Natural weathering studies of oil palm trunk lumber (OPTL) green polymer composites enhanced with oil palm shell (OPS) nanoparticles
© Islam et al.; licensee Springer. 2013
Received: 1 October 2013
Accepted: 30 October 2013
Published: 6 November 2013
In this study, a green composite was produced from Oil Palm Trunk Lumber (OPTL) by impregnating oil palm shell (OPS) nanoparticles with formaldehyde resin. The changes of physical, mechanical and morphological properties of the OPS nanoparticles impregnated OPTL as a result of natural weathering was investigated. The OPS fibres were ground with a ball-mill for producing nanoparticles before being mixed with the phenol formaldehyde (PF) resin at a concentration of 1, 3, 5 and 10% w/w basis and impregnated into the OPTL by vacuum-pressure method. The treated OPTL samples were exposed to natural weathering for the period of 6 and 12 months in West Java, Indonesia according to ASTM D1435-99 standard. Physical and mechanical tests were done for analyzing the changes in phenol formaldehyde-nanoparticles impregnated (PF-NPI) OPTL. FT-IR and SEM studies were done to analyze the morphological changes. The results showed that both exposure time of weathering and concentration of PF-NPI had significant impact on physical and mechanical properties of OPTL. The longer exposure of samples to weathering condition reduced the wave numbers during FT-IR test. However, all these physical, mechanical and morphological changes were significant when compared with the untreated samples or only PF impregnated samples. Thus, it can be concluded that PF-NP impregnation into OPTL improved the resistance against natural weathering and would pave the ground for improved products from OPTL for outdoor conditions.
Recently, plenty of oil palm trunk (OPT) and oil palm shell (OPS) as a lignocellulosic material is producing due to the increase of oil palm tree plantation (Lua and Guo 2001). This huge amount of lignocellulosic material is mostly considered as an agricultural waste. The shortage of timber supply in wood-based industries and the negative impact of the huge agricultural waste has drawn the attention of researchers to work on OPT (Abdul Khalil et al. 2010a) and OPS (Dagwa et al. 2012). However, the utilization of OPT and OPS has still not optimally done and has lower economic value. Numerous researches and development efforts have been undertaken to utilize the oil palm biomass like OPS for active charcoal (Arami-Niya et al. 2010), OPT for furniture (Abdul Khalil et al. 2012), and empty fruit bunches for pulping (Astimar et al. 2002).
The effort that led the use of OPT for zero waste; it is necessary to find out alternative measures that ensure the use of OPT inside buildings, lightweight construction materials and furniture. Impregnation of chemicals into OPT and its modification might be a way to do this. Thermosetting resin impregnation into wood was started in 1936 (Stamm and Seborg 1939) and continued until early twentieth century (Ryu et al. 1991). Impregnation of resin into non-wood specially into OPT has started in the recent years (Abdul Khalil et al. 2012; Bhat et al. 2010a). However, the synthetic resins and OPT experience photo-degradation upon exposure to water and sunlight, especially ultraviolet (UV) (Geburtig and Wachtendorf 2010). The photo-degradation of polymers originates from excited polymer-oxygen complexes, which are mainly produced by introducing catalyst residues, hydroperoxide groups, carbonyl groups, and double bonds during polymer manufacturing (Zou et al. 2008). It has been shown that lignin is the constituent of wood that is most likely to undergo photo-degradation, which leads to the radical induced depolymerization of lignin, hemicelluloses, and cellulose at wood surfaces (Ndiaye et al. 2008). Therefore, color fading, chalking, surface roughening, cracking, damage the wood microstructure and strength weakening of materials may caused by weathering, restricting treated OPTL to specific outdoor applications (Feist 1990). Evans et al. (1996) reported that depolymerization of lignin and cellulose caused by photo-oxidation and furthermore, degraded by physical and biological factors, and water. However, it has been reported that UV light cannot penetrate deeper than 75 μm though degradation occurs deeper than this in combination with other factors (Hon 2001). Therefore, the material climate determined by wood moisture content and temperature, and their dynamics (Gobakken and Lebow 2010).
The degradation mechanisms are very complex and are influenced by many factors, e.g., rain, solar radiation and temperature, and thus, difficult to improve the weather resistance properties of wood. However, modification of wood might improve the weather resistance of wood by reducing the oxidation reactions. Different chemical modification methods have been practiced to improve the weathering resistance of wood by blocking the hydroxyl groups of cell wall polymers (Macleod et al. 1995). It was found that impregnation of methyl methacrylate monomer followed by polymerization reduce the weathering effects (Feist 1990). It was also reported that the impregnation of nanoparticles into lumber improves the weathering resistance (Lei et al. 2010).
However, all these works were done with inorganic nanoparticles. Accordingly, it may be possible to improve the weather resistance by impregnation of organic nanoparticles into wood. To the best of our knowledge, no prior report has been made on the weathering resistance properties of organic nanoparticles impregnated lumber. Thus, the aim of this study is to demonstrate the effects of OPS nanoparticles impregnation with PF resin on the natural weathering properties of OPTL. Physical, mechanical, and morphological properties of OPS nanoparticles impregnated green OPTL polymer composites would be analyzed to find out the effects.
Materials and methods
Oil Palm Trunks (OPT) were collected from a local plantation of 30 years old from Western Indonesia. OPTs were sawn to produce samples having the dimension of 50 × 50 × 500 mm (radial, tangential and longitudinal, respectively). Only the inner part of the OPT having the density of 0.29 g cm-3 were used in the study. At least 180 samples were prepared for one experiment. The samples were kiln dried until the moisture content reached to 14% before impregnation.
Oil palm shells (OPS) were collected from a palm-oil processing mill in Banten, Indonesia in the form of chips. Nano sized particles were prepared from this OPS chips by high energy ball milling (Pulverisette, Fritsch, Germany) process for 30 hours with 170 rev min-1 rotation speed of the planet carrier.
Properties of PF resin
Viscosity @ 25°C (poise)
Specific Gravity @ 25°C
Resin Content @135°C (%)
Impregnation with OPS nanoparticles
PF resin was prepared at high molecular weight with a concentration of 15% w/w. Exactly 1, 3, 5 and 10% w/w OPS nanoparticles having the size of 50 to 100 nm was added to that PF resin for getting different concentrations of PF-NPI. The mixtures (PF resin and OPS nanoparticles) were compounded using twin screw extruder (Haake Model Rheodrive 500). The mixture was perfectly incorporated into the chamber to begin the process of impregnation. The PF-NPI was impregnated into OPTL by vacuum-pressure method. An initial vacuum was created for 15 minutes at 3 bar followed by pressure at 7 bar for 60 minutes, and then a final vacuum was created at 3 bar for 10 minutes. Untreated samples (without nano particle impregnation) were used as control.
Natural weathering test
The natural weathering test was done according to the ASTM D1435-99 standard. The samples, after impregnation of OPS nanoparticles with resin, were exposed to natural weathering for a period of 6 and 12 months from June 2012 to May 2013 at Bogor, West Java, Indonesia. Annual average temperature, relative humidity, UV intensity, rainfall and long radiation were 25.9°C, 81.7%, 856.5 cal m-2, 1,570 mm, and 67.2%, respectively in the experimental area. The area is situated 325 m above the mean sea level, and the experimental place was on the roof of a 15 m high building having no shadow from a neighboring obstruction. Samples were placed vertically on the roof and exposed to the various weathering factors, such as precipitation, sunlight, temperature, moisture and wind.
Testing of the materials
The OPS nano particles were analyzed by Scanning Electron Microscope (SEM) model ZEISS (type EVO 50, Germany), Transmission Electron Microscope (TEM) with a Philips CM12 instrument, and Fourier Transform Infrared (FT-IR) model Nicolet Avatar 360 (USA) for their structure, size and functional groups, respectively. Weight loss (%) of the treated samples was calculated according to ASTM D 3345-74 standard.
The physical properties, i.e. water absorption (WA), volumetric swelling coefficient (S), anti-swelling efficiency (ASE) and density, were measured according to BS EN 317: 1993, BS EN 317: 1993, and BS EN 325: 1993, respectively. Tensile properties, i.e. tensile strength (TS), tensile modulus (TM), and elongation at break (EB), and flexural properties, i.e. flexural strength (FS) and flexural modulus (FM) were measured by using a Instron (Model 5582, UK) Universal Testing Machine according to ASTM D 638 and ASTM D 790 standard, respectively. Impact strength (IS) was measured according to ASTM D256-04 standard by using a Ray Ran Universal Impact Pendulum (CS-1370). There were at least five replications for each type of test.
Univariate Analyses of variance (ANOVA) were done with linear models in a completely randomized design (CRD) by using SPSS version 16.0.
Results and discussion
Characterization of nano structured materials from OPS
Change of weight due to natural weathering
Effect of OPS nanoparticles impregnation into OPTL on weight loss and weight loss prevention ratio after 6 and 12 months of weathering
Weight loss (%)
Weight loss prevention ratios (%)
36.37 (0.85) Ab
26.84 (0.82) Aa
31.84 (1.00) Ab
24.95 (0.95) Aa
31.47 (0.91) Ab
18.93 (0.86) Ba
25.59 (0.95) Bb
22.06 (1.03) Aa
29.51 (1.04) Ab
A summary of the analysis of variance (p > 0.05) for concentration of nanoparticles and exposure time
Exposure time (ET)
C × ET
Weight loss prevention ratios were higher in PF-NPI compared to the only PF impregnated OPTL. The weight loss prevention ratio (%) was the highest when there were 5% nano particle impregnation for both 6 and 12 months of exposure. From this result, it is clear that the rate of weight loss is the function of time. It indicates that weathering occurs due to photo-degradation of lignin in the materials, and leaching of those degraded lignin fragments from the exposed sample surfaces (Bhat et al. 2010b). The impregnated nanoparticles and PF resins also undergo the leaching process.
As reported earlier, weight loss of the exposed surface of the weathered specimens was normally due to the formation of water soluble products in addition with gaseous and volatile products (Futo 1974;1976). The exposed samples then attacked by the microbes, which also reduced the weight (Bhat et al. 2010b). However, weathering varies with many factors like species of wood, density, and climatic conditions (amount of irradiation, rain action, wind) (Feist 1990;1983).
Change of functional groups due to natural weathering
Changes of FT-IR spectra due to the exposure to weathering condition at different exposure durations
Wave numbers (cm-1)
Assignments and Remarks
stretching vibrations of O-H bond in cellulose (Pandey and Pitman 2003)
CH2 asymetry stretching (Pandey and Pitman 2003)
2360-2358 (C = O stretching due to presence of carbondioxide) (Devi and Maji 2012)
- 1641 (amide (N-C = O) (Devi and Maji 2012)
- 1636 (C = O, C = C) (Devi and Maji 2012)
- 1639 (C = O, C = C) (Devi and Maji 2012)
1253 (Guaiacyl ring structure lignin) (Pandey and Pitman 2003)
1047-1046 (silicate minerals (Si-O bonds) (Georgokapoulos et al. 2003)
presence of poly hydroxyl groups (Klinkaewnarong and Maensiri 2010)
stretching vibrations of O-H bond in cellulose (Pandey and Pitman 2003)
CH2 asymetry stretching (Pandey and Pitman 2003)
- 1620 (OH bending) (Devi and Maji 2012)
- 1640 (amide (N-C = O) (Devi and Maji 2012)
- 1639 (OH stretching linked water to cellulose) (Pandey and Pitman 2003)
1462 (C-H deformation and aromatic ring vibration) (Sun et al. 1999)
destruction of the guaiacyl units (Sun et al. 1999)
- 1045 (silicate minerals (Si-O bonds) (Georgokapoulos et al. 2003)
- 1046 (silicate minerals (Si-O bonds) (Georgokapoulos et al. 2003)
- (CH deformation in cellulose) (Pandey and Pitman 2003)
poly hydroxy groups (Klinkaewnarong and Maensiri 2010)
3412-3413 stretching vibrations of O-H bond in cellulose) (Pandey and Pitman 2003)
(N-H stretching) (Pongjanyakul et al. 2009)
- 3435 (N-H stretching) (Pongjanyakul et al. 2009)
- 2925-2914 (CH2 asymetry stretching) (Pandey and Pitman 2003)
- 1637 (C = O, C = C) (Sun et al. 1999)
- 1606 (aromatic skeleton vibration in lignin) (Sun et al. 1999)
- 1640 (amide (N-C = O) (Devi and Maji 2012)
1467 (C-H deformations and aromatic ring vibrations) (Sun et al. 1999)
- 1118 (Aromatic skeletal and C-O stretching) (Sun et al. 1999)
- 1120 (stretching vibration of Si-O-Si linkage) (Galeener 1979)
1046-1045 (silicate minerals (Si-O bonds) (Georgokapoulos et al. 2003)
(CH deformation in cellulose) (Pandey and Pitman 2003)
- 613 (poly hydroxy groups) (Klinkaewnarong and Maensiri 2010)
- 610 (poly hydroxy groups) (Klinkaewnarong and Maensiri 2010)
The zbend of N2O (Klinkaewnarong and Maensiri 2010)
After exposure to weathering condition, various chemical reactions took place such as dehydration, hydrolysis, oxidation, decarboxylation, and transglycosylation resulting the changes in FT-IR spectra (Kocaefe et al. 2008). Photo-induced degradation of treated and untreated wood caused the main changes in the absorption intensity as were reported by Temiz et al. (2007). However, the intensity of the changes of these bands was related to the change of functional groups and chemical structure of the samples.
Several peaks in the stretching vibrations of O-H bond in cellulose at region (3419–3412) cm-1 in spectrum of samples, which were changed to peak at region (3415–3414 cm-1) and 3423–3413 after 6 and 12 months, respectively. These findings of decreased intensity at the peak with increasing exposure time were in consistent with the study carried out by Yildiz et al. (2011). They reported that weathering process caused more reduction in the range of 1720 to 1740 cm-1 (C = O stretching) than heat treatment at all treatment temperatures and durations, suggesting that there were decreasing photo-oxidation of wood surface after sunlight irradiation.
The absorption peak changes with the increase of nanoparticles concentration and duration of exposure. The PF-NPI OPTL had chemical changes in lignin and cellulose similar to that of acetylated wood as was studied by Feist et al. (1991). The study suggests that the observed reduction in weathering (weight loss) of PF-NPI OPTL may be a result of polymerization of both resin and nanoparticles. The free radical process may be disrupted during weathering when these components are polymer impregnated function as barrier and the weathering process is then retarded (Feist and Hon 1984).
Change of mechanical properties due to natural weathering
Effects of OPS nanoparticles impregnation on mechanical and physical properties due to the exposure to weathering condition at different exposure durations
Tensile strength (MPa)/month
Tensile modulus (GPa)/month
Elongation at break (%)/month
Flexural strength (MPa)/month
Flexural modulus (GPa)/month
Impact strength (k.J/m 2 )/month
Density (g/cm 3 )/month
Water absorption (%)/month
Swelling coefficient (%)/month
Antiswelling efficiency (%)/month
Several researchers have been proved that weathering reduced the mechanical properties (Bhat et al. 2010b; Esteves et al. 2008). They suggested that polymer degradation was mainly caused by chemical bond scission reactions in macro molecules. It was found that long-term exposed of the composites to elevated conditions affected the mechanical properties. Solar irradiance (UV component of the sunlight), relative humidity and temperature are the causal agents of this deterioration of natural fiber of impregnated samples (Lopez et al. 2006). The increase in the mechanical properties due to the chemical modification has been reported by several researchers. Bhat et al. (2010b) found that the flexural properties was attributed to the reaction of hydroxyl groups of cell wall polymers with the anhydrides, converting them into acetyl groups.
Change of physical properties due to natural weathering
Similar to mechanical properties, physical properties also changed with the exposure time of PF and PF-NP impregnated OPTL. The change of these properties with different exposure time to weathering condition is shown in Table 5. The change of these physical properties was the lowest for PF-NPI followed by PF impregnation and untreated OPTL which indicated that treatment enhanced the properties of OPTL. The density of PF impregnated OPTL decreased 23.8 and 52.4% for 6 and 12 months of exposure to the weathering condition. The density change was positively correlated with the nanoparticles concentration, however, inversely correlated with the exposure time. The PF-NPI decreased the water absorption (WA) for a concentration of 5% nanoparticles; however, higher nanoparticles concentration increased the water absorption. This might be because of the lower degree of crystallinity of OPS nanoparticles which leaded to higher water absorption of the sample. The reduced degree of water absorption due to the replacement of the hydroxyl groups with carbon atoms in the PF chains has also been reported by several researchers (Lopez et al. 2006; Abdul Khalil et al. 2010b). Abdul Khalil et al. (2010b) found an interesting result that the highest water absorption because of the presence of more hydroxyl groups in the parenchyma tissue that enabled more hydrogen bonding formation. The swelling coefficient (SC) increased with the exposure time, however, decreased with the increase of nanoparticles concentration up to 5%. While, the antiswelling effeciency (ASE) decreased with the increase of exposure time. The ASE increases linear with increasing concentration nanoparticles at each exposure time. Accordingly, it can be states that PF-impregnated at various concentration nanoparticles and periods may prevent the rate of swelling resulting from decay. The PF-impregnation with 5% nanoparticles exhibited the lowest ASE change than PF-impregnation with 0, 1, 3 and 10% nanoparticles. The only PF-impregnation exhibited the highest ASE change. Thus, nanoparticles can be widely used to treat the PF-impregnated OPT for increasing the dimensional stability.
Based on these results, the PF resin and nanoparticles in OPTL reduced the porosity and minimized the physical properties change of OPTL resulting from weathering. Moreover, the formation of wall polymers inside the cell wall enhances the physical properties of the OPT (Abdul Khalil et al. 2010b). Statistical analysis indicated that concentration of nano particles as well as exposure time significantly affected the studied physical properties. The interaction of C and ET had significant effect on density, while no significant effect on WA and ASE (Table 3).
Change of morphological properties due to natural weathering
Degradation of OPTL surfaces started at relatively low irradiation intensities having an fungi attack on the middle lamella whereas, higher intensities degraded the secondary cell walls (Fengel and Wegener 1989). The control sample showed loss of middle lamella, distortion of cell lumen and delamination of the cell wall after 6 months of exposure to weathering condition (Figure 5b). The OPTL cell wall was weathered and cell lumen became bigger than those of unweathered cell lumen which suggested the erosion of cell wall. Middle lamella appeared to be completely eliminated at the surface of OPTL after weathering with the presence of fungi in cell lumen of fiber. Similar findings were reported by Bhat et al. (2010b) where middle lamella showed holes, cell lumen were distorted and cell wall were degraded for Acacia mangium wood after 1 year of weathering.
On the other hand, distortion and erosion of fiber became more pronounced in the OPTL controls after 12 months of weathering (Figure 5c) particularly in the middle lamella and cell lumen. As mention earlier, fungi were found in the cell lumen after 12 months of weathering. Nevertheless, the middle lamella could still be clearly discerned in cell lumen of fiber after 6 months of weathering.
The changes of morphological properties of PF-NPI without nanoparticles after natural weathering are shown in Figure 5d–5f. The sample showed that the middle lamella could still be clearly discerned in these samples after 6 months of weathering (Figure 5e). Some defibrillation in the middle lamella and delamination in the cell wall was apparent in samples after 12 months of weathering (Figure 5f), but overall the changes were less pronounced than in OPTL impregnated samples. The effects of PF-impregnation could retard the formation of aromatic (lignin) radicals that initiate photo-oxidation. Alternatively, it is possible that resin matrix in OPTL scavenged free radicals preventing them from attacking lignin and cellulose. Such a suggestion is consistent with the observations by Nur Izreen et al. (2011) that benzoyl groups in wood obstruct free radicals and photostabilise polymers.
Oil palm trunk lumber was successfully prepared by impregnation of phenol formaldehyde with OPS nanoparticles by vacuum-pressure method. The OPS nanoparticles appear to increase the quality of OPTL when it is exposed to natural weathering. Among all the PF-NPI OPTL, the addition of 5% nanoparticles exhibited superior physical and mechanical properties after 12 months of natural weathering. Degradation of the polymer matrix occurred for all PF-NPI during natural weathering, however, no significant differences were observed for the variation of concentrations of nanoparticles. Thus, matrix degradation was independent from the concentration of nanoparticles, however, dependent on the weathering duration. Thus, the impregnation of PF and OPS nanoparticles were effective in retarding the degradation of OPTL against the natural weathering.
The author would like to thank Universiti Sains Malaysia (USM), Penang, Malaysia, for providing Research Grant no. RU-1001/PTEKIND/811195, and Ministry of Education, Malaysia for providing research grant # FRGS-203/PTEKIND/6711325. The author would also like to thank Prof. Pingkan Aditiawati, School of Life Sciences and Technology, Institut Teknologi Bandung, West Java-Indonesia and Prof. Y. S. Hadi, Department of Forest Product, Faculty of Forestry, Bogor Agricultural University, West Java, Indonesia, for providing the necessary facilities for preparing the part of the research during the sabbatical leave for the period of December 1, 2011 to August 31, 2012.
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