Natural Rubber Mixing Process

Rubber mixing uses the mechanical force of the rubber mixing machine to disperse various ingredients in the rubber evenly to form a rubber-based medium or rubber and specific materials that are compatible with it. It is a multi-phase colloidal dispersion system using a mixture of compounding components (compounding agents, other polymers) as the medium and compounding agents incompatible with rubber (such as powder fillers, zinc oxide, pigments, etc.) as the dispersed phase. The technical requirements for the mixing process are as follows: the compounding agents should be dispersed evenly so that the compounding agents, significantly reinforcing compounding agents such as carbon black, can achieve the best dispersion to ensure consistent rubber compound performance. The rubber material obtained after mixing is called "mixed rubber," and its quality significantly impacts further processing and product quality. Below, we mainly introduce the mixing of natural rubber, which is closely related to our airbag and fender products.

Natural rubber is the primary type of rubber used in raw rubber mastication. Good results can be obtained by using open mixers and internal mixers. When using an open mill for mastication, low-temperature (40-50°C) thin-pass (roller distance 0.5-1 mm) and segmented mastication methods are usually used to achieve the best results. When using an internal mixer to mold, the temperature should be below 155°C, and the time should be about 13 minutes. As the mastication time increases, the plasticity of the masticated rubber increases. However, do not over-refine; otherwise, the plasticity will become too high, and the physical and mechanical properties will decrease. In natural rubber mastication, accelerator M is often added as a peptizing agent to improve the mastication effect. Accelerator M is suitable for both open mill and internal mixer plasticizing. After natural rubber is masticated, to relax the rubber molecular chains (commonly known as fatigue recovery) and achieve uniform plasticity, it must be parked for a certain period (4 to 8 hours) before it can be used in the following process.

The main varieties of natural rubber currently used in China include domestic smoked rubber and standard rubber, imported smoked rubber, Malaysian standard rubber, etc. Since the initial Mooney viscosity of the rubber mentioned above differs, the required molding time is different to obtain the same plasticity. The order of the plasticizing time in order of length is imported tobacco sheet rubber>domestic tobacco sheet rubber>domestic standard rubber>Malaysian standard rubber. Constant viscosity and low viscosity standard Malaysian rubber, oil-extended natural rubber, tire rubber, and easy-to-operate rubber have low initial Mooney viscosity (generally less than 65) and can be mixed directly without plasticizing.

Natural rubber (NR) has comprehensive physical and mechanical properties, is highly elastic at room temperature, and has a rebound rate of over 50% to 85%. The natural rubber also has high mechanical strength, good flex fatigue resistance, slight hysteresis loss, low heat generation under multiple deformations, high tear strength, good wear resistance, and cold resistance, and good air tightness, waterproof, electrical insulation, and thermal insulation. Natural rubber has good alkali resistance and good resistance to general acids but is not resistant to concentrated and intense acids. Natural rubber also has good processing properties, mixing properties, good roll wrapping properties, raw rubber strength, and initial strength. It has high viscosity, good plasticity, usability, and wettability for compounding agents. The plasticity of the rubber increases quickly, and the heat generation is low. Therefore, it has good wettability to the compounding agent, the powder is devoured, and the dispersion is relatively easy; the mixing time is short, and the mixing operation process is easy to master. However, natural rubber is sensitive to mixing time. If the mixing time is too short, the surface of the mixed rubber will appear granular, making calendering and extrusion difficult; if the mixing time is too long, it will lead to over-refining. When mixing in an open mill, the roller temperature is between 45 and 55°C, and the front roller is five °C higher than the rear roller. Internal mixers often use a one-stage mixing method, and the debonding temperature is below 120°C.

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