THE CHANGE OF PROPERTIES OF ASPEN WOOD BY MECHANICAL TREATMENT – BY PRESSING

Józef Gaborik, Juraj Dudas
Department of Furniture and Wood Products, Technical University of Zwolen, Slovakia

ABSTRACT

In our paper we have concentrated on the study of chosen properties of less utilized wood of aspen. By purposeful treatment of its properties, it is possible to obtain the material, suitable for manufacture of furniture. In our study, we have concentrated on mechanical treatment of wood by pressing. We have studied strength properties of non-pressured and pressured aspen wood, namely its density, compression strength, bending strength with various moisture contents and compression degrees.

Key words: aspen, pressing, density, pressure, bending, stability.

INTRODUCTION

Mechanical properties of less utilized soft broadleaved species can be improved by compression of its parts that means, by increasing the amount of mass in the unit volume. Mechanical properties of compressed wood are changing depending upon the degree of compression and location of modularly rays. The wood strength will increase on condition that there is no substantial destruction of wood. In our paper we have concentrated mainly on investigation of mechanical characteristics (pressure and bending) of aspen wood pressed at three moisture levels in radial direction with three degrees of compression. Compressed and non-compressed aspen wood was conditioned to 12% moisture content and mechanical tests were made.

The studied problems are the part of the Grant Project No 1/0557/03 from VEGA programme.

MATERIAL AND METHODS

Investigation of mechanical characteristics (pressure and bending) of aspen wood was carried out on two basic groups:

• non-compressed wood,
• compressed wood.

Pressing was carried out in radial direction with three degrees of compression to 20, 30 and 50% of original thickness. Non-compressed and compressed wood was studied at three levels of moisture content (12, 16 and 30%). Prior to mechanical tests, the specimens were conditioned to 12% moisture content.

The dimensions of specimens for individual mechanical test are given in Figures 1 and 2.

The minimum bend radius was determined according to relation (1).

(1)

where: – y_max – the maximum defection, when comes to failure of specimen [mm],

– l_o² – the spacing between supports [mm].

The maximum deflection was determined according to Figure 3.

The bendability of wood was evaluated according to the coefficient of bendability, which was calculated according to the relation (2):

(2)

where: h – the thickness of specimen [mm].

Compression value – the degree of compression was determined in relation to the original size of specimen. After pressing to the determined value of compression, the pressure was released, the specimens were taken out of press and their dimensions were measured immediately. The specimens were pressed unidirectional in radial direction. The behaviour of aspen wood after compression and during conditioning is shown in Figure 4.

For calculation of the degree of compression was used the relation (3):

(3)

Individual tests were carried out on the RAUENSTEIN ZD 10/90 testing machine by the constant feed rate 19.6 mm/min. The bending test was carried out by the one-axial load with the loading forming radius r = 100 mm.

RESULTS AND DISCUSSION

The increase of wood density by mechanical treatment of aspen wood by pressing in radial direction with three degrees of compression (20, 30, 50%) at three moisture levels was (12, 16, 30%) was confirmed as follows:

• with increasing degree of compression the density of aspen wood increases,

• in the framework of one degree of compression, the increase in density depends on moisture; the lower the moisture is, the higher is the increase in density (Fig. 5).

• at initial 12% moisture content of wood is observed an increase of aspen wood density by 10.5% or even 30% in dependency on the degree of compression. At 16% moisture content there is an increase in density by 11.7 or even 28.5%, and at 30% moisture content there is an increase in density by 8.5 or even 16% (Fig. 5).

Compression of aspen wood changes its mechanical properties. Compression strength along the grains was changing as follows:

• compression strength has a downward tendency with increasing degree of pressing,

• at moisture contents 12% and 16% the compression strength reached higher values than with untreated wood,

• at the third degree of compression (50%) in all investigated cases was the compression strength lower than with untreated wood, although the wood density was higher (Fig. 6).

On the basis of acquired results we can state that 50 % compression causes a microdestruction in aspen wood, which is probably the reason of decrease in compression strength.

With bending strength the situation is better:

• Bending strength has also a downward tendency,

• But with all studied cases it reached higher values than with untreated wood,

• At compression to 10% and 20% and moisture contents 12% and 16% it was significantly higher than with untreated wood (Fig. 7).

An increase of bending strength of compressed aspen wood had a positive influence upon its bendability:

• bendability coefficient increases with increasing degree of compression,

• the best results were reached at the moisture level 16% – an increase of bend ability was from 8.8% to 40% (Fig. 8).

The coefficient of bend ability of untreated aspen wood at 12% moisture content was k_oh = 0.0274, and with treated aspen wood was between k_oh = 0.0298÷0.0384. These values are comparable with the values of beech wood, which reaches the coefficient of bend ability k_oh = 0.0343 [2] at w = 30%; 0.0312 [6]; 0.0337 [5]; 0.029 – 0.04 [3].

With untreated wood the stability of material plays an important role. After evaluation of stability results, which were reviewed on the basis of plastic deformations, we can state that with decreasing moisture contend and increasing degree of compression, the share of plastic deformations increases (Fig. 9).

CONCLUSIONS

Untreated aspen wood has not been used in the manufacture of furniture up to now. By modification of its properties, e.g. by pressing it is possible to improve its formability, which creates the possibilities of its use in manufacture of shaped furniture. Its bend ability is comparable with the bend ability of beech wood. By compression, the density increases as well as its compression strength and bending strength.

As follows from the results, compression should not exceed the 30 % value of its initial thickness. Although higher compaction increases the stability of wood, but probably causes micro destruction of wood, which was manifested by decreasing of its strength. The change of properties is also influenced by the moisture content of wood. The most suitable moisture contents seem to be the ones above 16 %.

The improved properties of compressed aspen wood predetermine it for combination with other tree species, e.g. in manufacture of shaped laminated wood.

REFERENCES

1. Gaborik J., 1995. Skúmanie vlastností plastifikovaného a komprimovaného dreva z hžadiska možností jeho tvarovania [Investigation properties plasticized and compressed wood in term of posibilities his shaping]. Kandidat. Diz. Pr. Zvolen, TU DF, [in Slovak].

2. Gaborik J., Dudas J., Gaff M., 2002. The facilities aspen wood from aspect of its shaping. Acta Fac. Xylol. TU Zvolen, 41-47.

3. Kulik J., 2003. Vyuzitie vysokofrekvenčnej energie pri plastifikácii mäkkých listnatých drevín za účelom ich ohýbania [Utilization of high-frequency energy for plastification soft broadleafed woods for the purpose their bending]. Dipl. Pr. Zvolen, TU DF [in Slovak].

4. Nemec ź., Sulan E., Zemiar J., 1986. Technológia výroby nábytku [Technology of furniture production]. Bratislava, ALFA [in Slovak].

5. Solar M., 2002. Plastifikácia dreva vysokofrekvenčným ohrevom pre účely jeho ohýbania [Plastification of wood by high-frequency heating for the purposes of his bending]. Diz. Pr. Zvolen, TU DF [in Slovak].

6. Stervens W. C., Turner U., 1970. Wood bending handbook. Stat. Office, London.

7. Zemiar J., Gaborik J., Solar M., Kotrady M., 1999. Tvárnenie dreva ohýbaním [Moulding of wood by bending]. Ved. Stud. 12/1999/A. Zvolen, TU DF [in Slovak].

Accepted for print: 06.09.2006

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