MECHANICAL PERFORMANCE OF THERMALLY MODIFIED BLACK PINE (PINUS NIGRA L.) WOOD
DOI:10.30825/5.EJPAU.20.2017.20.1

Vasiliki Kamperidou, Ioannis Barboutis
Laboratory of Wood Products and Furniture Technology, Faculty of Forestry and Natural Environment, Aristotle University of Thessaloniki, Greece

ABSTRACT

In this research, thermal modification of black pine (Pinus nigra L.) wood was conducted at 180 and 200°C, for 3–7 hours and some essential mechanical properties, such as modulus of rupture (MOR), modulus of elasticity (MOE), impact bending strength, compression strength, hardness in tangential and radial direction and surface roughness, were examined. It was observed that, as the intensity of treatment increases, the equilibrium moisture content and density of modified wood tend to decrease. The treatments of 180°C resulted in an improvement of bending strength, while treatments of 200°C caused a decrease. MOE of treated specimens increased regardless the duration or temperature. The treatments of 180°C increased the hardness, while at 200°C this improvement was limited to smaller increase and a reduction in subsequent treatments. Treatments of 180°C appeared to improve the impact bending strength, compared to control, while the treatments of 200°C demonstrated a strength decrease. Additionally, all the treatments improved the compression strength of pine, and referring to surface roughness, only the milder treatment managed to decrease it.

Key words: Thermal modification, Bending, Elasticity, Hardness, Mechanical properties, Roughness.

INTRODUCTION

The quality of the existing timber reserves seem to be gradually decreased, while the timber produced in plantations is characterized usually by lower biological durability and quality, compared to wood generated in virgin forests, creating a strong incentive for humanity to develop further technology and methods that will enhance the quality of this precious material. Mainly because of the fact that the other materials except wood, are most of them of finite ability of use, the need for sustainable use and reuse of materials, the improvement of its strength properties and increase of wood products lifetime appears increasingly urgent.

Although the timber is regarded as one of the best choices of construction material, it presents some drawbacks, which are inherent to its hygroscopic nature. The hygroscopicity of wood is often considered to be a disadvantage, because it is associated with the wood dimensions changes, resultant from the fluctuation of atmospheric humidity. As an anisotropic material, it exhibits different mechanical resistance and different dimensional change at different growth directions and additionally, it is susceptible to microorganism’s attacks and change of appearance, due to aging or exposure to ambient conditions [22].

To date, very resistant forest species such as tropical species mainly hardwoods, were chosen to be used, with the devastating environmental effects of harvest cuts. However, as the quality and quantity of these very durable tropical, and not only tropical, species has diminished severely, the wood industry is oriented in softwoods coming from managed forests or plantations, in which several protective techniques using a variety of wood preservatives have been applied [10].

For over 50 years the research community is engaged strongly with the modification methods of wood in order to find the answer to people’s concerns about the preservatives used today, and the increasing lack of qualitative timber. Aim of the investigations is the maximum improvement of wood properties, without generating a detrimental to the environment material after terminating its service life [10].

The properties and behavior of wood primarily depend on its chemical composition, thus most of the modification methods focus and rely on the change of its ingredients. Thermal modification is called the process of heat application in the material, in which the wood is thermally degraded and changes in chemical composition occur, responsible for the change of its properties. Thermal modification adds value to the existing timber and improves some physical characteristics, such as dimensional stability and biological resistance which could expand the service life of wood and its structures [16], without the use of chemicals, only by altering the cell wall structure. Compared to other wood modification methods, thermal modification is the oldest, most economic, but also one of the most advanced methods at research, as well as the commercial level, since it consists an alternative method that produces an environmentally friendly product [22].

As it is well known, the decrease in cell walls moisture content means an increase in the modulus of elasticity and strength of timber, whereas many other properties are also influenced (removal part of hydroxyl groups) [10]. The treatment temperature seems to be more important factor than the duration, and affects more the mass loss that occurs, as well as the modified wood properties [7]. Ates et al. [3] report that the treatment duration is a very crucial factor for the modulus of rupture (MOR), modulus of elasticity (MOE) and chemical composition of thermally modified wood, while the temperature affects more the physical properties.

The decrease in density of wood which appears because of the treatment is associated with the decrease of the mechanical properties, while lowering the moisture content of wood contributes to increasing wood resistance. Lekounougou et al. [16] reported that the thermal degradation of polysaccharides through the cleavage of multiple bonds, because of the high temperature, weakens or breaks the clusters of lignin-hemicellulose, in which the micro-fibrils of cellulose are immersed and retained, so as the loads not to be transferred and shared properly throughout the wood mass and a decrease in strength is being marked.

Purpose
The objective of the current study is to evaluate the effect of thermal treatment at 180 and 200°C for three different durations, on the mechanical properties of black pine (Pinus nigra L.) wood, in order to comprehend the behaviour and mechanical strength changes of this species after its thermal treatment, as well as elucidate the potential range of applications of the treated material in wooden structures and furniture.

MATERIAL AND METHODS

In the experiment that was conducted, boards of black pine (Pinus nigra L.) species of Greek origin were cut parallel to grain and placed for approximately 8 months in a climate chamber at a temperature of 20±2°C and 60±5% RH, in order natural drying to be achieved, until a constant weight, where an equilibrium moisture content (EMC) of 11.44% (0.172% standard deviation) was acquired. The mean density of pinewood before thermal treatment (the volume of wood was measured in moisture content levels mentioned) was 0.662 g/cm³ (0.015% St. dev.). The dimensions of the plates intended to participate in heat treatment was 35 mm thickness × 70 mm width × 400 mm length.

The thermal treatment of wood took place in a laboratory drying chamber, where the treatment was conducted at temperature of 180°C and 200°C, under atmospheric pressure conditions in the presence of air. The treatment lasted 3, 5 and 7 hours (counting 15 additional minutes each time for the recovery of temperature inside the chamber) and 10 plates were used in each treatment. According to the literature, the specific temperature levels and durations of treatments have not been tested so far and generally, there is only little information referring to this wood species behavior after thermal treatment. At the end of treatment duration, the plates were placed in glass desiccators to return gradually to ambient conditions and subsequently stacked in a conditioning chamber of stable conditions (humidity 60±5% and temperature 20±2°C). Only the material that was free of defects was selected to participate in the properties tests.

The plates of treated and untreated wood were cut to final dimensions for the properties testing, according to the relevant European and International standards and the final samples were conditioned for 1 month more till their constant weight was achieved. For each variable, 15 specimens were prepared.

After the conditioning of the samples that lasted about 2 months, the EMC and also the density of wood was determined again using the standards used ISO 3130:1975 and ISO 3131:1975.

The resistance tests to static bending forces were conducted in accordance to standard ISO 3133:1975 in Universal Testing Machine (Fig. 1a), with a movement speed of the loading piston of 5 mm/min. The loading was applied tangentially to specimens in the middle of their length (Fig. 1b).

The standard on which the test of the tangential and radial «Janka» hardness of the specimens was based, was ISO 3350:1975, using an Amsler Universal Testing machine having a piston speed of 6mm/min. The resistance test of the specimens to impact bending forces was conducted in accordance to ISO 3348:1975 standard, in Amsler Machine, by performing loading at the center of each specimen, perpendicularly to the tangential surface of the samples. The compression test of the specimens was conducted according to DIN 52185:1976 standard, in the same machine with a piston speed of 6mm/min. The surface roughness of the samples was determined using a profilometer apparatus (Mitutoyo Surftest SJ-301) using a diamond stylus device (Fig. 2). The measuring speed, the pin diameter and the top corner of the pin tool was 10 mm/min, 4 μm, and 90°, respectively. The roughness indexes values were determined with an accuracy of ± 0.01 μm.

The points of roughness measurement were randomly chosen on the surface of the samples. Measurements were implemented in a direction perpendicular to the fiber direction of the samples. Three roughness parameters were recorded, the mean arithmetic deviation of profile – Ra, the mean peak-to-valley height – Rz, and the root mean square deviation of the assessed profile – Rq (ISO 4287, DIN 4768). All the plates were subjected to sanding using the corresponding machine (bearing a 180 grade sandpaper). The plates were then cut to smaller dimensions (50 mm × 50 mm × 50 mm) [14, 15] for the roughness test, which was carried out in the tangential surface, because it usually presents lower roughness, compared to radial one [5]. For the statistical analysis of the results, SPSS Statistics PASW was used in order to analyze the variability of average resistance values using the method Bonferoni and Tamhane (One way ANOVA), as well as the Least Significant Difference method (LSD – two-way ANOVA), which examines the effect of two different independent variables (temperature and duration) on a constant dependent variable, aiming not only at the evaluation of the main effects of each independent variable, but also the influence of any interaction between them on the dependent variable.

RESULTS AND DISCUSSION

The results revealed that the EMC and density values of all the thermally modified samples were found to be lower compared to the corresponding control values, even in the case of the less severe treatment (180°C – 3 h) and as the intensity of the treatment increases, wood mark even lower values (Tab. 1, Fig. 3). Specifically, in the treatments of 180°C that lasted 3, 5 and 7 hours, EMC value decreased by 20.73, 22.48 and 26.27% respectively, while in the treatments of 200°C, EMC decreased by 29.89, 34.22 and 38.43%. This EMC decrease clearly declares that thermal treatment greatly affects the dimensional stability of wood and its moisture absorption capacity and could be possibly attributable to the reduction of the hydroxyls in wood mass.

Statistical analysis showed that the EMC values of control specimens presented a statistically significant difference from the respective values of longer duration treatments (180°C – 7 h) and treatments of 200°C (3, 5 and 7 h).

Furthermore, thermal treatment of 180°C that lasted 3, 5 and 7 hours, led to a density decrease of 0.76, 1.36 and 2.57%, respectively, while the treatments of 200°C that lasted 3, 5 and 7 hours, reduced the density value further by 9.37, 10.88 and 20.24% in relation to the density values of control. This density decrease, occurring as a result of the treatment, is potentially associated with the decrease in mechanical strength, while the EMC decrease with strength increase [10].

According to statistical analysis, the values of control and those of the two milder treatments (180°C – 3 and 5 h) were found to differ significantly from the values of treatments of 200°C that lasted 3, 5 and 7 h. The treatment of 180°C and 7 hours did not present statistically significant difference from the corresponding value of the treatment of 200°C and 3 h, although it is different from the two of higher intensity treatments (200°C – 5 and 7 h). It is remarkable that the density value of the last treatment differs considerably from the values of all the other treatments, including the control.

Several researchers have also recorded a decrease in EMC and density of pinewood after its thermal treatment on different conditions [3, 6, 11, 19] and more specifically, the species of Pinus nigra L. [2, 8].

According to the results, the treatments of 180°C appear to improve the modulus of rupture compared to the level of control values (Tab. 2). Especially, the treatment of 3 hours at 180°C caused the greatest strength increase, whereas MOR values decreased slightly increasing the duration to 5 and 7 hours, but without reaching the level of control values. By the treatments of 200°C, a slightly lower MOR value was obtained in relation to the control, which presented a decreasing tendency as the duration of treatment was increasing. More specifically, the treatments of 180°C that lasted 3, 5 and 7 h improved by 11.32, 8.22 and 3.55% MOR values, while treatments of 200°C (3, 5 and 7 h) presented a decrease in strength of 1.52, 6.02 and 16.97%, respectively.

As it is evident (Tab. 2), MOE values (measured during the process of bending strength test) increased because of the thermal treatment, compared to MOE values of control specimens. In treatments of 180°C, increasing the duration, from 3 to 5 and 7 hours, resulted in MOE increase of 1.48, 4.38 and 7.92% respectively, while in treatments of 200°C, the increase of the duration led to a restriction of MOE values enhancement to increase percentages of 7.49, 5.46 and 2.87% respectively. This MOE increase could be partly attributed to the reduction of amorphous areas of hemicelluloses – cellulose in modified wood and the fact that wood mass contains a smaller amount of bound water [4].

Statistically significant differences were found between MOR values of control and the respective values of the specimens treated in the mildest way (180°C – 3 h), as well as in the most intensive one (200°C – 7 h). MOR values of specimens of the treatment 180°C – 3 h appeared to differ significantly from the values of all treatments of 200°C. Significant differences were also identified between values of specimens treated at 180°C for 5 h and the respective values of the most intensive treatments (200°C – 5 and 7 h). Finally, the most intensive treatment (200°C – 7 h) presented significantly different values from all treatments, except the directly milder treatment (200°C – 5 h).

Two Way ANOVA analysis of MOR values showed that the influence of temperature is statistically significant and affects its variability by the percentage of 55%. The factor of duration also presented a statistically significant influence on MOR of pinewood, affecting its variability by 31.2%. The interaction between the factors temperature and duration was found weak, while the greatest influence of temperature was recorded (statistically significant) at the duration of 7 h.

Statistical analysis of One Way and Two Way ANOVA revealed that the values of MOE did not significantly differ statistically from each other and there was no interaction between time and temperature level as regards the elasticity of wood.

Several researchers have also recorded a decrease in MOR values of pine wood after its thermal treatment [1, 6, 11, 12, 17, 19] and a slight increase in elasticity of wood [6, 12]. Some researches revealed that the elasticity was not notably affected by the treatment [19], while several others recorded a slight decrease in modified pinewood MOE [1, 11, 12, 17]. More specifically, Kol [13] used Pinus nigra L. wood species modified with ThermoWood method (2 h, max. temperature: 212°C) and recorded 59.5% decrease in MOR values and to a lesser extent decrease in MOE (13%), while Sahin [20] who modified Pinus nigra Arnold. species using the procedure of ThermoWood, recorded a significant bending strength decrease and a slight decrease in elasticity of wood.

The results of hardness tests revealed that the hardness of black pine wood is improved mainly after the treatments of low intensity, while treatments of higher temperature and longer duration tend to considerably decrease the hardness compared to control levels (Tab. 3).

Generally, the hardness of control specimens was found to be higher in the radial surface compared to that of tangential. Thermal treatment, however, seems to improve to a greater extent the tangential hardness of the specimens compared to the radial one, resulting in almost all cases of treatments the tangential hardness to be found higher than that of radial. In particular, the tangential hardness of pine specimens modified thermally at 180°C for 3, 5 and 7 h were increased by 6.58, 15.13 and 12.20%, respectively, compared to the control level and the treatment of 200°C for 3 h caused 4.20% increase of tangential hardness, while the duration of 5 and 7 h appeared to decrease the hardness by 2.79 and 13.53%, respectively.

Regarding the radial hardness of specimens, it slightly increased (by 2.45%) only in the treatment of the lowest intensity (180°C – 3 h), while increasing the duration and temperature radial hardness decreased between 1.36 and 30%, compared to control level.

Statistical analysis revealed that the mean value of tangential hardness of control differs considerably from the value of specimens treated at 180°C – 5 h, 180°C – 7 h and 200°C – 7 h. Also, a significant difference was detected between the value of the mildest treatment (180°C – 3 h) and the values of the two most intensive treatments (200°C – 5 and 7 h). The treatments at 180°C for 5 and 7 h presented values significantly different from the corresponding values of treatments of 200°C, independently of the duration. Finally, the specimens of the most intensive treatment differed considerably from the specimens of all the other categories. As regards the radial hardness, the values of control specimens and the first two treatments (180°C – 3 and 5 h) were found to differ significantly from the corresponding values of treatments of 200°C, irrespective of the duration. Additionally, the treatment at 180°C for 7 h resulted in values that differed significantly only from the two most intensive treatments (200°C – 5 and 7 h).

Two Way Anova analysis showed that the influence of temperature on the tangential hardness of pine wood was found statistically significant and constitutes an essential factor of influence over the variation of hardness in the percentage of 51.4%. Also, statistically significant effect demonstrated the interaction between temperature and duration, in which the 30% of the hardness variability is attributed. The maximum effect of temperature factor was observed in treatments of 7 h duration, where the higher variability of hardness is also recorded. Two Way Anova, indicated that the effect of temperature on radial hardness was found statistically significant and constitutes an essential factor of influence over the hardness variability by 51%. Also, a statistically significant effect was demonstrated by the interaction between temperature and duration, in which a hardness variability of 15.5% is attributed and the duration appear to affect this variability by 15.9%. The maximum effect of temperature factor was observed in treatment duration of 7 h, where the higher variability was observed.

Kocaefe et al. [12] recorded an increase in tangential, radial and transverse hardness of pinewood after its thermal treatment, while the majority of researchers revealed a decrease of hardness owing to thermal treatment [1, 9, 11].

According to the impact bending test results (Tab. 3), thermal treatments of 180°C seem to increase the strength of pine specimens at impact forces, compared to the respective strength level of control specimens. Higher strength was detected in treatments of shorter durations (3 h), while increasing the treatment duration (5, 7 h), the strength seem to be decreased without though reaching the level of control. More specifically, treatment of 180°C for 3, 5 and 7h increased the impact bending strength by 41.81, 27.59 and 7.33%, respectively. This strength increase may be attributed to the fact that in the early stages of thermal treatment, new chemical bonds are formed in wood between the degradation products of hemicelluloses and lignin, forming a network of condensation products [21]. In contrast, thermal treatments of 200°C seem to decrease the strength of pine at impact forces compared to the strength level of control specimens, and this reduction tendency of resistance is inversely proportional to the treatment duration. Specifically, treatments of 200°C that lasted 3, 5 and 7h caused an impact bending strength decrease compared to control over 8.84, 25 and 30.60%, respectively.

The statistical analysis revealed that there are statistically significant differences between all the mean values of impact bending strength, except the cases between control specimens and specimens treated at 180°C for 7 h and 200°C for 3 h, as well as the case between the specimens of the two more intensive treatments (200°C – 5 h and 200°C for 7 h).

Two Way ANOVA showed that the effect of temperature factor on impact bending strength is statistically significant and the 92.2% of strength variability may be attributed up to this factor. Second most critical factor is the duration, which has significant influence on strength and may affect its variability by 73.9%. The interaction between temperature-duration is also statistically significant and affects the variability of impact strength by 18.4%. In treatments of 5 h was found the greatest variability of impact strength (statistically significant).

In corresponding experiments that have been carried out thermal modification has been found to cause a decrease in impact bending strength of pinewood specimens [11, 13, 14].

According to the results of compression strength tests, all the values of thermal treated specimens exhibited higher strength compared to control level (Tab. 3). Even the mildest treatment (180°C – 3 h) presented a considerable improvement of strength. Referring to treatments of 180°C, the longer the duration, the higher compression strength was marked. In particular, treatment at 180°C for 3, 5 and 7 h increased the compression strength of pine specimens by 16.42, 23.46 and 25.94, respectively, while treatments at 200°C for 3, 5 and 7 h caused a strength increase of 29.26, 29.76 and 20.63% compared to control specimens. Therefore, in treatments of 200°C, the compression strength of pine continues to increase due to the modification process of its mass, except the most intensive treatment (200°C – 7 h), which seems to slightly reduce the rate of strength increase, without approaching the control level. Since the compression strength is essential for the utilization of pine wood material, it is preferable the treatment duration at 200°C not to exceed 5 h.

According to the statistical analysis, the compression strength value of control specimens was found significantly different from the corresponding value of all the thermally modified specimen categories. Moreover, significant differences were recorded between the strength of specimens of the mildest treatment (180°C – 3 h) and the treatments of 200°C (3 and 5 h).

Two Way ANOVA showed that the interaction between temperature and duration exhibited the highest and statistically significant effect on the compression strength of pine wood, influencing the variability of strength by 27.4%. Also, the temperature affects the variability in the percentage of 12.6%, while the duration itself appears to have a weak effect on the strength level. The treatments which displayed statistically significant and the highest effect on compression strength were those of 3h.

Similar behavior of pine wood specimens was recorded by González-Peña and Hale [7] and Adewopo and Patterson [1], who found that in the early stages of thermal treatment, the compression strength presents an increase, whereas as the intensity of treatment and the consequent mass loss increase, the compression strength begin to record a decrease. Other researches revealed a clear increase of compression strength [11, 20] or a decrease [2] owing to thermal treatment, while some others declare that compression strength was not significantly influenced by thermal treatment, ground that hemicelluloses decomposition that takes place during treatment has a lower impact on compression strength, compared to cellulose and lignin [6].

The surface roughness test on black pine specimens showed that only the thermal treatments of low-intensity managed to reduce slightly the surface roughness, while raising the temperature and duration of treatment led to a strong increase of roughness, compared to the control level (Tab. 4). More specifically, Ra value, which constitutes the most important roughness indicator [5], presented a decrease in treatment at 180°C for 3 and 5 hours by 9.15 and 11.03%, respectively, which means that the treatment improved the surface texture of specimens, but when the duration reached 7 h (at the same temperature), Ra increased by 10.60%. Thermal treatments of 200°C caused further increase in Ra ranging from 22.79 to 75.28%, and therefore, the surfaces of specimens were characterized by higher roughness. The roughness increase of the specimens after its thermal treatment could be explained by the thermal degradation of wood, the structural lesions including conversion of wood chemical constituents and release of gaseous degradation products. Moreover, the dehydration reactions that occur already at the temperature of 140°C, cause reduction of hydroxyls and increase in brittleness and roughness of wood [5, 18].

According to statistical analysis, Ra values presented all significant differences among one another, except the cases between the values of control specimens and the milder treatments of 180οC (3, 5 and 7 h), which presented closer values. Also, the values of treatment 180°C – 7 h did not exhibit any significant differences compared to the next more intensive treatments 200°C – 3 and 5 h. Precisely the same differences were exhibited by the values of Rz and Rq indicators. Τwo Way ANOVA of Ra values showed that the effect of temperature was found to be statistically significant and affected its variability by 76.6%. The treatment duration also had a statistically significant effect on roughness level, affecting its variability by 67.2%. The interaction between the two factors was found also statistically significant, affecting the variability of roughness by 27.7%. In treatments of 7h, temperature factor presented a higher intensity of influence, which represents also a statistically significant effect.

Budakci et al. [5] recorded higher roughness indexes values in thermally treated specimens of pine wood, compared to control specimens, while Gündüz et al. [9] revealed also a decrease in roughness indexes levels due to thermal treatment, especially in the case of the mildest ones.

CONCLUSION

The most significant findings established during the present project are summarized below:

EMC percentage of all thermally modified pine specimens was found to be lower than the corresponding control value and as the treatment intensity increases, the higher EMC decrease is marked (20.73–38.43%). Additionally, thermal treatments led to a decrease in density of pinewood by 0.76–20.24%, and as the treatment intensity was increasing, the highest density decrease was recorded.

Treatments of 180°C for 3, 5 and 7 h caused MOR increase of 11.32, 8.22 and 3.55%, respectively, while the three treatments of 200°C exhibited strength decrease of 1.52, 6.02 and 16.97%, respectively. MOE values increased by 1.48, 4.38 and 7.92%, by the treatments of 180°C and 3, 5 and 7 h respectively, while in the treatments of 200°C the elasticity was enhanced compared to the respective control values to a lesser extent: 7.49, 5.46 and 2.87%, respectively.

In treatments of 180°C, the tangential hardness was increased by 6.58–15.13%, while in treatments of 200°C this hardness enhancement was 4.20%, turning into a decrease over the more intensive treatments (2.79–13.53%). The radial hardness increased by 2.45% only in the mildest treatment, while by increasing the treatment intensity the values decreased between 1.36 and 30%.

Treatments of 180°C appeared to improve the impact bending strength of pinewood compared to the control, by 41.81, 27.59 and 7.33%, while the treatments of 200°C caused a decrease of 8.84, 25 and 30.60%, respectively. Furthermore, all treatments increased the compression strength (16.42–29.76%), compared to control values.

Only the treatments of low intensity managed to reduce slightly (9.15 and 11.03%) the surface roughness of pinewood, while by increasing the treatment intensity, the roughness indexes values exhibited a strong increase compared to control values (10.60–75.28%).

In most cases, the effect of the temperature was found to be statistically significant and this factor influenced the variability of properties values at a higher proportion, compared to the factor of duration or the effect of the interaction between these two factors.

To sum up, thermal treatment enhances many crucial for the utilization of Pinus nigra L. species properties, while it mitigates some of its major drawbacks, especially in mildest treatments, introducing it to an extensive range of future applications. Since it is possible to foresee the loss of strength (in treatments of higher intensity), it can also be prevented or limited in order this loss not to constitute an obstacle to the further use of thermally modified wood, taking into account that this new material should not be used in construction or applications requiring high mechanical strength.

The best way of utilization of thermally modified pine is to participate in applications where full advantage of new improved properties could be taken, ensuring that the properties that deteriorate after treatment are not a priority in each application. Such examples could be: wooden doors/windows, floors, bathroom and kitchen furniture and structures, internal linings, decorative details indoor/outdoor, participation selectively to structures and outdoor furniture under roof and after additional protection etc.

REFERENCES

Accepted for print: 17.01.2017

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