THE MODIFICATION OF ASPEN WOOD FOR THE PURPOSE OF ITS SHAPING

Jozef Gaborik, Juraj Dudas, Milan Gaff

ABSTRACT

In our paper we have concentrated on determination of the influence of plastification on changes of properties of aspen wood for the purpose of its shaping. We were concentrated on the bendability, density and the stability of compressed wood.

Key words: aspen wood, plastification, steaming, HF-heating, compression, density, bendability, stability, deformation, hardness..

INTRODUCTION

Softwood broadleaved species are still at the periphery of interest in industrial utilization. Manufacture of certain shaped furniture units for embossing front faces of furniture or their use for flooring shows as one possibility of their prospective utilization. It is necessary to choose suitable methods of plastification and shaping for the given purpose.

In our work we have concentrated on determination of the influence of plastification on changes of properties of aspen wood (populus tremuloides) for the purpose of its shaping. We were investigating the maximum deflection, the coefficient of bendability, the change of density, the change of hardness and the stability of compressed wood. The change of properties was studied in several moisture levels.

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

METHODS

Determination of bending characteristics (max. deflection and coefficient of bendability) was carried out on the specimens with dimensions 20 x 20 x 300 mm with the growth rings orientation on the cross section which enabled their bending in radial and tangential direction.

The maximum deflection was investigated by bending test on the tensile testing machine Rauenstein ZD 10/90. From the obtained values of maximum deflection (y_max) was calculated the minimal radius of bending (R_min) and from it the coefficient of bendability (k_oh) according the following relations 1 and 2:

(1)

where: 1 – spacing between supports (240 mm)

(2)

Shaping by pressing – compacting was evaluated according to the changes of thickness, hardness and stability of compressed wood. For this purpose were used the specimens with dimensions 55 x 50 x 50 mm. The maximum compacting was up to 50% of the thickness of the sample.

The changes of density were evaluated in percentage in relation to the original density of wood.

The harness of wood was tested by Janko's method, i.s. by pressing a hemisphere in the given depth.

The stability was evaluated by investigating the elastic and plastic (permanent) deformations on the basis of measuring the change of specimen's thickness in the given time intervals, before pressing, after pressing and after stabilization to 12% moisture content.

The bending characteristics of aspen were investigated at 12% and 30% moisture levels and shaping by pressing was conducted at 8%, 16%, 30% and 100% moisture levels.

The obtained measured and calculated values of investigated characteristics are presented in the form of tables and graphs.

RESULTS AND DISCUSSION

Bending characteristics of aspen wood are compared with beech wood, which is our main raw material for the manufacture of bentwood furniture. Acquired values are given in Figure 1.

As in follows from the results, in both moisture levels with non-plastified wood, values of coefficient of bendability (1/k_oh) in radial and tangential direction are nearly the same, and vary from 1:28 to 1:36 with aspen and from 1:25 to 1:35 with beech (Figure 1). By plastification the values of coefficient of bendability have doubled. With aspen, they vary in the range from 1:16 to 1:18 and with beech wood they vary in the range 1:15 to 1:20 (Fig. 1). Similar results were achieved by (Gáborík 1995, Gáborík, Zemiar 1998, Solár 2002 ).

From the viewpoint of bendability, there are not considerable differences between aspen wood and beech wood. The used methods of plastification also did not significantly manifested themselves. However, high frequency heating significantly reduces the time of wood plastification without an influence on the quality of bending. By HF - heating we have reached 4–8 times shorter times of plastification than by steaming, which is a great advantage. As follows from the further analyses of our results, aspen wood is plastified faster than beech wood. From our experiments follows that the plastifications times with aspen were shorter by 15 min with steaming and 2 min with HF - heating compared with beech wood [2, 3, 6].

The properties of compressed-densified aspen wood are evaluated by the change of its density (Fig. 2), by hardness (Fig. 3) and by stability (Figs. 4, 5, 6).

Modification of wood density by compression is influenced by many factors, but we have focused on the degree of compression, moisture content of wood and the method of plastification.

With increasing degree of compression the density is increasing with all investigated cases. By compression of wood to 50% of its initial thickness was reached as much as 35% change of its density (Fig. 2). The basic density (ρ₀) of non-treated aspen wood was in pre range from ρ₀ = 354 kg*m^-3 to ρ₀ = 502 kg*m^-3. After compression – compaction it was increased up to ρ₀ = 420 – 543 kg.m^-3.

With decreasing of moisture content and increasing degree of compression, the density of compressed wood is increasing independently on its treatment.

As follows from the results of experiments, the used method of plastification has now significant influence on the change of density of aspen wood (Fig. 2).

With the change of wood density is closely connected the change of hardness, which is characterized by graph in Figure 3. The similar relation between density and hardness is also given by Uhlíř [8]. With increasing density the hardness of aspen wood increases proportional, too.

The stability was evaluated by comparing the deformations after pressing and after air-conditioning to 12% moisture content, in dependency on the degree of compression and the initial moisture content (Figs. 4–9). The graphs in the pictures 4, 6 and 8 show the condition of compressed material immediate after its taking out of the press. From the graphs follow that with decreasing initial moisture content and increasing degree of compression the portion of plastic-permanent deformations and the deformations elastic in time is increasing. At 50% compression their portion represents 25–39%.

The Figures 5, 7, 9 show the condition of specimens after their air-conditioning to 12% moisture content. We can see disappearing of deformation elastic in time and the influence of different initial moisture contents. In comparing of data in Figures (5, 7, 9) we can state that the method of plastification does not have a significant influence on stability, which was confirmed by permanent deformations.

The permanent deformations are approximately the same with both methods. They stabilized to the maximum value 15%. The change of plastic-permanent values is affected by moisture content and the degree of compression. With decreasing moisture content the portion of plastic deformations is increasing which causes an increase of the stability of material. With lower moisture contents (8% and 16%) with increasing degree of compression the values of permanent deformations are increasing (up to 15%), which simultaneously increases the stability of material. Whereas in higher moisture contents (30% and 100%) the degree of compression was not significantly manifested.

CONCLUSIONS

The results obtained from our experiments show, that aspen wood is suitable for the manufacture of some shaped units. It is characterized by bendability, which is comparable with the bendability of beech wood. The values of bendability coefficient of aspen wood are nearly the same in radial and tangential direction, which is also valid with beech wood. Plastification has doubled the bendability of wood in comparison with non-plastified wood. The different methods of plastification were not significantly manifested. However, by both methods aspen wood was plastified faster than beech wood.

From the viewpoint of compaction of wood, the higher effect was reached with lower moisture contents and higher degree of compression. Compression to 50% increased the density of wood by 1/3 in comparison with the initial one. With increasing density the hardness of wood raises, too, which is of great importance for the industry of flooring.

The dimensional stability is appropriately characterized by permanent-plastic deformations. This factor is significant for the sphere of wood decorating by embossing.

In investigating into dimensional stability of aspen wood the different way of treatment was not manifested. Similar as in compacting (by pressing) the highest values of plastic deformations were obtained with the lowest moisture contents and the highest compression of wood. In higher moisture contents the degree of compression was not manifested. The lower moisture content and increasing degree of compression has a positive effect on the increase of stability of material.

From the viewpoint of bendability the aspen wood is suitable, but it is unsuitable for an independent use in the manufacture of shaped-bended furniture units for its softness, lower strength, problematic machining (“fluffiness”), as well as its poor deformability in compression area in bending – formation of folds. Its use in combination with other tree species (e.g. beech) seems to be more prospective, i.e. in the manufacture of laminated – lamellar shaped furniture units, which enable to utilize its good bendability and eliminate its negative properties.

REFERENCES

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