Considering the average lengths before and after extrusion-injection molding , a reduction of about 2 times in the case of HF, 2.5 times for HF and 4 times for HF was observed. It follows that the reduction of fiber length was greater for longer initial fibers, similar to the results on simple PP/HF composites. Moreover, their length in the final composites was greater than for shorter initial fibers, which is consistent with previous results. For a deeper evaluation of fiber length reduction in PPM/HF composites after extrusion and injection molding, the length histograms of the initial fibers, collected after the mechanical treatment and before the incorporation in composites are given in Fig. 5. The representative images of the initial hemp fibers, HF, HF and HF are also shown in this figure. A thorough analysis of the initial fiber length shows that about 60% of HF are under 1mm in length, compared to 90% after processing, and only 27% in the case of HF and 4.5% in the case of HF, compared to 72% and 47% after processing. It follows that the proportion of “fines” is drastically increased in the case of longer fibers but it is maintained lower than 50% for HF. A side-by-side analysis of optical microscope images of HF before and after incorporation in composites highlights the intense defibrillation of the fibers. This is due to the concerted action of temperature and shear forces during processing in the double-screw extruder and injection molding machine. Average diameters of 91 ± 64, 99 ± 48 and 119 ± 69 m were obtained for the initial fibers, HF, HF and HF, after mechanical treatment. The lower dm value for HF and HF was caused by the more intensive mechanical treatment provided by the automatic cutting. The dm of HF decreased to 17.1 ± 11.1, 18.8 ± 9.8 and 23.8 ± 12.4 m after incorporation in PPM 30HF, PPM 30HF and PPM 30HF composites. Therefore, a rough calculation of the aspect ratio of the fibers,grow tent kit the length to diameter ratio, shows an increase from 12, 26 and 35 for mechanically treated HF,and to 33, 39 and 43 for HF in the composites containing these fibers.
Although both the length and the diameter of HF diminished after the melt processing of composites, the defibrillation seems more intense, which leads to an increase in the aspect ratio of the fibers. The analysis of fiber size after processing will be further used to understand the thermo-mechanical behavior of these composites.Natural fibers used as reinforcement of polymer matrices have been intensively investigated in recent decades and considered for diversified applications, from automotive, civil construction, sports, furniture, and packaging industries as well as in ballistic armors. The interest in the use of natural fibers ranges from their availability and renewability to intrinsic features, such as low density, biodegradability, cost-effectiveness, and low processing energy. In addition, the current need for replacing synthetic materials due to problems related to sustainability and non-renewable sources of energy led to a surging number of works to optimize their composite properties. Among the well-known natural fibers, the hemp fiber has historically been used in several applications such as textiles, manufacture of papers, and even in the pharmaceutical industry. Archaeologically, hemp is the oldest discovered natural fiber. The Columbia History of the World stated that the most antique relics of human industry are bits of hemp fabric discovered in tombs dating back to approximately 8000 BC. Nowadays, the plant Cannabis sativa, from which the industrial hemp fibers are extracted, is again widely grown in China, Europe, and Central Asia, after years of restriction in some countries. In fact, the C. sativa species produces less than 0.2 wt% of tetrahydrocannabinol , the, which is too low for use as a recreational drug, as in the case of marijuana, a popular denomination of Cannabis indica. The C. sativa fibers, responsible for keeping the trees upright, have its structural properties studied for reinforcement composites due to their remarkable strength and stiffness. Some historical applications corroborate its potential, such as when Henry Ford tried to adopt hemp-polymers composites in the automotive industry in 1941. Another example was the constructive details of the window frames and floor coverings, made of hemp fiber reinforced polymer composites used in facilities during the 2008 Beijing Olympics. Indeed, due to a relatively larger amount of cellulose and hemicellulose and lower microfibrillar angle,which are related to mechanical properties, the hemp fiber displays a remarkable tensile strength , elastic modulus and total strain. As such, after sisal, hemp is the world’s most applied natural fiber as reinforcement in composites.
However, a recognized disadvantage is the aforementioned variability in their properties, which is inherent to natural fibers in general and requires that each lot of fiber, obtained from a given supplier, to be preliminarily tested. This procedure will be carried out in the present work regarding basic mechanical and thermal properties. According to Shahzad, the main hemp fiber reinforced composite matrices are polypropylene and unsaturated polyester due to easy processing and cost. On the other hand, with comparable properties and cost of UP, epoxy is another thermoset polymer that has not been often used as a composite matrix for hemp fibers. As shown in our literature survey in Fig. 1, there is an emerging tendency to study epoxy composites reinforced with hemp fiber. In this decade an approximately exponential rise is occurring in the related number of articles. According to Fig. 1, today seven new publications already have a DOI number. In particular, flexural strength and modulus of 30 vol% of hemp fiber epoxy composites were found to surpass the corresponding flexural strength and modulus of polyester composites with an equal amount of same hemp fibers. In spite of the raising interest, Fig. 1, for epoxy composites reinforced with hemp fiber, no work has so far specifically investigated epoxy composites with hemp fabric. A surging area, where natural fiber/fabric reinforced polymer composites are attracting increasing attention, is that of personal ballistic protection. In addition to the numerous works cited in the aforementioned review papers, several recent publications reported on the ballistic protection provided by NFCs. As part of multilayered armor system , these NFCs display ballistic performance superior to commonly applied Kevlar and Dyneema laminates. Although much weaker than synthetic aramid and ultra-high molecular weight polyethylene, natural fibers possess the same capacity of absorbing the ballistic energy by capturing the fragments after the bullet impact against the MAS front ceramic. In view of these disclosed NFCs superior ballistic performance combined with lower density, cost effectiveness and sustainability, one might expect that hemp fabric reinforced epoxy composite could be used for ballistic protection. In a pioneer works, Wambua et al. investigated the response of 46 vol% of hemp fabric reinforced PP composite to ballistic impact by 1.1 g fragment simulating projectile . Their main result revealed an absorbed kinetic energy of 36 J associated with a limit ballistic impact velocity of 260 m/s.
Therefore, the primary objective of the present work was to investigate for the first time the ballistic performance of epoxy composites reinforced with up to 30 vol% of hemp fabric against 0.22 ammunition. As a preliminary investigation, the mechanical and thermal properties of the aforementioned composites are also investigated to characterize the specific reinforcement effect of our Brazilian supplied hemp fabric. The possible variability of results was statistically evaluated using the analysis of variance and the Tukey test.Table 1 shows the results for all conditions: composites reinforced with 10, 20, and 30 vol% of hemp fabric and neat epoxy, which was used as a control group. For better visualization, the average values were plotted in Fig. 4. All tested specimens were completely fractured, validating the results obtained as required by the standard. The graph in Fig. 4 presents a continuous increase in the impact energy along with the fabric volume fraction and a lower standard deviation for the plain epoxy resin compared to the composites due to the non-uniform proprieties of natural fiber. This behavior was observed in previous works for different composites and is expected as the amount of reinforcement is related to the increase of energy necessary to break the samples. Comparatively, the 30 vol% hemp fabric-epoxy composite presented only about 36% lower absorbed energy than the 30 vol% curaua-polyester composite, which, in another work, was presented as an acceptable ballistic performance. Fig. 5 shows the broken specimens after the Izod impact test. By visual analysis, the fracture surface of specimens with 0 and 10 vol%, and , respectively, reveal a smoother surface related to a brittle fracture tendency. On the other hand, the fracture surface of the samples reinforced with 20 and 30 vol% hemp fabric,indoor grow tent and gets more irregular as the volume fraction of reinforcement increases, which could indicate a brittleeductile transition. From the results, in Table 1 and Fig. 4, Table 2 presents the ANOVA analysis for the impact energy absorbed. The equality hypothesis with a confidence level of 95% was rejected, as F was higher than the Fc.The Tukey test honest significant difference for the absorbed impact of the plain epoxy result and the composite values above this result present a significant difference. Hence, the results in Table 3 show that the impact strength of the 30 vol% hemp fabric composites is the best amid the tested composites. It is important to note that the values are always greater than the HDS calculated, which suggests that the incorporation of hemp fabric provides an effective reinforcement to the epoxy resins for all volume fractions.
This points toward what was already presented in early works, in which the reinforcement contributed to a greater rupture surface area by interrupting or deviating the crack’s propagation.Table 4 presents the average results for the tensile strength of the hemp fabric-reinforced composites. From these values and the strain of the samples, it was also possible to calculate the elastic modulus. These results are plotted in Fig. 6 and , which correspond to tensile strength and elastic modulus, respectively, for better visualization. Table 4 also shows literature values for the tensile strength of the same neat epoxy resin. The results in Table 4 and Fig. 6 display relatively poor tensile properties for the 10 and 20 vol% hemp fabric reinforced composites comparatively with the neat epoxy resin. It suggests that these two fractions of fabric do not act as reinforcement when tensile loads are applied to the material. Consequently, the epoxy matrix bears most of the load applied during the test. As such, the hemp fabric incorporation has a negative effect on the composites and acts more as flaws in the material’s structure. However, the composites with 30 vol% hemp fabric improved considerably the tensile strength compared to the neat epoxy resin, which means that this amount of hemp fabric acts as an effective reinforcement for the material due to the mechanisms of restrict rupture of the fibers in the fabric that impart the tensile strength. This good performance, compared to other composites reinforced with hemp fibers, is an important parameter for the ballistic performance of the composites, as the tensile strength is associated with the penetration resistance of the target material. Also, it is possible to observe a considerable improvement of the elastic modulus for the composites reinforced with 30 vol% hemp fabric, which could be related to the higher stiffness of the hemp fiber. Although, the 10 and 20 vol% hemp fabric show lower values with a slight decrease attributed to a nonuniformity of the reinforcement’s properties. To better understand the behavior of the analyzed composites under tensile stress, Fig. 7 presents SEM images of the broken specimens, in which it can be noted different fracture mechanisms. The “river marks” present on the composites reinforced with 10 and 20 vol% hemp fabric, shown in Fig. 7 and , reveal low effectiveness of reinforcement of these composites due to limited fabric content and causes a brittle fracture for the material as the epoxy matrix bears most of the load.