**1. Introduction**

Petroleum-based plastics have been extensively used in numerous fields such as packaging bags, consumer goods, medical equipment, the automotive sector, and construction sites. The global production of plastics was valued at around 52.9 million tons in 2017. Asia commanded up to 31.4% of the global market in 2018, with a value of 16.61 million tons [1]. The World Wide Fund for Nature (WWF) reported that Malaysia was one of the top plastic consumers in Asia, with 16.8 kg of plastic consumption per person reported annually. In 2020, the consumption of plastic in Malaysia was 543.5 kilo tons [2]. Polyolefin plastics dominate 35 to 45 percent of the synthetic polymer produced in total [3]. The heavy usage of plastics produces a hefty amount of non-degradable wastes, which induces harmful effects on the ecosystem. The environmental pollution incurred due to the use of these traditional polymers has introduced the development of biodegradable polymers.

Biodegradable polymers such as polylactic acid (PLA) [4], polycaprolactone (PCL) [5], poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [6], polybutylene succinate (PBS) [7], and polybutylene adipate-co-terephthalate (PBAT) [8] were used by researchers to obtain cost-effective biocomposites with superior properties. PBS is considered as one of the promising alternatives because of its virtues in strength, toughness, excellent biodegradability, and good processing parameters [9]. Nevertheless, PBS shows insufficient impact strength and gas barrier issues for certain applications. This problem can be addressed by the physical blending of PBS with highly flexible PBAT [9]. PBAT is a 100% biodegradable

**Citation:** Yap, S.Y.; Sreekantan, S.; Hassan, M.; Sudesh, K.; Ong, M.T. Characterization and Biodegradability of Rice Husk-Filled Polymer Composites. *Polymers* **2021**, *13*, 104. https://doi.org/10.3390/ polym13010104

Received: 2 December 2020 Accepted: 24 December 2020 Published: 29 December 2020

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polymer exhibiting good thermal and mechanical properties [10]. The tensile strength is comparable with low-density polyethylene [11]. The studies conducted by Muthuraj et al. [12] showed good compatibility was achievable in the PBS/PBAT (40/60 wt%) blends, which was further proved by even dispersion of the PBS phase in the PBAT phase. However, poor cost performance existed when a comparison was done with a polymer like polyethylene (PE) and polypropylene (PP), thus restricting its wide-scale applications for practical usage. Hence in this work, high loading of waste material as fillers was used to reduce the cost of the production of biodegradable plastic. Blending with biodegradable materials was considered as an effective strategy to overcome the costs incurred during the material processing.

Natural fibers possess economic advantages in comparison with synthetic fibers. Besides this, they are lightweight, renewable, and biodegradable. One of the most commonly used biodegradable materials is starch. It is regarded as the optimal additive due to its cheap cost and availability. It also derives many advantages, such as renewability and complete biodegradability from nature [13,14]. However, starch is a hydrophilic polymer, while PBS and PBAT are hydrophobic polymers. Hydrophilic starch and hydrophobic polyesters are thermodynamically incompatible, having improper adhesion characteristics [15]. These properties can be enhanced by adding a plasticizer in starch to generate thermoplastic starch (TPS) [16]. The plasticizers such as water, glycerol, and polyvinyl alcohol are used to generate thermoplastic starch [15]. Considering this fact, the high tensile properties and thermal characteristics of the blend can be attained if TPS is well dispersed in PBS/PBAT matrix, hence possessing good phase interaction between TPS and PBS/PBAT [17].

Previous researchers have reported the potential of using waste material as filler in the composite fabrication [18–20]. Hence, rice husk waste was utilized in this study for the fabrication of a biodegradable polymer. It was reported that total rice production worldwide in 2018/2019 was valued around 495.9 metric tons [21]. The rice production approximately generates 123.87 metric tons of rice husk; out of that, some proportion is used for cattle feeding, while the remaining is dumped as waste in landfill and later burned openly. Burning of rice husk contributes to high CO<sup>2</sup> emission and environmental pollution, which further causes health issues [22]. Hence, utilization of this renewable agriculture waste material to form biodegradable polymers would resolve environmental issues, and hence contribute towards Sustainable Development Goal 12, which ensures sustainable consumption and production patterns. It could also be a way to resolve cost and waste disposal issues.

Several reported works on the utilization of rice husks in polymers are summarized in Table 1. As seen, the rice husk has been dominantly utilized in polyolefin rather than in bioplastics polymers. For bioplastic polymers, filler loading up to 5 and 30% have been utilized with PLA and PBAT, respectively. The PBAT/RH with a 70:30 weight ratio exhibited tensile strength, Young's modulus, and elongation at break as 14.5 MPa, 54 MPa, and 820%, respectively [19]. It possessed lower tensile strength and Young's modulus than PP/RH 70:30 wt%. Thus, more rice husk loading is required to achieve better tensile strength and Young's modulus. Hydrophilic rice husk and hydrophobic PBAT and PBS are thermodynamically incompatible, having poor miscibility. Therefore, in this work, rice husk waste (up to 40%) was used after modifying it with glycerol to form thermoplastic rice husk (TPRH). The outcome of 40% loading of TPRH with PBS/PBAT is investigated in this study.


**Table 1.** The summary of biodegradable polymer/rice husk composites with their respective composition, plasticizer, and mechanical properties.

AT: Alkaline treated, PP: polypropylene, PBAT: poly butylenes adipate-Co-terephthalate, PLA: polylactic acid.

The bare PBAT was soft, and therefore, blending with PBS to mold a product with a stiffness of 78.13 MPa [29] was an essential step. However, blending filler and polymers at nearly equal proportions resulted in an immiscibility issue. Hence, compatibilizer is required to reduce the interfacial tension and to form a co-continuous structure [30]. Therefore, in this work, compatibilizer such as MA and DCP was used to avoid phase separation and to promote an excellent interfacial adhesion for improved mechanical properties of PBS/PBAT with TPRH. The results of TPRH samples were compared with PBS/PBAT filled with TPS.
