SURFACE MODIFICATION TECHNIQUES USED IN NATURAL FIBERS

To enhance the interfacial adhesion of the fibers and matrix, several surface modification techniques are used which can be broadly classified into chemical and physical treatments. These treatments result in the physical and chemical changes in the surface layer of the fiber without altering the bulk properties.11

CHEMICAL SURFACE MODIFICATION

The main chemical techniques used to surface modify natural fibers are mercerization (alkali treatment), acetylation, acrylation treatment, isocyanate treatment, maleic anhydride treatment.

MERCERIZATION

Mercerization is the alkali treatment of natural fibers.12 Alkali treatment increases the surface roughness of the fibers which enables better mechanical interlocking at the interface and the amount of cellulose exposed on the surface. The following reaction takes place during alkali treatment:

The addition of aqueous sodium hydroxide to the natural fibers results in the ionization of hydroxyl group to alkoxide. Several studies conducted on mercerized natural fibers reveal that alkali treatment results in increased amount of amorphous cellulose at the expense of crystalline cellulose in the network structure.13

ACETYLATION

Acetylation of natural fibers is esterification method known for its plasticizing effect on the fibers.14 It is based on the reaction between hydroxyl groups present in the cell wall of lignocellulosic fibers with acetic or propionic anhydride, which takes place at elevated temperatures.

It has been shown that acetylation enhances the dispersion and dimensional stability of natural fibers in the polymeric matrix.13-15

ACRYLATION TREATMENT

Acrylation treatment, maleated polypropylene/maleic anhydride treatment, and titanate treatment of cellulosic fibers are also reported.16 Through these treatments, the surface energy of the fibers is increased, thereby providing better wettability and high interfacial adhesion.17

ISOCYANATE TREATMENT

Isocyanate consists of a -N=C=0 group which could react with the surface hydroxyl groups to form strong covalent bonds, which enhances the compatibility of the fibers with the polymeric matrix. Thermoplastic-natural fiber composites show superior properties when isocyanates are used. Isocyanate can act as a promoter or as an inhibitor of interaction.18

MALEIC ANHYDRIDE TREATMENT

Maleic anhydride can be used as a grafting monomer for biopolymer matrices. The highly reactive anhydride group reacts with hydroxyl groups to form grafted biopolymers. Maleic anhydride is also used as an adhesion promoter in biocomposite applications.19

PHYSICAL SURFACE MODIFICATION TECHNIQUES

Physical surface modification techniques used are corona, plasma, ultraviolet, heat treatments.

CORONA TREATMENT

Corona treatment uses plasma generated by the application of high-voltage difference between two sharp electrode tips using quartz at a low tempera- hire and atmospheric pressure. Oxygen-containing species are commonly used.20 It introduces surface roughness and increased surface polarity which is due to the introduction of new functional groups (carboxyl and hydroxyl groups) on the fiber surface. But application of corona treatment to a three- dimensional surface is not feasible.21-22

PLASMA TREATMENT

Plasma treatment is similar to corona treatment, but a vacuum chamber is used while the treatment is carried out. Continuous gas supply is enabled which enables the appropriate pressure and desired gas composition for the introduction of specific functionalities on the fiber surface.20 Plasma treatment results in increased fiber surface roughness and hydrophobicity of fibers, thereby increasing the interfacial adhesion.23 Also, interlaminar shear strength and flexural strength have been increased up to 35% and 30%, respectively, using plasma treatment.24

HEAT TREATMENT

Heat treatment is the heating of fibers at temperatures close to its degradation temperatures which brings about changes in physical, chemical, and mechanical properties of the fibers including water content, chemistry, cellulose crystallinity, degree of polymerization, and strength. Some of the chemical changes include chain scission, free radical production, and formation of carbonyl, carboxyl, and peroxide groups.20 The result of heat treatment depends on the time, temperature, and composition of gases present during the heat treatment.

APPLICATIONS OF NATURAL FIBERS

AUTOMOBILE INDUSTRY

Different natural fibers are extensively studied to explore their potential to be used as effective reinforcements for polymeric matrices in order to develop automobile components.25 Polylactic acid (PLA) is a biopolymer which could provide sufficient reinforcement along with natural fibers. These composites have been used for developing different several automobile components by using different types of natural fibers like cotton, hemp, kenaf, and man-made cellulose fibers (Lyocell) as reinforcements. Flax/Hemp fibers dispersed in an epoxy matrix exhibit the highest strength among the natural fiber composites. It also shows resistance to environmental degradation. Many automobile parts can be developed using these composites due to these properties. Cotton fibers have high-impact strength but exhibit low-tensile strengths. So, it is used as reinforcements in interior parts in cars and in safety helmets.

CONDUCTING COMPOSITES

Conducting cellulose composites have increasingly attracted attention not only due to their environment-friendly nature but also because of their flexibility, low weight, and ease of processing overpiezoceramic and magne- tostrictive materials. The basic function of the electroactive paper in such applications is to convert energy between electric and mechanical forms. Conducting polymers and their cellulose-based composites can be used in different devices, such as a polyaniline-based filter paper that acts as a sensor for acids, bases, and as endpoint indicators,26 heating devices,27 and actuators.28

Carboxy methyl cellulose, hydroxypropylcellulose (HPC), and acetoxy- propylcellulose (APC) are being used for the fabrication of electro-optical sensors from cross-linked combination ofHPC/APC29 30 and liquid crystalline solutions with and without a low-molecular-weight nematic liquid ciystal mixture. The use of cellulose in lithium batteries has also been elaborated to replace the polyolefin-based separator in rechargeable lithium ion batteries. A thin composite of cellulosic material (39-85 mm) used as a separator. These cellulosic separators exhibited good initial discharge capacity and capacity retention over 41 charge/discharge cycles.31

BARRIER APPLICATIONS

The impermeable crystalline domains present in the natural fibers generate a tortuous path which leads to slower diffusion rates and low permeability. From 1952, the filters used in cigarettes were made out of cellulose derivatives.32 A greater specific surface area and barrier properties of cellulose fibers enables 44% reduction in tar and a 35% reduction of the nicotine in smoke.25

DRUG-DELIVERY SYSTEMS

The limitation of poor mechanical properties of biodegradable polymers used for drug-delivery systems can be resolved by blending with different fillers, especially layered silicates like hydroxyl apatite. The chemical structures, molecular weight, composition, and synthesis conditions are parameters that influence the final morphology and drug-delivery nature of the polymer and its composites. Cellulose alone is frequently used as adhesive for drug- delivery systems, whereas its composites are gaining considerable interest in this area where the cellulose derivative, ethylcellulose, with a synthetic polymer such as polystyrene may be used for the release of water-soluble drugs, such as phenobarbital.33

Cellulose nitrate and cellulose acetate monolayer membranes containing thermotropic liquid crystals have been developed as thennoresponsive barriers using methimazole and paracetamol as hydrophilic and hydrophobic drugs, respectively.34

BUILDING AND CONSTRUCTION INDUSTRY

Biobased structural composites in construction industry are of significant importance for fencing, decking, siding, doors, windows, bridges, fiber cement, and so on. Advantages associated with the use of natural fibers to reinforce cement, known as fiber cement, include the availability of raw material from renewable sources, high-fiber-tensile strength, high modulus of elasticity, relatively low cost, and well-developed technology for fiber processing.35 Fiber cement presents improved toughness, ductility, flexural capacity, and crack resistance compared to nonfiber-reinforced, cement- based materials.25

Fiber reinforcements have two functions. Their primary function is to transfer the stresses and strengthen the composites. It also increases the toughness of the cement composites by means of energy-absorbing mechanisms. Different methods are available to make fiber-cement composites. In general, all processes utilize fiber concentrations of 8-12%.36~38

 
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