1. Introduction

In the past two decades, environmentally friendly biodegradable plastics have been receiving attention because of the need to diminish the worldwide pollution caused by petroleum-based synthetic plastics [1, 2]. Biodegradability by definition corresponds to the capacity of the material to be completely assimilated by indigenous micro-organisms in the ecosystem. Every year more than 300 million tons of plastics are produced for different applications [3]. In addition to nonrenewability of the synthetic plastics resource, disposing of those types of plastics in landfill will inevitably release toxins in the soil and underground water. Moreover, contaminants can be absorbed in food resources and eventually accumulated in the human body [4, 5].

Bioplastics from natural resources provide reliable and sustainable alternative to synthetic polymer. Starch is the most commonly studied materials as an eco-friendly polymer that is based on renewable plant material, fully biodegradable, and low cost [6, 7]. There are some challenges that oppose the development of starch-based plastics. Those challenges are mainly poor long-term stability, low water resistance, deterioration of mechanical properties due to moisture uptake, and the relatively fast biodegradability [8]. To overcome those challenges, natural lignocellulosic fibers have been widely utilized to improve the properties of the starchbased plastics [9]. Comparable to synthetic fibers, natural fibers are less dense in addition to being fully degradable. Furthermore, the reinforcement of lignocellulosic fibers significantly improves the mechanical properties of starch-based matrix [10]. Sisal, jute, kenaf, coir, wood, pulp, cellulose, bagasse, banana, orange, and flax fibers are lignocellulosic fibers that have been all studied and found to substantiate enhancement for the starch-based matrix's versatile properties [10–14]. The objective of this article is to review and conduct further analysis to understand the effect of high-fiber content (flax, palm, banana, bagasse, bamboo, and hemp) on mechanical and physical properties. Moreover, experimental results reported before will be compared with theoretical models, selecting the most appropriate model.

#### 1.1. Preparation and characterization

For flax, bagasse, banana, and date palm fiber (DPF) preparation took place by preheating the composite, pressing at 5 MPa and 150°C for 30 min [13–16]. Bamboo-reinforced starch composite was prepared by pressing at 20 MPa and 130 C for 5 min [17]. Hemp-reinforced starch composite was prepared by pressing at 10 MPa and 130 C for 5 min [18]. All fibers except bamboo and hemp were subjected to alkaline (NaOH) treatment before composite preparation. In this chapter, we will deal with tensile, moisture, and thermal results, and further details concerning experimental procedures can be found in the cited articles.
