1 Introduction

Medium density fiberboard (MDF) is an engineered wood product composed of fine lingo-cellulosic fibers combined with a synthetic resin and subjected to heat and pressure to form panels (Irle and Barbu 2010). To facilitate the chemical reaction and allow for reasonable press times, all production processes in the wood based panel industry employ similar methods of heating the mat. This is done by heated platens which are in contact with the mat surfaces. Heat then flows from the platens through the mat surfaces to the center by conduction (Bolton et al. 1989). Since volumetric heating of the mat is not undertaken, curing of the adhesive does not occur uniformly.

The adhesive is cured first on the mat surface and later in the central region. Most inorganic compounds and organic molecules such as polymers and wood are difficult to set in a vibration or excitation mode as they are poor conductors of heat and electricity. Consequently, the core of the mat attains cure temperature only after a significant amount of time has elapsed since subjected to temperature and pressure. Methods to enhance production by increasing the pressing time at a constant temperature or by increasing the press temperature keeping pressing time constant are followed.

Hot pressing is considered a costly unit operation. Reduced pressing time will have a positive impact on lowering the production costs and increasing the plant production capacity. Several methods have been developed to decrease the pressing time such as shortening closure time, high frequency heating, steam injection, increasing the curing rate of the adhesives, optimizing the press parameters. In the literature, the effect of using a high frequency heating source on mat pressing times has also been reported (Pound 1973; Maloney 1989 and Pereira et al. 2004). Application of nano-based materials opens up new aspects in the field of wood science. The effect of nano-silver impregnation on mechanical properties of heat-treated solid wood was carried by Taghiyari (2011). Taghiyari et al. (2011) studied the effect of nano-silver on the reduction of hot-pressing time. Ashori and Nourbakhsh (2009) studied the efficiency of nanoclay (layered silicate) as reinforcement filler in MDF boards. The efficiency of aluminum oxide nanoparticles as a sorbent for removing residual formaldehyde from urea formaldehyde resin was studied by Dudkin et al. (2006). Most metals are good conductors of heat as the atoms are set in a vibration mode of thermal excitation easily. The thermal properties of nanoparticles open up new possibilities for enhancement of heat transfer when added to the wood composite by mixing it with the resin. Thermal conductivity of the wooden panel can be higher, which can lead to shorter pressing time. As the particles are of nanometer size, the difficulties in sawing, nailing etc. are expected to cause fewer problems.

Hence experiments were conducted to estimate the curing time at a different weight concentration of nanoparticles when mixed with resin and wood fibers. The properties of MDF were estimated with an indirect measurement of the resin curing due to the addition of thermally conductive nanoparticles.

2 Experimental procedure

2.1 Materials

Fresh rubber wood (Hevea brasiliensis) fibers were obtained from Robin Resources (M) Sdn. Bhd. Urea–formaldehyde (UF) liquid resin was obtained from Dynea Malaysia Sdn. Bhd. The viscosity of the UF resin at 30 °C was 178 cp, pH 8.79, density 1.286 kg m−3 and gel time at 100 °C was 36 s. Aluminum oxide (Al2O3) nanoparticles were procured from Sigma Aldrich of 47 nm average particle size.

2.2 Mixing of Al2O3 nanoparticles with UF resin

A bench top overhead mixer IKA®-WERKE, model RW 20 DZM with a speed range of 72–2,000 RPM was used for pre-mixing Al2O3 nanoparticles with the UF resin at 1,800 RPM for 30 min. After mixing the Al2O3 nanoparticles and UF resin in a bench top mixer, the resin mixture samples were subjected to ultra-sonic treatment with NANO-LAB Ultrasonic probe dispersion QS1 system for 1 h for uniform mixing of nanoparticles in UF resin. The percentage weight fraction of Al2O3 nanoparticles were based on the weight fraction of oven dry wood fibers at the two levels 0.5 and 1.0 %.

2.3 Thermal conductivity of UF and UF/Al2O3 resins

The thermal conductivity of liquid UF resin and UF hybrids was calculated by using KS-1 sensor of KD2 Pro. It is a device manufactured by Decagon Devices to measure thermal conductivity of liquid samples. The liquid UF resin and hybrid samples are poured in the vials. The cap of the vial is equipped with a septum allowing direct insertion of the needle into the vial through the cap. In liquid samples, the KS-1 sensor needle was oriented vertically during the measurement to help prevent free convection.

2.4 Differential scanning calorimetry (DSC)

The curing process of UF/Al2O3 resin was investigated using differential scanning calorimetry (DSC, Q1000, TA instruments USA).The UF/Al2O3 resins of 6 mg were placed on T-zero hermetic aluminum crucibles. The samples were heated from 30 to 200 °C in an inert atmosphere of nitrogen, maintained at 50 ml min−1 flow rate, with an identical empty crucible used as a reference in the measurement process.

2.5 Fourier transform infrared spectroscopy

The FTIR transmittance spectra of UF and UF/Al2O3 resins were obtained with Perkin Elmer spectrum 100 in the spectral range of 500–4,000 cm−1 with a resolution of 2 cm−1 and 50 scans.

2.6 Scanning electron microscopy (SEM)

SEM analysis was done by using Phenom™ Scanning Electron Microscope with a magnification range of 24–24,000×, images up to 2,048 × 2,048 pixels to study the nanoparticles distribution in the MDF panels.

2.7 Preparation of MDF panels

The standard laboratory method was followed in the manufacturing of 12 mm thick MDF panels. A rotating drum blender was used for the mixing of wood fibers, Al2O3 nanoparticles and UF resin uniformly. The drum consists of steel dowels arranged in a zigzag pattern which was intended to facilitate tumbling and mixing with 18 RPM rotation speed. The Al2O3 nanoparticles resin mixture was sprayed onto the wood fibers with a spray gun. Then, the resin-sprayed fibers were formed into a 300 mm × 300 mm preformed panel which was then pre-pressed at 1.5 MPa pressure. Finally, the pre-pressed panel was hot-pressed at 180 °C for 360 s with a target thickness of 12 mm having a density of 800 kg m−3. Two sets of panels were prepared for each type of concentration. The first set of boards was used to find out the temperature profile inside the core of the mat during hot pressing and the second set of boards was used to determine the physical and mechanical properties. The experimental parameters such as moisture content of fibres, resin content, pressing time, platen temperature were maintained the same in all experiments. Table 1 gives details of the experiment conducted.

Table 1 Tab. 1 Number of samples taken for recording the core temperature and mechanical testingAnzahl der Platten, die zur Bestimmung der Kerntemperatur und der mechanischen Eigenschaften verwendet wurden
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2.8 Measurement of core temperature

The wood fiber mat was pre-pressed to half the thickness of the loose mat. The core temperature was measured by using the three K-type of thermocouples. These thermocouples were attached with data logger for storage of data. The thermocouple wires were kept 50 mm apart from each other. The minor deviations in the values from the thermocouples were due to slight difference in elevation of thermocouples location while hot pressing. The average of the three values was taken for the analysis. Three test specimens were prepared for each type, with 0.0, 0.5 and 1.0 % of nanoparticles.

2.9 Evaluation of MDF panel properties

All the MDF panels were sanded to remove the pre-cured surface layer and then cut into specific size specimens for the investigation of the panel properties. All the specimens were maintained at 60 % relative humidity and 20 °C for 3 weeks in a conditioner and then tested according to British Standards EN: 1993. Following number of samples were taken, for internal bonding (20 samples), thickness swelling (20 samples), modulus of rupture and modulus of elasticity (10 samples) and then their average value was taken for analysis.

3 Results and discussion

Figure 1 shows the development of the core temperature for the boards with Al2O3 nanoparticles and the control board (CB). At the beginning of the press cycle, there was a rapid rise in the core temperature which was due to a steep vapor pressure gradient (Bolton et al. 1989, Gupta 2007) that developed during the period of 15–70 s. At press time phase 70–130 s, a constant temperature in the central plane was observed which was due to phase change occurring in the board. The vapor formed is observed to exit from the edges of the board due to higher vapor pressure formed at the core. From the time period of 130 s, a gradual rise in the central plane was observed which was due to heat conduction in the board. As depicted in Fig. 4, the boards prepared with Al2O3 nanoparticles show higher thermal conductivity in comparison to the control boards.

Fig. 1 Abb. 1
figure 1

Core temperature profile of mat during hot-pressing of MDF panels

Verlauf der Kerntemperatur des Spankuchens beim Heißpressen von MDF-Platten

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The FTIR spectrum of the samples is shown in Fig. 2. A strong adsorption peak at 3,367.73 cm−1 was observed in all the samples. It represents the free –NH2 group and the peak intensity increases with the addition of nanoparticles. The peaks between 1,650 and 1,550 cm−1 reveal the presence of amide I, II, C=O, and CH2OH, CH3 group. The peaks between 1,400 and 1,360 cm−1 show the presence of CN group (Siimer et al. 2003). The 1,149 cm−1 peak is attributed to both asymmetric stretch of –N–CH2–N– and symmetric stretch of –C–O–C– of the ether linkage. The UF/Al2O3 resins show the same peaks as in pure UF with a higher order of transmittance intensities. The peak around 900–950 cm−1 is a characteristic peak for Al–O bond. It appeared in the samples containing UF/Al2O3 resins.

Fig. 2 Abb. 2
figure 2

FTIR spectra of UF and UF/Al2O3 resins

FTIR-Spektren von Harnstoffharz ohne und mit unterschiedlichen Al2O3 Anteilen

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DSC measurement results are very useful to estimate the chemical reaction kinetics of the UF resin. Figure 3 shows the DSC thermograph for UF and UF/Al2O3 resins at a heating rate of 15 °C min−1. The values of peak curing temperature and enthalpy of reaction is given in Table 2. It was observed that the peak curing temperature decreases as Al2O3 concentration increases. The enthalpy of reaction (ΔH) of UF resin containing Al2O3 shows increasing amount of heat released in comparison to pure UF. This means that more heat was generated during curing of UF resin. This enthalpy of reaction also contributed to the increase in heat transfer during the hot pressing of MDF.

Fig. 3 Abb. 3
figure 3

DSC exothermic thermographs of UF and UF/Al2O3 resins

Exothermische Thermographen der Differenzialrasterkalorimetrie von Harnstoffharz ohne und mit unterschiedlichen Al2O3 Anteilen

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Table 2 Tab. 2 Results of peak temperature and enthalpy of reaction through DSCMittels Differenzialrasterkalorimetrie bestimmte Maximaltemperatur und Reaktionsenthalpie
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Figure 4 shows the thermal conductivities of liquid UF and UF/Al2O3 resins. The pure UF resin has a thermal conductivity of 0.399 W mK−1. There is an increase of 2.7 and 3.75 % in the thermal conductivity with increasing weight percentage of Al2O3. Comparison of the internal bonding strengths of the different panels is shown in Fig. 5. The bonding strength increases with Al2O3 concentration which can be due to enhanced curing in the core layer of the mat. There is an increase in the bonding strength of 7.6, and 34.3 % with the loading of 0.5 and 1.0 % weight of nanoparticles as compared to standard panel.

Fig. 4 Abb. 4
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Thermal conductivity of liquid UF and UF/A2O3 resins using KD2 pro

Wärmeleitfähigkeit von flüssigem Harnstoffharz ohne und mit unterschiedlichen Al2O3 Anteilen gemessen mit KD2 pro

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Fig. 5 Abb. 5
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Internal bond strength for different MDF panels

Querzugfestigkeit verschiedener MDF-Platten

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There is no significant difference in MOR values of both types of MDFs as shown in Fig. 6. The MOE of MDF panels increases by 10.9 and 17.2 % of 0.5 % weight fraction and 1 % weight fraction loading of Al2O3 nanoparticles as shown in Fig. 7.

Fig. 6 Abb. 6
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Modulus of rupture (MOR) of different MDF panels

Biegefestigkeit (MOR) verschiedener MDF-Platten

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Fig. 7 Abb. 7
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Modulus of elasticity (MOE) of different MDF panels

Elastizitätsmodul (MOE) verschiedener MDF-Platten

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The thickness swelling (TS) is lower in the samples that have better bonding. Figure 8 shows thickness swelling (TS) results for all the samples. There is only slight reduction in TS percentage with the increase in nano particles percentage. The Al2O3 nanoparticles were well dispersed in the MDF panel as shown in Fig. 9. The nanoparticles were attached to wood fibers through the cross-linked UF resin.

Fig. 8 Abb. 8
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Thickness swelling of MDF panels with different weight percentage of Al2O3 nanoparticles

Dickenquellung von MDF-Platten mit unterschiedlichen Masseanteilen an Al2O3 Nanopartikeln

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Fig. 9 Abb. 9
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SEM micrograph indicating the Al2O3 nanoparticles (white arrows) and cured UF resin (black arrows) distribution in MDF

REM-Aufnahme, die die Verteilung der Al2O3 Nanopartikel (weiße Pfeile) und des gehärteten Harnstoffharzes (schwarze Pfeile) in der MDF-Platte zeigt

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4 Conclusion

In this study, Al2O3 nanoparticles were added to UF resin to improve the heat transfer during hot pressing of MDF. The data from thermal conductivity (KD2) and DSC have supported the claim. The mechanical properties such as internal bonding and MOE have also shown the improvement. The heat transfer improved due to the addition of Al2O3 nanoparticles and also from the exothermic curing reaction of UF/Al2O3 resin. The enhanced heat-transfer helped in the curing of UF throughout the mat, particularly in the core, resulting in the improvement of physical and mechanical properties of MDF panels.