Natural fibers physical treatment methods

The mechanical properties of composites are mainly influenced by the adhesion between matrix and fibers of the composite. As it is known from glass fibers, the adhesion properties could be changed by pretreatments of fibers. So special process, chemical and physical modification methods were developed. Moisture repel-lency, resistance to environmental effects, and, not at least, the mechanical properties are improved by these treatments. Various applications for natural fibers as reinforcement in plastics are encouraged. 

Native cellulose are commonly modified by physical, chemical, enzymic, or genetic means in order to obtain specific functional properties, and to improve some of the inherent properties that limit their utility in certain application. Physical/surface modification of cellulose are performed in order to clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. " Among the various pretreatment techniques, silylation, mercerization, peroxide, benzoylation, graft copolymerization, and bacterial cellulose treatment are the best methods for surface modification of natural fibers. 

Physical methods involve surface fibrillation, electric discharge (corona, cold plasma), etc. Cold plasma is already a very effective method to modify the siuface of natural polymers without changing their bulk properties. This method has been used for increasing adhesion and compatibility between two polymers (lorio et al. 1997 Petash et al. 1995 Tu et al. 1994). Physical treatments change stmctural and surface properties of the fiber and thereby influence the mechanical bonding with the matrix. 

The influence of physical and chemical treatment methods of natural fibers on mechanical properties was analyzed also. Additionally, the mechanical and other physical properties of the composite are generally dependent on the length, content, and dispersion of fibrous filler and processing parameters. 

The strength of the fiber is very important for natural fiber-reinforced composites. Polar natural fibers are inherently incompatible with hydrophobic polymers. Therefore, the surface modification of natural fibers is required to improve its physical and chemical properties. Various chemical treatment methods have been used to modify the surface properties of the natural fibers. Few methods of surface modification are described below ... 

The aforementioned studies show that plasma treatment can be a potential physical surface modification method to improve the mechanical properties of the natural fiber reinforced composites (NFRCs). However, supplementary research in this area is required to implement more energy-efficient and consistent treatment technologies. 

Essentially, physical methods are employed on natural fiber during processing in order to separate natural fiber bundles into individual filaments and also to modify the surface structure of the fibers so as to improve the use of natural fibers in composites. Physical methods can be divided into two categories viz (1) steam explosion and thermomechanical processes and (2) plasma, dielectric barrier techniques, radiation modification, ultrasonic treatment, and corona discharge. In an effort to impart and improve reactivity, these physical treatments have been used to modify thermoplastic polymeric films like polyethylene and polypropylene and thermosets, such as epoxy. 

The source of nanofiber is varied either from animal or plant that contains different amount of chemical composition. Thus, in nanofiber production, different natural fibers need to imdergo several chemical and physical treatments. In addition, every extraction method gave different results in term of quality and yields, hence other techniques of cellulose nanofiber production need to be explored. Furthermore, as found in previous research, combination of chemical, enzymatic, and physical... 

Therefore, a large number of studies on chemical and physical surface treatments of various natural fibers have been devoted not only to increasing the interfacial adhesion between the natural fiber and the polymer matrix but also to enhancing mechanical, thermal, and other properties of biocomposites consisting of different types of natural fibers and polymers [13-20]. Meanwhile, a few excellent papers have reviewed the surface modification of natural fibers for biocomposites [4, 11, 21, 22]. Many research results dealing with surface treatment of natural fibers and characterizing various properties of biocomposites with different modification methods as well as with different natural fibers and polymers have been reported in recent years. 

Physical methods for treating natural fibers before biocomposite processing involve electrical discharges such as cold plasma and corona, electron beam irradiation, ultraviolet (UV) treatment, and ultrasonic treatment,. Such physical approaches are of great interest because, in general, the processes are dry, clean, labor-friendly, environment-friendly, and fast in comparison with most of the chemical methods, which are wet processes. Under appropriate treatment conditions, they can effectively modify structural and surface characteristics of natural fibers, thereby improving the mechanical and thermal properties of biocomposites as well as enhancing the interfacial adhesion between the natural fibers and the polymer matrix. 

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