A Review on Potentiality of Nano Filler/Natural Fiber Filled Polymer Hybrid Composites

Abstract

The increasing demand for greener and biodegradable materials leading to the satisfaction of society requires a compelling towards the advancement of nano-materials science. The polymeric matrix materials with suitable and proper filler, better filler/matrix interaction together with advanced and new methods or approaches are able to develop polymeric composites which shows great prospective applications in constructions and buildings, automotive, aerospace and packaging industries. The biodegradability of the natural fibers is considered as the most important and interesting aspects of their utilization in polymeric materials. Nanocomposite shows considerable applications in different fields because of larger surface area, and greater aspect ratio, with fascinating properties. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors, such as aerospace, automotive, electronics, and biotechnology industries. Hybrid bio-based composites that exploit the synergy between natural fibers in a nano-reinforced bio-based polymer can lead to improved properties along with maintaining environmental appeal. This review article intended to present information about diverse classes of natural fibers, nanofiller, cellulosic fiber based composite, nanocomposite, and natural fiber/nanofiller-based hybrid composite with specific concern to their applications. It will also provide summary of the emerging new aspects of nanotechnology for development of hybrid composites for the sustainable and greener environment.

1. Introduction

Huge biomass, forestry and agriculture-based residues existing in nature are extensively utilized as a potential resource of materials for renewable energy in different sectors of industry. Many plants, crops and pods from agricultural sources are being regarded as an important source of viable natural fillers raw materials for polymer composite industries. Research, development, and progress of these bio-based materials might straightly support eco-system, leading to socio-economic development for cultivation, farming, and remote or rural areas in many of the developing and under developed countries as well. From a long time, natural or lignocellulosic fiberes have been extensively being used in the development and preparation of composites. Owing to increase in the concept of ecological safety and utilization of renewable materials towards greener society, the application of natural fibers in the industries as bio-filler/reinforcement materials in composites are considerably improved [1]. Natural fibers have attracted the interest of researchers, material scientists, and industries, owing to their specific advantages as compared to conventional or synthetic fibers from the past [2]. The attractive and possible advantages, such as reduced tool wear, low cost, and low density per unit volume and acceptable specific strength, along with their sustainable renewable and degradable features are some of the important properties of the natural fibers, which make them suitable to use as filler in polymer composites. Large and wide varieties of natural fibers that are being applied as fillers or reinforcement are well recognized. Synthetic fibers, such as carbon fibers and glass fibers, create severe ecological, and health hazard problems for the workers employed in manufacturing of their corresponding composites as compared to composites derived from natural fibers [2].

Natural fiber shows comparatively poor fiber/matrix interactions, water resistance, and relatively lower durability. The weaker interfacial or adhesion bonds between highly hydrophilic natural fibers and hydrophobic, non-polar organophilic polymer matrix, leads to considerable decrease in the properties of the composites and, thus, significantly obstructs their industrial utilization and production. However, several approaches and schemes have been established to supplement this deficiency in compatibility, including the introduction of coupling agents and/or various surface modification techniques [3]. The surface of the natural fibers can be modified and this can be achieved by physical, mechanical, and/or chemical means. For any composite, the circumstances for substantial reinforcement and virtuous properties are the homogeneous distribution of the reinforcing component, orientation, good adhesion, and relatively high aspect ratio.

Nano-particles are presently considered as a high-potential filler materials for the improvement of mechanical and physical properties of polymer composites [4]. As the nano scale fillers are usually free of defects, hence, their applications in the field of polymer composite area setup, new trends of prospect to overwhelm the restrictions of traditional/conventional micrometer scale. High matrix-filler interfacial area results because of uniform and homogeneous dispersion of nanoparticles are responsible for changing relaxation behavior, as well in ensuing the mechanical, molecular mobility, and thermal properties [5,6]. Generally, nano sized filler present in the minor zone whereas only few of the micro particles participate in the plastic zone deformation. This provides a way for the nanofillers to improve fracture and mechanical properties of the matrix having brittle property. Nanofillers, which possess greater aspect ratio (ratio of largest to smallest dimension) are of considerable interest, and, thus, show better reinforcement for the nanocomposites production [5]. Nanofillers are generally incorporated on a weight basis for the nanocomposite development [7]. The composite properties are greatly influenced by the specific surface area of nano fillers, which shows uninterrupted influence.

Nano fillers could be belongs to organic and inorganic in nature. The particles like silica (SiO₂), titanium dioxide (TiO₂), calcium carbonate (CaCO₃), or polyhedral oligomeric silsesquioxane (POSS), etc., are inorganic filler. However, the filler, such as coir nanofiller, carbon black and cellulosic nanofiller, and many others are derived organically and naturally represent organic nanfillers.

A perfect substitutes to traditional or conventional construction and buildings materials are the fiber reinforced polymer (FRP) composites due to a number of factors, including: higher strength and stiffness with reference to specific gravity; better resistance to corrosion, fire, acids, and natural hazardous environments; no conductivity and non-toxicity; longer service life and lower life-cycle costs; and higher fatigue strength and impact energy absorption capacity [8]. When natural FRPs are used to prepare construction or building modules, then, under developed or developing countries, with rural regions, will tend to forces the cultivation of required manufacturing crops and this would be empowering to address their own housing, poverty and financial issues without any outsider support [9]. From past few years virtually, everything became “nano”, even materials, which have been around for more than a hundred years, like carbon black, which have been extensively used as the reinforcement or fillers in rubbers. The general idea of nanocomposites is based on the concept of creating a very large interface between the nanosized-building blocks and the polymer matrix. Very often, the homogeneous distribution of the nanosized particles is problematic [7]. Nanocomposites, a high-performance material, exhibit unusual properties, combinations, and unique design possibilities [10]. Aggregation phenomena is a major subject in composites having spherical nanoparticles [7]. Traditional particulate-filled micro-composites are somewhat different from nanocomposites as their configuration/arrangement is often less complicated and the interactions results are more well defined. Natural fiber-reinforced composites and nanocomposites are more environmentally friendly, hence, more frequently applied to military applications, building and construction industries (partition boards, ceiling paneling), transportation (automobiles, railway coaches, aerospace packaging, consumer products), etc. [11].

Hybrid composite developed by various researchers, by combining natural fibers/natural fiber and natural fibers/synthetic fibers with epoxy, polyester, phenolic, poly vinyl ester, poly urethane resins, etc., are well established [2,12]. The environmental awareness attracted researchers to develop new composites with addition of more than one reinforcement from natural resources, such as natural fiber/natural fiber or natural fiber/nanofiller from organic sources as an alternative to synthetic fibers [13]. Hybridization involving the combination of nanofiller and natural fiber in the matrix results reduction of water absorption properties and increased in mechanical properties [14]. Several research works depicts all these facts. The mechanical and thermal properties of rice husk flour/high density polyethylene composites get improved by addition of small amount of nanoclay [15]. Mechanical and tribological performance of the date palm fiber/epoxy composites get enhanced by addition of graphite filler but high content of the graphite deteriorates the mechanical properties [16]. Natural fiber/nanofiller-based hybrid composites can be utilized in building and construction materials, transportation (automobiles, railway coaches, aerospace packaging, consumer products, etc., and also could be possible to produce acoustic insulator and extremely thermally stable materials.

2. Lignocellulosic Fibers

2.1. Source, and Classification of Lignocellulosic Fibers

Lignocellulosic fibers are natural fibers. Natural fibers are the most copious and renewable bio-based materials source in nature [17]. Some of the important aspects of biofibers, such as their species and origin, are listed in Table 1. Natural fibers are primarily based on their origins, either coming from plants, animals, or minerals fibers. All plant fibers are composed of cellulose, while animal fibers consist of proteins (hair, silk, and wool) [18]. The natural fibers are classified according to their origin or botanical type [19]. Complete classification of the various natural fibers is shown in Figure 1 [2]. Lignocellulosic fibers have been being used as reinforcing or filling materials for the past 3000 years, in association with polymeric materials. Biofibers are used for composites because of their low cost, ease of separation, lower density, higher toughness, enhanced energy recovery, reduced dermal, respiratory irritation, and significant biodegradability [20]. The stiffness and strength are provided by the natural fibers to the composites, also they are easily recyclable, and moreover, bio fibers will not be fractured when processing over sharp curvatures, unlike brittle fibers, such as glass. In terms of strength per weight of the material biofibers also compete perfectly when compared with conventional or traditional fibers, such as mica and glass, that are generally used for composites [21]. Natural fibers carbon dioxide neutrality is particularly attractive. Natural-fiber-based packaging materials possess various benefits over synthetic packaging materials, such as stiffness vs. recyclability and weight ratio [22].

Table 1. List of important biofibers. Reprinted with permission from Elsevier, 2008 [18].

Fiber Source	Species	Origin
Abaca	Musa textilis	Leaf
Bagasse	–	Grass
Bamboo	(>1250 species)	Grass
Banana	Musa indica	Leaf
Broom root	Muhlenbergiamacroura	Root
Cantala	Agave cantala	Leaf
Caroa	Neoglaziovia variegate	Leaf
China jute	Abutilon theophrasti	Stem
Coir	Cocosnucifera	Fruit
Cotton	Gossypium sp.	Seed
Curaua	Ananaserectifolius	Leaf
Date Palm	Phoenix Dactylifera	Leaf
Flax	Linumusitatissimum	Stem
Hemp	Cannabis sativa	Stem
Kapok	Ceibapentranda	Fruit
Kenaf	Hibiscus cannabinus	Stem
Oil Palm	Elaeisguineensis	Fruit
Piassava	Attaleafunifera	Leaf
Pineapple	Ananuscomosus	Leaf
Sisal	Agave sisilana	Leaf
Straw (Cereal)	–	Stalk
Sun hemp	Crorolariajuncea	Stem
Cadillo/urena	Urenalobata	Stem
Wood	(>10,000 species)	Stem

Table 1. List of important biofibers. Reprinted with permission from Elsevier, 2008 [18].

Fiber Source	Species	Origin
Abaca	Musa textilis	Leaf
Bagasse	–	Grass
Bamboo	(>1250 species)	Grass
Banana	Musa indica	Leaf
Broom root	Muhlenbergiamacroura	Root
Cantala	Agave cantala	Leaf
Caroa	Neoglaziovia variegate	Leaf
China jute	Abutilon theophrasti	Stem
Coir	Cocosnucifera	Fruit
Cotton	Gossypium sp.	Seed
Curaua	Ananaserectifolius	Leaf
Date Palm	Phoenix Dactylifera	Leaf
Flax	Linumusitatissimum	Stem
Hemp	Cannabis sativa	Stem
Kapok	Ceibapentranda	Fruit
Kenaf	Hibiscus cannabinus	Stem
Oil Palm	Elaeisguineensis	Fruit
Piassava	Attaleafunifera	Leaf
Pineapple	Ananuscomosus	Leaf
Sisal	Agave sisilana	Leaf
Straw (Cereal)	–	Stalk
Sun hemp	Crorolariajuncea	Stem
Cadillo/urena	Urenalobata	Stem
Wood	(>10,000 species)	Stem

Figure 1. Classification of natural and synthetic fibers. Reprinted with permission from Elsevier, 2011 [2].

Figure 1. Classification of natural and synthetic fibers. Reprinted with permission from Elsevier, 2011 [2].

2.2. Chemical Compositions and Properties of Natural Fibers

The chemical composition of natural fibers greatly depends on the type and nature of fiber. The overall properties of each fiber are influenced by the properties of each constituent [23]. The variation in chemical composition from plant to plant, and within different parts of the same plant are quite obvious [24]. The main and prime constituent of all cell walls are sugar based polymers (cellulose, hemicellulose) chiefly on dry basis [24]. The cell structure and chemical composition of natural fibers are quite complicated. Chemical composition of some important natural fibers are illustrated in Table 2 [25]. Natural fibers themselves regarded as the naturally occurring composites comprising mainly of helically wound cellulose micro fibrils, embedded in amorphous lignin matrix. Cellulose (a-cellulose), lignin, pectins, hemicellulose, and waxes are the major components of natural fibers. The component hemicellulose present in the natural fibers is regarded to be a compatibilizer between lignin and cellulose [26].

Table 2. Chemical compositions of some important natural fibers. Reprinted with permission from Elsevier, 2008 [25].

Natural Fibers	α-Cellulose	Lignin	Pentosans
Sisal	47–62	8–9	21–24
Pine apple	69.5	4.4	17.8
Wheat grass	21.4	–	–
Bamboo	26–43	21–30	15–26
Rice	28–48	12–16	23–28
Ryegrass	33–50	16–19	27–30
Sugar grass	32–48	19–24	27–32
Wheat straw	29–51	16–21	26–32
Kenaf bast	44–57	15–19	22–23
Jute	45–63	21–26	18–21
Oat straw	31–48	16–19	27–38
Esparto grass	33–48	17–19	27–32
Hemp bast	57–77	9–13	14–17
Barley	31–45	14–15	24–29
Wood grass	40–45	26–34	7–14
Marsh grass	25–27	–	–
Sorghum	27–31	12–16	23–28
Corn stalk	32–34	5–34	20–41
Coconut coir	28–29	16–45	7–23
Rush grass	26–29	–	–
Peanut hulls	35–36	32–33	19–20
Flax seed	43–47	21–23	24–26
Okra	23–35	–	–
Ramie bast	87–91	0.5–0.7	5–8
Sabai	–	22–23	24–25
Cotton linter seed hull	90–95	0.7–1.6	1–3
Reed fiber	44–46	22–24	20
Silver maple (Hardwood)	42.0	21.0	19.0
Groundnut husk	–	30.6	11.1
Seed flax tow	34.0	23.0	25.0

Table 2. Chemical compositions of some important natural fibers. Reprinted with permission from Elsevier, 2008 [25].

Natural Fibers	α-Cellulose	Lignin	Pentosans
Sisal	47–62	8–9	21–24
Pine apple	69.5	4.4	17.8
Wheat grass	21.4	–	–
Bamboo	26–43	21–30	15–26
Rice	28–48	12–16	23–28
Ryegrass	33–50	16–19	27–30
Sugar grass	32–48	19–24	27–32
Wheat straw	29–51	16–21	26–32
Kenaf bast	44–57	15–19	22–23
Jute	45–63	21–26	18–21
Oat straw	31–48	16–19	27–38
Esparto grass	33–48	17–19	27–32
Hemp bast	57–77	9–13	14–17
Barley	31–45	14–15	24–29
Wood grass	40–45	26–34	7–14
Marsh grass	25–27	–	–
Sorghum	27–31	12–16	23–28
Corn stalk	32–34	5–34	20–41
Coconut coir	28–29	16–45	7–23
Rush grass	26–29	–	–
Peanut hulls	35–36	32–33	19–20
Flax seed	43–47	21–23	24–26
Okra	23–35	–	–
Ramie bast	87–91	0.5–0.7	5–8
Sabai	–	22–23	24–25
Cotton linter seed hull	90–95	0.7–1.6	1–3
Reed fiber	44–46	22–24	20
Silver maple (Hardwood)	42.0	21.0	19.0
Groundnut husk	–	30.6	11.1
Seed flax tow	34.0	23.0	25.0

Hemicellulose is responsible for thermal degradation, moisture absorption, and biodegradation of the fiber as it shows least resistance but lignin is thermally stable and is greatly accountable for the UV degradation [23]. Phenylpropane derivative constitutes the lignin and it is an amorphous natural polymeric material that regulates the transference of fluid in the plant [17]. Fiber width and length is an important parameter of information used for comparing diverse variety of natural fibers. Thus, the fiber strength can also be an important factor in selecting a specific natural fiber for specific applications. The morphology and the anatomy of the aquatic plant fibers are quite different from those of terrestrial plant fibers [19]. The necessary strength and stiffness to the fibers are provided by the hydrogen bonds and other linkages. Fiber variability, crystallinity, strength, dimensions, defects, and structure are the important factors governing the properties of different natural fibers [23]. Table 3 shows the average diameter of frequently used natural fibers. All fibers are tinny in diameter or width except for bagasse, oil palm, and coconut [27]. The major drawback of natural fibers is their tendency of high water absorption and this make them inequitable with nonpolar polymer matrices [28]. Most of the resins are usually hydrophobic in nature and absorb little humidity. Table 3 shows the equilibrium moisture content of different natural fibers at 65% relative humidity (RH) and 21 °C [25]. The void content and non-crystalline parts of the fibers are determined by the humidity of the fibers. The strength or mechanical properties of the natural fibers are affected by the hydrophilic nature of the fibers. The characteristic properties of natural fibers are affected by many factors, such as climatic conditions, maturity, harvesting and collection time, retting degree, decortications, disintegration (mechanical, steam explosion treatment), fiber modification, textile, and technical processes [29]. Being natural in origin, the lignocellulosic fiber has several different stages of production, moreover, within each stage there are many factors that determined the quality of fiber, as shown in Table 4 [9].

Mechanical and physical properties of natural fibers provides important information that are required to know prior of its use to attain maximum level. Several efforts have been made to substitute the natural fiber composites in place of glass mostly both in non-structural and structural applications. Thus far, a large number of automotive parts and their components, which were formerly prepared with glass fiber composites, are recently being manufactured using environmentally friendly natural fibers and composites [30,31]. The mechanical properties of natural fibers are, relatively, much lower than those of glass fibers. Table 5 shows the mechanical properties of natural fibers and glass fibers. Researchers in many cases reported the comparison of mechanical and physical properties of natural fibers with E-glass [32]. Elongation to break is higher in the case of natural fibers than glass or carbon fibers, which promotes the composite performance.

Table 3. Average diameter and equivalent moisture content (EMC) of natural fibers [2,17,25].

Fibers	Diameters (μm)	EMC (%)
Oil palm Fronds	19.7	–
Oil palm EFB	19.1–25.0	–
Banana	12–30	15
Sisal	7–200	11
Pineapple Leaf	20–80	13
Ramie	18–80	9
Coconut husks	100–450	10
Bagasse	10–34	–
Jute	5–200	12
Hemp	10–500	9
Flax	5–600	7
Aloe	–	12
Wood	–	12
Abaca	–	9.5
Cotton	10–45	8
Kapok	–	10

Table 3. Average diameter and equivalent moisture content (EMC) of natural fibers [2,17,25].

Fibers	Diameters (μm)	EMC (%)
Oil palm Fronds	19.7	–
Oil palm EFB	19.1–25.0	–
Banana	12–30	15
Sisal	7–200	11
Pineapple Leaf	20–80	13
Ramie	18–80	9
Coconut husks	100–450	10
Bagasse	10–34	–
Jute	5–200	12
Hemp	10–500	9
Flax	5–600	7
Aloe	–	12
Wood	–	12
Abaca	–	9.5
Cotton	10–45	8
Kapok	–	10

Table 4. Factors effecting fiber quality at various stages of natural fiber production. Reprinted with permission from Elsevier, 2012 [9].

Stage	Factors Effecting Fiber Quality
Plant Growth	Species of plant
	Crop cultivation
	Crop location
	Fiber location in plant
	Local climate
Harvesting stage	Fiber ripeness, which effects:
	Cell wall thickness
	Coarseness of fibers
	Adherence between fibers and surrounding Structure
Fiber extraction stage	Decortication process
	Type of retting method
Supply stage	Transportation conditions
	Storage conditions
	Age of fiber

Table 4. Factors effecting fiber quality at various stages of natural fiber production. Reprinted with permission from Elsevier, 2012 [9].

Stage	Factors Effecting Fiber Quality
Plant Growth	Species of plant
	Crop cultivation
	Crop location
	Fiber location in plant
	Local climate
Harvesting stage	Fiber ripeness, which effects:
	Cell wall thickness
	Coarseness of fibers
	Adherence between fibers and surrounding Structure
Fiber extraction stage	Decortication process
	Type of retting method
Supply stage	Transportation conditions
	Storage conditions
	Age of fiber

Table 5. Properties of natural fibers in relation to those of E-glass. Reprinted with permission from 010 Publishers, 2005 [32].

Properties	Fiber
	E-glass	Hemp	Jute	Ramie	Coir	Sisal	Flax	Cotton
Density (g/cm3)	2.55	1.48	1.46	1.5	1.25	1.33	1.4	1.51
Tensile strength (MPa)	2,400	550–900	400–800	500	220	600–700	800–1,500	400
E-Modulus (GPa)	73	70	10–30	44	6	38	60–80	12
Specific (E/d)	29	47	7–21	29	5	29	26–46	8
Elongation at failure (%)	3	1.6	1.8	2	15–25	2–3	1.2–1.6	3–10
Moisture absorption (%)	–	8	12	12–17	10	11	7	8–25

Table 5. Properties of natural fibers in relation to those of E-glass. Reprinted with permission from 010 Publishers, 2005 [32].

Properties	Fiber
	E-glass	Hemp	Jute	Ramie	Coir	Sisal	Flax	Cotton
Density (g/cm3)	2.55	1.48	1.46	1.5	1.25	1.33	1.4	1.51
Tensile strength (MPa)	2,400	550–900	400–800	500	220	600–700	800–1,500	400
E-Modulus (GPa)	73	70	10–30	44	6	38	60–80	12
Specific (E/d)	29	47	7–21	29	5	29	26–46	8
Elongation at failure (%)	3	1.6	1.8	2	15–25	2–3	1.2–1.6	3–10
Moisture absorption (%)	–	8	12	12–17	10	11	7	8–25

3. Polymer/Matrix

The matrix may be metallic, ceramic or polymeric in origin. Polymers are of three types if classed with regard to degree of reticulation; thermoplastic polymers, thermosetting polymers, and rubbers. Thermoplastic polymers have little or no reticulation, are often solvent soluble, and melt easily. Rubbers are slightly cross linked, which prevents the chain from sliding when stretched and the last thermosetting polymers or thermosets are heavily cross linked, insoluble in solvents and also infusible [33]. In the composite industries most applicable and important thermosetting materials are phenolic resins (including phenol-formaldehyde); epoxy resins and unsaturated polyesters (UP); amino resins (e.g., melamine-formaldehyde and urea-formaldehyde). Unlike thermoplastic resins, cured thermosets will not melt and flow but soften when heated and, once formed, cannot be reshaped [19].

4. Nanofillers

In order to define nanometer scale items (10⁻⁹ m) the term nano is used. A nanometer is, therefore, equivalent to the billionth of a meter, or 80,000 times thinner than a human hair. The nanometer range covers sizes smaller than the wavelength range of visible light but bigger than several atoms [34]. Nanomaterials are categorized into three groups; nanotubes, nanoparticles, and nanolayers, depending on the number of measurements of the dispersed particles that are in the nanometer range [35,36]. Nano-particles regarded as the important potential filler materials for the enhancement of physical and mechanical properties of polymer matrix [4]. The unique nanometric size, capable of producing huge and vast specific surface areas, even more than 1000 m²/g, along with their other distinctive properties currently shows exhaustive research activities in the fields of engineering and natural sciences [37]. Nanofillers possess tendency to improve or adjust the altered or variable properties of the materials into which they are incorporated, such as fire-retardant properties, optical or electrical properties, mechanical and thermal properties, significantly, sometimes in synergy with conventional or traditional fillers. Nanofillers are incorporated in polymer matrices at rates from 1% to 10% (in mass) [38]. The diverse nanofillers that are used in nano composites are nanoclays, nano-oxides, carbon nanotubes, and organic nanofillers. However, according to the nanofillers the nanocomposites can be distributed, as classified in ISO/TS 27687 [39] and presented in Table 6. In the composite the incorporation of nanoparticles leads to changes in the following way, as shown in Figure 2.

Table 6. Classification of nanofillers with reference [39].

One-dimensional nanofiller	Dimension measurement (thickness)	Two-dimensional nanofiller	Dimension measurement (diameter)	Three-dimensional nanofiller	Dimension measurement (All dimension)
Plates < 100 nm		Nano tubes < 100 nm		Nanometric < 100 nm
Laminas < 100 nm		Nano fibers < 100nm		Silica beads
Shells < 100nm		–		–

Table 6. Classification of nanofillers with reference [39].

One-dimensional nanofiller	Dimension measurement (thickness)	Two-dimensional nanofiller	Dimension measurement (diameter)	Three-dimensional nanofiller	Dimension measurement (All dimension)
Plates < 100 nm		Nano tubes < 100 nm		Nanometric < 100 nm
Laminas < 100 nm		Nano fibers < 100nm		Silica beads
Shells < 100nm		–		–

Figure 2. Changes due to incorporation of nanoparticles in composites.

Figure 2. Changes due to incorporation of nanoparticles in composites.

5. Biocomposites

Composite materials are the most advanced and adaptable engineering materials. The perfect combination of a plastic polymeric matrix and reinforcing natural fibers produces composites, possessing the finest properties of each component. The term “natural fiber reinforced composite” usually refers to natural fibers in any sort of polymeric matrix (thermoplastic or thermoset; natural or synthetic). However, these composites found to be eco-friendly to a greater degree. These materials offer many of the same and equivalent advantages in terms of strength and toughness as conventional composites together with their own unique advantages including lower density [40], better matrix–fiber compatibility, and recyclability [41,42,43].

Performance of the natural fiber polymer composites influenced by several factors, such as fibers microfibrillar angle, defects, structure, physical properties, chemical composition, cell dimensions, mechanical properties and the interaction of a fiber with the polymer matrix. Thus, to understand the properties of natural fiber-reinforced composite materials, it is essential to recognize the mechanical, physical, and chemical composition/properties of natural fibers [3]. The most important matters in the development of natural fiber reinforced composites are (i) surface adhesion characteristics of the fibers, (ii) thermal stability of the fibers, and (iii) dispersion of the fibers in the case of thermoplastic composites [23]. The polarity characteristic of the natural fiber produces incompatibility difficulties with many polymers. Hydrophilic or polar characters of natural fibers produce composites with weak interface. Several chemical modifications or pretreatment of surface are being made to improve and enhance the adhesion or interfacial bonding between polymers and natural fibers [44,45]. Pretreatments of the natural fiber used to clean and unpolluted the fiber surface, to modify chemically the surface, decreases the rate of moisture absorption tendency, and to increase the external unevenness. The incorporation of natural fibers as filler or reinforcement produces significant changes in thermal stability of polymeric matrix. The manufacturing and the processing of these composites involves the collaboration of fibers and matrix at sufficiently high temperatures, hence, can lead to degradation of the bio-material, which results in unfavorable effects on the final properties [17]. Almost all production techniques can be used to manufacture the natural fiber-containing composites. Table 7 displays the list of manufacturing techniques used for composite preparation involving either thermoset or thermoplastic matrix.

Table 7. Techniques for the composite preparation with the polymer.

Processing of natural fiber-thermosetting composites	Processing of natural fiber-thermoplastic composites
Hand lay-up and spraying	Extrusion
Compression	Injection molding
Transfer	Thermoforming
Resin transfer molding	Compression method
Injection molding	Cold pressing
Compression injection	Heating
Pressure bag molding	Direct long fiber reinforced thermoplastics
Pultrusion	Filament Winding
Vacuum assisted resin transfer molding	Foam molding
Casting Injection molding	Rotational molding, calendaring
Polyurethane foam molding	Co-extrusion

Table 7. Techniques for the composite preparation with the polymer.

Processing of natural fiber-thermosetting composites	Processing of natural fiber-thermoplastic composites
Hand lay-up and spraying	Extrusion
Compression	Injection molding
Transfer	Thermoforming
Resin transfer molding	Compression method
Injection molding	Cold pressing
Compression injection	Heating
Pressure bag molding	Direct long fiber reinforced thermoplastics
Pultrusion	Filament Winding
Vacuum assisted resin transfer molding	Foam molding
Casting Injection molding	Rotational molding, calendaring
Polyurethane foam molding	Co-extrusion

A significant and effective method in formulating biocomposites of desired and superior properties include proficient chemical modification of fiber, efficient processing of fabrication techniques, and matrix modification by blending and functionalizing [18] (Figure 3).

Figure 3. Tricorner approach in designing of high performance biocomposites. Reprinted with permission from Elsevier, 2008 [18].

Figure 3. Tricorner approach in designing of high performance biocomposites. Reprinted with permission from Elsevier, 2008 [18].

6. Natural Filler Reinforced Polymer Nanocomposites

Nanocomposites considered to belong the groups called nanomaterials, where a nano-object (particle) is distributed into a matrix [38]. Generally, nanocomposite is a multiphase dense material in which at least one of its phase has either one, two or three measurements lower than 100 nm [46]. The nanocomposites exhibit unique characteristics and comparably better properties than conventional or traditional composites such as glass fiber reinforced composites [47]. Nowadays, a great deal of research and study are in progress towards various fillers to form a huge variety of nanocomposites. The nanofiller in nanocomposite material are the main components and can be constituted of inorganic/inorganic, inorganic/organic, or organic/organic sources. Polymer nanocomposites are polymers (thermoplastics, thermosets, or elastomers) that have been reinforced with small quantities (less than 5% by weight) of nano-sized particles having high aspect ratios (L/h > 300) [48]. The reinforcement of polymeric matrix materials (thermoplastics or thermosets) with nano-sized, such as nano-sized particles, carbon nano-tubes or intercalated layers to forms nanocomposites are considered as an attractive and active area of research. Polymer/layered nanocomposites, in general, can be classified into three different types, namely (i) Intercalated nanocomposites, (ii) flocculated nanocomposites, and (iii) exfoliated nanocomposites [49,50]. Considerably larger interfacial matrix material surface (interphase) are presented by nanocomposites, depicting properties quite dissimilar from the bulk polymer caused by high specific surface area of the nanofiller [5,6]. Thus, when the dimensions of polymer fiber materials are shrunken or reduced from micrometers to sub-microns or nanometers, numerous unique characteristics, such as flexibility in surface functionalities, greater surface area to volume ratio (this ratio for a nanofiber can be as large as 103 times of that of a micro-fiber), and superior mechanical performance (such as stiffness and tensile strength) tend to appear, compared with any other form of the material [4]. Recognizing the morphological and mechanical benefits of nanofillers, many researchers produced nanocomposites by using different polymeric matrix and reinforcing by a wide variety of clays, which exhibited improved and better properties. Reported work on nanocomposites are listed in Table 8 to make a review of technology of nano and micro-scale particle reinforcement regarding several polymeric fiber-reinforced systems including polyamide (PA), polyimide (PI), polyarylacetylene (PAA), poly(ether ether ketone) (PEEK), epoxy resin (ER), polyester, polyurethane (PU), and poly p-phenylenebenzobisoxazole(PBO). Researchers show the various processing techniques in nanocomposites and the characterization with their corresponding techniques [4,48,49,50,51,52]. Over conventional composites, nanocomposites display improvements in mechanical, electrical, thermal, and resistant (barrier) properties. Furthermore, the nano particles cause significant reduction in flammability and also preserve the transparency or clearness of the polymer matrix [24,53].

Table 8. List of reported work on nanocomposite type from different polymeric matrix with natural fiber/nanofiller.

Polymer Matrix	Natural Fiber/Nano Filler	References
Polyamide 11 (PA-11)	Nano clay	[54]
e-Caprolactam	Organo clay	[55]
Polyamide (PA-6)	Montmorillonite (MMT)	[56]
Polyamide (PA-6)	Organically modified MMT (OMMT)	[57,58,59]
Polyamide (PA-6)	Fe2O3 particles	[60]
Polyurethanes (PU)	Carbon nano tube	[61]
Polyaniline and sulfonated urethane	Carbon nano tube	[62]
Polypropylene (PP)	Nano clays	[63]
Polypropylene (PP)	Nano carbon fiber	[64]
Polypropylene (PP)	Nanoclays.	[65]
Poly(ethylene) (PE)	Carbon nanotube	[66]
Ultra-high MW poly(ethylene) (UHMWPE)	Carbon nanotube	[67]
Polystyrene (PS)	Carbon nanotube	[68]
Polystyrene (PS)	Carbon nanotube	[69]
Poly(ether ether ketone) (PEEK)	Nanoparticles of SiO2	[70]
Poly(ether ether ketone) (PEEK)	Carbon nanofibers CNFs	[71]
Poly(ether ether ketone) (PEEK)	organo-alkoxysilanes	[72]
Poly(ether ether ketone) (PEEK)	SiC nanoparticles.	[73,74]
Phenylethynyl-terminated imide (PI)	multi-walled carbon nanotube	[75]
Polyarylacetylene (PAA)	Carbon fiber	[76]
Polyarylacetylene (PAA)	carbon fiber/LiAlH4	[77]
Polyarylacetylene (PAA)	carbon fiber	[78]
Poly p-phenylenebenzobisoxazole (PBO)	SWNT (Single-walled nanotubes)	[79]
Epoxy resin (ER)	Coir-fiber nano filler	[80]

Table 8. List of reported work on nanocomposite type from different polymeric matrix with natural fiber/nanofiller.

Polymer Matrix	Natural Fiber/Nano Filler	References
Polyamide 11 (PA-11)	Nano clay	[54]
e-Caprolactam	Organo clay	[55]
Polyamide (PA-6)	Montmorillonite (MMT)	[56]
Polyamide (PA-6)	Organically modified MMT (OMMT)	[57,58,59]
Polyamide (PA-6)	Fe2O3 particles	[60]
Polyurethanes (PU)	Carbon nano tube	[61]
Polyaniline and sulfonated urethane	Carbon nano tube	[62]
Polypropylene (PP)	Nano clays	[63]
Polypropylene (PP)	Nano carbon fiber	[64]
Polypropylene (PP)	Nanoclays.	[65]
Poly(ethylene) (PE)	Carbon nanotube	[66]
Ultra-high MW poly(ethylene) (UHMWPE)	Carbon nanotube	[67]
Polystyrene (PS)	Carbon nanotube	[68]
Polystyrene (PS)	Carbon nanotube	[69]
Poly(ether ether ketone) (PEEK)	Nanoparticles of SiO2	[70]
Poly(ether ether ketone) (PEEK)	Carbon nanofibers CNFs	[71]
Poly(ether ether ketone) (PEEK)	organo-alkoxysilanes	[72]
Poly(ether ether ketone) (PEEK)	SiC nanoparticles.	[73,74]
Phenylethynyl-terminated imide (PI)	multi-walled carbon nanotube	[75]
Polyarylacetylene (PAA)	Carbon fiber	[76]
Polyarylacetylene (PAA)	carbon fiber/LiAlH4	[77]
Polyarylacetylene (PAA)	carbon fiber	[78]
Poly p-phenylenebenzobisoxazole (PBO)	SWNT (Single-walled nanotubes)	[79]
Epoxy resin (ER)	Coir-fiber nano filler	[80]

Bio-nanocomposites are regarded as an emerging and attractive groups of nanostructured materials that brings the expansion of the conception of biocomposites [81]. The two following ways used for defining the term bio-nanocomposites are (i) the nanocomposites materials developed from renewable and sustainable nanoparticles (e.g., cellulose whiskers and MFC) and petroleum-derived polymers like PP, PE, and epoxies, (ii) nanocomposites derived from biopolymers (e.g., PLA and PHA) and synthetic or inorganic nanofilers (e.g., carbon nanotubes and nanoclay), also come under bio-nanocomposites [82,83].

7. Hybrid Composites

The word “hybrid” is of Greek-Latin origin. Hybrid composites are the systems where one type of reinforcing or filler material is incorporated or added in a mixture of dissimilar or different matrices (blends) [84], or two or more reinforcing or filling materials are present in a single matrix [85,86] or, also, both approaches are combined. The integration of variety of fibers in a single matrix results in the development of hybrid biocomposites. Reinforcements have been incorporated either by: (i) intermingling of two types of short fibers thoroughly before incorporating them into the polymer in a mixer, or added alternately into the polymer with or without modification [87,88,89]. (ii) sandwiching of fibers or their mats or fabrics [20,89] or (iii) using non-woven or woven fabrics of both types of reinforcements, as in the case of glass fiber-LC fiber composite systems [20,89,90].

Hybrid biocomposites are usually designed and processed by the combination of a synthetic fiber and natural fiber (biofiber) in a matrix or with combination of two natural fiber/biofiber in a matrix [18]. The behavior of hybrid composites is a weighed sum of the individual components. The hybrid composite properties exclusively governed by the length of individual fibers, orientation, fiber to matrix bonding, content, extent of intermingling of fibers, and arrangement of both of the fibers. Rule of mixtures can be used to determine the properties of the hybrid system consisting of two components. Moreover, successful use of hybrid composites is determined by the mechanical, chemical, and physical stability of the fiber/matrix system. Several researchers developed hybrid composite by combining natural fibers with poly-urethane resins, phenolic, polyester, epoxy, poly vinyl ester, etc., as polymeric matrices. Table 9 shows reported and exclusive work on cellulosic/synthetic and cellulosic/cellulosic fibers reinforced hybrid composites.

Table 9. Reported work on hybrid composites.

Natural Fiber	Polymer Matrix	References
Palmyra/glass	Rooflite resin	[91,92]
Bamboo/glass	Vinyl ester	[93]
Jute/glass	Polyester (isothalic)	[94,95]
Coir/glass	Phenolic resin	[96]
Banana/kenaf	Polyester	[97]
Natural fiber/glass	Epoxy vinyl ester	[98]
Jute/biomass	Bisphenol-C-formaldehyde	[99]
Sisal/kapok	Unsaturated Polyester	[100]
Oil palm EFB/jute	Epoxy resin	[101]
Kenaf/glass	Epoxy resin	[102]
Cellulose/glass	Epoxy resin	[103]
Jute/cotton	Novolac phenolic	[104]
Jute/glass	Polypropylene (PP)	[105]
Flax/glass	Polypropylene (PP)	[106]
Kenaf/glass	Natural rubber	[107]
Cotton/waste silk	Polycarbonate (PC)	[108]
Wood flour/glass	Poly vinyl chloride (PVC)	[109]

Table 9. Reported work on hybrid composites.

Natural Fiber	Polymer Matrix	References
Palmyra/glass	Rooflite resin	[91,92]
Bamboo/glass	Vinyl ester	[93]
Jute/glass	Polyester (isothalic)	[94,95]
Coir/glass	Phenolic resin	[96]
Banana/kenaf	Polyester	[97]
Natural fiber/glass	Epoxy vinyl ester	[98]
Jute/biomass	Bisphenol-C-formaldehyde	[99]
Sisal/kapok	Unsaturated Polyester	[100]
Oil palm EFB/jute	Epoxy resin	[101]
Kenaf/glass	Epoxy resin	[102]
Cellulose/glass	Epoxy resin	[103]
Jute/cotton	Novolac phenolic	[104]
Jute/glass	Polypropylene (PP)	[105]
Flax/glass	Polypropylene (PP)	[106]
Kenaf/glass	Natural rubber	[107]
Cotton/waste silk	Polycarbonate (PC)	[108]
Wood flour/glass	Poly vinyl chloride (PVC)	[109]

Nano Filler/Natural Fiber Hybrid Composites

Natural fiber-reinforced polymer composites have established a huge attraction and concern as innovative material in several applications. Although, natural fiber–plastic composites have been commercialized, but their potentiality for use in many industries has been limited. Therefore, most studies in this area focus on improving the physical-mechanical properties and impact resistance of the composites. One way to improve the mechanical properties of natural-fiber-based composites is to produce hybrid composites by combining several types of reinforcement/filler such as nanoclay with polymers. The nanoclay materials are commonly selected for their particular dimension and high aspect ratio in most of the research work [110] (Table 10).

Table 10. Reported work on fiber/nanofiller hybrid composites.

Matrix	Filler	Reference
Polyamide (PA-6)	carbon fiber/nanoclay	[111]
Polyamide (PA-6)	carbon fiber/glass fiber/nanoclay	[111]
Polyamide (PA-6)	chopped glass fibers/hectorite-type clays (nano size)	[112]
Polyamide (PA-6)	glass fiber/layered silicate	[113]
Polyamide 12 (PA-12)	Carbon nanotubes and nanofibers	[114]
Polyester	Nanoclay/glass fiber	[115]
Polyvinyl ester	Organoclay/glass fiber mats	[116]
High density polyethylene(HDPE)	nanoclay/rice husk	[15]
Ethylene–propylene copolymer	Nano clay/cellulose	[79]
Polyarylacetylene (PAA)	carbon fiber/LiAlH4	[77]

Table 10. Reported work on fiber/nanofiller hybrid composites.

Matrix	Filler	Reference
Polyamide (PA-6)	carbon fiber/nanoclay	[111]
Polyamide (PA-6)	carbon fiber/glass fiber/nanoclay	[111]
Polyamide (PA-6)	chopped glass fibers/hectorite-type clays (nano size)	[112]
Polyamide (PA-6)	glass fiber/layered silicate	[113]
Polyamide 12 (PA-12)	Carbon nanotubes and nanofibers	[114]
Polyester	Nanoclay/glass fiber	[115]
Polyvinyl ester	Organoclay/glass fiber mats	[116]
High density polyethylene(HDPE)	nanoclay/rice husk	[15]
Ethylene–propylene copolymer	Nano clay/cellulose	[79]
Polyarylacetylene (PAA)	carbon fiber/LiAlH4	[77]

The hybridization of natural fiber with carbon [117], and mica [118] have produced encouraging results. The decrease in the water absorption and thickness swelling is observed by the hybridization of nanoclay with reed flour besides this it also upgraded the tensile properties of the whole system. Researchers also show the effect of coupling agent on mechanical and physical properties of reed flour/PP/nanoclay hybrid composite [119]. In other work, integration of nanoclay to the HDPE/rice husk system greatly enhanced their mechanical properties [15]. The study involving the hybrid effect of nanoparticles on the flexibility and stiffness on the hemp fiber/PP based hybrid composites was also found to be increased [120].

A substantial reduction in water absorption tendency and thickness enlargement by swelling get increased by nanoparticles loading also been reported in some cases. Researchers also studied the effect of compatibilization and incorporation of organoclay on mechanical and thermal properties of wood flour/HDPE composites systems, which emerges as an attention-grabbing work [121]. Researcher also found that hybridization of 5 wt% of nanoclay with micro-crystalline cellulose in the micro-crystalline cellulose reinforced ethylene–propylene (EP) copolymer shows remarkable changes in the Young’s modulus from 1.04 to 1.24 GPa [79]. In another study, effect of nanoclay dispersion on mechanical and physical properties of the wood–plastic–nanoclay hybrid composites, are also been reported [122]. The pronounced effect of compatibilization and organoclay incorporation on mechanical and thermal performance of the pine cone fiber hybrid polymer composite are also been studied by researchers [123]. Reduction in the stiffness/modulus of the systems, contrary to fiber, also notably observed. In another study a good dispersion and interfacial interaction of clay and pine cone fiber within the polymer are perfectly revealed by scanning electron microscopy in the clay/pine cone fiber hybrid polymer nanocomposite. The incorporation of nanoclay to the formed HDPE/wheat straw flour polymer composite also improved the properties of polymer composite including an increase in modulus and strength, heat resistance, low gas permeability, fire resistance and physical properties, even at the small scale of nanoclay [110]. The effects of azodicarbonamide (AZD) as the exothermic chemical foaming agent (CFA) and nanoclay on the physical, morphological and mechanical properties of nanocomposites produced from wheat straw flour (WSF) and HPDE were also studied [124]. The tendency of water absorption and thickness swelling properties of HPDE/WSF composites increased by adding AZD and a reduction by adding the NC into the matrix was witnessed. Impact resistance also decreased by adding NC and the CFA. The mechanical properties of HPDE/WSF composite get improved by adding 2 phr of NC, all these results are supported by SEM micrographs.

8. Applications and Challenges

Nanotechnologies are estimated to impact and influence at least $3 trillion in the worldwide economy by 2020. Furthermore, it has been expected that industries based on nanotechnology, considering nano particles, globally, might require at least six million workers to sustain them by the end of the decade [125]. Nanocomposites proposed perfections over conventional composites in mechanical, electrical, thermal, and resistance (barrier) properties [53]. A large variability of diversity in industrial applications for polymer nanocomposites includes automotive industry (interior-exterior panels, gas tanks and bumpers), construction industry (structural panels and building sections), aerospace (flame retardant panels and high performance components), electrical and electronics (printed circuit boards and variety of electrical components), in food packaging (containers and wrapping films) and in the cosmetics industry [49].

Construction is one of the most traditional and oldest industries, having been around for almost as long as mankind. Over time, significant technological changes have occurred, mainly in the way in which buildings are erected [126]. This is the most interesting application area, which relates to enhancing the functional properties of concrete, steel, wood, and glass, as the primary construction materials. Specifically, the embodiment of nanoparticles in the micro-matrices, or through coatings on the surface areas of these materials can improve their strength, stress tolerance, and durability [127]. Nanotechnology addresses environmental concerns in construction through many routes. The enabling nature of nanotechnology implies that it can provide traditional construction materials with completely new and eco-efficient functionalities [126].

Examples of nanotechnology-enabled applications in construction [127] are listed below:

• Long-lasting scratch-resistant floors using nano-structured materials;
• Super strong structural components using CNTs;
• Healthier indoor climates by nano-enabled filter technology;
• Antimicrobial steel surfaces using nano-scaled coatings;
• Improved industrial building maintenance using nano-enabled sensors;
• Lower energy consuming buildings using electrochromic “smart” windows;
• Self-cleaning low maintenance windows using new nano-scaled coatings.

Moreover in medical field polymer nano fibers extensively applied for the cure of burns or wounds of a humanoid skin, and also to designed haemostatic procedures and devices with certain special and exclusive features. Through, the aid of electric field, biodegradable polymers finest fibers, are able to directly sprayed/spun on to the damaged and injured area of skin to form a fibrous mat dressing (Figure 4), thus, assisting in healing wounds by boosting the development of normal skin growth and also help in eradication to reduce the formation of scar tissue.

Figure 4. Showing the use of nanofibers for wound dressing. Reprinted with permission from Elsevier, 2003 [128].

Figure 4. Showing the use of nanofibers for wound dressing. Reprinted with permission from Elsevier, 2003 [128].

Figure 5 shows some of the future potential of nanocomposite or polymer nanofibers. The promising prospective of development and utilization of nanofibers or nanocomposite are thought to be interesting or appealing investments, and are able to grab more attention from governments, academia, and the growing industry around the globe [128]. The pulp and paper industry is a materials industry which provides an excellent platform for developing nano-material fiber composites for use in higher value printing, barrier packaging, and intelligent communications media [34]. TiO₂, and the sucrose as the nano-additive can be added, revealing a much higher opacity for the synthetic sample [129].

Nanocomposite materials are also being used in cosmetic dentistry, and are the primary dental filling materials as they can be conveniently applied and yield a high esthetic quality. Nanocomposite materials are used to fabricate core and post systems and dental brackets, and are applied in various restorations (inlays, on-lays, veneers, and crowns). Novel liquid crystalline epoxy nanocomposites, which exhibit reduced polymerization shrinkage and effectively bond to tooth structures, can be applied in esthetic dentistry, including core and post systems, direct and indirect restorations, and dental brackets [130]. Although in manufacturing equipment the applications of nanotechnology are not truly recognizable to many customers, but they offer remarkable profits in the fabrication process. In the cosmetics industry the prime requirements, including easy-to-clean surfaces, low abrasion details, non-sticky materials for machinery, and value-added energy efficiency products, are a few illustrations of benefits gained by consuming nanotechnology in various manufacturing techniques [131]. The origination of cosmetic goods and products using nano carriers basically aims at improving the effectiveness of the product, decreasing the amount of constituents used with accompanying decrease in toxicity or irritation. This also aims to obtain better stability, increasing the infusion of the active material thus enables producing a more graceful product [131].

Figure 5. Potential applications of polymer nanofibers. Reprinted with permission from Elsevier, 2003 [128].

Figure 5. Potential applications of polymer nanofibers. Reprinted with permission from Elsevier, 2003 [128].

The application of nanotechnology in the automotive industry is diverse or manifold. They show diversity from energy conversion, light-weight construction, interior cooling, power train, surveillance control up to recycle potential, pollution sensing and reduction, driving dynamics and wear reduction etc. [132]. The concept of “nano in cars” conforms its assistances for quiet cars, CO₂-free engines, up to a mood-depending choice of colour, self-healing body and windscreens, safe driving and a self-forming car body. Figure 6 and Figure 7 show the potential areas of nanocomposite towards automotive applications. The automotive components in automotive industry includes nanoparticles such as dots, tubes, whisker, pores, layers, and fibers, either distributed or dispersed within a polymeric matrix material referred as “nanocomposites”.

Figure 6. Potential areas where nanotechnology can contribute to satisfy society demands in the automotive industry. Reprinted with permission from Elsevier, 2003 [132].

Figure 6. Potential areas where nanotechnology can contribute to satisfy society demands in the automotive industry. Reprinted with permission from Elsevier, 2003 [132].

Figure 7. Customer-specific requirements to future automobiles where nanotechnology has an impact. Reprinted with permission from Elsevier, 2003 [132].

Figure 7. Customer-specific requirements to future automobiles where nanotechnology has an impact. Reprinted with permission from Elsevier, 2003 [132].

Nanotechnology might revolutionize the food processing and food safety industries by creating more effective antimicrobial agents, providing high-barrier packaging materials. A stronger series of sensors that are able to detect even trace contaminants, impurities, microbes or gasses in packaged foods, can be made by considering polymer/clay nanocomposites, as high barrier packaging materials. In another study silver nanoparticles as potential antimicrobial agents, nanomaterial, and nanosensor-based assays for revealing food-relevant analytes (small organic molecules, gasses and food-borne pathogens) also been reported [133]. The well-established and recognized prospective applications of nanotechnology in virtually every segment of the food industry (Figure 8) from agriculture (e.g., pesticide, fertilizer or vaccine delivery; animal and plant pathogen detection; and targeted genetic engineering) to food processing (e.g., encapsulation of flavor or odor enhancers; food textural or quality improvement; new gelation, or viscosifying agents) to food packaging (e.g., pathogen, gas or abuse sensors; anti-counterfeiting devices, UV-protection, and stronger, more impermeable polymer films) to nutrient supplements (e.g., nutraceuticals with higher stability and bioavailability) are well known [133].

Figure 8. Some potential applications of nanotechnology in all areas of food science, from agriculture to food processing to security to packaging to nutrition and neutraceuticals. Reprinted with permission from Elsevier, 2011 [133].

Figure 8. Some potential applications of nanotechnology in all areas of food science, from agriculture to food processing to security to packaging to nutrition and neutraceuticals. Reprinted with permission from Elsevier, 2011 [133].

9. Conclusions

Polymer nanocomposites signify as the most encouraging and promising family of materials science from the last decades and consequently gained much attention due to their unique characteristics of enhancing the mechanical and barrier properties of construction, cosmetics, medical sciences, food packaging and many other composite-based industries. Nanocomposites obtained from polymeric matrix (thermoplastics or thermosets) reinforced with nano-sized fillers, such as nano-size particles, carbon nano-tubes or intercalated layers designated as a dynamic and active area of research. The synergistic reactions involving the matrix polymer and nanoparticle filler at the nanoscale level responsible for the enhanced properties; developed by the assimilation of slight quantity of nanofiller in polymer matrix. Hybrid filler filled composites tremendously delivers pronounced prospective in order to overwhelm many of the limitations such as the weak interfacial attractions between matrix polymer and the fiber, and characteristics moisture absorption properties of many natural fibers, thus hampering their applications in several industries. The nanoparticle reinforcement enhances the performance and properties, and, hence, shows great value for fiber-reinforced composite based industry. A great possibility has been shown by the incorporations of nanoparticle as the reinforcement of composites.

Future research on natural fiber/nano filler based hybrid composites not only driven by its automotive and construction applications but it required to explore further research on hybrid for aircraft components, rural areas and biomedical applications. However, more study and research remains to be achieved in order to recognized the possible ways of nano-reinforcement leading to major changes in material properties and their subsequent potential future applications in several composite based industries. There is need for more analysis of the different properties of natural fiber/nano filler based hybrid composites by modern equipment in the most of the areas covered in this review. The crucial success of outcomes of this research for future developments and wider acceptance of natural fiber/nano filler based hybrid composites in different applications depends on reliable scientific reports which confirm both the benefits and protective nature of nano materials. Apart from the benefits and safety aspects of nanotechnology-based products, public opinion, and attitudes towards nanotechnology are extremely important for the development of this emerging area.