**1. Introduction**

Conventional plastics bring so much convenience to our life. We consume plastics in our day-to-day life and in many applications, including food packaging and drinking straws [1]. Conventional plastic or fossil-fuel plastic has many unique characteristics that make it one of the most essential materials in manufacturing a product, such as having a wide range of processing temperatures, high chemical resistance, a high strength to weight ratio, ease of processing and low costs [2]. These properties indicate the versatility

**Citation:** Lai, D.S.; Osman, A.F.; Adnan, S.A.; Ibrahim, I.; Alrashdi, A.A.; Ahmad Salimi, M.N.; Ul-Hamid, A. On the Use of OPEFB-Derived Microcrystalline Cellulose and Nano-Bentonite for Development of Thermoplastic Starch Hybrid Bio-Composites with Improved Performance. *Polymers* **2021**, *13*, 897. https://doi.org/ 10.3390/polym13060897

Academic Editor: José Miguel Ferri

Received: 4 February 2021 Accepted: 12 March 2021 Published: 15 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of plastic materials, and therefore they are being used to substitute other materials such as metal and ceramic in certain applications. Unfortunately, plastic's great properties are sometimes overshadowed by its non-degradable properties which cause disposal problems and pollution on earth [3]. Nowadays, the plastic disposal problem is getting more severe. Thus, researchers are focusing on producing bio-plastics from natural sources such as starch which can be obtained from rice, corn, potato, and pea to solve the problem highlighted above.

Among all the biopolymers, starch has drawn significant attention from the researchers owing to its unique characteristics. Starch can be processed using the same processing techniques as conventional plastic, such as injection, extrusion and thermoforming. Starch is biodegradable and can be plasticized into a more useful form of plastic, which is called thermoplastic starch (TPS) [4]. However, to compete with conventional plastic, TPSbased plastics should possess comparable mechanical properties and barrier properties to conventional plastic. Yet, this is very difficult to achieve since thermoplastic starch inherited hydrophilic properties; thus, it has high sensitivity toward moisture absorption from the surroundings. The mechanical properties of thermoplastic starch decrease exponentially with the increase in humidity [5]. In order to improve the mechanical and barrier properties of thermoplastic starch, inorganic fillers or organic fillers are added to thermoplastic starch to increase its mechanical strength and barrier properties [6].

Palm oil is considered as one of the most important edible oils in the world, next only to soybean. However, palm oil extraction will generate an enormous amount of palm oil biomass wastes that consume up a large landfill area and endangering the environment [6]. Oil palm empty fruit bunch (OPEFB), as one of the major palm oil biomass wastes, contains a high percentage of cellulose of about 40–60%, which is the highest compared to kenaf, corn, and bagasse fiber [7]. Therefore, extraction of microcrystalline cellulose from the OPEFB is one of the effective strategies to add value the waste material.

In the search of new fillers for TPS that capable of enhancing its properties for various applications, the idea of using hybrid fillers (more than one type of filler) arrived due to the demands of consumers for biodegradable plastic packaging with combination of strength and toughness properties. The use of hybrid fillers may allow more comprehensive improvement in the properties of the TPS film by providing synergistic effect through the combination of both filler's properties [8].

In this study, we have investigated the efficiency of using hybrid nano-bentonite (in-organic nano-clay) and OPEFB derived microcrystalline cellulose (organic fiber) fillers in improving the mechanical properties of the TPS bio-composite plastic film, particularly its tensile strength and flexibility (toughness). The microcrystalline cellulose and ultrasonicated nano-bentonite were expected to have good interaction with the TPS due to their hydrophilic properties, thus enhancement in the TPS film's mechanical properties could be gained. However, the TPS bio-composite film's optimal strength can only be achieved when the nano-bentonite and microcrystalline cellulose are well dispersed in the matrix of the biopolymer. Nonetheless, nano-bentonite particles always tend to agglomerate when dispersed in the polymer matrix. To encounter this, ultra-sonication treatment is applied to improve dispersion of the nano-clay filler in a polymer matrix [6].

Tensile toughness property of a material relates to its capability to possess both high tensile strength and high flexibility when stretched. So far, there have been no published papers reporting an increase in both the strength and toughness of the bio-polymeric film with the addition of fillers; in fact, most articles report a decrease in the flexibility of the film along with the increase in its strength [9–14]. This is because high-modulus filler can restrict the molecular motions of the biopolymer matrices, lowering its elongation at break value and thus making it more brittle. Unfortunately, this characteristic is not suitable for flexible film packaging applications because the ability to stretch without breaking (tough) is a key requirement of such a film. Therefore, there is the need to discover new hybrid fillers that are capable of improving the toughness property of the bio-composite film by elevating both tensile strength and elongation at break values of the material. In this work, we have proved that enhancement in both tensile strength and elongation at break can be gained with the use of B/C hybrid fillers. As far as the bio-composite with a dual-type filler system (hybrid) is concerned, this discovery is the first to be reported. Furthermore, the dual hybrid filler system which comprised the OPEFB-derived microcrystalline cellulose and nano-bentonite was first to be introduced in the TPS bio-composite system. Both fillers were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Bio-composite films with several ratios of B/C filler were produced using the casting technique. A tensile test was employed to determine the TPS bio-composite films' mechanical properties containing a different ratio of microcrystalline cellulose and nano-bentonite. The optimal ratio of the OPEFB microcrystalline cellulose/nano-bentonite of the hybrid fillers that was capable of providing the best reinforcing and toughening effects to the TPS bio-composite film was determined.
