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Environmental concerns and increasing awareness about sustainability issues are driving the push to develop bio-based composites from renewable and sustainable resources. A clear understanding of fiber-matrix interaction at the interfaces is vital for the design and production of these composite materials because stress transfer between load-bearing fibers occurs at these interfaces, and this can determine their mechanical and thermal behaviors. A detailed study of different attributes such as processing technology parameters, types of biopolymers, and chemical and physical treatments is essential to effect interface modification and facilitate a better understanding of these composites from an application perspective. In this article, we attempt to critically review adhesion in biocomposites with special emphasis on the effects of various surface modifications and the processing technology adopted. The review also addresses various characterization techniques adopted for evaluating adhesion in biocomposites.

Biopolymers and biocomposites draw our attention in various applications due to their versatile properties. But the lack of good interfacial adhesion between the hydrophilic filler particles and hydrophobic polymer matrix make these composites less attractive for various applications. The use of adhesion promoters, additives or chemical modification of the filler surface can help to improve the interfacial adhesion between filler particles and polymer macromolecules and their dispersion in the matrix, and thus improve the overall mechanical performance of the composites. Interfacial adhesion between filler and matrix remains the key issue in terms of overall performance, since it influences the final properties of the composites. This review focuses on biopolymers, biomatrices, their modifications and processing techniques and characterization of interfacial modifications.

Polypropylene (PP)/banana fiber (BF) composites were fabricated from PP fiber and short BF by novel commingling method. The dynamic mechanical analysis (DMA) of the composites was performed with reference to BF loading and fiber surface treatments. By the incorporation of BF into the PP matrix, the storage modulus and loss modulus have been found to increase, whereas damping factor has been found to decrease. Glass transition temperature was found to increase with increase in BF loading. The viscoelastic properties of the composites were also found to depend on fiber surface treatments. The activation energy of the composites for the glass transition has been found to be increased by the increase in BF loading. Surface treatment of the BF further increased the activation energy of the composites, indicating a stronger interface for treated fiber composites. Scanning electron microscopy (SEM) photographs of the BF showed the physical changes induced by the surface treatments. Fourier transform infrared spectroscopy (FTIR) was used to ascertain the existence of the type of interfacial bonds. The use of theoretical equations to predict the storage modulus has also been discussed. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Hybridization of Banana fibers with glass fibers has been found to reduce the water absorption behavior of the composites in an earlier work by us. Banana fibers were hybridized with glass and different layering patterns were followed in the preparation of the composites. The effect of the various layering patterns on the water absorption of the composites was studied. It was found that water diffusion occurs in the composite depending on the layering pattern as well as the temperature. In all the experiments, it has been found that composites with an intimate mixture of glass and banana show the maximum water uptake except for temperature of 90°C. At 90°C the maximum water uptake is found to be for composites where there is one layer of banana and another layer of glass. The water uptake follows the same trend as that in all other temperatures till a time span of 4900 min is reached. The kinetics of diffusion was found to be Fickian in nature. The various thermodynamic parameters like sorption coefficient, diffusion coefficient. Enthalpy change, entropy change, and activation energy of the various composites were calculated. From all the calculations it has been concluded that layering pattern is an important parameter which controls the water absorption of the composites. The layering pattern Cg-b-g was found to have the lowest water uptake. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

The water sorption characteristics of banana fiber–reinforced polyester composites were studied by immersion in distilled water at 28, 50, 70, and 90°C. The effect of hybridization with glass fiber and the chemical modification of the fiber on the water absorption properties of the prepared composites were also evaluated. In the case of hybrid composites, water uptake decreased with increase of glass fiber content. In the case of chemically modified fiber composites, water uptake was found to be dependent on the chemical treatment done on the fiber surface. Weight change profiles of the composites at higher temperature indicated that the diffusion is close to Fickian. The water absorption showed a multistage mechanism in all cases at lower temperatures. Chemical modification was found to affect the water uptake of the composite. Among the treated composites the lowest water uptake was observed for composites treated with silane A1100. Finally, parameters like diffusion, sorption, and permeability coefficients were determined. It was observed that equilibrium water uptake is dependent on the nature of the composite and temperature. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3856–3865, 2004

The surface polarity of organically functionalized cellulose fibers has been characterized with linear solvation energy relationships. The Kamlet–Taft hydrogen-bond-donating ability, the hydrogen-bond-accepting ability, the dipolarity/polarizability, Gutman's acceptor number (an indicator of the electron-accepting ability), and Reichardt's ET (30) parameter have all been used to quantitatively represent the polarity of the chemically treated and untreated fiber surfaces. Three different probe dyes, cis-dicyanobis(1,10-phenanthroline)-iron(II), Michler's ketone, and aminobenzodifuranone, have been used as indicators to measure the polarity with correlation analysis of the energy of the ultraviolet–visible absorption maxima. Solvatochromic methods have been proven to be effective in characterizing chemically modified lignocellulosic fibers. Different silanes, NaOH, and long alkyl groups, used to modify the cellulose fiber surface, have changed the hydrogen-bond-donating ability of the fiber. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2546–2553, 2000