The word "rubber" comes from the indigenous word cau-uchu, meaning "weeping tree." Natural rubber is obtained from the latex that flows out when a rubber tree is tapped, which is then coagulated and dried.

Rubber is a basic raw material in the rubber industry and is widely used in the manufacture of tires, hoses, belts, cables, and various other rubber products.

When the bark of a rubber tree is cut, it secretes a large amount of sap containing rubber latex.

The rubber tree is native to South America, but after artificial transplantation, large numbers of rubber trees have also been grown in Southeast Asia. In fact, Asia has become an important source of rubber.

The basic production process of rubber products includes six fundamental steps: plasticating, mixing, calendering, extrusion, forming, and vulcanization.

The rubber processing technology mainly resolves the contradiction between plasticity and elasticity. Through various processing methods, elastic rubber is transformed into plasticated rubber with plasticity. Then, various compounding agents are added to make semi-finished products. Finally, through vulcanization, the plastic semi-finished products are transformed into rubber products with high elasticity and excellent physical and mechanical properties.

Natural rubber has high mechanical strength and good resilience. It is widely used in daily production, healthcare, transportation, industry, agriculture, scientific experiments, national defense, and other fields. When producing various products with natural rubber, sufficient plasticating is conducive to the incorporation and uniform dispersion of compounding agents, creating better conditions for subsequent processes such as mixing, calendering, extrusion, and forming. Compared with synthetic rubber, natural rubber is easier to plasticate. Why is this? And how should natural rubber be plasticated?

1. Reasons why natural rubber is easy to plasticate

The main component of natural rubber is cis-1,4-polyisoprene. The methyl conjugated effect in the isoprene structure reduces the bond energy of the molecular chain. Natural rubber with high molecular weight and a wide molecular weight distribution is easy to crystallize under elongation and is prone to chain scission under mechanical shear stress. The methyl group on the double bond of natural rubber activates the hydrogen atom on the α-methylene group, making it easy to undergo oxidative chain scission and degradation reactions. The free radicals generated after being cut by mechanical force have high stability, facilitating mechanical chain scission and oxidative cracking at high temperatures.

In summary, natural rubber is relatively easy to plasticate. Generally, different grades of natural rubber have different plasticating characteristics. Imported ribbed smoked sheets require longer plasticating time, followed by domestic ribbed smoked sheets, domestic standard rubber, and Malaysian standard rubber. Malaysian standard rubber grade 2LLYY1113-MXF requires the shortest plasticating time. Natural rubber with an initial Mooney viscosity of less than 65 usually does not need plasticating and can be directly mixed, such as oil-extended natural rubber and low-viscosity Malaysian rubber.

2. Methods of plasticating natural rubber

The plasticating process for natural rubber can be carried out in an open mill or in an internal mixer. When plasticating natural rubber in an open mill, the low-temperature, thin-pass method and the segmented plasticating method are generally used.

3. Conditions for plasticating natural rubber

When using the low-temperature, thin-pass method in an open mill, the plasticating temperature should be controlled between 40-50°C, and the roll gap between 0.5-1 mm. When plasticating natural rubber in an internal mixer, the rubber is typically plasticated at a temperature below 155°C for 10-13 minutes. The specific plasticating time and temperature should be determined based on the desired plasticity, avoiding excessive plasticity that would degrade the physical and mechanical properties of the vulcanized natural rubber.

To improve the plasticating effect of natural rubber, manufacturers can add a small amount of peptizing agent. For both open mill and internal mixer plasticating, accelerator M can be used as a peptizing agent. After plasticating, natural rubber needs to be left to rest for 4-8 hours to allow the rubber molecular chains to relax and achieve uniform plasticity. Only after sufficient resting can the natural rubber be used in the next process. In actual production, many natural rubber product manufacturers use reclaimed rubber to reduce costs; in such cases, it is necessary to ensure that the natural rubber is adequately plasticated.