Lignocellulosic fibers adhesion

The work of Valadez-Gonzalez et al. [37] was the only study reported so far to compare two methods, the SFP and the SFF, using a lignocellulosic fiber. In their work, the IFSS of henequen fibers embedded in polyethylene was separately evaluated by means of both SFP and SFF tests. The fibers were previously subjected to different surface chemical treatments aiming at improving the fiber/matrix adhesion. Table 9.3 shows the probable results obtained as a composition of Fig. 11 and Table 4 of Valadez-Gonzalez et al. [37] article. 

Still another micromechanical method for single fiber simulation of a polymer composite interfacial adhesion is the microbond test. This method was developed by Miller et al. [52] and initially applied for S3mthetic fibers. As mentioned by Craven et al. [53], the microbond test is suitable for any fiber that can carry only low loads. This is the particular case of silk, a strong natural fiber but with limited load bearing capacity due to diameters that can be finer than 50 pm. This could be the case of some lignocellulosic fibers such as the ramie with diameters of the order of 10 pm. 

For an extended review on current trends to characterize the fiber/matrix interphase as well as both direct and less direct methods for measuring the interfacial level of adhesion, the reader is referred to the work of Herrera-Franco and Drzal [34]. In this work, apart from the four methods and techniques already discussed (SFP, CLP, SFF, and microbond), all others have not been applied for lignocellulosic fibers. 

As mentioned previously, the main bottleneck in the broad use of these fibers in thermoplastics is the poor compatibility between the fibers and the matrix. The inherent high moisture sorption of lignocellulosic fibers certainly has an effect on their dimensional stability [28]. This may lead to the microcracking of the composites and degradation of mechanical properties [28]. Like other natural fibers, kenaf absorbs moisture due to its hydrophilicity. The key issue related to the development and production of natural fiber-reinforced composites is the interfacial adhesion between the fiber and polymer matrix. Because of their inherent dissimilarities, natural fibers/polymer matrix composites are not compatible and interfadal adhesion in these composites tends to be poor. The weak bonding at the interfaces between natural fibers and polymer matrix is surely a critical cause of the reduction of useful properties and performance of the... 

Moreover, obtaining composites filled with the lignocellulosic component with desired strength properties require consideration of many factors having an effect on the final macroscopic properties. The objectives of this study is to analyze the effect of the following factors such as the adhesion between the components, filler content, fiber length of fibrous filler, filler distribution, and the effect of processing parameters on the mechanical properties of composites. 

Many factors such as adhesion between components, fiber topography, and kinetic parameters of crystallization of semicrystalline matrix have been reported to influence transciystallinity. The transcrystallinity phenomenon in the natural fibers/polypropylene system is affected by the different type of chemical treatment of lignocellulosic materials. Moreover, the ability of natural filler to induce nucleation in polypropylene matrix is also dependent on the kind of chemical modification of surface fibers. Predominant nucleation ability was found for unmodified fibers. However, chemical modification of fiber surface slightly depressed the nucleation of polypropylene matrixes. 

The lignocellulosic materials mostly used as fillers in thermoplastic composites include wood flour, starch, rice husk and a wide variety of vegetable fibers available such as jute, sisal, flax, hemp, coir, banana, pineapple, among others. And whenever vegetable fiber reinforced thermoplastic composites with higher properties are needed, possible solutions include improved adhesion, better fiber orientation, and filler hybridization with synthetic fibers or mineral fillers. The latter solution is an intermediate alternative regarding environmental friendliness, cost, weight and performance compared to an all synthetic composite [12,26]. 

The treatments to improve fiber-matrix adhesion includes chemical modification of the lignocellulosic (anhydrides, epoxies, isocyanates, etc.), grafting of polymers onto the lignocellulosic and use of compatabilizers and coupling agents [38]. 

There are good adhesion characteristics of the cellulose fiber-cellulose ester interface. Cellulose derivatives have been overlooked as potential components in composites with lignocellulosics only a few studies have considered the use of cellulose esters as matrices in biocomposites. Cellulose esters are very well suited for use as matrix binders in natural fiber-based composites. Cellulose esters can be injection molded, extruded, blow and rotationally molded into structrual components, thermoformed from sheet, and extruded in films. Typical processing temperatrues lie between 180°... 

Plant based natural fibers are lignocellulosic, consisting of cellulose micro fibrils in an amorphous matrix of lignin and hemicellulose. To improve the incorporation of natural fibers into polymers and to have higher fiber/matrix interfacial adhesion, natural fibers can be altered by different physical and chemical treatments. 

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