MATERIALS AND METHODS

Materials: The softwoods Eugenia sp., Artocarpus rigidus, Artocarpus elasticus and Xylopia sp. and the hardwood Koompassia malaccensis were used for this study since June 2011 to end of December 2011. Chemicals used to impregnate these wood species were Nanoclays nanomer^® 1.30E, Montmorillonite (MMT) (Southern Clay Products, Inc. USA) and Phenol Formaldehyde resin (PF) (Merck, Germany). The purity grade of the chemicals were 99%.

Preparation of impregnation solutions: The impregnation solutions were prepared by adding 1% layered aluminosilicate nanofiller into the low viscosity phenol formaldehyde resin at a mixing speed of 2050 rpm for 20 min then nanoclays were mixed with the PF resin prepolymer to form impregnation solutions that were subsequently used to impregnate the wood species.

Manufacturing of wood polymer nanocomposites: Raw wood specimens were oven dried to constant weight at 103°C for 24 h. They were then placed into an impregnation chamber, in which there was no contact between samples and they were covered completely by nanoclay/PF mixtures. The system was evacuated to 60 mm Hg for 30 min. After that, compressed air was applied to the system and maintained at a pressure of 0.52 MPa for 30 min then released. The excess chemicals were wiped off the samples. Specimens were weighted and dried 24 h by air circulation, followed by oven drying at 90°C for 24 h. The excess polymers were then removed from the surface.

FT-IR spectroscopy analysis: The infrared spectra of the raw woods and WPC were recorded on a Shimadzu Fourier Transform Infrared Spectroscopy (FTIR) 81001 Spectrophotometer. The transmittance range of the scan was 370 to 4000 cm^-1.

The free- free flexural vibration testing: Determination of the dynamic Young’s modulus (E_d) was carried out using the free-free flexural vibration testing system. The specimen was held with AA thread according to the first mode of vibration. The specimen with an iron plate bonded at one end was set facing the electromagnet driver and a microphone was placed at the centre below the specimen. The frequency was varied in order to achieve a resonant or natural frequency. The E_d was calculated from the resonant frequency by using the following equation:

where, I is bd³/12, d is beam depth, b is beam width, l is beam length, f is natural frequency of the specimen, ρ is density, A is cross sectional area and n = 1 is the first mode of vibration, where m₁ = 4.730.

Determination of static Young’s modulus (E_s), modulus of elasticity (MOE) and modulus of rupture (MOR): Determination of E_s, MOE and MOR was carried out according to ASTM D-143 (2006). A Shimadzu Universal Testing Machine having a loading capacity of 300 kN was used for the test with the cross head speed of 2 mm/min. E_s was measured using the uniaxial compression test. The MOE and MOR were measured using the three point bending method and were calculated using the following equations, respectively:

(1)

(2)

Where:

Scanning electron microscopy analysis (SEM): The specimens were first fixed with Karnovsky’s fixative and then taken through a graded alcohol dehydration series. Once dehydrated, the specimen was coated with a thin layer of gold before being viewed on the Scanning Electron Microscope (JSM-6701F) supplied by JEOL Company Limited, Japan. The micrographs, taken at a magnification of 150X.

X-ray diffraction analysis (XRD): XRD analysis for raw wood and Wood Polymer Composites (WPC) were performed with a Rigaku diffractometer (CuK α radiation, λ = 0.1546 nm) running at 40 kV and 30 mA.

RESULTS AND DISCUSSION

Fourier transforms infrared (FTIR) spectroscopy analysis: FTIR spectra of raw wood and WPNC are shown in Fig. 1. The spectra were separated in two regions, namely: the OH stretching vibrations in 4000-2700 cm^-1 region and fingerprint region in 1800-400 cm^-1 region.

Fig. 1(a-b):	IR spectrum of (a) Raw wood and (b) WPNC

The stretching band of OH group and C-H group was 3300-4000 cm^-1 and 2800- 3000 cm^-1, respectively. The band region from 1000 to 1750 cm^-1 shown the superposition with sharp and discrete absorptions (Owen and Thomas, 1989). The absorption band 1508 cm^-1 is caused by lignin and the absorption located at 1735 cm^-1 is caused by hemicelluloses. This indicates the C = O stretch in non-conjugated ketones, carbonyls and ester groups (Owen and Thomas, 1989). The region between 1800 and 1100 cm^-1 comprises bands assigned to the main components from wood: cellulose, hemicelluloses and lignin. Clear difference can be detected in the infrared spectra, both in the different absorbance values and shapes of the bands and their location. The less xylan content in softwood is evidenced by a carbonyl band at 1735 cm^-1, for nanoclay/PF modified wood, this being shifted to a lower wave number value (1591 cm^-1). On the other hand, the decrease wave number of the peak, ascribed to C-O stretch of C-O-H in starch at 1317 and 1222 cm^-1 and C-O stretch of C-O-C in starch at 1027 cm^-1 confirmed the impregnation of nanoclay/PF wood sample due to the fact that plasticizer could form intense H-bonding interaction with the hydroxyl groups.

Dynamic Young’s modulus measurement: The dynamic Young’s modulus of the raw wood and WPNC, from the free-free flexural vibration testing system is shown in Fig. 2. Ten specimens were used for each species. The nanofiller with phenol formaldehyde monomer system increased the Young’s modulus, as seen in all species which was according to our previous work (Hamdan et al., 2010). After monomer impregnation the Young’s modulus of Eugenia sp., Xylopia sp., Artocarpus rigidus and Artocarpus elasticus, were significantly higher than that of raw one. On the other hand the Young’s modulus of Koompassia malaccensis was slightly higher due to their hard cell wall (due to the nanofiller effect on all wood species).

Fig. 2:	Dynamic Young’s modulus of raw wood, WPNC for all species

Fig. 3:	MOE of raw wood and WPNC

Modulus of Elasticity (MOE) and modulus of rupture (MOR) measurement: The MOE and MOR of raw wood and WPNC are shown in Fig. 3 and 4, respectively. The nanofiller phenol formaldehyde monomer system impregnation on the discerning wood species was investigated. The increment in MOE of the Eugenia sp. and Xylopia sp. were highest followed by, Artocarpus rigidus, Artocarpus elasticus and Koompassia malaccensis, respectively. WPNC yielded higher MOE compared to the raw wood because of the impact of nanofiller on wood species which is in accordance with other researchers (Cai et al., 2007, 2008).

From Fig. 3, the MOE of the Eugenia sp., Xylopia sp., Artocarpus elasticus and Artocarpus rigidus were significantly higher compared with raw wood. However in Koompassia malaccensis, (hardwood) there was no significant effect of nanomer impregnation but increment in MOE was slightly higher than that of our previous work.

Fig. 4:	MOR of raw wood and WPNC

In the wood specimens, phenol formaldehyde plasticizes on the cellulose and hemicellulose in wood cells which reduces the water molecules from the wood specimens. During the nanoclay phenol formaldehyde monomer impregnation nanofiller react as a binder which fill the void space in the wood specimens and increase it stiffness. For this reason the MOE of all WPNC was higher than that of raw wood (Fig. 3).

The MOR of Eugenia sp. WPNC was radically increased after nanofiller with phenol formaldehyde impregnation. The MOR also improved after nanofiller/phenol formaldehyde impregnation in accordance with previous research (Cai et al., 2007, 2008).

Figure 4 indicates that the MOR was significantly different for Xylopia sp., Artocarpus rigidus, Artocarpus elasticus and Eugenia sp. raw wood and WPC. The growth of MOR for Eugenia sp. was highest followed by Xylopia sp., Artocarpus rigidus, Artocarpus elasticus and Koompassia malaccensis, respectively. The value for Koompassia malaccensis (hardwood) WPNC was higher than that of raw one.

Static Young’s modulus (E_s) measurement: The Static Young’s modulus was determined from 10 repetitions, as summarized in Fig. 5. The highest increment of E_s value was observed in Eugenia sp. followed by Artocarpus rigidus, Artocarpus elasticus, Xylopia sp. and Koompassia malaccensis, respectively. Figure 5 indicates that, the increment of E_s between raw wood and WPNC for Eugenia sp. was significantly different. The increment of E_s in WPNC compared to raw wood was also reported by different researchers (Cai et al., 2007, 2008; Rahman et al., 2010). The nanofiller/phenol formaldehyde not only plasticized on the wood cell walls but also deeply increasing their lateral stability.

Fig. 5:	Static Young’s modulus of raw wood and WPNC

XRD analysis: Figure 6 presents the results of XRD for raw wood and WPNC. The spectrum corresponding to the raw wood and WPNC shows diffraction peaks at the following 2θ angles: 17°, 22.5° and 35° which correspond to the cellulose crystallographic planes 101, 002 and 040, respectively (Mulinari et al., 2010). The position of these peaks did not change which indicates that the structure of cellulose did not change in comparison with the raw wood. It is observed that the WPNC exhibited another five broad 2θ peaks at 43.74°, 49.21°, 51.13°, 55.65° and 72.87° which are due to increasing polymerization in the amorphous region.

These values can be attributed to modification of the wood by the nanofiller/phenol-formaldehyde prepolymer. The reason may be that the prepolymer has been injected into the lumen of wood cell. The crosslinking reaction occurred between the groups of prepolymer and the surface hydroxyl (OH) groups of wood. A quasi-crystalline form was generated in the XRD, but actually it may not represent an increase in the crystallinity of cellulose (Wu et al., 2004).

Scanning electron microscopy (SEM) analysis: Scanning Electron Micrographs (SEM) of raw wood and WPNC are shown in Fig. 7. The SEM micrographs showed that the raw wood surfaces were covered with an uneven layer with a number of void/hole spaces, while the treated wood surfaces were smooth (Fig. 7a, b). The smooth surface may be caused by the good penetration of the monomer mixture to the cell wall and vessels of the wood (Rahman et al., 2010). The SEM analysis indicated that the prepolymer was impregnated into cell wall and cell cavities of wood.

Fig. 6(a-b):	X-ray diffraction of (a) Raw wood and (b) WPNC

Fig. 7(a-b):	Scanning electron microscopy of (a) Raw wood and (b) WPNC

CONCLUSION

In the present study nanofiller/phenol formaldehyde WPC were investigated. FT-IR spectra indicate the decrease wave number of the peak, ascribed to C-O stretch of C-O-H in starch at 1317 cm^-1 and 1222 cm^-1 and C-O stretch of C-O-C in starch at 1027 cm^-1 confirmed the impregnation of nanoclay/PF wood sample due to the fact that plasticizer could form intense H-bonding interaction with the hydroxyl groups. The stiffness of the WPNC was significantly increased compared with raw wood. The MOE and MOR of WPNC were significantly increased for Eugenia sp., Xylopia sp., Artocarpus Rigidus and Artocarpus Elasticus respectively. The Young’s modulus of Eugenia sp. was significantly different between raw wood and WPNC. The XRD patterns of WPNC indicate that the crystallinity increases at the amorphous region due the monomer loading. The SEM micrograph of WPNC clearly shows the void space was filled by the monomer and removes the waxy substance. It can be concluded that nanofiller/phenol formaldehyde was significantly effective on Eugenia sp. followed by Xylopia sp., Artocarpus Rigidus and Artocarpus Elasticus wood species, respectively.