The relatively lower intensity of this 915 cm−1 band suggests that the epoxy resin was almost fully cured. All these epoxy bands predominate in the composites in Fig. 5. Additionally, few bands characteristic of the hemp fiber, Fig. 5, such as 2916, 1510, 1160 and 1030 cm−1 still appear in the composites spectra in Fig. 5, although with less intensity. This might be interpreted as an indication of active interaction between the epoxy and the hemp molecular groups. Similar results were also found for hemp fiber/polyester composites but corresponding discussion is beyond the scope of the present work. It is only worth mentioning that FTIR results for epoxy and polyester composites disclosed evidence of hemp fiber molecular interaction with both thermoset matrices, which contributes to its ongoing mechanical reinforcement. Table 1 presents the results of 3 points bending tests, in terms of flexural strength and modulus, for both epoxy and polyester composites reinforced with different volume fractions of continuous and aligned hemp fibers. In this table it is also presented flexural results for plain epoxy and polyester, as control specimens. The results in Table 1 are plotted in Figs. 6 and 7 for visual interpretation. In these figures it is worth noting that epoxy composites with 30 vol% of hemp fibers display a marked increase in both flexural strength and modulus. This characterizes an effective reinforcement. On the other hand,incorporation of hemp fibers into polyester composites does not improve the mechanical properties of the matrix. The statistical validation of these flexural results were analysed by ANOVA and Tukey test. Table 2 presents the values of P for ANOVA with just one factor. In this Table 5 % is the level of significance to determine if in fact exist differences between a given set of results. If P is greater than 0.05, No difference should exist in the behavior for any volume fraction of hemp fiber in the set of values. On the contrary, if P is smaller than 0.05 then, statistically, Yes, a difference must be considered.
The ANOVA analysis in Table confirmed that hemp fiber incorporation into epoxy composites causes a significant difference. In this case,trim bin tray as aforementioned, a marked reinforcement for 30 vol% of fibers shown in Figs. 6 and 7. An apparent contradictory assertion might exist for the possibility of significant difference, Yes, in the flexural modulus of polyester composites in Table 2. However, a close look in the corresponding results in Fig. 7 reveals a significant increase for 10 vol% of hemp fiber. The possible reinforcement at this volume fraction should be further investigated. The individual comparison between pairs of distinct volume fractions of hemp fibers in the flexural properties of epoxy and polyester composites was performed by the Tukey test and presented in Tables 3 and 4. In these tables the sign means that the values are statistically different while means statistically equal, with 95 % level of confidence. The results of the Tukey test in Tables 3 and 4 disclose some points worth mentioning. First, for both the flexural strength, Table 3, and modulus, Table 4,the values for 30 vol% hemp fiberin epoxy composites are marked different as compared to any other volume fraction . In association with the results in Table 1 and Figs. 6 and 7, this indicates a comparatively stronger reinforcement caused to the epoxy matrix by incorporation of 30 vol% of hemp fiber. Second, incorporation of hemp fiber into polyester matrix would not cause a sensible difference, except perhaps for a possible relatively small improvement in the flexural modulus for 10 vol% addition. For practical purposes this flexural modulus must be further confirmed. Table 5 presents the results of tensile strength and elastic modulus for both epoxy and polyester composites reinforced with different volume fractions of continuous and aligned hemp fibers. In this table it is also presented corresponding tensile results for plain epoxy and polyester, as control specimens. The results in Table 5 are plotted in Figs. 8 and 9 for visual interpretation. In both figures the tensile strength and elastic modulus of the epoxy composites are higher than those for polyester composites. Moreover, within the limits of precision , the 30 vol% hemp fiber/epoxy composites display values of strength and modulus superior to the plain epoxy. This is an indication of de facto mechanical reinforcement associated with this volume fraction of hemp fiber. By contrast, in composites with polyester matrices, the hemp fiber incorporation is not providing efficient reinforcement.
The large dispersion related to the standard deviation in Figs. 8 and 9, could be a reason. Indeed, the average values for 30 vol% hemp fiber/polyester composites are higher than corresponding ones for plain polyester. This must be further investigated. However, the following statistical analysis might contribute to clear this question. The data obtained in tensile tests and used to evaluate the results in Table 5, were subjected to the analysis of variance and Tukey test, similar to the flexural test data. Table 6 presents the ANOVA values of “P”. In this table, considering that 5 % is the level of significance,there ought to be a significant difference between results if P < 0.05. Reversely, if P > 0.05 then no difference might be attributed to the mechanical behavior of the distinct volume fractions of hemp fibers incorporated into the composite matrix. As shown in Table 6, based on the ANOVA hypothesis, the incorporation of hemp fibers into epoxy composites causes a statistically significant difference in the tensile strength and elastic modulus. In other words,the hypothesis that the values are the same can be rejected with 95 % confidence. According to the results in Table 5 and Fig. 8, this difference is associated with a reinforcement effect. The same applies for the tensile strength of polyester composites. However, there is no significant effect on the elastic modulus of polyester composites with incorporation of up to 30 vol% of hemp fibers. The statistic interpretation between volume fractions of hemp fiber in the tensile properties of epoxy and polyester composites is given by the Tukey test results in Tables 7 and 8. As shown in Table 7, only the results of 30 vol% hemp fiber incorporation can be considered different in confront with any other volume fraction for the tensile strength of epoxy composites. It is also worth noticing that, as for the elastic modulus of epoxy composites in Table 8, not only the 30 vol% but also the other hemp fibers incorporation are different than plain epoxy . Considering the results in Table 5 and Figs. 8 and 9, one can affirm that the 30 vol% hemp fiber indeed reinforces the epoxy matrix. Moreover, these hemp fiber epoxy composites have tensile properties substan-tially higher than those of polyester composites, Figs. 8 and 9, in which the Tukey tests results, Tables 7 and 8, are not supporting a reinforcement effect.
A reason for the difficult of hemp fibers to reinforce the polyester matrix, as emphasized in both flexural and tensile results, might be a poor fiber/matrix interaction. Fig. 10 shows SEM images of tensile fracture of 30 vol% hemp fiber in polyester composites. In these images the poor adhesion between hemp fiber and polyester matrix is indicated by arrows. As a final remark, it is worth mentioning that the ANOVA and Tukey test statistical analysis for both flexural and tensile results prove, for the first time, that the incorporation of 30 vol% hemp fiber in epoxy composites provides an effective reinforcement. Moreover, no evidence of reinforcement was statistically found for similar incorporation in polyester composites. This may contribute to a practical decision regarding the amount of hemp fiber and type of thermoset matrix to be applied in ballistic armors.The density of composites is mostly implied by the compaction stress applied during their production, among other factors. The influence of compaction on physical properties of composites has been highlighted by Balciunas in the case of hemp-sapropel composites compacted at 20, 40 and 60% of their initial volume. This study focuses on the composites obtained with hemp shiv and black liquor. Hemp shiv are lightweight aggregates with high porosity and interesting hygrothermal properties. This makes them perfectly suited for the development of bio-based insulating materials. They are the most commonly used aggregates in the literature for the production of building materials. Black liquor is a renewable binder from local industry, is better for the environment than using petroleum-based binder . More, pollen trim tray the use of black liquor leads to a better hygrothermal properties of bio-based composites than the use of petroleum-based binder as highlighted in previous study. Black liquor is readily available component since ten tonnes of black liquor are produced per tonne of pulp using the Kraft pulping process. Currently, black liquor is mainly used for the production of energy. However, efficient energy production is cumbersome and expensive. It is therefore important to seek to enhance this co-product in a different way. Due to its chemical composition , black liquor represents an interesting alternative for the synthesis of sustainable chemicals. For this study, the composites are produced with the same hemp shiv to black liquor ratio but seven different compaction stresses applied during forming step . Thus, this study investigates the effect of compaction conditions on density, porosity, mechanical, thermal and hygric performances of composites.For each kind of composites, six specimens are produced. Three of them are used for thermal and hygric characterization, the three others for mechanical characterization and measurement of skeleton density.
The same black liquor to hemp shiv dry mass ratio of 15% is used. This value is chosen in order to ensure a good cohesion. For preparation, the hemp shiv are mixed with the black liquor in a mixer with a flat paddle during 5 minutes. The mix is split into three parts to produce three specimens. Specimens are molded and compacted 5 times using an Instron 5988 testing machine fifitted with a upper plunger, to ensure a good particles arrangement. They are maintained under compression and heated , cooled to room temperature and demolded .Polypropylene – natural fiber composites are of great commercial interest because they are lightweight, environmentally friendly, show high performance/cost index, high flexibility, low abrasiveness and low impact on human health. The high flexibility of natural fibers, which are bent during processing and not fractured as in the case of mineral fibers, is especially interesting for automotive applications. Despite the intense study of these composites, the industrial application of PP/NF composites is not as large as expected. This is partially determined by the weak interfacial adhesion, characteristic to the polymer composites with NF. Fiber treatment or coupling agents are generally used to overcome this drawback. Chemical treatments of NF including alkali, acetyl, acryl, isocyanate or silane modification are the most popular. In particular, silane treatment of hemp fibers , with or without an alkali pretreatment shows some benefits on the mechanical properties of polymer composites containing these treated fibers. Nevertheless, natural fiber – polymer composites show lower mechanical properties compared to glass fiber – polymer counterparts due to the inherently lower mechanical properties of NF compared to GF. In addition, the treatment of the fibers is often expensive and harsh to the environment. Other approaches such as a higher aspect ratio or a better dispersion of the fibers were tried to ensure better mechanical properties in PP/NF composites, higher fiber length and aspect ratio ensuring better properties. Although continuous fiber composites show better mechanical performance, the short fiber composites are easier to process and cheaper and they are required for the fabrication of injection molded parts. Some works have shown that long fibers increased the stiffness and short fibers the ductility of polymers and the incorporation of both long and short fibers of the same origin may lead to optimized mechanical properties by self-hybridization. However, agglomerations may be more frequent for very long fibers, leading to poor processing and lower mechanical properties . For an efficient stress transfer from the polymer to the fibers, fiber length should be longer than a critical value. Fu and Lauke showed that the composite strength increases rapidly with the mean length close to the Lc value and does not vary anymore for very long fibers .