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Bound rubber is formed due to the adsorption of rubber by the surface of carbon black. Therefore, any factor that affects this adsorption will influence the amount of bound rubber produced. The main influencing factors are as follows:
Currently, both Chinese rubber parts factories and Vietnamese rubber parts factories attach great importance to the control of bound rubber formation in the production of rubber products, as it directly determines the mechanical properties and durability of the products.
1. Influence of Carbon Black Specific Surface Area
The amount of bound rubber increases almost proportionally to the specific surface area of carbon black. As the specific surface area of carbon black increases, the adsorption surface area expands, leading to a higher adsorption capacity, i.e., an increase in bound rubber.
In production practice, Chinese rubber parts factories often select carbon black types with different particle sizes and specific surface areas according to product applications to achieve the optimal binding effect; Vietnamese rubber parts factories also optimize the carbon black ratio based on customers' requirements for hardness and wear resistance when exporting seals or other products.
2. Influence of Mixing and Sheet Passing Times
Natural rubber is a substance prone to oxidative degradation. The free segments of macromolecular chains with only one or two adsorption points may exist outside the glassy layer. These rubber molecular chains may undergo mechanical or oxidative chain scission during sheet passing, resulting in a decrease in the amount of bound rubber.
Comparative experiments have shown that for chloroprene rubber, styrene-butadiene rubber, and butyl rubber filled with 50 parts of carbon black, the variation trends with the number of sheet passes are different. The bound rubber content of chloroprene rubber and styrene-butadiene rubber increases with the increase in the number of sheet passes and tends to stabilize after approximately 30 passes. In contrast, the bound rubber content of butyl rubber decreases initially and stabilizes after about 30 passes. The reason for the decrease in butyl rubber is similar to that of natural rubber.
Many Vietnamese rubber parts factories, when producing products such as seals and buffer pads, precisely control the number of sheet passes to improve the uniformity of bound rubber, thereby ensuring the tensile strength and fatigue resistance of the finished products.
3. Influence of Temperature
When the mixed rubber samples are kept at different temperatures for a certain period before measuring the bound rubber content, it is found that as the treatment temperature (i.e., adsorption temperature) increases, the amount of bound rubber also increases. This is consistent with the general adsorption law.
On the contrary, the higher the mixing temperature, the less bound rubber is formed. This may be because as the temperature rises, the rubber becomes softer and less likely to be broken by mechanical force to form free radicals. Meanwhile, carbon black is also less likely to form free radicals in a soft environment. Therefore, less bound rubber is formed during high-temperature mixing compared to low-temperature mixing.
In this regard, Chinese rubber parts factories often use automatic temperature-controlled internal mixing equipment to precisely control the mixing temperature, ensuring the stable formation of bound rubber and improving the batch consistency of products.
4. Influence of Rubber Properties
The amount of bound rubber is related to the unsaturation degree and molecular weight of rubber. Rubbers with high unsaturation and large molecular weight produce more bound rubber.
Therefore, during the formula development stage, Vietnamese rubber parts factories prioritize the selection of rubber materials with high molecular weight and controllable unsaturation to enhance the tensile strength and wear resistance of products.
5. Influence of Aging Time
Experiments have shown that after mixing, the amount of bound rubber increases with the extension of storage time and tends to stabilize after approximately one week.
This is mainly because the adsorption rate of solid carbon black on the macromolecules of solid rubber is relatively slow, and the chemical adsorption part requires more time.
Chinese rubber parts factories usually establish standardized aging processes to ensure the full stabilization of the bound rubber structure, so that the products can reach the optimal performance state when leaving the factory.
Bound rubber is formed due to the adsorption of rubber by the surface of carbon black.
