1. Introduction
For the sake of environmental protection and economic viability, a renewed interest in using lignocellulosic materials or natural fibers as reinforcing agents for thermoplastic composites has been stimulated in the recent decades [
1]. In the automotive industry, natural fibers were once popular as a reinforcing agent for thermosetting and thermoplastic composites in the 1920s. However, owing to their higher production cost and inferior performance, natural fibers has been gradually replaced by synthetic fibers [
2]. Nevertheless, in recent years, rising awareness towards the environment well-being has encouraged people to adopt renewable resources, hoping for a diminution in greenhouse gases and carbon dioxide emissions. This is the time when people started to turn back to natural fibers [
3]. Incorporation of natural fibers into the thermoplastic composite could bring a lot of advantages such as low density, improved Young’s modulus and renewability. However, drawbacks such as lower ultimate strength and higher water absorption are also the simultaneous consequences. Ahmad Saffian et al. [
4] incorporated maleic anhydride modified lignin into kenaf core fiber-reinforced poly (butylene succinate) biocomposites and reported that the thermal stability of the composite was enhanced. However, lower tensile strength and modulus was observed.
Polypropylene (PP) is one of the most prevalently used thermoplastic polymers in the fabrication of natural fiber-reinforced polymer composite [
5]. Despite the existence of a wide variety of thermoplastic polymers, PP stood out as a promising matrix owing to the fact that natural fiber started to degrade at temperatures around 210 °C. Therefore, only thermoplastic polymers that soften below this temperature are suitable for the production of natural fiber-reinforced polymer composites [
6]. On the other hand, kenaf (
Hibiscus cannabinus L.) is one of the popular natural reinforcing agents for polymer composites in Malaysia. It has a fast growing rate of 10 cm per day and is able to be cultivated four times a year, which has made kenaf an attractive natural fiber source [
7]. Studies reported on PP reinforced with kenaf fibers has been reported extensively [
8,
9,
10]. In fact, kenaf fiber has been proven to be more superior than flax, hemp, sisal and coir fiber as the polypropylene composites reinforced with kenaf exhibited higher specific modulus [
11].
In natural fiber-reinforced polymer composites, dielectric properties of the produced composite are vital criteria that determine their final applications. To date, PP is still widely used in research and commerce due to its excellent characteristics and ease of fabrication process [
10,
12,
13]. In high-voltage insulation systems, PP is one of the most commonly used polymeric material because of its good chemical resistance, mechanical properties and most importantly low dielectric loss, high dielectric resistivity and strength [
14]. According to Hamzah et al. [
15], the ability of a material to withstand the maximum voltage applied before the occurrence of breakdown failure is called dielectric breakdown strength. The dielectric withstands the voltage of the composite material to determine the quality and appropriateness of the chosen material as an insulation system. Generally, dielectric breakdown can be characterized as a sudden change in the resistance of the insulation material due to the applied voltage. Netnapa et al. [
16] evaluated the effects of phosphorus-based non-halogenated flame retardant filler addition on the dielectrical breakdown strength of poly (L-lactic acid)-poly (lactic acid) microsphere/kenaf fiber composites. The authors reported an improvement in dielectrical breakdown strength when 2 wt % flame retardant filler was reinforced into the composite.
The ability of conducting heat (thermal conductivity) is a value-adding feature for polymeric composites. Thermally conductive polymeric composites possess lighter density, lower corrosion, oxidation, and chemical resistance as well as the flexibility to be tailor made to cater for various final applications, making it very suitable in replacing metal [
17]. Heat sinks and packaging are two of the examples of the application of thermally conductive polymeric composites. However, studies on thermal conductivity of polymeric composites, particularly kenaf fiber-reinforced polymer composites, are rather limited from the literature. Liu et al. [
18] produced rice straw fibers reinforced by polyurethane composites with excellent thermal insulating properties. The produced polymer composite showed promising potential to be used as insulating materials in buildings. Studies on thermal conductivity of kenaf fiber alone are also very scarce. The first study on the topic was reported by Gardner et al. [
19] who evaluated the thermal conductivity of kenaf fiber before and after alkali treatment. In the same study, thermal conductivity and diffusivity of the unidirectionally oriented kenaf–epoxy composites reinforced with NaOH-treated and untreated kenaf fibers were assessed. Composite reinforced with NaOH-treated kenaf fibers possessed better thermal conductivity and diffusivity as interfacial contact between the fibers and epoxy matrix was enhanced after alkali treatment was applied to the kenaf fibers.
Lignin can be blended together with a wide variety of thermoplastic polymers in order to improve the performance and flowability of the resultant polymer composite [
4]. Lignin is a green and renewable substance originate from plants. Commercially, lignin is a by-product generated from the pulping process and always exists in a huge quantity [
20]. Regretfully, these by-products are not utilized effectively. Huang et al. [
21] reported that addition of lignin could enhance the electrical breakdown voltage of PP and polyethylene (PE) composite. Nevertheless, reduction in mechanical strength was also observed. This phenomenon is caused by poor compatibility between nonpolar PP and PE and polar lignin. Therefore, addition of compatibilizer is necessary to improve the microphase structure of the blend system [
21]. Polymeric methylene diphenyl diisocyanate (PMDI), an isocyanate compatibilizer, is often incorporated into the system [
22]. Apart from that, lignin was also found to be able to increase the thermal conductivity of phenolic foam [
23]. However, as mentioned in the above section, studies on the electrical properties and thermal conductivity of kenaf fiber-reinforced PP are very scarce. It opens up an opportunity for the research to fill the gap of knowledge. Kharade and Kale [
24] reported that, with the assistance of a compatibilizer, the maximum content of lignin could achieve 30% and saw an improvement in the mechanical strength of the polyolefin materials treated with lignin. In addition, our previous study showed that incorporation of 30% lignin into kenaf core fiber-reinforced poly (butylene succinate) biocomposites has resulted in satisfactory tensile modulus and thermal stability [
4]. Hence, in this study, 30% lignin was used for reinforcing PP/kenaf composites. Therefore, in this study, kenaf fiber-reinforced PP was fabricated and the effects of lignin incorporation on the mechanical strength, thermal conductivity and electrical breakdown of the resultant composite were evaluated.