3.1 Characteristics of hemp fibers after alkaline treatment
Mercerization is one of the chemical treatments of natural fibers most commonly used to reinforce the polymer matrix. The important modification resulting from alkaline treatment is the disruption of hydrogen bonding in the network structure, thereby increasing surface roughness. Additionally, alkaline treatment facilitates the removal of lignin, wax, and oils from the external surface of the fiber cell wall. FTIR technique was used to investigate the alkaline treatment influence on the hemp fiber structure. Figure 2 presents the FTIR spectra for both untreated and alkaline treated hemp fiber. A wide absorption band at 3308 cm− 1, referring to the OH band, was caused by cellulose and hemicellulose in the hemp fibers. The peak at 1732 cm− 1 was attributed to the presence of the carboxylic ester (C = O) in pectin, and waxes disappear after the alkaline treatment of hemp [30]. The peak referring to 2982 cm− 1 was related to the vibration of the symmetrical elongation of the C-H bond of the CH2 and CH groups of cellulose and hemicellulose [31]. It was evidence of partial removal of the hemicelluloses group from the fiber surface after the NaOH treatment. Moreover, compared to untreated hemp fiber, the peak intensity at 1249 cm− 1 of treated hemp fiber, which belongs to the C–O stretching of acetyl groups of lignin, was decreased [32], which was attributed to the removal of bad odor source, lignin [33].
Thermal stability of the short fibers is very important to prepare polymer/fiber composites during melting mixing. Generally, natural fiber will degrade at the melting temperature of the polymer, which limits natural fiber to be applied as reinforcement in composites, especially in thermoplastic composites where the processing temperature is as high as 150–220°C [29]. In this study, the thermal behavior of hemp and alkaline treated fibers was examined via thermogravimetric analysis (TGA) under a nitrogen atmosphere. As seen in Fig. 3, both treated and untreated fibers absorbed around 7% moisture below 110°C. The thermal stability of hemp fiber was raised from 322 to 344°C with alkaline treatment process. Additionally, residue amounts were calculated as 23 and 18% for untreated and treated fiber, respectively. This decrease addressing the removal of the lignin and other impurities on the surface, which lowers the degradation temperature, was aligned with the FTIR results given in Fig. 2.
During the alkaline treatment, the fibers are separated from one another (fibrillation), resulting in an increase in the effective surface area available for wetting by the matrix. Scanning electron microscopy is a powerful technique to analyze the change in surface morphology and topography in the microscales. Figure 4 presents the surface morphology of untreated and alkali-treated hemp fibers, respectively. In Fig. 4.a, the surface of untreated fiber is mostly in bundle form and covered with a layer of waxy substances such as lignin, pectin, and hemicellulose. After the alkali treatment (Fig. 4.b), fiber surface is cleaner than that of untreated fiber, and some degree of fiber separation is observed. This confirms the FTIR results and displays that some components are removed from the surface during the treatment process. Moreover, this fibrillation results in increased surface roughness which is necessary to provide additional interface interaction sites and, thus, to improve the mechanical of the polymer composite.
3.2 An ideal composition of hemp fiber reinforced PP compounds and their mechanical performance
A certain drawback of NFRCs is the incompatibility between the natural fiber and polymer matrix, resulting in poor mechanical properties. Therefore, alkaline treatment of the hemp fiber surface was employed for better adhesion between fiber and matrix. Apart from surface modification, the fiber content affects the performance of the NFRCs. The increase in natural fiber loading often leads to an increase in properties up to a certain loading amount [34]. Thus, the determination of the hemp fiber content yielding optimum performance of NFRCs is essential.
First, the tensile tests of homoPP samples reinforced with alkaline treatment hemp fiber at different loadings (10, 20, 30, and 40%) are performed, where the tensile strength and tensile modulus values are provided in Fig. 5.a and Fig. 5.b, respectively. Also, the tensile test results of strength, modulus, and strain at failure are tabulated with the amount of improvement compared to the neat homoPP in Table S1. The tensile strength was improved by 5.5%, 14.0%, 16.3%, and 23.6% compared to neat homoPP for 10%, 20%,30%, and 40% fiber loadings, respectively (Fig. 5.a). Similarly, the tensile modulus was improved by 39.1%, 89.9%, 118.2%, and 154.7% compared to neat homoPP for 10%, 20%,30%, and 40% fiber loadings, respectively (Fig. 5.b). Moreover, the tensile strain of the composite at any hemp fiber fraction was lower than neat homoPP, as shown in Figure S1.a. The change of the ductile behaviour of neat homoPP to the brittle can be observed clearly in the stress-strain curve of the compounds in Figure S1.b.
Then, the flexural tests are performed for the same sample configurations as the tensile tests. The flexural strength and flexural modulus results are provided in Fig. 5.c and Fig. 5.d, respectively, with tabulated results and corresponding improvements in strength, modulus, and strain at failure as in Table S2 and Figure S2.a. The flexural strength was improved by 11.9%, 18.6%, 20.3%, and 41.2% compared to neat homoPP for 10%, 20%,30%, and 40% fiber loadings, respectively (Fig. 5.c). Similarly, the flexural modulus was improved by 36.3%, 65.5%, 97.0%, and 100.6% compared to neat homoPP for 10%, 20%,30%, and 40% fiber loadings, respectively (Fig. 5.d). This conclusion was in line with other researchers' findings that flexural strength increases with fiber loading [35][36–38]. It is obvious from the Figure S2.b that homoPP behaves more brittle with increasing fiber content under flexural stress. Considering tensile and flexural test results, homoPP reinforced with 40% alkaline-treated hemp fiber is selected as the baseline for further investigation of the synergistic effect of MAPP as a compatibilizer and upcycled waste tire-driven GNP as co-reinforcement.
3.3 The integration of compatibilizer and GNP in hemp fiber reinforced PP compounds
Compatibilizers in polymer composites serve as a coupling agent both to overcome the dispersion issues and to improve the adhesion across the interface for enhanced mechanical strength by increasing the stress limit for the load transfer between the matrix and fiber [39, 40]. Moreover, GNP integration is another way to achieve further increment in the Young’s modulus and tensile strength of the composite. GNP is served as a co-reinforcement agent promoting effective load transfer when strong interfacial regions were achieved. The presence of GNP in the polymeric matrix structure has also the ability to change crystal structure due to the acting as nucleating agent [41].
This section investigates the effects of MAPP as a compatibilizer and upcycled waste tire-driven GNP as a co-reinforcement incorporated to homoPP reinforced with 40% alkaline-treated hemp fiber with tensile, flexural, and impact tests.
The change in tensile properties for tensile strength and modulus are given in Fig. 6.a and Fig. 6.b, respectively, with the inclusion of MAPP and GNP and tabulated results with the improvements for the tensile strength, modulus, and strain at failure were given in Table S1. From Fig. 6.a and Fig. 6.b, the tensile strength and modulus improvement were recorded for the MAPP and GPN inclusion together as 28 and 5.2% compared to 40 T-HF + homoPP, respectively. The effect of MAPP and GNP on tensile strain and stress-stress curve of 40 T-HF + homoPP is shown in Figure S3. The brittleness of 40 T-HF + homoPP was not changed dramatically by incorporation of MAPP and GNP .
Similarly, the effect of GNP and MAPP showed the same trend for the flexural strength and modulus values. For flexural strength and modulus MAPP, and GPN inclusion together yielded to increase of 35.8% and 6.1% compared to the baseline configuration, respectively (see Fig. 6.c and Fig. 6.d). Since the GNP has the main role in increasing the crystallinity during the injection process due to its nucleating agent effect [42] together with compatibilizing and reinforcing effect, the maximum tensile and flexural properties were attained for the compounds of 40 T-HF + MAPP + GNP + homoPP. In addition, there is no obvious effect of MAPP and GNP on the flexural strain of 40 T-HF + homoPP as seen in Figure S4.
Impact resistance plays an important role in the performance of composite subjected to regular shocking load or impact load. Figure 6.e represents the Charpy impact test results of the neat homoPP and homoPP compounds, including T-HF with MAPP and GNP. The impact strength of homoPP compound was strongly affected by the presence of the T-HF. The reinforcing effect of the short hemp fiber containing 40 wt% is indicated by an increase in the impact strength by approximately 40.4%, compared with neat homoPP compound. The test result showed an increase of impact energy of 40 wt % of T-HF + homoPP from 4.52 to 5.01 kJ m− 2 with MAPP addition (10.8% improvement). It has been reported that MAPP, as a compatibilizer can reduce water absorption and increase the modulus, hardness, and impact strength of the natural fiber reinforced compound [41]. The impact strength of the fabricated compound reaches a maximum value of 5.01 kJ m− 2 with the inclusion of MAPP together with GNP. To conclude, improvement in the impact properties was preserved for the 40 THF + MAPP + GNP + homoPP as in tensile and flexural properties by creating a synergy resulting to high mechanical performance.
3.4 Thermal characteristics of multi-scale hemp fiber reinforced homoPP composites
The mechanical properties of thermoplastic homoPP are in correlation with their microstructure and crystallinity state. Herein, the melting and crystallization behavior of hemp fiber reinforced compounds containing MAPP and GNP were determined through DSC analysis. The first cooling cycle for crystallization behavior and the heating cycle melting curves of samples have been shown in Fig. 7.a, and Fig. 7.b, respectively. Thermal parameters have been summarized in Table 2. The crystallization temperature of 40 T-HF + homoPP decreased by only 1°C by the implementation of MAPP and GNP, and this change can be neglected. DSC analysis shows no significant changes for the crystallization behavior of 40 T-HF + homoPP composites with MAPP and GNP inclusions. It can be seen from Fig. 7.b and Table 2 that melting peak temperatures of 40 T-HF + homoPP remain the same by MAPP and GNP implementations. According to the results, GNP and MAPP inclusions had no obvious effect on the thermal properties of the 40 T-THF + MAPP.
Crystallinity investigation can be conducted using the equation below:
$${X}_{C}=\left(\frac{\varDelta {H}_{M}}{\phi \varDelta {H}_{M}^{100\text{%}}}\right)\times 100$$
1
where \({X}_{C}\) is the degree of crystallization, \(\varDelta {H}_{M}\) is the melting enthalpy, \(\phi\) is the weight fraction of homoPP, and \(\varDelta {H}_{M}^{100\%}\) is the melting enthalpy of 100% crystalline homoPP. 207 J/g was taken for the melting enthalpy of the 100% crystalline homoPP in the calculations [43]. The crystallinity degrees of hemp-based compounds have been calculated according to Eq. (1) and listed in Table 2. The percentage crystallinity of 40 T-HF + homoPP was found as 40. MAPP addition led to rise in the percentage crystallinity by 4.9 %. Moreover,12.5 % increment as achieved by GNP addition. This can be explained as due to the nucleating ability of GNP which accelerated the crystallization process of homoPP [44]. Synergetic effect was established for the 40 T-HF + MAPP + GNP + homoPP having the 51.5% crystallinity. To conclude, the improvements in mechanical properties confirmed by the consequence of increased crystallinity of the prepared compound.
Table 2
Melting and crystallization parameters and percentage crystallinity of MAPP and GNP incorporated treated hemp fiber reinforced homoPP compounds
Sample | Melting peak temperature (°C) | Melting integral, ΔHm (J g− 1) | Crystallization peak temperature (°C) | Crystallization Integral, ΔHc (J g− 1) | Crystallinity, XC (%) |
40 T-HF + homoPP | 168.83 | -49.71 | 125.50 | 62.56 | 40.0 |
40 T-HF + MAPP + GNP + homoPP | 168.33 | -60.02 | 124.50 | 60.57 | 51.5 |
3.5 Benchmarking study of hemp fiber and glass fiber reinforced compounds
Polypropylene composites are very popular and widely used in various applications such as automotive, construction, and consumer products due to their excellent compromise in cost performance. They are often used in combination with one of the dominant synthetic fibers, glass fibers (GF), in different forms, such as short, long, or continuous [45]. Among them, long glass fiber reinforced thermoplastic granules are often preferred for different applications due to the advantages such as better mechanics at increased temperatures, less creep, and higher working absorption during impact stress. The pultrusion process, where the fiber bundle is pulled through a bath of thermoplastic melt and cut to the desired length, is the main manufacturing method for the long glass fiber reinforced polypropylene, and requires expensive tooling [46, 47]. The use of GF reinforced thermoplastics in industry brings with it some problems despite the many benefits that their utilization provides. Mainly, they are not capable of straightforward recycling at the end of their lifetime; moreover, they consume high energy, which results in CO2 emission during the fabrication process.
A new trend has been initiated using natural fibers in composite materials due to the variable prices of glass fiber and an increasing ecological awareness for the conservation of the natural resources. Thus, the new compound developed in this study was compared with the commercial long glass fiber (LGF) to report the ability of these materials to be replaced with commercially used one. First, usage of hemp fiber as a reinforcement material promotes sustainability and decreases the carbon footprint because of the benefits such as availability, biodegradability, and low cost. In addition, it could be produced by using thermokinetic mixer, which is a simple, fast, and cost-effective fabrication method compared to the pultrusion.
In order to compare the physical and mechanical properties of the hemp reinforced (40 T-HF + MAPP + GNP + homoPP) and reference (40 LGF + homoPP) composite materials, which are containing same amount of fiber, were listed in Table 3. Density of hemp-based composite is 14.5% less than the LGF composite which offer the opportunity for lightness. Additionally, the flexural strength and modulus of the 40 T-HF + MAPP + GNP + homoPP was 3% and 80.5% higher in comparison with the reference material, respectively. While, the tensile strength was preserved, the tensile modulus was increased by 27.3% using the newly developed compound formulation of this study.
The results confirmed that sample of 40 T-HF + MAPP + GNP + homoPP shown up with higher mechanical performance and lower density in comparison with 40 LGF + homoPP. On the other hand, this material meets the key demands of market such as sustainability, recyclability, cost-effectiveness and ease of processability. In addition, adapting GNP produced by upcycling into hemp composite will be an added value toward further enhancing the sustainability and productivity of industry as well as addressing an environmentally waste problem issue.
Table 3
Density and mechanical properties of GNP/hemp fiber reinforced composite and long glass fiber reinforced homoPP composite
Samples | Density (g/cm2) | Flexural Modulus (MPa) | Flexural Strength (MPa) | Tensile Modulus (MPa) | Tensile Strength (MPa) |
40 LGF-homoPP | 1.17 | 2504 ± 38 | 76.4 ± 0.1 | 4009 ± 81 | 55.0 ± 0.8 |
40 T-HF-+ MAPP + GNP + homoPP | 1.00 | 4520 ± 96 | 78.7 ± 1.6 | 5105 ± 422 | 54.4 ± 1.2 |
3.6 Cross-sectional morphology analysis of multi-scale hemp fiber reinforced homoPP composites
The morphology of the cross-sectional fracture surfaces of the neat homoPP and T-HF reinforced homoPP composites were studied by SEM, and the distribution of fibers as well as the interfacial interactions between the fibers and the polymer is observed. Figure 8.a shows the freeze-fractured surface of the neat homoPP. As expected, the freeze-fracture surface of neat homoPP is smoother compared to the fiber reinforced counterparts which were obtained after the tensile testing. SEM micrographs of fiber reinforced composites in Fig. 8.b display several fibers pulled out from the fracture surface and presence of the few voids. This indicates that the dominant failure mechanism is interfacial debonding [13]. In addition, the random distribution of the T-HFs in the matrix is clearly observed. However, high magnification image of 40 T-HF + MAPP + GNP + homoPP composite fracture surface in Fig. 8.d shows that polymer matrix covered the fibers, demonstrating the improved interfacial adhesion due to the synergistic effect of both MAPP and GNP inclusion.
3.7 Overmoulding process of hemp fiber reinforced compounds with continuous fiber reinforced bio-based prepregs
Composites for structural applications can be made by combining short and continuous fibers and/or combining different types of fibers. Short fibers are favored as reinforcement over continuous fibers due to their ease and speed of manufacture. While short fibers are good at resisting impact loads, the domains of continuous fiber reinforcement are better to support the flexural and tensile loads. Thus, hybridization of discontinuous fibers and continuous fibers reinforcement is necessary for composite production to gain the benefits of both two types of fibers. For this aim, bio-based hybrid composites using UD flax PP prepreg (continuous fiber source) and the developed compounds (short fiber source) were fabricated by overmoulding technique. Effectiveness of prepreg utilization in overmoulding procedure was also found in literature [26]. In this section, tensile, flexural and impact properties of overmoulded hybrid composites are investigated.
Figure 9 and Table S3 show the tensile properties of the overmoulded samples using UD Flax prepreg and prepared hemp reinforced homoPP. The improvements were calculated according to the results of the OM neat homoPP. After moulding, the homoPP material can be easily removed since it does not adhere to the matrix properly, one of the problematic subjects in the overmoulding composites, according to the literature [48, 49]. As shown in Figure S5, the presence of UD fibers in fabricated samples did not affect the failure type and also maintained the integrity of the composite during the tests. The overmoulded neat homoPP composite is comparatively ductile and resulted in prolonged elongation at the break without softening. As seen in Fig. 9.a, presence of T-HFs in the composite improved the tensile strength and modulus of the OM neat homoPP as 17 and 133%, respectively. Further increment was achieved by integration of MAPP and GNP added hemp fiber composites in the overmoulded samples. The sample of OM 40 T-HF + MAPP + GNP + homoPP improved the tensile strength and tensile modulus of OM neat homoPP by 41.6% and 151.7%, respectively.
Applying MAPP and GNP with treated hemp fiber in the overmoulded samples is also very effective in both flexural strength and modulus (see Table S4 and Figure S6). The flexural strength and modulus of overmoulded composite of the OM 40 T-HF + MAPP + GNP + homoPP were 76.6 and 5350 MPa, respectively as seen in Fig. 9.c and Fig. 9.d. It suggests a considerable influence of fiber arrangement of UD flax, MAPP and GNP interfacial effect during the test. As a result of this investigation, it was evident that hemp fiber overmoulded composites reinforced with MAPP and GNP had increased mechanical properties over their matrix materials and thus had better fiber matrix adhesion. This can be related to the fact that both GNP and homoPP adhere better to MAPP because of its altered chemical structure [50, 51].
The impact strengths of the overmoulded samples are reported in Fig. 9.e. In general, the fiber, the polymer matrix, and the interfacial bond strength of fiber reinforced polymer composites are the three factors that determine the toughness of the material [52]. It was observed that the Charpy impact strength of OM neat homoPP was enhanced in 15.66% of the presence of the treated hemp fiber. The maximum improvement was obtained for the OM 40 T-HF + MAPP + GNP + homoPP as 32.44% compared to OM neat homoPP. This value is 67.07% higher than the compound of 40 T-HF + MAPP + GNP + homoPP and proves the importance of the presence of UD fiber in the composite system. In conclusion, the mechanical performance of the composites, including T-HF, MAPP and/or GNP can be enhanced by the application of the overmoulding method with only one layer of UD Flax prepreg integration onto the surface of composites. It may be attributed to the using both UD fibers and high-performance composites as overmoulding material.
Investigating the interface morphology between the overmoulding material and the insert material is important in order to understand the interactions between the two parts of the hybrid composite. Figure 10 displays the cross-sectional SEM micrographs of the OM 40 T-HF + MAPP + GNP + homoPP composite after the tensile test with distinct features of both overmoulding material (40 T-HF + MAPP + GNP + homoPP) and the insert prepreg (UD-Flax). Insert prepreg mainly consists of long and unidirectional fibers with similar fiber diameters, as shown in Fig. 10. Moreover, it demonstrates the difference between the fracture mechanisms of overmoulding material and the insert prepreg and the interface area between the two. Under the tensile load, insert material fractures in a brittle manner, which can be observed from both the smoother surface and the tensile test results; meanwhile, the overmoulding material fractures in a slightly more ductile manner, indicated by the rougher surface. Furthermore, Fig. 10 shows the interface with proper adhesion between the two different materials, which is also supported by increased tensile strength (Fig. 9.a) due to compatible homoPP matrices of both materials.