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Borůvka, V., Gaff, M., Babiak, M., and Matúš, M. (2015). "Dimensional changes of veneer layered materials after cold pressing," BioRes. 10(4), 6663-6675.

Abstract

Dimensional changes in both non-densified and densified, thin, wooden components and layered materials after external pressing forces were released were evaluated in this work. Densification was carried out using a cold process on a semi-automatic hydraulic pressing machine. The specimens’ dimensional stabilities, focusing mainly on their residual plastic deformations, were monitored. The impacts of several factors, such as wood species, material thickness, densification degree, and their combinations, were analyzed. Results showed that, with increased degree of densification, the relative plastic deformations (pressing degree) usually decreased. With regard to the compositions explored, the best combination was a top poplar layer densified by 10% plus a bottom beech layer densified by 20%. The impacts of each of the factors on the pressing degree values proved to be significant; the least significant was the bottom beech layer thickness and degree of densification. The greatest practical benefits can be obtained using the recommended combinations of composite layers.


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Dimensional Changes of Veneer Layered Materials after Cold Pressing

Vlastimil Borůvka, Milan Gaff, * Marián Babiak, and Marko Matúš

Dimensional changes in both non-densified and densified, thin, wooden components and layered materials after external pressing forces were released were evaluated in this work. Densification was carried out using a cold process on a semi-automatic hydraulic pressing machine. The specimens’ dimensional stabilities, focusing mainly on their residual plastic deformations, were monitored. The impacts of several factors, such as wood species, material thickness, densification degree, and their combinations, were analyzed. Results showed that, with increased degree of densification, the relative plastic deformations (pressing degree) usually decreased. With regard to the compositions explored, the best combination was a top poplar layer densified by 10% plus a bottom beech layer densified by 20%. The impacts of each of the factors on the pressing degree values proved to be significant; the least significant was the bottom beech layer thickness and degree of densification. The greatest practical benefits can be obtained using the recommended combinations of composite layers.

Keywords: Veneer; Layered material; Densification; Pressing; Dimensional stability; Deformation

Contact information: Department of Wood Processing, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521 Czech Republic;

* Corresponding author: gaffmilan@gmail.com

INTRODUCTION

The term “thin-layered materials” includes all kinds of wood layers (veneers, lamellae, flooring components, etc.) up to thicknesses of 10 mm (Zemiar et al. 2009). Thin-layered materials are basic materials used for decorative and structural purposes when creating various more complex material types. They function either as the main structural element (in plywood and lamellar materials) or as a predominantly decorative covering on a bearing material (usually wood-based). In these cases, the surface veneers are important for the assurance of product/material quality.

In some cases, other properties are required in addition to the aesthetic requirements of the surface veneers. These include hardness, resistance to wear, smoothness, improved mechanical properties, or others (Kvietková et al. 2015a,b). Greater hardness than that of natural wood, and other wood properties inherent thereto— such as bending strength, modulus of elasticity, and density—can be achieved by means of pressing.

Pressing is a molding process during which external forces decrease the volume of a work piece; the material is densified (Zemiar et al. 2009). Wood pressing can differ in the wood pre-stamping treatment, the direction of the acting force, the output mode, as well as in other features (Nemec et al. 1986; Kafka 1989; Fekiač et al. 2015). Flat pressing was selected for the veneer pressing.

Neither the dimensional nor the shape changes taking place during pressing are permanent (Gáborík and Dudas 2006; Marko 2010; Gašparík and Barcik 2013). After the external forces are released, wood tends to revert to its initial shape and dimensions (Fig. 1). Unlike when it is placed under the long-term loading, elastic deformations are predominant only under short-term loading that is lower than the limit of elasticity, (Fig. 1). Therefore, under short-term loading, the proportionality limit should not be exceeded. Generally, the purpose of wood pressing is to obtain a product with stable dimensions and shape and higher density than that of natural wood (Lakes 2009).

Fig. 1. Creeping course: stress and deformation vs. loading/release time curve while not exceeding the proportionality limit

Dimensional stability is also the basis of shape stability. The goal of this research was to determine the dimensional changes occurring after the veneers were pressed under the selected conditions. This goal resulted from the application of the pressed veneers onto the surfaces of layered materials.

EXPERIMENTAL

Materials

Typical domestic representatives of both hard- and softwood species were chosen as the input materials for this study (Požgaj et al. 1997; Kurjatko et al. 2010): European beech (Fagus sylvatica L.) and Eurasian aspen (Populus tremula L.), respectively.

A multi-degree choice was used for the preparation of the specimens. The first degree consisted of the selection of suitable beech and aspen roundwood grown in the Sekier forest of the Technical University Forest Enterprise in Zvolen. Circular cutting yielded tangential pieces of timber 10 and 20 mm thick. The cut timber was dried spontaneously in cages such that the moisture was decreased below the fiber saturation point to between 20 and 25%. Subsequently, they were cut to 1-m lengths, the side surfaces were cleaned on a circular saw, and the pieces were divided to dimension stocks. Levelling and thickness equalizing were done. Next, the required number of dimension stocks were cold densified with pressurizing plates at 20 °C in two pressing steps; the thickness was reduced either by 10 or 20%. A polyurethane glue was applied to the selected stocks and groups were assembled (Liptáková and Sedliačik 1989). These groups were pressed with a UT6L (Italpresse, Italy) pressing machine (Fig. 2) under 0.6 MPa (eventually, 0.3 MPa) for 15 s (eventually, 90 min).

Fig. 2. Hydraulic pressing machine model UT6L

After pressing, specimens with dimensions of 60×60 mm were cut (Fig. 3). Subsequently, the specimens were conditioned to approximately 12% moisture in a Binder conditioning chamber (ED, APT Line II; Germany) at 65% relative humidity and 20 °C. The achievement of equilibrium moisture content was verified in accordance with ISO 3130 (1975).

Fig. 3. Test specimen

The experiments were carried out in individual layers (Fig. 4) as well as in various two-layer combinations (Fig. 5), created with densified/non-densified beech/aspen wood layers glued with polyurethane glue. There were a total of 48 groups, each consisting of 10 pieces. The group identification method is shown in Fig. 6. For two-layer groups, the top layer was always aspen.

Methods

The work methodology was based on the experimental verification of the pressed veneer-based materials’ dimensional changes. The test pieces were loaded with uniform pressure on their entire surface in the radial direction (Fig. 7). After this test, the deformations created on the test pieces (flexible, temporary flexible, and plastic) were monitored.

Fig. 4. Flowchart of specimens by the individual layers

Fig. 5. Flowchart of specimens of two-layer materials

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Fig. 6. Group identification method for specimens of both individual layers and two-layer materials

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Fig. 7. Test pieces (single- and two-layer) pressing

Testing was carried out on a Rauenstein ZDM 10/90 (Germany) tensile testing machine at pressures up to 60 kN for durations of approximately 2 min. The thickness of the specimens was measured immediately after releasing the pressurizing plates within an accuracy of 0.01 mm. After the test was over, the dimensional changes caused by the pressing were monitored after 0.5, 1, 2, and 24 h (Fig. 9). The thickness was measured at two marked points located 1.5 cm from the test piece margin (Fig. 8). The measured data were graphically illustrated during testing. Curves in the graph were used to identify the different types of deformations. Care was taken to make repeated measurements at the same points.

vykresy vlasta-Rozvržení1 (4)_3

Fig. 8. Test piece with measurement points

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Fig. 9. Deformation curve showing the entire course of thickness change, from compression until dimensional stabilization

Calculations and Evaluation

The moisture content of the specimens was determined using Eq. 1 according to ISO3130 (1975),

 (1)

where w is the moisture content of the specimens (%); mw is the mass (weight) of the test specimens at moisture content (kg); and m0 is the mass (weight) of the oven-dry test specimens (kg).

In addition to the individual deformation components of the total deformation, the relative plastic deformation values and the pressing degree were also computed from the measured values (Nemec et al. 1986), using Eq. 2,

 (2)

where εc is the deformation (%); lis the original dimension of the specimens in the pressing direction (mm); and lmin. is the minimum dimension (length) of the specimens in the pressing direction after loading (mm).

RESULTS AND DISCUSSION

As previously mentioned, neither the dimensional nor the shape changes that take place during pressing are permanent. After the external forces are released, the wood tends to revert to its initial shape and dimensions. The selected force acting on the specimens’ surface developed a pressure stress equal to approximately 17 MPa. For the radial pressure on diffuse porous wood species (which both beech and aspen are), this means that the proportionality limit was undoubtedly exceeded. Thus, the
proportionality limit, which is conventionally referred to as the breaking limit, was
undoubtedly exceeded (Regináč et al. 1990). The so-called “elastic-viscoplastic area” was achieved in such cases, as was the goal (Kunesh 1968). For aspen, the conventional breaking limit is approximately 3.1 MPa, and for beech it is approximately 7.0 MPa at 12% moisture content (USDA Forest Service 1999). For this stress type, the radial pressure determines the specific shape of the stress/deformation curve. It consists of three phases. During pressing, the third phase was achieved.

Fig. 10. Three-phase stress/deformation diagram for radial pressure applied to dispersed, porous wood species

The goal was to examine the responses of non-densified and densified thin components of wood and layered materials composed of beech and aspen wood after the pressing forces were released with regard to their deformations. To make correct comparisons, the data obtained from relative plastic deformation are most suitable, as their absolute values are affected by the various thicknesses of the test pieces (Table 1). Likewise, the deformation distribution to flexible and temporary flexible is not perfectly accurate since there is no unambiguous differentiation between reversible deformations, as far as the time line concerns. Therefore, the main feature monitored was the dimensional stability of the test pieces, primarily the residual plastic deformations. Eventually, the pressing degree (relative plastic deformation) was also monitored. The impacts of several factors such as the wood species, material thickness, degree of densification, and layer position were analyzed.

Table 1. Average Values and Portions of Elastic, Temporary Flexible, and Plastic Deformations for Both Individual Layers and All of Their Combinations

 

(D1 = flexible deformation; D2 = temporary flexible deformation; D3 = plastic deformation)

When comparing the layer thicknesses, the smallest absolute deformations were obviously found in the single layers, mostly those of beech with 5-mm thickness. For composite layers, the smallest absolute deformations were found in test pieces 5.10 to 5.0 and 5.10 to 10.20 mm. With increasing degree of densification, the relative plastic deformations decreased. In terms of the layer composition, the best combination was a top layer (aspen) densified by 10% and a bottom layer (beech) densified by 20%. The impact of each of the factors on the pressing degree was significant. The least significant was the impact of the bottom beech layer thickness and densification, as shown in Table 3 and in the graphic illustrations in Figs. 11 to 16. A similar impact was also demonstrated in terms of the factor’s contribution to the plastic deformation share of the total deformation (Table 4). The impact of each of the factors on the absolute plastic deformation was unambiguously significant (Table 2).

Some paradoxical results, mainly following the 10% densification of the bottom layer, may have been caused by the cold densification process.

Table 2. Impact of Factors on Absolute Plastic Deformation

Table 3. Impact of Factors on Relative Plastic Deformation

As shown in Fig. 11, the aspen wood (A) plastic deformation was the greatest, while that of the beech wood (B) was the lowest.

The relative plastic deformation values increased with increasing thickness of the growth top layer (Fig. 12).

Table 4. Impact of Factors on Plastic Deformation Share in Total Deformation

(for the meaning of 2-3-4-5, see Tables 2 and 3)

Fig. 11. Wood species’ effect on relative plastic deformation. A – Aspen, B – Beech, AB – Aspen and Beech. Data reported as mean ± SD Fig. 12. Effect of thickness of top layer on relative plastic deformation. Data reported as mean ± SD

Statistically significant decreases in the relative plastic deformation values were found with increasing top layer densification (Fig. 13).

As shown in Fig. 14, the lowest values of the monitored features were measured for bottom layer thickness equal to 10 mm.

Fig. 13. Effect of compression of top layer on relative plastic deformation. Data reported as mean ±SD Fig. 14. Effect of thickness of lower layer on relative plastic deformation. Data reported as mean ±SD

While the bottom layer densification increased, the relative plastic deformation values decreased in a statistically significant manner (Fig. 15). As shown in Fig. 16, a statistically significant decrease in the relative plastic deformation value took place with test piece densification. On the other hand, the monitored feature value increased in a statistically significant manner for the non-densified pieces.

Fig. 15. Effect of compression of lower layer on relative plastic deformation. Data reported as mean ± SD Fig. 16. Effect of compression and thickness of lower layer on relative plastic deformation. Data reported as mean ± SD

CONCLUSIONS

  1. When comparing the layer thicknesses, the smallest absolute deformations were found for single layers, mostly of beech with 5 mm thickness.
  2. For the composite layers, the smallest absolute deformations were in specimens 5.10-5.0 and 5.10-10.20.
  3. With increasing degree of densification, the relative plastic deformation decreased; as far as the layer composition is concerned, the best combination was of a top layer (aspen) densified by 10% and a bottom layer (beech) densified by 20%.
  4. The impact of each of the factors on the pressing degree value proved significant; the least significant was the bottom beech layer thickness and densification.
  5. The combinations 10-10 and 10-5 appeared unsuitable due to deformations reaching high values. This was caused by the aspen’s thicknesses and low degree of densification.
  6. The use of combinations of layers 5.10-5.0 and 5.10-10.20 is recommended. The top layer (aspen) was densified by 10%, while the bottom layer (beech) was either not densified or densified by 20%.
  7. One further recommendation is to carry out similar research using the same wood species but at densification rates from 30 to 50%.

ACKNOWLEDGMENTS

The authors are grateful for the support of the Internal Grant Agency of the Faculty of Forestry and Wood Science under project No. B05/15, “Properties of laminated materials based on wood and non-wood components.”

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Article submitted: June 2, 2015; Peer review completed: August 11, 2015; Revised version received and accepted: August 13, 2015; Published: August 19, 2015.

DOI: 10.15376/biores.10.4.6663-6675