Philippine Journal of Science 150 (1): 171-182, February 2021 ISSN 0031 - 7683 Date Received: 27 Jul 2020

Physico-mechanical Properties of Two Philippine Thermally Modified in a Steam Environment

Juanito P. Jimenez Jr.*, Michaela Germaine M. Macalinao, and Gilberto N. Sapin

Department of Science and Technology Forest Products Research and Development Institute College, Laguna 4031 Philippines

Two commercial – namely, “kauayan tinik” ( blumeana J.A. and J.H. Schultes) and giant bamboo [ asper (Schultes et.) Backer ex Heyne] – were treated by thermal modification (TM) or heat treatment in a steam environment using temperature and duration settings of 175 °C – 30 min and 200 °C – 30 min. The effect of TM and the presence and absence of node on the physical and mechanical properties of bamboo were studied. Physical [moisture content (MC), thickness swelling (TSw), water absorption (WA), and specific gravity] and mechanical (flexural, compression, and tensile strength) properties were determined. A factorial experiment in a completely randomized design (CRD) was used to analyze the data. Results showed that TM improved the dimensional stability of bamboo as evidenced by lower MC, TSw, and WA compared to control samples. Except for tensile strength, the flexural and compressive strength of heat-treated bamboo did not significantly change at 175 °C – 30 min compared to the control. The presence of nodes, on the other hand, significantly reduced the bamboo samples’ flexural and tensile strength. Generally, TM improved the dimensional stability of bamboo without significantly affecting the mechanical properties at 175 °C – 30 min. At 200 °C – 30 min, a slight reduction in strength was observed, especially for D. asper.

Keywords: B. blumeana, dimensional stability, D. asper, flexural strength, heat treatment, tensile and compressive strength

INTRODUCTION and decay fungi (Giron and Garcia 2005). One of the recent technologies to improve dimensional stability and Bamboo, once considered the “poor man’s timber,” durability of wood and bamboo against biodegrading is now known as the “wise man’s timber.” It has been agents is thermal modification (TM). recognized as one of the best alternative renewable construction materials as it is fast-growing, versatile, and TM is a heat treatment process, typically ranging from has comparable strength to wood and steel on a weight for 150–260 °C, aimed at increasing the durability, improving weight basis (Ogunbiyi et al. 2015). However, like wood, dimensional stability, and reducing hygroscopicity bamboo has its drawbacks of being a hygroscopic material of wood (FTA 2003; Hill 2006; Jimenez et al. that either shrinks or swells depending on the temperature 2011). However, severe degradation occurs at higher and moisture present in the air (Razal et al. 2012; Huang temperatures such as diminished strength and toughness et al. 2017) and being susceptible to the attack of insects (Boonstra et al. 2007). Thus, the temperature of less than 260 °C was recommended to avoid significant loss of *Corresponding Author: [email protected] strength in wood. [email protected]

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TM is done using a heated chamber with either steam or In the present study, two bamboo species, which are nitrogen to coat the wood and avoid burning due to high commercially used for engineered bamboo in the temperature (FTA 2003). An alternative TM process is by Philippines – namely, “kauayan tinik” ( immersing wood in a hot liquid such as crude vegetable J.A. and J.H. Schultes) and Giant bamboo [Dendrocalamus oil, which results in the fast and equal transfer of heat, asper (Schultes et.) Backer ex Heyne] – were treated by TM producing significant changes in the natural properties of or heat treatment in a steam environment using temperature wood (Rapp and Sailer 2000; Hill 2006). and duration settings of 175 C – 30 min and 200 °C – 30 min. The study determined the effect of the TM treatment European countries such as Finland (ThermoWood), and the presence of nodes on the physical and mechanical the Netherlands (Plato-Wood), France (Retified Wood), properties of the two bamboo species, which is important and Germany (Menz Holz) are some of the pioneering in the utilization in its raw form and in making engineered/ countries that have successfully used TM to improve laminated bamboo products. the characteristics of wood (Sandberg et al. 2017). In Asia, there have been efforts to improve the properties of bamboo through a similar TM process. In Japan, they developed the ecology dry system, which in principle is MATERIALS AND METHODS a TM method with a longer duration and lower chamber temperature (UNIDO 2003). In and Taiwan, on the other hand, they have bamboo carbonization that results Bamboo Collection and Sample Preparation in the caramelized color of bamboo used for a variety Twenty (20) mature culms each of B. blumeana and D. of laminated products such as flooring, furniture, and asper were collected from Cavinti and Luisiana, Laguna, handicrafts. The carbonization technology is also applied Philippines in the month of July. The bamboos were to bamboo scrimbers, which were developed in China in approximated by the farmers to be at least 3 yr old based the 1990s (Sharma et al. 2015a; Shangguan et al. 2016). on the culm appearance such as surface characteristics and In Indonesia, through the ITTO (2018) project, researchers color. Only the useful length of the poles for each bamboo developed the “ECO-SOTE,” a preservation technique was obtained while the topmost parts were excluded and in a closed chamber with smoke and high temperature left in the field. These were transported to the Department which results in blackened bamboos that are immune to of Science and Technology – Forest Products Research insect attack. In Ethiopia, another similar process called and Development Institute (DOST-FPRDI). ThermoBoo was developed (USAID 2014). The poles were cut equally into three sections Other countries in Asia like Malaysia, Vietnam, and the (bottom, middle, top) and allowed to be air-dried up Philippines have also studied bamboo extensively in its to their equilibrium moisture content (EMC) for 2 mo. unmodified state and are now considering the TM technology Morphological characteristics of the 10 poles’ sections for as a method to improve its properties. Several research works each species were measured and recorded. The defect-free have focused on studying the effect of the TM process on bottom portion of the poles was used in the study. The bamboo’s physical, mechanical, and chemical properties process flow and sample preparation of the experiments using various species and heating media. These include the are shown in Figure 1. studies done by Wahab et al. (2005), Nguyen et al. (2012), Bremer et al. (2013), Kamarudin and Sugiyanto (2012), and Thermal Modification (TM) Natividad and Jimenez (2015). The bottom part (1/3 of the culm’s total length) of all Although several research works have been done on the bamboo culms were used for the study. Samples for bamboo TM, no report discussed the effect of the presence TM measuring 116 cm long were cut using a bamboo pole cutter. For each TM treatment, 5–8 poles per batch of nodes on thermally modified bamboo since mechanical (depending on diameter) with inner segments (diaphragm) testing was often conducted on the internode of culms. intact were loaded in the cylindrical TM chamber. Shao et al. (2010) stated that nodes have an influence on the mechanical strength of the material, and Kappel et al. The TM process was done by allowing the steam produced (2003) claimed that the culm is supported by nodes during from a mini-boiler to enter the closed chamber. The three growth to avoid failure and buckling. A decrease in culm 4-kW heaters located at the bottom of the cylinder were strength is attributed to the nodes since it was found that then turned on to slowly increase to the desired treatment the length of the fibers located at the node was the shortest temperature. Pressure built up inside the chamber was (Liese and Grosser 1972; Shao et al. 2010). Taylor et al. automatically released when it reached 30 psi (2.11 kg/cm2). (2015) claimed that nodes may be a point of weakness but their mechanical effect on the culm would be more likely The samples were thermally modified in several batches to be observed in closely spaced nodes. using 175 °C – 30 min and 200 °C – 30 min settings. The

172 Philippine Journal of Science Jimenez et al.: Philippine Bamboos Thermally Vol. 150 No. 1, February 2021 Modified in a Steam Environment

Figure 1. Diagrammatic process flow and sample preparation in the study.

process was monitored for about 3 h from the ambient following ASTM D 143 (2014) with some modifications. temperature to the required treatment temperature and Sample specimens measuring actual culm wall thickness duration (about 2 h) up to the cooling down period (1 x 2.54 cm (width) x 10.16 cm (length) at 10 replicates per h after reaching the desired treatment temperature and treatment were prepared. Prior to submersion in water, all duration). The samples were taken out when the chamber samples (heat-treated and control) were oven-dried at 103 temperature fell below 50 °C. ± 2 ºC until constant weight. The oven-dry radial thickness and weight were measured using a Mitutoyo digimatic indicator and digital balance, respectively. The samples Physical Properties Tests were submerged horizontally with a weight restraint in a Preparation of specimens. After TM, the heat-treated vat half-filled with distilled water. After 24 h, the samples bamboo culms were unloaded and inspected for the were removed and suspended vertically in air to drain presence of defects caused by treatments. Defect-free for 10 min, then the excess surface water was wiped off thermally modified culms were segregated and conditioned with a dry cloth. The amount of water absorbed and TSw together with the control (air-dried bottom part of the pole) were calculated by using the difference of the weight samples for 1 mo inside a room with a temperature of 23 and thickness before and after submersion expressed as a ± 2 ºC and relative humidity (RD) of 65 ± 5%. Sample percentage of the initial weight (Wi) and thickness. preparation to get the required samples’ dimensions for the various physical and mechanical tests were done according Moisture content (MC) and relative density (RD). to Figure 1. Some poles were split using a manual bamboo Samples for MC and RD were cut from the tested flexural splitter to obtain slats for mechanical (flexural, tensile) and samples. Ten (10) replicates each for samples with nodes physical tests while others were cross-cut to get round culm and without nodes were used to determine the MC and for compression test. RD. Specimen size for the MC and RD was actual culm wall thickness x 25 mm (width) x 25 mm (length). The Hygroscopicity and dimensional stability. The specimen’s sample’s Wi was obtained using a digital top loading hygroscopicity (measured by WA) and dimensional balance while volume (Vm) was determined by water stability (measured by radial TSw) were determined,

173 Philippine Journal of Science Jimenez et al.: Philippine Bamboos Thermally Vol. 150 No. 1, February 2021 Modified in a Steam Environment displacement. After obtaining the Vm, the samples were treatment (control, 175 °C – 30 min, 200 °C – 30 min), oven-dried at 103 ± 2 ºC until constant mass/ oven-dry and characteristic (with node, without node or internode) weight (Wo) was attained. MC was computed as the loss of samples. Analysis of variance (ANOVA) per property in mass expressed in percent of the Wo. RD was obtained tested was made and means were separated using Tukey’s as the ratio of Wo over Vm divided by the density of water. honestly significant difference test.

Mechanical Properties Tests Flexural test. Internode and node samples with 10 replicates each measuring 35.56 cm length by 2.54 cm RESULTS AND DISCUSSION width by the sample’s actual thickness were prepared for the flexural test. Test procedures from ASTM D 143 Morphological Characteristics of B. blumeana and (2014) were followed with some modifications. The width D. asper and thickness were measured using Mitutoyo digital The results of the measurements of the morphological caliper and digimatic indicator, respectively, prior to characteristics of the two bamboos are shown in Table testing. The test speed of the Shimadzu universal testing 1. However, it should be noted that the measurements machine was 5 mm/min. A three-point bending test was were just indicative for the two species as morphological employed with the concentrated load applied in the center characteristics vary due to several growth factors such as of the span. For samples with nodes, the load was applied silvicultural management (Alipon et al. 2009) and site in the node, which was at the center of the span. Modulus location (Aguinsatan et al. 2019). of rupture (MOR) and modulus of elasticity (MOE) were The mean pole length of B. blumeana was shorter than the obtained parameters from the flexural test. D. asper, although the total internodes measured per Compression test. Ten samples for internode and node pole were the same. Quantity of internodes at the bottom samples in cylindrical shapes from each treatment were and middle showed that B. blumeana had one more prepared for the compressive strength (CS) test. The internode but lesser by two at the top compared to D. testing procedures followed were those of Alipon et al. asper. Internode length showed that, for both bamboos, (2009). The lengths of the compression specimens were the middle part had the longest internode compared with obtained by multiplying the culm wall thickness by 10. the bottom and top sections. Culm diameter showed a This is the standard being used at DOST-FPRDI for testing decreasing trend from bottom to top for both bamboos. bamboo culms. During the test, the load was applied at However, it was clearly seen that D. asper’s culm diameter a uniform rate of 1.3 mm/min until failure to get the was bigger than B. blumeana from bottom to top. The maximum crushing strength. mean top culm diameter of the former was even bigger than the bottom culm diameter of the latter. Culm wall Tensile test. Five each of the internode and node samples thickness showed that, for both bamboos, a decreasing from the treated culms and control were made into trend exists from bottom to top. D. asper’s culm wall dumbbell-shaped slats for the tensile strength (TS) test, thickness at the bottom was about twice that of B. following ASTM D 143 (2014) procedures. A test speed blumeana. Due to inherent variation within a pole, only of 1.3 mm/min was used. the bottom part was used in the evaluation of the physical and mechanical properties of the unmodified (control) and Experimental Design thermally modified bamboos. Statistical analysis using a two-factor (species and treatment) factorial for TSw and WA and three-factor Physical Properties factorial for MC, RD, MOR, MOE, CS, and TS in CRD For both species, thermally modified bamboo samples was carried out through SAS Analytics Pro. The three gave a darker color than the control (Figure 2). A factors were bamboo species (B. blumeana, D. asper), complex chemical reaction occurs when bamboo cell

Table 1. Mean measurements of the morphological characteristics of B. blumeana and D. asper.

Pole Internodes Internode length Culm diameter (cm) Culm wall thickness Internodes length (per section) (cm) (cm) Species (per pole) (cm) B M T B M T B M T B M T B. blumeana 1109 28 11 10 7 40 43 34 10.8 9.5 6.8 1.25 0.8 0.72 D. asper 1338 28 10 9 9 44 51 50 14.8 13.6 11.7 2.10 1.14 1.01 B – bottom; M – middle; T – top

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Figure 2. Color of thermally modified bamboo: B. blumeana (L) and D. asper (R). wall components similar to wood cell wall components (i.e. hemicelluloses, lignin, and extractives) are heated to elevated temperatures (Bekhta and Niemz 2003; Jimenez and Razal 2004; Sundqvist 2004; Nguyen et al. 2012; Bremer et al. 2013), resulting in brownish color similar to caramelized sugars. The culms’ color change may be considered an advantage as it hid gray streaks of staining fungi, which attacked some samples during the 2 mo of air-drying. In some cases, this could also mean that color staining during finishing would no longer be needed for products requiring dark bamboo, as the color of TM bamboo comes in different shades depending on the treatment temperature used in the process. However, this is considered a disadvantage for bamboo products requiring natural yellowish color. Hence, Figure 3. Powder post beetle attacked the untreated bamboo (top) the use of thermally modified bamboo would depend on but did not invade thermally modified bamboo (bottom) the end-use or products to be made by the manufacturers. in the 175 ºC – 30-min treatment. There was also a distinct smoky smell in the samples. An analysis of odor from thermally modified bamboo was Hygroscopicity and Dimensional Stability made by Sun et al. (2018). The smell, however, slowly The bamboos’ hygroscopicity, as measured by WA dissipated after several weeks. The smoky smell of the (Table 2), was affected by species and treatment. thermally modified bamboo might be one of the reasons Between B. blumeana and D. asper, the former had why during open storage in racks, they were not attacked 25.1% less WA than the latter (Table 3). This may by powder post beetles. In contrast, the untreated samples be attributed to the difference in their anatomical just beside them were heavily attacked, as evidenced by properties, such as the size of cell lumen and cell wall. numerous tiny exit holes made by the insects (Figure 3). Density variation between the two species likewise affected WA. This is similar to the findings of Xie et TM likewise resulted in surface checking and splitting for al. (2016), who concluded that higher density bamboo some poles in both species, especially those treated at a higher composites result in lesser WA. On the other hand, TM temperature (200 ºC). For every batch of treatment, almost significantly lowered the samples’ WA. Table 3 shows half showed these defects especially those at the bottom of the that TM at 175 and 200 °C resulted in 17.6–21.5% load since these were near the electric heaters at the bottom less WA than the control. In wood TM, this can be of the treatment cylinder. The occurrence of such defects was explained by the reduction in the free hydroxyl groups expected since the treatment temperature was so high. Such in the amorphous regions of the cellulose, as well as defects were observed in the kiln drying of bamboo, which the decomposition of hemicelluloses and the apparent used a lower temperature (Tang et al. 2013). increase in lignin content (FTA 2003; Jimenez and Razal 2004; Hill 2006; Cermak et al. 2015), which results in less bonding sites for water. In bamboo, the

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Table 2. ANOVA on water absorption (WA) and thickness swelling blumeana. The factors which explain the treated samples’ (TSw) of thermally modified B. blumeana and D. asper. improved dimensional stability are similar to the factors Source of affecting WA. %TSw %WA variation DF F-value P-value F-value P-value MC and RD Species 1 20.38 < 0.0001** 26.06 < 0.0001** MC variation, as shown by the ANOVA (Table 4), was Treatment 2 72.6 < 0.0001** 7.33 0.0015** significantly affected by the two main factors and their Species x 2 28.2 < 0.0001** 3.12 0.0523ns interactions: characteristics x treatment. Treatment The MC of samples with nodes was found to be **Significant at 1% level probability *Significant at 5% probability significantly higher than the samples without node (Table ns – not significant 5). This can be attributed to the wider cell lumen located in the nodal area of the bamboo (Huang et al. 2017), which

Table 3. Mean thickness swelling (TSw) and water absorption (WA) with species, treatment, and interaction as a source of variations. Source of variation TSw (%) WA (%) Species (S) B. blumeana 3.54B 18.04B D. asper 5.49A 24.09A Treatment (T) Control 8.10A 24.22A 175℃-30 min 3.48B 19.02B 200℃-30 min 1.98C 19.95B Interaction (S x T) B. blumeana x Control 4.53B 19.13A B. blumena x 175 °C – 30 min 3.51BC 17.30A B. blumeana x 200 °C – 30 min 2.28CD 17.69A D. asper x control 11.36A 29.31A D. asper x 175 °C – 30 min 3.44BC 20.73A D. asper x 200 °C – 30 min 1.68D 22.22A Means with the same letter are not significantly different according to Tukey’s post hoc test.

TM studies of Bremer et al. (2013) and Starke et al. stores more moisture than the internode. TM treatment (2016) show that these chemical composition changes also lowered the MC of the samples, as shown by the are also true. decreasing trend in Table 5. The MC was found lowest in 200 °C – 30 min setting for both bamboo species. The ANOVA (Table 2) shows that the treated culms’ significant decrease in the MC of treated samples may be dimensional stability, as measured by TSw, was affected due to the reduction of hydroxyl groups in the cell wall, by species, treatment, and the interaction of species and which resulted in the same chemical changes, which treatment. The difference in TSw between B. blumeana affected samples in the hygroscopicity and dimensional and D. asper was 35.5%, which is consistent with the result stability tests. The lower MC attained by the treated in WA, indicating that the former was more dimensionally samples is desirable for making engineered/laminated stable than the latter. Between the treatments (Table 3), the bamboo. Low-MC engineered bamboo products are control swelled more by 57% and 75.6% against the 175 expected to withstand cyclic changes in humidity over ºC – 30 min and 200 ºC – 30 min, respectively. Interaction long periods of time, thus ensuring little stress on the of the main factor (S x T) reveals that in the control, glue line. The interaction of characteristics and treatment D. asper swelled more than B. blumeana. However, was found significant in the control and the 175 ºC – 30 in thermally modified culms, the difference in TSw min treatment but not in the 200 ºC – 30 min treatment between the two species was insignificant. These findings (Table 5). This indicates that TM may have affected the validate Natividad and Jimenez’s (2015) TM study for B. nodes by causing the cells in them to come together

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Table 4. ANOVA on moisture content (MC) and relative density (RD) of thermally modifiedB. blumeana and D. asper. Source of variation %MC RD DF F-value P-value F-value P-value Species (S) 1 3.37 0.0692ns 158.4 < 0.0001** Characteristics (C) 1 44.43 < 0.0001** 0.58 0.4471ns S x C 1 3.37 0.0692ns 1.09 0.2996ns Treatment (T) 2 344.16 < 0.0001** 27.88 < 0.0001** S x T 2 1.75 0.1789ns 13.82 < 0.0001** C x T 2 8.82 0.003** 1.97 0.1445ns S x C x T 2 1.75 0.1789ns 0.91 0.4045ns **Significant at 1% level of probability *Significant at 5% level of probability ns – not significant and compacted. This is due to the reduction in hydroxyl their interaction. Table 5 shows the difference in RD of groups, which resulted in the nodes’ MC to equalize with the two bamboo species. RD of B. blumeana was higher that of the internodes. by 27.4% than that of D. asper. It is expected that RD would vary with the species since B. blumeana and D. RD variation, as shown by the ANOVA (Table 4), was asper differ greatly in their anatomical properties (Siam significantly affected by the species and treatment and

Table 5. Mean moisture content (MC) and relative density (RD) with species, characteristics, treatment, and interactions as a source of variations. Source of variation MC (%) RD Species (S) B. blumeana 7.55A 0.73A D. asper 7.26A 0.53B Characteristics (C) With node 7.94A 0.63A Without node 6.87B 0.64A Treatment (T) Control 10.22A 0.70A 175℃-30 min 6.85B 0.63B 200℃-30 min 5.14C 0.56C Interaction (S x T) B. blumeana x control 10.35A 0.77A B. blumena x 175 °C – 30 min 7.19A 0.70B B. blumeana x 200 °C – 30 min 5.11A 0.72AB D. asper x control 10.08A 0.64C D. asper x 175 °C – 30 min 6.52A 0.55D D. asper x 200 °C – 30 min 5.17A 0.41E Interaction (C x T) With node x control 10.87A 0.68A Without node x control 9.62B 0.73A With node x 175 °C – 30 min 8.29C 0.64A Without node x 175 °C – 30 min 5.95D 0.62A With node x 200 °C – 30 min 5.70E 0.56A Without node x 200 °C – 30 min 5.05E 0.56A Means with the same letter are not significantly different according to Tukey’s post hoc test.

177 Philippine Journal of Science Jimenez et al.: Philippine Bamboos Thermally Vol. 150 No. 1, February 2021 Modified in a Steam Environment et al. 2019). RD was also significantly affected by the that higher RD corresponded to higher MOE and MOR, as heat treatment – decreasing as treatment temperature exhibited by B. blumeana against D. asper. The anatomical increased. This result is expected because TM results in properties of the two species may have also significantly the degradation of some cell wall components, which contributed to the differences. Espiloy (1985) found out alters the culms’ chemical composition (Bremer et al. that for B. blumeana, fibrovascular bundle frequency is 2013; Starke et al. 2016). positively correlated with MOE but not with MOR. On the other hand, Mohmod et al. (1990) concluded that for some The interaction effect of species and treatment (Table 5) Malaysian bamboos, MOE is positively correlated with showed that the RD of D. asper. was more affected by the size of vascular bundle, fiber length, and fiber wall the treatment as shown in the downward trend of the RD thickness but negatively correlated with lumen diameter. while B. blumeana went down at 175 ºC – 30 min and On the other hand, MOR is negatively correlated with slightly went up again at 200 ºC – 30 min. The trend for fiber cell wall thickness but positively correlated with B. blumeana was perhaps also decreasing, although its lumen diameter. rate of decrease was lower than the rate of decrease in D. asper. The slight increase for B. blumeana at 200 ºC – 30 The presence of the node also significantly affected MOE min might be due not to the treatment but to the variability and MOR. Flexural samples with a node had 22% lower of the samples used since bamboo properties vary from MOE and 28% lower MOR than those without it. Since pole to pole, especially if coming from different clumps fiber length is correlated with MOE, a decrease in MOE and growth sites (Aguinsatan et al. 2019). on samples with a node is expected since fiber length located at the node is shorter than that at the internode Lower RD is seen as somehow advantageous for (Shao et al. 2010). The MOR in the studies of Espiloy engineered bamboo products, provided there is no (1985) and Mohmod et al. (1990) is not significantly corresponding significant reduction in the culms’ affected by fiber length, but they did not mention whether mechanical properties. Lower RD means lighter weight the samples tested had nodes. of the engineered bamboo products. Just like with other properties observed, TM treatment also significantly affected the MOE and MOR due to the Flexural Strength Properties resulting degradation of some cell wall components, which ANOVA (Table 6) shows that MOE and MOR were probably affected the samples’ cell lumen size, cell wall significantly affected by the three main factors: species, thickness, and fiber length. Studies on the effect of TM on characteristics/presence of nodes, and treatment. MOE and the anatomy of bamboo at different levels of temperature MOR were also significantly affected by the interaction are recommended to determine the extent of changes in effects of species x treatment and characteristics x the fiber cell wall and lumen dimensions. treatment. The interaction of species and treatment significantly Between the two bamboo species, it was expected that increased MOE of B. blumeana at 175 ºC – 30 min while there would be variation in their MOE and MOR since significantly decreasing that of D. asper at 200 ºC – 30 the two bamboos differed in the observed RD, as shown min. For MOR, B blumeana at 175 ºC – 30 min did not earlier in Table 5. Apparently, the result (Table 7) shows significantly change but at 200 ºC – 30 min, a significant

Table 6. ANOVA on the flexural strength of thermally modifiedB. blumeana and D. asper. Source of variation MOE (MPa) MOR (MPa)

DF F-value P-value F-value P-value Species (S) 1 21.51 < 0.0001** 68.01 < 0.0001** Characteristics (C) 1 37.06 < 0.0001** 30.85 < 0.0001** S x C 1 0.03 0.8569ns 0.02 0.8887ns

Treatment (T) 2 5.23 0.0068** 26.54 < 0.0001** S x T 2 9.09 0.002** 3.43 0.0359* C x T 2 4.69 0.0111* 7.04 0.0013**

S x C x T 2 0.39 0.6766ns 0.28 0.7565ns

**Significant at 1% level of probability *Significant at 5% level of probability ns – not significant

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Table 7. Mean MOE and MOR with species, characteristics, treatment, in the anatomical properties of the two species coupled and interactions as a source of variations. with the effect of high temperature. The higher MOE Source of variation MOE (GPa) MOR (MPa) exhibited by B. blumeana compared with D. asper and the improvement at 175 ºC – 30 min might have a positive Species (S) effect on the end-use application of B. blumeana, such as A A B. blumeana 7.91 93 for making bamboo musical instruments like Marimba D. asper 6.55B 64B (an idiophone musical instrument). For better acoustics Characteristics (C) property, higher MOE is desired because it is negatively With node 6.34B 63B correlated with damping (Manzo 2019). For D. asper, Without node 8.12A 88A decreasing MOE and MOR as temperature increased Treatment (T) would mean that it is wise to use only the TM treatment Control 7.32A 89A that would not severely degrade bamboo’s flexural strength, so as not to negatively affect the species’ target 175 °C – 30 min 7.76A 85A end-use such as for making engineered bamboo products B B 200 °C – 30 min 6.61 60 like flooring and furniture. Interaction (S x T) B. blumeana x control 7.14BC 98A The interaction of characteristics and treatment at 175 ºC – 30 min significantly changed MOE both for the B. blumena x 175 °C – 30 min 8.69A 105A with-node and without-node samples. For with node B. blumeana x 200 °C – 30 min 7.90AB 75BC samples, however, MOR was significantly reduced from B B D. asper x control 7.50 81 175 ºC – 30 min to 200 ºC –30 min. The degrading effect D. asper x 175 °C – 30 min 6.83C 65C of the node was observed, especially at 200 ºC – 30 min D. asper x 200 °C – 30 min 5.32D 46D for MOE and MOR. As the node is a part of the bamboo Interaction (C x T) culm and makes engineered bamboo distinguishable from With node x control 6.69C 88AB laminated wood, this would probably mean that at 175 ºC Without node x control 7.96AB 91A – 30 min, bamboo slats with nodes can be processed for lamination without significantly reducing the product’s With node x 175 °C – 30 min 7.24BC 74C MOE and MOR. Without node x175 °C – 30 min 8.28A 95A With node x 200 °C – 30 min 5.08D 43D Without node x 200 °C – 30 min 8.13AB 78C Tensile Strength Properties ANOVA (Table 8) shows that TS was significantly Means with the same letter are not significantly different according to Tukey’s post hoc test affected by just two main factors: the characteristics/ presence of nodes and treatment. Among the interactions, reduction in strength was observed. On the other hand, D. only the effect of species x treatment was observed. asper significantly decreased in MOR from 175 ºC – 30 The presence of a node significantly reduced the sample’s min up to 200 ºC – 30 min. TS by 63%. Table 9 shows that the presence of node As explained earlier, this might be due to the variation lowers the TS of bamboo due to the discontinuity of the

Table 8. ANOVA on the tensile and compression strength of thermally modifiedB. blumeana and D. asper. Source of variation Tensile strength (MPa) Compression strength (MPa) DF F-value P-value DF F-value P-value Species (S) 1 1.51 0.2247ns 1 32.86 < 0.0001** Characteristics (C) 1 53.99 < 0.0001** 1 2.18 0.1423ns S x C 1 1.99 0.1645ns 1 1.93 0.1678ns

Treatment (T) 2 4.43 0.172* 2 6.43 0.0023** S x T 2 3.79 0.0295* 2 2.82 0.638ns C x T 2 0.13 0.8770ns 2 1.02 0.363ns

S x C x T 2 0.7 0.5008ns 2 1.1 0.337ns **Significant at 1% level of probability *Significant at 5% level of probability ns – not significant

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Table 9. Mean TS and CS with species, characteristics, treatment, Compressive Strength Properties and interactions as a source of variations. ANOVA (Table 8) shows that the samples’ compressive Source of variation Tensile Compression strength was significantly affected only by two main strength strength factors: species and treatment. None of the interactions had (MPa) (MPa) significant effects. Between the two species, B. blumeana Species (S) had a 22% higher compressive strength than D. asper (Table B. blumeana 109A 59A 9). This is expected since the RD of the former is higher than the latter as mentioned earlier. However, it is surprising D. asper 101A 46B to find out that compressive strength was unaffected at 175 Characteristics (C) ºC – 30 min and even increased by 16% at 200 ºC – 30 With node 80B 50A min. The result of the present study is in agreement with Without node 130A 54A the related study by Sharma et al. (2015b), who found out Treatment (T) that semi-caramelized and caramelized bamboo has higher compressive strength than bleached and raw bamboo from Control 119A 49B Phyllostachys pubescens. The apparent increase might be 175℃-30 min 101B 51B attributed to the compaction of the fibrovascular bundle 200℃-30 min 95B 58A due to higher temperature treatment. It is recommended Interaction (S x T) that the results for compressive strength be validated with the same species and even with other species to determine B. blumeana x Control 120A 58A if there is really such a trend. B. blumena x 175 °C – 30 min 118A 58A B. blumeana x 200 °C – 30 min 90B 61A D. asper x control 118A 39A D. asper x 175 °C – 30 min 84B 44A CONCLUSIONS D. asper x 200 °C – 30 min 100AB 55A The present study showed that TM treatment generally Means with the same letter are not significantly different according to Tukey’s improves the dimensional stability of bamboo as post hoc test evidenced by lower MC, TSw, and WA compared to control samples. The color of bamboo changes from fibers when they connect to the node. Similar results were yellowish to dark brown as treatment temperature found by Shao et al. (2010) and Ribeiro et al. (2019). This increases. Except for tensile strength, other mechanical implies that in designing engineered bamboo products, it is properties such as flexural and compressive strength are best not to use the culm portion with the node that would generally not affected at 175 ºC – 30 min. Hence, it is be subjected to tensile forces. recommended to use only mild treatment to improve the physical and mechanical properties of bamboo. Similar to flexural properties, TM significantly reduced the TS of bamboo by 15 and 20% for 175 ºC – 30 min The nodal portion of the bamboo culms contains more and 200 ºC – 30 min, respectively. This is expected since moisture than the internodes, while mechanical strength there were changes in the chemical makeup of the culms properties such as flexural and tensile strength are due to the high-temperature treatment. Nonetheless, a significantly reduced when nodes are present. Between B. 15% reduction in strength at 175 ºC – 30 min would still blumeana and D. asper, generally, the former has better perhaps be acceptable as it is still above the minimum physical and mechanical properties than the latter both in strength requirement of 80 MPa for engineered bamboo the unmodified and thermally modified condition. (DTI-BPS 2015). The interaction effect of species and treatment significantly reduced TS of D. asper by 29% at 175 ºC – 30 min and ACKNOWLEDGMENTS 18% at 200 ºC – 30 min. It is assumed that there would be a decreasing trend due to the effect of TM but, apparently, The authors wish to thank the DOST-PCAARRD for the TS recovered at the higher treatment. The discrepancy providing the funds for the project entitled “Gluing of the results might be due to sample selection, preparation, and Finishing Characteristics of Thermally Modified or the inherently lower strength of the culms assigned to Bamboo” in which this study was a part of; the DOST- 175 ºC – 30 min. For B. blumeana, a decreasing trend in TS FPRDI management and staff for the support; the Plywood was observed although a significant reduction by 25% was Unit staff of EPDS for the assistance in the laboratory observed only at 200 ºC – 30 min. Between the two species, experiments and testing; and Ms. Ma. Socorro Dizon for the TS of B. blumeana was less affected at 175 ºC – 30 min. the statistical analysis.

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