Enhancement of mechanical properties of rubber compounds with hemp fibers

This study investigates the influence of natural hemp fibers, modified by alkali treatment, on the mechanical and rheological properties of rubber compounds. Hemp fibers were treated with different concentrations of NaOH (sodium hydroxide) to alter their surface characteristics and improve their compatibility with rubber matrices. These treated fibers were then incorporated into three types of rubber: natural rubber (NR), nitrile butadiene rubber (NBR), and epoxy-modified natural rubber (ENR). The study focuses on understanding the role of chemical modification in enhancing the performance of these rubber compounds by promoting better interaction between the hemp fibers and the rubber matrices. By using alkali-treated fibers, significant changes in the structural and mechanical properties of the composites were observed. The treatment process aimed to remove impurities like lignin and hemicellulose, which are naturally present in hemp fibers, while also improving their crystallinity and surface texture. These modifications helped create a stronger bond between the fibers and the rubber matrices, resulting in improved mechanical properties such as tensile strength and tear resistance. Additionally, the impact of alkali treatment on rheological behavior, including viscosity and flow properties of the rubber compounds, was thoroughly evaluated, highlighting the importance of fiber-matrix interactions. FTIR (Fourier Transform Infrared Spectroscopy) analysis revealed significant insights into the structural modifications of hemp fibers before and after alkali treatment. Untreated hemp fibers displayed characteristic peaks corresponding to the natural composition of the fibers, including components such as cellulose, hemicellulose, and lignin. These peaks were consistent with the inherent structure of raw hemp, reflecting the presence of functional groups like carboxyl and hydroxyl groups. However, substantial changes in the FTIR spectra were observed following alkali treatment, particularly with a 10% NaOH solution. The C=O stretching vibration peak at 1729 cm⁻¹, attributed to the carboxyl groups within the fibers, was no longer detectable after this treatment. This disappearance indicates the effective removal of these groups, which are associated with hemicellulose and lignin, components that contribute to the amorphous regions of the fibers. Moreover, the alkali treatment led to a noticeable decrease in the intensity of the peak at 3336 cm⁻¹, which corresponds to hydrogen bonding between carboxyl and hydroxyl groups. This reduction signifies that the treatment not only removes impurities and unwanted amorphous substances but also alters the surface properties of the fibers. These modifications reduce the abundance of certain functional groups, rendering the fibers more compatible with rubber matrices. As a result, the treated fibers exhibit improved potential for forming stronger interactions with the rubber, which is critical for enhancing the mechanical properties of the resulting composite materials. XRD (X-ray Diffraction) analysis provided further confirmation of the structural transformations induced by alkali treatment on hemp fibers. The diffraction patterns of untreated hemp fibers displayed characteristic peaks at angles of 14°, 16°, 23°, and 35° 2θ. These peaks are indicative of the natural composition of hemp, with the 14° and 16° peaks primarily associated with the amorphous regions of the fiber structure and the 23° peak linked to cellulose I, the dominant crystalline form of cellulose in untreated natural fibers. The amorphous regions in the untreated fibers largely consist of components such as hemicellulose and lignin, which contribute to the overall structural flexibility but reduce mechanical stability. Following alkali treatment with a 10% NaOH solution, notable changes were observed in the diffraction patterns. The intensities of the peaks at 14° and 16° decreased significantly, indicating a reduction in the amorphous content of the fibers. Simultaneously, the peak at 23° showed an increase in intensity, suggesting a marked enhancement in the crystallinity of the treated fibers. This improvement is attributed to the removal of amorphous components during alkali treatment, which reorganizes the cellulose molecules into a more ordered, crystalline structure. When the fibers were subjected to a stronger alkali treatment with 20% NaOH, the diffraction pattern revealed additional structural changes. A new peak emerged at 20°, signifying the formation of cellulose II, a more thermodynamically stable and crystalline form of cellulose. The crystallinity indices calculated for the fibers further corroborate these observations, with untreated fibers exhibiting a crystallinity index of 71.19%, which increased to 77.88% following 10% NaOH treatment. While 20% NaOH treatment resulted in a slight decrease in the crystallinity index to 73.60%, this change reflects a transition to a different crystalline form rather than a reduction in overall crystalline content. These findings underscore the effectiveness of alkali modification in improving the structural properties of hemp fibers. By enhancing crystallinity and reducing amorphous content, alkali-treated fibers become more suitable for use in composite materials, where improved mechanical and thermal properties are critical. The formation of cellulose II further highlights the potential of tailored chemical treatments to optimize the performance of natural fibers for advanced material applications. SEM (Scanning Electron Microscopy) analysis provided a detailed examination of the morphological transformations that hemp fibers underwent following alkali treatment. The surface of untreated hemp fibers was observed to be smooth and relatively uniform, with minimal roughness. This smoothness is characteristic of the natural composition of hemp fibers, where components such as lignin and hemicellulose form a protective layer around the cellulose microfibrils. These natural components contribute to the structural integrity of the raw fibers but also hinder effective bonding with polymer matrices due to their hydrophilic nature and lack of surface irregularities. Upon treatment with 20% NaOH, significant alterations in the fiber morphology were observed. The treated fibers displayed a markedly rougher surface, characterized by an irregular, fibrillar structure interspersed with micropores. This transformation can be attributed to the alkali-induced removal of amorphous components such as hemicellulose and portions of lignin. The removal of these non-cellulosic materials not only exposes the underlying cellulose microfibrils but also creates a more porous and textured surface, thereby increasing the fiber's effective surface area. The development of a rougher and more fibrillar structure has important implications for the performance of these fibers in composite applications. The increased surface roughness and porosity enhance the physical and chemical interactions between the fibers and the surrounding rubber matrix. This improved interfacial adhesion is critical for achieving superior load transfer and distribution within the composite, ultimately contributing to enhanced mechanical properties such as tensile strength, modulus, and tear resistance. These findings highlight the pivotal role of alkali treatment in tailoring the surface characteristics of hemp fibers for optimized performance in rubber-based composite materials. By modifying the fiber morphology to achieve greater compatibility with the rubber matrix, alkali treatment enables the development of composites with improved durability and mechanical robustness, making them suitable for a wide range of engineering and industrial applications. The incorporation of hemp fibers into rubber compounds resulted in a marked improvement in the mechanical properties of the materials. Tensile strength increased by up to 70%, and tear resistance showed substantial enhancements, with NBR exhibiting a remarkable 98% increase in tear resistance with untreated fibers and a 211% increase with alkali-treated fibers. Similarly, ENR compounds demonstrated a 29% improvement in tear resistance with alkali-treated fibers. These results underscore the significant potential of hemp fibers, especially those treated with alkali, to reinforce rubber materials and enhance their performance. The improvements in mechanical properties can be attributed to the increased fiber-matrix interaction facilitated by the surface modification of the fibers, as well as the enhanced crystallinity observed in the treated fibers. Viscosity measurements of the rubber compounds revealed complex behavior. For ENR and NBR compounds, the incorporation of 10% NaOH-treated fibers led to a reduction in viscosity compared to those containing untreated fibers. Interestingly, for NR compounds, an increase in viscosity was observed when NaOH-treated fibers were used, likely due to the interaction between the treated fibers and the rubber matrix, which could increase the network density and restrict flow. This complex relationship between fiber content, modification, and viscosity highlights the importance of optimizing fiber treatment and content to achieve the desired balance of rheological properties in rubber compounds. Hardness evaluations demonstrated a decrease in hardness for alkali-treated NR and NBR compounds compared to their untreated counterparts, while ENR compounds exhibited comparable hardness values regardless of fiber treatment. The reduction in hardness for alkali-treated fibers can be explained by the softening effect of NaOH treatment, which decreases the stiffness of the fibers and, consequently, the overall stiffness of the rubber compounds. The ability to tailor the hardness and other properties of rubber compounds using modified natural fibers presents a unique opportunity for the development of more sustainable and functional materials. Despite the significant improvements in mechanical performance, it was noted that deformation behavior deteriorated after 10% strain, suggesting that further optimization of fiber-matrix interactions is necessary. This could involve additional surface treatments or the use of coupling agents to enhance adhesion between the fibers and the rubber matrix. Such improvements would further optimize the performance of fiber-reinforced rubber composites for practical applications. In conclusion, this study demonstrates the considerable potential of alkali-modified natural hemp fibers as reinforcements in rubber compounds. The modification of the fibers enhances their mechanical properties, crystallinity, and surface characteristics, thereby improving the overall performance of the resulting composite materials. These findings offer valuable insights into the development of sustainable, high-performance rubber materials and pave the way for future research focused on optimizing fiber-matrix interactions and exploring additional surface treatments for further enhancement of composite properties. The integration of natural fibers, particularly when chemically modified, holds promise for advancing the field of sustainable materials and the development of more eco-friendly rubber products.