SOME METHODS FOR IMPROVING THE QUALITY OF WOOD USED IN PENCIL PRODUCTION

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

(Presented at the scientific final conference of the Chemical-Technological Faculty)

Wood, as a raw material for the pencil industry, must possess certain properties. The best wood possessing all the necessary qualities is Virginia juniper, or “pencil cedar,” which grows in America [1]. This raw material is used mainly by pencil factories in the USA and Czechoslovakia.

In the Soviet Union, Siberian cedar wood is used for pencil slats. The only factory supplying this raw material is the Tomsk Pencil Factory.

The thermo-ammonia method of treatment [2], which has existed since 1939 and is still used today, is far from perfect and leaves much to be desired. Improving the quality of the cut, color, and odor; reducing rejects due to pencil warping; and increasing the yield of pencil slats through the use of reaction wood—these are the tasks facing researchers.

The difficulty in solving these questions lies in the fact that wood is a heterogeneous, multicomponent raw material. It is a natural polymer of spatial structure, which does not dissolve without decomposition in any known solvent; moreover, coniferous wood, in particular Siberian cedar, contains large quantities of resinous substances which, filling the resin canals, constitute a serious obstacle to impregnation and staining of the wood [3].

The authors of the present article investigated an alkaline method for treating pencil slats. The best results were obtained with a 2-hour exposure in a 2% soda solution at a temperature of 147°C. Under these conditions the slats acquire a uniform brown color, and the pencil has good sharpening properties [4].

To improve the cutting quality of the pencil, it is important to achieve a uniform distribution of paraffin in the central and surface zones of the slat. GOST specifies a paraffin content in the center of not less than 4%, and at the surface not more than 10%. However, analyses of factory slats in most cases show too high a percentage of paraffin in the surface zone, which in pencil production is cut off and goes to waste, and a very small percentage of paraffin—1–1.5%—in the central zone. Reducing the surface tension of the medium by adding various surfactants to the molten paraffin [5] made it possible to obtain a positive

effect. Sulfonated kerosene, turbine oil, and transformer oil were used as surface-active substances [6]. The best results were obtained with the addition of sulfonated kerosene. In this case, in all 40 experiments the percentage of paraffin in the central zone lies within the limits of GOST; in order to obtain the desired result, it is quite sufficient to add 1.5% sulfonated kerosene relative to the amount of paraffin. Data from some experiments with sulfonated kerosene are given in Table 1.

Table 1

Experiments carried out under factory conditions with the addition of 1.5% sulfonated kerosene also gave good results.

It was of interest to trace the influence of wood moisture content on the distribution of paraffin in the slat [7]. At the factory, the slat enters the autoclave for thermo-ammonia treatment with an initial moisture content of 50–60%. In this connection, two variants were investigated:

a) the influence of the initial moisture content of the slat on the uniformity of paraffin distribution was determined. For this purpose, the slats were dried in a drying chamber at a temperature of 70–80°C to different moisture contents, and then passed through the entire factory cycle of upgrading: staining in an autoclave, paraffin treatment, and drying. The results of the analyses showed that, as the initial moisture content of the slat is reduced to a certain limit, the uniformity of paraffin distribution increases, and then, with a further decrease in moisture content, falls sharply. This is fully explained by the presence of air pockets in the pores of the wood which, at low moisture content, hinder the penetration of paraffin into the slat [8]. The results of some experiments are given in Table 2.

The most uniform distribution of paraffin in the surface and central zones, meeting GOST, is achieved at an initial slat moisture content of 15–30%;

b) in the second variant, the influence of different moisture contents on the process of paraffin treatment of stained slats was investigated. An experimental batch of slats, taken from the autoclave, was dried to different moisture contents in a thermostat at a temperature of 130–140°C.

The results of the analyses are given in Table 3.

The results of the analyses showed that drying after the autoclave has a favorable effect on the uniformity of paraffin distribution in the slat. Excessively high moisture content of the slat before paraffin treatment (more than 40%), as well as excessively low moisture content (less than 20%), interferes with uniform

distribution of paraffin. All analyses for paraffin showed that the paraffin content in the central zone is higher than in the surface zone.

Table 2

Table 3

Conclusions

1. Additions of surface-active substances have a favorable effect on the uniformity of paraffin distribution in the board. Of the substances studied, sulfonated kerosene is the most effective.

2. The initial moisture content of the board, as well as the moisture content of the steeped board before paraffining, has a great influence on the uniformity of paraffin distribution. The best results were obtained with an initial board moisture content of 15 to 30%, and the optimal moisture limits of boards dried after steeping lie within the range of 25 to 35%.

REFERENCES

1. D. N. Lektorskii. Impregnation of Wood. Goslestekhizdat, Moscow, 1940.
2. N. F. Gusev. Wood-Processing Industry, 11, p. 16, 1955.
3. Lewis E. Wise, Edwin C. Jahn. Wood Chemistry. Goslesbumizdat, vol. 2, 1960.
4. N. V. Karelina, I. P. Chashchin, V. V. Koyan, S. N. Karbainova. Refining pencil board by the soda method. In the present collection.
5. A. Schwartz, J. Perry. Surface-Active Agents. IL, 1953.
6. V. V. Guturina, G. P. Slavkin. Organic Chemistry and Flotation Reagents. Gosgortekhizdat, 1962.
7. Leont’ev. The influence of moisture on the physicomechanical properties of wood. Goslesbumizdat, 1962.
8. B. M. Buglai. Technology of Wood Finishing. Goslesbumizdat, 1962.