What is Rubber Mastication?
Mastication is an important step in the rubber compounding process. Before adding the compounding ingredients, rubber mastication is required.
Mastication is the process of softening or breaking down the molecular chains of the rubber particles by the means of mechanical shear such as grinding, mixing etc. Through rubber mastication, the viscosity of rubber is reduced to a suitable level. Natural rubber, which is obtained from rubber trees, have a very high molecular weight (MW), thus their molecular weight cannot be controlled like synthetic rubbers.
High Molecular weight means high viscosity, which renders the rubber hard, thus making it almost impossible to add any other powder or liquid ingredients to it at its original viscosity. Natural rubber requires controlled reduction in molecular weight, this molecular weight reduction process is the mastication process.
What does the Mastication Process do?
• Softens the rubber
• Ensure uniform mixing of compounding ingredients
• Improves flexibility and elasticity
• Improves physical properties such as tensile tear & abrasion resistance
• Better adhesion
How rubber mastication helps in adhesive manufacturing?
Mastication of natural rubber promotes intermolecular bonding within the rubber compound, leading to improved adhesion. It adheres to all surfaces and can be made for strong bonding applications. This is because masticated rubber has:
• High wettability of masticated rubber
• High surface contact
• High surface energy which creates more intermolecular interaction
• Increased tackiness
What kinds of Machinery is required for Rubber Mastication?
1) Two Roll Mill:
Two roll mills are widely used for rubber mastication as well as rubber mixing. The machine consists of two horizontally placed, counter-rotating rolls that are made of high-quality steel with smooth or grooved surface, one is the drive roll and the other is the front roll.
The drive roll is powered and rotates at a fixed speed, while the front roll rotates freely. The speed ratio between the two rolls can be adjusted to control the processing. The shearing action between the rolls provides mastication of rubber chains and provide uniform distribution of mixing.
Two roll mills are equipped with a heating and cooling system. Heating can soften the rubber for easier mastication, while cooling prevents excessive heat build-up and maintains the desired properties of rubber. Two Roll mills offer good control over the mastication and mixing process, allowing manufacturers to achieve desired properties in final rubber compounds.
2. Internal mixers:
Internal mixer, also known as the Banbury mixer, are enclosed mixers equipped with two rotating intermeshing rotors. Internal mixers are mainly used for plasticizing and mixing rubber intermittently in a closed state with adjustable temperature and pressure. It is mainly composed of an internal mixing chamber, and rotors. As the rotors rotate, they shear and knead the rubber, promoting mastication and ensuring through mixing of the ingredients.
An internal mixer is a kind of high-strength intermittent mixing equipment, which has been developed based on an open mixer. This mixer has a series of features superior to the open mill in the rubber mixing process, like larger mixing capacity, shorter time required, and higher production efficiency. It can better overcome dust flying, reduce the loss of compounding agents, improve product quality and work surroundings, safe and convenient operation, and reduce labour intensity.
3.Extruders:
Rubber extruders are commonly used for continuous mastication and shaping of rubber materials.
Extruders provide continuous processing, mixing and the ability to process large volumes of rubber compounds. The machine consists of a heated barrel and rotating screw inside. The screw is designed with various zones: feeding, compression, melting and metering sections.
Rubber compounds are added through the hopper, and the feeding system ensures a consistent supply of rubber compounds into the extruder for continuous processing. As the screw rotates, shear, compression, and heat generation occur. The die at the end of the extruder determines the final shape of the product.
Extruders are equipped with advanced control and automation system.
4.Calenders:
Calenders are machines used in the rubber industry for the processing and shaping of rubber compounds.
Calenders consist of 2 or more rollers arranged in line configuration.
Rubber material is fed into the nip between the rollers, and as it passes through, it undergoes mastication and is formed into a continuous sheet or a specific profile.
Calenders offers advantages such as continuous processing, excellent thickness control and the ability to achieve desired surface finishes. Single-calendering, multi-calendering, inverted L-calendering, offset calendering, and super calendering are the some of the calendering techniques.
5. Kneaders:
Dispersion Kneaders are specially designed for mixing a variety of materials ranging from low viscosity adhesive to high viscosity polymers. These versatile machines can be used in both rubber as well as plastic industries.
Dispersion kneaders perfectly control mixing and blending of raw material to form uniform compound mixtures ensuring better dispersion. Kneaders utilize a unique mixing mechanism that involves the combination of shear, compression, and elongation forces.
The Kneaders consist of a closed mixing chamber with a cylindrical or rectangular shape. The chamber is equipped with two counter-rotating, on-intermeshing rotors or blades. The rotors are of either sigma or z shaped. The rotors in a kneader rotate in opposite directions and create shearing, compression and promote mixing, mastication, and dispersion of additives.
In short, mastication is a preliminary stage to processing the raw rubber. And above mechanical equipment and additives (Processing aids, Plasticizers, softeners etc.) helps for mastication and thus improves the plasticity and reduces the viscosity.
