laboratory of TsNIILKhI [2] began work on a detailed study of acid methods for upgrading cedar. The possibility was studied of applying hydrolysis methods with dilute mineral acids (hydrochloric, sulfuric, and nitric) under various conditions.

In addition, since, according to data from one of the earlier works of the TsNIILKhI laboratory [2], oxyacids of the fatty series exerted a softening action on wood, which was intensified in the presence of mineral acids, the corresponding experiments were included in the research program.

In this connection, it became necessary to clarify the role of mineral acid and to study changes in the properties of wood under the action of acids.

The overwhelming mass of any wood, including Siberian cedar wood, consists of various carbohydrate compounds—chiefly cellulose. When mineral acids act on carbohydrates, hydrolysis of these compounds occurs first of all. In this process, depolymerization of cellulose molecules is observed; initially it is converted into hydrocellulose, cellulosodextrin, cellobiose, and finally into glucose.

Some mineral acids, such as nitric acid, simultaneously with hydrolysis also cause oxidation of the constituent parts of the wood. During the oxidation of cellulose in an acidic medium, oxycellulose of the reducing type is formed, with an increased copper number and high solubility in boiling dilute alkali.

The formation of both hydrocellulose and oxycellulose causes, depending on the processing conditions, a greater or lesser loss of fiber strength.

This circumstance was used to develop a method for upgrading pencil slats. The best results were obtained when cedar wood was treated with dilute hydrochloric acid and especially with a mixture of nitric acid and oxyacids (tartaric, citric, and lactic). Tests of the slats showed that, in terms of softness and uniformity when sharpened with a sharp knife, all the slats were very good, with those treated with acid of higher concentration being softer than the others. It was further found that, when the concentration of nitric acid in the solution was increased with a simultaneous decrease in the content of oxyacid, the finished slats gave a somewhat uneven cut when cut with a dull knife. All the slats had a uniform pink coloration.

These observations by D. N. Lektorskii, L. V. Gordon, and V. A. Baum [2] formed the basis of the present work. The objects of study were both normal wood and wood with continuous compression wood.

The working procedure was as follows.

The slats were placed in an aqueous solution containing 0.2% nitric acid and 0.5% tartaric acid, held for various periods under a vacuum of 85–90%, heated in the solution at atmospheric pressure for 4 hours at a temperature of 80°, and finally subjected to heat treatment at a temperature of 150° for 3 hours. From the slats treated in this way, black-graphite pencils were made, and then their sharpening capacity was determined on a special apparatus by the number of knife revolutions.

A total of 12 experiments were carried out. The results of some of them are presented in Table 1.

From these data it may be concluded that the best results with respect to sharpening capacity are achieved when the slats are treated under vacuum for 4 hours. However, under the specified conditions, uniform coloration is not achieved.

A very important problem is combating swelling, shrinkage, and warping during the impregnation and drying of wood, especially compression wood.

Swelling and shrinkage of wood can be reduced by applying four different treatment methods [3]:

1. Applying paint or hydrophobic chemicals to the surface.
2. Introducing into the cell walls substances that cause an increase in volume.

1. Blocking or replacement of the hydroxyl groups of cellulose and lignin by less polar groups.

2. Formation of cross-links between structural units [4].

Table 1

The work of Lektorskii, Fedorchukova, and Alekseeva [5] showed that, as a result of treating wood with a solution of urea or glycol, the material acquires the ability to undergo considerable shrinkage without the formation of cracks. The authors recommend impregnating the wood with 5–10% urea solutions or 1–5% glycol solutions, followed by treatment with live steam at a temperature of 140–160°C. As a result of the convergence of the wood fibers, the material treated in this way has high hardness.

The action of glycols on wood is apparently associated with swelling of the cell walls in polar compounds. Along with this, under conditions of a weakly acidic medium, the possibility is not excluded of partial alcoholysis of the lignocarbohydrate acetal, which should lead to a decrease in the number of cross-links [6]. It is further indicated that raising the impregnation temperature to 80°C shortens the treatment time and increases the amount of reagent introduced into the wood.

We conducted 36 experiments to determine the effect of treating pencil slats in a medium of 0.2% nitric acid, 0.5% tartaric acid, and with various amounts of ethylene glycol on changes in the sharpening properties of pencils. The experiments were carried out according to the method described above. The results of some experiments are given in Table 2.

Thus, the best sharpening properties are obtained when the wood is treated for 2 hours under vacuum in the presence of 30% ethylene glycol. It may also be noted that, under the indicated conditions, compression wood is stained more uniformly than normal wood.

Conclusions

1. The acid method can be successfully used for ennobling pencil slats made of Siberian cedar.

2. With ordinary treatment under vacuum, uniform staining of the slats is not achieved. To obtain a uniform pink color, additional impregnation of the slat under pressure is apparently necessary.

3. Slats made from compression wood are, as a rule, stained more uniformly than slats made from normal wood.

1. Treatment of the slat in the presence of ethylene glycol almost completely eliminates warping and cracking of the slats during impregnation and drying.

Table 2

REFERENCES

1. Krocker. German patent 224775, 27. I. 1909; 27. I. 1909; Buh-Bodwar, Holzkonservierung, 1922.

2. D. N. Lektorskii, L. V. Gordon, V. A. Baum. Upgrading cedar for the purpose of its use in pencil manufacture, Transactions of TsNIILKhI, issue VI, 1935.

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

4. A. I. Stamm. “Modern Plastics Encyclopedia,” p. 725, New York, Plastics Catalogue Corp., 1948.

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

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