Influence of gluing procedure on rheological properties of laminate elements.

Obucina, Murco ; Dzaferovic, Ejub ; Resnik, Joze 等

Abstract Influences of conventional hot (CH) and high frequency (HF) gluing on some relevant physical and mechanical properties, and creep at constant load and high humidity have been researched for laminate elements that made from beach peeled veneer. The Standard linear and four-parameter viscoelasticity Burger model were considered. Unknown model parameters were determined according to the experimental creep results and the method of the least squares. Obtained results for different gluing procedures were showed and discussed.

Key words: laminate elements, conventional hot gluing, HF gluing, creep, viscoelastic model.

1. INTRODUCTION

In wood industry gluing is considered to be the most spread technological procedure for wood bonding. In order to speed the gluing process, the glue and wood warming is carried out at proposed temperatures, depending on gluing regime.

The wood parts produced by HF gluing are often met in practice. The research has shown that HF glued parts have different deformations as compared with the hot glued parts made by conventional hot gluing. (Resnik, 1995).

For the analysis of stability of a wood construction product exposed to loading is important to know what kind correlation exists between deformations ([epsilon]) and stress ([sigma]). It was believed for a long time that linear correlation between stress and deformation was sufficient for product construction design (Hook's law). It was known that during long loading period wood-bending strength decreased. In the practice, the difference between behavior of wood products and the result carried out according to Hook's law has been shown. The ratio between stress and deformation was not linear in the whole area and deformation was not entirely recovered. It has been determined that the correlation between stress and deformation was not linear in the whole area and deformation was not entirely reversible. Wood behaves as a linear viscoelastic material up to certain load limit at constant moisture and temperature. (Hanhijarvi and Hunt, 1998); (Dinwoodie, 2000). In the present research of the data related to the relevant physical and mechanical properties of laminate elements, produced by different technological gluing procedures, were determined by experimental method.

2. EXPERIMENTAL DETAILS

Pealed beach veneer (Fagus Sylvatica L) of the 1st degree of quality, mostly with radial cut, size 550x550x2.1 mm and with 5.5% average moisture content was used for the construction of laminate elements. Melamine-urea-formaldehyde glue was used. Glue was spread over veneer manually by roller in quantity of 180 g/m2. The specific pressure of 1.8 MPa was applied for gluing. HF generator was used for HF gluing. Working frequency of the generator was 4.75-6.3MHz, nominal power 6-18 kW generating 800 V/cm strong electrical field between electrodes. During the gluing procedure the temperature between the third and fourth layer of veneer was measured and its flow chart is shown in Figure1.

[FIGURE 1 OMITTED]

The boards made by conventional hot gluing were marked as K and the boards made by HF gluing were marked as HF. After standard conditioning (T = 20 [degrees]C and f = 65%) specimens were cut out of each laminate elements board for determining relevant mechanical properties, as well as 15 specimens, 40 mm of width, for creep compliance test. Creep test was carried out on a device with four-wheel loading, where it was possible to perform creep test for 5 elements simultaneously (Figure 2). The device was placed in a chamber where relative humidity was maintained at a constant level (94 %) by axial fans forcing the air to flow over a container with saturated solution of water and [K.sub.2]S[O.sub.4]. The room temperature was 22.5 [degrees]C. Constant loading level was applied by weights (Figure 2 position 6). The mass of the weights together with press device was 35.94 kg, which is 17.5% of the maximum loading (sample fracture). By means of an electronic computer card A/D the processor records data in the first 10 seconds at the rate of 5 data/measurements per second, then one data/measurement per 2 seconds over the next 600 s and then one data/measurement per 600 s.

[FIGURE 2 OMITTED]

The theory of viscoelasticity defines standard mechanical models used for description of viscoelastic properties of the materials. The simplest models that can sufficiently describe linear viscoelastic behavior of wood at constant moisture content and temperature are the three-parameter and four-parameter Burger models, respectively (Skrzypek. and Hetnarski, 1993)

For the given loading situation, it is considered for viscoelastic materials that modulus of elasticity (E) is equivalent to relaxation modulus. For moderate loading values (compared to bending strength) and for the linear viscoelastic material behavior, the creep compliance is equivalent to the reciprocal value of elastic modulus [Obucina at al., 2006]

[DELTA]U(t) = 23/648 F x [l.sup.3]/I x J(t)

Where by: [DELTA]U(t)-linear visco-elastic deflection (m); F-force (N); l--reference lenghth (m); I--moment of inertia of the beam's cross section (m4), E-modulus of elasticity (MPa) Parameters of the three-parameter and four-parameter models, which describe visco-elastic behaviour of LVL elements, may be determined by application of the method of least squares.

3. RESULTS AND DISCUSSION

After the standard conditioning of the specimens some relevant physical and mechanical properties of laminate elements were determined according to the corresponding standard methods. Average values of these properties are shown in Table 1: The statistical analysis based on experimental data was carried out. The unknown obtained parameters for both models the corresponding experimental data and the calculated results are given by curves in Figure 3.

There are various processes occurring in laminate elements depending on the applied gluing procedure During the hot gluing, veneer layers warm up from outer layers towards the middle layers. Heat transfer to outer layers is from the hot pressing device. Depending on temperature, added heat and gluing pressure, the moisture in wood can evaporate. One part of moisture evaporates to the surrounding and the rest of it diffuses into wood gradually. Vapor moves toward the middle where the temperature is lower. As the heat is removed, vapor condenses and that produces higher moisture content and greater deformation in the outer layers comparing to the corresponding middle values.

During HF gluing the warming of veneer layers is fast and almost equally over the whole cross section. Because of different dielectric properties, glue is warmed faster than wood and a part of heat is transferred to the pressing device. A balanced temperature gradient enables fast and equal moisture distribution.

[FIGURE 3 OMITTED]

4. CONCLUSION

The following conclusions can be summarized from the described experiment:

* The thickness loss for laminate elements made by hot gluing was 1.7 % (absolute difference) greater than in the case of HF gluing. This difference is the result of twice longer pressing time for hot gluing procedure.

* Because of the same reason the specimens made by hot gluing had 4 % lower humidity and 2 % higher density.

* Lower humidity and higher density gave 6 % higher bending strength and small difference for elastic modulus 0.3 % for specimens made by hot gluing comparing to HF gluing.

* Analysis of creep curves and models unknown parameters pointed out that the creep values depend on the duration time of gluing procedure.

* Laminate elements made by HF gluing procedure had better rheological properties (smaller creep values), i.e. higher rheological parameters for both considered models.

* 4-parameter model was in better correlation with experimental data.

5. REFERENCES

Dinwoodie, J.M (2000). Timber its nature and behavior. London and New York, p. 245.

Hanhijarvi A., Hunt D. (1998) Experimental indication of interaction between viscoelastic and mechano-sorptive creep, Wood Science and Technology 32,

M. Obucina, E. Dzaferovic, R. Bajramovic, J. Resnik Influence of gluing technology on viscoelasticity properties of LVL; Wood Research. 51 (4):2006; 11-22

Resnik J., Tesovnik F. (1995) Thickness loss when gluing veneer sheets into boards in the hot press or by high frequency, Holz als Roh--und Werkstoff, 53: 113-115

Skrzypek J.J., Hetnarski R.B. (1993) Plasticity and Creep, theory, examples, and Problems, International Standard Book Number 0-8493-9936-X.

Table 1. Average values of physical and mechanical properties

Propert Unit HF CH

[rho] kg/[m.sup.3] 743.69 757.2
w % 9.85 9.49
TL % 5.4 7.1
[[sigma].sub.bs] MPa 129.9 137.8
[E.sub.m] MPa 13978 14015
[[sigma].sub.ss] MPa 5.05 4.83

 absolute ratio
Propert difference HF/CH

[rho] 13.58 0.98
w 0.36 1.04
TL 1.7 0.76
[[sigma].sub.bs] 7.9 0.94
[E.sub.m] 37 1.00
[[sigma].sub.ss] 0.22 1.05