UPGRADING OF PENCIL SLATS BY THE SODA PROCESS

N. V. KARELINA, I. P. CHASHCHIN, V. V. KOYAIN, S. N. KARBAINOVA

(Presented at the scientific final conference of the Faculty of Chemical Technology)

In the Soviet Union, cedar wood is used as the raw material for making pencils; in terms of its qualities, however, it does not meet the requirements imposed by pencil manufacturers. Pencil production requires wood possessing definite elasticity and coloration.

The purpose of this work is to alter the grinding properties and color of cedar pencil slats by an alkaline method. Calcined soda was used as this reagent.

In contrast to most materials subjected to dyeing, wood is extremely heterogeneous both in physical structure and in chemical composition. This heterogeneity is inherent not only in the wood of different species, with different density, coloration, content of tannins, resins, etc., but also in wood of the same species. The density of wood, its porosity, and its adsorption capacity depend on the age of the tree, the nature of the cut, the storage conditions of the wood, and a number of other factors.

In chemical composition, wood is a complex of highly reactive ingredients: cellulose, lignin, hemicellulose, and other compounds.

When various chemical substances act on wood, each of the ingredients reacts in its own way.

For the lignification of wood or other plant raw material, alkaline reagents began to be used in China as early as about 2000 years ago. Apparently, the use of alkalis for making paper from straw was first carried out in Europe at the beginning of the eighteenth century, and in 1708–1714 in Russia at the Bogoroditsk paper mill near Moscow [1].

Almost 50 years later, in 1765–1772, Schaeffer [2] used this method for treating barley straw with lime milk in open vessels at relatively low temperatures.

Only in the middle of the nineteenth century, in 1854, Mellier [2] patented a method of boiling straw in closed digesters at elevated temperatures and pressure. At almost the same time (1853–1956), the alkaline boiling method was first used to obtain cellulose from wood [2].

It was observed that, when wood chips are treated with 1–5% aqueous solutions of caustic soda in closed digesters at a temperature ...

At \(170\text{–}175^\circ\) and the corresponding pressure (5 atm), almost all the lignin and a large part of the polysaccharides of the wood accompanying the cellulose can be brought into solution [2].

In production, such treatment is called the soda cooking of cellulose, and the solution obtained in this process is called “black liquor.”

Lignin is most readily affected by hot alkalis; it is not an individual chemical compound, but a mixture of high-molecular-weight compounds containing a considerable number of methoxyl and hydroxyl groups.

It is assumed [3] that lignin reacts with alkali to form phenolate compounds that are readily decomposed by carbonic acid:

\[ \mathrm{R{-}OH + NaOH \rightarrow R{-}ONa + H_2O;} \]

\[ \mathrm{R{-}ONa + H_2O + CO_2 \rightarrow R{-}OH + NaHCO_3.} \]

Cellulose is the most resistant to the action of alkalis, but even it is noticeably destroyed at a high treatment temperature (\(160^\circ\)).

Most investigators [4, 5] believe that cellulose forms molecular compounds with alkali in aqueous solutions, of the type \((\mathrm{C_6H_{10}O_5})_2 \cdot \mathrm{NaOH}\).

Some authors [6], however, believe that cellulose reacts with alkali to form alcoholates by replacement of metal-hydrogen in the hydroxyl group, according to the reversible reaction:

\[ \mathrm{C_{12}H_{19}O_9(OH) + NaOH \rightarrow C_{12}H_{19}O_9(ONa) + H_2O.} \]

The chemistry of the interaction of cellulose with alkali has not been definitively established.

Hemicellulose is a mixture of polysaccharides similar in structure to cellulose, but possessing lower chemical stability.

When hot dilute solutions of alkalis act on hemicellulose (under cooking conditions), hexosans are hydrolyzed first of all, and the sugars formed in this process undergo further transformation in the alkaline medium. Pentosans, however, partially remain in the wood.

As a result of treating comminuted wood with a hot alkali solution, cellulose remains in the residue with an admixture of undissolved lignin and pentosans.

A characteristic feature of cellulose after alkaline treatment is its brown color, caused by the presence of coloring substances. The nature of these substances, their chemical composition, and their properties have not yet been sufficiently studied. At present, most investigators [7] arrive at the conclusion that these substances are lignin in character.

Experimental Procedure

Cedar slats with a moisture content of 45–50% are loaded into a closed autoclave and flooded with a 1–5% soda solution to \(2/3\) of the apparatus volume. The apparatus is heated electrically to \(147^\circ\mathrm{C}\) at a pressure of 4.3 atm, and the slats are held at this temperature for 1 to 4 hours. After this, the soda solution is drained through the lower outlet, and the apparatus is blown through with live steam at a pressure of 2 atm for 20 minutes. The slats are then quickly transferred to a bath containing a 5 cm layer of paraffin and a 20 cm layer of water at a temperature of \(89\text{–}90^\circ\mathrm{C}\). After 30 sec of paraffining, the slats are removed from the bath, the paraffin is allowed to drain off, and they are placed for 10 minutes in a thermostat at a temperature of \(110^\circ\mathrm{C}\). Finally, the slats are dried at a temperature of \(100^\circ\mathrm{C}\).

The sharpening properties of the slats obtained in this way, after pencils had been made from them, were determined on a special sharpening machine by the number of revolutions of the knife. In all, 21 experiments were carried out. The results of some of them are given in the table.

Table

Conclusions

The best results were obtained when the slats were treated with a 2% soda solution for 2 hours at a pressure of 4.3 atm and a temperature of 147°C.

Pencils made from these slats have the best sharpening properties (7.5 revolutions) and a more uniform brownish coloration.

A drawback of the method is the nonuniform course of leaching in different directions; this sometimes causes considerable fluctuations in the strength of the wood, as well as partial warping of the leached and dried slat.

Literature

1. Zaozerskaya. Essays on the History of Trade and Industry in Russia. (1928).

2. N. N. Nepenin. Production of Cellulose. (1940).

3. R. Klasson, B. Segefeld, Arkiv f. Kemi, 4, No. 6, (1911).

4. I. A. Makolkin, ZhOKh, 12, 365, (1942).

5. Z. A. Rogovin, N. N. Shorygina. Chemistry of Cellulose and Its Companions. Moscow, 166, 190, (1953).

6. H. Hibbert, E. Perciviol, E. Cuthbertson. J. Am. Soc., § 2, 3257, (1930).

7. N. I. Nikitin. Chemistry of Wood, p. 521, (1951).