Production and Characterization of Hybrid Polymer Composites Based on Natural Fibers Production and Characterization of Hybrid Polymer Composites Based on Natural Fibers

In this chapter, a review is made on the processing and properties of hybrid composites based on a polymer matrix and a blend of different natural (lignocellulosic) fibers. In particular, the processing methods are described and comparisons are made between the general properties with a focus on physical, mechanical and thermal properties. A discussion is presented on the effect of the polymer and fiber types, as well as reinforcement content. Properties improvement is also discussed using fiber surface treatment or the addition of coupling agents. Finally, auto‐hybrid composites are presented with conditions leading to a positive deviation from the rule of hybrid mixture (RoHM) model.


Introduction
Composites are materials containing at least two constituents, each one with different chemical composition. Their combination provides a new material with better functional properties than each of the components separately [1].
The main component in the composite is the matrix, which can be a metal, ceramic or polymer, while the other part is a reinforcement which can be in particulate, laminate, short fiber or long fiber form [2]. Composite materials are widely used in construction, aerospace, aircraft, medicine, electrical and automotive industries [2][3][4][5]. Here, a focus is made on fiber reinforced composites made from a polymer matrix reinforced with fibers having a natural origin [6].
is initially in a liquid form. So, lower viscosity improves fibers introduction and dispersion via different mixing equipment [18][19][20][21][22]. Fiber orientation as well as fiber content might improve mechanical properties in thermoset composites. Grass, leaf and bast fibers are more effective to increase the matrix mechanical properties, while surface treatment improves interfacial interactions. Table 2 summarizes some work on natural fiber thermoset composites with their manufacturing process, fiber content, fiber treatments and fiber source, as well as the main results obtained from each work.

Composites from Renewable and Sustainable Materials
The most common thermoplastic matrices used for natural fiber composites production are the different grades of polypropylene (PP) and polyethylene (PE), as well as polycarbonate (PC), nylon (PA), polysulfones (PSU), polyethylene terephthalate (PET) and polystyrene (PS). More recently, biopolymers such as polylactic acid (PLA) have gained interest to produce 100% biosourced materials [51][52][53][54][55]. Typical manufacturing processes for these composites are extrusion, injection, calendering, compression molding and thermoforming. Some advantages of using thermoplastic matrices are their recyclability and the production can be continuous [56][57][58][59][60][61]. Depending on the matrix, fiber and additives content, fiber treatment and manufacturing process, the mechanical and thermal properties of these composites can be adjusted as presented in Table 3, with the main results obtained.
The main objective of adding natural fibers in polymer matrices is to increase mechanical properties regardless of polymer and fiber type [21,26,31,40,52,54,55,[61][62][63][64][65][66][67][68]. Since natural fibers have lower density (1.2-1.5 g/cm 3 ) compared to synthetic/inorganic reinforcement such as glass fibers (2.5 g/cm 3 ), lightweight composites can be produced [28,69,70]. Nevertheless, lignocellulosic fibers are hydrophilic and polar which causes some incompatibility with the most common polymer matrices which are hydrophobic and nonpolar. This effect leads to poor mechanical properties due to a lack of interfacial adhesion between the fibers and the matrix. Furthermore, the high amount of hydroxyl groups available on the fiber surface is increasing water absorption, even when inside a composite [65,71,72]. These problems can be resolved by modification of the fibers surface such as mercerization (treatment in sodium hydroxide solution to remove lignins and hemicellulose) with subsequent addition of coupling agents [22,[73][74][75]. There is also the possibility to combine thermomechanical refining with coupling agent addition [71,72]. More recently, fiber treatment with a coupling agent in solution has been proposed [76].   Since natural fibers start to degrade at lower temperature (150-275°C) than most polymer matrices (350-460°C) [60,63,74,83,105], fiber mercerization and coupling agent addition were shown to improve the thermal stability of the fibers and therefore of the final composites [24,29,73,75,85,91,92].

Hybrid composites
To improve on the properties of natural fiber composites and/or overcome some of their limitations such as moisture absorption, thermal stability, brittleness and surface quality, the concept of hybrid composite was developed. The idea is to combine natural fibers with other fibers or particulate reinforcements, which can be of natural or synthetic origin such as glass fibers or rubber particles [15,51,63,[106][107][108][109]. The main purpose of blending different reinforcements is to obtain a material with better properties than using a single reinforcement. Assuming there is no chemical/physical interaction between each type of fibers, the resulting properties of hybrid composites (P H ) should follow the rule of hybrid mixtures (RoHM) given as [106,110,111]: where P C1 and P C2 are the properties of composite C1 and C2, respectively, while V C1 and V C2 are their respective volume fractions such that: Naturally, the model can be generalized for more than two types of reinforcement.
The final properties of hybrid composites depend are function of different factors [53,74,104,120], and Table 4 summarizes some of the most important mechanical and thermal properties of hybrid composites based on thermoset matrices. The effect of fiber and matrix type, as well as fiber surface treatment is reported with their mechanical properties and thermal degradation temperature. Similarly, Table 5 reports the corresponding information for hybrid composites based on thermoplastic matrices. In general, it is observed that combining natural fibers with inorganic reinforcements leads to improved thermal stability and impact strength, as well as higher flexural and tensile moduli. Moreover, Table 6 shows that water uptake decreases by combining two natural fibers from different sources, or using natural fibers with inorganic reinforcements in hybrid composites based on thermoplastics matrices.

Auto-hybrid composites
Composites reinforced with two sizes of the same type of reinforcement are referred to as autohybrid composites. As these composites only have a single type of reinforcement, they are easier to recycle. But most importantly, these materials were shown to exhibit a positive deviation from the RoHM depending on fiber concentration, weight ratio, size and type [64,102,147]. Nevertheless, the auto-hybridization effect seems to be more influenced by the total fiber content than coupling agent addition [64,147]. However, coupling agent addition is always important to improve tensile strength [102]. As total fiber content, fiber type and coupling agent content, all affect the level of deviation from the RoHM, and optimization of these parameters is a new challenging field of research to develop better composite performances. * MAPP was not used in auto-hybrid composites. Table 7. Overview of the different investigations on auto-hybrid composites based on natural fibers.

Conclusion
Natural fibers are now interesting alternative to replace synthetic fibers due their good specific properties (per unit weight). They have been used to develop different composites based on thermoset and thermoplastic matrices. As for any composite, their mechanical, thermal and physical properties are function of the properties of the matrix and the reinforcement, as well as fiber loading, fiber source and manufacturing process. Nevertheless, interfacial conditions are always important to optimize the general properties.
The main disadvantages of using natural fibers are water uptake, low thermal stability, as well as low mechanical properties due to fiber agglomeration and poor interfacial adhesion, especially at high concentration. The problem is usually more important in thermoplastics than thermosets due to their difference in initial resin viscosity. But most of the limitations associated to natural fiber composites can be controlled or overcome by the addition of coupling agents and/or fiber surface modifications.
Finally, another possibility to improve the properties of natural fiber composites is to add a second reinforcement to produce hybrid composites. These materials were shown to have improved mechanical and thermal properties over neat natural fiber composites as they follow the rule of hybrid mixture (RoHM) regardless of the matrix, manufacturing processing and fiber combination. Based on this concept, different class of materials was also developed such as all natural fiber hybrid composites (combination of two different natural fibers) and autohybrid composites (combination of two different sizes of the same fiber). The latter is highly interesting as positive deviations from the RoHM were reported. This is usually the case around 20 wt.% of total fiber content with around 30/70 short/long fiber ratio regardless of coupling agent addition, fiber type and processing method. This opens the door to a new field of investigation as several parameters can be controlled to optimize the final properties of the materials and to design new applications for these multi-functional composites.