Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Mould resistance, water resistance and mechanical properties of OHT-thermowoods Ladislav Reinprecht1, Zuzana Vidholdová2 1
Faculty of Wood Sciences and Technology, Technical University of Zvolen, SK-960 53 Zvolen, Slovakia,
[email protected]
2
Faculty of Wood Sciences and Technology, Technical University of Zvolen, SK-960 53 Zvolen, Slovakia,
[email protected]
Keywords: Thermowood, colza oil, moulds, swelling, soaking, compression strength, hardness
ABSTRACT Resistance of wood against decay, mould, and other types of biological damaging can be increased either with biocides, or by modification of its structure. Today various thermally treated thermowoods (ThermoWood, PlatoWood, OHT-wood, New-option-wood, etc.) with improved durability are applied for internal and also for external usage. The aim of this work was to modify beech wood (Fagus sylvatica L.) and sap part of pine wood (Pinus sylvestris L.) with OHT “oil-heat-treatment” process at temperatures of 180 °C and/or 220 °C, during 3 or 6 hours, using colza oil prepared from the seeds of Brassica napus. Laboratory prepared OHTthermowoods have been tested against the mixture of moulds (Aspergillus amstelodami, A. niger, Penicilllium brevicompactum, P. aurantiogriseum, Paecylomyces variotti, Chaetomium globosum and Trichoderma viride), and also against selected strains of Aspergillus and Penicillium. Growth of moulds on the surfaces of OHT-thermowoods was a milder comparing to natural woods, mainly if the treatment processes were carried out at higher temperatures and lasted longer time. They resistance was relatively the best against the strains of Aspergillus. All OHT-thermowoods better resisted to water – they had lower values of volume swelling and soaking. The compression strength parallel to grain changed at OHT-thermobeech (increased only at the most milder regime 180 °C/3h while at others decreased about 4-28 %), and at OHTthermopine increased about 3-29 %. The Brinell hardness of OHT-treatedwoods decreased maximally about 1.5-33 % at beech and 7-27 % at pine in a comparison to untreated woods. INTRODUCTION Thermal modification of hard- and soft-woods is a physical-chemical process at which are partially changed their structural characteristics and selected properties. Thermowood products, such as ThermoWood (Jämsä and Viitaniemi 2001, Mayes and Oksanen 2002), PlatoWood (Boonstra et al. 1998, Militz and Tjeerdsmna 2001), RetificatedWood (Vernois 2001), OHTWood (Rapp and Sailer 2001, Brischke and Rapp 2004), but also some others, have a better dimensional stability, lower hygroscopicity, higher durability – mainly better resistance against wood destroying fungi and insects, etc.. Their modulus of elasticity is obviously unchangeable, however their modulus of rupture in bending and other strength properties moderately fall off. A ratio of changes in the chemical and anatomical structure of wood, and subsequently in its physical, biological and mechanical properties is given either by the used method of thermal modification “e.g. regulation of temperature, time, pressure, air/nitrogen/steam conditions, etc.”, and either by the used species of wood and its initial moisture content (Tjeerdsmna et al. 1998, Yildiz et al. 2004, Hill 2006, Rapp et al. 2008).
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Wood species classified as non-durable or little durable against wood destroying fungi (with the 5th or 4th classes of durability by EN 350-2) are after thermal treatment at 180-220 °C during some hours classified as moderately durable or durable (with the 3rd or 2nd class of durability). Thermowood surfaces can better resist also to moulds, which was documented e.g. by Wang and Cooper (2005) at wood modified by the OHT-process using the soy oil or palm oil (in this work we tested the colza oil), but sometimes their mould resistance is only a very poor (Kartal 2007). EXPERIMENTAL METHODS Woods From beech (Fagus sylvatica L.) and sap-pine (Pinus sylvestris L.) have been prepared samples with the average densities: o-Beech = 729 kg/m3, o-Pine = 420 kg/m3, and these dimensions: - 50x20x5 mm (LxRxT) → mould tests (nBeech = 162 + 54, nPine = 162 + 54), - 30x20x20 mm (LxRxT) → swelling (nBeech = 30 + 5, nPine = 30 + 5), - 30x20x20 mm (LxRxT) → soaking (nBeech = 35, nPine = 35), - 30x20x20 mm (LxRxT) → compression strength parallel to grain (nBeech = 35, nPine = 35), - 30x20x20 mm (LxRxT) → Brinell hardness (nBeech = 60 +10, nPine = 60 + 10). OHT-process Wood samples have been dried at 103 2 °C into the oven dry state (mo, Vo, o), acclimatized 12 days in desiccators at 20 °C, then impregnated with pure colza oil (from the seeds of Brassica napus - Dr. Oils, Paradiso, Bzenec, Czech Republic) by Lowry at a pressure of 0.8 MPa lasting 3 hours, and finally undergone to the thermal modification in hot pure colza oil. Totally we have used 6-types of the OHT (oil-heat-treatment) process at the following temperatures and times: a) 180 °C / 3 hours b) 180 °C / 6 hours c) 220 °C / 3 hours d) 220 °C / 6 hours e) 180 °C / 3 hours + 220 °C / 3 hours f) 220 °C / 3 hours + 180 °C / 3 hours Colza oil retention Masses of all OHT-thermowoods (mT) were determined for calculation of the oil retention (ROil): ROil = (mT – mo)/Vo [kg/m3]
(1)
Mould tests Prepared OHT-thermowoods have been tested against moulds applying the Slovak Technical Standard STN 49 0604 (III./ test with the mixture of moulds), or variations of this Standard (I./ test with the Aspergillus strains; II./ test with the Penicillium strains): I. ASPERGILLUS: Aspergillus amstelodami Thom Church, and Aspergillus niger Van Theghem. II. PENICILLIUM: Penicillium aurantiogriseum Dierolax, and Penicillium brevicompactum Dierolax. III. MIXTURE OF MOULDS: A. amstelodami, A. niger, P. aurantiogriseum, P. brevicompactum, Chaetomium globosum Kunze ex Freis, Paecilomyces variotti Bainier, and Trichoderma viride Persson ex S.F.Gray. All these mould tests were carried out during 28 days in the Petri dishes 100 mm at t = 28 2 °C and = 95 2 %, and evaluated in the 7th, 14th, 21st and 28th day (Table 1, Fig. 1).
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Swelling and soaking tests Volume swelling of OHT-thermowoods was tested by the STN 49 0126, and evaluated in the ¼, ½, 1, 2, 3, 5, 8, 24 hour and then in the 2nd, 3rd, 4th, 7th, 14th, 21st and 28th day (Fig. 2). Soaking of distilled water into OHT-thermowoods was tested at the same samples by the STN 49 0104, and evaluated in the same time intervals as swelling (Fig. 3). Compression strength parallel to the grain test Untreated woods and OHT-thermowoods have been firstly conditioned at t = 20 2 °C and = 65 2 % into the moisture equilibrium state, and then the compression strength parallel to the grain was tested at them by the STN 49 0110 (Fig. 4). Brinell hardness test Untreated woods and OHT-thermowoods have been firstly conditioned at t = 20 2 °C and = 65 2 % into the moisture equilibrium state, and then the Brinell hardness (HB) was tested on their tangential surfaces using steel ball (D = 10 mm) and constant strength FBeech = 500 N or FPine = 200 N, measuring the diameter of ball-imprints (d in mm) into wood surfaces (Fig. 5). HB = 2 . F / {( . D) . [ D – (D2 – d2)]}
[MPa]
(2)
RESULTS AND DISCUSSION In this work we analyzed the efficacy of OHT-process carried out with colza oil on the mould resistance (Fig. 1) and selected physical properties (Fig. 2-5) of beech and pine woods. Retention of colza oil (ROil) into the OHT-thermo-beech samples was from 166 to 414 kg/m3, and into the OHT- thermo-sap-pine samples from 452 to 683 kg/m3. Oil retentions into sap-pine were similar as determined Spear et al. (2006). Growth of moulds on the surfaces of laboratory prepared OHT-thermowoods was partly suppressed, mainly if the treatment processes were carried out at a temperature of 220 °C during 6 hours, or if the combined treatment processes (e/ 180 °C/3h + 220 °C/3h; f/ 220 °C/3h + 180 °C/3h) were used (Table 1, Fig. 1). Efficacy of a shorter treatment process at 220 °C/3h and mainly of the processes carried out at 180 °C during 3 or 6 hours are from the practical point of view un-sufficient. Mould resistance of OHT-thermowoods increased especially against the strains of Aspergillus. Better results have been achieved with the beech in a comparison with the sap-pine. Table 1: Average growth degrees of moulds (GDM = 0-4) on wood surfaces in the 28th day OHT-process a/ 180 °C/3h b/ 180 °C/6h c/ 220 °C/3h d/ 220 °C/6h e/ 180 °C/3h + 220°C/3h f/ 220 °C/3h + 180 °C/3h Untreated wood
Aspergillus spp. Beech Sap-Pine 2.22 3.22 2.78 3.33 1.44 2.78 1.33 1.56 1.11 1.78 1 1.78 4 4
Penicillium spp. Beech Sap-Pine 4 3.67 4 3.56 3.67 3.56 2.33 3.33 3.44 3.22 2.78 3.22 4 4
Mixture of moulds Beech Sap-Pine 3.89 3.78 3.89 3.67 3.67 3.56 3.22 3.33 3.56 3.11 3 3.33 4 4
Note 1: Growth degree of moulds “GDM”: 0 none growth; 1 10 % of the wood surface 50x20 mm is covered with moulds, 2 25 % of the surface covered with moulds, 3 50 % of the surface covered with moulds, 4 50 % of the surface covered with moulds. Note 2: For each type of OHT-thermowood have been tested 9 samples (n = 9), so totally we tested 324 thermowood samples.
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market a) Beech Aspergillus spp.
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
Mixture of Moulds
Penicillium spp.
4,5 4,0
4,5
Mould growth degree
4,0
Mould growth dregree
Mould growth dregree
4,5
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
7 days
14 days
21 days
28 days
4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
7 days
14 days
21 days
28 days
7 days
14 days
21 days
28 days
b) Sap-Pine Aspergillus spp.
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
Penicillium spp.
4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
7 days
14 days
21 days
28 days
Mixture of Moulds
4,5
Mould growth dregree
4,0
Mould growth dregree
Mould growth dregree
4,5
4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0
7 days
180 °C/3h 220 °C/6h untreated wood
14 days
21 days
28 days
180 °C/6h 180 °C/3h + 220 °C/3h
7 days
14 days
21 days
28 days
220 °C/3h 220 °C/3h + 180 °C/3h
Figure 1: Kinetics of mould growth on the surfaces of OHT-thermowoods and untreated woods
Volume swelling of OHT-thermowoods (Fig. 2) and their soaking (Fig. 3) were lower in a comparison to untreated beech and sap-pine. Better results have been achieved with samples thermally treated at harder conditions (t 220 °C; 6 hours). Sap-Pine 20
18
18
volume swelling [%]
volume swelling [%]
Beech 20 16 14 12 10 8 6 4
16 14 12 10 8 6 4 2
2
0
0 0
100
200
300
400
500
600
hours 700
0
100
200
300
400
500
600
hours 700
600
hours 700
Figure 2: Kinetics of volume swelling of OHT-thermowoods and untreated woods Sap-Pine
140
140
120
120
water soaking [%]
water soaking [%]
Beech
100 80 60 40 20
100 80 60 40 20
0
0 0
100
200
300
400
500
600
hours 700
0
100
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Figure 3: Kinetics of water soaking of OHT-thermowoods and untreated woods Note 1: Each point in the Figures 2 and 3 is a average result from 5 samples (n = 5) Note 2: Legends in the Figures 2 and 3 are following: 180 °C/3h 220 °C/6h untreated wood
180 °C/6h 180 °C/3h + 220 °C/3h
220 °C/3h 220 °C/3h + 180 °C/3h
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Compression strength parallel to grain decreased at OHT-thermobeech about 4-28 % (increased only at the regime 180 °C/3h), while at OHT-thermopine increased about 3-29 % (Fig. 4). Sap-Pine Compression strength [MPa]
Compression strength [MPa]
Beech 100 90 80 70 60 50 40 30 20 10 0 180 °C/3h
180 °C/6h
220 °C/3h
220 °C/6h 180 °C/3h + 220 °C/3h + 220 °C/3h 180 °C/3h
100 90 80 70 60 50 40 30 20 10 0 180 °C/3h
untreated wood
180 °C/6h
220 °C/3h
220 °C/6h 180 °C/3h + 220 °C/3h + 220 °C/3h 180 °C/3h
untreated wood
Figure 4: Compression strength parallel to grain of OHT-thermowoods and untreated woods
Brinell hardness of OHT-treatedwoods decreased maximally about 1.5-33 % at beech and about 7-27 % at pine in a comparison to untreated woods (Fig. 5). Beech
Sap-Pine 70
Brinell hardness [MPa]
Brinell hardness
[MPa]
70 60 50 40 30 20 10
60 50 40 30 20 10 0
0 180 °C/3h
180 °C/6h
220 °C/3h
220 °C/6h 180 °C/3h + 220 °C/3h + 220 °C/3h 180 °C/3h
untreated wood
180 °C/3h
180 °C/6h
220 °C/3h
220 °C/6h 180 °C/3h + 220 °C/3h + 220 °C/3h 180 °C/3h
untreated wood
Figure 5: Brinell hardness of OHT-thermowoods and untreated woods
CONCLUSIONS
OHT-thermowoods a gently better resisted to moulds, more evidently at higher treatment temperature ( 220 °C) and time ( 6 hours), and especially against the Aspergillus. OHT-thermowoods better resisted to water – they had lower values of volume swelling and soaking. Mechanical properties of OHT-thermowoods, it means compression strength parallel to grain and especially Brinell hardness, were usually lower in a comparison to untreated woods.
Acknowledgements The authors wish to express their thanks to the Grant agency of the Slovak Republic (Grant No. 1/4377/07) and the COST Action E37 for the financial support of this work.
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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
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