FIRMOLIN : the mild way of thermal wood modification.

Transcription

1 FIRMOLIN : the mild way of thermal wood modification. Wim Willems FirmoLin Technologies BV Grote Bottel 7a, NL-5753 PE Deurne 1. Introduction Wood is exposed to a high level of risk of structural destruction during thermal modification treatments, when the process conditions cause absolute dryness (the oven-dry state) of wood. The anisotropic ring structure as well as possible natural wood defects, cause extremely high macroscopic stresses in oven-dried wood. Moisture reconditioning in the final process stage of the oven-dried wood will induce additional tensile stresses in the core of the timber, similar to those in casehardened timber (Figure 1). Internal stresses of this kind can lead to direct failure during production, or in service when exposed to variable humidity. Fig. 1: Stresses on reconditioning oven dried wood. The swelling of the timber will commence at the surface, but is restricted by the shrunken core, which will cause tension (minus signs) in the core and compression (plus signs) at the surface. The core tension causes internal checks, whereas compression closes surface checks (indicated by the arrow). Under varying environmental conditions surface checks may grow into the core tension region where they will inevitably cleave the timber along the rays. In a mild heat treatment the stress signs for core and surface are reversed. The compression in the core will then effectively stop the check propagation. In addition to these macroscopic stresses, there are increased microscopic stresses in the wood cell walls in response to humidity variations in service, since the free shrink/swell movement is restricted by much stronger molecular cross-links within the cellulose microfibrils, compared to normal wood (Gonzales-Peña & Hale 2005). Such cross-linking has a stabilizing effect on wood, but will break-down on the longer term (several years) under the action of humidity variations leading to wood failure. Dimensional stabilization of thermally modified wood is preferably obtained by removing the sites for water adsorption, rather than stiffening the cell wall by crosslinking. The latter cannot be prevented under oven-dry conditions. To obtain an improved kind of TMT (Thermally Modified Timber), the oven dry state was principally avoided in the so-called Firmolin process (firmo- Latin: strong, durable, -lin Abbreviated from -lignum. Latin: wood), (Willems 2006, 2009). Since

2 2009, a pilot production plant according to this principle is in use in the Netherlands at FirmoLin Technologies BV. This company for the production and sales of TMT is also active in research and development of the Firmolin technique and owns the intellectual rights of Firmolin. In the next chapter some details of the Firmolin process are theoretically explained. The advantages of the Firmolin process are illustrated with a number of practical applications. 2. Thermal wood modification revisited Wood chemical considerations. Thermal wood modification takes place at the ultra-structural level of the cell wall. The main constituents, (hemi-)cellulose and lignin, are modified by three main chemical reactions (Table 1, Fengel & Wegener 2003). The running order of these reactions is in accordance with the numbering in the table. On dry heating, reaction types 2 and 3 experience optimal conditions. The moisture needed for hydrolysis (reaction 1) is then, however, lost by evaporation, which must be compensated by long heating times at very high temperatures (>200 ºC). This problem was principally solved in the PLATO process (Tjeerdsma et al 1998) by employing separate treatment stages for hydrolysis in saturated steam (165 ºC) and for dehydration and curing in dry heat (185 ºC), with an intermediate lowtemperature conventional drying step. Burmester (1973) has shown in his FWD-process that thermal modification is possible in an autoclave with high pressure superheated steam treatments on kiln dried wood (MC=12-14%). In an early stage of this process acetic acid is liberated from the wood cell wall and remains trapped inside the timber by the action of the external steam pressure. Acetic acid acts as a catalyst on all three reaction types, which enables a fast modification at moderate temperature. At temperatures below 190 ºC very durable material can thus be produced (Ladner 2003; Tausch 2011; Willems et al 2014). Table 1: Wood chemical reactions during thermal modification. Reaction type Water Heat Catalyst 1 Hydrolysis Consumption Consumption Acid 2 Dehydration Production Consumption Acid 3 Cross-linking Production Production Acid 2.2. Wood physical considerations To avoid the evaporative loss of the acetic acid catalyst, the FWD-process requires a minimal gas pressure. To avoid the core stress problems shown in Figure 1, the moisture content of the wood should be kept sufficiently high. Both of these conditions are met in the Firmolin process by means of steam in hygroscopic equilibrium with the wood moisture. Typical Firmolin process conditions are indicated with the marked area in the temperature-relative humidity state diagram (Figure 2). FirmoLin Technologies BV 2 of 5

3 The associated pressures in this state diagram are indicated by isobars (p=1 to 10 bar abs.). Lines of constant moisture equilibrium (EMC) are also drawn, but must be interpreted with great care, because the changing wood composition during modification will significantly shift the moisture equilibria in time (Willems 2014). The typical Firmolin process conditions are clearly different dry heating processes with steam pressures of less than 3 bar (abs) and T> 200 C Technological considerations In production equipment the process conditions must be stable and uniformly controlled over the entire treatment space. An autoclave with this purpose was equipped with an accurate climate control system (figure 3). The autoclave is uniformly heated with thermal oil in a jacket around the vessel. The vapour is circulated inside the vessel by a fan. The humidity control acts via a separately controlled water reservoir temperature (Willems 2009). The climate variation can thus be controlled within narrow limits. The equipment has been shown to be able to heat kiln dried wood to high temperatures without degrade even in the presence of large structural defects (figure 6). Fig. 2: Process conditions for Firmolin treatments (marked area), determined by the conditions for controllable wood moisture content and controllable wood chemistry. Solid lines: steam isobars. Dotted lines: isosteres (constant EMC) Fig. 3: Schematic Drawing of the Firmolin Process. The controllability of the process is achieved by regulating the temperatures of the vessel wall and the water reservoir. 1 = Heating jacket of the autoclave, 2 = Thermal isolation, 3 = Fan, 4 = Timber stack, 5 = Heating element, 6 = Water reservoir, 7 = Water vapour. 5 FirmoLin Technologies BV 3 of 5

4 3. Results (Figures 4 7) Fig. 4: Spruce poles in large cross section. The absence of a stress distribution as shown in figure 1 can be shown with timber of large cross section. This picture shows a cross cut of a treated 140 mm diameter spruce pole (pith-free middle section of 4 m length) Firmolin, treated at 180ºC, free of internal and surface checks. Fig. 5: Treatment of pointed wooden stakes. The high steam pressure enables fast and homogeneous penetration of heat over the entire wood volume, independent of the diameter. Likewise, different timber dimensions (thickness) can be combined in one treatment batch with a uniform end result. The right hand stake was cut open after the treatment. The visible deep checks on the surface originate from air drying the pith containing stakes prior to the heat treatment. Fig. 6: Deformation- and check-free treatment, by the absence of drying gradients. This is evidenced in this beech board with a large structural defect, having a tendency to deform, which still remains stable. Fig. 7: Coloured heartwood zone of beech. The boundary between the coloured and white zones in heartwood of beech is not susceptible to check formation under Firmolin conditions. Arrows indicate the position of this boundary of the beech board (treated as 75 mm unedged boards, then re-sawn and planed to 65 mm thickness). FirmoLin Technologies BV 4 of 5

5 4. Conclusions The examples shown in chapter 3 prove the mild nature of Firmolin treatments, allowing completely new wood and product applications with thermally modified wood. The advantages of Firmolin are best utilised: 1. in exterior applications with extremely changing conditions (e.g. rain/sun on decking) because of the advantageous mechanical stress distribution in combination with the low shrinkage/swelling figure. 2. in solid wood applications (window/door frames, doors, poles), because of the possibility to treat wood in large dimensions without checks and distortion. 3. with wood species with a natural tendency to warp, which remain stable during the Firmolin treatment. References Burmester, A. (1973): Einfluß einer Wärme-Druck Behandlung halbtrockenen Holzes auf seine Formbeständigkeit. Holz als Roh- und Werkstoff 31, pp Fengel, D.; Wegener, G. (2003): Wood, Chemistry, Ultrastructure, Reactions, Verlag Kessel, D- Remagen, paperback, 613 p. González-Peña, M.M., Hale, M.D.C. (2007) The relationship between mechanical performance and chemical changes in thermally modified wood. Third European Conference on Wood Modification. H. Militz. Cardiff, University of Wales, Bangor: pp Ladner, C. (2003): Dauerhaftigkeit von modifiziertem Holz gegenüber holzzerstörenden Pilzen, Mster thesis, BOKU, Vienna Tausch, A. (2011): Pilzresistenz von Thermoholz - gravimetrische und mikroskopische Untersuchungen. PhD Thesis, Albert-Ludwigs University, Freiburg Tjeerdsma, B.F.; Boonstra, M.; Militz, H. (1998): Thermal modfication of non-durable wood species II. IRG/WP Document , 10 p. Willems, W. (2006): Patent NL ; US ; WO 2008/ A1 Willems W. (2009): A novel economic large-scale production technology for high-quality thermally modified wood. In: Proceedings of the 4th European Conference on Wood Modification, Stockholm, Englund F, Hill C, Militz H, Segerholm B (Eds). pp Willems, W. (2014): The hydrostatic pressure and temperature dependence of wood moisture sorption isotherms. Journal of Wood Science and Technology 48, pp Willems, W., Altgen, M., Militz, H. (2014) Comparison of fungal durability and equilibrium moisture content of heat-treated wood obtained in high versus low water vapour pressure reactor systems. International Wood Products Journal (invited paper). FirmoLin Technologies BV 5 of 5