Chapter 1 Introductory Chapter : Natural Fiber Plastic Composites-A Brief Review

The natural composites are classified mainly in three biomaterial categories: green composites, hybrid biocomposites, and textile biocomposites. In recent times, biological materials have become essential materials for the construction and automotive industry. Natural fibers and particles have been already used in various types of materials such as plastics, concrete, and textile products as strengthening part of the fiber/matrix combination. These composite materials have very good mechanical thermal and acoustical properties; therefore, they have been used in various engineering applications. The wood-plastic composites (WPCs) have been used in many application areas as automotives, constructions, marine, electronic and aerospace areas instead of fiber glass composites and steel materials. As a wood derivative, hemp fibers have been used in generating thermoplastic matrix composites. These composites can find its application area in the following five sectors: the first area is to modify some parts of the internal and external automobile structures and electric cars. The second area is to obtain strong cementation in building construction. Another area is the production of durable clothes for army suppliers. The fourth one is to produce small electric hand tools and the last one is to build supercapacitors in carbon nanosheets which are as strong as graphene. This review covers a general overview of the preparation phase (chemical procedures), test techniques (experiments) and results, and conclusions summaries (the gaining) of current studies on hemp fiber plastic composites. The data obtained by literature search of the 63 publications have been shown in Table A1 [1–63].


Introduction
The natural composites are classified mainly in three biomaterial categories: green composites, hybrid biocomposites, and textile biocomposites. In recent times, biological materials have become essential materials for the construction and automotive industry. Natural fibers and particles have been already used in various types of materials such as plastics, concrete, and textile products as strengthening part of the fiber/matrix combination. These composite materials have very good mechanical thermal and acoustical properties; therefore, they have been used in various engineering applications. The wood-plastic composites (WPCs) have been used in many application areas as automotives, constructions, marine, electronic and aerospace areas instead of fiber glass composites and steel materials. As a wood derivative, hemp fibers have been used in generating thermoplastic matrix composites. These composites can find its application area in the following five sectors: the first area is to modify some parts of the internal and external automobile structures and electric cars. The second area is to obtain strong cementation in building construction. Another area is the production of durable clothes for army suppliers. The fourth one is to produce small electric hand tools and the last one is to build supercapacitors in carbon nanosheets which are as strong as graphene. This review covers a general overview of the preparation phase (chemical procedures), test techniques (experiments) and results, and conclusions summaries (the gaining) of current studies on hemp fiber plastic composites. The data obtained by literature search of the 63 publications have been shown in

Scientific researches about natural hemp composites
The quality of the produced compounds depends on the elastic constants of the natural fibers used in the composition and the shape and size of the fibers as well as the properties of the matrix material. Factors influencing the strength of the composite material can be listed in more detail as follows: Morphic structure, chemical composition, density, thickness of wood plastic composites (WPCs) as well as the type and amount of bonding agent and fiber percentage used in composite material. According to the literature, the usage rate of the herb composites (WPCs) in the industrial areas is stated as follows: in the field of aviation 1%, in the area of consumer products 8%, in various fields 8%, in the field of electronics 10%, in the maritime field 12%, in the construction industry 26%, in the automotive sector 31%, and miscellaneous 4% [46].

General properties of natural fibers in plastic composites
Interface conditions have been influenced at the nanoscale level depending on the thermal sensitivity and the water content of the green materials in the process of preparing composite material from natural fiber embedded in a polymeric matrix. The natural fiber and the polymeric matrix interface features and the cell wall structure of the natural fibers influence: (i) the mechanical properties, (ii) the durability, and (iii) the recyclability of the industrially produced green composites. The literature survey results both on mechanical and chemical properties of the hemp fiber plastic composites and their usage in industrial areas have been listed in Table A1. Table A1 gives brief information on: (i) the aim of this research, (ii) the experimental methods used in compound production, and (iii) conclusions according to the obtained results. In Table A1; 63 research articles have been listed according to the fiber/matrix material characteristics. Of these 63 studies, 59 were related to fiber composites, while only 4 of them were related to particle composites. In the literature survey, the investigations were carried out in two groups:  Table 1 and Figure 1. The basic tests have been performed by tensile loading (23%) to obtain the Young's modulus and tensile strength of the composite material in addition to the microscopic visualizations (optical light microscope, SEM) (17%) to observe deformation patterns of the loaded specimens in micro and nanoscales.

Conclusions
The results obtained by the literature search were summarized below and very important keypoints about fiber and matrix compositions, the physical features of the hemp fibers and hemp fiber plastic composites were emphasized. The main results were as follows.
(1) It was found that tensile strength, Young's modulus, and impact strength of the hemp short fiber reinforced composites were increased in proportion to the increase in fiber content. (2) Flexural strength of the hemp fiber reinforced polylactide and unsaturated polyester composites were found to be decreased with increased fiber content. Additionally, flexural modulus was found to be increased in proportion to the increased fiber content. (3) The impact energy required to damage hemp composites was higher than in conventional laminates. (4) The deformation characterization of hemp/epoxy composites has been developed in three stages. (5) It was shown that natural fibers when compared to flexible fibers showed scattered and lower mechanical properties. (6) Minor variations in terms of the mechanical properties of the woody hemp core (WHC) cell walls were investigated at the nanoscale level. (7) The fiber/polymer interface was modified by using two functionalized chemical procedures simultaneously, and in this way, better adhesion capacity was obtained at the interface (between hemp fibers and thermoplastic matrix). (8) Variations obtained using nanodrawing tests showed slight variation in the cell wall properties, while the polymer composition was more variable. (9) Hemp fiber composites showed a greater resistance to crack formation and growth than glass fiber composites, although they had lower fatigue strength. (10) Testing natural fiber composites under low impact loading provides important information on the failure mechanisms of hemp (Cannabis sativa L.) fiber epoxy composites.
Characteristics of the deformed material such as matrix cracking, delamination, fiber breakage, and fiber pullout phenomenon were examined microscopically [15,16,25,44,58]. In literature, there were rare experimental studies on the characteristic determination of hemp cell wall structures. In a study on this subject, minor variations at the nanoscale level related to the mechanical properties of the cell wall have been identified [19]. The comparison between the hemp fiberpolypropylene matrix and hemp fiber-fabric reinforced polyester resin in terms of material properties were summarized in Refs. [30,46,47,60,62]. The graphical results on mentioned values were given in Figure 2.
A. Appendix The optimal production of hemp fibers (ACh) SEM, mass spectrometry, and temperature programmed chemical reaction High efficiency in pesticides removal on hemp fibers Kord [32] Mechanical properties of (PP)/hemp fiber Tensile and impact testing, and X-ray diffraction (XRD) testing The PP/hemp fiber composites Kavianiboroujeni et al. [33] The effects of different design parameters The study about fully biodegradable ("green") composite materials Tensile testing, impact testing, Fourier transform, infrared spectroscopy, X-ray diffraction, differential scanning calorimeter, and scanning electron microscopy