Rubber Mixing Process

Comparison Between Internal Mixers and Open Mills

Compared with open mills, internal mixers offer shorter mixing times, higher production efficiency, and easier operation during rubber compounding. They effectively reduce dust dispersion, significantly minimize the loss of compounding ingredients, and lower labor intensity. With continuous advancements in manufacturing processes, internal mixer technology and equipment have been continuously developed and improved.

The mixing process involves uniformly dispersing various compounding ingredients into rubber to form a multiphase colloidal dispersion system. In this system, rubber—or a compatible rubber–additive mixture—serves as the continuous medium, while incompatible additives act as the dispersed phase.

Fundamentals of Rubber Mixing

Rubber mixing is the process of using rubber processing machines to blend raw rubber or plasticized rubber with various compounding ingredients to produce a homogeneous rubber compound. This process uniformly disperses additives into the rubber matrix, forming a multiphase colloidal system in which particulate additives are dispersed phases and raw rubber is the continuous phase. Rubber mixing is the most critical process in rubber manufacturing.

1. Mixing Technology in Rubber Processing Using Internal Mixers

The process of uniformly dispersing compounding ingredients into raw or plasticized rubber using rubber processing machinery is known as mixing, and the resulting material is called a rubber compound. Mixing is the most important production process in rubber processing.

To improve physical and mechanical properties, enhance processing performance, and reduce production costs, various compounding ingredients—such as fillers, reinforcing agents, accelerators, vulcanizing agents, antioxidants, and scorch inhibitors—are added to rubber. These additives may be solid or liquid, and must be uniformly dispersed to ensure consistent compound properties.

Technical Requirements for Mixing

The primary technical requirement of mixing is uniform dispersion of compounding ingredients, especially reinforcing fillers such as carbon black, to ensure consistent compound performance. Poor mixing can result in defects such as scorch, blooming, poor calendering or extrusion behavior, and degraded product performance.

Stages of the Mixing Process

Rubber mixing consists of four stages:

  1. Incorporation (Wet-out / Powder Intake)
    Raw rubber undergoes shear and elongation, allowing additives to contact and enter the rubber matrix.

  2. Dispersion (Microscopic Dispersion)
    Additives are broken down into fine particles under mechanical force, increasing contact surface area.

  3. Mixing (Macroscopic Dispersion)
    Additives are evenly distributed without changing particle size.

  4. Plasticization
    Rubber becomes more workable through molecular breakdown.

Mixing Methods and Process Flow

Mixing methods are generally divided into open mill mixing and internal mixer mixing, both of which are batch processes and widely used in industry.

Typical mixing process flow:

  • Additive pre-treatment

  • Weighing of rubber and additives

  • Mixing

  • Quality inspection

2. Wetting and Dispersion Behavior of Rubber Compounding Ingredients

Due to the high viscosity of raw rubber, strong mechanical force is required to mix and disperse additives uniformly. Additives can be classified based on surface properties into:

  • Hydrophilic additives: carbonates, clay, zinc oxide, titanium dioxide, silica, etc.

  • Hydrophobic additives: carbon black and similar materials

Hydrophilic additives are difficult to wet with rubber, while hydrophobic additives are more easily wetted. Surface modification and the use of surfactants can improve dispersion. Surfactants act as intermediaries between rubber and additives, enhancing dispersion and stabilizing particle distribution.

The use of dispersants shortens mixing time, reduces energy consumption, improves dispersion uniformity, and significantly enhances product appearance and mold release performance.

Mixing Characteristics of Different Rubbers

  • Butyl Rubber (IIR): Difficult dispersion; requires higher mixing temperatures and multi-stage mixing.

  • EPDM: Good dispersion, low scorch tendency, but poor self-adhesion.

  • CSM (Chlorosulfonated Polyethylene): Thermoplastic behavior; stable and resistant to over-mixing.

  • Natural Rubber (NR): Excellent mixing performance, fast wetting and dispersion, but sensitive to over-mixing.

Carbon Black Dispersion Mechanism

Carbon black dispersion involves rubber penetrating carbon black agglomerates, forming rubber–carbon black clusters. These clusters are broken down by sufficient shear or tensile force until full dispersion is achieved. Adequate rubber viscosity is essential to overcome cohesive forces within carbon black agglomerates.

3. Preparation Before Rubber Mixing

Preparation includes:

  • Raw material inspection

  • Pre-treatment of additives

  • Accurate weighing

Additive pre-treatment may involve crushing, drying, screening, melting, filtering, or preparing masterbatches. Improper preparation can lead to poor dispersion, defects, and equipment damage.

Accurate weighing is critical and can be performed manually or automatically depending on production scale.

4. Common Equipment Used in Rubber Mixing

  • Open Mills: Early-stage equipment with exposed rollers; flexible but labor-intensive and less safe.

  • Internal Mixers: Enclosed systems offering high efficiency, safety, and automation, though with higher temperature control challenges.

5. Rubber Mixing Processes and Methods

Open Mill Mixing Stages:

  1. Rubber softening and heating

  2. Addition of compounding ingredients

  3. Homogenization and sheet-off

Internal Mixer Mixing Methods:

  • One-stage mixing

  • Two-stage mixing

  • Feed-assisted mixing

  • Reverse mixing

Two-stage mixing is preferred for compounds with high synthetic rubber content to avoid excessive heat buildup and premature vulcanization.

6. Characteristics of Rubber Mixing Processes

Mixing quality directly affects physical properties, Mooney viscosity, and vulcanization behavior. Temperature, time, and pressure control are critical.

  • High-temperature mixing: 130–150°C, no accelerators added

  • Medium-temperature mixing: 110–135°C, most commonly used

  • Low-temperature mixing: Used for easily mixed compounds

Proper control ensures optimal compound performance and stable downstream processing.