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. 2022 Mar 1;7(10):8364-8376.
doi: 10.1021/acsomega.1c05848. eCollection 2022 Mar 15.

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

Ekaroek Phumnok  1 , Parinya Khongprom  1   2 , Sukritthira Ratanawilai  1
Affiliations

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

Ekaroek Phumnok et al. ACS Omega. .

Abstract

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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Effects of the silica content (%) and the ball milling time (h) on the silica particle size (mm).
Figure 2
Figure 2
Particle size distributions of silica before and after milling.
Figure 3
Figure 3
Effects of the silica content (%) on the silica dispersion viscosity (cps).
Figure 4
Figure 4
Morphology of 20 phr wet-process (a), 20 phr dry-process (b), 35 phr wet-process (c), 35 phr dry-process (d), 50 phr wet-process (e), 50 phr dry-process (f), 65 phr wet-process (g), and 65 phr dry-process (h) silica-filled NR.
Figure 5
Figure 5
Effect of the silica content on the scorch time of the wet- and dry-process silica-filled NR.
Figure 6
Figure 6
Effect of the silica content on the curing time of the wet- and dry-process silica-filled NR.
Figure 7
Figure 7
Effect of the silica content on the hardness of the wet- and dry-process silica-filled NR.
Figure 8
Figure 8
Effect of the silica content on the stress at 300% deformation of the wet- and dry-process silica-filled NR.
Figure 9
Figure 9
Effect of the silica content on the tensile strength of the wet- and dry-process silica-filled NR.
Figure 10
Figure 10
Effect of the silica content on the elongation at break of the wet- and dry-process silica-filled NR.
Figure 11
Figure 11
Effect of the silica content on storage modulus (a), loss modulus (b), and tan δ (c) of wet- and dry-process silica-filled NR.
See this image and copyright information in PMC

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