Lignocellulosic composites processing, properties

Lignocellulosic Polymer Composites Processing, Characterization, and Properties... 

N.D. Yihnaz, Agro-residual fibers as potential reinforcement elements for biocomposites, in Lignocellulosic Polymer Composites Processing Characterization, and Properties, Wiley-Scrivener, 2014. 

In this context, the present work aims at examining comparatively (I) chemical compositions, heating values and surface properties of char samples obtained from slow pyrolysis of different lignocellulosic wastes, in relation to their potential as biofuel and/or for fiuther processing to produce activated carbons (2) pyrolysis kinetics of the selected wastes, necessary for the design of the reactors for char production. 

In this chapter we have reviewed some of the most important characteristics of cellulose and cellulose based blends, composites and nanocomposites. The intrinsic properties of cellulose such as its remarkable mechanical properties have promoted its use as a reinforcement material for different composites. It has been showed that cellulose is a material with a defined hierarchy that tends to form fibrillar elements such as elementary fibrils, micro fibrils, and macro fibers. Physical and chemical processes allow us to obtain different scale cellulose reinforcements. Macro fibers, such as lignocellulosic fibers of sisal, jute, cabuya, etc. are used for the production of composites, whereas nano-sized fibers, such as whiskers or bacterial cellulose fibers are used to produce nanocomposites. Given that cellulose can be used to obtain macro- and nano-reinforcements, it can be used as raw material for the production of several composites and nanocomposites with many different applications. The understanding of the characteristics and properties of cellulose is important for the development of novel composites and nanocomposites with new applications. 

Yet another limitation associated with the use of lignocellulosic fillers is the fact that the processing temperature of composites must be restricted to just above 200°C (although higher temperatures can be used for short periods of time), because of their susceptibility to degradation and/or the possibility of volatile emissions that could affect the composite properties. This limits the types of thermoplastics that can be used to polymers like polyethylene, PP, poly-vinyl chloride and polystyrene, which constitute, however, about 70 per cent of all industrial thermoplastics. Nevertheless, technical thermoplastics like polyamides, polyesters and polycarbonates, which are usually processed at temperatures higher than 250°C, cannot be envisaged as matrices for these types of conposite. 

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. 

Mucha and Krolikowski [39], studying the kinetics of isothermal crystallization of polypropylene, noticed that addition of a filler, e.g., wood flour, efficiently reduces the time of crystallization. This is desirable in the processing of composites as it reduces the injection-forming cycle and forms small spherulites improving the mechanical properties of composites. Transcrystalline structures are formed during polymer crystallization in the presence of lignocellulosic filler. 

The results of research described above prove univocally that the presence of lignocellulosic material in polymeric composites influences the crystallization process, altering the kinetic parameters, such as induction time, degree of crystallinity, conversion degree, etc., and characteristic temperatures. Using modification of the filler is not without the effect on crystallization of the matrix. An interesting phenomenon that has not been explained completely so far is the transcrystallization. Both the formation of transcrystallization and its effect on mechanical properties remain a disputable question among researchers. 

The literature reports also other methods for modification of lignocellulosic components to improve interphase adhesion. These comprise reactions with triazine derivatives [87], -octadecyl vinyl-sulfone [87], potassium permanganate [86, 88], as well as dicumil or benzoyl peroxides ([89] Jayamol [90]). Processing with peroxides in addition to significant improvement of mechanical properties of composites is also responsible for making processing of composite materials easier [86,91,92]. 

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