Eur. J. Wood Prod. (2009) 67: 237–239 DOI 10.1007/s00107-008-0300-2
BRIEF ORIGINALS · KURZORIGINALIA
Temperature and moisture content behaviour in microwave heated wood prior to bending – Mountain Ash (Eucalyptus regnans) Beat Studhalter · Barbara Ozarska · Graeme Siemon
Published online: 23 December 2008 © Springer-Verlag 2008
Subject The Cooperative Research Centre for Wood Innovations, within the University of Melbourne, has developed an innovative wood bending technology which involves the use of microwave softening of wood prior to bending, an automated process of wood bending suitable for “mass production” and microwave drying of bent components. This paper discusses the microwave softening of wood required prior to bending, in particular, temperature and moisture distribution and transmission in cross-sections of microwave heated wood. Optimal heating parameters for softening wood using microwave irradiation were also investigated.
1 Introduction An extensive research study has been undertaken by Cooperative Research Centre for Wood Innovations at the University of Melbourne aiming to investigate an innovative technology for the manufacture of bent components suitable for regrowth and plantation timber resources. The research involves the application of microwave technology for softening timber in order to make it plastic for bending, the development of an automated wood bending machine and microwave drying and stabilization of the bent components (Juniper 2008, Harris et al. 2007). B. Studhalter · B. Ozarska (u) School of Forest and Ecosystem Science, The University of Melbourne, 500 Yarra Boulevard, 3121 Richmond, Victoria, Australia e-mail:
[email protected] G. Siemon Forest Products Commission WA, Locked Bag 888, 6849 Perth Business Centre, Western Australia, Australia
This paper describes a research study undertaken with the aim to determine temperature distribution and transmission in cross-sections of wood during and after the heating process and to determine moisture distribution variations in the cross-section before and after the heating process. In addition, the heating process was investigated to determine optimal heating parameters for softening wood using microwave irradiation.
2 Materials and methods Timber used in this study was mountain ash (Eucalyptus regnans) from a natural regrowth stand in Victoria. Samples (25 mm × 25 mm × 350 mm) of three different moisture contents, 20, 35%, green (80–120%), were selected after conditioning to the required moisture content (MC). Each section was heated to either 85 or 105 ◦ C in a 5 kW/2.45 GHz online heating microwave applicator purposely designed and built. Two different heating times were applied, either slow heating or fast heating. With slow heating the timber was heated to 85 or 105 ◦ C within 24 seconds, while in fast heating the timber was heated within 12 seconds. The feed rate remained the same regardless of the heating time at 2.9 cm/s (1.75 m/min). Growth ring alignment was either parallel or perpendicular to the microwave propagation. Temperature was recorded continuously with 1.7 mm diameter fibre optic probes (FOP) in the middle of the specimens as well as on all four surfaces, using holes of 2.5 mm diameter. The moisture profile and moisture variations were determined before and after the heating process according to Australian Standard AS/NZ 1080.1:1997, using cores taken horizontally or vertically from the length of timber. These cores were each sliced into five segments and weighed be-
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fore and after oven-drying to estimate moisture content. This procedure was carried out on samples without applying microwave energy and immediately after microwaving.
3 Results and discussion The moisture profile before and after microwaving in the horizontal direction for each MC group is presented in Fig. 1. The vertical moisture profiles were shown to be similar to Fig. 1 Moisture profile in horizontal direction before and after microwaving Abb. 1 Feuchteprofil in horizontaler Richtung vor und nach Erhitzung in der Mikrowelle
Fig. 2 Mean temperature profile for all three MCs and two temperatures Abb. 2 Mittleres Temperaturprofil der drei Holzfeuchten und zwei Temperaturen
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Eur. J. Wood Prod. (2009) 67: 237–239
the horizontal profiles and therefore are not presented in this paper. The x-axis represents the location of the measurement in cross-section. Number one is located on the right surface and number five on the left surface, the surface exposed to the microwave. Figure 2 presents the temperature distributions for the three MC groups and the two targeted temperatures. Each box represents the temperature distribution within the cross section of the specimen as it was placed in the microwave tunnel.
Eur. J. Wood Prod. (2009) 67: 237–239
The analysis of the results reveals that the moisture profile does not change significantly during microwave heating and moisture loss is negligible. The MC has a significant effect on the temperature development; it affects both the heating and the cooling processes. Timber with lower MC can be heated more uniformly than wet timber and the energy requirement is not as high as for wet timber. The initial moisture content affected the temperature development significantly. The temperature in the centre of the specimen averaged 33, 45 and 106% higher than its surface temperatures for the different moisture content groups of 20, 35% and green respectively. The largest temperature gradient occurred in green timber, where the lowest surface temperature was 37 ◦ C while the central temperature was 105 ◦ C. The green timber cooled down significantly faster after microwaving than timber at the other two moisture contents, on average 100% faster than the lower moisture content groups of 20 and 35%. Microwaves heat wood very quickly from the inside to high temperatures, but the low temperatures on the surfaces are less satisfactory for the bending process. The desired
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temperature profiles for small cross-sectioned timber using microwave irradiation to obtain higher surface temperatures can be achieved by appropriate application of microwave equipment, energy and feed rate schedules. The fibre saturation point does have a significant effect on the heating process. The results suggest that the presence of free water in the wood cells has a significant influence upon the interaction between timber and microwave irradiation.
References Australian Standard: AS/NZ 1080.1:1997: Timber – Methods of test – Moisture content Harris GA, Brodie GI, Taube A (2007) Design of a microwave chamber for the purpose of drying of wood components for furniture. Conference paper for “The Recent Advances in High Power Microwave Research and Applications Conference”, 29–31 January 2008, Geelong, Melbourne, Australia Juniper LF (2008) Investigation into the mechanical behaviour of Eucalyptus regnans during microwave bending. PhD Thesis. The University of Melbourne
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