Influence of Kaolin particulate and Luffa cylindrica fiber on the mechanical properties polyester matrix

The reinforcement materials applied in this research are Luffa fiber and Kaolin filler, and the matrix is a polyester resin.

The Luffa cylindrical fiber was treated at 80°C with a 5% NaOH solution. It was washed with distilled water and dried at room temperature for 24 h from the outside surface after treatment to extract lignin, oil, and Luffa fiber wax, and then dried at room temperature. After drying in the sun for a few days, a fibrous mat (130 mm by 120 mm) was cut out of the outer core of the Luffa fruit shell, mounted further between two flat wooden plates, and straightened to an even thickness by applying a uniform compressive load with the mechanical bench vices for a few hours. Kaolin was used as a microparticulate filler and pure unsaturated polyester was used as the matrix material. Kaolin particulate was procured from the Department of Chemical Engineering, Ahmadu Bello University, Zaria, Nigeria.

To create the composite, a conventional process called the hand lay-up process was used (Plate 1). The hand lay-up method has been a widely studied method of manufacturing natural fiber–based composites because of its flexibility, cost-effectiveness, and durability, which is economically feasible for developing countries and less financially supported by universities and colleges. Fiber (Luffa) was varied at weight fraction (0, 5, 10, and 15%wt) for the fiber-reinforced composite, whereas for the hybrid composite filler (Kaolin) was kept constant at 6wt% fraction while the fiber (Luffa) was varied as in the mono-reinforced composite. The required quantity of polyester matrix and Kaolin was weighed using an electronic weighing balance and put in a 200-ml glass beaker. In order to prevent aggregation and to achieve a faster and more accurate distribution of the filler in the polyester resin matrix, the composite mixture was thoroughly stirred for 10 min using a long glass rod. Subsequently, methyl-ethyl-ketone-peroxide (MEKP) catalytic converter was introduced using a disposable syringe at a ratio of 10 ml of polyester to 0.2 ml of the catalytic converter and stirred for around 2 min, after which the cobalt naphthenate accelerator was applied at a rate of 10 ml of polyester to 0.1 ml of accelerator and stirred for another 2 min. The mold release layer was laid over the wooden molds prepared for the tensile, flexural, impact, and hardness tests of specimens (120 × 5 × 15 mm, 100 × 30 × 10 mm, 100 × 10 × 10 mm, and 10 × 10 × 10 mm respectively) to extract the composite material quickly and easily, and the release spray was applied to the inner surface of the mold. A thin layer of the mixture was poured after keeping the mold on the plywood, followed by the distribution of the fiber laminate onto the mixture. The mixture was applied over the fiber laminate, and the process was repeated in order to achieve the desired thickness. Care was taken to ensure correct wetting between the matrix and reinforcement before a compressive load of 150 KN was applied to ensure proper bonding and to push out trapped gases. The samples were allowed to cure for 72 h and the mold samples were taken for mechanical testing. The label and description of the composite samples are indicated in Table 1.

The effect of fiber loading in reinforced Luffa cylindrica fiber (LCF) composites on both with and without Kaolin filler (KF) is seen in Fig. 1. It was observed that the tensile strength of the composites decreases with the increase in fiber loading in both cases, i.e., with and without micro-filler. Tensile strength ranged from 12.11 to 9.17 MPa for the mono-reinforced composite whereas the hybrid composite shows values ranging from 15.32 to 11.46 MPa. It can also be observed that the maximum value of 12.11MPa was given for the mono-reinforced composites of C2 samples with 5wt of LCF with a 72.02% increase compared to the control sample C1 (0wt% of reinforcement), while for the hybrid composite of sample C5 with 5wt% LCF and 6wt% KF gave an optimum value of 15.32 MPa compared to other filled composites with 117.61% improvement in compared to C1 and 26.51% to C2. This can be attributed to the effectiveness of the addition of particulate to fiber-reinforced composites. The general expression of the results indicates that the composite material developed from materials based on Kaolin-Luffa has a higher tensile strength than that developed from materials based on Luffa cylindrica fiber only. Ichetaonye SI. et al. obtained similar findings where the tensile strength increased up 20wt% fiber loading before decreasing.

The flexural test measures the strength needed under three-point loading conditions for bending a beam. The data is also used to select materials for components that will carry loads without flexing. The results are as shown in Fig. 2 from which it can be seen that all the composites possess better flexural strengths than the control sample. It was also noticed that the composite of sample C7 with 15wt% LCF and 6wt% KF reinforcement displayed the best flexural strength of 101.25MPa comparing to other hybrid composites, with 183.62% and 90.7% improvement compared to control sample and sample C4 of mono-reinforce composites. This is due to the addition of Kaolin particulates. This also goes in line with the observation of Vinay et al. (2016) where C7 (15% fibers loading) exhibited maximum flexural strength (72 MPa) when compared with other filled and unfilled composites.

The impact test results shown in Fig. 3 indicate that in the Luffa fiber–reinforced composites materials, whether with or without Kaolin filler, fiber loading led to an improvement in the impact energy of the matrix material. It was observed that the impact energy of the composites increases with the increase in fiber loading in both cases, i.e., with and without micro-filler. Impact energy ranged from 0.15 to 0.23J/m for the mono-reinforced composite whereas the hybrid composite shows values ranging from 0.3 to 0.5 MPa. It was noted as well that the mono-reinforced composites of samples C4 with 15wt% of LCF gave the maximum value of 0.23J/m with a 56.5% improvement compared to control sample C1 (0wt% of reinforcement), while the hybrid composite of sample C7 with 15wt% LCF and 6wt% KF gave an optimum value of 0.5J/m compared to other filled composites with 80% improvement compared to C1 and 54% to C5. The strong bonding strength between micro fillers, matrix, and fiber and the stability of the molecular interface result in more energy being absorbed and distributed and more effectively prevent cracks from initiating early. This also goes in line with the observation of Vinay et al. (2016) where C7 (15% fiber loading) exhibited maximum impact strength (8 joules) when compared with other filled and unfilled composites.

Vickers hardness test has been performed on the composite samples. Figure 4 shows that compared to other filled and unfilled composites, the hardness value of hybrid composite of C7 sample with 15wt% LFC and 6wt% of KF reinforcement exhibited a maximum hardness number of 79.37 HVN. This may be attributed to the uniform dispersion of Kaolin particles and the decrease in the distance of interparticle in the matrix, which increases the indentation resistance of composites. It can be observed that with the rise in fiber filling, the hardness value of the composites increases in both cases, i.e., with and without micro-filler. This aligns with the findings of Vinay et al. (2016) where mechanical properties like hardness reached a peak at an optimal weight fraction before decreasing.