Introduction

Increasing demand of wooden products and limited supply of commercial timber is a serious concern of demand–supply chain and sustained natural forest health. The increasing shortage of regular timber has forced the wood based industries to utilise other non-commercial wood as alternative wood in order to satisfy the market (Sulaiman et al. 2009). Nowadays, the wood composite industries such as plywood, laminated timber, fiberboard, particleboard etc. are growing significantly due to their wide range of raw materials, consistent properties and reduced cost. Wood composite exhibits many advantages over solid wood like, smooth and uniform structure, knots free surface, dimensional stability, desirable sizes and thickness, and easier to work. Composite panels are being used widely to produce products such as furniture, cabinets, paneling and underlayment.

Various non-commercial, low quality woods are under intensive investigation for their commercial application in structural purposes. Malaysia’s economy relies on two such plantation crops i.e. oil palm tree (Elaeis guineensis) and rubberwood (Hevea brasiliensis). The oil palm tree was first introduced to Malaysia in 1871 and now it is one of the main agricultural cash crops of the country. Generally, the economic life span of oil palm tree is 25–30 years after that it is being replaced for the effective and efficient palm oil production. It has been estimated that 13.6 million tons of OPTs are produced from 100,000 hectares of plantation. In general, these oil palm remains are either left or burnt at the harvesting site to get rid of the waste. Despite being inexpensive and huge availability, only 20 % of OPTs are being utilized as raw material in wood-based industries (Anis et al. 2011). The low density and high carbohydrate content of OPTs are main reason of their restricted utilization. However, the OPT can be excellent source for various applications because of its huge availability and cost effectiveness. There is increasing interest of various wood modification technologies to improve the properties of wood without using any chemical. These modifications can be wood densification/compression or thermal modification.

Wood compressing is a process of wood density modification by modifying the cell structure by using mechanical method. Compressed wood was first produced in Germany in 1930s with the trade name of Lignostone (Seborg et al. 1956). Another example of compressed wood was developed in United States as a brand name of Compreg (Stamm and Seborg 1960) and Staypack (Seborg et al. 1956). Compreg is a resin-treated compressed wood that was produced by treating solid wood with water-soluble phenol formaldehyde resin and compressing it to a desired density and thickness (Kamke 2008). Staypack is like Compreg wood, compressed to a desired specific gravity and thickness without impregnating with resin. However staypack had many improved property but it absorbed moisture when exposed to humid environment called springback. Nowadays, wood compression technique is being applied to many solid wood, and wood veneer in a desire to improve the mechanical property (Adachi et al. 2004). In composite industries, the compressed veneers need lees adhesive due to nonporous, smooth surface of the veneers (Adachi et al. 2004; Bekhta et al. 2009). In compressed wood technique, low density wood can be modified to meet the desired strength required for structural applications. Bekhta et al. (2009) studied the compressed wood product and concluded that it was having higher strength and improved stiffness and hardness characteristics. Inoue et al. (1996) suggested that the pre-steaming treatment followed by compressing provides an additional advantage in compressed wood technique. The pre-steaming process dissolved the extractives and other chemical constituents present in the wood (Inoue et al. 1996). Furthermore, a pre-steaming treatment increases the degree of densification also by softening the cell structure of wood. The cells become more flexible by steam treatment, results in higher mechanical and physical properties of compressed wood (Blomberg et al. 2005).

However various studies have been done on oil palm trunk for their potential application in wood composites such as plywood and particleboard (Inoue et al. 1996; Unsal et al. 2009), but very little information is available for compressed OPT. So far compressed wood has exhibited an excellent strength properties in various solid wood (Bekhta and Marutzky 2007; Kamke 2008) but the application of this technique in OPTs is still in experimental stage. Therefore, in this study we investigated the pre-steaming technique to OPTs before compression and evaluated the properties of such compressed OPT. A positive success in compressed OPT can lead to an alternative raw material with huge and sustainable availability.

Materials and methods

Sample preparation compressed OPT manufacture

Oil palm trunks of an approximate age of 28 years old were harvested in a local plantation in Northern Malaysia. After harvesting, the trunks were immediately cut into small pieces with dimensions of 200 mm × 200 mm × 40 mm in a tangential direction. The samples were wrapped in plastic bags and kept in a freezer before testing to avoid possible contamination and deterioration. A steaming process was carried out in a chamber using an autoclave at a temperature of 130 °C. After steaming, the samples were dried in oven at temperature of 50 °C to reduce high water content in oil palm trunk during compression process. A low drying temperature was used to prevent warping of OPT sample. Ten replicates of compressed OPT samples were produced.

The oil palm trunk samples were compressed using a laboratory Molding Test Press, Model Fabricate GT-7014-A30, at a temperature of 200 °C and a pressure of 11.16 MPa for 60 min (Salim et al. 2012). Compressed OPT samples without steaming were coded as “compressed OPT” while compressed OPT samples with steaming were coded as “steamed compressed OPT”. All the compressed specimens were placed in a conditions chamber at temperature of 21 °C and relative humidity of 65 % before any test samples were cut.

Contact angle test

Contact angle of the compressed samples were measured to determine the wettability between the substrate and liquid. Wettability test was carried out using three liquids, namely water, urea formaldehyde and phenol formaldehyde adhesive. The method of contact angle determination was based on the study by Sulaiman et al. (2009). A video camera was employed to record the images of the droplets. The substrates were cleaned prior to the test to avoid any contamination that would affect the results of contact angle. Then, 10 μl of the water or adhesive was dropped manually using micropipette onto the surface of the panels. The image of the droplet was recorded using video camera for 0, 5, 10, 20, 40, 60 s. The angles of liquid droplet on the substrate were measured at a specified time interval of 10 s. A total of 5 replicates were used for the determination of contact angle for each type of liquid.

X-ray diffraction test

The compressed OPT samples were characterized with the aid of X-ray diffraction (XRD) and crystallinity index was calculated. High resolution (Hr) X-ray diffractometer (PANalytical X’Pert Pro MRD) from Rigaku diffractometer was used with Cu Kα1 radiation source (k = 1.5406 Å) generated at operating voltage and current of 40 kV and 30 mA, respectively. The Cu Kα1 radiation was filtered electronically with a thin Ni-filter. A 2θ angle range from 10° to 40° in reflection mode was scanned at 2°/min. The crystallinity index was calculated to quantify the crystallinity of the samples. The crystallinity index (CIr) is defined by Eq. (1);

$$ {\text{CrI }}\left( \% \right) \, = \, \left[ {\left( {I_{200} {-} \, I_{am} } \right)/I_{am} } \right] \times 100 $$
(1)

where I 200 is the peak intensity corresponding to crystalline and I am is the peak intensity of the amorphous fraction (Baskaran et al. 2012).

Chemical analysis of oil palm trunk

Different parts of the oil palm trunks were ground to pass through a 40-mesh screen size. The sampling and the preparation of wood for analysis were performed according to TAPPI-T257-cm-02 (2002), and the preparation of wood for chemical analysis was performed according to TAPPI-T264-cm-97 (1997). Extractive components were determined according to TAPPI-T204-cm-97 (1997) with a modification of the ethanol-toluene ratio of the solvent to 1:2. Holocellulose content was measured by the method of Wise et al. (1946).

Soil burial test

Soil burial test was conducted to study the durability of control sample, compressed OPT and steamed compressed OPT samples in uncontrolled exterior conditions. The soil burial test was conducted according to BS 1982: Part 2 (1990) standard, with some modification of sample size. Control specimens were oil palm trunk samples without any treatment. The samples were cut into dimensions of 20 cm length × 1 cm width and 2 cm thickness. Before soil burial, all the samples were put in the oven at a temperature of 105 °C for 24 h to determine their oven dry weight. The samples were buried to half of their length at Lembah Burung, Universiti Sains Malaysia for 3 months which is an open arena of test plot area. At every one month, the samples were taken out and cleaned from the soil before they were put in the oven at temperature of 105 °C for 24 h. This procedure was repeated until the tests were completed at the end of 3 months. The weight of each sample was taken and its weight loss was calculated based on oven dry weight according to Eq. (2);

$$ {\text{Loss in mass }}\left( \% \right) \, = \, \left[ {\left( {{\text{m}}1{-}{\text{m}}2} \right)/{\text{m}}2} \right] \times 100 $$
(2)

where loss in mass is weight loss (%), m1 is the initial oven dried weight (g), m2 is the final oven dried weight (g).

At the end of soil burial test, small specimens were prepared for scanning electron microscopy (SEM) analysis from each type of samples. Ultra-high resolution analytical Field Emission Scanning Electron Microscopy (FE-SEM) LEO Supra 50 VP was used for study. The specimens were mounted on SEM specimen stubs and coated with a thin layer of gold for examination.

Results and discussion

Wettability evaluation of steamed OPT

Figure 1 exhibits the contact angle of three different liquids namely water, urea formaldehyde and phenol formaldehyde on the surfaces of compressed OPT and steamed compressed OPT. The contact angle or spread diameter of droplet is function of liquid whereas the difference in contact angle for the same liquid is dependent properties of solid surface (de Campos et al. 2013). For example, high viscosity liquid should have greater contact angle in contrast to the low viscosity liquid. Due to this reason, water exhibited the least contact angle and phenol formaldehyde had the highest, depending to their viscosity. Furthermore, the results showed that the steamed compressed OPT samples had lower contact angel as compared to those of compressed OPT samples for all three types of liquids due to the difference in the surface properties of these two. In the visual observation it was found that the surface of compressed OPT samples was slightly rougher as compared to the steamed compressed OPT samples. Since the contact angle has reverse relationship with the surface roughness, this could be the reason for lower contact angle for compressed OPT as compared to steamed compressed OPT panels. Furthermore, in the steamed compressed OPT, the contact angle was observed significantly high in the beginning but this difference was found not significant when time increases. Since the cells of the steamed compressed OPT are very compact, less porous and more smooth, it resulted in high contact angle as compared to compressed OPT.

Fig. 1
figure 1

Wettability of compressed OPT and steamed compressed OPT against water, urea formaldehyde and phenol formaldehyde

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Crystallinity evaluation of steamed OPT

Figure 2 illustrated the X-Ray diffraction of steamed compressed OPT, compressed OPT and raw OPT specimens. The graph was plotted from diffraction peaks obtained around 20°–23° which is a typical diffraction pattern of the cellulose (Morán et al. 2008). An apparent increase in cellulose crystallinity in compressed OPT samples, particularly in steamed compressed OPT was observed. From Fig. 2, it is clear that steamed compressed OPT had the highest crystallinity index (CrI) followed by compressed OPT and raw OPT had the least CrI of 70.4, 58.7 and 40.6 %, respectively. The increase in CrI can be attributed to the splitting of lignin-hemicellulose linkages as well as new hydrogen bond formation in oil palm trunk due to steam treatment. A similar study was conducted by Ebringerova et al. (1993), and observed improved crystallinity in steam explosion. The steam treatment helps in desolation of easily excess lignin and hemicellulose of wood and generates new hydrogen bonding which results into increase in crystallinity (Negro et al. 2003). Gardner et al. (1993) investigated the hot-pressing conditions and attributed as the transition of amorphous polymers from the glassy state to the rubbery state was the reason for crystallinity improvement. During steaming and hot pressing, the amorphous lignin and hemicellulose are melt down and reorient themselves which results into improved crystallinity.

Fig. 2
figure 2

Crystallinity index of three different oil palm trunks

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Chemical composition evaluation of steamed OPT

Chemical composition of modified OPTs were calculated specifically holocellulose, extractives and ash contents. Steam compressed OPT had the highest percentage of extractives (21.45 %) followed by compressed OPT (19.58 %), steamed OPT (14.89) and the least were found in raw OPT (14.24). The relative change in chemical composition is illustrated in Fig. 3. However, the steam treated OPT alone shows the increment in extractives but the standard deviation error bar shows an overlapping trend with the raw OPT.

Fig. 3
figure 3

Chemical changes observed in different treated oil palm trunk

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The major increase in extractive is due to removal of water in polysaccharide degradation. According to Brito et al. (2008) at 180 °C temperature, new soluble products were derived from the heat of degradation of wood. Hakkou et al. (2006) also studied that the heat treatment of beechwood and observed an increase of extractive content at temperatures higher than 160 °C. Esteves et al. (2008) studied that application of high temperature during compression of OPT could drive new extractive compound resulting from degradation of structural components in the oil palm trunks. According to Yildiz et al. (2006) and Inari et al. (2007) heat treatment causes the degradation of hemicellulose content which is appeared in the form of extractives. Inari et al. (2007) had studied that holocellulose could be decreased after heat treatment. Widyorini et al. (2005) claimed that that hemicellulose was the most thermal-chemically sensitive and easy to degrade even at low temperature. Based on previous work, it was concluded that in steam compressed process extractive contents exhibited the inverse relation to holocellulose, i.e. holocellulose decreases with the increase of extractives. This could be related to steaming process which hydrolyzed the hemicelluloses of oil palm trunk before heating and pressing. According to Unsal et al. (2009), the extractive content can prevent the attack of bio-organism such as insect and microbes. The ash content for all the four samples were also calculated and found to be 2.13, 1.52, 1.32 and 1.46 % for raw OPT steamed OPT, compressed OPT and steamed compressed OPT respectively. The ash content was consisting of calcium, magnesium and silica.

Biodegradability evaluation

Soil burial test was conducted to study the biodegradability of treated and untreated OPT. The weight loss of test sample is an indication of durability; the samples loss the least weight is most durable. In a soil burial test, OPT sample is exposed to uncontrolled environment where many factors such as fungi, bacteria, termites, ants, bugs etc. are free to attack. Figure 4 elucidates the weight loss of all the three samples from first to third month. After three months, control samples exhibited the highest weight loss of 62.3 % where it was least for steamed compressed OPT. Moreover, the soil burial test is a decomposition of OPT sample by biological organism, steam compressed OPT shows the highest resistance. Since, OPTs are rich in starch and sugars which are highly desired food material of bio-organism, the damage was very high in control samples. However, in the compressed OPTs and steam compressed OPTs, various cell components such as hemicelluloses, starch and sugar are degraded partially due to high temperature which makes them unpalatable to bio-organism. This finding is also supported by a previous study which concluded OPT contains 2.4 % of starch in vascular bundles and 55 % in parenchyma (Tomimura 1992).

Fig. 4
figure 4

Weight loss of control oil palm trunk pre steaming (OPTS), compressed OPT and steam compressed OPT samples for first 3 months after soil burial

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A visual observation of retained samples buried for 3 months are presented in Fig. 5a, b. It illustrates, after 3 months of soil burial test, part of compressed OPTs panels buried in the soil were completely destroyed due to the biological attack whereas the remaining parts of the panels above the soil were partially degraded. The reason for the least damage to aerial parts is due to less bio-organism found in atmosphere compared to inside soil. The steamed compressed OPT samples exhibited the least damage compared to compressed OPT samples.

Fig. 5
figure 5

Retained samples a compressed OPT b steam compressed OPT, after buried for 1st, 2nd and 3rd months

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Scanning electron microscope study was conducted to see the microstructural changes in retained wood of steam compressed OPTs after 3 months. Figure 6a was taken at 50× whereas Fig. 6b was taken at 200×. However, all the component of compressed OPT was attacked by the organisms but it was observed more in parenchyma. It is believed that the parenchyma was attacked more easily due to presence of starch which is a priority food of micro-organism. This finding can be explained by the previous study on OPT that shows highest amount of starch (55 %) in parenchyma makes it more susceptible to microorganism (Tomimura 1992).

Fig. 6
figure 6

SEM image of steam compressed OPT a at 50× b at 200×, after 3 months of soil burial test. Circle, rectangle and arrow show the degraded parenchyma, degraded vessel and fungi growth respectively

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Conclusions

The compressed wood technique is an old and stablished technique to improve the properties of wood. Introducing a pre-steaming process gives an additional advantage to this technique. The steaming process before compressing caused the softening of lignin and hemicellulose of cell walls, results into smooth surface of compressed OPT. The XRD result showed an increase in crystallinity index of steam compressed OPT caused by the splitting of lignin-hemicellulose linkages and reorientation of hydrogen bond. The soil burial study showed that steamed compressed OPT is more resistant against micro-organism and exhibited least weight loss in an outdoor experiment. The improved resistance was assumed to be due to the degradation of hemicelluloses during steaming process which is an important food for the microorganism. The results exhibit the commercialization potential of improved compression technique which enhances the overall properties of OPT.