Biocomposite interfacial properties

Many research efforts to enhance the interfacial properties of biocomposites and ultimately to improve the mechanical and thermal properties and so forth have been performed more extensively by treating or modifying natural fibers before composite processing than by modifying polymer matrices. Natural fibers can be treated by chemical and physical approaches. 

It was reported that the efiect of surface modification of flax on the ILSS of two-directional flax fabric/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biocomposites [121]. The ILSS values of the surface-treated flax fabric/PHBV biocomposites were greater than that of with untreated fabric. The plasma-treated specimen exhibited ILSS values greater than those for the acetylated one in comparison with the untreated specimens. The explanation for this was that aliphatic moieties with greater hydrophobicity may be formed on the fiber surfaces by ethylene plasma. It was noted that both acetylation and plasma treatment played an important role in improving the interfacial properties of flax/PHBV biocomposites, suggesting that the ethylene plasma treatment was more efiective. 

It has been concluded from a number of studies that cellulose-based natural fibers must be chemically or physically surface treated to improve the interfacial properties of the biocomposites and consequently to increase their mechanical, thermal, and/or other relevant properties. 

Han, S.O., Cho, D., Park, W.H., and Drzal, L.T. (2006) Henequen/ poly(butylene succinate) biocomposites electron beam irradiation effects on henequen fiber and the interfacial properties of biocomposites. Compos. Interfaces, 13, 231-247. 

Lee and Wang [80] investigated the effects of lysine-based diisocyanate (LDI) as a coupling agent on the properties of biocomposites from PLA, poly (butylene succinate) (PBS) and bamboo fiber (BF). They observed that the tensile properties, water resistance, and interfacial adhesion of both PLA/BF and PBS/BF composites were improved by the addition of LDI, but thermal flow [81] was hindered due to cross-linking between polymer matrix and BF. Enzymatic biodegradability of... 

Hyun SL, Donghwan C (2008) Effect of natural fiber surface treatments on the interfacial and mechanical properties of henequen/polypropylene biocomposites. Macromol Res 16 411 17... 

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. 

Consequently, it is worth overviewing extensively current research efforts on the effects of surface treatment of natural fibers on the properties of biocomposites in terms of interfacial, static mechanical, dynamic mechanical, impact, thermal, physical, morphological, fracture behavior, and water absorption. In the present chapter, the description and information focus mostly on the results reported in recent years. 

After NaOH treatment, natural fibers of interest are normally washed or rinsed sufficiently with tap or distilled water in order to remove the excess of NaOH or unreacted NaOH for neutraUzation and then dried before use [8, 16, 19]. A large number of studies [12, 40-50] on the effect of alkaU treatment on the properties of biocomposites consisting of various natural fibers and polymer matrices have been extensively performed by many research groups. Some papers [51-53] explored the combination effect of alkali treatment and silane treatment to improve the mechanical properties through the interfacial modification between natural fibers and polymer matrix. 

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. 

The energy and frequency of UV radiation are smaller than those of X-rays but greater than those of visible light UV energy, which ranges from approximately 200-400 nm, can also be used to modify the natural fiber surfaces [113]. UV treatment may lead to an increase of the polarity of the fiber surfaces. As a result, the wettability of the fibers can be increased and the interfacial and mechanical properties of biocomposites can be increased. 

All polymer composites absorb substantial amounts of moisture or water in humid environment as well as in water. The most important concern in indoor and outdoor applications of natural fiber-based biocomposites with polymer matrices is their sensitivity to water absorption, which can reduce considerably their mechanical, physical, and thermal properties and performances. The water absorption of biocomposites results in the debonding or gap in the natural fiber-polymer matrix interfacial region, leading to poor stress transfer efficiency from the matrix to the fiber and reduced mechanical and dimensional stabilities as well [158]. It has been known that the hemiceUulose component in cellulose-based natural fibers may be mainly responsible for water absorption because it is more susceptible to water molecules than the crystalline cellulose component. Also, poor interfacial adhesion... 

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