Biocomposites, applications requirements

Some applications require composites in which phases of radically different properties have to be joined, such as ceramics with metals or rigid hydroxyapatite with highly deformable collagen. Such applications include the constmction of space vehicles (with a ceramic layer outside and a metaUic stmcture inside a vehicle), biocomposites, gas turbine blades, machining tools, attack vehicle armour and many others. 

Biocomposites have been an area of growing interest and a subject of active research for quite some time. This is due to both environmental concerns as well as anticipated future scarcity of oil and oil-derived products. A class of composites in which resins of natural origin have already had a commercial/industrial impact is in the field of rigid wood panels, such as particleboards. Natural-origin resins have already been used commercially for the last 30 years for these wood panels, and their use is still growing, although still relatively slowly [1]. In such an application, the binder is never more than 10% by weight of the whole composite panel. This is sufficient to conform to the performance and costs required by the wood panels industry and their respective product standards. 

The most common strategy to decrease the price or improve the properties of polylactide to fulfill the requirements of different applications is blending. Polylactide has been blended with degradable and inert polymers, natural and synthetic polymers, plasticizers, natural fibers and inorganic fillers. The most common blends include blends with other polyesters such as polycaprolactone or PLA/starch blends. Usually the compatibility between the two components has to be improved by addition of compatibilizers such as polylactide grafted with starch or acrylic acid (114,115). Recently a lot of focus was concentrated on the development of polylactide biocomposites, nanocomposites and stereocomplex materials. In addition various approaches have been evaluated for toughening of polylactide. 

The mechanical performance required of biocomposites is dependent on specific structural applications. Crude inferences can be made by comparing properties of materials that these biocomposites are intended to substitute. Mechanical data and/or allowable design values of wood and engineered wood products were used to evaluate potential applications of hemp fabric/cellulose acetate composites and hemp fabric/poly (hydroxybutyrate) composites (Table 13.2). From the comparisons, it can be inferred that these biocomposites, despite not passing the design values of wood structural material, can potentially substitute engineered wood products (of the same size) such as plywood and oriented strand boards to partially capture existing markets as crates, pallets, and formwork [38]. 

Apart from satisfactory mechanical properties, additional features are often required in product applications. One important requirement for construction applications is flame retardancy. Biocomposites are, however, flammable because they are composed of organic materials. To improve flame retardancy, addition of inorganic materials would be needed. Various flame retardants composing of halogens (Q, Br), heavy and transition metals (Zn, V, Pb, Sb), or phosphorus organic compounds may reduce the risk of combustion [39]. 

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