Results and discussion

For the determination of dynamic MOElv, the first vibration mode which represents its stiffness under compressive stress was measured, in consequence with the study on scantlings originating from Eucalyptus plantations (Hein et al. 2012). Table 1 shows the mean density, MOR, MOEsb, MOElv values of the results obtained from the investigated bark-based panels. Ratio values were determined from the division of MOElv by the MOEsb. Coefficient of determinations (R2) were linearly calculated to evaluate the correlation dependence of static MOEsb as a function of the dynamic MOElv on the specimens in each group of panels.

As indicated in Table 1 the bark-based panels density was calculated in the range of 350-400 kg/m³. A possible explanation for the increased mean density of the produced panels related to the target density of 350 kg/m³ could be the compression of bark particles during the hot pressing. Further, it was found that the addition of glass fibres exhibited opposite outcome, instead of the theoretically expected reinforcement on the mechanical properties of the bark panels. Additionally, the modulus of rupture was gradually decreasing by increasing the length of the glass fibres from 12 mm up to 30 mm. However, it seems that glass fibres did not occur any significant influence in the MOE, i.e. the stiffness of the investigated panels.

As illustrated by the results, the R² values in each group of panels were above 0.8 indicating strong correlation between the static and dynamic MOE measurements. The coefficients of determination resulted to be from 0.84 to 0.97. The only exception was in the case of control bark boards, in which the R² was defined as 0.50. In addition, As it is generally expected, the estimated MOElv are higher than the calculated MOEsb. This trend was verified for all the measured specimens. According to the findings, the mean

averages of the ratio values of the C_350, GF_12, GF_18, GF_24 and GF_30 panels were relatively high compared to an investigation on commercial wood-based panels (Poggi 2017).

Figure 1 Determination of MOE through the desctructive [a] and non-destructive tests [b]

Table 1 Reported mean values of the measured properties in this study. Standard deviations values are in brackets

In this study, there was an attempt to assess the calculation of MOE values of bark based panels with a common and simple set-up non-destructive test method. The predicted R² indicated comparatively strong correlations between the dynamic and static modulus of elasticity values. Therefore, determination of MOE low density reinforced bark-based panels through acoustic (resonance frequency) tests could potentially be feasible. However, higher amount of specimens is necessary to enhance and further verify the MOElv and MOEsb relationship through regression analysis statistics measurements. Results show the non-destructive testing for determination of mechanical properties needs further investigation. Testing method developed for structural material needs more sophisticated settings of parameters, but correlation could be found between tasting methods.

5. Acknowledgments

The described work was carried out as part of the „Sustainable Raw Material Management Thematic Network – RING 2017”, EFOP-3.6.2-16-2017-00010 project in the framework of the Széchenyi 2020 Program. This project is supported by the European Union, co-financed by the European Social Fund.

6. References

Irle, M. & Barbu, M-C., Wood-based panels: An introduction for specialists. Cost Action E49, Brunel University Press, 2010.

Blanchet, P., Cloutier, A. & Riedl, B., Particleboard made from hammer milled black spruce bark residues.

Wood Science and Technology 34, pp. 11-19, 2000.

Muszynski, Z. & McNatt J.D., Investigations on the use of spruce bark in the manufacture of particleboard in Poland. Forest Products Journal, 34(1), pp. 28-35, 1984.

Kain, G., Güttler, V., Barbu, M-C., Petutschnigg, A., Richter, K. & Tondi, G., Density related properties of bark insulation boards bonded with tannin hexamine resin. European Journal of Wood and Wood Products, 72(4), pp. 417-424, 2014.

Pásztory Z, Mohácsiné IR, Börcsök Z (2017) Investigation of thermal insulation panels made of black locust tree bark. Construction and Building Materials 147: 733-735

Ross, R.J., Nondestructive evaluation of wood. 2^nd ed., General Technical Report FPL-GTR-238, Madison WI, USA, pp. 1-5, 2015.

Cavalheiro, R.S., De Almeida, D.H., De Almeida, T.H., Christoforo, A.L. & Lahr, F.A.R., Estimation of modulus of elasticity in static bending of wood in structural dimensions as a function of longitudinal vibration and density. Current Journal of Applied Science and Technology, 26(1), pp.1-8, 2018.

Mochan, S., Moore, J. & Connoly, T., Using acoustic tools in forestry and the wood supply chain. FCTN018 Technical Note Forestry Commission, Online. 2009.

Niemz, P. & Mannes, D., Non-destructive testing of wood and wood-based materials. Journal of Cultural Heritage, 13S, pp. S26-S34, 2012.

Hein, P.R.G., Lima, J.T., Gril, J., Rosado, A.M. & Brancheriau, L., Resonance of scantlings indicates the stiffness even of small specimens of Eucalyptus from plantations. Wood Science and Technology, 46(4), pp. 621-635, 2012.

Maldonado, I.B., Gonzalez, G.I., Herrero, M.E. & Martitegui, F.A., Vibration method for the prediction of aging effect on properties of particleboard and fibreboard. Forest Products Journal, 62(1), pp. 69-74, 2012.

Ross, R.J. & Pellerin, R.F., NDE of wood-based composites with longitudinal stress waves. Forest Products Journal, 38(5), pp. 39-45, 1988.

Poggi, F., Bending properties of commercial wood-based panels by NDT methods. MSc Dissertation Linnaeus University, Växjö, Sweden, 2017.

III. RING – Fenntartható Nyersanyag-gazdálkodás Tudományos Konferencia 10-11 October 2019 - Sopron

INSULATION PANELS MADE FROM THERMALLY MODIFIED BARK

Zoltán Pásztory^a, Dimitrios Tsalagkas^a, Norbert Horváth^b, Zoltán Börcsök^a

a Innovation Center, University of Sopron, Sopron, Hungary, pasztory.zoltan@uni-sopron.hu

b Department of Wood Sciences, University of Sopron, Sopron, Hungary

Abstract: Bark insulation panels were made of pre-manufacturing thermally treated poplar bark. Bark chips were heat treated for one, two, and three hours at 180°C top temperature. The physical, mechanical and thermal properties of the panels were studied and compared to untreated ones. The target density was the same for every panel type. Thermal conductivity ranged from 0.064 – 0.067 W·m-1·K-1. The MOR and MOE showed a significant increase. The internal bond increased (27%) while the water absorption and thickness swelling decreased (53.8% and 69.1% respectively).

Keywords: bark panel, thermal insulation, pre-manufacturing thermal modification

1. Introduction

As most researchers have accepted climate change, reducing energy consumption has become increasingly important. One method for reducing energy demand for buildings is thermal insulation. On the other hand, the importance of natural-based, recyclable materials and solutions is increasing.

Therefore, many researches focusing on natural-based insulation materials, several natural and waste materials have been investigated including rice husk, sugar cane, coconut fiber, (Panyakaew and Fotios, 2008), cotton stalk fibers (Zhou et al., 2010), various grasses (Vėjelienė et al., 2011), papyrus (Tangjuank and Kumfu, 2011), pineapple (Tangjuank, 2011), jute (Fadhel, 2011), oil palm (Manohar, 2012), wool (Zach et al., 2012), wood ashes, cotton, animal hair (Rébék-Nagy and Pásztory, 2014), plant stalks, textile waste and stubble fibers (Binici et al., 2014) and straw (Volf et al., 2015), municipal solid waste (Faitli et al., 2015), geopolimer (Magyar et al., 2017). The thermal conductivity of insulation made of wood or other plant fibers ranged between 0.037 – 0.065 W·m^-1·K^-1 (Hurtado et al., 2016, Schiavoni et al., 2016, Veitmans and Grinfelds, 2016). Bark was also among the investigated materials (Kain et al., 2013, Pásztory and Ronyecz, 2013, Pásztory et al., 2017b).

The thermal modification of wood is well known. With heat treatment, dimensional stability and resistance against wood degrading organisms increases, while some strength properties decrease (Seborg et al., 1953; Rowel and Youngs, 1981; Hill, 2006). Many variables influence the results achieved during heat treatment including tree species, chamber design, treatment duration and temperature, closed or open system, medium, etc. (Rapp, 2001; Militz, 2002; Hill, 2006; Esteves and Periera, 2009; Navi and Sandberg, 2012; Sandberg and Kutnar, 2016).

Boards are heat treated mainly to reduce water uptake and thickness swelling, but in most cases the deterioration of the mechanical properties was observed when pre-manufacturing heat treatment was used (Lehmann, 1964; Tomek, 1966; Ohlmeyer and Lukowsky, 2004; Boonstra et al., 2006; Paul et al., 2006;

Mendes et al., 2013; Kwon and Ayrilmis, 2016; Lee et al., 2017).

Sometimes thermal post-treatment was used, but steam injection and post-treatment only work with isocyanate, PMUF, MUF, and phenol-formaldehyde adhesives (Ernst, 1967; Suchsland and Enlow, 1968;

Menezzi and Tomaselli, 2006; Boonstra et al., 2006; Ayrilmis et al., 2009; H’ng et al., 2012; Oliveira et al., 2017).

Thermal conductivity also decreases with thermal treatment (Sekino and Yamaguchi, 2010; Kol and Sefil, 2011; Korkut et al., 2013; Pásztory et al., 2017a). Similar processes can occur during heat treatment as the structure and the composition of the wood and bark are similar, but not the same.

The main goal of this investigation was to improve the mechanical properties and examine the thermal insulation property changes of insulation panels made of poplar bark. The secondary goal was to investigate the effect of thermal treatment duration on thermal conductivity and other parameters.

2. Materials and methods

‘Pannónia’ poplar clone (Populus ×euramericana (DODE)GUINIER cv. Pannónia) was studied, which clone is widespread in Hungarian plantations and favored by the wood industry because of its advantageous mechanical properties. Bark was collected from a nearby sawmill (TEAG PLC Wood Processing Plant). Inner and outer bark was not separated. The collected bark was hammer ground and dried to 8% moisture content. Particles smaller than 0.5 mm were fractionated from the chips.

A custom-made labor chamber was used for the treatment, which is not airtight, so steam escapes from the system during the treatment and so oxygen is present. The bark chips were heated from room temperature to 95°C in one hour, from 95°C to 130°C in another two hours, and then to the peak temperature of 180°C top in another 30 minutes. Three different treatment durations (constant temperature) were used which lasted one (T1), two (T2) and three (T3) hours (Figure 1). During cooling, the thermal inertia of the chamber was exploited; hence, the specimens were cooled to 25 °C in about 15 hours. Three panels were produced from each type.

Figure 1. Treated raw materials (C – control; T1 – one hour treatment; T2 – two hours treatment;

T3 – three hours treatment)

A laboratory hot press produced panels of 500×500×20 mm with the targeted density of 340 kg·m^-3 (Siempelkamp). The pressing time was 18 seconds per thickness millimeter, at 180 °C, with a pressure of 2.86 MPa, which was reduced after 120 seconds in three steps to release steam pressure inside the panel.

The physical and mechanical properties of the panels were examined. The thermal conductivity (λ) of all the panels was measured by a hot plate method. The temperature of the cold side was 5 °C and the hot side was 15 °C, with a mean temperature of 10 °C according to the standard (MSZ ISO 8301). To ensure parallel heat flow perpendicular to the surface of the panel, 15 cm of side insulation was used around the specimens. Before the thermal conductivity measurement, the panel had to reach a steady state, which was determined when the fluctuation of the last per minute measurement was under 0.002 W·m^-1·K^-1. The measuring equipment made one measurement every minute, and the average of the last 100 measurements was accepted as the measured result of the panel. The averages of the three data points collected were taken as the results.

Board moisture content and bulk density (𝜌) were calculated from ten samples taken from the panels. The thickness swelling (TS) and water absorption (WA) after immersion in water for 2 and 24 hours were calculated according to European standard EN 317 (1993). Ten 50×50 mm specimens were weighed and their thicknesses measured with an accuracy of 0.01 g and 0.1 mm, respectively. The samples were stored at 20°C and 65% relative humidity for seven days.

Bending strength, modulus of elasticity (MOR, MOE) (EN 310), and internal bond (IB) (EN 319) were tested using Instron 5506 universal testing machine. The specimens were prepared from different areas of the board and cut according to the EN 326-1 (1994) European standard.

The differences between the panels were evaluated with Statistica13 software. To find means of different treatment that significantly varied from each other, the Tukey-test was run on the raw data. On the basis of the differences and the identities of the different variables (MOR, MOE, TS, WA, EMC), the treatments were grouped to identify the similarities and differences between treatments.