In uence of kaolin ller on the mechanical properties of Luffa cylindrica/ polyester composite


 Polymeric materials are used in different industrial applications because they retain good environmental properties, low-cost, and easy to produce compared to conventional materials. This study investigated the effect of adding kaolin micro-filler (KF) on the mechanical properties of Luffa Fiber (LCF) reinforced polyester resin. Luffa cylindrica fiber treated with 5% NaOH, varied in weight fraction (5, 10, and 15%wt) was used to reinforce unsaturated polyester resin using hand lay-up method, whereas for the hybrid composite kaolin filler were kept constant at 6wt% fraction while the fibers varied as in the mono-reinforced composite. The samples were machined for mechanical and microstructural analysis. Analysis of the result revealed that the addition of kaolin has enhanced greatly the mechanical properties of Luffa-fibre based composites. The result reveal of the microstructure analysis, shows that there is an improvement in fiber-matrix adhesion.


Introduction
The demand for polymer composites reinforced or lled with natural bers and powders for various industrial applications (construction, automobiles, furniture, and sporting goods) is growing because they retain good environmental performance, low cost, and easy of production compared to traditional materials [1][2][3]. Low price, low weight and minimized impact on environmental pollution are the key reasons for the rapid development of polymer composite materials. Recent research efforts are aimed at nding alternative llers to replace inorganic llers [4]. In addition to inorganic llers, the use of natural bers (organic llers) also has the following advantages: low-cost, low density, non-abrasive, biodegradable, availability from natural resources, they are recyclable, and they are renewable natural resources [5][6]. Natural ber reinforced FRP can solve e ciency and environmental issues.
Luffa cylindrica is a naturally occurring cucumber vine in many countries. Luffa 's young cylindrical fruits are edible and contain a number of compounds, including ribosome-inactivating avonoids, phenols, triterpenes, and proteins. For medicinal purposes, such as immunostimulants and anti-in ammatory agents, the Luffa fruit has been used effectively [7]. Luffa sponge has been successfully used in the biosorption process of heavy metals in waste water and is an essential natural ber. This emerging cash crop has the full potential to improve the economies of developing nations [7][8][9][10]. There is 84% holocellulose, 66% cellulose, 17% hemi-cellulose, 15% legnine, 3.2% extractives, and 0.4% ashes in Luffa ber physical properties. The physical properties of Luffa ber are 820 kg / m3 mass, 25-60 µm diameter and 59.1 crystallinity index [11][12][13] In order to improve performance and reduce costs, llers are used with different commodities and engineering polymers. The use of inorganic mineral llers in plastic resins will increase the different physical properties of the material, such as mechanical strength and modulus. Generally, the mechanical properties of particle-lled polymer composites largely depend on the size, shape, distribution of ller particles in the polymer matrix, and degree of interfacial adhesion between ller and matrix [14][15][16][17].
Calcium carbonate, kaolin, mica and talc are most commonly used as llers to minimize production costs and enhance thermoplastic properties such as crystallinity, stiffness, stiffness, exural modulus, resilience, dimensional stability, electrical conductivity and thermal conductivity. In order to prepare particle composites, Al-Asade and Al-Murshdy studied the addition of kaolin into an unsaturated polyester matrix. The addition of 3-9% of kaolin to the unsaturated polyester resin indicates that the kaolin acts as a binder, and the resulting composite material acts as a particle strengthening agent, resulting in the improvement of the mechanical properties of the unsaturated polyester [18]. Ahmed et al. studied another study of kaolin composite polyester. In this study, a polymer composite made of diethylene glycol and untreated kaolin (based on PET waste derived from unsaturated polyester) was tested. Thermal and chemical methods have been carried out to process kaolin. These treatments affect the mechanical and electrical properties of kaolin lled polymer composites [19]. These reports motivated us to consider the inspiring possibility of incorporating the ne micron-sized kaolin particles in a composite comprising of luffa reinforced polyester to study their effects on the mechanical properties of the composites.
The hybridization of bers with llers has been utilized to enhance the properties of composites. A wise choice of matrix and reinforcing phase contribute to a composite with a combination of strength and modulus comparable to or even better than those of conventional metallic materials. Improving the properties of polymers and their composites by adding particulate ller materials in industrial and structural applications has shown great promise, and has recently attracted great attention. Sakthivel M et al., reseached on the feasibility of using luffa bers/coir as reinforcement for a polymer such as polypropylene in particulate form. They found that the addition of both reinforcement materials based on lignocellulose, resulted in improved mechanical properties, and there was continuous proof of consistency between the two materials [20]. Srinivasan C. studied the effect of ber treatment and addition of SiO2 nanoparticles on the properties of composite materials. Fiber treatment has been proven to improve the e ciency of the ber/matrix interface, and the mechanical properties can be improved by

Materials and quipment
The reinforcement materials applied in this research are luffa ber, kaolin ller, and the matrix is a polyester resin. They all come from local sourced. Equipment applied for characterization of composites are; Monsanto tensometer, Vickers hardness tester (model: MVI PC), and Charpy impact testing machine (model:412).

Reinforcement preparation
The Luffa cylindrical ber was treated at 80oC with a 5% NaOH solution. It was washed with distilled water and dried at room temperature for 1 day from the outside surface after treatment to extract lignin, oil, and luffa ber wax, and then dried at room temperature. After drying in the sun for a few days, a brous mat (130 mm by 120 mm) was cut out of the outer core of the luffa fruit shell, mounted further between two at 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 ller and pure unsaturated polyester was used as the matrix material. Micro particulate !Jer (kaolin) was procured from the department Chemical Engineering, Ahmadu Bello University, Zaria, Nigeria. Unsaturated polyester resin, hardener, sodium hydroxide pellets, acetone, and petroleum gel were obtained from Olasco Ltd, Zaria, Nigeria.

Composite fabrication
To create the composite, a conventional process called the hand lay-up process was used. The hand layup method has been a widely studied method of manufacturing natural ber-based composites because of its exibility, cost-effectiveness, and durability, which is economically feasible for developing countries and less nancially supported universities and colleges. Fiber (luffa) was varied at weight fraction (5, 10, and 15%wt) for the ber reinforced composite, whereas for the hybrid composite the ller (kaolin) was kept constant at 6wt% fraction while the ber (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 ller in the polyester resin matrix, the composite mixture was thoroughly stirred for 10 minutes 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 catalytic converter and stirred for around two minutes, 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 two minutes.
The mold release layer was laid over the wooden molds prepared for the tensile, exural, impact and hardness tests of specimens (120 x 5 x 15 mm, 100 x 30 x10 mm, 100 x 10 x 10 mm and 10 x 10 x 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 ber laminate on to the mixture. The mixture was applied over the ber 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 hours and the mold samples were taken for mechanical testing. The label and description of the composite samples are indicated in Table 1.

Flexural strength result
The exural test was carried out on specimens measuring 100.0 × 30.0 × 10.0 mm according to ASTM (D790) using a computerized Instron 3369 Universal Testing Machine with a load cell capacity of 50KN. The three-point bend exural test method was used with a cross-head speed of 5 mm/min and a span length of 65mm. Samples were positioned on the support span and the load was applied to the center by the loading nose producing three-point bending. The test was stopped at 5% de ection.

Hardness Test
The hardness test was carried out on each specimen according to ASTM C1327 with a dimension of 10.0 x 10.0 x 60 mm. The indentation technique using a Vickers diamond pyramid indenter on the microhardness tester was used. The measurement was done on the surface by applying 0.3kg load for 15seconds. Three Vickers hardness readings were taken for each sample and the average values for the test samples were used as the illustrative values.

Impact strength
The impact test was carried out on samples with a Charpy impact testing system with a capacity of 15J for polymer composites and 25J for metal composites at room temperature of model number 412. Test samples with a dimension of 100.0 x 10.0 x 10.0 mm were produced in accordance with ASTM 2000. The sample was placed on the machine prior to the test and the pendulum was released to calibrate the machine. The test samples were then horizontally clenched in a vice and the freely swinging pendulum given the requisite force to crack the bar. The value of the angle at which the pendulum swung before the test sample was broken corresponded to the value of the energy that was consumed when the sample was broken and was read from the machine's calibrated scale.

Tensile Results
The effect of ber loading in reinforced Luffa cylindrica ber (LCF) composites on both with and without Kaolin ller(KF) was seen in Fig. 1

Flexural strength
The exural 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 exing. The results are as shown in Fig. 2 from which it can be seen that all the composites possess better exural 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 exural strength of 120.34MPa comparing to other hybrid composites, with 183.62% and 90.7% improvement compared to control sample and sample C4 of monoreinforce composites. Due to the uniform distribution of ller materials and improved effective bonding between ller materials and matrix and strong polymer / ller interface adhesion, the incorporation of kaolin ller into different ber loadings of Luffa ber composites affects the exural strength of the composites.Similar observations have been studied in the literature of the in uence of the addition of micro ller on luffa ber-reinforced polymer composites

Impact strength
The impact test results shown in Fig. 3 indicates that in the Luffa ber-reinforced composites materials, whether with or without Kaolin ller, ber 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 ber loading in both cases, i.e. with and without micro-ller. 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 for the mono-reinforced composites of samples C4 with 15wt% of LCF gave the maximum value of 0.23J/m with a 130% improvement compared to control sample C1 ( 0wt% of reinforcement). While for the hybrid composite of sample C7 with 15wt% LCF and 6wt% KF gave an optimum value of 0.5J/m compared to other lled composites with 400% improvement in compared to C1 and 117.4% to C5. The strong bonding strength between micro llers, matrix and ber, and stability of the molecular interface results in more energy being absorbed and distributed, and more effectively prevents cracks from initiating early. Similar observations have been studied in the literature of the in uence of the addition of micro ller on luffa ber-reinforced polymer composites.

Hardness test
Vickers hardness test has been performed on the composite samples. Figure 4 shows that compared to other lled and un lled 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 interparticles in the matrix, which increases the indentation resistance of composites. It can be observed that with the rise in ber lling, the hardness value of the composites increases in both cases, i.e. with and without micro-ller. Similar observations of the in uence of the addition of micro ller on luffa ber-reinforced polymer composites have been reported in the literature. Figure 5, and Figure 6 shows the SEM micrographs of the composites material at different weight percentage composition llers. Figure 5(A and B) indicates good compatibility between the two llers and the matrix, good llers dispersion within the structure and also, good interfacial bonding between the llers and the matrix was observed. The good interaction between the llers and the matrix observed in gure 5. con rmed the e cient stress transfer between the llers and the matrix which resulted in the better stiffness and strength of the composite as observed in the results. Similar result were reported by Pracella et al., (2006). Good mixing and distribution of particulate in polymer matrix at higher composition of particulate reinforcement is always associated with some di culties which always leads to improper and poor wetting and thorough mixing of matrix and reinforcement as well as formation of air voids with the materials during and after casting. This effect can clearly be seen in the micrographs in gure 6 ( C and D). The air voids formed gave rise to the decrease in the properties of the material.

Conclusions
Luffa ber reinforced polyester composites lled with or without kaolin ller were investigated. The following nding was observed; 1. The addition of kaolin ller in the luffa ber matrix modi es the mechanical properties of polyester. 2. Hybridization of ber and ller gives better performing composite with 5wt%LCF-2wt%KF hybrid composite showcasing the best combination for tensile strength while 15wt%LCF-6wt%KF hybrid composite showcasing the best combination for impact energy, exural strength, and hardness test properties. 3. The current study reveals the promising potential of kaolin-luffa reinforced composites for industrial lightweight engineering and outdoor applications, including automotive parts and constructional panels Declarations Funding: self-sponsored Con icts of interest/ Competing interests: No con ict of interest.
Availability of data and materials: Materials are commercially available at a reasonable cost and quality result was obtained  Figure 1 Tensile Strength of the composites Figure 2 Flexural strength of the composites Figure 3 Impact energy of the composites Micrograpgh of Label C4 and C5 at 750x Micrograpgh of Label C6 and C7 at 750x