Eur. J. Wood Prod. (2009) 67: 383–396 DOI 10.1007/s00107-009-0332-2
ORIGINALS · ORIGINALARBEITEN
Thermal modification of consolidated oriented strandboards: effects on dimensional stability, mechanical properties, chemical composition and surface color C.H.S. Del Menezzi · I. Tomaselli · E.Y.A. Okino · D.E. Teixeira · M.A.E. Santana
Received: 12 August 2008 / Published online: 19 April 2009 © Springer-Verlag 2009
Abstract The objective of this paper was to propose a thermal post-treatment to improve the dimensional stability of oriented strandboard (OSB). Commercial OSB panels were obtained from an industrial batch and thermally treated in a single opening hot-press at two temperature levels (190 and 220 ◦ C) and three duration times (12, 16 and 20 min). Dimensional stability, mechanical properties, chemical composition and surface color were studied. The results pointed-out that the proposed treatment can be applied to significantly improve the OSB dimensional stability by reducing thickness swelling, water absorption, and equilibrium moisture content in comparison to the untreated board. The mechanical properties were partially affected with reduction in modulus of rupture and without any adverse effect on the other properties. Chemical degradation occurred, mainly in relation to hemicelluloses contents, reducing equilibrium moisture content. The board surface became darker and this characteristic was correlated with the observed properties improvement. Dimensional stability properties were affected by both temperature and duration C. Del Menezzi (u) Department of Forest Engineering, Faculty of Technology, University of Brasilia – UnB, Campus Darcy Ribeiro, 04357, 70919-970 Bras´ılia, Brazil e-mail:
[email protected] I. Tomaselli Forest and Wood Science Centre, Federal University of Paran´a – UFPR, 40234-910 Curitiba, Brazil
of the treatment, while the others mainly by temperature. The proposed thermal treatment can be recommended as a post-treatment to improve the OSB performance.
Zusammenfassung Ziel dieser Studie war es, OSB-Platten einer nachtr¨aglichen W¨armebehandlung zu unterziehen, um deren Dimensionsstabilit¨at zu verbessern. Handels¨ubliche OSB-Platten aus industrieller Fertigung wurden in einer Einetagen-Heißpresse bei zwei unterschiedlichen Temperaturen (190 und 220 ◦ C) unterschiedlich lang (12, 16 und 20 Min) w¨armebehandelt. Untersucht wurden die Dimensionsstabilit¨at, die mechanischen Eigenschaften, die chemische Zusammensetzung und die Oberfl¨achenfarbe. Die Ergebnisse zeigten, dass das vorgeschlagene Verfahren geeignet ist, durch Reduzierung der Dickenquellung, der Wasseraufnahme und der Gleichgewichtsfeuchte die Dimensionsstabilit¨at von OSB-Platten signifikant zu verbessern. Die mechanischen Eigenschaften waren davon teilweise betroffen. Die Biegefestigkeit nahm ab, die anderen Eigenschaften wurden nicht beeintr¨achtigt. Ein chemischer Abbau wurde festgestellt: im Wesentlichen bez¨uglich des Hemicellulosegehalts, wodurch die Gleichgewichtsfeuchte reduziert wurde. Die Holzoberfl¨ache wurde dunkler, und war mit den verbesserten Eigenschaften der OSB-Platten korreliert. Die Dimensionsstabilit¨at wurde sowohl von der Temperatur als auch von der Behandlungsdauer beeinflusst, w¨ahrend die anderen Eigenschaften nur von der Temperatur beeinflusst wurden. Die vorgeschlagene W¨armebehandlung kann als ein nachtr¨agliches Behandlungsverfahren zur Verbesserung der OSB-Eigenschaften empfohlen werden.
E. Okino · D. Teixeira · M. Santana Forest Products Laboratory – Brazilian Forest Service, Ministry of Environment, Av. L-4 Norte-SCEN, Trecho 2, lote 4, bloco B, 70818-900 Bras´ılia, Brazil
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1 Introduction The dimensional instability of wood is a major problem in the forest products industrialization. The hygroscopicity is transferred to all wood-based products. For boards like oriented strandboard (OSB) and particleboard (PB), in addition to wood hygroscopicity, there is also compression stresses imposed during hot-pressing. Board production needs hotpressing to provide particles intercontact, resin curing and board consolidation. The particle mat is pressed to reach the final board’s thickness, resulting in thickness reduction of 5 to 8 times. Thus, a great amount of compression stress will be retained, forcing the board to return to the original thickness. As the board comes in contact with any form of water, the compression stresses are released and, in combination with natural swelling of the wood, imparts alteration in the dimensional stability of the board, mainly thickness. In order to improve dimensional stability of the woodbased boards it is necessary to release the compression stresses and to modify the wood hygroscopicity. The former can be done by heating the board or the furnish above its viscoelasticity transition temperature (Tg), while the wood hygroscopicity can be reduced by imposing some chemical degradation. The hemicelluloses are the least heat stable wood polymers and their degradation can start above 150 ◦ C (Fengel and Wegener 1989) resulting in a weight loss of the wood. Likewise, a change in the degree of cellulose crystallinity (P´etrissans et al. 2003, Bhuiyan et al. 2001) has been observed being responsible for reducing wood hygroscopicity. Many researches concerning methods for improving dimensional stability of the board have focused on pretreatments of furnish (Mohebby et al. 2008, Paul et al. 2007, Paul et al. 2006, Boonstra and Tjeerdsma 2006). The principle is based on modifying the furnish hygroscopicity to make it less stiff to permit better accommodation of the furnish and consequently the stress during pressing, which can reduce then the latent compression stress. However, a severe thermal treatment of the furnish can reduce the quality of the wood bonding through the adverse impact on internal bonding (IB) and also on wood wettability, as observed by Boonstra and Tjeerdsma (2006), and P´etrissans et al. (2003), respectively. Paul et al. (2006) evaluated a very promising method to improve the manufacture of OSB, where strands of Pinus sylvestris were pre-treated at 220 and 240 ◦ C for 30 min. All dimensional stability properties were enhanced, conversely to bending properties and internal bonding. Recently, Winandy and Krzysik (2007) evaluated the effect of extended hot-pressing on MDF properties and observed that the longer the hot-pressing the better the dimensional stability. On the other hand, it is possible to improve the dimensional stability of consolidated boards applying a post-
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treatment (Suchsland and Xu 1991, Hsu et al. 1989). Del Menezzi and Tomaselli (2006) evaluated this treatment on laboratory made PF-bonded OSB. Results showed that thickness swelling and equilibrium moisture content could be reduced by 50 and 46%, respectively. Okino et al. (2007) employed this treatment to improve the performance of UF-bonded OSB from Cupressus glauca strands. Reduction in dimensional movement in heat-treated boards was reported and some resistance against wood fungi was obtained as well. Del Menezzi et al. (2008) observed that treated OSB maintained strength/stiffness after natural weathering exposure at a higher level than untreated one. An improvement in decay resistance was also observed. The versatility of this thermal post-treatment comes from the possibility to diminish the wood hygroscopicity and to release the compression stress at the same time. Del Menezzi and Tomaselli (2007) concluded that, in the Brazilian scenario, this is a cost-effective method for the OSB industry depending on the duration of treatment. They evaluated the economic feasibility and all studied economic variables showed very positive values. The final cost of the treatment was less than US$ 10.00/m3. According to the proposed scenario, 11–16 hot-presses with 15 openings would be necessary to treat 100 000 m3 /year. The proposed method can also be applied by the final consumer instead of the industry, as it had happened for other methods. This means a great advantage, mainly for OSB high volume consumers such as building companies, which can treat the board according to their needs. In this context, this paper aimed at deepening the study about thermal post-treatment, evaluating the effects of the temperature and duration of treatment on dimensional stability, mechanical properties, surface color and chemical composition of commercial OSB.
2 Materials and methods 2.1 Boards characteristics Forty-two commercial OSB boards were obtained from an industrial batch. One sample of 500 × 500 × 12.7 mm (w × l × t) dimensions was cut from each board. According to the manufacturer, the boards were made from Pinus taeda wood with a nominal density of 640 kg/m3, 19 kg/m3 of resins (11.4 kg/m3 of phenol-formaldehyde for external layers and 7.6 kg/m3 of isocyanate resin for the core layer), 230 g/m3 of wax and 40 ml/m3 of insecticide. The samples were measured, weighed to determine the density (D) and put into a conditioned room (20 ◦ C; 65% RH) until equilibrium moisture content was reached.
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2.2 Thermal treatment The conditioned boards were treated at two temperature levels, 190 and 220 ◦ C, for 12, 16 and 20 min in a laboratory single opening hot-press. Each combination of temperature-time was considered a group: T1190 ◦ C/12 min; T2-190/16; T3-190/20; T4-220/12; T5220/16 and T6-220/20. Another group was kept untreated as a control. Seven groups were evaluated, each one with six repetitions, totalizing 42 OSB. Prior to the treatment, a 5 × 5 cm2 specimen was cut from each board to determine the moisture content (MC). During the treatment, the pressure was applied just to assure contact between the press platens and the board (< 20 kPa). After the treatment, the boards were kept at room temperature to cool down and then weighed to calculate the loss of weight (LW). The boards returned to the conditioned room and were re-weighed after 45 days to determine the permanent loss of weight (PLW).
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ate a calibration curve for each sugar of interest and to determine the response factors. The calibration curve was a set of multi-component standards containing glucose, xylose, galactose, arabinose and mannose in the range of 0.01–3.00 mg/mL. The chromatographic system was controlled by a Varian® workstation. The neutralized hydrolysate specimens were analyzed using Bio-Rad Aminex column HPX-87P, 7.8 × 300 mm. The procedure for each specimen was done in duplicate and each hydrolysate was injected two times. Integrated peak areas from standard sugar solutions were used to compute the response factors. The instrumental conditions were set according to Okino el al. (2008). The determination of acid insoluble lignin was done according to Templeton and Ehrman (1995) and acid soluble lignin was conducted according to Ehrman (1996) using a UV spectrophotometer (Femto® Model 700 plus). 2.3.3 Color measurement
2.3 Material properties 2.3.1 Dimensional stability and mechanical strength The control and treated boards were evaluated according to ASTM D1037-98 for dimensional stability and mechanical properties and compared with CSA O437.0 (1993) standard. From each board four specimens were evaluated regarding dimensional stability properties: thickness swelling (TS), water absorption (WA) and equilibrium moisture content (EMC). The permanent thickness swelling (PTS) was the relationship between the board thickness before and after TS test and drying. For TS/WA/PTS evaluation 7.6 × 7.6 cm2 samples were used. Likewise, four specimens were obtained to evaluate modulus of rupture (MOR), modulus of elasticity (MOE), internal bonding (IB) and compression strength (COMP). For the static bending testing (MOR and MOE) and COMP half of the specimens were cut in parallel () direction and the other half in perpendicular (⊥) direction. 2.3.2 Chemical composition From each board a specimen (5 × 5 cm2 ) was cut to provide material for chemical analysis which was conducted according to Okino el al. (2008) and Santana and Okino (2007). The procedure is summarized as follows. The polysaccharides were hydrolyzed to their sugar monomers by sulfuric acid in a two-stage hydrolysis process. The hydrolysis of the extractive-free comminuted untreated and treated samples was quantified by HPLC. A series of sugar calibration standard solutions was prepared in deionized water containing five sugars in approximately the same proportions found in the wood, in order to cre-
The color of the boards was measured according to the CIE L ∗ a∗ b∗ System using the Datacolor Microflash D200 spectrophotometer. Four measurements were taken of each surface. The device was set for D65 illuminant and measurement angle of 10◦ for obtaining the variables lightness (L ∗ ) and the chromaticity coordinates a∗ (red-green axis) and b∗ (blue-yellow axis). These data were used to calculate the hue angle (h ∗ ) and the saturation (C). The color difference (ΔE ∗ ) and change in saturation (ΔC) were determined according to Charrier et al. (2002) and Okino et al. (2009). These variables were employed to compare the colorimetric values (L ∗ , a∗ and b∗ ) after and before the thermal treatment and to measure the effect of the proposed thermal treatment on the board color. 2.4 Statistical analysis Initially, the mean values of the properties were submitted to an analysis of variance (ANOVA) and the Dunnett test to evaluate the differences between the properties of the control and the treated boards at α = 0.05 of significance. Dunnett test compared the means of the control and treated boards pair to pair, instead of comparing the entire treatments. For the treated boards, further full factorial ANOVA (2 × 3) was run to identify the effects of temperature, time and interaction on the properties. For the variables affected by time Tukey HSD at α = 0.05 of significance was run to separate means. Further stepwise regression analysis was conducted for modeling the properties affected by the treatment using colorimetric and loss of weight as predictors. In this kind of regression, predictor variables were added and removed from the model to identify a useful group to compound the model.
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3 Results and discussion 3.1 Comparison between control and treated boards 3.1.1 Dimensional stability The board density (D) and moisture content (MC) were 643 kg/m3 and 9.2%, respectively. Figure 1 presents the results of the dimensional stability properties of the boards. It was observed that for all properties, the response of the treated boards was better than that of the untreated ones, indicating that the proposed thermal treatment modified the wood-water relationship. In general, thermal treatment
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meant an improvement of these properties. It was also observed that the improvement was higher as the temperature and time of the treatment were increased. The statistical analysis found significant differences between the evaluated dimensional stability properties. It was detected that even in the less severe treatment group, T1 (190 ◦ C, 12 min), there was a great reduction in TS2H, TS24H and EMC in comparison to the control boards. Thermal treatment was also effective to WA2H and WA24H, although in a lower magnitude. Thus, the proposed treatment reduced significantly the water absorption of the boards. The main objective of the treatment was to reduce the TS and, according to the results, it was achieved. Compar-
Fig. 1 Mean values, standard deviation and Dunnett test results for dimensional stability properties of control and treated boards. (Dashed line means maximum value for O-1 grade according to CSA O437.0 (1993); ∗ , ∗∗ significant at α = 0.05 and α = 0.01, respectively) Abb. 1 Mittelwerte, Standardabweichungen und Dunnett Signifikanzwerte verschiedener Messwerte zur Dimensionsstabilit¨at der behandelten und unbehandelten Kontrollplatten (die gestrichelte Linie bezeichnet den H¨ochstwert f¨ur O-1 Platten gem¨aß CSA O437.0 (1993); ∗ , ∗∗ signifikant bei α = 0,05 und α = 0,01)
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ing the mean values of TS24H control boards (19.2%) with T6 (9.7%), the most severe group, there was a reduction of approximately 49%. Thus, the treated board could comply with the Canadian Standard (CSA) requirement, which is 15% maximum. The hypothesis is that the thermal treatment released the internal compression stresses generated during hot-pressing of the board. This can be observed in the values of PTS. When the boards are soaked in water, the compression stresses are released and, in conjunction with wood natural swelling, determine the TS intensity. If the board is dried, the wood tends to return to its original thickness, but, the TS relative to the compression stress does not, and the thickness of the board remains higher than prior to the test. As can be seen in Fig. 1, the higher the thermal treatment temperature, the lower the PTS, indicating that thermal treatment reduced the level of compression stresses, providing a decrease in TS of the treated boards. These PTS values indicate that the thermal treatment almost totally eliminated the compression stresses of the treated board, as verified in treatments T5 (−89%) and T6 (−94%). These compression stresses can be released as a function of the nature of the wood polymers. As previously explained, these polymers are viscoelastic showing a glassy and a rubbery regime that depends on the temperature. Thus, when the lignin is below Tg, it is in a glassy state, whereas when it is above Tg it performs as a viscous polymer, less stiff. The thermal treatment provided this viscous condition, so during the treatment the board particle matrix lost stiffness and the compression stresses were released. Del Menezzi and Tomaselli (2006) argued that the stiffness reduction permits the stress release, because during the treatment the particle matrix becomes momentarily less stiff and
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rearranges at lower stress level. The thermal treatment reduced the WA values as can be observed in Fig. 1. However, the tendency was not remarkable as observed for the TS values. The less severe treatment (T1–T3) presented WA values sometimes near the control values, while others (T4– T6) were lower. It has been observed that WA is not directly affected by the thermal treatment (Mohebby et al. 2008), but by board density (Del Menezzi and Tomaselli 2006). The EMC was very positively affected by the thermal treatment. It can be seen in Fig. 1 that EMC values for treated boards were up to 46% lower than the control ones. This property relates the amount of water molecules adsorbed in the hydroxyl groups of the cellular wall, and moreover, it can be suggested that the thermal treatment reduced these sites or became less available. The thermal treatment can modify the structure of the wood polymers by transforming chemically or even degrading them. The reduction of the water adsorption sites may have occurred due to the polymers structural reorganization, such as cross-linking or by wood polymer degradation. Kolin and Janezic (1996) argue that the wood adsorption capacity can be affected by thermal treatment probably because it affects the accessibility of the hydroxyl groups in the amorphous area, thus reducing their capacity to connect water molecules resulting in a lower EMC. Tjeerdsma et al. (1998) affirmed that the cellulose and lignin can form cross-links, thus reducing the sites of water adsorption. The formation of this cross-linked net as a function of the thermal treatment was argued by Kosikova et al. (1999). Tjeerdsma et al. (2000) argued that hemicelluloses degradation, esterification and decrease of the cellulose crystallinity can also be mentioned as factors for hygroscopicity reduction as observed in thermally treated wood. Several authors
Fig. 2 Mean values and standard deviation for weight loss, permanent weight loss and density of the treated boards. (Different letters indicate that difference between means are statistically significant at α = 0.05 according to the Tukey HSD test) Abb. 2 Mittelwerte und Standardabweichungen des Masseverlustes, des permanenten Masseverlustes und der Dichte von behandelten Platten (unterschiedliche Buchstaben bedeuten, dass sich gem¨aß Tukey HSD Test die Mittelwerte bei α = 0,05 statistisch signifikant unterscheiden)
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(P´etrissans et al. 2003, Bhuiyan et al. 2001, 2000) have observed that thermal treatment affects the amorphous cellulose area reducing water adsorption and consequently reducing the EMC of the treated boards. The effect of thermal degradation can be observed by the loss of weight (LW and PLW) of the boards after treatment, as shown in Fig. 2. It is evident that the more severe the treatment the higher the loss of weight, without affecting the density of the treated board (DAT). The initial board MC was about 9.2%, while the LW varied from 9.3% (T1) to 11.9% (T6). It may be supposed that in treatment T1 the post-treatment only dried the boards, while in the other treatments some additional loss of components occurred. However, a definitive loss of component (PLW) could be observed for all treatments.
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3.1.2 Mechanical properties The results of flexural properties are presented in Fig. 3 while compression strength and internal bonding are presented in Fig. 4. It can be observed that thermal treatment resulted in a slight reduction of certain properties. However, there was no clear tendency for this effect; in other words, higher increasing temperature and time of treatment did not result in further reduction. The differences were more accentuated for the properties of treatment T6, the more drastic condition. However, some treatments resulted in higher values than the control. The behavior of these properties is very different, some groups are more resistant to thermal treatment, as seen for MOE and IB, while others are more sensitive, as for MOR and COMP.
Fig. 3 Means, standard deviation and Dunnett test results for flexural properties of control and treated boards (Dashed line means minimum requirement for O-1 grade according to CSA O437.0 (1993); ∗ significant at α = 0.05) Abb. 3 Mittelwerte, Standardabweichungen und Dunnett Signifikanzwerte der Festigkeits- und Steifigkeitseigenschaften der behandelten und unbehandelten Kontrollplatten (die gestrichelten Linien bezeichnen die Mindestanforderungen f¨ur O-1 Platten gem¨aß CSA O437.0 (1993); ∗ signifikant bei α = 0,05)
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Fig. 4 Mean values and standard deviation for compression strength and internal bonding of control and treated boards. (Dashed line means minimum requirement for O-1 grade according to CSA O437.0 (1993)). Abb. 4 Mittelwerte und Standardabweichung f¨ur die Druck- und Querzugfestigkeit der Kontroll- und behandelten Platten (gestrichelte Linie bezeichnet die Mindestanforderung f¨ur O-1 gem¨aß CSA O437.0 (1993))
The results of MOR⊥ did not present a clear tendency and treated boards (T1 and T2) showed higher values than the control, but no difference was identified between control and the other treatments (T1–T6). The MOE values of the heat treated boards, mainly in the perpendicular direction, were slightly higher than for the control. Yildiz et al. (2002) treated beech wood at 200 ◦ C for 10 h, and observed MOE values which were 15% higher than the ones of untreated wood. Kubojima et al. (2000) observed a tendency of increasing Young’s modulus for treatment up to 4 h, followed by a reduction for longer periods, while for MOR the same tendency was observed, but the decrease occurred after one hour. Other mechanical properties of the treated boards (COMP and IB) were not statistically different in comparison to the control (Fig. 4). The explanation for this low impact on the mechanical properties of the treated boards should take into account the two main components wood and adhesive. The reactions of thermal degradation of the wood are highly influenced by time, temperature, pressure, humidity and treatment method (Kubojima et al. 2000, Kozlik 1974). The thermal treatment may be considered as applied at mild condition, such as: short duration, low atmospheric pressure and low moisture content. It was expected that under these conditions, the chemical degradation of the board components may occur at a low level. It has been observed that mechanical behavior of the treated wood was directly related to the degradation of the hemicelluloses (Curling et al. 2001, Christiansen 1997). It was observed that IB mean values of the treated boards was higher than of the control, nevertheless no statistical difference was identified. Chow and Pickles (1971) argued that during wood thermal treatment certain extractives and lignin polymerization reactions can take place, and products with some adhesive behavior can be generated, increasing particle interbonding adhesion. This hypothesis can also be used to explain the low impact of the applied treatment
on the mechanical properties of the boards. Back (1987) reported that during the hardboard thermal treatment, interlacement polymers connections are formed and, in addition to the adhesive connections, they increase the mechanical properties. Ohlmeyer and Kruse (1999) observed a reduction in TS for PF- and UF-particleboard after hot-stacking. The authors argued that the adhesive post-curing improved the IB and consequently reduced the TS. Landrock (1985) stated that a high temperature resistant resin should have a high softening point and resist to the oxidation reactions and that such characteristics are present in the phenolic resin. Umemura and Kawai (2002) studied the resistance of the isocyanate resin to thermal degradation and observed a very good stability with only 7% of weight loss when submitted to 260 ◦ C for 20 min. 3.1.3 Chemical composition In Fig. 5, the results of lignin content and the following sugar unit contents are shown: glucan, xylan, galactan, arabinan and mannan. It can be observed that the content of galactan, arabinan and mannan is reduced as the treatment becomes more severe, while glucan and lignin remained almost unaffected. For longer periods (16’ and 20’) at 220 ◦ C (T5 and T6) the galactan was not detected. As these reductions of sugar content happened it can be inferred that total hemicelluloses content was also reduced. On the other hand, soluble lignin contents were homogeneous ranging from 32.7% (T5) to 34.6% (T3), while acid soluble lignin ranged from 0.50 (T6) to 0.59% (T1). Hsu et al. (1988) evaluated the sugar contents of steamed pre-treated Pinus contorta particles under 1.55 MPa pressure for 1, 2, 3 and 4 min. As the treatment was prolonged low variation in the lignin content was observed, but mannan and galactan were reduced by about 34% and 26%, respectively. The xylan content remained the same, but arabinose
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Fig. 5 Mean values, standard deviation and Dunnett test results for sugar components of control and treated boards (∗ , ∗∗ significant at α = 0.05 and α = 0.01, respectively) Abb. 5 Mittelwerte, Standardabweichungen und Dunnett Signifikanzwerte der Zuckerkomponenten der behandelten und unbehandelten Kontrollplatten (∗ , ∗∗ signifikant bei α = 0,05 und α = 0,01)
was improved. Boonstra and Tjeerdsma (2006) studied the chemical composition of heat-treated Pinus sylvestris. The results showed that total hemicelluloses content was reduced by about ≈ 22–49%, alpha-cellulose and Klason lignin increased ≈ 3–13% and ≈ 2–18%, respectively depending on the temperature of the treatment. Winandy and Krzysik (2007) evaluated the effect of prolonged hotpressing stage on MDF properties made from mixture of aspen, oak, maple and pine. The results indicated little or no noticeable changes in the contents of glucan, xylan and Klason lignin, but remarkable degradation of arabinan and galactan.
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The degradation of sugar contents observed can explain the small drop in MOR while the unchanged glucan content explains the unaffected MOE. According to Curling et al. (2001) bending property losses are highly associated with the kind of carbohydrate that is being degraded: loss of MOR corresponds to decrease in hemicelluloses, whereas dropping MOE corresponds to cellulose loss. It is wellknown that hemicelluloses are the least thermal stable wood polymers (Fengel and Wegener 1989, Stamm 1964) and that its thermal degradation begins at 150 ◦ C, while cellulose and lignin require higher temperature. According to Beall and Eickner (1970) lignin is the more stable wood poly-
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mer requiring a temperature above 250 ◦ C to start thermal degradation. The conditions of the applied post-treatment (short time, fast and dry conditions) were just enough to degrade hemicelluloses, while the cellulose and lignin contents remained intact. It is a very important advantage of the process, meaning that almost all original chemical constituents kept intact, degrading only the polymer which plays an important role in water adsorption and durability against fungi. Del Menezzi et al. (2008) evaluated the fungi resistance of this material. When treated boards were exposed to Trametes versicolor (white-rot) and Gloeophylum trabeum (brown-rot), the loss of weight was reduced by up to 35% in comparison to untreated boards. 3.1.4 Colorimetric variation Figure 6 presents results of the colorimetric variables. It is well reported that L ∗ is most affected by thermal treatment (Brischke et al. 2007, Charrier et al. 2002, Pincelli 1999, Mitchell 1988). The darkness of the board can also be evaluated by observing the changes in coordinates a∗ and b∗ , where they further dictate the trend in L ∗ . The coordinate
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a∗ shifts positively, which means that the board became redder, and coordinate b∗ shifts negatively indicating a loss of yellow color. Similar results regarding a∗ and b∗ behavior was found by Brischke et al. (2007) evaluating heat-treated wood from Picea abies. Schnabel et al. (2007) heat-treated three wood species in an industrial wood processing chamber according to three temperature intensities: low, medium and strong. The values of coordinate a∗ and b∗ were reduced as the treatment became more severe. As L ∗ , a∗ and b* were significantly affected by thermal treatment, the final color of the boards (ΔE ∗ ) was changed significantly as well as saturation (ΔC), results not shown here. The colorimetric variables might be more affected by temperature of the treatment instead of time. It can be observed that lightness was reduced as the treatment becomes more severe. According to Chow and Mukai (1972), the darkness is a result of the change in the α-cellulose color that is promoted probably by the oxidation reactions. Charrier et al. (2002) argued that during thermal treatment, these reactions also change the color of others compounds and that extractives migration has an important role. Ishiguri et al. (2003) suggested that extractives can be chemically changed as well.
Fig. 6 Mean values, standard deviation and Dunnett test results for color properties of control and treated boards (∗ , ∗∗ significant at α = 0.05 and α = 0.01, respectively, according to the Dunnett test) Abb. 6 Mittelwerte, Standardabweichungen und Dunnett Signifikanzwerte der Farbeigenschaften der behandelten und unbehandelten Kontrollplatten (∗ , ∗∗ signifikant bei α = 0,05 und α = 0,01 gem¨aß dem Dunnett Test)
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3.2 Effect of treatment temperature and time Tables 1 and 2 present mean values of the properties affected by temperature and time of the treatment according to the factorial ANOVA. It can be observed that 18 properties out of 25 evaluated were affected by temperature, but only seven were time-dependent. It was clear that the higher the temperature the better the dimensional stability. Thus, at 220 ◦ C treatment resulted in a significant improvement in the properties related to the dimensional stability (TS, WA and EMC) in comparison with 190 ◦ C. In other words, there was a reduction in water absorption/adsorption and thickness swelling. In spite of the low loss of weight (LW and PLW) the effect of the temperature was highly significant. However, this did not result in the reduction of the board density, once DAT was not affected by temperature. Consequently, the absence of the temperature effect on DAT can explain why the mechanical properties of the treated boards were only partly affected by temperature in comparison to the control. The MOR was also affected by treatment, but only the effect of the temperature was identified, while time did not affect it. The higher temperature (220 ◦ C) reduced the values of MOR by about ∼ = 12%. This reduction can be considered not pronounced in comparison with those observed by many researchers for solid wood (Bengtsson et al. 2002, Santos 2000, Kim et al. 1998). Kallander et al. (2001) evaluated the effect of the drying temperature at 120 ◦ C and observed that MOR was more affected than MOE. Matsumoto et al. 2001 observed that MOE was not affected at 160 ◦ C, while MOR was starting at 120 ◦ C. Bengtsson et al. 2002 also observed that MOE was slightly affected in structural pieces of thermally treated solid wood showing a reduction by 3.5% at 220 ◦ C and MOR was reduced by up to 50%. Since time of treatment did not affect the mechanical properties, this means that it is possible to use longer treatment times without adverse effects on these properties, even
Eur. J. Wood Prod. (2009) 67: 383–396 Table 2 Effect of time on the properties of the treated boards Tabelle 2 Einfluss der Behandlungsdauer auf die Eigenschaften der behandelten Platten Property
Time (min) Fcalc 12 16 20 TS24H (%) 13.9a 12.8b 11.9c 17.48∗∗ EMC (%) 7.2a 6.5b 5.9b 12.03∗∗ LW (%) 9.3a 10.1a 10.8b 24.15∗∗ PLW (%) 1.9a 2.4a 2.9b 25.08∗∗ L∗ 53.9a 51.9ab 51.0b 3.76∗ Glucan (%) 38.8a 38.8a 38.9b 9.42∗∗ Galactan (%) 0.8a 0.4b 0.2c 19.97∗∗ TS24H: thickness swelling after 24 h of water soaking; EMC: equilibrium moisture content; LW: weight loss; PLW: permanent weight loss; ∗∗ , ∗ significant at the level α = 0.01 and α = 0.05 levels, respectively. Different letters indicate that differences between means are statistically significant at α = 0.05 (according to the Tukey HSD test)
though very positive effects were detected in dimensional stability properties. The results obtained here differ partially from those obtained by Hsu et al. (1989). The authors applied thermal treatment in waferboard and observed that MOR was not affected by the time of treatment and a small elevation was observed in comparison with the untreated boards. However, in comparison with other methods for improving dimensional stability, mainly those regarding furnish pre-treatment, the present method is suitable due to the low adverse impact on the mechanical properties. Paul et al. (2006) observed reductions on MOE (6–30%) and MOR (35–50%) values of OSB made from pre-heated Pinus sylvestris strands. Boonstra and Tjeerdsma (2006) extensively evaluated the internal bonding of particleboards made from a mixture of Picea abies and Pinus sylvestris. The boards made from treated particles showed a drop in internal bonding, but after the boiling test, they presented similar or higher values than the boards made from non-treated particles. TS24H, EMC, LW and PLW were also affected by treatment time (Table 2). It can be observed that the longer the
Table 1 Effect of temperature on the properties of the treated boards Tabelle 1 Einfluss der Temperatur auf die Eigenschaften der behandelten Platten Temperature (◦ C) Fcalc Property Temperature (◦ C) Fcalc 190 220 190 220 TS2H (%) 6.6 4.2 68.38∗∗ L∗ 56.5 48.0 94.45∗∗ WA2H (%) 16.9 11.8 21.40∗∗ b∗ 28.8 23.5 101.25∗∗ TS24H (%) 14.8 10.9 198.75∗∗ C∗ 31.1 28.7 90.69∗∗ WA24H (%) 44.6 35.9 19.66∗∗ h∗ 67.8 63.8 81.11∗∗ EMC (%) 7.3 5.9 4.67∗∗ ΔE ∗ 4.5 13.9 78.33∗∗ LW (%) 9.6 10.8 106.54∗∗ ΔC 1.9 5.7 68.97∗∗ PLW (%) 1.6 3.2 175.47∗∗ Glucan (%) 38.2 39.2 70.71∗∗ PTS (%) 5.1 1.1 58.96∗∗ Xylan (%) 4.9 4.5 14.20∗∗ MOR (MPa) 31.2 28.1 6.19∗ Galactan (%) 0.86 0.21 128.96∗∗ TS2H, WA2H, TS24H, WA24H: thickness swelling and water absorption, after 2 and 24 h of water soaking; EMC: equilibrium moisture content; LW: weight loss; PLW: permanent weight loss; PTS: permanent thickness swelling; ∗∗ , ∗ significant at α = 0.01 and α = 0.05 levels, respectively Property
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treatment the lower the TS24H and EMC values, while for the variables related to the loss of weight (LW and PLW) opposite results were found. This effect was more evident for TS24H, showing three homogeneous groups in the Tukey test. It has been observed that dimensional stability properties were significantly affected by the duration of the treatment. Paul et al. (2007) pre-heated strands from Pinus sylvestris at 180, 200, 220 and 240 ◦ C for 30, 60 and 90 min and observed that the loss of weight was significantly improved as the temperature and time increased. Mohebby et al. (2008) pre-treated (120–180 ◦ C; 30–90 min) industrially obtained fiber to produce MDF and observed that TS24H was reduced with longer treatment. Chemical composition of the treated boards was affected by both temperature and time. The main affected wood polymers were the hemicelluloses, which imparted a slight improvement in glucan, but the lignin was almost unaffected. These results were expected in view of the relative low temperature of the post-treatment. Additionally, hemicelluloses degradation happens at fast rate and at lower temperature than cellulose and lignin as well. According to Sivonen et al. (2002), the deterioration of the hemicelluloses starts at or below 180 ◦ C, which produces acetic acid helping to degrade the structure of the hemicelluloses. Cellulose and lignin deteriorate at higher temperatures and at slower rate than hemicelluloses. The temperature of treatment had an effect on all studied color variables, except a∗ . The general observation as the temperature increases is that the board becomes darker, loses yellowness and saturation and its color changes pronouncedly. Bektha and Niemz (2003) studied the effect of thermal treatment on the change in color of wood from Picea abies. All variables were changed as the temperature of treatment was increased. The general trend of heattreated wood color was extensive darkening (L ∗ reduction) and reddening (a∗ increment) meaning a high degree of color change (ΔE ∗ higher). Bourgois et al. (1991) observed that color variables were dramatically changed from 210 to 240 ◦ C. According to Table 2, the duration of treatment affected only the variable L ∗ . This result differs from that presented by Sundqvist (2002), which studied the color change of Pinus sylvestris and identified that both temperature and time had an influence on the coloring process. The results presented here regarding the effect of temperature and time of treatment can bring practical advantages. The industry or final consumer may choose the suitable scheme of thermal treatment according to the needs and end-use of the board. If high dimensional stability is required, still keeping strength at a certain level, higher temperature for a shorter treatment time (T4 treatment) should be used. However, one must take into account that TS and EMC are time-sensitive properties. Then,
393
if a more stable board is required, the treatment must be prolonged and T6 schedule is recommended. Now, if bending strength is required with aesthetic quality, the suitable treatment is low temperature for a longer time (T3). 3.3 Interrelationship between material properties Table 3 presents the correlation among color parameters, chemical constituents and physical-mechanical properties that were affected by thermal treatment. All color parameters had a medium to strong correlation with all affected properties, but the variables L ∗ and ΔE ∗ presented the higher values of Pearson correlation. It is clear that as the board becomes darker, which means L ∗ reduction, the dimensional stability is improved. The same trend can be observed for the coordinate b∗ . Nevertheless, as the board color loses its yellowness, which means reduction in b∗ , the values of all dimensional stability properties are also reduced. The opposite behavior was observed for variables related to loss of weight (PL and PLW). According to Pellerin and Ross (2002), the measurement of color can be classified as a nondestructive method for evaluating wood properties. In this context, the results presented in Table 3 can be very useful for modeling the improvement of the dimensional stability through this method. In fact, some research has shown that color measurement is a suitable method to describe chemical changes in treated wood (Bourgois et al. 1991) and for quality control of thermally treated wood (Brischke et al. 2007, Schnabel et al. 2007), although a distinct result has been obtained by Johansson and Mor´en (2006). Figure 7 shows regression models to predict TS24H and EMC using L ∗ and PLW, respectively as predictors. The models were quite significant and could explain 63% of the TS24H and 89% of the EMC variability. These values can be considered satisfactory. However, when LW was included in the TS24H model (Eq. 1) the coefficient of determination (R2 ) was improved to 83% (F = 81.3∗∗) and a better model could be fitted. This means that the model may be used in the industry for predicting dimensional stability only by measuring weight loss and darkness of the treated boards. On the other hand, the inclusion of the variable L ∗ into the EMC model (Eq. 2) improved the R2 to 93% (F = 218.7∗∗). Del Menezzi et al. (2007) observed that stress wave velocity of treated boards was affected by changes in equilibrium moisture content and loss of weight, so it can also be used in quality control of thermal post-treatment process. TS24H = 19.5 + 0.184L ∗ − 1.63LW EMC = 13.7 − 1.35PLW+ 0.071L
∗
R2 = 0.831
(1)
R = 0.936
(2)
2
Several papers have shown that there is a relationship between color parameters and variables related to the loss of
13
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Table 3 Pearson correlation (r) among chemical constituents, color parameters and physical-mechanical properties affected by the thermal treatment Tabelle 3 Pearson Korrelationskoeffizienten (r) zwischen den durch die W¨armebehandlung ver¨anderten physikalisch-mechanischen Eigenschaften und den chemischen Bestandteilen bzw. den Farbparametern Property TS2H WA2H TS24H WA24H EMC PTS MOR Chemical Glucan –0.869∗ –0.954∗∗ –0.885∗∗ –0.885∗∗ –0.840∗ –0.638∗ –0.519 Xylan NS NS NS NS NS 0.443∗ NS Galactan 0.965∗∗ 0.909∗∗ 0.975∗∗ 0.975∗∗ 0.952∗∗ 0.732∗ 0.441 Arabinan 0.909∗∗ 0.858∗ 0.933∗∗ 0.933∗∗ 0.944∗∗ 0.577∗ 0.433 Mannan NS NS 0.801∗ 0.801∗ 0.865∗ NS NS Sol. Lignin 0.976∗∗ 0.908∗∗ 0.982∗∗ 0.981∗∗ 0.962∗∗ 0.613∗ 0.227 Color L∗ 0.779∗∗ 0.553∗ 0.792∗∗ 0.541∗ 0.568∗ 0.738∗ 0.541∗ a∗ 0.425∗ NS NS 0.528∗ 0.624∗ 0.501∗ NS b∗ 0.816∗∗ 0.611∗ 0.800∗∗ 0.614∗ 0.538∗ 0.797∗∗ 0.510∗ C 0.825∗∗ 0.629∗ 0.793∗∗ 0.637∗ 0.514∗ 0.813∗∗ NS h∗ 0.687∗ NS 0.753∗∗ NS 0.608∗ 0.632∗ 0.532∗ ΔE ∗ –0.680∗ –0.476∗ –0.748∗∗ –0.454∗ –0.610∗ –0.639∗ –0.500∗ ∗ ∗ ∗ ∗ ∗ ΔC –0.712 –0.497 –0.704 –0.515 NS –0.692 NS TS2H, WA2H, TS24H, WA24H: thickness swelling and water absorption, after 2 and 24 h of water soaking; EMC: equilibrium moisture content; PTS: permanent thickness swelling; ML: maximum load; ∗∗ ,∗ significant at the level α = 0.01 and α = 0.05 levels, respectively; NS: not significant
Fig. 7 Regression models to predict thickness swelling and equilibrium moisture content of treated boards Abb. 7 Regressionsgeraden zur Absch¨atzung der Dickenquellung und der Gleichgewichtsfeuchte der behandelten Platten
weight. To evaluate this aspect, stepwise regression analysis was run to model the loss of weight variables using color parameters as predictors. Equations 3, 4 and 5 are models that could be fitted to predict LW. These models were highly significant ( p < 0.000) and presented R2 ranging from 0.56 to 0.69. The PLW model (Eq. 6) had the same significance with R2 of 64.2%. LW = 17.5 − 0.212L ∗ + 0.154b∗ LW = 9.25 + 0.105ΔE
∗
PLW = 10.8 − 0.186L + 0.061b
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(3)
R = 0.568
(4)
R2 = 0.692
(5)
R = 0.642
(6)
2
LW = 14.13 − 0.081L ∗ + 0.056ΔE ∗ ∗
R2 = 0.638
∗
2
Brischke et al. (2007) studied extensively the possibility of employing color measurement as method for quality control of heat-treated wood from Picea abies, Pinus sylvestris and Fagus silvatica. Highly significant models with high R2 values (> 94.3%) could be fitted to explain the relationship between loss of weight and color variables. They observed that variables L ∗ , a∗ and b∗ had no linear relationship with LW, but when the cumulative effect of L ∗ plus b∗ was entered, linear models were obtained. They also concluded that color measurement of milled wood leads to a stronger correlation with the treatment intensity than measuring the solid wood. However, a discrepancy (R2 = 66.9%) between color variables and weight loss was ob-
Eur. J. Wood Prod. (2009) 67: 383–396
served for pine heartwood, which was attributed to the high resin content. Although in this present work relative good linear models could be fitted, the results obtained may be improved. Johansson and Mor´en (2006) concluded that color measurement was not a useful parameter for predicting loss of strength because the color of the treated wood was not homogeneous. It is well-know that the surface of the OSB does not present color homogeneity since some PF adhesive droplet is common, misleading the interpretation. Additionally, the variability of the color properties was larger for the more severe treatment than it was for the milder one, as can be observed in Fig. 6.
4 Conclusion The proposed post-thermal treatment can be applied to significantly improve the dimensional stability of OSB through reductions in thickness swelling, water absorption and equilibrium moisture content, in comparison to untreated boards. They were achieved either by the release of compression stresses previously imposed during hot-pressing and by some degradation of wood polymers. The mechanical properties were partially affected with a slight reduction only in the modulus of rupture without any drawback in other properties. The thermal treatment promoted degradation of the hemicelluloses content, but the cellulose and lignin contents remained practically unaffected. Although the board has lost weight after the treatment no effect on the density was identified. The treated board became darker, redder and lost yellowness. The dimensional stability properties were affected by both temperature and time of treatment, while other properties mainly by temperature. Thickness swelling and equilibrium moisture content of the treated board could be modeled satisfactorily using loss of weight and color variables as predictors. Finally, the proposed post-thermal treatment can be recommended to improve dimensional stability of consolidated OSB.
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