Physical modification, natural fiber

The quality of the fiber matrix interface is significant for the application of natural fibers as reinforcement fibers for plastics. Physical and chemical methods can be used to optimize this interface. These modification methods are of different efficiency for the adhesion between matrix and fiber. 

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. 

Identification of fibers is complicated by the presenee of many generie types and modifications of both man-made fibers and common natural fibers. The major generic types of manmade fibers are summarized in Table 12.29. Visual and microseopie examination together with simple manual tests remain the primary methods of fiber identification, though many new sophisticated instrumental methods are available that are based on the chemical and physical property differences among the fibers. These methods are able to distinguish between closely related fibers that differ only in chemical composition or morphology. 

Reinforcing fibers can be modified by physical and chemical methods. Chemical and physical modifications of natural fibers are usually performed to correct the deficiencies (described above) of these materials, especially to impart bonding and adhesion, dimensional stability, and thermoplasticity. Surface modification of natural fibers can be used to optimize properties of the interface. 

Based on the literature discussed in this chapter, it seems that surface modification of natural fibers is absolutely necessary to improve their thermal stability, dispersion in the polymer matrix, and compatibility with the polymer matrix. Physical and chemical methods reported have significantly modified the surface properties of the fibers as well as polymer matrices to improve the dispersion of the fibers and hence various properties of the polymers. Use of silane coupling agents and acetylation... 

Cellulose-based natural fiber can be oxidized using oxidizing agent with acidic catalyst. Oxidation reactions applied to cellulose in fiber for chemical modifications [43]. Oxidation reactions occur on cellulose selectively at particular position. The reaction of sodium metaperiodate with cellulose in wood fiber in the presence of sulfuric acid catalyst at 120°C and 85 KPa pressure yielded the oxidized product. Sodium metaperiodate reacts with hydroxyl groups of cellulose and produce 2,3-dialdehyde cellulose which improved the physical and mechanical properties of polymer composites [44]. 

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 ... 

In order to improve this characteristic, the plant fibers are subjected to several types of surface modifications including physical [1,6], chemical [4,5], or biological [7, 8] treatments. A review of the various types of surface modification undertaken to obtain natural fiber composites with mechanical properties comparable to those of their synthetic counterparts is presented in this chapter. 

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. 

Compared to physical and biological methods, more vigorous research is conducted in the realm of chemical modification of natural fibers. Among the various chanical surface modifications, alkalization, silanization, and grafting with anhydrides are the most commonly adopted methods. 

This chapter summarizes different surface modifications available to physically, chemically, or biologically modified natural fibers. Such a modification is required to improve the interaction between the fiber and matrix, which could attribute to better interlocking and in turn result in enhanced bulk mechanical properties of the resulting composites. 

Mukhopadhyay S, Fangueiro R. Physical modification of natural fibers and thermoplastic films for composites—a review. J Thermoplast Compos Mater 2009 22 (2) 135-162. 

PHYSICAL METHODS OF MODIFICATION OF THE NATURAL FIBERS FOR THE COMPOSITE PREPARATION... 

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. 

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. 

The improvement of the properties of biocomposites can also be attained by polymer matrix modification as well as by natural fiber modification. A right combination of natural fiber and polymer matrix modifications may give rise to a synergetic effect. For instance, the incorporation of organic-inorganic hybrid polymer matrix, nanostructured polymer matrix, blended polymer matrix, or chemically or physically modified polymer matrix into natural fiber reinforcement may be considered. 

S. Mukhopadhyay, and R. Fangueiro, Physical Modification of Natural Fibers and Thermoplastic Films for Composites - A Review. J. of Thermoplastic Composite Mater. 22 (2), 135-62 (2009). 

---