Research Progress of Natural Rubber Wet Mixing Technology

The performance of natural rubber (NR), a naturally occurring and sustainable material, can be greatly enhanced by adding different fillers to the NR matrix. The homogeneous dispersion of fillers in the NR matrix is a key factor in their ability to reinforce. As a novel method, wet mixing technology may effectively provide good filler dispersion in the NR matrix while overcoming the drawbacks of conventional dry mixing. This study examines the literature on wet mixing fillers, such as graphene, carbon nanotubes, silica, carbon black, and others, to prepare natural rubber composites. It also focuses on the wet preparation techniques and key characteristics of these fillers. Furthermore, the mechanism of filler reinforcement is also examined. To give guidance for the future development of wet mixing technology, this study also highlights the shortcomings of the current system and the urgent need to address them.

CFD simulation of silica dispersion/natural rubber latex mixing for high silica content rubber composite production

High silica contents rubber composites are favored in the green tire industry for their ability to reduce rolling resistance. However, achieving effective silica dispersion in natural rubber, particularly at high silica content, poses a challenge. In addition, the choice of impeller configuration significantly influences mixing performance, especially in commercial production, which requires large mixing tanks. Therefore, understanding the scaling-up process for this mixing system is essential. This research aims to investigate the mixing of silica dispersion in natural latex, specifically focusing on a high silica content regime. The flow characteristics of each liquid phase were simulated by employing the Computational Fluid Dynamics (CFD) approach, with a two-fluid model serving as the model based. Analyses were conducted on two variants of stirred tank reactors including four baffles and flat bottoms. Four configurations of Rushton turbine impellers were considered: four 90° blades (RT4-90), four 45° blades (RT4-45), six 90° blades (RT6-90), and six 45° blades (RT6-45). The simulations revealed that the 90° blade promoted the radial flow, while the 45° blades enhanced axial flow, through the process of diverting a significant proportion of the fluid above impeller, this regime effectively increases the liquid's velocity. Increasing the number of blades led to a more homogeneous velocity profile within the impeller region. Additionally, higher fluid velocity was observed in a larger mixing tank. In a smaller tank, the impact of impeller design (number and angle of the impeller) on mixing time was less pronounced. However, the mixing time decreased with the increasing blade number in a larger tank. In addition, the 45° blade angle tends to decrease the mixing time. The optimum design is the Ruston turbine with six blades set at a 45° angle. Furthermore, the upscaling criteria that were proposed by Norwood and Metzner were used into this inquiry. The suggested scaling criterion was consistently applied to the mixing of high silica natural latex, with no deviation exceeding 10%.

Effects of the surface chemical groups of cellulose nanocrystals on the vulcanization and mechanical properties of natural rubber/cellulose nanocrystals nanocomposites

Cellulose nanocrystals (CNCs), as the promising reinforcing fillers in the rubber industry, their surface chemical groups have vital effects on the vulcanization kinetics, cross-linking densities, and mechanical properties of rubber composites. Herein, CNCs with acidic carboxyl (CCA) and alkaline amino groups (CCP) were produced by modifying the sulfonic CNCs (CCS) in environment-friendly ways. Studies found the CCS and CCA with acid groups have obvious inhibiting effects on the vulcanization of natural rubber (NR), while CCP with alkaline amino groups accelerates the vulcanization of NR. Differential scanning calorimeter, Fourier transform infrared spectroscopy, and Electron paramagnetic resonance, etc. were performed to clarify the effecting mechanisms of CNCs surface groups on NR vulcanization. It was found that NR/CCS and NR/CCA nanocomposites vulcanize through radical reactions, and the surface acidic groups of CCS and CCA, i.e., hydroxyl, sulfonate, and carboxyl groups inactivate the sulfur radicals generated during vulcanization and depress the vulcanization activity. The amino groups of the polyethyleneimine of CCP promote the ring opening of sulfur (S₈) or the breaking of polysulfide bonds connected to NR molecular chains to form sulfur anion with a strong nucleophilic ability, which leads to the cross-linking of NR/CCP reacts via ionic reaction mainly. The vulcanization rate and cross-linking density of NR/CCP are improved by the ionic reaction. And benefiting from the higher cross-linking density and the reinforcement of CCP, NR/CCP had the best physical and mechanical properties. Our work elucidates the mechanism of the surface chemical groups of CNCs affecting NR vulcanization and may provide ideas for the preparation of high-performance rubber composites reinforced by CNCs.

Preparation of Natural Rubber Composites with High Silica Contents Using a Wet Mixing Process

A wet mixing process is proposed for filled rubber composites with a high silica loading to overcome the drawbacks of high energy consumption and workplace contamination of the conventional dry mixing process. Ball milling was adopted for preparing the silica dispersion because it has a simple structure, is easy to operate, and is a low-cost process that can be easily scaled up for industrial production. The response surface methodology was used to optimize the making of the silica dispersion. The optimum conditions for a well-dispersed silica suspension with the smallest silica particle size of 4.9 mm were an about 22% silica content and 62 h of ball milling. The effects of dry and wet mixing methods on the properties of silica-filled rubber composites were investigated in a broad range of silica levels from low to high loadings. The mixing method choice had little impact on the properties of rubber composites with low silica loadings. The silica-filled rubber demonstrated in this study, however, shows superior characteristics over the rubber composite prepared with conventional dry mixing, particularly with high silica loadings. When compared to silica-filled natural rubbers prepared by dry mixing (dry silica rubber, DSR), the wet mixing (for WSR) produced smaller silica aggregates with better dispersion. Due to the shorter heat history, the WSR exhibits superior curing characteristics such as a longer scorch time (2.2-3.3 min for WSR and 1.0-2.1 min for DSR) and curing time (4.1-4.5 min for WSR and 2.2-3.1 min for DSR). Additionally, the WSR has superior mechanical properties (hardness, modulus, tensile strength, and especially the elongation at break (420-680% for WSR and 360-620% DSR)) over the DSR. The rolling resistance of WSR is lower than that of DSR. However, the reversed trend on the wet skid resistance is observed.