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Natural fiber-reinforced polymer (NFRP) composites have obtained significance in recent times due to their wide application in different industries, such as automotive, aerospace, construction, home appliances, and ballistic vest because of the high cost of synthetic fibers and environmental issue. These natural fibers are cheaper and lighter but their mechanical properties are lower than the synthetic fibers. In addition, great efforts have been made to understand the phenomenon of weathering of polymeric materials, which ultimately result in the deterioration of the physical and mechanical properties of the polymer. This work aims to present some research in the degradation area of natural fibers and their composites. The components of NFRP and flexural strength and modulus are revealed in the first part. Afterwards, the second part will be devoted to review some methods of aging treatments applied to natural fiber composites. Eventually, a discussion about the optimal approaches for lifetime prediction of NFRPs will be drawn.

The objective of this work was to evaluate the influence of photodegradation by ultraviolet radiation in composites of epoxy matrix reinforced with sisal fabric in the energy absorption capacity against shots of 0.22-gauge lead projectile. The ballistic tests were performed at subsonic speed using a 150-bar compressed air draft system. The ballistic efficiency was evaluated using the residual velocity technique. Fourier transform infrared spectroscopy (FTIR) was also performed to observe changes after radiation exposure. Exposure to UV radiation, both at 75–225 h, caused changes in the color of the composite plates, in addition to optimizing the energy absorption capacity in level I events by NIJ 0101.04, absorbing about 93% of the projectile energy, showing to be a very easy and fast technique for improving the ballistic properties of composites which in turn are more economically viable than the commonly used synthetic materials.

The replacement of synthetic fibers to natural fibers has been the subject of intense research, particularly as applied in composites. Astrocaryum vulgare is an important palm tree employed for many people along the Amazonian region in handcrafts and other products, by traditional manipulation techniques. The aim of this work is to perform a density Weibull analysis of Astrocaryum vulgare (Tucum) fibers with different diameters. The results obtained may be a database of Astrocaryum vulgare fibers, helping future research that will address its application potential as an alternative fiber applied in polymer composites.

Since the twenty-first century began, environmental concerns related to energetic consumption and pollution have been gaining attention. In part, these could be associated with production and disregard synthetic materials. Using natural materials instead of synthetic aimed to become a trend, which has not happened. Natural lignocellulosic fibers (NLFs) were showed to be capable of replacing synthetic fibers in polymer composites. However, some limitations such as damage from heat can be considered a major constraint for wider application of NLFs/polymer composites. A novel strategy that is suggested to improve this property is the graphene oxide (GO) functionalization of NLFs. This work investigates the thermal behavior of epoxy/NLF composites, with and without GO functionalization. Two different amounts of reinforcement, low (20 vol%) and high (40 vol%), were dynamic mechanically investigated up to 160 °C. Investigated parameters revealed notable changes attributed to GO-functionalization effect on the NLF regarding viscous stiffness and damping capacity of the composite.

The use of natural lignocellulosic fibers (NLFs) as reinforcement in polymer matrix composites is being considered by many different industries, for instance, the automotive industry where components such as interior panel and cushion seat are being manufactured using NLFs. The piassava fiber is a less characterized NLF when compared to sisal or jute, although the properties displayed by the piassava fiber make them suitable as reinforcement to composite materials. Therefore, this work aimed to present a morphological analysis of the piassava fiber by scanning electron microscopy (SEM) as well as studied the spectroscopy aspects of this fiber by Fourier transform infrared (FTIR). The SEM analysis revealed differences in their shape and size as well as a rough aspect associated with a great number of protrusions. While the FTIR analysis showed a hydrophilic character of the fibers due to the presence of axil vibration of hydroxyl groups band between 3500 and 3300 cm−1.

Polyester and epoxy resins are thermosetting polymers extensively used as composite materials matrices. The advantage of the polyester resin in comparison with the epoxy matrix is its lower cost. On the other hand, its permeability can be considered a disadvantage, once as the solvents dry the resin becomes more porous. This represents a direct impact on its mechanical properties. Epoxy resin has no solvents and its polymerization occurs only when two components are mixed, resin and hardener. The processing of this type of polymer requires control of temperature and humidity during its production. Polyester resin continues to cure over time, are prone to cracking and breaking, while epoxy resin once cured retains its full properties. The current pullout tests were performed to compare the interfacial adhesion of fique fibers with epoxy and polyester polymer matrix. The results indicated a critical length approximately 65% higher for the fique fiber/polyester in comparison to fique fiber/epoxy as well as an interfacial strength 4.9 times higher, which may indicate stronger adhesion of fique fiber with epoxy resin.

Natural fibers have been investigated as composites reinforcement for technological applications. Several natural fiber composites were found to present similar ballistic performance as synthetic materials, such as KevlarTM in multilayered systems (MASs) against high-impact energy projectiles. MASs with a front ceramic followed by natural fibers, natural fabrics or natural non-woven mat such as curaua, jute, sisal, ramie, coir, piassava, fique, and sugarcane bagasse reinforcing polymer matrix composites attend the standards for personal ballistic protection against the threat of 7.62 mm ammunition. These natural fibers or fabrics or non-woven mats composites, as second MAS layer, succeed to capture the cloud of fragments resulting from the projectile impact on the ceramic layer with comparable ballistic efficiency as conventional synthetic aramid fiber composites with same thickness. In this work, the development of research dedicated to these natural fiber-based composites in MASs are overviewed and their advantages over synthetic materials are emphasized, especially the much lower cost.

Multilayered armor system (MAS) layers are composed of materials such as a front ceramic and a back composite that must show both high impact resistance and low weight. Recently, composites reinforced with natural fibers have been considered as MAS second layer thanks to their efficient ballistic performance associated with other advantages such as being cheaper and environmentally friendly. Among the natural fibers, the fique fiber has shown potential as reinforcement of polymer composites for engineering applications. Epoxy matrix composites reinforced with up to 25 vol.% of fique fiber were for the first time ballistic tested. For practical application as armor for personal protection, the layer of fique fiber composite presents not only a superior ballistic performance but also lightness and economical advantages over the conventional aramid fabric.

During the last decades, an increased interest has been reported in the use of polymeric matrix composites reinforced with natural fibers for engineering applications. The advantages on the use of these materials include the replacement of synthetic fibers for natural ones, taking advantage of biodegradable resources and contribute for environment issues. This work aims to evaluate four different fractions of natural fiber reinforcing epoxy matrix composites. The natural fiber used in this investigation was the fique fiber, which due to its properties can be considered a promising candidate for composite reinforcement application. Initially, the fique fabrics were cut into 150 mm × 120 mm dimensions and dipped into a metallic mold. The fractions considered were 15, 30, 40, and 50% of a plain fabric, corresponding to 1, 2, 3, and 4 layers, respectively. The composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray diffractograms revealed characteristics peaks associated with the fique fiber. It was also showed a relationship between the degree of crystallinity and the amount of reinforcement. Finally, the SEM analysis presented the morphological aspect of the composites as well as the differences associated with the increase of reinforcement.

Several works have been dedicated to study the ballistic response of multilayered armor systems (MAS), including those using natural fiber composites as second layer. These composites disclosed good ballistic performance, having economic and environmental advantages. In the present work, the objective was to investigate the effect of areal densities of each MAS layer, in the overall performance of a MAS with an alumina ceramic front, a curaua fabric reinforced composite filling, and an aluminum alloy backing the target. Ballistic “backface signature” tests were conducted, varying the areal densities of the MAS layers according to a Box-Behnken design. The results showed a similar influence of the ceramic and aluminum layers in the MAS “backface signatures”, although deformation and failure mechanisms were significantly different. The composite also showed significant participation in the overall MAS performance. Since the composite is cheaper and lighter, a balance minimizing the other layers would be advantageous.

Epoxy matrix composites reinforced with up to 10, 20 and 30 vol% of continuous and aligned natural mallow fibers and 30 vol% of mallow and jute hybrid fabric were for the first time ballistic tested as personal armor against class III 7.62 mm FMJ ammunition. The ballistic efficiency of these composites was assessed by measuring the dissipated energy and residual velocity after the bullet perforation. The results were compared with that in similar tests of aramid fabric (Kevlar™) commonly used in vests for personal protections. Visual inspection and scanning electron microscopy analysis of impact-fractured samples revealed failure mechanisms associated with fiber pullout, rupture and layers delamination. When compared to Kevlar™, the mallow fiber and hybrid fabric composite displayed practically the same ballistic efficiency. However, there is a reduction in both weight and cost, which makes both, mallow fiber and hybrid fabric composites, a promising material for personal ballistic protection.

Lignocellulosic natural fibers exhibit high variation in mechanical properties values due to their heterogeneity. Recent studies have shown that the dispersion of these properties depends on the diameter of the fiber. A possible explanation for the tendency of low mechanical properties with large diameter is the high probability of defects into the fiber. However, no study has yet investigated the relation between the diameter and defects of natural fibers. Hence, in the present work the total porosity of coir fiber (Cocos Nucifera L.) was estimated. A statistical analysis was carried out on a batch of about 100 fibers and the geometric density was measured by using a stereomicroscope. The closed and open porosity as well as the density of the fiber were quantified by helium pycnometry method. The values obtained were correlated with the diameter of each analyzed fiber. Results made it possible a more detailed knowledge of the porous structure of the fiber.