Key Stages of Rubber Processing and Their Industrial Importance
Rubber Processing Comprises Four Basic Steps:
Mastication, the first step, occurs when the elastomer is removed, and the molecules are broken down to provide a more effortless flow.
Mixing, the second step, is usually carried out immediately after the mastication process when additives are incorporated.
Shaping, the third step, occurs for the dense mass obtained, for example, either by extrusion or moulding.
Curing, the final step, occurs when the polymer molecules become interlinked and the shape of the dense mass is fixed.
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Mastication
Mastication and softening are both steps that are usually carried out in batches. The operation of rubber is done either on rubber mills or in a large enclosed mixing machine.
A prominent example of an enclosed machine is the Banbury, which is a registered trademark mixer. It comprises heavy steel counter-rotating paddles in an hourglass-shaped chamber and can hold up to a one-half ton of rubber.
Rubber mills usually hold two large horizontally opposed yet closely spaced steel cylinders, which are almost up to three meters or ten feet long. They are rotated slowly in opposite directions and at somewhat varied speeds.
In the mastication process, the rubber is sheared and softened. This occurs between the wall of the Banbury mixer and the paddles. Another approach is between the two cylinders in the rolling mill.
Mixing
Mixing is a process carried out on machines similar to those used in the mastication process, sometimes immediately after softening. A few various kinds incorporated into the base elastomer by a combined mixing and shearing action are- reactive materials, protective chemicals, oils, and fillers.
An enclosed Banbury-type mixer is known to produce up to one-half ton of the mixed compound within a few minutes. The compound is then sheeted out and coated with a soap release. This prevents the compound from sticking and then stored until used on the steel pallets that can hold up to one ton of rubber.
Shaping
Shaping of the mixture into the desired form occurs in multiple ways. Extruders produce long and continuous products such as tire treads, wire coverings, and tubing in this step. Extruders are used to build several profiles that are cut to the actual length later. Multi-Roll calendars are commonly used to make wide sheeting.
In injection and transfer moulds, the rubber mix is enforced through the channels into a mould chamber to acquire the required shape, cured under immense pressure.
Rubber tires are composed of multiple components- sidewall compound, cord plies, bead wire, tread, inner liner, and belt package. These components are brought and assembled as a complete whole tire. This occurs before the transfer to the curing press.
Curing
Curing is the final process that is carried out in pressurized steel moulds. It is heated either by electricity or steam to the temperature at which the interlinking reaction takes place.
The standard curing conditions are a few minutes at 160 °C (320 °F) temperature. Because heat penetrates slowly, thick articles must be allowed to cure for longer time intervals, several hours, and lower temperatures.
The pressure of one megapascal or 145 pounds per square inch or more is customarily imposed to maintain the wanted shape. This forces the trapped air to dissolve in the compound. A few other methods of curing the rubber mix upon the compound that took shape include steam heating in autoclaves or microwave radiation or passage through a heated bath of molten metal salts or a fluidized bed.
In these methods, the curing process is carried out at near atmospheric pressure.
What is Rubber Chemical Compound Processing?
The blending or compounding of polymers, specifically the elastomers, is done for two reasons. The first reason is to improve the base elastomer's technical properties and the second for an enhanced rubber chemical compound processing behaviour.
Rubber compounding is a standard method that refers to adding specific chemicals to raw rubber material to obtain the desired properties. The well-known chemicals that are often used are cross-linking agents, colourants, and anti-degradants.
The cross-linking agents must establish cross-linking agents to interconnect at the molecular level to improve elasticity and strength.
During the vulcanization or curing process, the unformulated elastomers are linked together. They form networks, increasing the strength and modulus and decreasing the hysteresis. Sulphur is a joint and widely used rubber compounding agent.
Reclaimed rubber is a rubber compounding ingredient. The scrap rubber undergoes a unique process before it can be reused. The rubber obtained as the by-product in the process is known as reclaimed rubber.
Rubber compounding processes using carbon particles often involve intensive mechanical processing.
FAQs on Rubber Processing Explained: Chemistry Principles & Applications
1. What is rubber processing and what does it involve?
Rubber processing refers to the series of industrial methods used to convert raw rubber, either natural or synthetic, into finished, usable products. The primary goal is to enhance its physical properties like elasticity, strength, and durability. The process generally involves four key stages: Mastication (softening the raw rubber), Compounding (mixing with additives), Shaping (like extrusion or calendering), and Vulcanization (curing).
2. What is the vulcanization of rubber?
Vulcanization is a crucial chemical process where raw rubber is heated with a curing agent, most commonly sulphur, along with accelerators and activators. This process creates chemical cross-links between the long polymer chains of the rubber. These cross-links prevent the chains from slipping past each other, transforming the soft, sticky raw material into a strong, elastic, and durable substance that maintains its shape.
3. Why is the vulcanization of rubber a crucial step in its processing?
Vulcanization is crucial because it fundamentally transforms the properties of raw rubber. Without it, natural rubber is:
- Soft and sticky, especially at high temperatures.
- Brittle when cold.
- Has low tensile strength and poor elasticity.
- Swells and dissolves in many organic solvents.
By creating a three-dimensional network structure, vulcanization imparts high elasticity, excellent tensile strength, resistance to temperature changes, and low water absorption, making rubber suitable for demanding applications like vehicle tyres and industrial hoses.
4. How does the molecular structure of natural rubber make vulcanization possible?
The molecular structure of natural rubber, which is cis-1,4-polyisoprene, is key to vulcanization. Each isoprene monomer unit in the polymer chain contains a double bond. These double bonds serve as active chemical sites. During vulcanization, sulphur atoms attack these sites, forming strong covalent cross-links between adjacent polymer chains. Without these reactive double bonds, the sulphur would have no points to attach to, and the cross-linking process could not occur.
5. What are the main stages in manufacturing a rubber product like a tyre?
The manufacturing of a complex rubber product like a tyre involves several distinct stages:
- Compounding: Raw rubber is mixed with various additives in a large mixer called a Banbury mixer. These additives include reinforcing agents like carbon black, softeners like oils, antioxidants, and the vulcanizing system (sulphur and accelerators).
- Shaping: The compounded rubber is processed into different shapes. Extrusion forms the tread and sidewalls, while calendering creates large sheets of rubber-coated fabric for the body.
- Building: These components are carefully assembled on a tyre-building machine to form a 'green' or uncured tyre.
- Curing: The green tyre is placed in a mould, inflated, and heated under pressure. This step, also known as vulcanization, cures the rubber and gives the tyre its final shape, including the tread pattern.
6. What is the primary difference between processing natural rubber and synthetic rubber?
The primary difference lies in their origin and initial preparation. Natural rubber is derived from the latex of the Hevea brasiliensis tree. This latex must first be collected, coagulated (usually with acid), cleaned, and dried to form raw rubber sheets or blocks. In contrast, synthetic rubbers like Styrene-Butadiene Rubber (SBR) are synthesised from petroleum-based monomers through chemical polymerisation reactions. While their initial sourcing is different, the subsequent processing stages like compounding, shaping, and vulcanization are largely similar for both types.
7. What role do additives like carbon black play in rubber processing?
Additives are not mere fillers; they are performance enhancers. Carbon black is the most important additive used as a reinforcing agent. When mixed with rubber, it dramatically increases the material's:
- Tensile strength and stiffness.
- Resistance to abrasion and tearing.
- Hardness and durability.
This reinforcement is critical for products like tyres that must withstand extreme wear and tear. Other additives include zinc oxide and stearic acid, which act as activators to make the vulcanization process more efficient.
