**3. Natural fiber polymer composites in structural purposes**

Natural fiber reinforced polymer composites are attracting widespread interest for purposes which are sensitive to the materials weight, because their strength and stiffness combine well with their low density, however, their toughness is a major concern [36]. NFPCs are prepared using plant fiber as fillers, which are hydrophilic in nature and do not interact well with hydrophobic polymer matrix [37–39]. NFPCs are a set of important materials developed for numerous areas application; medical, pharmaceutical, food packaging, electronics, aerospace, automobile, construction, building, transport and many more [40–43]. This is because of the many unique qualities these materials possess or can be designed to possess. Qualities which include, but are not limited to; light weight, resistant to chemical attraction, resistant to corrosion, ability to be molded to any shape, can be processed using existing technology, environmental friendliness and sustainability. That is why the interest in these materials has grown tremendously in the last two decades [36, 44–46].

Dweib et al. fabricated bio-based composites for roof structures in the form of paper sheets, entirely from cellulose fibers and soy oil-based resins [47]. These developed sheets were tested for structural unit beams and were established to have given the necessary strength and stiffness for consideration in roof construction. Also Bektas et al. manufactured panels with a density of 0.7 g/cm3 with a sunflower stalks percentage of 25, 50 and 75% fiber contents [48]. From the results of the mechanical tests, the panels were observed to have the required properties as required for general purpose-use particle board by normal standards.

Wood fibers/plastic composites have been used in large quantities for applications in window and door frames, decks, docks and molded panel components [49–51]. Natural fiber composites have been used to replace asbestos in the building industry, because of their health related issues [52]. The European Union policy tagged "end of life vehicle (ELV)" regulations, promulgated in 2003 and amended in 2005 and 2010, projected reduction of the final waste to be disposed at the end of life of vehicles to 5% by the year 2015. In the stated regulations, 85% of material used in manufacture of the vehicle must be recoverable through reuse or through recycling mechanically [49, 53]. This has generally increased interest and widespread use of NFPCs worldwide. It is noteworthy that this policy was promoted basically because of environmental and social concerns and not necessarily because of economic or technological reasons.

NFPCs durability and the availability of technology has allowed for large and complex shaped manufacturing of NFPCs, making them appealing in the automobile industries [54–56]. In Brazil, automobile industries consume, on average, 10–12.7 kg of natural fiber reinforcement per vehicle. These are circulated through the vehicle, such as rear door liners, front doors, boot liners, parcel shelves, sun roof interior shields and headrests [54]. Although NFPCs have gained tremendous interest in the industries, their applications are not unconnected to their environmental sustainability, low cost and renewability [57–60]. According to a review by Kiruthika [61], the challenge of replacing synthetic fibers completely in widespread applications is far from being overcome, with the improvement to the mechanical properties of composites being the major challenge.
