Effect of oil palm ash on the mechanical and thermal properties of unsaturated polyester composites

2.1 Material preparation

OPA was obtained from the palm oil mill Lumadan in Beaufort, Sabah. EFBs and shells were burnt in the industry for heat generation and to produce OPA. OPA were filtered to achieve the best filler particle size below 300 μm. OPA was then dried in an oven at high temperature of 250°C for 24 h and stored in a dry place. Commercial UPE (Sigma-Aldrich) and the hardener, methyl ethyl ketone peroxide (MEKP, Sigma-Aldrich) were supplied by Riang Ria Sdn Bhd (Kota Kinabalu, Sabah, Malaysia).

2.2 Preparation of biocomposites

The specimens were divided into different compositions of 0, 10, 20 and 30 vol% of OPA. The UPE matrix was mixed with 1 vol% hardener. Mixing of the matrix and filler were done using a mechanical stirrer for 1 h at 180 rpm. The mixture was poured into a mold with size 30 mm×30 mm×1 mm and pressed by using a cold press before curing in room temperature for 24 h. After curing, the specimens were taken from the glass mold and were cut into the standard tensile dimension using a hacksaw. The specimens for tensile testing were prepared following the usual standards.

2.3 Mechanical properties testing

The tensile test was performed according to ASTM D5083. The test was carried out with an universal testing machine (UTM) model GOTECH AI-7000-M (Taiwan) equipped with load capacity of 10 kN. Ten specimens were cut from the plates using a cutter where the size of specimens for the tensile test was 150 mm (length, L)×25 mm (width, W)×3 mm (thickness, T). The relative humidity (RH) and temperature in the testing room were maintained at 50% and 25°C, respectively.

2.4 Thermogravimetric analysis (TGA)

TGA was performed using a Perkin Elmer Thermogravimetric Analyzer TGA 6. Tests were conducted in the temperature range of 0–600°C with a heating rate of 10°C/min in a nitrogen atmosphere and a flow rate of 50 ml/min. Samples of 10.0–12.0 mg were heated in a sample pan.

2.5 Differential scanning calorimetry (DSC)

DSC measurement was performed using a Perkin Elmer differential scanning calorimetry DSC. Scans were conducted in the temperature range between 0 and 350°C at the heating rate of 10°C/min using nitrogen gas with the mean flow of 50 ml/min.

2.6 Measurement of density

The density (ρ) of specimens was determined. Initially, specimens were weighed (m) and then the volume (V) of the specimens was measured.

ρ=mV

where,

ρ=density of specimen

m=mass of specimen

V=volume of specimen

2.7 Water absorption test

A water absorption test was carried out according to ASTM D570. The edges of the samples were sealed with polyester resin and subjected to moisture absorption. The samples were dried at 80°C for 24 h. The specimens were weighed and recorded as Wd. The composite specimens were immersed in distilled water at room temperature until the water content reached saturation. The specimens were periodically taken out of the water, wiped with a clean dry cloth to remove the surface water, and weighed again within 1 min of removing them from the water in order to avoid any errors due to evaporation. The specimens were recorded as Wn.

The percentage of moisture absorption (Mt) was calculated using the following formula:

Mt=Wn−WdWd×100

where,

Wd=original dry weight (g)

Wn=weight after immersed (g)

2.8 Fourier transform infrared (FTIR) spectroscopy

FTIR spectroscopy was used in order to detect the presence of the functional groups in the composites. The spectra of the composites were obtained using an IR spectrometer (Perkin-Elmer Spectrum 100). About 2 mg of the sample was pressed into a disc of about 1 mm thick. The FTIR spectra of the sample were collected in the range of 4000–600 cm^-1.

2.9 Scanning electron microscope (SEM) analysis

Morphological studies of the UPE/OPA composites were carried out using a SEM JEOL JSM-5610LV. SEM was used to observe the fracture surface of the composite samples. Samples were observed with 20 kV electron rays at 300× magnification, diameter of 50 μm and with 60 s acquisition time.

3.1 Mechanical properties

Figure 2 shows the tensile strength, Young’s modulus and elongation at break of UPE/OPA composites with different composition of OPA (0, 10, 20 and 30 vol%). Based on Figure 2A, pure UPE (0 vol%) with tensile strength of 29.26 MPa decreased to 26.46, 19.03 and 12.18 MPa with the addition of OPA at 10, 20 and 30 vol%, respectively. The decline in tensile strength of UPE/OPA composites was due to the physical properties of OPA and the interaction between the filler and matrix of the polyester (10). The increase of filler content led to the reduced tensile strength of the UPE/OPA composites as the large particles present in OPA (50 μm) have a tendency to hold the particles that reacted in the wet conditions due to the low interaction between the matrix at the filler surface of the UPE. These concentrated particles have reduced the compatibility of matix filler in UPE (9). It is also shows that the ability to support the delivery pressure of UPE is weak, which can be clearly shown in the SEM analysis in Figure 2.

Figure 2:

Tensile strength, Young’s modulus and elongation at break of UPE/OPA composites.

Figure 2B shows the Young’s modulus of UPE/OPA composites. Different trends of Young’s modulus were observed where the Young’s modulus increases with the increasing of OPA content. Based on Figure 2B, it was found that the UPE/OPA composite of 30% of OPA content shows the highest Young’s modulus, i.e. 7.5 MPa. The addition of OPA has increased the stiffness matrix of UPE due to the Young’s modulus increases as the OPA content increased.

The elongation at the break of UPE/OPA composites is shown in Figure 2C. The addition of OPA causes the matrix to lose its elasticity properties where the addition of 10% of OPA shows the elongation at the break reduces it drastically. Suradi et al. (11) stated that the decreases of elongation at the break is due to the reduction in the amount of matrix as the elasticity properties are obtained from the matrix. Therefore, this indicates that the addition of OPA reducing the elasticity of the matrix, which led to the stiffer composites. UPE thermoset-based composites are generally more brittle and have lower elasticity than epoxy-based composites (12).

3.2 Thermal properties

Figure 3 shows the thermal behavior of the UPE/OPA composites. Thermogravimetric analysis is one of the techniques used to measure the weight change, thermal decomposition and thermal stability of a substance. According to the Figure 3, the weight loss is between the temperatures of 300°C and 450°C of UPE/OPA composites due to the decomposition of the main components of carbon in lignocellulosic (13). Based on the figure, UP70/OPA30 shows the highest reading in the composites caused by the nature of OPA itself contains inorganic elements that can withstand at high temperatures without changing in weight (14). Increasing of the filler contained in UPE shows a significant effect on the temperature. Bhat and Khalil (15), stated that OPA consist mostly of silica which has a resistance to high temperature. Thus, the thermal stability of the UPE/OPA composites increases with the increasing of OPA in the matrix. The thermal stability of 30 vol% of OPA is slightly higher than 10 and 20 vol% of OPA due to the deterrent effect of decomposition of products in a matrix (9). Therefore, the thermal stability of the composites increases as the filler content of OPA increased.

Figure 3:

DSC curve and TGA curve of UPE/OPA composites.

Figure 3 also shows the relationship between the decomposition of the DSC curve and the TGA curve of UPE/OPA composites that has occurred at the temperatures of 300°C and 400°C. The thermal properties of TGA and DSC analysis shows that UP70/OPA30 have stable thermal properties than the other composites of UPE/OPA. Based on DSC curves, it does not reveal any exothermic effect which obviously indicates that thermal degradation of UP in the absence of oxygen (nitrogen atmosphere) is related to the endothermic effect. Thermal stabilization was observed while increasing the filler content up to 30% but no value for the optimal thermal stabilization below that was observed. At low filler content, interfacial adhesion of polymer-filler dominates but the amount of exfoliated particles is not high enough to promote the thermal stability through char formation (16). Thus, when increasing the filler content, relatively more exfoliated particles are formed, char forms more easily and increases the thermal stability of the UP/OPA composite until 30% of OPA filler is reached. The glass transition temperatures of composites were increased but were almost similar due to gap between of the curves in Figure 3 being rather small.

3.3 Physical properties

The density and water absorption of OPA/UPE composites based on the different composition of OPA are shown in Table 1. The density of composite UPE100/OPA0 shows the highest density rather than the other composites of 10, 20 and 30 vol% of UPE/OPA. The experimental value of density of UPE/OPA composites were decreased when the concentration of OPA increased. This is due to natural characteristics of OPA that is light in weight and has high strength to weight ratio (17). In addition, the use of OPA as a filler in the manufacturing of composites has significant potential to reduce the use of more matrix in composite materials. For water absorption analysis, the water absorption increases as the OPA content increases. Water absorption of pure UPE (0 vol%) increases with the addition of OPA at 10, 20 and 30 vol% with 0.65, 0.99 and 2.37%, respectively. In general, the weight gain due to water absorption of all biocomposites linearly increases with the increase of the square root of the immersion time, following the Fickian diffusion process (6, 7). In a humid environment, moisture can penetrate easily between the bond of the filler and matrix, which forms an unstable composite dimension (18). This is because OPA is hydrophilic while UPE is hydrophobic. As the OPA are made of lignocellulosic materials, it thus shows the hydrophilic properties of the polymer (19). All of the contents of lignocellulosic materials play an important role in absorbing water as it consists of many OH groups which can form a water molecule. Thus, when the composites containing lignocellulosic materials used in the environment, it is easier for the composites to absorb water (20).

3.4 Spectroscopic characterization

FTIR spectroscopy is a powerful tool used to evaluate the specificity of functional groups that exist in each morphological region. FTIR for OPA/UPE composites are shown in the Figure 4. The peak at 3200–3650 cm^-1 indicates the O-H group (alcohol) present in the OPA/UPE composites. Sahari et al. (4) claimed that this peak refers to the stretching of the hydroxyl group and the hydrogen bonding available in the fibers’ structure. The peak at 1623.73 cm^-1 showed the H-O-H bending in pure UPE. However, with the addition of OPA a new peak observed at the 1000–1450 cm^-1 curve shows the addition of -CH₂ and -CH₃. In addition, at the height of 2100–2400 cm^-1 in Figure 4, it can be seen that the peaks become broader than the FTIR spectra of pure UP. This can be attributed to a combination of or the physical interactions between the functional groups (CH bond) on pure UP with filler OPA during the coating process.

Figure 4:

FTIR of UPE/OPA composites.

3.5 Analysis of SEM

Figure 5 shows the SEM analysis of UPE/OPA composites. There are clear differences in the morphology shown in different composition of OPA (0, 10, 20 and 30 vol%). As is shown in Figure 5A, the surface of the composite is totally smooth with only little unoccupied surface as it does not have any OPA. Whereas, Figure 5B shows few particles filled with UPE matrix, a lack of intensity and dispersion of filler in the matrix. The surface area of the UPE/OPA composite shown in Figure 5C is distributed with some filler particles of OPA. Next, Figure 5D shows a higher number of particles that filled up with the matrix of UPE as well as most of the particles agglomerates and many more of the unoccupied surface observed. The presence of the unoccupied surface in the UPE/OPA composites is due to the evaporation of moisture in natural fibers during the process of crystallization of UPE which is naturally exothermic. In a wet environment, it is easy to penetrate into the bonding between the fibers and the matrix, which makes the dimensional composites unstable because the natural fibers have hydrophilic properties while polymer has hydrophobic properties. Thus, the existence of the unoccupied surface caused by the evaporation of moisture trapped in UPE during the crystallization process is proven. To overcome this problem, modification of OPA with chemicals will reduced the degree of swelling and can enhance the mechanical properties of composites (21).

Figure 5:

SEM of UPE/OPA composites.