Development and characterization of EPDM rubber composites filled with natural expanded perlite and lignocellulose

In modern industrial production, enhancing material performance while ensuring environmental sustainability has become a critical area of focus in materials science and engineering. Rubber composites, recognized for their remarkable elasticity, flexibility, and durability, have found widespread applications across numerous industries, including automotive, construction, aerospace, and consumer goods. However, the inherent properties of rubber often require enhancement to meet specific performance demands, and the associated processing challenges necessitate innovative solutions. To address these challenges, researchers and manufacturers have increasingly incorporated fillers into rubber matrices. These fillers not only enhance material properties such as strength, stiffness, and thermal stability but also reduce production costs and improve processing efficiency. Each filler introduced into rubber imparts distinct characteristics, profoundly influencing the final properties of the composite. The selection of the type and amount of filler is crucial in tailoring rubber compounds for particular applications. Among the conventional fillers, carbon black has remained the most widely used due to its cost-effectiveness and exceptional reinforcing capabilities. Carbon black contributes to improved tensile strength, abrasion resistance, and elasticity in rubber composites. Despite its advantages, the production of carbon black involves significant environmental challenges, as it relies on the partial combustion of heavy hydrocarbons, resulting in a substantial carbon footprint. Alternative fillers, such as inorganic minerals, have emerged as viable substitutes. Precipitated silica, kaolin, titanium dioxide, and calcium carbonate are some of the most prominent inorganic fillers used to enhance the fundamental properties of rubber. Silica, in particular, has gained attention for its ability to provide an excellent balance of tear strength, modulus, wear resistance, aging resistance, and fatigue performance. Hybrid filler systems, such as combinations of carbon black and silica, have been shown to further enhance the mechanical properties of rubber composites, offering a versatile solution for demanding applications. Environmental concerns and global sustainability goals have catalyzed the search for eco-friendly alternatives to conventional fillers. The production of carbon black and silica not only involves energy-intensive processes but also generates hazardous by-products, posing environmental and economic challenges. The limitations associated with these materials highlight the critical need for sustainable fillers that reduce greenhouse gas emissions, minimize waste, and align with the principles of a circular economy. This study investigates the potential of natural and sustainable fillers, specifically expanded perlite (EP) and sunflower seed husks (SSH), in enhancing the performance of ethylene-propylene-diene monomer (EPDM) rubber matrices. Expanded perlite, a volcanic glass rich in silica, is abundant, cost-effective, and environmentally friendly. As a mineral filler, EP offers the potential to improve thermal and mechanical properties in rubber composites. Similarly, sunflower seed husks, an agricultural by-product, represent a renewable and biodegradable filler option. Rich in biopolymers such as cellulose, hemicellulose, and lignin, SSH provides a sustainable solution for waste management and eco-friendly material production. To explore the integration of EP and SSH into EPDM rubber composites, a two-stage production process was employed. Initially, the rubber compounds were prepared using an internal mixer, ensuring homogeneous dispersion of the fillers within the EPDM matrix. This was followed by milling on a two-roll open mill to achieve further uniformity and proper distribution of the additives. The compounds were then vulcanized under high temperature and pressure in a hydraulic press, yielding cured composites with various formulations of EP and SSH fillers. The impact of these fillers on the thermal, mechanical, and rheological properties of the composites was evaluated using a range of advanced characterization techniques. Thermal properties were assessed through thermogravimetric analysis (TGA), thermal aging tests, and thermal conductivity measurements, while surface morphology was examined using field emission-scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Rheological behavior and filler-matrix interactions were studied using a rubber process analyzer (RPA), with a particular focus on the Payne effect to evaluate filler-filler interactions. Fourier transform infrared (FTIR) spectroscopy was used to analyze filler-polymer interactions, and contact angle measurements were conducted to assess changes in the surface hydrophobicity of the composites. Additionally, crosslink density, a key parameter influencing the mechanical properties, was determined using the Lee-Pawlowski approach, providing insights into the network structure of the composites. The incorporation of expanded perlite significantly improved the thermal and mechanical properties of EPDM composites. Composites containing 15 phr (parts per hundred rubber) of EP demonstrated a 21% reduction in thermal conductivity compared to control samples, indicating enhanced insulation properties. Additionally, crosslink density increased by 20%, while hardness improved by 12%. However, a slight reduction in tensile strength and elongation at break was observed, which was attributed to the polarity mismatch between EP particles and the EPDM matrix. To address this limitation, surface modification techniques were employed to enhance the compatibility between EP and the EPDM matrix. Silane coupling agents, such as bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPT), bis(trimethoxysilyl)-polydimethylsiloxane (PDMS), and trimethoxysilylpropylmethacrylate (TMSPM), were utilized to modify the surface of EP. Among these, PDMS-modified EP produced the most significant improvements, with elongation at break increasing by 54% and tensile strength by 82%, demonstrating the effectiveness of surface treatments in optimizing filler performance. The addition of sunflower seed husks as a green filler also contributed positively to the properties of EPDM composites. When incorporated in varying amounts (e.g., 2.5, 5, and 10 phr), SSH enhanced mechanical properties such as tensile strength, elongation at break, and hardness. For instance, composites containing 5 phr SSH exhibited a 9% increase in elongation at break, while those with 10 phr SSH showed an 11% improvement in tensile strength. However, at higher filler levels, increased filler-filler interactions led to clustering, as indicated by the Payne effect, resulting in a reduction in flexibility. Thermal stability improvements were notable, with composites containing 5 phr SSH exhibiting 15.98% less weight loss at 400°C compared to SSH-free samples. This highlights the ability of SSH to enhance resistance to thermal degradation, making the composites suitable for high-temperature applications. A synergistic effect was observed when EP and SSH were used together in EPDM composites. The combined use of these fillers enhanced crosslink density and improved the overall network structure of the composites. This dual-filler approach demonstrates the versatility and compatibility of natural fillers with EPDM matrices, offering a tailored solution for optimizing the performance of rubber materials. By integrating natural fillers like EP and SSH, the study addresses the pressing need for sustainable materials in the rubber industry. These fillers not only provide environmental benefits but also improve the thermal, mechanical, and rheological properties of rubber composites, aligning with global efforts to mitigate climate change and reduce industrial waste. This study highlights the potential of expanded perlite and sunflower seed husks as sustainable, cost-effective, and high-performance fillers for EPDM rubber composites. The findings emphasize the environmental advantages of utilizing these renewable resources, promoting circular economy principles in material design. The adoption of these eco-friendly fillers reduces reliance on petroleum-based materials, paving the way for greener alternatives in the rubber industry. Additionally, incorporating natural fillers into rubber formulations fosters a shift towards more sustainable practices in material production, furthering efforts to minimize the environmental impact of manufacturing processes. The experimental results demonstrate that EPDM rubber composites incorporating EP and SSH exhibit remarkable potential for diverse applications across sectors such as automotive, construction, aerospace, and consumer goods. By optimizing filler content and employing surface modification techniques, the research provides a robust framework for advancing sustainable material science while addressing the environmental challenges of traditional rubber production. Future studies should further explore the integration of other renewable fillers and innovative surface treatments to enhance material properties, fostering the development of high-performance, eco-friendly rubber composites that align with global sustainability goals.